Research progress on tin-based anode materials for sodium ion batteries

Ju-Mei Liang Li-Juan Zhang De-Ge XiLi Jing Kang

Beijing Key Laboratory for Green Catalysis and Separation,College of Environmental and Energy Engineering,Center of Excellence for Environmental Safety and Biological Effects,Beijing University of Technology

作者简介：*Li-Juan Zhang is an Assistant Professor at Beijing University of Technology,China.She received her Ph.D.degree in Material Science and Engineering from Zhejiang University, China,in 2001,and she worked at Beijing University of Technology,China,as a postdoctoral researcher from 2003 to 2005. She is the author or co-author of over 40 publications.Her current research focuses on material science and electrochemistry in electrochemical devices (rechargeable Li-and Na-ion batteries,lithium primary battery,and fuel cell).e-mail:zhanglj1997@bjut.edu.cn;

收稿日期：12 November 2019

基金：financially supported by Beijing Municipal High Level Innovative Team Building Program (Nos.IDHT20170502,IDHT20180504);the 17 Connotation Development-Curriculum and Teaching Material Construction Quality Teaching Resources Project,Beijing University of Technology (No. KC2017BS020);

Research progress on tin-based anode materials for sodium ion batteries

Ju-Mei Liang Li-Juan Zhang De-Ge XiLi Jing Kang

Beijing Key Laboratory for Green Catalysis and Separation,College of Environmental and Energy Engineering,Center of Excellence for Environmental Safety and Biological Effects,Beijing University of Technology

Abstract：

Sodium ion batteries(SIBs) is considered as a promising alternative to the widely used lithium ion batteries in view of the abundant resources and uniform distribution of sodium on the earth.However,due to the lack of suitable anode and cathode materials,especially the anode materials with excellent performance,its practical application is trapped.In recent years,lots of attentions are devoted to developing new electrode materials with high sodium storage capacity and long life.In a large number of anode material libraries,tin-based materials with alloying reaction mechanism show great potential for application in high-energy SIBs due to their high theoretical specific capacity.In this paper,detailed and comprehensive research progress on tin-based anodes(including tin metal,tin alloy as well as its compounds) in recent years is summarized.Specific efforts to improve the electrochemical properties of tin-based anode materials are discussed.Moreover,the challenges and prospects of these anode materials are also proposed in this review.

Keyword：

Tin-based material; Anode; Sodium ion battery; Electrochemical performance;

Received： 12 November 2019

1 Introduction

With the growth of energy demand,the consumption of global fossil energy is increasing year by year,causing severe ecological and environmental problems such as global warming,air pollution and desertification.Therefore,clean energy such as solar energy,wind energy and tidal energy has drawn close attention [ 1] .Before these intermittent clean energy could be used effectively,largescale energy storage equipment plays an important role in the peak regulation for the electrical network.Electrochemical energy storage technology is one of effective means because of its high efficiency and long service life [ 2] .

Lithium-ion batteries (LIBs) have been widely used in mobile phones,notebook computers,digital cameras,power tools,and gradually expanded into new energy vehicles and large scale energy storage.It is highly anticipated in the battery field,possessing outstanding advantages such as high operating voltage,high energy density,long cycle life,no memory effect,low self-discharge rate [ 3] .However,due to the low reserves and uneven distribution of lithium on the earth,it is necessary to develop a new energy storage technology with low cost and high performance to replace LIBs [ 4] .

One response is to replace lithium with sodium,that is,to develop sodium ion batteries (SIBs).Sodium shares similar physicochemical properties with lithium.Accordingly,vast experience in LIBs technology can be inherited by SIBs.Compared with lithium,sodium has abundant reserves and low cost,as shown in Table 1 [ 5, 6] .In addition,since there is no alloying between sodium and aluminum,cheaper aluminum foil could be used instead of copper foil as the anodic current collector of SIBs,further reducing the cost of the battery.Therefore,SIBs with sodium ion as the charge carrier has greater application potential in large-scale energy storage system [ 7, 8] .

