DE102012022686A1 - Capacitive electrical storage for use in fossil fuel propelled vehicle, has permanent double-layers arranged in fixed semiconductor, where layers change as counter electrode, and are operated with working voltages larger than specific value - Google Patents

Abstract

The electrical storage has an electrically conductive electrode (2) of a high specific surface area contacting with a current collector (1), and another electrode (4) of another high specific surface area contacting with another current collector (5). Fixed, permanent double-layers are arranged in a fixed semiconductor (3) change as a counter electrode for transferring charges, where the double-layers are operated with working voltages larger than 10 V. A temperature-stable fabric layer (6) is made of glass fibers or oxide-ceramic fibers. The semiconductor is made of binary or ternary sulfides of elements e.g. bismuth, antimony, tin or copper or solid solutions or copper I halides, copper tin sulfides or copper chloride.

Description

The transition from fossil fuel-powered vehicles to electromobility requires power storage of very high energy density at economic prices, a problem that has not yet been solved. Comparable problem solving requires the desired transition of the supply of electrical energy from fossil fuels and nuclear energy to renewable energy production by wind turbines and photovoltaic electricity.

Regenerative power generation depends on the duration and intensity of solar radiation as well as wind speeds and is therefore not continuous. As a result, these forms of energy generation as such are not eligible for baseload. To meet the demand for the supply of energy you need very high storage capacities. So far, the storage is carried out insufficiently by pumped storage power plants, which have efficiencies of 80%. Studies at European level show that the construction of new pumped storage power plants in Europe is very limited; There are no geological and hydrological boundary conditions for the construction of large additional pumped storage power plants. All other possibilities of energy storage are not yet suitable for economically saving energies in the range of megawatts or even gigawatts.

The lack of economic power storage has also led to the undesirable situation that with the expansion of wind turbines and photovoltaic systems parallel power plants must be built, which must cover the current demand quickly with a decline in renewable electricity generation. These are essentially natural gas-fired power plants that can be started up quickly. Since the running costs of these power plants such as capital costs, maintenance or personnel continue to run during downtimes, these costs must be allocated to the terms. This makes their electricity more expensive the shorter their working hours are. The protection of the base load thus leads to a disproportionate increase in the total electricity costs as the proportion of regeneratively generated electricity increases, partly due to the standstill costs of the "stand-by power plants" and partly due to the higher electricity generation costs of the regenerative generation.

The development of economical storage systems is therefore the focus of scientific and technological work in all industrial nations.

Compressed air storage, despite heat recovery losses by 30 to 40%. They require extensive storage for thermal energy and large underground caverns for storing the compressed air. Such caverns do not exist in arbitrary volumes; but you also want to use caverns for storing natural gas as well as hydrogen or methane. Finally, there is too little suitable storage volume.

As further ways of storing electrical energy, the electrolysis of water to hydrogen and oxygen is discussed. The efficiency of this electrolysis is a maximum of 70%, because the energy fraction bound in the oxygen can not be used. As soon as the hydrogen is recycled by combustion in turbines, a loss of efficiency of 50 to 60% is incurred, which means a total loss of around 65%. If you wanted to convert the hydrogen back to electricity by means of a fuel cell, then the total loss would be slightly lower, by 55%. However, it has been found that fuel cell technology is uneconomical for the size of the quantities of electricity to be stored, and it has not even proven to be economical to drive vehicles in the kilowatt hour range.

Unfortunately, the chemical conversion of hydrogen with carbon dioxide to methane, which can be transported through existing pipeline networks and could be used as a cheap storage medium, associated with significant conversion losses. In the chain electricity-hydrogen-methane-electricity the total loss is about 65 to 75%.

The storage of energy in magnetic fields is limited to small amounts of energy. The storage capacity of superconducting magnetic fields is much too low, the superconductivity is also destroyed by high magnetic fields. Therefore, this type of energy storage has not come out over small demonstration plants in the last twenty years.

As a mechanical storage flywheels are able to provide a high performance in a short time and thus compensate for short-term energy losses. However, they can not store large amounts of energy.

As storage for large amounts of energy electrochemical storage are discussed, the electrolyte can be stored separately in tanks (redox flow principle). Several demonstration plants of redox flow batteries were built. Because of their lack of economic efficiency but so far no large plants were built.

In principle, reversible chemical reactions are carried out in a reversible battery, an accumulator, on electrodes, which are subject to the thermodynamics of chemical reactions. While an oxidation takes place at one electrode, an electrochemical reduction takes place at the counterelectrode. Even a very expensive reversible battery would be economical if it allowed an almost infinite number of charge and discharge cycles. Unfortunately, however, the chemical reactions occurring in each reversible battery are not completely reversible. Due to the thermodynamic conditions, undesirable by-products always occur, which concentrate with increasing number of cycles of charge and discharge and thus reduce the capacity of the battery from cycle to cycle. This includes chemical changes of the electrolytes as well as undesired oxidation states, as well as undesired changes on the electrode surfaces, in particular on the boundary layers separating the electrodes from the electrolyte or undesirable changes in the volume of the electrodes in the case of intercalation electrodes.

