Phase-separating active materials in lithium-ion batteries: a modelling overview with application to carbon-based anodes

Some active materials used in lithium-ion battery (LIB) electrodes undergo phase separation into Li-rich and Li-poor phases upon lithium intercalation. Typical examples are LiFePO4 (LFP) at the cathode and graphite at the anode. While phase separation enables useful features, such as a constant equilibrium potential as a function of state-of-charge, such behaviour is too complex to be modelled as solid-solution materials and necessitates a different approach. In this study we present a modelling framework, based on the phase-field approach and non-equilibrium thermodynamics principles [1], which reproduces phase separation at the particle scale in LIB electrodes. We discuss how the data obtained with classical techniques used to estimate solid-state diffusion coefficients, such as the galvanostatic intermittent titration technique (GITT) [2], must be revisited for this class of materials. In particular, simulations show that the relaxation behaviour of a phase-separating active material upon current interruption is not only a function of solid-state diffusion properties, but its time-scale depends also on the dynamics of moving interfaces between Li-rich and Li-poor phases. This has significant implications on the distribution of lithium within secondary particles because, upon fast lithiation, the Li-rich phase grows at the particle surface and prevents further lithiation. This is especially critical for graphite anodes, since the Li-rich phase (also known as stage I) at the particle surface promotes the plating of metallic lithium outside the particle [3]. On the other hand, experiments and simulations of a disordered carbon, which does not undergo phase separation, reveal an effectively faster solid-state diffusion and a more significant resistance to lithium plating even at high C-rate [4]. We finally discuss how these insights can guide researchers to improve battery materials and to design more efficient fast-charging protocols.