Ionic liquids as electrolytes for Li-ion batteries—An overview of electrochemical studies

The paper reviews properties of room temperature ionic liquids (RTILs) as electrolytes for lithium and lithium-ion batteries. It has been shown that the formation of the solid electrolyte interface (SEI) on the anode surface is critical to the correct operation of secondary lithium-ion batteries, including those working with ionic liquids as electrolytes. The SEI layer may be formed by electrochemical transformation of (i) a molecular additive, (ii) RTIL cations or (iii) RTIL anions. Such properties of RTIL electrolytes as viscosity, conductivity, vapour pressure and lithium-ion transport numbers are also discussed from the point of view of their influence on battery performance.     

Ionic conductivity and thermal property of solid hybrid polymer electrolyte composed of oligo(ethylene oxide) unit and butyrolactone unit

Various hybrid polymers composed of oligo(ethylene oxide) unit and butyrolactone unit, such as polymer blend, block copolymer and random copolymer, were synthesized by radical polymerization of poly(ethylene oxide) methyl ether methacrylate (PEOMA) and/or α-methylene-γ-butyrolactone (MBL), and the ionic conductivities and thermal properties of the solid polymer electrolytes using those hybrid polymers and LiN(SO₂CF₃)₂ were investigated. The solid polymer electrolyte using homopolymer of PEOMA (poly(PEOMA)) showed higher ionic conductivity and larger temperature dependence of ionic conductivity than those of the solid polymer electrolyte using homopolymer of MBL (poly(MBL)). The ionic conductivities of the solid hybrid polymer electrolytes were improved at especially low temperature region compared with each homopolymer electrolyte. This result indicates that the decline in the ionic conductivity at low temperature, which is characteristic of PEOMA unit, has been effectively reduced by combining with MBL unit. The poly(PEOMA) and the block or random copolymer electrolytes showed the higher thermal stability than the poly(MBL) and the polymer blend electrolytes.     

Various synthesis methods of aliovalent-doped ceria and their electrical properties for intermediate temperature solid oxide electrolytes

This article investigates the relationship between ionic conductivity and various processing methods for aliovalent-doped, ceria solid solution particles, as an intermediate temperature-solid oxide electrolyte to explain the wide range of conductivity values that have been reported. The effects of doping material and content on the ionic conductivity are investigated comprehensively in the intermediate temperature range. The chemical routes such as coprecipitation, combustion, and hydrothermal methods are chosen for the synthesis of ceria-based nanopowders, including the conventional solid-state method. The ionic conductivity for the ceria-based electrolytes depends strongly on the lattice parameter (by dopant type and content), processing parameters (particle size, sintering temperature and microstructure), and operating temperature (defect formation and transport). Among other doped-ceria systems, the Nd_0.2Ce_0.8O_2−d electrolyte synthesized by the combustion method exhibits the highest ionic conductivity at 600 °C. Further, a novel composite Nd_0.2Ce_0.8O_2−d electrolyte consisting of a combination of powders (50:50) synthesized by coprecipitation and combustion is designed. This electrolyte demonstrates an ionic conductivity two to four times higher than that of any singly processed electrolytes.     

Enhanced performance of a direct methanol alkaline fuel cell (DMAFC) using a polyvinyl alcohol/fumed silica/KOH electrolyte

A novel polymer-inorganic composite electrolyte for direct methanol alkaline fuel cells (DMAFCs) is prepared by physically blending fumed silica (FS) with polyvinyl alcohol (PVA) to suppress the methanol permeability of the resulting nano-composites. Methanol permeability is suppressed in the PVA/FS composite when comparing with the pristine PVA membrane. The PVA membrane and the PVA/FS composite are immersed in KOH solutions to prepare the hydroxide-conducting electrolytes. The ionic conductivity, cell voltage and power density are studied as a function of temperature, FS content, KOH concentration and methanol concentration. The PVA/FS/KOH electrolyte exhibits higher ionic conductivity and higher peak power density than the PVA/KOH electrolyte. In addition, the concentration of KOH in the PVA/FS/KOH electrolytes plays a major role in achieving higher ionic conductivity and improves fuel cell performance. An open-circuit voltage of 1.0 V and a maximum power density of 39 mW cm⁻² are achieved using the PVA/(20%)FS/KOH electrolyte at 60 °C with 2 M methanol and 6 M KOH as the anode fuel feed and with humidified oxygen at the cathode. The resulting maximum power density is higher than the literature data reported for DMAFCs prepared with hydroxide-conducting electrolytes and anion-exchange membranes. The long-term cell performance is sustained during a 100-h continuous operation.     

