Fast cycling of Li/LiCoO2 cell with low-viscosity ionic liquids based on bis(fluorosulfonyl)imide [FSI]

A charge–discharge cycling test of a Li/LiCoO₂ cell containing ionic liquids based on bis(fluorosulfonyl)imide ([FSI]⁻) as the electrolyte media, revealed significantly better rate properties compared to those of cells using conventional ionic liquids. The use of an 1-ethyl-3-methylimidazolium (EMI⁺) salt permitted the retention of 70% of the discharge capacity at a 4 C current rate. In contrast, similar performance of cells containing N-methyl-N-propylpyrrolidinium (Py₁₃⁺) and N-methyl-N-propylpiperidinium (PP₁₃⁺) salts of [FSI]⁻ was limited to operation at 2 and 1 C current rates, respectively. However, the charge/discharge cycling stability of the cell with Py₁₃[FSI] was much better than that of the cell using EMI[FSI].     

Ionic conduction in poly(vinyl chloride)/poly(ethyl methacrylate)-based polymer blend electrolytes complexed with different lithium salts

Poly(vinyl chloride)/poly(ethyl methacrylate)-based polymer blend electrolytes comprising propylene carbonate as a plasticizer and a lithium salt LiX (X = BF₄⁻, ClO₄⁻, CF₃SO₃⁻) are prepared by a solvent casting technique. The electrolytes are subjected to characterization by ionic conductivity, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetic/differential thermal analysis. The electrolytes that contain LiBF₄ exhibit maximum conductivity and are thermally stable up to 254 °C.     

PMMA-assisted synthesis of Li1−xNi0.5Mn1.5O4−δ for high-voltage lithium batteries with expanded rate capability at high cycling temperatures

In this work, poly(methyl methacrylate) (PMMA), a non-surfactant polymer was used to synthesize nonstoichiometric Li_0.82Ni_0.52Mn_1.52O_4−δ (0 ≤ δ ≤ 0.25) spinels. The presence of the polymer was found to be beneficial with a view to facilitating the use of the spinel in electrodes for lithium batteries. Thus, PMMA allowed spinel particles of a high crystallinity and uniform size and shape to be obtained, and particle size to be tailored by using an appropriate calcining temperature and time. By controlling these variables, spinels in nanometric, submicrometric and micrometric particle sizes were prepared and characterized by chemical analysis, X-ray diffraction, electron microscopy, thermogravimetry and nitrogen adsorptions measurements. The spinels were obtained as highly crystalline phases with lithium and oxygen deficiency and some cation disorder as revealed by chemical analysis, thermogravimetric and XRD data. Their electrochemical performance in two-electrode cells was tested at room temperature and 50 °C over a wide range of charge/discharge rates (from C/4 to 4C). Cell performance was found to depend on particle size rather than on structural properties. Thus, the spinel best performing at 50 °C was that consisting of submicrometric particles, which delivered a high capacity and exhibited the best capacity retention and rate capability. Particles of submicronic size share the advantages of nanometric particles (viz. the ability to withstand high charge/discharge rates) and micrometric particles (a high capacity and stability at low rates).     

Rechargeable lithium/sulfur battery with liquid electrolytes containing toluene as additive

The effect of incorporating varying amounts (in the range 2.5–10 vol%) of toluene additive in 1 M LiCF₃SO₃ in tetra(ethylene glycol) dimethyl ether (TEGDME) liquid electrolyte on the electrochemical performance of Li/S cell at room temperature was studied. Cyclic voltammetry measurements showed the same active voltage range for the electrolytes with and without toluene, consisting of an oxidation peak at 2.5 V and a pair of reduction peaks at 2.35 and 1.9 V. Electrolytes with toluene have higher redox currents resulting from increased ion mobility and ionic conductivity. Toluene addition enhanced initial discharge capacity; a maximum of 750 mAh g⁻¹ (45% of the theoretical specific capacity of sulfur) was obtained with 5% of toluene, which was 1.8 times that of the cell without toluene. The electrolyte with 5% toluene exhibited a stable cycle performance with the highest discharge capacity and charge–discharge efficiency. AC impedance analyses of Li/S cells with the electrolytes showed that toluene addition resulted in a lower initial interfacial resistance and a fast stabilization of electrode/electrolyte interfaces in the cell. Addition of toluene in low amounts is thus an effective means to enhance the electrochemical performance of 1 M LiCF₃SO₃ in TEGDME electrolyte in Li/S cells at room temperature.     

