Recent progress and challenges of high-loading cathodes for aqueous Zn-ion batteries

Bio-polymer binder

A commercial PVDF binder with symmetrical C-F functional groups possesses poor affinity with the active materials via weak van der Waals forces. Additionally, a toxic and high-cost N-methyl-2-pyrrolidone (NMP) solvent is required for a PVDF binder. Water-based bio-polymers, such as polysaccharides and proteins, contain rich polar functional groups (-COOH, -OH, -NH₂, etc.)^[67-70]. These polar functional groups can firmly anchor the active materials via robust chemical interactions. Besides, their advantages of low cost, abundant resources, and eco-friendliness draw wide attention of researchers. For example, water-based sodium alginate (SA) and xanthan gum (XG) have been explored as the binders for AZIBs^[71,72]. The water-based bio-polymer binders can make the electrode manufacturing process greener and more sustainable. Nevertheless, the significant swelling of bio-polymer binders in aqueous electrolytes will decrease their mechanical strength. Constructing a network structure is an effective strategy to address the swelling issue. At the same time, the robust network of polymer binders can dramatically enhance its mechanical properties to accommodate the internal stress in high-loading electrodes [Figure 3B and C]^[73-77]. The common reactions used to construct the robust network binders are displayed in Figure 3D-G, including multiple hydrogen bonding, ionic cross-linking, guest-host interaction, and covalent bonding network.

Conductive binder

The electronic/ionic conductivity of polymer binders plays an important role in the fast charge transfer kinetics in thick high-loading cathodes. Owing to the insufficient wetting and permeation of the electrolyte towards thick electrodes, the ionic transmission in the electrode is very sluggish, leading to low active material utilization rate and specific capacity [Figure 4A], especially under lean electrolyte. The hydrophilic groups (e.g., -COOH, -OH, and -NH₂) in polymer binders can contribute to the increased interfacial wetting between cathode and electrolyte^[78-80], and therefore boost interfacial Zn²⁺ transfer kinetics effectively. On the other hand, polymer binders with zincophilic functional groups (e.g., -COO^- and -SO₃^-) can act as an anionic polyelectrolyte to accelerate the Zn²⁺ diffusion [Figure 4B]^[81,82]. A water-soluble binder combining a polysaccharide SA with a hydrophobic polytetrafluoroethylene (PTFE) has been developed for high-performance MnO₂ cathode^[82]. The -COO^- in SA can facilitate the adsorption and transmission of Zn²⁺, while the hydrophobic PTFE can drive the fast de-solvation of hydrated Zn²⁺. As a result, the high Zn²⁺ diffusion coefficient as well as fast Zn²⁺ intercalation kinetics enables a high rate performance and specific capacity. Moreover, the SA/PTFE hybrid binder also helps to form a protective layer on MnO₂ cathode, which inhibits the disproportion reaction of Mn³⁺ and improves its stability. To achieve fast electronic conductivity in cathodes, electronic conductive polymers have been exploited as novel binders^[83,84]. The conductive polymers generally contain conjugated structures along the polymer backbone to achieve electron mobility. For example, a poly (3,4-ethylene dioxythiophene) : poly (styrene sulfonate) (PEDOT : PSS) system has been widely used as a conductive binder^[85]. The introduction of conductive binders in cathodes can also reduce the amount of inactive conductive carbon additive, and thus improve the energy density of the battery.

Figure 4. (A) The schematic to show key issues of Zn²⁺ transfer in thick cathodes; (B) The schematic of Zn²⁺ transfer in Zn²⁺-conducting polymer binders.