Development of low temperature Li-ion electrolytes for NASA and DoD applications

Both NASA and the US Army have interest in developing secondary energy storage devices with improved low temperature performance to meet the demanding requirements of space missions and man-portable applications. Lithium-ion systems have been identified as having the most promise due to their high specific energy density and wide operational temperature ranges from the use of organic solvent-based electrolytes, rather than aqueous-based systems. Initially, the SOA lithium-ion technology was limited to operation above −10°C, due primarilly to the fact that the electrolytes employed had high melting points and were highly viscous at low temperatures, resulting in low ionic conductivity. However, due to recent developments in electrolyte formulations at the Army and at JPL, improved low temperature performance of lithium-ion cells have been demonstrated, with efficient cell operation to temperatures as low as −30°C. This was achieved by developing multi-component solvent systems, based on mixtures of cyclic and aliphatic alkyl carbonates. In the course of investigating the viability of a number of advanced electrolyte systems, it was identified that the protective surface films which form on the carbonaceous-based anodes can strongly influence the low temperature capabilities of lithium-ion cells, in addtion to the ionic conductivity of the electrolyte. Thus, in order to optimize an electrolyte for low temperature applications, it is necessary to balance the inherent physical properties of the formulations (i.e. freezing point, viscosity, and ionic conductivity) with the observed compatibility with the chosen cell chemistry (i.e. the nature of the passivating films formed on the electrodes).     

La2Ce2O7 based materials for next generation proton conducting solid oxide cells: Progress,opportunity and future prospects

The development of efficient and environmentally-friendly technology is the only possible way to solve the existing energy and environmental crisis. Solid oxide cells (SOCs) technologies have huge potential in different technologies including energy conversion devices (fuel cells), hydrogen production (electrolysis), co-conversion, natural gas upgrading (conversion of C1 molecules), green synthesis of ammonia, hydrogen separation membrane, and sensors. In few years, great effort has been paid for the development of ionic conducting materials for SOCs. Since Iwahara's discovery of proton conducting oxides in the 1980s, there has been a significant advancement in materials research that has been responsible for the creation of high performance SOCs by modifying the material's properties, such as ionic conductivity, mixed ionic electronic conducting (MIEC) materials, and triple conducting oxides. Recently, La₂Ce₂O₇ (LCO) based materials with ionic (protonic and oxide ion) conductivity have been proposed as a new class of electrolyte for intermediate temperature solid oxide fuel cells as they exhibit high chemical stability, sufficient high ionic conductivity and low temperature sinterability compared to state-of-the-art proton conducting electrolytes. In order to engineer the materials properties, the fundamental understanding of materials such as true crystal structures, crystal structure tolerance ratio, hydration behavior, mechanism of ionic (oxide ions and protons) conduction, catalytic behavior, temperature of operation, ease of processing etc. is needed. All the above information is carefully reviewed for LCO based electrolyte materials with respect to SOCs operation which is not only acting as electrolyte but also as multifunctional properties.     

Functionalized imidazolium ionic liquids as electrolyte components of lithium batteries

Some basic properties and compatibility toward lithium electrode for electrolytes based on substituted imidazolium ionic liquid have been investigated. The ionic liquids having imidazolium cation substituted by methylcarboxyl or cyano group suffers from low conductivity. However, reversible lithium deposition–dissolution process was observed in electrolytes based on these electrolytes. In particular, lithium salt solution in cyanomethyl-substituted imidazolium ionic liquid provided similar cycle efficiency to conventional organic solvent electrolyte at constant-current condition. The mixed ionic liquid electrolyte containing the cyanomethyl-substituted ionic liquid also provided good cycle performance despite of containing large amount of 1-ethyl-3-methyl imidazolium (EMI)-based ionic liquid. Such mixed electrolyte system serves both the stability of lithium electrode process and valid conductivity for practical use.     

Enhanced conductivity of SDC based nanocomposite electrolyte by spark plasma sintering

Recently, ceria-based nanocomposites have been considered as promising electrolyte candidates for low-temperature solid oxide fuel cells (LTSOFC) due to their dual-ion conduction and excellent performance. However, the densification of these composites remains a great concern since the relative low density of the composite electrolyte is suspected to deteriorate the durability of fuel cell. In the present study, the ionic conductivity of two kinds of SDC-based nanocomposite electrolytes processed by spark plasma sintering (SPS) method was investigated, and compared to that made by conventional cold pressing followed by sintering (normal processing way). The density of solid electrolyte can reach higher than 95% of the theoretical value after SPS processing, while the relative density of the electrolyte pellets by normal processing way can hardly approach 75%. The structure and morphology of the sintered pellets were characterized by XRD and SEM. The ionic conductivity of samples was measured by electrochemical impedance spectroscopy (EIS). The results showed that the ionic conductivity of the two kinds of electrolytes treated with SPS was significantly enhanced, compared with the electrolyte pellets processed through the conventional method. The profile of impedance curve of the electrolytes was altered as well. This study demonstrates that the conductivity of SDC based nanocomposite electrolyte can be further improved by adequate densification process.     

