Properties Of Solid Electrolyte Interphase Research Articles

Decoupling Interfacial Kinetics Realizes 5C Fast Charging of Potassium‐Ion Batteries Using Graphite Anode

AbstractImproving interfacial kinetics is the key to realizing extreme fast charging (XFC) of graphite‐based potassium ion batteries (PIBs). The electrolyte engineering is commonly used for solid electrolyte interphase (SEI) design. However, this strategy adjusts both ion solvation structure and (de)solvation kinetics simultaneously, thus making it difficult to explicitly reveal the linkage between SEI properties and interfacial kinetics. Herein, the content of inorganic species in preformed SEI on graphite surface is precisely regulated and uncovered its critical role in improving the interfacial kinetics. The charge transfer kinetics on graphite/electrolyte interphase is found to be the rate limitation step upon XFC. Meanwhile, the increased inorganic species in SEI plays a decisive role in optimizing the charge transfer rather than the kinetics of naked K+ crossing SEI. Through unlocking the anodic charge transfer limitation with ultra‐inorganic rich SEI, the graphite//Prussian blue analogs full cells achieve a superior XFC ability (13 min charge to 80%) with a specific capacity of 103 mAh g−1 at 5 C. This work provides a fundamental understanding of the relationship between SEI properties and interfacial kinetics during XFC, which enables the rational design of SEI chemistry for fast‐charging PIBs.

Insights from Li and Zn systems for advancing Mg and Ca metal batteries.

The inherent limitations of lithium (Li)-ion batteries have sparked interest in exploring alternative technologies, especially those relying on metallic anodes: monovalent Li and divalent zinc (Zn), magnesium (Mg), and calcium (Ca) metals. In particular, Mg and Ca metal batteries offer significant advantages based on the natural abundance of their raw materials and high energy-storage capabilities resulting from the bivalency of the carrier ions. Yet, these battery systems are far from commercialization, and the lack of reliable electrolytes constitutes a primary concern. The formation of ion-insulating passivation layers on these metallic anodes and their inferior desolvation kinetics have long been recognized as formidable hurdles in the way of optimizing the electrolyte composition. These impediments call for innovative strategies in electrolyte engineering and an extensive analysis of the resulting solid-electrolyte-interphase (SEI) layer. In this review, we introduce recent pioneering studies of divalent Mg and Ca metal batteries that have been concerned with these issues. This review particularly focuses on drawing an analogy with Li and Zn metal batteries in terms of the relative advancement and by benchmarking against the strategies developed for these analogous systems. The areas of interest include a fundamental understanding of the thermodynamics and evolution of the morphology of metallic anodes, a correlation between the electrolyte and SEI compositions, state-of-the-art electrolyte strategies to realize reversible (de)plating of Mg and Ca, and new perspectives on the SEI properties and their relevance to corrosion and the calendar life. We finally encourage researchers in the community to delve into these emerging areas by linking with successful stories in the analogous systems, but identifying distinct aspects of Mg and Ca batteries that still require attention.

Temperature‐Mediated Dynamic Lithium Loss and its Implications for High‐Efficiency Lithium Metal Anodes

AbstractLithium (Li) metal has been strongly regarded as the ultimate anode option for next‐generation high‐energy‐density batteries. Nevertheless, the insufficient Coulombic efficiency induced by the extensive active Li loss largely hinders the practical operation of Li metal batteries under wide temperature range. Herein, the temperature‐mediated dynamic growth of inactive Li from −20 to 60°C via titration gas chromatograph measurements is quantitatively decoupled. Combined X‐ray photoelectronic spectroscopy, cryo‐transmission electronic microscopy, and scanning electronic microscopy methods depicted that both solid electrolyte interphase (SEI) characteristics and Li deposition compactness can be profoundly manipulated by working temperature. The elevation of temperature is found to fundamentally aggravate the parasitic reactions and deteriorate the spatial uniformity of SEI, yet promote the lateral growth of Li by kinetic reason. The opposite effects of temperature on SEI properties and Li deposition compactness can properly explain the intricate temperature‐dependent growth rates of SEI‐Li+ and dead Li0 capacity loss observed under titration gas chromatograph measurements. Design implications towards more stable Li metal anodes with higher reversibility can thus be yielded.

