Current collector engineering for advanced anode-free alkali metal batteries with liquid electrolyte

Abstract

Due to their high energy density and enhanced safety, anode-free alkali metal batteries (AFAMBs) have emerged as promising candidates for large-scale energy storage applications. However, their practical deployment is hindered by challenges such as uncontrolled dendrite growth and unstable solid electrolyte interphases (SEI), which significantly compromise cycling stability and coulombic efficiency. Recent advances in current collector engineering have shown considerable potential in mitigating dendrite formation and enhancing SEI stability. This review summarizes the latest developments in current collector design strategies aimed at addressing these issues, including materials optimization, crystal orientation regulation, porous structure design, and surface modification. The specific mechanisms for preventing dendrites and stabilizing the SEI are discussed in detail. Furthermore, the remaining challenges and future directions for current collector optimization in anode-free alkali metal batteries are discussed.

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Guangtong Sun

Guangtong Sun received his B.S. degree from Changsha University in 2023. He is now a master student at Dalian Maritime University. Currently, he conducts research at the Institute of Materials Science of Henan Academy of Sciences. His research focuses on current collect engineering for batteries.

Qiongqiong Lu

Qiongqiong Lu (H-Index 33) received his PhD degree at Technische Universität Dresden in 2022 and then conducted postdoctoral research at Leibniz Institute for Solid State and Materials Research (IFW) Dresden, Germany. He subsequently joined the Henan Academy of Sciences. His research focuses on functional materials for electrochemical energy storage applications. He has published over 70 peer-reviewed articles with more than 3600 citations.

Peixun Xiong

Peixun Xiong received PhD degree from Tianjin University in 2020. Then, he worked as a Postdoc Researcher in Prof. Ho Seok Park's group at Sungkyunkwan University from 2021 to 2023. He is currently working in Prof. Stefan Kaskel's group as an Alexander von Humboldt Research Fellow at Technische Universität Dresden. Dr Xiong's research focuses on the electrolyte/electrode interphase for high-performance electrochemical energy storage devices. He has co-authored over 100 journal articles, which received more than 8400 citations with an H-index of 48.

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1 Introduction

To address the greenhouse effect and air pollution caused by fossil fuel combustion, the development and utilization of renewable clean energy sources have become imperative.^1–4 However, the inherent intermittency and fluctuation of renewable clean energy can result in challenges such as grid frequency deviations and voltage instability.⁵ Energy storage systems play a vital role in alleviating these issues by enabling peak shaving and valley filling, thereby enhancing the stability of power systems.^6–10 Among various energy storage technologies, alkali-ion batteries (lithium-ion batteries, sodium-ion batteries, etc.) are becoming one of the most mature and widely adopted due to their high energy density.^11–18 In alkali-ion batteries, alkali metals such as lithium, sodium, and potassium have attracted significant attention as promising anode materials, owing to their high theoretical capacities and low redox potentials.^19–23 Nevertheless, the application of alkali metal anodes is associated with critical safety concerns. Furthermore, the excess alkali metal, typically with thicknesses exceeding 50 μm, substantially reduces the overall energy density of the battery. Anode-free alkali metal batteries (AFAMBs), which eliminate the need for an alkali metal anode, utilize only a thin current collector (thickness ≤ 10 μm) on the anode side. This design not only enhances energy density and safety but also simplifies the battery assembly process, thereby reducing manufacturing costs (Fig. 1).

Fig. 1 (a) Structural illustration of the configuration of different batteries. (b) Radar chart comparing the performance of different batteries. (AIBs: Alkali-ion batteries; AMBs: Alkali-metal batteries; AFAMBs: Anode-free alkali metal batteries).

2 Challenges of AFAMBs

In AFAMBs, during charging, alkali ions are electrochemically reduced and plated onto the current collector in metallic form, while during discharge, this alkali metal is reconverted into alkali ions.^24,25 During the deposition of alkali ions, the current collector's surface with non-atomically smoothness causes an uneven electric field, leading alkali ions to preferentially deposit at high electric field regions like tips, which leads to nonuniform alkali metal deposition and promotes the growth of dendritic structures.²⁶ This uneven deposition can cause significant internal volume expansion in the battery, and dendrite growth can pierce the separator, inducing internal short circuits.^27,28 Additionally, the deposited alkali metal can undergo irreversible side reactions (gas production and generation of passivation layer) with the electrolyte, resulting in loss of sodium and low coulombic efficiency.^29,30 In AFAMBs, under limited Na/K/Li in the cathode, the high reversibility of Na/K/Li is required and the volume change is more serious (Fig. 2). There is an urgent need to develop diverse strategies to effectively suppress dendrite and dead Na formation as well as build stable SEI. There are two main strategies: current collector engineering and electrolyte optimization. In this review, we will focus on current collector engineering in AFAMBs. Although previous reviews have summarized the current collectors engineering in anode-free batteries, the latest progress has not been fully covered and most of them summarize only a single type of anode-free alkali metal battery.^31–39 To address this gap, this review provides a broad overview of current collector engineering strategies aimed at achieving high-performance anode-free alkali metal batteries and adds the latest progress.

Fig. 2 Challenges of AFAMBs.

3 Current collector engineering for AFAMBs

In this section, we summarize current collector engineering for AFAMBs, including: (i) materials optimization; (ii) crystal orientation regulation; (iii) porous structure design; (iiii) surface modification. At the same time, current collector surface modification includes: (i) nucleation layer coating (ii) protective coating (Fig. 3).^31,40–42

Fig. 3 Schematic illustration of the current collector engineering for AFAMBs.

3.1 Materials optimization

Materials play a crucial role in governing the nucleation and deposition behaviors of alkali metal ions. According to electro-crystallization nucleation theory, the nucleation overpotential is inversely proportional to the alkali metal nuclei size and directly proportional to the cube of the nuclei area density.⁴³ Materials with alkali-metallophilicity characteristics can significantly reduce the nucleation overpotential. A lower nucleation overpotential results in larger nucleation sizes and a lower nucleation site density. In addition, a high overpotential accelerates the deposition rate of metal ions, often resulting in uncontrolled alkali metal deposition and growth. Therefore, a current collector with a high alkali-metallophilicity is preferred for achieving uniform and dense alkali metal deposition.

In lithium-metal batteries, Yan et al.⁴⁴ investigated the nucleation behavior of lithium on various metal substrates. They found that metals with definite solubility in lithium, such as Au, Ag, Zn, and Mg, exhibit no nucleation barriers. In contrast, metals with negligible lithium solubility, such as Cu and Al, present significant nucleation barriers (Fig. 4(a)). Therefore, current collectors with a definite solubility in lithium are preferred due to their low nucleation barriers. Furthermore, Pande et al.⁴⁵ conducted a computational screening of potential current collectors for AFLMBs using density functional theory (DFT) calculations. They found that current collectors with low diffusion activation energies and moderately strong binding with lithium can effectively suppress lithium dendrite formation. As a result, lithium alloys such as Li–Zn and Li–Ag emerge as promising current collector materials for AFLMBs due to their high specific energies and low nucleation overpotentials. Besides, Li et al.⁴⁶ found that the overpotential of Li deposition is inversely related to adhesion work, while uniform Li deposition requires a high adsorption energy and a low Li atom diffusion barrier. Furthermore, an alloy with atomically dispersed lithophilic sites was a promising current collector. For example, Liu et al.⁴⁷ proposed Cu₉₉Zn alloy as a current collector. The uniformly distributed atomic-scale Zn sites in Cu₉₉Zn serve as high-coverage nucleation seeds, enabling uniform Li nucleation. Therefore, lithium is homogeneously deposited across the entire current collector surface, promoting stable and dendrite-free electrodeposition (Fig. 4(b)). Recently, Ouyang et al.⁴⁸ prepared an ultralight composite current collector composed of polyacylsemicarbazide (PASC) substrate and sputtered Cu with a thickness of 5.2 μm and a surface density of 0.78 mg cm⁻² for AFLMBs. The polyacylsemicarbazide substrate enhanced interfacial interaction with the sputtered Cu layer, resulting in excellent interfacial stability, flexibility, and safety. The nanostructure of the copper layer on the surface of the composite current collector reduced the nucleation overpotential. In addition, the nanostructured PASC-Cu can enhance the lithium ions transport, improve the local ion concentration, and promote the formation of Li₃N-rich SEI, which improves the lithium plating/stripping reversibility and enhances the electrochemical performance.

