EP4684442A1 - Solid-state ionic conductor - Google Patents

Abstract

The present disclosure relates to a solid-state polycrystal with elevated ionic conductivity that provides an electrolyte for solid-state batteries including particular potassium ion batteries. The solid-state polycrystal is stable to atmospheric air and its manufacture follows a cheap and scalable process.

Description

P6633PC00 Solid-state ionic conductor Technical field The present invention relates to a solid-state polycrystal with elevated ionic conductivity that provides an electrolyte for solid-state batteries. The solid-state polycrystal is stable to atmospheric air and its manufacture follows a cost-efficient process. Background Fast inorganic solid-state oxide ionic conductors (ISOC) with high conductivities for alkali ions are promising solutions for several technological applications such as all-solid-state batteries, alkali-metal sulphur batteries and potentiometric gas sensors. Especially in case of solid-state batteries, using an ISOC instead of liquid ionic conductor and polymer separator offers several advantages such as higher thermochemical stability, wider temperature and potential range for operational conditions, safety improvement, higher energy densities and faster charging rates. In other solid-state devices such as potentiometric gas sensors, ISOC can be utilized to replace larger, more expensive and more power demanding optical gas sensing solutions. The conductivity of alkali ions within the ISOC, the cost of manufacturing as well as the stability of these compounds under different operational conditions are the key indicators for choosing the appropriate electrolyte. Li⁺-conducting ISOCs offer the highest conductivity amongst the rest of alkali ion conductors with Li-ion conductivities at room temperature from 10^-8 to 10^-3 S/cm. However, the high cost and scarcity of lithium, limits the potential of these materials for large-scale technological applications such as all-solid-state batteries. Conversely, ISOC with earth-abundant alkali ions constitute considerably more cost- effective solutions, with one example of a recently developed fast ionic conductor for K⁺ including honeycomb-layered tellurite K2Mg2TeO6. This material demonstrated very high ionic conductivities of 40 mS/cm and 0.01 mS/cm, measured under Argon, at 300 °C and 25 °C, respectively. However, K2Mg2TeO6 like most other K-ISOCs, suffered from hygroscopicity, and consequently needs to be stored under protective environments such as Argon. Similarly, KSi2P3 is another K-ion conductor candidate also illustrating remarkable bulk ionic conductivity values up to 2.6·10^-4 S/cm at room temperature, yet P6633PC00 this material was likewise sensitive to air and moisture, thus requiring to be handled under protective environment. There thus remain a need in the art for providing the next generation of inorganic fast solid-state oxide electrolytes with K-ion conductivity, which ideally should be highly stable under ambient conditions, displays fast K-ion conductivity in a relatively wide temperature range and involve a cost-effective manufacturing process that utilises earth- abundant and non-toxic elements. The present application sets out to meet this need by the invention disclosed herein. Summary Disclosed herein is a solid-state electrolyte that constitutes a fast inorganic solid-state oxide ionic conductor (ISOC). The solid-state electrolyte of the disclosure is a polycrystalline potassium magnesium silicate, structurally integrated by a network of corner-sharing polyhedra of SiO4 and MgO4 moieties, and as a result, each oxygen is bonded to both Si⁴⁺ and Mg²⁺ cations. The three-dimensional network of channels between the polyhedra provide migration channels for potassium cations, which are present in super-stoichiometric amounts. The potassium super-stoichiometry on the solid-state electrolyte of the disclosure is directly correlated with an increased ionic conductivity, in comparison to the stoichiometric analogue. Within the electrolyte solid-state structure, the potassium cations reside in highly meta- stable interstitial-sites, diffusing through channels, with very low activation energy barrier for migration. The low activation energy barrier enhances the potassium-ion conductivity of the solid-state electrolyte of the disclosure, in comparison to other solid-state oxide electrolytes reported in the prior-art, such as potassium beta-alumina solid-state electrolytes (K-BASE), KAlO2 and KFeO2. Furthermore, the K-ionic conductivity for the electrolyte exceeds 10^-7 S/cm at room temperature and the electrochemical performance remains largely unaffected upon exposure to atmospheric air, as shown in the examples herein. Since the crystal lattice in the corner-sharing framework of the solid-state electrolyte, in contrast to layered structures, provides exposed gaps that are too narrow to enable the inclusion of water molecules, stability to moisture is thereby achieved. Despite the elevated ionic P6633PC00 conductivity, the electronic conductivity is four orders of magnitude lower. The electrolyte features a high level of densification, as shown in the examples herein, which is for ensuring the optimal performance of the pellets and self-standing tapes in a variety of applications, most remarkably as alkali-ion conducting solid-state electrolytes for batteries, such as potassium- and/or sodium-ion conducting solid-state electrolytes for potassium and/or sodium batteries. The above presented features, in combination with the high level of densification of the material, provide optimal features for the development of solid-state batteries, such as solid-state alkali ion batteries and in particular a potassium-ion battery and/or sodium- ion battery, which comprises the solid-state electrolyte disclosed in the invention. One aspect of the invention is a solid-state electrolyte comprising an ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5). Another aspect of the invention is a solid-state electrolyte comprising a potassium-ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). Another aspect of the invention is a battery comprising: an ion conductive electrolyte layer, a positive electrode layer , and a negative electrode layer, wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises an ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). Another aspect of the invention is a potassium-ion battery comprising: a potassium-ion conductive electrolyte layer, a positive electrode layer , and a negative electrode layer, wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises a potassium-ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). P6633PC00 Another aspect of the invention is a sensor comprising the ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5), which is suitable for detecting gases such as SO₂, SO₃, NO, NO₂, CO, and CO₂. Another aspect of the invention is a sensor comprising the potassium-ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5), which is suitable for detecting gases such as SO₂, SO₃, NO, NO₂, CO, and CO₂. The electrochemical properties of the solid-state electrolyte allows for an additional use in a potentiometric sensor for gases, such as for SO₂. The sensitivity of a potentiometric sensor comprising the electrolyte of the invention has been demonstrated and proved to exhibit significantly superior detection limits. Another aspect of the invention is a method of manufacturing the solid-state electrolyte of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5), the method comprising the sequential steps: a) Mixing at least one source of potassium, at least one source of magnesium, and at least one source of silicon in a ratio such as to obtain a super-stoichiometric potassium ratio of K:Mg:Si corresponding to (2+X):1-(X/2):1 (0.0 < X ≤ 0.5), thereby forming a mixture, and b) Heating said mixture to a temperature between 600°C and 1000°C to obtain a sintered mixture. The manufacture method of the solid-state electrolyte of the disclosure is particularly suitable for industrial scalability because it involves the use of inexpensive and commercially accessible materials while at the same time the sintering temperature is 850°C, which is much lower than that of canonical K-based solid-state electrolytes reported in the prior art, such as K-BASE, of which the sintering temperature is 1400- 1700°C. Therefore, the more facile method of manufacture provides for a suitable implementation of large-scale production of the solid-state electrolyte of the disclosure. Description of Drawings P6633PC00 Figure 1 A: X-ray diffraction pattern of KMS-2 at room temperature (25 °C) under N₂ atmosphere. The 2θ reflections indicate KMS-2 adopts an orthorhombic structure of space group Pca2₁ at 25 °C. B: Schematics for the orthorhombic structure of KMS-2, modelled via the VESTA software. Figure 2 A: X-ray diffraction pattern of KMS-2 at 600 °C under N₂ atmosphere. The 2θ reflections indicate KMS-2 adopts a cubic structure of space group Fd-3m at 600 °C. B: Schematics for the cubic structure of KMS-2, modelled via the VESTA software. Figure 3 Comparison between the X-ray diffraction patterns of KMS-2 under N₂ at 25 °C (Cubic Fd-3m) and 600 °C (Orthorhombic Pca21). Figure 4 A: X-ray diffraction patterns of KMS-2 during progressive heating from 25 °C to 650 °C under pure N2. The X-ray diffraction patterns were recorded at 25 °C and after every 50 °C increment, starting from 100 °C. A transition from Pca21 phase to Fd-3m is observed between 200 °C and 250 °C. The cubic phase undergoes a progressive thermal expansion as the temperature rises above 250 °C. B: X-ray diffraction patterns of KMS-2 during progressive cooling from 650 °C to 25 °C under pure N2. The X-ray diffraction patterns were recorded at the same temperatures as for the heating screening. The cubic phase progressively shrinks as the temperature decreases from 650 °C. Between 250 °C and 200 °C, the reverse phase transition occurs, i.e. the orthorhombic Pca21 lattice is adopted. C: X-ray diffraction patterns of KMS-2 during progressive heating from 25 °C to 650 °C under atmospheric air. The X-ray diffraction patterns were recorded at 25 °C and after every 50 °C increment, starting from 100 °C. The 2θ reflections are identical to those recorded during the analogous heating under pure N2. D: X-ray diffraction patterns of KMS-2 during progressive cooling from 650 °C to 25 °C under atmospheric air. The X-ray diffraction patterns were recorded at the same temperatures as for the heating screening. The 2θ reflections are identical to those recorded during the analogous cooling under pure N₂. P6633PC00 Figure 5 A: SEM micrographs of the KMS-2 achieved from dry polished and thermally etched (30 minutes at 750 °C under atmospheric air) surface of the pellets sintered at 800 °C at low magnification level. B: SEM micrographs of the KMS-2 achieved from dry polished and thermally etched (30 minutes at 750 °C under atmospheric air) surface of for the pellets sintered at 800 °C at high magnification level. Figure 6 Ionic conductivity measurements on KMS-0, KMS-1, KMS-2, KMS-4 and KMS-5 at different temperatures between 25 °C and 600 °C. The chart shows that KMS-1, KMS- 2, KMS-4, and KMS-5 possess increased conductivity, compared to KMS-0. This difference becomes most notable below 300 °C Figure 7 A: Ionic conductivity measurements for KMS-2 under dry N2 and air at different temperatures between 25 °C and 600 °C. KMS-2 ionic conductivity is largely unaffected by the presence of moisture and/or O2 above 200 °C. B: DC potentiostatic polarization of 500 mV at room temperature under dry N2 performed on Pt/KMS-2/Pt symmetrical cells. Figure 8 Thermogravimetric analysis of a solid-state rod of KMS-2, measured under Ar atmosphere. After the removal of surface moisture, the normalised mass remains largely unaffected. Figure 9 X-ray photoelectron spectroscopy (XPS) high resolution binding energy spectrum measured for KMS-2 before (A-C) and after (D-F) an aging process as specified in Example 6, as well as after (G-I) a recovery process as specified in Example 6. A: 287-300 eV interval. The spectrum shows (from left to right) discernible K2p1/2, K2p3/2 and C1s peaks. B: 1297-1309 eV interval. The spectrum shows the peak corresponding to Mg1s. C: 95-107 eV interval. The spectrum shows a clear Si2p peak. P6633PC00 D: 287-300 eV interval. The spectrum shows (from left to right) discernible K2p_1/2, K2p_3/2 and C1s peaks. E: 1297-1309 eV interval. The peak associated to Mg1s is absent. F: 95-107 eV interval. The spectrum shows a clear Si2p peak. G: 287-300 eV interval. The spectrum shows (from left to right) discernible K2p_1/2, K2p_3/2 and C1s peaks. H: 1297-1309 eV interval. The spectrum again shows the peak corresponding to Mg1s. I: 95-107 eV interval. The spectrum shows a clear Si2p peak. Figure 10 Temperature dependence of ionic conductivity and comparison between the ionic conductivities of “KMS-2-Pristine”, “KMS-2-Aged” and “KMS-2-Recovered” samples, measured under dry N₂. The chart shows that the ionic conductivity of “KMS-2- Recovered” is partially restored, in comparison to “KMS-2-Pristine”, after the treatment. Figure 11 Sensitivity of type III potentiometric SO2 sensor (O2,Au|Ag|KMS-2|K2SO4|Pt|Au,SO2,O2) at 500 °C demonstrated via emf A: response/recovery times for 2 ppm SO2 step changes. B: dependence on the logarithm of SO2 concentration. C: schematics of the sensor assembly and performance mechanism. Figure 12 A: Galvanostatic cycling with Potential Limit (GCPL) measurement on Na/KMS-2/Na symmetrical cell, at 40 °C, current density of 0.1 mA/cm² and at metal plating/stripping depth of 0.025 mAh/cm². B: GCPL measurement on Na/KMS-2/Na symmetrical cell, at 40 °C, current density of 1 mA/cm² and at metal plating/stripping depth of 0.25 mAh/cm². Detailed description Definitions As used herein, the nomenclature KMS refers generally to compounds or compositions of general formula (I), P6633PC00 K_2+XMg_1-(X/2)SiO₄, (0.0 < x ≤ 0.5) (I) Within the context of the present invention, KMS-1 refers to the compound or composition of formula (I), wherein x = 0.1, and similarly for KMS-2, KMS-3, KMS-4 and KMS-5 as referred to herein. KMS or KMS-0 is used interchangeably herein and refers to the x = 0 compound K₂MgSiO₄. As used herein, the term “electrolyte” refers to a substance that allows the flow of electrical charges, i.e. electrical current, in the form of ion, between the anode and the cathode of a battery. The KMS polycrystal disclosed herein is described as “solid-state electrolyte”, since the compound is at solid state within the interval of temperatures it is designed to operate for such a