Metal-sulphur battery and manufacturing method thereof

Abstract

A metal-sulphur battery is disclosed, comprising an anode, a cathode comprising sulphur, an electrolyte and a separator. The electrolyte transports charged species between the anode and cathode during operation of the battery, and the separator is disposed between the anode and cathode so as to physically separate the anode from the cathode. The battery further comprises a layer of platelets disposed on a surface of the cathode facing the separator. The platelets may comprise functionalised graphite and / or graphene. In some embodiments, the platelets are contained in a matrix, for example a polymer such as polyethylene glycol (PEG) or polyethylene oxide (PEO). A method of forming a material for use as the cathode is also disclosed, comprising spraying a carrier liquid onto the surface of a cathode material, the carrier liquid comprising a suspension of said platelets, such that the platelets in said layer are orientated so as to lie substantially parallel to said surface of the cathode material.

Description

[0001] Metal-Sulphur Battery and Manufacturing Method Thereof

[0002] Technical Field

[0003] The present invention relates to metal-sulphur batteries and manufacturing methods thereof.

[0004] Background

[0005] Metal-sulphur batteries have become popular due to their relatively high capacities and energy densities. For example, Li-S batteries have a high theoretical capacity of 1675 milliamp hours per gram of sulphur (mAh / gs), leading to a theoretical energy density of over 2500 Watt hours per kilogram (Wh / kg) of anode and cathode. Sulphur also has the benefits of being a highly abundant material of low cost, with low toxicity. In recent years there has been interest in developing alternative metal anodes to avoid a high dependency on lithium, of which one promising example is sodium.

[0006] Sodium-sulphur (Na-S) batteries have a theoretical capacity of 1675 mAh / gsand could reach a theoretical energy density of 1300 Wh / kg of anode and cathode. In most batteries, liquid electrolytes are preferred because they provide the opportunity for fast ion transport, enabling metal ions to move quickly from the anode to the cathode during discharge and participate in redox reactions at the cathode, and to move back to the anode during charging to be converted to the solid metal (e.g. Li or Na), which is deposited back on the anode.

[0007] However, in practice metal-sulphur batteries cannot reach their theoretical capacity, mainly due to the loss of active materials during cycling of the battery. Sulphur batteries with liquid electrolytes suffer from the problem that sulphur and sulphides migrate from the cathode to the separator and anode during discharge, via the mechanisms of diffusion (for sulphur and sulphides) and electric field for the sulphides (which move to their counter-charged electrode, i.e. the anode during discharge). This migration during discharge, and further shuttling of polysulphides between the anode and cathode during charge, results in loss of active mass in the cathode during discharge, reduced specific capacity in discharge, low efficiency between discharge and charge, and reduced cyclability. It would therefore be desirable to have an improved mechanism for inhibiting the migration of species, such as sulphides, in sulphur-based batteries.

[0008] Summary of the Invention

[0009] According to a first aspect of the present invention, there is provided a metal-sulphur battery comprising an anode comprising a metal, a cathode comprising sulphur, an electrolyte for transporting charged species between the anode and cathode during operation of the battery, a separator disposed between the anode and cathode so as to physically separate the anode from the cathode, and a sulphide-inhibiting layer disposed between the cathode and the anode, the sulphide-inhibiting layer comprising a plurality of platelets.

[0010] In some embodiments according to the first aspect, the sulphide-inhibiting layer is disposed on a surface of the cathode, said surface being a surface that is exposed to the electrolyte.

[0011] In some embodiments according to the first aspect, the plurality of platelets comprise graphene platelets and / or graphite platelets.

[0012] In some embodiments according to the first aspect, the platelets are orientated so as to lie substantially parallel to said surface of the cathode.

[0013] In some embodiments according to the first aspect, the platelets comprise one or more dopants.

[0014] In some embodiments according to the first aspect, the one or more dopants comprises boron and / or nitrogen.

[0015] In some embodiments according to the first aspect, the platelets comprise a plurality of functional groups.

[0016] In some embodiments according to the first aspect, the plurality of functional groups include catalytic functional groups comprising a transition metal.

[0017] In some embodiments according to the first aspect, the catalytic functional groups comprise one or more of: B-N, FelX , C0N4, VN4 or WN4.

[0018] In some embodiments according to the first aspect, the sulphide-inhibiting layer comprises a matrix containing the platelets.

[0019] In some embodiments according to the first aspect, the matrix comprises a polymer.

[0020] In some embodiments according to the first aspect, said polymer comprises or consists of polyethylene glycol, PEG. In some embodiments according to the first aspect, said polymer comprises or consists of polyethylene oxide, PEO.

[0021] In some embodiments according to the first aspect, the cathode comprises sulphur encapsulated in a conductive material.

[0022] In some embodiments according to the first aspect, the conductive material comprises carbon.

[0023] In some embodiments according to the first aspect, the cathode comprises a pseudocapacitive material arranged to bind the platelets in the sulphide-inhibiting layer.

[0024] In some embodiments according to the first aspect, said pseudocapacitive material comprises poly(3,4-ethylenedioxythiophene) polystyrene sulphonate, PEDOT:PSS.

[0025] In some embodiments according to the first aspect, the electrolyte comprises silk fibroin or gelatine.

[0026] In some embodiments according to the first aspect, the anode comprises: lithium; sodium; aluminium; or magnesium.

[0027] According to a second aspect of the present invention, there is provided a method of manufacturing a metal-sulphur battery according to the first aspect, the method comprising disposing the sulphide-inhibiting layer between the cathode and the anode of the battery, the sulphide-inhibiting layer comprising the plurality of platelets.

[0028] In some embodiments according to the second aspect, disposing the sulphide- inhibiting layer between the cathode and the anode comprises disposing the sulphide- inhibiting layer on a surface of the cathode, said surface being a surface that is exposed to the electrolyte.

