A comparative study of agarose and sodium alginate-based gel polymer electrolytes for Zn-based batteries with CaV6O16·3H2O cathode

A comparative study of agarose and sodium alginate-based gel polymer electrolytes for Zn-based batteries with CaV6O16·3H2O cathode | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL Battery Energy This is a preprint and has not been peer reviewed. Data may be preliminary. 4 July 2025 V1 Latest version Share on A comparative study of agarose and sodium alginate-based gel polymer electrolytes for Zn-based batteries with CaV6O16·3H2O cathode Authors : Matteo Milanesi , Alessandro Piovano , Hamideh Darjazi , Xu Liu , Claudio Gerbaldi 0000-0002-8084-0143 [email protected] , and Giuseppe Elia Authors Info & Affiliations https://doi.org/10.22541/au.175163413.39831121/v1 Published Battery Energy Version of record Peer review timeline 572 views 210 downloads Contents Abstract Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Aqueous zinc-based batteries (ZIBs) are considered promising energy storage solutions, particularly targeting low-cost applications needed for levelling electricity production from renewable energy sources. However, numerous challenges need to be overcome to bring the technology to the market, chiefly including cathode dissolution, dendrite formation, hydrogen evolution reaction and zinc corrosion. The optimisation of the electrolyte, particularly the use of gel-polymer electrolytes (GPEs), is demonstrated as a viable approach to solve or mitigate such issues. In this respect, a comparative study of two GPEs based on biopolymers, agarose and sodium alginate, is here presented. Despite the fast and facile preparation procedure, the GPEs demonstrate to be strongly effective in suppressing dendrite and byproduct formation on zinc metal anodes, due to the abundant –OH groups along the chains in polymeric matrices. The electrochemical behaviour of GPEs is evaluated in terms of galvanostatic cycling in laboratory-scale zinc metal cells with a CaV6O16·3H2O cathode at low and high active material loadings of 2.5 and 5 mg cm-2, respectively. Resulting cycling performances in terms of specific capacity and rate capability is comparable (low loading electrodes) and even outperform (high loading electrodes) those obtained with a standard liquid electrolyte (2M ZnSO4) liquid electrolyte cell, thus accounting for the promising prospects of the bio-polymer GPEs as alternative green, sustainable electrolyte for next generation Zn-based batteries. A comparative study of agarose and sodium alginate-based gel polymer electrolytes for Zn-based batteries with CaV 6 O 16 ·3H 2 O cathode Matteo Milanesi 1,2 , Alessandro Piovano 1,2 , Hamideh Darjazi 1,2 , Xu Liu 3 , Claudio Gerbaldi 1,2 , Giuseppe A. Elia 1,2, * 1 GAME Lab, Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi, 24, 10129, Torino, Italy. 2 National Reference Center for Electrochemical Energy Storage (GISEL) - INSTM, Via G. Giusti 9, 50121, Firenze, Italy. 3 School of Energy and Environment & Z Energy Storage Center, Southeast University, 211189, Nanjing, China. Corresponding Author: Giuseppe A. Elia ( [email protected] ) ABSTRACT Aqueous zinc-based batteries (ZIBs) are considered promising energy storage solutions, particularly targeting low-cost applications needed for levelling electricity production from renewable energy sources. However, numerous challenges need to be overcome to bring the technology to the market, chiefly including cathode dissolution, dendrite formation, hydrogen evolution reaction and zinc corrosion. The optimisation of the electrolyte, particularly the use of gel-polymer electrolytes (GPEs), is demonstrated as a viable approach to solve or mitigate such issues. In this respect, a comparative study of two GPEs based on biopolymers, agarose and sodium alginate, is here presented. Despite the fast and facile preparation procedure, the GPEs demonstrate to be strongly effective in suppressing dendrite and byproduct formation on zinc metal anodes, due to the abundant –OH groups along the chains in polymeric matrices. The electrochemical behaviour of GPEs is evaluated in terms of galvanostatic cycling in laboratory-scale zinc metal cells with a CaV 6 O 16 ·3H 2 O cathode at low and high active material loadings of 2.5 and 5 mg cm -2 , respectively. Resulting cycling performances in terms of specific capacity and rate capability is comparable (low loading electrodes) and even outperform (high loading electrodes) those obtained with a standard liquid electrolyte (2M ZnSO 4 ) liquid electrolyte cell, thus accounting for the promising prospects of the bio-polymer GPEs as alternative green, sustainable electrolyte for next