Carboxymethyl Chitosan‐Modified Zinc Anode for High‐Performance Zinc–Iodine Battery with Narrow Operating Voltage

Reasonable regulation of iodine redox has gradually shown potential as a desirable cathodic reaction in zinc‐based batteries, but suffers from poor cyclic reversibility caused by uncontrollable side reactions. Also, the irregular growth of dendrites and unavoidable occurrences of hydrogen evolution reaction in H2O‐rich environment have become permanent topics in anodic zinc. Herein, a cross‐linked gel based on carboxymethyl chitosan is proposed and serves as an artificial electrolyte interphase for zinc anode (marked as Zn‐CMCS). Such a coating formed by crosslinking among a monodentate carboxyl group, a hydroxyl, an amino, and Zn2+ from adding solution closely adheres on the surface of the zinc foil with toughness, ductility, and ideal electrochemical kinetics. Additionally, its homogenized surface charge distribution provides a “flexible” substrate for zinc plating/stripping, resulting in a flat real‐time interface. While introducing I−/I0 conversion by matching adsorptive activated carbon on carbon fiber cloth (AC‐CFC) as cathode, the internal space restricted by CMCS gel enables the assembled Zn‐CMCS/AC‐CFC battery to exhibit a greatly improved reversibility under long‐cycling condition within 28 000 cycles (measured for more than 2 years) in a narrow operating voltage range of 0.23 V.


Introduction
Under the rapid development of the energy storage field, zinc-based batteries with ideal theoretical capacity, cost efficiency, and ecofriendly nature are now the emerging devices of energy storage. [1]Iodine redox, as a kind of halogen conversion, with reasonable control, has a narrow gap between charge-discharge medium voltage, thus becoming popular cathodic reaction choice for zinc-based batteries recently. [2]owever, high requirement in capacity implies massive production of I 2 , which facilitates the formation of polyiodide ions (e.g., I 3 À , I 5 À , I 7 À ), causing series problems. [3]Their poor reversibility reduces actual energy density, and concentration gradient prompts them migrating to the anodic interface and reacting with Zn 2þ , resulting in self-discharge and poor coulombic efficiency. [4]Accordingly, cathodic modification and electrolyte redesign are considered as the two effective strategies for figuring out shuttle effect caused by side reactions. [5]For the former, the internal pore size modulation or specific element doping can enhance the adsorptive ability of carbon-based cathode. [6]olyaniline is used to inhibit I 2 dissolution due to enhanced electrostatic interaction, while Co[Co 1/4 Fe 3/4 (CN)] 6 , Nb 2 AlT x enables the in situ I À /I 2 redox through reduced reaction energy barrier. [7]As for the latter, eutectic and water-in-salt electrolytes are carried out to emphasize the improvement of reaction reversibility by H 2 O activity inhibition. [2,8]hen it comes to aqueous electrolyte with a rich-H 2 O environment, the issues faced by anodic zinc are even more obvious.Both acidity of solution and higher H þ /H 2 reaction potential promote the hydrogen evolution reaction (HER) on the surface Reasonable regulation of iodine redox has gradually shown potential as a desirable cathodic reaction in zinc-based batteries, but suffers from poor cyclic reversibility caused by uncontrollable side reactions.Also, the irregular growth of dendrites and unavoidable occurrences of hydrogen evolution reaction in H 2 O-rich environment have become permanent topics in anodic zinc.Herein, a cross-linked gel based on carboxymethyl chitosan is proposed and serves as an artificial electrolyte interphase for zinc anode (marked as Zn-CMCS).Such a coating formed by crosslinking among a monodentate carboxyl group, a hydroxyl, an amino, and Zn 2þ from adding solution closely adheres on the surface of the zinc foil with toughness, ductility, and ideal electrochemical