Lithium ion separation membrane and associated method

Abstract

The present disclosure relates to a method of producing a dense inorganic lithium-ion selective membrane, preferably for use in electro-separation systems, comprising: forming an initial body comprising at least one solid-state electrolyte material which is selectively permeable to lithium ions and at least one inorganic filler; sintering the initial body to densify the at least one solid-state electrolyte material to form a sintered body, as well as membranes made by said method.

Description

LITHIUM ION SEPARATION MEMBRANE AND ASSOCIATED METHODTechnical Field

[0001] The present disclosure relates to a lithium ion separation membrane, and a method of forming same, in particular for use in electro-separation systems, as well as a method for recovering lithium by electrodialysis using said membrane.Background of the Disclosure

[0002] Global demand for lithium has increased rapidly over the past couple decades, with 540,000 metric tons of lithium produced worldwide in 2021. This demand is expected to continue to grow, with the World Economic Forum predicting that the global demand would surpass 3 million metric tons by 2023. This demand is primarily driven by advancements in electric vehicle and battery technology, both of which depend on lithium. There is growing concern that the supply of lithium will be unable to keep up with increasing demand.

[0003] Most commonly, lithium carbonate (LiiCOs) and lithium hydroxide (LiOH) are used for making lithium-ion battery cathodes. Currently, the majority of lithium carbonate and lithium hydroxide is produced from either minerals or lithium-rich brines. When produced from minerals such as spodumene, the mineral is mined, and then a series of crushing, roasting, milling, acid leaching, solvent extraction, and chemical precipitation processes follow. Lithium-rich brines are extracted from underground and evaporated repeatedly over a series of months to concentrate the brine, before a series of purification and chemical treatments are carried out. The energy cost and environmental impact of both these production routes are vast, and there is a need for alternative methods of lithium production or extraction to meet the growing demand without the environmental impact. Furthermore, these processes are limited to regions where there are either large deposits of lithium-bearing ores or the climate allows for open air evaporation over months.

[0004] Electro-separation of lithium from solutions has attracted some interest as a potential way to address these problems. Seawater remains a largely untapped resource of lithium, and electro-separation may also provide avenues for the recovery of lithium from lithium battery recycling processes. Electro-separation processes such as electrodialysis rely on applying a current to an ion exchange membrane in order to drive the transport of ions through the membrane. The ion exchange membrane is typically a porous film which enables the passage of ions through the channels formed by the pores. Existing electro-separationattempts for lithium have only ineffectively separated out lithium, require additional stages, and / or are unsuitable for scaling up to industrial scale production.Summary of the Invention

[0005] According to a first broad aspect, there is provided a method of producing a dense inorganic lithium-ion selective membrane, comprising: forming an initial body comprising at least one solid-state electrolyte material which is selectively permeable to lithium ions and at least one inorganic filler; sintering the initial body to densify the at least one solid-state electrolyte material to form a sintered body.

[0006] In some embodiments, the at least one inorganic filler is in an amount between 0.1 wt% to 2.5 wt%.

[0007] In some embodiments, the sintering of the initial body is carried out at a heating rate below about 5°C / min.

[0008] In some embodiments, the sintering of the initial body is carried out at a temperature between about 500°C and about 1500°C.

[0009] In some embodiments, the sintering is carried out in a series of heating steps.

[0010] In some embodiments, the heating steps are a series of unequal steps.

[0011] In some embodiments, the method further comprises subjecting the initial body to a pressure treatment prior to sintering.

[0012] In some embodiments, the method further comprises subjecting the initial body to a pressure treatment concurrently with sintering.

[0013] In some embodiments, the pressure treatment is carried out at a pressure of between about 10 MPa and 700 MPa.

[0014] In some embodiments, the at least one solid-state electrolyte material for forming the initial body is a powder with a first fraction of the powder having a smaller nominal particle size relative to a second fraction of the powder.

[0015] In some embodiments, the first fraction of the powder has a particle size in the range of about 0.2 pm to about 0.6 pm.

[0016] In some embodiments, the second fraction of the powder has a particle size between about 1 pm and 10 pm.

[0017] In some embodiments, the powder includes a third fraction with a larger particle size relative to both the first and second fractions of the powder.

[0018] In some embodiments, the third fraction has a particle size between about 50 and 150 pm.

[0019] In some embodiments, the at least one inorganic filler is a ceramic filler selected from one or more of: bentonite, aluminosilicate, silicate, alumina, titania, a low-melting point glass such as borosilicate or vanadate glass, hydroxyapatite, or a nanomaterial selected from one or more of graphene or boron nitride nanotubes.

[0020] In some embodiments, the method further comprises applying a surface treatment to at least one surface of the membrane to limit water ingress into the membrane, dehydrate hydrated ions, alter the surface charges on the membrane, and / or to increase hydrophobicity.

[0021] In some embodiments, the surface treatment comprises applying a coating or layer of at least one of: cellulose, PVA, PAN, PEO, graphene, titania, silver, silica, alumina, copper, zeolite, hydroxyapatite, or a zwitterionic compound.

[0022] In some embodiments, the surface treatment comprises a mechanical polishing of at least one surface of the membrane.

[0023] In some embodiments, the method further comprises applying a pattern to at least one surface of the membrane to control the flow profile across the patterned surface.

[0024] In some embodiments, the initial body is formed on a support substrate.

[0025] In some embodiments, the substrate is made of the solid-state electrolyte material or a film of woven or non-woven synthetic fabric, a porous rigid polymer, a porous ceramic, a woven ceramic, a porous graphite, or woven carbon fibres.

[0026] In some embodiments, the method further comprises locating the sintered body between layers of a porous separator material which is selective to monovalent ions.

[0027] In some embodiments, the method further comprises locating the sintered body between layers of a dehydrating material.

[0028] In some embodiments, the method further comprises adding at least one layer of a structural material

[0029] In some embodiments, the structural material is a porous glass fibre composite material.

[0030] In some embodiments, the sintered body is located in a rigid frame.

