Identifying the effect of fluorination on cation and anion redox activity in Mn based cation-disordered cathode.

Li-rich disordered rock-salt cathode (DRX) materials with advantage of low cost, long cycle life, nature abundant resource and high power and energy density attracted a great deal of scholarly attention. However, the poor cycle stability and the unclear realization of cation and anion redox activity in low-cost element system have severely hindered the construction of high-performance DRX. Herein, a promising class of Ti-Mn based cathode materials Li1.25Mn0.25Nb0.25Ti0.25O2 and Li1.25Mn0.25Ti0.5O1.75F0.25 were designed and successfully synthesized to construct high energy density DRX and investigate the effect of fluorination on cation and anion redox activity. The results show that both fluoridized and unfluoridized DRX possess a similar structure (Fm-3 m), but distinctly different charge/discharge profiles. The fluoridized cathode shows high initial charge/discharge capacity of 317.3/283.9 mAh g-1, specific energy density of 1370.4/735.5 Wh kg-1 and stable capacity retention with a discharge capacity of 202.6 mAh g-1 after 20 cycles at 20 mA g-1. Combining relevant spectroscopic results and HRTEM images, we revealed that the excellent cyclability of Li1.25Mn0.25Ti0.5O1.75F0.25 is rooted in the weakened adverse effects of moderated oxygen redox and the reduced Jahn-Teller distortion effect resulting from Mn3+, endowing the fluoridized DRX with better structural stability and larger Mn2+/Mn4+ reservoir. The strategy of constructing low cost oxyfluoride and the understanding of the mechanism of fluorination induced cation and anion redox activity would provide reference for the development of high-performance DRX materials.

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

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.