Effects of solvents and salts on the thermal stability of LiC6

Accelerating rate calorimetry (ARC) was used to study the thermal stability of Li_0.81C₆ in dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylene carbonate (EC), and an EC/DEC mixture as well as in LiPF₆- and LiBOB-based electrolytes. ARC results show that linear carbonates like DMC or DEC react strongly with Li_0.81C₆ and that robust passivating layers do not form. By contrast, the cyclic carbonate, EC, creates a robust passivating film that limits the rate of reaction between Li_0.81C₆ and EC as the temperature increases. X-ray diffraction shows that the addition of LiPF₆ to EC/DEC changes the surface film that forms on Li_0.81C₆ at elevated temperature to one dominated by LiF instead of lithium-alkyl carbonate or lithium carbonate. This increases the thermal stability of Li_0.81C₆ in LiPF₆ electrolyte compared to pure EC/DEC solvent. By an apparently similar mechanism, the addition of only 0.2 M LiBOB to EC/DEC greatly improves the thermal stability of Li_0.81C₆. ARC results for Li_0.81C₆ in pure and mixed salt LiPF₆ and LiBOB EC/DEC electrolytes of various molarities shed light on the reasons for the beneficial effect of the salts.     

Calorimetric investigation of the reactivity of the passivation film on lithiated graphite at elevated temperatures

Thermal storage of lithiated graphite electrodes has been performed between 40 and 90 °C for 8 h to 3 weeks. The results were compared for two separators: Celgard 2402 and a microporous PVdF membrane. The effects of storage on the capacity losses have been discussed with respect to the passivation film on the graphite electrodes in contact with the electrolyte solution EC:DMC:DEC (2:2:1)-1 M LiPF₆. The capacity loss shows a thermally activated character, which has been related to transformations of the passivation film at moderate temperatures. At higher temperatures, reaction of the intercalated lithium takes place, controlled by Li⁺-ion diffusion. DSC measurements were performed on passivated and lithiated graphite electrodes. Two peaks could be distinguished. An effect of the elevated temperature storage on the intensity and onset temperature of the first peak in DSC is evidenced. This peak could be attributed to the transformation of the passivation film. The second peak is due to the diffusion of lithium ions and the subsequent reaction with the liquid electrolyte.The effect of washing the electrode with DMC was thoroughly investigated. Our results allowed to attribute the transformation of the passivation film upon DSC analysis to a reaction taking place in the presence of LiPF₆.     

High flash point electrolyte for use in lithium-ion batteries

The high flash point solvent adiponitrile (ADN) was investigated as co-solvent with ethylene carbonate (EC) for use as lithium-ion battery electrolyte. The flash point of this solvent mixture was more than 110 °C higher than that of conventional electrolyte solutions involving volatile linear carbonate components, such as diethyl carbonate (DEC) or dimethyl carbonate (DMC). The electrolyte based on EC:ADN (1:1 wt) with lithium tetrafluoroborate (LiBF₄) displayed a conductivity of 2.6 mS cm⁻¹ and no aluminum corrosion. In addition, it showed higher anodic stability on a Pt electrode than the standard electrolyte 1 M lithium hexafluorophosphate (LiPF₆) in EC:DEC (3:7 wt). Graphite/Li half cells using this electrolyte showed excellent rate capability up to 5C and good cycling stability (more than 98% capacity retention after 50 cycles at 1C). Additionally, the electrolyte was investigated in NCM/Li half cells. The cells were able to reach a capacity of 104 mAh g⁻¹ at 5C and capacity retention of more than 97% after 50 cycles. These results show that an electrolyte with a considerably increased flash point with respect to common electrolyte systems comprising linear carbonates, could be realized without any negative effects on the electrochemical performance in Li-half cells.     

