Multi-phase formation induced by kinetic limitations in graphite-based lithium-ion cells: Analyzing the effects on dilation and voltage response

- During high-rate lithiation, overshooting phases are formed in graphite electrodes.
- Stage-2-formation, stage-1-formation and lithium plating are investigated.
- A simulation model for cell voltage during multi-phase formation is introduced.
- The model is capable of predicting kinetic limitations during lithiation.
- The amount of plated lithium on the surface of the negative electrode is estimated.

In this research work, dilation and voltage effects related with the formation and dissolution of overshooting phases in graphite electrodes are investigated and a modeling approach is proposed. These multi-phase effects are induced by kinetic limitations under high-rate conditions. Therefore, high-rate charge loads are applied to a graphite|NMC pouch cell at different temperatures, current rates and SOC ranges. Particularly at low temperatures, the dilation relaxation after the current phase is significantly influenced by multi-phase effects. For the first time, the dilation effects are also compared to the simultaneous voltage effects, which provide additional information about multi-phase-formation. By analyzing the time derivative of voltage during the relaxation phase subsequent to high-rate charge phases, individual multi-phase-features resulting from stage-2-relaxation, stage-1-relaxation and lithium plating are distinguished from each other. Finally, an electrochemical transmission line model for predicting and simulating multi-phase-effects is introduced. The model allows to simulate voltage responses both under heavy charge loads and during subsequent relaxation phases. Furthermore, it provides a representation of inhomogeneous graphite lithiation and allows to predict the onset of lithium plating. The approach is particularly promising with regard to fast charging applications, where the charge current needs to be limited to avoid lithium plating.

120