Effect of morphological change of copper-oxide fillers on the performance of solid polymer electrolytes for lithium-metal polymer batteries

Solid polymer electrolytes (SPEs) for Li-metal polymer batteries are prepared, in which poly(ethylene oxide) (PEO), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), and copper-oxide fillers are formulated. Their structural and electrochemical properties are analyzed when the morphology of the copper-oxide fillers has been modulated to spherical or dendritic structure. The ionic conductivity obtained by electrochemical impedance spectroscopy (EIS) has been increased to 1.007 × 10⁻⁴ S cm⁻¹ at 30 °C and 1.368 × 10⁻³ S cm⁻¹ at 60 °C, as the 5 wt% dendritic fillers have been added to the SPEs. This ionic conductivity value is 1.3 times higher than that of 5 wt% spherical filler-contained SPEs. The analyses of differential scanning calorimetry (DSC) and X-ray diffraction (XRD) indicate that the increase of ionic conductivity is due to the remarkable decrease of crystallinity upon the addition of copper-oxide filler into PEO matrix of SPEs. The fabricated SPEs with the dendritic copper-oxide fillers present a total ionic transference number of 0.99 and a lithium-ion transference number of 0.38. More importantly, it presents a stable potential window of 2.0–4.8 V at 25 °C and high thermal stability up to 300 °C. The specific discharge capacity of the prepared cell with the dendritic filler-contained SPEs is measured to be 51 mA h g⁻¹ and 125 mA h g⁻¹ under 0.1 current-rate (C-rate) at 25 °C and 60 °C, respectively. In this study, the ionic conductivity and the electrochemical performance of the PEO-based polymer electrolyte have been evaluated when morphologically different copper-oxide fillers have been incorporated into the PEO matrix. We have also confirmed the safety and the flexibility of the prepared solid polymer electrolytes when they are used in flexible lithium-metal polymer batteries (LMPBs).