Sustainable Zinc-Ion Battery Separators Based on Silica and Cellulose Fibers Derived from Coffee Parchment Waste

Abstract

Currently, electrochemical devices and portable electronic equipment play a significant role in people’s daily lives. Zinc-ion batteries (ZIBs) are growing rapidly due to their excellent safety, eco-friendliness, abundance of resources, and cost-effectiveness. The application of biomass as a polymer separator is gradually expanding in order to promote a circular economy and sustainable materials. This research focuses on the usage of cellulose fibers obtained from coffee parchment (CP) waste. The extracted cellulose fibers are produced via both mechanical and chemical methods. The sustainable separators are fabricated through vacuum filtration using a polymer filter membrane. The impact of incorporating silica particles and varying silica content on the physical and electrochemical properties of a cellulose-based separator is examined. The optimum amount of silica integrated into the cellulose separator is determined to be 5 wt%. This content led to an effective distribution of the silica particles, enhanced wettability, and improved fire resistance. The ZIBs incorporating cellulose/recycled silica at 5 wt% demonstrate exceptional cycle stability and the highest capacity retention (190% after 400 cycles). This study emphasizes the promise of sustainable polymers as a clean energy resource, owing to their adaptability and simplicity of processing, serving as a substitute for synthetic polymers sourced from fossil fuels.

1. Introduction

Recently, there has been a notable increase in the interest surrounding electrochemical energy storage devices, such as batteries and supercapacitors, largely attributed to their remarkable energy density, power performance, and longevity. These devices fulfill a wide range of functions, including electric vehicles (EV), smartphones, computing laptops, drones, electrical appliances, toys, and portable electronic devices [1,2,3,4]. Consequently, the effort to enhance the electrochemical performance of these devices has generated significant interest in investigating new substances and creative structural designs [5,6,7,8]. There is a growing emphasis on the development of more sustainable and environmentally friendly components for these devices, such as battery separators, biopolymer-based electrolytes, and current collector electrodes [9,10,11].

Lithium-ion batteries (LIBs) are the most popular choice for portable batteries, among many others, because of their lightweight design, portability, and high energy density [12,13,14]. However, there are many problems in the manufacture and use of LIBs, including safety risks, costs, and environmental effects [15,16,17]. Consequently, there is a necessity to innovate a substitute energy storage technology to replace LIBs. Despite the potential of next-generation zinc-ion batteries (ZIBs) as viable alternatives for conventional LIBs, due to their superior volumetric capacity of 5854 mAh/cm³ and high mass capacity (gravimetric capacity) of 820 mAh/g for the zinc (Zn) anode compared to 2061 mAh/cm³ and 3860 mAh/g for the lithium (Li) anode, ZIBs have demonstrated constrained performance in achieving the energy density limitations of LIBs in practical applications [18,19,20,21]. As a result, present research efforts are concentrated on the development of ZIB technology [22,23,24,25].

Batteries are made of materials that, in terms of the environment, may lead to issues in the future. Sustainability in batteries can be complicated, but it begins with replacing some of these components with sustainable ones [26]. Sustainable materials are those that have the least negative impact on future generations in terms of extraction, production, usage, longevity, and recycling [27]. Conventional ZIB separators are made of nonbiodegradable and thermally unstable polyolefin-based polymers such as polyethylene (PE) and polypropylene (PP). Researchers are increasingly focusing on the design and development of biomass-based separators as a means of reducing environmental contamination caused by PE or PP derived from fossil resources [28,29,30]. Cellulose derived from biomass is an eco-friendly, renewable, and biodegradable biopolymer that is cost-effective and exhibits excellent electrochemical properties, electrolyte wettability, thermal stability, biocompatibility, renewability, and high porosity [31,32,33]. This raw material is appropriate for producing a ZIB separator that meets the energy storage device requirements, especially when compared to traditional synthetic polymer separators [34]. However, cellulose exhibited significant flammability, characterized by a low limiting oxygen index (LOI) of around 18% [35]. The substantial interface impedance of cellulose restricts its use in ZIBs [36]. Therefore, utilizing flame-retardant and electrochemical modification strategies presents a promising method for broadening the application of cellulose in ZIBs [37].

Agro-industrial wastes, often known as agricultural residues, are one of the most abundant and sustainable sources of cellulose fibers [38,39]. Coffee, one of the world’s most recognized agricultural products, is widely eaten by millions of people every day. Coffee consumption has a substantial environmental impact because of the waste generated during manufacture [40]. Coffee parchment (CP) represents industrial residues produced during various stages of coffee processing, alongside other by-products like coffee hulls, defective beans, coffee silver skin, and spent coffee grounds [41,42]. CP refers to the yellowish by-product produced prior to roasting, arising from the drying and hulling stages in the wet processing of coffee cherries. The parchment surrounding coffee is referred to as an endocarp, which encases both halves of the coffee seed and constitutes 3.5–6.1% of the dry weight of the coffee cherry [43,44]. Lately, CP has emerged as a promising option for cellulose sourcing due to its relatively high content of a key biopolymer. The growing attraction to cellulose-based separators is attributed to their promising applications in a range of battery technologies [45]. Nevertheless, cellulose exhibits significant flammability, raising safety concerns in scenarios where the battery is subjected to misuse. When considering safety in battery applications, the development of flame-retardant cellulose separators is important [46].

