Effect of Li_x(MgCoNiCuZn)_1-xO as High-Dielectric Permittivity Fillers for Composite Polymer Electrolytes

Composite polymer electrolytes (CPEs) are leading hybrid solid-state materials to replace flammable liquid electrolytes in lithium-ion, lithium-metal, and lithium-sulfur batteries (LIBs). Their advantage lies in the possibility to combine the flexibility and chemical stability of polymers with the mechanical and electrochemical robustness of ceramics, enhancing the typically poor ionic conductivity (σ) and transference number of the organic component. A full understanding of the mechanisms of ion conduction in such systems is still not fully well understood resulting in a limitation to produce materials having optimum transport properties. In previous work, we have observed that the increase in ionic conductivity of CPEs is not proportional to the low weight load of fillers utilized, suggesting that their surface properties may be more influential than their bulk features. One of such properties is the filler nanoparticle dielectric permittivity, which describes the ability of a material to be polarized in an electric field. In this regard, the Anderson-Stuart model suggests that dielectric permittivity is inversely proportional to the strength of interaction between Li⁺ and ether-O-atom in a polymer like PEO. In consequence, our work focuses on evaluating CPEs containing fillers with different dielectric constants such as Li₆La₃ZrBiO₁₂ garnets (ε ~ 30) and high entropy oxides (HEO) such as Li_x(MgCoNiCuZn)_1-xO which are reported to have “colossal” dielectric constants reaching values of 10⁵ at 420°C and 20Hz. We propose to understand whether the relationship between dielectric constant and ionic conductivity is expressed as changes in the microstructure of the polymer/ceramic interface, improvement in the salt dissociation and/or immobilization of the anion on the surface of these fillers. Dielectric spectroscopy results show a good correlation of dielectric permittivity at low frequencies versus ionic conductivity, for example EIS analysis show a monotonic overall increase in ionic conductivity of membranes when decreasing EO:Li, each series showing a σ_max at 4% wt. HEO. More importantly, when decreasing the particles size from 2μm to 200nm, we saw an order of magnitude increase in σ to 5.3x10^-5 Scm^-1when keeping EO:Li as 20:1. To provide a mechanistic understanding of our observations, in addition to transport measurements, we utilize XRD, SEM, DSC, polarizing light microscopy and Raman spectroscopy to elucidate changes in polymer morphology and microstructure as well on Li-salt dissociation that are responsible for the improvements in ion transport observed.