Multifunctional binder

Multifunctional binders can act as multiple roles in electrodes, and address multiple issues simultaneously. For instance, in I₂ cathodes, the severe shuttle and sluggish conversion kinetics of polyiodide intermediates result in low active material utilization rate and coulombic efficiency^[86-88]. Therefore, the functional binder is expected to possess commendable chemisorption and catalytic capability. Wang et al.^[89] reported a bifunctional polyacrylonitrile copolymer binder (LA133) with strong iodine-chemisorption capability for shuttle-free aqueous Zn-I₂ batteries. Via theoretical calculation, the amide and carboxyl groups in LA133 binder can strongly bond to polyiodides to immobilize them in the cathode matrix [Figure 5A]. As a result, under a high mass loading of 7.82 mg cm^-2, the Zn-I₂ battery can still retain 83.3% of its initial capacity after 1000 cycles, which is much superior to the common PTFE binder [Figure 5B]. Owing to the poor iodine-chemisorption capability of PTFE binders, soluble I₃^- species diffuse toward the Zn anode side and react with zinc metal, causing severe Zn anode corrosion and roughening. As a result, serious side reactions on both cathode and anode occur, demonstrated by the scanning electron microscopy (SEM) images of cycled cathodes and Zn anodes [Figure 5C-F]. Yang et al.^[90] also proposed a Janus functional binder based on chitosan (CTS) molecules to hinder the serious shuttle effect of polyiodides. On the one hand, CTS molecules exhibit a robust double helix structure with strong mechanical properties to withstand the electrode swelling effect. On the other hand, the polar groups (-NH₂, -OH, and -O-) in CTS molecules show a strong affinity with iodine species to hinder their dissolution. Furthermore, these interactions enhance iodine catalytic conversion, effectively mitigating the undesired shuttle effect.

Figure 5. (A) The optimized adsorption configurations and corresponding binding energies of the PTFE binder and LA133 binder to I^-, I₃^-, and I₂ species; (B) Cycling performance of high-loading I₂ cathodes using LA133 binder and PTFE binder; (C and D) SEM images of the cycled cathodes using LA133 binder and PTFE binder, respectively; (E and F) SEM images of the cycled Zn anodes in Zn-I₂ batteries using LA133 binder and PTFE binder, respectively^[89]. Copyright 2024 The Authors. PTFE: Polytetrafluoroethylene; SEM: scanning electron microscopy.

This section summarizes the recent advances in the binder design for AZIBs. For constructing high-loading cathodes, the binders should first possess strong mechanical strength to withstand the electrode swelling and huge stress during electrode preparation and battery cycling. Multifunctional binders are a highly significant research direction to address multiple issues in the cathodes, which can further reduce the amount of inactive functional materials. Above all, water-based bio-polymer binders can make the electrode manufacturing process green and sustainable, and thus should be paid broad attention.

Slurry coating

Current collectors act as the electronic and mechanical support to the electrode coating, and play an important role in maintaining the physical structure of the electrode. Coating the cathode slurry into the 3D current collector is a straightforward way to achieve 3D structured electrodes. Carbon-based materials with the advantages of light weight and high electronic conductivity attract wide attention as 3D current collectors^[91,92]. For instance, Ma et al.^[91] constructed high-mass-loading VO₂ electrodes via combining commercial carbon fabric 3D current collectors and PEDOT : PSS conductive binders. Such a strategy provides fast electron transfer and offers sufficient void space to hold active materials and electrolytes. Therefore, the VO₂ loading could reach as high as 15 mg cm^-2 and the cathode can maintain a capacity of 275 mAh g^-1. Biomass, as the sustainable and low-cost raw materials, has been widely used to prepare carbonaceous materials^[93-96]. As shown in Figure 6A, Zhao et al.^[96] directly converted rattan into a carbon-based 3D current collector with directional channels and high compressive strength via an ultraviolet light irradiation-assisted delignification followed by a high-temperature carbonization. Then, a cathode slurry containing carbon nanotube (CNT)/α-MnO₂, conductive carbon, and PVDF binder (7:2:1) was uniformly coated into the as-prepared 3D current collector to achieve a high-mass-loading carbonized rattan CR/MnO₂ cathode, as shown in Figure 6B-F. Owing to the 3D architecture of the CR/MnO₂ cathode that can expose sufficient active sites for Zn²⁺ ion storage, high areal capacity can be achieved under high mass loading, as displayed in Figure 6G and H.

Figure 6. (A) Schematic illustration of the fabrication process of high-loading CR/MnO₂ cathode; (B and C) Top-view and (D and E) cross-section SEM images of CR/MnO₂ cathode; and (F) corresponding EDS mapping images; (G) Rate performance and (H) cycling performance of AZIBs with different MnO₂ cathodes^[96]. Copyright 2024 Elsevier Inc. SEM: Scanning electron microscopy; AZIBs: aqueous Zn-ion batteries; EDS: energy dispersive X-ray spectroscopy.

