Li5FeO4 was successfully synthesized via solid-state reaction using Li2O and Fe3O4 as precursors. The use of the proposed reactants allowed to lower the synthesis temperature to 430 °C. Li5FeO4 formation was found to be a two-step process, where the starting oxides partially react to produce LiFe5O8 and then LiFe5O8 reacts with Li2O to form Li5FeO4. The CO2 sorption properties of Li5FeO4 were investigated in the temperature range from room temperature to 750 °C. Time-resolved in situ high temperature synchrotron X-ray diffraction measurements coupled with Rietveld analysis showed that Li5FeO4 carbonation produces LiFeO2 and Li2CO3, and revealed a secondary reversible reaction occurring between 350 °C and 550 °C, in which LiFeO2 captures CO2 producing Li2CO3 and Fe2O3. The experimental results allowed to propose the mechanism of reaction for Li5FeO4 carbonation process.