Structure Of Solid Electrolyte Interphase Research Articles

Unraveling the Dynamics of Solid-Electrolyte Interphase (SEI) Formation on Lithium Metal: Insights from Multiscale Modeling

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.

(Invited) A Study of the SEI Layer Cross-Talk in Si/C Composite Li-Ion Anodes

Composite electrodes, especially silicon/carbon (Si/C) anodes, present significant opportunities for advancing the energy density of lithium-ion batteries (LIBs). However, challenges emerge, particularly in performance metrics when a high weight percentage of Si beyond 8-10 wt.% is employed 1. A nuanced understanding of the solid electrolyte interphase (SEI) in Si/C composites is imperative to overcome these limitations and enhance energy density. Conventional techniques such as ex-situ X-ray photoelectron spectroscopy and cryogenic electron microscopy face inherent drawbacks, including exposure to ultrahigh vacuum and the need for solvent washing, potentially altering the SEI layer 2. In contrast, nano-FTIR, utilizing Fourier transform infrared near-field spectroscopy, overcomes these challenges, enabling non-destructive probing of the SEI layer's chemistry and structure at room temperature within an inert atmosphere 3–5.In this study, we engineer custom-patterned electrodes featuring amorphous Si on atomically flat highly ordered pyrolytic graphite (HOPG) to emulate model composite electrodes. Leveraging nano-FTIR spectroscopy, near-field nanoscale white-light imaging, and atomic force microscopy measurements, our investigation reveals that the SEI on HOPG is enriched in organic lithium ethylene decarbonate (LiEDC), while the SEI on lithiated silicon unveils the presence of inorganic Li2CO3. Notably, we unveil a unique "mixed" SEI layer zone at the Si/HOPG interface, comprising a mixture of dominant SEI species from the surfaces of both the lithiated Si and the HOPG. The coexistence of these species suggests the formation of an SEI layer with presumably unique physicochemical properties, forming along Si-C interfaces in Si/C composite electrodes. These findings provide a comprehensive approach to studying model composite electrodes and offer valuable insights into optimizing the surface passivation of Si/C composite electrodes for high-performance LIBs. Acknowledgments This research was supported by the US Department of Energy (DOE)’s Vehicle Technologies Office under the Silicon Consortium Project directed by Brian Cunningham and managed by Anthony Burrell. References V. G. Khomenko and V. Z. Barsukov, Electrochim Acta, 52, 2829–2840 (2007).J. Wu, M. Ihsan-Ul-Haq, Y. Chen, and J. K. Kim, Nano Energy, 89, 106489 (2021).X. He, J. M. Larson, H. A. Bechtel, and R. Kostecki, Nat Commun, 13, 1398 (2022).I. Yoon, J. M. Larson, and R. Kostecki, ACS Nano, 17, 6943–6954 (2023).A. Dopilka, Y. Gu, J. M. Larson, V. Zorba, and R. Kostecki, ACS Appl Mater Interfaces, 15, 6755–6767 (2023).

Stable zinc anode solid electrolyte interphase via inner Helmholtz plane engineering

The inner Helmholtz plane and thus derived solid-electrolyte interphase (SEI) are crucial interfacial structure to determine the electrochemical stability of Zn-ion battery (ZIB). In this work, we demonstrate that introducing β-cyclodextrins (CD) as anion-receptors into Zn(OTf)2 aqueous electrolyte could significantly optimize the Zn anode SEI structure for achieving stable ZIB. Specifically, β-CD with macrocyclic structure holds appropriate cavity size and charge distribution to encase OTf- anions at the Zn metal surface to form β-CD@OTf- dominated inner Helmholtz structure. Meanwhile, the electrochemically triggered β-CD@OTf- decomposition could in situ convert to the organic-inorganic hybrid SEI (ZnF2/ZnCO3/ZnS‒(C-O-C/*CF/*CF3)), which could efficiently hinder the Zn dendrite growth with maintain the proper SEI mechanical strength stability to guarantee the long-term stability. The thus-derived Zn | |Zn pouch cell (21 cm2 size) with β-CD-containing electrolyte exhibits a cumulative capacity of 6450 mAh−2 cm−2 at conditions of 10 mAh cm−2 high areal capacity. This work gives insights for reaching stable ZIB via electrolyte additive triggered SEI structure regulation.

