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

Carbon dioxide (CO 2 ) is a greenhouse gas, the emission of which is a concern due to its contribution to global warming. The lithium–CO 2 battery has attracted attention as a means of CO 2 reduction and its effective utilization. Li–CO 2 batteries undergo discharge by the conversion of CO 2 into lithium carbonate (Li 2 CO 3 ), while charging is caused by the electrochemical decomposition of Li 2 CO 3 . Here, an iodine species was investigated as a bifunctional catalyst for both the discharge and charge processes. When the electrolyte in the Li–CO 2 battery contains a small amount of iodine, lithium iodide (LiI) is first formed at the cathode during the initial stage of discharge and subsequently CO 2 reduction occurs. The LiI that is formed accelerates CO 2 reduction. Li 2 CO 3 formed on the cathode during discharge is an insulator; therefore, the accumulation of Li 2 CO 3 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 Li 2 CO 3, because the decomposition potential of Li 2 CO 3 is 3.82 V. However, the redox potential of iodine in the trimethyl phosphate (TMP) electrolyte was greater than 3.8 V, so Li 2 CO 3 could be chemically decomposed by the iodine in TMP. The iodine mediator (3I 2 /2I 3 – ) in the Li salt–TMP electrolyte was confirmed to enhance the decomposition of Li 2 CO 3 under a low charge voltage.