1. Introduction

Encountering energy and environmental issues such as the heavy use of fossil fuel and relating pollution problem, human society necessitates the development of sustainable energy resources such as solar and wind energy. In this regard, energy storage systems to store electricity is a key technology to realize renewable future energy society. After continued development during the last decades, Li ion battery is currently widely used as the most promising energy storage system. However, the further development of Li ion battery’s energy density is limited by its redox chemistry exploiting heavy transition metal as redox center, which triggers the extensive research efforts to develop next-generation energy storage systems. Among them, Li–O₂ battery is considered the most promising candidate owing to its extremely high theoretical energy density. For the development of Li–O₂ battery, exploring catalysts to facilitate its electrochemical reaction is inevitable because of its sluggish cathode reaction. The introduction part provides the basics of Li–O₂ battery and redox mediators that are required to understand the following sections.