Understanding High-Voltage Behavior of Sodium-Ion Battery Cathode Materials Using Synchrotron X-ray and Neutron Techniques: A Review

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Introduction
To address global climate change by decreasing greenhouse gas emission, one of the decarbonization efforts is to widely use renewable clean energy such as solar, tidal, and wind energy.However, the integration of clean energy into the global grid network suffers from challenges in terms of stability and reliability due to the intermittent nature of renewable sources.The use of energy storage systems (ESSs) could mitigate this issue by storing electricity from renewable sources at off-peak times and discharging electricity at peak times [1,2].
Lithium-ion battery (LIB) technology is one of the most successful EESs and has been widely used in consumer electronics, electric vehicles (EVs), and electric grid networks [3][4][5].However, the increasing demands in many energy sectors, including energy grid systems and EVs, highlight the need for dependable and cost-effective EESs that can serve as reliable alternatives to lithium and other critical elements such as cobalt [6,7].Sodiumion battery (SIB) technology has been considered as a viable option to supplement LIB technology, especially in applications where high energy density is not a priority, such as grid ESSs [8,9].This is due to the plentiful supply of sodium and the identification of cathode active materials containing inexpensive and abundant elements such as copper, iron, and manganese [10,11].In addition, SIBs are closer to real-world applications than other "beyond-Li" battery chemistries including Zn, Mg, and Al due to the better kinetics of the monovalent working ion and the higher reversibility of working ions intercalation/deintercalation compared to the stripping/plating process.
Despite the growing expectations for SIBs, their practical application is restricted by their limited lifetime.Typically, cell degradation can originate from different parts, including the cathode, electrolyte, separator, and anode.The current challenges that SIB cathodes suffer from, especially at high voltage, can be summarized as follows [41]: (1) irreversible bulk phase transitions, which are accompanied by a large volume expansion and shrinkage as well as sluggish kinetics; and (2) surface-related degradation including surface instability in the air and interphasial side reactions between cathodes and electrolytes.A comprehensive and in-depth study of these fundamental issues is essential for developing high-voltage SIBs with an extended lifetime and higher energy density.Synchrotron X-ray and neutron scattering techniques significantly improve the efficiency and reliability of materials characterization by providing insight into the materials' bulk structure, electrode-electrolyte interphase evolution, and ionic motion dynamics [42][43][44][45].Herein, a systematic and comprehensive summary of the synchrotron-X-ray-and neutronscattering-based techniques used in characterizing high-voltage SIB cathode materials is introduced.We start with a brief overview of both synchrotron X-ray and neutron scattering techniques and their classifications.We then particularly highlight the fundamental scientific understanding of the high-voltage behavior of layered transition metal oxide SIB cathodes gained using these techniques.Polyanion-type cathodes, PBAs, and organic cathode materials are also discussed.Moreover, an outlook on the extension of synchrotron-X-ray-and neutron-scattering-based techniques regarding investigation into the cathode-electrolyte interphase (CEI), introduction of novel techniques (e.g., dark-field X-ray microscopy), and integration of morphology and phase characterizations is proposed for the study of high-voltage SIB cathode materials.Despite the growing expectations for SIBs, their practical application is restricted by their limited lifetime.Typically, cell degradation can originate from different parts, including the cathode, electrolyte, separator, and anode.The current challenges that SIB cathodes suffer from, especially at high voltage, can be summarized as follows [41]: (1) irreversible bulk phase transitions, which are accompanied by a large volume expansion

Brief Overview of Synchrotron Radiation
According to classical electrodynamics, synchrotron radiation is generated as a narrow tangent to the path of charged particles with a relativistic speed [42].Figure 2A shows a schematic of the basic components of a modern synchrotron facility.The acceleration of electrons begins by obtaining them from a source, such as a heated filament in an electron gun.Subsequently, these electrons are propelled forward through a linear accelerator (LINAC) before being introduced into a booster ring, where their acceleration continues.These electrons are then injected into the designated storage ring.Within this ring, bending magnet achromats are employed at arc sections to confine the electrons within a closed path.At the experimental hutch, the beamlines make use of the radiation emitted by insertion devices (IDs, e.g., wigglers, undulators) positioned at the straight sections between the arcs in the storage ring.The energy dissipated by the electrons due to the emission of synchrotron light is restored through radio frequency (RF) supplies.
mental scientific understanding of the high-voltage behavior of layered transition metal oxide SIB cathodes gained using these techniques.Polyanion-type cathodes, PBAs, and organic cathode materials are also discussed.Moreover, an outlook on the extension of synchrotron-X-ray-and neutron-scattering-based techniques regarding investigation into the cathode-electrolyte interphase (CEI), introduction of novel techniques (e.g., dark-field X-ray microscopy), and integration of morphology and phase characterizations is proposed for the study of high-voltage SIB cathode materials.

