Solid Electrolyte Interphase Research Articles

Unveiling degradation mechanisms of anode-free Li-metal batteries

Anode-free Li-metal batteries (AFLMBs), in which Li+ ions from the cathode are deposited on a Cu substrate and the deposited Li-metal serves as the anode, exhibit higher energy density compared to Li-metal batteries (LMBs). However, achieving stable cycle performance, even at moderate operating conditions, is difficult and has so far hindered their practical uses. In AFLMBs, the homogeneity of solid electrolyte interphase (SEI), initially created by electrolyte reduction on Cu substrate, is not maintained during Li-metal deposition, leading to uncontrolled electrolyte decomposition. The SEI is therefore not conserved, and uneven Li deposition morphology is induced on the Cu substrate and the eventual instability of SEI leads to the overall degradation of AFLMBs. Here, we report on the failure mechanisms of AFLMBs through a comparative study with LMBs using 3 M lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in N,N-dimethylsulfamoyl fluoride. Our investigation reveals that the SEI inhomogeneity in AFLMBs makes Li+ transport through SEI sluggish and non-uniform, triggering local compositional changes of the initially formed SEI on the Cu substrate and unwanted consumption of FSI– anion from the electrolyte. This work provides clear understanding to the interfacial engineering and important roles of Li-metal on the Cu substrate in AFLMBs, promising the creation of stable SEI, reversible electrochemical reaction of Li-metal, and interfacial stability of the cathode in LMBs.

Biomimetic surface design enables a resilient solid electrolyte interphase for high-performance anodes

Surface coating presents an effective methodology for mitigating the detrimental effects of large volume changes inherent to high-capacity anode materials (e.g. Si, SiOx). However, designs often prioritize the protection of internal active particles, inadvertently neglecting the intricate interplay between the coating layer and the external electrolyte which exhibits profound influences on the solid electrolyte interphases (SEIs). Inspired by the extracellular polymeric substance (EPS) protecting biological cells (e.g. yeast) from predation and chemical damages, we prepare a conducting polymer-based EPS system (CP-EPS) on a surface bilayer comprising soft carbon membranes and compact graphene walls, constructing the biomimetic cellular structure. The CP-EPS chemically interacts with electrolyte catalyzing the symbiosis of integrated LiF-enriched SEIs and physically provide sufficient resilience for SEIs. This resilient SEIs offer excellent reaction kinetics and roughness which protects the structural integrity of the particle and itself from pulverization and excessive SEI thickening. The prepared SiOx anode delivers a superior average coulombic efficiency of 99.4% over 200 cycles at 0.5C and a high reversible capacity of 730 mAh g-1 after 300 cycles at 2C.

Rate Capability of Fluorinated Catholyte-Based Li Primary Batteries Governed by Interfacial and Bulk Li+ Transport

Fluorinated catholytes based on fluoro–organosulfur reactants have recently been shown to offer a compelling design space for high-energy lithium (Li) primary batteries with ability to compete with carbon monofluoride (CFx) at low rates and moderate cell temperatures. However, rate capability is hindered as the operating temperature is lowered to room temperature, warranting further investigation. Here, we examined the origin of such loss in detail via impedance spectroscopy and 3–electrode cells. As temperature decreases, the Li anode disproportionately contributes to discharge overpotentials at moderate to high rates (0.3–1 mA cm−2), indicating that the Li solid electrolyte interphase (SEI) plays a pivotal role in governing performance. Despite a modest temperature change, the chemical composition of the SEI is significantly different at 25 °C than at 50 °C, with a lower oxygen/fluorine ratio at room temperature that impedes SEI Li+ transport. Furthermore, continuous growth in ionic resistance/viscosity of the catholyte throughout discharge are found to be the cause of cell “sudden death” at lower temperature and higher rates. This analysis indicates that development of these energy–dense primary battery chemistries for moderate-to-high–rate applications must focus on enhancing SEI Li+ conductivity and maximizing diffusivity of reactant species under solvent–lean conditions.

