Conductive biopolymer material

Abstract

A method of preparing an ionically conducting biopolymer material and a biopolymer material prepared or preparable by the method. The invention further relates to use of the biopolymer material as a s

Description

The present invention relates to method of preparing an ionically conducting biopolymer material and a biopolymer material prepared or preparable by said method. The invention further relates to use of the biopolymer material as a solid-state electrolyte as well as a battery wherein the solid-state electrolyte is a biopolymer material as defined herein. BACKGROUND OF THE INVENTION Biopolymers are naturally occurring polymers composed of monomeric units that are covalently bonded to form larger structures Examples of biopolymers include proteins and polysaccharides. Proteins are polymers of amino acids and serve as the building blocks for cellular structure and function. Polysaccharides, such as cellulose and starch, are polymers of monosaccharides and provide functions such as structural support and energy storage. Biopolymers are biodegradable and renewable materials which make them highly desirable for potential applications in sustainable development and green technology, offering eco-friendly alternatives to traditional synthetic polymers. Materials made using biopolymers, often referred to as “biopolymer materials”, are increasingly being developed and utilised due to their sustainability, biodegradability, and versatile properties. These materials are derived from natural sources such as plants, animals, and microorganisms, and can be tailored for a wide range of applications. Biopolymers possess the ability to form numerous hydrogen bonds, a characteristic that significantly influences their physical properties and functional capabilities. Hydrogen bonds occur when a hydrogen atom covalently bonded to a heteroatom, such as oxygen or nitrogen, interacts with another heteroatom. Biopolymers are rich in heteroatoms such as nitrogen and oxygen. This property is inherent in many biopolymers, including proteins and polysaccharides. For example, in proteins, hydrogen bonds are critical in maintaining secondary and tertiary structures, such as alpha helices and beta sheets, which are essential fortheir biological functions. These interactions are not only fundamental to the stability and functionality of biopolymers but also facilitate their ability to self-assembte, gel, and interact with other molecules. Many such biopolymers can form gels through the networks of hydrogen bonds, which play a critical role in their structure and functionality. The present inventor has discovered a new method of forming material derived from a combination of biopolymers, in the form of a tough and flexible soiid material having a very high degree of ionic conductivity in the solid-state. Ionic conductivity is a property of materials that quantifies their ability to conduct electric current via the movement of ions, and is typically measured siemens per centimetre iS / cm^T at 25 °C. This phenomenon is crucial in a wide range of applications, from energy storage and conversion devices like batteries and fuel cells. Ionic conductivity occurs when ions, migrate through a medium in response to an electric field. The efficiency of this process depends on the nature of the ions, the structure of the medium, and temperature. Ionic conductivity is a critical parameter in the functionality and performance of batteries, serving as the backbone for efficient energy storage and conversion. In batteries, the movement of ions between the anode and cathode through the electrolyte enables the flow of electric current, which is essential for the charging and discharging processes. High ionic conductivity ensures that ions can move swiftly and efficiently, reducing internal resistance and enhancing the battery's overall efficiency, capacity, and lifespan. This property is particularly important in solid-state batteries, where optimising ionic conductivity can lead to improvements in energy density, charging speed, and safety. By enabling efficient ion transport, materials with high ionic conductivity are pivotal in the development of high-performance batteries that power a wide range of applications, from portable electronics to electric vehicles and renewable energy systems. Solid-state electrolytes are materials that conduct ions in their solid phase. Unlike traditional liquid electrolytes, solid-state electrolytes offer significant safety advantages, such as reducing the risk of leaks and flammability, making batteries more stable and reliable. These electrolytes are typically composed of ceramics, glasses, or polymers that facilitate ionic movement while maintaining a solid structure. The high ionic conductivity, chemical stability, and mechanical strength of solid-state electrolytes are crucial for enhancing battery performance, enabling higher energy densities, faster charging times, and longer lifespans. The first inorganic solid-state electrolytes were discovered by Michael Faraday in the nineteenth century, these being silver sulphide (AgzS) and lead(ll) fluoride (PbF?). To this day, many solid-state electrolytes are inorganic materials that conduct ions through a lattice, such as garnets, argyrodite like compounds, perovskites, and alkali metal nitrides, halides, and hydrides. This class of solid-state electrolytes are capable of high ionic conductivity at room-temperature. However, they suffer from poor mechanical properties, such as brittleness and susceptibility to cracking and fracturing during cycling, as well as poor chemical stability with the potential for thermal runaway to cause fires. Additionally, many inorganic solid-state electrolytes are derived from toxic, highly reactive, and / or rare metals. Solid polymer electrodes are another class of solid-state electrolytes which are defined as a solvent-free salt solution in a polymer host material that conducts ions through the polymer chains. Examples include polycarbonates, polyesters, polynitriles, polyalcohols, polyamines, polysiloxane, and fluoropolymers. However, their ionic conductivity is lower than that of inorganic solid-state electrolytes and is often in the region of 10~6- 10~s Scm“1. Solid polymer electrodes also suffer from poor temperature sensitivity and may degrade significantly at lower temperatures, necessitating