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Table 1 A comparison of properties of sodium and lithium

So far,many types of negative electrode materials for SIBs have been reported,such as carbon materials (hard carbon [ 9, 10, 11, 12, 13, 14, 15, 16] ,soft carbon [ 17] ,graphene [ 18, 19, 20] ,N,S-doped carbon [ 21, 22, 23, 24, 25, 26, 27, 28] ,carbon nanotubes [ 29] ,carbon pellicles [ 30] ,carbon fibers [ 31, 32] ,3D-carbon [ 33, 34, 35, 36] ),organic compounds [ 37, 38, 39] ,transition metal oxides/sulfates (Fe-O/S [ 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50] ,Co-O/S [ 51, 52, 53, 54, 55, 56, 57, 58, 59] ,Ni/S [ 60, 61, 62, 63, 64, 65] ) [ 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65] .Carbon-based materials and organic compounds with the upsides of low price,simple preparation,low reaction platform and good cycle stability,unfortunately,have a low intrinsic capacity,which is not adequate for large-scale energy storage system.Transition metal oxides and sulfides have high theoretical specific capacity,but their poor conductivity restricts their development.

As a kind of alloy-type anode material,pure tin material has been widely concerned due to its high theoretical specific capacity of 847 mAh·g^-1.However,the volume expansion of 420%in the process of sodiation/desodiation makes the pure tin electrode smash rapidly,which leads to the loss of electrical contact between the active material and the collector,resulting in poor cycling stability and low initial Coulomb efficiency(ICE).In response to the above problems,numerous efforts have been devoted to designing novel materials,and three strategies could be summarized as follows:preparation of tin base alloy or compound materials,combining with carbon-based materials and building nanostructure.In this paper,the progress of the alloying reaction mechanism between Sn and Na was introduced at first,and then the recent development of tinbased materials was summarized.Finally,the personal perspectives of the further development of Sn-based materials as anode for SIBs were put forward.

2 Reaction mechanism of Tin in SIBs

It is believed that the reaction mechanism of Sn in SIBs is similar to that in LIBs,but the larger radius of sodium ion could lead to more complex reaction process.A lot of work has been done in this field.

Four voltage plateaus on the discharge curve were firstly observed by Ellis et al. [ 66] through electrochemical tests,and further proved to correspond to the alloying process of sodium and tin (Sn→NaSn₃→a-NaSn→Na₉Sn₄→Na₁₅Sn₄) by using in situ XRD,density functional theory(DFT) and Coulometric analysis.Huang et al.found that Na_xSn (x≈0.5) phase had been initially formed and then transformed into the final Na₁₅Sn₄ [ 67] .Transmission electron microscope (TEM) technology proves that 60%and 420%volume expansion,respectively,occur when Na_xSn (x≈0.5) and Na₁₅Sn₄ are formed.Chevrier and Ceder [ 68] calculated the sodium-embedded potential of Sn according to DFT.The results show that there are four voltage plateaus,and the crystal structures of corresponding phases are obtained from the inorganic crystal structure database (ICSD),namely NaSn₅,NaSn,Na₉Sn₄ and Na₁₅Sn₄.

Baggetto et al. [ 69, 70] used Sn films with different thicknesses to study the electrochemical behavior of Na-Sn systems,and found that the reaction mechanism would vary with cutoff voltage,electrode thickness,current density and the number of cycles.The existence of Na_0.6Sn phase,amorphous Na_1.2Sn phase and hexagonal Na₅Sn₂phase is proved by XRD,119Sn Mossbauer Spectroscopy,X-ray photoelectron spectroscopy (XPS) and DFT results.With the help of X-ray absorption spectroscopy,they found a new phase Na₇Sn₃,but no Na₉Sn₄ there because of the slow reaction kinetics of Na₉Sn₄ formation.Obrovac et al.studied the sodium storage mechanism of tin foil electrode with ex situ Mossbauer spectra and in situ XRD technology [ 71] .The results show that there are three phases Na₄Sn₄,Na₅Sn₂,and Na₁₅Sn₄ in the process of cyclic de/sodiation,as shown in Fig.1.

Stratford et al. [ 72] used a combination of theory and experiment to study the alloying mechanism of high-capacity tin anodes for SIBs.As shown in Fig.2,they found that the initial insertion of sodium ions into tin brings about the transformation of crystalline tin into a layered structure of NaSn₂.Then,NaSn₂ is decomposed into an amorphous phase with composition of approximately Na_1.2Sn,which is proved by the inverse Monte Carlo improvement of the ab initio molecular dynamics model.Further reaction with sodium would form Na_5-xSn₂ phase.The result of ²³Na solid-state nuclear magnetic resonance shows that the final product Na₁₅Sn₄ can accumulate more sodium in the form of non-stoichiometric compound (Na_15+xSn₄).

