Addition Of Fluoroethylene Carbonate Research Articles

Interfacial deterioration in highly fluorinated cation-disordered rock-salt cathode: Carbonate-based electrolyte vs. ether-based electrolyte

Cation-disordered rock-salts (DRX) emerge as an intriguing class of high-capacity cathode materials, garnering significant attention in the development of advanced energy-storage systems. Fluorination strategies have been raised to regulate redox and structure for better DRX electrochemical performance. However, research on the interfacial processes during fluorinated DRX cycling remains limited. Here, we evaluate a highly fluorinated DRX, Li2Mn2/3Nb1/3O2F (LMNOF), in different electrolytes to correlate its cycling performance with distinct interfacial evolutions. During cycling in the carbonate-based electrolyte, a thick Mn-containing CEI forms on the LMNOF surface, leading to gradual capacity decay. Intriguingly, the addition of fluoroethylene carbonate (FEC) would bring about surface oxygen loss, Mn-ion dissolution and formation of a fluorinated surface layer by reacting with the LMNOF surface, which critically deteriorate the cycling performance. In comparison, the ether-based electrolyte is compatible with the LMNOF surface, resulting in a more favorable cycling stability. This work sheds light on the interfacial deterioration mechanism of LMNOF cathode in traditional carbonate-based electrolyte, emphasizing the importance of using advanced non-reactive electrolytes to enhance DRX performance.

Comparative Investigation of Water-Based CMC and LA133 Binders for CuO Anodes in High-Performance Lithium-Ion Batteries.

Transition metal oxides are considered to be highly promising anode materials for high-energy lithium-ion batteries. While carbon matrices have demonstrated effectiveness in enhancing the electrical conductivity and accommodating the volume expansion of transition metal oxide-based anode materials in lithium-ion batteries (LIBs), achieving an optimized utilization ratio remains a challenging obstacle. In this investigation, we have devised a straightforward synthesis approach to fabricate CuO nano powder integrated with carbon matrix. We found that with the use of a sodium carboxymethyl cellulose (CMC) based binder and fluoroethylene carbonate additives, this anode exhibits enhanced performance compared to acrylonitrile multi-copolymer binder (LA133) based electrodes. CuO@CMC electrodes reveal a notable capacity ~1100 mA h g-1 at 100 mA g-1 following 170 cycles, and exhibit prolonged cycling stability, with a capacity of 450 mA h g-1 at current density 300 mA g-1 over 500 cycles. Furthermore, they demonstrated outstanding rate performance and reduced charge transfer resistance. This study offers a viable approach for fabricating electrode materials for next-generation, high energy storage devices.

The Impact of Fluoroethylene Carbonate Additive on Charged Sodium Ion Electrodes/Electrolyte Reactivity Studied Using Accelerating Rate Calorimetry

The effects of fluoroethylene carbonate (FEC) electrolyte additive on charged sodium ion electrode/electrolyte reactivity at elevated temperatures were investigated using accelerating rate calorimetry (ARC). The beneficial effect of FEC on cell lifetime was demonstrated using Na0.97Ca0.03[Mn0.39Fe0.31Ni0.22Zn0.08]O2 (NCMFNZO)/hard carbon (HC) pouch cells first prior to ARC measurements. Electrodes from these pouch cells were utilized as sample materials and 1.0 m NaPF6 in propylene carbonate (PC):ethyl methyl carbonate (EMC) (1:1 by vol.) was chosen as control electrolyte. Adding 2wt.% or 5wt.% FEC to the electrolyte does not significantly affect the reactivity of de-sodiated NCMFNZO compared to the control electrolyte (Figure A). However, the addition of FEC obviously changed the reactivity between sodiated HC and electrolytes, especially by showing a suppression on the exothermal behavior between 160°C and 230°C (Figure B). These results give a head to head comparison of the reactivity of FEC additive containing electrolytes with charged sodium ion electrode materials at elevated temperatures and show that the use of FEC at additive levels should not compromise the cell safety when extending cell lifetime. Figure 1

Influence of Vinylene Carbonate and Fluoroethylene Carbonate on Open Circuit and Floating SoC Calendar Aging of Lithium-Ion Batteries

