Stannate-Based Materials as Anodes in Lithium-Ion and Sodium-Ion Batteries: A Review

Binary metal oxide stannate (M2SnO4; M = Zn, Mn, Co, etc.) structures, with their high theoretical capacity, superior lithium storage mechanism and suitable operating voltage, as well as their dual suitability for lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), are strong candidates for next-generation anode materials. However, the capacity deterioration caused by the severe volume expansion problem during the insertion/extraction of lithium or sodium ions during cycling of M2SnO4-based anode materials is difficult to avoid, which greatly affects their practical applications. Strategies often employed by researchers to address this problem include nanosizing the material size, designing suitable structures, doping with carbon materials and heteroatoms, metal–organic framework (MOF) derivation and constructing heterostructures. In this paper, the advantages and issues of M2SnO4-based materials are analyzed, and the strategies to solve the issues are discussed in order to promote the theoretical work and practical application of M2SnO4-based anode materials.


Introduction
With the rapid development of human society in recent years, the need for energy has grown significantly. Environmental pollution caused by greenhouse gases such as carbon dioxide produced during the consumption of traditional fossil energy has prompted people to accelerate the development and use of green and clean energy [1][2][3]. However, renewable energy sources such as solar and wind are intermittent and diverse, and their large-scale expansion requires efficient and stable energy storage equipment [4][5][6]. Of all energy storage devices, alkali metal ion batteries are considered to be the most promising because of their high energy density, extended cycle life and environmental friendliness [7][8][9]. Since their introduction, lithium-ion batteries (LIBs) have become a mainstream energy storage device for mobile electronic devices, electric vehicles and other equipment [7,10]. However, commercial LIBs currently available are still unable to meet the requirements of large-scale energy storage devices such as electric vehicles and smart grids in terms of size, power output and energy density [11]. In the case of electric vehicles, there is diversity in the choice of cathode materials. BMW uses lithium nickel manganese cobalt oxide (NMC), while Tesla uses lithium nickel cobalt aluminum oxide (NCA), and electric vehicle companies in China often use lithium iron phosphate (LFP). Currently, carbonaceous materials are generally chosen as anode materials [12]. However, carbonaceous materials have a lower theoretical capacity (372 mAh·g −1 ) and take up about half the volume of the battery. Hence, higher-capacity anode materials need to be developed to replace carbon materials to match the high capacity of the anode materials and improve the energy storage capacity of LIBs. carbonaceous materials have a lower theoretical capacity (372 mAh·g −1 ) and take up about half the volume of the battery. Hence, higher-capacity anode materials need to be developed to replace carbon materials to match the high capacity of the anode materials and improve the energy storage capacity of LIBs. Nevertheless, the continued development and application of LIBs are leading to the rapid depletion of lithium resources, yet sodium resources are quite abundant. Therefore, SIBs, which use sodium resources as the main raw material, can be developed to replace a portion of LIBs, contributing to cost reduction and sustainability [13,14]. SIBs have a similar electrochemical storage mechanism to that of LIBs and can be adapted from LIB technology, allowing them to develop rapidly. Due to their significant advantage of lower cost, SIBs are likely to replace several LIBs as an important classification of energy storage devices. However, due to the relatively large radius of Na + (0.59 Å for Li + and 1.02 Å for Na + in radius), significant volume expansion problems occur during the sodiation/desodiation process, resulting in poor energy density and cycle stability [15][16][17]. Due to this issue, Na + is difficult to insert between the lattice layers of graphite; therefore, graphite anode material commonly used in LIBs cannot be applied to SIBs. Thus, selecting the right anode material is a key issue for both LIBs and SIBs.
Among the currently studied anode materials, which mainly include graphite, tin, germanium and silicon, a comparison of the capacity and volume variation of the elements in the IVA group is shown ( Figure 1). Although the theoretical specific capacity of Sn is not the highest of them all, the volumetric specific capacity of Sn is quite close to that of Si and Ge. In particular, Sn has a much higher electrical conductivity (9.17 × 10 4 S cm −1 ) than several other materials. Thus, Sn-based materials such as Sn, SnO2, SnS, M2SnO4 and their composites have been extensively researched as they can have a very high theoretical capacity and a suitable potential platform [18][19][20][21]. Due to conversion and alloying reactions, the theoretical capacity of low-cost Sn dioxide can reach 1494 mAh·g −1 , with the conversion reaction contributing 711 mAh·g −1 and the alloying reaction contributing 783 mAh·g −1 . This is much higher than that of commonly used graphite materials, making Snbased oxides a potential candidate for anode electrode materials [22][23][24]. As researchers investigated further, several serious problems with the Sn-based material were identified: (i) During the insertion/extraction of Li + and Na + , Sn fully converts to Li22Sn5 and Na15Sn4, undergoing severe volume expansion (260% and 420%, respectively). This causes pulverization of the active material, allowing it to break away from the collector and leading to electrode failure [25][26][27][28]. This is detrimental to the cycling stability and reversible capacity of the battery. (ii) The inherent poor electrical and ionic conductivity of the material results in slow reaction kinetics, which reduces high-magnification performance. (iii) The poor reversibility of the conversion reaction between Sn and Li2O or Na2O during the initial cycle results in a large initial irreversible capacity loss As researchers investigated further, several serious problems with the Sn-based material were identified: (i) During the insertion/extraction of Li + and Na + , Sn fully converts to Li 22 Sn 5 and Na 15 Sn 4 , undergoing severe volume expansion (260% and 420%, respectively). This causes pulverization of the active material, allowing it to break away from the collector and leading to electrode failure [25][26][27][28]. This is detrimental to the cycling stability and reversible capacity of the battery. (ii) The inherent poor electrical and ionic conductivity of the material results in slow reaction kinetics, which reduces high-magnification performance. (iii) The poor reversibility of the conversion reaction between Sn and Li 2 O or Na 2 O during the initial cycle results in a large initial irreversible capacity loss as well as a low initial coulombic efficiency (ICE) [29]. In response to these problems, several studies have shown that the doping of Sn-based materials with other transition metal elements to produce metal oxide M 2 SnO 4 can improve the properties [30][31][32]. On the one hand, due to the differences in expansion coefficients between different metal elements, redox reactions are enriched during the ion insertion/extraction process. This forms metal "M" or metal oxide "MO y " with different redox potentials, which can act as buffer substrates between each other to slow down the volume changes during cycling, thus ensuring the structural stability of the electrode material and reducing the capacity decay [33][34][35]. Furthermore, the conversion reaction of M can provide Sn with the required Li 2 O or Na 2 O, promoting the reversibility of the Sn conversion reaction. This improves the poor reversibility of the conversion reaction and reduces irreversible capacity loss [36]. On the other hand, combining different nanocrystals allows for the construction of heterogeneous structures at the contact interface. The internal electric field formed at the interface improves surface reaction kinetics, facilitating charge transfer and increasing material conductivity. For example, Mn 2 SnO 4 , a combination of Sn and Mn, has "metal-like conductivity" and improves the reaction kinetics [34,37]. In addition to overcoming the problems of tin-based materials, researchers have combined other modification strategies such as nanosizing, carbon compounding, heterostructuring and heteroatom doping.In recent years, stannatebased materials have gained great attention due to their superiority as anodes in LIBs and SIBs. In order to make stannate-based materials a viable alternative to anode materials, researchers have conducted a lot of exploration and proposed various modification strategies to address their problems. Table 1 provides a review of articles from the last decade on the research of different Sn-based materials. Among these excellent reviews, the main materials targeted include metallic Sn, SnO, SnO 2 , SnS, M 2 SnO 4 and their composites, as shown in Table 1. For example, Huang Bin et al. [38] summarized studies on the mechanisms of action and institutional design of Sn-based oxides, sulfides, alloys and their compound materials for the storage of alkaline earth metal ions, including an analysis of the lithium storage mechanism of Co 2 SnO 4 materials. Dong Sun's review [39] gives special attention to two tin-based materials, ZnSnO 3 and Zn 2 SnO 4 , outlining various synthetic strategies, formation mechanisms and morphology and summarizing their various applications. Although some of these excellent previous reviews mention the use of M 2 SnO 4 for LIBs and SIBs, there is a lack of systematic review articles that address the energy storage mechanisms and material design of these materials. However, M 2 SnO 4 cathode materials combining alloying and conversion reactions have shown great potential. It is therefore necessary to fill this gap by reviewing the rapid development of M 2 SnO 4 materials as negative electrodes in the field of rechargeable batteries. In this review, we provide an overview of research on M 2 SnO 4 as an anode in LIBs and SIBs. We systematically discuss the storage mechanisms, design of materials, and electrochemical properties of M 2 SnO 4 anode materials in each section ( Figure 2). In addition, we summarize and outline the future directions and prospects of M 2 SnO 4 -based anode materials.

