The chemo-mechanical instabilities in the cathode materials play a crucial role in the electrochemical performance of the alkali metal ion batteries.  During battery operation, alkali metals (Li, Na, and K) intercalate into the cathode structure, leading to a fundamental shift in the electronic and structural properties of the cathodes. Intercalation also leads to continuous volumetric changes in the cathodes, which leads to severe mechanical deformations and, eventually, particle fracture.  Furthermore, the interaction between organic electrolytes and cathode materials under an electric field could lead to the formation of cathode-electrolyte interphase (CEI) formation.  Operating the batteries at higher voltages is desirable for high energy density operations; however, it further escalates structural and interfacial instabilities on the cathodes, resulting in faster capacity decay and shortened lifetime of the batteries. Charging the batteries at faster rates is also needed for demanding applications such as electric vehicles, but diffusion-induced stress and associated metastable phase formations in the cathodes could also lead to severe capacity loss in the batteries.  Understanding the governing factors behind these interfacial and structural instabilities is crucial to designing robust battery materials. In this aspect, operando and non-destructive measurements are required to shed light on these instability mechanisms.

Here, I will start the talk by describing the principles of curvature measurement and digital image correlation (DIC) techniques for measuring stress and strains in the cathodes during battery operation. Both are optical and non-destructive techniques [1].  Curvature measurements provide information about stress generation in the electrodes with temporal resolution as a result of structural deformations and surface pressure build-up on the cathodes. DIC provides information about strain generations in the electrodes, with spatial and temporal resolution, as a result of volumetric changes in the composite electrodes during intercalation.   In this talk, I will present specific examples of how these operando techniques shed light on interfacial and structural instabilities in cathodes, such as amorphization during intercalation, the formation of CEI layers, and capacity loss at high voltages.  First, in-operando DIC measurements indicated that strain rates, rather than absolute strains, cause amorphization in the crystalline cathodes during ion intercalation [2].  Second,  the operando DIC, coupled with cryo X-ray photoelectron spectroscopy, determined the onset of the CEI formation on the cathode surface in organic electrolytes. [3] Furthermore, the role of the transition metals on the electrochemical performance of cathode structures and associated chemo-mechanical instabilities were investigated by combining electrochemical analysis, DIC, and spectroscopy techniques.  [4] The synchronization of in-operando curvature and DIC measurements also provided information about the governing forces behind the structural and interfacial instabilities in cathode electrodes at higher voltages. Overall, utilizing DIC and curvature measurements is a powerful tool to unravel charge storage mechanisms and governing forces behind interfacial and structural instabilities in electrode materials.