Effects of Electrolyte Solvent Composition on Solid Electrolyte Interphase Properties in Lithium Metal Batteries: Focusing on Ethylene Carbonate to Ethyl Methyl Carbonate Ratios

This study investigated the influence of variations in the mixing ratio of ethylene carbonate (EC) to ethyl methyl carbonate (EMC) on the composition and effectiveness of the solid electrolyte interphase (SEI) in lithium-metal batteries. The SEI is crucial for battery performance, as it prevents continuous electrolyte decomposition and inhibits the growth of lithium dendrites, which can cause internal short circuits leading to battery failure. Although the properties of the SEI largely depend on the electrolyte solvent, the influence of the EC:EMC ratio on SEI properties has not yet been elucidated. Through electrochemical testing, ionic conductivity measurements, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, the formation of Li2CO3, LiF, and organolithium compounds on lithium surfaces was systematically analyzed. This study demonstrated that the EC:EMC ratio significantly affected the SEI structure, primarily owing to the promotion of the formation of a denser SEI layer. Specifically, the ratios of 1:1 and 1:3 facilitated a uniform distribution and prevalence of Li2CO3 and LiF throughout the SEI, thereby affecting cell performance. Thus, precise control of the EC:EMC ratio is essential for enhancing the mechanical robustness and electrochemical stability of the SEI, thereby providing valuable insights into the factors that either enhance or impede effective SEI formation.

Revealing Lithium Nitrate-Mediated Solid-Electrolyte Interphase of Lithium Metal Anode via Cryogenic Transmission Electron Microscopy.

The cycle stability of lithium metal anode (LMA) largely depends on solid-electrolyte interphase (SEI). Electrolyte engineering is a common strategy to adjust SEI properties, yet understanding its impact is challenging due to limited knowledge on ultrafine SEI structures. Herein, using cryogenic transmission electron microscopy, we reveal the atomic-level SEI structure of LMA in ether-based electrolytes, focusing on the role of LiNO3 additives in SEI modulation at different temperature (25 and 50 °C). Poor cycle stability of LMA in the baseline electrolyte without LiNO3 additives stems from the Li2CO3-rich mosaic-type SEI. Increased LiNO3 content and elevated operating temperature enhance cyclic performance by forming bilayer or multilayer SEI structures via preferential LiNO3 decomposition, but may thicken the SEI, leading to reduced initial Coulombic efficiency and increased overpotential. The optimal SEI features a multilayer structure with Li2O-rich inner layer and closely packed grains in the outer layer, minimizing electrolyte decomposition or corrosion.

Inorganic Composition Modulation of Solid Electrolyte Interphase for Fast Charging Lithium Metal Batteries.

The solid electrolyte interphase (SEI) with lithium fluoride (LiF) is critical to the performance of lithium metal batteries (LMBs) due to its high stability and mechanical properties. However, the low Li ion conductivity of LiF impedes the rapid diffusion of Li ions in the SEI, which leads to localized Li ion oversaturation dendritic deposition and hinders the practical applications of LMBs at high-current regions (>3 C). To address this issue, a fluorophosphated SEI rich with fast ion-diffusing inorganic grain boundaries (LiF/Li3P) is introduced. By utilizing a sol electrolyte that contains highly dispersed porous LiF nanoparticles modified with phosphorus-containing functional groups, a fluorophosphated SEI is constructed and the presence of electrochemically active Li within these fast ion-diffusing grain boundaries (GBs-Li) that are non-nucleated is demonstrated, ensuring the stability of the Li || NCM811 cell for over 1000 cycles at fast-charging rates of 5 C (11mA cm-2). Additionally, a practical, long cycling, and intrinsically safe LMB pouch cell with high energy density (400Wh kg-1) is fabricated. The work reveals how SEI components and structure design can enable fast-charging LMBs.

