REVIEW 3 major objections 5 minor 2 references
Photoacoustic Imaging of Lithium Metal Batteries
T0 review · 3 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read This paper demonstrates that photoacoustic microscopy can image lithium metal protrusions inside a battery's glass fiber separator in three dimensions with micrometer resolution, offering a contrast mechanism for bulk lithium that…
desk verdict Solid proof-of-principle for PAM imaging of Li metal; quantitative claims are thin but the central imaging demo holds. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is the photoacoustic effect: a focused 532 nm pulsed laser (86 nJ, <2 ns, 1 kHz repetition) is absorbed by bulk lithium, which converts the absorbed light into heat and then into a broadband acoustic wave through rapid thermoelastic expansion. A custom needle hydrophone centered at 35 MHz picks up that wave, a matched filter of 20-60 MHz cleans the signal, and raster-scanning the sample on a three-dimensional stage builds volumetric images. What makes the contrast work is that lithium strongly absorbs visible light while the glass fiber separator and electrolyte do not, so the acoustic signal marks the metal itself.
What would settle it
Take a fresh Li/Li cell, record PAM images of the same region before and after cutting the sidewall, and compare them with a non-destructive three-dimensional map from synchrotron X-ray tomography or neutron imaging of the intact cell; if the protrusions appear only after cutting, or vanish when the cut face is lightly re-polished, the observed features are cutting artifacts rather than electrochemically grown lithium deposits.
Extended reading notes
Core claim
The paper's central claim is that the photoacoustic effect gives the lithium-metal-battery field a new way to see the metal itself. Because lithium has the third-lowest electron density of all elements, bulk Li is nearly invisible to electrons and X-rays, so established microscopies capture only surface decomposition products; PAM instead detects the acoustic wave generated when absorbed laser light heats and expands the metal. With a 532 nm pulsed source and a 35 MHz hydrophone, the authors image the cross-section of Li/Li symmetric cells and resolve Li protrusions inside the glass fiber separator. They demonstrate a lateral resolution of 3.3 µm, an axial resolution of 26 µm, and a penetration depth of about 160 µm in the separator at 86 nJ pulse energy, and they show that the lithium area fraction increases with charging current density and saturates above 0.5 mA/cm$^2$. The intended upshot is a proof-of-principle for high-contrast, depth-resolved, potentially in situ imaging of dendrite growth.
Load-bearing premise
The flat cross-section made by cutting the cell is assumed to represent the untouched interior, so the protrusions seen after charging are genuine lithium deposits and not damage, smearing, or displacement caused by the knife.
Editorial extensions
If this is right
- PAM provides depth-resolved three-dimensional maps of lithium protrusions inside the separator, information that optical microscopy cannot supply and that electron and X-ray methods struggle to obtain for bulk lithium.
- The lithium area fraction extracted from PAM images increases with charging current density and saturates above 0.5 mA/cm², giving a quantitative image-based metric for comparing deposition uniformity across cells and electrolytes.
- Because the demonstrated 256 × 256 image takes about 5 minutes, the authors argue that faster lasers and MEMS or hexagon-mirror scanners could push PAM toward real-time in situ monitoring of dendrite growth.
- The same contrast mechanism should extend to other light-absorbing metal anodes, including sodium and magnesium, and to lithium inside solid-state electrolytes, where dendrites can crack the electrolyte.
Reading between the lines
- A natural next step, not taken in the paper, is to use PAM's depth information to map where dendrites cross the separator and correlate those positions with the locations of short circuits.
- A testable extension would be to calibrate PAM signal amplitude against known lithium mass per area, so the images become quantitative deposition maps rather than morphology-only pictures.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper demonstrates photoacoustic microscopy (PAM) as a potential tool for imaging lithium metal protrusions in Li/Li symmetric cells. The authors built a home-built PAM system, calibrated lateral and axial resolutions (3.3 µm and 26 µm), measured a penetration depth of ~160 µm inside a glass fiber separator using a tungsten-wire phantom, and compared PAM with optical microscopy. They then imaged a Li/Li cell before and after charging at 1 mA/cm², showing protrusions after charging, and quantified a 'Li ratio' as a function of charging current density for five charged cells plus one uncharged control. The central claim is that PAM provides a new, label-free, three-dimensional imaging contrast for bulk Li metal with micrometer resolution and sufficient penetration to visualize Li inside a separator, with potential for in situ real-time imaging.
