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

Despites the high theoretical energy density, there still lacks of sufficient theoretical insight into the lithium -nitrogen (Li-N-2) 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-N-2 batteries. After the calculations on adsorption free energies (delta G(ads)) of N(2 )molecule on MPPcs, the Li-mediated NRR process was simulated by plotting the free energy profiles of LixN(2)* intermediates along two different pathways. It is found that only parts of the MPPcs could fully reduce N-2* into the final product Li6N2*, whose catalytic performance was compared by the discharge/charge overpotential (eta(DC)/eta(C)). Along with the end-on pathway, FePPc shows considerably low eta(DC) (0.37 V) and eta(C) (0.42 V), while along with the side-on pathway, ScPPc exhibits the lowest eta(DC) (0.24 V) and eta(C) (0.35 V) among all the screened MPPcs. The delta G(ads) of N-2 is found to be suitable indicator to evaluate the catalytic performance of MPPc in Li-N-2 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-N-2 batteries.