[0029] In some embodiments according to the second aspect, after disposing the sulphide- inhibiting layer on said surface of the cathode the method comprises: assembling the battery by disposing the separator between the cathode and the anode; and providing the electrolyte in a space between the cathode and the anode. According to a third aspect of the present invention, there is provided a method of manufacturing a material for use as the cathode in a metal-sulphur battery according to the first aspect, the method comprising: obtaining a cathode material comprising sulphur; and forming a layer of platelets on said surface of the cathode material by spraying a carrier liquid onto the surface of the cathode material, the carrier liquid comprising a suspension of said platelets, such that the platelets in said layer are orientated so as to lie substantially parallel to said surface of the cathode material.

[0030] In some embodiments according to the third aspect, after forming the layer of platelets on the surface of the cathode material, the method further comprises dividing the cathode material into a plurality of cathodes.

[0031] In some embodiments according to the third aspect, the cathode material is in the form of a sheet.

[0032] Brief Description of the Drawings

[0033] Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[0034] Figure 1 schematically illustrates a metal-sulphur battery, according to an embodiment of the present invention;

[0035] Figure 2 is a graph showing results of the first discharge at a rate of 0.05 C for case studies la, lb, 1c, Id, including metal-sulphur batteries according to embodiments of the present invention;

[0036] Figure 3 is a graph showing the results of galvanostatic charge-discharge (GCD) tests for the first discharge at a rate of 0.05 C for case studies 2a, 2b, 2c, 2d, according to embodiments of the present invention;

[0037] Figure 4 is a graph showing results of the first discharge at a rate of 0.05 C for case studies 2a, 2b, 2c, 2e, according to embodiments of the present invention;

[0038] Figure 5 is a graph showing cycling performance at different C-rates for case studies 2b, 2c, 2d, 2e, according to embodiments of the present invention;

[0039] Figure 6 is a graph showing results of GCD tests for the first discharge at a rate of 0.05 C for case studies 3a, 3b, 3c, 3d, according to embodiments of the present invention;

[0040] Figure 7 is a graph showing cycling performance at different C-rates for case studies 3a, 3b, 3c, 3d, according to embodiments of the present invention;

[0041] Figure 8 is a flowchart showing a method of manufacturing a metal-sulphur battery comprising a sulphide-inhibiting layer, according to an embodiment of the present invention; and Figure 9 is a flowchart showing a method of manufacturing a material for use as the cathode in a metal-sulphur battery, according to an embodiment of the present invention.

[0042] Detailed Description

[0043] In the following detailed description, only certain exemplary embodiments of the present invention have been shown and described, simply by way of illustration. As those skilled in the art would realise, the described embodiments may be modified in various different ways, all without departing from the scope of the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not restrictive. Like reference numerals designate like elements throughout the specification.

[0044] Referring now to Fig. 1, a metal-sulphur battery is schematically illustrated according to an embodiment of the present invention. The battery 100 comprises a cathode 110 comprising sulphur, an anode 120, an electrolyte 130 for transporting charged species between the anode 120 and cathode 110 during operation of the battery 100, a separator 140, and a sulphide-inhibiting layer 150 disposed between the cathode 110 and the anode 120, the sulphide-inhibiting layer 150 comprising a plurality of platelets 151. The electrolyte 130 may be any suitable liquid electrolyte, meaning any electrolyte that contains a metal ion salt dissolved in appropriate solvent system for any given implementation of the metal-sulphur battery 100. The electrolyte 130 may comprise silk fibroin or gelatine, which can assist in the trapping of polysulphides in the cathode 110.

[0045] In the present embodiment, the sulphide-inhibiting layer 150 is disposed on a surface of the cathode 110, said surface being a surface that is exposed to the electrolyte 130. In other embodiments the sulphide-inhibiting layer 150 may be disposed elsewhere, for example between the cathode 110 and the separator 140 at a distance from the surface of the cathode 110. In some embodiments, the sulphide-inhibiting layer 150 may be disposed on a surface of the separator 140. However, in comparison to disposing the sulphide-inhibiting layer 150 on the separator 140, an advantage of disposing the sulphide-inhibiting layer 150 on a surface of the cathode 110 (as shown in Fig. 1) is that the sulphide-inhibiting layer 150 can benefit from more effective electron transport to / from the cathode current collector. Consequently, any sulphur or sulphides trapped between the platelets of the sulphide-inhibiting layer 150 still takes part in the redox reactions as active cathode material due to the good electron conduction between the conductive layer 150 and the cathode 110, and the cathode 110 and conducting layer 150 being in close contact. In this respect, the platelets 151 may be formed of an electrically conductive material (e.g. graphene or graphite) to ensure good electron conduction between the conductive layer 150 and the cathode 110. By comparison, a sulphide-inhibiting layer 150 disposed on the separator 140 may still be capable of trapping polysulphides and protecting the anode 120 from a build-up of polysulphides, but the sulphur and polysulphides that collect on the separator 140 will remain inactive, i.e. will not participate in the redox reactions of the cathode as the high contact resistance with the rest of cathode prohibits electron transfer from the cathode.

[0046] The anode 120 can comprise any metal suitable for metal-sulphur batteries, and hence may also be referred to as a metal anode 120. In some embodiments the anode 120 comprises lithium, sodium, aluminium or magnesium, although in other embodiments the anode may comprise any other metal suitable for metal-sulphur batteries.

