WO2023111989A1 - Solid-state electrolyte and their uses - Google Patents

Abstract

The present disclosure relates to a solid-state electrolyte and their uses. In this application is disclosed a solid-state electrolyte of the formula K3OCl or of the formula 2K2O-KCl, or of the formula K3-2xMxOA, wherein 0 ≤ x ≤ 0.8; wherein M is selected from a list consisting of magnesium, calcium, strontium, barium, or combinations thereof; wherein A is selected from a list consisting of fluorine, chlorine, bromine, iodine, or combinations thereof, allowing the formation of a protective layer on both cathode and anode of said solid electrolyte.

Description

D E S C R I P T I O N

SOLID-STATE ELECTROLYTE AND THEI R USES

TECH NICAL FIELD

[0001] The present disclosure relates to a solid-state electrolyte and their uses. The present disclosure also relates to a potassium ion rich ferroelectric solid-state electrolyte and their use.

BACKGROUND

[0002] Over the years climate change and global warming pose an increasing danger to ecosystems, human health, and growing economies around the world. These changes are occurring due to the release of large amounts of greenhouse gases into the atmosphere from human activities. Mainly by burning fossil fuels to generate huge amounts of electricity and heat, as well as for transportation purposes. The burning of fossil fuels emits air pollutants that are extremely harmful to the environment and human health. Therefore, there is a great need to fulfil the gigantic demand for energy in an eco-friendly and sustainable manner [1-6], The development, production, and use of environment-friendly batteries are hence, the key to achieving a climate-neutral economy due to their important role in storing renewable energy and implementing zero-emission mobility [1-11],

[0003] Currently, the most popular rechargeable energy source is the Lithium-ion battery (LIBs). They are mainly used in portable electronic devices, electric and hybrid electric vehicles due to their high energy density, long life cycles, and high working electrical potential difference [10-15], However, to meet the development of large-scale high-capacity energy storage systems and reduce the cost of renewable energy storage, the development of alternative technologies with more economical and even better performance than that achieved by LIBs is crucial [8-9,16], The raw materials required for the production of LIBs, because of lack of abundance of lithium resources (0.0017 mass % in crust abundance), high cost to obtain, and uneven distribution in the Earth's crust, preclude the use of current LIBs as low-cost energy storage devices capable of storing energy from renewable energy sources, like solar or wind [16-19], Moreover, since commercial LIBs use liquid flammable electrolytes, these batteries cannot be freely operated at temperatures above 40°C due to the risk of thermal runaway, and thus battery explosion resulting in the release of toxic electrolyte derivatives [20-23],

[0004] Potassium-lon Batteries also have disadvantages. There is a poor diffusion of K⁺ ions in current solid electrolytes, which greatly slows down the reaction kinetics in solid-state KIBs. In addition, during the K⁺ ion de/intercalation process, the volume change of the electrode material due to the large K⁺ radius in KIBs will be larger than that of NIBs and LIBs [27-28], Due to the lower electrochemical potential of Potassium, a reduction of the solvent in the liquid electrolyte at the electrode surface in KIBs is possible, resulting in undesirable side reactions [27], Moreover, potassium, K itself has a lower melting point, c.a 63.5°C, than Na, c.a 98°C, and Li, c.a 180.5°C, and has a much higher reactivity especially with air components, i.e., oxygen and water vapor, which strongly decreases the operational safety of such batteries [27,29],

[0005] Therefore, it is important to obtain an efficient and safe electrolyte, which has a crucial part in forming protective layers on both the cathode (surface layer) and the anode (solid-electrolyte-interface layer (SEI)) [29,31],

[0006] Document CN111276734 discloses a solid electrolyte for conducting potassium ions, a preparation method and a potassium solid-state battery, and the chemical expression of the solid electrolyte is Ki-(_X/2)Ba_xTi2-xO4, where 0 < x < 2. In that document, K is an interlayer alkali metal ion; Ba is used as a high-valence cation (+2) for doping; a large number of vacancies are generated in crystal lattices of the solid electrolyte; a transmission channel of potassium ions is increased, and activation energy required by diffusion of the potassium ions is reduced, so that the ionic conductivity of the electrolyte is improved, the energy density of the battery is improved, necessary conditions are provided for preparation of the potassium solid-state battery, and meanwhile, compared with a liquid electrolyte in the prior art, the safety of the battery is greatly improved. However, in any point of this document it is anticipated the effect of self-charging corresponding to a potential step-up, which corresponds to a negative resistance, as mentioned in this description.

[0007] Document JP2019119667 discloses a potassium compound having excellent ion conductivity in a wide temperature range, a solid electrolyte for potassium ion secondary battery containing the potassium compound, and a secondary battery. However, in any point of this document it is anticipated the effect of self-charging corresponding to a potential step-up, which corresponds to a negative resistance, as mentioned in this description.

[0008] These facts are described to illustrate the technical problem solved by the embodiments of the present document.

GENERAL DESCRIPTION

[0009] The present disclosure relates to a solid-state electrolyte and their uses that are important for a more sustainable society and requires electrification.

[0010] Sodium and potassium ion-based electrolytes will likely play an important role in energy storage as these elements are very abundant. The latter cations and chloride are especially interesting since life on the planet is somehow based on biological transfers of these ions through cell membranes.

[0011] The subject matter of the disclosure also provides advantageous of getting an efficient and safe electrolyte, which has a crucial part in forming protective layers on both the cathode and the anode, allowing KIBs tobe provided in the commercial market in place of the currently used LIBs.

[0012] An aspect of the present disclosure relates to a solid-state electrolyte of the formula K3OCI or of the formula 2K2O-KCI, or of the formula K3-2xM_xOA, wherein 0 < x < 0.8; wherein M is selected from a list consisting of magnesium, calcium, strontium, barium, or combinations thereof; wherein A is selected from a list consisting of fluorine, chlorine, bromine, iodine, or combinations thereof, allowing the formation of a protective layer on both cathode and anode of said solid electrolyte.

[0013] In an embodiment for better results, the solid-state electrolyte composition comprises 0 < x < 0.8. [0014] In an embodiment for better results, it is described the solid-state electrolyte composition of the present disclosure wherein M is barium and A is chlorine.

[0015] In an embodiment for better results, it is described the solid-state electrolyte composition of the present disclosure wherein M is barium and A is iodine.

[0016] In an embodiment for better results, it is described the solid-state electrolyte composition of the present disclosure wherein M is calcium and A is iodine.

