Electrode Side Reactions, Capacity Loss and Mechanical Degradation in Lithium Ion Batteries

To further develop lithium ion batteries, it is important to improve the understanding of battery degradation mechanisms. In this presentation, we report the results of a series of experiments which helped establish a relationship between coulombic efficiency, side reactions, capacity loss, and mechanical degradation in lithium ion batteries. Graphite/Li(Ni_1/3Mn_1/3Co_1/3)O₂ (NMC) CR2325 full cells of two very different initial anode/cathode capacity ratios (R), both greater than 1, are assembled to investigate their electrochemical behavior. It is observed that, during electrochemical cycling, the cell with a higher R value displays a lower coulombic efficiency than that of a cell  with a smaller R value. The cell with the high R value is shown to have less initial capacity yet less capacity fade during cycling. This confirms the hypothesis that coulombic efficiency is not a perfect indicator of capacity fade. These experiments also show that electrolyte reactions on the anode affect the electrolyte reactions on the cathode and there is, thus, “cross talk” between the two electrodes. This work also supports the idea that capacity fade in graphite based cells is a result of the fracturing and repairing of the solid-electrolyte interphase (SEI) on the anode that leads to a lithium imbalance in full cells and that fracture is worse for cells where the anode particles are cycled to a greater percentage of the full capacity.