Thermodynamically tunable thermochemical materials

Abstract

The present application relates to thermochemical materials (TCMs) for energy storage. The TCM can be a metal salt with a chemical formula of Mx-bNbAy. ZH2O, in which M is a metal ion, N is a different metal ion from M, and A is an anion, and z is the number of water molecules associated with the salt. The metal salt may have more than two types of cations. The TCM demonstrates improved thermodynamic properties such as higher power density, higher energy density, and better cyclic stability.

Description

THERMODYNAMICALLY TUNABLE THERMOCHEMICAL MATERIALSCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application Serial No. 63 / 683,984 filed August 16, 2024, the entire content of which is hereby incorporated by reference.FIELD

[0002] The present application relates to thermochemical materials with improved energy storage properties.BACKGROUND

[0003] Thermochemical materials (TCMs) store and release energy through reversible chemical reactions. TCMs can potentially be used in various energy storage systems, such as solar thermal energy storage, industrial waste heat recovery, and building heating and cooling systems. It is a promising area of research and development in the field of energy storage, offering potential benefits in terms of energy density, efficiency, and long-term stability.

[0004] One group of TCMs is the metal salt, in which metal salt leverage the ability to undergo reversible chemical reactions that absorb or release heat. This approach offers several advantages over conventional thermal storage methods, such as the potential for efficient energy storage and retrieval.

[0005] Salt hydrates are salts that contain water molecules within their crystal structure. The storage mechanism relies on the endothermic and exothermic reactions of the salt hydrates. When the salt hydrate absorbs heat, it undergoes a phase change or chemical reaction that stores energy. Conversely, when the hydrate releases heat, it returns to its original state, thereby releasing the stored energy. For example, for endothermic reactions during charging, the salt hydrate absorbs heat and undergoes a transformation, such as dehydration. And for exothermic reactions, during discharging, the salt hydrate releases heat as it undergoes a reverse reaction, such as (re)crystallization or hydration.

[0006] Performance improvements of TCMs, such as improving their power density and energy density, are currently only explored via direct mixing / doping of different TCM powders. Such a method, however, comes with various limitations.

[0007] For examples, the complexity in processing, where achieving a uniform and homogeneous mixture of different powders can be difficult, which may lead to inconsistent properties and performance; the complicated synthesis, where the synthesis and processing techniques required to combine different powders can be more complex and time-consuming; the chemical incompatibility issues that can lead to undesirable reactions or degradation of material properties; the thermal expansion mismatch between different powders; the phase stability of the mixed / doped compositions, where different powders might have different coefficients of thermal expansion, leading to internal stresses, cracks, or delamination during thermal cycling; the decomposition, where some components may decompose or react with each other at high temperatures, affecting the overall stability and performance, and further leading to reduced long- term durability.

[0008] These limitations will invariably lead to performance trade-offs, such as compromised properties, where when enhancing one property (e.g., thermal conductivity) might lead to the degradation of another (eg., mechanical strength); and difficulties in optimization, where it can be challenging to optimize the mixture to achieve the desired balance of properties without compromising overall performance.

[0009] Thus, improvement is needed for the performance improvement of TCMs .SUMMARY

[0010] The present application relates to energy storage TCMs. In some embodiments, the TCM is a metal salt and has a chemical formula of. zH2O ; wherein M is a metal ion, N is a second metal ion, and A is an anion; wherein z is the number of water molecules associated with the salt; and wherein the metal salt demonstrates improved thermodynamic properties comparing to those of MxAy. zH2O . The metal ion (M) may comprise Ca2+, Mg2+, Na+, Cu2+, Zn2+, K+, Li+, Fe2+, Fe3+, Ni2+, Mn2+, Mn3+, and Al3+; the second metal ion (N) may comprise Ca2+, Mg2+, Na+, Cu2+, Zn2+, K+, Li+, Fe2+, Fe3+, Ni2+, Mn2+, Mn3+, and Al3+; and the the anion (A) may comprise oxalate, acetate, permanganate, dichromate, chromate, thiocyanate, cyanide, perchlorate, nitrite, bromide, iodide, sulphite, oxide, hydroxide, carbonate, sulphate, chloride, nitrate, phosphate, sulfide, fluoride, bicarbonate, malonate, succinate, glucarate, adipate, tartrate, suberate, fumarate, citrate, gluconate, and stearate. The metal salt may further comprise cations that are different from the metal ion and the second metal ion. The metal salt may have a mole ratio of M\N being lessthan 95:5, demonstrating a power density of greater than 50 W / kg, an energy density of greater than 400 kJ / kg, a cyclic stability for greater than 10 cycles, and / or an enthalpy increase per mole of water of greater than 5% compared to MxAy. zH2O . The present application also relates to energy devices that utilize the TCMs described herein.BRIEF DESCRIPTION OF THE FIGURES

