Space-Confined Electrochemical Reactions and Materials for High-Energy-Density Batteries

ConspectusLithium-ion batteries have demonstrated their profound impacts on global energy transition over the past few decades. Conventional Li-ion batteries operated based on topochemical Li+ intercalation in layered-structure electrode materials usually show highly stable and reversible electrode reactions yet limited energy-storage capability and cannot meet the increasing demands for electric cars and other emerging storage applications. Targeted at high-energy rechargeable batteries, materials that undergo lattice-unconstrained electrochemical storage reactions (e.g., conversion-type chalcogens, alloying-type silicon and their compounds, and Li metal) are able to host more Li ions per mass/volume unit and are considered the key to realize the "beyond Li-ion" technology. Nevertheless, the Li-uptake/release process in nonintercalation electrode materials is usually accompanied by rapid structure degradation, drastic volume variation and formation of an unstable electrochemical interface, which could result in performance fade, failure, and safety issues of the battery. By encapsulating nonintercalation electrode materials into a three-dimensional conductive framework, the composite electrode acquires much improved Li-storage performance in rechargeable batteries. In the previous works, the improved performance was ascribed to distinct advantages of space-confined electrode materials and reactions over the unconfined counterparts, including preserved structural and interfacial integrity, suppressed parasitic reactions, and efficient charge transfer. However, the fundamental physical chemistry behind the structure-performance correlation was rarely discussed.In this Account, we revisit the basic thermodynamics, from a classic system-environment view, of a space-confined electrochemical process occurring in a nonintercalation electrode material and compare it with the unconfined process to understand the advantageous effects of confined materials design on the Li storage. Based on our previous studies, we show the intrinsic challenges with respect to their open-architecture of the nonintercalation electrode materials, including chalcogen cathode materials (S, Se and Te), silicon-based anode materials (Si and SiOx), and finally, the Li-metal anode. We also highlight the recent advances on the ingenious design of spatially confined architecture and discuss the structure–performance correlations and the underlying mechanism. Finally, we look at future potentials and challenges of spatial confinement in designing high-performance electrode materials for the next-generation rechargeable batteries.