Electrochemical battery

Abstract

An electrochemical battery, comprising a sulphur cathode; a lithium anode; and a solid electrolyte between the sulphur cathode and the lithium anode, wherein the solid electrolyte has an argyrodite-type structure and comprises lithium, phosphorus, sulphur, and a halogen.

Description

Electrochemical battery Technical field

[0001] The present invention relates to an electrochemical battery having a solid electrolyte. Cross reference to related applications

[0002] This application claims priority from GB2318498.9 filed on 4 December 2023, the contents of which are hereby incorporated by reference. Background Art

[0003] All solid-state batteries (ASSB) are a promising development over conventional batteries which use a liquid electrolyte. For example, ASSB can solve many problems of liquid Li-ion batteries, such as flammability, limited voltage, unstable stable solid-electrolyte interface formation and poor cycling performance.

[0004] Conventional Li-ion batteries rely on the reversible intercalation of lithium ions into layered structures of the cathode and / or anode.

[0005] Solid-state Li metal batteries have drawn great research attention because of the non-flammable nature of the solid-state electrolytes (SE) and high energy density of Li metal anodes. The lithium metal anode of solid-state Li-metal batteries provides much higher energy density than Li-ion batteries which rely on lithium ion intercalation.

[0006] Some known examples of lithium ASSB have a Li anode and a cathode, with a SE between the anode and cathode. The capacity of a battery is theoretically determined by the anode capacity and the cathode capacity. However practical implementations of ASSBs have been limited by the cathode capacity. Furthermore, selection of an appropriate SE which is compatible with the cathode and the anode has been challenging due to a number of problems, such as electrochemical and chemical compatibility with the cathode / anode and poor conductivity. Many known SEs are not compatible with a lithium anode. Similar problems occur in anodes comprising a lithium alloy, rather than elemental lithium. Known SEs also show poor compatibility with known anodes and known cathodes.

[0007] Optionally, an SE may be dispersed within the cathode to improve the cathode Li+conductivity. However, incorporation of SE into the cathode reduces the energy capacity and energy density of the battery. Furthermore, incorporation of SEs into the cathode may be challenging if the SE is chemically incompatible with the cathode.

[0008] There have been no practical implementations of lithium-anode ASSBs having a high capacity and high energy density.

[0009] US 2023 / 0019090 A1 discloses a sulphide solid electrolyte for an all-solid secondary battery including lithium, phosphorus, sulphur, oxygen, and halogen atoms, wherein the sulphide solid electrolyte has an argyrodite-type crystal structure, the halogen atoms including chlorine and at least two of bromine, iodine, and fluorine, an atomic ratio of sulphur to oxygen in the sulphide solid electrolyte is about 4 or higher, and an atomic ratio of chlorine to the at least two of bromine, iodine, and fluorine is about 9 or higher. US 2023 / 0019090 A1 discloses that this SE may be used in a battery having a lithium anode and a cathode including one of the following cathode active materials: lithium transition metal oxide (e.g., lithium cobalt oxide (LCO), lithium nickel oxide, lithium nickel cobalt oxide, lithium nickel cobalt aluminum oxide (NCA), lithium nickel cobalt manganese oxide (NCM), lithium manganate, and lithium iron phosphate), nickel sulphide, copper sulphide, lithium sulphide, iron oxide, or vanadium oxide.

[0010] US 2023 / 0299333 A1 proposes a number of solid electrolytes having an argyrodite crystal structure comprising sulphur and oxygen, the argyrodite crystal structure having the formula: (Li1-kMk)7-(a+b)PS6-(a+b+x)OxClaBrb, wherein 0.5≤a / b≤1.5, 0<a<2, 0<b<2, 0<x≤l, 0≤k<l, and x<3-(a+b) / 2 are satisfied. Oxygen doping will cause the decrease of Li+conductivity of the SE.

[0011] US 2021 / 0320329 A1 discloses a solid electrolyte having a lithium-argyrodite structure for use in an all solid-state lithium ion battery.

[0012] WO 2023 / 100193 A1 discloses solid electrolyte particulates for use in ASSB. The solid electrolyte particulates comprise a core structure comprising a sulphide electrolyte and an outer layer covering the core structure, wherein the outer layer comprises an oxide electrolyte. The presence of the core reduces Li conductivity.

[0013] US 2021 / 0323824 A1 discloses a solid electrolyte having an argyrodite structure comprising Li, P, S, O and one or more selected from the group consisting of Cl, Br and I.

[0014] Matsuda et al., J. Phys. Chem. C 2022, 126, 14067−14074 discloses halogen-rich argyrodites for use in lithium ASSB batteries.

[0015] Zhichao Zhang et al.2022 J. Electrochem. Soc.169040553 discloses lithium argyrodite electrolytes for use in a lithium ASSB having a composite cathode obtained by mixing LiNbO3- coated LiCoO2cathode and Li5.4PS4.4Cl1.2Br0.4electrolyte (7:3 mass ratio).

[0016] Patel et al. Chem. Mater.2021, 33, 1435−1443 discloses use of a lithium argyrodite structure for ASSB.

[0017] US2024 / 088429 A1 discloses a solid electrolyte comprising Li5.4PS4.4Cl0.8Br0.8.

[0018] EP3511948B1 discloses the use of argyrodite solid electrolytes in lithium-ion batteries.

[0019] Li et al. “High-Entropy Lithium Argyrodite Solid Electrolytes Enabling Stable All-Solid- State Batteries” Angew. Chem. Int. Ed.2023, 62, e202314155 discloses a solid electrolyte comprising Li5.5PS4.5Cl0.8Br0.7in an all-solid-state battery comprising a nickel-rich oxide cathode and a In / InLi anode. Li et al. underscores the ionic conductivity of this electrolyte for the nickel- rich oxide and In / InLi battery chemistry.

[0020] The present invention aims to provide a lithium all solid-state battery with high capacity and high energy density. Summary of invention

[0021] The present invention provides an electrochemical battery, comprising: a sulphur cathode; a lithium anode; and a solid electrolyte between the sulphur cathode and the lithium anode, wherein the solid electrolyte has an argyrodite-type structure and comprises lithium, phosphorus, sulphur, and a halogen. A solid electrolyte having an argyrodite-type structure and comprising lithium, phosphorus, sulphur, and a halogen has been not previously been used in a battery having a sulphur cathode and a lithium anode. The inventors have discovered that a lithium electrochemical battery having a sulphur cathode and an argyrodite solid electrolyte exhibits superior electrochemical stability, has improved energy density and improved capacity.

[0022] In an embodiment, the solid electrolyte comprises at least two different halogens. This is particularly advantageous, because the Li+conductivity is increased when at least two different halogens are included. By introducing at least two different halogens, the activation energy for Li+diffusion through the solid electrolyte is reduced, and thus an improved ionic conductivity of Li+is achieved.

[0023] In an embodiment, the ratio of lithium to halogen in the solid electrolyte is less than 6:1, or preferably less than 5.9:1.1 or more preferably less than 5.8:1.2. According to this embodiment the ionic conductivity of the solid electrolyte is particularly high. When the ratio of lithium to halogen in the solid electrolyte falls outside these ranges, the ionic conductivity of the SE is reduced so that the full advantage of the invention may not be realised.

[0024] In an embodiment, the ratio of lithium to halogen in the solid electrolyte is more than 5.25:1.75, or preferably more than 5.55:1.65, or more preferably more than 5.4:1.6. According to this embodiment the ionic conductivity of the solid electrolyte is particularly high. When the ratio of lithium to halogen in the solid electrolyte falls outside these ranges, the ionic conductivity of the SE is reduced so that the full advantage of the invention may not be realised.

[0025] In an embodiment, the argyrodite-type structure of the solid electrolyte is represented by Formula 1, and wherein A, B C and D are halogens: Li7−x−y-z-w(PS4)(S2−x−y-z-wAxByCzDw) [Formula 1]. When the solid electrolyte is represented by Formula 1, the ionic conductivity of the solid electrolyte is particularly high and improved Li+conductivity is achieved.

[0026] In an embodiment having a solid electrolyte represented by Formula 1, 0.1<w<0.4 and 0.1<z<0.4. When this relationship is satisfied, the crystalline structure of the electrolyte is such that the activation barrier for Li+diffusion is particularly low, and therefore Li+conductivity is improved.

[0027] In an embodiment having a solid electrolyte represented by Formula 1, x, y, z and w satisfy the following condition x + y + z +w > 1, or preferably x + y + z +w > 1.1, and more preferably x + y + z + w > 1.2 and / or wherein x, y, z and w satisfy the following condition x + y + z + w < 1.8, preferably x + y + z + w < 1.75, more preferably x + y + z + w < 1.7. When at least two halogens are provided according to this ratio the crystalline structure of the electrolyte is formed such that the activation barrier for Li+diffusion is particularly low, and therefore Li+conductivity is improved.

[0028] In an embodiment, the argyrodite-type crystal structure of the solid electrolyte is represented by Formula 2: Li7−x−y(PS4)(S2−x−yClxBry) [Formula 2]. The present inventors have found that this crystal structure of the solid electrolyte is particularly suitable for use in lithium- sulphur batteries, because it shows excellent electrochemical compatibility with the lithium anode and shows particularly high Li+conductivity.

