The kinetics and product characteristics of oxygen reduction and evolution in LiO2 batteries

Abstract

Understanding the origin of substantial performance challenges limiting the practical development of Li-O2 batteries, such as low rate capability, limited cycle life (<100 cycles), and the large voltage polarization (0.6-1 V) on charge, requires improved understanding of chemical, electrochemical, morphological, and electronic processes occurring in the electrode. This chapter highlights current understanding of how the kinetics and reaction product characteristics in Li-O2 batteries during discharge and charge influence performance characteristics at the cell level. First, a brief overview of energy and power of various Li-O2 electrodes reported in the literature to date is presented for a range of O2 electrode materials and designs as a benchmark for what has been achieved at the laboratory scale. Next, we review chemical and morphological understanding of the oxygen reduction (discharge) process, with a particular focus on nanostructured carbon electrodes in 1,2-dimethoxyethane (DME) electrolyte. The kinetics of oxygen reduction and the influence of kinetics on the morphology and shape evolution of Li2O2 are discussed, including recent insights into the microscale structure and proposed growth mechanisms of toroidal crystalline Li2O2 at low currents or overpotentials. We next discuss the surface chemistry of discharged oxygen electrodes, including the morphology-dependent surface chemistry of Li2O2, reactivity between Li2O2 and the carbon electrode, reactivity between Li2O2 and ether-based electrolytes, and resulting parasitic products that form upon discharge and during subsequent cycling. In light of chemical instabilities present nearly universally in liquid cells, we highlight recent work utilizing in situ ambient pressure XPS (APXPS) to examine Li-O2 electrochemistry during battery operation in an all-solid-state cell. Finally, we discuss the influence of morphology and surface chemistry of the discharge product on the charging kinetics in carbon-nanostructured electrodes, where morphology-dependent Li2O2 surface chemistry and structure are found to significantly influence the overpotential required during oxidation. Combined chemical, electrochemical, morphological, and electronic understanding is increasingly important as researchers seek to develop improved O2 electrodes with increased round-trip efficiency and improved chemical/electrochemical reversibility approaching what is needed for practical devices.