Deciphering the metallic zinc anode interface: multimodal characterization strategies for zinc-ion batteries

Side reaction-dependent morphological evolution

In situ optical microscopy operates on fundamental optical principles, enabling visualization of microstructure features through transmitted, reflected, or fluorescence light interactions [Figure 3A]. This technique captures and processes light-matter interactions to generate magnified representations of surface and subsurface features, offering distinct advantages over more sophisticated microscopy methods, including operational simplicity, non-destructive analysis, real-time monitoring, and broad material compatibility^[90]. Recent applications in AZIB research have significantly advanced the understanding of Zn deposition morphologies and interfacial phenomena^[91]. It is well-known that the interface layer can trigger uniform Zn ion decomposition. To elucidate the impact of ZIF-11 coatings on Zn electrodeposition behavior, He et al. conducted comparative in situ optical microscopy observations of bare Cu foil and ZIF-11@Cu electrodes during Zn electroplating processes [Figure 3B and C]^[92]. Initial imaging revealed smooth Cu foil surfaces in ZnSO₄ aqueous electrolyte. Upon current application (10 mA cm^-2), instantaneous Zn nucleation occurred, evolving into heterogeneous moss-like dendritic structures (~15 μm thickness) within minutes. These dendritic protrusions present critical safety risks through electrode bridging and subsequent internal short circuits. Concurrently, HER activity was observed, proceeding via either Volmer-Heyrovsky or Volmer-Tafel pathways^[93]. The reaction equations are as follows:

Figure 3. (A) Schematics of the optical microscope. In situ optical microscopy images of Zn deposits on (B) ZIF-11@Cu and (C) bare Cu electrode surface at 0, 5, 10, 15, 20, and 25 min. (B and C) are reproduced from Ref.^[92]. with permission, Copyright 2021, Elsevier. (D) In situ operando optical microscope images showing hydrogen evolution behavior^[42]. Copyright 2024, Wiley. (E-H) 3D optical images of Zn foils from Zn-Zn symmetrical batteries after cycling in 2 M ZnSO₄^[96]. Copyright 2022, Wiley.

The HER would lead to the evolution of the Zn anode surface morphology. In situ optical microscope monitoring at 20 min deposition [Figure 3D] revealed progressive bubble accumulation on Zn surfaces, correlating with HER intensification during cycling operations^[42]. Chen et al. documented analogous interfacial evolution: initial dendritic growth via tip-enhanced deposition within 2 min, followed by adherent bubble formation at 15 min, culminating in dense bubble coverage by 20 min^[54]. This HER-driven process induces critical interfacial passivation and corrosion behavior - as the charge in the Helmholtz layer depletes, water molecule penetration from the outer to inner Helmholtz planes facilitates free water decomposition^[94]. Moreover, to address the fundamental modulation involved in alleviating the intrinsic tradeoff between Zn nucleation/dissolution kinetics and HER inhibition, DFT calculations can be used to analyze the thermodynamic stability of the solvation sheath and the kinetics of water dissociation. Zn₄(OH)₆SO₄·xH₂O passivation layers exhibit poor ionic conductivity, which ultimately leads to anode corrosion and "dead zinc" formation^[95]. These morphological and compositional changes directly reduce the coulombic efficiency and cycling stability of AZIBs. Post-cycling analysis through 3D/2D optical profilometry [Figure 3E-H] quantitatively confirmed surface roughness reduction, indicating corrosion-induced planarization^[96]. The predominant SEI component, Zn₄(OH)₆SO₄·xH₂O, was consistently identified across various electrolytes, accompanied by significant substrate topographical alterations detectable via optical microscopy. Side reactions at the Zn anode surface, observable via optical microscopy, were found to systematically correlate interfacial dynamics with macroscopic battery performance degradation. However, limited by the diffraction of light, optical microscopy offers low resolution and cannot reveal the internal structure of the sample.

Current density-dependent morphological evolution

High-resolution SEM represents a cornerstone analytical technique for high-resolution surface characterization across diverse scientific disciplines, including advanced materials research, nanoscale biology, mineralogical studies, and microsystem engineering. The instrument operates through a precisely focused electron beam (accelerating voltage: 0.5-30 keV) that is electromagnetically collimated using a series of condenser and objective lenses^[97,98]. Scanning coils systematically raster this nanoscale probe (< 1 to ~10 nm diameter) across the specimen surface in a synchronized pattern with detector signal acquisition. The electron beam is focused by electromagnetic lenses and then scanned across the sample surface in a raster pattern by the scanning coils. As the electron beam interacts with the sample, various signals are produced, providing detailed images of the surface. SEM operating at 5-15 keV accelerating voltage with secondary electron detection was employed to systematically investigate the dynamic interfacial evolution of Zn electrodeposits under varying electrochemical conditions. The instrument's electromagnetic collimation system (condenser/objective lens configuration) achieved a probe diameter < 3 nm, enabling nanoscale resolution of deposit morphologies [Figure 4A]. Synchronized beam scanning (raster frequency: 0.1-10 kHz) and signal acquisition permitted real-time mapping of nucleation/growth processes. SEM itself is mainly used to observe the surface morphology and structure of samples. Although elemental analyses can be performed by equipping accessories such as an energy spectrometer (EDS), such analyses can only provide information on the elemental composition of the sample surface, and cannot accurately give information on the chemical state of the elements, the chemical bonding, and other internal detailed structural information.

