Addition Of Fluoroethylene Carbonate Research Articles

A Computational Chemical Insight into Microscopic Additive Effect on Solid Electrolyte Interphase Film Formation in Sodium-Ion Batteries: Suppression of Unstable Film Growth by Intact Fluoroethylene Carbonate

Sodium (Na)-ion batteries (NIB) have recently attracted special attention as a substitute for Li-ion batteries due to the increase in the cost of Li. The performance of NIB with liquid electrolytes, for example, propylene carbonate (PC), is strongly dependent on a stable solid electrolyte interphase (SEI) film on the anode surface. According to a recent experiment, fluoroethylene carbonate (FEC) can be an efficient electrolyte additive to improve the SEI film formation in NIB. However, the molecular mechanism of such additive effect remains unknown. To investigate this mechanism, atomistic reaction simulations in PC-based electrolyte with and without FEC additives were performed using the hybrid Monte Carlo/molecular dynamics reaction method and successfully reproduced experimental observations such as the smaller irreversible capacity and the smoother SEI film in FEC-added electrolyte. Further, this study showed for the first time that intact FEC molecules can improve SEI film formation so as to enhance ...

Impact of FEC Additive on SEI Structure Formed on a Carbon Negative Electrode Studied By Hard X-Ray Photoelectron Spectroscopy

Solid electrolyte interphase (SEI) formed on a carbon negative electrode plays an important role in supporting the reversible Li ion intercalation phenomena during charge and discharge treatments. Since the chemical and electrochemical conditions of the electrolyte/electrode interface change during electrochemical lithiation and delithiation reactions, the structure of the SEI might change even after the formation of the SEI by the initial charge and discharge treatments, although details of the reaction mechanism remain unknown. We recently successfully analyzed the behavior of the SEI of a carbon negative electrode during electrochemical lithiation and delithiation reactions by using Hard X-ray photoelectron spectroscopy (HAX-PES) technique and indicated HAX-PES was useful for analyzing SEI structure. Additives can control the structure and components of SEI, leading to the improvement of battery performance. In the present study, we applied this method to understanding of the influence of the SEI forming additives. Lithiated and delithiated carbon anodes were prepared by using a coin-type cell with Li metal. The carbon anode was a composite graphite (active agents, carbon black, PVDF) coated on Cu foil. The electrolyte was 1 M LiPF6 in 90wt% ethylene carbonate / diethyl carbonate (EC / DEC) in a 1:1 volume ratio + 10 wt% fluoro ethylene carbonate (FEC). After charge and discharge, the cell was disassembled and the anode was then retrieved, washed with dimethyl carbonate repeatedly in an argon-filled glovebox (<1 ppm H2O, <1 ppm O2). The anodes were transferred from the glovebox to UHV measurement chamber without exposing to the air by using special transfer systems. The HAXPES measurements were carried out at the BL46XU beamline at SPring-8. The photon energy of incident x-rays was 8 keV. Figure 1 shows C 1s HAXPES spectra for the lithiated graphite anodes with the state of charge (SOC) 0%, 50%, 100% after twice charge and discharge cycles. The spectra were fitted with seven singlets centered at 283.5eV (Peak 1), 284.6 eV (Peak 2), 286.0eV (Peak3), 286.5 eV (Peak 4), 288.6 eV(Peak 5), 289.6 eV(Peak 6) and 290.5 eV(Peak 7). Peak 1 can be assigned to lithiated carbon LixC. Peak2 – Peak6 can be assigned to chemical components for lithium alkyl carbonate (LAC), the organic materials of SEI. Peak 7 can be associated with a carbonate component for the Li2CO3, a inorganic material of SEI. Peak 2 and Peak 7 include the hydrocarbon (Peak2) and organofluorine (Peak7) components for the PVDF binder. LixC was observed in the C 1s HAXPES spectra with all SOC conditions, whereas this peak wasn’t observed in the C 1s XPS spectra with Al K-α incident x-rays for the lithiated graphite anodes with SOC 50%, 100% after twice charge and discharge cycles (the C 1s XPS spectra were not shown). This result indicates that the probing depth of HAXPES (hν = 8keV) is larger than thickness of SEI, thus HAXPES detected whole SEI and a part of the LixC exists underneath SEI formed on all the lithiated graphite anodes in the depth direction. HAXPES technique allows us to analyze the variation of the chemical composition of SEI depending on electrochemical lithiation and delithiation reactions. Figure 2 shows the concentration ratio of LixC, LAC, and Li2CO3 depending on SOC determined by the C 1s HAXPES spectra. The ratio of LixC decreases up to SOC 100%, while those of LAC, and Li2CO3 increases up to SOC 100%. This variation shows the structural behavior of the SEI during electrochemical lithiation reactions after the formation of the SEI by the initial charge and cycles: SEI formed in the electrolyte with FEC additives continues growth and dissolution even after stable state is established. Figure 1

