LiPF6 based ethylene carbonate-dimethyl carbonate electrolyte for high power density electrical double layer capacitor

Electrochemical characteristics of the electrical double layer capacitor based on the two identical microporous carbide derived carbon C(TiC 950) electrodes in 1 M LiPF₆ ethylene carbonate-dimethyl carbonate (1:1 by volume) mixture have been studied using cyclic voltammetry and electrochemical impedance spectroscopy. Specific capacitance, phase angle, series and parallel resistances, characteristic time constant, energy and power densities etc. have been calculated and found to be dependent on the cell potential applied. Wide region of ideal polarisability ΔE ≤ 3.2 V, short characteristic time constant and high limiting capacitance 129 F g⁻¹, complex power and maximal energy and power density values have been obtained, indicating that this electrolyte can be used for high energy and power density supercapacitors. Additionally, the supercapacitors based on the partially graphitized C(VC) (applied as negatively charged electrode) and amorphous C(TiC 950) (applied as positively charged electrode) were completed and tested. The calculated energy and power densities were for asymmetrical C(VC 1100)|C(TiC 950)|1 M LiPF₆ + EC + DMC cell 26.2 Wh kg⁻¹ and 57.2 kW kg⁻¹, respectively, but for symmetrical C(TiC 950)|C(TiC 950)|1 M LiPF₆ + EC + DMC cell somewhat higher energy density 36.7 Wh kg⁻¹ and power density 83.6 kW kg⁻¹ values were established.     

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.     

Impact of “Super Acid” like filler on the properties of a PEGDME/LiClO4 system

Composite polymer electrolyte based on a new class of filler added to a PEGDME/LiClO₄ model system has been investigated. “Ceramic super acids” used consist of grafted SO₄²⁻ groups on Al₂O₃ particles surface obtained by calcinations route. Conductivity, DSC and FT-IR measurements performed on such composite electrolytes, when compared to the model PEGDME/LiClO₄ electrolyte, showed only slight improvement of their inner characteristics. In contrary, Li/Li symmetric cells study, by means of impedance spectroscopy, has presented a spectacular decrease of the interfacial resistance compare to the model electrolyte. This result opens a new pathway of investigation to master the lithium metal/polymer electrolyte interface.     

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.     

A study of electrochemical kinetics of lithium ion in organic electrolytes

Limiting current densities equivalent to the transport-controlling step of lithium ions in organic electrolytes were measured by using a rotating disk electrode (RDE). The diffusion coefficients of lithium ion in the electrolyte of PC/LiClO₄, EC : DEC/LiPF₆ and EC : DMC/LiPF₆ were determined by the limiting current density data according to the Levich equation. The diffusion coefficients increased in the order of PC/LiClO₄<EC : DEC/LiPF₆<EC : DMC/ LiPF₆ with respect to molar concentration of lithium salt. The maximum value of diffusivity was 1.39x10-5cm²/s for 1M LiPF₆ in EC : DMC=1 : 1. Exchange current densities and transfer coefficients of each electrolyte were determined according to the Butler-Volmer equation.     

LiClO4 doped cellulose acetate as biodegradable polymer electrolyte for supercapacitors

The possibility of producing a biodegradable polymer electrolyte based on cellulose acetate (CA) with varied concentration of LiClO₄ for use in supercapacitors has been investigated. The successful doping of the CA films has been analyzed by FTIR and DSC measurements of the LiClO₄ doped CA films. The ionic conductivity of the films increased with increase in salt content and the maximum ionic conductivity obtained for the solid polymer electrolyte at room temperature was 4.9 × 10^?3 Ω^?1 for CA with 16% LiClO₄. The biodegradation of the solid polymer electrolyte films have been tested by soil burial, degradation in activated sludge, and degradation in buffer medium methods. The extent of biodegradation in the films has been measured by AC Impedance spectroscopy and weight loss calculations. The study indicated sufficient biodegradability of the materials. A p/p polypyrrole supercapacitor has been fabricated and its electrochemical characteristics and performance have been studied. The supercapacitor showed a fairly good specific capacitance of 90 F g^?1 and a time constant of 1 s. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Supercapacitive properties of a nanowire-structured MnO2 electrode in the gel electrolyte containing silica

Nanowire-structured MnO₂ active materials were prepared by a chemical precipitation method and their supercapacitive properties for use in the electrodes of supercapacitors were investigated by means of cyclic voltammetry in an aqueous gel electrolytes consisting of 1 M Na₂SO₄ and fumed silica (SiO₂). The MnO₂ electrode showed a maximum specific capacitance of 151 F g⁻¹ after 1000 cycles at 100 mV s⁻¹ when using the gel electrolyte containing 3 wt.% of SiO₂, which is higher than 121 F g⁻¹ obtained when using the 1 M Na₂SO₄ liquid electrolyte alone.     

