Gel Polymer Electrolyte Enables Low-Temperature and High-Rate Lithium-Ion Batteries via Bionic Interface Design.

Anode-free lithium (Li) metal batteries (AFLBs) featured high energy density are viewed as the viable future energy storage technology. However, the irregular Li deposition and unstable solid electrolyte interphase (SEI) on anode current collectors reduce their cycling performance. Here, we propose a concept of anion-recognition electrodes enabled by anion-π interactions to regulate the inner Helmholtz plane (IHP) and electrolyte solvation chemistry for high-performance AFLBs. By engineering the electrodes with electron-deficient aromatic-π systems that possess high permanent quadrupole moment (Qzz), the anion-π interactions can be generated to concentrate the anions on the electrode surface and tune the IHP structure to construct a stable anion-derived SEI layer, thus achieving highly reversible Li plating/stripping process. Through designing various current collectors with different Qzz values, the intimate correlations among the surface charge of the electrode, competitive adsorption of the IHP, and SEI structures are demonstrated. Particularly, the modified carbon cloth current collector with a high Qzz value (+35.1) delivers a high average Li stripping/plating Coulombic efficiency of 99.1 % over 230 cycles in the carbonated electrolyte, enabling a long lifespan and high capacity retention of LiNi0.8Co0.1Mn0.1O2-based AFLBs with a commercial-level areal capacity (4.1 mAh cm-2).

Solid electrolyte interphase (SEI) is known to be heterogeneous which comprises inorganic and organic decomposition products. To assess the effects of heterogeneity on the stability of the SEI, we establish a mathematical model in simulating the stress of heterogeneous SEI. Comparing with the analytical solution of stress in the homogeneous film, the heterogeneity of SEI is identified to be essential in the stress calculation of the inorganic layer, which is proven decisive for the stability of SEI. Further, the peak tensile stress within the inorganic layer is found in the bilayer SEI. It generates at the interface between the active material and the inorganic layer when the battery is fully charged. In addition, the impacts of the interface parameter, modulus, Poisson’s ratio, thickness of SEI and lithiation properties of active material on SEI stress are systematically investigated. Based on the simulation results, this work provides insights into the stress analysis of the heterogeneous SEI, as well as suggestions in pursuing a well-designed SEI, i.e., a functional gradient heterogeneous SEI in which the inner layer close to the active material provides sufficient mechanical performance while the outer layer near the electrolyte performs good chemical and electrochemical behaviors, to enhance the battery performance.

Li-alloy based anode materials are very promising for breaking current energy limits of lithium-ion battery technologies. At present, their practical applications are mainly hindered by the solid-electrolyte interphase (SEI) stability issue. SEIs of Li-alloy anode materials exhibit insufficient surface passivation to prevent electrolyte penetration and suffer from cracking and peeling-off issues due to their poor tolerance to volume changes, consuming additional electrolyte and material for SEI repair. Here we propose a chemical SEI reinforcement strategy via introducing multiple functional components as SEI components, which have strong interactions with Li-alloy material surface and effectively improve the SEI stability, including its tolerance to volume changes and surface passivation during cycling. Using a modular synthetic approach, we realized a one-step fabrication of diverse components with designed structure and anchoring amount, allowing the facile construction and optimization of the SEI. Si nanoparticle (SiNP) materials were used as the platform for demonstrating this concept. After screening and optimizing the SEI structure using various pre-anchored components, one chemically reinforced SEI (CR-SEI), reinforced by a combination of two functional components with moderate anchoring amount shows significantly improved stability compared to the SEI derived from electrolyte with FEC additives. During cycling, the CR-SEI presents unique and stable chemical composition, resulted form the addtion of pre-designed functional components, which differs from the composition of conventioal SEI. Meanwhile, the CR-SEI also maintains good and durable morphology during cycling. After many cycles, only very limited “SEI accumulation” was observed in the TEM images of SiNP with CR-SEI and the CR-SEI remains thin and uniform. This is in contrast with the poor stability of conventional SEI. Large amount of SEI accumulation and SiNPs decrease in size were observed. Finally, owing to the improved SEI stability, SiNP electrode with CR-SEI presents significantly increased electrochemical performance, including increased 1st cycle Coulombic efficiency (CE), improved cycling CE and capacity retention, and limited increase in electrochemical impedance. Particularly, the SiNP electrode with CR-SEI present much better cycle life in the full-cell system than one containing electrolyte additive improved SEI. Moreover, this strategy has also been applied to GeNP SEI reinforcement with dramatic enhancement of the SEI stability, suggesting its viability for other Li-alloy materials. Owing to the facility and modularity of the synthetic approach, we are also able to further examine more functional components potentially suitable for SEI reinforcement.