Roll-to-roll Prelithiation of Li-ion Batteries Anodes Using Ultrathin Lithium Strips

The irreversible formation of solid electrolyte interphase (SEI) reduces lithium-ion batteries’ capacity and energy density (LIBs), prompting the prelithiation strategies to compensate for the Li loss. However, the accuracy of the prelithiation degree and the SEI formation under prelithiation conditions remain key issues, especially for practical industrial production. Herein, we demonstrate that roll-to-roll prelithiation of graphite anodes can be done using ultrathin lithium strips (6-μm-thick), and the prelithiation degree can be precisely tuned by the topological distribution. More importantly, the formation of SEI during the prelithiation process and its influence on the long-term cycling performance of the prelithiated LiFePO4 || graphite pouch cells was revealed. Afterward, the prelithiated pouch LiFePO4 || graphite cell at the optimized conditions exhibits the upgraded energy density after even 1200 cycles. This study shall provide an industrial-scale prelithiation technique for high-energy–density LIBs and a pioneering systematic investigation of SEI chemistry during prelithiation.

Microcrystalline-Fe2P4O12 as eco-friendly and efficient anode for high-performance dual-ion battery

Dual-ion batteries (DIBs) have garnered significant interest because of their cost-effectiveness and high level of safety. However, the anode typically utilized tends to show low cyclic stability in complete batteries when the only sources of lithium are primarily consumed in the electrolyte following the development of a solid electrolyte interphase (SEI) during the initial electrochemical process. Here, a simple approach is used to produce microcrystalline carbon-coated Fe2P4O12 at a mild temperature. This material has the ability to store lithium ions, resulting in a specific capacity of 215 mAh/g at a potential range of 1 to 2.5 V vs Li+/Li0. Furthermore, it demonstrates a remarkable coulombic efficiency of 99 % in the initial cycles, even in the absence of a solid electrolyte interphase (SEI) formation. On the other hand, the issue of the high voltage plateau of the graphite cathode is addressed by selecting a total of 4.2 V as the optimal voltage range in DIBs. Simultaneously, the composite Fe2P4O12 also demonstrates exceptional ion diffusion kinetics and reduced interfacial impedance due to its microcrystalline structure. The utilization of the proposed Fe2P4O12 anode addresses a crucial deficiency in the complete range of deep in-situ bioremediation techniques, hence expediting the industrialization process.

Experimental Investigation of Thermal Runaway Behaviour and Inhibition Strategies in Large-Capacity Lithium Iron Phosphate (LiFePO4) Batteries for Electric Vehicles

Large-scale lithium-ion batteries play an important role in the electric vehicle market and the thermal runaway (TR) behaviour of batteries is one of the significant threats to the passengers. In this study, we conducted a series of thermal abuse tests concerning single battery and battery box to investigate the TR behaviour of a large-capacity (310 Ah) lithium iron phosphate (LiFePO4) battery and the TR inhibition effects of different extinguishing agents. The study shows that before the decomposition of the solid electrolyte interphase (SEI) film, temperature consistency within the battery were maintained and it was advisable to use temperature detectors. With the destruction of the internal structure of the battery and the uneven temperature distribution at the subsequent battery reaction stage, a large amount of combustible gas was released when the safety valve was opened. The combustible gas fire detector is used to improve alarm reliability and provided a stable early warning response time. Based on this TR behaviour, a LiFePO4 battery box with a standard size of 1060 × 660 × 250mm was constructed. The TR inhibition effect of two different extinguishing agents (heptafluoropropane and perfluorohexanone) were investigated using a metal tube spraying method with a spraying dose of 1.8kg (spraying rate of 0.06kg/s) and a spraying pressure of 2.5MPa. The results showed that using either of these, satisfy the requirements for TR inhibition in large-capacity LiFePO4 batteries and the flames could be completely extinguished without any reignition, and the chain reaction of the surrounding batteries was effectively prevented. Moreover, heptafluoropropane was proven to have a shorter fire-extinguishing time, whereas perfluorohexanone had the advantage of inhibiting temperature rise. This study provides valuable experimental results for early warning and inhibition of TR in large-capacity LiFePO4 batteries for electric vehicles.

