Review—SEI: Past, Present and Future

Reduction at the anode can affect electrolyte decomposition, solid electrolyte interphase (SEI) formation and growth, and thus the lithium solvation/de-solvation near the SEI, and ultimately lead to various perilous side reactions such as inactive lithium formation. Lithium ions solvated in the electrolyte solution along with salt anions diffuse towards the surface of the electrode. At the charged surface, these solvated ions can undertake different pathways leading to various reductive decomposition products to subsequently form the SEI. The transfer of electrons from the electrode to the salt anions form inorganic SEI products. The SEI layer gradually thickens during repeated charge/discharge cycles due to electron exposure to electrolyte or electrolyte diffusion to the anode surface. This gradual thickening of SEI layer decreases active lithium ions, solvents, and salts and increases cell resistance and lowers the cell capacity and Coulombic efficiency. Essentially, the choice of electrolytes has a significant influence on the formation of an SEI and its underlying chemical and mechanical properties. Optimizing the electrolytes is crucial for an SEI formation since the properties of the SEI significantly affect the lithium-ion batteries’ cyclability, life time, capacity retention, high power density, rate capability, and safety. One approach of stabilizing the SEI is to utilize an electrolyte with high concentration of salt, also known as High-Concentration Electrolytes (HCEs). This approach modifies the Li+ solvation structure to form contact ion pairs (CIP) and aggregates (AGG) while decreasing solvent-separated ion pairs (SSIPs) so that the salt anion, such as FSI-, is preferentially decomposed to form a robust LiF-rich SEI. LiF is considered a beneficial SEI component to block electron transport. By introducing a diluent (a non-solvating solvent) in the HCE to form Localized High Concentration Electrolyte (LHCE), the disadvantages of the HCE, such as low ionic conductivity, high viscosity and high cost, can be minimized while retaining the highly concentrated salt-solvent clusters as they are in the HCE. LHCEs based on fluorinated solvents and diluents can further stabilize the electrode-electrolyte interface. Recent studies showed that the presence of fluorine in the SEI, either in the form of simple inorganic fluorides (LiF) or organofluoro-moieties, brought positive impacts such as expanded electrochemical stability window and high ionic transport. Fluorinated solvents can shift the oxidation stability to a higher voltage compared to their nonfluorinated counterparts. Fluorinated electrolytes enable a high lithium plating Coulombic efficiency and suppresses lithium dendrite formation to a greater extent. In this work we focused on identifying the selection rules for the diluent for designing LHCEs to preserve or improve the local high salt concentration clusters to facilitate the formation of an inorganic rich anion derivative film on the anode as well as to enhance ionic conductivity to enable fast charging. Some of the important properties to consider while selecting a diluent are: - diluent molecules must offer little or no solubility to the salt so that they have minimal participation in the solvation clusters, they must be readily miscible with the solvating solvent so that they dissolve and remove some solvent molecules from the clusters; effectively increasing the salt concentration in the solvation clusters, diluents should be distributed on the periphery of salt-solvent clusters, diluents should have low viscosity, to reduce the overall viscosity of the formulated electrolyte, which in turn improves the ionic conductivity since the low viscosity of diluents allow for higher mobility of the ionic clusters. We analyzed LHCEs consisting of different diluents and diluent molar ratios in a comparative fashion to understand their properties in retaining or improving the structures of the high concentration salt-solvent clusters and improving ionic conductivity. We varied the diluent molar ratio to understand its relationship to increasing salt concentration gradients in the center of the solvent-salt clusters. We also analyzed the relationship between diluent molar ratio and ionic conductivity and found that an optimum diluent molar ratio exists for which the ionic conductivity can be maximized. Our findings serve as design guidelines for practical applications of LHCEs.

