Li-ion batteries (LIBs) are widely used as energy storage media because of their high energy density, excellent power density, and slow self-discharge rates. In fact, they have been dominating the market of portable electronics since their launch in the 1990s. However, the fast-growing demand for large battery packs, especially those to be equipped on electric vehicles, poses severe safety concerns because the aprotic electrolytes contained are volatile, toxic, and flammable, and could potentially lead to fires and explosions. Solid-state batteries (SSBs) are promising alternatives to LIBs and the ultimate solution to the safety issues because of the non-flammability of the solid electrolytes (SEs). The compatibility between SEs with Li metal anodes and with high-voltage cathode...[ Read more ]

Li-ion batteries (LIBs) are widely used as energy storage media because of their high energy density, excellent power density, and slow self-discharge rates. In fact, they have been dominating the market of portable electronics since their launch in the 1990s. However, the fast-growing demand for large battery packs, especially those to be equipped on electric vehicles, poses severe safety concerns because the aprotic electrolytes contained are volatile, toxic, and flammable, and could potentially lead to fires and explosions. Solid-state batteries (SSBs) are promising alternatives to LIBs and the ultimate solution to the safety issues because of the non-flammability of the solid electrolytes (SEs). The compatibility between SEs with Li metal anodes and with high-voltage cathodes also lead to SSBs with higher energy densities compared with conventional LIBs. However, the performance of currently available SSBs is far below that of LIBs, the main reasons being the slow Li transport in SEs and the interfacial issues between SEs and electrodes. The slow Li transport in SEs leads to Ohmic resistance. The poor solid-solid contact, space charge layers, and impurities at the interfaces result in large interfacial impedance. All of these factors contribute to the unsatisfactory rate capability and poor cycling stability. In this thesis, we mainly focus on the Li diffusion in SEs and the interfacial issues between SEs and electrodes. We use atomistic simulations combined with thermodynamics to study the ion transport in the bulk of SEs and the stability of SEs and interfaces in SSBs. We use the insight from the simulations to experimentally design an SSB which is able to stably cycle at room temperature (RT).

This thesis is divided into three parts. In the first part, we use molecular dynamics (MD) simulations and density functional theory (DFT) to study the cation diffusion mechanisms in SEs. We study how different factors influence the ionic conductivity of SEs, including types of defect, defect concentration, and anion group dynamics. In the second part, we investigate the stability of SEs and their interfaces with electrodes. We use DFT-based phase diagrams to predict the reaction between SEs and the electrodes and to look for guiding principles to design interfaces in SSBs. In the last part, we use the knowledge obtained from the computations in the first two chapters to develop a plastic crystal material as the interlayer between SEs and electrodes in SSBs. We demonstrate that with the inclusion of such interlayer, a garnet-based ceramic-type SSB is able to cycle stably at RT.