Fig.1 a Room-temperature Mossbauer effect spectra of ex situ Na-Sn phases obtained from coin cells:Na₄Sn₄,Na₅Sn₂ and Na₁₅Sn₄,b XRD patterns of metallurgical alloy phases Na₄Sn₄,Na₉Sn₄ and Na₁₅Sn₄ prepared by annealing.Reproduced with permission [71] Copyright (2017)Elsevier B.V

Fig.2 Sodiated phased of tin.Reproduced with permission [72] .Copyright (2017) American Chemical Society

In short,there is no definite conclusion about the reaction mechanism of tin and sodium in the alloying process.There are many mesophases in the reaction process,and their composition and structure need to be further clarified.Moreover,the reaction process is influenced by many factors,such as the material itself and the test conditions,which make different researchers draw different conclusions.In the future,more complicated experiments and more advanced testing methods are needed to study the reaction mechanism.

3 Tin-based composites with nanostructures

3.1 Sn/C or Sn-M/C nanocomposites (M representing metal element)

3.1.1 Sn-C

Sn is generally not used alone as the anode materials for SIBs because of its huge volume change during the cycle.Composite of Sn and carbon materials not just improve the conductivity,but accommodate the volume expansion,which are beneficial for the cycling stability and rate capability.

Palaniselvam et al. [ 73] prepared Sn-C composite(SnNGnP) by supporting Sn on nitrogen-doped graphite nanoflakes through high-energy ball milling and hightemperature annealing.SnNGnP delivers first discharge and charge capacities of around 517 and 394 mA·h·g^-1,respectively,with a resulting ICE of 76.2%,This composites also show excellent cycle performance and rate performance with capacities of 452,437,429,405,384,and 253 mA·h·g^-1 at 0.05,0.10,0.20,0.50,1.00,and2.00 A·g^-1,respectively.This nitrogen-doped graphite nanoflakes can increase the electron conductivity of the electrode material,and its structural defects provide additional Na⁺storage sites.The synthesis steps,reaction mechanism and electrochemical performance of SnNGnP are shown in Fig.3.Sn@C composites with a core-shell structure by colloidal spray pyrolysis had been prepared and the preparation process is depicted as Fig.4 [ 74] .The carbon shell inhibits the volume expansion of the Sn during the cycle and the composites show capacities of 489.1,455.5,408.6,308.6,235.1,192.9,157.5 and115.1 mAh·g^-1 at 0.05C,0.1C,0.2C,0.5C,1C,5C,and10C,respectively.There is a reversible specific capacity of355.0 mAh·g^-1 after 200 cycles at 0.2C,which are much better than that of pure Sn.

Fig.3 a-c Synthesis steps for SnNGnP and d process of (de)sodiation in SnNGnP composite;e galvanostatic charge-discharge curves of SnNGnP,recorded at a current density of 50 mA·g^-1;f rate capability of SnNGnP electrode measured from 50 to 2000 mA·g-1;g cycle life of SnNGnP electrode.Reproduced with permission [73] Copyright (2019) John Wiley and Sons

Fig.4 Schematic of colloidal spray pyrolysis process for fabricating core-shell Sn@C particles.Reproduced with permission [74] Copyright(2018) American Chemical Society

Heteroatom-doped carbon fibers with one-dimensional structure are often used to support tin nanoparticles to prepare composite materials.Luo et al. [ 75] prepared 3D layered conductive Sn quantum dots in N,S co-doped carbon nanofibers wrapped in rGO volutes through an electrospinning process.When the current density is1 A·g^-1,there is still a reversible specific capacity of373 mAh·g^-1 after 5000 cycles.Sn nanoparticles on nitrogen-doped carbon nanofibers (Sn@NCNFs) composites were synthesized by electrostatic spinning technique and used as the anode of SIBs [ 76] .It shows the reversible specific capacity of exceeding 600 mAh·g^-1 after 200cycles at 0.1 C.The excellent performance of the material is mainly attributed to the following reasons:firstly,the composite has one-dimensional nanostructure,which can serve as ions and electrons channel,shorten the ion diffusion length,and provide a large number of active sites for the charge transfer reaction.Secondly,the material has porous structures,which can accommodate the volume change caused by the solidation and desolidation process.

