Neutron Reflectometry of Lithium‐Based Secondary Batteries

The rapid advancement of integrated circuits and artificial intelligence has accelerated the proliferation of smart devices, driving the demand for high‐performance lithium‐based secondary batteries (LSBs). Despite notable technological advancements, LSBs continue to face significant challenges, particularly regarding cycle life, power density, specific capacity, and long‐term stability. To optimize these batteries, advanced analytical techniques are crucial for probing fundamental processes such as lithium‐ion migration, electrode material transformations, and solid electrolyte interphase evolution. While techniques such as scanning electron microscopy, transmission electron microscopy, and X‐ray reflectometry offer valuable insights into surface morphology and structural properties, their sensitivity limitations to lighter elements like lithium and hydrogen hinder comprehensive analysis. In contrast, neutron reflectometry (NR) provides a distinct advantage, offering nondestructive, lithium‐sensitive, and deeply penetrating insights into the internal dynamics of LSBs. In situ NR facilitates real‐time monitoring of structural transformations at the electrode/electrolyte interface and lithium distribution during cycling, while enabling the quantification of chemical composition under operational conditions. This review examines the advantages of NR over conventional techniques, evaluates its strengths and limitations, and surveys recent applications of NR in LSB research. The potential of NR in advancing the development of high‐performance LSB technologies is also discussed.