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[Arghya Dutta](https://orcid.org/0000-0002-3769-7820), [Shoichi Matsuda](https://orcid.org/0000-0002-0640-3404)

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Application of Noninvasive Imaging Techniques for High Energy Density Lithium Metal Rechargeable BatteriesApplication of Noninvasive Imaging Techniques for HighEnergy Density Lithium Metal Rechargeable BatteriesArghya Dutta*[a] and Shoichi Matsuda*[a, b, c]Lithium metal batteries (LMBs) have the potential to exceed theenergy density of current lithium-ion batteries. Achieving thisrequires a thick positive electrode, a thin Li metal negativeelectrode, and minimal electrolyte-loading. Despite their prom-ise, high energy density LMBs with high-loading positiveelectrodes, thin Li, and low electrolytes face significantchallenges. A key issue is the high reactivity of Li metal withnonaqueous electrolytes, leading to the consumption of bothduring each cycle. This reaction causes insulating Li compoundsto accumulate, increases electrode porosity and thickness,depletes the electrolyte, raises cell impedance, and reducescapacity. Therefore, understanding the interphase evolution ofthe Li metal electrode is crucial to addressing cell failure. Whilevarious ex situ and in situ techniques have been used to studythese interphases, they often involve non-practical cell config-urations and sample-damaging preparation processes. In thisregard, noninvasive methods like X-ray and neutron-basedimaging are beneficial as they do not damage samples, can beused both in situ and ex situ, employ practical cell configu-rations, and enable long-term data collection. This reviewexplores recent advancements in X-ray and neutron-basedtechniques for characterizing high-energy LMBs, emphasizingtheir potential to improve understanding of interphasialdynamics and advance robust high-energy-density batteries.1. IntroductionIn response to the escalating energy demands of modernsociety, it is essential to develop high-performance energystorage systems to achieve sustainable energy solutions.[1] Fordecades, rechargeable lithium-ion batteries (LIBs) have proveneffective for powering portable electronic devices and haverecently emerged as the primary technology for electricvehicles. Nonetheless, the need for batteries with greaterenergy density is growing to enable electric vehicles to travellonger distances at reduced costs.[2,3] A promising strategy toincrease the energy density of LIBs is to substitute the graphiteelectrode, which has a theoretical specific capacity of372 mAhg� 1, with a lithium metal (Li) electrode.[4] Li metalelectrode provides an exceptional theoretical specific capacityof 3860 mAhg� 1, low redox potential (� 3.04 V versus thestandard hydrogen electrode), and low density (0.534 gcm� 3).Despite the promise, the commercialization of lithium metalbatteries (LMBs) faces significant challenges, primarily due tolow Coulombic efficiency (CE) and the formation of needle-likeLi dendrites, which can penetrate the separator and cause shortcircuits.[5–7] Although recent advancements in electrolyte for-mulations have significantly mitigated the dendrite issue, thelow CE remains a critical concern due to the high reactivity of Limetal.[7–12] Typically, the surface of Li metal is coated with anative film, and upon contact with a nonaqueous electrolyte, anew layer called the solid electrolyte interphase (SEI) forms.[13,14]Ideally, this SEI layer should protect the Li metal surface fromfurther reactions and stabilize the interphase. However, inpractice, the SEI is mechanically unable to withstand thesignificant volume changes during Li plating and strippingcycles. Consequently, the SEI cracks, exposes fresh Li surfaces,and triggers continuous SEI formation.[15] This repeated SEIformation depletes active Li, resulting in lower CE and reducedcycle life. Additionally, the formation of electrically discon-nected Li, or ’dead Li’, also reduces the CE and cycle life.[16,17]The accumulation of insulating SEI compounds and ‘dead Li’,causing electrode volume expansion, increased electrodeporosity, and entrapment of liquid electrolyte on the porouselectrode surface, leads to increased cell impedance, voltagepolarization, and premature cell failure.