Effect of 15-crown-5 on the charge transfer resistance at the polymer electrolyte/modified Li-electrode interface

The effect of 15-crown-5, which is applied immediately to pure and modified surface of a lithium electrode, on the charge transfer resistance at the electrode/polymer electrolyte interface is studied. The polymer electrolyte consists of a 1: 1 mixture of oligourethan dimethacrylate and polypropylene glycol monomethacrylate (20 wt %), an initiator (azobisisobutyronitrile) (2 wt %), and a 1 M LiClO₄ solution in gamma-butyrolactone (78 wt %). The conductivity of this gel electrolyte is 3 × 10^?3 S cm^?1. The temperature dependence of the impedance of the Li/gel electrolyte/Li electrochemical cells is measured for electrodes of four types. The activation energies for the charge transfer at the Li/electrolyte interface are calculated. It is found that, after treating the test lithium electrodes with 15-crown-5, the charge transfer resistance decreases, and in the case of the modified lithium surface, the activation energy for the process decreases by 1.8 times.     

Irreversible capacity of the amorphous silicon thin-film electrodes

Irreversible processes accompanying the lithium incorporation into amorphous thin-film silicon are investigated. It is shown that the irreversible processes occurred during the cathodic polarization result in the formation of passive film at the silicon surface. The passive film at silicon is close, in its composition, to the passive film at carbonaceous materials. However, unlike the carbonaceous electrodes, no effect of electrolyte composition on the irreversible capacity of the silicon electrodes is observed.     

Effect of fullerene coating on silicon thin film anodes for lithium rechargeable batteries

To improve the electrochemical performances of Si thin film anodes for lithium rechargeable batteries, fullerene thin films are prepared by plasma-assisted evaporation methods to be used as coating materials. Analyses via Raman and X-ray photoelectron spectroscopy indicate that amorphous polymeric films originated from fullerene are formed on the surface of the silicon thin film. The electrochemical performance of these fullerene-coated silicon thin film as an anode material for rechargeable lithium batteries has been investigated by cyclic voltammetry, charge/discharge tests, and electrochemical impedance spectroscopy. The fullerene-coated Si thin films demonstrated a high specific capacity of above 3,000 mAh g⁻¹ as well as good capacity retention for 40 cycles. In comparison with bare silicon anodes, the fullerene-coated silicon thin film showed superior and stable cycle performance which can be attributed to the fullerene coating layer which enhances the Li-ion kinetic property at the electrode/electrolyte interface.     

Electrochemical intercalation kinetics of lithium ions for spinel LiNi0.5Mn1.5O4 cathode material

The crystal structure and electrochemical intercalation kinetics of spinel LiNi_0.5Mn_1.5O₄ such as the resistance of a solid electrolyte interphase (SEI) film, charge transfer resistance (R _ct), surface layer capacitance, exchange current density (i ₀), and chemical diffusion coefficient are evaluated by Fourier transform infrared (FT-IR) and electrochemical impedance spectroscopy (EIS), respectively. FT-IR shows that LiNi_0.5Mn_1.5O₄ thus obtained has a cubic spinel structure, which can be indexed in a space group of Fd3m with a disordering distribution of Ni. EIS indicates that R _s is almost a constant at different states of charge. The thickness of SEI film increases with increasing of the cell voltage. R _ct values evidently decreases when lithium ions deintercalated from the cathode in the voltage range from OCV to 4.6 V, and R _ct value increases with increasing potential of deintercalation over 4.7 V. i ₀ varies between 0.2 and 1.6 mA cm^?2, and the solid phase diffusion coefficient of Li⁺ changed depending on the electrode potential in the range of 10^?11–10^?9 cm² s^?1.     

