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Development of advanced materials for next generation of rechargeable batteries
Li, Xuemin
Li, Xuemin
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2017
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2018-07-17
Abstract
The increasing demands for next generation of rechargeable batteries evoke the development of advanced electrode materials. For my thesis, I mainly study two systems of lithium/sodium-containing electrode materials, that is, alkali sulfides (M2S, M = Li and Na) cathodes and lithium silicides (LixSi) anodes. We develop an innovative method of synthesizing anhydrous and morphology controllable M2S nanocrystals (NCs) and demonstrate their potential as cathode materials for metal-sulfur batteries. Phasepure M2S-NCs are produced as precipitates during reactions between hydrogen sulfide (H2S) and metalorganic complexes (R-M) in organic solutions. The reaction is thermodynamically favorable and enables tuning the M2S-NC morphology via selecting different solvents and organic agents. In the ideal process, we have realized the net reaction of H2S + 2M → M2S + H2, which integrates three desirable tasks: abating the hazardous H2S, producing the wanted M2S-NCs, and recovering valuable delivering hydrogen (H2). All auxiliary reagents can be recycled directly without post-treatments. The electrode performance of synthesized Li2S-NCs (~100 nm) is superior to that of commercial Li2S micropowders in terms of capacity, cycling stability, output voltage, and voltage efficiency. This scalable approach provides an energy-efficient and environmentally-benign method to produce nanostructured cathode materials required in metal-sulfur batteries. Among all candidates scrutinized for the anode, silicon (Si) is one of the most promising materials due to its high capacity. However, the existing studies of its lithiated counterpart – lithium silicide (LixSi) are limited. In this thesis, three thermodynamically-stable phases of LixSi (x = 4.4, 3.75, and 2.33) plus nitride-protected Li4.4Si synthesized via the high-energy ball milling technique are systematically studied. All three LixSi phases show better performance than Si, with Li4.4Si being the best. When a nitride shell (LixNySiz) is created on the surface of Li4.4Si, the cycling stability is significantly improved. Increasing nitridation extent can further improve the capacity retention, while the initial capacity decreases due to the inactivity of some Si in LixNySiz. Moreover, the Coulombic efficiencies of all LixSi-based electrodes of the first cycle are significantly higher than that of the Si electrode. Our study demonstrates the promises and challenges of developing lithium silicide anodes.
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