[0011] FIG. 1 is a graph showing the shifts in phase lines between pure CaC2O4, and CaC2O4. H2O with 5 mol% of Ca2+substituted with Mg2+.

[0012] FIG. 2 is a graph showing the shifts in phase lines between pure CaC2O4, and CaC2O4. H2O with 20 mol% of Ca2+substituted with Mg2+.

[0013] FIG. 3 is a graph showing the cyclic stability for 30 hydration-dehydration cycles of the CaC2O4. H2O with 5 mol% of Ca2+substituted with Mg2+.DETAILED DESCRIPTION OF THE EMBODIMENTS

[0014] As discussed above, improvement on the existing thermochemical materials (TCMs) and TCM-based energy storage devices is needed to meet the demands of high-power industrial applications and to alleviate power grid congestion. The current state-of-the-art TCM systems typically operate at power densities in the range of approximately 50-100 W / kg, which limits their applicability in dynamic energy environments requiring rapid charge-discharge cycles.

[0015] These limitations are particularly pronounced in decentralized energy systems, where responsiveness and scalability are critical. Moreover, the integration of conventional TCMs into modular or mobile platforms is constrained by their low energy throughput and limited thermal responsiveness, making them less suitable for emerging use cases such as district heating, industrial process buffering, and renewable energy balancing.

[0016] The TCMs described in the present application exhibit a significant and unexpected enhancement in energy storage performance. Specifically, the energy density of the disclosed materials increases by approximately 7.8% to 30.0% compared to conventional energy storage technologies, including electrical batteries.

[0017] This improvement is attributed to molecular-level modifications that alter the hydrationdehydration thermodynamics of the base salt. Furthermore, the materials demonstrate a power density of up to 2500 W / kg, which enables fest thermal cycling and high-rate energy exchange,making them suitable for applications requiring rapid thermal response. In addition to power and energy density improvements, the materials show cyclic stability over 30 hydration-dehydration cycles, representing a durability increase of at least 200% relative to existing TCMs. This enhanced stability supports long-term deployment in systems with frequent cycling, such as chemical heat pumps and thermochemical reactors, without significant degradation in performance.

[0018] In some embodiments, the thermochemical behaviour of the disclosed materials is governed by a reversible hydration-dehydration mechanism, wherein water molecules are absorbed or released in response to thermal input. The underlying chemical transformation involves a transition between a hydrated salt form and its anhydrous counterpart, accompanied by a corresponding enthalpy change. The introduction of a second metal ion into the crystal lattice, via partial substitution of the primary metal ion, modifies the local coordination environment and alters the thermodynamic profile of the hydration reaction. This molecular-level substitution enables precise tuning of the enthalpy of hydration, phase transition temperature, and water uptake capacity. These changes are not merely compositional but result in measurable shifts in thermal behaviour, as evidenced by thermogravimetric analysis and Van’t Hoff-derived enthalpy values. The modified salts thus exhibit enhanced energy and power densities, as well as improved cyclic stability, making them suitable for high-performance thermal energy storage applications.Performances

[0019] In some embodiments, the thermochemical materials (TCMs) are assessed based on key performance indicators including power density, energy density, and cyclic stability. These parameters collectively define the operational efficiency, responsiveness, and durability of the material under repeated thermal cycling conditions. The evaluation of these metrics is essential for determining the suitability of the TCMs for integration into real-world energy storage systems, particularly those requiring rapid thermal response and long-term reliability.

[0020] Power Density: Power density refers to the amount of power generated or absorbed per unit mass (W / kg) during the hydration or dehydration process. It is a critical parameter in applications where fast thermal exchange is required, such as in load-following energy systems, industrial waste heat recovery, and thermal buffering for renewable energy sources. High power density enables the material to respond quickly to thermal input or demand, reducing system lag and improving overall energy throughput.

