Li-CO2 batteries involve a spontaneous conversion reaction by injecting CO2 into a cathode, and reversibly store energy without additional energy input through a charging and discharging process, thereby achieving carbon neutrality. Noble metal-based electrocatalysts have been actively considered to efficiently facilitate the Li-CO2 reaction with reduced overvoltage, yet the use of expensive noble metal catalysts is a barrier to developing this type of Li-CO2 battery. Here, the importance of minimizing the size of noble metal particle-based catalysts for Li-CO2 batteries is reviewed and emphasized. Comparisons of the performance of Li-CO2 cells with noble metal catalysts, such as Ru and Ir, showed that overpotential is lower when particle sizes are reduced to the single atom-scale. This indicates that the smaller the particle-to-atomic scale, the greater the catalytic activity. Recent diverse studies based on nano- or atomicscale Ru and Ir catalysts affecting catalytic activities of the Li-CO2 cell reaction are introduced. Other single atom catalyst candidates are also suggested for Li-CO2 battery applications.