The lithium-ion batteries (LIBs) are the most researched battery system nowadays. LIBs, since their commercialization in the 1990s, provide better gravimetric/volumetric energy density, higher voltage, and cycle life with lower self-discharge than previously developed battery systems. All those advantages made the LIB systems an excellent candidate as the power source for portable electronic devices, electric-powered vehicles, space vehicles, electricity grid storage, and future electric aviation.
However, there is a limitation to developing higher-capacity lithium-ion batteries as we approach the practical limit of the presently used cathodes, which makes today’s high-energy LIBs. Moreover, small-form-factor portable electric devices and large-scale applications of LIB systems for electric vehicles, space vehicles, electric and hybrid aircraft, and grid storage are all facing challenges of lower than required safety levels in today’s LIBs. Thus, developing new technologies and components of batteries with higher energy density and safety levels is the most desirable research & development topic. In this case, the lithium-sulfur battery (LSB) system is an excellent candidate for increasing the battery system's energy, beyond the energy storage limit of today’s LIBs. With ~650 Wh kg-1 of gravimetric energy density, Li-S battery (LSB) achieved more than two times the energy density of state-of-art LIBs (~250 Wh kg-1). Organic liquid electrolyte (OLE) is one of the essential components in LIBs due to its high ionic conductivity (10-2-10-1 S cm-1) and electrode wettability at ambient conditions. As the temperature rises, the lack of thermal stability and high flammability of OLEs becomes a significant challenge in designing a safe operable LIB. Even a moderately elevated temperature (>65°C) can severely diminish the useful capacity and cycle life and can pose thermal safety issues (such as fire and explosions). Pursuing safer electrolytes led battery researchers and manufacturers around the globe to a significant task in developing a high-conductivity, thermally-stable solid-state electrolyte (SE). Depending on material selection (polymer or inorganic ceramics or polymer-ceramic composite), the solid electrolyte can be incombustible, nonvolatile, nonflammable, and stable at elevated temperatures. Combining the concept of LSB (high energy) and SE (enhanced safety), researchers introduced high energy density, high safety all-solid-state batteries, particularly all-solid-state lithium metal batteries. My research involves understanding the performance and safety behavior of next-generation, high-energy, high-safety all-solid-state lithium batteries, including LSB and LIBs. In my study, we experimented with sulfur-infused carbon as high-capacity cathode materials. We infused the sulfur at different temperatures. We utilized carbon cloth, activated carbon on carbon cloth, and hierarchical porous carbon on carbon cloth as substrate. The cathodes were tested in the baseline liquid electrolyte-based lithium-sulfur battery. To increase the safety of the lithium-sulfur battery, we synthesized different solid electrolytes based on sulfides, such as lithium phosphorous sulfur bromine iodine (LPSBI) and lithium phosphorous sulfur chlorine (LPSCl). The selection of these Li+ conducting sulfides was based on different useful properties such as i) high Li+ conductivity, ii) high interfacial stability with lithium anodes, and iii) high compressibility required for cell fabrication at room temperature. For the synthesis of Li+ conducting sulfide solid electrolyte, we have developed a scalable synthesis route that includes material sintering in a furnace in an Ar glovebox and eliminated the risk of letting the material contact with air compared to the state-of-the-art procedure that involves sintering the materials in a volume constraint quartz tube. Learned the challenges of state-of-the-art rechargeable and primary LSBs. For the first time, we constructed and studied the performance of sulfide SE-based primary (non-rechargeable) LSBs. My research suggests that future research should address optimizing i) sulfur cathode loading, ii) stack pressure, iii) electrode kinetics to make solid-state lithium-sulfur a secondary battery. The lithium (Li) anode can undergo infinite volume change during the charge-discharge of LSBs. For example, if one starts with a Li thickness of 100 µm, during discharge thickness of the Li anode can vary from 100 µm to 0. This kind of Li volume change, especially when using SEs makes the pressure applied on the battery critical. Without proper pressure, the connectivity of LSB components (viz., anode, electrolyte, and cathode) will falter and make the battery dysfunctional. Thus, understanding the effect of pressure on the battery plays an important role in solid-state LSBs. So we studied the effect of pressure on lithium deposition (charge) and strapping (discharge) against an important sulfide SE (Lithium Phosphorus Sulfur Bromine Iodide, LPSBI). We adopted a unique charge/discharge protocol using asymmetric cell configuration and determined the maximum allowed stripping and deposition current density at various pressures. This research will facilitate future progress on rechargeable solid-state LSBs and other rechargeable solid-state LIBs. Finally, my research focused on understanding the safety (thermal, electrochemical, and environmental) of sulfide SE-based all-solid-state LIBs using high voltage cathode (lithium cobalt oxide, LiCoO2 and low voltage anode (graphite, C). Thermal safety has been evaluated using Differential Scanning Calorimetry (DSC) and electrical safety by monitoring the open circuit voltage of a fully charged battery at different temperatures up to 170°C. Environmental safety has been evaluated by measuring the quantity of released H2S gas. The thermal, electrochemical, and environmental safety data obtained on sulfide SE-based all-solid-state LIBs has been found superior to commercial-type organic LE-based LIBs.