Aging effect on the variation of Li-ion battery resistance as function of temperature and state of charge

Nowadays, lithium-ion batteries are widely employed in a lot of applications. Battery aging implies performance degradation of the battery itself. In particular, the battery aging causes capacity reduction and internal resistance increase. The capacity reduction mainly affects the energy that the battery can deliver in each cycle, while the increase of the internal resistance limits the power that the battery can instantaneously deliver. For this reason, the battery life is conventionally considered at its end when the capacity reaches 80% of the initial value or the resistance reaches 200% of the initial value. As is well known, the battery resistance changes with temperature and state of charge (SOC) and, even if this relationship was studied for new batteries, how this relationship changes with battery aging has not been studied yet. In this paper, the variation law of the internal resistance as a function of temperature and SOC at different aging conditions is analyzed. In particular, lithium battery cells were aged following a fixed protocol. During the aging process, electrochemical impedance spectroscopy was performed at different temperatures and SOCs to analyze the change of the battery impedance due to aging. By using the results of this experimental campaign, a mathematical model predicting how the internal battery resistance changes with temperature, SOC and aging is proposed. The effectiveness of the proposed model is validated by means of experimental tests and a chemical interpretation of phenomena is also provided.