Ionic conductivity and electrochemical properties of cross-linked solid polymer electrolyte using star-shaped siloxane acrylate

A star-shaped siloxane acrylate with a different number of repeating units of oligo(ethylene oxide) (EO) was synthesized as a cross-linker of solid polymer electrolytes. The cross-linked solid polymer electrolytes blended with the ionic conducting plasticizers, such as low molecular weight poly(ethylene oxide)dimethyl ether (PEGDME) were prepared by the in situ thermal curing of the star-shaped siloxane acrylate. Different morphologies of the cross-linked polymer electrolytes were observed according to the number of repeating units of EO (n) in the cross-linker. A micro-phase separated solid polymer electrolyte was obtained when the n of cross-linker was 1. When the n of cross-linker was larger than 1, homogeneously blended solid polymer electrolytes were prepared. The ionic conductivity was measured to be 6.3 to 7.8 × 10⁻⁴ S cm⁻¹ with 80 wt.% PEGDME at 30 °C. The ionic conductivity of the micro-phase separated solid polymer electrolyte was slightly higher than that of the homogeneously blended solid polymer electrolyte. The electrochemical stability window of the resulting solid polymer electrolyte could be extended to up to 4.8 V versus Li/Li⁺ reference electrode.     

Organic compounds with heteroatoms as overcharge protection additives for lithium cells

Various organic compounds with heteroatoms (N, O, F, Si, P, S) were tested as overcharge protection additives for 4-V class lithium cells. It was found that trimethyl-3,5-xylylsilane exhibited preferable oxidation potential (E_ox) as overcharge protection additive, and charge–discharge cycling efficiency (Eff) of lithium anode in electrolyte with arylsilanes was as high as tolyladamantanes, reported previously by us. From room temperature to 60 °C, E_ox of trimethyl-3,5-xylylsilane decreased only 0.07 V. Difference in E_ox among regioisomers of tolyltrimethylsilanes is smaller than that among tolyladamantanes. ¹H NMR and UV spectra suggest the steric repulsion between tolyl group and trimethylsilyl group in o-tolyltrimethylsilane is smaller than that of the related substituents of o-tolyladamantane.     

Lithium oxalyldifluoroborate/carbonate electrolytes for LiFePO4/artificial graphite lithium-ion cells

The electrolytes based on lithium oxalyldifluoroborate (LiODFB) and carbonates have been systematically investigated for LiFePO₄/artificial graphite (AG) cells, by ionic conductivity test and various electrochemical tests, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge test. The conductivity of nine electrolytes as a function of solvent composition and LiODFB salt concentration has been studied. The coulombic efficiency of LiFePO₄/Li and AG/Li half cells with these electrolytes have also been compared. The results show that 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte has a relatively higher conductivity (8.25 mS cm⁻¹) at 25 °C, with high coulombic efficiency, good kinetics characteristics and low interface resistance. With 1 M LiODFB EC/PC/DMC (1:1:3, v/v) electrolyte, LiFePO₄/AG cells exhibit excellent capacity retention ∼92% and ∼88% after 100 cycles at 25 °C and at elevated temperatures up to 65 °C, respectively; The LiFePO₄/AG cells also have good rate capability, the discharge capacity is 324.8 mAh at 4 C, which is about 89% of the discharge capacity at 0.5 C. However, at −10 °C, the capacity is relatively lower. Compared with 1 M LiPF₆ EC/PC/DMC (1:1:3, v/v), LiFePO₄/AG cells with 1 M LiODFB EC/PC/DMC (1:1:3, v/v) exhibited better capacity utilization at both room temperature and 65 °C. The capacity retention of the cells with LiODFB-based electrolyte was much higher than that of LiPF₆-based electrolyte at 65 °C, while the capacity retention and the rate capacity of the cells is closed to that of LiPF₆-based electrolyte at 25 °C. In summary, 1 M LiODFB EC/PC/DMC (1:1:3, v/v) is a promising electrolyte for LiFePO₄/AG cells.     

One ether-functionalized guanidinium ionic liquid as new electrolyte for lithium battery

One ether-functionalized guanidinium ionic liquid is used as new electrolytes for lithium battery. Viscosity, conductivity, behavior of lithium redox, chemical stability against lithium metal, and charge-discharge characteristics of lithium batteries, are investigated for the IL electrolytes with different concentrations of lithium salt. Though the cathodic limiting potential of the IL are 0.7 V vs. Li/Li⁺, the lithium plating and striping on Ni electrode can be observed in the IL electrolytes, and the IL electrolytes show good chemical stability against lithium metal. Li/LiCoO₂ cells using the IL electrolytes without additives have good capacity and cycle property at the current rate of 0.2 C when the LiTFSI concentration is higher than 0.3 mol kg⁻¹, and the cell using the IL electrolyte with 0.75 mol kg⁻¹ LiTFSI owns good rate property. The activation energies of the LiCoO₂ electrode for lithium intercalation are estimated, and help to analyze the factors determining the rate property.     

