NOVEL EUTECTIC ELECTROLYTES AND ELECTROCATALYSTS SEPARATORS FOR LITHIUM SULFUR AND LITHIUM SELENIUM DISULFIDE BATTERIES

Julio C. Zamora

doi:10.7273/000007253

Lithium-Sulfur (LSBs) batteries and Lithium Selenium disulfide (Li-SeS2) have been intensively studied as one of the most cost effective, environmentally benign, and high energy density energy storage systems for various applications, such as electric vehicles. However, like many emerging energy storage systems, both LSBs and Li-SeS2 batteries suffer from similar challenges such as poor rate capacity retention, short cycle life, and thermal/chemical degradation. These challenges are mainly caused by parasitic reactions relating to (i) dissolution/diffusion of active lithium polysulfide or polyselenide species known as the “shuttle effect” or (ii) decomposition of flammable ether-based solvents species forming a unstable irreversible passivation layer on the lithium metal anode reducing LSBs and Li-SeS2 electrochemical cycling performance. To address these two main issues, many efforts have been devoted towards the development of chemically/thermal stable nonflammable liquid electrolytes or designing functional layered separators to reduce parasitic reactions within both LSBs and Li-SeS2 batteries. This dissertation describes improved rate and prolonged electrochemical cycling performance pertaining to novel nonflammable sulfonamide deep eutectic solvent (DES) with the addition of fluorinated ether diluent (TTE) within Li-SeS2 battery. Under extreme thermal conditions (0oC to 60oC), we determined that increased concentration of TTE diluent within DES electrolyte maintained thermal stability and improved ionic conductivity (~10-3 S cm-1). Implementing experimental Raman spectroscopy and theoretical molecular dynamic (MD) simulations, the introduction of TTE diluent within DES electrolyte resulted in small aggregation of TFSI- anion in bidentate coordination with multiple Li+ ions. The study-determined DES-4TTE concentration resulted in optimal localized aggregate bidentate TFSI-Li+ ion coordination, improved overall bulk lithium-ion transport, and promoted uniform SEI deposition of LiF on lithium metal anode interface after Li-SeS2 cycling. In conjunction, Li-SeS2 cyclic voltammetry (CV) under various scan rate pertaining to DES-TTE electrolytes demonstrated overall increase in Li+ diffusion coefficient associated with insoluble/soluble lithium polysulfide/polyselenide redox kinetic reactions. We demonstrated that Li-SeS2 utilizing DES-4TTE electrolyte was able to retain specific capacity at high C-rate conditions; when compared to LSBs, this was determined by enhanced Li+ diffusion coefficient analysis of redox Li-SeS2 CV profiles. Benefiting from tunable porous structure of metal-organic frameworks (MOFs), via carbonization of bimetallic nickel-based Ni-M-MOF-74 (M= Fe, Cu, Zn) nanotemplate, we designed novel electrocatalyst nickel-based bimetallic alloy nitrogen doped carbon Ni-M-N-C (M=Fe, Cu, Zn) nanocomposite interlayered functional separator for LSBs. While comparing various bimetallic Ni-M-N-C (M = Cu, Fe, Zn) nanocomposites, Ni-Zn-N-C carbonized at 800oC obtained optimal carbon substrate surface defects, micro/mesoporous structure, and uniform distribution of isolated bimetallic Ni3.68 Zn0.32 moieties corresponding to enhanced lithium polysulfide electrocatalyst activity. Implementing LSBs (CV) under various scan rate, Ni-Zn-N-C had enhanced Li+ diffusion coefficient associated with kinetic oxidation/reduction of insoluble Li2S into Li2Sx (4 < x < 8) resulting in improved LSBs rate cycling retention capabilities and prolonged LSBs cycling under high current conditions. This work provides new strategies for the development of high-performance nonflammable lithium-ion electrolytes capable of maintaining thermal/chemical stability for high-energy density lithium-metal batteries with high-capacity cathodes systems. Such novel nonflammable sulfonamide deep eutectic liquid (DES) with fluorinated ether (TTE) can be utilized in other alkali metal (e.g. Na, K) salt mixtures for other low-cost Na-S or K-S batteries systems, which have prominent thermal degradation issues. In conjunction, this research demonstrates novel carbonization techniques of tunable multimetal MOF-74 nanotemplates as a viable precursor towards the development of various electrocatalyst nickel-based bimetallic alloy nitrogen doped carbon nanocomposite, which can be used as effective functional interlayer materials within LSBs.

NOVEL EUTECTIC ELECTROLYTES AND ELECTROCATALYSTS SEPARATORS FOR LITHIUM SULFUR AND LITHIUM SELENIUM DISULFIDE BATTERIES

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