In situ Raman spectroscopic–electrochemical studies of lithium-ion battery materials: a historical overview

Analysis of Structural Heterogeneity in Low-Rank Coal and Its Pyrolyzed Char Using Multi-Point Scanning Micro-Raman Spectroscopy

Understanding the changes in carbon structure during the mid-low-temperature pyrolysis of low-rank coal is important for efficient utilization. Raman spectroscopy is commonly used to analyze the structural order of carbonaceous materials, but traditional methods may overlook the heterogeneity of coal/char. This research explores the heterogeneity of char structure derived from low-rank coal at 700 °C through multi-point micro-Raman analysis. The analysis of parameters such as area (A), intensity (I), full width at half maximum (FWHM/W), and peak position (P) reveals that the carbon structure becomes less ordered as coal transforms into char due to the deposition of small molecules on the surface. The study emphasizes the benefits of multi-point detection for gaining in-depth insights into the structural evolution of carbonaceous materials. The increased standard deviation of Raman parameters indicates diverse structural characteristics resulting from pyrolysis at this temperature, which traditional methods may not capture effectively. The mapping method used in this research visually illustrates the distribution of carbon structures in the region.

Understanding the reaction mechanism and kinetics of photocatalytic oxygen evolution on CoOx-loaded bismuth vanadate

Photocatalytic water splitting for green hydrogen production is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Loading a co-catalyst is essential for accelerating the kinetics, but the detailed reaction mechanism and role of the co-catalyst are still obscure. Here, we focus on cobalt oxide (CoOx) loaded on bismuth vanadate (BiVO4) to investigate the impact of CoOx on the OER mechanism. We employ photoelectrochemical impedance spectroscopy and simultaneous measurements of photoinduced absorption and photocurrent. The reduction of V5+ in BiVO4 promotes the formation of a surface state on CoOx that plays a crucial role in the OER. The third-order reaction rate with respect to photohole charge density indicates that reaction intermediate species accumulate in the surface state through a three-electron oxidation process prior to the rate-determining step. Increasing the excitation light intensity onto the CoOx-loaded anode improves the photoconversion efficiency significantly, suggesting that the OER reaction at dual sites in an amorphous CoOx(OH)y layer dominates over single sites. Therefore, CoOx is directly involved in the OER by providing effective reaction sites, stabilizing reaction intermediates, and improving the charge transfer rate. These insights help advance our understanding of co-catalyst-assisted OER to achieve efficient water splitting.

Isotope Exchange Raman Spectroscopy (IERS): A Novel Technique to Probe Physicochemical Processes In Situ

A novel in situ methodology for the direct study of mass-transport properties in oxides with spatial and unprecedented time resolution, based on Raman spectroscopy coupled to isothermal isotope exchanges, is developed. Changes in the isotope concentration, resulting in a Raman frequency shift, can be followed in real time, which is not accessible by conventional methods, enabling complementary insights for the study of ion-transport properties of electrode and electrolyte materials for advanced solid-state electrochemical devices. The proof of concept and strengths of isotope exchange Raman spectroscopy (IERS) is demonstrated by studying the oxygen isotope back-exchange in gadolinium-doped ceria (CGO) thin films. Resulting oxygen self-diffusion and surface exchange coefficients are compared to conventional time-of-flight secondary-ion mass spectrometry (ToF-SIMS) characterization and literature values, showing good agreement, while at the same time providing additional insight, challenging established assumptions. IERS captivates through its rapidity, simple setup, non-destructive nature, cost effectiveness, and versatile fields of application and thus can readily be integrated as new standard tool for in situ and operando characterization in many laboratories worldwide. The applicability of this method is expected to consolidate the understanding of elementary physicochemical processes and impact various emerging fields including solid oxide cells, battery research, and beyond.