[0029] In an embodiment having a solid electrolyte represented by Formula 2, x and y may satisfy the following condition x + y > 1, or preferably x + y > 1.1, and more preferably x + y > 1.2. When the electrolyte comprises chlorine and bromine according to this ratio, the crystalline structure of the electrolyte is formed such that the activation barrier for Li+diffusion is particularly low, and therefore Li+conductivity is improved.

[0030] In an embodiment having a solid electrolyte represented by Formula 2, x and y may satisfy the following condition x + y <1.8, preferably x + y < 1.75, more preferably x + y < 1.7. When chlorine and bromine are provided according to this ratio the crystalline structure of the electrolyte is formed such that the activation barrier for Li+diffusion is particularly low, and therefore Li+conductivity is improved.

[0031] In an embodiment having a solid electrolyte represented by Formula 2, x may be equal to or greater than 0.55, preferably equal to or greater than 0.6, more preferably equal to or greater than 0.65, and / or x may be equal to or less than 0.9, preferably equal to or less than 0.85, morepreferably equal to or less than 0.8. When x satisfies this condition, the electrolyte exhibits particularly high Li+conductivity and compatibility with the cathode and anode, due to the crystal structure of the electrolyte.

[0032] In an embodiment having a solid electrolyte represented by Formula 2, y may be equal to or greater than 0.55, preferably equal to or greater than 0.6, more preferably equal to or greater than 0.65, and / or y may be equal to or less than 0.9, preferably equal to or less than 0.85, more preferably equal to or less than 0.8. When y satisfies this condition, the electrolyte exhibits particularly high Li+conductivity and compatibility with the cathode and anode, due to the crystal structure of the electrolyte.

[0033] In an embodiment, x is equal to y. The inventors have found that an argyrodite structure comprising equal amounts of chlorine and bromine results in the argyrodite crystal structure which shows particularly high Li+conductivity. Compositions of the argyrodite structure having equal amounts of chlorine and bromine have particularly high configurational entropies. The present inventors have found that the high configurational entropies of argyrodite structures lead to particularly high Li-ion conductivity.

[0034] In an embodiment, wherein the solid electrolyte is one of: Li5.5(PS4.5)(Cl0.75Br0.75), Li5.7(PS4.7)(Cl0.65Br0.65). In an embodiment, the solid electrolyte consists of Li5.5(PS4.5)(Cl0.75Br0.75) or Li5.7(PS4.7)(Cl0.65Br0.65), and optionally, incidental impurities. The inventors have found that these compounds are particularly effective solid electrolytes for a lithium-sulphur battery, and lead to a battery which shows high capacity conductivity and high electrochemical stability. These compositions have particularly high Li-ion conductivities and particularly high stability with the anode and the cathode.

[0035] In an embodiment, the Li+conductivity of the solid electrolyte is equal to or greater than 2 mS / cm and equal to or less than 11 mS / cm. The inventors have found that a solid electrolyte with a Li+conductivity in this range is particularly advantageous for a lithium solid-state battery having a sulphur cathode, because improved electrochemical kinetics are achieved at the cathode / electrolyte interface.

[0036] In an embodiment, the cathode layer comprises elemental sulphur. The inventors have found that when the cathode comprises elemental sulphur, the capacity of the electrochemical battery is improved, especially when compared to known cathodes.

[0037] In an embodiment, the cathode layer further comprises the solid electrolyte. By including the solid electrolyte within the cathode layer, the conductivity of the cathode layer can be improved and superior electrochemical kinetics are achieved.

[0038] In an embodiment, 10-50 vol.% of the cathode consists of sulphur. Providing 10-50 vol.% of sulphur in the cathode leads to a particularly good balance between utilising the high capacity of the sulphur cathode, without sacrificing the energy density of the battery. The remainder of the cathode volume may be occupied by other constituents, as discussed below, which may help improve the conductivity and the electrochemical kinetics of the cathode.

[0039] In an embodiment, the cathode further comprises a conductive additive, optionally, wherein 10-30 vol.% of the cathode comprises the conductive additive. By including 10-30 vol.% of conductive additive in the cathode, the total ionic conductivity of the sulphur cathode is improved while still maintaining excellent energy density.

[0040] In an embodiment, the conductive additive comprises carbon particles. Including carbon particles as the conductive additive has been demonstrated to lead to improvements in the electronic conductivity of the cathode.

[0041] In an embodiment, the cathode further comprises a binder, preferably an organic binder. By providing a binder, preferably an organic binder, the stability of the cathode is further improved.

[0042] In an embodiment, the lithium anode comprises lithium metal. Batteries with an anode comprising lithium metal have particularly high energy density. During discharge, dissolution of metallic lithium occurs at the anode. During charge, electrodeposition of lithium occurs at the anode.

[0043] In an embodiment, the solid electrolyte consists of lithium, phosphorus, sulphur, and halogen(s) in an argyrodite-type structure and incidental impurities. A solid electrolyte consisting of these elements has been shown to exhibit superior qualities to known argyrodite-type solid electrolytes.

[0044] In an embodiment, the lithium content of the solid electrolyte is homogenous between the anode and cathode. By providing a homogenous concentration of lithium within the solid electrolyte, the ionic conductivity of the solid electrolyte remains high thorough the entire thickness of the solid electrolyte layer. By removing any variations in the concentration of lithium within the solid electrolyte the total ionic conductivity of the electrolyte is improved.

[0045] The term “consisting of” is used herein to indicate that 100% of the composition is being referred to and the presence of additional components is excluded so that percentages add up to 100 volume percent. Unless stated otherwise, all amounts are given in volume percent (vol%). The term comprising is used in a non-exhaustive way, indicating that other components may be present.Figures

[0046] The invention will be more fully described, by way of example only, with reference to the accompanying drawings in which:

[0047] Figure 1 schematically illustrates a layered structure of a solid-state battery.

[0048] Figure 2a and Figure 2b show powder X-ray diffraction (XRD) data, illustrating the lattice spacing of a number of argyrodite samples with different Cl-to-Br ratios (abbreviated to ClxBry, or ClBrxwhen Cl:Br equals to 1:1).

[0049] Figure 3a and Figure 3b show unit cell parameters from Rietveld refinements, based on the results shown in Figures 2a and 2b. Figure 3c and Figure 3d show ionic conductivity of the synthesised products with different compositions, characterised by electrochemical impedance spectroscopy (EIS) tests.

[0050] Figures 4a-4c shows7Li solid-state NMR characterizations of sample Cl0.65Br0.65. Figure 4a shows the7Li spin-lattice relaxation (SLR) rates. Figures 4b and 4c show diffusion coefficients by pulse-field gradient (PFG) test at different temperatures, with dotted lines showing the respective fitting results.

[0051] Figure 5a shows current-voltage (CV) curves with step wise increased highest cut offs of unmodified argyrodite Cl1 (i.e. chlorine only), and Figure 5b shows that of modified argyrodite (Cl0.75Br0.75).

[0052] Figures 6a shows the X-ray photoemission spectroscopy (in situ-XPS) of unmodified argyrodite Cl1, Figure 6b shows the X-ray photoemission spectroscopy (in situ-XPS) of Cl0.65Br0.65, and Figure 6c shows the X-ray photoemission spectroscopy (in situ-XPS) of Cl0.75Br0.75, proving the stability of the materials to Li metal with self-limiting passivating SEI (solid electrolyte interface) layers forming stopping them from further reactions.

[0053] Figure 7a-c shows critical current density (CCD) test of Cl0.5Br0.5, Cl0.65Br0.65, Cl0.75Br0.75 respectively, and Figure 7d shows a comparative chart of CDD results for Cl0.5Br0.5, Cl0.65Br0.65, Cl0.75Br0.75 and unmodified argyrodite Cl1.

[0054] Figures 8a-b show long-time cycling performances in Li-metal symmetric cells of different products Cl0.65Br0.65 and Cl0.75Br0.75.

[0055] Figure 9a shows the rate performance at different current densities of the Li-S ASSB with Cl1 and Cl0.65Br0.65 as the SE respectively, with LiIn as the anode and a S loading of 3 mg cm-2. Figure 9b shows the corresponding charge-discharge curves of Li-S ASSBs with Li-In anodes. Figures 9c shows the cycling performance of Li-S cell with Cl0.65Br0.65 as the SE, LiIn as theanode and a S loading of 5 mg cm-2, at a current density of 0.5mAcm−2and Figure 9d shows the charge-dicharge curve of the initial cycle. Figures 9e shows the cycling performance of Li-S cells with Li metal as the anode and a S loading of 3 mg cm-2at a current density of 0.5mAcm−2. Figure 9f shows the areal capacity and S utilization of the Cl0.65Br0.65 cell compared with reported values in the literature, at a current density 2.5 mAcm−2of the Li-S cells with different SE materials.

[0056] Figure 10 shows Li-S cell cycling performance with Li metal anode and electrolyte Cl0.75Br0.75 at a current density of 0.5 mAcm−2.

[0057] Figure 11 shows a ternary phase diagram of a Li7-x-yPS6-x-yClxBry, with some of the best- performing compositions marked.

[0058] Figure 12 shows refinements conducted on the synchrotron XRD data of the modified argyrodite materials. Figure 13 shows SEI layer formation rate investigated for Cl0.65Br0.65, Cl0.75Br0.75 and Cl1.