3.1.2 Sn-M alloy

Compositing active metallic Sn with inactive metals,such as Ni,Cu,and Co,to form bimetallic nanoalloys is another effective way to accommodate volume expansion.In this kind of materials,Sn is responsible for providing high capacities,while other inactive metals act as conductive matrix to deal with the volume change and particles crushing during cycles.

Li et al. [ 77] synthesized Ni-Sn alloy spherical nanoparticles with different sizes,ranging from (3.9±0.7)to (4.6±0.6) nm,in a vacuum environment.At a current density of 0.1 A·g^-1,the alloy material shows capacity of160 mAh·g^-1 after 120 cycles.Interestingly,by comparing the performance of this alloy material as anode material in LIBs and SIBs,respectively,it is found that smaller size can bring higher capacitive contribution in sodium ion battery.Xie et al. [ 78] used a sputtering method to prepareβ-SnSb films as a model system.Electrochemical tests show that theβ-SnSb film has excellent cycle performance with first specific capacity of 700 mAh·g^-1 and 70%capacity retention after 150 cycles.The mechanism of cyclic de/intercalation of sodium ions are schematic illustrated in Fig.5,which are supported by in situ TEM results.Ma et al. [ 79] synthesized a bimetallic single-phase nanoporous (NP) SnSb alloy with a double continuous ligand channel structure by rapid solidification dealloying.The schematic illustration of the preparation process is shown in Fig.6.As the anode of SIBs,NP-SnSb alloy shows excellent sodium storage properties with a reversible specific capacity of 457.9 mAh·g^-1 at a current density of1 A·g^-1 after 150 cycles.It is believed that the unique nanopore structure,alloy effect and synergistic reaction are conducive to the excellent electrochemical performance of NP-SnSb.Most importantly,the sodium storage mechanism of the SnSb alloy was revealed by operating XRD and DFT calculations as follows:SnSb→Na(Sn,Sb)←→Na₉(Sn,Sb)4←→Na₁₅(Sn,Sb)4.Wang et al. [ 80] synthesized 3D porous Sn on Cu foil via electrochemical method.The 3D porous structure is beneficial to electrolyte penetration and Na⁺diffusion.In the half-cell test,there is a reversible specific capacity of about 700 mAh·g^-1 after400 cycles at a current density of 2.5 A·g^-1.An energy density of 311.7 Wh·kg^-1 and a long service life can be achieved in a full battery test.

Above mentioned synthesis strategies will help to further study the structural design of functional materials.At the same time,the research on the mechanism of alloy materials will provide theoretical support for its future application.

Fig.5 Schematic illustration of proposed phase transition ofβ-SnSb during sodiation-desodiation cycle.Reproduced with permission [78] Copyright (2018) American Chemical Society

Fig.6 Schematic illustration of one-step dealloying process for NP-SnSb alloy.Reproduced with permission [79] Copyright (2018)Elsevier Ltd

3.1.3 Sn-M-C

The combination of alloying and carbon composite is considered to further improve the sodium storage properties of tin based materials.

Huang et al. [ 81] synthesized a new kind of yolk shell composites with SnCo alloy as core and amorphous carbon as shell.Beyond 120 cycles at a current density of100 mA·g-1,the SnCo@C shows a reversible specific capacity of 317.4 mAh·g^-1,which is better than that of Sn@C.Youn et al. [ 82] synthesized uniformly dispersed SnSb nanoparticles with size of 3.5 nm on a nitrogendoped carbon composite (NC) by one-pot method.SnSb/NC material presents a reversible specific capacity of244 mAh·g^-1 after 200 cycles at a current density of0.1 A·g-1.The gradual solidation into the Sn and Sb species in the SnSb/NC electrode is proposed in this paper.Sn and Na₃Sb species generated at different potentials can act as inert matrix to each other,resulting in a reduction in volume expansion during cycling.

3.2 Sn-P composites

The alloy formed by tin and phosphorus is usually Sn₄P₃.Because of its high reversible capacity and low redox potential,Sn₄P₃ is considered to be one of the most promising anode materials for SIBs.However,the large volume expansion and tin aggregation during cyclic de/intercalation of sodium ions lead to poor cycle stability.Compounding with carbon-based materials or/and other elements is a main way to improve its performance.

