Redox Mechanism in Na-Ion Battery Cathodes Probed by Advanced Soft X-Ray Spectroscopy

Abstract

A Na-ion battery (NIB) device is a promising solution for mid-/large-scale energy storage, with the advantages of material abundance, low cost, and environmental benignity. To improve the NIB capacity and retainability, extensive efforts have been put into the developments of NIB electrode materials. The redox activities of the transition metal (TM)-based NIB electrodes are critical in defining the capacity and stability. Here, we provide a comprehensive review on recent studies of the redox mechanisms of NIB cathodes through synchrotron-based soft X-ray absorption spectroscopy (sXAS) and mapping of resonant inelastic X-ray scattering (mRIXS). These soft X-ray techniques are direct and effective tools to fingerprint the TM-3 d and O- p states with both bulk and surface sensitivities. Particularly, 3 d TM L -edge sXAS has been used to quantify the cationic redox contributions to the electrochemical property; however, it suffers from lineshape distortion for the bulk sensitive signals in some scenarios. With the new dimension of information along the emitted photon energy, mRIXS can address the distortion issue of in TM- L sXAS; moreover, it also breaks through the limitation of conventional sXAS on detecting unconventional TM and O states, e.g., Mn(I) in NIB anode and oxidized oxygen in NIB cathodes. The mRIXS fingerprint of the oxidized oxygen state enables the detection of the reversibility of the oxygen redox reaction through the evolution of feature intensity upon electrochemical cycling and thus clarifies various misunderstandings in our conventional wisdom. We conclude that, with mRIXS established as a powerful tool, its potential and power will continue to be explored for characterizing novel chemical states in NIB electrodes.

Laboratory Operando XAS Study of Sodium Iron Titanite Cathode in the Li-Ion Half-Cell

“…However, the soft X-ray energy region represents a challenge for operando studies, even at synchrotron radiation sources [ 43 , 44 , 45 ]. Therefore, the use of DFT theoretical approaches is a powerful addition to laboratory experimental studies [ 46 ].…”

Guidelines for Air-Stable Lithium/Sodium Layered Oxide Cathodes

“…However, many questions are still open, including the sequence of Na + (Li + ) extraction and H + insertion, the elements involved in the redox process, and to what extent does Na + (Li + )-H + exchange and TM oxidation participate in the reactions. Considering the severe structural changes during exposure and the coverage of various products on the particle surface, normal XAS and XPS might not be so reliable and other state-of-the-art characterizations are highly needed. − …”

Spatial and Temporal Analysis of Sodium-Ion Batteries

“…The TFY mode is subsurface-sensitive with a probing depth of 50−100 nm depending on the sample. 173 However, it usually suffers from self-absorption and saturation effects, which is minimized by iPFY. In contrast, TEY mode probes a depth of a few nanometers and partial electron yield is even more shallow, attributed to the limited escaping depth for electrons compared to photons depending on mean free path.…”

“…The former includes total fluorescence yield (TFY), partial fluorescence yield (PFY), and inverse partial fluorescence yield (iPFY), whereas the latter consists of total electron yield (TEY), partial electron yield (PEY), and Auger electron yield (AEY). The TFY mode is subsurface-sensitive with a probing depth of 50–100 nm depending on the sample . However, it usually suffers from self-absorption and saturation effects, which is minimized by iPFY.…”