Novel polymer electrolytes based on thermoplastic polyurethane and ionic liquid/lithium bis(trifluoromethanesulfonyl)imide/propylene carbonate salt system

Polymer electrolytes were prepared from thermoplastic polyurethane with addition of mixture of ionic liquid N-ethyl(methylether)-N-methylpyrrolidinium trifluoromethanesulfonimmide (PYRA_12O1TFSI), lithium bis(trifluoromethanesulfoneimide) salt and propylene carbonate. The electrolytes characterization was performed by thermogravimetric analysis, differential scanning calorimetry and scanning electron microscopy. The electrical properties were investigated in detail by impedance spectroscopy with the aid of equivalent circuit fitting of the impedance spectra. A model describing temperature evolution of ionic conductivity and the properties of electrolyte/blocking electrode interface was developed. The electrochemical stability of the electrolytes was studied by linear voltammetry. Our results indicate that the studied electrolytes have good self-standing characteristics, and also a sufficient level of thermal stability and a fairly good electrochemical window. The ionic conductivity increases with increasing amount of mixture, and the character of temperature dependence of conductivity indicates decoupling of ion transport from polymer matrix. For studied system, the highest value of ionic conductivity measured at room temperature was 10⁻⁴ S cm⁻¹.     

Preparation and performance of high-voltage electrolytes for supercapacitors

采用咪唑类离子液体1-乙基-3-甲基咪唑四氟硼酸盐（EMIBF₄）调制了两款耐压电解液并用于大容量圆柱式超级电容器中,考察了电容器的容量、内阻、循环等性能,分析了高压循环过程中电容器的发热行为。结果表明：相比商用耐压电解液,两款自制电解液均能一定程度提高电容器的能量密度,但是由于内阻的增加而引起功率密度有所下降。商用耐压电解液由于表面温升过快,难以在2.85 V及以上电压正常循环,而两款自制电解液均显著减少了表面温升,改善了电容器的高压循环能力。另一方面,降低电流密度可以有效控制超级电容器的表面温升速度,这使得各款电容器都能维持稳定的3 V限压循环,EMIBF₄/AN电解液甚至可以支持3.2 V上限循环,此时基于超级电容器总重量计算的最大能量密度与最大功率密度分别达到8.62 W·h/kg和16.18 kW/kg。     

Charge–discharge characteristics of LiCoO2/mesocarbon microbeads battery with poly(vinyl chloride)-based composite polymer electrolyte

Polyvinyl chloride (PVC)-based composite polymer electrolyte films consisting of PVC–LiCF₃SO₃–SiO₂ are prepared by the solution-casting method. The electrical properties of the electrolyte are investigated for ionic conductivity and its dependence on temperature. The electrolyte with the highest ionic conductivity is used to fabricate a LiCoO₂/PVC–LiCF₃SO₃–SiO₂/mesocarbon microbeads (MCMB) battery. The charge–discharge characteristics and performance of the battery at room temperature are evaluated to ascertain the effective viability, of these solid electrolytes in lithium-polymer batteries. Battery performances is also investigated at 313, 323 and 333 K.     

纳米锂镧锆钽氧粉体复合聚氧化乙烯制备的固态电解质电化学性能的研究

与采用液体电解液的传统二次锂离子电池相比,固态二次锂电池在高能量密度和安全性方面具有显著的潜在优势,近年来成为国内外的研究热点。作为固态二次锂电池的核心组成,固态电解质需要具备高离子电导率、宽电化学窗口、对锂稳定、力学性能优以及可抑制锂枝晶等特性。为达到以上要求,本工作探索制备了由纳米钽掺杂锂镧锆氧（LLZTO）粉体与聚氧化乙烯（PEO）复合的有机-无机复合固态电解质膜材料,对比研究了在有机物PEO中添加锂盐和不添加锂盐对固态电解质膜电导率及电化学特性的影响。发现在PEO-LLZTO复合电解质膜中,虽然PEO不导电,但界面处存在的渗流效应可极大提高膜的总电导率,室温离子电导率可达到2×104 S/cm。这一数值虽然略低于PEO-LiTFSI-LLZTO复合电解质膜（室温条件下电导率为6×104 S/cm）,但无锂盐添加的PEO-LLZTO复合电解质膜表现出较好的电化学稳定性和较强的抑制锂枝晶的能力。将PEO-LLZTO复合电解质膜与Li/LiFePO4和Li/LiFe0.15Mn0.85PO4组装成软包电池,在0.1 C、60 ℃的测试条件下可充分发挥正极材料的容量,并可稳定循环200次以上。     

