Stabilisation of lithiated graphite in an electrolyte based on ionic liquids: an electrochemical and scanning electron microscopy study

1-Ethyl-3-methylimidazolium-bis(trifluoromethylsulfonyl)imide (EMI-TFSI) is shown to reversibly permit lithium intercalation into standard TIMREX^® SFG44 graphite when vinylene carbonate (VC) is used in small amounts as additive. The best performance was obtained when 5% of VC was added to a 1 M solution of LiPF₆ in EMI-TFSI. Intercalation of lithium in the SFG44 graphite host was demonstrated over 100 cycles without noticeable capacity fading. The reversible charge capacity was around 350 mA h g⁻¹ and an only small irreversible capacity loss per cycle could be observed. Li₄Ti₅O₁₂ was used as counter electrode material. Scanning electron microscopy indicates the reduction of the electrolyte without graphite exfoliation in the neat electrolyte and the formation of a passivation film in the case of a VC-containing electrolyte. Other additives that were tested comprise ethylene sulphite and acrylonitrile which show also a positive effect, but a smaller one than vinylene carbonate. LiCoO₂ positive electrodes were cycled in a 1 M solution of LiPF₆ in EMI-TFSI with good charge capacity retention over more than 300 cycles, when Li₄Ti₅O₁₂ was used as counter electrode. The formation of a passivation film is proven on the LiCoO₂-electrodes, when the electrolyte contained VC, but not in the neat ionic liquid. Finally, the stable cycling of a full cell configuration is proven in this electrolyte system. An ammonium-containing ionic liquid (methyltrioctylammonium-bis(trifluoromethylsulfonyl)-imide, MTO-TFSI) is shown to permit the cycling of both, graphite and lithium cobalt oxide when VC is used as additive in small amounts, but at slightly elevated temperatures.     

Characterisation of the SEI formed on natural graphite in PC-based electrolytes

The origin of the different Li⁺ intercalation behaviour of raw and jet-milled natural graphite has been investigated. Jet-milled graphite is found to cycle reversibly in equal solvent mixture of propylene carbonate (PC) and ethylene carbonate (EC), whereas raw graphite does not. Using both Al Kα and synchrotron radiation (SR) Photoelectron Spectroscopy, new insight is obtained into the formation of the solid electrolyte interphase (SEI) on the two different graphite materials during electrochemical cycling in 1 M LiPF₆ in either PC:EC (1:1) or in PC with 5% vinylene carbonate (VC) as additive. Solvent reduction products are found at the surface of both raw and jet-milled graphite cycled in PC:EC (1:1), but differed in composition. The addition of VC reduces primarily the quantities of salt reaction products (LiF and Li_xPF_y compounds) and produces a mainly organic SEI layer. Electron diffraction from the edges for raw and jet-milled graphite particles shows a more disordered surface structure in the jet-milled particles than in the raw graphite. The more disordered surface structure can serve as a physical barrier hindering PC co-intercalation and facilitating the formation of a stable SEI layer.     

Compatibility of quaternary ammonium-based ionic liquid electrolytes with electrodes in lithium ion batteries

Electrochemical intercalation/deintercalation behavior of lithium into/from electrodes of lithium ion batteries was comparatively investigated in 1 mol/L LiClO₄ ethylene carbonate-diethyl carbonate (EC-DEC) electrolyte and a quaternary ammonium-based ionic liquid electrolyte. The natural graphite anode exhibited satisfactory electrochemical performance in the ionic liquid electrolyte containing 20 vol.% chloroethylenene carbonate (Cl-EC). This is attributed to the mild reduction of solvated Cl-EC molecules at the graphite/ionic electrolyte interface resulting in the formation of a thin and homogenous SEI on the graphite surface. However, rate capability of the graphite anode is poor due to the higher interfacial resistance than that obtained in 1 mol/L LiClO₄/EC-DEC organic electrolyte. Spinel LiMn₂O₄ cathode was also electrochemically cycled in the ionic electrolyte showing satisfactory capacity and reversibility. The ionic electrolyte system is thus promising for 4 V lithium ion batteries based on the concept of “greenness and safety”.     