3.2.1 Sn-P-C

Pan et al. [ 83] prepared Sn₄P₃/graphene aerogel (Sn₄P₃-GA) composites by low temperature rapid phosphating.The Sn₄P₃ nanoparticles with an average size of 8 nm are uniformly and tightly embedded on the GA surface,forming a unique 3D network structure.The unique 3D network structure as well as the synergistic effect between graphene nanosheets and Sn₄P₃ enables the Sn₄P₃-GA to have good electrochemical performance.At a current density of 0.1 A·g^-1,the specific capacity is 657 mAh.gafter 650 cycles.With good cycle performance and rate performance,it can be used as a negative electrode material for sodium ion batteries with great potential.

Wei et al.used PPy hollow fiber as a template to synthesize a composite material C@Sn₄P₃@HCF,and the SEM images of the as-repared samples at different steps are shown in Fig.7 [ 84] .The C@Sn₄P₃@HCF composite has a capacity of 625 mAh·g^-1 at a current density of100 mA·g^-1 and of 120 mAh·g^-1 at a current density of300 mA·g^-1.Its unique structure has the following advantages:(1) one-dimensional hollow structure shortens the diffusion path of electrons and electrolyte ions,thus improving the rate performance;(2) the synergistic effect of Sn₄P₃ quantum dots and N-doped hybrid hollow carbon fibers can increase the specific capacity;(3) the sandwich structure can provide sufficient void volume to store the volume change of the Sn₄P₃ quantum dots during cycling,which is beneficial to long-term cycle stability.

Choi et al. [ 85] prepared a core-shell structured Sn₄P₃-C (carbon) composite nanosphere by carbonization/reduction and phosphorylation of SnO₂-GCP (glucose-derived carbon-rich polysaccharide) nanospheres.The sodium storage properties of Sn₄P₃-C composite nanospheres with different nanometer sizes (about 30-300 nm) were studied.Among them,Sn₄P₃-C nanospheres with the size of140 nm shows a high specific capacity of 440 mAh·g^-1after 500 cycles at a high current density of 2 A·g^-1.

3.2.2 Sn-M-P-C

Lan et al. [ 86] synthesized the ternary compound phosphated copper-tin (Cu₄SnP₁₀) nanowires and Cu₄SnP₁₀/MWCNTs (multi-walled carbon nanotubes) composites by reflow method and ball milling,respectively.The Cu₄SnP₁₀/MWCNTs composite shows a stable specific capacity of 512 mAh.g^-1 at a current density of100 mA·g-1 after 100 cycles.When the current density reaches 1 A·g-1,the capacity of the Cu₄SnP₁₀/MWCNTs composite remains at 412 mAh·g^-1,showing good rate performance,which are shown in Fig.8.The reasons for such excellent performance are as follows:(1) the introduction of Cu helps to stabilize P (compared to Sn₄P₃) in the compound,thereby having a higher specific capacity;(2) the agglomeration of Sn in Sn₄P₃ is the main factor leading to the decrease of cycle performance,and the incorporation of copper can effectively alleviate the accumulation of Sn during the charge-discharge cycle,thereby improving the cycle performance;(3) coating the surface of Cu₄SnP₁₀ nanoparticles with MWCNTs further alleviates the volume expansion during the cycle.Good rate performance and cycle stability make Cu₄SnP₁₀/MWCNTs composites a good candidate for sodium ion battery anodes.

The ternary Sn₅SbP₃/C composites were synthesized via ball milling method by Zhang et al. [ 87] .The material consists of Sn₄P₃,SnSb and Sn nanoparticles (NPs).These nanoparticles are in close contact with each other and form a heterojunction in the conductive carbon matrix.The electrochemical performance of the ternary composite is better than that of binary Sn₄P₃/C or SnSb/C material.It was cycled for 200 cycles at a current density of500 mA·g^-1 and a specific capacity was maintained at431.6 mAh·g^-1.At a high rate of 1 A·g^-1,there was a reversible specific capacity of 370.5 mAh·g^-1 after 200cycles.The rate performance is also significantly higher than that of the binary Sn₄P₃/C and SnSb/C composites prepared under the same conditions.The excellent electrochemical performance of the ternary Sn₅SbP₃/C composite derived from the synergistic effect of multiphase nanostructures on the conductive network.

