EP4252302A1

EP4252302A1 - Solid state cell and associated manufacturing method

Info

Publication number: EP4252302A1
Application number: EP21805407.0A
Authority: EP
Inventors: Qianli Ma; Frank Tietz; Dina Fattakhova-Rohlfing; Olivier Guillon
Original assignee: Forschungszentrum Juelich GmbH
Current assignee: Forschungszentrum Juelich GmbH
Priority date: 2020-11-25
Filing date: 2021-10-28
Publication date: 2023-10-04
Also published as: US20230387455A1; WO2022111934A1; DE102020214769A1; JP2024500557A

Abstract

The invention relates to a solid state cell, a solid state battery, and an associated method for manufacturing a solid state cell. A solid state cell (10) has an electrolyte (20) which comprises NaSICON. The solid state cell (10) comprises a first electrode (12) located on a first region (21) of the electrolyte (20), and a second electrode (14) located on a second region (22) of the electrolyte (20). A continuous material layer (30) is located on at least a third region (23) of the electrolyte (20) on an outer surface (25) of the electrolyte (20). Alternatively, a chemical composition of the outer surface (25) in the third region (23) of the electrolyte (20) is modified. This makes it possible to effectively prevent the formation of filaments or dendrites and to operate at significantly increased current densities.

Description

Solid state cell and associated manufacturing process

description

The invention relates to a solid-state cell, a solid-state battery and a method for producing a solid-state cell.

Solid-state sodium batteries are considered to be promising energy storage devices because they offer advantages in terms of costs, availability of materials and operational safety compared to conventional lithium batteries with organic liquid electrolyte. However, in the known solid-state sodium batteries, just like in all other alkaline metal batteries, dendrite formation takes place, as described in the publication "Recent progress in solid-state electrolytes for alkaline-ion batteries" by Cheng Jiang et al. , Science Bulletin, Vol. 62, 2017, pp. 1473-1490.

Dendrites are electrochemical metal deposits that occur especially at high current densities above 1 mA cm ^-2 . These typically come from the electrodes and can result in a short circuit. The batteries can become unusable in this way, as described in the publication "Solid-state electrolyte materials for sodium batteries: towards practical applications" by F. Tietz and Q. Ma, ChemElectroChem, 2020, Vol. 7, No. 13. pp. 2693-2713, and Zhou et al. ACS Central Science, 2017, Vol. 3, No. 1, pp. 52-57.

A well-known sodium ion conductor is Nai _+x Zr ₂ (Si0 ₄ ) _x (P0 ₄ ) _3-x (0<x<3). Also known as NZSP, this substance crystallizes in rhombohedral or monoclinic structures. Such compounds are also referred to as NaSICON after the acronym of "Na Super ionic Conductor". Depending on the manufacturing process, an ion conductivity of NZSP of up to 5 * 10 ³ S cm ^-1 can be achieved, such as in DE 10 2015 013 155 A1 and in the publication "Room temperature demonstration of a sodium superionic conductor with grain conductivity in excess of 0.01 S cm ^-1 and its primary applications in symmetry battery cells” by Q. Ma et al., Journal of Materials Chemistry A, Vol. 7, 2019, pp. 7766 to 7776.

Compared to other solid-state electrodes, NaSICON already has improved robustness against dendrite formation. This is in the publication “Dendrite-tolerant all-solid-state sodium batteries and an important mechanism of metal self-diffusion” by Tsai et al. , Journal of Power Sources, Vol. 476, 2020, 228666. The current density at which stable operation of a symmetrical solid-state cell with the structure Na/solid electrolyte/Na is possible is therefore 1 mA cm ² . However, there is a need for further increased current densities in order to enable more efficient, commercially viable applications.

The object of the invention is to develop a further developed solid-state cell, a further developed solid-state battery and an associated manufacturing method.

The object is achieved by the solid-state cell according to claim 1 and by the solid-state battery and the method according to the independent claims. Refinements are specified in the dependent claims.

A solid-state cell with an electrolyte that includes NaSICON is used to solve the task. The solid state cell comprises a first electrode arranged on a first area of the electrolyte and a second electrode arranged on a second area of the electrolyte. A continuous layer of material is disposed on at least a third portion of the electrolyte on an outer surface of the electrolyte. Alternatively, a chemical composition of the outer surface is changed in the third region of the electrolyte.

