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Abstract: Lithium-ion batteries have been extensively employed in portable electronics and electric vehicles because of their high energy/power density. However, they inevitably suffer from severe energy/power losses in cold environments, especially when temperatures drop below −20 °C. Such poor low-temperature performance limits their applications for aeronautics/space missions, polar expeditions, and many military and civil facilities in cold regions, in which a battery operating temperature below −40 °C is typically required. Therefore, improving the battery performance (energy density, rate capability, and lifetime) at low temperatures requires exploiting new battery materials and chemistries to minimize the need for ancillary thermal systems.

The first part of this thesis investigates non-concentrated ether-based electrolytes for lithium-ion batteries with graphite anode. We demonstrate that ether-based electrolytes can achieve reversible graphite intercalation even at 1 M salt concentration by tailoring the solid-electrolyte interphase (SEI) structure through the preferential decomposition of anion and fluorinated additive. The optimized electrolyte enables lithium-ion batteries with outstanding fast-charging capability, long-term cycling stability, as well as wide-temperature operability down to −40 °C.

The second part of this thesis replaces the graphite anode with Li metal for high energy density and examines the importance of SEIs to the low-temperature performance of Li metal plating and stripping. We find that both solvent chemistry and solvation chemistry play critical roles in tuning Li+−anion coordination, which in turn promotes anion-derived SEIs with inorganic-rich compositions. By tailoring the anion distribution in the bulk electrolyte and hence the interphase, we demonstrate that Li metal can be cycled under extreme temperature conditions (e.g., −80 °C) with high Coulombic efficiencies.

In the final part, we present a series of lithium thiophosphate complex materials dissolved in organic solvents as high-capacity cathodes for lithium−sulfur (Li−S) batteries. The complexation chemistry can effectively accommodate discharge reaction products without precipitation. Therefore, major issues in Li−S batteries, such as sluggish redox kinetics, volume expansion, and voltage polarization, are mended. With the novel complexes as cathode materials, high specific capacity and excellent cycling stability are achieved at room temperature. Moreover, the highly reversible all-liquid electrochemical conversion enables wide-range cell operability down to −60 °C.

Thesis Committee: Prof. Weiyang (Fiona) Li (Chair), Prof. Jifeng Liu, Prof. William Scheideler, Prof. Hai Wang (Stanford University)