4f-5d orbital tag-team catalysis empowers high-loading zinc–iodine batteries

Domain: biology
Published: 2026-03-28
Editorial outcome: major_revision

Abstract
Conversion-type positive electrodes offer high theoretical energy density for next-generation energy storage, but their practical application is limited by severe shuttle effects and sluggish kinetics under high loadings. Achieving strong adsorption and fast redox kinetics is challenging, as the Sabatier principle implies that overly strong adsorption stabilizes intermediates and inhibits their conversion. Herein, we propose and validate an f–d orbital tag-team catalysis mechanism that overcomes this limitation within a model zinc||iodine system. Leveraging a machine learning-guided descriptor framework, we identify cerium single-atom catalysts as an optimal catalytic center for iodine conversion, surpassing d-block analogues. Density functional theory calculations reveal a tag-team catalytic mechanism in which Ce 5d orbitals strongly anchor iodine intermediates while near-Fermi-level Ce 4f orbitals introduce antibonding interactions that exert tunable electronic repulsion on the I−I bond. This orbital tag-team catalysis mechanism enables simultaneous stabilization of polyiodide intermediates and facilitation of bond activation, achieving an iodine loading of 44.7 mg cm−2 with an areal capacity of 10 mAh cm−2, and a practical pouch cell capacity at a high mass loading of 115 mg cm−2. This work bridges the orbital-level catalyst design with practical performance, offering a strategy for advancing high-loading conversion-type positive electrodes in battery systems.

Introduction
As global efforts intensify to integrate renewable energy on a large scale, the demand for safe, low-cost, and long-lifespan stationary energy storage systems has never been more pressing1-5. Among emerging candidates, batteries with conversion-type positive electrodes have garnered increasing interest due to their high theoretical energy densities6-8. Nevertheless, their practical implementation is hindered by sluggish redox kinetics and uncontrollable side reactions, challenges that are further exacerbated under the high mass loading required for grid-level applications9-11. Aqueous zinc||iodine (Zn||I2) batteries exemplify both the promise and the challenge of conversion-type systems12,13. With zinc offering a high theoretical capacity (~820 mAh g−1), iodine providing environmental benignity and a favorable redox potential (~1.3 V vs. Zn2+/Zn), and the intrinsic safety of aqueous electrolytes, Zn||I2 batteries present a compelling candidate for clean energy storage14-20.

Yet, under industrially relevant iodine loadings exceeding 5 mg cm−2, their performance declines rapidly due to severe shuttle effects and sluggish redox kinetics, stemming from the inherent challenge of simultaneously stabilizing iodine intermediates and facilitating I–I bond activation during cycling21,22. Although current strategies, such as physical confinement within carbon pores or ionic shielding through electrolyte additives23,24,25,26, offer partial mitigation to these issues, they remain inadequate for addressing the fundamental limitations imposed by the redox processes. Recent advances have explored the use of 3d-metal single-atom catalysts (SACs) (e.g., Fe, Co) to modulate iodine redox pathways through d–p orbital hybridization, a concept previously validated in sulfur reduction catalysis27,28. However, these catalysts face a fundamental constraint well-known as “Sabatier principle”, i.e., insufficient adsorption hinders I–I bond activation while excessive binding over-stabilizes intermediates and impedes catalytic turnover. The Sabatier principle imposes a dilemma for catalysis in conversion-type batteries, where bond activation is critical for quick reaction redox kinetics and strong adsorption is deemed necessary to prevent intermediate dissolution for high capacity retention (especially in high-loading positive electrodes).

To breach this adsorption–activation dilemma for iodine-conversion catalysis in Zn||I2 batteries, we constructed a machine learning (ML)-guided framework that links orbital features to iodine redox performance to rationally identify catalysts for practical Zn||I2 batteries. By integrating electronic parameters into a unified total descriptor (TD), cerium (Ce) was identified as a rare-earth center capable of concurrently stabilizing and activating intermediates. This data-driven screening led to the identification of an f–d orbital tag-team effect29, 30 in Ce SACs, a mechanism characterized by the functional decoupling of adsorption and activation into distinct orbital tasks. Herein, the Ce 5d orbitals engage in strong bonding interactions to anchor iodine species, while the partially filled Ce 4f orbitals near the Fermi level contribute antibonding interactions that activates the I–I bond. This orbital tag-team catalysis mechanism endows the Ce SACs/I2 positive electrode with the iodine loading of 44.7 mg cm−2 and the areal capacity of 6.2 mAh cm−2 over 100 cycles with 95% capacity retention. Notably, Ce SACs enable a near-linear relationship between areal capacity and I2 mass loading. Moreover, a practically assembled pouch cell with the Ce SACs/I2 positive electrode achieves a high areal capacity of 25.6 mAh cm−2 (115 mg cm−2) and exhibits promising calendar life, with less than 0.07% voltage decay per day over a 5-month period. This work establishes an orbital-level catalyst design framework for conversion-type batteries.

Results
ML-guided descriptor decoupling for catalyst design
To rationally design efficient SACs for Zn||I2 batteries, we built a ML framework consist of dataset construction, descriptor selection, performance prediction, and experimental validation (Fig. 1a), aiming at uncovering the intrinsic electronic factors that govern polyiodide adsorption and conversion. Unlike conventional approaches that focus primarily on thermodynamic outputs (e.g., adsorption energy or free energy), our method establishes physically meaningful descriptors that characterize the binding strength between the catalyst and iodine, as well as the ability to activate I–I bonds, thereby providing a more universal basis for catalyst screening.

We constructed a DFT-derived dataset of 3d, 4d, 5d, and 4f block SACs, from which structural and electronic features were extracted (Supplementary Tables 1, 2 and Supplementary Data 1–11). Through Spearman correlation analysis and linear regression fitting, two key descriptors were identified: Nd + Nf, quantifying the total number of d and f electrons near the Fermi level that contribute to orbital hybridization with iodine species, and ΔBader – Nf, representing the net charge polarization between the metal center and its ligands (Supplementary Note 1, Supplementary Figs. 1–3). These descriptors were used to predict two reaction-relevant properties: the adsorption energy of iodine species (ΔE*I), related to iodine immobilization and shuttle suppression, and the I–I bond length in *I3, reflecting bond cleavage efficiency. Notably, ΔBader – Nf shows a strong correlation with ΔE*I (R2 = 0.89), and Nd + Nf correlates with I–I bond length (R2 = 0.81), validating their predictive capability (Figs. 1b–c).

Moreover, while the two descriptors are correlated (Fig. 1d, R2=0.6), they are linked to distinct aspects of the reaction mechanism (Supplementary Fig. 4), allowing separate regulations of adsorption strength and bond activation. Among the candidates, Ce SACs stand out, achieving a strong charge polarization (ΔBader – Nf ≈ –3.48) and a low electron occupancy (Nd + Nf ≈ 2.0). We compare the electronic structures and polyiodide adsorption/conversion behaviors of a typical 3d metal SAC (Co–N4) with 4f metal SAC (Ce–N4) to elucidate a catalyst design strategy for overcoming the key challenges in Zn||I2 batteries (Figs. 1e, f). Co–N4, dominated by localized d orbitals, suffers from insufficient charge transfer to the coordinating N atoms (1.17 |e|), resulting in weak adsorption and sluggish conversion of polyiodides. In contrast, Ce–N4 engages in tag-team catalysis through the complementary and synergistic interactions of the d/f orbitals with iodine species, enabling stronger adsorption and more favorable redox behavior of polyiodides. The involvement of f orbitals enables independent control over charge transfer and orbital occupation, expanding the tunable parameter space for SACs and offering possibility for optimizing redox performance under practical iodine loading conditions.

Synthesis and structural characterization of SACs
To validate the descriptors of the SACs, we synthesized an array of transition metal-based SACs (M−N−C, M = Cu, Co, Fe, V, Zr, Nb, Mo, W, and Ce) through a scalable eutectic salt-assisted pyrolysis strategy31,32 (Figs. 2a, b). The precursor system strategically combines metallic salts with dual nitrogen-rich ligands, dicyandiamide and α-alanine, forming a low-melting eutectic complex in urea. The self-regulated coordination environment ensures atomic dispersion while maintaining M−N4 configuration during pyrolysis33-36. X-ray diffraction (XRD) patterns exhibit broad peaks at ~26° (graphitic carbon) without metallic/oxide phases (Fig. 2c), confirming exclusive atomic dispersion37,38. X-ray photoelectron spectroscopy (XPS) provides critical insights into the electronic states of metal centers and their coordination environments. For Ce SACs (Fig. 2d), the Ce 3d spectrum is deconvoluted into characteristic peaks corresponding to Ce3+ (885.1 eV and 905.2 eV), while the remaining peaks are attributed to Ce4+. The calculated Ce3+/Ce4+ atomic ratio is 91.17%, indicating partial reduction during pyrolysis39. This mixed oxidation state is considered beneficial to catalytic processes involving electron transfer40. The XPS spectra of other SACs show no zero-valence metallic particles (Supplementary Figs. 5a–f), illustrating the generality of the method for SAC synthesis41.

Scanning and transmission electron microscopy (SEM/TEM) images (Figs. 2e–g, Supplementary Figs. 5g–i and Supplementary Fig. 6) reveal porous carbon architectures with no visible metal nanoparticles, indicating successful single-atom anchoring. Elemental mappings (Supplementary Fig. 7) confirm uniform distribution of metal (Ce, Zr, and Co), carbon, and nitrogen elements, while energy-dispersive X-ray spectroscopy (EDS) quantifies the metal loading (2–4 wt%). Spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) directly visualizes isolated metal atoms (bright dots) embedded in the carbon matrix (Figs. 2h–j)42,43. Furthermore, to determine the local coordination environment of these isolated atoms, we performed synchrotron-based X-ray absorption spectroscopy (XAS). Quantitative analysis of the extended X-ray absorption fine structure (EXAFS) spectra confirms the dominant presence of a four-coordinate configuration for the metal centers (Supplementary Fig. 8 and Supplementary Tables 3, 4), which is consistent with our computational models.

