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-3d and O-p states with both bulk and surface sensitivities. Particularly, 3d 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.
Highlights
With the increasing penetration of the wind and solar energy into the power grid, large-scale energy storage devices are of severe demand to meet the intermittency of the renewable energy sources
In this review, we summarize the advancements of the cationic and anionic redox mechanisms in the Na-ion battery (NIB) electrodes that have been deciphered by SXS
With the new generation of diffraction limited light sources and further RIXS spectrometer upgrades into the spatial and temporal domains (Chuang et al, 2020), mRIXS will become more and more powerful and enable new opportunities for in-depth analysis of Na-ion battery (NIB) materials
Summary
With the increasing penetration of the wind and solar energy into the power grid, large-scale energy storage devices are of severe demand to meet the intermittency of the renewable energy sources. Deciphering the redox mechanisms of the NIB cathodes during the electrochemical cycling requires an incisive probe of the chemical states with elemental sensitivity, e.g., TMs for cationic redox and/or O for anionic redox. Compared with sXAS, mRIXS reveals a completely new dimension of information along the emitted photon energy (Yang and Devereaux, 2018) By this time, mRIXS has been demonstrated as a powerful tool to probe the redox states in the bulk electrodes for both TMs and O in the NIB cathodes. It is important to note that this review is not a general review of NIB materials; instead, it is a focused topical review on characterizations of the challenging and critical redox reactions through sXAS and mRIXS, with the emphasis on O redox reactions of NIB cathodes probed by mRIXS
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