Will in situ synchrotron-based approaches beat the durability issues of next-generation batteries?

The steady increase of electrical energy demand brings into the spotlight concerns of a ethical, environmental, societal and political nature, related to the usage of fossil fuels. Renewable energy, such as solar and wind, could replace hydrocarbons, but sustainability imposes their integration with reliable and efficient storage facilities. Electrochemistry plays a key role in the quest for better electrical energy storage devices in the whole power range from mW to MW, and its impact is growing at an impressive rate. Notwithstanding the huge amount of research and development efforts devoted worldwide to the fabrication of innovative electrochemical storage concepts, to date only a limited amount of traditional types of batteries are really available on the market and only a few pilot experiences have been reported for grid storage. The situation is certainly a complex one, but it is for sure that innovative battery concepts seem to have a hard time in attaining the technological readiness level required for widespread marketability. This is largely due to the fact that successful operation of a battery depends on the cooperation of an enormous number of physico-chemical, electrical, thermal and mechanical processes taking place over wide spatial and temporal ranges. Engineering the concertation of all these factors—many of which are elusive and counterintuitive—is a highly challenging interdisciplinary task, requiring a wide and firm knowledge basis. To give an idea of this complexity, below we list a few representative processes selected from the plethora of those ruling practical battery behaviour. Electrochemical kinetics of the primary interfacial redox processes is the key phenomenon controlling battery output: this depends on state-of-charge as well as on the charge–discharge history of the device. Electrodic reactions imply molecular level processes running at interfaces that may span a few nanometers, but they are coupled with concentration fields spreading from the microscopic to the device scale and implying multispecies mass transport controlled by diffusion, migration and often convection. In most cases, both electrodic kinetics and mass transport rely on peculiar physical characteristics of a material, such as particle size, phase composition and porosity distribution. Moreover, mechanical stresses can develop owing to volume changes in electrode active materials undergoing redox reactions. As far as the timescales are concerned, quasi-equilibrium processes such as double-layer charging can be completed in fractions of a ms while the time-constants of electrodic reaction are typically in the few-ms range and degradation events leading to capacity fade can occur on timescales from seconds to several days. Furthermore, interfacial kinetics, bulk chemistry, charge- and mass-transport, phase composition, component morphology, thermal and mechanical processes are generally coupled. In this scenario, the vision is that systematic and detailed understanding of the evolving material properties and chemistry, followed during battery operation, could provide the factual basis to develop rational guidelines for technological breakthroughs: hence the leading role of in situ and operando analytics. These approaches have several advantages compared to their ex situ counterparts since on the one hand they avoid a range of artefacts due to sample extraction, manipulation and transfer and on the other hand they allow the material to be observed in its real context (in situ) as well as with running processes (operando).

Among in situ approaches to battery studies, synchrotron-based ones have played a role of increasing importance over the past two decades, during which both battery science and synchrotron techniques have undergone rapid development, with a synergy that has indeed been growing, but can still develop with high potential for cross-fertilization between the two communities. Synchrotron sources, due to their high power and flux, are conceptually suited for rapid in situ studies with high lateral resolution, yielding structural, chemical and morphological information in times currently down to seconds, but potentially able to cover the timescales of chemical reactions and charge-transfer processes: depending on the specific kind of the experiment, the temporal resolution of classical synchrotron based measurements can range from milliseconds to hours. Experiments can now be designed to study nanometric regions as well as structural and chemical changes of single-particle during electrodic reactions. A suite of x-ray imaging techniques with chemical sensitivity, ranging from tomography to coherent diffractive approaches, complete the portfolio of tools for synchrotron-based battery studies. Of course, such sophisticated techniques can be implemented only at the cost of designing appropriate electrochemical cells, able to cope on the one hand with x-ray and sometimes electron transmission requirements, and on the other hand with current density distribution needs of a real battery. Finally, awareness of the possibility of beam damage is important and has to be excluded on the basis of positive experimental evidence. Essentially the whole range of wavelengths that can be generated with a synchrotron has been employed for battery studies: (i) synchrotron infrared offers high brightness and allows spatial resolution for high-precision FTIR mapping measurements; (ii) ultraviolet and soft x-rays are extensively used for chemical analysis at surfaces; (iii) hard x-rays are the most widely used radiation for probing bulk lattice and electronic structures. The soft x-ray absorption cross sections are typically two orders of magnitude larger than those of hard x-rays. These differences strongly impact the probing depth: a few hundred nm for soft x-rays and from µm to mm for hard x-rays. In addition to penetration depth, another key contrast between soft and hard x-rays in photon-in photon-out spectroscopies, is the difference in excitation and decay channels. In the case of core-level excitation processes, in addition to the typical coverage of the low Z (soft x-ray) and high Z (hard x-ray) elements, increased chemical sensitivity can be achieved by working at the L- rather than the K-edge of transition metals (TM). With photon-in-electron-out techniques, the probing depth varies from several Å to several nanometers, in the soft x-ray regime, to tens of nanometers with hard x-rays. A combination of different x-ray techniques—including diffraction, spectroscopy and imaging—is generally required in order to gain an insightful understanding of the structural, chemical and electronic properties of surfaces, interfaces and bulk of a battery component at multiple length scales.