Electrochemical performances of poly(3,4-ethylenedioxythiophene)-NiFe2O4 nanocomposite as electrode for supercapacitor

Poly 3,4-ethylenedioxythiophene (PEDOT)-based NiFe₂O₄ conducting nanocomposites were synthesized and their electrochemical properties were studied in order to find out their suitability as electrode materials for supercapacitor. Nanocrystalline nickel ferrites (5-20 nm) have been synthesized by sol-gel method. Reverse microemulsion polymerization in n-hexane medium for PEDOT nanotube and aqueous miceller dispersion polymerization for bulk PEDOT formation using different surfactants have been adopted. Structural morphology and characterization were studied using XRD, SEM, TEM and IR spectroscopy. Electrochemical performances of these electrode materials were carried out using cyclic voltammetry at different scan rates (2-20 mV/s) and galvanostatic charge-discharge at different constant current densities (0.5-10 mA/cm²) in acetonitrile solvent containing 1 M LiClO₄ electrolyte. Nanocomposite electrode material shows high specific capacitance (251 F/g) in comparison to its constituents viz NiFe₂O₄ (127 F/g) and PEDOT (156 F/g) where morphology of the pore structure plays a significant role over the total surface area. Contribution of pseudocapacitance (C_FS) arising from the redox reactions over the electrical double layer capacitance (C_DL) in the composite materials have also been investigated through the measurement of AC impedance in the frequency range 10 kHz-10 mHz with a potential amplitude of 5 mV. The small attenuation (∼16%) in capacitance of PEDOT-NiFe₂O₄ composite over 500 continuous charging/discharging cycles suggests its excellent electrochemical stability.     

Effects of particle size and electrolyte salt on the thermal stability of Li0.5CoO2

Accelerating rate calorimetry (ARC) has been used to compare the thermal stability of three Li_0.5CoO₂ materials with different particle sizes (diameters of approximately 0.8, 2, and 5 μm, respectively) when heated in 1.0 M LiPF₆ EC/DEC or 0.8 M LiBoB EC/DEC electrolytes. The thermal stability of Li_0.5CoO₂ was found to be related to its particle size. The larger the particle size, the higher the onset temperature of self-heating as measured by ARC. In the presence of sufficient reducing agent, EC/DEC solvent, Li_0.5CoO₂ can be reduced to Co₃O₄, CoO, eventually even to Co metal. The heats of reaction for each of these steps are 550, 270 and 540 J/g, respectively, measured per gram of Li_0.5CoO₂. The addition of LiPF₆ salt significantly decreases the reactivity of Li_0.5CoO₂ compared to pure EC/DEC solvent. The reactivity of Li_0.5CoO₂ is stronger in 0.8 M LiBoB EC/DEC than in LiPF₆ EC/DEC.     

Electric double layer capacitors with gelled polymer electrolytes based on poly(ethylene oxide) cured with poly(propylene oxide) diamines

Using a gel electrolyte for electric double layer capacitors usually encountered a drawback of poor contact between the electrolyte and the electrode surface. A gel electrolyte consisting of poly(ethylene oxide) crosslinked with poly(propylene oxide) as a host, propylene carbonate (PC) as a plasticizer, and LiClO₄ as a electrolytic salt was synthesized for double layer capacitors. Diglycidyl ether of bisphenol-A was blended with the polymer precursors to enhance the mechanical properties and increase the internal free volume. This gel electrolyte showed an ionic conductivity as high as 2 × 10⁻³ S cm⁻¹ at 25 °C and was electrochemically stable over a wide potential range (ca. 5 V). By sandwiching this gel-electrolyte film with two activated carbon cloth electrodes (1100 m² g⁻¹ in surface area), we obtained a capacitor with a specific capacitance of 86 F g⁻¹ discharged at 0.5 mA cm⁻², while the capacitance was 82 F g⁻¹ for a capacitor equipped with a liquid electrolyte of 1 M LiClO₄/PC. The capacitance decrease with the current density was less significant for the gel-electrolyte capacitor. We found that the less restricted ion diffusion near the electrolyte/electrode interface led to the smaller overall resistance of the gel-electrolyte capacitor. The high performance of the gel-electrolyte capacitor has demonstrated that the developed polymer network not only facilitated ion motion in the electrolyte bulk phase but also gave an intimate contact with the carbon surface. The side chains of the polymer in the amorphous phase could stretch across the boundary layer at the electrolyte/electrode interface to come into contact with the carbon surface, thus improving transport of Li⁺ ions by the segmental mobility in polymer.     