In-situ growth

In-situ growth of 3D-structured active materials on the conductive substrate is another effective strategy to construct 3D electrodes, which avoids the use of additional binder and conductive additive^[97-100]. As shown in Figure 7A-C, Chen et al.^[97] synthesized V₃O₇·H₂O nanoarrays (VOHAs) via facile one-step recrystallization of dissolved V₂O₅ from HCl solution on carbon-fiber paper, which was directly used as a binder-free cathode for AZIBs. The open pore structure of the VOHA cathode can boost the electrolyte infiltration and Zn²⁺ transfer. The dependence of the capacities on mass loading was studied as well. Figure 7D displays that the free-standing VOHA cathode can maintain maximal active material utilization with mass loading up to 5.0 mg cm^-2. When the mass loading is over 5.0 mg cm^-2, specific capacity drops with the increasing mass loading, owing to the decrease in the utilization rate of active materials. Therefore, the rational design for the electrode microstructure is vital. It should be further developed to effectively facilitate ionic/electronic transport and thus decrease such a capacity drop under increasing mass loading. Gao et al.^[98] reported a Na_0.55Mn₂O₄·1.5H₂O (NMO) with 3D varying thinness CNT networks (NMO/VTCNTs) as free-standing binder-free cathodes to simultaneously increase the mass loading of the active material and enhance the ionic/electronic conductivity of the electrode. NMO was firstly grown on low-thinness CNTs (LTCNTs, with a diameter of 5-10 nm) via a facile co-precipitation method to obtain NMO/LTCNTs composite. Then, the NMO/VTCNTs free-standing cathode was obtained via suction filtration of NMO/LTCNTs and high-thinness CNTs (HTCNTs, with a diameter of 110-150 nm) solution. The as-preparedNMO/VTCNTs cathodes are able to achieve a loading of about 5 mg cm^-2 and can maintain excellent electrochemical performance with a capacity of 158 mAh g^-1 after 1000 cycles at 2 A g^-1.

Figure 7. (A) Schematic of the one-step synthesis process for the binder-free VOHAs cathode; and (B and C) the corresponding SEM images; (D) The relationship between the capacity and mass loading of cathode materials^[97]. Copyright 2021 Elsevier Ltd. SEM: Scanning electron microscopy; VOHAs: V₃O₇·H₂O nanoarrays.

Novel manufacturing technology

The utilization of a novel electrode preparation process such as 3D printing has also attracted wide attention to construct high-loading electrodes^[101-104]. 3D printing technology shows unique manufacturing advantages, such as customized design, rapid prototyping, and structural optimization. Therefore, 3D printing technology has been widely used to fabricate various materials and devices in the fields of energy storage, electronic industry, etc. Yang et al.^[103] prepared a carbon-based 3D current collector [3D-printed carbon microlattices (3DP CMs)] by direct ink 3D printing and subsequent high-temperature annealing. Then, high-loading MnO₂ cathodes can be achieved via electrochemical deposition of MnO₂ on the 3DP CMs. Finally, 28.4 mg cm^-2 MnO₂ can be loaded on the 3D-printed current collector to deliver an areal capacity of 8.04 mAh cm^-2 at 0.1 mA cm^-2. As shown in Figure 8A and B, Ma et al.^[104] also manufactured a cellular and hierarchical porous Fe₅V₁₅O₃₉(OH)₉·9H₂O/reduced holey graphene oxide (FeVO/rHGO) cathode using the direct ink writing technology, where the ink consists of a composite of FeVO and rHGO with PVDF binder. The unique structure with periodic open macro-porous channels and hierarchical porous structures can boost the infiltration of electrolytes. As a result, the mass loading of FeVO/rHGO cathode with four printing layers can reach 24.4 mg cm^-2, which can give a remarkably high areal capacity of 7.04 mAh cm^-2 at 6 mA cm^-2 [Figure 8C].

Figure 8. (A) Schematic of direct ink writing fabrication process for the cellular FeVO/rHGO cathode; and (B) corresponding SEM image; (C) The plots of areal capacity of the FeVO/rHGO cathode with different printing layers at 6 mA cm^-2[104]. Copyright 2021 Wiley-VCH GmbH.

In summary, this section reviews the progress of high-loading cathodes based on 3D-structured electrodes. The 3D structured electrodes are commonly used to prepare high-loading cathodes in the laboratory study. However, compared to the commercial slurry-casting preparation process on planar current collector, these electrodes usually require additional preparation processes, which will increase the electrode manufacturing cost. Furthermore, the vast inner void space in the 3D structured electrodes will not only increase the electrolyte dosage but also decrease the volumetric energy density. Therefore, the future development of 3D structured electrodes should pay attention to sustainable and large-scale processing techniques. Moreover, the electrode microstructure should be optimized to facilitate ionic/electronic transport and minimize the excess inner void space simultaneously.