[0021] Increasing power density not only expands the range of viable applications but also enhances the economic feasibility of thermochemical systems. On a system level, higher power density reduces the need for auxiliary components such as oversized heat exchangers, buffer tanks, or active thermal management systems, thereby lowering both capital expenditure (CAPEX) and operational expenditures (OPEX). Additionally, high power density materials are particularly advantageous in modular or distributed energy systems, where compactness and responsiveness are critical for deployment in space-constrained or mobile environments.

[0022] In some embodiments, the power density is determined by extracting the enthalpy change associated with the hydration or dehydration reaction using the Van’t Hoff equation, fitted to experimental equilibrium data. The resulting enthalpy value is then multiplied by the rate of water uptake per unit time and normalized by the anhydrous mass of the TCM. This approach allows for a standardized comparison across different material compositions and substitution levels, and provides insight into the kinetic and thermodynamic contributions to the material’s performance.

[0023] In some embodiments, the TCMs of the present application demonstrate an enthalpy per mole of water of greater than 70 kJ / mol, e.g., greater than 72 kJ / mol, greater than 74 kJ / mol, greater than 76 kJ / mol, greater than 78 kJ / mol, greater than 80 kJ / mol, greater than 82 kJ / mol, greater than 84 kJ / mol, greater than 86 kJ / mol, greater than 88 kJ / mol, greater than 90 kJ / mol, greater than 92 kJ / mol, greater than 94 kJ / mol, greater than 96 kJ / mol, greater than 98 kJ / mol, greater than 100 kJ / mol, greater than 105 kJ / mol, greater than 110 kJ / mol, greater than 115 kJ / mol, or greater than 120 kJ / mol.

[0024] In some embodiments, the TCMs of the present application demonstrate a power density of greater than 50 W / kg, e.g., greater than 60 W / kg, greater than 70 W / kg, greater than 80 W / kg, greater than 90 W / kg, greater than 100 W / kg, greater than 110 W / kg, greater than 120 W / kg, greater than 130 W / kg, greater than 140 W / kg, greater than 150 W / kg, greater than 175 W / kg, greater than 200 W / kg, greater than 225 W / kg, greater than 250 W / kg, greater than 275 W / kg, greater than 300 W / kg, greater than 325 W / kg, greater than 350 W / kg, greater than 375 W / kg, greater than 400 W / kg, greater than 425 W / kg, greater than 450 W / kg, greater than 475 W / kg, or greater than 500 W / kg.

[0025] Energy Density: Energy density is a measure of the amount of energy stored in a given system or substance per unit volume or mass (J / kg or Wh / kg). It is an important parameter in various fields such as batteries, fuel cells, and fuels, as it determines the amount of energy that canbe stored and delivered by a given system. High energy density is particularly important for applications requiring long-duration energy supply in compact or lightweight forms.

[0026] In some embodiments, the energy density of the thermochemical material is determined by calculating the enthalpy change associated with the hydration or dehydration reaction, as derived from the Van’t Hoff equation fitted to equilibrium data. The extracted enthalpy value is then multiplied by the total water mass uptake capacity of the material and normalized by the anhydrous crystal density of the TCM. This approach accounts for both the intrinsic thermodynamic potential of the reaction and the material’s capacity to reversibly store water, providing a comprehensive measure of energy storage capability. The resulting energy density values can be used to benchmark the material against conventional thermal storage media and electrochemical batteries, and to guide material selection for specific system-level energy requirements.

[0027] In some embodiments, the TCMs of the present application demonstrate an energy density of greater than 400 kJ / kg, e.g., greater than 500 kJ / kg, greater than 600 kJ / kg, greater than 700 kJ / kg, greater than 800 kJ / kg, greater than 900 kJ / kg, greater than 1000 kJ / kg, greater than 1100 kJ / kg, greater than 1200 kJ / kg, greater than 1300 kJ / kg, greater than 1400 kJ / kg, greater than 1500 kJ / kg, greater than 1600 kJ / kg, greater than 1700 kJ / kg, greater than 1800 kJ / kg, greater than 1900 kJ / kg, greater than 2000 kJZkg, greater than 2100 kJ / kg, greater than 2200 U / kg, greater than 2300 kJ / kg, greater than 2400 kJ / kg, greater than 2500 kJ / kg, greater than 2600 kJ / kg, greater than 2700 kJ / kg, greater than 2800 kJ / kg, greater than 2900 kJ / kg, greater than 3000 kJ / kg, greater than 3100 kJ / kg, greater than 3200 kJ / kg, greater than 3300 kJ / kg, greater than 3400 kJ / kg, or greater than 3500 kJ / kg.

