An overview of aqueous zinc-ion batteries based on conversion-type cathodes

Aqueous Zn-S batteries

Sulfur is a typical insulator with a conductivity of 5 × 10^-28 S m^-1. Conductive carbon materials are usually used as the hosts of sulfur to improve the kinetics and reversibility of the electrode reaction. Ketjenblack^[26,41], activated carbon^[26], carbon nanotubes^[44], honeycomb carbon^[42,57], phosphorus-doped carbon sheets^[25] and nitrogen and phosphorus co-doped carbon foam^[24] have all been reported to be used in the preparation of sulfur/carbon composite electrodes. Luo et al.^[41]prepared a Ketjenblack/sulfur (KB-S) composite with a sulfur content of ~74.1% [Figure 2A] and assembled a Zn-S battery with the electrolytes of 1 M ZnCl₂, ZnSO₄, Zn(Ac)₂ and Zn(OTF)₂, respectively. It was found that the battery with 1 M ZnCl₂ delivered the highest discharge plateau of ~0.7 V and a specific capacity of 1668 mAh g^-1, leading to an outstanding energy density of 1083.3 Wh kg^-1. Unfortunately, this battery had a voltage hysteresis of 0.8 V during cycling and the specific capacity decayed to 33.6% of the initial capacity after four cycles. Although increasing the electrolyte concentration to 30 M could improve the cycle performance of the battery, it also resulted in a lower specific capacity and greater voltage hysteresis.

Figure 2. A: SEM images, elemental mapping, N₂ adsorption-desorption isotherms and pore size distribution of KB-S composite. B: Thermogravimetry (TG) curves, X-Ray Diffraction (XRD) patterns and N₂ adsorption-desorption isotherms of S@CNTs. C: Optical photographs, electronic conductivities and XRD patterns of S-Se solid solutions. Figures reproduced with permission from Luo et al.^[41], Li et al.^[44] and Li et al.^[24].

Li et al.^[44] prepared sulfur/carbon nanotube composites (S@CNTs) with different sulfur contents [Figure 2B] by a melt infiltration method and studied the effect of sulfur content on the performance of a Zn-S battery. The results showed that as the sulfur content increased from 30 to 60 wt.%, the discharge specific capacity decreased from 1088 to 659 mAh g^-1 and the voltage hysteresis increased from 0.72 to 0.82 V. In summary, the composite with 50 wt.% sulfur (S@CNTs-50) provides the best balance between the specific capacity of 1105 mAh g^-1 and the discharge plateau of 0.5 V at 0.1 A g^-1.

In addition to conductive carbon materials, selenium with a higher conductivity of 1 × 10^-3 S m^-1 can form an infinite solid solution with sulfur, thereby significantly improving the conductivity of electrodes. Li et al.^[24] studied the electrical conductivity of three sulfur-selenium solid solutions (SeS₁₄, SeS_5.76 and SeS_2.46) and demonstrated that with increasing selenium content, the electrical conductivity of the three solid solutions increased to 4 × 10^-22, 3 × 10^-9 and 2 × 10^-8 S m^-1, respectively [Figure 2C]. In order to further improve the conductivity of the electrodes, a 3D nitrogen-phosphorus co-doped carbon foam (3D-NPCF) was used as the substrate of the sulfur-selenium solid solutions. The authors analyzed the electron density, band structure and binding energy of different solid solutions through Density Functional Theory (DFT) calculations. It was found that Se doping can increase the electron cloud density and reduce the band gap of S atoms, thereby increasing the conductivity of the electrodes and reducing the overpotential of batteries. Furthermore, it can also reduce the breaking energy and binding energy of chemical bonds to enhance the electrode reaction kinetics.