Study of a High-Voltage NMC Interphase in the Presence of a Thiophene Additive Realized by Operando SHINERS

Improving the electrochemical properties and cycle life of high-voltage cathodes in lithium-ion batteries requires a deep understanding of the structural properties and failure mechanisms at the cathode electrolyte interphase (CEI). We present a study implementing an advanced Raman spectroscopy technique to specifically address the compositional features of interphase during cell operation. Our operando technique, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), provides a reliable platform to investigate the dynamics of the interphase structure and elucidate the compositional changes near the cathode surface. To improve the CEI properties, thiophene was introduced and investigated as an effective, high-voltage film-forming additive by largely diminishing the capacity fading triggered at high potentials in LiNi_1/3Co_1/3Mn_1/3O₂ cathodes. While the cells without thiophene show severe capacity fading, cells with an optimized concentration of thiophene exhibit a significant performance improvement. Operando SHINERS detects the presence of a stable CEI. The results suggest that the composition of the CEI is dominated by polythiophene and copolymerization products of ethylene carbonate with thiophene, which protects the electrolyte components from further decomposition. The formation mechanism of the polymeric film was modeled using quantum chemistry calculations, which shows good agreement with the experimental data.

3D High-Resolution Chemical Characterization of Sputtered Li-Rich NMC811 Thin Films Using TOF-SIMS

Massive demand for Li-ion batteries stimulates the research of new materials such as high-capacity cathodes, metal anodes, and solid electrolytes, which should ultimately lead to new generations of batteries such as all-solid-state batteries. Such material discovery often requires knowledge on lithium's content and local distribution in complex Li-containing systems, which is a challenging analytical task. The state-of-the-art time-of-flight secondary-ion mass spectrometry (TOF-SIMS) is one of the few chemical analysis techniques allowing for parallel detection of all sample components and representing their distributions in 3D with nanoscale resolution. In this work, we explore the outstanding potential of TOF-SIMS for comprehensive chemical and nano-/micro-structural characterization of novel Li-rich nickel manganese cobalt oxide thin films, which are potential cathode materials for the future generation batteries. Off-stoichiometric thin films of Li- and Ni-rich layered oxide with the composition of Li_xNi_0.8Mn_0.1Co_0.1O₂ (LR-NMC811, x > 1) were deposited using reactive magnetron sputtering. Such thin films do not contain any conductive additives or binders and therefore serve as model 2D systems to investigate compositional fluctuations, surface and interface phenomena, and their aging. TOF-SIMS revealed the presence of 400 ± 100 nm overlithiated grains and 100 ± 30 nm nanoparticles with an increased ⁷Li¹⁶O⁺ ion content in the buried part of LR-NMC811. The Li-rich agglomerates could potentially serve as Li reservoirs for compensating Li losses during cathode fabrication and cell operation. Interestingly, these sub-micron structures decomposed in time upon exposure to ambient conditions for 30 days.

koLayered Oxide Cathode-Electrolyte Interface towards Na-Ion Batteries: Advances and Perspectives

With the ever increasing demand for low-cost and economic sustainable energy storage, Na-ion batteries have received much attention for the application on large-scale energy storage for electric grids because of the worldwide distribution and natural abundance of sodium element, low solvation energy of Na⁺ ion in the electrolyte and the low cost of Al as current collectors. Starting from a brief comparison with Li-ion batteries, this review summarizes the current understanding of layered oxide cathode/electrolyte interphase in NIBs, and discusses the related degradation mechanisms, such as surface reconstruction and transition metal dissolution. Recent advances in constructing stable cathode electrolyte interface (CEI) on layered oxide cathode are systematically summarized, including surface modification of layered oxide cathode materials and formulation of electrolyte. Urgent challenges are detailed in order to provide insight into the imminent developments of NIBs.

