Peer review of: 4D imaging of lithium-batteries using correlative neutron and X-ray tomography with a virtual unr (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

4D imaging of Li-batteries using operando neutron and X-ray computed tomography in combination 
with a virtual unrolling technique 

This is an extremely interesting contribution to battery science made possible by the combination of 
two imaging techniques applied to a battery undergoing controlled discharge. The work shows 
procedures for inspecting electrode consumption spatially, hence will provide a route to more efficient 
battery construction. The “virtual unrolling” developed herein is a useful data analysis tool. Minor 
suggestions are made below with respect to achieving a consistent coordinate system.  

Notes: 
line 13: “high-resolved” should be “high-resolution” 
line 54: “known for its innovation in neutron imaging” This is an opinion. Please replace with a factual 
statement such as “known for its combination of high-resolution neutron imaging with step-wise 
sample evolution ” 

line 63: Fig 1d. The chirality (handedness) of the X-ray and neutron images differ. This is a known 
problem with state-of-the-art imaging systems where detectors are frequently exchanged. One of the 
data sets should be reflected about an axis. It would be helpful to rotate a data set about the axial 
axis (see triad in Fig 1d) to mutually register the spiral electrodes in the X-ray and neutron images. 

line 63: Fig 1a. The oblong objects in the cathode denote the “nickel current collecting mesh” (line 83). 
If a short note can be added to the already long Fig 1 caption to identify the oblong objects, that 
would be helpful. 

line 145  “A higher rate of lithium consumption …” This is a very interesting observation. Do the 
authors know the stoichiometric excess of lithium in this battery? That is, for the completely 
discharged battery, will 50% of initial lithium remain as unreacted lithium metal? An eyeball 
integration of the neutron images suggests 50% unreacted lithium. 

line 180 “The neutron images show a smaller volume expansion and electrode movement during 
discharging.” This brings up the point of sample statistics. The paper does not clearly state that two 
different batteries are imaged. There is no attempt to show that batteries are equivalent based on 
mass or electrical capacity. 

line 194: Fig 3b line plot. Tomogram 07 is not in the stacked line plot. Just curious, did the switch 
from cold to thermal neutrons create too much offset for normalization to the same scale as the other 
line plots? 

line 194: Fig 3. The x-axis in the line plots is (I think) a radial distance. Would it make more sense to 
set the origin of this distance to the battery axial center? We note that Fig 5 uses “length” as distance 
along an electrode. 

line 219: Let’s consider -045 image and line plots. The dips in the line plot at slices 36-44 at about 
160 mm and 190 mm should be visible in the -045 image. I think I can see the changes, but a call out 
with color rectangles (as done in Fig 2b “regions of strong…”) would be helpful. 

line 228: “result of the better Li-ion conductivity caused by the excess electrolyte at the bottom”. This 
points out the author’s failure to clearly establish a coordinate origin for the electrode length. I though 
origin was at the center per the axis triad in Fig 1d. Now, it appears the origin is the outer layer. This 
is confusing. 

line 250. The point of Fig 7, activity of electrodes at the very center of the battery, is obscured by the 
confusion of whether center of battery is a 0 mm or 220 mm. Interesting, the comment “late start” is 
a powerful indication of the complementary use of X-ray and neutron imaging. 

Great work folks! 

Les Butler 
Louisiana State University 
USA 
 

Reviewer #2 (Remarks to the Author): 

The manuscript by Ziesche and co-workers reports the temporally and spatially resolved imaging of a 
primary battery undergoing discharge using a combination of x-ray and neutron tomography. The 
sensitivities of x-ray to heavier elements and neutron to lighter elements complement each other in 
measurements, yielding a clearer picture of the structural evolution of various components in a 
primary battery with rolled electrodes. This study is a nice extension of the previous effort in applying 
in situ x-ray and neutron measurements to studying battery cells in operando, in particular the virtual 
unrolling representation of layered structures allows for semi-quantitative analysis of structural 
variations across the cell and over the time. 

As this Communication reports extending the application of a novel measurement methodology 
developed in the past years to studying batteries in operando, several noticeable deficiencies need to 
be addressed to elevate the caliber of the study using advanced techniques in major research facilities. 

1. The reported measurements are not real-time in-operando for neutron, and not truly multimodal or 
simultaneously probing the same location and the same event of the structure evolution toward 
achieving the goal of synergistic measurements of hard/soft matter. To take advantage of the 
complementary compositional sensitivity of x-ray and neutron, it is desirable to apply measurements 
to a single cell, or at least identical cells operating at identical conditions. 