Brief Overview of Synchrotron Radiation
According to classical electrodynamics, synchrotron radiation is generated as a narrow tangent to the path of charged particles with a relativistic speed [42].Figure 2A shows a schematic of the basic components of a modern synchrotron facility.The acceleration of electrons begins by obtaining them from a source, such as a heated filament in an electron gun.Subsequently, these electrons are propelled forward through a linear accelerator (LINAC) before being introduced into a booster ring, where their acceleration continues.These electrons are then injected into the designated storage ring.Within this ring, bending magnet achromats are employed at arc sections to confine the electrons within a closed path.At the experimental hutch, the beamlines make use of the radiation emitted by insertion devices (IDs, e.g., wigglers, undulators) positioned at the straight sections between the arcs in the storage ring.The energy dissipated by the electrons due to the emission of synchrotron light is restored through radio frequency (RF) supplies.(A) A schematic of the basic components of a modern synchrotron facility.(B) Photon energy and the wavelength spectrum with common synchrotron X-ray techniques and their applications in battery research (IR-infrared; UV-ultraviolet; sXAS-soft X-ray absorption spectroscopy; XRD-X-ray diffraction; PDF-pair distribution function; hXAS-hard X-ray absorption spectroscopy; XANES-X-ray absorption near-edge structure; EXAFS-extended X-ray absorption fine structure; RIXS-resonant inelastic X-ray scattering; STXM/TXM-scanning/transmission X-ray microscopy).
While different photon energy ranges can be accessed through synchrotron radiation [42] (Figure 2B) and have been utilized for studying battery materials, this work only focuses on synchrotron-based X-ray techniques for the discussion and understanding of SIB cathode materials failure mechanisms.X-rays can be classified as hard, "tender", or soft according to their different photon energy range [42].Hard X-rays have a photon energy range of a few kiloelectron volts (keV) to tens of keV while soft X-rays range from several tens of eVs to about 2 keV.The photon energy range of a "tender" X-ray is between the soft and hard X-ray ranges.Leveraging the different interactions between X-ray and matter, a combination of diverse X-ray techniques is preferred for disclosing detailed information on the surface, interphase, and bulk of battery materials (Figures 2B and 3).cathode materials failure mechanisms.X-rays can be classified as hard, "tender", or soft according to their different photon energy range [42].Hard X-rays have a photon energy range of a few kiloelectron volts (keV) to tens of keV while soft X-rays range from several tens of eVs to about 2 keV.The photon energy range of a "tender" X-ray is between the soft and hard X-ray ranges.Leveraging the different interactions between X-ray and matter, a combination of diverse X-ray techniques is preferred for disclosing detailed information on the surface, interphase, and bulk of battery materials (Figures 2B and 3).

Brief Overview of Neutron Technology
Neutrons, having no charge, possess a remarkable ability to penetrate through materials.Unlike X-rays, neutrons can traverse long distances without encountering significant scattering or absorption.Additionally, neutrons exhibit considerably lower levels of material damage compared to X-ray analysis [44,45].These unique characteristics provide neutrons with distinct advantages for battery studies, offering complementary information to X-ray studies.Due to its high sensitivity to light elements, especially Li, Na, and O, the neutron technology is able to distinguish them from other heavy elements, such as Ni, Mn, and Fe, which are common elements in SIB cathode materials [46].

Brief Overview of Neutron Technology
Neutrons, having no charge, possess a remarkable ability to penetrate through materials.Unlike X-rays, neutrons can traverse long distances without encountering significant scattering or absorption.Additionally, neutrons exhibit considerably lower levels of material damage compared to X-ray analysis [44,45].These unique characteristics provide neutrons with distinct advantages for battery studies, offering complementary information to X-ray studies.Due to its high sensitivity to light elements, especially Li, Na, and O, the neutron technology is able to distinguish them from other heavy elements, such as Ni, Mn, and Fe, which are common elements in SIB cathode materials [46].Some typical neutron scattering technologies are suitable for battery research including neutron diffraction (ND), small-angle neutron scattering (SANS), neutron total scattering combined with pair distribution function (PDF), inelastic neutron scattering (INS), quasielastic neutron scattering (QENS), and neutron imaging [46][47][48][49].Leveraging Bragg's law, ND is used to study the crystal structures of materials at an atomic scale.PDF is a mathematical function that characterizes the probability of finding two atoms separated by a particular distance, which offers information about the spatial distribution of atom pairs.The combination of PDF with neutron total scattering is able to give insights into the structural characteristics and chemical environments of disordered materials.SANS is especially suitable for determining the structure of materials at the nanoscale in terms of the size, shape, and spatial arrangement of particles.QENS provides insights into the dynamic behavior (e.g., motion and diffusion) of atoms and molecules within materials by examining the energy and momentum transfer of neutrons during scattering.By offering enhanced penetration and elemental contrast capabilities, neutron imaging allows for the visualization of certain elements' distribution.Typically, these neutron scattering techniques can serve as complementary support for synchrotron X-ray characterizations to obtain a comprehensive understanding of high-voltage cathode behavior (Figure 3).