The purpose of this study was to investigate the calendar aging of lithium-ion batteries by using both open circuit and floating current measurements. Existing degradation studies usually focus on commercial cells. The initial electrolyte composition and formation protocol for these cells is often unknown. This study investigates the role of electrolyte additives, specifically, vinylene carbonate (VC) and fluoroethylene carbonate (FEC), in the aging process of lithium-ion batteries. The results showed that self-discharge plays a significant role in determining the severity of aging for cells without additives. Interestingly, the aging was less severe for the cells without additives as they deviated more from their original storage state of charge. It was also observed that the addition of VC and FEC had an effect on the formation and stability of the solid electrolyte interphase (SEI) layer on the surface of the carbonaceous anode. By gaining a better understanding of the aging processes and the effects of different electrolyte additives, we can improve the safety and durability of lithium-ion batteries, which is critical for their widespread adoption in various applications.

Stable and Fast Ion Transport Electrolyte Interfaces Modified with Novel Fluorine- and Nitrogen-Containing Solvents for Ni-Rich Cathode Materials.

Ternary nickel-rich layered oxide LiNi0.8Co0.1Mn0.1O2 (NCM811) is recognized as a cathode material with a promising future, attributed to its high energy density. However, the pulverization of cathode particles, structural collapse, and electrolyte decomposition are closely associated with the fragile cathode-electrolyte interphases (CEI), which seriously affect the electrochemical performances of ternary high-nickel materials. In this paper, fluorine- and nitrogen-containing methyl-2-nitro-4-(trifluoromethyl)benzoate (MNTB) was selected, which was synergistically regulated with fluoroethylene carbonate (FEC) to generate a robust CEI film. The preferential decomposition of MNTB/FEC results in the formation of an inorganic-rich (Li3N, LiF, and Li2O) CEI film with uniformly dense and stable characteristics, which is conducive to the migration of Li+ and the stability of the NCM811 structure and enhances the cycling stability of the battery system. Simultaneously, MNTB effectively suppresses the adverse reaction associated with increased polarization caused by higher interface impedance due to conventional single FEC additives, further improving the rate capability of the battery. Moreover, MNTB/FEC can effectively eliminate HF, preventing its corrosion on the NCM811 cathode. Under the synergistic effect of MNTB/FEC, after 300 discharge cycles at a high cutoff voltage of 4.3 V and a current density of 1 C (2 mA cm-2), the discharge capacity of the NCM811||Li battery was 150.12 mA h g-1 with a capacity retention of 81.10%, while it was only 32.8% for the standard electrolyte (STD). The discharged capacity of the MNTB/FEC-containing battery was about 115.43 mA h g-1 at the high rate of 7 C, which was considerably higher than that of the STD (93.34 mA h g-1). In this study, the designed MNTB as a novel solvent synergistically regulated with FEC will contribute to the enhanced stability of NCM811 materials at high cutoff voltages and at the same time provide an effective modified strategy to enhance the stability of commercial electrodes.

Understanding the Solid-Electrolyte-Interface (SEI) Formation in Glyme Electrolyte Using Time-Of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS).

Lithium-ion batteries are commonly used for energy storage due to their long lifespan and high energy density, but the use of unsafe electrolytes poses significant health and safety concerns. An alternative source is necessary to maintain electrochemical efficacy. This research demonstrates new safe glyme-based electrolytes for silica/carbon (SiOx/C) nanocomposite derived from Australian rice husk (RH). The quality of SiOx/C was preserved by using deep eutectic solvent-based pre-treatment and single-step carbonization, which was confirmed through the X-ray analysis of the crystalline phase of silica. The electrochemical assessment of SiOx/C anode using various glyme-based electrolytes for LIBs was carried out. Among them, the resultant half cells based on diglyme electrolyte is superior to others with the first discharge capacity at 1274 mAh/g and a reversible discharge capacity of 759.7 mAh/g. Ex-situ SEM and Time-of-Flight Secondary Ion Mass Spectrometry (ToF- SIMS) analysis of the electrode indicated that diglyme not only improves the capacity but also sustains the electrode architecture for longer cycle life with more LiF-based components and also showed the absence of HF components. Importantly, the addition of fluoroethylene carbonate (FEC) additive enhanced the cycling stability. These results provide a new perspective on developing advanced SiOx/C anode using glyme electrolytes for Li-ion batteries.

