Chemical Science PERSPECTIVE Synchrotron radiation based operando Cite this: Chem. Sci., 2023, 14, 1641 characterization of battery materials All publication charges for this article Ashley P. Black, a Andrea Sorrentino, b François Fauth, b Ibraheem Yousef, b have been paid for by the Royal Society Laura Simonelli, b Carlos Frontera, a Alexandre Ponrouch, a Dino Tonti a of Chemistry and M. Rosa Palacı́n *a Synchrotron radiation based techniques are powerful tools for battery research and allow probing a wide range of length scales, with different depth sensitivities and spatial/temporal resolutions. Operando experiments enable characterization during functioning of the cell and are thus a precious tool to elucidate the reaction mechanisms taking place. In this perspective, the current state of the art for the most relevant techniques (scattering, spectroscopy, and imaging) is discussed together with the bottlenecks to address, either specific for application in the battery field or more generic. The former Received 7th August 2022 Accepted 11th December 2022 includes the improvement of cell designs, multi-modal characterization and development of protocols for automated or at least semi-automated data analysis to quickly process the huge amount of data DOI: 10.1039/d2sc04397a resulting from operando experiments. Given the recent evolution in these areas, accelerated progress is rsc.li/chemical-science expected in the years to come, which should in turn foster battery performance improvements. devices.2,3 Moreover, due attention has to be paid to the sensi- Introduction tivity of the technique (surface vs. bulk) and to time resolution, World transition towards a less fossil fuel dependent energy which will determine the feasibility of mapping in dynamic economy can be achieved by widespread deployment of energy processes. Beyond the use of standard state-of-the-art laboratory storage technologies at large (grid) and intermediate (vehicle) equipment, synchrotron radiation stands out as one of the most scales. In view of their efficiency and versatility, rechargeable powerful tools within battery research.4 It allows non-destruc- batteries are currently being considered for such applications, tive probing of materials with various depth sensitivities which face a different set of technical requirements (e.g. in allowing fast acquisition rates, large penetration depths, high terms of cost and lifetime), when compared to their use in spectral or spatial resolution, and access to techniques that are portable electronics. only possible with a continuous tuneable source over a wide Research efforts are thus on course worldwide to improve photon energy range. The use of operando techniques has currently available battery chemistries, such as Li-ion, while at intrinsic advantages, as they enable the detection of possible the same time looking for new concepts with high energy metastable intermediates and ensure characterization under density and/or advantages in terms of cost and sustainability. real conditions (non-equilibrium) avoiding the risk of ex situ Batteries being intrinsically complex devices,1 mastering sample evolution during manipulation. Operando materials science in general, and characterization techniques in experiments are thus crucial for both the elucidation of redox particular, is crucial to achieve progress in both research mechanisms in new technologies and also understanding of directions. failure and ageing processes for already commercial concepts, Measurements done in situ (inside the battery) or operando and thus their use is becoming widespread. However, compat- (taken during cell functioning) have lately boosted and ibility between the electrochemical cell architectures and the improved in spectral/spatial resolution for a myriad of tech- experimental set-ups may force some specic design features niques, including diffraction and a wide spectrum of spectro- and care has to be taken to ensure that these do not perturb the scopic and imaging techniques (and even combinations electrochemical response of the materials under investigation.5 thereof). Different length scales need to be probed: from Å to Synchrotron radiation involves a wide range of photon nm for surfaces/interfaces, and from tens of nm to mm for energies. X-rays with energies above ∼4 keV are called “hard” electrode materials, to reach mm for full electrodes and full X-rays, while those with much lower energies (and longer wavelengths) are called “so” X-rays. The former have a larger penetration (lower absorption cross section) probing depths a Institut de Ciència de Materials de Barcelona, ICMAB-CSIC, Campus UAB, 08193 from a few mm to the mm range, while the latter exhibit much Bellaterra, Catalonia, Spain. E-mail: rosa.palacin@icmab.es higher cross sections and penetration below a few mm of b CELLS – ALBA Synchrotron, 08290 Cerdanyola del Vallès, Catalonia, Spain © 2023 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2023, 14, 1641–1665 | 1641 Chemical Science Perspective probing depths, being therefore much more surface sensitive. “Tender” X-rays (TX) represent an intermediate region with energies between 1 and 4 keV. This perspective focuses on the state-of-the art of operando techniques using synchrotron radiation applied to batteries, covering from hard to so X-rays. The rst section describes the requirements for the experimental set-up, with emphasis on techniques where broader experience exists. They have been classied into three broad groups, which reect differences in the type of radiation/matter interactions and also data analysis approaches and requirements. The second section covers techniques that are based on elastic (kinetic energy conserved) scattering, including not only diffraction (probing long range Fig. 1 Typical penetration depth and spatial resolution (1D and 2D ordering) but also SAXS (Small Angle X-ray Scattering) and PDF measurements) for selected analytical techniques. The inset depicts 1/e penetration depths for various battery relevant materials as (Pair Distribution Function), the latter providing inputs at a function of energy (E), where e is Euler's number. TEM and SEM stand a local scale. The third section deals with absorption or for Transmission Electron Microscope and Scanning Electron emission based spectroscopic techniques, which access the Microscope, respectively. Reproduced from ref. 6 with permission, © electronic structure and enable additional chemical insights: Materials Research Society. XAS (X-ray Absorption Spectroscopy) which has been well developed to perform operando experiments using hard, so and tender X-rays and also other techniques which in this sense covered by a metal foil to enhance electronic conductivity. Note are less mature to date such as XPS (X-ray Photoelectron that when the same cell setup is to be employed for scattering Spectroscopy) or infrared spectroscopy. The fourth section is and XAS (X-ray Absorption Spectroscopy) experiments, thin devoted in general to imaging techniques and tomography, Kapton (polyimide) is used to minimize absorption, and the size which, despite intrinsic physical differences, typically