Apparatus for energy storage using high-entropy oxides with frequency-dependent dielectric properties

Abstract

Disclosed herein is a charge storage device comprising a first electrode, a second electrode, an intermediate layer disposed between the first and second electrodes, and a controller for controlling an operating frequency of the charge storage device, wherein each of the first and second electrodes comprise a high- entropy oxide (HEO), the intermediate layer comprises a HEO, or each of the first electrode, the second electrode and an intermediate layer comprises a HEO. The HEO has operating frequency dependent dielectric properties, and the controller is configured to selectively switch the operating frequency between a low frequency mode for energy storage and a high frequency mode for charging and discharging.

Description

[0001] APPARATUS FOR ENERGY STORAGE USING HIGH-ENTROPY OXIDES WITH FREQUENCY-DEPENDENT DIELECTRIC PROPERTIES

[0002] Technical Field

[0003] The present disclosure relates to advanced energy storage technologies. More particularly, the present disclosure relates to high-entropy oxides (HEOs) with frequency-dependent dielectric properties for energy storage applications such as in supercapacitors.

[0004] Background

[0005] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

[0006] High-entropy oxides (HEOs) represent an emerging class of materials that have attracted increasing interest from researchers worldwide due to their distinctive properties and potential applications. These materials are characterised by their entropy-stabilised nature, comprising different metal cations in equiatomic or near-equiatomic ratios, which combine to form a stable single-phase structure. First introduced in 2015, a growing body of research has explored the synthesis and properties of HEOs, leading to the successful fabrication of various singlephase oxides with different crystal lattice structures, including rock salt, spinel, fluorite, and perovskite types. The tunability of composition and structure in HEOs provides unique opportunities for tailoring their physical, chemical and functional properties for a wide range of applications in energy storage, catalyst, insulation and other functional applications.

[0007] Among them, rock-salt-type HEOs stand out as the most entropy-stabilised subset of HEOs with high dielectric properties, high lithium-ion conductivity and excellent catalytic performance. Several methods have been employed to synthesis HEOs, including sol-gel, hydrothermal, thermal plasma, ball-milling, solvothermal, solid-state synthesis, and others. While each of these techniques offers specific advantages depending on the target application, they typically involve complex processes and high-temperature sintering steps to stabilise the desired phase, which limits their scalability and feasibility for commercial production.

[0008] Modem technological advancements have significantly increased the demand for efficient and reliable energy storage systems. Various sectors, including renewable energy integration, electric mobility, and portable electronics, rely on energy storage devices that can address today's complex power requirements. Traditionally, energy storage solutions have been developed using conventional materials and well-established architectures that, while effective in some respects, struggle to meet the evolving performance expectations. The need for enhanced systems that not only store energy effectively but also provides rapid power delivery under diverse operational conditions is becoming increasingly evident. This growing interest in refined storage methods is prompting a broader exploration of novel compositions and configurations capable of balancing energy accumulation with dynamic energy release.

[0009] Industry and research communities are focusing on the development of energy storage devices that deliver both high energy storage capacity and swift power discharge. Modern applications require systems that can adapt to varying conditions. Achieving a harmonious balance between energy storage and rapid energy transfer is important for applications ranging from grid-scale storage to consumer electronics. In these environments, system efficiency, reliability, and operational flexibility are of great importance. Broadening the frequency response capabilities while maintaining high performance is necessary for advancing energy storage technologies and ensuring that they meet the stringent demands of today's applications.

[0010] Despite ongoing efforts in refining energy storage approaches, many conventional systems continue to face significant challenges. Established methods frequently exhibit performance limitations across the range of operational frequencies required by modern applications. In many cases, the materials and configurations in use do not adequately support dynamic switching between energy storage and power delivery functions. This imbalance can lead to reduced energy density, inefficiencies during charge and discharge cycles, and overall system instability. The limitations of these conventional systems underscore a major challenge: achieving a consistent, high- performance response that addresses both storage capacity and rapid energy discharge requirements in a single configuration.

[0011] A particularly pressing issue within the energy storage domain is the difficulty of tailoring component performance to effectively manage varying operational frequencies. Devices that need to support low-frequency energy accumulation while concurrently enabling high-frequency energy output often to encounter operational inconsistencies. This limitation hinders the ability to enhance both storage efficiency and response speed in applications where time-sensitive power delivery is important. There is a clear demand for innovative strategies that overcome these performance gaps. Addressing this challenge involves investigating new material structures and design methodologies that can dynamically adapt to diverse energy management scenarios, bridging the gap between basic material properties and the practical requirements of advanced energy storage systems.