Layered Oxide Cathode Materials
The sodium-based layered transition metal oxide Na x MO 2 (M: transition metal) consists of stacked edge-sharing MO 6 octahedral sheets.In accordance with Delmas' notation [13,50], Na x MO 2 can be categorized into O3 and P2 structures.In the O3 structure, the Na + ions are located in octahedral sites, whereas, in the P2 structure, they occupy prismatic sites.Typically, Na x MO 2 can be operated at a relatively high operating voltage based on transition metal redox couples (e.g., Ni 2+ /Ni 4+ ) [37,51,52].In addition, the participation of anionic (oxygen) redox activity leads to extra capacities at higher voltages [37].However, it is challenging to realize a long-lifetime high-voltage SIB with Na x MO 2 (Table 1) partially due to the degradation processes at the cathode side including structural transitions, oxygen loss, and interphasial reactions.Here, we discuss the use of synchrotron X-ray scattering and neutron scattering techniques to understand these Na x MO 2 failure mechanisms and move towards addressing them.Structural transitions: Phase transition of cathode materials, especially at high voltage, can be detrimental to the cell performance due to the potential changes of crystal structure and lattice parameters, which could result in the following: cracking, pulverization, a loss of active material, and inferior kinetics of ion transportation [37].Substitution and doping are widely used to suppress phase changes of Na x MO 2 during cell operation at a high voltage, especially when synchrotron X-ray [15,55,[61][62][63][64][65][66] and neutron scattering techniques [16,[67][68][69] are used to obtain a comprehensive understanding of their effects.Synchrotron X-ray diffraction (XRD) and ND are typically used for characterizing phase changes of Na x MO 2 as a function of potential.Tapia-Ruiz et al. [62] investigated the effects of Mg substitution on P2-type Na 2/3 Ni 1/3−x Mg x Mn 2/3 O 2 (0 < x < 0.2) with XRD to realize the suppression of the O2 phase by the formation of an OP4 phase above 4V.ND was utilized to explain the substituted sites of Ni 2+ from ordered [(Ni 2+ /Mn 4+ )O 6 ] honeycomb units by Mg 2+ .Liu et al. [16] studied the structure of a P3-type layered oxide Na 0.5 Ni 0.25 Mn 0.75 O 2 cathode, especially the nature of the high-voltage phase above 4 V, using in situ neutron diffraction (Figure 4A).They clearly showed the phase above 4 V (O3s phase) has a dramatically shorter interlayer distance when all sodium has been removed.The substitution or doping elements could be involved in the redox process and affect charge compensation mechanisms, which can be elaborated using synchrotron X-ray absorption spectroscopy (XAS).Liu et al. [15] developed the Fe/Ti co-substitution strategy to suppress P2-O2 phase transition and Na + /vacancy ordering at high voltages for P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 cathode material.They demonstrated the incorporation of an Fe 3+ / 4+ redox couple in the charge compensation using ex situ XAS (Figure 4B).XAS can also be used to study local atomic structure because it provides information on the short-range order of atoms surrounding the absorbing atom.With detailed analysis, one can determine the types of atoms surrounding the absorbing atom, bond distances, and coordination numbers.Leveraging extended X-ray absorption fine-structure (EXAFS) spectroscopy combined with fitting analysis, Bhange et al. [61] revealed unique local structure changes around Ni atoms due to the honeycomb ordering and size mismatch between Ni 2+ and Bi 5+ cations (Figure 4C).Irreversible oxygen redox activity: As a special case of structural transition, oxygen loss, is discussed here as an independent section because of its universality for NaxMO2 cathodes when charging to high voltage.To further increase the energy density of SIBs, it is urgent to utilize the extra capacity caused by the oxygen redox activity.However, large, irreversible capacity accompanied by oxygen loss and voltage decay typically occurs after the first charge process [70].Thus, to develop NaxMO2 cathode materials with highly reversible oxygen redox activity, fundamental studies of the redox reactions are required from the perspective of charge compensation mechanisms and rational materials design principles.X-ray absorption near-edge structure (XANES) spectroscopy has been demonstrated to be a reliable method of determining the redox contribution from oxygen (Figure 5A) [57,71,72].Structural modification using element substitution [73][74][75] or doping [12,14,72] has been reported to suppress oxygen redox activity and decrease voltage hysteresis.Ji et al. [73] reported that the substitution of Mg and Zn in a cathode material, Na0.67Mg0.1Zn0.1Mn0.8O2,could deliver an ultra-high capacity of ~233 mAh/g at 0.1 C by preventing the oxygen loss.Combining neutron PDF (Figure 5B) and resonant inelastic Xray scattering (RIXS, Figure 5C), they illustrated the migration of Mg 2+ and Zn 2+ out of the layer with consequent vacancy cluster structures that could trap O2 molecules when fully charged.Yu et al. [72] improved the reversibility and stability of O 2− by tuning the Na-O-TM structure to increase the energy level of the O 2p electronic state using a Zr 4+ doping strategy in an O3-type NaMn1/3Fe1/3Ni1/3O2 cathode material, which was proved by the O K-edge ex situ XAS results.The oxygen loss in oxygen redox cathodes can also be avoided by forming certain types of superstructures in the transition metal layers that can suppress transition metal migration [70].House et al. [76] demonstrated that the formation of Irreversible oxygen redox activity: As a special case of structural transition, oxygen loss, is discussed here as an independent section because of its universality for Na x MO 2 cathodes when charging to high voltage.To further increase the energy density of SIBs, it is urgent to utilize the extra capacity caused by the oxygen redox activity.However, large, irreversible capacity accompanied by oxygen loss and voltage decay typically occurs after the first charge process [70].Thus, to develop Na x MO 2 cathode materials with highly reversible oxygen redox activity, fundamental studies of the redox reactions are required from the perspective of charge compensation mechanisms and rational materials design