Lithium-ion batteries are the technology of choice for a broad range of applications due to their performance and long-term stability. The solid electrolyte interphase (SEI) is the key to merge these properties because it enables the combination of low-voltage anodes such as graphite with high performance liquid electrolytes. Direct contact between these phases results in continuous reduction of electrolyte solvent and salt. However, these reduction reactions produce solid SEI particles which quickly form a thin film during the first charge of a pristine graphite electrode. This thin film separates electrode and electrolyte, reducing the rate of electrolyte reduction considerably. However, this reaction is never absolutely suppressed and long-term SEI growth remains the biggest contributor to capacity fade in lithium-ion batteries. Our model describes the long-term evolution of a porous SEI [1]. It extends the approach of previous models [2,3] which describe SEI thickness evolution using a single rate-limiting transport mechanism. In this way, they avoid specifying reaction kinetics because the reaction rate equals the incoming throughput of SEI precursors. We instead consider all potentially rate-limiting transport mechanisms, namely electron conduction and solvent diffusion. These mechanisms transport SEI precursors through the film and its pores, as shown in figure 1. This allows us to trace the evolution of the SEI volume fraction in a spatially resolved way, see figure 2. The first model considers a single representative SEI formation reaction and predicts continuous growth of a porous SEI. Electron conduction drives film growth and thickness evolves with the square-root of time. We show that replacing electron conduction with alternative plausible mechanisms suggested in literature (such as diffusion of neutral lithium interstitials through the SEI [4]) leads to qualitatively similar results. Adding a second SEI formation reaction leads to the formation of a dual-layer SEI. The properties of the additional layer depend on the type of the second reaction, i.e., solvent reduction or reduction of SEI compounds. Furthermore, we predict an equilibrium relation between the thickness of the inner layer and the total SEI thickness. Different SEI profiles generated in our model are studied with impedance simulations of graphite anodes. All predictions above are observable with suitable experimental techniques, such as neutron reflectometry, and can be used for model validation. SEI porosity is an output of our model, when SEI growth is limited by electron conduction. However, structural constraints potentially impose a larger porosity onto the system. Our model can smoothly transition to the other rate-limiting transport mechanism which is solvent diffusion by considering these constraints. This enables us to systematically compare SEI formed by qualitatively different mechanisms. [1] Single, F., Horstmann, B., & Latz, A. (2016). Phys. Chem. Chem. Phys., 18, 17810–17814. doi:10.1039/C6CP02816K [2] Pinson, M. B., & Bazant, M. Z. (2012). Journal of the Electrochemical Society, 160(2), A243–A250. doi:10.1149/2.044302jes [3] Christensen, J., & Newman, J. (2004). Journal of The Electrochemical Society, 151(11), A1977. doi:10.1149/1.1804812 [4] Shi, S., Lu, P., Liu, Z., Qi, Y., Hector, L. G., Li, H., & Harris, S. J. (2012). Journal of the American Chemical Society, 134(37), 15476–15487. doi:10.1021/ja305366r Figure 1

We need batteries to get the best out of renewable energies and other low-carbon energy sources, and the most promising path forward is to invest in high-energy-density battery technologies. In this regard, lithium metal is the best candidate for the anode electrode as it is the lightest metal on earth and has the lowest electrochemical potential (-3.04 V vs. SHE). However, the challenge to overcome for mass commercializing lithium-metal batteries (LMB) is stabilizing the solid electrolyte interphase that naturally grows at the electrode-electrolyte interface to avoid uncontrolled reactions leading to lithium depletion. Since the successful introduction of lithium-ion batteries (LIB), it has been known that tuning of the SEI composition and morphology is fundamental to prolong the battery life span; a well-engineered SEI layer simultaneously serves as an electron barrier and favors Li+ ion conduction across it, ensuring proper battery performance. However, the engineering of the SEI layer in LMB is more complex than its counterpart LIB batteries, given that lithium metal is overwhelmingly more reactive than graphite and the overall Li+ ion current across it is orders of magnitude higher. The up-to-date knowledge on the SEI morphology indicates that inorganic phases produced upon the decomposition of lithium salts present in the electrolyte tends to grow dominantly buried within the SEI layer, acting as an intermediate between the surviving lithium metal and the outermost organic SEI phase grown after decomposition of solvent molecules. The effect of electrolyte additives and diluents is to tweak the SEI composition. The narrative built after years of experimentation tells that proper battery functioning depends heavily on the ability of the incoming Li+ ions to desolve near the SEI outermost layer without triggering further side reactions, penetrate the inner SEI, and deposit in the anode as reduced lithium. However, the intricacies of the elementary reactions and the stage where the reduction takes place still fall beyond detailed comprehension, even though the development of in situ and operando analytical techniques has brought us a level of understanding on the subject that was unimaginable a few decades ago.In this talk, we discuss the instrumental impact that multiscale modeling techniques have played in clarifying the elementary steps of the SEI formation on pristine lithium metal, providing a comprehensive understanding of the precursors formed in the early stages of Solid-Electrolyte Interphase (SEI) growth. For instance, density functional theory (DFT) calculations have revealed that electrolyte diluents, such as TTE, previously considered inert to lithium metal, actively modify the liquid electrolyte solvation structure by weakly interacting with Li+ ions. Additionally, these diluents decompose against lithium metal, releasing fluoride ions and inundating the SEI layer with LiF. Similarly, DFT-based molecular dynamics (MD) techniques, including ab initio (AIMD) and reactive classical molecular dynamics methods (ReaxFF MD, among others), have facilitated a picoseconds (ps) scale observation of the temporal evolution of these SEI precursors, propelling a mechanistic understanding of the dynamics of the SEI formation; it is now recognized that there is an initial and rapid electrolyte decomposition occurring within the first tens of picoseconds after contact with lithium metal, preceding a longer-scale mass transfer segregation process leading to the formation of a myriad of organic and inorganic microphases within the SEI structure. However, the accessible time and space windows to MD methods fall within few hundred ps, leaving out the study of the morphological aspects of the SEI over extended cycling. In this sense, we discuss through kinetic Monte Carlo (kMC) calculations the evolution of the SEI formation on lithium metal and confirm the critical role that dislocations and grain boundaries play in ensuring proper lithium plating and mobility within the SEI layer over cycling. We confirm that bulk lithium diffusion within the inorganic SEI microphases, such as LiF and Li2O, is significantly lower than lithium mobility across LiF/Li2O interfaces. We also discuss the impact of electrolyte tunning on SEI morphology and provide insights into developing an SEI formation strategy based on the electrolyte molecules electrochemical stability and their relative contents of heteroatom species, such as F and O, to seed in the SEI composition and morphology, which we believe could have a significant impact on the development of stable LMB batteries.

