Sodium ion batteries (SIBs) are a potential alternative to diversify the energy landscape, beyond Lithium-ion batteries (LIBs), due to their similar storage mechanism and easy technology transfer. Currently, the benchmark anodes for SIBs are hard carbons (HCs), since sodium ions do not intercalate into graphite. HCs can be produced from a variety of waste precursors and therefore are more sustainable and less geopolitically compromised than natural graphite, mainly concentrated in China. 

The electrochemical degradation of SIBs occurs at a higher rate than in LIBs. This can be attributed to the larger electrochemical reactivity of the HC anodes. A deeper operando understanding of the degradation mechanisms in SIBs, coupled with engineering of the materials and electrolyte to ensure that a better and more protective solid electrolyte interface (SEI) is formed, is needed for an accelerated scale up of this technology. In this talk I will show you some of the strategies we have developed for these aims.

Increasing the operating voltage of batteries brings additional challenges to the electrolyte, as it should be stable at higher potentials. Therefore, investigating the electrochemical stability window (ESW) of electrolytes is extremely important. While this could be considered an easy task with liquid electrolytes because mass transport limitation is seldom an issue, it is more complicated with solid polymer electrolytes. Common techniques used are cyclic voltammetry (CV) and linear sweep voltammetry (LSV). However, the main drawback of these methods is that the electrochemical stability of electrolytes is often arbitrarily determined and using conditions far from realistic. Therefore, alternative or complementary methods are required.

Herein, we investigate alternative techniques to determine the oxidative stability of solid polymer electrolytes. Staircase voltammetry (SV) is used to avoid the mass transport limitation and it is combined with electrochemical impedance spectroscopy to simultaneously detect changes in resistance and interfacial degradation. Synthetic charge–discharge profile voltammetry (SCPV) is used to apply the real voltage profile of the active material of interest. Finally, to include the effect of the active material, cut-off increase cell cycling method (CICC) has been developed where the upper cut-off voltage is gradually increased up to 5 V. Monitoring the voltage profile and the Coulombic efficiency can provide information of side reactions occurring at different voltages. The feasibility of these different methods has been investigated with two model solid polymer electrolytes: poly(ethylene oxide) and poly(trimethylene carbonate).[1]

As the world's need for energy storage grows at an accelerated pace, researchers are looking for new strategies to take batteries to the next level of performance, both in terms of capacity and durability, while improving safety. The focus has been on the discovery of new materials but also on the development of a wide range of sophisticated diagnostic techniques. This allows on the one hand to anticipate their behavior in the presence of electrolytes in various electrochemical devices and on the other hand to study the processes leading to the evolution of interfaces. Mastering the interface processes is a real challenge in electrochemistry, and it is crucial for the development of electrochemical devices.

One strategy to overcome this challenge is the implementation of sensing technologies (optical¹, acoustic²...) providing a real-time monitoring of the interfacial processes occurring in a battery. In this regard, interest in piezoelectric sensors employed in junction with electrochemical analysis, has also been growing steadily in the energy storage community.^3,4 In our group, we aim at describing this complex interface by operando piezoelectric sensors, during electrochemical cycling of battery electrodes. This methodology, commonly known as Electrochemical Quartz Crystal Microbalance (EQCM) based interface analysis,^4-6 is able to provide vital understanding of the positional cohabitation of ions and solvent molecules within the electrical double layer (EDL), which itself conveniently falls in the penetration depth of the acoustic shear wave emanating from the QCM sensor.

In this contribution, we will describe our piezoelectric sensing strategy to investigate the impact of electrolyte composition on the electrochemical performance. The objective is to establish an interface model for different electrolyte compositions, via a study of the various solvent/salt mixtures, differing in their dipole moment and size/weight, respectively. Our experimental methodology lifts the challenge in the probing the EDL’s composition and structuration: specifically, accessing to the identity and the repartition profile of the charge carriers, as well as the role of solvation. Our results exemplified for both Li and Na ion cells,^5,6 will be complemented by the recent fiber optic based sensing methodology,^6,7 which provides the information on the chemical evolution of this interface.

Operando measurements are a powerful approach to understanding the performance and degradation of batteries. Nuclear magnetic resonance (NMR) spectroscopy is well-suited to such measurements due to its non-invasive, non-destructive nature and its sensitivity to both structure and dynamics. However, it does not come without challenges. Among them are technical aspects such as achieving realistic battery cycling performance with a minimum amount of metal in the operando cell, as well as reducing the interference between the battery and NMR electrical circuits. Moreover, the resolution of operando NMR spectra is much lower than in conventional solid-state NMR due to the absence of magic-angle spinning and the signal overlap from different cell components.  