principles.X-ray absorption near-edge structure (XANES) spectroscopy has been demonstrated to be a reliable method of determining the redox contribution from oxygen (Figure 5A) [57,71,72].Structural modification using element substitution [73][74][75] or doping [12,14,72] has been reported to suppress oxygen redox activity and decrease voltage hysteresis.Ji et al. [73] reported that the substitution of Mg and Zn in a cathode material, Na 0.67 Mg 0.1 Zn 0.1 Mn 0.8 O 2 , could deliver an ultra-high capacity of ~233 mAh/g at 0.1 C by preventing the oxygen loss.Combining neutron PDF (Figure 5B) and resonant inelastic X-ray scattering (RIXS, Figure 5C), they illustrated the migration of Mg 2+ and Zn 2+ out of the layer with consequent vacancy cluster structures that could trap O 2 molecules when fully charged.Yu et al. [72] improved the reversibility and stability of O 2− by tuning the Na-O-TM structure to increase the energy level of the O 2p electronic state using a Zr 4+ doping strategy in an O3-type NaMn 1/3 Fe 1/3 Ni 1/3 O 2 cathode material, which was proved by the O K-edge ex situ XAS results.The oxygen loss in oxygen redox cathodes can also be avoided by forming certain types of superstructures in the transition metal layers that can suppress transition metal migration [70].House et al. [76] demonstrated that the formation of ribbon superstructure in Na 0.6 [Li 0.2 Mn 0.8 ]O 2 could inhibit Mn disorder, O 2 formation, and voltage hysteresis by synchrotron XAS (Figure 5D).Cathode-electrolyte interphasial reactions: It has been reported that cathode-electrolyte interphasial reactions can compromise SIBs lifetime by causing transition metal reduction and dissolution, electrolyte oxidation, heterogeneous surface reconstruction, and intragranular nanocracks [11,77,78].However, the complicated chemical species and some native surface film (e.g., NaOH, Na2CO3) on cathode surfaces make it difficult to discern the products from the electrochemical process [79].Here, we summarize the use of synchrotron X-ray techniques to diagnose the interphasial chemistries with the hope of providing scientific insights into stabilizing the interphase through cathode modification, electrolyte design, and artificial interphase fabrication.Soft XAS is a common technique used to investigate the surface chemical environments with both fluorescence yield (FY) and total electron yield (TEY) modes.Comparing the FY and TEY spectra of the same element, one can study the depth-dependent chemical changes because FY mode is used for probing the subsurface ( 50 nm) while TEY mode is used for studying the surface ( 10 nm) [77,80,81].Mu et al. [77] realized electrolyte oxidation on the cathode surface by Cathode-electrolyte interphasial reactions: It has been reported that cathode-electrolyte interphasial reactions can compromise SIBs lifetime by causing transition metal reduction and dissolution, electrolyte oxidation, heterogeneous surface reconstruction, and intragranular nanocracks [11,77,78].However, the complicated chemical species and some native surface film (e.g., NaOH, Na 2 CO 3 ) on cathode surfaces make it difficult to discern the products from the electrochemical process [79].Here, we summarize the use of synchrotron X-ray techniques to diagnose the interphasial chemistries with the hope of providing scientific insights into stabilizing the interphase through cathode modification, electrolyte design, and artificial interphase fabrication.Soft XAS is a common technique used to investigate the surface chemical environments with both fluorescence yield (FY) and total electron yield (TEY) modes.Comparing the FY and TEY spectra of the same element, one can study the depth-dependent chemical changes because FY mode is used for probing the subsurface (≈50 nm) while TEY mode is used for studying the surface (≈10 nm) [77,80,81].Mu et al. [77] realized electrolyte oxidation on the cathode surface by revealing a lower Ni oxidation state at the surface using TEY mode compared to FY mode at all states of charge (Figure 6A).Being similar to conventional XPS, synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES) is able to provide interphasial chemical information by analyzing the electronic properties of materials with an extended penetration depth and improved sensitivity.Doubaji et al. [82] demonstrated the interphasial chemistries evolution on the surface of Na x Co 2/3 Mn 2/9 Ni 1/9 O 2 during cycling using HAXPES (Figure 6B).Transmission X-ray microscopy (TXM), a non-destructive imaging technique, allows for the tracking of transition metal migration, which can be caused by interphasial reactions, through visualizing and analyzing the three-dimensional (3D) distribution of transition metal cations [83,84].For example, Rahman et al. [83] confirmed the Mn migration to the Na 0.9 Cu 0.2 Fe 0.28 Mn 0.52 O 2 particle surface during cycling by using TXM (Figure 6C).Although there are rare reports discussing the use of neutron scattering technology in studying CEI, there is plenty of scope for applying this technique in this area.We discuss it in the perspective section.
Batteries 2023, 9, x FOR PEER REVIEW 10 of 22 revealing a lower Ni oxidation state at the surface using TEY mode compared to FY mode at all states of charge (Figure 6A).Being similar to conventional XPS, synchrotron-based hard X-ray photoelectron spectroscopy (HAXPES) is able to provide interphasial chemical information by analyzing the electronic properties of materials with an extended penetration depth and improved sensitivity.Doubaji et al. [82] demonstrated the interphasial chemistries evolution on the surface of NaxCo2/3Mn2/9Ni1/9O2 during cycling using HAX-PES (Figure 6B).Transmission X-ray microscopy (TXM), a non-destructive imaging technique, allows for the tracking of transition metal migration, which can be caused by interphasial reactions, through visualizing and analyzing the three-dimensional (3D) distribution of transition metal cations [83,84].For example, Rahman et al. [83] confirmed the Mn migration to the Na0.9Cu0.2Fe0.28Mn0.52O2particle surface during cycling by using TXM (Figure 6C).Although there are rare reports discussing the use of neutron scattering technology in studying CEI, there is plenty of scope for applying this technique in this area.We discuss it in the perspective section.

