1. The effect of the working voltage window on the growth of the negative SEI film is that, due to the wide electrochemical window, the dissolution of positive transition metal elements is enhanced, and the dissolved transition metal elements migrate to the negative electrode surface, accelerating the growth of the negative electrode transition metal film. The results show that the negative electrode kinetics accelerate lithium decay, thus causing premature lithium precipitation at the negative electrode, leading to earlier nonlinear decay.
2. Effect of charge/discharge rate
Since the nonlinear degradation of lithium-ion batteries is primarily caused by lithium metal deposition on the negative electrode surface, the charge/discharge current is closely related to the occurrence of nonlinear degradation. The charging current has the greatest impact on nonlinear degradation; batteries charged at 1C show a trend of nonlinear degradation almost from the beginning. However, if the charging current is reduced to 0.5C, the time point of nonlinear degradation is significantly delayed. The effect of the discharge current on nonlinear degradation is almost negligible. This is mainly because with the increase in charging current, the polarization of the negative electrode also increases significantly, leading to a significantly increased risk of lithium release from the negative electrode. The deposited porous metal promotes and accelerates electrolyte decomposition. The decline in the dynamic performance of the negative electrode leads to the early occurrence of nonlinear degradation.
3. The effect of temperature
Temperature has a very important influence on the dynamic characteristics of the negative electrode, and therefore also has a significant impact on the time when nonlinear battery degradation occurs.
Batteries cycled at 35C exhibit a non-linear degradation at the very end. If we lower the battery voltage window to 3.17-4.11V, the degradation rates of batteries cycled at 35C and 50C are relatively consistent in the early stages, but at the end of their lifespan, batteries cycled at 35C begin to show a non-linear degradation. This is primarily due to the deterioration of battery kinetics at low temperatures, making the negative electrode more susceptible to lithium degradation, thus accelerating the growth of the SEI film and further worsening the negative electrode kinetics, leading to the non-linear degradation of early lithium-ion batteries.