Raman scattering enhancement of dielectric microspheres on silicon nitride film

Circulating light in the total internal reflection within dielectric spheres or disks is called the whispering gallery mode (WGM), which by itself is highly sensitive to its surface and capable of detecting viruses and single atomic ions. The detection site of the sensors using WGM is created by the evanescent light from the circulating light inside spheres. Here we report anomalous Raman scattering enhancement in dielectric microspheres on a silicon nitride (SiN) film. This Raman enhancement occurs at the periphery of the spheres, and a similar ring of light was also observed under a fluorescence microscope. This is caused by the light circulating around the dielectric spheres as in the WGM. We observed anomalously enhanced Raman spectrum at the periphery of 3 μm diameter polystyrene (PS) microspheres on a SiN film using confocal laser Raman microscopy. The wavelength intensity of this enhanced Raman spectrum was accompanied by periodic changes due to interference. These features may lead to the development of high-sensitive sensors and optical devices.

Review of Multifunctional Separators: Stabilizing the Cathode and the Anode for Alkali (Li, Na, and K) Metal-Sulfur and Selenium Batteries

Alkali metal batteries based on lithium, sodium, and potassium anodes and sulfur-based cathodes are regarded as key for next-generation energy storage due to their high theoretical energy and potential cost effectiveness. However, metal-sulfur batteries remain challenged by several factors, including polysulfides' (PSs) dissolution, sluggish sulfur redox kinetics at the cathode, and metallic dendrite growth at the anode. Functional separators and interlayers are an innovative approach to remedying these drawbacks. Here we critically review the state-of-the-art in separators/interlayers for cathode and anode protection, covering the Li-S and the emerging Na-S and K-S systems. The approaches for improving electrochemical performance may be categorized as one or a combination of the following: Immobilization of polysulfides (cathode); catalyzing sulfur redox kinetics (cathode); introduction of protective layers to serve as an artificial solid electrolyte interphase (SEI) (anode); and combined improvement in electrolyte wetting and homogenization of ion flux (anode and cathode). It is demonstrated that while the advances in Li-S are relatively mature, less progress has been made with Na-S and K-S due to the more challenging redox chemistry at the cathode and increased electrochemical instability at the anode. Throughout these sections there is a complementary discussion of functional separators for emerging alkali metal systems based on metal-selenium and the metal-selenium sulfide. The focus then shifts to interlayers and artificial SEI/cathode electrolyte interphase (CEI) layers employed to stabilize solid-state electrolytes (SSEs) in metal-sulfur solid-state batteries (SSBs). The discussion of SSEs focuses on inorganic electrolytes based on Li- and Na-based oxides and sulfides but also touches on some hybrid systems with an inorganic matrix and a minority polymer phase. The review then moves to practical considerations for functional separators, including scaleup issues and Li-S technoeconomics. The review concludes with an outlook section, where we discuss emerging mechanics, spectroscopy, and advanced electron microscopy (e.g. cryo-transmission electron microscopy (cryo-TEM) and cryo-focused ion beam (cryo-FIB))-based approaches for analysis of functional separator structure-battery electrochemical performance interrelations. Throughout the review we identify the outstanding open scientific and technological questions while providing recommendations for future research topics.

In-Depth Mechanism Understanding for Potassium-Ion Batteries by Electroanalytical Methods and Advanced In Situ Characterization Techniques

The advancement of potassium ion batteries (PIBs) stimulated by the dearth of lithium resources is accelerating. Major progresses on the electrochemical properties are based on the optimization of electrode materials, electrolytes, and other components. More significantly, the prerequisites for optimizing these key compositions are in-depth and comprehensive exploration of electrochemical reaction processes, including the evolution of morphology and structure, phase transition, interface behaviors, and K⁺ movement, etc. As a result, the obtained K⁺ storage mechanism via analyzing aforementioned reaction processes sheds light on furthering practical application of PIBs. Typical electrochemical analysis methods are capable of obtaining physical and chemical characteristics. The advent of in situ electrochemical measurements enables dynamic observation and monitoring, thereby gaining extensive insights into the intricate mechanism of capacity degradation and interface kinetics. By coupling with these powerful electrochemical characterization techniques, inspiring works in PIBs will burgeon into wide realms of energy storage fields. In this review, some typical electroanalytical tests and in situ hyphenated measurements are described with the main concentration on how these techniques play a role in investigating the potassium storage mechanism for PIBs and achieving encouraging results.