The uniqueness of this special issue with respect to highly qualified reviews and monographs that have recently appeared, lies in the following aspects: (i) comprehensive nature; (ii) focus on in situ approaches and (iii) presentation of spectral imaging methods. Similar publications, instead, tend to focus on a specific type of battery or on a given analytical method, neglecting a broader view that would help grasp the complementarity of synchrotron-based methods. Thus, the present work offers (see figure 1 for a synopsis): (i) a broad coverage of battery types, including advances in widely investigated systems and novel concepts; (ii) a range of experimental and data-processing methods in the realms of spectroscopy and imaging; (iii) an insightful description of cells and electrochemical protocols for in situ studies. Moreover, care has been taken to render each contribution as self-contained as possible, providing a readable account of the background both regarding electrochemistry and x-ray methods. We therefore believe that this publication on the one hand helps establish an effective link between the battery and x-ray communities, and on the other hand provides a compact and complete overview of the state-of-the-art.

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Regarding details of the works included in this publication, a first group of papers concentrates on XAS—both in the hard (hXAS [6, 8]) and soft (sXAS [1, 5]) x-ray regimes—and proposes in situ studies of lithium-ion batteries (LIB) [1, 6, 8], sodium-ion batteries [1] and catalysts for the oxygen evolution reaction (OER) [5]. One work describes the use of in situ near-ambient pressure x-ray photoelectron-spectroscopy (NAP-XPS) for the investigation of high-temperature direct methanol fuel cell anodes and anolytes [4]. A final group of papers presents a range of in situ imaging approaches: (i) micro-computed tomography (mCT) employed for the investigation of zinc-air (ZAB), sodium–air (Na–O₂), lithium–sulfur (Li–S) [02] and vanadium redox flow batteries (VRFB) [3]; (ii) soft-x ray fluorescence (sXRF) microspectroscopy [7, 9] and (iii) coherent diffractive imaging (CDI) [07], both applied to the study of ZABs. A brief overview of the key methods and results is provided below.

Operando hXAS—in both the XANES AND EXAFS regions—was used in [8] to obtain information on the modification at both Fe and Mn environments in mixed metal olivine LiFe_0.75Mn_0.25PO₄ during electrochemical lithiation/delithiation. Mixed metal olivines are of interest for LIB cathodes since the working potential of Fe-olivine can be increased by either modifying the structure, and thus the inductive properties, of the phosphate polyanions, or by partially substituting iron with other TMs. The substitution of Fe with Mn, however, tends to hinder the reversible insertion of lithium cations due to distortion at the Mn³⁺ centres formed during the charge. The operando XAS experiments described in [8] allowed to follow: (i) the redox processes involved in the electrochemical lithiation at both Mn and Fe centres and (ii) the modification of their coordination environment, obtaining direct information on the chemical-state and structural changes occurring during electrochemical operation. Quantitative analyses of potential-dependent spectra have been carried out with a combination of principal component analysis (PCA) and multivariate curve resolution (MCR). Even though Fe and Mn are selectively involved in redox processes in low and high voltage ranges, respectively, nevertheless XAS spectra at both K-edges were found to vary in the whole potential range, revealing their simultaneous evolution even when they are not directly participating in the redox process. In fact, while oxidation of Fe²⁺ to Fe³⁺ globally produces a simple shortening of the Fe–O, Fe–P and Fe–M distances, a major variation in the first coordination shell of the Mn atom could be observed. At higher potentials, Mn oxidation takes place, accompanied by a structural distortion that adequately explains the inhibition of complete lithiation in the Mn-modified oxide.