[0012] It would be desirable to provide an apparatus capable of ameliorating one or more of the abovementioned disadvantages with existing energy storage and accumulation technologies, or that at least provides a useful alternative.

[0013] Summary

[0014] In view of the limitations in prior art, apparatuses are herein described that use high-entropy oxides (HEOs) the dielectric constant of which are frequency dependent. These technologies facilitate rapid charge and discharge, which also maintaining stable energy storage conditions.

[0015] In one embodiment, the disclosure includes an apparatus for energy storage comprising a first electrode, a second electrode, and an intermediate layer disposed between them, wherein the first and second electrodes comprise a high-entropy oxide. In some embodiments, the apparatus further includes an operation frequency such that varying the frequency changes the dielectric properties of the electrodes, with the electrodes behaving as dielectric layers at a low frequency and as conductors at a high frequency. In some embodiments, the intermediate layer is implemented as either a dielectric layer or a high- entropy oxide layer, and in further embodiments, the high-entropy oxides form built-in p-n junction networks constituted by oxygen-rich grain shells and oxygen-poor grain centres.

[0016] Built-in p-n junction networks refer to structures within a material where regions of p-type and n-type semiconductors are formed inherently due to variations in composition or microstructure. In the context of high-entropy oxides (HEOs), these networks are created by oxygen-rich grain shells forming p-type regions and oxygen-poor grain cores constituting n-type regions. These junctions facilitate charge separation and transport, enhancing the material's electrochemical properties for applications such as energy storage.

[0017] In another embodiment, the disclosure includes an apparatus for energy storage comprising a first electrode, a second electrode, and an electrolyte layer disposed between them, wherein the electrolyte layer is a high-entropy oxide electrolyte. In some embodiments, the apparatus further includes an operation frequency whereby varying the frequency alters the dielectric properties of the electrolyte layer, such that the layer behaves as a dielectric at a low frequency and as an ion conductor at a high frequency. In some embodiments, each high- entropy oxide layer is formed from a plurality of constituent metal oxides in substantially equal atomic proportions and exhibits frequency-dependent dielectric properties.

[0018] In yet another embodiment, the disclosure includes a charge storage device comprising a first electrode, a second electrode, an intermediate layer disposed between them, and a controller for controlling an operating frequency of the charge storage device. In some embodiments, the first and second electrodes comprise a high-entropy oxide, or the intermediate layer comprises a high- entropy oxide, or all these layers comprise a high-entropy oxide. The controller is configured to selectively switch the operating frequency between a low frequency mode for energy storage and a high frequency mode for charging and discharging. In further embodiments, the controller includes a sensor for measuring a performance parameter of the supercapacitor, enabling the operating frequency to be adjusted based on the measured parameter.

[0019] In some embodiments, the disclosure includes a charge storage device wherein the high-entropy oxide is synthesised by mixing each of a plurality of metal oxide powders with deionised water, in a predefined molar ratio, to obtain a solution, stirring the solution, feeding the solution into a spray drying system at a predefined feed rate and evaporation temperature, to produce a spray-dried powder, and collecting and calcining the spray-dried powder, using a muffle furnace, at a predefined calcination temperature and ramping rate, to produce a plurality of calcined powder samples, wherein the plurality of metal oxide powders comprises at least five different metal oxide powders, and wherein each of the at least five different metal oxide powders occur in substantially equiatomic ratios.

[0020] In some embodiments, the disclosure includes a supercapacitor comprising the charge storage device.

[0021] In some embodiments, the disclosure includes a method for synthesising high- entropy oxides, comprising mixing each of a plurality of metal oxide powders with deionised water, in a predefined molar ratio, to obtain a solution, stirring the solution, feeding the solution into a spray drying system at a predefined feed rate and evaporation temperature, to produce a spray-dried powder, and collecting and calcining, using a muffle furnace, at a predefined calcination temperature and ramping rate, to produce a plurality of calcined powder samples, wherein the calcined powder samples comprise high-entropy oxides (HEOs) having operational frequency dependent dielectric properties.