In situ surface-enhanced Raman spectroelectrochemistry reveals the molecular conformation of electrolyte additives in Li-ion batteries

We report the mechanism of rhodamine B (RhB) acting as an electrolyte additive in Li/graphite cells. We show that the cycle performance and rate capability of graphite are enhanced in carbonate-based electrolytes containing 0.2 wt% RhB. By using silica-encapsulated Au nanoparticles, in situ surface-enhanced Raman spectroscopy (SERS) is applied to study the graphite/electrolyte interface. We find that the adsorption orientation of RhB molecules on the surface of graphite can be modulated by the applied potential: vertical adsorption at higher potentials while horizontal adsorption takes place at lower potentials. This behavior effectively suppresses the electrolyte solvent decomposition, as well as electrode corrosion while improving the Li⁺ diffusion. This work shows that SERS is a powerful tool for interfacial analysis of battery systems and provides new ideas for rational design of electrolyte additives.

Operando XPS: A Novel Approach for Probing the Lithium/Electrolyte Interphase Dynamic Evolution

The coupling protocols combining photoemission spectroscopy and other characterization methods such as electrochemical, electrical, optical, thermal, or magnetic paved the way to considerable progress in the field of materials science. Access to complementary data on the same object is relevant, but in the vast majority of cases, it is carried out sequentially and separately. This raises the complex question of the equivalence of the analyzed surfaces subjected to these different characterizations. In the frame of lithium ion battery technology (LIB), several techniques have been developed to follow in operando condition the reactivity of electro-active materials toward liquid or solid electrolytes. Besides the knowledge of the redox processes obtained using operando protocols, especially at the interfaces, some limitations associated with material sensitivity and/or the characterization techniques are still a breakdown to widen our understanding of the origin of the LIB performance degradation processes. Herein, we propose a new design of an operando cell adapted to perform X-ray photoemission spectroscopy (XPS) at the interface between electrode and electrolyte under electrochemical solicitations. To illustrate its performance, the crucial issues associated with the lithium metal interface have been scrutinized using Li/Li symmetrical cells and two types of ionic liquids, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (C₁C₆ImTFSI) and 1-hexyl-3-methylimidazolium bis(fluorosulfonyl)imide (C₁C₆ImFSI) laden with LiTFSI salt. Our original setup allowed us to follow-up the lithium surface reactivity toward these ionic liquid based electrolytes in open circuit voltage condition and under polarization. Beside the gain of time and the matter saving, we highlighted and optimized the blocking issues to perform accurate OXPS measurement for probing the evolution of the chemical structure and the surface potential change at the interface lithium/electrolyte in dynamic mode.

Persistent and reversible solid iodine electrodeposition in nanoporous carbons

Aqueous iodine based electrochemical energy storage is considered a potential candidate to improve sustainability and performance of current battery and supercapacitor technology. It harnesses the redox activity of iodide, iodine, and polyiodide species in the confined geometry of nanoporous carbon electrodes. However, current descriptions of the electrochemical reaction mechanism to interconvert these species are elusive. Here we show that electrochemical oxidation of iodide in nanoporous carbons forms persistent solid iodine deposits. Confinement slows down dissolution into triiodide and pentaiodide, responsible for otherwise significant self-discharge via shuttling. The main tools for these insights are in situ Raman spectroscopy and in situ small and wide-angle X-ray scattering (in situ SAXS/WAXS). In situ Raman confirms the reversible formation of triiodide and pentaiodide. In situ SAXS/WAXS indicates remarkable amounts of solid iodine deposited in the carbon nanopores. Combined with stochastic modeling, in situ SAXS allows quantifying the solid iodine volume fraction and visualizing the iodine structure on 3D lattice models at the sub-nanometer scale. Based on the derived mechanism, we demonstrate strategies for improved iodine pore filling capacity and prevention of self-discharge, applicable to hybrid supercapacitors and batteries.