Surface film formation on a graphite negative electrode in lithium-ion batteries: AFM study on the effects of co-solvents in ethylene carbonate-based solutions

In situ AFM observation of the basal plane of highly oriented pyrolytic graphite (HOPG) was performed before and after cyclic voltammetry in 1 mol dm⁻³ LiClO₄ dissolved in ethylene carbonate (EC), EC+diethyl carbonate (DEC), and EC+dimethyl carbonate (DMC) to clarify the effects of co-solvents in EC-based solutions on surface film formation on graphite negative electrodes in lithium-ion cells. In each solution, surface film formation involved the following two different processes: (i) intercalation of solvated lithium ions and their decomposition beneath the surface; and (ii) direct decomposition of solvent molecules on the basal plane to form a precipitate layer. The most remarkable difference among these solvent systems was that solvent co-intercalation took place more extensively in EC+DEC than in EC+DMC or EC. Raman analysis of ion-solvent interactions revealed that a lithium ion is solvated by three EC molecules and one DEC molecule in EC+DEC, whereas it is solvated exclusively by EC in EC+DMC and in EC, which suggested that the presence of linear alkyl carbonates in the solvation shell of lithium ion enhance the degree of solvent co-intercalation that occurs in the initial stage of the surface film formation.     

Behaviour of highly crystalline graphitic materials in lithium-ion cells with propylene carbonate containing electrolytes: An in situ Raman and SEM study

The microcrystalline flaked graphites SFG6 and SFG44 were evaluated with regard to their compatibility with propylene carbonate (PC) by in situ Raman microscopy and postmortem scanning electron microscopy (SEM) study. PC is employed as electrolyte component in lithium-ion batteries. However, when used with certain types of graphitic materials, exfoliation occurs. To compare the effects of exfoliation, the first lithium insertion properties of these graphitic materials were measured with in situ Raman microscopy. Lithium half-cells containing either 1 M LiClO₄ 1:1 (w/w) ethylene carbonate (EC):dimethyl carbonate (DMC) or 1:1 (w/w) EC:PC were investigated. The commencement of the exfoliation process was detected in SFG44 EC:PC by the appearance of a shoulder band at 1597 cm⁻¹ on the G-band (1584 cm⁻¹) below 0.9 V versus Li/Li⁺. The band (assigned as the exfoliation or E-band) at higher wavenumbers (1597 cm⁻¹) corresponded to solvated lithium ions intercalated into graphite. The in situ Raman spectra of SFG6 in EC:DMC or EC:PC and SFG44 in EC:DMC did not show the E-band and instead displayed regular lithium intercalation spectra.In situ Raman microscopy and SEM were further employed to study the exfoliation process observed for SFG44 in 1:1 (w/w) EC:PC, when the potential was held under steady-state conditions at 0.8, 0.6 and 0.3 V, respectively. A blue-shift in the E-band from 1597 to 1607 cm⁻¹ was observed as the potential was lowered. SEM images showed dissimilar degrees of exfoliation at these three potentials.     

Comparative study of the solid electrolyte interphase on graphite in full Li-ion battery cells using X-ray photoelectron spectroscopy,secondary ion mass spectrometry,and electron microscopy

Commercial lithium-ion cells with LiMn_1/3Ni_1/3Co_1/3O₂ as the positive electrode, graphite as the negative electrode and a LiPF₆-EC:PC:DEC electrolyte were cycled under several conditions, and the solid electrolyte interphase (SEI) on the graphite electrode was studied. Nuclear magnetic resonance (NMR) spectroscopy confirmed that LiPF₆-EC:PC:DEC electrolyte was used in the cell and the relative volume ratio between solvents was acquired via quantitative NMR. Secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and a dual-beam focused ion beam/scanning electron microscope have been used to characterize the thickness, morphology and chemical composition of complex SEI on graphite anodes. The advantages and limitations of the characterization techniques are discussed and the results compared to provide a more comprehensive analysis of the SEI.     

