Fluorine Doping for Improved Lithium and Manganese-Rich Layered Cathode Performance

Lithium- and manganese-rich (LMR) cathodes represent a promising cathode active material for the next-generation lithium-ion batteries (LIBs) due to their high energy density, low material cost, and structural stability across wide operating voltage window. However, the LMR-type cathode faces challenges involving high irreversible capacity due to oxygen release during the first formation cycle. In order to mitigate this drawback, we present a novel approach to surface modification by fluorine (F) doping on Li_1.15Ni_0.28Mn_0.57O_2-xF_x cathodes. The cathode powder was treated with various amounts of NH₄F, aiming for F concentrations of x = 0.01, 0.03, 0.05, and 0.1, followed by heat-treatment at 450 °C to achieve surface fluorination. Various characterization techniques were applied to investigate the impact of the F-doping on the electrochemical performance and thermal safety. The results showed that the F-doping led to the reductions of overall oxidation state of the transition metals (Ni and Mn) at the cathode surface region. The F-doping in Li_1.15Ni_0.28Mn_0.57O_2-xF_x promoted the formation of spinel phases, as evidenced by distinct peaks at 2.75 V_vsLi in the dQ/dV profiles, which is accompanied by reducing the Li₂MnO₃ phase. Such phase transitions in the cathode surface region contribute to the decrease in the irreversible capacity and enhanced thermal abuse tolerance of the Li_1.15Ni_0.28Mn_0.57O_2-xF_x cathodes. Our results suggest that the presence of pre-existing spinel phase on the surface of LMR cathode can be beneficial for LIB performances.