Mini-Review on the Regulation of Electrolyte Solvation Structure for Aqueous Zinc Ion Batteries

Abstract

Zinc as an anode, with low potential (−0.762 V vs. SHE) and high theoretical capacity (820 mAh g⁻¹ or 5854 mAh L⁻¹), shows great promise for energy storage devices. The aqueous zinc ion battery (ZIB) is known as a prospective candidate for large-scale application in the future due to its high safety, environmental friendliness, abundant zinc resources on earth, and low-cost advantages. However, the existence of zinc dendrites and side reactions limit the practical application of ZIBs. Therefore, a lot of effort has been made to improve the performance from aspects including the structure design and surface modification of zinc anodes, regulation of the electrolyte solvation structure, and design of the functional separator. In this review, we attempt to summarize recent advances on the regulation of the electrolyte solvation structure through a number of selected representative works from two aspects: high-concentration salt strategy and electrolyte additives. At the end of this review, the challenges and future development prospects are briefly outlined.

1. Introduction

The depletion of fossil energy and deterioration of the environment have led to the rapid development of sustainable clean energy, such as wind and solar energy. Thus, their proportion in the energy consumption structure has increased yearly. Due to the discontinuous and regional characteristics of clean energy, the development of energy storage devices has been driven [1,2,3]. The aqueous zinc ion battery (ZIB) with metal zinc as the anode exhibiting a high theoretical capacity and low reduction potential has attracted much attention. Meanwhile, ZIBs show the advantages of low cost with rich and cheap zinc resources, high energy density, and high safety, which has been regarded as one of the most favorable candidate systems for broad application prospects [4,5,6].

As early as 1986, Yamamoto et al. [7] made a preliminary exploration of the neutral secondary zinc manganese battery. In 2012, Kang et al. [8] proposed an environment-friendly and safe power battery structure consisting of MnO₂ as the cathode with zinc metal as the anode, and ZnSO₄ as the neutral electrolyte solution, which is called a “zinc ion battery” with the working principle of reversible intercalation/stripping of Zn²⁺ in MnO₂. These works opened the prelude to the research of ZIBs. Since then, researchers have invested a lot of effort in studying the energy storage mechanism of ZIBs and have proposed many feasible strategies, for example, Zn²⁺ insertion/extraction, H⁺/Zn²⁺ insertion/extraction, and chemical conversion reaction.

1.1. Zn²⁺ Insertion/Extraction Mechanism

The cathode materials of ZIBs are generally selected from nanomaterials with a layered or tunneled structure. The Zn ions are inserted/extracted in the cathode material during the working process of ZIBs. Kang et al. [8] found that the energy storage in Zn²⁺||α−MnO₂ cells can be ascribed to the reversible inserted/extracted reaction of Zn²⁺ in/from the lattice tunnel. The Zn anode can be successfully oxidized to Zn²⁺ during the discharge procedures and rapidly inserted in the cathode material with a laminar or tunnel structure after being transported through the electrolyte. Some of the Mn⁴⁺ is quickly reduced to Mn³⁺, but Mn³⁺ is prone to disproportionation reactions and converts to Mn⁴⁺ and Mn²⁺ due to its instability nature. Zn²⁺ would be removed from the cathode material during the charge process, pass through the electrolyte solution, and return to the anode for reduction to Zn. The Mn²⁺ in the electrolyte returns to the manganese vacancy and is oxidized to Mn³⁺ and Mn⁴⁺, and the original tunneling structure is restored [9]. Furthermore, Mai et al. [10] also confirmed the highly reversible single-phase reaction of Zn²⁺ insertion in vanadium-based materials using the operando technique and the corresponding qualitative analysis. The reaction equation of manganese-based materials can be described as follows:

$$\mathrm{Cathode} \mathrm{process}: {\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}+2{\mathrm{MnO}}_2\underset{}{\leftrightarrow }{\mathrm{ZnMn}}_2{\mathrm{O}}_4$$

(1)

$$\mathrm{Anode} \mathrm{process}: \mathrm{Zn}\stackrel{}{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}$$

(2)

1.2. H⁺ and Zn²⁺ Co-Insertion/Extraction Mechanism

The energy storage mechanism for another Zn²⁺||α-MnO₂ cell was proved to be the H⁺/Zn²⁺ co-insertion/extraction process. Wang et al. [11] characterized the diffusion kinetics of Zn²⁺ and H⁺ by the constant current intermittent titration technique (GITT). The H⁺/Zn²⁺ co-insertion mechanism was confirmed in a Mn-based cathode. In particular, H⁺ will first embed into the α-MnO₂ material in the discharge stage, and Zn²⁺ with relatively large radius will be vulnerable to electrostatic effects when entering the lattice gap. The embedding process can be slightly affected and will slowly embed into α-MnO₂ after H⁺ embedding, finally achieving reversible ion transport. During the discharge process, MnOOH and OH⁻ will generate from the reaction between MnO₂ and H₂O, while Zn²⁺ will embed in MnO₂ to provide capacity. The same energy storage mechanism was demonstrated by Chen et al. [12] for vanadium-based cathode materials.