purpose. Such an interval of temperatures ranges from 25 °C to 650 °C. The KMS polycrystal disclosed herein allows the flow of electrical charges in the form of ions, such as namely as K⁺ ions and/or as Na⁺ ions. As used herein, the term “active electrode material” refers to the component of the electrode that is involved in the electrochemical reactions that produce or store electrical energy. In other words, a substance or composition that allows the interstitial storage and release of mobile ions, such as positive electrode and negative electrode materials commonly known to persons skilled in the art of batteries and electrochemical cells. Opposed to active electrode materials, an electrolyte (see above) does not allow any storing or release of mobile ions, but only allow transfer/flow of such ions. A number of active electrode materials suited for working the present invention are defined herein. The electronic and spatial similarities between exemplary Na⁺ and K⁺ ions are well known in the field. Therefore, while the super-stoichiometric KMS electrolyte of the present invention is described herein as a conductor of in particular potassium (K⁺) ions, it should not be understood as applying exclusively to only potassium ions. The super- stoichiometric KMS electrolyte of the present invention may be considered a conductor of several ions, and in particular mono cationic ions such as the alkali ions and more so, especially sodium (Na⁺) ions due to the well-known similarities between Na⁺ and K⁺. While there are only mobile potassium ions inherently present in the super-stoichiometric KMS electrolyte of the invention, the combination with active electrode materials comprising other ions, such as an active electrode material comprising Na could release P6633PC00 mobile Na⁺ ions that through the three-dimensional network of corner-sharing polyhedra of SiO₄ and MgO₄ moieties found in the super-stoichiometric KMS are transportable through such migration channels. Example 8 and Figures 12A and 12B demonstrate this notion and support that ionic conductivity is not limited to potassium but also applies to other ions such as the alkali ions and in particular Na⁺ ions. It should therefore be readily understood by the skilled person that any reference herein to the super-stoichiometric KMS electrolyte as being potassium-ion conducting is also a reference to the same electrolyte being alkali-ion conducting and in particular sodium- ion conducting. As used herein, the term “space group” is to have the normal meaning used by skilled persons in the fields of chemistry and in particular crystallography and is thought of as referring to the crystallographic space groups of which there are 230 distinct space groups taking account of chiral space groups. This phrase is otherwise well-known and used in the field and requires no further explanation. As used herein, the term “battery” refers to a device capable of generating electric power, as a consequence of the flow of electric charge between the cathode and anode, which are spaced by an electrolyte material. Cathodes and anodes are terms well known, and frequently used, in the field by persons of ordinary skill and need no further clarification. The term “polycrystalline material” as used herein refers to a solid material that is integrated by multiple independent crystals of identical lattice, yet each of them presents an individual orientation in space. By the term “atmospheric air” is meant a gaseous composition comprised primarily by N2 and O2 and a percentage of relative humidity comprised anywhere between 0% and 100%. Atmospheric air may interchangeably be used with ambient air. The terms “phase transformation” and “phase transition” as used herein equally refer to a physical process throughout which a solid material structurally characterised by a well- defined crystalline space group undergoes a transformation that implies the material adopts, in its entirety, a different crystalline space group, whereas its chemical composition remains unchanged. The “phase transformations” or “phase transitions” the P6633PC00 KMS polycrystal undergoes, as described in the present disclosure, are induced by external physical factors, namely by a variation in the temperature. As used herein, the term “potentiometric sensor” refers to a sensor used to determine the concentration of a chemical species, i.e. analyte. This type of sensor produces an output signal in the form of an electrical potential difference between the working electrode and the reference electrode, said potential difference is mathematically correlated to the concentration of the target analyte in the sample. As described in the prior art, “potentiometric sensors” ´may be used for the detection of gaseous analytes, i.e. “potentiometric gas sensors”. A potentiometric gas sensor consists of two electrodes attached to both sides of a solid electrolyte. One electrode is exposed to the gas to be detected, while the reference electrode is facing a reference gas with a constant concentration. In type-I potentiometric gas sensors, the measured gas is converted to the predominant mobile ion of the solid electrolyte, i.e. the electrolyte should have a common species with the gas phase. Examples of type-I sensors include yttrium-stabilised zirconium – fast oxide-ion conductor – for the detection of O2. In type-II electrodes, the gaseous analyte reacts reversibly with the immobile ions in the electrolyte, forming an intermediate phase, separate or dissolved phase in the electrolyte. An equilibrium is thus established between the gas and the intermediate phase. A K2CO3-made electrolyte for the determination of CO2 constitute a type-II sensor. Type III is described as a sensor that sensor employs a thin auxiliary layer to detect different gaseous species of interest. The sensor is thus composed of an electrode of type-II and an ionic junction between the electrolyte and the auxiliary phase. The ionic junction allows the measurement of the concentration of the chemical species that are not present in the electrolyte. The term “M” as used herein refers to molarity or molar concentration. As used herein, the term “ball-milling” refers to a mechanical process, throughout which a solid material, or a mixture comprised by two or more solid materials, are grinded by a device termed “ball mill”. A “ball mill” as used herein comprises a hollow cylindrical shell rotating about its axis, the former being partially filled with balls that constitute the P6633PC00 grinding media. As the shell rotates, the balls are lifted up on the rising side of the shell and then they cascade down into the solid particles. The impact of the balls into the solid particles causes a reduction in their size. The term “pelletizing” as used herein refers to a mechanical process comprising the compression or molding of a solid material into the size of a compressed self-standing solid particle of spherical, rounded, oval or cylindrical shape termed pellet. The term “crushing” as used herein refers to a mechanical process comprising the use of a physical force to reduce the size, deform or compress solid particles. As used herein, the term “green pellet” refers to a pellet that is obtained just after pressing of material and is neither calcined nor sintered. It's a common terminology in ceramic processing and does not need further clarification. As used herein, the term “mother powder” refers to the mixture of e.g., KMS precursors with the same stoichiometric proportions that is used for the synthesis of a particular compound in a particular stoichiometry. It is a very well-known terminology in inorganic synthesis of inorganic materials with volatile elements. Abbreviations for chemicals The following chemicals are described herein, dimethoxyethane (DME), tetraethylene glycol dimethyl (TEGDME), diethyl carbonate (DEC), ethyl carbonate (EC), bis(trifluoromethanesulfonyl)imide (TFSI), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA), polyvinyl chloride (PVC), polyacrylonitrile (PAN), poly(methacrylic acid methyl ester) (PMMA), polyvinylidene fluoride (PVDF), poly(acrylic acid) (PAA) and polypropylene carbonate (PPC). The term “BASE” as used herein refers to beta-alumina solid-state electrolyte, which features a layered crystal structure that enables fast ion transport of alkali species, such as Na and K. The structure is a polyaluminate with channels along which the ionic alkali species of the solid can migrate. Should the structure contain sodium as alkali species, it is denoted as “Na-BASE”. Likewise, the K-containing analogue is termed as “K-BASE”. P6633PC00 As used herein, the term “carnegieite” refers to a sodium aluminium silicate of formula NaAlSiO₄. Other structures such as Na₂CaSiO₄ or Na₂MgSiO₄ are also known in the field as carnegieite-analogues since they crystallize with the same crystallographic space groups as carnegieite, namely orthorhombic Pbca at room temperature. The same is also true for K₂MgSiO₄ (KMS-0) which is a carnegieite-analogue structure that crystallizes at room temperature in orthorhombic Pbca. As used herein, the term “cristobalite” refers to a mineral polymorph of formula SiO₂. The crystal structure of cristobalite is cubic with space group Fd-3m. Chemical composition and structure One embodiment of the present disclosure provides for a solid-state electrolyte comprising an ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). In one embodiment, the solid-state electrolyte consists essentially of an ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5). One embodiment of the present disclosure provides for a solid-state electrolyte comprising a potassium-ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). In one embodiment, the solid-state electrolyte consists essentially of a potassium-ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). In one embodiment X is between 0.1 and 0.5, such as between 0.1 and 0.15, such as between 0.15 and 0.2, such as between 0.2 and 0.25, such as between 0.25 and 0.3, such as between 0.3 and 0.35, such as between 0.35 and 0.4, such as between 0.4 and 0.45, such as between 0.45 and 0.5. In one embodiment, X is selected from 0.1, 0.2, 0.3, 0.4 and 0.5 including any 0.01 integer therein between. In one embodiment X is 0.1. In one embodiment X is 0.2. In one embodiment X is 0.4. In one embodiment X is selected from within the range 0.2 to 0.4, preferably as 0.2 or 0.4. P6633PC00 In one embodiment, the polycrystalline material is characterized by an orthorhombic space group at room temperature. In one embodiment, the orthorhombic space group at room temperature is Pca2₁. As shown in the examples herein, X-ray diffraction analysis and subsequent data processing with computational methods enable the modelling of the unit cell and estimation for interatomic distances. The KMS electrolyte adopts an orthorhombic space group at room temperature, which is a differentiating factor compared to carnegiete, that features a Pbca crystal space group. In one embodiment of the present disclosure, the space group Pca2₁ of the KMS electrolyte described herein is characterized by the following lattice parameters: a (Å) = 10.9 to 11.1, b (Å) = 5.45 to 5.50, and c (Å) = 15.5 to 15.6 In one embodiment, the space group Pca21 of the KMS electrolyte described herein is characterized by the following lattice parameters: a (Å) = 10.9822±0.0005 b (Å) = 5.47305±0.0005, and c (Å) = 15.5487±0.0005. In one embodiment, the polycrystalline material is characterized by a crystal structure comprising corner-sharing polyhedra of non-alkali metal ions, such as corner-sharing tetrahedra comprising SiO4 and/or MgO4 moieties. In one embodiment, the polycrystalline material is characterized by corner-sharing tetrahedra comprising SiO4 and/or MgO4 moieties. Corner-sharing framework structures constitute a promising solution for the development of novel superior K⁺-conductors. The crystal lattice in the corner-sharing framework of the solid-state electrolyte, in contrast to layered structures, provides exposed gaps that are too narrow to enable the inclusion of water molecules, stability to moisture is thereby achieved. Furthermore, in corner-sharing frameworks, non-alkali-metal polyhedra display higher degrees of freedom, which results in more irregular interstitial space that, in turn, increases the interstitial site energy for alkali- metal-ions. High-energy alkali-metal ions in highly distorted sites show a lower activation P6633PC00 energy to the migration through interstices towards more symmetric coordinates, hence ultimately increasing the ionic conductivity. In one embodiment, the polycrystalline material is characterized by a phase transformation temperature between 100°C and 1000°C, such as between 100°C and 200°C, such as between 200°C and 250°C, such as between 250°C and 300°C, such as between 300°C and 350°C, such as between 350°C and 400°C, such as between 400°C and 500°C, such as between 500°C and 600°C, such as between 600°C and 700°C, and wherein the phase transformation temperature represents a transformation from an orthorhombic space group to a cubic space group. In one embodiment, the polycrystalline material is characterized by a phase transformation temperature between 200 °C and 250 °C, and wherein the phase transformation temperature represents a transformation from an orthorhombic space group to a cubic space group. In one embodiment, the polycrystalline material is characterized by a phase transformation temperature between 200 °C and 250 °C. In one embodiment, the phase transformation temperature represents a transformation from an orthorhombic space group to a cubic space group. In one embodiment, the polycrystalline material is characterized by a cubic space group at 300 °C. In one of the embodiments, the cubic space group at 300 °C is Fd-3m. As shown in the examples herein, X-ray diffraction analysis and subsequent data processing with computational methods enable the modelling of the unit cell and estimation for interatomic distances. The KMS polycrystal adopts cubic space group Fd-3m at 300 °C. The X-ray diffraction patterns recorded at several temperature values, as shown in the examples herein, show that upon temperature increase the KMS polycrystal undergoes a phase transformation between 200 °C and 250 °C, and crystallizes into higher symmetry cubic crystal structure with a space group Fd-3m and a refined lattice parameter of a = 7.79 Å. Upon further temperature increase above the phase transition temperature, the cubic Fd-3m crystal structure expands to a= 7.86 Å at 650 °C. The phase transition temperature featured by the KMS polycrystal, as described herein is lower than for other K-containing