Interfacial properties in energy storage systems studied by soft x-ray absorption spectroscopy and resonant inelastic x-ray scattering

Electrochemical energy storage systems, such as lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), play a significant part in sustainable energy applications.^1,2 In prevailing electrochemical energy storage devices, the functionality and stability of the interfaces and interphases between the electrodes and the electrolyte have been one of the central topics.^3–7 For example, in LIBs, the so-called solid electrolyte interphase (SEI) between the carbonate electrolyte and the graphite anode has been critical for the lifetime of commercial LIBs. For the other electrode, the cathode, the surface activities have been paid relatively less attention; however, the transition-metal (TM) dissolution and surface reconstruction have been known to be detrimental to the battery operations for various electrode materials.⁸ These interface behaviors are directly related to the impedance increase, capacity decrease, and exothermal reactions correlated with battery safety concerns. The issues related with surface properties of the pairing electrodes have become more important in recent years due to the demand of high-capacity and high-energy battery systems. The high operation voltage required for high-energy batteries often leads to electrolyte decomposition beyond the electrochemical stability window as well as the cathode instability issue from oxygen release and structural collapse. More importantly, at the charged state, the interactions between the highly oxidized cathode and the electrolyte may lead to cycling instability, battery decay, and safety issues. As a result, the surface and interface properties often define the stability of battery operation and determine the practicability of the high-energy battery systems.

Note that the interfacial behavior mostly involves complicated physical and electrochemical processes, which makes this research topic a daunting challenge.⁶ Up until now, although with lots of progress and achievements on understanding and controlling the anode SEIs,^9–12 a comprehensive and clear picture of the interface on electrodes has not yet been established on the detailed interfacial composition and structure, formation mechanism, functionality, and its influence on electrochemical performances. Over the years, tremendous and extensive interface investigations have been carried out with various techniques. Soft x rays generally exhibit shallow penetration depth, and corresponding soft x-ray spectroscopy techniques become a suitable choice for interface investigations,¹³ such as soft x-ray photoelectron spectroscopy (XPS)^14,15 and soft x-ray absorption spectroscopy (sXAS).¹⁶ Particularly, the ambient pressure x-ray photoelectron spectroscopy (APXPS) enables direct detection across the liquid–electrode interface under external bias,¹⁷ which demonstrates bright potential in energy storage device investigations. sXAS functions based on the core-level excitation and subsequent decay process, which exhibits elemental/orbital/chemical sensitivity. The energy range of soft x rays covers transition-metal (TM) L edges and O–K edges; as a result, sXAS can directly probe valence band related TM 3d and O 2p states and fingerprint detailed chemical species evolution. Based on different data collecting modes, sXAS offers two different probing depths of several nanometers from total electron yield (TEY) and hundreds of nanometers from total fluorescence yield (TFY) modes, which can be collected simultaneously and present contrast between the interface and bulk behaviors.¹⁸ These characteristics and advantages make soft x-ray absorption spectroscopy a powerful tool for interface research.