Especially the boundary layers in lithium-ion batteries (solid-electrolyte interface), which separate the electrolyte from the lithium electrode, are thermodynamically unstable. There are no higher-valued metal ion-containing networks from which boundary layers could be built, which are thermodynamically stable over metallic lithium with its extremely high reduction potential over time. All metal ions such as Al ³⁺ , Sc ³⁺ , Si ⁴⁺ , Ti ⁴⁺ or Zr ^{4+ used} to form the networks are not stable to metallic lithium; they are irreversibly reduced by the lithium, which damages the cell.

Networks containing only lithium as a cation and anions such as sulfide, phosphide, nitride or oxyphosphide nitride (LiPON) are thermodynamically stable to lithium. However, because of their low network density, which is formed only by the anion network, they have low mechanical stabilities and are not stable to organic electrolytes in which they swell. Even organic materials such as polymers are not stable.

Despite the intensive research and development, the thermodynamic boundary conditions mean that there is still no economic electrochemical power storage for the operation of vehicles as well as for the storage of electrical energy in public networks.

Electrical capacitors do not have the disadvantage of irreversible electrochemical reactions and therefore have much longer lifetimes. No masses are moved, just electrical charges. Unfortunately, capacitors contain even lower energy densities than electrochemical current storage. For example, the energy density of commercial electrolytic capacitors is 0.1 to 0.2 watt-hours per liter. Commercial double-layer capacitors, on the other hand, have energy densities in the range of 5 to 10 watt-hours per liter. New developments expect energy densities of up to 30 watt-hours per liter. Double-layer capacitors have an enormous capacity of around 10 microfarads per square centimeter or 100 farads per gram. A commercial double-layer capacitor with a capacity of 5,000 Farads and an operating voltage of 2.5 volts has a diameter of 76 millimeters at a height of 150 millimeters corresponding to a volume of about 0.7 liters, which is based on the energy content of a capacitor of ½ C × U ² gives an energy content of about 6-7 watt-hours per liter. The high capacitance results from the large area of the electrodes used with surfaces around 1,000 m ² per gram and the small spacing of the double layers by 0.4 to 10 nanometers. Often used as electrode material inexpensive pressed activated carbon.

The electric field within the bilayer is up to 5,000 volts / micron, a field strength to which a common dielectic would not be resistant. Here, however, the laws of atomic field strengths apply, which are determined by the properties of atomic and molecular bonds.

Unfortunately, the energy content of double-layer capacitors can not be particularly increased, because the usable working voltage due to the electrochemical resistance of the ions dissolved in the electrolyte and by the electrochemical resistance of the polar solvents used such as acetonitrile, dimethylacetamide, gamma-butyrolactone, ethylene carbonate, propylene carbonate or others to voltages of a maximum of 4 volts is limited. Organic electrolytes also have lower electrical conductivities than aqueous electrolytes. If you work with aqueous solvents such as alkali, so are the Conductivity greater, but the working voltage is further reduced to values around 1.2 to 1.5 volts because of the lower electrochemical stability of the aqueous electrolyte. As dissolved, the bilayers thawing salts such as lithium perchlorate, lithium tetrafluoroborate or tetraalkylammonium tetrafluoroborates are used.

It was therefore an object of the invention to find a power storage device that combines the advantages of the high capacity of a double-layer capacitor with a higher working voltage. If it were possible to increase the working voltage by a factor of ten from, for example, 2.7 volts to 27 volts at a constant capacitance, then because the working voltage enters the energy content quadratically, the energy density would be one hundred times higher, ie 600 to 700 watt hours per liter thus better than the best reversible lithium ion batteries.

Since energy storage is ultimately bound to mass and per kilowatt hour of energy content and the corresponding masses must be present, no or very small amounts of rare and expensive elements such as precious metals, rare earth metals, indium, gallium, germanium, selenium or tellurium should be used.

Toxicologically hazardous elements such as thallium, cadmium, mercury, lead or arsenic should be avoided. Also phosphorus in the form of metal phosphides should not be used. The metal phosphides form very toxic gaseous phosphines with moisture.

The object of the invention is achieved by using electrically conductive electrodes with very high specific surface areas to produce double layers according to the prior art, but instead of a liquid electrolyte working with a solid system and building permanent bilayers. Thus, electrochemical limitations of the operating voltage are bypassed and exploited the high electrical conductivity of a semiconducting material.

The inventive solution will be explained with reference to the attached sketch. The sketch does not reflect the geometric standards, it is only an explanation of the basic operation:
Analogous to double layer capacitors according to the prior art, the electrode units consist of the current collectors ( 1 ) and ( 5 ) and the conductive electrodes of high specific surface area ( 2 ) and ( 4 ). The electrodes are not in electrical contact with each other. If necessary, they are replaced by a temperature-resistant loose fabric or fleece ( 6 ) of glass fibers or a non-conductive ceramic material of fibers of, for example, alumina or mullite electrically separated.

