A separator for electrochemical device and a method of coating the same

Abstract

The present invention relates to a modified cellulose separator (270) for electrochemical devices comprising a bare cellulose substrate / membrane layer (210) coated with a plurality of coating layers applied on each side of said cellulose substrate layer using rollers (230) A first coating layer (220) is applied on both sides of the cellulose substrate and a second coating layer (260) is applied on the first coating layer (220) on both sides of the cellulose substrate using rollers (230). The first and second coating layers (220, 260) create a moisture barrier on the surface of the separator. The first coating layer (220) may comprise composite layer of Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and Calcium Oxide (CaO) and the second coating layer (220) may comprise covalently bonded layer of Polydimethylsiloxane (PDMS) and Calcium Oxide (CaO).

Description

A SEPARATOR FOR ELECTROCHEMICAL DEVICE AND A METHOD OF COATING THE SAMEFIELD OF INVENTION

[0001] The present invention relates to a separator for an electrochemical device, and more particularly, to a modified cellulose separator and a method of coating the same.BACKGROUND OF THE INVENTION

[0002] The subject matter discussed in the background section should not be assumed to be prior art merely because of its mention in the background section. Similarly, a problem mentioned in the background section or associated with the subject matter of the background section should not be assumed to have been previously recognized in the prior art. The subject matter in the background section merely represents different approaches, which in and of themselves may correspond to implementations of the claimed technology.

[0003] Batteries are typically constructed of electrodes, that is cathode and anode, and these electrodes are isolated by a separator. The separator is moistened with electrolyte and helps the movement of ions from cathode to anode on charge and reverse on discharge. Although, ions pass freely between the electrodes, the separator is an isolator with no electrical conductivity. Battery separators provide a barrier between the anode and the cathode while enabling the exchange of lithium ions from one side to the other.

[0004] Separator is a vital component of lithium-ion batteries (LIBs) due to its important roles in the safety and electrochemical performance of the batteries. Conventionally, the separator manufacturing has been dependent on fossil resources, however, presently cellulose and its derivatives are being used as sustainable battery separators owning to its numerous advantages. However, the cellulose based separator lack in multiple aspects.

[0005] Presently, majority of separator-modification technologies revolve around polypropylene and polyethylene -based separators. Even though, in the recent years, tremendous efforts have been made for the incorporation of cellulose-based derivatives as separator materials for Lithium-ion Batteries (LIBs), since the cellulose separators absorb moisture they are not suitable for the batteries.

[0006] The presence of moisture content affects the electrochemical performance and safety of LIBs is greatly impacted because of the decomposition of cathode materials, the decomposition of solid electrolyte interface (SEI), and the decomposition of electrolyte. A cellulose based separator is also hygroscopic, meaning it readily absorbs and retains moisture from the surrounding environment. This moisture sensitivity can affect the dimensional stability and mechanical properties of the separator. In high-humidity conditions or when exposed to moisture, such cellulose separators swell, leading to changes in pore structure and reduced performance. Thus, eliminating moisture is very important for better LIBs. Cellulose separators, being environmentally friendly and abundantly available is most preferred battery separator material. Cellulose separators have to be dried adequately in order to remove any trace of moisture from the separator. In spite of that, cellulose separators still tend to hold some moisture in the pores which interferes with the overall performance of the cell since the electrolyte tends to absorb the moisture from the separator, thus leading to its decomposition.

[0007] A cellulose based separator may also further have limited resistance to certain chemicals and electrolytes. They may be susceptible to degradation or swelling in the presence of certain solvents or aggressive electrolytes. This chemical vulnerability can lead to reduced separator integrity and performance over time, particularly in demanding or corrosive environments.

[0008] The cellulose based separator also do not possess the same level of mechanical robustness as most of the synthetic materials. When compared to synthetic polymer separators, cellulose separators may be more prone to tearing, punctures, or damage under mechanical stress or strain. This, in turn, impacts the durability and long-term reliability of the separator.

[0009] These cellulose separators usually have lower thermal stability in pristine form compared to certain synthetic materials. When exposed to high temperatures, cellulose can undergo thermal decomposition, leading to the loss of structural integrity and changes in pore structure. This reduced thermal stability can limit the operating temperature range and potentially affect the safety and performance of devices utilizing cellulose separators.