heating elements or operation at elevated temperatures. The biopolymer material of the present invention is useful as a solid-state electrolyte due to its high ionic conductivity in its soiid-state. The biopolymer material of the present invention achieves a high ionic conductivity, comparable to inorganic soiid-state electrolytes, whilst also having highly desirable mechanical properties and chemical stability. These properties make the biopolymer material of the present invention particularly well suited to use as a solid-state electrolyte, for example in a battery. SUMMARY OF THE INVENTION In a first aspect, the present invention provides a method of preparing an ionically conducting biopolymer material, the method comprising the steps of: a) providing an aqueous mixture comprising water, pectin, and gelatine; b) causing the aqueous mixture formed in step a) to undergo gelation to form a gel; and c) contacting the gel with an aqueous solution of at least one water soluble salt such that dissociated positive and negative ions of the at least one water soluble salt passively diffuse into the gel. In a second aspect, the present invention provides a biopolymer material prepared or preparable by the method as described hereinabove. In a third aspect, the present invention provides a biopolymer material formed from an intimate mixture of water, gelatine, and pectin, wherein dissociated ions of one or more water soluble salt species are present in the interstitial spaces created by hydrogen bonds between water and the strands of gelatine and pectin. in a fourth aspect, the present invention provides a battery comprising an anode, a cathode, and a solid-state electrolyte, wherein the solid-state electrolyte is a biopolymer material as described herein. In a fifth aspect, the present invention provides the use of a biopolymer material as described herein as a solid-state electrolyte. In a sixth aspect, the present invention provides a method of forming a battery, the method comprising providing an anode, a cathode, and a solid-state eiectrolyte in the form of a biopolymer as described herein in electrical communication with the anode and the cathode. BRIEF DESCRIPTION OF THE FIGURES Figure 1 depicts an electrochemical cell with stainless steel electrodes and the biopolymer material of the present invention as the solid-state electrolyte; Figure 2 depicts a Nyquist plot for Samples 1a and 1b of a biopolymer material of the present invention, listed in Table 1; Figure 3 depicts a Nyquist plot for Samples 2a and 2b of a biopolymer material of the present invention, listed in Table 1; Figure 4 depicts a Nyquist plot for Samples 3a and 3b of a biopolymer material of the present invention, listed in Table 1; Figure 5 depicts a Nyquist plot for Samples 4a and 4b of a biopolymer material of the present invention, listed in Table 1; Figure 6 depicts a cyclic voltammogram of a biopolymer material of the present invention using a potential range of from -6V to +6V and a scanning rate of 1 V / s, 0.5 V / s, and 0.1 V / s for samples 1a and 1 b, listed in Table 1; Figure 7 depicts a cyclic voltammogram of a biopolymer material of the present invention using a potential range of from -6V to +6V and a scanning rate of 1 V / s, 0.5 V / s, and 0.1 V / s for samples 2a and 2b, listed in Table 1; Figure 8 depicts a cyclic voltammogram of a biopolymer materia! of the present invention using a potential range of from -6V to +6V and a scanning rate of 1 V / s, 0.5 V / s, and 0.1 V / s for samples 3a and 3b, listed in Table 1; Figure 9 depicts a cyclic voltammogram of a biopolymer material of the present invention using a potential range of from -SV to +6V and a scanning rate of 1 V / s, 0.5 V / s, and 0.1 V / s for samples 4a and 4b, listed in Table 1; Figure 10 depicts a Scanning Electron Microscope image of a bio-polymer material of the present invention in a field view of 1.30 mm; and Figure 11 depicts a Scanning Electron Microscope image of a bio-polymer material of the present invention in a field view 150 pm. DETAILED DESCRIPTION Definition of Terms Forthe purposes of the present invention, the following terms as used herein shall, unless otherwise indicated, be understood to have the following meanings. Other terms that are not specifically defined below are to be understood as having their normal meaning in the art. Reference herein to a “biopolymer material” refers to a material which is at least partially derived from, or preferably substantially or exclusively derived from biopolymers. A “flexible” material, for example, a “flexible” biopolymer material is capable of being deformed by an applied force (for example a tensile, bending, compressive, twisting force etc) and returning substantially to its original form, following removal of the applied force and / or application of a counter force. Flexibility, in the context of this disclosure, thus has its usual meaning as being opposite to stiffness (which is instead characterised by an ability of a material to resist deformation in response to an applied force). The flexibility of a flexible biopolymer material, in the context of this disclosure, is preferably defined as having a stiffness of from 1 to 1000 mg.cm, preferably from 10 to 900 mg.cm, more preferably from 30 to 800 mg.cm, even more preferably from 40 to 700 mg.cm, even more preferably from 50 to 600 mg.cm, even more preferably from 60 to 500 mg.cm, even more preferably from 70 to 400 mg.cm, even more preferably from 80 to 350 mg.cm, even more preferably from 90 to 300 mg.cm, even more preferably from 100 to 250 mg.cm, even more preferably from 150 to 200 mg.cm, as measured using ASTM D1388. Reference herein to “powdered biomass” refers to any powdered plant matter or material derived therefrom as a result of physical and / or chemical processing performed on the plant matter. Reference herein to “room temperature” sometimes referred to as “rt” refers to a temperature of 25 “C. Reference herein to an “aqueous mixture” refers to a mixture of water and at least one other component. In this regard, the term “aqueous mixture” encompasses homogenous mixtures of water and water-soluble solute, i.e. solutions, as well as heterogenous mixtures of water and insoluble material, such as suspensions or dispersions, in addition to combinations thereof. “Gelatine”, also referred to as “gelatin”, is a natural biopolymer derived from