Lithium-Ion Storage
Transition metal stannate M2SnO4 is considered an ideal candidate for LIBs with high theoretical capacity values compared to current commercially available anode electrode

Lithium-Ion Storage
Transition metal stannate M 2 SnO 4 is considered an ideal candidate for LIBs with high theoretical capacity values compared to current commercially available anode electrode materials (graphite) [37,53]. This is mainly since the storage of lithium ions in M 2 SnO 4 materials (M 2 SnO 4 ; M = Zn, Mn, Co, etc.) is carried out by both a conversion reaction mechanism and an alloying-dealloying mechanism. The lithiation mechanism of M 2 SnO 4 materials is described by a four-stage reaction [23,33,[53][54][55][56][57]. As shown in Equation (1), the first step is an irreversible reaction, consisting specifically of the decomposition of M 2 SnO 4 into M and Sn and the formation of LiO 2 . Next, the second step is the alloying/dealloying reaction as expressed in Equation (2). This reaction allows Sn to bind up to 4.4 Li + , resulting in a high capacity, but also causing severe expansion of the Sn volume, which affects cycling stability. Finally, there is a conversion reaction between Sn and LiO 2 /SnO 2 and M and LiO 2 /MO 2, as indicated in Equations (3) and (4). The reversibility of the conversion reaction is a vital factor in maintaining the high capacity of the M 2 SnO 4 -based anode material.
During this lithiation/delithiation cycle, reactions (3) and (4) form two metals with different redox potentials (metal-lithium alloys). On the one hand, Reactions (3) and (4) can provide additional Li 2 O to Sn and Mrespectively, which constitutes a "synergistic effect" and promotes the reversibility of both [33]. On the other hand, the two act as a skeleton for each other to buffer the change in the volume of the material, ensuring the structural integrity of the material and reducing capacity degradation [34]. In addition, Figure 3 shows the CV curves of Mn 2 SnO 4 and Co 2 SnO 4 , whose responses correspond to the energy storage mechanism described above. It is due to the superiority of this lithium storage mechanism that the development of M 2 SnO 4 materials as anodes for LIBs is of great significance.
Molecules 2023, 28, 5037 6 of 24 storage mechanism that the development of M2SnO4 materials as anodes for LIBs is of great significance.

Nanostructures
Due to the advantages of nanosizing materials such as increasing the specific surface area and shortening the diffusion path of Li + , various structures of electroactive materials are designed and prepared on the nanoscale to improve the rate performance and cycle stability of batteries.
In terms of preparation methods, M2SnO4-based nanomaterials can be synthesized using a variety of techniques, including hydrothermal/solvothermal methods, solid-phase synthesis and sol-gel methods. Early studies by Irvine et al. [56] demonstrated the successful preparation of nanoscale M2SnO4 via solid-state synthesis. By comparing the Li + storage behavior of these materials, they found that M2SnO4 exhibited better reversibility when M=Mn or Zn and poorer capacity reversibility when M=Mg. More recently, hydrothermal methods have emerged as a popular approach for the preparation of M2SnO4based nanomaterials due to their advantages in terms of dispersion and ease of control. For instance, pure-phase Mn2SnO4 [58], Zn2SnO4 [59] and Co2SnO4 [23] nanoparticles prepared using this method have been shown to possess excellent electrochemical activity, with initial charge-discharge capacities of 1320, 1900.4 and 1533.1 mAh·g −1 , respectively.
Wang et al. [23] recently reported a comparison of the properties of Co2SnO4 nanocrystals prepared using hydrothermal (HT-Co2SnO4) and high-temperature solid-state (SS-Co2SnO4) methods. Their results showed significant differences in the electrochemical