In order to nearly triple the driving range of electric vehicles (EV), the US Department of Energy has set a goal of achieving an energy density of 500 Wh kg-1 in EV batteries, and the use of a lithium metal anode is essential to reaching this goal.(1) A key consideration in reducing the weight (and thereby increasing the energy density) of a lithium metal battery is its electrolyte, with the volume of electrolyte used and its salt concentration both affecting the overall battery weight. Unlike with the currently-used graphite anode, the highly reactive surface of a lithium anode varies widely from cycle to cycle, which irreversibly consumes the finite amount of electrolyte and causes lithium metal to suffer from low Coulombic efficiency. To account for Coulombic inefficiency caused by electrolyte depletion, many researchers "flood" their cells with electrolyte to prolong cycle life. However, commercial batteries will require smaller amounts of electrolyte in order to achieve weight and energy density goals. Additionally, the components that make up the solid electrolyte interphase (SEI) layer on the surface of the lithium metal are soluble in the electrolyte to varying degrees. As the more soluble components selectively dissolve into the electrolyte, reparative SEI grows in its place. During non-uniform lithium plating, new SEI also forms over the freshly-exposed surface area. Both of these processes consume more of the limited electrolyte. Factors such as the volume of electrolyte used and its salt concentration affect the stability of the SEI against dissolution while also altering the composition of the SEI that forms, which can have further impacts on cell performance. Here, three electrochemically-measurable factors are used to examine the stability of the SEI: formation rate (the rate at which the SEI forms or repairs itself each cycle), degradation rate (the rate at which the SEI dissolves or degrades during rest periods), and formation capacity (the total amount of charge dedicated to forming a stable SEI). Each of these factors will affect the consumption of electrolyte and ultimately dictate the amount of excess electrolyte needed to account for these losses. Three electrolyte volumes and three salt concentrations were chosen to examine the impact that these parameters have on SEI stability and Coulombic efficiency in the standard 3:7 ethylene carbonate/ethyl methyl carbonate with 2 % vinylene carbonate electrolyte with LiPF6 salt. Higher volumes of electrolyte resulted in increased SEI formation rates and dissolution rates. Differences in these factors at low and high salt concentrations were found to result from a change in the type of SEI that formed. References Battery500 Consortium to Spark EV Innovations: Pacific Northwest National Laboratory-led, 5-year $50M effort seeks to almost triple energy stored in electric car batteries, in Office of Technology Transitions Energy.gov (2016).

Lithium-metal batteries suffer from sluggish charge-transfer kinetics and unstable solid electrolyte interphase (SEI) layers under low temperatures, invariably leading to significant performance degradation. Herein, we propose a complementary “push and pull” electrolyte design strategy to controllably regulate the Li+ solvation structure. The pull factor is designed to enhance the interaction between Li+ and weakly solvating solvents, considering the electrostatic potential (ESP), Li+–solvent binding energy, solvent–solvent interactions, and F-donating ability. Furthermore, inorganic Li salt with a high Li+–anion binding energy is designed as a push factor to lower the Li+ desolvation barrier and weaken the destructive effects of the pull factor on the SEI, forming a high-ionic-conductive and stable SEI. As a result, stable Li plating/stripping with high Coulombic efficiencies of 99.4% and 98.5% are achieved under −20 and −40 °C, respectively, and the Li/LiFePO4 full cell can retain 73% of room-temperature capacity under −20 °C. These results demonstrate an effective electrolyte strategy for low-temperature Li-metal batteries.

Study of the formation of a solid electrolyte interphase (SEI) in ionically crosslinked polyampholytic gel electrolytes

Difunctional NH2-modified MOF supporting plentiful ion channels and stable LiF-rich SEI construction via organocatalysis for all-solid-state lithium metal batteries

A stable solid electrolyte interphase (SEI) is of great importance for battery electrodes in terms of cycling as well as for its shelf life. While SEI formation on silicon anodes is generally only studied after the first charge and discharge of cells and initial reaction of electrolyte, we show the formation of a liquid/solid SEI in symmetric cells with silicon electrodes in contact with carbonate and glyme-based electrolytes under close to open circuit conditions and its behavior during long-term ageing. Activation energies of SEIs were measured via temperature-dependent electrochemical impedance spectroscopy (EIS) to study the contribution of liquid/solid phases to ion transport. The effect of different solvents, salts, their concentrations, and final water content of the glyme-electrolyte on the SEI was studied in detail. SEIs formed in cells with glyme-based electrolytes are generally more porous than the ones in cells with carbonate-based electrolytes. The addition of vinylene carbonate to glyme electrolyte is shown to be beneficial for its SEI, as it causes lower and more stable SEI resistances over time. A small amount of water in glyme electrolytes causes a denser SEI without much change in SEI resistance.

