First principles investigation of the structural and electrochemical properties of Na4P2S6 and Li4P2S6

• Simulations show Kuhn's prepared Na4P2S6 (C2/m structure) to be meta-stable.
• Simulations find Na4P2S6 and Li4P2S6 to have the same disordered ground state structure.
• Geometric relationships between the stable and meta-stable structures of these materials are reported.
• Computed Na ion migration energies are generally more favorable than Li in these materials.

First principles simulations are used to examine the structural and physical properties of Na4P2S6 in comparison with its Li4P2S6 analog. Four model structures are considered including the C2/m structure recently reported by Kuhn and co-workers from their analysis of single crystals of Na4P2S6, and three structures related to the P63/mcm structure with P site disorder found in 1982 by Mercier and co-workers from their analysis of single crystals of Li4P2S6. The computational results indicate that both Na4P2S6 and Li4P2S6 have the same disordered ground state structures consistent with the P63/mcm space group, while the optimized C2/m structures have higher energies by 0.1 eV and 0.4 eV per formula unit for Na4P2S6 and Li4P2S6, respectively. In modeling ionic conductivity in these materials, activation energies for Na ion vacancy migration were computed to be smaller than the Li analogs in all of the structural models. Interestingly, the results also indicate that if Li4P2S6 could be prepared in the meta-stable C2/m structure, the Li ion vacancy migration would have very small barriers along certain channels. Simulations of Na4P2S6 (C2/m)/Na interfaces indicate that they may be slightly less reactive than Li4P2S6 (P63/mcm)/Li interfaces.