Role of operando microscopy techniques on the advancement of sustainable sodium-ion battery anodes

Abstract To build a sustainable society, the governments and industries are replacing fossil fuels for clean energy technologies, which produce electricity intermittently and need efficient energy storage systems. Thus, Na-ion batteries are the most attractive technologies as sodium is the most cost effective, earth-abundant and environment-friendly alternative. However, the design of anodic materials with reversible Na-ion (de)insertion represents one of the principal issues to improve in anodic electrodes. Typical ex-situ microscopy techniques may not correctly reveal the actual phenomena occurring at working conditions in NIB anodes, limiting the fundamental understanding of the anode performance. Therefore, monitoring in real-time the structural and chemical transformation of Na-ion anodes during (dis)charge cycling is a paramount objective. The development of operando methodologies, such as transmission electron, atomic force and light microscopies, has allowed to follow: phase transformation, growth of solid electrolyte interface and sodium dendrites. The progress on operando microscopy techniques indicates a promising route to acquire meaningful information to draw the path to improve the efficiency and sustainability of Na-ion anodes. This review emphasizes the need of operando techniques for NIBs, their state-of-the-art findings, cohesive understanding of various insertion, alloying and conversion reactions, and unanswered questions providing direction to futuristic research.