Solid State Voltammetry in Ionically Conducting Phosphazene ‐ LiSO3 CF 3 Films

Abstract

The electrochemical stability window of the solid polymer electrolyte MEEP4LiSO3CF3, MEEP = [NP(OC2H4OC2H4OCH3)2],,, at a glassy carbon electrode was determined to be +2.7 to -2.5V vs. Ag. These limits appear to arise from redox of the salt. Linear potential sweep voltammetry was performed on polymer solutions of ferrocene at a macroelectrode and on polymer solutions of [Os(phen)3]" ' and [Co(bpy)3] 2~ at a 10 txm Pt microelectrode. Diffusion coefficients for these electroactive solutes in the solid electrolyte are reported. The coefficients and activation barriers for diffusion of dilute solutions of [Co(bpy)3] 2~ in MEEP4LiSO3CF3 and in PEO~LiSO.~CF~ are the same. Plasticization of MEEP4~ LiSO3CF3 solutions by sorbed acetonitrile vapor enhances the ionic conductivity by ca. 300 times.The observations that alkali metal salt complexes of poly(alkene oxides) (1-3), such as poly(propylene oxide), PPO, and poly(ethylene oxide), PEO, have appreciable ionic conductivities led to substantial research efforts seeking to understand and to exploit this potentially useful transport property. Other ionically conducting polymer/salt systems have since been found (4), including that based on poly(bis-methoxyethoxyethoxy phosphazene), MEEP (5). Current research on these materials includes models that account for the temperature and concentration dependencies of ion mobility, experimental determination of transference numbers, synthesis of polymers that exhibit yet higher ionic conductivities, and the preparation of mixed ionic-electronic conductors (4).This paper examines solid-state, potential sweep voltammetry in MEEP,LiSO3CF3. That is, in electrochemical context the MEEP is the solvent and lithium triflate is the supporting electrolyte of a solid-state electrolyte solution within which we dissolve electroactive solutes. The notation MEEP4LiSO3CF3 designates a concentration ratio of four MEEP monomer units per LiSO3CF3 formula unit, which corresponds to sixteen ether oxygens per lithium ion. The P-O-C oxygen in the MEEP chain is not included in this count since NMR studies show it plays little or no role in cation solvation. The MEEP,LiSO3CF3 ratio corresponds to the polymer salt ratio with the highest ionic conductivity.It has been established (7) that PEO,6MX ion conductors undergo a melting transition at Tm (60~ and that their ionic conductivities increase sharply as this temperature is approached. Our initial studies (8) of solid-state voltammetry were in PEO16LiCF3SO3 containing dissolved, bulky electroactive complexes. We observed from the voltammetric currents that the diffusion coefficient of the complex [Os(phen)3] 2+ (phen = 9,10-phenanthroline) varied in a manner similar to the LiSO3CF3 ionic conductivity. That is, diffusion of the metal complex is very slow below Tin, increases rapidly near Tin, and is fast above Tin. Voltammetry of dissolved electroactive substances is thus a potentially useful tool for elucidating transport phenomena in polymer electrolytes (8b).The recently synthesized MEEP~MX complexes (5) ex...

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