kinetics.Additionally, its homogenized surface charge distribution provides a "flexible" substrate for zinc plating/stripping, resulting in a flat real-time interface.While introducing I À /I 0 conversion by matching adsorptive activated carbon on carbon fiber cloth (AC-CFC) as cathode, the internal space restricted by CMCS gel enables the assembled Zn-CMCS/AC-CFC battery to exhibit a greatly improved reversibility under long-cycling condition within 28 000 cycles (measured for more than 2 years) in a narrow operating voltage range of 0.23 V. in zinc plating.Then hydroxide (OH À ), whose relative concentration increases due to continuous H þ consumption, reacts with zinc foil to form Zn(OH) 2 and transforms to "dead" phase ZnO, resulting in passivation. [9]Also, thermodynamics reveals that uneven distribution of energy barriers leads to easy zinc dendrite formation, so electrolyte design and interface regulation around zinc foil are of great significance. [10]Researches show that increasing Zn 2þ concentration can adjust its solvated structure and reduce H 2 O retention on the anodic surface during plating, which promotes the development of water-in-salts, ionic liquids, eutectic solutions with limited H 2 O activity. [2,11]unctionalized additives are also an option, where the introduction of polyethylene glycol, diethyl, etc. homogenizes current density and the ethylene glycol, glucose, etc. enhance reaction kinetics by lowering the desolvated energy barrier of Zn 2þ . [12]n the other hand, Zn-based compounds with specific composition (e.g., In, Al) exhibit higher HER; a uniform plating is achieved in Zn|In due to good affinity between them. [13]oating with high electronic insulation (e.g., ZnF 2 , ZnO) and efficient ion transport path can suppress the HER while ensuring the stability of plating, by weakening activity and the number of H 2 O on the surface. [14]Carbon-based or hydrophilic-containing organic composite with certain thickness also serves as an artificial electrolyte interphase between anode and separator, of which the strong electronic conductivity uniformizes the electric field as well as the ideal mechanical strength provides a buffering effect. [15]These works highlight the importance of limiting side reactions on the anodic surface for achieving a stable zinc plating-stripping process.
In view of the problems mentioned in zinc anodic interface, an improvement strategy based on carboxymethyl chitosan (CMCS) crosslinked gel as artificial electrolyte interface on zinc anode is proposed.Such a gel coating on zinc foil (marked as Zn-CMCS) is formed by the crosslinked reaction among a monodentate carboxyl group, a hydroxyl, an amino, and Zn 2þ from adding ZnSO 4 solution, acting as a "flexible" medium with ideal electrochemical kinetics.Its homogenized surface charge distribution provides a buffer substrate for zinc plating/stripping, thus obtaining a smoother interface throughout the electrochemical process.While matching iodine-containing electrolyte (2 M ZnSO 4 þ 0.5 M KI) with adsorptive cathode (AC-CFC), the Zn-CMCS/AC-CFC battery exhibits an excellent long-term cycling stability of single-trunk coulombic efficiency close to 100% within 28 000 cycles (measured for more than 2 years) at 5 mA cm À2 in a narrow operating voltage range of 0.23 V.

Results and Discussion
The Zn-CMCS is prepared through a thorough crosslinking reaction at ambient temperature by pouring 1 M ZnSO 4 aqueous solution onto CMCS solution which is spread on the surface of zinc foil (See Supporting Information for the details and the optical images of evolution are shown in Figure S1, Supporting Information).According to the crosslinking mechanism, the polymer chain forms a multidimensional network by the ionic or covalent bonds with the "mediator." [16]The bonding potential between the empty orbital of Zn 2þ in ZnSO 4 and NH 2 , OH, COO À with abundant lone pair of electrons in the CMCS solution