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

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

锂离子电池电解质锂盐双草酸硼酸锂的研究进展

介绍了新型锂盐双草酸硼酸锂(LiBOB)的基本性质,归纳了其合成、检测和提纯方法,重点阐述了LiBOB在石墨类负极上的成膜性能,综述了LiBOB的溶解性和电导率、热稳定性及在固体电解质中的应用,总结了LiBOB基电解质的不足,指明了其未来的研究方向。     

Structural and electrochemical studies of a hexaphenylbenzene pyrolysed soft carbon as anode material in lithium batteries

XRD, SEM micrographs, BET analyses and typical electrochemical experiments (cyclic voltammetry, step voltammetry and Li insertion/deinsertion at constant current) have been carried out to characterize a new type of soft carbons obtained by pyrolysis of hexaphenylbenzene (HPB). By means of XRD and cyclic voltammetry at least three different type of sites for lithium storage were found. The first is graphite like type with d₀₀₂ graphene layer distance greater than pure graphite; the second is associated to disordered volumes among crystallities and the third is represented by Li sites at the hydrogen-terminated edges of hexagonal carbon fragments, characterized by higher energy in comparison with simple insertion sites. These last two types of sites are able to store some extra lithium, compared to pure graphite. BET analyses and cyclic voltammetries demonstrate the key role of the milling time on the characteristics and properties of this HPB pyrolysed carbon. Specific capacities shown by this pyrolysed material in Li coin-type cell have been also reported.     

Synthesis and electrochemical characterization of LiMn2O4 cathode materials for lithium polymer batteries

Spinel LiMn₂O₄ powders with sub-micron, narrow particle-size distribution, and phase-pure particles were synthesized at low temperatures from aqueous solution of metal acetate containing glyoxylic acid as a chelating agent by a sol-gel method. The effects of the calcination temperature and glyoxylic acid quantity on the physicochemical properties of spinel LiMn₂O₄ powders were examined with X-ray diffractometry (XRD), the Brunauer-Emmett-Teller (BET) method and scanning electron microscopy (SEM). Porous LiMn₂O₄ electrode was characterized electrochemically with charge/discharge experiments and A.C. impedance spectroscopy. The cycling performance of a Li/polymer electrolyte/LiMn₂O₄ cell has been discussed in terms of contact and interfacial resistance by A. C. impedance spectroscopy.     

Direct addition of lithium and cobalt precursors to Ce0.8Gd0.2O1.95 electrolytes to improve microstructural and electrochemical properties in IT-SOFC at lower sintering temperature

To improve the microstructural and electrochemical properties of gadolinium-doped ceria (GDC) electrolytes, materials co-doped with 0.5–2?mol% of lithium and cobalt oxides were successfully prepared in a one-step sol gel combustion synthesis route. Vegard's slope theory was used to predict the dopant solubility and the sintering behaviour. The charge and size of the added dopant influence the atom flux near the grain boundary with a change in the lattice parameter. In fact, compared to traditional multi grinding steps, sol gel combustion facilitates molecular mixing of the precursors and substitution of the dopant cations into the fluorite structure, considerably reducing the sintering temperature. Adding precursors of lithium and cobalt, as dopant, increases the GDC densification and reduces its traditional sintering temperature down to 1000–1100?°C, with an improvement of electrochemical properties. Impedance analysis showed that the addition of 2?mol% of lithium or 0.5?mol% of cobalt enhances the conductivity with a consequent improvement of cell performances. High total conductivities of 1.26·10^?1 S?cm^?1 and 8.72·10^?2 S?cm^?1 at 800?°C were achieved after sintering at 1000?°C and 1100?°C for ²LiGDC and ^0.5CoGDC, respectively.     

Physical and electrochemical properties of mix-doped lithium iron phosphate as cathode material for lithium ion battery

The Li_0.98Al_0.02FePO₄/C (2.0 wt.%) mix-doped composite had been synthesized by adding aluminum stearate to the react precursors through solid-state reaction. The mix-doping method does not affect the olivine structure of the cathode but greatly improves its kinetics in terms of capacity delivery, cycle life and rate capability. Such an enhancement of the electrochemical properties has been ascribed to the increase of intra- and inter-crystal electronic conductivity and the reduction of the particle size, these two effects being promoted by the co-existence of the lattice doping element (Al³⁺) and the non-lattice doping element (C). Overcharge test indicates that this composite has excellent safety performances.     

