Fluoroethylene Carbonate Research Articles

Ultrathin solid polymer electrolyte enabling mechanically-strong energy storing fiber reinforced composites

Structural batteries attract enormous research interest due to their advantages of integrated energy storage function in structure. Superior to the co-cured composite structural batteries based on glass fiber supported/reinforced liquid/low-strength polymer electrolyte, enhanced mechanical strength of solid polymer electrolyte would enable the facile fabrication of composite structural batteries through layer-by-layer stack up of composites and solid-state batteries. This work encloses a mechanically strong and self-supportive solid polymer electrolyte with a tensile strength of 9.28 MPa, significantly superior to the traditional PEO (Lithium salt) electrolyte. Systematic analysis of phase-microstructure-mechanical/electrochemical properties relationship of SPE with different fluoroethylene carbonate (FEC) amounts helps to find out a solid polymer electrolyte (SPE) with satisfactory ionic conductivity (0.35 × 10−4 S/cm) and high electrochemical stability window (4.6 V vs Li/Li+), based on which dimension-adaptable and mechanically-strong energy storing laminates which have high tensile strength of 343 MPa, tensile modulus of 30.6 GPa, flexural strength of 308 MPa, and high impact resistance have been successfully fabricated. The most surprising result is a capacity retention of as high as 67% of the original capacity in the 1st cycle and stable cycling of composite structural batteries over 50 cycles for the failed tensile sample, which points out its potential application in emergency rescue. We believe this work not only presents a facile method to fabricate solid polymer electrolytes but demonstrates the potential of this solid polymer electrolyte in facile fabrication of mechanically-strong composite structural batteries.

Boosting interfacial kinetics in extremely fast rechargeable Li-ion batteries with linear carbonate-based, LiPF6-concentrated electrolyte

Amidst the surging demand for battery-powered automobiles, it is crucial to tackle the safety risks of Li plating triggered by high cell polarization to achieve extremely fast charging (XFC) of Li-ion batteries. This study explores the impact of Li+ desolvation and solid-electrolyte interphase (SEI) chemistry on cell polarizations by utilizing linear carbonate (LC)-based, LiPF6-concentrated electrolytes (LPCEs). In the LC family, dimethyl carbonate (DMC) is thermodynamically preferred to facilitate desolvation kinetics, thereby lowering the charge-transfer barrier at the graphite anode. For effective graphite passivation and faster Li+ diffusion crossing the SEI, fluoroethylene carbonate (FEC) can help build up a thin and fluorinated SEI and reinforce the XFC cycling stability of graphite||NMC622 full cells (3.0 mAh cm−2; N/P ratio = 1.1), exhibiting 94.3% capacity retention over 500 cycles under a 10-min charging condition. The excellent XFC performance is practically validated using a 1.2-Ah pouch cell, demonstrating three times higher capacity retention over 200 cycles while suppressing Li plating-triggered cell swelling compared to conventional electrolytes. Unraveling the cell polarization governed by electrolyte chemistry provides valuable insights regarding future electrolyte designs for improving the XFC capabilities of Li-ion batteries.

Domino Reactions Enabling Sulfur-Mediated Gradient Interphases for High-Energy Lithium Batteries.

Silicon (Si)-based anodes are currently considered a feasible solution to improve the energy density of lithium-ion batteries owing to their sufficient specific capacity and natural abundance. However, Si-based anodes exhibit low electric conductivities and large volume changes during cycling, which could easily trigger continuous breakdown/reparation of the as-formed solid-electrolyte-interphase (SEI) layer, seriously hampering their practical application in current battery technology. To control the chemoelectrochemical instability of the conventional SEI layer, we herein propose the introduction of elemental sulfur into nonaqueous electrolytes, aiming to build a sulfur-mediated gradient interphase (SMGI) layer on Si-based anodes. The SMGI layer is generated through the domino reactions (i.e., electrochemical cascade reactions) involving the electrochemical reductions of elemental sulfur followed by nucleophilic substitutions of fluoroethylene carbonate, which endows the corresponding SEI layer with strong elasticity and chemomechanical stability and enables rapid transportation of Li+ ions. Consequently, the prototype Si||LiNi0.8Co0.1Mn0.1O2 cells attain a high-energy density of 622.2 W h kg-1 and a capacity retention of 88.8% after 100 cycles. Unlike previous attempts based on sophisticated chemical modifications of electrolyte components, this study opens a new avenue in interphase design for long-lived and high-energy rechargeable batteries.

