Peer review of: Unlocking the passivation nature of the cathode–air interfacial reactions in lithium ion batteries (2020)

Reviewer #1 (Remarks to the Author): 

In this manuscript, the authors reported on the chemical delithiation that results in the air-
instability of Ni-rich cathodes, and used the LiOH and disordered rocksalt structures to rationalize 
the surface passivation of the NMC cathodes. Many industrial and academic efforts have been 
made to increase the nickel content of NMC due to its higher capacity relative to Co and the limited 
stocks of Co, however, one of the remaining issues for commercializing high nickel NMC is the 
instability upon the air exposure. The use of dynamic, atomic-scale electron microscopy 
observations realizes the observations of each isolated reaction and thus delineates the origin of 
the air instability. The authors used the NMC cathodes as a model systems for this study, however, 
the results can possibly provide a broader guideline for tackling the air related issues of many 
advanced cathodes such as lithium rich cathodes because the fundamentals of chemical-driven 
delithiation may have a general applicability, so it satisfies the requirements to be published in a 
high-impact journal. The texts and figures were organized in a logical way, I recommend 
acceptance of the manuscript after some minor revisions. 

1.The current data presents a comprehensive picture of the passivation processes on the (003) 
terminated surface, to achieve effective surface protection, the edge planes of NMC need to be 
covered as well. However, the lithiation diffusion barriers can be different in the along-channel and 
across-channel directions. Would the edge planes, e.g., the (003) exit surfaces, be passivated as 
well? 

2.The real-time TEM observations suggest that the CO2 barely contribute to the initial stage of 
chemical delithiation, however, it would eventually alter the final products in the subsequent 
reactions with the passivation layers, what are the effects of such reactions on the developed 
passivation layers? Would this change the passivation nature of surface films? 

3.The TEM lamella used for the in situ experiments is prepared by FIB. Despite the final cleaning 
process performed at 2kV, a thin surface damage layer would be remaining there and the trace Ga 
could be embedded in the samples, how would the impurities affect the formation of the surface 
passivation layers? 

4.The details for the TEM experiments are missing. 

5.The format of references has many issues, please check and fix them carefully. 



Reviewer #2 (Remarks to the Author): 

The manuscript entitled “Unlocking the nature of the cathode-air interfacial reaction in lithium ion 
batteries” reports the in situ TEM observation of cathode surfaces exposed to various gas 
atmospheres. This paper claims that water vapor is the only active species that react with cathode 
surface forming a LiOH passive layer via chemical delithiation. Although the gas pressure used 
here is still much lower than the actual condition, the experimental data well corroborate the 
current understanding of air-instability of Ni-rich NMC cathodes. More interestingly, the authors 
show that a thin disordered rock-salt layer formed on the NMC811 surface can effectively mitigate 
the surface degradation by suppressing chemical delithiation. This observation may open a new 
venue for the development of novel high-energy and high-stability cathodes. Therefore, I 
recommend publication of this paper after the following issues are addressed properly. 

Comments: 
(1) The moisture sensitivity of Ni-rich cathodes and the formation mechanism of residual lithium 
compounds such as LiOH and Li2CO3 via reaction of delithiated lithium with water and CO2 have 
already been proposed and extensively studied in the literature. I don’t see this paper challenges 
the current understanding of surface degradation up on air-exposure. It rather corroborates the 
existing ex situ experiments data. In an actual air-storage situation, moisture in the air would 
initiate chemical delithiation to form a LiOH layer and subsequently, LiOH reacts with CO2 forming 
Li2CO3. The main text should be revised to address this. 

(2) An as-received pristine Ni-rich NMC particle is not even clean on the surface; LiOH and Li2CO3 
are present as a residual Li. The content and composition of such residual Li species also have a 
significant effect on the air-instability of NMCs. How were the NMC samples prepared and treated 
before the in situ TEM experiment? Please provide more information about the sample sources and 
the initial surface conditions should be characterized more carefully. 

(3) The stabilization of the cycled electrode is attributed to the disordered rock-salt layer. 
However, the surface of the multiple-cycled cathode would be already protected by a thick SEI 
layer, which may not be completely cleaned away by rinsing with the DMC solvent.