[0059] Figure 14 shows cross-section SEM images of the cold-pressed pellets of the argyrodite materials. Detailed Description

[0060] Solid-state lithium metal batteries have drawn great research attention because of the non- flammable nature of the solid-state electrolytes (SE) and high energy density of Li metal anode.

[0061] A plurality of cathodes has been proposed for use in Li ASSB, such as the cathodes identified in the prior art mentioned above. However, lithium ASSBs with these known cathodes exhibit relatively low capacity and energy density. Furthermore, many of the known cathodes use elements such as cobalt, which have low abundance and are not environmentally friendly.

[0062] The present inventors have identified sulphur as a promising candidate for a cathode because of its high capacity and high energy density. The inventors postulated that a battery having a lithium anode and a sulphur cathode may be particularly advantageous, because of its high energy density, high capacity, as well as natural abundance, low cost, and low environmental impact of lithium and sulphur.

[0063] A lithium-sulphur battery is based on the lithium / sulphur redox couple. Lithium dissolution occurs at the anode surface and lithium incorporation into polysulphide salts occurs at the cathode during discharge. This reaction is reversed during charge. The polysulphide salts may be formed on the surface of the cathode during discharge, and then reduced back to sulphur during charge. The main difficulty with this arrangement has been depletion of the cathode due tomigration of intermediate polysulphides. This is referred to as the “shuttling” effect, described below and this is what has limited development of this type of battery in the past.

[0064] During discharge, the lithium ions migrate from the anode to the cathode where the sulphur is reduced to lithium sulphide (Li2S). The semi-reaction of the cathode may therefore expressed as:

[0065] S + 2Li++ 2e- <=> Li2S

[0066] At the anode, dissolution of the metallic lithium occurs, with the production of electrons and lithium ions during the discharge and electrodeposition during the charge. The half-reaction may be expressed as:

[0067] Li <=> Li++ e-

[0068] The present inventors, by overcoming the shuttling difficulties of these type of batteries open up the possibility of batteries based on the lithium / sulphur redox couple. This allows batteries with lithium and sulphur to be produced. Because the atomic weights of lithium and sulphur are relatively low, a Li–S battery may advantageously be relatively light. Therefore, Li-S batteries of the invention are superior in terms of their high energy density (how much energy is stored per unit weight of the battery, expressed in Wh / kg) and capacity (total amount of electricity generated due to electrochemical reactions in the battery, typically expressed in Ah).

[0069] Furthermore, use of a sulphur cathode in ASSB has not been possible so far for a number of reasons.

[0070] A sulphur cathode has the drawback that is exhibits low Li+conductivity.

[0071] The inventors have discovered that this may be alleviated by using a SE having a particularly high Li+conductivity. However, finding a suitable SE for a Li-S ASSB has been challenging, because of the following problems with known SE. These problems are detrimental to battery performance.

[0072] First, known SE exhibit a Li+conductivity which is too low for use with a sulphur cathode. Second, known SE are incompatible with the lithium metal anode and are unstable at the SE / Li anode interface. Furthermore, known SE do not supress the formation of lithium dendrites through the SE, which can lead to an internal short circuit in the battery.

[0073] An additional challenge in lithium-sulphur batteries is the “shuttling effect” which can be detrimental to a Li-S battery. The “shuttling effect” is known from existing liquid electrolyte Li-S batteries.

[0074] In a lithium-sulphur battery, Li oxidation occurs with the formation of Li+ ions at the lithium anode, and the Li+ ions diffuse through lithium-conducting electrolyte towards the cathode. On the cathode, sulphur is reduced.

[0075] The reduction of sulphur to Li2S during discharge proceeds via a multi-step process in which a variety of intermediate polysulphides of the general formula Li2Snare formed, where n is between 1 and 8. These intermediate polysulphides can dissolve in the electrolyte and diffuse to the lithium anode. This so-called shuttling effect becomes problematic because these dissolved polysulphides can migrate back to the cathode during subsequent cycles, leading to loss of active material and reduced battery performance over time. This phenomenon can contribute to capacity fading, lower energy efficiency, and decreased overall battery life.

[0076] Therefore, incorporation of a sulphur cathode into a lithium ASSB is particularly challenging, as the detrimental impact of the shuttling effect should be avoided. A SE for a Li-S ASSB should preferably reduce polysulphide formation or dissolution through the SE, so that battery capacity can be retained during cycling of Li-S cells. Furthermore, the SE should preferably exhibit high ionic conductivity of Li+, good chemical stability with electrode materials, good electrochemical stability within the operating voltage range, and form a stable interface with the anode and with the cathode. Even more preferably, the SE should exhibit high ionic conductivity comparable to liquid electrolytes (at least 10 mS cm−1).

[0077] The present inventors have found and demonstrated an electrochemical battery which utilises the high capacity and energy density of a sulphur cathode in combination with an electrolyte which overcomes the problems outlined above. According to the present invention, there is provided an electrochemical battery, comprising: a sulphur cathode, a lithium anode, and a solid electrolyte between the sulphur cathode and the lithium anode, wherein the solid electrolyte has an argyrodite-type structure and comprises lithium, phosphorus, sulphur, and a halogen.

[0078] The structure of a lithium argyrodite structure is an argyrodite-type structure in Fm-3m space group, with Li+ions forming cages surrounding a sulphide anion. Some of the sulphide anion sites are substituted with the halogen atoms. Lithium argyrodites have the general formula Li7−x−y-z-w(PS4)(S2−x−y-z-wAxByCzDw), wherein A, B, C and D are halogens. This group of compounds may also be referred to as a “sulphide argyrodite” or a “lithium argyrodite” or “argyrodite” for brevity. During Li+diffusion, the Li+ “hop” between the cages, and this hopping is heavily influenced by the exact crystal structure. The present inventors have found that the crystal structure may be affected by factors such as anion species or anion site disorder, (i.e. the lattice / unit cell dimensions, and local deformations of the argyrodite structure can be affected).

[0079] The argyrodite-type structure may comprise any halogen, and may comprise a number of different halogens (for example 2, 3, 4 or more). The halogen may be one or more of fluorine, chlorine, bromine, iodine, etc. Optionally the halogens in the argyrodite may be chlorine and bromine. The two halogens are shown below to give the argyrodite superior properties. However because of the similarity between different halogens, it is predictable that other halogens will behave in a similar way.

[0080] The lithium anode may comprise elemental lithium, or may comprise a lithium alloy such as LiIn. While reference is made to a “lithium anode” through the description and claims, the “lithium anode” may comprise a lithium alloy. In the case that the lithium anode is comprised of a lithium alloy, the alloy may optionally consist of up to 15wt% alloying elements. During discharge, lithium ions transport from the anode to the cathode through the electrolyte. During charge, lithium ions are electrodeposited to the anode. In other words, the lithium anode may comprise lithium in a state of metal or in a state of an alloy, rather than comprising lithium which is in a state of a compound such as an oxide. The anode may consist of lithium in a state of metal or a state of an alloy, and optionally, incidental impurities.

[0081] The sulphur cathode may comprise amorphous sulphur, elemental sulphur, or sulphur in a molecular state such as S8. The sulphur cathode may consist of one of amorphous sulphur, elemental sulphur, or sulphur in a molecular state such as S8, and optionally, incidental impurities. During discharge, sulphur in the cathode is reduced to lithium sulphide (Li2S). During charge, the lithium sulphide is oxidised to S8. In the charged state, up to 50 vol. % of the cathode may consist of S8and the remainder may consist of the solid electrolyte and conductive additives.

[0082] An example of the present invention is shown on Figure 1, which shows a cathode layer 100 (may be referred to as “the cathode”), a solid electrolyte layer 200 (may be referred to as the “the solid electrolyte” or “the SE”), an anode layer 300 (may be referred to as “the anode”) and an optional current collector 400. In an embodiment the cathode layer 100 is in contact with the solid electrolyte layer 200. In an embodiment the solid electrolyte layer 200 is in contact with the anode layer 300. In an embodiment, a solid electrolyte interphase (SEI) layer may be present between the anode layer 300 and the solid electrolyte layer 200. The SEI layer may be formed on the anode by a chemical reaction between the anode and the SE. The SEI layer allows Li+ion transport while preventing further reaction between the electrolyte and the anode.

[0083] Another current collector may be included on top of the cathode layer 100 (on a side of the cathode layer opposite to the solid electrolyte layer 200). Other layers may be present.

[0084] The solid electrolyte layer is shown as an aggregation of spherical particles of the SE 102. However, the solid electrolyte layer may be a bulk layer, that is, the layer may be substantially free of pores. The solid electrolyte layer optionally consists of the argyrodite type structure and incidental impurities. Figure 1 shows that the cathode layer comprises spherical cathode material particles 101 and some particles of the SE 102, however, this configuration of the cathode is an optional embodiment. The cathode layer 100 may consist of a sulphur, or may comprise further constituents as discussed below with reference to optional features of the invention. The cathode layer 100 may be a bulk layer, that is, the layer may be substantially free of pores, rather than an aggregation of particles.