Stable operation of highly loaded pure Si-Fe anode under ambient pressure via carboxy silane-directed robust solid electrolyte interphase

Incorporation of higher content Si anode material beyond 5 wt% to Li-ion batteries (LIBs) is challenging, owing to large volume change, swelling, and solid electrolyte interphase (SEI) instability issues. Herein, a strategy of diacetoxydimethylsilane (DAMS) additive-directed SEI stabilization is proposed for a stable operation of Si-0.33FeSi2 (named as Si-Fe) anode without graphite, which provides siloxane inorganics and organics enrichment that compensate insufficient passivation of fluoroethylene carbonate (FEC) additive and reduce a dependence on FEC. Unprecedented stable cycling performance of highly loaded (3.5 mA h cm−2) pure Si-Fe anode is achieved with 2 wt% DAMS combined with 9 wt% FEC additives under ambient pressure, yielding high capacity 1270 mA h g−1 at 0.5 C and significantly improved capacity retention of 81% after 100 cycles, whereas short circuit and rapid capacity fade occur with FEC only additive. DAMS-directed robust SEI layer dramatically suppresses swelling and particles crossover through separator, and therefore prevents short circuit, demonstrating a possible operation of pure Si or Si-dominant anodes in the next-generation high-energy-density and safe LIBs.

Enhanced electrochemical performance of Bi2O3 via facile synthesis as anode material for ultra-long cycle lifespan lithium-ion batteries

The urgent demand for stable electrode materials, especially for the anode, arises in the pursuit of high-energy Li-ion batteries. This research focuses on bismuth oxide (Bi2O3) and uncovers its performance through a straightforward, commercially viable synthesis route, along with the optimization of binders and electrolytes. By employing a sodium carboxymethyl cellulose binder and fluoroethylene carbonate additives, the Bi2O3 anode demonstrates significantly enhanced performance compared to prior studies. It attains an impressive initial capacity of approximately 750 mA h g−1, exhibits excellent rate capability at 1000 mA g−1 and maintains stable cycling performance over 6000 cycles.

(Digital Presentation) Unraveling of the Morphology and Chemistry Dynamics in the FEC-Generated Silicon Anode SEI across Delithiated and Lithiated States

The silicon solid electrolyte interphase (SEI) faces cyclical cracking and reconstruction due to the ~350% volume expansion of Si which leads to shortened cell life during electrochemical cycling. Understanding the SEI morphology/chemistry and more importantly its dynamic evolution from delithiated and lithiated states is paramount to engineering a stable Si anode. Fluoroethylene carbonate (FEC) is a preferred additive with widely demonstrated enhancement of the Si cycling. Thus, insights into the effects of FEC on the dynamics of the resulting SEI may provide hints toward engineering the Si interface. Herein, ATR-FTIR, AFM, tip IR, and XPS probing all show pronounced relative invariance of the FEC-generated SEI compared to the FEC-free SEI between adjacent lithiated and delithiated states beyond the formation cycles. The SEI of Si thin film model surfaces in the baseline 1 M LiPF6 in EC:EMC (1:1) undergoes major morphological and chemical speciation swings between half-cycles while comparatively the SEI upon addition of FEC displays far less dynamic evolution. This morphology and chemistry stability of the FEC-SEI supports the enhanced cycling stability of silicon anodes in FEC-containing electrolytes. The experimental evidence gathered suggests that the FEC-SEI invariance is enabled by an elastomeric polycarbonate matrix that preserves the SEI integrity against the expansion of silicon upon lithiation. In turn, less electrolyte-consuming reconstruction occurs which manifests as and high LiF content from one half-cycle to the next. This work provides critical insights to enhance the silicon anode stability via targeted SEI engineering, namely that LiF protected by an elastomeric protective matrix may be key to buffering the unavoidable mechanical disruption. Figure 1