collagen, a protein found in animal connective tissues such as skin, bones, and cartilage. It is typically extracted through a process of partial hydrolysis, far example using acid hydrolysis, basic hydrolysis, or enzymatic hydrolysis, which breaks down the collagen into smaller, soluble fragments. Gelatine is typically derived from the hides or hoofs of large mammals such as pigs, cows, or horses, but may also be derived from other animais, such as fish. Any form of gelatine (e.g. powdered or sheet form) gelatine, derived from any animal source, may be used in connection with the present invention. “Pectin” is a naturally occurring polysaccharide found in the cell wails of plants, particularly abundant in fruits such as apples, citrus fruits, and berries. Pectin serves as a structural component in plants, helping to maintain cell wall integrity and plant rigidity. Pectin is composed primarily of D-galacturonic acid units, some of which may be esterified with methanol or ethanol. Slightly different structures of pectin may occur depending on the source. Appendant residues (such as D-xylose or D-apiose in the respective cases of xylogalacturonan and apiogaiacturonan) branching from a backbone of D-galacturonic acid residues may be present. Any form of pectin may be used in connection with the present invention. Detailed description of the Invention The present invention is based on the surprising discovery of a hitherto unknown gel comprising gelatine, and pectin. Without being bound by any particular theory, it is believed that the gel permits diffusion of dissociated ions from a water-soluble salt into interstitial spaces between the strands of gelatine and pectin. This has been found to create a solid phase biopolymer material having a high ionic conductivity and highly desirable physical properties that make it well suited to acting as a solid-state electrolyte. Additionally, the biopolymer material, and any breakdown products thereof, are safe, innocuous, and environmentally sound. Indeed, the biopolymer material serves a secondary use as a fertiliser once its useful lifetime as a solid-state electrolyte is complete. This is especiaily the case where the water-soluble salt is one which has utility as a fertiliser. In a first aspect, the present invention provides a method of preparing an ionically conducting biopolymer material, the method comprising the steps of: a) providing an aqueous mixture comprising water, pectin, and gelatine; b) causing the aqueous mixture formed in step a) to undergo gelation to form a gel; and c) contacting the gel with an aqueous solution of at least one water soluble salt such that dissociated positive and negative ions of the at least one water soluble salt passively diffuse into the gel. Step a) Typically, the aqueous mixture comprising water, pectin, and gelatine is prepared by combining an aqueous mixture comprising water and gelatine with an aqueous mixture comprising water and pectin to form a combined aqueous mixture comprising water, gelatine, and pectin. Typically, the aqueous mixture further comprises powdered biomass. Optionally the powdered biomass is added in the form of an aqueous mixture. Optionally the powdered biomass is added to the aqueous mixture comprising water, gelatine, and pectin. Nevertheless, the powdered biomass, optionally in the form of an aqueous mixture, may be added to the aqueous mixture comprising water and gelatine and / or the aqueous mixture comprising water and pectin prior to mixing. The aqueous mixture comprising water and gelatine preferably comprises from 5 to 30 wt.% gelatine as a percent of the mass of water, more preferably from 10 to 25 wt.% gelatine, for example 20 wt.%. The aqueous mixture comprising water and pectin preferably comprises from 0.5 to 10 wt.% pectin as a percent of the mass of water, more preferably 1 to 7.5 wt.% pectin, for example 5 wt.%. Typically, the same, or a similar, mass of water is used to prepare the aqueous mixture comprising water and pectin and the aqueous mixture comprising water and gelatine. Thus, the combined aqueous mixture comprising water, gelatine, and pectin typically comprises 2.5 to 15 wt.% gelatine as a percent of the mass of water, more preferably from 5 to 12.5 wt.% gelatine, for example 10 wt.%, and typically comprises from 0.25 to 5 wt.% pectin as a percent of the mass of water, more preferably 0.5 to 3.75 wt.% pectin, for example 2.5 wt.%. Typically, the w / w ratio of gelatine to pectin in the aqueous mixture comprising water, pectin, and gelatine will be from 10:1 to 1:1, preferably from 7.5:1 to 2.5:1, for example 5:1. Typically, the aqueous mixture comprising water and gelatine is formed by dissolving gelatine in water with stirring at an elevated temperature. For example, at a temperature of 30 °C or more, preferably 40 °C or more, more preferably 50 °C or more, for example 60 °C. Typically, heating and stirring is continued for at least 5 minutes, preferably 10 minutes, more preferably 20 minutes, even more preferably 40 minutes Typically, the aqueous mixture comprising water and pectin is formed by dissolving pectin in water with stirring at an elevated temperature. For example, at a temperature of 30 °C or more, preferably 40 °C or more, more preferably 50 °C or more, even more preferably 60 °C or more, even more preferably 70 °C or more, for example 80 °C. Typically, heating and stirring is continued for at least 5 minutes, preferably 10 minutes, more preferably 20 minutes, even more preferably 40 minutes. Alternatively, pectin and gelatine may be dissolved in the same aqueous solution simultaneously, or one by one, typically along with stirring and heating. For example, at a temperature of 30 °C or more, preferably 40 °C or more, more preferably 50 °C or more, even more preferably 60 °C or more, even more preferably 70 °C or more, for example 80 °C. Typically, heating and stirring is continued for at least 5 minutes, preferably 10 minutes, more preferably 20 minutes, even more preferably 40 minutes. Powdered biomass, for example, may be soluble or insoluble in water. For example, powdered biomass may comprise at least one of: lignin, tannin, ash, or a carbohydrate, such as cellulose, hemicellulose, starch, chitin, chitosan, glucan, alginic acid, hyaluronic acid, xanthan gum, or guar gum. The powdered biomass preferably has a mean (d50) particle size of less than 1000 pm, preferably of less than 800 pm, more preferably of less than 700 pm, even more preferably of less than 600 pm, even more preferably of less than 500 pm, even more preferably of from 50 pm to 500 pm, most preferably of from 100 pm to 300 pm. Particle size diameter (d50) may suitably be determined by means of a laser diffraction particle size analyser (e.g. a MicrotracS3500 Particle size analyser). The particle size impacts the texture of the biopolymer material. A smaller particle size provides a more uniform and smooth surface texture to the biopolymer material. Powdered biomass may be added in the form of an aqueous mixture comprising water and the powdered biomass, preferably wherein the aqueous mixture comprising water and the powdered biomass comprises from 5 to 50 wt. % of biomass, preferably from 5 to 30 wt.