Iodide based energy storage is a potential candidate to improve performance of hybrid supercapacitors and batteries. Here, the authors revisit the previous understanding and show that electrochemical oxidation of iodide results in solid iodine deposits stabilized by the confinement of nanoporous carbons.

Analytical Measurements to Elucidate Structural Behavior of 2,5-Dimethoxy-1,4-benzoquinone During Charge and Discharge

Organic compounds as electrode materials can contribute to sustainability because they are nontoxic and environmentally abundant. The working mechanism during charge-discharge for reported organic compounds as electrode materials is yet to be completely understood. In this study, the structural behavior of 2,5-dimethoxy-1,4-benzoquinone (DMBQ) during charge-discharge is investigated by using NMR spectroscopy, energy-dispersive X-ray spectroscopy, magnetic measurements, operando Raman spectroscopy, and operando X-ray diffraction. For both lithium and sodium systems, DMBQ works as a cathode accompanied with the insertion and deinsertion of Li and Na ions during charge-discharge processes. The DMBQ sample is found to be in two-phase coexistence state at the higher voltage plateau, and the radical monoanion and dianion phases have no long-distance ordering. These structures reversibly change into the original neutral phase with long-distance ordering. These techniques can show the charge-discharge mechanism and the factors that determine the deterioration of organic batteries, thus guiding the design of future high-performance organic batteries.

Novel Charging-Optimized Cathode for a Fast and High-Capacity Zinc-Ion Battery

A rechargeable aqueous zinc-ion battery (ZIB) is one of the attractive candidates for large-scale energy storage. Its further application relies on the exploitation of a high-capacity cathode and the understanding of an intrinsic energy storage mechanism. Herein, we report a novel layered K₂V₃O₈ cathode material for the ZIB, adopting a strategy of charging first to extract part of K-ions from vanadate in initial few cycles, which creates more electrochemically active sites and lowers charge-transfer resistance of the ZIB system. As a result, a considerable specific capacity of 302.8 mA h g^-1 at 0.1 A g^-1, as well as a remarkable cycling stability (92.3% capacity retention at 4 A g^-1 for 2000 cycles) and good rate capability, are achieved. Besides, the energy storage mechanism was studied by in situ X-ray diffraction, in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectroscopy. An irreversible K-ion deintercalation in the first charge process is proved. It is believed that this novel cathode material for the rechargeable aqueous ZIB and the optimizing strategy will shed light on developing next-generation large-scale energy storage devices.

Development of a half-cell for x-ray structural analysis of liquid electrolytes in rechargeable batteries

A half-cell of the rechargeable Li-ion battery was developed to characterize an electrolyte structure using high energy x-ray total scattering measurements in combination with a two-dimensional x-ray detector. The scattering pattern consisted of strong Bragg peaks from electrodes and diffuse scatterings from sapphire windows, in addition to a weak halo pattern from the electrolyte. By selectively removing the signals of the electrodes and windows using specific numerical procedures, we could successfully extract the structural information of the electrolyte, which was in reasonable agreement with the reference data obtained from the electrolyte in a glass capillary. The present demonstration with a half-cell is expected to shed new light on operand characterization of the electrolyte structure during charging and discharging.

High-Throughput Single-Nanoparticle-Level Imaging of Electrochemical Ion Insertion Reactions

Nanoparticle electrodes are attractive for electrochemical energy storage applications because their nanoscale dimensions decrease ion transport distances and generally increase ion insertion/extraction efficiency. However, nanoparticles vary in size, shape, defect density, and surface composition, which warrants their investigation at the single-nanoparticle level. Here we demonstrate a nondestructive high-throughput electro-optical imaging approach to quantitatively measure electrochemical ion insertion reactions at the single-nanoparticle level. Electro-optical measurements relate the optical density change of a nanoparticle to redox changes of elements in the particle under working electrochemical conditions. We benchmarked this technique by studying Li-ion insertion in hexagonal tungsten oxide (h-WO₃) nanorods during chronoamperometry and cyclic voltammetry. Interestingly, the optically detected current response revealed underlying processes that are hidden in the conventional electrochemical current measurements. This imaging technique may be applied to 13 nm particles and a wide range of electrochemical systems such as electrochromic smart windows, batteries, solid oxide fuel cells, and sensors.

