An advanced ion-exchange method using resin was employed to produce a novel cathode material, Fe<sup>3+</sup><sub><i>x</i></sub>Cr<sup>3+</sup><sub>2-<i>x</i></sub>Cr<sup>6+</sup><sub>4</sub>O<sub>15</sub> (0 ≤ <i>x</i> ≤ 2), where some of the Cr<sup>3+</sup> ions at the octahedral sites of Cr<sub>2</sub>O<sub>5</sub> were substituted with Fe<sup>3+</sup> ions. The battery cell test and X-ray photoelectron spectroscopy analysis of Cr<sub>2</sub>O<sub>5</sub>, Cr<sub>8</sub>O<sub>21</sub>, and Fe<sub>1.5</sub>Cr<sub>4.5</sub>O<sub>15</sub> indicate a change in the capacity from 210 to 280 and 350 mA h g<sup>-1</sup> with a change in the Cr<sup>6+</sup>/Cr<sup>3+</sup> atomic ratio from 2 to 3 and 8 for Cr<sub>2</sub>O<sub>5</sub>, Cr<sub>8</sub>O<sub>21</sub>, and Fe<sub>1.5</sub>Cr<sub>4.5</sub>O<sub>15</sub>, respectively. The discharge capacity of the compound with the crystallographic formula Fe<sup>3+</sup><sub>1.5</sub>Cr<sup>3+</sup><sub>0.5</sub>(Cr<sup>6+</sup>O<sub>4</sub>)<sub>2</sub>(Cr<sup>6+</sup><sub>2</sub>O<sub>7</sub>) is, by far, the highest reported capacity for transition metal oxide electrodes in the voltage range of 2.0-4.5 V vs Li<sup>+</sup>/Li<sup>0</sup>.