Interfacial properties between ionic-liquid-based electrolytes and lithium-ion-battery separator

For lithium salts, ionic liquids (ILs) are promising alternatives to conventional solvents in lithium-ion batteries (LIBs) due to a more favorable high-voltage operating window, and due to improved safety through reduction of flammability. Toward better understanding of wetting properties of IL-based electrolytes on a LIB separator, wetting properties affect electrochemical performance, experimental studies were made to determine the influence of solvent, lithium-salt type and salt concentration. Surface tensions and advancing contact angles were measured for two pure ILs ([C₄C₁im][BF₄] and [C₄C₁im][OTf]) and for four IL/alkylcarbonate solvent blends (1:1 mass ratio, [C₄C₁im][BF₄]/PC, [C₄C₁im][BF₄]/DMC, [C₄C₁im][OTf]/PC, and [C₄C₁im][OTf]/ DMC) with several concentrations of a lithium salt (LiClO₄, LiPF₆, and LiTFSI). A significant improvement of wettability of pure ILs was observed by adding DMC, while adding PC with surface tension higher than that of pure ILs is detrimental to wetting behavior. Contact angles decrease by adding LiTFSI but show almost no change upon addition of LiPF₆ or LiClO₄. Surface tensions follow the same trend as that for contact angles. Incorporation of TFSI⁻ anion gives favorable separator wettability. Estimates were made for interfacial properties of the separator (dispersive and polar components of the surface free energy for solid-vapor, for liquid–vapor, and for solid–liquid interfacial free energy).     

Effect of vinylene carbonate as additive to electrolyte for lithium metal anode

The cycling efficiencies and cycling performance of a lithium metal anode in a vinylene carbonate (VC)-containing electrolyte were evaluated using Li/Ni and LiCoO₂/Li coin type cells. The cycling efficiencies of deposited lithium on a nickel substrate in an EC + DMC (1:1) electrolyte containing LiPF₆, LiBF₄, LiN(SO₂CF₃)₂ (LiTFSI), or LiN(SO₂C₂F₅) (LiBETI) at 25 and 50 °C were improved by presence of VC. However, the lithium cycling efficiencies at low temperature (0 °C) decreased by adding VC to the EC+DMC (1:1) electrolyte. The deposited lithium at low temperature exhibited a dendritic morphology and a thicker surface film. The lithium ion conductivity of the VC derived surface film was lower than that of the VC-free surface film at low temperature. Therefore, we concluded that the cycling efficiency decreased with decreasing temperature. On the other hand, the cell containing VC additive has excellent performance at elevated temperature. The deposited lithium at 50 °C in the VC-containing electrolyte exhibited a particulate morphology and formed a thinner surface film. The VC derived surface film, which consists of polymeric species, suppressed the deleterious reaction between the deposited lithium and the electrolyte.     

Dimethyl sulfoxide-based electrolytes for rechargeable lithium batteries

Mixed solutions of dimethyl sulfoxide (DMSO) and low viscosity solvents have been examined as a solvent of the electrolyte for rechargeable lithium (Li) batteries. The electrolytic conductivities of LiClO₄. LiBF₄ and LiPF₆ were measured as a function of the solvent composition. Maximum conductivities were observed in the DMSO concentration ranges of 60–80 mol% for LiClO₄ and LiBF₄, and 20–60 mol% for LiPF₆. The highest conductivity of all examined systems was 1.6 × 10⁻² S cm⁻¹ in the solution containing 1,2-dimethoxyethane (DME) and LiPF₆ as the co-solvent and the electrolyte, respectively. Polarization behavior and charge-discharge characteristics of the lithium electrode were investigated in the DMSO-based solutions. The cycling efficiency was markedly dependent not only on the co-solvent but also the Li salt. The highest efficiency on the nickel substrate was observed in LiPF₆ (1 mol dm⁻³)/DMSO-DME (1:1 by volume). High rechargeability of Li was also expected in the solution containing LiClO₄ or LiBF₄ when aluminum was used as the substrate.     

Comparisons of graphite and spinel Li1.33Ti1.67O4 as anode materials for rechargeable lithium-ion batteries

The aim of this work was to compare the electrochemical behaviors and safety performance of graphite and the lithium titanate spinel Li_1.33Ti_1.67O₄ with half-cells versus Li metal. Their electrochemical properties in 1 M LiPF₆/EC + DEC (1:1 w/w) or 1 M LiPF₆/PC + DEC (1:1 w/w) at room and elevated temperatures (30 and 60 °C) have been studied using galvanostatic cycling. At 30 °C graphite has higher reversible capacity than Li_1.33Ti_1.67O₄ when using the LiPF₆/EC + DEC as electrolyte. At 60 °C graphite declines in cell capacity yet Li_1.33Ti_1.67O₄ remains almost unchanged. In a propylene carbonate (PC) containing electrolyte, graphite electrode exfoliates and loses its mechanical integrity while Li_1.33Ti_1.67O₄ electrode is very stable. An accelerating rate calorimeter (ARC) and microcalorimeter have been used to compare the thermal stability of lithiated lithium titanate spinel and graphite. Results show that Li_1.33Ti_1.67O₄ may be used as an alternative anode material offering good battery performance and higher safety.     