Adsorption and conversion kinetics of iodine species on SACs
To assess iodide adsorption capacities of various SACs under high iodine content (catalyst-to-iodine mass ratio = 1:9), I3− solutions were mixed with equal amounts of each catalyst. The solution treated with Ce SACs became nearly transparent, indicating stronger iodide adsorption, whereas those with other catalysts remained partially yellow (Fig. 3a). UV–vis spectra quantified this phenomenon, revealing that the characteristic I3− absorption peaks at 288 nm and 350 nm, were significantly weakened in Ce SAC-treated solutions compared to the control samples (Supplementary Figs. 9a–c). This result aligns with theoretical predictions based on adsorption energy calculations (Supplementary Fig. 10).

We further evaluated the kinetic advantages of Ce SACs, and selected Co SACs, a typical 3d transition metal SAC with moderate adsorption energies, for comparison. In situ electrochemical impedance spectroscopy (EIS) reveals similar trends for interfacial evolution across Ce SACs, Co SACs, and catalyst-free positive electrodes during Zn−I2 battery operation (Fig. 3b and Supplementary Figs. 11a, b). The interfacial resistance decreases during discharge (I2 → I3−/I5− → I−) but increases during charging (I− → I3−/I5− → I2), a phenomenon attributed to polyiodide shuttling that would subsequently induce positive electrode active material loss and zinc negative electrode corrosion. Notably, Ce SACs exhibit the lowest charge transfer resistance (Rct ≈ 15 Ω, compared to Rct > 30 Ω for Co SACs and Rct > 35 Ω for catalyst-free positive electrode), indicating faster charge transfer kinetics in the electrochemical reaction and reduced interfacial degradation. To unveil the underlying kinetic limitations, temperature-dependent EIS analysis was conducted to quantify the activation energy (Ea) of iodine reduction reactions (IRR) at different potential (Figs. 3c−e, Supplementary Fig. 12 and Supplementary Table 5). The conversion of I2 to soluble polyiodides (1.3 V vs. Zn2+/Zn) proceeds with a markedly low Ea of 0.126 eV for Ce SACs, indicative of accelerated reaction kinetics. In contrast, the subsequent polyiodide-to-ZnI2 transformation (1.1 V vs. Zn2+/Zn) exhibits a substantially higher Ea (0.141 eV), identifying I–I bond cleavage in intermediate species as the rate-determining step. Crucially, Ce SACs reduce the activation barrier for the critical I–I bond activation step by 16% relative to Co SACs (0.167 eV), directly correlating with accelerated reaction kinetics and suppressed polyiodide accumulation.

Ce SACs and Co SACs exhibit similar double-layer capacitance (Cdl) values (57.9 vs. 51.1 mF cm−2), indicating comparable active-site densities44,45 (Supplementary Fig. 13). Despite this, Ce SACs deliver improved geometric current than Co SACs, while the catalyst-free system shows negligible activity. Cyclic voltammetry (CV) and scan-rate-dependent studies were conducted to assess the catalytic performance of Ce and Co SACs, and catalyst-free systems46. At elevated scan rates (Figs. 3g–i and Supplementary Figs. 14, 15), Ce SACs maintain higher response current and minimal peak potential drift, highlighting the enhanced capability of Ce SACs to sustain rapid IRR kinetics and charge transfer efficiency, attributed to their electronic structure–mediated intermediate stabilization and lowered activation barriers. These results underscore the critical role of atomic-level Ce redox centers in accelerating interfacial electrochemical reactions in Zn||I2 batteries.

Relationships between TD and IRR activity
To validate the predicted orbital advantages of 5d/4f, we assembled and tested Zn||I2 batteries to evaluate the impact of f-block Ce SACs versus d-block counterparts. The battery with Ce SACs delivers an enhanced specific capacity of 256 mAh g−1 at 0.5 A g−1 (Fig. 4a), compared to other d-block SACs such as Zr, Nb, and Co with specific capacities of 250, 229, and 213 mAh g−1, respectively. Additionally, the I2 positive electrodes with Ce SACs display lower overpotential (12.5 mV, Supplementary Fig. 16) than Zr (13 mV), Nb (36.4 mV), and Co SACs (16.8 mV), indicating enhanced catalytic activity. The inverse differential capacity (dV/dQ) analysis (Supplementary Fig. 17) reveals that Ce SACs exhibit a pronounced and broadened peak during I3−→I− conversion, indicative of efficient IRR kinetics. In contrast, narrower and weaker peaks for d-block SACs (e.g., Co SACs) suggest sluggish activation and incomplete conversion. Rate capability tests across 0.5–10 A g−1 (Fig. 4b) further support these findings. By integrating the Sure Independence Screening and Sparsifying Operator (SISSO) method and ML approaches, TD was proposed to simultaneously account for polyiodide activation and shuttle suppression, expressed as:
TD = 0.594 (ΔBader – Nf) – 0.035 (Nd + Nf)
The TD shows predictive ability for both overpotential (R2 = 0.80) and specific capacity (R2 = 0.91), in agreement with experimental observations (Figs. 4c, d and Supplementary Table 6). The enhanced performance of Ce-based SACs is attributed to the low electron (Nd + Nf) and significant Ce–ligand charge polarization (ΔBader – Nf), which synergistically enhance catalytic activity, resulting in higher specific capacity and lower overpotential.

The long-term cycling stability of Ce SACs/I2 positive electrode was assessed to validate the descriptor-guided screening. At 0.5 and 2A g−1, the Ce SACs/I2 positive electrode retains capacities of 241 and 150 mAh g−1, respectively, with Coulombic efficiencies >99.9%, far exceeding those positive electrodes with Co SACs and catalyst-free positive electrodes (Supplementary Figs. 18a, b). Even at specific current of 10 A g−1 and 20 A g−1, the I2 positive electrode with Ce SACs demonstrates remarkably low capacity decay rates of 0.015% and 0.005% per cycle, respectively, underscoring their robustness for high-power applications (Fig. 4e and Supplementary Fig 18c). These results confirm that the Ce SACs facilitate rapid redox kinetics and sustained turnover, consistent with the trends predicted by the TD.

Addressing the practical challenge of high iodine loading, Ce SACs/I2 positive electrode sustain nearly linear scaling with areal capacity increasing from 1.75 to 10.06 mg cm−2. Moreover, well-defined redox plateaus were preserved across a broad current range (0.5–10 A g−1) (Supplementary Fig. 19). Beyond 10 mg cm−2, mechanical degradation from interfacial stress heterogeneity became pronounced. We have alleviated this phenomenon by strategically reducing the encapsulation pressure by 10%, enabling stable operation at 44.7 mg cm−2 with a high areal capacity of 10.2 mAh cm−2, while maintaining a near-linear capacity-loading proportionality at 0.5 A g−1 (Fig. 4f and Supplementary Fig 20a). Although ion diffusion limitations in thick electrodes disrupted linear scaling at high specific current (≥2 A g−1), the Ce SACs/I2 positive electrode exhibited stable cycling performance with 99% capacity retention over 130 cycles at 35 mg cm⁻2 under 0.5 A g−1, and 95% retention after 1,000 cycles at 20 mg cm⁻2 under 5 A g−1 (Figs. 4g, h and Supplementary Fig. 20b).

The electrochemical performance of Zn||I2 batteries under varying mass-loading conditions was systematically evaluated through pouch cell configurations. At a high loading of 28.8 mg cm−2, Ce SACs/I2 cells exhibit stable cycling performance, retaining 97% iodine utilization over 100 cycles with a near-unity Coulombic efficiency (99.9%) and stable voltage plateaus (Fig. 4i and Supplementary Fig. 21a). This contrasts sharply with catalyst-free systems, which suffer from rapid degradation (<50 cycles at 26.5 mg cm−2) due to incomplete iodine conversion and severe shuttle effects (Supplementary Fig. 21b). As the loading escalates to 115 mg cm−2, the Ce SACs/I2 positive electrode achieves a high areal capacity of 25.6 mAh cm−2 (total capacity: 414 mAh, N/P=5) (Supplementary Fig. 21c). Remarkably, the Ce SACs/I2 system maintains low self-discharge, retaining stable voltage platform (1.30 ± 0.01 V) over 5 months, while catalyst-free counterparts show sharp degradation (Supplementary Fig. 21d). These results demonstrate that the f–d orbital tag team in Ce SACs alleviates key limitations at high iodine loading and offers a promising pathway toward scalable Zn–I₂ batteries for grid-level energy storage (Supplementary Table 7).

After cycling, zinc negative electrode was characterized by SEM and XRD (Supplementary Fig. 22). Notably, when Co SACs or no catalysts were used, severe surface corrosion on the zinc surface was observed, indicative of rampant polyiodide shuttling. In contrast, smooth morphology with negligible byproduct was detected on zinc negative electrode paired with Ce SACs/I2 positive electrode (Supplementary Fig. 23). These observations indicate that the orbital-level regulation of Ce SACs not only optimizes intermediate stabilization and I–I bond activation, but also effectively suppresses polyiodide shuttling, thereby preserving zinc negative electrode integrity and enabling high areal capacity, rate capability, and long-term cycling stability under high iodine loading.

DFT and experimental insights into orbital tag-team catalysis mechanism
To directly correlate the iodine deposition dynamics with the failure mechanisms under high-loading conditions, Raman line scans were performed across the positive electrode cross-section (~820 μm thick) following 100 cycles at 2 A g−1 (Figs. 5a−c). In catalyst-free positive electrodes, polyiodide species (I3−/I5−) exhibited severe spatial heterogeneity, with obvious concentration gradients between the electrolyte interface and current collector vicinity. This aligns with the sluggish ion transport and non-uniform deposition mechanisms, exacerbating interfacial impedance and dead “I2” zone formation.

To gain deeper insight into positive electrode reaction mechanisms, in situ UV-vis of electrolyte was conducted. During Ce SACs/I2 electrode charge/discharge cycles, the characteristic absorption peaks of I3− (at 240 nm) progressively weakened during charging and intensified upon discharging, reflecting the reversible oxidation/reduction processes. The absence of detectable I3− signals confirmed efficient conversion of polyiodides (I3−→I−) and strong adsorption of intermediates at Ce active sites (Supplementary Fig. 24a). In contrast, Co SACs positive electrodes exhibited persistent I3− signals throughout cycling, indicating sluggish adsorption and incomplete conversion of polyiodides. Notably, the sustained high intensity of I3− peaks suggest severe I3− shuttle effects, which would also cause active material loss and rapid capacity fading (Supplementary Fig. 24b).

