M.A.K.L. Dissanayake

Ionic conductivity of plasticized(PEO)-LiCF3SO3 electrolytes

Many polymer electrolytes used in practical applications contain a low molecular wcight plasticizer. Plasticizers can be used to change the mechanical and electrical properties of polymer electrolytes by reducing the degree of crystallinity and lowering the glass transition temperature T g . In this study the ionic conductivity has been determined for a (PEO) 9 LiCF 3 SO 3 polymer electrolyte with added plasticizers. The samples were in the form of thin films with the thickness 0.1- 0.5 mm and ethylene carbonate (EC), propylene carbonate (PC) or a mixture of ethylene carbonate and propylene carbonate (EC:PC) were used as plasticizers. The ionic conductivity increases with increasing amount of plasticizer but the amount of plasticizer which can be added is limited since the films become too soft for use in practical applications. Without any plasticizer, the complex (PEO) 9 LiCF 3 SO 3 only has a conductivity of 2.5 x 10 -5 S cm -1 at 332 K. Adding 50% of the plasticizers by mol. weight of PEO to the (PEO) 9 LiCF 3 SO 3 complex yielded mechanically stable films with an ionic conductivity of 9.0 x 10 -4 S cm -1 with EC and 5.2 x 10 -5 S cm -1 with PC at the same temperature

Dye-sensitised photoelectrochemical solar cells with polyacrylonitrile based solid polymer electrolytes

Abstract Novel all solid state dye-sensitised photolectrochemical solar cells of the type, FTO–TiO 2 –dye–PAN, EC, PC, Pr 4 N + I − , I 2 –Pt–FTO, have been fabricated and characterised using current–voltage characteristics and action spectra. Liquid electrolyte generally used for such solar cells has successfully replaced by a quasi solid electrolyte comprised of polyacrylonitrile (PAN) with ethylene carbonate (EC) and propylene carbonate (PC) as plasticisers and Pr 4 N + I − /I 2 redox couple with tetrapropylammoniumiodide as the complexing salt. For the polymer electrolyte, the optimum conductivity of 2.95×10 −3 S cm −1 was obtained for the electrolyte composition, PAN:EC:PC=15:35:50 (wt.%). The short circuit current density ( J SC ) and the open circuit voltage ( V OC ) obtained for an incident light intensity of 600 W m −2 were 3.73 mA cm −2 and 0.69 V, respectively. This corresponds to an overall quantum efficiency of 2.99%. From the action spectrum, the maximum incident photon conversion efficiency (IPCE) of 33% was obtained for incident light of wavelength 480 nm.

Dielectric relaxation, ionic conductivity and thermal studies of the gel polymer electrolyte system PAN/EC/PC/LiTFSI

Abstract Dielectric relaxation, ionic conductivity and thermal properties have been measured for the gel polymer electrolyte system poly(acrylonitrile)/ethylene carbonate/propylene carbonate/lithium bis(trifluoromethanesulfone)imide (PAN/EC/PC/LiTFSI) and for its components in the frequency range from 1 MHz to 1.8 GHz and over a temperature range from −20 to 50 °C. DSC results suggest that EC/PC exists in two different environments within the gel network; as regions in which the EC/PC molecules subjected to pairing interactions by the CN group in PAN and also as regions consisting of “free” EC/PC molecules. Addition of PAN to the EC/PC/LiTFSI liquid electrolyte has increased the ionic conductivity. Out of the various PAN/LiTFSI composition ratios studied for the gel polymer electrolyte, the 6:1 composition ratio by weight gives the highest ionic conductivity. The room temperature (23 °C) conductivity of the gel electrolyte with this composition, PAN(15.4%)/EC(41.0%)/PC(41.0%)/LiTFSI(2.6%) (by weight) is 2.5×10−3 S cm−1. DSC results show that this composition has the most amorphous nature, above −105 °C. The e″ spectra of gel electrolytes with various compositions show the presence of a high-frequency peak in the 0.5-GHz region attributed to the α relaxation process and a peak/shoulder in the 10-MHz region attributed to the ion-pair relaxation. Li+ ion transport probably takes place in the vicinity of the PAN chains and the ion-pair relaxation frequency appears to reflect the dynamic environment in which the cations migrate. However, the coupling between the conductivity and the α relaxations, attributed to EC/PC molecules, appears to be weak. A model has been presented according to which the Li+ ions in the gel electrolyte appears to be solvated by both PAN (through CN) and EC/PC.

