Development of electrolytes for rechargeable zinc-air batteries: current progress, challenges, and future outlooks

Abstract

This review presents the current developments of various electrolyte systems for secondary zinc air batteries (SZABs). The challenges and advancements in aqueous electrolytes (e.g., alkaline, acidic and neutral) and non-aqueous electrolytes (e.g., solid polymer electrolyte, ionic liquids, gel polymer electrolyte, and deep eutectic solvents) development have been reviewed. Moreover, chemical and physical characteristics of electrolytes such as power density, capacity, rate performance, cyclic ability, and safety that play a vital role in recital of the SZABs have been reviewed. Finally, the challenges and limitations that must be investigated and possible future research areas of SZABs electrolytes are discussed.

Highlights

• Design and working mechanisms of rechargeable zinc air batteries.

• Investigation of various electrolyte systems for rechargeable zinc air batteries.

• Advances in the electrolyte technologies and stable electrolytes for rechargeable zinc air batteries.

1 Introduction

The growing demand for consumption of an electrical energy pose challenges to maintain a continuously available energy supply in various features of modern life in today's developed [1]. The problems connecting to environmental pollution has gained prominence as an urgent crisis due to the use of fossil fuels to produce electricity [2]. There have been many efforts to enhance the current energy storage systems. Solar and wind energy are emerging technology alternatives to fossil fuels [1, 2]. Until now, there has been a need for a dependable, safe, and an efficient energy storage approach to enhance energy conversion in power plants. Amongst existing methods, battery technology has the best chance of becoming an effective solution for the next generation of renewable energy uses as energy storage devices [2, 3]. Metal-air batteries (MABs) are currently regarded as desirable technologies for high energy density demands. In MAB, oxygen from the air is employed as the active material at the cathode. Using MABs offer many advantages due to a free, abundant, and simple design that provides a significant low cost as well as its ability to rise the energy density of the system [4]. These benefits of MAB increased the attention of researchers and companies in employing MAB as an energy storage device [5]. Hence, MAB is one promising alternative candidate for energy storage device as it provides large theoretical energy densities, very long shelf life, flat discharge voltages, low cost, environmentally benign, and high capacity that is typically independent of temperature. Some metals (e.g., calcium (Ca), iron (Fe), magnesium (Mg), lithium (Li), aluminum (Al), zinc (Zn) have been employed for the development of MABs. For each technology, a diverse composition and nature of electrolyte system is desired to attain the desired electrochemical reactions [6]. Based on the nature of their chargeability and electrolyte MABs are classified as non-rechargeable (primary) and electrically rechargeable (secondary) battery [7].

In spite of the great success of Li-ion batteries (LiBs) in the present market, the high costs and inadequate global resources of lithium restrict their industrial and practical applications [6]. Hence, alternative abundantly-available elements and low-cost must be developed, identified, and researched for commercially-feasible and promising applications [4]. Unlike other metals such as lithium, Zn is lighter, abundant, inexpensive, more stable, environmentally friendly, safer, high specific energy density (i.e., 5 times greater than Li-ion counterparts) and easily re-generate from aqueous electrolyte with an electrodepositing approach, make their employ as secondary batteries. Aqueous alkaline electrolytes have been employed for secondary Zn-based energy storage systems [8]. The electrolyte (i.e., neutral and aqueous alkaline electrolyte) and the Zn electrode limitation are strongly connected as a reactivity of Zn varies with the electrolyte. The development of the rechargeable zinc air battery (RZAB) (i.e., mainly alkaline electrolytes) has been developed due to their wide availability, high ionic conductivity and low cost in addition to other benefits like a low operating temperature. However, Zn dissolution and corrosion in alkaline aqueous electrolytes make obstacles for their further practical and commercially-feasible applications. [9]. Moreover, the little charge and discharge life cycle electrodes limit the commercialization and practicability of the rechargeable RZAB. The electrolytes are the "blood" of the batteries that regulate basic electrochemistry and provide ionic conductivity. They are one of the core components of RZAB because they show a vital part during the discharge–charge process. Moreover, they possess a noteworthy effect on RZAB rechargeability, operating voltage, lifetime, cyclic stability, power density, and safety. Therefore, electrolytes affect not only battery rechargeability and redox reactions, but also RZAB performance. Proper selection of the electrolyte system is the key features in achieving the long life cycle of Zn-anode, which in turn well enhances its rechargeability, cell discharge potential, and energy delivery [1]. Hence, to solve the problems of Zn anodization in aqueous electrolytes, an organic solvent can be used as an alternative electrolyte system. Stable Zn anodes (i.e., highly reversible) have been verified by employing aprotic electrolytes. The side reactions having a dendritic growth of Zn can be naturally attenuated by employing non-aqueous electrolytes because of the high stability (i.e., thermodynamic) of Zn anodes to organic solvents. However, the scientific community has recently shown a greater interest in finding alternative non-aqueous or aqueous electrolyte systems to reduce the Zn-electrode issues related with aqueous alkaline electrolytes [10, 11]. In general, this review is divided into five sections. Firstly, the introduction part is presented. Then the working mechanisms of RZAB is presented and followed by the challenges and development of different electrolytes. Then we explain the electrolyte characterization methods for RZABs and lastly, we discuss the conclusions and future prospects.

2 Working mechanisms of RZAB

The metallic Zn electrode, bi-functional air electrode (BAE), an aqueous potassium hydroxide (KOH) electrolyte, and porous separator are the most common components of RZABs. The BAE is composed from a bi-functional air catalyst (e.g., manganese oxide (MnO₂)) and a porous substrate to facilitate the oxygen-evolution reactions (OER) and oxygen-reduction reactions (ORR) [12] as illustrated in Fig. 1a and 1b. There are two types battery configuration in RZABs namely, a two electrode model system and tri-electrode model system [13]. The two-electrode model systems, the simple and most widely employed configuration, are analogous to primary Zn/air battery. The bi-functional electrode which is integrated with mixture of OER and ORR catalysts or a bi-functional oxygen electrocatalyst can replace the uni-functional ORR air–cathode. Hence, the ORR and OER electrochemical processes appear at the bi-functional air electrode through the discharging and charging processes correspondingly. A short life cycle of ORR catalysts could be deactivated at high voltages through the charging process. ZAB has an open circuit voltage value of ≈1.2 V in the discharging process. However, the desire charging voltage of ZAB is approximately 2.0 V or even higher because of the large potential of OER [14]. The high potentials could bring the corrosion and oxidation of oxygen electro-catalysts. The benefits of tri-electrode configuration compared to ORR and OER electrodes is that by using similar carbon electro-catalysts in the tri-electrode configuration leads to a well cycle recital than the analogous primary ZAB and two-electrode configurations [13, 15]. In tri-electrode configuration durable and active electro-catalysts on the cathode side are needed to catalyze ORR through discharge and OER through charge for RZARs [14, 16]. The air cathode and Zn anode redox reaction produces electricity as illustrated in Fig. 1 and Table 1. Once a battery is discharged, the metallic Zn anode oxidizes and reacts with the OH⁻ ion to produce zincate ions (Zn(OH)₄^–2). When these soluble (Zn(OH)₄^–2) in the electrolyte being supersaturated, they spontaneously decompose to produce an insoluble zinc oxide (ZnO) [3]. Oxygen must be used in the gas phase rather than the liquid phase since oxygen has a low solubility at atmospheric pressure [16]. The pressure difference of oxygen between the inside and outside of the cell permits the diffusion of atmospheric oxygen into the porous carbon electrode. In the alkaline electrolyte, the catalyst enables the reduction of oxygen to hydroxyl ions and then the electrons produce the Zn metal oxidation at the anode reaction. This process is known as a 3-phase reaction and it comprises electrolyte (liquid), catalyst (solid), and oxygen (gas) [5]. To complete the cell reaction, the hydroxyl ions generate travel from the air (cathode) to the Zn (anode). These overall steps during charging and discharging can be designated as the electrochemical reactions (Table 2).