Significance. If the central claim holds, the paper introduces a genuinely new imaging contrast for lithium metal in batteries, addressing a known limitation of electron and X-ray methods, which are largely insensitive to bulk Li. The demonstration of 3D visualization of Li protrusions inside a separator with micrometer lateral resolution is valuable to the battery community. The manuscript includes explicit calibration measurements (lateral resolution, axial resolution, and penetration depth) and provides a clear before/after charging comparison. However, the proof-of-concept is currently limited by the lack of an artifact control for the cutting procedure used to prepare the imaging surface and by the absence of statistical replication in the quantitative Li-ratio measurements. These issues must be addressed before the feasibility claim can be considered fully established.
major comments (3)
- [Section II and Fig. 5] The cutting procedure used to prepare the flat cross-sectional sidewall may compromise the core demonstration. The manuscript itself reports (Section II) that the imaged Li electrode thickness varied from less than 50 µm to up to ~300 µm versus the original ~240 µm, attributing this to mechanical damage during cutting. Because the cut surface is the imaging surface, the protrusions observed after charging (Fig. 5) could be influenced by cut-induced damage, such as preferential nucleation at the damaged edge or smearing of Li, rather than being representative of the pristine cell interior. Provide an artifact control that separates cutting effects from electrochemical deposition, for example by comparing cells cut before charging with cells cut after charging, or by imaging a surface prepared without mechanical cutting, to demonstrate that the observed protrusions are genuine electrochemical deposits and not artifacts of the sample preparation.
- [Section III, Fig. 6(c)] The quantitative Li-ratio versus current density is based on a single cell per charging condition with no error bars, and the analysis parameters are not fully specified. The Li ratio is defined as the proportion of area with Li in 2D MAP images, but the segmentation threshold used to decide which pixels are 'Li' and the Z-slab depth range over which the ratio is computed are not stated. These choices are load-bearing for the saturation claim (0.5 vs 1 mA/cm²) and for the proposed potential to deduce local current density. Specify the threshold and depth range, provide repeated measurements (at least three cells per condition) with error bars, or explicitly state that the curve is illustrative rather than quantitative.
- [Appendix B and Fig. 3] The penetration depth of ~160 µm inside the glass fiber separator is measured using tungsten wires, not lithium, and the justification for treating TW as a proxy for Li is partially confounded. In Appendix B, the photoacoustic signal amplitude of the TW is measured below a PET film, while the Li signal is measured without a PET film, so the top-100 amplitude ratio of 87% does not cleanly isolate the Li versus TW photoacoustic response because the PET film may attenuate the TW signal. Report the Li signal under the same PET-covered configuration, or otherwise justify that the ~160 µm depth applies to Li inside the GFS, since deep penetration is one of the paper's stated advantages (abstract and Section IV).
minor comments (5)
- [Section I] The statement that Li metal is 'neither visible to electrons nor to X-rays' overstates the case; the cited references themselves show cryo-EM and X-ray tomography imaging of Li-related structures. Rephrase to say that bulk Li metal is difficult to image with these modalities because of its low electron density.
- [Section II] The description 'thickness of the GFS was ~2 mm before soaked and <2 mm after soaked' is imprecise; please provide measured values or a range.
- [Section III, Fig. 6] The text says that Li thickness increased from ~0.11 mm to ~0.54 mm, but the images are maximum amplitude projections; please clarify how the thickness was measured from the 3D or 2D data.
- [Section III, Fig. 5] There is a tension between the report in Section II that cutting causes large thickness variations and the statement in Section III that the thin-layer Li electrode thickness 'before and after charging kept almost the same'; please reconcile these observations.
- [Appendix A] The damage-threshold calibration uses darkening in optical microscopy as the criterion for damage, which is somewhat subjective; consider reporting a quantitative metric such as a threshold on the change in reflectivity or PA signal.