Tailoring the Desired Surface Chemistry in Silicon-Based Lithium-Ion Batteries with Electrolyte Additives

The design of safe, high-energy batteries exhibiting prolonged lifetime greatly depends on understanding the surface reactions occurring on the electrode surfaces and the role of the solid electrolyte interphase (SEI) film. Most of the naturally forming SEI films demonstrate undesirable characteristics, such as instability and continuous surface reaction that decrease the capacity of Si-based Li-ion batteries (LIBs). An effective and economic way to modify the surface chemistry in LIBs rather than the Si architecture is by electrolyte additives. The objective of the present study is the examination of the concentration dependence of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) additives on the long-term electrochemical performance of LIBs, as well as the dependence of the SEI formation mechanism on the concentration of the VC and FEC additives. A significant improvement in the cycling performance of Si electrodes can be achieved under operation conditions of optimum VC and FEC concentration. To better understand the additive concentration effect, the reduction mechanisms of VC and FEC were investigated for both Si nanoparticle and Si(100) single crystal working electrodes. Ex-situ ATR-FTIR spectra of Si electrodes for 5% VC additive show a broad peak at a higher wavenumber (1831 cm–1) compared to the EC residual electrolyte (1799 and 1769 cm–1) (Fig. 1). Based on synthesized poly-VC standard spectra, the VC additive reduction compound is assigned to poly-VC or its oligomer, which is consistent with previous reports. However, for Si electrode and FEC additive, the original C=O stretch (1829 and 1802 cm–1) disappears or shifts to a lower wavenumber. The exact reduction product of FEC degradation has not been confirmed yet due to the lack of significant features other than the residual electrolyte. However, it is clear that FEC follows a different reduction path than VC. The reduction path of VC is polymerization with the reduction product remaining a “ring” structure (the C=O peak shifts to a higher wavenumber), whereas the reduction path of FEC is similar to that of ethylene carbonate (EC), leading to a reduction product of open “ring” structure, similar to that produced from EC reduction. More experiments are underway to confirm the different reduction paths of VC and FEC stated above. Acknowledgement This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Freedom CAR and Vehicle Technologies of the U.S. Department of Energy under contract No. DE-AC02-OSCH11231. Figure 1

Influence of Fluoroethylene Carbonate Additive Concentration on Silicon/Graphene Composite Anode

Silicon has been promising anode material in lithium ion battery due to higher theoretical capacity of 4200 mAh/g and abundance (1, 2). However, volume expansion and contraction of silicon particle over cycling lead to electrode deformation and poor cyclic performance. Irreversible capacity loss from solid electrolyte interface (SEI) film formation is also major factor to degrade cyclic performance (3). Generally SEI film is getting thicker due to continuous consumption of Li salt and reduction of carbonate electrolyte. Previous studies has demonstrated the effect of additives such as vinylene carbonate (VC), fluoroethylene carbonate (FEC), and other additives. FEC is found to be effective additive to improve the performance of silicon electrode (1, 4-6). In this study, we focused on influence of FEC additive concentration in carbonate electrolyte on electrochemical performance. Electrode morphology and surface chemistry of SEI was investigated with various FEC concentrations (3, 5, 10, 20 wt%) in 1.2M LiPF6 at EC/EMC (3/7, by wt) electrolyte. Electrodes were prepared by blending Si/graphene active material (XG sciences), graphene and poly acrylic acid (PAA) and casting the slurry on copper foil. In the presence of FEC additive, the better capacity retention with cycling, as shown in Figure 1, was obtained, which is consistent to previous report because of thinner and stable SEI film formation (5). FEC concentration effect on cycle performance of silicon electrodes can also be observed from Figure 1. Initially, cell containing 3wt% FEC has higher reversible capacity only until 20th cycle. The cells with other FEC concentration (5-20% FEC) have better capacity retention up to 69thcycles. Lower FEC concentration in Si/graphene electrode may not lead to enough SEI layer formation due to lack of the additive for longer cyclic life. In order to find out the FEC concentration related SEI properties, we conducted electrochemical impedance spectroscopy (EIS). Other surface analysis, such as SEM, EDX and Raman, will be reported. Optimum amount of FEC additive concentration will be discussed. Acknowledgement The silicon material is provided by XG sciences. The validation was conducted under Cell Analysis, Modeling, and Prototyping (CAMP) Facility at ANL. Support from David Howell and Peter Faguy of the U.S. Department of Energy’s Office of Vehicle Technologies is gratefully acknowledged.