Fig.7 SEM images of as-prepared samples at different steps:a PPy,b SnO₂@PPy,c C@SnO₂@PPy,d C@Sn@HCF,and e C@Sn₄P₃@HCF,f TEM image of PPy,g TEM image and h,i HRTEM images of C@Sn₄P₃@HCF.Reproduced with permission [84] Copyright (2019) John Wiley and Sons

Fig.8 a Cycle performance,b rate performance and c long cycling performance of Cu₄SnP₁₀/MWCNTs.Reproduced with permission [86] Copyright (2017) Elsevier Ltd

3.3 Sn-O/S/Se (VI group) composites

The VI group elements of O,S and Se in the periodic table can form SnO,SnO₂,SnS,SnS₂,and SnSe and other compounds with Sn.These compounds show decent Nastorage properties through the conversion reaction in the first step and the alloying reaction in the second step.The chemical equation for the reaction with sodium ions is as follows:

M stands for to O,S and Se.During discharge,SnMy first undergoes a conversion reaction with Na⁺to generate the corresponding metal elemental Sn.The metal elemental Sn has a higher reactivity,and then further alloys with Na⁺.Since there are many alloy phases in this process,there is currently no effective way to determine the composition of mesophase,but the final alloy phase is currently determined to be Na_3.75Sn.It is considered that the first step has poor kinetic performance,which seriously affects the efficiency of the first lap.

3.3.1 Sn-O comnposites

Zhao et al. [ 88] synthesized a SnO₂@CEM (carbonized eggshell membrane) composite with 3D structure by hightemperature carbonization and hydrolysis processes.The SnO₂@CEM provides a high reversible capacity of656 mAh·g^-1(at 0.1 A·g^-1) in the fifth cycle,close to 98%of its theoretical specific capacity.The high reversible capacity of 420 mAh·g^-1 was maintained after 200 cycles at 0.2 A·g^-1.This unique structural design offers the following advantages:(1) the independent CEM can provide a3D continuous conductive network and can be used directly as a current collector to replace the thick copper foil used in conventional current design;(2) CEMs have a remote,3D and continuous interpenetrating pore network that facilitates the access and diffusion of electrolytes and also,acts as a buffer matrix to mitigate mechanical stress during cycling;(3) direct anchoring of SnO₂ to CEMs allows electrons to be easily transferred from SnO₂ to CEMs.This enables the composites to be directly used as electrodes without bonding agents and conductive additives;(4) the separation and interleaving of SnO₂ nanosheets create more voids,prevent overlap of the nanosheets,and maximize surface area exposure to electrolyte,which provides a shorter solid phase ion diffusion length.In addition,CEMs are largely available for everyday use,making SnO₂@CEM a low cost for SIBs.

Qin et al. [ 89] obtained C@SnO₂@CHNSs composite material by freezing method.Ultrafine SnO₂ nanocrystals(2-5 nm) are tightly confined between the carbon shell and the hollow carbon core.This unique structure not only inhibits the migration and pulverization of tin dioxide,but also provides abundant void space to buffer volume changes.At the same time,it also supplements the electronic conductivity of the active material,thus realizing high conductivity and structural integrity of the entire electrode.The electrochemical performance results show that the composite material C@SnO₂@CHNSs exhibits extremely outstanding cycle performance.At a current density of 4.6 A·g^-1,a specific capacity of 200 mAh·g^-1could be maintained within 3000 cycles.

3.3.2 Sn-S comnposites

Wang et al. [ 90] obtained a flower-like SnS nanostructure for SIBs by a simple solvothermal method.The experimental evidence indicates that after 50 cycles,both the stepwise aggregation of Sn and the enrichment of Na₂S crystal lead to the capacity decline.Replacing the commonly used acetylene black conductive additive with multi-walled carbon nanotubes (MWCNT) greatly improves the cycle stability of the SnS electrode.A complete battery is assembled from a SnS/MWCNT anode and a P_2-Na_2/3Ni_1/3Mn_1/2Ti_1/6O₂ cathode.The initial energy density of the entire battery is 262 Wh·kg^-1,and after 40cycles,the entire battery can retain 71%of its initial capacity.

