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Lithium-ion batteries (LIBs) have been considered an easily accessible battery technology because of their low weight, cheapness, etc. Unfortunately, they have significant drawbacks, such as flammability and scarcity of lithium. Since the components of zinc-ion batteries are nonflammable, nontoxic, and cheap, AZIBs could be a suitable replacement for LIBs.
- aqueous zinc-ion battery
- flexible zinc-ion battery
- anode and cathode materials
1. Introduction
Lithium-ion batteries (LIBs) have attracted much attention to battery technology due to their low weight, high energy densities, and specific power. Unfortunately, LIBs have major drawbacks, such as energy density limits, high cost, toxic nature, and safety issues [1,2]. Therefore, large-scale applications of LIBs are still challenging because of these limitations. However, lead-acid- and nickel-cadmium-based batteries are currently dominant in the present battery energy storage market due to their low cost and durability [1,3,4,5]. They have limitations such as poor energy densities and environmental problems due to toxic electrodes. Moreover, various alkali metal cations (such as Na+ and K+) and multivalent charge carriers (such as Mg2+, Al3+, and Zn2+) have been investigated in aqueous electrolyte-based batteries. For example, aqueous zinc-ion batteries are particularly appealing since Zn has a large natural abundance, a low redox potential, a high theoretical capacity, intrinsic safety, and low toxicity [1,2,5,6,7,8,9].
Aqueous zinc-ion batteries (AZIBs) are facing challenges due to the deteriorating effect of cathode and anode materials [10,11,12,13,14,15]. Hence, these effects are responsible for lowering the coulombic efficiency and the specific capacity of AZIBs. Scientists are still working to overcome these issues regarding AZIBs [1,2,3,5,6,7,8,15,16,17,18,19,20]. Additionally, they are giving more attention to modifying cathodes, which can enhance the electrochemical performance. Materials with a spinel or layer structure, such as Mn-based, vanadium-based, and Prussian blue analogous, are attracting attention for modifying electrode materials for AZIBs [11,21,22,23,24,25,26,27,28]. Among them, Mn-based materials have gained a lot of interest in the cathode materials because of their diverse crystal structures, different valence states, and high-voltage platforms [26,29,30,31,32]. First, alkaline electrolytes acting as primary electrolytes were used in Mn-based materials in the 1860s. Unfortunately, several challenges appeared in the alkaline electrolyte system in ZIBs, such as the dendrite and byproducts of the Zn anode resulting in low coulombic efficiency and poor cycle performance. In 1988, Yamamoto et al. used a mild acid ZnSO4 electrolyte in α-MnO2 to enhance the electrochemical performance, although this approach did not attract much scientific attention [33]. Similarly, Wang et al. have shown exceptional results when they used a moderate acid ZnSO4 electrolyte in α-MnO2, and it has shown good reversibility and cyclability [34]. However, aqueous zinc-ion batteries (AZIBs) are still under investigation, and the complexity arising from aqueous electrolytes is hindering the creation of highly cyclic stable ZIBs. Scientists are looking for suitable electrolytes in place of aqueous electrolytes to reduce these issues, such as dendrite formation, cathode dissolution, etc. They are considering that electrolytes that have a lesser water content might reduce the parasitic reaction and could improve cyclic stability as well as electrochemical performance.
2. Zinc-Ion Batteries (ZIBs)
2.1. Advantages of ZIBs over Other Energy Storage Devices
ZIBs have attracted much attention because of their lower cost, high storage capacity, etc. In addition, ZIBs are less flammable than lithium-ion batteries (LIBs). Although sodium-ion and potassium-ion are cheaper than LIBs, they also have flammability issues, like LIBs [1,2,3,4,5]. Above all, ZIBs are a suitable candidate for future-generation battery technology. Unfortunately, several barriers exist in the way of further implementations involving side products (hydrogen production, dissolution, and contact resistance) and zinc dendrite growth at the anode. These issues are hindering the creation of sustainable ZIBs. Hence, developing strategies of anodes and electrolytes and the reaction mechanisms are crucial. These developments could reduce cathode erosion, zinc dendrites’ formation, hydrogen evolution, and other issues [7,8,9,10]. It is assumed from the literature review of ZIBs that electrolytes and durable cathodes and anodic materials could play a pivotal role in making sustainable ZIBs. As a result, considerably more attention should be directed to electrolyte development technologies to produce greatly extended durability and consistency.
