Peer review of: Impact of dual-layer solid-electrolyte interphase inhomogeneities on early-stage defect formation (2020)

Reviewers' Comments: 
 
Reviewer #1: 
Remarks to the Author: 
Si is believed to be the next generation anode material for Li-ion batteries and fundamental 
understanding of the fading mechanism is essential for developing better Si anodes. With recent 
advances in Si structure design and performance improvement, understanding and improving SEI 
resiliency are currently of growing interest. From this point, the work potentially can have a broad 
impact. The authors in this paper used operando FFDXM, AFM and ESM to help understand the fading 
mechanism of Si structure and interphase evolution with cycling. It is unique capability, yet I don’t 
find enough new knowledge beyond expectation to make it publishable in Nature Communications. 
Below please find my detail comments. 
First, I found it quite hard to understand some of the authors claims. What do the authors mean by 
early stage detection? The FFDXM signals start to show up in the 2nd cycle and after full lithiation. 
While Si has experienced expansion in the first lithiation and shrinkage in the first delithiation. What 
do the authors mean by “defects”, “structural defects”, what defects and defect of what? 
Second, the definition of SEI to me is the solid electrolyte interphase layer on top of the anode that 
has the specific function. With that in mind, I tend to believe the SEI is SEI after the 1st full lithiation 
process. It is good to study the formation process of the SEI. Yet, I think it is more appropriate to call 
“pre-SEI” or “early SEI” for what detected during the first lithiation process in the operando AFM and 
ESM characterization. 
Third, it is general belief that the SEI mostly forms in the first cycle and some in the 2nd cycle. The 
authors also showed support of SEI formation in 1st cycle in AFM measurement. Si structure change 
also happens in the first lithiation even though FFDXM start to show “defects” in the 2nd cycle. It is 
true that electrolyte decomposition and “early SEI” formation happen before Si lithiates around ~0.1V. 
While it is hard to provide the timeline/correlation for the “SEI” inhomogeneity and the Si structural 
change because until the formation of SEI after 1st full lithiation, it is not sure whether what formed 
on the electrode surface has the SEI functionality. I have reservation for the indication that SEI 
inhomogeneity leads to structural defects. Also, in spite of the unique in-operando study, the 
conclusions on SEI dual-layer structure and inhomogeneity have been reported by ex-situ AFM study 
of SEI in various systems. 
At last, the paper’s preparation needs to be further improved. For example, the minimum voltage cut 
off in Figure 2 and Figure 4 are not clearly labeled. Zoom-in figures are needed. The same for the 
current curve of the CV scan. 
Some of the detail information is missing. What is the beam size? What is the defect concentration? 
Can the authors provide some quantitative analysis on the inhomogeneity or structural defects? 
I am not sure why the authors claim the deformation is elastic (page 10). I would like to see some 
data support to the claim. 
Have the authors measured Young’s modulus of the model inorganic materials in the SEI inorganic 
layer? Is it consistent with what obtained in Figure 6? 
 
 
 
Reviewer #2: 
Remarks to the Author: 
The paper studies a Si-based LIB system by the combination of different in operando and ex situ 
techniques. These include a novel, high sensitivity, high spatial resolution, in operando technique, 
FFDXM, providing complementary information about local structural deformation within single-crystal 
Si electrode. This multi-technique study sheds light on the role of SEI inhomogeneities on early-stage 
defect formation within the Si electrode and highlights the significance of those defects on the 
subsequent larger scale deformation observable by non-local techniques. Such multi-technique in 
operando study is of critical importance to develop strategies to prevent early cell failure and to 
improve the cyclability of Si-based LIB’s. As such, the results presented in the manuscript will be of 
interest for others and will influence this field. 
 
I recommend the manuscript for publication with modifications. Please find my arguments below. 
 
The major claims of the paper: 
 
The authors include full field X-ray diffraction microscopy (FFDXM), a novel and promising new method 
into their study. This qualitative, in operando technique was combined with other in operando and ex-
vivo techniques (e.g. operando AFM, ESM, XPS, Young modulus analysis, 3D Reciprocal Space 
Mapping) to obtain direct evidence for early-stage structural defect formation in single-crystal pristine 
Si just below the lithiated Si layer. The early-stage defects appear just after the start of the SEI 
formation. The paper proposes a model describing the underlying processes based on direct 
observations. 
 
However, I am missing that the importance of the multi-technique approach is not highlighted in the 
paper. For example, mentioning only FFDXM in the abstract gives the impression that most of the 
important results have been obtained by this technique. I suggest the reformulation of the abstract in 
order to make it clear that such multi-technique approach, including in operando local methods, was 
crucial for this complex study. 
 
Certain ambiguous statements should be specified or reformulated. 
• For example in lines 30-31 it is claimed that “little attention has been paid to understanding the role 
of the SEI formed at the Si electrode surface”. However, e.g. the review of Zhang et al. “Design 
superior solid electrolyte interface on Silicon anodes for high-performance Lithium-ion battery” 
Nanoscale 11(41), 2019, cites quite a few papers on this subject. I recommend that the authors 
include this reference into their paper. 
• Another example (lines 38-40); the existence of a complex dual-layer SEI with a mostly inorganic 
inner-SEI layer and a more organic outer-SEI layer is not a new finding in itself (see the review of 
Zhang et al. and the papers cited therein). 
• I recommend that the authors claim clearly and unambiguously the new results obtained by their 
study in the context of already existing results. 
 
