Tuning oxygen redox chemistry of P2-type manganese-based oxide cathode via dual Cu and Co substitution for sodium-ion batteries

Abstract

High-voltage layered Mn-based oxide is promising high-capacity cathode via exploiting the oxygen redox chemistry. But such redox often suffer from irreversible oxygen loss, leading to severe voltage hysteresis, particle cracking and capacity decay. Hence, the complementary strategy of dual Cu and Co substitution is revealed to stabilizing the anion redox chemistry of P2-Na2/3(Mn-Ni-Cu-Co)O2 cathode under upper cut-off voltage. Dual substitution with Cu for Mn and Co for Ni forms stable Cu-O and Co-O octahedrons, which modulate the lattice structure with the shrinkage of TMO2 sheets and expansion of Na layer spacing to enhance the Na+ diffusion kinetics. Co substitution can raise high voltage region to tune the cut-off voltage up to 4.3 V and activate oxygen redox to weaken irreversible O2 loss. And Cu substitution can inhibit the O2 loss and suppress the voltage decay via the interaction between Cu 3d and O 2p orbitals. The P2-type Na0.67Mn0.6Ni0.2Cu0.1Co0.1O2 cathode induced by the complementary effect can boost long cycling life (82.07% capacity retention after 500 cycles) and remarkable rate capability under high upper cut-off voltage in half-cell. Furthermore, the full cells also exhibit outstanding rate capability with a high discharge capacity of 62.6 mAh g–1 at 20 C.