A new approach toward improved low temperature performance of Li-ion battery

We report a new approach toward formulating an electrolyte for low temperature operation of Li-ion batteries. The core of this new approach is to use LiBF₄ salt instead of LiPF₆, which is the chosen solute in the state-of-the-art Li-ion electrolytes. We found that although LiBF₄-based electrolyte has lower ionic conductivity than the LiPF₆ analogue, it provides improved low temperature performance. In particular, at −30 °C, a Li-ion cell with 1 m (mol/kg solvent) LiBF₄ dissolved in 1:1:3 (wt.) propylene carbonate (PC)/ethylene carbonate (EC)/ethylmethyl carbonate (EMC) mixed solvent delivers as high as 86% of capacity, in comparison to that obtained at 20 °C. Whereas the counterpart one, using LiPF₆, only retains 72%. Furthermore, the cell with LiBF₄-based electrolyte shows lower polarization at −30 °C. The above results suggest that the ionic conductivity of the electrolyte is not the only limitation to the low temperature operation of Li-ion batteries. Analysis of cell impedance reveals that the improved low temperature performance by LiBF₄ arises from a reduced charge-transfer resistance.     

Temperature dependence of the conductivity of plasticized poly(vinyl chloride)-low molecular weight liquid 50% epoxidized natural rubber solid polymer electrolyte

Characterizations were carried out to study on a new plasticized solid polymer electrolyte that was composed of blends of poly(vinyl chloride) (PVC), liquid 50% epoxidized natural rubber (LENR50), ethylene carbonate, and polypropylene carbonate. This freestanding solid polymer electrolyte (SPE) was successfully prepared by solution casting technique. Further analysis and characterizations were carried out by using scanning electron microscopy (SEM), X-ray diffraction, differential scanning calorimeter (DSC), Fourier transform infrared (ATR-FTIR), and impedance spectroscopy (EIS). The SEM results show that the morphologies of SPEs are compatible with good homogeneity. No agglomeration was observed. However, upon addition of salt, formation of micropores occurred. It is worth to note that micropores improve the mobility of ions in the SPE system, thus increased the ionic conductivity whereas the crystallinity analysis for SPEs indicates that the LiClO₄ salt is well complexed in the plasticized PVC-LENR50 as no sharp crystallinity peak was observed for 5–15% wt. LiClO₄. This implies that LiClO₄ salt interacts with polymer host as more bonds are form via coordination bonding. In DSC study, it is found that the glass temperature (T _g) increased with the concentration of LiClO₄. The lowest T _g was obtained at 41.6 °C when incorporated with 15% wt. LiClO₄. The features of complexation in the electrolyte matrix were studied using ATR-FTIR at the peaks of C=O, C–O–C, and C–Cl. In EIS analysis, the highest ionic conductivity obtained was 1.20?×?10^?3 S cm^?1 at 15% wt. LiClO₄ at 353 K.     

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 simple method of electrochemical lithium intercalation within graphite from a propylene carbonate-based solution

Electrochemical lithium intercalation within graphite from 1 mol dm^− 3 solution of LiClO₄ in propylene carbonate (PC) was investigated at 25 and − 15 °C. Lithium ions were intercalated into and de-intercalated from graphite reversibly at − 15 °C despite the use of pure PC as the solvent. However, ceaseless solvent decomposition and intense exfoliation of graphene layers occurred at 25 °C. The results of the Raman spectroscopic analysis indicated that the interaction between PC molecules and lithium ions became weaker at − 15 °C by chemical exchange effects, which suggested that the thermodynamic stability of the solvated lithium ions was an important factor that determined the formation of a solid electrolyte interface (SEI) in PC-based solutions. Charge–discharge analysis revealed that the nature of the SEI formed at − 15 °C in 1 mol dm^− 3 of LiClO₄ in PC was significantly different from that formed at 25 °C in 1 mol dm^− 3 of LiClO₄ in PC containing vinylene carbonate, 3.27 mol kg^− 1 of LiClO₄ in PC, and 1 mol dm^− 3 of LiClO₄ in ethylene carbonate.     