[18–21] Therefore, under-standing the interphasial evolution of the Li metal electrode iscrucial to addressing the interconnected problems of low CE,significant volume expansion, impedance growth, and shortcycle life.To investigate the interphasial processes on Li metalelectrodes, both qualitatively and quantitatively, various ana-lytical techniques have been applied. These include trans-mission electron microscopy (TEM), scanning electron micro-scopy (SEM), atomic force microscopy (AFM), optical microscopy(OM), X-ray photoelectron spectroscopy (XPS), and titration-[a] A. Dutta, S. MatsudaCenter for Green Research on Energy and Environmental Materials, NationalInstitute for Materials Science, 1–1 Namiki, Tsukuba, Ibaraki 305–0044,JapanE-mail: DUTTA.arghya@nims.go.jpMATSUDA.Shoichi@nims.go.jp[b] S. MatsudaNIMS-SoftBank Advanced Technologies Development Center, NationalInstitute for Materials Science, 1–1 Namiki, Tsukuba, Ibaraki 305–0044,Japan[c] S. MatsudaCenter for Advanced Battery Collaboration, National Institute for MaterialsScience, 1–1 Namiki, Tsukuba, Ibaraki 305–0044, Japan© 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbH.This is an open access article under the terms of the Creative CommonsAttribution Non-Commercial NoDerivs License, which permits use and dis-tribution in any medium, provided the original work is properly cited, the useis non-commercial and no modifications or adaptations are made.Wiley VCH Dienstag, 08.04.20252504 / 381023 [S. 3/11] 1Batteries & Supercaps 2025, 8, e202400504 (1 of 9) © 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbHBatteries & Supercapswww.batteries-supercaps.orgReviewdoi.org/10.1002/batt.202400504http://orcid.org/0000-0002-0640-3404http://crossmark.crossref.org/dialog/?doi=10.1002%2Fbatt.202400504&domain=pdf&date_stamp=2024-11-09based techniques etc.[22–35] Postmortem analyses using SEM andTEM offer high-resolution, site-specific insights into the inter-phasial morphologies of Li metal electrodes. However, thesample preparation required for these techniques often resultsin physical damage to the electrode surface, leading topotential inaccuracies. Furthermore, ex-situ methods cannotdeliver time-resolved information. While in situ SEM and TEMstudies can provide real-time visualization of electrode proc-esses, they typically require cell configurations that differsignificantly from actual batteries.[36–39] Additionally, the electronbeams used in SEM and TEM can cause substantial damage tothe sample under observation. In situ OM, although lessinvasive to the electrode surface structures, lacks sufficientspatial resolution.[24,40,41] Moreover, the cell configuration usedfor in situ OM, which often involves large quantities of electro-lyte and physically separated electrodes without stack pressure,does not represent practical battery cells. Moreover, the micro-scopic techniques fail to provide any quantitative correlationbetween active Li loss due to interphasial changes and capacityfade in the cell. XPS, on the other hand, provides quantitativeestimations of the SEI composition but is limited by its shallowpenetration depth of only a few nanometers from the surface.Titration-based techniques, in this regard, have emerged asmore precise quantitative tools for estimating Li loss due to SEIformation and dead Li accumulation.[26] Although each of thesetechniques has its own limitations, their combined applicationcan complement one another and provide a comprehensiveanalysis. However, despite the utility of these techniques incharacterizing the interphase of Li metal electrodes, theirapplicability is generally restricted to the initial few cycles. Thisrestriction hinders their ability to provide information on theevolution of the interphase over prolonged cycling periods.Furthermore, given the fragility of Li metal electrodes underextended cycling, it is crucial to avoid any physical damage tothe electrodes. This necessitates the introduction of noninvasivecharacterization techniques to ensure accurate and reliableanalysis.In this context, the characterization of the interphase usingnoninvasive techniques such as X-ray and neutron-basedimaging techniques is highly beneficial.[18,42–51] These techniquesoffer several advantages: they do not damage the samples, canbe employed both in situ and ex situ, closely resemble theactual cell configuration, and allow data acquisition overextended cycling periods. In this review, we discuss recentprogress in non-destructive analytical methods for the charac-terization of high-energy-designed liquid electrolyte-basedLMBs, focusing on their potential to enhance our understandingof interphasial dynamics and to drive the development of morerobust high-energy-density batteries. Given the extensivenumber of review articles that discuss the application of variousin situ and ex situ analytical techniques, this article specificallyhighlights the use of non-destructive methods in the study ofLMBs.[34,52–54] Despite significant advancements in all-solid-stateLMBs and the broad application of cutting-edge analyticaltechniques, the scope of this review is limited to liquidelectrolyte-based LMBs. The primary rationale for this focus isthe distinct degradation mechanisms between liquid electro-lyte-based and solid-state LMBs. By focusing on LMBs withliquid electrolytes, this review aims to enhance the under-standing of interphasial evolution at the Li electrode due to itscomplex interactions with liquid electrolytes and the subse-quent impact on the stability and performance of LMBs.2. Lithium Metal Degradation MechanismUnder High Energy Density ConditionIn this section, let us clarify the importance of a series oftechnological parameters, such as the amount of electrolyte,areal capacity, and Li utilization ratio for realizing the cell-levelhigh energy density rechargeable LMBs. The electrolyte-to-capacity ratio (E/C) is empirically utilized as a technologicalparameter for the cell design of LIB.