[0031] In some embodiments, the rigid frame is a porous glass fibre composite material.

[0032] In some embodiments, the rigid frame is a dense crystalline, glass ceramic, or glassy phase lithium-ion conductive solid-state electrolyte.

[0033] According to a second aspect, there is provided a dense lithium-ion selective membrane for electro-separation, comprising: an active layer comprising a densified network of a solid-state electrolyte material which is selectively permeable to lithium ions and at least one inorganic filler; the densified network of the solid-state electrolyte material allowing the selective transport of lithium ions through the membrane via vacancy, interstitial, and / or interstitial-substitutional exchange mechanisms.

[0034] In some embodiments, the inorganic filler is present in an amount between 0.1 wt% to 2.5 wt%.

[0035] In some embodiments, the active layer has at least one coating to limit water ingress into the membrane, dehydrate hydrated ions, alter the surface charges on the membrane, and / or to increase hydrophobicity.

[0036] In some embodiments, the at least one coating is selected from one or more of: cellulose, PVA, PAN, PEO, graphene, titania, silver, silica, alumina, copper, zeolite, hydroxyapatite, or a zwitterionic compound.

[0037] In some embodiments, the active layer further comprises a mechanically polished surface.

[0038] In some embodiments, the active layer includes a patterned layer for controlling the flow profile across the membrane surface.

[0039] In some embodiments, the active layer is supported by a substrate.

[0040] In some embodiments, the substrate is made of the solid-state electrolyte material or a film of woven or non-woven synthetic fabric, a porous rigid polymer, a porous ceramic , a woven ceramic, a woven graphite, or woven carbon fibres.

[0041] In some embodiments, the membrane has an ionic conductivity of at least 1 x 10’4S cm1.

[0042] In some embodiments, the membrane further comprises layers of a porous separator material, and wherein the active layer is located between layers of the porous separator material, wherein the porous separator material is selective to monovalent ions.

[0043] In some embodiments, the membrane further comprises layers of a dehydrating material, and wherein the active layer is located between layers of the dehydrating material.

[0044] In some embodiments, the membrane further comprises porous structural layers.

[0045] In some embodiments, the structural layers are a porous glass fibre composite material.

[0046] In some embodiments, the active layer is located within a rigid frame.

[0047] In some embodiments, the rigid frame is a porous glass fibre composite material.

[0048] In some embodiments, the rigid frame is a dense crystalline, glass ceramic, or glassy phase lithium-ion conductive solid-state electrolyte.

[0049] In some embodiments, the membrane is formed by a method according to the first aspect.

[0050] According to a third aspect, there is provided the use of a membrane according to the second aspect in a lithium-ion separation technique.

[0051] In some embodiments, an electric potential is applied to the membrane.

[0052] In some embodiments, the separation technique is electro-dialysis.

[0053] According to a fourth aspect, there is provided a method of recovering lithium from an aqueous feed solution containing lithium ions by electrodialysis, comprising: drawing at least a portion of the lithium ions from the feed solution through a membrane to form an aqueous product solution, wherein the membrane comprises at least one solid state electrolyte material which is permeable to lithium ions, and an inorganic filler.

[0054] In some embodiments, the inorganic filler is present in an amount between 0.1 wt% and 2.5 wt%

[0055] Other aspects, features, and advantages will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, which are a part of this disclosure and which illustrate, by way of example, principles of the inventions disclosed.Brief Description of the Figures

[0056] The present disclosure will become better understood from the following detailed description of various non-limiting embodiments thereof, described in connection with the accompanying figures, wherein:

[0057] FIGURE 1 shows an illustration of packing densities for a solid state electrolyte material prior to sintering.

[0058] FIGURE 2 shows a stylized illustration of a membrane structure according to an embodiment of the present invention.

[0059] FIGURE 3 shows an illustration of a cross-section of a membrane according to an embodiment of the present invention.

[0060] FIGURE 4 shows examples of membranes according to embodiments of the present invention with different ceramic fillers.

[0061] FIGURE 5 shows the XRD spectra for amorphous silica filler under different processing conditions.

[0062] FIGURE 6 shows a schematic diagram of an embodiment of a membrane according to the present invention.

[0063] FIGURE 7 shows SEM imagery of an embodiment of a membrane according to the present invention.

[0064] FIGURE 8A and 8B show example embodiments of membranes following use in electrodialysis.

[0065] FIGURE 9 shows the XRD spectra for a series of different LATP materials.Detailed Description

[0066] The present invention relates to a stable, selective, lithium-ion conductive inorganic membrane for use in liquid systems such as liquids with an aqueous component. The membrane may be particularly suited to the selective lithium ion capture and transfer from a liquid feed stream to a product stream, and even more particularly suited for electric field- driven lithium separation processes. In some embodiments, the membrane may be for use with electrodialysis applications, where a feed stream is passed through one or more membranes in series under the influence of an electric voltage in order to produce a product stream with a high concentration of lithium ions.

[0067] The membrane is formed of a dense solid-state electrolyte material, in particular an inorganic solid electrolyte, which is selectively permeable to lithium ions and an inorganic filler. Otherwise stated, the membrane is formed from an inorganic solid electrolyte and inorganic filler, without any organic materials. This is in contrast to many conventional membranes, which pair a solid-state electrolyte with, among other additives, at least one organic polymer. The addition of organic polymers results in a number of disadvantageous effects, of which the present membrane avoids. Organic polymers limit the temperature to which the membrane can be heated, preventing sintering and densification of the solid-state electrolyte network. Membranes incorporating polymers also lose efficiency due to the poor adhesion of the polymer to ceramic particles, causing channels and pores through which ions can bypass the solid state electrolyte. There are also environmental concerns, with legislation in many jurisdictions banning the use of fluorinated polymers which are commonly used in these membranes.

