Non-flammable lithium-ion electrolytes for secondary batteries

Abstract

Due to the escalating energy demands in contemporary society, lithium-ion batteries (LIBs) have garnered significant attention as high-energy storage devices. LIBs exhibit a seemingly superior energy density in comparison to other battery systems, allowing for the storage of more electric energy within the same volume. A conventional 18650 lithium-ion battery typically possesses a capacity ranging from 1200 to 3600 mAh and demonstrates an extended lifespan, capable of enduring at least 500 charge and discharge cycles. Moreover, LIBs exhibit elevated charge efficiency and manifest greater environmental friendliness when compared to lead-acid batteries. Nonetheless, several challenges persist in the current state-of-the-art technology. Notably, the cost of raw materials for LIB manufacturing remains relatively high. Furthermore, LIBs are susceptible to issues such as electrolyte leakage and flammability during the charge and discharge processes. Lastly, the existing charging rate of LIBs falls short of accommodating the demands of various applications. This thesis primarily addresses the concern surrounding the flammability associated with the electrolyte in lithium-ion batteries (LIBs). The utilization of solid-state electrolytes has garnered substantial global interest as it eliminates safety issues arising from the use of liquid organic electrolytes in LIBs. Among the various solid-state electrolytes investigated, garnet-type solid electrolytes have received considerable attention due to their stability in the presence of lithium metal, wide electrochemical window, and relatively high ionic conductivity. The focus of this research lies in the synthesis of a fluorine-doped lithium-stuffed garnet electrolyte. The study involves an investigation of the ionic conductivity and lithium metal wettability of two compositions, Li7-xLa3Zr2O12-xFx (x = 0, 0.25, 0.5, 0.75, 1.0) and Li6-x+2yLa2BaM2-yYyO12-xFx (M=Nb, Ta; x = 0, 0.1, 0.3, 0.5, 0.7, 0.9; y = 0.3, 0.4, 0.5, 0.6, 0.7). Additionally, the morphology and structural characteristics of these materials have been investigated. Among the compositions studied, Li6.5La3Zr2O11.5F0.5 showed the highest ionic conductivity of 1.40×10^(-4) S/cm and presented a morphology without any grain boundaries. Nevertheless, despite the advancements made in solid-state electrolytes, their performance remains inadequate in supporting battery functionality. Hence, this thesis also explores a new electrolyte system as an alternative approach. Termed as Localized Electrolyte (LE), it is derived from the structure of Localized High Concentration Electrolyte (LHCE). This system demonstrates reduced viscosity and improved ionic conductivity. Through comprehensive experimentation, we substantiate that LE exhibits exceptional capacity retention and enhanced discharge capacity compared to conventional EC/DEC electrolytes. Notably, LE's performance aligns well with commercial lithium-ion batteries, making it a compelling prospect. Crucially, LE demonstrates non-flammability, positioning it as a promising substitute for the previously discussed solid electrolyte counterparts.