[55] Even in the case of LMBs,the E/C plays a crucial role not only in cell-level energy densitybut also in cycle life. For example, Chen et al. explored therelationship between E/C and the cycle life of LMBs.[56] In theirstudy, they used an NMC622 electrode with an areal capacity of3.8 mAhcm� 2 and a 250 μm thick Li metal foil as the positiveand negative electrodes, respectively. As observed in Figure 1a,the LMB cell with an E/C of 25 gAh� 1 maintained stable capacityover 60 cycles before experiencing a sudden capacity drop. Instark contrast, an LMB cell with an E/C of 3 gAh� 1 showed asudden capacity drop after only around 10 cycles. They alsoexamined the relationship between E/C and cell-level energydensity in LMBs using a relatively thin Li metal foil with aArghya Dutta is currently serving as a NIMSSpecial Researcher at the National Institute forMaterials Science in Tsukuba, Japan. With overa decade of experience, he specializes in theresearch and development of electrochemicalenergy storage and conversion devices. Hiscurrent research endeavors are centeredaround the mechanistic study, failure analysis,and electrolyte design aimed at enhancingthe performance of lithium metal-based re-chargeable batteries.Shoichi Matsuda is a Team Leader at theNational Institute for Materials Science (NIMS),Japan. He studied chemistry in The Universityof Tokyo and received his PhD in 2015. Afterthat, he joined NIMS as an ICYS researcherand then got a tenured position in 2017. Hewas awarded the Young Researcher Award ofThe Electrochemical Society of Japan (2023),Young Scientists’ Award by the Commenda-tion for Science and Technology by the MEXTof Japan in 2024. His current research interestsare high energy density rechargeable batteriesand data-driven automated experiments.Wiley VCH Dienstag, 08.04.20252504 / 381023 [S. 4/11] 1Batteries & Supercaps 2025, 8, e202400504 (2 of 9) © 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsReviewdoi.org/10.1002/batt.202400504 25666223, 2025, 4, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202400504 by National Institute For, Wiley Online Library on [08/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensethickness of 50 μm. Figure 1b shows that with an E/C of25 gAh� 1, the cell-level energy density was less than150 Whkg� 1, despite the areal capacity being over 4 mAcm� 2.Conversely, with an E/C of 2 gAh� 1, the energy densityexceeded 300 Whkg� 1. These results clearly demonstrated thesignificant impact of E/C on the cycle life and energy density ofLMBs. It is also important to note that understanding thedegradation mechanism of high-energy density LMBs accuratelyrequires analyzing Li metal in an electrochemical cell environ-ment designed with appropriate technological parameters.Similar investigations into the relationship between E/C andcell-level energy density have been conducted for other LMBs,such as lithium-sulfur (Li� S) and lithium-oxygen (LOB) batteries,yielding comparable conclusions (Figure 1c).[57] In particular, inLOB, several groups have reported high-energy density cells,and it is clear that there is a very good correlation between thecell level energy density and E/C.In high energy density LMBs designed with a high-loadingpositive electrode, limited Li, and a lean electrolyte, thedegradation of the Li metal electrode becomes complex.[58] Thelimited reservoir of Li and a small amount of electrolyteirreversibly react with each other, leading to consumption ofboth during each cycle. As a result, cell failure can occur due toa shortage of Li, electrolyte, or both. Niu et al. studied thefailure mechanisms in a 1 Ah class LMB with a cell level specificenergy of 300 Whkg� 1, which is realized by a novel cell designfeaturing high mass loaded NMC622 positive electrode(3.8 mAhcm� 2), a thin metallic Li electrode (50 μm), and alimited electrolyte quantity (3 gAh� 1).[20] The LMB cell contain-ing 1.2 M lithium bis(fluorosulfonyl)imide (LiFSI) in a solventmixture of triethyl phosphate and bis(2,2,2-trifluoroethyl) ether(TEP/BTFE, 1 : 2 molar ratio) demonstrated stable charge/discharge performance over 200 cycles, retaining 86% of itscapacity. Although the initial thickness of the Li metal electrodewas only 50 μm, it significantly increased to 120–160 μm afterjust 50 cycles and nearly 200 μm after 200 cycles (Figure 2a–c).Ex situ SEM analysis of the Li metal electrode showedconsiderable volume expansion and increased porosity of theelectrode. Cross-sectional SEM revealed an expanded porousSEI layer on the electrode surface, with unused Li underneath,indicating that electrolyte depletion was the primary cause ofcell failure. In high-energy pouch cells, the limited Li andelectrolyte, combined with significant cell swelling, result ininadequate wetting of newly exposed Li