Chemical staining for fundamental studies and optimization of binders in Li-ion battery negative electrodes

Domain: chemistry
Published: 2026-02-17
Editorial outcome: major_revision

Abstract
The spatial distribution of binders in Li-ion battery electrodes is critical to electrode performance, yet remains challenging to visualise, limiting binder optimisation efforts to chemical modifications rather than spatial control. Here, we show an accessible approach to staining carboxymethyl cellulose and styrene butadiene rubber binders in graphitic and Si-based Li-ion electrodes with silver and bromine, enabling detailed electron imaging and precise spectroscopic quantification of the binder domain. Leveraging these methods, we perform binder-informed optimisation of electrode manufacturing, achieving a 14% reduction in electronic resistivity, suppression of binder migration during high-temperature electrode drying, and a 40% decrease in electrode ionic resistance. Furthermore, staining enables electrode-scale, high-resolution backscattered electron imaging of complex binder hierarchies, revealing multiple types of agglomerates and elusive nanoscale binder films. These films completely coat graphitic surfaces in pristine electrodes but shatter into highly inhomogeneous fragments after calendering in both research-grade and commercial electrodes, presenting new perspectives on interpreting common cycling stability and electrode performance issues. We show how binder staining can advance fundamental understanding, quality control and manufacturing optimisation of Li-ion electrodes, particularly those based on widely used water-processable binders.

Introduction
Polymeric binders are critical in providing mechanical stability to all Li-ion battery electrodes, but occupy less than 5% of the electrode volume1. It is well known that the spatial distribution of the binder within an electrode strongly influences the cohesion and adhesion of electrode coating, cycling stability2, electronic, ionic and thermal conductivity3–6 and, thus, the longevity, rate performance and cost of Li-ion batteries1,7. Particularly for Li-ion battery negative electrodes, however, mapping the small binder fractions remain highly challenging, impeding rational, binder-informed optimisation of electrode structure, performance and manufacturing. During electrode fabrication, for example, drying-induced binder migration can severely undermine subsequent electrode stability and rate performance8,9. To prevent this, industrial roll-to-roll manufacturing of Li-ion electrodes uses relatively slow drying in 30–100 m-long ovens that dominate the footprint of a coating line1,10,11 and require large capital investment (~105 k euro/m of drying line, based on linear fitting of the published data10). As opposed to trial-and-error, efficient development of new drying approaches relies on accurate binder mapping techniques, as demonstrated in a number of studies8,9,12–15. Furthermore, among the many, often elaborate, techniques for decreasing tortuosity and increasing the power density of battery electrodes16–19, changing binder distribution at the electrode scale presents an attractive9, yet mostly unexplored alternative – because of the complex, hard-to-characterise nature of the binder domain. At a finer scale, the micro-distribution and coverage of binders on the active particle surfaces similarly remain challenging to determine6,20–24, leaving a significant gap in understanding the effects of binders on key surface processes, such as Li intercalation25 or formation of solid electrolyte interphase (SEI)26–29.

Since the early 2000’s, the water-soluble sodium salt of carboxymethyl cellulose (Na-CMC) blended with emulsified styrene butadiene rubber (SBR) has become the now-standard binder mixture used in industrial graphite and graphite/silicon negative electrodes30, and is also being explored for next-generation Si negative electrodes31 and water-processable LiFePO4 positive electrodes32,33. Being dispersible in water rather than toxic N-methyl pyrrolidone (NMP), these binders considerably lowered the cost and complexity of negative electrode manufacturing, and improved cycling stability of these electrodes1,34. Contrary to the previous-generation polyvinylidene fluoride (PVDF) binder, however, SBR and CMC lack distinct, covalently bound elements that are traceable by readily accessible methods such as energy-dispersive X-ray spectroscopy (EDX) or backscattered electron imaging (BEI). Lacking a well-defined morphology, binders are also difficult to distinguish with scanning electron microscopy (SEM). Thus, there are only a few convincing demonstrations of mapping SBR and CMC, using, for example, thermogravimetry15, Raman spectroscopy35 (both requiring unpractically thick or concentrated samples) or more sophisticated time-of-flight secondary-ion mass spectroscopy36. Most recently, Lee et al. described staining of SBR with osmium tetroxide (OsO4) that could be detected by energy-selective backscattered electron imaging (EsB) or EDX spectroscopy to trace SBR migration in graphite electrodes during drying37,38. The extreme toxicity and volatility of OsO4, however, limit this approach to only the most specialised laboratories. Consequently, most of the works describing correlations between manufacturing conditions, electrochemical performance and binder distribution in graphite electrodes utilised EDX-traceable PVDF as a binder8,9,12–14, which has different mechanical properties, binding mechanism and can show exaggerated migration during drying compared with the commercially-used CMC/SBR binders15.

To circumvent these limits, here we describe two accessible binder staining methods that can be paired with EDX, BEI or EsB for rapid mapping and high-resolution imaging of aqueous binders in Li-ion battery electrodes. We demonstrate how these methods provide fundamental insights into binder organisation within the electrodes and enable binder-informed optimisation of Li-ion battery negative electrodes.

Results
Mechanism and specificity of staining reactions
Our concept of binder staining relies on exposing electrode samples to chemical agents that attach to the functional groups present within the binders but not in the other electrode components, as shown schematically in Fig. 1. Immersing graphite electrodes in aqueous AgNO3 solution leads to Ag+ ions binding to the carboxyl groups in CMC, forming a water-insoluble complex depicted in Fig. 1a. On the other hand, exposing the electrodes to Br2 vapour brominates the aliphatic sp2 carbons of SBR, as depicted in Fig. 1b. The latter reaction is similar to OsO4 staining but with significantly lower hazard, as indicated by a 217-times higher short-term safe exposure limit for Br2 compared with OsO439. Thus, the reactions can be performed using simple equipment — a Schlenk line for bromination (Fig. 1b and Supplementary Fig. 1), and a glass beaker and a vacuum desiccator for silverization (Fig. 1a). The mechanism of both reactions was confirmed using attenuated-total reflection infra-red spectroscopy (ATR-IR), X-ray photoemission spectroscopy (XPS) and EDX, showing diminished aliphatic C=C peaks in brominated SBR, a strong shift of -COO- peaks in silverized CMC and a substitution of Ag for Na, as discussed in the Supplementary Notes 1–2 and Supplementary Figs. 2–4. The high affinity of Ag+ for carboxylate groups is not limited to CMC—silverization was also successfully applied to carboxyl-rich sodium alginate and sodium polyacrylate — two other widely researched aquoeus binders with a superior performance in e.g. silicon negative electrodes31,34,40 (Supplementary Fig. 3). Note that because of the reactivity of Ag+ and Br2, staining and binder mapping should be applied on separate electrode areas to those used in electrochemical or other testing.

To test the specificity of the staining reactions, we performed them on individual electrode components, i.e. dried films of CMC and SBR and powders of graphite and carbon conductive additive (C45). EDX and XPS were used to quantify Br and Ag concentration in the first 1 μm (EDX) and 10 nm (XPS) of the stained materials, showing a large amount of Ag in Ag-CMC and of Br in Br-SBR, and little to no binding of these elements to the other materials, as presented in Supplementary Fig. 3. Encouraged by these results, we used EDX to test the selectivity of staining on five electrode coatings containing graphite, 1 wt% C45 and 4% wt. (CMC+SBR) binder mixture with the relative CMC:SBR mass ratio ranging from 4:0 to 0:4. Each electrode coating was delaminated from the current collector, stained, and homogenised by pulverising in a mortar to remove any influence of binder migration on the quantification of staining efficiency. Figure 1c, d shows BEI of silverized and brominated electrode coatings with either only 4% CMC or 4% SBR, together with Ag and Br content quantified with EDX in all coatings in Fig. 1e. In the atomic number-sensitive BEI, the presence of Ag and Br strongly brightened contrast to the remaining materials containing mainly C, O and H. This allows clear identification of threads of Ag-CMC in the CMC-only electrode and patches of Br-SBR in the SBR-only electrode in Fig. 1c, d, respectively. Figure 1e shows that the amount of Ag in the silverized electrodes scaled linearly with the CMC content across all the CMC:SBR ratios. High selectivity of Ag to CMC over SBR was confirmed by the 16-times more Ag in the 4% CMC electrode compared to the minute Ag quantity in the 4% SBR electrode. XPS analysis combined with detailed EDX mapping of non-homogenised electrodes presented in Supplementary Notes 2–3 and Supplementary Figs. 3–5 revealed that the small apparent reactivity of Ag to SBR was due to the reaction of Ag+ with leftover reagents used in the manufacturer’s SBR emulsion-polymerisation, such as ionic surfactants, persulfates and thiols41,42, whose presence was indicated by Na, K and S found in the top of both SBR-only electrode and pure SBR sample. This analysis also showed that in non-homogenised electrodes, the negative impact of these reagents on the Ag-CMC staining selectivity may be larger in the top electrode region, where these compounds appeared to concentrate.

The amount of Br in the brominated samples also scaled linearly with the SBR content in the electrodes, with 7-times more Br in the 4% SBR electrode compared to the 4% CMC electrode (Fig. 1e). The small amount of Br binding to CMC was due to surface oxidation of Na-CMC by Br2 and binding of Br to Na (see exemplary Hunsdiecker reaction scheme in Fig. 1b)43. The associated formation of NaBr was suggested by the 1:1 elemental ratio of Br to Na detected by local EDX mapping inside the porous, high surface-area CMC/C45 agglomerates, as well as the mostly ionic (Br−) form of bromine detected with XPS in the top few nanometres of brominated CMC, as presented in Supplementary Notes 2, 4 and Supplementary Figs. 3–4 and 6. The formation of NaBr on the CMC surface enabled BEI observation of CMC in brominated electrodes (Fig. 1d), similarly to Ag binding to CMC-COO− groups in silverized electrodes (Fig. 1c). The ionic binding of Br in brominated CMC was also stable under the SEM/EDX electron beam, contrary to the less stable Br bound to polymeric SBR, which progressively outgassed from the SBR-containing electrodes during continuous electron irradiation, as visible in time-resolved Br tracing in Fig. 1f. The extent of Br outgassing scaled linearly with SBR amount in the samples (Fig. 1g), suggesting it could be used to verify SBR quantification in brominated electrodes containing CMC — as we demonstrate below.

Staining bi-layered electrodes
Having established the high specificity of the staining reactions, we tested their sensitivity to varying binder fraction within graphite electrodes, as presented in Fig. 2. This was verified using a model, ~300 μm-thick bi-layered electrode spray coated onto a Cu foil using a method reported in our previous work refs. 44–46. By consecutively spraying and instantly drying two slurries with relatively high and low binder concentration, we designed this electrode to have a 4-times difference in binder fraction between a lower, binder-rich layer (with 4.5% SBR and 3.0% CMC by weight) and an upper, binder-lean layer (with 1.13% SBR and 0.75% CMC), keeping the overall binder content within the typical range of Li-ion electrodes. After fabrication and calendering, electrode fragments were stained with either Br2 or Ag+, cross-sectioned using Ar plasma milling, and analysed using EDX and BEI. Note that before silverization, the Ag-treated electrodes were detached from the current collector to avoid reaction of Ag+ with the Cu foil current collector, as explained in “Methods”.

The EDX maps of Ag-L and Br-L X-ray series presented in Fig. 2a show an expected bi-layered distribution of the staining elements, which is reflected in the derived through-thickness Ag and Br profiles in Fig. 2b. The profiles were individually ratioed and indicate 3.9±1.8 (silverized) and 3.1±0.4 (brominated) average element ratio Δ between the binder-rich and binder-lean layers, also summarised in Fig. 2e (note, ± corresponds to 95% confidence intervals). These average values are within 2.5% and 22% of the expected 4× ratio, with more undercounted Δ for the Br-stained SBR due to the larger susceptibility of SBR to drying-induced migration amplified by its weaker adsorption on graphite47, as verified in Supplementary Note 5 and Supplementary Fig. 7. The element ratios in individual samples were also within 5–23% (silverized) and 4–6% (brominated) of the average values, indicating good precision in single 1 mm-wide cross-section measurements. The addition of a surfactant (1 mM Triton X-100) to the AgNO3 staining solution benefited wetting and accuracy of Ag-CMC profiling through the binder-lean region of the samples, as discussed in the wetting analysis in Supplementary Note 6 and Supplementary Fig. 8. The higher complexity of liquid-phase silverization and lower intrinsic Ag signal in Ag-CMC (as quantified in Fig. 1e) could also explain the larger spread of silverized Δ’s compared to the brominated ones (Fig. 2e).

For the Br-SBR analysis, we verified its accuracy by following Br outgassing. This was done by subtracting Br-L traces derived from EDX maps acquired consecutively at increasing pixel dwell time (corresponding to the total mapping time and total electron dose). Fig. 2c shows that the differential (outgassed) Br-L profiles closely followed the absolute Br-L profiles, with a similar outgassing Δ of 3.5±0.9× averaged across the three samples (Fig. 2e). Also, for the absolute EDX Ag-L and Br-L profiling, increasing mapping time did not significantly change the Δ values - accurate through-thickness binder profiling across these ~350 μm-thick electrodes could be performed as rapidly as 8 min for Br and 24 min for Ag (Fig. 2d). Although generation of higher quality EDX maps required at least 1 h (e.g. 3 h for the maps in Fig. 2a), the mapping time could be reduced to just a minute using BEI (Fig. 2a), due to much higher yield of backscattered electrons than electron-generated X-rays48. By analysing the distribution of white pixels in BEI images after multilevel Otsu thresholding49, they could also be used for quantitative binder profiling. Except for two silverized samples, the BEI-derived pixel profiles in Fig. 2b matched the EDX elemental profiles and gave similar Δ values. However, cross-sectional BEI remained more uncertain due to its sensitivity to image artefacts (e.g. particle surface charging) and trace metallic contamination after ion polishing, as shown in Supplementary Figs. 9 and 10.

EsB imaging of local binder morphology
Having established the staining methodology, we investigated detailed local binder organisation inside graphitic electrodes by combining staining with EsB. EsB is a low beam energy-variant of BEI that utilises a polarised filtering grid in front of an in-lens backscattered electron detector to capture backscattered electrons originating only from a shallow depth of the analysed surface, with high compositional resolution50–52. The method has been used previously for mapping the general distribution of PVDF and OsO4-stained SBR in graphite electrodes, but without focusing on local binder morphology37,52.

Figure 3 shows secondary electron and EsB images of the interior of an uncalendered standard graphite electrode (with 2.2 wt% SBR and 1.5 wt% CMC) after silverization. The images were acquired at low and high magnification (Fig. 3a, b, respectively) and gradually decreasing beam voltages, corresponding to decreasing electron-sample penetration depth and increasing surface sensitivity in consecutive images48. At 10 kV, the EsB images show bright, few micron-sized agglomerates of stained CMC and C45 conductive carbon nanoparticles, localised mostly between graphite particles (denoted as α phase in Fig. 3a, b). Containing a large number of C45 nanoparticles, these agglomerates influence long-range interparticle electronic conductivity within the electrode53 (also see the “Correlating slurry mixing, CBD agglomeration and the electrical resistivity of graphite electrodes” section below). The contrast from these agglomerates at 10 kV originated from a relatively bulky stained binder within their composite structure. Consequently, this contrast mostly disappeared at lower beam voltages of 2.5 and 2 kV (shown in Fig. 3a), where the depth probed by backscattered electrons was reduced from a few hundred to a few dozen nanometres (see the simulations in Fig. 3c). At these lower beam voltages, a second agglomerate type containing tightly packed, dark SBR nanoparticles mixed with CMC and C45 became visible (β morphology in Fig. 3a, b). The differentiation between C45, CMC and SBR was achieved by comparative analysis of electrodes prepared without SBR or C45, presented in Supplementary Fig. 11. Due to the large concentration of SBR in these agglomerates, they provide mechanical cohesion and elasticity to the electrode54–56. Finally, the EsB images acquired at even lower beam biases of 1.5 kV and 1 kV surprisingly revealed a third, continuous-layer morphology of binder (termed γ in Fig. 3a, b) that coated large regions of graphitic surfaces and was not visible at larger beam voltages. This binder layer was also not visible in control EsB images of a non-stained electrode or of a silverized CMC-free electrode, being only visible in the CMC-containing electrodes (Supplementary Fig. 12). These comparator experiments confirmed that bright EsB contrast at 1.5 and 1 kV in Fig. 3 unambiguously corresponded to a continuous thin film of silverized CMC that also embedded some non-stained (dark) SBR nanoparticles, visible in Fig. 3b. These observations were reproduced in several hundreds of EsB images acquired on four separately-made graphite electrodes after staining, some of them shown in Supplementary Fig. 13.

