Mixed (ion and electron) conducting polymers, with applications in batteries

Abstract

A new method to fabricate 3D batteries using mixed (ion and electron) conducting polymers as electrolytes has been developed. The majority of the work was done using the polymer Poly(1,11-Di(N-pyrryl)-3,6,9-trioxaundecane) (PP2O3) because it demonstrated mixed conducting properties. Methods are presented for synthesising the monomer then polymerising either electrochemically to give films or chemically to give bulk samples or powders. The conductivities of both polymers were determined by Electrochemical Impedance Spectroscopy (EIS). For polymer films prepared electrochemically on conducting substrates, the conductivities were determined as a function of the p-doping level, using a cell containing a liquid electrolyte and an applied bias potential. The results were fitted with a transmission line model and revealed an electronic conductivity varying from 4.20 x10-10 to 1.69 x10-5 S/cm dependent on the doping level, and a relatively constant ionic conductivity of 1.74 x10-6 S/cm. Oxidative treatment by overdoping resulted in a reduction of electronic conductivity by a factor of about 50,000 times smaller at the potential of maximum conductivity, around 0.30 V with a small change in the ionic conductivity. Bulk samples of the chemically prepared polymer were examined between two blocking electrodes. The electronic conductivities of the as-prepared and chemically oxidized samples were both quite low, around 10-7 and 10-8 S/cm, whereas the ionic conductivity of both samples was around 10-5 and 10-6 S/cm. These values were slightly higher than those of the film samples due to a presence of PC plasticiser in bulk samples. Finally the polymers were tested as electrolyte/separators in lithium ion battery cells: the electrochemically treated film was found to be an effective separator between a 3D LiFePO4 positive electrode and a liquid lithium amalgam negative, and the chemically prepared materials showed a capacity of around 150 mAh per gram LiFePO4 in a conventional Li/LiFePO4 cell,. These experiments demonstrate a proof of concept for the use of mixed conducting polymers as electrolytes in lithium battery systems

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Identifiers

ORCID for John R. Owen: orcid.org/0000-0002-4938-3693

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