Fig. 4 (a) Voltage profiles of various materials with some solubility in Li during Li deposition at a current density of 10 μA cm². Reprinted with permission from ref. 44 Copyright (2016), Springer Nature. (b) Illustration of Li plating behavior on different substrates. Reprinted with permission from ref. 47. Copyright (2018), ELSEVIER. (c) Illustration of the sodium nucleation and deposition behavior on Cu or Zn current collectors. Reprinted with permission from ref. 50. Copyright (2022), IOP Publishing. (d) Summary of SEI observations. Reprinted with permission from ref. 51 Copyright (2023), American Chemical Society.

In sodium-metal batteries, Tang et al.⁴⁹ compared the Na affinity of Au, Sn, Sb, Cu, Cr and Mo elements by electrochemical measurements. It was found that the in situ formed Na–M (M = Au, Sn, Sb, etc.) alloy as a sodium-philic interface phase can reduce the sodium nucleation overpotential, which is conducive to the smooth deposition of sodium. However, Cr and Mo, which have the same lattice structure as sodium, cannot form alloys with sodium and therefore cannot be used to prepare sodium-philic current collectors for smooth sodium deposition. It is worth noting that repeated alloying–dealloying cycles may cause the pulverization of sodiophilic alloys, leading to interfacial degradation. Therefore, enhancing the adhesion of the sodiophilic interface is also crucial.⁴⁹ In addition, Liu et al.⁵⁰ employed Zn foil as a current collector. During the initial deposition process, the interaction between Zn and Na produces a sodiophilic NaZn₁₃ alloy, which work as nucleation sites guiding the uniform deposition of sodium, thus avoiding the growth of dendrites (Fig. 4(c)). In contrast, Cu foil and Al foil show a dendrite morphology. Furthermore, Copper et al.⁵¹ found that Zn foil is a promising current collector because of high lattice compatibility, low nucleation overpotential, and the ZnF₂-rich SEI formation. In contrast, the Cu foil showed large nucleation overpotentials and unfavorable corrosion reactions with HF, leading to rapid battery failure. Although traces of ZnF₂ were detected in the SEl for the α-brass current collector, the lower lattice compatibility and corrosion reaction between Cu and HF degraded the battery lifespan (Fig. 4(d)). Recently, Li et al.⁵² prepared Al–Sb eutectic alloy as a current collector employing the melting process. Sn metal and SnO₂ on the surface of the current collector have strong binding energy with sodium, so the current collector exhibits a low Na nucleation barrier, enabling uniform sodium deposition on the substrate surface, inhibiting dendrite growth.

3.2 Crystal orientation regulation

Because alkali-metal ions preferentially deposit at defect sites such as grain boundaries, uneven metal deposition occurs, which promotes dendrite formation. In contrast, a single-crystal current collector, which lacks grain boundaries, eliminates the preferred nucleation at these sites. This facilitates uniform alkali-metal deposition, resulting in a dendrite-free morphology.⁵³ Moreover, the single-crystal current collector influences the composition of the solid electrolyte interphase (SEI) through its interaction with solvent molecules and salt anions.⁵⁴ However, single-crystal current collectors are mostly prepared using high-temperature calcination, leading to high cost. The current collector with preferred orientation more favorable due to its simple preparation and low cost.

In lithium-metal batteries, because of the homogeneous physical and chemical properties of single-crystal Cu foil surfaces, a lateral growth model of Li was observed on the (110) plane of single-crystal Cu foil, leading to smooth and planar Li electrodeposition.⁵⁵ In addition, Kim et al.⁵⁶ found that he near-zero migration barrier of Li adatoms on single-crystal Cu(111) foils, which results in horizontal lithium growth and inhibits the dendrite growth. Additionally, Gu et al.⁵⁷ developed a Cu (100) preferred faceted current collector using an electrochemical faceting strategy. Due to its low lattice mismatch and strong binding effect with the lithium (110) plane, the Cu (100) current collector can guide the lithium (110) plane to grow parallel to the current collector, and achieve uniform lithium deposition and low nucleation barrier, thereby constructing a stable lithium metal-electrolyte interface phase, and reducing the side reactions between the anode and the liquid electrolyte. In addition, Li et al.⁵⁸ used textured reduced graphene oxide (rGO) (001) current collectors to lock the (110) face of lithium metal. The experimental and simulation results showed that due to the good in-plane lattice matching between the lithium (110) face and the rGO (001) face, each layer of lithium atoms grew along the (110) face, thus achieving planar lithium deposition with excellent reversibility (Fig. 5(a–d)). Furthermore, Hao et al.⁵⁴ demonstrated that tuning the crystal orientation of Cu foil can regulate the composition of SEI and its subsequent lithium deposition behavior. The strong adsorption of Cu (100) surface to anions can guide more anions to preferentially participate in the inner Helmholtz plane, further promoting the formation of a stable inorganic-rich SEI (Fig. 5(e)). As a result, single-crystalline Cu (100) achieves dendrite-free lithium deposition in a series of electrolytes with different anions, and improves the reversibility of lithium deposition/stripping and the electrochemical performance of the battery. Besides, Zhan et al.⁵⁹ fabricated a lithium salt-philic (220) Cu foil as a current collector. The Cu (220) enhanced lithium salt adsorption and catalyzed the decomposition of the salt into an inorganic-rich SEI. Due to its high ionic conductivity and mechanical properties, the inorganic-rich SEI enables fast ionic transport and prevent the dendrite formation.

Fig. 5 (a) Schematic illustration of the growth process of Li on the rGO substrate. (b–d) Top-view SEM images of the rGO substrate. Reprinted with permission from ref. 58 Copyright (2019), Wiley. (e) Schematic illustrations of the Li plating process on p-Cu, and s-Cu (100). Reprinted with permission from ref. 54 Copyright (2024), Wiley. (f) Theoretical mechanism of sodium deposition. Reprinted with permission from ref. 53 Copyright (2025), Springer Nature.

