Reconsideration of SEI stability: reversible lithium intercalation into graphite electrodes in trans-2,3-butylene carbonate

Lithium ion batteries with graphitic carbon anodes and LiCoO₂ cathodes are cycled reversibly in electrolytes based on trans-2,3-butylene carbonate (t-BC), even in the absence of ethylene carbonate. While the poor interfacial film (the solid electrolyte interface (SEI)) on the lithium electrode can be readily explained in terms of previous models of its stability, this highly reversible behavior of graphite is hard to account for. To explain this profound difference in the SEI stability of the two electrodes, we have taken into account the influence that the nature of the electrode (lithium metal versus graphite) and the type of the reaction site (basal plane versus edge sites) exert on the solvent reduction pathways.     

Interfacial reactions between graphite electrodes and propylene carbonate-based solutions: Electrolyte-concentration dependence of electrochemical lithium intercalation reaction

This study examines the electrochemical reactions occurring at graphite negative electrodes of lithium-ion batteries in a propylene carbonate (PC) electrolyte that contains different concentrations of lithium salts such as, LiClO₄, LiPF₆ or LiN(SO₂C₂F₅)₂. The electrode reactions are significantly affected by the electrolyte concentration. In concentrated solutions, lithium ions are reversibly intercalated within the graphite to form stage 1 lithium–graphite intercalation compounds (Li–GICs), regardless of the lithium salt used. On the other hand, electrolyte decomposition and exfoliation of the graphene layers occur continuously in the low-concentration range. In situ analysis with atomic force microscopy reveals that a thin film (thickness of ∼8 nm) forms on the graphite surface in a concentrated solution, e.g., 3.27 mol kg⁻¹ LiN(SO₂C₂F₅)₂/PC, after the first potential cycle between 2.9 and 0 V versus Li⁺/Li. There is no evidence of the co-intercalation of solvent molecules in the concentrated solution.     

Novel polymer electrolytes based on thermoplastic polyurethane and ionic liquid/lithium bis(trifluoromethanesulfonyl)imide/propylene carbonate salt system

Polymer electrolytes were prepared from thermoplastic polyurethane with addition of mixture of ionic liquid N-ethyl(methylether)-N-methylpyrrolidinium trifluoromethanesulfonimmide (PYRA_12O1TFSI), lithium bis(trifluoromethanesulfoneimide) salt and propylene carbonate. The electrolytes characterization was performed by thermogravimetric analysis, differential scanning calorimetry and scanning electron microscopy. The electrical properties were investigated in detail by impedance spectroscopy with the aid of equivalent circuit fitting of the impedance spectra. A model describing temperature evolution of ionic conductivity and the properties of electrolyte/blocking electrode interface was developed. The electrochemical stability of the electrolytes was studied by linear voltammetry. Our results indicate that the studied electrolytes have good self-standing characteristics, and also a sufficient level of thermal stability and a fairly good electrochemical window. The ionic conductivity increases with increasing amount of mixture, and the character of temperature dependence of conductivity indicates decoupling of ion transport from polymer matrix. For studied system, the highest value of ionic conductivity measured at room temperature was 10⁻⁴ S cm⁻¹.     

Galvanostatic cycling of lithium—titanium disulphide cells in propylene carbonate and propylene carbonate—acetonitrile electrolytes

The galvanostatic cycling of LiTiS₂ cells in 1M LiAsF₆/propylene carbonate (PC)—acetonitrile (AN), 1M LiAsF₆/PC and 1M LiClO₄/PC electrolytes is reported. For all electrolytes tested, the discharge capacity was always much larger on the first cycle than subsequent cycles. The capacity was also shown to be dependent on the surface area of the TiS₂ and the rate of charge and discharge. The general performance of the cell in LiClO₄/PC was a function of the condition of the lithium electrode. The cell performed best in the 1M LiAsF₆/PC-AN electrolyte with respect to charge/discharge rate, active material utilisation and cycle life. Although it was possible to cycle the cell in 1M LiAsF₆/PC-AN more than 1000 times at current densities ～ 1 mA cm^?2, the active material utilisation was < 10% after the first 25 cycles.     