To further enhance the redox kinetics of electrodes, iodine has been proposed as a redox mediator to increase the utilization of sulfur and reduce the overpotential of batteries. Li et al.^[44] used S@CNTs-50 as a cathode material and studied the performance of batteries with different electrolytes [1 M Zn(OTF)₂, 1 M ZnSO₄, 1 M Zn(Ac)₂ and 1 M Zn(Ac)₂ + 0.05 wt.% I₂]. The results showed that the battery assembled with the electrolyte of 1 M Zn(Ac)₂ with 0.05 wt.% I₂ exhibits the lowest voltage hysteresis of 0.72 V and the highest discharge specific capacity of 1105 mAh g^-1 [Figure 3A], with the energy density reaching 502 Wh kg^-1 based on S. In addition, the additive I₂ can also stabilize the plating and stripping behavior of the zinc anode, so that the battery has a capacity retention of 85% after 50 cycles at 1 A g^-1.

Figure 3. A: Charge and discharge curves of S@CNTs-50 cathode in different electrolytes. B: Charge and discharge curves of SeS₂@PCS in different electrolytes and calculated E_ads values of different ligands by DFT. C: Schematic diagram of the structure and the working mechanism of Zn-S batteries with a redox-ion charge-carrier electrode. D: Schematic diagram of the working mechanism of a cascade battery in the discharge process. Figures reproduced with permission from Li et al.^[44], Li et al.^[25], Wu et al.^[26] and Dai et al.^[42,57]. DFT: Density functional theory.

Li et al.^[25] constructed a Zn-S battery with a sulfur-selenium solid solution (SeS₂@PCS) electrode and a 1 M ZnSO₄ electrolyte with 0.1 wt.% I₂. The effect of I₂ was explored by DFT calculations. During the discharge process, the coordination effect of SeS₂ and the iodine species can enhance the hydrophilicity and reduce the reaction barrier, which enables SeS₂ to capture zinc ions at a higher potential, thus improving the reaction kinetics and discharge potential of Zn-S batteries. During the charging process, ZnS and ZnSe also have coordination effects with iodine species, and I³⁻ can oxidize S²⁻ and Se²⁻ spontaneously, thereby accelerating the oxidation process and reducing electrochemical polarization [Figure 3B]. Thus, the battery has a small voltage hysteresis of 0.41 V, a specific capacity of 1107 mAh g^-1, an energy density of 772 Wh kg^-1 based on SeS₂ and a capacity retention of 85% after 1000 cycles at 5 A g^-1.

The above-mentioned Zn-S batteries are all based on a two-electron redox reaction. Different from the typical two-electron reaction, Wu et al.^[26] reported a Zn-S battery with a four-electron electrode reaction realized by using Cu²⁺ as the redox-ion charge carrier [Figure 3C]. The battery was assembled with a KB-S cathode composite, a zinc foil anode, a 0.5 M CuSO₄ catholyte, a 0.5 M ZnSO₄ anolyte and an anion exchange membrane separator. It was shown that the active material undergoes the reaction process of S↔CuS↔Cu₂S in the discharge/charge process. Therefore, the battery has a discharge plateau of 1.15 V and a specific capacity of 2000 mAh g^-1 at 50 mA g^-1, resulting in an energy density of 547 Wh kg^-1 based on the cathode and 255 Wh kg^-1 based on the cathode and anode.

Dai et al.^[42] constructed a flexible Zn-S battery according to this mechanism [Figure 3C]. Specifically, a flexible zinc anode was prepared by electroplating, and a flexible sulfur/carbon (S@C) cathode was prepared using honeycomb carbon as the host of sulfur. Gelatin/ZnSO₄ and gelatin/CuSO₄ were prepared for the anolyte and catholyte, respectively. The cathode of this flexible battery also undergoes the redox of S↔CuS↔Cu₂S, and the battery exhibits a reversible specific capacity of 2016 mAh g^-1 at 0.1 A g^-1 and an outstanding energy density of 2372 Wh kg^-1 based on S. Dai et al.^[57] also reported another similar work, but interestingly, two flat discharge plateaus of 1.15 and 1.05 V appeared during the discharge process. In-situ XRD characterization confirmed that these two plateaus were attributed to the reaction mechanism of S↔CuS↔Cu₂S of the Zn-S battery and Cu²⁺↔Cu/Cu₂O of the Zn-Cu battery, respectively [Figure 3D]. As a result, the authors proposed the concept of a cascade battery. It is also found that Cu₂S, the discharge product of the first step of the cascade battery, can act as a catalyst for the second step of charging and discharging, thus improving the reaction kinetics of the Zn-Cu battery. Benefitting from the sequentially coupling two-step discharge process, the battery delivers an excellent specific capacity of 48 mAh cm^-2 and an energy density of 47.6 mWh cm^-2.

