Recent developments in insertion anode materials for Li-ion batteries

Abstract

Insertion anode materials have gained immense attention as commercial anode materials for lithium-ion batteries owing to their reversible Li-ion intercalation/deintercalation mechanism and highly stable crystal structure. Herein, the progress in the development of graphite, titanium-based anode materials, MXene (Ti₃C₂T_x), and other insertion anode materials is reviewed. The effect of modification strategies (such as nanostructure design, surface engineering, bulk phase engineering, and composite structure design) on the electrochemical properties of insertion anode materials is discussed. Studies have shown that the specific capacity and rate and cycling performances of these anode materials can be significantly improved by optimising their crystal structure, interface engineering, and conductive network construction. However, traditional insertion materials suffer from low specific capacity and high polarization at high rates. Further research is imperative to balance the energy density and dynamic performance of these materials to realise their widespread application in fast-charge and high energy density lithium-ion batteries.

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1. Introduction

The rapid transformation of the global energy structure and rising environmental awareness have led to a severe energy crisis and environmental challenges. In this context, the development of clean, efficient, and renewable energy storage and conversion technology has become one of the most urgent scientific and technological needs.^1–4 Lithium-ion batteries have emerged as a key component of innovative energy storage solutions because of their long lifespan, low self-discharge rate, and noticeably better energy density than conventional battery systems. Furthermore, lithium-ion batteries are frequently seen in innovative energy vehicles and portable smart devices.^5–12 The intercalation and deintercalation of lithium ions between the positive and negative electrodes are the primary mechanisms by which lithium-ion batteries store and release electrical energy.^13–17 As a key component of lithium-ion batteries, the negative electrode material has a decisive impact on their performance indicators such as charge and discharge efficiency, energy density, and safety. Depending on the mechanism of lithium-ion storage, negative materials can be divided into three categories: insertion anode materials (such as graphite, titanium-based negative materials, and MXene (Ti₃C₂T_x)), alloyed negative materials (such as Si, Ge, Sn, and their alloys), and converted negative materials (such as transition metal oxides and sulphides). Insertion anode materials offer the advantages of excellent cycle stability, high conductivity, good safety, and low cost; however, they suffer from high voltage lag.¹⁸ Although the converted anode materials exhibit high theoretical specific capacity, their cyclic stability and Coulomb efficiency are limited in practical applications, which hinders their commercialization.^19–21 Alloyed anode materials exhibit considerable volume expansion, limited electrical conductivity, and insufficient lithium-ion transport efficiency despite their high theoretical specific capacity and enormous application potential.^22–25

Significant advancements in the field of negative electrode materials have been made in recent years. Among them, insertion anode materials have become the most widely used commercial negative electrode materials because of their unique structure and excellent electrochemical stability. Nevertheless, the low theoretical specific capacity of commercial lithium-ion batteries limits their energy density. Therefore, research on the modification of insertion anode materials has become a focal point in both academic and industrial circles.²⁶

2. The characteristics of insertion anode materials

Insertion anode materials typically have a layered structure that allows the reversible insertion and extraction of lithium ions during the charging and discharging processes without significantly damaging their crystal structure, thus enabling efficient energy storage and conversion. Depending upon their elemental composition, insertion anode materials can be further classified into graphite-based anode materials, titanium-based materials, and MXene (Fig. 1). Among these, graphite has emerged as the most popular commercial anode substance owing to its stable operating voltage platform, excellent safety, and low cost.^26,27 Titanium-based materials are inexpensive, have a long cycle life, are highly safe, and charge and discharge quickly.^28–30 MXene (Ti₃C₂T_x), as a layered compound, has a flexible interlayer space and is able to accommodate a variety of ions under high charge and discharge rates. Due to its characteristics such as hydrophilic functional groups, excellent conductivity, and low ion and electron diffusion barriers, MXene is considered to be a very promising lithium-ion insertion electrode material.³¹

Fig. 1 The main classification of insertion anode materials. Crystal structure of Li₄Ti₅O₁₂. Reprinted with permission from ref. 29. Copyright 2012 Elsevier. Crystal structure of TiO₂. Reprinted with permission from ref. 30. Copyright 2022 Elsevier. Structure of MXene. Reprinted with permission from ref. 31. Copyright 2022 Elsevier.

