Boosting Zn||I2 Battery’s Performance by Coating a Zeolite-Based Cation-Exchange Protecting Layer

High-performance Zn||I2 batteries were established by coating zeolite protecting layers. The Zn2+-conductive layer suppresses I3− shuttling, Zn corrosion/dendrite growth. The Zeolite-Zn||I2 batteries achieve long lifespan (91.92% capacity retention after 5600 cycles), high coulombic efficiencies (99.76% in average) and large capacity (203–196 mAh g−1 at 0.2 A g−1) simultaneously. High-performance Zn||I2 batteries were established by coating zeolite protecting layers. The Zn2+-conductive layer suppresses I3− shuttling, Zn corrosion/dendrite growth. The Zeolite-Zn||I2 batteries achieve long lifespan (91.92% capacity retention after 5600 cycles), high coulombic efficiencies (99.76% in average) and large capacity (203–196 mAh g−1 at 0.2 A g−1) simultaneously. The intrinsically safe Zn||I2 battery, one of the leading candidates aiming to replace traditional Pb-acid batteries, is still seriously suffering from short shelf and cycling lifespan, due to the uncontrolled I3−-shuttling and dynamic parasitic reactions on Zn anodes. Considering the fact that almost all these detrimental processes terminate on the surfaces of Zn anodes, modifying Zn anodes’ surface with protecting layers should be one of the most straightforward and thorough approaches to restrain these processes. Herein, a facile zeolite-based cation-exchange protecting layer is designed to comprehensively suppress the unfavored parasitic reactions on the Zn anodes. The negatively-charged cavities in the zeolite lattice provide highly accessible migration channels for Zn2+, while blocking anions and electrolyte from passing through. This low-cost cation-exchange protecting layer can simultaneously suppress self-discharge, anode corrosion/passivation, and Zn dendrite growth, awarding the Zn||I2 batteries with ultra-long cycle life (91.92% capacity retention after 5600 cycles at 2 A g−1), high coulombic efficiencies (99.76% in average) and large capacity (203–196 mAh g−1 at 0.2 A g−1). This work provides a highly affordable approach for the construction of high-performance Zn-I2 aqueous batteries.


Introduction
While dominating the rechargeable (i.e., secondary) battery market with outstanding energy-/power-density and long lifespan, lithium-ion batteries (LIBs) are still seriously suffering from cost and, especially safety issues [1], due to the use of scarce/high-price elements (e.g., Li, Co) [2] and flammable organic electrolytes [3]. Therefore, the traditional aqueous Pb-acid batteries, despite low energy-density, short-lived and polluting-potential, are still widely adopted in many application scenarios where operational safety and/ or cost are the top priorities [4]. With the rapid development of smart grid and large-scale electrochemical energy storage devices, it becomes urgent to develop aqueous batteries that are simultaneously safe, low-cost, green, long-lasting, and high-performance [5].
Remarkably, iodine (I 2 ) cathode stores electrons through the direct conversion reaction between solid I 2 and soluble I − anions, providing a significant theoretical capacity of 211 mAh g −1 [22,23]. This reaction does not generate irreversible byproduct, thus is highly reversible and virtually passivation-free [24]. Even when the I 2 is etched or reduced by specific component in the electrolytes, the resulting I − species can still contribute capacity by oxidizing back to I 2 in following charge process, thanks to its high solubility and proper redox potential [25,26]. The major problems of this affordable cathode lie on the low electrical conductivity of I 2 , as well as the formation of soluble triiodide (i.e., I 3 − ) intermediate species via I 2 /I − complexing (I 2 + I − → I 3 − ) [27,28]. The I 3 − dissolving in electrolyte can easily penetrate through routine glass fibers (GFs) [29] or polypropylene [30] separators, and quickly react with the metallic Zn anode (by I 3 − + Zn → Zn 2+ + I − ), leading to fast I 2 loss and self-discharge [23,31].