[0068] Preferred solid-state electrolytes include NASICON, LISICON, perovskite, phosphate, or lithium- stuffed garnet-type materials. Especially preferred examples of solid- state electrolytes include but are not limited to Lithium Aluminium Titanium Phosphate (LATP), Lithium Lanthanum Titanate (LLTO), and Lithium Lanthanum Zirconate (LLZO). The key function of suitable solid- electrolytes is that they have mobile point defects in the material structure, enabling the migration of specific ions (in this case, lithium) through the material by vacancy, interstitial, or interstitial-substitutional exchange mechanisms. This is in stark contrast to conventional ion exchange membranes, which instead rely on the transport of ions through channels formed in the porous material of the membrane, and results in a higher ion selectivity. For convenience, these suitable solid-state electrolytes with these lithium selective properties will be referred to simply as solid-state electrolytes throughout this specification.

[0069] Solid-state electrolytes are known in the field of energy storage devices such as lithium-ion batteries. In these applications, the solid-state electrolytes are used as separators to prevent short-circuits. In these applications, the separator materials are typically in a porous form, and are rarely subjected to the atmosphere, water vapour, or aqueous solutions. In fact, the environments they operate in are kept free from these conditions in order to avoid material degradation or the formation of flammable or hazardous gases. These separators, accordingly, are completely unsuited to the environment in which the present membrane is intended for working in, where the electrolyte is exposed to aqueous solutions for extended periods of time. One particular issue when the aqueous solution is a lithium-rich brine or similar lithium source is the presence of Na+ions in the aqueous solution. These ions are known to cause rapid degradation of the membrane.

[0070] In order for a membrane to be suited for use in these aqueous environments for extended periods of time, the membrane must be liquid or watertight to prevent mixing of the feed and product streams, and must not leak, foul, erode, or scale over the lifespan of the membrane. It must also be resistant to degradation when Na+ions are present in the aqueous environment. This is in part achieved by the present dense inorganic solid-state electrolyte membrane.

[0071] The membrane is produced by forming an initial body, (which may be referred to as a green body) comprising at least a solid-state electrolyte material and at least one inorganic filler, and then subjecting the initial body to a sintering process or heat treatment, which causesthe material to undergo a number of beneficial changes, primarily densification, the resultant membrane showing improved transport of lithium ions across the membrane, as the densification results in reduced grain boundaries between aligned crystallites of the solid state electrolyte. Additionally, the densification results in greater mechanical strength and fewer voids or defects, increasing the suitability of the formed membrane for aqueous applications. The sintering process is possible because the membrane does not include any organic polymers, unlike conventional membranes, which would not survive the temperatures required to densify the membrane.

[0072] The at least one inorganic filler is preferably an inorganic ceramic filler, and preferably has similar or comparable thermochemical properties to the solid-state electrolyte so as to withstand the pressure and sintering process. The ceramic filler may induce a number of beneficial effects in the resultant membrane, for example, improving the interfacial and hardness properties of the membrane surface, increasing the scratch and indentation resistance of the membrane, and / or preventing or reducing erosion / degradation of the membrane during use in an electro-separation application. The addition of the ceramic filler may also act to fill pores and voids between particles of the solid-state electrolyte. These may provide an improved water-tightness of the membrane, and reduce or prevent fouling or scaling at the membrane surface. The at least one ceramic filler may also be chosen to improve the density of the resultant membrane, and / or to reduce the porosity and liquid tightness of the membrane. In preferred embodiments, the inorganic ceramic filler is mixed with the solid-state electrolyte to incorporate it within the crystal structure of the electrolyte. This may include mixing the filler and solid-state electrolyte and then melting and crushing the resultant mixture. In other embodiments, the filler may be milled with the electrolyte to form a mixture. In further embodiments, the filler may be uniformly and homogeneously dispersed as particles into particles of the solid-state electrolyte.

[0073] A known issue with heat treating solid-state electrolytes is that the electro-chemical properties required for use as a membrane can easily decline depending on a number of factors. Improper heat treatment can result in issues such as unfavourable grain size distribution, increased grain boundaries, increased porosity, microcracking, and the presence of unwanted secondary phases at grain boundaries. Lithium loss may also occur from the material when kept at an elevated temperature. Control over these parameters of the sintering process are thus important in avoiding these issues.

[0074] The temperature and duration of the sintering process are controlled to influence the grain size and diminish the thickness of grain boundary regions. The sintering process is preferably carried out on the solid-state electrolyte material at a temperature between 500°C and 1500°C in order to encourage rearrangement of crystallites and grain growth. If the membrane is heated too quickly, fast grain growth may occur , so microcracks, pores, and sites of local stress may form. As a result, the heating rate is preferably 5°C / min or less. A preferred heating rate is l°C / min. The hold / dwell time for the sintering treatment is chosen to control the rate of grain growth and to avoid the formation of microcracks or pores. The sintering process is preferably carried out with a hold time of not more than 12 hours, however as the reactive material is lost at elevated temperatures, the hold time is preferably less than 4, and even more preferably less than 1 hour. In some embodiments, the heating further comprises intermediate hold times, to facilitate crystal growth. For example, the temperature may be heated at l°C / min up to a temperature of 750°C, and then held at that temperature for 60 minutes before continuing heating to a second temperature and being held at that temperature for the rest of the sintering step.

[0075] To further reduce the adverse effects during the sintering process, the process is preferably carried out in air, or otherwise in a controlled atmosphere with around 20% oxygen. Air is preferred to maintain an oxidising environment. This may maintain ceramic oxide phases within the membrane rather than reduced phases of the material, which can cause imperfections and losses in ionic conductivity. In other embodiments, an inert atmosphere, such as argon, is used to suppress side reactions and material degradation, and to extend the feasible sintering time, that is to say the sintering time before enough reactive material is lost that the ionic conductivity is affected. Extending the available sintering time enables control of the microstructure to a greater degree, enabling the creation of ceramic-glassy phases which are thought to increase performance.