The interface reactions in electrochemical energy storage devices can be characterized through specific sXAS experiments. For example, on the anode surface, the formed SEI could be measured through O–K or C–K edge sXAS.¹⁹ Interfacial reaction products, such as carbonate species formation, can be well detected. On the cathode, surface passivation demonstrates TM valence state evolution, which could be detected by TM-L edge sXAS. Notably, at high voltages, oxygen participation in charge compensation, known as anionic redox reaction (ARR), further complicates the interfacial reactions and makes a clear characterization challenging. Compared with conventional XAS, recently developed resonant inelastic x-ray scattering (RIXS) mapping provides much improved chemical sensitivity by collecting the energy distribution curves of emitted photons.^20–24 The fundamental physical process and general comparison between sXAS and RIXS are shown in Fig. 1. For XAS, the unoccupied electronic states are mapped out by emitted photon counts or compensate current, which lead to the TFY and TEY data collection modes, respectively. However, for RIXS, the energy distribution of emitted photons is recorded, while the incident excitation energy is scanned through the absorption edges. Through the excitation and decay process, the changes in photon energy, momentum, and polarization can be transferred to intrinsic excitations of materials.²⁵ At each excitation energy, RIXS further resolves the single data point in XAS into an energy distribution curve, which hence provides a new dimension of information along emission energy.²⁶ Note that the photon-in–photon-out (PIPO) data collection mode makes RIXS mostly bulk sensitive. However, RIXS has been quickly established as a superior technique beyond conventional XAS in many aspects of battery studies. For example, RIXS has revealed the monovalent Mn in the battery electrode that cannot be detected by XAS;²⁷ it could also distinguish the intrinsic state of the oxidized oxygen in the battery electrode cycled at high potentials.^28–30 Note that such signals are mixed together with transition-metal characters in conventional sXAS results.³¹ Because this review focuses on the interfacial studies, the examples on RIXS studies in this paper are mainly for discussing the superior chemical sensitivity of RIXS for interfacial properties. Nonetheless, we note that the rapid development of the RIXS technique in the last couple of years has opened up new opportunities for employing this advanced tool for energy material studies, and the combined tool chest of conventional XAS and modern RIXS mapping often provides a powerful combination for interface properties in energy storage systems.

Other than the technical advances on the signal collection, in situ and operando soft x-ray spectroscopic measurements have also been much developed and become essential for studying the interface involving solid/liquid phases with dynamic response.^32–35 For in situ soft x-ray spectroscopy characterization, since soft x-ray spectroscopy exhibits shallow penetration depth and can only be conducted in vacuum chambers, specially designed cells are needed to seal liquid electrolytes.³⁶ Many works have showcased the advantages and power of in situ soft x ray for interface study.³⁷ Nevertheless, those works are generally challenging to handle experimentally and cannot exclude the potential radiation damage effect from long time data collection. The rapid development of solid-state batteries provides new opportunities for in situ soft x-ray characterization,^38,39 but corresponding research still needs more efforts. Overall, conventional ex situ characterizations have the advantages of better data quality, especially for quantitative analysis,⁴⁰ and relatively clean signal contributions, especially for elements such as O and C. However, samples for ex situ experiments need to be carefully controlled, so they could represent the true electrochemical states. On the other hand, in situ/operando experiments could maintain the non-equilibrium states, e.g., states that require an applied potential. As a result, we would like to stress the importance of both the in situ and ex situ characterizations, as each has its own technical advantages. We also note that protecting the samples from degradation, e.g., changes upon air or humidity exposure, is essential for many battery material studies. Additionally, ex situ experiments also require samples with locked electrochemical states with systematically evolving signals,^41,42 while in situ studies could be from one cell at different electrochemical states. In reality, however, ex situ studies of multiple samples, if properly controlled, are often more straightforward and less demanding compared with in situ/operando measurements of soft x-ray spectroscopy.

In this review, we provide a brief summary of different topics and applications regarding utilizing soft x-ray spectroscopy to study surfaces and interfaces in LIBs and SIBs, mostly focusing on sXAS and RIXS. Here, we first discuss the anode SEI characterizations, where the electrolyte decomposition and SEI layer formation have attracted much attention for decades. Then, we will move onto the cathode–electrolyte interface, where the intrinsic electrode fatigue from structural collapse and gas release are the research focus. Additionally, we show that although conventional XAS is capable of detecting many reaction products on the surface of battery and supercapacitor electrodes, it lacks the chemical sensitivity to measure the subtle effect at the interface if the signals are dominated by the signals from bulk materials that are always in the vicinity of the interface. We present a representative example of a model system on how RIXS could meet such a technical demand through its much improved chemical sensitivity. Finally, we provide our perspective of the technical developments and great potentials of advanced soft x-ray spectroscopy for studying interfacial problems in both liquid- and solid-state systems.