surfaces during cycling.Ensuring proper wetting of these surfaces is essential forcontinuous electrochemical reactions.The degree of cell swelling (Δt) was plotted against thecycle number (N) using logarithmic scales (Figure 2d). Theparameters log (N) and log (Δt) were found to be linearlyproportional with varying slopes, indicating two distinct regionsof Li metal structural evolution. In region I (the initial 50 cycles),the flat Li foil transformed into solid large Li particles coveredby SEI constituents, leading to rapid volume expansion of the Limetal electrode and quick cell thickening. Despite the forma-tion of Li particles, a compatible electrolyte and uniformpressure helped slow down cell swelling and extended thecycle life. In region II (subsequent long-term cycling), theexternal pressure maintained optimal contact between individ-ual Li particles, ensuring a percolation pathway for both ionsand electrons. Additionally, it drove the lean electrolyte to wetnewly formed Li surfaces, allowing electrochemical reactions tocontinue within the Li particles.Matsuda et al. examined the structural changes of Li metalelectrodes under low E/C conditions, where the areal capacitywas 4 mAhcm� 2 and the total electrolyte amount in the cellwas 10 μLcm� 2.[58] They fabricated Li jLi symmetric cells andanalyzed the structural changes of the electrodes using ex situSEM equipped with energy dispersive spectroscopic (EDS)analysis (Figure 2e-h). For the cell with a 200 μm thick Li metalelectrode, SEM images (Figure 2e, f) after the 10th Li depositionshowed an increase in thickness to 220 μm. The electrodecomprised three layers: (i) a 20 μm thick deposited Li layer, (ii) a30 μm thick porous matrix layer composed of SEI and dead Li,and (iii) a 170 μm thick unreacted Li layer. In the case where a20 μm thick Li foil was used as the electrode, the Li utilizationratio exceeded 90%. SEM analysis (Figure 2h) after the 10th Lideposition revealed that the electrode thickness reachedapproximately 90 μm, more than four times the initial Li foilthickness. Detailed examination showed the electrode consistedof three regions: (i) 5–10 μm sized metallic Li particles, (ii)Figure 1. (a) Cycling stability of coin cells with NMC622 cathodes of 3.8 mAhcm� 2, 250 μm Li foil anodes, and various amounts of electrolyte. (b) Calculatedenergy densities of Li jNMC622 pouch cells with a 50 μm Li-metal anode and various cathode loadings and various electrolyte contents. Reproduced withpermission from ref (56). Copyright 2019 Elsevier. (c) Calculated energy densities of 1 Ah pouch cells of different electrochemistry as a function of electrolyteamount (E/C). Reproduced with permission from ref (55). Copyright 2020 The Royal Society of Chemistry.Wiley VCH Dienstag, 08.04.20252504 / 381023 [S. 5/11] 1Batteries & Supercaps 2025, 8, e202400504 (3 of 9) © 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsReviewdoi.org/10.1002/batt.202400504 25666223, 2025, 4, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202400504 by National Institute For, Wiley Online Library on [08/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licenseaggregates measuring hundreds of nanometers, and (iii) micro-meter-sized voids. These voids were filled with electrolytesduring the Li dissolution/deposition reaction, facilitating Li-iontransport within the electrode.In high-energy-density LMBs designed with suitable techno-logical parameters, the significant volume expansion of Li metalelectrodes is a key factor in determining cycle life. Most studiesanalyzing the structural changes in Li metal electrodes rely onex situ SEM analysis. However, this method inevitably damagesthe electrodes during cell disassembly, cleaning, and dryingprocesses, preventing a clear identification of their actualcondition. Particularly for Li metal electrodes that have under-gone repeated charge/discharge cycles under practical cellconditions, they become very fragile due to increased porosity,and their structure easily collapses when released from the cellconstraints. Therefore, non-destructive analytical techniques areessential to accurately monitor and understand structuralchanges in Li metal electrodes. In the following sections, we willdiscuss the potential and recent advancements of X-ray andneutron-based imaging techniques for high energy density LMBanalysis.3. Application of X-Ray Imaging Technique inLMBsX-ray computed tomography (XCT) is widely recognized inmedical and scientific fields as a non-destructive imagingtechnique that derives contrast from the absorption coefficientsof different materials. The X-ray beam, attenuated by itsinteraction with the sample, is collected and processed usingadvanced algorithms to generate cross-sectional and three-dimensional (3D) images. Since its initial application in 1971,laboratory-based XCT technology has seen significant advance-ments, particularly in achieving spatial resolutions at thenanometer scale.[59–61] As a result, XCT has proven to be aninvaluable tool for evaluating batteries and related materials.