polycrystals reported in the prior art, such as KAlO2 and KFeO2, for which respective phase transition temperatures are 400-550 °C and 650 °C P6633PC00 In one embodiment of the present disclosure, the KMS polycrystal in the cubic Fd-3m crystal structure reverses to the initial orthorhombic Pca2₁ phase upon cooling of the sample below 250 °C. As shown in the examples herein, the X-ray diffraction patterns of the KMS polycrystal recorded at several temperature values, are, to a large extent, independent to the atmosphere used during the study, i.e. atmospheric air or N₂. The homogeneity between X-ray diffraction patterns under N₂ and atmospheric air indicates the absence of apparent phase decomposition under atmospheric air, as a result of O₂ and moisture, which is otherwise a problem observed with many other fast K⁺ conductors reported in the prior art. As shown in the examples herein, the thermogravimetric analysis further demonstrates the stability towards moisture of the KMS polycrystal, since neither hydrated phases, nor water intake were observed throughout the thermal process. In one embodiment of the present disclosure, the electrolyte is characterized by an alkali ion conductivity between 1·10^-7 S/cm and 1·10^-4 S/cm at room temperature (25 °C). In one embodiment of the present disclosure, the electrolyte is characterized by a potassium-ion conductivity between 1·10^-7 S/cm and 1·10^-4 S/cm at room temperature (25 °C). In one embodiment, the potassium-ion conductivity at room temperature (25 °C) is between 1·10^-7 S/cm and 1·10^-4 S/cm, such as 1·10^-7 S/cm and 5·10^-7 S/cm, such as 5·10^-7 S/cm and 1·10^-6 S/cm, such as 1·10^-6 S/cm and 5·10^-6 S/cm, such as 5·10^-6 S/cm and 1·10^-5 S/cm, such as 1·10^-5 S/cm and 5·10^-5 S/cm, such as 5·10^-5 S/cm and 1·10^-4 S/cm. In one embodiment of the present disclosure, the electrolyte is characterized by an alkali ion conductivity between 1·10^-7 S/cm and 1·10^-1 S/cm at 300 °C. In one embodiment of the present disclosure, the electrolyte is characterized by a potassium-ion conductivity between 1·10^-7 S/cm and 1·10^-1 S/cm at 300 °C. In one embodiment, the potassium-ion conductivity at (300 °C) is between 1·10^-7 S/cm and 1·10^-1 S/cm, such as 1·10^-7 S/cm and 1·10^-6 S/cm, such as 1·10^-6 S/cm and 1·10^-5 S/cm, such as 1·10^-5 S/cm and 1·10^-4 S/cm, such as 1·10^-4 S/cm and 5·10^-4 S/cm, such as 5·10^-4 S/cm and 1·10^-3 S/cm, such as 1·10^-3 S/cm and 5·10^-3 S/cm, such as 5·10^-3 S/cm and 1·10^-2 S/cm, such as 1·10^-2 S/cm and 5·10^-2 S/cm, such as 5·10^-2 S/cm and 1·10^-1 S/cm. P6633PC00 In one embodiment of the present disclosure, the electrolyte is characterized by a sodium-ion conductivity between 1·10^-7 S/cm and 3·10^-4 S/cm at 40 °C. In one embodiment, the ion conductivity is measured via electrochemical impedance spectroscopy (EIS). In one embodiment, the electrolyte is characterized by a relative density of 90±2% or more, such as 91±2% or more, such as 92±2% or more, such as 93±2% or more, such as 94±2% or more, such as 95±2% or more, such as 96±2% or more, such as 97±2% or more, such as 98±2%, preferably having a relative density of 95±2%. In one embodiment, the electrolyte is characterized by a relative density of 95±2%. The density is measured via gas pycnometer, as shown in the examples herein. The density measurements demonstrate the KMS solid-state electrolyte possess a high level of densification, which is crucial for ensuring the optimal performance of the pellets and self-standing tapes in a variety of applications, most remarkably as alkali-ion conducting solid-state electrolytes for batteries, such as potassium-ion conducting solid-state electrolytes for potassium batteries. In one embodiment, the electrolyte is characterized by an electronic conductivity between 1·10^-10 S/cm and 1·10^-6 S/cm at room temperature (25 °C). In one embodiment, the electronic conductivity at room temperature (25 °C) is between 1·10^-10 S/cm and 1·10^-6 S/cm, such as 1·10^-10 S/cm and 1·10^-9 S/cm, such as 1·10^-9 S/cm and 1·10^-8 S/cm, such as 1·10^-8 S/cm and 1·10^-7 S/cm, such as 1·10^-7 S/cm and 1·10^-6 S/cm. In one embodiment, the electrolyte is characterized by an electronic conductivity between 1·10^-10 S/cm and 1·10^-6 S/cm at 300 °C. In one embodiment, the electronic conductivity at 300 °C is between 1·10^-10 S/cm and 1·10^-6 S/cm, such as 1·10^-10 S/cm and 1·10^-9 S/cm, such as 1·10^-9 S/cm and 1·10^-8 S/cm, such as 1·10^-8 S/cm and 1·10^-7 S/cm, such as 1·10^-7 S/cm and 1·10^-6 S/cm. In one embodiment, the electronic conductivity is measured via chronoamperometry at 500 mV, but may be measured in other ways known to those skilled in the art. P6633PC00 As shown in the examples herein, the ionic conductivity of K-super-stoichiometric KMS- 1, KMS-2, KMS-4 and KMS-5 at temperatures of 300 °C or below, are higher than for the K-stoichiometric counterpart, KMS-0. The inclusion of super-stoichiometric amounts of potassium is thereby shown to increase the K-ionic conductivity. The temperature dependence of the recorded ionic conductivity values for KMS-1, KMS-2 and KMS-4, as shown in the examples herein, demonstrates that the latter compounds have a lower activation energy for K⁺ migration (0.36 eV), in comparison to the analogous value for stoichiometric KMS-0 (0.47 eV). Despite the elevated K-ionic conductivity, KMS-2 exhibits an electronic conductivity of ~ 5·10^-9 S/cm, approximately four orders of magnitude lower than the former parameter, as shown in the examples herein. Consequently, the solid-state electrolyte bears a K⁺ transference number close to unity, which is a desirable feature in development of potassium-ion batteries with solid-state electrolyte. Use in a sensor One embodiment of the present disclosure provides for the electrolyte of the disclosure, for use as a component in a sensor. An embodiment of the present disclosure is thus use of the electrolyte of general formula K2+XMg1-(X/2)SiO4 (0 < x ≤ 0.5) as a component in a sensor, such as a potentiometric sensor. One embodiment of the present disclosure is a sensor comprising the electrolyte of general formula K2+XMg1-(X/2)SiO4 (0 < x ≤ 0.5). In one embodiment, the sensor is a potentiometric sensor. In one embodiment the sensor is a type-III potentiometric sensor. In one embodiment the sensor is for sensing a gas, such as a gas selected from SO2, SO3, NO, NO2, CO, and CO2. In one embodiment the sensor is for sensing SO2. As shown in the examples herein, a type III potentiometric solid-state gas sensor for SO2 (balanced by synthetic air) detection was developed using KMS-2 as the electrolyte, porous K2SO4 layer in combination with porous Pt layer, as auxiliary sensing electrode, and air-exposed KMS-2 surface (below the porous silver) in combination with porous silver layer as the reference electrode (O2,Au|Ag| ^K2.2^Mg0.9^SiO4|K2SO4|Ag|Pt,SO2,O2). The operation principle of the electrode of the disclosure involves a thermodynamic equilibrium between solid-state electrolyte, SO2 in the gas and porous auxiliary sensing electrode (in this case Pt+K2SO4) via the reaction according to Eq. (I): P6633PC00 2^^{^} + ^^_^ + ^_^ + 2^^{^} ↔ ^_^^^_^ Eq (I) Another thermodynamic equilibrium is established on the silver-coated surface of the solid-state electrolyte between the solid-state electrolyte, O2 in the gas and porous reference electrode (here Ag) according to Eq. (II): 2^^{^} 1 + ^_^ + 2^^{^} _{↔ ^^^} Eq (II) 2 The establishment of the above thermodynamic equilibriums will fix the K⁺ chemical potential on the solid-state electrolyte surface in the interfaces electrolyte/sensing- electrode and electrolyte/reference-electrode and creates a chemical potential difference between the two electrodes. This chemical potential can be measured as an electrical potential via Nernst equation, Eq. (III): ^^ ^ The solid-state electrolyte layer (in the examples herein, KMS-2) provides the K⁺ for the abovementioned thermodynamic equilibrium reactions (Eqs. I and II). High ionic conductivity of K⁺ species within the solid-state electrolyte is a requirement for establishment of above thermodynamic equilibrium reactions and consequently fast response times, as well as stable sensing signal, which aligns with Nernst –type response (Eq. III). As shown in the examples herein, the potentiometric SO2 sensor constructed with the electrolyte of the disclosure enabled the determination of SO2 at 500 °C in concentrations in the range of 0-10 ppm, achieving a sensitivity of 74-89 mV/dec, and a response/recovery time between 2 and 120 minutes. Use in a battery One embodiment of the present disclosure provides for the use of the electrolyte of general formula K2+XMg1-(X/2)SiO4 (0 < x ≤ 0.5) as a component in a battery. In one embodiment, said battery is a solid-state battery or a semi-solid-state battery, but is in one embodiment preferably a solid-state battery. In one embodiment, said battery is an alkali-ion conducting battery, such as a sodium- ion battery or a potassium-ion battery. P6633PC00 One embodiment of the present disclosure provides for a battery comprising: An ion conductive electrolyte layer, a positive electrode layer, a negative electrode layer , and wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises an ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0 < x ≤ 0.5). In one embodiment of the present disclosure, the conductive polycrystalline material is capable of conducting alkali-metal ions, such as is alkali-ion conductive. In one embodiment, the conductive polycrystalline material is potassium-ion conductive and/or sodium ion conductive. In one embodiment of the present disclosure, the battery comprising the ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0 < x ≤ 0.5) is an alkali-ion battery, such as a potassium-ion battery and/or sodium-ion battery. In some embodiments of the present disclosure, the battery may be a mixed alkali-ion battery such as capable of conducting and intercalating two or more alkali ions, preferably wherein the two or more alkali ions comprise at least sodium and potassium. In one embodiment of the present disclosure, the alkali-ion battery is a potassium-ion battery. One embodiment of the present disclosure provides for a potassium-ion battery comprising: A potassium-ion conductive electrolyte layer, a positive electrode layer, a negative electrode layer , and wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises a potassium-ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0 < x ≤ 0.5). The electrochemical features of the solid-state electrolyte of the disclosure shown in the examples herein, namely ion and electron conductivities, densification, and stability to P6633PC00 ambient conditions, provide an optimal framework for the development alkali-ion batteries such as of K-ion batteries. In one embodiment the ion conductive electrolyte layer is a solid ion conductive electrolyte layer. In one embodiment the potassium-ion conductive electrolyte layer is a solid potassium-ion conductive electrolyte layer. In one embodiment, the ion conductive electrolyte layer is a liquid ion conductive electrolyte layer. In one embodiment, the potassium-ion conductive electrolyte layer is a liquid potassium-ion conductive electrolyte layer. In one embodiment, the ion conductive electrolyte layer is a solid/liquid hybrid ion conductive electrolyte layer. In one embodiment, the potassium-ion conductive electrolyte layer is a solid/liquid hybrid potassium-ion conductive electrolyte layer. In one embodiment, the ion conductive electrolyte layer is formed between the positive electrode layer and the negative electrode layer. In one embodiment, the potassium-ion conductive electrolyte layer is formed between the positive electrode layer and the negative electrode layer. In one embodiment the solid ion conductive electrolyte layer comprises one from the group consisting of: an inorganic solid-state electrolyte, a liquid electrolyte, a solid-state polymer electrolyte, and any of the foregoing, or a mixture thereof, as a composite in combination with the KMS electrolyte of the disclosure. In one embodiment the solid ion conductive electrolyte comprises KMS electrolyte of the present disclosure, alone or as part of a composite together with at least one selected from an inorganic solid-state electrolyte, a liquid electrolyte and a solid-state polymer electrolyte. In one embodiment the solid potassium-ion conductive electrolyte layer comprises one from the group consisting of: an inorganic solid-state electrolyte, a liquid electrolyte, a solid-state polymer electrolyte, and any of the foregoing, or a mixture thereof, as a composite in combination with the KMS electrolyte of the disclosure. In one embodiment the solid ion conductive electrolyte layer comprises an inorganic solid-state electrolyte. In one embodiment, the solid potassium-ion conductive electrolyte layer comprises a liquid electrolyte. In one embodiment the solid potassium-ion conductive electrolyte layer comprises an inorganic solid-state electrolyte. In one embodiment, the solid potassium- ion conductive electrolyte layer comprises a liquid electrolyte. In one embodiment the solid ion conductive electrolyte layer comprises a solid-state polymer electrolyte. P6633PC00 In one embodiment the solid potassium-ion conductive electrolyte layer comprises a solid-state polymer electrolyte. In one embodiment the battery according to the present disclosure comprises an inorganic solid-state electrolyte selected from the group consisting of K-BASE, K₂MgSiO₄, KAlO₂, KFeO₂, , KGaO₂, K_0.7Sr_0.15GaO₂ KAlSiO₄, K₂CaSiO₄, K₂Mg₂TeO₆, K₂ZnSiO₄, K₂Si₂P₃, K₂Sb₅P₂O₁₀, K_2.92Sb_0.92W_0.08S₄, K_0.59Mg_0.53Sb_0.47O₂, K₃SbS₄, K_1.9Fe_1.95P_0.05O₄, K_1.9Pb_0.05AlO₂, K_0.405Bi_0.865AsO₄, K_0.4Cd_0.3FeO₂, K₂Fe₄O₇, K_1.6Zn_0.8Ti_7.2O₁₆, K_0.72In_0.72Sn_0.28O₂, KBiO₃, K₃Sc(MoO₄)₃, KMgPO₄, K₂MgV₂O₇, K₂Mg₂Si₂O₇, K₂CaP₂O₇, K₂ZnGeO₄, K₂Mg₂(MoO₄)₃, K₄Mg(WO₄)₃, K₂CaPO₄F, Li₇La₃Zr₂O₁₂ (LLZO), Li_5.1Ga_0.32La₃Zr_2.25O₁₃, Li_6.25Ga_0.25La₃Zr₂O₁₂, Li_xLa_yTiO₃ (0.07 < x < 0.13; y=(2/3)-x) (LLTO), Li_0.45La_0.48TiO₃, Li₅La₃X₂O₁₂ (X= Nb or Ta), Li_xPO_yN_z (3 < x < 3.2; 3 < y < 3.5; 0 < z < 0.5; 3 < (y+z) < 4), Li_3.13PO_1.69N_1.39, Li_0.98PO_2.55N_0.50, Li₃PO₄, La_(1+x)Al_xTi_(2-x)(PO₄)₃ (0 < x < 2)(LATP), Li_1.3Al_0.4Ti_1.7(PO₄)₃, Li₁₀GeP₂S₁₂, Li₇P₃S₁₁, Na- BASE, NaM₂(PO₄)₃ (M = Ge, Ti, Zr), Na_1+xZr₂Si_xP_3-xO₁₂ (0 < X < 3), Na₂MgSiO₄, Na2CaSiO4, Na2ZnSiO4, Na2Mg2TeO6, Na2Mg2ZnO6, Na3PS4, Na4SiS4, Na3PSe4, Na10SnP2S12, Na3SbS4 and any combination thereof. In one embodiment the battery according to the present disclosure comprises an inorganic solid-state electrolyte selected from K-BASE, K2MgSiO4, KAlSiO4, K2CaSiO4, K2Si2P3, K2.92Sb0.92W0.08S4, K3SbS4, Na-BASE, Na1+xZr2SixP3-xO12 (0 < X < 3), Na2MgSiO4, and Na2CaSiO4 and any combination thereof. In one embodiment the battery according to the present disclosure comprises a liquid electrolyte selected from the group consisting of ether electrolyte, such as (AN(SO2F)2 in DME) or (ACF3SO3 in TEGDME), ester electrolyte, such as (APF6 in EC/DMC), (APF6 in PC) or (AN(SO2F)2 in EC/DEC), ATFSI in tetraglyme, wherein (A = K, Li, or Na), and aqueous electrolyte, such as (1 M KNO3 per 0.01 M HNO3), (1 M KNO3 pH = 2), 0.5 M K2SO4, 0.1 M KCl, , 22 M KCF3SO3, 3 M KCl, 30 M KFSI, , or 1 M KOH or a mixture thereof. In one embodiment, the battery according to the present disclosure comprises the liquid electrolyte ATFSI in tetraglyme wherein (A = K, Li, or Na). In one embodiment, the battery according to the present disclosure comprises a solid- state polymer electrolyte selected from the group consisting of (KTFSI+PEO), PEO- KAg4I5, PEO-KBrO3, PPC-KFSI, PEO-KFSI, PVP+PVA+KBrO3, PVC+KBrO3, PAN-KI, PMMA-KPF₆, PVA-KCl, (KTFSI+PEO). (LiTFSI+PEO), PEO-LiAg₄I₅, PEO-LiBrO₃, PPC- P6633PC00 LiFSI, PEO-LiFSI, PVP+PVA+LiBrO₃, PVC+LiBrO₃, PAN-LiI, PMMA-LiPF₆, PVA-LiCl, (LiTFSI+PEO).