Fig. 1

(Modified from reference [13]

Configurations of RZAB design: a two electrode and b tri-electrode

Table 1 Comparison of the performance of tri-electrode configuration of RZAB

Table 2 The redox reactions of RZAB battery during discharging and charging process [3]

3 Electrolyte developments

In RZAB system, during the discharge and charge process the electrolyte allows the ions to pass to the two electrodes and enable charge compensation and complete chemical reactions [22]. The RZABs performance is governed by the size and type of the ion, the nature of the electrolyte, the chemical and/or electrochemical and physical stabilities, the operating potential window, the interaction with electrode materials, and the solvent properties [23]. The best electrolyte for RZABs should have a good ionic conductivity, promising electrochemical stability, good thermal stability, and wide potential operating window, low viscosity, and inertness with electrode materials, environmentally friendly, low cost and non-flammability. These attributes, however, not available in a single electrolyte The electrolytes can principally be classified into aqueous (i.e. alkaline and neutral) and non-aqueous (i.e. gel polymer, ionic liquids, DES and solid polymer) systems based on the nature of precursor materials [4, 24].

3.1 Aqueous electrolyte

The redox reactions of Zn in RZABs are classified into alkaline, acid and neutral based on the aqueous electrolytes used:

$$ {\text{Alkaline}}\;{\text{reaction}}:{\text{2OH}}^{ - } + {\text{Zn}} \to {\text{H}}_{{2}} {\text{O}} + {\text{ ZnO }} + {\text{ 2e}}^{\_} $$

$$ {\text{Neutral}}\;{\text{reaction}}: {\text{Zn}}^{{{2} + }} \to {\text{Zn }} + {\text{ 2e}}^{\_} {\text{and}} $$

$$ {\text{Acidic}}\;{\text{reaction}}:{\text{ Zn}}^{{{2} + }} \to {\text{Zn }} + {\text{ 2e}}^{\_} $$

There are three types of aqueous electrolytes: alkaline, acid, and neutral, which are typically represented by KOH, sulfuric acid (H₂SO₄), and sodium sulfate (Na₂SO₄) correspondingly. The alkaline aqueous electrolytes are commonly used for RZABs application compared to acid or neutral electrolytes.

3.1.1 Alkaline electrolyte

Alkaline electrolytes are commonly employed for RZABs as they are widely available, inexpensive, operate at low temperature and have a high ionic conductivity [13]. Nevertheless, the main issues in alkaline electrolytes are dissolution of Zn, hydrogen evolution, dendrite growth, passivation, shape changes and precipitation of insoluble carbonates (Table 3) [25].

Table 3 Comparison electrolyte issues with different electrolyte systems for RZAB application [4, 5, 9, 25, 29, 39, 43, 56, 71, 82, 86]

Table 4 Major challenges in aqueous electrolyte and proposed solution for RZABs

Zn dissolution Zn electrode dissolution in the alkaline electrolyte (aqueous) happens in two stages. The first stage is Zn to Zn(OH)₄^–2 oxidation (Eq. 1) and the second stage is the precipitation of ZnO inside the dissolved Zn(OH)₄^–2 saturation in the solution (Eq. 2). The supersaturated or saturated Zn(OH)₄²⁻ formation in RZABs is associated with the excessive Zn solubility inside a strong alkaline electrolyte, due to passivation, precipitation of insoluble carbonates, Zn shape-change process and dendrite formation [4].

$$ {\text{4OH}}^{ - } + {\text{ Zn }}\left( {\text{s}} \right) \rightleftharpoons {\text{2e}}^{ - } + {\text{Zn}}\left( {{\text{OH}}} \right)_{{4}}^{{ - {2}}} \left( {{\text{aq}}} \right) $$

(1)

$$ {\text{Zn}}\left( {{\text{OH}}} \right)_{{4}}^{{ - {2}}} \rightleftharpoons {\text{2OH}}^{ - } + {\text{ZnO }}\left( {\text{s}} \right) $$

(2)

Zn passivation It deactivates the metal surface with the formation of a layer. It happens associated with the metal dissolution that creates an excess solubility of a Zn salt (solid) or the formation a compact solid film near the surface of the electrode owing to the hydroxide in the electrolyte [26]. The passivation film can reduce the discharge capacity of Zn anode's and power capability as well as creation of the OH⁻ ion diffusion barrier. On the other side, passivation happens at high rates of discharge; nevertheless, slow rates of discharge can also decrease Zn passivation that result a high quantity of the passive layer dissolution as more time is needed. In passive film formation, two main mechanisms have been postulated, namely the reactions in the solid state and the mechanism of dissolution and precipitation. In dissolution–precipitation, the Zn anode remains active until a critical value for zincate ion concentration is reached, and precipitation of Zn salt on the electrode surface occurs. In contrast, the mechanism of passivation processes in solid state reactions designates the direct zinc oxidation by polarization of anode [4, 26]. Additionally, the solid-state reactions mechanism was employed in potentiodynamic studies while the mechanism of precipitation and dissolution were largely employed for the galvanostatic measurements. The typical electrochemical reaction of anodes (Zn) in aqueous electrolytes (alkaline) can be described with Eq. (1) and (2) [17]. In chemical reaction (Eq. 2), passivation denotes to a supersaturated and strong alkaline electrolytes comprising of ZnO that deposit on the electrode (Zn) surface to create an insulating layer through the discharge process and closes the surface of electrode and influences its further reaction [27]. The film (insulating, ZnO passivation) reduces the battery capacity and the utilization of the electrode (Zn) as well as terminates the discharge process. Moreover, it avoids the metallic Zn reverse conversion and limit the rechargeability of the battery [26, 28].

Dendrite growth and deformation During the operation of RZAB in an electrolyte (alkaline), the Zn(OH)₄²⁻ appear through discharge and fully returns to Zn at the electrode (Zn) surface through charging as stated in reaction mechanisms of Eq. 1&2. This resulting in gradually change shape of the metal electrode as well as its surface roughness with uneven thicknesses and comparison of dendrite formation of different electrolytes presented in Table 3 [29, 30]. The uneven shape accumulates to form dendrites due to several charge and discharge cycles. Zn dendrites are usually appeared as needle-like and sharp morphologies that penetrate into the separators and resulting in short circuit (internal) by the relocation of Zn active material [4, 30].