Surface structural disordering in graphite upon lithium intercalation/deintercalation

We report on the origin of the surface structural disordering in graphite anodes induced by lithium intercalation and deintercalation processes. Average Raman spectra of graphitic anodes reveal that cycling at potentials that correspond to low lithium concentrations in Li_xC (0 ≤ x < 0.16) is responsible for most of the structural damage observed at the graphite surface. The extent of surface structural disorder in graphite is significantly reduced for the anodes that were cycled at potentials where stage-1 and stage-2 compounds (x > 0.33) are present. Electrochemical impedance spectra show larger interfacial impedance for the electrodes that were fully delithiated during cycling as compared to electrodes that were cycled at lower potentials (U < 0.15 V vs. Li/Li⁺). Steep Li⁺ surface-bulk concentration gradients at the surface of graphite during early stages of intercalation processes, and the inherent increase of the Li_xC d-spacing tend to induce local stresses at the edges of graphene layers, and lead to the breakage of C-C bonds. The exposed graphite edge sites react with the electrolyte to (re)form the SEI layer, which leads to gradual degradation of the graphite anode, and causes reversible capacity loss in a lithium-ion battery.     

Dialkoxyethane—propylene carbonate mixed electrolytes for lithium secondary batteries

The electrolytic characteristics of various 1,2-dialkoxyethane (DAE)-propylene carbonate (PC) mixed solvent electrolytes for Li secondary batteries have been examined. DAE[H₃C(CH₂)_nOC₂H₄O(CH₂)_nCH₃] is a low viscous, non-cyclic, aprotic solvent. As DAEs, dimethoxyethane (DME), diethoxyethane (DEE), and dibutoxyethane (DBE) were used. The conductivities of PC/DME and of PC/DEE showed maximum values around PC/DAE volume ratios of 1/1 and at 1M solute, due mainly to the high dielectric constant of PC and the low viscosity of DAE. The Li⁺ ion conductivity changed according to the DAE molecular volume. 1M LiAsF₆PC/DME (1/1) showed an approximately 2.6 times higher conductivity, 13.8 × 10^?3 ohm^?1 cm^?1, than PC alone. Lithium charge—discharge efficiency on the Li substrate increased with decreasing reactivity between Li and DAE, which would be expected from the oxidation potential for DAE. LiClO₄PC/DME and PC/DEE showed a greater than 90% Li cycling efficiency.     

Effect of particle size on lithium intercalation rates in natural graphite

The intercalation rate of Li⁺-ions in flake natural graphite with particle size that ranged from 2 to 40 μm was investigated. The amount of Li⁺-ions that intercalate at different rates was determined from measurement of the reversible capacity during deintercalation in 1 M LiClO₄/1:1 (volume ratio) ethylene carbonate–dimethyl carbonate. The key issues in this study are the role of particle size and fraction of edge sites on the rate of intercalation and deintercalation of Li⁺-ions. At low specific current (15.5 mA/g carbon), the composition of lithiated graphite approaches the theoretical value, x=1 in Li_xC₆, except for the natural graphite with the largest particle size. However, x decreases with an increase in specific current for all particle sizes. This trend suggests that slow solid-state diffusion of Li⁺-ions limits the intercalation capacity in graphite. The flake natural graphite with a particle size of 12 μm may provide the optimum combination of reversible capacity and irreversible capacity loss in the electrolyte and discharge rates used in this study.     

Temperature effect on the graphite exfoliation in propylene carbonate based electrolytes

Graphite exfoliation at a low potential has long been an issue for lithium-ion cells using a propylene carbonate (PC) based electrolyte. Two different mechanisms have been proposed in literature to explain this structural degradation. In this study, the initial lithium intercalation temperature is found to have a great impact on the extent of the graphite exfoliation. At an elevated temperature, the exfoliation can be largely suppressed and the irreversible capacity loss is reduced substantially. After the initial cycling at 50 °C, the graphite anode can be cycled in a PC-based electrolyte at room temperature without the exfoliation problem. It is also discovered that such a graphite anode gives rise to a specific capacity of over 372 mAh g⁻¹ at 50 °C and a room temperature capacity higher than that of a graphite anode with the initial lithium intercalation at room temperature. This finding sheds a new light on the exfoliation mechanism. It may lead to a simple cycling procedure that allows us to make rechargeable lithium-ion batteries with better safety and higher capacity.     

Reactivity and cycling behaviour of lithium in propylene carbonate-ethylene carbonate-dimethyl carbonate mixtures

Lithium reactivity was investigated in a propylene carbonate-ethylene carbonate-dimethylcarbonate (1:1:3 volume) mixture with CF₃SO₃Li and LiAsF₆ as the lithium salts. Resistance changes due to passivating layer formed on bulk lithium and electrodeposited lithium consumption were found to be diffusion-limited processes. Results indicate that lithium reactivity increases with the temperature. Moreover, it was shown that the addition of ethylene carbonate to propylene carbonate generally deteriorates both the lithium stability and the cycling characteristics, while the addition of dimethyl carbonate enhances these parameters.     