[0047] The separator 140 is disposed between the anode 120 and cathode 110 so as to physically separate the anode 120 from the cathode 110. The separator 140 may be in the form of a membrane. The separator 140 can be in physical contact with both the anode 120 and cathode 110, and in such embodiments the separator 140 may be formed of an electrically insulating material to avoid creating a short-circuit between the anode 120 and cathode 110. Examples of materials from which the separator 140 may be formed include, but are not limited to: a porous polymer insulator membrane; or a porous paper insulator membrane; and a porous glass fibre membrane.

[0048] The cathode 110 comprises sulphur, which may be in the form of elemental sulphur and / or a sulphur-containing compound, such as sulphide. The cathode 110 may further comprise a conductive material mixed with the sulphur, for instance in the form of sulphur encapsulated in the conductive material. The conductive material may comprise a carbon-based material, including but not limited to: carbon black, Super P (RTM) carbon, carbon nanotubes, graphene, carbon nanofibres, and carbon fibres. In some embodiments, the cathode 110 comprises sulphur or sulphide deposited in a carbon-fibre substrate, which may be a porous substrate such as a carbon-fibre mat or a carbon-fibre cloth. In some embodiments, the carbon fibre substrate may comprise activated carbon fibre. In some embodiments, the carbon fibre substrate may comprise electrospun carbon fibre mat or activated electrospun carbon fibre mat. In the present embodiment, as shown in Fig. 1, the cathode 110 comprises a plurality of porous conductive particles 111. The porous conductive particles 111 may for example comprise activated carbon particles, or hollow conductive particles with porous walls. The sulphur and / or sulphide may be contained in or on the porous conductive particles 111. In some embodiments the cathode 110 comprises a MXene, or M0S2 or any other conductive host, impregnated with sulphur and / or sulphide.

[0049] Continuing with reference to Fig. 1, in the present embodiment the cathode 110 comprises a pseudocapacitive material 112, which may be referred to as a "binding" or a "coating". For example, the pseudocapacitive binding and / or coating may be in the form of coating 112 on the surface of the porous conductive particles 111 which also binds the porous conductive particles 111 together, thereby acting as both a binding and a coating. In the present embodiment the binding and / or coating 112 comprises poly(3,4-ethylenedioxythiophene) polystyrene sulphonate, PEDOT:PSS. The pseudocapacitive binding and / or coating 112 can increase the specific capacity of the cell by promoting metal ion hopping and intercalation in the cathode 110. Without wishing to be bound by theory, it is believed that the pseudocapacitive binding and / or coating 112 increases the specific capacity due to links and interactions of the metal ions with the sulphur of PEDOT rings.

[0050] As shown in Fig. 1, the sulphide-inhibiting layer 150 comprises a plurality of platelets 151. In the present embodiment the platelets 151 are orientated so as to lie substantially parallel to said surface of the cathode 110. For example, such an arrangement may be obtained by spray-coating the plurality of platelets 151 onto a surface, such as a surface of the cathode 110. Without wishing to be bound by theory, it is believed that orientating the platelets 151 in this way (i.e. parallel to the surface of the cathode 110) increases the effectiveness of the sulphide-inhibiting layer 150 by acting as a physical barrier that inhibits the movement of sulphides from the cathode 110 to the anode 120, since sulphides forming at the cathode 110 must follow a highly tortuous path around and between the platelets 151 to pass through the sulphide-inhibiting layer 150. Also, by disposing the sulphide-inhibiting layer 150 on the surface of the cathode 110, an electronic contact between the platelets 151 in the layer 150 and the cathode 110 can be improved, further enhancing the effectiveness of the sulphide-inhibiting layer 150. Similarly, by ensuring that the conductive platelets 151 lie flat within the layer 150, excellent electronic conduction within the layer 150 can be achieved. This means that good electron transfer is ensured to any trapped polysulphides or any trapped sulphur in the sulphide-inhibiting layer 150, and consequently the trapped polysulphides and sulphur participate in the electron transfer-enabled, electrochemical redox reactions of the cathode, within the sulphide- inhibiting layer 150, which with its catalyst functional side groups accelerates the redox reactions.

[0051] The plurality of platelets may, for example, comprise graphene platelets and / or graphite platelets. In the present embodiment the plurality of platelets 151 are graphene platelets, but in other embodiments the plurality of platelets 151 may comprise a material other than graphene or graphite. For example, in some embodiments the plurality of platelets 151 may comprise IT-M0S2. The effectiveness of the sulphide-inhibiting layer 150 may be further enhanced by the addition of one or more dopants to the material from which the platelets 151 are formed. For example, the platelets 151 may be doped with boron and / or nitrogen (e.g. the platelets 151 may be formed from B-N doped graphene). B-N-doped graphene is particularly suited for use in the sulphide-inhibiting layer 150, as the B-N functional group has a high adsorption energy for polysulphides compared to other types of functional groups.

[0052] The platelets 151 may comprise other functional groups instead of or in addition to B- N functional groups, such as catalytic functional groups comprising a transition metal (e.g. FeN4, C0N4, VN4 or WN4). The inclusion of functional groups such as those listed above can further enhance the effectiveness of the sulphide-inhibiting layer 150, for example by helping the layer 150 to adsorb sulphur and soluble sulphides and to act as an electrocatalyst in redox reactions (e.g. reactions occurring at the cathode 110). These functional groups have two roles: (a) contribute to the trapping of polysulphides and sulphur via their high adsorption energy and (b) accelerate any electrochemical redox reactions of the polysulphides trapped in the sulphide-inhibiting layer.

[0053] One advantage of a layer 150 with conductive platelets 151 with functional groups, in comparison with a hypothetical alternative layer 150 comprising pure materials of such functional groups such as boron nitride nanotubes, is the high electronic conductivity of the functionalised graphene platelets, which facilitate good electron transfer from the cathode. An advantage of conductive platelets 151 with functional groups, in comparison with other formats of similar materials, such as B-N doped carbon nanotubes, is the high polysulphide trapping efficiency of the platelet format lying flat on a cathode surface, due to the high tortuosity of the platelets compared to the nanotubes, where the high tortuosity slows down the migration of polysulphides and contributes to their entrapment in layer 150.