Unveiling the Dynamics of Solid Electrolyte Interphase Evolution in Li-Ion Batteries Via Operando X-Ray Reflectivity: Experimental Insights for Advanced Energy Storage

The Solid-Electrolyte-Interphase (SEI) is a crucial component of Li-ion batteries (LIBs), as it passivates the highly reactive electrode from the electrolyte and helps to stabilize the electrode/electrolyte interface. However, the SEI formation and evolution can also limit the cycling performance and the capacity retention of the battery. Accurate electrochemical models must therefore account for the SEI and properly simulate it. In order to investigate experimentally the SEI, various techniques have been deployed, including electron microscopy1, X-ray photoelectron spectroscopy (XPS)2,3 and Nuclear magnetic resonance (NMR)4.Providing reliable information on the SEI evolution during cycling, and thus benchmarking models, is challenging with these ex-situ or destructive techniques, as they hardly allow in situ observation. On the other hand, synchrotron X-ray reflectivity (XRR) can be used for the operando investigation of interfacial phenomena. It provides information on the thickness, density, and roughness of the SEI, as well as their changes upon (de)lithiation or in out-of-equilibrium conditions. Moreover, operando XRR can be performed under controlled battery operating conditions, providing relevant information on the SEI. Recently, several works reported that operando XRR experiment can provide in situ structural information of the formation and the growth of SEI with sub-nanometer resolution during the cycling5–7.Herein, we report the results of operando XRR experiments performed at the beamline BM32 of the European Synchrotron Radiation Facility (ESRF) to investigate the SEI on Si anode using novel, sustainable fluorine-free electrolytes. Indeed, in the quest for environmentally friendly and safe LIBs, removing the fluorinated electrolytes that are toxic and release corrosive compounds is a necessary step. One way to enhance the safety and sustainability of LIBs is to replace the conventional fluorinated electrolytes by fluorine-free alternatives. A F-free composition based on the lithium bis(oxalato) borate (LiBOB) salt recently showed promising performance in commercially relevant cell configurations8. As with conventional electrolytes, the F-free electrolyte reacts at the surface of the electrodes during the first charge, thus forming the SEI. Characterizing the F-free SEI growth and stability is thus key to understand its behavior and to improve the cell performance for this promising green electrolyte.In this work, operando XRR experiments were performed in electrochemical cells with different electrolyte compositions: 1 M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 vol % (LP57), 0.7 M LiBOB in EC:EMC = 3:7 vol % (LiBOB), and 0.7 M LiBOB in EC:EMC = 3:7 vol % with 2 wt% vinylene carbonate (LiBOB-VC) during at least three cycles. XRR data show that the formations of SEI layers and their evolutions during the cycling for these two types of electrolytes are different. Furthermore, a layer with low electron density (ED) is still present after total delithiation in the LiBOB-cells, while the ED evolutions are generally reversible for LP57 cell. This suggests that the F-free SEI has different properties during cycling than the SEI formed with conventional salt LiPF6.In summary, operando XRR is a powerful technique for investigating the impact of new electrolyte chemistry and formulation on the SEI properties, which significally influence batteries performances. Moreover, the in situ and in real-time experimental insights with sub-nanometer resolution from this technique provides valuable information for the development of more accurate and predictive models for the SEI evolution. Its unique capabilities make it an essential tool for advancing the understanding of SEI behavior and designing more efficient and stable energy storage systems.Figure 1: (a) Principle of operando X-ray reflectometry. (b) XRR data and fit-derived EDP of LiBOB cell upon lithiation. Figure 1

(Battery Division Student Research Award Sponsored by Mercedes-Benz Research & Development) Quantitative SEI-Based Descriptors for Li metal Liquid Electrolyte Design