It has been shown that when solid-state cells are operated with an electrolyte containing NaSICON at high current densities, metallic sodium filaments are formed on the outer surface of the electrolyte. Like classic dendrite formation inside the electrolyte, these can lead to a short circuit and thus to the destruction of the cell. Such filaments form in particular at current densities above 1 mA cm ² . Experiments have shown that the formation of such filaments can be prevented by providing a continuous layer of material or by changing the chemical composition of the outer surface of the electrolyte. In other words, a particularly high tolerance to dendrite formation can be achieved. Current densities of more than 2 mA cm ² or 3 mA cm ² can be achieved in the cell according to the invention. No expensive chemicals and no complex technology are required to produce the solid-state cell according to the invention. A solid state cell is an electrical cell in which the electrodes and electrolyte are made of solid material. Solid state cells with an electrolyte containing NaSICON can also be referred to as solid state sodium cells. The solid cell is in particular a battery cell. Typically, the solid-state cell is a cell with a solid electrolyte and sodium ions as charge carriers, for example a sodium-air cell.

NaSICON is an acronym for "Na (Sodium) Super ionic Conductor" and includes substances with the formula M ^l ₁₊ 2 _w+x-y+z M ^ll _w M ^m _x (Zr,Hf) ^lv _2-wxy M ^v _y (Si0 ₄ ) _z (P0 ₄ ) _3-z . Here is M' Na. M", M ^m and M ^v are suitable divalent, trivalent and pentavalent metal cations, respectively. For example, M" can be Mg ²⁺ , Ca ²⁺ , Sr ²⁺ , Ba ²⁺ , Co ²⁺ and/or Ni ²⁺ . For example, M ^m can be Al ³⁺ , Ga ³⁺ , Sc ³⁺ , La ³⁺ , Y ³⁺ , Gd ³⁺ , Sm ³⁺ , Lu ³⁺ , Fe ³⁺ and/or Cr ³⁺ . For example, M ^v V ⁵⁺ , Nb ⁵⁺ and/or Ta ⁵⁺ .

NaSICON may further include substances with the formula Nai _+x Zr2Si _x P3- _x Oi2, 0 < x < 3. It can also include substances that are structurally built up according to the formula mentioned and in which a proportion of Na, Zr and/or Si is replaced by isovalent or equivalent elements. NaSICON are solids. NaSICON exhibit high sodium ion conductivity and negligible electron conduction. The electrolyte can consist of NaSICON and can therefore be referred to as NaSICON electrolyte. In particular, the electrolyte is a ceramic electrolyte. The outer surface of the electrolyte of the third region of the electrolyte may be a NaSICON surface which may be provided with a continuous layer of material or may have its chemical composition altered. In one embodiment, the electrolyte comprises or consists of Na 3.4 Zr 2.0 (SiO 4 ) 2.4 (PO 4 ) 0.6 .

The first area, the second area and the third area of the electrolyte are different areas. In one embodiment, these are different pages. In particular, each electrode is arranged in the respective area on the surface of the electrolyte. In particular, the areas are located on outer sides of the electrolyte and define portions of the outer surface of the electrolyte. In particular, the first area and the second area are arranged opposite one another. The third region is typically located between the electrodes. No electrode is arranged in the third area. The third region may include the entire outer surface of the electrolyte that does not covered by the electrodes. In one embodiment, the electrolyte is cylindrical and the third area is the lateral surface of the cylinder. The electrodes can be arranged opposite one another on the end faces of the cylinder. The electrolyte can be in the form of a pellet. Typically, the solid-state cell is built up in layers, so that an electrolyte layer is arranged between two electrode layers. This structure can be repeated.

A continuous layer of material can be arranged in the third region of the electrolyte. The layer of material provides a physical barrier between the material of the electrolyte and the surrounding atmosphere. In particular, the material layer is arranged in such a way that contact of the surrounding atmosphere with the electrolyte is prevented. The layer of material has no openings or interruptions, so that the electrolyte is completely shielded from the atmosphere. In particular, the material layer is arranged in all sections of the outer surface of the electrolyte that are not in contact with an electrode. The layer of material prevents the formation of sodium filaments, since metallic sodium cannot contact the electrolyte due to the layer of material. The layer of material can be referred to as a coating.

In one embodiment, the material layer or the changed chemical composition of the outer surface in the third area of the electrolyte serves to change or disrupt the three-phase system made up of electrode material, the surrounding atmosphere and the electrolyte material. The surrounding atmosphere can essentially consist of inert gas and/or noble gas, for example argon. It can contain oxygen in proportions of less than 1 ppm, in particular less than 0.5 ppm. An interaction of the electrode material with the atmosphere can be prevented by the solution according to the invention. For example, in conventional solid state cells, the sodium electrode material can be oxidized by the remaining oxygen, which can be prevented by the material layer or the changed chemical composition.

