DOI:
10.1039/D4QI03244F
(Research Article)
Inorg. Chem. Front., 2025,
12, 2245-2253
Functional modified separator with high-entropy material for high-performance Zn–I2 batteries†
Received
18th December 2024
, Accepted 17th January 2025
First published on 22nd January 2025
Abstract
The problems of soluble polyiodide shuttling and slow redox kinetics result in irrational cycling stability and restrict the further development of Zn–I2 batteries. In this paper, self-supporting layers containing high-entropy materials (HEMs) with adsorption–catalytic-conversion integrated functions were applied for the first time for separator modification of Zn–I2 batteries. Benefiting from the multi-elemental composition of HEMs, the widely distributed d-band centers improve the bonding between metals and molecules to realize the reduced adsorption energies to reactants or intermediates, giving the HEMs substantial highly active sites as adsorption and catalytic centers. This allows the polyiodides to be chemically anchored and catalytically converted by the modified layer once they are formed. The entire iodine conversion reaction proceeds efficiently without polyiodide shuttle and dissolution. As a result, satisfactory cycling stability was achieved. HEMs are proposed as separator modification materials with adsorption–catalytic-conversion synergies, presenting an effective strategy and new inspiration for realizing high-performance Zn–I2 batteries.
1. Introduction
In recent years, rechargeable aqueous metal-ion batteries have displayed enormous potential for large-scale energy storage due to their high safety, low manufacturing cost, environmental friendliness, and rewarding electrochemical properties.1–4 In various types of aqueous zinc ion batteries, the iodine cathode possesses a high theoretical specific capacity (211 mA h g−1 based on I0/I− conversion), suitable reaction potential (1.38 V vs. Zn/Zn2+), favorable reversibility, and fast redox kinetics compared with common metal oxide materials with ionic intercalation/deintercalation mechanisms. Nevertheless, the practical application of Zn–I2 batteries is still limited by the undesirable cycle life, which seriously hinders further development.5–7 In the aqueous electrolyte, Zn–I2 batteries exhibit a reversible conversion reaction of I2/I− with a charge storage mechanism, which may be accompanied by the generation of intermediate by-products polyiodides (I3− and I5−).8 The uncontrolled migration of these soluble polyiodide intermediates can lead to various undesirable side effects.9 On the one hand, the electrochemical redox reactions of I− and I3− at the cathode exhibit a high energy barrier, which can lead to slow reaction rates. On the other hand, the unfavorable by-product I3− rapidly diffuses from the cathode into the electrolyte, leading to severe overcharging and poor coulombic efficiency. Furthermore, weak immobilization of the intermediate product I3− leads to rapid migration of I3− through the separator to the surface of the zinc anode. The resulting corrosion of the zinc anode exacerbates the by-product generation and reduces the battery energy efficiency and lifetime. Therefore, these drawbacks greatly limit the wide application of Zn–I2 batteries.10
To address the above problems of Zn–I2 batteries arising from the uncontrolled shuttling of I3− from the cathode to the surface of the zinc anode, several targeted strategies have been investigated, including electrode material design, electrolyte regulation, and separator modification. For electrode design, one strategy is encapsulating iodine in a conductive host with high porosity to limit the I3−/I− reaction at the cathode surface.7,11 Although the carbon matrix can improve the electrochemical performance to some extent, the effect of carbon with weak adsorption capacity and low conductivity is limited.12 The subsequently proposed strategy of encapsulating reactive iodine using catalytically active host materials facilitates the slow I2 conversion reaction kinetics to some extent. Another approach is to construct hydrogel electrolytes or highly concentrated electrolytes for inhibiting I3− shuttling.13 Unfortunately, hydrogel electrolytes with low ionic conductivity and high-viscosity of high-concentration electrolytes severely limit their further application in Zn–I2 batteries.
Apart from the above research hotspots in Zn–I2 batteries, as an integral component of aqueous zinc ion batteries, the separator also exerts an essential influence on ion transport and the interface chemistry between the electrode and the electrolyte.14,15 Since the separator is a necessary route for soluble polyiodide shuttling, one of the most effective ways to block I3− shuttling is to prevent the unrestricted migration of I3− from the separator to the zinc anode side, thus preventing their side reactions at the anode.16 Therefore, functionalized modification of the separator is a straightforward and effective way to modulate the environment at the interface between the cathode and anode without involving complex electrode design.17,18 So far, studies targeting functionalized separator modification for inhibiting and confining the intermediate product I3− have been fewer, mainly focusing on introducing porous materials with suitable pore structures.19 However, relying only on the pore confinement and lack of chemical interaction may not help fully achieve the desired inhibition effect. Therefore, developing and constructing functionalized separators with significant chemical immobilization of polyiodides and effective restriction of I3− on the cathode side to block I3− shuttling are important to achieve highly reversible and stable Zn–I2 batteries.
