Morphology-Dictated Mechanism of Efficient Reaction Sites for Li 2 O 2 Decomposition.

In the pursuit of a highly reversible lithium-oxygen (Li-O 2 ) battery, control of reaction sites to maintain stable conversion between O 2 and Li 2 O 2 at the cathode side is imperatively desirable. However, the mechanism involving the reaction site during charging remains elusive, which, in turn, imposes challenges in recognition of the origin of overpotential. Herein, via combined investigations by in situ atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS), we propose a universal morphology-dictated mechanism of efficient reaction sites for Li 2 O 2 decomposition. It is found that Li 2 O 2 deposits with different morphologies share similar localized conductivities, much higher than that reported for bulk Li 2 O 2 , enabling the reaction site not only at the electrode/Li 2 O 2 /electrolyte interface but also at the Li 2 O 2 /electrolyte interface. However, while the mass transport process is more enhanced at the former, the charge-transfer resistance at the latter is sensitively related to the surface structure and thus the reactivity of the Li 2 O 2 deposit. Consequently, for compact disk-like deposits, the electrode/Li 2 O 2 /electrolyte interface serves as the dominant decomposition site, which causes premature departure of Li 2 O 2 and loss of reversibility; on the contrary, for porous flower-like and film-like Li 2 O 2 deposits bearing a larger surface area and richer surface-active structures, both the interfaces are efficient for decomposition without premature departure of the deposit so that the overpotential arises primarily from the sluggish oxidation kinetics and the decomposition is more reversible. The present work provides instructive insights into the understanding of the mechanism of reaction sites during the charge process, which offers guidance for the design of reversible Li-O 2 batteries.