HIGH-ENERGY-DENSITY LITHIUM-SULFUR BATTERIES USING GARNET SOLID ELECTROLYTE: PERFORMANCE AND CHARACTERIZATION
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There is every-growing clean energy storage systems demand to address the climate change challenges. The Lithium-Sulfur (Li-S) battery using solid-state electrolyte (SSE), therefore, are becoming a rising star to meet this requirement due to the low cost and high domestic availability of sulfur, exceptionally high theoretical energy density of sulfur chemistry, less flammability, the potential for suppression of the polysulfide shuttle, Li dendrite growth, low coulombic efficiency, and short circuiting in more conventional liquid electrolyte Li-S batteries. The most popular SSEs in Li-S battery field are Li10GeP2S12 (LGPS) and Li stuffed garnet type Li7La3Zr2O12 (LLZO) with a space group of Ia3 ̅d. LGPS has a high ionic conductivity of 1–10 mS/cm at room temperature (22℃), but the generation of H2S toxic gas when reacting with moisture and the instability with Li metal limit its applications. LLZO is a highly promising SSE for Li-S batteries due to its reasonably high ionic conductivity (0.1–1 mS/cm) at 22℃ and excellent chemical stability with Li metal. However, Li dendrite growth is still observed in LLZO. To overcome the potential Li dendrite growth issue, our group introduced a novel 3D porous/dense bilayer and porous/dense/porous trilayer LLZO structures that achieve an exceptional Li stripping/plating performance at a high current density of 10 mA/cm2 at 22℃ with no applied pressure. Although significant improvement has been done in mitigating the LLZO/Li anode interface, further work on stabilizing the sulfur/LLZO interface still needs to be done to achieve high energy density and stable cycling Li-S batteries. Through the studies in this dissertation, it was observed that La segregation to the LLZO surface on the sulfur cathode side led to Li-S battery charge failure. To address the issue, a PEO-based interlayer was applied on the cathode side to physically separate sulfur cathode and LLZO. Consequently, the Li-S batteries demonstrated a high initial discharge capacity of 1307 mAh/g at 22℃, corresponding to an energy density of 134 Wh/kg and 639 Wh/L. Next, since the PEO-based interlayer has a low ionic conductivity, an in-situ formed gel polymer electrolyte (GPE) was invented as a catholyte that had a high ionic conductivity of 3.5–5.6 mS/cm at 22℃. With an organic sulfur cathode (active material: sulfurized polyacrylonitrile) and a thin bilayer LLZO architecture, a very stable cycling using high sulfur loading (5.2 mg/cm2) was obtained for 60 cycles at a discharge current density of 0.87 mA/cm2 with a high initial discharge capacity of 1542 mAh/g, corresponding to an energy density of 223 Wh/kg and 769 Wh/L. In addition, using the same configuration and sulfur loading but using a different cell, 80% capacity retention for over 265 cycles was demonstrated at a discharge current density of 1.74 mA/cm2 at 22℃. In the third project, as a small amount of flammable organic liquid catholyte and/or GPE compromises on the safety of solid-state batteries, the proof-of-concept “all-solid-state” Li-S battery using LLZO electrolyte was first innovated through a novel three-phase sulfur cathode to meet the high safety demand of solid-state Li-S batteries. In addition, by using the same “all-solid-state” battery design and a 3D column LLZO architecture, a high energy density of 338 Wh/kg and 797 Wh/L was demonstrated.