Robust polymer electrolytes with fast ion-transport nanochannels constructed by carbon dots for long lifespan Li metal batteries

As an anode material in energy storage systems, Li metal has exceptional merits, including high specific capacity, low density, and low redox potential. However, challenges such as lithium dendrites and unstable solid electrolyte interphase (SEI) hinder the practical application of lithium metal batteries (LMBs). Solid polymer electrolytes (SPEs), notably PVDF-HFP, are expected to overcome the above problems because of their mechanical merits and safe features, but their limited ion transportation behaviors result in insufficient ionic conductivity at room temperature. Herein, we invent an approach to prepare high performance electrolytes composed by porous PVDF-HFP and functionalized carbon dots (CDs). Such porous PVDF-HFP polymer membranes are created by a subtle phase inversion process, possessing a sponge-like structure, vertical lithium-ion transport channels and CDs decorated surfaces, thus able to uptake the liquid electrolyte (LE) to form gel polymer electrolytes (GPEs). The CDs fillers are produced in large scale with adjustable surface states, which can enhance Li salts disassociation to release free Li ions. As a result, the optimal conductivity of the as-prepared GPEs at room temperature is up to 8.7 mS cm−1 and the lithium transference number reaches 0.64. Our GPEs endow Li symmetric batteries with very stable SEI and long cycling life over 2200 h, and help realize high capacities and retention rates of the LMBs using LiFePO4(LFP) or LiCoO2(LCO) cathodes.

Constructing stable solid electrolyte interphases to enhance the calendar life of anode-free lithium metal batteries

Advancement of anode-free lithium metal batteries (AFLMBs) has been appreciated due to their exceptional volumetric energy density and manufacturing simplicity. However, AFLMBs suffer from poor cycle performance due to the absence of excess lithium and current research lacks an understanding of lithium corrosion and calendar aging in AFLMBs. Here, we investigate the practical effects of chemical and galvanic corrosion on battery performance (calendar life) through a modified full-cell cycling experiment. The impact of galvanic corrosion is negligible, whereas chemical corrosion during a rest period reduces the capacity retention (CR) from 47% to 32% over 100 cycles. Consequently, a corrosion control strategy is implemented, substituting 1,2-dimethoxyethane with 1,2-dimethoxypropane (DMP), which has weaker lithium ion solvation properties due to its inductive effect. This approach results in the formation of a Li2O-rich solid-electrolyte interphase (SEI) layer, which effectively prevents direct contact of lithium with the electrolyte and the formation of additional SEI layers. Consequently, the decrease in CR due to chemical corrosion is considerably mitigated in DMP-based electrolyte, with only a slight decrease from 51% to 49%. This study provides insights into lithium corrosion in practical AFLMBs and offers guidelines for designing less-corrosive electrolytes to improve AFLMB performance for diverse industrial applications as energy-storage devices.

Rectifying solid electrolyte interphase structure for stable multi-dimensional silicon anodes

Rectifying solid electrolyte interphase structure for stable multi-dimensional silicon anodes

Optimizing Interface Chemistry with Novel Covalent Molecule for Highly Sustainable and Kinetics-Enhanced Sodium Metal Batteries

Metallic sodium has attracted increasing attention as an ideal anode material for next-generation high energy density and low-cost secondary batteries. However, it is highly desired yet remains challenging to improve their cycling stability and safety due to unstable solid electrolyte interphase and dendrite growth. Herein, a hybrid interface layer composed of Na2Se and Na3P is constructed on the surface of Na (Na@NPS) via in situ spontaneous reaction. The hybrid interface layer with merits of high sodiophilicity and high Na-ion conductivity can effectively induce homogeneous Na-ion flux distribution, accelerate the reaction kinetics and suppress decomposition of electrolyte components. Benefitting from the above advantages, the Na@NPS symmetric cell delivers a long cycle life (1000 h at 1 mA cm–2 and 1 mAh cm–2). Furthermore, the full cell coupling with Na3V2(PO4)3-based cathode provides an exceptionally long lifespan (1500 cycles) at 20 C with a capacity retention of 98.2% and high energy density (226 Wh kg–1). Therefore, the enhanced electrochemical performance illustrates the feasibility of the covalent molecule derived hybrid multifunctional interfaces in solving the irregular deposition of Na-ion and expediting reaction kinetics in Na metal batteries.