While the idea that fast charging batteries could be utilized in electric vehicles is enticing, current fast charging batteries face significant challenges that must be mitigated before this application is realized. Fast charging batteries currently suffer from capacity loss and degradation over time which not only affects battery life but poses a safety hazard. The current electrolyte, 1.2 M LiPF6 in 3:7 wt.% EC/EMC (Gen2), contains the salt LiPF6 that when exposed to moisture can form dangerous hydrofluoric acid and contains a carbonate-based solvent that when exposed to oxygen can ignite. In addition, batteries experience degradation over many cycles of fast charging which results in capacity fade and poor battery performance. However, previous studies have shown the potential for other electrolyte systems to mitigate these problems, specifically by using highly concentrated electrolytes1. These studies claim that using these electrolytes results in the formation of a solid electrolyte interphase (SEI) that passivates Li ions better than the Gen2 electrolyte. The SEI is a layer that forms on the anode within the battery and serves as a protective layer that allows for Li intercalation into graphite and can therefore protect against degradation. By using different electrolytes, a different SEI is formed, and different degradation mechanisms are observed. In this way, the SEI can be tuned to improve Li passivation and ionic conductivity by changing the electrolyte’s composition and concentration2. This research focuses on such alternative electrolyte systems and aims to characterize the different degradation mechanisms corresponding to each electrolyte. Favorable performance has been observed using 1.2 M LiFSI in 3:7 wt.% EC/EMC as an electrolyte. We are studying LiFSI in acetonitrile (AN) at higher concentrations than traditional Gen2 to quantify the SEI. We expect AN to form an anion derived SEI, a composition of which results in improved passivation of Li ions1. In this study, we characterize degradation of alternative electrolyte systems through x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Through these methods we can observe and quantify both crystalline, amorphous degradation products due to loss of Li inventory and loss of active material. XPS allows quantification of compositional changes in the SEI layer at the surface of the electrode. The SEM provides qualitative information on degradation such as visualizing lithium plating and dendrite formation. We implement quantitative XRD to measure the crystalline phases of the graphite anode, showing the degree of lithium intercalation and further quantifying degradation such as Li plating. Putting the results of these three characterization techniques together in tandem with electrochemical cycling data paints a picture of the degradation that happens on the anode of a fast-charging lithium-ion battery with respect to electrolyte composition.[1] Yamada, Y., Wang, J., Ko, S., Watanabe, E., & Yamada, A. (2019). Advances and issues in developing salt-concentrated battery electrolytes. Nature Energy. [2] Logan, E. R., & Dahn, J. R. (2020). Electrolyte Design for Fast-Charging Li-ion Batteries. Trends in Chemistry.