[0010] There is therefore a need in the art for techniques capable of addressing the above-mentioned shortcomings by providing a cellulose separator with improved moisturerepulsion, dimensional stability, electrolyte intake, ion conduction channels, and ion transport within the separator, and suppressing the side reactions resulting in better stability and electrochemical performance of the Li-ion batteries (LIBs).

[0011] It is therefore also desirable to provide techniques for modifying the cellulose separator of an electrochemical device, while addressing the aforesaid shortcomings of the conventional separators.OBJECTS OF THE INVENTION

[0012] An object of the present invention is to provide a modified cellulose separator having plurality of coatings on both sides of the separator.

[0013] Another object of the present invention is to provide a cellulose separator with improved moisture repelling capability.

[0014] The present invention also aims to improve a chemically stable cellulose separator with improved electrolyte uptake.

[0015] Yet another object of the present invention is to provide a cellulose separator with improved mechanical and thermal strength.

[0016] The present invention also aims to provide a cost-effective electrochemical cell and a method of manufacturing the same, several such electrochemical cells capable of being stacked together to form a battery.SUMMARY OF THE INVENTION

[0017] The summary is provided to introduce aspects related to electrochemical cells and method of manufacturing of the same. Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.

[0018] An aspect of the present invention relates to a separator for electrochemical devices comprising a cellulose substrate layer, a plurality of coating layers applied on each side of said cellulose substrate layer, at least a first coating layer applied on both sides of the cellulose substrate, at least a second coating layer applied on at least the first coatinglayer on both sides of the cellulose substrate, wherein the said at least first and second coating layers create a moisture barrier on the surface of the separator.

[0019] The first coating layer may comprise a composite Polyvinylidene fluoride-co- hexafluoropropylene (PVDF-HFP) and Calcium Oxide (CaO) and the second coating layer comprises CaO and Polydimethylsiloxane (PDMS). The PVDF-HFP binds the CaO particles to the cellulose substrate layer. The second coating layer is covalently bonded prior to application over the first coating layer, and the PDMS in the second coating layer is silanized by a silane coupling agent.

[0020] The separator after the coating layer are applied is calendared to a predetermined thickness, which, in a preferred embodiment may be a maximum of 23 pm. The coating layers applied to the cellulose separator, makes such modified separator hydrophobic, thus changing its inherent moisture retaining capabilities and in turn, repelling moisture.

[0021] Another aspect of the present invention relates to a method of coating a separator for electrochemical device, comprising the steps of preparing at least a composite layer of Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and Calcium Oxide (CaO), preparing at least a covalently bonded layer of Polydimethylsiloxane (PDMS) and Calcium Oxide (CaO). This is followed by applying the layer of PVDF-HFP / CaO composite on each side of a bare cellulose separator and then coating at least the layer of covalently bonded PDMS-CaO over the layer of PVDF-HFP / CaO composite layer on each side of the cellulose separator.

[0022] The PVDF-HFP, that is the first layer enhances the adhesion of CaO particles to the cellulose separator. In the second layer, the surface of the PDMS-CaO is activated and functionalised prior to covalent bonding. The PDMS surface in the second layer is silanized using a silane coupling agent, and the silane coupling agent includes at least 3- aminopropyltriethoxy silane (APTES)

[0023] Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS

[0024] The accompanying drawings constitute a part of the description and are used to provide further understanding of the present invention. Such accompanying drawings illustrate an embodiment of the present invention which is used to describe the principles of the present invention together with the description. The foregoing aspects and many of the advantages of this invention will become more readily appreciated as the same becomes better understood by the reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0025] Fig. 1 illustrates a mechanism of covalently bonding Polydimethylsiloxane (PDMS) and Calcium Oxide (CaO), in accordance with an embodiment of the present invention;

[0026] Fig. 2(a) illustrates application of composite Polyvinylidene fluoride-co- hexafluoropropylene (PVDF-HFP) and CaO layer on each side of the cellulose substrate, in accordance with an embodiment of the present invention;

[0027] Fig. 2(b) illustrates application of covalently bonded CaO and PDMS layer over the PVDF-HFP and CaO layer on the cellulose substrate, in accordance with an embodiment of the present invention;

[0028] Fig. 3 illustrates a representation of a coated cellulose separator, in accordance with an embodiment of the present invention;

[0029] Fig. 4(a) illustrates an uncoated cellulose separator incapable or repelling moisture, in accordance with an embodiment of the present invention, and

[0030] Fig. 4(b) illustrates a coated modified cellulose separator capable of repelling moisture, in accordance with an embodiment of the present invention;DETAILED DESCRIPTION OF THE INVENTION

[0031] The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. Each embodiment described in this disclosure is provided merely as an example or illustration of the present invention, and should not necessarily be construed as preferred or advantageous over other embodiments. The detailed description includes specific detailsfor the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details.

