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.     

Investigation of the anodic behavior of Al current collector in room temperature ionic liquid electrolytes

We have investigated the anodic behaviors of aluminum as a cathodic current collector for lithium ion batteries in several kinds of room temperature ionic liquids (RTILs) and EC + DMC solutions containing LiN(CF₃SO₂)₂ by cyclic voltammetry (CV), chronoamperometry (CA), scanning electron microscopy (SEM), energy dispersive X-ray (EDX) spectroscopy and X-ray photoelectron spectroscopy (XPS). Cyclic voltammetry and chronoamperometry data showed that the current density for aluminum foil with the RTIL electrolytes was less than that of aluminum foil in the EC + DMC solutions. Besides, much corrosion pits appeared on the aluminum foil surface after the electrochemical measurement in the EC + DMC solutions, while they were not observed on the aluminum foil with the RTIL electrolytes, suggesting that aluminum current collector was stable in the RTIL electrolytes. Further research by EDX and XPS analysis revealed that a good passivating film composed mainly of the products from the oxidation between aluminum and the anions of the RTIL electrolytes on the aluminum foil surface after the anodic polarization which suppressed the aluminum corrosion.     

Ionic conductivity and interfacial properties of nanochitin-incorporated polyethylene oxide-LiN(C2F5SO2)2 polymer electrolytes

Nanocomposite polymer electrolytes (NCPE) composed of poly(ethylene oxide) and nanochitin for different concentrations of LiN(C₂F₅SO₂)₂ (LiBETI) were prepared by a completely dry, solvent-free procedure using a hot press. The thermal stability of NCPE membranes was investigated by DSC and TG-DTA. The membranes were subjected to SEM, ionic conductivity and FTIR analysis. Li/NCPE/Li symmetric cells were assembled and the variation of interfacial resistance as a function of time was also measured. The surface chemistry of lithium electrodes in contact with NCPE revealed the formation of Li-O-C and LiN compounds. LiFePO₄/NCPE/Li cell was assembled and the cycling profile showed a well-defined and reproducible shape of the voltage curves thus indicating a good cycling behavior of the cell at 60 °C.     

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.     

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.     

Surface layer formation on Sn anode: ATR FTIR spectroscopic characterization

Surface layer formed on Sn thin film electrode in 1 M LiPF₆/EC:DMC electrolyte was characterized using ex situ FTIR spectroscopy with the attenuated total reflection technique. IR spectral analyses showed that the immersion of Sn film in the electrolyte resulted in a chemical interfacial reaction leading to the passivation of Sn surface with primarily PF-containing inorganic surface species and small amount of organics. When constant current cycling was conducted with lithium cells with Sn film electrode at 0.1-1.0 V vs. Li/Li⁺, the interfacial reaction between Sn and electrolyte appeared significantly intensified that the features of PF-containing species became enhanced and new IR features of organic species (e.g. alkyl carbonate/carboxylate metal salts and ester functionalities) were observed. The surface layer continued to form with cycling, partly due to non-effective surface passivation as well as particle pulverization accompanied by enlargement of active surface area. Comparative IR spectral analyses indicated that the interfacial reaction between Sn and PF₆⁻ anion played a leading role in forming the surface layer, which is different from lithiated graphite that had mainly organic surface species. The data contribute to a better understanding of the interfacial processes occurring on Sn-based anode materials in lithium-ion batteries.     

Room temperature molten salts as lithium battery electrolyte

In the present study, are reported investigations obtained with the room temperature molten salt (RTMS) ethyl-methyl-imidazolium bis-(trifluoromethanesulfonyl)-imide (EMI-TFSI) in order to use it as solvent in lithium battery. The thermal stability, viscosity, conductivity and electrochemical properties are presented. A solution of 1m lithium bis-(trifluoromethanesulfonyl)-imide (LiTFSI) in EMI-TFSI has been used to test the electrolyte in a battery with LiCoO₂ and Li₄Ti₅O₁₂ as respectively cathode and anode materials. Cycling and power measurements have been obtained. The results have been compared with those obtained with a molten salt formulated with a different anion, BF₄⁻ and with a conventional liquid organic solvent EC/DMC containing LiTFSI. The 1m LiTFSI/EMI-TFSI electrolyte provides the best cycling performance: a capacity up to 106 mAh g⁻¹ is still delivered after 200 cycles, with 1C rate at 25 °C.     