[0054] The sulphide-inhibiting layer 150 may comprise a matrix 152 that contains the platelets 151, as in the embodiment illustrated in Fig. 1. A matrix 152 can enhance the structural integrity of the sulphide-inhibiting layer 150 by helping to hold the platelets 151 in place, and / or by helping the layer 150 to bond more effectively to the surface on which it is disposed (e.g. a surface of the cathode 110). Examples of materials that can be used for the matrix include polymers, for example a solid polymer electrolyte such as polyethylene glycol (PEG) or polyethylene oxide (PEO), although in other embodiments other materials may be used in a matrix within the sulphide-inhibiting layer 150. A solid polymer electrolyte, such as PEG or PEO, can help to enhance the effectiveness of the sulphide-inhibiting layer 150 by facilitating the hopping of metal ions across the layer 150 (e.g. to enable the metal ions to react with sulphur and polysulphides in redox reactions during discharge of the cell). Embodiments of the invention are not limited to the use of PEG or PEO in a matrix 152 of the sulphide-inhibiting layer 150, and in other embodiments different materials may be used. For example, in some embodiments the sulphide-inhibiting layer 150 may comprise a matrix 152 comprising polyvinylidene fluoride (PVDF) or PEDOT:PSS.

[0055] Experimental results will now be discussed in relation to a number of case studies, including both embodiments according to the present invention and comparative examples to assist in understanding the inventive concept. As will become apparent from the following description, the provision of a sulphide-inhibiting layer 150 in a metal-sulphur battery (e.g. as shown in Fig. 1) can potentially increase the battery cell capacity by 50% (e.g. from 700 mAh / g of sulphur in the case of an Li-S cell up to 1050 mAh / g of sulphur), decrease overpotential and improve cyclability.

[0056] Referring now to Fig. 2, a graph is illustrated showing results of the first discharge at a rate of 0.05 C for four case studies, referred to as la, lb, 1c and Id, including metal- sulphur batteries according to embodiments of the present invention. In each of the case studies la, lb, 1c, Id, the battery comprises a lithium-sulphur (Li-S) battery.

[0057] CASE STUDY la:

[0058] In this comparative example, the metal-sulphur battery is an Li-S battery in the form of a coin cell. The battery comprises an electrolyte comprising a 1 molar (1 M) solution of Lithium bis(trifluoromethanesulfonyl)imide, referred to hereinafter simply as "LiTFSI", in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) DOL / DME at a ratio of 1: 1 in terms of volume ratio (v / v), with 0.25 M LiNCh. The battery comprises a NANOMYTE (RTM) BE-70E cathode manufactured by NEI Corporation, hereinafter referred to as a Nanomyte cathode with 70 weight percent (wt%) sulphur, Super P carbon black and 10 wt% polyvinylidene fluoride (PVDF) binder. The separator comprises a Celgard (RTM) 2400 membrane. As shown in Fig. 2, the first discharge at 0.05 C reaches 130.8 mAh / gs (milli Amp hours per gram sulphur).

[0059] CASE STUDY lb:

[0060] In this example embodiment, the metal-sulphur battery is an Li-S battery in the form of a coin cell. As with case study la, in case study lb the battery comprises an electrolyte comprising a I M solution of LiTFSI in DOL / DME at a ratio of 1: 1 v / v with 0.25 M LiNOs, and comprises a Nanomyte cathode with 70 wt% sulphur, Super P carbon black and 10 wt% PVDF binder. The separator comprises a Celgard 2400 membrane. Unlike case study la, the battery in case study lb comprises a sulphide- inhibiting layer comprising a plurality of platelets. In particular, the sulphide-inhibiting layer with boron-nitride (B-N) doped graphene platelets (hereinafter referred to as BNG platelets) is disposed on the separator by a spray-coating process using acetone solvent, on the side of the separator facing the cathode. The spraying solution contained a PVDF-type of binder that was at 10 wt% in the final BNG coating. As shown in Fig. 2, the first discharge at 0.05 C reaches 137.8 mAh / gs. This indicates that the presence of the sulphide-inhibiting layer on the separator increases the specific capacity of the cell, in other words, enabling more charge to be stored per gram of sulphur.

[0061] CASE STUDY 1c:

[0062] In this example embodiment, the metal-sulphur battery is an Li-S battery in the form of a coin cell. As with case study la, in case study 1c the battery comprises an electrolyte comprising a I M solution of LiTFSI in DOL / DME at a ratio of 1: 1 v / v with 0.25 M LiNOs, and comprises a Nanomyte cathode with 70 wt% sulphur, Super P carbon black and 10 wt% PVDF binder. The separator comprises a Celgard 2400 membrane. As with case study lb, the battery in case study 1c comprises a sulphide- inhibiting layer comprising a plurality of platelets. However, in case study 1c the sulphide-inhibiting layer comprising BNG platelets with 10 wt% PVDF binder is disposed on the cathode by a spray-coating process, on the side of the separator (i.e. a side of the cathode that is exposed to the electrolyte). It has also been confirmed via scanning electron microscopy (SEM) that the BNG platelets that were deposited via spraying lie flat on the surface of the Nanomyte cathode, as is illustrated schematically in Fig. 1. As shown in Fig. 2, the first discharge at 0.05 C reaches 185.6 mAh / gs, representing a 42% increase compared to the specific capacity of case study la.