High-Safety Electrolytes with an Anion-Rich Solvation Structure Tuned by Difluorinated Cations for High-Voltage Lithium Metal Batteries.

As next-generation energy storage devices, lithium metal batteries (LMBs) must offer high safety, high-voltage resistance, and a long life span. Electrolyte engineering is a facile strategy to tailor the interfacial chemistry of LMBs. In particular, the solvation structure and derived solid electrolyte interphase (SEI) are crucial for a satisfactory battery performance. Herein, a novel middle-concentrated ionic liquid electrolyte (MCILE) with an anion-rich solvation structure tuned by difluorinated cations is demonstrated to achieve ultrahigh safety, high-voltage stability, and excellent ternary-cathode compatibility. Novel gem-difluorinated cations first synthesized for prestoring fluorine on positively charged species, not only preferentially adsorb in the inner-Helmholtz layers, but also participate in regulating the Li+ solvation structure, resulting in a robust interphase. Moreover, these weak interactions in the Li+ solvation structure including anion-solvent and ionic liquid (IL) cation-solvent pairs are first revealed, which are beneficial for promoting an anion-dominated solvation structure and the desolvation process. Benefiting from the unique anion-rich solvation structure, a stable hetero-SEI structure is obtained. The designed MCILE exhibits compatibility with Li metal anode and the high-voltage ternary cathode at high temperatures (60°C). This work provides a new approach for regulating the solvation structure and electrode interphase chemistry of LMBs via difluorinated IL cations.

Fluorine-Terminated Self-Assembled Monolayers Grafted Graphite Anode Inducing a LiF-Dominated SEI Inorganic Layer for Fast-Charging Lithium-Ion Batteries.

The electrochemical kinetic processes of Li+ ions, including the desolvation of the Li+ ions from the electrolyte to the solid electrolyte interphase (SEI), the transportation of desolvated Li+ ions across the SEI, and the charge transfer at the interface between the SEI and graphite, determine the rate performance and cycling stability of the graphitic anode in lithium-ion batteries (LIBs). In this work, fluorine-terminated self-assembled monolayers were grafted on the surface of spherical graphite particles to regulate the chemical composition and structure of SEI formed on the graphite surface in the presence of conventional ester electrolytes. The comprehensive characterization and first-principles calculation results illustrate that a uniform LiF-dominated SEI film can be generated on the as-functionalized graphite anode due to the carbon-fluorine bonds' cleavage of fluorine-terminated self-assembled monolayers. The LiF-dominated SEI film is particularly beneficial for desolvated lithium-ion transport across the SEI, affording LiCoO2//graphite full cells with substantially enhanced fast-charging capability and cycle stability. This strategy should be potentially useful for modifying other anode materials to regulate the interfacial chemistry between the anode and electrolyte in lithium-ion batteries.

(Invited) Optimization of Carbonate Electrolytes for Lithium Metal Anodes

A solid electrolyte interphase (SEI) is generated on the anode of lithium ion batteries during the first few charging cycles. While the SEI generated for LiPF6/carbonate based electrolytes is stable on graphite anodes, the stability of the SEI is poor for LiPF6/carbonate based electrolytes with lithium metal anodes. However, modification of the carbonate based electrolytes via incorporation of alternative salts and/or electrolyte additives significantly improves the stability of the SEI and the cycle life of lithium metal anodes. Investigations of the SEI structure have been conducted via a combination of XPS, IR-ATR, SEM, and TEM. Mechanisms for the generation of the complicated mixture of products are presented along with the differences in the SEI structure and function.

