Thermally adaptive energy storage systems

Abstract

An energy storage system is disclosed. The energy storage system is thermally adaptive. The energy storage system comprises a negative electrode, a positive electrode comprising an active material, and a solid-based electrolyte separating the negative electrode from the positive electrode. The negative electrode is operable at a solid state, a molten state, and a mixture of solid and molten states. In some embodiments, the positive electrode is arranged in contact with a catholyte, and / or the catholyte is part of the active material of the positive electrode. The catholyte may be maintained in a liquid state across the span of temperatures at which the negative electrode is in the solid state, molten state, and mixture of solid and molten states.

Description

THERMALLY ADAPTIVE ENERGY STORAGE SYSTEMSCross-Reference to Related Application

[0001] This application claims priority from US application No. 63 / 600809 filed 20 November 2023 and entitled THERMALLY ADAPTIVE LIQUID SODIUM I SODIUM-ION HYBRID BATTERY CELL which is hereby incorporated herein by reference for all purposes. For purposes of the United States of America, this application claims the benefit under 35 U.S.C. §119 of US application No. 63 / 600809 filed 20 November 2023 and entitled THERMALLY ADAPTIVE LIQUID SODIUM I SODIUM-ION HYBRID BATTERY CELL which is hereby incorporated herein by reference for all purposes.Field

[0002] The invention pertains to energy storage systems, in particular, those that can be operated over a wide range of temperatures.Background

[0003] The global energy storage market is large and rapidly growing. By 2030, the global quantity of energy stored in energy storage devices is anticipated to increase to about 15 times the amount recorded in 2021 . Fueled by the rising electrification of society, battery research and development has experienced exponential growth over the last decade.

[0004] The increasing installation of renewable power sources such as wind and solar, which are naturally variable, has led to a rising demand for electricity storage. Batteries play a pivotal role in storing this energy for use when needed. Energy storage for applications such as electric grids, large-scale renewable energy storage, electric vehicles and critical loads are dominated by rechargeable lithium-ion batteries. However, material scarcity and geopolitical issues with lithium and other critical minerals (e.g., cobalt mining) are catalysts for research into new battery chemistries built with Earth-abundant materials that are less costly and more environmentally friendly.

[0005] The performance benchmarks against which batteries are measured include cost, cycle life, power, safety, temperature of operation and energy density. Sodium (Na) is an inexpensive and a widely available raw material. ZEBRA (Na / NiCI) and sodium-sulfur (Na / S) batteries are two mature sodium battery technologies which have been used in energy storage applications. ZEBRA and sodium-sulfur batteries are both rechargeable molten salt batteries, that achieve cell voltages of ~2.6 V and ~2.2 V, respectively. ZEBRA batteries, short for Zeolite Battery Research Africa Project, the initiative during which the technology was developed, employ a sodiumnickel chloride (Na / NiCI) chemistry while sodium-sulfur (Na / S) batteries operate based on a sodium-sulfur chemistry. These battery chemistries require high operating temperatures around 300-350°C for efficient cycling. The longer restart times and the necessity for high operating temperatures limit their use in certain applications as they have high energy demands.

[0006] Room temperature Na-ion batteries have been considered as low-cost alternative to Li-ion batteries. However, they suffer from lower volumetric and gravimetric energy density, lower power densities, and inferior cycling performance, which limits their application in different energy storage markets.

[0007] The inventors have recognised a general need for improved energy storage systems, in particular those that are thermally adaptive which can operate across a wide ranges temperature with minimal losses in performance.Summary

[0008] This application relates to an energy storage system. The energy storage system is thermally adaptive. The energy storage system comprises a negative electrode, a positive electrode comprising an active material, and a solid-based electrolyte separating the negative electrode from the positive electrode. The negative electrode is operable in a solid state, a molten state and a mixture of solid and molten states in transition. The positive electrode may be arranged in contact with a supply of catholyte and / or the catholyte may form part of the positive electrode. The catholyte may be maintained in a liquid state across the span of temperatures at which the negative electrode is in the solid state, the molten state, and the mixture of solid and molten states in transition.

[0009] In some embodiments, the temperatures at which the negative electrode is in the solid state, the molten state, and the mixture of solid and molten states are between -30°C and 300°C, and in some embodiments, between -20°C and 300°C, and in some embodiments, between 0°C and 200°C, and in some embodiments, between 20°C and 120°C, and in some embodiments, between 90°C and 110°C.

[0010] In some embodiments, the negative electrode is operable at a temperature within a first range of temperatures at which the negative electrode is in a solid state, and a temperature within a second range of temperatures at which the negative electrode is in a molten state. In some embodiments, the catholyte is maintained in a liquid state across the span of the temperatures within the first and second ranges of temperatures at which the negative electrode is in the solid state and the molten state respectively. In some embodiments, the negative electrode is operable at a transition temperature or a temperature within a third range of temperatures at which the negative electrode is in a mixture of solid and molten states in transition. In some embodiments, the catholyte is maintained in a liquid state at the transition temperature or across the temperatures within the third range of temperatures at which the negative electrode is in the mixture of solid and molten states in transition.

[0011] In some embodiments, the temperatures within the first range of temperatures are less about 100°C. In some embodiments, the temperatures within the second range of temperatures are above about 100°C.

[0012] In some embodiments, the temperatures within the first, second and third ranges of temperatures, and the transition temperature are between -30°C and 300°C, and in some embodiments, between -20°C and 300°C, and in some embodiments, between 0°C and 200°C, and in some embodiments, between 20°C and 120°C, and in some embodiments, between 90°C and 110°C.

[0013] Further aspects and example embodiments are illustrated in the accompanying drawings and / or described in the following description.

[0014] It is emphasized that the invention relates to all combinations of the above features, even if these are recited in different claims.Brief Description of the Drawings

[0015] The accompanying drawings illustrate non-limiting example embodiments ofthe invention.

[0016] FIG. 1 is a schematic diagram illustrating an energy storage system according to an example embodiment of the invention.

[0017] FIG. 2A is a graph of the specific capacity over cycle number from operating a FIG. 1 system comprising a sodium-based negative electrode and a positive electrode comprising sodium vanadium phosphate (referred to hereinafter as the “Na / NVP cell”) at a temperature of 130°C at rate of 1C.

[0018] FIG. 2B is a graph of voltage over capacity of the Na / NVP cell operating at a temperature of 130°C and at a rate of 1C.

[0019] FIG. 3 is a graph of the voltage over time of the Na / NVP cell operating in the power-thermal mode, showing thermal adaptability of the system between the ambient temperature and 130°C.

[0020] FIG. 4A is a graph of the specific capacity over cycle number of the Na / NVP cell operating at a temperature of 170°C at rates of 2C and 1C.

[0021] FIG. 4B is a graph of voltage over time of the Na / NVP cell operating at a temperature of 170°C at rates of 2C and 1C.

[0022] FIG. 5 is a graph of the voltage over time from operating a FIG. 1 system comprising a sodium-based negative electrode and a positive electrode comprising iron disulfide (referred to hereinafter as the “Na / FeS2 cell”) starting at a temperature of 120°C, followed by at a temperature of 150°C.

[0023] FIG. 6A is a voltage-time profile of Na I FeCh, Fe, NaCIO4 in an EMIM-OTf battery cell showing the initial activation cycle at 0.02 mA / cm2,

[0024] FIG. 6B is a voltage-time profile of Na I FeCh, Fe, NaCIO4 in an EMIM-OTf battery cell showing the cycles at 0.36 mA / cm2.

