A study on cobalt substitution in sodium manganese mixed-anion phosphates as positive electrode materials for Na-ion batteries

Sodium polyanionic materials provide various structural frameworks compared to their lithium counterparts, leading to diverse studies on novel polyanion-based electrode materials for sodium ion batteries. Na4M3(PO4)2P2O7 (M = Mn, Fe, Co, Ni, and Mg) is a new class of mixed-anion phosphates combined with two polyanion groups, (PO4)3− and (P2O7)4−. It is an attractive electrode candidate because of its stable cyclability and low volume changes upon cycling. Although Mn-based mixed-anion phosphate materials have shown a high redox potential of 3.8 V vs. Na+/Na, they have suffered from low electrical conductivity and structural distortion upon oxidation. Partial substitution of Mn by other divalent cations is an efficient strategy to suppress lattice distortion and to affect the phase transition process during electrochemical cycling. Here we study the cation substitution effect of the Mn-based mixed-anion phosphates on the phase stability and the kinetics of the electrochemical reaction. The electrochemical reversibility was found to be improved by certain cation substitution. In particular, Co substitution not only increased the operating voltage but also enhanced both cyclability and rate capability. Furthermore, we investigate the phase changes upon cycling using in situ synchrotron X-ray diffraction, supporting the positive effect of the mixed-anion phosphate structure through cation substitution.