Implications of the formation of small polarons in Li ₂O ₂ for Li-air batteries

Lithium-air batteries (LABs) are an intriguing next-generation technology due to their high theoretical energy density of ∼11 kWh/kg. However, LABs are hindered by both poor rate capability and significant polarization in cell voltage, primarily due to the formation of Li ₂O ₂ in the air cathode. Here, by employing hybrid density functional theory, we show that the formation of small polarons in Li ₂O ₂ limits electron transport. Consequently, the low electron mobility μ = 10 ^-10- 10 ^-9 cm2/V s contributes to both the poor rate capability and the polarization that limit the LAB power and energy densities. The self-trapping of electrons in the small polarons arises from the molecular nature of the conduction band states of Li ₂O ₂ and the strong spin polarization of the O 2p state. Our understanding of the polaronic electron transport in Li ₂O ₂ suggests that designing alternative carrier conduction paths for the cathode reaction could significantly improve the performance of LABs at high current densities.