Transport Properties and Local Ions Dynamics in LATP-Based Hybrid Solid Electrolytes

Hybrid solid electrolytes (HSEs), namely mixtures of polymer and inorganic electrolytes, have supposedly improved properties with respect to inorganic and polymer electrolytes. In practice, HSEs often show ionic conductivity below expectations, as the high interface resistance limits the contribution of inorganic electrolyte particles to the charge transport process. In this study, the transport properties of a series of HSEs containing Li_{(1+ x )} Al_x Ti_{(2- x )} (PO₄ )₃ (LATP) as Li⁺ -conducting filler are analyzed. The occurrence of Li⁺ exchange across the two phases is proved by isotope exchange experiment, coupled with ⁶ Li/⁷ Li nuclear magnetic resonance (NMR), and by 2D ⁶ Li exchange spectroscopy (EXSY), which gives a time constant for Li⁺ exchange of about 50 ms at 60 °C. Electrochemical impedance spectroscopy (EIS) distinguishes a short-range and a long-range conductivity, the latter decreasing with LATP concentration. LATP particles contribute to the overall conductivity only at high temperatures and at high LATP concentrations. Pulsed field gradient (PFG)-NMR suggests a selective decrease of the anions' diffusivity at high temperatures, translating into a marginal increase of the Li⁺ transference number. Although the transport properties are only marginally affected, addition of moderate amounts of LATP to polymer electrolytes enhances their mechanical properties, thus improving the plating/stripping performance and processability.