Monte Carlo simulations were used to estimate the thickness of the CMC layer based on its changing visibility at different EsB imaging voltages, as described in “Methods”. Figure 3c shows the simulated depth of electron backscattering events in an Ag-CMC layer on a graphite particle at different electron beam energies. As an approximate detectability criterion, we assumed that the proportion of the electrons backscattered in the Ag-CMC layer to the electrons backscattered in the underlying graphite should be much larger than 0.5 at 1 keV and 1.5 keV for detection, and similar or lower than 0.5 at 2 keV and 2.5 keV for no detection. The plots of the cumulative distribution of backscattered electrons in Fig. 3c show that this condition was met for an Ag-CMC thickness of only 10-15 nm, which is in the same range as the thickness derived from high resolution SEM/EsB imaging of a fractured graphitic particle (Supplementary Fig. 14), as well as the assumed uniform binder layer thickness in graphitic electrodes calculated by Landesfeind et al.57. Because the CMC layer was so thin, it closely followed the morphology and contours of the underlaying graphite surfaces. This is why the layer could not be distinguished from bare graphite using conventional secondary electron microscopy (both in this and in previous works), even at very low beam voltages (Fig. 3b). Also note that in the ion-polished samples, the CMC layer was completely masked by trace metallic contaminants deposited on graphitic surfaces after the polishing (Supplementary Fig. 10). The thin layer could also be observed in EsB images of silverized electrodes with an analogous polyacrylate binder (Supplementary Fig. 15). Previous works have postulated that binders form nanoscale layers on active particle surfaces in Li-ion electrodes6,58–61, but the visual evidence for such binder morphology, until now, was limited to only a few higly localized spectroscopic transmission electron microscopy (TEM) studies22–24.

Binder coverage on active particle surfaces
The EsB images in Fig. 3a, b were acquired from an internal, delaminated region of an electrode where the bright CMC layer appeared as a patchy layer, allowing for differentiation from dark uncoated graphite surfaces. To assess if delamination might have affected the binder distribution, we also acquired images from the electrode top (not delaminated) surface (Fig. 4). The images of the silverized electrode in Fig. 4a show that the CMC layer completely coated graphite particle surfaces, with additional brighter patches of Ag bound to CMC or SBR emulsifiers that migrated to the surface during electrode drying. These EsB images also show additional dark areas that were not stained by Ag+ but produced charging in the secondary electron images (see the marked areas in Fig. 4a). As shown in Fig. 4b, on bromination, these areas appeared bright in the intially-acquired EsB images and rapidly faded in subsequent images. Recalling the electron-induced outgassing of Br from Br-SBR, this allowed in identifying these patches as SBR. Additionally, high magnification images show that they consisted of self-assembled individual SBR nanoparticles (the same, less agglomerated SBR particles were also visible in the CMC layer in Fig. 3b). That SBR retains nanoparticle form in the electrode results from the mild mixing conditions required during slurry preparation to prevent SBR demulsification62. The island-like agglomeration of SBR compared to the uniform layer appearance of CMC also evidences their different affinity to graphite surfaces47. Furthermore, Br outgassing from the “free” (not agglomerated with CMC) brominated SBR nanoparticles differentiates them from the superficially brominated CMC layer, whose EsB contrast remained unchanged during consecutive image acquisitions (see the images of an internal part of the brominated electrode in Supplementary Fig. 16). The outgassing of Br-SBR also allowed for quantification of SBR particle coverage by subtracting a later-acquired EsB image from the first EsB image (see Methods). We estimate that the ~120 nm thick SBR agglomerates covered 8–9% of the apparent graphite surface area on top and within this electrode, with an additional 6% of the internal electrode surfaces covered by the SBR/CMC agglomerates. The similarity of binder morphologies detected with EsB inside silverized (Fig. 3) and brominated (Supplementary Fig. 16) electrodes also suggested that the binder distribution itself was not affected by the staining procedures.

During the manufacturing of Li-ion electrodes, slurry coating and drying are followed by compressing (calendering) the electrodes between two metal rolls. Calendering reduces inter-particle porosity and often improves electrode electrical conductivity63, but its effect on binder distribution is largely unknown. Thus, we analysed binder surface coverage after hot calendering the electrode to 30% porosity at 80°C. The EsB images from the top and interior of the calendered electrode in Fig. 4c show fracture and delamination of both the γ binder film and β SBR/CMC/C45 agglomerates. Fracture resulted in large, irregularly distributed areas of graphite particles where the previously near-complete CMC layer was shattered and partially delaminated. The remaining CMC patches covered only 32±5% of the particle surface, based on gaussian filtering and grey-level thresholding of five EsB images (Fig. 4e). On the top electrode surface directly exposed to the hot calender rollers, CMC coverage was even lower (21±3%, also visible in Fig. 4c). The shattering of the CMC layer was consistent with the reported brittleness of dried CMC64. These findings are not limited to laboratory-made electrodes—we also imaged uncycled commercial electrodes from LiFUN Technology (Xinma Industry Zone, Hunan Province, China). Figure 4d shows identical shattered CMC layers and SBR agglomerates in these electrodes, with over 60% of graphitic surfaces left bare and uncoated (Fig. 4e). Such high inhomogeneity in binder surface coverage can be expected to strongly affect surface processes on active material particles, such as Li+ (de)intercalation or formation of SEI, as explained in detail later.

Correlating slurry mixing, CBD agglomeration and the electrical resistivity of graphite electrodes
To demonstrate how binder mapping can benefit the development of Li-ion electrodes, we carried out two practical studies. First, we investigated the relationship between the conditions of slurry processing, the distribution of conductive carbon/binder domain (CBD) within the electrodes, and electrode electrical resistivity (Fig. 5). In the graphite electrodes prepared using our standard slurry mixing protocol shown in Fig. 5a, large CBD clusters could be readily detected after bromination or silverization with BEI (Fig. 5b) and EsB at 10 kV (Fig. 4a). Although the spatial density of these clusters was moderate (8.5 cluster per mm2), they had a large geometric area of more than 100 μm2/cluster, shown in the associated histogram in Fig. 5c. This implied that a fraction of binder-embedded C45 conductive carbon particles was ineffectively and inhomogeneously distributed across the electrode. To reduce CBD agglomeration, we fabricated electrodes of the same dry mass composition, but using a more concentrated CMC solution (increased from 1.5 to 3 wt%) during an initial mixing step. After subsequent mixing with graphite, the slurry was diluted with water to the same final water fraction as in the standard slurry, as shown in Fig. 5a. We hypothesized that because the amphiphilic CMC electrostatically stabilises solids dispersion in the slurry by adsorbing onto the graphite and carbon particle surfaces47, a more concentrated CMC solution in the first mixing step might increase CMC saturation on C45 surfaces, improving C45 dispersion in the slurry and in the final electrode. Figure 5b shows a BEI image of such a modified electrode after bromination, confirming the disappearance of large CBD clusters, which is also evident in the histogram in Fig. 5c. Interestingly, such a level of CBD de-agglomeration could not be achieved using zirconia balls during mixing of the standard 1.5 wt.% CMC slurry, indicating a stronger influence of the initial CMC concentration on the CBD dispersion (Supplementary Fig. 17). Recently, Park et al. similarly found that in increasing the initial CMC concentration during mixing with carbon black particles improved their dispersion in wet slurries65. Furthermore, four-point probe measurements of not-stained samples showed that reduced CBD agglomeration decreased mean electronic resistivity in both uncalendared and calendered electrode coatings by 14±2% (Fig. 5d), albeit with a somewhat larger uncertainty after calendering. While the absolute electronic resistivity of both electrode coatings was low (~0.1 Ω×cm) due to the omnipresence of electronically conductive graphite, these findings could help to optimise other electrodes based on more resistive active materials, e.g. LiFePO4 or sulfur, and similar binder/carbon additives. Future staining-assisted optimisation could investigate alternative pre-mixing methods of thinner, lower CMC-content slurries, or substituting C45 nanoparticles with carbon nanotubes or graphene for further resistivity decrease6.

Mitigating binder migration during rapid electrode drying through phase inversion
In the second application of binder staining, we explored phase inversion as a method to prevent binder migration during fast, high-temperature electrode drying (Fig. 6). Phase inversion is a roll-to-roll compatible process, where a polymer solution (e.g. PVDF in NMP) is immersed in a non-solvent (e.g. water) that destabilises the solution and causes rapid precipitation of the polymer, as schematically shown in Fig. 6a66. Several studies demonstrated that this process can improve manufacturability and cycling performance of >100 μm thick PVDF- and poly(ethersulfone)-based battery electrodes, partly due to induced directional porosity or altered PVDF morphology67–69. However, the effect of precipitating binders before electrode drying on final binder distribution has been difficult to assess. Therefore, we first screened 17 potential non-solvents and found that acetone and isopropanol rapidly precipitated CMC from aqueous solution, as shown in Supplementary Fig. 18. Then, we applied phase inversion to freshly-coated, wet graphite electrodes by dipping them for 3 min in isopropanol or acetone before drying at 120°C and calendering at 80°C, and compared them with a reference electrode processed identically but without phase inversion. Following staining of the binders, the EDX maps and binder profiles in Fig. 6b, c show that high-rate drying increased the CMC and SBR fraction at the top of the reference electrode. Blocking of the top surface pores with excessive binder obstructs the Li+ access to the electrode volume and leaves less binder at the interface with the current collector, lowering electrode adhesion9. Nonetheless, the migration was not as severe as in PVDF-based electrodes, in agreement with the recent results of Lee et al.37. Figure 6b shows that immersion in isopropanol further concentrated the binders at the top electrode surface by forming a dense polymer “skin layer”, likely associated with a relatively slow isopropanol permeation and delayed precipitation of the binder66,67. On the other hand, the electrode immersed in acetone showed a marked binder concentration near the current collector, possibly due to faster precipitation of CMC by acetone with 6× lower viscosity than isopropanol70. The greater binder fraction at the current collector allowed to extensively bend this relatively thick electrode without cracking, making it significantly more adherent to the current collector compared with the two other electrodes with the opposite binder distribution, as tested on not-stained samples (Fig. 6d). The EDX results were reproducible on separately made, processed and stained electrodes (Supplementary Fig. 19).

To explore effects on pore ionic resistance, we tested unstained samples using electrochemical impedance spectroscopy (EIS) in a non-intercalating electrolyte developed by Landesfeind et al.3,57. Figure 6e shows the Nyquist plots of the reference and acetone-processed electrode in which the pore resistance was derived by extrapolating the low-frequency, near-linear regions of the plots4. Figure 6f shows that the measured ionic resistance was 40% lower for the acetone-processed electrode compared to the reference electrode, with the actual resistance difference possibly even larger71, as discussed in "Methods". The process did not change the comparatively low electronic contact resistance of the coatings, indicated by the absence of a high-frequency semicircle in the Nyquist plots3,72 (highlighted in Supplementary Fig. 20). Considering that all electrodes had identical composition, thickness and total porosity, the marked reduction in ionic resistance was a sole consequence of the preferential binder (and porosity) distribution, in agreement with simulations9,73. As demonstrated by Morasch et al. on a high-temperature-dried, mechanically delaminated and upturned electrode with a similar final binder distribution, reduced pore resistance leads to significant improvements in electrode fast charging and discharging performance9, and likely extends electrode lifetime74,75. Our results show that the same preferential binder distribution and reduced ionic resistance could be realised with a much more practical and potentially up-scalable phase inversion, which simultaneously enables rapid electrode drying. Combining phase inversion with using spherical (instead of flaky) graphite or pore patterning techniques could further improve ionic transport in such electrodes5,16–19.

Staining limitations
Finally, we outline the present limitations of this study. First, the staining methods, particularly the liquid-phase silverization, are less applicable to electrodes containing easily-oxidizable materials (e.g. metallic, LiFePO4 or nano-Si particles), as these can rapidly undergo redox reactions with the staining elements, complicating image interpretation. Micro-Si or materials with an inert, stable surface passivation layer can be, however, suitable. This is shown in Fig. 7, which demonstrates successful staining on Si and C-coated SiOx microparticle-based electrodes but not on the more reactive LiFePO4 or nano-Si. For the same reason, silverization requires impractical detachment of any Cu current collector prior to staining, and the likely alteration of the electrode’s electrochemistry by Ag+ and Br2 staining may restrict its applications, for example, in operando studies or online manufacturing quality control. Also, despite the time- and accessibility advantages of BEI, this imaging mode has a limited applicability for mapping Ag/Br-stained electrodes containing other high-Z elements (e.g. Si) where the use of EDX may be necessary (see BEI vs EDX in Fig. 7c). The same limit applies to EsB, where the contrast remains largely relative, highly sensitive to surface contamination, and requires calibration (as demonstrated herein)50. Furthermore, to minimise geometrical artefacts, the cross-sectional binder analysis requires lengthy sample plasma cross-sectioning (~8–12 h for 100 μm-thick electrodes), which might be shortened using laser pre-cutting or resin impregnation and microtomy. Also, because Ag+/Br2 staining targets carboxylate and aliphatic C=C groups in the binders, processes that alter these groups beforehand (e.g. drying-induced cross-linking) may reduce Ag/Br uptake (which, however, could be exploited to quantify binder cross-linking). Consequently, bromination and silverization are not suitable for staining e.g. PVDF or PTFE binders (that lack these functional groups), but can be suitable for other binders, such as the carboxyl-rich alginates and polyacrylates (as shown in Supplementary Fig. 15). However, the accuracy of spatial mapping of these two binders in electrodes remains to be fully quantified.

Methods
Statistics
The uncertainty (±) of values reported in this work corresponds to their expanded 95% confidence intervals estimated with the Taylor Series Method or the Monte Carlo method, considering both random and systematic uncertainties of the values.

Chemicals
Artificial graphite (BTR S360-E3, flakes, nominal D50=16 μm), sodium carboxymethyl cellulose (degree of substitution=0.8–0.95, Ashland), SBR (40% wt. suspension in water), carbon conductive additive (C-NERGYTM SUPER C45, Imersys), bromine liquid (99.8%, Alfa Aesar), silver nitrate (99.9999% trace metal basis, Sigma Aldrich), silicon powders (325 mesh: 99% trace metal basis, Sigma Aldrich and <50 nm: 98%, Alfa Aesar), C/SiOx powder (High Capacity Silicon Based Anode Powder, MSE Supplies), ethylene carbonate (≥99%, acid <10 ppm, H2O <10 ppm, Sigma Aldrich) and dimethyl carbonate (anhydrous, ≥99%, Sigma Aldrich) were used as received. The as-received tetrabutylammonium perchlorate (for EC analysis, ≥99.0%, Merck-Supelco) was dried in a vacuum over silica gel at room temperature for 3 days prior to bringing it into an Ar-filled glove box to prepare the non-intercalating electrolyte for the EIS analysis. The water used for the preparation of aqueous solutions and electrode slurries was type II deionised water (0.5 μS/cm, TOC < 50 ppb). The battery-grade SBR was of a popular brand with known application in industrial Li-ion battery manufacturing, whose name is not disclosed for confidentiality.