Herein, we developed a sustainable cellulose-derived separator designed for use in ZIB applications. The cellulose fibers obtained from CP waste were produced through mechanical and chemical processes and were subsequently filtered using water as the medium. A facile filtration method is more accessible and cost-effective compared to the traditional techniques (such as electrospinning) used for petroleum-derived materials [47]. All extracted cellulose samples underwent characterization through Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Furthermore, the effect of adding silica or silicon dioxide (SiO₂) on the physical and electrochemical properties of a cellulose-based separator was investigated. The presence of SiO₂ particles allows for looser packing of cellulose fibers, resulting in a more porous and controllable structure [48]. The structure and properties of the SiO₂ particle containing a cellulose-based separator can be fine-tuned by modifying the SiO₂ particle content. To the best of our knowledge, this is the first report that addresses the cellulose/SiO₂ composite separators of ZIB. Our findings show that a sustainable composite separator is a very promising separator for greatly improving battery safety due to its high flame retardancy and excellent electrochemical properties. The present study highlights the potential of renewable polymers as a clean energy material due to their versatility and ease of processing, replacing synthetic polymers derived from fossil oil.

2. Experimental Section

2.1. Materials and Chemicals

CP waste is a source material abundant in cellulose fibers, provided from the Café Amazon Regional Store Operation Division and the Quality Analysis and Product Research and Development Department, New Energy Solutions, Thailand. Virgin SiO₂ powder (325 mesh size, 44 μm, high purity SiO₂ 99.72%) was acquired from Cernic International Co., Ltd., located in Nakhon Pathom, Thailand. Recycled SiO₂ was sourced from the Laboratory of the Department of Materials Science, Faculty of Science at Chulalongkorn University, located in Bangkok, Thailand. The particle size is an average of 1.14 μm, with a specific surface area of 5.26 m²/g, a pore volume of 0.0053 cm³/g, and an average pore diameter of 4.05 nm. The solid powder is composed of metal oxides, including SiO₂ 89.4%, calcium oxide (CaO) 3.42%, sodium oxide (Na₂O) 2.44%, titanium dioxide (TiO₂) 1.55%, aluminum oxide (Al₂O₃) 1.10%, and the remaining substances 2.09% (see Supplementary Information). This is obtained through the recycling of silicon (Si)-based solar panels using chemical leaching and selective precipitation processes. Zinc sulfate heptahydrate (ZnSO₄·7H₂O) was bought from KemAus™ (Cherrybrook, NSW, Australia) and has a purity of 99% analytical reagent (AR) grade. Zn metal foil with a purity of 99.99% was procured from Xiamen Aot Electronics Technology Co., Ltd., Xiamen, China. Manganese dioxide (MnO₂) was purchased from Alfa Aesar, Ward Hill, MA, USA. A 180 µm-thick carbon fiber paper with a density of 62 g/cm³ was obtained from The Fuel Cell Store located in Bryan, TX, USA. The carbon black (CB) utilized in this study was Vulcan^®XC-72, having a purity of 99.99%. It was supplied by Cabot Corporation, Boston, MA, USA. Additional chemicals and reagents were of analytical grade and employed without further purification.

2.2. Extraction of Cellulose Fibers from CP Waste

The 100 g of CP was crushed with a grinding mill at 30,000 rpm for 1 min and 3 cycles before passing through a sieve with a sieve diameter of 2 mm. It was thereafter rinsed with deionized water and dried in an oven set to 80 °C for 24 h. The untreated (UT) CP was then obtained. The cellulose fibers were extracted using a chemical treatment that was proposed in accordance with the approach reported by Zeleke et al. [49] and Brienza et al. [50], with minor modifications.

2.2.1. Dewaxing

The process was conducted to eliminate wax, fatty acids, tannins, sugars, and extractives from the UT sample utilizing a toluene/ethanol (2:1 v/v) mixture in a Soxhlet extractor (MiSUMi (Thailand) Co., Ltd., Rayong, Thailand) at 65 °C for 4 h. Then, the sample was washed with deionized water and dried in an oven at 80 °C for 24 h. The dewaxed (DW) CP was subsequently acquired.

2.2.2. Alkali Treatment

The process was carried out to remove hemicellulose and a trace quantity of lignin from the DW sample using a 7 wt% sodium hydroxide (NaOH) solution and a sample/solvent ratio of 1:20 m/v at 70 °C for 4 h. The sample was then filtered, rinsed with deionized water, and dried for 24 h in an oven set to 80 °C. The alkali-treated (AL) CP was later achieved.

2.2.3. Bleaching

The procedure aimed to get rid of lignin and any residual hemicelluloses from the AL sample employing a 10 wt% sulfuric acid (H₂SO₄) solution, with a sample/solvent ratio of 1:20 m/v, maintained at room temperature for a duration of 24 h. The sample underwent filtration, followed by rinsing with deionized water, and was afterward dried for 24 h in an oven maintained at 80 °C. The sulfuric acid-treated (SF) CP was then received. Finally, the cellulose fibers from the SF sample were purified by treating them with a 10% v/v hydrogen peroxide (H₂O₂) solution. The mixture was vigorously agitated for 4 h at 90 °C with a sample/solvent ratio of 1:20 m/v. The bleached (BL) CP or treated cellulose fibers were filtered, rinsed with deionized water until the pH was neutral, and dried in an oven at 80 °C for 24 h. Figure 1 illustrates the diagram of a coffee cherry along with the mechanical and chemical processes involved in extracting cellulose fibers from CP waste. A coffee cherry contains the skin, pulp, mucilage, CP, coffee silverskin, and coffee bean. The CP biomass is mostly constituted of cellulose, hemicellulose, lignin, and other inorganic materials. The extracted and purified cellulose fibers were obtained using the mechanical and chemical methods for separating fibers from CP waste.

