Unlocking the secrets of electrochemical interfaces is key to advancing energy storage materials, yet these processes remain a complex and critical challenge. Among the tools for probing these elusive regions, X-ray photoelectron spectroscopy (XPS) stands out for its ability to analyze electronic structures and characterize the solid-electrolyte interphase (SEI) formed between electrodes and electrolytes. Traditional ex-situ studies (conducted postmortem on electrodes cycled in liquid electrolytes) have showcased XPS as the go-to technique for SEI analysis, thanks to its probing depth (up to 10 nm) matching typical SEI thicknesses [1, 2]. However, these methods often miss transient or intermediate species, crucial for unraveling charge transfer dynamics.

For solid-state batteries, the challenge deepens: the SEI formed between lithium metal anodes and solid electrolytes is buried, inaccessible to direct analysis. Preparing ex-situ samples without disrupting the SEI [3] further complicates the picture. Understanding this buried interphase demands alternative approaches that can monitor its formation and evolution in real time.

Herein, a virtual charging operando XPS (OpXPS) technique was deployed to electrodeposit lithium directly on a halide-based solid electrolyte within the XPS chamber, enabling real-time tracking of SEI formation. Halide solid electrolytes are promising for next-generation all-solid-state batteries, thanks to their outstanding properties such as high ionic conductivity, oxidative stability and ductility. However, their reactivity with lithium metal remains a major challenge [4].

OpXPS revealed the formation of a dynamic mixed ionic-electronic conductive (MIEC) interphase between the halide electrolyte and lithium metal anode. Complementary Electrochemical Impedance Spectroscopy (EIS) and Distribution of Relaxation Times (DRT) analyses provided critical correlations between interfacial resistance and evolving chemical composition. These techniques demonstrate the powerful capabilities of OpXPS in decoding the complex dynamics of buried interfaces, paving the way for designing stable and efficient solid-state battery systems.

Zinc metal batteries (ZMBs) are promising candidates for low-cost, intrinsically safe, and environmentally friendly energy storage systems. However, the anode is plagued with problems such as the parasitic hydrogen evolution reaction, surface passivation, corrosion, and a rough metal electrode morphology that is prone to short circuits. One strategy to overcome these issues is understanding surface processes to facilitate more homogeneous electrodeposition of zinc by guiding the alignment of electrodeposited zinc. Using Scanning Electrochemical Microscopy (SECM), the charge transport rate on zinc metal anodes was mapped, demonstrating that manipulating electrolyte concentration can influence competing surface reactions and solid electrolyte interphase (SEI) formation in ZMBs. This work show that more extended high-rate cycling can be achieved using a 1 M ZnSO₄ electrolyte, and that these systems have a reduced tendency for soft shorts. Using XPS and Raman spectroscopy, it is demonstrated that an SEI is formed on zinc electrodes at neutral pHs, composed primarily of a Zn₄(OH)₆SO₄.xH₂O species attributed to local pH increases at the interface.  This experimental methodology studying metal battery electrodes is transferable to lithium metal and anode-free batteries, and other sustainable battery chemistries such as sodium, magnesium, and calcium.

      Understanding the electrolyte-electrode interface under realistic operating conditions is critical to designing new-generation battery materials. The past few decades of research have focused on developing a range of spectroscopic and imagining tools to enable the study of the chemical and structural changes at the electrolyte-electrode interface. Here, we introduce operando surface-enhanced infrared absorption spectroscopy (SEIRAS), a promising lab-based tool for gaining a molecular-level understanding of interfacial electrochemical processes.

      This talk will bring the most recent understanding of the electrochemical interface during the operation of energy storage applications using operando SEIRAS. We will highlight how the holistic information about the nature of the electrolyte-electrode interface can provide additional knobs to tune the battery performance, leading to further improvements in efficiency and stability for energy applications. Furthermore, fundamentals and experimental tips for operando SEIRAS will also be explained, which is essential for those interested in introducing operandoSEIRAS to probe your electrochemical system, especially battery applications.

In many applications device performance can be significantly influenced by defects, surfaces, and interfaces, all of which may evolve during operation. Alkali metal ion batteries are no exception, and understanding of how individual atomic scale motifs affect their performance is crucial for development of next generation battery materials. For this, atomic scale modelling play an important part, linking the experimentally observed behaviour to atomic scale mechanisms. In this talk, I will present how we can use the atomic scale modelling toolbox, and discuss the latest results from our group on tackling these challenges, with examples taken from our work on lithium, sodium and potassium ion batteries. Focusing on hard carbon anodes materials, and their interfaces with electrolyte, we will see how surface defects can lead to irreversible capacity and dendrite formation, the effect of pore structure on metal intercalation, and then extend this treatment to investigate the initial stages of the solid electrolyte interphase (SEI) formation. Finally, we will assess how changing the alkali metal ion affects the electrochemical behaviour, what this means for future battery design, and how atomic scale modelling can play an important role in battery manufacturing.