In situ Raman spectroscopy was also employed to investigate the mechanistic role of Ce SACs in iodine-based positive electrodes (Supplementary Fig. 25). For one thing, limited signals corresponding to I3− and I5− were observed during the evaluation of polyiodide diffusion toward the negative electrode side, indicating effective suppression of the shuttle effect. For another, lower energy barrier for iodine dissociation benefits I–I weakening, accelerating redox kinetics. In contrast, positive electrodes utilizing Co SACs or no catalyst exhibited two prominent peaks throughout the discharge/charge processes, signifying severe polyiodide shuttling, which contributes to capacity degradation. These results indicate that Ce SACs are effective in stabilizing iodine redox chemistry and suppressing detrimental shuttling phenomena.

To further elucidate the ML-guided screening results, we performed DFT calculations to examine the orbital-level mechanism underlying the performance of Ce SACs in IRR. Among the evaluated SACs (V, Fe, Co, Cu, Nb, Mo, W, Ce, Zr, La, Sm), Ce exhibits the strongest I2 chemisorption (−9.20 eV), correlating with its high d-band center (−0.037 eV) and efficient electronic coupling near the Fermi level (Supplementary Figs. 26a−c). This dual property facilitates both iodine capture and charge transfer, thereby promoting *I3 formation and I–I bond activation. Free energy profiles reveal that the rate-limiting step of IRR is the conversion from *I3 to *I (Supplementary Fig. 26d). While several SACs exhibit moderate energy landscapes, Ce SAC shows the most downhill profile with reduced activation barriers, aligning with its electronic configuration and consistent with the enhanced kinetics observed experimentally.

To gain deeper insight into the orbital interactions, we performed a combined orbital-level analysis using crystal orbital Hamilton population (COHP), projected density of states (PDOS), and natural adaptive orbitals (NAdOs). COHP results (Fig. 5d) indicate that Ce d−I p hybridization provides bonding interactions stabilizing *I3, while Ce f−I p hybridization introduces antibonding character that selectively weakens the I−I bond. PDOS further confirms the dual behavior that Ce d orbitals hybridize at deeper energy levels (~−4 eV), whereas Ce f orbitals overlap near the Fermi level to form partially occupied antibonding states (Fig. 5e). The orbital diagrams in Fig. 5f clarify the symmetry and roles of these interactions. Ce d and d orbitals bond with I p and p to stabilize the intermediate, while Ce f and f engage in antibonding combinations with I p orbitals. This energy-selective and symmetry-matched hybridization supports the f-d orbital tag-team behavior.

To visualize the real-space evolution of bonding, we analyzed the dominant NAdOs for each adsorption state (Fig. 5g, Supplementary Figs. 27−29). These orbitals are annotated with eigenvalues, representing their quantitative contribution to overall Mayer bond orders. Directionally overlap patterns, such as Ce d −I p bonding and Ce f −I p antibonding combinations are clearly resolved. In the *I3 intermediate, the leading NAdO displays delocalized antibonding features with a reduced eigenvalue, consistent with partial I−I bond activation. This coincides with the marked increase in Ce f-orbital contribution in the *I3 state (Fig. 5h, top), correlating directly with I−I bond destabilization. Notably, this intermediate also exhibits lowest Ce−I bond strength (Fig. 5h, bottom, Supplementary Table 8), confirming the intermediate’s dual character—anchored yet electronically primed for activation. This supports the view that Ce f−I p antibonding interactions are central to I−I activation at the rate-determining step.

Together, these insights reinforce the ML-derived descriptors and reveal the f-d orbital tag-team mechanism—driven by the adaptive interplay between Ce d and f orbitals—as a key design principle that enables simultaneous intermediate anchoring and dynamic bond activation, setting Ce SACs apart from conventional d-block catalysts in iodine redox catalysis.

Methods
Chemicals
Iodine (I2, >99.99%), urea (≥99.5%), α-alanine (≥98%), dicyandiamide (≥99%), polytetrafluoroethylene (PTFE, 60%), ammonium metavanadate (NH4VO3, ≥99%), iron(II) chloride hexahydrate (FeCl2·6H₂O, ≥98%), cobalt(II) chloride hexahydrate (CoCl2·6H2O, ≥99%), anhydrous copper(II) chloride (CuCl2, ≥99.5%), zirconium(IV) acetylacetonate (Zr(acac)4, ≥97%), sodium molybdate dihydrate (Na2MoO4·2H2O, ≥99%), niobium(V) chloride (NbCl5, ≥99%), sodium tungstate dihydrate (Na2WO4·2H2O, ≥99%), and cerium(III) nitrate hexahydrate (Ce(NO3)3·6H2O, ≥99.5%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. (China). Zinc sulfate heptahydrate (ZnSO4·7H2O, analytical grade) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Zinc foil (100 μm thickness) and porous carbon were supplied by Guangdong Canrd New Energy Technology Co., Ltd. (China). Carbon cloth (W1S1010, thickness ~0.32 mm), carboxylated multiwalled carbon nanotubes (MWCANT), zinc foil (99.99%, 100 µm thickness), glass fiber separators (GF/A, nominal thickness ~1.0 mm, average pore size ~1.6 μm) and polytetrafluoroethylene (PTFE) binder (60 wt% dispersion in water, average molecular weight ~106 g mol−1) were acquired from Kaide Technology Co., Ltd. (China), Shenzhen Suiheng Graphite Technology Co., Ltd (China) and Whatman (UK), respectively. Deionized (DI) water obtained from a Milli-Q System (Millipore) was used in all experiments. All chemicals and materials were purchased from commercial suppliers and used as received, without any further purification.

Preparation for SACs
SACs were synthesized through a dissolution-carbonization protocol. In a typical procedure, urea (2.7 g) melted in a metal bath at 150°C under continuous stirring. To the molten urea, 8 mmol of metal salt precursor (e.g., Ce(NO3)3·6H2O), α-alanine (600 mg), and dicyandiamide (900 mg) were sequentially added. The mixture was heated until a homogeneous transparent solution formed, transferred to an alumina crucible, and cooled to room temperature. Carbonization was conducted in a tubular furnace under a continuous argon flow (50 sccm). The temperature was ramped from ambient to 800°C at a heating rate of 5°C min−1, held at 800°C for 2 h, and then cooled to 200 °C over 2 h. The resulting carbonized product was ground into fine powder for further use.

Preparation of SACs/I2 electrodes
First, porous carbon and I2 were mixed in a mass ratio of 1:1 and placed in a Teflon-lined autoclave. The mixture was then sealed and heated at 100 °C for 6 hours to allow iodine to be confined into the porous carbon. Next, the iodine-impregnated porous carbon, SACs powder, MWCNTs, and PTFE binder were mixed in a mass ratio of 5.5:2:2:0.5, respectively. For catalyst-free electrodes, the proportion of SACs powder was replaced by an equivalent amount of MWCNTs to maintain the same overall composition. A small amount of ethanol was added during the grinding process to form a homogeneous paste-like mixture. The paste was subsequently rolled/calendered into uniform films using a calendering machine (Shenzhen Kejing Star Technology, China) at 25 °C with a roll speed of 1 m/min. Circular electrode discs with a diameter of 10 mm were then punched from the films and laminated onto carbon cloth current collectors. The assembled SACs/I2 composite electrodes were dried at room temperature for 12 hours to remove residual moisture before further use.

Electrochemical measurements
The electrochemical performance of Zn||I2 batteries was evaluated using CR2032 coin-type cells with all specific capacities and specific current normalized to the mass of iodide species in the positive electrode. Unless otherwise specified, the mass loading of the positive electrode was controlled within the range of 1–2 mg cm−2. A GF/A separator was punched into 16 mm discs, and zinc foil was cut into 10 mm discs. The electrolyte consisted of a 2M ZnSO4 aqueous solution, with a volume of 80 μL per 2 mg cm−2 of positive electrode mass loading, and this ratio was scaled accordingly for different loadings. Galvanostatic charge-discharge tests were conducted using a LAND (CT3002A, Wuhan, China) and a Neware battery (MIHW-200-160CH, Shenzhen, China) testing systems. CV and EIS were performed on a PMC-200 electrochemical workstation (AMETEK, USA). EIS measurements spanned a frequency range of 10−1 to 105 Hz with a 10 mV AC amplitude, collecting 10 data points per frequency decade. Prior to each EIS measurement, the cell was held at the open-circuit voltage for 5 min to attain a quasi-stationary state. All electrochemical tests, except for temperature-dependent EIS, were carried out at 25 °C ± 1 °C.

Pouch cells were assembled by stacking one double-side-coated SACs/I₂ electrode (5 × 5 cm2) between two zinc foils (5 × 5 cm2). Two glass fiber separators (6 × 6 cm2) and 2 M ZnSO4 aqueous electrolyte were used. The electrolyte volume was controlled at approximately 10 mL Ah−1 based on the theoretical capacity of the positive electrode. During assembly, the electrolyte was vacuum-injected into the stacked pouch, followed by a 30-second holding period for complete wetting. A gas-release procedure was performed before final sealing. Cells were cycled under an applied external stack pressure of 0.01 MPa. Electrochemical performance was evaluated within a voltage range of 0.6–1.5 V (vs. Zn2+/Zn) under ambient conditions. The self-discharge test was conducted at a mass loading of approximately 28 mg cm−2.

Characterization
Powder X-ray diffraction (XRD) analysis was performed on a Bruker D8 Advance diffractometer (Germany) equipped with Cu–Kα radiation (λ = 1.5418 Å). Scans were collected over a 2θ range of 10–80° at a rate of 10° min−1. Morphology and elemental distribution were examined using a Regulus 8100 field-emission scanning electron microscopy (Hitachi, Japan) coupled with energy-dispersive X-ray spectroscopy (EDS). Transmission electron microscopy (TEM) tests were performed using a JEM-2100F electron microscope (JEOL, Japan) with an accelerating voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) was conducted on a K-Alpha+ spectrometer (Thermo Fisher Scientific, USA). Binding energies were calibrated to the C 1s peak at 284.8 eV and analyzed using Avantage software. Atomic-resolution imaging was obtained via an ARM-200F spherical aberration-corrected TEM (JEOL, Japan). In situ Raman spectroscopy was acquired using a confocal Raman microscope (LabRAM HR Evolution, Horiba, Japan) with a 532 nm laser, by monitoring the SACs/I2 positive electrode surface through the quartz window of a custom two-electrode cell (IVIUM, Netherlands) during galvanostatic charge-discharge cycles. In situ UV-vis spectroscopy was conducted on an Evolution 220 spectrophotometer (Thermo Fisher, USA) using a 1 cm path length quartz cuvette as the electrochemical cell, with the positive electrode and negative electrode secured on opposite sides, ensuring absorbance values were within the optimal linear range.

