Denis Ostrovskii

A FTIR and Raman study of spontaneous reactions occurring at the LiNiyCo(1-y)O2 electrode/non-aqueous electrolyte interface

Spontaneous reactions occurring at the surface of LiNiyCo(1−y)O2-based electrodes during contact with non-aqueous organic electrolytes have been investigated by FTIR and Raman spectroscopy. It is found that several types of compounds and/or functional groups are formed on the electrode surface and that these compounds appear to be dependent on the type of electrolyte used. Thus, for a LiClO4-propylene carbonate (PC) electrolyte, the main reaction is formation of Li-carbonate, whereas in the case of LiPF6-ethylene carbonate/dimethyl carbonate (EC/DMC) electrolyte formation of P-, O- and F-containing compounds dominate. Spectroscopic data also show a variation of the LiNi(1−y)CoyO2 crystalline structure during storage in an electrolyte which probably is due to a spontaneous deintercalation of the Li ions. An analysis of the newly formed species is presented and possible reaction mechanisms are discussed.

Structure and properties of sulfonated poly [(vinylidene fluoride)–g-styrene] norous membranes porous membranes

The possibility of developing polyfvinylidene fluoride (PVDF)-based membranes as proton conductors has been investigated. Using electron beam radiation grafting and functionalisation, the composition of the membranes can be controlled. Membranes have been prepared from porous films of PVDF, and their structure and properties have been studied with Raman spectroscopy, wide angle X-ray scattering (WAXS), small angle X-ray scattering (SAXS), swelling tests, and by impedance spectroscopy. The grafting reaction of styrene, initiated in the amorphous regions and at the surfaces of the crystallites in the partly crystalline PVDF matrix, is very efficient, and high degrees of grafting can be achieved. Grafts are formed both from C–H and C–F branch pointsl Sulfonation can be accomplished to ca. 100%, and occurs mainly at the para-position of the phenyl rings. The hydration number was found to be independent of the degree of grafting, degree of sulfonation and crystallinity. The overall crystallinity decreased in the structure with the degree of grafting and sulfonation, partly owing to the diluting effect of the grafts, partly owing to efficient penetration of the grafts in the crystallites. The conductivity increased with the content of sulfonic acid groups and, in particular, with decreasing crystallinity. In the case of fully sulfonated membranes, a levelling-out of the conductivity was found at a degree of grafting of around 200%. Conductivities up to 120 mS cm–1 at room temperature were achieved.

Molecular and ionic interactions in poly(acrylonitrile)- and poly(methylmetacrylate)-based gel electrolytes

Vibrational Raman and Fourier transform IR (FTIR) spectroscopy has been used to determine molecular and ionic interactions in poly(acrylonitrile)—(PAN) and poly(methylmetacrylate)—(PMMA) based gel electrolytes, containing LiClO4 salt and a mixture of ethylene carbonate (EC) and propylene carbonate (PC) as a gelating agent. A number of model systems, representing different stages of gel preparation, as well as the pure components, have also been studied. Vibrational spectra reveal interactions between EC and PC molecules in EC–PC mixture via electric dipoles located on C=O groups. In the salt-doped EC–PC system, additional bands close to the ring breathing and bending fundamentals of the carbonate ring of EC and PC, respectively, are attributed to solvent molecules interacting with the cations via Coulombic forces, the satellite band frequencies being well correlated to the cationic potentials. In the PMMA-based gels, no spectroscopically detectable interactions between the polymer matrix and the electroly...

Reactivity of lithium battery electrode materials toward non-aqueous electrolytes: spontaneous reactions at the electrode-electrolyte interface investigated by FTIR

Abstract Spontaneous reactions occurring at the surface of LiNi 0.8 Co 0.2 O 2 and LiMn 2 O 4 -based electrodes during the storage in organic non-aqueous electrolytes have been investigated by diffuse reflectance FTIR technique. It is found that both materials spontaneously form different inorganic and organic compounds on their surface when in contact with electrolyte solutions. The nature of these self-acting reactions is moreover found to be similar to that of the processes occurring during electrochemical cycling of the electrodes. Reaction mechanisms and the final products depend on both electrode surface chemistry and the nature of electrolyte used. It appears that the spontaneous reactions are initiated by lithium deintercalation from the electrode active material. The influence of different factors, e.g. degree of lithiation of the active material, roughness of the electrode surface and temperature on the reaction rate is discussed.