In addition to the traditional elemental sulfur cathode, Zhao et al.^[43] reported a polysulfide cathode (LF-PLSD) based on the stabilization of a liquid membrane and explored its electrochemical behavior and performance in dilute, highly concentrated and gel electrolytes. The results showed that this electrode with a 3D network structure has good interfacial compatibility with the electrolyte and excellent structural stability during cycling. The battery assembled with 1 M Zn(TFSI)₂ has a specific capacity of 1146 mAh g^-1 based on the cathode, and the energy density and power density are up to 724.7 Wh kg^-1 and 432.5W kg^-1, respectively.

Because of the difference in the reaction mechanism of sulfur in water-based and organic electrolytes, research into aqueous Zn-S batteries is still in its infancy^[33]. Although Zn-S batteries have a theoretical specific capacity of 1675 mAh g^-1, the actual capacity based on a single electrolyte is still far lower. Dual electrolyte systems based on redox-ion charge carriers can significantly increase the discharge capacity and energy density, but these batteries have a complicated structure design and require an expensive anion exchange membrane as the separator, which is not conducive to the industrialization of Zn-S batteries. In particular, limited by the low conductivity of sulfur and the solid-solid reaction mechanism of aqueous Zn-S batteries, the kinetics and reversibility of the electrode reaction of Zn-S batteries are generally poor, resulting in significant voltage hysteresis. As a consequence, the aqueous Zn-S batteries currently reported all exhibit poor voltage efficiency and cycle life. Achieving faster electrode reaction kinetics and better cycle stability through the reasonable design of electrodes and optimization of electrolytes are critical issues for the development of aqueous Zn-S batteries. More attempts should be made to develop suitable catalytic matrixes, explore viable heterostructure electrodes and find new electrolyte formulations.

Aqueous Zn-I₂ batteries

Compared with sulfur, iodine has higher electrical conductivity (1 × 10^-7 S m^-1) and outstanding electrode reaction reversibility, so it is not only widely studied in redox flow batteries but has also drawn attention for static batteries. Although the solubility of iodine in water is negligible, the polyiodides derived from the combination of iodine and iodide can increase the solubility, leading to a shuttle effect similar to Li-S batteries, resulting in rapid capacity degradation and self-discharge behavior. With the aim of suppressing the dissolution and shuttle effect of iodine species, the solid-liquid electrode reaction of iodine is usually firmly confined in host materials. It has been reported that a variety of porous carbon materials can be used as the host of iodine in aqueous Zn-I₂ batteries, including carbon fiber cloth, carbon fiber paper and active carbon.

Pan et al.^[45] prepared an iodine/active carbon fiber cloth composite (I₂/ACF) with different contents of iodine by the method of solution adsorption and studied the influence of different iodine mass and electrolyte volume on the performance of the battery [Figure 4A]. They showed that the smaller the ratio of electrolyte volume to iodine mass (E/I), the better the rate capability and cycle performance of the battery. This was ascribed to the lower E/I being beneficial to accelerating the balance of iodine between the adsorption on the porous carbon and the dissolution in the electrolyte, thereby reducing the loss of iodine in the cathode. DFT calculations further demonstrated that iodine species are preferentially adsorbed on the surface of carbon rather than dissolving in the electrolyte, which helps to suppress the shuttle effect of polyiodides and the self-discharge behavior of batteries.

