Solid-state batteries (SSBs) with alkali metal anodes hold great promise as energetically dense and safe alternatives to conventional Li-ion cells. Whilst, in principle, SSBs have the additional advantage of offering virtually unlimited plating current densities, fast charges have so far only been achieved through sophisticated interface engineering strategies. With a combination of surface sensitive analysis, we reveal that such sophisticated engineering is not necessary in NaSICON solid electrolytes (Na3.4Zr2Si2.4P0.6O12) since optimised performances can be achieved by simple thermal treatments that allow the thermodynamic stabilization of a nanometric Na3PO4 protective surface layer. The optimized surface chemistry leads to stabilized Na|NZSP interfaces with exceptionally low interface resistances (down to 0.1 Ω cm2 at room temperature) and high tolerance to large plating current densities (up to 10 mA cm−2) even for extended cycling periods of 30 min (corresponding to an areal capacity 5 mAh cm−2). The created Na|NZSP interfaces show great stability with increment of only up to 5 Ω cm2 after four months of cell assembly.
Focus on Spectroscopy of Energy Materials
Guest Editors
Alan Chadwick, University of Kent, UK Alexis Grimaud, Collège de France, France
Scope
In the global search for better, cheaper, more efficient and environmentally friendly sources of energy production and storage the discovery and improvement of materials plays a major role. Key areas of research are in the fields of production (for example, in solar cells and photovoltaic devises), in usage (for example, in fuel cells) and storage (in batteries and supercapacitors). Materials chemists and physicists are exploring a whole new range of novel compounds to meet the growing demands. Underpinning the research is a wide range of spectroscopic techniques that are being used to characterise the new materials and to gain a fundamental understanding of the processes involved in their roles in the applications. The methods involve laboratory based techniques, such as IR, Raman, X-ray diffraction, etc., and those using powerful central facilities such as X-ray and neutron sources. The techniques have been used in research for many decades however it has been recently, in the last 20 years, which they have been crucial tools in energy research. This has involved the development of new procedures as the techniques have advanced and more efficient means for the collection of spectra as sources and computing power have increased.
Currently, the research in the spectroscopy of energy materials is at a particularly exciting stage and it is timely that JPhys Energy devotes this special issue to this area. The aim is to overview the field covering the present status of the techniques and a view to the future as new facilities come on to the scene. At this particular point in time the methods are powerful and the ability to perform in operando experiments are moving to the fore. Hence it is now becoming routine to probe structural and chemical changes of the components of a working device in real time, a prospect that not too long ago was relatively rare. This advance has been possible as the sources of radiation have become more intense and the radiation detectors have become more sensitive. In addition, it is now becoming increasingly common to use several spectroscopic probes simultaneously in experiments to provide a deeper understanding of the chemical and physical reactions.
No survey of a field would be complete without some attempt to look at the possible prospects for the future. In the field of energy materials it is a firm prediction that there will be rapid improvements and new developments of the techniques over the next decade. The capabilities of central facilities X-ray sources, methods used by the authors, are a good example of this prediction. Most synchrotron sources are now in the process of changing their magnet arrays to become 'fourth generation sources', often termed 'extremely brilliant sources'. This will increase the brilliance and coherence by some two orders of magnitude. Thus conventional experiments will be faster, focused beams will be sub-micron and a range of new techniques are expected to emerge. It is still early to predict the exact role that recently commissioned X-ray free electron lasers (XFEL), which produce very intense and very short bursts of light, but they will open up new possibilities. Furthermore, such rapid development will also exponentially increase the amount and the size of data generated, effect which is becoming even more pronounced with the recent boom for operando measurements. Hence, a transition is gradually expected between a manual data treatment to a more automated and assisted treatment, rendering the development of dedicated methodological and computational tools critical to the field.
We hope that this special issue will be both informative and interesting to a wide range of readers. A more challenging outcome will be fulfilled if some readers feel they should explore the use of some of the techniques in their own research projects.
