Cobalt Borate Complex With Tetrahedrally Coordinated Co2+- Promotes Lithium Superoxide Formation in Li-O2 Batteries

Abstract

The development of non-aqueous lithium-oxygen (Li-O₂) batteries is hindered by inefficient discharge product decomposition, side reactions with the electrolyte, and high charge overpotentials (>1 V). This study explores the use of sodium cobalt borate (Na₃CoB₅O₁₀, NCBO) with cobalt in tetrahedral geometry as an oxygen electrocatalyst for non-aqueous Li-O₂ batteries. The prepared cobalt borate exhibits an oxygen evolution reaction (OER) overpotential of 326 mV_RHE at a current density of 10 mA cm⁻² and a Tafel slope of 42 mV dec⁻¹ in 1 m KOH. Density Functional Theory (DFT) calculations identify the OH-covered (101) surface of NCBO as the preferred OER site, with an overpotential between 451 and 544 mV. In Li-O₂ batteries, the NCBO cathode demonstrates 200 cycles with an overpotential of 1.95 V and 56.00% round-trip efficiency at a capacity limit of 500 mA h g⁻¹, along with a smaller charge overpotential of 0.64 V at a capacity limit of 2000 mA h g⁻¹. Post-cycling analysis of NCBO electrodes reveals electronically conductive Lithium Superoxide (LiO₂) as the dominant discharge product. As revealed by DFT studies, the promising performance of NCBO in Li-O₂ batteries is attributed to its tetrahedral Co coordination, highlighting its potential for electrocatalytic applications.