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).     

Effects of electrolytes on the electrochemical performance of Si/graphite/disordered carbon composite anode for lithium-ion batteries

The influences of LiBF₄, LiClO₄, lithium bis(oxalato) borate (LiBOB), LiPF₆ with VC and without VC, and the mixed electrolytes composed of different ratios of LiBOB and LiPF₆ or LiClO₄ on the electrochemical properties of Si/graphite/disordered carbon (Si/G/DC) composite electrode were systematically investigated by constant current charge-discharge and electrochemical impedance spectra (EIS) techniques. Scanning electron microscopy (SEM) was used to observe the change of electrodes in morphology after given cycle numbers. X-ray photoelectron spectroscopy (XPS) was employed to understand the influences of different mixed electrolytes on the composition of SEI layers. The results showed that Si/G/DC composite electrode in the mixed electrolytes presented better electrochemical performance than in single electrolyte. The compactness and compositions of SEI layers intensively influenced the cycle performance of Si/G/DC composite materials. LiBOB and additive VC had a good synergistic effect on the formation of the dense SEI layers. In particular, Si/G/DC in 0.5 M LiBOB + 0.38 M LiPF₆ electrolytes containing VC exhibited a high reversible capacity and excellent cycle performance.     

Electrochemical impedance spectroscopic investigation of CO2 reduction on polyaniline in methanol

The ac response of polyaniline thin films on platinum electrodes was measured at different dc potentials during the CO₂ reduction in methanol/LiClO₄ electrolyte with a small amount of 0.5 M H₂SO₄. The complex capacitance curves were simulated and the data obtained were used to calculate kinetic parameters, based on the assumption that the thermodynamic potential E₀ is in the region of −0.2-−0.1 V versus saturated calomel electrode (SCE). With E₀=−0.2 V versus SCE and β=0.6, a j₀ value of ca. 10⁻⁴ A cm⁻² was found for the electroreduction of CO₂ on the polyaniline electrode.     

Impedance spectroscopy data for S2O8 anion electroreduction at Bi(1 1 1) plane

Electroreduction kinetics of the peroxodisulfate anions on the electrochemically polished Bi(1 1 1) single crystal electrode has been studied by impedance spectroscopy. Influence of the electrode potential, reaction intermediates, base electrolyte and reactant concentrations on the kinetic parameters of electroreduction has been established. Systematic analysis of the fitting results demonstrates the noticeable influence of adsorption of the reaction intermediate or reactant on the electroreduction rate of the S₂O₈²⁻ anions at the Bi(1 1 1) electrode. In the region of so-called “current pits” in the cyclic voltammetry curves, obtained by rotating disc electrode method, the mixed kinetics, i.e. the adsorption and “true” charge transfer limited steps have been established by impedance spectroscopy.     

The influence of preparation conditions on electrochemical properties of LiNi0.5Mn1.5O4 thin film electrodes by PLD

LiNi_0.5Mn_1.5O₄ thin films were prepared by pulsed laser deposition (PLD) on stainless steel substrates. The growth of the films has been studied as a function of substrate temperature and oxygen partial pressure in deposition, using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). Electrochemical properties of LiNi_0.5Mn_1.5O₄ thin film cathodes were investigated using cyclic voltammetry and galvanostatic charge/discharge against a lithium anode. The initial capacity and capacity retention of the films are highly dependent on the crystallinity and purity of the films. LiNi_0.5Mn_1.5O₄ thin films grown at 600 °C in an oxygen partial pressure of 200 mTorr are well crystallized with high purity, exhibiting excellent capacity retention between 3 and 5 V with a LiPF₆-based electrolyte.     

Effect of Lithium Perchlorate on the Optoelectrical and Thermal Properties of Poly(Vinylpyrrolidone)/Nano-Cesium Aluminate Solid Polymer Electrolytes

Solid polymer electrolyte (SPE) of polyvinylpyrrolidone (PVP) with varying amounts, namely, 5, 10, and 15?wt% of lithium perchlorate (LiClO₄) as an electrolyte and 8?wt% cesium aluminum oxide (CsAlO₂) nanoparticle have been fabricated by solution intercalation technique. The optoelectrical behaviors of the SPE films have been evaluated using UV–visible spectroscopy. The UV–visible spectral studies revealed the UV light-absorbing nature of NC films with considerable visible transparency. The chemical structure and morphological behaviors of PVP/8?wt% of CsAlO₂–LiClO₄ SPE films have been established by Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy, respectively. The AC conductivity of the SPEs was evaluated at room temperature by digital LCR meter in the frequency range 100 Hz–5?MHz. The thermal behaviors such as T_g and degradation patterns of the SPEs have been evaluated using differential scanning calorimetric analysis and thermogravimetric analysis, respectively.     