To restrain the free migration of I 3 − , the I 2 active materials are usually confined into porous matrix with high adsorption capability (e.g., active carbon [28][29][30]32] and MXene [33]) and even electrocatalytic ability (Co/Fe-hexacyanoferrate [27]). The nano-pore confining design restrains both I 3 − generation and migration, leading to much-improved cycle and shelf life [27,29]. At the same time, the AC (active carbon) and MXene matrix contribute not only additional capacity, but also prominent electrical conductivity, ensuring high I 2 -utilizing efficiencies even at high-rate charge/discharge [28]. Moreover, manipulating the electrolytes with novel zinc salts [29] or immobile anionic gelatinizing skeletons [34] prove also effective to suppress the I 3 − -shuttling from cathode to anode, by means of modulating either coordination [29] or electrostatic repulsion between I 3 − and the electrolyte [34], respectively. In the well-established Zn||I 2 flow batteries, the famous nafion cation-exchange membranes, as benchmark commercial separators, are usually employed to suppress the crossover migration of I 3 − anions [35]. Unfortunately, nafion membranes are, currently, too expensive to maintain the cost competitiveness of the Zn||I 2 batteries, even being robust and durable. To circumvent this dilemma, Zhou's group invented an artful ionic-sieve membrane separator based on Zn-BTC metal organic framework (MOF) [23]. This MOF separator can block not only I 3 − shuttling, but also parasitic reactions by regulating the electrolyte solvation structure. In Zn||I 2 batteries, almost all the parasitic reactions terminate on the surfaces of Zn anodes [16,17]. Therefore, modifying Zn anode's surface with protecting coatings should be one of the most straightforward and thorough approaches to synchronously restrain these detrimental processes [13,36,37]. An adequate protecting coating can virtually isolate Zn anodes from the aqueous electrolytes [38], effectively suppressing Zn corrosion/passivation, H 2 evolution and electrolyte consumption that associate with the reaction between Zn and electrolyte [16,39], as well as the quick self-discharge caused by reactions between I 3 − and Zn [40]. It means that  [23]. In other words, the coating should have a strong cation-exchange ability, in order to simultaneously achieve excellent charge/discharge performance and long shelf-life (i.e., low self-discharge rate) [22]. This is also the exact reason why nafion and MOF membrane separators have been used in Zn||I 2 batteries [23,35].
In this contribution, we propose a high performance and low-cost rechargeable Zn||I 2 secondary batteries with cheap aqueous ZnSO 4 electrolytes, by protecting the Zn anode with a zeolite-based cation-exchange coating. Zeolite, famous as the oldest molecular sieve, is a series of important inorganic microporous minerals featured with high cationic conductivity, low electronic conductivity and excellent stability, thanks to its unique aluminosilicate open framework [41]. In these materials, the replacement of some [SiO 4 ] tetrahedra by [AlO 4 ] imposes cavities and negative charges to the lattice framework, allowing the accommodation of mobile cations (such as zinc, lead, and cadmium) in the cavities. At the same time, the negatively-charged cavities can electrostatically forbid anions to pass through, achieving a precious cation-exchange ability at very low cost [42]. By simply coating a zeolite-based layer on Zn anode, the obtained Zeolite-Zn||I 2 batteries simultaneously achieved large capacity (196 mAh g −1 at 0.2 A g −1 ), high coulombic efficiencies (99.76 and 98.53% in average at 2 and 0.2 A g −1 , respectively), excellent cycling durability (91.92% capacity retention after 5600 cycles at 2 A g −1 , capacity decay rate: 0.0016% per cycle), and long shelf life (83% capacity retention after 50 h static resting). Compared with currently available strategies, this approach shows outstanding advantages in cost and environmental benignity, while delivering comparable performance. Moreover, density functional theory calculation suggests that microstructural optimization may be able to further improve the effectiveness of this strategy.