[51]In particular, the use of synchrotron-based XCT with high-brilliance beamlines enables faster imaging and greater sensi-tivity compared to that of laboratory-based XCT. This translatesto faster image acquisition, enhanced sensitivity, and the abilityto achieve higher spatial and temporal resolutions. Such power-ful synchrotron-based XCT offers several distinct advantages forthe analysis of LMBs.Sadd et al. utilized an operando synchrotron XCT (Figure 3a)to track the evolution of deposited Li metal and distinguish theformation of electrochemically inactive Li from the active bulkof Li microstructures.[49] The cell material was polyether etherketone, which minimized X-ray attenuation while providinggood sealing for the electrodes and electrolyte. Additionally,the middle section of the tube fitting was cut to reduce theouter diameter of the cell, and the electrodes were mounted.This novel electrochemical cell design, combined with a highflux synchrotron X-ray beam, enabled the fast acquisition oftomograms (63 seconds for a full tomogram) with enhancedcontrast between Li and surrounding components. All tomo-grams were collected within a field of view of 0.8 mm and avoxel size of 0.325 μm, resulting in a spatial resolution ofaround 1 μm. For operando XCT measurement, a Li/Cu half-cellwith 1.0 M Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in1,3-Dioxolane (DOL)/1,2-Dimethoxyethane (DME) was used, andthe cell was cycled at 0.5 mAcm� 2 (first cycle) and 1.0 mAcm� 2(second cycle). The reconstructed tomography (Figure 3b)showed that during the first cycle of plating at 0.5 mAcm� 2,needle-like microstructures were the primary form of depositedFigure 2. Cross-sectional SEM images of Li metal anodes consisting of Li foils on both sides of the Cu foil (8 μm thick) from Li j jNMC622 pouch cells(3.8 mAhcm� 2) (a) before cycling, (b) after 50 cycles, and (c) after 200 cycles. (d) The degree of cell swelling plotted as a function of the cycle number usinglog scales. Reproduced with permission from ref (20). Copyright 2019 Springer Nature. (e,f) Cross-sectional SEM and (g) EDS image of a 200 μm thick Lielectrode from a Li jLi symmetric cell after 10 cycles (4 mAhcm� 2). (h) Cross-sectional SEM image of a 20 μm thick Li electrode from a Li jLi symmetric cell after10 cycles (4 mAhcm� 2). Reproduced with permission from ref (58). Copyright 2023 American Chemical Society.Wiley VCH Dienstag, 08.04.20252504 / 381023 [S. 6/11] 1Batteries & Supercaps 2025, 8, e202400504 (4 of 9) © 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsReviewdoi.org/10.1002/batt.202400504 25666223, 2025, 4, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202400504 by National Institute For, Wiley Online Library on [08/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseLi, reaching heights up to 60 μm after 1 hour of plating. Duringthe stripping process, some of these needle-like structuresremained electrochemically active and nearly disappeared asstripping progressed. In contrast, some Li microstructures wereonly partially dissolved during stripping, with nearly half of thedeposits remaining on the copper substrate at the end of theprocess, directly highlighting the formation of inactive Li. Whenthe current density was increased to 1.0 mAcm� 2 in the secondcycle, rapid growth of a moss-like microstructure was observed.Another significant feature observed during plating at highcurrent density was the growth of a large Li microstructure,which combined both mossy and needle-like morphologies andreached a height of 90 μm after 1 hour of plating. This type ofLi microstructure is particularly hazardous due to its rapidvertical growth and the potential for small branches topenetrate a porous separator in the cell, leading to an internalshort circuit.Several groups have reported the structural analysis of Limetal electrodes using synchrotron-based XCT.[50] However,most of these studies used custom-made electrochemical cellsdesigned to maximize spatial and temporal resolution, adaptingto the limited beam time available at synchrotron facilities.Consequently, several key technological parameters are notwell controlled from the perspective of practical LMB celldesign. For instance, synchrotron-based XCT cells often containexcess electrolytes, thick glass fiber separators, and low arealcapacity.[62] These factors significantly influence the degradationmechanism of Li electrodes and the cycle life of LMBs, asdiscussed in Section 2. Therefore, to accurately understand thedegradation mechanism of Li metal electrodes, applying XCTtechniques to high energy density LMBs is crucial. In the field ofLIBs, many studies have explored synchrotron XCT applicationsfor cylindrical and/or pouch-type LIBs. In contrast, the use ofXCT techniques for analyzing LMBs with practical cell designs islimited, primarily due to the specific technical challengesinvolved in fabricating LMB cells. To our knowledge, theapplication of synchrotron XCT to LMBs with practically highenergy density has not yet been reported. In contrast, severalstudies have used laboratory-based XCT for analyzing highenergy density LMBs, although this method requires muchlonger measurement times to achieve a resolution comparableto synchrotron XCT.