Rare Metals2018年第3期

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.

参考文献

[1] Pan HL,Hu YS,Chen LQ.Room-temperature stationary sodium-ion batteries for large-scale electric energy storage.Energy Environ Sci.2013;6(8):2338.

[2] Palomares V,Serras P,Villaluenga I,Hueso KB,Carretero-Gonzalez J,Rojo T.Na-ion batteries,recent advances and present challenges to become low cost energy storage systems.Energy Environ Sci.2012;5(3):5884.

[3] Kang H,Liu Y,Cao K,Zhao Y,Jiao L,Wang Y,Yuan H.Update on anode materials for Na-ion batteries.J Mater Chem A.2015;3(35):17899.

[4] Zheng S,Xue H,Pang H.Transition-metal(Fe,Co,Ni)based metal-organic frameworks for electrochemical energy storage.Adv Energy Mater.2017;7(18):1602733.

[5] Li B,Zheng MB,Xue HG,Pang H.High performance electrochemical capacitor materials focusing on nickel based materials.Inorganic Chem Front.2016;3(2):175.

[6] Li B,Gu P,Feng YC,Zhang GX,Huang KS,Xue HG,Pang H.Ultrathin nickel-cobalt-phosphate 2D nanosheets for electrochemical energy storage under aqueous/solid-state electrolyte.Adv Funct Mater.2017;27(12):11.

[7] Yan Y,Gu P,Zheng SS,Zheng MB,Pang H,Xue HG.Facile synthesis of an accordion-like Ni-MOF superstructure for high-performance flexible supercapacitors.J Mater Chem A.2016;4(48):19078.

[8] Li C,Wang ZB,Wang Q,Gu DM.Recent advances in cathode materials for Li-S battery:structure and performance.Rare Met.2017;36(5):365.

[9] Kim SW,Seo DH,Ma X,Ceder G,Kang K.Electrode materials for rechargeable sodium-ion batteries:potential alternatives to current lithium-ion batteries.Adv Energy Mater.2012;2(7):710.

[10] Guo SH,Yu HJ,Jian ZL,Liu P,Zhu YB,Guo XW,Chen MW,Ishida M,Zhou HS.A high-capacity,low-cost layered sodium manganese oxide material as cathode for sodium-ion batteries.Chemsuschem.2014;7(8):2115.

[11] Yabuuchi N,Kubota K,Dahbi M,Komaba S.Research development on sodium-ion batteries.Chem Rev.2014;114(23):11636.

[12] Delmas C,Braconnier JJ,Fouassier C,Hagenmuller P.Electrochemical intercalation of sodium in Na_xCoO_2 bronzes.Solid State Ionics.1981;3-4(1):165.

[13] Berthelot R,Carlier D,Delmas C.Electrochemical investigation of the P2-Na_xCoO_2 phase diagram.Nat Mater.2011;10(1):74.

[14] Cao Y,Xiao L,Wang W,Choi D,Nie Z,Yu J,Saraf LV,Yang Z,Liu J.Reversible sodium ion insertion in single crystalline manganese oxide nanowires with long cycle life.Adv Mater.2011;23(28):3155.

[15] Jian Z,Zhao L,Pan H,Hu YS,Li H,Chen W,Chen L.Carbon coated Na_3V_2(PO_4)_3 as novel electrode material for sodium ion batteries.Electrochem Commun.2012;14(1):86.

[16] Duan WC,Zhu ZQ,Li H,Hu Z,Zhang K,Cheng FY,Chen J.Na_3V_2(PO_4)_3@C core-shell nanocomposites for rechargeable sodium-ion batteries.J Mater Chem A.2014;2(23):8668.

[17] Wang DX,Chen N,Li ML,Wang CZ,Ehrenberg H,Bie XF,Wei YJ,Chen G,Du F.Na_3V_2(PO_4)_3/C composite as the intercalation-type anode material for sodium-ion batteries with superior rate capability and long-cycle life.J Mater Chem A.2015;3(16):8636.

[18] Ma DL,Wang HG,Li Y,Xu D,Yuan S,Huang XL,Zhang XB,Zhang Y.In situ generated FeF3 in homogeneous iron matrix toward high-performance cathode material for sodium-ion batteries.Nano Energy.2014;10(3):295.

[19] Ali G,Oh SH,Kim SY,Kim JY,Cho BW,Chung KY.An open-framework iron fluoride and reduced graphene oxide nanocomposite as a high-capacity cathode material for Na-ion batteries.J Mater Chem A.2015;3(19):10258.

[20] Fang C,Huang YH,Zhang WX,Han JT,Deng Z,Cao YL,Yang HX.Routes to high energy cathodes of sodium-ion batteries.Adv Energy Mater.2016;6(5):18.

