Accelerated charging protocols for lithium‑ion batteries: Are fast chargers really convenient?

Abstract

The accelerated charging of lithium-ion cells is proposed as a case study that will facilitate the integration of several fundamental electrochemical concepts learned during a dedicated electrochemistry course. Lithium-ion cells are ubiquitous to our everyday life, and their presence will become even more pervading with the increase in number of electric vehicles and the wider penetration of renewable energy sources in our energy matrix. In this context, understanding conventional and fast charging of lithium ion cells and most importantly being able to relate the limitations of the charging process to fundamental electrochemical phenomena at the interphase level are key. By testing the charge and discharge of graphite/lithium cobalt oxide single cells at different C rates, a large array of experimental data is generated which will trigger the discussion. The applied current is interpreted as the imposed kinetics on the system. The voltage vs. time profiles are highly dependent on the applied current. Surprisingly, whilst during the CV regime the same voltage value is applied in all experiments, the resulting current vs. time profiles are different for experiments at varying C rates. The different profiles are interpreted in terms of activation, diffusion, and concentration overpotentials. The kinetics related both to electrochemical reactions and mass transport phenomena limit the maximum current that can be imposed to the system.