High-entropy materials (HEMs) are a new class of materials that have now made many advances in the fields of batteries, thermoelectricity, and catalysis.20–23 Unprecedentedly superior physicochemical properties have been obtained through interactions between components with high-mixed entropy values.24–26 Particularly noteworthy is the simultaneous superior catalytic and adsorption behavior of HEMs. As a classical index for evaluating the binding strength, the d-band center model provides important theoretical support for the catalytic activity and selectivity of HEMs. The huge compositional space of HEMs can provide a flexible platform for tuning the d-band centers.27 Through electronic structure tuning by changing the composition and strain induced by lattice distortion, the position of the d-band center in HEMs is efficiently shifted, which improves the bonding between metals and molecules to realize the reduced adsorption energies to reactants or intermediates. The combination of numerous elements in HEMs leads to abundant catalytically active atomic arrangements or surface microstructures, which induce different adsorption modes of the reactants and related intermediates.28 Such broad d-band centers in HEMs achieve a large range of adsorption energies with continuous coverage, allowing HEMs to typically feature multiple highly active sites for adsorption and catalytic centers.25 Thus, HEMs are a powerful option for anchoring and catalyzing polyiodides.
In this work, HEMs with integrated adsorption–catalytic-conversion functions were developed for the first time as functionalized separator-modified layers for Zn–I2 batteries. Benefiting from the wide d-band center nature of HEMs, the abundant adsorption sites and catalytically active centers on HEMs play an important role in both chemical anchoring and catalytic conversion of iodine. Firstly, through the chemical anchoring between the metals in HEMs and the polyiodides, the polyiodides are firmly anchored to the modified layer as soon as they leak from the cathode surface, avoiding the loss of active materials. As the dissolution and shuttling of polyiodides in the electrolyte are apparently inhibited, the corrosion of the zinc anode is effectively alleviated while the self-discharge behavior of the battery is suppressed. Moreover, HEMs act as electroactive sites for catalyzing the polyiodides, promoting rapid iodine conversion kinetics and enhancing active substance utilization. As a result, the assembled Zn–I2 full cell exhibits satisfactory electrochemical performance. Up to 220 mA h g−1 capacity was obtained at a current density of 0.5 A g−1, which is close to the theoretical capacity. At a high current density of 10 A g−1, an initial capacity of 164.2 mA h g−1 and a stable cycling of 30
000 cycles were obtained, accompanied by a coulombic efficiency close to 100%.
2. Results and discussion
2.1 Structural and compositional analysis of CNFs/HEO
Initially, carbonized nanofibers with a high-entropy oxide (CNFs/HEO) were synthesized with some modifications in reference to our previous methods, which were prepared by introducing five inorganic salts into fibers through the method of electrostatic spinning and high-temperature calcination.26 In contrast to our previous research, a newly composed high-entropy oxide was prepared by replacing and recombining new metal composition, including In, Zr, Yb, Y, and Tm. Among these metals, most of the metal elements are chosen to be rare metals due to the characterization of 4f orbitals with unfilled electrons and lanthanide contractions, which are rich in electronic energy levels and have unique chemical properties, providing favorable catalytic effects for many reactions and immobilization of a wide range of intermediates.29–31
Subsequently, the morphology of the prepared CNFs/HEO was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It is shown in Fig. 1a and b that CNFs/HEO with ultrafine nanoparticles on the surface are stacked on each other. Fig. S1† confirms the homogeneous distribution of the elements C, N, O, In, Zr, Yb, Y, and Tm on the surface of the fiber. Furthermore, the TEM image reveals the distribution of ultrafine nanoparticles within CNFs/HEO, as presented in Fig. 1c. The sample, characterized by a lattice fringe spacing of 0.2 nm corresponding to the (222) crystal surface of Yb2O3, is depicted in Fig. 1d using high-resolution TEM (HR-TEM). Additionally, the magnified TEM image and elemental mapping suggest a uniform distribution of the elements C, O, In, Zr, Y, Yb, and Tm throughout the entire material (Fig. 1e). Moreover, a cross-section SEM image of CNFs/HEO in Fig. S2† was observed to indicate that the thickness of CNFs/HEO is approximately 100 μm.
