Direct recycling of lithium-ion battery materials: separation and regeneration

Abstract

The rapid development of lithium-ion batteries (LIBs) has led to an urgent need for efficient recycling. Among current recycling strategies, direct recycling emerges as a promising alternative to conventional pyrometallurgy and hydrometallurgy, restoring valuable electrode materials while preserving the original crystal structure and minimizing energy consumption. However, direct recycling remains technically immature and faces challenges in practical cost, material purity, scalability, and process integration. This review summarizes recent advances in direct recycling of LIB active materials, following the workflow of disassembly, separation and regeneration, along with material degradation mechanisms. Various thermal, mechanical, and solvent-based separation methods are discussed in terms of their product size, efficiency, material compatibility, and cost. Regeneration methods, including solid-state sintering, solution-based, and molten salt regeneration, are discussed from their relithiation kinetics, processing conditions, upcycling strategies and electrochemical performance of restored materials. Finally, future perspectives are proposed to address current limitations towards scalable and cost-effective direct recycling technologies for sustainable LIB development.

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Jingjing Liu

Jingjing Liu is a PhD candidate in Materials Science and Engineering at Michigan State University, supervised by Dr Chengcheng Fang. She received her B.E. from Nanjing Tech University in 2019 and her M.E. from Southeast University in 2022. Her research focuses on advanced electrochemical energy systems, including energy storage materials, lithium-ion/metal batteries, fuel cells and advanced characterization techniques.

Ruixin Wu

Ruixin Wu is a PhD candidate in the Department of Mechanical, Aerospace, and Nuclear Engineering at Rensselaer Polytechnic Institute in Troy, USA. He previously worked as a research assistant at Michigan State University in East Lansing, USA, under the supervision of Prof. Chengcheng Fang. He received his M.Sc. in 2022 from the Hong Kong University of Science and Technology, Hong Kong, China. His research focuses on lithium metal anodes for solid-state batteries, interlayer design for anode-free solid-state batteries, solution-based synthesis of sulfide solid-state electrolytes, and the synthesis of sodium-ion battery cathode materials.

Junxiang Liu

Dr Junxiang Liu is a postdoctoral researcher in Argonne National Laboratory. He previously worked as a postdoctoral researcher at Michigan State University, under the supervision of Prof. Chengcheng Fang. He received his PhD from Nankai University and specializes in gaining a fundamental understanding of failure mechanisms and the rational design of electrode–electrolyte interfaces and interphases in rechargeable batteries. He is a recipient of the 2024 Argonne Impact Award and 2025 Outstanding Postdoctoral Performance Award.

Chengcheng Fang

Prof. Chengcheng Fang is an Assistant Professor in the Department of Chemical Engineering and Materials Science at Michigan State University. She received her PhD in Materials Science and Engineering from the University of California San Diego. Her research focuses on next-generation electrochemical energy storage, integrating advanced materials design, scalable manufacturing, and multiscale quantitative characterization across lithium/ sodium-based batteries, solid-state systems, liquid electrolytes, interfaces, and sustainable manufacturing. She is a recipient of the MIT Technology Review Innovators Under 35, the MSU Corporate Connector Award, and the Rising Star of Advanced Materials, recognizing her pioneering contributions and translational impact in battery technology.

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1 Introduction

The application of lithium-ion batteries (LIBs) has seen rapid growth in recent years. Global demand for LIBs is projected to rise from ∼300 GWh in 2020 to ∼2400 GWh by 2030, corresponding to an estimated production volume of 3 to 24 million tons (assuming 1 kWh ≈ 10 kg battery) (Fig. 1a).¹ Among them, ∼12% of the batteries will reach the end-of-life (EOL) between 2025 and 2040, highlighting the urgent need for spent battery recycling.^2,3 With efficient recycling, cost savings could reach $2–15 per kg (of battery). While recycling can reduce greenhouse gas (GHG) emissions by over 50% overall, its benefits are strongly chemistry-dependent, with clear GHG reductions for Ni/Co-rich cathodes but little or no advantage for LFP (Fig. 1b).^1,4,5 However, the actual benefits are highly dependent on recycling techniques and waste management during the recycling process. Maximizing both economic and environmental gains requires optimizing material recovery, reducing energy consumption, and controlling pollutants throughout the recycling workflow.

Fig. 1 (a) The projected LIB demand (Mt) and predicted LIB reaching EOL from 2020 to 2030.^1–3 (b) Savings ($ per kg) and avoided GHG emissions (kg kg⁻¹) by recycling NMC, nickel cobalt aluminum oxide (NCA), and LFP.^1,4,5 (c) The comparison among pyrometallurgy, hydrometallurgy, and direct cycling. Adapted from ref. 5. Copyright 2019 Springer Nature. (d) Illustration of the direct recycling workflow.

Various recycling techniques have been applied to restore battery materials, including pyrometallurgy, hydrometallurgy, and direct recycling.^6,7 Among them, hydrometallurgy and pyrometallurgy are the most widely commercialized. Hydrometallurgy soaks the electrode sheets in acid (sulfuric acid (H₂SO₄), hydrogen chloride (HCl)) and leaches the metal ions into soluble phase. The obtained solution undergoes several chemical precipitation reactions and sintering to restore the active materials.⁸ Hydrometallurgical recycling can achieve a high yield ratio and product purity, but it generates substantial wastewater, necessitating additional treatment steps that increase overall processing costs.⁹ Pyrometallurgy utilizes high temperature sintering to directly convert the entire electrode sheets into metallic alloys, which are then processed through hydrometallurgy to re-synthesize the materials. During the sintering, polymer binder and conductive agent decompose into gas, simplifying the operation of pyrometallurgical recycling but at the expense of high energy consumption, secondary pollution, and significant lithium (Li) loss.¹⁰ In contrast, direct recycling recovers the incumbent materials while preserving the crystal structure,¹¹ thereby increasing recovery efficiency for both Li and transition metal (TM) (Fig. 1c). Furthermore, less energy is consumed compared with pyrometallurgy and hydrometallurgy, providing a promising direction to expand the manufacturing recycling market.

Direct recycling involves multiple steps (Fig. 1d) from EOL battery to recovered battery materials. The spent batteries are first fully discharged and dismantled to separate components such as shells, separators, cathodes, and anodes (electrolytes attached to these parts). Strong binder-induced adhesion between active materials and current collectors greatly hinders peeling efficiency and reduces material purity. Therefore, effective separation, including solid-state, mechanical, or chemical methods, is essential for deactivating these bonds and liberating active materials for subsequent regeneration. Among the active materials, cathodes are the most valuable component to regenerate due to the precious elements, including TM such as nickel (Ni), cobalt (Co), and manganese (Mn), as well as Li, which is particularly important in lithium iron phosphate (LiFePO₄, LFP). However, the complexity of cathode materials, resulted from their diverse compositions,¹² surface chemistries,^13,14 multi-scale degradation mechanisms,^15,16 poses significant challenges for recovering the crystal structure and crystallinity.¹⁷ Thus, effective regeneration of spent cathode materials must address degradation across three distinct scales without sacrificing the original crystal structure: (1) atomic scale, involving issues such as cation mixing, anti-site defects, ion loss, and oxygen (O₂) evolution; (2) nanoscale, characterized by phase transitions, lattice distortions, and resistive interphases; (3) microscale, where mechanical degradation manifests as cracking, pulverization, and delamination. Overcoming these multiscale challenges requires regeneration strategies with thermodynamic-driven relithiation and cracked grain boundary migration, including solid-state sintering, solution-based and molten salt regeneration. During regeneration, lost ions are reintroduced into the lattice to repair atomic vacancies, while targeted thermal treatment promotes the migration and healing of cracked grain boundaries, thereby enhancing structural homogeneity and reducing interfacial resistance.

However, the commercialization of direct recycling remains challenging due to high costs, time-consuming manual dismantling, cathode heterogeneity, and low recovery yields relative to material demand. In this review, we focus on recent advances in direct recycling following the workflow of direct recycling (Fig. 1d). We first systematically discuss separation methods based on their underlying mechanisms and evaluate them from peeling efficiency, separation rate, product purity, structural integrity, and cost. Next, we summarize various strategies for restoring the crystal structure of both cathode and anode materials. An in-depth analysis of failure mechanisms from a material perspective, covering lithium cobalt oxide (LiCoO₂, LCO), lithium nickel manganese cobalt oxides (LiNi_xMn_yCo_1−x−yO₂, x + y + z = 1, NMC), and LFP, is performed, followed by a detailed study on their specific regeneration strategies, process optimization and upcycling approaches. Finally, we revisit the remaining challenges hindering the commercialization of direct recycling and provide an outlook to overcome them.

2 Active material separation

Efficient disassembly and separation are the foundation for a closed-loop, low-cost, and sustainable direct recycling, as they directly influence product purity, regeneration efficiency, and overall process cost. However, current separation processes are stepwise and heavily dependent on manual labor, resulting in low efficiency, reduced yield, and high operational costs.

Batteries at EOL will first be discharged for safety concerns and disassembled into four main parts: shell, separator, cathode, and anode.¹⁸ Liquid electrolytes remain attached to these solid components and are then removed through volatilization at 0–200 °C.¹⁹ Following electrolyte removal, the dried battery components are mechanically shredded and coarsely sorted by size to separate the electrode materials.²⁰ The mixture can contain diverse species due to the complex composition of electrode sheets, including active materials, conductive agents, binders and current collectors. Conductive agents can enhance electronic conductivity while inducing additional impurities during recycling. Binders, typically polymer-based materials like polyvinylidene difluoride (PVDF), provide strong binder adhesion to firmly attach active materials to the current collector. This bonding yields an active material–aluminum (Al) interface shear strength nearly twice its tensile strength,²¹ and the cathode–Al interfaces peel strength is about ten times that of anode–copper (Cu) interfaces.²² Therefore, anode materials can easily be obtained by mechanical methods such as striking and pulverizing,^23,24 whereas cathode separation is far more challenging.²⁵ The crushed pieces are roughly categorized into three size ranges: fragments larger than 2 mm mainly consist of Al foils and separators; those between 0.5 mm and 2 mm include Al and Cu foils; particles smaller than 0.5 mm can be further milled, and those below 0.075 mm are considered as electrode materials containing metal oxides and graphite.^26,27 These fine particles undergo refined separation to improve material purity, which is critical for the further structural recovery of active materials.²⁸

The process may remain impurities in the separated materials, such as electrolyte residues, conductive carbon, binder, current collector fragments, and mixed material phases, which hinder regeneration, introduce structural defects, and compromise the performance of recycled materials. The impact of impurities can be broadly categorized into three aspects. First, surface degradation, where impurities change the local reaction environment and disrupt surface stability. For example, residual graphite can create a reductive environment that counteracts the oxygen-rich conditions required for restoring layered cathodes (e.g., LCO and NMC).²⁹ In LFP recovery, excess carbon may cause partial deoxygenation or dephosphatation, promoting Fe⁰/Fe₂P secondary phases and lithium phosphate (Li₃PO₄) segregation.³⁰ In addition, PVDF decomposition may release hydrofluoric acid (HF), which further corrodes the cathode surface.³¹ Second, compositional contamination, including Al/Cu fragments and cross-contamination between electrodes, can be incorporated into the lattice during high-temperature regeneration, inducing cation disorder and secondary phase formation. For example, Al impurities may form aluminum oxide (Al₂O₃), while Cu contamination above ∼0.6 wt% can result in Li–Co–Cu–O phases and noticeable capacity degradation.^32,33 Third, non-uniform relithiation arises from residual electrolyte species and side reactions (e.g., HF reacting with Li salts), which can form inorganic surface layers such as lithium fluoride (LiF). These layers impede Li⁺ transport and lead to sluggish and spatially non-uniform relithiation.³⁴

To address these challenges, effective impurity management is required throughout direct recycling, involving the coordinated strategies across three different stages. At the upstream stage, impurity introduction is minimized during disassembly and separation by improving delamination efficiency and limiting cross-contamination between cathode and anode materials. Advanced separation techniques, such as high-pressure water delamination, cryogenic treatment, and microwave-assisted processes, have been developed to achieve efficient material separation while reducing impurity incorporation.³⁵ At the midstream stage, purification treatments are often applied prior to regeneration to remove contaminants, including thermal pretreatment and solvent-based processes to decompose binders and eliminate residual organics, thereby improving material purity and accessibility for subsequent regeneration.³⁶ At the downstream stage, strategies are designed to tolerate or compensate for residual impurities during regeneration. For example, eutectic salt systems in molten salt regeneration can promote the in situ removal of Al impurities through selective dissolution or chemical conversion, thereby suppressing their incorporation into the cathode lattice.³⁷ The integration of stage-specific impurity management strategies is essential to balance impurity control, process complexity, and scalability in practical direct recycling. Among them, the effectiveness of separation processes becomes particularly critical, as they directly determine impurity levels and downstream regeneration outcomes.

Therefore, the primary goal of separation is to efficiently isolate high-value components while minimizing structural damage, manual handling, material loss, and processing cost. Numerous separation methods have been developed, yet a systematic summary remains limited. This section focuses on cathode separation techniques, categorized into solid-state, mechanical, and solvent-based methods based on their mechanisms (Fig. 2), and briefly evaluates each from peeling efficiency, separation rate, material purity, structural preservation, and cost.

Fig. 2 Cathode material separation methods. (a) Used cathode consists of cathode materials and Al foil. The cathode materials need to be separated from Al foil before further regeneration. The cathode material separation methods can be classified into mechanical separation, solvent-based separation, and solid-state separation based on their working mechanism. (b) Comparison of the three separation methods in the metrics of efficiency, cost, scalability, purity and complexity.

2.1 Solid-state separation

Solid-state separation utilizes external intensive energy, such as heat and electrified field, to decompose or melt the PVDF. For thermal treatment, it is the most common and straightforward approach to deactivate PVDF, which has a melting point of 170–185 °C and decomposes above 350 °C. By sintering the pre-sorted powder at ∼600 °C for 5 hours, PVDF and conductive carbon are decomposed and gasified,³⁸ leaving pure active material. However, under a long-time heat treatment, the cathode active material may have Li volatilization³⁹ and irreversible crystal structure distortion.^40,41 Moreover, PVDF may release HF gas during decomposition and react with hydroxyl on metal oxide surface,³¹ inducing other impurities. Addressing these issues requires sintering aids to lower the reaction temperature and mitigate fluorine (F)-related impurities. For example, Wang et al.⁴² employed calcium oxide (CaO) as the reaction medium to lower PVDF decomposition temperature to 300 °C. During sintering, CaO acted as a heat reservoir and neutralized acidic hydrogen atoms in PVDF, preventing in situ fluorination reactions. Thus, the formation of C–F bonds further facilitated PVDF decomposition at reduced temperatures. Ross et al.⁴³ reported that excess lithium hydroxide monohydrate (LiOH·H₂O) as Li source can react with F released from PVDF to form stable phases such as LiF, thereby neutralizing harmful F-related byproducts and simultaneously relithiating the cathode material during sintering. Wang et al.⁴⁴ utilized aluminum chloride–sodium chloride (AlCl₃–NaCl) molten salt as the reaction medium which can achieve a separation temperature as low as 160 °C, benefiting from high heat storage during phase transitions. Optimal peeling conditions at 160 °C for 20 minutes resulted in a 99.8 wt% removal of cathode materials. Currently, pyrolysis pretreatment is often the initial step in separation due to its convenience and effectiveness to disable PVDF, typically combined with other mechanical or chemical methods to enhance separation efficiency.⁴⁵

For assistive electrified field, it can also provide intensive energy to delaminate the cathode materials. Gao et al.⁴⁶ demonstrated that a 15 watt laser can strip LCO cathodes by instantaneously decomposing or volatilizing PVDF and conductive carbon with a separation efficiency of 92.16%, resulting in high-purity regenerated cathode materials. However, it may induce severe Li loss and convert LCO into cobalt oxide (Co₃O₄) through localized heating, necessitating further relithiation. High-voltage pulsed discharge is another advanced technique. Tokoro et al.⁴⁷ applied pulsed energy to generate high-temperature plasma and shock waves that decomposed PVDF and reduced adhesion via Joule heating of the Al foil at 19.0 kA and 25 kV, achieving a 93.9% separation efficiency. A limitation of this method is localized melting of Al, causing at least 2.95% Al contamination.