$$\mathrm{Cathode} \mathrm{process}: {\mathrm{MnO}}_2{+\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\underset{}{\leftrightarrow }{\mathrm{MnOOH}+\mathrm{OH}}^{-}$$

(3)

$${\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}+2{\mathrm{MnO}}_2\stackrel{}{\leftrightarrow }2{\mathrm{ZnMn}}_2{\mathrm{O}}_4$$

(4)

$$\mathrm{Anode} \mathrm{process}: \mathrm{Zn}\stackrel{}{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}$$

(5)

1.3. Chemical Conversion Reaction Mechanism

Except for the aforementioned mechanisms, the chemical conversion reaction of α-MnO₂ and MnOOH was reported by Liu et al. [13] in a Zn||α-MnO₂ cell. MnOOH will form after α-MnO₂ reacts with H⁺ in water during the discharge process, while the remaining OH⁻ in water will take a couple with ZnSO₄ and H₂O, finally leading to the formation of ZnSO₄(Zn(OH)₂)₃·xH₂O, during which the manganese ions in α-MnO₂ will be converted from Mn⁴⁺ to Mn³⁺. MnOOH could be characterized by ex situ HRTEM with XRD tests, and zinc hydroxide sulfate in the discharge electrode was observed by NMR and XRD, presumably undergoing Equation (1.8) reactions [14]. The negative electrode undergoes the stripping and deposition of metallic zinc, and the reaction processes are as follows:

$$\mathrm{Cathode} \mathrm{process}: {\mathrm{H}}_2\mathrm{O}\stackrel{}{\leftrightarrow }{\mathrm{H}}^{+}{+\mathrm{OH}}^{-}$$

(6)

$${\mathrm{MnO}}_2{+\mathrm{H}}^{+}{+\mathrm{e}}^{-}\stackrel{}{\leftrightarrow }\mathrm{MnOOH}$$

(7)

$$\frac{1}{2}{\mathrm{Zn}}^{2+}{+\mathrm{OH}}^{-}+\frac{1}{6}{\mathrm{ZnSO}}_4+\frac{6}{x}{\mathrm{H}}_2\mathrm{O}\stackrel{}{\leftrightarrow }\frac{1}{6}{\mathrm{ZnSO}}_4{\left[\mathrm{Zn}{\left(\mathrm{OH}\right)}_2\right]}_3{.\mathrm{xH}}_2\mathrm{O}$$

(8)

$$\mathrm{Anode} \mathrm{process}: \mathrm{Zn}\stackrel{}{\leftrightarrow }{\mathrm{Zn}}^{2+}+2{\mathrm{e}}^{-}$$

(9)

Due to different crystal structures of the cathode materials, the energy storage mechanisms are different from each other, and zinc ions enter the cathode through a complex reaction. However, the energy storage mechanism of some other materials is even controversial, and the detailed investigation of the mechanism still needs to be further explored, which is essential for the development of high-performance materials and practical application for ZIBs. Apart from reaction mechanism, another research hotspot is the Zn anode, which deeply affects the electrochemical behaviors of ZIBs as well as the commercial use. Compared with other metal ion batteries (Li, Na, and K) [15], zinc is more stable in an aqueous environment. Nevertheless, there are more severe problems in the cycling process, and the solution condition to the anode problem is of great significance to developing ZIBs.

Over the past decade, research on ZIBs continues to pile up, which helps to stimulate the development of ZIBs. However, many bottlenecks still remain to be solved. During the charge and discharge process of ZIBs, the uneven deposition caused by the “tip effect”, lattice matching degree, and ion concentration leads to zinc dendrites on the Zn anode surface, and “dead zinc” forms when it is de-embedded [16,17,18,19]. Meanwhile, side-effects are inevitable due to the inherent disadvantages of aqueous batteries, consuming the electrolyte and anode material and seriously affecting the cycle life of ZIBs [20,21]. The above two serious issues are the main reasons that limit the electrochemical performance for aqueous ZIBs. The common ideas to suppress zinc dendrites in ZIBs can be divided into the following aspects: structural design of zinc anode [22,23], surface modification [24,25], electrolyte optimization [26,27], and research on the multi-functional design of separators [28,29]. For example, a 3D fiber network was prepared by constructing zincophilic Sn nanoparticles with nitrogen-enriched hollow carbon spheres via the electrospinning method, which can inhibit hydrogen evolution reactions (HERs) and promote uniform zinc deposition by regulating the flux of zinc ions [30]. Fabrication of an artificial SEI layer at the negative zinc electrode can control Zn²⁺ deposition homogeneously and physically block penetration of zinc dendrites into the diaphragm. Wang et al. carbonized polymer coatings containing F to construct inorganic CF layers with an electronic insulation feature and high ionic conductivity on the collector copper foil, capable of guiding dendrite-free Zn deposition [31]. Kang et al. [32] prepared inorganic nano-CaCO₃ coatings on the surface of the Zn anode to tune the uniform deposition of Zn²⁺ through nanopores [32]. Due to the complexity of structural design and interfacial modification and the difficulty of flexible application in realistic environments, researchers found that regulation of the solvation structure is more easily achieved. The structure design of the solvent can change the coordination environment of Zn²⁺ and the activity of water molecules to promote the stability as well as stabilize the electrochemical window of aqueous ZIBs. Herein, we attempt to summarize the current status of research from both a high-concentration strategy and additive engineering. Particularly, the high-concentration electrolyte strategy can alleviate the electrochemical window limitation of the ZIBs and inhibit the activity of the water molecules, which makes the solvation structure of Zn²⁺ reconfigured and is an effective way to adjust the solvation structure [33]. The electrolyte additive approach is facile, reliable, and highly efficient [34]. The existence of additives exhibits different interaction forces with Zn²⁺ and water molecules, which have significant effects on the tuning of the solvation structure. We believe that this review will give guidance for further investigations for high-performance ZIBs.