2.2. Zinc-Ion Batteries (ZIBs)—The Electrolytes and the Anode Materials
Zinc anode has issues such as HER (hydrogen evolution reaction), dendrite formation, and shape abnormalities, which could play a significant role in degrading the cyclic stability and performance of ZIBs. These issues lead to a smaller coulombic efficiency. However, the cyclic stability and performance vary depending on the anode materials. Therefore, its performance is still poorer. One of the most popular and successful methods to mitigate adverse reactions and the growth of zinc dendrites is to add additives to electrolytes [11,18,24,25,26,29,30,46,47]. Two types of additives are generally employed in aqueous ZIBs, organic and inorganic. For example, organic additives, including polyethyleneimine (PEI), polyethylene glycol (PEG), poly(vinyl alcohol) (PVA), and diethyl ether, can bind on the tips of zinc patches to reduce local current density. Other organic additives such as benzotriazole (BTA), tetrabutylammonium bromide (TBAB), potassium persulfate (KPS), dimethyl sulfoxide (DMSO), benzotriazole (BTA), and ethanol can mostly adsorb on the anode surface, which can considerably reduce species diffusion and strong corrosion resistance. Additionally, some organic additives, such as ethylene glycol (EG), glucose, and DMSO, could impact the solvent evaporation process of solvated Zn2+ ions [42,48,49,50,51,52,53,54]. For example, the use of vanillin as a bifunctional additive in an aqueous electrolyte to stabilize Zn electrochemistry was reported by Cheng et al. [55]. The Zn anode, which was dipped in 5 mM of vanillin modified with a 2 M ZnSO4 electrolyte, has demonstrated the storage capacity (10 mAh cm−2 at 1 mA cm−2), cycling stability (1 mAh cm−2 for 1000 h), and coulombic efficiency (99.8%).
Without the vanillin additive in ZnSO4 electrolytes, the apparent reaction between the aqueous solution and the thermodynamically unstable Zn anode incites parasitic reactions responsible for HER corrosion and a Zn4SO4(OH)6 xH2O byproduct. In addition, the horizontal movement of Zn2+ ions can invoke deposition on sporadically scattered prominent nuclei, which can limit surface energy and surface area, heterogeneous Zn plating/stripping, and promote dendritic development. The vanillin surface could not only cover active Zn from a free water attack through two-dimensional (2D) adsorption, but could also promote perpendicular Zn2+ migration and severely curb overgrown 2D diffusion, climaxing homogeneous and dendrite-free Zn building. Similarly, Qin et al. showed a hybrid electrolyte containing graphene oxide as a supplement that boosts consistent electric field distribution and eliminates Zn2+ nucleation over-potential, the smooth zinc electrodeposition layer, and reaction kinetics [56]. The symmetric zinc battery with graphene oxide addition consisted of a solid zinc anode that extended around 650 h at 1 mA cm−2 and kept a cycle life of 140 h at 10 mA cm−2. A 3D laser microscope was used to study the surface morphology of zinc anodes. In the initial condition, the electric field on the velvety surface of pure zinc foil was disseminated uniformly in the same order.