Polyanion Compounds
Polyanion-type compounds, built with (XO 4 ) n− (X = B, S, P, Si, etc.) units or their derivatives, have also attracted substantial research interest [36,37].They are able to exhibit higher operating potentials, which is due to the inductive effect of polyanion groups [85].The strong X-O covalent bonds dramatically improve the stability of lattice oxygen, thus increasing the thermal stability of polyanion-type cathode materials compared to layered oxide cathodes.However, polyanion-type cathode materials have a relatively low gravimetric specific capacity because of the heavy polyanion groups' presence.To increase the SIBs' energy density, it is important to explore high-voltage polyanion materials (e.g., phosphates, fluoride phosphates, pyrophosphates) to obtain more capacity.However, polyanion cathode materials can suffer from structural transition and severe interphasial reactions at high voltages, thus decreasing cell lifetime (Table 2).In addition, it is difficult to understand the charge compensation mechanisms because the transition metal redox couple varies significantly with different structures and polyanionic groups.To address these challenges, researchers have utilized state-of-the-art characterization techniques to investigate the micro/macro structure, chemical composition changes, and sodium storage mechanism of polyanion-type cathode materials.The use of ND (Figure 7A) and synchrotron XRD (Figure 7B) has been widely reported in the illustration of the structural transition of polyanion-type cathode materials, especially during high-voltage operation [93][94][95][96][97]. Leveraging in situ synchrotron XRD, Chen et al. [93] revealed a near-zero volume change for N-doped graphene-oxide-wrapped Na 3 V(PO 3 ) 3 N cathode materials during cycling.Wang et al. [94] developed a new cathode, Na 4 MnCr(PO 4 ) 3 , with a high specific capacity of 130 mAh g −1 .Its structural evolution with a sequence of single phase, two phases, and single phase during discharge was confirmed by the synchrotron XRD data.Synchrotron-based XAS has also been utilized to study transition metal redox activity to understand the charge compensation mechanisms for high-voltage polyanion-type cathode materials (Figure 7C) [88,93,98].Kawai et al. [98] demonstrated the activity of a Cr 4+ /Cr 3+ redox couple in Na 3-x Cr 2 (PO 4 ) 3 at 4.5 V vs. Na/Na + using Cr K-edge XANES spectra.When more transition metal redox couples were involved in the Na 4 MnCr(PO 4 ) 3 cathode, Zhang et al. [96] were able to reveal the underlying mechanism of the Mn 2+ /Mn 3+ , Mn 3+ /Mn 4+ , and Cr 3+ /Cr 4+ redox couples via XANES spectroscopy.
of a Cr 4+ /Cr 3+ redox couple in Na3-xCr2(PO4)3 at 4.5 V vs. Na/Na + using Cr K-edge XANES spectra.When more transition metal redox couples were involved in the Na4MnCr(PO4)3 cathode, Zhang et al. [96] were able to reveal the underlying mechanism of the Mn 2+ /Mn 3+ , Mn 3+ /Mn 4+ , and Cr 3+ /Cr 4+ redox couples via XANES spectroscopy.To develop high-voltage, friendly electrolyte systems for polyanion-type cathode materials, there have been many studies discussing the CEI [99][100][101][102][103][104].However, synchrotron X-ray and neutron scattering techniques have not been widely used in investigating the interphasial chemical evolutions for polyanion-type cathode materials.We discuss their potential applications in this area in the perspective section.