Certain statements are not convincing: 
• It is not clear how the information obtained by the different analytical techniques were linked 
together for obtaining the conclusion of the paper. The interpretation of multi-technique datasets is a 
real challenge; for example different spatial resolutions (from some nanometres to some hundred 
µm's), different sizes of the measured sample area, different information depths, and different 
volume- or surface-sensitivities have to be related to each other. Moreover, in operando and ex- vivo 
methods do not measure the same sample surface. The authors should include a discussion to 
describe how they handled this challenge for obtaining a reliable multi-technique data-set. 
• Lines 375-377: I find that this statement is not convincing and not well supported by the provided 
data. The paper includes no measurements on the Young modulus and lithium-ion mobility after the 
2nd and 3rd cycles, where the early-stage structural defects appear in the Si electrode. Moreover, the 
XPS technique does not provide information on the lateral compositional inhomogeneity of the 
measured SEI layer. All these techniques are surface sensitive compared to the high penetration depth 
of FFDXM. Could you please comment on the information depth of FFDXM? What thickness of the 
pristine Siwas probed by FFDXM? It is not obvious how these information can be linked together to 
obtain the conclusion cited by the authors (see the remark above). Could you please comment on 
this?  
A discussion would be useful concerning the validity of the obtained results and proposed model for 
other Si-based LIB systems; e.g. the constituents of the electrolyte will influence significantly the 
composition, thickness and morphology of the SEI. Could you please comment on the eventual 
presence of native SiO2 species on the single-crystal Si electrode surface, and its effect on the SEI 
composition? 
 
In general, the paper contains too much inaccurate expressions such as “little”, “fast”, “similar”, “low”, 
“a roughness of about..”. I recommend that the authors improve this by providing quantitative 
information whenever possible. 
 
Please find below some detailed remarks: 
 
Lines 103-105: 1 frame/s cannot be considered as “fast” measurement; today 100-1000 frame/s 
frame-rate is available with certain 2D detectors. What is the limiting factor of the measurement 
speed? Do the authors foresee faster studies e.g. to understand the dynamics of the abrupt changes 
occurring during (de)lithiation (Figure 2, 4)? 
 
Figure 2a-c and lines 152-164: I do not understand the interest of detailed analysis of the sum of the 
two individual defects (Int curves in the bottom of Fig 2a-c); the development and evolution of the 
individual defects during cycling is analysed in detail in the following paragraphs. I recommend 
shrinking or omitting this paragraph and the related subfigures, unless the author can illustrate the 
significant added value of this paragraph. 
 
The 1:4.3 aspect ratios of the FFDXM images are quite confusing. I recommend that the authors 
correct all these images and show them with 1:1 ratio. 
 
Figure 5: 
• It is quite confusing that images 5j-k do not show the same sample dimensions (they correspond to 
a 5 x smaller region) as images 5a-g. Moreover, it is not clear whether the spatial resolutions of 5a-g 
are the same as that of 5j-k. Could you please comment on this? How these surface sensitive 
information obtained from small (1-10 µm x 1-10 µm) sample areas can be linked and compared to 
the FFDXM-based information originating from 100 µm x 430 µm areas? 
• I do not see obvious “similar inhomogeneity” (line 289) between 5j and 5k, neither in the spatial 
distribution, nor in the relative variation of the measured values. Could you please define and quantify 
what you mean by “similar inhomogeneity” and provide a statistically sound quantitative comparison? 
• The statement of lines 372-374 indicates that “the inhomogeneous thickness and lithium-ion 
mobility observed in Fig. 5j and k are characteristic for the outer-SEI layer....” This is important 
information, which should be included into this paragraph (lines 284-297). 
 
Most of the images contain too much information. Moreover, most of the 2D images (e.g. 5f, 5j, 6c-g, 
S3) are of bad quality; the range of the represented values and the image contrast is not well 
adapted, some of the images seem to be unsharp (e.g. 6 f,g,j). As such, it impossible to check the 
related conclusions of the authors. Just two examples; in lines 267-268, for Fig. 5b they provide “a 
roughness of about 2.4 nm”, in lines 257-259 in Fig. S3 it is “a roughness of about 1.2 nm”. Neither of 
these values can be “seen” from these figures. The authors should provide the appropriate mean 
values (roughness, thickness etc) and their statistical fluctuation instead of such approximate values. 
 
Figure 6j is very confusing with two different X-axis, overlapping figure captions and X-axis title. 
Please include less information and re-organize this sub-figure. 
 Lines 384-385: “The density of these early stage defects was low, making them essentially invisible 
for non-local techniques that average over the entire sample.” Could you please quantify what “low” 
means? Have you performed such non-local measurements? If yes, which ones and what was the 
analytical sensitivity? 
 
Lines 443: “The rigidness of the thick Si substrates is necessary for high precision single-crystal 
diffraction measurements”. Does it mean that only thick (~200 µm) crystals can be studied with the 
method? This would strongly limit its possible application (e.g. excluding nanoparticles/thin-films?). 
Could you please comment on this? Could you please include a discussion on the limits of the 
experimental approach you used?