Low Lattice Energy Research Articles

Ionic Conduction in Sol-Gel Overcoats for Organic Photoreceptors

Silsesquioxanes are a class of silicone polymers that are useful as abrasion resistant overcoats for organic photoreceptors. We have manipulated the electrical properties of this sol-gel matrix by incorporating a solid electrolyte. Solid electrolytes, also referred to as solid ionic conductors, are materials in which electrical conductivity is provided by the motion of ions. We chose (3-glycidoxypropyl)trimethoxysilane (GPS) as the most suitable commercially available monomer to form the polymer matrix for lithium iodide migration. This epoxysilane contains two ether oxygen atoms for coordination to low lattice energy salts. A good correlation was observed between the level of GPS and the electrical properties of the coating, with increasing concentration of GPS in the silsesquioxane resulting in decreased resistivity. The silsesquioxane is a hard material and the resistivity remained relatively high, between 1010–1013 ohm-cm. However, this is a desirable resistivity for maintaining image quality with an overcoated photoreceptor.

New approaches to the design of polymer and liquid electrolytes for lithium batteries

All non-aqueous lithium battery electrolytes are Lewis bases that interact with cations. Unlike water, they do not interact with anions. The result is a high degree of ion pairing and the formation of triplets and higher aggregates. This decreases the conductivity and the lithium ion transference, and results in polarization losses in batteries. Approaches that have been used to increase ion dissociation in poly(ethylene oxide) (PEO)-based electrolytes are the use of salts with low lattice energy, the addition of polar plasticizers to the polymer, and the addition of cation complexing agents such as crown ethers or cryptands. Complexing of the anions is a more promising approach, since it should increase both ion dissociation and the lithium transference. At Brookhaven National Laboratory (BNL) we have synthesized two new families of neutral anion complexing agents, each based on Lewis acid centers. One is based on electron deficient nitrogen sites on substituted aza-ethers, wherein the hydrogen on the nitrogen is replaced by electron withdrawing groups such as CF 3SO 3–. The other is based on electron deficient boron sites on borane or borate compounds with various fluorinated aryl or alkyl groups. Some of the borane-based anion receptors can promote the dissolution of LiF in several solvents. Several of these compounds, when added in equivalent amounts, produce 1.2 M LiF solutions in DME, an increase in solubility of LiF by six orders of magnitude. Some of these LiF electrolytes have conductivities as high as 6×10 −3 S cm −1. The LiF electrolytes with borane anion acceptors in PC:EC:DEC solvents have excellent electrochemical stability. This has been demonstrated in small Li/LiMn 2O 4 cells.

Effect of added salt species on the ionic conductivity of PEO/sulfonamide salt hybrids

Addition of low molecular weight lithium salt improved the ionic conductivity of poly(ethylene oxide) (PEO) with the average molecular weight of 1000 having sulfonamide lithium salt on both ends (PEO 1900/SALi salt hybrid). These mixtures with the hybrid and a variety of lithium salts gave homogeneous viscous liquid. The DSC measurement revealed that the mixture was amorphous without the crystallization of PEO chains. Ionic conductivity of the mixture with hybrid and equimolar amount of LiClO 4 was 1.6×10 −4 S cm −1 at 50°C which was 3.8 times higher than that of the hybrid without the salt. Slight addition of the salt, which has low lattice energy such as LiClO 4, was effective for improvement of the ionic conductivity. Ionic conductivity of the mixture lowered by the increase of added salt concentration because of considerable increase of glass transition temperature ( T g). Among these, the mixture with lithium bis-(trifluoromethane sulfonyl)imide (LiTFSI) kept ionic conductivity high in a wide salt concentration without large elevation of the T g.

Preparation and polymerization of new organic molten salts; N-alkylimidazolium salt derivatives

N-vinylimidazolium tetrafluoroborate (VyImBF 4) was prepared by the neutralization and polymerized to obtain a model of molten salt polymers. After polymerization, obtained P(VyImBF 4) showed much lower ionic conductivity (2.0×10 −9 S cm −1 at 30°C) than the corresponding monomeric VyImBF 4 (1.0×10 −4 S cm −1 at 30°C). The conductivity increased with the increase of lithium salts (LiTFSI and LiBF 4) concentration up to 25 mol% to Im + unit. However, the ionic conductivity was decreased once at the concentration of 50 mol%. Then the conductivity increased again about one order at 60 mol% addition and reached a constant value for highly concentrated samples. All the samples were turbid when approximately more than 60 mol% salt was added, suggesting phase separation. The microscopic phase separation was effective to keep the ionic conductivity high in P(VyImBF 4). When LiCl was added to P(VyImBF 4), the maximum conductivity was found at 50 mol%. P(VyImBF 4)/LiCl mixture showed lower ionic conductivity than other mixtures did for any salt concentration. The salt having low lattice energy led to high ionic conductivity of the polymer salt.