[0032] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0033] The terms “or” and “and / or” as used herein are to be interpreted as inclusive or meaning any one or any combination. Therefore, “A, B or C” or “A, B and / or C” mean “any of the following: A; B; C; A and B; A and C; B and C; A, B and C.” An exception to this definition will occur only when a combination of elements, functions, steps or acts are in some way inherently mutually exclusive.

[0034] The present invention relates to a modified cellulose separator (270) with significant improvement in the properties of a conventional cellulose separator (210). The modified cellulose separator (270) is modified by the application of a composite coating comprising of polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP), Calcium Oxide (CaO) and, Polydimethylsiloxane (PDMS). The said coating makes the modified cellulose separator (270) improve the moisture control, dimensional stability, electrolyte intake, ion conduction channels, and ion transport within the separator, and suppressing the side reactions resulting in better stability and electrochemical performance of the Li-io batteries (LIBs).

[0035] Fig. 1 illustrates introduction of silane coupling agents to PDMS. Primarily, the PDMS surface is comprehensively cleaned, thereby removing any contaminants or residues that could interfere with the bonding process. Such cleaning is ensured using rinsing agents that may include solvents such as isopropyl alcohol or acetone. The cleaning process may be followed by a drying process. Following the drying process, a solution of desired silane coupling agent is prepared. A desired concentration of the solution may be achieved by dispersing or dissolving the silane coupling agent in a suitable solvent such as toluene or ethanol or the like. Such concentration of the solution may vary depending on the specific application and the reactivity of the silane coupling agent.

[0036] The surface functionalization of PDMS introduces functional groups or reactive species on the surface of PDMS, making it more receptive to covalent bonding with CaO. For CaO, silane coupling agents may be used to modify the surface of PDMS. Silane coupling agents have organosilane groups that can chemically react with the hydroxyl groups present on the surface of CaO, forming covalent bonds. The functionalization of PDMS can be achieved by using silane coupling agents containing suitable reactive groups, such as amino, epoxy, or carboxyl groups. Silane coupling agents may be introduced to PDMS (Polydimethylsiloxane) through a process called silanization or silane grafting. Silane coupling agents contain functional groups that can react with the PDMS surface, forming covalent bonds and providing a means for attaching other materials or modifying the surface properties.

[0037] The silane solution as prepared is then applied to the PDMS surface preferably by dipping or spraying the entire PDMS surface uniformly, and thus, coating the PDMS surface with the silane solution.

[0038] The silane coupling agent is kept at rest to react with the PDMS surface for a specified duration. This allows the functional groups of the silane to chemically bond with the surface of the PDMS, forming covalent bonds. The reaction time may vary depending on the specific silane coupling agent and reaction conditions.

[0039] The treated PDMS surface after a desired reaction time may be rinsed with a suitable solvent to remove any unreacted silane coupling agent or by-products. The surface may then be cautiously dried through air drying or by using a gentle stream of nitrogen or compressed air to ensure elimination of any residual solvent.

[0040] To activate the surface of CaO and PDMS and to create reactive sites (110, 120) for covalent bonding, surface oxidation may be done to activate the CaO surface. CaO may be oxidized by exposing it to high temperatures in the presence of an oxygen-rich atmosphere. This process, known as thermal oxidation, allows oxygen molecules to react with the CaO surface, resulting in the formation of calcium hydroxide (Ca(OH)2) or calcium carbonate (CaCOs) on the surface. The resulting hydroxyl groups (-OH) may serve as reactive sites (120) for covalent bonding with PDMS.