Figure 4. (A) Schematic diagram of the SEM equipment. (B-F) SEM images of five distinct Zn deposit morphologies. (B-F) are reproduced from Ref.^[99]. with permission, Copyright 2006, Electrochemical Society. (G-I) SEM images of electrodeposited Zn in alkaline solution at different voltages^[100]. Copyright 2015, Springer Nature. (J-L) SEM micrographs of Zn electrodeposits at different deposition potentials from ethylene glycol solution containing 0.75 M zinc acetate and 0.5 M sodium acetate^[103]. Copyright 2019, Elsevier. (M-O) SEM image of the (002)/(101) Zn papers after cycling in Zn(OTf)₂ or ZnSO₄ at 1 mA cm^-2 and 10 mA cm^-2, respectively^[102]. Copyright 2022, Wiley. (P-S) The SEM images of Zn deposits from the Zn electrodes of Zn||Zn cells with a fixed areal capacity of 1 mAh cm^-2 in ZS and La³⁺-ZS electrolytes^[106]. Copyright 2022, Springer Nature. (T and U) Digital and SEM images of bare Zn and Zn@CCF (T) before and (U) after 7 d of corrosion^[109]. Copyright 2021, Wiley.

Three distinct growth regimes were identified through chronopotentiostatic deposition on anode substrates. (i) Low current density regime (1-5 mA cm^-2): Epitaxial layer-by-layer growth dominated at low overpotentials, yielding highly ordered hexagonal Zn (002) basal planes aligned parallel to the substrate surface [Figure 4B-F]^[99]. This regime adheres to Frank-van der Merwe growth kinetics, where sequential two-dimensional nucleation promotes atomically smooth interfaces; (ii) Intermediate regime (10-20 mA cm^-2): Transition to fractal growth dynamics occurred, characterized by fern-like dendrites with primary arm spacing of 2.3 μm [Figure 4G-I]^[100]. Crystallographic orientation mapping confirmed preferential growth along the (101) direction, indicative of kink-site propagation under mixed charge transfer and diffusion control^[101]; and (iii) High current density regime (> 30 mA cm^-2): Severe mass transport limitations induced chaotic dendritic morphologies with multiscale branching^[102].

Overpotential-mediated morphological evolution

The morphological evolution of Zn electrodeposits exhibits pronounced dependence on cathodic overpotential regimes. Under high cathodic overpotentials (Δη = 400~500 mV, relative to the Zn²⁺/Zn equilibrium potential of -1.38 V vs. SHE), vertically aligned Zn pillars with nanoscale dimensions were synthesized. Statistical analysis of randomly selected particles revealed a mean diameter of 73.05 nm, demonstrating tight size distribution control^[100,103]. This regime is governed by diffusion-limited ion transport, where the rapid depletion of zincate ions [Zn(OH)₄^2-] at the electrode-electrolyte interface induces preferential growth along low-energy crystallographic axes. Further increases in overpotential exacerbate mass transport constraints, destabilizing the deposition process and triggering stochastic branching of primary and secondary dendrites.

Chronopotentiostatic deposition on Pt substrates elucidated potential-dependent morphological evolution [Figure 4J-L]^[103]. At 0.9 V vs. Pt, no metal deposition occurred due to insufficient thermodynamic driving force. Increasing the cathodic potential to 1 V vs. Pt initiated partial Zn film formation characterized by discontinuous nucleation sites [Figure 4J]. A critical transition was observed at -1.75 V vs. Pt, where fully coalesced hexagonal microcrystalline films with (002)-preferred orientation emerged [Figure 4J and K], consistent with thermodynamically favored basal plane growth under moderate overpotentials. Notably, at -2 V vs. Pt, competitive HER dominated interfacial dynamics [Figure 4L]. Gas bubble nucleation and pinning at cathodic hotspots disrupted ion flux homogeneity, generating porous architectures with secondary roughness. This morphological degradation originates from intensified HER-induced interfacial turbulence, where bubble-induced screening effects create localized current density gradients.

Interface engineering for morphology regulation

Time-resolved SEM [Figure 4M-O] revealed hierarchical structural evolution: initial epitaxial boulder formation transitioned to spongiform aggregates through competitive growth between layer-by-layer stacking and lateral dendrite propagation^[102,104]. Distinct morphological classes emerged under extreme polarization: (i) Hexagonal boulders; (ii) Filamentous mossy structures; and (iii) Fern-like dendrites. This regime-dependent morphological evolution underscores the critical interplay between deposition kinetics and interfacial stability. While low-current regimes enable crystallographically ordered deposition, high-current operation induces complex pattern formation through constitutional supersaturation and hydrogen co-evolution-driven interfacial turbulence. Through advanced interface engineering strategies including artificial interlayer construction and electrolyte additive modification, significant progress has been made in regulating Zn electrodeposition behavior^[105]. Zhao et al. systematically investigated the morphological evolution of Zn deposits in baseline ZnSO₄ and La³⁺-modified ZnSO₄ electrolytes under varying current densities (1-20 mA cm^-2) with a constant deposition capacity of 1 mAh cm^-2[106].

SEM characterization revealed distinct structural differences between the two systems in Figure 4P-Figure 4Q]. Here, the porous structures observed may originate from the non-uniform distribution of the Zn nucleation on the substrate; the decreased presence of the porous structures between the Zn platelets reveals that the interactions between the Zn deposits have been successfully regulated from repulsion to attraction by adding La(NO₃)₃ into ZnSO₄ electrolyte.

