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.