Recently, electrolyte modification, electric field regulation anode engineering, the artificial interface layer, and Zn ion transfer control approaches (Fig. 2) have been proposed to inhibit the formation of Zn dendrite [32,33,34,35,36].

• (a) Electrolyte modification This approach denotes the adoption of organic/inorganic additives, all-solid-state electrolytes, designing gel, and current dendrite permitted aqueous electrolytes (i.e. functionalize and substitute) as well as developing wholly organic electrolytes [31]. These methods permit the electrolytes (modified) to control migration of ion in both the micro space and bulk electrolytes near the electrolyte–electrode interface to achieve an identical distribution before plating of Zn. Hence, these methods accelerate growth of dents and Zn nucleation through electrostatic shielding and steric effects and they avoid Zn deposition on protruding parts [32].

• (b) Anode engineering The structure of anodes and physicochemical properties has a significant influence on Zn deposition. The electrode engineering approach mainly focused on developing current collectors or 3D electrodes and the conductive network (3D) can noticeably reduce the current density (localized) to hinder the Zn dendrites [32, 33].

• (c) The artificial interface layer This method can give a substantial role in dendrite protection. It follows the two strategies namely multimode-guided ion homogeneous diffusion and spatial shielding [34, 35]. The physical shielding approach comprises employing the interface layer to block the growth of a dendrite and resulting in a robust mechanical characteristic in the protective interface.

• (d) Electric field regulation This method drives the ions of Zn transfer to the interface between electrolytes and anodes as well as the nucleation of Zn on anodes. The control of electric field realizes from in-situ control of dendrite formation or electrode structure design. The charged separator, charging current and pulsed charging model control the electric field [36, 37].

• (e) Zn ion transfer control It plays a crucial part in the development of Zn on the nucleus. Zn ions uniform distribution results in similar Zn development rate on anodes. However, the non-uniform Zn seeding results in Zn ions concentration gradient which may be varying along the interfaces. A Zn-ion transfer accurate regulation is needed. The two approaches for accurate regulation comprises adding extra magnetic field and controlling the flow rate of electrolyte [38].

Fig. 2

The schematic view of the methods to inhibit Zn dendrite formation (recently proposed approaches) modified from reference [31]

Shape change The specific surface area and thickness of the electrode (Zn) could be altered after several charge and discharge cycles that gives rise to the capacity decay and densification of the battery [29]. This happens when the Zn²⁺ ions or dissolved Zn(OH)₄²⁻ are inside the electrolyte through the discharge reaction and redeposit onto dissimilar Zn electrode locations during charging. After numerous charge and discharge cycles, a loss of usable capacity and densification of the electrode occur. In summary, mechanistic investigations and modeling have ascribed to convective flows and uneven zones of reaction due to forces of electro-osmotic through the battery and shape change to the distribution of uneven current in the Zn electrode [39].

Insoluble carbonate formation The electrolyte attains interaction with the carbon dioxide (CO₂) as the alkaline electrolytes vulnerable to the air, which in turn reacts with the hydroxyl groups (OH⁻) and results formation of anions of insoluble carbonate or bicarbonate and comparison of carbonate formation presented in Table 3 [9].

$$ {\text{OH}}^{ - } + {\text{CO}}_{{2}} \to {\text{HCO}}_{{3}} $$

$$ {\text{OH}}^{ - } + {\text{HCO}}_{{3}}^{ - } \to {\text{H}}_{{2}} {\text{O }} + {\text{ CO}}_{{3}}^{{2}} $$

The formation of insoluble carbonates [29] results in a decrease in conductivity (ionic) due to the lower HCO₃⁻ or CO₃⁻² mobility than OH⁻ and blocking of air electrode microspore because of a low carbonate solubility that inhibits oxygen access and leads cathode performance degradation. Moreover, it creates the increase in electrolyte viscosity that causes a worse ORR of the bi-functional air electrode and complicating diffusion of oxygen into the electrolyte. Removal or reduction of the CO₂ amount in the air using filters from CO₂ absorbents via physical and/or chemical absorption improve the bi-functional air electrode cycle-life performance [4, 9]. A. R.Mainar et al. [9] employed calcium hydroxide (Ca(OH)₂)-lithium hydroxide (LiOH) and LiOH solid adsorbents for the removal of CO₂ from gas feed in ZABs (alkaline) in which the influence of filtering is determined with chemisorption and physisorption processes. Also, they mentioned that at ambient air at 409 ppm, the CO₂ amount is lowered to 20 ppm.

$$ {\text{CO}}_{{2}} \left( {\text{g}} \right) + {\text{2 LiOH }}\left( {\text{s}} \right) \to {\text{H}}_{{2}} {\text{O }}\left( {\text{g}} \right) + {\text{Li}}_{{2}} {\text{CO}}_{{3}} \left( {\text{s}} \right) $$

Hydrogen evolution Reaction (HER) It was associated with metal electrodes side reaction through the batteries charge–discharge process. Thermodynamically, Zn is unstable in alkaline solution as zinc has high negative reduction potential than hydrogen, resulting in the hydrogen gas evolution [30]. As shown Eq. (3) Zn/ZnO has standard reduction potential value of -1.26 V vs. SHE (standard hydrogen electrode), which is lower than the HER value (-0.83 V vs SHE) as presented in Eq. (4). The evolution H₂ consumes some amount of the electrons offered to the electrode (Zn) through charging process. Besides, it produces H₂ gas on Zn particles surface in aqueous media through consumption of the electrolyte by cycling, leading in swelling of battery. The unwanted side reactions may result to reduced life span and capacity fade of anode (Zn) [31].

$$ {\text{Zn}} + {\text{2H}}_{{2}} {\text{O}} \to {\text{ZnO}} + {\text{H}}_{{{2} }} \left( { - {1}.{26}\;{\text{V}}\;{\text{vs}}\;{\text{SHE}}} \right) $$

(3)

$$ {\text{H}}_{{2}} {\text{O}}^{ + } + {\text{2e}}^{ - } \to {\text{H}}_{{2}} + {\text{4OH}} - \left( { - 0.{83}\;{\text{V}}\;{\text{vs}}\;{\text{SHE}}} \right) $$

(4)

HER hinderance and corrosion rate reduction are strategies to add organic (inorganic) corrosion inhibitors to the electrolyte (electrode). This increases the anode HE and results in the formation of adsorption layer on the surface. Metal oxides, metal elements, salts and metal hydroxides are examples of inorganic corrosion inhibitors. The HER is repressed on metals surface with high HE and chemical stability over potential like cadmium, lead, indium, bismuth, nickel and mercury by using various coating, alloying, and additive materials.

