LiBOB-based gel electrolyte Li-ion battery for high temperature operation

In this work, we evaluated the chemical compatibility of 1.0m (molality) lithium bis(oxalate)borate (LiBOB) 1:1 (w/w) propylene carbonate (PC)/ethylene carbonate (EC) liquid electrolyte with lithium metal and spinel LiMn₂O₄ cathode using storage and cycling tests at high temperatures. Impedance analyses show that LiBOB and lithium are very compatible due to the formation of a stable passivation layer on the surface of lithium. Cycling tests of Li/Cu and Li/LiMn₂O₄ cells, respectively, show that lithium can be plated and stripped in LiBOB-based electrolyte with more than 80% cycling efficiency, and that this electrolyte can support LiMn₂O₄ cycling reversibly up to 60 °C without visible capacity loss. Using LiBOB-based liquid electrolyte and porous Kynar^® membrane, microporous gel electrolyte (MGE) Li-ion cells were assembled and evaluated. Results show that the MGE cell presents an improved cycling performance compared with a liquid cell, especially at elevated temperatures. It is confirmed that the LiBOB-based gel electrolyte Li-ion batteries can be operated at 60 °C with good capacity retention.     

Unique charge/discharge properties of carbon materials with different structures

The electrochemical properties of isotropic and anisotropic carbon fibers were studied using the two-electrode method in a 1 M LiPF₆ electrolyte solution dissolved in a 1:1 volume mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). Isotropic carbon fiber had a larger irreversible capacity than that of the other carbon fibers and might have many sites for the electrolyte decompositon and residual lithium ions. Active carbon fibers prepared from isotropic carbon fibers by steam gasiformation at 850 °C showed large specific surface areas. After steam gasiformation, the active carbon fibers changed into a more crystalline state and had electrochemical characteristics similar to the anisotropic carbon fibers. Impedance profiles of the carbon fibers were fitted by equivalent analogs of a three-resistance—capacitance (RC) circuit in series and the thickness of passivation layers in the carbon fibers was calculated.     

New electrolytes based on glutaronitrile for high energy/power Li-ion batteries

Glutaronitrile, CN[CH₂]₃CN, is evaluated as a co-solvent in thermally and (anodically) electrochemically stable electrolyte mixtures suitable for high energy/power Li-ion batteries. Linear sweep voltammetry scans indicate an electrochemical anodic stability of more than 6 V versus Li⁺/Li for the 1 M LiTFSI electrolytes. Glutaronitrile and its ethylene carbonate electrolyte solutions show high ionic conductivities and low viscosities reaching 5 mS cm⁻¹ and 7 cP, respectively, at 20 °C. Aluminum corrosion tests of the solutions showed an improved protective resistance up to 4.4 V. Lithium ion batteries incorporating graphite as an anode and LiCoO₂ as the cathode material were assembled using a glutaronitrile electrolyte mixture, whose stability on graphite was greatly enhanced by the use of ethylene carbonate as a co-solvent and Li (bioxalatoborate) (LiBOB) as a co-salt, and these cells showed moderately good discharge capacities with low capacity fade up to the 100th cycle.     

Production,performance and cost analysis of anode-supported NiO-YSZ micro-tubular SOFCs

In the present study, the anode-supported micro-tubular solid oxide fuel cells (MT-SOFCs) with an electrolyte thin interlayer were manufactured. The anode support tubes consisting of 56 wt% nickel oxide and 44 wt% YSZ (8 mol% yttria (Y₂O₃) stabilized zirconia (ZrO₂)) were produced by using the thermo-extrusion method, whereas the electrolyte and cathode layers were manufactured using the dip-coating method. The half-cells consisting of anode and electrolyte were manufactured by using two different methods. In the first method, the anode-support tubes were pre-sintered at 1200 °C, then covered with the electrolyte layer by using the dip-coating method and then exposed to second sintering at 1400 °C. In the second method, the anode and electrolyte layers were sintered together at 1400 °C (co-sintering) in order to produce the half-cells. The half-cells that were produced and then coated with cathode solutions by using the dip-coating method and the final cells were successfully produced at the end of the sintering at 1150 °C. The porosity and shrinkage percentage values of these MT-SOFCs differed from each other. The power densities of these cells were tested at 700 °C, 750 °C, and 800 °C by using H₂ gas as fuel and the results of the microstructural and cost analyses were compared.     

