Battery separator and method of manufacturing

Abstract

There is disclosed a solid state separator for battery, the solid state separator comprising: - a first layer including a pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre (CelNF) composite free-standing film; - a second layer including a porous polyolefin film; and - a third layer including a ceramic nanolayer provided in-between the first and second layers There is also disclosed a battery including such a separator as well as methods and a system for production their production.

Description

[0001] BATTERY SEPARATOR AND METHOD OF MANUFACTURING

[0002] The invention relates to a solid state separator for a battery. More specifically, the invention relates to a solid state separator including a pre-lithiated or pre- sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite freestanding film as well as a battery including such as separator. The invention also relates to a system and methods of producing a PPS cellulose nanofibre composite free-standing film.

[0003] The development of advanced separators holds the key to significantly enhancing the safety of batteries. Separators act as critical components within metal ion batteries such as Lithium (Li), Sodium (Na), and Zinc (Zn), preventing physical contact between the anode and cathode, which could lead to short circuits and thermal runaway. Innovations in separator technology, such as improved thermal stability, enhanced electrolyte compatibility, and the incorporation of flame-retardant materials, are pivotal in reducing the risk of battery failures. By focusing on the advancement of safer and more reliable separators, the battery industry can address the inherent safety concerns of lithium- and sodium-ion batteries, paving the way for their wider and more secure application across various sectors, including consumer electronics and electric vehicles.

[0004] Solid separators are vital for the safety, efficiency, and performance of Li and Na ion batteries. Originally, these batteries used porous polymers like polyethylene and polypropylene as separators, selected for their capability to enable ionic conductivity through electrolyte-filled pores and to maintain a physical barrier between the anode and cathode, thus preventing short circuits. However, these porous separators presented notable issues. They could lead to nonuniform lithium ion transport and create localized electrical fields that may encourage the formation of lithium dendrites, particularly around the pores. Additionally, their tendency to shrink under high temperatures could result in short circuits, and their structural integrity was at risk of being compromised by punctures.

[0005] In response to the limitations of traditional porous separators, there has been a notable trend towards the development of non-porous solid separators in lithium-ion batteries. Unlike their microporous counterparts, non-porous separators lack any form of pores or openings within their structure. This design is instrumental in curbing the formation of lithium dendrites - sharp, needle-like projections that can emerge on lithium metal anodes. These dendrites pose significant risks, including the potential for short circuits and safety hazards. Additionally, their robust structure makes the battery more resistant to physical damages. By integrating non-porous separators, it's possible to enhance both the capacity and energy density of lithium-ion batteries. This improvement is particularly beneficial for devices employing lithium metal anodes, including wearable technology, drones, and electric vehicles, where performance and safety are paramount. These advanced separators offer improved thermal stability and mechanical strength at high temperatures, greatly enhancing battery safety.

[0006] However, the transition to non-porous materials requires overcoming several challenges, including the development of new solid electrolytes that can conduct lithium ions without liquid electrolytes and scalability to large-scale production. These materials must possess high ionic conductivity, electrochemical stability, and compatibility with electrode materials. Moreover, adapting manufacturing processes to accommodate these new materials and ensuring scalability for mass production are essential steps for the widespread adoption of non-porous separators in lithium-ion batteries.

[0007] Transitioning to non-porous materials for lithium-ion batteries involves navigating through a set of complex challenges. Key among these is the innovation of new solid electrolytes capable of facilitating lithium ion movement without the need for traditional liquid electrolytes. These novel materials must exhibit exceptional ionic conductivity, maintain electrochemical stability, and be fully compatible with existing electrode materials. Additionally, the manufacturing processes must evolve to integrate these advanced materials effectively, en- suring that they can be produced at scale. Achieving scalability for mass production is crucial to enable the broad implementation of non-porous separators in the Li- and Na-ion battery industries.

[0008] The solid separator made by pre-lithiated polyphenylene sulfide (PPS) has shown promising properties such as high Li ion conductivity, thermal stability, and chemical resistance (US2022037696), owning to PPS being a high- performance thermoplastic polymer known for its exceptional resistance to heat, chemicals, and corrosion, as well as its rigidity and dimensional stability. However, due to its stiffness, it is difficult to process it to thin film. The thin film has relatively low mechanical strength, and mechanical reinforcement is necessary.

[0009] As research in this area continues to advance, the development of non-porous solid separators is poised to significantly enhance the safety, capacity, and overall performance of Li- and Na-ion batteries. This progress is expected to open new possibilities for their application, ranging from consumer electronics to electric vehicles and energy storage solutions, marking a significant leap forward in battery technology.

[0010] The invention has for its object to remedy or to reduce at least one of the drawbacks of the prior art, or at least provide a useful alternative to prior art.

[0011] The object is achieved through features, which are specified in the description below and in the claims that follow.

[0012] The invention is defined by the independent patent claims. The dependent claims define advantageous embodiments of the invention.