[0025] FIG. 7 is a graph of the discharge capacity over cycle number, showing the cycling of Na I FeC , Fe, NaCIO4 in an EMIM-OTf battery cell at a rate of 3C at 150 °C.Detailed Description

[0026] Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements havenot been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive sense.

[0027] It is to be understood that the presently disclosed subject matter is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims.

[0028] This invention pertains to an energy storage system, such as a cell or battery. One embodiment of the energy storage system comprises a sodium-based negative electrode, a positive electrode, and a solid-state electrolyte separating the sodium- based negative electrode and the positive electrode. A catholyte is optionally arranged in contact with the positive electrode.

[0029] The energy storage system is thermally adaptive. As used herein, a “thermally adaptive system” means a system that is capable of operating over a wide range of temperatures with minimal losses in performance, and can continue to function during temperature transitions.

[0030] In some embodiments, the energy storage system described herein is designed to operate over a wide range of temperatures, from subzero temperatures of about -20°C to about 300°C. Over this temperature range, the sodium-based negative electrode is in a solid state below about 100°C and is in a molten state above about 100°C. The catholyte may be maintained in a liquid state across a range of temperatures which span the transition of the sodium-based negative electrode between the molten state and the solid state, and vice versa.

[0031] Proof-of-concept experiments demonstrate that cycling stability under fast charge and discharge can be achieved from operating the disclosed energy storage system at a wide range of temperatures under conditions in which the sodium-based negative electrode is in a solid state, in which the sodium-based negative electrode is in a molten state, and in which the sodium-based negative electrode is in a mixture of the solid and molten states in transition.Example embodiments of the energy storage system

[0032] FIG. 1 is a schematic diagram illustrating an energy storage system 10 according to an example embodiment of the invention. In some embodiments, theenergy storage system 10 comprises a sodium-based negative electrode 14, a positive electrode 16, and a solid-state electrolyte 20 separating the sodium-based negative electrode 14 from the positive electrode 16. In some embodiments, the solid- state electrolyte 20 is arranged in contact with the sodium-based negative electrode 14. In some embodiments, the solid-state electrolyte 20 is arranged spaced-apart from the positive electrode 16. In some embodiments, the solid-state electrolyte 20 is arranged in contact with the positive electrode 16.

[0033] The sodium-based negative electrode 14 is configured to function over a wide range of temperatures. The sodium-based negative electrode 14 is operable in a solid state, a molten state and a mixture of solid and molten states in transition. In some embodiments, the temperatures at which the sodium-based negative electrode 14 is in the solid state, the molten state, and the mixture of the solid and molten states in transition are in the range of from about -30°C to about 300°C, and in some embodiments, from about -20°C and to 300°C, and in some embodiments, from about 0°C to about 200°C, and in some embodiments, from about 20°C to about 120°C, and in some embodiments, from about 90°C to about 110°C. Over the range of temperatures of from -30°C to 300°C, sodium is in a solid state below about 100°C and is in a molten state above about 100°C. During operation of the system 10, the sodium-based negative electrode 14 acts as a hybrid anode that is operable between -30°C and 300°C (in some embodiments, between about -20°C and about 300°C, and in some embodiments, between about 0°C and about 200°C, and in some embodiments, between about 20°C and about 120°C, and in some embodiments, between about 90°C and about 110°C) by being in solid and molten states or a mixture of solid and molten states in transition at different temperatures and time intervals.

[0034] In some embodiments, the sodium-based negative electrode 14 comprises an active material comprising sodium 24. The active material 24 may comprise pure sodium, or a sodium-alloy. In some embodiments, the sodium-alloy comprises sodium being mixed with one or more metals and / or non-metals. The one or more metals and / or non-metals may be any suitable elements, including but are not limited to potassium (K), tin (Sn), caesium (Cs), indium (In), lead (Pb), bismuth (Bi), magnesium (Mg), aluminum (Al), and / or silicon (Si).

[0035] The negative electrode 14 may be assembled with the active material 24 or assembled free or substantially free of the active material 24. In some embodiments, the negative electrode 14 is assembled with the active material 24, thereby constructing a negative electrode 14 that is in a “charged state”. In some embodiments, the negative electrode 14 is assembled free or substantially free of the active material 24, thereby constructing a negative electrode 14 that is in a “discharged state”. In such “anode-free” configuration, the active material comprising sodium 24 will begin to form at the negative electrode 14 after the initial charging process, referred to as battery formation.

[0036] In some embodiments, a current collector is arranged in contact with the active material comprising sodium 24. In some embodiments, the current collector is arranged adjacent to, or spaced-apart from, the solid-state electrolyte 20. In some embodiments, the current collector is arranged in contact with the solid-state electrolyte 20. The current collector may be formed from one or more materials comprising iron (Fe), chromium (Cr), copper (Cu), aluminum (Al), zinc (Zn), nickel (Ni), carbon (C) and / or titanium (Ti). The current collector 26 may be formed from pure metal, or a metal alloy. In some example embodiments, the current collector is formed from a material comprising stainless steel, a carbon-based material such as carbon and non-carbon steel, and brass. In some embodiments, the current collector is porous. In such embodiments, the current collector comprises a mesh or a foam. In some embodiments, the current collector is non-porous.

[0037] In some embodiments, the active material comprising sodium 24 is loaded into a free volume of a porous current collector. The active material comprising sodium 24 may be loaded from about 20% to about 100% of the free volume of the porous current collector, and in some embodiments, from about 20% to about 60%. In some embodiments, the remaining free volume of the porous current collector, after being loaded with the active material comprising sodium 24, is at least about 40%, and in some embodiments, at least about 40% to about 80%.

[0038] The average thickness of the active material comprising sodium 24 may be in the range of from about 50 pm to about 50 nm, and in some embodiments, from about 50 pm to about 1000 pm, and in some embodiments, from about 50 pm to about 500 pm, and in some embodiments, from about 100 pm to about 300 pm, andin some example embodiments, about 200 pm.

[0039] A coating is optionally applied on a surface of the current collector of the negative electrode 14. The coating may comprise any suitable chemical composition which may improve surface contact between the current collector and the active material comprising sodium 24, and / or improve surface contact between the current collector and the solid-state electrolyte 20. In some embodiments, the coating comprises tin (Sn), lead (Pb), bismuth (Bi), and combinations thereof.

[0040] In some embodiments, the system 10 comprises a solid-state electrolyte 20 that acts as both: (i) a separator, arranged to separate the negative electrode 14 from the positive electrode 16; and (ii) a sodium ion conductor, configured to conduct sodium ions between the electrodes 14, 16. The solid-state electrolyte 20 may separate the catholyte from the negative electrode 14. In some embodiments, the system 10 is free or substantially free of a liquid anolyte solution.

[0041] The solid-state electrolyte 20 may be made of an ionic conductive material comprising ceramic, metal, glass, polymer, and combinations thereof. In some embodiments, the solid-state electrolyte 20 comprises a structure formed by one layer comprising one type of the ionic conductive material or by one layer comprising a homogenous mixture of different types of the ionic conductive materials. In some embodiments, the solid-based electrolyte 20 comprises a structure formed by a plurality of layers arranged in a stack. Each of the layers in the stack may comprise one type of the conductive material or a homogenous mixture of different types of the conductive materials.

[0042] In some embodiments, solid-state electrolyte 20 comprises a porous support. The porous support may be arranged in contact with the one or more layers of the ionic conductive materials. The one or more layers of the ionic conductive materials may be deposited on the porous support. In some embodiments, the porous support is formed from a material that is non-metallic, i.e. , compounds that do not contain any metal element or metalloid elements. In some embodiments, the porous support comprises an ion-conductive material. A desirable porous support is one which possesses good ionic conductivity and minimal electronic conductivity.