3. Graphite-based anode materials

Graphite is a stable carbon allotrope whose layered structure (layer spacing 0.335 nm) allows for the reversible insertion and removal of Li⁺.^32–36 Yazami et al. used graphite as a negative electrode material for lithium-ion batteries for the first time, a breakthrough that laid the foundation for subsequent research.^37–40 In 1991, SONY Corporation successfully developed graphite-based lithium-ion batteries for commercial applications for the first time. The following equation describes how lithium ions intercalate in graphite (1):

Li_xC_n ↔ xLi + xe⁻ + C_n (1)

Real-time structural evolution of graphite during lithiation was studied using in situ X-ray diffraction (XRD) (Fig. 2(a)). Graphite undergoes a series of solid-solution phase transitions from primary graphite to stage IV, a biphasic transition from stage IV to stage III, a solid-solution phase transition during stage III, a biphasic transition from stage III to stage II, and a biphasic transition from stage II to stage I.

Fig. 2 The lithium storage mechanism of graphite anodes. (a) In situ XRD patterns of graphite during the lithiation process. (b) Voltage curve of graphite during the lithiation process. Schematic of different stages of graphite: (c) Rudorff–Hofmann model, (d) Daumas–Hérold model, and (e) localized domain model. Reprinted with permission from ref. 41. Copyright 2022 Wiley-VCH.

Fig. 2(b) shows the voltage curve of graphite obtained during the lithiation process in a previous study, showing the voltage changes occurring during the structural transition in different stages. When Li⁺ ions pass through the solid electrolyte interface (SEI) and reach the surface of graphite, they insert into the graphene layer, forming a series of hierarchical structures (Fig. 2(c)). It has been shown that Li⁺ ions occupy the interlayer of each of the four layers of graphene alternately in stage IV to stage I (LiC₂₄ to LiC₆). However, the transition of ions is not smooth, suggesting that the rapid charging ability of graphite is limited by its inherent dynamic properties. Fig. 2(d) shows the possible deformation or folding of the graphene layer during lithium intercalation, which may lead to a local stress concentration. The two hierarchical models show the same behaviour in the long-range ordered structure. However, in the short-range region, these models show different behaviours, which needs to be further investigated using microstructure-sensitive techniques. During the lithiation process, the ordered structure of graphite gradually transforms into a disordered one, forming dislocations and microdomains, which contain different phase structures. Local pressures brought on by an unequal distribution of Li are the primary source of the graphene layer's deformation. Although the phase structures appear to be ordered on the macro scale, they are not uniform on the micro scale (Fig. 2(e)). When graphite lithiates or delithiates, these structural flaws and phase changes can be reversed.⁴¹

Failure in graphite mainly stems from its electrochemical and structural characteristics. During the cycling process, the SEI film formed on the surface of graphite ruptures and regenerates because of volume expansion/contraction and continues to consume active lithium and electrolytes, resulting in capacity attenuation.⁴² At the same time, in the case of overcharge, low temperature or high rate charge, the lithium-ion intercalation rate lags behind the electron transport, and the lithium not embedded in time can easily precipitate and form dendrites on the graphite surface, which may not only puncture the diaphragm and cause short circuit or thermal runaway, but also aggravate the pulverization of the material (the layered structure undergoes volume changes because of repeated lithium embedding/removal and finally breaks and loses electrical contact).^43,44 In addition, under high temperature or pressure conditions, the compatibility of graphite with the electrolyte (e.g. carbonate solvent) decreases, the gas produced in the side reaction leads to battery expansion, and the low working potential of graphite (approximately 0.1 V vs. Li⁺/Li) approaches the lithium deposition threshold, further amplifying the risk of lithium analysis.⁴⁵ To solve these problems, researchers often improve the structural stability and interface compatibility using various techniques such as surface engineering, bulk engineering, and composite structure design.

3.1 Surface engineering

An efficient method for enhancing graphite anode materials’ electrochemical characteristics is surface engineering. The poor ionic conductivity of SEI films, particularly during rapid charging, significantly hinders the insertion of Li⁺, leading to increased interface resistance and lithium plating. This, in turn, reduces the rate performance and lifespan of graphite anode materials.^46–49 Therefore, the modification of the graphite surface to achieve the effective isolation of the anode material and electrolyte and the optimisation of the structure and composition of the SEI film are necessary to improve the performance of graphite anodes.^50,51 The surface modification of graphite is mainly achieved by surface coating and doping.^52–57 Popular surface coating materials include organic compounds,⁵⁸ metallic compounds, inorganic compounds,^59,60 polymers, and organic/inorganic composites. In addition, surface doping (with elements such as nitrogen, fluorine, etc.) is an important strategy for the modification of graphite anodes. Moreover, the surface activity of graphite anodes can be optimised by heterogeneous doping.^61,62