Computational methods
Spin-polarized DFT calculations were performed using VASP with projector augmented wave (PAW). The Perdew-Burke-Ernzerhof (PBE) functional within the Generalized Gradient Approximation (GGA) was used to represent the exchange correlation potential. The cutoff energy of the plane wave basis is set to 520 eV, and a 6×6×1 Monkhorst-Pack k-point mesh were applied. A vacuum spacing of 25 Å was introduced along the z direction to avoid spurious interactions. The electronic and ionic convergence thresholds were set to 1.0×10−5 eV/atom and 1.0×10−2 eV/Å, respectively. van der Waals interactions between the substrate and iodine species were treated using the DFT-D3 method with Becke–Johnson damping. All geometric optimizations, adsorption energies, thermodynamic corrections, and d-band center calculations were obtained from the spin-polarized ground state, ensuring a physically accurate description of the open-shell configurations commonly exhibited by transition-metal SACs47,48. The adsorption energy (Eab) of Ix on the catalyst surface was calculated as follows49:
Eab = Ecat+Ix − Ecat − EIx (1)
where Ecat+Ix, and Ecat, and EIx are the total energies of the catalyst with and without Ix adsorption, respectively. E Ix is the free energy of the I−, I2, or I3−. The d-band center (εd) of a transition metal was calculated as follows50:
εd = ∫ Ef −∞ E×f(E)dE / ∫ Ef −∞ f(E)dE (2)
where f(E) is the density of states and Ef is the energy of the Fermi level. The differential enthalpy of adsorption for the different surfaces was calculated using the DFT total energies and the thermal correction to enthalpy change (∆Hcorr(T)) at a specific temperature T:
∆Hads = ∆E + ∆Hcorr(T) (3)
For bonding analyses, including crystal orbital Hamilton populations (COHPs), projected density of states (PDOS), and natural adaptive orbitals (NAdOs) calculations, the optimized spin-polarized structures were placed into a 2×1×1 supercell and evaluated in a closed-shell configuration during post-processing, as implemented in the CP2K simulation package and MultiWFN program51 52. This step is necessary because the NAdO method operates on spin-paired density matrices and does not support open-shell wavefunctions53. No further structural optimization was carried out under the closed-shell setting, and the qualitative bonding trends were confirmed to be consistent with the spin-polarized electronic structures.

Machine learning methods
The dataset is provided in CSV format and contains sample identifiers, multi-dimensional structural/electronic descriptors, and the corresponding target variable. Prior to modeling, we constructed a feature matrix 𝐗 ∈ ℝ𝑛×𝑝 from a predefined set of numerical descriptors and applied Min–Max normalization to scale each feature to [0,1] according to 𝑥′ = (𝑥 −𝑥min)/(𝑥max −𝑥min), thereby reducing the influence of differing units and value ranges. The dataset was then randomly split into training/validation/test sets using a fixed random seed for reproducibility. To assess monotonic associations and feature redundancy, Spearman rank correlation coefficients were computed on the normalized features and visualized as a correlation heatmap to evaluate collinearity. In addition to the linear regression model used as the primary, interpretable baseline, we benchmarked several classical machine-learning models, including random forest and support vector machine (SVM) regressors, under the same data split and evaluation protocol. We also conducted ablation studies by systematically removing or retaining specific groups of descriptors (e.g., structural vs electronic features and selected key feature subsets) to quantify their individual contributions and to verify the robustness of the identified structure–property relationships. Finally, the normalized feature matrix was visualized using a bubble plot (features on the x-axis and samples on the y-axis), where bubble size is proportional to ∣ 𝑣 −0.5 ∣ and color is encoded by a piecewise gradient centered at 𝑣 = 0.5, enabling intuitive comparison of relative differences across samples and features.