Most solid-state separation techniques can effectively decompose PVDF and conductive carbon, achieving high separation efficiencies. They are relatively simple to operate without advanced equipment. Nonetheless, the complete removal of PVDF and conductive carbon from cathodes remains challenging, limiting material purity. Furthermore, precise control of reaction conditions is essential, as excessive energy input can damage the cathode crystal structure or cause additional Li loss.

2.2 Mechanical separation

Mechanical separation first uses mechanical force to split cathode materials from the current collector and then separates PVDF and active materials according to their intrinsic properties, such as density, strain, and electronic conductivity.⁴⁸ The traditional method of grinding the cathodes into small pieces is typically followed by separation techniques such as magnetic eddy currents or forth flotation,^26,49 particularly in large-scale recycling. For the magnetic eddy current method, it works based on the electrical conductivity differences between organic binders and various nonferrous metals (mainly Al foils and electrode material).⁵⁰ As metallic particles move along a conveyor through a time-varying magnetic field, eddy currents are induced within them. The magnitude of these currents depends on the conductivity of each material⁵¹ and force particles to move in different trajectories, which can be influenced by factors such as detachment angle, feed rate, and the speed of the magnetic roller.^52,53 Magnetic eddy current method offers a high separation rate and low cost for fragments smaller than 0.25 mm. However, it achieves a relatively modest separation efficiency of ∼90%, with material purity remaining ∼85%.⁵⁴

Froth flotation is usually used to remove the graphite phase from coarsely separated materials.^55,56 After grinding, a fine fraction (−0.075 mm) of the mixed LIB particles enables high-purity graphite and cathode active materials.^26,27 Froth flotation can be subsequently applied to separate these components based on their differences in hydrophobicity.⁵⁷ The bubbles generated by flotation agents act as carriers to absorb hydrophobic graphite particles at the solid–liquid–gas interface, separating them from the more hydrophilic cathode materials. Although froth flotation achieves high separation efficiency, it is typically combined with other separation techniques due to limitations to separable species. Moreover, binders can hinder flotation by causing partial floatability of cathode materials thus decreasing the purity,⁵⁶ which can be improved by a pyrolysis pretreatment prior to flotation. For example, Li et al.⁵⁷ introduced a roasting flotation to enrich the metal content in the concentrate. Roasting at 500 °C for 1 hour optimally increased the enrichment ratio while preventing graphite oxidation. Besides the roasting flotation,⁵⁸ other strategies like Fenton reagent-assisted flotation^59,60 and grinding flotation^61–63 can also robust the separation efficiency. Despite its low cost and recent advancements achieving separation efficiencies over 95%, froth flotation inherently suffers from fine particle entrainment by bubbles, which remains a limiting factor for further improvement.⁶⁴

Low-temperature treatments can also accelerate the exfoliation process. Immersing the grinded cathode in liquid nitrogen (N₂) can induce cracks due to different cold-contraction rates between cathode materials and Al foil. Moreover, PVDF exhibits significant stiffening at around −40 °C,^65,66 resulting in brittle behavior and weakening the bonding between cathode materials and the Al foil. Cryogenic grinding has demonstrated impressive peel-off rates. Wang et al.⁶⁷ pretreated cathode materials in liquid N₂ for 5 minutes, followed by cryogenic ball milling, achieving a peel-off efficiency of 87.29% within just 30 seconds of grinding. To further enhance separation efficiency, Liu et al.^66,68 combined cryogenic grinding with froth flotation, achieving a separation efficiency of 89.83% after 9 minutes of grinding. Cryogenic grinding clearly enables rapid and efficient separation, while the high cost of maintaining the low operating temperature (∼235 K) poses a significant barrier to large-scale application.⁶⁹

The separation methods discussed above all involve grinding and blending of spent electrode sheets, leading to the destruction of Al foil and impurities into the cathode material. To maximize the purity of recycled cathode materials, emerging techniques focus on binder deactivation while preserving the integrity of the Al foil. For example, Ji et al.⁷⁰ introduced a pressure washing system without involving chemical reagents or heat treatment, in which high-pressure water delivered through a nozzle generated significant shear stress that can break PVDF adhesion and achieve efficient separation in a short processing time (∼74 min m⁻³) with peel-off efficiency exceeding 90%. The process produces no secondary waste solution, but the high-pressure water can penetrate and damage the Al current collector, thereby increasing the risk of Al impurity.⁶⁹ Instead of intense mechanical force, ultrasonic treatment offers a mild approach that avoids Al foil chipping. He et al.⁷¹ demonstrated that high-power ultrasound induced cavitation and acoustic pressure on the cathode surface. During the rarefaction stage, negative pressure in the solvent generated numerous microbubbles, facilitating the detachment of cathode material. Under the control of suitable temperature, solution, ultrasonic power, ultrasonic time, the peel-off efficiency can be up to ∼99%. However, the separation efficiency significantly decreased below 90% at less than 70 °C or short ultra-sonic duration.

Mechanical separation methods can achieve efficient separation within a relatively short time. Most mechanical methods generate minor secondary pollution, and they are low-cost and easy to operate. Therefore, mechanical separation is a highly promising approach for large-scale recycling and mass production. However, a major limitation is the relatively lower purity of recovered materials, as Al foil fragments and anode active materials can mix with the cathode active materials.

2.3 Solvent-based separation

Solvent-based separation has been widely employed to separate cathode active materials by dissolving either the binder or the current collector in a suitable solvent without damaging cathode active materials. N-Methyl-2-pyrrolidone (NMP) is the commonly used solvent for binder removal, which is chemically stable with PVDF and allows reuse due to its strong dissolving capacity.⁷² However, the separation efficiency is only ∼90%, which is still insufficient for large-scale applications. To improve the separation efficiency of NMP, He et al.⁷¹ introduced ultrasonic agitation and heating to the NMP dissolving process, generating bubbles that peel off cathode materials, raising the separation efficiency to ∼99%. Another major issue of NMP remains its intrinsic toxicity,⁷³ making it less environmentally friendly. Ethylene glycol (EG), a widely used chemical in antifreeze and polymer precursor,^74,75 has been used as a green and mild solvent for PVDF. EG can overcome the strong van der Waals forces of PVDF and efficiently disrupt the hydrogen bonds between PVDF and Al foil.⁷⁶ However, the peeling-off rate is limited by the low solubility and the high viscosity of EG, requiring longer treatment times or elevated temperatures to enable rapid exfoliation.^77,78 Besides NMP and EG, other organic solvents have been developed to achieve efficient separation,⁷⁹ such as N,N-dimethylacetamide (DMAC),⁸⁰ N,N-dimethylformamide (DMF),⁸¹ dimethyl sulfoxide (DMSO)⁸² and ethanol.⁸³ Among them, DMAC has demonstrated the highest efficiency in dissolving PVDF.⁸⁴ Although most organic solvents are green and non-toxic, their costs are much higher than inorganic solutions. Furthermore, additional reducing agents or external assistance, such as high-temperature treatment or mechanochemical activation, are often required to ensure high separation efficiency and peeling rates.^85,86

In hydrometallurgical processes, deep eutectic solvents (DES) are gradually replacing the traditional leaching acid of a mixture of H₂SO₄ and hydrogen peroxide (H₂O₂) for safer, cleaner, and more controlled recycling processes.^75,87,88 DES are homogeneous systems formed by mixing a hydrogen bond donor (HBD) with a hydrogen bond acceptor (HBA),⁸⁹ combining the advantages of both organic and inorganic solvents. A commonly used DES is a mixture of choline chloride (ChCl) as HBA and EG as the HBD, which simultaneously dissolves metal ions and binders, necessitating subsequent active material re-synthesis via precipitation or electrodeposition.^90–93 Recent studies highlight the potential to optimize the composition, mole ratio and leaching conditions of the DES for selectively deactivating PVDF without attacking the active materials.^90,94,95 For example, Wang et al.⁹⁵ achieved a peeling efficiency of ∼99.86% using a DES composed of ChCl and glycerol (molar ratio 2.3 : 1) at 190 °C for 15 minutes. The DES generates multiple hydrogen bonds that stabilize the solvent, while [Cl]⁻ and other hydrogen-donating groups interact with the –CH₂ and –CF₂ groups of PVDF. These interactions weaken the C–F bonds, promoting their oxidation into hydroxyl or carbonyl groups, thereby deactivating PVDF and facilitating efficient material separation. Also, X-ray diffraction (XRD) showed no significant changes in the NMC crystal structure before and after DES treatment, confirming that the DES did not damage the cathode material. Optimized DES can selectively deactivate PVDF, enabling efficient separation of cathode materials from Al foil under mild and environmentally friendly conditions. However, achieving high selectivity without dissolving TM oxides remains challenging, requiring precise control of DES composition and process parameters.

Besides dissolving PVDF, dissolving Al foil is another approach to liberate cathode materials as Al foil readily dissolves in strong alkali.^96–98 Senćanski et al.⁹⁶ demonstrated that boiled 5 M sodium hydroxide (NaOH) solution can dissolve the Al current collector within 30 minutes, achieving a peeling-off rate twice that of NMP leaching and a yield ratio of ∼100%. Also, the byproduct sodium aluminate (NaAlO₂) in the NaOH solution can be further processed to recover Al metal through precipitation. Kong et al.⁹⁹ investigated the effects of operating conditions on the dissolution rate and found that ultrasonic power (288 W) significantly reduced the required NaOH concentration to 10 wt% at 30 °C. However, an additional sintering step is necessary to completely remove residual PVDF and conductive carbon from the solid residue.

The solvent-based separation method is relatively simple and mild with high separation efficiency. Most solvents can operate separation under room-temperature (RT), reducing energy consumption, and can be reused across multiple cycles without complex equipment, making the process economical. Moreover, most solvents can directly deactivate PVDF, preventing HF generation and F diffusion into cathode active materials during subsequent regeneration. However, solvent-based separation may cause secondary pollution and organic solvent residue even after multiple washing.²⁸

Overall, solid-state, mechanical, and solvent-based separation differ in efficiency, cost, and scalability (Fig. 2b). Solid-state methods are simple and scalable but limited in efficiency and impurity control. Mechanical methods are cost-effective and suitable for large-scale processing but often require additional purification. Solvent-based approaches provide higher selectivity and purity but involve higher cost and more complex processing. In practice, different methods are often combined and applied at different stages of the separation to optimize the process.

3 Cathode active material regeneration

Cathode active materials degrade across multiscale during electrochemical cycling. Therefore, cathode active material regeneration is the pivotal step in direct recycling to replenish ion disorder (atomic level), recover phases (nm level) and repair morphology (µm level). In this context, three commonly used terms in battery recycling literature refer to distinct processes. Relithiation refers to the replenishment of Li loss during cycling, aiming to restore the Li stoichiometry. Reconstruction describes the recovery towards the original crystal structure by reversing phase transitions or disorder. Regeneration represents a broader concept that encompasses the recovery of overall electrochemical performance by addressing degradation across multiple scales, including relithiation and reconstruction.

Solid-state sintering, solution-based and molten salt regeneration are three main strategies for cathode materials direct recycling (Fig. 3). This section focuses on LCO, NMC, and LFP, covering their failure mechanisms, regeneration mechanisms, condition optimization, upcycling approaches, as well as their respective advantages, limitations, and future outlooks.

Fig. 3 Multiscale degradation of cathode materials and direct recycling strategies for regeneration. Cathode active materials degrade through structural and chemical changes at atomic, nano, and micron scales over cycling. To restore electrochemical performance, direct recycling approaches aim to replenish the Li loss, reconstruct the crystal structure and repair the morphology. Three regeneration approaches, solid-state sintering, solution-based, and molten salt, are compared based on key metrics including diffusion kinetics, temperature, controllability, scalability, economic viability, and structure restoration.

3.1 Cathode failure mechanisms

3.1.1 LiCoO₂. LCO is one of the most widely used cathode materials in LIBs since it was proposed in 1981.^100,101 LCO crystallizes in an α-NaFeO₂-type layered structure (space group R3m), where Li and Co atoms are coordinated with oxygen to form edge-sharing LiO₆ and CoO₆ octahedra, providing a theoretical capacity of 274 mAh g⁻¹.¹⁰² However, its practical capacity is limited to ∼140 mAh g⁻¹ due to the charging voltage constraint required to suppress Co dissolution.¹⁰³ The overall electrochemical reaction of LCO is described in (eqn (1)). During charging, Li⁺ is progressively extracted from the lithium layers, inducing phase transformation from the initial hexagonal structure through H1, H2, M1 (monoclinic), H3, M2, and finally to the O1 (orthorhombic) phase.¹⁰⁴

LiCoO₂ ⇌ Li_1−xCoO₂ + xLi⁺ + xe⁻ (1)

LCO has an anisotropic lattice contraction along the c-axis during delithiation, with charge compensation by the Co³⁺/Co⁴⁺ redox couple.¹⁰⁵ The increased electronic repulsion between adjacent Co layers distorts the CoO₆ octahedra, reducing the thermodynamic stability of the lattice and weakening Co–O bonds.¹⁰⁶ Under high voltage (>4.4 V), further oxidation of Co beyond Co⁴⁺ becomes thermodynamically unfavorable, leading to partial oxidation of lattice oxygen (O) into O₂ and further destabilization of the structure.¹⁰⁷ Simultaneously, the local structural rearrangements, O deficiency, and interfacial side reactions can reduce nearby Co⁴⁺ into Co²⁺, facilitating lattice gliding and migration into Li layers. This Li/Co cation mixing further promotes the formation of Co₃O₄-type spinel phase, which imposes a higher energy barrier for Li⁺ diffusion, hindering its reinsertion and resulting in Li loss.¹⁰⁸

Such irreversible Li loss during cycling can lead to the overlap of Co t_2g and O 2p energy levels, promoting cathode electrolyte interphase (CEI) formation through electrolyte decomposition driven by surface Co²⁺.¹⁰⁹ The electrolyte further reacts with the cathode surface, particularly at high voltages, forming a resistive CEI layer that consumes Li⁺. Also, electrolyte oxidation at high voltages can generate HF, accelerating Co dissolution.¹¹⁰

The anisotropic lattice strain induced by CoO₆ octahedra distortion leads to particle fracture and microcrack formation,¹⁰⁵ exposing fresh LCO surfaces to the electrolyte, enhancing parasitic reactions and increasing interfacial impedance.¹¹¹ Furthermore, crack propagation causes loss of electronic contact and disrupts Li⁺ transport pathways within particles, ultimately degrading material.

To enable effective direct recycling of LCO, regeneration strategies should focus on restoring Li stoichiometry, repairing structural distortions, and suppressing Co dissolution. Relithiation, oxygen replenishment, and surface reconstruction coatings can help recover lattice integrity and mitigate CEI-related resistance.

