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The anode/electrolyte interface behavior, and by extension, the overall cell performance of sodium‐ion batteries is determined by a complex interaction of processes that occur at all components of the electrochemical cell across a wide range of size‐ and timescales. Single‐scale studies may provide incomplete insights, as they cannot capture the full picture of this complex and intertwined behavior. Broad, multiscale studies are essential to elucidate these processes. Within this perspectives article, several analytical and theoretical techniques are introduced, and described how they can be combined to provide a more complete and comprehensive understanding of sodium‐ion battery (SIB) performance throughout its lifetime, with a special focus on the interfaces of hard carbon anodes. These methods target various length‐ and time scales, ranging from micro to nano, from cell level to atomistic structures, and account for a broad spectrum of physical and (electro)chemical characteristics. Specifically, how mass spectrometric, microscopic, spectroscopic, electrochemical, thermodynamic, and physical methods can be employed to obtain the various types of information required to understand battery behavior will be explored. Ways are then discussed how these methods can be coupled together in order to elucidate the multiscale phenomena at the anode interface and develop a holistic understanding of their relationship to overall sodium‐ion battery function. [ABSTRACT FROM AUTHOR]

Amorphous Al2O3 film that naturally exists on any Al substrate is a critical bottleneck for the cyclic performance of metallic Al in rechargeable Al batteries. The so‐called electron/ion insulator Al oxide slows down the anode's activation and hinders Al plating/stripping. The Al2O3 film induces different surface properties (roughness and microstructure) on the metal. Al foils present two optically different sides (shiny and non‐shiny), but their surface properties and influence on plating and stripping have not been studied so far. Compared to the shiny side, the non‐shiny one has a higher (~28 %) surface roughness, and its greater concentration of active sites (for Al plating and stripping) yields higher current densities. Immersion pretreatments in Ionic‐Liquid/AlCl3‐based electrolyte with various durations modify the surface properties of each side, forming an electrode‐electrolyte interphase layer rich in Al, Cl, and N. The created interphase layer provides more tunneling paths for better Al diffusion upon plating and stripping. After 500 cycles, dendritic Al deposition, generated active sites, and the continuous removal of the Al metal and oxide cause accelerated local corrosion and electrode pulverization. We highlight the mechanical surface properties of cycled Al foil, considering the role of immersion pretreatment and the differences between the two sides. [ABSTRACT FROM AUTHOR]

The formation of the solid electrolyte interphase (SEI) on HC composite electrodes plays a crucial role in enhancing the performance and operational stability of sodium (Na+) ion batteries. It has been demonstrated that for HC anodes improved electrochemical performance, e. g., increase in coulombic efficiency (CE) and improved rate performance have been achieved in ether‐based electrolytes. Here, we investigate spray‐coated HC composite electrodes charged at low and high current rates in 1 M sodium hexafluorophosphate (NaPF6) in diglyme using half‐cell experiments. The pristine and cycled HC anodes were examined in terms of conductivity and their electrochemical properties after cycling. In 1 M NaPF6 ether‐based electrolyte, the spray‐coated HC composite electrodes (film thickness approx. 22.0 μm with an active mass loading of approx. 2.0 mg cm−2) reached a discharge capacity of 431 mA h g−1 at 0.1 C that stays constant for 40 cycles, which is substantially higher than that obtained in carbonate‐based electrolytes. We investigated the formed interphase using conductive atomic force microscopy (c‐AFM), scanning electrochemical microscopy (SECM), X‐ray photoelectron spectroscopy (XPS) and time‐of‐flight secondary‐ion mass spectrometry (ToF‐SIMS), revealing distinct differences for longer cycling and at varying current rates which indicates that the properties of the formed SEI layers are influenced by the formation conditions. [ABSTRACT FROM AUTHOR]

Sodium‐ion batteries are among the most promising alternatives to lithium‐ion batteries. Hard carbon (HC) electrodes have been recognized as suitable active anode material for mono‐valent ion batteries. Here, we present a simple and cost‐effective spray‐coating process to prepare HC composite electrodes on copper current collectors with different binder (sodium carboxymethyl cellulose, CMC) content and different HC particle sizes. The spray‐coated electrodes were evaluated and tested in 1 M sodium perchlorate (NaClO4) in propylene carbonate (PC) in dependence of the CMC content with and without fluoroethylene carbonate (FEC) as additive, and the performance was also compared to doctor bladed HC electrodes. Spray‐coated anodes in Na half‐cells revealed improved capacity during the first cycles compared with doctor bladed anodes with similar thicknesses. Time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) studies were performed, which revealed a significant increase of inorganic fluoro‐compounds in the formed solid electrolyte interphase (SEI) when FEC was present as additive. In addition, first single electrode microcalorimetry studies on spray‐coated thin HC composite electrodes yielded an entropy of the sodiation process of 80 J mol−1 K−1 at high state of charge (SoC), comparable to that of bulk Na deposition. [ABSTRACT FROM AUTHOR]