Enhancing charge density around oxygen via electronic structure modulation to inhibit voltage fading in Na-based...

Na-based oxygen-active cathode materials are promising candidates to achieve high energy density sodium-ion batteries, due to their ultra-high specific capacity (>200 mAh g^-1) originated from the anion oxygen redox reaction. However, the rebarbative voltage fading issue during electrochemical process is difficult to solve, which largely reduces the energy density and hampers the practical application. Herein, an electronic structure modulation strategy is presented to address this issue by introducing Zr⁴⁺ into the lattice. By means of theoretical calculation, it is confirmed that introduction of Zr⁴⁺ into the lattice enhances the negative charge around oxygen atom, which improves the antioxidant ability and reduces the formation of oxygen vacancies. Advanced electron energy loss spectroscopy identifies severe oxygen release and reduction of Mn oxidation state in un-modified cathode. However, such process is significantly suppressed after modification. As a consequence, the modified cathode material exhibits largely promoted voltage retention with maintaining 95.10% at 0.2C for 40 cycles and 90.10% at 0.5C for 100 cycles, which represent a significant improvement compared to the control sample (82.00% at 0.2C and 72.59% at 0.5C). This work paves a way to control voltage fading by electronic structure modulation.