Effects of Fluorine Doping on Nickel-Rich Positive Electrode Materials for Lithium-Ion Batteries

Three fluorine-doped lithium nickel oxide samples series (LiNiO_2−xF_x, LiNi_1−xMg_xO_2−xF_x; Li_1+x/2Ni_1−x/2O_2−xF_x) were prepared and investigated. It is suggested that fluorine was introduced into the lattice structure during the calcination. As fluorine is introduced into LiNiO_2−xF_x and LiNi_1−xMg_xO_2−xF_x the percentage of Ni (or Ni and Mg) in the Li layer increases for x > 0.05. However, adding excess Li in Li_1+x/2Ni_1−x/2O_2−xF_x sucessfully balances the charge differential introduced by fluorine doping therefore very little Ni²⁺ was created and the lithium layers remain “uncontaminated” by other metals. Data from Li/LiNiO_2−xF_x, Li/LiNi_1−xMg_xO_2−xF_x and Li/Li_1+x/2Ni_1−x/2O_2−xF_x cells mirror the percent of cation mixing as determined by X-ray diffraction (XRD) and Rietveld refinement in each case. In situ XRD of Li_1.1−xNi_0.9O_1.8F_0.2 shows no multipule phase transitions which further suggests fluorine was successfully doped into the lattice. Acclelerating rate calorimetry (ARC) experiments show a potential safety advantage brought by fluorine doping. pH titration was used to explore if residual LiF (if any) at the surface converted to other lithium compounds (LiOH, Li₂O or Li₂CO₃). No evidence of residual LiF was found.