[0063] CASE STUDY Id: In this example embodiment, the metal-sulphur battery is an Li-S battery in the form of a coin cell. As with case study la, in case study Id the battery comprises an electrolyte comprising a I M solution of LiTFSI in DOL / DME at a ratio of 1: 1 v / v with 0.25 M LiNOs, and comprises a Nanomyte cathode with 70 wt% sulphur, Super P carbon black and 10 wt% PVDF binder. The separator comprises a Celgard 2400 membrane. The battery in case study Id comprises the same sulphide-inhibiting layer as in case study lb, disposed on the separator, and also comprises the same sulphide- inhibiting layer as in case study 1c, disposed on the side of the cathode facing the separator. As shown in Fig. 2, the first discharge at 0.05 C in Figure 1 reaches 171.6 mAh / gs, which again represents a significant increase compared to the specific capacity of case study la.

[0064] The results from case studies lb to Id therefore demonstrate that the presence of a sulphide-inhibiting layer comprising a plurality of platelets, in this case BNG platelets, can increase the specific capacity of the battery relative to a comparative example without any such layer (i.e. case study la). The most significant increase is seen in case study 1c, where a layer of BNG platelets is disposed on the surface of the cathode only without any such layer on the surface of the separator. However, case studies lb and Id also demonstrate that a layer of BNG platelets on the surface of the separator still provides an increase in the specific capacity relative to the comparative example in case study la.

[0065] Referring now to Figs. 3 to 5, graphs are illustrated showing experimental results for five further case studies, 2a to 2e. In each of the case studies 2a to 2e, the battery comprises a lithium-sulphur (Li-S) battery. Figure 3 is a graph showing the results of galvanostatic charge-discharge (GOD) tests for the first discharge-charge cycle at a rate of 0.05 C for case studies 2a, 2b, 2c, 2d. Figure 4 is a graph showing results of the first discharge-charge cycle at a rate of 0.05 C for case studies 2a, 2b, 2c, 2e, according to embodiments of the present invention. Figure 5 is a graph showing cycling performance in terms of specific capacity of discharge (left-hand-side vertical axis) versus cycle number in scheduled GOD cycling at different C-rates as indicated by the C-rate schedule broken line (right-hand-side vertical axis) for case studies 2b, 2c, 2d, 2e, according to embodiments of the present invention.

[0066] CASE STUDY 2a:

[0067] Case study 2a is a comparative example taken from the scientific literature [McBrayer, J.D. JCESR Li-S battery research, Sandia National Laboratories - SAND2016-6712PE (2016)]. The metal-sulphur battery in this case is an Li-S battery in the form of a coin cell. As with case studies la to Id, the electrolyte in case study 2a comprises a 1 M solution of LiTFSI in DOL / DME at a ratio of 1 : 1 v / v with 0.25 M LiNCh. The cathode in case study 2a comprises 60 wt% sulphur, in combination with Ketjenblack (RTM) EC- 600JD (Lion Corporation, Japan), hereinafter referred to as "KB", and 10 wt% PVDF binder. The battery in case study 2a also comprises a Celgard separator. As shown in Fig. 3, the first discharge reaches 675 mAh / gs.

[0068] CASE STUDY 2b:

[0069] In this example embodiment, the metal-sulphur battery is an Li-S battery in the form of a coin cell. As with case studies la to Id and 2a, the electrolyte in case study 2b comprises a I M solution of LiTFSI in DOL / DME at a ratio of 1: 1 v / v with 0.25 M LiNOs, and the separator comprises a Celgard 2400 membrane. The cathode comprises 45.4 wt% sulphur, in combination with KB and 10 wt% PEDOT:PSS binder. This cathode is in the format of a coating, hereinafter referred to as the "cathode coating", which was formed on the surface of the current collector aluminium foil using the "doctor blade" technique for forming a film with well-defined thickness. Although aluminium foil is used as the current collector in this case study, in other embodiments a different material may be used as the current collector. As shown in Fig. 3, the first discharge reaches 744.6 mAh / gs to 1.7 V. The effect of the PEDOT:PSS binder is to reduce the overpotential in the low voltage plateau section from around 0.2-0.25 V to around 0.15 V. A third voltage plateau is also observed for case study 2b in Fig. 3, in the discharge range from 1.8 to 1.7 V. Without wishing to be bound by theory, it is hypothesised that this third voltage plateau is caused by the pseudocapacitance of PEDOT:PSS interacting with Li+ions. Figure 5 shows the cycling performance of the cell in terms of the specific capacity in discharge, when the Li-S cell is cycled in the specific schedule of C-rates presented by the broken black line (RHS vertical axis for C-rate). At C-rates > 0.5 C, the specific capacity drops to a negligible value due to resistance-induced voltage drop.

[0070] CASE STUDY 2c:

[0071] In this example embodiment, the metal-sulphur battery is an Li-S battery in the form of a coin cell. The electrolyte comprises a 1 M solution of LiTFSI in DOL / DME at a ratio of 1 : 1 v / v with 0.8 M LiNOs, and the separator comprises a Celgard 2400 membrane. The cathode comprises 45.4 wt% sulphur, in combination with KB and 10 wt% PEDOT:PSS binder. As with case study 2b, this cathode coating was formed on the surface of the current collector aluminium foil using the doctor blade technique. In addition, a sulphide-inhibiting layer comprising BNG platelets was sprayed on the cathode surface with 10 wt% PEO binder, where the molecular weight of PEO was Mv = 100,000. Testing demonstrated that the PEO (polyethylene oxide) does not dissolve in the electrolyte. The BNG sulphide-inhibiting layer was of negligible (i.e. not detectable) weight on the 19 mm cathode disc weighed on a balance of ±0.1 mg precision. As shown in Fig. 3, the first discharge reaches a specific capacity of 932.2 mAh / gs to 1.7 V and 1008 to 1.6 V, which is 35% higher than for CASE STUDY 2b (without the thin BNG sprayed layer on the cathode). Figure 5 also exhibits significantly improved cycling performance for case study 2c compared to case study 2b, at all tested C-rates.