generate of the window is oen reduced to avoid decreasing stack pres- the largest datasets. Last but not least, a nal Discussion and sure as much as possible. Outlook section provides the authors' view on the evolution of In the case of SAXS (small angle X-ray scattering), some cell the eld and the workarounds that may be developed in the next designs used for conventional diffraction can be used and at few years. cell geometry is not an issue, as the angular range is very limited and hence the X-ray path is almost identical for all scattering angles measured. The main concern is to avoid window Electrochemical cell & setup materials which can contribute to the signal and hence materials not having long range structural order (polymers or requirements glassy carbon) cannot be used in this case. Care has to be taken Besides generic electrochemical cell requirements (corrosion in choosing window materials not exhibiting Bragg peaks in the resistance, good electric contact, tightness to avoid liquid detector range. To this purpose, oen aluminium or mica is electrolyte leakage or exposure to air of any sensitive samples), used. the setups used for operando X-ray scattering measurements Some cell set-ups can be employed for different techniques need to be compatible with prolonged exposure to the beam, (e.g. diffraction and XAS) and facilitate complementary studies having the lowest possible absorption, although the latter is (multi-modal characterization), while in other cases, specic much less critical when using synchrotron geometries are required (e.g. cylindrical or radially symmetric radiation. A relevant factor is the photon energy, which largely cells for tomographic imaging). Customized cell setups were affects the penetration depth (see Fig. 1), and eventual vacuum common in the early days of operando studies and new requirements. In general, for all the experiments which require improved designs are being continuously developed8 transmission, the electrochemical cell has to be designed to be considering adaptability to somewhat different concepts (e.g. all as X-ray transparent as possible optimizing the choice of solid state cells). Yet, a gradual evolution to more standardized materials and component thickness. congurations has taken place,9–11 with high energy Rigid material windows ensure stack pressure and hence experiments allowing measurements on conventional setups good electric contact. This is an oen overlooked aspect, but (pouch, coin and cylindrical). An interesting approach to enable uniform pressure is also critical for reliable electrochemical the use of these cells with lower beam energies is to modify data, especially over extended cycling periods. Beryllium is the them, such as laser thinning the top coin case to 50 mm. This most transparent metal and widely used window, its drawbacks has enabled an operando long duration ageing study for >100 being high cost, toxicity and lack of stability at high potentials cycles by simply testing the cell (becoming itself oxidized), the latter being usually alleviated by continuously in the beamline hutch and periodically placing it the use of an additional foil (e.g. aluminium). Yet, other thin under the beam.12 Conventional setups enable improved metal foils are also an option,7 provided that absorption is not representativity with respect to real commercial cells, but they too high, or even insulating materials (glass), that can also be may be limiting in some cases (e.g. spectroscopy cannot be 1642 | Chem. Sci., 2023, 14, 1641–1665 © 2023 The Author(s). Published by the Royal Society of Chemistry Perspective Chemical Science performed unless suitable windows are implemented). Overall relevant data regarding real battery operation under very higher exibility can only be achieved for specically designed different environments as temperature will impact kinetics and setups13–15 and can also pave the way to multimodal can also induce phase transitions within the electrodes. More- characterization coupling different techniques. These on over, these studies can also provide inputs on temperature purpose developed cells can allow the use of a third electrode, driven degradation reactions. In effect, cycling to temperatures for independent monitoring of the potential at the positive and of up to 70 °C is a common means to accelerate ageing negative ones, opening also the possibility of performing phenomena in batteries.24 Ideally, the cell designs should not electrochemical impedance spectroscopy on a single differ much from those used at room temperature to ensure the electrode.16 Regardless the type of cell, the high ux of representativity of the electrochemical behaviour. The main synchrotron radiation allows fast data collection and thus it issue in this case is to be able to heat/cool the cell to a homo- enables high-throughput experiments measuring multiple cells, geneous and controlled temperature. Note that the thermal and for this, special sample holders have been designed.13,15,17 expansion of the unit cells of the materials under study does Sample preparation is another relevant aspect, whose also need to be considered and that it provides a suitable importance is not always emphasized.10 First of all, non-active indirect way to calibrate the cell temperature. While heating cell components (such as binder, carbon and electrolyte) do could be a priori easier via an electrical resistance, interference bring in a contribution, e.g. background increase at low with the electrochemical response cannot be excluded. Since diffraction angles due to scattering by an amorphous carbon a crucial aspect is ensuring that the probed sample (and not additive, the position of which is dependent on the electrolyte.18 only the housing) is thermalized, a circulating uid inside the Metal current collectors (typically Cu and Al for negative and cell body appears as the most attractive option enabling both positive electrodes in the Li-ion technology) will also result in heating and cooling (Fig. 2). Other options could be to work sharp peaks in diffraction studies, which can mask the evolution of some overlapping peaks corresponding to the sample under study. The collector of the same element of investigation in the counter electrode is hiding the information obtainable by XAS, and a hole in the collector in correspondence with the beam path can be foreseen to avoid the overlapping signals. The total absorption of a cell (considering windows, electrodes, separator, and electrolyte) needs to be adjusted in the case of XAS measurements in transmission mode to minimize the total absorption by approaching an absorption jump of 1 for the element of interest. In particular, operando XAS at relatively low energies needs to minimize the X-ray absorption along the beam path and are oen more accessible in uorescence mode. For S K-edge operando XAS studies, for example, S compounds have been embedded in a Cu mesh to allow the penetration of the X-ray beam.19 Electrode preparation mimicking commercial protocols is a priori desirable (e.g. tape casting), as electrochemical performance will be improved. Yet, this can be a drawback for