Surface structures and electrochemical characteristics of surface-modified carbon anodes for lithium ion battery

Petroleum coke and those heat-treated at 1860 °C, 2100 °C, 2300 °C 2600 °C and 2800 °C (abbreviated as PC, PC1860, PC2100, PC2300, PC2600 and PC2800) were fluorinated by elemental fluorine of 3 × 10⁴ Pa at 200 °C and 300 °C for 2 min. Natural graphite powder samples with average particle sizes of 5 μm, 10 μm and 15 μm (abbreviated as NG5μm, NG10μm and NG15μm) were also fluorinated by ClF₃ of 3 × 10⁴ Pa at 200 °C and 300 °C for 2 min. Transmission electron microscopic (TEM) observation revealed that closed edge of PC2800 was destroyed and opened by surface fluorination, which increased the first coulombic efficiencies of PC2300, PC2600 and PC2800 by 12.1–18.2% at 60 mA/g and by 13.3–25.8% at 150 mA/g in 1 mol/dm³ LiClO₄–ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume). Light fluorination of NG10μm and NG15μm increased the first coulombic efficiencies by 22.1–28.4% at 150 mA/g in 1 mol/dm³ LiClO₄–EC/DEC/PC (PC: propylene carbonate, 1:1:1 in volume).     

Electrochemical properties of PEO/PMMA blend-based polymer electrolytes using imidazolium salt-supported silica as a filler

In this study, the composite polymer electrolytes (CPEs) were prepared by solution casting technique. The CPEs consisted of PEO/PMMA blend as a host matrix doped with LiClO₄. Propylene carbonate (PC) was used as plasticizer and a small amount of imidazolium salt-supported amorphous silica (IS-AS) as a filler was prepared by the sol–gel method. At room temperature, the highest conductivity was obtained for the composition having PEO–PMMA–LiClO₄–PC–4wt. % IS-AS with a value of 1.15 × 10^?4 S/cm. In particular, the CPE using the IS-AS filler showed a higher conductivity than any other sample (fumed silica, amorphous silica). Studies of differential scanning calorimetry and scanning electron microscopy indicated that the ionic conductivity increase was due to an expansion in the amorphous phase which enhances the flexibility of polymeric chains and the homogeneous structure of CPEs. It was found that the ionic conductivity and interfacial resistance stability of CPEs was significantly improved by the addition of IS-AS. In other words, the resistance stability and maximum ambient ionic conductivity of CPEs containing IS-AS filler were better than CPEs containing any other filler.     

Low-temperature performance of Li-ion cells with a LiBF<Subscript>4</Subscript>-based electrolyte

The effect of LiBF₄ on the low-temperature performance of a Li-ion cell was studied by using a 1:1:1 (wt) EC/DMC/DEC mixed solvent. The results show that the LiBF₄-based electrolyte has a 2- to 3-fold lower ionic conductivity and shows rather higher freezing temperature compared with a LiPF₆-based electrolyte. Owing to electrolyte freezing, cycling performance of the Li-ion cell using LiBF₄ was significantly decreased when the temperature fell below –20 °C. However, impedance data show that at –20 °C the LiBF₄ cell has lower charge-transfer resistance than the LiPF₆ cell. In spite of the relatively lower conductivity of the LiBF₄-based electrolyte, the cell based on it shows slightly lower polarization and higher capacity in the liquid temperature range (above –20 °C) of the electrolyte. This fact reveals that ionic conductivity of the electrolytes is not a limitation to the low-temperature performance of the Li-ion cell. Therefore, LiBF₄ may be a good salt for the low-temperature electrolyte of a Li-ion cell if a solvent system that is of low freezing temperature, high solubility to LiBF₄, and good compatibility with a graphite anode can be formulated. Electronic Publication     

Hydrolysis lignin-based organic electrode material for primary lithium batteries

The possibility of using hydrolysis lignin as a cathode material for primary lithium batteries has been demonstrated for the first time. The electroconductivity, morphology, and element composition of hydrolysis lignin have been investigated by means of the methods of impedance spectroscopy, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The main parameters and the behavior of lignin-based lithium batteries were studied using two electrolyte systems: 1 M LiBF₄ in γ-butyrolacton and 1 M LiClO₄ in propylene carbonate. The chemical composition of cathode materials upon battery discharge down to 0.9 V was studied by the methods of X-ray photoelectron spectroscopy and infrared spectroscopy. The suggestions on possible electrochemical reactions occurring in the lithium/hydrolysis lignin system were made on the basis of the products composition analysis.     

