Conductivity and nuclear spin relaxation in superionic glasses, polymer electrolytes and the new polymer-in-salt electrolyte

The phenomenology of electrical relaxation in superionic glasses and their melts, and in salt-in-polymer electrolytes is reviewed, and then recent observations on nuclear spin lattice relaxation of mobile species like ⁷Li⁺ are examined in the same conceptual frame work are examined. To shed light on the origin of difference found in superionic glasses between the correlation times, τ_c, for fluctuations causing spin lattice relaxation and those, τ_σ, allowing conductivity relaxation, ⁷Li nuclear spin resonance was studied across a binary liquid system of [polypropylene oxide + Li salt(s)]. In the polymer-rich solutions, the two correlation times are found to have the same value but above the (salt-in-polymer) (ppolymer-in-salt) boundary, a gap starts to open up. The value of T₁ at its minimum is found to be almost independent of solution composition, implying that the Li⁺ cations find comparable sites as polymer replaces salt in the solution, To observed the behavior in polymer-free salt melts, salt mixtures had to be used to gain access to the viscous liquid regime. The study of such liquid salts reveals a difference in τ_c and τ_σ of 1.1 orders of magnitude, comparable to that found in glassy superionics. The results are discussed in terms of the serial decoupling of different relaxation modes observed in fragile liquids during cooling towards the glass transition, and are used to support the assertion that conductivity in the 'polymer-in-salt' electrolytes is dominated by Li⁺ migration.