Improvement of electrochemical characteristics of natural graphite negative electrode coated with polyacrylic acid in pure propylene carbonate electrolyte

In order to improve the negative electrode characteristics of a graphite electrode in a propylene carbonate (PC)-containing electrolyte, we have prepared a graphite negative electrode coated with a water-soluble anionic polymer as a binder for composite graphite electrodes. The electrochemical characteristics of the coated graphite were evaluated by cyclic voltammetry and charge–discharge cycle tests. The coated graphite negative electrode showed a stable Li⁺ ion intercalation/deintercalation reaction without the exfoliation of the graphene layers caused by the co-intercalation of the PC solvent in the LiClO₄/PC solution. The charge–discharge characteristic of the coated graphite negative electrode in a PC-containing electrolyte was almost the same as that in ethylene carbonate-based electrolyte.     

Effects of cyclic carbonates as additives to γ-butyrolactone electrolytes for rechargeable lithium cells

γ-Butyrolactone (GBL) has a high boiling point, a low freezing point, a high flashing point, a high dielectric constant and a low viscosity. GBL is a very preferable solvent for lithium ion batteries. However, GBL readily undergoes reductive decomposition on the surface of the negative electrodes, and it forms a solid electrolyte interphase (SEI) with a large resistance. It is causing deterioration of battery performances. In this work, effects of cyclic carbonates as additives to GBL electrolytes were investigated. As these carbonates, ethylene carbonate (EC), propylene carbonate (PC), vinylene carbonate (VC), vinylethylene carbonate (VEC), and phenylethylene carbonate (PhEC) were investigated using LiCoO₂/graphite cells. The effects of these additives were evaluated from the viewpoints of improvement of the battery performance and suppression of the reductive decomposition of GBL. VC, VEC and PhEC were effective to suppress the excess reductive decomposition of GBL. Battery performances were improved and the following results were obtained from the electrochemical measurements of LiCoO₂/graphite cells with GBL-based electrolytes. Residual capacity was high in the order of VEC > VC > PhEC. Rate capability was high in the order of PhEC > VC > VEC. These additives have advantages and disadvantages. By optimizing electrolyte formulation, the performances of Li-ion batteries using GBL-based electrolytes will be improved further.     

Toward high performance solid-state lithium-ion battery with a promising PEO/PPC blend solid polymer electrolyte

A promising solid polymer blend electrolyte is prepared by blending of poly(ethylene oxide) (PEO) with different content of amorphous poly(propylene carbonate) (PPC), in which the amorphous property of PPC is utilized to enhance the amorphous/free phase of solid polymer electrolyte, so as to achieve the purpose of modifying PEO-based solid polymer electrolyte. It indicates that the blending of PEO with PPC can effectively reduce the crystallization and increase the ion conductivity and electrochemical stability window of solid polymer electrolyte. When the content of PPC reaches 50%, the ionic conductivity reaches the maximum, which is 2.04 × 10⁻⁵ S cm⁻¹ and 2.82 × 10⁻⁴ S cm⁻¹ at 25°C and 60°C, respectively. The electrochemical stability window increases from 4.25 to 4.9 V and the interfacial stability of lithium metal anode is also greatly improved. The solid-state LiFePO₄//Li battery with the PEO/50%PPC blend solid polymer electrolyte has good cycling stability, which the maximum discharge specific capacity is up to 125 mAh g⁻¹ at a charge/discharge current density of 0.5 C at 60°C.     