[21] DiVincenzo DP,Mele EJ.Cohesion and structure in stage-1graphite intercalation compounds.Phys Rev B(Condens Matter).1985;32(4:2538.

[22] Ding J,Wang H,Li Z,Kohandehghan A,Cui K,Xu Z,Zahiri B,Tan X,Lotfabad EM,Olsen BC,Mitlin D.Carbon nanosheet frameworks derived from peat moss as high performance sodium ion battery anodes.ACS Nano.2013;7(12):11004.

[23] Tang K,Fu LJ,White RJ,Yu LH,Titirici MM,Antonietti M,Maier J.Hollow carbon nanospheres with superior rate capability for sodium-based batteries.Adv Energy Mater.2012;2(7):873.

[24] Farbod B,Cui K,Kalisvaart WP,Kupsta M,Zahiri B,Kohandehghan A,Lotfabad EM,Li Z,Luber EJ,Mitlin D.Anodes for sodium ion batteries based on tin-germanium-antimony alloys.ACS Nano.2014;8(5):4415.

[25] Liu YC,Zhang N,Jiao LF,Tao ZL,Chen J.Ultrasmall Sn nanoparticles embedded in carbon as high-performance anode for sodium-ion batteries.Adv Funct Mater.2015;25(2):214.

[26] Darwiche A,Marino C,Sougrati MT,Fraisse B,Stievano L,Monconduit L.Better cycling performances of bulk Sb in Na-ion batteries compared to Li-ion systems:an unexpected electrochemical mechanism.J Am Chem Soc.2012;134(51):20805.

[27] Wang YX,Lim YG,Park MS,Chou SL,Kim JH,Liu HK,Dou SX,Kim YJ.Ultrafine Sn02 nanoparticle loading onto reduced graphene oxide as anodes for sodium-ion batteries with superior rate and cycling performances.J Mater Chem A.2014;2(2):529.

[28] Patra J,Chen HC,Yang CH,Hsieh CT,Su CY,Chang JK.High dispersion of 1-nm Sn02 particles between graphene nanosheets constructed using supercritical CO_2 fluid for sodium-ion battery anodes.Nano Energy.2016;28(3):124.

[29] Zheng Y,Zhou TF,Zhang CF,Mao JF,Liu HK,Guo ZP.Boosted charge transfer in SnS/SnO_2 heterostructures:toward high rate capability for sodium-ion batteries.Angew Chem Intl Ed.2016;55(10):3408.

[30] Zhou TF,Pang WK,Zhang CF,Yang JP,Chen ZX,Liu HK,Guo ZP.Enhanced sodium-ion battery performance by structural phase transition from two-dimensional hexagonal-SnS_2 to orthorhombic-SnS.ACS Nano.2014;8(8):8323.

[31] Xu XM,Niu CJ,Duan MY,Wang XP,Huang L,Wang JH,Pu LT,Ren WH,Shi CW,Meng JS,Song B,Mai LQ.Alkalineearth metal vanadates as sodium-ion battery anodes.Nat Commun.2017;8(4):11.

[32] Choi JW,Aurbach D.Promise and reality of post-lithium-ion batteries with high energy densities.Nat Rev Mater.2016;1(5):16013.

[33] Jache B,Adelhelm P.Use of graphite as a highly reversible electrode with superior cycle life for sodium-ion batteries by making use of Co-intercalation phenomena.Angew Chem Intl Ed.2014;53(38):10169.

[34] Wen Y,He K,Zhu YJ,Han FD,Xu YH,Matsuda I,Ishii Y,Cumings J,Wang CS.Expanded graphite as superior anode for sodium-ion batteries.Nat Commun.2014;5(5):10.

[35] Stevens DA,Dahn JR.High capacity anode materials for rechargeable sodium-ion batteries.J Electrochem Soc.2000;147(4):1271.

[36] Alcantara R,Jimenez-Mateos JM,Lavela P,Tirado JL.Carbon black:a promising electrode material for sodium-ion batteries.Electrochem Commun.2001;3(11):639.

[37] Komaba S,Murata W,Ishikawa T,Yabuuchi N,Ozeki T,Nakayama T,Ogata A,Gotoh K,Fujiwara K.Electrochemical Na insertion and solid electrolyte interphase for hard-carbon electrodes and application to Na-ion batteries.Adv Funct Mater.2011;21(20):3859.

[38] Cao YL,Xiao LF,Sushko ML,Wang W,Schwenzer B,Xiao J,Nie ZM,Saraf LV,Yang ZG,Liu J.Sodium ion insertion in hollow carbon nanowires for battery applications.Nano Lett.2012;12(7):3783.