When Zn2+ began to build up from a selected vicinity of the zinc floor, a distinguishable electric-powered subject turned established, and numerous outcomes confirmed that the neighborhood electric-powered subject after the dendrite tip turned into as much as two times that of a starting electrodeposit in the long run, mainly due to the formation of zinc ions within the dendrite tip. Those indentations or dendrites popped up as purple regions within the 3D laser image. Besides that, because the electrodeposit has become more potent and has been cut down the center into thinner dendrites, the depth of the electrical subject ought to attain three instances or more than the preliminary electric-powered subject, indicating that the end effect enhanced until the battery demonstrated excessive impedance. Additionally, because of the inclination to lessen the floor power and decrease transmission paths, the presently exposed zinc coating has become the ultimate electrodeposition location. Using GO as an electrolyte addition, on the other hand, may completely repair the dendrite problem. The bonding among GO debris in addition to Zn could not prevent distinguishable electric-powered fields from forming and could perhaps simply show the dissemination of the electrical subject; however, it may additionally confirm that charged Zn2+ operates right away to the anode floor, resulting in a steady zinc deposition layer. Additionally, Mantia et al. tested the morphology and kinetics of zinc electrodeposition in a 0.5 M ZnSO4 solution with the use of branched polyethyleneimine (BPEI) as an electrolyte additive [53]. When BPEI is present, the electrodeposited layers undergo modifications from laminated hexagonal huge crystals to a compact layer, not using an essential boom form. Furthermore, it was discovered that deposition of BPEI at the substrate’s floor influences the kinetics of zinc electrodeposition and slows grain expansion, favoring nucleation overgrowth. As a result, BPEI guarantees that conductivity is calmly dispersed and that the deposited layer is uniform.
The partial overlaying of zinc by zinc hydroxide or zinc oxide in the seventh cycle is noted to be the cause of this phenomenon. This prevents extra zinc from being electrodeposited, resulting in a vast preliminary over-potential that favors hydrogen formation. The minimum overall performance is observed when 30 ppm BPEI is added, whereby the overall performance is 88%. For the 4 primary cycles, the electrodeposition overall performance is 100% with the presence of 300 ppm BPEI. Following this, the performance is decreased to 95% and remains constant for the trial. As a result, raising the additive from 30 to 300 ppm can increase the zinc electrodeposition overall performance from 88% to 95%. Furthermore, BPEI adsorption does not affect the oxidation over-potential, however it does affect the cathodic over-potential, which might be 10 mV more with the presence of 30 ppm BPEI than with the natural electrolyte. The upward thrust in cathodic over-potential reaches 60 mV in the presence of 300 ppm BPEI.
2.3. Aqueous Zinc-Ion Batteries (AZIBs)—Cathode Materials
Sustainable cathodic materials of ZIBs require the following characteristics, such as a proper structure, excellent structural consistency, ideal working voltage, electrochemical stability, high energy storage density, and reduced cost and environmental consequences [1,2,3,4,5]. However, there are still several obstacles suppressing the advancement of this field. The most common issues are cathode dissolution byproducts. Scientists are trying to reduce these issues to make sustainable AZIBs. There are several cathode materials, such as manganese-based, vanadium-based, other materials-based, etc. Manganese-based material is a promising cathode material due to its lower cost, low toxicity, low combustible, high abundance, high storage capacity, and high ionic conductivity. Furthermore, the structural properties of MnO2 (α, β, γ, δ, λ) provide some adequate pathways and surfaces for the transport and accumulation of other metal ions, including Zn ions. Chain-type, tunnel-type, and layered-type structures are interconnected to form MnO6 octahedra as the fundamental structural component by connecting vertices or edges [30,57,58,59,60]. These structural versatilities of MnO2 usually lead to making excellent cathode materials.
The electrochemical execution shifted concurring with the different gem shapes. These tracts make the MnO2 reasonable for economical application in super-capacitors and lithium-ion batteries. For example, Yuan et al. showed that a manganese-based metal-organic system (MOF) can be utilized as a moved forward cathode for ZIBs [68]. The oxygen atoms of two neighboring -COO– are utilized to implement coordination unsaturation of Mn. Its moderately high reactivity and quick electrochemical response energy are upheld by its ideal unsaturated coordination degree, which offers prevalent Zn2+ transport and electron trade all through repetitive charging/discharging cycles. Due to the aforementioned characteristics, this MOF-based anode incorporates a huge capacity of 138 mAh g−1 at 100 mA g−1 and a long lifespan (93.5% capacity holding after 1000 cycles at 3000 mA g−1).
