Strategies to Understand Li-ion Cell Failure and Improve Lifetime

Abstract

The skyrocketing production of electric vehicles means fewer batteries for energy storage and a strain on Li, Ni, and Co metal supplies. Therefore, improving the lifetime of Li-ion cell chemistries is crucial to enable vehicle-to-grid applications that can support the grid, and maximizing the energy output of Li-ion cells. This thesis considers ways to understand Li-ion cell failure and improve the lifetime. First, the impact of cycling conditions on the lifetime of Li(Ni0.8Mn0.1Co0.1)O2 (NMC811) cells was studied, and various post-mortem characterizations tools were used to probe cell degradation. Second, we examined the role that the graphite negative electrode material plays in the lifetime of NMC811 cells, and proposed a cell design that combines competitive graphite materials with ideal cycling conditions for NMC811 to yield long-lived cells. Next, we studied the impact of electrolyte, Li excess, particle size, and NMC blending of the performance and degradation of Li1+xMn2-xO4 (LMO) cells at different temperatures. This led to the identification of an optimal LMO composition (i.e., x in Li1+xMn2-xO4), and development of mixed salt electrolyte systems that can hinder Mn dissolution and improve cell lifetime. Using this knowledge, we designed cells with large “single-crystalline” LMO particles and mixed salt electrolytes with competitive lifetime. Finally, we pivoted from the LMO system and used the mixed salt electrolytes to improve the high-temperature cycling performance of LiFePO4 cells.