In particular, the material layer is essentially gas-tight. For example, the material layer is impermeable to air and/or oxygen. This applies in particular to the entire temperature range in which the solid-state cell can be operated. It can apply in a range from -50 °C to 100 °C. The material layer is in particular watertight and/or vapor-tight. In particular, the material layer is designed in such a way that per cm ² area at a pressure difference of 10 ⁵ Pa less than 1 cm ³ gas loss per year, for example less than 1 cm ³ gas loss in 10 years occurs.

In one embodiment, the material layer consists of an inert material. This means a material that does not undergo any chemical reactions under the given conditions. In particular, no chemical reactions take place between the material layer and the electrolyte, the electrode material and/or a surrounding atmosphere. In particular, no chemical reactions take place between the material layer and metallic sodium. In particular, the material layer lies flat against the surface of the electrolyte.

In one embodiment, the layer of material is electrically non-conductive. Typically it has an electrical conductivity which is less than 10 ⁴ S/m, in particular less than 10 ⁸ S/m and in one example less than 10 ¹ ° S/m.

In one embodiment, the material layer comprises or consists of a synthetic resin, in particular an epoxy resin. Synthetic resins such as epoxy resins are readily available, easy to use and inexpensive. In one embodiment, the material layer comprises at least one resin from the group comprising polyethylene resins, polypropylene resins, polybutene resins, polyvinyl chloride resins, polystyrene resins, phenolic resins, epoxy resins, lauxite resins, furan resins. The resins mentioned are well suited to prevent the formation of sodium filaments.

The chemical composition of the surface in the third area of the electrolyte can be altered. This means that the chemical composition of the surface differs from the chemical composition of the electrolyte material. This can be realized, for example, by the presence of at least one substance at certain points, in sections or throughout, which has a composition that differs from the material of the electrolyte. This substance can be arranged on the surface and/or in the material forming the surface. The surface whose chemical composition has changed can be designed in such a way that particles or sections with a different chemical composition are arranged on a surface consisting of the electrolyte material. The surface whose chemical composition has changed can be designed in such a way that the material forming the surface contains at least one other substance in addition to the electrolyte material. The chemical composition can be changed Carrying out a chemical reaction on the surface of the electrolyte. For example, a chemical reaction of the electrolyte material with a strong acid or base can take place. The surface that has changed in its chemical composition can have changed properties compared to a hypothetical, unmodified surface.

The effectiveness of the solution according to the invention can be checked in a simple manner by operating a symmetrically constructed Na/NaSICON/Na cell, for example, with increasing current density until a short circuit occurs. In this way, in particular on the basis of the current density achieved, the tolerance to the formation of sodium filaments can be determined.

In one configuration, the chemical composition of the outer surface is changed in that a salt is arranged on the surface in the third region of the electrolyte. In particular, the salt is arranged in the form of microparticles, nanoparticles and/or salt crystals. The salt can be placed in the third region, for example, by applying an aqueous salt solution and evaporating or evaporating the water. In one embodiment, the salt forms a continuous layer on the surface of the electrolyte. However, this is not absolutely necessary to solve the task. In particular, the salt changes the chemical composition of the surface of the electrolyte so that no sodium filaments are formed during operation of the solid-state cell.

In a further embodiment, the salt is at least one salt from the group comprising LiCl, KBr, Mgl, Ca(N0 ₃ ) ₂ , SrS0 ₄ , Ba(HS0 ₄ ) ₂ , FeHP0 ₄ , Ni(HC0 ₃ ) ₂ , CU(C ₂ H ₃ 0 ₂ ) ₂ arranged. In one embodiment, a salt containing no sodium is provided.

In one embodiment, a sodium salt is arranged as the salt. In particular, the sodium salt is selected from the group comprising NaCl, NaBr, NaI, NaNO ₃ , Na ₂ SO ₄ , NaHS0 ₄ , Na ₃ PO ₄ , NaH ₂ PO ₄ , Na ₂ HP0 ₄ , Na ₂ CO ₃ , NaHC0 ₃ , _NaC2 _H3 _O2 . The sodium salts mentioned can be present individually or in any desired mixture. Sodium salts are readily available and effectively prevent filament formation.

In one configuration, the first electrode and/or the second electrode comprises metallic sodium. The electrode material thus comprises at least one electrode metallic sodium. The first electrode and/or the second electrode may be made of an alloy containing sodium. The first electrode and/or the second electrode may contain Na and Sn, In, K and/or C. In particular, the at least one electrode consists of metallic sodium. Typically, the anode comprises or is made from sodium.

