Spinel LiMn2O4 was modified by an aqueous fluoride modification method (AFMM). It is found that tiny crystals of potassium manganese trifluoride (KMnF3) grew on the surface of the spinel particles during the electrochemical process. The quantity and the integrality of the KMnF3 Crystals were controlled by varying current density and processing time. The mechanisms of the formation of KMnF3 were discussed. The harmful two-phase reaction of LiMn2O4 upon cycling was suppressed after the cathode was processed by AFMM. The chemical diffusion coctficients (D-s) of Li+ were measured by a potential relaxation technique (PRT). The values of D-s obtained in this way are of the order of 10(-10) - 10(-9) cm(2)/s.