Peer review of: 3D microstructure design of lithium-ion battery electrodes assisted by X-ray nano-computed tomo (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

First, let me congratulate the authors for doing a very comprehensive and thorough work on 
investigating transport and diffusion at both particle and electrode level by quantitatively resolving the 
micro (nano)structure of the composite electrode using X-ray CT. The results will be very useful to the 
lithium ion battery community in advancing the knowledge of electrode design specifically meeting the 
requirements such as fast charging, capacity utilization and addressing issues such as capacity 
degradation. and thermal safety. Now I will provide specific examples justifying my observation. 

1. This is a paper that discusses all aspects and important parameters pertaining to heterogeneity in 
Li-ion transport in LIB electrodes, (i) Variation of State of lithiation (SoL), (ii) Charge transfer and 
current density distribution at the pores and interface at various C-rates, (iii) effect of graded 
electrode porosity and particle size distribution and finally (iv) the effect of calendering on the 
performance. 

2. In addition, the manuscript provides practical suggestion on how to optimize the power and energy 
at high discharge rates by optimizing the microstructure such as gradient porosity. 

3. Overall there is tremendous value on observing Li-ion transport across the through plane 
(thickness) in 3D and observing the variation of transport parameters near the separator till the 
current collector. 

4. Regarding the technique although X-ray CT has been used extensively by others but dual scan 
superposition (DSS) methods brings much needed fidelity and resolution to resolve the microstructure 
for undertaking 4D X-ray CT. 

Couple of points- 
1. Fig. 2f and h- It's interesting that authors have discussion of the shape (aspect ratio) and sphericity 
distribution. This was for the pristine or uncycled electrode. Its possible that under long term cycling 
the majority of particles will deviate from spherical shape due to stress etc.. Similar effect has been 
reported from XANES tomography which has even higher spatial resolution ~ 30nm [Nano Lett. 2014, 
14, 4334−4341]. This provides a good discussion theme for discussion for the changes in particle 
morphology with cycling which can effect the porosity, density etc.. 

2. Fig. 3- Charge transfer- Jct across the electrolyte-particle interface : this doesn't show much 
variation with C-rates unlike electrolyte concentration or Li-ion flux ? 



Reviewer #2 (Remarks to the Author): 

This manuscript reports a dual-scan superimposition technique to image the features of the NMC 
cathode material by X-ray tomography. Electrochemical model, built based on the features obtained 
from imaging, is employed to study how the micro pores, particle heterogeneity (size, distribution) 
and the calendaring affect the electrode’s performance (capacity, rate performance and/or energy 
density). However, many questions are remained as unclear in terms of data interpretation before it 
can be considered to be published. 

1. In line 79, the authors introduce a dual-scan superimposition method. Details have been given in 
the paragraph about the second scan. How about the first scan? What are the main differences 
between the first and second scans? Experimental parameters about the first and second scans are 
also missing in the Methods section, which should be added. 

2. As far as the referee understand, Fig. 1a and 1b come from the results of X-ray micro-CT from a 
cylindrical battery, while Fig. 1e to 1g are from the results of cathode sheet prepared by doctor blade 
from X-ray nano-CT. It is misleading if these cathode sheets are not directly obtained from a 
cylindrical battery as the red lines show in Fig.1. In addition, the experimental parameters of the X-
ray micro CT are missing in the Method section. 

3. The authors mentioned that “conservation of species (i.e., Li+, PF6-, e- and Li) was enforced in the 
entire simulation domain”. But several species are not constant during the electrochemistry. For 
instance, in the simulation domain Li+ ions are inserted in the NMC particles and electrons are 
provided from the other electrode and are consumed in this domain. How do the authors explain this? 

4. Regarding the figures in this manuscript, 1) the region of R1 to R4 in Fig.7 is not clearly indicated in 
Fig. 7a-c; 2) In Fig. 8, the labels number 1-4 on the dashed lines might not be correct, as number 2 
and 3 are labeled on one dashed line (pink). 