Introduction Recently, there has been an increasing demand for high-energy-density lithium-ion batteries (LIBs) that operate under broad temperature conditions (–40 to 70°C). As an electrolyte solvent, ethylene carbonate (EC) has been essential to form a solid electrolyte interphase (SEI), which effectively suppresses side reactions such as graphite exfoliation and continuous electrolyte decomposition on negative electrodes(1). However, EC is not only flammable but also susceptible to severe oxidative and reductive decomposition at high temperatures, thereby limiting the operational temperature of LIBs to around 55°C(2). There has been no alternative solvent that has (i) SEI-forming ability, (ii) nonflammability, and (iii) high-temperature stability. Hence, it has been challenging to realize the high-temperature and safe operation of LIBs.In this study, we designed and synthesized a novel phosphate solvent, FDPO, which is nonflammable and can form stable SEI on graphite. FDPO has a structural feature similar to LiPO2F2, which can improve SEI properties and high-temperature performance of batteries(3-4). We investigated the charge-discharge performances of negative and positive electrodes for LIBs with FDPO-based electrolytes at high temperature. Experimental methods Electrolytes were prepared by mixing LiPF6, FDPO, and methyl 2,2,2-trifluoroethyl carbonate (FEMC), which is known as a low viscous and flame-retardant solvent. Charge-discharge tests were conducted using coin cells with the natural graphite|Li cell configuration. The current density of 186 mA g-1 (2C) was applied from the 4th cycle after pre-conditioning three cycles at a current density of 37.2 mA g-1 (10C). The cut-off voltage range was set to 2.5 V – 0.01 V. Results and discussion Figure 1 shows the charge-discharge performances of natural graphite|Li coin cells at 70°C. The cell with the commercial electrolyte exhibited severe capacity decay to ca. 142 mAh g-1 after 50th cycles. On the other hand, the cell with FDPO-based electrolyte retained a high reversible capacity of ca. 343 mAh g-1. In addition, the charge-discharge efficiency was over 99.6% in the FDPO-based electrolyte, indicating suppressed electrolyte decomposition even at 70°C. Based on these results, the FDPO itself is thermally stable and also forms a thermally stable SEI on graphite, thereby suppressing capacity degradation under high-temperature conditions. In the presentation, the results of X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry analyses for the SEI derived from FDPO-based electrolytes, along with the charge-discharge performances of positive electrodes, will be discussed. References (1) Wang, A. et al., npj Computational Materials, 2018, 4, 15.(2) Lu, L. et al., J. Power Sources, 2013, 226, 272.(3) Yang, G. et al., RSC Adv, 2017, 7, 26052.(4) Kuang, S. et al., ACS Appl. Mater. Interfaces, 2022, 14, 19056-19066. Acknowledgment A part of this work was supported by JST GteX (JPMJGX23S3). Figure 1

The charge/discharge efficiency and the cycle life of lithium-ion batteries (LiBs) depends on a large number of competing interfacial processes. Probably the best-known interface in LiBs is the solid-electrolyte-interphase (SEI), which is a passivating layer formed at the negative electrode by electrolyte decomposition during the first cycles.[1–3] Since the SEI stability is a deciding factor with regards to cycle life, understanding and improving the SEI formation process is crucial. In order to yield an effective SEI, commercial battery cells undergo an extensive so-called formation procedure after cell assembly, which generally consist of multiple voltage-holds and current-steps at various temperatures.[4,5] During the first cycles of a LiB, the SEI is generated by the reduction products of the commonly used carbonate-based electrolyte components, such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).[6,7] Additionally, electrolyte additives are commonly added to form an SEI layer with increased cycling stability and reduced irreversible capacity loss during the first cycle.[6,8–10] Among a variety of additives, vinylene carbonate (VC) is the most prominent. Since the reduction of VC initiates at higher reduction potentials compared to alkyl carbonate solvents (e.g., EC), it is reduced preferentially during the first charge and thus strongly affects the SEI composition.[6,11] In the present study we investigated the effect of the formation temperature in a range between 10-60 °C on the performance characteristics of full-cells with a graphite anode (MAGE) and an NCM831205 cathode as well as of MAGE/Li half-cells, both with LP572 (1 M LiPF6 in EC:EMC 3:7wt/wt + 2wt% VC) electrolyte. These include the first-cycle irreversible capacity loss, the negative electrode impedance build-up, the gas evolution during formation, the rate capability, and the cycle life.Employing electrochemical impedance spectroscopy using a µ-reference electrode allowed us to determine the graphite intercalation resistance (R Int), representing the sum of the SEI resistance (R SEI) and of the charge transfer resistance (R CT), after two 0.1C formation cycles at a given temperature. The impedance spectrum was recorded at 25 °C and 40% SOC referenced to 190 mAh/gNCM reversible capacity. As depicted in Figure 1, both R Int of the graphite electrode in the MAGE/NCM83125 full-cell and the first-cycle irreversible Li-loss determined a MAGE/Li half-cell increase with increasing formation temperature.Additionally, on-line electrochemical mass spectrometry (OEMS) was applied to identify and quantify the gaseous reduction products for LP572 in a MAGE/NCM831205 full-cell during one 0.1C formation cycle at different temperatures. As displayed in Figure 1, the relative increase of the total gas evolved, which is the sum of hydrogen, ethylene, carbon monoxide, and carbon dioxide, follows the relative increase of the intercalation resistance. Next to the overall gas amounts, changes in the quantity of the individual gases give insights into the highly temperature dependent reduction mechanisms of VC. Acknowledgement: The authors thank BMW AG for their financial support.

