Lithium-ion Differential Thermal Analysis Studies of the Effects of Long-Term Li-ion Cell Storage on Electrolyte Composition and Implications for Cell State of Health

Li-ion cells being developed for long lifetime applications are often subjected to storage tests at various states-of-charge and various temperatures. Storage is interrupted from time to time for reference performance tests so that cell capacity and impedance can be checked. These reference performance tests give no information about any compositional changes that may have occurred in the electrolyte. Lithium-ion differential thermal analysis applied to cells after years of storage can be used to determine if the electrolyte has changed significantly due to unwanted reactions with the electrode materials or if little to no change has occurred. Here, Li-ion differential thermal analysis is used to study electrolyte changes in a more-or-less “yes/no” manner for single crystal NMC532/graphite cells stored between 3.67 and 4.3 V at 20, 40 and 55 °C for up to five years. Such measurements can be used to give confidence about lifetime predictions. Several such cells are detailed here, with correlation between degree of cell degradation and degree of change in electrolyte composition. Relationships are shown between degradation and evolution of state of electrolyte in elevated temperature and voltage storage experiments.