facilitates the formation of artificial solid electrolyte interphase (SEI) on the surface of zinc foil (Figure 1a).Raman spectra is used to initially observe the triggering of the crosslinking reaction, in which the symmetric S-O stretching vibration of the sulfate group with Zn 2þ (V 1 (SO 4 2À ) with Zn 2þ ) appearing at 980 cm À1 indicates the capture and association of Zn 2þ and SO 4 2À during the gelation of CMCS. [17]The inheritance of V as (O-H) (3217 cm À1 ) and V s (O-H) (3429 cm À1 ) also reveals the higher content and activity of water molecules in CMCS gel compared with CMCS solution (Figure 1b, S2, Supporting Information).The specific interactions of each group in the crosslinked structure are analyzed by Fourier-transform infrared (FTIR) spectra.In general, the free SO 4 2À exhibits four fundamental vibrations, which are distinguished as nondegenerate (V 1 , 980 cm À1 ), doubly degenerate (V 2 , 455 cm À1 ), and triply degenerate (V 3 , 1119 cm À1 and V 4 , 622 cm À1 ) vibrations.Among then, only a small bulge of V 3 (SO 4 2À ) is observed in 1 M ZnSO 4 , and the increased intensity and blueshift of the peak at 1619 cm À1 corresponding to the bending vibration of H 2 O reflects their extremely high activity in dilute solution (Figure S3, Supporting Information). [18]hen, the FTIR spectra of CMCS gels treated with different concentrations of ZnSO 4 solutions (denoted as CMCS-0.5,CMCS-1.0, and CMCS-2.0)are carried out to investigate the complexation of CMCS and Zn 2þ .As the concentration of the solution used for treatment increases, the peaks of CMCS solution at 1070 (secondary stretching of O-H), 1640 (asymmetrical stretching of COOÀ), and 3387 (stretching of O-H, N-H) cm À1 are gradually redshifted in CMCS-0.5 and CMCS-1.0,indicating the interaction of ÀCOOH, ÀOH, ÀNH 2 groups with Zn 2þ . [19]lso, the peaks at 603, 982 cm À1 matching V 4 (SO 4 2À ), V 1 (SO 4 2À ) are detected, further verifying the presence of SO 4 2À in gel.Interestingly, the peak of COOÀ symmetrical stretching reappears and blueshifts in CMCS-2.0,combined with the V (O-H, N-H) at 3197 cm À1 , demonstrating that the ÀCOOH group replaces ÀNH 2 for bidentate chelation with most of the Zn 2þ (Figure 1c). [20]Thereby, the complex structures of Zn 2þ and CMCS chains are proposed, in which Zn 2þ in CMCS-0.5,CMCS-1.0 is coordinated with a monodentate carboxyl group, a hydroxyl, and an amino.While most of Zn 2þ in CMCS-2.0 is coordinated with bidentate carboxyl group through oxygen tetrahedron, it displays a similar structure as zinc acetate (Figure S4, Supporting Information). [21]ased on these, the CMCS-1.0gel with more suitable Zn 2þ chemical environment during the cross-linking reaction is taken as the object of further investigation, and the zinc foil coated by it is denoted as Zn-CMCS.
The situation of gel coating on the surface of Zn-CMCS is performed, in which the CMCS gel uniformly covers the zinc foil (about 100 μm in thickness) with thickness of 41 μm and forms fine ripples due to inevitable surface tension (Figure 1d, S5, Supporting Information).The uniform distribution of Zn 2þ on the CMCS has been proved now according to the energy dispersive X-ray spectroscopy results of the CMCS coating film (Figure S6, Supporting Information), and the thermogravimitry analysis analysis of CMCS indicates a 10.36% of water content (Figure S7, Supporting Information).The mechanical strength of the coating allows it to be peeled from Zn-CMCS, and the optical images further reflect its near-transparent macroscopic morphology (Figure S8, Supporting Information), and the CMCS coating was flexible (Figure S9, Supporting Information).On the other hand, the contact angles of the two underpure aqueous solutions are employed to detect their surface wettability, which decreases to 10°for Zn-CMCS compared with 101°for bare Zn, heralding the enhanced capture ability of water molecules by the CMCS gel coating (Figure 1e).