[0072] CASE STUDY 2d:

[0073] In this example embodiment, the metal-sulphur battery is an Li-S battery in the form of a coin cell. The electrolyte, cathode and separator are the same as in case study 2c, except for the addition of 0.8 wt% silk fibroin in the electrolyte. As shown in Fig. 3, the first discharge reaches a specific capacity of 1343.3 mAh / gs to 1.7 V and 1372 mAh / gs to 1.6 V, which is 80% higher than for case study 2b (without the thin BNG sprayed layer on the cathode and without silk fibroin in the electrolyte). Figure 5 also exhibits significantly improved cycling performance at low C-rates below 0.5 C but inferior performance at 0.5 C and above, compared to case study 2c. Without wishing to be bound by theory, it is hypothesised that the addition of silk fibroin assists the BNGs in the trapping of polysulphides in the cathode, which can be attributed to the coordination number of polysulphides with the silk fibroin molecules. At higher rates, it is believed that the silk fibroin causes a resistance-induced voltage drop or traps Li+ions due to be transported to the cathode and lowers the rate of Li+ion transport, rendering it equivalent to a C-rate below 0.5 C.

[0074] CASE STUDY 2e:

[0075] In this example embodiment, the metal-sulphur battery is an Li-S battery in the form of a coin cell. The electrolyte, cathode and separator are the same as in case study 2c, except that the cathode coating comprises 55 wt% sulphur. As shown in Fig. 4, the first discharge reaches a specific capacity of 932 mAh / gs to 1.7 V, which is the same as case study 2c but with 10 wt% more sulphur than case study 2c. The larger amount of sulphur in the cathode seems to yield a longer second voltage plateau, but with the voltage falling faster in this plateau. Without wishing to be bound by theory, this behaviour can be attributed to the precipitation of larger amount of U2S2 and U2S raising the cell resistance. Figure 5 also exhibits that although the initial specific capacity in case study 2e is the same as that for case study 2c, it falls quickly as cycling is performed at low C-rates, but it reaches the specific capacity of case study 2c at IC-rate. In any case, the cycling performance of case study 2e with the sul phide-i nh ibiting BNG layer on the cathode is better than that of case study 2b (which does not include any such layer of BNG platelets). Since case study 2e has 55 wt% sulphur compared to 45 wt% sulphur for case study 2b, this means that the specific capacity of case study 2e with respect to the total cathode mass is even better than that of case study 2b.

[0076] From the results discussed above with reference to Figs. 3 to 5, it can be seen that the provision of a sulphide-inhibiting layer comprising BNG platelets sprayed on a KB- based cathode can raise the specific capacity of the cell by as much as 35%. When aided by the addition of silk fibroin in the electrolyte, the combined effect of the sulphide-inhibiting BNG layer on the cathode and the silk fibroin in the electrolyte can raise the specific capacity by a total of 80%. SEM-based examination of the KB-based cathodes in case studies 2c and 2d have demonstrated that the BNG platelets lie flat on the KB cathode and remain in-situ after GOD cycling and opening of the cells, demonstrating that the PEO binder used for the sulphide-inhibiting BNG layer in these embodiments is robust and does not dissolve in the electrolyte, maintaining the adherence of the BNG platelets on the cathode coating.

[0077] Referring now to Figs. 6 and 7, graphs are illustrated showing experimental results of galvanostatic charge-discharge (GOD) tests for four further case studies, 3a to 3d. In each of the case studies 3a to 3d, the battery comprises a sodium-sulphur (Na-S) battery with a KB-based cathode. Figure 6 is a graph showing results of GOD tests for the first discharge-charge cycle at a rate of 0.05 C for case studies 3a, 3b, 3c, 3d, according to further embodiments of the present invention. Figure 7 is a graph showing cycling performance in terms of specific capacity (left-hand-side vertical axis) versus cycle number in scheduled GOD cycling at different C-rates as indicated by the C-rate schedule broken line (right-hand-side vertical axis) for case studies 3a, 3b, 3c, 3d, according to embodiments of the present invention.

[0078] CASE STUDY 3a:

[0079] In this example embodiment, the metal-sulphur battery is a Na-S battery in the form of a coin cell. The electrolyte is a 1 M solution of NaTFSI in tetraethylene glycol dimethyl ether (TEGDME). The separator comprises a Celgard 2400 membrane. The cathode comprises 44 wt% sulphur in combination with KB and 10 wt% PVDF binder, and a cathode coating was fabricated using the doctor blade technique. As shown in Fig. 6, the first discharge reaches 30 mAh / gs to 0.5 V. However, the subsequent charge curve is very long to 918 mAh / gs. Without wishing to be bound by theory, it is suggested that this behaviour may be attributed to the heavy shuttling of polysulphides that migrated to the anode during the first discharge. This sulphide migration to the anode could also explain the low specific capacity in discharge, due to loss of active mass in the cathode. Figure 7 shows the cycling performance of the cell in terms of the specific capacity in discharge, when the Na-S cell is cycled in the specific schedule of C-rates presented by the broken black line (RHS vertical axis for C-rate). At C-rates > 0.5 C, the specific capacity drops to a negligible value due to resistance-induced voltage drop. The lower specific capacity of the Na-S cells compared to that of the equivalent Li-S cells is due to the fact that the sodium sulphides are less soluble in the Na-S cell electrolyte(s) than is the case for the lithium sulphides in the Li-S cell electrolyte DOL:DME.

