Presence Of Fluoroethylene Carbonate Research Articles

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.

Organic Cathode with Dual-Type Multielectron Reaction Centers for High-Energy-Density Lithium Primary Batteries.

Organic electrode materials are composed of abundant elements, have diverse and designable molecular structures, and are relatively easily synthesized, promising a bright future for low-cost and large-scale energy storage. However, they are facing low specific capacity and low energy density. Herein, we report a high-energy-density organic electrode material, 1,5-dinitroanthraquinone, which is composed of two kinds of electrochemically active sites of nitro and carbonyl groups. They experience six- and four-electron reduction and are transformed into amine and methylene groups, respectively, in the presence of fluoroethylene carbonate (FEC) in the electrolyte. Drastically increased specific capacity and energy density are demonstrated with an ultrahigh specific capacity of 1321 mAh g-1 and a high voltage of ∼2.62 V, corresponding to a high energy density of 3400 Wh kg-1. This surpasses the electrode materials in commercial lithium batteries. Our findings provide an effective strategy to design high-energy-density and novel lithium primary battery systems.

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.

Effect of Temperature on Silicon-Based and Silicon-Rich Anodes for Li-Ion Batteries

Several governments are committed to fight against climate change and, reducing the pollution derived from the utilization of fossil fuel powered cars is considered as a part of the solution. Therefore, environmentally friendly electric vehicles (EVs) keep on gaining importance [1], [2] . Nevertheless, in order to extend the autonomy range of EVs to that achieved by internal combustion cars, next-generation lithium-ion batteries (LIBs) with larger specific capacities and subsequent higher energy densities are required.Replacing graphite, the anode widely used in current LIBs, with other materials capable of providing much larger capacities, is a well-known strategy to boost the specific capacity of this technology [3], [4] . In this sense, silicon is one of the most promising materials, not only because it can deliver a capacity (~3579 mAh/g) approximately 10 times that of graphite (~370 mAh/g) [5], [6] , but also because it is an affordable substitute anode material for LIBs, since it is the second most abundant element in the earth’s crust [7] .Numerous scientific investigations have been carried out in silicon anodes for LIBs, however, a parameter that has not been assessed and would be crucial in its real performance in EVs is the effect that temperature would have over its cyclability. To determine such effect, we have conducted galvanostatic tests with silicon-based and silicon-rich anodes in half-cells at different temperatures. Besides, the absence or presence of fluoroethylene carbonate (FEC) as electrolyte additive has also been evaluated. The different capacities and cycle-lifes observed, along with the possible kinetic mechanisms behind them, will be here discussed. We gratefully acknowledge support from the U. S. Department of Energy (DOE), Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. The U.S. government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the government. [1] M. Armand and J-M. Tarascon. Nature 2008, 451, 652-657. Building better batteries. [2] C. C. Chan. EEE 1999 International Conference on Power Electronics and Drive Systems, PEDS’99, Hong Kong. The Past, Present and Future of Electric Vehicle Development. [3] W-J. Zhang, J. Power Sources 2011, 196 (1), 12-24. A review of the electrochemical performance of alloy anodes for lithium-ion batteries. [4] V. Etacheri, R. Marom, R. Elazari, G. Salitra and D. Aurbach. Energy Environm. Sci. 2011, 4, 3243-3262. Challenges in the development of advanced Li-ion batteries: a review. [5] X. Su, Q. Wu, J. Li, X. Xiao, A. Lott, W. Lu, B. W. Sheldon and J. Wu. Adv. Energy Mater. 2014, 4, 1300882. Silicon-Based Nanomaterials for Lithium-Ion Batteries: A Review. [6] H. Wu and Y. Cui. Nanotoday 2012, 7(5), 414-429. Designing nanostructured silicon anodes for high energy lithium ion batteries. [7] U. Kasavajjula, C. Wang and A. J. Appleby. Journal of Power Sources 2007, 163, 1003. Nano- and bulk-silicon-based insertion anodes for lithium-ion secondary cells.