Demand on batteries with high energy density is surging. Conventionally, carbon materials have been used as an anode active material, but recently silicon materials are likely to be incorporated to form a composite anode in lithium-ion batteries (LIBs) for higher energy density. However, LIBs which include silicon materials in anode tend to show a low cycle performance especially the ratio of silicon materials in the anode increases. Commercial LIBs still have difficulty in incorporating large amount of silicon materials especially under the requirements where multiple aspects like safety, charging rate and energy density should be totally met.One potential pathway to improve the cycle performance in LIBs with silicon materials is to tune the nature of solid electrolyte interphase (SEI), a thin film which forms at the interface between anode active material and electrolyte. The nature of SEI is particularly important for LIBs with silicon materials because such silicon active materials show large volume changes during battery operation and SEI is dynamically reconstructed. It has been qualitatively reported so far that LiF is an important component for better battery performance. Our work suggests that the ratio of LiF in initially formed SEI is a crucial factor to realize improved cycle performance in such LIBs.In our work, anode with silicon oxide (SiOx)-graphite composite is considered. We investigated the relationship between electrochemical performance of the cell with such composite electrode and the nature of the film estimated by characterization and molecular simulation. We systematically changed the composition of liquid electrolyte to form various types of SEI on anode surface. For characterization, both fresh and aged cells with different electrolytes were dismantled, and SEI components were extracted by designated solvents, then NMR (nuclear magnetic resonance) was applied to identify the SEI species and quantify the amounts of major SEI components. Comparison between the evolution of SEI amount and an impedance element in electrochemical impedance spectroscopy during aging indicates that degradation of the cells is closely related to the growth of SEI. To further deepen understanding the role of LiF, molecular dynamics simulation was carried out, where the mixture of LiF and typical organic SEI was modelled. Correlation between calculated mechanical property and measured electrochemical performance of the cells were revealed. In our presentation, the requirements that SEI should satisfy will be discussed from the insight of electrochemical measurements, NMR characterization and modeling approaches.

Lithium-ion battery (LIB) technology plays a critical role the clean energy sector, with applications in electronic devices, electric and hybrid vehicles, and stationary energy storage. In order to improve battery efficiency and performance, novel materials must be developed and studied. Battery anode materials are of particular interest, and silicon has been selected for materials research due to its high theoretical storage capacity as well as the extensive existing knowledge regarding silicon material processing and fabrication. The initial cycling of a LIB, regardless of anode material, results in the formation of a solid electrolyte interphase (SEI) layer between the electrolyte and the anode. This passivating layer, consisting of decomposition products from the electrolyte solution, is critical to the reliability and performance of the battery. The SEI must be both electronically insulating and ionically conductive: permeable to lithium ions yet resistant to electron flow. The resistance can be inhomogeneous along the anode plane or perpendicular to it. To our knowledge, no characterization technique has been capable of measuring spatial variation in SEI resistivity. In order to measure SEI resistivity with nanometer-scale resolution, our group has utilized scanning spreading resistance microscopy (SSRM) with a specialized atomic force microscopy (AFM) system installed in an argon glove box to minimize sample exposure to oxygen and water. The setup employs a doped diamond coated silicon probe that exhibits high wear resistance and low electronic resistance. Through application of a sample-probe bias voltage while utilizing variable forces on the probe in AFM contact mode, the technique allows for the measurement of SEI resistivity laterally and vertically. The vertical depth of measurement is controllable by the applied probe force (as well as raster scan line density) and is thus capable of milling away SEI material to measure resistivity at defined depths. Subsequent AFM height measurement allows for measured resistance to be associated with the depth of measurement. The electronic system attached to the system measures current through the probe-sample with a logarithm-scale amplifier, which can be directly converted to resistance or resistivity. Our measurements of resistance vs. depth for SEIs formed on single-crystal silicon wafers [001] after a single cycle of lithiation/delithiation demonstrate strong trends for decrease in resistance as the probe penetrates deeper levels of the SEI. Such plots provide an interesting basis for the study of SEIs formed under different cycling conditions with distinct electrolyte solutions. Figure: (a) Schematic diagram of electronic setup of the described system, (b) resistance map at a depth of 43 nm on a SEI formed on a single-crystal silicon wafer [001], with a single cycle of lithiation (0.01 V for 5 hours) and delithiation (1.0 V for 5 hours) in a battery cell containing ethylene carbonate (EC) and diethyl carbonate(DEC) electrolytes, (c) topography map displaying the SEI depth of resistance mapping. Figure 1