Cathodically induced antimony for rechargeable Li-ion and Na-ion batteries: The influences of hexagonal and amorphous phase

Cathodic corrosion, a green electrochemical method, has been employed to obtain Sb nanomaterials utilized as anode material for lithium-ion batteries and sodium-ion batteries. Interestingly, two different corrosion mechanisms are found, coming from the impact of electrolyte, resulting in the formation of hexagonal and amorphous Sb in aqueous and organic solution, respectively. With the help of water-soluble carboxymethyl cellulose binder and the electrolyte additive fluoroethylene carbonate, both hexagonal and amorphous Sb electrodes exhibit good cycling stability when utilized as anode materials for lithium-ion batteries and sodium-ion batteries. Additionally, both the hexagonal and amorphous Sb electrodes show very good rate capability in lithium-ion batteries. Even at high current density (2000 mA g−1), the hexagonal and amorphous Sb give reversible capacities of 422 and 379 mA h g−1, respectively. Surprisingly, when used as anode materials for sodium-ion batteries, the hexagonal Sb electrode exhibits a good rate performance of 632, 625, 569, 515 and 426 mA h g−1 at a current density of 100, 200, 500, 1000, and 2000 mA g−1, respectively. However, limited rate performance is observed from the amorphous Sb electrode in case of sodium-ion battery due to the large impedance.

SEI-component formation on sub 5 nm sized silicon nanoparticles in Li-ion batteries: the role of electrode preparation, FEC addition and binders.

Silicon is a promising negative electrode for secondary lithium-based batteries, but the electrochemical reversibility of particularly nanostructured silicon electrodes drastically depends on their interfacial characteristics, commonly known as the solid electrolyte interface (SEI). The beneficial origin of certain electrolyte additives or different binders is still discussed controversially owing to the challenging peculiarities of interfacial post-mortem investigations of electrodes. In this work, we address the common difficulties of SEI investigations of porous silicon/carbon nanostructures and study the addition of a fluoroethylene carbonate (FEC) as a stabilizing additive as well as the use of two different binders, carboxymethyl cellulose/styrene-butadiene rubber (CMC/SBR) and polyacrylic acid (PAA), for the SEI formation. The electrode is composed of silicon nanocrystallites below 5 nm diameter allowing a detailed investigation of interfacial characteristics of silicon owing to the high surface area. We first performed galvanostatic long-term cycling (400 times) and carried out comprehensive ex situ characterization of the cycled nanocrystalline silicon electrodes with XRD, EDXS, TEM and XPS. We modified the preparation of the electrode for post-mortem characterization to distinguish between electrolyte components and the actual SEI. The impact of the FEC additive and two different binders on the interfacial layer is studied and the occurrence of diverse compounds, in particular LiF, Li2O and phosphates, is discussed. These results help to understand general issues in SEI formation and to pave the way for the development of advanced electrolytes allowing for a long-term performance of nanostructured Si-based electrodes.

