Battery Components Derived from Silica-Depleted Rice Hull Ash (SDRHA)

Abstract

The global momentum targeting development of new energy sources has undergone rapid growth to alleviate environmental concerns, prompting demands for more sophisticated energy storage techniques. However, the true sustainability of current energy storage devices is questionable. The environmental impact of batteries extends to heavy metal-containing active materials and significant greenhouse gas emissions during processing and operation. To address these challenges, the integration of biowaste-based materials in battery components offers a pathway towards truly green and sustainable energy sources, ensuring closed carbon cycles. In this dissertation, rice hull ash (RHA) has been investigated as a biomass source providing access to novel battery components. The first part of this thesis discusses extracting SiO2 from RHA using an approach that generates distillable spirosiloxane. RHA consists of >90 wt. % amorphous SiO2 intimately mixed with unburned carbon. This method enables adjusting SiO2:C ratios simply by reacting RHA with hexylene glycol (HG) and a base catalyst to distillatively extract SiO2 to produce silica-depleted RHA (SDRHA) with SiO2 contents of 40–65 wt.%. Thereafter, the focus is on detailed studies using the SDRHA without adding extra carbon for direct carbothermal reduction. The preserved structures and coincidently eliminated alkali impurities permit the use of the resulting composites as components for electrochemical energy storage devices among other applications. The second part describes exploring multiple materials derived from SDRHA for Li+ storage. SDRHA40-60 is found to contain hard carbon (HC) structures. Electrochemical Li+ storage performance in SDRHA40-60 reveals significant contributions from the porous structures of HC in SDRHA through surface capacitive behavior and diffusion into the inner areas. Layered, cubic SiC produced from SDRHA provides another potential anode for LIBs. SDRHA-derived SiC w/wo HC (SiC/HC, SiC/O), exhibit unexpected capacity increases eventually to >950 mAh/g and >740 mAh/g, respectively, after 600 cycles. Further investigation into the lithiation mechanisms finds partial cubic to hexagonal phase transformation offers larger interstitial spaces/defects and subsequently higher capacities on cycling. Furthermore, it was found that introducing a relatively small amount of graphite to SiC/HC composites promotes rates of capacity increases while retaining sustainability. The positive effects from the coincidentally formed HC are demonstrated. Coincidentally, the feasibility of using SiNxOy as electrodes is also described. Preliminary characterization suggests elongation along the a axis in orthorhombic Si2N2O resulting in specific capacities >400 mAh/g. Introducing external Li into Si2N2O using Li2CO3 finds coincidental formation of lithium silicon oxynitride during carbonitriding. The 3D lithium conduction channels facilitate Li+ diffusion in the electrodes and consequently improve their rate performance. Experimental post-mortem studies provide insights into understanding the Li+ diffusion pathways. The last part of this thesis covers extended findings on stabilizing performance of the cobalt-free, high-voltage LiMn1.5Ni0.5O4 (LMNO) spinel cathode. By ball-milling LMNO with flame-made nanopowder (NPs, e.g., LiAlO2, LATSP, LLZO) electrolytes, the coated composites mitigate the well-recognized Mn2+ dissolution issues. Comprehensive characterization provides supportive explanation for different NPs showing various effects on the LMNO-composite cathode performance compared to pristine LMNO. The positive effects of improving Li+ diffusivities and forming protective layers are discussed in detail. Overall, the findings in this dissertation provide fundamental insights toward incorporating RHA and its derivatives in the loop to produce next-generation LIBs. The impletion of biowaste-based materials and cobalt-free cathode are two essential strategies to achieve sustainable electrification by reducing the world's dependence on mining natural sources and attaining carbon neutrality.