UC San Diego

Great advances have been made in lithium-ion batteries (LIBs) since they were first invented in 1970s. High energy density, fast charging rate and wide temperature operation have been achieved, which paves their way to broad applications in portable electronic devices, electric vehicles (EVs) and grid energy storage. However, despite their rapid performance improvement in recent years, the safety issue is still the sword of Damocles of LIBs due to their inherent flammability and thermodynamically non-equilibrium operation, which can lead to catastrophic consequences once they are exposed to extreme conditions such as overheating, shorting or cracking. In this thesis, strategies to foreclose potential safety hazards in LIBs have been explored in the level of material design. By integrating thermo-responsive polymer switching composites (TRPS) into the LIBs, it can successfully inhibit the heat generation when cells suffer from thermal abuses, which can prevent the occurrence of thermal runaway. To make TRPS materials more attractive in practical applications, we synthesized nanospiky Ni as the conductive fillers for the TRPS materials, which not only successfully reduce the thickness and improve the energy density of cell equipped with TRPS, but also boost the electrical conductivity of TRPS. To make TRPS materials can be seamlessly transferred to the industrial scale, we developed a solvent-based fabrication method to prepare PE-based TRPS materials on current collectors which can be directly integrated into the electrodes production process. We revealed that the size distribution range of conductive fillers plays critical role for the temperature response capability. The processing parameters such as substrate temperature, drying out time can affect the distribution of fillers in the polymer matrix, which determines the properties of TRPS films. We obtained TRPS films in a large size with uniform quality and demonstrated the protective effect in pouch cells. We also designed a general synthetic method to obtain carbides with the desired morphology. Systematic investigations were conducted by characterizations and calculations to understand the phase and morphology evolution process of as-prepared tungsten carbide. A spherical tungsten carbide was synthesized and employed as fillers for TRPS, which boosts the conductivity of WC-based TRPS materials. Overall, this dissertation offers a deep understanding and guidance for designing and fabrication of TRPS materials, taking one step further for the industrial production and real applications of TRPS to provide more safety protection of LIBs.