Dual-Salts Electrolyte with Fluoroethylene Carbonate Additive for High-Voltage Li-Metal Batteries

The combination of Li-metal anode and high-voltage cathode is regarded as a solution for the next-generation high-energy-density secondary batteries. However, a traditional electrolyte is either incompatible with the Li-metal anode or vulnerable to high voltage. This work reports a 1 M dual-salts Localized-High-Concentration-Electrolyte with Fluoroethylene carbonate (FEC) additive. It enables stable cycling of Li||LiNi0.8Co0.1Mn0.1O2 (NMC811) battery, which shows 81.5% capacity retention after 300 cycles with a charge/discharge current density of 1 C and a voltage range of 2.7–4.4 V. Scanning electron microscopy (SEM) images show that this electrolyte not only largely reduced Li dendrites and ‘dead’ Li on anode surface but also well protected the microstructure of NMC811 cathode. Possible components of both solid-electrolyte interlayer (SEI) and cathode-electrolyte interlayer (CEI) were characterized by energy-dispersive X-ray spectroscopy (EDX). The result illustrates that FEC protected Li salts from decomposition on the anode side and suppressed the decomposition of solvents on the cathode side.

Effects of Me-Solvent Interactions on the Structure and Infrared Spectra of MeTFSI (Me = Li, Na) Solutions in Carbonate Solvents-A Test of the GFN2-xTB Approach in Molecular Dynamics Simulations.

We investigated the performance of the computationally effective GFN2-xTB approach in molecular dynamics (MD) simulations of liquid electrolytes for lithium/sodium batteries. The studied systems were LiTFSI and NaTFSI solutions in ethylene carbonate or fluoroethylene carbonate and the neat solvents. We focused on the structure of the electrolytes and on the manifestations of ion-solvent interactions in the vibrational spectra. The IR spectra were calculated from MD trajectories as Fourier transforms of the dipole moment. The results were compared to the data obtained from ab initio MD. The spectral shifts of the carbonyl stretching mode calculated from the GFN2-xTB simulations were in satisfactory agreement with the ab initio MD data and the experimental results for similar systems. The performance in the region of molecular ring vibrations was significantly worse. We also found some differences in structural data, suggesting that the GFN2-xTB overestimates interactions of Me ions with TFSI anions and Na+ binding to solvent molecules. We conclude that the GFN2-xTB method is an alternative worth considering for MD simulations of liquids, but it requires testing of its applicability for new systems.

New Insight into the Role of Fluoro-ethylene Carbonate in Suppressing Li-Trapping for Si Anodes in Lithium-Ion Batteries

New Insight into the Role of Fluoro-ethylene Carbonate in Suppressing Li-Trapping for Si Anodes in Lithium-Ion Batteries

Fluorinated High-Voltage Electrolytes To Stabilize Nickel-Rich Lithium Batteries.