The Mechanism of SEI Formation on Single Crystal Si(100), Si(110) and Si(111) Electrodes

Subsequent to our previous studies on the SEI formation mechanism on the single crystal silicon (100) surface, here we report on complementary studies of the SEI formation on Si surfaces with the crystal orientations (111) and (110). The differences in electrochemical behavior of the different crystal orientations are discussed - especially with regard to the effect of the SEI forming electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) added to ethylene carbonate (EC)/diethyl carbonate (DEC) based electrolytes. Fourier transform infrared spectroscopy (FTIR) of the SEI during early stages of SEI formation and physico-chemical investigations (wetting behavior) indicate a strong dependence of the chemical composition of the SEI on the surface orientation and the electrolyte composition during the early stages of lithiation of Si. However, at a higher lithiation degree less difference in the chemical composition of the SEI can be observed. These findings are in agreement with those made for the SEI formation on the Si(100) surface.

Effect of Vinylene Carbonate and Fluoroethylene Carbonate on SEI Formation on Graphitic Anodes in Li-Ion Batteries

Binder free (BF) graphite electrodes were utilized to investigate the effect of electrolyte additives fluoroethylene carbonate (FEC) and vinylene carbonate (VC) on the structure of the solid electrolyte interface (SEI). The structure of the SEI has been investigated via ex-situ surface analysis including X-ray Photoelectron spectroscopy (XPS), Hard XPS (HAXPES), Infrared spectroscopy (IR) and transmission electron microscopy (TEM). The components of the SEI have been further investigated via nuclear magnetic resonance (NMR) spectroscopy of D2O extractions. The SEI generated on the BF-graphite anode with a standard electrolyte (1.2 M LiPF6 in ethylene carbonate (EC) / ethyl methyl carbonate (EMC), 3/7 (v/v)) is composed primarily of lithium alkyl carbonates (LAC) and LiF. Incorporation of VC (3% wt) results in the generation of a thinner SEI composed of Li2CO3, poly(VC), LAC, and LiF. Incorporation of VC inhibits the generation of LAC and LiF. Incorporation of FEC (3% wt) also results in the generation of a thinner SEI composed of Li2CO3, poly(FEC), LAC, and LiF. The concentration of poly(FEC) is lower than the concentration of poly(VC) and the generation of LAC is inhibited in the presence of FEC. The SEI appears to be a homogeneous film for all electrolytes investigated.

Solid electrolyte interphases at Li-ion battery graphitic anodes in propylene carbonate (PC)-based electrolytes containing FEC, LiBOB, and LiDFOB as additives

Density functional theory is used to investigate the reactivity, reduction and effect of electrolyte additives such as fluoroethylene carbonate (FEC), lithium bis(oxalate) borate (LiBOB) and lithium difluoro(oxalato) borate (LiDFOB) in propylene carbonate (PC)-based Li-ion battery electrolytes. The structural, thermodynamical and calculated infra-red vibrational analyses indicate that FEC additives reduce prior to PC, providing stable SEI film formation near the graphite anode. The reduction and reaction mechanisms of LiBOB and LiDFOB influence the SEI film composition at the graphite surface. These additives contribute to stable SEI film formation without degradation of the anode structure by PC co-intercalation.

FEC-LiTFSI-Pyr&lt;sub&gt;1A&lt;/sub&gt;TFSI Ionic Liquid Electrolyte to Improve Low Temperature Performance of Lithium-Ion Batteries

N-methyl-N-allylpyrrolidinium bis (trifluoromethanesulfonyl) imide (PYR1ATFSI) with substantial supercooling behavior is synthesized to develop low temperature electrolyte for lithium-ion batteries. Additive fluoroethylene carbonate (FEC) in LiTFSI/PYR1ATFSI/EC/PC/EMC is found that it can reduce the freezing point. LiFePO4/Li coin cells with the FEC-PYR1ATFSI electrolyte exhibit good capacity retention, reversible cycling behavior at low temperatures. The good performance can be attributed to the decrease in the freezing point and the polarization of the composite electrolyte.