(NaTFSI+PEO), PEO-NaAg₄I₅, PEO-NaBrO₃, PPC-NaFSI, PEO-NaFSI, PVP+PVA+NaBrO₃, PVC+NaBrO₃, PAN-NaI, PMMA-NaPF₆, PVA-NaCl, , (NaTFSI+PEO) and any combination thereof. In one embodiment, the battery according to the present disclosure comprises the solid-state polymer electrolyte (KTFSI+PEO). In one embodiment, the battery according to the present disclosure comprises a positive electrode layer selected from a solid positive electrode layer, a liquid positive electrode layer and a solid-liquid hybrid positive electrode layer. In one embodiment the positive electrode layer is a solid positive electrode layer comprising: a. a carbonaceous material such as carbon black, b. a binder such as PVDF or PAA, c. at least one inorganic solid-state electrolyte selected from the group consisting of K-BASE, K2MgSiO4, KAlO2, KFeO2, KGaO2, K0.7Sr0.15GaO2 KAlSiO4, K2CaSiO4, K2Mg2TeO6, K2ZnSiO4, K2Si2P3, K2Sb5P2O10, K2.92Sb0.92W0.08S4, K0.59Mg0.53Sb0.47O2, K3SbS4, K1.9Fe1.95P0.05O4, K1.9Pb0.05AlO2, K0.405Bi0.865AsO4, K0.4Cd0.3FeO2, K2Fe4O7, K1.6Zn0.8Ti7.2O16, K0.72In0.72Sn0.28O2, KBiO3, K3Sc(MoO4)3, KMgPO4, K2MgV2O7, K2Mg2Si2O7, K2CaP2O7, K2ZnGeO4, K2Mg2(MoO4)3, K4Mg(WO4)3, K2CaPO4F, Li7La3Zr2O12 (LLZO), Li5.1Ga0.32La3Zr2.25O13, Li6.25Ga0.25La3Zr2O12, LixLayTiO3 (LLTO) (0.07 ≤ x ≤ 0.13; y±0.05 = (2/3)- x), Li0.45La0.48TiO3, Li5La3X2O12 (X= Nb or Ta), LixPOyNz (3.0 ≤ x ≤ 3.2; 3.0 ≤ y ≤ 3.5; 0.0 ≤ z ≤ 0.5; 3 ≤ (y+z) ≤ 4), Li3.13PO1.69N1.39, Li0.98PO2.55N0.50, Li3PO4, La1+xAlxTi2-x(PO4)3 (0 ≤ x ≤ 2), Li1.3Al0.4Ti1.7(PO4)3, Li10GeP2S12, Li7P3S11, Na-BASE, NaM2(PO4)3 (M = Ge, Ti, Zr), Na1+xZr2SixP3-xO12 (0 ≤ X ≤ 3), Na2MgSiO4, Na2CaSiO4, Na2ZnSiO4, Na2Mg2TeO6, Na2Mg2ZnO6, Na3PS4, Na4SiS4, Na3PSe4, Na10SnP2S12, and Na3SbS4, and d. an active electrode material comprising one or more of: i. a sulphur-type positive electrode material selected from pure sulfur, K2Sx (x= 1, 2, 3, 5), Na2Sx (x= 1, 2, 3, 5), Li2Sx (x= 1, 2, 3, 5), and PAN-S, preferably K2Sx (x= 1, 2, 3, 5) and/or PAN-S, ii. an inorganic-type positive electrode material selected from K₂Ni₂TeO₆, K₃CoO₂, K_0.3MnO₂, K₂FeSiO₄, KFeSiO₄, K₂CoNiTeO₆, P6633PC00 KFePO₄F, K₂NiO₂, K₂CuP₂O₇, K₂FeSiO₄, KFeSi₂O₆, KMnPO₄, K₂NiFe(CN)₆·1.2H₂O, K_1.85Fe_0.33Mn_0.67[Fe(CN)₆]_0.98·0.77H₂O, K_0.22V_1.74O_4.37·0.82H₂O, KM^IIFe^III(CN)₆(M = Mn, Fe, Co, Ni, and Zn), NaCoO₂, NaNiO₂, NaMnO₂, NaFeO₂, Na_7/9Cu_2/9Fe_1/9Mn_2/3O₂, Na₃V₂(PO₄)₃, Na₂VTi(PO₄)₃, Na_0.66Mn_0.66Ti_0.34O₂, Na_0.44MnO₂, Na₂LiV₂(PO₄)₃, Li₄Ti₅O₁₂, LiFePO₄, LiMn₂O₄, LiNiMnCoO₂, LiNiCoAlO₂, LiCoO₂, preferably K₂Ni₂TeO₆, K₂FeSiO₄ and KFeSi₂O₆, and iii. an organic-type positive electrode material selected from A_2+xC₆O₆ (0 < X < 4), (A = Na, K, Li). In one embodiment the positive electrode layer is a liquid positive electrode layer comprising as the active electrode material one or more of a sulphur-type positive electrode material selected from pure sulphur, K2Sx (x= 1, 2, 3, 5), Na2Sx (x= 1, 2, 3, 5), Li2Sx (x= 1, 2, 3, 5), preferably K2Sx (x= 1, 2, 3, 5) , optionally in combination with a liquid electrolyte selected from the group consisting of ether electrolyte, such as (AN(SO2F)2 in DME) or (ACF3SO3 in TEGDME), ester electrolyte, such as (APF6 in EC/DMC), (APF6 in PC) or (AN(SO2F)2 in EC/DEC), ATFSI in Tetraglyme, and aqueous electrolyte, such as (1 M KNO3 per 0.01 M HNO3), (1 M KNO3 pH = 2), 0.5 M K2SO4, 0.1 M KCl, 22 M KCF3SO3, 3 M KCl, 30 M KFSI, or 1 M KOH, wherein (A = K, Li, or Na). In one embodiment the positive electrode layer is a hybrid positive electrode layer comprising any combination of solid positive electrode layers as defined herein above, and liquid positive electrode layers as defined herein above. In one embodiment the negative electrode layer is selected from a solid negative electrode layer, a liquid negative electrode layer and a hybrid negative electrode layer. In one embodiment the negative electrode layer is a solid negative electrode layer comprising one or more of: a) a metal selected from K, Li, Na, Al or an alloy thereof, preferably K or a ternary K-Na-Li alloy, and P6633PC00 b) a carbon-type negative electrode selected from graphite and hard carbon. c) a silicon type negative electrode selected from silicon, crystalline silicene or zintle (potassium silicides, sodium silicides, lithium silicides and calcium silicides). In one embodiment, the negative electrode layer is a liquid negative electrode layer comprising one or more of: a metal selected from K, Li, Na, Al or an alloy thereof, preferably K or a ternary K-Na-Li alloy. In one embodiment, the negative electrode layer is a hybrid negative electrode layer comprising any combination of solid negative electrode layers as defined herein above, and liquid negative electrode layers as also defined herein above. In one embodiment the positive electrode layer is a solid positive electrode layer, and the negative electrode layer is a solid negative electrode layer. A sensor One embodiment of the present disclosure provides for a sensor comprising the potassium-ion conductive polycrystalline material described herein. An embodiment of the present disclosure is this a sensor comprising the ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). In one embodiment, the sensor of the disclosure is suitable for detecting a gas selected from the group consisting of SO2, SO3, NO, NO2, CO, and CO2. In one embodiment, the sensor of the disclosure is suitable for detecting SO2. In one embodiment, the sensor of the disclosure is a potentiometric sensor, such as a type-III potentiometric sensor. In one embodiment, the sensor of the disclosure has a sensitivity of 74-89 mV/dec at 500 °C. The sensitivity of the potentiometric sensor integrated by the electrolyte exceeded, by a factor of 10, the sensitivity reported in the prior art by sensors incorporating lithium lanthanum zirconium oxide (LLZO). P6633PC00 In one embodiment, the sensor of the disclosure has a detection limit of 1-10 ppm for SO₂ at 500 °C, such as 2 ppm, such as 4 ppm, such as 6 ppm, such as 8 ppm, such as 10 ppm, such as any concentration between 1 and 10 ppm. In one embodiment, the sensor of the disclosure has a response/recovery time between 2 and 120 minutes for SO₂ at 500 °C, such as 2 to 3 minutes, such as 3 to 5 minutes, such as 5 to 15 minutes, such as 15 to 45 minutes, such as 45 to 120 minutes. Manufacture One embodiment of the present disclosure, provides for a method of manufacturing a solid-state electrolyte, said method comprising the sequential steps: a. Mixing at least one source of potassium, at least one source of magnesium, and at least one source of silicon in a ratio such as to obtain a super-stoichiometric potassium ratio of K:Mg:Si corresponding to (2+X):1-(X/2):1 (0.0 < X ≤ 0.5), thereby forming a mixture, and b. Heating said mixture to a first temperature between 600 °C and 1000 °C to obtain a sintered mixture. The solid-state electrolyte of the disclosure can be manufactured from inexpensive, abundant and non-toxic starting materials. The method of manufacture for the solid-state electrolyte of the disclosure provides for a general and common procedure to access all the stoichiometries comprised within the interval given for “x” in formula (I). Variations on the stoichiometry can be achieved by simply adjusting the relative proportions between starting materials, K2CO3, SiO2 and MgO at the start of the process. In one embodiment, the method of manufacturing the solid-state electrolyte of the disclosure further comprises, prior to step b, a step of ball-milling said mixture. Exemplary, oxide precursors may in one embodiment be mixed in ethanol and wet-milled via a planetary ball mill, using cups and balls made from zirconia, at 150 rpm for 2 to 12 hours, such as 4 to 8 hours, such as 6 hours. Alternative solvents for ball-milling are water, isopropanol, methanol, ethyl acetate, and cyclohexane. In one embodiment of the present disclosure, the method of manufacturing the solid- state electrolyte of the disclosure further comprises, subsequent to step b, a step of crushing the sintered mixture into a fine powder, pelletizing said fine powder, and heating P6633PC00 said pelletized powder to a second temperature between 600°C and 1000°C. The crushing of the sintered mixture into a fine powder may be done using the ball-milling procedure described herein above. In one embodiment the step of crushing, pelletizing and heating to a second temperature is repeated at least 3 times, such as 4 times, such as 5 times, such as any one of 6, 7, 8, 9, and 10 times. In one embodiment, the first and second temperature are a temperature below 1000 °C, such as below 950 °C, such as below 900 °C, such as below 850 °C, such as below 800 °C. In one embodiment, the first and second temperature are a temperature below 850 °C. In one embodiment, the second temperature is a temperature below 1000 °C, such as below 950 °C, such as below 900 °C, such as below 850 °C, such as below 800 °C, such as below 750 °C, such as below 700 °C, such as below 650 °C. In one embodiment, the first temperature in step b, is below 850 °C, such as below 800 °C. Furthermore, the first and second temperatures are lower than for the analogous manufacturing process of canonical K-based solid-state electrolytes reported in the prior art, such as K-BASE, which is sintered at a temperature of 1400-1700 °C. Therefore, the more facile method of manufacture provides for a facile implementation of large-scale production of the solid- state electrolyte of the disclosure. In one embodiment, the pelletizing, comprised in the method of manufacturing the solid- state electrolyte of the disclosure, is performed by application of uniaxial pressure in the range of 50 MPa to 100 MPa, such as in the range of 50 MPa to 60 MPa, such as in the range of 60 MPa to 70 MPa, such as in the range of 70 MPa to 80 MPa, such as in the range of 80 MPa to 90 MPa, such as in the range of 90 MPa to 100 MPa, preferably in the range of 70 MPa to 90 MPa. In one embodiment, the pelletizing, comprised in the method of manufacturing the solid-state electrolyte of the disclosure, is performed by application of uniaxial pressure in the range of 70 MPa to 90 MPa. P6633PC00 In one embodiment, the at least one source of potassium is a salt of potassium or an oxide of potassium, such as selected from the group consisting of KF, KCl, KBr, KI, KNO₃, K₂CO₃, K₃PO₄, K₂SO₄, KClO₄, KClO₃, K₂O, and KOH. In one embodiment, the at least one source of potassium is K₂CO₃ or K₂O. In one embodiment, the at least one source of magnesium is a salt of magnesium or an oxide of magnesium, such as selected from the group consisting of MgF₂, MgCl₂, MgBr₂, MgI₂, Mg(NO₃)₂, MgCO₃,Mg₃(PO₄)₂, MgSO₄, Mg(ClO₄)₂, Mg(ClO₃)₂, MgO, and Mg(OH)₂. In one embodiment, the at least one source of magnesium is MgO. In one embodiment, the at least one source of silicon is a halide of silicon or an oxide of silicon, such as selected from the group consisting of SiCl₄ and SiO₂. In one embodiment of the present disclosure, the at least one source of silicon is SiCl4 and/or SiO2. In one embodiment, the method of manufacturing the solid-state electrolyte of the disclosure is performed in an ambient atmosphere. In one embodiment, the method of manufacturing the solid-state electrolyte of the disclosure is performed in inert atmosphere, such as consisting essentially of nitrogen or argon or any mixture thereof. In one embodiment, the method of manufacturing the solid- state electrolyte of the disclosure is performed in inert atmosphere, such as consisting essentially of nitrogen. In one embodiment, the method of manufacturing the solid-state electrolyte of the disclosure is performed in inert atmosphere, such as consisting essentially of argon. P6633PC00 Examples Example 1: synthesis of KMS solid-electrolytes via solid-state method The stoichiometric amounts of the starting materials K₂CO₃ (Sigma-Aldrich Corporation Supplier product number: 342890), MgO (Riedel-de Haën) and SiO₂ (Sigma-Aldrich Corporation Supplier product number: 209619) were adjusted accordingly to afford the different compositions of KMS solid-electrolyte, determined by value “X” on formula (I). Table 1. Stoichiometric amounts of the starting materials to synthesise the various KMS solid-electrolytes. X-value K2CO3 (mol) MgO (mol) SiO2 (mol) 0.1 2.1 0.95 1 0.2 2.2 0.90 1 0.3 2.3 0.85 1 0.4 2.4 0.80 1 0.5 2.5 0.75 1 Methods The KMS solid-electrolytes are synthesised via solid-state method. The oxide precursors, K₂CO₃, MgO and SiO₂, are mixed in ethanol and wet-milled via a planetary ball mill (Retsch Planetary ball mill PM 400), using cups and balls made from zirconia, at 150 rpm for 6 hours. Subsequently, the milled slurry is dried, and the resulting powder was pelletized using a uniaxial press (70 MPa), for better contact and interdiffusion between the powder particles during calcination. The green pellets are calcined at 600 °C for 24 hours under air. Afterwards, the calcined pellets are pulverized (by mortar and pestle), milled (150 rpm in 6 hours), pelletized, and calcined via the abovementioned procedure three more times. At the subsequent iterations, the calcination temperature was increased to 700 °C and to 800 °C respectively. Furthermore, during calcination, regardless of the temperature, the pellets were embedded within the mother powder inside capped alumina crucibles to protect the samples from potassium evaporation. After calcination at 800 °C and subsequent pulverizing and milling of the samples, the resulting powders were uniaxially pressed (90 MPa) into pellets (solid-state