稀有金属(英文版) 2018,37(03),167-180

High-performance anode materials for Na-ion batteries

De-Liang Cheng Li-Chun Yang Min Zhu

Key Laboratory of Advanced Energy Storage Materials of Guangdong Province,School of Materials Science and Engineering,South China University of Technology

收稿日期：22 November 2017

基金：financially supported by the Fund for Innovative Research Groups of the National Natural Science Foundation of China (No.NSFC51621001);the National Natural Science Foundation of China (No.51671089);Guangdong Natural Science Funds for Distinguished Young Scholar (No.2017B030306004);the Fundamental Research Funds for the Central Universities (No.2017ZD011);

High-performance anode materials for Na-ion batteries

De-Liang Cheng Li-Chun Yang Min Zhu

Key Laboratory of Advanced Energy Storage Materials of Guangdong Province,School of Materials Science and Engineering,South China University of Technology

Abstract：

Na-ion batteries are considered a promising alternative to Li-ion batteries for large-scale energy storage systems due to their low cost and the natural abundance of Na resource. Great effort is making worldwide to develop high-performance electrode materials for Na-ion batteries,which is critical for Na-ion batteries. This review provides a comprehensive overview of anode materials for Na-ion batteries based on Na-storage mechanism: insertion-based materials, alloy-based materials, conversion-based materials and organic composites. And we summarize the Nastorage mechanism of those anode materials and discuss their failure mechanism. Furthermore, the problems and challenges associated with those anodes are pointed out,and feasible strategies are proposed for designing highperformance anode materials. According to the current state of research, the search for suitable anode materials for Na-ion batteries is still challenging although substantial progress has been achieved. Nevertheless, we believe that high-performance Na-ion batteries would be promising for practical applications in large-scale energy storage systems in the near future.

Keyword：

Na-ion batteries; Anode materials; Na-storage mechanism;

Author： Min Zhu e-mail:memzhu@scut.edu.cn works in School of Materials Science and Engineering, South China University of Technology. He received his Ph.D. at Dalian University of Technology in 1988. He had served as "Croucher Visiting Scholar", "Humboldt Research Fellow", "Invited researcher" in City University of Hong Kong, Technical University Berlin, University of Tokyo, National Institute for Materials (Japan), University of Sydney. He was awarded Distinguished Young Scientist Fund from NSFC,Changjiang Professor by MOE. He also served as chief scientist of "973 project" and Innovative Research Groups of the NSFC. His research includes hydrogen storage materials, lithium storage materials, mechanical alloying and nanomaterial and shape memory alloys. He has published more than 250 papers in international peer-reviewed journals, own more than 20 patents including 3 PCT patents. He has edited《Advanced Materials for hydrogen storage》,《Advanced Materials for clean energy》,《Functional Materials》,《New Materials Science and Technology》,《Engineering Materials》, He won University Innovation Prize of China in Automobile Field from GM China in 2010, Natural Science Award (first grade) from Guangdong Provincial Government in 2012 and Technology Invention Award (first grade) from Ministry of Education in 2016. He is the editorial board member of《Acta Metallurgica Sinica》,《Rare Metals》,《J. Energy Chemistry》,《Energy Storage Science and Technology》, And he is also the steering committee member of the International symposium of Metal-Hydrogen, International symposium Hydrogen and Energy.;

Received： 22 November 2017

1 Introduction

With the increasing environment and resources problems caused by using fossil fuel,a variety of renewable energy sources,such as the wind and sun,are growing rapidly [ 1] .Thus,a large-scale energy storage systems becomes extremely important to modulate intermittent renewable resources and integrate them into the smart grid safely and smoothly [ 2, 3, 4, 5, 6, 7, 8] .

As an energy storage device,Li-ion batteries (LIBs)have been successfully utilized as a power source in portable electronic devices and electric vehicles due to their excellent electrochemical performance.However,large-scale application of LIBs would force us to consider the growing price,low abundance and unevenly distribution of Li resource [ 9] .Na and Li are in the same main group,showing similar chemical properties.Thus,being considered as a very promising alternative to LIBs,Na-ion batteries (NIBs) have attracted great attention due to low cost and natural abundance of Na resource recently [ 10, 11] .