As state-of-the-art (SOA) lithium-ion (Li-ion) batteries approach their specific energy limit (∼250 Wh kg-1), layer-structured, nickel-rich (Ni-rich) lithium transition metal oxide-based cathode materials, e.g., LiNi0.8Mn0.1Co0.1O2 (NMC811), have attracted great interest owing to their practical high specific capacities (>200 mAhg-1). Coupled with their high average discharge voltages (∼4 V vs Li/Li+), Ni-rich cathode-based lithium batteries possess a great potential to achieve much higher specific energies (>350 Wh kg-1 at the cell level) than the SOA Li-ion counterparts. In addition, Ni-rich oxides are low-cost battery cathode materials due to their low cobalt contents. However, Ni-rich cathode-based lithium batteries suffer quick capacity degradations upon cycling, particularly at high upper cutoff voltages (e.g., ≥4.5 V vs Li/Li+), due to crystal structure changes of the active cathode materials and parasitic side reactions at the electrolyte/electrode interfaces. In this study, a fluorinated-solvent-based, high-voltage stable electrolyte (HVE), i.e., 1 M Li bis(trifluoromethanesulfonyl)imide (LiTFSI) in fluoroethylene carbonate (FEC), bis(2,2,2-trifluoroethyl) carbonate (FDEC), and methyl (2,2,2-trifluoroethyl) carbonate (FEMC) with Li difluoro(oxalate)borate (LiDFOB) additive, was formulated and evaluated in Li/NMC811 battery cells. To the best of our knowledge, this class of electrolyte has not been investigated for Ni-rich cathode-based lithium batteries. Li/NMC811 cells with HVE exhibited a superior long-term cycle performance stability, maintaining ∼80% capacity after ∼500 cycles at a high cutoff voltage of 4.5 V (vs Li/Li+) than a baseline carbonate-solvent-based electrolyte (BE). The superior cycle stability of the Li/NMC811 cells is attributed to the inherently high-voltage stability of HVE, supported by the physical and electrochemical analyses. This conclusion is supported by our density functional theory (DFT) modeling where HVE shows a less tendency of deprotonation/oxidation than BE, leading to the observed cycle stability. The findings in this study are important to help tackle the technical challenges facing Ni-rich cathode-based lithium batteries to realize their high energy density potentials with a long cycle life.

Co-Intercalation-Free Ether-Based Weakly Solvating Electrolytes Enable Fast-Charging and Wide-Temperature Lithium-Ion Batteries.

Ether-based electrolytes are competitive choices to meet the growing requirements for fast-charging and low-temperature lithium-ion batteries (LIBs) due to the low viscosity and low melting point of ether solvents. Unfortunately, the graphite (Gr) electrode is incompatible with commonly used ether solvents due to their irreversible co-intercalation into Gr interlayers. Here, we propose cyclopentyl methyl ether (CPME) as a co-intercalation-free ether solvent, which contains a cyclopentane group with large steric hindrance to obtain weakly solvating power with Li+ and a wide liquid-phase temperature range (-140 to +106 °C). A weakly solvating electrolyte (WSE) based on CPME and fluoroethylene carbonate (FEC) cosolvents can simultaneously achieve fast desolvation ability and high ionic conductivity, which also induces a LiF-rich solid electrolyte interphase (SEI) on the Gr anode. Therefore, the Gr/Li half-cell with this WSE can deliver outstanding rate capability, stable cycling performance, and high specific capacity (319 mAh g-1) at an ultralow temperature of -60 °C. Furthermore, a practical LiFePO4 (loading ≈25 mg cm-2)/Gr (loading ≈12 mg cm-2) pouch cell with this WSE also reveals outstanding rate capability and stable long-term cycling performance above 1000 cycles with a high Coulombic efficiency (≈99.9%) and achieves an impressive low-temperature application potential at -60 °C.

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.

Understanding the Electrolyte Chemistry Induced Enhanced Stability of Si Anodes in Li-Ion Batteries based on Physico-Chemical Changes, Impedance, and Stress Evolution during SEI Formation

The volume expansion/contraction of Si-based anodes during electrochemical lithiation/delithiation cycles causes a loss in mechanical integrity and accrued instability of the solid electrolyte interphase (SEI) layer, culminating into capacity fade. Electrolyte additives like fluoroethylene carbonate (FEC) improve SEI stability, but the associated causes still under debate. This work reveals some of the roles of FEC via post-mortem observations/analyses, operando stress measurements and a comprehensive study of the impedance associated with the formation/evolution of SEI during lithiation/delithiation. Usage of 10 vol.% FEC as electrolyte additive leads to significant improvements in cyclic stability, Coulombic efficiency and facilitates smoother/compact/crack-free surface/SEI, in contrast to the cracked/pitted/uneven surface upon non-usage of FEC. Operando stress measurements during SEI formation reveal compressive stress development, followed by loss in mechanical integrity, upon non-usage of electrolyte additive, in contrast to insignificant stress development associated with SEI formation upon usage of FEC. The EIS model proposed here facilitates good fit with the impedance data at all states-of-charges, with the SEI resistance and capacitance exhibiting expected variations with cycling and the SEI resistance progressively decreasing with cycle number in the presence of FEC. By contrast, in the absence of FEC, severe fluctuations observed with the SEI resistance and capacitance indicate instability.