Prussian Blue Analogue and Organic Materials
Due to the advantages of excellent redox activity, low cost, and elemental Earth abundance, Prussian blue (PB), its analogues (PBAs), and organic materials have been widely investigated as promising cathode active materials for SIBs.However, their average working voltage is not as satisfactory as the one from transition metal oxides and polyanionic compounds.To further improve cell energy density, recently emerging efforts have been devoted to increasing the working voltage of PBAs and organic cathode materials from To develop high-voltage, friendly electrolyte systems for polyanion-type cathode materials, there have been many studies discussing the CEI [99][100][101][102][103][104].However, synchrotron X-ray and neutron scattering techniques have not been widely used in investigating the interphasial chemical evolutions for polyanion-type cathode materials.We discuss their potential applications in this area in the perspective section.

Prussian Blue Analogue and Organic Materials
Due to the advantages of excellent redox activity, low cost, and elemental Earth abundance, Prussian blue (PB), its analogues (PBAs), and organic materials have been widely investigated as promising cathode active materials for SIBs.However, their average working voltage is not as satisfactory as the one from transition metal oxides and polyanionic compounds.To further improve cell energy density, recently emerging efforts have been devoted to increasing the working voltage of PBAs and organic cathode materials from the perspective of tuning the materials' structure [105][106][107].Although synchrotron X-ray and neutron scattering have less often been used with a focus on investigating high-voltage PBAs and organic cathodes, it is urgent to highlight the application of these advanced characterizations in these existing cathode chemistries with the hope of increasing the use of these techniques in future high-voltage PBAs and organic cathodes.
PBAs, typically represented by a chemical formula of A x M 1 [M 2 (CN) 6 ] y 1-y zH 2 O, consist of more than 100 family members with different crystal phases [38].Here, A represents an alkali metal while M1 and M2 represent transition metals.Typically, these transition metals are bonded with a C≡N − anion to form a three-dimensional open structure for hosting working ions (i.e., A).The sign of " " represents the structural vacancy associated with the lack of M 2 (CN) 6 group.The electrochemical activity with electron transfer of PBA cathode materials is typically associated with the valence change of M 1 and M 2 .Due to the crystallinity and the involvement of transition metals, synchrotron XRD and XAS are typically used in characterizing PBAs to investigate bulk structure changes and redox mechanisms, respectively [108][109][110][111]. To extend the cell lifetime, it is important to eliminate crystal water from PBAs.Wang et al. [111] illustrated phase transformation for a dehydrated Na 2−x FeFe(CN) 6 sample during cycling in situ by synchrotron XRD (Figure 8A).Seok et al. [110] studied the structure of a PBA using synchrotron XRD when iron was substituted with nickel (PBN).They demonstrated that the addition of nickel could dramatically decrease the volumetric expansion, thus improving cycling stability.In addition, the added nickel was electrochemically inactive, which was proved by the XANES data (Figure 8B).X-ray total scattering combined with the PDF technique has also been used to study the local structure and coordination geometry of PBAs.Xu et al. [112] identified the structure difference in a short range (r < 10 Å, Figure 8C) between cubic structured nickel hexacyanoferrate (NiHCF) and monoclinic NiHCF using synchrotron X-ray total scattering with PDF analysis.This could have been caused by the lattice distortion and Fe−C≡N−Ni skeleton twisting in the monoclinic phase.As a complementary technique, ND combined with refinement has been used in characterizing the bulk structure of PBAs.Using ND, Peng et al. [113] identified the structure of a rust-derived Prussian blue as an SIB cathode material with the Na + occupying the 8c and 24d positions.Leveraging ND with the refinement of the oxygen occupancies, Nielsen et al. [114] reported that water molecules do not induce phase transitions for Na 2 Fe [Fe(CN) 6 ]•zH 2 O.Other neutron scattering techniques also need to be applied in studying PBAs to obtain more insights into further high-voltage chemistries, which are discussed in the perspective section.Organic cathode materials, typically consisting of light elements (e.g., C, H, O, N), can be divided into n type, p type, and bipolar type according to their different redoxactive units or moieties with redox mechanisms [33,39].Typically, many n-type organic materials involve an initial reduction process that leads to the formation of a negatively charged state to combine with metal cations.Usually p-type organic materials undergo an oxidation process to combine with anions in the electrolytes.By incorporating the two characteristics described above, bipolar-type organic materials exhibit the capability to undergo either oxidation or reduction processes.Due to the lack of redox activities from  [111], copyright 2022 Wiley.(B) Operando XANES spectra at Ni K-edge of PBN during discharging process.Reproduced from ref. [110], copyright 2020 American Chemical Society.(C) PDF pattern of cubic (purplish blue) and monoclinic (orange) structured NiHCF.Reproduced from ref. [112], copyright 2019 Wiley.
Organic cathode materials, typically consisting of light elements (e.g., C, H, O, N), can be divided into n type, p type, and bipolar type according to their different redox-active units or moieties with redox mechanisms [33,39].Typically, many n-type organic materials involve an initial reduction process that leads to the formation of a negatively charged state to combine with metal cations.Usually p-type organic materials undergo an oxidation process to combine with anions in the electrolytes.By incorporating the two characteristics described above, bipolar-type organic materials exhibit the capability to undergo either oxidation or reduction processes.Due to the lack of redox activities from transition metals, the use of synchrotron X-ray techniques to characterize typical organic cathodes has not attracted substantial interest.However, the proper use of synchrotron X-ray combined with ab initio molecular dynamics (MD) simulations is also able to illustrate short-range structural features of organic materials at different states of charge.Kuan et al. [115] reported a novel small molecule, hexaazatrianthranylene embedded quinone (HATAQ, Figure 9A), as an SIB cathode material with 99% capacity retention over 5000 cycles at a high rate of 60 A g −1 .Leveraging ex situ synchrotron total X-ray scattering measurements (Figure 9B) with theoretical modeling, they illustrated the sodium storage mechanism by elaborating the appearance of Na-C, Na-O, and Na-N correlations.

Conclusions and Perspectives
High-voltage materials have shot to notability as a promising research area for SIBs because of their advantage of improving cell energy density with Earth-abundant elements.However, they face several serious problems such as bulk structure transition, lattice oxygen irreversibility, and cathode-electrolyte interphase evolution.Different strategies have been performed to improve the performance of high-voltage SIB cathode materials, such as element substitution or doping, rational electrolyte design, and artificial interphase fabrication.Simultaneously, state-of-art characterization techniques need to be applied to comprehensively understand the relationship between electrochemical performance and the evolution of the bulk and interphase during high-voltage operation.
In this review, synchrotron-X-ray-and neutron-scattering-based characterization techniques applied in high-voltage SIB cathode materials with both ex situ and in situ To improve the working potential of organic cathodes, it is feasible to combine organic redox species with transition-metal-containing compounds.Shimizu et al. [116] investigated CPL-4 ([Cu 2 (pzdc) 2 (azpy)], pzdc = pyrazine-2,3-dicarboxylate, azpy = 4,4azopyridine, Figure 9C) as an SIB cathode material, which is a combination of aromatic azo compounds with metal carboxylates.The Cu element in this cathode material facilitates the use of synchrotron X-ray characterization to illustrate the redox reaction mechanism.The operando Cu K-edge XANES spectra (Figure 9D) were collected to demonstrate the reversible redox reaction of Cu 2+ /Cu + .The use of neutron characterization in organic cathode materials needs to be extended, which is discussed in the perspective section.