Novel SEI formation of maleimide-based additives and its improvement of capability and cyclicability in lithium ion batteries

This investigation elucidates three maleimide (MI)-based aromatic molecules as additives in electrolyte that is used in lithium ion batteries. The 1.1 M LiPF₆ in ethylene carbonate (EC):propylene carbonate (PC):diethylene carbonate (DEC) (3:2:5 in volume) containing MI-based additives can prompt the formation of a solid electrolyte interface (SEI); and inhibit the entering into the irreversible state during lithium intercalation and co-intercalation. The reduction potential is 0.71-0.98 V versus Li/Li⁺ as determined by cyclic voltammetry (CV). The morphology and element analysis of the positive and negative electrode after the 100th charge-discharge cycle are examined by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS). Moreover, the MI was used in lithium ion batteries and provided 4.9% capacity increase and 16.7% capacity retention increase when cycled at 1C/1C. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries. MI is decomposed electrochemically to form a long winding narrow SEI strip on the graphite surface. This novel SEI strip not only prevents exfoliation on the graphite electrode but also stabilizes the electrolyte. The MI-based additive also ensures respectable cycle-ability of lithium ion batteries.     

Methyl phenyl bis-methoxydiethoxysilane as bi-functional additive to propylene carbonate-based electrolyte for lithium ion batteries

Methyl phenyl bis-methoxydiethoxysilane (MPBMDS) was prepared and its effects were investigated as an additive in 1.0 mol dm⁻³ LiPF₆-propylene carbonate (PC)/dimethyl carbonate (DMC) (1:1, v/v) electrolyte for lithium ion batteries. The electrochemical properties of the electrolyte with MPBMDS were characterized by discharge/charge tests, cyclic voltammetry, electrochemical impedance spectroscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The addition of MPBMDS can effectively prevent the decomposition and the co-intercalation of PC. In addition, burning tests showed that the addition of 4–13 wt.% MPBMDS to the bare PC-based electrolyte effectively reduces the flammability. This eco-friendly compound provides a new promising direction for the development of bi- or multi-functional additives for lithium ion batteries.     

Novel amphiphilic polymer gel electrolytes based on (PEG‐b‐GMA)‐co‐MMA

Amphiphilic conetwork–structured copolymers containing different lengths of ethylene oxide (EO) chains as ionophilic units and methyl methacrylate (MMA) chains as ionophobic units were prepared by free radical copolymerization and characterized by FTIR and thermal analysis. Polymer gel electrolytes based on the copolymers complexed with liquid lithium electrolytes (dimethyl carbonate (DMC) : diethyl carbonate (DEC) : ethylene carbonate (EC) = 1 : 1 : 1 (W/W/W), LiPF₆ 1.0M) were characterized by differential scanning calorimetry and impedance spectroscopy. A maximum ion conductivity of 4.27 × 10^?4 S/cm at 25^oC was found for the polymer electrolyte based on (PEG2000‐b‐GMA)‐co‐MMA with long EO groups. Moreover, the effect of temperature on conductivity of the amphiphilic polymer electrolytes obeys the Arrhenius equation. The good room temperature conductivity of the polymer electrolytes is proposed to relate to the enhancement in the amorphous domain of the copolymers due to their amphiphilic conetwork structure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