In this contribution, we demonstrate how the above challenges can be addressed and that operando NMR can provide information that may be difficult to obtain with other analytical techniques. We will focus on Li-ion batteries, presenting ⁷Li operando NMR data obtained on NMC811/graphite and LNO/graphite cells. Two aspects will be discussed in detail: Unwanted Li metal plating on graphite and the changes of structure and Li dynamics in the cathode materials. 

Li metal deposition on graphite is an unwanted process which does not only accelerate capacity fade, but also poses a serious safety hazard. Since NMR spectroscopy is an excellent technique for the unambiguous detection of Li metal, we employ it here for studying the Li plating process in operando under different cycling conditions, with varying charging current, voltage and temperature.[1] 

The Ni-rich cathode materials LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811)[1] and LiNiO₂ (LNO) are studied during charge and discharge, illustrating their structural changes as well as the changes of Li-ion mobility with state-of-charge. For the first time, we present well-resolved operando NMR data of LNO/graphite cells. We use this data to refine the description of phase transformations in LNO during (de)lithiation, in particular with respect to changes of Li mobility. Open questions regarding the ⁷Li solid-state NMR spectra of Li_xNiO₂ are discussed as well.

In this invited talk I will present our latest advances in Na and Al based batteries. In particular I will discuss advances in anodes for Na ion batteries from atomistic understanding to upscale at pauch cell level. These will be based on both hard carbons from biomass as well as Sn/C composites. I will pay particular atnetion to the SEI development in Na-ion batteries using different electrolytes and aditives based on analysis using electrochemistry coupled with mass spectroscopy to detect the gases evolved as well as ToF-SIMS and XPS on the electrodes to understand the chemical composition of the SEI.I will also discuss Al dual ion batteries and in particular the interface on the Al metal  due to corosion because of the electrolytes as well as the role of the curent colectors and the stainless steel casing of coin cells. 

Various in operando characterisation btechniquyes such as solid state NMR as well as Raman will be discussed for both na and al systems.

The current strong interest in electromotive mobility and the need to transition to an energy grid with sustainable storage devices has led to a renewed interest in sodium ion batteries (SIBs). Hard carbons (HCs) are promising candidates for high-capacity negative electrode materials in SIBs. Their high capacities, however, are often accompanied with high irreversible capacity losses during the initial cycles,[1] while low initial losses are accompanied with moderate capacities.[2] In the DialySorb project, which is funded by the German Ministry for Research and Education, we are aiming at a local separation of reversible sodium storage and irreversible losses in novel synthetic carbon anodes, using a core-shell concept.

Advanced analytical techniques are employed to identify structure-performance relation of the core-shell anodes and will be presented in the present presntation.

Literature:

[1] Mehmood A., Ali G., Koyutürk B., Pampel J., Chung K. Y., Fellinger, T.-P., Nanoporous nitrogen doped carbons with enhanced capacity for sodium ion battery anodes, Energy Storage Materials, 2020, 28, 101-111.

[2] Matsukawa, Y.; Linsenmann, F.; Plass, M. A.; Hasegawa, G.; Hayashi, K.; Fellinger, T.-P. Gas sorption porosimetry for the evaluation of hard carbons as anodes for Li-and Na-ion batteries, Beilstein J. Nanotechnol. 2020, 11, 1217–1229.

Silicon is a promising active material for anodes in lithium-ion batteries owing to its high theoretical capacity (3579 mAh/g)¹. However, it is well known that the compound contends with large volume changes during cycling, creating cracks in the material and therefore limiting the lifetime and capacity of the cell. To cope with these volume changes, this project aims to develop self-healing binders to improve the cycling stability of lithium-ion batteries.

Both hydrogen bonds and dynamic covalent bonds have previously been used to modify binder systems to get self-healing properties through reversible cross-linking with the binder in the electrode. While the hydrogen bonds have eminent reversibility, the dynamic covalent bonds provide high mechanical stability^2–4. Both of which is desirable properties to implement in the polymeric binder system.