Preparation of Zn-Based Zeolite and Zn-Based Zeolite Coated Zn Foil (Zeolite-Zn)
The commercial artificial zeolite (average size: ~ 10 μm) was provided by Shanghai Aladdin Bio-Chem Co., Ltd. XRD assessment indicates the powder is a mixture of the FAU framework (JCPDS No. 38-0241) and ETR framework type zeolite (JCPDS No. 71-1557, Fig. S1). To prepare the Zn 2+ -exchanged zeolite, 1 g pristine zeolite was added into 80 mL deionized (DI) water containing 0.5 g ZnSO 4 (corresponding to a concentration of 1 M, AR grade, Aladdin Bio-Chem Co., Ltd), and mechanically stirred for 6 h. After thoroughly washing and drying, the Zn 2+ -exchanged zeolite powder was mixed with polyvinylidene difluoride (PVDF, weight ratio: 8:2, as binder) in proper amount of N-methyl pyrrolidone (NMP) solvent by grinding. The resulting slurry was uniformly coated on bare Zn foils and dried at 60 ℃ overnight. The zeolite/PVDF coated Zn samples were named as Zeolite-Zn below.

Preparation of the I2@AC Composite
The I 2 @AC composite cathode was prepared via an I 2 sublimation method [28,29]. Briefly, 0.5 g I 2 and 0.5 g activated carbon (AC) were thoroughly mixed by grinding. Afterward, the mixed powder was sealed in a hydrothermal reactor and heated at 90 ℃ for 4 h. During heating, the I 2 was thermally sublimated and infused into the pores of the activated carbon. After natural cooling, the porous carbon enveloped I 2 (I 2 @AC) composite was obtained (Fig. S2).

Preparation of I 2 @AC Cathode Electrode
The cathode coating slurry was fabricated by mixing the I 2 @ AC powders, acetylene black (conductive agent) and polyvinylidene difluoride (PVDF, binder) at a weight ratio of 7:2:1 with proper amount of N-methyl pyrrolidone (NMP) as solvent. The resulting slurry was uniformly coated on graphite paper (GP, current collector) and dried at 40 ℃ for 12 h. The I 2 @AC-coated GP was cut into Φ16 mm discs and used as cathodes for further Zn||I 2 battery assembly. To clarify whether or not the I 2 in the I 2 @AC can be dissolved by the NMP solvent, we collected the TG curves of the dried slurry in a dynamic nitrogen atmosphere within 30-800 °C (Fig. S3). Up to 300 °C, the dried slurry demonstrates a I 2 sublimation weight loss of 33.62%, in good line with the theoretical value of 35.00% (I 2 @AC: acetylene black: PVDF weight ratio of 7:2:1, 50% I 2 in the I 2 @AC powder). This TG curve evidently confirms the survival of the I 2 from dissolution during electrode preparing process, thanks to the strong interactions between the AC matrix and the infused I 2 (see Fig. S2e, pay attention that the I 2 transformed from crystalline into amorphous after loading into the AC matrix). Above 300 °C, the 8.94% weight loss between 310 and 500 ℃ can be attributed to the decomposition of PVDF [43]. We also prepared pure AC cathodes following the same preparation process, in order to determine the capacity contribution from the AC component.

Zn||Zn Symmetrical Cell Assembly
Bare-and Zeolite-Zn foils (50 μm in thickness) were firstly cut into Φ16 mm discs. Then, two Zn discs (bare-or Zeolite-Zn) were separated by a glass fiber paper (Φ = 19 mm) thoroughly wetted by 1 M ZnSO 4 electrolyte, and then packed into a 2032-type button cell. To standardize the measurement, a fixed amount (120 μL) of electrolyte was used in each coin cell. All batteries were assembled in ambient air atmosphere.

Zn||I 2 Battery Assembly
The Zn||I 2 batteries were assembled with the GP-supporting I 2 @AC discs as cathodes, glass fiber papers as separators, and Zeolite-Zn (or bare-Zn) discs as anodes, in a form of CR2032 coin cell. The assembling processes were completed in ambient air atmosphere, with a 1 M ZnSO 4 aqueous electrolyte.