In AFSMBs, Liu et al.⁵³ constructed Al (100) single-crystal current collector based on the grain boundary migration theory by high temperature calcination, The Al (100) single crystal current collector eliminated the diffusion resistance of sodium ions at the grain boundary, reduced the sodium nucleation overpotential and interface diffusion energy barrier, increased the sodium ion transmission rate, and showed uniform and reversible sodium deposition ability (Fig. 5(f)). Besides, Wu et al.⁶⁰ prepared a F-rich surface of the (100)-oriented Al substrate by annealing followed by fluorination with hydrofluoric acid. The fluorine-rich surface of the (100) – oriented aluminum substrate offers high-affinity nucleation sites that facilitate uniform sodium metal deposition. Meanwhile, this surface also promotes the generation of a robust NaF-rich SEI layer. Since NaF has a high Young's modulus and good ionic conductivity, it can inhibit dendrite growth and accelerate Na ion transport, thereby improving electrochemical performance. Although the application of single-crystal current collectors can effectively regulate uniform alkali metal deposition and thus inhibit dendrite growth, the complex preparation process with high cost makes this strategy difficult to apply commercially on a large scale.

3.3 Porous structure design

To mitigate dendrite formation and to buffer the volumetric expansion that accompanies alkali-metal deposition, researchers have implemented current collectors with porous architectures. In accordance with Sand's equation, high-porosity current collectors lower the local current density owing to their large surface area, which facilitates charge dispersion and delays dendrite formation.⁶¹ Meanwhile, current collectors with low density and superior mechanical behavior should be considered. However, alkali metals tend to deposit on the surface of conductive hosts, so guiding their deposition into the interior of the porous current collector is crucial. In this regard, researchers have developed a variety of different structures, such as foams, gradients, and Janus.^62–66

In anode-free Li-metal batteries (AFLMBs), lots of porous current collector was proposed to prevent the dendrite formation and accommodate the volume expansion. Among metal current collectors, electrodeposited copper foil enables the formation of well-defined structures, making it promising as a current collector. For example, Umh et al.⁶⁷ prepared Cu dendritic superstructure using the dynamic H₂-bubble template method. This hierarchical current collector features engineered macropores and an enhanced specific surface area. Li-metal deposition follows the hierarchical architecture, thus preventing the vertical Li dendrite growth. Compared with other methods for fabricating 3D metallic current collectors, this method is environmentally friendly and cost-effective, and can obtain a hierarchical structure without using any additional organic or inorganic templates, so it has great application prospects. In addition, Zhang et al.⁶⁸ prepared ultrathin porous Cu film using complexing-agent electrolytes at precise temperatures. The Cu film with three-dimensional hierarchical micro and nanoscale porosity contributes to the homogenization of the local electric field, thereby suppressing dendrite formation and promoting a stable SEI generation. At the same time, by changing the temperature of the complexing-agent electrolyte, different pore structures can be obtained. These micropores can accelerate the transmission of lithium ions and have enough space to accommodate lithium, thereby alleviating the volume expansion of the anode during the cycle. Since lithium tends to deposit on the surface of conductive current collectors, designing a current collector with a gradient in lithium affinity is advantageous for guiding lithium deposition into the porous structure of the collector and thus preventing the dendrite formation on the surface. For instance, Gong et al.⁶⁹ constructed a carbon nanotube skeleton current collector with gradient lithium affinity. Using physical vapor deposition, the pre-lithiation of carbon nanotubes was achieved, inducing the formation of gradient lithium affinity, enabling lithium ions to be preferentially deposited at the bottom of the skeleton with high affinity, inhibiting the generation of dendrites on the surface (Fig. 6(b)). Although a structured current collector can help prevent dendrite formation, it is also essential to tailor the stable solid electrolyte interphase (SEI) formation for minimizing side reactions between lithium and the electrolyte. Li et al.⁷⁰ prepared a CuCl pre-coated engineered micro-hole-grid copper reservoir. The CuCl induced a LiCl-concentration gradient SEI formation, which enables Li-ionic conductivity and stabilizes the interface. In addition, the Cu nanoparticle derived from the CuCl reduction as nucleation sites guides the Li deposition into the pores of the copper reservoir, thus preventing dendrite formation (Fig. 6(a)). In addition, Kwon et al.⁷¹ utilized an atomically defective carbon current collector, which fosters a uniform and ultrathin SEI formation. Additionally, it promotes the lateral distribution of lithium nuclei across the current collector surface, thereby guiding uniform Li deposition. In addition, this defect has good interfacial compatibility in anode-free batteries due to its low Fermi level characteristics, which can inhibit the lowest unoccupied molecular orbital (LUMO) of electron transfer to the electrolyte and endow lithium atoms with strong binding force through electron transfer, thus addressing the issues of dendrite growth and uneven SEI on the current collector surface.

Fig. 6 (a) Li deposition behavior during the charging process on three-dimensional MHG structures compared to two-dimensional Cu foils. Reprinted with permission from ref. 70. Copyright (2022), ELSEVIER. (b) Schematic illustration of lithium-ion deposition on LCu and LC. Reprinted with permission from ref. 69. Copyright (2020), American Chemical Society. (c) Representation for the combination of resilient Ti₃C₂T_x|CNT NAFs. Reprinted with permission from ref. 73. Copyright (2023), ELSEVIER.

In anode-free sodium metal batteries (AFSMBs), Liu et al.⁷² developed a porous Al current collector. The interconnected porous structure provides an increased surface area for Na nucleation and promotes a more uniform Na⁺ flux distribution, leading to uniform Na plating and inhibiting dendrite growth. In addition, Al does not alloy with Na. It is advantageous over Cu current collectors in terms of cost and weight. Besides, Kandula et al.⁷³ designed mechanically robust MXene|carbon nanotube (CNT) nano-accordion frameworks (NAFs) capable of accommodating sodium deposition without dendrite formation. These microcellular MXene|CNT-NAFs feature abundant micron-sized pores and uniformly distributed sodium nucleation sites, lowering the overpotential and promoting homogeneous Na deposition. Among cycling, the resilient nano-accordion structures undergo reversible compression and expansion driven by the capillary forces of Na nucleation and dissolution, thereby enabling long-term operation with minimal volumetric fluctuation. Shown in Fig. 6(c). In addition, Li et al.⁷⁴ fabricated mesoporous nitrogen-doped carbon nanofibers uniformly embedded with tin clusters (SnNCNFs) as a current collector. The hybrid host exhibits exceptional sodiophilicity, enabling rapid sodium infusion and achieving a Na nucleation overpotential of only 2 mV. The porous architecture accommodates high sodium loading and facilitates Na deposition uniformity, supporting highly reversible sodium plating and stripping. Additionally, Xu et al.⁷⁵ prepared carbon nanofiber with Zn-Nx active sites, which enable Na⁺ laterally diffusion and planar growth as well as enhance the adsorption of PF₆⁻ and thus forming a stable inorganic-rich SEI. In addition, An et al.⁷⁶ designed N,P-codoped carbon macroporous fibers incorporated with sodiophilic CoP nanoparticles, which effectively suppresses dendrite growth as a result of decreased local current density and more uniform Na⁺ flux distribution.

In AFPMBs, Li et al.⁷⁷ showed a hierarchically porous carbon nanofibers embedded with uniformly dispersed binary active sites comprising nitrogen and zinc clusters. The porous host combined with Zn-containing binary potassiophilic sites provides low density and void space to accommodate high potassium loading, and offers strong potassiophilicity facilitating preferential potassium nucleation. In addition, this host can also regulate the local electric field, thus promoting uniform potassium plating, inhibiting dendrite growth. Although the porous hosts can reduce the local current density and accommodate the alkali metal deposition, the high specific surface area will intensify the side reactions between the alkali metal and the electrolyte.