[0022] Brief description of the drawings

[0023] Embodiments of the claimed invention will now be described, by way of nonlimiting example, with reference to the drawings in which : Figure 1 shows a schematic of a high-entropy-oxide (HEO)-based supercapacitor, according to a preferred embodiment of the claimed invention.

[0024] Figure 2 shows a schematic of domain segregation within HEO grains, according to an aspect of the claimed invention.

[0025] Figure 3 shows a schematic of a charge storage device comprising a first electrode, a second electrode, a HEO dielectric layer, and a frequency controller module, according to a preferred embodiment of the invention.

[0026] Figure 4 shows X-ray diffraction (XRD) spectra of calcined HEO powder samples.

[0027] Figure 5 shows scanning electron microscopy (SEM) images of pellet samples, in 5,000x and 20,000x magnification.

[0028] Figure 6 shows energy-dispersive X-ray spectroscopy (EDS) maps of identified elements of a pellet sample with short shoot time, and comprises (a) an electron image, element maps of (b) O, (c) Mg, (d) Ni, (e) Co, (f) Cu, (g) Zn, and (h) a comparison between the Cu and Zn elements.

[0029] Figure 7 shows EDS maps of a second HEO site with short shoot time, and comprises (a) an electron image, element maps of (b) 0, (c) Mg, (d) Ni, (e) Co, (f) Cu, and (g) Zn.

[0030] Figure 8 shows EDS maps of a third HEO site with short shoot time, and comprises (a) an electron image, element maps of (b) 0, (c) Mg, (d) Ni, (e) Co, (f) Cu, and (g) Zn.

[0031] Figure 9 shows EDS maps of a first (as first illustrated in Figure 6) HEO site with long shoot time, and comprises (a) an electron image, element maps of (b) 0, (c) Mg, (d) Ni, (e) Co, (f) Cu, and (g) Zn.

[0032] Detailed description Energy storage technologies play an important role in addressing the growing demand for efficient and sustainable energy solutions. As the world transitions towards renewable energy sources, electric vehicles, and advanced electronics, the need for high-performance energy storage systems becomes increasingly important. Supercapacitors, known for their high-power density and rapid charge / discharge capabilities, have emerged as promising candidates for various applications. However, despite their advantages, supercapacitors face significant challenges in terms of energy storage capacity, efficiency, and adaptability to fluctuating energy demands.

[0033] Current supercapacitor technologies often struggle with limitations such as reduced energy density compared to batteries, which impacts their ability to store energy over extended periods. Additionally, the charge / discharge rates of conventional supercapacitors may not be optimal for all applications, particularly those requiring rapid energy delivery or high-power density. Furthermore, existing materials used in supercapacitors may not provide the versatility needed to operate efficiently across a wide range of frequencies, limiting their effectiveness in diverse applications.

[0034] The present disclosure addresses these challenges by introducing a novel approach to energy storage using high-entropy oxides (HEOs) with frequencydependent dielectric properties. This method leverages the distinct characteristics of HEOs to enhance the performance of supercapacitors, offering a wide range of dielectric constants and moderate dielectric losses across frequencies from about 1 kHz to about 1 MHz. By integrating HEOs into the electrode materials of supercapacitors, either as active materials or additives, the approach optimises energy storage capacity, power density, and efficiency across a broad spectrum of operating frequencies. This innovative method provides tailored energy storage solutions, enabling rapid charge / discharge rates and improved energy efficiency, thereby overcoming the limitations of traditional supercapacitor technologies.

[0035] High-entropy oxides (HEOs) as used in the present invention shall be understood to be generally characterised by their entropy-stabilised nature. HEOs typically comprise five, or more than five, different metal cations. The metal cations may be in equiatomic (i.e. atomic species are present in equal proportions or ratios) or near equiatomic ratios, which combine to form a stable single-phase crystal structure. In high-entropy oxide (HEO) systems, "near- equiatomic" typically refers to compositions in which the molar fraction of each principal element lies within about 5 to about 10 atomic % of an equiatomic ratio. HEOs have a chemical formula in the form of (MIM2M3M4MS)XOX, where Mi, M2, M3, M4 and Ms each refer to a different metal cation.