2. Solvation Structure

The concept of SEI film, first introduced in 1979, is a passivation layer covering the surface of the electrode material by a reaction between the electrode material and the solid–liquid phase interface of the electrolyte [35]. As an interfacial layer, this passivation layer has the characteristics of a solid electrolyte. However, with the in-depth study of SEI membranes, it is found that some phenomena cannot be fully explained by SEI theory. For example, when dimethyl sulfoxide (DMSO) was added to the Zn(TFSI)₂ electrolyte solution [36], DMSO could significantly increase the proportion of TFSI⁻ anions in the inner solvation sheath of Zn²⁺. TFSI anions would be preferentially reduced before Zn deposition to form a fluorine-containing SEI layer in situ on the Zn anode. In this case, the synergistic mechanism of SEI theory and solvation theory work together to adjust the flux of Zn²⁺ to achieve dendrite-free Zn deposition. The addition of tetramethylurea (TMU) to Zn(OTf)₂ was able to adjust the solvation structure of Zn²⁺ while forming a unique inorganic–organic bilayer SEI to improve the transport kinetics of Zn²⁺ and promote the uniform deposition of Zn²⁺ [37]. Due to this consideration, the introduction of the solvation structure theory can be well explained. The solvation theory can be traced back to 1981. In the past decades, the study of the solvation structure of electrolytes with solvent–solute interactions has gradually intensified [38,39,40,41,42]. Interaction between the cation/anion and the additives and solvent in aqueous electrolyte will form a relatively stable sheath-like structure, which is called the solvation structure [43]. In aqueous electrolytes, water molecules exist in two forms: solvation and free water. Compared with anions, metal cations are more easily combined with water molecules to form larger hydrated metal ions with a more muscular solvent sheath layer [43,44]. As can be noted from Figure 1b,c, compared to other metal ions, Zn²⁺ will form a larger radius of hydrated zinc ions than other metal ions and non-metal carriers given the smaller ionic radius (Li⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, and NH₄⁺), and generate the solvation form of Zn[(H₂O)₆]²⁺ [45]. Significantly, the deposition process of Zn involves three steps and is presented in Figure 1a: (1) liquid-phase mass transfer, (2) Zn[(H₂O)₆]²⁺ desolvation, and (3) substrate nucleation. Particularly, the solvation H₂O will not form hydrogen bonds at the electrode and electrolyte interfaces and can easily shed protons, which will be reduced more preferentially than Zn²⁺, thus inducing HER. Subsequently, the local pH value will increase in the electrolyte solution, causing side reactions and leading to anode corrosion (Figure 1d) [46,47,48].

The first two steps of the Zn deposition process mentioned above are related to the applied electrolyte, which are the critical factors affecting the zinc dendrites. Additionally, the solvent structure would have a significant effect on the performance of the ZIBs, which has been confirmed by more and more research. By adjusting the solvation structure of Zn²⁺, generation of Zn dendrites and side reactions as well as anodic corrosion can be feasibly suppressed so that it can diffuse in two dimensions on the electrode surface and electrolyte to realize homogeneous deposition of Zn²⁺. As illustrated in Figure 2, commonly used methods for optimizing electrolytes through structure design can be divided into two aspects including the high-salt-concentration strategy and electrolyte additives.

2.1. High-Concentration “Water-in-Salt” Strategy

Conventional aqueous electrolytes, influenced by the voltage of water decomposition, show a narrow electrochemical window (~1.23 V) [49]. The water activity will decrease and the electrochemical stability window will enlarge in aqueous electrolytes with the increase in salt concentration [50,51]. Meanwhile, the coordination environment of metal ions can be tuned and the number of solvation water molecules of metal cations can be reduced by employing high-concentration salt. The solvent sheath layer of metal ions is occupied by anions, which reduces the required energy for the desolvation of metal ions. The active water molecules reaching the electrode and electrolyte surface are reduced, which is able to prohibit the evolution of H₂ and the corrosion phenomenon, and inhibit the occurrence of side reactions of the metal anode, effectively promoting the cycle life of ZIBs [52,53].