fundamental studies on the redox mechanisms of specic materials with particles exhibiting a morphology prone to preferential orientation (e.g. TiS2 platelets).18 In such a case, simple mixtures of the material with carbon black will be preferred to enable the observation of 00l reections. Although common battery technologies use liquid (either aqueous or organic solvent based) electrolytes, the development of batteries using solid electrolytes is currently a hot topic. Indeed, concepts involving lithium metal as the negative elec- trode would a priori enable very high energy densities and improved safety with respect to state-of-the-art Li-ion tech- nology. The use of solid electrolytes brings about additional parameters to consider for operando experiments such as the Fig. 2 Image of a thermalized cell21 designed taking the one reported need to maintain external pressure.20 in ref. 22 as the basis, modified to enable fluid circulation through the main cell body (top). Diffraction patterns achieved using a similar setup Besides the control of pressure, efforts are also currently for Na3V2(PO4)2F3 in sodium cells at C/10 and 0 °C, which enabled the underway to extend experiments beyond ambient conditions. phase diagram to be followed upon Na+ deintercalation in 2D (a) and Their interest arises from the need to understand the inuence 3D (b) plots. Reproduced from ref. 23 with permission, © WILEY-VCH of temperature in the redox mechanism. They can bring in Verlag GmbH & Co. © 2023 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2023, 14, 1641–1665 | 1643 Chemical Science Perspective with cylindrical cells, which could be placed inside a cryostat or batteries operating in parallel.17,33 Indeed, intermittent sample a hot blower, as in this case the geometry would enable the probing enables following several cells simultaneously (which combination with other techniques, such as tomography. usually offers enough data points, unless the aim is to follow In addition to the generic interest in elucidating the inu- short-lived intermediates), while at the same time reducing ence of temperature in the redox mechanism, these experi- beam exposure and hence minimizing potential beam damage. ments may be even more relevant in specic battery In addition to that, fast data collection can be used as a mesh technologies operating at high temperature, such as Li metal mapping tool in a single cell for tracking inhomogeneities (see solid polymer electrolyte cells, commercialised using LiFePO4 below). as the positive electrode, which operate at ca. 80 °C.25 In this Better energy and angular resolution should enable a more case, a capillary cell setup was used, wrapped in a heating band, accurate renement of the crystal structures of the phases which enabled the control of temperature and spatially resolved observed from the data collected operando.33 In addition, energy phase distribution. The results allowed inferring the differences tunability enables optimizing the powder diffraction conditions between the structural changes taking place close to the elec- (operate in transmission and minimize absorption by the cell trolyte and those occurring in the vicinity of the current casing), while also offering the possibility to couple powder collector. With respect to cooling, the study of the redox diffraction and hard X-ray absorption techniques.34 mechanism in graphite electrodes at 0 °C has also been re- The emergence of high energetic and high brilliance 3rd ported, with the stage sequence observed being different from generation synchrotron sources, together with improved optics the one taking place at room temperature, which is relevant to (Kirkpatrick–Baez mirror setups and refractive lenses), has understand the behaviour of Li-ion cells in cold climates.26 made possible to standardly decrease beam sizes, in the Experiments at different temperatures may also be relevant for micrometer range rst, and ultimately down to the diffraction new battery technologies, for instance, experiments down to limit in the very hard regime (<20 nm at ∼100 keV). These −40 °C cooling pouch cells with liquid N2 were carried out to get conditions open the way to operando XRD microdiffraction, further insights into the intercalation of chloroaluminate in which was typically used to study the phase evolution of indi- graphite electrodes in aluminium batteries.27 vidual LiFePO4 nanoparticles and provided fundamental insights into the rate dependence of the redox mechanism for Elastic scattering based techniques this compound, widely used in commercial cells.35 In this case, the small beam illuminated a nite number of grains whose Diffraction Bragg reection spots could be discriminated. This approach X-ray powder diffraction appears as the most mature operando has recently enabled following the lithium concentration across X-ray based analysis technique within the battery eld. The rst a 80 mm thick graphite electrode, detecting and quantifying report dates back to 1978 using a specially designed cell with heterogeneities.36 One step further consists in probing a single a CuKa radiation laboratory diffractometer.28 This technique particle using XRD-tomography based techniques allowing the spread quickly in the last few decades concomitant with the visualization of cracks and strains in LiNi0.8Mn0.1Co0.1O2 development of various cell designs, some becoming commer- induced during battery operation and contributing to capacity cial, and continuous improvements of laboratory diffractome- fading.37 ters (higher energy derived from MoKa or AgKa radiation and Most oen experiments in a synchrotron are carried out in higher throughput related to the use of rotating anodes and transmission mode, with the incident beam along the electrode solid-state position sensitive detectors). stacking direction. While this enables simpler alignment of the Since most battery electrode materials are crystalline, cell, it prevents the detection of inhomogeneities, which can powder diffraction can provide time-resolved information on arise in thick electrodes and are relevant in commercial the evolution of the phase composition (e.g. formation of solid batteries, especially at high operation rates. A possible alter- solutions and phase transitions), the average crystal structures native is to direct the beam parallel to the battery layers, in (variations in the cell parameters, (de-)inserted ion localization grazing incident geometry.38 This geometry enables tuning in the crystal structure and site occupancy) and the micro- energy and/or changing the incident angle and can be useful for structure (micro-strains and coherent diffraction domain size) depth proling experiments. This can be extremely useful to get occurring during the redox reactions involved in battery oper- a better understanding on how electrochemical reactions ation. The emergence of synchrotron and dedicated instru- propagate but additional considerations apply as misalignment mentation (optics and experimental station) expanded the effects can easily interfere with the interpretation of results. range of operation by increasing the ux on the sample, Probing the electrode