In [6], operando hard and tender x-ray XAS measurements of LIB and Li–S batteries, accompanied by quantitative analyses of the XANES and the EXAFS parts of the at the Fe, Co and S K edge spectra, were employed to follow the behaviour of a Co-containing hexacyanoferrate-based cathode and S speciation during both charge and discharge. In the LIB cathode, the redox role of the two metals and its impact on structure modification upon the potassium release and the lithium insertion during the charge and discharge of the battery were clarified. Fe was found to be electrochemically active, while Co is not taking part in the process, but the number of Fe–C–N–Co linear chains of the Fe-haxacyanocobaltate structure increases during the first charge. Cycling was shown not to be fully reversible in terms of structural modification of the ordering within the cathode material.

Li–S batteries are emerging high-energy-density devices—based on lightweight Li⁰ and S⁰ electrodic materials, with great potential for automotive applications. Their electrochemical mechanisms include complex solid–liquid–solid transformations—involving the initial and final solid phases S₈ and Li₂S as well as intermediate polysulfides dissolved in the electrolyte -, that can be appropriately characterized by XAS. The different sulfur compounds formed at the cathode (elemental sulfur; crystalline Li–S; Li-polysulphide chains of different lengths; S⁶⁺ present in the electrolyte) and their relative amounts could be identified during charge and discharge from the characteristic energy positions of the sulfur edge and pre-edge resonances. The spectral trends for polysulphides, elemental S and Li₂S are symmetrically inverted during the charge and discharge, even though the system was shown to not be completely reversible.

Li et al [1] reviews quantitative sXAS at the TM L-edge of a range of LIB cathode materials, representative of the three key classes of state-of-the-art oxide-based materials as well as prospective systems for Na-ion batteries, operating like LIBs on the basis of electrochemical insertion, but implementing safer and cheaper active compounds. According to the specific systems, quantitative analyses were performed by fitting either with combinations of measured reference spectra or with theoretically computed ones. In the LiFePO₄ olivine, the changes of chemical state of Fe upon electrochemical lithiation and delithiation highlighted subtle deviations from the neat two-phase transformation occurring in purely chemical processes. The Mn and Ni L-edge spectra of spinel LiNi_0.5Mn_1.5O₄ cathodes in different electrochemical conditions were quantitatively analyzed, allowing to pinpoint a stable Ni³⁺ intermediate. Moreover, the contrast between surface and bulk signals allowed by fluorescence and electron-yield detection modes showed the formation of electrochemically inactive Ni²⁺ at the cathode surface. The chemical state of Mn was also followed in Na_0.44MnO₂ electrodes for novel Na-ion batteries, cycled to different electrochemical states. The expected Mn³⁺/⁴⁺ redox couple was assessed, but in addition a strong surface Mn²⁺ signal was observed and correlated with capacity fade. Moreover, an operational potential range was identified in which such a detrimental species does not form. For layered LiCoO₂, exhibiting a solid solution type transformation during electrochemical cycling, the Co L-edge was found to exhibit a progressive shift, that could be followed accurately by modeling with Co³⁺ and Co⁴⁺ spectra.