A novel high temperature stable lithium salt (Li2B12F12) for lithium ion batteries

Basic properties and battery performances of the novel high temperature stable lithium salt (Li₂B₁₂F₁₂, Dilithium Dodecafluorododecaborate; Li₂DFB) were studied using a Mn-based cathode and anode composed of a hard carbon and graphite mixture. The effect of co-solvents (mainly linear carbonate in electrolyte formulation of PC/EC/co-solvent (5/30/65 vol% mixture)) on conductivity, viscosity, charge-discharge capacities, rate performance, temperature performance, cycle life and storage life at 60 °C was investigated. Conductivity of Li₂DFB electrolyte increased with reducing its viscosity by changing co-solvent and increasing the volume of the higher dielectric solvent. Li₂DFB electrolytes showed comparable discharge capacity and columbic efficiency against LiPF₆ electrolyte. Li₂DFB electrolytes improved the storage life and cycle life of a Mn-based cell at 60 °C.     

Polymer electrolytes containing guanidinium-based polymeric ionic liquids for rechargeable lithium batteries

The electrochemical properties of solvent-free, quaternary polymer electrolytes based on a novel polymeric ionic liquid (PIL) as polymer host and incorporating 1g13TFSI ionic liquid, LiTFSI salt and nano-scale silica are reported. The PIL-LiTFSI-1g13TFSI-SiO₂ electrolyte membranes are found to be chemically stable even at 80 °C in contact with lithium anode and thermally stable up to 320 °C. Particularly, the quaternary polymer electrolytes exhibit high lithium ion conductivity at high temperature, wide electrochemical stability window, time-stable interfacial resistance values and good lithium stripping/plating performance. Batteries assembled with the quaternary polymer electrolyte at 80 °C are capable to deliver 140 mAh g⁻¹ at 0.1C rates with very good capacity retention.     

Application of Si-C-O glass-like compounds as negative electrode materials for lithium hybrid capacitors

The Si-C-O glass-like compound (a-SiCO) was applied to a negative electrode of a lithium hybrid capacitor (LHC) with activated carbon positive electrodes. The performance as a negative electrode (by a three-electrode system) and LHC (by a two-electrode system) was evaluated in LiClO₄ (EC-DEC) and LiBF₄ (PC) electrolytes. With a-SiCO reversible insertion/extraction of lithium ions at high current densities (0.5-2.0 A g⁻¹) was possible. By prior short-circuiting of the negative electrode with lithium metal in the electrolytes for appropriate periods, the charge/discharge performance of the assembled LHC compared favorably with an electric double layer capacitor (EDLC) made of the activated carbon used for LHC. The cycle performance of the LHC was better but the capacitance was smaller in the LiBF₄ (PC) electrolyte than in LiClO₄ (EC-DEC) electrolyte. Smaller capacitance is mainly due to lower electric conductivity and higher viscosity of LiBF₄ (PC) electrolyte than LiClO₄ (EC-DEC) electrolyte. The energy density of the assembled LHC reached a maximum of about three times that of EDLC, with the power density comparable to that of the EDLC.     

Analysis of SEI formed with cyano-containing imidazolium-based ionic liquid electrolyte in lithium secondary batteries

Two kinds of cyano-containing imidazolium-based ionic liquid, 1-cyanopropyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CpMI-TFSI) and 1-cyanomethyl-3-methylimidazolium-bis(trifluoromethanesulfonyl)imide (CmMI-TFSI), each of which contained 20 wt% dissolved LiTFSI, were used as electrolytes for lithium secondary batteries. Compared with 1-ethyl-3-methylimidazolium-bis(trifluoromethane-sulfonyl)imide (EMI-TFSI) electrolyte, a reversible lithium deposition/dissolution on a stainless-steel working electrode was observed during CV measurements in these cyano-containing electrolytes, which indicated that a passivation layer (solid electrolyte interphase, SEI) was formed during potential scanning. The morphology of the working electrode with each electrolyte system was studied by SEM. Different dentrite forms were found on the electrodes with each electrolyte. The SEI that formed in CpMI-TFSI electrolyte showed the best passivating effect, while the deposited film formed in EMI-TFSI electrolyte showed no passivating effect. The chemical characteristics of the deposited films on the working electrodes were compared by XPS measurements. A component with a cyano group was found in SEIs in CpMI-TFSI and CmMI-TFSI electrolytes. The introduction of a cyano functional group suppressed the decomposition of electrolyte and improved the cathodic stability of the imidazolium-based ionic liquid. The reduction reaction route of imidazolium-based ionic liquid was considered to be different depending on whether or not the molecular structure contained a cyano functional group.     

