Peer review of: Tuning lithium-peroxide formation and decomposition routes with single-atom catalysts for lithiu (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The manuscript NCOMMS-19-33424 by Song et al. discusses performance of Co electrocatalyst 
encapsulated by polymer in Li-O2 cells. The study includes variety of characterization methods to 
characterize, however, a major question regarding true reversibility of the cells is not discussed. 
 
Common electrochemical tests such as galvanostatic cycling or cyclic voltammetry do not provide any 
conclusive information regarding the quantity (or degree) of formation and decomposition of reversible 
products. Researchers often assume that if discharge and charge capacities are “balanced” this means 
reversible batteries. It has however shown in many literatures that oxygen batteries can easily suffer 
formation of side products formed via parasitic reactions. Therefore, it is crucial to quantify the 
reversibility of reactions. Some of analytical techniques such DEMS (to quantify the amount of 
consumed/formed O2) and in-situ XRD (with some added reference materials to quantify the amount 
of reactions products) can be useful. 
 
Therefore, without quantifying the main reaction products it is not possible to reveal the performance 
of the proposed catalyst. Thus, the manuscript in its current format is not suitable for publication. 
 
 
Reviewer #2 (Remarks to the Author): 
 
The manuscript described a single-atom catalyst in Li-O2 batteries, reducing the discharge and charge 
overpotentials and improve the cycling efficiency. The cell performance is quite nice, however, I think, 
the claims are not well supported by the data and I am not convinced by this work. The current 
version cannot be accepted from my perspective and for the authors, the following issues need to be 
solved. 
1. The authors claimed they have synthesized a single-atom catalyst, however, it is a bit too much to 
claim it as single-atom catalyst purely based on HAADF-STEM result. In fact, the PXRD stills shows a 
bit Co signal in Fig. 2g and Figures in SI, and a higher resolution one is needed. XANES, EXAFS, X-ray 
absorption spectra data, etc. are necessary to prove the idea of single-atom catalyst. The bond length, 
angle, charge state, tec. between Co and N need further discussion. 
2. As comparisons, the cell performance in this manuscript is among the average in the catalyst in Li-
O2 batteries. It is not fair simply compare this with commercial Pt/C catalyst. The Pt/C is not designed 
for Li-O2 batteries and the authors should compare with other catalysts in Li-O2 batteries to prove the 
importance and benefits of this single-atom catalyst. 
3. The measurements in Q1 should be done on the catalysts after cycling to confirm whether the 
catalyst has been poisoned or not. 
4. The quantification of Li2O2 is necessary as well after the 1st, 5th and 50th discharge and charge to 
measure how much Li2O2 has been formed and decomposed during cycling. 
5. The PXRD quality is low and the signal and noise ratio should be optimised. For FTIR, it would be 
good to have the band from 4000 cm-1 to 400 cm-1 to demonstrate whether LiOH and other 
carbonates exists. 
6. How can this catalyst have effect both on discharge and charge? The authors need to clarify the 
mechanism with more evidence. 
7. I do not like studies in Li-O2 batteries where the discharge capacity claimed to be over 15000 
mAh/g, while cycling at a 1000 mAh/g capacity. Why not have the full capacity cycling? I don’t 
understand Fig. 3c, voltage vs. capacity in different rates? As the authors claimed this superior rate 
performance, then why not cycling at relatively higher rates with a much bigger capacity? 
In general, without solid evidence of single-atom catalyst plus the average cell performance, I think it 
is too brave to prove the single-atom catalyst idea and the authors have not convinced me the 
importance of this studies.