An electrochemical and Raman spectroscopical study of gel polymer electrolytes for lithium batteries

Abstract The characterisation of transport properties and molecular interactions in two different gel-type polymer electrolytes are described and discussed. The gel electrolytes were based on poly(acrylonitrile) (PAN) and poly(methylmethacrylate (PMMA), respectively, the polymer hosts containing an ethylene carbonate–propylene carbonate (EC–PC) solution of LiClO 4 . The two systems show comparable conductivities though large differences are developed in the interfacial resistance at the lithium metal electrode. The molecular interactions were investigated using Raman spectroscopy for different salt–solvent compositions. The spectroscopic data are important for the clarification of difference in reactivity towards the lithium metal electrode, and, thus, for the applicability of the two types of membranes in rechargeable lithium batteries.

Optical properties of endohedral Li@C60

Abstract UV–VIS, IR and Raman investigations have been carried out on purified Li@C60 produced by low energy ion implantation of C60. Two endohedral fractions can be isolated that have very different optical properties and stabilities. Here we show that the first endohedral fraction is a dimer species and the second fraction is the monomer. The IR spectra of sublimed monomer films show a prominent peak at around 1.1 μm corresponding to the first electronic transition of the C60− anion. Raman spectra show the vibrational–rotational movement of the Li inside the carbon cage.

Water sorption properties of and the state of water in PVDF-based proton conducting membranes

Abstract A new proton conducting material, poly(vinylidene fluoride) (PVDF) grafted with polystyrene (PS) and sulfonated (PVDF–SPS), has been investigated with regard to water uptake and the state of water in the material. The water uptake was estimated for the materials exposed to liquid water, in the temperature range between 22 and 100°C, and to water vapour of different water activities. The water uptake from saturated water vapour was found to be close to that from liquid water. The state of the absorbed water has been studied by Raman spectroscopy. We found that in the porous materials the state of water is similar to that of bulk water whereas in the non-porous samples a significant part of the absorbed water differs from that of the bulk.

The effect of additives on the ionic conductivity performances of perfluoroalkyl sulfonated ionomer membranes

Abstract Improvements of ionic conductivity of perfluoroalkyl sulfonated polymer membranes by a film recasting method with several kinds of additives are reported. Nafion film was liquefied by dissolving in ethanol/water (50/50) at 230°C in a high pressure vessel to make a 10% polymer solution. This solution was cast on a glass petri dish with a solvent dimethylformamide, together with some additives, and the film was dried in an oven at 70°C for 2 h, then at 130–150°C in a vacuum. The resulting film was cut and mounted on an electrochemical cell for AC impedance measurements. The conductivity of the films was compared with that of original Nafion in dry and wet states. The effective increase in ionic conductivity was observed for three kinds of additives: the moisture absorbing reagents such as molecular sieve, organic sulfonate compounds such as alkylbenzene sulfonates, and structure aligning copolymers with ion exchange sites. These results offered supporting data to the criteria for designing high performance ion conducting membranes, which were proposed in a previous study based on the ion and water transport model in Nafion membranes [1] .

Raman study of water in Nafion-117 membranes

Abstract We report on a Raman scattering study of the state of water in the acid form of the perfluorosulfonate exchanger membrane Nafion-117 in H 2 O- and D 2 O-wet conditions. The data reveal that in Nafion-117 (molecular weight 1100), as distinct from Nafion with higher molecular weights (1200 and 1400), practically all water molecules participate in OH · · · H bonding rather than linking to the fluorocarbon matrix. Moreover, the shifts of the OH and OD bands in Nafion-1100 towards higher frequencies as compared to Nafion of larger equivalent weights (1200 and 1400) implies that the state of water in Nafion-117 is more bulk-like than in the higher molecular weight membranes.

Cation coordination in ion-conducting gels based on PEO-grafted polymers

Abstract Ionic conducting polymer gels prepared from PEO-grafted acrylates, ethylene carbonate (EC), dimethyl carbonate (DMC), and LiPF 6 are studied by means of infrared and Raman spectroscopy. It is found that the presence of grafted PEO chains substantially changes the coordination of lithium cations from ‘cation–solvent’ to ‘cation–polymer’. Spectroscopic studies show that Li + –PEO coordination is strongly favored in competition with the solvent molecules and dominates completely at ether oxygen to Li–cation ratios (O:Li) of 10:1 and 5:1. At an O:M of 4:1 a solvent–cation interaction arises, in agreement with previously reported MD calculations of a preferred coordination number of 5 in similar systems. The results suggest that the complex interactions which determine long time stability and ionic transport properties can be designed by functionalization of the polymer backbone in PMMA-based gel electrolytes.