| 초록 |
Polyanionic compounds as positive electrode materials for rechargeable batteries have been widely investigated due to high operating voltage and stable crystal structure. Sodium polyanionic materials provide various structural frameworks as compared to lithium counterpart, 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-, and recently reported to be an attractive electrode candidate because of low volume changes upon cycling. While a Mn-based mixed anion phosphate shows a high redox potential of 3.8 V vs. Na+/Na, it has been suffered from low electrical conductivity and structural distortion induced by the Jahn-Teller Mn3+ ions upon oxidation. Partial substitution for Mn by other divalent cations in the mixed-anion phosphate structure is one of the efficient strategy to suppress lattice distortion and to affect phase transition process during electrochemical cycling. Here we study the cation substitution effect of the Mn-based mixed-anion phosphates on phase stability and the kinetics of electrochemical reaction. The electrochemical reversibility of the Mn-based mixed-anion phosphates is found to improve by certain cation substitution, in particular, cobalt substitution not only increased the operating voltage but also enhanced both cyclability and rate capability. We furthermore investigate the phase changes of the mixed-anion phosphates upon cycling through in situ synchrotron X-ray diffraction, supporting the positive effect of the mixed-anion phosphate structure by cation substitution. |
