First-principles calculation of the stabilities of LMP/LAMP lithium superionic conductors against sodium-ion exchange in seawater

TL;DR

This study uses first-principles electronic structure calculations to evaluate the stability of lithium superionic conductors against sodium-ion exchange in seawater, revealing how composition and cell volume influence resistance.

Contribution

It provides novel insights into how substituting elements in lithium phosphate compounds enhances their stability against sodium exchange in seawater environments.

Findings

Resistance to sodium exchange increases with decreasing cell volume.

Only hypothetical LiSi2(PO4)3 is stable against sodium exchange among pure LMP compounds.

Aluminum substitution improves stability, enabling stabilization of LAMP compounds against sodium exchange.

Abstract

Electronic structure calculations carried out to estimate the free energies of Na(aq)+ exchange for lithium in LiM2(PO4)3 (LMP, M=Si4+,Ge4+,Ti4+,Sn4+,Zr4+) and Li1+xAlxM2-x(PO4)3 (LAMP; M=Ge4+,Ti4+, between 0 and 0.5) compounds in seawater. The calculations show that resistance to sodium-ion exchange increases with decreasing cell volume. For the pure LMP compounds, only the hypothetical LiSi2(PO4)3 is predicted to be stable against sodium ion exchange in aqueous solution. The calculations indicate that increasing the extent of Al3+ substitution for M4+ in the LAMP compounds increases the resistance to exchange, and that both LAGP and LATP can be stabilized against sodium exchange for x greater than or equal to approximately 0.5 Li1+xAlxM2-x(PO4)3.