[0043] In some embodiments, the solid-state electrolyte 20 comprises an ionconducting ceramic material. In some example embodiments, the solid-stateelectrolyte 20 comprises a NASICON-type structure, represented by the formula Nai+xZr2SixP3-xOi2, or variations thereof. In some embodiments, x is between 0 and 3. In some example embodiments, the solid-state electrolyte 20 comprises beta-alumina (BASE), or variations thereof.

[0044] The average thickness of the solid-state electrolyte 20 may be in the range of from about 500 nm to about 750 pm, and in some embodiments, from about 500 nm to about 300 pm, and in some embodiments, from about 1 pm to about 100 pm, and in some example embodiments, about 1 pm.

[0045] A coating is optionally applied on the solid-state electrolyte 20. The coating may comprise any suitable chemical composition which may improve surface contact between the solid-state electrolyte 20 and the negative electrode 14 and / or the positive electrode 16. Coating of the solid-state electrolyte 20 may serve to improve surface contact between the solid-state electrolyte 20 and the active material comprising sodium 24. The improved surface contact may improve wetting of the surface of the solid-state electrolyte 20 by the molten sodium during cycling of the system. In some embodiments, the coating comprises tin (Sn), lead (Pb), bismuth (Bi), and combinations thereof.

[0046] The solid-state electrolyte 20 may comprise any suitable morphology. In some embodiments, the morphologies of the one or more conductive materials deposited on the porous support may be particles comprising shapes such as one or more of round, oval, polygonal, etc. The morphology of the solid-state electrolyte 20 may be adjusted to improve surface contact between the solid-state electrolyte 20 and / or the negative electrode 14 and / or the positive electrode 16. Improving surface contact between the solid-state electrolyte 20 and / or the negative electrode 14 and / or the positive electrode 16 may desirably improve wetting of the electrodes 14, 16, thereby reducing charge-transfer resistance for sodium plating or striping reactions and / or enabling reliable low-temperature operation of the system 10.

[0047] The positive electrode 16 comprises an active material 28. In some embodiments, the positive electrode 16 is arranged in contact with a catholyte 36. In some embodiments, the catholyte 36 is mixed with the active material 28 of the positive electrode 16. In some embodiments, the catholyte 36 is homogeneously mixed with the active material 28. In such embodiments, the catholyte 36 forms partof the positive electrode 16.

[0048] The active material 28 may comprise any suitable one or more materials that can undergo one or more of the following electrochemical reactions which enable the storage and release of electrical energy in the system 10:- intercalation reactions;- conversion reactions;- displacement / replacement reactions;- alloying / dealloying reactions;- redox reactions.

[0049] In some embodiments, the active material 28 comprises an intercalating material. In some embodiments, the active material 28 comprises sodium vanadium phosphate. In some example embodiments, the active material 28 comprises a polyanion-type of sodium vanadium phosphate intercalating material which is represented by the formula NaxM2(PC>4)3.

[0050] In some embodiments, the active material 28 comprises one or more materials which undergo an electrochemical conversion reaction. In some embodiments, the active material 28 comprises a mixture comprising a metal and sodium salt. The metal may comprise one or more of iron (Fe), titanium (Ti), manganese (Mn), nickel (Ni), copper (Cu), and zinc (Zn). The sodium salt may for example be sodium chloride, but any other suitable sodium salts may be selected. In the discharged state, the metal may be in a solid state, and the salt may be partially or completely dissolved in a catholyte 36. In the charged state, the oxidized metal may be partially dissolved and partially precipitated in the form of salt, and the sodium salt may be fully dissolved.

[0051] In some embodiments, the active material 28 comprises a mixture comprising a halogen oxyanion metal salt and a transition metal. In some embodiments, the halogen oxyanion metal salt comprises a metal perchlorate. In some example embodiments, the metal perchlorate comprises sodium perchlorate (NaCIO4). In some example embodiments, the transition metal comprises iron (Fe). In some embodiments, the halogen oxyanion metal salt comprises a mixture of a chlorine oxyanion metal salt and iodine-based salts. During discharging and charging, the halogen species and the metal may change oxidation states, which generatescapacity. A soluble transition metal complex salt may also be formed.

[0052] In some embodiments, the active material 28 comprises transition metal oxides, sulfides, aluminum, phosphates, inorganic complex salts, and mixtures thereof. Examples of transition metal oxides that may be selected for use as the active material 28 include one or more of manganese (Mn), vanadium (V), iron (Fe), nickel (Ni), titanium (Ti), copper (Cu), zinc (Zn), and cobalt (Co). Non-limiting examples of an inorganic salt complex that may be selected for use as the active material 28 is Prussian blue and / or Prussian white. In some example embodiments, the active material 28 comprises iron disulfide (pyrite, FeS2).

[0053] In some embodiments, the positive electrode 16 additionally comprises one or more additives. The one or more additives may be mixed in with the active material 28 of the positive electrode 16. The one or more additives may be added to improve the electrical conductivity of the active materials 28. In some embodiments, the one or more additives comprise carbon black and / or graphite.

[0054] In some embodiments, a current collector is arranged in contact with the active material 28. The current collector may be positioned adjacent to, or spaced-away from, the solid-state electrolyte 20. In some embodiments, the current collector is arranged in contact with the solid-state electrolyte 20. The current collector may comprise an electrically conductive material. In some embodiments, the current collector 30 is made of a material comprising carbon.

[0055] A catholyte 36 may be arranged to contact the positive electrode 16. The catholyte 36 directs a flow of ions between the positive electrode 16 and the solid- state electrolyte separator 20. In some embodiments, the catholyte 36 is contained in the system 10. In some embodiments, the catholyte 36 is supplied to the positive electrode 16. In some embodiments, a catholyte reservoir is flowingly connected to the system 10. A flow pump may be arranged to supply a flow of the catholyte 36 to the positive electrode 16 through a catholyte supply connection. In some embodiments, the catholyte 36 is exposed to a reservoir containing liquid catholyte. In some embodiments, the reservoir is an open reservoir for fluid around the positive electrode 16. In some embodiments, the catholyte 36 separates the positive electrode 16 from the solid-state electrolyte 20.

[0056] In some example configurations of the system 10, the positive electrode 16 isarranged in contact with the solid-state electrolyte 20 at one side of the positive electrode 16. A catholyte reservoir may be arranged at the opposite side of the positive electrode 16.

[0057] The catholyte 36 may be maintained in a liquid state across a range of temperatures which span the transition of the negative electrode 14 between the molten state and the solid state and vice versa. In some embodiments, catholyte 36 is maintained in a liquid state across a range of temperatures between about -30°C and about 300°C, and in some embodiments, between about -20°C and about 300°C, and in some embodiments, between about 0°C and about 200°C, and in some embodiments, between about 20°C and about 120°C, and in some embodiments, between 90°C and about 110°C. The catholyte 36 may comprise one or more undissolved particles. The undissolved particles may comprise undissolved active material and / or salts. Undissolved particles in the catholyte 36 may act as a reserve, which may improve cycle life of the system 10. In some embodiments, one or more precipitates may be formed in the catholyte 36 during the cycling of the system 10 when the system 10 is at a first temperature. The one or more precipitates may be dissolved in the catholyte 36 during the cycling of the system 10 when the system is at a second temperature which is at a temperature higher than the first temperature.

[0058] In some embodiments, one or both of the ionic conductivity and vapor pressure of the catholyte 36 are adjusted to optimize the performance of the catholyte 36 in the range of operating temperatures of the system 10.