2.3. Preparation of Sustainable Separators from Cellulose Fibers and Silica Particles

Figure 2 schematically depicts the preparation process for the sustainable separators from cellulose fibers and silica particles. Briefly, the cellulose filter paper was initially beaten in a high-speed blender at 30,000 rpm for 2 min to produce a 0.5 wt% cellulose fiber suspension in distilled water. The extracted cellulose fiber from CP waste of 5 wt% was then added in suspension, along with SiO₂ particles containing 0, 1, 2, 5, and 10 wt% of extracted cellulose fiber, and stirred continuously for 2 h. The biopolymer slurry was then applied to create a sustainable separator from cellulose fibers and silica via vacuum filtration with a PP filter membrane (pore size of 0.44 μm and diameter of 10 cm) [51]. The cellulose-based separator and cellulose-based composite separator were separated and dried in an oven at 80 °C for 24 h. Finally, they were compressed using a roll press machine to maintain the separator thickness at 60 μm and stored in a desiccator.

2.4. Characterization Methods of Samples

2.4.1. Extracted Cellulose Fibers

The FT-IR spectra for the extracted cellulose fibers were obtained using a VERTEX 70v instrument from Bruker (Rosenheim, Germany), which is equipped with a single-bounce attenuated total reflectance (ATR) setup. The dried samples underwent examination in transmission mode across the wavelength spectrum ranging from 4000 to 400 cm⁻¹, employing 32 scans for each spectrum and achieving a resolution of 6 cm⁻¹.

The thermal properties of cellulose fibers were investigated using a Pyris 6 TGA from Perkin Elmer (Artisan Technology Group ^®, Champaign, IL, USA). A sample weighing 15 mg was placed in aluminum pans and heated by scanning from 50 to 620 °C at a heating rate of 15 °C/min, with a nitrogen flow rate of 20 mL/min.

The crystallinity of the fibers was examined using a wide-angle LabX XRD-6100 from Shimadzu (Kyoto, Japan). The diffractometer, working with monochromatic Cu-Kα radiation (λ = 1.542 Å) at 30 kV and 20 mA, was operated with a step size of 0.04° over the range of 10° to 40° at ambient temperature.

The morphology analyses of the sample following various treatments were conducted using a MIRA3 SEM from TESCAN (Brno, Czech Republic), which operates at an acceleration voltage of 5 kV. The surface samples required a gold coating process using a vacuum sputter coater to enhance their electrical conductivity. All samples were ground into a fine powder, followed by drying in an oven at 80 °C for 24 h, and were subsequently stored in a desiccator prior to characterization [40].

2.4.2. Cellulose-Based Composite Separators

The elemental composition of carefully chosen sections in the cellulose-based composite separator was investigated using energy dispersive X-ray spectroscopy (EDX) in conjunction with SEM to verify the successful incorporation of SiO₂ particles. The samples were prepared by coating a gold layer through ion sputtering techniques.

The measurement of the contact angle between a water droplet and sustainable separators was taken with a Portable Contact Angle Meter PGX+ from iboson Technology Co., Ltd. (Zhubei City, Hsinchu County, Taiwan) at room temperature.

The flammability test involved directly igniting the separators measuring 5 × 2 cm² [37], while capturing the changes in the samples on camera throughout the combustion experiment at 10, 20, and 30 s.

2.5. Development and Assembly of Batteries

2.5.1. Zn//Zn Symmetric Cells

The Zn//Zn symmetric cells were constructed using Zn metal foil for both the anode and cathode materials in an ordinary coin cell configuration. The Zn foil was shaped into circles with a diameter of 1.6 cm, followed by a washing process using acetone in an ultrasonic machine operating at 120 Hz for 15 min. The symmetric cells were built utilizing Zn foil as the electrode components, integrated by the synthesized sustainable separators and 300 μL of ZnSO₄ serving as polymer separators. In the end, the cells underwent compression using a sealing machine prior to the electrochemical testing.

2.5.2. ZIBs

For ZIBs, the construction involved the use of Zn metal foil as the anode material within a standard coin cell setup. In the fabrication of cathode materials, a combination of MnO₂, CB, and polyvinylidene fluoride (PVDF) was used in a weight ratio of 70:20:10. This mixture was then dispersed in N-methyl-2-pyrrolidone (NMP) as the solvent and stirred continuously for 2 h until the slurry achieved homogeneity [52]. The resulting slurry mixtures were spread on carbon fiber paper using a doctor blade to achieve a controlled thickness of 30 μm, followed by drying at 60 °C for 12 h in a vacuum oven. All electrodes have been cut into circular shapes with a diameter of 1.6 cm. Coin cells featuring Zn//MnO₂/CB/PVDF were assembled using cellulose-based composite separators and ZnSO₄ as the supporting electrolyte. Figure 3 presents a schematic representation of ZIBs described in this study.

2.6. Electrochemical Evaluation

The testing for plating and stripping performed on the Zn//Zn symmetric cells employed cellulose-based composite separators. The experiments were conducted utilizing a battery testing machine specifically engineered for modules and packs (BTS4000 Series) from Neware Technology Ltd. (Shenzhen, China), under atmospheric pressure and across various current densities (0.5, 1, 2, 3, 5, and 8 mA/cm²) [53].