% of biomass. The powdered biomass or the aqueous mixture comprising water and the powdered biomass may further comprise activated carbon and / or carbon black. The addition of powdered biomass, activated carbon, and / or carbon black improves mechanical properties. One or more surfactants may also be added to the mixture formed in step a), for example non-ionic surfactants, such as alcohol ethoxylates, alkyl phenol ethoxylates, fatty acid ethoxylates, fatty amine ethoxylates, polysorbates, alkyl glucosides, sucrose esters, sorbitan esters, or polyethylene glycol esters; preferably, sorbitan esters and polysorbates, more preferably polysorbate 80 (Tween 80 (RTM)) or sorbitan oleate (Span 80 (RTM)j. These types of surfactants do not carry any charge but enhance the ability of a liquid to spread across or penetrate a surface. This helps distribute water within the biopolymer material and form a greater number of hydrogen bonds through filling in the interstitial spaces and enhancing ionic conductivity. Preferably, the one or more surfactants are added in an amount to provide from 1 to 10 wt.% of the resulting biopolymer material, preferably 2.5 to 7.5 wt.%, more preferably 4 to 6 wt.%, for example 5%. Step b) The aqueous mixture formed in step a) will undergo gelation to form a gel if left at room temperature. The gelation process is thought to occur due to abundant hydrogen bonding between the pectin, the gelatine, and optionally any further hydrogen bond donating or accepting additives, such as powdered biomass. The mechanical properties, such as flexibility, of the gel can be controlled by the nature and amount of the powdered biomass added in step a). The aqueous mixture formed in step a) is typically left at room temperature for 1 hour or more, preferably 2 hours or more, more preferably 3 hours or more, even more preferably 4 hours or more, even more preferably 6 hours or more. Reduced temperature may also be used to aid gelling, for example, a reduced temperature of from 0 °C to 20 °C, preferably from 0 °C to 15 °C, more preferably from 0 °C to 10 °C, even more preferably from 0 °C to 5 °C. The time and temperature necessary for the gel to form will depend on the dimensions of the gel. The aqueous mixture formed in step a) is typically poured into a mould or cast, or onto a flat surface, depending on the desired dimensions of the gel. Injection moulding may also be used to form specific shapes. Step c) An aqueous solution of at least one water soluble salt may be prepared by dissolving one or more water soluble salt in water, thereby forming a solution of dissociated ions of the water-soluble salt. Preferably, the at least one water soluble salt is at least one inorganic water-soluble salt. Preferably, the at least one water soluble salt comprises a salt or mixture of salts which are suitable for use as a plant fertiliser. This provides the additional advantage that the biopolymer material can be repurposed as a fertiliser once it is to be disposed of, without the need for any further processing. The biopolymer material may also be prepared from a choice of water-soluble salt with a view to ultimately being repurposed as fertiliser for a specific crop, i.e. where the at least one water soluble salt is a water-soluble salt, or combination of water-soluble salts, well suited to act as a fertiliser to a specific crop. As the other components of the biopolymer material are gelatine, pectin, and water, the biopolymer material is fully biodegradable and can be easily discarded without causing any environmental hazard. Additionally, the gelatine, pectin, and naturally derived additives such as carbon black, activated carbon, and / or powdered biomass components may also contribute to providing soil nutrients in the case that the biopolymer material is repurposed as a fertiliser at the end of its lifetime. As would be appreciated, any wear and tear caused to the biopolymer materia! as a normal consequence of repeating charging and discharging, for example, degradation of the biopolymers, loss of interstitial spaces through processes such as sintering, crystallisation of the w'ater-soluble salts, would not negatively impact the exhausted biopolymer material's ability to act as a safe and environmentally friendly fertiliser. Furthermore, the biopolymer material of the present invention will dissolve in hot water. Many plastics and polymers are biodegradable, however, this still requires an often extended composting process. The present biopolymer material can simply be dissolved in hot water with no further processing, which provides a cheaper and simpler way to dispose of the material after its useful lifetime. For example, the at least one water soluble salt may comprise one or more of: sodium ions, potassium ions, magnesium ions, calcium ions, and ammonium ions; and / or one or more of: chloride ions, nitrate ions, nitrite ions, phosphate ions, citrate ions, acetate ions, sulphate, and gluconate ions. These are all examples of positive and negative ions, respectively, that are commonly found in fertilisers. For example, the at least one water soluble salt may comprise one or more of: potassium citrate, magnesium citrate, calcium acetate, calcium gluconate, Na2SO4, Na3PO4, Na2HPO4, NaH2PO4, K3PO4, K2HPO4, KH2PO4, K2SO4, K2SO4-2MgSO4, KCI, KNO2, KNO3, NaNOs, Ca(NO3)2, CaCI2, CaSO4, Ca(H2PO4)2, MgSO4, MgCI2, NH4NO3, (NH4)2HPO4, (NH4)4SO41 NH4H2PO4) CuSO4, ZnSO4 MnSO4, and FeSO4. These are all examples of water-soluble salts that are commonly used as fertilisers. Preferably, the at least one water soluble salt is a sulphate salt, more preferably selected from (NH4)2SO4, K2SO4, MgSO4, CaSO4, ZnSO4, CuSO41 MnSO4, FeSO4, and Na2SO4. Sulphate fertilisers are especially advantageous in providing sulphur in a readily available form, enhancing plant growth, and improving crop yields. Typically, the aqueous solution of at least one water soluble salt comprises from 5 wt.