It should be noted,however,that energy density of NIBs will always be lower than that of LIBs.First,the atomic weight of Na (23 g·mol^-1) is larger than that of Li(6.9 g·mol^-1).Second,the redox potential of Na/Na⁺(2.71 V vs.SHE) is higher than that of Li analog.Moreover,the radius of Na⁺(0.102 nm) is about 40%larger than that of Li⁺(0.076 nm),and this intrinsic characteristic is a barrier to reversible sodiation/desodiation of electrode materials.However,energy density is not a critical issue in the field of large-scale energy storage systems.Thus,developing NIBs with a similar working principle as LIBs for large-scale energy systems is still a reasonable alternative and of great importance.

Important performance characteristics of batteries,such as capacity,cycling stability and operation voltage,are mainly determined by the electrochemical performance of the electrode materials.Therefore,the major challenge in advancing NIBs technology lies in finding high-performance electrode materials including anode and cathode materials.Many materials have been developed for the cathode of NIB s,e.g.,layered transition metal oxides Na_xMO₂ (M=Co,Mn,Ni,etc.) [ 12, 13, 14] ,phosphates(NaFePO₄,Na₃V₂(PO₄)3,etc.) [ 15, 16, 17] ,fluorides [ 18, 19] .And,a lot of efforts are still made to develop better cathodes for NIBs and the readers are referred to Refs. [ 9, 20] .

With respect to anode materials for NIB s,it is a great challenge to seek suitable anode materials with high performance,such as appropriate voltage window,high reversible capacity and stable cycling performance.Unlike Li,Na metal cannot be directly used as an anode considering its safety hazard and unstable passivation layer in most organic electrolytes at room temperature [ 3] .Besides,graphite,the conventional anode for LIB s,is electrochemically less active in NIBs due to the mismatching of the interlayer distance to the larger Na⁺radius [ 21] .Therefore,in recent years,non-graphitic carbons [ 22, 23] ,alloys [ 24, 25, 26] ,and transition metal compounds [ 27, 28, 29, 30, 31] are developed and emerging as promising anode materials for NIBs.Figure 1 plots capacity and voltage for present anode materials in NIBs [ 32] .Based on reaction mechanism,anode materials for NIBs can been classified into three types:(1) insertion-based materials,(2) alloy-based materials,(3) conversion-based materials.And currently organic composites have attracted growing attention as anodes for NIBs due to their excellent electrochemical performance.In this review,we focus on recent research progress in NIBs anodes.In particular,this review discusses the Na-storage and failure mechanisms of the selected anodes and points out the problems and challenges faced by those anodes.Moreover,feasible strategies are suggested for designing high-performance anode materials.

2 Insertion-based material

Insertion-based materials mainly include carbon-based materials and titanium-based compounds.The structure of insertion-based materials can insert/extract a certain amount of Na⁺during charge/discharge process,keeping the structure unchanged.

Carbon-based materials are the most studied anodes for NIBs due to their natural abundance and renewability,in addition to resembling to LIBs.However,graphite,as the most popular anode for LIBs,is electrochemically less active in NIBs due to the mismatching of the interlayer distance to the larger Na⁺radius [ 21] .Jache and Adelhelm [ 33] demonstrated that this limitation can be circumvented by using co-intercalation phenomena in a diglyme-based electrolyte.The resulting compound is a Stage-I ternary intercalation compound,and the graphite showed a superior cycle life with capacity close to 100 mAh.g-1 for 1000cycles and coulombic efficiency>99.87%.Encouragingly,Wang's group successfully prepared expanded graphite with an enlarged interlayer lattice distance of0.43 nm through a process of oxidation and partial reduction on graphite,as shown in Fig.2a.As an anode for NIBs,it showed a high capacity of 284 mAh·g^-1 at 20·g^-1and maintained 136 mAh·g^-1 at 100 mA·g^-1 after 1000cycles [ 34] .