[0036] In some embodiments, the HEO is (MgCoNiCuZn)O, which comprises both p- type and n-type conducting components. The HEO may also be (MgNiCoFeMn)O, (MgNiCuZnMn)O, (MgFeCoMnCu)O, (NiCoCuZnMn)O, (MgNiCuFeZn)O, (CoNiCuFeMn)O, (MgMnNiCuZn)O, (MgCoNiZnFe)O, (MgNiZnMnFe)O, or (MgNiCuZnAI)O with entropy-stabilised rock-salt phases. These HEO systems possess a high concentration of structural defects, which may contribute to polarisation mechanisms.

[0037] In some embodiments, where the HEO is (MgCoNiCuZn)O, defects typically arise from each of the constituent oxides. CuO, NiO and CoO are typically known for their intrinsic p-type conductivity. This may be driven by the vacancies in Cu, Ni and Co, respectively, which in turn tends to generate hole carriers. Conversely, ZnO and MgO are typically known for their intrinsic n-type conductivity, primarily due to oxygen vacancies and zinc interstitials (in the case of ZnO) that donate electrons to the conduction band. These oxygen vacancies act as electron donors, providing free electrons that enhance the n-type behaviour. Hence, both p-type and n-type conducting components may be observed in (MgCoNiCuZn)O.

[0038] A mixture of the five or more cations giving rise to a high configurational entropy, stabilises the mixture into a single-phase structure, even at high temperatures. Due to the high entropy, HEOs typically have high ionic conductivity, enhanced thermal stability and tuneable electronic / magnetic junction.

[0039] In some embodiments, the high-entropy oxides (HEOs) are prepared via a spray-drying process. Beginning with analytical grade metal oxide precursors, the metal oxide powders may be mixed with a 1 : 1 molar ratio (i.e. equimolar) in 250 mL of deionised water. The molar ratios may also be in the range of about 10 to about 20 atomic % variation from equimolar ratios for near- equiatomic HEOs. This variation may be larger for non-equiatomic HEOs. The deionised water may be obtained from a lab water system. A solution is obtained, and kept stirring at room temperature for one hour until a clear solution was obtained. The solution may have a concentration of 40 g / L. These metal oxide precursors may be cobalt nitrate hexahydrate, nickel nitrate hexahydrate, zinc nitrate hexahydrate, copper nitrate hexahydrate, or magnesium nitrate hexahydrate. The metal oxide precursors may be used without additional purification steps, or they may be further purified before use.

[0040] The well-mixed metal oxide precursor solution may then be fed into a spray drying system at a feed rate of 100 mL / h and 220 °C evaporation temperature. Feed rates of between 50 mL / h and 30000 mL / h may be used for spray-drying. Evaporation temperatures of between about 100 °C and 500 °C may be used for spray-drying. The spray-dried powder was then collected and calcined using a muffle furnace. Calcination may occur at temperatures of 500, 700, 900 and 1000 °C for 4 h, with a ramping rate of 5 °C / min to obtain high-entropy oxide powder samples. Calcination may also occur at temperatures of between about 400 °C and about 1500 °C, to give rise to high-entropy oxide powder samples.

[0041] Advantageously, the spray-drying process is straightforward and efficient, offering a potential for large-scale production of HEOs with greater ease and lower processing costs.

[0042] For microstructural and dielectric characterisation, the HEO pellets are typically fabricated via a two-step calcination process to avoid crack formation during calcination. Spray-dried powder may first be calcined at an intermediate temperature of 400 °C in a muffle furnace for 120 min with a temperature ramping rate of 5 °C / min. 0.3 g of the resulting powder was measured and pressed into a pellet using a 13 mm diameter KBR. die mold with a hydraulic press at a force of 1 ton, for 20 min (compaction time). The obtained pellet was further calcined at 1000 °C for four hours with a 5 °C / min ramping rate. Disclosed herein is a high-entropy oxide (HEO)-based supercapacitor (100). Figure 1 shows a schematic of a HEO-based supercapacitor (100), according to an embodiment of the claimed invention. The HEO-based supercapacitor may be used as an apparatus for energy storage. In some embodiments, the HEO- based supercapacitor (100) comprises a first electrode (102), a second electrode (104) and an intermediate layer (106) disposed between the first electrode (102) and the second electrode (104). Each of the first electrode (102) and the second electrode (104) may comprise a HEO having operational frequency dependent dielectric properties.