Data availability
All data are available in the main text and the supplementary information. Source data are provided with this paper. The VASP-format optimized structures54 of the M-N-C catalysts and the relevant adsorption intermediates, as well as code that support the findings of this study are openly available in Zenodo at https://doi.org/10.5281/zenodo.17629945. A README file is included in the repository, detailing system requirements, installation steps, and instructions for reproducing the results.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this work, Chen et al. present a compelling demonstration of f-d orbital synergy in cerium single-atom catalysts for high-loading zinc-iodine batteries. The study stands out for its sophisticated integration of machine learning guidance with deep orbital-level analysis, offering a coherent strategy to overcome the persistent adsorption–activation dilemma in conversion-type cathodes. The proposed \"tag-team\" mechanism is well-supported by a combination of theoretical calculations and experimental validations, and the achievement of record-high iodine loading and areal capacity in practical pouch cells underscores the practical relevance of this fundamental insight. I therefore recommend acceptance in Nat Commun after the following points are considered.\n1. A more detailed comparison with other f-block SACs (e.g., La, Sm) would help clarify the unique advantages of Ce and strengthen the rationale behind its selection.\n2. The discussion on mechanical degradation at loadings >10 mg cm-2 is mentioned. A further analysis of electrode architecture or strategies to mitigate stress heterogeneity could strengthen the practical narrative.\n3. The XPS data indicates a mixture of Ce3+ and Ce4+. In-depth discussion on their respective roles in the catalytic cycle would provide deeper mechanistic insight, e.g., which state is more critical for adsorption vs. charge transfer?\n4. The variable-temperature EIS analysis, used to determine activation energies, requires the equivalent circuit model and fitting errors to be included in the SI to ensure methodological rigor.\n5. Several experimental details are suggested to be added for reproducibility: the electrolyte volume in coin and pouch cells, and the specific configuration for the in situ Raman/UV-vis cells.\n6. Direct visualization of cycled separators (e.g., through photographs or SEM) would provide additional convincing evidence for the suppressed shuttle effect in Ce SAC-based cells.\n7. The reported specific capacity (~242 mAh g-1) exceeds the theoretical value for the I₂/I⁻ redox couple (211 mAh g-1). The authors should clarify the origin of this excess capacity (e.g., contribution from carbon, normalization method, or other redox processes).\n8. For the high-loading pouch cell, a calculation of the iodine utilization rate (areal capacity / theoretical capacity based on I2 loading) would help quantify the efficiency of active material use.", "reviewer_2": "Chen et al. report an article titled “4f-5d orbital tag-team catalysis empowers high-loading zinc–iodine batteries”. The authors use ML to identify Ce single atom catalysts (SACs) that interact with iodine. From their analyses, the authors argue that their new material is practical for batteries.\nAlthough this article is probably publishable after major revisions, there are some imprecisions that should be cleared up prior to publication:\nMajor comments:\n1. The authors introduce the terminology of “tag team” catalysts. I think such terminology should be clarified. I had not heard this term before. It seems the authors are introducing this new terminology to describe f/d interactions.\n2. In their results section, the authors state “Unlike conventional approaches that focus primarily on thermodynamic outputs (e.g., adsorption energy or free energy), our method establishes physically meaningful descriptors that characterize the binding strength between the catalyst and iodine, as well as the ability to activate I–I bonds, thereby providing a more universal basis for catalyst screening” and then immediately follow this up with “These descriptors were used to predict two reaction-relevant properties: the adsorption energy of iodine species (ΔE*I), related to iodine immobilization and shuttle suppression, and the I–I bond length in *I3, reflecting bond cleavage efficiency.” Is the point that the authors are trying to make that understanding *why* the adsorption energy from a chemical perspective is what they are doing? I would argue that what the authors have done here is not that different, because neither the bader charge nor the number of d/f electrons near the fermi level can be readily be controlled.\n3. There is a lot of description around Zn-I batteries, but then the article talks mainly about Ce-SACs. I am no battery expert, but how these two are related is not clear to me from the early parts of the article. The first time where this is discussed is on page 14. I would urge the authors to give some background on this.\n4. “Moreover, the two descriptors vary independently and are associated with distinct aspects of the reaction mechanism (Fig. 1d)”. To me, it appears that these descriptors are highly correlated, at least in rank. Thus, saying that they vary independently appears incorrect.\n5. For Co SACs and other SACs with 3d metals, the authors need to consider open shell electron configurations, which are likely and probable for 3d metals. The work of Kulik has done this on SACs.\n6. When the authors consider SACs computationally, they assume certain coordination environments. There is no direct proof that the synthesized SACs have these coordination environments. Do the authors have reasoning for this?\n7. The authors should provide all their calculated geometries on zenodo, instead of the images they currently provide.", "reviewer_3": "This manuscript presents a comprehensive study on the design of a cerium single-atom catalyst (Ce SAC) for high-loading zinc-iodine batteries, addressing the trade-off between adsorption strength and redox kinetics, which is a fundamental challenge in conversion-type cathodes. The proposed “f-d orbital tag-team catalysis” mechanism is innovative and well-supported by a combination of machine learning screening and DFT calculations. The experimental results, particularly the high areal capacity and pouch cell demonstration, are impressive and highlight the practical relevance of the work. Overall, this manuscript is well-organized with solid data presented, contributing to the understanding of f-block catalysts and designing of high-loading conversion-type cathodes. I recommend its publication in Nature Communications with the following suggestions:\n1. To further strengthen the proposed \"f-d orbital tag-team\" mechanism, it would be helpful to provide additional theoretical analysis, such as charge density difference or Bader charge calculations, to more directly quantify the contribution of Ce 4f orbitals during I–I bond cleavage.\n2. Equivalent circuit fitting of the EIS data, along with relevant parameters and error analysis, would help better support the conclusions regarding charge transfer resistance and interface evolution.\n3. Clarifying the physical state of iodine (e.g., crystalline or amorphous) within the carbon host through XRD or Raman spectroscopy. In addition, quantifying the capacity contribution from the carbon substrate itself would help accurately attribute the electrochemical performance to iodine redox.\n4. Including the UV-Vis calibration curve and fitting details used for polyiodide concentration analysis in the Supplementary Information would improve the reproducibility and reliability of the adsorption measurements.\n5. Have the authors explored how different Ce SAC loadings influence the iodine redox kinetics? A correlation between catalyst loading and electrochemical performance could provide valuable insight for further optimizing the material design.\n6. SEM or TEM images of the cathode before and after cycling would be useful to evaluate the morphological and structural stability of the electrode under operating conditions.\n7. To better situate the high-loading performance within a practical context, it would be beneficial to include cycling stability and efficiency data at moderate loadings (e.g., 2–10 mg cm-2).\n8. A direct performance comparison with other recently reported Zn–I2 systems (particularly in terms of areal capacity, mass loading, and cycling stability) would help readers better appreciate the advances made in this work."}, "author_response": "Point-by-point response to the reviewers’ comments\nReviewer #1:\nIn this work, Chen et al. present a compelling demonstration of f-d orbital synergy in cerium\nsingle-atom catalysts for high-loading zinc-iodine batteries. The study stands out for its\nsophisticated integration of machine learning guidance with deep orbital-level analysis, offering\na coherent strategy to overcome the persistent adsorption–activation dilemma in conversion-\ntype cathodes. The proposed \"tag-team\" mechanism is well-supported by a combination of\ntheoretical calculations and experimental validations, and the achievement of record-high\niodine loading and areal capacity in practical pouch cells underscores the practical relevance of\nthis fundamental insight. I therefore recommend acceptance in Nat Commun after the following\npoints are considered.\nReply: We sincerely appreciate the reviewer’s positive evaluation and constructive feedback.\nBelow are our detailed responses to each comment.\n1. A more detailed comparison with other f-block SACs (e.g., La, Sm) would help clarify the\nunique advantages of Ce and strengthen the rationale behind its selection.\nReply: We thank the reviewer for this insightful comment. To clarify the differences among f-\nblock SACs and to justify the choice of Ce, we performed a detailed comparison of how the d–\nand f–orbital characteristics of Ce, La, and Sm govern their catalytic behavior toward I\n2\nactivation. As shown in Fig. R1a–c, Ce exhibits the highest d-band center among the tested f-\nblock metals, giving adsorption energies for I *, I *, and I* that fall within the Sabatier optimum.\n2 3\nIn contrast, La binds these intermediates too weakly, while Sm over-stabilizes them and hinders\nturnover.\nCrucially, the enhanced activity of Ce originates from its partially filled and moderately\ndelocalized 4f1 orbital. As revealed by the orbital composition (Fig. R1e), Ce is the only metal\nwhose 4f orbital directly participates in the I * interaction. This participation introduces a\n3\nsignificant f-orbital antibonding component, which weakens the I–I bond within I *, lowers the\n3\ntransition-state energy, and facilitates I–I activation. La lacks such an f channel (4f0), while the\n1\nhighly localized Sm 4f5 orbital cannot contribute effectively.\nThis mechanism is supported by the Mayer bond orders (Fig. R1f), which show that Ce\nmaintains the optimal M–I bond strength. In addition, the reaction enthalpy diagram (Fig. R1d)\nfurther confirms this trend by revealing the lowest overall energy barrier for Ce.\nTogether, these results demonstrate that Ce uniquely combines a favorable d-band position\nwith an f-orbital-derived antibonding interaction, making it the most active f-block SAC for I\n2\nactivation.\nFigure R1. a–c, Correlation between the d-band centers of Ce SAC with Ce3+, La SAC, Sm\nSAC, and Ce SAC with Ce4+ and the adsorption energies for I * (a), I * (b) and I (c)\n2 3 *\nintermediates. d, Enthalpy diagram for the I reduction reaction on the above SACs. e, Orbital\n2\ncomposition of the above metal d and f states and f, total Mayer bond orders in SAC+I *\n3\nstructures, as the rate-determining step.\n2\n2. The discussion on mechanical degradation at loadings >10 mg cm-2 is mentioned. A further\nanalysis of electrode architecture or strategies to mitigate stress heterogeneity could strengthen\nthe practical narrative.