Cai et al. prepared a graphite electrode coated with p-sulfonic polyallyl phenyl ether (AG@SPAPE) via in situ electrochemical polymerization (Fig. 3(a)).⁶³ The SPAPE coating formed an artificial SEI film on the electrode surface, which effectively inhibited the excessive decomposition of the electrolyte, significantly improving the battery performance. Jeong et al. deposited Co₂P nanoparticles onto the surface of natural graphite (NG) through a thermogenic phase transition process (Fig. 3(b)).⁶⁴ The Co₂P nanoparticles modified the physical and chemical properties of the NG surface, forming an interface layer rich in Li₃P and containing a small amount of Co on the surface. This improved the anode's ability to charge quickly by lowering the contact resistance. Ko et al. proposed a simple fluorine-heat treatment method to prepare fluorine-doped flake graphite (F-FG). The resulting F-FG anode showed significantly improved lithium ion behaviour, high specific capacity and excellent high rate performance and cyclic stability.⁶⁵ Fluorine treatment enhances the charge transfer kinetics properties of the active sites, which is conducive to the formation of a thin and stable SEI and can effectively inhibit the growth of lithium dendrites (Fig. 3(c) and (d)).

Fig. 3 Characterization of graphite anode materials modified by surface engineering. (a) Flowchart for the surface modification of graphite with SPAPE. Reprinted with permission from ref. 63. Copyright 2024 Elsevier. (b) Schematic for the synthesis of Co₂P coated with natural graphite (NG). Reprinted with permission from ref. 64. Copyright 2024 Elsevier. SEI formation and morphological changes on the surface of (c) SG and (d) F-FG. Reprinted with permission from ref. 65. Copyright 2024 Elsevier.

3.2 Bulk engineering

Bulk engineering of graphite can be effectively performed by tailoring its microstructure and defect content.^66–69 It is possible to greatly increase the active sites for lithium-ion storage and hence improve its performance by increasing the number of flaws in the graphite structure.^70–74

Qin et al. proposed a novel method for bulk engineering of graphite using a high-current pulsed electron beam (HCPEB) to improve the energy storage performance of lithium-ion batteries.⁷⁵ Microstructural analysis showed that during the HCPEB irradiation, graphite particles transformed in situ into self-supporting graphene nanosheets. Furthermore, a variety of structural defects, such as Stone–Wales and double-vacancy defects, were created as a result of the elevated temperature (Fig. 4(a) and (b)). The results showed that the modified SEI film exhibited higher stability than the unmodified graphite, thus significantly improving the cycling performance of the battery (Fig. 4(c)).

Fig. 4 Characterization of graphite anode materials modified by bulk engineering. Schematic showing the lithium storage of (a) graphite and (b) self-supporting defect graphene. (c) Rate performance of G12 and M-G12 electrodes. Reprinted with permission from ref. 75. Copyright 2024 Elsevier. (d) Schematic of ArF laser action on a control sample. (e) Change in grain morphology after PLA. (f) Cyclic performances of graphite (S0) and laser annealed graphite as negative electrodes for Li-ion batteries under optimised parameters (S2). Reprinted with permission from ref. 69. Copyright 2023 Elsevier.

Khosla et al. performed controlled engineering of the microstructure and defect content of graphite via pulsed laser annealing (PLA) (Fig. 4(d)) to increase its Li⁺ ion capture site density.⁶⁹ As shown in Fig. 4(e), the following alterations are brought about by the PLA treatment: (1) graphite particle surface step and groove development, which increases the charge and rate of Li⁺ intercalation; (2) removal of inactive polyvinylidene fluoride (PVDF) binders on the top of and between the graphite particles, reducing their obstruction to Li⁺ migration; and (3) introduction of carbon vacancies in the (0001) crystal plane, providing additional Li⁺ storage sites. After the treatment, the capacity of the battery increases from an average of 360 mA h g⁻¹ to 430 mA h g⁻¹ (Fig. 4(f)).

