CO₂ Crossover to the Li Anode and Its Implications on the Solid Electrolyte Interphase Composition in a Rechargeable Li–CO₂ Battery

A major fundamental challenge in a Li–CO₂ battery, which has not received due attention in comparison to the intricacies of CO₂ redox, is the CO₂ gas crossover toward the Li anode. The CO₂ crossover during battery operation changes the Li chemical state and the composition of the proximate solid electrolyte interphase (SEI). Systematic ex situ Raman spectroscopy and X-ray diffraction investigations confirm continuous active Li metal depletion with successive cycles due to CO₂ (or CO₂/O₂) crossover. Irrespective of the gas, our studies convincingly reveal active Li metal conversion to majorly LiOH and Li₂CO₃, with LiOH being the dominant phase at higher cycle numbers. Depth profile X-ray photoelectron spectroscopy reveals additional SEI components, viz., Li₂O, Li₂S, LiF, and CF₃, which exhibit a depth-dependent spatial distribution along with Li₂CO₃ and LiOH. Analytical chemical investigations based on metalation reactions quantitatively reveal irreversible loss of active Li metal with successive cycling.