\nReply: We thank the reviewer for this valuable suggestion. We agree that a deeper analysis of\nstrategies to mitigate stress heterogeneity is crucial for practical high-loading electrodes. The\npost-cycling SEM analysis revealed that the significant performance decay at high mass\nloadings (>10 mg cm⁻2) is primarily due to micro-crack formation within the thick electrode\n(Figure R2). This is caused by the repeated volume changes and stress concentration during\ncycling, which is exacerbated when the electrode thickness increases without a proportional\nadjustment in the stack pressure. These cracks degrade the ionic/electrical contact pathways,\nleading to increased impedance and rapid failure.\nFigure R2. SEM image of the Ce SACs/I cathode cross-section.\n2\n3. The XPS data indicates a mixture of Ce3+ and Ce4+. In-depth discussion on their respective\nroles in the catalytic cycle would provide deeper mechanistic insight, e.g., which state is more\ncritical for adsorption vs. charge transfer?\nReply: We thank the reviewer for this important point. The coexistence of Ce3+ and Ce4+ in\nXPS is expected for f-block systems with flexible valence states. DFT calculations enable us to\ndistinguish their mechanistic roles.\nCe3+ is the catalytically active state for iodine adsorption and I–I bond activation. As shown\nin Fig. R1a–c, Ce3+ exhibits a higher d-band center and therefore provides moderate, optimally\nbalanced adsorption energies for I *, I *, and I*. More importantly, Ce3+ retains a partially filled\n2 3\nand moderately delocalized 4f¹ orbital. Orbital composition analysis (Fig. R1e) reveals that this\n4f¹ state directly contributes an antibonding interaction with the I–I fragment in I *, exerting\n3\n3\ntunable electronic repulsion that weakens the I–I bond and lowers the transition-state barrier.\nThis f-orbital antibonding effect is unique to Ce3+ and is responsible for efficient charge\nredistribution during I–I activation.\nIn contrast, Ce4+ plays a minimal catalytic role. The Ce4+ state lacks 4f electrons (4f⁰\nconfiguration), resulting in no f-orbital overlap with I *. Its d-band center is also significantly\n3\nlower, leading to weaker and less effective adsorption. Consistently, the reaction enthalpy\ndiagram (Fig. R1d) shows that Ce4+ has substantially higher barriers and cannot stabilize the\nkey I * intermediates. Thus, Ce4+ primarily acts as a spectator state rather than an active site for\n3\ncharge transfer or bond activation.\nOverall, Ce3+ provides both (i) an optimal adsorption strength arising from its elevated 5d\nstates and (ii) a crucial charge-transfer pathway enabled by its near-Fermi-level 4f¹ antibonding\ninteraction, making it the catalytically dominant state. In contrast, Ce4+ does not significantly\ncontribute to either function. These results clearly show that the catalytic function of the Ce\ncenter is governed predominantly by the Ce3+ state.\n4. The variable-temperature EIS analysis, used to determine activation energies, requires the\nequivalent circuit model and fitting errors to be included in the SI to ensure methodological\nrigor.\nReply: We greatly appreciate the reviewer’s comment. In the revised manuscript, we have\nincluded the equivalent circuit diagram, detailed the testing parameters, and reported the fitting\nerrors. These additions provide a clearer understanding of the activation energy calculations.\nFigure R3. a, Temperature-dependent EIS of Ce SACs/I cathode at 1.1 V. b, Activation\n2\nenergies derived from Arrhenius plots of Ce SACs/I and Co SAC/I cathodes at 1.1 V. c,\n2 2\nActivation energies for Ce SACs/I and Co SAC/I cathodes at 1.3 V and 1.1 V, respectively.\n2 2\na, b, Nyquist plots and equivalent circuits of Ce SACs (a) and Co SACs (b) at 1.3 V vs. Zn2⁺/Zn\nunder varying temperatures (25–45 °C). c, Arrhenius plots derived from EIS data at 1.3 V. d,\nNyquist plots of Co SACs at 1.1 V vs. Zn2⁺/Zn under varying temperatures (25–45 °C).\nTable R1. Fitted R values for Zn-I cells with Ce SACs/I and Co SACs/I electrodes at 1.1 V\nct 2 2 2\nand 1.3 V under different temperatures.\nTemperature 25°C 30°C 35°C 40°C 45°C\nR (Ω) 6.11 5.57 4.88 4.52 3.70\nct\nCe SACs (1.3V)\nError(%) 3.69 4.33 4.30 4.34 5.74\nR (Ω) 19.49 14.87 11.92 10.97 8.92\nct\nCo SACs (1.3V)\nError(%) 2.48 2.75 2.79 2.66 3.05\nR (Ω) 0.62 0.42 0.32 0.19 0.14\nct\nCe SACs (1.1V)\nError(%) 3.08 2.84 4.12 4.96 6.54\nR (Ω) 0.84 0.59 0.40 0.22 0.12\nct\nCo SACs (1.1V)\nError(%) 2.45 2.05 3.79 5.84 6.70\nWe have supplemented the equivalent circuits in the Revised Manuscript and the Revised\nSupporting Information.\nLine 180-183, Page 10: “To unveil the underlying kinetic limitations, temperature-dependent\nEIS analysis was conducted to quantify the activation energy (E ) of iodine reduction reactions\na\n(IRR) at different potential (Figs. 3c−e, Supplementary Fig. 12 and Supplementary Table 5).”\n5. Several experimental details are suggested to be added for reproducibility: the electrolyte\nvolume in coin and pouch cells, and the specific configuration for the in situ Raman/UV-vis\ncells.\nReply: We thank the reviewer for carefully noting these details. The revised Experimental\nSection now specifies the electrolyte volumes for both coin and pouch cells, as well as the\nconcentration and path length used in UV– vis analysis.\nUpdates in the Revised Manuscript:\nPage 23 in the Revised Manuscript: The electrolyte used was a 2M ZnSO aqueous solution,\n4\nwith a volume of 80 μL per 2 mg cm−2 of cathode mass loading, and this ratio was scaled\naccordingly for different loadings.\nPage 24 in the Revised Manuscript: In situ Raman spectroscopy was acquired using a\nconfocal Raman microscope (LabRAM HR Evolution, Horiba, Japan) with a 532 nm laser, by\nmonitoring the SACs/I cathode surface through the quartz window of a custom two-electrode\n2\ncell (IVIUM, Netherlands) during galvanostatic charge-discharge cycles.\nPage 24 in the Revised Manuscript: In situ UV-vis spectroscopy was conducted on an\nEvolution 220 spectrophotometer (Thermo Fisher, USA) using a 1 cm path length quartz\ncuvette as the electrochemical cell, with the cathode and anode secured on opposite sides,\nensuring absorbance values were within the optimal linear range.\n6. Direct visualization of cycled separators (e.g., through photographs or SEM) would provide\nadditional convincing evidence for the suppressed shuttle effect in Ce SAC-based cells.\nReply: We thank the reviewer for this insightful suggestion. To directly verify the suppression\n6\nof the shuttle effect, we have conducted additional cycling tests and captured optical images of\nthe separators from cells with and without the Ce SACs catalyst. As shown in Figure R5, the\nseparator in the Ce SACs-based cell remains significantly lighter, indicating less polyiodide\npermeation.\nFigure R5. Optical images of separators disassembled from cells with Ce SACs/I and catalyst-\n2\nfree electrodes.\nUpdates in the Revised Manuscript:\nWe have supplemented the testing details in the revised Supporting Information.\nLine 280-283, Page 16: “Notably, when Co SACs or no catalysts were used, severe surface\ncorrosion on the zinc surface was observed, indicative of rampant polyiodide shuttling. In\ncontrast, smooth morphology with negligible byproduct was detected on zinc anode paired with\nCe SACs/I cathode (Supplementary Fig. 23).”\n2\n7. The reported specific capacity (~242 mAh g-1) exceeds the theoretical value for the I₂/I⁻\nredox couple (211 mAh g-1). The authors should clarify the origin of this excess capacity (e.g.,\ncontribution from carbon, normalization method, or other redox processes).\nReply: We thank the reviewer for this insightful suggestion. To directly verify the extra capacity\ncontribution, we have conducted additional rate tests of the porous carbon. As shown in Figure\nR6, the porous carbon matrix contributes less than 50 mAh g-1 of non-faradaic capacity.\ncorresponding Coulombic efficiency at various current densities. (b) Galvanostatic charge-\ndischarge voltage profiles at a current density of 0.5 A g-1.\n8. For the high-loading pouch cell, a calculation of the iodine utilization rate (areal capacity /\ntheoretical capacity based on I loading) would help quantify the efficiency of active material\n2\nuse.\nReply: We sincerely thank the reviewer for highlighting this important point. We thank the\nreviewer for this important suggestion. Based on the measured areal capacity (~6 mAh cm−2)\nand iodine loading (28.8 mg cm−2), the calculated specific capacity of the pouch cell is 205\nmAh g−1, corresponding to an iodine utilization efficiency of approximately 97% relative to the\ntheoretical capacity of iodine (211 mAh g−1). We have incorporated this quantitative analysis\ninto the Revised Manuscript to emphasize the practical significance of our tag-team catalysis\nstrategy.\nUpdates in the Revised Manuscript:\nLine 255-258, Page 14: “At a high loading of 28.8 mg cm−2 (~2.4 C), Ce SACs/I cells exhibit\n2\nexceptional stability, retaining 97% iodine utilization over 100 cycles with a near-unity\nCoulombic efficiency (99.9%) and stable voltage plateaus (Fig. 4i and Supplementary Fig. 21a).”\nChen et al. report an article titled “4f-5d orbital tag-team catalysis empowers high-loading zinc–\niodine batteries”. The authors use ML to identify Ce single atom catalysts (SACs) that interact\nwith iodine. From their analyses, the authors argue that their new material is practical for\nbatteries. Although this article is probably publishable after major revisions, there are some\nimprecisions that should be cleared up prior to publication:\nReply: We sincerely appreciate the reviewer’s positive evaluation and constructive feedback.\nBelow are our detailed responses to each comment.\nMajor comments:\n1. The authors introduce the terminology of “tag team” catalysts. I think such terminology\nshould be clarified. I had not heard this term before. It seems the authors are introducing this\nnew terminology to describe f/d interactions.\nReply: Thank you for this insightful comment. The term \"tag-team catalysis\" is introduced to\ndescribe a mechanistic paradigm, referenced in our manuscript (Nature 456, 841 (2008);\nScience 322, 77-80 (2008)), where discrete catalytic components function cooperatively yet\naddress distinct mechanistic steps.\nIn our study, this concept is extended to the orbital level. The \"f-d orbital tag-team\"\nmechanism describes the functional decoupling of two critical aspects in iodine\nelectrochemistry: The Ce 5d orbitals drive strong chemisorption of polyiodides, essential for\nsuppressing the shuttle effect, while the Ce 4f orbitals facilitate I-I bond activation by\nantibonding interactions near the Fermi level, enabling fast conversion kinetics.\nUnlike conventional synergism, which often enhances a single function, this “orbital tag-\nteam” mechanism resolves the inherent competition between strong adsorption and facile\nactivation. This division of labor allows the catalyst to bypass the Sabatier-type dilemma, where\nstrong adsorption typically poisons active sites. Consequently, simultaneous immobilization\nand efficient conversion of intermediates are achieved, which is pivotal for the high-loading\nperformance demonstrated.\nLine 68-71, Page 4: “This data-driven screening led to the discovery of a unique f–d orbital\ntag-team effect1,2 in Ce SACs, a mechanism characterized by the functional decoupling of\nadsorption and activation into distinct orbital tasks.”\n2. In their results section, the authors state “Unlike conventional approaches that focus primarily\non thermodynamic outputs (e.g., adsorption energy or free energy), our method establishes\nphysically meaningful descriptors that characterize the binding strength between the catalyst\nand iodine, as well as the ability to activate I–I bonds, thereby providing a more universal basis\nfor catalyst screening” and then immediately follow this up with “These descriptors were used\nto predict two reaction-relevant properties: the adsorption energy of iodine species (ΔE*I),\nrelated to iodine immobilization and shuttle suppression, and the I–I bond length in *I3,\nreflecting bond cleavage efficiency.” Is the point that the authors are trying to make that\nunderstanding *why* the adsorption energy from a chemical perspective is what they are doing?\nI would argue that what the authors have done here is not that different, because neither the\nbader charge nor the number of d/f electrons near the fermi level can be readily be controlled.