Since conventional anodes (e.g., graphite) exhibit Li chemical potential higher than the LUMO (lowest unoccupied molecular orbital) of the carbonate electrolyte, continuous electrolyte reduction and interface reactions may well take place on the anode surface. Such thermodynamic instability on the anode was once the bottleneck challenge for the practical development of LIBs. To overcome the problem, a stable solid electrolyte interphase (SEI) layer formation is essential or even prerequisite to enable a long cycle of LIBs.¹⁰ However, the interface reaction between the anode and the electrolyte is very complicated. Building up the full picture of the SEI layer then calls for systematic and comprehensive characterizations. In the past decades, XPS has been widely utilized to investigate SEI layer composition and its evolution with the battery cycle, and the influence of electrolyte additives on the stability of SEI can further be established.⁴³ By combining the ion etching strategy or selecting the excitation x-ray energy, XPS can be further utilized to present the SEI component distribution with depth profiles. Based on the extensive efforts, the so-called “mosaic model” of SEI has been widely accepted presently, but it is still not fully established experimentally and faced with many challenges.⁶ 

With the elemental and surface sensitivity, soft x-ray absorption spectroscopy can provide unique and valuable insights into disentangle factors determining the interface reactions. One typical example is clarifying the influence of crystal orientation on SEI formation. By comparing the sXAS collected on two types of Sn single crystals, (100) oriented Sn and (001) oriented Sn, distinguished SEI components can be detected [Figs. 2(a) and 2(b)].⁴⁴ While SEI on Sn (100) mostly consists of porous Li₂CO₃, which mainly results from LiPF₆ salt decomposition on the surface, SEI on Sn (001) consists mostly of LiF and organic molecules, which can be attributed to carbonated electrolyte decomposition. The distinguished SEI components and morphology demonstrate different electrochemical behaviors during long cycles. While the dense LiF can act as a passivation layer, porous Li₂CO₃ cannot prevent further electrolyte decomposition.

With the deepening of the investigation, it was further found that cathode electrolyte interphase (CEI) demonstrates dynamic response with the charge–discharge process, called “breathing effect” [Figs. 2(c) and 2(d)]. With a selected Cu electrode as a model template, the SEI formation and evolution can be investigated.⁴⁵ By comparing with reference spectra from lithium ethylene dicarbonate (LEDC) and lithium acetate (H₃C–COO–Li), the carbonyl formation on the Cu anode can be verified experimentally, such as semi-carbonate, oxalate, or carboxylate species. As verified from the C–K edge and O–K edge absorption peak evolution, carbonate species get decomposed from SEI during delithiation and reformed during subsequent lithiation. The redox reversibility of nascent carbonate species in SEI leads to thickness oscillating with cycle, which is also supported by TOF-SIMS. Moreover, sXAS demonstrates that the overall SEI thickness gradually increases and the surface gets passivated with extended cycles. Such a dynamic response and the chemical profile of the SEI layer were also verified on the C–ZnFe₂O₄ electrode by comparing XAS and XPS with different probing depths.⁴⁶ Partial reversible alkyl lithium carbonate (∼5 nm to 7 nm) formation at SEI can act as a Li reservoir and contribute to extra capacity of electrodes.