[0041] Once the surfaces of both CaO and PDMS are functionalized, they can be brought into contact under appropriate conditions to allow for covalent bond formation. Thereactive groups on the surfaces can react with each other, forming covalent bonds between CaO and PDMS. The reaction conditions, such as temperature, time, and the presence of catalysts, may vary depending on the specific reactive groups and their reactivity.

[0042] In a preferred embodiment, silanization of PDMS may be done using (3- aminopropyl)triethoxysilane (APTES) as the silane coupling agent, as described in Fig. 1. APTES contains an amino group that may react with the hydroxyl groups on the PDMS surface, forming covalent bonds.

[0043] The PDMS surface in accordance with the preferred embodiment is cleaned by rinsing the PDMS surface with ethanol thereby removing any contaminants or residues, and may then be rinsed with deionized water to remove residual ethanol. The cleaned PDMS surface is then dried using a flow of compressed air. The APTES solution may be prepared in a petri dish by diluting the APTES with ethanol. The preferred concentration may range between 1% to 10%. The cleaned PDMS surface is dipped in the APTES solution and allowed to react by keeping it at rest for a specific duration forming a silane layer on the PDMS surface.

[0044] Further, in accordance with the preferred embodiment, CaO is heated in an oven in air or on a hot plate at a specific temperature (e.g., 300-400°C) for a specific duration (for example, one hour) to activate the CaO surface. This thermal activation process removes surface impurities and enhances reactivity of the surface.

[0045] The thermally activated CaO particles may be dispersed in ethanol to create a slurry. The silanized PDMS is then to be fully immersed in the in the CaO slurry. This silanized PDMS may be allowed to soak in the CaO slurry for a specific duration (for example, one hour) to facilitate interaction and adhesion between the silanized PDMS and CaO particles.

[0046] Once the above-mentioned process is concluded, the silanized PDMS is removed from the CaO slurry and the excess slurry is allowed to drain off. The resulting covalently bonded PDMS / CaO is then placed in an oven or on a hot plate at an appropriate temperature (for example, 80-120°C) for a specific duration (for example, one to two hours) to evaporate the solvent and promote curing.

[0047] A composite material comprising covalently bonded PDMS and CaO is achieved. The APTES-silanized PDMS provides a chemically bonded interface with the CaO particles, resulting in improved adhesion and durability. The surface activation of CaO leads to introduction of hydroxyl groups on the surface of CaO. These surface hydroxyl groups can then react with the functional groups of the silanized PDMS, leading to the formation of covalent bonds. The chemical structure of CaO (calcium oxide) does not typically include a hydroxyl group (OH) as it is a compound composed of calcium and oxygen. However, during thermal treatment or exposure to moisture, CaO can react with water to form calcium hydroxide (Ca(OH)2) through a process called hydration. The reaction can be represented as:CaO + H2O Ca(OH)2

[0048] In the above exhibited reaction, water molecules (H2O) react with CaO to produce calcium hydroxide (Ca(OH)2). Calcium hydroxide contains hydroxyl groups (OH) attached to the calcium atom. The structure of calcium hydroxide consists of a calcium cation (Ca2+) bonded to two hydroxide anions (OH”).

[0049] The reactive silane molecules on the PDMS surface can react with the surface hydroxyl groups on the CaO particles, resulting in the formation of covalent Si-O-Ca bonds.

[0050] According to Fig. 1, in an APTES (3-aminopropyltriethoxysilane) silanized PDMS, the silanol group is located at the end of the silane molecule. APTES is an organosilane compound that consists of a triethoxysilane group (-Si(OC2H5)3) and an aminopropyl group (-CH2CH2CH2NH2) connected by a linker. During the silanization process, APTES reacts with the surface of PDMS, resulting in the attachment of the silane molecule to the PDMS surface. The triethoxysilane group undergoes hydrolysis upon exposure to moisture, resulting in the formation of a silanol group (-Si-OH) and ethanol (CH3CH2OH) as a byproduct. The silanol group (-Si-OH) on the APTES silane serves as the reactive site (110) for subsequent chemical reactions. It can participate in condensation reactions with other silanol groups or with hydroxyl groups on other surfaces, such as CaO particles, to form covalent bonds. Overall, the silanol group in APTES silanized PDMS is present on the APTES molecule itself, enabling it to participate in covalent bonding reactions with other materials, CaO in this case.