[39] Wang YX,Chou SL,Liu HK,Dou SX.Reduced graphene oxide with superior cycling stability and rate capability for sodium storage.Carbon.2013;57(6):202.

[40] Xu Y,Lotfabad EM,Wang HL,Farbod B,Xu ZW,Kohandehghan A,Mitlin D.Nanocrystalline anatase Ti02:a new anode material for rechargeable sodium ion batteries.Chem Commun.2013;49(79):8973.

[41] Yang YC,Ji XB,Jing MJ,Hou HS,Zhu YR,Fang LB,Yang XM,Chen QY,Banks CE.Carbon dots supported upon N-doped Ti02 nanorods applied into sodium and lithium ion batteries.J Mater Chem A.2015;3(10):5648.

[42] Ni JF,Fu SD,Wu C,Zhao Y,Maier J,Yu Y,Li L.Superior sodium storage in Na2Ti307 nanotube arrays through surface engineering.Adv Energy Mater.2016;6(11):8.

[43] Rudola A,Saravanan K,Devaraj S,Gong H,Balaya P.Na2Ti6013:a potential anode for grid-storage sodium-ion batteries.Chem Commun.2013;49(67):7451.

[44] Mortazavi M,Ye Q,Birbilis N,Medhekar NV.High capacity group-15 alloy anodes for Na-ion batteries.Electrochem Mech Insights.2015;285(6):29.

[45] Chevrier VL,Ceder G.Challenges for Na-ion negative electrodes.J Electrochem Soc.2011;158(9):A1011.

[46] Ellis LD,Hatchard TD,Obrovac MN.Reversible insertion of sodium in tin.J Electrochem Soc.2012;159(11):A1801.

[47] Wang JW,Liu XH,Mao SX,Huang JY.Microstructural evolution of tin nanoparticles during in situ sodium insertion and extraction.Nano Lett.2012;12(11):5897.

[48] Nam DH,Kim TH,Hong KS,Kwon HS.Template-free electrochemical synthesis of Sn nanofibers as high-performance anode materials for Na-ion batteries.ACS Nano.2014;8(11):11824.

[49] Cheng YY,Huang JF,Li RZ,Xu ZW,Cao LY,Ouyang HB,Li JY,Qi H,Wang CW.Enhanced cycling performances of hollow Sn compared to solid Sn in Na-ion battery.Electrochim Acta.2015;180(7):227.

[50] Liu J,Wen YR,van Aken PA,Maier J,Yu Y.Facile synthesis of highly porous Ni-Sn intermetallic microcages with excellent electrochemical performance for lithium and sodium storage.Nano Lett.2014;14(11):6387.

[51] Xiao LF,Cao YL,Xiao J,Wang W,Kovarik L,Nie ZM,Liu J.High capacity,reversible alloying reactions in SnSb/C nanocomposites for Na-ion battery applications.Chem Commun.2012;48(27):3321.

[52] Luo W,Gaumet JJ,Mai LQ.Antimony-based intermetallic compounds for lithium-ion and sodium-ion batteries:synthesis,construction and application.Rare Met.2017;36(5):321.

[53] Zhou XS,Dai ZH,Bao JC,Guo YG.Wet milled synthesis of an Sb/MWCNT nanocomposite for improved sodium storage.J Mater Chem A.2013;1(44):13727.

[54] Ko YN,Kang YC.Electrochemical properties of ultrafine Sb nanocrystals embedded in carbon microspheres for use as Na-ion battery anode materials.Chem Commun.2014;50(82):12322.

[55] Liu J,Yu LT,Wu C,Wen YR,Yin KB,Chiang FK,Hu RZ,Liu JW,Sun LT,Gu L,Maier J,Yu Y,Zhu M.New nanoconfined galvanic replacement synthesis of hollow Sb@C yolk-shell spheres constituting a stable anode for high-rate Li/Na-ion batteries.Nano Lett.2017;17(3):2034.

[56] Kim Y,Park Y,Choi A,Choi NS,Kim J,Lee J,Ryu JH,Oh SM,Lee KT.An amorphous red phosphorus/carbon composite as a promising anode material for sodium ion batteries.Adv Mater.2013;25(22):3045.

[57] Song J,Yu Z,Gordin ML,Hu S,Yi R,Tang D,Walter T,Regula M,Choi D,Li X,Manivannan A,Wang D.Chemically bonded phosphorus/graphene hybrid as a high performance anode for sodium-ion batteries.Nano Lett.2014;14(11):6329.