Besides XAS, RIXS provides further energy resolution along the emission energy scale at resonant energy, which can be well utilized for SEI research. Augustsson et al. utilized RIXS to investigate the SEI layer on the graphite anode cycled in different electrolytes.¹⁹ With selective excitation energy, the RIXS spectra of SEI can be well matched with reference compounds, including lithium oxalate (Li₂C₂O₄), lithium succinate (LiO₂CCH₂CH₂CO₂Li), and lithium methoxide (LiOCH₃). By specially designed background subtracting and quantitative fitting, the C–K edge emission of SEI species can be well separated from graphite and further simulated by using 0.1 Li-oxalate, 0.45 Na-succinate, and 0.45 Li-methoxide. Zhang et al. further utilized RIXS to study the electronic structure and chemical bonding of graphene oxide–sulfur (GO–S) nanocomposite.⁴⁷ GO is verified to partially reduce from S incorporation, and the interaction between GO and S can further stabilize S bonding during cycle. These pioneering works demonstrate the power of RIXS in detailed and insightful SEI research. With the further development of synchrotron techniques, it is foreseeable that RIXS will play a more significant role for future SEI research not only qualitatively but also quantitatively. Meanwhile, with the utilization of synchrotron-based x-ray microfluorescence (uXRF),⁴⁸ two-dimensional spatial resolved RIXS mapping across the anode surface can be even anticipated for SEI research.

Early studies of “conventional” voltage range typically suggest that cathodes do not suffer much parasitic reactions on the surfaces because the lithium chemical potential of most prevailing cathodes lies within the electrochemical stability window of the carbonate electrolyte.⁴⁹ However, recent push toward high voltage batteries has led to fervent debates on the cathode electrolyte interphase (CEI) layer formation, which demonstrates as dramatic structural reconstruction at the particle surface, electrolyte decomposition, as well as distinguished TM valence states between the cathode surface and the bulk.^50–53 Moreover, high voltage cycling may further trigger oxygen participation in charge compensation, and the chemically active high valence oxygen makes the cathode interface reaction even more complicated.^54,55 These interface instability issues and induced CEI layer formation are found directly related to the electrochemical performances, such as internal impedance increase, voltage fade, and capacity decay.

In previous reports, TM mostly demonstrates a lower valence state at the cathode surface compared with that of the bulk, which indicates surface redox reaction against the carbonate electrolyte. The sXAS characterizations further indicate that the CEI layer demonstrates dynamic TM valence variation upon charge–discharge, which can be quantified with delicate spectra fitting. In spinel LiNi_0.5Mn_1.5O₄, lower valence Mn²⁺ at the cathode surface is quantified to flourish during charge and decrease during discharge.^56,57 Particularly, the Mn²⁺ increase accompanies high valence Mn⁴⁺ at the electrode facing electrolyte, indicating that surface Mn²⁺ is correlated with the interface reactions between the higher valence Mn⁴⁺ and the liquid electrolyte at the charge state [Figs. 3(a)–3(c)]. These findings are in contrast to the conventional scenario that Mn²⁺ comes from the Mn³⁺ disproportion reaction, which mostly takes place at the discharge state. However, in the case of Na_0.44MnO₂, Mn²⁺ evolution at the cathode surface is quite different, which increases dramatically at the discharge state below 2.6 V but decreases at the charge state [Fig. 3(d)].⁴¹ Regulating the cycling voltages can well improve the electrochemical cycling stability of the assembled battery. Such a behavior has also been found in the NaLi_0.5Mn_0.5O₂ electrode, which agrees with the Mn³⁺ disproportion scenario and is in contrast to the spinel LiNi_0.5Mn_1.5O₄ case. The contrasting results can be attributed to detailed cathode property differences, including the crystal structure, chemical composition, or even cycle parameters, which indicate the complexity of interface reactions in various systems. Furthermore, dynamic TM ion dissolution from the cathode, migration in the electrolyte, and redeposition on the anode/cathode have been verified in the past years.⁵⁸ Such a dynamic response may further affect the experimental results, and more in-depth studies are hence necessary on the surface TM state variation mechanism.