electrolytes) with the preferred size, followed by cold isostatic pressing (300 MPa) and finally sintered at 850 °C for 20 hours under air. During sintering, the solid-state electrolytes should be P6633PC00 embedded within the mother powder inside capped alumina crucibles, to protect the samples from potassium evaporation. Example 2: phase transition study on KMS-2 via X-ray diffraction pattern analysis Materials and methods X-ray diffraction patterns of KMS-2 pulverized pellets were recorded at different temperature values from 25 °C up to 650 °C; individually under both N₂ and atmospheric air (Figures 4A-D). Samples measured under atmospheric air were pulverized and then stored under atmospheric air (relative humidity 30-40%) at room temperature, inside capped bottles for 60 days prior measurement. Samples measured under N₂, were initially vacuum packed within moisture barrier bags and pulverized immediately before XRD characterization. The X-ray diffraction analyses on the powder samples were performed with a Rigaku SmartLab diffractometer using Cu-Kα radiation (1.5406 Å). The Rietveld refinement method on the diffraction patterns was conducted using FullProf software suite for determining crystallographic parameters, as well as phase analysis. The schematics of the crystallographic structures and the estimation for interatomic distances were realized via the VESTA software. Results X-ray diffraction pattern The X-ray diffraction pattern in Figure 1A, shows that KMS-2 adopts an orthorhombic structure with space group Pca21 (Figure 1B) at room temperature, unlike the typical low temperature structures of carnegieite-analogues, which crystallize in orthorhombic Pbca. Upon temperature increase, KMS-2 undergoes a phase transformation between 200 °C and 250 °C, and crystallizes into higher symmetry cubic crystal structure with a space group Fd-3m (Figures 2A and 2B) and a refined lattice parameter of a = 7.79 Å. The comparison of the 2θ reflections between both crystal structures is displayed in Figure 3. As anticipated, the high-temperature KMS-2 cubic carnegieite-analogue structure expands with increasing temperature from 250 °C to 650 °C, and the refined lattice parameter increases to a= 7.86 Å accordingly. During the cooling under both atmospheres (Figures 4B and 4D), the high-temperature KMS-2 cubic carnegieite- P6633PC00 analogue structure reversibly shrinks and the former value of lattice parameter “a” is re- established, namely a = 7.79 Å at 250 °C. Upon further temperature decrease below 250 °C the crystal phase transforms into the orthorhombic Pca2₁ crystal structure. The transformation temperature observed in this work is significantly lower than the values of 400-550 °C and 650 °C reported for KAlO₂ and KFeO₂, respectively. This lower phase transformation temperature will result in relatively higher ionic conductivity for KMS-2, compared to KAlO₂ and KFeO₂, specially at the standard operational temperatures for K-based solid state battery applications (~ 300 °C) and potentiometric solid-state gas sensors (~ 500 °C). As shown in Figures 4A-D, the X-ray diffraction patterns obtained under ambient air compared to those obtained under an atmosphere of N₂ at various temperatures are highly similar, which indicates the absence of any apparent phase decomposition in atmospheric air as a result of O₂ and moisture, which is otherwise a problem observed with many other fast K⁺ conductors. Conclusion KMS-2 displays an orthorhombic Pca21 phase at room temperature and undergoes a reversible phase transition, between 200 °C and 250 °C, into the cubic phase Fd-3m. The lower phase transition temperature, in comparison to previously reported solid-state electrolytes, will result in a relatively higher ionic conductivity at lower temperatures. KMS-2 is a stable alternative for a potassium-ion conducting electrolyte for applications in atmospheric air, since it does not exhibit any structural changes upon moisture and/or O2 exposure. Example 3: microstructure and density analyses on KMS-2 Materials and methods Microstructural characterization was performed via Scanning Electron Microscopy (SEM) (ZEISS Merlin, Carl Zeiss, Germany). The surface of the samples was dry polished and thermally etched under air at 750 °C for 30 minutes. A combination of secondary and in- lens electron signal, as well as an acceleration voltage (UEHT) of 2-5 kV, was used for acquiring the micrographs. The density measurements were conducted with a Micromeritics AccuPyc 1340 Gas Pycnometer under Helium at 25 °C. P6633PC00 Results Figures 5A-B display the Scanning Electron Microscopy (SEM) micrographs, at different magnifications, for the KMS-2 surface (dry polished and thermally etched under air at 750 °C for 30 minutes) of pellets that were sintered at 800 °C. The images reveal that the pellets have a high level of densification, even at this relatively low sintering temperature, which suggests an almost quantitative elimination of voids and pores in the material. This observation is supported by the density measurement results obtained via gas pycnometer, which showed a relative density of 95±2% with a value of 2.28±0.01 g/cm³. Conclusions Analysis of KMS-2 via SEM and density measurements pinpoint a high level of densification, which is crucial for ensuring the optimal performance of the pellets and self-standing tapes in a variety of applications, most remarkably as potassium-ion conducting solid-state electrolytes for potassium batteries. Example 4: electrochemical characterization of KMS-1, KMS-2, KMS-4 and KMS-5 and comparison to KMS-0 Materials and methods The dense sintered pellets with the diameter and thickness of ~10 mm and 1-2 mm respectively, were prepared for the electrical characterization of KMS. The Pt as K- blocking electrode was coated on both sides of the pellets. The ionic conductivity of the samples was measured via electrochemical impedance spectroscopy (EIS) using a Solarton 1260 impedance analyzer in the frequency range between 0.06 Hz and 1 MHz, under open circuit condition with an AC amplitude of 50 mV, and in a two-electrode configuration. During the measurement, the samples were squeezed between two Pt grids with the application of a small external load to provide better contact. The test- house was place inside a close-ended tube furnace, and measurements were conducted from room temperature to 600 °C under N2 (pO2= 3·10^-5 bar, relative humidity 2-4% at the inlet) and air (relative humidity 30-40% at the inlet) and wet-air (relative humidity higher than 90% at the inlet). The measurement was carried out for 4-8 samples at each temperature and atmosphere combination and the average value was reported as the conductivity. The same samples were also analyzed via DC potentiostatic polarization P6633PC00 (chronoamperometry) of 500 mV at room temperature under dry N₂ in order to evaluate electronic conductivity. Results The results of ionic conductivity measurements on KMS-0, KMS-1, KMS-2, KMS-4 and KMS-5 are provided in Figure 6. By changing the chemical composition from stoichiometric KMS-0 to potassium super-stoichiometric and magnesium deficient KMS- 1, KMS-2, KMS-4 and KMS-5 an increase of the ionic conductivity, especially at temperatures below 300 °C, upon increasing “x” value is observed. Furthermore, temperature dependence of conductivity values demonstrates that the super- stoichiometric compounds (x = 0.1, 0.2, 0.4, 0.5) have significantly lower activation energy associated with K⁺ transport kinetics (0.36 eV), in comparison to the analogous value for stoichiometric KMS-0 (0.47 eV). A comparison between the ionic conductivity in Pt/KMS-2/Pt symmetrical cells under dry N2 (dry protective environment relative humidity between 2-4 % at the inlet) and under air (ambient condition 30-40% relative humidity) is presented in Figure 7A. KMS-2 exhibited high ionic conductivities under dry N2, up to 5·10^-5 S/cm and 2 × 10^-2 S/cm at room temperature and 300 °C, respectively. Temperature dependence of ionic conductivity in KMS-2 (measured under N2) was found to have a relatively low activation energy, 0.36 eV. Neither non-abundant elements, such as Sb and Te, nor metals with multi-ionization states, such as Fe, were used to achieve such a level of conductivity. While KMS-2 has demonstrated competitive levels of ionic conductivities compared to canonical K-BASE (potassium beta alumina solid-state electrolyte), the former compound only sintered at 850 °C, which is significantly lower than the typical sintering temperature for K-BASE (1400-1700 °C), making it a cheaper and more scalable alternative for industrial applications. The results of DC potentiostatic polarization of Pt/KMS-2/Pt cell at room temperature under dry N2 are shown in Figure 7B. This measurement is used to have an assessment on the electronic conductivity of KMS-2 samples. KMS-2 exhibits very low electronic conductivity of ~ 5·10^-9 S/cm, which is roughly four orders of magnitude lower than the measured ionic conductivity at room temperature, and consequently a K⁺ transference number close to unity. An ionic transference number close to unity is a necessary feature in development of potassium-ion batteries with solid-state electrolyte. P6633PC00 Conclusion The ionic conductivity of KMS-2 is higher than reported values in the literature for classical K-ion conducting solid-electrolytes, such as K-BASE. Therefore, these data show the potential of KMS as a fast K⁺ conductor. Besides, this material can be processed and handled under atmospheric air, which is not the case for many of the previously reported fast solid-state K⁺ conductors, such as K₂Mg₂TeO₆, KSi₂P₃ or K₃SbS₄. Example 5: thermogravimetric analysis on KMS-2 Materials and methods The thermogravimetric analysis ((TG) analyser (STA 409 CD – Simultaneous TG-DSC) NETZSCH – Gerätebau GmbH, Germany) on KMS-2 was performed by applying a heating ramp rate of 10 °C/min from 25 °C to 700 °C, dwell-time at 700 °C and a cooling ramp rate 10°C/min from 700 °C to 25 °C, under Ar atmosphere. Results The thermogravimetric analysis graph, shown in Figure 8, provides insights into the thermal behaviour of KMS-2. The graph shows a significant decrease in mass throughout the initial heating ramp, which can be attributed to the removal of surface moisture and the decomposition of hydrated phases. This trend continued until 200 °C. Thereafter, the rate of mass reduction slowed down. As the temperature increased further to 700 °C, the rate of mass reduction stabilized. The stabilized mass observed at 700 °C suggests that the sample had reached a thermodynamically stable state. Upon cooling the sample from 700 °C to room temperature, no significant weight gain was observed. Conclusion The constant weight of the KMS-2 sample throughout the thermogravimetric analysis demonstrates the stability of the sample and the absence of hydrated phase formation and water uptake. Example 6: stability under ambient conditions Materials and methods Sintered pellets of KMS-2 were initially vacuum packed within moisture barrier bags before characterization (KMS-pristine). A group of sintered pellets stored under air P6633PC00 (relative humidity 30-40%) at room temperature for 60 days before the measurement (KMS-2-Aged) as well as another set of KMS-2-Aged samples that have been heat- treated for 24 hours at 600 °C under dry N₂ (KMS-2-Recovered), were analysed using X-ray photoelectron spectroscopy (XPS). The XPS analysis was performed with an ESCALAB 250Xi X-ray photoelectron spectrometer microprobe (Thermo Fisher Scientific, East Grinstead, UK) using a monochromatic Al-Kα (1486.6 eV) X-ray source with a 300 µm spot size and a take-off angle of 90° from the surface plane. A dual-beam source in the form of a combined ion/electron gun was employed on the surface for charge compensation. The sputter depth profiling was also conducted using monatomic Ar⁺ ion beam with kinetic energy of 1000 eV. For every step, the etching time was 5 seconds and 5-6 etching steps were carried out during the depth profiling. Elemental composition on the surface was determined via survey spectra of 0-1350 eV. High- resolution binding energy spectra for K 2p_1/2/K 2p_3/2 (288-304 eV), Mg1s (1296-1309 eV) and Si2p (95-110 eV) was also attained for further characterization. The peak positions on the spectra were calibrated by analysing Au4f7/2 (84.0 eV) peak acquired from the surface of a gold foil before each measurement. Results As shown in Figure 9, the spectra display peaks related to the binding energies of K⁺, Mg²⁺ and Si⁴⁺. The peaks in the energy ranges of 290-294 eV, 294-297 eV, 1300-1306 eV and 98-106 eV are associated with the K2p1/2, K2p3/2, Mg1s and Si2p binding energies respectively. In fact, the Mg1s peak in KMS-2-Aged sample (Figure 9E) is almost non- existent, which indicates that a thin K-rich/Si-rich layer exists on the surface. The surface composition change is possibly caused by the formation of an amorphous hydrated phase, as previously reported for similar stuffed cristobalite silicates. It is interesting to note that, after the recovery process, as shown in the spectra obtained for KMS-2- Recovered, Mg content on the surface is observed to increase (Figure 9H). It is evident from the reappearance of Mg1s peak that the surface composition partially reforms during recovery. Considering the mobility of all the ions in this composition, this phenomenon is most probably due to downward diffusion of K⁺ and consequent surface dissolution of the surface segregated phase. The minor-extent degradation, which occurs in the surface of KMS-2 upon exposure to atmospheric air, be largely reversed via heat treatment at 600 °C under N2 for 24 hours, as shown in Figure 10. It should be noted that the KMS-2-Aged samples in Figure 9, in P6633PC00 addition to the aging process under air at room temperature in sintered pellet form for 60 days, were also additionally exposed to wet-air for 300 hours to have an exaggerated degradation. KMS-2-Aged samples then underwent the heat-treatment recovery process after the exaggerated degradative process. Most probably the amorphization, or formation of hydrated phases taking place on the KMS-2 surface during aging, created a thin superficial layer with lower ionic conductivity, which can then be partially recovered via the secondary heat treatment. However, this is different from performance recovery of layered structures, in which, moisture sensitivity can be reduced via heat- treatment/drying at high temperatures and consequent decrease in inter layer spacing. It should be noted that, the performance recovery from the degraded hydrated phase has also been reported on non-oxide superionic conductors such as Na₃SbS₄, via