Evaluation of high-entropy (Cr, Mn, Fe, Co, Ni)-oxide nanofibers and nanoparticles as passive fillers for solid composite electrolytes

• (Cr,Mn,Fe,Co,Ni) HEO are evaluated as fillers in solid composite electrolytes (SCEs). • The effects of morphology, oxide grain and crystallization degree are investigated. • The incorporation of HEOs enhances the performance of PEO/LiTFSI-based SCEs. • Well crystallized HEO NFs with highly defective surface outperform other fillers. • Using these NFs gives a 4.7 v stability window and a 0.37 Li + transference number. Solid-state electrolytes (SSEs) could represent the key to solve safety issues of lithium-ion batteries (LIBs). Among them, those obtained by homogenously dispersing inorganic nanofillers into a polymer matrix combine advantages of all SSE typologies. In this work, high-entropy (Cr,Mn,Fe,Co,Ni) oxide (HEO) with different morphology (nanoparticles or nanofibers) are evaluated as passive fillers for the preparation of composite polyethylene oxide (PEO)-based SSEs. By varying their preparation conditions (calcination at 400 or 800 °C for 0.5 or 2 h, followed by rapid cooling) different size and crystallization degree of the oxide grains are obtained. The results of the electrochemical testing of the PEO/HEO composites evidence the crucial role of the filler microstructure and morphology. The best results in terms of electrolyte resistance (22.5 Ω), electrochemical stability window (4.7 V), Li + transference number (0.37) and ionic conductivity (3.0⋅10 −4 S cm −1 at 65 °C) are obtained by using well crystallized HEO nanofibers with highly defective surface. The suitability of the most promising composite for practical applications is validated by successfully using it in full cell with commercial high-voltage cathode materials.