Durability Aspects of Fast Charging, Mechanical Constraint, and Inhomogeneity in Lithium-Ion Batteries

The development of lithium-ion batteries with higher energy and power density, better safety, and lower cost has significantly contributed to the increased market share of electric vehicles (EVs) in the last decade. However, the expectations of end-users of EVs still require a continuous quest for better performance. One important end-user expectation is the ability of the battery to be charged rapidly, but the durability of lithium-ion batteries could be affected by the fast charging. Hence, detailed investigations are required to understand the extent and mechanism of the degradation for an optimized battery usage and material development. In order to meet the high energy and power required in EVs, multiple large-format cells are connected in series and in parallel. Such a condition leads to an uneven distribution of temperature, pressure, and current in a cell or among cells that may cause locally inhomogeneous ageing and accelerate the global battery ageing. This thesis investigates the effects of charging rate, charging protocol, and external compression on battery durability. The impacts of inhomogeneities induced by cell design constraint, and uneven compression and temperature distributions are also addressed. The studies are based LiNi1/3Mn1/3Co1/3O2/graphite cells. Cell housing for a controlled pressure and temperature application was developed. Electrochemical and material characterization techniques were used in the investigation. The results show that fast charging at a rate equivalent to full charging in 20 minutes (3C rate) or less accelerates battery ageing. The ageing rate is less sensitive to charging rate in a longer charging time, i.e. at 2C and below, where it is determined more by factors such as the extent of full charging. In all cases, the capacity loss is limited by the cyclable lithium loss. External compression of a battery in an optimum range reduces ageing, but compression above or below the optimum range accelerates ageing. Lithium-ion batteries age non-uniformly. Cycling induces an increase in the impedance at the outer radius of curvature of a prismatic cell jellyroll, associated with a loss of contact between the current collector and the electrode coating. An unfavorable current distribution induced by uneven temperature distribution can accelerate battery ageing. QC 20180419