[0080] CASE STUDY 3b:

[0081] In this example embodiment, the metal-sulphur battery is a Na-S battery in the form of a coin cell. As in case study 3a, the electrolyte comprises a 1 M solution of NaTFSI in TEGDME. The separator comprises a Celgard 2400 membrane. The cathode comprises 45 wt% sulphur in combination with KB and 10 wt% PEDOT:PSS binder, where the cathode coating was fabricated on current collector aluminium foil using the doctor blade technique. In addition, a sulphide-inhibiting layer comprising BNG platelets was sprayed on the cathode with 10 wt% PEO binder, where the molecular weight of PEO was Mv= 100,000. The BNG sulphide-inhibiting layer was of negligible (i.e. not detectable) weight on the 19 mm cathode disc weighed on a balance of ±0.1 mg precision. As shown in Fig. 6, the first discharge reaches a specific capacity of 688 mAh / gs to 0.5 V, which is 23 times higher than for case study 3a (with PVDF binder in cathode and without the sulphide-inhibiting layer sprayed on the cathode). Figure 7 also exhibits significantly improved cycling performance at low C-rates (0.05 and 0.1C) for case study 3b compared to case study 3a.

[0082] CASE STUDY 3c:

[0083] In this example embodiment, the metal-sulphur battery is a Na-S battery in the form of a coin cell. The electrolyte, cathode and separator are the same as in case study 3b, except for the addition of 0.8 wt% silk fibroin in the electrolyte. As shown in Fig. 6, the first discharge reaches a specific capacity of 489 mAh / gs to 0.5 V, which is less than for case study 3b (without fibroin). However, the battery in case study 3c exhibits a higher first voltage plateau in the first 300 mAh / gs, but exhibits negligible capacity above 0.05 C-rate as shown in Fig. 7.

[0084] CASE STUDY 3d: In this example embodiment, the metal-sulphur battery is a Na-S battery in the form of a coin cell. The electrolyte, cathode and separator are the same as in case study 3b, except for the addition of 0.8 wt% gelatine in the electrolyte. As shown in Fig. 6, the first discharge reaches a specific capacity of 725.9 mAh / gs to 0.5 V, which is 24 times higher than for case study 3a (with PVDF binder in cathode, without the sulphide-inhibiting layer sprayed on the cathode, and without gelatine in the electrolyte). Also, as shown in Fig. 7 the battery of case study 3d exhibits negligible capacity above 0.05 C-rate.

[0085] From the results discussed above with reference to Figs. 6 and 7, it can be seen that the provision of a sulphide-inhibiting layer comprising BNG platelets sprayed on a KB- based cathode can raise the specific capacity of Na-S batteries by as much as 23 to 24 times, compared to a similar battery but without such a sulphide-inhibiting layer on the cathode.

[0086] Referring now to Fig. 8, a flowchart is illustrated showing a method of manufacturing a metal-sulphur battery comprising a sulphide-inhibiting layer, according to an embodiment of the present invention. First, in step S801 a sulphide-inhibiting layer 150 is disposed between the cathode 110 and the anode 120 of the battery 100. The sulphide-inhibiting layer 150 comprises a plurality of platelets 151, as described above with reference to Figs. 1 to 7. In the present embodiment, step S801 involves disposing the sulphide-inhibiting layer 150 on a surface of the cathode 110, said surface being a surface that is exposed to the electrolyte 130. Hence, the battery that is manufactured in this embodiment comprises a layer of platelets 151 (i.e. the sulphide-inhibiting layer 150) on the surface of the cathode 110, similar to the batteries described above with reference to case studies 1c, Id, 2c, 2d, 2e, 3b, 3c and 3d. However, as noted above, in some embodiments the sulphide-inhibiting layer 150 may be disposed elsewhere, for example on a surface of the separator 140 as in case study lb. Accordingly, in such embodiments step S801 may involve disposing the sulphide-inhibiting layer 150 somewhere other than on a surface of the cathode 110.

[0087] Next, in step S802 the battery is assembled by disposing the separator 140 between the cathode 110 and the anode 120, and in step S803 the electrolyte is provided in the space between the cathode and the anode (in other words, the battery 100 is filled with electrolyte 130 in step S803).

[0088] Referring now to Fig. 9, a flowchart is illustrated showing a method of manufacturing a material for use as the cathode in a metal-sulphur battery, according to an embodiment of the present invention. First, in step S901 a cathode material comprising sulphur is obtained. Here, any suitable base material may be used for the cathode, including but not limited to the materials described above in case studies la to 3d. The cathode material may be in the form of a sheet, such that the resulting cathode has a relatively high surface area exposed to the electrolyte relative to the total mass of the cathode. Alternative, the cathode may be in the form of coating on the current collector foil, as is the case in the presented example cases. However, other forms of cathode are possible. Next, in step S902 a layer of platelets 151 is formed on the surface of the cathode material by spraying a carrier liquid onto the surface of the cathode material, the carrier liquid comprising a suspension of said platelets.

[0089] As a result of the spraying process, the platelets 151 in said layer 150 are orientated so as to lie substantially parallel to said surface of the cathode material. As discussed above with reference to Fig. 1, it is believed that this orientation of the platelets enhances the effectiveness of the sulphide-inhibiting layer 150 by acting as a physical barrier that inhibits the movement of sulphides from the cathode 110 to the anode 120, since sulphides forming at the cathode 110 must follow a tortuous path around and between the platelets 151 to pass through the sulphide-inhibiting layer 150.

[0090] Then, in step S903 the cathode material comprising the surface layer of platelets is divided into a plurality of parts, each of which can be used as a cathode in a separate battery. In some embodiments step S903 may be omitted, for example, if the cathode material that is obtained in step S901 is already the correct size and shape for use as a cathode in a battery.