To produce the current memory according to the invention, the electrodes ( 2 ) and ( 4 ) by a molten salt ( 3 ) infiltrated above its melting point, the temperature of the entire assembly being higher than the melting temperature of the molten salt. About the electrically conductive current collector ( 1 ) and ( 5 ) is now applied by the current source S, a DC electrical voltage, whereby with respect to the surfaces of the electrodes ( 2 ) and ( 4 ) a double layer is produced, in the sketched example by the arrangement of negatively charged anions of the molten salt ( 3 ) against the positively charged electrode ( 2 ) as well as positively charged cations with respect to the negatively charged electrode ( 4 ). The applied voltage is chosen so low that no electrochemical reactions on the electrodes ( 2 ) and ( 4 ) and at the current collectors ( 1 ) and ( 5 ) occur. While maintaining the applied voltage, the assembly is allowed to cool. If the arrangement undershoots the melting point of the molten salt, the molecular arrangement of its interfaces is frozen, and with respect to the electrodes ( 2 ) and ( 4 ), a permanent double layer of ions is obtained, a ferroelectric double layer.

At the same time as falling below the melting point of the molten salt ( 3 ), the remaining salt beyond the bilayers, the matrix material, passes into the crystalline and electrically semiconducting state, whereby a solid and conductive counterelectrode is added to ( 2 ) and ( 4 ) arises. The matrix material consists of the electrically highly conductive semiconductor, which allows the movement of charge carriers between the bilayers.

Compared to a double layer in a liquid, a solid double layer has great advantages:
It is largely insensitive to electrochemical reactions. Transfers of electrons are due to the fixed distance between the bilayers and the electrodes ( 2 ) and ( 4 ) under medium voltages, this would require shocks between the electrodes and the ions forming the bilayers. It can therefore also be explained that the metal-semiconductor contacts blocking the current flow in the power electronics under the influence of high electrical voltages no subject to electrochemical changes. The voltage capability of the inventive memory is approximately equal to the reverse voltage of a metal-semiconductor contact or Schottky contact, which in the case of silicon is about 30 volts.

The used semiconductors ( 3 ) preferably have high band gaps of more than 1 electron volt (eV). The bandgap of silicon is comparatively 1.1 eV. The higher bandgap allows higher reverse voltages. In place of the lightly doped or insulating thin layer between the metal and the N-type semiconductor of a high-voltage rectifier according to the principle of a Schottky diode occurs in the energy storage devices of the invention, the high-resistance layer of ions. This additionally enables higher operating voltages, the magnitude of which is estimated to be between 40 and 100 volts.

Even higher working voltages are possible if, as electrode material ( 2 ) does not use carbon with its work function around 4.8 to 4.9 eV, but materials with a work function lower than that of the material ( 3 ). In case of the N-conduction of the material ( 3 ) is thus a transfer of electrons ( 3 ) to ( 2 ) additionally difficult.

Such materials with very high specific surface area and low work function are, for example, titanium carbide with 3.4 to 3.8 eV or manganese dioxide with 2.8 to 3 eV as work function (US Pat. "Electron Affinity and Work Function of Pyrolytic MnO2 Thin Films Prepared from Mn (C2H3O2) 2 4H2O", M.I. et al., Solid State Ionics, Proceedings of the 10th Asian Conference, June 12-16, 2006, pp 193-200 ). Their preparation, for example by pyrolysis of the metal carboxylates such as manganese acetate in the presence of atmospheric oxygen or by pyrolysis of the titanium alkoxides such as titanium tetrabutoxide under the exclusion of oxygen, is state of the art.

Overall, operating voltages should be achievable that correspond to those of power rectifiers with values greater than 100 volts. At the same time, the ferroelectric double layer gives a higher local dielectric constant, which contributes to an additional increase in capacitance.

Another advantage of the capacitive memory of the invention is that because of the lack of any solvent, the number of ions per volume and area is significantly greater than in prior art double layer capacitors. In addition, the ions are not surrounded by solvent molecules. As a result, the charge density of the double layers of the memories according to the invention is higher than that of conventional double-layer capacitors. This achieves higher energy contents.

At the same time, the field strength in the vicinity of the bilayers drops faster due to the stronger Debye shield, which further increases the breakdown field strength.

The materials ( 3 ) consist of metal cations and anions. The large anions in comparison to the metal cations are much easier to polarize than the smaller metal cations because of their larger electron shell and the resulting easier deformability of their electron shell. Higher electronic polarizability, together with the shift polarization of the ions, contributes to the easier formation of an ionic double layer. This is opposite to the electrode ( 2 ) produces a very dense bilayer of highly polarizable anions.

The significantly lower electronic polarizability of the positive metal cations leads to the double layer which causes the current collector to have negative potential ( 5 ), is formed as a comparatively less sharply demarcated double layer. The formation of a diffuse double layer at this point is favorable, because thereby the crystal lattice of the semiconductor is less disturbed and the electrical conductivity is hardly lowered. Thus, it may be possible that between the electrode ( 4 ) and the semiconductor ( 3 ) forms an electrically conductive ohmic contact. Thus, there is the possibility that a series connection of two capacitors with halving the capacity is avoided, which approximately doubles the capacity of the arrangement.