Sodium is readily available and inexpensive. In addition, metallic sodium allows a particularly high specific capacity of the solid-state cell. The specific capacity can reach up to 1166 mAh g ^_1 . In addition, a minimum potential of -2.71 V compared to the standard hydrogen electrode can be achieved by metallic sodium.

In one configuration, the first electrode and the second electrode are designed in the same way. This means that the electrodes have the same shape and/or the same material. In other words, the solid cell is a symmetrically constructed cell. For example, both electrodes can be metallic sodium electrodes. Thus, the solid-state cell can have the structure Na-NaSICON-Na. Symmetric cells are often used simplistically to study dendrite formation because they are easier to fabricate than complete cells with anode, cathode, and electrolyte, and their dendritic behavior is the same as that of complete cells. In this way, further disadvantages due to deficiencies in the cathode material can be avoided, so that current densities above 1 mA cm ^-2 are possible. For a technical application, however, the complete cells mentioned are particularly relevant. The cathode of such a cell can be made of Na3V2P3<Di2, for example.

In one embodiment, the continuous layer of material is a polymer layer. Polymers are electrically non-conductive, do not react with metallic sodium and can easily be applied to the NaSICON surface without gaps. They are inexpensive, easy to process and, due to the wide range of compositions available, offer good opportunities for customizing desired properties.

Another aspect of the invention is a solid state battery. The solid-state battery comprises at least one solid-state cell according to the invention. A solid state battery is a rechargeable battery in which the electrodes and electrolyte are solid material exist. The solid state battery may include one or more solid state cells.

Another aspect of the invention is a method of making a solid state cell. The solid state cell has an electrolyte that includes NaSICON. The solid state cell has a first electrode arranged on a first area of the electrolyte and a second electrode arranged on a second area of the electrolyte. The method includes placing a continuous layer of material on an outer surface in a third region of the electrolyte. Alternatively, the method includes changing a chemical composition of the outer surface in the third region of the electrolyte. All the features, embodiments and effects of the solid-state cell described at the beginning also apply accordingly to the method and vice versa.

In one embodiment, a continuous glass layer is placed on the outer surface of the electrolyte. Glass has high availability, high hardness and extremely low electrical conductivity. In one embodiment, heating to a temperature above 400° C. can take place to fix the glass on the surface of the electrolyte. The glass layer can contain one or more components from the group consisting of _S1O2 , Na2Ü, CaO, K2O, SrO, BaO, _B2O3 , P2O5, Al2O3.

In one embodiment, the material is placed by immersing the electrolyte in a liquid or gel-like material. For example, molten glass or liquid to gel polymer can be used. The layer of material can be arranged by coating. In particular, the layer thickness of the material layer is significantly greater than 1 μm, for example in the range between 0.1 mm and 10 mm.

In particular, the method for producing the solid-state cell is carried out in such a way that the composition of the electrolyte material in the sections of the electrolyte that are not on the outer surface of the electrolyte remains at least essentially unchanged. In other words, a chemical change in the composition occurs at most in the section of the outer surface of the electrolyte. If changes occur in sections of the outer surface of the electrolyte in the first or second region, these sections can undergo a mechanical Are subjected to surface treatment so that a surface is made or restored from the electrolyte material.

In one embodiment, the chemical composition of the outer surface is changed by applying a saline solution containing a salt dissolved in a solvent to the outer surface. The solvent is removed so that salt particles form on the outer surface.

The arrangement of the salt on the surface of the electrolyte by means of a dissolved salt is easy to carry out. For example, the surface can be moistened with the solvent. The solid-state cell according to the invention can thus be produced in a particularly simple and technically uncomplicated manner. In particular, the salt particles are formed in the form of microparticles, nanoparticles and/or salt crystals. The solvent is characterized in that it is suitable for dissolving the salt. The solvent can be water, for example.

As described, salt can change the chemical composition of the surface of the electrolyte in such a way that sodium filaments are not formed. It is possible, but not absolutely necessary, for one or more chemical reactions to take place between the salt and the NaSICON.

In one embodiment, the solvent is removed by evaporation or vaporization. In this way, the chemical composition of the surface can be changed in a technically uncomplicated manner. For example, salt crystals can be arranged on the surface in a simple manner.

In one embodiment, the salt solution is applied and/or the solvent is removed in such a way that nanoparticles of the salt are formed on the surface. In the case of evaporation or evaporation, this is achieved in particular by suitably setting at least one of the parameters temperature, duration and atmospheric humidity or vapor pressure of the solvent. A particularly thin layer that effectively prevents the formation of filaments can be produced in a simple manner.