Deep eutectic solvent with film-forming fluoroethylene carbonate as a nonflammable electrolyte for lithium metal batteries

Nonflammable deep eutectic solvents (DESs) are potential electrolytes for Li metal batteries. However, their practical application is prevented by their degradation with Li metal at low voltages. Here, we use fluoroethylene carbonate (FEC), a film-forming compound, as a cosolvent in DES-based electrolytes, demonstrating that compatibility with Li metal can be improved due to the formation of a robust and compact solid electrolyte interphase layer on the anode surface. The optimal electrolyte (50%DES) exhibits the Li+ conductivity of 1.58 mS cm−1 at room temperature. After the introduction of 50%FEC, the DES electrolyte shows a considerable reduction in viscosity from 1206.3 mPa s−1 to 20.4 mPa s−1 while retaining high flame-retardant efficiency. The Li/LiFePO4 cell with 50%DES electrolyte exhibits a reversible discharge capacity of 140.7 mAh g−1 at 50 mA g−1 and a capacity retention of 94.3 % after 200 cycles. The good cycling stability of the cell in 50%DES electrolyte can be attributed to the formation of a robust SEI layer containing inorganic-rich compounds (LiF and Li3N) with rapid Li+ transport on the Li metal anode that can improve interfacial stability and suppress Li dendrite growth.

Thick Architected Silicon Composite Battery Electrodes Using Honeycomb Patterned Carbon Nanotube Forests

To meet the growing performance demands for personal electronics and electric vehicles the energy density of lithium-ion batteries can be increased by incorporating thicker electrodes. We present thick “honeycomb” electrodes based on patterned, vertically aligned carbon nanotubes (CNTs) on Cu foils. Thick electrodes are created by Si deposition on >100 μm tall honeycomb patterned CNTs. Si-CNT electrodes are cycled in half-cells, demonstrating electronic connection between the Si and Cu foil via the aligned CNTs. For ~4.7 mAh cm−2 capacity the honeycomb patterning improves capacity retention (78%) over 30 cycles compared to non-patterned electrodes (58%). We attribute this improvement to the honeycomb’s ability to accommodate Si expansion, thereby reducing cracking that causes active material loss and solid electrolyte interphase instability, and to provide pathways for Li-ion transport into the electrode. The Si-CNT electrode capacity is further increased to 20 mAh cm−2 by increasing the Si loading. Finally, a fluoroethylene carbonate containing electrolyte is used to increase cell lifetime. Here, the honeycomb electrodes have a higher areal (~10.2 mAh cm−2) and retained (65%) capacity over 180 cycles, and exhibit superior rate performance to their non-patterned counterparts. Our work demonstrates the role of patterning in enabling aligned CNTs as a robust template for thick battery electrodes.

Transition from Vogel-Fulcher-Tammann to Arrhenius Ion-Conducting Behavior in Poly(Ethyl Acrylate)-Based Solid Polymer Electrolytes via Succinonitrile Plasticizer Addition

Solid polymer electrolytes (SPEs) offer potential advantages over liquid electrolytes, including flexibility, safety, and processability. However, they suffer from low room-temperature ionic conductivity. Recently, it has been reported a poly(ethyl acrylate) based (polyEA) SPE, by incorporating 50 wt% of succinonitrile (SN) solid plasticizer, 30 wt% of lithium salt and 5 wt% of fluoroethylene carbonate additive, which achieves a high room-temperature ionic conductivity of 1.01 × 10−3 S cm−1 (Nat. Nanotechnol, 2022, 17, 768-776). This novel SPE exhibits stability against Li0 and anodic stability up to 4.9 V vs Li+/Li. However, the specific mechanism responsible for its high ionic conductivity remains elusive. In this work, by adjusting the weight ratio of SN in the SPE, a transition from Vogel-Fulcher-Tammann to Arrhenius ion-conducting behavior is observed. It is demonstrated that the addition of SN leads to the gradual decoupling of Li-ion from the polymer backbone and its coordination with SN, as revealed by 6Li solid-state nuclear magnetic resonance spectroscopy. As a result, Li-ion migration primarily occurs through SN rather than the segmental motion of the polymer backbone. Performances of the SPE in Cu||Li, Li||Li and a LiFePO4||Li pouch cells are shown to demonstrate the commercial viability of this SPE in Li0-anode solid-state batteries.

