Al and Fe-containing Mn-based layered cathode with controlled vacancies for high-rate sodium ion batteries
Abstract: Mn-based layered oxides as one of the most promising and cost-effective cathode candidates for sodium-ion batteries still face great challenge to achieve high capacity with long cycle life under high-rate current simultaneously. In this work, we propose an effective strategy by a combination of liquid N2 quenching and aliovalent doping to get new layered cathode materials. As evidenced by in-situ synchrotron X-ray diffraction, time-of-flight powder neutron diffraction and solid-state 23Na nuclear magnetic resonance techniques, the proposed synthesis methods allow tuning the transition metal ions vacancies and enhance Mn4+/Mn3+ redox center of P2-type Mn-based materials. Our results demonstrate that such an optimized structure significantly enhances the deliverable capacity, Na+ mobility and electronic conductivity of the materials. Furthermore, the effects of aliovalent doping elements and different cooling approaches on the long-range structure, local environment and electrochemical performance are comprehensively studied by comparing a wide range of doped Na0.67MxMn1-xO2 (M = Li, Mg, Al, Fe) materials. The optimized Na0.67Al0.1Fe0.05Mn0.85O2 material exhibits a remarkably high initial capacity of 202 mAh g−1 among ever reported P2-type layered oxides within 2–4 V, a stable capacity retention of 81% after 600 cycles and outstanding rate capability of the specific capacity up to 122 mAh g−1 at 1200 mA g−1.