Employment of [Amim] Cl in the effort to upgrade the properties of cellulose acetate based polymer electrolytes

Transparent thin film polymer electrolytes were prepared by solvent casting technique with the doping of environmental-friendly ionic liquid, 1-allyl-3-methylimidazolium chloride ([Amim] Cl) into the matrix formed by cellulose acetate (CA) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). The ionic conducting nature of this system improves significantly from the order of 10^?7–10^?2 S cm^?1 upon increasing doping of [Amim] Cl content till a maximum of 4.68 × 10^?2 S cm^?1 is attained for the composition CA:LiTFSI:[Amim] Cl (14:6:80 wt%). The improving trend in ionic conductivity results from the bond weakening between the connecting atoms in the crystalline region that induces to the increase in amorphous counterpart fractions in the CA matrix. This observation was proved via the accountancies in the reduction of relative viscosity, root mean square value and increase in void as increase in [Amim] Cl doping. The resultant phase conversion hence permits immense lithium ion (Li⁺) fluidity along the polymer backbone and assisting the improvement in ionic conductivity. The thin film polymer electrolyte is found to be elastic in the presence of crystalline fraction and radically deforms upon the chains diffusion into the amorphous fraction. The linear curvatures of the Arrhenius plot justify the conductivity improvement as via the increasing frequency of Li⁺ ions hopping as the temperature increases. The increasing addition of [Amim] Cl diminishes both the heat-resistivity and thermal stability of CA:LiTFSI:[Amim] Cl matrix.     

Lithium intercalation in thin amorphous-silicon films

Electrochemical intercalation of lithium in thin films of amorphous hydrogenated silicon (a-Si:H), deposited at temperatures of 100 and 250°C on stainless-steel substrates, is studied. It is shown that the discharge capacity of films of identical thicknesses manufactured at a temperature of 250°C is greater than that for films produced at of 100°C. Dependence of the discharge capacity of the films manufactured at 250°C on their thickness is examined. It is established that an increase in the film thickness leads to acceleration of the decrease in the discharge capacity in the course of cycling. At a current density of 0.175 mA cm^?2, the discharge capacity of films 0.25 and 1.35 μm thick equals nearly 2 Ah g^?1 in a third cycle, whereas in a hundredth cycle it amounts to 1.10 and 0.37 Ah g^?1, respectively. The diffusion coefficient for lithium in the films is equal to ～10^-13 cm²s^?1.     

Electrolytic molybdenum sulfides for thin-layer lithium power sources

The active molybdenum sulfide compound Mo₂S₃, which should be considered as a cathode material for thin-layer rechargeable power source, has been produced by electrolysis. Using impedance spectroscopy and potential relaxation method after current interruption, the kinetic parameters of lithium intercalation in electrolytic Mo₂S₃ have been obtained. Activation energy of Li⁺ migration in electrolyte (13.76 kJ/mol), charge transfer through the Mo₂S₃ electrode/electrolyte interface (38.8 kJ/mol), and Li⁺ diffusion in a solid phase (57.3 kJ/mol) have also been established. Taking into account the coefficient data of charge mass transfer in a solid phase and the reaction rate coefficient of charge transfer through the interface electrode/electrolyte within the temperature range 20–50 °C, the stage of Li⁺ transfer in a solid phase has been determined as a limiting stage for lithium intercalation in electrolytic molybdenum sulfide Mo₂S₃.     

Hydrogen Evolution Efficiency at Illuminated Silicon

The change of kinetic characteristics of photoelectrochemical hydrogen evolution at p-type silicon in acid aqueous electrolyte solutions under prolonged continuous illumination is studied. It is shown that, during the transfer of full charge Q _t < 150 C/cm² through the silicon/electrolyte interface, the interrelation between the reciprocal time of the charge transfer into electrolyte and the steady-state current remains linear. In this case, a Tafel-like relation links the interfacial charge to the steady-state electrode current. Passing current through the electrode even further results, at Q _t > 350 C/cm², in breaking-down the direct relation between the current and the charge transfer time, despite the electrode's retaining high photosensitivity. The effect is probably caused by significant energy and structure distortions in the surface layer of silicon.     

Thin film nonstoichiometric chromium oxide-based cathode material for rechargeable and primary lithium batteries