Lithium ion conductivity of gel polymer electrolytes containing insoluble lithium tetrakis(pentafluorobenzenethiolato) borate

Lithium ion conducting gel polymer electrolytes composed of insoluble lithium tetrakis(pentafluorobenzenethiolato) borate (LiTPSB), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) and ethylene carbonate–propylene carbonate mixed solvent (EC–PC) were prepared and their ionic conductivities and electrochemical stabilities were investigated. Ionic conductivity was largely dependent on the contents of EC–PC and LiTPSB. Gel polymer electrolyte containing optimized content of 50 (LiTPSB)–50 (PVDF-HFP/EC–PC (13:87 wt.%)) exhibited ionic conductivity of 4 × 10⁻⁴ S cm⁻¹ at 30 °C, lithium ion transference number of 0.33 and anodic oxidation potential of 4.2 V.     

Functional interface of polymer modified graphite anode

Graphite electrodes were modified by polyacrylic acid (PAA), polymethacrylic acid (PMA), and polyvinyl alcohol (PVA). Their electrochemical properties were examined in 1 mol dm⁻³ LiClO₄ ethylene carbonate:dimethyl carbonate (EC:DMC) and propylene carbonate (PC) solutions as an anode of lithium ion batteries. Generally, lithium ions hardly intercalate into graphite in the PC electrolyte due to a decomposition of the PC electrolyte at ca. 0.8 V vs. Li/Li⁺, and it results in the exfoliation of the graphene layers. However, the modified graphite electrodes with PAA, PMA, and PVA demonstrated the stable charge–discharge performance due to the reversible lithium intercalation not only in the EC:DMC but also in the PC electrolytes since the electrolyte decomposition and co-intercalation of solvent were successfully suppressed by the polymer modification. It is thought that these improvements were attributed to the interfacial function of the polymer layer on the graphite which interacted with the solvated lithium ions at the electrode interface.     

Vinylene carbonate and vinylene trithiocarbonate as electrolyte additives for lithium ion battery

Vinylene carbonate (VC) and vinylene trithiocarbonate (VTC) are studied as electrolyte additives in two kinds of electrolytes: (1) propylene carbonate (PC) and diethyl carbonate (DEC) (1:2 by weight) 1 mol dm⁻³ LiPF₆; (2) ethylene carbonate (EC) and DEC (1:2 by weight) 1 mol dm⁻³ LiPF₆. Characterization is performed by cyclic voltammetry, impedance spectroscopy, scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and half cell tests. Cyclic life is better in either electrolyte with VC than either electrolyte with/without VTC. SEM shows VC and VTC both form well developed passivation films on the graphite anode, but the films with VTC are thicker than with VC. EIS shows the VTC films have significantly higher charge transfer resistance. The VTC film in PC fails to protect against exfoliation. XPS indicates VTC has different reaction pathways in PC relative to EC. In EC/DEC, VTC forms polymeric C-O-C-like components and sulfide species (C-S-S-C, S and C-S-C). In PC/DEC, VTC does not form polymeric species, instead forming a film mainly containing LiF and Li₂S. It appears that a thinner polymeric film is preferential. The specific data herein are of interest, and the general conclusions may help development of improved additives for enhanced Li-ion battery performance.     

In situ neutron radiography of lithium-ion batteries: the gas evolution on graphite electrodes during the charging

In situ neutron radiography (NR) was used to study the gas evolution on graphite electrodes in lithium-ion cells containing different PVDF-based gel-type electrolytes. The amount of gas bubbles and channels was calculated by image analysis. Gas production was extremely high in the case of the electrolyte containing ethylene carbonate (EC) and propylene carbonate (PC) (2:3, w/w), 1 M LiClO₄. About 60% of the electrode surface consisted of the gas phase which resulted in an inhomogeneous local current distribution. In contrast, the electrolyte containing EC and γ-butyrolactone (GBL) (1:1, w/w), 1 M LiBF₄ only showed a small increase of the gas volume between the electrodes of about 3%. In situ NR also revealed the displacement of the electrolyte due to gas evolution and volume changes of the electrodes.     

