Interface-rich mixed P2 + T phase NaxCo0.1Mn0.9O2 (0.44 ≤ x ≤ 0.7) toward fast and high capacity sodium storage

Abstract

In this work, we report a novel design of mixed P2 + T phase Na_xCo_0.1Mn_0.9O₂ (0.44 ≤ x ≤ 0.7) with interface rich characteristics for high performance sodium storage, whereby the tunneled T phase offers fast Na ion diffusivity and excellent structural stability, the layered P2 phase contributes to high specific capacity, and the P2 + T phase interface offers additional channels and active sites for charge transfer and excess charge storage. Consequently, the as-prepared Na_xCo_0.1Mn_0.9O₂ (0.44 ≤ x ≤ 0.7) demonstrates an excellent discharge capacity of 219 mA h g⁻¹ at a current rate of 0.1C (1C = 176 mA g⁻¹), and retains 117 mA h g⁻¹ even at a high rate of 5C. This outstanding performance is significantly superior to that of pure T-type Na_0.44Co_0.1Mn_0.9O₂ and P2-type Na_0.7Co_0.1Mn_0.9O₂, and also outperforms state-of-the-art manganese-based cathodes. The kinetic analysis indicates the drastically improved Na⁺ diffusion coefficient of the P2 + T phase, which is 6 and 200 times that of the pure T and P phases, respectively. The good reversibility and structural stability is evidenced through an ex situ XRD study.