a secondary heat treatment at 200 °C under vacuum, though the mechanism for this behaviour is not clear. Conclusion The structural behaviour of the KMS-2 solid-electrolyte shows the potential for utilising this compound in ceramic processing techniques under ambient laboratory conditions, and subsequently reverse the degradative effect via heat treatment of the material under reduced-moisture-content atmospheres. Example 7: potentiometric sensing for SO2 Materials and methods A type III potentiometric solid-state gas sensor for SO2 (balanced by synthetic air) detection was developed using KMS-2 as the electrolyte; porous K2SO4 layer, in combination with porous Pt layer as auxiliary sensing electrode; and air-exposed KMS- 2 surface (below the porous silver vide infra), in combination with porous silver layer as the reference electrode (O2,Au|Ag|KMS-2|K2SO4|Pt|Au,SO2,O2), as shown in the scheme in Figure 11C. In type III potentiometric gas sensors, the gas of interest (here SO2) is in thermodynamic equilibrium with an ionic species within the auxiliary sensing electrode (here immobile SO4^2- in K2SO4). Results Electromotive force (emf) dependence on the SO2 concentration at 500 °C, emf response/recovery times for 2 ppm SO2 step changes, as well as schematic illustration of potentiometric sensing mechanism for SO₂ are shown in Figures 11A and 11B P6633PC00 respectively. At 500 °C and SO₂ concentrations in the range of 0-10 ppm, the sensitivity of the sensor is estimated to be 74-89 mV/dec, which is in very good agreement with the theoretical sensitivity for a two-electron reaction (77 mV/dec) based on Nernst equation (Eq. III). Conclusion The sensor in this study, within the same temperature and SO₂ concentration range, demonstrated similar or superior sensitivity (by a factor of 10), as well as comparable response/recovery time (2-120 minutes) compared to type III potentiometric sensors based on lithium lanthanum zirconium oxide (LLZO) electrolytes and similar sulphate sensing electrodes. Example 8: cycling of symmetrical cells – metal electrode plating/stripping Materials and methods The ability of metal electrode plating/stripping is one of the key indicators for utilizing solid-state-electrolytes for all-solid-state-battery or semi-solid-state-battery applications. This test is usually conducted by Galvanostatic Cycling with Potential Limit (GCPL) of symmetrical cells fabricated by sandwiching a solid-state-electrolyte between two metal electrodes comprising metal ions suitable for conduction through the electrolyte solid- state electrolyte. The aim of this measurement is to demonstrate two key functionalities of particular importance for battery applications, a) alkali-metal ions can be transported through the electrolyte and b) alkali-metal can be incorporated into and disbanded from battery electrodes during operation. For this purpose, a KMS-2 disc with a dimeter of 1 cm and thickness of 120 micrometers is sandwiched between two Na-foils to fabricate a Na/KMS-2/Na symmetrical cell. Sequentially, this cell was assembled into a CR2032 coin cell inside an argon filled glovebox with oxygen and moisture levels of less than 0.1 ppm. The Na electrode is chosen here as one of the alkali-metal ions (Na⁺ and K⁺) that is identified that can be transported through the KMS-2. The assembled symmetrical coin cell was analyzed outside of glovebox, using the BioLogic VMP3 multichannel P6633PC00 potentiostat (BioLogic Science Instruments) at 40 °C following GCPL methodology according to the manufacturer’s instructions. During charging (plating), metal ions from the electrolyte are attached to the surface of the Na electrode by the influence of the applied electric field. The metal ions are reduced by the electrons provided from the external circuit (generated in an external power source) and deposited on the metallic anode surface. As more ions are deposited and the metallic anode layer thickens, the electrical energy is stored in the form of chemical potential energy. During discharge (stripping), the reverse process occurs and metals in the electrode layer are oxidized and diffuse into the electrolyte layer releasing electrons to the external circuit. Flow of electrons in the outer circuit can be utilized as electrical work. Successful plating and stripping is crucial in enabling reversible charge and discharge cycles within the solid-state cell. The metal plating/stripping cycles were conducted using current density of 0.1 mA/cm² and 1 mA/cm² as well as the potential range of -1 V to 1 V. The results of cycling Na/KMS- 2/Na symmetrical cell at metal plating/stripping depth of 0.025 mAh/cm² (current density of 0.1 mA/cm²) and 0.25 mAh/cm² (current density of 1 mA/cm²) are shown in Figures 12A and 12B, respectively. In both cases, stable and repeatable metal plating/stripping cycling performance, low overpotential and low interface resistance can be observed within the time frame of the experiment (30.000 seconds), which supports the usefulness of KMS-2 as a solid-state electrolyte for various solid-state- or semi-solid-state-battery applications. Items 1. A solid-state electrolyte comprising a potassium ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). 2. A solid-state electrolyte according to item 1, consisting essentially of a potassium ion conductive polycrystalline material of general formula K2+XMg1-(X/2)SiO4 (0.0 < x ≤ 0.5). 3. The electrolyte according to any one of items 1 or 2, wherein X is selected from 0.1, 0.2, 0.3, 0.4 and 0.5 P6633PC00 4. The electrolyte according to any one of the preceding items, wherein X is selected from within the range 0.2 to 0.4, preferably as 0.2 or 0.4. 5. The electrolyte according to any one of the preceding items, wherein the polycrystalline material is characterized by an orthorhombic space group at room temperature. 6. The electrolyte according to item 5, wherein the orthorhombic space group is Pca2₁ at room temperature. 7. The electrolyte according to item 6, wherein the space group Pca2₁ is characterized by the following lattice parameters: a (Å) = 10.9 to 11.1, b (Å) = 5.45 to 5.50, and c (Å) = 15.5 to 15.6 8. The electrolyte according to any one of items 6 to 7, wherein the space group Pca21 is characterized by the following lattice parameters: a (Å) = 10.9822±0.0005 b (Å) = 5.47305±0.0005, and c (Å) = 15.5487±0.0005 9. The electrolyte according to any one of the preceding items, wherein the polycrystalline material is characterized by a crystal structure comprising corner- sharing polyhedra of non-alkali metal ions, such as corner-sharing tetrahedra comprising SiO4 and/or MgO4 moieties. 10. The electrolyte according to any one of the preceding items, wherein the polycrystalline material is characterized by a phase transformation temperature between 100 °C and 1000 °C, such as between 100 °C and 200 °C, such as between 200 °C and 250 °C, such as between 250 °C and 300 °C, such as between 300 °C and 350 °C, such as between 350 °C and 400 °C, such as between 400 °C and 500 °C, such as between 500 °C and 600 °C, such as between 600 °C and 700 °C, and wherein the phase transformation temperature P6633PC00 represents a transformation from an orthorhombic space group to a cubic space group. 11. The electrolyte according to any one of the preceding items, wherein the polycrystalline material is characterized by a phase transformation temperature between 200 °C and 250 °C, and wherein the phase transformation temperature represents a transformation from an orthorhombic space group to a cubic space group. 12. The electrolyte according to any one of the preceding items, characterized by a potassium ion conductivity between 1·10^-7 S/cm and 1·10^-4 S/cm at room temperature (25 °C) when measured via electrochemical impedance spectroscopy (EIS). 13. The electrolyte according to any of the preceding items, wherein the polycrystalline material is characterized by a cubic space group at 300 °C, wherein the cubic space group is Fd-3m. 14. The electrolyte according to any one of the preceding items, characterized by a potassium ion conductivity between 1·10^-7 S/cm and 1·10^-1 S/cm at 300 °C when measured via electrochemical impedance spectroscopy (EIS). 15. The electrolyte according to any one of the preceding items, characterized by a relative density of 90±2% or more, such as 91±2% or more, such as 92±2% or more, such as 93±2% or more, such as 94±2% or more, such as 95±2% or more, such as 96±2% or more, such as 97±2% or more, such as 98±2%, preferably having a relative density of 95±2%. 16. The electrolyte according to any one of the preceding items, characterized by an electronic conductivity between 1·10^-10 S/cm and 1·10^-6 S/cm at room temperature (25 °C) when measured via chronoamperometry at 500 mV. 17. The electrolyte according to any one of the preceding items, characterized by an electronic conductivity between 1·10^-10 S/cm and 1·10^-6 S/cm at 300 °C when measured via chronoamperometry at 500 mV. P6633PC00 18. The electrolyte according to any one of the preceding items, for use as a component in a sensor. 19. The electrolyte according to item 18, wherein the sensor is a potentiometric sensor. 20. The electrolyte according to any one of items 18 to 19, wherein the sensor is for sensing a gas, such as a gas selected from SO₂, SO₃, NO, NO₂, CO, and CO₂. 21. A potassium-ion battery comprising: a potassium-ion conductive electrolyte layer, a positive electrode layer, a negative electrode layer, and wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises the potassium ion conductive polycrystalline material of any one of items 1 to 17. 22. The battery according to item 21, wherein the potassium-ion conductive electrolyte layer is formed between the positive electrode layer and the negative electrode layer. 23. The battery according to any one of items 21 to 22, wherein the potassium-ion conductive electrolyte layer is a solid potassium-ion conductive electrolyte layer. 24. The battery according to item 23, wherein the solid potassium-ion conductive electrolyte layer comprises at least one selected from the group consisting of: an inorganic solid-state electrolyte, a liquid electrolyte, a solid-state polymer electrolytes, and any one of the foregoing as a composite in combination with the KMS electrolyte of any one of items 1 to 17. 25. The battery according to item 24, wherein the inorganic solid-state electrolyte is selected from the group consisting of K-BASE, K2MgSiO4, KAlO2, KFeO2, KGaO2, K0.7Sr0.15GaO2 KAlSiO4, K2CaSiO4, K2Mg2TeO6, K2ZnSiO4, K2Si2P3, K2Sb5P2O10, K_2.92Sb_0.92W_0.08S₄, K_0.59Mg_0.53Sb_0.47O₂, K₃SbS₄, K_1.9Fe_1.95P_0.05O₄, K_1.9Pb_0.05AlO₂, P6633PC00 K_0.405Bi_0.865AsO₄, K_0.4Cd_0.3FeO₂, K₂Fe₄O₇, K_1.6Zn_0.8Ti_7.2O₁₆, K_0.72In_0.72Sn_0.28O₂, KBiO₃, K₃Sc(MoO₄)₃, KMgPO₄, K₂MgV₂O₇, K₂Mg₂Si₂O₇, K₂CaP₂O₇, K₂ZnGeO₄, K₂Mg₂(MoO₄)₃, K₄Mg(WO₄)₃, K₂CaPO₄F, Li₇La₃Zr₂O₁₂ (LLZO), Li_5.1Ga_0.32La₃Zr_2.25O₁₃, Li_6.25Ga_0.25La₃Zr₂O₁₂, Li_xLa_yTiO₃ (LLTO) (0.07 ≤ x ≤ 0.13; y±0.05 = (2/3)-x), Li_0.45La_0.48TiO₃, Li₅La₃X₂O₁₂ (X= Nb or Ta), Li_xPO_yN_z (3.0 ≤ x ≤ 3.2; 3.0 ≤ y ≤ 3.5; 0.0 ≤ z ≤ 0.5; 3 ≤ (y+z) ≤ 4), Li_3.13PO_1.69N_1.39, Li_0.98PO_2.55N_0.50, Li₃PO₄, La_1+xAl_xTi_2-x(PO₄)₃ (0 ≤ x ≤ 2), Li_1.3Al_0.4Ti_1.7(PO₄)₃, Li₁₀GeP₂S₁₂, Li₇P₃S₁₁, Na-BASE, NaM₂(PO₄)₃ (M = Ge, Ti, Zr), Na_1+xZr₂Si_xP_3-xO₁₂ (0 ≤ X ≤ 3), Na₂MgSiO₄, Na₂CaSiO₄, Na₂ZnSiO₄, Na₂Mg₂TeO₆, Na₂Mg₂ZnO₆, Na₃PS₄, Na₄SiS₄, Na₃PSe₄, Na₁₀SnP₂S₁₂, and Na₃SbS₄. 26. The battery according to any one of items 24 to 25, wherein the inorganic solid- state electrolyte is selected from K-BASE, K₂MgSiO₄, KAlSiO₄, K₂CaSiO₄, K₂Si₂P₃, K_2.92Sb_0.92W_0.08S₄, K₃SbS₄, Na-BASE, Na_1+xZr₂Si_xP_3-xO₁₂ (0 < X < 3), Na2MgSiO4, and Na2CaSiO4. 27. The battery according to item 24, wherein the liquid electrolyte is selected from the group consisting of ether electrolyte, such as (AN(SO2F)2 in DME) or (ACF3SO3 in TEGDME), ester electrolyte, such as (APF6 in EC/DMC), (APF6 in PC) or (AN(SO2F)2 in EC/DEC), ATFSI in Tetraglyme, and aqueous electrolyte, such as (1 M KNO3 per 0.01 M HNO3), (1 M KNO3 pH = 2), 0.5 M K2SO4, 0.1 M KCl, 22 M KCF3SO3, 3 M KCl, 30 M KFSI, or 1 M KOH, wherein (A = K, Li, or Na). 28. The battery according to item 24, wherein the solid-state polymer electrolyte is selected from the group consisting of (KTFSI+PEO), PEO-KAg4I5, PEO-KBrO3, PPC-KFSI, PEO-KFSI, PVP+PVA+KBrO3, PVC+KBrO3, PAN-KI, PMMA-KPF6, PVA-KCl, (LiTFSI+PEO), PEO-LiAg4I5, PEO-LiBrO3, PPC-LiFSI, PEO-LiFSI, PVP+PVA+LiBrO3, PVC+LiBrO3, PAN-LiI, PMMA-LiPF6, PVA-LiCl, (NaTFSI+PEO), PEO-NaAg4I5, PEO-NaBrO3, PPC-NaFSI, PEO-NaFSI, PVP+PVA+NaBrO3, PVC+NaBrO3, PAN-NaI, PMMA-NaPF6, PVA-NaCl, such as is preferably (KTFSI+PEO), (LiTFSI+PEO), or (NaTFSI+PEO). P6633PC00 29. The battery according to any one of items 21 to 28, wherein the positive electrode layer is selected from a solid positive electrode layer, a liquid positive electrode layer and a hybrid positive electrode layer. 30. The battery according to item 29, wherein the positive electrode layer is a solid positive electrode layer comprising: a. A carbonaceous material such as carbon black, b. A binder such as PVDF or PAA, c. at least one inorganic solid-state electrolyte selected from the group consisting of K-BASE, K₂MgSiO₄, KAlO₂, KFeO₂, KGaO₂, K_0.7Sr_0.15GaO₂ KAlSiO₄, K₂CaSiO₄, K₂Mg₂TeO₆, K₂ZnSiO₄, K₂Si₂P₃, K₂Sb₅P₂O₁₀, K_2.92Sb_0.92W_0.08S₄, K_0.59Mg_0.53Sb_0.47O₂, K₃SbS₄, K_1.9Fe_1.95P_0.05O₄, K_1.9Pb_0.05AlO₂, K_0.405Bi_0.865AsO₄, K_0.4Cd_0.3FeO₂, K₂Fe₄O₇, K_1.6Zn_0.8Ti_7.2O₁₆, K_0.72In_0.72Sn_0.28O₂, KBiO₃, K₃Sc(MoO₄)₃, KMgPO₄, K₂MgV₂O₇, K2Mg2Si2O7, K2CaP2O7, K2ZnGeO4, K2Mg2(MoO4)3, K4Mg(WO4)3, K2CaPO4F, Li7La3Zr2O12 (LLZO), Li5.1Ga0.32La3Zr2.25O13, Li6.25Ga0.25La3Zr2O12, LixLayTiO3 (LLTO) (0.07 ≤ x ≤ 0.13; y±0.05 = (2/3)- x), Li0.45La0.48TiO3, Li5La3X2O12 (X= Nb or Ta), LixPOyNz (3.0 ≤ x ≤ 3.2; 3.0 ≤ y ≤ 3.5; 0.0 ≤ z ≤ 0.5; 3 ≤ (y+z) ≤ 4), Li3.13PO1.69N1.39, Li0.98PO2.55N0.50, Li3PO4, La1+xAlxTi2-x(PO4)3 (0 ≤ x ≤ 2), Li1.3Al0.4Ti1.7(PO4)3, Li10GeP2S12, Li7P3S11, Na-BASE, NaM2(PO4)3 (M = Ge, Ti, Zr), Na1+xZr2SixP3-xO12 (0 ≤ X ≤ 3), Na2MgSiO4, Na2CaSiO4, Na2ZnSiO4, Na2Mg2TeO6, Na2Mg2ZnO6, Na3PS4, Na4SiS4, Na3PSe4, Na10SnP2S12, and Na3SbS4, and d. an active electrode material comprising one or more of: i. a sulphur-type positive electrode material selected from pure sulfur, K2Sx (x= 1, 2, 3, 5), Na2Sx (x= 1, 2, 3, 5), Li2Sx (x= 1, 2, 3, 5), and PAN-S, preferably K2Sx (x= 1, 2, 3, 5) and/or PAN-S, ii. an inorganic-type positive electrode material selected from K2Ni2TeO6, K3CoO2, K0.3MnO2, K2FeSiO4, KFeSiO4, K2CoNiTeO6, KFePO4F, K2NiO2, K2CuP2O7, K2FeSiO4, KFeSi2O6, KMnPO4, K2FeGeO4, KVP2O7, K0.71Cu[Fe(CN)6]0.72·3.7H2O, K0.6Ni1.2Fe(CN)6·3.6H2O, K2Fe^II[Fe^II(CN)6]·2H2O, Fe^III[Fe^III(CN)6], K2NiFe(CN)6·1.2H2O, K1.85Fe0.33Mn0.67[Fe(CN)6]0.98·0.77H2O, K0.22V1.74O4.37·0.82H2O, KM^IIFe^III(CN)6(M = Mn, Fe, Co, Ni, and Zn), NaCoO₂, NaNiO₂, NaMnO₂, NaFeO₂, Na_7/9Cu_2/9Fe_1/9Mn_2/3O₂, P6633PC00 Na₃V₂(PO₄)₃, Na₂VTi(PO₄)₃, Na_0.66Mn_0.66Ti_0.34O₂, Na_0.44MnO₂, Na₂LiV₂(PO₄)₃, Li₄Ti₅O₁₂, LiFePO₄, LiMn₂O₄, LiNiMnCoO₂, LiNiCoAlO₂, LiCoO₂, preferably K₂Ni₂TeO₆, K₂FeSiO₄ and KFeSi₂O₆, and iii. an organic-type positive electrode material selected from A_2+xC₆O₆ (0 < X < 4), (A = Na, K, Li). 