Abstract
At present, the ever-increasing energy demand and continuous carbon dioxide emissions have brought severe challenges to human survival and development. The recently proposed Li-CO2 battery (LCB) is considered one of the revolutionary candidates to simultaneously solve the above problems because of its dual functions of energy storage/conversion and carbon dioxide utilisation. The research of LCB will also promote the development of Li-air batteries, considering CO2 as an inevitable cathode reactant component of Li-air batteries. However, the development of LCB is still in its infancy, and two main issues need to be solved urgently: (1) There is still a lack of highly efficient cathode electrocatalysts for overcoming the high charging potential and improving poor reversibility of LCB to enhance its practical applications; (2) Due to the lack of effective characterisation methods, the reaction mechanism of LCB is still unclear, which severely limits further performance optimisation and applications.
To address the above problems, in this project a versatile on-chip LCB platform was designed and fabricated to simultaneously achieve efficient catalyst screening and in situ probing of product chemical composition and morphology evolution. Through this platform, we identified the E-beam deposited Pt catalyst as one of the most efficient catalysts, which demonstrated the smallest overpotential (~0.55 V) and can greatly promote the LCB performance, from a series of candidates (Pt, Au, Ag, Cu, Fe, Ni, In-implanted HOPG). The high reversibility of Pt-based LCB was further indicated by the in situ Raman and the in situ atomic force microscopy (AFM) characterisation. The lithium carbonate and carbon were reversibly generated and decomposed during the discharge and charge process. Further density functional theory (DFT) calculations studied the best possible reaction pathways on Pt(111) basal plane, promoting the understanding of the electrochemical processes of Pt-based LCBs. Three possible pathways were calculated in this thesis. According to the calculation results, the pathway which involved the *LiC2O4 intermediate possessed the smallest Gibbs free energy change of the rate-determining step and was regarded as the most possible pathway for lithium carbonate and carbon formation and decomposition on Pt(111) basal plane.
To further validate the practical application of the as-screened-out efficient catalyst. Pt-based Li-CO2 coin cells and pouch cells were assembled. The as-assembled coin cells delivered a large specific discharge capacity of 41470 mAh g-1, a low overpotential of 0.35 V, and a high energy efficiency of 89.5% at the current density of 100 mA g-1. Further ex-situ Raman and SEM characterisations detected the lithium carbonate generation and decomposition during the discharge and charge process, suggesting the high catalytic activity of the Pt catalyst. The as-assembled pouch cells demonstrated a low overpotential of 0.56 V, a low charging potential of 3.0 V, and a high energy efficiency of around 85% at the current density of 30 mA g-1. Further working atmosphere study indicated that the moisture of ambient air further degraded the performance of the battery rapidly, which hinders its practical application and needs further improvement.
In addition to the as-screened catalysts in this thesis, more types of efficient catalysts, such as single-atom catalysts, needed to be screened in the future via this on-chip platform. More generally, this versatile and reliable platform can be modulated and broadly applied to other systems, such as metal-gas batteries, opening opportunities for rapid and scalable screening, accurate testing, and mechanism investigation. For the practical application of Li-air batteries in ambient air, some future works are of great significance, including developing highly efficient catalysts, developing selective membranes for electrolyte protection, and exploring effective Li anode protection methods.