Figure 4. A: Schematic illustration of the structure of a Zn-I₂ battery with an I₂/ACF cathode. B: Configurations of I₂, ZnI₂ and Zn(I₃)₂ by DFT calculations and schematic illustration of the reaction in the I₂/CC cathode. C: Schematic diagram of NPC fabrication. D: Schematic diagram of I₂-Nb₂CT_X fabrication. E: Schematic diagram of PBAs/I₂ fabrication and reaction. F: Schematic illustration of PANI-I₂ fabrication. G: Schematic illustration of the structure of the Zn-I₂ battery with double-layered cathodes. Figures reproduced with permission from Pan et al.^[45], Li et al.^[58], Yu et al.^[50], Ma et al.^[27], Li et al.^{[ 52]}, Zeng et al.^[51] and Lin et al.^[59]. NPC: Nitrogen-doped porous carbon ; DFT: density functional theory.

Bai et al.^[46] prepared an activated carbon cloth (ACC)/I₂ composite cathode by infusing I₂ into nanoporous ACC by heat treatment and investigated its reaction mechanism in a 1 M ZnSO₄ electrolyte. Benefiting from the confinement effect of the porous structure of ACC on the iodine species, both Cyclic Voltammetry tests and Raman tests confirmed the direct conversion process of I₂/I⁻ without the formation of triiodide intermediates, thus greatly mitigating the self-discharge behavior and enhancing the cycle stability of the battery, which exhibited a specific capacity of 255 mAh g^-1 at 0.5 C and a capacity retention of 90% after cycling 1500 cycles. Li et al.^[58] reported a flexible Zn-I₂ battery based on an iodine/microporous carbon cloth composite (I₂/CC) and explored the effect of pore structure on the reaction mechanism of the battery via experiments and DFT calculations. The results showed that the microporous structure of the carbon cloth (pore size of ~0.63nm) could accommodate I₂ and ZnI₂ but not Zn(I₃)₂ (minimal diameter of ~0.95 nm) [Figure 4B]. Therefore, there were no polyiodides detected in the charging and discharging process of the battery. In contrast, a cathode material made of non-porous carbon KS6 and Super P could be detected the formation of polyiodides in the discharge process. Benefitting from the confinement effect of the microporous structure, a battery assembled with 2 M ZnSO₄ exhibited a specific capacity of 355 mAh g^-1 and an ultrahigh energy density of 400 Wh kg^-1.

The confinement effect of non-polar carbon materials on iodine species is limited. In order to enhance the interaction between carbon materials and iodine species, heteroatom doping has been proven to be an effective method^[47,50,60]. Yu et al.^[50] prepared a nitrogen-doped porous carbon as the host of iodine by a simple pyrolysis method [Figure 4C] and demonstrated that heteroatom doping can enhance the adsorption capability of carbon for iodine species. In particular, the interaction between iodine and graphite N (N-Q) was the strongest compared with its counterparts of N-6, N-5 and N-O, which could significantly improve the rate capability and cycle stability. Therefore, a battery assembled with a 1 M ZnSO₄ electrolyte delivered an operating voltage of 1.37 V, a specific capacity of 345.3 mAh g^-1 and a capacity retention of 80.9% after 10000 cycles at 10 C. Lu et al.^[47] prepared an N and S co-doped 3D graphene foam (3D-NSGF) and employed it as the scaffolds of both the cathode and anode of a Zn-I₂ battery. Benefiting from the increased defects and surface polarity derived from the doping of N and S, the specific surface and mesopores of the 3D-NSGF with a honeycomb architecture showed a certain degree of improvement. Hence, this 3D-NSGF exhibited an excellent confinement effect on iodine and could improve the transfer efficiency of electrons and ions, boosting the electrochemical performance of the cathode and anode simultaneously.