Given the advantages of low cost, non-toxicity, good solubility, and stable electrochemical properties of zinc sulfate (ZnSO₄) in an aqueous solution, it is regarded as one of the common zinc salts at this stage and widely used in aqueous ZIBs. In the mixed electrolyte (ZnSO₄+MnSO₄), Raman spectra show that the broad peaks indicating free water molecules are gradually suppressed (Figure 3a), and Zn⁺-coordinated ambient water molecules are reduced as the electrolyte concentration of ZnSO₄ increases from 2 M to 4.2 M (Figure 3b,c). It is able to restrain side reactions and improve the cyclic stability of zinc metal anodes. A half-cell of Zn||Cu showed low polarization in dilute electrolyte (2 M ZnSO₄ + 0.1 M MnSO₄) with Zn plating/stripping cycles over 1000 h at 0.2 mA cm⁻² current density (Figure 3d) [54]; the average coulombic efficiency (ACE) for cycling >120 was 97.54%; while in 4.2 M ZnSO₄ and 0.1 M MnSO₄ mixed electrolyte, it was able to reach 99.21% (Figure 3g). Another full cell of Zn||MnO₂ showed advanced long-cycle behavior under a measurement of more than 1200 cycles at 938 mA g⁻¹ (Figure 3h), with an ACE close to 100% and 88.7% capacity retention. The dissolution of the cathode material would be inhibited via addition of MnSO₄ and a synergistic effect with its solvation structure was created to achieve a stable, reversible, and high-performing Zn||MnO₂ full cell [55]. The Zn anode in 3 M ZnSO₄ electrolyte exhibited an impressively advanced coulombic efficiency (>99%) [56], excellent reversibility, as well as good rate capability, achieving >8000 cycles, which significantly prolonged the cycle life of Zn with increasing current density (Figure 3e,f).

Another example of zinc salt is Zn(TFSI)₂ with high electrochemical stability and ionic conductivity. Wang and coworkers [15] introduced the concept of “water-in-salt” in ZIBs for the first time by using the electrolyte with high concentration (20 m LiTFSI + 1 m Zn(TFSI)₂), resulting in an electrolyte pH of 7 (Figure 4a). According to the FTIR and ¹⁷O−NMR spectra (Figure 4b,c), it can be found that with increase in the concentration of Li ions, the water molecules tend to form solvent sheaths with Li ions more. When the concentration of LiTFSI ≥20 m (see in Figure 4d), the water molecules are strictly confined to the Li ions’ solvation structure, and such a solvent sheath layer of Zn ions is changed from the original partition to be occupied by anions, and TFSI⁻ surrounds Zn²⁺. The altered solvation structure of zinc ions avoids direct contact between the zinc metal anode and the water molecules of the solvent sheath layer, thus significantly inhibiting the HER phenomenon and the formation of by-products. The metallic Zn cathode exhibits superb electrochemical stability and deposition/stripping reversibility. The highly concentrated electrolyte results in dendrite-free zinc plating/stripping of about 100% coulombic efficiency with no observed by-product formation throughout the application. As for another highly concentrated solution with 20 m LiTFSI+ 1 m Zn(OTf)₂ [57], Zn²⁺ prefers to combine with O(TFSI⁻) compared to O(water). Compared with low-concentration electrolytes, it shows good reversibility of galvanizing/zinc stripping, less polarization in long cycles, and long cycle life (Figure 4e–h). It can form a stable SEI film (Figure 4i–j), which makes the galvanized Cu surface uniform and flat. The capacity retention of the Zn||LiMn₂O₄ full cell is 92% with an average CE about 99.62% after 300 cycles. A Zn||P(4VC₈₆-stat-SS₁₄) full cell exhibited an ultra-long lifetime in 4 M Zn(TFSI)₂ electrolyte [58]. A high reversible capacity about 184 mAh g⁻¹ was maintained with a capacity retention rate of 83% after 48,000 continuous cycles.