Electronic to ionic conductivity of glasses in the Li2O-V2O5-TeO2 system

Abstract Glasses in the 3TeO 2 x Li 2 O(1− x )V 2 O 5 system have been synthesised for 0≤ x ≤1. Conductivity data obey an Arrhenius type behaviour between room temperature and the glass transition temperature. The isothermal conductivity curves exhibit a sharp minimum near x =0.5. Values of activation energies and pre-exponential factors in the V 2 O 5 rich region are lower compared to those in the alkali oxide rich region implying that the conductivity mechanisms in these two regions are different. Experimental values suggest that an electronic conductivity by polaron hopping between V VI and V V cations would prevail in the first region. In the alkali oxide rich region, ionic conductivity would operate by an interstitial pair mechanism between non-bridging oxygens. The very low conductivity expected for x =0.5 could be explained by assuming that, for this composition, both ionic and electronic paths would not percolate any more.

A novel gel polymer electrolyte based on polyacrylonitrile (PAN) and its application in a solar cell

A PAN-based gel polymer electrolyte with possible iodide ion conductivity was prepared by incorporating a mixture of Pr4N+I−, iodine, EC and PC in PAN. Out of various compositions prepared and characterised, the sample with composition PAN (13%):EC (31%):PC (45%):Pr4N+I− (7%):I2 (4%) by weight ratio, exhibited the maximum room temperature (25°C) conductivity of 2.95×10−3 S cm−1. The predominantly ionic nature of the electrolyte was established by using the dc polarisation technique. The temperature dependence of ionic conductivity follows the VTF behaviour, indicating the amorphous nature of the electrolyte. Dye-sensitised photoelectrochemical solar cells prepared using this electrolyte exhibited an open circuit voltage (Voc) of 0.69 V, a short circuit current (Isc) of 3.73 mA cm−2 for an incident light intensity of 600 W m−2 yielding an overall quantum efficiency of 2.99%.

Efficiency enhancement in dye sensitized solar cells using gel polymer electrolytes based on a tetrahexylammonium iodide and MgI2 binary iodide system

Quasi-solid-state dye-sensitized solar cells have drawn the attention of scientists and technologists as a potential candidate to supplement future energy needs. The conduction of iodide ions in quasi-solid-state polymer electrolytes and the performance of dye sensitized solar cells containing such electrolytes can be enhanced by incorporating iodides having appropriate cations. Gel-type electrolytes, based on PAN host polymers and mixture of salts tetrahexylammonium iodide (Hex4N(+)I(-)) and MgI2, were prepared by incorporating ethylene carbonate and propylene carbonate as plasticizers. The salt composition in the binary mixture was varied in order to optimize the performance of solar cells. The electrolyte containing 120% Hex4N(+)I(-) with respect to weight of PAN and without MgI2 showed the highest conductivity out of the compositions studied, 2.5 × 10(-3) S cm(-1) at 25 °C, and a glass transition at -102.4 °C. However, the electrolyte containing 100% Hex4N(+)I(-) and 20% MgI2 showed the best solar cell performance highlighting the influence of the cation on the performance of the cell. The predominantly ionic behaviour of the electrolytes was established from the dc polarization data and all the electrolytes exhibit iodide ion transport. Seven different solar cells were fabricated employing different electrolyte compositions. The best cell using the electrolyte with 100% Hex4N(+)I(-) and 20% MgI2 with respect to PAN weight showed 3.5% energy conversion efficiency and 8.6 mA cm(-2) short circuit current density.

