In Situ Liquid-Cell Transmission Electron Microscopy Insights Into Lithium-Ion Battery Materials Degradation: Challenges and Emerging Solutions.

Lithium-ion batteries (LIBs) have revolutionized energy storage technologies, yet their performance, safety, and durability remain constrained by complex degradation processes that limit their ability to meet growing demands. Addressing these challenges requires a deep understanding of the underlying mechanisms. Liquid cell transmission electron microscopy (LC-TEM), has emerged as a transformative platform for probing dynamic electrochemical processes, offering direct, multiscale insights from the crystal lattice to the full cell. Continuous advances in LC architectures, together with cutting-edge TEM capabilities, have positioned LC-TEM at the forefront of investigations of LIB degradation. Nevertheless, intrinsic challenges associated with liquid environments and individual cell architecture continue to limit the ability to resolve deeper details of degradation processes, creating a critical bottleneck. This review provides an overview of LIB performance challenges, recent TEM progress, and the evolution of LC-TEM designs, emphasizing their strengths, limitations, and applications across key LIB materials. Emerging directions such as advanced microfabrication, correlative imaging, and machine learning integration are highlighted as promising pathways to expand the technique's reach and overcome current constraints. Finally, strategies are proposed to bridge in situ observations with macroscopic battery behavior. The review outlines that these emerging directions extend benefits beyond LIBs to other electrochemical and functional materials.