Self-standing Li_1.2Mn_0.6Ni_0.2O₂/graphene membrane as a binder-free cathode for Li-ion batteries

Lithium-rich transition-metal layered oxides (LROs), such as Li_1.2Mn_0.6Ni_0.2O₂, are promising cathode materials for application in Li-ion batteries, but the low initial coulombic efficiency, severe voltage fade and poor rate performance of the LROs restrict their commercial application. Herein, a self-standing Li_1.2Mn_0.6Ni_0.2O₂/graphene membrane was synthesized as a binder-free cathode for Li-ion batteries. Integrating the graphene membrane with Li_1.2Mn_0.6Ni_0.2O₂ forming a Li_1.2Mn_0.6Ni_0.2O₂/graphene structure significantly increases the surface areas and pore volumes of the cathode, as well as the reversibility of oxygen redox during the charge-discharge process. The initial discharge capacity of the Li_1.2Mn_0.6Ni_0.2O₂/graphene membrane is ∼270 mA h g^-1 (∼240 mA h g^-1 for Li_1.2Mn_0.6Ni_0.2O₂) and its initial coulombic efficiency is 90% (72% for Li_1.2Mn_0.6Ni_0.2O₂) at a current density of 40 mA g^-1. The capacity retention of the Li_1.2Mn_0.6Ni_0.2O₂/graphene membrane remains at 88% at 40 mA g^-1 after 80 cycles, and the rate performance is largely improved compared with that of the pristine Li_1.2Mn_0.6Ni_0.2O₂. The improved performance of the Li_1.2Mn_0.6Ni_0.2O₂/graphene membrane is ascribed to the lower charge-transfer resistance and solid electrolyte interphase resistance of the Li_1.2Mn_0.6Ni_0.2O₂/graphene membrane compared to that of Li_1.2Mn_0.6Ni_0.2O₂. Moreover, the lithium ion diffusion of the Li_1.2Mn_0.6Ni_0.2O₂/graphene membrane is enhanced by three orders of magnitude compared to that of Li_1.2Mn_0.6Ni_0.2O₂. This work may provide a new avenue to improve the electrochemical performance of LROs through tuning the oxygen redox progress during cycling.