3.1.2 Acidic electrolytes

Acidc electrolytes (e.g., phosphoric acid (H₃PO₄), nitric acid (HNO₃), sulfuric acid (H₂SO₄), and hydrochloric acid (HCl) have a short lifetime and require a suitable carrier and catalyst to overcome the difficulties associated with catalyst activity of the air electrode. They are rarely employed in the manufacture of RZABs [4]. The acid electrolytes have different electrochemical behavior with alkaline electrolytes. The formation of dendrite starts at larger current density values in acid electrolytes (aqueous) in comparison with the solutions of alkaline electrolyte. Moreover, Zn surface passivation happens only in the occurrence of film formers [40]. In non-complexing acidic and neutral solutions, there are two consecutive charge transfer stages in the electrode reaction (Zn/Zn²⁺) as presented in Eqs. 5 and 6. Zn⁺_ad is an intermediate material which adsorb on the metal surface.

$$ {\text{Zn}} \leftrightarrow {\text{Zn}}^{ + }_{{{\text{ad}}}} + {\text{e}} $$

(5)

$$ {\text{Zn}}^{ + }_{{{\text{ad}}}} \leftrightarrow {\text{Zn}}^{{ + {2}}} + {\text{e}}^{ - } $$

(6)

3.1.3 Neutral electrolytes

They are a potential opponent of the alkaline electrolytes for RZABs as they can reduce formation of Zn dendrite and prevent carbonate formation. The difference between alkaline and neutral electrolytes lies in diverse reactions with electrodes (Zn). The reactions of Zn dissolution through the discharging are represented as in Eq. 5–6 and Eq. 7–9 in the alkaline and neutral electrolytes, respectively [41]. Supersaturated Zn(OH)₄²⁻ are not available in the formation of dendrite inside the neutral electrolytes,. The neutrality of the electrolyte pH reduces the solubility of Zn and results in a very low CO₂ absorption that leads in the extended shelf life of the cell. However, it results in a poor cycling stability. Furthermore, the non-corrosive and nontoxic nature of the neutral electrolytes make them safer for batteries for wearable and flexible electronics [42, 43]. Electrolytes comprising ammonium chloride (NH₄C1), stannous chloride (SnCl₂), potassium chloride (KC1), mercuric chloride (HgCl₂), bismuth chloride (BiCl₃), mercurous chloride (Hg₂Cl₂), lead chloride ( PbCl₄), tin (IV) chloride (SnCl₄), zinc chloride (ZnCl₂), cadmium chloride (CdCl₂), magnesium chloride (MgCl₂), lead chloride (PbCl₂), ammonium chloride ( NH₄Cl), ammonium nitrite (NH₄NO₃), lithium chloride ( LiCl), ammonium hydroxide (NH₄OH), ammonium sulfate (NH₄)₂SO₄, potassium sulfate (K₂SO₄), potassium nitrate (KNO₃), sodium sulfite (Na₂SO₃) and sodium sulfate (Na₂SO₄) are considered neutral and offer wide working potential windows of less than 2 V and a high buffering capacities that can be employed in mitigation of corrosion [4, 44]. They have the ability to reduce dendrite formation and carbonation of the electrolyte. The electrolyte pH can be regulated to 7 with KCl, KNO₃, Na₂SO₃, K₂SO₄ and Na₂SO₄ while it can be adjusted to < 5 by employing ammonium salts such as NH₄NO₃ and NH₄Cl. However, the neutral electrolyte usually has smaller conductivity (ionic) and very small concentration of OH. Consequently, the air cathode electrochemical reactions in ZABs, the ORR through discharging, and the OER during charging tend to be kinetically slower in neutral media resulting in a significant performance reduction of the air electrodes and limits the power density of ZABs [41].

$$ {\text{4OH}}^{ - } + {\text{Zn}} \rightleftharpoons {\text{Zn}}\left( {{\text{OH}}} \right)_{{4}}^{{{2} - }} + {\text{2e}}^{ - } $$

(7)

$$ {\text{Zn}}\left( {{\text{OH}}} \right){4}^{{2}} \rightleftharpoons {\text{2OH}} - + {\text{H}}_{{2}} {\text{O}} + {\text{ZnO}} $$

(8)

$$ {\text{H}}_{{2}} {\text{O}} + {\text{ Zn}} \rightleftharpoons {\text{H}}^{ + } + {\text{ZnOH}} + {\text{e}}^{ - } $$

(9)

$$ {\text{ZnOH}} \rightleftharpoons {\text{ZnO}} + {\text{H}}^{ + } + {\text{e}}^{ - } $$

(10)

$$ {\text{H}}_{{2}} {\text{O}} + {\text{ZnO}} \rightleftharpoons {\text{2OH}}^{ - } + {\text{Zn}}^{{{2} + }} $$

(11)

3.1.4 Air cathode in aqueous electrolytes

It consumes O₂ in the air through discharging process by ORR and releases O₂ through battery charging by OER. In BAE reactions both ORR and OER are the crucial parameters for the efficiency of aqueous RZABs [3]. The BAE contains a bifunctional air catalyst (e.g., MnO₂) and a porous substrate to enable the ORR and OER [1].

3.1.4.1 Reaction mechanisms of OER/ ORR in alkaline electrolytes

In alkaline electrolytes the ORR proceeds through two pathways namely a 4e⁻ and 2e pathway. In the 4e⁻ pathway the O₂ is reduced to OH⁻ (Eq. 12). In the 2e⁻ pathway firstly O₂ is reduced to HO⁻² (Eq. 13) and then another pathway into OH⁻ (Eq. 14) or disproportionation of H₂O₂ (Eq. 13) [41]. In ZABs, the 2e- pathway is not needed as the intermediates of H₂O₂ are very reactive and could decompose, leading in a lower faradic efficiency.

$$ {\text{O}}_{{2}} + {\text{2H}}_{{2}} {\text{O}} + {\text{4e}}^{ - } \to {\text{4OH}}^{ - }\, \left( {0.{4}0{1}\;{\text{V}}\;{\text{vs}}.\;{\text{SHE}}} \right) $$

(12)

$$ {\text{O}}_{{2}} + {\text{H}}_{{2}} {\text{O}} + {\text{2e}}^{ - } \to {\text{OH}}^{ - } + {\text{HO}}^{{ - {2}}}\, \left( { - 0.0{76}\;{\text{V}}\;{\text{vs}}.\;{\text{SHE}}} \right) $$

(13)

$$ {\text{H}}_{{2}} {\text{O}} + {\text{HO}}_{{2}}^{ - } + {\text{2e}}^{ - } \to {\text{3OH}}^{ - }\, \left( {0.{878}\;{\text{V}}\;{\text{vs}}.\;{\text{SHE}}} \right) $$

(14)

$$ {\text{2 HO}}_{{2}}^{ - } \to {\text{2OH}} - + {\text{O}}_{{2}} $$

(15)

H₂O acts as a proton donor inside alkaline electrolytes. The 4e⁻ pathway for ORR starts through O₂ adsorption at an active catalytic site ( ∗) (Eq. 16) and then remaining reactions proceed as presented from (Eq. 17–20) [45]. Also, the O₂ may directly dissociate (Eq. 21) and followed by (Eq. 17–20).

$$ {\text{O}}_{{2}} + * \leftrightarrow {\text{O}}^{ * }_{{2}} $$

(16)

$$ {\text{O}}^{ * }_{{2}} + {\text{H}}_{{2}} {\text{O}} + {\text{e}}^{ - } \leftrightarrow {\text{OH}}^{ - } + {\text{HOO}}^{ * } $$