In a further embodiment, the chemical composition of the outer surface is changed in that a chemical reaction of the Surface is done with an acid or base. Alternatively or in addition, the surface undergoes a chemical reaction with at least one substance from the group comprising Al2O3, ZrÜ2, S1O2, MgO. The chemical reaction takes place in particular at a temperature above 1000°C. As a result of the chemical reaction, at least one substance that is different from the material forming the surface can be embedded in edge regions of the electrolyte.

In particular, a reaction with a strong acid or a strong base can take place. In this case, the reaction can take place at room temperature and can therefore be carried out technically in a simple manner. In one embodiment, the surface undergoes a chemical reaction with at least one base from the group comprising NaOH, LiOH, KOH, Ca(OH) ₂ , Sr(OH) ₂ , Ba(OH) ₂ .

In one embodiment, the surface undergoes a chemical reaction with at least one acid from the group consisting of H3BO3 and H2S1O3. In one embodiment, the surface undergoes a chemical reaction with an acid that is solid at room temperature and/or during the chemical reaction.

In one embodiment, a liquid or pasty material is arranged on the surface of the electrolyte for the arrangement of the continuous material layer on the surface of the electrolyte. The continuous material layer is created by hardening of the pasty material.

Hardening means hardening of the pasty material into a solid layer of material, regardless of the underlying mechanism. Curing can take place, for example, by chemical reaction or physical processes such as evaporation.

In one embodiment, placing a paste material is accomplished by placing a quantity of the paste material on at least a portion of the surface and spreading the material over the surface. In particular, what is meant is a substantially continuous and/or uniform distribution.

In one embodiment, the liquid or pasty material contains a polymer that is designed to harden on the surface of the electrolyte. For example, a gel-like polymer that hardens into a solid polymer layer can be used. For example, furan resin or organic silicate resins can be used that harden at an elevated temperature between 60°C and 200°C. Resins are good at preventing the formation of sodium filaments. In one embodiment, the method includes increasing the temperature to cure the polymer. This embodiment makes it possible to produce the solid-state cell according to the invention in a particularly simple and technically uncomplicated manner.

In one embodiment, the liquid or pasty material includes a synthetic resin, for example an epoxy resin, and a hardening agent. For example, the liquid or pasty material contains a curable resin. Curable resins are synthetic resins that can be cured to form thermosets. Reaction resins are meant in particular.

The resin and the curing agent can be easily mixed together at room temperature and then applied to the surface of the electrolyte. During curing, a dense and hard polymer layer is formed, which is well suited to prevent the formation of sodium filaments.

In one embodiment, after changing the chemical composition of the outer surface or after arranging the continuous layer of material, the following steps occur: Optionally, the first area and/or the second area of the electrolyte is subjected to a mechanical surface treatment in order to remove any impurities. This is done, for example, in order to free the material of the electrolyte from any impurities. The first electrode is arranged on the first area of the electrolyte and the second electrode is arranged on the second area of the electrolyte. The two electrodes can be arranged at the same time.

In an alternative sequence, an electrode is placed on the electrolyte followed by changing the chemical composition or placing the continuous layer of material. In particular, this relates to the cathode, for example a Na V PsOi cathode. For example, a Na V PsOi cathode is placed on one of the first and second regions of an NZSP electrolyte, and then the surface of the third region is machined. The sodium anode can then be placed on the other of the first and second portions of the electrolyte. The mechanical surface treatment can remove applied salt solution, salt particles remaining after removal of the solvent and/or arranged material from the first and the second region. Freeing the material of the electrolyte from any impurities means in particular the removal of such residues. The salt particles or layer of material placed in the third area are not removed. The mechanical surface treatment can take place, for example, by grinding, polishing and/or cleaning. The electrodes are arranged in a respective first and second region of the electrolyte, in particular after the pasty material has hardened.

A further aspect of the invention is the use of a solid-state cell according to the invention, in particular as a stationary energy store, for example for storing electrical energy from renewable energy sources such as wind or solar energy. A further aspect of the invention is a particularly stationary energy store, for example for storing electrical energy from renewable energy sources such as wind or solar energy, which comprises a solid-state cell according to the invention.

Exemplary embodiments of the invention are also explained in more detail below with reference to figures. Features of the exemplary embodiments can be combined with the claimed subject matter individually or in a plurality, unless stated otherwise. The claimed scopes of protection are not limited to the exemplary embodiments.

Show it:

FIG. 1: a schematic sectional drawing of a conventional solid-state cell, FIG. 2: a first embodiment of a solid-state cell according to the invention,

FIG. 3: a second embodiment of a solid-state cell according to the invention,

FIG. 4: a third embodiment of a solid-state cell according to the invention,

Figure 5: an operational diagram of a conventional solid state cell,

FIG. 6: an operational diagram of a solid-state cell according to the invention, and FIG. 7: an operational diagram of another solid-state cell according to the invention.