Rethinking the Role of Formerly Sub-Sufficient Industrial/Synthesized SEI Additive Compounds - a New Perspective

In order to improve the performance of lithium-ion batteries (LIBs), novel electrolytes are of primary importance. Recently, fluorinated cyclic phosphazene derivatives in combination with fluoroethylene carbonate (FEC) are mentioned in the literature as a promising electrolyte additive combination, which can decompose to form a dense, uniform, and thin protective layer on the surface of the anode and cathode electrode.[1,2] Additionally, suppressing further electrolyte decomposition and electrode corrosion, thus protecting the structural destruction of the electrodes, are mentioned within this electrolyte composition.[1–3] Furthermore, galvanostatic charge and discharge experiments with different cell composition materials demonstrate that fluorinated cyclic phosphazene compounds as additional additive material tend to improve cycling stability.[1,3,4] Although the electrochemical aspects of cyclic fluorinated phosphazene compounds combined with FEC are briefly introduced, it is still not fully clear how these two compound classes interact constructively during operation mode. Thus, the positive synergistic effect of FEC/Hexafluorocyclotriphosphazene (HFPN)-derivatives on the electrochemical performance during cell operation is not enlightened. The focus of this study is to investigate the complementary effect of FEC and ethoxy(pentafluoro)cyclotriphosphazene (EtPFPN) as additive compounds in an aprotic organic electrolyte in LiNi0.5Co0.2Mn0.3O (NCM523) SiOx/C full cells. Furthermore, the formation mechanism of lithium ethyl methyl carbonate (LEMC)-EtPFPN interfacial products and the reaction mechanism of lithium alkoxide with EtPFPN are proposed and supported with DFT measurements. Additionally, a new effect of FEC regarding the SEI formation will be introduced. The EtPFPN decomposition compounds in the electrolyte after the SEI formation have been investigated via gas chromatography-mass spectrometry (GC-MS) and gas chromatography-high resolution mass spectrometry (GC-HRMS). The electrode electrolyte interface investigation of the SEI has been performed via in-situ shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) and scanning electron microscopy (SEM). Constant current cycling is conducted, and in-situ Raman measurements characterize the deposition of electrolyte components and LEMC-EtPFPN traces on the SiOx/C anode material during the SEI formation. Finally, the interplay between EC, EMC, Li-alkoxide, LEMC, FEC, and EtPFPN has been visualized schematically via a reaction mechanism postulated based on analytical data of the electrolyte.[1] A. Ghaur, C. Peschel, I. Dienwiebel, L. Haneke, L. Du, L. Profanter, A. Gomez‐Martin, M. Winter, S. Nowak, T. Placke, Adv Energy Mater 2023, 2203503.[2] J. Liu, X. Song, L. Zhou, S. Wang, W. Song, W. Liu, H. Long, L. Zhou, H. Wu, C. Feng, Z. Guo, Nano Energy 2018, 46, 404–414.[3] Q. Liu, Z. Chen, Y. Liu, Y. Hong, W. Wang, J. Wang, B. Zhao, Y. Xu, J. Wang, X. Fan, L. Li, H. bin Wu, Energy Storage Mater 2021, 37, 521–529.[4] Y.-H. Liu, M. Okano, T. Mukai, K. Inoue, M. Yanagida, T. Sakai, J Power Sources 2016, 304, 9–14.