[0059] In some embodiments, the catholyte 36 comprises a liquid catholyte. In some embodiments, the liquid catholyte 36 comprises one or more salts. In some embodiments, the one or more salts comprise one or more of sodium tetrachloroaluminate (NaAICU), sodium halogen oxyanion (NaCICk), halide salt such as sodium chloride (NaCI), and transition metal halide salts, such as aluminum chloride (AICI3). For example, in some embodiments, NaCI is mixed with AICI3 to produce a suitable catholyte 36 which comprises a suitable ratio of NaCI : AICI3. In some example embodiments, the catholyte 36 comprises a molten mixture comprising NaCI and AICI3 having a molar ratio of about 37 : 63 NaCI : AICI3. Different molar ratios of NaCI : AICI3 may be used to adjust the properties (e.g., ionic conductivity, compressibility, stability, etc. ) of the catholyte 36 to optimize theperformance of the catholyte 36. NaCI may be mixed with any suitable one or more alkali metal chlorides to produce the catholyte 36.

[0060] In some embodiments, the catholyte 36 is a solution comprising one or more salts dissolved in a solvent. The solvent may comprise one or more of an organic solvent, an ionic liquid, a deep eutectic solvent, a monomer, and a polymer.

[0061] Non-limiting examples of organic solvents that may be selected include a diol and triol alcohol such as propylene glycol, ethylene glycol, glycerol, etc., organic carbonates such as ethylene carbonate, propylene carbonate, and combinations thereof.

[0062] An ionic liquid are salts which comprise organic cations such as imidazolium and / or pyridinium salts complexed with inorganic and / or organic anions such as sulfonate- and / or phosphate-based anions. In some example embodiments, the ionic liquid comprises 1-ethyl-3-methylimidazolium trifluoromethanesulfonate.

[0063] In some embodiments, the polymer comprises a molten polymer. In some example embodiments, the polymer comprises polyethylene oxide (PEO).

[0064] In some example embodiments, the solvent is selected from one or more of the following liquids:1-ethyl-3-methylimidazolium trifluoromethanesulfonate;1-ethyl-3-methylimidazolium dimethyl phosphate;1 :9 molsodium bis(fluorosulfonyl)imide / N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide;- propylene carbonate;- ethylene carbonate;- glycerol;- ethylene glycol;- propylene glycol; and- choline chloride : glycerol, e.g., 1 : 2.5 molar ratio.

[0065] The solvent may be produced from one or more of the above listed liquids. The molar ratio of each of the liquids to form the solvent may be adjusted to optimize one or more of cost, cycle life, power level, safety, temperature of operation, and / or energy and power density of the energy storage system 10.

[0066] Non-limiting examples of catholyte 36 that may be used to operate the energystorage system 10 across a range of temperatures which spans the transition of the negative electrode 14 between the molten state and the solid state, and vice versa include:- sodium perchlorate (NaCIO4) dissolved in an ionic liquid;- sodium perchlorate dissolved in molten polyethylene oxide;- sodium perchlorate and iron chloride dissolved in an ionic liquid (1 -ethyl-3- methylimidazolium trifluoromethanesulfonate);- sodium perchlorate and iron chloride dissolved in propylene carbonate; and- sodium chloride dissolved in glycerol, etc.

[0067] In some embodiments, the system 10 is free or substantially free of a catholyte 36.

[0068] A power source 40 may be connected to apply an electrical charge to the sodium-based negative electrode 14 and the positive electrode 16. The power source 40 applies a potential difference between the sodium-based negative electrode 14 and the positive electrode 16.

[0069] In some embodiments, an external heat source is electrically connected to the system 10. The heat source may be configured to supply a source of heat to the system 10 during operation. In some embodiments, one or more heat sources are connected to supply a source of heat to the positive electrode 16 and / or negative electrode 14. In some embodiments, a heat source is connected to supply a source of heat to the catholyte reservoir containing the catholyte. The catholyte may be heated in the catholyte reservoir before being supplied to the positive electrode 16.

[0070] In some embodiments, the system 10 is heated internally. The charging and / or discharging of the system 10 generates heat in the system 10. In some embodiments, the heating of the system 10 is attained through the charging and / or discharging of the cell (i.e. , internal heating) alone or in combination with one or more external heat sources (i.e., external heating). The heating ramp rate may be controlled through one or both of these heating means.

[0071] A plurality of the energy storage system 10 may be provided. The plurality of energy storage systems 10 may be connected in parallel, in series or both to form a battery. Such battery may for example be used in electric grid storage.

[0072] In operation, during charging of the energy storage system 10, the negativeterminal of the power source 40 may be connected to the sodium-based negative electrode 14. The positive terminal of the power source 40 may be connected to the positive electrode 16. The power source 40 supplies electrons to the sodium-based negative electrode 14. A reduction reaction takes place at the sodium-based negative electrode 14. An oxidation reaction takes place at the positive electrode 16. In some embodiments, the oxidation reaction at the active material 28 of the positive electrode 16 releases a supply of sodium ions (Na+). The sodium ions may pass through the solid-state electrolyte 20 to reach the sodium-based negative electrode 14. The sodium ions are reduced at the sodium-based negative electrode 14 to form sodium metal during charging.

[0073] In some embodiments, the negative electrode 14 is assembled free or substantially free of the active material comprising sodium 24. In such “anode-free” configuration, the sodium ions that are released from the oxidation reaction at the positive electrode 16 forms the active material comprising sodium 24 at the negative electrode 14.

[0074] During discharge, for example, when an electrical load is arranged to connect the sodium-based negative electrode 14 and the positive electrode 16, a reduction reaction takes place at the positive electrode 16. An oxidation reaction takes place at the sodium-based negative electrode 14. The oxidation reaction at the active material comprising sodium 24 of the sodium-based negative electrode 14 releases a supply of sodium ions. The sodium ions may pass through the solid-state electrolyte 20 to reach the positive electrode 16. The sodium ions may be stored at the positive electrode 16.Example applications of the energy storage system

[0075] The energy storage system 10 is operable at a wide range of temperatures under conditions in which the sodium-based negative electrode 14 is in a solid state and in which the sodium-based negative electrode 14 is in a molten state, advantageously allowing the system to be used for both high power and ambient application regimes. The system 10 provides the ability to: (i) start or operate at lower temperatures, i.e., allowing the system to “cold start”, and (ii) thermal cycle, i.e. , subjecting the system to repeated cycles of high and low temperatures.

[0076] For example, for high power applications, the system 10 may be operated at a temperature greater than 100°C, or between 100°C and 150°C. The system 10 may begin operating in a fully charged state. During rest, heat may be turned off to improve energy efficiency. The system may be cooled to a temperature between ambient temperature and the operating temperature. Such temperature may depend on factors such as the rest period and thermal insulation. Discharging of the system 10 may be started at any temperature between ambient temperature and the operating temperature, thereby allowing the system to cold start. The heating ramp rate can be controlled by the rate of discharge of the cell and / or external heating. For example, a ramp rate of 1-5 °C / min (and in some embodiments, a ramp rate of less than 1 °C / min) can be used to reach a temperature of greater than 100°C with or without external heating using the described system 10.