Lithium metal has attracted extensive attention as a promising anode material for high-performance lithium batteries. However, an unstable and fragile solid electrolyte interphase (SEI) could be formed by the side reactions between Li metal and liquid electrolytes and induce the formation of dendritic growth. Here, we propose a facile strategy to fabricate fluorine- and nitrogen-donating gel polymer electrolytes (FN-GPEs) via the polymerization of hexafluorobutyl methacrylate (HFBMA) and triazole-based methacrylate (TBMA). A uniform porous structure is formed through the regulation of the ratio of HFBMA and TBMA in the polymer matrix, which is beneficial for absorbing the liquid electrolyte and enhancing the ionic conductivity. More significantly, the FN-GPEs containing abundant fluorinated and nitrided chains could form a stable SEI in situ with the components of LiF and Li3N. This facile strategy can achieve the complementary advantages of LiF- and Li3N-based SEI protective layers and significantly enhance the interfacial stability. Consequently, the FN-GPEs exhibit high porosity, high ionic conductivity, enhanced Li-ion transference number, and a wide electrochemical window, which endow the Li/Li symmetrical cells with excellent cyclic stability, and the LiFePO4/Li batteries show good long-term cycling stability. This work provides a new avenue to construct a stable SEI in situ on Li metal via molecular design to achieve high-performance LMBs.

With the rapid advancement of energy storage technologies and the widespread adoption of electric vehicles (EVs), the demand for high-energy-density rechargeable batteries is growing exponentially. Lithium metal batteries (LMBs) have emerged as promising candidates for next-generation energy storage due to their high specific energy, exceeding 350 Wh kg⁻¹. However, conventional LMBs employing liquid electrolytes suffer from limited cycle life and poor safety, primarily due to non-uniform lithium deposition and dendritic lithium growth. Therefore, constructing a robust and stable solid electrolyte interphase (SEI) is crucial to suppress parasitic side reactions and inhibit dendrite formation. Gel polymer electrolytes (GPEs) have gained attention as an effective strategy, as they can modulate the Li⁺ solvation sheath through interactions with functional groups in the polymer matrix, facilitating the formation of a stable, anion-derived, inorganic-rich SEI. In this study, a multifunctional acrylate crosslinker-based GPE was designed to regulate the Li⁺ solvation environment and promote the formation of a stable SEI. Fourier-transform infrared (FT-IR), Raman, and nuclear magnetic resonance (NMR) spectroscopy revealed a more aggregated solvation configuration of Li⁺ in the GPE. This optimized solvation structure leads to enhanced lithium plating morphology and stable SEI formation, as confirmed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Furthermore, the acrylate-based GPE exhibits a higher Li⁺ transference number than conventional liquid electrolytes. As a result, Li/GPE/LiNi₀.₈Co₀.₁Mn₀.₁O₂ cells achieved superior electrochemical performance, retaining 70% of their initial capacity after 160 cycles at 1.0 C under a high mass loading of 20 mg cm⁻² at 25 °C. These findings underscore the potential of acrylate-based GPEs to enable the practical application and commercialization of high-energy-density LMBs.