[0017] In an embodiment for better results, it is described the solid-state electrolyte composition of the present disclosure wherein M is barium and A is selected from a list consisting of chlorine, iodine, or combinations thereof.

[0018] In an embodiment for better results, it is described the solid-state electrolyte composition wherein M is magnesium and A is selected from a list consisting of chlorine, iodine, or combinations thereof.

[0019] In an embodiment, it is described the solid-state electrolyte composition wherein x is zero and A is selected from a list consisting of chlorine, iodine, or combinations thereof.

[0020] In an embodiment, it is described the solid-state electrolyte composition wherein M is barium, A is chlorine, and x is 0.010.

[0021] In an embodiment, it is described the solid-state electrolyte composition wherein M is barium and A is chlorine and x is 0.10.

[0022] In an embodiment, it is described the solid-state electrolyte composition wherein M is barium, A is chlorine and x is 0.015.

[0023] In an embodiment, it is described the solid-state electrolyte composition wherein M is barium, A is chloride, and x is 0.005.

[0024] In an embodiment, the solid-state electrolyte composition comprises the mixture with a polymer.

[0025] In an embodiment, the solid-state electrolyte composition is deposited or mixed with cellulose.

[0026] In an embodiment, the present discosure it is disclosed electrochemically, electrostatically, and structurally novel electrolytes, K3CIO and K_2.99Ba_0.005CIO, and compare their performance with Na₃CIO and Na_2.99Ba_0.005CIO in symmetric and asymmetric structural electrode-less cells, such as K/ K_2.99Ba_0.005CIO in cellulose membrane/K, Na/Na_2.99Ba_0.005CIO in cellulose membrane/Na, AI/K_2.99Ba_0.005CIO composite/Cu, and AI/Na_2.99Ba_0.005CIO composite/Cu at temperatures that range from - 45 to 65°C, respectively. An ab initio molecular dynamics followed by band structure determination using DFT, hybrid simulations and experiments allowed to compare the amorphous character of the structures, bandgap, and electron localization function for both K3CIO at 25°C and Na₃CIO at 37°C, temperatures at which preliminary studies indicate that these compounds are already amorphous. As in Na⁺-based electrolytes, the ferroelectric character of the K⁺-based electrolytes is well recognizable, especially at - 45°C where the relative real permittivity achieves 10¹³ in K/K_2.99Ba_0.005CIO in cellulose membrane/K symmetric cells, for an ionic conductivity of =120 mS/cm. As in Na⁺-based electrodes-less structural battery cells, self-charge and self-cycling phenomena are also demonstrated reinforcing the ferroelectric nature of the A3CIO (A = Li, Na and K) family of electrolytes. These electrolytes based on K⁺ and Na⁺ conduction behaviour are of very high importance in energy harvesting and storage, as well as the biologic world.

[0027] With no conventional cathode and anode needed, potassium and sodium- based solid-state electrolytes are ideal for structural batteries applications [20,40-48], With collectors-electrodes such as aluminum or zinc and carbon or copper to fix the difference in chemical potentials and a Na- or K-based solid-state electrolyte that fixes the capacity of the cell by plating on the collectors, large extensions of, for example, coaxial-beam shaped cells, can be applied to the interior of a vehicle, on a wall, in industrial facilities, and databanks with the multiple functionalities of storing energy, harvesting wasted heat and thermal energy and being protective against mechanical impacts.

[0028] In an embodiment, it is disclosed new electrolytes, K3CIO and K_2.99Ba_0.005CIO, and compared their properties with Na₃CIO and Na_2.99Ba_0.005CIO electrolytes in both symmetric and asymmetric structural electrode-less cells. Electrochemical properties of Na_2.99Ba_0.005CIO electrolytes measured in symmetric cells with blocking electrodes have been firstly reported by Braga et al. in [49],

[0029] Ab initio molecular dynamics structural studies followed by band structure determination by DFT and hybrid simulations allowed the comparison between the amorphous nature of the structures, bandgap, and electron localization functions for K3CIO at 25°C and Na₃CIO at 37°C, temperatures at which preliminary studies indicate that the compounds are already amorphous.

[0030] It is also described a ferroelectric electrolyte comprising the solid-state electrolyte composition previously described.

[0031] It is also described an energy storage device comprising the solid-state electrolyte composition previously described.

[0032] In an embodiment, the energy storage device comprises a battery.

[0033] In an embodiment, the energy storage device comprises a capacitor.

[0034] It is also described an energy harvesting device comprising the solid-state electrolyte composition previously described.

[0035] In an embodiment, the energy harvesting device comprises a negative resistance.

[0036] In an embodiment, the energy harvesting device comprises a negative capacitance.

[0037] In an embodiment, the energy harvesting device comprises a tunnelling phenomenon.

[0038] In an embodiment, the energy harvesting device comprises a thermoelectric device.

[0039] In an embodiment, the energy harvesting device comprises a photovoltaic device.

[0040] In an embodiment, the energy harvesting device comprises harvesting electrical energy at a constant temperature.

[0041] It is also described in this application, a solid-state electrolyte of the formula K3-3XQXRA, wherein 0 <x < 0.8; wherein Q is a cation Q³⁺ and is selected from a list consisting of boron, aluminium, galium, indium, and combinations thereof, and R is oxygen or sulfur and A is selected from a list consisting of fluorine, chlorine, bromine, iodine, and combinations thereof.

[0042] In an embodiment, it is described the solid-state electrolyte formula wherein Q is aluminum.

[0043] In an embodiment, the solid-state electrolyte composition comprises 0 < x < 0.8. BRI EF DESCRI PTION OF THE DRAWI NGS

[0044] The following figures provide preferred embodiments for illustrating the disclosure and should not be seen as limiting the scope of invention.

[0045] Figure 1: Schematic representation of cyclic voltammetry analysis (CV) vs temperature for K_2.99Ba_0.005CIO and Na_2.99Ba_0.005CIO all-solid state ferroelectric electrolytes at 0.1 mV/s in K/ K_2.99Ba_0.005CIO in cellulose/K and Na/ Na_2.99Ba_0.005CIO in cellulose/Na symmetric cells. Data obtained at open-circuit voltage (OCV) (a) Average current; (b) Capacitance; and (c) Permittivity.