Electrode fabrication
Typical graphite electrodes were prepared from liquid slurries mixed using an Interionics Thinky ARE-250 planetary mixer in 35 mL polypropylene pots. First, 6 g of 1.5% wt. aqueous Na-CMC was mixed with 60 mg of C45 powder at 500 rpm for 1 min and 2000 rpm for 10 min. Then, 5.71 g of graphite powder was added and mixed at 500 rpm for 1 min and 2000 rpm for 10 min, followed by degassing at 2200 rpm for 3 min. Note that in the CBD agglomeration study (in “Correlating slurry mixing, CBD agglomeration and the electrical resistivity of graphite electrodes”), the graphite mixing was divided into two 5 min intervals with an additional 1 min 500 rpm mixing step in between, for full comparability with the modified procedure described in the same section. Finally, 0.33 g of 40% wt. aqueous SBR suspension was added and mixed at 500 rpm for 5 min. The as-obtained slurry with a total solid mass content of 49.3% was then blade-casted onto 10 μm-thick, isopropanol-wiped and dried Cu foil, using a digital adjustable applicator (QC Laboratory Equipment Ltd.), and dried in a fan convection oven for 30 min at 50°C. The typical electrodes had a dry mass composition of 95.21±0.06 wt% graphite, 2.22±0.02 wt% SBR, 1.51±0.01 wt% CMC and 1.05±0.06 wt% C45, based on the weight of the materials used in 10 separately-made slurries. The electrodes were then cut into 20 mm-wide rectangular stripes and calendered using Sumet CA3 hot calender (150 mm drums width) operating at 80°C, 0.2 m/min pass speed and 35 N/mm set force (corresponding to 263 N/mm of force normalised to the sample width), to produce 30% porous electrodes (standard deviation = 1%, n = 28).

Bromination of electrodes
The electrodes were brominated after vacuum-drying by exposing them to vapours of Br2 in an air-free setup depicted in Supplementary Fig. 1. The reaction was carried out inside a small glass reactor at 50 oC and Br2 pressure of 5–8×10−2 mbar for 270 s, followed by outgassing of excess Br2 under active vacuum (~2×10−2 mbar). The temperature and time were chosen to ensure no condensation of Br2 vapours inside the reactor and to facilitate complete reaction with SBR while minimising the processing time. Br2 vapours were provided by the small Br2 pool placed in a separate glass vial immersed in a small water bath (~20°C) and connected to the pre-evacuated and heated reactor (~2×10−2 mbar, 50°C) under static vacuum. The bromination conditions were applied without any further optimisation. After bromination, the initial samples were kept in a N2-sealed vessel until EDX/BEI analysis to avoid humidity-induced hydrolysis of brominated SBR; however, our later EDX tests also indicated no significant loss of Br from the electrodes in air for at least two weeks prior to the EDX/BEI analysis.

Staining electrodes with Ag+ (aq)
The electrodes were silverized by immersing them in a crystallizer containing 0.5 M AgNO3 for 3 min (depicted in Fig. 1a). For the CMC analysis of homogenized and bi-layered electrodes (in “Mechanism and specificity of staining reactions and Staining bi-layered electrodes”, respectively), the 0.5 M AgNO3 solution was freshly mixed with 1 mM Triton X-100 non-ionic surfactant prior to staining to help with penetration of the solution into all of the electrode, as discussed in detail in Supplementary Note 6 and Supplementary Fig. 8. During all silverizations, the crystallizer with AgNO3-soaked electrodes was briefly placed in a desiccator under mild vacuum to facilitate penetration of AgNO3 solution within the electrodes. After that time, the electrodes were taken out of the solution, washed with copious water and dried. Following silverization of the initial samples, there were visual signs of corrosion of the Cu foil and micron-sized Ag particles in random areas in the SEM images of the samples. This points to galvanic replacement of Ag+ with Cu, according to Eq. (1):

2Ag+ (aq) + Cu(s) → 2Ag0(s) + Cu2+ (aq) (1)

To prevent it, prior to staining further electrodes, they were first carefully delaminated and transferred from the Cu foil support onto adhesive carbon SEM discs supported on a polypropylene film that was inert to Ag staining.

Characterization
Secondary electron microscopy, BEI and EDX spectroscopy were performed using Zeiss Merlin field-emission SEM paired with Oxford Instruments (OI) XMax 150 mm2 and windowless XMax 80 mm2 EDX detectors operating in double detector mode, as well as Hitachi TM3030 table-top SEM paired with OI 30 mm2 EDX detector equipped with SATW polymer window (used for BEI in “Correlating slurry mixing, CBD agglomeration and the electrical resistivity of graphite electrodes” and for the EDX analysis of stained individual electrode components, presented in Supplementary Information). Typical EDX acquisitions were performed at an acceleration voltage of 10 kV and a spectral resolution of 5 eV/channel using TruMap mode, following energy-calibration against a 99.9% pure Cu standard. Elemental quantification was performed using factory standards provided in the OI Aztec software version 6.1. Systematic uncertainty of the EDX quantification was verified using freshly cleaned Ag wire and dried C-coated KBr pellet with 99.9% and ≥99% trace metal-basis purity, respectively, giving 1.3±1.0% and 3.2±1.5% relative bias for Ag and Br quantification, respectively. Considering ~4% uncertainty due to the roughness/porosity of the graphitic samples48, we estimate the relative combined systematic uncertainty of Ag and Br quantification at the applied conditions and 95% confidence level as ±8.4% and ±10.4%, respectively. All EsB images were acquired at a filtering grid voltage of −513 V.

Attenuated total reflection infra-red spectroscopy (ATR-IR) was performed using a Varian Excalibur FTS 3500 FT-IR spectrometer equipped with a DTGS detector and Specac Golden Gate ATR accessory (monolithic diamond window, single bounce, 45° incidence angle, KRS-5 lenses) operating in air. For each sample, the background-corrected spectra were averaged from 256 scans acquired at 2.5 kHz speed and 4 cm−1 resolution.

XPS was performed using a Thermo Scientific K-Alpha spectrometer using a monochromatized Al K-alpha X-ray beam with an energy of 1486.7 eV and a voltage of 12 kV. For survey scans, a pass energy of 200 eV and a step size of 1 eV were used. For narrow scans, a pass energy of 50 eV and a step size of 0.1 eV were used. The binding energy of the spectra was corrected using the fitted saturated C-H peak at 285.0 eV. Spectra processing was done using CasaXPS 2.3.2777.

Electrical resistivity of electrode coatings was measured using an Ossila four-point probe system equipped with spring-loaded soft contact cylindrical golden probes with 0.48 mm diameter and 1.27 mm spacing. Because the technique requires samples to be supported by non-conductive substrates, to measure uncalendared coatings, they were slurry-casted on polypropylene sheets and cut into 30×30 mm samples after drying. To measure calendered coatings, coatings were first fabricated on Cu foils and calendered to 228 μm thickness and 32% porosity at 80°C, delaminated from Cu foils, cut to 25×25 mm squares and measured on top of a glass substrate. Each sample was measured 10 times, rotating it by 90° between the measurements, and the results were averaged and normalised to sample thickness (the measurements are summarised in Supplementary Table 1).

To determine the pore ionic resistivity, the electrodes were first cut into 16 mm-diameter discs and dried under active vacuum at 120°C overnight. After drying, the electrodes were brought to an Ar-filled glove box (H2O and O2 level below 10 ppm) without breaking the vacuum and assembled in symmetric 2-electrode cells using EL-CELL PAT-Cell test cells equipped with a 260 μm-thick, 91% porous Whatman GF/A glass fibre separator. The electrolyte was 100–140 μL of 10 mM tetrabutylammonium perchlorate in 1:1 wt. ethylene carbonate/dimethyl carbonate. Care was taken to maximise the alignment of the top electrode directly above the bottom electrode by using a vacuum pen. EIS was performed using a Biologic SP-150 potentiostat in an air-conditioned room maintained at 21±1°C. The measurements were done at the DC potential equalling the open circuit potential of each cell, in the 100 kHz–10 mHz frequency range, using an AC potential amplitude of 15 mV, averaging five measures per frequency and performing three full frequency scans for each cell (with a resting period of 2 h in between the measurements). Before the first measurement, the cells were allowed to equilibrate at the open circuit potential for 8 h. The electrodes had an average loading of 14±1 mg/cm2 and 30±3% porosity. Note that most recently, Bieneck et al. showed that the pore ionic resistance determined with symmetric-cell EIS can be strongly influenced by the spatial distribution of the double layer capacitance (Cdl) of carbon black, and its contribution to the overall Cdl of the electrode71. Based on the mass composition of our electrodes and the BET surface area of the graphite and C45 carbon black (1.47 and 45 m2/g, respectively), the estimated contribution of C45 to the total Cdl of the electrodes was ~25%. The analysis of Figure 9 in Bieneck’s paper shows that for such C contribution, the pore ionic resistance of the reference electrode (with CBD concentrated close to the separator) determined with EIS may be underestimated by ~12% and that of the acetone-processed electrode (with CBD concentrated close to the current collector) may be overestimated by ~23%, implying that the actual pore ionic resistance difference between these electrodes may be ~57% instead of the apparent 40%.

EDX data processing
All the EDX maps and derived elemental profiles were acquired and processed using Oxford Instruments Aztec 6.1 SP2 software, using signal filtering at 99% or 95% confidence threshold. The combined EDX maps in Figs. 2 and 6 were prepared by binning Ag-L and Br-L elemental maps with a factor of 2, followed by clipping 5% of the high intensity pixels and stacking with C and Cu maps using weighting settings of 1.0 (Ag or Br) and 0.4 (C and Cu) for an optimum visibility of the staining elements against the sample matrix. Note that the maps of silverized samples did not include Cu, as these samples were detached from the Cu foil prior to staining and analysis. All the EDX Br-L and Ag-L line profiles were checked for EDX geometrical artefacts by following the reference C-K signals, which should not show a significant gradient through the thickness of electrode coatings. In one sample (silverized reference electrode in the phase inversion study), a 35% positive linear gradient was present in the C-K signal from the bottom to the top of the coating. Hence, the Ag-L signal was corrected by subtracting this gradient using Eq. (2):

ycorr = y − aS(x − x0) (2)

where y and ycorr is the raw and corrected Ag-L signal, respectively, x is the distance (in μm), a is the linear regression slope (gradient) of the C-K signal vs x, fitted between x0 = 50 μm and xend = 137 μm (the part of the EDX scan corresponding to the electrode coating), and S is the scaling factor (the ratio of the integrated Ag-L to integrated C-K signals). The raw and corrected Ag-L and reference C-K signals are provided in the Source Data file.

Binder quantification from BEI and EsB images
To derive the vertical distribution of binders from BEI images, the images were segmented using the Multi Otsu Threshold plugin78 in the Fiji 2.14.079 package. Most of the images were segmented using three levels corresponding to the bright stained binder phase, grey graphite phase and black interparticle pore phase. In a few images with visible strong charging artefacts at the graphite particle edges, the number of levels was set to 4 to account for these artefacts, giving a smaller error in the binder phase detection.

To quantify CBD agglomerate size distribution from the BEI images in the “Correlating slurry mixing, CBD agglomeration and the electrical resistivity of graphite electrodes”, the images were first processed using Smart Blur (setting 50) in CorelDRAW 2021. Then, they were thresholded in Fiji using Sauvola local threshold (radius = 100, parameter 1 = 0.2, parameter 2 = 7) followed by the particle size analysis using Fiji’s default “Analyze Particles” function, considering agglomerates with at least 1 μm2 surface area.

For quantification of CMC surface coverage from EsB images of silverized electrodes, for the images with high EsB noise, they were first de-noised using the default Fiji smooth filter. This was followed by selectively removing dark interparticle pores from the analysis area using Phansalkar's local threshold algorithm (radius 100, parameter 1 = 0.7, parameter 2 = 0). This was followed by applying several local and general threshold algorithms (e.g. Bernsen, Niblack, Moments, Huang) on copies of the image and averaging the results. For quantification of SBR coverage from EsB images of brominated electrodes, two EsB images acquired at the first and 3rd–5th scan (before and after Br outgassing, respectively) were first registered using the affine SIFT algorithm80, cropped on edges, and de-noised using median and non-local means filter (sigma = 5, smoothing factor = 1). The Yen threshold was applied on the nth EsB image to quantify SBR/CMC agglomerates, which were subsequently removed from the region of interest in the 1st EsB image. Phansalkar's local threshold was used to remove dark interparticle pores, followed by subtracting the nth image from the 1st image and applying Otsu or Yen threshold to quantify the surface SBR agglomerate areas. The macro used for quantification is presented in Supplementary Note 7.