Ionic Conduction in Sol-Gel Overcoats for Organic Photoreceptors

Silsesquioxanes are a class of silicone polymers that are useful as abrasion resistant overcoats for organic photoreceptors. We have manipulated the electrical properties of this sol-gel matrix by incorporating a solid electrolyte. Solid electrolytes, also referred to as solid ionic conductors, are materials in which electrical conductivity is provided by the motion of ions. We chose (3-glycidoxypropyl) trimethoxysilane (GPS) as the most suitable commercially available monomer to form the polymer matrix for lithium iodide migration. This epoxysilane contains two ether oxygen atoms for coordination to low lattice energy salts. A good correlation was observed between the level of GPS and the electrical properties of the coating, with increasing concentration of GPS in the silsesquioxane resulting in decreased resistivity. The silsesquioxane is a hard material and the resistivity remained relatively high, between 1010-1013 ohm-cm. However, this is a desirable resistivity for maintaining image quality with an overcoated photoreceptor.

Crystal structure predictions for acetic acid

Possible crystal structures of acetic acid were generated, considering eight space groups and assuming one molecule in the asymmetric unit. Our grid-search method was compared with a Monte Carlo approach as implemented in the Biosym/MSI Polymorph Predictor. This revealed no sampling deficiencies. A large number of possible crystal structures were found (∼100 within only 5 kJ/mol), including the experimental structure. Energy minimizations were done with a united-atoms force field (GROMOS), an all-atoms force field (AMBER), and a potential that describes the electrostatic interactions with distributed multipoles (DMA). In all cases, the experimental structure had a low lattice energy. The number of hypothetical crystal structures was reduced considerably by removing space-group symmetry constraints, or by a primitive molecular dynamics shake-up. Nevertheless, sufficient structures of equal or lower energy compared with the experimental structure remained to suggest that other factors need to be considered for genuine structure prediction. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 459–474, 1998

The synthesis and crystal structure of Rb4TmI6: Lattice energy calculations on networks of condensed cubes

Single crystals of Rb4TmI6 were obtained by heating a mixture of RbI, Tm, and HgI2 in Ta ampoules. Using single crystal X-ray diffraction, the crystal structure of Rb4TmI6 was assigned to the K4CdCl6 structure type; space group R3̄c (no. 167); a=14.159(2) Å, c=17.46(3) Å; Z=6, R=0.056; Rw=0.098 (F>4σ(F)) for 450 observed reflections with 2θmax=45°. This structure features a one-dimensional chain along the c axis, which is composed of alternating, face-sharing [TmI6] trigonal antiprisms (distorted octahedra) and distorted [RbI6] trigonal prisms. Alternatively, Rb4TmI6 is a three-dimensional network of TmI6-centered Rb8 distorted cubes which share vertices according to the formula [(TmI6)Rb8/2]. This viewpoint establishes its relationship to the tetragonal Tl4HgBr6 structure type. Lattice energy and extended Hückel calculations were carried out on several models of A4MI6 with s0 and s2 A+ cations: the rhombohedral K4CdCl6 structure type always shows the lower lattice energy, but a s2–s2 attractive interaction (via sp hybridization) enhances the stability of the tetragonal Tl4HgBr6 structure type.

Reactions of Carboxylic Acids on Oxides: 2. Bimolecular Reaction of Aliphatic Acids to Ketones

The reaction of aliphatic carboxylic acids over oxidic catalysts has been studied. Ketones are the main product in this reaction. Up to now there has been no agreement in the literature concerning the mechanism of this ketonization reaction. In the case of acetic acid, it appears that the ketone can be formed via two different routes. On oxides with a low lattice energy, bulk acetates are formed, decomposition of which leads to acetone. On oxides with a high lattice energy, the reaction to acetone takes place on the surface and leaves the bulk structure of the catalyst unaltered. The surface reaction to ketones probably proceeds via an intermediate that is oriented parallel to the surface and that has chemical interactions with the catalyst via both the carboxyl group and the α-carbon of the alkyl group. For the latter interaction abstraction of an α-hydrogen atom is required. The alkyl group of this intermediate can react with a neighboring carboxylate to give the ketone. The remaining carboxyl group forms CO2. The intermediate is very likely to be in pseudoequilibrium with the corresponding ketene.