Fig.1 Relationship between capacity and voltage for present NIBs anode materials [32]

Fig.2 a Diagram of Na-storage in graphite and expanded graphite [34] ,b diagram of Na-storage in hard carbon [21] ,c charge/discharge curves versus Na of hard carbon (E-voltage,Q-capacity),a ethylene carbonate,b propylene carbonate,c butylenes carbonate [37] ,d scanning electron microscopy (SEM) image of hollow carbon nanowires [38] ,e SEM image of carbon nanosheets [22] ,f transmission electron microscopy (TEM)image of hollow carbon nanospheres [23] ,g-i SEM image,high-resolution transmission electron microscopy (HRTEM) image and cycling performance of graphene [39]

Recently,hard carbon has been widely used as anodes for NIBs due to its highly disordered structure and large interlayer distance.As shown in Fig.2b,the structure of hard carbon is composed of carbon layers and micropores.Stevens and Dahn [ 35] first reported hard carbon as anode for NIBs,which achieved a high reversible capacity of300 mAh·g^-1.Alcantara et al. [ 36] reported carbon black as anode for NIB s,which achieved a high reversible capacity of 200 mAh·g^-1.The presence of disordered layers and the low density of the carbon black favor the reversibility.Figure 2c shows the typical charge/discharge curves of hard carbon,which display two distinct features:a sloping region and a long plateau region which can be assigned to the insertion of Na⁺between parallel layers and into micropores of hard carbon,respectively [ 37] .

Although hard carbon exhibits a relative high capacity of 200-300 mAh·g^-1,the high-rate capability is insufficient due to the low graphitization of hard carbon.Through structure optimizing,such as constructing nanostructure,the Na-storage performance of carbon-based materials could be improved.For example,hollow carbon nanowires derived from the hollow polyaniline (Fig.2d) [ 38] ,carbon nanosheets derived from the biomass precursor (Fig.2e) [ 22] ,hollow carbon nanospheres derived from the glucose(Fig.2f) [ 23] ,were demonstrated effective ways to improve the Na-storage performance of carbon-based materials.Recently,graphene,a unique one-atom-thick layered 2D carbon-based material,with superior flexibility and excellent conductivity,has also been studied as anode for NIBs.Wang et al. [ 39] prepared reduced graphene oxide by simple modified Hummer’s method,delivering a reversible capacity of 217.2 mAh·g^-1 at 40 mA·g^-1 and a capacity retention of 141 mAh·g^-1 over 1000 cycles.

Currently,various titanium-based compounds,including TiO₂,Na₂Ti₃O₇,Na₂Ti₆O₁₃,have been explored as a promising insertion-based host due to their low cost,low operation voltage and excellent cyclability.Xu et al. [ 40] first reported that the anatase TiO₂ nanocrystalline was able to insert Na⁺and delivered a high reversible capacity of150 mAh·g^-1 for 100 cycles (Fig.3a).Yang et al. [ 41] fabricated N-doped TiO₂ nanorods decorated with carbon dots (N-TiO₂/C-dots) by heteroatom doping to enhance the Na-storage performance of the TiO₂ (Fig.3b).Recently,layered sodium titanium oxide,Na₂Ti₃O₇,was examined as anode for NIBs due to its large theoretical specific capacity(310 mAh·g^-1) and low working voltage (0.3 V vs.Na⁺/Na).Li’s group [ 42] reported that surface engineered Na₂Ti₃O₇ nanotube arrays can serve as efficient anode for NIBs.They delivered high reversible capacity of221 mAh·g^-1 and exhibited superior cycling and rate performance,retaining 78 mAh·g^-1 at 10 C over 10,000cycles (Fig.3c).Another layered sodium titanium oxide,Na₂Ti₆O₁₃,was also studied as a promising Na insertion host.Rudola et al. [ 43] prepared Na₂Ti₆O₁₃ nanorods by a soft-template method.When used as the anode for NIBs,they showed a reversible capacity of more than40 mAh·g^-1 (corresponding to 0.85 Na⁺insertion per formula unit) and a superior cycling stability with 85%capacity retention for 5000 cycles at 20 C,but the capacity is quite low (Fig.3d).

As noted above,the main challenge for insertion-based materials is the limited insertion Na⁺,which will result in relatively low capacity.And much effort has been devoted to increasing the active sites for Na⁺,such as constructing nanostructure or hollow structure,which can be synthesized by pyrolysis,hydrothermal,template method,etc.

