In Situ Stress and Strain Detection in Lithium-ion Batteries: A Review of Optical, Acoustical, and Electrical Techniques.

Lithium-ion batteries (LIBs) have emerged as the key devices for energy storage, powering a global shift toward electrification in transportation and portable electronics. However, the relentless pursuit of higher energy densities for LIBs is fundamentally challenged by chemo-mechanical degradation, a process during which electrodes undergo volume changes during cycling, which in turn precipitates internal stress and limits battery life span. While the importance of these chemo-mechanical phenomena is widely recognized, a comprehensive review that systematically categorizes the diverse landscape of in situ detection techniques for stress and strain detection has been conspicuously absent. Herein, we address this gap by introducing a framework that organizes these techniques based on their physical modality: optical, acoustic, and electrical. Within this framework, we critically evaluate key methods, including optical laser beam position detection and multi-beam optical stress sensor, acoustic ultrasonic wave and acoustic emission, and electrical resistance strain gauge and in situ expansion meter. For each method, we detail its working principle, compare its advantages, and showcase key research applications that provide insight into improving battery performance. Finally, we outline the main challenges and future directions for in situ stress and strain detection in lithium-ion batteries.