The Nyquist plot derived from electrochemical impedance spectroscopy (EIS) was obtained for coin cells of ZIBs. The measurements were performed using a chemical impedance analyzer IM3590 from Hioki (Nagano, Japan) at an alternating current (AC) voltage of 10 mV, spanning a frequency range from 1 Hz to 100 kHz.

The cyclic voltammetry (CV) of batteries was tested via a Potentiostat/Galvanostat (Autolab PGSTAT204) from Metrohm AG (Herisau, Switzerland) at a scan rate of 1 mV/s, covering a voltage range of 0.8 to 2.0 V during the initial three cycles.

The galvanostatic charge–discharge (GCD) cycle tests were carried out working with a battery testing machine equipped with BTS-7.1 software from Neware Technology Ltd., based in Shenzhen, China. The capacity performance of ZIBs utilizing cellulose-based composite separators was examined across the potential voltage range of 0.6 to 2.0 V at various current densities of 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 A/g, with a total of five cycles conducted for each current density.

3. Results and Discussion

3.1. Characterization of Extracted Cellulose Fibers

CP waste consists of α-cellulose (44 wt%), hemicellulose (25 wt%), lignin (30 wt%), and fiber (1 wt%), forming the outer layer that protects the coffee seed [54]. It underwent a sequence of chemical modification processes, which included dewaxing, alkali treatment, acid treatment, and bleaching. Each stage led to significant changes in both the chemical composition and visual appearance of the material, especially regarding the particle coloration. It was easily observable without any devices, as demonstrated in Figure 4. At the beginning, the UT sample displayed a deep brown color attributed to the presence of cellulose, hemicellulose, lignin, wax, and various impurities. Following the dewaxing process, the removal of waxes and surface impurities led to a clearer visibility of cellulose fibers, which in turn resulted in a little lighter shade. The subsequent alkaline treatment process resulted in the dissolution of some hemicellulose and lignin, which contributed to a more pronounced brown coloration, signifying structural changes within the fiber network [55]. After treatment with the acid process, the remaining hemicellulose and lignin went through more destruction, leading to a lighter and more yellowish hue of the substance. In the end, the bleaching process produced a bright yellowish-white fiber, signifying the effective elimination of the majority of non-cellulosic elements and the enhancement of purified cellulose fiber [56]. In addition, the yield of extracted cellulose reached approximately 90, 80, 65, and 50 wt% following dewaxing, alkali treatment, acid treatment, and bleaching, respectively.

FT-IR spectroscopy serves as a powerful method for assessing the modifications in chemical structure that take effect throughout various treatment stages [57], as well as for analyzing the functional groups present in the extracted cellulosic substances. Figure 5 illustrates the FT-IR spectra of cellulose fibers treated by dewaxing, alkaline treatment, acid treatment, and bleaching processes. It exhibited distinct absorption patterns that correspond with the specific chemical functional groups present in the treated materials. The spectra displayed a wide absorption peak at 3311 cm⁻¹, which is associated with O–H stretching, a feature typical of cellulose, hemicellulose, and lignin. A peak at 2980 cm⁻¹ signified C–H stretching of the sample, whereas the signal at 2400 cm⁻¹ was linked to carbon dioxide (CO₂) in this instrument. The peaks observed at 1792 cm⁻¹ and 1633 cm⁻¹ correspond to C=O stretching vibrations, which are associated with hemicellulose and the aromatic rings of lignin, respectively. The absence of these peaks in samples subjected to alkaline treatment, acid hydrolysis, and bleaching processes indicates a successful elimination of hemicellulose and lignin. Furthermore, the peak corresponding to the ester bond near 1294 cm⁻¹, which is commonly observed in hemicellulose and lignin, was absent in these treated samples. This demonstrates that the chemical extraction methods successfully eliminated non-cellulosic elements, thereby improving the purity of cellulose fibers. The results align with earlier studies on the extraction of cellulose from biomass and agricultural byproducts [58,59], which found a significant reduction or elimination of hemicellulose and lignin-associated functional groups after chemical treatments.

The biomass samples and their components were characterized through TGA to investigate the thermal degradation characteristics of the samples [60]. The TGA and derivative thermogravimetry (DTG) thermograms of cellulose fibers subjected to different chemical modification processes, as illustrated in Figure 6, indicated notable changes in their thermal behavior. All samples demonstrated initial moisture loss within the temperature range of 50–150 °C. During the first decomposition stage (200–330 °C), significant weight loss was observed exclusively in the UT and DW samples, suggesting the presence of hemicellulose. Conversely, samples that underwent alkaline treatment, acid treatment, and bleaching exhibited no decomposition within this range, indicating successful elimination of hemicellulose during the dewaxing process. The second decomposition stage (230–400 °C) was associated with the degradation of cellulose, which was notably evident in all chemically treated samples, thereby affirming the integrity of the cellulose structure. At approximately 500 °C, the degradation of lignin was observed solely in the UT and DW samples, indicating their elevated lignin concentration. The findings are consistent with the previous research [61,62], demonstrating that chemical treatments effectively eliminate non-cellulosic constituents, including hemicellulose, lignin, wax, and other elements in agro-industrial residues.

