In summary, our study employs first-principles DFT calculations to investigate the intricate electrocatalytic processes involved in reaction intermediates within Mg-CO2 batteries, with a particular focus on ruthenium oxide (RuO2) as a representative cathode catalyst. Our findings reveal intriguing aspects, such as the enhanced Mg adsorption energies that activate inert CO2, resulting in the discharge product MgC2O4 with lower overpotential compared to MgCO3 on the RuO2 (211) surface. While MgC2O4 is thermodynamically favored as a discharge product, our investigation suggests its subsequent decomposition into MgCO3, MgO, and C. Bader charge analysis highlights the negligible electron transfer for CO2, rendering it inactive and making the formation of carbonate (CO3)/oxalate (C2O4) intermediates via C-C coupling thermodynamically unfavorable. Remarkably, our results demonstrate significant electron transfers from the catalytic surface to the MgC2O4/MgCO3 products illustrating improved adsorption characteristics. These findings have the potential to enhance catalytic activity and reduce the free energy in the formation of final discharge products in Mg-CO2 batteries. Additionally, we carefully examine the electrochemical free energy profiles of the most favorable reaction pathways, quantifying discharge, and charge overpotentials as 1.3 V and 1.35 V, respectively. Overall, these findings underscore the critical importance of deliberate catalyst design for cathode materials in addressing performance limitations encountered in non-aqueous rechargeable Mg-CO2 batteries.