State of charge dependency of the overvoltage generated in commercial Li-ion cells

The overvoltage that is produced in the cells under operation limits the capacity and power they can deliver. A detailed study about the mechanisms that contribute to that overvoltage and thus to their lifetime is required for optimizing the use of batteries as well as their manufacturing process. We investigate galvanostatic discharge at low and moderate rates in an LCO-NMC/graphite cell in order to quantify the ohmic voltage drop and activation and concentration polarizations. For doing so, we compare half cells to full cell overvoltages. We find that the ohmic drop and concentration polarization dominate at high rates and low rates, respectively. Moreover, we track the evolution of concentration polarization with State-of-Charge (SoC) and we observe that there exists a relationship between diffusion and phase transformations. Specifically, we validate experimentally that initial stages of a phase formation are not dominated by diffusion. Phase transitions are commonly evaluated by incremental capacity analysis. However, we determine that it is more appropriate to obtain that information from the full cells by the overvoltage analysis. Furthermore, we suggest that the working SoC range can be optimized from the overvoltage analysis by avoiding the particular SoCs at which the most detrimental phase transitions take place.