3.1.2 LiNi_xMn_yCo_1−x−yO₂. Even though NMC has the same O3-type crystal structure as LCO, it is more complex due to multiple redox pairs of Ni, Mn and Co. The reaction formula is shown in (eqn (2)). The Co is partially substituted with other TM, composing common compositions of LiNi_0.33Mn_0.33Co_0.33O₂ (NCM111), LiNi_0.5Mn_0.3Co_0.2O₂ (NMC532), LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622), and LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811).¹¹² Increasing the Ni content enhances the specific capacity and energy density due to the more Ni²⁺/Ni⁴⁺ redox couples. However, higher Ni concentrations reduce structural and thermal stability,¹¹³ as Ni-rich materials are more prone to cation mixing (Li/Ni disorder), O₂ release and microcracking under high-voltage operation.

LiTMO₂ ⇌ Li_1−xTMO₂ + xLi⁺ + xe⁻ (2)

NMC is a layered structure with Li layers in 3a sites and TM layers in 3b sites stack-up.¹¹⁴ Multiple TM redox reactions are involved during delithiation, resulting in heterogeneous phase transitions from the initial H1 phase to intermediate hexagonal phases with different oxygen arrangements (H2 and H3), and eventually to a non-layered M phase.¹¹⁵ These transitions induce anisotropic lattice shrinkage, leading to the accumulation of microstrain.¹¹⁶ A higher Ni content further amplifies lattice volume changes during cycling, generating increased internal stress that eventually promotes crack formation.¹¹³ Moreover, Ni⁴⁺–O bonds are thermodynamically unstable and tend to undergo reduction to the more stable Ni²⁺ state, accompanied by lattice O₂ release.¹¹⁷ The resultant Ni²⁺ can easily migrate from the crystallographic TM layer (3b sites) into the Li⁺ layer (3a sites) due to the similar atomic radius of Ni²⁺ (0.69 Å) and Li⁺ (0.76 Å),¹¹⁸ leading to cation mixing.

The cation mixing causes irreversible Li loss, reduces the Li⁺ mobility, and leads to the crystal structure collapse from layer to spinel and ultimately to a completely disordered rock-salt phase.^119,120 Meanwhile, Ni-rich cathodes exhibit poor surface stability due to their electronic structure. The t_2g orbitals of Ni and Mn lie closer in energy to the O 2p band than those of Co, resulting in stronger TM–O orbital hybridization and higher covalency.¹²¹ This facilitates the generation of reactive O species, which readily react with electrolyte and result in the formation of a resistive CEI, gas evolution, and dissolution of TM.¹²²

Compared to single crystal LCO cathodes with micron level particles, polycrystal NMC consists of secondary particles formed from nanosized primary crystallites. Heterogeneous phases and anisotropic microstrain make NMC more susceptible to mechanical degradation. The accumulated strain initiates microcracks in the core of primary crystallites, and then propagates across neighboring particles, leading to secondary particle cracking, particularly at high voltage.¹¹⁵ Also, the higher surface area of polycrystalline NMC increases its exposure to the electrolyte and promotes parasitic reactions. Consequently, single-crystalline NMC with long-range, ordered atomic arrangement, has emerged as a more favorable cathode material in recent research.¹²³

Direct recycling of NMC should target the recovery of Li content, minimization of cation mixing, and reconstruction of the layered structure degraded by Ni migration and O loss. Controlled relithiation, high-temperature phase healing, surface re-coating and morphology upcycling are essential to restore electrochemical activity and long-term cycling stability.

3.1.3 LiFePO₄. LFP was first introduced in 1996 and is widely used for its excellent thermal stability, long cycle life, and safety.¹²⁴ LFP has an orthorhombic crystal structure composed of alternating layers of FeO₆ octahedra and PO₄ tetrahedra, featuring a 1D pathway for Li⁺ diffusion along the [010] direction. During delithiation, the Fe²⁺/Fe³⁺ redox is triggered with single phase transition to iron phosphate (FePO₄), leading to a volume change of ∼7% due to the same olivine structure of LFP and FePO₄. The reaction is:

LiFePO₄ ⇌ Li_1−xFePO₄ + xLi⁺ + xe⁻ (3)

The two-phase reaction between LFP and FePO₄ induces spatially anisotropic phase transformations, generating local lattice strain and transient Li vacancies during cycling. Thus, Fe²⁺ from adjacent FeO₆ octahedra can migrate into the vacant Li sites within the 1D diffusion channels, owing to similar ionic radius of Fe²⁺ (0.76 Å) and Li⁺ (0.74 Å).¹²⁵ This cation migration is thermodynamically favorable as accumulated structural defects and non-uniform local phase lower the energy barrier for Li–iron (Fe) site exchange (∼0.7 eV).¹²⁶ The resulting Li–Fe antisite defects partially obstruct Li⁺ diffusion pathways, reduce Li⁺ mobility, distort the crystal structure and progressively degrade the electrochemical performance of the cathode.

Simultaneously, HF produced from parasitic electrolyte decomposition reactions can dissolve Fe²⁺ into the electrolyte. The dissolved Fe²⁺ can migrate toward the anode, destabilizing the solid electrolyte interphase (SEI) and further depleting reversible Li. At the microscale, heterogeneous phase transition of LFP and FePO₄ accumulate mechanical stress despite the small volume change during cycling, eventually leading to particle cracking, decreased interfacial kinetics, and disrupted Li⁺ migration.¹²⁷

Therefore, direct recycling of LFP should address Li–Fe antisite defect removal, Li replenishment, and reactivation of isolated particles caused by Fe migration and mechanical cracking. Thermal or chemical treatments in reductive atmosphere combined with surface modifications can repair Li⁺ diffusion channels and restore electronic percolation for improved LFP performance.

3.2 Cathode material regeneration

3.2.1 Solid-state sintering regeneration. Solid-state sintering regeneration is a straightforward method to recycle spent cathode materials through intensive thermal energy. The degraded cathode materials are homogenously mixed with Li sources such as LiOH, lithium carbonate (Li₂CO₃), and the mixture is treated at high temperatures under a controlled atmosphere to compensate for the Li loss, recover the crystal structure, and repair surface cracks. However, this method is relatively coarse and difficult to precisely control the chemical stoichiometry and particle morphology.
3.2.1.1 Process optimization. Numerous studies have been conducted to optimize the solid-state sintering process, aiming to enhance lithiation efficiency, achieve more precise control over the particle properties, eventually improve the electrochemical performance of regenerated cathodes.
3.2.1.1.1 LCO. Nie et al.¹²⁸ investigated the effects of sintering temperature on the crystal structure of LCO. They found that regeneration at 800–950 °C eliminated Co₃O₄ impurities in spent LCO and facilitated the LCO layered structure, improving Li⁺ intercalation and electrochemical properties as I₀₀₃/I₁₀₄ increased. 900 °C was identified as the optimized sintering temperature since it showed the lowest cation mixing level. However, the crystal structure deteriorated when the regeneration temperature was further raised to 950 °C due to the Li dissolution. Chen et al.³² studied the impacts of Li source amount on recycled LCO structure and cell performance. They revealed that the Co₃O₄ impurities disappeared until the Li/Co ratio was beyond 1.0, and the optimized ratio was 1.0–1.2. The possible reason is that when the Li/Co ratio is below 1, insufficient Li hinders the complete reaction with Co₃O₄ to produce LCO; conversely, when the ratio exceeds 1.2, excess Li remains as inactive residue in the cathode material, compromising the electrochemical performance. Additionally, they examined the effects of Al and Cu contamination and identified the critical concentration of Cu < 0.6 wt% and Al < 0.4 wt% to ensure an optimized cycling performance.
3.2.1.1.2 NMC. Meng et al.¹²⁹ regenerated NMC111 with a mechanochemical activation process before the solid-state sintering. 20% excessive Li₂CO₃ was mixed with spent NMC111 and then put into intensive ball milling. Ball milling at 500 rpm for 4 hours effectively reduced the particle size, shortened the diffusion path, and facilitated Li⁺ diffusion during the sintering at 800 °C. This process resulted in a smaller Li/TM mixing ratio and a more ordered layered crystal structure (Fig. 4a). Montoya et al.¹³⁰ proposed a two-step sintering approach for NMC111 recycling, enabling Li source activation while mitigating insufficient sintering time or temperature. The cathode was regenerated through a two-step lithiation process at 350 °C for 4 hours followed by 650 °C for 4 hours, demonstrating an improved discharge capacity compared to a single-step annealing process at 650 °C for 8 hours. They claimed that single-step heating led to inadequate relithiation or structural degradation, while multi-stage processes enhance material homogeneity by optimizing Li activation and integration. The two-step annealing NMC111 exhibited a discharge capacity of 154 mAh g⁻¹ at C/3, which was higher than that of the one-step annealing sample and comparable to the pristine NMC111. For Ni-rich layered structure recovery, oxygen partial pressure plays a crucial role in restoring the crystal structure. Shi et al.¹³¹ demonstrated that the NMC111 samples recycled in air and pure oxygen had similar Li/Ni cation mixing. However, for NMC532, the sample sintered in air had higher cation mixing than that in oxygen, indicating that low oxygen partial pressure hindered the conversion of the rock-salt and spinel phase to the layered structure, preventing effective lithiation and promoting TM migration.

Fig. 4 Progress on solid-state regeneration. (a) Process optimization. Ball milling of spent NMC and lithium salt before sintering can mechanochemically activate NMC and increase the crystallinity, while the sintering temperature can modify particle size. Reprinted with permission from ref. 129. Copyright 2019 Elsevier. (b) Morphology control. Secondary agglomerate NMC can be broken down into nanoparticles after ball milling activation and further upgraded into single crystal after sintering. Reprinted with permission from ref. 136. Copyright 2022 American Chemical Society. (c–e) Upcycling. (c) Diffusion energy barrier of F in bulk spent-LCO. Reprinted with permission from ref. 138. Copyright 2024 Wiley. (d) Wavelet-transform EXAFS results of spent LFP and regenerated LFP. Reprinted with permission from ref. 140. Copyright 2024 Wiley. (e) Infrared spectroscopy (IR) map of the organic lithium salt Li₂DHBN from thermogravimetry (TG)-IR coupling measurements. Reprinted with permission from ref. 141. Copyright 2023 Springer Nature. (f) RHT techniques. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and corresponding fast Fourier transform (FFT) pattern of the RHT-regenerated LCO. Also shown are a schematic of the roll-to-roll ultrafast cathode repair method and a comparison of energy consumption and operation time among different regeneration methods. Reprinted with permission from ref. 144. Copyright 2023 American Chemical Society.

3.2.1.1.3 LFP. Song et al.¹³² recovered LFP from the mixture with fresh LFP. The impurities of FePO₄, iron oxides (FeO and Fe₃O₄) were detected from the spent LFP, and they disappeared when sintering temperature increased to 700 °C. However, when the temperature increased to 800 °C, LiF peaks appeared alongside the main LFP phase, primarily due to the generation of HF from the incomplete decomposition of PVDF during separation. Also, excessive fresh LFP can improve the crystallization of FePO₄ and ferric oxide (Fe₂O₃), eliminating impurity phases and inhibiting PVDF decomposition. Except for sintering temperature control, another method to prevent HF is to completely pyrolyze PVDF before the Li supplementary. Liang et al.¹³³ applied the preheat on spent LFP at 450 °C for 2 hours before regeneration. The Fourier transform infrared spectroscopy (FTIR) revealed that the peaks corresponded to the CF₂ stretching vibration of PVDF disappeared after heat treatment, while the main peaks of LFP were stable, indicating successful pyrolysis of PVDF and preservation of LFP olivine structure.
3.2.1.2 Single crystal. For particle size control, single crystal materials have become a prominent focus of current research. Conventional polycrystalline cathodes are prone to structural degradation and cracking as charged to high voltage, leading to capacity fading and reduced battery lifespan.¹³⁴ Single crystal cathodes, characterized by the absence of grain boundaries, exhibit enhanced structural integrity and improved cycling stability. Thus, more studies focus on single crystal cathodes recycling for high-voltage application.

The direct recycling of single crystal cathode materials presents several challenges, primarily due to Li⁺ diffusion kinetic limitations, structural degradation, and phase instability. Thus, it requires larger force to overcome diffusion barrier while maintaining the crystal structure. In solid-state sintering regeneration, controlling temperature, duration, and Li source are crucial for uniform relithiation. Guan et al.¹³⁵ mixed single crystal NMC631 with Li₂CO₃ in Li/TM = 1.05 and sintered at 950 °C for 11 hours. The regeneration process for single crystal cathodes is similar to that of conventional polycrystalline cathodes, but they require significantly higher temperatures and longer sintering times due to the longer Li⁺ diffusion paths in single crystals. A recent approach involves upcycling polycrystals into single crystals by adding additional mechanical force. Dong et al.¹³⁶ combined ball mailing, a mechanical activation process, with solid-stating sintering and achieved single crystal NMC532. As shown in Fig. 4b, secondary polycrystalline particles were broken down into smaller primary particles during the mechanical activation process. Subsequent sintering at 500 °C for 5 hours yielded single crystal NMC532 particles, with the pretreatment step facilitating their formation by lowering both the necessary sintering temperature and time. They further applied a lithium aluminate (LiAlO₂) coating on the surface of the recycled cathode, increasing the discharge capacity from 121.96 mAh g⁻¹ to 132.52 mAh g⁻¹ at 5C.

While single crystal cathodes offer superior structural stability and cycling performance compared to their polycrystalline counterparts, their direct recycling poses significant challenges due to limited Li⁺ diffusion and structural rigidity. Overcoming these barriers requires additional energy to promote the grain boundary migration during solid-state sintering. Thus, the direct recycling of single crystal particles is not achieved solely through conventional methods but often involves elements of upcycling to enhance material performance and restore functionality.