Sodium‐metal batteries (SMBs) are regarded as key for next‐generation energy storage due to their high theoretical energy and potential cost‐effectiveness. However, Na‐metal systems remain challenged by several factors, including uncontrollable metallic dendrite growth for liquid systems and unstable solid electrolyte interphase (SEI) with most organic electrolytes, which limit the feasibility of SMBs. Here, the authors detail the interrelated degradation mechanisms. Afterward, the recent advances in improving the electrochemical performance of SMBs are highlighted. The strategies can be subdivided into the following taxonomy: The utilization of three‐dimensional (3D) conductive skeletons; the introduction of protective layers; the development of compatible electrolyte systems; and the employment of alloy anodes. Designing deposition substrates for the “‘hostless”’ Na has a great effect on reducing the current density and buffering the plating‐stripping volume expansion. The artificial interface approach would introduce efficient mechanical modulus or film‐forming type barriers, which can inhibit detrimental parasitic reactions. Improved solvent‐electrolyte combinations and additives tailored for Na will promise to suppress Na dendrite growth by obtaining a more stable SEI. The formation of alloy phase with Na can tune the bulk properties of the metal per se. Finally, the research challenges and possible development directions of SMBs are also discussed.

Considering the usage of smart phones, electrical vehicles, and power sources for grid storage application, lithium ion battery (LIB) operating under harsh circumstances have become a serious issue due to the sluggish ionic conductivity and increased viscosity of electrolyte at low temperatures. Ethylene carbonate (EC) is a widely used electrolyte component of commercial LIB owing to the suitable electrochemical stability window, high dielectric constant, and especially, its ability to form stable solid electrolyte interphase (SEI) on the graphite anode, which prevents continuous electrolyte decomposition and resulted in excellent cycling stability on graphite anode. However, its high freezing point (36℃), greatly increases electrolyte solidification temperature and hurts ionic conductivity at low temperatures, thereby leading to a narrow operation temperature range for LIB using EC-based electrolytes. Recently, plenty of efforts have been made to extend LIB operating temperature range, using techniques such as self-heating method with extra Ni-foil in specially designed cell configuration1, or electrolyte modifications by introducing low melting point solvents to increase ionic conductivity at low temperatures. Electrolyte modification is the most effective approach due to its low cost, high flexibility, and convenience. It can easily alter electrolyte physical and chemical properties, affect SEI chemical compositions, and result in positive impact on battery performance. Particularly, SEI is the interphase between anode and electrolyte, which allows Li ion transportation and insulates electron transfer, protects anode surface and prevents electrolyte decomposition. Many different research groups have made a lot of progress of electrolyte modification, including adjusting solvent salt combinations, fluorinating of solvents, and developing novel additives.2 Nevertheless, there are still many challenges need to be addressed for stable low temperature operation.Herein, considering the research importance and current progress of low temperature electrolyte, we screened several organic compounds as co-solvents or additives to improve low temperature performance while keeping other properties of current LIB. For example, propylene carbonate (PC) has comparable dielectric constant and electrochemical window as EC, and much lower freezing point (-49℃), making it very promising candidate for low temperature electrolyte.3 Unfortunately, PC cannot be used as single solvent for LIBs because it usually co-intercalates into graphite layer and cause graphite exfoliation. In order to prevent co-intercalation of PC, a stable SEI needs to be formed. We developed several new electrolytes using different salts, solvents and additives, which have higher reduction potential than PC intercalation voltage and enable stable SEI formation, making PC-based low temperature electrolyte application possible. In addition, advanced characterization techniques were used to characterize the interphase properties to reveal underlying mechanism of improved electrochemical performances by modifying electrolyte composition. These results will be presented at meeting.Reference C.-Y. Wang, Nature, 2016, 529, 515.G. Zhu, Journal of Power Sources, 2015, 300, 29-40.Q. Li, Applied Materials & Interfaces, 2017, 9, 42761-42768. AcknowledgementThe work carried out at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the US Department of Energy through the Applied Battery Research for Transportation (ABRT) program.

A stable solid-electrolyte interphase (SEI) is crucial for cycling reversibility of Na-ion batteries by mitigating continuous side reactions. So far, the severe SEI dissolution leads to low Coulombic efficiency (CE) and short cycle life. Meanwhile, the quantified relationship between SEI components and their solubility remains unclear. In this work, we establish the direct correlation between SEI components and SEI solubility, and quantify that the solubility of organic-rich SEI is 3.26 times of inorganic-rich SEI. We further propose a feasible strategy to preform inorganic-rich insoluble SEI and demonstrate a practical hard carbon (HC)||NaMn0.33Fe0.33Ni0.33O2 full cell in a commercial electrolyte of 1 M NaPF6 in propylene carbonate (PC) with 80.0 % capacity retention for 900 cycles, and achieve a record-high average CE of 99.95 % for a practical Na-ion full cell. This study provides an effective strategy of preforming insoluble SEI to suppress its dissolution towards highly reversible Na-ion batteries.