Anion receptor for enhanced thermal stability of the graphite anode interface in a Li-ion battery

The thermal stability of the solid electrolyte interphase (SEI) formed on a graphite anode has been enhanced by adding an anion receptor, tris(pentafluorophenyl)borane (TPFPB), to the electrolyte. The investigated electrolyte was LiBF₄ in a 2:1 mixture of ethylene carbonate (EC) and diethyl carbonate (DEC). Two concentrations of TPFPB have been investigated, 0.2 and 0.8 M. Galvanostatic cycling and differential scanning calorimetry (DSC) were used to study the effect of TPFPB on the electrochemical performance and thermal stability of graphite anodes. The best performance is obtained for a graphite anode cycled in an electrolyte with 0.2 M TPFPB: cyclability is improved, and the onset temperature for the first thermally activated reaction is increased by more than 60 °C up to 140–160 °C. X-ray photoelectron spectroscopy (XPS) has been used to examine the composition of the SEI formed in the different electrolytes; the improved performance for the graphite cycled with 0.2 M TPFPB is attributed to a reduced amount of LiF in the SEI.     

Ionic processes in solid-electrolyte passivating films on lithium

The electrochemical behaviour of a Li electrode in solutions of LiAlCl₄ in thionyl chloride, LiBF₄ in γ-butyrolactone and LiClO₄ in the mixed solvent propylene carbonate (PC) + dimethoxyethane (DME) in the process of cell storage has been investigated by the methods of electrode impedance spectroscopy and pulse voltammetry. Analogous studies have been carried out in PC + DME solution with the Li electrode coated with a specially formed protecting film of Li₂CO₃. The results have been compared with those obtained earlier for other lithium electrochemical systems. The general regularities of the Li electrode electrochemical kinetics attributed to the process of Li⁺ ion transport through a passivating film coating a lithium surface have been discussed. Received: 22 February 1999 / Accepted: 20 June 1999     

The single crystal metal-non-aqueous solvent interface: Underpotential deposition of Cd on Ag (111) in propylene carbonate

The underpotential deposition of cadmium on Ag (111) in propylene carbonate using either lithium perchlorate or tetra-n-butylammonium perchlorate as supporting electrolytes has been found to yield sharp and well defined voltammetric peaks. The results obtained in aqueous LiClO₄ and tetraethylammonium perchlorate solutions for the same UPD-host metal system were, however, found to be different from those in propylene carbonate, particularly in the case of LiClO₄ for which the voltammetry was characterized by broad and ill-defined features. This novel solvent-induced effect has been attributed to differences in the extent of specific adsorption of the perchlorate anion in water and PC and/or to solvent-metal substrate interactions.     

Empirical formula for the concentration dependence of the conductivity of organic electrolytes for lithium power sources in the vicinity of a maximum

To optimize the compositions of liquid organic electrolytes for lithium power sources, it is useful to have the dependence of the conductivity on the lithium salt concentration in a convenient analytical form. An empirical formula was suggested on the basis of the modified Kohlrausch equation for the concentration dependence of the conductivity of organic electrolytes in the vicinity of a maximum. The accuracy of this equation was checked on solutions of LiBF₄ in propylene carbonate; LiClO₄ in ethylene carbonate; and LiPF₆ in ethylene carbonate/diethyl carbonate (1: 1), ethylene carbonate/ethylmethyl carbonate (1: 1), and ethylene carbonate/methyl acetate (1: 1) at different temperatures. The calculated data are in good agreement with experiment for all the systems. The new empirical formula allows the determination of the maximum conductivity of organic electrolytes based on a few points with good accuracy, which is very important in choosing the electrolyte salt concentration in practice.     

