Enhancing the performance of solid-state supercapacitors using Li⁺-garnet polymer hybrids as electrolyte

The present work delves into the synthesis and characterization of Li⁺-ion garnet polymer composites and their novel electrolytic application in solid-state supercapacitors (SSCs). For this, Niobium (Nb) substituted cubic garnet LLZNO (Li_6.75La₃Zr_1.75Nb_0.25O₁₂) was synthesized and reinforced into a PEO matrix, leading to an optimized composition of 12LiClO₄ + 88{(PEO)_0.55 (LLZNO)_0.45}, with high ionic conductivity ∼2 × 10^-4 Ω^-1 cm^-1 at 30 °C. Supercapacitors were then assembled using these electrolyte membranes with high surface-area (∼1800 m² g^-1) activated carbon electrodes. The SSC interface was subsequently tuned by forming a micro-gel layer at the electrode-electrolyte interface to achieve high performance parameters. For this, dimethylformamide (DMF) was incorporated in a minimal amount of ∼5 µL cm^-2 at the interface. The SSCs with ∼3 V stability window exhibit high performance parameters, viz. specific capacitance ∼143 F g^-1 at 2 V/1 mA, specific power ∼1303 W kg^-1, and specific energy ∼24 Wh kg^-1. The SSCs exhibit ∼60% and ∼53% capacitance retention after 15 000 and 100 000 galvanostatic charge-discharge cycles, respectively. The temperature tolerance of the SSCs was tested over a wide temperature range (-10 °C to 50 °C), and the performance parameters exhibit excellent reproducibility. Investigation reveals that these SSCs are predominantly of the electric-double-layer type. Finally, the practical use of fabricated SSCs is demonstrated by glowing two parallel-connected 3 V LEDs for ∼30 minutes.