Application of Film-forming Additives for Ionic Liquid Based Electrolyte

N-methyl-N-propylpiperidinium bis(trifluoromethanesulfonyl)imide was synthesized for the application in ionic liquid based electrolytes in lithium ion battery, 10% vinylene carbonate (VC) and 10% 1,3-propane sultone (PS) were added to the electrolyte system respectively as additives to improve the property of solid electrolyte interface and cyclic performance. The results of cyclic voltammetry showed that homogenous and compact solid electrolyte interface film formed on graphite electrode which was detected by observing the morphology of cycled graphite anode. Charging and discharging performance of LiFePO4/Li half cell was tested in the electrolyte with or without additives. The initial specific discharging capacities were increased to 129.4 and 123.0 mA×h/g by the addition of VC and PS, respectively, compared with that of additive-free electrolyte. The discharging retentions were 88.9% and 84.6% in electrolyte containing VC and PS after 10 cycles.     

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.     

Effect of vinyl acetate plus vinylene carbonate and vinyl ethylene carbonate plus biphenyl as electrolyte additives on the electrochemical performance of Li-ion batteries

For application to Li-ion batteries, we studied the electrochemical behavior and thermal stability of the following two combinations of binary electrolyte additives in a triphenylphosphate (TPP)-containing ionic electrolyte: vinyl acetate (VA) plus vinylene carbonate (VC), and vinyl ethylene carbonate (VEC) plus biphenyl (BP). Mesocarbon microbeads (MCMB) and LiCoO₂ were used as the anode and cathode materials, respectively. Cyclic voltammetry (CV), differential scanning calorimetry (DSC), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) were used for the analyses. These results confirmed the capability of the VEC + BP electrolyte additive to improve the cell performance and electrolyte thermal stability in TPP-containing solutions in Li-ion batteries.     

High temperature lithium cells using conversion oxide electrodes

Oxidized stainless steel electrodes containing chromium oxides without any conducting additives or binder have been successfully cycled at high temperatures (up to 100 °C) in organic solvent-based electrolytes with high reversibility. Cycling at high temperature results in an enhancement of the capacity at lower voltages, which is maintained upon cycling. After studying different electrolyte candidates, the best results were obtained using lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in ethylene carbonate.     

Rechargeable Li/LiFePO4 cells using N-methyl-N-butyl pyrrolidinium bis(trifluoromethane sulfonyl)imide-LiTFSI electrolyte incorporating polymer additives

We have incorporated polymer additives such as poly(ethylene glycol) dimethyl ether (PEGDME) and tetra(ethylene glycol) dimethyl ether (TEGDME) into N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR₁₄TFSI)-LiTFSI mixtures. The resulting PYR₁₄TFSI + LiTFSI + polymer additive ternary electrolyte exhibited relatively high ionic conductivity as well as remarkably low viscosity over a wide temperature range compared to the PYR₁₄TFSI + LiTFSI binary electrolytes. The charge/discharge cyclability of Li/LiFePO₄ cells containing the ternary electrolytes was investigated. We found that Li/PYR₁₄TFSI + LiTFSI + PEGDME (or TEGDME)/LiFePO₄ cells containing the two different polymer additives showed very similar discharge capacity behavior, with very stable cyclability at room temperature (RT). Li/PYR₁₄TFSI + LiTFSI + TEGDME/LiFePO₄ cells can deliver about 127 mAh/g of LiFePO₄ (74.7% of theoretical capacity) at 0.054 mA/cm² (0.2C rate) at RT and about 108 mAh/g of LiFePO₄ (63.4% of theoretical capacity) at 0.023 mA/cm² (0.1C rate) at −1 °C for the first discharge. The cell exhibited a capacity fading rate of approximately 0.09-0.15% per cycle over 50 cycles at RT. Consequently, the PYR₁₄TFSI + LiTFSI + polymer additive ternary mixture is a promising electrolyte for cells using lithium metal electrodes such as the Li/LiFePO₄ cell reported here. These cells showed the capability of operating over a significant temperature range (∼0-∼30 °C).     