ZnCl₂ with significant solubility is often chosen as a high-concentration electrolyte. A highly reversible full cell of V₂O₅||Zn was fabricated by Tang et al. [59] with a ZnCl₂ “water-in-salt” electrolyte (WISE) to extend its life at high-temperature storage. V₂O₅ cathodes in dilute electrolytes experience severe lattice swelling from 4.4 to 13.9 Å (319%) due to the co-insertion of hydrated zinc ions, with reduced active material, resulting in capacity loss and reduced cycling performance. An ultrahigh capacity of about 302 mAh g⁻¹ can be achieved by a V₂O₅||Zn full cell with conventional electrolyte (1 M ZnSO₄), and the capacity decreases rapidly. In contrast, cycling in 30 m ZnCl₂ exhibits a maximum capacity of 341 mAh g⁻¹, which shows about a 9.5% decay after 300 cycles (Figure 5a,b), demonstrating ultra-high cycling stability within 1000 cycles (Figure 5c). The ZnCl₂ WISE (30 m) can prevent co-insertion of water molecules, and inhibit dissolution of vanadium-based materials, resulting in an expanded electrochemical window, which inhibits the protrusion of zinc dendrites, and improves electrochemical performance. (Figure 5d). A mixed electrolyte was formed by adding 5 m LiCl to 30 m ZnCl₂, in which hydrogen bonds between H₂O can be broken by Li salt, reduce the free water molecules around Zn²⁺, and limit the solvation process of Zn²⁺ ions to achieve a higher CE of the mixed double-ion (5 m LiCl + 30 m ZnCl₂) WISE than that of a single salt (30 m ZnCl₂) [60]. Zhang and coworkers [61] found that the solubility of ZnCl₂ was able to reach 31 m at room temperature and proposed using a 30 m ZnCl₂ solution as an electrolyte for ZIBs to enhance the reversibility of the Zn anode. Distinct from 5 m ZnCl₂ (Figure 5e), a symmetric cell of Zn||Zn exhibited excellent cycle stability when plating/stripping at 0.2 mA cm⁻² for 10 min in a 30 m ZnCl₂ electrolyte solution. The viscosity of the electrolyte solution increases with concentration increases and electrical conductivity decreases. However, the ion migration number of Zn²⁺ is higher in the highly concentrated electrolyte than that in the lowly concentrated one. A lower degree of hydrolysis slows down Zn(OH)₂/ZnO formation (Figure 5f), and the electrochemical window and pH also increase with concentration, indicating that the hydrolysis of Zn²⁺ is inhibited (Figure 5g) and inhibits the HER, which brings higher reversibility of the zinc metal anode as well as tunes the uniform deposition of Zn²⁺ (Figure 5h,i).

Table 1. Recent publications focused on high−concentration electrolytes in ZIBs.

Electrolyte	Current Density	CE	Lifespan	Ref.
3 M ZnSO4	20 mA cm−2, 1 mAh cm−2	100%	800 h	[56]
3 M ZnSO4 + 2 M LiCl	0.2 mA cm−2, 2 mAh cm−2	__	170 h	[68]
4.2 M ZnSO4 + 0.1 M MnSO4	0.5 mA cm−2, 1 mAh cm−2	99.21%	1000 h	[54]
3 M Zn(CF3SO3)2	0.1 mA cm−2, 0.1 mAh cm−2	100%	800 h	[64]
1 m Zn(TFSI)2 + 20 m LiTFSI	0.2 mA cm−2, 0.033 mAh cm−2	100%	170 h	[15]
30 m ZnCl2	0.2 mA cm−2,0.035 mAh cm−2	95.4%	600 h	[61]
30 m ZnCl2 + 5 m LiCl	2 mA cm−2, 4 mAh cm−2	99.7%	4000 h	[60]
2.4 m Zn(ClO4)2	1 mA cm−2, 1 mAh cm−2	99%	3000 h	[69]
8 M NaClO4 + 0.4 M Zn(CF3SO3)2	1 mA cm−2, 1 mAh cm−2	__	200 h	[70]

summarizes the electrochemical properties of zinc anode using highly concentrated electrolytes. With the development of ZIBs, a range of zinc salts used in ZIBs were investigated, including ZnCl₂, ZnSO₄, Zn(Ac)₂, ZnF₂, Zn(TFSI)₂, and Zn(CF₃SO₃)₂ [62,63,64,65]. In summary, the addition of highly concentrated salt solutions can, on the one hand, directly reduce the number of water molecules, disrupt the coordination equilibrium of Zn²⁺ in aqueous solvent, and replace the H₂O molecules in the Zn²⁺ solvation structure. On the other hand, interaction between anionic or cationic water molecules of some highly concentrated salt solutions is greater than that between water molecules and Zn²⁺, and Zn²⁺ shares the solvation, reducing the energy required for the desolvation of Zn²⁺. Alternatively, a new solvation sheath can be formed directly to suppress the HER phenomenon and regulate the deposition of Zn²⁺ homogeneously. However, although highly concentrated electrolytes can promote the coulombic efficiency of Zn plating/stripping and are beneficial for the deposition of dendrite-free Zn, some unavoidable problems still remain to be solved. The concentrated electrolyte is highly polarized even at 0.2 mA cm⁻² and 0.03 mAh [66,67]. Its viscosity increases with the concentration [61,64], resulting in reduced ionic conductivity of the electrolyte. In addition, the high cost of the concentrated electrolyte can raise the cost of ZIBs, losing the advantage of the low cost of aqueous batteries. All these issues can limit the practical application of highly concentrated electrolytes in ZIBs. Another neglected point is the effect of the high concentration strategy on the cathode material. ZIB cathode materials suffer from the issues of material dissolution and structural collapse. However, based on the complexity of the energy storage mechanism of cathode materials, the current focus of research hotspots is the modification of the anode, but not much research is focused on the impact of high concentration on the cathode materials, so more in-depth research is in urgent need.