[0028] Cyclic stability described the longevity, efficiency, and overall performance of the energy storage system. It is an important performance parameter in the following aspects.

[0029] Durability and Longevity: Thermochemical batteries often undergo numerous charge and discharge cycles. High cyclic stability ensures that the materials involved maintain their effectiveness and structural integrity over many cycles, reducing the need for frequent replacements or maintenance.

[0030] Material Degradation: Good cyclic stability minimizes degradation of the reactive materials, which helps in preserving the battery’s performance and extending its operational life.

[0031] Consistent Performance: Maintaining stable cyclic behavior ensures that the battery consistently performs at its optimal efficiency. If cyclic stability is poor, the capacity and efficiency of the battery can degrade over time, leading to reduced energy storage and conversion efficiency.

[0032] Reduced Replacement Costs: Improved cyclic stability reduces the frequency of component replacement, which lowers the overall cost of operation and maintenance for the battery system.

[0033] Consistent Operation: Stable cyclic performance contributes to the reliable operation of the battery, which is essential for safety and predictable performance in practical applications.

[0034] Cyclic stability is measured by assessing the stability of performance in terms of the mass increase and decrease by cyclic hydration and dehydration, respectively, as well as the corresponding power, and by assessing the cyclic structural integrity, including volume change, pulverization and fragmentation. Rehydration is done by changing temperature / vapor pressure conditions such that the TCM crossovers the phase transition line to a region of higher hydrated state.

[0035] Tn some embodiments, the TCMs of the present application demonstrate a cyclic stability for greater than 10 cycles, e.g., greater than 15 cycles, greater than 20 cycles, greater than 25 cycles, greater than 29 cycles, greater than 32 cycles, greater than 34 cycles, greater than 36 cycles, greater than 38 cycles, greater than 40 cycles, greater than 42 cycles, greater than 44 cycles, greater than 46 cycles, greater than 48 cycles, or greater than 50 cycles.

[0036] One aspect to note is the trade-off between energy density and power density. The tradeoff between energy density and power density is a result of the inherent limitations and properties of the materials and designs used in energy storage systems. Optimizing for one typically involves compromising the other because the mechanisms that allow for high energy storage capacity often impede rapid energy release, and vice versa.

[0037] For example, for ion transport, high energy density materials, such as those with thick electrodes or densely packed structures, can store more ions but can also slow down ion transport This slower movement of ions limits the rate at which energy can be released (lower power density). Conversely, materials with structures that allow for fast ion transport (e.g., porous, or nanostructured materials) can release energy quickly (high power density) but may have lower overall ion storage capacity (lower energy density).

[0038] And for electrical conductivity, materials with high electrical conductivity can support rapid electron transport, which is essential for high power density. However, achieving high conductivity often requires materials that may not have the highest energy storage capacity.Furthermore, thermal management and efficiency also limits the ability of achieving a material with both high energy and power density. For example, high power density operation generates more heat due to increased current flow and resistance. Managing this heat without degrading the materials or the device’s performance is challenging.

[0039] Applicants have surprisingly found the following advantages of the described TCMs, comparing to other existing materials. For example, higher energy density when compared to conventional thermal storage methods, such as sensible heat storage; better energy storage efficiency; longer term storage, without significant losses over time, as the reactions are reversible and can be maintained over long periods; and better temperature flexibility, wherein the thermochemical materials can be operated at a wide range of temperatures, making this technology adaptable to various applications.

[0040] This allows the improvement of the thermodynamic properties of TCMs through molecular substitution. The power density range that can be realised outcompetes the state-of-the-art thermochemical materials and electrical batteries in performance and price.Embodiments

[0041] In some embodiments, the present application relates to a TCM that is a metal salt. The metal salt may have the chemical formula of MxAy. zH2O , wherein M is a metal ion, A is an anion, x is the number of metal cation per formula unit, y is the number of anions per formula unit, and z is the number of water molecules of crystallization associate with each formula of the salt When in dehydrated form, z can equal to zero (0).

