Understanding the different aging trends of usable capacity and mobile Li capacity in Li-ion cells

摘要

While the usable capacity is limited by the internal resistance, the available amount of mobile Li at open circuited voltage (OCV) conditions is not influenced by any change in internal resistance over aging. The difference in the aging trends of usable capacity and available amount of mobile Li at OCV conditions is analyzed.A method is suggested and validated in order to determine the available amount of mobile Li using a standard characterization procedure without constant voltage (CV) step at discharge. By applying the method to calendrical aged cell tests, aging trends for mobile Li and usable capacity are analyzed.Additionally, a model concept is presented, which explains the decrease of internal resistance at the end of discharge and therefore, the different aging trends. Considering lithiation depended over-potentials for anode and cathode and change in electrode balancing due to loss of mobile Li, it can be shown that the internal resistance at end of a constant-current (CC) discharge is decreasing with loss of mobile Li. The model concept is parametrized with half-cell measurements and validated with the previously analyzed calendric aging tests.