material at different cell locations and contributing to a better signal-to-noise ratio and also enabling depth proling are also possible using a pencil size beam a much faster acquisition time. The former allows spotting of coupled to energy dispersive diffraction.39,40 In this case, specic weak peaks, which can enable the detection of otherwise elusive detectors able to discriminate energy are required and data phases induced by ion ordering29,30 or the formation of incom- treatment is also somewhat more complex but the advantage is mensurate structures.31 Faster acquisition opens the possibility that the probed volume, dened by incoming and scattered to perform experiments at much higher rates such as 60C (ref. beams, remains constant, which is particularly valuable when 32) but more importantly, it pushed towards the development of dealing with very small beams. high throughput setups for serial data acquisition on several 1644 | Chem. Sci., 2023, 14, 1641–1665 © 2023 The Author(s). Published by the Royal Society of Chemistry Perspective Chemical Science The maturity of the operando X-ray diffraction approach does also manifest in the development of multimodal approaches coupling it to other complementary techniques (oen also operando and hence using cell setups compatible with both). A meaningful illustration of this methodology is the combination of energy dispersive diffraction and X-ray tomography, which have enabled the visualization of the degradation of the InLi alloy electrode/Li10SnP2S12 electrolyte interface in all solid-state cells41 and to get further insights into the mechanism of anion intercalation in aluminium cells using graphite positive electrodes.42 With the improvement of synchrotron facilities, experiments at high energy (∼80 keV) have enabled penetration depths on the order of 1 mm with very good spatial resolution (ultimately related to focusing optics). One of its main advantages is the possibility to use real commercial cells in the experiments, such as pouch LiNixMnyCo1−x−yO2/hard carbon44 or 18650 LiFePO4/ graphite for which phase transitions at both electrodes could be followed at a relatively fast rate (4C).45 In the former case, the lattice expansion of the Cu current collector enabled measuring the temperature and probing different regions of the cell allowed the drawing of 2D maps, which despite limited spatial resolution (0.3 mm, beam size) facilitated the detection of inhomogeneities (see Fig. 3). Last but not least, it is also worth mentioning that recent operando studies under optimized conditions at 107.7 keV (0.1173 Å) have permitted to follow the homogeneity of the lithium plating/stripping process in an “anode-free” pouch cell despite the low scattering factor of lithium.46 Pair distribution function analysis (PDF) Besides diffraction, involving only Bragg scattering and hence Fig. 3 (a) Experimental setup for in situ grid mapping (white dots) and providing long-range average structural information, the operando position tracking (red diamonds: P1, P2, and P3) for graphite/ consideration of total scattering (including also diffuse scat- LiNi0.5Mn0.3Co0.2O2 (NMC532) single layer pouch cells. (b) Contour plot for NMC532 and LixC6 reflections of interest collected at P3 during tering) and the corresponding pair distribution function (PDF) charge and discharge. (c) Diffraction line scan across the pressure provides local information. It describes the probability of interface (black dotted line in (a)). (d) The evolution of voltage during nding two atoms separated by a certain distance in the the 10th cycle together with selected NMC peaks and cell volumes, material under investigation, which is particularly useful to with marker sizes indicating phase fractions. Reproduced from ref. 43 characterize systems in which no long-range order exists, and with permission, © Elsevier. hence diffraction is not an option. Information deduced from PDF can also in some cases13 complement results achieved with SAXS (see the next section). This technique can be also are challenging because a low, constant and reproducible considered complementary to extended X-ray absorption ne background is required to extract reliable data, and this makes structure (EXAFS), in which the local structure is probed around the cell design much more critical than for diffraction, which an absorber (see the corresponding section). focuses on sharper Bragg peaks.48 In order to achieve data over a relevant range of momentum This technique does also place additional requirements on transfer (Q) to reach low r values, high energies (>45 keV) and the cells to be employed. If using high energy radiation and large angular coverage of the detector are required, which are a small sample to 2D detector distance (typically 100 mm), at only available in some specic beamlines. Otherwise, the situ- cells will result in different absorption at high angles and thus ation would be similar to laboratory X-ray sources for which the a cylindrical geometry is more suitable. In addition to that, energy is limited (maximum 25 keV with an Ag source) and the a blank cell needs to be measured to remove its contribution. ux much lower. Nonetheless, despite these issues, Pair Data treatment can also be complex. The small box modelling Distribution Function analysis is being increasingly used.47 Its approach requires the crystal structure as an input and then the result is a histogram of pair distances in the real space, but B (Debye–Waller) factors or selected atom positions can be Fourier transformation is required due to data being collected rened. Discrepancy in PDF calculated values with respect to in the reciprocal space. Operando total scattering experiments the average crystal structure can be attributed to local effects © 2023 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2023, 14, 1641–1665 | 1645 Chemical Science Perspective such as slight variations of bond distances. Another possibility graphite based composites as negative electrodes)52,53 and also is big box modelling including all atoms in a given volume and in the mechanistic study of alternative chemistries such as the adjusting their positions to match the experimental data. Yet, Li–O2 concept.54 In the former case, monitoring the silicon this is only viable for small nanosized particles with a limited particle active volume by SAXS concomitant with the stage number of atoms and involves a signicant computing time. formation in graphite inferred from WAXS enabled the degree Overall, besides the limitations explained above, the experi- of lithiation in each component (affected by the cycling rate) to mental setup is not signicantly different from that of be independently followed, and also the assessment of the conventional powder diffraction, and both depth proling or effect of pre-lithiation. In the latter, the growth mechanism of combination with other techniques (e.g. tomography)49 are also Li2O2 upon oxygen reduction in weakly solvating electrolytes is possible. Nonetheless operando PDF insights are especially investigated, with results seemingly dismissing previous beliefs valuable for materials with a disordered