Material Characterization
X-ray powder diffraction (XRD) was carried out on a D/ max-2500/PC X-ray diffractometer with Cu Kα radiation (λ = 0.15418 nm). The surface morphology and element mapping of the samples were characterized by a JEOL JSM-7610F field emission scanning electron microscope (SEM) equipped with an energy-dispersive spectroscope (EDS). The electrolyte/anode contact angles were measured by an optical contact angle and interface tension meter (CA, CA100C, Innuo, China) at room temperature in air, and a 10 μL droplet of 1 M ZnSO 4 electrolyte was used in the experiment. Thermogravimetry analysis (TGA) was carried out on a Shimadzu TGA-60 analyzer, within 30-700 °C in a dynamic nitrogen atmosphere (100 mL min −1 ). In this test, the heating rate is fixed to 10 °C min −1 . The UV absorption spectra of different electrolytes were measured on a Persee TU-1810 UV-Vis spectrophotometer. The Brunauer-Emmett-Teller (BET) specific surface areas of the AC and I 2 @AC powders were determined by N 2 sorption method on an ASAP 2460 physical adsorption analyzer (Micromeritics, America) at 77.3 K. Before BET testing, the samples were outgassed in a vacuum at 120 °C for 2 h. The pore size distributions were derived from the adsorption branch using the Barrett-Joyner-Halenda (BJH) model.

Electrochemical Measurements
Galvanostatic charge-discharge (GCD), Coulombic efficiency (CE) and rate capability tests of all cells were performed on a LAND-CT2001A battery-testing instrument. The Zn||Zn symmetrical cells were GCD cycled at a current density of 2.5 mA cm −2 with an areal capacity of 2.5 and 10 mAh cm −2 (corresponding to 10 and 40% depth of discharge, DOD, respectively). Bare-Zn||bare-Cu and Zeolite-Zn||Zeolite-Cu cells were also assembled to explore the influence of the zeolite-based coating on Coulombic efficiency (CE, the ratio of Zn stripping capacity to plating capacity). The Zn||I 2 battery was cycled in a GCD manner between 0.5 and 1.6 V at either 0.2 or 2 A g −1 (1C = 211 mAh g −1 ). Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), potentiostatic polarization and Tafel curves were collected on a CHI-660E electrochemical workstation in a two-electrode configuration. The EIS spectra were collected within a frequency range of 10 -2 -10 6 Hz under a bias of 10 mV (vs. Zn/Zn 2+ ). To determine the specific ionic conductivity contributed by Zn 2+ ion, a potentiostatic polarization method was utilized with a 10 mV bias applied to Zeolite-Zn/Zeolite-Zn and Zn/Zn symmetric cells. In the Tafel test, a Zn foil was used as the working electrode, while another Zn foil as the counter/reference electrode.

Computational Methods
The positions occupied by atoms in the crystal structure of the substances in this study were generated by the Supercell Program [44]. For fractional occupation as well as fractional occupation structures, the ten structures with the lowest Coulomb energy were obtained by random sampling and Coulomb energy calculation. And the substances were screened using the original cell structure; the experimentally 1 3 obtained lattice parameters were used to construct the solidstate electrolyte structures. All first-principles calculations are performed with the projector augmented wave (PAW) potential [45,46] and the Vienna Ab initio Simulation Package (VASP) [47]. The structures screened by Supercell Program were subjected to structural relaxation using VASP software to take the lowest energy structures. Structural relaxation was achieved with a total energy of 10 -5 eV and a force of 0.01 eV Å −1 as convergence criteria. The truncation energy is uniformly set to 520 eV during the calculation.