The Rs of KMO cathodes does not change particularly during cycling, but it significantly increases for α-MnO2 cathodes, firmly fused with the capacity decrease. Islam et al. also improved the electrochemical performance of MnO2 by carbon coating [27]. Other than manganese-based cathodes, vanadium oxide (V2O5) has been studied extensively as a cathode material for lithium-, sodium-, and zinc-ion batteries. The researchers found some issues such as poor ionic conductivity, low capacity, and structural instability of the V2O5. Several attempts have been made to solve these problems. Yan et al. [70] improved Li insertion capability into V2O5 by adding Na ions. It is not only effective for lithium-ion batteries, but this technique is also applicable for other metal-ion batteries. The addition of metal ions (Na+, K+, Li+) act as pillars and enhance the structural stability of V2O5 in AZIBs [46,47,71,72,73]. Though vanadium-based cathodes offer high capacity, they fail to reach high voltage. In this situation, hybrid ion batteries can reach high potential [74].
3. Flexible Zinc-Ion Battery
Zinc has been recognized as one of the most promising anode materials because of its high theoretical capacity, large abundance, low cost, high corrosion resistance in aqueous electrolytes, non-toxicity, low oxidation/reduction potential, environmental friendliness, and good reversibility. However, aqueous zinc-ion batteries (AZIBs) confront some unexpected issues, including water-mediated parasitic reactions that speed up zinc dendrite formation, the dissolution of cathode materials, and the construction of byproducts on the cathode. Hence, these issues arise from the parasitic response, hampering an aqueous zinc battery’s capacity and cyclic stability. Scientists around the globe are working on resolving these issues. They have employed several strategic solutions, such as changing electrolytes, surface engineering, and structural/hierarchical design. Moreover, electrical vehicles and portable devices have been transformed by smart energy storage. The rate of penetration into flexible electronic markets by the present smart energy storage technologies is remarkable. Largely flexible devices require additional requirements, such as bendable, twistable, stretchy, and ultrathin batteries, to adjust mechanical deformation under working conditions. Flexible batteries are essential power sources for these devices. Currently, flexible zinc-ion batteries have attracted more attention for their low cost and considerable energy density [75,76,77,78].
Polymer/hydrogel electrolytes such as poly(vinyl alcohol) (PVA) and polyacrylamide (PAM) can play a significant role in resolving these issues because of the lower content of water in polymer/hydrogel electrolytes. In addition, solid–solid contact between electrodes and the electrolyte rather than a solid–liquid interface can create a significant barrier for preventing the formation of dendrites and the dissolution of cathode materials. Meanwhile, the stable solid–solid interface has unexpected capabilities such as self-healing, elasticity, anti-freezing, and thermal sensitivity [69,75,76,79,80].
Although polymer/hydrogel electrolytes in flexible zinc-ion batteries have achieved tremendous advancements in battery technology, they have some drawbacks, such as ionic activities, length of time effects of polymer/hydrogel electrolytes, ubiquitous charge-transfer mechanisms at the solid–solid interface are unclear, etc. Understanding the mechanisms of polymer gel electrolytes in electrochemical performance is still vague [69,75,76,79,80].
Polymer/Gel Electrolytes
Polymer electrolytes for ZIBs are classified into solid polymer electrolytes, new hydrogel electrolytes, and hybrid polymer electrolytes. In this context, polymer electrolytes surpass aqueous electrolytes to prevent zinc dendrite development and reduce cathode dissolution. In addition, the polymer electrolytes’ additional functionalities lessen parasitic weight invasion in the separator and enhance mechanical performance. Solid polymer electrolytes have more mechanical strength than hybrid polymer electrolytes, although hydrogel electrolytes have a competitive ionic conductivity compared to aqueous electrolytes. Due to their stable structure, solid polymer electrolytes are expected to limit zinc dendrite formation the most. In contrast, hydrogel electrolytes are projected to have a weaker ability to prevent dendrite formation due to the water content in the hydrogel. Combining these benefits, hybrid polymer electrolytes that offer the optimal balance between these viewpoints are critical for further commercialization. They provide high ionic conductivities and multi-competitive functionalities and mechanical behaviors. A three-dimensional, double-cross-linked gelatin and sodium alginate hydrogel imbibed with ZnSO4 aqueous solution was used as an electrolyte membrane for flexible Zn-ion batteries, according to Huang et al. [79]. They discovered that the three-dimensional, double-cross-linked gelatin and sodium alginate hydrogel had better electrochemical behavior, easy fabrication, electrochemical stability, and Zn anode suitability [78,99].