Thermalgravimetry–mass spectrometry studies on the thermal stability of graphite anodes with electrolyte in lithium-ion battery

Thermal reactions of a lithiated graphite anode in 1 M LiPF₆-ethylene carbonate (EC)/dimethyl carbonate (DMC) (50:50 vol.%) in the temperature range 40–320 °C were investigated by TG–MS analysis. Studies by TG–MS during thermal reactions detected a small exothermic peak around 140 °C due to CO₂ (m/z = 44) evolution, which suggests partial destruction of the SEI formed on the graphite and/or decomposition of the electrolyte through the SEI. In addition, the main exothermic reaction above 280 °C, which is associated with simultaneous evolution of C₂H₄O (m/z = 44), is caused by direct reaction of the lithiated graphite with solvent.     

Electrolytic characteristics of asymmetric alkyl carbonates solvents for lithium batteries

Asymmetric alkyl carbonate solvents (ACS) have been used as components of liquid electrolyte systems designed for Li-ion cells. Four ACS were selected: methyl-propyl carbonate (MPC), ethyl-propyl carbonate (EPC), methyl-isopropyl carbonate (MiPC) and ethyl-isopropyl carbonate (EiPC). The common features of all these ACS are a low melting point and a low viscosity, enhancing electrolytes conductivity toward low temperatures. The viscosity and the conductivity (salt: LiPF₆, 1 M) of the ACS and their mixtures with ethylene carbonate (EC, 50% v/v), were studied as a function of the temperature (T). Arrhenius types plots of the logarithm of the conductivity versus 1/T reveals that ACS and ACS/EC mixtures are vitreous at low temperatures. The electrochemical and cycling behaviors of a graphite anode and a LiCoO₂ cathode have been evaluated using coin cells with a Li counter electrode. The charge and discharge capacities have been determined as a function of the cycle number. MiPC which builds an highly stable surface film on the graphite electrode, can be used as a single-solvent electrolyte with only a slight decrease in capacity of the LiCoO₂ cathode. All ACS/EC mixtures exhibit good filming properties at the negative electrode and no capacity loose at the positive electrode. The ability of some of the electrodes–electrolyte systems to undergo increased rates of discharge (C/5 to C/2) has been also evaluated.     

A novel composite gel polymer electrolyte for rechargeable lithium batteries

Composite polymer electrolyte (PE) films comprising of thermoplastic polyurethane (TPU) and polyacrylonitrile (PAN) (denoted as TPU–PAN) have been prepared by two different processes. Scanning electron microscope (SEM) of the films reveal the differences in morphology between them. The electrochemical properties of composite electrolyte films incorporating LiClO₄–propylene carbonate (PC) were studied. TPU–PAN based gel PE shows high ionic conductivity at room temperature. Thermogravimetric analysis informs that the composite electrolyte possesses good thermal stability with a decomposition temperature higher than 300 °C. Electrochemical stability in the working voltage range from 2.5 to 4.5 V was evident from cyclic voltammetry. Cycling performances of Li/PE/LiCoO₂ cells were also performed to test the suitability of the composite electrolyte in batteries.     

Effect of Cu2O coating on graphite as anode material of lithium ion battery in PC-based electrolyte

Cuprous oxide-coated graphite was synthesized by a polyol reduction process and analyzed by scanning electron microscopy, charge–discharge measurements and cyclic voltammetry. Cu₂O exists at the surface of graphite in the form of nanoparticles and nanorods. The coated cuprous oxide layer acts as a protective layer separating graphite from the propylene carbonate (PC)-based electrolyte solution, and greatly suppresses PC decomposition and graphite exfoliation in PC-based electrolyte systems.     

Poly(ethyl methacrylate) and poly(2-ethoxyethyl methacrylate) based polymer gel electrolytes

New poly(ethyl methacrylate) and poly(2-ethoxyethyl methacrylate) gel electrolytes containing immobilised lithium perchlorate solution in propylene carbonate were prepared by UV radical polymerisation. Materials exhibit high ionic conductivity up to 0.23 mS cm⁻¹ and long-term stability of chemical and mechanical properties. Both materials keep their suitable conductivity above −20 °C. The effect of material composition, temperature, cross-linking agent and salt concentration on the electrochemical and mechanical properties were studied using impedance spectroscopy and cyclic voltammetry. The accessible electrochemical window of both polymer electrolytes was estimated from −2.1 to 1.5 V versus Cd/Cd²⁺. Impedance measurements showed almost one-order increase of conductivity when ethylene dimethacrylate was used as a cross-linking agent in comparison with the polymer electrolyte without agent.     