Chao et al. [ 91] used a rapid carbon-plasma method to grow robust hierarchical graphene (hG) uniformly on tin sulfide nanobelt networks.After circulating 1600 cycles at the current density of 3 A·g^-1,the hG@SnS composite material shows a specific capacity of more than600 mAh·g^-1,and with a capacity retention rate of 93%.A reasonable explanation for such excellent performance is shown in Fig.9.It is believed that the 3D crosslinked structure of the uniform graphene coating provides a permanent continuous electronically conductive network that promotes electrode reaction kinetics.Besides,hG wrapped mesoporous SnS and internal pores effectively accommodate the volume expansion,ease strain,avoid accumulation of SnS during the de/intercalation process,and make the membrane electrode strong and stable.Kim et al. [ 92] prepared a Sn-S hybrid nanocomposite uniformly distributed on reduced graphene oxide by a simple hydrothermal method and a carbothermal reduction method.The unique reduction process of the material minimizes the grain size to below 2 nm,overcomes the low electronic conductivity of sulfide,and alleviates the volume expansion (≈420%) of Sn in the alloying reaction process.

3.3.3 Sn-Se comnposites

Zhang et al.synthesized SnSe/NSCs (Nanosheet Clusters)by citric acid polymerization with a reaction time of less than 300 s [ 93] .This material exhibits excellent electrochemical performance and rate performance as a negative electrode material for SIBs.At a current density of25 mA·g^-1,the reversible capacity is 738 mAh·g^-1,which is more than 94%of its theoretical capacity(780 mAh·g^-1).Even at very high discharge/charge current densities of 10/20/30/40 A·g^-1,the SnSe/NSCs electrode can still provide discharge specific capacity of200/106/32/16 mAh·g^-1,respectively.Specifically,the synthesis method is low in cost and simple in process,which is beneficial to industrial production.

Cheng et al. [ 94] synthesized SnSe/FLG (few-layered graphene) composites by plasma milling.In this material,the form of Sn-C bond and Se-C bond makes SnSe nanoparticles have strong affinity with FLG matrix,thus inhibiting the aggregation and detachment of SnSe even if it is cycled for a long time.The mechanisms are illustrated in Fig.10.As a negative electrode material for sodium ion batteries,SnSe/FLG provides high reversible capacities of488,460,403,396,342 and 285 mAh·g^-1,respectively,at0.1,0.2,0.5,1.0 and 2.0 A·g-1,showing excellent cycle performance and rate performance.

Fig.9 a High-rate long-term cycling stability of SnS,rGO/SnS and hG@SnS at 3 A·g^-1,b schematic illustration of three types of electrode configurations and their structural evolutions.Reproduced with permission [91] Copyright (2018) John Wiley and Sons

Fig.10 Schematic illustration of a fabrication of SnSe/FLG and b discharge/charge process of SnSe/FLG with and without Sn-C and Se-C Co-bonding.Reproduced with permission [94] Copyright(2019) American Chemical Society

Ren et al. [ 95] synthesized a SnSe/NC (nitrogen-doped carbon) composite with a nanobelts structure (Fig.11).It has a reversible specific capacity of 723 mAh.g^-1 after 50cycles at a current density of 25 mA·g^-1,and a reversible specific capacity of 258 mAh·g^-1 after 200 cycles at a high rate of 2 A·g-1 (Fig.12).They believe that the excellent electrochemical performance comes from the following reasons:firstly,the distance between the plates is relatively wide,which is favorable for repeated solidation/desolidation process.Secondly,the conductive NC layer effectively limits the aggregation of SnSe during the cycle and improves the conductivity of the composite.Thirdly,the strong interfacial interaction between SnSe and NC through the Sn-C bond significantly enhances the charge transfer and structural stability of SnSe/NC electrode.The novel SnSe/NC nanobelt riveting structure exhibits enhanced pseudocapacitive sodium storage through a fast,efficient Faradaic redox reaction on the SnSe (sub) surface.DFT calculations show that the presence of N heteroatoms in the NC matrix has an important effect on the formation of Sn-C bonds and stability of SnSe/NC structure.

Zhao et al. [ 96] synthesized SnSe quantum dots (E-SnSe) coated with nitrogen-doped carbon nanofibers through electrostatic spinning,high-temperature selenization,and high-energy ball milling.The E-SnSe electrode has a specific discharge capacity of 268 mAh·g^-1 after 750cycles at 2 A·g^-1 and performs significant cycle durability at a high rate of 5 A·g^-1.The excellent cycle stability could be attributed to the 3D conductive carbon network structure.Assembled Na₃V₂(PO₄)3@C//E-SnSe full battery shows an output voltage of higher than 2.0 V,an ultra-long cycle life of 1500 times and a high reversible discharge capacity of 100 mAh·g^-1 at 1 A·g^-1.