The effects of CMCS coating on the interface condition between electrolyte and anode were first considered.Zn 2þ transference number (t Zn 2þ ) in 2 M ZnSO 4 þ 0.5 M KI was carried out by chronoamperometry (CA) curves at 100 mV with electrochemical impedance spectroscopy (EIS) before and after polarization of symmetric cells, which was 0.72 and 0.74 in Zn-CMCS and bare Zn, respectively (Figure 2a, S10, Supporting Information).It is indicated that both reduced interior space and interface status change caused by the coating exhibit limited decay in electrochemical properties of ions and electrons. [22]hen, zinc plating/stripping on the anode was further characterized, in which in the equilibrium potential (53.7 vs. 74.8mV) and corrosion current (12.4 vs. 45.2 mA) of Zn-CMCS were revealed by Tafel curves, indicating the step-down rate and degree of corrosion reaction due to coating (Figure 2b, S11, Supporting Information).This phenomenon was ubiquitous in the anodic gel coating attributed to the shielding effect brought about by a regular and compact network in a crosslinked structure. [23]or this reason, a wider electrochemical stability window of Zn-CMCS (3.06 V vs. 2.56 V in bare Zn) was marked in linear sweep voltammetry (LSV) curves, and the activity of HER was suppressed according to its slowing slope (Figure 2c).On the other hand, Zn/Cu half cell with bare zinc or Zn-CMCS (15 mm in diameter) and Cu foil (12 mm in diameter) were assembled to evaluate the efficiency of the zinc plating/stripping process.Coulombic efficiencies of around 80% in both initial and coated anodes gradually increased due to the infiltrated activation, where Zn-CMCS with enhanced hydrophilia was more efficient (98% vs. 96% of bare Zn in the tenth cycle) (Figure 2d).Notably, cells with bare Zn showed obvious significant vibration in coulombic efficiency after 26 cycles, while the stable operation with smooth galvanostatic voltage curve over 180 cycles collected in Zn-CMCS revealed its durability (Figure S12, Supporting Information).Symmetric cells under galvanostatic condition were further measured; both had ideal rate capabilities in the range between 0.25 to 2.5 mA cm À2 with a weakened polarization in Zn-CMCS (72.1 vs. 87.3mV in bare Zn) (Figure 2e).In the case of long-term cycling at 1.25 mA cm À2 , a drastic increase in overpotential (from 81.9 mV to 108.6 mV) of Zn/Zn cell was observed within 200 h.After replacing bare Zn with Zn-CMCS, an overpotential of 42.2 mV was obtained which exhibited nearly constant potential within 1300 h (Figure 2f ).Moreover, a similar infiltrated activation was captured in bare Zn at low current density of 0.25 mA cm À2 , which took about 40 h (Figure S13, Supporting Information).
In situ optical microscope observation provides a visual view of interfaces in bare Zn and Zn-CMCS with 2 M ZnSO 4 þ 0.5 M KI during zinc plating.From Zn-CMCS, a direct deposition of Zn on the base (Zn foil) rather than flexible deposition on  CMCS coating was observed, with a uniform interface (Figure 3a).It is worth noting that due to the high transparency of CMCS coating, it cannot be directly observed under an optical microscope and can be verified by further experiments (Figure S14, Supporting Information).In contrast to bare Zn, a bulky galvanized coating was detected from 2 h and continued to expand thereafter, which was attributed to the sensitive electric field differential-induced preferred orientation (Figure 3b). [24]In addition, scanning electron microscopy (SEM) images were adopted to observe the anodic side of symmetric cells after operating for 250 h, and the results obtained in bare Zn once again confirmed its uneven plating/stripping process.The flakes corresponding to zinc hydroxyl sulfate were caused by active water molecules in the interfacial reaction environment (Figure 3d).These phenomena were effectively resolved after the introduction of CMCS coating; therefore, a smooth anodic surface was obtained after the coating lifted (Figure 3c,e).The chemical stability of anode was further examined by immersing in the electrolyte for 3 days.Zn-CMCS among them exhibited a relatively flat surface with some attached nanoparticles, while bare Zn presented uneven agglomeration of nanosheets, which once again verified the key role played by CMCS coating in homogenizing charge distribution (Figure S15, Supporting Information). [25]So far, the CMCS coating has shown advantages in promoting and stabilizing zinc plating/stripping.Considering the frequently reported poor Coulombic efficiency of iodine redox in aqueous electrolytes caused by uncontrollable side reactions and further shutting effects, such a crosslinked gel layer had the potential of polyiodide buffering and electrolyte confinement.Therefore, full-cell assembly with AC-CFC was used as the adsorptive cathode to explore the redox reversibility of iodine-containing electrolyte.It was worth noting that the AC without impurities in AC-CFC uniformly formed the surface of whole block carbon fiber cloth with a thickness of 65 μm (Figure S16, Supporting Information).Cyclic voltammetry (CV) curves between 0.7 and 1.6 V (vs.Zn 2þ /Zn) were carried out, in which the redox peaks located at 1.14/1.20V correspond to the I À /I 0 transfer.Also, the active side reactions in the H 2 Orich environment were still unavoidable, which was confirmed by the additional peaks at 1.30/1.37V (Figure S17, Supporting Information).In general, flexible CMCS coating provides excellent contact and buffer for interfacial reactions, so the operating voltage was greatly narrowed to explore its potential for efficient energy storage.When the operating voltage was narrowed to 1.00-1.23V (the interval width is only 0.23 V), a direct and completely reversible I À /I 0 conversion was obtained.Such an improvement in Coulombic efficiency at the appropriate sacrifice of capacity was considered acceptable (Figure 4a).