In addition to porous carbon materials, other materials with continuous channels have also been reported as the substrate of iodine. Li et al.^[52] fabricated I₂/MXene composite electrode (I₂-Nb₂CT_X) using electrochemical deposition to confine iodine in the layers of the MXene [Figure 4D]. Benefiting from the good conductivity of MXene and the adsorption to iodine species, the cathode could improve the redox kinetics and suppress the shuttle effect. The battery exhibited good rate capability and ultralong cycle stability with a power density of 23505 W kg^-1, an energy density of 259.3 Wh kg^-1 and a cyclic life of 23000 cycles. Ma et al.^[27] also reported a composite electrode (PBAs/I₂) based on a Prussian blue analogue Co[Co_1/4Fe_3/4(CN)₆] [Figure 4E]. The ordered pore structure of the composite and the synergistic effect of Fe and Co were conducive to the confinement of iodine. Furthermore, the electrocatalysis of transition metals could also facilitate the transport of electrons and ions, improving the redox kinetics of the electrode. Therefore, the battery showed a specific capacity of 236.8 mAh g^-1 and an energy density of 305 Wh kg^-1 at 0.1 A g^-1.

The materials reported above mainly absorb iodine through the physical interaction with iodine species. In addition to physical adsorption, chemical adsorption has also been reported to effectively confine iodine and suppress the shuttle effect of polyiodides. Polymer materials that have a good interaction with iodine usually show good adsorption to iodine species. In particular, polyvinylpyrrolidone (PVP) has been used as the substrate of iodine in Al-I₂^[61], Li-I₂^[62] and Mg-I₂^[63] batteries. However, the poor conductivity of PVP is not beneficial for the improvement of reaction kinetics, which hinders its further application. Zeng et al.^[51] prepared an iodine-doped polyaniline cathode (PANI-I₂) and verified that most of the iodine was chemically doped in the PANI chain by ex-situ Raman and XPS tests [Figure 4F]. During the cycle process, the polyiodides are confined to the polymer backbone through electrostatic interaction, which can effectively suppress the dissolution and shuttle effect of iodine species. Therefore, the battery assembled with 2 M ZnSO₄ has a reversible capacity of 160 mAh g^-1 and a capacity retention of 79% after 700 cycles at 1.5 A g^-1. Lin et al.^[59] constructed a double-layered cathode (CC-PPy-120) containing a conductive layer and an adsorption layer, a layer of polypyrrole (PPy) on the carbon cloth by in situ electropolymerization [Figure 4G]. While ensuring good electrical conductivity, this composite cathode could also reversibly absorb polyiodide ions through the electrostatic interaction between the PPy chain and the iodine species, thereby suppressing the shuttle effect and improving the Coulombic efficiency and cycle stability of the battery.

The optimization of the electrolyte also has a significant effect on alleviating the dissolution and shuttle effect of iodine species. The species^[48] and concentration^[64] of zinc salts, the pH of the electrolytes^[65] and additives in the electrolytes^[60,66] have all been reported to have a significant impact on the Coulombic efficiency, kinetic performance or cycle stability of aqueous Zn-I₂ batteries. Li et al.^[48] reported a Zn-I₂ battery with 2 M Zn(CF₃SO₃)₂ as the electrolyte. DFT calculations demonstrated that the absolute value of the binding energy of I₂ and CF₃SO₃²⁻ is distinctly lower than that of SO₄²⁻, meaning that the interaction of I₂ and CF₃SO₃²⁻ is more reversible, leading to a remarkable improvement in the utilization of iodine [Figure 5A]. In addition, the interaction of Zn(CF₃SO₃)₂, ZnI₂ and I₂ can also form a dense SEI film on the surface of the Zn anode, which can protect the zinc anode and suppress the zinc dendrite growth. In consequence, this battery exhibited an excellent energy density of 237 Wh kg^-1 and a capacity retention of 75% after cycling 9000 cycles at 5 A g^-1. Wang et al.^[60] designed a Zn-I₂ battery with overcharge protection capability by adding 35 mg mL^-1 of poly(2-vinylpyridine) (P2VP) to a 1 M ZnSO₄ electrolyte. In the overcharged state, due to the increase in pH, P2VP undergoes a deprotonation effect accompanied by a rapid transition from a hydrophilic soluble state to a hydrophobic gel state [Figure 5B]. This contributes to a sharp increase in the solution resistance and the charge transfer resistance of the electrolyte, thereby reducing the overcharge capacity to 6% of the initial capacity to avoid the failure of the battery. When the pH of the electrolyte was recovered, P2VP could transform into a soluble state again, allowing the battery to resume normal operation.