In operation, the inventive capacitors have fundamental differences from prior art electrical double layer capacitors:
While the conductive connection between the two series-connected double-layer capacitors according to the prior art takes place by ion conduction, according to the invention it comes about by electron or hole conduction. The charge storage is achieved by the movable charges on opposite surfaces of a capacitor. In the double-layer capacitors according to the state In the art these are the induced charges on the surfaces of the electrodes of high surface area and the bilayers formed by the mobile ions opposite charge. In the case of the capacitors according to the invention, these are also the induced charges on the electrodes of high specific surface area ( 2 ) and ( 4 However, the active mating surfaces form the interfaces between the semiconductor matrix and the ferroelectric layers. The rigid, non-conducting ferroelectric layers perform the function of a dielectric with high dielectric constants.

The invention is explained in more detail below:
As a current collector ( 1 ) and ( 5 ) a metal or alloy is used whose melting point is higher than the temperature at which the high surface area electrodes ( 2 ) and ( 4 ) with the molten salt ( 3 ) are infiltrated. This may be, for example, a sheet or foil of an iron alloy from which a lawn of carbon nanofibers grows in a carbon-containing atmosphere according to the prior art. This advantageously results in a very low-resistance contacting of ( 2 ) on ( 1 ) as well as from ( 4 ) on ( 5 ).

But it can also be the materials of the current collector ( 1 ) and ( 5 ) on, for example, layers of the pressed high-surface area electrode material ( 2 ) and ( 4 ) spray on.

As electrodes ( 2 ) and ( 4 For example, carbon materials such as activated carbon, carbon nanotubes, carbon nanofibers, graphene, manganese dioxide, titanium carbide or other very high surface area, electrically conductive materials may be used in the art.

Of central importance is the material ( 3 ). As material ( 3 ), an ionic compound is used which is a semiconductor at temperatures below its melting point, but above the melting point, a molten salt with conventional ion-conducting properties without electronic line components. Such materials are essentially the sulfides of the elements bismuth, antimony, tin and copper and the halides of monovalent copper. Selenides and tellurides such as Ag ₂ Se, Tl ₂ Se, NITE ₂ or NITE are semiconducting compared to sulfides in the liquid state.

The semiconductors ( 3 ) should have the lowest possible melting temperatures to allow infiltration of the electrodes at the lowest possible temperatures. They should have the highest possible band gap to allow the highest possible working voltages. Finally, they should have the lowest possible specific electrical resistances in order to obtain an energy store with the lowest possible internal resistance and thus high power density.

Many of the sulfides in question are known from the literature. They have already been investigated as to whether they can be used as active materials for photovoltaic cells or for thermoelectric modules because of their semiconducting properties. A selection of semiconducting sulfides with literature data is shown in the table following the text.

In addition to binary compounds such as Bi ₂ S ₃ , Sb ₂ S ₃ , Cu ₂ S or SnS and ternary compounds of the systems Cu-Bi-S, Cu-Sb-S, Cu-Sn-S or Bi-Sn-S are used come. Especially the system Cu-Bi-S is rich in single compounds. By contrast, no stable compounds are known from the Bi-Sn-S system.

The system Bi-Sb-S also does not form ternary compounds, there are only the solid solutions (Bi ₂ S ₃ ) _x (Sb ₂ S ₃ ) _(1-x) whose melting points are between those of bismuth sulfide and antimony sulfide ,

In general, sulfides with antimony have lower electrical conductivities than sulfides with bismuth.

After a first look at the table, the compound Cu ₃ BiS ₃ (= 3Cu ₂ S · Bi ₂ S ₃ ) with its low melting point of 527 ° C appears to be the most attractive for the application. It is thus very close to the melting point minimum of the system Cu ₂ S-Bi ₂ S ₃ at 523 ° C. However, compared to bismuth sulfide, Bi ₂ S ₃ , its electrical conductivity is at least sixfold lower, which would lower the performance of the capacitor. On the other hand, the semiconducting ternary copper-tin sulfides Cu ₂ SnS ₃ (= Cu ₂ S · SnS ₂ ) and Cu ₄ SnS ₄ (= 2Cu ₂ S · SnS ₂ ) are considerably more attractive. They have low resistance values, but their melting temperatures are 840 and 860 ° C, respectively.

The in the literature "Transport Properties of Bi2S3 and the Ternary Bismuth Sulfides KBi0.33S10 and K2Bi8S13", G. Kanatzidis et al., Chem. Mater. Vol. 9, No. 7, 1997, pp 1655-1658 The ternary compound K ₂ Bi ₈ S ₁₃ (= K ₂ S · 4Bi ₂ S ₃ ) described represents one type of potassium sulfide-highly doped bismuth sulfide, with part of the Bi ^{3+ being} replaced by K ⁺ . The room temperature conductivity is attractive 0.003 ohm cm, the congruent melting point at 713 ° C and thus is basically also a suitable material for the application. However, it has a narrower band gap of less than 0.5 eV ( "Synthesis and Thermoelectric Properties of New Ternary Sulfides KBi6, 33S10 and K2Bi8S13", MG Kanatzidis et al., Chem. Mater. 1996, 8, pp 1465-1474 ). The narrow band gap points to a lower and thus more disadvantageous dielectric strength than for Bi ₂ S ₃ with its band gap of 1.3 eV.

With respect to the lower melting temperature of 546 ° C. and the higher band gap of 1.7 to 1.8 eV, a reproducible doping of antimony sulfide would be to 1 ohm.cm without loss of N-conduction and without particular lowering of the band gap would lead, ideally.