Characterization of the Zeolite-Based Protecting Layers
Before coating, the pristine zeolite powder underwent a Zn 2+ -exchange treatment in 1 M ZnSO 4 for 6 h, in order to provide immediate Zn 2+ supply for smooth Zn stripping/plating ( Fig. S1a-b) [35,48]. Afterward, the zeolite-based protecting layers were deposited on Zn foils via a simple knife coating method, with a blend slurry of the Zn 2+ -exchanged zeolite and PVDF binder (weight ratio: 8:2) in NMP solvent. As revealed by SEM observation, the layers are very dense in the vertical direction, without any detectable penetrating holes/cracks ( Fig. 1a and S4a-b), even exhibiting a rough top surface (Fig. S4c-d). The large surface roughness [49,50] and the hydrophobic nature of the PVDF binder [51] endow the zeolite-Zn a much larger contact angle (CA) than the bare Zn foil (113° vs. 76°, Fig. 1b), very favorable to isolate the underneath Zn foil from the corrosive aqueous electrolyte. As a result, the corrosion potential of the Zn foils shifts from − 17 to − 15 mV after zeolite-layer coating, along with a lower corrosion current (from 62 to 49.7 μA, Fig. 1c), indicating a suppressed corrosion rate on the Zeolite-Zn foils [13,52].
To further confirm the anticorrosive ability of the protecting layer, a static self-corrosion experiment was performed by soaking a bare-and Zeolite-Zn foil in 1 M ZnSO 4 electrolytes for 15 days. Postmortem XRD, Raman and SEM analyses (Figs. 1d-e and S5a-c) clearly show the formation of Zn 4 (OH) 6 SO 4 ·5H 2 O and even ZnO on the surface of the soaked bare-Zn [15,16,23]. On the other hand, the generation of corrosion products and the surficial morphology evolution of the underneath Zn are remarkably suppressed by the zeolite-based protecting layer (Fig. S5d-f), suggesting a slow corrosion rate. Probably, the anticorrosive ability of the layer stems from not only the physical isolation of electrolyte from Zn anodes, but also the desolvation of hydrate Zn 2+ by the zeolite cavities. As revealed by first principles calculation (Fig. S6), the lowest migration barriers of bare and hydrate Zn 2+ in the FAU-/ETR-type zeolites are determined to be 0.0086/0.3821 and 0.5002/1.4648 eV, respectively, indicating the much higher migration difficulty of hydrate Zn 2+ than its bare counterparts. Therefore, the pores of FAU and ETR zeolite are helpful to remove the solvation sheath of hydrate Zn 2+ , due to the very different migration barriers between the bare and hydrate Zn 2+ .
Based on theoretical calculation, we also investigate the transport behaviors of I 3 − ions within the zeolite lattice. The migration barriers of I 3 − are 0.59/1.60 eV in FAU-/ETRtype zeolite, respectively (Fig. S7), much higher than those of the bare and hydrate Zn 2+ . The even harder migration of I 3 − indicates that the zeolite layer can not only protect Zn from water-induced corrosion, but also suppress the quick self-discharge caused by I 3 − shuttling. The quite different migration barriers in different zeolites further highlight the significant influence of zeolites' structure on cation sieving and Zn 2+ desolvation. This topic is worthy for additional in-depth study.
