Computational screening on two-dimensional metal-embedded poly-phthalocyanine as cathode catalysts in lithium-nitrogen batteries

Abstract

Despites the high theoretical energy density, there still lacks of sufficient theoretical insight into the lithium-nitrogen (Li-N2) batteries, which limits their further development. Here we performed computational studies on screening of the two-dimensional metal-embedded poly-phthalocyanines (MPPcs) for electro-catalysis of Li-mediated nitrogen reduction reaction (NRR) in cathode of Li-N2 batteries. After the calculations on adsorption free energies (ΔGads) of N2 molecule on MPPcs, the Li-mediated NRR process was simulated by plotting the free energy profiles of LixN2* intermediates along two different pathways. It is found that only parts of the MPPcs could fully reduce N2* into the final product Li6N2*, whose catalytic performance was compared by the discharge/charge overpotential (ηDC/ηC). Along with the end-on pathway, FePPc shows considerably low ηDC (0.37 V) and ηC (0.42 V), while along with the side-on pathway, ScPPc exhibits the lowest ηDC (0.24 V) and ηC (0.35 V) among all the screened MPPcs. The ΔGads of N2 is found to be suitable indicator to evaluate the catalytic performance of MPPc in Li-N2 batteries, which should be negative and close-to-zero. This work not only proposes FePPc and ScPPc as the promising cathode catalysts, but also provides theoretical insight into the microscopic mechanism in Li-N2 batteries.