[0046] Figure 2: Graphic representation of potentiostatic electrochemical impedance spectroscopy (EIS) and chronopotentiostatic (CP) analysis vs temperature for K_2.99Ba_0.005CIO and Na_2.99Ba_0.005CIO all-solid state ferroelectric electrolytes in K/K_2.99Ba_0.005CIO in cellulose/K and Na/Na_2.99Ba_0.005CIO in cellulose/Na symmetric cells. (a) EIS for the K-based cell; (b) EIS for the Na-based cell; (c) CP for K-based cell at 0°C; (d) CP for K-based cell at -40°C.

[0047] Figure 3: Graphic representation of potentiostatic EIS for AI/K_2.99Ba_0.005CIO composite/Cu (foil and mesh) and AI/Na_2.99Ba_0.005CIO composite/Cu ferroelectric coaxial structural batteries at 0, 25, and 40°C. Nyquist plots for (a) AI/K_2.99Ba_0.005CIO composite/Cu (foil); (b) Zoom of (a); (c) AI/K_2.99Ba_0.005CIO composite/Cu (mesh); (d) Zoom of (c); of the AI/Na_2.99Ba_0.005CIO composite/Cu (foil); (f) Zoom of the(e).

[0048] Figure 4: Graphic representation of cyclic voltammetry for AI/K_2.99Ba_0.005CIO composite/Cu (foil and mesh) and AI/Na_2.99Ba_0.005CIO composite/Cu (foil) ferroelectric asymmetric coaxial structural batteries, (a) Permittivity for AI/K_2.99Ba_0.005CIO composite/Cu (foil); (b) Permittivity for AI/K_2.99Ba_0.005CIO composite/Cu (mesh); (c) Permittivity for AI/Na_2.99Ba_0.005CIO composite/Cu; (d) Cyclic voltammetry for AI/K_2.99Ba_0.005CIO composite/Cu (foil) at 40°C and rate 0.1 mV/s (first cycle); (e) Cyclic voltammetry for AI/K_2.99Ba_0.005CIO composite/Cu (foil) at 40°C and rate 0.1 mV/s (first cycle) (second cycle); (f) Cyclic voltammetry for AI/K_2.99Ba_0.005CIO composite/Cu (mesh) at 40°C and 0.1 mV/s (first cycle); (g) Cyclic voltammetry for AI/K_2.99Ba_0.005CIO composite/Cu (mesh) at 40°C and 0.25 mV/s (second cycle); (h) Cyclic voltammetry for AI/Na_2.99Ba_0.005CIO composite/Cu at 40°C and 0.1 mV/s (first cycle); (i) Cyclic voltammetry for AI/Na_2.99Ba_0.005CIO composite/Cu at 40°C and 0.1 mV/s (second cycle). [0049] Figure 5: Graphic representation of typical discharge curve for all-solid-state AI/K_2.99Ba_0.005CIO composite/Cu (mesh) coaxial structural battery that was connected, at =25°C, to a 1.8 kQ resistor after 22.8 h and then heated to approximately 42.5°C; (a) full discharge; (b) to (e) details of the periodic potential and temperature vs time.

[0050] Figure 6: Graphic representation of typical curve for AI/Na_2.99Ba_0.005CIO composite/Cu coaxial structural battery at approximately 40°C when the battery-cell was set to discharge with a resistor; (a) Na⁺-electrolyte based coaxial-cell connected to a 26.6 kΩ resistor at t = 0 and correspondent equivalent circuit after 8 months of selfcharge; (b) Photograph of a demonstrative coaxial cell as well as the possible schematic representation of the feedback current accountable for self-charge, and self-cycle, observed when the cells are set to discharge with a load; Poincare two maps feedback model used to understand the feedback leading to self-charge in the present ferroelectric electrolyte based cells [63]; (c) Na⁺-electrolyte based coaxial-cell connected to a 26.6 kQ resistor at t = 0 and correspondent equivalent circuit after more than one and half years of self-charge.

[0051] Figure 7: Graphic representation of Ab-initio simulations for K3CIO at 25°C (298 K) and Na₃CIO at 37°C (310 K). (a) PDF; (b) XRD for CuKa radiation; electron localization function of three (001) Miller plans; (c) and (d) for K3CIO and (e) for Na₃CIO.

DETAILED DESCRI PTION

[0052] This disclosure relates to a potassium ion rich ferroelectric solid-state electrolyte and their use.

[0053] In an embodiment, the subject matter of the disclosure relates to a solid-state electrolyte of the formula K₃OCI or of the formula 2K₂O-KCI, or of the formula K_3-2xM_xOA, wherein 0 < x < 0.8; wherein M is selected from a list consisting of magnesium, calcium, strontium, barium and their combinations thereof, and; wherein A is selected from a list consisting of fluorine, chlorine, bromine, iodine, and their combinations thereof. In an additional embodiment for formulas K₃OCI or 2_K2O-KCI or K_3-2xM_xOA, x can be 0 < x <

0.8.

[0054] In an embodiment, two different types of all-solid-state ferroelectric electrolytes with enhanced electrochemical and thermal properties were synthesized, for comparison effect, ( with A = Na, K) and their properties were compared in different cell configurations K/K_2.99Ba_0.005CIO in cellulose membrane/K, Na/Na_2.99Ba_0.005CIO in cellulose membrane/Na, AI/K_2.99Ba_0.005CIO composite/Cu, and AI/Na_2.99Ba_0.005CIO composite/Cu.

[0055] In an embodiment, the synthesis of the dry glass Na⁺ or K⁺ based solid-state electrolytes were realized in compliance with the protocol presented by Braga et al. [49], The precursors NaCI (>99 %, Merck) or KCI (99.5%, PanReac AppliChem), Na(OH) (>99%, Merck) or K(OH) (85.7%, Alfa Aesar), and Ba(OH)₂ (94-98 %, Alfa Aesar) were mixed with deionized water before letting them react and dry between 230 and 250°C. The solid- state electrolytes A_2.99Ba_0.005CIO_1-x(OH)_x with A = Na or K were subsequently dried to 230 - 250°C to eliminate the hydroxide phases and obtain the glassy A_2.99Ba_0.005CIO (A = Na, K). Once in its final configuration, the solid-state electrolytes were ground for 45 minutes at 350 rpm using a ball milling machine with a hermetically closed Agate container and balls with a diameter of 20 mm.