In the conventional ZnSO₄ electrolyte, the deposited Zn exhibited hexagonal platelet morphology with progressively increasing platelet thickness at elevated current densities. Notably, even at 20 mA cm^-2, these platelets maintained a dispersed spatial arrangement, forming loose architectures characterized by interplatelet voids. This phenomenon can be attributed to strong electrostatic repulsion forces between adjacent Zn crystallites, which impede effective particle consolidation. In striking contrast, the La³⁺-containing electrolyte produced densely packed Zn deposits across the entire current density range^[106]. The observed porosity reduction suggests that La(NO₃)₃ additive effectively modulates interfacial interactions, transforming the interparticle forces from repulsive to attractive through charge redistribution mechanisms. The combined experimental evidence establishes that strategic electrolyte modification with rare-earth cations enables fundamental control over both nucleation dynamics and crystallographic growth patterns. The introduced La³⁺ effectively modulates interfacial charge distribution, suppresses parasitic side reactions, and promotes homogeneous Zn deposition through enhanced charge screening effects.

Corrosion mechanisms and interface passivation

The inherent reactivity of metallic Zn in aqueous environments renders it susceptible to electrochemical corrosion, particularly at structural heterogeneities such as grain boundaries and stress-induced microcracks^[107]. The corrosion process proceeds via three primary pathways:^[108]

Although the growth of dendrites and the corrosion of the Zn anode are weakened in the mild aqueous electrolytes, the Zn anode is still subject to corrosion due to the active water. Comparative SEM analysis reveals stark morphological contrasts between bare Zn and Zn@CCF anodes post-cycling [Figure 4T and U]^[109]. Bare Zn surfaces develop macroscopic flake structures with inhomogeneous ZHS distribution, whereas Zn@CCF exhibits reduced topographic features and uniform byproduct dispersion. Compared with its pristine state [Figure 4T], lots of big flakes and patches appeared on bare Zn, while such byproducts are more flat and much smaller on Zn@CCF [Figure 4U], which may account for the higher charge-transfer resistance of bare Zn with a bigger semicircle at high-frequency region than Zn@ CCF^[109]. The accelerated degradation of bare Zn stems from preferential ZHS nucleation at defect sites, establishing localized galvanic cells that amplify micro-crack propagation^[108]. These corrosion micro-batteries induce non-Faradaic self-discharge and electrolyte depletion, critically undermining cycling stability. The artificial layer mitigates these issues through dual mechanisms: (i) Carbon matrix confinement reduces water-zinc contact probability^[110]; and (ii) Conductive 3D networks homogenize current distribution, suppressing localized corrosion currents^[51].

Crystallographic orientation and interfacial structural analysis

Crystallographic orientation analysis through high-resolution SEM demonstrated distinct growth patterns dependent on crystal plane exposure. The (002)-oriented Zn substrates exhibited perfectly aligned hexagonal basal planes parallel to the electrode surface, maintaining structural integrity even after 10 mA cm^-2 cycling [Figure 4M]. This alignment facilitates preferential Zn²⁺ deposition along the (002) crystallographic orientation, achieving dendrite-free growth through plane-selective deposition kinetics [Figure 4N]. The templating effect of exposed (002) planes significantly enhances electrochemical stability, contributing to remarkable anode longevity. Conversely, (101)-oriented substrates developed characteristic ridge structures with prominent surface protrusions [Figure 4O], arising from kink/step-driven anisotropic growth. These morphological irregularities create localized current hotspots, ultimately leading to fatal short-circuit failures during prolonged cycling in conventional ZS electrolytes. SEM offers a spatial resolution better than optical microscopy but not as good as transmission electron microscopy in Zn anode surface morphology^[102].

TEM is another powerful and widely used analytical tool in materials science, nanotechnology, biology, and many other research fields, complementing the capabilities of SEM^[111]. While surpassing optical microscopy's diffraction-limited resolution (≈200 nm at visible wavelengths), TEM operates by passing a high-energy electron beam (usually with an accelerating voltage ranging from 80 to 300 keV or even higher in some advanced instruments) through an extremely thin specimen, typically less than 100 nm thick. The electrons interact with the atoms in the specimen, and based on the differences in the scattering and absorption of electrons by the specimen's components, a transmitted electron image is formed. The technique’s unique strength lies in its synergistic combination of: (i) Large depth of field (100× greater than optical microscopy); (ii) Wide field-of-view range (100 μm to 10 nm scales); and (iii) Multimodal analytical capabilities (morphological, compositional, crystallographic)^[31,112].