Surface engineering

Surface engineering with conducting materials has been demonstrated as an effective approach to improve the electronic conductivity and structural stability of active materials^[105-108]. Zhang et al.^[106] reported a graphene-wrapped H₁₁Al₂V₆O_23.2 nanobelt (HAVO@G) composite, which was prepared by a hydrothermal method and freeze-drying technique. The uniform graphene coating on the surface of HAVO nanobelts can improve electrical conductivity and inhibit the dissolution of the active material into electrolyte, thus leading to enhanced reaction kinetics and structural stability. As a result, the HAVO@G electrode exhibits excellent cycling stability under a high mass loading of 15.7 mg cm^-2, delivering a high reversible capacity of 131.7 mAh g^-1 after 400 cycles at 2 A g^-1. Recently, MXenes have attracted extensive attention in AZIBs, owing to their high metallic conductivity and excellent hydrophilicity^[109,110]. Furthermore, compared to carbonaceous materials, MXenes possess high density to achieve high-tap-density cathode composite, and thus can improve the volumetric energy density of AZIBs. Shi et al.^[110] prepared high-density Ti-MXene (Ti₃C₂T_x)@MnO₂ micro-flowers via a gas-phase spray drying approach, where MnO₂ nanoparticles are encapsulated in the MXene nanosheets. The Ti₃C₂T_x@MnO₂ micro-flowers not only show high tap density of 1.52 g cm^-3 but also possess fast 3D conductive architecture. As a result, under a mass loading of 8.0 mg cm^-2, satisfactory rate property and cycling stability can still be achieved.

Lattice regulation

Lattice regulation of the active material is considered as another effective way to enhance their ionic/electronic conductivity and structural stability^[111-115]. Doping/insertion of metal ions is one of the common lattice matrix regulation strategies. Yang et al.^[112] reported a Mn_2.5V₁₀O₂₄·5.9H₂O (MVOH) cathode material, in which inserted Mn²⁺ can regulate the electronic structure and defective state of the MVOH material. The inserted Mn²⁺ can form new coordination bonds with O atoms in the [VO] layer to regulate the electronic structure, resulting in high electronic conductivity and structural stability [Figure 9A]. Simultaneously, resulting from the insertion of low-valence state Mn²⁺, abundant O vacancies occur in the MVOH, and thus the electrostatic interactions between Zn²⁺ and the cathode host can be weakened, decreasing Zn²⁺ diffusion energy barrier [Figure 9B]. As a result, the rate property and cycling stability of MVOH cathodes can be obviously improved, as shown in Figure 9C. In particular, the high-loading (~12 mg cm^-2) MVOH cathode with synchronously promoted electronic and ionic transport can deliver a high areal capacity of 3.42 mAh cm^-2 after 850 cycles with a remarkable capacity retaining rate of 91.4 %. Zong et al.^[113] reported the two-dimensional topological Bi₂Se₃ with Bi-vacancies derived from Cu doping (Cu-Bi_2-xSe₃) through a one-step hydrothermal process. Density functional theory (DFT) calculations indicate that the formation energy of Bi vacancies for Cu-Bi_2-xSe₃ is dramatically lowered compared to the pure Bi₂Se₃ sample (1.1 eV vs. 3.7 eV). Importantly, the formation of Bi vacancies can effectively enhance the electronic conductivity and reduce the Zn²⁺ migration barrier, resulting in more carrier reversible insertion. Consequentially, the specific capacity of high-loading Cu-Bi_2-xSe₃ cathodes can be maintained as high as 231.5 mAh g^-1 and 170.2 mAh g^-1 after 50 cycles under the mass loading of 6.59 mg cm^-2 and 10.18 mg cm^-2, respectively.

Figure 9. (A) Calculated PDOS and TDOS profiles; and (B) the possible Zn²⁺ migration pathways and calculated diffusion energy barrier in the crystal lattice of cV₂O₅, VOH, and MVOH; (C) Electrochemical performance of the high-loading V-based cathodes^[112]. Copyright 2023 Elsevier B.V. MVOH: Mn_2.5V₁₀O₂₄·5.9H₂O; VOH: V₁₀O₂₄·6.9H₂O; PDOS: partial density of states; TDOS: total density of states.