[0042] The metal ion M can be from any metal in the periodic table. Non-limiting exemplary metal ions include, e.g., Ca2+, Mg2+, Na+, Cu2+, Zn2+, K+, Li+, Fe2+, Fe3+Ni2+, Mn2+, Mn3+, and Al3+.

[0043] In some embodiments, the metal ion M is selected from the group consisting of Ca2+, Mg2+, Na*, K*, Li*, Zn2+, Al3+, Fe2+, Fe3+, Ni2+, Ni3+, Mn2+, Mn3+, Mn4+, Mn6+, Mn7+, Cu*, Cu2+, Co2+, Co3+, Pb2+, Pb4+, Sn2+, and Sn4+.

[0044] The anion A can be any anion that can form a salt with the cation M. Non-limiting exemplary anions include, e.g., oxalate, acetate, thiocyanate, cyanide, perchlorate, nitrite, bromide,iodide, sulphite, oxide, hydroxide, carbonate, sulphate, chloride, nitrate, phosphate, sulfide, fluoride, and bicarbonate.

[0045] In some embodiments, the anion A is selected from the group consisting of oxalate, acetate, thiocyanate, cyanide, perchlorate, nitrite, bromide, iodide, sulphite, oxide, hydroxide, carbonate, sulphate, chloride, nitrate, phosphate, sulfide, fluoride, and bicarbonate.

[0046] In some embodiments, the metal ion of the metal salt is partially substituted by a second metal ion. The second metal ion is different from the metal ion that already existed in the metal salt. The metal salt with two types of metal ions may have the chemical formula of Mx-bNbAy. ZH2O, where b is the number of metal cation of N per formula unit.

[0047] In some embodiments, x is in the range from 1 to 10, e.g., x is greater than 1, greater than 2, greater than 3, greater than 4, greater than 5, greater than 6, greater than 7, greater than 8, or greater than 9.

[0048] In some embodiments, b is in the range from 0.01 to x., e.g., b is greater than 0.01, greater than 0.05, greater than 0.1, greater than 0.2, greater than 0.5, greater than 1, greater than 2, greater than 3, greater than 5, or greater than 7.

[0049] In some embodiments, y is in the range from 1 to 10., e.g., greater than 1, greater than 2, greater than 3, greater than 4, greater than 5, greater than 6, greater than 7, greater than 8, or greater than 9.

[0050] In some embodiments, z is in the range from 0 to 20, e.g., greater than 1, greater than 2, greater than 3, greater than 5, greater than 7, greater than 10, greater than 12, greater than 15, greater than 18, or grater than 19.

[0051] In some embodiments, the second metal ion N is selected from the group consisting of Ca2+, Mg2+, Na+, K+, Li+, Zn2+, AP+, Fe2+, Fe3+, Ni2+, Ni3+, Mn2+, Mn3+, Mn4+, Mn6+, Mn7+, Cu", Cu2+, Co2+, Co3+, Pb2+, Pb4+, Sn2+, and Sn4+.

[0052] In some embodiments, the metal ion of the metal salts is partially substituted with two or more different types of cations.

[0053] The principle behind the preparation method is molecular substitution. For metal salts, the molecular substitution can either replace all the metal ions with a second metal ion, or only partially replace. There are a few methods for molecular substitution, such as solution methods, which comprises dissolving the salt in a solution containing the second metal ion can facilitate substitution through ion exchange processes; electrochemical methods, which comprises applyingan electric current to drive the substitution of metal ions in a salt; and solid-state reactions, which comprises heating a mixture of the original salt and a second metal compound to promote molecular substitution.

[0054] The degree of substitution can be characterized by the amount of metal ion being replaced by the second metal ion. In some embodiments, the metal salts have a degree of substitution of greater than 0.2 mol%, e.g., greater than 0.5 mol%, greater than 0.7 mol%, greater than 1 mol%, greater than 2 mol%, greater than 3 mol%, greater than 5 mol%, greater than 8 mol%, greater than 10 mol%, greater than 12 mol%, greater than 15 mol%, greater than 18 mol%, greater than 20 mol%, greater than 22 mol%, greater than 25 mol%, greater than 28 mol%, greater than 30 mol%, greater than 35 mol%, greater than 38 mol%, greater than 40 mol%, greater than 45 mol%, greater than 50 mol%.