Energy density requirements for next-generation batteries make the Li metal anode a key candidate to replace graphite in Li-ion batteries due to Li’s improved capacity (3,860 mAh/g vs. 372 mAh/g). However, Li anodes display lower Coulombic efficiency (CE, <99.9%) than graphite (>99.95%), resulting in faster loss of reversible Li over cycling. This shortfall derives from parasitic Li-electrolyte reactions that lead to accumulation of inactive Li0 and electrolyte-derived byproducts, which result in the formation of a native solid electrolyte interphase (SEI) on Li. Despite the importance of the SEI, its properties and composition are challenging to probe experimentally due to its exceedingly low amount (sub-μmolLi/cm2/cycle at >99% CE). Consequently, the framework on which the understanding of SEI composition and function was built has largely been qualitative, and descriptors to CE have overwhelmingly focused on electrolyte-, as opposed to SEI-based, metrics.To bridge this gap, this collective work focuses on advancing experimental methodologies to precisely quantify (1) functional properties of native SEIs on Li and (2) their chemical composition, both in order to explore their relationships with CE. First, we use a combination of self-consistent measurements involving EIS and CV to measure rates of Li+ exchange across these native interfaces, revealing that this functional property varies across electrolytes, increasing from low to high CE. These experiments further reveal an evolution of Li+ exchange with cycling unique to high-CE electrolytes and that is tightly linked to rate capability. In particular, we find that SEIs of high-CE electrolytes can support Li+ exchange rates in excess of 1 mA/cm2, and increasing to >>10 mA/cm2 after SEI formation and cycling. In order to (2) explore which SEI phases tend to be favored at high CE, we leveraged the quantitative power of analytical tools such as GC, NMR and ICP-AES to measure byproducts of SEI formation. We developed a custom operando GC experiment to measure and chemically-resolve gas evolution in situ during Li cycling. With sub-nmol/min resolution, these revealed, quantitatively, that SEI formation reactions that release CO or CO2, thus leaving behind a decarbonylated/decarboxylated byproduct in the SEI are associated with higher CE. We also expanded our chemical quantification studies to postmortem cells using a titration scheme that was capable of quantifying with ~nAh resolution SEI phases such as ROCO2Li, Li2C2, Li2C2, LiF and P-containing phases, in addition to the inactive Li0 that accumulates on the anode after cycling. In carbonate-based electrolytes, it was found that salt-derived phases (LiF and P) comprised only ~5% of the SEI, with solvent-derived products being the majority of the SEI. Furthermore, it was found that Li2C2, a previously-unmeasured phase on Li, shows a strong anti-correlation with CE. More recently, we further expanded our titration strategy to include other previously-unmeasured phases in leading high-CE electrolytes. Altogether, chemical quantification of the SEI reveals quantitative “SEI fingerprints” that are unique to each electrolyte. Taken together, these quantitative SEI-based descriptors enable a new avenue for electrolyte engineering, informed by a fundamental understanding of the “optimal” Li SEI and their associated capacity loss mechanisms at high CE.

Synergistic Impact of Electrolyte Components: Interplay between the Novel Phosphorus-Based Additives and Conducting Salt Advancing NMC811||SiC Cell Performance

A lithium-ion battery (LIB) typically consists of an anode, a lithiated transition metal oxide cathode, and an electrolyte formulation containing inorganic lithium conducting salt in aprotic organic solvents/co-solvents, and functional additives. In-depth research is being conducted in order to develop novel chemistries for the next-generation LIBs. Within the line, silicon-based anodes and nickel-rich cathodes are recognized as two feasible next-generation electrode materials. The long-term galvanostatic cycling performance of resulting cell chemistries relies on the effectivity and robustness of solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) formed during the initial cycles and originate from the galvanostatic cycling conditions, as well as the complex interplay between electrolyte and corresponding electrodes .[1] To enhance the SEI and CEI properties, implementing functional electrolyte additives into baseline electrolyte has been shown to be one of the most effective and cost-favorable strategies. Film-forming additives sacrificially reduce and/or oxidize at corresponding electrodes to form protective layers (SEI and CEI).Previous research has shown that phosphorus-based additives improve electrode |electrolyte interfaces stability and simultaneously enhance cell flame-retardant capability advancing the overall cell performance.[2,3] In this report, we indicated part of the above-mentioned properties originate from the synergy between lithium hexafluorophosphate (LiPF6) which is the state-of-the-art (SOTA) electrolyte conducting salt in LIBs and considered functional additives. Here, we demonstrated the functionality of 2-phenoxy-1,3,2-dioxaphospholane (PhEPi) as a novel film-forming additive[4] and 2,2,4,4,6-pentafluoro-6-(2,2,2-trifluoroethoxy)-1,3,5,2λ5,4λ5,6λ5-triazatriphosphinine (CF3PFFPN) as a new flame-retardant additive, synthesized for the first time by our group. The presence of PhEPi and CF3PFPN at the optimum concentrations in the baseline electrolyte (1 M LiPF6 in EC: EMC 3:7 + 8 % VC) enhances the performance and safety of the resulting NMC811||Si-graphite (20% Si) cell chemistry. Impact of phospholane-based additive was confirmed by substituting LiPF6 with LiBF4 conducting salt and complementary post mortem analysis. This study along with improved cell chemistry, provided a better understanding and deeper insights into the synergistic interplay and impact of considered electrolyte components on the resulting NMC811||Si-graphite cell chemistry. [1] C. Wölke, B. A. Sadeghi, G. G. Eshetu, E. Figgemeier, M. Winter, I. Cekic-Laskovic, Advanced Materials Interfaces 2022, 9, DOI 10.1002/admi.202101898.[2] N. Von Aspern, D. Diddens, T. Kobayashi, M. Börner, O. Stubbmann-Kazakova, V. Kozel, G. V. Röschenthaler, J. Smiatek, M. Winter, I. Cekic-Laskovic, ACS Applied Materials and Interfaces 2019, 11, 16605–16618, DOI 10.1021/acsami.9b03359.[3] N. von Aspern, C. Wölke, M. Börner, M. Winter, I. Cekic-Laskovic, Journal of Solid State Electrochemistry 2020, 24, 3145–3156, DOI 10.1007/s10008-020-04781-1.[4] B. A. Sadeghi, C. Wölke, F. Pfeiffer, M. Baghernejad, M. Winter, I. Cekic-Laskovic, Journal of Power Sources 2023, 557, 232570, DOI 10.1016/j.jpowsour.2022.232570.

