<i>Operando</i> Monitoring the Real-Time pH Evolution of Electrolytes in V-Based AZIBs via a Balloon-like Bent Fiber-Optic Sensor

Vanadium-based aqueous zinc-ion batteries (V-AZIBs) face significant challenges in terms of capacity degradation, zinc dendrite growth, and electrode corrosion; they are affected by the pH fluctuations of the electrolyte during cycling. In this study, a balloon-like bent fiber-optic sensor (BBFOS) was embedded into the electrode/electrolyte interface of a pouch cell, and light signals were collected during charging/discharging and further converted to pH values. Finally, a smart and nondestructive operando device was designed for operando real-time monitoring of the pH evolution of the electrolytes in V-AZIBs. The operando testing results showed a strong correlation between pH evolution and capacity decay in pouch cells and clarified the relationship between pH evolution mechanisms and capacity degeneration in V-AZIBs. Furthermore, the pH stability and the reversible layered chemical mechanism during the charging/discharging process were revealed by using operando BBFOS and operando X-ray powder diffraction analysis, respectively. The highly accurate and nondestructive operando device can promote the progress of AZIBs and other battery systems.