An Exploration of the Sodium Solid Electrolyte Interphase in Glyme Electrolytes with Carbonate Additives

The utilization of alkali metal anodes is hindered by an inherent instability in organic electrolytes. Sodium (Na) is of growing interest due to its high natural abundance, but the carbonate electrolytes that are popular in lithium systems are unable to form a stable solid electrolyte interphase (SEI) with a sodium metal electrode. However, the glyme (chain ether) electrolytes produce thin, predominantly inorganic SEI at sodium metal interfaces. Using half-cell and symmetric cell analysis, we identify diglyme (G2) as the best performing of the glymes, balancing the high nucleation barrier of the short glyme (G1) and the high plateau overpotential of the long glyme (G4). Based on mesoscale modeling, we capture the critical dependence of the nucleation and early-stage growth morphology on a competing set of mechanisms, including the reduction kinetics on the substrate/newly deposited Na and the surface mobility of these deposits. Through in situ optical microscopy, the onset and growth of Na dendrites is revealed in glyme electrolytes, and the addition of small quantities (~10% volume/volume) of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) to G2 is shown to facilitate uniform sodium plating characteristics in the optical cell, presumably through alterations to SEI composition. X-ray photoelectron spectroscopy (XPS) analysis reveals that the FEC additive results in an SEI with similar atomic composition to that formed in G2 alone, whereas addition of EC to G2 results in an entirely different SEI composition, despite the molecular similarity of the carbonate additives. We have determined that the SEI formed by glyme alone may not support extensive or extreme cycling conditions, but the addition of FEC provides a much more robust SEI at the Na metal surface to facilitate numerous consistent sodium plating and stripping cycles.

Multifunctional Additive Ethoxy(pentafluoro)Cyclotriphosphazene Enables Safe Carbonate Electrolyte for SiOx-Graphite/NMC811 Batteries

The silicon or silicon monoxide (SiO x )-graphite/nickel-rich (≥80% Ni) LiNi x Mn y Co z O2 (x+y+z = 1, NMC) cell chemistry is widely regarded as one of the most promising chemistries for next-generation electric vehicle batteries for due to its higher energy density compared to state-of-the-art graphite/Ni-rich NMC batteries. However, the drastic volume change during lithiation and delithiation of silicon poses a significant challenge to realize its high capacity with a practical cycle life. Besides performance degradation, it is desirable to address safety risks stemming from, e.g., the high flammability of carbonate solvents and oxygen release from highly delithiated Ni-rich NMC materials.To address these problems, one of the most effective methods is to develop advanced electrolytes that combine flame-retardant properties and excellent electrochemical performance. Generally, in consideration of the balanced trade-offs between cost, scalability, safety, and electrochemical compatibility, the use of flame retardants is a very promising approach to realize the next-generation nonflammable electrolytes.However, it is still a huge challenge to optimize the electrolyte composition to stabilize Si-based and Ni-rich lithium-ion batteries.Here, the compatibility of the flame retardant ethoxy(pentafluoro)cyclotriphosphazene (PFPN) as additive for carbonate electrolytes is studied, for the first time, in the SiO x -graphite/NMC811 full cells. [1] A synergistic effect is found between PFPN and lithium hexafluorophosphate (LiPF6) for the flame retardancy, and a modified electrolyte (1.35M LiPF6 in ethylene carbonate (EC):ethyl methyl carbonate (EMC) (3:7 by vol.) with 10 wt.% fluoroethylene carbonate (FEC) and 5 wt.% PFPN) displays a very low self-extinguishing time of only 3 s/g while also maintaining a high ionic conductivity of 8.4 mS/cm at 25 °C. Furthermore, the morphology evolution and interphase composition of both anode and cathode are investigated, gaining important insights into the decomposition mechanism of PFPN to stabilize the electrode-electrolyte interfaces. Meanwhile, the wetting properties of the modified electrolyte are also improved due to the excellent wettability of PFPN. As a result, half and full coin cells with SiO x -graphite anode and NMC811 cathode display better capacity retention during long-term cycling with the modified electrolyte than with 1M LiPF6 in EC:EMC (3:7 by vol.) with 10 wt.% FEC. In addition, the enhanced cycling performance with the modified PFPN-contained electrolyte is also confirmed for 1 Ah SiO x -graphite/NMC811 pouch cells cycled under various pressures. These results open a promising avenue for the development of safe and high-performance electrolytes for silicon-based lithium-ion batteries.

