Investigating the Oxygen Reduction Reaction in Lithium-Oxygen Cells
Authors: Sankarasubramanian Shrihari , Singh Nikhilendra , Seo Jeongwook , Prakash Jai , Mizuno Fuminori 
Published: February 2020

The rechargeable Lithium (Li)-Oxygen (O2) battery has great potential as a high energy battery system due to its high theoretical specific energy of 3505 Wh Kg-1, which is significantly higher than current Li-ion cells.1,2 However, to succeed as a practical replacement for Li-ion cells, Li-O2 cells must overcome known problems such as (i) poor cycle life, (ii) high charge and discharge overpotentials (iii) low coulombic efficiency and (iv) low power capability.3 In recent studies, the poor cycle life and high overpotentials could be traced to the oxidative instability of the electrolyte in conjunction with electrode passivation by lithium peroxide (Li2O2).4-6 Early theoretical predictions for the Oxygen Reduction Reaction (ORR) at the cathode, envisioned a Li2O2 covered surface after the initial cycle and further ORR occurring on the deposited Li2O2, while also predicting large overpotentials in accordance with the experimental observations.7,8 However, the issue of electron transfer across poorly conducting Li2O2 had to be explained using mechanisms such as d-band bending and poloron hopping.7,9

                The use of classical electrochemical techniques, such as Rotating Disk Electrode (RDE) and Rotating Ring-Disk Electrode (RRDE) to study the kinetics and catalytic activity of various surfaces has been common for aqueous ORR.10-12 However, application of such techniques to the Li-O2 ORR has been challenging due to surface changes during peroxide deposition, previously reported anomalous Tafel behavior, and extreme sensitivity to experimental conditions.13,14 To date, no clear mechanistic understanding or explanation of the ORR has been established, via theoretical simulations or electrochemical experiments.

                Here, we intend to establish such a mechanistic understanding of the ORR by combining information from theoretical simulations and classical electrochemistry. We report first principles, Density Functional Theory (DFT) modeling of the Li-O2 ORR on carbon, noble metals and their alloys under vacuum and in solvents, along with the correlation between intermediate species binding energies and favored reaction pathways from amongst 1e-, 2e- and 4e- mechanisms. Further, classical electrochemical techniques were utilized to experimentally study (and support) the reaction kinetics and mechanisms via RDE and RRDE experiments, which are difficult to explain through theoretical simulations alone. It is the authors’ hope that such use of theoretical simulations and classical electrochemistry in tandem, will serve as a guide towards future experimental studies; especially those directed towards the screening of potential catalytic surfaces for efficient ORR.

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