[0056] In an embodiment, the first type of cells that were prepared to characterize the electrochemical properties of these Li-free all-solid-state electrolytes were the symmetrical coin cells, with Na- or K-metal electrodes. The CR2032 coin-cells were selected to host symmetric cells manufactured with disks of pure A alkali-metal electrodes with a reference diameter of 8 mm. The alkali-metal disks were cut from the raw material chunks of Na (>99%, Sigma-Aldrich) or K (>98%, Sigma-Aldrich). The interleave electrolyte-separator was a nonwoven cellulose layer impregnated with the correspondent A⁺- based electrolyte A_2.99Ba_0.005CIO (A = Na, K) with a diameter of 16 mm and thickness of ~1 mm. The separators’ disks were soaked in a slurry composed of absolute ethanol (>99.5 %, VWR chemicals) mixed with the electrolyte powders. The separators were then let to dry overnight in the Ar-dry glovebox at 70°C before proceeding with the assembly of the cells. [0057] In an embodiment, as described previously, all materials were handled in an Ar-dry glovebox with O₂ % < 1.0 ppm; extra attention was given to the metals by removing the oxide layer on the exposed surface before cutting.

[0058] In an embodiment, an all-solid-state coaxial structural battery design was selected as an application for the proposed electrolytes. In this configuration, introduced by Danzi et al. [20], the electrolyte was mixed with the thermoplastic Polyvinyl Acetate (PVAc) (C₄H₆O₂)_n in a 4 A_2.99Ba_0.005CIOi-x(OH)_x: 1 PVAc ratio. This coaxial structural battery design is composed of a [90/0/ +45/-45]s outer shell of carbon fiber reinforced plastic (CFRP) fabricated using the T800-736LT 100 gsm. The tubular structure works as a host for the coaxial battery fabricated with a copper with a thickness of 0.127 mm from Alfa Aesar as a positive electrode/current collector co-cured to the CFRP outer shell while in the axis of the circular beam it is placed a 4 mm diameter rod of commercial aluminum as the negative electrode/current collector. The gap between the two was then filled with the electrolyte-based mixture previously described. The geometries and further details used are the same already disclosed in the document WO2022/243970 and in Danzi et al. [20]

[0059] Electrochemical analysis

[0060] In an embodiment, the electrochemical performance of the all-solid-state electrolytes and correspondent cells was then evaluated via a series of electrochemical tests.

[0061] Four types of tests were adopted, cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), chronopotentiometry (CP), and electrochemical discharge.

[0062] The CV, EIS, and CP tests with the symmetric coin-cells were performed using a Biologic SP-240 potentiostat/galvanostat/impedance spectroscope in an Ar-dry glove box. A Biologic VMP-300 potentiostat/galvanostat/impedance spectroscope, and a Gamry reference 3000 potentiostat/galvanostat/ZRA were used for testing the structural coaxial batteries.

[0063] The CV experiments were carried out to determine the capacitance of the different cell configurations here presented as well as the relative real permittivity or dielectric constant of the electrolytes. All the experiments were carried out superimposing ± 0.1 V to the initial differential potential corresponding to the OCV of the cell. The CR2032 symmetric cells with the alkali-metal electrodes were tested in a temperature range of -45°C to 70°C for Na_2.99Ba_0.005CIO and -45°C to 40°C for K_2.99Ba_0.005CIO. The lower temperature is the minimum temperature achievable in the freezer unit integrated within the glove box, while the upper range has been defined considering a safety margin from the melting point of the alkaline metals here used. To each temperature corresponds ten CV cycles at 0.1 mV/s. Temperature equilibration was achieved unequivocally. The CV analyses of the asymmetric cylindrical cells were instead carried out at three different temperatures only: 0°C, room temperature, approximatively 25°C, and 40°C. These tests were performed varying the scan rate from 0.1 mV/s up to 50 mV/s and the values of the average current were measured at the measured OCV value of the cell.

[0064] The EIS analyses were performed to determine the internal resistance of the distinct types of cells. All the experiments were carried out with an alternate current AC with an amplitude of ±10 mV for an initial differential potential corresponding to the OCV of the cell and the frequency range was from 3.0 MHz down to 200 mHz.

[0065] Chronopotentiostatic cycles with a constant current of 2 mA/cm² were performed at -40, -20, -10, 0, 10°C to assess the K/K_2.99Ba_0.005CIO and Na/Na_2.99Ba_0.005CIO interfaces' behaviours and compare internal resistances obtained with this DC method versus obtained with EIS using AC. The cycles were carried on for (2+2) h.

[0066] Electrochemical discharges were performed with external physical resistors that were connected to the coaxial cells inside the sand or silicon-oil bath at approximately 25 or 40°C. The use of external physical resistors is intended to control unequivocally the discharge of the cell, and also the self-charge and self cycling, not making it dependent on the amplifier of the potentiostat or the variation of the internal resistance of the cell. The measuring instrument works solely as a voltmeter connected in parallel with the external resistor.

The capacitance of the cell was calculated at OCV, using the <i> vs dV/dt slope and Eq.

1:

where i is the average current at OCV, C is the capacitance of the cell, and dV/dt is the voltage rate.

[0067] From the capacitance in Eq. 1, the real relative permittivity or dielectric constant εr of the dielectric in the double-layercoin cells is determined. The thickness of the electrolyte-cellulose membrane ferroelectric dielectric separator is d, and the cross-section surface area is A which obeys the condition required by Gauss's law d² « A, (2)

where ε₀ is the permittivity of the vacuum. In the CR2032 coin-cells studied herein, A = 0.5 cm² and d = 0.1 cm.

[0068] The conductivity is obtained from the resistance R using, (3)

[0069] The resistance was determined from the impedance spectroscopy data using the equivalent circuit discussed in [49] when possible or simply by identifying the real passive resistance on the Re(Z) axis or using Ohm's law V = RI on chronopotentiometry data.

[0070] From the capacitance in Eq. 1, the dielectric constant in the coaxial cells is determined,

(4)

where the length of the cell, b the external radius, and a the internal radius (b > a) of the ferroelectric electrolyte.

[0071] The resistance towards the radial ionic movement is then attained, (5)

which allows to calculate the ionic conductivity in coaxial cells. In the cells here studied, .