Unfortunately, the introduction of alkali metal ions into the antimony sulfide structure analogous to the bismuth compound, as in the case of the compound KSb ₅ S ₈ (= K ₂ S · 5Sb ₂ S ₃ ), does not lead to a substantial increase in the electrical conductivity. Although the compound is semiconducting and has a pleasingly high band gap of 1.8 eV and an advantageously low congruent melting temperature of 441 ° C, its resistivity is greater than 10 ⁸ ohm cm ( "Phase Change Materials For Storage Media," JB Wachter et al, Journal of Solid State Chemistry, 180 (2007) pp 420-431 ).

Among the sulphides known in the literature, bismuth sulphide is, in addition to the copper-tin sulphides, a compound which is well suited to the application. Solid bismuth sulfide is an intrinsic N-type semiconductor with a density of 6.78 grams / cc. It melts congruently at a relatively low melting temperature of 760 to 775 ° C and exhibits among the known sulfidic semiconductors one of the highest electrical conductivities at room temperature. At room temperature, the resistivity is about 0.05 ohm.cm. From the stoichiometric composition small excesses of bismuth can be tolerated, but not a sulfur surplus. ( "Transport Properties of Bi2S3 and the Ternary Bismuth Sulfides KBi6, 33S10 and K2Bi8S13", MG Kanatzidis et al., Chem. Mater. Vol. 9, No. 7, 1997, 1655-1658 ).

It is not advantageous to lower the melting point of bismuth sulfide by the addition of antimony sulfide, because thereby the electrical conductivity drops sharply.

Compared to the complex ions of double layer capacitors according to the prior art with large complex cations such as Teraalkylammoniumionen the sulfide ions of the metal sulfides have smaller radii, whereby the volume and surface density is additionally increased advantageously.

The transition from a molten salt to the semiconductor can be observed very easily with bismuth sulfide:
Taking a quartz container with the melt from an oven above the melting temperature, the bismuth sulfide shows up as a water-clear, slightly yellowish liquid, completely transparent and of low viscosity.

But as soon as the melting temperature is reached at a point on the container wall as a result of the cooling, there forms a metallic silvery shiny and the light-reflecting layer of the semiconducting bismuth sulfide, the typical sign of free conduction electrons.

The second group of suitable materials ( 3 ) are the halides of monovalent copper, copper iodide CuJ, copper bromide, CuBr and copper chloride, CuCl. The following list shows the features that are essential for the application: material melting temperature density bandgap el. polarizability ° C g / cm ³ eV 10 ^-24 cm ³ Cul 606 5.67 3.1 7.1 (J) CuBr 492 4.77 3.1 4.8 (Br ^- ) CuCl 422 4.14 3.4 3.7 (Cl ^- )

In contrast to the sulfides, the copper halides advantageously have lower melting temperatures, lower densities, higher band gaps and higher electronic polarizabilities Anions on. The electronic polarizability of Cu ⁺ should not be greater than the polarizability of the potassium ion at 0.8 × 10 ^-24 cm ³ , rather it should be lower because of the smaller inner radius. The Cu ⁺ ion has a radius of 60 to 80 picometers, the K ⁺ ion to a 150 picometers. However, the electrical conductivities of the pure copper halides are lower than those of sulfides such as bismuth sulfide. Copper iodide has the largest cost because of the use of iodine, moreover decomposes copper iodide above its melting point with appreciable speed. When using copper iodide there is also the danger of ionic conduction in the solid state under the influence of high electric fields.

With the higher electronic polarizabilities of the halide ions, when the copper halides are used, the negatively charged double layer can be exposed to the electrode ( 2 ) in high quality. Because of the broader band gap, higher working voltages than the sulfides are to be expected.

Of the copper I halides, copper I chloride is most attractive because of its low density, lowest melting point among copper halides, high thermal stability, lowest cost, and technical accessibility. To increase its electrical conductivity, it is doped with up to three mole percent zinc chloride, ZnCl ₂ , whereby resistivities specific to room temperature of around 6 ohm.cm are achieved ( Rajami K. Vijayarayhavan, Thesis, School of Electronic Engineering, Dublin, July 2011 - Investigation of doped cuprous halides for photovoltaic and display applications ). Thus, an advantageous n-conductive material for the inventive application is obtained.

From its comparatively low melting point at 420 ° C., zinc phosphide, Zn ₃ P ₂ , would also be present as a semiconductor ( 3 ) suitable material. It has a density of 4.55 grams / cm ³ , its band gap is 1.5 eV. Zinc phosphide, however, is questionable in terms of toxicology because of the phosphidione. In addition, the resistivity is relatively high at 100 to 300 ohm.cm. Analogous reasons argue against the use of copper cyanide, CuCN (melting point 473 ° C) and copper thiocyanate (melting point 1.084 ° C, band gap 3.6 eV).

To the high-area electrode ( 2 ), the charge is connected to the positive pole. Thus, the positive electrode opposite to the negative hochpolarisierbaren sulfide ions S ^2- or halide ions. Due to their high electronic polarizability, these ions form a sharply defined double layer, which results in a particularly high capacitor capacity. The sulfide ions, like most large ionic anions, have a high polarizability. According to the Pauling scale, the electronic polarizability of sulfide ions increases to 10.2 × 10 ^-24 cm ³ . The electronic polarizability of the metal ions such as that of the bismuth ions Bi ³⁺ is below that of barium ions (140-160 picometers) with a value of 1.55 × 10 ^-24 cm ³ because of their smaller inner radius (100-110 picometers).

