Solid Electrolytes Derived From Precursors and Liquid-Feed Flame Spray Pyrolysis Nano-Powders to Enable the Assembly of All-Solid-State-Batteries

Abstract

Solid electrolytes enable several next-generation energy storage systems (designs) including solid oxide fuel cells, super capacitors, and batteries. State-of-the-art battery technologies depend highly on the discovery of electrically insulating solids with high ionic mobilities. Current Li-ion batteries (LIBs), using traditional organic liquid electrolytes, suffer from poor electrochemical and thermal stabilities, leakage and flammability. Hence, replacing liquids with solid electrolytes offers multiple possibilities for developing new battery chemistries and designs. Solid electrolytes enhanced thermal stabilities provide opportunities to design new architectures that simplify battery configurations and reduce the peripheral mass of traditional LIBs. For example, the battery pack can be redesigned to minimize thermal management systems and overpressure vents are typically installed to overcome the challenges of using flammable liquid electrolytes. Furthermore, solid electrolytes facilitate adoption of newer battery chemistries. The development of state-of-the-art Li-S and Li-air batteries will benefit greatly from the use of solid electrolytes. In this dissertation, we investigate the design, synthesis, characterization, and performance of polymer and inorganic solid electrolytes to enable the assembly of all-solid-state batteries (ASSBs). Key properties that determine the utility of solid electrolyte include high ionic conductivity (>10-6 S/cm), high transference number (≈ 1), low electrical conductivity (>10-8 S/cm), wide electrochemical stability window (0-5 V vs Li/Li+), good chemical and thermal stability, excellent mechanical properties, low cost, ease of fabrication, eco-friendliness, and simple device integration. Our first study is to develop polymer precursor electrolytes that offer properties anticipated to be similar or superior to (lithium phosphorous oxynitride, LiPON) glasses. Such precursors offer the potential to be used to process LiPON-like thin glass/ceramic coatings for use in ASSBs. LiPON glasses provide a design basis for the synthesis of sets of oligomers/polymers by lithiation of OP(NH2)3-x(NH)x [from OP(NH)3], OP(NH2)3-x(NHSiMe3)x and [P=N]3(NHSiMe3)6-x(NH)x. Treatment with selected amounts of LiNH2 provides varying degrees of lithiation and Li+ conducting properties commensurate with Li+ content. Polymer electrolytes impregnated in/on Celgard exhibit Li+ conductivities up to ~1×10-5 S cm-1 at room temperature and are thermally stable to ≈150 °C. A Li-S battery assembled using a Li6SiPON composition polymer electrolyte exhibits an initial reversible capacity of 1500 mAh gsulfur-1 and excellent cycle performance at 0.25 and 0.5 C rate over 120 cycles at room temperature. We then show the versality of co-dissolution of poly(ethylene oxide) (PEO, Mn 900k) with LixPON and LixSiPON polymer systems at ratios of approximately 3:2 followed by casting provides transparent, solid solution films 25-50 µm thick, lowering PEO crystallinity, and providing measured impedance values of 0.1-2.8 mS/cm at ambient. These values are much higher than simple PEO/Li+ salt systems. These solid solution polymer electrolytes (PEs) are: (1) thermally stable to 100 °C; (2) offer activation energies of 0.2-0.5 eV; (3) suppress dendrite formation and (4) enable the use of lithium anodes at current densities as high as 3.5 mAh/cm2. Galvanostatic charge/discharge cycling of SPAN/PEs/Li cell (SPAN = sulfurized, carbonized polyacrylonitrile) shows discharge capacities of 1000 mAh/gsulfur at 0.25 C and 800 mAh/gsulfur at 1 C with high columbic efficiency over 100 cycles.