Tailoring the nucleation and growth routes of discharge products for lithium-oxygen batteries through the facet engineering of Ni2P catalysts

Abstract

Li-O2 batteries (LOBs) with high theoretic energy densities are regarded as potential energy storage devices. However, the insulative discharge product, Li2O2, would accumulate on the cathode, restricting the depth of discharge and raising barriers for charge. The rational catalyst design is proposed as an efficient strategy to tailor growth routes and morphology of Li2O2, and consequently address the above issue. Herein, porous Ni2P nanorods with exposed (0001) or (101¯0) facets are prepared on Ni foams through the heat treatment of Ni2P2O7 arrays in a PH3 atmosphere. The exposed facets of Ni2P nanorods could be well controlled through the variation of the hydrothermal reaction time for Ni2P2O7. Employed as free-standing cathodes for LOBs, Ni2P nanorods with exposed (0001) facets exhibit superior electrochemical performance than those nanorods with exposed (101¯0) facets due to the interaction between exposed facets and the nucleation, growth of LiO2 or Li2O2, consequently tuning the morphology of discharge products. The high electron transfer rate and weak adsorption energy of LiO2 on exposed (0001) facets could promote the generation rate of the (solvated) LiO2, inducing the formation of large Li2O2 sheets (over 30 μm) and further growth of flower-like nanoflake assemblies (∼500 nm) on the surface of Li2O2 sheets assignable to the variation of the surface environment of the cathode during the discharge process, exhibiting more Li2O2 deposited at full coverage of cathode surface. It is proposed that the facet engineering is a feasible way to improve the electrochemical performance of cathode catalysts for LOBs.