Prediction of electrolyte viscosity for aqueous and non-aqueous systems: Results from a molecular model based on ion solvation and a chemical physics framework

Electrolyte viscosity is a macroscopic property, although its foundation lies on molecular-scale interactions between solvent and ionic species. A comprehensive understanding of viscosity behavior with respect to solvent composition, salt concentration and temperature is only possible with correct interpretations of molecular interactions and related quantities. This work introduces a new methodology for predicting electrolyte viscosity under a wide range of conditions, based on molecular, physical, and chemical properties. The general formalism is universal for aqueous and non-aqueous systems alike. Although the immediate application of the resultant model is candidate electrolytes for lithium ion batteries, other applications abound in the areas of industrial fluids, biological systems, and other electrochemical systems whose performance characteristics are tied to viscosity. Viscosity predictions are compared to experimental data for a number of electrolytes, demonstrating exceptional accuracy of predictions over wide temperature ranges and broad ranges of salt concentration.     

Improving ionic conductivity of polymer-based solid electrolytes for lithium metal batteries

Because of its superior safety and excellent processability, solid polymer electrolytes (SPEs) have attracted widespread attention. In lithium based batteries, SPEs have great prospects in replacing leaky and flammable liquid electrolytes. However, the low ionic conductivity of SPEs cannot meet the requirements of high energy density systems, which is also an important obstacle to its practical application. In this respect, escalating charge carriers (i.e. Li⁺) and Li⁺ transport paths are two major aspects of improving the ionic conductivity of SPEs. This article reviews recent advances from the two perspectives, and the underlying mechanism of these proposed strategies is discussed, including increasing the Li⁺ number and optimizing the Li⁺ transport paths through increasing the types and shortening the distance of Li⁺ transport path. It is hoped that this article can enlighten profound thinking and open up new ways to improve the ionic conductivity of SPEs.     

Synthesis and electrochemical properties of lithium methacrylate-based self-doped gel polymer electrolytes

In this study, a strategy for synthesizing lithium methacrylate (LiMA)-based self-doped gel polymer electrolytes was described and the electrochemical properties were investigated by impedance spectroscopy and linear sweep voltammetry. LiMA was found to dissolve in ethylene carbonate (EC)/diethyl carbonate (DEC) (3/7, v/v) solvent after complexing with boron trifluoride (BF₃). This was achieved by lowering the ionic interactions between the methacrylic anion and lithium cation. As a result, gel polymer electrolytes consisting of BF₃-LiMA complexes and poly(ethylene glycol) diacrylate were successfully synthesized by radical polymerization in an EC/DEC liquid electrolyte. The FT-IR and AC impedance measurements revealed that the incorporation of BF₃ into the gel polymer electrolytes increases the solubility of LiMA and the ionic conductivity by enhancing the ion disassociations. Despite the self-doped nature of the LiMA salt, an ionic conductivity value of 3.0 × 10⁻⁵ S cm⁻¹ was achieved at 25 °C in the gel polymer electrolyte with 49 wt% of polymer content. Furthermore, linear sweep voltammetry measurements showed that the electrochemical stability of the gel polymer electrolyte was around 5.0 V at 25 °C.     

Electrochemical behavior of biphenyl as polymerizable additive for overcharge protection of lithium ion batteries

Electrochemical properties and working mechanism of biphenyl as a polymerizable electrolyte additive for overcharge protection of lithium ion batteries are studied by microelectrode voltammetry, charge-discharge measurements and SEM characterization of the overcharged cell’s components. The experimental results reveal that biphenyl can electrochemically polymerize at the overcharge potential of 4.5-4.75 V (versus Li/Li⁺) to form a layer of conductive film on the cathode surface and the polymer deposits may develop to penetrate the separator to reach the anode surface, resulting an internal short-circuit to prevent from the cell voltage runaway. On the other hand, the electro-oxidative polymerization of biphenyl produces excessive gas and heat, which help to enhance the sensitivities of electric disconnecting devices. In addition, it is also found that the use of biphenyl as an electrolyte additive does not significantly influence the normal performances of the lithium ion batteries.     

