Study of Synergistic Effects of Cu and Fe on P2-Type Na0.67MnO2 for High Performance Na-Ion Batteries

P2-type Na_0.67MnO₂ with a stable structure and an open framework can provide numerous channels for fast Na⁺ de/intercalation, for which it is considered to be advantageous in application of the cathode material for Na-ion batteries. However, the complex phase transition occurring during cycling and the lattice distortion triggered by the Jahn-Teller effect severely restrict its development. Herein, the modified Na_0.67MnO₂ with Cu or Fe single-element doping as well as Cu and Fe double-element doping was synthesized by the sol-gel method, and the effects of doping on the crystal structure and electrochemical performances of Na_0.67MnO₂ were studied. It was demonstrated that the phase of the material did not change after the introduction of Cu and Fe elements, and the cycling stability and rate performance were greatly improved by Cu and Fe double-doping owing to their synergistic effect. The Na_0.67Mn_0.92Fe_0.04Cu_0.04O₂ (NMFCO) cathode delivers discharge specific capacities of 110.5 mA h g^-1 at 5 C and 91.8 mA h g^-1 at 10 C and exhibits the high-capacity retention of 94.35% at 1 C and 90.68% at 5 C after 100 cycles. Overall, this study offers a guiding direction for accelerating the modification of P2-type Na_0.67MnO₂ as a cathode active material for high performance Na-ion batteries.