 |
| Fig. 1 (a and b) SEM images and (c) TEM images of CNFs/HEO. (d) Magnified HR-TEM image of CNFs/HEO. (e) TEM image and the corresponding elemental mapping images of C, O, In, Zr, Y, Yb, and Tm of CNFs/HEO. (f) XRD Rietveld refinement pattern of CNFs/HEO. (g) Ratio of the metal elements in CNFs/HEO. | |
To explore the structure and phase composition of CNFs/HEO, X-ray diffraction (XRD) was tested and presented in Fig. 1f. The diffraction peaks corresponding to the standard card of Yb2O3 (JCPDS No. 43-1037) are obvious, and the sharp peaks indicate favorable crystallinity. The corresponding crystallographic data is listed in Table S1.† To clarify the crystal structure of CNFs/HEO, the most stable crystal configuration of HEO is determined from theoretical calculations (Fig. S3†). In addition, two peaks located at 1360 and 1597 cm−1 can be seen in the Raman spectrum corresponding to the D peak (disordered carbon) and G peak (graphitized carbon), respectively (Fig. S4†). The intensities of the D peak and G peak (ID/IG) of CNFs/HEO and CNFs/Yb2O3 were calculated to be greater than 1, which indicates that they share a high degree of graphitization. It also provides a certain basis for fine electron transfer and high electrical conductivity.32,33 Then, the elemental composition in CNFs/HEO was further investigated by X-ray photoelectron spectroscopy (XPS). Five different metallic elements were detected, as presented in Fig. S5.† Besides, it can be illustrated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) that the contents of five metallic elements in CNFs/HEO are similar, as presented in Fig. 1g. This similar composition of elemental ratios is in accordance with the requirements for high-entropy compounds, which require near-equal ratios of five and more elemental species.
2.2 Kinetic analysis and comparison of CNFs/HEO and CNFs/Yb2O3
To investigate and compare the effects of CNFs/HEO and CNFs/Yb2O3 modified layers on the reaction kinetics of Zn–I2 batteries, they were assembled between the cathode and the separator in the coin cells and subjected to subsequent electrochemical tests. Initially, the catalytic effects of CNFs/Yb2O3 and CNFs/HEO toward I2/I− were examined utilizing cyclic voltammetry (CV). As can be seen from the CV curves in Fig. 2a, significantly higher current density was achieved by the CNFs/HEO cell than that of the CNFs/Yb2O3 cell, which implies a higher electrochemical activity of the CNFs/HEO cell and favors to provide a higher capacity performance. Furthermore, markedly lower polarization voltage is realized by the CNFs/HEO cell, which can be attributed to the modified interface with higher catalytic activity towards iodine and consequently promotes favorable redox reaction kinetics. The superior specific capacity and reduced polarization voltage of CNFs/HEO are further evidenced by the constant current charge–discharge (GCD) curve presented in Fig. 2b. In the rate test, CNFs/HEO consistently shows higher specific capacities than CNFs/Yb2O3 across all tested current densities (Fig. 2c). At an initial current density of 0.5 A g−1, a capacity of 220.3 mA h g−1 was achieved by CNFs/HEO. Upon returning to the initial current density, CNFs/HEO maintains a highly steady specific capacity, indicating robust reversibility and favorable rate behavior.
