Benefits of Copper and Magnesium Cosubstitution in Na_0.5Mn_0.6Ni_0.4O₂ as a Superior Cathode for Sodium Ion Batteries

Transition metal oxides are considered to be one kind of the most promising cathode materials for sodium ion batteries. Here, P2-type Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 cathode material was designed and synthesized by a sol–gel method for the first time. The cosubstitution of copper and magnesium in Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 inhibits the P2–O2 phase transition and enhances lattice spacing to reduce the resistance of sodium ion deintercalation and intercalation, which is beneficial to the improvement of electrochemical properties. A Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 electrode delivers an initial specific capacity of 126.1 mAh g–1 with a high average voltage of 3.6 V (2–4.6 V) and 96.7% capacity retention after 100 cycles at 0.1C. More importantly, the full cells using this cathode material and hard carbon as anode exhibit initial reversible specific capacity of 70.8 mAh g–1 with energy density of 226.56 Wh kg–1 and high capacity retention of 90.1% after 200 cycles at 0.5C. Therefore, Na0.5Mn0.6Ni0.2Cu0.1Mg0.1O2 is able to be a cathode material with high operation voltage, good stability, high energy density, and excellent rate performance for sodium ion battery applications.