The working mechanism of nonaqueous Li-O₂ battery (LOB) technology is based on the redox reaction between O₂ and peroxide (Li₂O₂). Within the past 10 years, our group closely keeping up with the development pace of LOBs, which can be generally separated into two stages: the initial “solid catalyst” period and the advanced “redox mediator” period. However, among the varying scientific and engineering challenges, the employment of gaseous O₂ (as active species) becomes the most essential obstacle against the practical application of LOB, since the gas open system requires burdensome O₂ storage/purification devices. Besides, the O₂ crossover and volatilization of electrolyte also trigger us to design a non-O₂ sealed cell environment (illustrated in Fig. 1).

Recently, battery researchers become infatuate in triggering the lattice oxygen activity from oxide state to a peroxo-like state within transition metal (TM) oxides cathodes (Li/Na-rich cathodes, P2-type layered cathodes, etc.), which is so-called O-related anionic redox activity. Even though being stabilized by the covalent TM-O bond, the practical potential (typically beyond 4.0 V vs. Li/Li⁺) presents far more polarized beyond the thermodynamic equilibrium potential of peroxide decomposition (around 3.0 V vs. Li/Li⁺). As a result, irreversible O₂ evolution and related structural distortion would seriously influence the rechargeability. Moreover, dragged by the heavy TM-ion within lattice, the specific capacity triggered by O-related anionic redox presents very limited (below 100 mAh/g_~cathode). Thus, why not give up using lattice TM, but only introduce trace amount of TM nanoparticles to warp the surface of pure Li₂O and restrain the O₂ evolution? Followed by this idea, we took nearly 2 years and tried to control the Li₂O/Li₂O₂ (oxide/peroxide) conversion within a non-O₂ redox region.

By pre-embedding Li₂O nanoparticles into an iridium-graphene (Ir-rGO) catalytic substrate, we successfully controlled the charging potential within a moderate region to avoid over-polarization (see Fig. 2). By controlling the charging depth (400 mAh/g_~cathode), we restrained the irreversible O₂ evolution and superoxide production. Being stabilized by the inter-metallic Li_2-xO₂-Ir compound formed on the particle/substrate interface, we finally confined the O₂-free reversible Li₂O/Li₂O₂ interconversion within sealed cell environment. Although, the deep catalysis mechanism still requires further atomistic investigations, we have in mind that our design route would open up a new field for the development of anionic redox related battery researches, and triggers a design direction for synergistically combining the electrode and electrocatalyst materials, engineering the next-generation high-energy-density battery technologies.

Read the complete story at Nature Catalysis.