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Li–CO2/O2 batteries, a promising energy storage technology, not only provide ultrahigh discharge capacity but also capture CO2 and turn it into renewable energy. Their electrochemical reaction pathways' ambiguity, however, creates a hurdle for their practical application. This study used copper selenide (CuSe) nanosheets as the air cathode medium in an environmental transmission electron microscope to in situ study Li–CO2/O2 (mix CO2 as well as O2 at a volume ratio of 1:1) and Li–O2 batteries as well as Li–CO2 batteries. Primary discharge reactions take place successively in the Li–CO2/O2–CuSe nanobattery: (I) 4Li+ + O2 + 4e− → 2Li2O; (II) Li2O + CO2 → Li2CO3. The charge reaction proceeded via (III) 2Li2CO3 → 4Li+ + 2CO2 + O2 + 4e−. However, Li–O2 and Li–CO2 nanobatteries showed poor cycling stability, suggesting the difficulty in the direct decomposition of the discharge product. The fluctuations of the Li–CO2/O2 battery's electrochemistry were also shown to depend heavily on O2. The CuSe‐based Li–CO2/O2 battery showed exceptional electrochemical performance. The Li–CO2/O2 battery offered a discharge capacity apex of 15,492 mAh g−1 and stable cycling 60 times at 100 mA g−1. Our research offers crucial insight into the electrochemical behavior of Li–CO2/O2, Li–O2, and Li–CO2 nanobatteries, which may help the creation of high‐performance Li–CO2/O2 batteries for energy storage applications. [ABSTRACT FROM AUTHOR]