A series of LiI/acetamide phase transition electrolytes and their applications in dye-sensitized solar cells

A new series of electrolytes composed of LiI and acetamide have been investigated in dye-sensitized solar cells (DSSCs). These electrolytes melt at about 50 °C and their ionic conductivities vary drastically below and above the melting points (T_m). They tend to form large crystals at low temperature, leading to poor penetration and contact within porous TiO₂ anode film. This shortage is improved by introducing nano-SiO₂ particles into the electrolyte. A total conversion efficiencies (η) of 0.3% at 35 °C and 4.2% at 75 °C are achieved respectively under AM 1.5 simulated solar light illumination when a LiI/acetamide (1:16) electrolyte with 8 wt% nano-SiO₂ is used. It is expected that the DSSC using phase transition electrolyte could show high efficiency for operation at high temperature and high stability for storage at low temperature.     

Electrodeposition of iron oxide nanorods on carbon nanofiber scaffolds as an anode material for lithium-ion batteries

Iron oxide film with spaced radial nanorods is formed on the VGCF (vapor-grown carbon nanofiber) scaffolds by means of anodic electrodeposition. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy show that the iron oxide film deposited on the VGCF surface is α-Fe₂O₃ and consists of spaced radial nanorods having 16-21 nm in diameter after annealing at 400 °C. Galvanostatic charge/discharge results indicate that the α-Fe₂O₃/VGCF anode (970 mAh g⁻¹) has higher capacity than bare α-Fe₂O₃ anode (680 mAh g⁻¹) at 10 C current discharge. VGCF scaffolds fabricated by electrophoretic deposition favor the electron conduction, and the spaced radial nanorods on VGCFs facilitate the migration of lithium ion from the electrolyte. Electrochemical reactions between α-Fe₂O₃ and lithium ion are therefore improved significantly by this tailored architecture.     

The effect of electrolyte temperature on the passivity of solid electrolyte interphase formed on a graphite electrode

The effect of electrolyte temperature on the passivity of solid electrolyte interphase (SEI) was investigated in 1 M LiPF₆-ethylene carbonate/diethyl carbonate (50:50 vol.%) electrolyte, using galvanostatic charge-discharge experiment, and ac-impedance spectroscopy combined with Fourier transform infra-red spectroscopy, and high resolution transmission electron microscopy (HRTEM). The galvanostatic charge-discharge curves at 20 °C evidenced that the irreversible capacity loss during electrochemical cycling was markedly increased with rising SEI formation temperature from 0 to 40 °C. This implies that the higher the SEI formation temperature, the more were the graphite electrodes exposed to structural damages. From both increase of the relative amount of Li₂CO₃ to ROCO₂Li and decrease of resistance to the lithium transport through the SEI layer with increasing SEI formation temperature, it is reasonable to claim that, due to the enhanced gas evolution reactions during transformation of ROCO₂Li to Li₂CO₃, the rising SEI formation temperature increased the number of defect sites in the SEI layer. From the analysis of HRTEM images, no significant structural destruction in bulk graphite layer was observed after charge-discharge cycles. This means that solvated lithium ions were intercalated through the defect sites in the SEI, at most, into the surface region of the graphite layer.     

Low temperature performance of graphite electrode in Li-ion cells

We studied low temperature performance of Li/graphite cell. Results show that capacity of the graphite electrode falls significantly in the temperature range of 0 to −20 °C. When lithiation and delithiation are both carried out at −20 °C, graphite only retains 12% of the room temperature capacity. However, delithiation capacity of graphite increases to 92% of the room temperature value if the lithiation is carried out at room temperature. We believe that the poor low temperature performance of the cell is due to slow kinetics of lithium ion diffusion in graphite rather than low ionic conductivity of electrolyte and solid electrolyte interface (SEI) on the graphite surface. During lithiation and delithiation processes, lithium ion has the similar apparent chemical diffusion coefficient of 10⁻⁹-10⁻¹⁰ cm²/s at 20 °C, depending on the state of lithiation of graphite. We observed a dramatic decrease in lithium ion diffusivity in the temperature range of 0 to −20 °C, and that at low temperatures of <−20 °C, lithium ion has higher diffusivity in the delithiated graphite than in the lithiated one. We also observed that temperature dependence of cycling behavior of the Li/graphite cell follows the change of lithium ion diffusivity.     

Electrochemical stability of thin film LiMn2O4 cathode in organic electrolyte solutions with different compositions at 55 °C

A survey of the electrochemical stability of electrostatic spray deposited thin film of LiMn₂O₄ was performed in LiClO₄-EC-PC, LiBF₄-EC-PC, and LiPF₆-EC-PC solutions at 55 °C. The solution resistance, the surface film resistance, and the charge-transfer resistance were all found to depend on the electrolyte composition. Among the LiX-salts studied, the lowest charge transfer-resistance, and surface layer resistance were obtained in LiBF₄-EC-PC solution. There is no major influence of the electrolyte solution compositions upon lithium ion transport in the LiMn₂O₄ bulk at 55 °C. The diffusion coefficient of lithium in the solid phase varied within 10⁻¹⁰-10⁻⁸ cm² s⁻¹ in the three solutions. In general, it seems that in LiBF₄ solutions, the surface chemistry is the most stable in the three solutions examined, and hence the electrode impedance in LiBF₄ solutions was the lowest. In LiPF₆ solutions, HF seems to play an important role, and thus, the electrode impedance is relatively high due to the precipitation of surface LiF.     