[0013] In a first aspect, the invention relates to a solid state separator for ta battery, the solid state separator comprising:

[0014] - a first layer including a pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre (CeINF) composite (PPSCC) free-standing film;

[0015] - a second layer including a porous polyolefin film; and

[0016] - a third layer including a ceramic nanolayer provided in-between the first and second layers. In one embodiment, the second layer may include polyethylene (PE) or polypropylene (PP). Both PE and PP has been shown to be useful for preventing thermal runaway, enhancing mechanical strength and for suppressing the reduction and oxidation of the first layer of the solid-state battery separator.

[0017] The second layer may have a thickness in the range in the range 5-20 pm, preferably 5-10 pm. A thin layer is desirable for maintaining a high energy density in batteries, while a thicker layer is desirable for enhanced mechanical stability. 5-20 pm, and in particular 5-10 pm, has been found to be an optimal trade-off in certain applications.

[0018] In one embodiment, the first layer may include PPS, CeINF, and polytetrafluoroethylene (PTFE) powder with a weight ratio of 89-97%: 0.1-5%: 3-8%, preferably 90-94%: 1-3%: 4-6%, respectively. A smaller fraction of well- dispersed CeLNF holding a threshold of fiber network in the separator enhances the mechanical properties as well as Li-ion transport, but without generating too many pores in the solid separator. In addition of a small fraction of PTEF, the fibrous structure helps processability to produce free-standing film, but without damaging the ion transport properties of PPS film.

[0019] In one embodiment, the ceramic nanolayer may have a thickness in the range 30-300 nm.

[0020] The first layer may have a thickness of 5-45 pm, preferably 10-30 pm, and even more preferably 15-25 pm.

[0021] In one embodiment the first layer may have a porosity lower than 10%, preferably lower than 3%. The porosity may be measured by means of N2 or CO2 absorption.

[0022] The third layer may include one or more of the following:

[0023] - alumina;

[0024] - a Li-rich ceramic, such as Li salts, Li rich cathode materials, Lithium Phosphorous Oxynitride (UPON) or lithium titanate; and

[0025] - a highly porous material, such as carbon nanospheres or metal-organic framework (MOF) materials. Thickness of the separator may be in the range of 10-60 pm, preferably 15-30 pm, and even more preferably 20-25 pm. As for the first layer, the total thickness of the separator is a trade-off between the energy density of the battery in which the separator will be used and its mechanical stability.

[0026] In one embodiment the solid state separator may have a width in the range of 5 -160 cm, where the upper end of the range may correspond to a width in an industrial-scale process, while lower end of the range may correspond to a typical width in a laboratory- scale process.

[0027] Addressing the challenges identified in previous approaches, this invention introduces a solid separator designed to significantly enhance battery performance. The invention provides a multifunctional solid state separator for batteries, such as Li- or NA-ion batteries, including a multilayer structure. The top layer is the PPS-cellulose nanofibre composite "free standing" film, and the bottom layer is the porous polyolefin, such as PE, layer. Between the two layers is provided a ceramic nanolayer. The solid state separator according to the first aspect of the invention has exceptional mechanical strength while at the same time its method of manufacturing has been significantly simplified.

[0028] This design ensures excellent compatibility and wettability with electrolytes, facilitating efficient ion transport. Additionally, the separator boasts superior thermal characteristics, with PPS's high melting point of 280-285°C contributing to its robust and stable performance. This feature, combined with the material's ability to prevent thermal runaway, significantly enhances battery safety. This non-porous PPSCC film, i.e. the first layer, is designed to selectively permit the passage of lithium or sodium ions, thereby reducing dendrite growth. It achieves this by promoting a uniform ion diffusion and eliminating any pores, which effectively halts the formation of lithium dendrites. The composite film is produced from a blend of pre-lithiated polyphenylene sulfide (PPS) powder, cellulose fibres (CeINF), and polytetrafluoroethylene (PTFE) powders, forming a thin, solid membrane separator. The production process, as will be presented in the following, involves orienting and heating the mixture of pre-lithiated or pre-sodiated PPS powder, cellulose fibres, and PTFE in a supersonic dry airflow, subsequently pressing it into a free-standing film with a precisely controlled thickness. During this process, the PTFE is transformed into fibres, and a small amount of cellulose nanofibre, ranging from 0.01 to 3 weight percent, is incorporated into the composite. This addition not only enhances the thermal elongation properties of the thermoplastic PPS powder but also significantly boosts the mechanical strength of the free-standing film, thereby improving the controllability of the automatic manufacturing process.