(17)

$$ {\text{HOO}} * + {\text{ e}}^{ - } \leftrightarrow {\text{OH}}^{ - } + {\text{O}} * $$

(18)

$$ {\text{H}}_{{2}} {\text{O}} + {\text{O}} * + {\text{e}}^{ - } \leftrightarrow {\text{OH}}^{ - } + {\text{HO}} * $$

(19)

$$ {\text{HO}} * + {\text{ e}}^{ - } \leftrightarrow * + {\text{OH}}^{ - } $$

(20)

$$ {\text{O}}_{{2}} + * + * \leftrightarrow {\text{ O}} * + {\text{ O}} * $$

(21)

ORR happens through the charging of the battery and the reverse stages of the ORR are shown in Eq. 16–21. For instance, in alkaline electrolytes firstly the active catalytic site ( ∗) reacts with OH⁻ to form HO ∗ and O ∗ (Eq. 19–20). The O₂ is created through two possible reaction pathways based on the O ∗ surface coverage. Two adjacent sites of O ∗ mix directly to give O₂ at larger O ∗ coverage (Eq. 21). Moreover, at smaller coverage of O ∗ , the O ∗ could react with OH⁻ and results in the formation of form HOO ∗ (Eq. 18), followed by the decomposition reaction and results in O₂ production (Eqs. 16 and 17).

3.1.4.2 Reaction mechanisms of OER/ORR in neutral electrolytes

In neutral electrolytes, the OER/ORR mechanisms are the basis in developing effective O₂ electrocatalysts. The ORR/OER occurs at centers of redox metal on the electrocatalysts surface [41]. Therefore, metal ions or metals with various oxidation state values are desirable and the bonds of M–O created through the reactions must be neither too weak nor too strong. This technique applies to all electrocatalysts (ORR/OER) and employed in near-neutral or neutral electrolytes [46]. The OH⁻ and H⁺ are the chief reactants for ORR in alkaline and acidic electrolytes, correspondingly. However, H⁺/OH⁻ concentration is low in neutral electrolytes resulting in lower ORR. Therefore, in neutral electrolytes the ORR might be analogous to acidic electrolytes which proceeds with Eq. 16 and followed through Eq. 22–25:

$$ {\text{O}}^{ * }_{{2}} + {\text{ H}}^{ + } + {\text{ e}}^{ - } \leftrightarrow {\text{HOO}}^{ * } $$

(22)

$$ {\text{HOO}}^{ * } + {\text{H}}^{ + } + {\text{e}}^{ - } \leftrightarrow {\text{H}}_{{2}} {\text{O}} + {\text{O}}^{ * } $$

(23)

$$ {\text{H}}^{ + } + {\text{O}} * + {\text{ e}}^{ - } \leftrightarrow {\text{HO}}^{ * } $$

(24)

$$ {\text{HO}}^{ * } + {\text{H}}^{ + } + {\text{e}}^{ - } \leftrightarrow * + {\text{H}}_{{2}} {\text{O}} $$

(25)

3.1.4.3 Electrocatalysts for aqueous RZABs

Effective electrocatalysts of O₂ are vital to allow ZABs by long lifetime and high-power density. For instance, electrocatalysts of Pt-based materials are active for ORR, however, they have limited OER catalytic activity [16]. Among the possible candidate materials, metal oxides are comparatively inexpensive, environmentally friendly, and exhibit considerably high activity in alkaline electrolytes. Moreover, many metal oxides are preferable for OER catalysts and are appropriate for bifunctional catalysts. Amongst them, MnOx has attracted much interest due to its particularly rich redox chemistry of Mn, low cost, environmental compatibility chemical components and crystal phases. For instance, comparison of charge/discharge conditions among the noble metal oxide and metal catalysts (Pt, Pd, Ru, MnO₂, RuO₂, and PdO) showed that MnO₂ is a better candidate of the cathode material with higher values of specific capacities and cycling capabilities [24]. It was found that the activity of ORR for MnOx depends on the chemical composition in the order of Mn₅O₈. The catalytic recital depends on the ability of the cation to provide various valence states, particularly in the redox couples formation in the potential regions of ORR/OER [43]. MnOx was the primary bifunctional metal oxide of the O₂ electrocatalyst under the neutral electrolytes [47] but the lower ionic conductivity and OH concentration in neutral electrolytes resulted in a decreased catalytic activity. The catalytic characteristics of various MnO₂ stages indicated that that γ-MnO₂ is the most active stage in neutral electrolytes while, α-MnO₂ is the utmost active stage in alkaline electrolytes [48].

3.1.4.4 Advance strategies of alkaline electrolytes

The performance of Zn-electrodes (e.g. Columbic efficiency, battery capacity, and cycle life) can be improved through passivation, removal of the dendrite growth, change of shape, and evolution of hydrogen that has an effect on alkaline electrolytes using solutions (additive) for instance, electrode and electrolyte additives as well as 3D structures. For instance, J. Y. et al. [29, 49] proposed the electrochemical performance improvement of a Zn electrode in a zinc sulphate or zinc nitrate aqueous electrolyte by using activated carbon. They reported that well-developed and rich pores of activated carbon were suppressed the growth of Zn passivation products and dendrites, confirming the electrochemical activity of Zn electrodes. Mohammad et al. [50] also investigated that an uper-P addition to a porous electrode (Zn) and tested the assembled RZABs recital inside the electrolyte (6 M KOH). It showed that the Zn electrode (i.e., 2wt % super-P) provided a power density and a specific discharge capacity of 20 mWcm⁻² and 776 mAhg⁻¹ respectively as a result of the Zn particles arrangement and powder of super-P on the Zn-electrode (porous). Moreover, Lee et al. [5] also used a Zn electrode coated with a thin aluminum oxide (Al₂O₃) layer that inhibit self-discharge and enhance the efficiency of electrochemical values in RZABs. Similarly, AR.Mainar et al.[47] revealed that the shelf life of the Zn electrode increases an anion-exchange ionomer layer that was applied in a homogeneous layer. Also, the use of electrolyte additives has proven to be an effective method for overcoming the problems associated with Zn electrodes, for instance, Sun, Kyung EK, et al. [30] showed that the occurrence of polyethylene glycol in an electrolyte decreases the dendrite formation and rate of kinetics for Zn electrodeposition. Wu T.H. et al. [7] also employed inorganic corrosion inhibitors such as indium (In), bismuth (Bi) and lead (Pb) for hydrogen evolution reduction and addition of polymer like polyvinyl alcohol, poly(vinylidene fluoride) and carboxymethyl cellulose for enhancement of the change in shape of Zn electrodes. Additionally, a green solvent (e.g. ethanol) has been employed for an electrolyte (alkaline) additive to alleviate the problems with electrochemical performance of a RZAB, for example, S. Hosseini et al. [48] showed that alcohols transformation in alkaline solutions resulted in ions of alkoxide that compete with OH⁻ ions in coordination into Zn²⁺ ions, confirming that addition of alcohol leading in a significant reduction in the formation of ZnO. S.Hosseini et al. [51] in a different work also investigated the use of potassium hydroxide (KOH)/ Dimethyl sulfoxide (DMSO) as a super base system (i.e. a reactivity and physical characteristics of KOH/DMSO giving an extraordinary basicity with a pKa value of 30–32 owing to a combination between the two bases). Moreover, KOH/DMSO system enhances the surface area of the Zn particles that allows for more electrochemical reactions of Zn in the electrolyte (alkaline) and improves the electrochemical performance of the Zn anode. Hence, rising Zn electrodes surface area reduces the possibility in formation of dendrite through charging [52, 53]. Wu T.-H. et al.[7] also examined a novel Zn (3D-wired) design that can withstand the charge/discharge cycles of 80 at high currents and result in no dendrite growth. Furthermore, R. Khezri et al. [54] revealed that a combination of potassium persulfate (K₂S₂O₈ or KPS) with an alkaline electrolyte can significantly improve electrochemical properties and the performance of RZABs. Table 4 presents the major challenges in aqueous electrolyte and proposed solution for RZABs [7, 9, 13, 16, 48, 51, 55]. Table 5 presents the summary of recently investigated electrolyte (alkaline) additive for ZABs [33, 35, 53,54,55].