Electrochemical cell studies based on non-aqueous magnesium electrolyte for electric double layer capacitor applications

Performances of electric double layer capacitors (EDLCs) based on an activated carbon electrode with acetonitrile (ACN), propylene carbonate (PC), or a ternary electrolyte, i.e., PC:ethylene carbonate (EC):diethyl carbonate (DEC), at 1 mol dm⁻³ of magnesium perchlorate [Mg(ClO₄)₂] salt have been investigated. The electrochemical responses were studied by impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge experiments at 25 °C in a three-electrode configuration. For a comparative evaluation, lithium perchlorate (LiClO₄) salt-based systems were also evaluated. All the observed results showed typical EDLC characteristics within the potential range between 0 and 1 V vs. an Ag/Ag⁺ reference electrode. The Mg-based systems exhibited similar or rather better performances than the corresponding Li-based electrolytes; in particular, the rate capability of Mg-based ACN and PC electrolytes was much better than the corresponding Li-based electrolytes, indicating the high accessibility and utility of activated carbon pores by solvated Mg ions.     

Charge-discharge rate of spinel lithium manganese oxide and olivine lithium iron phosphate in ionic liquid-based electrolytes

Rate capability of Li/spinel LiMn₂O₄ or olivine LiFePO₄ positive electrode cells containing mixed imidazolium ionic liquids electrolytes has been investigated under comparison with conventional organic solvent electrolyte and piperidinium ionic liquid. The LiMn₂O₄ electrode provides variation of rate capabilities among the ionic liquid electrolytes, while ionic liquid electrolytes provide similar extent of capacity degradation under high rate compared with organic solvent electrolyte for LiFePO₄ electrode. Such differences in electrolyte dependences of the rate capabilities can be explained in relation to parameters in the high frequency resistances on AC impedance, assumed as interfacial resistances. The rate capability of LiMn₂O₄ is somewhat related to the activation energy of the high frequency resistance, while for LiFePO₄ the resistance value appears to contribute to the rate capability.     

Ternary polymer electrolytes containing pyrrolidinium-based polymeric ionic liquids for lithium batteries

The electrochemical properties of solvent-free, ternary polymer electrolytes based on a novel poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide polymeric ionic liquid (PIL) as polymer host and incorporating PYR₁₄TFSI ionic liquid and LiTFSI salt are reported. The PIL-LiTFSI-PYR₁₄TFSI electrolyte membranes were found to be chemically stable even after prolonged storage times in contact with lithium anode and thermally stable up to 300 °C. Particularly, the PIL-based electrolytes exhibited acceptable room temperature conductivity with wide electrochemical stability window, time-stable interfacial resistance values and good lithium stripping/plating performance. Preliminary battery tests have shown that Li/LiFePO₄ solid-state cells are capable to deliver above 140 mAh g⁻¹ at 40 °C with very good capacity retention up to medium rates.     

Comparison of the electrochemical properties of LiBOB and LiPF6 in electrolytes for LiMn2O4/Li cells

The electrochemical stability and conductivity of LiPF₆ and lithium bis(oxalato)borate (LiBOB) in a ternary mixture of ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) were compared. The discharge capacities of LiMn₂O₄/Li cells with the two electrolytes were measured at various current densities. At room temperature, LiMn₂O₄/Li cells with the electrolyte containing LiBOB cycled equally well with those using the electrolyte containing LiPF₆ when the discharge current rate was under 1 C. At 60 °C, the LiBOB-based electrolyte cycled better than the LiPF₆-based electrolyte even when the discharge current rate was above 1 C. Compared with the electrolyte containing LiPF₆, in LiMn₂O₄/Li cells the electrolyte containing LiBOB exhibited better capacity utilization and capacity retention at both room temperature and 60 °C. The scanning electron microscopy (SEM) images and the a.c. impedance measurements demonstrated that the electrode in the electrolyte containing LiBOB was more stable. In summary, LiBOB offered obvious advantages in LiMn₂O₄/Li cells.     