The polymer membrane was well-staged between the V2O5/CNT cathode and the Zn/graphite paper anode [79]. The battery capacity is 251 mAh g−1 after the first cycle and 188 mAh g−1 after 200 cycles, with a coulombic efficiency of over 99.8%. The cell can fold 55 cycles under a load of 1470 times the device’s weight, maintaining 74% of its original capacity. Similarly, poly (3,4-ethylene dioxythiophene) polystyrene sulfonic acid (PEDOT: PSS) is incorporated into polyaniline (PANI) on carbon nanotubes (CNTs) for electrochemical behavior and cycle stability [75]. It shows excellent activity in improving electrochemical behavior. Although the discharge capacity has been shown to gradually decrease with increasing current density, the high conductivity of the tCNTs-PA-PE cathode describes a large capacity of 145 mAhg−1 even at a sharp current density of 10 Ag−1 and exemplary cycle stability, with the reversible capacitance of 113 mAh g−1 over 1500 cycles, and remarkable 100% coulombic efficiency.
The Zn powder carbon film coated on the Zn anode was expected to prevent the abnormal growth of dendrites and promote the performance of Zn. Zhi et al. have shown a stretchable zinc-ion battery (ZIB) with double-helix electrodes and a cross-linked polyacrylamide (PAM) electrolyte [76]. It exhibits a high specific volume, volumetric energy density (302.1 mAh g−1 and 53.8 mWhcm−3, respectively), and excellent cycle stability (98.5% capacity retention after 500 cycles). In addition, the semisolid-state yarn ZIB showed excellent stretchability (up to 300% elongation) and excellent water tightness (high-capacity retention of 96.5% after 12 h of underwater operation).
Higher efficiency may be due to the consistency of the electrodes and the high ionic conductivity of the PAM-based electrolyte. The deformability test was very sustainable. The load was evaluated after the trigger. This was almost 97.2% under normal conditions, and it was found that 93.6% of the capacity was stable even after discharge
The hybrid polymer electrolyte was found to have a coulombic efficiency of over 99% and capacity retention of 96% after 1000 cycles at 2 Ag−1 according to Parkin et al. [80]. As the sampling rate increased from 0.1 to 1 mVs−1, the electrochemical response under capacitive control became much more sensitive. The charge/discharge rates of SAPAM, SA, and AZIB were studied with gradual current densities ranging from 0.1 to 5 Ag−1 over 5 cycles for each current density. As a result, the SAPAM battery achieved an amazing 305 mAh g−1 at 0.1 Ag−1, and the coulombic efficiency (CE) was 96.2%, while the CE of the original SA battery was 91.6% at 258 mAhg−1. In addition, Niederberger et al. used a polydimethylsiloxane (PDMS) substrate and a polyacrylamide (PAM) hydrogel electrolyte, and a capacity of 176.5 mAh g−1 was achieved after 120 cycles at various stress levels of up to 50% [100].
