Reversible Hybrid Aqueous Li-CO₂ Batteries with High Energy Density and Formic Acid Production

Metal-CO2 batteries, an attractive technology for both energy storage and CO2 utilization, are typically classified into organic Li(Na)-CO2 batteries with a high energy density/output voltage and aqueous Zn-CO2 batteries with flexible chemical production. However, achieving both high-efficiency energy storage and flexible chemical production is still challenging. In this study, a reversible hybrid aqueous Li-CO2 battery is developed, integrating Li with an aqueous phase, which exhibits not only a high operating voltage and energy density but also highly selective formic acid production. Based on a Li plate as the anode, NaCl solution as the aqueous electrolyte, solid electrolyte Li1.5Al0.5Ge1.5P3O12 (LAGP) as a separator and Li+ transporter, and a bifunctional Pd-based electrocatalyst as the cathode, the resulting battery shows a high discharge voltage of up to 2.6 V, an outstanding energy conversion efficiency of above 80 %, and remarkable selectivity of CO2-to-HCOOH conversion of up to 97 %. The related reaction mechanism is proposed as CO2+2 Li+2 H+ reversible arrow HCOOH+2 Li+.