Specifically, the focus of this work has been to utilize the reversibility of borate ester bonds and couple them with the polymer binder poly(vinyl alcohol) (PVA). The electrochemical performance of the cells has been investigated, where the borate ester-based binders have shown an increased capacity compared to the PVA binder alone. Besides the performance, understanding the self-healing mechanism of these binders and possible degradation reactions from these functionalities is key to further improving the system and cycle life. Therefore, the mechanism behind the self-healing was investigated through FTIR measurements at room and elevated temperatures. Furthermore, the electrochemical stability of the functional groups was investigated in order to compare the impact of dynamic covalent bonding, especially at the low operating voltages of silicon. This purposes that the self-healing functionalities are electrochemically stable and do not contribute to increased degradation in the cell.

For increasing the energy density, lithium metal is considered the "holy grail" anode material due to its high theoretical capacity (3860 mA h g-1) and lowest electrochemical potential (-3.04 V vs the normal hydrogen electrode) ​[1]​. However, the commercialisation of a lithium metal anode is hindered by several issues including safety concerns and rapid lithium inventory loss ​[2]​. To address these challenges, research has focused on electrolyte modification strategies and, more recently, 3D current collectors ​[3]​. Here we propose using free-standing, electro-spun, lignin-derived carbon fibre as a sustainable, high performance lithium anode host structure to replace the conventional current collector. 

Optimisation of the fibre fabrication method, in particular the carbonisation process, resulted in an average coulombic efficiency greater than 99% over 100 cycles, demonstrating low lithium inventory loss. In addition, these fibres promote cycling stability and inhibit dendrite formation as demonstrated by extended lifetimes, and a low and stable voltage hysteresis.  

Increasing the carbonisation temperature from 700°C to 1000°C decreases the amount of solid-electrolyte interface (SEI) formed initially on the fibre surface, with electrochemical impedance spectroscopy (EIS) confirming a decrease in SEI resistance as well as an improvement in SEI stability for fibres carbonised at higher carbonisation temperatures. Increasing the carbonisation temperature from 700°C to 1000°C further increases the pre-plating plateau capacity, defined as the capacity in the plateau region of the voltage curve before the lithium metal nucleation point is reached. Ex-situ nuclear magnetic resonance spectroscopy (NMR) indicates that, in this region, lithium intercalates in ordered crystal regions, while small lithium clusters form in the closed porosity. An increased pre-plating plateau capacity appears to have a positive effect on the fibre lithiophilicity.  

Overall, an improvement in cycling performance at increased carbonisation temperatures has been attributed to a combination of decreased SEI resistance and increased fibre lithiophilicity. In detail, the reduced SEI resistance ensures homogeneous lithium-ion flux and low deposition overpotential, which favours large, uniform and low surface area lithium deposits. The increased lithiophilicity further homogenises the lithium-ion flux by electrostatic interaction ​[4]​.  

Finally, the lithium deposition behaviour and cycling performance at different carbonisation temperatures were studied in relation to the varying fibre properties such as heteroatom content, structural order, as well as open and closed porosity ​[5]​. This knowledge will help us to tune the fibre structure to optimise performance, and ultimately minimise the amount of excess lithium needed in the electrode for improved safety and sustainability.  

The metallic lithium (Li) represents the most promising anode material among the next generation of solid-state lithium batteries [1]. An efficient strategy to achieve durable and effective Li-anode batteries is by engineering the solid-electrolyte interphase (SEI) with purposely designed molecules. To this aim, the vinylene carbonate (VC) is one of the most used additives in conventional electrolytes. Some recent experiments proved that the VC promotes the formation of a stable and protective SEI layer between Li metal and electrolyte [2, 3]. Unless the well-known SEI composition, it is difficult to control the VC reactivity, that involves dissociation and polymerization at the electrode surface. Therefore, to dissect these tangled processes, here we present new atomistic insights on VC-Lithium SEI formation via first-principles calculations by Density Functional Embedding Theory (DFET) [4,5], see Fig. 1. Such approach has potentialities for modeling complex reactions at hybrid interfaces in electrocatalysis: it is well suited to combine the best feasible approaches for molecular species (in this case, hybrid HF-DFT for VC molecules and derivatives) and for Li metal electrode (semi-local GGA density functional). Our results highlight different VC dissociation pathways, with formation of reactive radical species and localized cluster of Li2O and Li2CO3. The use of hybrid-DFT-in-DFT embedding is crucial for obtaning energy barriers and qualtitative results in agreement with experiments [3,6]. Overall, the energetics and structural features of these intermediates improve the current understanding of SEI formation process and can be exploited to drive the reactions toward the desired interfacial properties.

