Zhang et al. ^[1][61] investigated the PVA-based gel electrolyte containing different wt.% KOH to Zn/MnO₂ batteries. The conductive characteristics of the gel electrolytes closely resemble those of KOH aqueous solutions. Initially, the conductivity increased with an elevation in KOH concentration due to the reduction in the crystalline phase but then decreased at high KOH concentrations because of the restricted ionic mobility ^[2][62]. The resulting battery achieved good cycling performance.

Zhu et al. ^[3][63] fabricated an elastic PGE film using a solution polymerization method consisting of 0.02 wt.% K₂S₂O₈, 16.75 wt.% acrylic acid, and 83.23 wt.% aqueous KOH solution, which was optimized for the reaction. The utilization of PGE electrolytes in Zn/Air, Zn/MnO₂, and Ni/Cd cells showed that the PGE film had chemical and electrochemical stability comparable to that of aqueous alkaline solutions.

In a study conducted by Gaikwad et al. ^[4][64], a stretchable MnO₂-zinc cell was fabricated using a gel electrolyte based on polyacrylic acid (PAA). The cell utilized off-the-shelf compliant silver fabric as a current collector, which was embedded with MnO₂ and Zn particles. Remarkably, the cell demonstrated a discharge capacity of 3.775 mAh cm⁻² that was fully maintained even under a high strain of 100%.

Lee et al. ^[5][65] proposed a poly(acrylamide-co-acrylic acid) gel electrolyte for the Zn–Ni secondary battery. The gel electrolyte was prepared through a straightforward process by dissolving P(AAm-co-AAc) in an alkaline solution and subsequently gelling the mixture. Considering the conductivity and viscosity characteristics of the gel polymer electrolyte, it was observed that the ionic conductivity slightly decreases with an increase in P(AAm-co-AAc) concentration, while the viscosity increases with an increase in P(AAm-co-AAc) concentration. To optimize the gel electrolyte, the concentration of P(AAm-co-AAc) was fixed at 6% by weight, resulting in a conductivity of 4.8 × 10⁻¹ S cm⁻¹. The utilization of the gel polymer electrolyte significantly improved the capacity retention of the cell. After 60 cycles, the capacity retention reached about 88% (310 mAh g⁻¹) in comparison to approximately 40% retention (125 mAh g⁻¹) observed with a regular alkaline electrolyte Ni-Zn cell under the same cycling conditions. This highlights the superior performance of the gel polymer electrolyte in enhancing the stability and capacity retention of the Zn–Ni secondary battery. The improved cycling performance was attributed to the lower reactivity of polymer electrolytes compared with liquid electrolytes and the suppression of dendrite growth by gel polymer electrolyte as well as the zinc dissolution confinement in alkaline environments by gel polymer electrolytes.

Xinying et al. ^[6][66] developed a 3D cross-linked GPE consisting of poly(acrylamide-potassium acrylate) (P(AM-KA)), zinc alginate, and KOH. Benefiting from the covalent-bonded network and the abundant ion ligand complex, the GPE has favorable ion transport and adsorption, effectively mitigating dendrite growth in the Ni-Zn battery. As a result, the battery showed remarkable stability over 10,000 cycles with a capacity retention of 88.96%.