Monte Carlo simulations
Monte Carlo simulations were performed using Casino 5.3.3.04 software81 for a layer of Ag-CMC on a graphite substrate, using ELSEPA scattering cross-sections82 and a modified Bethe equation by Joy and Luo for calculating stopping power83. The simulations were performed using 106 electrons formed into a beam of 5 nm diameter and a Gaussian profile (1.65 nm2 variance), at primary electron energies of 1, 1.5, 2, 2.5 and 10 keV and a lower energy cutoff of 513 eV to reflect the EsB filter grid voltage of -513 V used in the experiments. The thickness of the Ag-CMC layer was varied between 1 and 20 nm, and the thickness of the graphite substrate was set at 1 μm (for 1–2.5 keV simulations) or 10 μm (for the 10 keV simulation). The stoichiometry of Ag-CMC used in the simulation was Ag802C7617H6615O10824, which corresponds to a Na-CMC chain with 102 glucopyranose units and a degree of substitution of 0.82, where all the Na bound to carboxylate groups was substituted with Ag (this stoichiometry was also found to have a similar atomic composition as the one determined with EDX on silverized CMC). The Ag-CMC density was set at 1.96 g/cm3, based on the average reported true density of Na-CMC of 1.5 g/cm3 and assuming the same molar volume of Ag-CMC as Na-CMC. The cumulative distributions of maximum penetration depth of backscattered electrons were calculated based on all the simulated electrons, while 104 electrons were considered for plotting backscattered electron trajectories. These trajectories were extracted from the simulation data files into .csv files using a Python code generated with OpenAI ChatGPT-484 and verified by the first author for accuracy. The code is provided in Supplementary Note 8.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The authors present an accessible approach to staining carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) binders in graphitic Li-ion electrodes with silver and bromine. This strategy enables detailed electron imaging and precise spectroscopic quantification of the binder domain and achieves a 14% reduction in electronic resistivity and a 40% decrease in electrode ionic resistance. Furthermore, staining enables unprecedented electrode-scale, high-resolution backscattered electron imaging of surprisingly complex binder hierarchies, revealing multiple types of agglomerates and hardly-ever-seen nanoscale binder films. These films completely coat graphitic surfaces in pristine electrodes but shatter into highly inhomogeneous fragments after calendaring in both research-grade and commercial electrodes, presenting new perspectives on interpreting common cycling stability and electrode performance issues. Although the study is interesting and presented results are good. I recommend the publication of this study after addressing my comments, which are given below;\n1.Why are carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) specifically amenable to staining with silver and bromine? Do functional groups (e.g., carboxyl in CMC, double bonds in SBR) directly interact with these staining agents?\n2.Could alternative binders (e.g., PVDF, PAA) be stained similarly, or is this method unique to CMC/SBR due to their chemical structures?\n3.The authors claimed that the presented strategy achieved a 14% reduction in electronic resistivity and a 40% decrease in electrode ionic resistance. Does the staining process directly cause the reported 14% electronic resistivity and 40% ionic resistance improvements, or are they attributed to binder redistribution insights enabled by the method? Clarify causation to avoid misinterpretation.\n4.Is the choice of CMC/SBR driven by their widespread use in commercial graphite anodes, making the method more applicable to real-world systems? How scalable is this staining protocol for quality control in high-throughput electrode production? Highlight time, cost, and compatibility challenges with industrial workflows (e.g., solvent safety, drying times).\n5.How does the CMC/SBR ratio influence staining efficiency? Does a higher SBR content alter bromine uptake due to its hydrocarbon structure?\n6.Does the mechanical flexibility of SBR or the rigidity of CMC affect the spatial distribution or adhesion of staining agents?\n7.Are CMC/SBR inherently difficult to image via electron microscopy due to low conductivity or contrast, necessitating this staining method? How does staining mitigate these issues?\n8.What advantages do CMC/SBR offer over other binders in balancing stainability, electrode performance, and processability?\n9.Does the staining process alter the ionic/electronic conductivity of the binder domain, potentially affecting rate capability or cycle life measurements? Are there risks of silver/bromine residues introducing side reactions (e.g., redox activity) during electrochemical cycling?\n10.Is this staining method applicable to next-gen anodes (e.g., silicon, lithium metal) where binders face harsher mechanical/chemical stresses? Is this staining technique compatible with operando studies of working electrodes, or is it limited to ex-situ/post-cycled samples?", "reviewer_2": "The authors employed silver and bromine staining to enhance the visualization of binder components in graphite anodes for lithium-ion batteries under electron imaging. This innovative approach produced exceptionally clear electron microscopy images, a result that is rarely reported in existing studies. However, the impact and significance of this finding are not adequately conveyed or discussed. Therefore, I recommend that the manuscript undergo revisions before being considered for publication or resubmission. Below are several minor issues that need to be addressed:\nFigure 1(c): The authors present backscattered electron images (BEI) of silverized graphitic electrodes containing either 1.5 wt.% CMC or 2.3 wt.% SBR. What was the rationale for using different binder contents for the same silver-stained graphite electrodes? Could this variation influence the quality or interpretation of the electron imaging results?\nBromination Process: The manuscript describes the bromination of SBR in the electrodes, but it does not mention the bromination process for Na-CMC. Since both binders were brominated, it is important to include the bromination process for Na-CMC in the manuscript for consistency and clarity.\nFigure Captions: In the figure captions for Figure 1 and Figure 2, the authors use two different labeling formats for subfigures [A) or (A)]. I suggest adopting a consistent labeling format throughout the manuscript to improve clarity and presentation.\nSection 2.4 Alignment: At the beginning of Section 2.4, the text appears misaligned. The authors should correct the alignment to ensure proper formatting.\nViscosity and CBD Area Reduction: In Section 2.4, the authors use 3.0 wt% Na-CMC in the initial mixing step to reduce the area of the conductive carbon/binder domain (CBD). Given that the viscosity difference between 1.5 wt% and 3.0 wt% Na-CMC is substantial, the authors might want to consider using ball milling or mortar grinding for the preliminary mixing of the electrode slurry. This could further enhance the reduction of the CBD area without altering the mechanical stirring conditions. I recommend including this experiment in the manuscript as a potential improvement.\nReview of Anode Active Materials: While the manuscript provides a detailed introduction to the types of binders used in lithium-ion battery anodes, it lacks a review of the anode active materials. I suggest adding a section reviewing lithium-ion battery anode active materials in the introduction, referencing relevant studies such as Chemical Engineering Journal, Volume 499, 2024, 156360 (ISSN 1385-8947).\nStreamlining the Manuscript: The manuscript could benefit from a more streamlined structure and clearer presentation of the results. Some sections, particularly the results and discussion, are dense and would be more accessible if broken down into smaller, more digestible segments with clearer headings and subheadings.", "reviewer_3": "The author develops an accessible approach of staining carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) binders in graphitic Li-ion electrodes with silver and bromine, enabling detailed electron imaging and precise spectroscopic quantification of the binder domain. Notably, the staining reactions of Br2 vapour is similar to OsO4 staining but with significantly lower hazard, as indicated by a 217-times higher short-term safe exposure limit for Br2 compared with OsO4. This work can provide a guidance for slurry processing and electrode preparation of graphite anode. Therefore, this work has a chance to be published after major revise:\n1. As shown in Figure 1A, the schematic diagram shows that the Ag+ can replace the Na+ in Na-CMC. It is unscientific, because the Ag+ tends to undergo cross-linking reactions with hydroxyl and carboxyl groups rather than substitution reactions.\n2. In Figure S4, the EDX and X-ray photoemission spectroscopy (XPS) were used to quantify Br and Ag concentration. However, the measurement results of EDX and XPS are usually inaccurate when compared with Inductively Coupled Plasma (ICP). It is better to provide the ICP measurement of Br and Ag concentration.\n3. As illustrated in Figure 5A, the author claims that the more concentrated CMC solution in the first mixing step might improve the final C45 dispersion in the electrode. What’s the specific mechanism? Generally, the concentrated binders solutions are not conducive to dispersion of electrode materials.\n4. It is better to provide a optimized concentration of CMC solution in the first mixing step, which is important for the slurry processing in Li-ion battery industry.\n5. In Figure 6, when the polymer solution (e.g. PVDF in N-methyl pyrrolidone) is immersed in a non-solvent (e.g. water), the adhesion of polymer slurry on Cu foil will greatly decrease and even detach from the Cu foil surface. This phenomenon will increase the contact resistance of electrode materials and further decrease the electrochemical performance of Li-ion battery. However, the author reports a opposite result by attributing to the reduced pore resistance? I think that it is hard to convince. The author should provide more discussions and comparations in this section by supplementing experiment.\n6. As we know, the Br2 vapours will corrode the Cu foil, CMC and SBR during bromination of electrodes at high temperature, which is is very detrimental to the electrochemical performance of Li-ion battery. Therefore, how to avoid this phenomenon during practical application?\n7.The article contains some technical terms and abbreviations (such as EDX, BEI, EsB, etc.). While these are helpful for concise expression, they may pose comprehension barriers for non-specialist readers. It is suggested that the authors provide detailed explanations when these terms first appear.\n8.Although the authors have explained the mechanism of the staining reactions, the descriptions of the optimization processes for some key parameters are not detailed enough. For example, the rationale behind the selection of reaction temperature, time, and bromine vapor concentration in the bromination process has not been fully elaborated. It is suggested that the authors provide more detailed information on these optimization processes to help readers better understand and reproduce the method.\n9.While the authors have observed the complex distribution patterns of binders within the electrodes, the in-depth analysis of how these patterns affect the electrochemical performance of the electrodes is not sufficiently thorough. For example, regarding the role of nanoscale binder films during charge-discharge processes and the impact of non-uniform binder distribution on battery cycling stability, it is recommended that the authors conduct a more in-depth investigation in conjunction with the results of electrochemical testing.\n10.While the article mentions the advantages of CMC and SBR binders over traditional PVDF binders in terms of staining, the comparative analysis with other potential staining methods may not be comprehensive enough. It is suggested that the authors include a comparison with other staining methods in the discussion section or elaborate on the universality of this staining approach.\n11.In presenting key data, such as quantitative analysis of binder distribution, the authors provided EDX and BEI images for only part of the electrodes, which may not fully represent the entire electrode. It is recommended to increase the image coverage to more comprehensively show the overall characteristics of the electrodes. For example, larger - area images could be provided, or multiple images taken at different positions and stitched together to ensure data integrity and representativeness.", "reviewer_4": "In this manuscript, authors suggested a new method that staining carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) binders in graphitic Li-ion electrodes with silver and bromine, enabling detailed electron imaging and precise spectroscopic quantification of the binder domain. By using this method, the authors could achieve the suppression of binder migration during high-temperature electrode drying, and a 40% decrease in electrode ionic resistance for unprecedented electrode-scale, high- resolution backscattered electron imaging of surprisingly complex binder hierarchies, revealing multiple types of agglomerates and hardly-ever-seen nanoscale binder films. In conclusion, the rich information provided by binder staining can shed new light on fundamental electrode processes and extend optimization routes of negative Li-ion electrodes. Therefore, I suggest this manuscript to be published in journal \"Nature Communications \" after minor revisions according to the following aspects:\n1. The authors conducted special analysis for SE, EDX, BEI. If possible, please provide XPS mapping data for not only elemental analysis but also chemical state analysis.\n2. In Fig4e, the authors provided graphite surface coverage data to confirm binder distribution. Detailed process to get surface coverage data should be described.\n3. In this manuscript, the authors only considered graphite as an anode materials. Please provide possibility to use this method to the other electrode materials.\n4. In Fig3a, the authors showed images of internal electrode surface exposed by scotch tape peel-off and subsequent silverization, imaged at different electron beam voltages with surface sensitivity. How does the surface sensitivity work with electron beam voltages?\n5. The authors claimed that they could reduce 14% of electronic resistivity and decrease 40% of ionic resistance. What is possible way to increase each value?"}, "author_response": "NCOMMS-25-04022 Chemical staining – a toolbox for fundamental studies and practical\noptimization of binders in negative Li-ion electrodes\nDear Reviewers,\nThank you for your time and valuable suggestions on our manuscript Chemical staining – a\ntoolbox for fundamental studies and practical optimization of binders in negative Li-ion\nelectrodes. We have thoroughly addressed your comments, including performing a number of\nnew experiments as requested, and modified our manuscript accordingly. Because of the article\nsize limit for Nature Communications, we have kept modifications to the main article concise\nwithout comprising clarity. Below, we reproduce each of the comments together with our\nresponse, and the related changes in the manuscript are highlighted in yellow.\nReviewer #1 (Remarks to the Author):\nThe authors present an accessible approach to staining carboxymethylcellulose (CMC) and\nstyrene butadiene rubber (SBR) binders in graphitic Li-ion electrodes with silver and\nbromine. This strategy enables detailed electron imaging and precise spectroscopic\nquantification of the binder domain and achieves a 14% reduction in electronic resistivity\nand a 40% decrease in electrode ionic resistance. Furthermore, staining enables\nunprecedented electrode-scale, high-resolution backscattered electron imaging of\nsurprisingly complex binder hierarchies, revealing multiple types of agglomerates and\nhardly-ever-seen nanoscale binder films. These films completely coat graphitic surfaces in\npristine electrodes but shatter into highly inhomogeneous fragments after calendaring in both\nresearch-grade and commercial electrodes, presenting new perspectives on interpreting\ncommon cycling stability and electrode performance issues. Although the study is interesting\nand presented results are good. I recommend the publication of this study after addressing\nmy comments, which are given below;\n1. Why are carboxymethylcellulose (CMC) and styrene butadiene rubber (SBR) specifically\namenable to staining with silver and bromine? Do functional groups (e.g., carboxyl in CMC,\ndouble bonds in SBR) directly interact with these staining agents?\nand SBR staining with Br. To improve clarity we have slightly modified the first sentence\nand we provide the key chemical reaction equations in Fig. 1a and 1b, showing bonding of\nAg+ to the -COO-groups of the CMC, and of Br to the C=C groups of SBR:\nspecifically attach to the functional groups present within the binder structure but not in the\nother components of the electrode, as shown schematically in Figure 1. Immersing graphite\nelectrodes in aqueous AgNO solution leads to Ag+ ions binding to the carboxyl groups in CMC,\n3\nforming a water-insoluble complex depicted in Fig. 1a. On the other hand, exposing the\nelectrodes to Br vapour brominates the aliphatic sp2 carbons of SBR, as depicted in Fig 1b.\nupdated with EDX) data confirming changes in the carboxylate bands of the CMC upon\nsilverization, and in the C=C peaks of SBR upon bromination. We have now also added XPS\nC 1s, Ag 3d and Br 3d core level analysis (Supporting Note S2 and Supporting Figure S4)\nthat also support these findings. These observations confirm the direct interaction of the\nstaining agents with the carboxylate groups of CMC and aliphatic C=C groups of SBR.