Tisnsb, a Promising Anode Material for Li-Ion Batteries? the Role of the Electrode/Electrolyte Interphase

Conversion materials for Li-ion batteries, such as Sb1 and Sn-based2 compounds, have attracted much intense scientific attention for their high storage capacities. Among conversion materials, TiSnSb has been developed as a negative electrode for Li-ion batteries.3 This material can reversibly take up 6.5 Li per formula unit which corresponds to a specific capacity of 580 mAh/g with noteworthy high rate capabilities.4 As the working potential of this active material is out of the electrochemical window of classical electrolytes like alkylcarbonates mixtures, the formation of a protective and stable passivation film (the solid electrolyte interphase or SEI)5 is required. As a large volume change occurs during lithiation/de-lithiation of a conversion material, a dynamic and unstable behavior of the SEI layer is expected upon cycling6, which will drastically limit the cycle life of the electrode.At this time, little information can be found about the formation and composition of the SEI layer on conversion electrodes. With the aim to study the electrode/electrolyte interphase, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) were performed on TiSnSb electrodes in association with electrochemical studies. In order to improve the performances of this material, SEI builder additives have been added to the standard electrolyte (1M LiPF6in EC/PC/3DMC). As vinylene carbonate (VC) and fluoroethylene carbonate (FEC) have been already recognized as efficient SEI builders for graphite and silicon negative electrodes, they have been employed in this study to modify the SEI composition and morphology.XPS quantitative analysis and EIS results, lead first to the conclusion that the thickness and the resistance of the SEI layer formed at the TiSnSb surface was lower at high cycling rate (4C) than low cycling rate (C/2). Adding additives, like FEC to the standard electrolyte containing already VC, has also a strong impact on the SEI formation: FEC/VC-containing electrolyte form a thinner SEI layer and the corresponding impedance measurements supported the same conclusion. Moreover, the partial dissolution of the SEI layer occurs during the electrode de-lithiation7, and the continuously growing in thickness of the SEI layer upon cycling confirms the dynamic and unstable behavior of the SEI layer already reported for other conversion materials. Nevertheless, the use of both VC and FEC additives permits to limit the SEI growth and to perform extensive cycling before failure (500 cycle).In order to overcome the SEI growth drawback, a pyrrolidinium based ionic liquid (IL) has been used as electrolyte instead of the alkylcarbonates mixture. As a matter of fact, the IL-containing electrolyte was found not only to improve the cycling performances of TiSnSb electrode but also to decrease the cumulative capacity losses upon cycling. XPS studies reveal that the nature of the SEI species which are responsible of the enhanced cycleability.In conclusion, an efficient way to improve the cycling performances of TiSnSb, and possibly other conversion electrodes, is to modify the electrolyte formulation. The use of a more stable IL as solvent or co-solvent and adding efficient additives are the best means to stabilize the SEI layer which is strongly related to the specific capacity and cycleability of this type of active material.References :1. L. Monconduit et al., J. Power Sources, 107 (2002) 74.2. J. Wolfenstine et al., J. Power Sources, 109 (2002) 230.3. H. A. Wilhelm et al., Electrochem Commun24 (2012) 89.4. Peled, E. J. Electrochem. Soc.126 (1979) 2047.5. C. Marino et al., J. Phys. Chem. C117 (2013) 19302.6. M. Stjerndahl et al., Electrochim. Acta 52 (2007) 4947.

Facile synthesis of ammonium vanadium oxide nanorods for Na-ion battery cathodes

A simple and versatile method for preparation of NH4V4O10 nanorods is developed via a simple hydrothermal route. NH4V4O10 nanorods display better cycling stability than NH4V4O10 microflowers as a cathode material for sodium-ion batteries because of the changes in crystalline structure, which would be in favor of superior discharge capacity. Furthermore, the enhancement of electrochemical performance for NH4V4O10 nanorods at high current rates is offered in addition of fluoroethylene carbonate to electrolyte. Such a good performance results from the improvement of reaction kinetics and Na-ion transfer rate.

Structure and electrochemical performance of LiCoO2 cathode material in different voltage ranges

LiCoO2 sample prepared by high-temperature solid state calcination shows a typical hexagonal structure with a single phase and fine particle size distribution. The high-voltage electrolyte with additive fluoroethylene carbonate (FEC) has been used. Electrochemical results show that the initial discharge capacities of the prepared LiCoO2 cathode are 157.7, 169.5, 191.0, and 217.5 mAh g−1 in the voltage ranges of 3.0–4.3, 3.0–4.4, 3.0–4.5, and 3.0–4.6 V, respectively. The capacity increases, while the initial coulombic efficiency and capacity retention decrease with increasing the charge cutoff voltage. The capacity retention is only 10.4 % after 200 cycles at 1C rate in the voltage range of 3.0–4.6 V. X-ray diffraction measurements confirm structural changes of the layered material in the different voltage ranges. A phase transition from the O3 to the H1-3 phase can be observed when LiCoO2 is charged above 4.5 V. The AC impedance analysis indicates that the resistances (R (sf+b), R ct) of the prepared LiCoO2 rapidly increase when the cell is charged to higher voltage. The amount of dissolved Co into the electrolyte also greatly increases with increasing the charge cutoff voltage.