generation Zn-based batteries. Keywords: Zinc battery, gel polymer electrolyte, bio-polymer, agarose, sodium alginate, V-based cathode INTRODUCTION Electrochemical energy storage plays a crucial role in addressing climate change and facilitating the green transition by enabling the efficient storage and deployment of renewable energy. Because renewable sources like solar and wind are naturally intermittent, dependable energy storage is essential to capture surplus power during peak generation and supply it when demand rises. Batteries, the preferred power source for electric vehicles, also play a vital role in renewable energy systems by offering efficient and scalable storage solutions. In this respect, rechargeable lithium-ion batteries (LIBs) are nowadays the leading solution due to their high energy densities, low self-discharge rates, and long operational lifespans 1 . Thanks to their versatility, they can be used in a wide range of applications, from portable electronics to automotive, and large-scale storage from renewables. However, current LIBs depend on scarce, expensive, and even hazardous materials. In particular, the use of cobalt and nickel in the preparation of cathode materials is extremely critical, as well as the use of natural graphite at the anode and lithium in the whole system; while these are the primary critical raw materials (CRMs), other materials like copper, aluminum, and various electrolytes and separators are also needed in the battery manufacturing process 2 . The development of alternative electrochemical storage systems based on abundant and non-critical elements is considered a suitable solution to ease the resource concerns associated with LIB technology 3 . In particular, alternative electrochemical storage systems based on more abundant elements, such as Na + , K + , Mg 2+ , Ca 2+ , Zn 2+ and Al 3+ have been widely investigated in recent years, and notably, the Na-ion battery technology successfully reached the market 45, . Zinc (Zn), possessing a high theoretical capacity of 820 mAh g -1 or 5855 mAh cm -3 , is considered very interesting for realizing low-cost and sustainable electrochemical storage systems using water-based electrolytes, being also sustainable-by-design as they use 6 . The Zn 2+ /Zn redox process takes place at a higher potential (–0.76 V vs SHE) compared to other metals such as Mg 2+ (–2.37 V vs SHE), Ca 2+ (–2.87 V vs SHE) and Al 3+ (–1.66 V vs SHE), which reduces the cell operative voltage, but allows the use of water-based electrolytes with a reduced competition with the hydrogen evolution reaction (HER) side process. Using water-based electrolytes with mild-acidic or neutral pH minimises such parasitic reactions. In addition, It brings an intrinsic enhanced safety, together with a boost in ionic conductivity, compared to organic electrolytes generally used in LIBs or NIBs. Thanks to its dielectric constant and dipolar moment, water exhibits acid-base behaviour and solvation capabilities that only this solvent has. Moreover, ion-pairing and ion triplets formation in organic electrolytes is much more severe than in water electrolytes, which has detrimental effects on the rate capabilities and cycle life of the batteries 7 . For these reasons, aqueous zinc-ion batteries (ZIBs) stand out as one of the best candidates for future sustainable electrochemical energy storage systems 78, . Besides above detailed advantages, several challenges still hinder the commercialization of ZIBs, which chiefly include the anode stability, the electrolyte performance, and the development of cathode materials. Zinc anode can be subjected to corrosion and dendrite formation. Dendrite formation and growth are associated with the uneven deposition of Zn, given by differences in electric field strength on the surface, gradients in ion concentration and surface energy 9 . Uncontrolled dendrite growth can lead to dead zinc or separator piercing and cell failure. As briefly mentioned above, thermodynamically, HER inevitably occurs during zinc deposition, although its reaction rate is strongly mitigated under a mild acidic pH environment. HER can cause several issues like lower Coulombic efficiency (CE), electrolyte depletion, H 2 bubble formation with subsequent uneven metal deposition and fluctuation in the pH value of the electrolyte, with possible corresponding byproduct formation 8 . The cathode side is also affected by a series of detrimental effects. The major issue, as reported for Mn-based, V-based and Prussian Blue analogue-based cathodes, is the dissolution into the electrolyte during cycling with structural degradation, which worsens the electrochemical properties 10 . Additionally, byproduct