% to 40 wt.% of the one or more water soluble salt, preferably from 10 wt.% to 35 wt.%. When the gel formed in step b) is contacted with the aqueous solution of at least one water soluble salt, a process of passive diffusion allows the dissociated positive and negative ions of the at least one water soluble salt to diffuse into the gel. The gel formed in step b) is rich in hydrogen bonds which form part of the gelatine, pectin, and optionally powdered biomass. The polymeric strands of gelatine, pectin, and optionally powdered biomass also form hydrogen bonds to each other to form interstitial spaces between the polymeric strands which are rich in hydrogen bond acceptors and donors. Once the positive and negative ions of the at least one water soluble salt diffuse into the gel, they may be stabilised in their charged form in these interstitial spaces. This means that the gel may act like a solvent, insofar as dissociated positive and negative ions of the at least one water soluble salt diffuses through part of the solid phase of the material formed upon contact of the gel and the aqueous solution of at least one water soluble salt in step c). Figures 10 and 11 show the morphological structure of biopolymer material using Scanning Electron Microscope. Figures 10 and 11 show distribution of interstitial spaces, as black spots, throughout the sample. The size distribution of these interstitial spaces varies between 5 pm and 20 pm representing voids between the biopolymer chains. interstitial spaces act as pathways for the movement of ions. Larger and more connected interstitial spaces facilitate easier and faster diffusion of ions through the biopolymer matrix. In contrast, smaller or less connected interstitial spaces can impede ionic movement, slowing down the diffusion process. The ionic conductivity of a biopolymer material is heavily dependent on the availability and characteristics of interstitial spaces. High ionic conductivity requires a network of well-connected interstitial spaces that allow ions to move freely. Biopolymers with fewer or poorly connected interstitial spaces exhibit lower ionic conductivity. Interstitial spaces determine the extent to which ions can interact with the polymer chains. In polymers with larger interstitial spaces, ions have less frequent interactions with the polymer chains, potentially leading to faster movement. In polymers with smaller interstitial spaces, ions may interact more frequently with the chains, which can slow down their movement due to increased friction and binding interactions. When a biopolymer swells in the presence of a solvent, the interstitial spaces typically increase in size, providing more room for ions to move. This is particularly important in ion-conducting polymers like those used in electrolytes for batteries and fuel ceils. Enhanced swelling increases the ionic mobility and overall ionic conductivity. Preferably, the aqueous solution of at least one water soluble salt may further comprise a polyol plasticiser, for example, glycerol. The polyol plasticiser is also rich in hydrogen bond doners and acceptors and so may be similarly incorporated into interstitial spaces between the polymeric strands of the gel by passive diffusion. The inclusion of molecules of polyol plasticiser impacts the physical properties of the gel allowing for a more flexible biopolymer material to be formed in step c). The addition of glycerol has been found to be particulariy beneficial in improving the flexibility of the biopolymer material. Flexibility is a useful property for solid-state electrolytes as it provides improved mechanical stability which reduces cracking and better accommodates any volume changes that occur in the electrodes during battery charge and discharge cycles. Furthermore, flexibility improves the interfacial contact of a solid-state electrolyte so that it may conform more easily to the surfaces of the electrodes, reducing interfacial resistance. Flexibility also provides advantages in terms of versatility of battery design. The incorporation of glycerol also reduces the freezing point of the biopolymer material, thus allowing use at lower temperatures. Typically, the aqueous solution of at least one water soluble salt comprises from 5 wt.% to 40 wt.% of the polyol plasticiser, preferably from 10 wt.% to 35 wt.%. The w / w ratio of the at least one water soluble salt and the polyol plasticiser is typically from 1:3 to 3:1, preferably 1:2 to 2:1, more preferably 1:1.5 to 1.5:1, for example 1:1. Other examples of polyol plasticisers that are suitable for use in the present invention include, for example, ethylene glycol, propylene glycol, sorbitol, mannitol, xylitol, erythritol, maltitol, isomalt, lactitol, butylene glycol, and pentylene glycol. Contacting the gel with an aqueous solution of at least one water soluble salt can be achieved for example by pouring the solution of at least one water soluble salt over the gel, by submerging the gel in the solution of at least one water soluble salt, by spraying the gel with the solution of at least one water soluble salt, or combinations thereof. Contacting the gel with the solution of at least one water soluble salt will achieve passive diffusion of ions from the solution of at least one water soluble salt into the gel so long as the surface of the gel is in contact with the solution of at least one water soluble salt. Typically, the gel is contacted with the solution of at least one water soluble salt for at least 1 hour, preferably at least 2 hours, more preferably at least 4 hours, even more preferably at leasts hours, even more preferably at least 12 hours, even more preferably at least 18 hours, even more preferably at least 24 hours. The time taken for step c) to be completed will depend on the thickness of the gel, the surface area of the gel which is available for contact with the solution of at least one water soluble salt, and the nature of the at least one water soluble salt. This process may suitably take place at room temperature, although elevated