Between the electrode ( 4 ) and the semiconductor ( 3 ) can form an ohmic contact due to the more diffuse double layer of the less polarizable metal ions, which prevents the transfer of electrons from the electrode ( 4 ) in the material ( 3 ) allows.

To promote this transfer of electrons this means for n-type materials ( 3 ) that the work function of the surface of the electrode ( 4 ) must be lower than that of the material ( 3 ). The work functions of the metal sulfides such as bismuth sulfide are at 5 eV, that of copper chloride at 6.2 eV, that of carbon fortunately at 4.6 to 5 eV, whereby the escape of electrons from the electrode ( 4 ) in the semiconductor ( 3 ) in the case of activated carbon as electrode material ( 4 ) would not be hindered.

Used as material ( 3 ) uses a P-type semiconductor, the ratios at the interface between the semiconductor ( 3 ) and the negative electrode ( 4 ) in the case of an ohmic contact:
While the conditions at the positive electrode remain substantially unchanged, arises at the interface of the current collector ( 4 ) to P-type semiconductor ( 3 ) a blocking diode surface. Electrons enter the P-type semiconductor and neutralize the near-surface holes, widening the blocking depletion layer and arresting further current flow. An ohmic transition between the electrode ( 4 ) with a high specific surface area and the material ( 3 ) is not possible. For this reason, as semiconductors ( 3 ) preferred those of the n-type conductivity.

Should it prove in the design and manufacture of the capacitive energy storage devices according to the invention that the infiltration of the electrode ( 2 ) should be too expensive due to too small pore sizes, so it is according to the capacitor formula energy content = ½C × U ² , after which the stored energy is proportional to the capacity, but proportional to the square of the voltage, by reducing the surface to a part the capacity to dispense in favor of a higher working voltage.

Due to the low mechanical elongation at break of the sulfides and the copper halides, the construction of the inventive energy storage device in the form of stacked cells is preferable to the conventional winding cells. Owing to the high electrical conductivity of the semiconductors, a greater layer thickness of the electrodes than the prior art (FIG. 2 ) and ( 4 ) not disadvantageous.

Bismuth sulfide is easily prepared by reacting bismuth with technically pure sulfur under inert conditions. Low-cost bismuth with a purity of 99.5% as well as desulfurization with very pure and cheap technical sulfur can be used.

For the production of laboratory quantities, for example, the stoichiometric amounts of the elements are heated in an evacuated quartz ampoule, for which the material is heated in a furnace linearly over ten hours from room temperature to 900 to 1000 ° C and then maintained at 900 to 1000 ° C for 5 hours ( DE 10 2005 008 865 ).

In the absence of a great need to date, technical quantities of bismuth sulphide are currently not produced on a ton scale. After all, the world production of bismuth metal, preferably as a by-product of the production of lead, tungsten, zinc or tin in 2010 was around 16,000 tonnes. For metallic bismuth with a purity of 99.99%, the 2010 price was around $ 20 / kg.

Laboratory methods are not very suitable for the synthesis of tonnages on a scale. Wet-chemically, bismuth sulfide can be prepared by reacting bismuth salts such as chloride with soluble sulfides such as sodium sulfide or less expensive sodium hydrosulfide. Bismuth chloride is an intermediate in metallurgical processes for the isolation and purification of metallic bismuth anyway. For purification bismuth chloride is distilled (boiling point 447 ° C). The water-insoluble bismuth sulfide is filtered off or centrifuged off, washed and dried. However, this synthetic route leads to by-product salts, and there is also the disadvantage of partial hydrolysis of the sulfide.

More technically relevant is therefore the synthesis of the elements:
For the free enthalpy of formation from the elements, the relationship delta G ⁰ = -193,450 + 101,0 × T (J / mol) is given in the literature. Thereafter, the free enthalpy of formation at 400 ° C is about -125 kJ / mol, at 800 ° C still around -85 kJ / mol. The enthalpy of fusion of the bismuth sulfide is 79.37 kJ / mol. The activity of sulfur in bismuth sulfide at 850 ° C is around 0.007, resulting in a sulfur vapor pressure of about 2 Pa ( "The Bi-S (bismuth-sulfur) system, J.-C. Lin et al., Journal of Phase Equilibria, Vol. 2, 1996, pp 132-139 ).

Under an initial pressure of 10 ^-4 mm Hg evaporate from 10 grams of bismuth sulfide at 650 ° C per hour around 0.4 grams, preferably sulfur ( GG Gospodinov et al., "Study Of The Behavior Of Bismuth Sulfides, Antimony Sulfides And Antimony Selenide Under Sublimation In A Vacuum," NASA Technical Translation F13, 461, March 1971 ). This means that the synthesis must always be carried out in a closed container.