 |
| Fig. 2 (a) CV curves, (b) GCD curves and (c) rate performance of CNFs/HEO and CNFs/Yb2O3 batteries. (d) CV curves at different scan rates, (e) log(i)–log(v) plots for specific peak current, and (f) capacitive and diffusion contribution versus scan rate curve of CNFs/HEO batteries. (g) Arrhenius plots for the I2 reduction process on the cathodes with CNFs/HEO and CNFs/Yb2O3 modified layers, and unmodified batteries. (h) Potentiostatic deposition curves of I2 on the cathode with CNFs/HEO (the inset reflects the surface state after iodine deposition). (i) AFM image of the cathode with CNFs/HEO after depositing I2. | |
For deeply analyzing the electrochemical reaction kinetics of the electrodes, CV curves of CNFs/HEO cells at various scan rates were recorded (Fig. 2d). When the scan rate increases, the CV curves maintain similar shapes, indicating favorable reversibility and stable redox behavior at the CNFs/HEO modified interface. By analyzing and fitting, the arbitrary coefficient b values of the two redox peaks were calculated to be between 0.5 and 1. This range demonstrates that the reaction is featured by both the diffusion process of Zn2+ and the capacitive behavior (Fig. 2e). The capacitive contribution at the CNFs/HEO interface is greater than that at the CNFs/Yb2O3 interface at scan rates of 1, 1.2, 1.5, 1.8, and 2 mV s−1 (Fig. S6† and Fig. 2f). The superior capacitive behavior of the CNFs/HEO battery may come from the favorable catalytic ability to the iodine transformation, which facilitates faster reaction kinetics. Moreover, the relationship between the charge transfer resistance (Rct) and the temperature was explored to determine the activation energies (Ea) of CNFs/Yb2O3 and CNFs/HEO modified interfaces, and unmodified separators for assisting the iodine reduction process (Fig. 2g, S7 and Fig. S8†). The Ea value for the CNFs/HEO interface is significantly lower than that for CNFs/Yb2O3 and unmodified separators, suggesting a substantial contribution from the polyactivity core in facilitating iodine deposition. To directly assess the impact of the CNFs/HEO modified interface on iodine deposition, constant potential deposition tests were performed (Fig. 2h and Fig. S8†). The results demonstrate a notably increased current at the CNFs/HEO interface compared to CNFs/Yb2O3, which benefit from the enhanced catalytic effects and improved deposition kinetics of iodine by CNFs/HEO. A more uniform and dense distribution of iodine is shown in the inset in Fig. 2h and Fig. S8.† Also, atomic force microscopy (AFM) images in Fig. 2i and Fig. S9† further corroborate that the catalytic action of the CNFs/HEO interface promotes more effective and uniform iodine deposition.
2.3 Reaction mechanism analysis of the CNFs/HEO battery
To provide a further exploration of the iodine conversion reaction mechanism, a series of measurements were conducted. Firstly, the mechanism of the iodine conversion was tested using the in situ Raman spectroscopy (Fig. 3a and b). The electron transfer between iodine and carbon materials facilitates the formation of I3− and I5− symmetric stretching bands, as evidenced from the peaks observed at 110 and 162 cm−1.34 Throughout the charging and discharging process, the presence of I3− and I5− at the CNFs/Yb2O3 interface consistently explains the adverse reversibility of the entire reaction process and the sluggish conversion of polyiodides. However, I3− and I5− are only retained at the fully charged state in the CNFs/HEO battery, indicating the rewarding reversibility of the overall process. Benefiting from the satisfying catalytic activity of HEO, the iodine conversion reaction was accelerated, thereby impeding the shuttling of polyiodides. Subsequently, XPS tests were employed to visualize the elemental valence state of iodine at different charge/discharge states, further elucidating the reaction mechanism of the iodine conversion. As depicted in Fig. 3c, iodine initially exists primarily in the monomer form. When discharged, the iodine monomer gains electrons to form I− in the −1 valence state. When discharged to an intermediate potential, I− and I2 coexist on the electrode (Fig. 3d). Upon charging to 1.6 V, the −1-valent iodine was transformed into a 0-valent iodine monomer (Fig. 3e). During the entire charging and discharging process, the mutual conversion of I− and I2 occurs rapidly and completely. Almost no polyiodides were detected, indicating high reversibility of the iodine conversion reaction. The corresponding discharge/charging curve is displayed in Fig. 3f. Besides, in situ UV-vis spectroscopy tests were performed to validate the reaction mechanism and the important role of CNFs/HEO. As shown in Fig. 3g, peaks of interconversion of I− and I2 are found to exist during the discharge process. Notably, undesired peaks of I3− are absent, suggesting that there is no dissolution or shuttling of polyiodide throughout the process.