[0076] This sintering process and the associated densification may not only improve the watertightness of the membrane, but may improve the ionic conductivity as individual particles of the material grow together and form Li-ion conducting paths along the grain core and grain boundaries. The reduced amount and / or thickness of grain boundaries throughout the material, which are known to slow the transport of Li-ions through the material, also may contribute to improving the overall ionic conductivity of the resultant membrane.

[0077] The membrane may also undergo a pressure treatment in order to further densify the membrane. Processes such as dry pressing, isostatic pressing, and roll compaction are all examples of pressure treatments which are suitable to reduce or eliminate pores from the membrane. These pressure treatments may also increase the overall strength of the membrane, as well as the erosion resistance (attributed to the smaller number of pores and grain boundaries) by reducing interfacial defects and limiting sites for crack formation and chemical attack. The removal or reduction in number of grain boundaries may also act to improve the ionic conductivity of the membrane. The pressure treatment may also act to regulate stresses throughout the membrane, avoiding areas of localised stress which may result in membrane failure.

[0078] Preferably, the pressure treatment is carried out at a pressure between approximately 10 MPa and 700 MPa, preferably between 50 MPa and 250 MPa, most preferably between 100 and 200 MPa, although other pressures may be applicable depending on which solid-state electrolyte is used, and the presence of any additives or filler materials. This pressure treatment has been found to improve the green strength of the material, the strength prior to the sintering process, which makes handling and processing during the rest of the method easier. Lower erosion of the membrane has been observed in embodiments which have received a pressure treatment. In some embodiments, the pressure treatment is carried out prior to the sintering process, that is to say the pressure treatment is carried out on the initial body. In other embodiments, the pressure treatment may be carried out simultaneously with the heat treatment, for example using techniques such as hot isostatic pressing (HIP). In these embodiments, the required temperature to densify the material may be reduced, and the lithium loss from the solid-state electrolyte at these temperatures may be reduced accordingly. In these embodiments, the sintering temperature may be carried out under pressure between 500°C and 1200°C.

[0079] The density of the membrane can also be improved by controlling the particle sizes of the solid-state electrolyte in the initial body of the membrane. For example, the solid-state electrolyte may be wet or dry-milled to achieve a desired range of particle sizes. In some embodiments, the solid-state electrolyte may be mixed with the inorganic filler prior to being milled, so as to form a mixture of the solid-state electrolyte and filler. For example, LATP and SiOi may be ball milled, to produce an initial body with the preferred particle size. Preferably, the particle sizes of the solid-state electrolyte are chosen to encourage and maximize theamount of shrinkage that occurs when the membrane is sintered, as this results in a denser membrane. In preferred embodiments, the powder of the solid state electrolyte comprises a first fraction of particles in a first size range and a second fraction of particles in a second size range larger than the first size range. This allows for a greater packing density relative to a single particle size. Assuming spherical particles, the maximum packing density for identically sized spheres is 74%. When at least two particle sizes are used, the smaller sized particles can fill void spaces between the larger sized particles, allowing for packing densities of 90% or greater. FIGURE 1 illustrates this behaviour, with a mix of a first fraction of particles (12) with a size of 0.4 pm and a second fraction of particles (11) with a size of 1 pm showing increased packing density and contact points between particles relative to either the first or second fraction on their own. The high surface energy of the smaller particles are thought to drive necking and coalescing processes during the initial sintering process, fill voids as they grow in an intermediate stage of sintering, and may collapse pores in a final stage of sintering, resulting in an effective reduction of grain boundary defects and increased ionic conductivity.

[0080] In preferred embodiments, a first fraction of the material particles is milled to have a small primary size in the range of 0.2 to 0.6 pm, with a D50 (defined as the mean or average particle size) of 0.4 pm and a second fraction of the material particles is milled to have a larger primary size in the range of 1 to 10 pm with a D50 of 3 pm. The solid state electrolyte may also be milled to a third fraction, with a size larger than the first and second fractions, in the range of 50 to 150 pm with a D50 of 50 pm. Additionally, by providing differently sized fractions, the number of contact points between particles can be maximised, which both improves sintering and reduces grain boundaries, resulting in improved ionic conductivity.

[0081] This method results in a membrane with a structure comprising a densified network of solid electrolyte crystallites, which extend across the membrane matrix. An example of a densified network is shown in FIGURE 2. In this example, the crystallites are pure LATP unit cells. These unit cells possess transport pathways which allow selective lithium migration through interstitial sites or channels in the lattice. Lithium is selectively transported through the lattice due to a charge and size exclusion principle, and LATP is an example of a NASICON structure with vacancies sized to fit lithium's ionic radius and charge density. The grain boundaries between these crystallites is minimised during densification, improving the transport of lithium ions through the membrane. Otherwise stated, densification of the membrane caused in part by sintering, pressure treatments, and particle size control results ina network of solid-state electrolyte crystallites which extend continuously from one end of the membrane to the other, so that lithium ions can 'hop' from vacancy to vacancy through the network of solid-state electrolyte crystallites to pass through the membrane.

[0082] The term densify is intended to refer to a process in which the density of the material is increased, that is to say, pores and other voids are reduced and / or removed from the material, increasing the mass to volume ratio. The term densified is intended to refer to a material which has undergone a densification process, resulting in fewer voids / pores relative to the undensified material.

