Operando liquid cell electron microscopy of discharge and charge kinetics in lithium-oxygen batteries

Abstract Despite the promising future of lithium-oxygen (Li-O 2 ) battery in replacing conventional lithium ion battery for high-energy applications, the complicated reaction mechanisms determining the sluggish discharge-charge kinetics have not been fully understood. Here, utilizing in situ liquid transmission electron microscopy, the (electro)chemical fundamentals in a working Li-O 2 battery is explored. During discharge, the nucleation of Li 2 O 2 is observed at the carbon electrode/electrolyte interface, and the following growth process exhibits Li + diffusion-limited kinetics. Nucleation and growth of Li 2 O 2 are also observed within the electrolyte, where there is no direct contact with the carbon electrode indicating the existence of non-Faradaic disproportionation reaction of intermediate LiO 2 into Li 2 O 2 . The growth of Li 2 O 2 isolated in the electrolyte exhibits O 2 - diffusion-limited kinetics. Li 2 O 2 at the carbon electrode surface and isolated in the electrolyte are both active upon charging and gradually decomposed. For Li 2 O 2 particles rooted at the carbon electrode surface, the decomposition starts at the electrode/Li 2 O 2 interface indicating electron-conduction limited charge kinetics. For Li 2 O 2 isolated within the electrolyte, surprisingly, a side-to-side decomposition mode is identified indicating the non-Faradaic formation of dissolvable O 2 - , whose diffusion in the electrolyte controls the overall charge kinetics. This work reveals further details of underlying mechanisms in a working Li-O 2 battery and identifies various limiting factors controlling the discharge and charge processes.