Towards low-cost, high energy density Li₂MnO₃ cathode materials

The extremely sluggish Li-ion diffusion rate at the activation plateau (∼4.5 V) in the initial charging is an activation bottleneck of Li₂MnO₃, which seriously restricts its discharge capacity (energy density) and rate performance. Herein, a targeted strategy is proposed where a few fluorines are substituted for oxygen (Li₂MnO_3-xF_x, x = 0.00, 0.015, 0.03, 0.045). F-substitution reduces the deintercalation barrier of Li⁺ from the crystal structure by weakening the Li-O bond, and improves the electronic transmission performance by inducing more Mn³⁺ and oxygen vacancies. Therefore, a major breakthrough is made for the activation bottleneck as the lithium-ion diffusion coefficient at 4.5 V is increased by more than two orders of magnitude. Meanwhile, F-ions inhibit the grain growth along the (001) direction and decrease the primary particle sizes. In the narrow electrochemical window of 2.0-4.6 V, a first discharge capacity of 299 mA h g^-1 with a high energy density of 934 W h kg^-1 at 0.05 C, and a capacity of 200 mA h g^-1 at 1 C with a retention ratio of 92% after 100 cycles are achieved.