[0029] The present disclosure directs the PPSCC film multifunctional. The multilayer designed a thin polyolefin, polyethylene (PE) or polypropene (PP) layer porous layer with a thickness, typically ranging from 3-10 micrometres. It provides the function of thermal runaway prevention: In the case of excessive heat, the pores close and ion transport effectively shuts down with increasing temperature until the PE layer melts at about 105-130°C, and PP layer at 130-170°C. In addition, the PPSCC film has a very good thermal stability and keeps excellent dimensional stability up to 280 °C, preventing direct contact of the anode and cathode, thus significantly reducing the short circle potential. PPS exhibits a low flammability rate according to the UL94 standard, achieving a V-0 rating. This indicates a high level of fire resistance, providing an additional safety feature. This design prevents the battery from reaching the thermal runaway threshold and potentially venting with flame, thereby enhancing safety.

[0030] The present disclosure directs the PPSCC film multifunctionality by engineering a coating layer between the PPSCC and polyolefin layer, i.e. the third layer between the first and second layers. The coating layer may e.g. include alumina, Li rich ceramic, such as Li salts, Li rich cathode materials, Lithium Phosphorous Oxynitride (UPON) and lithium titanate, highly porous materials, such as carbon nanospheres and MOF materials. A solution containing the above materials is used as glue to laminate the polyolefin and PPSCC layer to each other. The third layer formed after drying increases the dimensional stability of the polyolefin layer. The third layer may include alumina and / or lithium-rich materials or compounds, such as Li salts, Li rich cathode materials, Lithium Phosphorous Oxynitride (LiPON), lithium titanate, provide Li supplementation. In lithium batteries, particularly lithium-ion batteries, additional lithium is sometimes introduced to compensate for lithium loss during the initial charging and to enhance overall battery performance. MOF materials and / or carbon nanospheres may also be used in the coating layer and may increase the holding of electrolytes and improve the compatibility with electrodes.

[0031] In a second aspect, the invention relates to a battery including a solid state separator according to the first aspect of the invention.

[0032] In one embodiment, the battery may be a Li or Na battery.

[0033] In a third aspect the invention relates to a method for producing a freestanding, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film (PPSCC), the method including the steps of:

[0034] - mixing and milling metal chloride dissolved in deionized water, tetrachloro-p- benzoquinone (TCBQ) and PPS powder to form a mixed slurry;

[0035] - transferring the mixed slurry to a reactor for hydrothermal treatment;

[0036] - washing and filtering the hydrothermally treated mixture in a centrifuge;

[0037] - drying the filtered solution to disintegrate PPS powder;

[0038] - mixing the disintegrated PPS powder with cellulose fibres and polytetrafluoroethylene (PTFE) powder at a temperature at which the PTFE powder is in a glass state to obtain a mixed solution;

[0039] - griding the mixed solution in dry, compressed air;

[0040] - compacting the grinded, mixed solution through one or more rollers to obtain a film; and

[0041] - passing the film through a hot calendering machine to obtain the freestanding, pre-lithiated polyphenylene sulfide (PPS) cellulose nanofibre composite film.

[0042] In a fourth aspect, the invention relates to a method for producing a freestanding, pre-lithiated polyphenylene sulfide (PPS) cellulose nanofibre composite film, the method including the steps of:

[0043] - mixing and milling metal chloride dissolved in deionized water, tetrachloro-p- benzoquinone (TCBQ), PPS powder and cellulose fibres to form a mixed slurry;

[0044] - transferring the mixed slurry to a reactor for hydrothermal treatment;

[0045] - washing and filtering the hydrothermally treated mixture in a centrifuge;

[0046] - drying the filtered solution to disintegrate PPS powder;

[0047] - mixing the disintegrated PPS powder with polytetrafluoroethylene (PTFE) powder at a temperature at which the PTFE powder is in a glass state to obtain a mixed solution;

[0048] - griding the mixed solution in dry, compressed air;

[0049] - compacting the grinded, mixed solution through one or more rollers to obtain a film; and

[0050] - passing the film through a hot calendering machine to obtain the freestanding, pre-lithiated polyphenylene sulfide (PPS) cellulose nanofibre composite film.

[0051] In one embodiment the metal chloride may be one or more of the following:

[0052] - lithium chloride;

[0053] - sodium chloride;

[0054] - magnesium chloride; and aluminum chloride.

[0055] In one embodiment the PPS powder may be a linear chain and end crosslinked semi-crystalline PPS powder, with a degree of crystallinity > 60% in the range 1-25 pm, preferably 1-5 pm.

[0056] The mass ratio of metal chloride, DIW and TCBQ to the total mass of PPS may be substantially 1 :30:5: 100.

[0057] In one embodiment the hydrothermal treatment may be performed at a reaction temperature of approximately 210 °C, for 1-3 hours, preferably around 2 hours, and a vapour pressure of 60-70 bar.

[0058] In one embodiment, mixing of PPS powder with cellulose fibres and PTFE may takes place at a temperature of 45-50°C.

[0059] In one embodiment the griding may performed in an air jet mill pre-heated by compressed air to a temperature of 40-50 °C.

