A new chemical oxidation method was proposed to mimic the crystalline structure change of cathode materials during charge-discharge process by chemical treatment as an efficient experimental data collection method for exploring cathode materials for lithium batteries by informatics such as machine learning. LiNi0.5Mn1.3Ti0.2O4 prepared by the solid-state reaction method was soaked in 2 mol/L HNO3 for 10 days, and the chemical delithiation about 53% was confirmed. Furthermore, from the information obtained by synchrotron XRD and XAFS measurements, it could be predicted that the chemical oxidation reaction was accompanied by delithiation. In HCl and H2SO4, which are different acids, the lattice parameter decreased with decreasing Li composition as in the case of HNO3. However, from the value of the metal-oxygen octahedral volume calculated from the crystal data obtained by structure refinement, it was concluded that the Li+ to H+ ion-exchange reaction rather than the oxidation reaction proceeded in HCl.