Morphology-Dictated Mechanism of Efficient Reaction Sites for Li₂O₂ Decomposition

In the pursuit of a highly reversible lithium-oxygen(Li-O-2) battery, control of reaction sites to maintainstable conversionbetween O-2 and Li2O2 at the cathodeside is imperatively desirable. However, the mechanism involving thereaction site during charging remains elusive, which, in turn, imposeschallenges 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 universalmorphology-dictated mechanism of efficient reaction sites for Li2O2 decomposition. It is found that Li2O2 deposits with different morphologies share similarlocalized conductivities, much higher than that reported for bulkLi(2)O(2), enabling the reaction site not only atthe electrode/Li2O2/electrolyte interface butalso at the Li2O2/electrolyte interface. However,while the mass transport process is more enhanced at the former, thecharge-transfer resistance at the latter is sensitively related tothe surface structure and thus the reactivity of the Li2O2 deposit. Consequently, for compact disk-like deposits,the electrode/Li2O2/electrolyte interface servesas the dominant decomposition site, which causes premature departureof Li2O2 and loss of reversibility; on the contrary,for porous flower-like and film-like Li2O2 depositsbearing a larger surface area and richer surface-active structures,both the interfaces are efficient for decomposition without prematuredeparture of the deposit so that the overpotential arises primarilyfrom the sluggish oxidation kinetics and the decomposition is morereversible. The present work provides instructive insights into theunderstanding of the mechanism of reaction sites during the chargeprocess, which offers guidance for the design of reversible Li-O-2 batteries.