[0077] For ambient applications, the system 10 may be operated at ambient temperature, or at a temperature between ambient temperature and 100°C. The system 100 may be heated to greater than 100°C after some cycles of operation (in some embodiments, after hundreds of cycles of operation). The heat liquefies the sodium-based negative electrode 14 (or in particular, the active material 24) from solid to molten form, removing dendrites which may be undesirably formed on the negative electrode 14. The removal of dendrites improves safety and cycle life of the system 10. Isolated sodium may be undesirably created at the negative electrode 14 during operation of the system 10. Such isolated sodium may for example, be created during earlier operation of the system 10 which did not provide appropriate electronic or ionic conductive contact between one or more components of the system 10. This can result in low performance of the system 10. The system 10 is configured to liquefy such isolated sodium, which allows the sodium to flow. The melted isolated sodium may be mixed in with the sodium at the active material 24, and thus allow such sodium to become useful again. The ability to remove such isolated sodium has the advantages of at least improving capacity and / or extending the cycle life of the system 10. The high temperature step in this application may be a maintenance step to extend the cycle life of the system. The system 10 preferably remains in a fully charged state for more than 30 minutes at a given temperature of about 100°C, after which the system 10 may be operated again at temperatures of below 100°C.

[0078] The invention is further described with reference to the following specific examples, which are not meant to limit the invention, but rather to further illustrate it.EXAMPLESExample 1- Na / NVP, Temperature: 20°C to 130°C

[0079] An energy storage system 10 of the type illustrated in FIG. 1 was used in this example. Metallic sodium was used as the active material 24 for the negative electrode 14. Na-intercalating material was used as the active material 2428 for the positive electrode 16. The Na-intercalating material was sodium vanadium phosphate (NVP) - Na3V2(PO4)3. This Na-intercalating material was mixed with a conductive agent, carbon black, and dispersed in acetone using an ultrasonic bath, p-alumina membrane (BASE) was used as the solid-state electrolyte 20. The obtained ink was coated on carbon paper and the electrodes 14, 16 were transferred to the argon-filled glovebox after drying at 130°C in a vacuum oven for 12 hours. NVP mass loading was 3-5 mg cm-2, while the surface area of the electrodes 14, 16 were 0.4 cm2. 15 pL of catholyte 36, 1 M NaCIC>4 in ionic liquid (1-ethyl-3-methylimidazolium trifluoromethanesulfonate), was pipetted on the carbon paper electrode 16 and was left for 5 minutes to wet the electrode 16. The cell was assembled in a glovebox by placing the Na electrode 14 at the bottom of the cell, followed by the separator (BASE 20) and catholyte-wet positive electrode 16. After placing a 316 stainless steel spacer and a spring on top of the positive electrode 16, the cell was closed by hand tightening the metallic caps on either side of the cell. Based on the spring constant and the compression height, the applied force was estimated to be between 0.2 and 0.3 MPa.

[0080] The cell was operated at 130°C in an oven located in the glovebox using galvanostatic charge-discharge steps at a 1C rate. The voltage-capacity profile of the cell shows flat charge and discharge plateaus characteristic for a NVP positive electrode and very small plateau overvoltage implying low internal resistance of the cell and high-power capabilities (FIG. 2B). The operating discharge voltage of the cell was 3.35 V. Capacity was stable during the first 60 cycles with no visible decrease, indicating potential for excellent cycle life (FIG. 2A).

[0081] A cell of the same composition and architecture was tested to investigatefeasibility for the high-power application in which the system typically operates between 100°C and 150°C but during the idle state when the cell was not being operated, heating is off to improve energy efficiency and save the energy. To enable fast response of the system, the battery cell chemistry has to be able to start delivering energy at a reasonable rate at any temperature between ambient and operating temperature, simultaneously with heating back to an operating temperature above 100°C. This protocol was simulated in FIG. 3, where the cell operated at 130°C during the first charge and then, simultaneously, heating was turned off and the cell was left at idle state for 4 hours. During this cooling period, cell temperature decreased to 28°C. At this temperature (close to the ambient), the cell was operated at C / 5 for 15 min, after which heating was turned on at 2°C / min. The temperature ramped back to 130°C and cycled in a stable manner. Overvoltage decreased as the temperature was ramping up, showing a decrease of cell internal resistance with temperature. This test shows that the cell can operate at ambient temperatures at decent current densities but, also, that it is thermally adaptive, i.e. , the cell has the ability of switching between ambient and operating temperatures of greater than 100°C.

[0082] A graph of the specific capacity with cycle number, and voltage over time of this Na / NVP cell operating at 170°C at 2C and 1C rates is illustrated in FIGS. 4A and 4B respectively. The results show that at higher temperatures, the cell can operate at higher C-rates, improving the power while maintaining high specific capacity. At 25°C, the cell was able to operate at C / 5.Example 2 - Na / FeS2, T 120°C to 150°C

[0083] In this experiment, a similar battery cell as the one described in Example 1 was used but in this experiment, iron disulfide (pyrite, FeS2) was used as the active material 3028 of the positive electrode 16, and a mixture of sodium chloride and aluminum chloride was selected as the catholyte 36. All other components of the cell and method of preparation are the same as described in Example 1. Catholyte was prepared by mixing anhydrous NaCI and AICI3 powders in the ratio 37 : 63 and heating them at 180°C for 30 min. The obtained compact mass was grinded with a mortar and pestle and mixed with the positive electrode active material (FeS2) andcarbon black in the ratio of 1 :8:1 and spread on one side of the BASE solid electrolyte. The cell was tested at 120°C and 150°C at a C / 12 rate between 1.2 V and 3 V. At the operating temperature, catholyte is in the molten state and has significantly higher ionic conductivity than in the solid state. The voltage-time profile (FIG. 5) shows significant improvement in capacity when the cell operates at the higher temperature, reaching almost 80 mAh / g.Example 3 - Na / NaCIC>4 + Fe

[0084] In this experiment, the same negative electrode 14 and the same solid-state electrolyte 20 as the ones described in Examples 1 and 2 were used, but several active material of the positive material / catholyte combinations were used to prepare different battery cells with very similar performance, voltage capacity profile and specific capacity of active materials. The tested positive active material / catholyte combinations were:1) Iron (II) chloride (FeC ) and iron metal powder, in mol ratio of 1.2 : 0.1 , mixed with an electrolyte consisting of 1 M sodium perchlorate (NaCIO4) in 1 -ethyl-3- methylimidazolium trifluoromethanesulfonate (EMIM-OTf);2) 0.4 M FeCI2, 1 M NaCIO4in EMIM-OTf;3) Fe powder, 1 M NaCIO4in EMIM-OTf;4) Fe powder, 1 M NaCIO4in propylene carbonate (PC);5) FeCI2, 1 M NaCIO4in PC; and6) Fe powder and FeCh in 1 M NaCIO4PC.

[0085] The active materials in these combinations are Fe and perchlorate (CIO4_) ion because they change their oxidation states with charge and discharge of the battery.

[0086] FIGs. 6A, 6B and 7 show the data from operating the first positive active material / catholyte combination (FeC , Fe, NaCIO4in EMIM-OTf). Activation of the cell occurred over about 110 hours at 150°C through a slow initial cycle (FIG. 6A). The cell was then cycled at increasingly high current densities of up to 3.4 mA / cm2and representative voltage-time curves (characteristic after the first two cycles) are shown in FIG. 6B. The maximum capacity reached was ~352 mAh / g, based on the mass of sodium perchlorate and iron, at a current density of 0.04 mA / cm2, indicating a high theoretical capacity for this chemistry.

[0087] After these tests, a current density of 1 .72 mA / cm2was chosen for long-term cycling (FIG. 7) resulting in charge-discharge cycle times of about 0.7 hrs (which was equivalent to about 30). As can be seen from FIG. 7, over 1800 cycles only 11 % capacity loss was observed compared to the initial cycle. Long cycle life is crucial for stationary energy storage systems and this result shows a great potential in this regard. Additionally, high C-rate indicates that the cell is capable of fast energy delivery through discharge and charge steps, i.e. , high power.

