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Sodium chromium oxide, NaCrO2, exhibits promising features as a cathode electrode in Na-ion batteries, yet it encounters challenges with its capacity fading and poor cycle life. NaCrO2 undergoes multiple phase transitions during Na ion intercalation, eventually leading to chemical instabilities and mechanical deformations. Here, we employed the digital image correlation (DIC) technique to probe electrochemical strain generation in the cathode during cycling via cyclic voltammetry and galvanostatic cycling. The electrode undergoes significant irreversible mechanical deformations in the initial cycle, and irreversibility decreases in the subsequent cycles. During desodiation and sodiation, the electrode initially undergoes volume contraction at lower state-of-(dis)charge followed by expansions at a higher state-of-(dis)charge. The similar progression between strain and capacitive derivatives points out the phase-transition-induced deformations in the electrode. The evolution of cumulative irreversible strains with cycling time indicates the irreversibility rising from the formation of cathode-electrolyte interphase layers. The study demonstrates valuable insights into mechanical deformations in NaCrO2 electrodes during battery cycling, which is critical to engineer mechanically robust cathodes for Na-ion batteries. 

The battery chemistry must be diversified to achieve a sustainable energy landscape by effectively utilizing renewable energy sources. Alkali metal-ion, all-solid-state, metal-air batteries, and multivalent batteries offer unique cost, safety, raw material abundance, energy, and power density solutions. However, realizing these “Beyond Li-ion batteries” must uncover their working principles and performance & property relationship. In this aspect, mitigating chemo-mechanical instabilities in the structure and surface of the electrodes plays a crucial role in their performance. Unfortunately, the coupling between electrochemical and mechanical interactions is often poorly understood due to a lack of operando characterization. This review article explains the working principles of curvature measurement and digital image correlation for measuring stress and strain generations in battery materials. We provided specific examples of how these operando mechanical measurements shed light on instabilities in alkali-metal ion electrodes, solid electrolytes, Li-O2 batteries, and aqueous Zn-ion batteries. Operando mechanical measurements offer an effective way to map changes in the physical fingerprint of the battery materials, therefore providing crucial information to elucidate instabilities in battery materials. 

Layer-structured Na intercalation compounds such as NaxMO2 (M=Co, Mn, Cr) have attracted much attention as cathode materials for sodium-ion batteries due to their high volumetric and gravimetric energy densities. Among them, NaCrO2 with layered rock salt structure is one of the promising cathodes since NaCrO2 has a desirable flat and smooth charge/discharge voltage plateau.1 In addition, NaCrO2 has the highest thermal stability at charged state which makes it a potentially safer cathode material.2 The NaCrO2 exhibits a reversible capacity of 110 mAh g-1 with good cycling performance.3 However, the transition metal oxide (TMO) cathode materials in NIBs undergo severe chemo-mechanical deformations which leads to capacity fade and poor cycling and is the limiting factor of NIBs. The electrochemical characterization and examination of the electrode structure were the primary focus of several investigations. To improve the lifespan and performance of electrode materials for Na-ion batteries, it is vital to comprehend how Na ions impact the chemo-mechanical stability of the electrodes. In this talk, we will discuss the driving forces behind the structural and interfacial deformations on NaCrO2 cathodes. Digital image correlation measurements were conducted to probe strain evolution in the electrode during cycling. The free-standing composite NaCrO2 electrode was used for stain measurements in custom-cell assembly. The battery was cycled against Na metal in 1 M NaClO4 in PC. The first part of the study involves structural and interfacial deformations in the lower voltage range of 2.3 V to 3.5 V where x<0.5 in NaxCrO2. And the second part focuses on the structural and interfacial deformations in the voltage range of 2.3 V to 4.7 V where x>0.5 in NaxCrO2. In the preliminary studies, we observed that the initial insertion of Na ions leads to negative strain evolution (contraction) in the electrode, followed by expansions in the electrode at a higher state of discharge. Similar phenomena are also observed during charge cycles, where extraction of Na results in an initial contraction in the electrode, followed by expansion at a higher state of charge. Understanding the mechanisms behind chemo-mechanical deformations will allow to tune structure & material property for better electrochemical performance.