\nReply: Thank you for raising this important point, which allows us to clarify the conceptual\nadvancement of our work. The primary contribution of our study lies in establishing and\nvalidating a predictive framework that directly correlates intrinsic electronic properties with\ncatalytic performance for iodine redox in Zn–I batteries, moving beyond traditional post-hoc\n2\nthermodynamic analysis.\nWhile conventional screening requires computing ΔE*I for each candidate through\ndemanding adsorption-state calculations, our framework demonstrates that the intrinsic\nelectronic structure of the pristine catalyst can reliably predict both adsorption energy\nand bond activation capability. This enables rapid performance estimation without requiring\nfull adsorption-state computations for each new candidate, significantly accelerating the\ndiscovery process.\nRegarding descriptor controllability, while fundamental atomic properties are fixed, the\nelectronic environment of a catalytic site can be effectively modulated through coordination\nengineering. Our descriptors serve as quantitative indicators of this tunable electronic\n10\nenvironment, wherein variations in coordination number (e.g., M–N to M–N ) or the\n4 3\nintroduction of heteroatoms (e.g., S/O ligands) alter both Bader charge and local electron\ndensity. This tunability underpins the broader applicability of our framework, suggesting\nits potential extension not only to other coordination motifs but also to related conversion-type\nsystems such as lithium–sulfur and lithium–selenium batteries, where similar adsorption–\nactivation challenges exist, though further validation would be required.\nIn summary, our descriptor framework enables the prediction of key catalytic properties\ndirectly from a pristine catalyst's electronic structure, bypassing intensive adsorption\ncalculations and enabling a more rational design strategy.\n3. There is a lot of description around Zn-I batteries, but then the article talks mainly about Ce-\nSACs. I am no battery expert, but how these two are related is not clear to me from the early\nparts of the article. The first time where this is discussed is on page 14. I would urge the authors\nto give some background on this.\nReply: Thank you for raising this important point regarding the connection between Zn-I\n2\nbatteries and our focus on Ce-SACs. The single-atom catalysts (SACs) are precisely the key\ncomponent engineered into the cathode of the Zn-I battery to address the core challenges of\n2\niodine electrochemistry (shuttle effect and slow reaction kinetics) that we discussed. In response,\nwe have revised the introduction and description of Fig. 1e, f (and Figure R7) to better illustrate\nthe linkage from the very beginning.\nSpecifically, the introduction now establishes the following context on page 3: “Recent\nadvances have explored the use of 3d-metal single-atom catalysts (SACs) (e.g., Fe, Co) to\nmodulate iodine redox pathways through d–p orbital hybridization, a concept previously\nvalidated in sulfur reduction catalysis. However, these catalysts face a fundamental constraint\nwell-known as “Sabatier principle”, i.e., insufficient adsorption hinders I–I bond activation\nwhile excessive binding over-stabilizes intermediates and impedes catalytic turnover.” This\nbackground directly motivates our machine learning (ML) approach to discover more optimal\ncatalysts like Ce-SACs that can overcome this limitation.\nFurthermore, to make this contrast visually immediate, we have revised the description of\n4\nfollows: “We compare the electronic structures and polyiodide adsorption/conversion behaviors\nof a typical 3d metal SAC (Co–N ) with 4f metal SAC (Ce–N ) to elucidate a catalyst design\n4 4\nstrategy for overcoming the key challenges in Zn–I batteries (Figs. 1e, f). Co–N , dominated\n2 4\nby localized d orbitals, suffers from insufficient charge transfer to the coordinating N atoms\n(1.17 |e|), resulting in weak adsorption and sluggish conversion of polyiodides. In contrast, Ce–\nN engages in tag-team catalysis through the complementary and synergistic interactions of the\n4\nd/f orbitals with iodine species, enabling stronger adsorption and more favorable redox behavior\nof polyiodides”.\nFigure R7. a, b, Schematic illustration of polyiodide adsorption and conversion on Co SACs\n(a) and Ce SACs (b), respectively.\n4. “Moreover, the two descriptors vary independently and are associated with distinct aspects\nof the reaction mechanism (Fig. 1d)”. To me, it appears that these descriptors are highly\ncorrelated, at least in rank. Thus, saying that they vary independently appears incorrect.\nReply: Thank you for this insightful comment. We should agree that the two descriptors are not\nfully independent (Figure R8), but are not “highly correlated”, either. More quantitatively, the\ntwo descriptors (total d/f electron count and metal-ligand charge polarization) have a moderate\ncorrelation with an R2 of 0.6. Crucially, we find that they are functionally distinct and\neffectively link to independent aspects of the reaction mechanism. Our conclusion is based on\nthe following evidence:\nN +N (total d/f electron count) is the primary descriptor for the I-I bond activation step,\nd f\n12\nshowing a strong correlation with the I–I bond length (R2=0.81) but a much weaker one with\nadsorption energy (R2=0.58).\nConversely, ΔBader-N (metal-ligand charge polarization) is the dominant descriptor for the\nf\nadsorption step, strongly predicting adsorption energy (ΔE*I, R2=0.89) while poorly predicting\nthe I–I bond length (R2=0.48) in Figure R9. It underscores that the descriptors, while somewhat\ncorrelated, are associated with distinct and non-interchangeable aspects of the catalytic process.\nFigure R8. a, Correlation between the adsorption energy of I⁻ (ΔE ) and ΔBader – N for\n*I f\ndifferent SACs. b, Correlation between the I–I bond length in *I and N + N for different SACs.\n3 d f\nc, The relationship between two descriptors: N + N and ΔBader − N.\nd f f\nFigure R9. (a) Correlation between the adsorption energy of I⁻ (ΔE ) and N + N for different\n*I d f\nSACs. (b) Correlation between the I–I bond length in *I and ΔBader – N for different SACs.\n3 f\nWe have revised the relevant sentence to include the quantitative correlation coefficient (R2 =\n0.6), providing a more precise description of the descriptor relationship.\nUpdates in the Revised Manuscript:\nLine 110-112, Page 6: “Moreover, while the two descriptors are correlated (Fig. 1d, R2=0.6),\n13\nthey are linked to distinct aspects of the reaction mechanism (Supplementary Fig. 4), allowing\nseparate regulation of adsorption strength and bond activation.”\n5. For Co SACs and other SACs with 3d metals, the authors need to consider open shell electron\nconfigurations, which are likely and probable for 3d metals. The work of Kulik has done this\non SACs.\nReply: Thank you for raising this critical point regarding the importance of open-shell\nconfigurations for 3d-metal SACs. Using open-shell configurations in SAC calculations is\ncritical for correctly describing systems with unpaired electrons. We would like to clarify that\nall our DFT calculations for Co and other 3d-metal single-atom sites were performed\nexactly using spin-polarized, open-shell configurations, just as you commented. For\nclearance, we have now explicitly stated this point in the section of computational details in the\nrevised manuscript. Furthermore, we have duly cited the references from Prof. Kulik's group\nand others on open-shell 3d-metal SACs.\nUpdates in the Revised Manuscript:\nLine 453-463, Page 24: “Spin-polarized DFT calculations were performed using VASP with\nprojector augmented wave (PAW). The Perdew-Burke-Ernzerhof (PBE) functional within the\nGeneralized Gradient Approximation (GGA) was used to represent the exchange correlation\npotential. The cutoff energy of the plane wave basis is set to 520 eV, and a 6×6×1 Monkhorst-\nPack k-point mesh were applied. A vacuum spacing of 25 Å was introduced along the z direction\nto avoid spurious interactions. The electronic and ionic convergence thresholds were set to\n1.0×10-5 eV/atom and 1.0×10-2 eV/Å, respectively. van der Waals interactions between the\nsubstrate and iodine species were treated using the DFT-D3 method with Becke–Johnson\ndamping. All geometric optimizations, adsorption energies, thermodynamic corrections, and d-\nband center calculations were obtained from the spin-polarized ground state, ensuring a\nphysically accurate description of the open-shell configurations commonly exhibited by\ntransition-metal SACs47,48.”\nLine 470-481, Page 25: “The differential enthalpy of adsorption for the different surfaces were\ncalculated using the DFT total energies and the thermal correction to enthalpy change (∆H )\ncorr(T)\n14\nat a specific temperature T:\n∆𝐻 = ∆𝐸 +∆𝐻 (3)\n(cid:3028)(cid:3031)(cid:3046) (cid:3030)(cid:3042)(cid:3045)(cid:3045)((cid:3021))\nFor bonding analyses, including crystal orbital Hamilton populations (COHPs), projected\ndensity of states (PDOS), and natural adaptive orbitals (NAdOs) calculations, the optimized\nspin-polarized structures were placed into a 2×1×1 supercell and evaluated in a closed-shell\nconfiguration during post-processing, as implemented in the CP2K simulation package and\nMultiWFN program51,52. This step is necessary because the NAdO method operates on spin-\npaired density matrices and does not support open-shell wavefunctions53. No further structural\noptimization was carried out under the closed-shell setting, and the qualitative bonding trends\nwere confirmed to be consistent with the spin-polarized electronic structures.”\nReferences\ncatalyzed electrochemical reduction of carbon dioxide in water. 2. mechanism from first\nprinciples. J. Phys. Chem. A 114, 10174-10184 (2010).\nopen-shell transition-metal catalysts. ACS Catal. 10, 15033-15047 (2020).\npackage - Quickstep: Efficient and accurate electronic structure calculations. J. Chem.\nPhys. 152, 194103 (2020).\n33, 580-592 (2012).\nÁ. Bond Order Densities in Real Space. J. Phys. Chem. A 124, 339-352 (2020).\n6. When the authors consider SACs computationally, they assume certain coordination\nenvironments. There is no direct proof that the synthesized SACs have these coordination\nenvironments. Do the authors have reasoning for this?\nReply: Thank you for raising this important point. The coordination environments in our\ncomputational models are based on experimental evidence. For the representative Co and Ce\nanalysis. Specifically, the fitting of the Extended X-ray Absorption Fine Structure (EXAFS)\nspectra, as shown in Figure R10b and d, provides direct evidence for the first coordination shell.\nThe results confirm a coordination number of approximately 4 for both metals, which directly\nvalidates our tetra-coordinated computational models. For other metal SACs studied\ncomputationally, while not directly characterized here, extensive literature on the molten-salt\nannealing method consistently confirms that it predominantly yields tetra-coordinated (M-N )\n4\nsingle-atom structures31,32. In summary, our computational models are grounded in both direct\nexperimental validation and consistent literature reports, ensuring their relevance to the\nsynthesized SACs.\nFigure R10. a. Synchrotron radiation analysis of Co and Ce SACs. (a, c) X-ray absorption near-\nedge structure (XANES) spectra for Co and Ce SACs, respectively. (b, d) Extended X-ray\nabsorption fine structure (EXAFS) spectra for Co and Ce SACs, respectively.\n2 2\nSample Path N R/(Å) σ /Å ∆E/eV R factor\nCo SACs Co-N 4.00 1.946 0.015 0.077 0.009\nTable R3. Curve fit parameters of Ce K-edge EXAFS for Ce SACs.\nSample Path N R/(Å) σ2/Å2 ∆E/eV R factor\nCe SACs Ce-N 4.00 2.021 0.010 -0.100 0.020\nUpdates in the Revised Manuscript:\nWe have supplemented the experimentally determined coordination structures of the Co and Ce\nSACs from XAS analysis in the revised Manuscript and Supplementary information.\nLine 156-161, Page 9: “Furthermore, to determine the local coordination environment of these\nisolated atoms, we performed synchrotron-based X-ray absorption spectroscopy (XAS).\nQuantitative analysis of the extended X-ray absorption fine structure (EXAFS) spectra confirms\nthe dominant presence of a four-coordinate configuration for the metal centers (Supplementary\nFig. 8 and Tables 3, 4), which is consistent with our computational models.”