First Principles Study on Phase Stability in Li-Rich Layered Li2MnO3-LiMO2 (M=Mn, Ni, Co)

Lithium–rich layered oxides Li₂MnO₃-LiMO₂ have attracted much attention as cathode materials in Li-ion battery recently because of their higher capacity (larger than 250 mAh/g), increased energy density and structural stability at high voltage. However, in order to be commercialized, there are still some unclear issues (structure complexity and reaction mechanisms) and challenges (first-cycle irreversible capacity loss, poor rate performance and voltage fade with cycling which is related to structural evolution).

Regarding their structure complexity, there is significant debate in the literature so far on whether these Lithium–rich layered cathode material form homogeneous solid solutions or Li₂MnO₃ nano-composite within a LiMO₂matrix. It is important to get a clear picture of the actual detail structure, to understand and control their electrochemical performance. And without a detailed knowledge of the structure, it will be problematic to further explore failure mechanisms with cycling.

Recently in our research, the atomic order in Li-rich layered composites with composition xLi₂MnO₃-(1-x)LiMO₂ is investigated with first-principles calculations using Generalized Gradient Approximation (GGA) +U method. The random solid solution structure model between Li₂MnO₃ and LiMO₂ are created by Special Quasirandom Structures (SQSs) theory. After calculating the mixing energy, the miscibility gap is achieved and phase separation between Li₂MnO₃ and LiMO₂ are predicted. Further study on the coherency strain energy shows that the strain energy is relatively small compared to mixing energy. Phase separation cannot be inhibited by strain.