Enabling Long‐term Cycling Stability of Na3V2(PO4)3/C vs. Hard Carbon Full‐cells

AbstractSodium‐ion batteries are becoming an increasingly important complement to lithium‐ion batteries. However, while extensive knowledge on the preparation of Li‐ion batteries with excellent cycling behavior exists, studies on applicable long‐lasting sodium‐ion batteries are still limited. Therefore, this study focuses on the cycling stability of batteries composed of Na3V2(PO4)3/C based cathodes and hard carbon anodes. It is shown that full‐cells with a decent stability are obtained for ethylene carbonate/propylene carbonate electrolyte and the conducting salt NaPF6. With cathode loadings of 1.2 mAh/cm2, after cell formation discharge capacities up to 92.6 mAh/g are obtained, and capacity retentions >90 % over 1000 charge/discharge cycles at 0.5 C/0.5 C are observed. It is shown that both, the additive fluoroethylene carbonate and impurities in the electrolyte, negatively affect the overall discharge capacity and cycling stability and should therefore be avoided. Remarkably, the internal resistances of well‐balanced and well‐built cells did not increase over 1500 cycles and 5 months of testing, which is a very promising result regarding the possible lifespan of the cells. The initial loss of active sodium ions in hard carbon remains a major problem, which can only be partially reduced by proper balancing.

The Impact of Fluoroethylene Carbonate Additive on Charged Sodium Ion Electrodes/Electrolyte Reactivity Studied Using Accelerating Rate Calorimetry

The effects of fluoroethylene carbonate (FEC) electrolyte additive on charged sodium ion electrode/electrolyte reactivity at elevated temperatures were investigated using accelerating rate calorimetry (ARC). The beneficial effect of FEC on cell lifetime was demonstrated using Na0.97Ca0.03[Mn0.39Fe0.31Ni0.22Zn0.08]O2 (NCMFNZO)/hard carbon (HC) pouch cells first prior to ARC measurements. Electrodes from these pouch cells were utilized as sample materials and 1.0 M NaPF6 in propylene carbonate (PC):ethyl methyl carbonate (EMC) (1:1 by vol.) was chosen as control electrolyte. Adding 2 wt% and 5 wt% FEC to the electrolyte does not significantly affect the reactivity of de-sodiated NCMFNZO compared to the control electrolyte. However, the addition of FEC obviously changed the reactivity between sodiated HC and electrolytes, especially by showing a suppression on the exothermal behavior between 160 °C and 230 °C. These results give a head to head comparison of the reactivity of FEC additive containing electrolytes with charged sodium ion electrode materials at elevated temperatures and show that the use of FEC at additive levels should not compromise the cell safety when extending cell lifetime.

Differing Electrolyte Implication on Anion and Cation Intercalation into Graphite.

Emerging dual-graphite batteries (DGBs) capture extensive interest for their high output voltage and exceptional cost-effectiveness. Yet, developing electrolytes compatible with both the cathode and anode stands to be a tremendous challenge, and how electrolyte impacts anion and cation intercalation into graphite remains inexplicit or controversial. Herein, we have evaluated the performance of graphite anode and cathode in typical ethyl methyl carbonate (EMC) based electrolytes and unveiled their electrode-electrolyte interphase using Cryogenic transmission electron microscopy (Cryo-TEM). The addition of fluoroethylene carbonate (FEC) brings substantial improvement in cycle stability and Coulombic efficiency for both the graphite cathode and anode, but its implication on cation and anion intercalation differs. FEC is involved in anodic side reactions to produce a LiF-embedded solid-electrolyte interphase layer. It is much thinner and more uniform than that formed in the electrolyte without FEC, which is correlated with less graphite exfoliation and enhanced stability. As for the graphite cathode, both basal and edge planes are largely bare, and only few scattered byproducts are found. In addition, we also reveal layer bending and local lattice disordering of the graphite cathode based on multiple Cryo-TEM images, which are speculated to be caused by high lattice strain induced by anion intercalation and local oxidation under high voltage. The absence of cathode-electrolyte interphase (CEI) layers overturns the paradigm of attributing cathodic performance to CEI features and is regarded as a fundamental reason for severe self-discharge of graphite cathode. FEC helps to alleviate graphite exfoliation issues and enhance cycle stability, and we ascribe it to weakened solvation, which means reduced probability of solvent co-intercalation during charging, rather than compositional changes of cathodic byproducts.

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.