Table 5 Summary of the alkaline electrolyte additive for ZABs

3.2 Non-aqueous electrolytes

Exploration of the electrolytes (non-aqueous) may offer another chance that simplify the Zn anode electrochemistry and can expand the electrochemical window as well as solve the limitations in aqueous electrolytes of RZABs [52]. The main benefits of non-aqueous electrolytes compared with aqueous electrolytes include that they are potentially able to compensate for some of the deficiencies such as dendrite formation, passivation, carbonization, hydrogen evolution, and water evaporation that led to electrolyte drying. In addition, they can offer a broad window of electrochemical behavior and higher thermal stability [9, 30, 50].

3.2.1 Gel polymer electrolyte (GPE)

GPEs have been recently developed to replace the liquid electrolytes. It comprises a salt, polymer network, and solvents. The polymer network is essential because it envelops the liquid part that comprises the solvent/s and salt and prevents their escape. Hence, GPEs don’t possess a leakage issues like the liquid electrolytes illustrated in Table 3 [56]. Moreover, GPEs don’t leak and dry out like the aqueous electrolytes. GPEs also possess a good adhesive properties, mechanical strength, and electrochemical stability [2]. However, the use of GPEs in RZABs is still not feasible in practical applications because of the poor solubility of Zn salts, high viscosity and flammability and volatility of organic solvents (e.g., DMSO, ethylene carbonate, and propylene carbonate). Due to the volatile and flammable properties of these organic they are inappropriate in employing them for ZABs (open systems) [57]. The widely used polymer matrix for GPEs comprise poly(ethylene oxide) (PEO), poly(vinyl chloride) (PVC), poly(methyl methacrylate) (PMMA), poly(vinylidenefluoride-hexafluoropropylene)(PVDF-HFP), poly(vinylidene fluoride) (PVDF), poly(acrylic acid) (PAA) and poly(acrylonitrile)(PAN). Generally, a framework of polymer in GPEs is employed as host material and provides a large mechanical integrity. Numerous conditions such as fast segmented motion of polymer chain, wide electrochemical window, high molecular weight, low glass transition temperature, promoting the dissolution of salts and high degradation temperature should be met by polymers to be considered as good hosts [58].

3.2.1.1 GPE polymeric host materials

In preparation of GPE the host polymer (i.e., at least one) that acts like a base matrix is required before inclusion of any other materials. Poly(vinyl alcohol) (PVA), PVC, PEO, PAN, PAA, poly(ethyl methacrylate) (PEMA), PVDF, PMMA) and PVDF-HFP are some of the host polymer that are usually employed in GPEs preparation [59].

PVA:It is an optimistic material for host polymer because of its unique properties such as good mechanical strength, cost-effectiveness, tensile strength, good optical properties, non-toxicity, excellent film-forming properties, preparation, good chemical and thermal stability, good flexibility, high abrasion resistance, and biocompatibility, [59, 60]. The physical properties of PVA differ depending on the degree of polymerization and alcoholysis [61]. PVA also contains a large number of functional hydroxyl groups (hydrophilic) that can improve water absorption. Recently, PVA has been examined as a gel electrolyte in ZABs for flexible energy devices owing to a promising mechanical characteristics [60].

PAA It is used as a battery binder owing to the hydrogen bond formation within the carboxyl groups produced by the hydroxyl groups [61]. PAA appears to be a promising host polymer for electrolytes (alkaline) for ZABs. In one-pot synthesis approach, precursors and cross-linkers as well as the various concentrations of additives and monomers (e.g. ZnO in alkaline solution) polymerize to produce GPEs [62].

Poly (acrylamide-co-acrylic acid) (P-(AM-co-AA) Numerous mixings of polymers frequently employed as host in the GPEs for a planar ZAB. Gel electrolyte (alkaline) host of (P-(AM-co-AA)), representative example of copolymerization polymer that can be applied in a planar ZAB, has an improved capacity of water retention and superior atmospheric stability as well as higher ionic conductivity than the often employed gel electrolyte (alkaline) (i.e. PVA) [63].

PEO It is among the popular host polymer that has been broadly explored owing to a good high capacity in salt complexation, dimensional stability, good corrosion resistance, mechanical flexibility, high conductivity (ionic) in amorphous state, chemical stability and acceptable commercial cost [59]. Nevertheless, the low ionic conductivity at ambient temperature because of its tendency to crystallize at lower temperature is the limitation of PEO for using as a host polymer. Table 6 presented the recently reported polymer matrix materials for GPE in ZAB [58, 63,64,65,66,67,68].

Table 6 The polymer matrix materials for GPE in ZAB

Table 7 Classification of DESs

The morphology of surface and cross section of the polymer matrix decides directly the performance of GPE. By observing the morphology of the surface and cross section with Scanning electron microscopy (SEM), XRD (x-ray defraction spectroscopy), Raman spectra, Electrochemical impedance spectroscopy (EIS) and FT-IR spectra, non-porous or porous polymer structures can be observed as presented in Fig. 3 [68]. Furthermore, the shapes, structures and sizes of the pores in the membranes can also be observed.

Fig. 3

Characterization of the GPEs. Optical images (a–c), SEM image of PVAA-graphene oxide (GO) (d), XRD patterns (e), FT-IR spectra (f), and Raman spectra of GO and different gel polymers (g), electrolyte and water uptake behaviors of PVA, PVAA-GO and PVAA (h), electrolyte retention capabilities of the GPEs (i), ionic conductivities of the GPEs (j). Reproduced in permission from ref. [68]

In general, the high-quality GPEs is supposed to satisfy three primary requirements: performance, durability, and price in Fig. 4 [59]. The performance level of GPEs is related to the material's ionic conductivity and its reliability or durability should be sufficient to withstand normal operating conditions over time. Also, the cost of the GPE devices must be low in order for them to be widely available.

