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Nano-Micro Lett. (2022) 14:82 the coating should be able to selectively block I3− while smoothly conducting Zn2+ [23]. In other words, the coat- ing should have a strong cation-exchange ability, in order to simultaneously achieve excellent charge/discharge perfor- mance 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||I2 batteries [23, 35]. In this contribution, we propose a high performance and low-cost rechargeable Zn||I2 secondary batteries with cheap aqueous ZnSO4 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 conduc- tivity, low electronic conductivity and excellent stability, thanks to its unique aluminosilicate open framework [41]. In these materials, the replacement of some [SiO4] tetra- hedra by [AlO4] 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, achiev- ing a precious cation-exchange ability at very low cost [42]. By simply coating a zeolite-based layer on Zn anode, the obtained Zeolite-Zn||I2 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 reten- tion after 50 h static resting). Compared with currently avail- able strategies, this approach shows outstanding advantages in cost and environmental benignity, while delivering com- parable performance. Moreover, density functional theory calculation suggests that microstructural optimization may be able to further improve the effectiveness of this strategy. 2 Experimental and Caculation 2.1 Materials Preparation and Device Assembly 2.1.1 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 Page 3 of 13 82 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 Zn2+-exchanged zeolite, 1 g pristine zeolite was added into 80 mL deionized (DI) water containing 0.5 g ZnSO4 (cor- responding 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 Zn2+-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 °C overnight. The zeolite/PVDF coated Zn samples were named as Zeolite-Zn below. 2.1.2 Preparation of the I2@AC Composite The I2@AC composite cathode was prepared via an I2 subli- mation method [28, 29]. Briefly, 0.5 g I2 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 °C for 4 h. During heating, the I2 was thermally sublimated and infused into the pores of the activated car- bon. After natural cooling, the porous carbon enveloped I (I @AC) composite was obtained (Fig. S2). 2 2 2.1.3 Preparation of I2@AC Cathode Electrode The cathode coating slurry was fabricated by mixing the I2@ AC powders, acetylene black (conductive agent) and poly- vinylidene 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 °C for 12 h. The I2@AC-coated GP was cut into Φ16 mm discs and used as cathodes for further Zn||I2 battery assembly. To clarify whether or not the I2 in the I2@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 I2 sublimation weight loss of 33.62%, in good line with the theoretical value of 35.00% (I2@AC: acetylene black: PVDF weight ratio of 7:2:1, 50% I2 in the I2@AC powder). This TG curve evidently confirms the survival of the I2 from dis- solution during electrode preparing process, thanks to the 13PDF Image | Boosting Zn Battery by Coating a Zeolite‐Based Cation‐Exchange
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