[0051] Thereby, achieving a covalently bonded CaO / PDMS structure, R and R' represent organic groups or alkyl chains attached to the silicon (Si) atoms in the PDMS molecule. The silicon (Si) atoms in the silanized PDMS react with the hydroxyl groups (OH) on the CaO surface to form covalent bonds. The oxygen (O) atoms from the silanol groups on the PDMS and the CaO surface participate in the bond formation.

[0052] Fig. 2(a) illustrates an embodiment where a first layer (220) of polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) is applied to a layer of cellulose membrane / substrate (210) on each side of the said substrate using a set of rollers (230). The first layer (220) on the cellulose substrate (210) will be comprising of a composite of PVDF- HFP / CaO composite. PVDF-HFP contributes to binding CaO particles and helping them stick to the cellulose substrate’s surface through several mechanisms. PVDF-HFP possesses inherent adhesive properties due to its polar nature and the presence of functional groups, such as fluorine and oxygen atoms. These polar groups can interact with the surface of CaO particles, promoting adhesion. The polar interactions, including dipole-dipole interactions and hydrogen bonding, between PVDF-HFP and the surface of CaO facilitate the binding of the particles. PVDF-HFP also has good wetting properties, thus, it can spread and cover the surface of CaO particles effectively. The presence of PVDF-HFP on the surface of CaO allows for better contact between the particles and the substrate, enhancing adhesion.

[0053] Further, the PVDF-HFP layer is capable of forming a thin film or coating on the surface to which it is applied. This film can encapsulate the CaO particles and provide a protective layer. The film formation helps in binding the particles together and improves their adherence to the substrate. Additionally, the PVDF-HFP film can provide barrier properties, preventing the release or leaching of the CaO particles. PVDF-HFP infiltrates into the voids and irregularities on the surface of the substrate. As it solidifies or cures, PVDF-HFP mechanically interlocks with the substrate and the CaO particles, enhancing the overall adhesion strength.

[0054] The PVDF-HFP after coming into contact with CaO (calcium oxide), interacts primarily based on ionic and polar interactions. Calcium oxide (CaO) is an ionic compound composed of calcium cations (Ca2+) and oxide anions (O2). In the presence of water or moisture, CaO can undergo hydrolysis, forming calcium hydroxide (Ca(OH)2). When PVDF-HFP interacts with CaO or Ca(OH)2, the polar groups in PVDF-HFP, such as the fluorine atoms and oxygen atoms, can interact with the calcium cations through ionicbonding. The fluorine atoms in PVDF-HFP are electronegative, and they can attract the positively charged calcium ions. PVDF-HFP is a polar copolymer due to the presence of fluorine atoms and oxygen atoms in its structure. These polar groups can interact with the oxygen atoms in CaO or Ca(0H)2 through dipole-dipole interactions or hydrogen bonding. The oxygen atoms in PVDF-HFP can form polar interactions with the oxygen atoms in the CaO compound.

[0055] Referring now to Fig. 2(b), where an embodiment illustrates the second layer (260) comprising of covalently bonded CaO and PDMS has the primary function of improving the electrochemical, mechanical and physical properties of the cellulose separator (210). PDMS and CaO will be bonded covalently in this layer (260). To develop covalent bonding between CaO and PDMS, specific surface modifications or chemical treatments are performed. Firstly, the surface of CaO and PDMS may be activated to create reactive sites (110, 120) for covalent bonding. Surface oxidation will be undertaken to activate the CaO surface. CaO may be oxidized by subjecting it to high temperatures in the presence of an oxygen-rich atmosphere. This process, known as thermal oxidation, allows oxygen molecules to react with the CaO surface, resulting in the formation of calcium hydroxide (Ca(OH)2) or calcium carbonate (CaCOs) on the surface. The resulting hydroxyl groups (-OH) can serve as reactive sites (120) for covalent bonding with PDMS.

[0056] Further, surface functionalization introduces functional groups or reactive species on the surface of PDMS, making it more receptive to covalent bonding with CaO. For CaO, silane coupling agents modify the surface of PDMS. Silane coupling agents have organosilane groups that chemically react with the hydroxyl groups present on the surface of CaO, forming covalent bonds. The functionalization of PDMS is achieved by using silane coupling agents containing suitable reactive groups, such as amino, epoxy, or carboxyl groups. Silane coupling agents may be introduced to PDMS (Polydimethylsiloxane) through a process called silanization or silane grafting. Silane coupling agents contain functional groups that can react with the PDMS surface, forming covalent bonds and providing a means for attaching other materials or modifying the surface properties.