Interestingly, the metal ion doping/intercalation can boost the reaction kinetics via an activation effect. For example, a co-intercalation strategy of Na⁺ and Cu²⁺ was introduced into δ-MnO₂ (NCMO) by Gao et al.^[116]. On the one hand, the co-intercalation of Na⁺ and Cu²⁺ synergistically modulates the electronic structure of δ-MnO₂ to achieve rapid electronic transfer. On the other hand, the activation effect of Cu²⁺ towards the Mn²⁺/Mn⁴⁺ two-electron conversion is amplified by the pre-intercalation of Na⁺, resulting in improved specific capacity. Therefore, the NCMO cathode can exhibit a specific capacity as high as 576 mAh g^-1 under the mass loading of 1.0 mg cm^-2 and an areal capacity of 2.1 mAh cm^-2 under the high mass loading of 10.9 mg cm^-2.

Molecule/atom engineering

Molecule insertion is another promising approach to expand the interlayer spacing of the cathode host and boost Zn²⁺ transport kinetics^[117-119]. Yao et al.^[118] reported a MoS₂/C₁₉H₄₂N⁺ (CTAB) superstructure, in which the MoS₂ monolayer is alternately separated by the CTAB layer. Owing to the insertion of CTAB, the interlayer spacing of the layered MoS₂ host increases significantly from 0.63 nm to 1.0 nm [Figure 10A and B]. Specially, due to the close-knit interactions between MoS₂ monolayers and CTAB layers, the stability of the layered structure can be strengthened. Meanwhile, the soft CTAB layer, as a hyperelastic buffer, can effectively alleviate expansion stress and suppress structural collapse, which has been indicated by the in-situ synchrotron X-ray diffraction (SXRD) tests in Figure 10C. The high-resolution transmission electron microscopy (HRTEM) images during cycling further reveal the breathing of the interlayer distance between 0.98 nm and 1.17 nm upon Zn²⁺ insertion/extraction [Figure 10D and E]. Therefore, the MoS₂-CTAB electrode shows enhanced cycling stability with a capacity retention of as high as 92.8% at 10 A g^-1 over 2100 cycles. Even under a large mass loading of 11.5 mg cm^-2, the MoS₂-CTAB electrode can cycle stably over 50 cycles at 1 A g^-1 [Figure 10F]. Moreover, as the mass loading increases, an almost linear increase in areal capacity can be achieved.

Figure 10. HRTEM images of (A) pristine MoS₂ and (B) MoS₂-CTAB; (C) In-situ SXRD tests of the MoS₂-CTAB superstructure during the Zn²⁺ insertion/extraction processes; HRTEM images of MoS₂-CTAB (D) after charging and (E) after discharging; (F) The electrochemical performance of the MoS₂-CTAB superstructure under different mass loading^[118]. Copyright 2022 American Chemical Society. HRTEM: High-resolution transmission electron microscopy; SXRD: synchrotron X-ray diffraction; CTAB: C₁₉H₄₂N⁺.

Apart from the high intrinsic ionic conductivity of the cathode host materials, ion transport highway in the electrode level is also of great importance. Jiang et al.^[120] reported that a sharp capacity shrinkage under high loading can be caused by the formation of vast alkaline zincate [Zn₄SO₄(OH)₆·nH₂O, ZSH] on the surface of MnO₂ cathode, which blocks the ionic transfer pathway. The H⁺/Zn²⁺ co-intercalation in MnO₂ cathodes, which is widely demonstrated, can lead to H⁺ consumption and thus shift the acid-base balance in the aqueous electrolyte to continuously produce OH^-. Under high loading, the massive consumption of H⁺ can lead to sharp increase of local pH and vast ZSH generation on the cathode/electrolyte interface. The ZSH can fully cover the high-loading MnO₂ cathode, leading to a stagnant electrochemical reaction. To address this issue, they introduced the positively charged interstitial carbon into the lattice matrix of MnO₂ (IC@MnO₂). DFT calculations indicate that IC@MnO₂ possesses lower affinity to SO₄^2- and OH^- than that of bare MnO₂, and thereby suppresses the ZSH generation and covering effect. Hence, under a high mass loading of 10 mg cm^-2, the IC@MnO₂ cathode can deliver a much higher active material availability and specific capacity than the MnO₂ electrode (176.4 mAh g^-1vs. 108.5 mAh g^-1).