3.2.1.3 Upcycling. Solid-state sintering involves high temperature reactions and ion diffusion. The structure reconstruction can be used for synchronous material modification to further improve the cathode material properties and electrochemical performance.
3.2.1.3.1 Doping. Doping is commonly utilized towards increasing crystal structure stability and enhancing high-voltage performance. Wang et al.¹³⁷ reported magnesium (Mg) and Al co-doping into spent LCO, acting as ‘pillars’ between the CoO₆ layers and improving its structural stability. High-resolution transmission electron microscopy (HRTEM) revealed the occupation of Mg and Al mainly in the Li sites and the induced lattice disorder. The intercalation of Mg/Al atoms expanded the interlayer spacing between Co layers from 0.256 to 0.295 nm, mitigating the lattice contraction by H1-3 phase transition as charging to high voltage over 4.55 V. The regenerated LCO achieved 220.4 mAh g⁻¹ at C/5 with 79.7% capacity retention after 300 cycles. Besides cation doping, halogens¹³⁸ and large molecular groups¹³⁹ are also employed during the solid-state sintering. Lei et al.¹³⁸ created gradient F-doped subsurface in LCO. Existing oxygen vacancies in spent LCO decrease the diffusion energy barrier of F from 1.73 eV to 0.61 eV (Fig. 4c) and promote rigid CoO₅F phase in CoO₆ octahedra, which enhances the structural stability of F-gradient LCO. The regenerated LCO with 0.2 wt% F dopant achieves 154.4 mAh g⁻¹ at 5C with 88.3% capacity retention after 500 cycles (3–4.5 V). Fan et al.¹³⁹ introduced stoichiometric ratio metal source and ammonium dihydrogen phosphate (NH₄H₂PO₄) into spent NMC532. They reported that the doping of large molecular groups PO₄³⁻ enlarged the interlayer spacing of NMC532, thus increasing the electrostatic repulsion between TMO₆ octahedrons, stabilizing the crystal structure and facilitating Li⁺ transport. The optimized dopant NMC532 with 0.02 M PO₄³⁻ reached 170.7 mAh g⁻¹ at 1C and maintained 83.2% capacity retention after 300 cycles.
3.2.1.3.2 Coating. Coating is an efficient way to improve structure stability and inhibit side reactions. Cao et al.¹⁴⁰ used tannic acid to form a compact coating layer with abundant functional groups, creating a mildly acidic environment to reduce Fe³⁺ and recover the anti-sites vacancies. Another additive thiourea provided nitrogen (N) and sulfur (S) source as dopant in the carbon coater to enhance electronic conductivity. The extended X-ray absorption fine structure (EXAFS) results showed that the regenerated LFP had a shorter Fe–O bonds length than the spent LFP, indicating a stronger Fe–O bonding strength and more stable structure (Fig. 4d). The regenerated LFP reached 141.3 mAh g⁻¹ at 1C and delivered 500 cycles with 85% capacity retention, surpassing the commercial LFP.
3.2.1.3.3 Multifunction Li source. Multifunctional Li salts have emerged as a more cost-effective and efficient alternative to monofunctional Li sources or additives in direct recycling. Ji et al.¹⁴¹ developed a multifunctional organic lithium salt 3,4-dihydroxybenzonitrile dilithium (Li₂DHBN) to regenerate spent LFP cathodes, which can effectively replenish Li vacancies and create a reductive atmosphere to prevent Fe³⁺ formation. The salt decomposed into two inorganic salts to replenish Li loss (Fig. 4e): the initial decomposition occurs at ∼200 °C, transforming Li₂DHBN into Li₂CO₃. At temperatures above 700 °C, the salt further decomposed and formed lithium oxide (Li₂O). Such decomposition also created a conductive carbon layer on the surface, enhancing Li⁺ and electron transport. The restored LFP showed excellent cyclability at high charging rate and fast redox kinetics in low temperature (−20 °C), enabling 110 mAh g⁻¹ at 5C with a high retention of 88% after 400 cycles.
3.2.1.4 Rapid heat treatment (RHT). Conventional solid-state sintering spans several hours to recover cathode materials. Therefore, RHT techniques, such as high temperature shock (HTS), flash Joule heating (FJH), carbothermal shock (CTS) have been employed in direct recycling to expedite regeneration, significantly reducing processing time, minimizing energy consumption, and improving material properties. RHT applies abrupt high-temperature pulse, including laser heating, Joule heating, plasma or microwave heating, to deliver substantial energy within seconds to overcome Li⁺ migration barriers.¹⁴² It has been demonstrated that controlled RHT processes can effectively restore the crystal structure of NMC, LCO and LFP cathodes, achieving near-original electrochemical properties.^125,143,144 Their key features comparison is shown in Table 1.

Table 1 Comparison between three RHT recycling methods

Feature	High-temperature shock	Flash Joule heating	Carbothermal shock
Energy source	Preheated surface	Joule heating	Carbon-based reducing agent + thermal shock
Peak temperature	1500–2500 °C	2500–3000 °C	1500–3000 °C
Duration	Seconds to minutes	Milliseconds to seconds	Milliseconds to seconds
Atmosphere	Inert gas or vacuum
Scalability	More scalable	Small samples	Moderate

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HTS heats materials indirectly using a preheated surface to induce a rapid thermal shock, such as hot metal plate or plasma. Guo et al.¹²⁵ reported that HTS enabled rapid atomic-scale Li–Fe reordering in LFP, restoring structure and enhancing electrochemical performance for sustainable battery regeneration. Simulation results revealed that the Li and Fe diffusion energy barriers depended on the concentration of Li vacancies: at low vacancy levels, Fe reordering occurred readily with a lower energy barrier (0.19 eV), whereas high vacancy concentrations induced severe lattice distortion due to Fe³⁺ formation, raising the energy barrier to 1.12 eV. Thus, the pre-lithiation before HTS promoted defect repair and Li–Fe reordering by lowering migration barriers. Rapid heating supplied sufficient kinetic energy to overcome the energy barrier for Li–Fe antisite defect reordering within seconds while minimizing undesired side effects, such as impurity diffusion and phase degradation. The 1.6 Ah pouch cells made of the HTS-regenerated LFP remained 97.56% capacity retention after 300 cycles at 1C cycling.

FJH applies direct electrical current to a conductive material, rapidly increasing its temperature through Joule heating. Chen et al.¹⁴³ reported a FJH method combined with magnetic separation to recover fresh cathode materials from spent electrodes, followed by solid-state relithiation. The FJH rapidly removed the binder, CEI, conductive carbon, and metal impurities while largely preserving the cathode crystal structures and hierarchical morphologies, enabling their further reconstruction into new electrodes. The FJH process achieved a high metal recovery yield of ∼98% within only milliseconds. This approach has been demonstrated to be feasible for layered LCO, NMC, and graphite materials, with the regenerated cathodes exhibiting cycling performance and capacities comparable to those of commercial counterparts.

CTS utilizes a carbon source as the conductive agent in a high-temperature shock process to transport direct current. Yin et al.¹⁴⁴ demonstrated one-step regeneration of spent LCO cathodes by CTS within 8 seconds. The TEM images revealed a heterogeneous mixture of Li_1−xCoO₂ and Co₃O₄ in the spent LCO (Fig. 4f), whereas the spinel Co₃O₄ phase was fully converted into an ordered LCO layered structure after ultrafast CTS treatment. The recovered LCO delivered 123.9 mAh g⁻¹ at C/5, remaining 80% capacity after 300 cycles. They further proposed a roll-to-roll concept for large-scale cathode recycling (Fig. 4f), in which a mixed precursor comprising spent cathode material and Li source undergoes high-temperature treatment within a heating zone, rapidly restoring the crystalline structure and electrochemical performance. Enabled by RHT, this approach offers strong compatibility with existing industrial protocols and shows great potential for ultrafast, continuous roll-to-roll regeneration of cathode materials.

3.2.1.5 Summary. Solid-state sintering is a scalable direct recycling method for restoring cathode structures but requires high temperatures, long processing times, and precise parameter control. Key challenges for industrial adoption include process optimization, cost reduction, and consistent material quality. Advanced RHT enables replenishment within seconds but risks structural uncontrollability, oxygen deficiency, and irreversible cation mixing. While defective engineering can benefit materials like LFP, it is less suitable for layered cathodes unless delicate precursor design. Electrochemical performance reported across different studies is evaluated under varying materials, voltage windows, C-rates, cell configurations, and testing protocols, which limit direct quantitative comparability. Accordingly, the data summarized in Tables 2 and 4–6 are intended to highlight general trends and underlying mechanisms rather than provide strict quantitative benchmarks. To enhance transparency, the corresponding testing conditions are provided alongside the reported performance metrics. Ongoing research should integrate complementary techniques to simultaneously enhance product quality and lower costs.

Table 2 Summary of the solid-state sintering method for cathodes direct recycling

Recycling method	Cathode material	Recycling process	Electrochemical performance	Ref.
Solid-state sintering	LCO	Mixed with Li2CO3 (Li/Co = 1.05), 900 °C, 12 h, air	• 152.4 mAh g^−1@30 mA g^−1, 3–4.3 V• 80 cycles@98.35% capacity retention	128
Solid-state sintering	LCO	Mixed with Li2CO3 (Li/Co = 1.0), 850 °C, 10 h	• 150.3 mAh g^−1@C/10, 3–4.3 V• 100 cycles@93.21% capacity retention	32
Solid-state sintering	NMC333	(1) Ball milling, 500 rpm, 4 h, mechanochemical activation	• 165 mAh g^−1@C/5, 2.5–4.3 V• 100 cycles@80% capacity retention	129
		(2) Mixed with 20% Li2CO3, 800 °C, 4 h
Solid-state sintering	NMC333	(1) Mixed with LiOH·H2O, 350 °C, 4 h	• 188 mAh g^−1@C/3, 3–4.3 V• 50 cycles@96% capacity retention	130
		(2) 650 °C, 4 h
Solid-state sintering	NMC111	Mixed with 5% Li2CO3, 850 °C, 12 h, air	• 153.3 mAh g^−1@1C, 3–4.3 V• 100 cycles@81.8% capacity retention	131
Solid-state sintering	NMC532	Mixed with 5% Li2CO3, 850 °C, 12 h, O2	• 153.3 mAh g^−1@1C, 3–4.3 V• 100 cycles@84.93% capacity retention	131
Solid-state sintering	LFP	Mixed with 10% fresh LFP, 700 °C, 8 h	• 142 mAh g^−1@C/10, 2.5–4.1 V• 100 cycles@95.07% capacity retention	132
Solid-state sintering	LFP	(1) 450 °C, 2 h	• 139 mAh g^−1 at C/5, 2.0–4.3 V• 100 cycles@94.96% capacity retention	133
		(2) Mixed with Li, Fe, phosphate (P) salts, 650 °C, 10 h, N2
Solid-state sintering	LFP	(1) 450 °C, 2 h, Ar	• 147 mAh g^−1 at 1C, 2.5–4.3 V• 500 cycles@90% capacity retention	145
		(2) Mixed with Li2CO3, C6H12O6, C2H6O, ball mill at 200 rpm, 2 h
		(3) 700 °C, 6 h, Ar/H2
Solid-state sintering	LFP	(1) Mixed with lithium stearate, and creatine, 250 °C, 3h; 700 °C, 5 h	• 146.2 mAh g^−1 at 1C, 2.5–4.3 V• 200 cycles@95.83% capacity retention	146
Solid-state sintering	Single crystal NMC631	(1) Mixed with 5% Li2CO3	• 184.6 mAh g^−1 at 1C, 3–4.4 V• 200 cycles@95.12% capacity retention	135
		(2) 950 °C, 11 h
Solid-state sintering	Single crystal NMC532	(1) Ball milling, 450 rpm, 6 h	• 167.5 mAh g^−1 at 1C, 3–4.5 V• 100 cycles@99.7% capacity retention	136
		(2) Mixed with 15% Li2CO3, 950 °C, 10 h, air
Solid-state sintering	Mg/Ti doped, Al-coated LCO	(1) Mixed with 1% magnesium oxide (MgO), 1% titanium dioxide (TiO2), 3%Li2CO3, ball milling, 200 rpm, 1 h; 100 °C, 15 h; press into pellet, 1000 °C, 10 h, air	• 174.29 mAh g^−1 at 1C, 3–4.5 V• 200 cycles@96.77% capacity retention	147
		(2) Ultrasonic treatment of 0.25 wt% aluminum isopropoxide (C9H21AlO3) in 50 mL ethanol, 30 min; stir with deionized (DI) water (4 : 1 v/v) and 4 g precursor from last step, 6 h
		(3) Water bath, 80 °C, stirred, 10 h
		(4) 650 °C, 3 h
Solid-state sintering	F doped LCO	Mixed with 10% Li2CO3 and 0.2% ammonium fluoride (NH4F), ball milling, 400 rpm, 2 h; 450 °C, 5 h, 900 °C, 8 h, air	• 154.4 mAh g^−1 at 5C, 3–4.5 V• 500 cycles@88.3% capacity retention	138
Solid-state sintering	PO4^3− doped NMC532	(1) Mixed with stoichiometric LiOH, nickel oxide (NiO), manganese oxide (MnO2) and 0.02 M NH4H2PO4, 450 °C, 5 h, O2	• 170.7 mAh g^−1 at 1C, 2.7–4.3 V• 300 cycles@83.2% capacity retention	139
		(2) 850 °C, 15 h, O2
Solid-state sintering	F-doped carbon layer LFP	Mixed with 5% Li2CO3 and 2.5 wt% PVDF, 650 °C, 2 h, N2	• 141.5 mAh g^−1 at 1C, 2.5–4.2 V• 100 cycles@99.6% capacity retention	148
Solid-state sintering	N/S-doped carbon layer LFP	(1) Mixed with thiourea, tannic acid and DI water, stir, 2 h then heat 140 °C, 7 h	• 141.3 mAh g^−1 at 1C, 2.5–4.3 V• 500 cycles@85% capacity retention	140
		(2) Centrifuged, collected, 80 °C dried
		(3) Mixed with glucose and ethyl alcohol, grind, 2 h
		(4) 600 °C, 5 h, hydrogen (H2)/argon (Ar)
Solid-state sintering	Carbon coated-LFP	Mixed with 5 wt% Li2DHBN, 800 °C, 6 h, H2/Ar	• 110 mAh g^−1 at 5C, 2.5–4.3 V• 400 cycles@88% capacity retention	141
Solid-state sintering	Na/S-doped LCO	Mixed with lithium lignosulfonate, 850 °C, 6 h, air	• 150 mAh g^−1 at 10C, 3–4.6 V• 300 cycles@89.72% capacity retention	149
High temperature shock	F-doped carbon layer LFP	(1) React with 0.08 M LiOH solution, 45 °C, 0.5 h	• 142 mAh g^−1 at 1C, 2.5–3.7 V• 300 cycles@97.56% capacity retention	125
		(2) Mixed with DI water and 1% wt Li2CO3 (Li : Fe : P = 1.02 : 1 : 1), 500 rpm
		(3) Dry at 80 °C, 12 h
		(4) HTS, 720 °C, 20 s (100 V/72 A), Ar
Flash Joule heating	LCO	(1) Mixed with conductive agents	• 132.09 mAh g^−1 at C/5, 3–4.2 V• 100 cycles@96.9% capacity retention	143
		(2) Loaded into a quartz tube (graphite rods and copper wool as the electrodes and spacers)
		(3) Flash Joule, ∼2850 K, 300 ms (120 V), Ar
Carbothermal shock	LCO	(1) Mixed with excessive Li2CO3 and alcohol, 6 h	• 123.9 mAh g^−1 at C/5, 3–4.3 V• 300 cycles@80.71% capacity retention	144
		(2) Pressed into pellet after drying at 80 °C
		(3) Rapid Joule heating, 1440 K, 8 s, Ar
		(4) Washing and dry

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3.2.2 Solution-based regeneration. Solution-based regeneration is a cost-effective method for recovering spent cathode materials.¹⁵⁰ Unlike hydrometallurgy, it barely emits toxic gases or requires prolonged high-temperature sintering. Instead, it treats the degraded cathode materials in Li-containing solution to replenish Li. The process typically consists of two main steps: relithiation, which supplies lost Li, followed by post-annealing to restore the crystal structure. Hydrothermal, ionic liquid (IL), and redox mediator (RM) regeneration are all classified as solution-based regeneration. Their differences are compared in Table 3. These techniques enable precise control over particle size and morphology by tuning various parameters, such as solution temperature, pressure, reaction time, solution pH, solvent, concentration, and additives. However, the scalability of solution-based regeneration remains limited by its inherently batch-based nature, which poses challenges for continuous, large-scale production.¹⁵¹

Table 3 Differences between three solution-based regeneration methods

	Hydrothermal	Ionic liquid	Redox mediator
Lithiation mechanism	Direct chemical lithiation via Li^+ in aqueous solution	Direct lithiation in non-aqueous ionic liquid	Indirect lithiation via redox couple mediator and electron shuttles
Solvent	Water	Ionic liquid	Organic or inorganic solvent with redox-active species
Temperature	140–200 °C	25–150 °C	25–100 °C
Post-treatment	Wash and post-annealing
Cost	Low	High	Moderate

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Table 4 Summary of the solution-based regeneration for cathodes direct recycling