3.3 Composite materials

The use of composite materials has demonstrated efficacy in augmenting the electrochemical performance of graphite-based anodes.^76–79 By combining graphite with functional materials with high specific capacity, its inherent drawbacks can be effectively overcome and the synergistic optimisation of electrode material properties can be realised.⁸⁰ Various materials such as silicon,^81–85 metals⁸⁶ and their compounds have been widely studied for preparing composites with graphite.^77,87–91

Karuppiah et al. deposited silicon nanowires on graphite (Gt-SiNW) using a one-pot method (Fig. 5(a)).⁹² The resulting composite showed a uniform distribution of SiNWs. The electrode was kept from being ground up during cycling because of the neat arrangement of graphite sheets, which also made room for silicon's volume growth throughout the charge and discharge procedures (Fig. 5(b)). Gt-SiNWs had outstanding electrochemical performance in the experiment, retaining up to 87% of its capacity after 250 cycles at 2C (Fig. 5(c)). Tao et al. successfully synthesised graphene and lithium–iron–phosphate (LFP) composites.⁹³ The composite electrode's rate performance and cycle stability were noticeably better. Combining the structural stability of LFP with the high conductivity of graphene enhances the electrode's electron transport capacity and lithium-ion transport efficiency while also enhancing its electrochemical performance at high rates (Fig. 5(d)–(f)).

Fig. 5 Characterization of material composite graphite anode materials. (a) Schematic showing the synthesis of SiNWs on graphite. (b) SEM image of Gt-SiNWs. (c) Cycling performance of pristine SiNWs, Gt-SiNWs, and Gt-mix-SiNWs at a rate of C/5. Reprinted with permission from ref. 92. Copyright 2024 American Chemical Society. (d) Schematic of LFP-HCDG composite spheres and transportation of electrons inside the LFP-HCDG electrode. (e) SEM image of LFP-HCDG. (f) Rate performance of the LFP-HCDG, LFP-GO, and commercial LFP electrodes. Reprinted with permission from ref. 93. Copyright 2020 Wiley-VCH.

4. Titanium-based anode materials

4.1 Lithium titanate

Li₄Ti₅O₁₂ (LTO) is a member of the cubic space group Fd3m and exhibits a spinel structure.^94–97 As illustrated in Fig. 6(a), lithium ions preferentially occupy the tetrahedral 8a sites and partially populate the octahedral 16d sites during the lithiation process, whereas Ti⁴⁺ ions primarily occupy the remaining 16d sites.⁹⁸ Three Ti⁴⁺ ions are reduced to Ti³⁺ during lithiation. Through reversible redox processes, this arrangement enables each formula unit to hold three lithium ions, providing a theoretical capacity of 175 mA h g⁻¹.^99–101 This process can be described using the following equation:

Li₄Ti₅O₁₂ + 3Li⁺ + 3e⁻ ↔ Li₇Ti₅O₁₂ (2)

Fig. 6 Modification of Li₄Ti₅O₁₂ anode materials and their electrochemical performance. (a) Crystal structure of Li₄Ti₅O₁₂. Reprinted with permission from ref. 29. Copyright 2012 Elsevier. (b) Schematic showing the structural evolution of the quenched-LTO anode. (c) Rate performance of IWQ-LTO and pristine LTO. Reprinted with permission from ref. 126. Copyright 2022 Wiley-VCH. (d) Schematic for the preparation of LTO/rGO/SnO₂ nanocomposites. (e) Rate performances of LTO, LTO/rGO, and LTO/rGO/SnO₂ nanocomposites. Reprinted with permission from ref. 134. Copyright 2023 Wiley-VCH.

The LTO's high potential (∼1.55 V vs. Li/Li⁺) enhances the initial Coulomb efficiency during cycling and encourages Li⁺ diffusion at the interface by preventing the development of the SEI film as a result of electrolyte reduction.^102–107 At the same time, it mitigates the safety issues associated with lithium plating.¹⁰⁸ However, its wide bandgap and slow Li⁺ diffusion lead to low intrinsic conductivity, which limits its performance in fast charging applications.^109–114 To overcome this problem, nanostructure regulation of LTO is widely regarded as an effective method to improve its fast charging performance.