[0060] The step of compacting the grinded, mixed solution through one or more rollers may include producing a film with a thickness of 1.0-3.0 mm, preferably 1.0-1.5 mm. In one embodiment, the method may further include the step of passing the film through the hot calendering machine, including the step of cold-rolling the film at a roller temperature of approximately 40-70 °C, preferably 45-55 °C, to obtain a film with a thickness of 0.3-0.5 mm. In one embodiment the hot calendering machine may have a roller temperature of 125-130°.

[0061] A friction of the roller may be in the range of 1-20%, preferably 8-12%.

[0062] The line load may be in the range of 500-250 N / mm, preferably 350-370 N / mm.

[0063] The speed may preferably be 100 m / min or higher when the process is up- scaled to an industrial process. Though the experiments presented below have mainly been done in controlled small-scale set-ups, the speed and line load / roll pressure can be increased without influencing the parameters of the previous steps of the method to any significant degree.

[0064] In a fifth aspect the invention relates to a method of producing a freestanding, pre-lithiated polyphenylene sulfide (PPS) cellulose nanofibre composite film according to the method of the third or fourth aspects of the invention, the method further comprising the steps of:

[0065] - laminating a film of porous polyolefin to the PPS cellulose nanofibre film by means of a glue including a ceramic nanolayer.

[0066] In a sixth aspect, the invention relates to a system for carrying out the method according to the third, fourth or fifth aspects of the invention, the system comprising: :

[0067] - primary storage tanks for metal chloride, deionized water, tetrachloro-p- benzoquinone (TCBQ), PPS powder and optionally cellulose fibre,

[0068] - a first mixer for mixing content from the primary storage tanks into a mixed slurry;

[0069] - a reactor for hydrothermal treatment of the mixed slurry;

[0070] - a centrifuge for washing and filtering the hydrothermally treated slurry;

[0071] - means for drying the filtered solution to disintegrate PPS powder;

[0072] - secondary storage tanks for disintegrated PPS powder, polytetrafluoroethylene (PTFE) powder and optionally cellulose nanofibres (if not included in the primary storage tanks);

[0073] - a second mixer for mixing content from the secondary storage tanks;

[0074] - a grinder for grinding the mixed solution in compressed air; - one or more pairs of rollers for compacting the grinded solution into a film;

[0075] - a hot calendaring machine through which the compacted film is passed to obtain a free-standing, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film; and optionally

[0076] - a laminator to add a film of porous polyolefin to the free-standing, pre- lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film by means of a ceramic nanolayer glue to obtain a solid state battery separator.

[0077] Below follows an exemplary implementation of a method according to the third aspect of the invention in the form of a continuous manufacturing process for the large-scale production of the PPSCC layer of a solid separator disclosed herein. This exemplary process encompasses several stages: a) Mixing of powders to achieve a uniform blend. b) Pre-treating mixed powders in an autoclave reactor, preparing it for subsequent stages. c) Filtering pre-treated, mixed powders using a centrifuge to separate and refine the components. d) Drying the filtered, mixed powders in a fluidized bed reactor for optimal moisture content. e) Final, thorough mixing of pre-treated Polyphenylene Sulfide (PPS) powder with celluslose nanofibres and PolyTetraFluoroEthylene (PTFE) powder to ensure even distribution. f) Conveying the final, mixed solution into a high-speed shearing machine using preheated compressed air, with the airflow rate reaching supersonic speeds to fiberize the powder. g) Extruding the fiberized powder through a roller compactor to form pellets. h) Processing the pellets through an automatic multiple rollers machine, which involves sequential cold and hot calendering / rolling steps to finalize the self-supporting PPSCC film.

[0078] The outcome of this exemplary process is a substantially defect-free, freestanding film with a thickness ranging from 5 to 45 pm, typically 10-30 pm and a width varying between 5 to 160 cm. This innovative manufacturing ap- proach ensures high-quality, consistent production of solid separators suitable for large-scale applications.

[0079] As used herein, the terms of "lithium battery" and "sodium battery", refer to all types of Li and Na batteries known as, but not limited to, rechargeable or secondary Li, Na and Zn ion batteries, metal batteries etc.

[0080] In the following is described an example of an embodiment illustrated in the accompanying drawings, wherein:

[0081] Fig. 1 shows a schematic system according to the sixth aspect of the invention.

[0082] Any positional indications refer to the position shown in the figures.

[0083] In the figures, same or corresponding elements are indicated by same reference numerals. For clarity reasons, some elements may in some of the figures be with-out reference numerals.

[0084] Fig. 1 shows an exemplary and highly schematic system 1 according to the sixth aspect of the invention. The system 1 as shown is adapted to produce a solid state separator according to the first aspect of the invention by means of methods according to the fourth and fifth aspects of the invention.

