Bifunctional Catalytic Activity of Iodine Species for Lithium–Carbon Dioxide Battery

Carbon dioxide (CO2) is a greenhouse gas, the emission of which is a concern due to its contribution to global warming. The lithium–CO2 battery has attracted attention as a means of CO2 reduction and its effective utilization. Li–CO2 batteries undergo discharge by the conversion of CO2 into lithium carbonate (Li2CO3), while charging is caused by the electrochemical decomposition of Li2CO3. Here, an iodine species was investigated as a bifunctional catalyst for both the discharge and charge processes. When the electrolyte in the Li–CO2 battery contains a small amount of iodine, lithium iodide (LiI) is first formed at the cathode during the initial stage of discharge and subsequently CO2 reduction occurs. The LiI that is formed accelerates CO2 reduction. Li2CO3 formed on the cathode during discharge is an insulator; therefore, the accumulation of Li2CO3 produces a passivation layer, which leads to charging at high overpotential (ca. 4.5 V vs Li+/Li). Iodine with a redox potential below 3.5 V vs Li+/Li cannot decompose Li2CO3, because the decomposition potential of Li2CO3 is 3.82 V. However, the redox potential of iodine in the trimethyl phosphate (TMP) electrolyte was greater than 3.8 V, so Li2CO3 could be chemically decomposed by the iodine in TMP. The iodine mediator (3I2/2I3–) in the Li salt–TMP electrolyte was confirmed to enhance the decomposition of Li2CO3 under a low charge voltage.