EXTENDED LIFETIME AND INCREASED ENERGY DENSITY STUDIES FOR LI-ION CELLS

Abstract

Strategies to improve Li-ion cells involve decreasing cost, increasing lifetime, increasing energy density, and increasing safety. This thesis presents work from two studies aimed at extending cell lifetime and improving Li-ion cell energy density. The first study analyzes Li-ion cells throughout their lifetime to construct a timeline of performance, electrolyte composition, and electrode changes. The matrix included LiNi0.5Mn0.3Co0.2O2 (NMC532)/graphite and LiNi0.6Mn0.2Co0.2O2 (NMC622)/graphite pouch cells with excellent performing electrolyte mixtures, both cycling and storage protocols at 40 °C and 55 °C with both 4.3 V and 4.4 V upper cutoff potentials. This study found that under conservative conditions, minimal change to electrolyte and electrode compositions occurred. Many cell metrics and electrolyte transformations, such as fraction of transesterification, gas production, and transition metal dissolution, appeared to have a constant rate of change in this 12-month observation period. In most cases, results from cells after three to six months of testing could be used to reasonably estimate the status of the cells (electrolyte composition, gas production, etc.) at 12 months. The second work presented considers the cycling and performance effects due to higher electrode loadings. The short and long-term cycling results showed capacity retention and parasitic reactions comparable to regular loading cells. While cells were limited to low charge and discharge rates, such work indicates increased stack energy density is achievable via increasing electrode loading with minimal cycle life penalty. Impacts of the work presented in this thesis shows, under some conditions, that minimal change to electrolyte is observed in long-term aged cells and that high electrode loadings can increase the stack energy density without penalty.