Stress-Induced Interface Instability during Ion Intercalation in Battery Compounds

Abstract

Misfit stress generated by lattice mismatch between the lithiated and delithiated phases upon ion (de)intercalation plays a key role in cycling-induced mechanical degradation and capacity fading of battery electrodes. As a general phenomenon, misfit stress also leads to interface instability. A well-known example is the stress-induced island growth or the Stranski-Krastanov growth mode of epitaxial thin films, where the misfit strain between thin film and substrate destabilizes the uniform film growth. In this work, we predict that similar phenomenon can also occur during the ion (de)intercalation process in battery compounds, in which the misfit stress triggers the morphological instability of the moving phase boundary. This phenomenon results in non-uniform (de)intercalation, leading to capacity under-utilization and battery degradation and even failure due to stress concentration, and therefore needs to be carefully mitigated to improve battery performance. In our theoretical analysis, we consider the kinetic evolution of a sinusoidal perturbation applied to an initially flat interface between the lithium-rich and lithium-poor phases upon (de)lithiation under constant overpotential or flux conditions. The perturbation growth rate is calculated from the linear stability analysis. For interface-limited phase boundary movement, there exists a critical wavelength above which perturbation will growth. On the other hand, two critical wavelengths are identified for diffusion-limited phase transformation, and only perturbations with wavelengths between them will grow with time. The growth exponents predicted by the linear stability theory agree well with phase-field simulations. Based on the analysis, a stability diagram for lithium (de)intercalation is constructed in the space of particle size, applied overpotential (or intercalation flux) and interface location, as shown in Figure 1. Such diagram provides guidance on the conditions at which stress-induced Li intercalation instability can be avoided. A useful insight from our study is that the interface is most susceptible to instability when it is near the electrode particle surface, i.e. at the beginning of (dis)charge process. Accordingly, we propose a (dis)charging strategy to suppress the instability, in which a high Li flux pulse is applied at the initial (de)lithiation stage to stabilize the interface until it moves into the particles. We compare interface evolution in this (dis)charging protocol against during galvanostatic cycling in phase-field simulations. It shows that the proposed strategy can effectively prevent interface instability, which results in an increase in the (dis)charge capacity and reduction in localized stress hotspots. Figure 1