31. The battery according to item 29, wherein the positive electrode layer is a liquid positive electrode layer comprising as the active electrode material one or more of a sulphur-type positive electrode material selected from pure sulphur, K₂S_x (x= 1, 2, 3, 5), Na₂S_x (x= 1, 2, 3, 5), Li₂S_x (x= 1, 2, 3, 5), preferably K₂S_x (x= 1, 2, 3, 5) , optionally in combination with a liquid electrolyte selected from the group consisting of ether electrolyte, such as (AN(SO₂F)₂ in DME) or (ACF₃SO₃ in TEGDME), ester electrolyte, such as (APF₆ in EC/DMC), (APF₆ in PC) or (AN(SO2F)2 in EC/DEC), ATFSI in Tetraglyme, and aqueous electrolyte, such as (1 M KNO3 per 0.01 M HNO3), (1 M KNO3 pH = 2), 0.5 M K2SO4, 0.1 M KCl, 22 M KCF3SO3, 3 M KCl, 30 M KFSI, or 1 M KOH, wherein (A = K, Li, or Na). 32. The battery according to item 29, wherein the positive electrode layer is a hybrid positive electrode layer comprising any combination of solid positive electrode layers as defined in item 30, and liquid positive electrode layers as defined in item 31. 33. The battery according to any one of items 21 to 32, wherein the negative electrode layer is selected from a solid negative electrode layer, a liquid negative electrode layer and a hybrid negative electrode layer. 34. The battery according to item 33, wherein the negative electrode layer is a solid negative electrode layer comprising one or more of: a) a metal selected from K, Li, Na, Al or an alloy thereof, preferably K or a ternary K-Na-Li alloy, b) a carbon-type negative electrode selected from graphite and hard carbon, and c) a silicon type negative electrode selected from silicon, crystalline silicene or zintle (potassium silicides, sodium silicides, lithium silicides and calcium silicides). P6633PC00 35. The battery according to item 33, wherein the negative electrode layer is a liquid negative electrode layer comprising one or more of: a metal selected from K, Li, Na, Al or an alloy thereof, preferably K or a ternary K-Na-Li alloy. 36. The battery according to item 33, wherein the negative electrode layer is a hybrid negative electrode layer comprising any combination of solid negative electrode layers as defined in item 34, and liquid negative electrode layers as defined in item 35. 37. The battery according to any one of items 21 to 36, wherein the positive electrode layer is a solid positive electrode layer, and the negative electrode layer is a solid negative electrode layer. 38. A sensor comprising the potassium ion conductive polycrystalline material of any one of items 1 to 17. 39. The sensor according to item 38, wherein the sensor is suitable for detecting a gas selected from the group consisting of SO2, SO3, NO, NO2, CO, and CO2. 40. The sensor according to any one of items 38 to 39, wherein the sensor is a potentiometric sensor, such as a type-III potentiometric sensor. 41. The sensor according to any one of items 38 to 40, wherein the sensor has a sensitivity of 74-89 mV/dec at 500 °C. 42. The sensor according to any one of items 38 to 41, wherein the sensor has a detection limit of 1-10 ppm for SO2 at 500 °C. 43. The sensor according to any one of items 38 to 42, wherein the sensor has a response/recovery time of 2-120 minutes for SO2 at 500 °C. 44. A method of manufacturing the solid-state electrolyte according to any one of items 1 to 17 the method comprising the sequential steps: P6633PC00 a. Mixing at least one source of potassium, at least one source of magnesium, and at least one source of silicon in a ratio such as to obtain a super-stoichiometric potassium ratio of K:Mg:Si corresponding to (2+X):1-(X/2):1 (0.0 < X ≤ 0.5), thereby forming a mixture, and b. Heating said mixture to a first temperature between 600 °C and 1000 °C to obtain a sintered mixture. 45. The method according to item 44, further comprising prior to step b, a step of ball- milling said mixture. 46. The method according to any one of items 44 to 45, further comprising subsequent to step b., a step of crushing the sintered mixture into a fine powder, pelletizing said fine powder, and heating said pelletized powder to a second temperature between 600 °C and 1000 °C. 47. The method according to item 46, wherein the step of crushing, pelletizing and heating is repeated at least 3 times, such as 4 times, such as 5 times, such as any one of 6, 7, 8, 9, and 10 times. 48. The method according to any one of items 44 to 47, wherein said first and second temperature is a temperature below 1000 °C, such as below 950 °C, such as below 900 °C, such as below 850 °C, such as below 800 °C. 49. The method according to item 48, wherein said first and second temperature is a temperature below 850 °C. 50. The method according to any one of items 46 to 49, wherein pelletizing is performed by application of uniaxial pressure in the range of 50 MPa to 100 MPa, preferably 70 MPa to 90 MPa. 51. The method according to any one of items 44 to 50, wherein the at least one source of potassium is a salt of potassium or an oxide of potassium, such as selected from the group consisting of KF, KCl, KBr, KI, KNO3, K2CO3,K3PO4, K2SO4, KClO4, KClO3, K2O, and KOH. P6633PC00 52. The method according to any one of items 44 to 51, wherein the at least one source of potassium is K₂CO₃ or K₂O. 53. The method according to any one of items 44 to 52, wherein the at least one source of magnesium is a salt of magnesium or an oxide of magnesium, such as selected from the group consisting of MgF₂, MgCl₂, MgBr₂, MgI₂, Mg(NO₃)₂, MgCO₃,Mg₃(PO₄)₂, MgSO₄, Mg(ClO₄)₂, Mg(ClO₃)₂, MgO, and Mg(OH)₂. 54. The method according to any one of items 44 to 53, wherein the at least one source of magnesium is MgO. 55. The method according to any one of items 44 to 54, wherein the at least one source of silicon is a halide of silicon or an oxide of silicon, such as selected from the group consisting of SiCl₄ and SiO₂. 56. The method according to any one of items 44 to 55, wherein the at least one source of silicon is SiCl4 and SiO2. 57. The method according to any one of items 44 to 56, wherein the method is performed in ambient conditions. 58. The method according to any one of items 44 to 56, wherein the method is performed in inert atmosphere, such as consisting essentially of nitrogen or argon.

P6633PC00 Items 2 1. A solid-state electrolyte comprising a potassium ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5). 2. A solid-state electrolyte according to item 1, consisting essentially of a potassium ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5), wherein X is selected from 0.1, 0.2, 0.3, 0.4 and 0.5. 3. The electrolyte according to any one of the preceding items, wherein the polycrystalline material is characterized by an orthorhombic space group at room temperature, and wherein the orthorhombic space group is Pca2₁ at room temperature characterized by the following lattice parameters: a (Å) = 10.9 to 11.1, b (Å) = 5.45 to 5.50, and c (Å) = 15.5 to 15.6 4. The electrolyte according to any one of the preceding items, wherein the polycrystalline material is characterized by a crystal structure comprising corner- sharing polyhedra of non-alkali metal ions, such as corner-sharing tetrahedra comprising SiO4 and/or MgO4 moieties. 5. The electrolyte according to any one of the preceding items, wherein the polycrystalline material is characterized by a phase transformation temperature between 200 °C and 250 °C, and wherein the phase transformation temperature represents a transformation from an orthorhombic space group to a cubic space group. 6. The electrolyte according to any of the preceding items, wherein the polycrystalline material is characterized by a cubic space group at 300 °C, wherein the cubic space group is Fd-3m. 7. The electrolyte according to any one of the preceding items, characterized by a potassium ion conductivity between 1·10^-5 S/cm and 1·10^-4 S/cm at room temperature (25 °C) P6633PC00 8. The electrolyte according to any one of the preceding items, characterized by a potassium ion conductivity between 1·10^-4 S/cm and 1·10^-1 S/cm at 300 °C. 9. The electrolyte according to any one of the preceding items, characterized by a relative density of 95±2% or more. 10. The electrolyte according to any one of the preceding items, characterized by an electronic conductivity between 1·10^-10 S/cm and 1·10^-6 S/cm at room temperature (25 °C) and also at 300 °C when measured via chronoamperometry at 500 mV. 11. The electrolyte according to any one of the preceding items, for use as a component in a potentiometric sensor, wherein the sensor is for sensing a gas, such as a gas selected from SO₂, SO₃, NO, NO₂, CO, and CO₂. 12. A method of manufacturing the solid-state electrolyte according to any one of items 1 to 10, the method comprising the sequential steps: a. Mixing at least one source of potassium selected from K2CO3 or K2O, at least one source of magnesium being MgO, and at least one source of silicon selected from SiCl4 and SiO2, in a ratio such as to obtain a super- stoichiometric potassium ratio of K:Mg:Si corresponding to (2+X):1- (X/2):1 (0.0 < X ≤ 0.5), thereby forming a mixture, and b. Heating said mixture to a first temperature between 600 °C and 1000 °C to obtain a sintered mixture. 13. The method according to item 12, further comprising subsequent to step b., a step of crushing the sintered mixture into a fine powder using a planetary ball mill with cups and balls made from zirconia at 150 rpm for 6 hours, pelletizing said fine powder, and heating said pelletized powder to a second temperature between 600 °C and 1000 °C, wherein the step of crushing, pelletizing and heating is repeated at least 3 times, and wherein said first and second temperature is a temperature below 850 °C. 14. A potassium-ion battery comprising: a potassium-ion conductive electrolyte layer, P6633PC00 a positive electrode layer, a negative electrode layer, and wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises the potassium ion conductive polycrystalline material of any one of items 1 to 10. 15. A potentiometric sensor comprising the potassium ion conductive polycrystalline material of any one of items 1 to 10, wherein the sensor is suitable for detecting a gas selected from the group consisting of SO₂, SO₃, NO, NO₂, CO, and CO₂, and wherein the sensor is characterised by a detection limit of 1-10 ppm for SO₂ at 500 °C.

Claims

P6633PC00 Claims 1. A solid-state electrolyte comprising an ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5). 2. A solid-state electrolyte according to claim 1, consisting essentially of an ion conductive polycrystalline material of general formula K_2+XMg_1-(X/2)SiO₄ (0.0 < x ≤ 0.5). 3. The electrolyte according to any one of claims 1 or 2, wherein X is selected from 0.1, 0.2, 0.3, 0.4 and 0.5 4. The electrolyte according to any one of the preceding claims, wherein X is selected from within the range 0.2 to 0.4, preferably as 0.2 or 0.4. 5. The electrolyte according to any one of the preceding claims, wherein the polycrystalline material is characterized by an orthorhombic space group at room temperature. 6. The electrolyte according to claim 5, wherein the orthorhombic space group is Pca21 at room temperature. 7. The electrolyte according to claim 6, wherein the space group Pca21 is characterized by the following lattice parameters: a (Å) = 10.9 to 11.1, b (Å) = 5.45 to 5.50, and c (Å) = 15.5 to 15.6 8. The electrolyte according to any one of claims 6 to 7, wherein the space group Pca21 is characterized by the following lattice parameters: a (Å) = 10.9822±0.0005 b (Å) = 5.47305±0.0005, and c (Å) = 15.5487±0.0005 9. The electrolyte according to any one of the preceding claims, wherein the polycrystalline material is characterized by a crystal structure comprising corner- P6633PC00 sharing polyhedra of non-alkali metal ions, such as corner-sharing tetrahedra comprising SiO₄ and/or MgO₄ moieties. 10. The electrolyte according to any one of the preceding claims, wherein the polycrystalline material is characterized by a phase transformation temperature between 100 °C and 1000 °C, such as between 100 °C and 200 °C, such as between 200 °C and 250 °C, such as between 250 °C and 300 °C, such as between 300 °C and 350 °C, such as between 350 °C and 400 °C, such as between 400 °C and 500 °C, such as between 500 °C and 600 °C, such as between 600 °C and 700 °C, and wherein the phase transformation temperature represents a transformation from an orthorhombic space group to a cubic space group. 11. The electrolyte according to any one of the preceding claims, wherein the polycrystalline material is characterized by a phase transformation temperature between 200 °C and 250 °C, and wherein the phase transformation temperature represents a transformation from an orthorhombic space group to a cubic space group. 12. The electrolyte according to any one of the preceding claims, characterized by a potassium-ion conductivity between 1·10^-7 S/cm and 1·10^-4 S/cm at room temperature (25 °C) when measured via electrochemical impedance spectroscopy (EIS). 13. The electrolyte according to any of the preceding claims, wherein the polycrystalline material is characterized by a cubic space group at 300 °C, wherein the cubic space group is Fd-3m. 14. The electrolyte according to any one of the preceding claims, characterized by a potassium-ion conductivity between 1·10^-7 S/cm and 1·10^-1 S/cm at 300 °C when measured via electrochemical impedance spectroscopy (EIS). 15. The electrolyte according to any one of the preceding claims, characterized by a relative density of 90±2% or more, such as 91±2% or more, such as 92±2% or more, such as 93±2% or more, such as 94±2% or more, such as 95±2% or more, P6633PC00 such as 96±2% or more, such as 97±2% or more, such as 98±2%, preferably having a relative density of 95±2%. 16. The electrolyte according to any one of the preceding claims, characterized by an electronic conductivity between 1·10^-10 S/cm and 1·10^-6 S/cm at room temperature (25 °C) when measured via chronoamperometry at 500 mV. 17. The electrolyte according to any one of the preceding claims, characterized by an electronic conductivity between 1·10^-10 S/cm and 1·10^-6 S/cm at 300 °C when measured via chronoamperometry at 500 mV. 18. The electrolyte according to any one of the preceding claims, wherein said ion conductive polycrystalline material is alkali ion conductive, such as potassium- ion conductive and/or sodium ion conductive. 