Improved thermal stability of graphite electrodes in lithium-ion batteries using 4-isopropyl phenyl diphenyl phosphate as an additive

To enhance the thermal stability of graphite electrodes for lithium-ion batteries, 4-isopropyl phenyl diphenyl phosphate (IPPP) was investigated as an additive in the electrolyte of 1.0 M LiPF₆ in ethylene carbonate and diethyl carbonate (1:1 in weight). The electrochemical performance of Li/IPPP-electrolyte/C half cells was evaluated. The thermal behavior of Li _x C₆ and Li _x C₆-IPPP-electrolytes were examined using a C80 micro-calorimeter. Electrolytes with 5 and 10% IPPP improve the thermal stability of the graphite electrode in the tests. The electrochemical performance of Li/IPPP-electrolyte/C cells is not degraded by the addition of this amount of IPPP to the electrolyte.     

Effects of particle size on the thermal stability of lithiated graphite anode

The thermal behavior of fully lithiated natural graphite flakes with different particle sizes has been investigated using differential scanning calorimetry (DSC). For DSC measurements, a fully lithiated graphite anode was heated in a hermetically sealed high pressure pan with a poly vinylidene diflouride (PVdF) binder and 1 M LiPF₆ solution in ethylene carbonate (EC)-diethyl carbonate (DEC) mixture. It has been founded that the particle size has a strong influence on the thermal stability of the lithiated graphite anode. The heat generation due to the solid electrolyte interface (SEI) decomposition increases with decreasing the particle size. The onset temperatures for exothermic reactions after initial SEI decomposition appear to be lower for graphite electrodes with smaller particle sizes. This is attributed to a thermal induced delithiation facilitated by reduced diffusion path and higher surface area in smaller graphites. The structural changes in graphites during DSC scan have been investigated by ex situ X-ray diffraction (XRD) and Raman spectrometer.     

Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes

Electrochemical intercalation of lithium into a natural graphite anode was investigated in electrolytes based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion. Graphite electrode was less prone to forming effective passivation film in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Reversible intercalation/de-intercalation of TMHA cations into/from the graphene interlayer was confirmed by using cyclic voltammetry, galvanostatic measurements, and ex situ X-ray diffraction technique. Addition of 20 vol% chloroethylenene carbonate (Cl-EC), ethylene carbonate (EC), vinyl carbonate (VC), or ethylene sulfite (ES) into the ionic electrolyte resulted in the formation of solid electrolyte interface (SEI) film prior to TMHA intercalation and allowed the formation of Li-C₆ graphite interlayer compound. In the ionic electrolyte containing 20 vol% Cl-EC, the natural graphite anode exhibited excellent electrochemical behavior with 352.9 mAh/g discharge capacity and 87.1% coulombic efficiency at the first cycle. A stable reversible capacity of around 360 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. Mechanisms concerning the significant electrochemical improvement of the graphite anode were discussed. Ac impedance and SEM studies demonstrated the formation of a thin, homogenous, compact and more conductive SEI layer on the graphite electrode surface.     

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.     

A comparison of solid electrolyte interphase (SEI) on the artificial graphite anode of the aged and cycled commercial lithium ion cells

The commercial lithium ion cells with LiCoO₂ as cathode, artificial graphite as anode and 1 M LiPF₆/EC-DEC-EMC (ethylene carbonate-diethyl carbonate-dimethyl carbonate) (1:1:1, v/v/v) with additives (1 wt.% vinylene carbonate (VC) + 1 wt.% propylene sulfite (PS)) as electrolyte were aged at 60% and 100% state of charge (SOC) for 6 months at room temperature and the corresponding cycle performance was measured. Charge/discharge results showed that the capacity retentions after 100 cycles were in the order of fresh cell >60% SOC > 100% SOC. The composition of SEI on the anode was analyzed by X-ray photoelectron spectroscopy (XPS) and the sulfur atom in PS was used as a tagged atom in XPS analysis. The results suggested that the transformation of organic species to inorganic species and the species containing sulfur atom from the reduction of PS was dissolved for the cells aged at 60% and 100% SOC. The SEM and XPS surface and depth profile analysis showed that the increase of the thickness of SEI layer and the variation of compositions on storage or cycling, is one of the most important reasons that results in the deterioration of the cycle performance of commercial lithium ion cells aged at 60% and 100% SOC at room temperature for 6 months.     

