Enhanced Ion mobility in Aluminosilicate/Polysiloxane Network Polyelectrolytes
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ABSTRACT A new series of polysiloxane-based single-ion conductors was prepared. These contain solvating oligoether sidechains and covalently linked aluminosilicate or alkoxy/siloxy-aluminate anions attached to the polysiloxane backbone. Of these two systems, the polymers containing aluminosilicate [(SiO) 4AI]" anions show higher room temperature conductivities (10' S/cm) than those with alkoxy/siloxyaluminate [(SiO) 2(CH 20) 2A1] anions (10` S/cm). The incorporation of longer covalent tethers between the alkoxy/siloxyaluminate anion and the polymer backbone results in enhanced room temperature conductivities at high ion loadings. Differential scanning calorimetry data provide a rationale for the high conductivity.
INTRODUCTION A challenge in polymer electrolyte research is the preparation of single-ion conductors (polyelectrolytes) which conduct as well as simple polymer-salt complexes, where both cations and anions are free to diffuse. Interest in this work arises from potential applications and fundamental studies. These materials restrict the mobility of one of the charge-carrying species so that the build-up of concentration gradients in the electrolyte is suppressed.' Transport number measurements indicate that in many polymer-salt complexes, the anion is more mobile than the cation, with cation transference numbers (the fraction of the total charge that is carried by cations, expressed as t, = a±/[a+ + c.]) in the range of only 0.2 to 0 .6."2 In battery applications, concentration gradients arise due to the high anion mobility in simple polymer-salt complexes. In this situation a limiting current is established beyond which salt depletion will occur at the cathode/electrolyte interface. This depletion is observed at a time inversely proportional to a-, so a larger t+ leads to a higher limiting current, a reduced salt depletion, and higher cell efficiency.' In polyelectrolytes, the covalently-bound anion can move only as quickly as the rest of the polymer framework, so t. is effectively 1. From a fundamental standpoint, the absence of anion mobility also simplifies studies of ion transport mechanisms in polymer electrolytes. To achieve high ion mobility, the coulombic interaction must be weak between the cation and the bound anion of a polyelectrolyte, and the polymer framework must be both flexible and capable of solvating cations. Previous studies in our laboratory and elsewhere demonstrate that aluminate, alumino(semi)silicate, and aluminosilicate anions may be used to form extended linear and network structures with polyethylene glycol (PEG)." 9 The best conductivity of these polyelectrolytes is above 10-6 S/cm at 25 'C. Further enhancement of an order of magnitude is obtained by addition of a cation-encapsulating agent (2.2.2 cryptand). The room temperature conductivities of these polyelectrolytes increases in the order aluminate < alumino(semi)silicate < aluminosilicate, corresponding to the addition of Si-O linkages around the Al center. 6 The SiO linkages serve to increase polymer flexibility near the
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