Peer review of: Structural evolution at the oxidative and reductive limits in the first electrochemical cycle of Li1.2 (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

This work reports a very detailed investigation on the structural change during the deep charge and 
deep discharge of lithium-rich cathode material. It was found that the migration of Mn, coupled with 
the loss of oxygen, is closely associated with the extra capacity offered by the lithium-rich cathode. 
Detailed structural characterization also shed light on the phase transformation pathway when deeply 
delithiated at 4.8V and overlithiated at 1.2 V. 

In the discussion session, the authors also discussed and application potential of the extra capacity. It 
will be helpful if the authors also comment on the negative side of the extra wide potential swing for 
real applications. 

Overall, this is a well prepared work with deep insight. It is recommended for publication with minor 
revision. 


Reviewer #2 (Remarks to the Author): 

This work reports the phase evolution in Li-rich layered oxide under harsh condition. The authors 
report carefully designed experiments and the most distinct finding is a densified phase in the material 
bulk. The formation of a new phase under extreme oxidation is clear. However, the claim of this new 
phase in the bulk, not simply on the surface as reported before, is questionable. Regardless, the 
concern is the significance of the discovery of this densified phase, which results in no benefit in cell 
performance at all. All the other claims in this work, regarding the strategy to mitigate the first cycle 
irreversible capacity and recover the structural ordering by annealing are known for Li-rich layered 
oxides. This manuscript is not suitable for the publication at Nat. Commun. 

Below are the questions and comments on the technical aspects of this work: 

Regarding the claim of densified phase in the bulk material, the experimental evidence is not 
convincing. This claim is mainly drawn based on Rietveld refinement and STEM. For Rietveld 
refinement, the authors present two scenarios according to the chemical formula determined by ICP 
and OEMS, one has oxygen vacancies and the other does not, instead, it represents a densified phase 
with increased M/O ratio. The authors argue the second scenario is the case based on lattice 
parameters. What is the determining factor to exclude the first scenario? Also, how can the author 
draw the conclusion it is a bulk phenomenon rather than surface based on Fig. 1f? Where is the STEM 
image taken in the bulk rather than showing a schematic bulk in Fig. 1f? What is the scale in these 
STEM images being considered? 

A whole-paragraph discussion in Page 7 is based on Fig. S4 and Fig. S5. The quality of the STEM 
images presented is very poor. The contrast here almost makes it impossible to follow the discussion 
on TM occupancy. What is the particle size of the pristine material? The bulk information presented in 
Fig. S4c is at most 10 nm. 

The authors discussed the effect of OCV at high voltage (4.8 V), it is important to compare the 
electrochemistry of this material at various testing protocols. From Fig. S16, OCV or CV at 4.8 V 
results in a significant capacity drop and some voltage decay. What is the origin of such key difference? 
What is the correlation with phase A and A’? 

For comparison, what is the STEM of phase A at the end of 4.6 V charge? Do the authors anticipate 
the contrasting observation between phase A and phase A’ (both bulk and surface) in comparison of 
4.6 CC and 4.8 CC-CV? 

The authors claimed phase A’ becomes reversible after the first charge. In Fig. S10, does Mn migration 
only remain at the surface or in the bulk as well? If in the bulk, the Mn cation trapped in the interlayer 
octahedral sites in Figure S10 should be highlighted to guide the readers. 



Reviewer #3 (Remarks to the Author): 

The paper by Yin et al reports on a study of a prototypical Li-excess compound 
(Li1.2Ni0.13Co0.13Mn0.54O2) at extreme states of charge and discharge. The study reveals the 
emergence of a new phase, referred to as A’, when the compound is held at 4.8 volt for 5 hours. The 
authors refine the A’ phase as being O3 with some Mn having migrated from the transition metal layer 
to the Li layer. Irreversible capacity loss when charged to high voltage is recovered when the 
compound is cycled and held at very low voltages, leading to the formation of what the authors refer 
to as P’ and then P’’. The discovery of the A’ phase at high voltages and the peculiar capacity recovery 
at low voltages are to my knowledge new results and provide invaluable clues to enable a better 
understanding of the electrochemistry of Li-excess compounds. 

The paper could be suitable for publication in Nature Communications after the authors address the 
following points: 

1. Throughout the paper the A’ phase is referred to as a “densified phase”. While this may be true, no 
clear evidence is provided in the paper. The arguments used to conclude that the A’ phase is 
“densified” are not convincing or logical (bottom of page 6). As I understand the argument, it is 
concluded that A’ is densified since its unit cell volume is smaller than that of the pristine phase. Most 
layered intercalation compounds contract at high states of charge (certainly along their c-axis), and 
this occurs without any densification (i.e. transition metal cations filling vacant sites and oxygen 
vacancies diffusing to the surface). More convincing evidence would be based on a careful refinement 
of site occupancies. If the authors insist on referring to the A’ phase as a “densified phase” throughout 
the paper (as is the case in the current version), they need to provide stronger evidence for this 
statement. 

2. The twinning at high states of charge is interesting and likely of importance to better understanding 
structural changes that occur at high states of charge. However, the paper is not clear in its 
description of the twinned structure. For example, it refers to “the mirror-twinned rhombohedral 
structure with the (001) twin plane.” However it is not clear whether the (001) indices are for a 
rhomborhedral unit cell, or for the hexagonal unit cell of the O3 crystal. Can the authors consider 
adding a schematic that illustrates the crystallography of the twins and how they may relate to the 
original A phase? 

3. In the refinement of the A’ phase, was tetrahedral Mn occupancy considered? Very few details are 
provided about the refinements. Are the authors able to speculate on the oxidation state of the 
tetrahedral Mn in the surface layer? It is not clear why the authors rule out tetrahedral Mn in the bulk 
phase. It seems that similar features that are attributed to tetrahedral Mn at the surface are also 
apparent in the bulk (Figure 1f and S4). 

4. The HAADF-STEM images (e.g. S4) appear to show void formation, similar to that observed by Yan 
et al Nature Nanotechnology 602 (2019). Can the authors comment on this (i.e. confirm or rule out 
that voids have in fact emerged within the bulk of the A’ phase)? 

5. Can the authors comment on the recent study by Taylor et al JACS, 141, 7333 (2019), which also 
shows recovered capacity at very low voltage (~1.7 V). In this study though, there was clear evidence 
of the reduction of peroxo-molecules. Could something similar be occurring in Li-excess Mn containing 
compounds? 

6. The authors measure O2 release and provide quantitative values. Can the authors translate these 
numbers into an estimate of how much the crystals have densified?