Theoretical tuning of Ruddlesden–Popper type anti-perovskite phases as superb ion conductors and cathodes for solid sodium ion batteries

Abstract

It is very important and yet extremely challenging to develop solid-state electrolytes for safe sodium ion batteries, largely due to sodium ions being significantly larger than lithium ones. Here in this work we have carried out systematic modelling, using a materials genome approach in the framework of density functional theory (DFT), to formulate a new system of ion conductors and compatible cathodes. Through iso-valent substitution of both the anion and cation sites in a Na₄OI₂ compound with a Ruddlesden–Popper type phase based on sheets of anti-perovskite structural units, or an anti-Ruddlesden–Popper phase (ARP), we have identified a series of stable layer-structured phases, with the general formula Na_4−cLi_cAX₄ (A = O and/or S; X = I and/or Cl), as remarkable electrolytes and high capacity cathodes to enable solid sodium ion batteries. The optimized Na₃LiS_0.5O_0.5I₂ compound is a marvellous Na⁺ conductor, with an extremely low activation energy for Na⁺ transportation (0.12 eV) and a high Na⁺ conductivity of 6.3 mS cm⁻¹ at standard room temperature (298 K). This superb solid electrolyte does not react with the sodium anode, and formation of layer-structured phases due to its sodium depletion leads to compatible cathode materials with high voltage plateaus to enable full batteries with high energy densities.