LiAsF6 in propylene carbonate—acetonitrile for primary lithium batteries

1M LiAsF₆ in 50% v/v propylene carbonate—acetonitrile (PC-AN) is an electrolyte solution which offers improved cathode utilization, improved energy efficiency and more stable discharge voltages when used in primary lithium batteries with MnO₂, TiS₂, Cu₂S, CuS, MoO₃, V₂O₅, V₆O₁₃ and NbSe₃ cathodes. Incorporation of solvated Li⁺ into the cathodic material may be part of the cathodic process, and the lower viscosity and lower molar volume of acetonitrile, which is a solvator of Li⁺ in PC/AN mixtures, are thought to be responsible for the major improvement over LiAsF₆ in PC as electrolyte solution for these cells.     

Novel ionic plastic crystal-polymeric ionic liquid all-solid-state electrolytes for lithium ion batteries

离子塑性晶体作为一类新型的固态电解质材料,近年来受到研究人员的极大关注。本文合成了一种新型离子塑性晶体：N,N-二甲基吡咯双氟磺酰亚胺（P₁₁FSI）,并将其与吡咯阳离子离子液体聚合物-聚二甲基二烯丙基铵双氟磺酰亚胺（PILFSI）和锂盐（LiFSI）复合制备了P₁₁FSI-PILFSI-LiFSI全固态电解质。采用差示扫描量热法、热重分析、阻抗测试、线性扫描伏安法及对称锂电池测试等一系列表征技术对全固态电解质的热性能和电化学性能进行了系统研究。所制备的电解质膜具有好的柔韧性和热稳定性,高的离子电导率和电化学稳定性,以及与金属锂良好的界面相容性。将全固态电解质应用于Li/LiFePO₄电池中,在50℃、0.2 C充放电倍率时,电池放电比容量在60次循环后仍可达151.1 mA·h/g,容量保持率为96.8%;且在0.5 C、1.0 C倍率下放电比容量仍然高达138.1 mA·h/g和128.1 mA·h/g,展现出高的放电比容量,好的循环性能和倍率性能,有望应用于全固态锂离子电池中。     

An imidazolium based ionic liquid electrolyte for lithium batteries

An electrolyte for lithium batteries based on the ionic liquid 3-methy-1-propylimidazolium bis(trifluoromethysulfony)imide (PMIMTFSI) complexed with lithium bis(trifluoromethysulfony)imide (LiTFSI) at a molar ratio of 1:1 has been investigated. The electrolyte shows a high ionic conductivity (∼1.2 × 10⁻³ S cm⁻¹) at room temperature. Over the whole investigated temperature range the ionic conductivity is more than one order of magnitude higher than for an analogue electrolyte based on N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (Py₁₄TFSI) complexed with LiTFSI and used here as a benchmark. Raman results indicate furthermore that the degree of lithium coordinated TFSI is slightly lower in the electrolyte based on PMIMTFSI and thus that the Li⁺ charge carriers should be higher than in electrolytes based on Py₁₄TFSI. An ionic liquid gel electrolyte membrane was obtained by soaking a fibrous fully interconnected membrane, made of electrospun P(VdF-HFP), in the electrolyte. The gel electrolyte was cycled in Li/ionic liquid polymer electrolyte/Li cells over 15 days and in Li/LiFePO₄ cells demonstrating good interfacial stability and highly stable discharge capacities with a retention of >96% after 50 cycles (∼146 mAh g⁻¹).     

Electrochemical intercalation of lithium into polypyrrole/silver vanadium oxide composite used for lithium primary batteries

Hybrid composites of polypyrrole (PPy) and silver vanadium oxide (SVO) used for lithium primary batteries were chemically synthesized by an oxidative polymerization of pyrrole monomer on the SVO surface in an acidic medium. The composite electrode exhibited higher discharge capacity and better rate capability as compared with the pristine SVO electrode. The improvement in electrochemical performance of the composite electrode was due to PPy which accommodates lithium ions and also enhances the SVO utilization. Chronoamperometric and ac-impedance measurements indicated that lithium intercalation proceeds under the mixed control by interfacial charge transfer and diffusion. The enhanced SVO utilization in the composite electrode results from a facilitated kinetics of interfacial charge transfer in the presence of PPy.     

The cycling behaviour and stability of the lithium electrode in propylene carbonate and acetonitrile electrolytes

The conductivity and chemical stability with lithium of various electrolytes containing propylene carbonate (PC) and acetonitrile (AN) were determined. Addition of AN improved the conductivity of LiClO₄/PC and LiAsF₆/PC electrolytes, and the LiAsF₆/PC-AN electrolyte showed remarkable chemical stability in contact with lithium. The lithium cycling efficiency was determined on nickel and aluminium substrates in the various electrolytes over a range of current density. While the efficiencies observed on nickel substrates were very poor for all AN-containing electrolytes, efficiencies approaching those for electrolytes containing only PC were obtained with the LiAsF₆/PC-AN electrolyte at low current densities (～1 mA cm^?2) on aluminium substrates. It was concluded that the LiAsF₆/PC-AN electrolyte had generally favourable characteristics and may prove suitable for primary battery applications.     