Finally, L-edge sXAS, that in the case of TMs is a direct probe of 3D states, thus conveying information of spin states, has been used for the study of Na-ion battery cathode materials, such as Na_2−xFe₂(CN)₆, exhibiting mixed spin states, with possible spin state transitions during electrochemical operation. The electrochemical cycling behaviour, exhibiting two characteristic plateaus, has been rationalized on the basis of sXAS spectra and multiplet calculations in terms of two types of iron atoms in different spin states, explicitly indicating that the spin state of the TM could define the electrochemical properties of a cathode material.

Operando sXAS was used in [5] to address water-splitting related topics. In particular, sXAS at the L-edge of Cu, Ni and Co, was employed to study the electro-deposition and OER electro-catalytic performance of thin TM oxides on Au substrates. The experiments were performed in a wet flow cell with Si₃N₄ window in fluorescence-yield detection mode. The dependence of the electronic structure on potentials and polarization times was followed by sXAS measurements and interpreted on the basis of DFT modeling, allowing to identify the dominating chemical species and their coordination environment. Combining sXAS-based estimates of the TM valence and occupancy of the 3D orbitals and electrochemical measurements, the OER activity was correlated with the oxidative dissolution of the TM films, resulting in the loss of oxygen from the metal-oxide lattice.

Recent instrumental advancements allow XPS measurements at near-ambient pressures, enabling a range of in situ and operando studies in electrochemistry. In [4] NAP-XPS was employed for the in situ study of Pt₃Ru DMFC anodes, operated in a cell with a phosphoric acid-doped hydrocarbon membrane, enabling operation at high temperature with low methanol crossover. In situ electrochemistry was performed in a two-electrode configuration under different polarization conditions and in different H₂O/methanol gas ambients. XPS spectra were collected in the electrolyte and anode regions. Quantitative analyses of the Pt/O atomic ratios as a function of potential and gas ambient composition led to the conclusion that potential-induced flooding of the catalytic layer takes place as a result of the migration of phosphoric acid-solvated phosphate anions to the surface of Pt particles. Such flooding is enhanced by the presence of methanol. Moreover, potential-induced dissociation of phosphoric acid was assessed, leading to the accumulation of phosphate anions in anolyte, that can enhance phosphate adsorption, acting as a catalytic poison. Concerning the electro-catalyst, a strong dependence was found of the Pt/Ru elemental ratio on applied potential, suggesting that the surface of Pt₃Ru particles is enriched with Ru under all investigated conditions. In H₂O ambient, Pt surface atoms are in the fully reduced state in the whole potential interval studied, probably protected by Ru segregation. Addition of methanol leads to potential-dependent partial oxidation of Pt. Since such oxidation cannot be attributed to the action of methanol alone, it can be explained by the changes in the phosphoric acid electrolyte and by the stronger flooding of the catalytic layer. Ru, even at the most cathodic potentials investigated is present as a mixture of Ru(0) and RuO₂, to which Ru(IV) hydroxides add at the most anodic potentials. The presence of methanol strongly affects the chemical state of Ru, resulting in a decrease in the contribution of metallic Ru, and an increase in the Ru(IV) species, which are believed to be the catalytically active ones.

The electrolyte and electro-catalyst changes observed by in situ NAP-XPS are expected to be detrimental to the fuel cell operation and having been able to pinpoint them can guide modifications of the materials, leading to a better performance of these devices.

