The Intriguing Nature of Fluorine Doping on Li₂CuO₂ and the Reasons behind the Inhibition of Oxygen Evolution

Oxygen evolution is a major contributor to the degradation of lithium metal oxide cathode materials during initial charging, leading to low initial capacities and poor cyclability. The introduction of different ions to modify the electrochemical behavior of these materials and combat oxygen evolution is commonplace; while the benefits of these modifications are evident, the nature of these modifications are not fully understood. Herein, a multiangle approach is utilized to elucidate the effects of fluorine doping on the evolution of oxygen in Li₂CuO₂. Through XRD, the presence of fluorine is confirmed in the synthesized samples via the detection of the contraction in the unit cell of Li₂CuO₂ in the a and c directions, and the results are verified through solid-state ¹⁹F NMR. As a result of F-doping, the electrochemical results suggest a modification of the bulk properties, showing an increased reversible capacity. Li₂CuO₂ only discharges 83 mAh g^-1 on the 10th cycle, while the cuprate doped with 5.0 mol % fluorine achieved a discharge capacity of 133 mAh g^-1. This improvement is attributed to the presence of F in the structure of Li₂CuO₂, as determined via in situ DEMS experiments. Lastly, ab initio calculations further support the hypothesis that fluorine doping changes the density of states close to the Fermi level, altering the bulk properties and the likelihood for oxygen evolution.