UC San Diego

The decarbonization of the global energy system depends on electrochemical energy storage that is at once higher in energy, faster to charge, safer, and able to operate across a wide temperature range. While conventional lithium-ion batteries (LIBs) have been widely adopted for consumer electronics and, more recently, electric vehicles, graphite-based LIBs remain bounded on two fronts: power and energy density. The first bound is on power density. Through a study of commercial lithium iron phosphate/graphite cells cycled under fast-charging conditions, this work shows that cell failure is driven jointly by the sluggish kinetics of lithium-ion intercalation into graphite and by the continuous growth of the solid electrolyte interphase (SEI), with the balance between these mechanisms shifting with charge rate and temperature. The second bound is on energy: because the usable energy density of the cell is capped by the intercalation chemistry of the graphite anode. As such, lithium metal anode is among the most promising candidates for high- energy-density batteries (> 500 Wh kg-1, > 1000 Wh L-1), owing to its high theoretical capacity, low reduction potential, and low density, but its commercialization is limited by short cycle life from continuous dendrite growth and by safety concerns arising from porous electrodeposition. As a common denominator in battery systems, electrolyte engineering remains the most direct means of addressing these issues, simultaneously governing the rate of lithium-ion transport through the bulk and across the interface and dictating the chemistry of the anode interphase. Yet the two objectives are often in tension: high-concentration and localized-high concentration electrolytes form favorable, anion-derived interphases but do so at the cost of high viscosity, reduced ionic conductivity, and poor wettability of thick electrodes, underscoring the need for an electrolyte that delivers fast transport and a stable interphase concomitantly.These limits motivate the central strategy of this dissertation: the design of liquefied gas electrolytes (LGEs), whose low viscosity, wide liquid range, and weakly coordinating character relax transport limitations while stabilizing reactive metal surfaces. Building on this approach, an aggregate-rich electrolyte that combines the high-voltage stability of an ionic liquid with the fluidity of the liquefied gaseous solvent, fluoromethane (FM), is shown to enable a thin lithium metal anode to cycle stably against a high-voltage nickel-rich cathode, retaining high conductivity and low-temperature operation.The same design principle is then generalized from lithium to sodium. Sodium is attractive for its natural abundance, and its lower charge density yields comparatively weak ion–solvent binding that pairs naturally with the weakly coordinating nature of liquefied gas solvents; exploiting this complementarity, a LGE is developed based on a similar concept of localized highly concentrated electrolytes that lowers the sodium-ion desolvation barrier and sustains fast transport, enabling stable sodium metal cycling deep into low-temperature regimes. Taken together, these studies establish that rationally engineered LGEs can unlock high-energy density systems with the use of alkali metal anodes while maintaining competitive power density and rate capability.