3.4 Surface modification

Since alkali metals deposit on the surface of the current collector, the surface properties of the collector play a crucial role in regulating alkali metal deposition and SEI. Therefore, it is important to modify the current collector surface to guide uniform alkali metal deposition and form a stable SEI. Current collector surface modification strategies generally include two main approaches: (1) introducing alkali metal-philic materials to serve as nucleation sites, thereby promoting uniform alkali metal deposition and suppressing dendrite formation; (2) incorporating protective layers to stabilize the interface and mitigate side reactions between the electrolyte and the alkali metal.

3.4.1 Nucleation layer coating. Due to their strong affinity for alkali metals, alkali metallophilic materials can effectively lower the nucleation overpotential and can serve as a nucleation layer coating, thus guiding the uniform deposition of alkali metals and suppressing dendritic growth.⁷⁸ Moreover, conductive coatings contribute to the homogenization of the local electric field and ion flux, achieving uniform alkali metal deposition.

In AFLMBs, Zhang et al.⁷⁹ employed a thin Sn layer as a seeding interlayer to facilitate lithium plating on a Cu substrate. During the initial deposition stages, Li reacts with the Sn layer to form the Li–Sn alloy. Due to the strong chemical affinity between Li and the Li–Sn alloy, the pre-deposited tin interlayer enhances Li adhesion to the copper substrate and promotes a uniform deposition morphology. What's more, Guan et al.⁸⁰ introduced a nanostructured 3D Cu@Sn architecture, which can promote efficient electron conduction, reduce local current density, limit lithium deposition, and mitigate volume expansion during cycling. In addition, the uniform tin deposited by magnetron sputtering was converted into lithium-tin alloy through an in situ alloying reaction and integrated into the 3D copper framework, which can fully contact the liquid electrolyte, thereby promoting the rapid diffusion of lithium ions (Fig. 7(a)). Besides, Cheng et al.⁸¹ fabricate an Ag@3D-Cu current collector, where the 3D structure reduces local current density as well as provides abundant lithium nucleation sites and extensive internal channels to accommodate deposited lithium, enabling Li deposits to homogeneously throughout the Ag@3D-Cu internal architecture. Furthermore, Weldeyohannes et al.⁸² developed a porous polyimide film with the back of a gold-sputtered (PI@Au) as the current collector. This design directs initial Li nucleation and subsequent growth away from the separator-facing side, referred to as away-from-separator growth. Such backside deposition effectively mitigates the risk of dendrite penetration into the separator, thereby enhancing battery safety. Besides, Kang et al.⁸³ demonstrate that benzotriazole (BTA) modifies the copper surface. DFT simulations demonstrated that the nitrogen atoms of BTA molecules have a higher affinity for Li⁺ than Cu atoms, which is beneficial for Li⁺ nucleation and leads to dendrite-free lithium deposition on BTA-modified Cu foil, contributing to improved reversible Li⁺ plating/stripping. To reduce the nucleation barriers and achieve lateral growth, Co and N co-doped carbon collector with hard base (N sites) and soft acid sites (carbon matrix) was proposed by Zhu et al.⁸⁴ The N sites help in adsorbing solvated Li⁺ and reduce the nucleation barrier. Meanwhile, the carbon matrix facilitates lateral growth kinetics of Li atoms by enhancing Li-substrate interaction (Fig. 7(b)).

Fig. 7 (a) Schematic representation of the of Li plating. Reprinted with permission from ref. 80. Copyright (2021), ELSEVIER. (b) Schematic diagram of the three key processes in lithium deposition and the structure of HBSA-Co Sas. Reprinted with permission from ref. 84. Copyright (2018), Royal Society of Chemistry. (c) Schematic diagrams of the formation mechanism of unstable Na–Sn alloy interfaces and stable microalloying welding interfaces and their impact on sodium plating/stripping. Reprinted with permission from ref. 94. Copyright (2022), Springer Nature. (d) Schematic of K deposition behavior on Cu and Cu–OSe NWs. Reprinted with permission from ref. 95. Copyright (2024), ELSEVIER.

In AFSMBs, Dahunsi et al.⁸⁵ prepared nanoscale carbon-coated Al as a current collector. The carbon–aluminum junction sites interface facilitated more uniform sodium deposition and significantly reduced overpotentials. Besides, Li et al.⁸⁶ employed a graphite carbon-coated Al to regulate the Na deposition. Meanwhile, stable SEI and CEI were achieved by adding boron-containing salts. This effectively inhibits the formation of dead Na or Na dendrite, repairs the cracks formed during sodium insertion and extraction, protects the integrity of the electrode, and also prevents side reactions with the electrolyte. In addition, Zhu et al.⁸⁷ utilized a plasma-treated carbon-coated Al (p-Al@C) as a current collector to guide the uniform Na deposition. Meanwhile, a weakly solvated double salt electrolyte is used to facilitate the formation of a B/F-derived inorganic-species-rich SEI, which also helps to suppress the dendrite growth and reduce the side reactions. Furthermore, Cai et al.⁸⁸ prepared a nanofiber Zn on aluminum foil (Zn@Al) using the magnetron sputtering method. The intrinsic sodiophilicity of zinc and the formation of NaZn₁₃ after cycling exhibit significantly stronger affinity for sodium, effectively lowering the nucleation overpotential and promoting uniform sodium deposition. Furthermore, the 3D Zn layer with increased surface electrochemical activity contributes to reduced local current density, promotes a more homogeneous electric field distribution, thereby guiding the uniform Na deposition. In addition, Ruan et al.⁸⁹ constructed a hard carbon-derived interfacial layer on an Al current collector, taking advantage of its low dielectric constant and strong interaction with sodium, achieving uniform sodium nucleation, thereby minimizing active Na loss during cycling. Besides, Zhang et al.⁹⁰ fabricated Cu₃P nanowires on copper foil (Cu₃P@Cu), which reduces the Na diffusion barrier and induces uniform Na nucleation. This is because Cu₃P will form Na₃P after the sodiation process, which has lower sodium adsorption energy and diffusion energy, so that sodium is smoothly deposited on the surface of Cu₃P current collector, avoiding the growth of dendrites. Recently, sodiophilic substrates, such as Sn, Zn, and Ag, have also demonstrated the ability to promote the formation of inorganic-rich SEI layers (e.g., NaF, Na₂O), due to their strong adsorption affinity toward PF₆⁻ anions. The high ionic conductivity and Young's modulus of these inorganic-rich SEI components contribute to uniform sodium deposition, thereby enhancing the electrochemical performance of AFSMBs.^91–93 For example, Xie et al.⁹⁴ prepared sodiophilic nano-zinc coating on carbon-coated Al (Al@C–Zn). They found that Al@C–Zn induces a thin and robust organic/inorganic hybrid SEI formation (Fig. 7(c)). This avoids the formation of dead Na and allows for reversible Na stripping/platting. Besides, Hao et al.⁵⁴ found that Ru also accelerated the decomposition of NaPF₆, thereby forming an ultra-thin NaF-rich SEI layer with a high Young's modulus and ionic conductivity, thereby inhibiting dendrite growth and facilitating the Na⁺ transform.