Conclusions and Perspectives
High-voltage materials have shot to notability as a promising research area for SIBs because of their advantage of improving cell energy density with Earth-abundant elements.However, they face several serious problems such as bulk structure transition, lattice oxygen irreversibility, and cathode-electrolyte interphase evolution.Different strategies have been performed to improve the performance of high-voltage SIB cathode materials, such as element substitution or doping, rational electrolyte design, and artificial interphase fabrication.Simultaneously, state-of-art characterization techniques need to be applied to comprehensively understand the relationship between electrochemical performance and the evolution of the bulk and interphase during high-voltage operation.
In this review, synchrotron-X-ray-and neutron-scattering-based characterization techniques applied in high-voltage SIB cathode materials with both ex situ and in situ methods are summarized systematically.These characterization techniques are related to crystal structure, chemical composition and distribution, and valence state changes.The crystalstructure-related techniques (e.g., XRD, ND, PDF) have been proven as powerful tools to track the phase transformation and local atomic structures during the electrochemical processes.Composition-and valence-state-related techniques, such as RIXS, HAXPES, and XAS, can provide valuable information about potential reaction pathways and charge compensation mechanisms.Morphology-and element-distribution-related techniques, such as TXM, are able to provide detailed information about the internal structure, morphology, and elemental distribution of materials at the micrometer or nanometer scale.Despite the advantages of these characterizations mentioned above, there is plenty scope to use these characterizations to address the challenges of high-voltage SIB cathodes, especially when synchrotron X-ray and neutron scattering techniques are updated.We would like to summarize a few specific and practical directions for further research using synchrotron-X-rayand neutron-scattering-based techniques (Scheme 1).HAXPES, and XAS, can provide valuable information about potential reaction pathways and charge compensation mechanisms.Morphology-and element-distribution-related techniques, such as TXM, are able to provide detailed information about the internal structure, morphology, and elemental distribution of materials at the micrometer or nanometer scale.Despite the advantages of these characterizations mentioned above, there is plenty scope to use these characterizations to address the challenges of high-voltage SIB cathodes, especially when synchrotron X-ray and neutron scattering techniques are updated.We would like to summarize a few specific and practical directions for further research using synchrotron-X-ray-and neutron-scattering-based techniques (Scheme 1).
Scheme 1. Schematic illustration of future efforts in studying high-voltage behavior of SIB cathodes using synchrotron X-ray and neutron scattering technology.
1. Investigating CEI in high-voltage polyanion-type cathode: Investigations into the CEI of polyanion-type cathode materials have attracted substantial interest.However, most of the reported results in this area are from lab-based characterizations, which suggests that there is plenty of scope for applying synchrotron X-ray and neutron scattering techniques.As a successful example, the use of synchrotron-based soft XAS with both FY and TEY mode was able to reveal the transition metal activity in the CEI of layered transition metal oxide cathodes.More similar advanced techniques are encouraged for application in the investigation of the CEI of high-voltage polyanion-type cathodes to obtain more insights with the hope of extending cell lifetime; 2. Extending neutron scattering techniques: In the research area of high-voltage SIB cathode materials, ND has been widely used to study the bulk structure of layered transition metal oxide cathodes and polyanion-type cathodes.There remains wide scope for using neutron scattering techniques in CEI studies.SANS is a powerful tool for obtaining the size and shape information of materials at the nanometer scale which has been successfully used in studying the chemical nature, morphology, and Scheme 1. Schematic illustration of future efforts in studying high-voltage behavior of SIB cathodes using synchrotron X-ray and neutron scattering technology.

1.
Investigating CEI in high-voltage polyanion-type cathode: Investigations into the CEI of polyanion-type cathode materials have attracted substantial interest.However, most of the reported results in this area are from lab-based characterizations, which suggests that there is plenty of scope for applying synchrotron X-ray and neutron scattering techniques.As a successful example, the use of synchrotron-based soft XAS with both FY and TEY mode was able to reveal the transition metal activity in the CEI of layered transition metal oxide cathodes.More similar advanced techniques are encouraged for application in the investigation of the CEI of high-voltage polyaniontype cathodes to obtain more insights with the hope of extending cell lifetime; 2.
Extending neutron scattering techniques: In the research area of high-voltage SIB cathode materials, ND has been widely used to study the bulk structure of layered transition metal oxide cathodes and polyanion-type cathodes.There remains wide scope for using neutron scattering techniques in CEI studies.SANS is a powerful tool for obtaining the size and shape information of materials at the nanometer scale which has been successfully used in studying the chemical nature, morphology, and size evolution of the solid-electrolyte interphase (SEI) in LIBs [117,118].Neutron reflection testing is able to probe the structure and kinetics at the materials' interphase by detecting the reflected intensity as a function of angle, which has been used to study the structure and composition gradients of SEI in LIBs [119,120].These successful examples from LIBs suggest the possibility of involving these neutron-scatteringbased techniques in SIB CEI studies from different perspectives; 3.
Exploring new techniques: To accelerate the understanding of the SIB cathode's behavior at high voltage, more synchrotron-X-ray-and neutron-based complementary techniques are needed to obtain a more complete picture of the material's structure, dynamics, and electrochemical behavior.For example, the introduction of dark-field Xray microscopy [121] could complement TXM in characterizing material morphology by enhancing contrast with selective detection of scattered X-rays.Especially, darkfield X-ray microscopy [121] is highly sensitive to surface features, which could provide valuable information about the interphase morphology.Due to the sensitivity to the dynamic behavior of nanoscale features, the use of X-ray photon correlation spectroscopy (XPCS) could provide information about the movement and fluctuations of nanoparticles and crystal domains [122,123].This is important for understanding working ion diffusion, phase transitions, and structural heterogeneities or defects on the nanosecond to microsecond timescales; 4.
Bridging materials characterizations to electrode performance: Although there have been substantial efforts towards understanding high-voltage SIB cathode materials using synchrotron X-ray and neutron scattering techniques, it is challenging to directly link the materials characterization results with the electrode, as well as cell device performance partially due to the disconnection between morphology and phase characterization.One finds it is difficult to perform transmission microscopy and diffraction for the same batch of particles to obtain morphology and phase change information simultaneously.It would be ideal if we could collect morphology and structural phase information for a controllable number of particles at the same time in the near future.By studying the behavior of these particles statistically, one could establish a model to predict the electrode performance with the aid of artificial intelligence, which could also guide the selection of the materials synthesis methodology.

Figure 2 .Figure 2 .
Figure 2. Schematics on the introduction of synchrotron techniques used in battery research.(A) A schematic of the basic components of a modern synchrotron facility.(B) Photon energy and the wavelength spectrum with common synchrotron X-ray techniques and their applications in battery research (IR-infrared; UV-ultraviolet; sXAS-soft X-ray absorption spectroscopy; XRD-X-rayFigure 2. Schematics on the introduction of synchrotron techniques used in battery research.(A)A schematic of the basic components of a modern synchrotron facility.(B) Photon energy and the wavelength spectrum with common synchrotron X-ray techniques and their applications in battery research (IR-infrared; UV-ultraviolet; sXAS-soft X-ray absorption spectroscopy; XRD-X-ray diffraction; PDF-pair distribution function; hXAS-hard X-ray absorption spectroscopy; XANES-X-ray absorption near-edge structure; EXAFS-extended X-ray absorption fine structure; RIXS-resonant inelastic X-ray scattering; STXM/TXM-scanning/transmission X-ray microscopy).