Unraveling the Complex Temperature-Dependent Performance and Degradation of Li-Ion Batteries with Silicon-Graphite Composite Anodes

Competing effects of graphite and Si result in a complex temperature dependent performance and degradation of Li-ion batteries with Si-graphite composite anodes. This study examines the influence of varying the Si content (0 to 20.8 wt%) in Si-graphite composite anodes with consistent areal capacity and N/P ratio in full cells containing NMC622 cathodes. One hundred pilot-scale double-layer pouch cells were built and cycle aged in the temperature range from −10 to 55 °C. Electrochemical characterization demonstrated that increasing Si contents enhance capacity and mitigate internal resistance at low temperatures. On the other hand, high Si contents decrease charge-discharge energy efficiency and cycle life, particularly at elevated temperatures. Post-mortem analysis of aged electrodes, including physico-chemical characterization (scanning electron microscopy, energy-dispersive X-ray analysis, thickness measurements) and cell reconstruction revealed significant solid electrolyte interphase growth and increased loss of active material in anodes with high Si content. The optimum temperature for longest cycle life as derived from Arrhenius plots decreased from 30 °C for graphite anodes to 10 °C for cells with moderate Si content up to 5.8 wt%. These findings allow the design of optimized cells by balancing the Si content versus operating temperature in order to achieve lowest cell aging.

High-efficient sodium compensation enabled by dual-carbon coupling catalyst strategy for sodium-ion batteries

Sodium oxalate (Na2C2O4), a green, cost-effective, and high-capacity additive candidate, suffers from a decomposition voltage that largely exceeds the cutoff voltage of currently common layered oxides and polyanionic compounds, limiting its availability. This work constructs a composite structure of carbon-coated Na2C2O4 coupled with a catalyst-loaded Ketjen black (NCO@C@MMNKB) using a low-temperature carbon-coated assisted ball milling technique. The results show that the decomposition potential of Na2C2O4 is significantly reduce from 4.42 to 3.91 V. Meanwhile, correspondingly the full cell energy density is enhanced from 135.0 to 236.4 Wh Kg−1. Further investigations show that the by-products of the sodium compensation additive exhibit different “gas effects” on the formation of the cathode and anode solid electrolyte interphase (CEI and SEI) films. This provides a practical thought to promote similar additives towards practicality to improve the overall performance of SIBs.

Quantifying the Effect of Degradation Modes on Li-ion Battery Thermal Instability and Safety

Understanding the thermal stability of lithium-ion (Li-ion) cells is critical to ensuring optimal safety and reliability for various applications such as portable electronics and electric vehicles. In this work, we demonstrate a combined modeling and experimental framework to interrogate and quantify the role of different degradation modes on the thermal stability and safety of Li-ion cells. A physics-based Li-ion cell aging model is developed to describe the underpinning role of degradation mechanisms such as Li plating, solid electrolyte interphase growth, and the loss of electrode active material on the resulting capacity fade during cycling. By incorporating mechanistic degradation descriptors from the aging model, we develop a degradation-aware cell-level thermal stability framework that captures key safety characteristics such as thermal runaway (TR) onset temperature, self-heating rate, and peak TR temperature for different cycling conditions. Additionally, we perform electrochemical and accelerating rate calorimetry (ARC) experiments to evaluate the thermo-kinetic parameters associated with the various exothermic reactions during TR of pristine and aged Li-ion cells. Through a synergistic integration of thermo-electrochemical characteristics from the ARC experiments and degradation insights from the cell aging model, the proposed aging-coupled safety framework provides a baseline to quantify the thermal stability of Li-ion cells subject to a wide range of operating conditions and degradation scenarios.