[0043] Figure 2 shows a schematic of domain segregation within HEO grains, according to an aspect of the claimed invention. The HEO grains possess an observed strong variation in dielectric properties with frequency. Such an observation may be explained by the schematic of domain segregation within HEO grains. Grain boundaries (202) and domain interfaces (204) have been marked out accordingly. These interfaces, in turn, facilitate two key polarisation mechanisms.

[0044] In a first polarisation mechanism, also referred to as a primary polarisation mechanism, these domain interfaces serve as regions where charge carriers (i.e. electrons and holes) can accumulate, resulting in an interfacial polarisation. Interfacial polarisation may also be referred to as Maxwell-Wagner-Sillars polarisation. Interfacial polarisation notably enhances the dielectric constant at low frequencies due to the charge separation at domain interfaces. Additionally, the random distribution of domains results in a random arrangement of p-n- type junctions between them, which amplifies charge accumulation at domain interfaces and consequently enhances the material's dielectric response. Such a complex distribution of p-type and n-type regions produces a distinctive dielectric behaviour. This distinguishes the HEOs from simple binary or ternary oxides.

[0045] The p-n junctions at the interfaces of semiconductor domains in HEOs contribute to interfacial polarisation by creating localised electric fields that promote charge separation and accumulation. The primary parameters influencing this behaviour include band alignment at interfaces, charge carrier density differences, and junction capacitance. The multi-element nature of HEO introduces a range of band offsets and junction capacitances due to the varying electronic properties of the constituent cations. The diversity in HEOs amplifies interfacial polarisation due to multiple relaxation mechanisms occurring simultaneously. The presence of interfaces with different band alignments and local chemistries results in synergistic contributions to the dielectric constant.

[0046] In a second polarisation mechanism, also referred to as a secondary polarisation mechanism, the grain boundaries in HEO act as barriers to carrier mobility at low frequencies, promoting charge accumulation at the boundaries and facilitating space charge polarisation across grains. Similar polarisation mechanisms and tuneable material designs have been employed in advanced functional materials, exhibiting dielectric relaxation for multifunctional applications.

[0047] At low frequencies, free charge carriers have sufficient time to respond to the applied electric field, activating both the primary and secondary polarisation mechanisms. In contrast, at higher frequencies, these polarisation mechanisms weaken as the charge carriers can no longer keep pace with the rapid oscillations of the electric field. However, these polarisation mechanisms may also contribute to an undesirable high dielectric loss observed at lower frequencies. The accumulation and trapping of charges lead to energy dissipation, typically as heat, thereby increasing the dielectric loss.

[0048] This effect may be further intensified by Zn precipitation at the grain boundaries, which enhances charge accumulation in these regions. It appears that resistivity in grain boundaries contributes more to high dielectric losses than the grains themselves. Conversely, the reduced charge accumulation at higher frequencies results in lower energy dissipation. This may explain the significantly lower dielectric loss observed at higher frequencies.

[0049] While metals (e.g. Cu) tend to segregate near grain boundaries, when placed in a binary and multicomponent system, this behaviour appears to be reversed in HEOs. In a five-component HEO system, which may take the form of (MIM2M3M4M5)XOX, where Mi, M2, M3, M4 and Ms each refer to a different cation, metals such as Zn preferentially occupy the regions near grain boundaries, while Cu tends to remain within the grains. This may have a significant impact on the polarisation mechanism and contributes significantly to the strong frequency dependence observed in the HEO system and the elevated dielectric loss at low frequencies.

[0050] At low frequencies, interfacial polarisation at p-n domains boundaries and space charge polarisation at grain boundaries act synergistically, both contributing to charge accumulation at the interfaces. This behaviour aligns with the Maxwell- Wagner-Sillars theory, commonly used to analyse dielectric behaviour. Space charge polarisation often leads to significantly higher dielectric losses, which are orders of magnitude larger than the dielectric response due to molecular fluctuations, with the extent of these losses increasing in materials with more prominent phase boundaries.

[0051] Conversely, interfacial polarisation may exhibit the highest dielectric constant with the lowest dissipation factor among polarisation mechanisms. The significantly higher dielectric loss observed at low frequencies suggests that space charge polarisation is the dominant mechanism, contributing more substantially to the observed effects, as compared to interfacial polarisation.