crystal structure for about the formation of a passivating conformal Li2O2 lm which diffraction cannot provide information. Besides innova- which would intrinsically limit the achievable electrochemical tive materials, such as transition metal oxides with a disordered capacity. rock salt type structure used as positive electrodes in lithium ion cells,48 one particularly interesting case is non-graphitizable hard carbon, which has a structure consisting of graphene Absorption and emission based nanosheets with signicant turbostratic disorder. Hard carbon spectroscopic techniques is the negative electrode material of choice in sodium-ion batteries, a technology being already in the manufacture phase Absorption spectroscopy by several start-up companies and subject of intensive study. In The application of XAS to materials science was for many years this case, despite the difficulties in nding a suitable structural overshadowed and underestimated when compared to X-ray model and the overlaps from scattering by the electrolyte,50 diffraction techniques. This was due to both XRD enabling meaningful data could be achieved which enabled to correlate precise crystal structure determination and the lack of pyrolysis temperature with the presence local structural defects extremely bright X-ray sources required to accurately determine and the electrochemical performance while also shedding light the X-ray absorption coefficient of a material as a function of on the currently controversial redox mechanism. photon energy. Major advancements in the theory for extended X-ray absorption ne structure (EXAFS) triggered the need for dedicated storage rings that led to the construction of the rst Small angle X-ray scattering (SAXS) synchrotrons during the decade of the 80's.55,56 Small angle elastic X-ray scattering enables probing bulk A great advantage of XAS is that it is element specic and microstructural features in a range of 1–300 nm (e.g. micropo- probes short-range order, giving structural and chemical rosity and particle size), which provide signals at very low angles information of not only crystalline samples but also amorphous (typically below 1°) when compared to diffraction. As in the case materials, liquids, gases, and complexes in solution, including of diffraction laboratory equipment is available, the benets of highly diluted systems. The X-ray absorption near-edge struc- synchrotron radiation being that the higher ux enables much ture (XANES) range of the spectrum corresponds to the elec- faster experiments, which may be relevant to get a deeper tronic conguration and it is sensitive to the oxidation state of understanding of kinetic aspects. It also enables accessing the element. Moreover, as the excitation energy is just sufficient longer lengths, so that some Bragg peaks can be observed if the to allow the photo-ejected core electron to reach the continuum, sample is crystalline and hence complementary information the photo-ejected electron wavelength is relatively large with can be achieved (note that this is sometimes also termed WAXS, respect to the interatomic distances, and the global XANES Wide Angle X-ray Scattering to stress its contrast with SAXS). shape can be used as a ngerprint of chemical species. Instead, This technique can be used to study the pore dynamics during the EXAFS region of the spectrum corresponds to photo-ejected redox operation in low crystallinity electrode materials, such as core-electrons of relatively high energy, with wavelengths hard carbons. Yet, data treatment is not straightforward and comparable to the interatomic distances. In this region, the requires not only evaluation and subtraction of the cell EXAFS oscillations can be modelled in the single scattering component contributions but also the use and validation of approximation obtaining quantitative information on the reliable models and sophisticated data analysis.51 When SAXS- interatomic distances, kind and number of atoms surrounding WAXS combined experiments are carried out, two detectors are the absorber and their relative disorder. Typically, EXAFS used, one collecting the SAXS signal and the other the Bragg studies are performed at room temperature, for example as diffraction (with a limited angular range and hence not suitable a function of state-of-charge, to access information on the for crystal structure determination). An additional issue is that charge compensation mechanism. Interesting complementary given the need to access large d spacing values, X-ray energies in information can be achieved by ex situ temperature dependent the medium range are most suitable (e.g. 10 keV), and hence EXAFS studies. Such measurements allow the local bond absorption from the sample can be a problem. strength and structural disorder to be quantitatively accessed, This technique has been useful in probing reaction mecha- both parameters affecting the nal electrochemical properties. nisms both in materials which are already at the commerciali- In particular, the local bond strength depends on the absorber- zation scale (or close to) for the Li-ion technology (e.g. silicon– ligand orbital overlap, which affects the redox process. In this 1646 | Chem. Sci., 2023, 14, 1641–1665 © 2023 The Author(s). Published by the Royal Society of Chemistry Perspective Chemical Science sense, it has been recently proposed that a larger degree of Numerous examples of operando XAS studies on the electronic overlap between the transition metal and oxygen electronic and structural properties of active electrode materials can be states in Li-rich NMC increases the rigidity of the oxygen sub- found in specic literature60,68–73 and reviews.62,65,74 lattice upon lithium deintercalation and results in a more Experimental advances and improvements of synchrotron reversible oxygen redox.57,58 Following the evolution of the radiation sources lead to the development of Quick scanning EXAFS oscillations as a function of the temperature enables the EXAFS (QEXAFS), which pushed the acquisition time of a single experimental elucidation of changes in the local structure spectrum in real-time from tens of minutes down to the sub- resulting from the redox process. Moreover, low temperature second regime. It has become an ideal tool for operando inves- EXAFS data allow the quantication of the static disorder, tigations of the kinetics of electrochemical reactions in which affects the diffusion coefficients of the intercalating ions. batteries.77 The sub-second QEXAFS technique is based on For instance, both local static disorder and bond strength have specialized X-ray monochromators, which enable rapid and been found to inuence capacity in NaxCoO2.59 continuous energy scans by smoothly oscillating the mono- Whenever possible, XAS spectra are recorded in trans- chromator crystals.78,79 The achievable temporal resolution is mission to achieve a higher signal-to-noise ratio. This allows for limited by the mechanical stability of the monochromator, the a fast scanning speed and is bulk sensitive, as the beam crosses response times of the detectors, and the available photon ux. the full sample or cell. In contrast, uorescence