Transition-metal based layered compounds are considered as promising cathode materials in sodium-ion batteries (SIBs), in which sodium ions (Na⁺) can occupy the interstitial sites. [1] In addition to the typical transition-metal (cationic) redox activities, the anions in layered compounds, such as the lattice oxygen or sulfur, may also participate in the electrochemical redox reactions during sodiation/desodiation process. [2] Although layered oxides are very promising cathode materials for SIBs, there are still limitations such as voltage fade due to ion migration and loss of molecular oxygen during overcharging. [3,4] Therefore, replacing oxygen with sulfur and designing layered sulfide compounds with reversible anionic redox activities are promising for next generation of cathode for SIBs.

Titanium (Ti), as a transition metal element with abundant resources, has attracted considerable attention. In our study, we synthesized a series of Ti-based layered sulfide compounds and investigated them as cathode materials in SIBs. Through the utilization of several synchrotron radiation techniques, phase transitions and redox processes during sodiation and desodiation are comprehensively explored. Furthermore, electrolyte-dependent electrochemical behaviors of layered sulfide compounds are also investigated. These investigations can offer valuable insights into the working mechanisms of Ti-based sodium layered sulfide compounds, providing reference information for the design of layered sulfide cathode materials for SIBs in the future.

Materials play a crucial role in the circular economy, and their selection and supply are currently driven primarily by cost. However, considering that different minerals and materials feature on various global critical materials lists, alternatives may be necessary. Lithium, graphite, phosphate, silicon, and cobalt, for example, are listed as critical materials by the EU and the UK, highlighting supply chain risks and their economic importance. By embracing the principles of sustainability, including reduce, reuse, recover, and recycle, we can gradually reduce our reliance on these materials. Sodium-ion batteries are being explored as a viable alternative, allowing for the substitution of lithium, cobalt, and graphite with sodium, iron, and hard carbon, respectively. This substitution significantly reduces material costs. However, it is important to consider other sustainability factors, such as recyclability and duration of use, in-life phases and additionally the current higher embedded carbon emissions per kilowatt-hour (kWh) reported for sodium-ion batteries.

Furthermore, this study examines design considerations for disassembly in sodium-ion batteries. We discuss recycling routes, materials recovery, and potential reuse cases for these batteries. The recovery rates are often influenced by the design of the cells and electrodes. To maximize the performance and recovery rates of materials, we propose specific routes for electrode manufacturing, compositions, and design considerations. By implementing these strategies, we aim to enhance the overall sustainability of sodium-ion battery technologies and promote effective recycling and reuse.

The development of high voltage and high capacity Na-ion cathodes is one of the key challenges for the successful realization of high energy density sodium ion batteries.¹ Among the Na-ion cathodes explored so far, NASICON-Na₃V₂(PO₄)₃ cathode is attractive because of its high intercalation voltage (3.45 V vs. Na⁺/Na⁰), moderate capacity (~120 mAh g^-1) and excellent rate capability. Further, other cations are substituted in the place of vanadium of the NVP lattice to seek the participation multiple redox centres, thereby enhancing intercalation capacities. Recently, our group has developed a series of Na₃V₂(PO₄)₃-type cathode in which vanadium cations are partially replaced by different cations such as Mn²⁺, Mg²⁺, Al³⁺ and In³⁺.^2-3 The Mn-rich Na_3.75V_1.25Mn_0.25(PO₄)₃ cathode has displayed highest capacity and rate performances (100 and 89 mAh g^-1 at 1 and 5C rates, respectively) due to its modulated V- and Mn-redox centers and optimum bottleneck size. The substitution of Mg²⁺ and Al³⁺ into the Na₃VMn(PO₄)₃-type cathode has enhanced its rate performances and cycling stability. The In-substituted Na₃VIn(PO₄)₃ cathode has exhibited asymmetric multi-redox V⁵⁺/V⁴⁺/V³⁺ operation during Na-ion (de)intercalation reaction. During the talk, we will highlight the importance of chemical tunning to enhance the electrochemical performances of NASICON cathodes.

The transition from fossil fuels to renewable energy sources requires economic and sustainable electrochemical energy storage solutions with high energy and power density. With the aim of providing a promising alternative to the voluminous intercalation anodes (negative electrode) of state-of-the-art Li-ion batteries, we propose anode-free cells with alkali-metal anodes and solid electrolytes. Such cells require no intercalation material in the anode and no addition of alkali metal during cell assembly. This increases the energy density by up to 35% and reduces production costs [1].