[0052] In some embodiments, the operating frequency has a low frequency and a high frequency. The first electrode (102) and the second electrode (104) behave as dielectric layers at the low frequency and as conductors at the high frequency. The low frequency may be about 1 kHz, and the high frequency may be about 1 MHz. The low frequency may be between about 10 Hz to about 100 kHz, and the high frequency may be between about 1 MHz to about 10 GHz.

[0053] In some embodiments, the intermediate layer is a dielectric layer.

[0054] In some embodiments, the intermediate layer is a HEO layer with dielectric properties.

[0055] In some embodiments, the HEOs of the first electrode (102) and the second electrode (104) comprise built-in p-n junction networks formed by oxygen-rich grain shells and oxygen-poor grain centres. The oxygen-rich grain shells may occur at the p-type semiconductor domains, such as at Cu, Ni and Co. The oxygen-poor grain centres may occur at the n-type semiconductor domains, such as at Zn and Mg.

[0056] Also disclosed herein is an apparatus for energy storage (100) comprising a first electrode (102), a second electrode (104) and an electrolyte layer (106), disposed between the first electrode (102) and the second electrode (104), wherein the electrolyte layer (106) is a high-entropy oxide (HEO) electrolyte having operational frequency dependent properties.

[0057] In some embodiments, the operating frequency has a low frequency and a high frequency, the electrolyte layer behaving as a dielectric at the low frequency and as an ion conductor at the high frequency. The low frequency may be about 1 kHz, and the high frequency may be about 1 MHz. The low frequency may be between about 1 kHz to about 100 kHz, and the high frequency may be between about 0.1 MHz to about 100 MHz.

[0058] According to an aspect of the claimed invention, each HEO layer may be formed from a plurality of constituent metal oxides, in substantially equal (or near equal) atomic proportions and exhibiting frequency-dependent dielectric properties.

[0059] Disclosed herein is also a supercapacitor comprising a first electrode, a second electrode, an intermediate layer disposed between the first and second electrodes, and a controller for controlling an operating frequency of the supercapacitor. As set out above, either each of the first electrode and the second electrode comprises a high-entropy oxide (HEO), the intermediate layer comprises a HEO, or both. The HEO in each case has operating frequency dependent dielectric properties, such that charge / discharge and charge storage preferentially occur at different operating frequencies. Figure 3 shows a schematic of a charge storage device, in the form of a supercapacitor, according to a preferred embodiment of the invention. The frequency controller module may be configured to selectively switch the operating frequency of the device between a low frequency mode for energy storage and a high frequency mode for charging and discharging.

[0060] In some embodiments, the controller comprises a sensor for measuring a performance parameter of the supercapacitor, the controller being configured to adjust the operating frequency based on a measured performance parameter (e.g., during charging, an amount of charge in the supercapacitor or that the supercapacitor is fully charged (switch from high frequency to low frequency), that the charge of the supercapacitor needs to be used (switch from low frequency to high frequency), a resistance across the supercapacitor (indicating an amount of charge held by the supercapacitor) and others). The controller is an external electronic module designed to regulate the operating frequency of the supercapacitor. It selectively switches between low-frequency operation (e.g., about 1 kHz) for energy storage mode, where the HEO layers behave primarily as dielectrics, and high-frequency operation (e.g., about 1 MHz) for charge / discharge mode. The controller may include a frequency generator and switching circuitry, enabling dynamic modulation of the dielectric response in real time. This frequency control enables the supercapacitor to adapt its functionality based on application requirements, offering tuneable performance across different operational modes.

[0061] Example 1 - Characterisation

[0062] A Bruker D8 Advance X-ray diffractometer with Cu K radiation (A = 1.54 A) was employed to obtain the X-ray diffraction (XR.D) spectrum of the samples. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) measurements were also conducted with a JEOL-6700 SEM model, operating at an accelerated voltage of 15 kV with an EDS detector. The samples were hot-etched using a high-temperature furnace at 900 °C with a heating rate of 10 °C / min and a holding time of 10 min. For measurement of the dielectric properties, silver electrodes were printed on both sides of the HEO pellets and dried at 130 °C for 10 minutes. Capacitance and dielectric loss tangents (tan 5) were then measured.

