Pure Liquid Electrolyte Research Articles

Emerging Strategies for Gel Polymer Electrolytes with Improved Dual-Electrode Side Regulation Mechanisms for Lithium-Sulfur Batteries.

Although GPE still has the risk of shuttling due to the incomplete removal of liquid electrolytes compared to SPE, which has the most promise of eliminating polysulfide shuttling, researchers have made abundant efforts to eliminate as much of the polysulfide shuttling effect as possible while retaining the unique advantages of GPE. For example, physical barrier to polysulfides by improving the pore size of GPE or fabricating multidimensional structures by different preparation methods. Further chemical adsorption of polysulfides by adding nanofillers to increase polar sites to create polar-polar interactions with polysulfides or to create Lewis acid-base interactions. However, although chemical adsorption can indeed highly immobilize polysulfides, it still brings disadvantages such as loss of active material. Therefore, other researchers have employed GPE with ion-selective permeability that has electrostatic repulsive force or steric hindrance to polysulfides to better inhibit polysulfide shuttling. However, modifying only the cathode side is not enough to enhance this overall properties of Li-S cells. These problems of poor Li+ transport, lithium dendrite growth, and poor SEI due to uneven lithium ion deposit on Li anode side still affect the overall performance of Li-S cells. Therefore, a GPE to improve these problems on the Li anode side is summarized below. Compared with an all-solid electrolyte, GPE, which has a partially liquid electrolyte, clearly has advantages such as strong interfacial contact, good anode interface compatibility, and high flexibility. However, it is still not comparable to the ionic conductivity, etc. of pure liquid electrolyte only. Therefore, the problems on the lithium metal anode side are mainly focused on the lithium ion transport problems and the problems of lithium dendrite growth and inhomogeneous SEI at the lithium anode interface. Facing the problems in these two aspects, researchers have given many improvement solutions respectively. For the lithium ion transport problem, researchers have instead provided pathways for lithium ion transport by adding amorphous nanofibers or nanofillers to reduce the crystallinity of the polymer and improve the ionic conductivity. Alternatively, the migration number of lithium ions can be increased by limiting the anions in the electrolyte. As for the interfacial problems of lithium anodes, researchers have effectively suppressed the growth of lithium dendrites or inhomogeneous lithium plating/stripping phenomena mainly by adding nanofillers to increase the mechanical strength of GPE or by participating in the generation of SEI.

Ionic Conductivity Enhancement in UHMW PEO Gel Electrolytes Based on Room-Temperature Ionic Liquids and Deep Eutectic Solvents.

Physical gels made of poly(ethylene oxide) (PEO) and deep eutectic solvents urea-Li bis(trifluoromethanesulfonyl)imide (TFSI) and ethylene glycol/LiTFSI, or pyrrolidinium ionic liquid solutions PYR13TFSI-LiTFSI and PYR14TFSI-LiTFSI, are prepared by a fast, single-step process, which involves no auxiliary solvents or intermediates and is reproducible and scalable. The properties of these gels are studied as a function of the PEO content and its molecular weight and the nature of the liquid electrolyte. The gels prepared with a low concentration (1–5 wt %) of ultrahigh molecular weight (UHMW) PEO are tough, stretchable materials which resemble soft elastomers and are also self-healing and transparent. Their rheology shows the conventional behavior of physical polymer gels, so that the higher the molecular weight of PEO, the lower the polymer concentration needed to produce the gel. However, the ion conductivities and diffusivities of the gels are striking, in many cases being equal to or significantly higher than those of pure liquid electrolytes. This ion conductivity enhancement is the highest for the lowest PEO concentration with the highest molecular weight. This unprecedented molecular weight dependence of conductivity and diffusivity is the result of two combined effects: the liquid electrolyte chemical structure modification as a consequence of the addition of PEO and the development of elastic networks, where ion mobility and rheology are uncoupled when the PEO added is of UHMW.

Accelerated lithium-ion conduction in covalent organic frameworks.

We design a porous separator that consists of a covalent organic framework (COF), the ionic conductivity of which is 8 times higher than that of the most well-established separator of lithium-ion batteries. Moreover, the effective ionic conductivity is even higher than that of the corresponding pure liquid electrolyte.

A safe quasi-solid electrolyte based on a nanoporous ceramic membrane for high-energy, lithium metal batteries

The use of lithium metal as the anode for Lithium Metal Batteries (LMB) requires having solid or quasi-solid electrolytes able to block dendrites formation during cell cycling. Here we reported on a hybrid electrolyte membrane based on nanostructured yttria-stabilized-zirconia, sintered by means of High Pressure-Field Assisted Sintering Technique (HP-FAST) in order to retain proper nano-porosity, and activated with a standard LiPF6-EC-DMC solution. By a thorough physico-chemical and functional characterization, we demonstrated that the liquid is effectively nano-confined in the ceramic membrane, and the resulting quasi-solid electrolyte is non-flammable. A remarkable conductivity value of 0.91 mS cm−1 was observed at room temperature, with activation energy of 0.2 eV, and cation transference number, t+ = 0.55, substantially higher than that of the pure liquid electrolyte. The hybrid electrolyte showed electrochemical stability up to 5.5 V vs. Li+/Li, and excellent resistance to dendrite formation for more than 350 cycles in a Li/electrolyte/Li symmetrical cell. A full cell Li/electrolyte/LiMn2O4 showed more than 90 mAh g−1 at 2C for more than 120 cycles. These very promising results indicated that nano-porous ceramic hybrid electrolytes may be conveniently used in LMB.