Depending on the operating temperature and the materials used, the alkali-metal anodes can be in a solid or a liquid state. Solid alkali-metal anodes suffer from dendrite formation and limited power capabilities. In contrast, liquid alkali-metal anodes, when combined with solid electrolytes, allow for high power charging and discharging, as well as high areal capacities [2,3]. However, a major challenge with liquid anodes is alkali-metal management, which involves storing and releasing the liquid alkali metal within the anode compartment during cell cycling, accommodating the volumetric changes within the anode, and ensuring good electrical contact between the alkali metal and the solid electrolyte [4]. Effective alkali-metal management is crucial to achieving cycling stability and efficient utilization of all active materials at low overpotentials and low operating temperatures [5]. Currently, wetting and phase change phenomena of alkali metals under battery-relevant conditions are not sufficiently understood to guide the design of coatings and capillary structures for effective alkali-metal management. Therefore, here we propose a series of dedicated experiments on the wetting behavior of alkali metals on a variety of materials.

The development of next generation battery chemistries beyond Li-ion is of crucial importance to achieving a zero emissions economy by 2050, and batteries are expected to play a pivotal role in the electrification of a range of sectors, including transport, aerospace and grid-scale storage. Li-S batteries are a particularly attractive option, due to their projected high energy density, low cost, operating temperature range, and safety; however, sulfur, on its own, is a poor electrode material due to its extremely low conductivity (5 x 10^‑30 S cm^‑1). Various forms of carbon have been explored as the host material for sulfur cathodes because they are conductive, lightweight and mechanically robust; furthermore, depending on the structure and functionality, employing carbon hosts can address other performance degradation mechanisms arising from volume expansion, polysulfide shuttling, sluggish kinetics and poor contact during cycling. In this work, free-standing carbon fibre electrodes 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 electrodes with high sulfur loadings and improved redox kinetics. In addition, enhanced polysulfide interactions result in the suppression of shuttling and sulfur inventory loss, leading to greater capacity retention over long-term cycling. The ability to tailor these hierarchical nanostructures therefore makes this process a promising route to achieving new cathode materials for Li‑S technologies.

Solid-state batteries have undergone significant progress in the recent years, mainly driven by the development of fast solid conductors. Fulfilling the ambitious promise of high gravimetric and volumetric energy density still represents several challenges, covering the processing of materials into components, cell integration and performance. Even fast ionic conductors such as oxides, sulfides or halides dare with resistive and dynamic solid-solid interfaces which are still the main bottleneck to develop high-performance devices.

Such interfaces appear naturally when processing the materials into components or even when stacking components during cell assembly. Some examples are the active material/solid electrolyte interface present at the positive electrode, the solid electrolyte/Li metal anode interface and even between electrolyte particles after densification. Such type of resistive interfaces can be minimized to a great extent from the careful choice of materials chemistry, optimization of microstructure and processing strategies.

Interfaces in SSBs can also be dynamic as a result of the different processes taking place during cell cycling. This leads to even more complex challenges, such as volume expansion and void formation at the anode during lithium electro-platting and dissolution, lithium dendrite propagation across the solid electrolyte, cracking of the positive material from continuous redox processes and resistive interfaces formed from poor electrochemical stability between solid electrolyte and electrodes. These complex interfaces may adversely affect the Li transport, often leading to a fast decrease on the cell performance.

In this talk we will analyze key solid-solid interfaces of different electrolytes present within the cell and how they affect to the performance and ionic transport; from redox processes at the positive electrode and electrochemical stability with Li metal electrodes to dendrite propagation. Besides, we will discuss on how the engineering of interfaces may improve cell performance and paves the way towards the desired anode-less concept.

The need for application specific materials is ever increasing. Materials design plays a pivotal role in this, and the future technologies cannot be realised without efficient materials. The development and optimization of high-performance anode materials for alternative and complementary battery technologies to lithium ion is a crucial challenge for the sustainable energy revolution. ^1–3 Atomic scale computational modeling using density functional theory (DFT) allows for efficient and targeted materials design from the nanoscale up, and is increasingly used by the scientific community as an integral part of material development.^4–6 Computational modeling does not only provide fundamental insight, but also direct input for experimental synthesis, often where quantities cannot readily be obtained by other means. In this talk, I will give an overview of the work we conducted on carbon materials for sodium and potassium batteries using an intelligent materials design framework combing computational and experimental methods. ^7–11 Through DFT simulations, we identified performance limiting and enhancing defect structures, and probed the effect of morphology on battery performance. By simulating the metal ion behaviour at different carbon motifs, the role of the atomic scale structure on the anode performance could be shown.