[0063] The real and imaginary parts of the dielectric permittivity (E) were determined. The dielectric permittivity is a complex quantity that is expressed by the following equation:

[0064] E = E' - j£" where E' and E" represent the dielectric constant and the dielectric loss of the material, respectively. The dielectric constant reflects how the material stores energy, or its polarising capability, while the dielectric loss signifies the energy dissipation of the material, E" was calculated using the following: s" = s' tan 5 tan 6, the dielectric loss tangent, measures the energy dissipation in a dielectric material when it is subjected to a varying electric field, such as in capacitors or high-frequency electronics.

[0065] Example 2 - X-ray diffraction analysis

[0066] Figure 4 shows the X-ray diffraction (XR.D) spectra of calcined HEO powder samples, calcined at different temperatures. It was observed, during calcination, that the colour of the samples changed from grey to black. This suggests changes in the electronic structure. At 700 °C, the sample exhibited a multiphase mixture of rock salt and other minor peaks of tenorite (CuO), spinel (CO3O4) and wurtzite (ZnO) phases. As the temperature of calcination increases above 900 °C, the minor phases vanished, and powders were transformed into a single-phase rock salt face-centred cubic (FCC) structure with space group of Fm3m.

[0067] The observed phase has been classified as the most entropy-stabilised subset of HEOs. Diffraction peaks at 36.69, 42.66, 61.88, 74.18 and 78.060are consistent with the lattice planes of (111), (200), (220), (311) and (220), of the HEOs.

[0068] Figure 4 also suggests that the crystallinity of the HEO powder increases as the calcination temperature increases, as evidenced by the narrowing and sharpening of diffraction peaks, as the temperature increases from 500 °C to 1000 °C. This suggests that a calcination temperature of 1000 °C would yield the most crystalline HEO powder, among the four calcination temperatures. Advantageously, the two-step calcination process was also found to aid in preparing HEO pellets, without the need to add any binder component to the powder.

[0069] Example 3 - Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray spectroscopy (EDS) analysis

[0070] Figure 5 shows the scanning electron microscope (SEM) images of the fabricated pellet samples under two different magnifications - 5,000x and 20,000x. Irregular particles of different sizes were observed to be combined when HEO powder forms into the pellet form. The grain boundaries were also clearly observed in the magnified image.

[0071] Figure 6(a) shows an electron image of a pellet sample, and Figures 6(b)-(g) show the energy-dispersive X-ray spectroscopy (EDS) elemental maps of the pellet samples, providing information on the elemental distribution. Six elemental maps were obtained for each scan, namely for the five metals (Mg, Ni, Co, Cu and Zn) and oxygen (O).

[0072] Figures 6(b)-(g) suggest that all the elements are rich in different domains within the grains of the microstructure, and notably, Zn occurs mostly in the grain boundaries. Further, Figure 6(h) illustrates a comparative elemental positioning of Cu and Zn elements in electron images, indicating that Zn tends to occupy regions near the grain boundaries, compared to other cations like Cu, which remain in the grain.

[0073] Figures 7-9 show further EDS maps of three separate HEO sites, with short, short, and long shoot times, respectively, for the elements of O, Mg, Ni, Co, Cu and Zn. The short shoot times provide sharper spatial resolution, allowing clearer visualization of domain segregation and subtle variations in elemental distribution. In contrast, while the long shoot time results in a higher signal intensity and more crowded appearance due to increased X-ray counts, similar domain segregation features can still be discerned. This indicates that short shoot times are more effective in resolving such fine-scale heterogeneities.

[0074] The cation-rich domain segregation is mainly driven by thermodynamic and kinetic factors. At elevated temperatures, dynamic defect interactions, including vacancy redistribution and charge compensation processes, facilitate cation mobility, leading to segregation as the system seeks to minimise its free energy through strain relief and defect stabilisation. The segregation behaviour is further driven by competition for defect sites and local bond distortions. It is reported that the multi-element co-decoration of lattice defects in high-entropy oxides may give rise to unique segregation behaviours driven by the thermodynamic and kinetic competition among the interacting species.

[0075] Table 1 below presents the average dielectric property measurements of the HEO pellet samples under both high-frequency (1 MHz, 1 V) and low-frequency (1 kHz, 1 V) electrical inputs. The dielectric properties of the synthesised HEO pellets were evaluated to assess their potential applicability.