3 Alloy-based materials

Alloy-based materials have been proved to be promising anode materials due to their high theoretical capacity and low working voltage.As shown in Fig.4a,b,based on theoretical calculations and experimental results,the Nastorage voltage plateaus of alloy-based materials are relatively low (<1 V).And they can alloy with Na to form Narich alloy phases,yielding a much higher capacity than carbon-based materials,e.g.,Sb (Na₃Sb,660 mAh·g^-1),Sn(Na₁₅Sn₄,847 mAh·g^-1),P (Na₃P,2596 mAh·g^-1),Ge(NaGe,369 mAh·g^-1).However,Na alloys with alloybased materials accompanied by large volume change of200%-400%,as shown in Fig.4c,d,resulting in the pulverization and poor cyclability of electrode materials during the charge/discharge process [ 44, 45] .In recent years,much research effort has been devoted to reducing the negative effect of large volume change.First,nanostructures or hollow structures are prepared,which can better accommodate the large volume change and facilitate the Na⁺transport.Second,carbon materials are introduced to composite with the alloy-based anodes.And the carbon matrix can act as a buffer to suppress the volume change and particle aggregation of the alloy during the sodiation/desodiation process.In addition,the carbon phase can also enhance the electrical conductivity of the electrodes.With the same consideration as that for compositing with carbon,M-(Sn,Sb,P,Ge) intermetallics were fabricated,where M is an electrochemically inactive/active component.The M component acts as an effective matrix to alleviate volume variation and aggregation of the alloy during the charge/discharge process.

Ceder’s group reported that Na insertion into the crystalline Sn occurred in four steps,i.e.,forming NaSn₅,NaSn,Na₃Sn and Na₁₅Sn₄ (Fig.5a) [ 45] .Obrovac et al.demonstrated that several Na-Sn phases were formed during sodiation of Sn and the alloy formed at full sodiation was crystalline Na₁₅Sn₄ (Fig.5b) [ 46] .In addition,in situ TEM analysis showed that different phase transitions occurred in the insertion of Na⁺into Sn nanoparticles [ 47] .Sn nanoparticles were first sodiated to form amorphous NaSn₂ via a two-phase reaction (Fig.6a),then was sodiated to several Na-rich amorphous phases and finally to crystalline Na₁₅Sn₄ via one-phase reaction (Fig.6b,c).The in situ TEM analysis also showed that Sn nanoparticles are expanded by about 420%after full sodiation,and this volume change is detrimental to the cycling performance of Sn (Fig.6d).

Fig.3 a Charge/discharge profiles of anatase TiO₂ nanocrystals at 50 mA·g^-1 [40] ,b cycling performance of N-TiO₂/C-dots at 2 C [41] ,c cycling performance of Na₂Ti₃O₇ nanotube arrays at 10 C [42] ,and d cycling performance of Na₂Ti₆O₁₃ nanorods at 20 C [43]

Kwon et al.synthesized Sn nanofibers (Fig.7a) as anode for NIBs using a simple electrochemical deposition process.And they exhibited excellent cycling performance,with a high retention of 95.09%of the initial charge capacity after 100 cycles.The excellent cycling performance is mainly attributed to the high mechanical stability of the nanofibers [ 48] .Wang et al.compared the electrochemical performance of the hollow Sn with that of solid Sn,and the hollow Sn nanoparticles presented improved cycling stability (Fig.7b),which is due to the excellent structural stability of the hollow structure [ 49] .Liu et al. [ 25] prepared the Sn@C composite (Fig.7c) using an aerosol spray pyrolysis method,and it exhibited a stable capacity of approximate to 415 mAh·g^-1 after 500cycles at 1000 mA·g^-1 (Fig.7d).The superior electrochemical performance is due to the synergetic effects between the well-dispersed ultra-small Sn nanoparticles and the conductive carbon network.Yu and co-workers synthesized highly porous Ni₃Sn₂ microcages as anode for NIBs by a facile template-free solvothermal method,it exhibited highly stable capacity,with a reversible capacity of approximate 270 mAh·g^-1 retained at 1 C after 300cycles (Fig.7e) [ 50] .In the system,inactive Ni could effectively suppress the volume change and aggregation of the Sn alloy during the sodiation/desodiation process.Liu and co-workers prepared the SnSb/C composite as anode for NIBs using a simple high-energy mechanical milling method.The anode demonstrated a high initial reversible capacity of 544 mAh·g^-1 and a high capacity retention of80%over 50 cycles [ 51] .In the system,the coexisting Sn and Sb phases formed in situ during the charge/discharge processes form a self-supporting network and maintain the integrity and conductivity of the whole electrode material.