[0083] At least one ceramic filler may be chosen to improve the density of the resultant membrane. The particle size of many materials of varying hardness and glassy structures can more easily be controlled, for example by fracturing, precipitating, melting, and / or atomizing said material to a desired particle size. Preferably, the ceramic filler is provided at nanometre to micron-sized particle sizes. As discussed above, including a fraction of smaller particles in the green body has been found to result in a denser membrane. These ceramic filler materials are typically lower cost than the solid-state electrolyte, and may additionally result in costsaving benefits when producing the membrane. These ceramic fillers are also preferably non- conductive, so as to not interfere with ion transfer through the solid-state electrolyte. For example, bentonite, aluminosilicates such as kaolin and zeolites, silicates, alumina and titania have all been found to be suitable materials for use as a ceramic filler, owing in part to the relatively easy tailoring of the particle sizes, and their non-conductive nature. In preferred embodiments, the ceramic filler is a silicate, such as SiOi- A schematic of an example of the effect of such a filler is shown in FIGURE 3. In this diagram, the membrane is composed primarily of a solid-state electrolyte, in this case an LATP active layer network (21), with a mesoporous zeolitic silicate layer (22) filling the pores and reducing the porosity. This prevents liquid from the feed solution (23) passing through the membrane and entering the concentrate solution (24), while lithium ions are able to pass through the LATP active layer. Silicates are particularly preferred as the ceramic filler as they provide significant stability to the resultant membrane in the presence of strong brines. Without wishing to be bound by theory, it is thought that the silicates influence the crystal structure and grain boundaries of LATP and may form a glassy phase during sintering that bonds the particles.

[0084] The at least one ceramic filler may be chosen to reduce the porosity and liquid tightness of the resultant membrane. When chosen for this purpose, the ceramic filler ispreferably a low-melting point inorganic material. When the initial body undergoes sintering, the low-melting point inorganic material softens or melts, flowing into and filling voids in the membrane body. When a low-melting point inorganic material is used as a filler, the temperature of the sintering process may be reduced relative to when only a solid-state electrolyte is present in the green body, while still producing the required dense body. Additionally, membranes formed with a low-melting point inorganic material as a filler material have been found to significantly limit permeation of feed solution into and through the produced membrane. Examples of low-melting point inorganic materials suitable for use as a ceramic filler include borosilicate and vanadate glasses. One specific example is the addition of Li2.9B0.9S0.1O31 glass (LBSO) which has been found to undergo transformation into a liquid phase during sintering. This liquid phase has been found to effectively coat the grains and fill the pores of the material, facilitating improved densification of the solid electrolyte material.

[0085] A potential issue with forming ceramic electrolytes is their relatively low fracture toughness, which can result in cracking during handling and installation in, for example, an electrodialysis cell assembly. To address this, the at least one inorganic filler may be chosen to improve mechanical strength and the microstructure of the resultant membrane. The solid-state electrolyte material may have impurities which cause segregation and / or cracking to occur at grain boundaries when the solid-state electrolyte is sintered. These segregated regions limit the transport of lithium ions between grains, and reduce the ionic conductivity of the membrane. Again, silicate is preferred as a filler as it can act as a sintering additive, inhibiting grain growth and / or crack formation and thus improving the conductivity of the membrane. Another example filler material is reduced graphene oxide, which has been found to increase the fracture toughness of the resultant material. Preferably, these filler materials are added in an amount not more than 5 wt%, as a higher amount may result in decreased ionic conductivity through the material. Preferably, the filler is in the range of 0.1 to 2.5 wt%, more preferably in the range of 0.5 and 1.5 wt%, and even more preferably in the range of 0.5 to 0.6 wt%.

[0086] Lithium-ion conductive materials often exhibit highly resistant grain boundaries which strongly limit the total ionic conductivity. At least one inorganic filler may be chosen to increase the ionic transfer of lithium ions through the membrane and / or increase the selectivity for lithium ions. When chosen for this purpose, the fillers are chosen to provide additional ion conduction pathways, for example additional lithium-ion conducting ceramics. For example,Li A I S i CM may be used as a filler to increase the speed of ionic transfer between grains of the solid state material. In some embodiments, the ceramic filler chosen is hydroxyapatite (HAP). HAP comprises closely bonded calcium atoms, phosphate tetrahedral units, and hydroxide groups. Calcium ions can escape from the HAP structure to form negative potential vacancies, attracting lithium cations. These negatively charged surface charges within the membrane can thus attract lithium cations and improve the lithium-ion availability and transfer through the membrane. In other embodiments, the filler is graphene. Graphene is a two-dimensional layer of carbon atoms arranged in a honeycomb structure, and is known for its advantageous properties, including its high electrical conductivity. When graphene is used as a filler, it disperses across the membrane, increasing the electronic and lithium ion conductivity of the membrane as a whole. Graphene may also provide improved anti-fouling properties. Alternatively, in other embodiments, the filler may be chosen to limit conductivity through the filler, and instead force increased conductivity through appropriate pathways, namely through the solid-state electrolyte. For example, boron nitride nanotubes are a non-conductive material which may be added as a filler.

[0087] The membrane may further comprise at least one surface treatment, protective layer, coating, or grafting. For ease of understanding, these will collectively be referred to as surface treatments. Surface treatments may be applied to the membrane for a number of reasons, such as to prevent or limit liquid ingress into or through the membrane, increase the mechanical strength of the membrane, prevent or limit reactions between the membrane and the feed solution, dehydrate hydrated ions in the feed solution, increase hydrophobicity of the membrane, or to protect the membrane from fouling or scaling. In other embodiments, the surface treatment may include a physical treatment such as mechanical polishing of the membrane surface to reduce surface defects.

[0088] In some embodiments, the material chosen for the surface treatment results in more than one of these benefits. In some embodiments, the membrane may be spray coated with a polymer such as cellulose, polyvinyl alcohol (PVA), polyethylene oxide (PEG) or polyacrylonitrile (PAN). Preferably, the coating is PEO, as PEO may offer superior performance in coating applications that require high durability, flexibility, moisture resistance and thermal stability over other polymer coatings. In other embodiments, a cured coating of inorganic particles such as titania, silver, silica, alumina, or copper, or a cured coating of graphene is applied. These have been found to improve the overall mechanical strength,hydrophobicity, and anti-fouling properties of the membrane, while simultaneously reducing the porosity and permeability. The surface treatment may also comprise a sealant, in particular around the edges of the ceramic phase of the membrane, which fills the open pores which occur at these areas to prevent water ingress. In these embodiments, the sealant may comprise a low- temperature melting glass, which may be the same low-temperature melting glass added as a filler.