[0088] In summary, one aspect of the invention relates to a thermally adaptive energy storage system, comprising: a negative electrode comprising a sodium-based active material; a positive electrode; a solid-state electrolyte material separating the negative electrode from the positive electrode; and optionally, a catholyte comprising sodium salt in a solvent, where the catholyte separates the positive electrode from the solid-state electrolyte; wherein the storage system is capable of operating over a temperature range that comprises the negative electrode in a solid state, a solid / liquid slurry state, a molten state, or some combination thereof.

[0089] The range of temperatures at which the energy storage system is capable of operating at may be in the range of from -30°C to 300°C, or 0°C to 200°C, or 20°C to 120°C, or -30°C to 160°C.

[0090] The sodium-based active material in the negative electrode may comprise pure sodium or a sodium alloy containing potassium (K), tin (Sn), cesium (Cs), indium (In), lead (Pb), and bismuth (Bi), or mixtures thereof.

[0091] The negative electrode may be assembled without active material, thereby having an “anode-free” architecture, such that the battery cell is assembled in the discharged state.

[0092] In some embodiments, the solid-state electrolyte material comprises ceramic, metal, glass, polymer, or mixtures thereof. The solid-state electrolyte material may comprise one or more components wherein the mass fraction of each component is between 0 and 1 . The material components can be arranged in differentmorphologies including but not limited to a homogenous mixture of different ratios and layered structures. The solid-state electrolyte material may be deposited on a porous support. In some embodiments, the material comprises an ion conducting ceramic. In some embodiments, the materials comprise beta-alumina (BASE) or NASICON Nai+xZr2SixP3-xOi2(0 < x < 3).

[0093] The solid-state electrolyte material may be configured to have improved surface contact with the positive electrode by applying a coating to the positive electrode or the solid-state electrolyte. The solid-state electrolyte material may be configured to have improved surface contact with the negative electrode by applying a coating to the negative electrode or the solid-state electrolyte. The solid-state electrolyte may be configured to have improved surface contact with the positive electrode by modifying the morphology of the solid-state electrolyte in contact with the positive electrode. The solid-state electrolyte may be configured to have improved surface contact with the negative electrode by modifying the morphology of the solid- state electrolyte in contact with the negative electrode.

[0094] In some embodiments, the coating comprises tin (Sn), lead (Pb), bismuth (Bi), or some combination thereof.

[0095] In some embodiments, the positive electrode comprises an active material, a current collector and additives. The current collector may be an electrically conductive material. The conductive material may be carbon. The active material may comprise an intercalating material. In some embodiments, the active material comprises sodium vanadium phosphate. In some embodiments, the active material comprises transition metal oxides, sulfides, aluminum, phosphates, inorganic complex salts, or mixtures thereof. The transition oxide metal may comprise manganese (Mn), vanadium (V), iron (Fe), nickel (Ni), titanium (Ti), copper (Cu), zinc (Zn), and cobalt (Co), or mixtures thereof. In some example embodiments, the active material comprises iron disulfide (pyrite, FeS2). In some example embodiments, the inorganic complex salt comprises Prussian blue. In some embodiments, the additives comprise carbon black, graphite, or mixtures thereof.

[0096] In some embodiments, the positive electrode is configured to be exposed to an open reservoir (i.e., no containment) of the catholyte comprising sodium salt.

[0097] In some embodiments, the catholyte is a liquid in the temperature range of -30- 300°C, or in the temperature range of 0°C to 200°C, or in the temperature range of 20°C to 120°C, or in the temperature range of -30° C to 160°C.

[0098] In some embodiments, the catholyte is a liquid and solid mixture in the temperature range of -30 - 300°C, or in the temperature range of 0°C to 200°C, or in the temperature range of 20°C to 120°C, or in the temperature range of -30°C to 160°C.

[0099] In some embodiments, the catholyte comprises molten Na-salt or a mixture of salts. In some embodiments, the catholyte comprises NaAICU, NaCI, AICI3, or mixtures thereof.

[0100] In some embodiments, the catholyte comprises Na-salt or a mixture of salts dissolved in a solvent. The solvent may be an organic solvent, an ionic liquid, a polymer, water or mixtures thereof. An example polymer may be polyethylene oxide.

[0101] In some embodiments, the energy storage system is configured to operate with an external heater or heating source and / or with in-situ heat generated with or without an external heating source.Interpretation of Terms

[0102] Unless the context clearly requires otherwise, throughout the description and the claims:• “comprise”, “comprising”, and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”;• “connected”, “coupled”, or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements; the coupling or connection between the elements can be physical, logical, or a combination thereof;• “herein”, “above”, “below”, and words of similar import, when used to describe this specification, shall refer to this specification as a whole, and not to any particular portions of this specification;• “or”, in reference to a list of two or more items, covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list;• the singular forms “a”, “an”, and “the” also include the meaning of any appropriate plural forms. These terms (“a”, “an”, and “the”) mean one or more unless stated otherwise;• “and / or” is used to indicate one or both stated cases may occur, for example A and / or B includes both (A and B) and (A or B);• “approximately” when applied to a numerical value means the numerical value ± 10%;• where a feature is described as being “optional” or “optionally” present or described as being present “in some embodiments” it is intended that the present disclosure encompasses embodiments where that feature is present and other embodiments where that feature is not necessarily present and other embodiments where that feature is excluded. Further, where any combination of features is described in this application this statement is intended to serve as antecedent basis for the use of exclusive terminology such as "solely," "only" and the like in relation to the combination of features as well as the use of "negative" limitation(s)” to exclude the presence of other features; and• “first” and “second” are used for descriptive purposes and cannot be understood as indicating or implying relative importance or indicating the number of indicated technical features.

[0103] Words that indicate directions such as “vertical”, “transverse”, “horizontal”, “upward”, “downward”, “forward”, “backward”, “inward”, “outward”, “left”, “right”, “front”, “back”, “top”, “bottom”, “below”, “above”, “under”, and the like, used in this description and any accompanying claims (where present), depend on the specific orientation of the apparatus described and illustrated. The subject matter described herein may assume various alternative orientations. Accordingly, these directional terms are not strictly defined and should not be interpreted narrowly.

[0104] Where a range for a value is stated, the stated range includes all sub-ranges of the range. It is intended that the statement of a range supports the value being at an endpoint of the range as well as at any intervening value to the tenth of the unit of the lower limit of the range, as well as any subrange or sets of sub ranges of the range unless the context clearly dictates otherwise or any portion(s) of the statedrange is specifically excluded. Where the stated range includes one or both endpoints of the range, ranges excluding either or both of those included endpoints are also included in the invention.

[0105] Certain numerical values described herein are preceded by "about". In this context, "about" provides literal support for the exact numerical value that it precedes, the exact numerical value ±5%, as well as all other numerical values that are near to or approximately equal to that numerical value. Unless otherwise indicated a particular numerical value is included in “about” a specifically recited numerical value where the particular numerical value provides the substantial equivalent of the specifically recited numerical value in the context in which the specifically recited numerical value is presented. For example, a statement that something has the numerical value of “about 10” is to be interpreted as: the set of statements:• in some embodiments the numerical value is 10;• in some embodiments the numerical value is in the range of 9.5 to 10.5; and if from the context the person of ordinary skill in the art would understand that values within a certain range are substantially equivalent to 10 because the values with the range would be understood to provide substantially the same result as the value 10 then “about 10” also includes:• in some embodiments the numerical value is in the range of C to D where C and D are respectively lower and upper endpoints of the range that encompasses all of those values that provide a substantial equivalent to the value 10.