Nanostructures
Due to the advantages of nanosizing materials such as increasing the specific surface area and shortening the diffusion path of Li + , various structures of electroactive materials are designed and prepared on the nanoscale to improve the rate performance and cycle stability of batteries.
In terms of preparation methods, M 2 SnO 4 -based nanomaterials can be synthesized using a variety of techniques, including hydrothermal/solvothermal methods, solid-phase synthesis and sol-gel methods. Early studies by Irvine et al. [56] demonstrated the successful preparation of nanoscale M 2 SnO 4 via solid-state synthesis. By comparing the Li + storage behavior of these materials, they found that M 2 SnO 4 exhibited better reversibility when M = Mn or Zn and poorer capacity reversibility when M = Mg. More recently, hydrothermal methods have emerged as a popular approach for the preparation of M 2 SnO 4 -based nanomaterials due to their advantages in terms of dispersion and ease of control. For instance, pure-phase Mn 2 SnO 4 [58], Zn 2 SnO 4 [59] and Co 2 SnO 4 [23] nanoparticles prepared using this method have been shown to possess excellent electrochemical activity, with initial charge-discharge capacities of 1320, 1900.4 and 1533.1 mAh·g −1 , respectively.
Wang et al. [23] recently reported a comparison of the properties of Co 2 SnO 4 nanocrystals prepared using hydrothermal (HT-Co 2 SnO 4 ) and high-temperature solid-state (SS-Co 2 SnO 4 ) methods. Their results showed significant differences in the electrochemical properties of these materials (Figure 4c). After 50 cycles, the discharge capacity of SS-Co 2 SnO 4 was 112.8 mAh·g −1 with a capacity retention rate of only 12.2%, which is much lower than that of HT-Co 2 SnO 4 (555.9 mAh·g −1 with a capacity retention of 50.3%). SEM and TEM characterization revealed that HT-Co 2 SnO 4 consisted of uniform spherical nanoparticles with diameters ranging from 80 to 100 nm, while SS-Co 2 SnO 4 had an irregular shape with smaller particles (about 100 nm) coexisting with larger particles (1-2 µm) (Figure 4a,b). The particle size of the nanocrystals may be the main reason for the difference in electrochemical properties, with smaller nanoparticles obtained by the hydrothermal method exhibiting better electrochemical properties compared to those obtained by the high-temperature solid-state method.  In addition, some unique nanometer-size structures have been designed, such as hollow skeletons [60], hollow nanospheres [54], core-shell nanostructures [61], nanowires [62] and nanoplates [62], which are favorable for buffering volume expansion and maintain structural stability. Using a sol-gel method combined with phase separation, Wang et al. [60] synthesized macroporous Co2SnO4 with a hollow skeleton (Figure 5a,b) using polyacrylic acid (PAA) as the phase separation agent. As an anode for LIBs, the macroporous Co2SnO4 exhibited high capacity retention (115.5% at 200 mAh·g −1 after 300 cycles) and provided an ultrahigh specific capacity (921.8 mAh·g −1 at 1 A·g −1 ). Similarly, Zhang et al. [54] prepared hollow nanospheres (Figure 5c,d) using typical hydrothermal synthesis and heat treatment to serve as an anode for LIBs, which exhibited characteristics such as high multiplicative performance and cycling stability; even at a current density of 1 A·g −1 , they still could achieve the reversible specific capacity of 442 mAh·g −1 after 60 cycles. In contrast to the above preparation methods, one-dimensional nanowires are usually prepared using the vapor phase method. Zn2SnO4 nanowires (Figure 5e,f)) were synthesized directly on stainless steel substrates by the vapor transport method. Constant current cycling studies of Zn2SnO4 nanowires at voltages ranging from 0.005 to 3 V and currents of 120 mA·g −1 have shown a reversible capacity of 1000 (±5) mAh·g −1 , which is almost constant for the first 10 cycles and decays to 695 mAh·g −1 through 60 cycles thereafter [62]. The design of these special shapes provides additional cushioning space, relieves volume expansion and protects the stability of the structure. In addition, some unique nanometer-size structures have been designed, such as hollow skeletons [60], hollow nanospheres [54], core-shell nanostructures [61], nanowires [62] and nanoplates [62], which are favorable for buffering volume expansion and maintain structural stability. Using a sol-gel method combined with phase separation, Wang et al. [60] synthesized macroporous Co 2 SnO 4 with a hollow skeleton (Figure 5a,b) using polyacrylic acid (PAA) as the phase separation agent. As an anode for LIBs, the macroporous Co 2 SnO 4 exhibited high capacity retention (115.5% at 200 mAh·g −1 after 300 cycles) and provided an ultrahigh specific capacity (921.8 mAh·g −1 at 1 A·g −1 ). Similarly, Zhang et al. [54] prepared hollow nanospheres (Figure 5c,d) using typical hydrothermal synthesis and heat treatment to serve as an anode for LIBs, which exhibited characteristics such as high multiplicative performance and cycling stability; even at a current density of 1 A·g −1 , they still could achieve the reversible specific capacity of 442 mAh·g −1 after 60 cycles. In contrast to the above preparation methods, one-dimensional nanowires are usually prepared using the vapor phase method. Zn 2 SnO 4 nanowires (Figure 5e,f)) were synthesized directly on stainless steel substrates by the vapor transport method. Constant current cycling studies of Zn 2 SnO 4 nanowires at voltages ranging from 0.005 to 3 V and currents of 120 mA·g −1 have shown a reversible capacity of 1000 (±5) mAh·g −1 , which is almost constant for the first 10 cycles and decays to 695 mAh·g −1 through 60 cycles thereafter [62]. The design of these special shapes provides additional cushioning space, relieves volume expansion and protects the stability of the structure.
(Figure 5e,f)) were synthesized directly on stainless steel substrates by the vapor trans method. Constant current cycling studies of Zn2SnO4 nanowires at voltages ranging 0.005 to 3 V and currents of 120 mA·g −1 have shown a reversible capacity of 1000 (±5) mAh which is almost constant for the first 10 cycles and decays to 695 mAh·g −1 through 60 c thereafter [62]. The design of these special shapes provides additional cushioning spac lieves volume expansion and protects the stability of the structure. Figure 5. SEM images of (a,b) Co 2 SnO 4 hollow skeletons [60]. Copyright 2022, MDPI. TEM images of (c,d) Zn 2 SnO 4 hollow nanospheres [54]. Copyright 2014, Royal Society of Chemistry. TEM images of (e,f) Zn 2 SnO 4 nanowires [62]. Copyright 2013, American Chemical Society.
Nanosizing the material and designing it with suitable structures, especially hollow, porous and unique structures, has been shown to effectively improve the bulk expansion of stannate-based materials.
Previously, Qi and colleagues [61] synthesized a core-shell nanostructure of Co 2 SnO 4 @C using glucose as the carbon source. The uniform carbon layer (5-10 nm) coated on the surface of the Co 2 SnO 4 material (Figure 6a) significantly improved the electrochemical performance of the electrode material. The capacity retention rate of Co 2 SnO 4 @C was much higher than that of pure-phase Co 2 SnO 4 in terms of cycling performance (Figure 6b). EIS test results (Figure 6c) showed that the diameter of the semicircle in the mid-frequency region of Co 2 SnO 4 @C was smaller than that of Co 2 SnO 4 , indicating that it had a smaller charge transfer resistance. Similarly, a core-shell nanorod structure of Zn 2 SnO 4 @C was synthesized using a similar method. The nanorod is composed of continuous stripes with the same orientation and has a lattice spacing of about 0.26 nm and a uniform carbon coating layer of 5-10 nm (Figure 6d) [65]. The electrochemical performance of carbon composite materials was compared with pure-phase materials (Figure 5e,f), and it was found that the combination of carbon materials can effectively improve the stability and charge transfer impedance of the electrode. The presence of this carbon coating layer serves as a buffer base to enhance structural stability on one hand and improves electron conductivity on the other hand to accelerate charge transfer on the surface. Therefore, it can exhibit superior electrochemical performance. mance of carbon composite materials was compared with pure-phase materials (Figu 5e,f), and it was found that the combination of carbon materials can effectively impro the stability and charge transfer impedance of the electrode. The presence of this carb coating layer serves as a buffer base to enhance structural stability on one hand and i proves electron conductivity on the other hand to accelerate charge transfer on the surfa Therefore, it can exhibit superior electrochemical performance.  Compared with traditional carbon materials, graphene materials possess excellent electrical conductivity, flexible structure and high specific surface area. In particular, they exhibit greatly improved electrochemical properties when composited with metal oxide anode materials [71,74]. Typically, a Zn 2 SnO 4 /graphene nanohybrid in flake form was prepared using the in situ hydrothermal method. In this anode system, GNSs act as a buffer to mitigate the volume change and as a separator to inhibit the aggregation of nanoparticles, thus improving the cycling stability. Additionally, the addition of GNSs provides a two-dimensional conductive channel for the Zn 2 SnO 4 nanocrystals, enhancing the rate capability [68]. It is worth mentioning that graphene and M 2 SnO 4 are combined via an electrostatic mechanism, which generally manifests itself as positively charged M 2 SnO 4 nanoparticles being uniformly wrapped in negatively charged graphene under the influence of electrostatic forces [71]. Therefore, some method is needed to positively charge the surface of M 2 SnO 4 to bind to the negatively charged graphene. For example, Co 2 SnO 4 nanoparticles (Co 2 SnO 4 NPs) were dispersed in CoCl 2 solution so that they adsorbed Co 2+ for positive charge [71]. Similarly, a Co 2 SnO 4 hollow cube (Co 2 SnO 4 HC) was modified with aminopropyltriethoxysilane (APTES) so that its surface was functionalized and positively charged (Figure 7a) [67]. As the uniform wrapping of graphene sheets (Figure 7b,c) effectively alleviates the volume change, the electrodes can maintain excellent conductivity throughout the discharge/charging process, so Co 2 SnO 4 HC@rGO can provide more than 1000 mAh·g −1 capacity at 100 mA·g −1 after 100 cycles, while Co 2 SnO 4 NPs@rGO can achieve the large reversible capacity of 1037.9 mAh·g −1 after 200 cycles. In addition to the above-listed Co 2 SnO 4 , the graphene-coated Zn 2 SnO 4 (Figure 7d,e) obtained by electrostatic interaction also exhibits enhanced properties compared to the pure-phase material [69].
sheets (Figure 7b,c) effectively alleviates the volume change, the electrodes can maintain excellent conductivity throughout the discharge/charging process, so Co2SnO4 HC@rGO can provide more than 1000 mAh·g −1 capacity at 100 mA·g −1 after 100 cycles, while Co2SnO4 NPs@rGO can achieve the large reversible capacity of 1037.9 mAh·g −1 after 200 cycles. In addition to the above-listed Co2SnO4, the graphene-coated Zn2SnO4 (Figure 7d,e) obtained by electrostatic interaction also exhibits enhanced properties compared to the pure-phase material [69]. In contrast to the direct wrapping of graphene sheets on stannate-based material described above, the work of Rehman et al. [53] designed a bouquet-like nanocomposite structure in which graphene sheets were embedded in nanoparticles containing manganese and tin to achieve a high degree of bonding. The strategy benefits from a unique porous nanostructure in which high electronic pathways provided by graphene sheets are used to enhance electronic conductivity and uniformly distributed nanoparticles accelerate the kinetic reaction with lithium ions. Moreover, the graphene sheet also limits the growth of stannate nanoparticles, reducing the grain size. Due to the unique structure and conductive network of Mn2SnO4@GS, the Mn2SnO4@GS anode material exhibits excellent rate and cycling In contrast to the direct wrapping of graphene sheets on stannate-based material described above, the work of Rehman et al. [53] designed a bouquet-like nanocomposite structure in which graphene sheets were embedded in nanoparticles containing manganese and tin to achieve a high degree of bonding. The strategy benefits from a unique porous nanostructure in which high electronic pathways provided by graphene sheets are used to enhance electronic conductivity and uniformly distributed nanoparticles accelerate the kinetic reaction with lithium ions. Moreover, the graphene sheet also limits the growth of stannate nanoparticles, reducing the grain size. Due to the unique structure and conductive network of Mn 2 SnO 4 @GS, the Mn 2 SnO 4 @GS anode material exhibits excellent rate and cycling performance. At a current density of 400 mA·g −1 , a specific capacity of about mAh·g −1 can be achieved after 200 cycles, while at a high current density of 2500 mA·g −1 , a specific capacity of about 455 mAh·g −1 can still be provided (Figure 8a). Two-dimensional graphene sheets and one-dimensional carbon nanotubes are the stars of the carbon materials world. Carbon nanotubes (CNTs) are frequently used as carbon materials compounded with metal oxides due to their superior electrical conductivity, high aspect ratio and large specific surface area [75][76][77]. Three-component Mn 2 SnO 4 @MWCNTs composites with cubic particles and high porosity anchored on carbon nanotubes were synthesized through a facile hydrothermal method [73]. It is clearly demonstrated by the impedance spectra that due to the synergy between Mn 2 SnO 4 and MWCNTs, the electron transfer resistance in the LIBs is reduced [78], resulting in a much faster charge transfer in Mn 2 SnO 4 @MWCNTs than in bare Mn 2 SnO 4 (Figure 8b).
In recent years, metal-organic frameworks (MOFs) have been widely investigated as materials that contain both metal and carbon sources with easily controlled structures [79][80][81][82], offering a new approach to the preparation of carbon-coated materials using the pyrolysis of MOFs [83,84]. Shi et al. [33] designed carbon-coated Mn 2 SnO 4 nanomaterials with a two-dimensional combined micro/nanoscale configuration using a two-step carbonization process based on Mn-based metal-organic frameworks (Mn-MOFs) as a precursor. The Mn-MOFs play a role in providing both the Mn source and the flake-like porous carbon matrix. When comparing the cyclic and rate performance of Mn 2 SnO 4 @C with MnO/SnO 2 @C which has the same elemental composition, it was concluded that the "synergistic effect" in Mn 2 SnO 4 [36] and the unique 2D structure brought by the MOF precursors contribute to the reversibility of the lithium storage reaction (Figure 8c). In contrast to the two-step method described above, Yue et al. [85] directly synthesized a Zn-Sn binary MOF (ZT-MOF) and then used rapid calcination under a reducing atmosphere to obtain Zn 2 SnO 4 @C/Sn composites. Interestingly, this work explored the effect of calcination time on the electro-chemical properties of the material and found that the material calcined for 1 min had the best performance, mainly since metal Sn grew into large particles as the calcination time increased, which transformed the lithium storage process from surface-controlled to diffusion-controlled.