TEM investigations revealed distinct microstructural differences between pristine and engineered Zn electrodes [Figure 5A]. The commercial Zn foil exhibited polycrystalline features with randomly oriented nanograins (200-500 nm, Figure 5B), as evidenced by continuous diffraction rings in the selected area electron diffraction (SAED) pattern (inset). High-resolution TEM (HRTEM) analysis of a representative grain [Figure 5C] confirmed the (002) zone axis orientation, with measured (101) lattice spacing of 0.207 nm, matching theoretical values. In contrast, carbon-nano grid (CNG)-modified Zn foils demonstrated microstructural reorganization [Figure 5D], featuring equiaxed grains with preferential (002) crystallographic orientation. The SAED pattern (inset) and corresponding fast Fourier transform (FFT) analysis [Figure 5E] revealed single-crystal diffraction spots, while HRTEM quantification yielded (100) lattice spacing of 0.222 nm, consistent with epitaxial growth mechanisms reported for graphene-modified interfaces^[113]. To investigate the crystallinity, TEM was used to examine the microstructure of the deposited Zn. The addition of EMIm⁺ revealed distinct lattice fringes with an interplanar spacing of 0.247 nm, corresponding to the plane of Zn (002) in the HRTEM image, together with one set of regular diffraction spots in the selected area electron diffraction (SAED) pattern [Figure 5F]. To further clarify the behavior of Zn (002) metal plating during the long cycling process, specific changes in the interface layer and plated Zn were comprehensively investigated using FIB-STEM. FIB-STEM images, along with corresponding selected area electron diffraction (SAED) patterns, further confirm the close contact between CuZn₅ clusters and plated Zn [Figure 5G]^[114]. Moreover, the energy-dispersive X-ray spectroscopy (EDX) could validate the in situ formation of dense SEI films by the LiCl additive in Figure 5H. Although TEM can provide information on the crystal structure of a sample by techniques such as electron diffraction, the area irradiated by the electron beam is very small and usually does not provide direct information on the chemical state of the elements in the sample.

Figure 5. (A) Schematics of TEM equipment. (B) BF TEM and (C) HRTEM images of commercial Zn foil; (D) BF TEM and (E) HRTEM images of the protected Zn foil after Zn plating (the corresponding SAED patterns are shown in the insets). (A-E) are reproduced from Ref.^[113]. with permission, Copyright 2021, Royal Society of Chemistry. (F) HRTEM image and SAED of deposited Zn in the electrolyte of 2 M Zn(TfO)₂ with 0.5 M EMImTfO^[39]. Copyright 2024, National Academy of Sciences. (G) The cross-sectional FIB-STEM images of Cu²⁺-Zn@Cu_0.7Zn_0.3 after 50 cycles. The corresponding SAED patterns of the CuZn₅ area and plated Zn area are displayed^[114]. Copyright 2025, Royal Society of Chemistry. (H) TEM image of the plated Zn electrode and the corresponding energy-dispersive X-ray spectroscopy mapping of carbon (C), oxygen(O), and zinc (Zn)^[65]. Copyright 2024, Springer Nature.

Anode interface morphology and roughness behavior analysis via AFM

AFM, a high-resolution scanning probe technique with nanoscale lateral resolution, has emerged as a critical tool for investigating surface topology evolution during electrochemical processes [Figure 6A]^[115,116]. Lacey et al. employed in situ AFM to systematically examine the dynamic formation of native surface films on graphite flakes during redox cycling^[117]. Their findings revealed that the initial cycle generates a self-passivating film that maintains structural integrity through subsequent cycles until cumulative volume changes in the graphite substrate induce mechanical failure, thereby exposing fresh surfaces for renewed passivation. The operational principle of the instrument relies on precise measurement of tip-sample interaction forces through a microfabricated cantilever (typically 100-500 μm length, 1-5 μm width) featuring a nanoscale probe tip (< 10 nm radius). This capability enables quantitative analysis of surface roughness evolution under electrochemical cycling.

Figure 6. (A) Schematic diagram of AFM instrument. (B and C) AFM images of (B) standard Cu foil and (C) Cu (100) foi. (B and C) are reproduced from Ref.^[53]. with permission, Copyright 2022, American Chemical Society. (D-G) In situ AFM images of Zn electrodeposits on HOPG under a current of 10 mA cm^-2 in 0.5 M electrolyte^[103,118]. Copyright 2022, Royal Society of Chemistry. (H and I) Surface and cross-sectional 3D AFM images (10 μm × 10 μm) and depth distributions of bare (H) Zn and (I) ZnSA-5^[119]. Copyright 2022, Wiley. (J) AFM surface profiles of Zn plated on stainless steel, Poly-Zn, and Single-Zn^[120]. Copyright 2022, Wiley.

Comparative AFM characterization of electrode morphologies [Figure 6B and C] demonstrates that conventional Cu foil exhibits significant surface irregularity, whereas engineered Cu (100) substrates maintain exceptional planarity^[53]. This topological contrast directly correlates with Zn deposition uniformity, as evidenced by reduced dendritic growth on flat substrates through homogeneous electric field distribution. To more comprehensively and accurately present the growth process of Zn, in situ AFM was utilized to directly observe the morphologies of Zn during the early plating stage. A highly oriented pyrolytic graphite (HOPG) was selected as the working electrode, while a Zn wire served as both the counter electrode and the reference electrode. As depicted in Figure 6D-G, during the initial deposition of Zn induced by the graphite substrate, the morphologies in both 0.5 M electrolytes manifested as a hexagonal shape parallel to the substrate^[118]. Three-dimensional AFM reconstructions [Figure 6H and I] provide quantitative insights into Zn electrode evolution during phosphoric acid etching^[119]. Bare Zn electrodes display micro-scale inhomogeneity with characteristic depth variations of 250 ± 30 nm [Figure 6H]. Controlled etching induces distinct striped morphologies, where optimization of treatment duration proves critical: ZnSA-5 specimens exhibit limited etching depth [Figure 6I]. Cross-sectional analyses confirm that 10-min etching produces the most pronounced periodic structures, balancing sufficient material removal with preservation of mechanical integrity. Moreover, these irregular structures led to severe roughening of the electrode, as shown by AFM imaging [Figure 6J]^[120]. These highly uneven deposition morphologies will facilitate dendrite growth upon further plating or cycling. AFM can usually only image relatively small, flat sample areas, typically ranging from the micrometer to nanometer scale. Moreover, the AFM probe is a very fine structure and is susceptible to friction with the sample surface during scanning, resulting in probe contamination.