Fig. 4

Schematic representation of fundamental requirement of GPE for RZAB modified from reference [61]

Recently, the following methods have been proposed to fundamental requirements of GPE for RZAB applications.

• (a) Ionic conductivity (good): The porosity, degree of crystallinity, and polymer's ability to absorb solvent within the matrix determine a good ionic conductivity of GPEs.

• (b) Cationic transference number (high): It is also a vital parameter in influencing the efficiency of GPEs. In single salts GPEs, the cation mobility relative to the anion has a wide range of cationic transference.

• (c) Chemical and thermal stability (good): The GPEs must sustain the high conductivity (ionic) value over a wide range of temperature while maintaining a structurally stable manufacturing, storage, mobile assembly and use in order to be effective. Inorganic particles (e.g. tin oxide (TiO₂), silicon dioxide (SiO₂) or aluminum oxide (Al₂O₃)) can be added to GPEs to improve their thermal stability [69].

• (d) Mechanical properties (good): GPE must not undergo deformation and mechanically stable that ought to expose the battery stability as a GPE is inserted between the anode and the cathode.

• (e) Cost (low): For storage technologies, cost-effective GPEs are highly required to overwhelmed the existing energy and environmental challenges. It makes it critical for a convenient fabrication process.

3.2.2 Solid polymer electrolyte (SPE)

SPEs are created with inorganic salts solution in a functional polymers (polar) and then develop in solid electrolyte that conduct an ion [70]. Electrostatic forces are the primary cause of interactions between metal ions and polymer polar groups that results a coordinating bonds formation. The polymer-metal ion interactions are influenced with the characteristics of the functional groups (i.e. polymer backbone distance and compositions), degree of branching, charge and properties of metal cation, counter ions and molecular weight. SPE system is solvent-free and comprises of a salt without any liquid phase and a host polymer. The SPE conductivity is not large like the GPE system as the ions travel through the polymer without any support of organic solvent or water. Poly(carbonate-ether), PAN, and PEO are examples of SPEs [71]. In ZABs, the SPEs or thin-film electrolytes may have several potential benefits over current electrolytes including enhancement in energy efficiency, electrical rechargeability shelf life, and operating temperature range. SPEs, ionic conductive solids (i.e. macromolecules with heteroatoms), permit dissolution and diffusion of one or more salts in the existence of an applied electric field. SPEs also provide a number of advantages including a good mechanical strength with some deformability, simplicity of handling, allowing a thin film fabrication, reducing the problems associated with electrode corrosion and increasing battery life, avoiding the problems linked with battery leakage, and eliminating air electrode flooding [2, 4]. The main disadvantages of SPEs are a low conductivity (ionic) at room temperature, low solubility of the Zn salts, high interfacial resistance, and the creation of ZnO passive layers between the electrode and SPE that limits the ability to perform a reversed charging reaction [72]. Ion-conduction mechanisms studies (theoretical) have shown that polymers having a more amorphous structures and lower glass transition temperatures (Tg) could have higher conductivity (ionic) [23]. Hence, for the reduction of the Tg and achieving a high ionic conductivity value a material design approaches like copolymer synthesis and crosslinking have been proposed somewhere else [23, 73].

3.2.2.1 Design strategies for improving the recital of polymer electrolytes (PEs)

Various methods can be used to rise the performance and characteristics of PEs including doping, addition of inorganic fillers, polymer blending, incorporation of plasticizers, use of copolymers (comb-branched) (Fig. 5).

Fig. 5

Modified from reference [62]

Schematic representation of various strategies for achieving high performance PE.

(a) Blending Polymer blending is created by mixing a minimum of two polymers (i.e. by chemical bonding or without forming chemical bonding) or by forming a physical mixing of two or more polymers or copolymers [74,75,76].

(b) Plasticizers These are additives that improve the ionic conductivity at room temperature by increasing plasticity that allows for a greater ionic mobility [75, 76].

(c) Copolymers These are polymers formed by cross-linking a minimum of two different sorts of monomers [77].

(d) Cross-linked polymers In SPE, the high ionic conductivity value achieves due to the amorphous property of the polymer matrix at low temperature. The cross-linked PEs have a fully amorphous structure and a good ionic conductivity at room temperature [59].

(e)Fillers: Incorporation of a small particle size and fillers (i.e. electrochemically inert) into matrices of the polymer are an alternative method for improving the performance of SPE [78].

3.2.3 Room temperature ionic liquids (RTILs)

They comprise a large asymmetric cations and weakly coordinating anions which melt near or at room temperature [48]. Some of the advantages of RTILs include a high thermal stability, high conductivity (ionic), broad stability of the electrochemical windows and low vapor pressure [4]. A room temperature ionic liquid (IL) has limitations as it requires a high purity material. The impurities of the IL (i.e. even in trace amounts), influence various properties such as physical properties, energy density of the battery (i.e. due to the dual-electron reaction mechanism of RTILs), price, sensitivity to moisture, viscosity, and synthesis approach (non-environmentally friendly). Moreover, the lower ionic conductivity value of ILs is still challenging as compared to KOH, hence a significant further study is still required to elucidated the characteristics and improve their performance [24, 79]. Among the various approaches, incorporating water into RTILs reduces the electrolyte viscosity and the activation barrier needed for Zn deposition as well as assisting in a stable cycling. The commonly employed cations of ILs are imidazolium (e.g. [EMIM]⁺), ammonium (e.g. [DEME]⁺), pyrrolidinium (e.g. PYR14), and sulfonium (e.g. [Me₃S]⁺). ILs can be grouped into three kinds (i.e. based on the different compositions of anions and cations), namely protic, aprotic, and zwitterion [80]. RTILs also can be classified into first, second and modern-generation based on the simplicity of the reactivity ILs with water (Fig. 6) [4]. The 1st generation ILs (e.g. Haloaluminate anion (AlCl₄)⁻) is sensible to moisture and can be controlled by anhydrous condition. The 2nd generation ILs (e.g. tetrafluoroborate ([BF4⁻]) or hexafluororophosphate ([PF6⁻]) with water are less reactive and they absorb moisture that affects the chemical and physical behaviors of the liquid. The modern-generation ILs (e.g. perfluoroalkyl phosphate [FAP⁻]) and bis(trifluoromethanesulfonyl) imide [TFSI⁻]) are more hydrophobic anions and moisture sensitive than the 1st and 2nd generation of ILs [81]. A suitable choice of the room-temperature ILs to be employed as electrolytes in RZABs is highly reliant on a vital understanding of how contents of water affects the physical and electrochemical performance of the cathode and anode reactions [82].