A Vibrational Spectroscopic Study of Ion Solvation in Lithium Perchlorate/Propylene Carbonate Electrolyte

Abstract

The infrared (IR) and Raman spectra of propylene carbonate (PC) containing various concentrations of LiClO₄ have been measured and analyzed. The difference in spectra of PC with and without LiClO₄ was attributed to the interaction of the PC molecules and lithium ions. This interaction occurs mainly on the carbonyl oxygen atom of the PC molecule. The ring deformation, symmetric ring deformation, carbonyl stretching and stretching of ring oxygens for PC are sensitive to this interaction. The solvation number of Li⁺ is also calculated. On the other hand, the structure of the ClO^? ₄ is also affected by PC molecule, forming the solvent separated ion pairs.     

Laser Raman and conductivity studies of plasticized polymer electrolyte P(ECH-EO):propylenecarbonate:<Emphasis Type="Italic">γ</Emphasis>-butyrolactone:LiClO<Subscript>4</Subscript>

The plasticized polymer electrolytes composed of poly(epichlorohydrin-ethyleneoxide) (P(ECH-EO)) as host polymer, lithium perchlorate (LiClO₄) as salt, γ-butyrolactone (γ-BL), and propylene carbonate (PC) as plasticizer have been prepared by simple solution casting technique. The effect of mixture of plasticizers γ-BL and PC on conductivity of the polymer electrolyte P(ECH-EO):LiClO₄ has been studied. The band at 457 cm⁻¹ in the Raman spectra of plasticized polymer electrolyte is attributed to both the ring twisting mode of PC and the perchlorate ν ₂(ClO₄⁻) bending. The maximum conductivity value is observed to be 4.5 × 10⁻⁴ S cm⁻¹ at 303 K for 60P(ECH-EO):15PC:10γ-BL:15LiClO₄ electrolyte system. In the present investigation, an attempt has been made to correlate the Raman and conductivity data.     

Solid polymer electrolytes I,preparation, characterization,and ionic conductivity of gelled polymer electrolytes based on novel crosslinked siloxane/poly(ethylene glycol) polymers

A series of crosslinked siloxane/poly(ethylene glycol) (Si–PEG) copolymers were synthesized from the reactive methoxy‐functional silicone resin (Si resin) and PEGs with different molecular weights via two kinds of crosslinking reactions during an in situ curing stage. One of the crosslinking reactions is the self‐condensation between two methoxy groups in the Si resin, and another one is an alkoxy‐exchange reaction between the methoxy group in the Si resin and the OH group in PEG. The synthesized crosslinked copolymers were characterized by Fourier transform infrared spectroscopy, DSC, and ¹³C NMR. The crosslinked copolymers were stable in a moisture‐free environment, but the Si? O? C linkages were hydrolyzed in humid conditions. The gel‐like solid polymer electrolytes (SPEs) were prepared by impregnating these crosslinked Si–PEG copolymers in a propylene carbonate (LiClO₄/PC) solution. The highest conductivity reached 2.4 × 10^?4 S cm^?1 at 25 °C and increased to 8.7 × 10^?4 S cm^?1 at 85 °C. The conductivities of these gel‐type SPEs were affected by the content of LiClO₄/PC, the molecular weights of PEGs, and the weight fraction of the Si resin. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2051–2059, 2004     

Solvation environment of lithium ion in a LiBF<Subscript>4</Subscript>–propylene carbonate system in the presence of 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid studied by NMR and quantum chemical modeling

Solutions of lithium and 1-ethyl-3-methylimidazolium tetrafluoroborates ([emim][BF₄]) in propylene carbonate (PC) were studied by the high-resolution NMR method on ¹H, ⁷Li, ¹¹B, ¹³C, and ¹⁹F nuclei. The degree of solvation of lithium ions was determined by measuring selfdiffusion coefficients by pulse-field-gradient spin echo NMR method on ¹H, ⁷Li, and ¹⁹F nuclei. The hydrodynamic radii of solvated Li⁺ cations were estimated by the Stokes–Einstein equation. The model structures of the solvation complexes of Li⁺ ion with propylene carbonate molecules and BF ₄ ^– anion and their associates with ionic liquid components were calculated in terms of the density function theory. The calculated values of the chemical shifts were compared with the experimental data. PC molecules were predominantly bound to the Li⁺ cation, while LiBF₄–[emim][BF₄]–PC (1: 4: 4) electrolyte had a maximum conductivity of 9.5 mS cm^–1 at 24 °С compared to the compositions of a lower content of the solvent.     