Investigation of Gas Evolution by Operando GC/MS from SEI Forming Additives in Lithium-Ion-Batteries

Due to the chemical reactivity of the electrolyte, electrolyte decomposition often occurs at the electrode-electrolyte interface during cycling, which can lead to a loss in LIB performance. On the anode side, however, the formation of a stable Solid Electrolyte Interphase (SEI) is essential for the safe operation of Lithium-ion batteries (LIBs). The SEI layer, which is permeable for Li+ ions, protects the electroactive materials from direct contact with the electrolyte, thereby inhibiting further electrolyte decomposition. In the formation cycles of fresh cells, electrolyte degradation is primarily attributed to the SEI formation processes at the anode. Several literature reports have confirmed that the newly formed interphase consists of organic and inorganic decomposition products of the electrolyte.1 To increase battery performance, cycle life and safety, it is essential to form a stable SEI.2 A common strategy to stabilize the SEI is to use additives in the electrolyte composition. The additives should exhibit higher reduction potentials than other electrolyte components to enable preferential reduction and incorporation into the SEI. The most common SEI forming additives are fluoroethylene carbonate (FEC) and vinylene carbonate (VC). They both decompose during the formation cycle to form a polymeric layer at the electrode surface.The electrolyte decomposition reactions which lead to SEI formation are usually accompanied by gaseous side products. The evolving gas species form a complex gas mixture which can be separated by gas chromatography (GC) into their individual components and subsequently identified by mass spectrometry (MS). While operando GC/MS gives information on the volatile side products of the electrolyte decomposition reactions, X-ray photoelectron spectroscopy (XPS) can be used to identify the degradation products that form on the surface of the electrode and are incorporated into the SEI. The combination of the two techniques allows a more comprehensive picture of SEI formation processes to be drawn.In this work, operando GC/MS of Lithium-ion full cells with NMC 811 cathodes and graphite anodes was used to compare the gas species evolving during the formation reactions in an electrolyte with the composition of 1M LiPF6 in EC/DEC 1:1 with FEC and VC electrolyte additives in a ratio of 1w%, respectively. In a comparative study we present the decomposition pathways of the electrolytes with and without SEI forming additives by analysing the composition of the gas phase as a function of cell potential. We observe that the complex gas mixture consists of carbonate components originating from transesterification reactions and evaporation of the volatile electrolyte. In addition, inorganic components such as carbon monoxide and carbon dioxide are produced, which mainly stem from the degradation of cyclic carbonates. Furthermore, the gas phase consists of saturated and unsaturated hydrocarbons which can be ascribed to the linear carbonate components as well as ether and carbonyl components. We have also observed that it is possible to distinguish between volatile compounds which form during the charge and discharge processes, respectively. The increased gas evolution at the beginning of the formation cycle confirms that a substantial part of the passivation layer is formed during charge. Finally, operando GC/MS was also used to monitor the evolution of the decomposition products during battery operation at overcharge conditions.Acknowledgement:The author gratefully acknowledges the FFG (Austrian Research Promotion Agency) for funding this research within project No. 879613

Insights into Electrolyte Reactivity and Solid Electrolyte Interphase Formation at the Li Metal Anode Surface from DFT Simulations