FIG. 1 shows a conventional solid-state cell with a first electrode 12, a second electrode 14 and an electrolyte 20 arranged in between Electrolyte 20 is designed as a solid electrolyte. The first electrode 12, second electrode 14 and electrolyte 20 are each circularly cylindrical in shape and extend about the central axis 28 in this non-limiting example . The first electrode 12 rests in the first region 21 of the electrolyte 20 illustrated above, which defines a first side of the electrolyte 20 . The second electrode 14 rests in the second region 22 of the electrolyte 20 shown below, which defines a second side of the electrolyte 20 . The third area 23 of the electrolyte 20 corresponds to the peripheral lateral surface of the circular-cylindrical electrolyte 20. The solid-state cell 10 shown here tends to form filaments or dendrites, so that the current density that can be achieved during operation / depending on the electrolyte material is limited to a maximum of 1 mA cm ^-2 .

Figures 2 to 4 show different embodiments of solid-state cells 10 according to the invention. According to a basic structure, each of the solid-state cells 10 comprises a first electrode 12, a second electrode 14 and an electrolyte 20 arranged between them. The sizes and proportions of the electrodes 12, 14 and the electrolyte 20 in Figures 2 to 4 are not to scale. The radial extent of the electrodes can be at least substantially the same as or greater than the radial extent of the electrolyte.

The electrolyte 20 is designed as a NaSICON solid electrolyte. The first electrode 12, the second electrode 14 and/or the electrolyte 20 can be designed in the shape of a circular cylinder and can extend around the central axis 28. The first electrode 12 rests against the first side of the electrolyte 20 illustrated above. The second electrode 14 rests against the second side of the electrolyte 20 shown below. In the case of a circular-cylindrical electrolyte 20, the third side of the electrolyte 20 corresponds to the peripheral surface area. The first and/or the second electrode can consist of sodium. The electrodes can be designed in the same way.

Typically, the electrodes 12, 14 have a smaller cross-section than the electrolyte 20. In other words, the electrolyte 20 projects horizontally beyond the electrodes 12, 14 on both sides in the orientation shown here, as illustrated in the figures. In particular, the cathode is thicker than the anode. In the embodiment shown in FIG. 2, a continuous material layer 30 is arranged on the outer surface 25 of the electrolyte 20 in the third region 23 of the electrolyte 20 . This is shown on both sides of all sections of the surface of the electrolyte 20 in the sectional drawing. The material layer 30 is arranged circumferentially and completely covers the surface 25 of the third region 23 of the electrolyte 20 . It prevents the formation of dendrites or metallic filaments on the surface 25. In this way, a short circuit is prevented and operation of the solid-state cell at higher current densities is possible. The third region 23 comprises sections pointing radially outwards, a section pointing in a first axial direction or in the direction of the first electrode 12 and a section pointing in the other axial direction or in the direction of the second electrode 14 . It includes all areas of the electrolyte 20 that are not contacted by an electrode 12, 14. In one embodiment, the material layer 30 contacts the first electrode 12 and/or the second electrode 14 such that the electrolyte 20 is completely spatially isolated from the surrounding atmosphere.

In a first example of a solid-state cell 10 constructed according to FIG. 2, a continuous polymer layer is arranged in the surface 25 of the third region 23 of the electrolyte 20 . A powder of Na _3.4 Zr _2, o( _Si0 ₄ ) _2.4 (P0 ₄ ) 0.6 was produced. The circular-cylindrical electrolyte 20, which is also referred to as a pellet, was produced from the powder. These steps were carried out according to DE 102015 013 155 A1 and Journal of Materials Chemistry A, Vol. 7, 2019, pp. 7766 to 7776, to which reference is made here. The pellet has a diameter of 10 mm and a thickness of 2 mm. The relative density of the pellet is > 95%.

The commercially available polymer resin Epoxy 2000 and the associated hardening agent from Cloeren Technology GmbH were used in the production of the solid-state cell 10 according to the invention. 6.8 g polymer resin and 3.2 g curing agent were mixed. The paste obtained was evenly applied to the entire outer surface 25 of the third region 23 of the electrolyte 20 . This was done with a cotton swab. It should be noted that the first area 21 and the second area 22 of the electrolyte 20 remain free of the paste. If contamination occurs in the first area 21 and/or the second area 22, it must be removed, in particular by mechanical surface treatment such as wiping, brushing or grinding. The first area 21 and the second area 22 are here, for example, circular sections of the surface of the electrolytes. After about 8 hours the paste had cured to a solid polymeric resin. Sodium electrodes were then applied in a glove box to the mutually opposite areas 21, 22 of the electrolyte 20 and pressed on with a force of 1 kN. Operational data of the solid state cell manufactured in this way are shown in FIG. Operating data of a conventional solid-state cell, which was produced according to the first example but without the polymer coating described, is shown in FIG.