Antimony (Sb) is also a promising alloy anode for NIBs [ 52] .Darwiche et al. [ 26] explored the Na-storage mechanism of pure Sb by in situ XRD analysis.As shown in Fig.8a,Na was inserted into crystalline Sb to form intermediate phases Na_xSb,which were mostly amorphous and finally to form hexagonal Na₃Sb.However,during charge/discharge process,alloying of Sb accompanied by a large volume change of 293%resulted in the pulverization and poor cyclability of the electrode.Zhou et al. [ 53] obtained the Sb/multi-walled carbon nanotube (Sb/MWCNT)nanocomposite by wet-milling method,as anode for NIBs,the Na-storage performance of the as-obtained nanocomposite was significantly improved,which showed high reversible capacities (>400 mAh·g^-1) and stable cycling performance over 100 cycles.The Sb/C nanocomposite was prepared by one-pot spray pyrolysis,as shown in Fig.8b,and ultrafine Sb nanocrystals were uniformly distributed in a carbon matrix with a microsphere morphology.As anode for NIBs,it exhibited a high capacity of621 mAh·g^-1 at a rate of 300 mA·g^-1 and a high capacity retention of 90%over 100 cycles [ 54] .Recently,our group prepared a novel Sb@C nanosphere anode with yolk-shell structure via a nanoconfined galvanic replacement route.As shown in Fig.8c,the yolk-shell microstructure consisted of Sb hollow yolk completely protected by a well conductive carbon thin shell.The substantial void space in these hollow Sb@C yolk-shell particles allowed the full volume expansion of inner Sb,while the outside carbon shell maintained the framework of the anode.As for Nastorage,these yolk-shell Sb@C particles maintained a reversible capacity of approximate 280 mAh·g^-1 at1000 mA·g^-1 after 200 cycles (Fig.8d) [ 55] .

Fig.4 Theoretical calculated Na-storage voltage profiles of a group-14 alloy elements,b group-15 alloy elements,and theoretical calculated Na-storage volume change of c group-14 alloy elements and d group-15 alloy elements [44,45]

Fig.5 a Voltage curves of the first desodiation and second sodiation of sputtered tin,superimposed on predicted DFT voltage curve [45] ;b in situ X-ray diffraction (XRD) pattern of Sn after full sodiation [46]

Fig.6 a-c In situ TEM analysis of several Na-Sn phases during sodiation process of Sn nanoparticles;d schematic illustration of structural evolution of Sn nanoparticles during sodiation process [47]

As anode for NIB s,phosphorus (P) provides a very high theoretical capacity of 2596 mAh·g^-1 and an appropriate redox potential of about 0.4 V versus Na⁺/Na.Kim et al. [ 56] explored the Na-storage mechanism of the P/C composite through ex situ XRD analysis,which showed that the product was Na₃P after full sodiation.However,same as other alloybased anode materials,P also undergoes large volume expansion.In addition,P has a quite low electrical conductivity (1×10^-14 S·cm^-1).Wang et al.synthesized the P/G hybrid with excellent Na-storage performance through highenergy ball milling,which retained a high capacity of above1700 mAh·g^-1 at 260 mA·g^-1 after 60 cycles (Fig.8e) [ 57] .Sn₄P₃ was first reported as an anode for NIBs by Lee'et al.and exhibited a high reversible capacity of 718 mAh·g^-1 with negligible capacity fading over 100 cycles [ 58] .The excellent electrochemical performance of Sn₄P₃ was attributed to synergistic Na-storage reactions of the Sn and P components,where the Sn phase worked as electronic channels to enhance the electrical conductivity of the P component,while the elemental P and Na_xP acted as a shielding matrix to relieve the volume expansion.Qian et al. [ 59] prepared the Sn₄P₃/C composite by ball milling,and in comparison with the Sn/Cand P/C anode,the Na-storage performance of the Sn₄P₃/C anode was improved significantly (Fig.8f).

Reversible sodiation/desodiation was also examined for Ge and In alloy.Baggetto et al. [ 60] reported the reversible Na-storage behavior of Ge thin film,the behavior was accompanied with a reversible capacity close to350 mAh·g^-1,which matched the value expected for the formation of NaGe.However,ex situ XRD patterns of Ge film revealed that the product after full sodiation was amorphous.Abel et al. [ 61] synthesized nanocolumnar Ge thin films as anode for NIBs by evaporative deposition.The reversible capacity of the nanocolumnar films was 430 mAh·g^-1 and retained 88%of initial capacity after 100 cycles at 0.2 C.Recently,the In thin films were examined as anode for NIBs,they exhibited a reversible capacity of about 100 mAh·g^-1,despite the theoretical specific capacity was 467 mAh·g^-1.An ex situ XRD pattern revealed that the sodiated phase was the mixture of Naln and In even after being short-circuited for40 h at 65℃ [ 62] .This indicates that In-based materials have poor sodiation kinetics and are not appropriate for NIBs.