[0085] A sulphide argyrodite SE having an argyrodite-type structure and comprising lithium, phosphorus, sulphur, and at least one halogen exhibits high ionic conductivity. The crystal structure of the argyrodite SE can be modified by the constituent elements, and the inventors have found that lithium, phosphorus, sulphur, and at least one halogen lead to particularly advantageous properties of the SE, such as high ionic conductivity. This makes it a surprisingly superior SE, as this SE shows a good electrochemical stability with the sulphur cathode and the lithium anode, and is stable solid electrolyte interphase (SEI) layer when contacting Li metal. The SEI is a layer formed on the surface of the anode from the electrochemical reduction of the electrolyte, and plays an important role in the long term cyclability of the battery.

[0086] Surprisingly the shuttling effect is supressed by use of the argyrodite of the present invention. It is thought this is because the diffusion of the polysulphides in the argyrodite is low.

[0087] The sulphide argyrodite SE also supresses growth of lithium dendrites through the structure, which is one of the main problems with batteries having a lithium anode. This was shown experimentally by carrying out critical current density experiments, as shown on Figures 7a- 7d (and discussed in detail below). This dendrite-suppressing behaviour was not expected.

[0088] According to the present invention, a halogen-rich argyrodite structure is provided, which shows significant improvement on ionic conductivity without compromise of interfacial and electrochemical stability. The inventors have found that providing an argyrodite structure having lithium, phosphorus, sulphur, and at least one halogen lowers the Li+diffusion barrier and improves the Li conductivity. This is shown experimentally below, for example with reference to Figure 3 and Figure 4 (and discussed in detail below).

[0089] The solid electrolyte may comprise at least two different halogens. By introducing different halogen anions, the activation energy for Li+diffusion can be lowered because of the higher polarizability of the SE, and thus an improved ionic conductivity can be achieved. This isshown experimentally below, for example with reference to Figure 3 (and discussed in detail below). The halogens can be any one or more of Group 17 elements of the periodic table, such as fluorine (F), chlorine (Cl), bromine (Br), iodine (I), etc. Furthermore, introducing different anions increases formation entropy (with the more complex compositions), resulting in a crystalline structure that can tolerate more defect content. Argyrodite structures with high-entropy exhibit improved ionic conductivity compared to their lower-entropy counterparts.

[0090] The ratio of lithium to halogen in the solid electrolyte may be less than 6:1, or preferably less than 5.9:1.1 or more preferably less than 5.8:1.2. As the relative amount of halogen to lithium is increased, the activation energy can be lowered because of the higher polarizability, and thus an improved ionic conductivity can be achieved. When the relative amount of halogen to lithium is too low, the ionic conductivity is reduced. This is shown experimentally below, for example with reference to Figure 3 and Figure 9 (and discussed in detail below). When the ratio of halogen X to lithium is too high, LiX impurities precipitate out. Precipitation of LiX in the SE is detrimental to battery performance, as LiX precipitates reduce the effective volume of SE in the battery and lead to lower ionic conductivity of the SE. Precipitation of LiX impurities is discussed in further detail with reference to figure 2.

[0091] The ratio of lithium to halogen in the solid electrolyte may be more than 5.25:1.75, or preferably more than 5.55:1.65, or more preferably more than 5.4:1.6. When the relative amount of lithium to halogen is too low, the critical current density (i.e. the current density at which the cell short-circuits) decreases. The improved compatibility of a SE according to the present invention with the anode is described with reference to Figures 9a-9f, as described below. A SE having a lithium to halogen ratio of 6:1 shows inferior compatibility with the lithium anode, when compared to a SE having a lithium to halogen ratio of 5.5:1.5.

[0092] In an embodiment of the invention, the argyrodite-type crystal structure of the solid electrolyte is represented by Formula 1, wherein A, B C and D are halogens: [Formula 1] Li7−x−y-z-w(PS4)(S2−x−y-z-wAxByCzDw).

[0093] Providing halogen-rich argyrodite structure can lead to particularly high ionic conductivity, making this SE particularly suitable for a battery having a sulphur-cathode. Furthermore, by providing an argyrodite structure without oxygen, the Li+ionic conductivity of the argyrodite structure is improved, because oxygen doping of the argyrodite decreases Li+ ionic conductivity.

[0094] Optionally, w and z in Formula 1 satisfy the following relationship: 0.1<w<0.4 and 0.1<z<0.4. Optionally, x, y, z and w in Formula 1 satisfy the following condition x + y + z +w > 1,or preferably x + y + z +w > 1.1, and more preferably x + y + z + w > 1.2 and / or wherein x, y, z and w satisfy the following condition x + y + z + w < 1.8, preferably x + y + z + w < 1.75, more preferably x + y + z + w < 1.7. The inventors have found that providing a structure having a mixture of halogens which satisfy any one or more of these conditions achieves the highest Li+conductivity, while being compatible with the lithium anode. For example, an argyrodite structure having structure Li6.4PS5.4Cl0.3Br0.3(x + y + z +w=0.6) exhibits lower conductivity than Li5.5PS4.5Cl0.75Br0.75(x + y + z +w=1.5), and Li5.2PS4.2Cl0.9Br0.9(x + y + z +w=1.8) exhibits lower conductivity than Li5.5PS4.5Cl0.75Br0.75(x + y + z +w=1.5). This is described with reference to Figure 3 (discussed in detail below).

[0095] In an embodiment of the invention, the argyrodite-type crystal structure of the solid electrolyte is represented by Formula 2:

[0096] [Formula 2] Li7−x−y(PS4)(S2−x−yClxBry).

[0097] Formula 2 is a specific example of Formula 1 where w and z have been set to 0, and halogens A and B correspond to Cl and Br.

[0098] The values of x and y may satisfy the following condition x + y > 1, or preferably x + y > 1.1, and more preferably x + y > 1.2 and / or optionally the following condition x + y < 1.8, preferably x + y < 1.75, more preferably x + y < 1.7. For example, an argyrodite structure having structure Li6.4PS5.4Cl0.3Br0.3(x + y + z +w=0.6) exhibits lower conductivity than Li5.5PS4.5Cl0.75Br0.75(x + y + z +w=1.5) and Li5.7PS4.7Cl0.65Br0.65(x + y + z +w=1.3) and Li5.2PS4.2Cl0.9Br0.9(x + y + z +w=1.8) exhibits lower conductivity than Li5.5PS4.5Cl0.75Br0.75(x + y + z +w=1.5) and Li5.7PS4.7Cl0.65Br0.65(x + y + z +w=1.3). This is described with reference to Figure 3 (discussed in detail below). In the present disclosure, compounds according to Formula 2 may be referred by reference to their chlorine and bromine content as “Cl x Br y” for brevity. For example, Li5.7PS4.7Cl1Br0.3is referred to as Cl1Br0.3 and Li6PS5Cl is referred to as Cl1. In the present disclosure, an argyrodite compound according to Formula 2 where x is equal to y is referred to as “Cl x Br x”. For example, Li5.5(PS4.5)(Cl0.75Br0.75) is referred to as Cl0.75Br0.75 and Li5.7(PS4.7)(Cl0.65Br0.65) is referred to as Cl0.65Br0.65.

[0099] Optionally, x may be equal to or greater than 0.55, preferably equal to or greater than 0.6, more preferably equal to or greater than 0.65, and / or x is equal to or less than 0.9, preferably equal to or less than 0.85, more preferably equal to or less than 0.8. Chlorine-rich argyrodite structures (particularly having chlorine in these amounts) show significant improvement on ionic conductivity without compromise of interfacial and electrochemical stability.

[0100] Optionally, y may be equal to or greater than 0.55, preferably equal to or greater than 0.6, more preferably equal to or greater than 0.65, and / or y may be equal to or less than 0.9, preferably equal to or less than 0.85, more preferably equal to or less than 0.8. By introducing bromide (particularly in these amounts), the activation energy for Li+diffusion is lowered because of the higher polarizability, and thus an improved ionic conductivity can be achieved.

[0101] In an embodiment, the argyrodite-type crystal structure of the solid electrolyte may be represented by Formula 3:

[0102] [Formula 3] Li7−x−y(PS4)(S2−x−y−pOpClxBry).

[0103] The values of x, y and p may satisfy the following condition x + y +p > 1, or preferably x + y +p > 1.1, and more preferably x + y + p > 1 and / or optionally the following condition x + y +p < 1.8, preferably x + y +p < 1.75, more preferably x + y +p < 1.7. According to an embodiment comprising an oxygen-doped argyrodite, the air-stability and moisture-resistance of the SE can be improved. Furthermore, chemical stability with the sulphur cathode can be improved.

[0104] A halogen-rich argyrodite, wherein the halogens consist of chlorine and bromine has exceptionally high Li+ conductivity and stability with the lithium anode.

[0105] Preferably, x may be equal to y. A synergistic balance of the effects of chlorine and the effects of bromine may be achieved when chlorine and bromine are provided in equal amounts.

[0106] For example, the SE may be one of Li5.5(PS4.5)(Cl0.75Br0.75) or Li5.7(PS4.7)(Cl0.65Br0.65). Li5.5(PS4.5)(Cl0.75Br0.75) is referred to as Cl0.75Br0.75 and Li5.7(PS4.7)(Cl0.65Br0.65) is referred to as Cl0.65Br0.65.

[0107] Preferably, the Li+conductivity of the solid electrolyte may be equal to or greater than 2 mS / cm and equal to or less than 11 mS / cm. Solid electrolytes according to the present invention exhibit conductivity values in this range.

