Nonaqueous Lithium–Oxygen batteries: Reaction mechanism and critical open questions

Nonaqueous lithium–oxygen (Li–O2) batteries have received intensive research attention owing to their potential to provide gravimetric energy density 2–5 times that of conventional Li-ion batteries. However, Li–O2 batteries are suffering from poor cycle life, low rate capability and poor round-trip efficiency. In-depth understanding on the reaction and degradation mechanism is the prerequisite for rational electrode and electrolyte design for efficient and long-life Li–O2 batteries. In this review, we analyze recent progress in oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) mechanism. We discuss on-going debates and open questions related to OER processes including the reaction intermediates, the origin for the side reactions, the reaction interface and the catalysis mechanism. We provide insights into the sources of discrepancies and highlight future research directions in deepening the understanding of Li–O2 reactions and strategies in mitigating the severe instability between the reaction intermediates and cell components for efficient and long-life Li–O2 batteries.