Theoretical Study Of Ion Transport Properties In Na₁₁M₂PS₁₂-type Solid-state Electrolyte

All-solid-state batteries have the advantages of high energy density,long cycle life and high safety,and have become a current research hotspot.Solid-state electrolyte is the core of all solid-state battery,so the key to the development of all-solid-state battery is the design of solid-state electrolyte with high ionic conductivity.However,designing solid-state electrolytes with high ionic conductivity requires a comprehensive understanding of their determinants as well as the transport mechanisms.Experiments show that the Na₁₁Sn₂PS₁₂superionic conductor has a high room-temperature sodium ion conductivity,which is the most valued performance as a solid-state electrolyte.In addition,its other properties are relatively stable,and has become a new generation of solid electrolyte research hotspot.Subsequent theoretical studies of such electrolytes suggest that there is polyanion rotation in Na₁₁Sn₂PS₁₂electrolytes and that this rotation is coupled to cations,thereby promoting cation migration.The mechanism by which such rotation promotes cation transport is called paddlewheel mechanism or cog-wheel mechanism,which can be vividly understood as the paddle wheel rotation drives cation transport.But there are very few studies on the attribution of this rotation.Inspired by the improvement of ion conductivity by isovalent ion doping in lithium ion batteries,we performed ab initio molecular dynamics（AIMD）simulation on the Na₁₁M₂PS₁₂（M=Sn,Ge,Si）phase,where M represents the substitution of IV-A group elements.The ab initio molecular dynamics simulations of the three materials at 300 K,600 K,750 K,900 K,1050 K and 1200 K,respectively.Analysis of the motion trajectory found that the conductivity of the three materials showed a trend of Na₁₁Sn₂PS₁₂<Na₁₁Ge₂PS₁₂<Na₁₁Si₂PS₁₂at high temperature,while the trend was opposite at low temperature Na₁₁Sn₂PS₁₂>Na₁₁Ge₂PS₁₂>Na₁₁Si₂PS₁₂.Further analysis found that PS₄polyanion rotation existed in these materials,but MS₄polyanion hardly rotated.Combined time correlation function and sodium ion transport quantity statistics and charge-fluctuation analysis,the results show that the transport of sodium ion is directly related to the charge-fluctuations of MS₄polyanion.Inspired by the relationship between energy variance and hot capacity,the similar relationship between charge-fluctuation and differential capacitance is found,and the concept of differential capacitance is first applied to the simulation of solid electrolyte.Among the three materials,the MS₄polyanion itself and the differential capacitance between it and sodium ion are different,which can promote the rotation of PS₄and accommodate more sodium ion transmission in it,thus affecting the level of electrical conductivity.These studies can provide a basic and comprehensive understanding of the conductivity differences of Na₁₁M₂PS₁₂-type materials,and is expected to provide new ideas for the optimization and design of solid-state batteries.