Motivated by this, cyclic performance under galvanostatic condition was first evaluated.An areal capacity of 1.25 mA h cm À2 at 1 mA cm À2 was performed in Zn-CMCS/AC-CFC battery, which maintained no attenuation within 200 cycles and a single-turn coulombic efficiency of 100%.Represented charge-discharge curves also reflected that the single I À /I 0 redox contributed all the reversible capacity within the narrow operating voltage interval of 0.23 V (Figure 4b).Meanwhile, such as ideal electrochemical performance was also reflected in rate capability, in which the average areal capacities of 1.22, 1.13, 1.08, 0.95, 0.76, 0.62, and 0.54 mA h cm À2 were obtained between the current density of 0.5 and 15.0 mA cm À2 .Combined with the coulombic efficiency close to 100% in the whole stage, it showed that the introduction of CMCS coating strengthened the fast electrochemical kinetic and high reversibility of iodine redox reaction in the narrow operating voltage range (Figure 4c, S18, Supporting Information).Fortunately, the Zn-CMCS/AC-CFC battery still exhibited the areal capacity of 1.04 mA h cm À2 after 28 000 cycles in the long-term cycling measurement of increasing current density (5 mA cm À2 ), which was no attenuation (compared with 0.94 mA h cm À2 in the first cycle) and showed severe polarization during the test period of up to 2 years.The performance of Zn/AC-CFC battery with bare Zn as anode was also explored, which only showed an areal capacity of 0.23 mA h cm À2 with poor charge-discharge curves, indicating its unsuitability for operation in this voltage range (Figure 4d).Control tests were carried out on coin cells and pouch cells, respectively, and the results indicated that the battery performance was greatly improved with the narrow electrochemical window after the introduction of CMCS coating.When the adsorptive area of cathode was enhanced to 2 Â 2 cm 2 , the assembled pouch cell showed a reversible capacity of 0.74 mA h cm À2 after 800 cycles at 5 mA cm À2 (Figure 4e).Similarly, the pouch cell loaded with bare zinc loses its capacity after 220 cycles.So far, the electrochemical enhancement brought by CMCS coating was significant, which provided a guarantee for high reversible operation of the battery in a narrow voltage range.
The CV curves of scan rate from 0.1 to 2.0 mV s À1 were collected to further study the evolution of iodine redox kinetics, in which the only I À /I 0 redox peaks of single-electron transfer moved to higher (lower) voltage as the scan rate accelerated.The values of b were calculated as 0.67 and 0.66 during the charge and discharge process, respectively, suggesting that the redox of iodine was cocontrolled by diffusion and capacitance. [26]urthermore, the capacity contribution was decomposed into ionic diffusion and surface capacitance, in which the proportion contributed by capacitance gradually increased from 56.8% at 0.1 mV s À1 to 92.1% at 2.0 mV s À1 , indicating the efficient reactivity of I À /I 0 conversion at the interface between electrolyte and cathode (Figure S19, Supporting Information). [2,27]On the other hand, the EIS spectra indicates the stable charge transfer resistance over 200 cycles (Figure S20, Supporting Information).A rapid increase around ten cycles was observed corresponding to the wetting process of the electrolyte, which subsequently decreased, indicating ideal kinetic stability of I À /I 0 conversion and I 2 adsorption.