Recycling method	Cathode material	Recycling process	Electrochemical performance	Ref.
Hydrothermal	LCO	(1) 5.0 M LiOH solution, vapor pressure, ∼3.0 °C min^−1, 200 °C, 20 h	• 144.0 mAh g^−1@C/5, 3–4.3 V• 40 cycles@92.2% capacity retention	152
		(2) Wash, dried at 80 °C, 10 h
Hydrothermal	LCO	(1) 2 M LiOH solution, ultrasonic (work 5 s, stop 2 s), 120 °C, 6 h	• 132.8 mAh g^−1@C/5, 2.7–4.3 V• 40 cycles@96.61% capacity retention	153
		(2) Wash, dried at 80 °C
Hydrothermal	LCO	(1) 4 M LiOH or a 1 M LiOH + 1.5 M Li2SO4 solution, 200 °C, 4 h, wash, dry	• 148.2 mAh g^−1@1C, 3–4.3 V• 100 cycles@91.2% capacity retention	154
		(2) 850 °C, 5 °C min^−1, 4 h
Hydrothermal	LCO	(1) 4 M LiOH solution, microwave-assistance, 220 °C, 45 min, centrifugation, wash, dry	• 151.5 mAh g^−1@1C, 3–4.3 V• 100 cycles@94.5% capacity retention	172
		(2) 800 °C, 5 °C min^−1, 4 h
Hydrothermal	Li1.4Al0.4Ti1.6(PO4)3-coated LCO	(1) 4 M LiOH solution, 220 °C, 4 h, wash, dry	• 166 mAh g^−1@1C, 3–4.4 V• 100 cylces@93.0% capacity retention	155
		(2) Mixed with LATP, ball milling
		(3) 800 °C, 5 °C min^−1, 4 h
Hydrothermal	Ni–Mn co-doping LCO	(1) 1 g LCO mixed with 0.51 mmol of nickel and manganese acetates (Mn : Ni = 3 : 1) in 10 mL DI water	• 160.23 mAh g^−1@1C, 3–4.35 V• 100 cylces@91% capacity retention	173
		(2) Add 1.275 mmol of urea as precipitation agent
		(3) Mix, 30 min
		(4) Mixed with 20 mL solution containing 3.0 M LiOH and 3.0 M KOH, 190 °C, 16 h
		(5) Centrifugation, 900 °C, 4 h, air
Hydrothermal	LiNi0.15Mn0.15Co0.7O2	(1) 4 M LiOH solution, 220 °C, 2 h, stir at 300 rpm, wash, dried at 80 °C, 24 h	• 150.7 mAh g^−1@C/3, 2.8–4.4 V• 50 cylces@91.2% capacity retention	157
		(2) 850 °C, 4 h, air
Hydrothermal	NMC111	(1) 4 M LiOH solution, 220 °C, 4 h, wash	• 158.4 mAh g^−1@1C, 3–4.3 V• 100 cylces@77.4% capacity retention	131
		(2) Mixed with 5% Li2CO3, 850 °C, 5 °C min^−1, 4 h, O2
Hydrothermal	NMC532	(1) 4 M LiOH solution, 220 °C, 4 h, wash	• ∼155 mAh g^−1@1C, 3–4.3 V• 100 cylces@82.8% capacity retention	131
		(2) Mixed with 5% Li2CO3, 850 °C, 5 °C min^−1, 4 h, O2
Hydrothermal	NMC532	(1) 4 M Li^+ solution, 225 °C, 8 h, wash, dry at 120 °C in vacuum	• 157 mAh g^−1@C/10, 3–4.2 V• 1500 cylces@80% capacity retention (C/2, 8 Ah pouch cell)	174
		(2) 800 °C, 5 °C min^−1, 3 h, air
Hydrothermal	NMC622	(1) 2 g of harvested material, 75 mL of concentrated 4 M LiOH and 4 mL of 30% H2O2, 125 °C, 30 min, dry at 120 °C in vacuum	• 175 mAh g^−1@C/10, 3–4.2 V	174
		(2) 800 °C, 5 °C min^−1, 3 h, O2
Hydrothermal	NMC111	(1) 0.1 M LiOH + 3.9 M KOH solution, 220 °C, 2 h, wash, dried at 120 °C, vacuum	• 150.5 mAh g^−1@C/3, 3–4.3 V• 50 cylces@93.3% capacity retention	159
		(2) Mixed with 5% LiOH, 750 °C, 4 h
Hydrothermal	NMC532	(1) 3 M LiOH solution, dry at 150 °C, 4 h	• 136.7 mAh g^−1@1C, 2.8–4.25 V• 100 cylces@90% capacity retention	175
		(2) 810 °C, 5 °C min^−1, 3 h
Hydrothermal	NMC532	(1) 200 mg of spent NMC532, 20 mL DI water, 8 mL ammonium hydroxide, 180 °C, 6 h, wash, dry at 60 °C in vacuum	• 152 mAh g^−1@C/2, 2.5–4.3 V• 100 cylces@90% capacity retention, 200 cylces@76% capacity retention	158
		(2) Mix with LiOH, 500 °C, 5 °C min^−1, 2 h; 850 °C, 5 °C min^−1, 10 h, air
Hydrothermal	NMC622	(1) 200 mg of spent NMC532, 20 mL DI water, 8 mL ammonium hydroxide, 180 °C, 6 h, wash, dry at 60 °C in vacuum	• 167 mAh g^−1@C/2, 2.5–4.3 V• 100 cylces@90% capacity retention, 250 cylces@75% capacity retention	158
		(2) Mix with LiOH, 500 °C, 5 °C min^−1, 2 h; 800 °C, 5 °C min^−1, 10 h, air
Hydrothermal	Nb-doped NMC532	(1) 4.83 g of spent NMC532, 60 mL of 2 M LiOH, 150 °C, 12 h, wash, dry	• 150 mAh g^−1@C/5, 2.5–4.3 V• 200 cylces@75.3% capacity retention	176
		(2) Ball mill in anhydrous ethanol with Li2CO3 and Nb2O5 at 400 rpm, 6 h
		(3) 500 °C, 8 h, air
Hydrothermal	LFP	(1) 5 g of spent LFP, L/S = 6 mL g^−1, Li^+ (Li salt Li2SO4·H2O) = 12 g L^−1, 1.0 mL of hydrazine hydrate, 200 °C, 3 h	• 141.9 mAh g^−1@1C, 2.2–4.2 V• 200 cylces@98.6% capacity retention	162
		(2) Wash, dry at 80 °C, 10 h
Hydrothermal	LFP	(1) 3 g of spent LFP mixed with 30 mL 0.08 M tartaric acid, 20 mL 0.2 M Li^+ (Li salt LiOH·H2O) solution, 200 °C, 3 h, wash, dry	• 114.96 mAh g^−1@5C, 2.0–4.3 V• 200 cycles@94.7% capacity retention	163
		(2) Concentrated at 95 °C; saturated sodium carbonate (Na2CO3) was added to recover Li
		(3) 700 °C, 2 h, Ar
Hydrothermal	LFP	(1) Mixed with 0.2 M LiOH and 0.08 M citric acid solution, 80 °C, 5 h, wash, dry	• 102 mAh g^−1@10C, 2.5–3.8 V• 300 cycles@∼92% capacity retention	164
		(2) Mixed with excess 4% Li2CO3, 600 °C, 5 °C min^−1, 2 h, N2
Hydrothermal	LFP	(1) 5 g of spent LFP, L/S = 6 mL g^−1, Li^+ (Li salt Li2SO4) = 9 g L^−1, 6 g of Na2SO3, 200 °C, 6 h	• 135.9 mAh g^−1@1C, 2.2–4.2 V• 100 cycles@>99% capacity retention	166
		(2) Wash, dry, 8 h
Hydrothermal	LFP	(1) 1 g of spent LFP, 1 mmol Li2C2O4, 5 mL DI water, 100 °C, 4 h	• 106 mAh g^−1@10C, 2.2–4.1 V• 400 cycles@79.34% capacity retention	177
		(2) Wash, dry, 80 °C, 12 h (LFP-Hydro)
		(3) 1 g of LFP-Hydro, 50 mg tartaric acid in 10 mL DI water, stir, 15 min, wash, dry, 80 °C, 12 h
		(4) 600 °C, 4 h, Ar
Hydrothermal	Cu-doped LFP	(1) Mixed with 0.05 M LiOH, 0.025 M Na2SO3, 150 °C, 24 h, filter, wash, dry	• 130.03 mAh g^−1@C/5, 2.5–3.65 V• 100 cycles@92.36% capacity retention	165
		(2) Mixed with cupric nitrate (Cu(NO3)2) (3 wt%) and glucose (5 wt%), 600 °C, 4 h, N2
Hydrothermal	N-doped carbon layer LFP	(1) Spent LFP (100 mg) mixed with PVP and filled with 15 mL ethanol, lithium acetate (CH3COOLi) (15 mg) was added after stirring for 5 h	• ∼95 mAh g^−1@10C, 2.5–4.3 V• 1000 cycles@76.8% capacity retention	167
		(2) 180 °C, 5 h, centrifuged, collected, vacuum drying at 120 °C for 12 h
		(3) 700 °C, 5 °C min^−1, 5 h, Ar
Hydrothermal	Lignin-chain zinc (Zn) complex composite LFP	(1) Mixed with LiOH solution according to Li loss (ICP Li : Fe : P = 1.05 : 1 : 1), stirred for 0.5 h at 25 °C	• ∼40 mAh g^−1@5C, 1.8–3.9 V (full cell with gr)• 1000 cycles@78.7% capacity retention	168
		(2) Mixed with glucose (10 wt%), washed and dried at 80 °C
		(3) 650 °C, 5 h, Ar
Hydrothermal	LFP/microwave-reduced graphene oxide (MWrGO)	(1) Graphite, freeze-dried overnight, placed in a microwave reactor, 3–5 s, 1000 W to get MWrGO	• 161.4 mAh g^−1@C/5, 2.5–4.2 V• 100 cycles@94.9% capacity retention	178
		(2) MWrGO (−) placed in 0.01 wt% poly(diallyldimethylammonium chloride) (PDDA), sonication
		(3) 5 wt% MWrGO (+) and spent LFP mixed with LiOH, L-ascorbic acid solution (molar ratio of 1 : 1 : 0.5), stir for 1 h, transferred into Teflon microwave reactors, 150 °C, 60 min
Hydrothermal	LFP/reduced graphene oxide (RGO)	(1) 1 g spent graphite and 0.5 g sodium nitrate (NaNO3) added into a three-necked flask with 23 mL concentrated H2SO4 in ice-bath	• 150.4 mAh g^−1@C/2, 2.5–4.2 V• 300 cycles@∼100% capacity retention	179
		(2) Potassium permanganate (KMnO4) was added slowly into the mixture, <3 °C, stir for 1 h
		(3) Three-necked flask was transferred to a water-bath, 38 °C, stir for 1 h
		(4) 46 mL DI water was added into mixture, 95 °C, stir for 0.5 h
		(5) 140 mL DI water of 40 °C and 15 mL 30% vol. H2O2 was added
		(6) The GO powder was washed, dispersed and dried
		(7) Spent LFP mixed with 30 mL LiOH, AA (L-ascorbic acid) and dodecyl benzene sulphonic acid sodium (SDBS, C18H29SO3Na) solution with the molar ratio of 1 : 3 : 3 : 1, stirred for 0.5 h, mixed with 5% regenerated GO, transferred into Teflon microwave reactors, 160 °C, 6 h
Hydrothermal	AlPO4/Li3PO4 coated-LFP	(1) 0.2 M LiOH and 0.08 M tea polyphenols solution, 120 °C, 10 h, wash, dry	• 134.96 mAh g^−1@2C, 2.5–4.2 V• 400 cycles@92.1% capacity retention	180
		(2) Mixed with Al(NO3)3, NH4H2PO4, and 2 wt% Li2CO3, stirred at 80 °C until dry
		(3) 600 °C, 5h, N2
Ionic liquid	NMC111	(1) Mixed 2.5 mmol degraded NMC111 and 2.5 mmol LiBr in 2.5 mL [C2mim][NTf2], 150 °C, 6 h	• 125 mAh g^−1@1C, 3–4.3 V• 100 cycles@∼88% capacity retention	169
		(2) Filtration, washed, acetone, ethanol, dried at 100 °C, 2 h
		(3) 500 °C, 4 h
Ionic liquid	LFP	(1) Mixed 0.5 g spent LFP and 0.3 g L-AA in 7 g [BMIM][BF4], stirred for 10 min, 60 °C, 2 h, Ar	• 138.5 mAh g^−1@1C, 2–4 V• 300 cycles@92.1% capacity retention	181
		(2) Added 0.1 g LiBr, 210 °C, 8 h, Ar
		(3) Centrifuged, washed, acetone, DI water, ethanol, dried at 80 °C for 6 h under vacuum
		(4) 700 °C, 2 h, Ar
Redox mediator	NMC111	(1) Mixed 0.5 M DTBQ in DME, stirred overnight	• 157 mAh g^−1@C/10, 3–4.3 V	170
		(2) Added Li^0 to electrolyte, stirred, removed Li^0 before next step
		(3) Mixed 7.5 mL lithiated DTBQ electrolyte with 5 g spent NMC111
		(4) Sealed a Li^0 chip in Celgard separator and added in the mixture
		(5) Stirred for 60 min
		(5) Wash, DME
		(6) 850 °C, 5 °C min^−1, 4 h, air
Redox mediator	LFP	(1) Mixed 4 mmol Li chip and 4 mmol Py in 20 mL DME, stirred for 1 h	• 160.1 mAh g^−1@C/2, 2.7–4.2 V• 100 cycles@∼100% capacity retention	171
		(2) Mixed 0.5 g degraded LFP (3.22 mmol) with 5.6 mL Py–Li (0.2 mol L^−1), stirred for 15 min
		(3) Centrifugation, dried at 80 °C overnight, air

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3.2.2.1 Hydrothermal regeneration. The hydrothermal method is a conventional solution-based synthesis technique that facilitates chemical reactions in aqueous solutions, typically conducted in a sealed vessel such as an autoclave at 120–200 °C. In direct recycling of cathode materials, hydrothermal methods regenerate spent cathodes through controlled dissolution, ion exchange, and subsequent recrystallization. This method enables efficient microstructure restoration and fast relithiation due to high ion diffusivity in solution.
3.2.2.1.1 LCO. Kim et al.¹⁵² first applied a hydrothermal method to renovate LCO in a concentrated LiOH solution. The relithiation process was conducted in polytetrafluoroethylene (PTFE) vessels placed inside an autoclave, where vapor pressure was generated during the reaction. The hydrothermal treatment was carried out in a 5.0 M LiOH solution at 200 °C for 20 hours. The resulting precipitate was thoroughly washed and dried, yielding well-crystallized particles of ∼20 µm composed of nano-sized, aggregated grains (Fig. 5a), formed through a “dissolution–precipitation” mechanism. Raman spectroscopy and X-ray absorption near edge structure (XANES) analyses confirmed that the recovered LCO successfully replenished Li content and restored the transition metal valence state. Zhang et al.¹⁵³ investigated the effects of temperature, reaction time, and ultrasonic irradiation on the recovery. Spent LCO was placed in a 2 M LiOH solution and subjected to ultrasonic irradiation and stirring under varying temperatures and durations. XRD results revealed that hydrothermal relithiation at 120 °C for 6 hours yielded the most well-defined crystal structure, delivering 132.8 mAh g⁻¹ at C/5 with 96.61% capacity retention after 40 cycles. However, hydrothermal treatment alone is insufficient to fully restore the crystal microstructure and crystallinity; post-annealing is more effective in refining structural and surface. Shi et al.¹⁵⁴ added a short post-annealing after the wet-chemical process. LCO powders were hydrothermally treated in 4 M LiOH or a 1 M LiOH + 1.5 M lithium sulfate (Li₂SO₄) solution at 220 °C for 4 hours. The powders were further annealed at 800 °C for 4 hours. The Co₃O₄ impurities have been totally removed after wet processing, but it displayed poor cycling stability due to severe cation mixing (Fig. 5b). After sintering, the particle surface became smoother and distributed evenly. The recovered LCO with short annealing showed 148.2 mAh g⁻¹ at 1C and achieved 91.2% capacity retention after 100 cycles. To further shorten the reaction time, microwave was combined with hydrothermal.