High capacity lithium-ion electrodes made from earth-abundant materials are highly desirable for large-scale energy storage applications. High entropy materials are a relatively new class of materials that have only recently started to be explored in applications ranging from battery electrodes[1],[2] to solid electrolytes[3] and electrochemical catalysts[4]. High entropy materials comprise of five or more elements randomly arranged in the lattice, with equal probability of occupying the same sites.[5] This disorder, or high configurational entropy, endows the materials with unusual chemical and electronic properties that are still being uncovered.[6] In this study, we present two novel high entropy sulfides including wurtzite Cu₇Mg₂Sn₂ZnGeS₁₃ and zinc-blende Cu₃AlGaInZnS₇, as conversion-type lithium-ion cathode materials with extremely high capacities.

Our results show high capacities of ~1000 mAh g^-1 at 50 mAh g^-1 which is ca. 15% higher than the theoretical capacity of the binary sulfide CoS₂, an extensively studied material as a lithium-ion conversion material.[7] The plateaus in the discharge curves of both materials at ~ 1.7 V and ~0.8 V can be attributed to the reduction of the high entropy sulfide to metal and Li₂S, while in the charging curve, the anodic peaks at ~2.0 V and ~ 2.4 V are assigned to a two-step resulfidation process.[8] These processes are largely reversible, with a variance of not more than +/- 10 mA g^-1 between the charge and discharge capacities . At higher current densities, from 100 – 500 mA g^-1, the specific capacity degrades quickly, but unexpectedly recovers in the last five cycles when the current density returns to 50 mA g^-1. The capacity fade can be attributed to lithium-aluminium alloy formation which occurs at 0.23 V – visible as a low-voltage plateau in the final stage of discharge, as well as SEI formation. Current work is focused on replacing the aluminium current collector with copper foil and carrying out galvanostatic charge-discharge tests over a narrower potential window. Further work will focus on characterising the phases being formed and experimenting with different carbon and electrolyte additives to stabilise the SEI.

This study demonstrates versatile novel high entropy materials with high capacities that could lead to the development of more energy dense lithium-ion batteries using earth abundant transition metals. The findings of this study may be applied to a variety of unexplored high-entropy materials for a range of electrochemical energy conversion/storage applications thanks to their desirable properties, tunability and easy synthesis methods.

Molybdenum disulphide (MoS₂) is seeking to replace graphite as the negative electrode in lithium-ion batteries (LIB), due to the natural abundance of bulk 2H MoS₂ and its high theoretical capacity (670 mAh/g). However, bulk MoS₂ suffers from rapid loss of performance during the first 50 cycles when fully discharged (0.01 V). Aiming to understand the degradation mechanism at play in bulk MoS₂ electrodes, coin cells with electrode material coatings of 10 μm, 40 μm, and 80 μm were opened after lithiation to 0.80 V, 0.40 V, and 0.01 V. Cells lithiated to 0.01 V and then delithiated to 3.00 V were also studied. The cycled electrodes revealed a tri-colored inhomogeneous concentric ring pattern, which remains present during initial delithiation and further cycling. Thicker electrode coatings resulted in a more noticeable ring pattern, which was mimicked by surface corrugation on the Li foil counter electrode. Ex-situ characterization involving scanning electron microscopy (SEM), X-ray diffraction (XRD), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS) was carried out on the 40 μm electrodes to analyze the different regions of the ring pattern. All three segments were covered by a solid electrolyte interphase (SEI) made up of fluoride, carbon, and oxygen. The central segment of the electrode is made up of 2H MoS₂, largely unaltered by the lithiation process. The middle segment is composed of predominantly 1T phase MoS2 and a small presence of 2H phase MoS₂. The outer segment has a thick SEI cover, with MoS₂ underneath whose phase could not be confirmed. Overall, it is postulated that the ring pattern occurs due to poor contact between the bulk MoS₂ electrode, separator, and Li counter electrode within the coin cell configuration. Thus, a significant fraction of the bulk MoS₂ material is unavailable for lithiation, especially in thicker electrodes.