[0055] In some embodiments, the wherein the mole ratio of M:N is less than 95:5, e.g., less than 90:10, less than 85:15, less than 80:20, less than 75:25, less than 70:30, less than 65:35, less than 60:40, less than 55:45, less than 50:50, less than 45:55, less than 40:60, less than 35:65, less than 30:70, less than 25:75, less than 20:80, less than 15:85, less than 10:90, or less than 5:95.

[0056] In one embodiment, the TCM is CaC2O4with 5 mol% of Ca2+being substituted by Mg2+.

[0057] In one embodiment, the TCM is CaC2O4-H2O with 5 mol% of Ca2being substituted by Mg2+.

[0058] In one embodiment, the TCM is CaC2O4with 20 mol% of Ca2+being substituted by Mg2+.

[0059] In one embodiment, the TCM is CaC2O4-H2O with 20 mol% of Ca2+being substituted by Mg2+.

[0060] In one embodiment, the TCM is CaC2O4with 5 mol% of Ca2+being substituted by Cu2+.

[0061] In one embodiment, the TCM is CaC2O4-H2O with 5 mol% of Ca2+being substituted by Cu2+.

[0062] In some embodiments, the TCM is CaC2O4-H2O with 1 mol% being substituted by a second metal ion selected from Mn2+, Mn3+, Mn4+, Mn6+, Mn7+, Fe2+, Fe3+, Co2+", Co3+, Cu+, Cu2+, Pb2+", Pb4+, Sn2+, Sn4+, Zn2+, Ni2+, and Ni3+.

[0063] In one embodiment, the TCM is a metal oxalate in which the metal ions can be any positively charged ion of any charge magnitude in which foreign ions are introduced to affect the thermodynamic properties.

[0064] In one embodiment, the metal ions of the TCM are partially substituted by a second metal ion to affect the material thermodynamic properties.

[0065] In one embodiment, where the metal ions of the TCM are partially substituted by two or more types of metal ions to affect the material thermodynamic properties.

[0066] In one embodiment, where the metal ions of the TCM are partially substituted by similar ions of different charges to affect the material thermodynamic properties.

[0067] In one embodiment, where the metal ions of the TCM are introduced without substitution to affect the material thermodynamic properties.

[0068] In one embodiment, where the TCM with substituted metal ions demonstrates a different energy density compared to the TCM without substituted metal ions.

[0069] In one embodiment, where the TCM with substituted metal ions demonstrates a different power density compared to the TCM without substituted metal ions.

[0070] In one embodiment, where the TCM with substituted metal ions demonstrates a different water loading compared to the TCM without substituted metal ions.

[0071] Tn one embodiment, where the TCM with substituted metal ions demonstrates a different enthalpy per mole of water compared to the TCM without substituted metal ions.

[0072] In one embodiment, where a heat storage system comprises one of more of the TCMs described in the application.

[0073] In one embodiment, where a heat storage system comprises one of more of the TCMs described in the application, and is used in beds in convective thermochemical reactors and in vacuum reactors.

[0074] In one embodiment, where a heat storage system comprises one of more of the TCMs described in the application, and is used in a chemical heat pump.Examples

[0075] The Examples below are prepared by solid solution method in which ions from the base TCMs are substituted by other ions to affect the thermodynamic properties and enhance the materials key performance indicators.

[0076] Example 1: CaC2O4with 5 mol% of Ca ions substituted for Mg ions was synthesized according to the following method. A diluted solution of CaC2O4was added dropwise to a premade solution of MgCh and CaCh under stirring at 70 °C for 1 hour. Afterwards the solution is filtrated using qualitative filter paper, washed with water, and dried at 60 °C overnight.Thermogravimetric analysis of Example 1 shows a shift in phase line (FIG. 1) while being stable for 30 hydration-dehydration cycles. A shift in thermogravimetric analysis indicates a change in the thermal properties of the material, which can result from changes in, e.g., composition, phase transitions, and / or reactions. Such a shift can help with the understanding these changes and their implications for the TCM’s behavior and application.

[0077] Example 1 also shows an enthalpy per mole of water increase of 8.8% compared to pure CaC2O4made under similar conditions: 80.4 kJ / mol water versus 73.9 kJ / mol water (8.8%). The power of the material is determined as 2500 W / kg.