By designing nanostructures with different dimensions, it is possible to decrease the Li⁺ diffusion channel and improve the contact area between the electrolyte and the electrode active material, which enhances LTO's fast charging capabilities.^115,116 Various nanostructures of LTO, containing nanoparticles in zero dimensions, nanowires in one dimension, nanoribbons and nanorods, nanosheets in two dimensions, three-dimensional nanospheres, and nanoarrays, have been developed successfully.^117–121

Defect engineering provides an efficient approach to mitigate the inherent low conductivity of LTO and its limited lithium-ion diffusion kinetics through ion defect induction and charge compensation.^122–124

Yan et al. quantitatively regulated the oxygen vacancy content in LTO by subjecting it to high pressure and temperature.¹²⁵ Experimental results showed that the as-prepared LTO anode containing oxygen vacancies showed significantly improved high rate performance with a specific capacity of 176 mA h g⁻¹ at a high rate of 20C. Su et al. used in situ synchrotron radiation analysis and atomic resolution microscopy (Fig. 6(b)) to examine the presence of oxygen-rich positions and the occurrence of cation redistribution in LTO after ice water quenching treatment.¹²⁶ This structural change enables the irreversibly extracted Li⁺ ions to participate in a reversible cycle, thereby significantly improving the electrochemical properties of the material. The as-prepared LTO negative electrode exhibited a sustained specific capacity of 202 mA h g⁻¹ over the voltage range of 1.0–2.5 V (Fig. 6(c)) and excellent high rate performance and cycle stability, thus overcoming the drawback of poor cycle stability in traditional defective electrodes.

LTO's low specific capacity and poor electrical conductivity are the primary obstacles to its practical implementation. To overcome these limitations, LTO is often combined with highly conductive and high-capacity materials.^127–129 For example, when combined with carbon materials^130–132 and metal compounds,^133,134 LTO shows enhanced rate performance and cycle life in addition to markedly better electrical conductivity and structural stability. This strategy provides an important technical path for the development of high-performance anode materials for lithium-ion batteries and effectively improves the performance of LTO.

Wang et al. successfully synthesised LTO/rGO/SnO₂ nanocomposites by in situ electrostatic self-assembly and hydrothermal reduction processes (Fig. 6(d)).¹³⁴ The resultant composite exhibited a higher rate of diffusion of lithium ions, enhanced structural stability, and increased total electrical conductivity in comparison to LTO. The LTO/rGO/SnO₂ nanocomposite showed significantly improved high rate properties (Fig. 6(e)).

4.2 Titanium dioxide

Titanium dioxide (TiO₂) has gained immense attention in the field of electrochemistry owing to its unique physical and chemical properties. As shown in Fig. 7, common TiO₂ phases include octavite, titanite, rutile, TiO₂ (R), TiO₂ (H), and TiO₂ (B), all of which can reversibly intercalate/deintercalate lithium ions during electrochemical processes.^135–142

Fig. 7 Typical polymorphs of TiO₂ as indicated. Reprinted with permission from ref. 30. Copyright 2022 Elsevier.

The intercalation/deintercalation of Li⁺ in the TiO₂ phase usually occurs at 1.4–1.8 V (vs. Li/Li⁺) and can be described as follows:³⁰

TiO₂ + xLi⁺ + xe⁻ ↔ Li_xTiO₂ (3)

However, the primary obstacle impeding TiO₂'s practical use is its low electronic conductivity and Li⁺ diffusion capacity, which lead to subpar rate performance. This challenge can be overcome by regulating the size of TiO₂ to the nanometre level and designing nanostructures with different morphologies, such as mesoporous structures,¹⁴³ hollow microspheres,^144,145 nanoparticles,¹⁴⁶ nanotubes,¹⁴⁷ nanorods,¹⁴⁸ nanowires, and nanosheets.¹⁴⁹ These nanostructures greatly improve the electrochemical performance of TiO₂ by increasing its specific surface area, decreasing its Li⁺ diffusion channel, and expanding its area of contact with the electrolyte.^150–154

Doping is a useful method for increasing TiO₂'s conductivity and electrochemical performance. By introducing foreign elements (such as nitrogen, carbon, transition metals, etc.) into the lattice structure of TiO₂, its band structure can be adjusted to increase the carrier concentration, which leads to a significant increase in its electrical conductivity.^155–157 At the same time, doping can introduce defects or form new active sites in TiO₂, further enhancing its electrochemical reactivity.^158–160

Choi et al. devised a novel synthesis method (Fig. 8(a) and (b)) that preferentially introduced interstitial nitrogen (N) inside TiO₂ while preserving oxygen vacancies (Vo).¹⁶¹ In contrast to traditional N doping methods, this method enables N to be concentrated inside the material rather than on the surface, thus maximising the effect of N doping. Studies have shown that interstitial N is more beneficial to improve the conductivity of TiO₂ than substitutional N. N-doped TiO₂ materials prepared by this method show significantly improved electrochemical properties.