Fig. 5 Progress on solution-based regeneration. (a and b) Process optimization. The post-annealing after hydrothermal regeneration can refine the (a) crystal structure and (b) particle morphology. Reprinted with permission from ref. 152 and 154. Copyright 2004 Elsevier. Copyright 2018 Royal Society of Chemistry. (c–f) Mechanism investigation. (c) Pseudo-second-order kinetic model. (d) Langmuir isotherm model. (e) Schematic of the hydrothermal relithiation mechanism. (f) HRTEM image of the cathode particle regenerated via solid-state sintering, revealing the coexistence of layered and spinel phases, which may introduce additional barriers to Li⁺ diffusion. Reprinted with permission from ref. 157. Copyright 2023 Elsevier. (g) Upcycling. Illustration of Li⁺ diffusion pathways in NMC532. Reprinted with permission from ref. 158. Copyright 2023 American Chemical Society. (h–j) Ionic liquid regeneration. (h) Reaction mechanism, (i) FTIR of [C₂OHmim][NTf₂] and (j) cycling performance of ionothermal regenerated NMC. Reprinted with permission from ref. 169. Copyright 2020 Wiley. (k–m) Redox mediator regeneration. (k) Reaction mechanism, (l) effects of solution concentration and reaction time on recycled NMC capacity, (m) FTIR spectra of a pure DME solvent and the 0.5 M DTBQ in DME before and after exposure to Li⁰ chip. Reprinted with permission from ref. 170. Copyright 2021 American Chemical Society.

The upcycling of LCO during hydrothermal regeneration is also preferable to improve electrochemical performance. Wang et al.¹⁵⁵ applied interface engineering to enhance the high-voltage performance of regenerated LCO. After hydrothermal relithiation, the sample was mechanically mixed with 2% lithium aluminum titanium phosphate (Li_1.4Al_0.4Ti_1.6(PO₄)₃, LATP) and sintered at 800 °C for 4 hours. At temperatures above 700 °C, LCO reacted with LATP to form composite surface phases, including Co₃O₄/cobalt aluminate (Co₂AlO₄), cobalt titanium oxide (CoTiO₃), and Li₃PO₄, acting as protective layers to suppress side reactions and enhance structural stability under high voltage. The regenerated LCO delivered superior performance at 4.4 V, achieving a discharge capacity of 166 mAh g⁻¹ and retaining 93.0% of its capacity after 100 cycles at 1C.

3.2.2.1.2 NMC. NMC cathode regeneration is complicated by TM non-stoichiometry and cation mixing with the formation of surface rock-salt and spinel phases. The surface heterogeneity hinders Li⁺ transport, making the reverse to the layered structure thermodynamically and kinetically challenging. Hydrothermal synthesis effectively produces uniform, highly ordered NMC layered oxides with controlled, rapid reaction kinetics driven by the high Li⁺ diffusivity in solution environment, enabling efficient relithiation with lower energy use.¹⁵⁶ However, post-annealing is required to fully incorporate adsorbed Li⁺ into the lattice. Chan et al.¹⁵⁷ studied the hydrothermal relithiation mechanism based on simulation. The pseudo-second-order fit indicated that hydrothermal relithiation is governed by chemisorption via electron sharing or exchange (Fig. 5c). Monolayer adsorption, consistent with the Langmuir model (Fig. 5d), suggested each Li⁺-deficient site accommodates one Li⁺, requiring high Li⁺ concentration for effective adsorption. Based on adsorption kinetics and redox behavior, the relithiation begins with Li⁺ chemisorption on deficient sites, releasing water and oxygen (Fig. 5e). However, wet treatment alone cannot fully restore structure, as some Li⁺ remain surface-adsorbed due to the spinel or rock-salt phases that hinder Li⁺ penetration into bulk (Fig. 5f). Therefore, post-annealing is required to provide additional thermal energy for Li⁺ to overcome barriers and integrate into lattice sites.

Shi et al.¹³¹ supported this mechanism through a systematic comparison between LCO, NMC111 and NMC532 recovered by hydrothermal method and solid-state sintering. The inductively coupled plasma optical emission spectrometry (ICP) showed that both NMC111 and NMC532 had more sluggish Li⁺ transport kinetics than LCO, requiring higher temperature during hydrothermal relithiation (NMC 220 °C vs. LCO 180 °C). Also, NMC recovered by hydrothermal treatment followed by post-annealing exhibited lower cation mixing than that recovered by solid-state sintering in pure O₂, with no rock-salt phase observed in HRTEM. To further overcome the high energy barriers from spinel and rock-salt phases of NMC, Jia et al.¹⁵⁸ developed a new strategy of using transition metal hydroxide (TM(OH)₂) as an intermediate. Simulations revealed that the 2-TM Li⁺ transport channels (2.13 eV) from cation disorder have larger migration barrier than 1-TM transport channels in pristine NMC (1.37 eV), leading to inhomogeneous Li⁺ transport during relithiation (Fig. 5g). Therefore, they added ammonium hydroxide in a hydrothermal process to convert the spinel and rock-salt phases into TM(OH)₂ first, which can be topotactically transformed into NMC due to their structural similarity. The depth profiling analysis of X-ray photoelectron spectroscopy (XPS) and HRTEM confirmed uniform Li⁺ distribution and layered structure in topotactic hydrothermal transformed NMC throughout surface to bulk, unlike gradient profiles from solid-state sintering. NMC532 recovered from the hydroxide delivered 152 mAh g⁻¹ at C/2, remaining 90% after 100 cycles and 76% after 200 cycles. This strategy can also be broadly applied to various layered cathode materials, including NMC532, NMC622, and LCO.

Although a high Li⁺ concentration is required for effective hydrothermal relithiation, strategies exist to reduce raw material costs. Xu et al.¹⁵⁹ investigated the effects of various hydrothermal conditions on the cycling performance of NMC111, including temperature, duration, solution composition, Li source compensation, and post-annealing temperature. They identified the most cost-effective and optimized relithiation protocol as hydrothermal treatment at 220 °C for 2 hours in a solution containing 0.1 M LiOH and 3.9 M potassium hydroxide (KOH), followed by post-annealing with 5% excess LiOH at 750 °C for 4 hours. Replacing 4 M LiOH with 0.1 M LiOH + 3.9 M KOH remained compatible with NMC111 due to the preferential insertion of Li⁺ over K⁺, attributed to higher reduction potential and smaller ionic radius of Li⁺.

3.2.2.1.3 LFP. Performance degradation in LFP cathodes is mainly due to Li vacancies and Li–Fe antisite defects.¹⁶⁰ A high activation energy of 1.4 eV is needed for Fe²⁺ to return to their original M2 sites due to strong electrostatic repulsion from Fe³⁺.¹⁶¹ Thus, the reduction of Fe³⁺ is the core of LFP regeneration. Various reductive agents have been used in previous studies, including hydrazine hydrate,¹⁶² tartaric acid,¹⁶³ citric acid,¹⁶⁴ sodium sulfite (Na₂SO₃),^165,166 ethanol¹⁶⁷ and glucose.¹⁶⁸ Most concerns are still the reductive agents' cost. Xu et al.¹⁶⁴ used citric acid as the reductive agent to spontaneously reduce Fe³⁺ and replenish Li⁺ in spent LFP. Gibbs free energy calculations revealed that the reaction between FePO₄ and citric acid is thermodynamically favorable, significantly reducing the hydrothermal duration and cost. ICP analysis showed that lowering the solution temperature to 80 °C had minimal impact on the relithiation kinetics. Also, XRD confirmed full reduction of Fe³⁺ within 5 hours. Highly crystalline LFP with further reduced anti-site defects was obtained after short annealing, delivering 102 mAh g⁻¹ even at 10C with minor degradation after 300 cycles.

Surface coating is a common upgrading strategy to enhance the intrinsically low electrical conductivity of LFP while effectively utilizing organic additives. Jia et al.¹⁶⁷ used ethanol as a reducing agent, leveraging its electron-donating tendency upon oxidation. XPS confirmed effective Fe³⁺ reduction to Fe²⁺ at 180 °C for 5 hours, progressing from the surface into the bulk material. To further prevent the oxidation of Fe²⁺ during cycling, polyvinylpyrrolidone (PVP) was introduced to form a heterogeneous interface with N-doped carbon, modifying the d-band structure of recycled LFP. According to d-band center theory, shifting the d-band away from the Fermi level reduces antibonding state occupation, enhancing Fe–O bond strength and immobilizing Fe²⁺ to prevent Li–Fe antisite defects. DFT simulations confirmed a raised d-band center, strengthened Fe–O bonding, and reduced Li⁺ diffusion barriers in the modified structure. Thus, the upcycled LFP exhibited faster Li⁺ transport, minimal phase transformation, and enhanced performance, delivering over 90 mAh g⁻¹ at 10C and retaining 76.8% capacity after 1000 cycles.

3.2.2.2 Ionic liquid regeneration. IL regeneration is a moderate-temperature chemical approach for restoring Li content in degraded battery cathode materials using Li-containing IL as both the solvent and Li source. IL is made of bulky, asymmetric organic cations and anions, allowing them to stay liquid at low temperatures. In a typical process, spent cathode powders are mixed with a Li salt dissolved in a stable ionic liquid, such as [C₂mim][NTf₂], [C₄mim][NTf₂], [C₂OHmim][NTf₂],¹⁶⁹ [BMIM][BF₄], and heated to moderate temperatures under an inert atmosphere. The high thermal and electrochemical stability of IL provides a non-volatile and highly ionic conductive medium. During relithiation, Li⁺ from the IL intercalates into the cathode structure, driven by the chemical potential gradient and the inherent mobility of Li⁺ in the IL. IL relithiation operates under mild reaction conditions without proton attack, but it suffers from relatively slow lithiation kinetics, potential contamination from IL residues, and the high cost or limited recyclability.

Wang et al.¹⁶⁹ introduced an ionothermal regeneration method for direct recycling of spent NCM111 cathodes, leveraging ILs and lithium halides as the reaction medium and Li source, respectively. The optimal relithiation was achieved using recyclable IL containing LiBr in [C₂OHmim][NTf₂] at 150 °C for 6 h. After treatment, the cathode was separated, and ILs were recovered for subsequent cycles, demonstrating IL recyclability. The ionicity of the ionothermal medium enables Li⁺ insertion into the TM oxide layer structure without compromising particle morphology (Fig. 5h). The hydrophilic hydroxide groups in [C₂OHmim] improved Li salt solubility and interaction with degraded NMC, and lithium bromide (LiBr) was superior to lithium chloride (LiCl) due to its lower oxidation potential, allowing Br⁻ to be oxidized to Br₂ (1.065 V) more readily than Cl⁻ to Cl₂ (1.358 V). Moreover, FTIR confirmed that the [C₂OHmim][NTf₂] remained chemically stable during the ionothermal relithiation process at 150–250 °C (Fig. 5i). All the key functional group peaks were preserved without any degradation, indicating excellent thermal stability and recyclability (98.7%) of the IL. The NMC111 relithiated by [C₂mim][NTf₂] delivered a discharging capacity of 125 mAh g⁻¹ at 1C, remaining ∼88% retention after 100 cycles (Fig. 5j).

3.2.2.3 Redox mediation regeneration. RM regeneration is a direct recycling method that uses soluble redox-active species to shuttle electrons and Li⁺ between a reductant (e.g., Li⁰) and degraded cathodes. RM is first reduced by the reductant, then oxidized by transferring Li⁺ to cathode (Fig. 5k), enabling Li⁺ reinsertion via repeated redox cycles. RM regeneration can be operated under ambient temperature and pressure, driving relithiation spontaneously by electrochemical potential without costing external energy. However, its efficiency is limited by redox potential constraints and sluggish kinetics. Also, chemical RMs can induce side reactions at higher potentials and promote the formation of passivation layers on the cathode surface, which may hinder further Li⁺ insertion and reduce overall efficiency.

Park et al.¹⁷⁰ developed a mild RM regeneration for NMC111. They regenerated spent NMC111 through a spontaneous redox reaction using 3,5-di-tert-butyl-o-benzoquinone (DTBQ) as the RM in dimethoxyethane (DME) solvent, with Li⁰ as the reductant. DTBQ was pre-reduced by Li⁰ to form a lithiated DTBQ solution, after which the Li⁰ strip was removed. The lithiated DTBQ solution was then mixed with spent NMC111 and another fresh Li⁰ chip enclosed in a Celgard separator to prevent particle crossover while allowing ion transport. The mixture was stirred for 10–60 minutes at RT. Afterwards, the relithiated NMC111 powder was washed with DME and annealed in air at 850 °C for 4 hours. The DTBQ concentration and reaction time were tuned, with 0.5 M DTBQ and 60 minutes showing optimal results (Fig. 5l). FTIR showed that the C=O and C=C peaks of Li–DTBQ disappeared, while new peaks corresponding to C–O*, C–C* radicals, and aromatic C=C bonds emerged, indicating Li⁺ coordination and electron transfer to the quinone ring (Fig. 5m). The Li–DTBQ complex then transferred Li⁺ and electrons to the spent NMC111, enabling relithiation and reduction of TM ions. Redox potential is a key criterion in selecting RMs, as it governs both the driving force and selectivity of the relithiation reaction. The redox potential of RM should be positioned between the Li⁰ potential (0 V vs. Li/Li⁺) and the cathode operating potential (typically 3.5–4.5 V vs. Li/Li⁺) to ensure efficient mass transfer from Li⁰ to the cathode while minimizing parasitic reactions. Aromatic Li complexes such as biphenyl lithium (Biph-Li) and naphthalene lithium (Naph-Li) have low reduction potentials (<0.5 V vs. Li⁺/Li), making them strong lithiation agents for Li compensation. However, their high reducing power risks overlithiation, causing irreversible crystal damage and interface degradation in cathode materials. Based on this RM design strategy, Wu et al.¹⁷¹ screened controllable reductive polycyclic aromatic hydrocarbons (PAHs) and identified pyrene lithium complex (Py–Li) as suitable RM in DME. Py–Li provided sufficient driving force to insert Li⁺ into delithiated LFP without triggering overlithiation or side reactions due to a suitable reduction potential of ∼0.82 V vs. Li/Li⁺. Nuclear magnetic resonance (NMR) supported that pyridine was successfully coordinated with Li⁺, forming a soluble and labile complex that readily dissociated to release Li⁺. As a result, Li⁺ can rapidly intercalate into the cathode lattice, enabling regeneration of spent LFP within minutes.

3.2.2.4 Summary. Solution-based regeneration is cost-effective and efficient, but output stability is influenced by numerous factors. The required conditions and environments vary widely across materials, and diverse additives have been explored to improve regeneration performance. However, no clear standards or guidelines exist for their selection. Also, aqueous or organic media may damage surface-sensitive materials. Future research should theoretically decouple the roles of different compositions in the solution, elucidating and quantifying their effects and design rationality, thereby identifying optimal conditions based on performance and cost. Furthermore, the batch-to-batch nature of solution-based regeneration limits its applicability for rapid commercial use, underscoring the need for innovative designs that enable continuous production.

