Designing and Modifying of Lithium-Rich Layered Compound Based on Anion Redox
Authors: Xia Dingguo , Li Biao , Ma Jin , Zuo Yuxuan , Yan Huijun 
Published: February 2020

Lithium ion batteries have been increasingly urged with high energy density and long cycle life to meet the increasing requirements for portable electronics, use of renewable energy and electric vehicles. The availability of the appropriate cathode material is crucial for the development of lithium ion batteries with high energy density. Lithium rich manganese-based layered oxides (LRMO), proposed by Amine, are considered to be the next generation promising cathode material owing to their high capacity and low cost. However, capacity fading and voltage decay hinder its application in practice. To overcome these problems, tremendous efforts have been made to realize electrochemical performance improvement, including partial element doping and the coating of active materials. The high capacity of LRMO is contributed by multiple electron process consisted of metal and oxygen redox. Many previous work demonstrated that the active oxygen redox is caused by the overlap between O 2p band and transition metal (TM) 3d band. Though providing extra capacity, the oxygen redox also brings the unstability of layered structure when gives excess electrons out.
In our works, we proposed a general strategy to tune the electronic structure of layered oxides to improve the cycle performance by introducing polyanions into the local structure. We incorporated boron atoms into LRMO structure and found that boron atoms show high covalence with oxygen than metal atoms. This undermined the covalence of TM-O bond and lowered the top level of oxygen 2p band, which decreased overlap area between TM 3d band and O 2p band. We at last demonstrated a better LRMO cathode with higher stability (capacity retention is 94% at 20 mA/g for 80 cycles).
The oxygen redox is essential for the multiple electron process providing high capacity. However, anionic redox has also been proved in conventional layered oxides like LiCoO2 and LiNi1/3Co1/3Mn1/3O2, but they show terrible stability and deliver capacity less than 200 mA h/g though with high theoretical capacity of about 270 mA h/g. This provokes a question about why these two types of layered oxides show different stability though with similar layered structure. In order to figure it out, we took a series of in situ investigation to study the geometric and electronic structural evolution of Li2RuO3 during charging/discharging, a typical lithium rich layered oxide with both of cation and anion redox. We found that lithium rich oxides show higher stability than conventional layered structure due to the superlattice structure. The extra lithium atoms in the transition metal layer will make the structure flexible and facilitate the distortion caused by oxygen redox upon deep delithiation. This enhances the stability of layered structure during lithiation and delithiation.
Anionic redox can be deemed as a double-edged sword since it can provide extra capacity in the price of structural stability. Only when the anions give proper amounts of electrons can ensure high capacity and acceptable stability. Therefore, controllable anionic redox is required to develop optimal cathodes. We presented a work by doping different amounts of boron atoms in Li2RuO3. We found that Li2RuBxO3 (B=0, 0.02, 0.05, 0.1) show distinctive charge/discharge behavior since boron atoms can tune the electronic structure of oxygen and change the redox path. The obtained materials show different reversibility due to the variant redox path. This work is of great importance for the instruction of modification of such lithium rich layered oxides.
Furthermore, anionic redox is instructive for the designing of new kinds of lithium rich high capacity electrodes. As studied in our work, we aware that the recipe of high capacity electrodes is multiple electron process, flexible structure and enough lithium ions. In addition to oxygen, sulfur is probably accessible to the multiple electron process as well. Hence, we developed lithium rich layered sulfides with high capacity of 200 mA h/g, which has a lower voltage and higher stability and safety compared with LRMO, and mild voltage decay is presented. This work will open a new scope for the development of high capacity electrodes.
In summary, anionic redox is mysterious and needs to be deeply investigated, and it provides us a new direction for the modifying and designing of cathode materials. From oxygen to sulfur to selenium to tellurium, anions give amounts of possibilities for the developing of high capacity electrodes.