Calendar and cycle life of lithium-ion batteries (LIBs) are predominantly dictated by the passivating nature of the solid electrolyte interphase (SEI) film growth on battery anodes. Graphitic anodes exhibit stable SEI growth which enables ubiquitous commercialization of LIBs. In contrast, silicon anodes exhibit poorly passivating SEI characteristics including rapid thickness and composition changes during cycling (“breathing”) and sensitivity to electrolyte composition and surface functionalization. In this work, we develop a chemically complex continuum-level multicomponent, multiphase coupled thermodynamics-reaction-transport SEI model to unravel the mechanisms of SEI growth in LIB anodes. Atomistic calculations in conjunction with experimental datasets are utilized to ascertain the dominant decomposition pathways towards predominant SEI components like Li2EDC, Li2O, Li2CO3, LiMC, LiF etc. and inform the continuum model predictions. Furthermore, experimental voltage-hold measurements are utilized to validate the SEI model parasitic currents/composition evolution profiles enabling prediction of voltage regimes for stable SEI growth with favorable SEI composition. The development of such chemically complex SEI models will aid predictive electrolyte screening for the growth of stable and passivating SEI layers on next generation anodes like silicon.Figure 1(a) shows a schematic of the detailed SEI model incorporating species and charge transport through the pore and solid phases of the SEI for LEDC formation. Figure (b) showcases experimental leakage currents during V-hold at 100 mV and 250 mV for Li-Si half cells and the detailed SEI model fits. Figure (c) shows the SEI composition profiles predicted by the model and the formation of inner inorganic (Li2CO3, Li2O) - outer organic (LEDC) bilayer SEI. Interestingly, inorganic LiF from FEC decomposition is predicted to form throughout the SEI. Figure 1

The solid electrolyte interphase (SEI) is a layer that forms at the anode-electrolyte interface in lithium-ion batteries. The layer forms due to voltage instability of the electrolyte at low anode potentials but serves to passivate the electrolyte to protect against further uncontrolled decomposition. In theory, the SEI is self-limiting, but in reality, continued growth over the battery’s lifetime leads to capacity fade, poor rate capability, and eventually cell death. Though significant progress in recent years has improved the SEI’s function and stability, poor understanding of its most basic chemistry impedes “rational design” of SEI layers for advanced battery applications. Understanding and quantifying the elementary chemistry of the SEI is made challenging by the layer’s thickness (4 to ca. 100 nm), chemical sensitivity, mechanical fragility, and complex chemistry (upwards of 100 reactions have been proposed). These factors combine to make modeling the SEI's fundamental growth and evolution chemistry a significant challenge. Both the computational tools to model the SEI chemistry and the experimental data required to validate such models are all too rare.This talk will present two operando measurements of the SEI grown on a non-intercalating thin film tungsten anode: depth profiling via neutron reflectometry (NR) and SEI mass uptake data via quartz crystal microbalance with dissipation monitoring (QCM-D) taken during cyclic voltammetry cycling (Figure 1).[1,2] NR results directly observe the hypothesized two-layer SEI structure, with a thin, dense inorganic “inner” layer close to the anode and a thicker, porous organic “outer” layer closer to the electrolyte. QCM-D results show the dynamic evolution of the SEI, with heavier SEI products formed at higher potentials, followed by reactions forming lighter SEI products at lower potentials. Both techniques observe “SEI breathing,” whereby soluble SEI products exit the SEI during sweeps to more positive potentials. Interrogation of the results provides new insight into the detailed chemistry of SEI formation and evolution and a platform for validating numerical simulations of the detailed SEI chemistry. Such detailed models can identify prominent reaction pathways and key chemical species present in the SEI which, in turn, can help guide the design of thermally, mechanically, and chemically stable SEI layers for advanced batteries.[1] Rus, E.D., Dura, J.A., “In situ neutron reflectometry study of solid electrolyte interface (SEI) formation on tungsten thin-film electrodes.” ACS Appl. Mat. Int., 11(50), 2019, p. 47553—47563.[2] Lee, C.H, Dura, J.A., LeBar, A., DeCaluwe, S.C., “Direct, operando observation of the bilayer solid electrolyte interphase structure: Electrolyte reduction on a non-intercalating electrode.” J. Power Sources, 412, 2019, p. 725—735. Figure 1

Chemical and morphological structure of solid electrolyte interphase (SEI) plays a vital role in lithium-ion battery (LIB), especially for its cycleability and safety. To date, research on SEI is quite limited due to 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 as well as Fe3O4 electrode surfaces were revealed. Our results showed that SEI layer formed by reductive decomposition of the EC/DMC electrolyte was principally composed of alkyl carbonates ROCO2Li, while the SEI layer formed by FEC/DMC electrolyte was mainly composed of LiF. Only trace of Li2CO3was found in the SEI layer formed from FEC/DMC electrolyte. The dense and hard nature of SEI formed by FEC/DMC electrolyte can protect the graphite against dendrite formation and could be the reason for better cyclability of FEC/DMC electrode as compared to those of EC/DMC based electrolyte. For the Fe3O4 anode, it was found that Fe3O4 electrode was unable to form a stable SEI layer on the electrode surface which resulting in electrolyte decomposition. FEC-based electrolyte can help to improve the performance of Fe3O4 anodes in lithium ion batteries but its protective effects are far from perfect. To accelerate the application of Fe3O4 or other metal oxide anodes in lithium ion batteries, better electrolytes and sophisticated carbon coating techniques are needed warrant formation of a stable SEI layer. Conventional method to evaluate electrolytes are usually complex and tedious procedures. Thus, combination of in situ electrochemical AFM and ex situ XPS could potentially serve as a fast diagnostic tool to evaluate the properties and quality of SEI formed on different electrodes from diverse electrolytes and additives.