The XRD technique was employed to characterize and evaluate the changes occurring in the crystalline structure throughout all stages of the extraction process. The elevated crystallinity of cellulose fiber stands as a key factor that significantly influences its thermal stability and mechanical characteristics [63,64], thereby shaping its viability for use in biocomposite separators. Figure 7 displays the XRD patterns observed at various stages of CP waste extraction. All samples exhibit two distinct crystalline peaks at 2θ = 16.14° (110) and 21.86° (200), which correlate with the characteristic structure of cellulose I. This observation suggests that the crystalline structure of cellulose I in CP waste remained functioning after all extraction processes. No distinct diffraction peaks of lignin, hemicellulose, amorphous and defective crystalline cellulose, soluble substances, or other various components were found following the bleaching step. As a result, the XRD outcomes support the findings from FT-IR and TGA investigations. Moreover, the extracted cellulose fibers exhibit greater intensity when compared to biomass. The acid hydrolysis of the bleaching method targets the amorphous regions, resulting in an increased crystallinity index of the materials [65].

Figure 8 illustrates the SEM images of the surfaces of UT, DW, AL, SF, and BL fibers, focusing on the progressive changes resulting from chemical treatment on the surface areas of the cellulose fibers. The surface of raw cellulose fibers consists of lignin and multiple non-cellulosic components, leading to a rough and irregular texture. The partial elimination of wax and contaminants found on the fibers following the dewaxing procedure. After undergoing alkali treatment, the bundles of cellulose fibers are divided into individual fibers, causing a significantly smoother surface on the cellulose fibers due to the removal of non-cellulosic components [66]. It is important to observe that the surface of the cellulose fibers appears to be more exposed when applying acid treatment. The observed phenomenon could result from the partial removal of non-cellulosic constituents such as hemicelluloses, lignin, wax, and various impurities from the initial materials, which in turn may cause minor fibrillation of the treated materials [67]. The bleaching process effectively eliminates any residual lignin that may remain. The surface of extracted cellulose fibers exhibits a smoother texture, even following the bleaching process. The presence of a chemical agent in the bleaching step led to the oxidation of lignin, which resulted in the solubilization of lignin in the solvent medium [68]. Hence, the application of dewaxing, alkaline treatment, acid hydrolysis, and bleaching techniques successfully broke down the ether and ester linkages that connect lignin and hemicellulose, resulting in a smoother, purer, and more distinct fiber network. The decrease in cellulose fiber size enhances the surface-to-volume ratio, facilitating the development of polymer separators through improved fiber packing and appropriate porosity. Additionally, the modified distribution of fibers following all treatments leads to better mechanical characteristics and increased electrolyte uptake, because the open and porous surface promotes ion transport by reducing ionic resistance within the battery system.

3.2. Characterization of Cellulose-Based Composite Separators

The cellulose fiber extracted from CP waste was utilized to produce a sustainable separator, and the surface morphology structure was examined using SEM. The cellulose-based separator exhibited a smooth surface and was free from impurities, which demonstrates its homogeneity and excellent uniformity [69,70]. In the framework of this study, the incorporation of silica particles into a cellulose-based separator at varying filler concentrations of 1, 2, 5, and 10 wt% resulted in significant transformations in surface morphology. The addition of SiO₂ at 1, 2, and 5 wt% to the cellulose separator revealed that the small silica particles were evenly distributed on the fiber surfaces without any aggregation, reflecting an effective distribution of the silica particles (Figure 9 (left)). Upon increasing the SiO₂ content to 10 wt%, major agglomeration of large particles or silica clusters was observed in multiple parts of the separator surface. Furthermore, the EDX spectrum of the cellulose-based separator modified with silica, as shown in Figure 9 (right), clearly displays distinct peaks corresponding to the elements found in SiO₂, including peaks for Si and oxygen (O). Their growth was proportional to the incorporation of SiO₂ particles into the cellulose separator. The separators containing 5 and 10 wt% silica had significantly increased Si contents. This demonstrates that the biopolymer separator was successfully functionalized with silica particles. Based on the information presented above, the cellulose-based composite separators with 5 wt% silica will be selected for further investigation of their properties.

The wettability, or contact angle, of cellulose-based separators with different materials is investigated through static water contact angle (WCA) measurements. Upon the addition of a water droplet to the surface of the pure cellulose separator, the droplet is absorbed instantly since it has a contact angle being nearly zero (0°). Similar behaviors are observed on the surface of both the cellulose/silica separator and the cellulose/recycled silica separator (Figure 10), meaning that they exhibit superhydrophilic characteristics. A sample with a contact angle below 10° is typically regarded as a highly hydrophilic substance [71]. A contact angle value approaching 0° and the ability to spread without forming droplets or angular profiles on the surface indicate that the material has outstanding electrolyte absorption properties [72]. The incorporation of both virgin and recycled silica did not affect the hydrophilicity of cellulose fibers, as silica naturally possesses surface silanol groups (–Si–OH) that can establish hydrogen bonds with water molecules, thereby facilitating thorough water dispersion. While recycled silica could have structural irregularities or impurities due to the recycling process, the maintained 0° contact angle indicates an adequate presence of hydroxyl groups (–OH) on the surface of the material, facilitating effective electrolyte absorption in cellulose-based composite separators.

The flame-retardant properties of the sustainable separators are carefully linked to the safety of the battery. The flammability testing of the cellulose-based separators constructed from various materials is illustrated in Figure 11. Upon ignition, the cellulose-based separators ignited quickly. This behavior resembled that of conventional separators derived from polyolefin materials [73,74]. The pure cellulose separator exhibited a tendency for ignition owing to its carbon-dense composition and numerous hydroxyl groups, which are particularly sensitive to oxidation when exposed to flames. In comparison, cellulose-based composite separators (including both virgin and recycled silica) ignited at a slower rate than the pure cellulose separator. This enhancement is due to the inclusion of silica particles, which are non-combustible and have a high melting point, functioning as thermal insulators and preventing the transfer of heat and gases required for combustion. Additionally, silica plays a role in absorbing thermal energy from the flame, which helps to further inhibit the spread of fire. These findings highlight the essential function of silica in improving the fire resistance of battery separators.