2.2. Functional Additives

As mentioned above, side reactions and Zn dendrites are the main issues that need to be addressed in ZIBs, which can result in corrosion, HER, and passivation. Different types of functional electrolyte additives can inhibit side reactions and growth of zinc dendrites by different mechanisms. In recent years, the role of additive engineering has been extensively studied in ZIBs.

Tetrabutylammonium sulfate (TBA₂SO₄) is an effective, low-cost, non-toxic cationic surface-activator-type electrolyte additive. Zhu et al. [71] proposed to add TBA₂SO₄ to the 2 M ZnSO₄ electrolyte with a small amount about 0.029 g L⁻¹, and the positively charged TBA⁺ gathered at the negatively charged zinc “bulge” by electrostatic adsorption, forming an electrostatic shield and limiting the tip effect. TBA⁺ causes Zn²⁺ deposition in adjacent flat areas (Figure 6a,b), inducing uniform zinc deposition through a unique zincophobic repulsion mechanism. A full cell of Zn||MnO₂ with TBA₂SO₄ exhibits a cycle stability of about 300 times at 1 A g⁻¹ with a 94% capacity retention. Wang et al. [62] found that by adding 2 v.% of diethyl ether (Et₂O), Zn²⁺ will shift to other regions during zinc deposition. Zn||MnO₂ cells in 0.1 M Mn(CF₃SO₃)₂ and 3 M Zn(CF₃SO₃)₂ mixed aqueous solution with Et₂O as an additive were able to cycle 4000 times at 5 A g⁻¹ with a 97.7% capacity retention rate (Figure 6c,d), showing excellent cycling performance. Because Et₂O molecules can preferentially adsorb on the prominent tip, eliminating the “tip effect” and flattening the zinc anode surface (Figure 6e), it can largely restrain growth of zinc dendrites. Guo et al. [63] added a small amount (25 × 10⁻³ M) of Zn(H₂PO₄)₂ salt to 1 M Zn(CF₃SO₃)₂ electrolyte. From the Raman spectrum, it can be seen that SEI−Zn has bonds between Zn and O, and P and O, compared with bare−Zn, which demonstrates the formation of the SEI (Figure 6f). As shown in Figure 6g, Zn(H₂PO₄)₂ can generate a dense, stable, and highly Zn²⁺−conducting SEI film on the negative surface, ensuring uniform and rapid Zn²⁺ migration and achieving dendrite-free growth of zinc metal, which also is confirmed by the XRD pattern (Figure 6h). Even if the coating breaks down, the Zn(H₂PO₄)₂ in electrolyte will still form a new interfacial layer, thus solving the coating failure problem. Under actual test conditions with Zn(H₂PO₄)₂ addition, the Zn||V₂O₅ full cell exhibits a capacity retention of about 94.4% after 500 cycles.

The abovementioned additives can be broadly classified as ionic and nonionic, capable of achieving dendrite-free zinc deposition by electrostatic shielding or by forming SEI films. Improving the performance of ZIBs also requires avoiding side reactions. In the aqueous electrolyte, the stable form of Zn²⁺ is [Zn(H₂O)₆]²⁺ requires high energy for desolvation during the deposition process. Researchers have found that additives can adjust the solvation structure. Replacing H₂O molecules in [Zn(H₂O)₆]²⁺ with other ions or molecules with weaker solvation structure effects can reduce the desolvation energy as well as prevent the side reactions [46].