To elucidate the improved cycle reversibility promoted by Zn-CMCS/AC-CFC battery, X-ray photoelectron spectroscopy (XPS) was employed to inquire the chemistry changes on the surface of AC-CFC cathode.The characteristic peaks at 631.2 and 619.7 eV associated with elemental I 0 in I 3d spectrum appeared with charging, which directly proved the adsorption of oxidation product (I 2 ) by AC and then gradually weakened with the rereduction of I 2 .Meanwhile, an attenuation of the signal was observed after cycling in spectrum peaks at 285.8 and 284.8 eV indexed to C─C and C═C bonds, respectively, which portended the adsorption of I 2 on the surface obscuring part of the signal as well as weakened the structural order of AC (Figure S21, Supporting Information). [28]The ex situ X-ray diffraction pattern was carried out to observe the anodic interface, in which characteristic peaks represented two zinc hydroxy sulfate phases with different contents of crystal water (Zn 4 SO 4 (OH) 6 •H 2 O [39-0690] and Zn 4 SO 4 (OH) 6 •5H 2 O [39-0688]) that appeared after 100 cycles in bare Zn.It dominated after 1700 cycles, indicating that the active H 2 O in electrolyte was deeply involved in the plating/ stripping process of unprotected anodic zinc.The underlying zinc in Zn-CMCS almost retained the original phase within 1700 cycles, reflecting a single plating/stripping process at the interface (Figure S22, Supporting Information).
Based on these, the introduction of CMCS coating on zinc foil led to a milder anodic surface.But more importantly, its presence in the battery greatly improved the stability of energy storage reaction.After matching with adsorptive cathode, the assembled Zn-CMCS/AC-CFC battery realized a highly reversible I À /I 0 conversion within a narrow operating voltage range.

Conclusion
In summary, a CMCS cross-linked gel was coated on zinc foil serving as an artificial electrolyte interphase.The CMCS coating was formed by a crosslinked reaction based on a monodentate carboxyl group, a hydroxyl, and an amino from the solution with Zn 2þ from 2 M ZnSO 4 solution.It features flexibility, ductility, and favored electrochemical properties, including ionic transfer number, ionic conductivity, and wettability.Combined with the homogenization of surface charge distribution, such a coating provides a "buffer" for zinc plating/stripping, resulting in a smoother interface.While matching iodine-contained electrolyte (2 M ZnSO 4 þ 0.5 M KI) and cathode with the adsorptive medium (AC), the assembled Zn-CMCS/AC-CFC battery exhibited an excellent cycling stability within 28 000 cycles at 5 mA cm À2 (measured for more than 2 years) as well as coulombic efficiencies close to 100% at current density from 0.5 to 15 mA cm À2 in a narrow operating voltage range of 0.23 V.This strategy of stabilizing interfacial reactions by introducing a flexible artificial electrolyte interphase with a certain thickness between electrodes provides a reference for the development of zinc-based batteries.Importantly, narrowing the operating voltage range based on specific redox conversion also sheds light on the design of highefficiency halogen batteries.

Figure 2 .Figure 3 .
Figure 2. a) CA curves at 100 mV of Zn-CMCS/Zn and Zn/Zn batteries, respectively.b) Tafel curves at 10 mV s À1 of Zn-CMCS/Zn and Zn/Zn batteries.c) LSV curves at 5 mV s À1 of Zn-CMCS/CFC and Zn/CFC batteries.d) Coulombic efficiencies of zinc plating/stripping in Zn-CMCS/Cu and Zn/Cu batteries.Galvanostatic e) rate capabilities and f ) cycling performances at 1.25 mA cm À2 of Zn-CMCS/Zn and Zn/Zn batteries.