有机电解液对锂离子电池合金型负极材料性能的影响

锂离子电池合金型负极材料的研究得到了广泛的关注,但是合金电极与电解液相互作用的研究非常少。本文采用电镀和热处理相结合的方法制备出Cu₆Sn₅合金薄膜电极,研究了各种电解液对电极性能的影响。研究结果表明,合金电极在LiN(CF₂SO₂)₂(LITFSI)为溶质的电解液中表现出比在常用的以LiPF₆作为溶质的电解液中更高的容量和更好的循环性能。合金薄膜电极在1mol·L^-1 LITFSI/EC∶DEC(1∶2)电解液中具有更小的反应电阻和更大的反应电流密度,锂离子在电极上插入和脱嵌的可逆性良好,反应电阻只有在1mol·L^-1 LiPF₆/PC电解液中的1/10。研究结果表明,乙烯碳酸酯(EC)由于在充放电过程中会形成固体电解质界面（SEI）膜,能大幅度提高材料的电化学性能,在锂离子电池中是不可或缺的。     

Effect of surface modification using various acids on electrodeposition of lithium

Sulface modification of lithium was carried out using the chemical reaction of the native film with acids (HF, H₃PO₄, HI, HCl) dissolved in propylene carbonate (PC). The chemical composition change of the lithium surface was detected using X-ray photoelectron spectroscopy. The electrodeposition of lithium on the as-received lithium or the modified lithium was conducted in PC containing 1.0 mol dm^–3 LiClO₄ or LiPF₆ under galvanostatic conditions. The morphology of electrodeposited lithium particles was observed with scanning electron microscopy. The lithium dendrites were observed when lithium was deposited on the as-received lithium in both electrolytes. Moreover the dendrites were also formed on the lithium surface modified with H₃PO₄, HI, or HCl. On the other hand, spherical lithium particles were produced, when lithium was electrodeposited in PC containing 1.0 mol dm^–3 LiPF₆ on the lithium surface modified with HE However spherical lithium particles were not obtained, when PC containing 1.0 mol dm^–3 LiClO₄ was used as the electrolyte. The lithium surface modified by H₃PO₄, HI, or HCl was covered with a thick film consisting of Li₃PO₄, Li₂CO₃, LiOH, or Li₂O. The lithium surface modified with HF was covered with a thin bilayer structure film consisting of LiF and Li₂O. These results clearly show that the surface film having the thin bilayer structure (LiF and Li₂O) and the use of PC containing 1.0 mol dm^–3 LiPF₆ enhance the suppression of dendrite formation of lithium.     

Performance of lithium tetrafluorooxalatophosphate in methyl butyrate electrolytes

Methyl butyrate (MB) has been investigated as a co-solvent for lithium-ion battery electrolytes to improve the performance at low temperature (?10 to ?30 °C). The cycling performance of graphite/LiNi_1/3Co_1/3Mn_1/3O₂ cells with 1.2 M lithium tetrafluorooxalatophosphate (LiFOP) in 2:2:6 EC/EMC/MB was compared to 1.2 M LiPF₆ in both 3:7 EC/EMC and 2:2:6 EC/EMC/MB. The LiFOP/MB electrolyte has a good operational temperature window and comparable cycling performance to the LiPF₆ electrolyte at both room temperature and low temperature (?10 °C). However, after accelerated aging the LiFOP/MB electrolyte has worse performance at very low temperature (?30 °C) compared to LiPF₆ electrolytes. Ex-situ surface analysis was conducted by scanning electron microscopy, X-ray photoelectron spectroscopy, and Fourier transfer infrared spectroscopy to provide insight into the performance differences.     

Electrochemical properties of carbide-derived carbon electrodes in non-aqueous electrolytes based on different Li-salts

Electrochemical characteristics of carbide-derived micro/mesoporous carbon material C(TiC) (prepared from TiC) have been studied in 1 M LiClO₄, 0.5 M LiClO₄ + 0.5 M LiPF₆, and 1 M LiPF₆ electrolyte solutions in ethylene carbonate–dimethyl carbonate solvent mixture (1:1 by volume), by using cyclic voltammetry (CV), constant current charge/discharge and electrochemical impedance spectroscopy (EIS). Region of ideal polarizability, values of series capacitance and resistance, charge transfer resistance and capacitance, and other characteristics dependent on the electrolyte anion chemical composition have been established. The dependence of Li⁺ ion intercalation characteristics and solid electrolyte interface (SEI) formation on the salt anion composition have been established and discussed. It was found that the three electrolytes studied are comparatively weak candidates for long-lasting high energy and power density supercapacitors.     