[0072] In an embodiment, ultimately, discharge curves of cylindrical asymmetric cells were performed with physical resistors of 1.8 and 26.6 kQ for AI/K_2.99Ba_0.005CIO composite/Cu (foil and mesh) and AI/Na_2.99Ba_0.005CIO composite/Cu (foil), respectively. Simulations

[0073] Molecular Dynamics (MD) enables the development of a model in which the molecules are continuously moving. It simulates the behavior of a limited number of molecules, confined to a given volume and interacting with one another through a given pair potential [51],

[0074] In an embodiment, in an Ab-initio Molecular Dynamics (AMD) run the forces calculated in a given geometry step are used to update the atomic positions. The system dynamics, i.e. the ionic movements, are subject to Newton mechanics while the forces acting on the ions are calculated from ab initio using a self-consistent electronic density, known as Hellmann-Feynman forces, as implemented in Vienna Ab Initio Software Package (VASP) [52], Periodic boundary conditions are assumed and allow that a molecule, which leaves the volume across one face, re-enters across the opposite phase. In the statistical ensemble, the various states of the system differ in positions and velocities of the component's particles. The space of all possible system states is of dimension 6N for N particles is termed the phase space.

[0075] In an embodiment, a system at imposed pressure P, and temperature T, is represented by an isothermal-isobaric ensemble. This ensemble plays an important role as chemical reactions are usually carried out under constant pressure conditions. The number of molecules and the pressure are identical for all system states belonging to the ensemble, but they differ in total energy, which is a fluctuating variable in this ensemble. Each state j of the isothermal-isobaric ensemble occurs with a probability proportional to e^-Ej/kBT where kB is the Boltzmann constant and E_j is the total energy (kinetic plus potential) of the system in state j. Here we use the symbol E for total energy (kinetic and potential). The Boltzmann factor e^-Ej/kBT expresses that low energy states are favored when compared to high-energy states. Increasing temperature broadens the energy distribution in the ensemble and, as consequence, the average energy is increased. [0076] In an embodiment, Ab initio molecular dynamics (AMD) as implemented by VASP [52] was used to simulate a closed system thermostatted to a heat bath at a constant temperature. The studied systems were (K₃CIO)₂₇ and (Na₃CIO)₂₇ that were left to relax. The initial structures were the optimized structures at the correspondent temperatures after performing microcanonical simulation NV'E (with volume V' and total energy E constant) from the crystalline optimized structure (antiperovskite, cubic Pm-3m). Isothermal-isobaric simulations NPT were performed to set the volume at the correspondent temperature. The temperatures, 25 and 37°C (298 and 310 K) for (K₃CIO)₂₇ and (Na₃CIO)₂₇ respectively, were chosen to assure that the system is in the amorphous state, immediately above its transition to amorphous.

[0077] The simulations were based on density functional theory (DFT) and the GGA- PBE [53,54] exchange correlation functional for describing the interactions and hybrid functional HSE06, both for the structures obtained with AMD. It is noteworthy that hybrid functional HSE06 is more precise in what concerns the computation of the band structure of a semiconductor.

[0078] Electronic structure calculations were conducted with a planewave cutoff energy of 400 or 500 eV. The electronic iterations convergence was 10^-5 eV using the Normal (blocked Davidson) algorithm and real and reciprocal space projection operators. The requested k-spacing for non-local exchange was 0.5 Å^-1 which leads to a lxlxl mesh or 0.3 Å^-1 which leads to a 2x2x2 mesh. Using first-order Methfessel-Paxton smearing with a width of 0.2 eV.

[0079] An AMD with 4 fs time step was used and velocities were rescaled every time step to maintain a constant temperature. The simulations extended for at least 360 fs.

[0080] The pair distribution functions (PDF) g(r) versus the interatomic distances r and x-ray diffraction (XRD) patterns for CuKa source radiation were also obtained for controlling the non-periodic character of the amorphous structures. The electron localization functions were also established to determine the nature of the bonding, the dipole character, and the dynamics of the electrons favored by the possible dynamics of the ions in these semiconductors at a certain temperature.

[0081] It is important to emphasize that if an event is observed during the AMD relaxation, the correspondent manifestation should be observed in a real environment. However, processes with slow dynamics will not be observed in AMD, even if occurring in Nature.

[0082] In an embodiment, one of the most striking features in the A2.99M0.005CIO (A = Li, Na, K, and M = Mg, Ca, Sr, Ba) family of glassy electrolytes is their ferroelectric character. A ferroelectric material polarizes spontaneously and its polarization can be reversed by the application of an electric field [55], The ferroelectric phenomenon is of a quantum mechanics nature but its coherence is maintained at the classical scale. In face of a novel electrolyte of the same family, one of the most interesting properties to determine is the relative real permittivity or dielectric constant at different temperatures, as illustrated in Figure 1. The permittivity was then determined by cyclic voltammetry (CV) using non-blocking alkali-metal electrodes. The permittivity is higher at all temperatures than ever found before [50,56] and, for that, different factors may contribute: (1) the presence of non-blocking electrodes for which the electrolytes of this family have a very good affinity, not exhibiting the formation of an SEI layer but forming a layer of electrolyte leading to plating; (2) the low voltage rate used in the CV experiments allowing ferroelectric polarization of the cells; (3) the drying efforts conducted in the glove box to avoid hydroxide crystalline phases formation which are detrimental to fast ionic conduction necessary to the polarization optimization process; (4) the affinity these electrolytes show towards the presence of a non-woven cellulose matrix [57,58], Cellulose is a dielectric (ε_r = 1.0 to 1.5) material with (OH)- groups along the fibers that can attract the alkali mobile cations and facilitate both polarization and ionic conduction [57-59], It was previously highlighted that K⁺ is the key charge carrier in plants, and therefore, there might be a relationship between the presence of cellulose and the ionic enabled polarization in the K3CIO family of electrolytes and similarly in the Na₃CIO. Cellulose is a very relevant structural component of the primary cell wall of green plants.

[0083] In an embodiment, it is clear that the symmetric cell with potassium shows a huge permittivity which is higher at -45°C, refuting the variation trend of the sodium- based permittivity. The sodium-based cell, on the other hand, shows a transition temperature at approximately -20°C, which was observed in previous works of the group with different types of cells with Na_2.99Ba_0.005CIO and not fully characterized yet. Another feature worth mentioning is the polarization current at OCV of the K/K_2.99Ba_0.005CIO in cellulose/K cell, which assumes the value of 3 mA/cm² at -45°C for a capacitance of approximately 15 Farad, as illustrated in Figure 1. From -45 to 40°C the permittivity of the K-based ferroelectric-dielectric separator varies from 4xl0¹³ to 10¹³, as illustrated in Figure Id. The permittivity of the Na-based ferroelectric-dielectric separator in the Na/ Na_2.99Ba_0.005CIO in cellulose/Na cell varies from 8x10¹¹ to 2x10¹³ which is an equally elevated permittivity but not as outstanding as that obtained with the K/K_2.99Ba_0.005CIO in cellulose/K symmetric cell.