temperature may be used to accelerate the process, for example a temperature of 30 °C or more, preferably 40 °C or more, more preferably 50 °C or more, even more preferably 60 °C or more may be used. Typically, the weight ratio of gekaqueous solution of at least one water soluble salt is from 1:0.5 to 1:10, preferably from 1:0.6 to 1:5, most preferably from 1:0.7 to 1:3, for example 1:1. Once step c) is complete it is preferable that the biopolymer material formed is dried to remove excess water. Drying may be achieved by methods known in the art, for example the biopolymer material may be left to dry at room temperature for at least 1 hour, preferably at least 2 hours, more preferably at least 4 hours, even more preferably at least 6 hours, even more preferably at least 12 hours. The biopolymer may also be dried using exposure to airflow or turbulent air, dried in an oven using heating and / or fanning, or dried in a dehydrator. Further drying may be achieved in combination with any of these methods by contacting the partially dried material with an absorbent material and optionally applying pressure to the partially dried material with the absorbent material. Drying may persist for anywhere between 1 hour and 72 hours, preferably from 6 hours to 24 hours. Even after drying to remove excess water, water molecules will be present in some interstitial spaces between the pectin, gelatine, and any other biopolymers present such as powdered biomass. The resulting biopolymer material may therefore contain an amount of water which impacts fire retardant properties onto the biopolymer material. Typically, the water content of the dried biopolymer material after drying is complete is less than 35 % w / w, preferably less than 30 % w / w, more preferably less than 25 % w / w, most preferably less than 20 % w / w. In a second aspect, the present invention provides a biopolymer material prepared or preparable by the method as described hereinabove. In a third aspect, the present invention provides a biopolymer material formed from an intimate mixture of water, gelatine, and pectin, wherein dissociated ions of one or more water soluble salt species are present in the interstitial spaces created by hydrogen bonds between water and the strands of gelatine and pectin. Optionally, a biopolymer material formed from an intimate mixture of water, gelatine, and pectin, and powdered biomass. The intimate mixture of water, gelatine, pectin, and optionally powdered biomass provides interstitial spaces between polymeric strands of gelatine, pectin, and powdered biomass, if present. These interstitial spaces are rich in hydrogen bond donors and acceptors and are thus able to stabilise dissociated positive and negative ions of the water-soluble salt species. Preferably, water is also present in the interstitial spaces between the strands of gelatine, pectin, and any other biopolymers present such as powdered biomass. The presence of water causes the biopolymer material to have fire retardant properties. This is especially useful in battery applications as this provides enhanced safety and thermal stability, which prevents thermal runaway during charging or discharging. Preferably, a polyol plasticiser, more preferably glycerol, is also present in the interstitial spaces between the strands of gelatine, pectin, and any other biopolymers present such as powdered biomass. The presence of the plasticiser imparts increased flexibility onto the biopolymer material. A higher concentration of glycerol will aiso allow for a greater number of hydrogen bonds to water molecules which will further contribute towards fire retardant properties. Typically, the biopolymer material of the second or third aspect may further comprise powdered biomass, preferably wherein the powdered biomass is as described in the first aspect or in any embodiment described in relation thereto. Typically, the biopolymer material of the second or third aspect may further comprise powdered activated carbon and / or carbon black. The water-soluble salt species of the biopolymer material of the second or third aspect may be as defined in the first aspect or in any aspect or in any embodiment described in relation thereto. Typically, the biopolymer material of the second or third aspect has an ionic conductivity of at least 2 x 10~3 S / cm at 25 °C, preferably wherein the biopolymer material has an ionic conductivity of at least 3 x 10'3 S / cm at 25 °C, more preferably at least 4 x 10’3 S / cm, even more preferably at least 5 x 10-3 S / cm, even more preferably at least 7.5x10-3 S / cm, most preferably at least 1 x 10'2 S / cm. Typically, the interstitial spaces of the biopolymer material of the second or third aspect have a size distribution of from 5 pm and 20 pm, or a median diameter (d50) of from 5 pm and 20 pm, for example, as measured by Scanning Electron Microscopy. In a fourth aspect, the present invention provides a battery comprising an anode, a cathode, and a solid-state electrolyte, wherein the solid-state electrolyte is a biopolymer material as described herein. In a fifth aspect, the present invention provides the use of a biopolymer material as described herein as a solid-state electrolyte. Preferably, use of a biopolymer material as described herein as a solid-state electrolyte in a battery, such as a battery as described herein. Use of the biopolymer material as described herein as a solid-state electrolyte typically takes the form of use of the biopolymer material as a solid-state electrolyte in a battery, such as a battery as described herein. Use of said battery involves discharging the battery to power an electrical device, such as an electrical device as described herein. In a sixth aspect, the present invention provides a method of forming a battery, the method comprising providing an anode, a cathode, and a solid-state electrolyte in the form of a biopolymer as described herein in electrical communication with the anode and the cathode. Typically, the method further comprises applying a voltage to the battery, thereby charging the battery. During charging, the anode is oxidised, releasing electrons, and the cathode is reduced, absorbing electrons. Typically, charging is performed with a battery charger, which typically draws power from an external source, such as mains electricity or a solar panel. Typically, a protective architecture is provided to at least partially encase the anode, cathode, and biopolymer material. The invention will now be described by reference to the following non-limiting Examples and the Figures. EXAMPLES Example 1 - Standard Procedure for the Preparation of a Biopolymer Material of the Present Invention 200mL of water was heated to a temperature of about 60°C and 40g of gelatine (e.g. gelatine available from Special Ingredients or MM Ingredients, or other sources of pork, or beef gelatine produced commercially and available for food use with specification as 240 Bloom unflavoured powdered gelatine) was added with continuous stirring for 40 minutes until all the gelatine was dissolved. 