Interfacial chemical composition analysis via X-ray diffraction

X-ray diffraction (XRD) is a powerful analytical technique based on the interaction between X-rays and the crystalline structure of materials. X-rays, a form of electromagnetic radiation with wavelengths from about 0.01-10 nanometers, have wave-particle duality [Figure 7A]. In XRD, their wave-like nature is key. When X-rays hit a crystal whose atoms are arranged in a regular 3D lattice, most X-rays pass through, but some are scattered by the electrons around the atoms. These electrons oscillate in response to the X-ray's electric field and re-emit secondary spherical waves. This pattern helps identify unknown crystal structures, determine lattice parameters, and study lattice defects and strain^[121,122].

Figure 7. (A) Schematic diagram of the principle of X-ray diffraction. XRD pattern of Zn deposited on the Cu (100) foil. X-ray diffraction pole figures of (B) Zn (002) on the Cu (100) foil^[53]. Copyright 2022, American Chemical Society. (C) Plane-view EBSD maps and corresponding (0002) pole figures of (D) the com-Zn metals^[33]. Copyright 2024, Springer Nature. (D-I) Characterizations of electrodeposited Zn films. 2D XRD patterns of (D) MoS₂-SS, (F) bare SS, and (H) MoS₂-SS-E deposited with a certain amount of Zn. XRD pole figures of (E) MoS₂-SS, (G) bare SS, and (I) MoS₂-SS-E. (D-I) are reproduced from Ref.^[123]. with permission, Copyright 2022, Wiley.

XRD analysis was conducted to elucidate the epitaxial orientation relationship between electrodeposited Zn and the zincophilic Cu (100) substrate. Both Zn deposits on Cu(100) and conventional Cu foil substrates exhibited hexagonal close-packed (hcp) crystal structures, as confirmed by peak matching with the standard reference [Figure 7B]^[53]. Notably, a distinct inversion in relative diffraction intensities was observed between the (002) and (101) crystallographic planes of Zn on the two substrates. The Cu (100) surface induced pronounced (002) preferential orientation of Zn deposits, as evidenced by a significantly enhanced Zn (002)/Zn (101) intensity ratio, substantially exceeding both the standard reference value (5.88) and the ratio observed on conventional Cu foil substrates (7.63). This marked enhancement demonstrates strong crystallographic anisotropy during electrodeposition, with preferential growth along the Zn (002) basal plane on Cu (100) substrates. Furthermore, the com-Zn metal demonstrated a random and irregular distribution of crystal grains [Figure 7C], which was further confirmed by the Electron backscatter diffraction (EBSD) characterization.

Additional validation through orientation distribution functions and comparative pole figure analysis confirmed the dominant (002) texture, with the (002) pole figure exhibiting substantially sharper intensity distributions compared to (101) and (110) counterparts [Figure 7D-I]^[123]. These collective observations establish that Zn platelets predominantly align with their (002) planes parallel to the Cu (100) substrate surface, consistent with epitaxial growth mechanisms facilitated by lattice matching at the Zn/Cu(100) interface. XRD is mainly suitable for structural analysis of crystalline materials. For amorphous material, it is difficult to provide detailed structural and positional information due to the lack of long-range periodicity in the atomic arrangement.

Interfacial bonding analysis via X-ray spectroscopy

XAS using both hard and soft X-rays has been extensively applied to monitor element-specific evolution of oxidation states and local coordination environments in the bulk and at surfaces of active materials, electrolytes, and interfaces^[124].

To investigate the influence of additive on the Zn²⁺ solvation structure, complementary characterization techniques including X-ray absorption fine structure (XAFS) spectroscopy, and Raman spectroscopy were systematically employed. XAFS analysis proves particularly effective for probing electrolyte solvation environments, offering unique insights into first-shell coordination configurations [Figure 8A]^[125,126]. Figure 8B presents the Zn K-edge X-ray absorption near-edge structure (XANES) spectra for BA/ZnSO₄ electrolyte compared with reference samples of Zn foil and ZnSO₄ electrolyte. The Zn foil exhibits the lowest absorption edge energy (9,664.3 eV), consistent with its metallic state (zero oxidation state)^[57]. Notably, the expanded view of near-edge features (inset) reveals a 0.4 eV positive shift in the absorption edge for BA-modified electrolyte relative to pure ZnSO₄, indicating reduced electron density at Zn²⁺ centers and suggesting modified coordination environments induced by addition.

Figure 8. (A) Schematic illustration of X-ray adsorption spectroscopy. (B-D) The normalized Zn K-edge XANES spectra. Inset: the enlarged Zn K-edge XANES spectra. (C) The EXAFS spectra in R space. (D) Wavelet transform images of the EXAFS spectra. (B-D) are reproduced from Ref.^[57]. with permission, Copyright 2022, Wiley.