formation, usually zinc salts, can occur at the cathode surface, increasing interfacial charge transfer resistance and lowering cycle life. Different approaches have been proposed in the literature to solve or at least mitigate these problems, involving anode engineering 11 , development and modifications of novel cathodes 12 , and electrolyte optimization 13 . Since parasitic reactions and dendrite growth occur at the electrode-electrolyte interface, the role played by the electrolyte is, without any doubt, fundamental. Since zinc salt has a direct influence on various electrolyte properties such as pH value, ionic conductivity, working voltage window, and the reversibility of the Zn anode, several salts, including ZnSO₄, ZnCl₂, Zn(ClO₄)₂, Zn(NO₃)₂, Zn(CH₃COO)₂, Zn(TFSI)₂, and Zn(CF₃SO₃)₂, have been employed as electrolytes in ZIBs 14 . Electrolyte additives have been widely investigated to improve electrochemical performances and minimize degradation processes involving the electrolyte 15 . These include ionic species (e.g., Mn 2+ 16 , Al 3+ 17 ), organic compounds like citric acid or naphthalene 18 , as well as metals and inorganic compounds, such as tin oxide 19 and phosphoric acid 20 . Besides liquid electrolytes, gel polymer electrolytes (GPE) demonstrated to strongly improve the anode reversibility, CE and cycle life of the battery. GPEs feature a 3D porous network composed of polymeric chains in which ions can move with more organised ion migration compared to liquid aqueous electrolytes. It promotes uniform ion flow and reduces the formation and growth of dendrites, also thanks to the mechanical resistance exerted on protrusions on zinc anode 2122, . Functional groups along the polymeric chains play a pivotal role. Hydroxyl and carboxyl groups, for example, can interact with free H 2 O with high electrochemical activity, converting it into bond water 23 . This phenomenon implies that a lower amount of free water will be available for HER, and the modification of the solvation structure of Zn 2+ will also be reflected by lower overpotentials related to desolvation processes. Moreover, GPEs can be coupled with advanced and well-engineered liquid electrolytes, combining the advantages of liquid and polymer electrolytes. Many polymeric matrices based on polyvinyl alcohol (PVA 24 ), polyacrylamide (PAM 2526, ), poly(acrylic acid) (PAA 27 ) and other polymers have been reported in the literature 22 . Particularly, bio-polymer based gel electrolytes also possessing all the advantages of GPEs have attracted great attention, since their low cost, low toxicity and environmental friendliness allow for their exploitation in many applications in which bio-compatibility is fundamental 28 . Among all the natural polymers, polypeptides and polysaccharides are the most commonly used to prepare bio-sourced GPEs, such as chitosan 29 , agar 3031, , alginate 32-34 , cellulose 635, , gelatin 36 and xanthan gum 3738, . Following an initial screening, agarose and alginate were identified as the most suitable candidates a comparative study due to their rapid and straightforward preparation procedures, scalability potential and capacity to form self-standing membranes, and are used in this work as polymeric hosts for 2M ZnSO 4 liquid electrolyte. Self-standing GPEs with high ionic conductivity and improved zinc deposition in Zn//Zn symmetric cells were obtained using facile and easy scalable preparation procedures 3940, . SEM and EDX analysis also confirmed these findings. The rate capability of GPEs was also evaluated in full cells using a novel CaV 6 O 16 3H 2 O cathodic material never tested with bio-polymer-based GPE. Both bio-polymer-based GPEs delivered good specific discharge capacities, comparable with the results obtained from the liquid electrolyte or even better in terms of cyclability as in the case of high-loading electrodes, which accounts for their promising prospects in developing novel, stable and sustainable aqueous Zn-based batteries conceived for large-scale (seasonal) energy storage applications. EXPERIMENTAL SECTION GPEs preparation procedures All chemicals used in this work were purchased from Sigma-ALDRICH. Agarose gel polymer electrolyte (AG-GPE) was obtained by adapting a preparation procedure reported in the literature 39 . A solution of agarose in water with a ratio of 1:15 in weight was prepared and stirred at 90 °C until complete polymer dissolution, yielding a clear and homogeneous solution. The solution was then cast using a doctor blade, allowing for better control of the thickness, with a thickness of 1 mm and soaked in 2M ZnSO 4 . Sodium alginate gel polymer electrolyte (SA-GPE) was also obtained