Spectroscopic and Computational Evaluation of Electrochemical Stability of Electrolyte Solutions; Solvents, Electrolytes and Their Concentration Dependence

Since the introduction of Lithium-ion batteries (LIBs) into commercial use, great improvements have been achieved in their performance such as energy density, durability, and safety. However, there still remain many technical issues to meet the increasing demands for a longer driving range of electric vehicles, a larger storage for renewable energy. Research and development of electrode active materials are extensively progressing to respond those market needs, and R&D for electrolyte solutions are also active in order to utilize the fullest extent of the newly developed electrode materials.Various kinds of characteristics are required for electrolyte solutions in LIBs. Ideally, the electrochemical stability in a wide potential range is needed, but almost all the electrolyte solutions reductively decompose on negative electrodes, and the decomposition products precipitate on the electrode to form a protective surface film, which is referred to as SEI (solid electrolyte interphase). Because the nature of SEI is influenced by chemical structures of electrolyte solutions, the solvents and lithium salts need to be combined properly to realize LIBs with better performance. Although the conventional electrolyte solutions show satisfactory performance to some extent, there still remain some disadvantages; instability against moisture and temperature, and volatility and flammability of carbonate ester solvents. Great efforts have been made to overcome those disadvantages in the conventional electrolyte solutions, for example, utilization of ionic liquid, polymer electrolytes and inorganic solid state electrolytes, and recently, highly concentrated electrolyte solution gathers a lot of attention.Based on these background, solid electrolyte interphase (SEI) formed on graphite in conventional and highly concentrated electrolyte solutions was thoroughly characterized by a combined experimental and computational study. The comprehensive understanding revealed that a chemical composition of SEI, as well as the solvation structures unstable to reductive potential, can be predicted by a profound understanding of density functional theory (DFT) calculation results of the solvates containing a counter anion. The solvation structures were determined by Raman spectroscopy to evaluate electron affinity (EA) and LUMO of the solvates containing an anion by DFT calculation. The chemical composition of SEI was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS), and the results were compared with a prediction based on the calculation. The formation mechanism of SEI during charge and discharge process can be partly estimated by the combination of experiment and theoretical calculation. These results indicate that electrolyte solutions can be efficiently designed by predicting the physicochemical properties of SEI through the more effective utilization of DFT calculation. Thus, a combination of DFT prediction and experimental analysis is proved to be an effective approach to reveal SEI formation mechanisms, and can be utilized as a versatile approach to develop new solvents, electrolyte salts, and additives, and to design electrolyte solutions with appropriate concentration.It is expected that the oxidative stability and reaction mechanism of electrolyte solutions at the positive electrode can be estimated using similar approaches. Here, it is necessary to predict oxidative stability using ionization potentials (IP), rather than EA. In the same way, other solvents than carbonate, such as ethers and sulfones, which have been difficult to put into practical use in conventional electrolytes, can also be the target to be studied. By analyzing the solvation structure and estimating its chemical stability, we believe it will be possible to get many insights on the reactions that proceed at the interface between the electrode and the electrolyte. Figure 1