Circularity Check
No significant circularity: the paper reports direct measurements with independent calibrations; no predicted quantity is fitted from the same data by construction.
full rationale
This is an experimental proof-of-concept paper, not a derivation, so the circularity patterns based on fitted parameters or self-citation chains do not apply. The key quantities—lateral resolution (3.3 µm), axial resolution (26 µm), penetration depth (~160 µm), and Li ratio trends—are obtained from direct measurements with independent calibration targets (razor blade edge, carbon fiber, tungsten wires) rather than from fitting a model to the battery data and then presenting that fit as a prediction. The penetration-depth calibration substitutes tungsten wires for Li, but Appendix B explicitly measures the photoacoustic signal amplitudes of both and reports a ratio of 87%, making the substitution an experimentally justified proxy rather than a tautology. The only self-references (refs 25 and 26, by co-author S.-L. Chen) are cited for standard PAM system design and edge-spread-function fitting; these are not load-bearing for the central imaging demonstration. The manuscript's own admission that cutting changed the Li electrode thickness from ~240 µm to values between <50 µm and ~300 µm raises a legitimate experimental concern about whether the imaged protrusions are representative of an intact cell, but that is a question of sample-preparation validity and artifact control, not circularity: the paper's before-charge image and after-charge image are independent measurements and the conclusion is not obtained by assuming the conclusion. No circular step can be exhibited from the text, so the score is 0.
Assumptions & free parameters
free parameters (2)
- Li ratio segmentation threshold =
not reported
- Depth range (Z-slab) for Li ratio =
not reported
assumptions (4)
- domain assumption Photoacoustic effect in Li metal produces detectable ultrasound at 532 nm with the stated pulse energy and detection chain.
- domain assumption Tungsten wire PA amplitude is representative of Li PA amplitude for penetration depth calibration.
- domain assumption The cut cross-section surface is representative of the bulk Li protrusion structure.
- standard math Standard photoacoustic signal generation and detection physics (thermoelastic expansion, acoustic propagation) apply without modification.
Cite this review
Pith. "Pith review of Photoacoustic Imaging of Lithium Metal Batteries." pith.science (2026). https://pith.science/paper/IEDTIMS3
@misc{pith2026191000097,
author = {Pith},
title = {Pith review of: Photoacoustic Imaging of Lithium Metal Batteries},
year = {2026},
howpublished = {\url{https://pith.science/paper/IEDTIMS3}},
note = {Machine review of arXiv:1910.00097}
}
read the original abstract
We demonstrate that photoacoustic microscopy (PAM) can be a potential novel imaging tool to investigate the Li metal dendrite growth, a critical issue leading to short circuit and even explosion of Li metal batteries. Our results suggest several advantages of PAM imaging of Li metal batteries: high resolution (micrometers), 3D imaging capability, deep penetration in a separator, and high contrast from bulk Li metal. Further, PAM has potential for in situ real-time imaging of Li metal batteries.
Figures
Figures from the paper (2 more)
Reference graph
Works this paper leans on
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[4]
Direct observation of lithium dendrites inside garnet-type lithium-ion solid electrolyte,
Y. Ren, Y. Shen, Y. Lin, and C.-W. Nan, “Direct observation of lithium dendrites inside garnet-type lithium-ion solid electrolyte,” Electrochem. Commun. 57, 27–30 (2015). 5. S. Chandrashekar, N. M. Trease, H. J. Chang, L. S. Du, C. P. Grey, and A. Jerschow, “7Li MRI of Li batteries reveals location of microstructural lithium,” Nat. Mater. 11, 311–315 (201...
work page 2015
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[19]
Advances in real-time multispectral optoacoustic imaging and its applications,
A. Taruttis and V. Ntziachristos, “Advances in real-time multispectral optoacoustic imaging and its applications,” Nat. Photonics 9, 219–227 (2015). 20. J. Y. Kim, C. Lee, K. Park, G. Lim, and C. Kim, “Fast optical-resolution photoacoustic microscopy using a 2-axis water-proofing MEMS scanner,” Sci. Rep. 5, 7932 (2015). 21. S. Hu, “Listening to the brain ...
work page 2015
Reviewed August 14, 2026 · model on record in the stance chip above.
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