COMPOSITE POLYMER ELECTROYTES FOR ALL-SOLID-STATE LITHIUM BATTERIES USING NANOSTRUCTURED CERAMIC GARNET FILLERS

摘要

The Li ion conducting garnet Li_7La_3Zr_2O_(12) (LLZO) has attracted substantial interest as a solid electrolyte for next generation, all-solid-state lithium batteries on account of its relatively high ionic conductivity of ～10~4 S cm~(-1), non-reactivity with lithium, and wide voltage stability window (＞ 5 V vs. Li/Li~+). Although more than a decade has passed since LLZO was first reported, the application of LLZO as a ceramic electrolyte in all-solid-state batteries is still met with several practical challenges due to its brittle nature and the difficulty of forming good contacts or interfaces with electrodes. Recently, the incorporation of LLZO as nanostructured ceramic fillers within solid polymer electrolytes has received great interest. The application of nano-sized particles as ceramic fillers has already been demonstrated to be effective for enhancing the mechanical stability and ionic conductivity of polymer-based solid electrolytes, but these fillers have mostly consisted of spherical particles of inert or "passive" components without intrinsic Li~+ conductivity. Recent studies using LLZO-embedded into polymer films have revealed different degrees of effectiveness, indicating that more careful design of the composite polymer electrolytes (CPEs), including optimization of the LLZO filler properties and more detailed mechanistic study of the Li~+ transport pathways, may be needed for the development of CPEs with high ionic conductivity. In our work, we show that by incorporating only 5 wt% of electrospun LLZO nanowires into a polyacrylonitrile-LiCIO_4 matrix, the room temperature ionic conductivity of the composite is increased 3 orders of magnitude to 1.31 × 10~(-4) S/cm. CPEs made using LLZO nanoparticle and AI_2O_3 nanowire fillers are also studied to elucidate the role of filler type (active vs. passive), LLZO composition (undoped vs. doped), and morphology (nanowire vs. nanoparticle) on the CPE conductivity. It is demonstrated that both intrinsic Li~+ conductivity and nanowire morphology are needed for optimum performance. Subsequent studies using solid-state nuclear magnetic resonance and synchrotron X-ray fluorescence microscopy are used to investigate the Li transport pathways and understand the dispersion of the nanowires within the composite films.