Figure 3 .
Figure 3. Overview of the capabilities of synchrotron X-ray and neutron scattering characterization techniques for the study of cathode materials and CEI in rechargeable battery systems.

Figure 3 .
Figure 3. Overview of the capabilities of synchrotron X-ray and neutron scattering characterization techniques for the study of cathode materials and CEI in rechargeable battery systems.

Figure 4 .
Figure 4. Examples of synchrotron X-ray or neutron scattering techniques used in characterizing structure transitions of Na x MO 2 cathode materials.(A) In situ ND patterns of the Na 0.5 Ni 0.25 Mn 0.75 O 2hard carbon pouch cell collected while cycling at 10 mA g −1 .Reproduced from ref [16], copyright 2020 Royal Society of Chemistry.(B) Ex situ Fe K-edge XANES spectra for Na 0.66 Ni 0.23 Fe 0.1 Mn 0.57 Ti 0.1 O 2 electrode.Reproduced from ref. [15], copyright 2021 Elsevier.(C) Ni K-edge EXAFS spectra of Na 3 Ni 2 BiO 6 samples at different states of charge.Reproduced from ref. [61], copyright 2016 Royal Society of Chemistry.

Figure 5 .
Figure 5. Examples of synchrotron X-ray or neutron scattering techniques used in characterizing oxygen redox activity of Na x MO 2 cathode materials.(A) O K-edge of ex situ XANES spectra for P2-Na 2/3 [(Ni 0.5 Zn 0.5 ) 0.3 Mn 0.7 ]O 2 .Reproduced from ref. [57], copyright 2020 Wiley.(B) Neutron PDF patterns of Na 0.67 Mg 0.1 Zn 0.1 Mn 0.8 O 2 collected at the OCV and 4.5 V.The low r region labeled by a light-blue square was analyzed to obtain information on the shrinkage of the O-O bond length.(C) O K-edge RIXS measurements taken on Na 0.67 Mg 0.2 Mn 0.8 O 2 , Na 0.67 Zn 0.2 Mn 0.8 O 2 , and Na 0.67 Mg 0.1 Zn 0.1 Mn 0.8 O 2 charged to 4.5 V at 531 eV.Reproduced from ref. [73], copyright 2023 Elsevier.(D) O K-edge XAS (right panel) and RIXS spectra (left panel) recorded at an excitation energy of 531 eV for ribbon-ordered Na 0.6 [Li 0.2 Mn 0.8 ]O 2 at different states of charge (i.e., pristine, 4.5 V, and 2 V).Reproduced from ref. [76], copyright 2019 Nature Springer.

Figure 7 .
Figure 7. Examples of synchrotron X-ray techniques used in characterizing polyanion-type cathode materials.(A) Observed and calculated scattering intensity versus scattering vector Q for ND pattern of Na 2.5 Fe 1.75 (SO 4 ) 3 at 620 K. Reproduced from ref. [95], copyright 2016 American Chemical Society.(B) Ex situ synchrotron XRD patterns of Na 4 MnCr(PO 4 ) 3 at various states of charge.Reproduced from ref. [94], copyright 2020 Wiley.(C) Cr K-edge XANES spectra of Na 3 Cr 2 (PO 4 ) 3 /acetylene black electrode in the first charging process.Reproduced from ref. [98], copyright 2018 American Chemical Society.

Batteries 2023, 9 ,
x FOR PEER REVIEW 15 of 22 investigated CPL-4 ([Cu2(pzdc)2(azpy)], pzdc = pyrazine-2,3-dicarboxylate, azpy = 4,4azopyridine, Figure9C) as an SIB cathode material, which is a combination of aromatic azo compounds with metal carboxylates.The Cu element in this cathode material facilitates the use of synchrotron X-ray characterization to illustrate the redox reaction mechanism.The operando Cu K-edge XANES spectra (Figure9D) were collected to demonstrate the reversible redox reaction of Cu 2+ /Cu + .The use of neutron characterization in organic cathode materials needs to be extended, which is discussed in the perspective section.

Figure 9 .
Figure 9. Examples of synchrotron X-ray techniques used in organic SIB cathode materials.(A) Chemical structure of the small-molecule HATAQ; (B) synchrotron X-ray PDFs of pristine and sodiated HATAQ.Reproduced from ref. [115], copyright 2022 Royal Society of Chemistry.(C) Schematic representation of the pillar-layered structures of CPL-4; (D) operando Cu K-edge XANES spectra of the CPL-4 cathode during the first discharging process.Reproduced from ref. [116], copyright 2022 American Chemical Society.

Figure 9 .
Figure 9. Examples of synchrotron X-ray techniques used in organic SIB cathode materials.(A) Chemical structure of the small-molecule HATAQ; (B) synchrotron X-ray PDFs of pristine and sodiated HATAQ.Reproduced from ref. [115], copyright 2022 Royal Society of Chemistry.(C) Schematic representation of the pillar-layered structures of CPL-4; (D) operando Cu K-edge XANES spectra of the CPL-4 cathode during the first discharging process.Reproduced from ref. [116], copyright 2022 American Chemical Society.

Table 1 .
A summary of the selected electrochemical performance of high-voltage SIBs with Na x TMO 2 .

Table 2 .
A summary of the selected electrochemical performance of high-voltage SIBs with polyanion cathodes.