In-situ and operando atomic force microscopy are powerful tools to investigate various energy storage and energy conversion systems such as batteries, fuel cells, or electrocatalytic systems. By utilizing this method, the solid electrolyte interface (SEI) formation as well as Li intercalation and deposition on anode materials have been elucidated.^[1,2,3]  Various degrees of heterogeneity are found depending on the exact system under investigation. Importantly, the local mechanical properties of the interfaces that are obtained simultaneously with the topography and are critically discussed in this presentation. Furthermore, the dependency of mechanical properties on the state-of-charge is outlined.^[4]

Next to morphological and mechanical information, a full understanding of the local electronic conductivity of electrode materials is of utmost importance. In this contribution, limitations of the conductivity of electrospun carbon nanofibers (CNFs) are presented with respect to the carbonization temperature.^[5] A large fraction of the surface of CNFs are found to be not conductive, critically depending on the carbonization temperature. The detected current signals indicate electrically well-interconnected fibers; hence, poor interconnections or heterogeneities of CNF mats are not the limiting factor for an ideal macroscopic conductivity.

Batteries are expected to play a pivotal role in the electrification of a range of sectors, including transport, aerospace and grid-scale storage. Of the next generation battery chemistries, Li-S batteries are a particularly attractive option due to their projected high energy density, low cost, and operating temperature range. However, the chemistry of such systems is complex, involving the electrochemical conversion between two insulating species (S and Li₂S), dissolution and shuttling of soluble intermediates, uncontrolled deposition of cathode and anode species, and large volume expansion; if not carefully regulated, these processes can lead to gradual capacity fade at best, explosive cell failure at worst. Here, we discuss the development of free-standing carbon fibre electrodes that are conductive, lightweight and mechanically robust, and their role in addressing degradation mechanisms arising from volume expansion, polysulfide shuttling, dendrite formation and inventory loss. The fibres were prepared via electrospinning of biomass precursors followed by further heat treatment. The porosity, functionality and conductivity can be tailored by varying the precursor and carefully tuning the treatment conditions, to enable free-standing cathodes with high sulfur loadings, improved redox kinetics and enhanced polysulfide interactions to suppress shuttling and sulfur inventory loss. As anode supports, the carbon fibres provide a lithiophilic substrate for a homogeneous lithium-ion flux and low deposition overpotential, which favours large, uniform and low surface area lithium deposits. Employing UV/vis spectroscopy and optical microscopy to observe operando cell processes, we correlate the surface and structural properties of the carbon fibre electrodes with the ability to suppress polysulfide shuttling and control Li deposition and plating, to allow us to further tune the fibre properties to optimise cycling performance. The assembled cell demonstrates greater capacity retention over long-term cycling than conventional Li-S cathode and anode substrates; additionally, the free-standing configuration allows significant gains in energy density by dispensing with traditional electrode components including the binder, conductive additive and metallic current collector, making this process a promising route to achieving new high-energy-density electrode materials for Li‑S technologies.

The structural accommodations, essential to the processes of insertion of the Li-ion inside the materials of the battery, are strongly dependent on the kinetics of the electrochemical reaction. Thus, the quantification of these structural inhomogeneities, their interfaces and their evolutions according to the state of charge and the electrochemical cycling regime is the key to better understand the processes at the origin of the degradation of the retentions in capacity for the Li-ion batteries. For example, knowledge of the spatial distribution of phase, orientation, grain boundary, and strain is crucial to obtain a complete picture of the phenomena occurring during material operation.

Recently, new in situ/operando analytical tools have been developed to monitor structural and chemical transformations, which have allowed important advances in the knowledge of dynamical processors. Liquid cell TEM is a developing technique that allows us to apply the powerful capabilities of the electron microscope to image and analyze materials immersed in liquid. The liquid/bias cell (Protochips) consists of silicon nitride windows on silicon support called E-chip, which separates the liquid from the vacuum of the microscope and confines it in a thin enough layer for TEM imaging. The importance of liquid cell microscopy in electrochemistry is that liquid cell experiments allow direct imaging of key phenomena during battery operation and relate structural and compositional changes to electrochemical behaviors. Different interesting results on monitoring dynamical processes occurring in a wide variety of electrochemical systems, such as LiFePO4, NMC811, LMNO and solid state will be presented here.

4D-STEM techniques yielding structure and strain maps were used to locally probe the crystallographic information of active material crystals and correlate it to the spatial occupancy of Li-ion during the electrochemical cycle. Liquid mass spectrometry analysis was also used to monitor the evolution of the liquid electrolyte during the formation of SEI on the surface of the negative electrode. Structural refinement of an individual cathode grain was achieved using 3DED (electron diffraction tomography) techniques revealing changes in lattice parameters after an in situ electrochemical delithiation process. The ability to couple emerging analytical techniques with liquid electrochemical cell TEM paves new way in energy material characterization, in particular in the study of the dynamic phenomena occurring during the operation which are until now inaccessible to the primary particle scale.