Researchers unveil mystery of operating voltages in Li-CO2 batteries

A team led by Prof. TAN Peng from the University of Science and Technology of China (USTC) has renewed the understanding of the operating voltages of lithium-carbon dioxide (Li-CO₂) batteries, providing a new strategy for the next generation of Li-CO₂ batteries. Their work was published in Proceedings of the National Academy of Sciences of the United States of America through direct submission.

Li-CO₂ batteries can turn CO₂ into carbonate and carbon while outputting electric energy, therefore possessing the advantage of both energy storage and CO₂ utilization. Previous study generally reported that the operating voltage of Li-CO₂ batterie is about 2.6 V, which is similar to that of Li-O₂ batteries. However, this assumption has been facing increasing questions about whether slow CO₂ reduction reaction (CO₂RR) can generate such high voltages.

To cast light on the above question, Prof. TAN Peng’s team built an electrochemical test system for the Li-flowing CO₂ battery, ensuring a pure CO₂ environment. The carbon nanotube (CNT) electrode, catalyst-loaded carbon nanotube (RuO₂/CNT) and non-carbon nanotube (RuO₂/NiO) all indicated that the Li- CO₂ battery operates at about 1.1 V and that the CO₂RR rate is much lower than the oxygen reduction reaction. The team determined the equilibrium potential to be about 2.82 V using galvanostatic current intermittent titration technique.

After analyzing the product, the team proposed that the discharge products at 1.1 V is a mixture of crystalline Li₂CO₃, amorphous Li₂CO₃ and amorphous C, verifying the four-electron transfer mechanism (Li⁺+ CO₂+ 4e⁻→ Li₂CO₃+ C). This mechanism theoretically predicts an equilibrium potential of 2.8 V, which is consistent with the test results. The products analysis showed that the four-electron transfer proceeds slowly, compliant with the characteristics of low voltage system and inert CO₂. Moreover, Using the transmission electron microscope (TEM), the team found that under electron beam irradiation, small particles in the products began to grow through the phagocytosis of amorphous materials and merging with other particles. In this process, amorphous substance gradually transformed into crystalline state. Therefore, the TEM image in some previous studies was probably not the natural discharge products but the products of electron beam irradiation.

To find out the source of the high voltage, the research team further investigated the effects of decoupled air components and operating conditions on the battery performance. Raising the voltage plateaus to 1.8-2.0 V by 1% O₂ and 500 ppm H₂O, the team didn’t detect byproducts like LiOH and Li₂O₂  in the discharge products. However, the morphology and crystallinity of Li₂CO₃ showed significant difference. O₂ and H₂O lowered the potential energy barrier and alleviated electrode passivation by changing the generation path of Li₂CO₃, thus accelerating the reaction and raising the discharge voltage plateaus. Based on the decoupling analysis, the slight air residue or leakage in the test device could lead to higher voltage plateaus and is extremely difficult to detect.

This work suggested that for the development of the next generation Li-CO₂ batteries, researchers need to conduct mechanism study in pure CO₂ environment and develop compatible components like catalysts, electrolytes and electrodes.