Sm0.5Sr0.5CoO3 + Sm0.2Ce0.8O1.9 composite cathode for cermet supported thin Sm0.2Ce0.8O1.9 electrolyte SOFC operating below 600 °C

The cathode is a key component in low temperature solid oxide fuel cells. In this study, composite cathode, 75 wt.% Sm_0.5Sr_0.5CoO₃ (SSC) + 25 wt.% Sm_0.2Ce_0.8O_1.9 (SDC), was applied on the cermet supported thin SDC electrolyte cell which was fabricated by tape casting, screen-printing, and co-firing. Single cells with the composite cathodes sintered at different temperatures were tested from 400 to 650 °C. The best cell performance, 0.75 W cm⁻² peak power operating at 600 °C, was obtained from the 1050 °C sintered cathode. The measured thin SDC electrolyte resistance R_s was 0.128 Ω cm² and total electrode polarization R_p(a + c) was only 0.102 Ω cm² at 600 °C.     

Electrochromic devices employing methacrylate-based polymer electrolytes

Poly(ethyl methacrylate) (PEMA)- and poly(2-ethoxyethyl methacrylate) (PEOEMA)-based polymer gel electrolytes with entrapped solutions of lithium perchlorate in propylene carbonate (PC) were prepared by direct, UV-initiated polymerization. The electrolytes were studied using electrochemical methods and they exhibit good ionic conductivity (up to 0.7 mS cm⁻¹ at 20 °C) as well as electrochemical stability up to 2.5 V vs. Cd/Cd²⁺ (5.1 V vs. Li/Li⁺) on gold electrode. The electrolytes have thermal stability up to 125 °C. The electrolytes were successfully tested as ionic conductors in the electrochromic device FTO/WO₃/Li⁺-electrolyte/V₂O₅/FTO using coupled optoelectrochemical methods to discuss the relationship between the electrolyte composition and parameters such as change of transmittance, response time and stability. The transmittance change Δτ was found to be 30-45% at 634 nm.     

Ionic liquid doped PEO-based solid polymer electrolytes for lithium-ion polymer batteries

The influence of adding the room-temperature ionic liquid 1-ethyl-3-methyllimidazolium bis(trifluoromethylsulfonyl)imide (EMImTFSI) to poly(ethylene oxide) (PEO)–lithium difluoro(oxalato)borate (LiDFOB) solid polymer electrolyte and the use of these electrolytes in solid-state Li/LiFePO₄ batteries has been investigated. Different structural, thermal, electrical and electrochemical studies exhibit promising characteristics of these polymer electrolyte membranes, suitable as electrolytes in rechargeable lithium-ion batteries. The crystallinity decreased significantly due to the incorporation of ionic liquid, investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The ion–polymer interaction, particularly the interaction of cations in LiDFOB and ionic liquid with ether oxygen atom of PEO chains, has been evidenced by FT-IR studies. The polymer electrolyte with ～40 wt% of ionic liquid offers a maximum ionic conductivity of ～1.85 × 10^?4 S/cm at 30 °C with improved electrochemical stabilities. The Li/PEO-LiDFOB-40 wt% EMImTFSI/LiFePO₄ coin-typed cell cycled at 0.1 C shows the 1st discharge capacity about 155 mAh g^?1, and remains 134.2 mAh g^?1 on the 50th cycle. The addition of the ionic liquid to PEO₂₀-LiDFOB polymer electrolyte has resulted in a very promising improvement in performance of the lithium polymer batteries.     

Charge and discharge characteristics of a commercial LiCoO2-based 18650 Li-ion battery

We studied the charge and discharge characteristics of commercial LiCoO₂-based 18650 cells by using various electrochemical methods, including discharging at constant power, ac impedance spectroscopy, and dc-voltage pulse. At 20 °C, these cells deliver 8.7–6.8 Wh of energy when discharged at a power range of 1–12 W between 2.5 and 4.2 V. Ragone plots show that the effect of discharge power on the energy is significantly increased with decreasing of the temperature. For example, energy of the cell is entirely lost when the temperature downs to −10 °C and the discharge rate still remains at 10 W. Impedance analyses indicate that the total cell resistance (R_cell) is mainly contributed by the bulk resistance (R_b, including electric contact resistance and electrolytic ionic conductivity), solid electrolyte interface resistance (R_sei), and charge-transfer resistance (R_ct). Individual contribution of these three resistances to the cell resistance is greatly varied with the temperature. Near room temperature, the R_b occupies up to half of the cell resistance, which means that the rate performance of the cell could be improved by modifying cell design such as employing electrolyte with higher ionic conductivity and enhancing electric contact of the active material particles. At low temperature, the R_ct, which is believed to reflect cell reaction kinetics, dominates the cell resistance. In addition, galvanosatic cycling tests indicate that the charge and discharge processes have nearly same kinetics. The performance discrepancy observed during charging and discharging, especially at low temperatures, can be attributed to these two factors of: (1) substantially higher R_ct at the discharged state than at the charged state; (2) asymmetric voltage limits pre-determined for the charge and discharge processes.