Na2CoSiO4 as a cathode material for sodium-ion batteries: structure, electrochemistry and diffusion pathways.

The importance of developing new low-cost and safe cathodes for large-scale sodium batteries has led to recent interest in silicate compounds. A novel cobalt orthosilicate, Na2CoSiO4, shows promise as a high voltage (3.3 V vs. Na/Na+) cathode material for sodium-ion batteries. Here, the synthesis and room temperature electrochemical performance of Na2CoSiO4 have been investigated with the compound found to yield a reversible capacity greater than 100 mA h g-1 at a rate of 5 mA g-1. Insights into the crystal structures of Na2CoSiO4 were obtained through refinement of structural models for its two polymorphs, Pn and Pbca. Atomistic modelling results indicate that intrinsic defect levels are not significant and that Na+ diffusion follows 3D pathways with low activation barriers, which suggest favourable electrode kinetics. The new findings presented here provide a platform on which future optimisation of Na2CoSiO4 as a cathode for Na-ion batteries can be based.