Fluorinated co-solvent electrolytes enable lithium metal batteries to operate at low temperatures

Enhancing the energy output of batteries in low temperatures is critical to broadening the application areas of advanced electronic devices, which can be accomplished by utilizing high-voltage cathodes to match lithium metal anode, optimizing electrolyte electrochemical windows and forming stable solid electrolyte interphases to provide facile ion-transmission kinetics at low temperature. Herein, we demonstrate a fluorinated ester co-solvent electrolyte, which applies 1 M LiPF6 in an ethyl 4,4,4-trifluoro butyrate/fluoroethylene carbonate (FEC) (4/1 by volume ratio) that jointly satisfies high voltage cathode and lithium metal anode reversibility at required ambient. The performance of the battery is due to the rapid dissociation of Li+ and the formation of a fluorine-rich inorganic electrolyte interface in the fluorinated solvent. Therefore, the fluorinated ester co-solvent electrolyte system can provide 209.9 mAh g−1, 194.8 mAh g−1, and 175.5 mAh g−1 when discharged at room temperature, −20 ℃ and −40 ℃, respectively, those far exceeds the performance of carbonate electrolytes. This work advances the development of low-temperature lithium metal batteries.

A CaI2-Based Electrolyte Enabled by Borate Ester Anion Receptors for Reversible Ca-Organic and Ca-Se Batteries.

Passivating solid electrolyte interphases (SEIs) in Ca metal anodes constitute a long-standing challenge, as they block Ca2+ transport and inhibit reversible Ca deposition/stripping. Current solutions focus primarily on boron/aluminum-based electrolytes to mitigate such interfacial issues by producing Ca2+-conductive species, yet the complex synthetic procedure of these salts restricts the widespread application. Moreover, whether any inorganic phases possess decent Ca2+ conductivity within SEIs remains ambiguous. Herein, we report that a commercially available CaI2-dimethoxyethane electrolyte supports reversible Ca/Ca2+ redox reactions via forming CaI2-involved SEI, inspired by our density functional theory calculations where CaI2 species is predicted to possess the lowest Ca2+ diffusion barrier among a range of inorganic phases. We further materialize this finding by introducing a serial of borate ester anion receptors, resulting in the formation of CaI2/borides hybrid SEIs with an enhanced Ca2+ conductivity. Consequently, the resultant electrolytes realize a 7-fold reduction in deposition/stripping overpotential compared to anion receptor-free one, allowing for the construction of reversible Ca-metal full cells with high-capacity selenium and organic cathodes.

A CaI2‐Based Electrolyte Enabled by Borate Ester Anion Receptors for Reversible Ca−Organic and Ca−Se Batteries

AbstractPassivating solid electrolyte interphases (SEIs) in Ca metal anodes constitute a long‐standing challenge, as they block Ca2+ transport and inhibit reversible Ca deposition/stripping. Current solutions focus primarily on boron/aluminum‐based electrolytes to mitigate such interfacial issues by producing Ca2+‐conductive species, yet the complex synthetic procedure of these salts restricts the widespread application. Moreover, whether any inorganic phases possess decent Ca2+ conductivity within SEIs remains ambiguous. Herein, we report that a commercially available CaI2‐dimethoxyethane electrolyte supports reversible Ca/Ca2+ redox reactions via forming CaI2‐involved SEI, inspired by our density functional theory calculations where CaI2 species is predicted to possess the lowest Ca2+ diffusion barrier among a range of inorganic phases. We further materialize this finding by introducing a serial of borate ester anion receptors, resulting in the formation of CaI2/borides hybrid SEIs with an enhanced Ca2+ conductivity. Consequently, the resultant electrolytes realize a 7‐fold reduction in deposition/stripping overpotential compared to anion receptor‐free one, allowing for the construction of reversible Ca‐metal full cells with high‐capacity selenium and organic cathodes.

Tuning Reaction Kinetics of Fluorinated Molecules to Construct Robust Solid Electrolyte Interphases on SiOx Anode.