Fig. 8 Modification of TiO₂ anode materials and their electrochemical performance. (a) Schematic for the synthesis of INR-TiO₂ and ONR-TiO₂. (b) N doping process in each synthesis route. Reprinted with permission from ref. 161. Copyright 2020 Elsevier. (c) Schematic showing the synthesis of N, P-C, TiO₂ HM and N, P-C@TiO₂. (d) Rate performance of N, P-C, TiO₂ HM and N, P-C@TiO₂. Reprinted with permission from ref. 172. Copyright 2022 Elsevier.

It has been shown that the electrochemical performance of materials based on titanium dioxide is greatly impacted by the introduction of oxygen vacancies. These crystallographic defects facilitate lattice expansion through increased interplanar spacing, while simultaneously enhancing charge carrier mobility by lowering interfacial resistance in electron transfer processes. The altered electronic structure brought about by oxygen shortage also enhances electrical conductivity and speeds up reaction kinetics.¹⁶² Hao et al. prepared blue TiO₂ nanoparticles with oxygen vacancies (B-TiO_2−δ) as anode materials for lithium-ion batteries.¹⁶³ Results from experiments indicated that after 500 cycles at 1C, the B-TiO_2−δ electrode could sustain a lithium storage capacity of 335 mA h g⁻¹, which is much higher than the intrinsic kinetic limit of TiO₂ materials. The synergistic interaction between the diffusion control and surface capacitance mechanisms is responsible for this improved lithium storage performance.

TiO₂ exhibits low Li⁺ diffusion capacity and electronic conductivity, which lead to poor high rate performance. Forming compounds of TiO₂ with conductive materials such as carbon is an efficient approach to solve this problem.^164–171 The surface's carbon layer can prevent TiO₂ nanoparticles from clumping together, increasing their electrical conductivity.

Zhao et al. used pollen as the precursor material to prepare a N, P co-doped porous three-dimensional carbon skeleton @TiO₂ nanoparticle hybrid (N, PAC@ TiO₂) via a template-assisted sol–gel method (Fig. 8(c)).¹⁷² The hybrid that resulted had a huge specific surface area and a multistage porous hollow structure with lots of redox-active spots. The N, PAC@TiO₂ negative electrode demonstrated reversible capacities of 687.3 and 440.5 mA h g⁻¹ for 200 cycles at current densities of 0.1 and 1 A g⁻¹, respectively (Fig. 8(d)). This excellent performance of the hybrid anode can be attributed to its hierarchical porous hollow structure and the synergistic effect of N and P co-doped carbon and TiO₂, which improved the storage capacity of Li⁺, accelerated the reaction kinetics, and stabilised the electrode structure and interface during the charging and discharging processes.

4.3 Other titanium-based anode materials

In addition to the abovementioned titanium-based insertion anode materials, Zhao et al. designed a novel titanium-based insertion anode material – TiNCl (Fig. 9(a)).¹⁷³ The material had a rock salt TiN skeleton and a Cl-terminated laminar structure. It also demonstrated high Li⁺ intercalation capacity, which can be attributed to the interlayer voids terminated by the duplex TiN block and Cl⁻. The TiN block provided a fast electron transport channel, and the two-dimensional interlayer space enabled pseudo-capacitive insertion. Studies have shown that Li⁺ transmits through the pyramid site of Cl⁻ coordination in TiNCl with a significantly lower energy barrier (0.06 eV) than that in TiO₂ (0.47 eV). This unique structure of TiNCl allows for the separation of electron and lithium-ion transport paths, thus increasing the material's electrochemical performance.

Fig. 9 Preparation and electrochemical performance of TiNCl and its derivatives. (a) Schematic for the synthesis of TiNCl and its derivatives. (b) Rate performance of TiNCl and its derivatives at different current densities. Reprinted with permission from ref. 173. Copyright 2022 Wiley-VCH.

To further optimise the performance of TiNCl, a TiO₂-coated TiNCl composite was prepared via ammonolysis and heat treatment. During the deep lithiation/delithiation process, TiO₂ evenly encapsulated TiNCl, effectively preventing the collapse of the layered structure. In addition, TiO₂ not only acted as a structural support, but also enhanced the interface's electron transport. Experimental results showed that the TiNCl-TiO₂ heterostructure demonstrated an impressive 804 mA h g⁻¹ at a current density of 0.1 A g⁻¹. In addition, the anode material showed excellent rate performance (Fig. 9(b)).