4. Anodes for AZIBs and AFZIBs
As previously stated, complications arising from undesired Zn dendrites would lead to prolonged stripping/plating treatments, resulting in water consumption and irreversible negative effects, which could be responsible for degrading the coulombic efficiency (CE) accompanied by an inferior operation life [1,2,3,4,5]. When the Zn dendrite grows at a steady rate, the separator may develop short circuits, posing a security issue. Due to the usage of excessive Zn metal, the discharge rate and consumption proportion of Zn metal anodes are usually poor and do not represent the full energy density. Published studies presently suggest that Zn dendrites may be prevented by modifying the surface of the Zn anode, electrodepositing Zn on 3D nanostructures, adjusting the separator composition, and changing the electrolyte [6,7,8,9,10]. Although such solutions can generally help stabilize cells, the problem of establishing Zn dendrites remains unsolved. Nonetheless, these anode materials have low capacitance, have irreversible phase shifts, and their capacitance drops rapidly during Zn2+ insertion/extraction, which severely limits their use. As a result, the creation of an improved insertion anode material for ZIB with a strong structure, high capacitance, and higher power density was very attractive. Hur et al. has used a simple spin-coating process to protect the Zn anode using a protective thin layer of highly polar poly (vinylidene difluoride) (β-PVDF) [103]. The β-PVDF layer is robust and has adjusted the Zn stripping/plating process while withstanding erosion. In the asymmetric cell test, the resulting β-PVDF-coated Zn anode (β-PVDF@Zn) outperformed both bare Zn metal and α-PVDF@Zn at a low overvoltage of 40 mV at 2000 h of operation. In addition, PVDF has extended the life of the entire cell to 4000 cycles while maintaining excellent cycle stability. Two redox peaks of 0.59 and 0.78 were clearly visible in the first cycle, after which a slightly dark pair of reduction/oxidation peaks in the range 0.9 to 1.1 diverged in each cell. However, after 3 cycles, all peaks in these profiles were integrated into 0.59/0.78 and 0.89/1.14, respectively, according to the Zn2+ two-step intercalation/deintercalation process.
Even after 20 cycles for each period of growing the current rate, the capacity bounced back to 265 mAh g−1 when the current density unexpectedly plummeted to 1 Ag−1. The β-PVDF cell outperformed the others in terms of rate and cycling, completely overtaking their uninteresting traits. Finally, at a rate of 1 Ag−1, the long-term cycle life was assessed. A cathode material (MnO2) ultrathin layer is first coated upon the surface of the carbon cloth as active sites for in situ nucleation and guided nucleation and growth of metal Zn, according to Shao et al. [104]. Consequently, the CC@MnO2-UTF@Zn anode can be cycled in an aqueous electrolyte and keep its cycling performance consistent during repeated Zn deposition/stripping procedures. After 100 cycles, the complete cell with the Zn plate anode shows a more extensive capacity loss, with 157 mAh g−1 (41.8% capacity retention). Long-term cycling performance was also assessed at 1 Ag−1. The complete cell with the CC@MnO2-UTF@Zn anode still has a high coulombic efficiency of almost 100% and a specific capacity of 186 mAh g−1 after 300 cycles (81.0% of capacity retention). The creation of freestanding, super versatile, and conductive carbon nanotube (CNT)/paper scaffolds to maintain zinc metal anodes has been described by Wang et al. [104]. The scaffold-stabilized zinc anodes had low polarization strengths, a long cycling life of over 1800 h, and superior charging–discharging capabilities.
Zinc-ion batteries/hybrid capacitors with ultralong cycle lifetimes were also successfully produced, due to the scaffold-stabilized zinc anodes’ robust electrochemical stability and reversibility [105]. The V2O5|Zn(CF3SO3)2|Zn@CNTS ZICs have been charged/discharged at 3 Ag−1 for 2000 cycles and the activated carbon|Zn(CF3SO3)2|Zn@CNTS ZIBs have been charged/discharged at 2 Ag−1 for 7000 cycles. Both have an excessive degree of cyclical balance. A unique 2D ultrathin-layered zinc orthovanadate array cathode, a Zn array anode supported with the aid of conductive porous graphene foam, and a gel electrolyte were used by Fan et al. to develop a high-end, ultra-strong, flexible, quasi-solid-country zinc-ion battery [82]. Each electrode has a nanoarray construction that ensures high-fee functionality and prevents dendrite formation.
This entry is adapted from the peer-reviewed paper 10.3390/nano12223997
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