 |
| Fig. 3 Charge/discharge curves and in situ Raman spectra of (a) CNFs/Yb2O3 and (b) CNFs/HEO cells. I 3d XPS spectra in the CNFs/HEO cell under (c) initial state, (d) discharging, (e) charging states and corresponding (f) discharge and charging curves. (g) In situ UV-vis spectra of the CNFs/HEO cell during discharging. UV-vis spectra of (h) CNFs/Yb2O3 and (i) CNFs/HEO immersed in ZnSO4 electrolyte containing I3−. Diffusion of the I2 solution for different time intervals monitored in an H-type cell with (j) CNFs/Yb2O3 and (k) CNFs/HEO. Diffusion of the I3− solution for different time intervals monitored in an H-type cell with (l) CNFs/Yb2O3 and (m) CNFs/HEO. | |
Subsequently, the ability of CNFs/HEO and CNFs/Yb2O3 to adsorb and immobilize iodine species was initially compared. Firstly, adsorption experiments of I2 and I3− by CNFs/HEO and CNFs/Yb2O3 substantiate the efficient adsorption capacity of iodine species by the CNFs/HEO modified interface. As shown in Fig. 3h and i, after immersing CNFs/HEO and CNFs/Yb2O3 into the electrolyte containing I3− for 24 h, the signals of I3− absorption peaks on CNFs/HEO are quite weak, which indicates that the adsorption of iodine species on CNFs/HEO is strong. Thereafter, shuttle experiments of iodine species were further tested. The color change of the solution on both sides of the membrane was observed after separating the electrolyte containing iodine species from the pure electrolyte by CNFs/HEO and CNFs/Yb2O3 and leaving them for some time. As presented in Fig. 3j–m, shuttling of iodine species is observed in the H-type electrolytic cell with CNFs/Yb2O3, whereas it is absent in the case of CNFs/HEO. These experiments underscore the pivotal role of the CNFs/HEO modified interface in inhibiting the shuttling of iodine species, thereby laying the groundwork for favorable electrochemical performance.
2.4 Theoretical and experimental analysis of iodine species fixation by CNFs/HEO
Subsequently, a series of experiments and theoretical analyses were conducted to verify the fixation of iodine species by CNFs/HEO. First of all, CNFs/HEO and CNFs/Yb2O3 were immersed in the electrolyte containing I−, I2, and I3−, respectively. The XPS spectra of various metal elements in the materials were examined. Peak positions of most metal elements in CNFs/HEO were observed to shift, suggesting the bonding ability and interaction with iodine species, as presented in Fig. 4a–c.35,36 Conversely, peak positions of Yb in CNFs/Yb2O3 show minimal shifts, which indicates its low bonding ability with iodine species (Fig. 4d).18,36 These results suggest that HEO possess capability to bond with iodine species, which is important for inhibiting the loss and shuttling of iodine species. Additionally, the phase composition in CNFs/HEO remains unchanged after cycling (Fig. S10†). Shifts in metal element peaks in the XPS spectra are not due to the change in the phase composition.37 Furthermore, the work function distributions of CNFs/HEO and CNFs/Yb2O3 depicted in Fig. 4e highlight the unique electronic structure conferred by HEO. This structure allows CNFs/HEO to exhibit a broader distribution of the work functions compared to CNFs/Yb2O3, suggesting potential advantages of HEO in this context.