In recent years, reversible ligand oxygen participation in charge compensation, known as anionic redox couple (ARR), has attracted wide attention, which demonstrates an extraordinary capacity of ∼250 mAh/g.⁵⁹ The novel reaction scheme has made interface reactions even more complicated. Previously, oxygen participation at high voltage is widely believed to be harmful to the battery cycle since higher valence oxygen is chemically unstable and may trigger oxygen gas release from the crystal lattice. Such radical oxygen release mostly occurs at the cathode surface, which further leads to structural collapse, TM dissolution, cation migration, or even safety concern. On the other hand, high valence oxygen may further trigger interface reactions. Particularly, the redox reaction between oxidative high valence oxygen and reductive carbonate electrolyte may take place well. Moreover, a high voltage cycle upon ARR involves higher valence TM, which may further bring about a potential catalytic effect on electrolyte decomposition or trigger redox reactions at the interface between the cathode and the electrolyte. Note that it is challenging to experimentally characterize ARR, and conventional XAS cannot disentangle the signals of intrinsic oxidized O in the lattice from the strong TM-O hybridization. In contrast, RIXS provides extra sensitivity to chemical states with a new dimension of information along emission energy.⁶⁰ A specific RIXS feature can be directly verified fingerprinting the bulk oxygen redox, which could be completely buried in conventional sXAS.^21,61,62 Presently, Li rich cathodes are still hindered by several application-wise issues, such as sluggish kinetics, large hysteresis, and voltage fade, where ARR plays a significant role.^63,64 However, strategies including surface modification are demonstrated effective to stabilize surface oxygen and benefit electrochemical cycle performances,⁶⁵ further indicating the key role of the surface effect in ARR exploitation. As can be inferred, RIXS will play an increasingly important role in the future development of ARR-active cathodes, in company with its rapid development on synchrotron facilities worldwide.^66,67 With the pursue of the next generation of LIBs with higher working voltage and larger Li capacity, the interface reactions on the cathode maybe even more serious. The detailed reactions can be influenced by the cathode structure, chemical composition, and activated ARR at high voltage. Furthermore, the cathode surface reaction can be entangled with anode surface reactions in the well-sealed battery.⁶⁸ Such correlation has been verified in the case of the TM dissolution–migration–redeposition procedure between the cathode and the anode, and the dynamic and integrated interface layer evolution still calls for more research.^58,69

In addition to LIBs and SIBs discussed above, soft x-ray spectroscopy can be widely utilized for studying the surfaces and interfaces in other electrochemical devices, such as fuel cells, solar cells, and electro-catalysis studies.^70,71 A striking example is on revealing an intriguing interface on the electrode surface of a supercapacitor with an aqueous electrolyte. Aqueous electrochemical devices are highly safe, less expensive, and environmental friendly but are limited by the narrow potential window within HER (hydrogen evolution reaction) or OER (oxygen evolution reaction).⁷² With layered Mn₅O₈, Shan et al. obtained a pseudocapacitor electrode with a wide and stable potential window of ∼3.0 V [Fig. 4(a)]. With the surface and elemental sensitive O–K edge sXAS, hydroxylated species formation and Mn–O coordination change on the electrode surface can be verified. By virtue of Mn₅O₈ background subtraction, sXAS of the surface hydroxylated layer can be further obtained. Compared with reference samples and theoretical calculations, the obtained surface hydroxylated layer demonstrates strong ice-like ordering with perfectly aligned H-bond and O–O direction but presents a much longer O–O distance after cycle. Based on their research, the interplay between the Mn²⁺ terminated surface and the hydroxylated interface layer favors mitigating gas evolution and leads to the wide stability window, meanwhile providing a facile pathway for sodium-ion transport. Such interface layer formation finally demonstrates high energy and power performances and a long cycle retention of 85% after 25 000 cycles.