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

Improved performance of LiMn0.8Fe0.2PO4 by addition of fluoroethylene carbonate electrolyte additive

The addition of electrolyte additives is an effective strategy for tuning the property of the electrolyte to engineer the electrode/electrolyte interface, and there exist obvious discrepancies regarding the effect of fluoroethylene carbonate (FEC) as an electrolyte additive on the performance of cathodes. Herein FEC is introduced into the electrolyte of the LiMn0.8Fe0.2PO4/Li cell and its effect on the properties of the LiMn0.8Fe0.2PO4 is investigated. It is found that the addition of FEC in the electrolyte has a positive effect on the performance of the LiMn0.8Fe0.2PO4 cathode, which can be attributed to the reduced products generated by the interfacial side-reactions on the LiMn0.8Fe0.2PO4 cathode surface and the decreased metal dissolution in the FEC-containing electrolyte, thanks to the higher oxidation resistance of FEC and the easier and stronger binding of FEC and PF6−.

Synergistic dual electrolyte additives for fluoride rich solid-electrolyte interface on Li metal anode surface: Mechanistic understanding of electrolyte decomposition

Improving the quality of the solid-electrolyte interphase (SEI) layer is highly imperative to stabilize the Li-metal anodes for the practical application of high-energy–density batteries. However, controllably managing the formation of robust SEI layers on the anode is challenging in state-of-the-art electrolytes. Herein, we discuss the role of dual additives fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO2F2, LiPF) within the commercial electrolyte mixture (LiPF6/EC/DEC) considering their reactivity with Li metal anodes using density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. Synergistic effects of dual additives on SEI formation mechanisms are explored systematically by invoking different electrolyte mixtures including pure electrolyte (LP47), mono-additive (LP47/FEC and LP47/LiPF), and dual additives (LP47/FEC/LiPF). The present work suggests that the addition of dual additives accelerates the reduction of salt and additives while increasing the formation of a LiF-rich SEI layer. In addition, calculated atomic charges are applied to predict the representative F1s X-ray photoelectron (XPS) signal, and our results agree well with the experimentally identified SEI components. The nature of carbon and oxygen-containing groups resulting from the electrolyte decompositions at the anode surface is also analyzed. We find that the presence of dual additives inhibits undesirable solvent degradation in the respective mixtures, which effectively restricts the hazardous side products at the electrolyte-anode interface and improves SEI layer quality.

Effect of Fluoroethylene Carbonate Electrolyte Additives on the Electrochemical Performance of Nickel-Rich NCM Ternary Cathodes

It has been well established recently that fluorinated electrolyte additives such as fluoroethylene carbonate (FEC) could promote the formation of LiF-based solid electrolyte interphases that can stabilize lithium metal anodes. Meanwhile, the impact of FEC additives on the cathode side, particularly for the high energy density nickel-rich LiNi1–x–yCoxMnyO2 (NCM) ternary cathodes, remains unclear. In this study, we investigated the structural and chemical composition of a cathode electrolyte interphase (CEI) and its electrochemical performance to elucidate the effect of FEC additives on the LiNi0.9Co0.05Mn0.05O2 (NCM90) cathode for high energy lithium-ion batteries. It is discovered that the FEC additive in carbonate electrolyte (BE-FEC) can produce a LiF-based CEI, which could stabilize the NCM90 surface and improve the cycle performance at low cut-off voltage. The formation of a thick LiF layer under high cut-off voltage and high rate has been observed to result in increased polarization and slower Li+ transport kinetics, ultimately leading to a deterioration in battery performance. On the other hand, in a carbonate electrolyte (BE) and under low voltage, the unstable Li2CO3-based CEI components on the NCM90 surface with an intermediate rock salt phase in between contribute to poor long-term performance and reduced reliability. While under high voltage, the BE sample shows superior electrochemical performance due to the formation of a thin LiF layer from the decomposition of LiPF6. Our work provides a comprehensive understanding of the role of FEC in the CEI of nickel-rich cathodes, offering practical guidance for the design of electrolytes for high-energy high-voltage nickel-rich cathodes.