How Relevant Are SEI Studies from Half Cell Samples to Understand Aging Mechanisms in Potassium Batteries? An in-Depth Photoelectron Spectroscopy Study

Owing to a shared common technological basis, monovalent K-ion battery (KIB) systems follow in the footsteps of Li-ion batteries (LIBs) with the aim to develop a broader portfolio of available battery technologies and to put them on a more sustainable material basis. For example, both cations intercalate into a graphite host structure, which can be used in combination with carbonate-based electrolytes as negative electrode. Especially in half cells, the degradation of graphite electrodes in KIBs is significantly accelerated in comparison to LIBs. Previous surface studies demonstrated that the formation of a durable and protective solid electrolyte interphase (SEI), that is paramount for long cycle life in alkali-ion batteries, is a major bottleneck for KIBs because of less benign SEI properties[1][2].Recently reported crosstalk phenomena in half cell configurations due to the highly reactive potassium counter electrode make the analysis of surface layers a particular challenge [2][3]. In this presentation, a detailed characterization of the SEI formation on graphite was performed in both half and full cell configurations by in-house and synchrotron-based photoelectron spectroscopy (PES) in order to demonstrate fundamental differences in surface layer composition under the influence of a reactive counter electrode. While it is generally acknowledged that PES results show differences in SEI composition and thickness between both configurations, our results clearly demonstrate that in case of KIBs the changes become severe. This challenges interpretations of SEI layer properties made in half cells, as they are distinctly different from those in full cells. More importantly, this also affects interpretations on the major electrolyte degradation pathways in these systems. Using PES depth profiling and complementing results with gas chromatography, as well as DFT calculations, delivers new insights on the irreversible processes in KIBs. Reference s : [1] Naylor, et al., ACS Appl. Mater. Interfaces. 11 (2019) 45636–45645.[2] Allgayer et al., ACS Appl. Energy Mater. 5 (2022) 1136–1148.[3] Hosaka et al., ACS Energy Lett. 6 (2021) 3643–3649.

Unveiling the Mystery of LiF within Solid Electrolyte Interphase in Lithium Batteries.

Over the past decades, significant advances have been made in lithium-ion batteries. However, further requirement on the electrochemical performance is still a powerful motivator to improve battery technology. The solid electrolyte interphase (SEI) is considered as a key component on negative electrode, having been proven to be crucial for the performance, even in safety of batteries. Although numerous studies have focused on SEI in recent years, its specific properties, including structure and composition, remain largely unclear. Particularly, LiF, a common and important component in SEI, has sparked debates among researchers, resulting in divergent viewpoints. In this review, the recent research findings on SEI and delve into the characteristics of the LiF component is aim to consolidated. The cause of SEI formation and the evolution of SEI models is summarized. The distinctive properties of SEI generated on various negative electrodes is further discussed, the ongoing scholarly controversy surrounding the function of LiF within SEI, and the specific physicochemical properties about LiF and its synergistic effect in heterogeneous components. The objective is to facilitate better understanding of SEI and the role of the LiF component, ultimately contributing to the development of Li batteries with enhanced electrochemical performance and safety for battery communities.

Tailoring chemical composition of solid electrolyte interphase by selective dissolution for long-life micron-sized silicon anode

Micron-sized Si anode promises a much higher theoretical capacity than the traditional graphite anode and more attractive application prospect compared to its nanoscale counterpart. However, its severe volume expansion during lithiation requires solid electrolyte interphase (SEI) with reinforced mechanical stability. Here, we propose a solvent-induced selective dissolution strategy to in situ regulate the mechanical properties of SEI. By introducing a high-donor-number solvent, gamma-butyrolactone, into conventional electrolytes, low-modulus components of the SEI, such as Li alkyl carbonates, can be selectively dissolved upon cycling, leaving a robust SEI mainly consisting of lithium fluoride and polycarbonates. With this strategy, raw micron-sized Si anode retains 87.5% capacity after 100 cycles at 0.5 C (1500 mA g−1, 25°C), which can be improved to >300 cycles with carbon-coated micron-sized Si anode. Furthermore, the Si||LiNi0.8Co0.1Mn0.1O2 battery using the raw micron-sized Si anode with the selectively dissolved SEI retains 83.7% capacity after 150 cycles at 0.5 C (90 mA g−1). The selective dissolution effect for tailoring the SEI, as well as the corresponding cycling life of the Si anodes, is positively related to the donor number of the solvents, which highlights designing high-donor-number electrolytes as a guideline to tailor the SEI for stabilizing volume-changing alloying-type anodes in high-energy rechargeable batteries.