\nThe mechanism of both reactions was confirmed using attenuated-total reflection infrared\nspectroscopy (ATR-IR), X-ray photoemission spectroscopy (XPS) and energy-dispersive X-ray\nspectroscopy (EDX), showing diminished aliphatic C=C peaks in brominated SBR, a strong shift of\n-COO- peaks in silverized CMC and substitution of Ag for Na, as discussed in Supporting Notes S1-\nS2 and Supporting Figures S2-S4.\nWe also provide extended discussion on the mechanism of the staining reactions in\nSupporting Note S1:\nThe mechanism of staining reactions was first assessed by ATR-IR. Figure S2 shows ATR-IR spectra\nof pristine and freshly brominated SBR (Fig. S2A), as well as spectra of pristine CMC and CMC after\nexposure to Ag+ (Fig S2B). As expected, the spectra of SBR before and after bromination indicated\ndiminishing intensity of aliphatic C=C peaks, particularly the trans peak at 967 cm-1, vinyl peaks at\n909, 995 and 1640 cm-1 and cis peak at 1403 cm-1, with the appearance of a broad shoulder around\n1225 cm-1, which could be ascribed to the C-H wagging vibrations within the CH -Br group.1–3\ncm-1 can be indicative of the partial decomposition of C-Br bonds in air.4 This reaction did not occur\nat high enough rate to be of concern for EDX mapping of binders, with no significant lowering of Br\n2\nconcentration in samples exposed to air for up to a year. Bromination of a thick and compact SBR\nfilm resulted in bromination of the top 1-2 μm part of the film, as evident from the backscattered\nelectron image of the sample cross section (Figure S3).\nThe ATR spectra of CMC in Fig. S2B showed four distinctive regions: 3700-3000 cm-1 related to\nvibrations of OH groups (linked to pyranose rings of CMC, physiosorbed water and intra- and\nintermolecular hydrogen bonds), 3000-2800 cm-1 related to vibrations of CH and CH groups, 1800-\n2\n1200 cm-1 related to vibrations of carboxylate groups, and < 1200 cm-1 related to vibrations of\npyranose rings and glycosylic linkages between them.5–7 The spectra showed strong interactions\nbetween the complexing metal ions and carboxylate groups of CMC upon staining with Ag+,\nas\nevident by the downshifting of the peaks at 1588 cm-1 and 1414 cm-1 associated with asymmetric\nand symmetric vibrations of COO- groups. The separation between these two peaks could also be\nrelated to the structure of the carboxylate-metal complex.8,9 The peak separation of 174 ± 2 cm-1\nfor Na-CMC and 154 ± 4 cm-1 for Ag-CMC indicated that Ag binding changed the metal-CMC\ncoordination from bidentate bridging to bidentate chelating geometry.8,9 Although there were no\nsignificant changes in the hydroxyl region between the samples, a fuller analysis of this region may\nbe obstructed by the presence of adsorbed water.\n2. Could alternative binders (e.g., PVDF, PAA) be stained similarly, or is this method unique\nto CMC/SBR due to their chemical structures?\nYes, our investigations of sodium alginate and polyacrylate showed successful binding of Ag\nto these polymers, as mentioned in Section 2.1:\nThe high affinity of Ag+ for carboxylate groups was not limited to CMC – silverization was also\nsuccessfully applied to carboxyl-rich sodium alginate and sodium polyacrylate – two other widely\nresearched aquoeus binders with a superior performance in e.g. silicon anodes31,34,40 (Supporting\nFig. S3).\nTo strengthen further, we have now added a new result on EsB imaging of PAA in graphitic\nelectrodes in Section 2.3 (EsB imaging of local binder morphology):\nThe thin layer morphology could also be observed in EsB images of silverized electrodes with an\nanalogous polyacrylate binder (Supporting Figure S15).\nWe have also clarified this aspect further in the comment to the Limitations discussion:\nconsisting of 1.5 wt.% polyacrylic acid (PAA), 95.2 wt.% graphite, 2.3 wt.% SBR and 1 wt.% C45. The\nimages were taken at 10 kV (top left) or 1 kV (all the other images) of primary electron beam voltage\nand show that PAA adapted the same type of morphologies on graphite as CMC. The high\nmagnification images at the bottom show fractured graphite particle and estimated PAA layer\nthickness, based on 4 (EsB) and 5 (SE) measurements along the dotted lines. The average PAA layer\nthickness is similar to the one of CMC in an analogous electrode (Figure S14).\nAlso, because Ag+/Br staining targets carboxylate and aliphatic C=C groups in the binders, processes\n2\nthat alter these groups beforehand (e.g. drying-induced cross-linking) may reduce Ag/Br uptake\n(which, however, could be exploited to quantify binder cross-linking). Consequently, bromination and\nsilverization are not suitable for staining e.g. PVDF or PTFE binders (that lack these functional groups)\nbut can be suitable for other binders, such as the carboxyl-rich alginates and polyacrylates (as shown\nin Supporting Figure S15). However, the accuracy of spatial mapping these two binders in electrodes\nremains to be assessed.\n4\n3.The authors claimed that the presented strategy achieved a 14% reduction in electronic\nresistivity and a 40% decrease in electrode ionic resistance. Does the staining process\ndirectly cause the reported 14% electronic resistivity and 40% ionic resistance\nimprovements, or are they attributed to binder redistribution insights enabled by the method?\nClarify causation to avoid misinterpretation.\nThank you for this comment – based on this and a few other points raised by other reviewers,\nwe realized there was some confusion about the staining concept. The staining is performed\non electrode samples cut out from the electrode sheets only for binder analysis – it is not\napplied on the part of electrode sheets that are used for electrochemical testing (nor for\nmaking batteries). Thus, all reported differences in properties (e.g. electronic/ionic\nconductivity) of differently processed electrodes are a result of the processing itself (e.g.\npreparation of slurries from dilute vs. concentrated CMC, or drying with or without phase\ninversion) – and not the staining. We now clarify at the end of the paragraph introducing the\nstaining concept (section 2.1):\nThe high affinity of Ag+ for carboxylate groups is not limited to CMC – silverization was also\nsuccessfully applied to carboxyl-rich sodium alginate and sodium polyacrylate – two other widely\nresearched aquoeus binders with a superior performance in e.g. silicon anodes31,39 (Supporting\nFigure S3). Note that because of the reactivity of Ag+ and Br ,staining and binder mapping\n(aq) 2\nshould be applied on separate electrode areas to those used in electrochemical or other testing.\nWe have also clarified it in Section 2.5 (CBD agglomeration study):\nFurthermore, four-point probe measurements of not-stained electrodes showed […]\nAnd Section 2.6 (phase inversion study):\nTo explore effects on pore ionic resistance, we investigated unstained electrodes using\nelectrochemical impedance spectroscopy (EIS) in a non-intercalating electrolyte developed by\nLandesfeind et al.3,54\nWe also added a note to the Limitations section:\nFor the same reason, silverization requires impractical detachment of any Cu current collector prior\nto staining, and the likely alteration of the electrode’s electrochemistry by Ag+ and Br staining may\n2\nrestrict its application, for example, in operando studies or on-line manufacturing quality control.\n5\n4. Is the choice of CMC/SBR driven by their widespread use in commercial graphite anodes,\nmaking the method more applicable to real-world systems? How scalable is this staining\nprotocol for quality control in high-throughput electrode production? Highlight time, cost,\nand compatibility challenges with industrial workflows (e.g., solvent safety, drying times).\nYes, we focused on CMC/SBR because of their widespread adaptation in commercial\ngraphitic electrodes, and because of the lack of convenient methods for analysing their spatial\ndistribution within these electrodes. We explain in the Introduction:\nSince the early 2000’s, the water-soluble sodium salt of carboxymethyl cellulose (Na-CMC)\nblended with emulsified styrene butadiene rubber (SBR) has become the now-standard binder\nmixture used in industrial graphite and graphite/silicon negative Li-ion electrodes,30 and is also\nbeing explored for next-generation Si anodes31 and water-processable LiFePO cathodes.32,33 Being\n4\ndispersible in water rather than toxic N-methyl pyrrolidone (NMP), these binders considerably\nlowered cost and complexity of anode manufacturing, and improved anode’s cycling stability.1,34\nContrary to the older-generation polyvinylidene fluoride (PVDF) binder, however, SBR and CMC\nlack distinct, covalently bound elements that are traceable by readily accessible methods such as\nenergy-dispersive X-ray spectroscopy (EDX).\nPlease note, that in our approach we did not choose CMC and SBR as the binders applicable\nfor the staining approach (i.e., we did not first develop the staining methods, and then chose\nthe binders that it could be applied to). Instead, we first realized that there are no practical\nmethods for mapping CMC/SBR (the two most common binders in commercial Li-ion\nanodes), and based on that knowledge, we then designed these staining methods to analyse\nparticularly these (and other analogous) binders.\nAs for the scalability and industrial application, we now include a note on the large area\ncross-sectioning preparation in the Limitations section:\nTo minimize geometrical artifacts, the cross-sectional binder analysis requires lengthy sample\nplasma cross-sectioning (~8-12 hours for 100 μm-thick electrodes), which might be shortened using\nlaser pre-cutting or resin impregnation and microtomy.\nFollowing that, we have included brief discussion in the Discussion and Summary (Section\n3):\nStaining combined with BEI, in particular, is simple and attractive for binder-informed\noptimization and off-line quality control of electrodes in industrial settings, considering availability\n6\nof BEI even in table-top SEMs. The cost of implementing these methods comprises capital and\noperational cost of EDX/BEI-equipped SEM, while the staining itself utilizes simple equipment and\nvery small quantities of relatively cheap and re-usable chemicals (here, 0.3 g AgNO or 0.6 g Br per\n3 2\nbatch of up to 5 samples). With optimization of sample-cross sectioning, the complete staining and\nthrough-thickness EDX analysis time may be reduced from 15 to 5 h/sample and down to 3\nh/sample for BEI analysis. Note that the staining and analysis not requiring cross-sectioning, such\nas top-down CBD agglomeration check with BEI, can be already performed at < 1 h/sample.\nNote the only solvent used in the processing was water. We have also only focused on the\ncross-sectioning time as it was the limiting factor for the total processing time. The drying\ntime after silverization (30 min) was insignificant compared to sample cross-sectioning time\n(10-12 h for 100 um cross-sections).\n5.How does the CMC/SBR ratio influence staining efficiency? Does a higher SBR content\nalter bromine uptake due to its hydrocarbon structure?\nWe have performed an additional staining campaign on newly-made electrodes with different\nCMC+SBR ratios and a total binder mix of 4% wt. Multiple areas of delaminated electrodes\nwith a total area of ~20 cm2/coating were stained with Ag+ or Br using he protocols already\n2\ndescribed, and the amount of Ag or Br was analysed with EDX. To remove any influence of\nbinder migration and to increase representativeness of the average staining efficiency, prior to\nEDX analysis, we homogenized each batch of samples by pulverizing them in a mortar,\nfollowed by mounting the powders onto 12 mm-diameter SEM sticky carbon discs. EDX\nanalysis was conducted on at least 5×1.6 mm2 random areas per sample. We additionally\nverified the accuracy of our EDX Ag and Br factory-standardized quantification routine usinf\nfreshly cleaned >99.9% pure Ag wire and a >99% pure KBr pellet, which gave very small\nrelative Ag and Br quantification biases of only 1.3% and 3.2%, respectively.\nof a graphite electrode immersed in aqueous AgNO during silverization, together with a reaction\n3\nscheme of Ag+ binding to Na-CMC. b Photograph of supported electrode fragments during\nbromination, together with a reaction scheme of Br binding to SBR (primary reaction) and Na-CMC\n(side reaction). c Backscattered electron imaging (BEI) of silverized and homogenized graphitic\nelectrodes containing either 4 wt.% CMC or 4 wt.% SBR. d BEI of brominated and homogenized CMC-\nand SBR-only graphitic electrodes. e Atomic % of Ag and Br in stained and homogenized electrodes\ncontaining 4 wt.% of CMC+SBR mixture with different CMC:SBR ratios, determined with EDX at 10 kV.\nEach point represents Ag/Br content averaged from at least five 1.46×1.09 mm area measurements. f\nEDX Br content in brominated electrodes during continuous irradiation with SEM/EDX electrons,\nnormalized to the Br content averaged over the first 10 seconds of the experiment. For each sample,\n152×114 μm area was exposed to 5 kV, 2 nA electron beam current and EDX spectra were continuously\nacquired every 3.5 seconds. g Final normalized Br content in the electrode fragments, averaged\nbetween 170-200 seconds of the experiment in f. The error bars and shaded areas in e-g represent\n95% confidence intervals of the average values and linear regression fits, respectively. The samples in\nc-e were silverized in 0.5M AgNO with an addition of 1 mM Triton X-100 surfactant, as explained in\n3\nthe Methods. The contrast of BEI images was equalized for comparable visibility of graphite and\nstained binders.\ncoatings, and similarly, a linearly increasing amount of Br with the amount of SBR in the\ncoatings, with R2 between 0.978-0.994. The two formulations containing either only 4% wt.\nCMC or 4% wt. SBR additionally allowed us to quantify selectivity. We found ~16× larger\namount of Ag binding per unit mass of CMC compared with SBR, and ~7× larger amount of\nBr binding per unit mass of SBR compared with CMC in the electrodes. These values are\nalso similar to the values derived from the slopes and intercepts of a linear fits to data over\nthe composition range (16× and 5.5× selectivity, respectively). This confirmed that within the\nanalysed binder content (which is representative to the typical binder content in commercial\nelectrodes), staining efficiency/selectivity is largely independent on the CMC:SBR ratio.\nUsing the set of brominated electrode coatings with known, linearly changing SBR content,\nwe additionally quantified the correlation between the amount of SBR in the electrodes and\nthe degree of electron-induced Br outgassing from Br-SBR after brominating the samples\n(which we previously quantified only in two samples containing either 1.5% CMC or 2.3%\nSBR). We found a linearly increasing degree of Br outgassing with the increasing SBR\ncontent (R2 = 0.944), showing that Br outgassing can be used for selective quantification of\nSBR content in the samples containing CMC.\nThis new analysis is more robust than the previous analysis of only two electrodes with either\n1.5 wt.% CMC or 2.3% wt. SBR so we have substituted the new results for the previous ones\n– updating Figure 1 and the corresponding discussion and moving the previous results to the\nSupporting Information.\n6.Does the mechanical flexibility of SBR or the rigidity of CMC affect the spatial distribution\nor adhesion of staining agents?\nBecause both bromination and silverization are performed using highly mobile reagents (Br\n2\nvapours and Ag+ ions dissolved in water), we do not expect mechanical properties of CMC\nand SBR to affect spatial distribution and adhesion.\n7. Are CMC/SBR inherently difficult to image via electron microscopy due to low\nconductivity or contrast, necessitating this staining method? How does staining mitigate\nthese issues?\ndue to a combination of factors, depending on the mode employed:\n- amorphous structure\n- low fraction\n- nanometer dimensions of the binders (in some areas)\n- degradation by the electron beam\n- chemical composition highly similar to graphite\n- insufficient surface and chemical sensitivity of backscattered electron detectors\nParticularly in the case of secondary electron imaging in the SEM, distinguishing between\nmaterials requires them to have different morphology, but binder morphology largely\ntemplates the graphite.\nFor imaging with chemically-sensitive back-scattered electron imaging BEI, both SBR and\nCMC are chemically highly similar to the graphite but staining them with much higher\natomic number Br and Ag, as we show, readily allows for BEI and EDX.\nWe have now condensed this information in the paragraph in the Introduction:\nContrary to the previous-generation polyvinylidene fluoride (PVDF) binder, however, SBR and CMC\nlack distinct, covalently bound elements that are traceable by readily accessible methods such as\nenergy-dispersive X-ray spectroscopy (EDX) or backscattered electron imaging (BEI). Lacking a well-\ndefined morphology, binders are also difficult to distinguish with scanning electron microscopy\n(SEM) techniques.\n8.What advantages do CMC/SBR offer over other binders in balancing stainability, electrode\nperformance, and processability?\nWe have now added a short note on that to the introduction:\nBeing dispersible in water rather than toxic N-methyl pyrrolidone (NMP), these binders have\nlowered the cost and complexity of anode manufacturing, additionally improving cycling\nstability.1,34\n9.Does the staining process alter the ionic/electronic conductivity of the binder domain,\npotentially affecting rate capability or cycle life measurements? Are there risks of\n10\nsilver/bromine residues introducing side reactions (e.g., redox activity) during\nelectrochemical cycling?