A dual-modification strategy for P2-type layered oxide via bulk Mg/Ti co-substitution and MgO surface coating for sodium ion batteries

Abstract

P2-type materials are regarded as competitive cathodes for next generation sodium ion batteries. However, the unfavorable P2 → O2 phase transition usually leads to severe capacity decay. Moreover, the cathode material always suffers from destruction of surface crystal structure caused by trace amount of HF. In this study, a dual-modification method containing Mg/Ti co-doping and MgO surface coating is designed to solve the defects of P2-type Na_0.67Ni_0.17Co_0.17Mn_0.66O₂ cathode. Results turn out that the P2 structure can be stabilized via Mg/Ti co-substitution and MgO layer could effectively prevent the surface from corroding by HF and promote migration of Na⁺. Moreover, the as-prepared MgO-coated Na_0.67Ni_0.17Co_0.17Mn_0.66Mg_0.1O₂ exhibits improved electrochemical performance than the raw material. It delivers 111.6 mAh g^-1 initial discharge capacity and maintains 90.6% at high current density of 100 mA g^-1 within 2-4.5 V, which has been obviously enhanced than that of Na_0.67Ni_0.17Co_0.17Mn_0.66O₂. The significant improvement can be attributed to the synergistic effect of Mg/Ti co-substitution and MgO surface coating. This dual-modification strategy based on the synergetic effect of Mg/Ti co-doping and MgO surface coating might be a resultful step forward to develop cathode materials for sodium ion batteries.

Keywords: Electrochemical performance; Mg/Ti co-substitution; MgO surface coating; Sodium ion batteries.