Direct in situ measurements of electrical properties of solid-electrolyte interphase on lithium metal anodes.

The solid-electrolyte interphase (SEI) critically governs the performance of rechargeable batteries. An ideal SEI is expected to be electrically insulative to prevent persistently parasitic reactions between the electrode and the electrolyte and ionically conductive to facilitate Faradaic reactions of the electrode. However, the true nature of the electrical properties of the SEI remains hitherto unclear due to the lack of a direct characterization method. Here we use in situ bias transmission electron microscopy to directly measure the electrical properties of SEIs formed on copper and lithium substrates. We reveal that SEIs show a voltage-dependent differential conductance. A higher rate of differential conductance induces a thicker SEI with an intricate topographic feature, leading to an inferior Coulombic efficiency and cycling stability in Li∣∣Cu and Li∣∣LiNi0.8Mn0.1Co0.1O2 cells. Our work provides insight into the targeted design of the SEI with desired characteristics towards better battery performance.

Continuum-level modeling of Li-ion battery SEI by upscaling atomistically informed reaction mechanisms

Understanding and controlling solid-electrolyte interphase (SEI) formation to stabilize cell performance is a significant challenge for next-generation Li-ion battery technologies. In recent years, computational modeling has become an essential tool in providing fundamental insights into SEI properties and dynamics. However, neither atomistic nor continuum-level approaches alone can capture the complexities of SEI chemistry across all relevant length and time scales. In this work, a continuum-level model is developed that is informed by reaction mechanisms obtained from first-principle calculations. The atomistically informed continuum-level model is used to understand electrolyte degradation, including the decomposition of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and fluoroethylene carbonate (FEC). The model presented here is the most chemically complex continuum-level SEI model in the literature to date. The SEI model is calibrated against experimental irreversible leakage currents and shows qualitative agreement with expected SEI growth trends. The model framework is expected to accelerate fundamental understanding of SEI formation, facilitate mechanism development feedback, and dynamically interact with experimental insights.

Electrolyte Strategies Facilitating Anion-Derived Solid-Electrolyte Interphases for Aqueous Zinc-Metal Batteries.

Rechargeable aqueous zinc-metal batteries (AZBs) are a promising complimentary technology to the existing lithium-ion batteries and the re-emerging lithium-metal batteries to satisfy the increasing demands on energy storage. Despite considerable progress achieved in the past years, the fundamental understanding of the solid-electrolyte interphase (SEI) formation and how its composition influences the SEI properties are limited. This review highlights the functionalities of anion-tuned SEI on the reversibility of zinc-metal anode, with a specific emphasis on new structural insights obtained through advanced characterizations and computational techniques. Recent efforts in terms of key variables that govern the interfacial behaviors to improve the long-term stability of zinc anode, i.e., Coulombic efficiency, plating morphology, dendrite formation, and side-reactions, are comprehensively reviewed. Lastly, the remaining challenges and future perspectives are presented, providing insights into the rational design of practical high-performance AZBs.

Elucidating Degradation Mechanisms of Silicon‐graphite Electrodes in Lithium‐ion Batteries by Local Electrochemistry

AbstractThe integrity of the solid electrolyte interphase (SEI) formed on the negative electrode of lithium‐ion batteries (LIB) is especially critical for the performance of next‐generation LIBs comprising silicon‐carbon based electrode materials. The protecting character of the SEI is compromised due to volume expansion and shrinking during de/intercalation of Li ions leading to irreversible changes upon long‐term cycling. Scanning electrochemical microscopy (SECM) is proposed as local electrochemical technique to investigate the degradation mechanisms of advanced negative electrodes. The impact of charge/discharge cycling on the SEI properties on Si−C electrodes was investigated, and the sensitivity of SECM successfully reveals inhomogeneities at an early stage of the cycling already at about 5 cycles. Macroscopic EIS measurements and evaluation of the coulombic efficiency may result in misleading interpretations of degradation. SECM is demonstrated to be a powerful and complementary technique for revealing μm‐heterogeneities in the SEI surface reactivity after a few charge/discharge cycles.