Metal–air batteries have attracted great interest as promising energy storage technologies with distinct energy density advantages, among which the Zn–air technology has been mainly focused on due to its low operation cost, safety, and better striping/plating ability. The most commonly used configuration for a zinc–air battery consists of a zinc anode, an alkaline electrolyte, and an air cathode, typically made from a porous and carbonaceous material. Concentrated aqueous alkaline solutions have been used as the electrolyte by virtue of their better kinetics and catalytic activity, especially KOH, which shows better properties of high ionic conductivity, high activity, large oxygen diffusion coefficient, good low-temperature performance, and good solubility of carbonate by-products ^[7][8][9][67,68,69]. In the process of discharging a zinc–air battery, oxygen molecules permeate the porous air cathode and undergo reduction to hydroxyl ions, facilitated by the catalyst layer on the cathode. At the same time, electrons are generated through the electrochemical oxidation of zinc. During the recharge process, oxygen is evolved and diffuses out of the cathode, while Zn²⁺ are deposited back onto the anode. In rechargeable batteries, it is crucial to have an oxygen electrode that exhibits dual catalytic activity for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), whereas primary zinc–air batteries, which are not designed for recharging, typically employ ORR electrocatalysts that are specialized for one specific reaction. Solid-state zinc–air batteries exhibit better cycling performance since the mitigation of aqueous electrolyte volatilization improves access to oxygen on cathodes, which increases the transportation rate in the cathode. Moreover, polymer electrolytes offer a wider electrochemical stability window and enhanced mechanical robustness. These properties contribute to an extended battery shelf life and an expanded operating temperature range. Therefore, besides the common required properties like high ionic conductivity, the electrolyte should also cut down the content of volatile solvents. Vassal et al. ^[10][70] were the pioneers in introducing a potassium hydroxide-based alkaline solid polymer electrolyte, specifically a copolymer of poly(epichlorohydrin) and poly(ethylene oxide) known as P(ECH-co-EO), into zinc–air cells. This innovative electrolyte enabled the cell to deliver a high current density of 14 mA cm⁻² at a discharge voltage of 0.8 V. Additionally, it demonstrated exceptional performance by sustaining a current density of 30 mA cm⁻² at the same voltage and operating at a temperature of 60 °C, surpassing the limitations of PEO/KOH/H₂O electrolytes that tend to melt at this temperature.

PVA is one of the most commonly used polymer electrolytes in solid-state zinc–air batteries thanks to its excellent chemical stability, hydrophilicity, high dielectric constant, and high alkali tolerance ^[11][71]. The high humidity absorption of PVA due to –OH groups promotes salt solvation and thereby enhances ionic conductivity ^[12][13][14][72,73,74].

Liu et al. ^[15][58] investigated the development of flexible al-solid-state Zn–air batteries using various components such as a free-standing nano-porous carbon nanofiber film-based air cathode, zinc foil anode, alkaline PVA gel electrolyte, and pressed nickel foam current collector (to enhance conductivity). Although the PVA gel electrolyte had limited ionic conductivity and caused high contact resistance, negatively impacting the charge–discharge performance, the battery demonstrated excellent flexibility and cycling stability. Notably, the battery maintained this stability even when subjected to substantial bending or folding, demonstrating its robustness and flexibility. Yue et al. ^[16][75] synthesized a double network hydrogel using agar, graphene oxide, and PVA for Zinc–air batteries to demonstrate both good mechanical strength (388 kPa) and ionic conductivity (75 mS cm⁻¹). Furthermore, an agarose biopolymer matrix was developed to directly dissolve in KOH solution, thus avoiding the use of petroleum-based plastics ^[17][76].

However, PVA-based electrolytes are often plagued by very poor mechanical properties and insufficient ion-transport capability, which harms the electrochemical performance and mechanical flexibility. Zhi’s group ^[18][77] designed an alkaline-tolerant dual-network PANa-cellulose hydrogel through simple radical polymerization. The battery exhibited remarkable properties of super-stretchability (stretched up to 800% in a flat shape and 500% in a fiber-shaped configuration) and a high-power density of 108.6 mW cm⁻², which was superior to batteries with PVA electrolytes because of the better ionic conductivity when soaked in 6 M KOH. Joohyuk Park et al. ^[19][78] used gelatin as an electrolyte for zinc–air cells, and the electrolyte had comparable and higher ionic conductivity even at lower KOH concentrations (0.56 wt.%) compared with previously reported KOH-based GPEs. The gelatin electrolyte met the high requirement of robustness in cable-type flexible zinc–air batteries. Similarly, a cross-linked double-network GPE with carboxymethyl chitosan, acrylamide, and sodium acrylate was proposed, showing a good electrochemical property over a wide temperature range (−20–80 °C) ^[20][39].

Other studies on solid-state alkaline zinc rechargeable batteries are summarized in Table 1.