High-Performance PEO-Based All-Solid-State Battery Achieved by Li-Conducting High Entropy Oxides

A rock-salt-structured Li-conducting high entropy oxide was prepared and utilized as an active filler in a polyethylene oxide (PEO)-based solid-state composite electrolyte. X-ray diffraction and high-resolution transmission electron microscopy were adopted to analyze the crystal structure of the high entropy oxide containing 20% of Li ions (HL20). The HL20 was crystallized in the Fm3̅m space group with Li⁺ ions located at the center of the MO₆ octahedra. The ionic conductivity of the composite membrane at 30 °C reaches 3.44 × 10^-5 S cm^-1. The inflection point of activation energy of the membrane with HL20 decreases by 5 °C compared with that of the pure PEO membrane. In the galvanostatic plating/stripping test, the Li||Li symmetric batteries could be cycled at a current density of 200 μA cm^-2 for over 1200 h with an overpotential of 140 mV. The Li||LiFePO₄ full battery could be charged/discharged at 0.5 C for 100 circles with a high capacity retention rate of 91%. Excellent rate performance is also achieved at lower temperatures and higher rates, showing the superiority of HL20 as an active filler. This work sheds light on the development of high entropy oxide as a new type of fast ionic conductor, promoting the practical application of all-solid-state batteries at a lower temperature.