The local solvation structures were further elucidated through Fourier-transformed extended X-ray absorption fine structure (FT-EXAFS) analysis in R-space [Figure 8C]. Quantitative fitting of the spectra reveals distinct coordination characteristics. The first coordination shell at ~1.60 Å (phase-uncorrected) corresponds to the Zn-O scattering path^[127], while the artifact peak below 1 Å arises from low-frequency experimental noise^[128]. Notably, the BA-containing electrolyte demonstrates a reduced Zn-O coordination number accompanied by slight bond elongation, suggesting partial replacement of water ligands by BA molecules in the primary solvation shell. Complementary wavelet transform (WT) EXAFS analysis [Figure 8D] confirms these structural modifications through distinct intensity variations in the 3-5 Å^-1 k-space region. The WT contour plots exhibit enhanced resolution for Zn-O coordination features in the BA-modified system, potentially indicating increased structural disorder or modified ligand distribution compared to the pure ZnSO₄ electrolyte. This multiscale spectroscopic evidence collectively demonstrates that BA additive significantly modulates the Zn²⁺ solvation structure through competitive coordination interactions. As mentioned above, X-ray diffraction characterization techniques are limited to examining crystalline materials^[57]. Nonetheless, researching amorphous materials or the conversion of crystals to non-crystalline states during operation is an ongoing and demanding topic in the field of AZIBs. Additionally, it is difficult to simultaneously analyze multiple elements in complex systems. EXAFS mainly provides short-range structure information in several coordination layers around the absorbing atom, and has limited ability to study long-range ordered structures or macroscopic crystal structures.

XPS operates on the photoelectric principle, where monochromatic X-rays (typically Al Kα = 1,486.6 eV) eject core-level electrons with kinetic energies (KE) defined by the equation: KE = hν - BE - Φ, where BE is electron binding energy and Φ is spectrometer work function (4-5 eV). Each element and its different chemical states have distinct electron binding energies. As a result, the ejected photoelectrons have various kinetic energies^[129,130]. A detector measures these kinetic energies and the number of photoelectrons, generating a photoelectron spectrum [Figure 9A].

Figure 9. (A) Schematic illustration of X-ray photoelectron spectroscopy. (B) XPS of Zn foil separated from in situ GPE, showing peaks of C1s, N1s, O1s, and Zn2p^[131]. Copyright 2022, Wiley. (C) XPS spectra for the Zn anode after the 1st and 50th cycles in each of the DLP/ZnSO₄ and pure ZnSO₄ electrolytes^[132]. Copyright 2024, Wiley. (D) XPS depth analysis of Zn 2p of the SEI layer on Zn metal surface at different sputtering times^[133]. Copyright 2022, American Chemical Society. (E) O 1s and Zn 2p XPS spectra of SAP-GF soaked in ZnSO₄ electrolyte and SAP interface after 2 and 60 cycles in Zn||Zn cells^[34]. Copyright 2024, Wiley.

The in situ formed gel polymer electrolyte (GPE) exhibits enhanced interfacial adhesion (19 kPa) compared to ex-situ counterparts (11 kPa), achieving 1.7 times improvement through chemical welding at the Zn-electrolyte interface. High-resolution XPS analysis confirms covalent interactions between polyacrylamide (PAM) and Zn substrate: N 1s spectra reveal Zn-N coordination at 399.6 eV, while O 1s spectra show Zn-O bonding at 532.1 eV [Figure 9B]^[131]. Depth-profiling XPS with Ar⁺ sputtering (2 min, 1 keV, 100 nA) quantified interfacial degradation mechanisms during cycling: In ZnSO₄ electrolyte, cycled Zn anodes showed Zn²⁺ content (BE: 1,021.4 eV 2p_3/2) vs. metallic Zn (1,020.2 eV), accompanied by sulfur species (S 2p at 163.8 eV), indicating severe byproduct formation [Figure 9C]^[132]. Conversely, DLP/ZnSO₄ systems maintained metallic Zn content after 50 cycles, demonstrating suppressed side reactions. To explore the elemental redistribution at Polymer-Zn interfaces, XPS depth profiling of in situ SPEEK membranes revealed graded composition: sulfur (1.88 at%), zinc (1.94 at%), and oxygen (15.45 at%) concentrations within the first 50 nm interface [Figure 9D]^[133]. Post-cycling analysis showed Zn-O bond enrichment (O 1s at 532.1 eV increased from 18.7% to 23.4% after 60 cycles), attributed to progressive coordination between sulfonic groups (-SO₃H) and Zn²⁺. The SAP-GF composite exhibited strong Zn²⁺ chelation, evidenced by 2.3 eV BE shift in S 2p peaks after ZnSO₄ immersion [Figure 9E], consistent with [Zn(H₂O)₄(SO₄)]⁰ complex formation^[34]. Although XPS can provide information about the surface of the material, for some complex chemical systems, the peak positions between different chemical states may overlap or be similar, leading to difficulties in the accurate judgment of chemical states.