[18,19] Consequently, real-time operandomeasurements are not feasible with laboratory-based XCT.However, laboratory-based XCT offers the advantage of easieraccess to machine time, allowing for more flexible experimentdesigns. This is in contrast to synchrotron XCT, where access islimited by beamline availability and scheduling, necessitatingcareful planning and coordination.Dutta et al. reported the application of laboratory-basedXCT on 300 Whkg� 1 class LMBs (Figure 4a).[18] These LMBs werefabricated with high areal-capacity NMC811 positive electrodes(30 mgcm� 2, 6.6 mAhcm� 2), thin lithium metal negative electro-des (50 μm, 9.8 mAhcm� 2), and lean electrolyte conditions(10 μLcm� 2). As a result, the E/C was less than 2 gAh� 1, meetingthe requirements for high energy density LMBs, as described inSection 2. X-ray CT analyses of the pouch-type LMB cells werecarried out with a source voltage and power of 140 kV and10 W, respectively. The measurement time was approximately12 hours, and the pixel resolution of the obtained images was3.385 μm. The fabricated LMB cells were cycled at differentdischarge rate conditions. Notably, the cell cycled at a higherdischarge current density (3 mAcm� 2) showed better cycle lifethan the one cycled at a lower discharge rate (0.6 mAcm� 2)(Figure 4b). To elucidate the detailed mechanism behind thisdischarge rate-dependent cycle life difference, XCT analysis wasperformed. Under low-rate discharge conditions, the initialthickness of the 50 μm Li-metal electrode increased to approx-imately 112 μm after 20 cycles of plating/stripping (Figure 4c).Conversely, under high-rate discharge conditions, the electrodethickness increased to 75 μm after 20 cycles (Figure 4d). Theseresults clearly revealed that the thickness growth of the Lielectrode is substantially suppressed when the cell is dis-charged at a relatively higher rate. The magnified cross-sectional XCT image and selected slice images (Figure 4e) of theFigure 3. (a) Schematic illustration of the operando X-ray tomographic microscopy setup. (b) 3D renderings of segmented Li and identification of isolatedregions, after plating and after stripping at 0.5 mAcm� 2 for 1 h. Reproduced with permission from ref (49). Copyright 2023 Springer Nature.Wiley VCH Dienstag, 08.04.20252504 / 381023 [S. 7/11] 1Batteries & Supercaps 2025, 8, e202400504 (5 of 9) © 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsReviewdoi.org/10.1002/batt.202400504 25666223, 2025, 4, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202400504 by National Institute For, Wiley Online Library on [08/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons LicenseLi electrode from the low-rate discharged cell showed a three-layered structure in the Li metal electrode: (1) the top layer nearthe separator, (2) the middle layer, highlighted by a whitedashed line, and (3) the bottom layer attached to the Cu foil.Considering the initial thickness of the Li metal electrode was50 μm, the bottom layer could be identified as the unusedmetallic Li foil. The middle and top layers could be identified ascomposites of SEI compounds and dead Li. The slice XCT imagerevealed the formation of several tens of micrometer-sizedparticles in the middle layer, indicating that dead Li was themain component of this layer. In contrast, the XCT image of thetop layer did not show a clear structure, suggesting that SEIcompounds are the main component in this layer.4. Application of Neutron Imaging Techniquesin LMBsUnlike X-rays, which primarily interact with the electron cloudsof atoms, neutrons are uncharged particles that interact withatomic nuclei. This fundamental difference gives neutron-basedimaging techniques several unique advantages. While X-rayattenuation is significantly affected by the atomic number, withheavier elements causing greater attenuation, neutron attenu-ation is largely independent of atomic number. As a result,neutron imaging (NI) is a highly effective tool for analyzing LMBcomponents, owing to the high neutron sensitivity to low-Zmaterials, such as Li and hydrogen, which are difficult tovisualize by X-ray-based imaging techniques (XIs).