 |
| Fig. 4 XPS spectra of (a) Yb, Zr, and Tm, (b) Y, (c) In in CNFs/HEO. (d) Yb in CNFs/Yb2O3. (e) work function obtained from the Kelvin Probe Force Microscopy of CNFs/Yb2O3 and CNFs/HEO. (f) Gibbs energy of the I2 reduction reaction. (g) The dissociation dynamics of I3−. (h) ICOHP and (i) COHP of various metal–I3− bonds. (j) Binding energies of HEO and Yb2O3 to I−, I2, and I3−. (k) Schematic diagram of the density of states (DOS) for different unary materials and high-entropy materials. (l) Total density of states and (m) partial projected density of states (PDOS) of each element in Yb2O3 and HEO. | |
Moreover, theoretical calculations were carried out to gain a deeper understanding of the interaction mechanisms of HEO on the iodine conversion reaction. The energy changes throughout the whole reaction are further presented in Fig. 4f. All the reactions are exothermic. The ΔG value for HEO is more negative than that of Yb2O3 in the reaction step, suggesting a greater catalytic activity for HEO. It is indicated that HEO serves as an effective catalyst in promoting iodine multiple electron transfer reactions. Besides, the decisive step of the entire reaction is the conversion of I3− to I− through the transfer of two electrons (I3− + 2e− → 3I−). Fig. 4g illustrates the lower energy barrier associated with HEO, which accelerates the decomposition of I3−. It explains the absence of I3− in the cell with CNFs/HEO during the reaction. Subsequently, the integrated crystal orbital Hamilton population (ICOHP) values of Yb2O3 and HEO were further computed. The majority of the –ICOHP values of metal-I in HEO are more negative than those of Yb–I in Yb2O3, indicating stronger bonds formed between HEO and the iodine species (Fig. 4h).38,39 Additionally, the bonding stability of metal elements with iodine in Yb2O3 and HEO was further assessed using the crystal orbital Hamilton population (COHP). Positive values denote bonding contributions, while negative values denote antibonding contributions. Fig. 4i demonstrates that all metals in HEO exhibit increased neighborhood filling to the bonding orbitals and decreased neighborhood filling to the antibonding orbitals, while the bonding orbitals and antibonding orbitals of Yb in Yb2O3 have comparable values, suggesting stronger interactions between HEO and the iodine species. Fig. 4j presents the adsorption energies of Yb2O3 and HEO for I−/I2/I3−. Lower calculated adsorption energies show greater adsorption capacity of the material for iodine species. The adsorption energies of HEO for I−/I2/I3− are all notably lower than those of Yb2O3, which indicates superior adsorption capacity of HEO for iodine species. These theoretical calculations collectively suggest the CNFs/HEO modified interface possesses a stronger ability to anchor with I3− and catalyzes iodine conversion compared with CNFs/Yb2O3, which plays an important role in mitigating polyiodide shuttling and dissolution to enhance the cycling stability.
To investigate the reasons for the favorable catalytic activity and immobilization of HEO towards polyiodides, a subsequent analysis of the electronic structure of HEO was carried out. As illustrated in Fig. 4k, a more continuous electronic structure is conferred to HEO compared to unitary materials due to the almost infinite combination of coordination sites in HEO. This continuous electronic structure gives the high-entropy material a wider distribution of adsorption energy. Also, the continuous adsorption energy facilitates the immobilization and catalysis of a wide range of iodine species, resulting in desirable electrocatalytic properties. Besides, the infinite combination of multiple elements leads to continuous coverage of a large distribution of d-band centers compared to a single metallic element composition. The total density of states was further determined using DFT simulations, as presented in Fig. 4l. Compared with single-phase Yb2O3, HEO clearly crosses the Fermi energy level, which suggests the enhanced electron transfer capability due to the synergistic effect of multiple metals. Subsequently, the partial projected density of states of various metal elements in HEO and single-phase Yb2O3 are also displayed (Fig. 4m). The results indicate that the elements in HEO exhibit different d-band centers, which will be conducive to forming a continuous and broad model of d-band center for the catalysis and immobilization of various intermediates.22,40,41
Subsequently, for a more intuitive and clear representation of the role of CNFs/HEO in Zn–I2 batteries, a schematic diagram is displayed in Fig. 5. As a separator-modified layer, self-supporting CNFs/HEO were assembled between the cathode and the separator. Due to the synergistic effect of multiple elements in the high-entropy nanoparticles, the unique atomic arrangement or surface microstructure induces different adsorption modes of reactants and related intermediates, resulting in a favorable chemical immobilization ability to polyiodides by the modified layer. As a result, inhibited polyiodide shuttling is realized, which greatly alleviates the corrosion of the zinc anode by polyiodides to enhance the stability and durability of the electrode. Additionally, HEO acts as electroactive sites for catalyzing polyiodides due to their unique chemical composition and surface electronic structure, which promotes the fast conversion kinetics of iodine in the Zn–I2 battery. The catalyzed reaction proceeds under a highly reversible I2/I− with no intermediate I3− leaked, which improves the utilization of the active substance and ensures the stability of the battery.