Papers
Despite the challenges in achieving its full theoretical capacity of reversible extraction of two Li ions, the Li2FeSiO4 (LFS) cathode shows a remarkable cycling stability once its low electronic conductivity is addressed. By studying the local structure around the iron during electrochemical cycling using in situ x-ray absorption spectroscopy (XAS), it is possible to gain insight into the factors which determine the electrochemical properties of this material. In order to practically perform in situ XAS studies, the charge/discharge of LFS was maximized using two approaches: (a) reducing the particle size of LFS samples from micro-scale to nano-scale in order to reduce the diffusion path for intercalating ions; and (b) applying a conductive coating to each nanoparticle to facilitate electron transfer. A family of LFS materials was synthesized and characterized using x-ray diffraction, and scanning electron microscopy with energy dispersive analysis for structural and morphological analysis, as well as cyclic voltammetry and cycling tests for electrochemical performance diagnosis. This material was then characterized by in situ XAS. The results provide insight into the stable electrochemical performance of LFS and suggest new synthetic routes to reaching the theoretical capacity.
Although batteries represent a key tool for sustainable development, their working processes, in terms of reaction mechanisms, side reactions, ion transport and formation of a solid–electrolyte interface, are not yet fully understood. In this respect, operando experiments are of enormous importance for providing hints on the relevant chemical species that form 'while a battery is working'. X-ray absorption spectroscopy (XAS) has for a long time been the standard in the investigation of local structures of materials. In this regard, applied operando can provide invaluable information on the working mechanisms of batteries. In this review, after introductory paragraphs concerning battery chemistry and the principles of XAS, some of the most important developments in operando XAS applied to battery science are considered. Emphasis is given to Li-metal, Na-ion, Li/sulfur and all solid-state batteries. Related and advanced techniques, such as resonant inelastic x-ray scattering and high-resolution fluorescence-detected x-ray absorption spectroscopy are discussed as well. Suggestions are offered for planning an XAS experiment at the synchrotron radiation source, and finally, some considerations concerning future developments are presented.
The dioxygen molecule has two bound states, singlet and triplet, which are different in energy, lifetime, and reactivity. In the context of oxygen electrocatalysis as applied to fuel cells and water splitting the involved O2 is typically considered to be exclusively in its triplet ground state. However, applying spin-conservation rules for the transformation between triplet O2 and singlet OH−/H2O reaction intermediates predicts an additional free energy barrier associated with the required spin flip. As a result, for conditions under which both can form, the formation of triplet dioxygen from the singlet OH−/H2O might be slower than the formation of singlet O2. Correspondingly, singlet O2 might be more active than triplet O2 in the oxygen reduction reaction. Here, we discuss the possible existence and influence of singlet oxygen in oxygen electrocatalysis. Some perspectives for studying singlet oxygen in oxygen electrocatalysis are also provided.
The consistent fabrication of high performance α-Fe2O3 photoanodes for the oxygen evolution reaction remains a challenge. We work towards resolving this issue by developing in situ variable temperature Raman spectroscopy as a means to better understand the formation of α-Fe2O3, using the conversion of γ-FeOOH to α-Fe2O3 under varied gaseous environments as a model case. The sensitivity of Raman spectroscopy to structural changes provides mechanistic insights that are not readily available in more conventional approaches, such as thermal gravimetric analysis and differential scanning calorimetry. The Raman spectra are combined with conventional thermal analyses to interpret the photoelectrocatalytic performance of a series of α-Fe2O3 photoanodes prepared by systematic variation of a three-stage annealing protocol. The combined results suggest that protohematite, a form of α-Fe2O3 where trapped hydroxyl ligands are balanced by Fe(III) vacancies, forms between 200 °C and 400 °C in a reaction environment-dependent fashion. This protohematite is shown to be remarkably persistent once formed, degrading photoelectrocatalytic performance. This research advances understanding of the γ-FeOOH to α-Fe2O3 structural transformation, illustrates a powerful method to study solid state phase transitions, and provides guidance for the synthesis of high quality α-Fe2O3 from a convenient precursor.