200mL of water was heated to a temperature of about 80°C and 8g of pectin (e.g. pectin available from Special Ingredients, Mr P Ingredients, Intra laboratories, or other sources produced commercially and available for food use usually derived from orange, lemon, lime, and grapefruit peels, or as a by-product of apple juice and cider production) was added with continuous stirring for 40 minutes until all the pectin was dissolved. The two solutions were mixed together with continuous stirring for another 20 minutes at 80°C. This mixture was poured into a mould to create a film of 1mm and left in a cool place for 4 hours to gel. A 30 wt.% solution of a 1:1 w / w mixture of the water-soluble salt and glycerol was poured into the gelatine / pectin gel containing mould and left for 12 hours to soak. The sample was removed from the mould and left to dry at room temperature for 12 hours. A flexible biopolymer material was obtained. The mechanical properties and strength of biopolymer materia! can be further enhanced by adding powdered biomass or organic fibres. After two solutions (gelatine and pectin) are mixed together with continuous steering for 20 minutes at BOAT powdered biomass derived from plant or animal sources, or man-made fibres produced from organic (carbon-containing) compounds, can be added at maximum 10% wt. Dry or wet powder particulate materials derived from organic sources, which can include plant-based, animal-based, or synthesised organic compounds, can be added at maximum 10% wt. Example 2 - Measurement of Ionic Conductivity A number of biopolymer material samples were prepared as described in Example 1 to provide biopolymer materials in the form of approximately 12 mm diameter membranes. Samples were prepared using potassium citrate (Samples 1a and 1b), aluminium potassium sulphate (samples 2a and 2b), potassium sulphate (samples 3a and 3b), and potassium citrate with surfactants (samples 4a and 4b), as the water-soluble salt, and the samples contained either no further additive, or 5% wt of activated carbon. The biopolymer materials were homogeneous and free of defects. The thickness was measured by digital callipers. The biopolymer materials were placed in electrochemical cells with stainless stell electrodes (using a three-electrode setup: working electrode, counter electrode, reference electrode as depicted in Figure 1). The cell was assembled with the working electrode (where the biopolymer material is mounted), counter electrode, and reference electrode. The electrodes were connected to a potentiostat / galvanostat with electrochemical impedance spectroscopy capabilities. The frequency response analyser was set up for electrochemical impedance spectroscopy measurements. The ionic conductivity (Scm-1) of the samples were measured and the results are shown in Table 1 below. Table 1. Ionic conductivity results Sample Water soluble salt Ionic conductivity (Scm'1) la Potassium citrate 8.33E-04 1b Potassium citrate 8.42E-04 2a Aluminium potassium sulphate 6.87E-05 2b Aluminium potassium sulphate 3.95E-05 3a Potassium sulphate 6.30E-06 3b Potassium sulphate 3.35E-06 4a Potassium citrate with 5%wt surfactant Span 80 (RTM) 7.59E-03 4b Potassium citrate with 5%wt surfactant Tween 80 (RTM) 2.27E-03 Electrochemical Impedance Spectroscopy Measurements The same set up was used to measure the electrochemical impedance spectroscopy of samples with different water-soluble salts, as shown in Table 1, using an Emstat 4S potentiostat. An AC voltage in the range of 5 to 10 mV was applied across the biopolymer material and the voltage frequency was swept through a range of from 1 MHz to 0.1 Hz. The impedance responses (real and imaginary parts) were measured as a function of frequency and plotted to make Nyquist plots (imaginary impedance vs. real impedance). These Nyquist plots were used to determine the bulk resistance of the samples to calculate ionic conductivity. The high-frequency intercept on the real axis corresponds to the bulk resistance % of the ionic conductor. The Nyquist plots for all samples listed in Table 1 can be seen in Figures 2 to 5, respectively. At the constant sample thickness and area, the lower bulk resistance would mean higher ionic conductivity. Equivalent circuit modelling was used to fit the impedance data and extract the resistance associated with ionic conduction. The ionic conductivity (a) can be calculated using the relation: g-L / R-A where L is the thickness of the biopolymer, R is the resistance obtained from the impedance fitting, and 4 is the area of the electrode. The same set up was used to measure the electrochemical stability window for the same samples as shown in Figures 6 to 9 using an Emstat 4S potentiostat. The potential range for cyclic voltammetry was set to from -6V to +6V. The cyclic voltammetry scan was begun, and the current response recorded as a function of the applied potential. Multiple cycles were performed to ensure reproducibility and stability of the biopolymer. The current vs potential curve was plotted (cyclic voltammogram). The peaks corresponded to the oxidation and reduction processes. For ionic conductivity, the capacitive behaviour and any redox peaks provide insights into ionic transport and electrochemical activity. Figures 6 to 9 show the cyclic voltammetry results using a scanning rates of 1 V / s, 0.5 V / s, and 0.1 V / s. The increased scanning rate altered the current readings, however, the electrochemical stability window (along x axes) remained practically unchanged. A wider electrochemical stability window indicates the biopolymer electrolyte can be used in high-voltage applications without decomposing. Figures 6 to 9 show the stability window around 6V, spreading from -3V to 3V.