Electrolytes for hybrid carbon-MnO2 electrochemical capacitors

Different aqueous-based electrolytes have been tested in order to improve the electrochemical performance of hybrid (asymmetric) carbon/MnO₂ electrochemical capacitor (EC). Chloride and bromide aqueous solutions lead to the formation of Cl₂ and Br₂ respectively upon oxidation of the corresponding salt, thus limiting the useful electrochemical window of the MnO₂ electrode and producing gas evolution (in the case of chloride salts) detrimental to the cycling ability of an hybrid device. For sulfate and nitrate salts, MnO₂ electrode exhibits a 20% increase in capacitance when lithium is used as the cation compared to sodium or potassium salts, probably due to partial lithium intercalation in the tunnels of α-MnO₂ structure. The higher ionic conductivity and solubility of LiNO₃ has led to the investigation of this electrolyte in carbon/MnO₂ supercapacitor compared to standard hybrid cell using K₂SO₄. A lower resistance increase was evidenced when the temperature was decreased down to −10 °C. Long term cycling ability of carbon/MnO₂ supercapacitor was also evidenced with 5 M LiNO₃ electrolyte.     

Characterization of poly(vinylidenefluoride-co-hexafluoropropylene)-based polymer electrolyte filled with TiO2 nanoparticles

Nanoscale TiO₂ particle filled poly(vinylidenefluoride-co-hexafluoropropylene) film is characterized by investigating some properties such as surface morphology, thermal and crystalline properties, swelling behavior after absorbing electrolyte solution, chemical and electrochemical stabilities, ionic conductivity, and compatibility with lithium electrode. Decent self-supporting polymer electrolyte film can be obtained at the range of <50 wt% TiO₂. Different optimal TiO₂ contents showing maximum liquid uptake may exist by adopting other electrolyte solution. Room temperature ionic conductivity of the polymer electrolyte placed surely on the region of >10⁻³ S/cm, and thus the film is very applicable to rechargeable lithium batteries. An emphasis is also be paid on that much lower interfacial resistance between the polymer electrolyte and lithium metal electrode can be obtained by the solid-solvent role of nanoscale TiO₂ filler.     

Advanced experimental analysis of the reaction of Ca(OH)2 with HCl and SO2 during the spray dry scrubbing process

This work evaluates both the removal efficiencies of HCl and SO₂ at different points in a spray dryer using Ca(OH)₂ as the absorbent. The operating conditions were specified in terms of the temperature of the flue gas (200-300 °C), the HCl concentration (120-1000 ppm), the SO₂ concentration (150-500 ppm) and the amount of CaCl₂ added (10-30 wt.%).The experimental results showed that the SO₂ removal efficiencies were higher in the presence of HCl (120-500 ppm) than in the absence of HCl at 250 °C and 20% relative humidity (RH). However, the removal efficiency of SO₂ decreased as the HCl concentration increased. The removal efficiency of SO₂ also increased with the amount of CaCl₂ in the spray dryer.     

Comb-shaped single ion conductors based on polyacrylate ethers and lithium alkyl sulfonate

Comb-shaped single ion conductors have been synthesized by (1) sulfonation of small molecule chloroethyleneglycols, which, after ion exchange to the Li⁺ salt were then converted to the acrylate by reaction with acryloyl chloride and copolymerized with polyethylene glycol monomethyl ether acrylate (Mn = 454, n = 8) (PAE₈-co-E₃SO₃Li); (2) sulfonation of chloride end groups grafted on to prepolymers of polyacrylate ethers (PAE₈-g-E_nSO₃Li, n = 2, 3). The highest conductivity at 25 °C of 2.0 × 10⁻⁷ S cm⁻¹ was obtained for the PAE₈-co-E₃SO₃Li with a salt concentration of EO/Li = 40. The conductivity of PAE₈-g-E₃SO₃Li is lower than that of PAE₈-co-E₃SO₃Li at similar salt concentrations, which is related to the incomplete sulfonation of the grafted polymer that leads to a lower concentration of Li⁺. The addition of 50 wt.% of plasticizer, PC/EMC (1/1, v/v), to PAE₈-g-E₂SO₃Li increases the ambient conductivity by three orders of magnitude, which is due to the increased ion mobility in a micro-liquid environment and an increase concentration of free ions as a result of the higher dielectric constant of the solvent. A symmetrical Li/Li cell with an electrolyte membrane consisting of 75 wt.% PC/EMC (1/1, v/v) was cycled at a current density of 100 μA cm⁻² at 85 °C. The cycling profile showed no concentration polarization after a break-in period during the first few cycles, which was apparently due to reaction of the solvent at the lithium metal surface that reacted with lithium metal to form a stable SEI layer.     