Review of Recent Development of In Situ/Operando Characterization Techniques for Lithium Battery Research

The increasing demands of energy storage require the significant improvement of current Li-ion battery electrode materials and the development of advanced electrode materials. Thus, it is necessary to gain an in-depth understanding of the reaction processes, degradation mechanism, and thermal decomposition mechanisms under realistic operation conditions. This understanding can be obtained by in situ/operando characterization techniques, which provide information on the structure evolution, redox mechanism, solid-electrolyte interphase (SEI) formation, side reactions, and Li-ion transport properties under operating conditions. Here, the recent developments in the in situ/operando techniques employed for the investigation of the structural stability, dynamic properties, chemical environment changes, and morphological evolution are described and summarized. The experimental approaches reviewed here include X-ray, electron, neutron, optical, and scanning probes. The experimental methods and operating principles, especially the in situ cell designs, are described in detail. Representative studies of the in situ/operando techniques are summarized, and finally the major current challenges and future opportunities are discussed. Several important battery challenges are likely to benefit from these in situ/operando techniques, including the inhomogeneous reactions of high-energy-density cathodes, the development of safe and reversible Li metal plating, and the development of stable SEI.

In Operando Probing of Lithium-Ion Storage on Single-Layer Graphene

Despite high-surface area carbons, e.g., graphene-based materials, being investigated as anodes for lithium (Li)-ion batteries, the fundamental mechanism of Li-ion storage on such carbons is insufficiently understood. In this work, the evolution of the electrode/electrolyte interface is probed on a single-layer graphene (SLG) film by performing Raman spectroscopy and Fourier transform infrared spectroscopy when the SLG film is electrochemically cycled as the anode in a half cell. The utilization of SLG eliminates the inevitable intercalation of Li ions in graphite or few-layer graphene, which may have complicated the discussion in previous work. Combining the in situ studies with ex situ observations and ab initio simulations, the formation of solid electrolyte interphase and the structural evolution of SLG are discussed when the SLG is biased in an electrolyte. This study provides new insights into the understanding of Li-ion storage on SLG and suggests how high-surface-area carbons could play proper roles in anodes for Li-ion batteries.

Optical spectroscopy as a tool for battery research

The following compendium reviews the development and establishment of optical spectroscopy as an analytical method for battery material components and electrochemical reactions. The interaction of light with matter is a sensitive and non-destructive way to characterize any sample state, i.e. solids, liquids or gases. Special attention is devoted to infrared and ultraviolet spectroscopy, covering a wavelength range from 12 μm to 200 nm, as well as Raman scattering spectroscopy, in order to excite different vibrational/rotational lattice modes and transitions of valence electrons. This allows an insight into structural properties, chemical composition, oxidation states or kinetic processes. The development of spectroelectrochemical in situ cells allows the investigation of various battery components, e.g. working and counter electrode, separator, electrolyte as well as interfaces between these components. These powerful tools allow the evaluation of the functionality, stability and safety aspects of an electrochemical storage cell.

Evolution of Reactions of a Fluoride Shuttle Battery at the Surfaces of BiF3 Microclusters Studied by In Situ Raman Microscopy

Fluoride shuttle batteries (FSBs), which utilize defluorination of metal fluorides and fluorination of the resultant metals, are expected to have high energy densities. In situ Raman microscopy was conducted during FSB reactions of a nearly-2D cluster of orthorhombic BiF₃ microparticles partly embedded in a gold-plated film (o-BiF₃ /gold). At a high overpotential, defluorination of the surface of an o-BiF₃ particle (or cluster) was almost completed within approximately 120 s. At a low over potential, defluorination proceeded from the contours of the cluster that was in contact with the gold to the center of the cluster, suggesting that the rate-limiting process was electronic diffusion. Conversely, fluorination proceeded uniformly at the surface of the cluster to form BiF₃ with a cubic structure (c-BiF₃ ). The results will lead to the establishment of a strategy for efficient use of active materials with low electronic and ionic conductivities.

