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

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 antiRuddlesden- Popper phase (ARP), we have identified a series of stable layer-structured phases, with the general formula Na4-cLicAX4 (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 Na3LiS0.5O0.5I2 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.