Electrosyntheses of poly(neutral red), a polyaniline derivative

The electrochemical oxidation of neutral red in 0.5 mol dm⁻³ H₂SO₄ solution was carried out by using repeated potential cycling between −0.20 and 1.20 V (versus SCE). The polymer film was electrochemically deposited on a platinum anode and had an electrochemical activity in the solution of 0.5 mol dm⁻³ Na₂SO₄ with pH ≤ 4.0. The result from the X-ray photoelectron spectroscopy (XPS) experiment shows that the anions can be doped into the polymer film during the electropolymerization reaction of neutral red. The scanning electron microscopy (SEM) micrograph shows the surface of poly(neutral red) film deposited on the platinum foil is covered with a micro-structured network of mass interwoven fibers with a diameter of 2-4 μm. A straight fiber of the unsystematic micro-fibers is longer than 0.4 mm. The UV-vis spectrum and infrared spectrum (IR) of the polymer are different from those of the monomer.     

Properties of nano-crystalline LiMn2O4 thin films deposited by pulsed laser deposition

Properties of LiMn₂O₄ thin films deposited on polished stainless steel substrates at 400 °C and 200 mTorr of oxygen by pulsed laser deposition have been characterized by electrochemical measurements and physical analyses. The film was mainly composed of nano-crystals less than 100 nm. A maximum specific capacity of 141.9 mAh/g cycled between 3.0 and 4.5 V with a current density of 20 μAh/cm² has been achieved. The film exhibited an excellent cycling stability up to 500 cycles. The low charge-transfer resistance at high potentials as revealed by AC impedance resulted in high charge/discharge potential and more capacity. The effect of overdischarge was limited and Jahn-Teller effect was overcome to a significant extent in this nano-crystalline film. Ex situ XRD, Raman and XPS provided supporting evidence in the changes in structure, reactivity and cycling stability of nano-crystalline LiMn₂O₄ film cathodes under different charge/discharge states and cycling tests. SEM images also revealed the stability of the surface topography after a long-term cycling test.     

A novel polymer electrolyte based on PMAML/PVDF-HFP blend

A gel polymer electrolyte based on the blend of poly(methyl methacrylate-co-acrylonitrile-co-lithium methacrylate) (PMAML) and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was prepared and characterized. The synthesized PMAML were characterized by FTIR and NMR, respectively, and the surface morphology of the PMAML and PVDF-HFP blend membrane was also observed by scanning electron microscope (SEM). The electrochemical properties of composite electrolyte membranes were studied. The ionic conductivity of the polymer electrolyte composed of 75 wt.% 1 M LiBF₄ in ethylene carbonate (EC) and dimethyl carbonate (DMC) (EC:DMC=1:1 by weight) was about 2.6×10⁻³ S cm⁻¹ at ambient temperature. The electrochemical window of the polymer electrolyte was about 4.6 V determined from the linear sweep voltammetry plot. The lithium ion polymer batteries were assembled by sandwiching gel polymer electrolyte between LiCoO₂ cathode and mesophase carbon fibre (MPCF) anode. Charge-discharge test results display that lithium ion batteries with these gel polymer electrolytes have good electrochemical performance.     

Effects of the surface treatment of the Al2O3 filler on the lithium electrode/solid polymer electrolyte interface properties

The lithium deposition-dissolution process in solid polymer electrolytes containing Al₂O₃ filler treated under different conditions has been investigated comparing with the ionic conduction behavior of the electrolyte. The composite electrolytes were prepared from poly(ethylene oxide) (PEO), LiBF₄ and α-Al₂O₃ filler by using a dry process, where the surface of α-Al₂O₃ was beforehand modified by a wet process. The exchange current densities, i₀, of the lithium electrode process in P(EO)₂₀LiBF₄ with and without Al₂O₃ filler were determined by a micro-polarization method. The temperature dependence of i₀ provided similar values for activation energy, ca. 25 and 70 kJ mol⁻¹ in both temperature regions above and below 60 °C, respectively. The effect of the surface treatment of the filler on the lithium electrode process gave a different tendency from that on the ionic conductivity. The Al₂O₃ surface treated by alkali solution enhanced the electrode process to the largest extent among the fillers used here, while it led to rather poor cycling stability in voltammetry. The enhanced reaction rate at the lithium electrode/solid polymer electrolyte interface has probably resulted in the improved ion dissociation by the surface groups of the Al₂O₃ filler.