[0106] Specific examples of systems, methods and apparatus have been described herein for purposes of illustration. These are only examples. The technology provided herein can be applied to systems other than the example systems described above. Many alterations, modifications, additions, omissions, and permutations are possible within the practice of this invention. This invention includes variations on described embodiments that would be apparent to the skilled addressee, including variations obtained by: replacing features, elements and / or acts with equivalent features, elements and / or acts; mixing and matching of features, elements and / or acts from different embodiments; combining features, elements and / or acts from embodiments as described herein with features, elements and / or acts of other technology; and / oromitting combining features, elements and / or acts from described embodiments.

[0107] As will be apparent to those of skill in the art upon reading this disclosure, each of the individual embodiments described and illustrated herein has discrete components and features which may be readily separated from or combined with the features of any other described embodiment(s) without departing from the scope of the present invention.

[0108] Any aspects described above in reference to apparatus may also apply to methods and vice versa.

[0109] Any recited method can be carried out in the order of events recited or in any other order which is logically possible. For example, while processes or blocks are presented in a given order, alternative examples may perform routines having steps, or employ systems having blocks, in a different order, and some processes or blocks may be deleted, moved, added, subdivided, combined, and / or modified to provide alternative or subcombinations. Each of these processes or blocks may be implemented in a variety of different ways. Also, while processes or blocks are at times shown as being performed in series, these processes or blocks may instead be performed in parallel, simultaneously or at different times.

[0110] Various features are described herein as being present in “some embodiments”. Such features are not mandatory and may not be present in all embodiments. Embodiments of the invention may include zero, any one or any combination of two or more of such features. All possible combinations of such features are contemplated by this disclosure even where such features are shown in different drawings and / or described in different sections or paragraphs. This is limited only to the extent that certain ones of such features are incompatible with other ones of such features in the sense that it would be impossible for a person of ordinary skill in the art to construct a practical embodiment that combines such incompatible features. Consequently, the description that “some embodiments” possess feature A and “some embodiments” possess feature B should be interpreted as an express indication that the inventors also contemplate embodiments which combine features A and B (unless the description states otherwise or features A and B are fundamentally incompatible). This is the case even if features A and B are illustrated in different drawings and / or mentioned in different paragraphs, sections or sentences.

[0111] It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions, omissions, and sub-combinations as may reasonably be inferred. The scope of the claims should not be limited by the preferred embodiments set forth in the examples, but should be given the broadest interpretation consistent with the description as a whole.

Claims

WHAT IS CLAIMED IS:1 . An energy storage system comprising: a negative electrode; a positive electrode comprising an active material; and a solid-based electrolyte separating the negative electrode from the positive electrode, wherein the negative electrode is operable in a solid state, a molten state and in a mixture of solid and molten states.

2. The energy storage system as defined in claim 1 , wherein the temperatures at which the negative electrode is in the solid state, the molten state and in the mixture of solid and molten states are between -20°C and 300°C.

3. The energy storage system as defined in claim 1 , wherein the temperatures at which the negative electrode is in the solid state, the molten state and in the mixture of solid and molten states are between 0°C and 200°C.

4. The energy storage system as defined in claim 1 , wherein the temperatures at which the negative electrode is in the solid state, the molten state and in the mixture of solid and molten states are between 20°C and 120°C.

5. The energy storage system as defined in claim 1 , wherein the temperatures at which the negative electrode is in the solid state, the molten state and in the mixture of solid and molten states are between 90°C and 110°C.

6. The energy storage system as defined in any one of claims 1 to 5, the positive electrode is arranged in contact with a catholyte and / or the catholyte is part of the positive electrode.

7. The energy storage system as defined in claim 6, wherein the catholyte is maintained in a liquid state across the span of temperatures at which the negative electrode is in the solid state, molten state, and mixture of solid and molten states,8. The energy storage system as defined in claim 6 or 7, wherein the catholyte comprises a mixture comprising one or more salts and a solvent.

9. The energy storage system as defined in claim 8, wherein the one or more salts comprise one or more sodium salts and / or a mixture of metal salts.

10. The energy storage system as defined in claim 8 or 9, wherein the one or more salts are selected from one or more of a halide salt, transition metal halide salt, and halogen oxyanion metal salt.11 . The energy storage system as defined in any one of claims 8 to 10, wherein the one or more salts are selected from one or more of sodium tetrachloroaluminate (NaAICU), sodium halogen oxyanion (NaCIO4), sodium chloride (NaCI), and aluminum chloride (AICI3).

12. The energy storage system as defined in claim 11 , wherein the one or more salts comprise a mixture of sodium chloride and aluminum chloride.

13. The energy storage system as defined in claim 12, wherein the mixture of sodium chloride and aluminum chloride additionally comprises one or more of an alkali metal salt and earth-alkali metal salt.

14. The energy storage system as defined in any one of claims 8 to 13, wherein the solvent is selected from one or more of an organic solvent, ionic liquid, deep eutectic solvent, monomer, and polymer.

15. The energy storage system as defined in any one of claims 8 to 14, wherein the solvent is selected from one or more of 1-ethyl-3-methylimidazolium trifluoromethanesulfonate, 1-ethyl-3-methylimidazolium dimethyl phosphate, 1 :9 molsodium bis(fluorosulfonyl)imide / N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, propylene carbonate, ethylene carbonate, glycerol, ethylene glycol, propylene glycol, and choline chloride : glycerol, and polyethylene oxide.

16. The energy storage system as defined in any one of claims 8 to 15, wherein the one or more salts comprise sodium perchlorate and / or iron chloride, and thesolvent comprises an ionic liquid.

17. The energy storage system as defined in claim 16, wherein the ionic liquid comprises 1 -ethyl-3-methylimidazolium trifluoromethanesulfonate.

18. The energy storage system as defined in any one of claims 8 to 15, wherein the one or more salts comprise sodium perchlorate and / or iron chloride, and the solvent comprises propylene carbonate.

19. The energy storage system as defined in any one of claims 8 to 15, wherein the one or more salts comprise sodium chloride, and the solvent comprises glycerol.

20. The energy storage system as defined in any one of the preceding claims, wherein the active material of the positive electrode comprises a mixture of metal and sodium salt.21 . The energy storage system as defined in claim 20, wherein the metal comprises one or more of iron (Fe), titanium (Ti), manganese (Mn), nickel (Ni), copper (Cu), and zinc (Zn).

22. The energy storage system as defined in any one of claims 1 to 19, wherein the active material of the positive electrode comprises a halogen oxyanion metal salt and a transition metal.

23. The energy storage system as defined in claim 22, wherein the halogen oxyanion metal salt comprises sodium perchlorate (NaCIO4).

24. The energy storage system as defined in claim 22 or 23, wherein the halogen oxyanion metal salt comprises a mixture of a chlorine oxyanion metal salt and iodine- based salts.

25. The energy storage system as defined in any one of claims 22 to 24, wherein the transition metal comprises iron (Fe).

26. The energy storage system as defined in any one of claims 1 to 19, wherein the active material of the positive electrode comprises an intercalating material.

27. The energy storage system as defined in claim 26, wherein the active material of the positive electrode comprises one or more of transition metal oxide, sulfide, aluminum, phosphate, and inorganic complex salt.

28. The energy storage system as defined in claim 27, wherein the phosphate comprises sodium vanadium phosphate.

29. The energy storage system as defined in claim 27 or 28, wherein the inorganic complex salt comprises Prussian blue and / or Prussian white.

30. The energy storage system as defined in any one of claims 27 to 29, wherein the sulfide comprises iron disulfide (FeS2).31 . The energy storage system as defined in any one of the preceding claims, wherein the positive electrode additionally comprises one or more additives mixed with the active material of the positive electrode.

