Predicting the Effects of Capacity Fade and Thermal Behavior of Silicon Anodes in Lithium-Ion Batteries

Abstract

Silicon is one of the most promising anode materials for Li-ion batteries, but large volume expansion, pulverization, and strains accelerate electrode disintegration and lead to capacity fade. Our research addressed the problems in silicon anodes in lithium-ion batteries and through modeling proposed solutions. Previous work has used additives and novel electrolytes to create a stable SEI (solid electrolyte interface) to suit silicon surface interaction. Hence, we used a separator which is a porous matrix filled with electrolyte made up of lithium hexafluorophosphate (LiPF6) dissolved in a 3:7 liquid mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). We concentrated the work on the growth and kinetics of SEI layer. Since capacity loss is in part dependent on the cell materials, two different electrodes, LiNi0.8Co0.15Al0.05O2 (NCA) and LiNi1/3Mn1/3Co1/3O2 (NMC 111), were used in combination with silicon to study capacity changes using simulations in COMSOL version 5.5. The results of these studies provide insight into the effects of anode particle size and electrolyte volume fraction on the behavior of silicon anode-based batteries with different positive electrodes. It was observed that the performance of a porous matrix of solid active particles of silicon anode could be improved when the active particles were 150 nm or smaller. The range of optimized values of volume fraction of the electrolyte in the silicon anode were determined to be between 0.55 and 0.40. The silicon anode behaved differently over the cycle time with NCA and NMC cathodes. However, NMC111 gave a high relative capacity in comparison to NCA and proved to be a better working electrode for the proposed silicon anode structure. We also discuss the role of electrode structural characteristics on the thermal behavior of lithium-ion batteries. Preliminary modeling runs have employed a 1D lithium-ion battery coupled to a two-dimensional axisymmetric model using silicon as the battery anode material. The two models are coupled by the heat generated and the average temperature. Our study is focused on the silicon anode particle sizes, and it is observed that silicon anodes with nano-sized particles reduced the temperature of the battery in comparison to anodes with larger particles. These results are discussed in the context of the relationship between particle size and thermal transport properties in the electrode.