Fluoroethylene Carbonate As an Promising Additive for γ-Butyrolactone based Electrolytes

Compared to the actual state of the art electrolyte (SOTA) for lithium ion batteries (LIBs), a novel electrolyte with better safety and lower costs and at the same time equal electrochemical performance was developed and investigated in this work. This electrolyte consists of a mixture of γ-Butyrolactone (GBL) as a solvent with fluoroethylene carbonate (FEC) as solid electrolyte interphase (SEI) additive and lithium tetrafluro borate (LiBF4) as electrolyte salt. Raman spectroscopy measurements indicated that the salt is completely dissociated by the solvent GBL which is the reason for the enhanced ionic conductivity compared to the SOTA electrolyte. The oxidation stability of this mixture is sufficient for the use in actual LIBs. Cyclic voltammetry with a T44 graphite electrode revealed the positive influence of the SEI additive FEC. The addition of FEC (1 % by wt.) enabled fully reversible de-/ intercalation of lithium ions into graphite. Furthermore, 1 M LiBF4 in GBL + FEC (1 % by wt.) improved the coulombic efficiency of T44 graphite in the first cycle of constant current cycling experiments and in addition, no capacity fading was observed after 50 cycles. On NCM electrodes a stable cycling was observed as well, obtaining similar performance compared to the SOTA electrolyte, even at an increased cut-off potential of 4.6 V vs. Li/Li+.

Study on Trimethylboroxine and Fluoroethylene Carbonate As Electrolyte Additive for High Voltage LiCoPO4 Cathode Materials

High voltage cathode materials are promising technologies for increasing the energy density and thus increasing the range of electric vehicles. LiCoPO4 is known as a possible high voltage material, which is electrochemically active at 4.8 V vs. Li/Li+ with a theoretical capacity of 167 mAh/g [1], corresponding to a theoretical energy density of ~800 Wh/kg. Unfortunately, this material exhibits poor performances on cycling in commonly used electrolytes containing LiPF6.Recently, Markevich et al. [2] explained that LiPF6 reacts with LiCoPO4 through a nucleophilic attack of the F- anion in solution on the P atoms of LiCoPO4 material, causing capacity fade of the battery. In their recent work, the use of an electrolyte containing LiPF6 with trimethylboroxine (TMB) and fluoroethylene carbonate (FEC) allows to maintain a capacity of 100 mAh/g after 100 cycles [3]. From XPS studies, they conclude that FEC creates a better SEI layer, but the effect of TMB is not yet clear.In the present work, an optimization of the amounts of TMB and FEC additives in LP30 electrolyte for LiCoPO4 electrodes is carried out. The best results are obtained with an electrolyte containing 0.5%wt TMB on the one hand and with 5%wt FEC on the other hand. For a better understanding of the effect of those additives, we used cyclic voltammetry (CV), impedance measurements (EIS) and on-line electrochemical mass spectrometry (OEMS) [4]. For each technique, carbon electrodes (Super C65, TIMCAL) and LiCoPO4 electrodes with in-house synthesized LiCoPO4are analyzed.CV experiments with OEMS show that FEC and TMB decompose completely at high potential during the first charge of the cell. Impedance measurements on electrolytes containing only FEC confirm the assertion from Sharabi et al. [3]that this compound forms a better SEI layer. Moreover, an EIS study with different amounts of TMB will be presented in order to explain the low performance of an electrolyte with a high TMB content.Finally, OEMS experiments on the initial charging of the battery until 5.5 V will be presented for optimized electrolytes with TMB and FEC. For example, figure 1 shows the OEMS results for the anodic decomposition of an electrolyte with 0.5%wt TMB on a carbon electrode. During the decomposition of TMB at 4.8-4.9 V, a product with the mass/charge-ratio 49 is detected and identified as BF3. A correlation between the XPS results from Sharabi et al. [3] and this experiment will be done.

