Emerging characterization techniques for delving polyanion-type cathode materials of sodium-ion batteries

Polyanion-type cathode materials have grown in leaps and bounds and become one of the promising candidates for metal-ion batteries since the successful case of LiFePO4 in lithium-ion batteries, which own stable crystal structure, high thermal stability, good ionic conductivity, adjustable voltage and chemical composition. However, further exploration is requisite, such as, the change of crystal/electronic structure, reaction mechanism, and structure evolution during charge/discharge processes, which results from variety of crystal types and redox centers, anion and cationic doping/substitution, as well as transition metal ion migration in polyanion-type materials. In this review, we focus on the advanced characterization techniques referred in polyanion-type cathode materials of sodium-ion batteries, mainly consist of the structure-related, morphology-related, composition-related techniques and in-situ/operando techniques during charge/discharge processes. The respective detection mechanisms, scope of application, information available and limitations of each technique are discussed in detail, and the latest developments of these characterization techniques used in polyanion-type materials are summarized. Advanced characterization techniques play a crucial role in understanding the reaction mechanisms of electrode materials, and can provide an important guiding principle for designing high-performance polyanion-type cathode materials and further optimizing the battery systems of sodium-ion batteries.