[0057] According to Fig. 2(b), the covalently bonded CaO and PDMS layer (260) is then applied over the CaO / PVDF-HFP layer (220) that is applied over both sides of the cellulose membrane (210) using a set of rollers (230). The PVDF-HFP / CaO / PDMS coating improves moisture repulsion of the cellulose separator (210). The coated / modified celluloseseparators (270) are also dried for comparatively less time than the uncoated bare separator before fabrication. The ceramic matrix PVdF-HFP / CaO layer (220) improves the safety of the cell by improving its thermal as well as mechanical stability. The coating improves the wettability and electrolyte uptake of the cellulose separator (210). Since moisture in the separator is controlled, this promotes aqueous slurry coating of the electrodes and may also help reduce toxic-NMP based coating. Significantly, the cellulose separator (210) reduces the drying time required (usually 48 hrs for the pristine cellulose separator (210)) and hence the cost of the cell manufacturing will also be reduced as a consequence of elimination of the drying step.

[0058] The covalently bonded composite of PDMS (Polydimethylsiloxane) and CaO (calcium oxide) renders a combination of unique properties and the potential synergistic effects that arises from their interaction. PDMS and CaO have distinct properties that can be advantageous in different applications. PDMS exhibits flexibility, thermal stability, low surface energy, and biocompatibility. CaO, on the other hand, exhibits properties such as high melting point, good electrical insulating characteristics, and chemical reactivity. By combining PDMS and CaO in a composite, the material properties may be tailored to achieve desired characteristics that neither material possesses individually. For example, the composite can exhibit improved thermal stability from PDMS and enhanced electrical insulation from CaO.

[0059] Advantageously, the combination of PDMS and CaO results in a plurality of synergistic effects, where the composite exhibits properties that are superior to the sum of its individual components. For illustration, the addition of CaO particles can enhance the mechanical strength and hardness of the PDMS matrix, improving its load-bearing capacity and dimensional stability. Furthermore, the PDMS matrix can provide flexibility and influence resistance, which can complement the inherent brittleness of CaO. This synergistic interaction can lead to a composite material with improved overall performance compared to each component alone.

[0060] The PDMS-CaO composite also exhibits multi-functional properties, combining the benefits of both materials in a single composite structure. For example, the composite can possess thermal stability, good electrical insulation, resistance to chemical degradation, and enhanced mechanical properties. This multi-functionality can open up opportunities for applications in various fields such as electronics, energy storage, biomedicine, and coatings.

[0061] A material incorporating CaO in the composite is preferred because CaO exhibits higher chemical compatibility with cellulose separators (210) compared to others. CaO can form strong chemical bonds with cellulose fibers, leading to improved adhesion and compatibility. This chemical compatibility is crucial for creating a stable and durable composite structure. CaO also has a higher surface reactivity, ioninc conductivity and porosity. These properties facilitate better electrolyte intake and absorption into the cellulose separator (210). The highly porous structure of CaO allows for improved electrolyte penetration, ensuring efficient ion transport throughout the separator. This enhanced electrolyte intake can contribute to better performance and electrochemical properties in energy storage devices. Ionic conductivity is important for separators in electrochemical devices, as it enables efficient ion transport between the electrodes. The higher ionic conductivity of CaO can contribute to lower internal resistance, improved charge / discharge rates, and enhanced overall device performance. CaO even has a higher surface reactivity, which allows for easy surface modification and functionalization. This surface reactivity enables the introduction of functional groups or chemical species that can enhance specific separator properties. For example, the introduction of functional groups through surface modification can improve wettability, electrochemical stability, or compatibility with other components in the system. CaO is generally more cost-effective and readily available making it industrially viable. Calcium oxide is widely used in various industries and can be sourced economically in large quantities. This cost advantage may make CaO a preferred choice for large-scale production or applications that require cost efficiency.