As described above,the fatal challenge for alloy-based materials is the large volume change during cycling process,which will result in pulverization of electrode materials and rapid capacity decay.And much research effort has been devoted to alleviating the large volume change,such as designing nanostructure or introducing a second phase as a buffer matrix.

Fig.7 a SEM image of Sn nanofibers [48] ,b cycling performance of hollow Sn and solid Sn nanoparticles [49] ,c TEM image of Sn@C composite [25] ,d cycling performance of Sn@C composite [25] ,and e cycling performance of porous Ni₃Sn₂ microcages [50]

4 Conversion-based materials

Conversion-based materials mainly include metal oxides,sulfides,selenides,etc.The conversion-based materials are considered as promising anode materials for NIBs due to their high theoretical capacity,e.g.,Fe₂O₃(1007 mAh·g^-1),SnO₂ (1387 mAh·g^-1),SnS₂(1135 mAh·g^-1),and MOS₂ (672 mAh·g^-1).The reaction mechanism can be summed up into two types according to the electrochemically inactive or active metal in the conversion compounds.If the metal is an inactive element,such as Fe,Co,Ni,Cu,conversion-based materials can react with Na⁺through a one-step conversion reaction:

Fig.8 a Operando XRD patterns of Sb anodes [26] ,b TEM image of Sb/C nanocomposite [54] ,c TEM image of hollow Sb@C yolk-shell nanosphere,d cycling performance of hollow Sb@C yolk-shell nanosphere [55] ,e cycling performance of the P/C composite [57] ,and f cycling performance of the Sn₄P₃/C,Sn/C,P/C [59]

where M is a transition metal,X is an anion,and n is the number of the transition metal ion in MX.If the metal is an active element,such as Sn,Sb,materials can react with Na⁺via a conversion reaction (Reaction 1) and a further alloying reaction (Reaction 2).

These anodes show high capacities due to the multielectron reactions [ 63, 64] .However,they suffer from large volume change during cycling,which results in degradation of the electrodes and severe capacity fading.In addition,the initial coulombic efficiency (ICE) of conversion-based anode materials is low due to the trap of Na⁺in Na_nX and the formation of solid electrolyte interphase (SEI).

Fe₂O₃ and Fe₃O₄ are attractive anodes due to their high theoretical capacity and low cost.Jiang et al. [ 65] reported Fe₂O₃ thin film as anode for NIB s,which delivered a reversible capacity of 386 mAh·g^-1 at 100 mA·g^-1 over200 cycles,and confirmed that Na uptake/extract is in the way of reversible conversion reaction.Kim et al.investigated Fe₃O₄ nanoparticles as anode for NIBs,the Fe₃O₄with alginate binder delivered a reversible capacity of248 mAh·g^-1 at 83 mA·g^-1 after 50 cycles [ 66] .Zhou and co-workers prepared Fe₂O₃@GNS composite using a nanocasting technique.As an anode for NIBs,it exhibited an initial capacity of 535 mAh·g^-1 and 75%capacity retention after 200 cycles at 100 mA·g^-1 [ 67] .However,the ICE was low.Recently,CuO [ 68] ,Co₃O₄ [ 69] and NiO [ 70] were also explored as anode for NIBs and showed good Na-storage performance.