Determining the Composition and Structure of a High-Performing SEI Layer for Lithium-Mediated Nitrogen Reduction to Ammonia

The industrial synthesis of ammonia (NH3) via the Haber-Bosch process has enabled mass production of fertilizer and the human population growth to 8 billion.1 NH3 also shows promise as a long-term, energy-dense fuel amenable to widespread distribution.2 However, the Haber-Bosch process operates at high temperatures and pressures in centralized facilities and accounts for 1.3% of global carbon emissions.1,2 To ensure a food-secure future and realize NH3 as a truly carbon-free fuel, we must develop an NH3 synthesis alternative that operates at ambient conditions and can be driven by renewables.One of the most promising NH3 synthesis method under mild conditions is the electrochemical, Li-mediated process (Li-NRR). In this process, Li is electrodeposited from solution and reacted with N2 to form Li3N. A proton shuttle, e.g., ethanol, protonates Li3N to form NH3.3 The electrodeposited Li forms a solid-electrolyte interphase (SEI) layer that controls N2, Li+, and H+ transport and is key to system performance. The use of fluorinated Li salts, e.g., LiBF4, have recently advanced Li-NRR system performance. Improved selectivity has been attributed to changes in the SEI layer, but evaluating SEI structure and composition remains a significant challenge.3 Neutron-based characterization methods, complementary to X-ray methods, offer unique benefits to study SEI layers containing light elements such as Li- and H; for example, an element’s neutron scattering cross section does not depend on its atomic number.4 In this work, we studied the formation of the SEI layer in-situ before and after applying -0.3 mA/cm2 in galvanostatic experiments (chronopotentiometry, CP) on a copper (Cu) working electrode. We utilized a combination of deuterated and non-deuterated solvents and proton donors to distinguish their contributions to the SEI. We found that the SEI forms two layers: a compact, lithium rich layer, and a diffuse layer with carbonaceous species (Figure 1).4 We also identified the presence of boron in the layers, indicated by neutron absorption. Finally, we observed more proton-rich species in experiments containing the non-deuterated H+ donor, corroborating previous studies that indicate that Li-ethoxide may be a major component of the diffuse SEI (Figure 1b-c).5 Compared to LiClO4-based electrolytes, the structure of these LiBF4-derived layers is consistent even after repeated current cycling, indicating a more stable SEI structure and suggesting a relationship between SEI stability and increased Li-NRR performance.References Nørskov, J. et al. DOE Roundtable Report (2016).Chang, F. et al. Advanced Materials 33, 2005721 (2021).Li, S. et al. Joule 6, 2083–2101 (2022).Blair, S.J., Doucet, M. et al. ACS Energy Lett., 7, 6, 1939-1946 (2022).Steinburg, K. et al. Nature Energy, 8, 138-148 (2023). Figure 1: (a) reflectivity curve before and after 4 mins constant applied current at -0.3 mA/cm2 (chronopoteniometry, CP) to plate Li using 1 M LiBF4 with 1 vol% ethanol, (b) resulting scattering length density (SLD) profile, and (c) schematic of layers indicated in SLD profile Figure 1

Dual-Additive Approach for Tuning Si-Based Anode Interphase Realized By Operando ATR-FTIR