Review: in‐depth historical and current review; 261 refs.

An in-depth historical and current review is presented on the science of lithium-ion battery (LIB) solid electrolyte interphase (SEI) formation on the graphite anode, including structure, morphology, composition, electrochemistry, and formation mechanism. During initial LIB operation, the SEI layer forms on the graphite surfaces, the most common anode material. The SEI is essential to the long-term performance of LIBs, and it also has an impact on its initial capacity loss, self-discharge characteristics, rate capability, and safety. While the presence of the anode SEI is vital, it is difficult to control its formation and growth, as they depend on several factors. These factors include the type of graphite, electrolyte composition, electrochemical conditions, and temperature. Thus, SEI formation and electrochemical stability over long-term operation should be a primary topic of future investigation in the LIB development. This article covers the progression of knowledge regarding the SEI, from its discovery in 1979 to the current state of understanding, and covers differences in the chemical and structural makeup when cell materials and components are varied. It also discusses the relationship of the SEI layer to the LIB formation step, involving both electrolyte wetting and subsequent slow charge–discharge cycles to grow the SEI.

Lithium ion battery (LIB) performance degradation is a complex process involving multiple overlapping, but still poorly understood mechanisms. Processes occurring at the anode-electrolyte interphase, commonly known as the “solid-electrolyte interphase” (SEI) in LIBs are known to stabilize cell performance and have been extensively studied. Similar processes occurring at the cathode-electrolyte interphase (CEI) are less well studied, however, CEI processes are known to form products which drive cell degradation. How these products form, and hence, how they can be controlled remains an active area of research. Multiple processes are known to occur at the CEI including oxidation of the electrolyte, dissolution of transition metals and gas evolution both from electrolyte decomposition as well as decomposition of the cathode material itself. Recent work has shown that electrolyte oxidation processes in the presence of metal oxide cathode materials may occur through multiple mechanisms including direct electron transfer from the electrolyte to the electrode as well as the potential chemical oxidation of the electrolyte by reactive species leaving the cathode at high voltage.Scanning electrochemical microscopy (SECM) is a powerful scanning probe technique that can be used to characterize near surface electrochemical processes under steady state conditions. The technique is particularly well suited to study short-lived reactive species as well as the presence of chemical reactions coupled to electron transfer processes. Our work uses these capabilities to help unravel some of the complex processes occurring as part of the electrolyte oxidation process at the CEI. We use SECM to explore electrolyte oxidation processes occurring at non-intercalating electrodes (Pt and glassy carbon) and compare to companion studies using model metal oxide cathode materials. This allows isolated study of direct electron transfer between the electrode and electrolyte and potential chemical oxidation processes that may occur at the metal oxide surface. Studies of these two processes appear to show both similarities and differences between apparent chemical and electrochemical oxidation routes for the LIB and results will be shared in this presentation.

