Advanced characterizations and measurements for sodium-ion batteries with NASICON-type cathode materials

NASICON (Na superionic conductor)-type cathode materials for sodium-ion batteries (SIBs) have attracted extensive attention due to their mechanically robust three-dimensional (3D) framework, which has sufficient open channels for fast Na+ transportation. However, they usually suffer from inferior electronic conductivity and low capacity, which severely limit their practical applications. To solve these issues, we need to deeply understand the structural evolution, redox mechanisms, and electrode/electrolyte interface reactions during cycling. Recently, rapid developments in synchrotron X-ray techniques, neutron-based resources, magnetic resonance, as well as optical and electron microscopy have brought numerous opportunities to gain deep insights into the Na-storage behaviors of NASICON cathodes. In this review, we summarize the detection principles of advanced characterization techniques used with typical NASICON-structured cathode materials for SIBs. The special focus is on both operando and ex situ techniques, which help to investigate the relationships among phase, composition, and valence variations within electrochemical responses. Fresh electrochemical measurements and theoretical computations are also included to reveal the kinetics and energy-storage mechanisms of electrodes upon charge/discharge. Finally, we describe potential new developments in NASICON-cathodes with optimized SIB systems, foreseeing a bright future for them, achievable through the rational application of advanced diagnostic methods.