Dual-additive approach for tuning Si-based anode interphase realized by Operando ATR-FTIRMasoud Baghernejad1, Matthias Weiling1, Felix Pfeiffer1, Diddo Diddens1, Lars Frankenstein2, Christian Lechtenfeld2 1Helmholtz-Institute Münster, IEK-12, Forschungszentrum Jülich GmbH, Corrensstraße 46, 48149 Münster, Germany 2MEET Battery Research Center, Institute of Physical Chemistry, University of Münster, Corrensstraße 46, 48149 Münster, GermanySilicon is a promising next-generation anode material, offering a high gravimetric capacity. However, silicon-based anodes suffer from significant volume expansion during lithiation that can cause particle pulverization and capacity fade, continuous solid-electrolyte interphase (SEI) (re-)formation, and constant electrolyte consumption. To alleviate these issues, the effect of 2-Sulfobenzoic anhydride (2-SBA) as an SEI electrolyte additive was investigated through complementary electrochemical and operando spectroscopic techniques.The electrochemical performance was evaluated through galvanostatic charge/discharge cycling. The results show that the optimum concentration of 2-SBA improves the cycling stability and capacity retention of NMC811||AG+20% SiOx (4.5 – 2.8 V) cells to more than 100 cycles compared to 70 cycles with the baseline electrolyte (BE) before cell failure. Introducing lithium difluorophosphate (LiDFP), a common additive for high-voltage applications, to the electrolyte containing 2-SBA further increased the cycle life to 160 cycles. Interestingly, LiDFP alone in the electrolyte did not lead to improved electrochemical performance or longer cycle life of the cells compared to BE, proving a beneficial dual-additive approach of combining inorganic and organic compounds for sufficient SEI formation on AG+20% SiOx anodes. The analysis of the electrochemical performance concludes that the improved SEI structure formed by 2-SBA and LiDFP mitigates pulverization. In this regard, the SEI structure is further analyzed using operando ATR-FTIR spectroscopy.Operando ATR-FTIR spectroscopy proves to be a powerful technique in battery research, particularly for investigating SEI. The ATR mode's surface sensitivity enables studying the composition of the SEI without bulk signal interference. In this work, we employed the internal reflection mode of the ATR-FTIR spectroscopy, for which the electrode of interest has to be IR permeable. Furthermore, by variation of the incident angle of the IR beam, the penetration depth can be varied to obtain an interphase profile from electrode to bulk electrolyte. In this study, the SEI composition on the silicon anode in the presence of BE and the corresponding additives, 2-SBA and LiDFP, was investigated under real-working conditions. The results elucidate that 2-SBA is accumulated and decomposed via ring-opening on the silicon surface to form a stable SEI layer. However, in the presence of LiDFP and 2-SBA, a hybrid organic and inorganic SEI layer is formed, which proves to enhance electrochemical performance and cycle life. The findings of this study provide valuable structural insights into the silicon particle surface for the development of high-voltage silicon-based lithium-ion batteries with Ni-rich cathodes.

Investigating the Reaction Mechanism of Vinylene Carbonate Additive in Lithium Ion Batteries Using X-Ray Photoelectron Spectroscopy

The rechargeable Li-ion battery is an enabling technology which facilitates the electrification of the automotive industry and reduces the demand for fossil fuels. During the charge and discharge of a battery, a solid-electrolyte interphase (SEI) forms between the liquid electrolyte and the solid negative electrode as a result of electrolyte degradation. The chemical and physical stability and the functionality of the SEI is a key determining factor of battery performance. The chemical composition of the SEI is mainly controlled by the choice of solvent and salt used, but can be manipulated by the addition of electrolyte additives that influence the electrolyte degradation process.Vinylene carbonate (VC) is an additive used in many commercial Li-ion battery configurations. The addition of a small amount of VC to ethylene carbonate (EC) containing electrolytes has been demonstrated to drastically improve several performance metrics for batteries including cycle life, coulombic efficiencies, capacity retention and thermal stability. The success of VC is widely attributed to its ability to form a thinner, denser and more robust SEI, which better protects the electrode surface and prevents further electrolyte consumption. This improvement in stability has been linked to the presence of poly(VC) in the SEI, but the mechanism by which VC reacts in EC-containing electrolytes is still an area of ongoing study. Whilst computational methods have suggested several potential decomposition pathways of electrolytes containing VC, it is yet to be convincingly demonstrated experimentally. Whilst there is a general consensus about ‘What?’ forms in the SEI on the addition of VC to EC-containing solvent, this work aims to confirm ‘Where?’ these components are in the interface and the currently unanswered questions of ‘How?’ and ‘Why?’ they form.Using highly surface sensitive photoelectron spectroscopy (PES) measurements of the SEI deposited at various potentials, we identify the differences in composition and structure that result from the addition of VC additive to EC-containing electrolytes. The SEI formation is studied using a smooth Au surface as the model working electrode and a fluorine free EC:diethyl carbonate (DEC) (1:9) 1 M LiClO4 electrolyte in an attempt to isolate the impact of VC on the SEI. A potential step procedure allows analysis of the working electrode surface by PES at several progressively more negative potentials on the first charge. Through systematic variation of components such as the concentration of VC, the ratio of EC:DEC and swapping Au for other electrode materials such as Cu or Si, the interaction between VC, EC and the electrode surface is revealed. The differences in SEI composition and structure formed in each environment and at each potential step are correlated to quartz crystal microbalance (QCM) data in order to identify potential reaction mechanisms and formation processes. The results play a key role in understanding the reaction pathway of VC-containing electrolytes and will accelerate development of more efficient electrolyte additives.