Every commercial lithium-ion battery undergoes formation at the end of the manufacturing process [1]. The formation process is time and capital-intensive, motivating battery manufacturers to develop new formation recipes to decrease formation time while preserving battery lifetime and safety [2]. Yet, despite its importance, a general framework for modeling formation of commercial lithium-ion battery systems is lacking, hindering the innovation cycle. While first-principles solid electrolyte interphase (SEI) formation modeling approaches exist [3], such models are often computationally expensive and challenging to validate, limiting their application towards predicting macroscopic variables relevant to commercial lithium-ion batteries such as capacity loss and resistance growth over life. Semi-physics based (i.e. semi-empirical, or phenomenological) approaches are popular [4], but such models are often parameterized using cell data collected after formation has already completed, making them unsuitable for capturing the initial stages of the SEI formation process.This work presents a framework for developing semi-physics based models of SEI formation and growth dynamics for use in commercial lithium-ion battery systems. The model enables real-time prediction of lithium consumption during the formation (i.e. first charge) process, which can be extended to track the lithium consumption over the entire lifespan of the cell. By explicitly considering the electrolyte reduction reaction dynamics during the first cycle, the model enables the study of how different formation protocols influence SEI passivation properties [5] and hence battery lifetime. The model leverages half-cell near-equilibrium potential curves to track the evolution of lithium stoichiometries and electrode potentials at each electrode, during and after the formation process. The model thus enables real-time tracking of the SEI-forming electrolyte reduction rates which can be used to estimate the quantity of lithium consumed to create the SEI. Model parameters are tuned against experimental electrode expansion measurements collected on NMC622|graphite pouch cells (Figure 1), which resolves both the reversible (i.e. lithiation-induced) and irreversible sources of electrode expansion [6],[7]. Model-predicted lithium consumption rates and electrode expansions are validated against an experimental dataset consisting of three different formation protocols which includes cycle life testing until the end of life.The modeling framework we propose enables practical pathways for bridging the electrochemistry of battery formation to macroscopic variables related to battery safety and lifetime, including total capacity loss, resistance growth, gas generation (during SEI and due to electrolyte oxidation at the positive electrode), and copper dissolution rates.[1] S. J. An, J. Li, C. Daniel, D. Mohanty, S. Nagpure, and D. L. Wood, “The state of understanding of the lithium-ion-battery graphite solid electrolyte interphase (SEI) and its relationship to formation cycling,” Carbon N. Y., vol. 105, pp. 52–76, 2016.[2] Y. Liu, R. Zhang, J. Wang, and Y. Wang, “Current and future lithium-ion battery manufacturing,” iScience, vol. 24, p. 102332, Apr. 2021.[3] A. Wang, S. Kadam, H. Li, S. Shi, and Y. Qi, “Review on modeling of the anode solid electrolyte interphase (SEI) for lithium-ion batteries,” npj Computational Materials, vol. 4, no. 1, 2018.[4] L. von Kolzenberg, J. Stadler, J. Fath, M. Ecker, B. Horstmann, and A. Latz, “A four parameter model for the solid-electrolyte interphase to predict battery aging during operation,” J. Power Sources, vol. 539, p. 231560, Aug. 2022.[5] P. M. Attia, S. J. Harris, and W. C. Chueh, “Benefits of fast battery formation in a model system,” J. Electrochem. Soc., vol. 168, no. 5, p. 050543, 2021.[6] P. Mohtat, S. Lee, J. B. Siegel, and A. G. Stefanopoulou, “Towards better estimability of electrode-specific state of health: Decoding the cell expansion,” J. Power Sources, vol. 427, pp. 101–111, July 2019. Figure 1

Site-specific microstructural analysis combined with interphase design is employed to understand the origin of solid-state electrolyte failure, and to promote extended electrochemical stability. For example, stable anode-free all-solid-state battery (AF-ASSB) with an argyrodite is achieved by tuning wetting of lithium metal on “empty” copper current-collector by coating it with a thin composite film of thermodynamically stable Li2Te and Cu microparticles. Cryo-FIB sectioning demonstrates a dense and uniform electrodeposit, with minimal voiding or dendrites at the collector-SSE interface. Unmodified Cu current-collector promotes inhomogeneous Li electrodeposition/dissolution, leading to a thick non-uniform solid electrolyte interphase (SEI) interspersed with voids. As another example, thin intermetallic Li2Te–LiTe3 bilayer derived from 2D tellurene stabilizes the Li-argyrodite SEI, allowing state-of-the-art electrochemical performance with conventional foils. The Li2Te–LiTe3 bilayer impedes SSE decomposition and suppresses voiding. Unmodified Li–LPSCl undergoes reduction decomposition that extends deep into the SSE, causing reactivity-induced voids in both metal and SEI. For the first time, short-circuiting Li metal dendrite is directly identified, employing 1.5 mm diameter "mini" symmetrical cell and cryogenic focused ion beam (cryo-FIB) electron microscopy. The branching sheet-like dendrite traverses intergranularly, filling the interparticle voids and forming an SEI around it. A separate study on dendrites in garnet LLZTO demonstrates that the compact's grain size distribution and internal porosity critically affect electrical short-circuiting, indicating the importance of local electronic properties. Lithium dendrites propagate intergranularly through regions where LLZTO grains are statistically smaller than the bulk average. Metal also accumulates in the otherwise empty pores of sintered compact present along the dendrite path. Related Work: Mechanical Milling - induced Microstructure Changes in Argyrodite LPSCl Solid-State Electrolyte Critically Affect Electrochemical Stability, Advanced Energy Materials. 2024, in-press. Dendrite Growth—Microstructure—Stress—Interrelations in Garnet Solid-State Electrolyte. Advanced Energy Materials. 2024, 2303062. https://doi.org/10.1002/aenm.202303062Stable Anode‐Free All‐Solid‐State Lithium Battery through Tuned Metal Wetting on the Copper Current Collector. Advanced Materials . 2022 Nov 29:2206762.Tuned Reactivity at the Lithium Metal–Argyrodite Solid State Electrolyte Interphase. Advanced Energy Materials . 2023 Dec;13(46):2301338.

