Hierarchical Reactions Involving Dynamic Structural Changes in Lithium-Ion Batteries during Overcharge Using Advanced Analytical Techniques

Abstract

Abstract High-Ni cathode materials are promising for lithium-ion batteries compared to conventional cathode materials because of relatively high energy density and low cost. High-Ni cathodes are inherently thermally unstable, and when used in large scale batteries, they can cause serious problems such as thermal runaway and associated destruction of the internal structure. For practical use, it is necessary to understand the internal phenomena when they are incorporated into large scale batteries. We examined the stacked electrode structural change inside pouch cells with approximately 1 Ah capacity during over charging, which includes LiNi0.8Mn0.1Co0.1O2, LiNi0.6Mn0.2Co0.2O2, and LiCoO2 athodes, by using synchrotron-based operando X-ray radiography. In addition, the bulk crystalline, electronic, and local structures of these materials as well as surface electronic structure changes during overcharging were also analyzed. We found that as the amount of Ni-content in the cathodes increase, destabilization of surface oxygen and structural phase transitions occur at smaller state of charge, leading to faster and more severe collapse of the stacked electrode structure inside the pouch cells. This finding of a relationship of hierarchical structural changes from the cathode surface to the pouch cell is useful for the safe use of lithium-ion batteries with high energy density containing high-Ni cathodes.