\nReferences\nmultimetallic designs. Nat. Mater. 21, 681-688 (2022).\neffective epoxidation of styrene. Adv. Mater. 32, 2000896 (2020).\n7. The authors should provide all their calculated geometries on zenodo, instead of the images\nthey currently provide.\nReply: Thank you for this valuable suggestion. In response, we have uploaded the key\ncomputational geometries from this work to a Zenodo repository. This dataset includes the\noptimized structures of the M-N-C single-atom sites and relevant adsorption intermediates in\nVASP format (CONTCAR files). The repository is publicly available at\nhttps://doi.org/10.5281/zenodo.17629945. A corresponding data availability statement,\n17\nincluding this DOI link, has been added to the revised manuscript.\nUpdates in the Revised Manuscript:\nLine 501-503, Page 26: “The VASP-format optimized structures of the M-N-C catalysts and\nthe relevant adsorption intermediates that support the findings of this study are openly available\nin Zenodo at https://doi.org/10.5281/zenodo.17629945.”\nThis manuscript presents a comprehensive study on the design of a cerium single-atom catalyst\n(Ce SAC) for high-loading zinc-iodine batteries, addressing the trade-off between adsorption\nstrength and redox kinetics, which is a fundamental challenge in conversion-type cathodes. The\nproposed “f-d orbital tag-team catalysis” mechanism is innovative and well-supported by a\ncombination of machine learning screening and DFT calculations. The experimental results,\nparticularly the high areal capacity and pouch cell demonstration, are impressive and highlight\nthe practical relevance of the work. Overall, this manuscript is well-organized with solid data\npresented, contributing to the understanding of f-block catalysts and designing of high-loading\nconversion-type cathodes. I recommend its publication in Nature Communications with the\nfollowing suggestions:\nReply: We sincerely appreciate the reviewer’s positive evaluation and constructive feedback.\nBelow are our detailed responses to each comment.\n1. To further strengthen the proposed \"f-d orbital tag-team\" mechanism, it would be helpful\nto provide additional theoretical analysis, such as charge density difference or Bader charge\ncalculations, to more directly quantify the contribution of Ce 4f orbitals during I–I bond\ncleavage.\nReply: We thank the reviewer for this insightful suggestion. Following the comment, we added\nthe charge density difference plots in Fig. R11 to visualize the real-space electron rearrangement\nduring adsorption of I *, I *, and I* on Ce SAC. Although Ce exhibits some electron depletion\n2 3\nin all three cases, which reflects normal metal–iodine polarization, the spatial pattern of this\nredistribution differs markedly among the intermediates. For I ∗ and I*,\nthe electron depletion\n2\naround Ce is relatively isotropic, reflecting conventional adsorption mainly governed by Ce-5d\npolarization. In contrast, the I * complex shows a distinctly directional and fragmented\n3\nredistribution of electrons along the I-I axis. This anisotropic pattern is consistent with the\nparticipation of Ce 4f orbitals, whose localized nature promotes directional antibonding\ninteractions that weaken the I-I bond.\n3 2\nthat I-I activation in I * is not driven by simple charge transfer. Instead, it arises from a local\n3\norbital-driven antibonding interaction, consistent with the proposed f–d orbital tag-team\nmechanism. This interpretation is supported by the PDOS and COHP analyses (Fig. 5 in\nManuscript), which distinguish the contributions of Ce 5d and 4f states. The charge density\ndifference plots further confirm the unique directional electron redistribution in the I *\n3\nconfiguration.\nFigure R11. Charge density difference plots for SACCe interacting with (a) I *, (b) I *, and (c)\n2 3\nI, where red and blue denote electron accumulation and depletion, respectively.\n2\n2. Equivalent circuit fitting of the EIS data, along with relevant parameters and error analysis,\nwould help better support the conclusions regarding charge transfer resistance and interface\nevolution.\nReply: We greatly appreciate the reviewer’s comment. In the revised manuscript, we have\nincluded the equivalent circuit diagram, detailed the testing parameters, and reported the fitting\nerrors. These additions provide a clearer understanding of the activation energy calculations.\nFigure R12. a, Temperature-dependent EIS of Ce SACs/I cathode at 1.1 V. b, Activation\n2\nenergies derived from Arrhenius plots of Ce SACs/I and Co SAC/I cathodes at 1.1 V. c,\n2 2\nActivation energies for Ce SACs/I and Co SAC/I cathodes at 1.3 V and 1.1 V, respectively.\n2 2\na, b, Nyquist plots and equivalent circuits of Ce SACs (a) and Co SACs (b) at 1.3 V vs. Zn2⁺/Zn\nunder varying temperatures (25–45 °C). c, Arrhenius plots derived from EIS data at 1.3 V. d,\nNyquist plots of Co SACs at 1.1 V vs. Zn2⁺/Zn under varying temperatures (25–45 °C).\nTable R4. Fitted R values for Zn-I cells with Ce SACs/I and Co SACs/I electrodes at 1.1 V\nct 2 2 2\nand 1.3 V under different temperatures.\nTemperature 25°C 30°C 35°C 40°C 45°C\nR (Ω) 6.11 5.57 4.88 4.52 3.70\nct\nCe SACs (1.3V)\nError(%) 3.69 4.33 4.30 4.34 5.74\nR (Ω) 19.49 14.87 11.92 10.97 8.92\nct\nCo SACs (1.3V)\nError(%) 2.48 2.75 2.79 2.66 3.05\nR (Ω) 0.62 0.42 0.32 0.19 0.14\nct\nCe SACs (1.1V)\nError(%) 3.08 2.84 4.12 4.96 6.54\nR (Ω) 0.84 0.59 0.40 0.22 0.12\nct\nCo SACs (1.1V)\nError(%) 2.45 2.05 3.79 5.84 6.70\nWe have supplemented the equivalent circuits in the Revised Manuscript and the Revised\nSupporting Information.\nLine 180-183=-3, Page 10: “To unveil the underlying kinetic limitations, temperature-\ndependent EIS analysis was conducted to quantify the activation energy (E ) of iodine reduction\na\nreactions (IRR) at different potential (Figs. 3c−e, Supplementary Fig. 12 and Supplementary\nTable 5).”\n3. Clarifying the physical state of iodine (e.g., crystalline or amorphous) within the carbon\nhost through XRD or Raman spectroscopy. In addition, quantifying the capacity contribution\nfrom the carbon substrate itself would help accurately attribute the electrochemical performance\nto iodine redox.\nReply: We thank the reviewer for this insightful suggestion. To directly verify the physical state\nof iodine and the extra capacity contribution, we have conducted XRD analysis and additional\nrate tests of the porous carbon. As shown in Figure R14 and Figure R15, the iodine is in an\namorphous form within the porous carbon, and the porous carbon matrix contributes less than\n50 mAh g⁻¹ of non-faradaic capacity.\nFigure R14. XRD patterns of pristine activated carbon (AC) and iodine-loaded activated\ncarbon (AC+I ).\n2\ncorresponding Coulombic efficiency at various current densities. b, Galvanostatic charge-\ndischarge voltage profiles at a current density of 0.5 A g⁻¹.\n4. Including the UV-Vis calibration curve and fitting details used for polyiodide concentration\nanalysis in the Supplementary Information would improve the reproducibility and reliability of\nthe adsorption measurements.\nFigure R16. a, UV-Vis absorption spectra of the solution. b, The standard calibration curve of\nI ⁻ concentration.\nWe have supplemented the calibration curve in the Revised Supporting Information.\nLine 166-169, Page 10: UV–vis spectra quantified this phenomenon, revealing that the\ncharacteristic I − absorption peaks at 288 nm and 350 nm, were significantly weakened in Ce\n5. Have the authors explored how different Ce SAC loadings influence the iodine redox\nkinetics? A correlation between catalyst loading and electrochemical performance could\nprovide valuable insight for further optimizing the material design.\nReply: We thank the reviewer for this insightful suggestion. We performed cyclic voltammetry\n(CV) measurements with I cathodes at different Ce SAC loadings (4 wt%, 8 wt%, and 16 wt%).\ndemonstrated the smallest polarization and the highest current density, confirming it as the\noptimal loading for iodine redox kinetics.\nscan rate of 0.2 mV/s.\n6. SEM or TEM images of the cathode before and after cycling would be useful to evaluate\nthe morphological and structural stability of the electrode under operating conditions.\nReply: We thank the reviewer for this valuable suggestion. The SEM images show well-\npreserved electrode morphology after cycling, demonstrating excellent structural stability with\nno visible degradation.\nFigure R18. SEM images of the Ce SACs/I₂ cathode (a) before cycling and (b) after 100 cycles\nat 2 A g⁻¹.\nUpdates in the Revised Manuscript:\nWe have supplemented the testing details in the revised Supporting Information.\nLine 280-283, Page 16: “Notably, when Co SACs or no catalysts were used, severe surface\ncorrosion on the zinc surface was observed, indicative of rampant polyiodide shuttling. In\ncontrast, smooth morphology with negligible byproduct was detected on zinc anode paired with\nCe SACs/I cathode (Supplementary Fig. 23).”\n2\n7. To better situate the high-loading performance within a practical context, it would be\n24\nbeneficial to include cycling stability and efficiency data at moderate loadings (e.g., 2–10 mg\ncm-2).\nReply: We thank the reviewer for this valuable suggestion. To address it directly, we have\nconducted additional cycling stability tests under moderate mass loadings, specifically at 4 mg\ncm-2 (under current densities of 0.5, 2, and 10 A g g-1) and at 8 mg cm-2 (under 0.5 and 2 A g-\n1). As shown in the new figure below, the cells exhibit exceptional long-term cycling stability\nover 30,000 cycles. These comprehensive data, combined with our previously reported high-\nloading performance, provide a complete view of our material's capability across various\npractical scenarios and firmly underscore its robustness and high-rate potential.\nFigure R19. a, Cycling stability and Coulombic efficiency of the electrode at a moderate mass\nloading of 4 mg cm-2 under various current densities (0.5, 2, and 10 A g-1). b, The areal capacity\nretention over 30,000 cycles at a high current density of 10 A g⁻¹ and a mass loading of 4 mg\ncm-2.\nUpdates in the Revised Manuscript:\nWe have supplemented the testing details in the revised Supporting Information.\nLine 242-245, Page 13: “Addressing the practical challenge of high iodine loading, Ce SACs/I\n2\ncathode sustain nearly linear scaling with areal capacity increasing from 1.75 to 10.06(cid:3031)mg(cid:3031)cm−2.\n10(cid:3031)A(cid:3031)g−1) (Supplementary Fig. 19).”\n8. A direct performance comparison with other recently reported Zn–I2 systems (particularly\nin terms of areal capacity, mass loading, and cycling stability) would help readers better\nappreciate the advances made in this work.\nReply: We thank the reviewer for this valuable suggestion. As recommended, we have now\nincluded a direct performance comparison with other state-of-the-art Zn-I systems in the\n2\nrevised supplementary manuscript, which is presented in Supplementary Table 7.\nTable R5. Summary of the electrochemical performance of Ce SACs/I in comparison with\n2\nprevious reports, with a focus on mass loading, current density, areal capacity, capacity\nretention, and cycling stability.\nCurrent\nMass Areal Capacity\nCycling\nCathode loading density capacity retention Ref.\nnumber\n(mg cm‒2) (A g‒1) (mAh cm‒2) (%)\n35 0.5 6.1 96 137 This\nCe SACs/I\n2\n4 10 0.8 74 31700 work\nB-Fe-NC/I 1 2.11 0.2 99 10000 [2]\nFe SAC-MNC/I 1.2-1.8 0.5 0.2-0.3 81 50000 [5]\nI /NiSAs-HPC 1 1 0.2 94 10000 [7]\n2\nCMK-3@I 1.2-1.5 0.2 0.2 86 10000 [12]\nNZ-aNC@I 5.5 5 0.8 93 1500 [15]\nWe have supplemented the testing details in the revised Supporting Information.\nLine 265-267, Page 14: “These results demonstrate that the f–d orbital tag team in Ce SACs\nalleviates key limitations at high iodine loading and offers a promising pathway toward scalable\nZn–I batteries for grid-level energy storage (Supplementary Table 7).”\n2\n27"}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors revised the manuscript accordingly, it is publishable in the present form.", "reviewer_2": "The reviewers have adequately addressed my comments.", "reviewer_3": "All my concerns have been addressed and the manuscript can be accepted now."}, "author_response": ""}]

Decision letter