acquisition QEXAFS has been employed to study nonequilibrium phase mode shows typically lower signal-to-noise ratios and requires transitions in LiNi0.33Mn0.33Co0.33O2 at high rates, enabling to slower scanning speeds. It is used to investigate non-X-ray determine that the redox process on Ni ions occurs to a larger transparent samples or highly diluted systems.60 Alternatively or extent at higher C rates, which is consistent with the benets in complementarily, XAS measurements in total electron yield rate capability observed for high Ni content materials (Fig. 4a).75 (TEY) mode benet from a high signal-to-noise ratio, fast XAS has also been used to investigate transition metal high- scanning speed and surface sensitivity. The fundamental prin- entropy oxides (HEOs), a class of conversion-type electrode ciples of the XAS method and progress of the instrumentation materials studied for Li-ion batteries that integrate at least ve as well as the data analysis of both XANES and EXAFS have been elements into a single phase to increase the congurational covered in detail in several recent reviews such as ref. 61 and 62. entropy, which is expected to improve electrode reversibility Hard X-ray. Hard X-ray XAS enables the monitoring of the and cycle life. The reaction products of these HEOs are nano- chemical and structural changes taking place during redox metric, defective and poorly crystalline and thus difficult to processes. It has been extensively applied operando to the study probe via XRD. The complex charge storage mechanism of of battery materials since the early 90's with pioneering inves- spinel type (CrMnFeNiCu)3O4 HEOs has been unveiled by tigation of alkaline MnO2 batteries63 and organosulde poly- operando quick XAS enabling to conrm that all ve elements meric electrodes for Li and K ion technologies.64 One of the contribute to the charge capacity to different extents at each main reasons why this technique has been so widely used in the state of charge, the processes being in some cases non- or poorly battery eld is related to the fact that it is element specic and reversible, especially the reoxidation of Cu0 to Cu2+ (Fig. 4b).76 the most common electrode materials contain rst-row (3d) So and tender X-ray. Compared with hard X-ray, so X-ray transition metals as redox centres, with K-edge energies lying absorption spectroscopy (sXAS)80 is a more direct and efficient between 4.5 and 10 keV which is of easy access. In addition, the experimental probe of the electronic states near the Fermi level. large penetration depths in the hard X-ray range allow the bulk sXAS is typically measured in TEY, accessing surface sensitivity of the material to be probed and the measurements can be (a few nm). When the so X-ray uorescence yield (FY) is made in transmission mode. Several studies have enabled the simultaneously available, as in the harder X-ray regime, it allows unveiling of the charge compensation mechanism and local surface and bulk contributions to be discerned. However, the structure evolutions taking place during electrochemical deli- short penetration depth of so X-rays requires dedicated oper- thiation and lithiation of layered oxides in general and in ando set-ups designed to operate in high vacuum environments. materials containing more than one active redox element in Strategies to overcome the vacuum requirement are for instance particular, such as NMCs. Early studies following the XANES of the use of cells with integrated X-ray transparent windows the K-edge of Ni, Co and Mn in the operating voltage (up to ∼4.6 (Si3N4, SiO2, Si or graphene) or making perforations on the V) enabled the conclusion that manganese remains as Mn4+ and current collector, which enabled in situ measurements on NMCs that the charge compensation mechanism involves mainly the and LiFePO4 electrode materials.81 Another original approach oxidation of Ni2+ to Ni4+ ions.65 The element specicity of XAS deserving mention is the intentional generation of a gas bubble enables pinpointing which of the potentially redox active from the electrolyte inside a microuidic electrochemical cell elements undergoes oxidation or reduction at each state of with high intensity X-rays, which enabled the study of the solid charge and has enabled recently the understanding of the electrolyte interphase (SEI) on silicon electrodes in situ through underlying anionic redox process that governs the hysteresis transmission so XAS (Fig. 5).82 Last but not least, the devel- and kinetics of Li-rich electrode materials such as Li1.2[- opment of frequency modulated methods enabled decoupling Ni0.132+Co0.133+Mn0.544+]O266 and Li2RuO3.67 In addition to that, and measuring the tiny TEY current generated by the secondary EXAFS provides quantitative information on the local structure electrons at the electrode/electrolyte interface from the current around an absorber, enabling to directly access local strains and circulating through the working electrode under vacancies and hence is an ideal complement to diffraction data. © 2023 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2023, 14, 1641–1665 | 1647 Chemical Science Perspective Fig. 5 (a) Illustration of a novel approach to investigate the SEI on silicon thin-film electrodes in situ through transmission sXAS and (b) extraction of the SEI spectrum from the overshadowing electrolyte spectrum and subtraction of the bubble and beam damage.82 Operando XAS on Li/S or Na/S batteries19,85,86 has shed light Fig. 4 Two examples of operando quick-scanning XAS: (a) XANES on sulfur speciation during the redox process and on the results of LiNi0.33Mn0.33Co0.33O2 at Ni, Co, and Mn K-edges mechanisms that govern dissolution and deposition processes throughout oxidation at a 30C rate. Ni K-edge energy shifts as (Fig. 6). The different polyanionic species formed during cell a function of nominal lithium content x in NMC during oxidation at operation are easily distinguishable by linear combination ts different rates (1C, 10C, and 30C). Reproduced from ref. 75 with (LCFs) as their absorption edges. Spatially resolved XAS has permission ©John Wiley&Sons. (b) Voltage vs. capacity profile for the (CrMnFeNiCu)3O4 HEO electrode, and corresponding XAS spectra been used to differentiate between the intermediate species which enable changes in the oxidation state/coordination for the present in the positive electrode and the separator of an oper- involved metal ions to be inferred as a function of the electrode state ating Li/S cell during the rst and second cycles87 and time- of charge.76 resolved operando XAS experiments with sub-second time resolution provided insights into the effect of the discharge rate in the redox mechanism.88 Finally, it is also worth mentioning electrochemical cycling, providing a platform to investigate that P K-edge XAS on LiFePO4 electrodes could also be followed solid/liquid interfaces by operando sXAS.83,84 via operando TX-XAS.89 The tender X-ray (TX) range, in which the K-edges of In addition to the absorption techniques mentioned above several relevant elements used in batteries, such as Na, Mg, which are based on electronic transitions to unoccupied