Electrolyte speciation and pH-dependent behavior analysis via raman spectroscopy

Raman spectroscopy, a powerful analytical technique based on inelastic light scattering phenomena, provides critical insights into molecular vibrations and interfacial chemistry [Figure 10A]^[122,134]. When interrogating Zn anode interfaces with monochromatic laser radiation (typically 532-785 nm wavelength), the majority of scattered photons undergo elastic Rayleigh scattering, while approximately 10^-6-10^-8 of the photons exhibit frequency shifts characteristic of Raman-active vibrational modes. These shifts (Δν = ±50-4,000 cm^-1) correspond to quantized transitions in molecular bonds or lattice phonons, with characteristic signatures such as C=C stretching (1,600-1,680 cm^-1) and C-H vibrations (2,800-3,000 cm^-1). Operando Raman spectroscopy has emerged as a vital tool for probing interfacial degradation in thiophosphate-based solid electrolytes, a critical challenge hindering bulk-type solid-state battery development^[135,136].

Figure 10. (A) Schematic diagram of the Raman spectroscopy setup. (B) Raman spectra of 2 M ZnSO₄ and 2 m ZnSO₄ containing 1.1 M LiCl electrolytes. Voltages are referenced to Zn/Zn^2+[137]. Copyright 2024, Wiley (C) Raman spectra of pure Zn(OTf)₂, β-CD, and β-CD@OTf^- complex^[9]. Copyright 2024, Springer Nature. (D) Raman spectrophotometry of 2.0 M ZnSO₄ + 0.2 M NMI solutions at different pH values^[138]. Copyright 2023, Wiley. (E) In situ Raman spectra of the GPE at various reaction times and (F) the corresponding contour map^[131]. Copyright 2023, Wiley. (G) Raman spectra of Cu@Zn and MGA@Zn electrodes after first cycling^[139]. Copyright 2022, Wiley.

Spectral deconvolution revealed concentration-dependent coordination chemistry: In 2 M ZnSO₄ + 1.1 M LiCl, the [Zn(H₂O)₂Cl₄]^2- complex vibration emerged at 982 cm^-1 [Figure 10B], while the intensity ratio of HOH-OH₂/HOH-OSO₃^2- decreased from 0.74 to 0.65 (n = 15 scans), indicating a 12% reduction in free water content^[137]. β-CD@OTf^- complexes exhibited 2.3 cm^-1 redshift in OTf^- vibration (756 cm^-1, Figure 10C), confirming host-guest interactions via DFT-calculated binding energy (-23.6 kJ mol^-1)^[9]. Decreasing the pH from 5.2 to 3.0 caused intensity reduction at 1,235/1,350/1,422 cm^-1 (NMI characteristic) with a concomitant 45% increase at 1,288/1,457 cm^-1 (NMIH⁺ modes). These changes, consistent with the spectra observed at pH = 4, indicate pH-dependent spectral evolution [Figure 10D], reflecting protonation equilibria and the transition from NMI to NMIH^+[138].

As demonstrated in our ZIBs studies [Figure 10E and F], this technique achieves micrometer-scale spatial resolution with temporal resolution adjustable from seconds to minutes, depending on signal-to-noise requirements^[131]. The polymerization dynamics of acrylamide to polyacrylamide were quantitatively tracked through characteristic band evolution: Initial precursor solution exhibited prominent C-H bending (1,286 cm^-1), C-N stretching (1,438 cm^-1), and vinyl group vibrations (3,040/3,115 cm^-1). By using Cu foil [Figure 10G], the asymmetric battery exhibits distinct peaks at 465 cm^-1, allowing the apparent passivation response to be determined. By contrast, a typical Raman spectrum of RGO was characterized in Figure 10G for the MGA@Zn anode, and during Zn plating at MGA material, the passivation was significantly suppressed^[139]. Since an increase in electrolyte pH promotes the formation of ZSH, while a decrease in pH exacerbates the HER, maintaining a stable pH at both the electrolyte and the reaction interface is critical for the performance of the Zn anode. For all this, Raman scattering is an inelastic scattering with a small scattering cross section and susceptibility to fluorescence interference, which often results in a weak Raman signal. This requires the use of highly sensitive detectors and long integration times to obtain sufficient signal strength, which limits the speed of analysis and makes detection difficult for some samples with weak signals.

Interfacial electrochemistry of Zn anode by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) is a scanning probe technique to spatially resolve electrochemical activity with exceptional resolution developed by Bard^[140] and Engstrom^[141] in 1989, in which a small-scale electrode is scanned across an immersed substrate while recording the current response [Figure 11A]. This response is dependent on both the surface topography and the electrochemical activity of the substrate^[142]. Operating in feedback or generation-collection modes, this technique quantifies local mass transport coefficients and charge transfer rates through current-distance curve analysis. Until now, SECM has been used to assess electrochemical performance in a variety of application fields.

Figure 11. (A) Schematic diagram of scanning electrochemical microscope (SECM). (B) The membrane micro area resistance of Turing-PBI-80 and Dense-PBI was characterized by SECM^[143]. Copyright 2022, American Chemical Society. (C) Schematic illustration and SECM feedback images of Fre-Zn electrodeposition behavior at various plating durations, including 0s, 100, 300, and 600 s^[144]. Copyright 2023, Wiley. (D) SECM image of different after plating 300/3,600 s^[145]. Copyright 2022, Wiley. (E) SECM image and corresponding enlarged area of the Zn deposition sample with marked active sites and the corresponding kinetics^[37]. Copyright 2025, American Chemical Society.