3.2.3 Molten salt regeneration. Although solution-based regeneration offers faster kinetics than solid-state methods, it can sometimes lead to undesirable side reactions such as TM leaching or structural degradation due to the aqueous environment and harsh conditions. Furthermore, solution-based relithiation can have fast surface kinetics in aqueous, pressurized environments, but this does not necessarily translate to effective bulk relithiation. The solution medium often has limitations to penetrate Li⁺ deep into particle bulk due to its relatively low chemical potential (concentration), especially in degraded or dense materials.¹⁸² In contrast, molten salt benefits from the proton-free and Li rich environment, enabling more controlled and efficient Li⁺ transport into the bulk of cathode materials under air atmosphere. By immersing the cathode materials in a molten Li salt, such as LiCl, lithium nitrate (LiNO₃) at medium temperatures (typically 300–600 °C), Li⁺ can quickly diffuse and penetrate the host crystal lattice.¹⁸³ The proton-free, oxidative environment and fast kinetics of molten salt regeneration make it suitable for restoring layered oxide cathode materials, such as LCO, NMC.¹¹²

3.2.4 LCO. Yang et al.¹⁸⁴ chose LiOH–KOH–Li₂CO₃ mixture as the eutectic salt. A LiOH–KOH eutectic was the basic molten salt system, resulting in a lower melting point than the individual component. Li₂CO₃ was added as an additional Li source due to its higher reactivity compared to LiOH. The degraded LCO was added to the molten eutectic salt bath and maintained at 500 °C for 8 hours. The strong oxidative environment of LiOH–KOH–Li₂CO₃ effectively broke down residual carbon and organic binders through spontaneous reaction, while the Li-rich, uniform molten phase enhanced Li⁺ transport, promoting relithiation. After regeneration, the cracks on LCO were repaired, and the particles became smooth, leading to a capacity improvement from 70 mAh g⁻¹ to 150 mAh g⁻¹, which maintains 92.5% retention after 200 cycles at C/5.

To further improve the structure stability and crystallinity of LCO, Gao et al.¹⁸⁵ developed Ca²⁺-assisted molten salt relithiation. Minor CaO was introduced into eutectic salts, followed by moderate heating at 750 °C. The addition of CaO expanded the metastable dissolution region of LCO and raised the relithiation temperature from 470 °C to 540 °C, promoting highly ordered crystal formation and improved Li⁺ diffusion for structural repair. Differential scanning calorimetry (DSC) indicated that CaO facilitated Ostwald ripening, leading to the precipitation of isotropic crystals with uniform size and improved structural ordering. Moreover, HAADF-STEM revealed calcium (Ca) atoms within the Li intercalation layer, partially compensating for Li vacancies formed during cycling (Fig. 6b). Simulations indicated that Ca²⁺ doping modifies Li⁺ pathways and enhances diffusivity by creating localized internal electric fields from charge imbalance and improving connectivity between adjacent Li sites.

Table 5 Summary of the molten salt regeneration for cathodes direct recycling

Recycling method	Cathode material	Recycling process	Electrochemical performance	Ref.
Molten salt	LCO	(1) 50 g LiOH–KOH–Li2CO3 (salt molar ratio = 3 : 7 : 0.5) mixture, heat at 500 °C, 3 °C min^−1	• 144.5 mAh g^−1@C/5, 2.75–4.5 V• 200 cycles@92.5% capacity retention	184
		(2) 1 g degraded LCO added to molten salt, 8 h, under air
		(3) Wash, centrifuged, dried at 80 °C for 12 h
Molten salt	LCO	(1) The spent LCO was heated at 300 °C, 8 h and 500 °C, 16 h in LiOH–KOH with 0.05 mol LiNO3, under air, 840 mL min^−1	• 149.1 mAh g^−1@C/5, 2.75–4.25 V• 100 cycles@93% capacity retention	195
		(2) Wash, centrifuged, dried at 80 °C for 12 h
Molten salt	LCO	(1) Spent LCO, potassium chloride (KCl), LiNO3, and CaO (mass ratio of CaO : LCO = 0.015), mixed, 750 °C, 12 h, air	• 128.1 mAh g^−1@2C, 3–4.3 V• 300 cycles@79.7% capacity retention	185
		(2) Wash, dried at 60 °C for 12 h
Molten salt	NMC532	(1) Spent NMC532, LiNO3, LiOH (salt molar ratio = 3 : 2), mixed, heat at 300 °C, 4 h	• 149.3 mAh g^−1@1C, 3–4.3 V• 100 cycles@90.15% capacity retention	192
		(2) Wash, DI water
		(3) Mixed with 5% Li2CO3, 850 °C, 5 °C min^−1, 4 h, O2
Molten salt	NMC532	(1) Spent NMC532, KCl, potassium nitrate (KNO3), LiNO3 (molar ratio = 1 : 8 : 8 : 0.8), mixed, heat at 750 °C, 5 °C min^−1, 12 h, air	• 152.2 mAh g^−1@C/5, 2.75–4.25 V• 100 cycles@95.5% capacity retention	196
		(2) Wash, DI water, dried at 80 °C, 12 h
Molten salt	NMC532	(1) Spent NMC532, LiOH, Li2CO3 (molar ratio = 0.95 : 0.84 : 0.16), mixed, heat at 5 °C min^−1, 440 °C, 5 h; 850 °C, 12 h	• 146.3 mAh g^−1@1C, 2.8–4.3 V• 200 cycles@89% capacity retention	197
Molten salt	NMC532	(1) Spent NMC532, LiOH, Li2CO3 (molar ratio = 7 : 3), mixed, heat at 500 °C, 5 h; 800 °C, 6 h	• 152.92 mAh g^−1@C/2, 2.8–4.3 V• 100 cycles@96% capacity retention	198
Molten salt	From NMC532 to single crystal LiNi0.665Mn0.195Co0.14O2	(1) Spent NMC532, Ni0.83Mn0.09Co0.08(OH)2, LiOH, Li2SO4 (molar ratio = 1 : 1 : 2.5 : 0.5), mixed, heat at 10 °C min^−1, 900 °C, 5 h; 860 °C, 15 h, air; cooling, −2 °C min^−1, 300 °C; nature cooling	• 160 mAh g^−1@1C, 2.8–4.3 V, 30 °C• 200 cycles@95% capacity retention	190
		(2) Wash, DI water, dried at 70 °C, air
		(3) 700 °C, 4 h, air
Molten salt	From NMC532 to single crystal LiNi0.8Mn0.12Co0.08O2	(1) Spent NMC532, Ni0.83Mn0.09Co0.08(OH)2, LiOH, Li2SO4 (molar ratio = 1 : 1 : 2.5 : 0.5), mixed, heat at 10 °C min^−1, 780 °C	• 175 mAh g^−1@1C, 2.8–4.3 V, 30 °C• 200 cycles@85% capacity retention	190
		(2) Wash, DI water, dried at 70 °C, air
		(3) 700 °C, 4 h, air
Molten salt	NMC532	(1) Spent NMC532, LiNO3, LiOH, CH3COOLi (salt molar ratio = 9 : 6 : 10), mixed, heat at 400 °C, 4 h	• 160 mAh g^−1@C/2, 3–4.3 V• 100 cycles@93.7% capacity retention	199
		(2) Soak, DI water, ultrasonic 30 min
		(3) 5% Li2CO3, 850 °C, 6 h, O2
Molten salt	From NMC111 to NMC622	(1) Spent NMC111, nickel nitrate hexahydrate (Ni(NO3)2·6H2O), LiCl, NaOH (mole ratio = 1 : 2.4 : 0.86 : 1.2), mixed, dried at 110 °C, vacuum; heat at 5 °C min^−1, 300 °C, 5 h; 800 °C, 5 h, air	• 140 mAh g^−1@200 mA g^−1, 3–4.3 V• 100 cycles@92.8% capacity retention	200
		(2) Wash, dried at 110 °C, vacuum
		(3) 600 °C, 5 °C min^−1, 2 h, air
Molten salt	From NMC111 to single crystal NMC622	(1) Spent NMC111, NiO, LiOH (mole ratio = 1 : 0.67 : 0.67), mixed, 20% excess LiOH and 10% excess Li2SO4 added to flux, heat at 10 °C min^−1, 900 °C, 10 h, O2	• 125 mAh g^−1@5C, 2.5–4.3 V• 100 cycles@77.59% capacity retention	201
Molten salt	NMC532	(1) Spent NMC532, LiI, LiOH (mole ratio = 1 : 1.65 : 1.35), mixed, 5 wt% dicobalt trioxide (Co2O3), 5 wt% MnO2 added, 200 °C, 4 h; 850 °C, 5 h	• 137 mAh g^−1@C/2, 2.5–4.3 V• 300 cycles@73.0% capacity retention	194
Molten salt	NMC532	(1) Spent NMC532, 3 wt% Al, LiNO3, LiOH (salt molar ratio = 3 : 2), 300 °C, 4 h	• 135.93 mAh g^−1@1C, 2.8–4.3 V• 200 cycles@89.6% capacity retention	202
		(2) Wash, dried at 60 °C
		(3) 5 wt% LiOH, 800 °C, 8 h
Molten salt	NMC532	(1) Spent NMC532, LiOH, LiNO3, lithium salicylate (LiC7H5O3) (mole ratio = 10 : 2 : 3 : 5), 300 °C, 4 h	• 136.9 mAh g^−1@C/2, 2.5–4.3 V• 200 cycles@88.2% capacity retention	203
		(2) Wash, DI water, ethyl alcohol, dried at 70 °C, 10 h
		(3) 5% Li2CO3, 850 °C, 6 h
Molten salt	Single crystal NMC811	(1) Spent NMC811, LiOH·H2O, NaCl (mole ratio = 1 : 2 : 0.4), mixed, 850 °C, 15 h	• 155.8 mAh g^−1@1C, 3–4.3 V• 200 cycles@86.5% capacity retention	188
		(2) Wash, DI water
		(3) 5 wt% LiOH, 800 °C, 10 h
Molten salt	NMC532	(1) 0.2 g spent NMC532, 9.9 mg LiOH, 42.8 mg LiNO3, 132.5 mg LiC7H5O2, 5 °C min^−1, 300 °C, 4 h	• 145.7 mAh g^−1@C/2, 2.5–4.3 V• 300 cycles@70.1% capacity retention	204
		(2) Wash, centrifuge, dry for 12 h
		(3) 0.25% Li2CO3, 850 °C, 5 °C min^−1, 6 h
Molten salt	LFP	(1) Spent LFP, LiNO3, ferrous oxalate (FeC2O4) (mole ratio = 1 : 0.5 : 0.1), mixed, 10 wt% of sucrose added, 300 °C, 2 h, Ar	• 145 mAh g^−1@C/2, 2.5–4.2 V• 100 cycles@90% capacity retention	191
		(2) Wash, DI water, 80 °C, 12 h
		(3) 650 °C, 6 h

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Fig. 6 Progress on molten salt regeneration. (a) Process optimization. Temperature effects on Li content and particle size. Reprinted with permission from ref. 188. Copyright 2024 Wiley. (b) Material modification. Annular bright-field (ABF)-STEM, HAADF-STEM, ICP result of Ca²⁺ doped LCO. Reprinted with permission from ref. 185. Copyright 2024 Elsevier. (c) Upcycling. Ni, Mn and Co relative concentrations distribution over a particle slice revealed by XRF. Reprinted with permission from ref. 190. Copyright 2022 Elsevier. (d) Eutectic salt. Phase diagram and TG-DTG curves of the LiI–LiOH eutectic salt system. Reprinted with permission from ref. 194. Copyright 2022 American Chemical Society.

Table 6 Summary of the anode material direct recycling

Recycling method	Recycling process	Electrochemical performance	Ref.
Flash Joule sintering recycling	(1) Heating the graphite via the flash system (Tmax = 2850 K, 1 s, 120 V, 1.3 Ω)	• Recycled graphite‖LFP• 131.1 mAh g^−1 at C/5• 400 cycles@77.3% capacity retention (C/2)	209
	(2) Leach at 60 °C for 3 h in 0.1 M HCl
Pretreatment and catalytic graphitization	(1) Leach at 35 °C overnight in 2 M sulfuric acid	• 358 mAh g^−1 at C/10• 500 cycles@245.4 mAh g^−1 (1C)	210
	(2) Stir acid-leached powder, cobalt nitrate, and alcohol mixture until alcohol evaporates
	(3) 900 °C, 4 h, N2
Alkali etching	(1) Alkali roasting, 300 °C, 2 h, air (KOH : NaOH = 5 : 1)	• 336.9 mAh g^−1 at C/10• 100 cycles@99.7% capacity retention	211
	(2) Leach at 60 °C for 2 h with stirring in 2 M H2SO4 (60 mL) and H2O2 (40 mL, 30 wt% in H2O)
	(3) 1100 °C, 2 h, N2 (leached powder and pyrolytic)
Alkaline roasting	(1) Alkali roasting, 350 °C, 2 h, Ar (spent graphite : NaOH = 1 : 2)	• 361.2 mAh g^−1 at 0.05A g^−1• 200 cycles@368.1 mAh g^−1 (0.1 A g^−1)	212
	(2) Leach in DI water
Microwave heating	(1) Microwave heating, 800 W, 20–30 s, N2	• 353.5 mAh g^−1 at C/10• 100 cycles@320 mAh g^−1 (C/2)	213
	(2) Graphite mixed with DI water (1 : 5), go through CO2 (60 mL min^−1) with stirring (600 rpm)
	(3) Recover Li salt: heat and evaporate lithium solute
Hydrometallurgical	(1) Leach in 5 M sulfate acid and 35 w/w% H2O2	• 359.3 mAh g^−1 at C/5• 100 cycles@84.63% capacity retention	214
	(2) Alkali roasting, 500 °C, 40 min with NaOH
Boric acid leaching and short annealing	(1) Leach in 5 wt% boric acid solution (1 g graphite and 2 mL acid solution)	• 330 mAh g^−1 at C/3• 100 cycles@333 mAh g^−1	215
	(2) 1050 °C, 1 h, N2
Solid-state sintering	(1) 750 °C, 5 h, Ar	• 310 mAh g^−1 at C/2• 1000 cycles@93% capacity retention	216
	(2) Wash, dimethyl carbonate (DMC)
	(3) 750 °C, 5 h, Ar
Electrolysis	(1) Graphite plate as positive electrode, spent graphite as negative electrode, Na2SO4 solution as electrolyte	• 353.5 mAh g^−1 at C/10• 100 cycles@320 mAh g^−1 (C/2)	217
	(2) 10 cm of inter-electrode distance, 1.5 g L^−1 of electrolyte concentration and 30 V of voltage, separation for 25 min
Leaching combined with sintering	(1) Used graphite, 98.3% H2SO4, 20% of total mass DI water, mixed, stir for 20 min	• 349 mAh g^−1 at C/10• 50 cycles@98.9% capacity retention	218
	(2) 200 °C, 24 h
	(3) Leaching in 200 g per L H2SO4 solution
	(4) Wash, DI water, washing until PH = 7
	(5) 1500 °C, 2 h, Ar

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3.2.5 NMC. Molten salt becomes disordered IL at medium temperatures, weakly solvating Li⁺ that transports through vehicular diffusion and ion coordination mechanisms, diffusing rapidly into the Li slab of cathode lattice driven by the concentration gradient.¹⁸⁶ Also, the molten salt environment facilitates oxygen incorporation and TM reoxidation, further stabilizing the layered structure. Thus, it is preferable to solid-state sintering or solution-based regeneration of NMC.

