Electrochemical and thermal features of aging lithium-ion batteries cycled at various current rates

In the present work, three types of battery samples with 90% state of health (SOH) are prepared, to demonstrate the electrochemical and thermal features of aging lithium-ion batteries after the long-term cycling at various rates (1C, 2C and 4C), by reference to the fresh battery. With the aging of batteries, battery capacity declines stably; meanwhile, the phenomena of electrolyte decomposition and lithium depositing on the electrode materials' surface gradually prominent. As a result, the thermal stability of both electrode materials scraped from aged batteries decreases compared with the fresh battery, which further worsens the thermal runaway behavior of full batteries when over-heated. Moreover, the severity aggravates as the current rate increases. In addition, a more severe thermal runaway phenomenon is also demonstrated by the aged batteries. A parameter (α) is defined here to determine the advancement ratio of the ignition time and thermal runaway time during thermal runaway testing for the aged batteries, referred by the fresh battery. In virtue of the rapid evolution of thermal runaway, the aged batteries experience a gentler mass loss during this process, especially for the 4C cycled one.