[0108] The cathode layer may comprise elemental sulphur. This may be advantageous over cathodes comprising sulphur which is not found in elemental form (e.g. in a compound such as nickel sulphide or copper sulphide). This is because elemental sulphur exhibits high capacity and energy density. According to the present invention, the cathode may comprise sulphur in a form that enables the sulphur to be reduced to lithium sulphide (Li2S) during discharge and oxidised to S8during charge.

[0109] According to the present invention, the anode may comprise lithium or a lithium alloy in metallic form. The anode may be metallic. The lithium anode may consist of lithium, may be elemental lithium may be metallic lithium. There may be impurities in the anode. The anode may comprise 80wt% or more lithium, preferably 90wt% or more lithium. The anode may comprise thesolid electrolyte. When the anode comprises the solid electrolyte, the anode may consist of elemental lithium (or lithium alloy) and the solid electrolyte. This is contrast to cathodes used for Li-ion batteries, in which Li ions are intercalated into the cathode / anode during charging. The cathode layer 100 may further comprise the solid electrolyte. For example, as shown in Figure 1, the cathode layer 100 may comprise cathode material regions 101 and solid electrolyte regions 102. While spherical particles are shown in Figure 1, the shape of the solid electrolyte regions and any other constituents of the cathode layer 100 which may be present are not necessarily spherical and may be of any shape. By incorporating the electrolyte into the cathode, the conductivity of the cathode is improved and electro kinetics of the battery are improved.

[0110] For example, 10-50 vol.% of the cathode may consist of sulphur. Accordingly, the cathode may include further constituents such as the solid electrolyte. When 10-50 vol.% of the cathode consists of sulphur, the capacity of the cathode is suitably high to achieve a battery with high capacity, but the cathode may comprise further additives (such as those discussed below) to improve other qualities of the battery. In a preferred embodiment the cathode consists of 10 to 30vol.% as this strikes a good balance between high conductivity and high capacity. For example, in the charged state, 10 to 50 vol.% (or preferably, 10 to 30 vol. %) of the cathode may consist of elemental sulphur. The remainder of the cathode may consist of the solid electrolyte, and optionally, a conductive additive.

[0111] The cathode may further comprise a conductive additive. Accordingly, the conductivity of the cathode is improved without sacrificing the total capacity and energy density of the battery. For example, optionally, 1-20% of the cathode may consist of the conductive additive, for example, 18 vol.%.

[0112] The conductive additive may be carbon particles, such as Ketjenblack EC-600JD conductive carbon black. In an example, 18 vol% of Ketjenblack EC-600JD conductive carbon black may be added to the cathode.

[0113] The cathode may further comprise a binder, preferably an organic binder. This may improve the structural stability of the cathode. The binder may comprise one or more binders selected from the following: polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyethylene glycol (PEO), carboxymethyl cellulose (CMC), poly-dioxolane (poly-DOL), a poly- carbonates, nitrile butadiene rubber (NBR), poly(acrylic acid) (PAA).

[0114] Optionally, the cathode may comprise materials other sulphur. For example, the cathode may comprise an oxide. Optionally, the cathode may consist of an oxide material and / or incidental impurities.

[0115] The solid electrolyte may consist of lithium, phosphorus, sulphur, and at least one halogen in an argyrodite-type structure and incidental impurities. The structure may comprise a single halogen, such as chlorine. Alternatively, the structure may comprise a mixture of different halogens, such as a mixture of chlorine and bromine for example. The advantageous effects of including a mixture of halogens, such as chlorine and bromine, are discussed with reference to the results shown on Figures 5a,5b and Figure 9.

[0116] Optionally, the lithium content of the solid electrolyte is homogenous between the anode and cathode. A homogeneous content of lithium means that any region of the electrolyte has the same, or substantially the same, concentration of lithium. There may be some local deviations of lithium concentration due to defects which may be present in the argyrodite structure (e.g. voids, interstitial defects, impurities). In an embodiment, there are no further phases present, such as oxide phases which are sometimes included in the electrolytes according to the prior art. When the lithium concentration is homogeneous, high lithium conductivity is achieved throughout the electrode because there are no lithium-depleted regions which may limit the conductivity of the electrode.

[0117] The advantageous effects of an electrochemical battery will now be described with reference to Figures 2-10, showing data collected from cathodes / batteries according to the present invention, as well as comparative examples.

[0118] The influence of Cl-to-Br ratio on the unit cell of the argyrodite structure and on the Li+conductivity can be seen from Figures 2a, 2b and 3a-3d. Figures 2a and 3a-3b respectively show the powder X-ray diffraction (XRD) data and electrochemical impedance spectroscopy (EIS) obtained based on five samples of halide-rich (Li5.7PS4.7Cl1.3−xBrx) argyrodite structures with various Cl-to-Br ratios from Cl1.3Br0to Cl0Br1.3.In other words, Figures 2a, 3a-3b show data for compounds Li5.7PS4.7Cl1.3, Li5.7PS4.7Cl1Br0.3, Li5.7PS4.7Cl0.65Br0.65, Li5.7PS4.7Cl0.3Br, Li5.7PS4.7Br1.3referred to as Cl1.3, Cl1Br0.3, Cl0.65Br0.65, Cl0.3Br1, Br1.3. Figures 2b and 3c-3d respectively show the powder X-ray diffraction (XRD) data and electrochemical impedance spectroscopy (EIS) obtained based on five samples of halide-rich (Li7−2xPS6-2xClxBrx) argyrodite structures with various values of x from Cl0.3Br0.3 to Cl0.9Br0.9. In other words, Figures 2b and 3c-d show data for the compounds Li5.2(PS4.2)(Cl0.9Br0.9), Li5.5(PS4.5)(Cl0.75Br0.75), Li5.7(PS4.7)(Cl0.65Br0.65), Li6(PS5)(Cl0.5Br0.5), Li6.4(PS5.4)(Cl0.3Br0.3). Namely, Figures 2a and 2b show XRD patterns for compounds satisfying Formula 2. The different compositions are shown on the right hand side of the Figures, corresponding to the XRD patterns from the top to the bottom of the figure. The compositions are referred to by their content of Cl and Br for brevity. In Figures 2 and 2b, theXRD results for multiple compounds are superimposed on one graph of intensity vs 2-Theta, with the baseline of each set of results being offset on the intensity axis for ease of comparing the XRD results of the compounds.

[0119] To make the samples, the following method was followed. Precursor powders of Li2S, P2S5, LiCl and LiBr with stoichiometric ratios were mixed and ground in an agate mortar, and transferred to air-tight ball-milling jars. The mixtures were mechanically milled at 750 rpm for 16 hours, then pelletized and annealed for 10 hours at various temperatures ranging from 375 to 550 ◦C. The final products can be collected by grinding the pellets. All the process was conducted in Ar-filled gloveboxes.

[0120] According to XRD patterns as shown in Figure 2a, the main phase of the product is the argyrodite phase, with impurities of LiBr peaks showing up in Br-rich compounds Cl0.3Br1and Br1.3due to insufficient Br-tolerance of the structure. Figure 2b shows the XRD patterns for the following compositions (from top to bottom): Li5.2(PS4.2)(Cl0.9Br0.9), Li5.5(PS4.5)(Cl0.75Br0.75), Li5.7(PS4.7)(Cl0.65Br0.65), Li6(PS5)(Cl0.5Br0.5), Li6.4(PS5.4)(Cl0.3Br0.3) labelled as Cl0.9Br0.9, Cl0.75Br0.75, Cl0.65Br0.65, Cl0.5Br0.5, Cl0.3Br0.3 respectively. The 2-Theta peaks at ~29° and ~32 ° (marked with a dot) are characteristic peaks indicating the presence of LiBr. The 2-Theta peaks at ~33 ° and ~49 ° are characteristic peaks of LiCl (marked with a star). The XRD for Cl0.9Br0.9 indicates the presence of LiBr and LiCl peaks, meaning that LiBr and LiCl precipitates are formed in Cl0.9Br0.9, i.e. Li5.2(PS4.2)(Cl0.9Br0.9). For Cl0.75Br0.75, there is almost no intensity at the characteristic LiBr and LiCl peaks, indicating that formation of LiBr and LiCl precipitates in Cl0.75Br0.75 is almost completely suppressed. The XRD plot for Cl0.65Br0.65 shows that there is no intensity at the position of the LiBr and LiCl peaks, indicating that the formation of LiCl and LiBr is completely suppressed for Li5.7(PS4.7)(Cl0.65Br0.65).

[0121] LiBr and / or LiCl precipitates are formed when the amount of Br and / or Cl in a halogen- rich argyrodite is too high. For example, when x and y in Formula 2 are greater than 0.75, precipitation of LiBr and / or LiCl may be particularly likely.

[0122] The presence of LiBr and / or LiCl in the solid electrolyte may decrease the total ionic conductivity of the solid electrolyte, so it may be preferable to avoid the presence of LiBr / LiCl precipitates. Thus, compositions having a lithium to halogen ratio which is too low may have lower ionic conductivity making them less suitable for use as an electrolyte of a battery. For example, it may be expected that compositions with a lithium to halide ratio of less than 5.4:1.5 may lead to lithium halide precipitates. Compositions such as Cl0.75Br0.75 and Cl0.65Br0.65 aretherefore better candidates for a solid electrolyte than Cl0.9Br0.9. Cl0.65Br0.65 may be particularly advantageous, as it shows no precipitation of LiBr or LiCl.