This section summarizes the rational cathode material design strategies for high-loading cathodes. On the one hand, the conductive surface coating can significantly improve electronic conductivity and suppress the collapse of active materials. On the other hand, the lattice matrix regulation of the cathode host effectively adjusts their electronic structure, interlayer spacing, and structural stability. The integrated strategy combining surface coating and lattice regulation should be developed to enhance both apparent and intrinsic conductivity of cathode materials, and thus effectively decrease the capacity shrinkage upon increasing mass loading. For insertion-type cathode materials, the H⁺/Zn²⁺ co-intercalation can result in massive ZSH by-products and severe blocking effect. It should be paid wide attention to avoid such covering effect and ensure fast ion transfer on the cathode/electrolyte interface, especially under high mass loading.

Artificial coating

Constructing a solid protective layer is an effective approach to homogenize the Zn²⁺ flux and shield side reactions, as shown in Figure 11A. Various artificial protective films have been constructed by blade-coating, spin-coating, atomic layer deposition and other processes^[121-128]. Zhu et al.^[121] constructed a multifunctional water-glass artificial protective layer containing rich Si-O hydrophilic groups on Zn anode surface via a spin-coating method. The vast polar Si-O functional groups can effectively improve the interfacial wettability, facilitate the even electric field distribution, and guide uniform Zn deposition along the (002) plane. Hong et al.^[122] coated a cation-exchange membrane of perfluorosulfonic acid on the Zn anode surface, which contains abundant sulfonic acid groups. This cation-exchange membrane can electrostatically repulse the SO₄^2- anions while accelerating the uniform Zn²⁺ ion transport. Therefore, the formation of Zn₄SO₄(OH)₆·5H₂O by-products can be suppressed and the homogenization of Zn²⁺ distribution on the Zn anode surface can be achieved.

Chen et al.^[125] constructed a polyvinyl alcohol (PVA)@SO₄^2- receptor (SR)-ZnMoO₄ multifunctional composite coating on Zn anode with a solid electrolyte interphase (SEI)-like structure. The PVA@SR outer layer with good flexibility can not only improve the coating stability but also effectively bind SO₄^2- to hinder its access to the Zn surface. Meanwhile, it finds that the ZnMoO₄ inorganic component with high ionic binding energy can effectively enhance the ionic conductivity of the composite coating. Based on these advantages, the Zn anode exhibits high reversibility with a coulombic efficiency of 99.42% and shows a superior high-capacity cycling performance of 1,700 h at 5 mA cm^-2/5 mAh cm^-2. To shield H₂O molecules, Zhou et al.^[128] construct a hydrophobic-zincophilic bifunctional layer on Zn anodes, which is synthesized by free radical polymerization of PVDF and acrylic acid. The hydrophobic framework acts as a barrier layer to isolate the Zn anode from the active H₂O molecules in aqueous electrolyte, and thus suppresses the HER and corrosion reactions. Meanwhile, the zincophilic functional groups (-COOH) on the sidechain can form effective transfer pathways for Zn²⁺, and thus guide uniform Zn deposition. As a result, even under high current density and areal capacity (12 mA cm^-2 and 6 mAh cm^-2), Zn||Zn symmetrical cells can cycle stably for 1,000 h. In sharp contrast, the symmetrical cells with bare Zn anodes are unbearable to harsh conditions and fail after only several cycles.

Under large-capacity cycling, huge volume change and stress can be generated on the interface of Zn anode/protective layer, due to the massive Zn plating/stripping. Therefore, to maintain the long-term function of the artificial protective layer, the self-adapting/self-healing ability is of great importance^[129-132], as shown in Figure 11B and C. Through dynamic or reversible chemical interactions, the self-adapting/self-healing coating can be achieved. For instance, Guo et al.^[130] reported a self-adaptable poly(dimethylsiloxane) (PDMS)/TiO_2-xcoating to accommodate the volume change and inhibit dendrite growth, in which PDMS possesses high self-adaptability resulting from the dynamic and reversible B-O bonds. Hong et al.^[131] designed zinc alginate hydrogel as a dynamic coating and Zn²⁺ redistributor on Zn anode via in-situ cross-linking effect. Therefore, the Zn anode with a self-adapting interface can accommodate the volume change and repair the possible ruptures to achieve long-term cycling stability. We also constructed a mechanically robust and self-healing protective coating, featured by dual cross-linking dynamic network between SA and Zn²⁺/lithium polysilicate^[132]. On the one hand, the dual cross-linking network with strong mechanical properties is highly resistant to the deformation of Zn anodes. On the other hand, when cracks occur in the network coating, it can effectively self-repair to durably protect the Zn anode.