In AFPMBs, Cui et al.⁹⁵ used non-fully selenized Cu–OSe nanowires (NWs) as potassiophilic subtract, whose active sites provide a low energy barrier for K nucleation. In addition, the low potassium ion transport energy promotes the easy diffusion behavior of K on the surface of Cu–OSe NWs (Fig. 7(d)). Besides, Zhao et al.⁹⁶ prepared potassiophilic graphene-modified Al foils (Al@G) via a roll-to-roll plasma-enhanced chemical vapor deposition (PECVD) technique. Benefiting from the strong interfacial adhesion (10.52 N m⁻¹) and high surface energy (66.6 mJ m⁻²), the Al@G facilitates a uniform potassium deposition process and minimizes the loss of potassium. Besides, Yu et al.⁹⁷ use Ni-modified carbon nanofibers coated with Al as a current collector. Due to its high potassium affinity, Ni guides the uniform potassium deposition. In addition, the Ni-modified carbon nanofibers increase the work function of Al, which prevents solvent reduction and forms an inorganic-rich SEI film with high elasticity.

3.4.2 Protective layer coating. Although the presentation of a nucleation site layer can regulate the alkali-metals uniform deposition, the side reactions are not inhibited. Therefore, the introduction of a protective layer is also essential to inhibit direct contact between the alkali metal and the electrolyte, thereby reducing side reactions.^98–100

In AFLMBs, ionic-conductive polymer was used as a protective layer coating due to its flexible and robust properties. For instance, Chen et al.¹⁰¹ used a polyethylene oxide (PEO) film-coated Cu as a current collector. The PEO coating facilitates a thin and robust SEI generation, thus promoting controlled Li incorporation and mitigating uncontrolled side reactions. Consequently, the modified Cu foil demonstrated highly stable lithium cycling, maintaining an average CE of approximately 100% after 200 cycles and exhibiting minimal overpotential of ∼30 mV at 0.5 mA cm⁻². Furthermore, the AFLMBs with the LiFePO₄ cathode exhibited an average CE of 98.6% and a capacity retention rate of 30% at a 0.2C rate after 200 cycles. In addition, Abrha et al.¹⁰² prepare a β-poly(vinylidene difluoride) (PVDF) coating on Cu via electrospinning. β-PVDF adsorption is thermodynamically favorable on the Li surface, effectively directing Li cation flux. In addition, the interaction between β-PVDF and plated lithium leads to the generation of a robust LiF-rich interface, avoiding direct contact between the electrolyte and the current collector, reducing the occurrence of side reactions. After undergoing electrochemical activation (TEA) via five cycles at 60 °C, the Cu@β-PVDF‖NMC cell shows capacity retentions of 68.36% and 78.45% at the 20th cycle at 25 °C and 60 °C at 0.2 mA cm⁻². Besides the polymer materials, the inorganic materials as a protective layer coating are also favored, because their high Young's modulus can prevent the dendrite formation. For example, Chen et al.¹⁰³ investigated SiO_x coating as a protective layer. A uniform and porous SiO_x coating was transformed from the silicone-based tape by using a laser. This coating achieves a superior average CE of 99.3% at 2.0 mA cm⁻² with 2.0 mA h cm⁻². AFLMBs show an average CE of 99.2% and a capacity retention of 45.6% after 100 cycles at 0.9 mA cm⁻². Compared to other coating methods, preparation coatings using lasers is dry, rapid, and avoids the use of toxic organic solvents and extensive drying times, making laser conversion a promising coating method for practical applications. To combine the advantages of the nucleation layer and protective layer, the dual-layer structured interface is favored to simultaneously prevent the dendrite formation and side reactions. For instance, Temesgen et al.¹⁰⁴ fabricated a double-layer structured interlayer on Cu foil. The lithiophilic Ag nanoparticles encapsulated in polydopamine (Ag@P) on the bottom layer, acting as lithium nucleation seeds, effectively reduce nucleation overpotential by forming alloys with Li, guiding both the Li nucleation and deposition processes. Meanwhile, the upper layer composed of PVDF-HFP and LiTFSI facilitates the formation of a robust, densely packed anion-derived SEI, therefore effectively preventing the side reactions. AFLMBs with an NMC cathode show a capacity retention of 41.44% after 70 cycles at 0.2 mA cm⁻². Besides, Wang et al.¹⁰⁵ fabricated a highly ordered hollow ZnO matrix and a surface-coated LiPON layer. The ZnO matrix provides sufficient cavities and lithiophilic sites to promote uniform lithium plating/stripping within the cavities, while the LiPON layer maintains a solid electrolyte interface that is stable from mechanical and electrochemical damage. As a result, lithium is confined within the cavities, and the overall anode shape is effectively controlled during long-term and high-rate cycling. The assembled half-cell operates stably for 335 cycles with a coulombic efficiency of 98.83% at 1.2 mA cm⁻² with 0.6 mA h cm⁻². The full cell using a modified LiFePO₄ (mLiFePO₄) cathode exhibits a lifespan of 150 cycles and a high energy density of 435 Wh kg⁻¹ with a CE of 98.01% at 2C rate. In addition, the full cell using an NCM523 cathode exhibits a CE of 98.23% at 1C rate for 200 cycles. Furthermore, forming a dual-layer structured interphase through the reaction between the coating and the lithium is more promising. For instance, Xia et al.¹⁰⁶ proposed Cu₂O nanoparticles decorated on Cu current collectors. Due to the reaction between Cu₂O and Li, metallic Cu nanoparticles and Li₂O were produced. These in situ generated Cu nanoparticles form continuous electronic pathways and substantially increase the electrode's effective surface area, thereby facilitating uniform Li nucleation. The formed lithium oxide functions as a stable artificial SEI component. Therefore, the half-cell maintains an average CE of 99.16% over 150 cycles at 2 mA cm⁻² with 2 mA h cm⁻², while the AFLMBS exhibits a capacity of 41% over 100 cycles with an average CE of 99.24% at 0.5C. Besides, Wang et al.¹⁰⁷ prepared an ultrathin (250 nm) triethylammonium germanate (TEG) coating on Cu foil. Owing to enhanced adsorption energy with Li, the derived tertiary amine and Li_xGe alloy facilitate the adsorption, nucleation, and deposition of lithium ions (Fig. 8(a)). As a consequence, asymmetric cell shows a CE of 99.3% for 250 cycles at 1 mA cm⁻² with 1 mA h cm⁻², and AFLMBs with 16.9 mg cm⁻² LFP loading deliver a capacity retention of 40.2% after 250 cycles at 1.36 mA cm⁻². Recently, Ouyang et al.¹⁰⁸ also proposed deoxyribonucleic acid (DNA) as a coating on Cu foil. Due to the strong interaction between base pairs in DNA and Li, homogeneous Li⁺ flux and enhanced deposition kinetics were achieved (Fig. 8(b)). Consequently, asymmetric cell demonstrates a CE of 99.2% for 425 cycles at 1 mA cm⁻² with 1 mA h cm⁻², and AFLMBs with 20.3 mg cm⁻² LFP show a capacity retention of 30% over 400 cycles at 0.5C.

Fig. 8 (a) Schematic illustration of the anode-free battery based on a TEG-modified Cu current collector. Reprinted with permission from ref. 107 Copyright (2023), Wiley. (b) Schematic illustration of an anode-free Li metal battery using a DNA-modified Cu current collector. Reprinted with permission from ref. 108 Copyright (2024), Wiley. (c) Diagrams of the SMBs with different interfaces. Reprinted with permission from ref. 109 Copyright (2023), Wiley.