[0076] Table 1 : Dielectric property measurements

[0077] Notably, the dielectric constant and capacitance at low frequency (479.07 and 287.3 pF, respectively) are significantly higher compared with the high- frequency measurements (28.31 and 17.31 pF, respectively). A similar frequency-dependent trend is observed in the dielectric loss tangent values of 7.17 for low-frequency and 0.62 for high-frequency input.

[0078] The strong frequency-dependent behaviour may be attributed to two levels of polarisation mechanisms present within the HEO microstructure. Elemental domain segregation observed in EDS establishes interfaces between p-type semiconductor domains (Cu, Ni and Co) and n-type domains (Zn and Mg) within the HEO grains, creating six distinct p-n domain interfaces within the grains.

[0079] It will be appreciated that many further modifications and permutations of various aspects of the described embodiments are possible. Accordingly, the described aspects are intended to embrace all such alterations, modifications, and variations that fall within the spirit and scope of the appended claims.

[0080] Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

Claims1. An apparatus for energy storage, comprising: a first electrode; a second electrode; and an intermediate layer disposed between the first and second electrodes, wherein each of the first electrode and second comprises a high-entropy oxide (HEO) having operating frequency dependent dielectric properties.

2. The apparatus of claim 1, wherein the operating frequency has a low frequency and a high frequency, the first electrode and the second electrode behaving as a dielectric layers at the low frequency and as an conductors at the high frequency.

3. The apparatus of claim 1 or 2, wherein the intermediate layer is a dielectric layer.

4. The apparatus of claim 1 or 2, wherein the intermediate layer is a HEO layer.

5. The apparatus of any one of claims 1 to 4, wherein the HEOs of the first electrode and the second electrode comprise built-in p-n junction networks formed by oxygen-rich grain shells and oxygen-poor grain centres.

6. An apparatus for energy storage, comprising: a first electrode; a second electrode; and an electrolyte layer disposed between the first and second electrodes, wherein the electrolyte layer is a high-entropy oxide (HEO) electrolyte having operating frequency dependent dielectric properties.

7. The apparatus of claim 6, wherein the operating frequency has a low frequency and a high frequency, the electrolyte layer behaving as a dielectric at the low frequency and as an ion conductor at the highfrequency.

8. The apparatus of any one of claims 1 to 7, wherein each HEO layer is formed from a plurality of constituent metal oxides in substantially equal atomic proportions and exhibiting frequency-dependent dielectric properties.

9. A charge storage device comprising: a first electrode; a second electrode; an intermediate layer disposed between the first and second electrodes; and a controller for controlling an operating frequency of the charge storage device; wherein : each of the first electrode and second electrode comprises a high-entropy oxide (HEO); the intermediate layer comprises a HEO; or each of the first electrode, second electrode and intermediate layer comprises a HEO, wherein the HEO has operating frequency dependent dielectric properties, wherein the controller is configured to selectively switch the operating frequency between a low frequency mode for energy storage and a high frequency mode for charging and discharging.

10. The charge storage device of claim 9, wherein the controller comprises a sensor for measuring a performance parameter of the supercapacitor, the controller being configured to adjust the operating frequency based on a measured performance parameter.

11. A charge storage device according to claim 9 or 10, wherein the HEO is synthesised by: mixing each of a plurality of metal oxide powders with deionised water, in a predefined molar ratio, to obtain a solution;stirring the solution; feeding the solution into a spray drying system at a predefined feed rate and evaporation temperature, to produce a spray-dried powder; and collecting and calcining the spray-dried powder, using a muffle furnace, at a predefined calcination temperature and ramping rate, to produce a plurality of calcined powder samples, wherein the plurality of metal oxide powders comprises at least five different metal oxide powders, and wherein each of the at least five different metal oxide powders occur in substantially equiatomic ratios.

12. A supercapacitor comprising the charge storage device of any one of claims 9 to 11.

13. A method for synthesising high-entropy oxides, comprising: mixing each of a plurality of metal oxide powders with deionised water, in a predefined molar ratio, to obtain a solution; stirring the solution; feeding the solution into a spray drying system at a predefined feed rate and evaporation temperature, to produce a spray-dried powder; and collecting and calcining, using a muffle furnace, at a predefined calcination temperature and ramping rate, to produce a plurality of calcined powder samples, wherein the calcined powder samples comprise high-entropy oxides (HEOs) having operational frequency dependent dielectric properties.

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