Evaluating temperature dependent degradation mechanisms of silicon-graphite electrodes and the effect of fluoroethylene carbonate electrolyte additive

Understanding and overcoming the relatively high rate of degradation observed with silicon (Si)-based anodes in lithium (Li)-ion batteries (LiBs) is crucial for developing cells with increased energy density. The capacity fade of Si-graphite (Gr) electrodes in electrolytes with and without 10 wt% fluoroethylene carbonate (FEC) is explored during cycling at temperatures of 25, 45, and 70 °C, focusing on the complex degradation modes arising from loss of active material, loss of Li inventory, and cell resistance growth. We first show the common half-cell testing cannot be used at elevated temperatures as the performance of Li metal overshadows the behavior of Si-Gr electrode. Symmetric cell configuration successfully eliminates the effect of foreign materials, and reveals that while the presence of FEC provides better passivation of Si, it also results in greater parasitic reactions at 70 °C and faster capacity fade. The electrolyte without FEC, on the other hand, shows higher capacity retention at 70 °C. Finally, we propose a multi-step test protocol which can successfully deconvolute the information on loss of Li inventory and of active material. By combining half-cell and symmetric cell test methods, we demonstrate that loss of active material is reduced at higher temperatures and the dominant mechanisms of capacity loss for Si-Gr electrodes at elevated temperatures is loss of Li inventory.

X-ray photoelectron spectroscopy as a probe for understanding the potential-dependent impact of fluoroethylene carbonate on the solid electrolyte interface formation in Na/Cu2Sb batteries

The solid electrolyte interface (SEI) forms from electrolyte decomposition during the initial discharge of half-cell batteries and is affected by the presence of electrolyte additives. Breaking down the initial discharge into stages, defined by voltage cut offs, can help discover the role of additives in SEI growth. In this study, X-Ray Photoelectron Spectroscopy (XPS) is used to analyze the SEI formed on electrodeposited, binder free Cu2Sb thin films in sodium ion half-cell batteries. The presence of fluoro-ethylene carbonate (FEC), an electrolyte additive known to enhance battery lifetime, has a significant effect on the carbon 1s XPS spectra. The concentration of oxygenated carbon environments are dramatically decreased when FEC is added to the system. These environments were present in samples without FEC before significant electrochemistry was observed, potentially displaying the reactivity of sodium metal with conventional carbonate electrolytes to form the initial components of the SEI before the battery is cycled. The differences observed when FEC is added are likely the chemical environments of the SEI that have the dramatic effect on battery performance. Interestingly, these results suggest that the critical aspects of SEI formation are determined before the active material is sodiated, with FEC playing an integral role.

Effect of temperature on capacity fade in silicon-rich anodes

Coin half-cells containing 80 wt% silicon electrodes are assembled and cycled at the ~C/10 rate in the temperature range of 25–55 °C. To the best of our knowledge, this is the first time that the effect of temperature is reported for such high-silicon-containing cells. Two different electrolytes are used in this study, a baseline electrolyte and the baseline electrolyte +10 wt% fluoroethylene carbonate (FEC). Analysis of the capacity vs. cycle count data by curve fitting reveals that the addition of FEC markedly affects the capacity loss mechanism. Without FEC, the kinetic rate law for the capacity loss mechanism can be described as the sum of two logistic growth models. With the addition of FEC, the rate law depends on ln(t). Clearly, the addition of FEC has a profound effect on the mechanism of capacity loss. Interestingly, X-ray photoelectron spectroscopy (XPS) shows that the composition of the solid electrolyte interphase (SEI) layer changes markedly from mostly organic to mostly inorganic in the presence of FEC and how it varies at the different temperatures tested, especially in the absence of FEC.

Monitoring the morphological changes of Si-based electrodes by X-ray computed tomography: A 4D-multiscale approach