[0085] A plurality of storage tanks / dispensing pods is provided, where a first storage tank 2 includes pre-lithiated PPS powder, a second storage 4 tank includes tetrachloro-p-benzoquinone (TCBQ), a third storage tank 6 includes metal chloride (xCI), a fourth storage tank 8 includes deionised water (DIW) and a fifth storage tank 10 includes cellulose nanofibres (CeINF). It should be noted that is this specific embodiment, the CeINF is added and mixed into the solution from the start, rather than at the later fibrillation step.

[0086] One of more first dispensing pumps 12 are provided to dispense content from the various tanks 2-10 at desired amounts into a first resonance acoustic mixer (RAM) 14, with a not shown grinding medium. The grinding medium may in certain embodiments be Teflon-coated balls, such steel balls, or ceramic balls. A first RAM dispensing pump 15 is provided to move the mixed slurry from the RAM 14 into a closed reactor 16, here in the form of an autoclave, for hydro- thermal treatment. The autoclave 16 is provided with not shown means for controlling in- and outfeed. From the autoclave 16, the mixed slurry is further supplied to a peeler centrifuge 18, via an autoclave dispensing pump 17. In the peeler centrifuge 18, the slurry is washed to a neutral pH and filtered to a "cake". By means of a centrifuge dispensing pump 20, the filtered cake is transferred to fluidized bed reactor 22 to disintegrate agglomerated particles, now in the form on PPS powder with CeINF. Via a fluidized bed dispensing pump 24, the PPS powder with CeINF is fed to a sixth storage tank 26 in which it is mixed with xCI. A seventh storage 28 tank includes PTFE powder. Via cold room dispensing pumps 30, 32 content from the sixth and seventh storage tanks 26, 28, respectively, are sucked into a second RAM 33. The second RAM 33, sixth and seventh storage tanks 26, 28 and the cold room dispensing pumps 30, 32 are provided in a cold room 34, implying that the PTFE powder is in glass state.

[0087] The mixed solution from the second RAM 33 is fed to a jet mill 36 via a second RAM dispensing pump 37, together with compressed air from a rotary screw air compressor 38. The air is pharmaceutical grade dry compressed air according to the standard ISO 8573-1 and class 1-2-1 (particles-water-oil). The rotary screw air compressor 38 receives air from the fluidized bed reactor via a cooler 40. During the air jet milling process a size reduction of PPS powder particles takes place due to grinding combined with shear forces, whereby PTFE molecular chains extend and open to form fibres. The PTFE fibres form a net-like binding structure on the surface of the PPS particles, giving them a cotton candy-like powder appearance. The cotton candy-like powder is collected by a gas filter.

[0088] The powder from the air jet mill 36 is dispensed via a jet mill dispensing pump 42 into a screw feeder array 44. The screw feeder array 44 feeds the powder, now in the form strips / ribbons to a sequence of vertical gap-controlled cold rollers at room temperature. In the shown embodiment, the sequence includes three primary pairs of rollers 46 operating in parallel and feeding the resulting film 50 to a secondary pair of rollers 48, generating a film with a thickness in the order of 1 mm. The resultant, rather thick film, is then passed through a plurality of horizontally arranged rollers 52 with gradually reduced gaps between them and at temperature of approximately 50°C. The resulting film after the horizontal rollers has a thickness in the order of 0.5 mm and is substantially free of defects.

[0089] After the horizontal rollers 52, the film 50 is passed to a hot calendaring machine 54 at approximately 130°C. A pressure between rollers 56 of the calendaring machine 56 is set to 45 kN, corresponding to a line load of 360N / mm. The friction between the rollers 56 is 10% and the speed 2.5 m / min. Friction is defined as friction in the roller bearings and may be controlled by individually controlling the speed of the rollers 56.

[0090] The final PPSCC film 50 is then trimmed in an edge trimming machine 57 before a ceramic nanolayer glue, stored in an eight storage tank 58, is applied to the film in a glue applicator machine 60. Glue is dispensed from the eight storage tank 58 by means of a glue dispensing pump 62. The film 50 with the glue is then laminated to a PE / PP film 64 in a laminator 66. The PE / PP film 64 is stored on a reel 68. The resultant film 3 passes through a slitting array 70 before it is stored on a storage reel 72 in its desired length and width.

[0091] The process is executed and controlled from a control unit 74.

[0092] Further examples of methods according to the invention and reference examples are presented in the following. In particular, Examples 1, 3 and 7-8 correspond to methods according to various aspect of the invention, while examples 2, 4-6 and 9 are included for reference.

[0093] Example 1 :

[0094] This embodiment provides a process for fabricating a multifunctional solid Li battery separator, which includes the following steps:

[0095] (1) Powder mixture

[0096] Dissolved metal chloride(xCI) in deionized water (DIW), tetrachloro-p- benzoquinone (TCBQ) and polyphenylene sulfide (PPS) powder are mixed and milled in resonance acoustic mixer with grinding medium. The mixed slurry is thereafter transferred into a closed reactor for hydrothermal reaction. After the reaction, the powder is washed to a neutral pH and filtered in vertical peeler centrifuge. The filtered cake is dried in a fluidized bed dryer to disintegrate the agglomerated particles which is then PPS powder.