Unraveling propylene oxide formation in alkali metal batteries.

The increasing need for electrochemical energy storage drives the development of post-lithium battery systems. Among the most promising battery types are sodium-based battery systems. However, like its lithium predecessor, sodium batteries suffer from various issues like parasitic side reactions, which lead to a loss of active sodium inventory, thus reducing the capacity over time. Some problems in sodium batteries arise from an unstable solid electrolyte interphase (SEI) reducing its protective power. While it is known that the electrolyte affects the SEI structure, the exact formation mechanism of the SEI is not yet fully understood. Here we follow the initial SEI formation on sodium metal submerged in propylene carbonate with and without the electrolyte salt sodium perchlorate. We combine X-ray photoelectron spectroscopy, gas chromatography, and density functional theory to unravel the sudden emergence of propylene oxide after adding sodium perchlorate to the solvent. We identify the formation of a sodium chloride layer as a crucial step in forming propylene oxide by enabling precursors formed from propylene carbonate on the sodium metal surface to undergo a ring-closing reaction. We identify changes in the electrolyte decomposition process, propose a reaction mechanism to form propylene oxide and discuss alternatives based on known synthesis routes.

Suppression of Co(II) ion deposition and hazards: Regulation of SEI film composition and structure

Despite the presence of LiF components in the solid electrolyte interphase (SEI) formed on the graphite anode surface by conventional electrolyte, these LiF components primarily exist in an amorphous state, rendering them incapable of effectively inhibiting the exchange reaction between lithium ions and transition metal ions in the electrolyte. Consequently, nearly all lithium ions within the SEI film are replaced by transition metal ions, resulting in an increase in interphacial impedance and a decrease in stability. Herein, we demonstrate that the SEI film, constructed by fluoroethylene carbonate (FEC) additive rich in crystalline LiF, effectively inhibits the undesired Li+/Co2+ ion exchange reaction, thereby suppressing the deposition of cobalt compounds and metallic cobalt. Furthermore, the deposited cobalt compounds exhibit enhanced structural stability and reduced catalytic activity with minimal impact on the interphacial stability of the graphite anode. Our findings reveal the crucial influence of SEI film composition and structure on the deposition and hazards associated with transition metal ions, providing valuable guidance for designing next-generation electrolytes.

Pure Amorphous and Ultrathin Phosphate Layer with Superior Ionic Conduction for Zinc Anode Protection.

Fast and uniform ion transport within the solid electrolyte interphase (SEI) is considered a crucial factor for ensuring the long-term stability of metal electrodes. In this study, we present the fabrication of ultrathin artificial interphases consisting of a zinc phosphate nanofilm with pure amorphous characteristics and a surfactant overlayer. The thickness of the interphases can be precisely controlled within the range of a few tens of nanometers. We explore the impact of artificial SEI structure, including thickness and crystallinity, on its protective capabilities. The pure amorphous phosphate layer with optimized nanoscale thickness is found to provide an abundance of short and isotropic ion migration pathways and a low diffusion energy barrier. These features facilitate rapid and homogeneous Zn2+ transportation, resulting in compact and planar zinc deposition. Meanwhile, the hydrophobic alkyl moieties of the overlayer prevent disassociation of water at the interface. As a result, this nanofilm endures ultralong cycling stability with a low overpotential and enables high Zn plating/stripping reversibility. The Zn||MnO2 full cell shows a stable cycle life for 700 cycles under practical conditions of lean electrolyte, high areal capacity cathode, and limited Zn excess. These findings provide insights into the design and optimization of SEI layers for protection of metal anodes.