[0089] At least one surface treatment may also be selected to further functionalise or improve the performance of the membrane. For example, a coating may be selected to increase the hydrophobicity, increase or decrease the surface charges on the membrane, and / or to alter the wettability of the membrane. For example, coatings of graphene, zeolite, hydroxyapatite, and / or zwitterionic compounds may be applied to alter the surface charges and push ions away to increase the lithium ion selectivity of the membrane.

[0090] In some embodiments, the initial body is formed as a thin layer or sol-gel of the solid-state electrolyte and optional fillers on a substrate. The substrate may be a porous mechanical support substrate of the solid state electrolyte material or the filler material, a low- cost porous inorganic substrate, or a flexible inorganic support such as a woven inorganic substrate. For example, the support substrate may be formed from a film of a woven or nonwoven synthetic fabric, a porous rigid polymer such as nylon, a porous ceramic such as alumina, a woven ceramic such as silica, a porous graphite, or woven carbon fibres. Membranes with thicknesses of less than 10 pm have been fabricated by forming multiple thin layers on a porous mechanical support, thicknesses of 10 to 250 pm when formed as a dense coating on a support, and thicknesses of between 60 pm to 1 cm when formed as a dense film of on a substrate of the porous electrolyte.

[0091] In some embodiments, the initial body is formed by tape casting or ink-jet printing. In these embodiments, the initial slurry subjected by these methods comprises the solid-state electrolyte material, a dispersant, and a solvent such as toluene, and optionally at least one inorganic filler as discussed above. The initial body is then subjected to pressure and / or sintering treatments to produce the dense, inorganic membrane.

[0092] In some embodiments, the initial body forms a layer or layers of a multi-layered membrane. These layers, comprising the at least one solid-state electrolyte material, may be referred to as active layers. In some embodiments, the active layer is embossed, bonded, orotherwise patterned to alter the local flow characteristics at the surface of the active layer. This has been found to improve ion transfer and can reduce scaling and / or fouling at the membrane surface.

[0093] Preferably, the active layer or layers are sandwiched between layers of a dehydrating material. This is beneficial because hydronium ions may compromise the solid- state electrolyte material's stability, and these dehydrating layers prevent hydronium ions from reaching the active layers. These dehydrating layers may also improve the lithium ion selectivity via the following process. Lithium ions have a large hydration radius, larger than the radii of competing ions such as sodium and potassium. In contrast, lithium ions have a smaller dehydrated radius than these cations, but a similar radius to dehydrated magnesium ions. By dehydrating the lithium ions, they can more easily transfer into and through the active material relative to competing ions.

[0094] The multi-layered membrane may also comprise structural layers to provide additional strength to the membrane and to protect the active layers from breaking due to strain or pressure, such as layers of a porous glass fiber composite. Additionally or alternatively, an active layer may be framed, flanked, or otherwise surrounded by a structural material in order to further reinforce the membrane. These may be, for example, a rigid glass or plastic material, or in some embodiments may be a lithium-ion selective solid-state electrolyte material.

[0095] The multi-layered membrane may also include further porous separator layers for their antifouling properties or to increase the ionic selectivity of the membrane. For example, commercially available monovalent ion selective membranes may form additional layers of the membrane, in order to limit multivalent ions such as magnesium from reaching the active layer.

[0096] The present disclosure will become better understood from the following experimental data and example of a non-limiting embodiment.

[0097] To assess the impact of particle size of the solid-state electrolyte material in the green body on the density of the sintered membrane, the vol% shrinkage of green bodies formed of LATP with differing particle sizes at a sintering temperature of 800°C and 1200°C was assessed. A higher vol% shrinkage represents a better densified membrane. The results are tabulated below:

[0098] These results show that for a sintering temperature of 800°C, the smaller particle sizes produce worse results compared to the 10 pm. When sintered at 1200°C however, the 1 pm particle size provides the greatest shrinkage and thus the greatest densification.

[0099] To assess the impact of a filler material on the resultant membrane, membranes were fabricated of LATP (only), LATP with kaolin as filler, and LATP with bentonite as filler. The results are shown in FIGURE 4. The LATP only membrane failed, showing the improved mechanical strength properties provided by the inorganic filler present in the green body.

[0100] To assess the impact of the pressure and sintering process on a filler material, in particular amorphous silica, the XRD spectra were obtained for the amorphous silica unsintered, sintered at 1200°C, and hydraulically pressed and sintered at 1200°C. The results are shown in FIGURE 5. For the sintered (only) silica (51), crystallinity patterns can be seen which are comparable to the amorphous unsintered silica (52). In contrast, the hydraulically pressed silica (53), when sintered under the same conditions, shows high degrees of crystallinity, indicating the formation of cristobalite. This has a number of beneficial effects on the membrane, including improved density, ionic conductivity, and mechanical performance.

[0101] As an example embodiment, a multi-layered membrane was formed, a schematic of which is shown in FIGURE 6. The membrane consists of an active layer in the form of a disk of sintered LATP (51) which is held in a rigid frame of porous glass fiber composite material (52). It will be understood that in other embodiments, other rigid frames made of glasses or polymers may be used to provide the structural support. Porous separator layers (53a) and (53b) are arranged above and below the active layer and frame, and these layers are sandwichedbetween cover layers (54a) and (54b) of the same glass fiber composite material. The layers are formed with orientations orthogonal to each other. SEM imagery of the active layer of sintered LATP and the active layer in context of the other layers is shown in FIGURE 7.

[0102] In another embodiment, a multi-layered membrane similar to as shown in FIGURE 6 is formed, however the rigid frame is constructed from a lithium-ion conducting solid-state material. The lithium-ion conducting solid-state material, for example LLTO, is preferably in a dense crystalline, glass ceramic, or glassy phase, and forms a liquid tight frame around the membrane. The term glass ceramic refers to intermediates of glass and ceramics, comprising between about 50% and 95% crystallinity with the remaining being residual glass phase). Glass ceramics are preferred as the glass phase provides an amorphous surface for improved stability while the crystalline phase enables effective lithium ion transfer. As with the active layer of the membrane, the size of the grains and degree of crystallinity can be chosen based off the selection of sintering temperature, heating rate, and hold / dwell time.