of 24
performance. At a current density of 400 mA·g −1 , a specific capacity of about mAh·g −1 can be achieved after 200 cycles, while at a high current density of 2500 mA·g −1 , a specific capacity of about 455 mAh·g −1 can still be provided (Figure 8a). Two-dimensional graphene sheets and one-dimensional carbon nanotubes are the stars of the carbon materials world. Carbon nanotubes (CNTs) are frequently used as carbon materials compounded with metal oxides due to their superior electrical conductivity, high aspect ratio and large specific surface area [75][76][77]. Three-component Mn2SnO4@MWCNTs composites with cubic particles and high porosity anchored on carbon nanotubes were synthesized through a facile hydrothermal method [73]. It is clearly demonstrated by the impedance spectra that due to the synergy between Mn2SnO4 and MWCNTs, the electron transfer resistance in the LIBs is reduced [78], resulting in a much faster charge transfer in Mn2SnO4@MWCNTs than in bare Mn2SnO4 (Figure 8b).
In recent years, metal-organic frameworks (MOFs) have been widely investigated as materials that contain both metal and carbon sources with easily controlled structures [79][80][81][82], offering a new approach to the preparation of carbon-coated materials using the pyrolysis of MOFs [83,84]. Shi et al. [33] designed carbon-coated Mn2SnO4 nanomaterials with a two-dimensional combined micro/nanoscale configuration using a two-step carbonization process based on Mn-based metal-organic frameworks (Mn-MOFs) as a precursor. The Mn-MOFs play a role in providing both the Mn source and the flake-like porous carbon matrix. When comparing the cyclic and rate performance of Mn2SnO4@C with MnO/SnO2@C which has the same elemental composition, it was concluded that the "synergistic effect" in Mn2SnO4 [36]  In summary, incorporating carbon materials through various methods to optimize material composition has proven to be an effective approach for mitigating volume expansion and enhancing the electrical conductivity of M 2 SnO 4 materials. This approach has been widely adopted in the development of M 2 SnO 4 -based anode materials.