by adapting a preparation procedure reported in the literature 40 . A solution of sodium alginate and water with a ratio of 1:20 in weight was prepared to facilitate homogeneous mixing using a thicky mixer. Once the homogeneous solution was obtained, it was cast using the doctor blade with a 1mm thickness. The casted solution was then directly wetted with 2M ZnSO 4 to allow Zn 2+ ionic crosslinking. As the final step, the obtained SA-GPE was soaked in 2M ZnSO 4 . Thickness of both AG- and SA-GPEs ranged between 500 and 700 µm. Investigation of Zn electrode morphologies after cycling Scanning electron microscopy (SEM) images were collected with 5 keV electrons using an in-lens detector of a Zeiss SUPRA 40 (Zeiss SMT, Oberkochen, Germany) field emission electron microscope (FESEM). Evaluation of zinc electrode morphologies with SEM was performed after plating and stripping in Zn//Zn symmetric cells cycled at 2 mA cm -2 with 2 mAh cm -2 capacity for 100 cycles. The recovered zinc electrodes were washed three times with ethanol before characterisation. Electrochemical characterisation of GPEs Electrochemical behaviour of GPEs and 2M ZnSO 4 were evaluated housing the materials in two-electrodes electrochemical test cells (ECC-Std by EL-Cell, Germany). Zinc metal foils were cut into 10 mm diameter discs, and Whatman GF-A was used as the separator for 2 M ZnSO 4 liquid electrolyte, while for AG-GPE and SA-GPE, no separators or spacers were used. Ionic conductivity was evaluated by electrochemical impedance spectroscopy (EIS) of symmetric SS//SS cells within a frequency range of 500 kHz to 100 mHz and applying a sinusoidal voltage signal (DV) of 20 mV. The values of conductivity (σ) were calculated using the σ = l/(RA) equation, where l represents the thickness, A the contact area of the electrolyte and R the bulk resistance obtained from EIS measurement. The electrochemical stability window (ESW) was evaluated using linear sweep voltammetry (LSV) of Zn//SS cells in the potential range of 0 to 3 V vs. Zn 2+ /Zn at 0.1 mV s -1 . CV tests were performed in Zn//Ni cells between -0.3 and 1 V vs. Zn 2+ /Zn with a sweep rate of 0.2 mV s -1 . Reversible plating and stripping capability was tested in Zn//Zn symmetric cells by galvanostatic cycling at various current densities of 0.25, 0.5, 1, and 2 mA cm -2 with a total exchanged capacity of 0.25 mAh cm -2 . After the various current density steps, long-term cycling capability was evaluated at 0.5 mA cm -2 . For the evaluation of zinc electrode morphologies by SEM analysis, electrodes were cycled at 2 mA cm -2 with 2 mAh cm -2 capacity for 100 cycles. Electrochemical characterization of Zn|| CaV 6 O 16 ·3H 2 O full cells Electrochemical characterisation of full ZIB cells was performed in ECC-Std two-electrodes cells. The CaV 6 O 16 3H 2 O cathode material was synthesized as reported in a previous work 41 . The cathode slurry was prepared by mixing the active material CaV 6 O 16 ·3H 2 O , the electronic conducting additive (Timcal C-65 carbon black powder) and the polymeric binder poly(vinylidene fluoride) - PVDF (Solvay Solef) binder with a weight ratio of 7:2:1 in N-methyl-2-pyrrolidone (NMP) solvent. The slurry was cast on GDL (AvCarb EP40T) and cut into 10 mm diameter discs. The electrodes (active mass loading of approximately 2.5 and 5 mg cm -2 ) were then dried in a Buchi oven overnight at 70 °C and then transferred in the glove-box for further cell assembly. Ambient temperature galvanostatic charge-discharge measurements of the laboratory-scale ZIBs were performed at different current densities (from 50 to 2000 mA g –1 ) in the 0.2 to 1.6 V vs. Zn 2+ /Zn voltage range. RESULTS AND DISCUSSION Electrochemical characterisation of gel polymer electrolytes Both AG-GPE and SA-GPE were prepared with fast and facile preparation procedures as briefly illustrated in Figure 1 3940, . The gelation of the two bio-polymers occurs through different mechanisms. In AG-GPE gelation occurs during the cooling process. After casting, agarose forms single or double helix structures through physical crosslinking that bonds one with another, contributing to the formation of the 3D network of the gel electrolyte. Sodium alginate chains instead consist of mannuronic acid (M unit) and guluronic acid (G) organised in segments rich in G units, segments rich in M units and segments in which G and M units alternate 3233, . Divalent cations, such as Zn 2+ , play the crucial role of ionic crosslinkers between the carboxylate groups at the guluronic acid units (G units) promoting the formation of a highly interconnected polymeric framework of the gel polymer electrolyte. Ionic conductivity of GPEs was evaluated by EIS