| Metal alloy based materials（Sn,Sb,Bi,etc.）are considered as a potential anode materials for sodium ion batteries（SIBs）because of their high theoretical specific capacity,high tap density,moderate working potential and high electronic conductivity.However,the huge volume changes during the charge and discharge process and serious performance degradation under high current density of metal alloys limit their practical application for SIBs.Furthermore,the failure mechanism of metal Sb for SIB anodes is not completely clear,which also poses a great obstacle to improve its sodium storage performance.Thus,exploring the failure mechanism of metal Sb-based composites as negative electrode materials for SIBs,and designing and preparing appropriate metal alloy-based nanocomposites both have great significance to realize their long cycle and high rate performance.In this paper,a high-performance SIB anode material was constructed with the help of micro-nano structure design based on the metal Bi.And other metal with high theoretical specific capacity（Sn and Sb）are introduced into the metal Bi matrix to form binary alloy-based nano materials(Sn-Bi two-phase alloy and BixSb1-x solid solution alloy),so as to improve the actual specific capacity of the overall composite based on stable cycle performance and high rate performance of metal Bi.In addition,the electrochemical dissolution phenomenon,capacity attenuation and failure mechanism of Sb found in Sb-Bi alloy is explored deeply and systematically.The main research contents are as follows:（1）To improve the long cycle performance and rate performance of metal Bi under high current density,three-dimensional and multilayered Bi@nitrogen doped carbon nanosheets composite（ML-Bi@NCSs）was designed and prepared.The three-dimensional hierarchical structure（0D（?）2D（?）3D）is formed by stacking two-dimensional N-doped carbon nanosheets（NCS）embedded with zero dimensional Bi nanospheres.The three-dimensional heterostructure not only can provide enough space to buffer the huge volume change of Bi nanoparticles,but also provide a also provide a steric and continuous conductive network,and ensure the efficiently electrical contact between the active material and the conductive substrate.Moreover,the open layered structure also can contribute to the rapid penetration of electrolyte and ion diffusion,The electrode material shows good rate performance(30 A g-1: 288 m Ah g-1).Moreover,the specific capacity of 347 and 309 m Ah g-1 can still be provided after 5000 and 10000 cycles at 10 and 20 A g-1,respectively.Its comprehensive electrochemical performances are higher than most reported Bi-based composites.This study provides reference for designing the negative electrode materials for SIBs with ultra-stable cycle performance and ultra-fast charging rate.（2）To improve the overall specific capacity of metal Bi-based composites,Sn-Bi two-phase alloy was constructed by introducing Sn with higher theoretical specific capacity(847 m Ah g-1).And it grow between the layers and on the surface of expanded graphite（EG）successfully,forming the Sn-Bi alloy/expanded graphite composites（Sn-Bi/EG）with different Sn/Bi molar ratios.In this composite,the smaller Sn-Bi alloy nanospheres can be used to reduce the huge mechanical stress caused by sodiation/de-sodiation reaction,and the flexible graphite sheets can also be used to well buffer the huge volume changes of Sn-Bi nanoparticles.After comparison,the rate properties and actual specific capacity of Sn-Bi/EG composites can be flexibly adjusted with the change of alloy element composition.Among them,Sn-Bi（1:1）/EG electrode not only inherits the good rate performance of Bi,but also possesses the high specific capacity of Sn.It can release a high specific capacity of396 m Ah g-1 under the high-rate test condition(5 A g-1).Moreover,it can still maintain the high specific capacity of 389 and 376 m Ah g-1 after 1500 cycles at 2 and5 A g-1,respectively.This work provides an important reference for further exploration and research of multicomponent alloys for sodium storage,and also has certain significance for promoting the practical application of alloy negative materials for SIBs with high energy density and fast charging performance.（3）To improve the overall specific capacity of metal Bi-based composites and explore the capacity attenuation mechanism of Sb-Bi alloy in DME-based electrolyte,Three dimensional Sb-Bi/nitrogen doped porous carbon（3D Sb-Bi/N-PC）composites with different Sb/Bi molar ratios were successfully prepared by template method.Among them,3D Bi3Sb1/N-PC electrode shows an optimal rate performance(30 A g-1:383 m Ah g-1).Moreover,it still can provide high specific capacities of 297 and 245 m Ah g-1 after 6000 cycles at a big current density of 5 and 10 A g-1 with a low capacity attenuation rate of 0.0031% and 0.0054% per cycle,respectively.After studying the disassemble components of the cell,black solid precipitations containing Sb element was found on the separator close to metal Na,indicating the dissolution and loss of active Sb from the Cu current collector.Because the three-dimensional carbon skeleton can well buffer the volume changes of Sb-Bi nanoparticles during the alloying/dealloying processes,which indicates that the dissolution and deactivation of Sb in DME electrolyte is another reason for the capacity attenuation in addition to the above volume expansion problem.Moreover,with the increase of Bi element in Sb-Bi alloys,the loss of proportions of "active antimony" on the current collector become much less,indicating that the Bi crystal framework has a strong fix effect on Sb atoms.（4）To further study the capacity attenuation and failure mechanism of metal Sb,static experiments and electrochemical tests on metal Sb half cells with various electrolyte systems（Ethers and esters containing different sodium salts）were carried out by us.Among them,no serious self-discharge and chemical dissolution phenomena were found in the metal Sb half cells equipped with various electrolytes.And the electrochemical dissolution of metal Sb was found to start at the starting point of desodiation（about 0.7 V）during charging process with ether electrolyte only.Through the characterization of disassembled cell components,it can be seen that the metal Sb on the original electrode was oxidized into antimony ions and dissolved into the electrolyte during the charging process.And the dissolved Sb element can pass through the separator to negative electrode and form a black solid precipitation of Na Sb（OH）6.With the increase of electrochemical cycles,the Sb containing deposits on the separator close to metal Na also gradually increased.And the Sb containing deposits would no longer participate in the subsequent electrochemical cycles to form "dead antimony".The finding demonstrates that the electrochemical dissolution of Sb-based materials in ether electrolyte and their migration/deposition to the negative electrode is one of the key factors leading to the loss of active substances and the rapid decline of capacity.This study further revealed the electrochemical dissolution process of Sb containing metal negative electrode,and ruled out the influence of sodium salt on the electrochemical dissolution phenomenon to a certain extent,and extended the range of electrochemical dissolution of Sb containing metal negative electrode to ether electrolyte. |
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