The liquid electrolyte in typical batteries consists of alkali salt dissolved in organic or aqueous solvents. In these electrolytes, the solvation shell can be viewed as the intrinsic interface between alkali ions and solvent molecules, which is directly related to the intercalation and deintercalation process and fundamentally determines the interface reactions.¹⁰ However, detecting such an “interface” of the solvation shell is extremely difficult because they are buried in the electrolyte solvent, and more importantly, the shell consists of exactly the same chemical components as in the bulk material surrounding it. Figure 4(b) shows the sXAS results of KCl solution in water with different salt concentrations. Indeed, not much difference can be detected from sXAS although with different salt concentrations. The overall consistency in sXAS is because the “interface signal” from the solvation shell is well buried in the signal from the overwhelming non-coordinated solvent molecules. With the photon-in–photon-out scheme, in situ RIXS can be conducted in the sealing cells, which directly probes the local electronic structure of aqueous KCl solutions. The effect of ion solvation on the reorganization of the hydrogen bond network can be directly detected through the clear variation of the RIXS spectra [Fig. 4(c)], which is correlated with ultrafast molecular dissociation of H₂O molecules and gradual slow-down of proton dynamics in the KCl solution.⁷³ The molecular level probe via soft x-ray spectroscopy provides valuable information on the detailed solvent molecular configuration in the liquid electrolyte, and such information is key to understand the dynamic electrochemical process with battery cycle. These findings from RIXS open up new opportunities for studying Li salt solvation and desolvation in the liquid electrolyte, which are fundamentally and practically significant for future electrochemical energy storage device development.

Compared with ex situ characterizations, in situ studies can mimic the real-world chemistry in batteries if handled properly. In situ characterization exhibits a unique advantage in probing solid–liquid interfaces or even solid–liquid–gas interfaces.^{13,15,74–76} In situ soft x-ray spectroscopy experiments are generally more challenging compared with hard x-ray techniques.^77,78 This is because the energy range of soft x rays is ∼200 eV to 2000 eV, which has shallow penetration depth in atmosphere and can only be conducted in a vacuum chamber. Since the liquid electrolyte evaporates easily and is incompatible with the vacuum atmosphere, well-sealed model cells can be one promising choice. Note that conventional cells for in situ hard x-ray spectroscopy cannot be directly transferred to in situ soft x-ray characterization, and the x-ray transmission window selection becomes the key. This window not only separates the liquid electrolyte from the vacuum atmosphere but also allows the soft x-ray penetration. With specially designed thin film windows of Si₃N₄, carbon, aluminum, etc., which typically have a thickness of tens of nanometers, a “static cell” or “flow cell” can be assembled for in situ soft x-ray characterizations [Figs. 5(a) and 5(b)].^36,78–81 Several works have been reported with in situ soft x-ray absorption spectroscopy. With a UHV-compatible in situ static cell, Arthur et al. studied the Mg deposition behavior with Mg–K edge XAS, the presence of an interfacial Mg intermediate can be verified at voltage below the equilibrium Mg/Mg²⁺ potential.⁸² Note that the photon-in–photon-out fluorescence mode can be directly probed within the well-sealed cells but achieve mostly bulk information. Meanwhile, the extraction of the pure interface layer signal via electron yield is very challenging and needs detailed experimental design. With the combination of a piezo-chopper and lock-in amplifier scheme, the electric double layer across the Au–H₂O interface can be obtained, which evolves with bias voltage. Based on the in situ interface probe, the hydrogen bonding direction at the interface region can be statistically calculated.³⁷ 

With the development of diverse solid-state electrolytes (SSEs), solid-state batteries (SSBs) become promising choice for future applications.⁸⁴ SSBs avoid flammable carbonate electrolytes and favor superior safety performances, which is key to future large scale applications. Meanwhile, SSEs demonstrate a large electrochemical stability window (there is still some dispute on this topic), which is widely accepted to support cathodes to even higher voltage. More importantly, SSEs exhibit larger mechanical shear modulus, which may well suppress Li dendrite growth. Therefore, higher voltage cathodes and Li metal anodes can be utilized in SSBs, which favor high-energy density batteries far beyond conventional LIBs.