Potassium-ion batteries (PIBs) recently emerged as a next-generation energy storage technology, utilizing abundant and inexpensive potassium as the charge carrier cation. PIBs operate by an analogous mechanism to lithium-ion batteries (LIBs), with reversible potassium intercalation in anode and cathode through an inorganic salt - organic solvent electrolyte medium. Despite its larger size, potassium exhibits several electrochemical advantages over sodium, including a higher affinity for intercalation into graphitic (carbonaceous) anodes, forming a stage-one KC₈ structure in graphite for a specific capacity of 279 mAh g^-1. This thesis aims to provide a thorough foundation for PIB carbon anodes, through a comprehensive experimental approach combining electrode synthesis, advanced material characterization and electrochemical-analytical techniques.<br>Safety concerns have consistently plagued LIBs despite almost three decades of widespread commercialization. Thermal runaway of LIBs can initiate as early as 80°C from exothermic breakdown of the solid electrolyte interphase (SEI) layer that covers the carbon anode surface. The subsequent reaction of lithiated carbon with electrolyte solvent leads to cathode decomposition and oxygen release for cell gassing and combustion. This thesis investigates the thermal runaway behavior of graphite anode for PIBs via differential scanning calorimetry analysis, determining the effect of electrode material, state-of-charge, and cycling history on heat generation. Notably, the PIB system emits significantly less heat overall than for LIBs, albeit an earlier and more intense onset reaction at 100°C raises safety concerns. Strategies to mitigate this exothermic reaction are presented, including electrode binder manipulation to improve graphite particle coverage and enhance SEI layer stability.<br>To further evaluate the practicality of PIBs, the electrochemical behavior of graphite anode was investigated from 0 - 40°C operating temperature, in comparison to standard LIBs. The poor rate capability of potassium is attributed to sluggish solid-state diffusion and augmented cell impedance, where 3-electrode studies revealed dramatic polarization of the potassium metal counter electrode at low temperatures. Accelerated cell aging at elevated temperatures is attributed to SEI layer growth induced by the 61% volumetric expansion of graphite during potassiation, as well as the extreme reactivity of potassium metal. A full-cell system with a Prussian blue nanoparticle cathode and graphite anode showed enhanced rate performance at low temperatures by removing potassium metal counter electrode. These results provide valuable mechanistic insight for potassium intercalation in graphite and offer a practical evaluation of temperature dependent electrochemical performance for PIBs.<br>Supplementary research includes the exploration of carbon nanofi bers electrospun from polyacrylonitrile precursor with subsequent pyrolysis as PIB anode. The design of an amorphous, low density carbon with a nanoscale one dimensional morphology enables mitigation of the 61% volumetric expansion of graphite during potassiation. Remarkable stability (2000 charge-discharge cycles) is thus achieved by preventing electrode pulverization, SEI layer growth, and impedance rise during cycling. Electrochemical analysis revealed a pseudo-capacitance mechanism, enabling rapid charging through surface storage of potassium that could be enhanced by surface functionalization via plasma oxidation treatment. Moreover, two dimensional MXene transition carbonitride sheets were explored as PIB anode with X-ray diffraction and X-ray photoelectron spectroscopy used to study structural changes during potassium insertion.<br>Finally, the effect of particle morphology was investigated for LIB carbon anodes, wherein commercial graphite is compared with synthesized spherical and spiky carbons. Intercalation dynamics, side reaction rates (e.g. SEI growth), self-heating, and thermal runaway behavior were studied through a combination of electrochemical analysis and modeling by a fi nite volume method. Spherical particles outperform irregular commercial graphite by eliminating unstructured inhomogeneities that lead to non-uniform current distributions. Interestingly, spiky particles offer a nontrivial response, where the ordered irregularities enhance intercalation dynamics to prevent degradation at extreme operating conditions. These fi ndings emphasize the importance of tailoring particle morphology and structure in promoting desired LIB behavior and suppressing unwanted problems.