Ionic Conductivity of Composite Polymer Electrolyte: Clarifying the Role of the Interface with Nonconductive Particles

Solid‐state electrolytes are considered as an enabler for batteries with extended energy density. Polymers are promising materials but show insufficient ion transport properties, a limitation that can be lifted by the fabrication of composite polymer electrolytes. Nevertheless, the role of the inorganic particles in tuning the conduction properties is often unclear, especially when inert fillers are added. Herein, poly(trimethylene carbonate) and alumina particles: α‐Al 2 O 3 , γ‐AlOOH and γ‐Al 2 O 3 are mixed by two dispersion methods to prepare a series of composites. The correlations between microstructure, surface chemistry, and transport properties are studied, which are evaluated by combining synchrotron nanotomography, solid‐state nuclear magnetic resonance, and electrochemical impedance spectroscopy, respectively. An increase of the conductivity by a factor of 1.9 ± 0.6 (1.4 ± 0.4) is observed for α‐Al 2 O 3 (γ‐AlOOH) while no beneficial effect of dispersion is seen for γ‐Al 2 O 3 . It is found that dispersion of the particles is crucial, with an interfacial area between polymer matrix and inorganic fillers increased by a factor of 1.3–3 when the particles are dispersed. Moreover, the density of surface groups is estimated at the interface and correlations are found to the changes in conductivity, showing that interactions at the interface of the particles are one key parameter driving the composite performance.