3.3.4 Sn-O/S/Se-M-C

Lu et al. [ 97] used a microwave-assisted hydrothermal method to prepare Co-doped SnO₂/C clusters.At a current density of 50 mA·g^-1,a high specific capacity of576 mAh·g^-1 can be obtained after 250 cycles.Even at a high current density of 1600 mA·g^-1,it gives a specific capacity of 242 mAh·g^-1.Such excellent electrochemical performance comes from the cobalt dopant which can suppress the coarsening of SnO₂,increase the total conductivity and improve the reversibility of the conversion reaction from Sn to SnO₂.The material has high firstround efficiency and high rate performance,and is expected to become a potential sodium ion anode material.

Fig.11 a Schematic illustration of preparation process of SnSe/NC hybrid nanobelts;SEM images of b ZnSe(DETA)0.5,c ZnSe/NC and d SnSe/NC nanobelts.Reproduced with permission [95] Copyright (2018) Elsevier Ltd

Fig.12 a Cycling performance and coulombic efficiency,b rate capability of SnSe/NC and pure SnSe electrodes.Reproduced with permission [95] Copyright (2018) Elsevier Ltd

Tang et al. [ 98] used a simple hydrothermal method to prepare SnSe_0.5S_0.5/C nanocomposites,which exhibits high specific capacity and long-term cycle stability in both LIBs and SIBs.For LIBs,the specific capacity is 625 mAh·g^-1at 500 mA·g^-1 after 1500 cycles.For SIBs,the specific capacity is 430 mAh·g^-1 at 200 mA·g^-1 after 100 cycles.These results indicate that the new tin sulfide selenium(SnSe_0.5S_0.5) material is likely to be an excellent anode material for lithium ion and sodium ion storage.Fu et al. [ 99] prepared a Cu₂SnS₃/reduced graphene oxide (CTS/RGO) composite by hydrothermal method.The composite consists of ultrafine CTS nanoparticles uniformly fixed on the surface of RGO flakes.Thanks to its unique structural characteristics,CTS/RGO exhibits good cycle stability and rate performance at a high current density of 3200 mA·g-1.Anchoring ultrafine CTS nanoparticles on the surface of RGO sheets with high conductivity and flexibility can significantly enhance the electrochemical reaction kinetics and effectively alleviate the volume effect of active materials.The improvement of electrochemical performance is attributed to the unique design of ternary materials and the rapid kinetics of nanostructures.

Chen et al. [ 100] synthesized nanostructured Cu-doped SnSe as a negative electrode material for SIBs by surfactant-free solvothermal method.The SnSe composite with10%Cu-doped has the smallest particle size among all samples and shows the best electrochemical performance with a reversible specific capacity of 375 m·h·g^-1 after270 cycles at 1 A·g-1.Even at a high current density of5A·g-1,it still shows a specific capacity of 304 mAh·g^-1after 1000 cycles.Such excellent electrochemical performance comes from the following three aspects:(1) Cu doping can significantly reduce the contact resistance and charge transfer resistance,thereby improving its rate performance;(2) the doping of copper reduces the average size of SnSe particles,which can reduce the capacity attenuation caused by volume changes;(3) the sample doped with 10 wt%copper exhibits a unique layered structure,which helps to accommodate volume changes during charge/discharge to obtain better structural stability,and also helps to reduce the diffusion length of sodium ion and accelerate electrochemical reaction kinetics.

4 Outlook

Sn-based anode materials have attracted much attention due to their high specific capacity and low operating voltage.Although great progress has been made in improving high-capacity and long-life anodes,there are still major challenges on alleviating volume change,inhabiting aggregation and activating the dynamics of de/intercalation of sodium ion.Composite with conductive materials,preparation of electrochemically active/inactive composite materials,and synthesis of nano materials with special morphology are still the main means to achieve high capacity,stable cycle,and excellent rate performance of tin-based materials.The detailed reaction mechanism of Sn-based anode is not clear at present.More detailed experiments and advanced test methods need to be designed to deeply understand the process of sodium insertion/removal,which will be beneficial to further improve the electrochemical performance of tin-based anode materials.With the development of electrode materials,we believe that the SIBs with rich resources and low price will play an important role in the field of energy storage.

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