P2-Na_2/3MnO₂ by Co Incorporation: As a Cathode Material of High Capacity and Long Cycle Life for Sodium-Ion Batteries

The P2-Na_2/3MnO₂ compound is one of the attractive cathodes for sodium-ion batteries due to its high initial capacity and abundance of Na and Mn elements in nature. The existence of Mn³⁺ Jahn-Teller ion, however, impedes electrode performance for long term. Here, we challenge to minimize the effect of the Jahn-Teller distortion caused by Mn³⁺ in the structure, via substitution of Mn³⁺ by Co³⁺ in P2-Na_2/3[Mn_1-xCo_x]O₂ (x = 0-0.3). The P2-Na_2/3[Mn_0.8Co_0.2]O₂ compound substantializes the electrochemical performance with a capacity of about 175 mAh g^-1 (26 mA g^-1) and retained over 90% of its initial capacity for 300 cycles at 0.1 C (26 mA g^-1) and 10 C (2.6 A g^-1). The operando X-ray diffraction study indicates that a single-phase reaction is associated with the insertion of sodium ions into the structure, accompanied by a small volume change of approximately 3%. Furthermore, ex situ X-ray diffraction and high-resolution transmission electron microscopy results show that the crystal structure remained after 300 continuous cycles. It is believed that such good electrode performances attribute to the structural stabilization assisted by the presence of Co³⁺ in the crystal structure. Our finding provides a way to take advantage of low-cost Mn-rich cathode materials for sodium-ion batteries.