[0097] (2) Fibrillation

[0098] Pre-lithiated PPS powder, Cel NF, and PTFE powder are mixed according to the weight percentage of 91%: 3%: 6% in a resonance acoustic mixer evenly without shear forces to powder, and the mixing process is carried out under the temperature condition that polytetrafluoroethylene is in a glass state.

[0099] The preheated pharmaceutical grade dry compressed air according to the standard ISO 8573-1 and class 1-2-1 (particles-water-oil) from a rotary screw air compressor is injected into a spiral air jet mill.

[0100] The powder is introduced into the air jet mill grinding chamber through a feeder with high-speed dry compressed air. During the air jet milling process a size reduction of PPS powder particles takes place due to grinding combined with shear forces, whereby PTFE molecular chains extend and open to form fibres. The PTFE fibres form a net-like binding structure on the surface of the PPS particles, giving them a cotton candy-like powder appearance. The cotton candy-like powder is collected by a gas filter.

[0101] (3) Film making:

[0102] The powder then enters roller pre-com pacters through a feeding mechanism to make thick strips or ribbons. These strips or ribbons are then directly fed into multiple sequential vertical automatic gap-controlled cold rollers at room temperature to generate a film with a thickness from 1.0 to 1.5mm.

[0103] The resultant thick film is then passed through the multiple horizontal rollers which is controlled by automatic constant force and speed at a friction of 15% and at a temperature of 50°C. The gaps of the multiple rollers are 1000, 900, 700, 600, and 500 pm, respectively. The thickness of the final defect-free film was 520 pm. The resultant film is passed to hot horizontal calendering which is controlled by a constant force and speed at a friction of 10% temperature of 130° C. The film thickness after the hot calendering was 15 pm. This thickness was obtained with a roll pressure of 45 kN, corresponding line load of 360 N / mm width, and at a speed of 2.5 m / min. When upscaling production, higher speeds should be used, such as in the order of 100 m / min or even higher.

[0104] The produced separator was then cut into the desired widths and the outer edges were cut off to make a separator including a PPS layer with cellulose nanofibres.

[0105] (4) PE layer Lamination

[0106] To make prepare the solid separator for use in Li and NA batteries, a polyolefin layer is attached to the PPS layer to prevent thermal runaway and enhance the mechanical strength.

[0107] A PE layer or PP layer is laminated to one side of the PPS layer to increase the mechanical strength of the solid separators. The PE layer or PP layer was adhered to the PPS with ceramic glue with alumina colloids. The laminated layer was finally dried in air at a temperature of 120 °C. The ceramic glue becomes a coating layer of AI2O3.

[0108] The conductivity of lithium ions was determined using a multichannel constant potential instrument (PARSTAT MC, AMETEK), employing electrochemical methods. The measured lithium ion conductivity was found to be 2.5 x 10^-4 S / cm2. The tensile strength of the films was evaluated in accordance with the ASTM D882 standard test method, resulting in a measured tensile strength of 19.4 Mpa.

[0109] The thermal stability of the multilayer film was tested at 130°C. The PE layer was melted, but the PPS layer was kept in its original dimension, Demonstrating its prevention of thermal runaway and thereby increased safety in solid state batteries.

[0110] The porosity measured by N2 adsorption was 3%. The gas tightness of the film was measured by the ASTM F2622 standard test method. No gas permea- bility was observed. It suggests that the nonporous solid separator is obtained with a small fraction of closed pores.

[0111] Example 2:

[0112] All the steps are similar to one in Example 1, except that the fibrillation process only PPS powder and PTFE powder are mixed according to the weight percentage of 94%: 6%, i.e. without the cellulose fibers.

[0113] It has been shown that it is difficult to make a film with a thickness from 1.0 to 1.5 mm by the first step of the film-making process in Example 1. Several times rounds of pre-compacting through a feeding mechanism to make thick strips or ribbons may be useful in that regard.

[0114] The measured lithium-ion conductivity was found to be 2. lx 10^-4 S / cm / K2.

[0115] The tensile strength was 14.1 Mpa.

[0116] Example 3:

[0117] All the steps similar to Example 1, except that in this embodiment there was no PE layer Lamination step whereby the final product includes only the PPSCC freestanding film, without the PE lamination layer. The PPSCC film has a 5.1 Mpa tensile strength, which is much lower than 19.1 Mpa tensile strength of the multilayer structure. This demonstrates the impact of the third and second layers on the tensile strength of the multilayer structure.

[0118] Example 4:

[0119] All the steps are similar to the ones in Example 2, except that in this embodiment there was no PE layer Lamination, whereby the final product includes only the PPS freestanding film, without the PE lamination layer and without the cellulose nanofibres. The free-standing PPS film without cellulose nanofibre and the PE lamination layer has tensile strength of 0.4 Mpa., The tensile strength is significantly lower than in the previous examples, making the winding of a thin PPS film very difficult.