5. In line 463, the authors state that “a video has been added…”. Unfortunately, the referee did not 
see such a video in the manuscript tracking system. 

6. Suggested minor things: 1) Reference 13 and 15 are the same. 2) The preferential pathway of 
electron shown in Fig. S3 is not correct as electrons prefer to migrate along the carbon matrix instead 
of passing through the NMC particles. 



Reviewer #3 (Remarks to the Author): 

The manuscript entitled “Advanced Design of Lithium-ion Battery Electrodes assisted by 4D X-ray 
Nano-computed Tomography and Modelling” should be accepted to Nature Communications with 
revisions. 

The manuscript can be subdivided into four main sections. The first section introduces a novel dual-
scan superposition process to discriminate the binder phase from pore space in a 
LiNi1/3Mn1/3Co1/3O2 cathode. The technique implements a second X-ray nano-CT scan. The 
resulting X-ray measured microstructures capture detailed binder features previously measured using 
scanning electron microscopy. The paper’s second section incorporates these microstructures into 3D 
electrochemical models. Initial modeling results using the reconstructed data indicates the particle 
distribution and orientation. Additionally, the model predicts how Li-ion transport losses throughout 
the electrode cause inhomogeneous electrode utilization. The manuscript’s third section models a 
modified electrode particle distributions to predict the influence of particle-grading. Finally, an 
experimental time-lapse X-ray nano-CT experiment is used to quantify the effects of electrode 
calendaring. 

The following are major revisions that need to be addressed: 
1. Lines 100 and 448-449: Three-body thresholding is difficult, and can be influenced by ambiguities 
in the images and subjective decisions. The paper would benefit from further discussion to explain and 
interpret the three-body thresholding method. If there are three obvious peaks in the gray-scale data 
that clearly identify the respective regions, then this can just be stated in the manuscript. Otherwise, 
the thresholding scheme needs to be further documented. 
2. Line 210-211, 454-455 and 38 in Supplemental: The authors implement dilute solution theory to 
model the Li-ion transport. The Poisson-Nernst-Planck equations used in the present study result in 
different solution-phase potential drops than those derived from concentrated solution theory [1]. 
Concentrated solution theory is commonly used in Li-ion battery modeling, usually following the 
approach developed by Newman and colleagues. The paper must correct the citation in line 38 of the 
Supplemental material, which incorrectly references the Doyle-Fuller-Newman model as implementing 
dilute solution theory. Additionally, the authors found that the local discrepancy in electrolyte potential 
and charge-transfer current density are closely related (Lines: 216-217). Supporting text is needed in 
the paragraph (Lines 190-223) that discusses how using concentrated solution theory may affect the 
reported modeling results. 

The following are minor revisions that need to be addressed: 
3. Line 3: The first author Xuekun Lu is associated with the Electrochemical Innovation Lab twice (i.e., 
there are two number ones above their name). 
4. Line 111: What is the program/calculation used to extract the “volume-averaged tortuosity factor”. 
5. Figure 6 d-e, g-h, and j-l need to state the C-rate and at what time the data is taken. 
6. Lines 250-252 and 256-257: Particle size reduction (or morphological dilation) is nontrivial in a 
three-phase media. Additional text is required that discusses how the particle size is reduced near the 
separator “while maintaining the same particle volume fraction”. Additional text is also required to 
support what the authors mean by “morphological dilation.” The authors state that they shrink the 
CBD domain. Does this create more solid/electrolyte active lithiation sites? 
7. Line 286-287: The authors state: “decreasing the CBD volume while maintaining the mass loading 
constant.” The meaning of this sentence is unclear. To keep the mass loading constant did the authors 
increase the NMC volume fraction in this region? Do the authors instead mean that the NMC loading is 
held constant, while the CBD volume fraction is reduced? 
8. Figure 9k illustrates the porosity and tortuosity as a function of strain. How are these parameters 
calculated? Are they averaged over the entire domain or just the cut plane illustrated in Fig. 9d-f. 
9. Line 437: The word “voxel” is oddly typeset. 
10. References: There are sometimes odd spaces before and after “– “. For example, see Line 487. 
11. Line 491 (References): There are dots where there should be dashes. 
12. Figure S7: What is the x-axis? 

