Kinetically Stable Oxide Overlayers on Mo₃P Nanoparticles Enabling Lithium–Air Batteries with Low Overpotentials and Long Cycle Life

The main drawbacks of today's state-of-the-art lithium-air (Li-air) batteries are their low energy efficiency and limited cycle life due to the lack of earth-abundant cathode catalysts that can drive both oxygen reduction and evolution reactions (ORR and OER) at high rates at thermodynamic potentials. Here, inexpensive trimolybdenum phosphide (Mo₃ P) nanoparticles with an exceptional activity-ORR and OER current densities of 7.21 and 6.85 mA cm⁻² at 2.0 and 4.2 V versus Li/Li⁺ , respectively-in an oxygen-saturated non-aqueous electrolyte are reported. The Tafel plots indicate remarkably low charge transfer resistance-Tafel slopes of 35 and 38 mV dec⁻¹ for ORR and OER, respectively-resulting in the lowest ORR overpotential of 4.0 mV and OER overpotential of 5.1 mV reported to date. Using this catalyst, a Li-air battery cell with low discharge and charge overpotentials of 80 and 270 mV, respectively, and high energy efficiency of 90.2% in the first cycle is demonstrated. A long cycle life of 1200 is also achieved for this cell. Density functional theory calculations of ORR and OER on Mo₃ P (110) reveal that an oxide overlayer formed on the surface gives rise to the observed high ORR and OER electrocatalytic activity and small discharge/charge overpotentials.