Poly(ammonioalkanesulfonate) Blends with Polar Organic Species and Alkali Metal Salts: Structure, Glass Transition and Ionic Conductivity

Because the dipole moment of its zwitterionic side group is very high (μ∼23 D), poly[3-(N,N-diethyl-N-(5-methacryloyoxy-3-oxopentyl)-ammonio) propanesulfonate] affords a unique polar host matrix possessing a strong solvation power towards a variety of polar or ionic guest species. Water, glycerol, liquid ethylammonium nitrate, triethylammoniopropanesulfonate are all good plasticizers with a fairly similar efficiency of ΔTg∼−2°C/mol% of additive, while a dizwitterion behaves as a weak antiplasticizer. The stoichiometric blends of the polyzwitterion with alkali metal salts of low enough lattice energy such as thiocyanates, trifuoromethanesulfonates, iodides, perchlorates, tetrafluoro or tetraphenylborates, are amorphous systems showing a single glass transition, with plasticization or antiplasticization effects depending on the salt nature. Microphase separation systematically occurs in these binary systems but long-range order is observed only in some cases, with development of lamellar (I−) or hexagonal (SCN−) structures. Conductivity increases and the dielectric constant of the material decreases as salt is added. The activation energies of the conductivity are not strongly affected either by the state of the material, glassy or viscoelastic, or by the salt nature. © 1997 John Wiley & Sons, Ltd.

Salt exchange reaction via competitive solvation by poly(ethylene oxide) oligomers

Polyethers, such as poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) were used as polymer solvents. PEO interacted with smaller cations more strongly, and formed stable complexes with the salts having lower lattice energy. In a mixed salt system, solvation state of cations should reflect the degree of ion-dipole interaction with PEO. As a result, the salt, which has higher lattice energy and larger cation, was phase separated by the addition of the other salt which was solvated stronger by PEO. A competitive solvation of salts was applied to phase separate the target salt from the PEO oligomer solution. Furthermore, the exchange of cations or anions of the phase separated salts was also observed during salt mixing. PEO was concluded to have an ability to solubilize certain salts preferentially and selectively reflecting the characteristics of the salts.

Comparative study of poly(ethylene oxide) electrolytes made with LiN(CF 3SO 2) 2, LiCF 3SO 3 and LiClO 4: Thermal properties and conductivity behaviour

The phase transitions in mixtures of poly(ethylene oxide) (PEO) ( M n = 4 × 10 3) with LiN(CF 3SO 2) 2, a lithium salt of low lattice energy recently discovered by Armand and coworkers, are compared with those of the PEOLiCF 3SO 3 and PEOLiClO 4 systems. As expected, this new salt is more soluble in PEO than LiCF 3SO 3 and LiClO 4. It forms crystalline complexes similar to those of LiClO 4, but its mixtures made in 10 1 and 8 1 molar ratios (EO units/salt) remained completely amorphous, even after a long standing at −5°C. In agreement with Armand and coworkers, these electrolytes exhibit conductivity magnitudes greater than 10 −5 S cm −1 at 25°C. A comparison made at higher temperatures (50 and 100°C) with melted or supercooled electrolytes containing LiClO 4 in the same molar ratios is slightly in favour of LiN(CF 3SO 2) 2. The solubility curve of LiClO 4 has been characterized over a wide range of temperatures. A thermodynamical analysis of this curve yields negative values for both the enthalpy and entropy of mixing of the liquid components.

The Effect of Anion Vacancies on the Tribological Properties of Rutile (TiO2-x), Part II: Experimental Evidence

Friction and wear tests were completed with the (001) and (110) planes of single crystal rutile (TiO 2-x ) specimens sliding against selected ceramic counterfaces, in well-defined crystallographic directions. The purpose of the experiments was to investigate the environmental influences on anion vacancy formation, as related to a recent hypothesis connecting oxygen substoichiometry with predictable variations in the tribological properties of rutile. The data were obtained with two, entirely different test machines operating at various loads, speeds, temperatures, sliding directions and durations, as well as test specimen atmospheres. The results independently confirmed the predicted, anion vacancy-controlled formation of certain low and high lattice (strain) energy crystallographic shear systems (i.e., Magnèli phases) and that their generation is overwhelmingly environment-dependent. The stoichiometry-controlled lattice energy of these rutile phases influences the surface and bulk shear strength (τS) of the oxide single crystal. τS values were calculated for the (001)[110] surface as a function of rutile stoichiometry. The τS of (001)[110] rutile at an estimated stoichiometry of TiO 1.93–1.98 was as low as 8 MPa, equivalent to the τS of run-in MoS2 films in high vacuum.

Polymer solid electrolytes - an overview

Polymers containing solvating heteroatoms form conductive complexes with low lattice energy alkali salts. Despite the apparent complexity of the phase diagram and the influence of preparation history, only the amorphous domains show an appreciable mobility. With low interfacial resistance and a broad stability window, these materials open a new field in solid-state electrochemistry. A still debated question is the extend of anion mobility and how it limits performances for DC utilizations.