[0103] The membrane is especially suited to electro-separation techniques, in particular electrodialysis. If conventional membranes comprising a solid-state electrolyte are used in an electrodialysis process for extraction of lithium from brines or other sources, an issue arises from the presence of Na+ions in the brine, which causes rapid degradation of the membrane. This is not a consideration when solid state electrolytes such as LATP are used in lithium-ion storage battery applications, as there is no significant amount of Na+ions in contact with the solid state electrolyte.

[0104] Further experiments were carried out to investigate the effects of different amounts of inorganic filler material in the initial body. For these experiments, the inorganic filler material was SiOi and the solid state electrolyte material was LATP. These experiments found that when SiOi is included, the sintered body forming the membrane is comparatively much more resistant to Na+ions in a feed solution of an electrodialysis process for recovering lithium from aqueous solutions.

[0105] The SiOi component of the solid-state electrolyte material prior to sintering is a key factor in improving the performance of the LATP membrane in terms of robustness and selectivity of lithium ions against other monovalent and divalent ions. It is believed this is due to the reduction of grain boundary defects by the silicate species thereby resisting other monovalent or divalent ions, thereby increasing the selectivity and mechanical integrity instrong brine conditions. The silicate species in the LATP membrane matrix are ideally in the range of 0.1 to 2.5 wt%, preferably in the range of 0.5 to 1.5 wt%, and more preferably in the range of 0.5 to 0.6 wt%. The results of the experiments are tabulated below:

[0106] These results show the improved selectivity and reduced flux of the membrane with 0.6 wt% silica added relative to the membrane without any silica added. Additionally, the 25 wt% silica shows reduced selectivity and flux relative to both other membranes, indicating that the inclusion of high amounts of silica is detrimental to the membrane properties.

[0107] The degradation resistance provided by the inorganic filler can be seen in FIGURE 9A and FIGURE 9B. Both images show a membrane after 46 hours use for electrodialysis. The membrane shown in FIGURE 9A does not include any silicate material and is heavily eroded. In contrast, FIGURE 9B shows an image of a membrane which was formed using LATP with 0.5 wt% silica following the same electrodialysis conditions for 96 hours. No erosion is shown.

[0108] Prior to sintering the initial body, some or all of the SiO2may be present in particles separate from the LATP particles. In these embodiments, the SiO2is preferably uniformly and homogenously dispersed throughout the LATP particles. However, performance of the membrane is has been found to improve when some, or preferably all, of the SiO2is spread within the LATP particles prior to the sintering operation. This may be carried out, for example,by melting the S1O2 and LATP material, and then crushing the cooled materials so that the SiOi is present within the crystal structure of LATP itself.

[0109] FIGURE 11 shows the XRD patterns of six different commercial LATP materials, which vary slightly in terms of the ratio of elements within LATP, where LATP is Lii+XAlxTii-X(PO4)3 with 0.3 < x < 0.5. The commercial LATP samples have been termed LATP-1 (111) LATP-2 (112), LATP-3 (113), LATP-4 (114), LATP-5 (115) and LATP-6 (116). Materials identified as LATP-4 and LATP-5 differ from the other samples in that they incorporate silicates by mixing silica into LATP and then melting and crushing the material or by adding silica and LATP to a ball mill and milling to form an LATP-silica mixture. The LATP-4 (114) and LATP-5 (115) spectra exhibit SiCh crystal planes which are indicated by the peaks (at a 20 value of 21.5) marked with asterisks.

[0110] In the foregoing description of certain embodiments, specific terminology has been resorted to for the sake of clarity. However, the disclosure is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes other technical equivalents which operate in a similar manner to accomplish a similar technical purpose.

[0111] In this specification, the word “comprising” is to be understood in its “open” sense, that is, in the sense of “including”, and thus not limited to its “closed” sense, that is the sense of “consisting only of’. A corresponding meaning is to be attributed to the corresponding words “comprise”, “comprised” and “comprises” where they appear.

[0112] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as, an acknowledgement or admission or any form of suggestion that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0113] In addition, the foregoing describes only some embodiments of the invention(s), and alterations, modifications, additions and / or changes can be made thereto without departing from the scope and spirit of the disclosed embodiments, the embodiments being illustrative and not restrictive.

[0114] Furthermore, invention(s) have described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention(s). Also, the various embodiments described above may be implemented in conjunction with other embodiments, e.g., aspects of one embodiment may be combined with aspects of another embodiment to realize yet other embodiments. Further, each independent feature or component of any given assembly may constitute an additional embodiment.

Claims

The claims defining the invention are as follows:

1. A method of producing a dense inorganic lithium-ion selective membrane, comprising: forming an initial body comprising at least one solid-state electrolyte material which is selectively permeable to lithium ions and at least one inorganic filler; sintering the initial body to densify the at least one solid-state electrolyte material to form a sintered body.

2. The method of claim 1, wherein the at least one inorganic filler is in an amount between 0.1 wt% to 2.5 wt%.

3. The method of either claim 1 or 2, wherein the sintering of the initial body is carried out at a heating rate below about 5°C / min.

4. The method of any one of the preceding claims, wherein the sintering of the initial body is carried out at a temperature between about 500°C and about 1500°C.

5. The method of any one of the preceding claims, wherein the sintering is carried out in a series of heating steps.

6. The method of claim 5, wherein the heating steps are a series of unequal steps.

7. The method of any one of the preceding claims, further comprising subjecting the initial body to a pressure treatment prior to sintering.

8. The method of any one of the preceding claims, further comprising subjecting the initial body to a pressure treatment concurrently with sintering.

9. The method of any one of claims 7 to 8 wherein the pressure treatment is carried out at a pressure of between about 10 MPa and 700 MPa.

10. The method of any one of the preceding claims, wherein the at least one solid-state electrolyte material for forming the initial body is a powder with a first fraction of thepowder having a smaller nominal particle size relative to a second fraction of the powder.