3.3. Electrochemical Testing of Zn//Zn Symmetric Cells

The investigation of the plating and stripping techniques for Zn//Zn symmetric cells utilizing sustainable separators is illustrated in Figure 12. The influence of separator type and silica content in cellulose-based composite separators on the performance of Zn//Zn symmetric cells was examined. The experiments were performed at various current densities, ranging from 0.5 to 8 mA/cm², with consistent charge and discharge durations of 15 min for each cycle. The aim was to evaluate the impact of silica particle incorporation on zinc-ion transport, dendrite suppression, and the overall electrochemical stability. The findings demonstrated that the cellulose-based composite separator containing 5 wt% silica showcased excellent performance. It displayed a notably lower overpotential and exhibited smooth, stable hysteresis profiles throughout zinc plating and stripping across 150 h. The silica particles contributed to improved mechanical stability, enhanced electrolyte retention, and increased ion conductivity [75,76]. In particular, silica acted as a stabilizing component, promoting even distribution of Zn²⁺ ions and inhibiting the formation of zinc dendrites, which is essential for ensuring symmetric cell stability during prolonged cycling. In contrast, the cellulose-based composite separator with 1 and 2 wt% silica demonstrated only slight enhancements, probably because the silica content was insufficient to establish a stable and reinforcing network. On the other hand, the addition of 10 wt% silica resulted in significant aggregation, which increased internal resistance and obstructed zinc-ion transport. Furthermore, the concentration of 1 wt% silica is inadequate to achieve the required enhancements in ionic conductivity, mechanical strength, and dendrite inhibition. The Zn//Zn symmetric cell experiences uncontrolled Zn deposition, interfacial instability, and early failure, resulting in a voltage drop and the battery’s electrochemical death. Thus, the appropriate silica content in the cellulose-based separator was determined to be 5 wt%. Due to the material’s excellent ability to achieve an optimal balance between structural integrity, ionic mobility, and electrochemical performance. This emphasizes the potential of silica particle-boosted cellulose separators in enhancing safety and efficiency within Zn//Zn symmetric cell systems.

Following 190 h of using pure cellulose-based and cellulose/silica composite-based separators in Zn//Zn symmetric cells at a steady current density of 3 mA/cm², the surfaces of the Zn electrodes were analyzed using SEM. Figure 13a illustrates that the surface morphology of the Zn electrode within the pure cellulose-based separator system exhibits a highly rough surface, irregular crystal growth, and some parts display the development of zinc dendrites. The formation of dendrites occurs due to irregular zinc deposition throughout the charging stage of zinc-based batteries, leading to an increased probability of short-circuiting. The growth of dendrites plays a crucial role in determining the cycle life and performance stability of batteries [53,77]. The morphology analysis of the zinc electrodes in a cellulose-based composite separator with 5 wt% silica (Figure 13b) revealed a smoother and more continuous zinc sheet surface when compared to the pure cellulose separator. The presence of a dense zinc deposition layer, along with a lack of dendrite formation or irregular crystal growth, suggests a high level of control and uniformity in the growth of zinc crystals [78,79]. Consequently, it can be inferred that incorporating silica particles into the cellulose separator enhances ion distribution and optimizes zinc deposition, leading to a more stable electrode surface structure regarding long-term performance and safety.

3.4. Electrochemical Testing of ZIBs

The interaction at the boundary between a cellulose-derived separator and metallic electrodes plays a crucial role in determining battery efficiency [80]. The assessment of interfacial resistance between the separator composed of liquid electrolyte-soaked cellulose fibers and the metal electrodes was carried out through the analysis of the EIS spectra of ZIBs. The EIS measurements were conducted to obtain a comprehensive understanding of recombination and charge transport in batteries [53]. Figure 14 shows the Nyquist plots of ZIBs employing different cellulose-based composite separators. Spectroscopic observations were performed over a frequency range of 1 Hz to 100 kHz, using an AC voltage of 10 mV under atmospheric pressure conditions. The separator made from cellulose and 5 wt% recycled silica demonstrated the least internal resistance compared to all other separators. This suggests that it possesses superior ionic conductivity and implies that the separator design efficiently promotes the movement of Zn²⁺ ions. Moreover, the pronounced incline in the low-frequency region, associated with Warburg impedance, further validates the improved ion diffusion characteristics. In comparison, the cellulose/silica 5 wt% and pure cellulose separators exhibited similar initial real resistance and slope, although they presented slightly higher overall impedance than the cellulose/recycled silica 5 wt% composite separator. The results indicate that recycled silica particles can significantly improve the electrochemical performance of the cellulose composite separator in ZIBs.

