Aging and Homogenized Mechanical Character of Quasi-Statically Charged Gr-Si and NMC Based Electrodes Using Damage Material Modeling

Abstract

Silicon-based, high-energy-density electrodes show severe microstructural degradation due to continuous expansion and contraction upon charging and discharging. This mechanical degradation behaviour affects the cell’s lifetime by changing the microstructure morphology, altering transport parameters, and active volume losses. Since direct experimental observations of mechanical degradation are challenging, we develop a computer simulation approach that is based on real three-dimensional electrode microstructures. By assuming quasi-static cycling and taking into account the mechanical properties of the electrode’s constituents we calculate the heterogeneous deformation and resulting morphological changes. Additionally, we implement an ageing model that allows us to compute a heterogeneously evolving damage field over multiple cycles. From the damage field, we infer the remaining electrode capacity. Using this technique, an anode blend of graphite particles and silicon carbon composite particles (SiC-C) as well as a cathode consisting of Lithium-Nickel-Manganese-Cobalt Oxide with molar ratio of 8:1:1 (NMC811) are studied. In a two-level homogenization approach, we compute, firstly, the effective mechanical properties of silicon composite particles and, secondly, the whole electrode microstructure. By introducing the damage strain ratio, the degradation evolution of the graphite SiC-C anode blend is studied for up to 95 charge-discharge cycles. With this work, we demonstrate an approach to how mechanical damage of battery electrodes can be treated efficiently. This is the basis for a full coupling to electrochemical simulations.