5. MXene (Ti₃C₂T_x)-based anode materials

MXenes are a class of two-dimensional layered materials made up of different elements and have gained immense attention in the field of two-dimensional materials over the past few years. Their general molecular formula is M_n+1X_nT_x (n = 1–4), where M represents a transition metal (such as Ti, V, Mo, Nb, etc.), X represents carbon and/or nitrogen, and T represents functional groups on the surface of MXene (such as –O, –F, –OH, etc.).^174–177 MXene-based materials demonstrate distinctive advantages in electrochemical applications due to their surface-terminating hydrophilic moieties, exceptional electronic conductivity, and low activation energies for ionic/electronic diffusion. These characteristics synergistically enhance interfacial charge transfer kinetics and mass transport efficiency, making them promising candidates for advanced energy storage devices (e.g., supercapacitors and batteries) and high-performance sensing platforms.^178–181

Typical MXene materials, such as Ti₃C₂T_x, have flexible interlayer space that allows for a variety of ions to be charged and discharged at rapid rates. Moreover, Ti₃C₂T_x is an effective lithium ion insertion electrode material with a hydrophilic functional group, high conductivity, and low ion and electron diffusion barriers.^182–188 As shown in Fig. 10, two redox pairs have been observed in Ti₃C₂T_x, corresponding to their two-layer spacing. To verify whether the intercalation and deintercalation of Li⁺ take place successively between the layers, in situ XRD examination is employed.¹⁸⁹

Fig. 10 CV curves of Ti₃C₂T_x. Reprinted with permission from ref. 189. Copyright 2020 American Chemical Society.

Although MXene shows significant advantages as a material for lithium-ion batteries’ negative electrodes, the effect of its surface chemical groups on its electrochemical performance cannot be ignored. These groups may block ion migration and limit redox reactions. Lu et al. successfully removed the –F group in MXene and transformed the OH group into an –O terminal in a hydrogen environment (Fig. 11(a)).¹⁹⁰ Experimental results showed that annealing treatment significantly improved the transfer kinetics of lithium ions between the electrolyte and electrode and reduced the interface charge transfer impedance. According to the electrochemical test, the annealed MXene's specific capacity was roughly twice that of the unannealed substance (Fig. 11(b)).

Fig. 11 Modification of MXene anode materials and their electrochemical performance. (a) Preparation of a low F-Ti₃C₂ flexible independent thin film electrode. (b) Rate performance and the corresponding Coulomb efficiency of MX-H₂ and MX. Reprinted with permission from ref. 190. Copyright 2019 Elsevier. (c) Schematic for the preparation of Ti₃C₂T_x @GDY heterojunction nanocomposites. (d) Rate performance and the corresponding Coulomb efficiencies of Ti₃C₂T_x@GDY heterojunction nanocomposites and Ti₃C₂T_x at different current densities. Reprinted with permission from ref. 209. Copyright 2024 Elsevier. (e) Synthesis of three-dimensional porous Fe₃O₄@SnO₂/MXene composites. (f) Rate performance of Fe₃O₄, Fe₃O₄@SnO₂, Fe₃O₄/MXene-10 and three-dimensional porous Fe₃O₄@SnO₂//MXene-10 composites. Reprinted with permission from ref. 210. Copyright 2024 Elsevier.

In addition, MXene shows strong self-assembly capability because of its abundant micro/nanostructure, which makes it an ideal substrate for building composite materials.^191–195 To further improve the electrochemical performance of MXene, MXene-based composites are used. Common materials used for preparing such composites include inorganic substances,^196–200 organic–inorganic hybrid materials²⁰¹ and metal oxides.^202,203 When combined with MXene, these materials show excellent electrical conductivity, structural stability, and electrochemical activity, thus significantly increasing the application potential of MXene in the energy field.^204–208