Chen et al.¹⁸⁷ compared the molten salt and traditional solid-state sintering method for spent NMC. XPS revealed that molten salt recovered NMC exhibited homogenous surface with less Ni²⁺, while traditional solid-state sintering remained F-related impurities from CEI decompose. Also, molten salt recovered NMC had larger grain size (∼2 µm), indicating a secondary particle growth due to rapid ion diffusion and grain boundary migration. Qin et al.¹⁸⁸ reported that higher temperatures and longer duration promoted NMC811 grain coarsening in LiOH–NaCl molten salt. However, there was significant surface Li volatilization when the treatment temperature reached 900 °C. Additionally, focused ion beam (FIB)-TEM revealed fully connected secondary particles of recovered NMC811 (Fig. 6a). These observations suggested that NMC811 was completely relithiated and reconstructed into single crystal, driven by enhanced mass transport within LiOH–NaCl molten salt.

Beyond morphology control and grain growth, molten salt relithiation enables value-added upcycling. Low-Ni NMC compositions such as NMC111 and NMC532 dominate commercial use due to their structural stability and mature manufacturing processes but offer limited energy density.¹⁸⁹ In contrast, Ni-rich layered oxides like NMC622 and NMC811 are preferred for future applications. As the retired low-Ni NMC increases alongside growing demand for high-Ni NMC, upcycling provides a critical pathway to bridge this gap. Qian et al.¹⁹⁰ introduced additional TM(OH)₂ into LiOH–Li₂SO₄ eutectic salts. The TM ions and Li⁺ diffuse simultaneously into the NMC lattice at elevated temperature, transiting NMC111 into NMC622 or NMC811 depending on the feedstock stoichiometry. The recovered particles exhibited well-defined morphology, uniform crystallinity, and a homogeneous distribution of Ni, whereas X-ray fluorescence (XRF) tomography revealed inhomogeneous spatial distributions of Mn and Co (Fig. 6c). This may be attributed to differences in the solubility and diffusivity of Ni, Co, and Mn in the molten salt system of LiOH–Li₂SO₄, which affected the bulk mass transport and crystal growth kinetics. Despite this, ICP showed that the recovered NMC622 and NMC811 reached target TM content from electrode level to particle level, as well as good cycling performance in both half cells and pouch-type full cells.

3.2.6 LFP. While the oxidative atmosphere in molten salt offers benefits on layered oxide cathode recovery, it is detrimental for LFP, which requires reductive atmosphere to avoid Fe³⁺. Thus, a reductive atmosphere must be coupled with molten salt to suppress the oxidation. For example, Liu et al.¹⁹¹ used sucrose as a reducing agent during the LiNO₃-based molten salt relithiation of LFP to maintain its structural integrity. The recovered LFP delivered a specific capacity of 145 mAh g⁻¹ at C/2, representing more than a 13% improvement compared to the spent LFP. However, the high cost of molten salt treatment makes it less practical for recycling LFP. Instead, LFP is suited for wet chemicals due to its lower processing cost and milder reaction conditions.

3.2.7 Molten salt system design. The Li salts used in molten salt regeneration typically have low melting points. For example, LiOH melts at ∼462 °C, and LiNO₃ melts at ∼255 °C.¹⁹² However, single-component Li salts are rarely used in molten salt regeneration due to their limited Li⁺ mobility, high viscosity, and poor thermal stability.¹⁹² To achieve low melting points, high relithiation kinetics, and good thermal stability, binary or ternary eutectic salt systems have been developed.

In pure salt, cations and anions form a stable lattice with a characteristic melting point. Introducing other salts with different ionic radius or lattice parameters disrupts this lattice, lowering its stability and favoring liquid-phase formation at reduced temperatures due to lower Gibbs free energy.¹⁹³ For example, Ma et al.¹⁹⁴ proposed the eutectic lithium iodide (LiI)–LiOH salt, which crystallizes at the minimum melting point of ∼176 °C, corresponding to the eutectic point at the intersection of the liquidus curves in the binary phase diagram (Fig. 6d). However, TG and derivative thermogravimetric (DTG) showed that the eutectic salt and NMC532 mixture mainly replenish Li and reconstruct surface lattice oxygen between 315–650 °C, which is still a high temperature for efficient regeneration (Fig. 6d). Additionally, the use of dual Li salts results in elevated costs that challenge commercialization.

To reduce costs and enhance Li⁺ diffusivity, potassium (K) and sodium (Na) salts can partially substitute Li salts. Non-Li salts reduce molten salt viscosity to enhance Li⁺ mobility and dissolve metals, facilitating simultaneous impurity removal. Also, K⁺ and Na⁺ are unlikely to be incorporated into the cathode lattice due to their larger ionic radius. For example, Yang et al.¹⁹⁵ demonstrated that LiOH–KOH eutectic salts with LiNO₃ and air as oxidants can effectively remove impurities, including graphite, carbon black, PVDF. Gibbs free energy calculations confirmed that reactions between the impurities and LiNO₃/O₂ are thermodynamically favorable, thereby suppressing side reactions with LCO. With increasing oxygen partial pressure, the impurity removal reactions were promoted. Therefore, the treated LCO exhibited a smooth surface with no impurity residues, delivering a discharge capacity of 149.1 mAh g⁻¹ at C/5.

3.2.8 Summary. The molten-salt regeneration has highest relithiation kinetics and requires no specialized equipment, enabling practical implementation. However, excessive Li salt increases chemical costs, and residual deposits on the cathode surface can raise interfacial resistance, promote side reactions, and accelerate capacity fading, necessitating additional washing and post-annealing steps. Thus, commercialization of molten salt requires material innovations and salt reuse to reduce the eutectic salt cost, simplify the processing and avoid impurities.

4 Anode graphite direct recycling

Graphite is the most common anode material in LIB industry due to its low electrochemical potential, high theoretical capacity (372 mAh g⁻¹), good mechanical stability, strong thermal resistance, low cost and accessibility. So far, synthetic graphite boost production surpassing natural graphite and is preferable by LIB fabrication, attributing to its high purity and better thermal stability.²⁰⁵ Despite the relatively higher prices, it's still affordable as it was priced at about $890 per metric ton.²⁰⁶ Thus, less attention was given to graphite anode recycling. The anode fabrication cost is ∼10% in LIBs,²⁰⁷ so it's still significative to recycle spent graphite anode from economical and eco-friendly perspectives. Also, precious battery metal resources such as Li, Ni, Mn, Co still distribute from the spent graphite layered structure to the surface of SEI,^208,209 which is valuable to extract.

The successful recycling of graphite anode involves the deactivation of binders, the extraction of Li, the removal of SEI, trapped salt, and other impurities. Similar to cathode recycling, pyrometallurgy and hydrometallurgy are the mainstream for anode active material recycling. However, commercial interest has been limited due to the high cost of these processes and the relatively low market price of synthetic graphite. Recently, numerous advanced strategies have been developed to enable cost-effective anode recycling. Chen et al.²⁰⁹ proposed a flash Joule heating method for recycling spent graphite anodes, which applied a high current to the spent anode, generating rapid Joule heating to above 2000 K in less than 1 s due to the high resistivity of residual impurities (Fig. 7a). In contrast, the low-resistivity graphite remained at a significantly lower temperature, in accordance with Joule's law. Under such high temperatures, surface impurities such as SEI components, Li salts, and other volatile compounds were gasified, leaving behind graphite and transition metals (higher boiling points above 3000 K). A subsequent acid-leaching removed residual transition metals, enabling a clean, reusable graphite anode. Chen et al.²¹⁰ introduced a two-stage strategy that combined pretreatment with catalytic graphitization to regenerate graphite (Fig. 7b). First, spent graphite was annealed at high temperature followed by acid leaching using H₂SO₄ to eliminate impurities, preparing it for structural restoration. Then, the mixture of Co salt and graphite was further catalytically graphitized, with the Co ions acting as catalysts to promote reformation of crystalline graphite by repairing defects. The Co-catalyzed graphite exhibited significantly enhanced electrochemical performance, achieving ∼207.3 mAh g⁻¹ at 1C and 245.8 mAh g⁻¹ after 500 cycles. Da et al.²¹¹ introduced alkaline etching to regenerate spent graphite. The spent graphite was etched by KOH–NaOH at 180–300 °C, which can effectively remove acid-insoluble impurities embedded in the graphite matrix. After purification, the clean graphite was coated by pitch to rebuild graphitic crystallinity and grow epitaxially on the underlying graphite surface during a subsequent short, low-temperature annealing. TEM revealed that this epitaxial growth not only refined the graphitic layer but also reconstructed Li⁺ transport channels between the regenerated surface layers and the original graphite bulk (Fig. 7c). Liu et al.²¹² replaced hazardous acid-based roasting with an alkaline source NaOH, avoiding toxic HF emissions by chemically transforming LiF into sodium fluoride (NaF) and LiOH at moderate temperatures (Fig. 7d). Following the roasting, Li was further recovered via simple water leaching, producing nearly quantitative yields without harsh acids. The graphite left behind maintained excellent electrochemical performance of 370.8 mAh g⁻¹ at 0.05 A g⁻¹, paving the way for scalable, eco-friendly anode recycling with high resource efficiency.

Fig. 7 Progress on graphite anode regeneration. Workflow and mechanism of (a) flash Joule heating. Reprinted with permission from ref. 209. Copyright 2022 Wiley. (b) Pretreatment + catalytic graphitization. Reprinted with permission from ref. 210. Copyright 2022 Elsevier. (c) Alkaline etching. Reprinted with permission from ref. 211. Copyright 2021 American Chemical Society. (d) Alkaline roasting. Reprinted with permission from ref. 212. Copyright 2022 American Chemical Society.

In addition, microwave heating,²¹³ hydrometallurgical,²¹⁴ boric acid leaching,²¹⁵ and solid-state sintering²¹⁶ all paved the way to achieve anode recycling. Recent progress of graphite anode recycling is summarized in Table 6. However, artificial graphite itself is inexpensive, so the key to anode direct recycling is keeping the overall cost lower than producing new graphite while delivering clear industrial benefits. The process must balance cost, performance, and sustainability to be commercially viable.

5 Conclusions and outlook

In this review, we discussed recent progress in the direct recycling of LIBs. Following the typical workflow of direct recycling, we first introduced various separation methods to obtain active materials from spent batteries. We then discussed degradation mechanisms and regeneration techniques for cathode materials (LCO, NMC, LFP), highlighting process optimization, relithiation mechanism and upcycling strategies. Additionally, we summarized direct recycling approaches for graphite anodes, emphasizing the challenges and opportunities in morphology control and yield efficiency.

Direct recycling is considered as a promising approach for recovering active materials in LIBs without sacrificing original crystal structure, requiring lower energy and chemical input than pyrometallurgy and hydrometallurgy. However, direct recycling is still constrained at laboratory or pilot scale due to both technical and cost limitations. Research efforts should be devoted toward the following aspects in the future:

(1) Cost reduction: separation and sorting are crucial to ensure the effectiveness of further regeneration, however, they involve extensive manual labor and are highly time-consuming, particularly when transitioning from battery pack to active material level. Battery packs incorporate complex circuitry, and their designs vary significantly across manufacturers due to differing priorities in maximizing energy density and ensuring operational safety. Moreover, spent LIBs typically retain residual energy, necessitating pretreatment to fully discharge and ensure safe disassembly before manual disassembly-further contributing to overall costs. Addressing these issues requires innovations in battery pack design to facilitate EOL processing (such as tool-free disassembly), reduce labor intensity and improve recycling efficiency. Furthermore, with the rapid development of artificial intelligence (AI), intelligent systems capable of integrating residual energy detection, automative discharging and disassembly have significant potential to enhance both the safety and cost-effectiveness of LIB direct recycling.

(2) Product quality: the separation process may still overlook certain impurities, and the regeneration feedstock often contains a mixture of coating layers, dopants, binders, and electrolyte decomposition fragments. Thus, achieving high product purity remains challenging. The impurities are often introduced during collection, storage, and disassembly, and are difficult to remove without compromising the target product. In addition, the heterogeneous nature of spent LIBs, which varies in chemistry, aging state, and manufacturer design, further complicates the purification process. Therefore, ensuring product quality at the industrial scale requires rigorous control of upstream processes, which increases operational complexity and cost. Overcoming these purity challenges demands the development of efficient separation technologies and real-time monitoring systems. Specifically, process-integrated purification strategies and closed-loop feedback mechanisms could improve material quality consistency.

(3) Upcycling: a key challenge in direct recycling lies in the material mismatch between cathodes currently in use and those required to meet future energy density demands. This discrepancy creates a gap between future applications and existing battery waste streams. Therefore, strategies for upcycling, transforming lower-performance recycled cathodes into high-value materials, are essential for maximizing the economic potential for direct recycling. For example, adding TM(OH)₂ into the feedstock enables the upcycling of low-nickel NMC into high-nickel NMC.¹⁹⁰ However, the inhomogeneous diffusivity and solubility of TM from the surface to the bulk has led to uneven stoichiometry, concentration gradients, and internal particle strain, ultimately undermining structural integrity. Thus, detecting micron-level particle integrity in real time and controlling diffusivity of different ions over varying grain sizes and morphology are critical. AI-driven models can analyze sensor data from spectroscopy or tomography in real time and dynamically adjust processing parameters to maintain material quality. Furthermore, closed-loop control systems can automatically adjust temperature, atmosphere, and precursor feeding based on detected deviations, enhancing the consistency and scalability of direct recycling processes.

(4) Life cycle assessment (LCA) and techno-economic analysis (TEA): LCA and TEA are critical in guiding the optimization and practical implementation of direct recycling. While lab-scale studies focus on material recovery and electrochemical performance, LCA provides a system-level evaluation of the environmental impacts, including energy consumption, GHG emissions, and resource utilization across the entire process chain.²¹⁹ Meanwhile, TEA assesses economic feasibility by quantifying process costs, capital investment, and operational efficiency.²²⁰ They optimize the process by identifying steps with the highest energy demand and cost within the recycling workflow, thereby redirecting efforts toward the most impactful stages. For example, intensive thermal or solvent-based processes may improve material recovery but increase energy demand and environmental burden. Integrating LCA and TEA into protocol design enables trade-off analysis and comprehensive decision-making, ensuring environmental and economic viability as well as technical performance. Future efforts should couple LCA and TEA with process design in a closed-loop framework to enable data-driven optimization.

To bridge the gap between lab-scale studies and industrial implementation, a general roadmap toward direct recycling industrialization can be outlined. First, safe and automative pretreatment is required, including battery discharge, diagnostic sorting, and efficient disassembly to reduce labor intensity and handle diverse material feedstocks. Second, scalable separation and regeneration processes should be coupled with impurity control, green processing strategies, and real-time monitoring to ensure robust product quality. Third, industrialization requires continuous and closed-loop process design, including standardized evaluation metrics, pilot-scale validation, and LCA/TEA. In the long term, transparent and automated recycling systems, supported by battery passports and AI-driven robotics, will further enable a sustainable recycling ecosystem.²²¹

Building on this roadmap, the goal of direct recycling is successful scale-up, which requires process optimization and precise control of key parameters through industrial-grade equipment. Cost-effective and automative pretreatment of the spent batteries is essential to manage diverse cell designs, while innovations in cell designs and intelligence systems can reduce labor demands. A comprehensive assessment of direct recycling and advanced upcycling is necessary from the perspective of technical, economic, environmental impacts, validating the feasibility of large-scale production. In addition, establishing standardized evaluation metrics is critical to quantitatively compare and benchmark different direct recycling strategies. Key indicators, such as material recovery efficiency, energy consumption, process cost, and product quality, should be systematically defined and reported to guide technical optimization and support the transition from lab-scale demonstrations to industrial implementation. Finally, establishing pilot plants is crucial to demonstrate process stability and economic viability, while robust supply chains and policy compliance support smooth scaling-up.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

The authors acknowledge the financial support from Michigan State University.

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