Basic Properties And Improvement Of Ionic Conductivity Of Ternary Sodium Chloride Solid Electrolyte

All-solid-state sodium-ion batteries（ASS-SIB）are one of the ideal alternatives to lithium-ion batteries due to their tremendous cost advantages.Like all-solid-state lithium-ion batteries,the key question of research on ASS-SIB is the development of solid electrolytes with high Na-ion conductivity and good stability.Recently,a class of ternary lithium halides,especially ternary lithium chlorides,has shown excellent performance as a solid electrolyte and has received wide attention,becoming an important direction in the research of solid electrolytes for lithium batteries.However,the study of ternary sodium chloride solid electrolytes is still in its initial stage,and only a few ternary sodium chlorides such as Na₃XCl₆（X=In,Sc）and Na₂Zr Cl₆ have been reported,and the intrinsic ionic conductivity is very low,so it is unclear whether they can be used as solid electrolytes.Based on this,the effect of structural modulation and heterovalent ion doping on the performance improvement of Na₃XCl₆（X=In,Sc）and Na₂Zr Cl₆ materials was investigated at the microscopic level by using first-principles calculations and strategies such as structural design and ion doping,and the transport mechanism of Na ions in these materials was revealed.The main research results are as follows:（1）Using the elemental substitution method,the structure of Na₃XCl₆（X=In,Sc）with different phases was systematically designed besides the experimentally synthesized structure,and its stability and basic physical properties were predicted.The computational results show that all predicted P3^—m1 and C2/m phases are stable in thermodynamics and kinetics.Secondly,calculations of the electronic structure,mechanical and thermodynamic properties reveal that among all phases of Na₃XCl₆（X=In,Sc）materials,the P3^—m1 phase exhibits outstanding fundamental properties with a wide electrochemical window（～2.6 V）,good mechanical ductility（B/G～2.0）and thermal conductivity（～1.3 W/（m·K）,at room temperature）.These excellent basic properties favor its use as a solid electrolyte for ASS-SIB.（2）The transport properties of Na ions in Na₃XCl₆ materials with C2/m and P3^—m1 phases were systematically investigated using BVSE and ab initio molecular dynamics.It is shown that the structure has a large influence on the transport of Na ions.The C2/m and P3^—m1 phases exhibit two-dimensional and three-dimensional Na ion diffusion channels,respectively,and the activation energies of the C2/m and P3^—m1phases are 0.27-0.33 e V and 0.20-0.26 e V,the ionic conductivity are 0.66-2.83 m S/cm and 3.09-11.54 m S/cm at room temperature,respectively.On the one hand,all these structures are superior to the experimentally synthesized P3^—1c and P2₁/n phases.On the other hand,the improvement of the transport properties of Na ions is more significant in the P3^—m1 phase compared to C2/m.（3）The effect of lower cost Al element doping on the basic properties and ion transport of Na₂Zr Cl₆ was investigated.The doped system Na_2+xZr_1-xAl_xCl₆（0≤x≤0.5）is shown to have high structural stability by formation energy calculations and molecular dynamics simulations.Electronic structure calculations show that although the band gap of the doped system is narrowed compared to the intrinsic band gap of4.5 e V,it still has a band gap of up to 3.7 e V when the doping concentration reaches50%,which can effectively impede electron conduction.On the other hand,thermodynamic calculations reveal that Al doping has almost no negative impact on the mechanical and thermal properties of the material.Molecular dynamics simulations show that the activation energy and the ionic conductivity at room temperature for all doped systems are 0.22-0.32 e V and 0.43-6.24 m S/cm,respectively,with very significant improvement in transport properties compared to the intrinsic material with 0.45 e V activation energy and 0.01 m S/cm conductivity,and the Na ions change from a one-dimensional diffusion channel along the c-direction to a three-dimensional channel.In addition,the system shows the best Al ion doping concentration at x=0.33,and the doped system has the lowest activation energy（0.22 e V）and the highest room temperature ionic conductivity（6.24 m S/cm）.In summary,structural design and heterovalent ion doping are effective strategies to improve the ionic conductivity of ternary sodium chloride,and this research idea will provide a reference and basis for the experimental preparation of high-performance sodium halide electrolytes.