 |
| Fig. 5 Schematic diagram of CNFs/HEO modified interface in maintaining the stability of Zn–I2 batteries. | |
2.5 High-performance Zn–I2 full batteries
To explore the practical application of CNFs/HEO, full batteries were assembled and compared employing CNFs/Yb2O3 and CNFs/HEO modified interfaces, respectively. Fig. S11† presents the morphological characterization of the cathode. The long-cycle performance of the full battery was tested at different current densities. In fact, issues such as sluggish iodine redox kinetics and polyiodide dissolution/shuttle become more pronounced at lower current densities. This is evidenced in Fig. 6a, where, at a current density of 0.5 A g−1, the CNFs/Yb2O3 battery shows a lower initial capacity and experiences significant capacity decay after a period of cycling. However, the CNFs/HEO battery displays negligible capacity decay and demonstrates a robust cycling stability over 200 cycles. Besides, at a high current density of 10 A g−1, the CNFs/HEO battery was stably cycled over 30
000 cycles (Fig. 6b). It has achieved a coulombic efficiency close to 100% and a capacity retention much better than the CNFs/Yb2O3 battery. Fig. 6c and Table S2† display the results of the stable cycling performance of the materials reported in the literature compared with CNFs/HEO, indicating the satisfactory performance by CNFs/HEO. Furthermore, the cycling performance of the cells at high loading was further tested, as displayed in Fig. 6d. It is found that the CNFs/HEO cells exhibit a higher capacity and slower the capacity degradation, implying the important role of CNFs/HEO in maintaining the stability of the cells at high loadings.
 |
| Fig. 6 Long-term cycling performance at (a) 0.5 A g−1 and (b) 10 A g−1. (c) Comparison results of the cycling performance of the CNFs/HEO cell with the reported literature. (d) Cycling performance at high mass loading. Voltage changes of CNFs/Yb2O3 and CNFs/HEO cells after being fully (e) charged and (f) discharged. (g) Photos of flexible pack batteries successfully lighting up electronic appliances. (h) Cycling performance of the pouch cells under different bending states with the CNFs/HEO modified layer. | |
To further compare the stability of the electrodes, self-discharge tests were conducted. The CNFs/HEO battery exhibits a slower voltage change and greater coulombic efficiency compared to the CNFs/Yb2O3 battery, as illustrated in Fig. 6e and f. This improvement can be attributed to the capability of CNFs/HEO to immobilize iodine by bonding, thereby preventing its shuttle to the zinc anode, consequently enhancing the coulombic efficiency of the battery. Furthermore, SEM images and the corresponding elemental mappings of the zinc anode after cycling in the CNFs/HEO battery and CNFs/Yb2O3 battery are presented in Fig. S12 and S13.† These images reveal that the surface of the zinc anode in CNFs/HEO battery appears considerably smoother and flatter, with a reduced presence of other elements corresponding to the by-products. These findings indicate the vital role of CNFs/HEO in mitigating polyiodide shuttling and maintaining electrode stability. Finally, to investigate the practical potential of CNFs/HEO, flexible soft pouch cells were assembled for electrochemical performance testing. As shown in Fig. 6g, the assembled soft-packed battery successfully powers an electronic watch. Furthermore, the cycling stability of the soft-packed battery was evaluated under various bending states, as presented in Fig. 6h. Remarkably, even after 75 cycles of bending from 180° back to the flat state, the battery maintains robust cycling performance. These results suggest promising applications for CNFs/HEO in soft power devices.
3. Conclusions
In conclusion, an interfacial layer containing a high-entropy material was applied for the first time for the separator modification of Zn–I2 batteries. The nature of the wide d-band centers, owing to the multi-element synergistic effect of the high-entropy material effectively modulates the distribution and role of the active centers, resulting in the distribution of a large number of highly active adsorption and catalytic sites. The polyiodides were captured and catalytically converted by the modified layer once they were generated from the cathode, further preventing the shuttling of polyiodides to the zinc anode. This allows the iodine conversion reaction process to proceed in a highly iodine-utilizing manner without I3− being detected in the electrolyte, which ensures the cell stability and improves the utilization of the active substance. As a result, a favorable specific capacity of 135.4 mA h g−1 (10 A g−1) and 30
000 cycles were achieved. The separator modification scheme in this study presents an attractive candidate for regulating the efficient electrochemical conversion of iodine species and thus preventing the leakage of polyiodides. HEMs with integrated adsorption–catalytic-conversion features offer novel references and inspirations for achieving long-life Zn–I2 batteries.
Data availability
The data supporting this article have been included as part of the ESI.†
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (22172023, 22275031, 22375033 and 22305032), the Science and Technology Development Plan Project of Jilin Province (20240602117RC), and the More Acid and Metal Nanomaterials Science and Technology Innovation Center of Jilin Province.
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