32. The energy storage system as defined in claim 31 , wherein the one or more additives comprise carbon black and / or graphite.

33. The energy storage system as defined in any one of the preceding claims, further comprising a current collector arranged to contact the active material of the positive electrode.

34. The energy storage system as defined in claim 33, wherein the current collector comprises an electrically conductive material.

35. The energy storage system as defined in claim 34, wherein the electrically conductive material comprises one or more of carbon, titanium, aluminum and stainless steel.

36. The energy storage system as defined in any one of claims 33 to 35, wherein the current collector of the positive electrode is arranged in contact with the solid- state electrolyte or spaced-apart from the solid-state electrolyte.

37. The energy storage system as defined in any one of the preceding claims,wherein the negative electrode comprises an active material comprising sodium.

38. The energy storage system as defined in claim 37, wherein the active material of the negative electrode comprises pure sodium.

39. The energy storage system as defined in claim 37, wherein the active material of the negative electrode comprises a sodium alloy.

40. The energy storage system as defined in claim 39, wherein the sodium alloy comprises a mixture comprising sodium and one or more of potassium (K), tin (Sn), cesium (Cs), indium (In), lead (Pb), and bismuth (Bi).41 . The energy storage system as defined in any one claims 37 to 40, further comprising a current collector arranged in contact with the active material comprising sodium of the negative electrode.

42. The energy storage system as defined in claim 41 , wherein the current collector is arranged in contact with the solid-state electrolyte or spaced-apart from the solid-state electrolyte.

43. The energy storage system as defined in claim 41 or 42, wherein the active material of the negative electrode is loaded into a free volume of the current collector.

44. The energy storage system as defined in claim 43, wherein the active material of the negative electrode is loaded into about 20% to about 100% of the free volume of the current collector.

45. The energy storage system as defined in claim 43 or 44, wherein the active material of the negative electrode is loaded into about 20% to about 60% of the free volume of the current collector.

46. The energy storage system as defined in any one of claims 41 to 45, wherein the current collector is formed of a material comprising one or more elements comprising iron (Fe), chromium (Cr), copper (Cu), aluminum (Al), zinc (Zn), nickel (Ni), carbon (C) and / or titanium (Ti).

47. The energy storage system as defined in any one of claims 41 to 46, wherein the current collector comprises a porous material.

48. The energy storage system as defined in claim 47, wherein the porous material comprises a mesh or foam.

49. The energy storage system as defined in any one of claims 37 to 48, wherein the active material of the negative electrode has an average thickness in the range of from about 50 pm to about 50 mm.

50. The energy storage system as defined in any one of the preceding claims, wherein the negative electrode further comprises a coating applied on the current collector of the negative electrode.51 . The energy storage system as defined in claim 50, wherein the coating comprises one or more of tin (Sn), lead (Pb), and bismuth (Bi).

52. The energy storage system as defined in any one of claims 1 to 36, wherein the negative electrode is free or substantially free of an active material comprising sodium in an as assembled state of the system.

53. The energy storage system as defined in any one of the preceding claims, wherein the solid-state electrolyte is arranged in contact with the negative electrode.

54. The energy storage system as defined in any one of the preceding claims, wherein the solid-state electrolyte is made of one or more layers of ionic conductive materials.

55. The energy storage system as defined in claim 54, wherein the ionic conductive material comprise one or more of ceramic, metal, glass, and polymer.

56. The energy storage system as defined in any one of the preceding claims, wherein the solid-state electrolyte comprises a structure formed by one layer comprising one type of the conductive material or a homogenous mixture of different types of the ionic conductive materials.

57. The energy storage system as defined in claim 56, wherein the solid-state electrolyte comprises a structure formed by a plurality of layers arranged in a stack.

58. The energy storage system as defined in claim 57, wherein each one of the plurality of layers comprise one type of the ionic conductive material or a homogenous mixture of different types of the ionic conductive materials.

59. The energy storage system as defined in any one of claims 53 to 58, wherein the solid-state electrolyte additionally comprises a porous support arranged in contact with the one or more layers of ionic conductive materials.

60. The energy storage system as defined in claim 59, wherein the porous support is formed from a non-metallic material.61 . The energy storage system as defined in any one of claims 54 to 60, wherein the conductive material comprises an ion-conducting ceramic material.

62. The energy storage system as defined in claim 61 , wherein the ion-conducting ceramic material comprises a NASICON-type structure, and variations thereof.

63. The energy storage system as defined in any one of the preceding claims, wherein the solid-state electrolyte comprises beta-alumina (BASE), and variations thereof.

64. The energy storage system as defined in any one of the preceding claims, wherein the solid-state electrolyte comprises one or more of a NASICON-type structure, beta-alumina (BASE), and variations thereof.

65. The energy storage system as defined in any one of the preceding claims, wherein the average thickness of the solid-state electrolyte is in the range of from about 500 nm to about 750 pm.

66. The energy storage system as defined in any one of the preceding claims, wherein the average thickness of the solid-state electrolyte is in the range of from about 500 nm to about 300 pm.

67. The energy storage system as defined in any one of the preceding claims, wherein the solid-state electrolyte further comprises a coating applied on a surface thereof.

68. The energy storage system as defined in claim 67, wherein the coating comprises one or more of tin (Sn), lead (Pb), and bismuth (Bi).

69. The energy storage system as defined in any one of claims 6 to 68, further comprising a catholyte reservoir, the catholyte reservoir flowingly connected to supply the catholyte to the positive electrode.

70. The energy storage system as defined in any one of claims 6 to 68, wherein the catholyte is contained in the system.71 . The energy storage system as defined in any one of the preceding claims, further comprising a heat source connected to supply heat to the system.

72. The energy storage system as defined in claim 71 , wherein the heat source is connected to supply heat to one or both of the positive electrode and the negative electrode.

73. The energy storage system as defined in claim 71 or 72, wherein the heat source is connected to supply heat to the catholyte reservoir.

74. The energy storage system as defined in any one of the preceding claims, further comprising a power source connected to apply an electrical energy to the positive electrode and the negative electrode.

75. The energy storage system as defined in any one of the preceding claims, wherein the system is configured to generate a supply of heat during the charging and / or discharging of the system.

76. The energy storage system as defined in any one of the preceding claims, wherein the system is free or substantially free of a liquid anolyte.

77. An energy storage system comprising: a negative electrode; a positive electrode comprising an active material;a solid-based electrolyte separating the negative electrode from the positive electrode; and a catholyte being arranged in contact with the positive electrode and / or being part of the active material of the positive electrode, wherein the negative electrode is operable in a solid state, a molten state and in a mixture of solid and molten states, and wherein, the catholyte is maintained in a liquid state across the span of temperatures at which the negative electrode is in the solid state, molten state, and mixture of solid and molten states.

78. The energy storage system as defined in claim 77, wherein the span of temperatures at which the negative electrode is in the solid state, molten state and mixture of solid and molten states are between -20°C and 300°C.

79. The energy storage system as defined in claim 77, wherein the span of temperatures at which the negative electrode is in the solid state, molten state and mixture of solid and molten states are between 0°C and 200°C.

80. The energy storage system as defined in claim 77, wherein the span of temperatures at which the negative electrode is in the solid state, molten state and mixture of solid and molten states are between 20°C and 120°C.81 . The energy storage system as defined in claim 78, wherein the span of temperatures at which the negative electrode is in the solid state, molten state and mixture of solid and molten states are between 90°C and 110°C.

82. The energy storage system as defined in any one of claims 77 to 81 , wherein the system is free or substantially free of a liquid anolyte.

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