[0120] Example 5: All the steps are similar to the ones in Example 2, except that in this:

[0121] The resultant 520 pm thick film was passed to the hot horizontal calendering which is controlled by a constant force. The average film thickness of 7, 11 and 19 pm was obtained at the roll pressure of 30 kN (rather than 45kN as in Example 2), a friction of 10% at 130° C, and a speed of 0.5, 1.5 and 2.5 m / min, respectively, demonstrating the effect of the roll pressure and speed on the final thickness of the PPS film.

[0122] Example 6:

[0123] All the steps are similar to one in Example 2, except that in this:

[0124] The resultant 520 pm film was passed to the hot horizontal calendering which is controlled by a constant gap, in contrast to constant force in the previous Examples A PPS film thickness of 40-50 pm was obtained at the roll gap of 10 pm, A speed of 0.5 m / min, a friction of 10%, and the surface temperature of the roller at 130° C, demonstrating that it may be difficult to make the final PPS film sufficiently thin if controlling by a constant gap instead of force.

[0125] Example 7:

[0126] All the steps were similar to the ones in Example 1, except that in this embodiment:

[0127] The powder entered the powder rollers pre-compacter through a feeding mechanism to make thick strips / ribbons, and these ribbons are then directly fed into multiple sequential vertical automatic gap-controlled rollers at 50 °C, as in Example 1, to generate a thick film ranging from 1.0 to 1.5mm.

[0128] Pin holes were observed on the final 15 pm after the hot calendering, which demonstrates the importance of performing the pre-compacting at room temperature.

[0129] Example 8:

[0130] All the steps are similar to the ones in Embodiment 1, except that in this embodiment the PPS powder was pre-sodiated: The procedure is similar to Embodiment 1 step 2, except that, in this embodiment, polyphenylene sulfide powder containing 600 ppm of Na, cellulose fibre, and PTFE powder were mixed according to the weight percentage of 91%: 3%: 6%. The measured porosity was 9%. The pores were filled with NaBF4 electrolytes, and the measured Na ion conductivity was found to be lx 10^-4 S / cm2.

[0131] Example 9:

[0132] All the steps are similar to one in Embodiment 1, except that in this embodiment for the fibrillation process was for fabricating a separator for a Zn battery:

[0133] The procedure is similar to Embodiment 1 step 2, except that, in this embodiment, pre-zincified polyphenylene sulfide powder, cellulose fibre, and PTFE powder are mixed according to the weight percentage of 91%:3%:6%. The measured porosity was 6%. The pores were filled with IM Zn(CF3SO3)2 electrolytes and the measured Zn ion conductivity was found to be 5*10-3S / cm-1. A person skilled in the art will understand that the figures are just principal drawings. The relative proportions of individual elements may also be distorted.

[0134] It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise", and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

[0135] The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

C l a i m s1. Solid state separator for a battery, the solid state separator comprising:- a first layer including a pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre (CeINF) composite free-standing film;- a second layer including a porous polyolefin film; and- a third layer including a ceramic nanolayer provided in-between the first and second layers.

2. Solid state battery separator according to claim 1, wherein the second layer includes polyethylene (PE) and / or polypropylene (PP).

3. Solid state battery separator according to claim 1 or 2, wherein the thickness of the second layer is in the range 3-15 pm, preferably 5-10 pm.

4. Solid state separator according to any one of the preceding claims, wherein the first layer includes PPS, CeINF, and polytetrafluoroethylene (PTFE) powder with a weight ratio of 89-97%: 0.1-5%: 3-8%, preferably 90-94%: 1-3%: 4-6%, respectively.

5. Solid state separator according to any one of the preceding claims, wherein the ceramic nanolayer has a thickness in the range 30-300 nm.

6. Solid state battery separator according to any of the preceding claims, wherein the first layer has a thickness of 5-45 pm, preferably10-30 pm, and even more preferably 15-25 pm.

7. Solid state battery separator according to any one of the preceding claims, wherein the first layer has a porosity lower than 10%, preferably lower than 3%.

8. Solid state battery separator according to any one of the preceding claims, wherein the third layer includes one or more of the following:- alumina,- a Li-rich ceramic, such as Li salts, Li rich cathode materials, lithiumphosphorous Oxynitride (UPON), or lithium titanate,- a highly porous material, such as carbon nanospheres or metalorganic framework (MOF) materials.

9. Solid state separator according to any one of the preceding claims, where in the separator has a thickness in a range of 10-60 pm, preferably 15-30 pm, and even more preferably 20-25 pm.

10. Solid state separator according to any one of the preceding claims, where in the solid state separator has a width in the range of 5 -160 cm.