\nAs detailed in Answer 3 above, staining is not applied to electrodes subject to any other\ntesting than binder mapping, so it does not affect cyclability of actual electrodes.\n10.Is this staining method applicable to next-gen anodes (e.g., silicon, lithium metal) where\nbinders face harsher mechanical/chemical stresses? Is this staining technique compatible\nwith operando studies of working electrodes, or is it limited to ex-situ/post-cycled samples?\nWe have now fabricated new CMC/SBR-based electrodes made of SiOx/graphite,\nmicro-\nSi/graphite and LiFePO4, stained them with Ag and Br and investigated by EsB/BEI/EDX.\nWe also verified reactivity of the AgNO with nano- and micropowders of Si and and C-\n3\ncoated SiOx. We have included the results in the new Figure 7 and Limitations section:\nFirst, the staining methods, particularly the liquid-phase silverization, are less applicable to\nelectrodes containing easily-oxidizable materials (e.g. metallic, LiFePO or nano-Si particles),\n4\nas these can rapidly undergo redox reactions with the staining elements, complicating image\ninterpretation. Micro-Si or materials with an inert, stable surface passivation layer can be,\nhowever, suitable. This is shown in Figure 7 that demonstrates successful staining on Si and\nC-coated SiO microparticle-based electrodes but not on the more reactive LiFePO or nano-\nx 4\nSi. For the same reason, silverization requires detachment of any Cu current collector prior\nto staining, which is often impractical in an industrial setting, and the likely alteration of the\nelectrode’s electrochemistry by Ag+ and Br staining may restrict its application, for example,\n2\nin operando studies or on-line quality control.\nThe above comment also covers that the method will not be compatible with reactive and/or\nwater sensitive metals such as Li metal.\nAs for the operando studies, we do not recommend performing them on stained electrodes\ndue to the side-effects of the staining (current collector oxidation by Br , introduction of\n2\nelectrochemically-active Ag+ to the system), which will likely alter original electrochemical\nbehaviour of the electrodes, making such studies not representative for the actual behaviour\nof Li-ion cells. For the same reasons, we do not perform staining on electrodes subject to EC\ncycling, as explained in the Limitations note above and in the comment to Question 1.3.\n4\npowders of Si and C-coated SiO with Ag/Si ratio averaged from 5 measurements per sample. b BEI\nx\nof silverized Si and C/SiO powders, showing average particle diameter (D) and surface-to-volume\nx\nratio (S/V). Due to the large S/V, the reactivity of nano-Si with Ag+ will impede staining selectivity\ntowards CMC. c Top down secondary electron (SE), BEI, EDX and high magnification EsB images of\ninternal surfaces of silverized micro-Si and C/SiO electrodes containing 20 wt.% graphite, 2.3% SBR,\nx\n1.5% CMC and 1% C45. The EDX maps show that Ag was bound and accurately found in CBD carbon-\nbinder clusters (highlighted in the SE insets). The EsB images show good contrast of CMC nanolayer\non C/SiO but poor on Si (see CMC on graphite particles as internal contrast standard). d EsB images\nx\nof brominated micro-Si and C/SiO electrodes with the same composition as in c, together with\nx\ncolorized differential Br outgassing maps of SBR processed using Supplementary Code S1. e EsB\nimages of silverized and brominated LiFePO electrodes with 1.5 wt.% CMC and 2.3 wt.% SBR. During\n4\nsilverization, Fe2+ reduced Ag+ forming large Ag dendrites. Bromination resulted in formation of LiBr\nthat deliquesced in air forming liquid droplets visible in the inset photograph. Both reactions obscure\nbinders’ detection in the LiFePO -based electrodes.\n4\nThe authors employed silver and bromine staining to enhance the visualization of binder\ncomponents in graphite anodes for lithium-ion batteries under electron imaging. This\ninnovative approach produced exceptionally clear electron microscopy images, a result that\nis rarely reported in existing studies. However, the impact and significance of this finding are\nnot adequately conveyed or discussed. Therefore, I recommend that the manuscript undergo\nrevisions before being considered for publication or resubmission. Below are several minor\nissues that need to be addressed:\n1. Figure 1(c): The authors present backscattered electron images (BEI) of silverized\ngraphitic electrodes containing either 1.5 wt.% CMC or 2.3 wt.% SBR. What was the\nrationale for using different binder contents for the same silver-stained graphite electrodes?\nCould this variation influence the quality or interpretation of the electron imaging results?\nWe originally chosen 1.5 wt.% CMC and 2.3 wt.% SBR to mimic industrial practice but we\nhave now investigated staining of electrodes with both equal binder contents, and a wider\nrange of CMC:SBR ratios. We have compared the staining efficiency EDX data of the new\nelectrodes (with 4% binders) to the original ones, and the Ag / Br quantities normalized to\ndifferent binder content were very similar at the same analysis conditions (which is a\nconsequence of strongly linear dependency between Ag/Br content and CMC/SBR content,\nas established in the new experiments described in the updated Figure 1). In updated Fig. 1\n(also attached above, Page 8), we have now provided BEI images of the 4% CMC or SBR\nelectrodes, in which the binder morphologies are analogous to the previous BEI images of\n1.5% CMC / 2.3% SBR electrodes.\n2. Bromination Process: The manuscript describes the bromination of SBR in the electrodes,\nbut it does not mention the bromination process for Na-CMC. Since both binders were\nbrominated, it is important to include the bromination process for Na-CMC in the manuscript\nfor consistency and clarity.\nWe have had new, detailed discussion with research chemists with relevant experience and\nwe now provide the potential reaction equation in Fig. 1a: M+-assisted Hunsdiecker\nbromination of carboxylic groups in the Na-CMC followed by air-hydrolysis of the\nbrominated organic product and leaving NaBr as the predominant Br-containing compound in\nbrominated Na-CMC, as now explained in Section 2.1:\n2\nbinding of Br to Na (see exemplary Hunsdiecker reaction scheme in Fig. 1b).42\nFurther investigation would require detailed (e.g. NMR) analysis that is outside the scope of\nthis work - hence, we note exemplary reaction scheme.\n3. Figure Captions: In the figure captions for Figure 1 and Figure 2, the authors use two\ndifferent labelling formats for subfigures [A) or (A)]. I suggest adopting a consistent\nlabelling format throughout the manuscript to improve clarity and presentation.\nWe have now changed all the figure captioning formats for follow the standard adopted\nacross Nature publications\n4. Section 2.4 Alignment: At the beginning of Section 2.4, the text appears misaligned. The\nauthors should correct the alignment to ensure proper formatting.\nWe have checked now to ensure the alignment is correct.\n5. Viscosity and CBD Area Reduction: In Section 2.4, the authors use 3.0 wt% Na-CMC in\nthe initial mixing step to reduce the area of the conductive carbon/binder domain (CBD).\nGiven that the viscosity difference between 1.5 wt% and 3.0 wt% Na-CMC is substantial, the\nauthors might want to consider using ball milling or mortar grinding for the preliminary\nmixing of the electrode slurry. This could further enhance the reduction of the CBD area\nwithout altering the mechanical stirring conditions. I recommend including this experiment in\nthe manuscript as a potential improvement.\nFollowing this insightful suggestion, we fabricated an electrode using the standard method\n(with initial CMC concentration of 1.5 wt.%) but with addition of zirconia balls to the mixing\npot to see if they could provide better CBD dispersion. Milling balls provided only a\nmoderate decrease in CBD clustering, from 8.5 clusters/mm2 (reference electrode) to 4.8\nclusters/mm2, but still quite far from 0.4 clusters/mm2 in the electrode prepared from the\nmodified slurry with initially-concentrated (3%) CMC. We have now described this in the\nsame section:\nFigure 5B shows a BEI image of a modified electrode after bromination, confirming the\ndisappearance of large CBD clusters, which is also evident in the histogram in Fig. 5C.\nduring mixing of the standard 1.5 wt. % CMC slurry, indicating stronger influence of the initial\nCMC concentration on CBD dispersion (Supporting Figure S17).\nFigure S15. Comparison of carbon-binder domain (CBD) agglomeration in electrodes made from\nstandard (1.5 wt. % CMC initial) slurry, the same type of slurry with an addition of two zirconia balls\nto the mixing pot, or the modified (3.0 wt.% CMC initial) slurry. (A) Representative color-enhanced\nbackscattered electron images (BEI) of the top electrode surfaces, showing CBD as the bright blue\nphase; (B) Histograms showing size distribution of the CBD clusters in the three types of electrodes.\nBEI image segmentation and CBD clustering analysis was done according to Supporting Procedure\nS2.\nFollowing this and the similar Question 4.10, we also included it in the comment:\nFuture staining-assisted electrode optimization could investigate alternative pre-mixing\nmethods of thinner, lower CMC-content slurries, or substituting C45 nanoparticles with\ncarbon nanotubes or graphene for a further resistivity decrease.6\n6. Review of Anode Active Materials: While the manuscript provides a detailed introduction\nto the types of binders used in lithium-ion battery anodes, it lacks a review of the anode\nactive materials. I suggest adding a section reviewing lithium-ion battery anode active\nmaterials in the introduction, referencing relevant studies such as Chemical Engineering\nJournal, Volume 499, 2024, 156360 (ISSN 1385-8947).\nThank you for directing us to this article. We are not sure if the above reference is correct, as\nthe one we found using the above data (“Robust self-healing ion-conductive interlocking\n15\ndual-network binder based on gradient dynamic bonding for advanced SiO anodes in Li-Ion\nbatteries” by Weng et al.) is not a review of anode active materials but describes a\nfunctionalized PAA binder for SiO anodes. Still, it is an interesting demonstration of using a\nfunctionalized cross-linked and self-healing PAA binder in SiO anodes, thus we have\nincluded it in the Introduction as one of the references on PAA binders for silicon-based\nanodes:\nThe high affinity of Ag+ for carboxylate groups is not limited to CMC – silverization was also\nsuccessfully applied to carboxyl-rich sodium alginate and sodium polyacrylate – two other widely\nresearched aquoeus binders with a superior performance in e.g. silicon anodes31,33,40 (Supporting\nFig. S3).\n7. Streamlining the Manuscript: The manuscript could benefit from a more streamlined\nstructure and clearer presentation of the results. Some sections, particularly the results and\ndiscussion, are dense and would be more accessible if broken down into smaller, more\ndigestible segments with clearer headings and subheadings.\nWe have now additionally split Results sub-section 2.3 (describing EsB imaging of binders\non active particle surfaces) into two parts (2.3 describing EsB detection of the binders and\nMonte Carlo estimation of CMC layer coverage, and new section 2.4 describing estimation of\nsurface coverage by the binders in as-fabricated and calendered electrodes). The editing\ninstructions of Nature Communications mention that the Discussion and Conclusions should\nnot be split into sub-sections, thus we abstain from further dividing this part of the\nmanuscript.\nThe author develops an accessible approach of staining carboxymethylcellulose (CMC) and\nstyrene butadiene rubber (SBR) binders in graphitic Li-ion electrodes with silver and\nbromine, enabling detailed electron imaging and precise spectroscopic quantification of the\nbinder domain. Notably, the staining reactions of Br2 vapour is similar to OsO4 staining but\nwith significantly lower hazard, as indicated by a 217-times higher short-term safe exposure\nlimit for Br2 compared with OsO4. This work can provide a guidance for slurry processing\nand electrode preparation of graphite anode. Therefore, this work has a chance to be\npublished after major revise:\n1. As shown in Figure 1A, the schematic diagram shows that the Ag+ can replace the Na+ in\nNa-CMC. It is unscientific, because the Ag+ tends to undergo cross-linking reactions with\nhydroxyl and carboxyl groups rather than substitution reactions.\nWhile we initially also suspected OH to play role in Ag binding to CMC, ATR-IR has shown\nno differences in the 3200 cm-1 OH band in the material after Ag staining. Recently, we have\nalso attempted Ag+ staining of polyvinyl alcohol (PVA) that is rich with OH groups but lacks\ncarboxylate groups, and we found no significant Ag+ binding to this polymer after subsequent\nwashing with water (the EDX atomic Ag amount normalized to C+O was 0.0015 in PVA\ncompared to 0.10 in CMC). Furthermore, EDX and XPS analysis of CMC before and after\nAg staining showed complete substitution of Ag for Na after silverization. Thus, our\nexperimental evidence suggests that the critical interaction that stabilizes Ag+ binding to\nCMC is the strong complexation by the COO- groups with simultaneous substitution of Na+\n(that is washed away during the subsequent water rinse), which is highlighted in Fig. 1a. We\nhave now added this information to the first paragraph:\nThe mechanism of both reactions was confirmed using attenuated-total reflection infrared\nspectroscopy (ATR-IR) X-ray photoemission spectroscopy (XPS) and energy-dispersive X-ray\nspectroscopy (EDX), showing diminished aliphatic C=C peaks in brominated SBR, a strong shift of -\nCOO- peaks in silverized CMC and a substitution of Ag for Na in the EDX spectrum of Ag-CMC, as\ndiscussed in detail in Supporting Notes S1-S2 and Supporting Figures S2-S4.\n2. In Figure S4, the EDX and X-ray photoemission spectroscopy (XPS) were used to quantify\nBr and Ag concentration. However, the measurement results of EDX and XPS are usually\n17\ninaccurate when compared with Inductively Coupled Plasma (ICP). It is better to provide the\nICP measurement of Br and Ag concentration.\nThis suggestion motivated us to measure additional electrodes with a varying CMC/SBR ratio\n(also see our answers to Questions 1.5 and 2.1 above). The primary reason we used EDX for\nquantification of Br and Ag concentration was that this is the same method used for the\nfollowing binder mapping in the electrodes, thus it represents the same level of accuracy as\nthe primary method used for binder analysis in the electrodes. While ICP-MS is certainly a\nmore accurate method than EDX, it cannot be applied to spatially map binders in electrodes,\nbeing additionally difficult to apply for analysing difficult-to-digest polymers, such as dried\nSBR or SBR on graphite. Additionally, we have now verified accuracy of Ag and Br EDX\nquantification on a pure Ag wire and KBr pellet, using the same EDX conditions as the ones\nused for the Ag and Br concentration analysis in Section 2.1. There was only a very small\nbias of 1.3% and 3.2 % for Ag and Br quantification, respectively, showing that EDX\nquantification was already sufficiently accurate. We have now expanded the EDX data to\ninclude the 4% relative error due to sample roughness/porosity, giving the complete\nuncertainty levels at the 95% confidence level within 10% relative. We have updated the\nMethods with the description of this characterization:\nElemental concentration was calibrated using factory standards provided in the OI Aztec software\nversion 6.1. Systematic uncertainty of the EDX quantification was verified using freshly cleaned\nAg wire and dried C-coated KBr pellet with 99.9% (trace metal-basis) and ≥99% purity,\nrespectively, giving 1.3±1.0% and 3.2±1.5% relative bias for Ag and Br quantification, respectively.\nConsidering ~4% uncertainty due to the roughness/porosity of the graphitic samples,47 we\nestimate the relative combined systematic uncertainty of Ag and Br quantification at the applied\nconditions and 95% confidence level as ±8.4% and ±10.4%, respectively.\nThe new experiments presented in updated Section 2.1. and Fig. 1 were also now measured\non multiple electrodes with changing binder content, providing significantly higher statistical\nvalue than the previous ones. The new experiments showed an expected linear trend between\nAg or Br concentration and CMC or SBR content in the electrodes. The EDX measurements\nwere shown to be accurate enough to provide required selectivity and sensitivity. Note that all\nthese experiments were measured on at least 5×1.4 mm2 random spots from large (20 cm2)\nhomogenized samples of the electrode coatings.\n18\n3. As illustrated in Figure 5A, the author claims that the more concentrated CMC solution in\nthe first mixing step might improve the final C45 dispersion in the electrode. What’s the\nspecific mechanism? Generally, the concentrated binders solutions are not conducive to\ndispersion of electrode materials.\nWe kindly direct you to our discussion in the same section, which we have now extended to\nclarify the mechanism of CBD de-agglomeration:\nWe hypothesized that because the amphiphilic CMC electrostatically stabilizes solids dispersion in\nthe slurry by adsorbing onto the graphite and carbon particle surfaces,47 a more concentrated CMC\nsolution in the first mixing step might increase CMC saturation on C45 surfaces, improving C45\ndispersion in the slurry and in the final electrode.\nThis is equivalent to acknowledging that CMC acts as a surfactant in the slurry (as evident\nfrom the zeta-potential studies in e.g. Ref 47), adsorbing on C45 and graphite surfaces. By\nincreasing the concentration of the surfactant, we increase its saturation on the solid surfaces\nduring mixing, ensuring that it prevents the solids from agglomerating to each other.