[0084] In an embodiment, the conductance, and consequently the ionic conductivity of the cells, should be related to their permittivity as the polarization of the ferroelectricelectrolyte is partially enabled by the hopping of the mobile cations; yet, the full conductance of the symmetric cell might not be synchronized with the permittivity as not only other phenomena might influence the conduction process across the cell, but also the relaxation phenomena might play an important role as observed before in relaxation oscillators obtained with these families of electrolytes [60] as will be analyzed hereafter.

[0085] In an embodiment, from the EIS data, using Eq. 3, it is possible to determine the conductivity of the K/K_2.99Ba_0.005CIO in cellulose/K cell, σ = 191 mS/cm (-20°C) to 127 mS/cm (-40°C) and 150 mS/cm (25°C), which includes all resistances such as the bulk ionic conductivity and interfaces as it was impossible to determine just the bulk ionic conductivity from the Nyquist plot. The interfaces' resistance, including the K- metal/stainless steel, is predominant.

[0086] In an embodiment, for the Na/Na_2.99Ba_0.005CIO in cellulose/Na cell, σ = 171 mS/cm (-40°C) to 69 mS/cm (0°C), also which includes all resistances. This latter analysis shows a possible lag between the transition that is observed at -20°C in the CV measurements and 0°C in EIS measurements.

[0087] In an embodiment, the chronopotentiometry measurements show that results agree with the EIS at 0°C, the CP of the K/K_2.99Ba_0.005CIO in cellulose/K cell returns an ionic conductivity of σ = 130 mS/cm while EIS returns σ = 181 mS/cm . It is noteworthy that a DC method such as CP usually delivers lower conductivity than an AC method such as EIS, due to the nature of the applied current and its effects on the material being a function of the current's frequency.

[0088] In an embodiment, another feature worth highlighting on the CP measurements shown in Figures 2c and 2d is the tendency to a preferential direction for the fastest conduction of the ions which was progressive from 10 to -40 °C and the selfoscillation that only arises at -40°C.

[0089] In an embodiment, the asymmetric AI/K_2.99Ba_0.005CIO composite/Cu and AI/ Na_2.99Ba_0.005CIO composite/Cu ferroelectric coaxial structural batteries have shown much higher resistance than the symmetric cells as reflected in Figure 3. For the asymmetric cells, the theoretical OCV is given by the difference between the chemical potential of the electrodes, which in these embodiments commercial Al and pure Cu are used. The aluminum chemical potential is likely affected by the oxide layer that inevitably forms on its surface, and therefore, the OCV is 1.15 to 1.2 V for K⁺-based and 1.05 to 1.13 V for Na⁺-based asymmetric cells. The OCV of the asymmetric cell may also have a noticeable contribution from the polarization of the electrolyte according to Landau-Devonshire's theory for a ferroelectric material [50,55], The highest conductivity (σ_{K+- coaxial cell, Cu mesh} ⁼ 0.17 mS/cm is attained for AI/K_2.99Ba_0.005CIO composite/Cu (mesh) coaxial battery, at 40°C, but it is very similar for all cells at the same temperature &K⁺- coaxtai ceil, cu foil = 0.14 mS/cm, although slightly lower for AI/ Na_2.99Ba_0.005CIO composite/Cu σNa⁺- coaxial ceil, cu foil = 0.10 mS/cm. Nonetheless, at 0°C, the internal resistance of the AI/ Na_2.99Ba_0.005CIO composite/Cu corresponding to a conductivity σσa⁺- coaxiai ceil, cu foil = 0.0014 mS/cm is similar to that of the AI K_2.99Ba_0.005 CIO composite/Cu coaxial (foil and mesh) batteries σ_{K+- coaxial ceil, cu foil} = 0.0026 mS/cm and (σ_{K+- coaxial cell, Cu mesh} ⁼ 0.0016 mS/cm (SI, Table S2).

[0090] In an embodiment, the temperature dependency of the resistance for the AI/K_2.99Ba_0.005CIO composite/Cu asymmetric cell does not follow the permittivity dependency obtained with the K/K_2.99Ba_0.005CIO in cellulose/K symmetric cell and not even the resistance obtained with it. The conductivity σ_{K+- coaxial ceil, cu foil} = 0.048 mS/cm, σ_{K+- coaxial cell, Cu mesh} = 0.018 mS/cm, and σ_{Na+- coaxial cell, cu foil} = 0.033 mS/cm was attained at 25°C for AI/K_2.99Ba_0.005CIO composite/Cu (foil and mesh) and AI/Na_2.99Ba_0.005CIO composite/Cu, respectively. Although a symmetric cell, conversely to an asymmetric, reflects the properties of the electrolyte, as shown in Table 1 below, for the differences between cells, several factors might influence: (a) the presence of moisture during fabrication which is more difficult to avoid even after the cell's treatment in the argon filled glove box as the shape of the cell does not facilitate the release of moisture; (b) the presence of a thermoplastic to aggregate and facilitate contact among powders and between electrolyte and metals, instead of cellulose; (c) the less affinity to plate K and Na on Cu, than to plate on the correspondent alkali metal; and finally (d) the pressure deficiency, not enough to keep the electrolyte in contact with the collectors. To the highest frequencies' Cole-Cole semicircles, correspond a much smaller resistance than to the lower frequencies' semicircles. The lower frequency part of the Nyquist plots is attributed to the interfaces and the higher frequencies to the bulk electrolyte in which the ions are freer to move. Therefore, it is more accurate to compare the impedance in the symmetric cells with the impedance correspondent to the semicircles at the highest frequencies, as illustrated in Figure 3 and is shown on Table 1 below. Both electrolytes (K⁺ and Na⁺-based) in the coaxial asymmetric batteries show the same conductivity at 0°C =0.02 mS/cm. At 25 and 40°C, the K_2.99Ba_0.005CIO composite demonstrates a conductivity that is approximately one order of magnitude higher than Na_2.99Ba_0.005CIO composite at the same temperatures, as shown in Table 1 below. How the coaxial cells behave electrochemically and electrostatically at 25 and 40°C, seems to be much more related to the ionic conductivity than to the dielectric constant, as demonstrated hereafter.