X-ray mCT lends itself ideally to in situ battery studies since it allows to gain detailed direct-space insight into performance-limiting degradation processes such as dendrite formation, morphology changes of metal anodes, loss of liquid electrolyte and mass-transport processes. mCT is an appealing method for battery research for the following key reasons: (i) it is a non-destructive method, allowing ultra-low energy input during analysis; (ii) spatial resolution is of a few µm with conventional sources and of a fraction µm with synchrotron light; (iii) the duration of a tomography scan is between some tenths of microseconds and several hours, depending on the x-ray source, on the imaging setup, as well on the specimen of interest and electrochemical polarization protocol; (iv) use of monochromatized beams allows spectroscopic imaging. In [2] a selection of examples is reported regarding metal-air (specifically Na–O₂ and Zn–O₂) and Li–S batteries, while [3] focuses on VRFBs. Among diverse types of electrochemical energy storage constructs, the metal–air battery concepts exhibit storage potentialities ranging from low-power portable consumer electronics, to automotion and home applications, as well as to mini- and power grid. The Zn-air chemistry is highly appealing for the following reasons: (i) high energy density (more that twice that of the LIB technology); (ii) intrinsic safety (not flammable, no thermal runaway); (iii) environmental friendliness (no toxic redox reagents, potentially no toxic additives); (iv) affordability (0.25% of LIB). The Na-air system exhibits still higher energy density, but exhibits two key drawbacks: (i) Na is unstable in aqueous ambient and (ii) it cannot be recharged if a water-based electrolyte is employed. Flow batteries are among the most attractive solutions for grid-scale energy storage owing to their chemical and structural simplicity: (i) the redox reagents are all in the liquid state and the electrolyte is water-based; (ii) no electro-catalysts are required; (iii) the electode materials are simply carbon felts. In operando mCT experiments reported in [2] have visualised the precipitation of NaO₂ into the porous cathode structure of a Na–O₂ battery: this process can lead to pore clogging, resulting in electrolyte or O₂ shortage. As far as Zn–O₂ batteries are concerned, tomographic data show that it is possible to discriminate accurately between Zn, the anodically active material, and the liquid electrolyte, indicating that this approach is suitable to follow in situ the Zn shape changes of secondary batteries, that crucially impair their durability. Moreover, mCT allowed the direct observation of liquid electrolyte transport into the pores of the GDL, giving rise to the formation of liquid droplets, that can lead to O₂ starvation. Finally, operando x-ray radiography of Li–S batteries revealed the formation of macroscopic β-sulfur dendrites at the end of the charge step and ex situ tomography of the same system revealed that they are distributed deep inside the carbon cathode, resulting in loss of active material during cycling.

As far as VRFB are concerned, their technology, though appealing, is still at the initial stages and C-felt electrode durability problems—related to corrosion, wetting and compression issues –have not been clarified yet. In this scenario, mCT is a valuable diagnostic tool for the monitoring of damage, agglomeration and distortion of the carbon fibres and for the visualisation of electrolyte penetration under different operating conditions and stages of cycle lifetime. Jervis et al [3] describes in detail the design of a miniature flow cell with rotational symmetry and equipped with facilities to apply different compression levels, for in situ mCT work. A selection of experimental results shows the visualization of incomplete wetting of the felt by electrolyte, that not only reduces the active volume available for the redox reactions, but can also cause local fluctuations in electrolyte concentration, leading to local starvation and undesired side reactions. Segmented images allow to quantify precisely observables such as wetting amounts, pore size distribution, porosity, tortuosity, surface area and contact angles, that—through structure-based modeling—allow the estimation of key operating quantities, like the pressure drop through the felt and local reactant concentrations.

Bozzini et al [7] reports the application of in situ spatially resolved soft x-ray fluorescence microspectroscopy in addressing electrochemical fabrication issues of ZAB cathodes. In scanning sXRF microscopy, a monochromatized microprobe x-ray beam is focused by diffractive optics to a spot ca. 100 nm in diameter and the specimen is raster-scanned to obtain 2D images, allowing to obtain morphological and chemical information jointly. Lithographed transmission wet cells have been developer for soft x-ray work in transmission: these cells contain Si₃N₄-supported electrodes, designed to ensure a tailored current density distribution, that also act as x-ray windows confining a thin layer of electrolyte. The study described in [7] concentrates on the electro-deposition of Mn–Co/polypyrrole nonocomposite ORR electro-catalysts for ZABs and alkaline fuel cells. These materials are synthesized electrochemically by a sequence of anodic and cathodic pulses: this complex process—though effective for fabrication purposes—presents several ill-defined mechanistic aspects regarding the definition of the chemical state of the electrodeposited metals and their space distribution. In situ identical-location sXRF microspectroscopy disclosed the development of elemental distributions during growth, allowing to identify growth instability processes and localization effects. The unique information gathered by spectromicroscopy allowed to correlate the specific electro-deposition mechanisms of Mn and Co together with polypyrrole in the relevant electrochemical system (i.e. precipitation of the former and direct reduction of the latter metal) with the actual global and local time-dependent metal distributions. This information can be fed into appropriate models of electro-deposition dynamics [9] in view of attaining a knowledge-based quantitative design of functional materials for electrochemical energetics. Specifically [9], focuses on quantitative use of in situ dynamic sXRF microspectroscopy images. Quantitative analysis consists in identifying the parameters of a morphochemical model of electrochemical phase formation by means of an original map identification algorithm. In particular, the morphology and manganese distribution changes have been followed during ageing under operating conditions. The availability of quantitative information on space-time evolution of electrode components in terms of model parameters, on the one hand can be employed for material optimization and on the other hand allows efficient use of the wealth of information generated by dynamic multidimensional analytics.

