Reaction Mechanism of Na-Ion Deintercalation in Na2CoSiO4

Sodium transition metal silicates are potential candidate electrode materials to enable two-electron redox per transition metal ion center. Yet, the electrochemical reaction mechanism remains elusive despite the widely reported electro-chemical activity for this class of materials as intercalation cathodes for Na-ion batteries. Adopting monoclinic Na2CoSiO4 as a model compound, we used high-resolution synchrotron X-ray diffraction (XRD) and X-ray pair distribution function (PDF) analysis to elucidate the structure of the partially desodiated Na2-xCoSiO4 phases for the Co3+/Co2+ redox couple. The appearance of satellite reflections in the intermediate Na1.5CoSiO4 and NaCoSiO4 phases manifests the formation of modulated structures, which are induced by Na+/vacancy and Co2+/Co3+ charge orderings. Accounting for these structural orderings is important to understand the function and performance of sodium transition metal silicate electrodes.