Investigation of the Phase Transition Mechanism in LiFePO₄ Cathode Using In Situ Raman Spectroscopy and 2D Correlation Spectroscopy during Initial Cycle

The phase transition of the LiFePO₄ and FePO₄ in Li-ion cell during charging-discharging processes in the first and second cycles is elucidated by Raman spectroscopy in real time. In situ Raman spectroscopy showed the sudden phase transition between LiFePO₄ and FePO₄. Principal component analysis (PCA) results also indicated that the structural changes and electrochemical performance (charge-discharge curve) are correlated with each other. Phase transition between LiFePO₄ and FePO₄ principally appeared in the second charging process compared with that in the first charging process. 2D correlation spectra provided the phase transition mechanism of LiFePO₄ cathode which occurred during the charging-discharging processes in the first and second cycles. PCA and 2D correlation spectroscopy are very helpful methods to understand in situ Raman spectra for the Li-ion battery.

Application of Operando X-ray Diffractometry in Various Aspects of the Investigations of Lithium/Sodium-Ion Batteries

The main challenges facing rechargeable batteries today are: (1) increasing the electrode capacity; (2) prolonging the cycle life; (3) enhancing the rate performance and (4) insuring their safety. Significant efforts have been devoted to improve the present electrode materials as well as to develop and design new high performance electrodes. All of the efforts are based on the understanding of the materials, their working mechanisms, the impact of the structure and reaction mechanism on electrochemical performance. Various operando/in-situ methods are applied in studying rechargeable batteries to gain a better understanding of the crystal structure of the electrode materials and their behaviors during charge-discharge under various conditions. In the present review, we focus on applying operando X-ray techniques to investigate electrode materials, including the working mechanisms of different structured materials, the effect of size, cycling rate and temperature on the reaction mechanisms, the thermal stability of the electrodes, the degradation mechanism and the optimization of material synthesis. We demonstrate the importance of using operando/in-situ XRD and its combination with other techniques in examining the microstructural changes of the electrodes under various operating conditions, in both macro and atomic-scales. These results reveal the working and the degradation mechanisms of the electrodes and the possible side reactions involved, which are essential for improving the present materials and developing new materials for high performance and long cycle life batteries.

In situ analytical techniques for battery interface analysis

Interface is a key to high performance and safe lithium-ion batteries or lithium batteries.

Dynamic Raman imaging system with high spatial and temporal resolution

There is an increasing need to study dynamic changing systems with significantly high spatial and temporal resolutions. In this work, we integrated point-scanning, line-scanning, and wide-field Raman imaging techniques into a single system. By using an Electron Multiplying CCD (EMCCD) with a high gain and high frame rate, we significantly reduced the time required for wide-field imaging, making it possible to monitor the electrochemical reactions in situ. The highest frame rate of EMCDD was ∼50 fps, and the Raman images for a specific Raman peak can be obtained by passing the signal from the sample through the Liquid Crystal Tunable Filter. The spatial resolutions of scanning imaging and wide-field imaging with a 100× objective (NA = 0.9) are 0.5 × 0.5 μm² and 0.36 × 0.36 μm², respectively. The system was used to study the surface plasmon resonance of Au nanorods, the surface-enhanced Raman scattering signal distribution for Au Nanoparticle aggregates, and dynamic Raman imaging of an electrochemical reacting system.