The application of nonstoichiometric chromium oxide-based thin film cathodes in lithium rechargeable and primary batteries operating at high rates has been demonstrated. Films of varying composition have been obtained by anionic Cr (VI) species electrodeposition on a 1X18N10Т grade stainless steel cathode from fluoride-containing electrolytes. The effect of film doping by Li⁺ ions during its electrosynthesis has been сonsidered. As-prepared films were studied by scanning and transmission electron microscopies, 3D optical profiler, thermogravimetric analysis, chemical analysis, and X-ray diffraction (XRD). The main phase components of the electrodeposited films regardless of Li⁺ in an electrolyte are Cr₂O₃, α-CrOOH, β-CrOOH, and metallic chromium as shown by XRD pattern refinement. The electrochemical reduction rate in a non-aqueous electrolyte (1 M LiClO₄ in PC/DME) correlates with the chromium oxide-hydroxide component content of film. Primary CrO _x -Li CR2325 mock-up cathode coating can be discharged in a pulsed mode at 10 Ω external resistance with 80–84 mA cm^?2 current densities for 10–100 ms. Thin film cathodes electrodeposited in the presence of lithium ions become rechargeable when the lithium content of the film reaches 0.02 wt.%. Mock-ups of CR2325 coin battery with a thin film cathode doped with lithium ions can be discharged more than 40 times with 136 mAh g^?1 specific capacity, 461 Wh kg^?1 specific energy and 154 W kg^?1 power density at 30 kΩ external resistance. The simplicity of thin film preparation makes this technology promising for thin film lithium batteries.     

Evaluating the electrochemical properties of PEO‐based nanofibrous electrolytes incorporated with TiO2 nanofiller applicable in lithium‐ion batteries

In the present work, nanofibrous composite polymer electrolytes consist of polyethylene oxide (PEO), ethylene carbonate (EC), propylene carbonate (PC), lithium perchlorate (LiClO₄), and titanium dioxide (TiO₂) were designed using response surface method (RSM) and synthesized via an electrospinning process. Morphological properties of the as‐prepared electrolytes were studied using SEM. FTIR spectroscopy was conducted to investigate the interaction between the components of the composites. The highest room temperature ionic conductivity of 0.085 mS.cm^?1 was obtained with incorporation of 0.175 wt. % TiO₂ filler into the plasticized nanofibrous electrolyte by EC. Moreover, the optimum structure was compared with a film polymeric electrolyte prepared using a film casting method. Despite more amorphous structure of the film electrolyte, the nanofibrous electrolyte showed superior ion conductivity possibly due to the highly porous structure of the nanofibrous membranes. Furthermore, the mechanical properties illustrated slight deterioration with incorporation of the TiO₂ nanoparticles into the electrospun electrolytes. This investigation indicated the great potential of the electrospun structures as all‐solid‐state polymeric electrolytes applicable in lithium ion batteries.     

EQCM studies of composition and electrochemical performance of film prepared by electrochemical reduction of sodium ferrate

A solid film was prepared by electrodepositing on a gold-film-coated quartz crystal electrode in Na₂FeO₄ solution, and characterized in 1?M LiClO₄/propylene carbonate (PC) + 1, 2-dimethoxyethane (DME; 1:1 by volume) electrolyte using electrochemical quartz crystal microbalance (EQCM). The EQCM experimental and X-ray photoelectron spectroscopy results indicate that the composition of the electrodeposited solid film prepared in the potential range of 0.18 to ?0.57?V vs. Ag/AgCl is FeOOH; and almost 1?mol lithium ions can be intercalated into and then extracted from 1?mol FeOOH film during discharge/charge process in 1?M LiClO₄/PC + DME electrolyte. The discharge/charge experiment indicates that the specific capacity of FeOOH film stabilizes at a value close to its theoretical specific capacity after 20 cycles, and FeOOH film maintains a specific capacity of about 300?mAh?g^?1 at the end of 170 cycles. It is therefore concluded that the FeOOH film has a good electrochemical cycle ability in 1?M LiClO₄/PC + DME electrolyte.     

A theoretical study of lithium absorption in amorphous and crystalline silicon

Lithium absorption in silicon is studied at the DFT level. By means of the developed method of modeling the structure of amorphous silicon, including with impurities, it is shown that with an increase in the lithium concentration intermediate amorphous Li _x Si _y phases, up to the crystalline Li₁₅Si₄ phase, form in crystalline silicon. An increase in the silicon cell volume, as it is filled with lithium, is calculated. A nonlinear dependence of silicon voltage on lithium intercalation is found. The lithium diffusion coefficient in crystalline silicon at a low lithium concentration is calculated and it is demonstrated for amorphous silicon that lithium diffusion is substantially accelerated by the lattice deformation inherent in amorphous silicon. The calculated values can be used in the production of high-capacity lithium ion batteries with a silicon anode.     