Most metal oxides consisting of active metal elements can deliver high reversible capacities.For example,Wang and co-workers showed that the octahedral SnO₂nanocrystals (～60 nm),prepared by a hydrothermal method,delivered a reversible capacity of about500 mAh·g^-1 with excellent cyclability [ 71] .The improved cyclability is attributed to the retardation of the aggregation of Sn during cycling by the Na₂O matrix.Ex situ TEM analysis revealed that the Na⁺was firstly inserted into the SnO₂ to generate NaSnO₂ and then inserted into NaSnO₂ to form Na₂O and Sn,finally Sn alloyed with Na⁺to form Na_xSn (Fig.9a).Wang et al. [ 27] employed SnO₂/reduced graphene oxide (RGO) as a NIBs anode,which displayed a reversible capacity of 330 mAh·g^-1 (at80 mA·g¹) with capacity retention of 81.3%after 150cycles (Fig.9b).Confined ultra-small SnO₂ nanoparticles in the pores of mesoporous carbon (SnO₂@C) were prepared and tested as anode for NIBs,an initial reversible capacity of 780 mAh·g^-1 is achieved with unprecedented capacity retention of 80%and 54%after 100 and 4000cycles at 1800 mA·g^-1,respectively (Fig.9c) [ 72] .Recently,our group prepared the (SnO_x-Sn)@FLG composite via facile oxygen plasma-assisted milling method,the in situ formed FLG and residual Sn nanoparticles improved the electrical conductivity of the composite,meanwhile,alleviated the aggregation of SnO_x nanoparticles and relieved the volume change during the cycling.As an anode material for NIB s,the composite exhibited a high capacity retention of 82.6%at 100 mA·g^-1 after 250 cycles(Fig.9d) [ 73] .

Fig.9 a Schematic illustration of Na-storage mechanism for initial discharge process of SnO₂ nanocrystals [71] ;cycling performance of b SnO₂/RGO composite (80 mA·g^-1) [27] ,c SnO₂@C composite (1800 mA·g^-1) [72] and d (SnO_x-Sn)@FLG composite (100 mA·g^-1) [73]

It is believed that the M-S bonds in metal sulfides are weaker than the corresponding M-O bonds in metal oxides,which could be kinetic ally favorable for conversion reactions [ 74] .Recently,various transition metal sulfides such as CoS [ 75] ,FeS₂ [ 76, 77] ,SnS [ 74] ,SnS₂ [ 78] ,MoS₂ [ 79] and Sb₂S₅ [ 80] have shown promising Na-storage performance.MoS₂,a typical layered material,has been intensively investigated as an anode for NIBs.Su et al. [ 81] synthesized few-layer MoS₂ nanosheets using a ultrasonic exfoliation technique,the MOS₂ nanosheets demonstrated good Na-storage performance because the few-layer structure can relieve the strain and decrease the barrier for Na⁺intercalation.Chen’s group reported MoS₂/C microspheres as anode for NIBs,in which ultrathin MoS₂nanosheets (～2 nm) with enlarged interlayers(～0.64 nm) are homogeneously embedded in mesoporous carbon microspheres.The as-synthesized MoS₂/C microspheres demonstrated long cycling stability (390 mAh·g^-1after 2500 cycles at 1000 mA·g^-1) [ 79] .The superior electrochemical performance is due to the uniform distribution of ultrathin MoS₂ nanosheets in mesoporous carbon frameworks,which not only improves the electronic and ionic transport,but also alleviates pulverization and aggregation of MoS₂ nanosheets during cycling.Cao et al.prepared a SnS-C nanocomposite by a simple high-energy mechanical milling method [ 74] .The SnS-C electrode exhibited a high Na-storage capacity (568 mAh·g^-1 at20 mA·g^-1) and excellent cycling stability (capacity retention of 97.8%over 80 cycles).Ex situ XRD result confirms a sequential conversion and alloying-dealloying reaction mechanism of the SnS-C electrode during the cycling [ 74] .Zhang et al. [ 82] reported SnSe/C nanocomposite synthesized by high-energy ball milling as highperformance anode material for NIBs,which maintained324.9 mAh·g^-1 at 500 mA·g^-1 in the 200th cycle.

As described above,conversion-based materials possess the advantage of high capacity,but they also suffer from large volume change during sodiation/desodiation process and relatively low ICE,which would result in rapid capacity decay.Nanostructuring and compositing have been demonstrated effective way to settle above problems.However,the redox potentials versus Na⁺/Na of conversion-based materials are relatively high (>1 V),which is a detrimental shortcoming for conversion-based anode materials and influences their practical application.Therefore,it is essential to explore conversion-based anode materials with low redox potentials or cathode materials with high redox potentials,achieving full cells with high operation voltage.