Heterogeneous Structures
The coupling between different nanocrystals builds heterogeneous structures at the contact interface, and the resulting Mott-Schottky heterogeneous junction generates an internal electric field at the interface, which improves the electrochemical dynamics [86,87]. However, the rational design and controlled synthesis of nano-heterostructured anode materials with high performance is still a challenge.
The yolk-shell structure is a classical form of a heterogeneous structure. Ju et al. [88] described the preparation of Mn-Sn-O-C composites with yolk-shell heterostructures by simple spray pyrolysis at various temperatures, where the shell and core portions were MnO-Mn 2 SnO 4 -C and Sn-Mn 2 SnO 4 -C, respectively. The Mn-Sn-O-C prepared at 900 • C had good structural stability with a discharge capacity of 784 mAh·g −1 over 100 cycles at a current density of 1 A·g −1 (Figure 9a-e). Similar to the yolk-shell structure, Tian et al. [89] used a three-step method combining co-precipitation, hydrothermal treatment and carbonization to design multi-yolk-shell SnO 2 /Mn 2 SnO 4 @C nanoboxes. In this report, it is shown that the SnO 2 /Mn 2 SnO 4 heterostructure on the one hand produced lattice distortions in the internal material to improve thermodynamic stability, and on the other hand promoted reaction kinetics by hindering the coarsening of Sn, inducing a redistribution of electrons between SnO 2 and Mn 2 SnO 4 and accelerating the diffusive adsorption of Li + through the internal electric field at the heterogeneous interface. Benefiting from the properties brought about by the heterogeneous structure, SnO 2 /Mn 2 SnO 4 @C nanoboxes as anode materials for LIBs exhibit a large reversible capacity (1293 mAh·g −1 at 0.2 A·g −1 after 100 cycles) and a stable long cycle performance (more than 549 cycles at 2 A·g −1 ). Zhuang et al. [34] reported a novel heterogeneous composite material with excellent rate performance and cyclability by constructing a sandwich structure of graphene hollow spheres confined to a Mn2SnO4/SnO2 heterostructure (Mn2SnO4/SnO2@SG) as an anode for LIBs. Microscopically, the Mn2SnO4 and SnO2 nanoparticles form a heterogeneous structure next to each other between the graphene spherical shells; macroscopically, the structure appears to be a sandwiched hollow sphere. The Mn2SnO4/SnO2 Mott-Schottky heterojunction generates a strong electric field at the interface, which greatly facilitates the electronic/ionic transport kinetics and therefore shows excellent rate properties (823.8 mAh·g −1 at 5 C) (Figure 9f-h).
The heterogeneous structure is a modification method that involves introducing other materials compositionally to construct special structures. This approach has significantly improved the electrochemical performance of M2SnO4 anode materials. However, the process of constructing these structures can be complex and challenging. Zhuang et al. [34] reported a novel heterogeneous composite material with excellent rate performance and cyclability by constructing a sandwich structure of graphene hollow spheres confined to a Mn 2 SnO 4 /SnO 2 heterostructure (Mn 2 SnO 4 /SnO 2 @SG) as an anode for LIBs. Microscopically, the Mn 2 SnO 4 and SnO 2 nanoparticles form a heterogeneous structure next to each other between the graphene spherical shells; macroscopically, the structure appears to be a sandwiched hollow sphere. The Mn 2 SnO 4 /SnO 2 Mott-Schottky heterojunction generates a strong electric field at the interface, which greatly facilitates the electronic/ionic transport kinetics and therefore shows excellent rate properties (823.8 mAh·g −1 at 5 C) (Figure 9f-h).
The heterogeneous structure is a modification method that involves introducing other materials compositionally to construct special structures. This approach has significantly improved the electrochemical performance of M 2 SnO 4 anode materials. However, the process of constructing these structures can be complex and challenging.