[0091] Table 1 - resistances and conductivities for the electrolyte in coaxial structural asymmetric batteries at different temperatures

[0092] In an embodiment, the permittivity of the asymmetric cells shows a different trend with temperature; it is higher at higher temperatures, as illustrated in Figure 4 at 40°C, the permittivity is >10^1:L for AI/K_2.99Ba_0.005CIO composite/Cu (foil) which is approximately two orders of magnitude lower than the permittivity of the symmetric cell K/K_2.99Ba_0.005CIO composite/K at the same temperature. The ratio between the ionic conductivities of the symmetric/asymmetric cells is >3xl0³ at 40°C.

[0093] In an embodiment, the permittivity depends on the applied external field rate reflected on the applied potential rate, as illustrated in Figure 4 and, as discussed in [20], this dependency is due to the possibility of polarizing more efficiently at lower frequencies, such as 0.1 mV/s, than at 50 mV/s.

[0094] In an embodiment, the CV curves of Figures 4c and 4d show an oxidation reaction at OCV (hollowed circles) + 0.15 V which is displaced from the correspondent reduction reaction of 0.20 V, as illustrated in Figure 4c, corresponding to the difference between the charge and discharge plateau voltage and reflecting the effect of the internal resistance. [0095] In an embodiment, another interesting feature that is not observed in symmetric cells and here only shown in AI/K_2.99Ba_0.005CIO composite/Cu is the negative resistance demonstrated in Figures 4c to 4f and which is -66 Q in the CV of Figure 4d. The negative resistance attained while charging corresponds to the tunneling of electrons from the electrolyte to the negative electrode as shown herein later when referring to Poincare feedback, leading to an increase in the chemical potential of the negative electrode. The negative resistance is in agreement with the phenomena illustrated in Figure 5, when a cell set to discharge connected to a 1.8 kQ resistor is heated and starts to self-charge, as illustrated in Figure 5a. Besides these ferroelectric electrolyte-based cells, the other single cell that shows "real" (not circuit driven) negative resistances in l-V curves are the tunnel diodes [61], A tunnel diode or Esaki diode is a type of semiconductor diode that shows an effective negative resistance due to electron tunneling, which is a quantum mechanical effect.

[0096] In an embodiment, non-linear phenomena corresponding to self-oscillations with different periods may arise and vary with the bath's temperature increase. Conversely, the oscillation of the potential of the cells seems to determine the oscillation of the temperature of the cell, as illustrated in Figure 5b. After 92.5 h, the lowest oscillation potential was in phase with the lowest oscillation temperature, but the highest potential spike corresponded to halfway up to the highest temperature of the matching peak, as illustrated in Figures 5c and 5d. After 270 h, once only two different periods for the pulsating potential are observed, the potential spike becomes in phase with the temperature peak. When four different periods corresponding to four different shaped peaks could be analyzed, to a maximum A = 0.16 V corresponded a AT=1.7 °C. After 488 h the potential and temperature synchronized with just the same ~1 h period observed for both. This latter synchronization is another phenomenon demonstrative of emergence arising in complex systems [62],

[0097] In an embodiment, it is important to emphasize that self-charging, corresponding to a potential step-up, corresponds to a negative resistance as illustrated in the CVs of Figures 4c to 4f. In Figure 5a, it is observed a self-charge of A = 0.77 V for the AI/K_2.99Ba_0.005CIO composite/Cu (mesh) cell after 24.1 h, corresponding to Tbath/ceii = 16°C, while the cell is set to discharge connected to the 1.8 kQ resistor. The resistor was connected to the cell after 22.8 h. The output current can be calculated using Ohm s law V = Rextl where R_ext =1800 Q. The average current was approximately 0.5 mA.

[0098] In an embodiment, a similar self-charging phenomenon as illustrated in Figure 5 is shown in Figure 6 for an AI/Na_2.99Ba_0.005CIO composite/Cu coaxial cell that had been shown in [20] that has overcome 5330 h self-charging uninterruptedly. After 1104 h, as illustrated in Figure 6a, the self-cycling voltage amplitude is reduced spontaneously, which is likely to have contributed to the potential rise from 1.28 to 1.53 V, thereafter. With the reduced intensity of the oscillations, less energy is spent transforming a DC phenomenon into an AC, and hysteresis is avoided. For this cell set to discharge with a 26.6 kQ external resistor, the internal resistance resulting in the Joule effect is overcome by a negative resistance due to the feedback electron-current phenomenon that is likely to take place at the surface of the semiconductor ferroelectric-electrolyte leading to self-charge, as illustrated in Figure 6b. As illustrated in Figure 6c, it is possible to illustrate that this phenomenon was maintained for at least more than one and half years. The latter topologic conduction does not screen the charge accumulation at the double-layer capacitors (EDLCs) formed at the interfaces to align the electrode/electrolyte electrochemical potentials or Fermi levels. In other words, the conduction of electrons does not annihilate the EDLCs as the potential, not only is not reduced, but increases from 1.06 to 1.53 V. The phenomenon previously described may be schematically illustrated in the Figure 6b. The two maps feedback Poincare model is considered to be a suitable model to describe the processes taking place in the ferroelectric-electrolyte based cells when set to discharge with an electrical load [63],

[0099] In an embodiment, Ab-initio simulations allow to access the structure of unknown materials and their properties. Standing on our previous experience of both simulations and experiments with ferroelectric electrolytes of the A3CIO family with A = Li, Na, the disordered structure of K3CIO at 25°C (298 K) was simulated and its PDF, XRD, electron localization function, band structure, and energy of formation were calculated. The structure was firstly simulated by AMD, as described previously. The same procedure was taken for Na₃CIO at 37°C (310 K) to be able to compare structures and properties of these electrolytes thought to be amorphous at these temperatures. [00100] In an embodiment, by the analyses of Figures 7a and 7b, it is concluded that the K3CIO electrolyte at 25°C (298 K) is much more disordered than the Na₃CIO at 37°C (310 K). The third neighbors in the PDF of K3CIO are not recognized but those in Na₃CIO are still identifiable within a certain broad range of r.

[00101] In an embodiment, the electron localization functions may shed light on how the ferroelectric-electrolyte structures become disordered with temperature-forming polymer-like chains of (A - 0)_n- aligned dipoles.

[00102] In an embodiment, one of the most intriguing features of the simulated electron localization functions is that observed in the center of Figure 7c and in the Figure 7d where a K⁺ may attract another K⁺ bending the lattice formed by the (K - O - K)_n chains. This phenomenon is similar to the mechanism subjacent to the conduction of electron pairs (Cooper pairs).