Figure 5. A: Structures and binding energies of different complexes by DFT calculations and the initial charge and discharge curves in different electrolytes. B: Schematic illustration of self-protection function of a Zn-I₂ battery and electrolyte resistances based on different concentrations of P2VP. C: Schematic diagram of Zn-I₂ battery configuration with hydrogel electrolytes. D: Schematic illustration of the structure of alginate-based hydrogels and Zn-I₂ battery with an alginate-based hydrogel electrolyte. E: Schematic diagram of Zn-I₂ batteries with different separators and optical images of the resistance property against triiodide of different separators. Figures reproduced with permission from Li et al.^[48], Wang et al.^[60], Soni et al.^[49], Shang et al.^[28] and Yang et al.^[29]. DFT: density functional theory.

Compared with liquid electrolytes, the reasonable design of gel electrolytes cannot only suppress the dissolution of iodine in cathodes but also mitigate the side reactions and dendrite growth on anodes. Sonigara et al.^[49] used an amphiphilic block copolymer (PEOn-PPOm-PEOn) with 0.5 M ZnSO₄ and 1 M KI + 0.1 M I₂ to prepare a gel electrolyte and a gel catholyte, respectively [Figure 5C]. The unique core-shell structure of this electrolyte can confine anions to the PPO region but allows zinc ions to rapidly migrate by the interaction with PEO, which can not only suppress the shuttle effect of iodide species but also endow the electrolyte with excellent ionic conductivity (> 1 mS cm^-1), thereby enabling the specific capacity of the battery to reach 228 mAh g^-1 at 0.5 C. Based on this work, this team also reported an iodine-enriched porous metal-organic gel catholyte (MOG-I)^[53]. The unique microporous structure of the MOG catholyte and the strong polarity of the central metal can effectively inhibit the diffusion of iodine species, which enables the battery to exhibit a specific capacity of 231.6 mAh g^-1 at 0.5 C and a capacity retention of 98.5% after cycling for 1500 cycles at 1 C. Shang et al.^[28] designed a hydrogel electrolyte based on sodium alginate [Figure 5D], which can suppress the shuttle effect of triiodide while rapidly transporting zinc ions, thereby alleviating the corrosion and dendrite growth of the zinc anode. A quasi-solid Zn-I₂ battery assembled with this hydrogel delivered a specific capacity of 188 mAh g^-1 at 0.2 A g^-1 and a capacity retention of 66.8% after cycling 2000 cycles at 2 A g^-1.

Another effective method for suppressing the dissolution and shuttle effect of iodine is through separator modification. Tangthuam et al.^[67] synthesized a series of polyelectrolyte membranes using different ratios of carboxymethyl cellulose (CMC) and polyvinyl alcohol (PVA) and used them as the separators of Zn-I₂ batteries. The results showed that with increasing CMC content, the polyelectrolyte membrane has a more compact morphology, a lower ion conductivity and a stronger blocking effect on polyiodides. However, unfortunately, all polyelectrolyte membranes can not completely suppress the shuttle effect of polyiodides. Yang et al.^[29] prepared a multifunctional metal-organic framework membrane, which not only suppressed the migration of triiodide to the anode but also changed the solvation structure of zinc ions to depress the water activity [Figure 5E]. Based on this synergistic effect, the side reactions and dendrite growth of the anode could be significantly suppressed. This Zn-I₂ battery delivered a specific capacity of 203 mAh g^-1 and a capacity retention of 86.4% after cycling 6000 cycles at 1.92 A g^-1.