Synergetic Control of Li+ Transport Ability and Solid Electrolyte Interphase by Boron‐Rich Hexagonal Skeleton Structured All‐Solid‐State Polymer Electrolyte

High Li+ transference number electrolytes have long been understood to provide attractive candidates for realizing uniform deposition of Li+. However, such electrolytes with immobilized anions would result in incomplete solid electrolyte interphase (SEI) formation on the Li anode because it suffers from the absence of appropriate inorganic components entirely derived from anions decomposition. Herein, a boron‐rich hexagonal polymer structured all‐solid‐state polymer electrolyte (BSPE+10% LiBOB) with regulated intermolecular interaction is proposed to trade off a high Li+ transference number against stable SEI properties. The Li+ transference number of the as‐prepared electrolyte is increased from 0.23 to 0.83 owing to the boron‐rich cross‐linker (BC) addition. More intriguingly, for the first time, the experiments combined with theoretical calculation results reveal that BOB− anions have stronger interaction with B atoms in polymer chain than TFSI−, which significantly induce the TFSI− decomposition and consequently increase the amount of LiF and Li3N in the SEI layer. Eventually, a LiFePO4|BSPE+10% LiBOB|Li cell retains 96.7% after 400 cycles while the cell without BC‐resisted electrolyte only retains 40.8%. BSPE+10% LiBOB also facilitates stable electrochemical cycling of solid‐state Li‐S cells. This study blazes a new trail in controlling the Li+ transport ability and SEI properties, synergistically.

In Situ Spectroscopic Probing of Oxygen Crossover Effects on Solid Electrolyte Interphase in Aprotic Lithium‐Oxygen Batteries

AbstractThe solid electrolyte interphase (SEI) on lithium metal anodes (LMA) plays a critical role in affording a long lifespan required for aprotic lithium‐oxygen (Li–O2) batteries. Nevertheless, the crossover of oxygen from the cathode to the anode, an inevitable phenomenon for most of the current Li–O2 batteries, and its effects on the formation and operation of SEI on LMA remain less explored. In this work, a mechanistic study of the SEI formation at a model Cu/dimethyl sulfoxide (DMSO) interface in the presence of oxygen is presented. Direct spectroscopic evidence coupled with theoretical calculation reveals that oxygen can alter the SEI formation pathway and result in distinct SEI properties. Specifically, oxygen can inhibit the fission of the C–S bond of DMSO solvent and therefore reduce the formation of unstable SEI components (e.g., C≡C species) and volatile products (e.g., C2H6 and H2). Thus, the SEI formed under oxygen is more uniform and of less voids, and enables improved electrochemical performance of LMA. This work presents new insights into the oxygen crossover effects on SEI chemistry and is beneficial for designing better LMA/electrolyte interface for future Li–O2 batteries.

Enhancing Li cycling coulombic efficiency while mitigating “shuttle effect” of Li-S battery through sustained release of LiNO3

In practical lithium-sulfur batteries (LSBs), the shuttle effect and Li cycling coulombic efficiency (CE) are strongly affected by the physicochemical properties of solid electrolyte interphase (SEI). LiNO3 is widely used as an additive in electrolytes to build a high-quality SEI, but its self-sacrificial nature limits the ability to mitigate the shuttle effect and stabilize Li anode during long-term cycling. To counteract LiNO3 consumption during long-term cycling without using a high initial concentration, inspired by sustained-release drugs, we encapsulated LiNO3 in lithiated Nafion polymer and added an electrolyte co-solvent (1,1,2,2-tetrafluoroethylene 2,2,2-trifluoromethyl ether) with poor LiNO3 solubility to construct high-quality and durable F- and N-rich SEI. Theoretical calculations, experiments, multiphysics simulations, and in-situ observations confirmed that the F- and N-rich SEI can modulate lithium deposition behavior and allow persistent repair of SEI during prolonged cycling. Hence, the F- and N-rich SEI improves the Li anode cycling CE to 99.63% and alleviates the shuttle effect during long-term cycling. The lithium anode with sustainable F- and N-rich SEI shows a stable Li plating/stripping over 2000 h at 1 mA cm−2. As expected, Li||S full cells with this SEI achieved a long lifespan of 250 cycles, far exceeding cells with a routine SEI. The Li||S pouch cell based on F- and N-rich SEI also can achieve a high energy density of about 300 Wh kg−1 at initial cycles. This strategy provides a novel design for high-quality and durable SEIs in LSBs and may also be extendable to other alkali metal batteries.