[0123] Rietveld refinement results of the powder X-ray diffraction (XRD) data show that the lattice expands with more bromine content because of the larger radius of bromide anion (see Figures 3a and 3b). Ionic conductivity of the compounds was tested with electrochemical impedance spectroscopy (EIS) on respective cold-pressed pellets. The ionic conductivity increases with the increasing bromine content, with peaks at 6.9 mScm−1of the sample Cl0.65Br0.65, and decreases with more bromide incorporated, as can be seen on the left-hand-side axes of Figures 3c and 3d. The activation energy of Li+ion diffusion decreases with more bromide content, resulted from the higher polarizability of bromine atoms, as can be seen on the right-hand-side axes of Figures 3c and 3d.

[0124] Argyrodite compounds which contain a mix of Cl and Br can deliver higher ionic conductivity because of the synergic effect of chlorine and bromine that can bring in high structural disorder and large polarizability at the same time. Figure 3d shows that Li5.5PS4.5Cl0.75Br0.75and Li5.7PS4.7Cl0.65Br0.65have higher conductivity values than Li6PS5Cl0.5Br0.5. In a preferable embodiment, particularly high values of Li+ionic conductivity can be achieved when the Cl-to-Br ratio reaches 1:1.

[0125] Modified argyrodite products with various contents of halide ion and fixed Cl-to-Br ratio of 1:1 were synthesised, denoted as ClBr0.3 to Cl0.9Br0.9, representing Li6.4PS5.4Cl0.3Br0.3to Li5.2PS4.2Cl0.9Br0.9compounds respectively. Cl0.9Br0.9 shows high impurity content of LiBr, and ClBr0.3 exhibits low ionic conductivity, which are dismissed from further discussion. The conductivity increases from 2.8 to 10.3 mS cm−1 with the increasing halide content from Cl0.5Br0.5 to Cl0.75Br0.75, and the activation energy of Li-ion diffusion decreases from 0.3 to 0.26 eV, showing that higher halide content improves the ionic conductivity. The increased conductivity and decreased activation energy are due to changed structural features in the crystalline structures. Structural disorder is stabilized and additional Li sites between the Li-cages lower the Li-ion migration barrier. The ionic conductivity achieved is superior to known sulphide argyrodite materials.

[0126] Figure 3d demonstrates that Li+ionic conductivity for Li6(PS5)(Cl0.5Br0.5) is 2.9 mS cm-1, the ionic conductivity is 6.9 mS cm-1for Li5.7(PS4.7)(Cl0.65Br0.65) and the ionic conductivity is 10.3 mS cm-1for Li5.5(PS4.5)(Cl0.75Br0.75), while the activation energy of Li ions is 0.26eV, 0.28 and 0.30 respectively.

[0127] The increased conductivity and decreased activation energy are caused by changes in the crystal structures, where more Li vacancies are introduced, and additional Li sites between the Li cages lower the Li ion hopping distances and migration barriers.

[0128] Further refinements conducted on the synchrotron XRD data of the modified argyrodite materials, as shown on Figure 12, show that the chloride, bromide and sulphide distribute almost evenly between the two anion sites. The anion sites are the sites occupied by the sulphur atoms in an argyrodite structure, for example, as defined in Formula 1 or Formula 2. The high formation entropies of Cl0.65Br0.65 and Cl0.75Br0.75 make these materials in solid-solution state and more tolerant towards structural defects.

[0129] The configurational entropy of the argyrodite compound is maximised when the amount of chlorine in the argyrodite structure is equal to the amount of bromine in the argyrodite structure. For example, the configurational entropy of Cl0.65Br0.65 and Cl0.75Br0.75 (ΔSconf) is 2.02R and 2.09R, respectively. Here, entropy is expressed in terms of the gas constant R which has a value of 8.314 JK-1mol-1.

[0130] In order to show the superior Li+-conducting properties of these halide-rich argyrodite materials, solid-state NMR techniques of spin-lattice relaxation (SLR) and pulse-field gradient (PFG) were applied on a Cl0.65Br0.65 sample. These data are shown schematically in Figures 4a- 4d.

[0131] Figures 4a-4d show7Li solid-state NMR characterizations of sample Cl0.65Br0.65. Figure 4a shows the NMR spectra at different temperatures (shortest peak observed at highest temperature and highest peak observed at lowest temperature). Figure 4a shows spin-lattice relaxation (SLR) rates at different temperatures. Figures 4b shows fitting results of the PFG NMR tests at various temperatures, and the activation energy can be calculated with the fitted7Li diffusion coefficients with the Arrhenius equation, as shown in Figure 4c.

[0132] The short-range Li+hopping activation energy in sub-nanometre scale can be tested with spin-lattice relaxation (SLR) rates at various temperatures when fitted with Bloembergen-Purcell- Pound (BPP) model. The7Li PFG NMR can detect the medium-range diffusion coefficient of7Li in micrometre scale, and the activation energy can be calculated with the7Li diffusion coefficient at different temperatures with the Arrhenius equation. As shown in Figs.4a-4c, the activation energy values are calculated to be 0.26 eV from SLR and PFG NMR respectively. The well- matched values are only slightly lower than the electrochemical result (0.28eV) from EIS tests possibly because of the grain-boundary resistance, further proving the good Li+conducting properties of the material.

[0133] Figures 5a and 5b show current-voltage curves for a half cell having a mixture of carbon and SE as the cathode, SE as the separator layer and LiIn as the anode. Figure 5a shows the obtained curves for a Cl1 sample (comparative example) and Figure 5b shows the obtained curves for a Cl0.75Br0.75 sample (an embodiment of the invention).

[0134] Each CV scan was carried out from 1 V to the highest cutoff values which starts from 2.3V and increases by a step of 0.1V after every two cycles. The first increase of current shows up at 1.3 V which represents the reduction reaction of phosphorus. When the cutoff voltage reaches 2.5V, an oxidation reaction starts happening as proved by the current increasing around 2.5 V and two reduction peaks around 1.6 and 2 V during the respective reverse scan, which are the typical redox reaction potentials for S / S2−. There is little difference between the redox reaction potentials of various samples, proving that the electrochemical stability window of different samples are almost uninfluenced by the different halide-content or the incorporation of bromide. The argyrodite participates in the redox reaction reversibly while maintaining the Li-diffusion paths if the cutoff voltages of the Li-S cells are properly set up, resulting in an insignificant capacity decay of the Li- S batteries during cycling. The uninfluenced electrochemical stability windows show that the modified argyrodite samples can also be applied in Li-S batteries without significant degradation during cycling. The differences in current density of the redox peaks are due to the different specific areas of the materials.

[0135] The interfacial stability to Li metal can be analysed by an in-situ X-ray photoemission spectroscopy (XPS) technique, with a polished SE pellet attached on a thin Li metal foil and the other side of the pellet exposed to an electron beam for different time while the core-level photoemission spectra are acquired. Figures 6a, 6b and 6c show the solid electrolyte interphase (SEI) evolution of the SE pellets during continuous Li plating, and the showing up of Li metal peak around 52 eV (marked as *) that keeps increasing with more current proves that the self-limiting passivating SEI layer can stop further reaction between the SE and Li metal, which indicates the SE materials are well compatible with Li metal anodes. The SEI passivating layer is a passivation layer formed on the surface of the anode produced by electrolyte decomposition. The SEI layer is formed on the electrode surface. The SEI layer controls Li+diffusion from the electrolyte to the anode, and it prevents further electrolyte decomposition and ensures continued electrochemical reactions. Figures 6b and 6c show the solid electrolyte interphase (SEI) evolution of a Cl0.65Br0.65 sample and a Cl0.75Br0.75 sample (according to the present invention). Figure 6a shows the solid electrolyte interphase (SEI) evolution of a Cl1 sample (comparative example).

[0136] The appearance of the Li metal peak (marked by an asterisk in Figures 6a, 6b and 6c) is indicative of the formation of a passivating SEI. As is can be seen, the passivating SEI layer is formed for both the Cl0.65Br0.65 sample and Cl0.75Br0.75 sample according to the present invention.

[0137] Interfacial stability between argyrodite compositions and Li metal may also be characterised by coulometric titration time analysis (CTTA). Figure 13 shows the SEI layer formation rate investigated via CTTA with 1.225 mAh Li plating steps and a 50mVOCV cutoff potential for Cl0.65Br0.65, Cl0.75Br0.75 and Cl1. The upper line of the graph in Figure 13 correspond to the CTTA results for Cl0.75Br0.75, the middle line corresponds to Cl0.65Br0.65 and the bottom line corresponds to Cl1. In the CTTA tests, 1.225 mAh of Li was plated on the SE pellet, followed by an open-circuit voltage (OCV) period until the voltage increased to 50mV (vs. Li+ / Li), at which point another 1.225 mAh of Li was plated. The OCV time period represents the time required for the plated Li to be fully consumed by side reactions with the SE to form SEI layer. As shown in Figure 13, gradually increasing OCV time periods result in the formation of progressively more passivating SEI layers for all argyrodites.