To experimentally examine the I 3 − -blocking ability of the zeolite-based coating, we specifically designed a spectrophotometry test by making good use of the characteristic optical absorption (centered at 288 and 354 nm) and intense brown color of this anion [25]. Firstly, I 3 − -pregnant ZnSO 4 electrolyte (with 0.5 mM KI + 0.5 mM I 2 ) was employed to simulate the electrolyte in real Zn||I 2 batteries [23]. Then, identically-sized Zeolite-Zn and bare-Zn foils were separately immersed into the I 3 − -pregnant solution. After 36 h, the brown color of the bare-Zn soaking solution, along with the absorption bands of I 3 − , had thoroughly faded, suggesting the completely reduction of I 3 − by metallic Zn (Fig. 1f). In contrast, considerable amount of I 3 − survived in the Zeolite-Zn soaking solution, as revealed by the brown color and the relatively strong residual absorption bands of I 3 − (Fig. 1g), indicating the remarkable I 3 − -blocking ability of the zeolite protecting layer. Moreover, the I 3 − -blocking ability of the zeolite layer can also be confirmed by another visually-monitored shuttling experiment, as shown in Fig. 1h [53]. In the H-shape quartz container, the left and right chambers are filled with brown I 3 − (0.1 M KI and 0.1 M I 2 ) and colorless ZnSO 4 solutions, respectively. These chambers are severed by either a bare or a zeolite-coated commercial Celgard separator. With a bare Celgard separator, the colorless ZnSO 4 solution in the joint-neck position of the left chambers turns to light yellow after only 10 min. Afterward, the color gradually deepens into orange-brown and occupies the whole upper layer of the solution, due to the fast penetration of I 3 − through the porous separator [23]. In striking contrast, the chamber isolated by the zeolite-coated Celgard separator keeps nearly colorless after even 20 h, thanks to the suppression of I 3 − -shuttling by the protecting layer.
Besides high anticorrosive and I 3 − -blocking performance, an adequate Zn-anode protecting layer needs also a high Zn 2+ ionic conductivity to ensure smooth Zn striping/ plating on the anodes. To determine this property, a freestanding zeolite layer is sandwiched between two stainless steel current collectors after electrolyte wetting. With the thickness and EIS data (electrochemical impedance spectra, Fig. 2a  It is worth noting that, the EIS-deduced ionic conductivity is a total value contributed by all the charge carriers, including both cations and anions [54]. To determine the specific conductivity contribution by Zn 2+ , the transference numbers of Zn 2+ (TZn 2+ ) were tested by combining potentiostatic polarization and EIS measurements (Fig. 2b-c) [48,55]. In the potentiostatic polarization test, the bias voltage (ΔV = 10 mV) stimulates a large initial current (I 0 ) at the beginning, which gradually decreases to a smaller steady-state current (I ss ) due to the establishment of stable concentration polarization in the vicinity of the electrode. With the assistance of EIS-determined charge transfer impedances (R 0 , R ss , Fig. S8), the Zn 2+ transference number (TZn 2+ ) of the protecting layer can be calculated following Eq. (2) [56]: Compared to the Zn||Zn symmetric cell, the Zeolite-Zn||Zeolite-Zn cell is small in both I 0 (23.7 vs. 116.9 μA) and I ss (13.5 vs. 17.0 μA, Fig. 2b), because of the suppression of anion-contributed ionic conductivity [54]. As a result, the T Zn 2+ of the symmetric cells is significantly improved from 0.15 to 0.53 by the zeolite layer. The simultaneous achievement of high ionic conductivity and large Zn 2+ transference number indicate that the zeolite layer is well-performed in both Zn 2+ -conducting and I 3 − -blocking [48]. In addition, the layer's excellent anti-corrosive ability awards the Zeolite-Zn electrodes high striping/plating reversibility, as shown by their higher Columbic efficiency (99.1% vs. 93.9% in average) and much longer cycling lifetime (200 vs. 29 cycles) than the Zn electrodes (Figs. 2d and S9). The dramatically improved striping/plating stability of the Zeolite-Zn electrodes can also be confirmed by the galvanostatic charge/discharge (GCD) cycling test. As shown in Fig. 2e, the Zn||Zn symmetric cell failed after ~ 140 h during test with stepwise increasing current densities. While the Zeolite-Zn cell easily passed through the 200 h test, with only a minor increase in overpotential from 67.0 to 94.8 mV (Fig. S10), thanks to the high ionic conductivity of the protecting layer. At a relatively large DOD (depth of discharge) of 10% (Eq. S1), the zeolite-based protecting layer dramatically prolongs the lifetime of the symmetric cells by more than 7 times (from 64 to 460 h), while remaining the overpotentials comparable (Figs. 2f and S11). Fitting results of bare-and Zeolite-Zn electrodes' EIS plots indicate that the Zeolite-based coating slightly elevates the electrode's equivalent resistance (R s ) from 1.41 to 2.32 Ω cm −2 , due to electric insulation of the layer (Fig. S12, Table S1). At the same time, the coating also reduced the charge-transfer resistance (R ct ) from 157.26 to 140.50 Ω cm −2 , probably because of the down-sized Zn nuclei and the modified Zn striping/plating kinetics [14,52]. The competition between enlarged R s and reduced R ct results in the comparable overpotentials. At even an ultrahigh DOD of 40%, the lifetime of the Zeolite-Zn cell is still overwhelmingly longer than its bare-Zn counterpart (280 vs. 48 h, Fig. S13). The capability of cycling at high current density and large DOD is crucial for the achievement of high power and energy outputs.