Paddle-wheel versus percolation mechanism for cation transport in some sulphate phases

Abstract Lithium sulphate and a few other compounds have high temperature phases which are both solid electrolytes and plastic crystals (rotor phases). Three types of experiments are here considered in order to test the validity of a “paddle-wheel mechanism” that has been proposed for cation conductivity in these phases. A single-crystal neutron diffraction study has been performed for cubic lithium sulphate. The refinement of the data gives a very complex model for the location of the lithium ions. There is definitely a void at and near the octahedral (1/2, 1/2, 1/2) position. 90% of the lithium ions are located at the tetrahedral 8c-sites (1/4, 1/4, 1/4), although significantly distored in the directions of the four neighbouring sulphate ions. The remaining 10% of the lithium ions are refined as an evenly distributed spherical shell which is sorrouding the sulphate ions. The lithium ions are transported along a slightly curved pathway of continous lithium occupation to a corressponding to a distance of about 3.7 A. Thus, lithium transport occurs in one of the six directions [110], [1 1 0], [101] etc. The electrical conductivity has been studied for solid solutions of lithium tungstate in cubic lithium sulphate. The conductivity is reduced in the one-phase region, while it is increased in a two phase (solid-melt) region. There are pronounced differenes between the rotor phases and other phases concerning how partial cation substitution affects the electrical conductivity of solids solutions. Regarding self and interdiffusion, all studied mono- and divalent cation are very mobile in the rotor phases, which lack the pronounced correlation with ionic radii that is characteristic for diffusion in other of solid classes of solid electrolytes. The quoted studies are to be considered as strong evidence against a percolation model proposed by Secco.

Copper-ion conducting solid-polymer electrolytes based on polyacrylonitrile (PAN)

Two copper-ion conducting solid-polymer electrolyte systems based on polyacrylonitrile (PAN) have been synthesized and characterized using DC polarization tests and impedance measurements. The system with 21 mol% PAN: 30 mol% EC: 45 mol% PC: 04 mol% CuCNS has a room temperature conductivity of 3.30 × 10 -5 S cm -1 and an activation energy of 0.25 eV. The conductivity versus temperature plot obeys an Arrhenius type variation. It is predominantly an ionic conductor with negligible electronic conductivity. It has a high anionic transference number (t - = 0.80) due to CNS ions and a low cationic transference number (t + = 0.20) due to Cu + ions. The system with 20 mol% PAN: 41 mol% EC: 34 mol% PC: 5 mol% CuTf has a room temperature conductivity of 4.10 × 10 -3 S cm -1 and an activation energy of 0.14 eV. It obeys the VTF equation. The system appears to be a mixed conductor with a cationic (Cu 2+ /Cu + ) transference number of + = 0.50 and an electronic transference number of t e . = 0.50 with negligible anionic conductivity. Both systems yielded free standing stable polymer electrolyte films.

Ionic conductivity of glasses in the system Li2OP2O5TeO2

Abstract Ionic conductivity of Li + conducting glasses with a macromolecular network made of PO 4 tetrahedra and TeO 4 bipyramids have been measured. Glass pellets, according to the formula 0.4 Li 2 O 0.6[ xP 2 O 5 (1 − x ) Te 2 O 4 ] were obtained by room temperature quenching of melts in a brass mould. The glass forming domain was limited to 0.4 ≤ x ≤ 0.8 and the glass transition temperatures were between 240 °C and 320 °C. Ionic conductivities below T g obey Arrhenius relationships with an almost constant pre-exponential factor A with a value of log A (S K cm −1 ) = 4.3 ± 0.6. At constant temperature conductivity increases with x and a peak is observed at x = 0.6.

Ionic conductivity of solid solutions of α-Li2SO4 with Li2WO4: Strong evidence for the paddle wheel mechanism of ion transport

Abstract Ionic conductivity of α-Li 2 SO 4 with up to 4 mol% Li 2 WO 4 has been measured using quartz capillary U-tubes and complex impedance technique. A significant drop in conductivity and increase in activation energy is observed for 2, 2.5, 3 and 3.5 mol% Li 2 WO 4 compositions while 4 mol% Li 2 WO 4 composition shows an increased conductivity. Based on a recent phase diagram of the binary system, the results are interpreted as convincing evidence for the “Paddle Wheel” mechanism of ion transport in α-Li 2 SO 4 .