[63] Addition-ally, because the nucleus is much smaller than the electroncloud, neutrons are only weakly absorbed by many commonmaterials. As a result, NI can effectively probe structures hiddenwithin dense metallic environments. Another significant advant-age of NI is its sensitivity to isotopes, which allows for isotopiclabeling—a powerful technique where specific isotopes withinbattery components are selectively reported.Although NI complements XI by excelling at visualizingmaterials and features that are challenging for X-rays, thisadvantage is counterbalanced by certain limitations. Theinteraction between neutrons and atomic nuclei is inherentlyweak, resulting in lower neutron fluxes compared to X-raysources. Consequently, NI often experiences lower countsreaching the detector, leading to higher noise levels and lowersignal-to-noise ratios in the images. This necessitates longerexposure times to achieve comparable image quality to XI. Interms of spatial resolution and temporal resolution, neutron-based analysis cannot compete with X-ray-based analysis simplyfrom the viewpoint of structural analysis of metallic Li electro-des.Song et al. demonstrated the use of time-resolved NI tostudy the dendrite growth of Li, a major cause of short circuitsin Li-metal batteries, and the dynamic redistribution processesof Li during plating and stripping.[42] They used a custom-madecell (Figure 5a) with the cell body having an inner diameter of10.6 mm, which fits the neutron beam size. To maximize theamount of deposited Li on the anode side, a LiMn2O4 positiveelectrode with high mass loading (thickness: 800 μm, capacityover 20 mAhcm� 2) was utilized. A deuterated electrolyte (d-ethylene carbonate and d-dimethyl carbonate containing LiPF6(1 M) in a 3 :7 volume ratio) was used to reduce the incoherentscattering of hydrogen. In addition, to enhance the contrastbetween the Li metal deposits and the Li metal electrode,isotopic 7Li was utilized, which has a higher attenuationcoefficient than natural Li. The measurements were carried outusing a polychromatic cold neutron beam with an intensity of2.2×106 ncm� 2 s� 1 The 2D radiographic image with an effectiveresolution of approximately 100 μm could be obtained within ameasurement time of 30 sec (Figure 5b). In comparison, it takesmore than 10 hours to obtain a 3D tomographic image. The 3DFigure 4. (a) Photographic image of XCT measurement set up. (b) Capacity vs. Cycle-life plot of a Li jNMC811 cell. Cross-sectional XCT images of pouch cellAfter 20th cycle at (c) low-rate discharge (0.6 mAcm� 2) and (d) high-rate discharge (3 mAcm� 2) under low-rate charge (0.6 mAcm� 2) conditions. (e) Magnifiedcross-section XCT images of pouch cell after 20th cycle at low-rate discharge. Reproduced with permission from ref (18). Copyright 2023 Wiley-VCH.Wiley VCH Dienstag, 08.04.20252504 / 381023 [S. 8/11] 1Batteries & Supercaps 2025, 8, e202400504 (6 of 9) © 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsReviewdoi.org/10.1002/batt.202400504 25666223, 2025, 4, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202400504 by National Institute For, Wiley Online Library on [08/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensetomographic image after the cell charged for 30 hours, at whichthe estimated amount of deposited Li metal was 80 μm, isshown in Figure 5c. The white/yellow regions in the positiveelectrode/separator represent the distribution and morphologyof natural Li in the form of LiMn2O4 particles and LiPF6 salt inthe electrolyte. The 7Li metal electrode remains invisible due toits low neutron absorption cross-section. Notably, the dendriticformation of deposited Li metal was clearly observed, revealingthe effectiveness of NI techniques for structural analysis of Lielectrodes. Although 3D tomographic image provides visual-ization evidence of dendrite growth and Li distribution, thedynamic features could only be revealed by real-time 2Dradiography during battery operation. Figure 5d shows time-resolved in situ neutron radiography during the chargingprocess. Due to the high neutron attenuation coefficient of Li,regions with high Li concentration show strong attenuation ofthe incident neutron beams, resulting in reduced transmittedintensity through these areas. Conversely, regions with low Liconcentration exhibit weak attenuation, leading to increasedtransmitted intensity. As a result, the transmitted intensitythrough the high Li concentration region appeared dark,colored by a dark blue region. In contrast, the region with lowLi concentration appeared bright, which is colored by red/whiteregions. With the progress of the charging reaction, there canbe seen the deposition of Li metal deposits at the separator/Limetal electrode interface, which is colored as blue regions. Inaddition, the depletion of Li from the positive electrode regionis also observed in the form of the appearance of red/whiteregions.Ziesche et al. reported the application of neutron topo-graphic analysis of commercial CR2-type primary LMB with aheight of 27 mm and width of 15.6 mm diameter (Figure 6a).