TiSnSb, a Promising Anode Material for Li-Ion Batteries: The Role of the Electrode/Electrolyte Interphase

Conversion materials for Li-ion batteries, such as Sb1 and Sn-based2 compounds, have attracted much intense scientific attention for their high storage capacities. Among conversion materials, TiSnSb has been developed as a negative electrode for Li-ion batteries.3 This material can reversibly take up 6.5 Li per formula unit which corresponds to a specific capacity of 580 mAh/g with noteworthy high rate capabilities.4 As the working potential of this active material is out of the electrochemical window of classical electrolytes like alkylcarbonates mixtures, the formation of a protective and stable passivation film (the solid electrolyte interphase or SEI)5 is required. As a large volume change occurs during lithiation/de-lithiation of a conversion material, a dynamic and unstable behavior of the SEI layer is expected upon cycling6, which will drastically limit the cycle life of the electrode.At this time, little information can be found about the formation and composition of the SEI layer on conversion electrodes. With the aim to study the electrode/electrolyte interphase, X-ray photoelectron spectroscopy (XPS) and electrochemical impedance spectroscopy (EIS) were performed on TiSnSb electrodes in association with electrochemical studies. In order to improve the performances of this material, SEI builder additives have been added to the standard electrolyte (1M LiPF6 in EC/PC/3DMC). As vinylene carbonate (VC) and fluoroethylene carbonate (FEC) have been already recognized as efficient SEI builders for graphite and silicon negative electrodes, they have been employed in this study to modify the SEI composition and morphology.XPS quantitative analysis and EIS results, lead first to the conclusion that the thickness and the resistance of the SEI layer formed at the TiSnSb surface was lower at high cycling rate (4C) than low cycling rate (C/2). Adding additives, like FEC to the standard electrolyte containing already VC, has also a strong impact on the SEI formation: FEC/VC-containing electrolyte form a thinner SEI layer and the corresponding impedance measurements supported the same conclusion. Moreover, the partial dissolution of the SEI layer occurs during the electrode de-lithiation7, and the continuously growing in thickness of the SEI layer upon cycling confirms the dynamic and unstable behavior of the SEI layer already reported for other conversion materials. Nevertheless, the use of both VC and FEC additives permits to limit the SEI growth and to perform extensive cycling before failure (500 cycle).In order to overcome the SEI growth drawback, a pyrrolidinium based ionic liquid (IL) has been used as electrolyte instead of the alkylcarbonates mixture. As a matter of fact, the IL-containing electrolyte was found not only to improve the cycling performances of TiSnSb electrode but also to decrease the cumulative capacity losses upon cycling. XPS studies reveal that the nature of the SEI species which are responsible of the enhanced cycleability.In conclusion, an efficient way to improve the cycling performances of TiSnSb, and possibly other conversion electrodes, is to modify the electrolyte formulation. The use of a more stable IL as solvent or co-solvent and adding efficient additives are the best means to stabilize the SEI layer which is strongly related to the specific capacity and cycleability of this type of active material.References :1. L. Monconduit et al., J. Power Sources, 107 (2002) 74.2. J. Wolfenstine et al., J. Power Sources, 109 (2002) 230.3. H. A. Wilhelm et al., Electrochem Commun 24 (2012) 89.4. Peled, E. J. Electrochem. Soc. 126 (1979) 2047.5. C. Marino et al., J. Phys. Chem. C 117 (2013) 19302.6. M. Stjerndahl et al., Electrochim. Acta 52 (2007) 4947.

Fluoroethylene Carbonate Addition Effect on Electrochemical Properties of Mixed Carbonate-based Organic Electrolyte Solution for a Capacitor

In this paper, organic solvent electrolytes were prepared by a mixture of propylene carbonate (PC), dimethyl carbonate (DMC), tetraethylammonium tetrafluoroborate (TEABF4) as a salt, and by containing a different content of fluoroethylene carbonate (FEC) as an additive agent. The aim of this paper is to evaluate the ionic properties of propylene carbonate (PC)/dimethyl carbonate (DMC) mixtures as solvents for a capacitor application, in view of improving the electrochemical performances. The bulk resistance and interfacial resistance of the mixture electrolytes were investigated using an AC impedance method. The morphology of carbon-based electrodes which were contained in different electrolytes was analyzed by scanning electron microscopy (SEM) method. From the experimental results, by increasing the FEC content, capacitance of electrodes was increased, and the interfacial resistance was decreased. In particular, by a content of 2 vol % FEC in 0.2 M TEABF4 PC/DMC solvent, the electrolyte showed the superior capacitance. However, when FEC content exceeds 2 vol %, the capacitance was decreased and the interfacial resistance was increased.