Zhou et al. successfully synthesised Ti₃C₂T_x@GDY heterostructures using an in situ growth process (Fig. 11(c)).²⁰⁹ This unique synthesis method effectively reduced the self-agglomeration of the Ti₃C₂T_x layer, increased the layer spacing, and generated an intrinsic electric field. The Ti₃C₂T_x@GDY heterostructure exhibited a specific capacity of 464.4 mA h g⁻¹ at a current density of 0.1 A g⁻¹. In addition, the heterostructure exhibited high cycling stability and rate performance as compared to the Ti₃C₂T_x nanosheets (Fig. 11(d)). Duan et al. prepared an Fe₃O₄@SnO₂/MXene-10 composite with a three-dimensional porous structure via synchronous etching, coating and electrostatic adsorption (Fig. 11(e)).²¹⁰ This composite material showed excellent rate performance (Fig. 11(f)). The enhanced electrochemical behavior originates from the multifunctional collaboration between SnO₂ and MXene components. SnO₂ efficiently absorbed the structural stress of FeO₄ during the cycle, increased electron transport, and caused electron reduction at the interface to restrict volume expansion. In contrast, MXene provided pseudocapacitance and facilitated the construction of a very adaptable and conductive network structure, which effectively inhibited the agglomeration and accumulation of the Fe₃O₄ particles.

6. Conclusions

Significant advancements have been made in the creation of innovative insertion anode materials with enhanced performance for lithium-ion batteries in recent years. Graphite-based materials prepared using surface coating and structural modification show improved high rate performance and cycle life. Fast-charging graphite can achieve 80% charge in 15 min. Nanostructuring, doping, and composite conductive technology have been used to improve the stability of LTO at extreme temperatures. Thus, LTO subjected to these treatments is widely used in energy storage and electric buses. Titanium dioxide is used in special energy storage batteries; however, owing to its poor conductivity and low lithium-ion diffusion rate, it has not yet been commercialised on a large scale. MXene is an emerging negative electrode material because of its ultra-high electrical conductivity and rapid ion transport between layers. However, MXene has several disadvantages such as high preparation cost, complex preparation process, easy oxidation, and the need for large-scale electrode processing technology (such as slurry dispersion).

Because of their stable structure and low volume deformation, insertion anode materials show good cycle stability and safety in lithium-ion batteries. The electrochemical properties of these materials can be significantly improved through nanostructure design, surface modification, and composite structure design. For example, the zero-strain properties of LTO enable it to maintain structural integrity over long cycles, while the high electrical conductivity and open interlayer channels of two-dimensional materials such as MXene provide new pathways for fast lithium-ion transport. Nevertheless, compared to alloy or conversion materials, these materials often have a lower theoretical capacity. In addition, the high cost of these materials restricts their large-scale applications. Therefore, research on interface control and multi-scale structural design should be done to further enhance these materials’ overall performance.

Future research on insertion anode materials can focus on the following: (1) developing novel high-capacity layered materials to break through the traditional theoretical capacity limits; (2) constructing multilevel composite structures with improved ion/electron conductivity through heterogeneous interface design; (3) combining in situ characterisation and theoretical calculations to elucidate the dynamic intercalation of lithium ions and mechanism of interfacial side reactions; and (4) for graphite anode materials, efforts should be focused on the innovation of green and low-cost manufacturing and preparation processes and the development of efficient graphite recycling methods. For other insertion anode materials, low-cost, sustainable large-scale preparation processes should be explored to promote the industrial application of these insertion anode materials. In addition, the use of emerging technologies such as solid electrolyte and pre-lithium may provide a new pathway to optimise the energy density and safety of these materials. It is anticipated that further advancements in insertion anode materials will create the groundwork for the creation of high-power, long-life lithium-ion batteries in the future.

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.

Author contributions

J. Z. and Q. S. made equal contributions as co-first authors, jointly designing the review framework and writing the review. X. L., J. W. and C. Z. coordinated data collection, including consulting relevant literature and conducting classification. C. L., Z. W., M. M., L. L. and A. B. arranged and organized the figures and contributed to revising the manuscript. Y. S. R. Y. and M. H. R. are responsible for supervising all stages of the project, from determining the review topic to finalizing the manuscript, to ensure scientific rigor.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

M. H. R. thanks the National Natural Science Foundation of China (grant no. 52071225), the European Union's Horizon Europe Research and Innovation Program under grant agreement no. 101087143 (Electron Beam Emergent Additive Manufacturing (EBEAM)) and the Research Excellence for Region Sustainability and High-tech Industries (RE-FRESH) program (project no. CZ.10.03.01/00/22_003/0000048) via an operational program transition. A. B. thanks Norway Grants project no. 2019/34/H/ST8/00547 through the National Science Centre. Q. S. thanks the Jiangsu Funding Program for Excellent Postdoctoral Talent.

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