Mixed Divalent Polymer Electrolytes
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MIXED DIVALENT POLYMER ELECTROLYTES
MICHAEL COLE, ROGER J.LATHAM, ROGER G.LINFORD and MARK H.SHELDON Department of Chemistry, Leicester Polytechnic, P.O.Box 143, Leicester LEI 9BH, UK.
ABSTRACT Some features of interest for divalent polymer electrolytes are described. Attention is focussed on two contrasting roles for cations: providing chemical cross-links within the polymer matrix and offering a source of mobile charge carriers. The possible merits of using mixtures of two types of cation, one type for each role, is discussed. Alternative mixed salt systems containing two anions with a common cation are considered. Another class of mixed polymer electrolyte systems involving two divalent cations in conjunction with two monovalent anions is also examined in the context of illustrating competitive anion-cation interactions within the polymer matrix by EXAFS local structure techniques. Results for the systems PE0 1 5 :ZnI 2 /Mg(CIO 4 ) 2 , PEOI 5 :ZnI 2 /Mg(CF 3 SO 3 ) 2 , PEO 1 5 :CaI 2 /ZnI 2 and PEO1 5 :CaI 2 /CaBr 2 are discussed. INTRODUCTION Despite the wide-spread interest since c.1975 in lithium polymer electrolytes which arises from their suitability for technological application, it is interesting to note that divalent polymer electrolytes were first studied approximately ten years earlier (1]. In chemical terms as the enhanced interest in divalent, as opposed to monovalent systems manifests itself in several ways. The predominant monovalent cations of interest lie in Group IA of the periodic table where a monotonic increase in ionic radius and concomitant decrease in charge density occurs in the order: lithium-caesium. By contrast divalent species offer a richness of chemistry, with structural and bonding implications, that makes them more attractive candidates for scientific study. The relative behaviour in the vertical columns in the periodic table is perhaps no more interesting than for the monovalent species, but comparison across the first row of the d-block elements affords a test-bed for a subtle exploration of competing factors. The focal subset which is currently receiving attention [2, 31 is the group: iron, cobalt, nickel, copper and zinc. It is also useful to compare the behaviour of this group with the non-d-group divalent cations, magnesium, calcium, cadmium, mercury and lead. Within the focal group the ionic radii hardly change, and yet the coordination chemistry is varied, both in its geometry and the strength of metal-ligand interaction. The alternative divalent cations referred to are substantially different in size (d-block focal group 69-74 pm; magnesium 66 pm; calcium and cadmium 98 pm; mercury 110 pm; lead 120 pm) and in some cases have greater propensity towards covalent structures. The focal group (excluding zinc) together with mercury and lead also exhibit interesting redox behaviour. These general trends are summarised in Table 1.
Mat. Res. Soc. Symp. Proc. Vol. 210. 01991 Materials Research Society
264
Relative Characteristics of Divalent Cations (adapted from Ref.2) h = i
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