Current state-of-the-art characterization techniques for probing the layered oxide cathode materials of sodium-ion batteries

Layered transition-metal oxides have been extensively pursued as promising cathodes for sodium-ion batteries (SIBs) by virtue of their two-dimensional Na-diffusion channels and high theoretical capacities. Nevertheless, irreversible phase transitions, structural instability, and moisture sensitivity place obstacles in their way to approach higher performance. Aiming at tackling these challenging issues, an in-depth understanding of the structural evolutions, morphology changes, composition and valence variations, as well as the electrode/electrolyte interface reactions upon battery cycling is of vital importance. Current state-of-the-art characterization techniques can gain valuable insights into the elusive reaction mechanisms, yield an overall picture of the battery configurations, and provide a guideline for the design of new electrode materials. Herein, the latest progresses on the applications of advanced analytical techniques to probe the Na-storage layered oxide cathodes are comprehensively summarized. In-situ or operando techniques are highlighted in this review to directly link the real-time structure, morphology, composition information with the electrochemical response, and the electrochemical measurements are also mentioned in selected examples. Special attention is paid to the detection principle of each technique and what valuable information can be obtained. Finally, the future developments of layered oxides towards highperformance SIB cathode materials with the help of advanced diagnostic methods are well prospected.