Designing an advanced P2-Na0.67Mn0.65Ni0.2Co0.15O2 layered cathode material for Na-ion batteries

Abstract

A high performance layered P2-Na_0.67Mn_0.65Ni_0.2Co_0.15O₂ cathode material for sodium ion batteries with high rate capability and excellent long-life cyclic performance has been successfully designed and synthesized by a simple sol–gel method. In comparison with the reported Na_0.7MnO₂, the designed P2-Na_0.67Mn_0.65Ni_0.2Co_0.15O₂ cathode material can be charged and discharged in an extended voltage range of 1.5–4.2 V and shows reversible capacities of 155, 144, 137, 132 and 126 mA h g⁻¹ at different current densities of 12, 24, 48, 120 and 240 mA g⁻¹, respectively. Even at high current densities of 480 (2C), 1200 (5C) and 1920 mA g⁻¹ (8C) it can still deliver capacities of 117, 93 and 70 mA h g⁻¹, respectively, which are much higher than those of the recently reported Na_0.5[Ni_0.23Fe_0.13Mn_0.63]O₂. In addition, the Na_0.67Mn_0.65Ni_0.2Co_0.15O₂ cathode material also displays an excellent capacity retention ca. 85% and 78% after 100 cycles at 0.05C and 0.5C, respectively. It is also proposed that Mn⁴⁺ may be “activated” in a low voltage range, especially below 2.0 V, which contributes to the additional capacity. The Na-ion diffusion coefficient, D_Na+, is ca. 10⁻¹⁴ cm² s⁻¹ as calculated by the PITT and the discharge diffusion coefficient is a little larger than the charge one. The designed Na_0.67Mn_0.65Ni_0.2Co_0.15O₂ shows great potential as a cathode material for sodium ion batteries.