Sodium-Storage Mechanism And Electrochemical Performances Of Tin-based Microparticles

With the rapid development of portable electronics,electric vehicles,and smart grids,there is an increasing demand for electrochemical energy storage devices with high energy density,environmental friendliness,and safety.Although lithium-ion batteries(LIBs)are the current mainstream equipment in energy storage devive,the limited reserves and uneven distribution of lithium resources in nature make them unable to meet the demand of market right now.Therefore,sodium-ion batteries(SIBs)are back in the spotlight in the post-LIBs era.Compared with LIBs,SIBs have many advantages,such as:(1)Sodium salts are abundant in crust and ocean,which greatly reduce the material costs.(2)Because the Stokes diameter of Na+in the electrolyte is smaller,a lower concentration of the sodium salt could provide a high ionic conductivity.Therefore,it is possible to use a low concentration of sodium salt in the electrolyte.(3)Since Na and A1 do not form an alloy,Al can be used as the current collector of negative electrodes.Compared with the case of Cu foil,using Al foil as the current collector not only reduces the types of electrode materials,but also lowers material cost and battery weight.Although SIBs have many advantages,they also face many problems.One of them is the higher standard reduction potential of Na+/Na(-2.71 V vs SHE)compared with Li+/Li(-3.04 V vs SHE),resulting in a lower energy density of SIBs.Therefore,searching anode materials with a high theoretical specific capacity and a low operating voltage is crucial for improving the energy density of SIBs.Sn is considered as one of these anode materials due to its high theoretical specific capacity(847 mAh g-1),low operating voltage(～0.3 V),and good electrical conductivity.However,similar to other alloy mechanism materials,Sn suffers a huge volume change(～420%)during the sodiation/de-sodiation process,which results in severe particle rupture,inevitable capacity loss,and poor cycling stability.In order to solve these problems,numerous strategies have been proposed,such as size controlling down to nanometers,preparation of porous structures,and carbon coating/encapsulation.Albeit these strategies made marked progress,they bring forth new issues,such as a low initial coulombic efficiency,a low tap density,and a high production costs.The use of microparticles can effectively mitigate these problems,but there are very few strategies that can be applied to microparticles,especially for tin-based materials.Because of their extremely low melting point,size control,structural control,and carbon coating of Sn-based microparticles are difficult.In this paper,several novel strategies are proposed to improve the sodium-storage performance of tin-based microparticles.The in-depth mechanism is explored by theoretical calculations and in-situ characterization techniques.The details are shown as follows:(1)Voltage-modulated structural stress for enhanced electrochemical performances.Electrode characterization,density functional theory(DFT)and finite element analysis(FEA)show that the crucial phase transition of Sn/NaSn3 is an important cause of surface cracks,particle aggregation and battery failure.Therefore,eliminating this phase transition by controlling the voltage window successfully extends the cycle life of μ-Sn from～40 cycles to 2500 cycles(2 A g-1).Moreover,this strategy is applicable to other alloy materials,such asμ-Bi in potassium-ion batteries.This strategy provides a simple method to achieve excellent performance without complicated preparation procedures,expensive reagents and laborious handling.(2)Micron-sized α-Sn was used as anode material for sodium-ion batteries,and its sodium storage mechanism was revealed.Compared with the case of commonly used β-Sn,the unique crystal structure of α-Sn makes it the relatively smaller volume change,smaller energy barrier for sodium intercalation,stronger binding with polymer binders,and more uniform stress distribution.More importantly,α-Sn undergoes amorphous transformation during charging and discharging,which promotes the uniform distribution of stress.Therefore,α-Sn exhibits excellent electrochemical performance,which is much superior to the previously widely used β-Sn.Even α-Sn microparticles without any treatment can maintain a capacity of 451 mAh g-1 after 3500 cycles at a current density of 2 Ag-1.In rate performance,its capacity can still remain at 464 mAh g-1 at a large current density of 4 A g-1.The full cell assembled with the NVP/rGO cathode can maintain a specific capacity of 458 mAh g-1(based on anode)at a current density of 1 A g-1 after 200 cycles.(3)SnBi binary metal microspheres(μ-SnBi)were prepared by heat-assisted sonication at low temperature.This material combines the advantages of Sn(high specific capacity)and Bi(high rate and long cycle performances),and exhibits extremely excellent sodium storage performance.More importantly,the initial coulombic efficiency is as high as 90.6%.The reversible capacity remains at 541 mAh g-1 after 3000 discharge/charge cycles at a current density of 2 A g-1.And it has a relatively outstanding rate capability.The specific capacity at 10 A g-1 is about 393 mAh g-1.At the same time,it can be stably cycled for 200 times at a high loading of 7.6 mg cm-2,and the area specific capacity is 3.7 mAh cm-2.The pouch battery with a capacity of 12 mAh prepared with NVP/rGO as the positive electrode andμ-SnBi as the negative electrode can maintain a reversible capacity of 364 mAh g-1 after 100 cycles at a current density of 0.1 A g-1.