Polymer-Based Organic Batteries

The storage of electric energy is of ever growing importance for our modern, technology-based society, and novel battery systems are in the focus of research. The substitution of conventional metals as redox-active material by organic materials offers a promising alternative for the next generation of rechargeable batteries since these organic batteries are excelling in charging speed and cycling stability. This review provides a comprehensive overview of these systems and discusses the numerous classes of organic, polymer-based active materials as well as auxiliary components of the battery, like additives or electrolytes. Moreover, a definition of important cell characteristics and an introduction to selected characterization techniques is provided, completed by the discussion of potential socio-economic impacts.

In Situ AFM Imaging of Solid Electrolyte Interfaces on HOPG with Ethylene Carbonate and Fluoroethylene Carbonate-Based Electrolytes

Chemical and morphological structure of solid electrolyte interphase (SEI) plays a vital role in lithium-ion battery (LIB), especially for its cyclability and safety. To date, research on SEI is quite limited because of the complexity of SEI and lack of effective in situ characterization techniques. Here, we present real-time views of SEI morphological evolution using electrochemical atomic force microscopy (EC-AFM). Complemented by an ex situ XPS analysis, fundamental differences of SEI formation from ethylene carbonate (EC) and fluoroethylene carbonate (FEC)-based electrolytes during first lithiation/delithiation cycle on HOPG electrode surface were revealed.

Effects of Solute-Solvent Hydrogen Bonding on Nonaqueous Electrolyte Structure

We investigate the source of Raman background signal commonly misidentified as fluorescence in nonaqueous electrolytes via a variety of spectroscopies (Raman, fluorescence, NMR) and find evidence of hydrogen-bonding interactions. This hydrogen bonding gives rise to broadband anharmonic vibrational modes and suggests that anions play an important and underappreciated role in the structure of nonaqueous electrolytes. Controlling electrolyte structure has important applications in advancing in operando spectroscopy measurements as well as understanding the stability of high concentration electrolytes for next-generation electrochemical energy storage devices.

Low temperature in situ Raman spectroscopy of an electro-generated arylbis(arylthio)sulfonium ion

Low temperature in situ Raman spectroscopy detects reactive intermediate cations generated by the electrochemical oxidation in organic chemistry.

Single nanowire electrode electrochemistry of silicon anode by in situ atomic force microscopy: solid electrolyte interphase growth and mechanical properties

Silicon nanowires (SiNWs) have attracted great attention as promising anode materials for lithium ion batteries (LIBs) on account of their high capacity and improved cyclability compared with bulk silicon. The interface behavior, especially the solid electrolyte interphase (SEI), plays a significant role in the performance and stability of the electrodes. We report herein an in situ single nanowire atomic force microscopy (AFM) method to investigate the interface electrochemistry of silicon nanowire (SiNW) electrode. The morphology and Young's modulus of the individual SiNW anode surface during the SEI growth were quantitatively tracked. Three distinct stages of the SEI formation on the SiNW anode were observed. On the basis of the potential-dependent morphology and Young's modulus evolution of SEI, a mixture-packing structural model was proposed for the SEI film on SiNW anode.

Insitu Raman study of lithium-ion intercalation into microcrystalline graphite

The first and second order Raman spectra of graphite during the first lithiation and delithiation have been investigated in a typical lithium-ion battery electrolyte. In situ, real-time Raman measurements under potential control enable the probing of the graphitic negative electrode surface region during ion insertion and extraction. The experimental results reveal the staging formation of a single particle within a free standing graphitic electrode. In particular, the in situ behaviour of the double resonance 2D band during the lithiation and delithiation of graphitic carbon has not been previously reported. The 2D band was observed to shift from 2681 to 2611 cm⁻¹ and the band shape transformed into a single Lorentzian from 0.24 to 0.15 V vs. Li/Li⁺, providing further information on the electronic structure and C–C bonding of stage 3 and 4 graphite intercalation compounds. The behaviour of the 2D band is in keeping with the Daumas–Hérold model of electrochemically derived intercalation, where the graphene layers are flexible and deform around domains of intercalating lithium ions.