Visualization and quantification of the morphological changes of Si-based electrodes occurring upon cycling are essential for better understanding their degradation mechanism and for optimizing their formulation. In this context, in-situ and ex-situ X-ray computed tomography (XRCT) analyses are here performed on Si-based electrodes for different cycling steps and at different scales ( i.e. from the composite electrode level down to the Si particle one). Three different cell configurations and four different X-ray sources (one laboratory and three synchrotrons) have been used and their impact on the image resolution/quality and the segmentation of the different solid, electrolyte and gas phases of the composite electrodes is highlighted. From these complementary XRCT analyses with a voxel size ranging from 0.8 to 0.05 μm, key morphological features have been studied such as (i) the volume expansion/contraction of the electrode, (ii) the dynamics of the electrode macrocracking, (iii) the initial solid electrolyte interphase (SEI) layer growth and related formation of gas and consumption of electrolyte, which strongly depend on the presence of fluoroethylene carbonate in the electrolyte, (iv) the evolution of the electrode porosity and macrocrack volume fraction/connectivity/width after prolonged cycling. • Morphological changes of Si-based electrodes are studied by in- and ex-situ XRCT. • XRCT analyses are performed at different scales and spatial resolutions. • The impact of the cell configuration and X-ray source is highlighted. • The electrode volume change, macrocracking and SEI formation are characterized.

Ethylene carbonate-free electrolytes for Li-ion battery: Study of the solid electrolyte interphases formed on graphite anodes

We investigated the Solid Electrolyte Interphase (SEI) formed onto graphite using adiponitrile/dimethyl carbonate-based electrolytes with either lithium difluoro(oxalato)borate (LiDFOB) or lithium bis(fluorosulfonyl)imide (LiFSI), with or without fluoroethylene carbonate (FEC) as SEI-forming additive, by a combination of X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and scanning electron microscopy. Without FEC, LiDFOB leads to a more protective SEI layer than LiFSI. FEC leads to improvements in both cases and, in combination with LiDFOB, allows forming an SEI richer in LiF with an overall impedance lower than without FEC. It also prevents the dissolution of the SEI upon cycling. For the graphite electrodes cycled with the LiFSI electrolytes, the interface is greatly influenced by the presence of FEC. Nevertheless, with or without FEC, the SEI layer thicknesses decreases upon cycling. In presence of FEC though, this effect is mitigated, but localized exfoliation of graphite was observed after 50 cycles.

Artificial Solid Electrolyte Interphase Formation on Si Nanoparticles through Radiolysis: Importance of the Presence of an Additive

In the context of energy transition, irradiation is a powerful tool to mimic quickly the modification of electrode materials upon charge/discharge cycles in lithium-ion batteries. In this study, the evolution of the surface of silicon nanoparticles upon irradiation in two electrolytes, containing or not fluoroethylene carbonate (FEC), was studied. In the presence of FEC, irradiation leads to the formation of a homogeneous layer of a few nanometers thick, covering the whole surface of the nanoparticles. The formation of an artificial solid electrolyte interphase (SEI) layer through radiolysis is thus achieved. Without FEC, only patches of degradation products are formed on the nanoparticle surfaces for the same irradiation dose. In the absence of FEC, LixPFyOz salts are formed. In the presence of FEC, LixPOy, LiF, and Si–F bonds are generated. In both cases, the interphase contains Li2CO3 and a polymer containing ethylene carbonate units. Slightly different polymers are formed at the surface of nanoparticles in the presence or absence of FEC, i.e., more cross-linked in the former case. The elastomeric properties of the polymer formed in the presence of FEC are thought to be responsible for the formation of the homogeneous layer on the Si surfaces, leading to the generation of an artificial solid SEI through the radiolysis process. This SEI, however, prevents the efficient transfer of Li+ ions, and more work is required to optimize its intrinsic (electro)chemical properties.

Effect of temperature on silicon-based anodes for lithium-ion batteries

Silicon is considered a promising anode candidate to replace graphite in lithium-ion (Li-ion) batteries for electric vehicle (EV) applications, given its high capacity (approximately 10 times higher than graphite) and, consequently, the higher energy density it can provide. However, to ensure that silicon can be used in EVs, it is necessary to determine whether it can withstand the temperature changes that an EV can experience. For this purpose, we evaluate the electrochemical performance of silicon-graphite (Si-Gr) half-cells at 4 different temperatures and explore the effect of adding fluoroethylene carbonate (FEC) to the electrolyte. We observe that cells cycled without FEC are temperature-sensitive; that is, the specific capacity, cyclability, and coulombic efficiency of Si-Gr vary as a function of the temperature applied. Conversely, the presence of FEC in the cell stabilizes the electrochemical behavior of Si-Gr, diminishing its sensitivity to temperature and providing similar reversible capacity, coulombic efficiency, and cycle-life. In this manuscript, we provide an explanation of the reasons causing this phenomenon along with other side effects of temperature, as proved by several characterization techniques.