Revisiting Anode Fast-Charging Capability with Solid Electrolyte Interface Using Cryogenic Transmission Electron Microscopy

The Li+ ion diffusion through the solid electrolyte interphase (SEI) has been widely considered as one of the limiting steps for the Li fast (de)intercalation process. However, a comprehensive understanding of the kinetic limitation of SEI on anode fast-charging remains elusive. Using H-phase (monoclinic) Nb2O5 (H-Nb2O5) as the anode material, we comprehensively studied the fast charging behaviors on both “inorganic SEI free” and “inorganic-rich SEI” anodes with cryogenic transmission electron microscopy (cryo-TEM) and X-ray photoelectron spectroscopy (XPS). The results reveal that there is a negligible difference on the electrochemical performance including fast-charging capability and cycling stability between these H-Nb2O5 anodes in spite of significant discrepancy of SEI structure (e.g. thickness and chemical component). Further, it was observed using cryo-TEM that the discrete decoration of the individual inorganic particles (e.g. Li2O) and amorphous LiNxOy species in the direct SEI does not constitute a dense solid layer. From this study, we conclude a pore diffusion mechanism is dominated across the whole direct SEI layer with those porous organic components, which is extremely faster than the lithium ion knock-off diffusion in the inner inorganic components. Thus, the SEI structures have much less influence on the limitation of the lithium ion transport kinetics than those as commonly accepted if an inner dense inorganic layer is not formed. Our results provide a refreshed understanding on the fundamentals of the lithium ion transport in SEI, and will guide future design of battery materials for fast-charging.

(Invited) Multiscale Interfacial Heterogeneity Explored by Advanced Spectroscopy and Imaging for Batteries Beyond Lithium-Ion

Keywords: solid-liquid interface, solid-state battery, surface-enhanced Raman spectroscopy (SERS), tip-enhanced Raman spectroscopy (TERS), neutron tomography, sulfide solid-state electrolyte, Raman imagingThe ever-growing needs for energy storage systems require batteries with even higher power and energy density with extended life and enhanced safety beyond the current Li-ion battery technologies. Fulfilling these needs requires new battery chemistries, including high-energy-density electrode materials, solid-state electrolytes, and efficient interphases to mitigate the side reactions between the electrodes and the electrolytes.Charge transport across the electrode and electrolyte interface is believed to be one of the charge/discharge rate-limiting steps. Although the composition, morphology, and structure of solid-electrolyte interphase (SEI) have been extensively studied, probing its evolution at the nanoscale is challenging to a large extent. Plasmon-enhanced Raman spectroscopy (PERS) is promising to solve this challenge. PERS is the resonant oscillation of conduction electrons at the interface between negative and positive permittivity molecules stimulated by incident light. It includes surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS). In the first part of this talk, I will show that gap-mode SERS is ideal to probe salt solvation structure in the immediate vicinity (< 20 nm) of the electrode/electrolyte interface. SERS lacks the nanoscale resolution in the sample plane. This can be compensated by TERS. TERS analysis on cycled high-energy silicon anodes indicates that the nanometer scale SEI "islands" are unevenly distributed. Even for the same SEI "island", the composition is different from point to point with inter-point distance smaller than 10 nm.All-solid-state lithium metal batteries (SSBs) promise specific energy >500 Wh/kg. A solid-state electrolyte (SSE) plays an irreplaceable role in reaching such an energy-density goal. Sulfide-based SSEs have emerged as a prominent class of soft ionic conductors that have comparable room-temperature ionic conductivity to their liquid electrolyte counterparts. However, when paring with a high voltage cathode, such as lithium nickel manganese cobalt oxide (NMC), the (electro)chemical instability of the sulfide SSE at the electrode/SSE interfaces becomes a major challenge. The interfacial instability can result in >50% initial capacity loss in a Li/sulfide SSE/NMC battery, thereby keeping the sulfide SSEs from commercialization. In the second part of this talk, I will show how we synergistically use neutron computed tomography and in situ Raman imaging, with clustering analysis to track lithium displacement at the interface of a sulfide-based SSB. Acknowledgment This research was conducted at Oak Ridge National Laboratory, managed by UT Battelle, LLC, for the U.S. Department of Energy (DOE) and is supported by Asst. Secretary, Energy Efficiency and Renewable Energy (EERE), Vehicle Technologies Office (VTO) through the Advanced Battery Materials Research (BMR) Program. This research used resources at High Flux Isotope Reactor, a DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory.