General thoughts: 
13. The final section in the manuscript (Lines 330-381) seems tacked-on as opposed to a continuation 
of the paper. This section could be removed. Alternatively, the section could be strengthened by 
providing clear context relative to the body of the paper.. 
14. The figures are not colorblind friendly. The same tone of red and green are commonly used in the 
same plot. 

[1] A.M. Bizeray, D.A. Howey, and C.W. Monroe. Resolving a discrepancy in diffusion 
potentials with a case study for Li-ion batteries. J. Electrochem. Soc., 163 E223-E229 (2016). 



Reviewer #4 (Remarks to the Author): 

This paper deals with some suggestions to improve the electrode design of NMC LIBs. To achieve this, 
a combination of visual 3D tomography technique, electrical measurement of coin cells and a physical 
chemical model in COMSOL was applied. To analyse the full structure of the NMC electrode a 
superimposition technique of high attenuation and low attenuation regions were overlaid. The model 
was used to identify better performances of the electrode while changing the ratio between pores and 
binder and applying graded particle sizes near the separator. Also the influence of calendaring of the 
NMC electrode is studied. 
The final results of the paper regarding the calendaring for different cell types (high power / high 
energy) and the graded particle approach are relevant and presented well. However, the structure of 
the paper is very confusing. Some subsections may help to follow the different analysis and small 
connecting paragraphs between the sections would improve continuous comprehension and 
readability. The title is about 4D tomography, normally the fourth dimension in the literature is time, 
but here no time elapsed measurement is shown. 

First Part: 3D characterisation 
The author point out, that X-ray can be used to get insight of material structure. But there are some 
limits and a novel technique was used to overcome the limits. 
• This novel technique is not explained in detail. If it is about using a short exposure time and a long 
exposure time and overlaying this images, it is not novel it is like HDR images. In addition, is this 
process before or after reconstruction? What is the meaning of high confidence in the context of this 
measurement? 
• Why didn’t you use synchrotron X-ray to achieve a high contrast and resolution tomography? 
• [p2; line 38] How is it possible to remove the NMC particles within a sample? If this is a physical 
process how do you ensure that it is not changing the structure? 
• How can you be sure that the reconstructed data has no artefacts, void zones or noise? (compare 
180° rotation vs. 360° rotation) 
• More information about the reconstruction process is needed 

Second Part: Microstructure and performance correlation 
[p.5; line 4] Coin cells were prepared to show the correlation between model and experiment. 
• Information about the coin cells experiment is missing (amount of measured coin cells, at which 
operating temperature). In addition, it is not clear if the model has the same material parameters as 
the used coin cell materials. 

Third Part: Design and optimisation of next-generation electrodes 
Some subsections would improve the readability (procedure and steps, results and impacts and finally 
suggestions) 
It is shown, that the shrinking of the CBD (or increasing the porosity) has a high impact of the 
electrical performance and the capacity. 
• Hence the quality (noise) and threshold of the reconstructed data has even a big influence on the 
results. This is not discussed and leads to a not trustworthy model. (250nm shrinking = 4 voxels = 
within measuring error?) 
• [p11. Line 29] This is the case in the most of BMSs 

Fourth Part: Combining 4D X-ray nano-CT and electrochemical performance 
This should be the main part related to the title. 
• It is not explicit said, what is the fourth dimension? If its time, there are no experimental data or in-
situ measurements. 

Fifth Part: Conclusion 
• [p.14 line 15] This is not a new but maybe a relevant insight.