[0138] Figure 13 shows that for each test time, a higher charge accumulates for Cl0.75Br0.75 than for Cl0.65Br0.65. For each test time, a higher charge accumulates for Cl0.65Br0.65 than for Cl1, indicating a correlation between bromine content and SEI layer thickness. In other words, a higher bromine content leads to a thicker SEI layer. The present inventors have determined that one cause of a thicker SEI is the presence of LiBr, which precipitate when the bromine content is too high.

[0139] While Cl1 may have greater interfacial stability with a Li anode than Cl0.75Br0.75 and Cl0.65Br0.65 (as demonstrated in Figure 13), the ionic conductivity of Cl0.65Br0.65 is more than 20% greater than the ionic conductivity of Cl1 (see Figure 3b). Cl0.65Br0.65 strikes a balance between high ionic conductivity and good interfacial stability with the lithium anode.

[0140] Figures 7a-c shows critical current density (CCD) experiments conducted on Cl0.5Br0.5, Cl0.65Br0.65 and Cl0.75Br0.75 in symmetric Li-metal cells. These experiments illustrate the compatibility of the SE with Li metal anodes. As the current density increases to the CCD value, Li dendrites would grow and penetrate through the separator layer, resulting in the short-circuit of the cell. The CCD value is influenced by the characteristic properties of the materials and interfacial stability between the SE layer and Li metal. Figure 7d shows the CDD values for various compositions of the argyrodite. In Figure 7d, the composition labels are written in short- hand for brevity. Namely, in Figure 7d the label Cl0.5Br0.5 means “Cl0.5Br0.5”, the label Cl0.65Br0.65 means “Cl0.65Br0.65” and the label Cl0.75Br0.75 means “Cl0.75Br0.75”. Asshown in Figure 7d, as halide content increases, the CCD value increases to 2.5 mA cm−2 for Cl0.65Br0.65, and then decreases to 1.3 mA cm−2for Cl0.75Br0.75, which shows significant improvement from 0.6 mA cm−2for unmodified argyrodite (Cl1 –comparative example). Cl0.65Br0.65 exhibits the highest critical current density and thus the greatest resistance to lithium dendrite growth. Lithium dendrites cause short-circuits in the battery, so the prevention of lithium dendrite growth is key in ensuring good battery performance.

[0141] Thus, in addition to striking an optimum balance between high ionic conductivity and good interfacial stability, Cl0.65Br0.65 also exhibits the greatest dendrite resistance.

[0142] Halide-rich argyrodites may contain impurities (such as pores) which are introduced during the manufacturing process. It may be preferable to reduce or prevent the occurrence of pores. The present inventors have discovered that Cl0.65Br0.65 shows reduced porosity and impurity levels, when compared to Cl0.75Br0.75. This leads to improved CDD of Cl0.65Br0.65 when compared to Cl0.75Br0.75.

[0143] The decreased porosity of Cl0.65Br0.65 relative to other argyrodite compositions is confirmed by cross-section SEM images of cold-pressed pellets of the argyrodite materials, as shown in Figure 14a-f. Figure 14a-f show the SEM image and the associated porosity of the image.

[0144] Figure 14a shows an SEM image of commercial Li6PS5Cl, Figure 14b shows the SEM image of Li6PS5Cl made according to the present invention, Figure 14c shows the SEM image of Li5.7PS4.7Cl1.3, Figure 14d shows an SEM image of Li6(PS5)(Cl0.5Br0.5), Figure 14e shows an SEM image of Li5.7PS4.7Cl0.65Br0.65and Figure 14f shows an SEM image of Li5.5(PS4.5)(Cl0.75Br0.75). Commercial Cl1 particles (Figure 14a) have a size of approximately 5 μm size, while Cl1 made via a method according to the present invention (Figure 14b) have a size of approximately 1 μm. The relative density of the cold-pressed pellet increases from 96.5% for Cl1 to 98.9% for Cl0.65Br0.65 and then drops to 96.2% for Cl0.75Br0.75. Thus, while the Cl0.75Br0.75 sample may exhibit higher ionic conductivity than Cl0.65Br0.65, Cl0.65Br0.65 exhibits reduced porosity. Short- circuiting can occur due to Li deposition along flaws (such as cracks, grain boundaries, and pores) inside SEs, exerting stress that will act to grow the flaw and fracture the SE once a critical stress is exceeded. It has further been reported that higher current densities lead to increased stress, increasing the likelihood of a short circuit. According to linear elastic fracture mechanics, the critical stress to cause failure is inversely proportional to the square root of flaw length. Thus, the higher CCD of Cl0.65Br0.65 is a result of its improved microstructure (Figure 16 e), displaying significantly reduced porosity (and, therefore, reduced flaw length and improved resistance tofailure) compared to the other compositions. Therefore, SEs with higher density (i.e. reduced porosity) are better candidates for a solid electrolyte of an ASSB as they are more mechanically robust. Considering that the relative density of Cl0.65Br0.65 is higher than Cl1 with similar particle size distribution, the better microstructure of Cl0.65Br0.65 suggests a reduced mechanical hardness of Cl0.65Br0.65 compared to other compositions.

[0145] Thus, Cl0.65Br0.65 is optimised for use as a solid electrolyte in terms of its ionic conductivity, interfacial stability with a lithium anode, low porosity and resistance to the formation of LiBr / LiCl precipitates.

[0146] As shown on Figures 8a and 8b, the Li symmetric cells of Cl0.65Br0.65 and Cl0.75Br0.75 cycle well under current density 0.5 mA cm−2for over 800 hours without short-circuit happening (i.e. without dendrite growth). The difference could be due to slightly different crystallinity or interfacial stability. The high CCD values make mixed-halide-rich argyrodite materials more possible to be applied in all-solid-state Li-metal batteries.

[0147] An example of a method which can be used to make a cathode is as follows. Milling and / or mixing sulphur and carbon black (conductive additive) and optionally the SE to obtain a mixture. Annealing the mixture to obtain the cathode. Preferably, the mixture is annealed such that the contact between the sulphur and carbon black particles is maximised (i.e. porosity is reduced). Sulphur content in the cathode may be determined by thermal gravimetry analysis (TGA) in flowing Argon. The sulphur may be amorphous after annealing.

[0148] The cathode may be stacked with the SE and a lithium anode to obtain an electrochemical cell.

[0149] Performances of a Li-S battery comprising a Cl0.65Br0.65 material as the SE (according to the present invention) and Cl1 unmodified argyrodite as the SE (comparative example) Figures 9a- f. Figures 9a, 9b show the rate performance at current densities of 0.5 mA cm−-2.5 mA cm−of the Li-S cells with different SE materials (Cl1 and Cl0.65Br0.65). In Figure 9a, the capacity results for Cl1 correspond to the darker bottom line at between 15 and 20 cycles on Figure 9a and the results of Cl0.65Br0.65 correspond to the second to bottom line on Figure 9a at between 15 and 20 cycles. The top two lines on Figure 9a correspond to the Coulombic efficiency results, with the darker lower line at between 15 and 20 cycles corresponding to Cl1 and the top lighter line corresponding to the results of Cl0.65Br0.65.

[0150] Figure 9c shows the cycling performance of the Cl0.65Br0.65 cell with a LiIn anode. The squares correspond to areal capacity results, and the circules correspond to the Colombin efficiency results. The C0.65lBr0.65 cell delivers over 6mAhcm−2 areal capacity with specific capacity over1300mAhg-1(sulfur utilization of 79%) after 50 cycles. - Figure 9d shoes the charge-discharge curve of the initial cycle. Figure 9e shows the cycling performance of the Li-S cells with Li anodes at a current density of 0.5mAcm-2, with an S loading of 3 mg cm-2. The capacity results for Cl1 correspond to the bottom darker line in Figure 9e and the results of Cl0.75Br0.75 correspond to second to last line on Figure 9e. The Coulombic efficiency results in Figure 9e are represented by dots, with the upper set of results corresponding to Cl1 and the lower set of results corresponding to Cl0.65Br0.65. Figure 9f shows areal capacity and S utilization of the ClBr0.65 cell compared with reported values in the literature.

[0151] In the charge-discharge cycle experiments, in order to avoid the degradation of sulphide argyrodite structure, especially the reduction of phosphorus at low potential (below 1.3 V) that might influence the structure frame and ionic conducting pathways, the cutoff potentials of the cells are set to be 1.62-3.02 V vs. Li+ / Li (1.0-2.4 V vs LiIn / In). With a sulphur loading of 3 mg cm−2, a stack pressure of 50MPa at 30ºC, both the materials can help the cell to achieve a capacity of above 1500 mAh / g near the theoretical capacity (1672 mAh g−1) at a current density of 0.5 mA cm−2(0.1 C), with a high areal capacity of over 4.5 mAh cm−2. The Cl0.75Br0.75 cell could deliver a capacity of 1525, 1464, 1271 and 900 mAh g−1at current densities of 0.5, 1, 2.5 and 5 mA cm−2respectively, and back to 1519 mAh g−1when current density decreases to 0.5 mA cm−2. The excellent areal capacity of 2.7 mAh cm−2at ultra-high current density of 5 mA cm−2(1.67 A g−1) could only be achieved with the solid-state electrolyte with high ionic conductivity, outstanding from all Li-S ASSB systems considering the relatively high sulphur loading, low temperature, low stack pressure, high current density and high capacity retention. When cycling at a current density of 0.5 mA cm−2, the Cl0.75Br0.75 cell can retain above 98 percent of the maximum capacity after 20 cycles, in comparison to a capacity retention of only 95 percent of unmodified argyrodite cell. Both the unmodified argyrodite and Cl0.75Br0.75 cell can cycle well at a current density of 2.5 mA cm−2(0.5 C) without obvious capacity decay, showing that no significant argyrodite degradation happens at such high current density.