In the embodiments shown in FIGS. 3 and 4, a chemical composition of the outer surface 25 in the third region 23 of the electrolyte 20 is changed in each case. This is realized in FIG. 3 by salt crystals which are arranged on the surface 25 . In particular, these are nanoparticles made from a sodium salt. The third area 23 is designed analogously to FIG.

In a second example of a solid state cell produced according to FIG. 3, the electrolyte is produced as a NaSICON pellet as in the first example above. A saturated salt solution was prepared from 5.0 g NaCl (Merck, 99%) and 10 g distilled water at room temperature. The saline solution was evenly applied to the entire outer surface 25 of the third region 23 of the electrolyte 20 . This was done with a cotton swab. It should be noted that the first area 21 and the second area 22 of the electrolyte 20 remain free of the saline solution. In the event of any contamination, the procedure is the same as in the first example above. Drying took place at 60° C. for 0.5 hours, during which the water evaporates so that only NaCl remained on the surface 25. The sodium electrodes were applied as in the first example above. Operational data of the solid-state cell manufactured in this way are shown in FIG.

In FIG. 4, the chemical composition of the outer surface 25 of the electrolyte 20 is changed by a chemical reaction of the surface 25 with an acid, a base, or a substance from the group comprising Al2O3, ZrC>2, SiO2, MgO is. The material of the electrolyte 20 has a different chemical composition on the outer surface 25 compared to the inside. This is shown schematically in that particles made of a material 34 that differs from the material of the electrolyte 20 are present on the outside of the electrolyte 20 in the third area 23 . The changed chemical composition of the surface 25 effectively prevents the formation of filaments or dendrites. The depiction of the particles of salt 32 and the material 34 in FIGS. 3 and 4 is purely schematic and not to scale. It cannot be ruled out that the salt 23 and/or the material 34 forms a continuous and/or closed layer at least in some areas.

The operating diagrams of solid state cells shown in FIGS. 5 to 7 show the time T in minutes on the horizontal axis, the electrical voltage U in volts on the vertical axis shown on the left and the current density / in mA cm ^-2 on the vertical axis shown on the right.

FIG. 5 shows the operation of a conventional solid-state cell at a constant current density / of 2 mA cm ^-2 at a temperature of 25°C. It can be seen that the voltage U increases continuously up to a time T of about 25 minutes. The voltage U reaches a local maximum value of about 0.18 V and then drops steeply. An irregular course of tension follows. The voltage drop after the maximum value is the result of a short circuit 40, which is shown with an arrow in FIG. The formation of Na filaments on the surface of the electrolyte leads to electrical contact between the two electrodes and thus to the behavior shown in FIG. It can be seen that a permanent use of this solid state cell at a current density / of 2 mA cm ^-2 is not possible.

FIG. 6 shows the operation of a solid-state cell according to the invention according to FIG. 2 over a large number of charging cycles at a temperature of 25.degree. The solid-state cell was operated galvanostatically, i.e. with a constant current intensity. A moderate force of 1 kN was applied to the electrodes to improve the contact between the electrodes and the electrolyte. The operation took place at a current density / represented by dashed lines, which alternately amounts to 2 mA cm ^-2 and -2 mA cm ^-2 . It is shown that stable voltages U above 0.08 V or below -0.08 V are achieved. It becomes clear that a permanent operation of this solid state cell at a current density / at 2 mA cm ^-2 is possible.

In a comparable illustration, FIG. 7 shows the galvanostatic operation of a solid-state cell according to the invention as shown in FIG. 3. Operation took place over a large number of charging cycles at a temperature of 25.degree. The Electrodes applied a moderate force of 1 kN to improve contact between the electrodes and the electrolyte. The current density / represented by dashed lines was 3 mA cm ^-2 and -3 mA cm ^-2 in alternation. It turns out that stable voltages U in the range of 0.15 V or -0.15 V can be achieved. It becomes clear that a permanent operation of this solid state cell at a current density / at 3 mA cm ^-2 is possible.