In AFSMBs, Wang et al.¹⁰⁹ prepared sodium formate (HCOONa) on Cu current collector (Fig. 8(c)). HCOONa interface work as a robust SEI to facilitate dendrite-free Na deposition and prevent contact between Na from the electrolyte to reduce side reactions. As a result, AFSMBs with a 10 mg cm⁻² NVP loading exhibit a capacity retention of 88.2% over 400 cycles at 0.5C.

Overall, the above research work demonstrates the feasibility and great prospects of the strategy of current collector modification in improving the electrochemical performance of AFAMBs. Finally, the progress of typical AFAMBs performance is summarized in Table 1.

Table 1 Comparison of the performances of asymmetric cells and the full cells

	Electrode	Electrolyte	Coulombic efficiency of asymmetric cell	Anode-free batteries performance	Ref.
AFLMBs	Au NP@carbon	1 M LiPF6 in EC/DEC (1 : 1) 1% VC + 10% FEC	98% for 300 cycles at 0.5 mA cm^−2 with 1.0 mA h cm^−2	—	44
	Cu99Zn on Cu	1 M LiPF6 in EC/DEC	98% for 180 cycles at 0.5 mA cm^−2 with 1 mA h cm^−2	—	47
	Cu (100)	1 M LiTFSI/DME-DOL (1/1, V/V) with 2 wt% LiNO3	99% for 400 cycles at 2 mA cm^−2 with 1 mA h cm^−2	—	57
	rGO (001) CNT	1 M LiPF6 in EC/DEC	99% for 300 cycles at 1 mA cm^−2 with 1 mA h cm^2	—	58
	Cu (100)	1 M LiTFSI in DOL/DME with 2 wt% LiNO3	99.17% for 800 cycles at 1 mA cm^−2 with 1 mA h cm^−2	—	54
	Cu (110)	1 M LiPF6 in EC/DMC (1 : 1 in volume)	98% for 100 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with NCM523 cathodes, the initial discharge specific capacities is 144.9 mA h g^−1 after 300 cycles at 1C rate with capacity retention of 76.71%	55
	Cu (110)	1 M LiPF6 in EC/DMC (1 : 1 in volume)	92% after 100 cycles at 1.0 mA cm^−2 with 1.0 mA h cm^−2	—	55
	Cu (111)	1.3 M LiPF6 in ECDEC + 10 wt% FEC	97.2% for 10 cycles at 0.5 mA cm^−2 with 3.5 mA h cm^−2	Coupled with 5.3 mA h cm^−2 LiNi0.8Co0.1Mn0.1O2 (NCM 811) cathode delivers a capacity retention of 80% after 120 cycles at 0.2C	56
	Cu (220)	1 M LiPF6 in EC : DEC + 10 vol% FEC + 1 vol% VC	From 99.2% to 99.5% after 400 cycles at 0.5 mA cm^−2 with 1 mA h cm^−2	Coupled with 1.5 mA h cm^−2 NCM 811 shows a capacity retention of 61% after 100 cycles at 0.2C	59
	Cu dendritic superstructure	1 M LiTFSi in 1,3 DOL/1,2-DME (1 : 1 by volume) with 5 mM Li2S8	95% for 140 cycles at 0.5 mA cm^−2 with 1 mA h cm^−2	—	67
	Porous Cu	1 M LiTFSI and 2% LiNO3 in DOL and DME (1 : 1)	96% for 150 cycles at 0.5 mA cm^−2 with 1 mA h cm^−2	Coupled with LiFePO4 cathodes (4.5 mg cm^−2), after 100 cycles at 0.2C rate with 98.3% CE and capacity retention from of 55.5%	68
	Carbon nanotube skeleton	1 M LiTFSI in DME/DOL = 1 : 1 vol% with 1% LiNO3	98% for 200 cycles at 0.5 mA cm^−2 with 1 mA h cm^−2	Coupled with LiFePO4, after 500 cycles at 1C with N/P ratio of 1.43 with 86% capacity retention and CE of 95.07%	69
	MHG-Cu-CuCl	1 M LiTFSI in DOL/DME = 1 : 1	99% for 400 cycles at 1 mA cm^−2 with 2 mA h cm^−2	Coupled with LiFePO4, capacity from 125 mA h g ^−1 to 95 mA h g^−1 after 100 cycles at 1.05 mA cm^−2 with 99.5% CE and 76.6% capacity retention	70
	Atomically defective carbon current collector	1 M LiPF6 EC/DEC + 10% FEC + 1% VC	92.1% for 60 cycles at 2 mA cm^−2 with 5 mA h cm^−2	Coupled with LiNi0.8Mn0.1Co0.1O2 with areal capacity (4.2 mA h cm^−2), after 50 cycles under lean electrolyte conditions (E/C of 4.0 μl mAh^−1) with 90% capacity retention	71
	Li–Sn@Cu	1 M LiPF6 dissolved in a 1 : 4 (wt.) FEC and EMC	88.1% for 10 cycles at 0.5 mA cm^−2 with 0.5 mA h cm^−2	Coupled with LiNi0.85Co0.10Al0.05O2, after 80 cycles with 93% CE at 0.1 mA cm^−2	79
	Ag@3D-Cu	1 M LiTFSI and 1 wt% LiNO3 in DOL/DME (v/v = 1 : 1)	—	Coupled with Li2S (14.6 mg cm^−2), capacities from 573.1 mA h g^−1 to 252.6 mA h g^−1 after 40 cycles at 0.1C with 44.1 capacity retention	81
	PI@Au	1 M LiTFSI dissolved in DOL/DME (1 : 1 vol%) with 2 wt% LiNO3	96.55% for 160 cycles at 0.5 mA cm^−2 with 2 mA h cm^−2	Coupled with LiFePO4 after 340 cycles at 0.5 mA cm^−2 with 20% capacity retention and 98.7% CE	82
	BTA-Cu	1 M LiTFSI in a solvent mixture of DOL/DME (1 : 1 vol%) with 2% LiNO3	99.0% for 350 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with LiFePO4 after 200 cycles at 1C of 33% DOD with 98.5% CE	83
	HBSA-Co SA collector	1M LiPF6 in EC : DEC : DMC = 1 : 1 : 1 by volume, with 1% LiNO3	99.2% for 657 hours at 0.1 mA cm^−2 with 2 mA h cm^−2	Coupled with 15 mg cm^−2 NCM811 deliver an initial capacity of 209.2 mA h g^−1 with a capacity retention of 79.8% after 150 cycles at 0.1C	84
	Cu@PEO	1M LiTFSI, DME/DOL (v/v 1/1) and 2 wt% LiNO3	∼100% for 200 cycles and low voltage hysteresis (∼30 mV) at 0.5 mA cm^2	Coupled with LiFePO4 after 200 cycles at 0.2C with 98.6% CE and 30% capacity retention	101
	Cu@β-PVDF	1 M LiPF6 EC/DEC (1 : 1 v/v)		Coupled with LiNi1/3Mn1/3Co1/3O2, at the 20th cycle, with 68.36% capacity retention and CE of 99.22% for subsequent cycling at 25 °C and a capacity of 78.45% and CE of 97.51% for subsequent cycling at 60 °C at 0.2 mA cm^−2	102
	LI-SiOx	4 M LIFSI/DME	99.3% for 250 cycles at 2 mA cm^−2 with 2 mA h cm^−2	Coupled with LiFePO4, after 100 cycles at 0.9 mA cm^−2 with 45.6% capacity retention and 99.2% CE	103
	Cu/Ag@P-APF	1M LiPF6 in EC : DEC (1 : 1 v/v ratio) plus 5% FEC	97.30% for 60 cycles at 0.2 mA cm^−2 with 1 mA h cm^−2	Coupled with LiNi1/3Mn1/3Co1/3O2 (14.59 mg cm^−2), after 70 cycles at 0.2 mA cm^−2 with 40% capacity retention and 98.15% CE	104
	Li4.4Sn@Cu	6 M LiFSI in DME	99.6% for 500 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with LiNi0.8Mn0.1Co0.1O2 (4 mA h cm^−2), after 50 cycles with 355 W h kg^−1 under 2 g A h^−1 with 85.5% capacity retention	110
AFSMBs	Cu@Sn-NPs	1 M NaOTf-diglyme with 0.01 M NaTFSI	99.9% for 2000 cycles at 2 mA cm^−2 with 1 mA h cm^−2	Coupled with Na3V2(PO4)3, the capacity after 100 cycles at 0.118 mA g^−1, equivalent to 0.2 mA cm^−2 with 92.2% capacity retention	49
	Cu@Sb-MPs	1 M NaOTf-diglyme with 0.01 M NaTFSI	99.9% for 1700 cycles at 2 mA cm^−2 with 1 mA h cm^−2	Coupled with Na3V2(PO4)3, the capacity after 100 cycles at 0.118 mA g^−1, equivalent to 0.2 mA cm^−2 with 96.2% capacity retention	49
