Magnetic field-assisted vertically aligned NiFe2O4 nanosheets in composite solid polymer electrolytes for advanced all solid-state lithium metal batteries

Two-dimensional materials (2D Ms) as fillers have been applied in polyethylene oxide (PEO)-based electrolyte to enhance the low ionic conductivity and poor interface compatibility. However, the randomly dispersed fillers in PEO matrix result in anisotropy of Li⁺ transportation and insufficent ionic conductivity. Herein, NiFe₂O₄ (NFO) nanosheets are firstly introduced in polymer matrix to form vertically aligned NFO-PEO (ANFO-PEO) composite solid-state electrolytes (CSEs) through magnetic field-assisted alignment strategy. The vertically aligned NFO/PEO interface in CSEs can construct oriented Li⁺ transport channels and maximize the utilization of high in-plane conductivity. Meanwhile, the NFO nanosheets with abundant oxygen vacancies could effectively anchor TFSI⁻ to promote the dissociation of Li salts. Furthermore, the optimized Li⁺ transport flux in CSEs enables homogeneous Li deposition and effectively mitigates the growth of dendrites. Owing to the synergistic effects, the ANFO-PEO CSEs exhibit high ionic conductivity (9.16 × 10⁻⁴ S cm⁻¹ at 60 °C) and stable potential window up to 5.0 V vs Li/Li⁺. Therefore, LiFePO₄ in the full cell and pouch cell with ANFO-PEO CSEs could deliver excellent cycling performance (92.78 % capacity retention after 1000 cycles at 0.5C; 96.88 % capacity retention after 105 cycles at 0.1C).