Solid electrolyte interphase (SEI) is a critical component of lithium ion batteries, yet is poorly characterized and not well understood. We use plane wave density functional theory simulations to perform a comprehensive study of the degradation reaction mechanisms of four electrolyte solvents and additives, ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and vinylene carbonate (VC), at the Li (001) metal anode surface. The calculations provide fundamental insights into electrolyte decomposition and spontaneous SEI formation. Utilizing automated workflows implemented in the Schrödinger Materials Science Suite, we compute the adsorption energies and calculate the reaction energies for each decomposition pathway. Furthermore, we employ nudged elastic band calculations to compute the decomposition reaction barriers and provide mechanistic insights into the onset of SEI formation. We analyze trends in decomposition reaction energies and the relationships between the reaction energies and the activation barriers for the various electrolyte molecules as a function of applied bias potential. We find that the preferred decomposition pathways are different for the solvent molecules (EC and PC) than for the additives (FEC and VC). Calculations also suggest that applied bias reduces decomposition reaction barriers thus enhancing SEI formation. Such a comprehensive dataset of reaction energies and activation barriers provides useful benchmarks for the development and validation of reactive machine learned force fields for modeling advanced battery chemistries at a larger scale.

Co-Intercalation-Free Fluorinated Ether Electrolytes for Lithium-Ion Batteries

Lithium-ion batteries are widely used to power portable electronics because of their high energy densities and have shown great promise in enabling the electrification of transport. However, the commercially used carbonate-based electrolytes are limited by a narrow operating temperature window and suffer against next generation lithium-ion battery chemistries such as silicon-containing anodes. The lack of non-carbonate electrolyte alternatives such as ether-based electrolytes is due to undesired solvent co-intercalation that occurs with graphitic anodes. Recently, fluorinated ether solvents have become promising electrolyte solvent candidates for lithium metal batteries but their applications in other battery chemistries have not been studied. In this work, we synthesize a group of novel fluorinated ether solvents and study them as electrolyte solvents for lithium-ion batteries. Using X-ray diffraction (XRD) and solid-state nuclear magnetic resonance (ssNMR), we show that fluorinated ether electrolytes support reversible lithium-ion intercalation into graphite without solvent co-intercalation at conventional salt concentrations. To the best of our knowledge, they are the first class of ether solvents that intrinsically suppress solvent co-intercalation without the need for high or locally high salt concentration. In full cells using graphite anode, fluorinated ether electrolytes enable much higher energy densities compared to conventional glyme ethers, and better thermal stability over carbonate electrolytes (operation up to 60°C). As single-solvent-single-salt electrolytes, they remarkably outperform carbonate electrolytes with fluoroethylene carbonate (FEC) and vinylene carbonate (VC) additives when cycled with graphite-silicon composite anodes. Using X-ray photoelectron spectroscopy (XPS), NMR and density functional theory (DFT) calculations, we show that fluorinated ethers produce a solvent-derived solid electrolyte interphase, which is likely the key to suppressing solvent co-intercalation. Rational molecular design of fluorinated ether solvents will produce novel electrolytes that enable next generation lithium-ion batteries with higher energy density and wider working temperature window.

Formation and Suppression of Toxic Organofluorophosphates in Lithium Ion Batteries: Making the High-Voltage Additive Lithium Difluorophosphate Viable for Commercial Applications

Organofluorophosphates (OFPs) are one of the major concerns regarding the safety of Lithium Ion Batteries (LIBs) due to their neurotoxic properties. The formation through the hydrolysis of the conducting salt LiPF6 and subsequent reactions (reductive/oxidative) with the cyclic and linear carbonates is a major drawback in terms of commercial application of LIBs. The additive Lithium Difluorophosphate, which is literature known and beneficial for high-voltage LIBs due to its transition metal (TM) capturing abilities, accelerates the formation of toxic OFPs even further as it also is a hydrolysis product of LiPF6. The suppression of the formation of toxic OFPs through a synergetic dual additive approach with fluoroethylene carbonate (FEC) is presented in this study. Here, the thermal- and electrochemical formation of OFPs are successfully inhibited with the addition of 2 wt.% of FEC to the electrolyte containing LiDFP (1 wt.%). Characterization of OFPs is conducted via high performance liquid chromatography mass spectrometry and the toxicological capabilities of the aged electrolyte is assed through an inhibition-evaluation of Acetylcholinesterase lyophilizate. Combined, these two methods illustrate the successful inhibition of OFP formation in FEC/LiDFP dual-additive electrolytes. Further, this dual-additive approach does not deteriorate the performance of LiDFP as single additive making the FEC/LiDFP additive combination a possible new benchmark for high-voltage LIBs.