Although considered as promising candidates for lithium-ion secondary batteries, spinel LiMn2O4 cathodes suffer from significant capacity decay owing to the Jahn-Teller effect, dissolution of Mn and lattice oxygen loss during the charge/discharge process, preventing their wider use. In this work, we realize that F-doping at small concentrations could improve the battery voltage and reduce the capacity decay using an atomistic model. For voltage, F-doping improves the voltage to about 4.4 eV under large delithiation. For capacity decay, it retards capacity decay owing to the reduced lattice oxygen loss. The larger Gibbs free energy of oxygen release after F-doping indicates harder lattice oxygen loss. In addition, although F-doping makes the average valence of Mn lower, the existence of Mn4+ during delithiation exerts a positive effect by reducing the Jahn-Teller effect. However, since the Mn3+ ions in the spinel structure could induce Jahn-Teller distortion, the effect of F-doping on Jahn-Teller distortion is determined by the competition between Mn4+ and Mn3+. The atomistic mechanism of F-doping in the performance of LiMn2O4 offers new insight in developing spinel lithium manganese oxide cathode materials with superior performance.