	NaZn13	1 M NaPF6 in DIGLYME	99.87% for 450 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with Na3V2(PO4)3, after 40 cycles at 0.5C with 98.5% capacity retention	50
	Al–Sb	1 M NaPF6 in DIGLYME	99.97% for 4000 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with 10 mg cm^−2 NVP loading demonstrated a capacity of 97.7 mA h g^−1 with a capacity retention of 81% after 100 cycles at 0.5C	52
	Zn	1 M NaPF6 in DIGLYME	98.9% for 200 cycles at 0.2 mA cm^−2	—	51
	Al (100)	1M NaPF6-DME	99.9% for 500 cycles at 2 mA cm^−2 with 2 mA h cm^−2	Coupled with Na3V2(PO4)3, after 100 cycles at 1.755 mA cm^−2 with a discharge capacity of 68.0 mA h g^−1	53
	Porous Al	1 M NaClO4 in EC/DEC (1 : 1)	99.9% for 1000 cycles at 0.5 mA cm^−2 with 0.25 mA h cm^−2	Coupled with Na3V2(PO4)3, after 350 cycles at 0.1 mA cm^−2 with a capacity rentation of 50%	111
	MXene|CNT-NAFs	1 M NaPF6 in DIGLYME	99.7% for 450 cycles at 1.0 mA cm^2 with 1.0 mA h cm^2	Coupled with Na3V2(PO4)3@C, after 5000 cycles at 10.0C with a capacity rentation of 52.3%	73
	Na–SnNCNF	1 M NaPF6 in DIGLYME	99.96% for 2000 cycles at 3 mA cm^−2 with 3 mA h cm^−2	Coupled with Na3V2(PO4)3, after 1000 cycles at 1 A g^−1 with capacity retention of 92.1%	74
	C@Al	1 M NaPF6 in DIGLYME	Initial efficiency of 98.8% and exceeding 99.9% for at least 200 cycles at 2.0 mA cm^−2 with 3.0 mA h cm^−2	Coupled with Na3V2(PO4)3, after 100 cycles with capacity retention of 93%	85
	3D Zn@Al	1M NaPF6-DME	99.9% for 1200 cycles at 0.5 mA cm^−2 with 0.5 mA h cm^−2	Coupled with Na3V2(PO4)3 (1.8 mg cm^−2), after 100 cycles with capacity retention of 98.8%	88
			99.6% for 1000 cycles at 1 mA cm^−2 with 1 mA h cm^−2
	F-A-Al	1 M NaPF6^−EC/DMC (1 : 1 v/v)	CE of 86% for 10 cycles at 0.5 mA cm^−2 with 0.5 mA h cm^−2	Coupled with Na3V2(PO4)3 (11.3 mg cm^−2), after 50 cycles with capacity retention of 46.1% and 98% CE	60
	p-Al@C	0.6 M NaOTF + 0.4 M NaBF4-G2	99.88% for 100 cycles at 0.5 mA cm^−2 with 0.5 mA h cm^−2	Coupled with Na3V2(PO4)3, an capacity of 67 mA h g^−1 after 200 cycles at 0.1C and CE of 99%	87
	HCOONa-modified Cu foil	1 M NaPF6 in DIGLYME	99.9% for 840 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with Na3V2(PO4)3 (10 mg cm^−2), after 400 cycles at 0.5C and CE of 99.97% and 88.2% capacity retention	109
	Zn	1 M NaPF6 in DIGLYME	99.9% for 500 cycles at 2 mA cm^−2 with 3 mA h cm^−2	Coupled with Na3V2(PO4)3, after 100 cycles with capacity retention of 90%	112
	PC-CFe	1 M NaPF6 in DIGLYME	99.6% for 500 cycles at 10 mA cm^−2 with 10 mA h cm^−2	Coupled with Na3V2(PO4)3 (10 mg cm^−2), after 100 cycles with 103 mA h g^−1 at 1 mA cm^−2 and 97% capacity retention	113
	Ag@C	1 M NaClO4 EC/PC + 5% FEC	99% for 500 cycles at 2 mA cm^−2 with 2 mA h cm^−2	Coupled with Prussian White, after 800 cycles at 0.5C and 56% capacity retention	114
	Carbon/Al	1 M NaPF6 in DIGLYME	99.8% for 1000 cycles at 0.5 mA cm^−2 with 0.25 mA h cm^−2	Coupled with presodiated FeS2, after 40 cycles with 400 Wh kg^−1 at 0.125 mA cm^−2 and 85% capacity retention	115
	Carbon black	1 M NaPF6 in DIGLYME	98% for 200 cycles at 0.325 mA cm^−2	Coupled with Na3V2(PO4)3, after 100 cycles with 318 Wh kg^−1 at 0.33C and 82.5% capacity retention	116
	Cu@Au	1 M NaSO3CF3 in DIGLYME	99.8% for 300 cycles at 2 mA cm^−2 with 1 mA h cm^−2	Coupled with presodiated FeS2, after 50 cycles at 0.1 A g^−1 with 56% capacity retention	117
	Macroporous nanowebs	1 M NaPF6 in DIGLYME	99.8% for 1600 cycles at 0.5 mA cm^−2 with 0.5 mA h cm^−2	Coupled with Na1.5VPO4.8F0.7, after 100 cycles with 380 Wh kg^−1 at 0.1 A g^−1 and 95% capacity retention	118
	Al–Cu@C	1 M NaPF6 in DIGLYME	99.8% for 600 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with Na3V2(PO4)3/C, after 50 cycles at 1C with 99.5% CE and a capacity rentation of 78%	119
	NST-Na	1 M NaPF6 in DIGLYME	99.4% for 30 cycles at 1 mA cm^−2 with 15 mA h cm^−2	Coupled with Na3V2(PO4)3, after 1000 cycles at 1C with 296 Wh kg^−1 and 99.9% CE and 91% capacity retention	120
	Cu2NiZn@CNT	1 M NaPF6 in DIGLYME	99.4% for 500 cycles at 2 mA cm^−2 with 2 mA h cm^−2	Coupled with NaVPO4F (11.7 mg cm^−2), after 200 cycles at 0.5C with 351.6 Wh kg^−1 and 93.7% capacity retention	121
	FCTF	1 M NaPF6 in DIGLYME	99.6% for 400 cycles at 2 mA cm^−2 with 1 mA h cm^−2	Coupled with Na3V2(PO4)3, after 400 cycles at 2C with 99.7% CE and 56% capacity retention	122
AFPMBs	Al@G	4 M KFSI in DME	99% for 500 cycles at 0.5 mA cm^−2 with 0.5 mA h cm^−2	Coupled with K–FeS2, after 30 cycles at 0.1 A g^−1 with a capacity of 35%	123
	Cu–OSe (NWs)	4 M KFSI in DME	98.95% for 600 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with K0.5MnO2 (9.8 mg cm^−2), at 100 mA g^−1, after 200 cycles with a capacity retention of 63.5%	95
	Bi/CTG@Cu	1.0 M KFSI in DME	99.4% for 1100 cycles at 0.5 mA cm^−2 with 0.5 mA h cm^−2	Coupled with K1.69Fe[Fe(CN)6]0.90, after 200 cycles with a capacity of 109.1 mA h g^−1 at 0.1 A g^−1	124
	CNFNi@Al	1 M KFSI/DEE	99.2% for 400 cycles at 0.5 mA cm^−2 with 1 mA h cm^−2	Coupled with MnHCF, after 50 cycles at 100 mA g^−1 with 89 capacity retention	97
	Cu6Sn5@Cu	4 M KFSI in DME	99.69% for 300 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with PTCDA, after 30 cycles with a reversible specific capacity of 88.4 mA h g^−1 and 69.4% capacity retention	125
	MSCNF	1M KFSI in EC/DEC, (1 : 1, v/v)	99.99% for 200 cycles at 0.5 mA cm^−2 with 1 mA h cm^−2	Coupled with K0.220Fe[Fe(CN)6]0.805, after 600 cycles at 500 mA g^−1 with a discharge capacity of 470 mA h g^−1 and 60% capacity retention	77
	Cu foil	3 M KTFSI in DME	99.5% for 90 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with K0.51V2O5, after 130 cycles at 0.5 A g^−1 with 99% CE and 97% capacity retention	126
	Cu foil	0.4 M KPF6-DME with 2 vol% PDMS additive	99.1% for 100 cycles at 1 mA cm^−2 with 1 mA h cm^−2	Coupled with Potassiated PTCDA, after 50 cycles at 0.2C with 97% CE and 82% capacity retention	127