[0062] Fig. 3 illustrates an embodiment of the modified separator comprising of a plurality of coatings of PVdF-HFP / CaO / PDMS layer on a bare cellulose separator (210) with a maximum thickness of 16 pm. The modified cellulose separator (270) is dried and calendared to achieve maximum thickness which may be as thin as 23 pm. The separator is then dried under vacuum for 24 hours to eliminate any excess solvent and moisture that may have collected on the separator’s surface.

[0063] The bare cellulose separator (210) with a thickness of 16pm is sandwiched between coating layers of PVdF-HFP / CaO / PDMS coating on both sides of the cellulose separator (210). The coated separator is dried for solvent removal in vacuum and thencalendared. The separator is dried again before fabricating the cell. The separator is then tested with NMC 811 cathode and Graphite as anode.

[0064] The bare cellulose separator (210) as illustrated in Fig. 4(a), when used within the same set up as a modified cellulose separator (270) takes a long time to attain the upper cut-off voltage of 4.2 V. This is due to the interference of the moisture that has already been absorbed into the pores of the cellulose separator (210).

[0065] However, looking at Fig. 4(b), replacing the bare cellulose separator (210) with the PVDF-HFP / CaO / PDMS coated separator, which is the modified cellulose separator (270), the modified separator (270) reduces this time for charging and gives higher coulombic efficiency compared to that of a bare cellulose separator (210). The bare cellulose separator is supposed to show (210) higher fading at higher C-rate whereas this expected to reduce when a PVDF-HFP / CaO / PDMS separator is used.

[0066] In view of the present disclosure which describes the present invention, all changes, modifications and variations within the meaning and range of equivalency are considered within the scope and spirit of the invention. It is to be understood that the aspects and embodiment of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiment may be combined together to form a further embodiment of the disclosure.

Claims

CLAIMS:

1. A separator (270) for electrochemical devices comprising: a cellulose substrate layer (210); a plurality of coating layers applied on each side of said cellulose substrate layer (210); at least a first coating layer (220) applied on both sides of the cellulose substrate (210); at least a second coating layer (260) applied on at least the first coating layer on both sides of the cellulose substrate (210), wherein the said at least first and second coating layers create a moisture barrier on the surface of the separator.

2. The separator (270) as claimed in claim 1, wherein at least the first coating layer (220) comprises composite Polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) and Calcium Oxide (CaO).

3. The separator (270) as claimed in claim 1, wherein at least the second coating layer (260) comprises CaO and Polydimethylsiloxane (PDMS).

4. The separator (270) as claimed in claim 2, wherein the PVDF-HFP binds the CaO particles to the cellulose substrate layer (210).

5. The separator (270) as claimed in claim 3, wherein the second coating layer (260) is covalently bonded prior to application over the first coating layer (210).

6. The separator (270) as claimed in claim 2, wherein the PDMS in the second coating layer (260) is silanized by a silane coupling agent.

7. The separator (270) as claimed in claim 1, wherein the said separator (270) is calendared to a predetermined thickness.

8. The separator (270) as claimed in claim 7, wherein the maximum thickness of the coated separator is 23 pm.

9. The separator (270) as claimed in claim 1 , wherein the said plurality of coating layers makes the separator hydrophobic.

10. A method of coating a separator for electrochemical device, comprising the steps of: preparing at least a composite layer (220) of Polyvinylidene fluoride-co- hexafluoropropylene (PVDF-HFP) and Calcium Oxide (CaO); preparing at least a covalently bonded layer (260) of Polydimethylsiloxane (PDMS) and Calcium Oxide (CaO); applying at least the layer of PVDF-HFP / CaO composite on each side of a bare cellulose separator (210); coating at least the layer of covalently bonded PDMS -CaO (260) over the layer of PVDF-HFP / CaO composite layer (220) on each side of the cellulose separator (210).

11. The method as claimed in claim 9, wherein the PVDF-HFP layer (220) enhances the adhesion of CaO particles to the cellulose separator (210).

12. The method as claimed in claim 9, wherein the surface of the PDMS -CaO is activated and functionalised prior to covalent bonding.

13. The method as claimed in claim 9, wherein the PDMS surface is silanized using a silane coupling agent.

14. The method as claimed in claim 12, wherein the silane coupling agent includes at least 3 -aminoprop yltriethoxysilane (APTES).

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