Cation doping have been widely applied for the development of energy storage/conversion materials. However, due to extensive works such as combinatorial synthesis and data-driven material explorations, the effectiveness of cation doping is reaching the limitations. Instead of cation doping, anion doping is one of the most promising new strategies for energy material development, although tuning of anion composition is technically difficult due to mismatch of ionic size and volatility. To overcome such difficulties, we recently develop electrochemical fluorine doping reactor and succeeded to introduce F defect into the oxide host structure [1]. In this work, fluorine doping into an oxide-based lithium-ion battery cathode is examined by using our electrochemical F doping reactor, to understand influences of anion defect on battery performance.

LiMn₂O₄ was synthesized by solid state reaction method from Li₂CO₃ and Mn₂O₃. For F-doping, oxygen-deficient LiMn₂O_4-d was prepared by annealing 1%O₂ at 700^oC. F-doping reactor was composed of Pt/La_0.9Ba_0.1F_2.9/PbF₂-Pb, and the oxygen-deficient LiMn₂O_4-d was placed onto the Pt current collector. The reactor was uniaxially pressed and heated at 250^oC, and then fluorine was electrochemically introduced the small compartment where LiMn₂O_4-d exist. The amount of supplied F was controlled by electric charge passing through the reactor. F-doped LiMn₂O₄ was characterized by XRD, XPS, TOF-SIMS. The composite cathode was prepared from the cathode active maerials:PVDF:Carbon = 70:10:20, and the structure of the battery test cell was the composite cathode/1M LiPF₆ EC:DMC=1:1/Li metal. The battery performance was evaluated by HJ1001SD8 (Meiden Hokuto Corp.) at 25^oC.

After F-doping treatment, no impurity phase was observed, and the F-doped LiMn₂O₄ maintained the original spinel structure. Clear F-1s peak was detected from F-doped LiMn₂O₄ by XPS and its intensity increases with increase the amount of supplied F. The uniformity of the fluorine distribution in the LiMn₂O₄ particle was checked by TOF-SIMS. While F intensity of the particle surface was slightly higher than that of the bulk part, clear F signal was confirmed at the deep inside of the particle. This means our F pumping treatment successfully introduce fluorine defect not only on the surface but also into the particle inside.

To understand the influence of anion defect species, oxygen vacancy and fluorine defect in this study, charge discharge measurement was performed on the oxygen-stoichiometric LiMn₂O₄, the oxygen-deficient LiMn₂O_4-d and F-doped LiMn₂O₄. Among them, the oxygen-stoichiometric LiMn₂O₄ showed the best capacity retention upon charge discharge cycles. The oxygen-deficient LiMn₂O_4-d showed significant capacity degradation upon cycles, suggesting that oxygen vacancy significantly degrade battery performance [2]. On the contrary, F-doped LiMn₂O₄ showed better cycle stability than the oxygen-deficient LiMn₂O_4-d, suggesting that F defect improve the battery performance. These indicate suitable tuning of anion defect structure can be a promising new strategy for battery material development.