Phase stability, electrochemical stability and ionic conductivity of the Li[subscript 10±1]MP[subscript 2]X[subscript 12] (M = Ge, Si, Sn, Al or P, and X = O, S or Se) family of superionic conductors

Abstract

We present an investigation of the phase stability, electrochemical stability and Li[superscript +] conductivity of the Li[subscript 10±1]MP[subscript 2]X[subscript 12] (M = Ge, Si, Sn, Al or P, and X = O, S or Se) family of superionic conductors using first principles calculations. The Li[subscript 10]GeP[subscript 2]S[subscript 12] (LGPS) superionic conductor has the highest Li[superscript +] conductivity reported to date, with excellent electrochemical performance demonstrated in a Li-ion rechargeable battery. Our results show that isovalent cation substitutions of Ge[superscript 4+] have a small effect on the relevant intrinsic properties, with Li[subscript 10]SiP[subscript 2]S[subscript 12] and Li[subscript 10]SnP[subscript 2]S[subscript 12] having similar phase stability, electrochemical stability and Li[superscript +] conductivity as LGPS. Aliovalent cation substitutions (M = Al or P) with compensating changes in the Li[superscript +] concentration also have a small effect on the Li[superscript +] conductivity in this structure. Anion substitutions, however, have a much larger effect on these properties. The oxygen-substituted Li[subscript 10]MP[subscript 2]O[subscript 12] compounds are predicted not to be stable (with equilibrium decomposition energies >90 meV per atom) and have much lower Li[superscript +] conductivities than their sulfide counterparts. The selenium-substituted Li[subscript 10]MP[subscript 2]Se[subscript 12] compounds, on the other hand, show a marginal improvement in conductivity, but at the expense of reduced electrochemical stability. We also studied the effect of lattice parameter changes on the Li[superscript +] conductivity and found the same asymmetry in behavior between increases and decreases in the lattice parameters, i.e., decreases in the lattice parameters lower the Li[superscript +] conductivity significantly, while increases in the lattice parameters increase the Li[superscript +] conductivity only marginally. Based on these results, we conclude that the size of the S[superscript 2−] is near optimal for Li[superscript +] conduction in this structural framework.