at room temperature. Table 1 reports the conductivity values of the prepared GPEs and the 2 M ZnSO 4 solution. Indeed, the 2 M ZnSO 4 showed an ionic conductivity of 3.5×10 -2 S cm -1 , SA-GPE of 2.5×10 -2 S cm -1 , while AG-GPE achieved the highest value of 3.9×10 -2 S cm -1 among the electrolytes studied. The excellent ionic conductivities of SA-GPE and AG-GPE, comparable to the results obtained with the 2M ZnSO 4 , indicate that ion movement is unaffected or hindered by the polymeric network. The obtained conductivity values are in line with those reported for similar systems in the literature 30333940, and are suitable for applications in electrochemical storage systems operating at high current densities at ambient conditions. Table 1 Comparison of ionic conductivity, Coulombic efficiency (CE) of zinc metal and electrochemical stability window (ESW) for the electrolytes under study. Ionic conductivity 3.5×10 -2 S cm -1 2.5×10 -2 S cm -1 3.9×10 -2 S cm -1 CE 30% 55% 76% ESW 1.7 V 1.9 V 2 V Linear sweep voltammetry (LSV) was used to evaluate the electrochemical stability window (ESW) of the two GPEs and the 2M ZnSO 4 electrolyte. Figure 2a shows the current response of the Zn//SS cells employing the 2M ZnSO 4 (blue line), SA-GPE (yellow line) and AG-GPE (green line) electrolytes, evidencing ESWs of 1.7, 1.9, 2 V, respectively, defined considering a limit leakage current, associated with the OER process, of 10 µA cm -2 . Results evidence an extended ESW of the prepared GPEs compared to the liquid electrolyte, indicating enhanced stability towards oxidative (anodic) potentials, most likely associated with the reduced availability of free water for the OER process 42 . The cyclic voltammetry of the Zn//Ni cells (Figure 2b) shows the characteristic behaviour of metal deposition processes. The measurement evidences a higher peak current for the liquid electrolyte. However, by calculating the CE by integrating the area of the peak current, the liquid 2M ZnSO 4 electrolyte has the lowest efficiency (30%), while both the SA-GPE and AG-GPE showed improved efficiencies of 55% and 76%, respectively. Metal anode-electrolyte compatibility and stability are key parameters in an electrochemical cell, particularly in aqueous batteries. In aqueous media, zinc electrodes can be affected by several detrimental effects like corrosion, passivation, dendrite formation and HER; all phenomena in which the pH of the electrolyte also plays a crucial role 43,8 . Thus, electrolytes under study were tested for their reversible plating and stripping behaviour in Zn//Zn symmetrical cells to evaluate zinc deposition and stripping stability and reversibility. The results obtained for 2M ZnSO 4 , SA-GPE, and AG-GPE are shown in Figure 3a, Figure 3b, and Figure 3c, respectively, while the direct comparison of the three systems upon long-time cycling is shown in Figure 3d. Figure 3 Rate capability test of Zn//Zn cells at various current densities from 0.25 to 2 mA cm −2 . a) 2M ZnSO 4 . b) SA-GPE. c) AG-GPE. d) Long-term Zn plating-stripping voltage profiles at 0.5 mA cm -2 The 2M ZnSO 4 liquid electrolyte based cell showed a stable plating and stripping behaviour up to 1 mA cm -2 with a voltage polarisation of about 100 mV at 0.25 mA cm -2 . However, during cycling at 2 mA cm -2 , a drop in voltage polarisation can be clearly noted, most likely due to uneven zinc stripping deposition and dendrite formation, as also evidenced in SEM images shown in Figure 4c-d. Non-homogeneous deposition and dendrite formation are well documented in the literature for liquid electrolytes 44,45 . Several factors, such as ion concentration gradients, surface energy and electric field strength, influence the nucleation of zinc. An example is the ”tip effect”, whereby a localised higher electric field promotes preferential deposition of zinc cations in some regions at the zinc electrode surface 46 . As the deposition process continues, the area close to the tip becomes depleted of zinc cations, leading to a variation of Zn 2+ concentration on the surface of the electrode that further exacerbates dendrite growth. It is worth mentioning that dendrites can lead to dead zinc or short-circuiting of the cell due to separator piercing. Additionally, they provide more sites where HER, corrosion, and passivation could occur. On the contrary, by using the prepared GPEs, it is evident that there is a more stable plating and stripping behaviour with respect to the 2 M ZnSO 4 liquid electrolyte. Moreover, the test carried out using the GPEs shows a lower voltage polarisation at all of the tested current densities. Amongst the two developed GPEs, AG-GPE showed an almost ideal voltage profile