Impedance Analysis of Inherently Redox‐Active Ionic‐Liquid‐Based Photoelectrochemical Cells: Charge‐Transfer Mechanism in the Presence of an Additional Redox Couple

An intensive electrochemical impedance study was carried out to understand the charge‐transfer processes in photoelectrochemical (PEC) cells based on ionic liquid (IL) electrolytes. Three different electrolytes were utilized to understand the role of redox species as well as the medium on the charge‐transfer mechanism. The negligible diffusion resistance, despite the presence of two different redox species in the case of Fe(CN)₆^?4/?3 in IL, was explained on the basis of charge transfer between species of two different redox couples. Accordingly, the redox species are not required to travel through the bulk of the electrolyte for the removal of accumulated charges, as short‐range charge transfer between the IL and the Fe(CN)₆^?4/?3 species facilitates the removal of accumulated charges. It is also shown that PEC cells utilizing dual redox couples are highly stable with larger photoelectrochmeical windows, >3 V.     

Evolution of an iron passive film in a borate buffer solution (pH 8.4)

The evolution under open-circuit conditions of iron passive films formed at 0.8 V_SCE in a borate buffer solution at pH 8.4 was investigated with electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. The composition of the freshly formed passive film as determined by X-ray photoelectron spectroscopy (XPS) was found to be in agreement with a bilayer model, where the inner layer is composed mainly of iron oxide and the outer layer consists of a hydrated material. Results of XPS measurements also showed that the open-circuit breakdown of passive films was consistent with a reductive dissolution mechanism. When the iron electrode reached an intermediate stage in the open-circuit potential decay (approximately −0.3 V_SCE), the oxide film, containing both Fe(II) and Fe(III), was still protective. The impedance response in this stage exhibited a mixed control by charge transfer at the metal/film and film/solution interfaces and diffusion of point defects through the film. At the final stage of the open-circuit potential decay (approximately −0.7 V_SCE), the oxide film was very thin, and the ratio of Fe³⁺/Fe²⁺ and O²⁻/OH⁻ had decreased significantly. The impedance response also exhibited a mixed charge-transfer–diffusion control, but the diffusion process was related to transport of species in the electrolyte solution resulting from dissolution of the oxide film.     

K2Mn4O8/Reduced Graphene Oxide Nanocomposites for Excellent Lithium Storage and Adsorption of Lead Ions

Ion diffusion efficiency at the solid–liquid interface is an important factor for energy storage and adsorption from aqueous solution. Although K₂Mn₄O₈ (KMO) exhibits efficient ion diffusion and ion‐exchange capacities, due to its high interlayer space of 0.70 nm, how to enhance its mass transfer performance is still an issue. Herein, novel layered KMO/reduced graphene oxide (RGO) nanocomposites are fabricated through the anchoring of KMO nanoplates on RGO with a mild solution process. The face‐to‐face structure facilitates fast transfer of lithium and lead ions; thus leading to excellent lithium storage and lead ion adsorption. The anchoring of KMO on RGO not only increases electrical conductivity of the layered nanocomposites, but also effectively prevents aggregation of KMO nanoplates. The KMO/RGO nanocomposite with an optimal RGO content exhibits a first cycle charge capacity of 739 mA h g^?1, which is much higher than that of KMO (326 mA h g^?1). After 100 charge–discharge cycles, it still retains a charge capacity of 664 mA h g^?1. For the adsorption of lead ions, the KMO/RGO nanocomposite exhibits a capacity of 341 mg g^?1, which is higher than those of KMO (305 mg g^?1) and RGO (63 mg g^?1) alone.     

Analysis of degradation of electrolytic Fe, Co, Ni sulfides and their graphitized analogs in lithium battery using impedance spectroscopy

Changes in parameters of complex-plane plots are analyzed for electrolytic Me _x S _y electrodes (Me = Fe, Co, Ni) and their graphitized analogs contacting electrolyte (ethylene carbonate, dimethyl carbonate, 1 M LiClO₄) in the case of degradation in a layout lithium battery at cycle 15. Impedance measurements in the high- and medium-frequency region of complex-plane plots revealed the key role of the surface solid-phase film and also of resistance of charge transfer through the sulfide material/electrolyte interface in degradation of Me _x S _y electrodes under cycling in the potential range of 2.80?0.02 V vs. Li/Li⁺. The activation energy is determined for processes of charge transfer through the electrode/electrolyte interface.     