Rechargeable lithium/sulfur battery with suitable mixed liquid electrolytes

The suitability of some single/binary liquid electrolytes and polymer electrolytes with a 1 M solution of LiCF₃SO₃ was evaluated for discharge capacity and cycle performance of Li/S cells at room temperature. The liquid electrolyte content in the cell was found to have a profound influence on the first discharge capacity and cycle property. The optimum, stable cycle performance at about 450 mAh g⁻¹ was obtained with a medium content (12 μl) of electrolyte. Comparison of cycle performance of cells with tetra(ethylene glycol)dimethyl ether (TEGDME), TEGDME/1,3-dioxolane (DIOX) (1:1, v/v) and 1,2-dimethoxyethane (DME)/di(ethylene glycol)dimethyl ether (DEGDME) (1:1, v/v) showed better results with the mixed electrolytes based on TEGDME. The addition of 5 vol.% of toluene in TEGDME had a remarkable effect of increasing the initial discharge capacity from 386 to 736 mAh g⁻¹ (by >90%) and stabilizing the cycle properties, attributed to the reduced lithium metal interfacial resistance obtained for the system. Polymer electrolyte based on microporous poly(vinylidene fluoride) (PVdF) membrane and TEGDME/DIOX was evaluated for ionic conductivity at room temperature, lithium metal interfacial resistance and cycle performance in room-temperature Li/S cells. A comparison of the liquid electrolyte and polymer electrolyte showed a better performance of the former.     

On the capacity fading of LiCoO2 intercalation electrodes:: the effect of cycling, storage, temperature, and surface film forming additives

The electrochemical behavior and surface chemistry of LiCoO₂ intercalation cathodes as a function of cycling and storage at 25, 45, and 60 °C was studied. The standard solutions for this work comprised ethylene carbonate (EC), ethyl-methyl carbonate (EMC), (1:2) and 1 M LiPF₆. The effect of two surface film-forming additives, vinylene carbonate (VC) and an organo-borate complex (denoted as Merck's AD25) in solutions was also explored. We analyzed temperature-dependent processes of surface film formation on the cathodes, which increase their impedance upon cycling and storage, thus making their electrochemical kinetics sluggish. We also analyzed cobalt dissolution from the cathodes at 25, 45 and 60 °C. The apparent capacity fading of the LiCoO₂ electrodes is attributed mostly to changes on their surface, rather than to bulk degradation. There are signs that the presence of HF in solutions may play a major negative role. Hence, as the electrode's surface/solution volume ratio is higher, the capacity fading of the LiCoO₂ electrodes should be lower. The main tools for this study were cyclic voltammetry, chronopotentiometry, impedance spectroscopy, electrochemical quartz crystal microbalance (EQCM), IR-spectroscopy, XRD, XPS, and SEM.     

The function of vinylene carbonate as a thermal additive to electrolyte in lithium batteries

The role of vinylene carbonate (VC) as a thermal additive to electrolytes in lithium ion batteries is studied in two aspects: the protection of liquid electrolyte species and the thermal stability of the solid electrolyte interphase (SEI) formed from VC on graphite electrodes at elevated temperatures. The nuclear magnetic resonance (NMR) spectra indicate that VC can not protect LiPF₆ salt from thermal decomposition. However, the function of VC on SEI can be observed via impedance and electron spectroscopy for chemical analysis (ESCA). These results clearly show VC-induced SEI comprises polymeric species and is sufficiently stable to resist thermal damage. It has been confirmed that VC can suppress the formation of resistive LiF, and thus reduce the interfacial resistance.     

