Influence of Electrode Formulation in Silicon Microparticle Electrodes for Lithium-Ion Batteries

Abstract

Commercialization of silicon anodes could be a critical step towards increasing electric vehicle adoption among consumers. Silicon has the potential for three times the volumetric capacity, ten times the gravimetric capacity, are more amenable for fast-charge, and can be made using similar manufacturing processes as the incumbent, widely commercialized graphite anodes. To achieve such high capacities, silicon forms amorphous alloys with lithium during lithiation. To accommodate the alloying lithium, the silicon will expand up to three times its original volume, kicking off a cycle of degradation involving particle cracking and irreversible solid electrolyte interphase growth. Although nanosizing or nano-structuring can circumvent this degradation pathway, nanomaterials are not suitable for near-term commercialization due to high material cost and safety concerns. Therefore, to meet battery cell cost targets of $75/kWh, development efforts have shifted towards improvement of microparticle silicon electrodes. Many milestones in the development of silicon anodes were enabled by breakthroughs in electrode formulation. The most prominent of these have been the advancements in binder chemistry and conductive additive integration. Comparatively few studies are devoted to understanding the influence of electrode formulation – the relative ratios of electrode components – on electrode properties and performance. This thesis examines the influence of formulation on structure-properties-performance relationships in silicon microparticle electrodes for lithium-ion batteries. We track how formulation affects porosity, mechanical properties, and electrical conductivity, and identify correlations between these characteristics and electrochemical performance. Full cells with industrially relevant capacity loadings (4.5 mAh cm-2 at beginning of life) were evaluated using three critical performance metrics: fast-charge capability, capacity retention, and gravimetric energy density. We build upon the literature by utilizing diffraction techniques to detect silicon amorphization, thus utilization, and understand how this is affected by formulation and cycling. We also utilize first-of-its-kind operando magnetic dilatometry to explore the effects of formulation, capacity ratio, and electrolyte selection on cell expansion in coin cells. We track reversible and irreversible cell expansions. Our measurements reveal that reversible expansions scale with cell discharge capacity and are most sensitive to the mechanical properties of the electrode. Meanwhile, irreversible expansions are a symptom of cell degradation mechanisms, with electrolyte composition showing the strongest influence. Ultimately, these measurements shed light on how volumetric energy density evolves with cycling—an important metric for battery integration. The insights gained through the course of this work are expected to inform continued electrode and cell optimizations, guide definition of product specifications for commercialization, and serve as a catalyst for future research and development of silicon anode electrodes for lithium-ion batteries.