Measurement of transference numbers for lithium ion electrolytes via four different methods, a comparative study

We report here on comparative measurements of cationic transference numbers of some lithium battery related electrolytes including lithium tetrafluoroborate in propylene carbonate, lithium hexafluorophosphate in blends of ethylene carbonate/diethyl carbonate and ethylene carbonate/propylene carbonate/dimethyl carbonate, as well as lithium difluoromono (oxalate) borate in an ethylene carbonate/diethyl carbonate blend via four different methods. Whereas three electrochemical methods yield transference numbers decreasing with concentration in accordance with electrostatic theories, valid for low to intermediate concentrations of the electrolyte, nuclear magnetic resonance spectroscopy measurements show increasing transference numbers with increasing concentration. The discrepancy is attributed to effects of ion–ion and ion–solvent interaction.     

Application of pulse methods to the determination of the electrochemical characteristics of lithium intercalates

The transfer processes proceeding in insertion electrodes with surface control on the application of a potential or current step are considered theoretically. The theoretical relationships have been verified by the determination of the kinetic and diffusion parameters of electrochemical lithium intercalation into thin carbon films. The overall electrode polarization is divided, both theoretically and experimentally, into the kinetic component, related to hindered ion transfer in the passive surface layer, and the diffusion one, related to decelerated lithium diffusion in the carbon matrix. The polarization dependence of kinetic current is shown to obey the same regularities that the current-potential function of the lithium electrode. The concentration dependences of the surface layer parameters and the diffusion coefficient of lithium in carbon have been determined.     

Effects of heteroatoms on electrochemical performance of electrode materials for lithium ion batteries

Recent studies of lithium ion batteries focus on improving electrochemical performance of electrode materials and/or lowering cost. Doping of active materials with heteroatoms is one promising method. This paper reviews the effects of heteroatoms on anode materials such as carbon- and tin-based materials, and cathode materials such as LiCoO₂, LiNiO₂, LiMn₂O₄ and V₂O₅. There are favorable and unfavorable effects, which depend on the species and physicochemical states of heteroatoms and the parent electrode materials. In the application of lithium ion batteries advantageous factors should be exploited, unwelcome side effects should be avoided as far as possible. Considerable gains towards improved electrochemical performance of the electrode materials have been achieved. Nevertheless, there are still problems needing further investigation including theoretical aspects, which will in the meanwhile stimulate the investigation for better electrode materials.     

A quantum-chemical study on the boron centers in nonaqueous electrolyte solutions and polymer electrolytes

Quantum-chemical calculations were performed to study the effect of Lewis acid centers introduced to liquid or polymer electrolytes as boric acid esters. Particular attention has been paid to the modeling of solvent effects on ion–ion and anion-acid center interactions. Calculated complexation energies for lithium salts and polymerizable boric acid esters with diols in different solvents were analyzed and related to available conductivity data.     

Ion transport properties of lithium ionic liquids and their ion gels

A new series of lithium ionic liquids were prepared by introducing of two electron-withdrawing trifluoroacetyl groups in borate salts containing two methoxy-oligo(ethylene oxide) groups in the structures. Successive substitution reactions of oligo-ethylene glycol monomethyl ether and trifluroacetic acid from LiBH₄ yielded the lithium salts, which were clear and colorless liquids at room temperature. The fundamental physicochemical properties, such as density, thermal property, viscosity, ionic conductivity, self-diffusion coefficients, and electrochemical stability, were measured. The lithium ionic liquids had self-dissociation ability and conducted ions even in the absence of organic solvents. New polymer electrolytes, named ‘ion gels’, were prepared by radical cross-linking reactions of a poly(ethylene oxide-co-propylene oxide)tri-acrylate macromonomer in the presence the lithium ionic liquid. An increase in the glass transition temperatures (T_g) of the ion gels was very small even with increasing lithium ionic liquid concentration, and the T_g's were lower than that of the ionic liquid itself. The ionic conductivity of the ion gels surpassed that of the lithium ionic liquid in the bulk at certain compositions.     