Electrolyte engineering

Electrolyte design is a promising strategy to suppress the H₂O-induced side reactions and achieve uniform planar Zn deposition. Various electrolyte optimization strategies have been developed, including adjusting Zn salt type, exploiting functional additives, introducing co-solvents, and constructing eutectic solvents and concentrated electrolytes^[133-138]. Exploring electrolyte additives is an economical and scale-up approach to improve the stability of Zn anodes. Figure 12A displays the common functions of the electrolyte additives, including adsorption and shielding, regulating hydrogen bonding network, reconstructing Zn²⁺ solvation structure, and in-situ forming SEI film. The electrolyte additives can form a layer of protective film via adsorption or reaction on the Zn surface to shield side reactions and adjust Zn²⁺ flux. Li et al.^[139] introduced an electrolyte additive N,N-dimethylformamidium trifluoromethanesulfonate (DOTf) in the 2M ZnSO₄ aqueous electrolyte. Interestingly, in water, the DOTf is dissociated to neutral ion pairs triflic acid and N,N-dimethylformamide, resulting in the formation of Zn²⁺-conducting nanostructured SEI on the Zn surface. Such a SEI layer not only excludes H₂O but also induces nanoscale nucleation-growth process of Zn, leading to near 100% coulombic efficiency for the Zn||Cu asymmetric cell under 4 mA cm^-2/4 mAh cm^-2. Zeng et al.^[140] reported the in-situ formation of a dense and stable SEI layer with high ionic conductivity by introducing Zn(H₂PO₄)₂ into the electrolyte. The formed SEI not only hinders the side reactions by isolating Zn anode from the bulk electrolyte but also enables uniform Zn²⁺ transport. Hence, homogeneous Zn deposition and stable Zn plating/stripping have been achieved, as shown in Figure 12B and C. Finally, Figure 12D displays that the Zn||V₂O₅ full cell can give a long cycling lifespan under high loading and low N/P ratio. We also exploited penta-potassium triphosphate (KTPP) as a novel trifunctional electrolyte additive to tune the electrode/electrolyte interface^[141]. Firstly, the KTPP electrolyte additive can induce a Zn²⁺-conducting and mechanically robust protective film on the Zn anode surface. Secondly, the KTPP can complex with Zn²⁺ to reconstitute their dissolution sheath structure. Finally, the K⁺ cations from KTPP can adsorb on the Zn anode tips acting as a dynamic shielding layer to regulate Zn²⁺ flux. Consequentially, Zn||Zn symmetric cells achieve dramatically improved cycling stability and a high cumulative capacity of 6,400/7,200 mAh cm^-2 at 40/20 mA cm^-2. Li et al.^[142] used L-cysteine to construct a self-assembled multilayer, acting as a buffer layer on the Zn surface. The molecular multilayer dynamically replenished the defects caused by the change in electrode morphology during the cycling process, and thus achieved dendrite-free Zn anodes with excellent stability.

Figure 12. (A) Schematic illustration of function mechanism for the electrolyte additive. SEM images of Zn electrodes after cycling for 400 h in the (B) baseline electrolyte (BE) and (C) the designed electrolyte (DE) at 1 mA cm^-2/1 mA h cm^-2; (D) Cycling stability of Zn||V₂O₅ cells under practical conditions^[140]. Copyright 2021 Wiley-VCH GmbH. SEM: Scanning electron microscopy.

The coordinated H₂O within the Zn²⁺ solvation sheath and active free water in the bulk electrolyte cause severe HER and side reactions. Several electrolyte additives/co-solvents have been designed to reconstruct the Zn²⁺ solvation sheath and hydrogen bonding network in the electrolyte^[143-148]. For example, You et al.^[143] presented a cost-effective and eco-friendly aqueous electrolyte consisting of formamide (FA) and zinc acetate. FA, as both a hydrogen bonding acceptor and donor, can powerfully anchor H₂O molecules via a double-site anchoring configuration [Figure 13A] and thereby break the original hydrogen bonding network. As a result, the free water activity is reduced to decrease the HER. Meanwhile, the freezing point of the electrolyte is significantly decreased to < -40 °C [Figure 13B]. Moreover, by adding FA into the electrolyte, uniform Zn deposition can be achieved by regulating the Zn²⁺ solvation structure. As a result, the Zn||Zn symmetric cell and Zn||polyaniline (PANI) full cell with the as-designed electrolyte display impressive cycling stability at -30 °C, as shown in Figure 13C and D. We exploited the sustainable natural disaccharide, D-trehalose (DT), as a multifunctional co-solvent to massively reconstruct Zn²⁺ coordination structure and hydrogen bonding network of the electrolyte^[144]. DT molecules contain a great number of polar -OH groups, which can act as a facile platform to form strong interactions with Zn²⁺ and H₂O molecules. Especially, the massive strong hydrogen bonds between DT and H₂O molecules can not only significantly reduce the H₂O activity but also suppress their rearrangement under low temperatures. Consequently, at 5 mA cm^-2/5 mAh cm^-2, the Zn||Zn symmetric cell using DT30 can stably cycle over 800 h, which is much superior compared to the cell without DT co-solvent.

