Lithium and sodium diffusion in solid electrolyte materials of AM2(PO4)3(A = Li, Na, M = Ti, Sn and Zr)

Abstract

Electrolytes with a high ionic conductivity are the prerequisite for the success of solid state rechargeable ion batteries. In this paper, density functional theory (DFT) calculations are applied in combination with a climbing-image nudged elastic band (CI-NEB) method to obtain the diffusion barriers of the lithium (Li) and sodium (Na) in stoichiometric AM 2( PO 4)3 (A = Li, Na, M = Ti, Sn and Zr) compounds. In the AM 2( PO 4)3, Li and Na ions occupy the interstitial sites, M1, which is coordinated by a trigonal antiprism of oxygen, and M2, which has a distorted eight-fold coordination. Results show that the diffusion barriers are closely related with the average bond length of A–O bond when the atom A occupies the M2 site in the AM 2( PO 4)3 compounds, and the diffusion barriers decrease with increasing the average bond length. Among the various types of the AM 2( PO 4)3 compounds, the AM 2( PO 4)3 has a good potential for solid electrolytes due to its low diffusion barriers for the Li and Na.