Lithium Rich Layered Sulfides As a New-Type Cathode for Lithium Ion Batteries
Authors: Li Biao , Xia Dingguo , Ma Jin , Zuo Yuxuan , Yan Huijun , Wei Hang 
Published: February 2020

Recent years more and more attention are paid on the exploration of lithium rich high capacity electrodes since the growing demand for high energy density lithium ion batteries for electric vehicles. In addition to the well-known lithium rich manganese based layered oxides, several lithium rich systems were proposed like ruthenium, molybdenum, and niobium based layered oxides[1-4]. Based on these work, we studied the recipe for the high capacity layered oxide cathodes and summarize it to be three aspects, namely sufficient lithium ions, multiple electron process, and flexible structure, of which the importance for the stabilization of the layered structure was demonstrated in our recent work[5]. The multiple electron process is usually realized by both the redox of metal and oxygen in these lithium rich oxides.
However, though the oxygen ions can provide extra electrons accounting for the high capacity, excess oxidation of oxygen ions will result in destabilization of the structure and large irreversible capacity. Furthermore, the activation of the oxygen redox often happens at high voltage range, proposing quite high requirements for the electrolytes and leading to the unsafety concerns. Sulfur would be more superior choice than oxygen since polysulfide ions is more stable than peroxides or superoxides. On the other hand, sulfides have relatively lower voltage than oxides and provides access for the multiple electron process happening at a lower voltage range. The higher top of sulfur p state also will increase the electron mobility and thus the conductivity of the cathode. So the developing of lithium rich layered sulfides is a feasible route for the development of high capacity cathodes.
In this work, we have developed a kind of lithium rich layered sulfides Li1.2Ti0.6Co0.2S2 for the first time based on the layered Li2MnO3-like structure Li2TiS3. Layered Li2TiS3 can’t deliver substantial capacity since the sole redox of sulfur will destabilize the structure. However, after we introduced active transition metal ions cobalt in the layered structure to substitute Ti atoms, the compound Li1.2Ti0.6Co0.2S2 can deliver capacity of more than 200 mA h/g at 20 mA/g. The first charge can only deliver 100 mAh/g which is contributed only by sulfur redox. On the later discharge, the capacity was increased to 200 mA h/g with a new redox of Ti4+/3+ is activated. The as-prepared new lithium rich layered sulfides can contribute more than 150 mA h/g for 100 cycles. The average voltage is only 2.4 V and will offer large safety than existing cathodes. This work will open a new area for the developing of lithium rich high capacity electrodes.

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Figure 1