11. Solid state battery including a solid state separator according to any one of the preceding claims.

12. Solid state battery according to claim 11, wherein the solid state battery is a Li- or Na-ion battery.

13. A method for producing a free-standing, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film, the method including the steps of:- mixing and milling metal chloride dissolved in deionized water (DIW), tetrachloro-p-benzoquinone (TCBQ) and PPS powder to form a mixed slurry;- transferring the mixed slurry to a reactor for hydrothermal treatment;- washing and filtering the hydrothermally treated mixture in a centrifuge;- drying the filtered solution to disintegrate PPS powder;- mixing the disintegrated PPS powder with cellulose fibres and polytetrafluoroethylene (PTFE) powder at a temperature at which the PTFE powder is in a glass state to obtain a mixed solution;- griding the mixed solution in dry, compressed air;- compacting the grinded, mixed solution through one or more rollers to obtain a film; and- passing the film through a hot calendering machine to obtain the free-standing, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film.

14. A method for producing a free-standing, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film, the method including the steps of:- mixing and milling metal chloride dissolved in deionized water, tetrachloro-p-benzoquinone (TCBQ), PPS powder and cellulose fibres to form a mixed slurry;- transferring the mixed slurry to a reactor for hydrothermal treatment;- washing and filtering the hydrothermally treated mixture in a centrifuge;- drying the filtered solution to disintegrate PPS powder;- mixing the disintegrated PPS powder with polytetrafluoroethylene (PTFE) powder at a temperature at which the PTFE powder is in a glass state to obtain a mixed solution;- griding the mixed solution in dry, compressed air;- compacting the grinded, mixed solution through one or more rollers to obtain a film; and- passing the film through a hot calendering machine to obtain the freestanding, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film.

15. The method according to claim 13 or 14, wherein the metal chloride is one or more of the following:- lithium chloride;- sodium chloride;- magnesium chloride; and aluminum chloride.

16. The method according to any one of claims 13-15 wherein the PPS powder is a linear chain and end cross-linked semi-crystalline PPS powder, with a degree of crystallinity > 60% in the range 1-25 pm, preferably 1- 5 pm.

17. The method according to any one of claims 13-16, wherein the mass ratio of metal chloride, DIW and TCBQ to the total mass of PPS is substantially 1 :30:5: 100.

18. The method according to any one of claims 13-17, wherein the hydro- thermal treatment is performed at a reaction temperature of approximately 210 °C, for two hours and a vapour pressure of 60-70 bar.

19. The method according to any one of claims 13-18, wherein the mixing of PPS powder with cellulose fibres and PTFE takes place at a temperature of 45-50°C.

20. The method according to any one of claims 13-19, wherein the griding is performed in an air jet mill pre-heated by compressed air to a temperature of 40-50 °C.

21. The method according to any one of claims 13-20, wherein the step of compacting the grinded, mixed solution through one or more rollers includes producing a film with a thickness of 1.0-3.0 mm, preferably 1.0- 1.5 mm.

22. The method according to any one of claims 13-21, wherein the method, prior to passing the film through the hot calendering machine, includes the step of cold-rolling the film at a roller temperature of 40-70 °, preferably 45-55°, to obtain a film with a thickness of 0.3-0.5 mm.

23. The method according to any one of claims 13-22, wherein the hot calendering machine has a roller temperature of 120-140°C, preferably 1250-130°C.

24. The method according to claim 23, wherein friction in rollers of the calendaring machine is controlled in a range of 1-20%, preferably 8-12%, while the line load is in a range of 500-250 N / mm, preferably 350-370 N / mm,25. Method of producing a solid state separator for a battery, the method including producing a free-standing, pre-lithiated or pre-sodiated poly-phenylene sulfide (PPS) cellulose nanofibre composite film according to the method of any one of claims 12-24, the method further comprising the steps of:- laminating a film of porous polyolefin to the PPS cellulose nanofibre film by means of a glue including a ceramic nanolayer.

26. System for carrying out the method of claim 25, the system comprising:- primary storage tanks for metal chloride, deionized, water tetrachloro- p-benzoquinone (TCBQ), PPS powder and optionally cellulose fibre,- a mixer for mixing content from the primary storage tanks into a mixed slurry;- a reactor for hydrothermal treatment of the mixed slurry;- a centrifuge for washing and filtering the hydrothermally treated slurry;- means for drying the filtered solution to disintegrate PPS powder; -secondary storage tanks for disintegrated PPS powder, polytetrafluoroethylene (PTFE) powder and optionally cellulose nanofibres (if not part of the primary storage tanks);- a mixer for mixing content from the secondary storage tanks;- a grinder for grinding the mixed solution in compressed air;- one or more pairs of rollers for compacting the grinded solution into a film;- a hot calendaring machine through which the compacted film is passed to obtain a free-standing, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film; and optionally- a laminator to add a film of porous polyolefin to the free-standing, pre-lithiated or pre-sodiated polyphenylene sulfide (PPS) cellulose nanofibre composite film by means of a ceramic nanolayer glue to obtain a solid state battery separator.

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