Surface structure and electrochemical characteristics of natural graphite fluorinated by ClF3

Natural graphite samples with average particle sizes of 5, 10 and 15 μm (NG5 μm, NG10 μm and NG15 μm, respectively) were fluorinated by ClF₃ (3 × 10⁴ Pa) at 200 and 300 °C for 2 min. X-ray photoelectron spectra of surface-fluorinated samples showed that surface fluorine concentration increased with increase in the particle size of graphite and reaction temperature. Small amounts of chlorine were also detected in all the fluorinated samples. Raman spectra of original and surface-fluorinated samples indicated that the surface disordering was increased for NG10 μm and NG15 μm. Surface areas were decreased by the fluorination for NG5 μm and NG10 μm but unchanged for NG15 μm. The mesopores with diameter of 1.5-2 nm increased while those of 2-3 nm decreased for all the samples. First coulombic efficiencies for NG10 μm and NG15 μm were highly increased by surface fluorination in 1 mol/dm³ LiClO₄-EC/DEC/PC (EC: ethylene carbonate, DEC: diethyl carbonate, PC: propylene carbonate) solution.     

Effect of adding ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate on the coordination environment of Li<Superscript>+</Superscript> ions in propylene carbonate,according to data from IR spectroscopy and quantum chemical modeling

The effect ionic liquid (IL) 1-ethyl-3-methylimidazolium tetrafluoroborate has on the coordination environment of Li⁺ cations in carbonate solvents is studied by means of IR spectroscopy and quantum chemical modeling using the example of propylene carbonate (PC). LiBF₄ is used as the lithium salt. This system is promising for use as an electrolyte in lithium power sources (LPSs), but the mechanism of ionic conductivity by Li⁺ ions in such systems has yet to be studied in full.     

Effect of ionic conductivity of a PAN–PC–LiClO<Subscript>4</Subscript> solid polymeric electrolyte on the performance of a TiO<Subscript>2</Subscript> photoelectrochemical cell

A nanoparticle TiO₂ solid-state photoelectrochemical cell has been fabricated. The effect of ionic conductivity of a solid electrolyte of polyacrylonitrile (PAN)–propylene carbonate (PC)–lithium perchlorate (LiClO₄) on the performance of a photoelectrochemical cell of indium tin oxide (ITO)/TiO₂/PAN–PC–LiClO₄/graphite has been investigated. A nanoparticle TiO₂ film was deposited onto ITO-covered glass substrate by controlled hydrolysis technique. A solid electrolyte of PAN–LiClO₄ with PC plasticizer prepared by solution casting technique was used as a redox couple medium. The room temperature conductivity of the electrolyte was determined by AC impedance spectroscopy technique. A graphite electrode was prepared onto a glass slide by electron beam evaporation technique. The device shows a photovoltaic effect under illumination. The short-circuit current density, J _sc, and open-circuit voltage, V _oc, vary with the conductivity of the electrolyte. The highest J _sc of 2.82 μA cm⁻² and V _oc of 0.56 V were obtained at the conductivity of 4.2 × 10⁻⁴ Scm⁻¹ and at the intensity of 100 mW cm⁻².     

Ionic conductivity of hybrid films composed of polyacrylonitrile,ethylene carbonate,and LiClO4

The electrical conductivity of hybrid films consisting of polyacrylonitrile (PAN), ethylene carbonate (EC), and LiClO₄ was investigated. In these films, EC and LiClO₄ are found to be molecularly dispersed in PAN, forming solid solutions over a wide composition range. The ionic character of the electrical conductivity is demonstrated. The conductivity is not correlated with the content of LiClO₄ or of PAN, but primarily with the mole ratio [EC]/[LiClO₄] in the films. An increase in the [EC]/[LiClO₄] ratio enhances the conductivity. When the ratio is about 2, the conductivity attains 10^?4–10^?5 S cm^?1 at 25°C. This change in conductivity results from a change in carrier mobility. PAN makes the films solid without decreasing the carrier mobility. In the hybrid films, the carrier mobility and the macroscopic viscosity are not related by Walden's rule. The high conductivity is due to regions in the film characterized by a low microscopic viscosity. This is determined by the mole ratio [EC]/[LiClO₄] and largely controls the carrier mobility.