Superior Rate Capability and Cycling Stability in Partially Cation-Disordered Co-Free Li-Rich Layered Materials Enabled by an Initial Activation Process

Abstract

Li-rich layered materials that have Co-free and Mn-rich 3d-transition metals have the potential to increase the achievable energy density of batteries because they are inexpensive and yield high capacity by exploiting an additional oxygen redox reaction. However, these have low electrochemical activity and sustainability, with severe voltage fade, rapid capacity decay, and poor rate capability. Here, we report sustainable cycling stability and fast rate capability of Co-free Li2MnO3-based Li-rich layered materials that are governed by the electrochemical activation process during the 1st cycle and that this process can be controlled by the degree of the cation disordering in the pristine material. From the comparative study of two samples that have different degrees of cation disordering in the same composition, an increase in cation disordering in the pristine material strongly improves its tolerance to structural changes in the bulk and on the surface during the activation process at the 1st cycle, leading to less structural changes for subsequent cycles. As a result, high electrochemical activity and superior rate capability in subsequent cycles can be achieved even with the cation disordering in the pristine. Furthermore, we verified the findings by developing an additional material that had higher cation disordering in the pristine structure than the samples tested and showing that the additional sample has improved rate capability and cycle retention. This understanding that sustainable electrochemical characteristics are governed by an activation process in the 1st cycle, which can be controlled by a structural feature of the pristine material, will be useful in the design of low-cost, Li-rich layered materials that can achieve sustainable high energy density and fast rate capability for Li-ion batteries.