In Situ Diffuse Reflectance Infrared Fourier-Transform Spectroscopy Investigation of Fluoroethylene Carbonate and Lithium Difluorophosphate Dual Additives in SEI Formation over Cu Anode

The synergetic effect of fluoroethylene carbonate (FEC) and lithium difluorophosphate (LiPO2F2) dual additives on the cycling stability of lithium metal batteries has been previously reported. This study applies in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) to examine the impact of these two additives on SEI species formation over Cu anode using a base electrolyte of LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC). The results indicate that all electrolyte components and additives can be electrochemically reduced over the Cu anode following a potential sequence of LiPO2F2 > FEC > EC > DEC. The results illustrate that LiPF6 likely interacts with the Cu anode upon contact, resulting in LixPFy, which can lead to a reduction peak at ∼1.44 V in CV. With the base electrolyte, reduced species from LixPFy lead to the formation of alkyl phosphorus fluorides (RPF), which can be suppressed by the presence of FEC and/or LiPO2F2. Similar to previous reports, FEC reduction in the 1st lithiation cycle leads to the continuous formation of poly(FEC), while EC is electrochemically reduced to (CH2OCO2Li)2 and Li2CO3 and DEC is reduced to CH3CH2OCO2Li and Li2CO3. With only the LiPO2F2 additive, the redox of LiPO2F2 can be found in CV with LixPOy as the possible reduced product. In addition, Li2CO3 formation from EC and DEC reduction was relatively suppressed by the presence of LiPO2F2. The simultaneous presence of the FEC additive can suppress the redox of LiPO2F2 and partly the decomposition of LiPF6 likely via the preferential adsorption of FEC on Cu. Similar DRIFTS observations are found over the Li anode. The electrolyte with dual additives demonstrates a possible advantage from poly(FEC) and LixPOy species formation, suppressing the reduction of LixPFy, EC, and DEC though not completely.

The Effects of Silicon Anode Thickness on the Electrochemical Performance of Li-Ion Batteries

The electrode configuration is an important element in the development of Li-ion cells. The energy density is proportional to the loading of the active material. Therefore, increasing the electrode thickness is the simplest way to achieve higher capacities. In this paper, we compare the effects of three different thicknesses of Ag-decorated Si electrode anode (HCSi) on the electrochemical performances such as the SEI layer formation, impedances, and mass capacitances. We prepared three different silicon electrode thicknesses to optimize the electrodes: 20, 40 and 60 µm and measured in situ galvanostatic electrochemical impedance spectroscopy (GEIS). Using GEIS, we studied the intercalation mechanism of Li+ ions in detail and found that despite having the same capacities (≈3500 mAh g−1), the thinnest electrode, HCSi20, allows diffusion of Li+ ions into the bulk, whereas thicker layers prevent smooth diffusion into the bulk of the silicon electrode due to increased layer resistance. The Voigt model was used to analyze the anomaly of the frequency dependence of the measured impedance, in which, the classical Randles circuit is connected in series with one or two R ‖ C parallel combinations. One R ‖ C circuit could be the result of the SEI formation, and the second R ‖ C circuit could be the contribution of Li. To increase the number of charge and discharge cycles, we improved the electrolyte by adding fluoroethylene carbonate (FEC), which reduced the capacity of the HCSi20 electrode to 50% of the initial capacity (≈3500 mAh g−1) after 60 cycles, whereas it dropped to 20% of the initial capacity after 10 cycles without the addition of FEC.

Solid–Electrolyte Interface Formation on Si Nanowires in Li-Ion Batteries: The Impact of Electrolyte Additives

The morphological changes of Si nanowires (Si NWs) cycled in 1:1 ethylene–carbonate (EC)/diethyl–carbonate (DEC) with or without different additives, fluoroethylene carbonate (FEC) or vinylene carbonate (VC), as well as the composition of the deposited solid–electrolyte interphase layer, are investigated by a combination of experimental microscopic and spectroscopic techniques. Scanning electron microscopy and optical spectroscopy highlight that the NW morphology is better preserved in samples cycled in the presence of FEC and VC additives compared to the additive-free electrolyte. However, only the use of FEC is capable of slightly mitigating the amorphization of silicon upon cycling. The solid electrolyte interphase (SEI) formed over the Si NWs cycled in the additive-free electrolyte is richer in organic and inorganic carbonates compared to the SEI grown in the presence of the VC and FEC additives. Furthermore, both additives are able to remarkably limit the degradation of the LiPF6 salt. Overall, the use of the FEC-additive in the carbonate-based electrolyte promotes both morphological and structural resilience of the Si NWs upon cycling thanks to the optimal composition of the SEI layer.