2. Thus the comparison and complementarity are compromised with the high c-rate in x-ray 
measurements and low c-rate in neutron measurements. There are unique limitations and advantages 
of each technique, overlap of certain operation conditions are needed. 

3. The outlier date point using thermal neutron could be addressed with more rigor, and better 
quantification is possible through normalization using the flux and cross-section ratios of thermal and 
cold neutrons. 

4. The statistical significance of the data and observation is not sufficiently addressed. The extents of 
the generalization of the failure mechanism as observed in this study are not assessed. 

5. It is necessary for better quantification of experimental measurements to be more relevant to the 
electrochemical characteristics of batteries. The comparison between the amount of transported Li and 
the state of the charge and the correlation between the volume change and the change of Li content 
could be addressed. 

6. In general, the focus of the study is mostly on engineering aspects of primary batteries. It is 
desirable to apply novel measurement techniques to the state of the art battery materials or new 
battery architectures and addressing the most relevant scientific issues in advancing battery 
technology. 

Recommendation -- the manuscript could be further improved. The scheme and outcome could fit to a 
journal more specialized on methodology or toward electrochemical research community. 



Reviewer #3 (Remarks to the Author): 

This is an interesting paper detailing a novel combination of imaging methods to extract insightful 
information about the battery discharge process. This kind of work is an important contribution 
towards the goal of understanding the connections between manufacturing/production methods and 
discharge performance. The method gives significant insight into transient, spatially resolved 
behaviors relevant to battery performance. While not done here, using these techniques to study 
transient discharge behavior in different battery designs could offer impactful new insights as to why 
some designs or manufacturing methods result in superior capacity/cycle life, which in many cases can 
currently only be hypothesized with the help of end-state battery imaging or computational modeling. 
I suggest some minor revisions before publication: 

1. Lines 178-182: 
a. Can you be sure that the differences in electrode expansion between medium and high rate 
discharges are due to cracking and not due to differences in imaging technique and/or image 
postprocessing? Is resolution sufficient enough to confidently report these volume expansion numbers 
without more statistics? Even a +- reporting of spread throughout the length of the electrode to 
accompany the average value reported would give these measurements more weight. It may be 
stated elsewhere, but how does the pixel size compare to the volume expansion: how much error is 
introduced if one pixel of expansion is incorrectly included/not included in the expansion measurement? 
b. What is the theoretical/measured expansion of pure LiMnO2 upon lithiation and how does that 
compare to these measured values? This may give insight into how much porosity (and therefore Li+ 
transport through the pore network) is decreasing due to swelling, at least in the medium C-rate case 
where there is less cracking. 
c. You chose to report averaged volume increase, yet for the other metrics in subsequent figures you 
show the values as a function of length around the spiral cathode. Can you state why? Is the volume 
increase uniform around the cell or was there sinusoidal behavior like you see in Figure 6? For 
example, do you see more significant electrode expansion opposite the tab current collector to support 
the conclusion in line 214/215 and 218? 
2. Line 190-191: Since this statement is repeated several times throughout the paper during the 
analysis, it should be more detailed by explaining why this statement is true (A lower compression 
results in a more porous electrode, allowing for easier lithium diffusion through the pore network as 
well as providing more room for mesoscale electrode expansion into the pores as well as macroscale 
expansion via cracking of a less dense/strong composite material.) Up to this point in the paper, there 
has been no mention of electrode porosity, so it may not be clear why the statement is true. 

3. Line 249. You mention mechanisms, but only state one mechanism. Lithium may not necessarily 
have to diffuse through the cathode material to get to the cathode endings. Early in discharge, there 
may be no incentive for the lithium to transport out to the cathode endings through the electrolyte 
given that there is much closer cathode material available. As the state of charge of the nearby 
cathode material increases and approaches capacity, the longer transport through the electrolyte to 
the cathode endings (low SOC) likely becomes the most favorable path. I would think a longer 
transport path through electrolyte to low-SOC cathode is the likely mechanism here, rather than 
lithium diffusion through the active material as you state. 

4. While not directly related to this work, the impact on computational battery modeling could be 
mentioned somewhere in the introduction/conclusions. The transient, 3D spatially-resolved image data 
of lithiation and electrode swelling presented here would be invaluable to the modeling field that is 
quite starved for such detailed model validation data. For transient discharge simulations, typically 
only global measurements are available (discharge curves, macroscale pouch swelling, etc.).