Fig. 6

Summary of the classification room temperature ILs based on reactivity of water modified from reference [4, 83]

3.2.4 Deep eutectic solvents (DES)

Abbot and his colleagues developed a cheap and an ecofriendly room temperature DESs which are alternatives to RTILs at the beginning of the 19th century [83, 84]. The concept of DES is more frequently described as a combination of compounds (two or more) with a smaller melting/freezing point than the pure components and not always restricted to a specific ratio of compounds. This idea opens a wide range of options for DES with tunable characteristics [85]. The interaction of the anion with a complexing agent (i.e., a bond donor of hydrogen) is denoted as DESs. This interaction results in an effective size increase and a shield in the interaction with the cation as well as the reduction of the mixtures melting point [2]. The cation melting point decreases with the increases in asymmetry. The strength of the hydrogen bond of the various counter ions (negatively charged) affects the freezing points. DESs can dissolve a range of metal oxides due to protons or electrons acceptance (donation) behaviors for the formation of hydrogen bonds that provide an excellent dissolution characteristics [57]. DESs have various advantages over other solvents including nontoxicity, synthetically accessible, high thermal stability, less expensive, wider electrochemical window, environmentally friendly, chemical inertness with water (easy storage), and biodegradability presented in Table 3. Moreover, DESs exhibit some unusual and intriguing properties such as reversible thermos-chromic behavior with metal chlorides and dissolution of poorly soluble compounds with bio-availability [84]. One of the key demerits of DESs is the viscosity values (range from 50 to 8500 cP) while the IL viscosities range from 10 to 500 cP and comparison of viscosities with different electrolytes illustrated in Table 3 [57, 86]. These viscosity values are influenced by the temperature, chemical behavior of the DES constituents and the water content [87]. The use of hydrogen bond donor or small cations result in the formation of the low viscosity DESs. Table 7 presented the recentely classification of deep eutectic solvents [87,88,89,90]

4 Electrolyte characterization methods

The various techniques employed in a physicochemical characterization of electrolyte include SEM [91], thermogravimetric analysis (TGA) [92], FTIR, XRD [93], cyclic voltammetry (CV) [94, 95], conductometery measurement [96], viscosity measurement [97] and refractometer measurement [98] illustrated in (Fig. 7).

Fig. 7

The schematic view of characterization of electrolyte materials for ZABs modified from reference [94]

5 Summary and future outlook

In summary, RZABs have many unique advantages including environmental friendliness, intrinsic properties and low cost that illustrates a high potential for large-scale energy storage materials for practical applications. In this review, the issues and potential solutions in electrolyte materials development for RZABs were summarized. In RZABs, aqueous alkaline electrolytes play a vital role in the solubility of Zn. It generates a supersaturated or saturated Zn(OH)₄²⁻ which is strongly linked to the formation of dendrite, Zn passivation, carbonate precipitation and anode shape change. These issues could be solved by employing a suitable pH and using appropriate additives. There are many ongoing research efforts in developing non-aqueous electrolytes as a substitute to aqueous electrolytes for use in secondary ZABs. In this paper, the four different kinds of electrolytes (i.e., non-aqueous) have been explored. PEs can well relieve the electrode material dissolution and provide a stable electrochemical recital due to the limited availability of water. Traditional aqueous electrolytes have the leakage and electrolyte evaporation issues that can be avoided. In the meantime, to achieve improved mechanical properties, some PEs can be used. However, the commercialization of PEs for RZABs still faces significant challenges resulting from a low Zn salt solubility, high viscosity, flammability, and high interfacial resistance. The addition of inorganic fillers, polymer blending, use of copolymers (comb-branched), incorporation of plasticizers and doping (i.e., in comparison with non-aqueous electrolyte) and using a room-temperature ILs electrolyte systems (i.e., having high thermal stability, low vapor pressure and high values of ionic conductivity) are effective methods to enhance the characteristics and recital of PEs. However, PE is less common for industrial applications and generally uses pure and expensive materials. Hence, DES-based non-aqueous RZABs is attracting more attention because it is environmentally friendly and low cost than room temperature ionic liquids. In general, the RZABs for the practical application still faces significant challenges and inspires more research efforts to realize its electrochemical performances and adoption. The highest existing importance and future prospects lead to provide a special stress to the advances in the electrolyte technologies and requires developing a wide-ranging approach and progress in environmentally friendly, low-cost, highly efficient, and stable electrolytes for RZAB.

Moreover, to increase the recital of the electrolyte, the following issues seem to require immediate attention.

• Electrolytes solubility: The conducting salt and its additives must be dissolved in the electrolyte. If any of these components reaches at the end of their solubility range, they will precipitate and cease to participate in the electrochemical process. In general, RZAB's interfacial stability, solvation structure, cathode dissolution, cost, safety, and overall performance of the electrochemical are all influenced by the electrolyte formulation of concentrations, salts, additives and solvents. Hence, more effort should be devoted in developing an advanced formula for the electrolyte that capable in supporting large-scale RZAB application.

• Electrolyte transport properties: To facilitate ion transport, the electrolyte should be liquid within the temperature range of operation and has a low viscosity. A low vapor pressure should also be provided. Ionic conductivity is a critical metric for evaluating electrolyte performance. The aqueous liquid conductivity of electrolyte is generally high due to strong dissolving ability, high salt concentration and low dynamic viscosity. Due to higher solvent viscosity and poorer dissolving ability, the conductivity of a non-aqueous liquid electrolyte is lower than that of an aqueous electrolyte. The ionic conductivity value of a solid-state electrolyte is high. So, raising the reaction rate of electrodes require the conductivity improvement of the electrolyte and it is influenced with the ionic migration rate, free ion number, and the valence of the negative and positive charges. Therefore, considering the viscosity of materials is vital to enhance the ionic conductivity values of the electrolytes.

• Stability of temperature: The temperature stability of the electrolyte can be a major issue when using RZABs in harsh situations like a high or low-temperature environment. It comprises two features i.e., the electrolyte and battery system thermal stability when the electrode and electrolyte interrelate with each other. The electrolyte thermal stability is influenced with the composition of solvent, salt, and additive. It is often characterized with DSC/ TGA methods. In a battery system, the thermal stability is more complex regard to the interaction between electrode and electrolyte materials. Once the battery self-heating process is started due to external influences like heating/overcharge, a series of chemical reactions including the decomposition of electrolyte and electrode materials interaction will occur. These reactions result in high internal pressure inside the battery and even leads to an explosion. Hence, investigation of the thermal properties of materials is vital to ensure the safety of the battery.

• Interfacial compatibility The long-term cycle stability of RZABs is dependent on interface compatibility. Good interface friendly with the electrodes can be achieved by selecting and regulating the electrolyte. The compositions of the electrolyte directly influence the composition of solid electrolyte interface (SEI) or interface passive layer of the anode and cathode, which further influences the recital of the cell. Nevertheless, the SEI formation mechanisms in diverse electrolyte systems for RZABs and their modification through optimization of the electrolyte are still unknown. Besides, one of the main problems for practical use of RZABs is the high interface contact resistance value between electrodes and electrolyte.

• Investigating the structure of electrolytes The basic structure electrolyte has a significant impact on the electrolyte basic characteristics and the electrochemical recital of RZABs. The solvation sheath structures (e.g., bond length, mayer bond level and de-solvation energy etc.) are critical properties and establishes the electrode material interfacial compatibility. However, the present understanding of these characteristics is still inadequate because of the restrictions of characterization approaches and the electrolyte system complexity. Thus, it is valuable and meaningful to examine essential structures of the electrolytes and how these factors influences the recital of the electrochemical materials.