Achieving a minimal ionic resistance between two electrodes is essential for the best performance of ZIBs. The ionic conductivity (σ) of the pure cellulose separator was evaluated alongside that of the cellulose/silica composite separator after being immersed in liquid electrolyte. The conductivity was determined from the high-frequency intersection of the Nyquist plot with the Z’ axis. At room temperature, the ionic conductivity of the prepared cellulose/recycled silica 5 wt% separator saturated with liquid electrolyte can reach up to 0.274 mS/cm, surpassing that of the pure cellulose separator (0.193 mS/cm) and the cellulose/silica 5 wt% separator (0.195 mS/cm). The cellulose/recycled silica has slightly higher ionic conductivity due to its high electrolyte absorption capability and well-interconnected pore morphology [80,81,82]. Overall, the cellulose/silica composite separator demonstrates significantly improved zinc-ion migration, which is beneficial for improving battery performance.

The design of polymer composite separators not only contributes to enhanced physical and chemical properties but also improves ion transfer distances and establishes efficient pathways for ion movement, leading to superior electrochemical performance in batteries [83]. Figure 15 presents the CV curves of ZIBs employing sustainable separators derived from pure cellulose and cellulose/silica composite materials. The CV test was conducted for the third cycle at atmospheric pressure, applying a scan rate of 1 mV/s and a voltage range covering from 0.8 to 2.0 V. The CV plot reveals clear oxidation and reduction peaks, particularly around 1.6 V for oxidation and between 1.4 and 1.6 V for reduction. This implies that a redox reaction occurred, involving the movement of metal ions from the cellulose-based separator to the battery electrodes. All separators displayed sharp peaks with high current levels, indicating rapid charge transfer and successful redox reactions in ZIBs. Notably, the pure cellulose separator demonstrated the highest peaks in comparison to polymer composite separators. The cellulose/silica 5 wt% separator exhibited better performance compared to the cellulose/recycled silica 5 wt% separator in both oxidation and reduction processes, mostly at 1.6 V, indicating enhanced charge storage capacity and redox efficiency. The CV curve for cellulose/silica showed a broad peak and an extensive area, meaning it has considerable charge storage potential. The internal structure of the separator may exhibit a porous architecture, providing an optimal space for enhanced ion diffusion. In contrast, the cellulose/recycled silica separator displayed narrower and shorter peaks, suggesting a more restricted charge transfer. This limitation is probably due to the irregularity in the particle size or distribution of SiO₂ inside the cellulose/recycled silica separator, which impacts the conductivity of metal ions [84,85]. The data demonstrate outstanding electrochemical stability, as evidenced by the absence of substantial variations in the CV curve throughout all voltage tests, highlighting the exceptional electrochemical resilience of all separators.

Figure 16a–c provide the charge–discharge curves of ZIBs employing pure cellulose and cellulose/silica composite separators at current densities of 0.05, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0 A/g, with a total of five cycles performed for each current density. Analysis of the GCD curves reveals that the separator composed of cellulose and 5 wt% recycled silica exhibits markedly superior electrochemical performance compared to both the cellulose/silica 5 wt% separator and the pure cellulose separator. Particularly in terms of specific capacity, the cellulose/recycled silica separator demonstrates the highest value across all current densities. This phenomenon suggests that the recycled silica improves the structure of the cellulose-based separator, making it more suitable for the transmission of metal ions. This modification leads to the formation of a porous network and a more uniform distribution of silica particles, ultimately resulting in increased efficiency in charge storage and ion conduction [86,87]. Furthermore, the consistency of the voltage curve pattern of the cellulose/recycled separator demonstrated a more reliable charge–discharge performance, particularly within the voltage range pertinent to the electrochemical interaction between zinc and the electrolyte. The findings clearly demonstrated that the composite separator made from cellulose and recycled silica could greatly enhance the charge–discharge performance of the battery cells, outperforming both the cellulose/silica separator and the pure cellulose separator. Consequently, the results from the GCD align with the outcomes obtained from the EIS studies.

The final test involved evaluating the extended cycling stability and capacity retention of a ZIB utilizing a pure cellulose separator and cellulose/silica composite separators (Figure 16d). This was conducted at a constant current density of 1.0 A/g and within a voltage range of 0.6 to 2.0 V over a duration of 400 cycles. The findings indicated that the composite separator made from cellulose and 5 wt% recycled silica exhibited superior cycle stability and the highest capacity retention values (achieving 190% after 400 cycles). The unusually high capacity retention of over 100% in ZIBs with cellulose/recycled silica separators is primarily due to electrochemical activation during early cycling. Recycling silica improves Zn transport and electrolyte wettability, allowing for gradual access to more active sites in the MnO₂ cathode. Furthermore, the formation of a stable cathode-electrolyte interface (CEI) layer may help to reduce Mn dissolution and side reactions. These effects, when combined, result in increased capacity over time rather than degradation [88,89]. The examination of the cellulose/recycled silica composite separator reveals a consistent capacity, exhibiting only slight fluctuations when compared with the cellulose/silica separator and the pure cellulose separator. The pure cellulose separator showed the least cycle stability, with a continuous decrease in capacity retention values (reaching 30% after 400 cycles). The sudden drop in capacity retention observed in pure cellulose and cellulose/silica separators is primarily due to electrochemical and structural instability caused by prolonged cycling. Pure cellulose separator lacks the mechanical reinforcement and interfacial functionality required to withstand volume changes and dendrite formation, resulting in separator degradation or zinc dendrite penetration. When commercial silica is added to the cellulose/silica separator, its limited surface reactivity and compatibility with cellulose result in weak interface adhesion, providing insufficient support to maintain ionic transport and mechanical integrity. Commercial silica excludes beneficial dopants and defects found in recycled silica, leading to less efficient Zn²⁺ transport and higher internal resistance. These issues can accelerate side reactions, passivation, and loss of active material, resulting in the observed break and sudden performance drop. This observation indicates that the cellulose/recycled silica separator features a well-structured polymer network, facilitating an even distribution of SiO₂, decreasing electrolyte degradation, and minimizing the formation of zinc dendrites. The integration of recycled silica particles into the cellulose, rich in functional groups, significantly enhances the mechanical strength of the sustainable separator, facilitates the movement of zinc ions toward the electrodes during current supply and charging stages [53,90,91], and minimizes the possibility of zinc dendrite formation. Hence, the incorporation of silica particles into cellulose-based separators may enhance the characteristics of ZIBs and address the primary challenge associated with them, leading to a significantly extended working lifespan.

