Metal Oxygen Batteries

Metal−oxygen batteries have received significant attention owing to their potential to provide higher gravimetric energy density than conventional lithium−ion batteries. However, the development of metal−oxygen batteries are still in its early stage. Fundamental studies on the reaction/degradation mechanisms and material designs to improve the energy efficiency and cycling stability are of critical importance.

  • Mechanism of Oxygen Evolution Reaction (OER) and Catalytic Processes

We study the charging mechanism and the working mechanism of peroxide oxidation catalysis. We develop high-temporal resolution online electrochemical mass spectrometry (OEMS) to real-time monitor the gas consumption and evolution during the operation of the Li-O2 batteries (Fig. 1a).1 We show that solid catalysts (e.g. ruthenium Ru), effectively reduce the charge potential of Li-O2 batteries to evolve O2­ without the need of soluble species (Fig. 1b). Our work provides direct evidence of solid−solid nonaqueous OER catalysis and suggests that catalyst’s ability to stabilize solid reaction intermediates of Li2O2 oxidation is a key governing factor for OER activity.

Fig. 1. (a) Design of online electrochemical mass spectrometry1,2. (b) The efficacy of solid catalyst in solid-state Li-O2 batteries.1

  • Role of redox mediator in suppressing charging instabilities of lithium-oxygen batteries:

An alternative approach to reduce OER overpotential is using redox mediator. We show that charging Li-O2 batteries with redox mediators (using LiBr as a model system) significantly reduces parasitic gas evolution and improves oxygen recovery efficiency (Fig. 2a).2 We attribute the enhance mechanism to the transformation from electrochemical pathways to chemical pathways, which bypasses the formation of highly reactive intermediates upon electro-oxidation of Li2O2 (Fig. 2b). Such transformation reduces self-amplifying degradation reactions of electrode and electrolyte in Li-O2 cells. Our finding suggests that transforming electro-oxidation to chemical-oxidation is a promising strategy to simultaneously mitigate charging side reactions and achieve low overpotential for the Li-O2 batteries.

Fig. 2. Voltage and gas evolution profiles during charging with and without LiBr.2 Proposed reaction scheme with and without LiBr.2

Reference:

1 Wang Y., Liang Z., Zou Q., Cong G. and Lu Y.C.*, "Mechanistic Insights into Catalyst-Assisted Non-Aqueous Oxygen Evolution Reaction in Lithium-Oxygen Batteries" Journal of Physical Chemistry C, 2016, 120 (12), pp 6459–6466 Link

Liang Z. and Lu Y.C.*, "Critical Role of Redox Mediator in Suppressing Charging Instabilities of Lithium-Oxygen Batteries" Journal of the American Chemical Society, 138 (24), pp 7574–7583, 2016 Link