Electrochemical performance of nonflammable polymeric gel electrolyte containing triethylphosphate

Nonflammable polymeric gel electrolyte has been prepared by immobilizing 1 M LiBF₄/EC + DEC + TEP (55:25:20, v/v/v, EC: ethylene carbonate, DEC: diethyl carbonate and TEP: triethylphosphate) solution in poly(vinylidene fluoride-co-hexafluoro propylene) (PVdF-HFP) where TEP acts as a fire-retardant solvent in the gel electrolyte. The polymeric gel electrolyte has a high value of ionic conductivity of 1.76 mS cm⁻¹ at 28 °C. Thermal safety calorimetry (TSC) experiments show good thermal stability of the gel electrolyte. Cyclic voltammetry and charge/discharge cycling tests were performed on LiMn₂O₄/gel electrolyte and graphite/gel electrolyte half cells. The gel electrolyte works well for graphite/LiMn₂O₄ cell although some improvement in the cycleability of the graphite electrode is still needed.     

Ethyl isocyanate—An electrolyte additive from the large family of isocyanates for PC-based electrolytes in lithium-ion batteries

To avoid solvent co-intercalation into graphite, the presence of a solid electrolyte interphase (SEI) is required. This film is formed via the reductive decomposition of electrolyte species, i.e. a film forming electrolyte additives. In this contribution we focus on an isocyanate compound, ethyl isocyanate (EtNCO) which performs well in a propylene carbonate electrolyte at both graphite anode and LiCoO₂ cathode. EtNCO is investigated by in situ Fourier transform infrared (FTIR) spectroscopy. We conclude that the formation of a radical anion via electrochemical reduction of the electrolyte additive is the initiating step of the SEI formation process. The electro-polymerization of isocyanate monomers in small additive amounts in the PC electrolyte is critically discussed.     

Electrochemical capacitors with plasticized gel-polymer electrolytes

Solid electrolytes which comprise lithium and magnesium triflate ionic salts in polyacrylonitrile (PAN)-based gels are used to fabricate electrochemical double-layer capacitors in conjunction with ethylene carbonate (EC) and propylene carbonate (PC) as plasticizers and high-density graphite (HDG) as polarizable electrodes. The conductivity of the solid electrolytes is around 10⁻³ S cm⁻¹ at ambient temperatures. Composites which consist of PAN+EC+PC+electrolyte are electrochemically stable over a wide potential range. Cells of the type: HDG/PAN–EC–PC–Li and Mg-triflate/HDG exhibit single-electrode discharge capacitance values of 480 and 383 μF cm⁻², respectively, with attractive charge–discharge characteristics.     

High‐performance solid PEO/PPC/LLTO‐nanowires polymer composite electrolyte for solid‐state lithium battery

Solid polymer composite electrolyte (SPCE) with good safety, easy processability, and high ionic conductivity was a promising solution to achieve the development of advanced solid‐state lithium battery. Herein, through electrospinning and subsequent calcination, the Li_0.33La_0.557TiO₃ nanowires (LLTO‐NWs) with high ionic conductivity were synthesized. They were utilized to prepare polymer composite electrolytes which were composed of poly (ethylene oxide) (PEO), poly (propylene carbonate) (PPC), lithium bis (fluorosulfonyl)imide (LiTFSI), and LLTO‐NWs. Their structures, thermal properties, ionic conductivities, ion transference number, electrochemical stability window, as well as their compatibility with lithium metal, were studied. The results displayed that the maximum ionic conductivities of SPCE containing 8 wt.% LLTO‐NWs were 5.66 × 10^?5 S cm^?1 and 4.72 × 10^?4 S cm^?1 at room temperature and 60°C, respectively. The solid‐state LiFePO₄/Li cells assembled with this novel SPCE exhibited an initial reversible discharge capacity of 135 mAh g^?1 and good cycling stability at a charge/discharge current density of 0.5 C at 60°C.