Cheng, Yingwen Y.
Ph.D. (Doctor of Philosophy)
Department of Chemistry and Biochemistry
Batteries with higher energy density and safe properties are demanded in modern society. This dissertation comprehensively studies the materials and interfaces of electrochemical energy storage and conversion for lithium-sulfur (Li-S) batteries as well as aqueous zinc-manganese dioxide (Zn-MnO2) batteries to fundamentally meet the renewable energy demands.This dissertation is divided into 2 sections. The first section (chapter 2-4) describes the electrodes and electrolyte modification for Li-S batteries. Specifically, chapter 2 describes sulfur cathodes’ modification by utilization of Fe3O4 and Nitrogen(N)-doped carbon cloth as sulfur host. Results show that synergistic effects of Fe3O4 and N-carbon bring strong adsorption toward lithium polysulfide and ensure nearly complete conversion of short-chain polysulfide to Li2S during discharge, as well as Li2S solids activation without a noticeable overpotential during charge. This novel cathode material deliver a specific capacity of 1316 mAh g−1 at 0.1C and stable cycling for 1000 cycles at 0.2C under a high sulfur loading of ≈4.7 mg cm−2. Chapter 3 describes simple sulfur host (Fe/Fe3C@Fe-N-C, Fe@Fe-N-C, Fe3C@Fe-N-C) synthesis method by one-step pyrolysis of a novel Fex[Fe(CN)6]y/polypyrrole composite at different temperatures. We revealed that the polar Fe3C strongly adsorbs polysulfide whereas the Fe particles catalyze fast polysulfide conversion. This work provides new insights on the functional mechanism of advanced sulfur hosts, which could eventually translate into new design principles for practical Li–S batteries.
Chapter 4 describes the electrolyte modification by utilizing of tris(2,2,2-trifluoroethyl) borate (TFEB) as additive for high S-loading and lean-electrolyte batteries. Results show that TFEB strongly interacts with lithium polysulfides forms highly active complex. This assembled Li-S batteries delivered dramatically improved performance metrics under practically relevant mass S-loading (~5 mg/cm2), lean electrolyte condition (E/S ratio: ~4) reaching a specific capacity of 1050mA/g at 0.1C with ~86% capacity retention among 100 cycles.The second section (chapter 5-6) describes the cathode and electrolyte manipulation for aqueous Zn-MnO2 batteries. Specifically, chapter 5 describes the modulating MnO2 interface with self-adhering alkylphosphonic layers. In this project, we utilized modifier organic molecules butylphosphonic acid (BPA) with phosphonic functional group as coating layer on MnO2 surfaces. The coating layer not only improve interfacial charge transfer of H+ ions but also effectively reduce dissolutive loss of Mn2+ from MnO2 during battery cycling, thus much improved the Zn-MnO2 batteries’ cyclic performance (>60% capacity retention for 400 cycles). Chapter 6 describes novel electrolyte utilization for aqueous Zn-MnO2 batteries. In this work, we use of simple Zn(ClO4)2 aqueous electrolytes for all-weather Zn–MnO2 batteries even down to −60 °C. And we identified that subfreezing cycling shifts the reaction mechanism on the MnO2 cathode from unstable H+ insertion to predominantly pseudocapacitive Zn2+ insertion. The Zn2+ insertion at −30 °C is faster and much more stable than at 20 °C, and delivers ≈80% capacity retention for 1000 cycles without Mn2+ additives.
Gao, Siyuan, "Materials and Interfaces of Electrochemical Energy Storage and Conversion" (2023). Graduate Research Theses & Dissertations. 7145.
Northern Illinois University
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