Multifunctional surface structure with enhanced electron and ion conductivities for superior electrochemical performance of Co-free Li-rich Mn-based layered oxide cathodes

Abstract

Co-free Li-rich Mn-based layered oxides (LRMLOs) are attractive cathode materials for lithium-ion batteries due to their low cost, high energy density and environmental friendliness. However, they also suffer from the drawbacks of low initial Coulombic efficiency, rapid voltage fading and capacity decay, and poor rate performance as the general LRMLO cathode, which prevent their practical applications. In the present work, a multilayer surface coating composed of Li2B4O7/B2O3 and a spinel heterostructure rich-in oxygen vacancies are in-situ formed at the surface of the Co-free LRMLO particles by a facile thermal reduction treatment with NH4B5O8. The multilayer surface improves greatly the electron and ion conductivities, and plays significantly synergistic roles on suppressing oxygen release, reducing the dissolution of Mn and Ni elements, retarding the structure transformation from layered to spinel, and preventing the side reaction of the active material with electrolyte during cycling for the cathodes. As a result, the overall electrochemical properties of the cathodes are significantly improved. With an optimized addition of 10 wt% NH4B5O8, the initial Coulombic efficiency of the cathode is as high as 90.0%, and a capacity of 275 mAh g-1 is obtained at 0.1 C (1 C = 200 mA g-1) with the capacity retention of 98.5% after 100 cycles. A capacity of 194 mAh g-1 is maintained after 300 cycles at 1 C, with the capacity retention of 92.8%. A capacity of 117 mAh g-1 is obtained at a high rate of 20 C. In addition, the voltage fading during cycling is also greatly inhibited. The structure evolution of the cathode material during cycling is systematically investigated and the mechanism for the improved electrochemical properties is proposed. The present work provides a new surface structure design strategy and the method for enhancing the overall electrochemical performance of Co-free Li-rich Mn-based layered oxide cathodes.