Na2S Treatment and Coherent Interface Modification of the Li-Rich Cathode to Address Capacity and Voltage Decay.

Abstract

Li-rich and Mn-based layered oxides are the most promising candidates for next-generation high energy density cathode materials. However, inherent problems including poor rate performance, continuous capacity degradation, and voltage fading hinder their commercial utilization. Herein, a lattice- and interfacial-modified Li1.2Mn0.54Co0.13Ni0.13O2 with a pristine-layered bulk structure, Na- and S-doped transition phase, and epitaxially grown Na2Mn (SO4)2 (C2/c symmetry) layer were constructed by Na2S treatment. The monoclinic Na2Mn(SO4)2 not only acts as an interface protective layer, alleviating the harmful electrode-electrolyte reactions, but also promotes formation of oxygen vacancy in the layered structure, enhancing reversibility of oxygen redox. The Na and S surface lattice doping leads to enhanced Li+ diffusion and alleviates the chance of oxygen release. With the positive effects provided by the stable interfacial layer and lattice modification, the modified cathodes with moderate Na2S treatment shows alleviated capacity and voltage decay and enhanced electrochemical kinetics. Especially, the washed cathode with 3 wt % Na2S treatment delivers a discharge specific capacity of 305 at 0.1 C and 219 mA h g-1 at 1 C, as well as 93.15% capacity retention and 88.20% voltage retention after 200 cycles at 1 C.