The use of in situ keyhole coherent diffractive imaging (kCDI) for ZAB studies is reported in [7]. In kCDI the illuminated area of the sample, defined by diffractive optics, is ca. 35 µm in diameter and diffracted x-rays are collected by a CCD camera and elaborated with appropriate algorithms for image reconstruction, yielding nanometer space resolution. Moreover, by scanning the beam energy, spectral CDI can be obtained, allowing to reconstruct local sXAS absorption and phase spectra. In [7] kCDI has been applied to the same Mn–Co/polypyrrole electro-deposition system investigated by sXRF microscopy in the same paper, yielding nanometric resolution and providing complementary absorption and phase contrast imaging modes, the latter enabling to follow the morphochemical development of very tiny, weakly absorping features. The joint information derived from kCDI and sXRF mapping allows measurement of otherwise inaccessible observables that are a prerequisite for electro-deposition modeling and control of the dynamic localization processes, on which the quality and durability of the electro-catalysts depend.

In conclusion, this special issue collects a focused and comprehensive selection of novel visions of in situ and operando synchrotron-based battery studies. On the one hand, unique physical insight—gained with advanced analytics—is offered on novel materials for more mature electrochemical storage technologies, such as Li-ion batteries, direct methanol fuel cells and water splitting, and on the other hand pioneering in situ studies are reported of advanced batteries such as Li–S, Na-ion, metal-air and redox flow types. As far as synchrotron-based techniques are concerned, this publication features both instrumental and methodological advances in classical techniques, such as hXAS and sXAS and novel methods, such as near-ambient pressure XPS, and presents a special focus on in situ imaging (tomography) and spectral imaging methods (sXRF microscopy and coherent diffractive imaging). The global message of this special issue is thus that, thanks to the implementation of a suite of synchrotron-based spectroscopies, microspectroscopies and imaging methods, it is possible to correlate integral performance indicators with fundamental physicochemical and structural descriptors of the state of battery components in situ and during device operation. In order to overcome the limitations imposed by the absorption of x-rays of the energy relevant to the specific experiment, special cells had to be developed: cell design on the one hand has to cope with the constraints of the specific technique and on the other hand must reproduce the real battery operation—in terms of material combinations and current density distribution—as faithfully as possible. The corpus of results obtained with these model batteries and presented in this collective work demonstrates the great potential of both established and emerging x-ray based analytics for the study of virtually any type of electrochemical energy storage device and we envisage that the applications of synchrotron methods for these studies can be further developed and refined in terms of: (i) assessment of space-dependent processes, such as the evolution of the distribution of electroactive or obnoxious elements and the localization of over-voltages; (ii) rationalization of damaging mechanisms at the submicron scale; (iii) acquisition of chemical-state information on couplings of electro-active and electro-catalytic materials; (iv) achievement of full operando, dynamic capabilities. Finally, it is worth pointing out that the data deluge generated by high-throughput, multidimensional microspectroscopies will require numerical tools able to provide both transparent physicochemical insight into complex morphochemical distributions and effective data-reduction capabilities.