Alasdair M. Christie

Increasing the conductivity of crystalline polymer electrolytes

Polymer electrolytes consist of salts dissolved in polymers (for example, polyethylene oxide, PEO), and represent a unique class of solid coordination compounds. They have potential applications in a diverse range of all-solid-state devices, such as rechargeable lithium batteries, flexible electrochromic displays and smart windows. For 30 years, attention was focused on amorphous polymer electrolytes in the belief that crystalline polymer:salt complexes were insulators. This view has been overturned recently by demonstrating ionic conductivity in the crystalline complexes PEO6:LiXF6 (X = P, As, Sb); however, the conductivities were relatively low. Here we demonstrate an increase of 1.5 orders of magnitude in the conductivity of these materials by replacing a small proportion of the XF6 - anions in the crystal structure with isovalent N(SO2CF3)2 - ions. We suggest that the larger and more irregularly shaped anions disrupt the potential around the Li+ ions, thus enhancing the ionic conductivity in a manner somewhat analogous to the AgBr1-x I x ionic conductors. The demonstration that doping strategies can enhance the conductivity of crystalline polymer electrolytes represents a significant advance towards the technological exploitation of such materials.

Selection of new Kynar-based electrolytes for lithium-ion batteries

Abstract New electrolyte solution compositions have been identified for use in lithium-ion batteries after gelling with an appropriate quantity of Kynar polymer. Since the Li + conducting medium is largely the liquid electrolyte component, the assessment of these solutions as suitable lithium-ion cell candidates were investigated before adding the polymer. Selected electrolyte solutions were then used in the preparation of polymer gels. The specific conductivities of Kynar-based gels were determined as a function of salt concentration and polymer concentration. Optimised self-supporting polymer films, based on mixtures of ethylene carbonate (EC), ethylmethyl carbonate (EMC) and lithium hexafluorophosphate (LiPF 6 ) or lithium tetrafluoroborate (LiBF 4 ), showed good high current density cycling performance when used as separators in coke and Li 1− x Mn 2 O 4 (spinel) half-cells.

Temperature Dependence of the Electrical Resistance of Sound and Carious Teeth

Temperature variations are expected to influence measurement error in electrical resistance of teeth. It was the aim of this study to determine the changes in electrical behavior of extracted human teeth due to temperature changes in the range of room temperature to intra-oral temperature. Nine extracted teeth were selected, and the occlusal or an approximal surface was chosen for measurement. Carious involvement of the surfaces ranged from sound to cavitated. Electrical impedance spectroscopy sweeps in a frequency range of about 100 kHz to 10 Hz were completed at selected temperatures between 22°C and 40°C. After fitting the data to equivalent circuits that yielded parameter values for components of the equivalent circuit, we calculated the dc bulk resistance (Rb ). The temperature dependence of Rb of the surfaces with different carious involvement was very similar, and the mean drop of Rb from 20 to 35°C was 45% (SD 2%). It was concluded that the electrical resistance of sound and carious tooth surfaces is inversely related to temperature.

Measurement of the apparent lithium ion transference number and salt diffusion coefficient in solid polymer electrolytes

Measurements of the apparent cation transference number of a solid polymer electrolyte by a voltammetric technique are compared with results from the steady-state dc polarization method. The combination of measurements made at a microelectrode and at a macroelectrode, performed in the absence of supporting electrolyte, yields both the apparent transference number θ, and the salt diffusion coefficient D. Lithium cationic transference numbers in a series of macroscopically solid gels were determined as a function of salt concentration.

Improving the performance of graphite anodes in rechargeable lithium batteries

Abstract A low cost graphite was examined as a negative electrode for rechargeable lithium batteries. The use of an electrolyte solution consisting of LiPF6 (1 mol dm−3) in ethylene carbonate (EC) and dimethyl carbonate (DMC) at a volume ratio of 2:1 resulted in a capacity loss of 35% on the first cycle. When small quantities of dimethyl pyrocarbonate (DMPC) were added to the binary electrolyte system, the capacity loss on the first cycle was only 18% and thereafter a practical capacity value of 357 mA h g−1 was sustained for more than 50 cycles, representing more than 2000 h of cycling.

The lithium/polymer electrolyte interface

Abstract Kinetics and mechanisms for the electrodeposition of lithium from liquid and solid polymer electrolytes formed from lithium salts or from lithium salt complexes have been studied using chronoamperometric and cyclic voltammetric methods at a microelectrode. The type of nucleation and crystal growth process was determined as a function of polymer electrolyte composition. Exchange current densities, coulombic stripping efficiencies and lithium corrosion rates were evaluated.

LiN(CF3SO2)2 Kynar gels at carbon negative electrodes

Abstract Electrolyte solutions, comprising of LiN(CF 3 SO 2 ) 2 dissolved in solvent mixtures of ethylene carbonate (EC) and either dimethyl carbonate (DMC) or ethylmethyl carbonate (EMC), were used to prepare films after gelling with an appropriate quantity of Kynar™ polymer. The specific conductivity of each film is comparable with liquid electrolyte solutions at ambient temperatures. Half-cells assembled with each gel film and graphite from different sources are shown to have good cyclability with practical capacities of around 350 mA h g −1 at C/8. Cells constructed using coke as the active electrode material were also cycled successfully at a high charge/discharge rate of C/4 (1 mA cm −2 ) with practical capacities of greater than 200 mA h g −1 . An EC:EMC:Kynar-based cell was successfully cycled at 0°C using this electrode material. Although an increase in the resistance of the passivating layer was observed at 0°C, the cell sustained a capacity of 230 mA h g −1 at a rate of C/50.

The structure of poly(ethylene oxide)8 : NaBPh4 from a single crystal oligomer and polycrystalline polymer

We show that the structure of a polymer electrolyte may be solved by growing single crystals of an oligomeric (short chain) complex which provided an adequate starting model for refinement of the equivalent polymeric structure using powder diffraction: the efficacy of this method has been demonstrated by determining for the first time the structure of an 8 : 1 complex, poly(ethylene oxide)(8) : NaBPh(4).