Claims

1. A method of preparing an ionically conducting biopolymer material, the method comprising the steps of:a) providing an aqueous mixture comprising water, pectin, and gelatine;b) causing the aqueous mixture formed in step a) to undergo gelation to form a gel; andc) contacting the gel with an aqueous solution of at least one water soluble salt such that dissociated positive and negative ions of the at least one water soluble salt passively diffuse into the gel,2, The method of Claim 1, wherein the aqueous solution of at least one water soluble salt further comprises a polyol plasticiser, for example, glycerol, such that the polyol plasticiser passively diffuses into the gel during step c); preferably wherein the aqueous solution of at least one water soluble salt comprises from 5 wt.% to 30 wt.% of the polyol plasticiser.

3. The method of Claim 1 or Claim 2, wherein the aqueous mixture of step a) further comprises powdered biomass, preferably wherein the powdered biomass comprises at least one of: lignin, tannin, ash, or a carbohydrate, such as cellulose, hemicellulose, starch, chitin, chitosan, glucan, alginic acid, hyaluronic acid, xanthan gum, or guar gum.

4. The method of Claim 3, wherein the powdered biomass has a mean (d50) particle size of less than 1000 pm, preferably of less than 800 pm, more preferably of less than 700 pm, even more preferably of less than 600 pm, even more preferably of less than 500 pm, even more preferably of from 50 pm to 500 pm, most preferably of from 100 pm to 300 pm.

5. The method of any one of the preceding claims, wherein the aqueous mixture of step a) further comprises activated carbon and / or carbon black.

6. The method of any one of the preceding claims, wherein the gel is submerged in the aqueous solution of at least one water soluble salt, or wherein the gel is sprayed and covered with the aqueous solution of at least one water soluble salt during step c).

7. The method of any one of the preceding claims, wherein the at least one water soluble salt comprises a salt or mixture of salts which are suitable for use as a plant fertiliser.

8. The method of any one of the preceding claims, wherein the at least one water soluble salt comprises one or more of: sodium ions, potassium ions, magnesium ions, calcium ions, and ammonium ions; and / or wherein the at least one water soluble salt comprises one or more of: chloride ions, nitrate ions, nitrite ions, phosphate ions, citrate ions, acetate ions, and gluconate ions.

9. The method of any one of the preceding claims, wherein the at least one water soluble salt comprises one or more of: potassium citrate, magnesium citrate, calcium acetate, calcium gluconate, Na2SO4, Na3PO4, Na2HPO4, NaH2PO4, K3PO4, K2HPO4, KH2PO4, K2SO4, K2SO4-2MgSO4, KCI, KNO2, KNO3, NaNO3, Ca(NO3)2, CaCh, CaSO4, Ca(H2PO4)2, MgSO4, MgCI2, NH4NO3, (NH4)2HPO4, (NH4)4SO4, NH4H2PO4, CuSO4, ZnSO4 MnSO4, and FeSCu; preferably one or more of (NH4)2SO4, K2SO4, MgSO4, CaSO4, ZnSO4, CuSO4, MnSO4, FeSO4, and Na2SO4.

10. The method of any one of the preceding claims, wherein step a) comprises combining an aqueous mixture comprising water and gelatine with an aqueous mixture comprising water and pectin to form a combined aqueous mixture comprising water, gelatine, and pectin, preferably wherein the aqueous mixture comprising water and gelatine comprises from 5 to 20 wt.% gelatine; and / or wherein aqueous mixture comprising water and pectin comprises from 0.5 to 5 wt.% pectin.

11. The method of any one of the preceding claims, wherein step a) comprises adding the powdered biomass to an aqueous mixture comprising water, gelatine, and pectin.

12. The method of Claim 11, wherein the powdered biomass is in the form of an aqueous mixture comprising from 5 to 50 wt. % of biomass, preferably from 5 to 30 wt.% of biomass.

13. The method of any one of the preceding claims, wherein the aqueous solution of at least one water soluble salt comprises from 5 wt.% to 30 wt.% of the one or more water soluble salt.

14. The method of any one of the preceding claims, wherein the weight ratio of gekaqueous solution of at least one water soluble salt is from 1:0.5 to 1:10, preferably from 1:0.6 to 1:5, most preferably from 1:0.7 to 1:3.

15. A biopolymer material prepared or preparable by the method of any one of Claims 1 to 14.

16. A biopolymer material formed from an intimate mixture of water, gelatine, and pectin, wherein dissociated ions of one or more water soluble salt species are present in the interstitial spaces created by hydrogen bonds between water and the strands of gelatine and pectin.

17. The biopolymer material of Claim 16, wherein a polyol plasticiser, preferably glycerol, is also present in the interstitial spaces between the strands of gelatine and pectin.

18. The biopolymer material of Claim 16 or Claim 17, wherein one or more of:i) the biopolymer material further comprises powdered biomass, which is preferably as defined in Claim 3 or Claim 4;ii) the biopolymer material further comprises activated carbon and / or carbon black; and / oriii) the water-soluble salt species is as defined in any one of Claims 7 to 9.

19. The biopolymer material of any one of Claims 16 to 18, wherein the biopolymer material has an ionic conductivity of at least 2 x 10'3 S / cm at 25 °C.

20. The biopolymer material of Claim 19, wherein the biopolymer material has an ionic conductivity of at least 3 x 10-3 S / cm at 25 °C, preferably at least 4 x 10-3 S / cm, more preferably at least 5 x 10 3 S / cm, even more preferably at least 7.5 x 10~3 S / cm, most preferably at least 1 x 10~2 S / cm.

21. A battery comprising an anode, a cathode, and a solid-state electrolyte, wherein the solid-state electrolyte is a biopolymer material according to any one of Claims 15 to 20.

22. The battery of Claim 21, wherein the solid-state electrolyte is a single continuous phase, and is in contact with both the anode and the cathode.23, Use of a biopolymer material according to any one of Claims 15 to 20 as a solid-state electrolyte.

24. A method of forming a battery, the method comprising providing an anode, a cathode, and an electrolyte in the form of a biopolymer material of any one of Claims 15 to 20 in electrical communication with the anode and the cathode.

25. The method of claim 24, further comprising applying a voltage to the battery.

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