[43]The cell consisted of an MnO2 positive electrode and a Li metalnegative electrode supplied by Duracell. The tomographic NIwas carried out using a polychromatic cold neutron beam witha neutron flux of 2.7×106 ncm� 2 s. A camera with 2048×2048pixels was utilized, resulting in a pixel size of 12.9 μm. Thedischarge process was interrupted during tomography acquis-ition due to the long exposure time of 8 h per tomogram.Figure 6b revealed the orthogonal slices of the neutron tomo-gram, where the Li electrode and the excess electrolyte in themiddle of the cell are clearly visible. With the progress of thedischarge process, there can be seen the inhomogeneity of Liremoval from the Li metal electrode (blue arrow in Figure 6b).In addition, Inhomogeneous electrolyte consumption is alsoobserved as shown in the yellow arrow. Figure 6c shows thevertical cross-sectional neutron tomographic image obtainedduring the discharge process with different SOC. With theprogress of the discharge process, there can be clearly seen adecrease in electrolyte stored in the middle part of the cell. Itshould be noted that during the discharging process the excesselectrolyte, which is mainly stored in the middle core of the cell,is consumed where a part may be used for forming an excessamount of SEI. The excess electrolyte helps to compensate forthe electrolyte consumption and maintain the Li-ion conductiv-ity between the electrodes during operation. During discharg-ing, the excess electrolyte is consumed steadily and disappearsfrom the inner region of the cell.Figure 5. (a) Photographic image of the netron imaging cell. (b) 2D radiographic image after 30 sec and C) 3D tomographic image after 12 h. (d) 2D evolutionof Li distribution with different charging time. Reproduced with permission from ref (42). Copyright 2019 American Chemical Society.Wiley VCH Dienstag, 08.04.20252504 / 381023 [S. 9/11] 1Batteries & Supercaps 2025, 8, e202400504 (7 of 9) © 2024 The Authors. Batteries & Supercaps published by Wiley-VCH GmbHBatteries & SupercapsReviewdoi.org/10.1002/batt.202400504 25666223, 2025, 4, Downloaded from https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/batt.202400504 by National Institute For, Wiley Online Library on [08/09/2025]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License5. Summary and OutlookIn this review article, we overview the applications of non-invasive characterization techniques used to study the degrada-tion of LMBs. These techniques aim to address key issues suchas the morphological evolution of Li metal, interphase growth,and quantitative assessment of cell degradation withoutcausing physical damage to cell components. Non-invasivetools have successfully captured the microstructures of Li,revealing the heterogeneous growth of Li metal in threedimensions. Both laboratory-based and synchrotron XCT techni-ques have provided detailed insights into complicated struc-tural changes of Li electrodes. These observations help usunderstand Li growth behaviors and interphase formation,which are crucial for identifying the root causes of cell failure,such as Li depletion or electrolyte consumption. However, lab-based XCT suffers from low resolution, while synchrotron XCT,though resolving this issue, faces the challenge of limitedbeamtime availability in many cases. Moreover, XCT techniquesstruggle with detecting low atomic number elements. NI is ahighly effective tool for analyzing the degradation mechanismof LMB, owing to the high neutron sensitivity of low-Z materials,such as Li and hydrogen. Although the NI technique requireslonger measurement time compared to the XI technique. NI canprovide complements of XI by excelling at visualizing materialsand features that are challenging for X-rays, such as concen-tration change of Li ions and movement of the electrolyte.Despite the promising potential of Li metal electrodes, practicalLMBs face several challenges, including an insufficient under-standing of degradation mechanisms. Recent advancements instate-of-the-art techniques have shed light on some of thesemechanisms. Each technique has its limitations, but collectively,they have significantly enriched our understanding of Li metalelectrodes. To develop high-energy-density LMBs, focusing onnon-invasive characterization techniques using practical cellconfigurations is essential. Complementary use of multiple toolscan provide a more comprehensive understanding of cellprocesses, overcoming individual technical limitations.Conflict of InterestsThe authors declare no conflict of interest.Keywords: Battery analysis · Noninvasive techniques · XCT ·Neutron imaging · Li-metal electrode[1] M. Armand, J.-M. Tarascon, Nature 2008, 451, 652.[2] J. W. Choi, D. Aurbach, Nat. Rev. Mater. 2016, 1, 16013.[3] X. Shen, H. Liu, X.-B. Cheng, C. Yan, J.-Q. Huang, Energy Storage Mater.2018, 12, 161.[4] D. Lin, Y. Liu, Y. Cui, Nat. Nanotechnol. 2017, 12, 194.[5] X. Zhang, A. Wang, X. Liu, J. Luo, Acc. Chem. Res. 2019, 52, 3223.[6] X. Gao, Y.-N. Zhou, D. Han, J. Zhou, D. Zhou, W. Tang, J. B. Goodenough,Joule 2020, 4, 1864.Figure 6. (a) An illustration of the studied Li/MnO2 CR2 primary cell from Duracell. (b) Horizontal and (c) vertical slices of neutron tomograms. Reproducedwith permission from ref (43). 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Introduction 2. Lithium Metal Degradation Mechanism Under High Energy Density Condition 3. Application of X-Ray Imaging Technique in LMBs 4. Application of Neutron Imaging Techniques in LMBs 5. Summary and Outlook Conflict of Interests