Aging-Driven Composition and Distribution Changes of Electrolyte and Graphite Anode in 18650-Type Li-Ion Batteries

A series of low-temperature studies on LiNi_0.80Co_0.15Al_0.05O₂ 18650-type batteries of high-energy type with different stabilized states of fatigue is carried out using spatially resolved neutron powder diffraction, infrared/thermal imaging, and quasi-adiabatic calorimetry. In-plane distribution of lithium in the graphite anode and frozen electrolyte in fully charged state is determined non-destructively with neutron diffraction and correlated to the introduced state of fatigue. An independent electrolyte characterization is performed via calorimetry studies on variously aged 18650-type lithium-ion batteries, where the shape of the thermodynamic signal is evolving with the state of fatigue of the cells. Analyzing the liquid electrolyte extracted/harvested from the studied cells reveals the decomposition of conducting salt to be the main driving factor for fatigue in the electrolyte degradation.