Electrode properties of P2-Na_2/3Mn_yCo _{1- y}O₂ as cathode materials for sodium-ion batteries

The development of high-performance Na-ion intercalation electrodes has been required recently because Na-ion batteries hold much promise for inexpensive and efficient energy storage, which can be deployed in a power grid. For both optimization and better understanding of the electrode materials, it is indispensable to clarify the relationship between the electronic state and electrochemical properties systematically. In this work, we studied the electrochemical properties of P2-Na_2/3Mn_yCo _1-yO₂ in detail. A series of the P2 phases was successfully synthesized by the conventional solid-state reaction. The solid solution P2 compounds showed that the redox potential of Co⁴⁺/Co ³⁺ and Mn⁴⁺/Mn³⁺ shifts systematically by the transition-metal substitution. The charge-discharge cycle tests revealed that with increasing y the initial specific capacity increases while the cycle stability degrades. The origin for the cycle degradation was analyzed by the electrochemical impedance spectroscopy, which evidenced that the substitution of Co for Mn accelerates the formation of the passivating layer at the electrode surface.