states, Al, Si, and P, are located is unfortunately not commonly X-ray emission spectroscopy (XES) is especially suited to probe accessible. Indeed, it is oen too high for sXAS beamlines and the occupied states, including the valence electrons. Probing too low for most hard X-ray XAS beamlines. However, the emission lines one can infer not only on the emitter's oxidation penetration depths are longer that those of so X-rays, so that and local spin state, but also on the ligand nature. Studies conventional coin cells can be employed for operando combining XES and XAS revealed that Mn3+/Mn4+ play an active measurements provided that the window is thin and counterbalancing role during oxygen redox reactions in lithium- composed of light elements, with Be, Si3N4 or Kapton being and manganese-rich layered oxides.90 Operando XAS enables the most commonly used. On the other hand, the attenuation also multi-technique approaches combining XANES and EXAFS length is too low to measure in transmission mode and with sXAS, XES, and even other inelastic scattering techniques, therefore FY is preferred. such as resonant inelastic X-ray scattering (RIXS) or X-ray 1648 | Chem. Sci., 2023, 14, 1641–1665 © 2023 The Author(s). Published by the Royal Society of Chemistry Perspective Chemical Science transition metals, despite its lower reliability to probe the bulk when compared to XAS in transmission mode.91 Among the techniques considered in this perspective paper, this is the only one based on the detection of electrons instead of photons. It originates from the photoelectric effect, the emission of elec- trons with kinetic energy Ekin aer irradiation of photons with energy hn, according to the conservation of energy: hn − Ekin = Efin − Ein = BE + F (1) where h is the Planck constant, and En and Ein are the nal and initial energy of the sample, whose difference for solids is conventionally split at the Fermi energy into a Binding Energy (BE) and a work function (F). The BE is an intrinsic property of the electronic structure of the investigated compound, while the work function is sensitive to its surface electric dipoles. The number of electrons as a function of their kinetic energy essentially reproduces the shape of the density of electronic states (DOS). It is important to stress that binding energy shis in XPS may be not only of chemical, but also of electrostatic origin,92 when a dipole exists between an atom and the sample ground. Thus, for the correct interpretation of shis when non- electron conducting layers are involved (such as electrolytes in operando experiments), the cell conguration must be taken into account.93 Since the right part of eqn (1) is determined by the material, the actual Ekin range is controlled by the chosen hn photon energy. While electrons interact with any atom following a universal curve,94,95 the penetration depth of X-rays strongly Fig. 6 (a) Scheme of coin cells allowing measuring S K-edge oper- depends on the elements they interact with, in particular when ando in fluorescence mode. (b) Sulfur K-edge XANES evolution as a function of the state of charge followed operando in Na/S batteries they are specically absorbed to excite transitions. Nonetheless, and (c) selected spectra at various states during the first cycle with the electron inelastic free mean path tends to be much shorter positions for reference compounds highlighted by vertical dotted than the penetration depth of X-rays of the same energy (1 nm at lines. Reproduced from ref. 19, © John Wiley&Sons. 500 eV and 10 nm at 10 keV). Even in gaseous phases, the free path is short, and therefore for sufficient electron collection either ultra-high vacuum (UHV) or short paths at higher pres- Raman scattering (XRS), the latter giving access to so X-ray sure are necessary. The availability at synchrotrons of hard X-ray absorption edges by exploiting hard X-rays, i.e. with bulk photoelectron spectroscopy (HAXPES) has signicantly sensitivity.74 Complementary studies involving sXAS and RIXS expanded the sampling depth, overcoming some of the impor- were especially useful to probe light elements such as oxygen. tant limitations of lab-scale XPS. In fact, the high surface This enabled the correlation of the mechanism of generation of sensitivity makes challenging even ex situ characterization. O2− and dimerization to form (O2)n− with the localization of Surfaces are easily affected by interaction with the atmosphere electrons–holes primarily on the oxygen atoms in Li1.2Ni0.13- and adsorbates or reacted surface layers need to be removed Co0.13Mn0.54O2. By investigating the local environment around aer introducing the sample in a vacuum by either heating or the transition metal ions using XANES it was evidenced that gas sputtering with an ion gun. Even when applied carefully, Mn4+ plays a specic role in trapping the holes on the oxygen such methods will hardly leave an exact representation of atoms that ultimately determine the ne balance between a fresh surface of the electrode material that may be of interest. oxygen loss and reversible oxygen redox in the operation of For the same risk of altering composition and redox states, these electrode materials.66 caution must be used in depth proling studies assisted by sputtering.96 The non-destructive depth proling offered by Photoelectron spectroscopy varying the photon energy with a synchrotron source is partic- Photoelectron spectroscopy is a well-established technique for ularly attractive, as the electrode surface is oen the subject of surface analysis that has become widely applied for the deter- interest (investigation of passivation and SEI layers, which may mination of the chemical environment for the different exhibit a complex composite gel-like structure). In these cases, elements present in a sample. Although mostly indicated for manipulation may still largely affect surfaces, and ensuring studying surfaces, its widespread availability at the laboratory representativity is not straightforward. To exclude the impact of scale has resulted in its use to determine the redox state of environmental contamination, the integration of chambers © 2023 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2023, 14, 1641–1665 | 1649 Chemical Science Perspective enabling to perform electrochemical experiments and handle samples directly connected to the analyzer chamber has been proposed.97 However, introducing surfaces wet with complex organic/inorganic mixtures directly into a vacuum pos- es a problem to the SEI integrity. Near ambient pressure photoelectron spectroscopy (NAPP) has mitigated this problem. By differential pumping, it is possible to keep a higher pressure close to the sample surface, so that fast drying is avoided, and the liquid electrolyte can be measured during hours simulta- Fig. 7 Schematic representation of the original dip-and-pull method neously to the solid electrode.98 Other approaches for SEI (a) and two variants (b and c) for XPS analysis of solid–liquid interfaces. analysis have addressed the formation of electrochemical Some of the electrons generated at the interface cross the liquid meniscus and are collected by the