Comparative study of interfacial behavior for polyphosphazene based polymer electrolytes and LiPF6 in EC/DMC against lithium metal anodes

The characteristics of polyphosphazene based polymer electrolytes were investigated in contact to lithium metal electrodes with regard to interface stability, SEI, current-overpotential relation and kinetics of lithium deposition and dissolution. The comparative study concerned three electrolyte types: a) the pure liquid electrolyte 0.7 M LiPF6 in 1:1 (v:v) EC/DMC as reference electrolyte, b) freshly prepared dry electrolyte membranes made from poly[bis(2-(2-methoxyethoxy)ethoxy) phosphazene] (MEEP) with dissolved LiPF6 and c) a MEEP-based gel containing MEEP with 50 wt% of the above liquid electrolyte. A combination of DC current-voltage analysis, electrochemical impedance spectroscopy and optical microscopy was used to study lithium cycling, time dependent galvanostatic lithium plating/stripping and overpotential variation. A detailed model is presented which explains the observed voltage-time profile and the accompanying interface changes considering the comprehensive influences of electrolyte decomposition, changes of the interface morphology, the increase of surface roughness induced by dendrite formation (microscopically monitored) and the intermediately forming SEI (analyzed by impedance spectroscopy).

Nanostructured Polymer Particles as Additives for High Conductivity, High Modulus Solid Polymer Electrolytes

For the next generation of safe and high energy rechargeable lithium metal batteries, we introduce nanostructured polymer particles of asymmetric miktoarm star copolymers as additives to liquid electrolytes for use as solid polymer electrolytes (SPE). The mechanical properties of the resulting SPEs are dramatically improved compared to the pure liquid electrolyte (the elastic modulus increased by up to 8 orders of magnitude), while the ionic conductivity was maintained close to that of the pure liquid electrolyte. In particular, the addition of 44 wt % miktoarm stars, composed of ion conducting poly(ethylene oxide), PEO, arms that complement stiff insulating polystyrene arms, PS ((PS)n(PEO)n, where n = 30 the number of arms), in a low molecular weight PEO doped with lithium bis(trifluoromethane)sulfonamide (LiTFSI), resulted in SPEs with a shear modulus of G′ ∼ 0.1 GPa and ion conductivity σ ∼ 10–4 S/cm. The SPEs show a strong decoupling between the mechanical behavior and the ionic conductivity as G′ rem...

Molecular interactions in high conductive gel electrolytes based on low molecular weight gelator

Organic ionic gel (OIG) electrolytes, also known as gel electrolytes or ionogels are one example of modern functional materials with the potential to use in wide range of electrochemical applications. The functionality of OIGs arises from the thermally reversible solidification of electrolytes or ionic liquids and their superior ionic conductivity. To understand and to predict the properties of these systems it is important to get the knowledge about the interactions on molecular level between the solid gelator matrix and the electrolyte solution. This paper reports the spectroscopic studies (FT-IR, UV–Vis and Raman) of the gel electrolyte based on low molecular weight gelator methyl-4,6-O-(p-nitrobenzylidene)-α-d-glucopyranoside and solution of quaternary ammonium salt, tetramethylammonium bromide. The solidification process was based on sol-gel technique. Below characteristic temperature, defined as gel to sol phase transition temperature, Tgs, the samples were solid-like and showed high conductivity values of the same order as observed for pure liquid electrolytes. The investigations were performed for a OIGs in a wide range of molar concentrations of the electrolyte solution.

Efficient quasi-solid-state dye-sensitized solar cells employing polyaniline and polypyrrole incorporated microporous conducting gel electrolytes

With an aim of elevating liquid electrolyte loading, ionic conductivity, and electrocatalytic activity of gel electrolyte toward triiodides, freeze-dried microporous poly(acrylic acid)–poly(ethylene glycol) (PAA–PEG) matrix is employed to uptake conducting polymers, such as polyaniline (PANi) and polypyrrole (PPy). An ionic conductivity of 19.18 mS cm−1 at 65 °C is obtained from PAA–PEG/PANi conducting gel electrolyte which is nearly similar to 19.22 mS cm−1 from pure liquid electrolyte. The conducting polymers can form interconnected channels within insulating microporous PAA–PEG matrix, therefore, the reduction reaction of triiodide ions in the dye-sensitized solar cells (DSSCs) can be extended from Pt/gel electrolyte interface to both interface and three-dimensional framework of microporous conducting gel electrolyte. The resultant DSSCs from PAA–PEG/PANi and PAA–PEG/PPy conducting gel electrolytes display power conversion efficiencies of 7.12% and 6.53%, respectively, which are much higher than 5.02% from pure PAA–PEG-based DSSC. The new concept along with easy fabrication promises the microporous conducting gel electrolytes to be good alternatives in efficient quasi-solid-state DSSCs.