Heteroatom Doping
Doping heteroatoms into the lattice of a material can generate a large number of active sites and improve electronic conductivity. This approach can be applied to M 2 SnO 4 nanomaterials and carbon-based materials to enhance their electrochemical properties. Incorporating heteroatoms into these materials holds great potential for improving their performance in various applications [90].
Wang et al. [91] reported for the first time the fabrication of nanocomposites (Co-ZTO-G-C) consisting of ultrafine (3-5 nm) Co-doped Zn 2 SnO 4 nanoparticles, graphene nanosheets and amorphous carbon layers using a one-step hydrothermal method. Since the radii of Co 2+ (0.074 nm) and Zn 2+ (0.074 nm) are so close to each other, the substitution of Co 2+ for Zn 2+ in the lattice of Zn 2 SnO 4 is favorable, and no other byproducts such as cobalt oxide were found by X-ray diffraction analysis, which also proves the successful doping of Co 2+ . In particular, the crystallinity of the Co-ZTO-G-C nanocomposite is significantly higher than that of the other two nanocomposites, indicating that Co doping enhances the crystallinity of the Zn 2 SnO 4 nanoparticles. The doping of N atoms into carbon-based materials such as N, P, B and S is also a common strategy. Wan et al. [37] doped N atoms into carbon and encapsulated Sn@Mn 2 SnO 4 nanoparticles in N-doped carbon to be used as anode materials for LIBs. The doping of N atoms enhanced the electrical conductivity of the carbon shell and improved the lithium storage capacity.
Whether through the direct introduction of Co + into nanomaterials to improve their crystallinity or the introduction of N atoms into composite carbon materials to increase active sites and improve electrical conductivity, doping heteroatoms has proven to be a feasible means of modifying M 2 SnO 4 anode materials.

Sodium Ion Storage
The sodium storage mechanism of the M 2 SnO 4 material in SIBs involves a multi-step electrochemical redox reaction, which can be expressed by the following Equations (5)-(8) [24,57,92,93]. The initial step is an irreversible decomposition reaction as shown in Equation (5), which specifically includes the decomposition of M 2 SnO 4 to M and Sn and the formation of NaO. The second step is the alloying/dealloying reaction expressed in Equation (6), where the formation of Na x Sn causes the expansion of the volume. The final Equations (7) and (8) The introduction of a second metal can enrich redox reactions compared to monometallics [35], and M and MO can be used as "substrates" to attenuate the effect of volume expansion on cycling performance caused by Na x Sn generated by alloying reactions during cycling. For example, Huang et al. [94] developed CoMoO 4 @NC using two metals (Co and Mo) as an SIB anode material, which exhibited a long cycle life and maintained a specific capacity of 190 mAh·g −1 after 3200 cycles even at a high current density of 1 A·g −1 .

Nanostructures
The use of nanostructures can effectively shorten the diffusion path of Na + , facilitate sodiation/desodiation, and reduce the adverse effects of volume expansion during cycling.
Park et al. [57] used the hydrothermal method to directly synthesize high-purity Zn 2 SnO 4 nanowires and for the first time used them as an anode material for SIBs to investigate their electrochemical properties. In an SIB, Zn 2 SnO 4 has a reversible capacity of 306 mAh·g −1 after 100 cycles at 0.1 C and maintains a high coulombic efficiency of about 99%. Nanowires (NWs) composed of a cubic (Zn 2 SnO 4 ) phase have a homogeneous morphology, and the NWs are able to form a network to mitigate the volume change, which improves the cycling performance and extends the battery life. In this study, the working mechanism in Zn 2 SnO 4 was also investigated by cyclic voltammetry (CV), I-V combined with ex situ XRD, and it was found that the alloying reaction did not proceed completely enough to reach the Na x Sn alloy and lead to capacity reduction, while the reversible conversion reaction helped the capacity recovery. The above findings are consistent with previous studies on Sn and SnO 2 [54,95].

Composited with Carbon Materials
Combining Sn-based materials with carbon materials and designing superior structures can effectively improve the material conductivity as well as the structural stability of electrodes and therefore has received a lot of attention.
Graphene is an excellent carbon material. N. Kalaiselvi's team [92] designed Mn 2 SnO 4 nanoporous cubes with graphene and used them for the first time as an anode material for SIBs. To prepare Mn 2 SnO 4 /graphene (MSO/G), the prepared Mn 2 SnO 4 was sonicated with graphene in an ethanol solution, followed by oven drying and calcination under an argon atmosphere to ensure complete wrapping and adhesion of graphene sheets to the Mn 2 SnO 4 nanocubes. In the SIB, the MSO/G composite anode exhibited high electrochemical performance at various current rates in terms of cycling capacity (257 mAh·g −1 after 100 cycles at 100 mA·g −1 ) and rate capacity (211 mAh·g −1 at 500 mA·g −1 ) and reached 106 mA h g −1 even after 1000 cycles at a high current density of 1 A·g −1 . The presence of graphene networks and MSO nanocube voids greatly mitigates the shock caused by volume changes, ensuring that the composite anode material can maintain structural stability even under long cycling and enhancing the sodium storage capacity.