Influence of ionic additives NaI/I2 on the properties of polymer gel electrolyte and performance of quasi-solid-state dye-sensitized solar cells

The ionic additives NaI/I₂ in polymer gel electrolyte not only provide cations, but also affect the liquid electrolyte absorbency of the poly(acrylic acid)-poly(ethylene glycol) hybrid, which results in the change of ionic conductivity of polymer gel electrolyte and the photovoltaic performance of quasi-solid-state dye-sensitized solar cell. With the optimized components of liquid electrolyte containing 0.5 M NaI, 0.05 M I₂, 0.4 M pyridine, 70 vol.% γ-butyrolactone and 30 vol.% N-methylpyrrolidone, a 4.74% power conversion efficiency of quasi-solid-state dye-sensitized solar cell was obtained under 100 mW cm⁻² (AM 1.5) irradiation.     

Properties of Li-graphite and LiFePO4 electrodes in LiPF6–sulfolane electrolyte

Sulfolane (also referred to as tetramethylene sulfone, TMS) containing LiPF₆ and vinylene carbonate (VC) was tested as a non-flammable electrolyte for a graphite |LiFePO₄ lithium-ion battery. Charging/discharging capacity of the LiFePO₄ electrode was ca. 150 mAh g⁻¹ (VC content 5 wt%). The capacity of the graphite electrode after 10 cycles establishes at the level of ca. 350 mAh g⁻¹ (C/10 rate). In the case of the full graphite |1 M LiPF₆ + TMS + VC 10 wt% |LiFePO₄ cell, both charging and discharging capacity (referred to cathode mass) stabilized at a value of ca. 120 mAh g⁻¹. Exchange current density for Li⁺ reduction on metallic lithium, estimated from electrochemical impedance spectroscopy (EIS) experiments, was j_o(Li/Li⁺) = 8.15 × 10⁻⁴ A cm⁻². Moreover, EIS suggests formation of the solid electrolyte interface (SEI) on lithium, lithiated graphite and LiFePO₄ electrodes, protecting them from further corrosion in contact with the liquid electrolyte. Scanning electron microscopy (SEM) images of pristine electrodes and those taken after electrochemical cycling showed changes which may be interpreted as a result of SEI formation. No graphite exfoliation was observed. The main decomposition peak of the LiPF₆ + TMS + VC electrolyte (TG/DTA experiment) was present at ca. 275 °C. The LiFePO₄(solid) + 1 M LiPF₆ + TMS + 10 wt% VC system shows a flash point of ca. 150 °C. This was much higher in comparison to that characteristic of a classical LiFePO₄ (solid) + 1 M LiPF₆ + 50 wt% EC + 50 wt% DMC system (T_f ≈ 37 °C).     

In situ Li nuclear magnetic resonance observation of the electrochemical intercalation of lithium in graphite; 1st cycle

We show a continuous, in situ nuclear magnetic resonance (NMR) experiment on a lithium/graphite electrochemical cell. The objective is to study a commercial graphite currently used as negative electrodes in secondary lithium batteries. A plastic cell is made, with metallic lithium as the counter electrode and 1 mol dm⁻³ LiPF₆/ethylene carbonate (EC) + diethylcarbonate (DEC) electrolyte. The reversible capacity is 346 mAh/g and the irreversible capacity 55 mAh/g, measured in the galvanostatic mode, at a rate of C/20 (20 h for the theoretical capacity of LiC₆) for the first cycle. We show the first discharge and the first charge of the cell inside the magnet and record simultaneously and regularly (in real time) static ⁷Li NMR spectra. As expected, we observe the quadrupolar lines characteristic of the lithium graphite intercalation compounds (GICs). During the discharge, the two types of in-plane densities of Li are successively found that correspond to the dilute LiC₉, then to the dense LiC₆ configuration; during the charge, we observe the successive decrease of these states. The galvanostatic curve helps to identify the stages NMR signature and the stages coexistence.     

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.