A novel carbon-coated LiCoO2 as cathode material for lithium ion battery

Using a commercially available LiCoO₂ as starting material, a surface-modified cathode material was obtained by coating it with a nano layer of amorphous carbon. The carbon-coated LiCoO₂ was characterized by X-ray diffraction analysis, scanning electronic microscopy, transmission electronic microscopy, electrochemical impedance spectroscopy and measurement of charge/discharge behavior. Results show that the carbon-coated LiCoO₂ displays marked lower charge transfer resistance, higher lithium ion diffusion coefficient and much better rate capability than the original LiCoO₂. It also indicates promising application of lithium ion batteries in the areas requiring charge and discharge at high rate.     

Study of lithium ion intercalation/de-intercalation into LiNi1/3Mn1/3Co1/3O2 in aqueous solution using electrochemical impedance spectroscopy

The mechanism of lithium ion intercalation/de-intercalation into LiNi_1/3Mn_1/3Co_1/3O₂ cathode material prepared by reactions under autogenic pressure at elevated temperatures method is investigated both in aqueous and non-aqueous electrolytes using electrochemical impedance spectroscopy (EIS) technique. In accordance with the results obtained an equivalent circuit is used to fit the impedance spectra. The kinetic parameters of intercalation/de-intercalation processes are evaluated with the help of the same equivalent circuit. The dependence of charge transfer resistance (R _ct), exchange current (I ₀), double layer capacitance (C _dl), Warburg resistance (Z _w), and chemical diffusion coefficient (D _Li+) on potential during intercalation/de-intercalation is studied. The behavior of EIS spectra and its potential dependence is studied to get the kinetics of the mechanism of intercalation/de-intercalation processes, which cannot be obtained from the usual electrochemical studies like cyclic voltammetry. The results indicate that intercalation and de-intercalation of lithium ions in aqueous solution follows almost similar mechanism in non-aqueous system. D _Li+ values are in the range of 10^?8 to 10^?14?cm²?s^?1 in aqueous 5?M LiNO₃ and that in non-aqueous 1?M LiAsF₆/EC+DMC electrolyte is in the order of 10^?12?cm²?s^?1 during the intercalation/de-intercalation processes. A typical cell LiTi₂ (PO4)₃/5?M LiNO₃/LiNi_1/3Mn_1/3Co_1/3O₂ is constructed and the cycling stability is compared to that with an organic electrolyte.     

Kinetics of electrochemical lithium intercalation into thin tungsten (VI) oxide layers

The methods of galvanostatic intermittent titration, cyclic voltammetry, and electrode impedance spectroscopy are used to study the behavior of tungsten (VI) oxide film electrodes free of binding and conducting additives in the course of reversible lithium intercalation from nonaqueous electrolyte at 25°C. The studies are performed for electrodes with different degrees of crystallinity at the variation of the lithium concentration in intercalate from zero to 0.017 mol/cm³. Lithium diffusion coefficient is in the range of 10^?11–10^?16 cm²/s. The concentration dependences of the intercalation-layer transport parameters are analyzed, the equivalent circuit versions are discussed, and results obtained by different methods are compared.     

Li/LiFePO4 batteries with room temperature ionic liquid as electrolyte

Room temperature ionic liquid (RTIL) was prepared on basis of N-methyl-N-butylpiperidinium bis(trifluoromethanesulfonyl)imide (PP₁₄TFSI), which showed a wide electrochemical window (?0.1–5.2 V vs. Li⁺/Li) and is theoretically feasible as an electrolyte for batteries with metallic Li as anodes. The addition of vinylene carbonate (VC) improved the compatibility of PP₁₄TFSI-based electrolyte towards lithium anodes and enhanced the formation of solid electrolyte interphase film to protect lithium anodes from corrosion. Accordingly, Li/LiFePO₄ cells initially delivered a discharge capacity of about 127 mAh g^?1 at a current density of 17 mA g^?1 in the ionic liquid with the addition of VC and showed better cyclability than in the neat ionic liquid. Electrochemical impedance spectroscopy disclosed that the addition of VC enhanced Li-ion diffusion and depressed interfacial resistance significantly.