Fire-Retardant Phosphate-Based Electrolytes for High-Performance Lithium Metal Batteries

Rechargeable lithium metal batteries (LMBs) are considered as promising candidates for high-energy storage systems, but their practical applications are plagued by the severe safety concerns and poor cyclability. Here we report a fire-retardant electrolyte consisting of 2.8 M lithium bis(trifluoromethanesulfonyl)imide in triethyl phosphate (TEP) with 10 vol % fluoroethylene carbonate (FEC) for high-performance LMBs. This concentrated system with almost no free solvent molecules and anions can alleviate the electrolyte reactivity toward Li anode and improve the anodic stability up to 5.0 V. FEC cosolvents as functional additives can enhance the ionic conductivity and reduce the viscosity of the concentrated electrolyte. Moreover, the presence of FEC in the TEP-based electrolyte favors to form a stable LiF-rich solid electrolyte interphase, suppressing the parasitic reactions between Li and TEP solvent and enabling a dendrite-free cycling of Li anode. A Li||LiFePO4 battery using this electrolyte shows a hig...

Impact of alginate and fluoroethylene carbonate on the electrochemical performance of SiO–SnCoC anode for lithium-ion batteries

The electrochemical performance of SiO–SnCoC composite anode for high-energy lithium-ion batteries was evaluated with particular emphasis on the impact of Alginate as a polymeric binder, as well as fluoroethylene carbonate (FEC) as electrolyte additive. It was found that the presence of FEC and the use of alginate pH 3 as binder help to improve the electrochemical stability of the composite anode, showing the best electrochemical performance with a high specific capacity, great capacity retention, and excellent coulombic efficiency. Particularly, the high precision self-discharge current study revealed that the alginate binder slowed down the parasitic reactions between the lithiated anode and the non-aqueous electrolyte.

The Effect of Solvent on the Capacity Retention in a Germanium Anode for Lithium Ion Batteries

A thin and mechanically stable solid electrolyte interphase (SEI) is desirable for a stable cyclic performance in a lithium ion battery. For the electrodes that undergo a large volume expansion, such as Si, Ge, and Sn, the presence of a robust SEI layer can improve the capacity retention. In this work, the role of solvent choice on the electrochemical performance of Ge electrode is presented by a systematic comparison of the SEI layers in ethylene carbonate (EC)-based and fluoroethylene carbonate (FEC)-based electrolytes. The results show that the presence of FEC as a cosolvent in a binary or ternary solvent electrolyte results in an excellent capacity retention of ∼85% after 200 cycles at the current density of 500 mA g−1; while EC-based electrode suffers a rapid capacity degradation with a capacity retention of just 17% at the end of 200 cycles. Post analysis by an extensive use of X-ray photoelectron spectroscopy (XPS) was carried out, which showed that the presence of Li2O in FEC-based SEIs was the origin for the improved electrochemical performance.

Evaluation of double-layer density modulated Si thin films as Li-ion battery anodes

Double-layer density modulated silicon thin films which contain alternating low and high density Si film layers were fabricated by magnetron sputtering. Two different samples consisting of alternating layers of high-density/low-density and low-density/high-density Si thin film layers were investigated as anode electrodes in Li-ion batteries. Si thin film in which the terminating layer at the top is low density Si layer-quoted as low-density/high-density film (LD/HD)- exhibits better performance than Si thin film that has high density layer at the top, -quoted as high-density/low-density (HD/LD). A highly stabilized cycling performance with the specific charge capacities of 2000 mAh g−1 at the 150th cycle at C/2 current density, and 1200 mAh g−1 at the 240th cycle at 10 C current density were observed for the LD/HD Si anode in the presence of fluoroethylene carbonate (FEC) electrolyte additive.

Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

In the emerging market of electric vehicles (EVs), the development of batteries with higher energy density and improved cycle-life is essential.1 Silicon-based anodes can significantly improve the energy density due to the huge specific capacity of ~3600 mAh/gSi, (corresponding to the Li15Si4 phase2) which is roughly ten times larger compared to conventional graphite electrodes (372 mAh/gC 3). However, the alloying of silicon with lithium is accompanied by large structural changes, resulting in a volume increase by 310% upon full lithiation.2 This causes fast degradation of the capacity due to particle cracking, loss of electric and ionic conductivity, increasing electrode polarization and continuous electrolyte consumption.4-6 In this work we will investigate the consumption of the electrolyte additive fluoroethylene carbonate (FEC) which is known to significantly improve the lifetime of Li-ion batteries with silicon anodes.7 Fig. 1 shows the cycling stability and coulombic efficiency of Si/C composites (40%wt nano-Si, 40%wt VGCF fibers, and 20%wt LiPAA binder; 1.2 mgSi/cm2) cycled in coin cells with metallic lithium counter electrodes and LP57 electrolyte containing various amounts of FEC. A striking feature is a rapid capacity drop which is delayed to higher and higher cycle numbers as more FEC is added to the electrolyte. By the use of 19F-NMR spectroscopy we demonstrate that the rapid capacity drop is due to the total consumptions of FEC. Its depletion causes a significant increase of the cell polarization, leading to the rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of µmolFEC/mgelectrode) in the cell. The lifetime of silicon anodes is therefore determined by the specific amount of FEC rather than by the FEC concentration. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in the literature, a new reductive decomposition mechanism for FEC is proposed yielding CO2, LiF, Li2O, Li2CO3, H2 and a partially cross-linked polymer. Fig. 1. (a) Coulombic efficiency and (b) specific lithiation capacity vs. cycle number of Si-Li coin cells with 75 µL LP57 electrolyte containing different amounts of FEC. The first three cycles are conducted at C/10 followed by cycling at C/3. The theoretical capacity is 1440 mAh/gelectrode and the specific amount of FEC (in µmolFEC/mgelectrode) is specified in the figure.

Understanding the Roles of Electrolyte Additives on the Surface Modification Mechanism of Silicon-Based Anodes in Lithium-Ion Batteries

Silicon is considered as a promising candidate of the anode material for the next generation lithium-ion batteries, due to its substantially higher gravimetric and volumetric capacity compared to conventionally used graphite. Electrolyte additives, fluoroethylene carbonate (FEC) and vinylene carbonate (VC) particularly, are usually applied for silicon-based lithium-ion batteries to tackle the problems such as poor capacity retention and low coulombic efficiency.1-3 In the present work, the decomposition mechanism of FEC as well as the surface modification of Si electrodes were investigated.4 It was observed that the FEC additive degrades at a higher reduction potential prior to the other carbonate solvents. A conformal solid electrolyte interphase (SEI), which consists of the decomposition products of FEC, is instantaneously formed on the silicon electrode as a result. This stable SEI layer sufficiently limits the emergence of cracks and suppresses the additional SEI formation from the decomposition of other electrolyte components. Both of these effects lead to a slower increase in polarization, thus a slower capacity fading, for the FEC containing cell. The differences in the SEI layer, formed with or without the presence of FEC, are schematically demonstrated in Fig 1. Besides, it was also observed that the LiPF6 decomposition can be influenced with the presence of FEC, and this was further studied with a combination of X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance (NMR) techniques. We also demonstrated that by forming an effective SEI layer with the FEC and VC electrolyte additives, the silicon electrodes can be equally well functioning with a novel lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI)-based electrolyte as with the state-of-art LiPF6-based electrolyte.5 Figure 1

Consumption of Fluoroethylene Carbonate (FEC) on Si-C Composite Electrodes for Li-Ion Batteries

