Nitrogen-Doped Lithium Lanthanum Titanate Nanofiber-Polymer Composite Electrolytes for All-Solid-State Lithium Batteries

Abstract

Currently cation doping is common for improving ionic conductivity of metal oxide-based lithium-ion conductors. In this work, anions (nitrogen ions) have been doped to perovskite Li_3xLa_{2/3 – x}TiO₃ (LLTO) nanofibers by heat treatment in the ammonia-containing atmosphere, and substituted for oxygen anions partially in the perovskite structure. Density-functional theory (DFT) calculation results reveal that nitrogen doping weakens the bonding of Li ions on the A sites in perovskite ABO₃ structure and allows for larger lattice distortion, reducing the activation energy for Li-ion hopping in both the forward and backward jumping directions. Experimental results have also confirmed that nitrogen doping has improved ionic conductivity of LLTO. Nitrogen-doped LLTO nanofibers have been incorporated with a poly (vinylidene fluoride)-co-hexafluoropropylene (PVDF-HFP) polymer to form a solid-state composite electrolyte, which exhibits ionic conductivity of 3.8 × 10^–4 S·cm^–1 at room temperature and an electrochemical stability window of up to 4.9 V vs Li|Li⁺. The all-solid-state Li metal|composite electrolyte|LiFePO₄ lithium batteries, which employ nitrogen-doped LLTO nanofibers, show better rate capability and cycling stability at room temperature than the counterparts with pristine LLTO nanofibers.