Non-Technical Summary This CAREER project, supported by the Solid State and Materials Chemistry Program in the Division of Materials Research, advances fundamental insights into eventually designing higher energy, higher power, and more stable rechargeable batteries to meet the demand of the rapid-growing electric vehicle and energy storage market. Energy storage is a vital technology to enable the widespread adoption of renewable energy and to accelerate the technological advancement towards negative CO2 emission. The Li ion battery technology represents one of the most important energy storage technologies. Further development of Li ion batteries calls for more fundamental studies that can reveal reaction mechanisms and inform the design of new materials. Despite many years of materials development, most commercial Li-ion batteries still rely on several cathode materials that are derived from intercalation materials discovered in the 1980s. In these conventional materials, there are defined pathways for Li ions to transport. Recently, there have been exciting discoveries in new battery materials with disordered Li ion transport pathways. Unfortunately, these materials exhibit inferior battery performance compared to conventional materials, although theoretically they should provide much higher capacity. This project uses advanced experimental methods to develop fundamental understanding of electrochemical processes in these new disordered materials. The successful outcom