Introducing fluorinated electrolyte additives to construct LiF-rich solid-electrolyte interphase (SEI) on Si-based anodes is proven an effective strategy for coping with its massive volume changes during cycling. However, most current research on fluorine-containing additives focuses on their thermodynamics of decomposition, lacking studies on the correlation between the molecular structure of additives and their decomposition kinetics. Herein, two fluorinated ester additives, diethyl fluoromalonate (F1DEM) and diethyl 2,2-difluoromalonate (F2DEM) were designed and synthesized. Through combining a wealth of characterizations and simulations, it is revealed that despite the similar reduction thermodynamics, the favorable reduction kinetics of single-fluorinated F1DEM facilitate a LiF-rich layer during the early stage of SEI formation, contributing to the formation of a more robust SEI on SiOx anode compared to the difluorinated F2DEM. Consequently, the proposed additive achieves excellent cycling stability (84 % capacity retention after 1000 cycles) for 5 Ah 21700 cylindrical batteries under practical testing conditions. By unveiling the role of reaction kinetics, a long-overlooked aspect for the study of electrolyte additives, this work sheds light on how to construct a stable SEI on Si-based anodes.

Tuning Reaction Kinetics of Fluorinated Molecules to Construct Robust Solid Electrolyte Interphases on SiOx Anode

AbstractIntroducing fluorinated electrolyte additives to construct LiF‐rich solid‐electrolyte interphase (SEI) on Si‐based anodes is proven an effective strategy for coping with its massive volume changes during cycling. However, most current research on fluorine‐containing additives focuses on their thermodynamics of decomposition, lacking studies on the correlation between the molecular structure of additives and their decomposition kinetics. Herein, two fluorinated ester additives, diethyl fluoromalonate (F1DEM) and diethyl 2,2‐difluoromalonate (F2DEM) were designed and synthesized. Through combining a wealth of characterizations and simulations, it is revealed that despite the similar reduction thermodynamics, the favorable reduction kinetics of single‐fluorinated F1DEM facilitate a LiF‐rich layer during the early stage of SEI formation, contributing to the formation of a more robust SEI on SiOx anode compared to the difluorinated F2DEM. Consequently, the proposed additive achieves excellent cycling stability (84 % capacity retention after 1000 cycles) for 5 Ah 21700 cylindrical batteries under practical testing conditions. By unveiling the role of reaction kinetics, a long‐overlooked aspect for the study of electrolyte additives, this work sheds light on how to construct a stable SEI on Si‐based anodes.

Stabilizing the Solid Electrolyte Interphase of SiOx Negative Electrodes: The Role of Fluoroethylene Carbonate in Enhancing Electrochemical Performance

Stabilizing the Solid Electrolyte Interphase of SiOx Negative Electrodes: The Role of Fluoroethylene Carbonate in Enhancing Electrochemical Performance

Altering the Solid Electrolyte Interface Through Surface-Modification of Lithium Metal Anode for High-Voltage Lithium Battery

Abstract The physical structure and chemical composition of the solid electrolyte interphase (SEI) affect the performance of the lithium metal anode. The tuning of the chemical composition and structure of the SEI through the surface modification of the lithium metal anode has been conducted. A series of dicarboxylic acids, oxalic acid, malonic acid, succinic acid, glutaric acid, and adipic acid have been utilized to modify the surface of the lithium anode. Physical characterization methods have been employed to study the surface morphology and chemical composition of the SEI. Symmetrical (Li/Li) and asymmetrical (NMC622/Li) cells with pristine lithium and surface modified lithium electrodes have been assembled and tested. NMC622/Li cell with surface modified lithium shows improved performance compared to that of pristine lithium. Malonic acid-treated lithium outperforms all the electrodes by retaining 141 mAh/g specific capacity even after 100 cycles of charge-discharge. XPS depth profiling analysis reveals that the SEI on the MA-Li contains evenly distributed organic and inorganic components which are responsible for the performance of MA-Li.