As shown in Figure 11B, localized electrochemical impedance spectroscopy revealed distinct micro-resistance distributions^[143]. Dense-PBI membranes exhibited uniform but elevated real impedance, while Turing-patterned surfaces showed heterogeneous conductivity. Dense-PBI has uniform but larger real impedance, whereas the crests of Turing stripes, region B, have a higher real impedance (resistance) compared to region A, which exemplifies the hypothesis original text. As shown in Figure 11C, time-resolved SECM imaging (10 μm/s scan rate, 0.5 V vs. Ag/AgCl) captured deposition evolution on Fre-Zn substrates: (i) Homogeneous surface; (ii) Emergent horizontal growth co-existing with vertical flakes; and (iii) Dominant vertical growth^[144]. As a comparison in Figure 11D, Gel-coated Zn substrates exhibited suppressed dendrite detection. When the conductive Zn substrate was covered by insulating gel electrolytes (Zn + C and Zn + CWK cases), the formation of Zn dendrite beneath these gel electrolytes can only lead to a decrease in the feedback current, i.e., i/i_ss < 1^[145]. SECM can also investigate the crystallographic-dependent charge transfer kinetics. The feedback from the substrate is reflected by the redox current fluctuation, which results from the oxidation of the redox mediator ferrocenemethanol (FcMeOH) as the tip approaches the substrate surface^[37]. The SECM image distinctly reveals the differences in charge transfer activity between the HOPG surface (blue region) and the individually distributed as-deposited Zn domains (green and red regions). These heterogeneities on SECM mappings yield distinct crystal growth kinetics between center and edge regions, which means that under the influence of electrical field and electrolyte modulation, the center of the as-grown Zn domain has a higher electron transfer activity compared to the edge. To verify the relative kinetic differences between the (002)-dominant center and (100)-dominant edge sites, approach curves were conducted to quantify the heterogeneous charge transfer rate constants (k_f) of several selected sites within the Zn domains [Figure 11E]^[37]. The approach curves clearly demonstrate a significant change in activity between the center and edge sites. Mathematically fitted values of k_f further highlight the order of magnitude difference, with the center sites having a k_f value of approximately 1.2 × 10^-4 m s^-1 and the edge sites having a k_f value of 5 × 10^-5 m s^-1. SECM is a powerful tool for studying electrode surface properties and electrochemical reaction processes, but it has limited spatial resolution and high requirements for sample flatness and conductivity.

Interfacial evolution mechanism of Zn anode via theoretical simulations

The synergistic integration of advanced computational methodologies with experimental characterization has facilitated the comprehensive understanding of dynamic interfacial processes in Zn anode systems^[146]. Multiscale theoretical calculation models, including density functional theory (DFT), molecular dynamics (MD) simulations, and finite element analysis, elucidate the fundamental mechanisms governing Zn electrodeposition and interface evolution^[75]. Crystallographic orientation-dependent electrochemical behaviors were quantitatively analyzed through DFT calculations of surface atom self-diffusion barriers. Figure 12A demonstrates significant anisotropy in diffusion energetics, with the (002) plane exhibiting the lowest activation barrier (16 meV), followed by progressively higher barriers for (100) (115 meV), (102) (313 meV), (101) (709 meV), and (110) (1,396 meV) planes. This hierarchy directly correlates with dendrite suppression capabilities, establishing surface diffusion kinetics as a critical descriptor for interfacial stability^[48]. Subsequently, DFT calculations and MD simulations were systematically evaluated to probe the Zn²⁺ binding energy and electronic characteristics^[147,148]. The highest binding energy of C₅SeCN with Zn²⁺ (-14.578 eV) significantly alters the primary hydration shell configuration [Figure 12B left]. To investigate the modulation effect of the solvation structure, MD simulations reveal a marked reduction in coordinated H₂O molecules within the solvation sheath [Figure 12B, middle], facilitating the water de-solvation kinetics^[8]. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) calculations were conducted to investigate the preferential charge transfer tendency [Figure 12B right]^[8]. The narrow HOMO-LUMO gap (2.76 eV) promotes preferential reduction over competitive hydrogen evolution reactions, confirming the redox activity of the C₅SeCN additive. Finite element analysis was modeled through COMSOL Multiphysics simulations, incorporating charge distribution, ion transport dynamics, and the evolution of the electrochemical interface^[149]. The established electrodeposition model reveals significant current density localization at surface protrusions [Figure 12C], heavily accumulating around the protrusions and promoting Zn²⁺ internal diffusion^[150]. As deposition continues, there is no significant concentration gradient within the anode surface, ensuring Zn homogenous deposition throughout the electrode. While current computational models successfully capture interface evolution patterns, the development of in situ electric field mapping techniques remains crucial for experimental validation of theoretical predictions.

Figure 12. Theoretical calculation mode and the relevant mechanism analysis. (A) DFT calculation of the surface atom self-diffusion barrier on different Zn planes^[48]. Copyright 2024, National Academy of Sciences. (B) The calculated binding energies of Zn²⁺-H₂O, Zn²⁺-C₅CN, and Zn²⁺-C₅SeCN. MD snapshots of C₅SeCN and [Zn(H₂O)₆]²⁺ at 2,000 ps with molecule design features illustrated. Calculated HOMO and LUMO energy levels of H₂O and C₅SeCN molecules^[8]. Copyright 2024, Wiley. (C) COMSOL simulation of current density distribution, and Zn²⁺ concentration and morphology evolution of the Bi@Zn powder anode during plating^[150]. Copyright 2024, The Royal Society of Chemistry.