and lowest voltage polarization values of 20, 50, 85 and 115 mV at 0.25, 0.5, 1, and 2 mA cm -2 , respectively, while the liquid electrolyte showed voltage polarisations of 110 mV, 115 mV, 130 mV and 150 mV at the same current densities. In Figure 4d, the comparison between 2 M ZnSO 4 , SA-GPE and AG-GPE upon long-term cycling at 0.5 mA cm 2 is shown. Both GPEs outperformed the liquid electrolyte based cell in terms of long-term performance. SA-GPE maintained the stripping and deposition for around 425 hours; however, it evidences a sudden increase in polarisation, most likely associated with uneven deposition of the zinc, leading to cell failure due to short circuit related to dendrite formation. Instead, AG-GPE showed superior cycling stability, maintaining a low and almost constant polarisation for more than 3800 hours, with a very stable voltage profile suggesting a smooth and homogeneous deposition of the zinc. The superior performances obtained by the AG-GPE are ascribed to this GPEs ability to mitigate parasitic reactions and promote an efficient and stable Zn-metal deposition and stripping has been ascribed to the synergistic effects of two contributions related to the polymer matrix and its functional groups. The first one is the guiding effect of polymeric chains and their functional groups on the diffusion of zinc cations. Carboxylate groups of sodium alginate restrain zinc cations movement, hindering their diffusion in regions of the electrodes where the ”tip effect” occurs, implying the slowing and mitigation of dendrites growth 40 . The second one is the capability of GPEs to perturb and alter the solvation structure of Zn 2+ thanks to the presence of -OH groups along the polymeric chains. The interaction of -OH groups through hydrogen bonds with water severely limits the mobility and hydration effect of Zn 2+ , contributing to stable electrodeposition 39 . In addition, -OH groups can also limit the free diffusion of SO 4 2- lowering the formation of byproducts. SEM imaging of zinc electrodes after 100 cycles of plating and stripping at 2 mA cm -2 with a capacity of 2 mAh cm -2 (see Figure S1 in the Supporting Information) was performed to evaluate the influence of the electrolyte composition on the zinc deposition morphology. Figure 4 shows SEM images of the zinc electrode in pristine condition (Figure 4a-b), and after cycling in 2M ZnSO 4 (Figure 4c-d), SA-GPE (Figure 4e-f) and AG-GPE (Figure 4g-h), while the table reports results obtained from EDX analysis (corresponding images with EDX maps are shown in the Supporting Information as Figure S2, S3 and S4). Figure 4 SEM images of Zn foils morphologies after 100 cycles at 2 mAcm -2 with capacity of 2 mAhcm -2 . a-b) Pristine zinc, c-d) 2M ZnSO 4 , e-f) SA-GPE, g-h) AG-GPE. As expected from the electrochemical results, the Zn metal recovered by the cell using the 2 M ZnSO 4 electrolyte exhibited an uneven deposition morphology, with a widespread presence of flake-like dendrites at all the magnifications (Figure 4c-d). On the contrary, SA-GPE (Figure 4e-f) and AG-GPE (Figure 4g-h) resulted in a relatively uniform surface deposition of the zinc after cycling. In particular, the Zn electrodes recovered from the cell using SA-GPE exhibited tree-like structures uniformly distributed on the surface, along with flakes that have smaller dimensions than those found in the liquid electrolyte. The Zn electrodes recovered from the cell using AG-GPE showed a uniform formation of plates and plates-like structures with negligible amounts of flake-like dendrites; in agreement with the plating-stripping results (Figure 3), it accounts for its ability to promote even zinc deposition, preventing the formation of dendrites. The different surface morphologies observed for the GPEs suggest that the polymeric matrix also plays a role in the deposition of Zn metal and, as a consequence, of the resulting morphology of zinc electrodes. EDX measurements were carried out to evaluate the elemental composition at the Zn electrode surface. Figure 4 shows the elemental composition, revealing a notable difference of oxygen and sulfur content. The presence of these elements on the electrode surface arises from the formation of byproducts like zinc hydroxysulfate (Zn 4 (OH) 6 SO 4 ⋅5H 2 O) 32 . Zn 4 (OH) 6 SO 4 ⋅5H 2 O formed at the Zn electrode surface, where water from the solvation sheaths of Zn 2+ and diffused SO 4 2- participate in parasitic redox reactions 47 . These reactions and their byproducts contribute to dendrite formation and zinc consumption. Compared to the liquid electrolyte, both GPEs present a lower amount of oxygen (O %wt: 17.27 and 9.12 for SA-GPE and AG-GPE, respectively), and