[0152] With the great compatibility with Li metal anodes of Cl0.65Br0.65 and Cl0.75Br0.75, functional all-solid-state Li-S batteries with Li metal as the anode can be operated with the modified argyrodite materials at 30ºC.

[0153] Figure 10a shows the capacity of a Li-S cell (with Cl0.75Br0.75 as the SE and Li metal as the anode) as a function of cycle number. As it can be seen, the battery can cycle well at a current density of 0.5 mA cm−2, showing a capacity of 1500 mAh g−1without noticeable capacity decay after 50 cycles.

[0154] Characterisations of the samples described above were carried out as follows.

[0155] X-ray diffraction (XRD) patterns were collected with a Rigaku MiniFlex benchtop X-ray diffractometer with the 2-theta range from 10-90 degree. Synchrotron XRD patterns were collected at the synchrotron X-ray source on the I11 beamline at Diamond Light Source, UK, with the Mythen2 Position Sensitive Detector (PSD), two data collections of 20 seconds each were taken at angles 0.25 degrees apart, then summed to account for gaps in the detector coverage. Rietveld refinements of the patterns were done with the GSASII37 and topas38 softwares. Scanning electron microscopy (SEM) images of the powder products were conducted with a TESCAN MIRA4 SEM machine. Solid-state nuclear magnetic resonance (ssNMR) spectroscopies were collected using a 4mm wide-range probe on a Bruker Biospin AvanceIII spectrometer operated at a magnetic field of 9.4T. The temperatures were calibrated with KBr before the tests. For in situ-XPS measurements, polished SE pellets were attached on a Li foil with a diameter of 5 mm on a stain-less steel foil, which were attached on the XPS holder with conductive carbon tape. The SE surface was exposed to an electron dose of 30 μAAcquired spectra were then fitted to Gaussian–Lorentzian and Voigt line shapes (or Doniach–Sunjic line shapes for metallic Li components, where asymmetry was found to be significant), after application of a Shirley background, using CasaXPS software.

[0156] The ionic conductivity was tested with electrochemical impedance spectroscopy (EIS) with the frequency ranging from 1M to 400 mHz. The test was conducted on a pellet that was fabricated with the SSE powder under 500 MPa for 10 min, with two Nickel electrodes as the current collectors. Critical current density (CCD) of the solid-state electrolytes in lithium metal symmetric cells was tested in a home-made cell mould with diameter of 10 cm, the stack pressure was kept at 15 MPa and the temperature was kept at 30ºC. During the tests, a constant capacity of 1 mAh was kept for each charge or discharge step with the current density increasing by a step of 0.1 mAcm−2.

[0157] Li-S cells were fabricated with the home-made PEEK battery moulds. To synthesise the cathode composite, sulphur powder (sublimed,100 mesh, 99.5 percent, Alfa Aesar) and Ketjenblack EC-600JD conductive carbon black with the weight ratio of 2:1 were ball-milled in sealed grinding bawls at 500 rpm for 6 hours, annealed at 155 ºC for 2 hours and then mixed with various SE powder by mechanical milling at 500 rpm for 5 hours.100 mg of synthesized SE powder were pelletized at 200 MPa for 2 min, then the cathode composite powder and Li-In powder were added onto the sides of the SE pellet and densified at 500 MPa for 10 min. An In foil with a thickness of 200 um was attached on the Li-In side and the cell was pressed at 100 MPa for 2 min to make sure a good contact between the layers. All the procedures were conducted in Ar-filled gloveboxes. The cells were dwelled at 60 ºC for 1 hour. A stack pressure of 50 MPa and a constant temperature of 30 ºC were applied during the electrochemical tests.

[0158] A lithium-sulphur battery with Li5.7PS4.7Cl0.65Br0.65as the SE exhibits a capacity of 1481mAhg-1at a current density of 0.5mAcm-2and high rate performance delivering 840mAhg-1at a current density of 5mA cm-2. An Li-S ASSB with a Li metal anode was shown to cycle well without short circuits, delivering an areal capacity of 4.3mAh cm-2at a current density of 0.5mAcm-2, surpassing reported values.

Claims

Claims 1. An electrochemical battery, comprising: a sulphur cathode; a lithium anode; and a solid electrolyte between the sulphur cathode and the lithium anode, wherein the solid electrolyte has an argyrodite-type structure and comprises lithium, phosphorus, sulphur, and a halogen.

2. The electrochemical battery according to claim 1, wherein the solid electrolyte comprises at least two different halogens.

3. The electrochemical battery according to claim 1 or 2, wherein the ratio of lithium to halogen in the solid electrolyte is less than 6:1, or preferably less than 5.9:1.1 or more preferably less than 5.8:1.

2.

4. The electrochemical battery according to any of claims 1 to 3, wherein the ratio of lithium to halogen in the solid electrolyte is more than 5.25:1.75, or preferably more than 5.55:1.65, or more preferably more than 5.4:1.

6.

5. The electrochemical battery according to any of claims 1 to 4, wherein the argyrodite-type structure of the solid electrolyte is represented by Formula 1, and wherein A, B, C and D are halogens: [Formula 1] Li7−x−y-z-w(PS4)(S2−x−y-z-wAxByCzDw).

6. The electrochemical battery according to claim 5, wherein 0.1<w<0.4 and 0.1<z<0.

4.

7. The electrochemical battery according to claim 5 or 6, wherein x, y, z and w satisfy the following condition x + y + z +w > 1, or preferably x + y + z +w > 1.1, and more preferably x + y + z + w > 1.2 and / or wherein x, y, z and w satisfy the following condition x + y + z + w < 1.8, preferably x + y + z + w < 1.75, more preferably x + y + z + w < 1.7.

8. The electrochemical battery according to claim 5, wherein the argyrodite-type crystal structure of the solid electrolyte is represented by Formula 2: [Formula 2] Li7−x−y(PS4)(S2−x−yClxBry).

9. The electrochemical battery according to claim 8, wherein x and y satisfy the following condition x + y > 1, or preferably x + y > 1.1, and more preferably x + y > 1.

2.

10. The electrochemical battery according to any one of claims 8 or 9, wherein x and y satisfy the following condition x + y < 1.8, preferably x + y < 1.75, more preferably x + y < 1.

7.

11. The electrochemical battery according to any of claims 5 to 10, wherein: x is equal to or greater than 0.55, preferably equal to or greater than 0.6, more preferably equal to or greater than 0.65, and / or x is equal to or less than 0.9, preferably equal to or less than 0.85, more preferably equal to or less than 0.

8.

12. The electrochemical battery according to any of claims 5 to 11, wherein: y is equal to or greater than 0.55, preferably equal to or greater than 0.6, more preferably equal to or greater than 0.65, and / or y is equal to or less than 0.9, preferably equal to or less than 0.85, more preferably equal to or less than 0.

8.

13. The electrochemical battery according to any of claims 5 to 12, wherein x is equal to y.

14. The electrochemical battery according to any of claims 1 to 13, wherein the solid electrolyte is one of: Li5.5(PS4.5)(Cl0.75Br0.75), Li5.7(PS4.7)(Cl0.65Br0.65).

15. The electrochemical battery according to any of claims 1 to 14, wherein the solid electrolyte is Li5.7(PS4.7)(Cl0.65Br0.65).

16. The electrochemical battery according to any of claims 1 to 15, wherein the Li+conductivity of the solid electrolyte is equal to or greater than 2 mS / cm and equal to or less than 11 mS / cm.

17. The electrochemical battery according to any of claims 1 to 16, wherein the cathode layer comprises elemental sulphur.

18. The electrochemical battery according to any of claims 1 to 17, wherein the cathode layer further comprises the solid electrolyte.

19. The electrochemical battery according to any of claims 1 to 18, wherein 10-50 vol.% of the cathode consists of sulphur.

20. The electrochemical battery according to any of claims 1 to 19, wherein the cathode further comprises a conductive additive, optionally, wherein 1-20 vol. % of the cathode consists of the conductive additive.

21. The electrochemical battery according to claim 20, wherein the conductive additive comprises carbon particles.

22. The electrochemical battery according to any of claims 1 to 21, wherein the cathode further comprises a binder, preferably an organic binder.

23. The electrochemical battery according to any of claims 1 to 22, wherein the lithium anode comprises lithium metal.

24. The electrochemical battery according to any of claims 1 to 23, wherein the solid electrolyte consists of lithium, phosphorus, sulphur, and one or more halogens in an argyrodite- type structure and incidental impurities.

25. The electrochemical battery according to any of claims 1 to 24, wherein the lithium content of the solid electrolyte is homogenous between the anode and cathode.

26. The electrochemical battery according to any of claims 1 to 25, wherein the solid electrolyte consists of one phase.

27. The electrochemical batter according to any of claim 1 to 26, further comprising a passivating layer between the solid electrolyte layer and the anode.

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