Multifunctioning Electrocatalytic Cathodes for Lithium-Air and Lithium-Oxygen Batteries.

Abstract

Despite its historical success, currently, the limited energy density in Li-ion batteries restricts the electrifying of transportation into small and medium-scale vehicles. On the contrary, Li- O2 batteries (LOBs) and Li-air batteries (LABs) with larger theoretical energy density are capable of powering heavy-duty transportation, given they overcome the sluggish oxygen kinetics. One of the possible ways to achieve this goal is by introducing novel bifunctional electrocatalysts at the battery cathode, enhancing the cycle life and the discharge capacity of the LABs and LOBs via promoting oxygen reaction kinetics. My PhD study is focused on developing transition metal derivative-based novel bifunctional catalysts to achieve superior charge densities and cyclability in LABs with relatively lower expenses. Following the mentioned rationale, self-standing cathode structures were developed by grafting binary oxide of NiCo2O4, sulfide of NiCo2S4, phosphide of NiCoP, and their heterogeneous phases on Ni foam. During the study, different physicochemical, morphological, and electrochemical analytical techniques were analysed to determine the suitability of the synthesised materials for catalysing oxygen kinetics. The performance of developed cathode structures was mainly assessed by cycling the assembled aprotic LABs with different current ratings, while CV and EIS studies were also conducted to obtain a further understanding of the battery performance. The examined strategy of optimization of eg-orbital occupancy in binary metal oxides through stoichiometric adjustments was found to be effective in enhancing the electrocatalytic activity in binary oxides. During the study, the highest discharge capacity of 25162 mAgh-1 at a current density of 400 mAg-1 was obtained for the heterogeneous hollow catalytic microstructure of NiCoP/ NiCo2S4/ NiCo2O4. The in-situ synthesized NiCo2O4/ NiCoP hybrid structure lasted exceeding 400 cycles at a current density as high as 800 mAg-1 highlighting the success of the study advancing beyond the state-of-the-art knowledge. In both cases, morphology and the orientation of the discharged Li2O2 were able to be altered through the modulation of the electronic structure of the catalyst, allowing undisturbed diffusion of Li+ towards the cathode, allowing high discharge depths and extended cyclability.