Due to these advantages, SSBs have attracted wide attention but are still hindered with various obstacles such as poor interface contact and large surface resistance.^85,86 SSEs further confront chemical instability and interface reactions against both electrodes.^87–89 Soft x-ray spectroscopy can play a significant role in fingerprinting interface reactions in SSBs. One example is the chemical reaction of garnet LLZO against moisture air.⁹⁰ Li₂CO₃ formation at the LLZO surface can be detected by sXAS and verified to be the fundamental origin of large interface resistance. With the contrast from difference probe depth, Li₂CO₃ thickness can be further estimated below 100 nm. Surface polishing can effectively remove Li₂CO₃ and dramatically improve interface properties. With sXAS, the same research group further found that the interface resistance of LLZO is correlated with grain size and Al dopant distribution,⁹¹ and the reaction path can be established by combining theoretical calculations.

In the meantime, SSBs provide promising model systems for in situ investigations because they do not require the specific cell to conceal the liquid and/or gas in the soft x-ray vacuum systems, which make SSBs great candidates for in situ soft x-ray characterizations. The in situ sXAS experiment was demonstrated in SSB systems with polymer PEO as SSE many years ago [Fig. 5(c)].⁸³ With in situ and operando sXAS, distinguished dynamic response can be detected on LiNi_1/3Co_1/3Mn_1/3O₂ (NMC) and LiFePO₄ (LFP). Whereas NMC responds immediately to the electrochemical cycle and follows the overall state of charge (SOC), LFP demonstrates strong relaxation and the SOC gradient effect. The in situ characterizations provide valuable insights into the dynamic response of battery operation, and the setup can be well utilized for investigating other electrodes. The employment of SSBs makes various experiments possible for in situ soft x-ray studies of some challenging issues, e.g., the buried solid–solid interface between the electrode and the electrolyte. With controlled thin film deposition, the electrode thickness matching probe depth of soft x ray could be designed and manufactured, and both electrode materials and the interfacial behaviors in SSBs could be detected through in situ/operando experiments. This also provides unique opportunities for RIXS experiments to detect the bulk states of electrode materials under in situ/operando conditions.²⁶ 

In this review, we provide an overview of soft x-ray spectroscopy on the interfacial research in electrochemical energy storage devices. We focus on four different topics in studying interfacial phenomena in battery systems through sXAS and RIXS techniques: (i) the anode SEI, (ii) the cathode surface, (iii) the liquid electrolyte system, and (iv) in situ/operando experiments. For the anode SEI studies, we show that sXAS could detect the surface chemical species through which different SEI formation mechanisms and their dynamics could be detected. For the cathode surface, we argue that recent efforts toward high-energy batteries drive the surface of the transition-metal oxide based electrode unstable and behave distinctly from the bulk electrode behavior. In particular, at the highly charged/oxidized states, the surface of the oxide cathode often displays the counter-intuitive low oxidation state of the TMs. This is associated with the fervent debates on the oxygen oxidation reactions at high potentials, which triggers different kinds of reactions and remains elusive. We then demonstrate that RIXS, through its new dimension of information of the emission photon energy, provides superior chemical sensitivity for detecting the subtle changes in the solvation shell, which consists of the same solvent molecules as in the bulk electrolyte and is thus undetectable through conventional sXAS. At the end, we discuss the developments of in situ/operando soft x-ray experiments and provide our perspective on the bright future of sXAS/RIXS techniques for studying SSB systems. The soft x-ray spectroscopic technique has witnessed significant improvements over the last two decades since the third generation synchrotron light source became available. At this time, the new generations of synchrotron light sources with high-brightness diffraction-limited storage rings have been planned in many countries, which will further advance the technical capabilities of synchrotron techniques for tackling the grand challenges in energy storage materials.

Spectroscopic data collection used resources of the Advanced Light Source, which is a U.S. Department of Energy Office of Science User Facility, under Contract No. DE-AC02-05CH11231. Works in China were supported by the National Basic Research Program of China (Grant No. 2015CB921502) and the 111 Project (No. B13029).