[0091] Embodiments of the present invention have therefore been described in which a thin layer (e.g. a nanolayer) of platelets (e.g. BNG platelets) is provided in a metal-sulphur battery. For example, the layer of platelets may comprise a solid polymer electrolyte binder (e.g. PEG). The layer has been shown to be most effective when disposed on the surface of the cathode, although in some embodiments the layer of platelets may be disposed elsewhere in the battery, for example on the surface of the separator. The layer of platelets can inhibit the migration of soluble polysulphides from the cathode to the anode. In embodiments in which the layer of platelets is disposed on the cathode and is thereby in good electronic conduction with the rest of the circuit to which the battery is connected, the sulphur and sulphides trapped between the platelets (and binder, if present) can participate in the electrochemical redox reactions in the cathode. The inclusion of a layer of platelets in this way can potentially increase the cell capacity of an Li-S battery by as much as 50%, as well as decreasing overpotential and improving cyclability. The improvement is even more significant for a Na-S battery, raising the cell capacity by 23 times. Without wishing to be bound by theory, these effects are believed to be the result of several mechanisms taking place at the platelet layer: (a) the layer attracts polysulphides trying to leave the cathode (to move to anode) via the adsorption energy of its functional groups and the high tortuosity of its flat-lying platelets on the cathode surface, (b) the layer has good electron transport from cathode (i.e. when disposed on the surface of the cathode) and therefore offers the benefit to the sulphur and polysulphides trapped there to participate in electrochemical redox reactions, and (c) the layer can act as an electrocatalyst.

[0092] Whilst certain embodiments of the invention have been described herein with reference to the drawings, it will be understood that many variations and modifications will be possible without departing from the scope of the invention as defined in the accompanying claims.

Claims

Claims1. A metal-sulphur battery comprising: an anode comprising a metal; a cathode comprising sulphur; an electrolyte for transporting charged species between the anode and cathode during operation of the battery; a separator disposed between the anode and cathode so as to physically separate the anode from the cathode; and a sulphide-inhibiting layer disposed between the cathode and the anode, the sulphide-inhibiting layer comprising a plurality of platelets.

2. The metal-sulphur battery of claim 1, wherein the sulphide-inhibiting layer is disposed on a surface of the cathode, said surface being a surface that is exposed to the electrolyte.

3. The metal-sulphur battery according to claim 1 or 2, wherein the plurality of platelets comprise graphene platelets and / or graphite platelets.

4. The metal-sulphur battery according to claim 1, 2 or 3, wherein the platelets are orientated so as to lie substantially parallel to said surface of the cathode.

5. The metal-sulphur battery according to any one of the preceding claims, wherein the platelets comprise one or more dopants.

6. The metal-sulphur battery according to claim 4, wherein the one or more dopants comprises boron and / or nitrogen.

7. The metal-sulphur battery according to any one of the preceding claims, wherein the platelets comprise a plurality of functional groups.

8. The metal-sulphur battery according to claim 7, wherein the plurality of functional groups include catalytic functional groups comprising a transition metal.

9. The metal-sulphur battery according to claim 8, wherein the catalytic functional groups comprises one or more of: FelX , C0N4, VN4 or WN4.

10. The metal-sulphur battery according to any one of the preceding claims, wherein the sulphide-inhibiting layer comprises a matrix containing the platelets.

11. The metal-sulphur battery according to claim 10, wherein the matrix comprises a polymer.

12. The metal-sulphur battery according to claim 11, wherein said polymer comprises or consists of polyethylene glycol, PEG.

13. The metal-sulphur battery according to claim 11, wherein said polymer comprises or consists of polyethylene oxide, PEO.

14. The metal-sulphur battery according to any one of the preceding claims, wherein the cathode comprises sulphur encapsulated in a conductive material.

15. The metal-sulphur battery according to claim 14, wherein the conductive material comprises carbon.

16. The metal-sulphur battery according to any one of the preceding claims, wherein the cathode comprises a pseudocapacitive material arranged to bind the platelets in the sulphide-inhibiting layer.

17. The metal-sulphur battery according to claim 16, wherein said pseudocapacitive material comprises poly(3,4-ethylenedioxythiophene) polystyrene sulphonate, PEDOT:PSS.

18. The metal-sulphur battery according to any one of the preceding claims, wherein the electrolyte comprises silk fibroin or gelatine.

19. The metal-sulphur battery according to any one of the preceding claims, wherein the anode comprises: lithium; sodium; aluminium; or magnesium.

20. A method of manufacturing a metal-sulphur battery according to any one of the preceding claims, the method comprising: disposing the sulphide-inhibiting layer between the cathode and the anode of the battery, the sulphide-inhibiting layer comprising the plurality of platelets.

21. The method according to claim 20, wherein disposing the sulphide-inhibiting layer between the cathode and the anode comprises disposing the sulphide-inhibiting layer on a surface of the cathode, said surface being a surface that is exposed to the electrolyte.

22. The method according to claim 21, wherein after disposing the sulphide- inhibiting layer on said surface of the cathode the method comprises: assembling the battery by disposing the separator between the cathode and the anode; and providing the electrolyte in a space between the cathode and the anode.

23. A method of manufacturing a material for use as a cathode in a metal-sulphur battery according to any one of claims 1 to 19, the method comprising: obtaining a cathode material comprising sulphur; and forming a layer of platelets on said surface of the cathode material by spraying a carrier liquid onto the surface of the cathode material, the carrier liquid comprising a suspension of said platelets, such that the platelets in said layer are orientated so as to lie substantially parallel to said surface of the cathode material.

24. The method according to claim 23, wherein after forming the layer of platelets on the surface of the cathode material, the method further comprises: dividing the cathode material into a plurality of cathodes.

25. The method according to claim 23 or 24, wherein the cathode material is in the form of a sheet.

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