Most current aqueous Zn-I₂ batteries are based on the two-electron redox reaction of I₂/I⁻, with a discharge plateau of ~1.3 V and a theoretical capacity of 211 mAh g^-1. Iodine is a typical polyvalent element and the batteries based on it exhibit a much higher discharge voltage and specific capacity if the redox reaction of iodine based on high valence can be activated. Li et al.^[39] prepared a halogenated MXene cathode (Ti₃C₂I₂) via heat treatment and constructed a Zn-I₂ battery with an electrolyte containing F⁻ or Cl⁻. Under the interaction between I⁺ and F⁻ or Cl⁻ verified by in-situ Raman characterization, the I⁺/I₂ redox is successfully activated and stabilized. In the optimized electrolyte of 2 M ZnCl₂ with 1 M KCl, the battery exhibited two discharge plateaus of 1.65 and 1.30 V [Figure 6A], as well as good reaction reversibility and cycle stability, with a cycle life of 2800 cycles at 3 A g^-1 and an energy density of 467 Wh kg^-1 based on I and 280 Wh kg^-1 based on Ti₃C₂I₂. Zou et al.^[38] employed an electrolyte of extremely concentrated ZnCl₂ and LiCl with a diluent of acetonitrile (ACN) to successfully realize a voltage plateau at ~1.71 V of I⁺/I₂ redox [Figure 6B]. When the molar ratio of the three materials was 19:19:8, the battery showed a specific capacity of 594 mAh g^-1, an energy density of 750 Wh kg^-1 and a capacity retention of 82% after cycling 6000 cycles at 2 A g^-1.

Figure 6. A: CV and Galvanostatic charge-discharge (GCD) curves of Zn||Ti₃C₂I₂ battery with different electrolytes. B: Charge and discharge curves of Zn-I₂ batteries with different electrolytes and typical voltage-capacity curve of 19-19-8 system and the corresponding in-situ Raman spectrum. Figures reproduced with permission from Li et al.^[39] and Zou et al.^[38]. CV: Cyclic voltammetry.

In view of the good reaction reversibility of active materials and the capacity contribution of host materials, the actual discharge capacities are generally close to the theoretical capacity or even exceeds the theoretical capacity of aqueous Zn-I₂ batteries, which has a long cycle life and considerable energy density. However, the energy densities of Zn-I₂ batteries are usually calculated based on the active materials. The rational design of the cathode structure to increase the mass loading and utilization rate of iodine to accomplish better overall energy density plays a pivotal role in the practical application of Zn-I₂ batteries. Another key factor restricting the improvement of the energy density of Zn-I₂ batteries is their inferior discharge plateau of ~1.3 V. Although it has been reported that the average discharge voltage of the batteries can be increased by activating the I⁺/I₂ redox, the employment of high-concentration electrolytes significantly raises the manufacturing cost of the batteries. Therefore, it is a more feasible method for developing new cathode substates or optimizing electrolytes to introduce a new redox reaction in a low-concentration electrolyte with the aim of achieving a higher operating voltage and energy density.

Aqueous Zn-Br₂ batteries

The liquid-liquid reaction mechanism of bromine endows it with faster reaction kinetics^[55] and therefore attracts extensive research on redox flow batteries. However, for static batteries, liquid Br₂ makes the structure design of batteries very challenging. Biswas et al.^[68] employed the sedimentation effect by gravity of bromine to design a minimal architecture Zn-Br₂ battery without a separator. The battery was constructed using 2 M ZnBr₂ as the electrolyte, porous carbon foam on the bottom of the battery and carbon cloth on the top of the battery as the cathode and anode current collector, respectively [Figure 7A]. During the charging process, due to the density of Br₂ being larger than that of the electrolyte, Br⁻ is oxidized to Br₂ and then gradually precipitates into the interior and on the surface of the porous carbon foam, while Zn²⁺ is reduced to Zn on the surface of the carbon cloth. If zinc dendrites are formed and contact the cathode, the Br₂ on the surface of the porous carbon foam reacts with them and converts them into Zn²⁺ to be dissolved in the electrolyte, thereby preventing the battery from short-circuiting. Lee et al.^[30] also reported a similar work. In order to enhance the ability of cathode to adsorb bromine, they assembled the battery employing nitrogen-doped microporous carbon on graphite felt as the cathode current collector [Figure 7B]. It was shown that nitrogen-doped microporous carbon exhibited excellent adsorption to bromides and polybromides, therefore significantly inhibiting the cross-diffusion and shuttle effect of bromine species. Therefore, the battery delivered excellent cycle stability with a VE value of 84% after 1000 cycles.