Characteristics of ionic liquid-based electrolytes for chip type aluminum electrolytic capacitors

Since ionic liquids (ILs) possess several attractive properties, including chemical and thermal stability, nonflammability, high ionic conductivity, and negligible vapor pressure, a new electrolyte system based on ILs has been proposed for chip type aluminum electrolytic capacitors. Four ILs based on imidazolium/pyrrolidinium cations and maleate/phthalate anions have been synthesized and their thermal stabilities have been examined. The 25 wt.% solutions of the four ILs in gamma-butyrolactone (GBL) solvent were prepared as electrolytes of chip type aluminum electrolytic capacitors. The conductivity, sparking voltage and thermal stability of these electrolytes have been systematically investigated. The results revealed that the four IL-based electrolytes exhibited high conductivity. Furthermore, the conductivity of maleate anion-based electrolytes is higher than that of phthalate anion-based electrolytes, whereas the high-temperature durability of phthalate anion-based electrolytes in conductivity is superior to that of maleate anion-based electrolytes irrespective of pyrrolidinium or imidazolium cation. However, this type of electrolytes could only be utilized in low-voltage type capacitors due to their comparatively low sparking voltage. In addition, the capacitors utilizing the four IL-based electrolytes show excellent thermal stabilities during a reflow soldering process.     

Development of PVA based micro-porous polymer electrolyte by a novel preferential polymer dissolution process

A micro-porous polymer electrolyte based on PVA was obtained from PVA–PVC based polymer blend film by a novel preferential polymer dissolution technique. The ionic conductivity of micro-porous polymer electrolyte increases with increase in the removal of PVC content. Finally, the effect of variation of lithium salt concentration is studied for micro-porous polymer electrolyte of high ionic conductivity composition. The ionic conductivity of the micro-porous polymer electrolyte is measured in the temperature range of 301–351 K. It is observed that a 2 M LiClO₄ solution of micro-porous polymer electrolyte has high ionic conductivity of 1.5055 × 10⁻³ S cm⁻¹ at ambient temperature. Complexation and surface morphology of the micro-porous polymer electrolytes are studied by X-ray diffraction and SEM analysis. TG/DTA analysis informs that the micro-porous polymer electrolyte is thermally stable upto 277.9 °C. Chronoamperommetry and linear sweep voltammetry studies were made to find out lithium transference number and stability of micro-porous polymer electrolyte membrane, respectively. Cyclic voltammetry study was performed for carbon/micro-porous polymer electrolyte/LiMn₂O₄ cell to reveal the compatibility and electrochemical stability between electrode materials.     

Preparation and electrochemical characterization of polymer electrolytes based on electrospun poly(vinylidene fluoride-co-hexafluoropropylene)/polyacrylonitrile blend/composite membranes for lithium batteries

Apart from PEO based solid polymer electrolytes, tailor-made gel polymer electrolytes based on blend/composite membranes of poly(vinylidene fluoride-co-hexafluoropropylene) and polyacrylonitrile are prepared by electrospinning using 14 wt% polymer solution in dimethylformamide. The membranes show uniform morphology with an average fiber diameter of 320-490 nm, high porosity and electrolyte uptake. Polymer electrolytes are prepared by soaking the electrospun membranes in 1 M lithium hexafluorophosphate in ethylene carbonate/dimethyl carbonate. Temperature dependent ionic conductivity and their electrochemical performance are studied. The blend/composite polymer electrolytes show good ionic conductivity in the range of 10⁻³ S cm⁻¹ at ambient temperature and good electrochemical performance. All the Polymer electrolytes show an anodic stability >4.6 V with stable interfacial resistance with storage time. The prototype cell shows good charge-discharge properties and stable cycle performance with comparable capacity fade compared to liquid electrolyte under the test conditions.     