Synthesis and electrochemical performance of three-dimensional ordered hierarchically porous Li4Ti5O12 for high performance lithium ion Batteries

In this work, a three-dimensional ordered hierarchically porous (3DOHP) Li₄Ti₅O₁₂ that possesses inner-particle mesopores resulting from a soft template method and a three-dimensional ordered macroporous (3DOM) Li₄Ti₅O₁₂ using polystyrene spheres as a hard template have been synthesized. Both 3DOM Li₄Ti₅O₁₂ and 3DOHP Li₄Ti₅O₁₂ have ordered macropores and interconnected skeletons with a regular periodicity revealed by SEM and TEM observations. The specific surface area of 3DOHP Li₄Ti₅O₁₂ is up to 135 m² g^?1 which is much higher compared with that of 3DOM Li₄Ti₅O₁₂ because of the existence of inner-particle mesopores. Attributed to the higher surface area and smaller crystal grain size, more excellent cycle performance and rate capability are obtained for 3DOHP Li₄Ti₅O₁₂ compared to 3DOM Li₄Ti₅O₁₂. In addition, the hierarchically porous structure of 3DOHP Li₄Ti₅O₁₂ can meet rapid insertion and deinsertion of lithium ion even at extremely high rate. It is apparent that 3DOHP Li₄Ti₅O₁₂ has a lower total resistance and faster Li⁺ diffusion coefficient compared to 3DOM Li₄Ti₅O₁₂ according to electrochemical impendence spectroscopy analysis.     

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

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

Electrochemical studies of ferrocene in a lithium ion conducting organic carbonate electrolyte

We carried out a detailed study of the kinetics of oxidation of ferrocene (Fc) to ferrocenium ion (Fc⁺) in the non-aqueous lithium ion conducting electrolyte composed of a solution of 1 M LiPF₆ in 1:1 EC:EMC solvent mixture. This study using cyclic (CV) and rotating disk electrode (RDE) voltammetry showed that the Fc⁰/Fc⁺ redox couple is reversible in this highly concentrated electrolyte. The ferrocene and ferrocenium ion diffusion coefficients (D) were calculated from these results. In addition, the electron transfer rate constant (k⁰) and the exchange current density for the oxidation of ferrocene were determined. A comparison of the kinetic data obtained from the two electrochemical techniques appears to show that the data from the RDE experiments are more reliable because they are collected under strict mass transport control. A Tafel slope of c.a. 79 mV/decade and a transfer coefficient α of 0.3 obtained from analysis of the RDE data for ferrocene oxidation suggest that the structure of the activated complex is closer to that of the oxidized specie due to strong interactions with the carbonate solvents. The experiments reported here are relevant to the study of redox reagents for the chemical overcharge protection of Li-ion batteries.     

Anodic behavior of aluminum in organic solutions with different electrolytic salts for lithium ion batteries

The anodic polarization behavior of aluminum (Al) as a current collector of lithium (Li) ion battery has been investigated in organic electrolyte solutions containing different lithium salts. The Al current collector has suffered serious corrosion in the solution containing Li(CF₃SO₃)₂N (LiTFSI) under an anodic polarization condition, whereas, it was anodically stable in the LiPF₆ solution. In the solution of Li(C₂F₅SO₂)₂N (LiBETI), the Al anode showed an intermediate character between those in the LiPF₆ and LiTFSI solutions. The corrosion behavior of the Al electrode was much influenced by its surface condition. The addition of LiPF₆ in the imide-salts (LiTFSI and LiBETI) solutions suppressed the anodic corrosion of Al. The results of electrochemical quartz crystal microbalance (EQCM) experiments proved that the anodic processes on Al in the organic electrolytes consist of the formation of surface films and their dissolution. The X-ray photoelectron spectroscopy (XPS) analysis suggests that the anodic stability of the Al electrode in the imide-salts solutions containing LiPF₆ is associated with the formation of a fluoride (AlF₃)-rich film on the Al surface.