[0078] Example 2: CaC2O4with 20 mol% of Ca ions substituted for Mg ions was synthesized according to the following method. A diluted solution of Na2C2O4was added dropwise to a premade solution of MgCb and CaCh under stirring at 70 °C for 1 hour. Afterwards the solution is filtrated using qualitative filter paper, washed with water, and dried at 60 °C overnight. Thermogravimetric analysis of Example 2 shows a shift in phase line (FIG. 2) with an enthalpy per mole of water increase of 6.5% compared to pure CaC2O4made under similar conditions: 78.7 kJ / mol water versus 73.9 kJ / mol water (6.5%).

[0079] Example 3: CaC2O4with 5 mol% of Ca ions substituted for Mg ions was synthesized according to the following method. A diluted solution of Na2C2O4was added dropwise to a premade solution of MgCh and CaCh under stirring at 70 °C for 1 hour. The diluted solution of Na2C2O4is more concentrated than those of Examples 1 and 2. Afterwards the solution diluted and filtrated using qualitative filter paper, washed with water, and dried at 60 °C overnight. Thermogravimetric analysis of Example 3 shows a shift in phase line. An enthalpy per mole of water increase of 29.9% compared to pure CaC2O4made under similar conditions.

[0080] Example 4: CaC2O4with 20 mol% of Ca ions substituted for Mg ions was synthesized according to the following method. A diluted solution of Na2C2O4was added dropwise to a premade solution of MgCh and CaCh under stirring at 70 °C for 1 hour. The diluted solution of Na2C2O4is more concentrated than those of Examples 1 and 2. Afterwards the solution diluted and filtrated using qualitative filter paper, washed with water, and dried at 60 °C overnight. Thermogravimetric analysis of Example 4 shows a shift in phase line. An enthalpy per mole of water increase of 7.8 % is seen compared to pure CaC2O4made under similar conditions.

[0081] Example 5: CaC2O4with 5 mol% of Ca ions substituted for Cu ions was synthesized according to the following method. A diluted solution of Na2C2O4was added dropwise to apremade solution of CuCh and CaCb under stirring at 70 °C for 1 hour. The diluted solution of Na2C2O4is more concentrated than those of Examples 1 and 2. Afterwards the solution diluted and filtrated using qualitative filter paper, washed with water, and dried at 60 °C overnight. The thermogravimetric analysis of Example 5 shows a shift in phase line. An enthalpy per mole of water increase of 16.9 % is seen compared to pure CaC2O4made under similar conditions.

[0082] Example 6: In some embodiments, a TCM is prepared according to the synthesis methods described in Examples 1-5, wherein CaC2O4comprises a partial substitution of Ca ions with Mg ions. The degree of substitution may be 0.2 mol%, 0.5 mol%, 0.7 mol%, 1 mol%, 5 mol%, 10 mol%, 20 mol%, 40 mol%, 50 mol%, 60 mol%, 75 mol%, or 90 mol%. In terms of ranges, the degree of substitution may range from 0.2 mol% to 90 mol%, e.g., from 0.2 mol% to 0.5 mol%, from 0.5 mol% to 0.7 mol%, from 0.7 mol% to 1 mol%, from 1 mol% to 5 mol%, from 5 mol% to 10 mol%, from 10 mol% to 20 mol%, from 20 mol% to 40 mol%, from 40 mol% to 50 mol%, from 50 mol% to 60 mol%, from 60 mol% to 75 mol%, or from 75 mol% to 90 mol%. Such substitution enables tuning of the thermochemical transition temperature within a range of 0 to 5 °C relative to unsubstituted CaC2O4

[0083] Example 7: In some embodiments, a TCM is prepared according to the synthesis methods described in Examples 1-5, wherein CaC2O4comprises a partial substitution of Ca ions with 1 mol% of a second metal ion selected from Mn2+, Mn3+, Mn4+Mn6+, Mn7+, Fe2+, Fe3+Co2+", Co3+, Cu+, Cu2+, Pb2+, Pb4+, Sn2+, Sn4+, Zn2+, Ni2+, and Ni3+. Such substitution enables tuning of the thermochemical transition temperature within a range of 0 to 5 °C relative to unsubstituted CaC2O4