A novel approach on ionic liquid-based poly(vinyl alcohol) proton conductive polymer electrolytes for fuel cell applications

The preparation of proton conducting-polymer electrolytes based on poly(vinyl alcohol) (PVA)/ammonium acetate (CH₃COONH₄)/1-butyl-3-methylimidazolium chloride (BmImCl) was done by solution casting technique. The ionic conductivity increased with ionic liquid mass loadings. The highest ionic conductivity of (5.74 ± 0.01) mS cm⁻¹ was achieved upon addition of 50 wt% of BmImCl. The thermal characteristic of proton conducting-polymer electrolytes is enhanced with doping of ionic liquid by showing higher initial decomposition temperature. The most conducting polymer electrolyte is stable up to 250 °C. Attenuated total reflectance-Fourier Transform Infrared (ATR-FTIR) confirmed the complexation between PVA, CH₃COONH₄ and BmImCl. Polymer electrolyte membrane fuel cell (PEMFC) was fabricated. This electrochemical cell achieved the maximum power density of 18 mW cm⁻² at room temperature.     

Phase transformation and electrical properties of bismuth oxide doped scandium cerium and gadolinium stabilized zirconia (0.5Gd0.5Ce10ScSZ) for solid oxide electrolysis cell

Scandium cerium and gadolinium stabilized zirconia (SCGZ) is among the zirconia-based electrolytes which exhibits high ionic conductivity. However, stabilization of high conducting cubic phase is needed to maintain the ionic conductivity during prolonged operation. In this study, 1–2 mol% of Bi₂O₃ is added in the SCGZ to enhance the stability of the cubic phase. The phase characterization, sintering behavior, and electrochemical performance of the electrolyte-supported cells are studied. Bi₂O₃ is found to act as both phase stabilizer and sintering aid. Doping of Bi₂O₃ results in a partial decrease of unwanted rhombohedral phase (Sc₂Zr₇O₁₇) which is a low conducting phase. Increasing Bi₂O₃ content also significantly increases electrolyte densification. The unwanted rhombohedral phase is found to form at high sintering temperature. Thus, the lowest sintering temperature which is able to provide sufficiently dense electrolyte is required. In the present work, adding 2 mol% of Bi₂O₃ in the SCGZ helps reduce sintering temperature to 1350 °C with sufficiently high relative density (>96.5%). The ionic conductivity of the electrolyte is also improved with adding Bi₂O₃.     

Investigation of the sudden drop of electrolyte conductivity at low temperature in ceramic fuel cell with Ni0·8Co0·15Al0·05LiO2 electrode

The sudden drop of ionic conductivity of GDC (Gd_0.1Ce_0·9O_1.95) electrolyte in ceramic fuel cells with NCAL (Ni_0·8Co_0·15Al_0·05LiO₂) as electrode at low temperature was studied. It is found that the peak power density (PPD) of the cell with GDC electrolyte decreases linearly with the decreasing of the operation temperature above 400 °C. However, when the operation temperature drops to 400 °C, the cell PPD decreases significantly. EIS results show that the ionic conductivity of the electrolyte decreases linearly with the decrease of cell operating temperature. When the temperature decreases to approximately 400 °C, the ionic conductivity of the electrolyte decreases from 0.251 S cm^?1 at 425 °C to 0.026 S cm^?1 at 400 °C. The rapid decrease of the electrolyte ionic conductivity is considered to be the direct cause of the sudden decrease of the PPD. According to the results of XPS, FTIR and TG-DSC, LiOH/Li₂CO₃ formed in the NCAL anode diffuses into the electrolyte and melts at 419 °C or above, which is the reason for the high ionic conductivity of the electrolyte. The reason for the sudden drop of ionic conductivity is that LiOH/Li₂CO₃ and other compounds solidify in molten salts below 419 °C.     

Development of co-sintering process for anode-supported solid oxide fuel cells with gadolinia-doped ceria/lanthanum silicate bi-layer electrolyte

Gadolinia-doped ceria (GDC) and lanthanum silicate (LS) are expected to be promising materials for electrolytes of solid oxide fuel cells (SOFCs) because of their high ionic conductivities at intermediate temperatures. However, performance degradation of SOFCs is caused by current leakage through GDC and poor densification of LS. In the present study, LS was used as a blocking layer for preventing the current leakage of GDC electrolyte. Thermal shrinkage measurements and scanning electron microscopy (SEM) observation suggested that the addition of Bi₂O₃ in LS electrolyte (LSB) contributed to the decrease in the sintering temperature of the LS and improved densification of the GDC/LS bi-layer electrolyte. Consequently, the open-circuit voltage for the cell with GDC/LS and GDC/LSB bi-layer electrolytes increased effectively in comparison with that of the cell with GDC single-layer electrolyte. The electrical conductivity and fuel cell characteristics were compared among the cells with GDC, GDC/LS, and GDC/LSB electrolytes.