5 Organic composites

Organic composites have attracted growing attention due to their high capacity,low cost and abundant resource.The reported organic materials mainly include carboxylatebased,biomolecular-based and schiff-based compounds.Hu et al. [ 83] reported that the disodium terephthalate delivers a reversible capacity of 250 mAh·g^-1,corresponding to a two-electron transfer at an average voltage of0.45 V.And the sodiation was demonstrated proceeding by ionic bonding to the oxygen ions [ 84] .Disodium salt of2,5-dihydroxy-1,4-benzoquinone was recently proposed as a promising anode for NIBs,which displayed a capacity of265 mAh·g^-1 at 0.1 C rate at approximately 1.2 V versus Na⁺/Na and a long cycle life over 300 cycles at 1 C rate [ 85] .Recently,Armands group reported that the polymeric schiff base has a low discharge voltage below 1 V versus Na⁺/Na and delivers a capacity of 350 mAh·g^-1 at a current density of 26 mA·g^-1 [ 86] .Although the organic composites provide respectable storage capacity,the enhancement of rate and cycling performance is in high demand.A few strategies were proposed including molecular tuning,surface coating and polymerization [ 87, 88, 89, 90] .

6 Electrolytes

Optimizing the electrolyte is important for the actual application of NIBs.Palacin’s group recently evaluated three kinds of salt (NaClO₄,NaTFSI,NaPF₆) combined with different solvents (propylene carbonate,ethylene carbonate,dimethyl carbonate,dimethoxyethane,diethyl carbonate,tetrahydrofuran,triglyme and their mixtures) by testing their thermal stability,conductivity,viscosity and electrochemical window [ 91] .They found that NaClO₄ or NaPF₆ in the EC:PC solvent mixture is the best electrolyte for the hard carbon anode.Kim et al. [ 92] reported that natural graphite is not only capable of Na intercalation but also exhibits outstanding performance using ether-based electrolytes.Chen et al.reported bulk Bi anode with surprisingly high Na-storage performance in combination with glyme-based electrolytes.And the controllable formation of SEI between the electrolyte and the electrode material is also important for a good electrolyte system,which affects the Na-storage performance of anode materials greatly [ 93] .Yang’s group achieved outstanding performance on carbon anodes using ether-based electrolyte [ 94] .This outstanding performance is ascribed to the formation of stable,thin,compact,uniform and ion conducting SEI film.Recently,an optimized electrolyte system with electrolyte additive fluoroethylene carbonate (FEC),which can inhibit the decomposition of the electrolyte and stabilize the SEI layers,is also fatal for enhancing the electrochemical performance of materials.Darwiche et al. [ 26] found that the FEC can lead to the formation of a stable SEI layer,which achieved good coulombic efficiency and high stability for the Sb anode.Wang et al. [ 27] demonstrated that the FEC additives can lead to the formation of high-quality SEI layer on the active materials,which improved the cycling performance for the SnO₂ anode.However,few SEI studies have been performed for NIB s,and the obtained results are far from conclusive.

7 Summary and outlook

With the increasing demand for large-scale energy storage and steep consumption of Li resource,NIBs have been considered a promising candidate to replace LIBs due to their low cost and the abundance of Na resource.The anode is an important component of NIBs,and its electrochemical performance is critical for NIBs.Therefore,in this review,we not only focus on the recent development of anode materials for NIBs,but also summarize the Nastorage mechanism of these anodes and discuss their failure mechanism.Furthermore,the problems and challenges are pointed out,and feasible strategies are proposed for designing high-performance anode materials.Although the capacity of insertion-based materials is relatively low,the advantages of abundant resource,low cost and excellent cyclability make them a great potential for commercial application.And through the optimization of morphology structure,the Na-storage performance can be improved significantly.Alloy-based materials possess extremely high capacity,especially for phosphorus with a theoretical capacity of 2596 mAh·g^-1,while the volume expansion during cycling is quite huge.And much effort has been devoted to alleviating the huge volume change,such as designing nanostructure or introducing a second phase as a buffer matrix.Conversion-based materials possess the advantage of high capacity,but they also suffer from large volume change during cycling.And,nanostructuring and compositing have been demonstrated effective ways to settle the problem.Moreover,the ICE is relatively low and the redox potential is high,which influence their practical application,and the performance enhancement needs to be further studied.In addition,organic composites with low cost and high storage capacity have also emerged as a new promising direction for future NIBs technology.Finally,searching a suitable electrolyte system is an important strategy to improve the electrochemical performance of anode materials.Although the search for suitable anode materials for NIBs is still challenging,we believe that high-performance NIBs would be promising for practical applications in large-scale energy storage systems in the near future.

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