Heterogeneous Structures
According to the related reports, a heterogeneous structure constructed by combining bimetallic and carbon materials introduces defects, disorders and heterogeneous interfaces that can provide effective channels for the diffusion and adsorption of Na + , thus promoting the kinetics of the reaction [96][97][98].
Tian et al. designed novel nanoboxes with multiple yolk-shell structures (denoted as SnO 2 /Mn 2 SnO 4 @C) containing heterogeneously structured SnO 2 /Mn 2 SnO 4 nanoparticles as the yolk and phenolic resin-derived carbon as the shell. The procedure for the preparation of SnO 2 /Mn 2 SnO 4 @C is represented in Figure 8a. Specifically, it includes the following steps: firstly, MnCl 2 , SnCl 4 and NaOH are used as raw materials to obtain MnSn(OH) 6 nanoboxes using the co-precipitation method; after that, the phenolic resin is wrapped on the surface of MnSn(OH) 6 by hydrothermal reaction to obtain PR-MnSn(OH) 6 nanoboxes; finally, heat treatment is performed to obtain SnO 2 /Mn 2 SnO 4 @C products. During the heat treatment, MnSn(OH) 6 was transformed into heterostructured SnO 2 /Mn 2 SnO 4 nuclei and PR was carbonized into carbon shells. As an SIB anode, it provides a large reversible capacity of 203 mA h g −1 after 100 cycles. In addition, according to the EIS spectra and fitting results, the SnO 2 /Mn 2 SnO 4 @C anode has a low R ct value (30.55 Ω) and a large D Na coefficient (8.72 × 10 −14 ). The excellent electrochemical performance is mainly attributed to the following: the heterogeneous structure of SnO 2 /Mn 2 SnO 4 effectively mitigates the volume expansion of Sn→Na x Sn and improves the reversibility of conversion and alloying, the lattice distortion and the redistribution of charges at the heterogeneous interface accelerate the migration of Na + , and the hollow structure of SnO 2 /Mn 2 SnO 4 @C nanoboxes and the phenolic resin-derived carbon shell attenuate the crushing and agglomeration of the material.

Heteroatom Doping
The doping of heteroatoms (N, S, B, P, etc.) into carbon materials can provide more reactive sites and thus effectively improve the electrochemical properties of the materials, so this method of modification at the atomic level has gained great attention [99,100].
Using a typical synthesis method of hydrothermal treatment, carbon encapsulation and high-temperature treatment, Wan et al. [37] prepared the composite Sn@Mn 2 SnO 4 -NC containing Mn 2 SnO 4 nanoparticles with Sn encapsulated in an N-doped carbon layer. Figure 10a shows synthesis path of Sn@Mn 2 SnO 4 -NC. In an SIB, the introduction of heteroatoms improves the electrical conductivity of the material, facilitates electron/ion transfer and accelerates the reaction kinetics, and the hollow structure combined with the carbon material provides the space needed for volume expansion/contraction and protects the material from crushing. The electrochemical reaction of Sn@Mn 2 SnO 4 -NC in SIBs is mainly controlled by pseudocapacitance according to the CV curves (Figure 10b,c). In addition, the capacitance contribution of SIBs is higher than that of LIBs at the same scan rate, which is mainly attributed to the fact that the radius of Na + is larger than that of Li + , which hinders the insertion of ions. The doping of heteroatoms (N, S, B, P, etc.) into carbon materials can provide more reactive sites and thus effectively improve the electrochemical properties of the materials, so this method of modification at the atomic level has gained great attention [99,100].
Using a typical synthesis method of hydrothermal treatment, carbon encapsulation and high-temperature treatment, Wan et al. [37] prepared the composite Sn@Mn2SnO4-NC containing Mn2SnO4 nanoparticles with Sn encapsulated in an N-doped carbon layer. Figure 10a shows synthesis path of Sn@Mn2SnO4-NC. In an SIB, the introduction of heteroatoms improves the electrical conductivity of the material, facilitates electron/ion transfer and accelerates the reaction kinetics, and the hollow structure combined with the carbon material provides the space needed for volume expansion/contraction and protects the material from crushing. The electrochemical reaction of Sn@Mn2SnO4-NC in SIBs is mainly controlled by pseudocapacitance according to the CV curves (Figure 10b-c). In addition, the capacitance contribution of SIBs is higher than that of LIBs at the same scan rate, which is mainly attributed to the fact that the radius of Na + is larger than that of Li + , which hinders the insertion of ions.
Similarly, Kim et al. [99] reported the synthesis of Zn2SnO4 nanoparticles with uniformly encapsulated nitrogen-doped carbon layers using dopamine as a single carbon and nitrogen source, and the thickness of the carbon layer was adjusted by tuning the degree of polymerization of dopamine. The presence of the N-doped carbon cladding layer enhances the ionic/electronic conductivity, alleviates the volume expansion during electrode cycling, and prevents the aggregation of nanoparticles and the adverse effects caused by contact with the electrolyte.

Conclusion and Outlook
In conclusion, we have presented a comprehensive analysis of the development of M2SnO4-based nanomaterials as anodes for LIBs and SIBs. By incorporating other metallic elements into Sn-based materials to form M2SnO4 compounds (where M=Zn, Mn, Co, etc.), researchers have enriched the redox reactions involved in energy storage and created metal "M" or metal oxide "MO" buffer substrates. This effectively mitigates the issue of volume expansion during charge/discharge cycles. Tables 2 and 3 summarizes a range of M2SnO4-based anode materials, categorized by synthesis techniques, morphological structures and electrochemical performance in LIBs and SIBs. Similarly, Kim et al. [99] reported the synthesis of Zn 2 SnO 4 nanoparticles with uniformly encapsulated nitrogen-doped carbon layers using dopamine as a single carbon and nitrogen source, and the thickness of the carbon layer was adjusted by tuning the degree of polymerization of dopamine. The presence of the N-doped carbon cladding layer enhances the ionic/electronic conductivity, alleviates the volume expansion during electrode cycling, and prevents the aggregation of nanoparticles and the adverse effects caused by contact with the electrolyte.

