Interplay between charging and plating in graphite electrodes via phase-field modelling and operando optical microscopy

Rapid charging of lithium-ion batteries within minutes poses a significant technological challenge, as it leads to notable anode degradation and accelerates battery ageing. This degradation, primarily during fast charging, is closely linked to mesoscopic electrochemical phenomena. A key problem is lithium plating, the deposition of metallic lithium on active material particles, a process promoted by phase-separation dynamics in graphite anodes. Our study introduces a robust pseudo-2D electrochemical modelling framework [1], based on the phase field approach and non-equilibrium thermodynamics [2]. This model is validated using operando high-resolution optical microscopy to elucidate the rate-dependent spatial dynamics of phase separation, alongside lithium plating and stripping in graphite electrodes during charge and relaxation phases, respectively. Predictions from the mesoscopic model are used to corroborate the parameterisation and validate the phase field framework against both optical and electrical data, particularly under conditions of fast charging at varied current densities (2 and 4 mA cm-2). Its application extends to observing multistage phase separation processes, inter/intra-particle lithium exchange, and dynamics of plated lithium back-intercalation. Importantly, the model provides an insight into the material, revealing the distribution of graphite's state of lithiation throughout electrode thickness and particle radius (Figure 1a). Additionally, it enables direct observation of the distribution of intercalation, plating, and stripping reaction currents across the electrode, thereby clarifying the interaction between phase separation and lithium plating-stripping dynamics (Figure 1b). The modelling framework and its parameterisation are then implemented in 3D microstructure-resolved models, to inform the design of advanced fast charge protocols with rational rest steps to suppress plating.