\n4. It is better to provide a optimized concentration of CMC solution in the first mixing step,\nwhich is important for the slurry processing in Li-ion battery industry.\nAs noted, we provide the optimized concentration of CMC solution in the first mixing step as\n3% wt.\nTo reduce CBD agglomeration, we fabricated electrodes of the same dry mass composition, but\nusing a more concentrated CMC solution (increased from 1.5 to 3 wt.%) during an initial mixing\nstep. […] Fig. 5b shows a BEI image of such modified electrode after bromination, confirming\nthe disappearance of large CBD clusters, which is also evident in the histogram in Fig. 5c.\nFurthermore, four-point probe measurements of non-stained electrodes showed that reduced\nCBD agglomeration decreased mean electronic resistivity in both uncalendared and calendered\nelectrode coatings by 14±2% (Fig. 5d) […].\nWe now specify the concentration of CMC in both slurries in the caption of Figure 5. The\nconcentration of CMC solution in the modified slurry mixing protocol is also annotated in the\nfirst mixing step on the mixing protocol schematic in Fig. 5a.\n19\n5. In Figure 6, when the polymer solution (e.g. PVDF in N-methyl pyrrolidone) is immersed\nin a non-solvent (e.g. water), the adhesion of polymer slurry on Cu foil will greatly decrease\nand even detach from the Cu foil surface. This phenomenon will increase the contact\nresistance of electrode materials and further decrease the electrochemical performance of Li-\nion battery. However, the author reports a opposite result by attributing to the reduced pore\nresistance? I think that it is hard to convince. The author should provide more discussions\nand comparations in this section by supplementing experiment.\nWe did not observe the adhesion problems mentioned above – in fact, we observed a strong\nincrease in adhesion between the electrode coating and the Cu foil after phase inversion in\nacetone (as mentioned in the text and now rephrased for better clarity):\nOn the other hand, the electrode immersed in acetone showed a marked binder concentration\nnear the current collector, possibly due to faster precipitation of CMC by acetone with 6× lower\nviscosity than isopropanol.71 The greater binder fraction at the current collector allowed bending\nof the relatively thick electrode without cracking, and it was significantly more adherent to the\ncurrent collector than the electrodes with the opposite binder distribution (Fig. 6d).\nBy using binder mapping, this was directly correlated with the increased concentration of the\nbinders (that help with the adhesion) close to the current collector after this treatment (Fig. 6).\nIn the EIS measurements, we also did not observe high-frequency semicircles in Nyquist\nplots of reference and phase-inverted electrodes that are characteristic to electrode contact\nresistance, showing that phase inversion did not increase this already-low resistance (see e.g.\nLandesfeind et al., J. Electrochem. Soc., 163 (2016), DOI: 10.1149/2.1141607jes, or\nLandesfeind et al., J. Electrochem. Soc., 164 (2017), DOI: 10.1149/2.0131709jes). We have\nnow included this information in the text and provided a supporting figure that compares\nNyquist plots of these graphitic electrodes with a NMO/Al cathode measured separately in\nour group, in which contact resistance is clearly evident:\nAlso, the process did not change the comparatively low electronic contact resistance of the\ncoatings, as indicated by the absence of a high-frequency semicircle in the Nyquist plots3,72 (also\nsee Supporting Figure S19).\nphase-inverted graphite/Cu electrodes described in this work and (B) calendered Na MnO /Al\n0.7 2\nelectrodes measured separately in our group. Insets show high frequency, low impedance regions of the\nspectra. The measurements were performed at open circuit potential, in symmetric cell configuration, in\n10 mM tetrabutylammonium perchlorate in 1:1 wt. EC:DMC, in 100kHz – 10 mHz frequency range. The\nhigh-frequency resistive/capacitive semicircle in the Na MnO /Al electrode represents the contact\n0.7 2\nresistance between the electrode coating and Al current collector that is characteristic to battery\nelectrodes supported on metallic foils with a native oxide layer.23,24 The lack of semicircles in the Nyquist\nplots of the graphite/Cu electrodes shows that the contact resistance was below the detection limit in\nboth the reference- and acetone-phase-inverted graphite electrodes.\nWe are unsure what mechanism could lead to the potential delamination concerns mentioned\nabove – we could also not find reports of such an issue in the previous works describing\napplication of phase inversion in electrode manufacturing. These other works (which we also\ncite in our text) reported similar observation as ours – an increased adhesion strength of the\nphase-inverted coatings to the current collector. This was previously attributed to e.g.\ndifferences in crystallinity of the PVDF binder in phase-inverted coatings or low strain\nassociated with rapid precipitation of binders during phase inversion in the liquid phase, as\nopposed to the precipitation in a moving drying and shrinking front during conventional\nslurry drying (see e.g. Harks et al., J. Power Sources, 441 (2019), DOI:\n10.1016/j.jpowsour.2019.227200; Yang et al., Adv. Funct. Mater., 26 (2016), DOI:\n10.1002/adfm.201604229; Wu et al., ACS Nano, 15 (2021), DOI: 10.1021/acsnano.1c06491).\nPlease also note that we attribute lowered pore ionic resistance (which is different to\nelectronic contact resistance) to the changed through-thickness distribution of the binders in\nthe electrodes phase inverted in acetone:\nmarked reduction in ionic resistance was a sole consequence of the preferential binder (and\nporosity) distribution, in agreement with simulations.9,73\n6. As we know, the Br2 vapours will corrode the Cu foil, CMC and SBR during bromination\nof electrodes at high temperature, which is is very detrimental to the electrochemical\nperformance of Li-ion battery. Therefore, how to avoid this phenomenon during practical\napplication?\nThank you for this correct observation. As we explained in our answer to Question 1.3. above\nand our now-included clarification in Section 2.1, staining is performed on separate electrode\npieces for binder mapping only and is not applied to electrode samples subject to other testing\n(e.g. EIS).\nNote that because of the reactivity of Ag+ and Br ,staining and binder mapping should be\n(aq) 2\napplied on separate electrode areas to those used in electrochemical or other testing.\nWe have also clarified it in Section 2.5 (CBD agglomeration study):\nFurthermore, four-point probe measurements of non-stained electrodes showed […]\nAnd Section 2.6 (phase inversion study):\nThe greater binder fraction at the current collector allowed bending of the relatively thick\nelectrode without cracking, and it was significantly more adherent to the current collector than\nthe electrodes with the opposite binder distribution (Fig. 6d).\nTo explore effects on pore ionic resistance, we tested unstained electrodes using electrochemical\nimpedance spectroscopy (EIS) in a non-intercalating electrolyte developed by Landesfeind et\nal.3,54\nWe have now also included a note in the Limitations:\nFor the same reason, silverization requires impractical detachment of any Cu current collector\nprior to staining, and the likely alteration of the electrode’s electrochemistry by Ag+ and Br may\n2\nrestrict its application, for example, in operando studies or on-line manufacturing quality control.\n22\n7.The article contains some technical terms and abbreviations (such as EDX, BEI, EsB, etc.).\nWhile these are helpful for concise expression, they may pose comprehension barriers for\nnon-specialist readers. It is suggested that the authors provide detailed explanations when\nthese terms first appear.\nWe have analysed the article again to confirm these terms are explained in their first\nappearance in the article. EDX, EsB, BEI, CMC, SBR, PVDF, NMP, SEI are first mentioned\nand explained in the Introduction. XPS, ATR-IR are introduced and explained in Section 2.1\nand EIS in Section 2.6 (previous Section 2.5). We have additionally re-explained the most\nimportant terms at the beginning of the relevant Results sections (e.g. EDX, BEI are\nexplained again in 2.1 and EsB in 2.3).\n8.Although the authors have explained the mechanism of the staining reactions, the\ndescriptions of the optimization processes for some key parameters are not detailed enough.\nFor example, the rationale behind the selection of reaction temperature, time, and bromine\nvapor concentration in the bromination process has not been fully elaborated. It is suggested\nthat the authors provide more detailed information on these optimization processes to help\nreaders better understand and reproduce the method.\nThank you for this suggestion. We have now extended the Methods section to provide more\ndetails on the processing parameters. Please note that the applied conditions of bromination\nwere used without any further optimization – e.g. the provided bromine vapor pressure was\nnot chosen by us, but was simply a consequence of the initial vacuum in the system,\ntemperature of the Br pool and the volume of the Schlenk apparatus:\n2\nfree setup depicted in Figure S1. The reaction was carried out inside a small glass reactor at 50 oC\nand Br pressure of 5-8×10-2 mbar for 270 s, followed by outgassing of excess Br under active\n2 2\nvacuum (~2×10-2 mbar). The temperature and time were chosen to ensure no condensation of Br\n2\nvapour inside the reactor and to facilitate complete reaction with SBR, while minimising the\nprocessing time. Br vapour was provided by a small Br pool placed in a separate glass vial\n2 2\nimmersed in a small water bath (~20 oC) and connected to the pre-evacuated and heated reactor\n(~2×10-2 mbar, 50 oC) under static vacuum. The bromination conditions were applied without any\nfurther optimization. After bromination, samples were kept in N -sealed vessel until EDX/BEI\n2\nanalysis to avoid humidity-induced hydrolysis of brominated SBR; however, our later EDX tests also\n23\nindicated no significant loss of Br from the electrodes in air for at least two weeks prior to the\nEDX/BEI analysis.\nFor silverization, we now also included a note on the use of the surfactant in the experiments\nfor bulk silverization and through-thickness mapping of the electrodes, which was previously\nmissing from the description.\nRecently when reproducing the bi-layer electrode silverization (which previously already\ngave similar results to the bromination of these electrodes) to address Question 3.11 of the\nsame Reviewer, we realized that the original bi-layered silverization experiment was done\nwith an addition of a small amount of non-ionic surfactant (1 mM Triton X-100) in the\nAgNO solution (the surfactant was not used in any further work, as its solution with AgNO\n3 3\nwas found to have a limited shelf life). However, recent investigations show that the addition\nof the surfactant had some benefits for the wetting and accuracy of Ag mapping in the CMC-\nlean regions of the bi-layered electrodes, due to their specific composition and preparation\nroutine. With this knowledge, we now include surfactant use during the silverization of\nelectrodes presented in the updated Section 2.1. and Fig. 1, as these electrodes required an\nanalogous preparation routine to the bi-layered electrodes. The surfactant was only beneficial\nin the specific order of sample preparation, staining and then analysis. We have now provided\nan extended discussion and experimental data on this in the Supporting Note S5 and\nSupporting Figure S7.\nWe have now included this information in the main article – in Section 2.2:\nThe addition of a surfactant (1 mM Triton X-100) to the AgNO staining solution benefited wetting\n3\nand accuracy of Ag-CMC profiling through the binder-lean region of electrodes, as dicussed in the\nwetting analysis in Supporting Note S6 and Supporting Figure S8.\nas well as in the Methods:\nThe electrodes were silverized by immersing them in a crystallizer containing 0.5M AgNO for 3\n3\nminutes (depicted in Fig. 1A). For the CMC analysis of homogenized and bi-layered electrodes\n(Mechanism and specificity of staining reactions and Staining bi-layered electrodes sections\nrespectively), the 0.5M AgNO solution was freshly mixed with 1 mM Triton X-100 non-ionic\n3\nsurfactant prior to staining to help with penetration of the solution into all of the electrode, as\ndiscussed in detail in Supporting Note S6 and Supporting Figure S8.\nthickness and top-down binder analysis. B) Top-down BEI images of top and internal (bulk) surfaces\nof an electrode silverized without or with the surfactant. The electrode contained 1.5 wt.% CMC and\nno SBR. C) EDX Ag quantification in internal- and top surfaces of the same type of electrode silverized\nwithout or with the surfactant. D) EDX maps of carbon (brown) and silver (teal) in bi-layered\nelectrodes silverized without and with an addition of 1 mM Triton X-100 non-ionic surfactant to the\n0.5M AgNO staining bath. The electrodes contained four times more CMC and SBR in the lower layer\n3\ncompared with the upper layer. E) Ag-L profiles derived from the EDX maps, showing the ratio (Δ) of\nthe Ag-L signal between the binder-rich and -lean layers.\n25\n9.While the authors have observed the complex distribution patterns of binders within the\nelectrodes, the in-depth analysis of how these patterns affect the electrochemical\nperformance of the electrodes is not sufficiently thorough. For example, regarding the role of\nnanoscale binder films during charge-discharge processes and the impact of non-uniform\nbinder distribution on battery cycling stability, it is recommended that the authors conduct a\nmore in-depth investigation in conjunction with the results of electrochemical testing.\nWe agree fully that these results warrant new detailed investigations on the roles of binder\ndistribution on battery cycling stability and performance, as we have also extensively\ndiscussed in the Discussion and Conclusions. Such proper in-depth analysis, however, will\nnecessitate broad testing campaign with carefully designed comparative experiments\nreproduced on many battery cells to adhere to modern standards in scientific battery\ncommunity, which would exceed the scope, time and size constraints of this single article.\nBecause methods for mapping and visualizing binders in battery electrodes are limited, yet\nurgently sought (as proven by the interest we have received from the academic community\nand several big battery OEMs) the article focuses on the new binder analysis methods, and\nalready provides several comprehensive application scenarios (Sections 2.3-2.6). However, as\na new method we prioritise extensive consideration of the method validation (Sections 2.1-\n2.2). Together with the additional experiments and new data that was requested and we have\nnow added, the article is now the maximum size for Nature Communications. Therefore,\nbattery performance investigations and implications will be described in a subsequent\npublication.\n10.While the article mentions the advantages of CMC and SBR binders over traditional\nPVDF binders in terms of staining, the comparative analysis with other potential staining\nmethods may not be comprehensive enough. It is suggested that the authors include a\ncomparison with other staining methods in the discussion section or elaborate on the\nuniversality of this staining approach.\nThe only other binder staining technique that we are aware of that has been properly\ndescribed in the literature is the OsO staining of SBR by Lee et al, that we mention in the\n4\ndetected by energy-selective backscattered electron imaging (EsB) or EDX to trace SBR migration\nin graphite electrodes during drying.37,38 The extreme toxicity and volatility of OsO , however,\n4\nlimits this approach to only the most specialized laboratories.\nWe also discuss how SBR staining by Br compares with this method at the beginning of\nThe latter reaction is similar to OsO staining but with significantly lower hazard, as indicated by a\n4\n217-times higher short-term safe exposure limit for Br compared with OsO .39\n2 4\nWe have now also included discussion of staining other binders at the end of the Limitations\nsection:\nAlso, because the staining methods rely on the presence of carboxylate and aliphatic C=C groups\nin the binders, any process that affects these groups prior to staining (e.g. drying-induced cross-\nlinking) may lower the fraction of Ag and Br that reacts with the binders (note, however, that this\ncan be also beneficial for quantification of binder cross-linking). Consequently, bromination and\nsilverization are not suitable for staining e.g. PVDF or PTFE binders (that lack these functional groups)\nbut can be suitable for other binders, such as the carboxyl-rich alginates and polyacrylates. However,\nthe accuracy of spatial mapping these two binders in electrodes remains to be fully quantified.\nAdditionally, in the same section, we have now included details on staining electrodes based\non other-than graphite active materials, such as micro- and nano-Si, carbon-coated SiOx and\nLiFePO , including new Figure 7 (also attached above, page 12):\nelectrodes containing easily-oxidizable materials (e.g. metallic, LiFePO or nano-Si particles), as\n4\nthese can rapidly undergo redox reactions with the staining elements, complicating image\ninterpretation. Micro-Si or materials with an inert, stable surface passivation layer can be,\nhowever, suitable. This is shown in Figure 7 that demonstrates successful staining on Si and C-\ncoated SiO microparticle-based electrodes but not on the more reactive LiFePO or nano-Si.