Suppression of electrochemical decomposition of propylene carbonate at a graphite anode in lithium-ion cells

The decomposition of propylene carbonate (PC) at a graphite anode in lithium-ion cells is suppressed remarkably by choosing a proper mixing ratio of PC with co-existing solvents. For example, the decomposition of PC is essentially inhibited using PC-diethyl carbonate (DEC), PC-methylethyl carbonate (MEC) or PC-dimethyl carbonate (DMC) with 1:4 (v/v) mixed solvent solution containing a volume % of PC of less than 25%. Conductivity measurements show that all PC molecules can be solvated to Li⁺ ions as Li(PC)₂ in these mixed solvents, where 1.0 M LiPF₆ and 25 vol. % of PC are used. This suggests that the solvated PC molecules are not decomposed at graphite anode.     

Reversibility of lithium intercalation in lithium and sodium phyllomanganates

The cycling of sodium and lithium phyllomanganates in liquid lithium batteries was investigated both by galvanostatical and potentiostatical methods. Slow-scanning voltammograms show the occurrence of a single-phase reaction extending from 3.1 to 2.6 V on discharge for both compounds. On cycling, the voltammogram of Li phyllomanganate smears out, while the current peak narrows in the case of Na. In both cases, the initial capacity of ∼240 Ah/kg drops continuously on cycling between 2 and 4 V. X-ray diffraction shows an important disordering with cycling, with the possible emergence of a cubic-packed, spinel-like structure.     

LiTFSI-BEPyTFSI as an improved ionic liquid electrolyte for rechargeable lithium batteries

We report an electrochemical study of solutions of lithium bis(trifluoromethanesulfonyl)imide, LiTFSI, in a N-n-butyl-N-ethylpyrrolidinium bis(trifluoromethanesulfonyl)imide, BEPyTFSI. We show that these ionic liquid solutions have stability towards lithium metal electrode which allows various electrochemical tests, including impedance spectroscopy and voltammetry. The ionic conductivity and lithium transference number, of the order of 10⁻³ S cm⁻¹ and 0.4, respectively, make these solutions suitable for application as electrolytes in advanced lithium batteries. A prototype of these batteries, having lithium iron phosphate as the cathode, showed good performance in terms of charge–discharge efficiency and rate capability. The results reported in this work, although preliminary, are encouraging in supporting the practical interest of this LiTFSI-BEPyTFSI class of lithium conducting ionic liquids.     

Solid polymer electrolytes with sulfur based ionic liquid for lithium batteries

The electrochemical properties of a solid hybrid polymer electrolyte for lithium batteries based upon tri-ethyl sulfonium bis(trifluorosulfonyl) imide (S₂TFSI), lithium TFSI, and poly(ethylene oxide) (PEO) is presented. We have synthesized homogenous freestanding films that possess low temperature ionic conductivity and wide electrochemical stability. The hybrid electrolyte has demonstrated ionic conductivity of 0.117 mS cm⁻¹ at 0 °C, and 1.20 mS cm⁻¹ at 25 °C. At slightly elevated temperature ionic conductivity is on the order of 10 mS cm⁻¹. The hybrid electrolyte has demonstrated reversible stability against metallic lithium at the anodic interface and >4.5 V vs. Li/Li⁺ at the cathodic interface.     

One ether-functionalized guanidinium ionic liquid as new electrolyte for lithium battery

One ether-functionalized guanidinium ionic liquid is used as new electrolytes for lithium battery. Viscosity, conductivity, behavior of lithium redox, chemical stability against lithium metal, and charge-discharge characteristics of lithium batteries, are investigated for the IL electrolytes with different concentrations of lithium salt. Though the cathodic limiting potential of the IL are 0.7 V vs. Li/Li⁺, the lithium plating and striping on Ni electrode can be observed in the IL electrolytes, and the IL electrolytes show good chemical stability against lithium metal. Li/LiCoO₂ cells using the IL electrolytes without additives have good capacity and cycle property at the current rate of 0.2 C when the LiTFSI concentration is higher than 0.3 mol kg⁻¹, and the cell using the IL electrolyte with 0.75 mol kg⁻¹ LiTFSI owns good rate property. The activation energies of the LiCoO₂ electrode for lithium intercalation are estimated, and help to analyze the factors determining the rate property.