Figure 7. A: Schematic illustration and optical photographs of the minimal architecture Zn-Br₂ battery without a separator. B: Schematic diagram of the structure and mechanism of the membraneless and flowless Zn-Br₂ battery with an NGF electrode. C: Schematic diagram of fabrication of Br-Ti₃C₂T_X and crystal structure of Br-Ti₃C₂T_X and adsorption energy of Br species on Ti₃C₂T_X and the charge-of-density patterns of Br species on Ti₃C₂T_X. D: Schematic diagram of the structure of the Zn-Br₂ battery with TPABr additive. E: Schematic illustration and electrochemical stability window of the Zn-Br₂ battery with the alkaline-acid hybrid electrolytes. Figures reproduced with permission from Biswas et al.^[68], Lee et al.^[30], Li et al.^[31], Gao et al.^[32] and Yu et al.^[54].

Although there is a higher requirement for the substrates of liquid bromine, the substrates currently reported are still porous carbon materials, such as porous carbon foam^[68], nitrogen-doped microporous carbon^[30], ordered mesoporous carbon CMK-3^[32], porous carbon powder^[54], VC/carbon textile^[69] and so on. These carbon materials have an unsatisfactory confinement effect on bromine species. In addition to porous carbon materials, Li et al.^[31] prepared a Br-containing MXene cathode (Br-Ti₃C₂T_X) by electrodeposition [Figure 7C] and analyzed the interaction between bromine species and MXene by DFT calculations. The results showed that there was a good affinity and electron transfer ability between bromine species and MXene, which not only restricts the movement of bromine species, inhibiting the shuttle effect, but also improves the reaction kinetics, leading to good rate capability and cycle stability. The battery assembled with the electrolyte of 1 M Zn(OTF)₂ exhibits a discharge plateau of 1.75 V, a specific capacity of 179.6 mAh g^-1, an energy density of 259 Wh kg^-1 based on Br and a capacity retention of 81% after 2000 cycles at 2 A g^-1.

As well as the confinement of bromine through the structure design of cathodes, electrolyte optimization is another alternative method for confining bromine species. Gao et al.^[32] added a surfactant of 0.2 M TPABr to the electrolyte of 0.5 M ZnBr₂ to stabilize the bromine species and assembled a static Zn-Br₂ battery with CMK-3 as the cathode [Figure 7D]. Benefitting from the strong complexing ability of TPA⁺ to bromine species, soluble tribromide can be converted into insoluble TPABr₃, thereby greatly suppressing the shuttle effect of polybromide and the self-discharge behavior of the battery. In addition, TPA⁺ can impede the dendrite growth by adsorbing on the surface of the zinc anode during the charging process, thus improving the Coulombic efficiency and cycle life of the battery. Under the synergistic effect of the chemical adsorption of TPA⁺ additives and the physical adsorption of the CMK-3 cathode, the Zn-Br₂ battery has a discharge voltage of 1.7 V and a specific capacity of 487 mAh g^-1 at 0.1 A g^-1. It can cycle stably for 11000 cycles with a capacity retention of 63% at 5 A g^-1.

The high discharge voltage and theoretical capacity of aqueous Zn-Br₂ batteries are conducive to increasing the energy density of batteries. However, the liquid bromine has severely impeded the research progress of Zn-Br₂ static batteries. Due to the liquid-liquid reaction mechanism of Zn-Br₂ batteries, many hosts that have a good confinement effect on iodine have limited effect on bromine. As a result, there is an urgent need to develop more effective matrix materials to achieve a good restraint effect on bromine species. The cathode materials currently reported are almost all based on the physical interaction between the host materials and bromine species. It may be an effective method to develop new hosts based on chemical interactions, such as polymer materials. In addition, reasonable electrolyte additives can also confine bromine species from another perspective, so multifunctional additives should also get more attention. The separator design can simultaneously mitigate the dissolution of cathodes and the corrosion and dendrite growth of anodes, but this has not been reported in Zn-Br₂ batteries. This is also a development direction worthy of future study.