The electrolyte additive fluoroethylene carbonate (FEC) is known to significantly improve the lifetime of Li-ion batteries with silicon anodes. In this work, we show that FEC can indeed improve the lifetime of silicon-carbon composite anodes but is continuously consumed during electrochemical cycling. By the use of 19F-NMR spectroscopy and charge/discharge cycling we demonstrate that FEC is only capable to stabilize the cell performance as long as FEC is still remaining in the cell. Its total consumption causes a significant increase of the cell polarization leading to a rapid capacity drop. We show with On-line Electrochemical Mass Spectrometry (OEMS) that the presence of FEC in the electrolyte prohibits the reduction of other electrolyte components almost entirely. Consequently, the cumulative irreversible capacity until the rapid capacity drop correlates linearly with the specific amount of FEC (in units of μmolFEC/mgelectrode) in the cell. The latter quantity therefore determines the lifetime of silicon anodes rather than the concentration of FEC in the electrolyte. By correlating the cumulative irreversible capacity and the specific amount of FEC in the cell, we present an easy tool to predict how much cumulative irreversible capacity can be tolerated until all FEC will be consumed in either half-cells or full-cells. We further demonstrate that four electrons are consumed for the reduction of one FEC molecule and that one carbon dioxide molecule is released for every FEC molecule that is reduced. Using all information from this study and combining it with previous reports in literature, a new reductive decomposition mechanism for FEC is proposed yielding CO2, LiF, Li2O, Li2CO3, H2 and a partially cross-linked polymer.

Quantification of the Mass and Viscoelasticity of Interfacial Films on Tin Anodes Using EQCM-D.

Electrochemical quartz crystal microbalance coupled with dissipation (EQCM-D) is employed to investigate the solid electrolyte interphase (SEI) formation and Li insertion/deinsertion into thin film electrodes of tin. Based on the frequency change we find that the initial SEI formation process is rapid before Li insertion but varies significantly with increasing concentration of the additive fluoroethylene carbonate (FEC) in the electrolyte. The extent of dissipation, which represents the film rigidity, increases with cycle number, reflecting film thickening and softening. Dissipation values are almost twice as large in the baseline electrolyte (1.2 M LiPF6 in 3:7 wt % ethylene carbonate:ethyl methyl carbonate), indicating the film in baseline electrolyte is roughly twice as soft as in the FEC-containing cells. More importantly, we detail how quantitative data about mass, thickness, shear elastic modulus, and shear viscosity in a time-resolved manner can be obtained from the EQCM-D response. These parameters were extracted from the frequency and dissipation results at multiple harmonics using the Sauerbrey and Voigt viscoelastic models. From these modeled results we show the dynamic mass changes for each half cycle. We also demonstrate that different amounts of FEC additive influence the SEI formation behavior and result in differences in the estimated mass, shear modulus and viscosity. After three cycles, the film in baseline electrolyte exhibits a 1.2 times larger mass change compared with the film in the FEC-containing electrolyte. The shear elastic modulus of films formed in the presence of FEC is larger than in the baseline electrolyte at early stages of lithiation. Also with lithiation is a marked increase in film viscosity, which together point to a much stiffer and more homogeneous SEI formed in the presence of FEC.

Improved Performance of the Silicon Anode for Li-Ion Batteries: Understanding the Surface Modification Mechanism of Fluoroethylene Carbonate as an Effective Electrolyte Additive

Silicon as a negative electrode material for lithium-ion batteries has attracted tremendous attention due to its high theoretical capacity, and fluoroethylene carbonate (FEC) was used as an electrolyte additive, which significantly improved the cyclability of silicon-based electrodes in this study. The decomposition of the FEC additive was investigated by synchrotron-based X-ray photoelectron spectroscopy (PES) giving a chemical composition depth-profile. The reduction products of FEC were found to mainly consist of LiF and −CHF–OCO2-type compounds. Moreover, FEC influenced the lithium hexafluorophosphate (LiPF6) decomposition reaction and may have suppressed further salt degradation. The solid electrolyte interphase (SEI) formed from the decomposition of ethylene carbonate (EC) and diethyl carbonate (DEC), without the FEC additive present, covered surface voids and lead to an increase in polarization. However, in the presence of FEC, which degrades at a higher reduction potential than EC and DEC, instantaneously a conformal SEI was formed on the silicon electrode. This stable SEI layer sufficiently limited the emergence of large cracks and preserved the original surface morphology as well as suppressed the additional SEI formation from the other solvent. This study highlights the vital importance of how the chemical composition and morphology of the SEI influence battery performance.