Figure 13. (A) Calculated binding energies of H₂O-H₂O, H₂O-FA, H₂O-OAc^-, FA-FA, and FA-OAc^- pairs; (B) The DSC curves of the electrolytes with different volume ratios of H₂O and FA; Cycling stability with the ZHF50 electrolyte at -30 °C for (C) Zn||Zn cell and (D) Zn||PANI cell^[143]. Copyright 2023 The Royal Society of Chemistry. FA: Formamide; DSC: differential scanning calorimetry.

The competitive HER will lead to the localized high-concentration OH^- on the Zn anode surface, which combine with Zn²⁺ and SO₄^2- to form ZSH by-products^[149-152]. The insoluble ZSH not only covers the active surface of Zn anodes but also causes the loss of active Zn, leading to fast failure of Zn anodes. Moreover, for the practical application of AZIBs, the low N/P ratio is indispensable. The loss of active surface and active Zn is fatal under limited usage of Zn anode in practical AZIBs. To address this issue, we proposed an effective reactivation strategy for the main by-product ZSH on Zn anodes enabled by the catalysis-coordination chemistry^[153]. We found that the urea electrolyte additive can preferentially adsorb on the produced ZSH by-products [Figure 14A]. Then, it can suffer from catalytic hydrolysis to generate strong ligand NH₃. The NH₃ rapidly adsorbs on the ZSH surface and coordinates with the Zn²⁺ ion to form soluble [Zn(NH₃)₄]²⁺, resulting in the timely decomposition of ZSH, as shown in Figure 14B. Based on this strategy, the reversibility of Zn anodes has been obviously improved, and the cumulative capacity of the Zn||Zn symmetric cell reaches as high as 10,600 mAh cm^-2 at 40 mA cm^-2 [Figure 14C]. For practical application, the as-prepared electrolyte can act as a reactivation solution to repair the waste AZIBs, as shown in Figure 14D and E.

Figure 14. (A) Calculated adsorption energies of H₂O, urea, and NH₃ molecules on the Zn (002) and ZSH (001) crystal planes; (B) Schematic demonstration of the reactivation of ZSH@Zn by urea additive; (C) Cycling stability of Zn||Zn symmetric cell in the designed electrolyte ZHU at 40 mA cm^-2/2 mAh cm^-2; (D) Model diagram of the reactivation process for the waste battery under practical application; (E) Cycling performance of Zn||LiFePO₄ full cell before and after the reactivation^[153]. Copyright 2024 Elsevier Inc.

To match the high-areal-capacity cathodes, this section reviews the recent progress of Zn anodes under large capacity based on surface coating and electrolyte engineering. The artificial surface coating as a solid barrier can effectively shield the active water and regulate the Zn²⁺ flux to achieve uniform and dendrite-free Zn deposition. In-situ and ex-situ methods have been developed to construct the functional surface coatings. The Zn²⁺ coordination structure and hydrogen bonding network of the electrolyte can be adjusted by electrolyte engineering to effectively regulate the deposition behavior of Zn and decrease the activity of water. However, under practical conditions, massive Zn stripping/plating can cause huge volume change and internal stress. Therefore, to accommodate the large stress, the mechanical property and self-adapting/self-healing ability of the artificial protective layer should be given considerable attention. For current study, flooded electrolyte is commonly used to achieve optimal cycling stability. Under lean electrolyte, the optimal electrolyte composition should be reevaluated. Under a low N/P ratio, the loss of active Zn will result in fast capacity fading of the AZIBs. Therefore, the development of an effective reactivation strategy for dead Zn is an important and urgent research direction.