11. The method of claim 10, wherein the first fraction of the powder has a particle size in the range of about 0.2 pm to about 0.6 pm.

12. The method of claim 11, wherein the second fraction of the powder has a particle size between about 1 pm and 10 pm.

13. The method of any one of claims 10 to 12, wherein the powder includes a third fraction with a larger particle size relative to both the first and second fractions of the powder.

14. The method of claim 13, wherein the third fraction has a particle size between about 50 and 150 pm.

15. The method of any one of the preceding claims, wherein the at least one inorganic filler is a ceramic filler selected from one or more of: bentonite, aluminosilicate, silicate, alumina, titania, a low-melting point glass such as borosilicate or vanadate glass, hydroxyapatite, or a nanomaterial selected from one or more of graphene or boron nitride nanotubes.

16. The method of any one of the preceding claims, wherein the method further comprises applying a surface treatment to at least one surface of the membrane to limit water ingress into the membrane, dehydrate hydrated ions, alter the surface charges on the membrane, and / or to increase hydrophobicity.

17. The method of claim 16, wherein the surface treatment comprises applying a coating or layer of at least one of: cellulose, PVA, PAN, PEO, graphene, titania, silver, silica, alumina, copper, zeolite, hydroxyapatite, or a zwitterionic compound.

18. The method of claim 16, wherein the surface treatment comprises a mechanical polishing of at least one surface of the membrane.

19. The method of any one of the preceding claims, wherein the method further comprises applying a pattern to at least one surface of the membrane to control the flow profile across the patterned surface.

20. The method of any one of the preceding claims, wherein the initial body is formed on a support substrate.

21. The method of claim 20, wherein the substrate is made of the solid-state electrolyte material or a film of woven or non-woven synthetic fabric, a porous rigid polymer, a porous ceramic, a woven ceramic, a porous graphite, or woven carbon fibres.

22. The method of any one of the preceding claims, wherein the method further comprises locating the sintered body between layers of a porous separator material which is selective to monovalent ions.

23. The method of any one of the preceding claims, wherein the method further comprises locating the sintered body between layers of a dehydrating material.

24. The method of any one of the preceding claims, wherein the method further comprises adding at least one layer of a structural material25. The method of claim 24, wherein the structural material is a porous glass fibre composite material.

26. The method of any one of the preceding claims, wherein the sintered body is located in a rigid frame.

27. The method of claim 26, wherein the rigid frame is a porous glass fibre composite material.

28. The method of claim 27, wherein the rigid frame is a dense crystalline, glass ceramic, or glassy phase lithium-ion conductive solid-state electrolyte.

29. A dense lithium-ion selective membrane for electro- separation, comprising: an active layer comprising a densified network of a solid-state electrolyte material which is selectively permeable to lithium ions and at least one inorganic filler; the densified network of the solid-state electrolyte material allowing the selective transport of lithium ions through the membrane via vacancy, interstitial, and / or interstitial-substitutional exchange mechanisms.

30. The membrane of claim 29, wherein the inorganic filler is present in an amount between 0.1 wt% to 2.5 wt%.

31. The membrane of either claim 29 to 30, wherein the active layer has at least one coating to limit water ingress into the membrane, dehydrate hydrated ions, alter the surface charges on the membrane, and / or to increase hydrophobicity.

32. The membrane of any one of claims 29 to 31, wherein the at least one coating is selected from one or more of: cellulose, PVA, PAN, PEO, graphene, titania, silver, silica, alumina, copper, zeolite, hydroxyapatite, or a zwitterionic compound.

33. The membrane of any one of claims 29 to 32, wherein the active layer further comprises a mechanically polished surface.

34. The membrane of any one of claims 29 to 33, wherein the active layer includes a patterned layer for controlling the flow profile across the membrane surface.

35. The membrane of any one of claims 29 to 34, wherein the active layer is supported by a substrate.

36. The membrane of claim 35, wherein the substrate is made of the solid-state electrolyte material or a film of woven or non-woven synthetic fabric, a porous rigid polymer, a porous ceramic , a woven ceramic, a woven graphite, or woven carbon fibres.

37. The membrane of any one of claims 29 to 36, wherein the membrane has an ionic conductivity of at least 1 x 10’4S cm1.

38. The membrane of any one of claims 29 to 37, wherein the membrane further comprises layers of a porous separator material, and wherein the active layer is located between layers of the porous separator material, wherein the porous separator material is selective to monovalent ions.

39. The membrane of any one of claims 29 to 38, wherein the membrane further comprises layers of a dehydrating material, and wherein the active layer is located between layers of the dehydrating material.

40. The membrane of any one of claims 29 to 39, wherein the membrane further comprises porous structural layers.

41. The membrane of claim 40, wherein the structural layers are a porous glass fibre composite material.

42. The membrane of any one of claims 29 to 41, wherein the active layer is located within a rigid frame.

43. The membrane of claim 42, wherein the rigid frame is a porous glass fibre composite material.

44. The membrane of claim 43, wherein the rigid frame is a dense crystalline, glass ceramic, or glassy phase lithium-ion conductive solid-state electrolyte.

45. The membrane of any one of claims 29 to 44, wherein the membrane is formed by a method according to any one of claims 1 to 28.

46. Use of a membrane according to any one of claims 29 to 45 in a lithium-ion separation technique.

47. The use according to claim 46, wherein an electric potential is applied to the membrane.

48. The use according to either claim 45 or 46, wherein the separation technique is electrodialysis.

49. A method of recovering lithium from an aqueous feed solution containing lithium ions by electrodialysis, comprising: drawing at least a portion of the lithium ions from the feed solution through a membrane to form an aqueous product solution, wherein the membrane comprises at least one solid state electrolyte material which is permeable to lithium ions, and an inorganic filler.

50. The method of claim 49, wherein the inorganic filler is present in an amount between 0.1 wt% and 2.5 wt%

Images