Much research related to organic substances as common additives already exists. Li et al. [72] added ethylene glycol (EG) to the ZnSO₄ electrolyte and found that it is effective in improving Zn²⁺ reversible deposition (Figure 7a). The red-shift in the OH stretches H₂O (3200~3400 cm⁻¹), and OH stretched bending of C−OH (1460~1465 cm⁻¹) shows a blue shift and can be noted from the Raman spectrum (Figure 7b), indicating that the electron density of OH in H₂O is lower and the electron density of C−OH in EG is higher, and the hydroxyl group of EG has a solid force to generate hydrogen bonds with H₂O, which can effectively weaken the force between Zn²⁺ and H₂O. Experiments and theoretical calculations show that with the increase in EG content, the OH stretching vibration range from 3000 to 3500 cm⁻¹ can lead to a significant redshift, and H₂O bending vibration in the range of 1600~1700 cm⁻¹ is slightly blueshifted in the FTIR spectrum (Figure 7c) [73,74]. Pan et al. [75] used ab initio calculations to analyze the solvation energy of different Zn²⁺ solvation sheaths in ethylene glycol/water solutions. They found that the solvation energy was ranked as [Zn(H₂O)_m (EG)_n]²⁺ < [Zn(H₂O)₆ ]²⁺ < [Zn(EG)₃ ]²⁺, indicating that the interaction force between EG and zinc ions was greater than that between Zn²⁺ and water, disrupting the solvation of Zn²⁺ and effectively inhibiting the occurrence of the HER (Figure 7d). As shown in Figure 7e, the Zn||Zn cell with a 3 M ZnSO₄/H₂O/68 v.% EG electrolyte achieves an extremely long cycle life of 2668 h at 0.5 mA cm⁻². It is attributed that the solvation structure of Zn²⁺ can be controlled by the EG molecule. Figure 7f shows that the radius of [Zn(EG)₃ ]²⁺ is larger than that of [Zn(H₂O)₆ ]²⁺, which increases the spatial site resistance of Zn²⁺ transfer, limits the diffusion of Zn²⁺, and increases the over potential of zinc deposition, thus refining the grain size of zinc as well as avoiding rapid growth of zinc to produce dendrites [76,77]. A study found the addition of dimethyl sulfoxide (DMSO) to ZnCl₂ solution [78] (Figure 7g). H₂O in a zinc ion solvation sheath can be replaced by DMSO because of its (29.8) higher Gutmann donor number and larger dielectric constant than that of H₂O (18), and O had a smaller electron density in DMSO [79,80]. With the addition of DMSO, interaction between solvation water and Zn²⁺ can be weakened, which facilitates the desolvation process of zinc ions and can suppress the decomposition of active H₂O, improving the zinc plating and stripping coulombic efficiency. The solvation DMSO can lead to SEI layer formation in situ on the anode surface, which allows passage of zinc ions and hinders the passage of water molecules (Figure 7h). The Zn||Zn symmetric cell in such a mixed electrolyte exhibited a cycle life of more than 1000 h. The cycle life was improved by a factor of 2.5 compared to that of the ZnCl₂−H₂O symmetric cell. Zn||Ti half-cells increased the CE to 99.0% and finally 100% within 30 cycles, exhibiting a stable over potential of ~24 mV, confirming the ability of the DMSO additive in improving the CE of plating/stripping. Qiao et al. [81] added 50 v.% methanol to the ZnSO₄ electrolyte. They found that methanol can insert into the internal Zn²⁺ solvation sheath owing to the high dielectric constant and small size (37.2) [82,83] and can disrupt the coordination equilibrium of Zn²⁺ by adjusting the amount of methanol addition (Figure 7i). The methanol molecules will first interact with H₂O in the first layer of the Zn²⁺ solvated sheath layer and form hydrogen bonds. In addition, the high wettability between methanol and the Zn metal anode makes it easier for methanol molecules to adsorb on the surface of Zn metal than solvent water, inducing the realization of dendrite-free Zn²⁺ uniform deposition (Figure 7j). When the size of methanol molecules becomes larger, they will eventually insert into the inner layer of the Zn²⁺ solvent sheath, the activity of water will decrease, and both hydrogen precipitation reactions and side reactions will be inhibited (Figure 7k,l). Yang et al. [84] demonstrated by introducing N-methyl-2-pyrrolidone (NMP) polar additives to ZnSO₄ that the addition of organic solvents containing carbonyl groups contributes to stabilizing the hydrogen bonding network of water and the structural remodeling of Zn²⁺−solvation, and such a synergistic effect helps to inhibit dendrite formation and water-induced parasitic reactions.

By investigating the influence of different organic additives on the Zn²⁺ solvation structure, researchers have provided an electrolyte regulation approach to achieve highly reversible Zn anodes and cells. Except for organic additives, salt additives are considered as another effective way to promote the electrochemical performance of Zn anode. Zhang et al. [90] used trifluoromethanesulfonate (OTf⁻) anions and ethylenediaminetetraacetic acid (Y⁴⁻) anions to form a mixed electrolyte (BE + 100 mM) consisting of 1 M Zn(OTf)₂ + 100 mM Na₄Y, and the double anion can change the Zn²⁺ solvation sheath to stabilize the Zn anode (Figure 8a–c). As shown in Figure 8d–f, this coordination environment can reduce the solvation water content and activity, inhibiting HER, and homogenizing the Zn²⁺ plating process. In addition, this solvation structure allows decomposition of ligand anions to generate an in situ organic–inorganic SEI on the Zn surface. The resulting Zn²⁺ conducting SEI layer can lead to separation between Zn and the electrolyte, thereby inhibiting side reactions derived from H₂O. As a result, the Zn||Zn cell has a coulomb efficiency of 99.7% at 1.0 mA cm⁻² and 1.0 mAh cm⁻² for more than 1600 h, demonstrating excellent long-term stability (Figure 8g). The Zn||Cu cell is highly reversible, possessing a coulomb efficiency of 99.7% at more than 400 cycles at 1.0 mA cm⁻² (Figure 8h). Chen et al. [91] demonstrated that via adding halogen ions to the Zn²⁺ solvation structure, the issues of dendrite growth and hydrogen precipitation reactions could be overcome. Through electrolyte regulation consisting of ammonium halide and zinc acetate, I⁻ can combine with Zn²⁺ to convert the conventional Zn(H₂O)₆²⁺ to ZnI(H₂O)₅⁺, where I⁻ can transfer electrons to H₂O, and water molecules in the solvation structure would be reduced by inhibiting the reduction in the lowest unoccupied molecular orbital (LUMO) energy level and reducing the electron loss, thus improving the reduction stability and, thus, inhibiting HER (Figure 8i). The formation of a dynamic electrostatic shielding layer accompanied with NH₄⁺ can inhibit the growth of dendrites (Figure 8j–k).