[58] Kim Y,Kim Y,Choi A,Woo S,Mok D,Choi NS,Jung YS,Ryu JH,Oh SM,Lee KT.Tin phosphide as a promising anode material for Na-ion batteries.Adv Mater.2014;26(24):4139.

[59] Qian J,Xiong Y,Cao Y,Ai X,Yang H.Synergistic Na-storage reactions in Sn4P3 as a high-capacity,cycle-stable anode of Na-ion batteries.Nano Lett.2014;14(4):1865.

[60] Baggetto L,Keum JK,Browning JF,Veith GM.Germanium as negative electrode material for sodium-ion batteries.Electrochem Commun.2013;34(9):41.

[61] Abel PR,Lin YM,de Souza T,Chou CY,Gupta A,Goodenough JB,Hwang GS,Heller A,Mullins CB.Nanocolumnar germanium thin films as a high-rate sodium-ion battery anode material.J Phys Chem C.2013;117(37):18885.

[62] Webb SA,Baggetto L,Bridges CA,Veith GM.The electrochemical reactions of pure indium with Li and Na:anomalous electrolyte decomposition,benefits of FEC additive,phase transitions and electrode performance.J Power Sources.2014;248(6):1105.

[63] Lu Y,Li B,Zheng SS,Xu YX,Xue HG,Pang H.Syntheses and energy storage applications of M_xS_y(M=Cu,Ag,Au)and their composites:rechargeable batteries and supercapacitors.Adv Funct Mater.2017;27(44):28.

[64] Li Z,Ding J,Mitlin D.Tin and tin compounds for sodium ion battery anodes:phase transformations and performance.Acc Chem Res.2015;48(6):1657.

[65] Jiang YZ,Hu MJ,Zhang D,Yuan TZ,Sun WP,Xu B,Yan M.Transition metal oxides for high performance sodium ion battery anodes.Nano Energy.2014;5(8):60.

[66] Kumar PR,Jung YH,Bharathi KK,Lim CH,Kim DK.High capacity and low cost spinel Fe_3O_4 for the Na-ion battery negative electrode materials.Electrochim Acta.2014;146(5):503.

[67] Jian ZL,Zhao B,Liu P,Li FJ,Zheng MB,Chen MW,Shi Y,Zhou HS.Fe_2O_3 nanocrystals anchored onto graphene nanosheets as the anode material for low-cost sodium-ion batteries.Chem Commun.2014;50(10):1215.

[68] Wang LJ,Zhang K,Hu Z,Duan WC,Cheng FY,Chen J.Porous CuO nanowires as the anode of rechargeable Na-ion batteries.Nano Res.2014;7(2):199.

[69] Rahman MM,Glushenkov AM,Ramireddy T,Chen Y.Electrochemical investigation of sodium reactivity withnanostructured Co_3O_4 for sodium-ion batteries.Chem Commun.2014;50(39):5057.

[70] Sun WP,Rui XH,Zhu JX,Yu LH,Zhang Y,Xu ZC,Madhavi S,Yan QY.Ultrathin nickel oxide nanosheets for enhanced sodium storage and lithium storage.J Power Sources.2015;274(6):755.

[71] Su DW,Wang CY,Ahn H,Wang GX.Octahedral tin dioxide nanocrystals as high capacity anode materials for Na-ion batteries.Phys Chem Chem Phys.2013;15(30):12543.

[72] Jahel A,Ghimbeu CM,Darwiche A,Vidal L,Hajjar-Garreau S,Vix-Guterl C,Monconduit L.Exceptionally highly performing Na-ion battery anode using crystalline SnO_2 nanoparticles confined in mesoporous carbon.J Mater Chem A.2015;3(22):11960.

[73] Cheng DL,Liu JW,Li X,Hu RZ,Zeng MQ,Yang LC,Zhu M.A highly stable(SnO_x-Sn)@few layered graphene composite anode of sodium-ion batteries synthesized by oxygen plasma assisted milling.J Power Sources.2017;350(1):1.

[74] Wu L,Lu H,Xiao L,Qian J,Ai X,Yang H,Cao Y.A tin(ii)sulfide-carbon anode material based on combined conversion and alloying reactions for sodium-ion batteries.J Mater Chem A.2014;2(39):16424.

[75] Li QD,Li L,Owusu KA,Luo W,An QY,Wei QL,Zhang QJ,Mai LQ.Self-adaptive mesoporous CoS@alveolus-like carbon yolk-shell microsphere for alkali cations storage.Nano Energy.2017;41(11):109.

[76] Kim TB,Choi JW,Ryu HS,Cho GB,Kim KW,Ahn JH,Cho KK,Ahn HJ.Electrochemical properties of sodium/pyrite battery at room temperature.J Power Sources.2007;174(2):1275.