analyzer. Reproduced from ref. 116. interfaces by in situ (UHV) step-by-step deposition of solid electrolytes on top of electrode materials, which allows the study of dipole formations by following the shis of the char- acteristic peaks of the substrate and electrolyte.99 Operando XPS availability of beamlines capable of HAXPES and near ambient experiments would eliminate the effects of variable ambient conditions. One example is the HIPPIE beamline at the MAX IV contamination, but are more difficult to envisage. To observe laboratory that is designed for in situ and operando NAPP effects comparable to the application of a bias, the charging experiments at the solid–liquid (dip-and-pull setup) interfaces produced by irradiation of an electron gun has been exploited to under full electrochemical control.112,113 In addition, the radi- generate a so-called “virtual electrode”100 and explore interac- olysis problems, discussed in the imaging section, represent tions between metallic lithium and sulde solid electrolytes. a major challenge.114 As for other spectroscopic methods, the The most straightforward option for truly operando photo- imaging mode has also been demonstrated operando, enabling emission measurements is using solid state cells. One early for instance differentiation of lithium extraction rates from example was based on a Na b′′-alumina plate to study the elec- different facets of 10 mm LiCoO2 single particles, suggesting tronic effects of Na intercalation on a top layer of in situ that the electrode rate capability can be improved by controlling prepared TiS2.101 More recent work studied the reactivity to the crystal habit.115 oxygen of RGO deposited onto a solid glass-ceramic NASICON- Most of these studies are facilitated by tender and hard X- type electrolyte in an ambient pressure XPS chamber. The rays with higher penetration depths (several tens of nm), resulting solid-state Li/O2 battery positive electrode demon- enabling operando experiments under more realistic conditions strated the intrinsic instability of most carbon defects to the in terms of electrodes and electrolytes. On the other end, the use oxygen radicals.102 Some operando studies with laboratory XPS of so X rays, even if increasing the surface sensitivity, allows could reveal chemical and electrochemical degradation at the much higher sensitivity to the lightest elements, including interface between the two components of the electrode and lithium, whose photoionization cross section is weak with sulde solid electrolyte.103–105 The method is based on the laboratory radiation. observation of an electrode fabricated as a composite between the electrode active material and the solid electrolyte, so that both components and their interface are observed simulta- Infrared spectroscopy neously. However, with the availability of HAXPES, it is now Fourier transform infrared (FTIR) spectroscopy is based on the possible to study full solid state batteries. With a 10 nm Al absorption of infrared light by the molecules present in the collector on top of a 35 nm LiCoO2 layer, the P 1s signal of the sample of interest. It is a non-destructive technique (photon garnet solid state electrolyte beneath the positive electrode energy < 1 eV) and particularly useful for non-crystalline organic could be observed, in addition to the signal of the remaining compounds for which absorption occurs when the frequency of elements Al, Li, Co, and O.106 With liquid electrolytes, the the incident IR light coincides with the vibrational frequency of feasibility of cells containing ionic liquids has been demon- a bond. Infrared spectra can thus constitute a ngerprint for strated in UHV.107 Instead, the use of more conventional organic some of the components of electrolytes, the SEI and organic or aqueous electrolytes is still limited, although technical electrode materials. Operando measurements would therefore advances have been possible with the introduction of graphene enable the investigation of not only the evolution of the cation windows for the observation of liquids108 and the availability of solvation structure in the electrolyte117,118 but also the redox liquid ow cells supporting graphene or Si3N4 windows that mechanisms and/or degradation processes for organic elec- with a typical thickness of 25–50 nm can be crossed by electrons trodes.119 Unfortunately, the study of the formation of the SEI emitted from harder X-rays.109 Such experiments are becoming and related passivation layers is limited by the fact that they are frequent especially in catalysis.110 Another approach for liquid commonly very thin (between 5 and a few tens of nm) and electrolytes, requiring NAPP, is the “meniscus XPS” or “dip and consist of compounds containing functional groups similar to pull” method.111 It consists in pulling the electrode out of the those present in the electrolyte they originate from, which is electrolyte so that it forms a nm-thick layer. These congura- present inside the cell. tions are depicted in Fig. 7. Examples of experiments involving Despite that fact, coupling synchrotron radiation with FTIR liquid electrolytes are still scarce, mainly because of the limited microspectroscopy (SR-mFTIR) has great potential to study 1650 | Chem. Sci., 2023, 14, 1641–1665 © 2023 The Author(s). Published by the Royal Society of Chemistry Perspective Chemical Science operando battery materials, as the synchrotron infrared source terms of data treatment as the signal related to the electrolyte is 100–1000 times brighter than a conventional thermal (e.g. could mask the one originating from the system to be Globar) sources.120 Moreover, its high brightness (i.e. ux investigated. density) allows smaller regions (3–10 mm) to be probed in Another important parameter to account for in ATR-FTIR microscopy using different operation modes like transmission measurement mode is the probing depth (dp), typically ranging or reection, suitable for operando battery analysis with an between a few hundreds of nm and 10–15 mm.119 acceptable signal-to-noise ratio.121 l In addition to the brightness advantage, the synchrotron dp ¼ pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi (2) 2p n1 sin2 q  n2 2 2 source has other unique features. For example, the broad band of the synchrotron light enables FTIR spectroscopy to exploit the full infrared spectrum expanding from the far- to near- with (l) the wavelength of the light, (q) the angle of inci- infrared regions of the electromagnetic spectrum. This is dence, and (n1) and (n2) the refraction indices of the IR trans- essential for determining the physical properties of solid-state parent window and the probed medium, respectively (Fig. 7). components, to study surface science, as well as for the char- Therefore, a higher penetration depth can be achieved using acterization of the chemical composition.122 a low refractive index transparent window and measuring in the Other advantages over the conventional thermal sources are low frequency region. However, most of the signal is associated a pulsed time structure and a high degree of polarization, which with the small fraction of the electrode/electrolyte (