Conclusions and Outlook
In conclusion, we have presented a comprehensive analysis of the development of M 2 SnO 4 -based nanomaterials as anodes for LIBs and SIBs. By incorporating other metallic elements into Sn-based materials to form M 2 SnO 4 compounds (where M = Zn, Mn, Co, etc.), researchers have enriched the redox reactions involved in energy storage and created metal "M" or metal oxide "MO" buffer substrates. This effectively mitigates the issue of volume expansion during charge/discharge cycles. Tables 2 and 3 summarizes a range of M 2 SnO 4 -based anode materials, categorized by synthesis techniques, morphological structures and electrochemical performance in LIBs and SIBs.   To improve the electrochemical performance of M 2 SnO 4 as a battery anode material, researchers have explored various approaches, starting with pure-phase M 2 SnO 4 -based compounds. A primary focus has been on reducing the size of M 2 SnO 4 materials to the nanoscale and designing various nanostructures (e.g., nanorods, nanowires, nanosheets, hollow nanospheres) to increase specific surface area and shorten Li + and Na + diffusion paths. Nanosized M 2 SnO 4 materials consistently exhibit superior electrochemical properties compared to their larger counterparts. Three synthesis methods were discussed in the previous section: high-temperature solid phase, hydrothermal/solvothermal and vapor phase. Of these, hydrothermal synthesis is widely used due to its ability to produce high-purity materials with small particle sizes and controllable morphologies.
Subsequently, researchers have combined M 2 SnO 4 with various carbon carriers to form carbon composites. In these composites, carbon carriers are encapsulated on the M 2 SnO 4 nanostructure in the form of lamellae or shells. Carbon materials have been compounded with M 2 SnO 4 using electrostatic forces to combine graphene with M 2 SnO 4 , high-temperature carbonization to encapsulate organic carbon sources (e.g., dopamine, fructose) on the surface of M 2 SnO 4 , and pyrolysis of MOF materials. The incorporation of carbon has a dual effect: it buffers the volume expansion of the active material during charge/discharge cycles and improves the electrical conductivity of the composite. As a result, hybrid M 2 SnO 4 -based materials with carbon exhibit enhanced electrochemical performance as anode materials.
Other researchers have coupled M 2 SnO 4 with different nanomaterials to create heterogeneous structures with contact interfaces between different nanocrystals. These structures include yolk-shell, sandwich and multi-yolk-shell nanobox structures. The materials chosen for coupling are typically corresponding metal oxides and carbon carriers. The lattice distortion and charge redistribution at the heterogeneous interface between the two nanomaterials facilitate rapid Li + and Na + migration, thus improving reaction kinetics. Additionally, the unique structures created by this strategy can mitigate the crushing and agglomeration of active materials and enhance battery cycling stability.
Although nanosizing enhances the mass-specific capacity of materials, it concurrently diminishes their volumetric loading. Additionally, excessive carbon content may result in inadequate capacity. Modification strategies, such as the formation of heterostructures or doping with foreign elements, often necessitate intricate synthesis procedures. Consequently, selecting suitable modification techniques for distinct materials is of paramount importance. Tables 2 and 3 provide detailed information on recent developments in M 2 SnO 4 -based anode materials for LIBs and SIBs. These tables summarize the composition, synthesis methods, material morphology and electrochemical properties of these materials.
As performance requirements for M 2 SnO 4 -based anode materials continue to increase, researchers are focusing on reducing particle size and studying the physicochemical properties of these materials in greater depth. However, many conventional experimental tools and characterization methods have significant limitations. Simulation calculations have emerged as an important tool in materials research and are widely used in the design of electrode materials, as well as in the calculation, prediction, validation, optimization and simulation of properties. For material design and preparation, simulation can provide an effective way to avoid unnecessary experiments. For mechanistic studies, it can elucidate the factors influencing electrochemical reactions and reveal microscopic mechanisms at the atomic level. Thus, simulation calculations will be a powerful aid in the study of M 2 SnO 4 -based anode materials.
Volume expansion and its associated problems remain major challenges in the development of M 2 SnO 4 -based materials. To address these issues, researchers are exploring the compounding of stannate materials with various other materials (including carbon carriers, metals and their oxides) and creating unique and refined morphological structures to improve and maintain the high capacity of stannate-based anode materials. Reducing grain size to less than 10 nm could be an effective strategy in terms of material size. In terms of material composition, the heterogeneous interface between different materials and the strength of chemical bonding should be considered. In terms of material morphology, a reasonable layout between the conductive network formed by the carbon material, active material and extra buffer space provided by hollow porous structures is crucial. In terms of heteroatom doping, introducing multiple heteroatoms into a suitable lattice structure could be a direction for future development. For example, while striving for the high weight capacity that nanosizing brings, it is important to also consider the volume capacity required for practical applications.
At the industrial application level, we suggest that using simple preparation methods to obtain M 2 SnO 4 @C composites by combining lower-cost carbon materials with M 2 SnO 4 is the most promising direction. Although these low-cost carbon materials do not have the exceptional properties of graphene or carbon nanotubes, they can effectively mitigate volume expansion, prevent shedding and aggregation and improve the cycling stability of stannate-based anode materials. Additionally, more attention should be paid to comprehensive performance rather than solely pursuing excellence in one aspect. M 2 SnO 4 -based materials are already widely used as anode materials in LIBs and SIBs due to their high theoretical capacity. Similarly, potassium-ion, magnesium-ion and calcium-ion batteries have similar reaction mechanisms, and M 2 SnO 4 -based materials could be potential candidates for their anode materials. In addition to these applications, M 2 SnO 4 -based materials may also have potential in emerging energy storage technologies. For example, lithium-ion hybrid capacitors (LICs) are a new type of energy storage device that combines a pre-lithiated anode from an LIB with an electric double-layer capacitor (EDLC)-type cathode. This device combines the high energy density of LIBs with the high power density and long cycle life of EDLCs and is considered to be a highly promising energy storage device. Stannate-based anode materials with high capacity are potential candidates for LIC anodes.
Despite the many challenges in the development of M 2 SnO 4 -based materials, we believe that they have great potential for use in the next generation of high-performance LIBs and SIBs. We hope that this review will be helpful in the development of M 2 SnO 4based anode materials.