Oxygen Vacancy-Rich Ultrathin Co3O4 Nanosheets as Nanofillers in Solid-Polymer Electrolyte for High-Performance Lithium Metal Batteries

The development of high-performance solid-polymer electrolytes (SPEs) is a key to the practical application of lithium metal batteries (LMBs). The use of two-dimensional (2D) inorganic nanofiller is an efficient way to build poly(ethylene oxide) (PEO)-based SPEs with high ionic conductivity and stability. Herein, a series of 2D oxygen vacancy-rich Co3O4-y−x (x = 1, 2 and 3) with well-defined 2D nanostructures, a high surface area and controllable oxygen vacancy contents (Co3O4-y) was synthesized via a facile self-assembly method and NaBH4 reduction. When the 2D Co3O4-y−x (x = 1, 2 and 3) nanosheets are introduced as nanofillers in PEO-based SPEs, they can interact with the PEO to form a three-dimensional (3D) PEO/Co3O4-y film with uniform Li+ distribution and vertical diffusion channels, as well as strong adsorption of NO3− from LiNO3 electrolyte salt at the defective sites. As a result, the PEO/Co3O4-y−2 film reached a high ionic conductivity of 4.9 × 10−5 S cm−1, high Li+ a transference number of 0.51 and a wide electrochemical window over 4.6 V at 80 °C. The PEO/Co3O4-y−2 film enables the Li||PEO/Co3O4-y−2||LiFePO4 cell to deliver a high reversible capacity of 117.7 mAh g−1 at 2 C and to maintain 126.7 mAh g−1 at 1 C after 250 cycles with an initial capacity retention of 87.9%.