UC San Diego

The increasing demand for electric vehicles and portable devices has revealed the potential of lithium-ion batteries (LIBs). However, conventional graphite-based LIBs fall short in providing sufficient energy density and power density, which hinders the development of electric vehicles and electric aircraft. Lithium metal anode material emerges as one of the most promising candidates for high energy density batteries (> 500 Wh/kg and > 1000 Wh/L) due to its exceptional theoretical specific capacities, lowest reduction potential, and low density. Nonetheless, the commercialization of lithium metal anodes faces challenges such as limited cycle lives caused by continuous dendrite growth and safety concerns arising from porous electrodeposited structures. Electrolyte engineering represents the most efficient approach to establish a compatible anode/electrolyte interphase (SEI) at a fundamental level. Despite significant research on the development of high-concentration electrolytes and localized high-concentration electrolytes, both suffer from reduced ionic conductivities and poor wettability towards thick electrodes. Liquefied gas electrolytes (LGEs) offer a promising alternative to enable high energy density lithium metal anodes due to their low viscosity, inherent pressure, electrochemical stability, and high fluorine content available for donation. However, it is crucial to prioritize addressing challenges related to the pressurized nature, relatively lower Li+/solvent coordination, and the flammability of fluoromethane (FM)-based electrolytes.To enhance salt solubility and expand the range of compatible salts, we propose replacing FM with the simplest ether, dimethyl ether (Me2O), due to its similar physical properties and more polar functional group, which potentially enables stronger Li+ solvation. When integrated with carbon monofluoride electrodes, Me2O-based LGE exhibits excellent performance at ultra-low temperatures, reaching as low as -70°C, and competitive fast-rate capabilities up to 6.25 C. However, low-concentration Me2O electrolytes face challenges in terms of relatively poor oxidative stability and flammability. To address safety concerns, we enhance the safety features of liquefied gas electrolytes by incorporating Me2O with other fire-extinguishing gas agents. The addition of fluorinated fire-extinguishing gases such as 1,1,1,2 tetrafluoroethane (TFE) and pentafluoroethane (PFE) significantly improves the safety of the formulated electrolytes. By utilizing the concept of localized highly concentrated electrolytes, we have developed a fire-extinguishing TFE-PFE-based LGE that enables stable Li/NMC622 cycling over 200 cycles at a cutoff voltage of 4.2 V. Moreover, these electrolytes can be successfully recollected after use, contributing to the sustainability of the LGE. In parallel studies, we have observed unique characteristics of Me2O-based electrolytes when high salt concentrations are maintained. Such electrolytes can maintain a liquid state under ambient conditions, facilitating electrolyte preparation and significantly reducing operating pressure, thereby lowering costs. The obtained saturated LiFSI-Me2O electrolyte exhibits excellent stability in Li-metal plating and stripping, achieving over 99.2% Coulombic efficiency for 1000 cycles. When combined with an SPAN electrode, the electrochemical performance of Li/SPAN demonstrates competitiveness due to the low solubility of polysulfides and the SEI derived from the salt. In conclusion, by introducing Me2O-based LGE, we present a promising direction for achieving high energy density, improved safety, ultra-low temperature operation, and sustainability in multiple Li-based batteries.