The current density at which stable operation of a symmetrical solid-state cell with the structure Na / solid electrolyte / Na is possible has hitherto been at most 1 mA cm ^-2 . This value was achieved using a solid electrolyte made of Na _3.4 Zr _2, o ( _Si0 ₄ ) _2.4 (P0 ₄ ) 0.6 (Journal of Power Sources, Vol. 476, 2020, 228666). The use of other solid electrolytes led to significantly lower current densities of 0.3 mA cm ^-2 and below. The current density can by the solution according to the invention when using a solid electrolyte made of Na _3.4 Zr _2, o (SiC> ₄ ) _2.4 (P0 ₄ ) o _{, 6} to 2 to 3 mA cm ^-2 , ie by a factor of 2 to 3, to be increased.

Reference List

solid state cell 10

First electrode 12

Second electrode 14

Electrolyte 20

First area 21

Second area 22

Third area 23

surface 25

Central axis 28

material layer 30

salt 32

items 34

tension

current density

Time T (min)

short circuit 40

Claims

Expectations

1. Solid state cell (10) with an electrolyte (20) comprising NaSICON, with a first electrode (12) arranged on a first region (21) of the electrolyte (20) and with a first electrode (12) on a second region (22) of the electrolyte ( 20) arranged second electrode (14), characterized in that a continuous material layer (30) is arranged on at least a third region (23) of the electrolyte (20) on an outer surface (25) of the electrolyte (20) or that a chemical Composition of the outer surface (25) in the third region (23) of the electrolyte (20) is changed.

2. Solid-state cell (10) according to the preceding claim, characterized in that the chemical composition of the surface (25) is changed in that a salt (32) is arranged on the surface (25).

3. Solid-state cell (10) according to the preceding claim, characterized in that a sodium salt is arranged as the salt (32), the sodium salt being selected in particular from the group consisting of NaCl, NaBr, NaI, NaNOs, Na ₂ SO ₄ , NaHS0 ₄ , Na ₃ PO ₄ , NaH ₂ PO ₄ , Na ₂ HPO ₄ , Na ₂ CO ₃ , NaHCOs, NaC ₂ H ₃ O ₂ .

4. solid state cell (10) according to any one of the preceding claims, characterized in that the first electrode (12) and / or the second electrode (14) comprises metallic sodium.

5. solid state cell (10) according to any one of the preceding claims, characterized in that the first electrode (12) and the second electrode (14) are configured identically.

6. solid state cell (10) according to any one of the preceding claims, characterized in that the continuous material layer (30) is a polymer layer.

7. solid-state battery, comprising at least one solid-state cell (10) according to any one of the preceding claims.

8. A method for producing a solid-state cell (10) with an electrolyte (20) comprising NaSICON, with a first electrode (12) arranged on a first area (21) of the electrolyte (20) and with a first electrode (12) on a second area (22 ) The electrolyte (20) arranged second electrode (14), comprising

- arranging a continuous layer of material (30) on an outer surface (25) in a third region (23) of the electrolyte (20), or changing a chemical composition of the outer surface (25) in the third region (23) of the electrolyte (20 ).

9. Method according to the preceding claim, characterized in that the chemical composition of the surface (25) is changed by applying a salt solution containing a salt (32) dissolved in a solvent to the surface (25) and removing the solvent is formed so that salt particles on the surface (25).

10. The method according to the preceding claim, characterized in that the solvent is removed by evaporation or evaporation.

11. The method as claimed in one of the two preceding claims, characterized in that the salt solution is applied and/or the solvent is removed in such a way that nanoparticles of the salt (32) are formed on the surface (25).

12. The method according to any one of the four preceding claims, characterized in that changing the chemical composition of the surface (25) takes place in that a chemical reaction of the surface (25) takes place with an acid or base and / or that a chemical reaction of Surface (25) with at least one substance from the group comprising Al2O3, ZrÜ2, S1O2, MgO at a temperature above 1000 ° C.

13. The method according to any one of the five preceding claims, characterized in that for the arrangement of the continuous material layer (30) on the surface (25) of the electrolyte (20), a liquid or pasty material on the surface (25) of the electrolyte (20) is arranged and the continuous material layer (30) is formed by hardening of the pasty material.

14. The method according to the preceding claim, characterized in that the liquid or pasty material comprises a synthetic resin, for example an epoxy resin, and a curing agent.

15. The method according to any one of the seven preceding claims, characterized in that after changing the chemical composition of the surface (25) or after arranging the continuous material layer (30), optionally the first area (21) and/or the second area (22 ) of the electrolyte (20) is subjected to a mechanical surface treatment in order to remove any impurities, the first electrode (12) is arranged on the first region (21) of the electrolyte (20) and the first electrode (12) is arranged on the second region (22) of the electrolyte (20 ) the second electrode (14) is arranged.