3.5. Integration of Ceramic Particles in Sustainable Separator

The integration of ceramic substances into biomass-derived separators presents numerous advantages in the field of battery technology. In contrast to biomass, functional ceramic materials exhibit distinct characteristics, including superior thermal stability and remarkable mechanical strength [92,93,94]. The properties of these materials keep them excellent for improving the efficiency and safety of biomass-based separators in battery applications [95]. The improved performance of the cellulose/SiO₂ composite separator is attributed to two main factors: (i) the abundant Si-OH groups on SiO₂ particles interact with Zn²⁺ ions, improving ion transport and promoting uniform Zn deposition; and (ii) the added SiO₂ improves the mechanical strength of the membrane, helping to suppress Zn dendrites. Furthermore, adding recycled SiO₂ from Si-based solar panels (via chemical leaching and selective precipitation) to cellulose-based separators improves ZIBs performance and lifetime more effectively than commercial SiO₂ due to its unique structural and chemical properties. Recycled SiO₂ has a high surface area of 5.26 m²/g, a pore volume of 0.0053 cm³/g, and an average pore diameter of 4.05 nm, leading to improved electrolyte uptake and Zn²⁺ ion transport (Figure 17). The surface of recycled powder contains functional groups and trace dopants (e.g., CaO, Na₂O, TiO₂, Al₂O₃) that enhance interactions with cellulose fibers and Zn²⁺ ion coordination. These characteristics contribute to more uniform zinc deposition and inhibit harmful dendrite growth. The improved interface stability between the separator and the electrolyte boosts cycle stability and reduces side reactions. Thus, this work contributes to sustainability by utilizing waste materials while achieving better or comparable electrochemical performance for batteries.

Polymer-based separators offer benefits in all-organic batteries, such as enhanced flexibility, adjustable characteristics, and compatibility with organic electrode materials [96].

Table 1. A comparative analysis of polymer separators for multiple battery systems, including fabrication techniques, specific capacities, cyclic stabilities, and ionic conductivities.

Polymer Separator	Fabrication Technique	Specific Capacity (mAh/g)	Cyclic Stability	Ionic Conductivity (mS/cm)	Reference
Cellulose/Recycled SiO2/ZnSO4	Solution/Vacuum Filtration	24	190% After 400 Cycles	0.274	This Work
Cellulose/Virgin SiO2/ZnSO4	Solution/Vacuum Filtration	13	78% After 400 Cycles	0.195	This Work
Coffee Silverskin Cellulose (CSC)/Polyacrylamide (PAM)/ZnSO4	In Situ Polymerization	37	50% After 200 Cycles	9.10	[53]
Cellulose-PE/Al2O3/Lithium Hexafluorophosphate (LiPF6)/Ethylene Carbonate (EC)/Dimethyl Carbonate (DMC)	Slurry Coating	110	100% After 200 Cycles	0.502	[97]
Methacrylate Polymer/1-Butyl-3-Methylimidazolium bis(Trifluoromethyl Sulfonyl)Imide (BMImTFSI)	In Situ Polymerization	24	77% After 1000 Cycles	740	[98]
Poly(Ethylene Oxide) (PEO)/Lithium Perchlorate (LiClO4)	Solution Casting	8	86% After 30 Cycles	0.39	[99]
Poly(Vinylidene Fluoride-co-Hexafluoropropene) (PVDF-HFP)/LiPF6	Phase Inversion	80	82% After 1000 Cycles	2.40	[100]

highlights a comparison of several polymer separators based on fabrication technique, specific capacity, cyclic stability, and ionic conductivity across various battery systems. The electrochemical properties of 5 wt% cellulose/recycled silica identified in this work align with the values published by prior researchers [53,97,98,99,100].

4. Conclusions

In summary, we developed a sustainable separator that exhibits outstanding wettability, flame resistance, and prolonged cycling stability, suitable for producing safe and cost-effective ZIBs. The cellulose-based composite separators were fabricated utilizing silica particles along with cellulose fibers extracted from CP waste. The incorporation of 5 wt% recycled silica into the cellulose separator led to an improvement in the electrochemical characteristics of the cellulose/recycled silica composite separators, surpassing those of both the pure cellulose separator and the cellulose/virgin silica composite separators. The ZIBs containing 5 wt% cellulose/recycled silica exhibited an ionic conductivity of 0.274 mS/cm, showed excellent cycle stability, with the maximum capacity retention of 190% after 400 cycles. The cellulose/recycled silica separators have high electrolyte absorption capacity and well-connected pore shape, promoting the mobility of Zn²⁺ ions in batteries. As a result of these advantages, the cellulose/silica separator is a very interesting choice for ZIB separators, and as-fabricated cellulose-based composite separators may also have applications in other electrochemical devices such as supercapacitors and LIBs.