Effect of lattice fluoride and borohydride on the electrochemical performances of NaAlCl4 solid electrolyte

NaAlCl₄ solid electrolyte is a highly promising solid-state sodium-ion electrolyte with significant application potential. However, most studies on the modification of such material have focused on the impact of cation doping on performance, and the effects of anion doping remain unknown. In this work, fluoride (F⁻) and borohydride (BH₄⁻) were successfully introduced into NaAlCl₄ using a mechanical ball milling method, resulting in the synthesis of NaAlCl_4−xF_x (x = 0, 0.1, 0.3, 0.5, and 0.7) and NaAlCl_4−x(BH₄)_x (x = 0.1 and 0.3). These derivatives maintain the same orthorhombic structure as NaAlCl₄. Electrochemical measurements validate the feasibility of F⁻ and BH₄⁻ doping for modifying the NaAlCl₄ electrolyte. X-ray photoelectron spectroscopy (XPS) analyses demonstrate that the fluorinated Na/NaAlCl_3.5F_0.5 interphase formed during cyclic voltammetry (CV) measurements, this prevented further side reactions. This work reveals that F⁻ and BH₄⁻ doping exhibit similar electrochemical effects, holds profound practical significance for future research in modifying halide-based sodium ion solid electrolytes.