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 Li_1.2Mn_0.54Co_0.13Ni_0.13O₂ with a pristine-layered bulk structure, Na- and S-doped transition phase, and epitaxially grown Na₂Mn (SO₄)₂ (<i>C</i>2/<i>c</i> 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.