Energy Storage Materials: Mitigating the Degradation of Alloy-based Anodes for Lithium and Sodium Ion Batteries
Creators: Woodard, Jasper C.
Publisher: name: University of Alberta Library
Year: 2023

<p>Lithium ion batteries have become a ubiquitous technology for consumer electronics and electric vehicles. Alloying anodes, such as silicon and tin, offer large increases in volumetric energy density compared to conventional graphite anodes. However, the alloying mechanism is accompanied by an expansion of almost 300 %, and repeated expansion and contraction over many cycles leads to particle fracture and pulverization, electrical isolation, and build up of the solid electrolyte interphase (SEI). Electrical isolation of anode material leads to catastrophic capacity loss, while continuous SEI build up causes low Coulombic efficiency during cycling and reduced capacity retention in full cells.<br>
Many strategies have been employed to mitigate the deleterious effects of silicon and tin expansion. Nano-sized silicon structures may reduce the degree of fracturing upon expansion, and mixing of nanoscale silicon with polymer binders and conductive additive can prevent electrical isolation after pulverization. Silicon particle pulverization may also be affected by the formation of the terminal lithium-silicide phase at room temperature, c-Li15Si4. The reaction of electrolyte to form SEI can be pre-empted by adding sacrificial additives, such as fluoroethylene carbonate (FEC), which form a stable and passivating SEI on the anode surface. An artificial SEI (a-SEI) might also be employed on silicon anodes prior to cell assembly, with the same goal of passivating the surface and preventing further electrolyte decomposition and SEI build up. Another approach is the use of mixed anodes, which combine graphite with either silicon or tin for lithium ion batteries. Incorporating graphite dilutes the impacts of cell fracturing and of SEI accumulation, while graphite also prevents the alloying active material from becoming electrically isolated.</p>

<p>This thesis describes four approaches to better understand and mitigate material fracture and continuous SEI build-up. Three of the strategies are performed on silicon anodes in lithium ion batteries, while the final project focuses on porous tin anodes for lithium and sodium ion batteries. In Chapter 2, we examine the effects of forming the c-Li15Si4 phase in nano-sized silicon. Contrary to planar thin films of silicon, the c-Li15Si4 phase is not associated with greater capacity loss, but it does seem to promote more SEI build-up and reduced Coulombic efficiency, which is especially detrimental to lithium ion full cells. Chapter 3 and Chapter 4 focus on efforts to improve the SEI layer on silicon nanoparticles through the use of a covalently bound a-SEI and electrolyte additives, respectively. We successfully form a-SEI on the surface of silicon anodes. Perfluorinated a-SEI passivates the silicon surface, and improves the Coulombic efficiency over 100 cycles, but still leads to lower overall capacity retention due to poor adhesion with the more flexible polar polymer binders. The relative electrochemical performances of other a-SEI functionalities, such as polyethylene oxides and vinyl ethylene carbonate, depend heavily on the size of silicon and the thickness of the oxide layer to which they are being compared. The SEI can also be manipulated using sacrificial electrolyte additives. We demonstrate that additives containing an alkyne or alkene may plausibly form covalent bonds with the surface of exposed silicon through in-situ electrografting. However, an SEI formed through in-situ electrografting leads to much worse capacity retention than an SEI formed using FEC, potentially due to the self-healing properties of intermolecular interactions between the SEI and silicon surface in accommodating the colossal expansion of the silicon anode.</p>

<p>Chapter 5 focuses on the use of porous tin anode material for lithium ion (LIB) and sodium ion batteries (NIB). Mixed electrodes of porous tin with graphite and porous tin with hard carbon were used for LIBs and NIBs, respectively, and compared to commercial micron sized tin powder. Pores within tin provide space to accommodate expansion during lithiation or sodiation, while active carbon material dilutes the impacts of tin expansion and fracturing. The addition of up to 10 % p-Sn is associated with larger specific capacity, while still maintaining &gt; 80 % capacity retention over 100 cycles. However, each marginal increase in p-Sn content is associated with lower Coulombic efficiency over all 100 cycles. Porous tin had improved rate capabilities compared to micron sized commercial tin during early cycles, but this distinction dissipated over the course of 100 cycles as micron sized tin fractures and is reduced in size.</p>