[00103] In an embodiment, cooper pairs yielded by a positively charged lattice. In Figure 7d, it is demonstrated how the oxygen anions' electron polarization changes extending towards another oxygen anion by the attraction action of the potassium cations. This mechanism may result in a topological current observed in these ferroelectricelectrolytes leading to self-charge and self-cycling.

[00104] In an embodiment, the same type of mechanisms should occur in Na₃CIO, although this electrolyte might need to be doped with Ba²⁺ or being at a higher temperature to have its structural disorder increased. Both electrolytes, K3CIO at 25°C and Na₃CIO at 37°C, are semiconductors, as shown in Table 2 below. It is worth mentioning that the semiconductor and thermoelectric Bi₂Te₃ which is also a topologic insulator shows a bandgap of 0.390 eV (DFT).

[00105] Table 2. Ab-initio simulation-based data for K3CIO and Na₃CIO

[00106] In an embodiment, the A2.99M0.005CIO (A = Na, K, and M = Ba) family of ferroelectric glassy electrolytes were studied employing symmetric coin-cells and asymmetric coaxial electrode-less batteries. The features that stand out are the huge permittivity associated with very small resistance (high ionic conductivity) in the alkali metal symmetric cell where the electrolyte is embedded in a cellulose matrix that may also contribute to the polarization of the electrolyte.

[00107] In an embodiment, another feature that sticks out is the self-charge and selfcycling behavior of the cell containing K_2.99Ba_0.005CIO composite, especially for approximately 40°C. This cell can step increase 0.8 V by having the sand/silicone bath increase its temperature by 17°C from 25-26°C to 42°C. Moreover, as for self-charge, i.e. a negative resistance, quantum signatures such as those expressed on charge/discharge self-cyclings may reflect emergent phenomena attributed to complex systems.

[00108] In an embodiment, the symetric cells' optimal features set a goal for the optimization of the asymmetric cells. The strategy may pass by using collectorelectrodes that show higher affinity to K and/or Na plating, by synthesizing different composites, and by using pressure to obtain better contact between the electrolyte and the current collector. Another possibility is the use of traditional cathode materials reinforcing the battery character of the cell. A rectification strategy with diodes or capacitors or just 3 min charges will attenuate or avoid the less practical self-cycling phenomena while discharging for 24h.

[00109] In an embodiment, and for better results, aluminium and other cations of the type Q³⁺ can be used. In these embodiments, the solid-state electrolyte has the formula K_3-3XQ_XRA, wherein x is from 0 to 0.8, Q is a cation Q³⁺ and is selected from a list consisting of boron, aluminium, galium, indium, and combinations thereof, R is oxygen or sulfur and A is selected from a list consisting of fluorine, chlorine, bromine, iodine, and combinations thereof. [00110] In an embodiment, for better results, a combination of the elements named as Q and A of the formula K_3-3XQ_XRA can be used, such as for example in K_2.985AI_0.005CI_0.5I_0.5O.

[00111] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[00112] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.

[00113] The above described embodiments are combinable.

[00114] The following claims further set out particular embodiments of the disclosure.

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Claims

C L A I M S

1. A solid-state electrolyte of the formula K₃OCI or of the formula 2K₂O-KCI, or of the formula K3-2xM_xOA wherein 0 < x < 0.8; wherein M is selected from a list consisting of magnesium, calcium, strontium, barium, or combinations thereof; wherein A is selected from a list consisting of fluorine, chlorine, bromine, iodine, or combinations thereof.

2. The solid-state electrolyte composition according to the previous claim, wherein 0 < x < 0.8.

3. The solid-state electrolyte composition according to any of the previous claims, wherein M is barium and A is selected from a list consisting of chlorine, iodine, or combinations thereof.

4. The solid-state electrolyte composition according to any of the previous claims, wherein M is magnesium and A is selected from a list consisting of chlorine, iodine, or combinations thereof.

5. The solid-state electrolyte composition according to any of the previous claims, wherein x is zero and A is selected from a list consisting of chlorine, iodine, or combinations thereof.

6. The solid-state electrolyte composition according to any of the previous claims, wherein M is barium and A is chlorine.

7. The solid-state electrolyte composition according to any of the previous claims, wherein M is barium and A is iodine.

8. The solid-state electrolyte composition according to any of the previous claims, wherein M is calcium and A is iodine.

9. The solid-state electrolyte composition according to any of the previous claims, wherein M is barium, A is chlorine, and x is 0.010.

10. The solid-state electrolyte composition according to any of the previous claims, wherein M is barium and A is chlorine and x is 0.10.

11. The solid-state electrolyte composition according to any of the previous claims, wherein M is barium, A is chlorine and x is 0.015.

12. The solid-state electrolyte composition according to any of the previous claims, wherein M is barium, A is chloride, and x is 0.005.

13. The solid-state electrolyte composition according to any of the previous claims, comprising the mixture with a polymer.

14. The solid-state electrolyte composition according to any of the previous claims, wherein said composition is deposited or mixed with cellulose.

15. A ferroelectric electrolyte comprising the solid-state electrolyte composition described in any of the claims 1 to 14.

16. An energy storage device comprising the solid-state electrolyte composition described in any of the claims 1 to 14.

17. The energy storage device according to the previous claims, comprising a battery.

18. The energy storage device according to any of the claims 16 to 17, comprising a capacitor.

19. An energy harvesting device comprising the solid-state electrolyte composition described in any of the claims 1 to 14.

20. The energy harvesting device according to claim 19, comprising a negative resistance.

21. The energy harvesting device according to any of the claims 19 to 20, comprising a negative capacitance.

22. The energy harvesting device according to any of the claims 19 to 21, comprising a tunnelling phenomenon.

23. The energy harvesting device according to any of the claims 19 to 22, comprising a thermoelectric device.

24. The energy harvesting device according to any of the claims 19 to 23 , comprising a photovoltaic device.

25. A solid-state electrolyte of the formula K_3-3xQ_xRA wherein 0 < x < 0.8; wherein Q is a cation Q³⁺ and is selected from a list consisting of boron, aluminium, galium, indium, and combinations thereof;

R is oxygen or sulfur and

A is selected from a list consisting of fluorine, chlorine, bromine, iodine, and combinations thereof.

26. The solid-state electrolyte composition according to the previous claim, wherein 0 < x < 0.8.