Investigation of Silicon-Based and Multicomponent Electrodes for High Energy Density Li-ion Batteries

Abstract

Li-ion batteries have enabled the widespread adoption of portable electronics and are becoming the technology of choice for electric vehicles and grid storage. One of the most promising ways to accommodate this demand is to increase the energy density and cycle life of battery electrode materials. Key strategies promoted in the literature include the use of nickel-rich cathodes as well as high-capacity anodes like silicon and lithium metal. While these materials enable a high energy density, this advantage is often counterbalanced with deficits such as poor stability and high cost. Multicomponent electrodes refer to strategies that try to leverage the relative advantages of different materials to offer an attractive compromise of energy density, cost, and cycle life. This thesis has investigated various aspects of multicomponent electrodes with a special emphasis on silicon-based anodes and high-entropy materials. Silicon (Si) is the second-most abundant element on earth and has one of the highest gravimetric capacities. However, silicon anodes are notorious for their poor cycle stability. Herein, improvements in the stability of silicon-based electrodes are achieved with multicomponent composite strategies involving the use of nanostructured spherical silicon. The nanosilicon is studied in high-fraction (80 wt% Si) and low-fraction (≤20 wt% Si) formulations to investigate both failure mechanisms and practical capacity retention, respectively. Composite strategies in which nanosilicon is encapsulated within a Li₄Ti₅O₁₂ ceramic or MOF-derived carbon matrix are shown to deliver superior capacity retention compared to simple composites of silicon and graphite. Considerable attention is given to the selection of a water-soluble binder and its role in electrochemical stability and electrode cohesion in high-loading silicon electrodes. It is found that unmodified high-molecular-weight sodium carboxymethyl cellulose offers better capacity retention compared to xanthan gum or low-molecular-weight binders. The high-entropy design strategy has created a diverse and largely unexplored set of multicomponent oxides and alloys with great potential as electrode materials. This strategy is applied to the family of layered cathodes, where the synthesis and electrochemical properties of the best-performing Li(NiCoMnTiFe)₁O₂ are reported. Despite the low Ni content, the cathode delivers a high initial capacity with unique overlithiation stability despite being charged to 4.4 V. Throughout the thesis, Operando XRD is used to reveal important insight into the lithiation mechanisms of the multicomponent electrodes including intercalation-based graphite, alloying-based silicon, and a novel organic azaacene.