Effect of Electrochemical and Mechanical Properties of SEI on Dendritic Growth during Lithium Deposition on Lithium Metal Electrode

Abstract

In the past three decades, lithium-ion batteries experienced a long series of successful improvements, which nowadays make them a widespread technology. Intercalation electrodes, like graphite in the negative electrode, drove large part of this success. On the other hand, they constitute a limit in terms of energy density, which makes them unlikely to match the increasing demands set by renewable energies and especially electric mobility. Thus, it becomes fundamental to investigate new materials that overcome these limits. Lithium metal represents the optimal candidate for the negative electrode, due to its high theoretical capacity (3860 mAh.g-1) and low potential (-3.04 V SHE). On the other hand, the major drawback of this technology is the formation of dendrites, which are lithium structures that form during its deposition. They can cause thermal runaway and internal short-circuits, potentially leading to explosion. Moreover, during lithium stripping, some parts of the lithium electrode tend to be isolated in the electrolyte. This phenomenon causes a reduction of the electroactive lithium quantity thus a limited lifetime of the cell. Therefore, a deeper comprehension of the interfacial phenomena involved during lithium deposition and stripping is necessary to make lithium metal a viable and competitive option as a negative electrode, fostering the transition towards the next generations of batteries.The proposed model starts from the stability analysis of the electrode interface proposed by Newman and Monroe [1] that considers the effect of the mechanical properties on the lithium deposition. Different from it, the presented model studies a time-dependent variation of the surface and it includes a pseudo-2D representation of the Solid Electrolyte Interface (SEI) component. The approach to model the SEI is based on the work of Liu and Lu [2]; and it includes both SEI creation, due to deposition side reactions, and SEI thickness deformation due to the change in geometry of the underlying electrode. In addition, the model considers the effect of the mechanical properties of the SEI on the surface evolution of the electrode. Therefore, starting from the initial surface geometry and the electrochemical and mechanical properties of the components, the model is able to predict the conditions that favors dendritic growth. The figure shows lithium electrodeposition, starting from the same surface defect, on the lithium metal electrode in a liquid electrolyte. After the same quantity of lithium is deposited, it is possible to distinguish different patterns of surface evolution, due to the difference in magnitude of the current density applied: a quasi-uniform growth at low current density (left), a mossy behavior at medium current density (center) and a branching pattern at high current density (right). The difference in dendritic regime is strictly connected to the local change in the SEI thickness, as shown in the figure, which confirms the importance of this component, often neglected in the literature of dendrites models.Since SEI has a major impact on dendritic growth, a consistent set of experiments has been designed to provide its parameters to the model. Electrochemical Impedance Spectroscopy (EIS) was applied to find the electrochemical parameters of the SEI (e.g. SEI diffusion coefficient, SEI resistivity), while Atomic Force Microscopy (AFM) was conducted to find its mechanical properties (e.g. SEI Young’s modulus). The proposed model has the goal of guiding the experiments in finding ways to avoid or curtail dendrites formation, being able to simulate the effects on dendrites growth of operative conditions, SEI and electrolyte compositions, electrode surface defects and coatings. Starting from the model, innovative ways to increase the lifetime of the lithium metal anodes will be analyzed during the presentation; including coatings, surface morphology modification and pulse charging.[1] C. Monroe, J. Newman, Journal of the Electrochemical Society 151(6), A880-A886 (2004)[2] G. Liu, W. Lu, Journal of the Electrochemical Society 164(9), A1826-A1833 (2017) Figure 1