反钙钛矿Li_3OX(X=F,Cl,Br)快离子导体的密度泛函研究

DFT investigations on antiperovskite Li_3OX(X=F, Cl, Br) superionic conductors

作为一种很有希望的固体电解质材料,新近发现的Li_3OX(X=F,Cl,Br)系列材料因较高的离子电导率和宽的电化学窗口受到广泛关注,然而众多研究指出,此类反钙钛矿结构材料相对于分解产物Li_2O和LiX,室温下为亚稳态。本工作利用第一性原理计算方法研究了Li3OX(X=F,Cl,Br)系列材料的晶体结构和电子结构,结果表明它们都具有立方晶胞和较宽的电化学窗口。进一步结合声子谱计算和准简谐近似方法研究了3种反钙钛矿材料的晶格动力学稳定性以及热力学稳定条件,结果表明,不同种卤素成分材料的热稳定性有较大差异,热力学稳定条件也不尽相同,Li_3OF在模拟的温度和压力范围内始终无法热力学稳定,同时高温利于Li_3OCl相的稳定,而Li_3OBr则倾向于在高压下热力学稳定,这些差异可能是由于卤素原子与氧原子的相对大小差异导致。模拟结果与实验合成Li_3OCl的条件定性符合,一定程度上解释了亚稳态Li_3OX反钙钛矿材料能够合成的原因。

As a family of promising inorganic solid electrolyte in csrystallized phase, the antiperovskite superionic conductor with general formula of LiaOX（X=F, C1,Br） received much attention since the day they were sncssesfully synthesized. However many researches pointed out that LiaOX is thermaldynamically metastable with respect to decomposition to Li2O and LiX. In this paper a comparative study is made between the members of this family by means of first-principle calculations with together the lattice-dynamic simulation under the quasi harmonic approximation. The result shows that they share the same atomic configuration with a cubic unit cell and a wide electrochemical window. However the Gibbs formation energy at different temperature and pressure reveals their distinct theromodynamic stability wich may root in their different radius ratios to the oxygen atom. The simulation results are in qualitive agreement with the experimental condition under which the Li3OCl are synthesized and explain the reason why the thermodynamically meta-stable antiperovskite LiaOX can exist to some extend.