sulphur (S %wt: 2 and 0.55 for SA-GPE and AG-GPE, respectively). It suggests that GPEs can reduce the formation of byproducts, substantially improving the cyclability of zinc anodes, and particularly agarose in AG-GPE, having the lowest amounts of O and S, can substantially mitigate side reactions at the surface of the Zn anode electrode. These findings are also confirmed by XRD analysis of zinc anodes after cycling for similar GPEs reported in literature 548, ,525, . Both SEM and EDX analyses confirm plating and stripping results clearly showing an improved deposition capability with limited formation of byproducts and dendrites in GPEs based cells, thanks to the already described effects, role on deposition of polymeric backbone and functional groups of the electrolytes. The electrochemical behaviour of the prepared SA-GPE and AG-GPE was confirmed in laboratory-scale Zn-ion cells, using CaV 6 O 16 ·3H 2 O (CVO) cathode material, and compared with 2M ZnSO 4 liquid electrolyte. Notably, to our knowledge, CVO was never tested before with these bio-polymer based GPEs in Zn-based cells 41 . Zn//CVO electrochemical cells were assembled with two different active material loadings of 2.5 mg cm -2 (low) and 5 mg cm -2 (high . The galvanostatic cycling tests were performed using different specific currents ranging from 50 to 2000 mA g -1 ; after the rate capability test at various current densities, the cells were cycled to 100 cycles at fixed 500 mA g -1 . Figure 5 a, b and c show the galvanostatic charge-discharge curves for 2 mg cm -2 cathode loading for 2 M ZnSO 4 , SA-GPE and AG-GPE, respectively. Figure 5 d shows the rate capability for the low loading cell (2 mg cm -2 ) active material loading while Figure 5 e shows the same test for the high loading cell (5 mg cm -2 ). Representative voltage profiles recorded from the cycling test performed with the low loading electrode, Figure 5 a-b-c, evidencing two different plateaus, indicate that intercalation of Zn 2+ in CVO cathode occurs through a multistep process; moreover, H + co-insertion in vanadium-based cathodes was also already reported 4149, . The capacity and storage mechanism of vanadium-based cathodes is controlled by both capacitive and diffusive processes 50 . Reversibility, structural stability and high capacities of these materials depend on crystalline polymorphs, particle size and components 51 . Common V 2 O 5 is subjected to structural degradation upon repeated intercalation and de-intercalation of Zn 2+ . In recent years, the insertion of alkali and transition metals in the cathode structure has emerged as a possible solution. Metals like Na 52 , Li 53 , Zn 54 and Ca 55 act as pillaring agents in the layered structure of the cathode significantly improving structural stability. Specifically, Ca contributes to increased interlayer spacing, promoting easy and efficient insertion of Zn 2+ 3856, . Additionally, structural H 2 O has been shown to enhance and improve electrochemical performances of V-based cathodic materials. H 2 O molecules intercalated within the layered structure also play a crucial role in widening interlayer distances and improving structural flexibility. Moreover, the electrostatic shielding effect of Zn 2+ interactions with the cathodic host material guarantees a facile Zn 2+ diffusion 57 58 59 . Figure 5 shows the rate capability tests of the electrolytes assembled in cells with two different active material loadings. At low 2.5 mg cm -2 loading, both SA-GPE and AG-GPE delivered good specific discharge capacities, similar to the capacities delivered by the liquid electrolyte. In the first cycle, 2 M ZnSO 4 , SA-GPE and AG-GPE delivered 273, 271 and 283 mAh g -1 with coulombic efficiencies of 97, 92 and 95 %, respectively, that gradually increased upon cycling, demonstrating overall a good reversibility. The irreversible capacity in the first cycles likely derives from the parasitic reaction of O 2 reduction 60 . Although the de-aeration of 2M ZnSO 4 was conducted by N 2 bubbling, all cells were assembled in open air environment, leading to some unavoidable presence of dissolved oxygen. In particular, SA-GPE delivered slightly lower specific capacities than 2 M ZnSO 4 at 100, 200, 500 and 1000 mA g -1 , while similar rate capabilities were obtained at 2000 mAg -1 , as well as upon the more prolonged cycling at 500 mAg -1 . AG-GPE showed rate capabilities almost identical to that of the liquid electrolyte at the various current densities, while during the prolonged cycling at 500 mA g -1 , AG-GPE delivered about 20 mAh g -1 more than ZnSO 4 . At high 5 mg cm -2 cathode loading (Figure 5d), the initial irreversible capacity remains evident and