The electrochemical performances of zinc anode with various electrolyte additives are summarized in

Table 2. Recent publications focused on organic additives in ZIBs.

Electrolyte Addictive	Current Density	CE	Lifespan	Ref.
68 vol.% Ethylene glycol	0.5 mA cm−2, 0.5 mAh cm−2	__	2668 h	[75]
50 vol.% Methanol	1 mA cm−2, 0.5 mAh cm−2	99.7%	__	[81]
Polyethylene glycol	1 mA cm−2, 1 mAh cm−2	99.6%	650 h	[85]
Dimethyl sulfoxide	1 mA cm−2, 1 mAh cm−2	99.73%	2100 h	[86]
PEO	1 mA cm−2, 1 mAh cm−2	98.7	700 h	[87]
Diethyl ether + Ethylene glycol	20 mA cm−2, 1 mAh cm−2	98%	700 h	[88]
Propanediol	0.2 mA cm−2, 0.2 mAh cm−2	98.9%	1000 h	[89]
Diethyl ether	20 mA cm−2, 1 mAh cm−2	__	250 h	[62]

. In summary, electrolyte additives have obvious effects inhibiting side reactions and dendrites of ZIBs. Significantly, the selection of additives should consider three aspects: (1) Gutmann donor number of solvent additives, which can replace H₂O in the solvation sheath of Zn²⁺ when the donor number is larger than H₂O and can maintain Zn²⁺ ion transfer kinetics to some extent [92]. (2) Interaction with H₂O, which can form hydrogen bonds with H₂O, reduce H₂O activity, and inhibit H₂O reduction. (3) In situ formation of SEI film, which should form a dense, self-healing Zn²⁺-conducting SEI film to prevent water penetration into the zinc anode. Furthermore, the electrolyte additives also have a vital effect on the cathode material. For example, the addition of triethyl phosphate (TEP) in the Zn(OTF)₂-H₂O electrolyte in Zn||V₂O₅ batteries can keep the pH of the electrolyte solution stable and hinder the dissolution of V₂O₅ to some extent, which has strong interactions with Zn²⁺ and H₂O molecules. The TEP will enter the inner layer of the solvation sheath of Zn²⁺ and break the hydrogen bonding network between water molecules, thus reducing the activity of H₂O and avoiding the reaction between V₂O₅ and H₂O [93]. Another report mentioned that introducing polyethylene glycol (PEG) into the electrolyte can reduce the content of free water molecules and inhibit the H⁺ insertion to the LiV₂(PO₄)₃ (LVP) cathode through the strong interaction between PEG and H₂O, which is beneficial for suppressing the phase change of LVP. Significantly, much attention should also be paid to the cathode materials for the development of water-based ZIBs.

3. Challenges and Future Development Prospects

This paper reviews two approaches to optimize the electrolyte from a solvation structure perspective, and summarizes the effects on the anode in terms of both the inhibition of zinc dendrites and reduction in side reactions. The current research on high-concentration strategies and electrolyte additives has achieved some results, but they are far from sufficient. For the high-concentration strategy of ZIBs, the mechanism affecting the evolutionary transformation of the solvation structure needs further insight through some in situ characterization and theoretical calculations. In addition, based on the practicality of the high-concentration strategy, it is necessary to explore more suitable salt solutions, including high ionic conductivity, good thermodynamic stability, low cost, ability to achieve compatibility with both poles of the battery system, and a good balance between high concentration and viscosity. ZIBs work differently with various electrolyte systems (alkaline, neutral, and weak acid) with different cathode materials for electrolyte additives. The working mechanism of electrolyte additives and their effects on cathode materials need to be further disclosed. In addition, some of the electrolyte additive types studied so far are costly and toxic, increasing the battery system’s flammability. The type of electrolyte and its mechanism of action need to be considered when adjusting the solvation structure of Zn²⁺.

The research on the electrolyte engineering for ZIBs has yet to mature and is more in the laboratory stage. In view of the existing problems, the future research directions of electrolyte engineering can include the following: (1) The influence of the electrolyte system on the cathode material. The working mechanism of the electrolyte is based on the energy storage mechanism of the battery system because the energy storage mechanism is different for various cathode materials. Research targeting cathode materials will exhibit great potential. (2) Development of quasi-solid or all-solid electrolytes. In order to avoid the hazards caused by active water molecules and with the development of flexible devices, it is promising to explore solid or gel electrolytes with environmental friendliness, low cost, high ionic conductivity, and non-toxicity. (3) Exploring new multifunctional electrolyte additives. Current research on electrolyte additives shows the great potential of electrolyte additives. Research to explore more different additives is necessary, depending on the demand. Despite many difficulties visible to the naked eye, we firmly believe that water-based ZIBs will continue to evolve and provide a viable path for future energy storage methods.