To explore the reason why the Zeolite-Zn electrodes are more long-lasting, the cycled bare-and Zeolite-Zn electrodes were disassembled from the asymmetric cells, and thoroughly rinsed to remove the attached electrolyte salt and GF separator. The fragmentary and holey edge of the cycled bare-Zn electrode, along with its rough concertina-like surficial morphology, evidently reveal the dynamic redistribution of Zn caused by the uneven dendritic deposition (Fig. 3a-c and S14a-b) [14]. In addition, the repeated striping processes may break some Zn dendrites at their roots, resulting in many isolated "dead" Zn debris losing capacity contribution (Fig. S15) [57,58]. During cyclic test, the accumulation of both Zn dendrites and debris on the bare Zn electrode pierce though batteries' separators and provoke short-circuit failure [57,59]. In contrast, the cycled Zeolite-Zn electrode remains a decent shape integrity, as well as a compact surficial morphology without any detectable holes or isolated fragments (Fig. 3d-f and S15c-d). In fact, the zeolite/PVDF composite protecting layers should be able to profoundly modify Zn striping/plating behaviors through multiple mechanisms [14], including uniformizing Zn 2+ flux, increasing deposition nuclei density, downsizing deposits' dimension, confining deposition position and even dielectric effect [60,61]. Furthermore, the zeolite-based coating can also suppress the electrolyte-induced corrosion and surface passivation [16,17], as evidenced by the weak XRD and Raman signals of the corrosion products (e.g., ZnO, Zn(OH) 2 , and Zn 4 (OH) 6 SO 4 ·H 2 O, Fig. 3g-h). In short, the multifunctional Zeolite-Zn electrodes perform simultaneously well in anticorrosion, I 3 − -blocking, Zn 2+ -conducting, and dendrite suppression (Fig. 3i).

Electrochemical Performance of Zn||I 2 Batteries
The outstanding performance of the zeolite-based protecting layers encourages us to further explore their influence on Zn||I 2 full batteries. As shown in Fig. 4a-b, the battery with a Zeolite-Zn anode delivers a high capacity of 195 mAh g −1 at 0.1 A g −1 , along with a very small polarization voltage of 54 mV (Fig. S16) (i.e., voltage difference between charge and discharge plateaus), thanks to the high ionic conductivity of both the protecting layer and aqueous electrolyte. At an ultrahigh current density of 5 A g −1 , the capacity still reaches up to 125.4 mAh g −1 , indicating an excellent rate capability ( Fig. 4a-b). Pay attention that the capacities of the I 2 @AC cathodes are based on the weight of I 2 , considering the low-capacity attribution from the AC component (~ 8% at 0.2 A g −1 , Figs. S17 and 4c). This capacity is convenient to reflect the utilizing efficiency of the I 2 redox reactions, and therefore is widely adopted by different researchers [28,29,62]. In fact, the capacity contribution from AC component should be even lower than the tested value, due to I 2 loading and coverage. Figure 4c-d further depicts influence of the zeolite protecting layer on the capacities and CEs of Zn||I 2 full battery. At 0.2 A g −1 , the bare-Zn anode battery delivers a decent and stable capacity of ~ 178.1 mAh g −1 , thanks to the excessmass design of the Zn anode [13,63]. However, this battery failed after only 104 cycles (Fig. S18a-b), most possibly 1 3 because of the dendrite-induced short-circuit failure. Even before failure, the battery had already demonstrated very low CEs (85.9% in average), due to the parasitic reactions including I 3 − shuttling and Zn corrosion/passivation. On the other