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4 Summary and perspectives

This review summarizes recent advances in current collector engineering for AFAMBs, with a focus on strategies such as materials optimization, crystal orientation regulation, porous structure design, and surface modification. Although these approaches have demonstrated certain effectiveness, several challenges remain to be addressed for the practical application of AFAMBs (Table 2). For instance, in terms of materials optimization, while Zn foil shows promise in guiding uniform Na deposition and regulating SEI composition, developing thin Zn foils with sufficient mechanical strength for practical AFAMBs remains a challenge. Single-crystal current collectors exhibit excellent electrochemical performance in AFAMBs. However, their high cost and complex preparation process limit large-scale application. Porous structures help reduce local current density and accommodate alkali metal deposition, but their high surface area may lead to increased side reactions. Surface modification appears effective in stabilizing the interphase, yet overly thick coatings can hinder ionic conductivity, especially at high current densities. Future research should prioritize several key areas to advance AFAMBs. These systems present complex, system-level challenges, where electrolyte optimization is as critical as current collector engineering. Synergistic development of both components is essential for achieving high electrochemical performance. Additionally, advanced in situ and operando characterization techniques are needed to gain deeper insights into interfacial dynamics. Scalable fabrication methods and strategies that bridge the performance gap between laboratory-scale cells and practical pouch or full cells are also crucial. Currently, most research focuses on the performance of coin cells, which provides limited reference value for optimizing the performance of practical batteries. It is essential to evaluate performance under more realistic and demanding conditions, including high rates, high mass loading, low temperatures, low N/P ratios, reduced electrolyte volumes, and other extreme environments. Moreover, the assembly and testing of pouch cells are necessary, especially considering that pouch cells typically operate without external pressure. Their performance under such conditions must be systematically assessed. The significant differences in structure and operating conditions between coin cells and pouch cells lead to distinct failure mechanisms, often resulting in low coulombic efficiency and limited cycle life for practical pouch cells. Finally, a multidisciplinary approach, integrating materials science, electrochemistry, and manufacturing engineering, will be pivotal in accelerating the commercialization of AFAMBs and enabling their deployment in electric vehicles and grid-scale energy storage.

Table 2 Advantages and disadvantages of current collector engineering strategies for AFAMBs

Strategy	Effect	Approach	Advantage	Disadvantage
Materials optimization	Regulate the alkali metal deposition and SEI component	Pure metal	Simple fabrication	Ultrathin and high mechanical strength required; interface instability
		Alloy	Uniform alkali metal deposition	Complex fabrication approach; interface instability
Crystal orientation regulation	Regulate the uniform alkali metal deposition and SEI component	Single crystal	Diffusion resistance on grain boundaries eliminated	High cost; low mechanical strength
		Preferential crystal face	Simple fabrication and low cost	Difficulty in precise synthesis; diffusion resistance on grain boundaries remained
Porous structure design	Reduce local current density and accommodate the alkali metal deposition	Porous metal	High mechanical strength	Reduced volumetric energy density; more electrolyte required; acceleration of side reactions
		Porous nonmetal	Enhanced gravimetric energy density	Reduced volumetric energy density; more electrolyte required; high mechanical strength required
Surface modification	Regulate the alkali metal nucleation/deposition and prevent the side reactions	Nucleation layer	Uniform alkali metal deposition	Surface deposition; side effects still existing
		Protection layer	Prevented side reaction	High ionic conductive and mechanical strength required

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Author contributions

P. X. and Q. L. and G. Z. conceived and initiated the project. G. S. and X. W. and W. P. and L. X. carried out the literature survey, organized all the reference data, and prepared the figures. G. Z. and P. Y. helped discuss the mechanism. The manuscript was written through the contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing interests.

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

We gratefully acknowledge the financial support provided by the Joint Fund of Henan Province Science and Technology R&D Program (grant no. 225200810093, 235200810004), the Science and Technology Development Project of Henan Province (grant no. 242102241042, 242102210215), the Natural Science Foundation of Henan Province (No. 242300420031, 252300420045, 242300420688), the Provincial State-owned Capital Operation Budget Expenditure Project in 2024 (project no. Yucaiqi[2024]10), the Fundamental Research Fund of Henan Academy of Sciences (No. 20250617001, 240617060), the Scientific and Technological Research Project of Henan Academy of Sciences (No. 20252317008), and the Startup Research of Henan Academy of Sciences (grant no. 231817001, 231817067, 232017013). P. Xiong acknowledges the Alexander von Humboldt Foundation for a postdoctoral fellowship.

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