19. The electrolyte according to any one of the preceding claims, wherein said ion conductive polycrystalline material is potassium-ion conductive. 20. The electrolyte according to any one of the preceding claims, wherein said ion conductive polycrystalline material is sodium ion conductive. 21. Use of the electrolyte according to any one of the preceding claims as a component in a sensor. 22. The use according to claim 21, wherein the sensor is a potentiometric sensor. 23. The use according to any one of claims 21 to 22, wherein the sensor is for sensing a gas, such as a gas selected from SO2, SO3, NO, NO2, CO, and CO2. 24. A battery comprising: an ion conductive electrolyte layer, a positive electrode layer, a negative electrode layer, and P6633PC00 wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises the conductive polycrystalline material of any one of claims 1 to 20. 25. The battery according to claim 24, wherein the conductive polycrystalline material is potassium-ion conductive and/or sodium ion conductive. 26. The battery according to any one of claims 24 to 25, wherein said battery is a potassium-ion battery or a sodium-ion battery. 27. The battery according to claim 26, wherein said battery is a potassium-ion battery. 28. The battery according to claim 26, wherein said battery is a sodium-ion battery. 29. The battery according to claim 27, wherein said battery comprises: an ion conductive electrolyte layer being a potassium-ion conductive electrolyte layer, a positive electrode layer, a negative electrode layer, and wherein at least one of the electrolyte layer, the positive electrode layer, and the negative electrode layer comprises the ion conductive polycrystalline material of any one of claims 1 to 20. 30. The battery according to any one of claims 24 to 29, wherein the ion conductive electrolyte layer is formed between the positive electrode layer and the negative electrode layer. 31. The battery according to any one of claims 24 to 30, wherein the ion conductive electrolyte layer is a solid ion conductive electrolyte layer, such as the KMS electrolyte of any one of claims 1 to 20. 32. The battery according to claim 31, wherein the solid ion conductive electrolyte layer comprises at least one selected from the group consisting of an inorganic solid-state electrolyte, a liquid electrolyte, a solid-state polymer electrolytes, the P6633PC00 KMS electrolyte of any one of claims 1 to 20, and any one of the foregoing as a composite in combination with the KMS electrolyte of any one of claims 1 to 20. 33. The battery according to claim 32, wherein the inorganic solid-state electrolyte is selected from the group consisting of K-BASE, K₂MgSiO₄, KAlO₂, KFeO₂, KGaO₂, K_0.7Sr_0.15GaO₂ KAlSiO₄, K₂CaSiO₄, K₂Mg₂TeO₆, K₂ZnSiO₄, K₂Si₂P₃, K₂Sb₅P₂O₁₀, K_2.92Sb_0.92W_0.08S₄, K_0.59Mg_0.53Sb_0.47O₂, K₃SbS₄, K_1.9Fe_1.95P_0.05O₄, K_1.9Pb_0.05AlO₂, K_0.405Bi_0.865AsO₄, K_0.4Cd_0.3FeO₂, K₂Fe₄O₇, K_1.6Zn_0.8Ti_7.2O₁₆, K_0.72In_0.72Sn_0.28O₂, KBiO₃, K₃Sc(MoO₄)₃, KMgPO₄, K₂MgV₂O₇, K₂Mg₂Si₂O₇, K₂CaP₂O₇, K₂ZnGeO₄, K₂Mg₂(MoO₄)₃, K₄Mg(WO₄)₃, K₂CaPO₄F, Li₇La₃Zr₂O₁₂ (LLZO), Li_5.1Ga_0.32La₃Zr_2.25O₁₃, Li_6.25Ga_0.25La₃Zr₂O₁₂, Li_xLa_yTiO₃ (LLTO) (0.07 ≤ x ≤ 0.13; y±0.05 = (2/3)-x), Li_0.45La_0.48TiO₃, Li₅La₃X₂O₁₂ (X= Nb or Ta), Li_xPO_yN_z (3.0 ≤ x ≤ 3.2; 3.0 ≤ y ≤ 3.5; 0.0 ≤ z ≤ 0.5; 3 ≤ (y+z) ≤ 4), Li_3.13PO_1.69N_1.39, Li_0.98PO_2.55N_0.50, Li₃PO₄, La_1+xAl_xTi_2-x(PO₄)₃ (0 ≤ x ≤ 2), Li_1.3Al_0.4Ti_1.7(PO₄)₃, Li₁₀GeP₂S₁₂, Li₇P₃S₁₁, Na-BASE, NaM2(PO4)3 (M = Ge, Ti, Zr), Na1+xZr2SixP3-xO12 (0 ≤ X ≤ 3), Na2MgSiO4, Na2CaSiO4, Na2ZnSiO4, Na2Mg2TeO6, Na2Mg2ZnO6, Na3PS4, Na4SiS4, Na3PSe4, Na10SnP2S12, and Na3SbS4. 34. The battery according to any one of claims 32 to 33, wherein the inorganic solid- state electrolyte is selected from K-BASE, K2MgSiO4, KAlSiO4, K2CaSiO4, K2Si2P3, K2.92Sb0.92W0.08S4, K3SbS4, Na-BASE, Na1+xZr2SixP3-xO12 (0 < X < 3), Na2MgSiO4, and Na2CaSiO4. 35. The battery according to claim 32, wherein the liquid electrolyte is selected from the group consisting of ether electrolyte, such as (AN(SO2F)2 in DME) or (ACF3SO3 in TEGDME), ester electrolyte, such as (APF6 in EC/DMC), (APF6 in PC) or (AN(SO2F)2 in EC/DEC), ATFSI in Tetraglyme, and aqueous electrolyte, such as (1 M KNO3 per 0.01 M HNO3), (1 M KNO3 pH = 2), 0.5 M K2SO4, 0.1 M KCl, 22 M KCF3SO3, 3 M KCl, 30 M KFSI, or 1 M KOH, wherein (A = K, Li, or Na). 36. The battery according to claim 32, wherein the solid-state polymer electrolyte is selected from the group consisting of (KTFSI+PEO), PEO-KAg4I5, PEO-KBrO3, PPC-KFSI, PEO-KFSI, PVP+PVA+KBrO₃, PVC+KBrO₃, PAN-KI, PMMA-KPF₆, P6633PC00 PVA-KCl, (LiTFSI+PEO), PEO-LiAg₄I₅, PEO-LiBrO₃, PPC-LiFSI, PEO-LiFSI, PVP+PVA+LiBrO₃, PVC+LiBrO₃, PAN-LiI, PMMA-LiPF₆, PVA-LiCl, (NaTFSI+PEO), PEO-NaAg₄I₅, PEO-NaBrO₃, PPC-NaFSI, PEO-NaFSI, PVP+PVA+NaBrO₃, PVC+NaBrO₃, PAN-NaI, PMMA-NaPF₆, PVA-NaCl, such as is preferably (KTFSI+PEO), (LiTFSI+PEO), or (NaTFSI+PEO). 37. The battery according to any one of claims 24 to 36, wherein the positive electrode layer is selected from a solid positive electrode layer, a liquid positive electrode layer and a hybrid positive electrode layer. 38. The battery according to claim 37, wherein the positive electrode layer is a solid positive electrode layer comprising: a. A carbonaceous material such as carbon black; and/or b. A binder such as PVDF or PAA; and/or c. at least one inorganic solid-state electrolyte selected from the group consisting of K-BASE, K2MgSiO4, KAlO2, KFeO2, KGaO2, K0.7Sr0.15GaO2 KAlSiO4, K2CaSiO4, K2Mg2TeO6, K2ZnSiO4, K2Si2P3, K2Sb5P2O10, K2.92Sb0.92W0.08S4, K0.59Mg0.53Sb0.47O2, K3SbS4, K1.9Fe1.95P0.05O4, K1.9Pb0.05AlO2, K0.405Bi0.865AsO4, K0.4Cd0.3FeO2, K2Fe4O7, K1.6Zn0.8Ti7.2O16, K0.72In0.72Sn0.28O2, KBiO3, K3Sc(MoO4)3, KMgPO4, K2MgV2O7, K2Mg2Si2O7, K2CaP2O7, K2ZnGeO4, K2Mg2(MoO4)3, K4Mg(WO4)3, K2CaPO4F, Li7La3Zr2O12 (LLZO), Li5.1Ga0.32La3Zr2.25O13, Li6.25Ga0.25La3Zr2O12, LixLayTiO3 (LLTO) (0.07 ≤ x ≤ 0.13; y±0.05 = (2/3)- x), Li0.45La0.48TiO3, Li5La3X2O12 (X= Nb or Ta), LixPOyNz (3.0 ≤ x ≤ 3.2; 3.0 ≤ y ≤ 3.5; 0.0 ≤ z ≤ 0.5; 3 ≤ (y+z) ≤ 4), Li3.13PO1.69N1.39, Li0.98PO2.55N0.50, Li3PO4, La1+xAlxTi2-x(PO4)3 (0 ≤ x ≤ 2), Li1.3Al0.4Ti1.7(PO4)3, Li10GeP2S12, Li7P3S11, Na-BASE, NaM2(PO4)3 (M = Ge, Ti, Zr), Na1+xZr2SixP3-xO12 (0 ≤ X ≤ 3), Na2MgSiO4, Na2CaSiO4, Na2ZnSiO4, Na2Mg2TeO6, Na2Mg2ZnO6, Na3PS4, Na4SiS4, Na3PSe4, Na10SnP2S12, and Na3SbS4; and/or d. an active electrode material comprising one or more of: i. a sulphur-type positive electrode material selected from pure sulfur, K2Sx (x= 1, 2, 3, 5), Na2Sx (x= 1, 2, 3, 5), Li2Sx (x= 1, 2, 3, 5), and PAN-S, preferably K2Sx (x= 1, 2, 3, 5) and/or PAN-S; and/or P6633PC00 ii. an inorganic-type positive electrode material selected from K₂Ni₂TeO₆, K₃CoO₂, K_0.3MnO₂, K₂FeSiO₄, KFeSiO₄, K₂CoNiTeO₆, KFePO₄F, K₂NiO₂, K₂CuP₂O₇, K₂FeSiO₄, KFeSi₂O₆, KMnPO₄, K₂FeGeO₄, KVP₂O₇, K_0.71Cu[Fe(CN)₆]_0.72·3.7H₂O, K_0.6Ni_1.2Fe(CN)₆·3.6H₂O, K₂Fe^II[Fe^II(CN)₆]·2H₂O, Fe^III[Fe^III(CN)₆], K₂NiFe(CN)₆·1.2H₂O, K_1.85Fe_0.33Mn_0.67[Fe(CN)₆]_0.98·0.77H₂O, K_0.22V_1.74O_4.37·0.82H₂O, KM^IIFe^III(CN)₆(M = Mn, Fe, Co, Ni, and Zn), NaCoO₂, NaNiO₂, NaMnO₂, NaFeO₂, Na_7/9Cu_2/9Fe_1/9Mn_2/3O₂, Na₃V₂(PO₄)₃, Na₂VTi(PO₄)₃, Na_0.66Mn_0.66Ti_0.34O₂, Na_0.44MnO₂, Na₂LiV₂(PO₄)₃, Li₄Ti₅O₁₂, LiFePO₄, LiMn₂O₄, LiNiMnCoO₂, LiNiCoAlO₂, LiCoO₂, preferably K₂Ni₂TeO₆, K₂FeSiO₄ and KFeSi₂O₆; and/or iii. an organic-type positive electrode material selected from A_2+xC₆O₆ (0 < X < 4), (A = Na, K, Li). 39. The battery according to claim 37, wherein the positive electrode layer is a liquid positive electrode layer comprising as the active electrode material one or more of a sulphur-type positive electrode material selected from pure sulphur, K2Sx (x= 1, 2, 3, 5), Na2Sx (x= 1, 2, 3, 5), Li2Sx (x= 1, 2, 3, 5), preferably K2Sx (x= 1, 2, 3, 5) , optionally in combination with a liquid electrolyte selected from the group consisting of ether electrolyte, such as (AN(SO2F)2 in DME) or (ACF3SO3 in TEGDME), ester electrolyte, such as (APF6 in EC/DMC), (APF6 in PC) or (AN(SO2F)2 in EC/DEC), ATFSI in Tetraglyme, and aqueous electrolyte, such as (1 M KNO3 per 0.01 M HNO3), (1 M KNO3 pH = 2), 0.5 M K2SO4, 0.1 M KCl, 22 M KCF3SO3, 3 M KCl, 30 M KFSI, or 1 M KOH, wherein (A = K, Li, or Na). 40. The battery according to claim 37, wherein the positive electrode layer is a hybrid positive electrode layer comprising any combination of solid positive electrode layers as defined in claim 38, and liquid positive electrode layers as defined in claim 39. 41. The battery according to any one of claims 24 to 40, wherein the negative electrode layer is selected from a solid negative electrode layer, a liquid negative electrode layer and a hybrid negative electrode layer. P6633PC00 42. The battery according to claim 41, wherein the negative electrode layer is a solid negative electrode layer comprising one or more of: a) a metal selected from K, Li, Na, Al or an alloy thereof, preferably K or a ternary K-Na-Li alloy; and/or b) a carbon-type negative electrode selected from graphite and hard carbon; and/or c) a silicon type negative electrode selected from silicon, crystalline silicene or zintle (potassium silicides, sodium silicides, lithium silicides and calcium silicides). 43. The battery according to claim 41, wherein the negative electrode layer is a liquid negative electrode layer comprising one or more of: a metal selected from K, Li, Na, Al or an alloy thereof, preferably K or a ternary K-Na-Li alloy. 44. The battery according to claim 41, wherein the negative electrode layer is a hybrid negative electrode layer comprising any combination of solid negative electrode layers as defined in claim 42, and liquid negative electrode layers as defined in claim 43. 45. The battery according to any one of items 24 to 44, wherein the positive electrode layer is a solid positive electrode layer, and the negative electrode layer is a solid negative electrode layer. 46. A sensor comprising the ion conductive polycrystalline material of any one of claims 1 to 20. 47. The sensor according to claim 46, wherein the sensor is suitable for detecting a gas selected from the group consisting of SO2, SO3, NO, NO2, CO, and CO2. 48. The sensor according to any one of claims 46 to 47, wherein the sensor is a potentiometric sensor, such as a type-III potentiometric sensor. 49. The sensor according to any one of claims 46 to 48, wherein the sensor has a sensitivity of 74-89 mV/dec at 500 °C. P6633PC00 50. The sensor according to any one of claims 46 to 49, wherein the sensor has a detection limit of 1-10 ppm for SO₂ at 500 °C. 51. The sensor according to any one of claims 46 to 50, wherein the sensor has a response/recovery time of 2-120 minutes for SO₂ at 500 °C. 52. The sensor according to any one of claims 46 to 50, wherein the sensor has a response and/or recovery time of 2-120 minutes for SO₂ at 500 °C. 53. A method of manufacturing the solid-state electrolyte according to any one of claims 1 to 20 the method comprising the sequential steps: a. Mixing at least one source of potassium, at least one source of magnesium, and at least one source of silicon in a ratio such as to obtain a super-stoichiometric potassium ratio of K:Mg:Si corresponding to (2+X):1-(X/2):1 (0.0 < X ≤ 0.5), thereby forming a mixture, and b. Heating said mixture to a first temperature between 600 °C and 1000 °C to obtain a sintered mixture. 54. The method according to claim 53, further comprising prior to step b, a step of ball-milling said mixture. 55. The method according to any one of claims 53 to 54, further comprising subsequent to step b., a step of crushing the sintered mixture into a fine powder, pelletizing said fine powder, and heating said pelletized powder to a second temperature between 600 °C and 1000 °C. 56. The method according to claim 55, wherein the step of crushing, pelletizing and heating is repeated at least 3 times, such as 4 times, such as 5 times, such as any one of 6, 7, 8, 9, and 10 times. 57. The method according to any one of claims 53 to 56, wherein said first and second temperature is a temperature below 1000 °C, such as below 950 °C, such as below 900 °C, such as below 850 °C, such as below 800 °C. P6633PC00 58. The method according to claim 57, wherein said first and second temperature is a temperature below 850 °C. 59. The method according to any one of claims 55 to 58, wherein pelletizing is performed by application of uniaxial pressure in the range of 50 MPa to 100 MPa, preferably 70 MPa to 90 MPa. 60. The method according to any one of claims 53 to 59, wherein the at least one source of potassium is a salt of potassium or an oxide of potassium, such as selected from the group consisting of KF, KCl, KBr, KI, KNO₃, K₂CO₃,K₃PO₄, K₂SO₄, KClO₄, KClO₃, K₂O, and KOH. 61. The method according to any one of claims 53 to 60, wherein the at least one source of potassium is K2CO3 or K2O. 62. The method according to any one of claims 53 to 61, wherein the at least one source of magnesium is a salt of magnesium or an oxide of magnesium, such as selected from the group consisting of MgF2, MgCl2, MgBr2, MgI2, Mg(NO3)2, MgCO3,Mg3(PO4)2, MgSO4, Mg(ClO4)2, Mg(ClO3)2, MgO, and Mg(OH)2. 63. The method according to any one of claims 53 to 62, wherein the at least one source of magnesium is MgO. 64. The method according to any one of claims 53 to 63, wherein the at least one source of silicon is a halide of silicon or an oxide of silicon, such as selected from the group consisting of SiCl4 and SiO2. 65. The method according to any one of claims 53 to 64, wherein the at least one source of silicon is SiCl4 and SiO2. 66. The method according to any one of claims 53 to 65, wherein the method is performed in ambient conditions. P6633PC00 67. The method according to any one of claims 53 to 65, wherein the method is performed in inert atmosphere, such as consisting essentially of nitrogen or argon. 68. Use of the electrolyte according to any one of claims 1 to 20 as a component in a battery, 69. The use according to claim 68, wherein said battery is a solid-state battery or a semi-solid-state battery. 70. The use according to claim 69, wherein said battery is a solid-state battery. 71. The use according to any one of claims 68 to 70, wherein said battery is an alkali- ion conducting battery, such as a sodium-ion battery or potassium-ion battery. 72. The use according to any one of claims 68 to 71, wherein said battery is a potassium-ion battery. 73. The use according to any one of claims 68 to 71, wherein said battery is a sodium- ion battery.