To D. Cubicciotti, "The Bismuth-Sulfur Phase Diagram", J. Phys. Chem., 1962, 66 (6), pp 1205-1206 Upon heating a stoichiometric mixture of sulfur and bismuth, the sulfur first melts (melting point about 115 ° C), then the bismuth (melting point 271 ° C). Shortly after the bismuth is melted, a highly exothermic reaction takes place and a gray, sponge-like mass is obtained. The reactivity of bismuth with sulfur is so high at low temperatures that a pure product of high electrical conductivity can be produced from the incompletely fully reacted sponge in solid phase at, for example, 500 ° C. ("Transport Properties ....", see above). This method of preparation can be carried out without great effort in boxes which are heated in an oven in an inert or sulfur-containing atmosphere. Their advantage is that the dissociation pressure, 200 to 300 ° C below the melting point, is very low and that low losses of sulfur in the range of up to one mole percent can be tolerated.

It is more advantageous, without a preliminary reaction, to proceed directly from a bismuth powder which is produced from the melt state, as is customary, for example, in powder metallurgy, with the exclusion of oxygen by spraying. Such a powder is sulfided in a sulfur atmosphere, whereby the reaction temperature is steadily raised, but always remains below the melting point of the reaction mixture. The melting temperature of the reaction mixture increases steeply: Even at a sulfur content of about 10 atomic percent, the melting point is 500 ° C. This method avoids exothermic overheating and oxidation and / or hydrolysis of the product. Because of the sulfur atmosphere the dissociation of the bismuth sulfide is suppressed or excluded according to the dissociation equilibrium.

From a process engineering point of view, a synthesis in the melt is more economical due to the much higher reaction rate and the resulting smaller reactor volumes:
In a small pressure-tight continuous reactor, a stirred reactor or better a flow tube, a bismuth sulfide prepared elsewhere is introduced and melted. In this melt, the stoichiometric amounts of bismuth and sulfur melt are injected at separate locations under elevated pressure, after which the reactants, diluted in the melt, come with controllable exothermic reaction. All parts contacted with the reactants and the melt must have material surfaces which are inert to the reaction conditions, for example borated, carburized or nitrocarburized surfaces of high temperature resistant metallic materials.

With a bismuth sulfide density of 6.8 g / cc, a price of $ 20 / kg for metallic bismuth (99.99% purity, the price of 99.5% purity is expected to be lower), a bismuth content of 81% by weight for bismuth sulfide Neglecting the volume of the electrodes and a conservatively estimated energy density of 0.7 kWh per liter, the material cost per kilowatt hour would be around $ 160.

Because of the lower density of 4.86 g / cc and the lower proportion of high-priced tin (production world 2011: around 300,000 tons), which also costs about $ 20 / kg like bismuth (Cu: around $ 8 / kg), the raw material costs for Cu ₂ SnS ₃ (indirect band gap around 1 eV, direct band gap around 1.7 eV) at $ 70 / kWh.

The raw material costs for the more attractive of its higher band gap and the lower tin content Cu ₄ SnS ₄ (indirect band gap at 1.2 eV, direct band gap around 1.8 eV, density 4.56 grams / ccm) are at 60 $ / KWh. However, the melting temperatures of these semiconductors are about one hundred degrees higher than those of the bismuth sulfide and the electrical conductivities are lower by a factor of ten.

Because of the higher reaction enthalpies, the preparation of these compounds in solid phase is preferable.

For this purpose, the single phase in the liquid phase alloys are sprayed with the compositions Cu ₂ Sn or Cu ₄ Sn from the melt and the finely divided powder reacted with increasing the temperature in the solid phase with gaseous sulfur. Since in this case the enthalpies of reaction are higher than in the reaction with bismuth, complete conversions at temperatures of up to 500 ° C. are obtained by this method.

Copper chloride has the lowest cost of raw materials at about $ 30 / kWh. There are technical processes for the production of copper chloride. For example, it is according to the patent GB 1 284 110 by adding copper and introducing chlorine into a given melt of cuprous chloride. In this case, the addition of the necessary amounts of dopants, for example by the addition of zinc, the copper-I-chloride are doped simultaneously. Table: Literature data of a selection of sulfidic semiconductors. The electrical conductivities vary depending on the manufacturing conditions and crystal form. The value for Bi ₂ S ₃ is derived from a material obtained by the reaction of bismuth with sulfur at 500 ° C. sulfide melting point spec. Resistance at RT bandgap cable type ° C Ohm × cm eV Bi ₂ S ₃ 760-775 about 0.05 1.3 n Sb ₂ S ₃ 546 about 10 ⁸ 1.8 n Sb ₂ S ₃ 546 30-300 (film, C-doped) 1.8 n SnS 882 20-100 1.3-1.6 p Cu ₂ S 1100 0.03-0.4 1.2 p Cu ₂ SnS ₃ 856 0.1-20 about 1 indir, 1.7 you p Cu ₄ SnS ₄ 833 0.1-2 approx. 1.2 ind., 1.8 dir p Cu ₃ BiS ₃ 527 30-100 1.4 p K ₂ Bi ₈ S ₁₃ 713 0,003 <0.5 n KBi _6,33 S ₁₀ 0.02 <0.5 n Cu ₂ ZnSnS ₄ 980-990 0.1-100 1.5 - 1.7 p Cu ₃ SbS ₃ 610 102-10 ⁶ 1.5 p CuSbS ₂ 553 30-3,000 (film) 1.5 p KSb ₅ S ₈ 441 Approximately 108 1.8 n Bi ₂ S ₃ / SnS Eutekt. 650 Bi2S3 / Sb2S3 546-775 (no compounding, solid solutions) n Cu ₂ S / SnS only simple eutectic, in the solid state no miscibility