Electrochemical characteristics of phase-separated polymer electrolyte based on poly(vinylidene fluoride-co-hexafluoropropane) and ethylene carbonate

A polymer electrolyte based on microporous poly(vinylidene fluoride-co-hexafluoropropane) (PVdF-HFP) film was studied for use in lithium ion batteries. The microporous PVdF-HFP (Kynar 2801) matrix was prepared from a cast of homogeneous mixture of PVdF-HFP and solvents such as ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). After evaporation of DMC and EMC, a sold film of the PVdF-HFP and the EC mixture was obtained. EC-rich phase started its formation in the PVdF-HFP/EC film at EC content of about 60 wt.% based on the total weight of PVdF-HFP and EC. The formation of the new phase resulted in the abrupt increase of the porosity of the PVdF-HFP matrix from 32 to 62%. The ionic conductivity of the film soaked in 1 M LiPF₆-EC/DMC=1/1 was significantly increased from order of 10⁻⁴ S/cm to order of 10⁻³ S/cm at the EC content of 60 wt.%. Thermal and spectroscopic investigations showed that most of the EC interact with PVdF-HFP with the EC content being below 60 wt.%. MCMB/polymer electrolyte/LiCoO₂ cells employing the microporous PVdF-HFP polymer film showed stable charging/discharging characteristics at 1C rate and good rate capability.     

Electrochemical performance of nanoporous Si as anode for lithium ion batteries in alkyl carbonate and ionic liquid-based electrolytes

Nanoporous Si was obtained by means of metal-assisted chemical etching. Li ion insertion–extraction was tested by voltammetric and galvanostatic electrochemical cycling in conventional 1 M LiPF₆ ethylene carbonate/dimethyl carbonate EC/DMC and in 1 M LiTFSI 1-butyl-1-methyl-pyrrolidinium bis (trifluoromethyl) sulfonylimide [BMP] [TFSI] electrolytes. The nanoporous Si demonstrated high reversibility when cycled in 1 M LiPF₆ EC/DMC electrolyte and showed superior activity compared to the non-structured sample. In contrast to the organic carbonate electrolyte, the material cycling in ionic liquid media showed reduced capacity and reversibility of the Li ion exchange. The latter results were discussed in terms of the high viscosity of the ionic liquid and ineffective cathodic passivation of the Si substrate in the ionic liquid-based electrolyte. Scanning electron microscopy imaging showed minor morphological changes due to the large volume change during Li insertion. No signs of crack formation and propagation were detected during the time span of the measurement.     

The influence of lithium salt on the interfacial reactions controlling the thermal stability of graphite anodes

The thermal stability of graphite anodes used in Li-ion batteries has been investigated, with the influence of electrolyte salt under special scrutiny, LiPF₆, LiBF₄, LiCF₃SO₃ and LiN(SO₂CF₃)₂ in an ethylene carbonate (EC)/dimethyl carbonate (DMC) solvent mixture. Differential scanning calorimetry (DSC) showed exothermic reactions in the temperature range 60-200 °C for all electrolyte systems. The reactions were coupled to decomposition of the solid electrolyte interphase (SEI) and reactions involving intercalated lithium. The onset temperature of the exothermic reactions increased with type of salt in the order: LiBF₄<LiPF₆<LiCF₃SO₃<LiN(SO₂CF₃)₂. X-ray photoelectron spectroscopy (XPS) was used to identify surface species formed prior to and after the exothermic reactions, to clarify different thermal behaviour for different salts. The decomposed SEI's in LiCF₃SO₃ and LiN(SO₂CF₃)₂ electrolytes were found to be mainly solvent-based, including lithium alkyl carbonate decomposition to stable Li₂CO₃ and the formation of poly(ethylene oxide) (PEO)-type polymers. In the LiBF₄ and LiPF₆ systems, decomposition was governed by salt reactions, which decomposed the salts and resulted in the main product LiF.