Ethylene carbonate—propylene carbonate mixed electrolytes for lithium batteries

Electrolytic characteristics of propylene carbonate (PC)ethylene carbonate (EC) mixed electrolytes were studied, compared with those in PC electrolytes. Conductivity and Li charge—discharge efficiency values increased with EC contents increasing. For example, 1 M LiClO₄ECPC (EC mixing molar ratio; [EC]/[PC] = 4) showed the conductivity of 8.5 ohm^?1 cm^?1, which value was 40% higher than that in PC. Also, 1 M LiClO₄ECPC([EC]/[PC] = 5) showed the Li charge—discharge efficiency of 90.5% at 0.5 mA cm^?2, 0.6 C cm^?2, which value was ca. 25% higher than that in PC. ECPC mixed electrolytes were considered to be practically available for ambient lithium batteries in regard to the high Li⁺ ion conductivity and also high Li charge—discharge efficiency.     

Gel Polymer Electrolyte with Anion‐Trapping Boron Moieties via One‐Step Synthesis for Symmetrical Supercapacitors

A novel gel polymer electrolyte (GPE) which is based on new synthesized boron‐containing monomer, benzyl methacrylate, 1 m LiClO₄/N,N‐dimethylformamidel liquid electrolyte solution is prepared through a one‐step synthesis method. The boron‐containing GPE (B‐GPE) not only displays excellent mechanical behavior, favorable thermal stability, but also exhibits an outstanding ionic conductivity of 2.33 mS cm^?1 at room temperature owing to the presence of anion‐trapping boron sites. The lithium ion transference in this gel polymer film at ambient temperature is 0.60. Furthermore, the symmetrical supercapacitor which is fabricated with B‐GPE as electrolyte and reduced graphene oxide as electrode demonstrates a broad potential window of 2.3 V. The specific capacitance of symmetrical B‐GPE supercapacitors retains 90% after 3000 charge–discharge cycles at current density of 1 A g^?1.     

Electrochemical investigation of LiMn2O4 cathodes in gel electrolyte at various temperatures

A composite lithium battery electrode of LiMn₂O₄ in combination with a gel electrolyte (1 M LiBF₄/24 wt% PMMA/1:1 EC:DEC) has been investigated by galvanostatic cycling experiments and electrochemical impedance spectroscopy (EIS) at various temperatures, i.e. −3<T<56 °C. For analysis of EIS data, a mathematical model taking into account local kinetics and potential distribution in the liquid phase within the porous electrode structure was used. Reasonable values of the double-layer capacitance, the exchange-current density and the solid phase diffusion were found as a function of temperature. The apparent activation energy of the charge-transfer (∼65 kJ mol⁻¹), the solid phase transfer (∼45 kJ mol⁻¹) and of the ionic bulk and effective conductance in the gel phase (∼34 kJ mol⁻¹), respectively, were also determined. The kinetic results related to ambient temperature were compared to those obtained in the corresponding liquid electrolyte. The incorporated PMMA was found to reduce the ionic conductivity of the free electrolyte, and it was concluded that the presence of 24 wt% PMMA does not have a significant influence on the kinetic properties of LiMn₂O₄.     

Design of an “all solid-state” supercapacitor based on phosphoric acid doped polybenzimidazole (PBI) electrolyte

The effectiveness of phosphoric acid doped polybenzimidazole as a polymer electrolyte membrane to fabricate an all solid-state super capacitor has been explored using hydrous RuO₂/carbon composite electrodes (20 wt.%) of surface area 250 m² g⁻¹ with many intrinsic advantages. The electrochemical evaluation of these super capacitors through cyclic voltammetry, charge/discharge and impedance measurements demonstrate the utility of this type of thin, compact and flexible supercapacitor capable of functioning at 150 °C to yield a maximum capacitance of about 290 F g⁻¹ along with a life of more than 1,000 cycles. A power density of 300 W kg⁻¹ and energy density of 10 Wh kg⁻¹ have been accomplished although the equivalent series resistance (ESR) of about 3.7 Ω needs to be reduced further for high rated applications.