A comparative study of charge transport in quasi-solid state dye-sensitized solar cells using polymer or nanocomposite gel electrolytes

To improve the photovoltaic and durability performance of quasi-solid dye-sensitized solar cells (DSCs), we added selected gelators to the liquid electrolyte. Metal-oxide nanoparticles or fluorine polymers were employed to solidify 3-methoxypropionitrile-based liquid electrolytes containing an ionic liquid, 1,2-dimethyl-3-propyl imidazolium iodide, as a source of iodide. The electrochemical impedance spectroscopy and cyclic voltammetry were employed to study the kinetics of electrochemical and photoelectrochemical processes, including the elucidation of the electronic and ionic processes occurring in the various quasi-solid-state solar cells. Among tested gelators, a cell with TiO2 solidified electrolyte (P25, 17.5wt%) exhibited the best photovoltaic performance with power conversion efficiency 7.65% under simulated sunlight (100mWcm−2). The DSCs with a TiO2 solidified electrolyte showed endurance of stability superior to that of a pure liquid electrolyte.

Inorganic solid/organic liquid hybrid electrolyte for use in Li-ion battery

This study focuses on preparing a hybrid electrolyte, the combination of 90wt% inorganic solid and 10wt% organic liquid, for lithium based rechargeable batteries to illustrate the effect of electrode/electrolyte interfacing on electrochemical performance. The inorganic solid electrolyte selected is Li1.3Ti1.7Al0.3(PO4)3, and the Li-ion conducting organic liquid electrolyte selected is 1M LiPF6 in EC:DEC. Because of the addition of Li-ion conducting liquid between the solid electrode and solid electrolyte, the hybrid electrolyte cell minimizes the ineffective solid-on-solid interfaces common in all-solid-state cells. It is also expected that using a liquid electrolyte at the point of contact between the solid electrolyte and the electrode will adjust for the volume change of the electrode during Li insertion/extraction. As a result, the electrochemical performance of the hybrid electrolyte cell is superior to that of a solid electrolyte cell and is also competitive to that of a pure liquid electrolyte coin cell. Another advantage of the hybrid electrolyte cell observed in this work is that this system behaves as a self-safety device when sudden, higher temperatures are applied.

Poly(vinylpyridine- co-styrene) based in situ cross-linked gel polymer electrolyte for lithium-ion polymer batteries

A gel polymer electrolyte (GPE) was successfully prepared by means of an in situ cross-linking reaction of poly(2-vinylpyridine-co-styrene) and oligo(ethylene oxide) with epoxide functional groups at 65 °C without using a polymerization initiator. A stable gel polymer electrolyte could be obtained by adding only 1% of a polymer gelator. The ionic conductivity of the GPE containing 99 wt% of liquid electrolyte was measured to be ca. 10 −2 S/cm at the ambient temperature. The ionic conductivity of the resulting GPE was comparable to that of a pure liquid electrolyte. The electrochemical stability window of the prepared gel polymer electrolytes was measured to be 5.2 V. The test cell carried a discharge capacity of 133.2 mAh/g at 0.1 C and showed good cycling performance with negligible capacity fading after the 200th cycle, maintaining 99.5% coulombic efficiency throughout 200 cycles. The resulting gel polymer electrolyte prepared by in situ thermal cross-linking without a polymerization initiator holds promise for application to on the high power lithium-ion polymer batteries.

Swollen Polymethacrylonitrile Urethane Networks for Lithium Batteries

Copolymers of methacrylonitrile are reported. Surprisingly, their curing through urethane cross-links acts as an internal plasticization. This cross-linking prevents any dissolution or leakage, up to 90°C, when the networks are swollen by liquid organic electrolytes. These provide high conductivities, and the increase in resistivity with respect to pure liquid electrolytes remains ⩽2. The electrochemical investigation shows a clear improvement of stability in reduction as compared to polyacrylonitrile. The first evaluations of lithium insertion in graphite are encouraging. © 2002 The Electrochemical Society. All rights reserved.

Lithium-7 NMR and ionic conductivity studies of gel electrolytes based on poly(methylmethacrylate)

Gel electrolytes synthesized from poly(methylmethacrylate), ethylene carbonate, propylene carbonate and various lithium salts [LiClO 4, LiAsF 6, or LiN(CF 3SO 2) 2] have been investigated by differential scanning calorimetry, electrical conductivity, and 7Li, 19F and 75As NMR spectroscopy. Although the ionic conductivities of the gels approach those of liquid electrolytes above room temperature, the NMR results indicate that the immediate environments of both the cations and anions differ significantly in the gel and in the liquid. Thus the presence of microscopic regions of pure liquid electrolyte in the gel can be ruled out.