is attributed to the parasitic reactions previously described. As the cathode loading increases, the amount of zinc participating in plating and stripping at the anode also rises, due to the greater capacity of the cathode to accommodate zinc ions. Testing rate capabilities under high-loading conditions is particularly important, as it better represents practical operating conditions and helps at avoiding overestimations of performance that may arise from low-loading configurations. In such cases, limited zinc utilization at the anode and reduced electrochemical and mechanical stresses at the electrode/electrolyte interfaces can mask degradation phenomena. Conversely, higher loadings amplify these effects, accelerating failure mechanisms and enabling their earlier detection. Even under these demanding conditions, the gel polymer electrolytes (GPEs) exhibited superior rate capability and consistently high coulombic efficiency across all tested current densities, outperforming the liquid electrolyte. Notably, the cell containing 2 M ZnSO 4 electrolyte failed after the 90 th cycle, whereas both AG-GPE and SA-GPE sustained operation until the end of the experiment. These results demonstrate that the use of GPEs significantly enhances the stability of the charge–discharge processes, thereby improving both the efficiency and the cycling life of the cells. CONCLUSIONS In this work, a comparative study is presented of two bio-based gel polymer electrolytes (GPEs) to be used in the next generation of green, sustainable aqueous Zn-based batteries (ZIB). They were prepared via simple and rapid procedures, and demonstrated their ability to enhance electrochemical performance in both symmetric and full cells employing a novel cathode material not previously tested with GPEs in laboratory-scale ZIB. The SA-GPE and AG-GPE exhibited high ionic conductivities of 2.5 × 10 -2 and 3.9 × 10 -2 S cm -1 , respectively, which are comparable to that of conventional liquid electrolytes. Owing to the synergistic effects of the polymer chains guiding Zn 2+ diffusion and the presence of abundant hydroxyl groups that modify Zn 2+ solvation structures, both electrolytes enabled stable Zn plating/stripping at high current densities (0.5 mA cm -2 ), maintaining remarkably stable performance for 400 h with SA-GPE and over 3500 h with AG-GPE. The enhanced electrode–electrolyte interfacial stability was further confirmed by SEM and EDX analyses, which revealed suppressed dendrite formation and reduced by-product accumulation. Interestingly, the surface morphology of zinc electrodes differed depending on whether alginate- or agarose-based GPEs were used, indicating a strong influence of the polymer matrix on the Zn deposition mechanism. In full cell configurations with CVO cathodes, both GPEs delivered specific capacities and Coulombic efficiencies comparable to those achieved with 2 M ZnSO 4 at low cathode loadings. However, under higher loading conditions, the ZnSO 4 -based cell failed prematurely, highlighting the superior stability, efficiency, and cycle life imparted by the GPEs, consistent with the results observed in symmetric cells, and demonstrating the promising prospects of the newly developed materials for their practical application in next-generation, sustainable and high-performing Zn-based batteries conceived for large-scale (seasonal) energy storage. Acknowledgement The HIPERZAB project (https://hiperzab.eu/en) has received funding from the European Union’s EIC research and innovation programme under grant agreement No 101115421. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union. 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Information & Authors Information Version history V1 Version 1 04 July 2025 Peer review timeline Published Battery Energy Version of Record 2 Nov 2025 Published Copyright This work is licensed under a Non Exclusive No Reuse License. Collection Battery Energy Keywords energy storage gel polymer electrolyte zn based battery Authors Affiliations Matteo Milanesi Politecnico di Torino View all articles by this author Alessandro Piovano Politecnico di Torino View all articles by this author Hamideh Darjazi Politecnico di Torino View all articles by this author Xu Liu Southeast University View all articles by this author Claudio Gerbaldi 0000-0002-8084-0143 [email protected] Politecnico di Torino View all articles by this author Giuseppe Elia Politecnico di Torino View all articles by this author Metrics & Citations Metrics Article Usage 572 views 210 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Matteo Milanesi, Alessandro Piovano, Hamideh Darjazi, et al. 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