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102005008865 [0065]
• GB 1284110 [0078]

Cited non-patent literature

• "Electron Affinity and Work Function of Pyrolytic MnO2 Thin Films Prepared from Mn (C2H3O2) 2 4H2O", M.I. et al., Solid State Ionics, Proceedings of the 10th Asian Conference, June 12-16, 2006, pp 193-200 [0028]
• "Transport Properties of Bi2S3 and the Ternary Bismuth Sulfides KBi0.33S10 and K2Bi8S13", G. Kanatzidis et al., Chem. Mater. Vol. 9, No 7, 1997, pp 1655-1658 [0045]
• "Synthesis and Thermoelectric Properties of New Ternary Sulfides KBi6, 33S10 and K2Bi8S13", MG Kanatzidis et al., Chem. Mater. 1996, 8, pp 1465-1474 [0045]
• "Phase Change Materials For Storage Media", JB Wachter et al, Journal of Solid State Chemistry, 180 (2007) pp 420-431 [0047]
• "Transport Properties of Bi2S3 and the Ternary Bismuth Sulfides KBi6, 33S10 and K2Bi8S13", MG Kanatzidis et al., Chem. Mater. Vol. 9, No 7, 1997, 1655-1658 [0048]
• Rajami K. Vijayarayhavan, Thesis, School of Electronic Engineering, Dublin, July 2011 [0056] "Investigation of doped cuprous halides for photovoltaic and display applications".
• "The Bi-S (bismuth-sulfur) system, J.-C. Lin et al., Journal of Phase Equilibria, Vol. 2, 1996, pp 132-139 [0068]
• GG Gospodinov et al., "Study Of The Behavior Of Bismuth Sulfides, Antimony Sulfides And Antimony Selenide Under Sublimation In A Vacuum," NASA Technical Translation F13, 461, March 1971. [0069]
• D. Cubicciotti, "The Bismuth-Sulfur Phase Diagram", J. Phys. Chem., 1962, 66 (6), pp 1205-1206 [0070]

Claims (8)

Capacitive high energy density electrical storage with a current collector ( 1 ) contacted electrically conductive electrode ( 2 ) of high specific surface area and a second electrode of high specific surface area ( 4 ) connected to a second current collector ( 5 ), characterized in that they contain solid, permanent bilayers and that these solid bilayers are transformed into a solid semiconductor ( 3 ) as a counter electrode for charge transfer and that they can be operated at working voltages greater than 10 volts. Capacitive electrical storage according to claim 1, characterized in that the double layers and the semiconductor ( 3 ) between the electrodes of high specific surface area ( 2 ) and ( 4 ) and the current collectors ( 1 ) and ( 5 ) consist of a single material, which is a pure ion-conducting molten salt above its melting point, but below its melting point exists as a semiconductor of high electrical conductivity. Capacitive electrical storage according to claims 1 and 2, characterized in that the solid, permanent double layers are produced by placing the electrodes of high specific surface area ( 2 ) and ( 4 ) above the melting temperature of the semiconductor material ( 3 ) with the melt of the semiconductor material ( 3 ) infiltrates or soaks in the melt state via the current collector ( 1 ) and (5) applies an electrical voltage which is insufficient to trigger electrochemical reactions in the memory, but which have double layers opposite to the electrodes of high specific surface area ( 2 ) and ( 4 ) and, while maintaining the applied voltage, cooling the assembly, the melt matrix of ( 3 ) goes below the melting point in the semiconducting state. Capacitive electrical storage according to claims 1 to 3, characterized in that the semiconductor ( 3 ) between the electrodes ( 2 ) and ( 4 ) and the current collectors ( 1 ) and ( 5 ) consists of binary or ternary sulfides of the elements bismuth, antimony, tin or copper or their solid solutions or of copper (I) halides. Capacitive electric storage according to claims 1 to 4, characterized in that the semiconductor ( 3 ) between the high surface area electrodes ( 2 ) and ( 4 ) and the current collectors ( 1 ) and ( 5 ) of bismuth sulfide, Bi ₂ S ₃ , copper-tin sulfides of the formulas Cu ₂ SnS ₃ , Cu ₄ SnS ₄ or copper chloride, CuCl. Capacitive electrical storage according to claims 1 to 5, characterized in that between the semiconductor ( 3 ) and the surfaces of the electrode ( 4 ) and the electricity collector ( 5 ) there is an ohmic contact. Capacitive electrical accumulator according to claims 1 to 6, characterized in that the positive electrode of high specific surface area ( 2 ) is made of a material whose work function is at least 0.5 electron volts lower than that of the semiconductor ( 3 ). Capacitive electrical storage according to claims 1 to 7, characterized in that the electrodes ( 2 ) and ( 4 ) by a temperature-resistant fabric layer ( 6 ) are electrically isolated from each other from glass fibers or oxide ceramic fibers.

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