Reforming the Uniformity of Solid Electrolyte Interphase by Nanoscale Structure Regulation for Stable Lithium Metal Batteries.

The stability of high-energy-density lithium metal batteries depends on the uniformity of solid electrolyte interphase (SEI) on lithium metal anodes. Rationally improving SEI uniformity is hindered by poorly understanding the effect of structure and components of SEI on its uniformity. Herein, a bilayer structure of SEI formed by isosorbide dinitrate (ISDN) additives in localized high-concentration electrolytes was demonstrated to improve SEI uniformity. In the bilayer SEI, LiNx Oy generated by ISDN occupies top layer and LiF dominates bottom layer next to anode. The uniformity of lithium deposition is remarkably improved with the bilayer SEI, mitigating the consumption rate of active lithium and electrolytes. The cycle life of lithium metal batteries with bilayer SEI is three times as that with common anion-derived SEI under practical conditions. A prototype lithium metal pouch cell of 430 Wh kg-1 undergoes 173 cycles. This work demonstrates the effect of a reasonable structure of SEI on reforming SEI uniformity.

Reforming the Uniformity of Solid Electrolyte Interphase by Nanoscale Structure Regulation for Stable Lithium Metal Batteries

AbstractThe stability of high‐energy‐density lithium metal batteries depends on the uniformity of solid electrolyte interphase (SEI) on lithium metal anodes. Rationally improving SEI uniformity is hindered by poorly understanding the effect of structure and components of SEI on its uniformity. Herein, a bilayer structure of SEI formed by isosorbide dinitrate (ISDN) additives in localized high‐concentration electrolytes was demonstrated to improve SEI uniformity. In the bilayer SEI, LiNxOy generated by ISDN occupies top layer and LiF dominates bottom layer next to anode. The uniformity of lithium deposition is remarkably improved with the bilayer SEI, mitigating the consumption rate of active lithium and electrolytes. The cycle life of lithium metal batteries with bilayer SEI is three times as that with common anion‐derived SEI under practical conditions. A prototype lithium metal pouch cell of 430 Wh kg−1 undergoes 173 cycles. This work demonstrates the effect of a reasonable structure of SEI on reforming SEI uniformity.

Enhancing long-term cycling stability of lithium-ion batteries with prelithiated MXene@SiO2 anodes

The pulverization and volume expansion of silica (SiO2) particles during the continuous lithiation/delithiation process have great impacts on the long-term cycling stability for lithium-ion batteries (LIBs). Herein, a stable and robust solid electrolyte interphase (SEI) containing abundant LiF and Li2CO3 species was realized through MXene integration and prelithiation for SiO2 electrodes. The as-generated SEI displayed well electron insulator capability and fast Li+ transport kinetics owing to the synergistic effect of LiF and Li2CO3, as well as the excellent mechanical properties with the contributions of MXene layers. Therefore, the prelithiated MXene@SiO2 anodes achieved a high capacity of 380.2 mAh g−1 and a long cycling stability with a capacity retention of 88.3% after 600 cycles at the current density of 100 mA g−1. This work provided a deep understanding on the SEI structure and its efficiency on the cycling stability for high energy anodes.