hand, the battery with a Zeolite-Zn anode achieves a higher initial capacity of 203.0 mAh g −1 (Fig. 4c), which  Fig. 4d). Since I 3 − shuttling is the main reason accounting for self-discharge of Zn||I 2 batteries, the I 3 − -blocking protecting layer should also be able to improve shelf life of the batteries. As exhibited in Fig. 4e, the bare-Zn battery loses 12.2% and 49.1% of its capacity after 10 and 50 h opencircuit resting, respectively, due to the fast consumption of the shuttling I 3 − by the metallic Zn anode [23]. On the contrary, the Zeolite-Zn battery loss only 17.0% of its initial capacity after 50 h resting, indicating a nearly 3 times slower self-discharge rate (Fig. 4f). At a high rate of 2 A g −1 , the Zeolite-Zn battery demonstrates extraordinary cycling stability (91.92% capacity retention after 5600 cycles) and CEs (99.76% in average, Fig. 4g), corresponding to an extremely slow capacity decay rate of 0.0016% per cycle, whereas the bare-Zn battery delivers not only low CEs (97.09% in average), but also short cycling lifetime (failed at 1015 cycles). The long battery lifetime enabled by the zeolite-based protecting layer has also been readily achieved in the Zn(AC) 2 electrolyte, indicating the excellent reproducibility of this strategy (Fig. S19). The achievement of stable and high areal capacity is another necessary precondition for practical application of battery systems. To highlight the application potential of this strategy, we further constructed a Zeolite-Zn||I 2 full battery with an ultrahigh I 2 mass loading of 13.3 mg cm −2 on the cathode. At a current density of 0.2 A g −1 , this battery achieves an initial specific capacity of 134.2 mAh g −1 (areal capacity: 3.6 mAh, Fig. S20), corresponding to a Zn utilization coefficient of 7.2%. The capacity keeps almost unchanged within a testing period of 950 cycles. We further test the batteries connected in series or in parallel to mimic  Fig. 4 a GCD curves and b rate performance of the Zeolite-Zn||I 2 battery. c Cycling performance and d Coulombic efficiencies (CEs) of the Zn||I 2 batteries with either bare-or Zeolite-Zn at 0.2 A g −1 . e-f Electrochemical aging test (static resting after fully charge state) of Zn||I 2 batteries with either bare-Zn or Zeolite-Zn anode. g Capacity and CE evolution of Zn||I 2 batteries at current density of 2 A g −1 with either bare-Zn or Zeolite-Zn anode 1 3 practical application conditions. At all conditions, the batteries deliver expected capacity and voltage outputs (Fig. S21).

Conclusion
In summary, we develop a high performance, low-cost and intrinsically safe rechargeable Zn||I 2 aqueous batteries, by means of comprehensively suppressing parasitic reactions on the Zn anodes with a zeolite-based cation-exchange protecting layer. On the one hand, the multifunctional zeolite-based layer allows smooth crossover migration of Zn 2+ , which Zn is deposited uniformly and rapidly. One the other hand, zeolite-based cation-exchange protecting layer can effectively block electrolyte and anions from passing through, and effective inhibit dendrite growth, Zn corrosion/passivation, and self-discharge. Thanks to the multifaceted merits of this protecting layer, the resulting Zeolite-Zn||I 2 battery simultaneously achieves a high capacity (203-196 mAh g −1 at 0.2 A g −1 ), a high CE (99.76% in average at 2 A g −1 ), a long-term cycling stability (91.92% capacity retention after 5600 cycles at 2 A g −1 ). This work provides a new approach for the achievement of high-performance aqueous Zn||I 2 batteries.