Modification And Electrochemical Performance Of Sn-based Oxide Anode Materials For Sodium-ion Batteries

Sn-based oxides have been widely studied and applied in SIBs anode due to their high theoretical specific capacity,low cost and high safety.Unfortunately,the low intrinsic electro-conductivity together with the tremendous volume change（⁴20%）will lead to continuous pulverization of active materials and even loss of electrical contact with the current collector,deteriorating the cyclability and rate performance of Sn-base oxides.Construction of nanostructured Sn-based oxides and Sn-based oxide composites are two effective ways to solve the problems of Sn-based oxides.However,in the current research related to the Sn-based oxide nanocomposites,there still remains several problems such as monotonous type of composite（focusing on the combination with carbon materials）,poor interfacial bonding between the components,sluggish ion transfer kinetics and so on.Therefore,how to synthesize the Sn-based oxide nanocomposites with tight interfical bonding and fast charge transfer kinetics remains a huge challenge.Nb-based oxides show excellent structural stability and tight combination with Sn-based oxides,which will help to alleviate the mechanical stress caused by Sn-based oxides during sodium storage process.In addition,the heterojunction formed between carbon and transiton metal sulfides shows fast charge transfer rate at the interface,which will help to boost the charge transfer dynamics of Sn-based oxide nanocomposites.In this work,aiming at solving the problems of low electronic conductivity and tremendous volume change of Sn-based oxide and building the high-performance SIBs anode materials,we proposed a simple high-temperature calcination method to prepare mulberry-like Sn₂Nb₂O₇/SnO₂nanoparticles anchored on 3D carbon networks（M-Sn₂Nb₂O₇/SnO₂@3DC）nanocomposite and 3D carbon-MoS₂ VSHs anchored with ultrasmall SnO₂nanoparticles（SnO₂@3DC-MoS₂ VSHs）nanocomposite with the assistance of template of 3D NaCl assembly.The effect of different raw material ratios on the phase and microstructure has been systematically investigated.Moreover,their sodium storage performances and mechanisms have also been explored.It was found that the amount of Nb precursor determined the crystal structure and the morphology of the products.The M-Sn₂Nb₂O₇/SnO₂@3DC nanocomposite synthesized under the optimized preparation condition have delivered a reversible specific capacity of 302 mAh g^-1 at 100 mA g^-1 even after 100 cycles when evaluated as a sodium-ion battery anode.Meanwhile,the electrode of M-Sn₂Nb₂O₇/SnO₂@3DC nanocomposite has also exhibited a superior cycling stability at high rates,namely,a specific capacity of 130 mAh g^-1 was obtained at 5.0A g^-1 and maintained⁹6.3%even after 5000 cycles.The addition of Nb-based oxides results in the unique“self-buffering”effect.After the first dicharge process,in-situ formed amorphous Na_xNb₂O₅ substrate can not only buffer the large mechanical stress which is originated from the tremendous volume change but also lead to the capacitive-controlled Na⁺reaction behavior which enables fast Na⁺transfer kinetics.As a result,the electrode of M-Sn₂Nb₂O₇/SnO₂@3DC nanocomposite can demonstrate an excellent prolonged cycling capability at high rates.The optimized synthesis process of SnO₂@3DC-MoS₂ VSHs nanocomposite is that SnCl₄,（NH₄）₆Mo₇O₂₄,CH₄N₂S,C₆H₈O₇ and NaCl were taken as the raw materials with a molar ratio of 1:1:20:200 between Sn,Mo,C and NaCl to prepare the freeze-drying precursor gel,followed by a high-temperature calcination process at 750 ^oC for 2 h under Ar.It is worth noting that the obtained SnO₂@3DC-MoS₂VSHs exhibited an remarkably enhanced rate capability.As the current density increased from 0.1 to 0.2,0.5,1.0,2.0,5.0 and 10 A g^-1,the reversible capacities were 499.3,468.6,414.4,380.8,347.3,303.6 and 238.8 mAh g^-1,respectively.The3D carbon-MoS₂ VSHs not only can effectively restrain the aggregation of active materials,well accommodate their volume variation upon cycling and thus maintain structural integrity of the overall electrode,but also highly enhance the electron and ion transfer kinetics across the interface.Consequently,the electrode of SnO₂@3DC-MoS₂ VSHs nanocomposite can demonstrate the outstanding sodium storage performances.