[0084] In some embodiments, the partial substitution of the first metal ion with the second metal ion at low concentrations (e.g., 1 mol%) enables adjusting of the thermochemical transition temperature of the resulting material. This substitution can shift the transition temperature by approximately from 0 to 5 °C relative to unsubstituted TCM, depending on the identity and oxidation state of the substituting ion. Such tunability allows for precise control over the thermal behavior of the material, which may be advantageous in applications requiring specific thermal thresholds, such as thermal energy storage, temperature-sensitive release systems, or thermally activated chemical processes. The thermochemical transition temperature of the TCM can be from 0 to 5 °C , e.g., from 0.2 °C to 0.8 °C, from 0.6 °C to 1.4 °C, from 1.0 °C to 1.7 °C, from 1.3 °C to 2.9 °C, from 1.8 °C to 2.6 °C, from 2.0 °C to 3.0 °C, from 2.5 °C to 3.8 °C, from 2.9 °C to 4.1 °C, from 3.0 °C to 4.2 °C, from 3.5 °C to 4.9 °C, or from 4.0 °C to 5.0 °C.

[0085] Changes may be made in the above compositions and processes without departing from the scope hereof. It should thus be noted that the matter contained in the above description or shown in the accompanying drawings should be interpreted as illustrative and not in a limiting sense. The following claims are intended to cover all generic and specific features described herein, as well as all statements of the scope of the present method and system, which, as a matter of language, might be said to frill therebetween.

Claims

CLAIMSWhat is claimed is:

1. A thermochemical material composition for energy storage, the composition comprising: a metal salt having the chemical formula of Mx-bNbAy. zH2O wherein M is a metal ion, N is a second metal ion different from M, and A is an anion; wherein x is the number of M per formula unit, y is the number of anions per formula unit, b is the number of N per formula unit, z is the number of water molecules associated with the metal salt.

2. The composition of claim 1, wherein the metal ion (M) comprises Ca2+, Mg2+, Na*, Cu2+, Zn2+, K*, Li*, Fe2+, Fe3+, Ni2+, Mn2+, Mn3+, and Al3+.

3. The composition of claim 1, wherein the second metal ion (N) comprises Ca2+, Mg2+, Na+, Cu2+, Zn2+, K+, Li+Fe2+, Fe3+, Ni2+, Mn2+, Mn3+, and Al3+.

4. The composition of claim 1, wherein the anion (A) comprises oxalate, acetate, permanganate, dichromate, chromate, thiocyanate, cyanide, perchlorate, nitrite, bromide, iodide, sulphite, oxide, hydroxide, carbonate, sulphate, chloride, nitrate, phosphate, sulfide, fluoride, bicarbonate, malonate, succinate, glucarate, adipate, tartrate, suberate, fumarate, citrate, gluconate, and stearate.

5. The composition of claim 1, wherein the metal salt further comprises cations that are different from M and N.

6. The composition of claim 1, wherein the mole ratio of M;N is less than 99.8:0.2, preferably 95:5.

7. The composition of claim 1, wherein the composition demonstrates a power density of greater than 50 W / kg.

8. The composition of claim 1, wherein the composition demonstrates an energy density of greater than 400 kJ / kg.

9. The composition of claim 1 , wherein the composition demonstrates a cyclic stability for greater than 10 cycles.

10. The composition of claim 1, wherein the composition demonstrates an enthalpy increase per mole of water of greater than 5% compared to MxAyzH2O .

11. The composition of claim 1, wherein the metal salt is CaC2O4-H2O with from 0.2 to 90 mol% of Ca2+substituted by Mg2+.

12. The composition of claim 1, wherein the metal salt is CaC2O4-H2O with 5 mol% of Ca2+substituted by Mg2+13. The composition of claim 1, wherein the metal salt is CaC2O4-H2O with 20 mol% of Ca2+substituted by Mg2+.

14. The composition of claim 1, wherein the metal salt is CaC2O4-H2O with 5 mol% of Ca2+substituted by Mg2+and having a cyclic stability for at least 30 hydration-dehydration cycles.

15. The composition of claim 1, wherein the metal salt is CaC2O4-H2O with 1 mol% of Ca2+substituted by the second metal ion selected from the group consisting of Mn2+, Mn3+, Mn4*, Mn6+, Mn7*, Fe2’, Fe3+, Co2+, Co3+, Cu*, Cu2+, Pb2+, Pb4*, Sn2+, Sn4*, Zn2+, Ni2+, and Ni3+.

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