Earth-Abundant Tri-Molybdenum Phosphide Nanocatalyst for the Next Generation of Lithium Batteries

Successful demonstration of the lithium-air (Li-air) battery technology, known as a potential alternative to lithium ion batteries due to its high theoretical energy density, can contribute to electrification of transportation sector as well as solar/wind powerplants to resolve their intermittency issue. The advancement of this technology requires a cathode catalyst to drive oxygen reduction and evolution reactions (ORR and OER) happening during discharge and charge processes, respectively, at high rates at low overpotentials. This can lead to low potential gap, high efficiency, and long cycle life due to reversible formation/decomposition of lithium peroxide (Li 2 O 2 ) at the cathode surface. Recently developed catalysts systems such as noble metals, bimetallic catalysts, carbon-based catalysts, transition metal oxides, and transition metal dichalcogenides have been studied to achieve such performances with incremental improvements. Here, we are presenting trimolybdenum phosphide (Mo 3 P) nanoparticles as an earth-abundant and stable catalyst with outstanding structural and electronic properties at surface active sites studied for ORR and OER. Our electrochemical results indicate ORR and OER current densities of 7.21 mA/cm 2 at 2.0 V vs Li/Li + (ORR) and 6.85 mA/cm 2 at 4.2 V vs Li/Li + (OER) for Mo 3 P nanoparticles in a non-aqueous electrolyte. Tafel plot analysis for this catalyst show slopes of 35 and 38 mV/dec for ORR and OER, respectively, suggesting a faster charge transfer kinetics, as well as ORR and OER onset potentials of 4 and 5.1 mV that are the lowest values yet reported. Moreover, our turnover frequency (TOF) calculation, actual catalytic activity, indicates up to 7 times higher activity of Mo 3 P nanoparticles for both ORR and OER compared to state-of-the-art catalysts used for the same application. We have tested the catalytic performance of Mo 3 P nanoparticles in our custom designed Li-air battery cell working at the actual air environment. The results indicate that this catalyst works perfectly together with electrolyte and lithium anode to achieve an energy efficiency of 90.2% and potential gap of 350 mV at a capacity of 500 mAh/g, surpassing the performances of state-of-the-art Li-air batteries. We have also performed various characterization techniques such as scanning electron microscopy (SEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), in-situ differential electrochemical mass spectroscopy (DEMS) to elucidate the nature of the product in our battery cell. The results confirm fully reversible formation and decomposition of lithium peroxide (Li 2 O 2 ) as the only discharge product. Furthermore, density functional theory (DFT) calculation suggests that the observed ORR and OER activities are due to the formation of a kinetically stable oxide overlayer on the Mo-terminated Mo 3 P (110) surface sites. The high performance, inexpensive catalyst found in our work can indeed contribute to development of efficient energy storage systems, specifically Li-air batteries, to speed up the global energy transition.