\nx 4\n11.In presenting key data, such as quantitative analysis of binder distribution, the authors\nprovided EDX and BEI images for only part of the electrodes, which may not fully represent\nthe entire electrode. It is recommended to increase the image coverage to more\ncomprehensively show the overall characteristics of the electrodes. For example, larger -\n27\narea images could be provided, or multiple images taken at different positions and stitched\ntogether to ensure data integrity and representativeness.\nFollowing this suggestion, we performed a campaign of reproduction tests, and we have now\nincluded four additional reproductions of brominated and silverized bi-layered samples in\nSection 2.2. and updated Figure 2. These tests showed good reproducibility (binder ratio\nvalues in individual samples within 3-23% of the averaged values), showing that measuring\nsingle 1 mm-cross sections can give representative. Since We have updated the figure and\ntext accordingly with the discussion on the precision and accuracy of through-thickness\nbinder profiling. Following the extended discussion on Br outgassing from the new testing\ncampaign on homogenized electrodes with different SBR:CMC ratio in Section 2.1 and Fig.\n1, in Section 2.2. and Figure 2 we now also include comparison of SBR profiling from the\nabsolute EDX Br distribution and the differential (outgassed) Br distribution. Also, having\nnow more cross-sectional BEI data, we have modified the BEI through-thickness\nquantification routine with a more generalized approach of multilevel Otsu thresholding,\ngiving a more complete picture on the BEI quantification uncertainty.\nAg (turquoise) or Br (cyan) and contrast-equalized backscattered electron images (BEI) of silverized and\nbrominated electrodes with bi-layer binders distribution. b EDX- and BEI-derived through-thickness profiles of\nAg and Br, showing ratios (Δ) of each signal between the binder-rich and binder-lean layer. c EDX-Br profiles of\nthe third brominated sample acquired consecutively at an increasing pixel scanning time (total electron dose),\ntogether with Br outgassing profiles obtained by subtracting 5th scan profile from the 1st and 3rd scan profiles.\nd\nEffects of EDX mapping time (pixel dwell time) on the accuracy of Δ values derived from the absolute Ag and Br\nsignals (points) during the analysis of the 2nd silverized and 3rd brominated samples, respectively, also showing\noutgassed Br Δ (horizontal bars) calculated between consecutive Br scans. e Comparison of Δ values derived\nusing EDX, BEI and Br outgassing in all the samples. Weighted means (columns) were averaged from the data of\nindividual samples (points). The error bars correspond to Student’s t-95% confidence intervals. All images were\nacquired at 10 kV and post-processed as described in Supplementary Methods.\nmeasurements, we also now include in Supporting Figure S18 two matching reproductions of\nSBR mapping and profiling in separately made, stained and analysed reference and acetone-\nphase inverted electrodes (the phase inversion study).\nFigure S19. EDX maps of Br and C in reference and phase inverted in acetone electrodes after cross-\nsectioning and Br staining, together with through-thickness Br-L distribution profiles derived from\nthe EDX maps. The profiles exclude the brominated Cu current collector region. The electrodes\nwere separately made and identically processed as the electrodes described in the phase inversion\nsection of the main article.\nIn this manuscript, authors suggested a new method that staining carboxymethylcellulose\n(CMC) and styrene butadiene rubber (SBR) binders in graphitic Li-ion electrodes with silver\nand bromine, enabling detailed electron imaging and precise spectroscopic quantification of\nthe binder domain. By using this method, the authors could achieve the suppression of binder\nmigration during high-temperature electrode drying, and a 40% decrease in electrode ionic\nresistance for unprecedented electrode-scale, high- resolution backscattered electron\nimaging of surprisingly complex binder hierarchies, revealing multiple types of agglomerates\nand hardly-ever-seen nanoscale binder films. In conclusion, the rich information provided by\nbinder staining can shed new light on fundamental electrode processes and extend\noptimization routes of negative Li-ion electrodes. Therefore, I suggest this manuscript to be\npublished in journal \"Nature Communications \" after minor revisions according to the\nfollowing aspects:\n1. The authors conducted special analysis for SE, EDX, BEI. If possible, please provide XPS\nmapping data for not only elemental analysis but also chemical state analysis.\nThank you for this suggestion.\nWe have now provided detailed XPS chemical state analysis of C 1s, Ag 3d and Br 3d core\nlevels of CMC and SBR before and after staining, together with survey spectra elemental\nquantification discussion in Supporting Figs. S3 and S4 and Supporting Note S2. This is now\nreferenced in Section 2.1 in the discussion on the mechanism of staining:\nThe mechanism of both reactions was confirmed using attenuated-total reflection infrared\nspectroscopy (ATR-IR), X-ray photoemission spectroscopy (XPS) and energy-dispersive X-ray\nspectroscopy (EDX), showing diminished aliphatic C=C peaks in brominated SBR, a strong shift of -\nCOO- peaks in silverized CMC and substitution of Ag for Na, as discussed in Supporting Notes 1-2 and\nSupporting Figures 2-4.\n(Gr) and C45 carbon conductive additive after silverization and bromination, determined with EDX\nand XPS. The figure also shows EDX quantification of Ag in silverized sodium alginate (Na-Alg) and\nsodium polyacrylate (Na-PAA). B) XPS spectra and C) elemental quantification of CMC before and\nafter bromination and silverization. D) XPS spectra and E) elemental quantification of SBR before and\nafter bromination and silverization. The error bars correspond to 95% confidence intervals based on\nmeasurements of at least 5 spots for EDX and 3 spots for XPS. In A), the 95% confidence intervals\nalso include systematic quantification uncertainty - 10% relative for EDX and 30% relative assumed\nfor XPS. EDX was acquired at 15 kV beam voltage. Note that the elemental quantification in C) and E)\nexcludes Si found in CMC-Br, SBR-Br and SBR-Ag (1.8 at%, 0.5 at% and 9.0 at%, respectively).\nA) C 1s region of CMC, silverized CMC and brominated CMC; B) Ag 3d region of silverized CMC and\nSBR, together with a spectrum of metallic Ag foil used to create empirical Ag0 peak model (the\nresults of fitting Ag foil with asymmetric peak shapes are shown for informative purpose only); C) C\n1s analysis of SBR, brominated SBR and silverized SBR; D) Br 3d region of brominated SBR and\nbrominated CMC. For each sample, binding energy was corrected by referencing to the fitted C 1s C-\nC/C-H peak at 285.0 eV. Dark grey lines represent summed fitting envelopes. Note that due to close\nproximity of some of the peaks (e.g. C=C and C-H, O-C=O and O-C-O, Ag0 and Ag+), the peak models\nand quantitative analysis of C 1s and Ag 3d regions have a significant level of uncertainty. The C 1s\nand Br 3d spectra were processed with the universal polymer Tougaard background and the Ag 3d\nspectra with the iterated Shirley background that stabilized the Ag 3d fitting model.\n33\n2. In Fig4e, the authors provided graphite surface coverage data to confirm binder\ndistribution. Detailed process to get surface coverage data should be described.\nWe have included detailed description and algorithm for quantifying binder coverage from\nEsB images in the Supporting Method S2 and Supporting Code S1. We have referenced the\nprocedure in the caption of Figure 4:\nAll images were acquired at 1 kV of beam voltage and post-processed as described in the\nSupporting Method S2.\nWe have now also included a reference to the Method S2 and Code S1 in the Characterization\nsection of the Methods:\nA detailed description of EDX, BEI and EsB image post-processing, including the process for\nquantification of binder surface coverage, is included in Supporting Method S2 and Supporting\nCode S1.\n3. In this manuscript, the authors only considered graphite as an anode materials. Please\nprovide possibility to use this method to the other electrode materials.\nTo address this question and similar Questions 1.10 and 3.10, we have now included\nadditional testing results of staining on C/SiO , nano- and micro-Si and LiFePO -based\nx 4\nelectrodes in new Figure 7 (also attached above, page 12) and Limitations section:\nFirst, the staining methods, particularly the liquid-phase silverization, are less applicable to\nelectrodes containing easily-oxidizable materials (e.g. metallic, LiFePO or nano-Si particles), as\n4\nthese can rapidly undergo redox reactions with the staining elements, complicating image\ninterpretation. Micro-Si or materials with an inert, stable surface passivation layer can be,\nhowever, suitable. This is shown in Figure 7 that demonstrates successful staining on Si and C-\ncoated SiO microparticle-based electrodes but not on the more reactive LiFePO or nano-Si.\nx 4\n4. In Fig3a, the authors showed images of internal electrode surface exposed by scotch tape\npeel-off and subsequent silverization, imaged at different electron beam voltages with surface\nsensitivity. How does the surface sensitivity work with electron beam voltages?\n2.3 and provided a reference to Goldstein’s handbook on electron microscopy, where the\ndependency of probing depth on the primary electron beam voltage is described in detail:\nThe images were acquired at low and high magnification (Fig. 3A and 3B, respectively) and\ngradually decreasing beam voltages, corresponding to decreasing electron-sample penetration\ndepth and increasing surface sensitivity in consecutive images.48\nThe beam voltage-probing depth dependence is also mentioned in the following text:\nConsequently, this contrast mostly disappeared at lower beam voltages of 2.5 and 2 kV (shown in\nFig. 3a), where the depth probed by backscattered electrons was reduced from a few hundred to\na few dozen nanometers (see the simulations in Fig. 3c).\n5. The authors claimed that they could reduce 14% of electronic resistivity and decrease 40%\nof ionic resistance. What is possible way to increase each value?\nFor electronic resistivity decrease, we have added a comment at the end of the Correlating\nslurry mixing… section\nFuture staining-assisted optimization could investigate alternative pre-mixing methods of thinner,\nlower CMC-content slurries, or substituting C45 nanoparticles with carbon nanotubes or\ngraphene for further resistivity decrease.6\nFor pore ionic resistance, we have added a comment at the end of Phase Inversion section:\nCombining phase inversion with using spherical (instead of flaky) graphite or pore patterning\ntechniques could further improve ionic transport in such electrodes.16–19,76\nchanges to the article:\n- Corrected the statement on the origin of commercial electrodes described in line 301\nand shown in Fig. 4. In our recent communication with our collaborators, we learned\nthat these electrodes, albeit commercial and industrially-made, were not from an\nelectric vehicle as we were previously told, but originated from LiFUN Technologies\n(Xinma Industry Zone, Hunan Province, China). This company is a popular\nmanufacturer and commercial supplier of industrially-made electrodes and cells, also\nused extensively as commercial electrode and cell benchmarks by other research\ngroups. Therefore, we corrected and specified the origin of these electrodes in line\n301 to be fully accurate, but there is no change to our findings - our methods working\nequally well for commercial and laboratory- and/or in-house produced electrodes, and\nthe same binder morphologies in both types of electrodes.\nThese findings are not limited to laboratory-made electrodes – we also imaged uncycled\ncommercial electrodes from LiFUN Technology (Xinma Industry Zone, Hunan Province,\nChina).\n- Modified the figures and captions to adhere to the style of Nature Communications\n- Added “Si-based” to “graphitic Li-ion electrodes” in the abstract to reflect the updated\ntests on Si-based electrodes:\nHere, we present an accessible approach of staining carboxymethylcellulose (CMC) and\nstyrene butadiene rubber (SBR) binders in graphitic and Si-based Li-ion electrodes with silver\nand bromine[…]\n- Specified the mixer type (centrifugal planetary mixer) used for slurry mixing in\nFigure 5.\n- Corrected the Δ value for the previously-shown Ag-stained bi-layered sample in\nSection 2.2 from Δ = 4.4x to Δ = 4.8x. The previous value was calculated based on\nEDX Ag/(C+O) profiles (adapted during the initial stages of our work). Later, during\npreparation of the manuscript, we adapted absolute Ag-L (and Br-L) profiles to\ndescribe binder cross-sectional distribution, as they accurately represented drop of the\nelement signals at the top and bottom edges of the cross-sections (as visible in Figs 2\nand 6). As we realized the originally-presented Ag Δ corresponded to the previous\ncalculation routine, we corrected it to represent Δ based on absolute Ag-L profile and\nto be consistent with all the other Δ that are based on the absolute Ag-L (or Br-L)\nsignal profiles.\n36\n- Substituted discussion on EDX/XPS testing staining specificity on individual battery\nmaterials at the beginning of Section 2.1 with a shorter and more streamlined version:\nEDX and X-ray photoemission spectroscopy (XPS) were used to quantify Br and Ag\nconcentration in the first 1 μm (EDX) and 10 nm (XPS) of the stained materials, showing large\namount of Ag in Ag-CMC and of Br in Br-SBR, and little- to no binding of these elements to\nthe other materials, as presented in Supporting Fig. S3.\n- Moved the note on the need of delamination of electrode coatings subject to\nsilverization to Section 2.2 (because in the new homogenized electrode analysis in\nSection 2.1., both brominated and silverized electrodes were delaminated from the\ncurrent collector before the staining and homogenization).\n- Removed the note on contrast-enhancing role of Ag and Br in cross-sectional BEI of\nbi-layered electrodes from Section 2.2, because such Ag/Br effect was already\nmentioned in Section 2.1.\n- Condensed analysis of SBR migration in a bi-layered electrode made with no SBR in\nthe top layer, previously mentioned at the end of Section 2.2, with the original results\nof non-homogenized SBR-only electrode analysis (previously shown in Section 2.1).\nThese analysis are now presented together in the discussion on differences in drying-\ninduced migration of SBR and CMC, in Supporting Information and referred to in\nSection 2.2:\nThese average values are within 2.5% and 22% of the expected 4× ratio, with more\nundercounted Δ for the Br-stained SBR due to the larger susceptibility of SBR to drying-\ninduced migration amplified by its weaker adsorption onto graphite47, as verified in\nSupporting Note S5 and Supporting Figure S7.\n- Added a brief discussion on the accuracy of pore ionic resistance determination with\nEIS in symmetric setup to the Methods section, considering the findings of Bieneck et\nal published this month in J. Electrochem. Soc. 172 (2025) (DOI: 10.1149/1945-\n7111/adde17):\nNote that most recently, Bieneck et al. showed that the pore ionic resistance determined\nwith symmetric-cell EIS can be strongly influenced by the spatial distribution of the double\nlayer capacitance (C ) of carbon black, and its contribution to the overall C of the\ndl dl\nelectrode.72 Based on the mass composition of our electrodes and the BET surface area of\nthe graphite and C45 carbon black (1.47 and 45 m2/g, respectively), the estimated\ncontribution of C45 to the total C of the electrodes was ~25%. The analysis of Figure 9 in\ndl\nBieneck’s paper shows that for such C contribution, the pore ionic resistance of the\ndl\n37\nreference electrode (with CBD concentrated close to the separator) determined with EIS\nmay be underestimated by ~12% and that of the acetone-processed electrode (with CBD\nconcentrated close to the current collector) may be overestimated by ~23%, implying that\nthe actual pore ionic resistance difference between these electrodes may be ~57% instead\nof the apparent 40%.\n- Updated the Conflict of Interest statement with a note on a patent application that was\nfiled on the methods described in this paper:\nThe authors declare the following competing interests: Oxford University Innovation Limited\nfilled provisory patent application 2511810.0 claiming inventorship of Stanislaw P.\nZankowski and Patrick S. Grant for aspects of the described method. The application is\npending review.\nWith these changes and additions, we hope we have fully addressed all the Reviewers’\nquestions. Once again, we would like to thank all the Reviewers for your time and providing\nvaluable suggestions, and we are looking forward to your responses.\n38"}, {"round": 2, "reviewer_comments": {"reviewer_2": "This study, for the first time, achieves nanoscale visualization of aqueous binders in Li-ion battery electrodes through chemical staining coupled with high-resolution imaging, unveils their microstructural evolution mechanisms, and offers a fresh paradigm for electrode performance optimization and manufacturing control. The authors have provided a thorough response to the reviewers’ comments and expanded the dataset; the paper can be accepted for publication.\nI have reviewed the authors' responses and find that they have satisfactorily addressed all the reviewers' comments. I have no further comments.", "reviewer_3": "The revised manuscripts can be published by Nature communications.\nThe revised manuscript has addressed referee #1's comments, and therefore can be accepted by Nature Communications."}, "author_response": ""}]

Decision letter