Probing the Mechanically Stable Solid Electrolyte Interphase and the Implications in Design Strategies.

The inevitable volume expansion of secondary battery anodes during cycling imposes forces on the solid electrolyte interphase (SEI). The battery performance is closely related to the capability of SEI to maintain intact under the cyclic loading conditions, which basically boils down to the mechanical properties of SEI. The volatile and complex nature of SEI as well as its nanoscale thickness and environmental sensitivity make the interpretation of its mechanical behavior many roadblocks. Widely varied approaches are adopted to investigate the mechanical properties of SEI, and diverse opinions are generated. The lack of consensus at both technical and theoretical levels has hindered the development of effective design strategies to maximize the mechanical stability of SEIs. Here, the essential and desirable mechanical properties of SEI, the available mechanical characterization methods, and important issues meriting attention for higher test accuracy are outlined. Previous attempts to optimize battery performance by tuning SEI mechanical properties are also scrutinized, inconsistencies in these efforts are elucidated, and the underlying causes are explored. Finally, a set of research protocols is proposed to accelerate the achievement of superior battery cycling performance by improving the mechanical stability of SEI.

Interface Engineering with Nonsacrificial Perfluorinated-Anion Additives for Boosting the Kinetics of Lithium-Ion Batteries.

Though lithium-ion batteries (LIBs) have seen a meteoric rise in worldwide deployment over the last decade, they should be further advanced in constant demand of higher rate capability and wider temperature adaptability. A solid electrolyte interphase (SEI) is the essential part of LIBs, determining the charge-discharge performance and degradation behavior. Herein, improvement of the SEI properties is achieved by regulating the electrochemical double layer structure with a nonsacrificial electrolyte additive, that is, lithium nonafluoro-1-butanesulfonate. The anion adsorption of the additive affects the decomposition behavior of other additive and solvent species, and the generated SEI at the graphite electrode becomes thinner and more uniform, leading to decreased impedance and finally resulting in improved energy efficiency, power capability, and fast charging performance of the graphite/NCM811 cell. Furthermore, the low-temperature cycleability at -20 °C is considerably enhanced with no dendritic Li metal deposition at the negative electrode surface. A mechanistic study on the interfacial phenomena and the effect is carried out by using various theoretical and experimental methods, such as density functional theory calculations, electrochemical quartz crystal microbalance, and transmission electron microscopy. Consequently, the approach of SEI modification with the nonsacrificial electrolyte additive can be one of the effective ways to advance LIB technology in future.

Prefabrication of "Trinity" Functional Binary Layers on a Silicon Surface to Develop High-Performance Lithium-Ion Batteries.

The silicon (Si) anode is widely recognized as the most prospective next-generation anode. To promote the application of Si electrodes, it is imperative to address persistent interface side reactions caused by the huge volume expansion of Si particles. Herein, we introduce beneficial groups of the optimized binder and electrolyte on the Si surface by a co-dissolution method, realizing a "trinity" functional layer composed of azodicarbonamide and 4-nitrobenzenesulfonyl fluoride (AN). The "trinity" functional AN interfacial layer induces beneficial reductive decomposition reactions of the electrolyte and forms a hybrid solid-electrolyte interphase (SEI) skin layer with uniformly distributed organic/inorganic components, which can enhance the mechanical strength of the overall electrode, restrain harmful electrolyte depletion reactions, and maintain efficient ion/electron transport. Hence, the optimized Si@AN11 electrode retains 1407.9 mAh g-1 after 500 cycles and still delivers 1773.5 mAh g-1 at 10 C. In stark contrast, Si anodes have almost no reserved capacity at the same test conditions. Besides, the LiNi0.5Co0.2Mn0.3O2//Si@AN11 full-cell maintains 141.2 mAh g-1 after 350 cycles. This work demonstrates the potential of developing multiple composite artificial layers to modulate the SEI properties of various next-generation electrodes.