[77] Liu ZM,Lu TC,Song T,Yu XY,Lou XW,Paik U.Structure-designed synthesis of FeS_2@C yolk-shell nanoboxes as a high-performance anode for sodium-ion batteries.Energy Environ Sci.2017;10(7):1576.

[78] Qu BH,Ma CZ,Ji G,Xu CH,Xu J,Meng YS,Wang TH,Lee JY.Layered SnS_2-reduced graphene oxide composite-a high-capacity,high-rate,and long-cycle life sodium-ion battery anode material.Adv Mater.2014;26(23):3854.

[79] Lu YY,Zhao Q,Zhang N,Lei KX,Li FJ,Chen J.Facile spraying synthesis and high-performance sodium storage of mesoporous MoS_2/C microspheres.Adv Funct Mater.2016;26(6):911.

[80] Lu YY,Zhang N,Jiang S,Zhang YD,Zhou M,Tao ZL,Archer LA,Chen J.High-capacity and ultrafast Na-ion storage of a self-supported 3D porous antimony persulfide-graphene foam architecture.Nano Lett.2017;17(6):3668.

[81] Su DW,Dou SX,Wang GX.Ultrathin MoS2 nanosheets as anode materials for sodium-ion batteries with superior performance.Adv Energy Mater.2015;5(6):6.

[82] Zhang ZA,Zhao XX,Li J.SnSe/carbon nanocomposite synthesized by high energy ball milling as an anode material for sodium-ion and lithium-ion batteries.Electrochim Acta.2015;176(5):1296.

[83] Zhao L,Zhao JM,Hu YS,Li H,Zhou ZB,Armand M,Chen LQ.Disodium terephthalate(Na_2C_8H_4O_4)as high performance anode material for low-cost room-temperature sodium-ion battery.Adv Energy Mater.2012;2(8):962.

[84] Wu XY,Jin SF,Zhang ZZ,Jiang LW,Mu LQ,Hu YS,Li H,Chen XL,Armand M,Chen LQ,Huang XJ.Unraveling the storage mechanism in organic carbonyl electrodes for sodium-ion batteries.Sci Adv.2015;1(8):9.

[85] Zhu ZQ,Li H,Liang J,Tao ZL,Chen J.The disodium salt of2,5-dihydroxy-1,4-benzoquinone as anode material for rechargeable sodium ion batteries.Chem Commun.2015;51(8):1446.

[86] Castillo-Martinez E,Carretero-Gonzalez J,Armand M.Polymeric Schiff bases as low-voltage redox centers for sodium-ion batteries.Angew Chem Intl Ed.2014;53(21):5341.

[87] Park Y,Shin DS,Woo SH,Choi NS,Shin KH,Oh SM,Lee KT,Hong SY.Sodium terephthalate as an organic anode material for sodium ion batteries.Adv Mater.2012;24(26):3562.

[88] Wang S,Wang L,Zhu Z,Hu Z,Zhao Q,Chen J.All organic sodium-ion batteries with Na_4C_8H_2O_6.Angew Chem Intl Ed.2014;53(23):5892.

[89] Wan F,Wu XL,Guo JZ,Li JY,Zhang JP,Niu L,Wang RS.Nanoeffects promote the electrochemical properties of organic Na_2C_8H_4O_4 as anode material for sodium-ion batteries.Nano Energy.2015;13(2):450.

[90] Luo C,Zhu Y,Xu Y,Liu Y,Gao T,Wang J,Wang C.Graphene oxide wrapped croconic acid disodium salt for sodium ion battery electrodes.J Power Sources.2014;250(5):372.

[91] Ponrouch A,Marchante E,Courty M,Tarascon JM,Palacin MR.In search of an optimized electrolyte for Na-ion batteries.Energy Environ Sci.2012;5(9):8572.

[92] Kim H,Hong J,Park YU,Kim J,Hwang I,Kang K.Sodium storage behavior in natural graphite using ether-based electrolyte systems.Adv Funct Mater.2015;25(4):534.

[93] Wang CC,Wang LB,Li FJ,Cheng FY,Chen J.Bulk bismuth as a high-capacity and ultralong cycle-life anode for sodium-ion batteries by coupling with glyme-based electrolytes.Adv Mater.2017;29(35):7.

[94] Zhang J,Wang DW,Lv W,Zhang SW,Liang QH,Zheng DQ,Kang FY,Yang QH.Achieving superb sodium storage performance on carbon anodes through an ether-derived solid electrolyte interphase.Energy Environ Sci.2017;10(1):370.