Controlling SEI Formation on SnSb‐Porous Carbon Nanofibers for Improved Na Ion Storage

Porous carbon nanofiber (CNF)-supported tin-antimony (SnSb) alloys are synthesized and applied as a sodium-ion battery anode. The chemistry and morphology of the solid electrolyte interphase (SEI) film and its correlation with the electrode performance are studied. The addition of fluoroethylene carbonate (FEC) in the electrolyte significantly reduces electrolyte decomposition and creates a very thin and uniform SEI layer on the cycled electrode surface, which an promote the kinetics of Na-ion migration/transportation, leading to excellent electrochemical performance.

A Comparative Study of Vinylene Carbonate and Fluoroethylene Carbonate Additives for LiCoO2/Graphite Pouch Cells

Vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are compared as electrolyte additives for LiCoO2/graphite pouch cells using the ultra high precision charger (UHPC) at Dalhousie University, an automated storage system, electrochemical impedance spectroscopy (EIS) and long term cycling. Both VC and FEC are useful additives that improve couloumbic efficiency (CE), reduce charge end point capacity slippage, improve long-term cycling and reduce self-discharge during storage compared to cells with control electrolyte. Increasing the concentration of VC over 2% causes a dramatic increase in charge transfer resistance at the negative electrode surface, while the same effect is not observed for FEC. Therefore larger concentrations of FEC can be added to the electrolyte without this problem. However, when 4 or 6% FEC is used, greater gas generation during extended cycling at 40°C is detected. When only a single additive of VC or FEC is used in these LCO/graphite pouch cells tested at 40°C, a concentration of between 2% and 4% VC appears to be optimum as that provides high CE, low charge end point capacity slippage, a small increase in charge-discharge polarization with cycling and a small self-discharge during storage. The VC content would be optimized between these limits to trade off lifetime for rate capability.

An easy way for preparing high performance porous silicon powder by acid etching Al–Si alloy powder for lithium ion battery

We developed a novel, simple method to prepare porous silicon powder by acid etching Al–Si alloy powder. .The morphology and structure of the as-obtained material were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscope (TEM), and BET methods. It was found that the porous silicon powder (size about 15μm) had a spongy structure, consisting of silicon nanobars with diameter about 50nm and length of 1.5μm. Its specific surface area was 102.8m2g−1. The electrochemical properties of porous silicon electrode were evaluated by measuring voltammograms and charge and discharge curves. The porous silicon electrode with ratio of porous Si powder:Super P:binder=1:1:1 was tested in button style lithium/Si cell. It was found that due to its ability to promote the formation of primal SEI film on the surface of electrodes, additive fluoroethylene carbonate (FEC) had an effect to improve the charge and discharge cycle stability of porous silicon electrodes. In solution 1M LiPF6, EC:DMC=1:1 (V/V) containing 15% FEC, the first charge and discharge capacities of porous silicon electrode were 3450mAhg−1 Si and 2072mAhg−1 Si respectively, at current density 100mAg−1. The discharge capacity retained 66% as 1368mAhg−1 Si after 258 charge and discharge cycles. In 1M LiPF6/EC:DEC=1:1 (V/V) solution, the charge and discharge capacities of porous silicon electrode in first cycle were 3396mAhg−1 Si and 2537mAhg−1 Si respectively. At 69th cycle, the discharge capacity remained 59% as 1497mAhg−1 Si. The high electrochemical performance of porous silicon powder could be attributed to its porous structure, which provides enough tiny space to buffer the huge volume change of Si anode during charging and discharging processes. The nano-size Si bars benefited the diffusion process of lithium in Li–Si alloy. Moreover, the firm connection between Si nanobars in spongy porous structure prevented the breakdown of porous Si particles. This new advanced method for preparing high performance porous Si material is simple and inexpensive, presenting a promising prospect for practical application.