Method for producing a pre-product, a battery cell and a battery

Abstract

The invention relates to a method for producing a pre-product for a battery cell, to a method for producing a battery cell, to a method for producing a battery, and to a battery cell. In a method for producing a pre-product (1) for a battery cell (2), Li and Na are mechanically mixed with one another in the solid state. In this way, a three-dimensional lattice structure can be produced which allows particularly low impedance and thus particularly good electrical transfer.

Description

[0001] Process for producing a precursor, a battery cell and a battery

[0002] Description

[0003] The invention relates to a method for producing a precursor for a battery cell, a method for producing a battery cell, a method for producing a battery and a battery cell.

[0004] Lithium or sodium batteries have a high energy density, which is why they are widely used and continue to be the subject of diverse research activities. Lithium and sodium metal electrodes are lightweight, enable particularly high energy densities, and exhibit low self-diffusion. However, these electrodes often exhibit high polarization during electrochemical cycling. This reduces rapid charging capacity and can lead to dendrite formation, which can cause irreversible damage to the cell and pose a safety risk. For this reason, carbon electrodes are currently frequently used. These electrodes do not exhibit these problems but have a significantly lower energy density.

[0005] To capitalize on the advantages of Li and Na metal electrodes, various concepts have been developed to improve the electrical transition between the separator and the electrode. One approach is to arrange three-dimensional structures or intermediate layers that reduce polarization, for example, made of polymer or ceramic. This can counteract dendrite formation. However, the production of such structures or layers is time-consuming and expensive. Furthermore, such structures or layers cannot be implemented in every cell concept, for example, in the case of solid-state cells, as additional support structures may be necessary.

[0006] It is an object of the invention to provide a precursor, a battery cell and a battery in a simple manner which can enable a good electrical transition at lithium and sodium electrodes.

[0007] The object is achieved by the method for producing a precursor for a battery cell according to claim 1, as well as by the method for producing a battery cell, the method for producing a battery, and the battery cell according to the independent claims. Advantageous embodiments are specified in the subclaims.

[0008] To solve the problem, a process for producing a precursor for a battery cell is used in which Li and Na are mechanically mixed together in the solid state.

[0009] It has been shown that mechanical mixing of Li and Na in the solid state can produce a mixture in which the two substances are finely dispersed. This is not possible in the melt, for example, because large, coherent, separate phases of the two metals form there. Mechanical mixing is simple and cost-effective. In this way, a three-dimensional lattice structure can be produced, which enables particularly low impedance when used in a battery cell.

[0010] A precursor for a battery cell is a product that, in its manufactured form and / or after suitable further processing, can be used as part of a battery cell. In particular, the precursor is a precursor for an electrode, preferably for a negative electrode.

[0011] The finely distributed mixture can be used in or as part of a metal electrode of a battery cell. The finely distributed mixture can be further processed to produce an at least substantially homogeneous mixture in which the Na atoms are substantially uniformly distributed in the Li and / or the Li atoms are substantially uniformly distributed in the Na. An at least substantially homogeneous three-dimensional structure, in particular in the form of a lattice, can be produced from the finely distributed mixture. As explained below, such a structure can be used particularly advantageously in a metal electrode for a battery cell.

[0012] In a battery cell produced in this way with a metal electrode in which Li and Na are finely distributed or at least essentially homogeneous, one of the two materials can be electrochemically removed from the other material during charging or discharging of the cell, leaving the other substance behind, in particular as a spatial lattice in which corresponding free spaces remain. During opposite charging or discharging, the substance that has been removed in the meantime can accumulate again in the free spaces. The electrical transition of such an electrode is no longer limited to the surface, but takes place via the three-dimensional lattice structure. In this way, the current density is reduced and a particularly high electrochemical performance is enabled, which is reflected, for example, in a high charging rate. Dendrite formation is effectively prevented, at least under normal operating conditions. The more finely distributed orThe more homogeneously the two metals are mixed, the more evenly the current flow is distributed over the three-dimensional structure and the higher the achievable electrochemical performance.

[0013] A battery within the meaning of the invention basically means a rechargeable battery or secondary battery. It can basically be a battery with a solid or liquid electrolyte. In particular, this means a battery in which the mobile ions are Na ions (hereinafter referred to as Na battery) or a battery in which the mobile ions are Li ions (hereinafter referred to as Li battery). The metal of the mobile ions is the electrochemically active metal that participates in the redox reaction of the battery cell. In particular, the electrode of such a battery cell comprises the respective electrochemically active metal. Typically, according to the invention, the electrode comprises the respective other metal. The electrode can additionally contain admixtures such as additives and / or impurities.

[0014] A metal electrode is an electrode made of metal or a metallic material, such as an alloy. This electrode is primarily a negative electrode, thus acting as an anode during battery discharge.

[0015] Li in the solid state and Na in the solid state are mixed together. This typically takes place in an environment that has a temperature below the melting point of Li and below the melting point of Na. The temperature of at least the environment of the substances to be mixed typically remains below the stated melting points during mixing, so that the mixture is produced in the solid state. The mechanical energy input can lead to local heating or to heating of the substances to be mixed and / or the produced mixture. Typically, however, the temperature of the substances to be mixed and / or the produced mixture always remains below the stated melting points. Mixing is done mechanically. This means that mechanical force is applied or introduced to produce the mixture.Typically, mechanical force is the largest or most significant energy input leading to the production of the mixture. It is not excluded that additional energy is input through one or more other means. In one embodiment, at least 75%, in particular at least 85%, and in one exemplary embodiment, at least 95% or 100% of the input energy is input by mechanical force. The resulting mixture can also be referred to as an alloy.

[0016] In particular, the mixing is carried out in such a way that a mixture is formed in which Li and Na are finely distributed. For example, coherent phases or particles of Li and / or Na in the respective other metal are no larger than 1 pm, in particular no larger than 0.3 pm, and preferably no larger than 0.1 pm. In a particularly advantageous embodiment, the mechanical mixing is carried out in such a way that coherent phases of a metal are no larger than 50 nm, preferably 10 nm, and particularly preferably 1 nm. The size refers to the maximum extent of the largest particle of the respective metal.

[0017] In particular, mixing takes place at room temperature. This reduces the technical complexity and the risk of metals reacting with atmospheric contaminants, for example.

[0018] Different processes for mechanical mixing are possible in order to achieve a finely distributed mixture. In principle, any process which produces a finely distributed mixture is possible. For example, mechanical alloying can be carried out in which Li and / or Na are present in powder form and are intensively mechanically processed together and / or ground together in a mill to produce a homogenized mixture which can possibly be homogenized down to the atomic level. In another example, mixing can be carried out by cold welding. In this process, high pressure is exerted on the materials at a comparatively low temperature, e.g. at room temperature, under which pressure they bond firmly in a similar way to a weld. This results in a finely distributed mixing of the materials. In a further example, cold forming can be carried out, in particular multiple times, in which the metals are mixed together.In particular, mechanical mixing takes place in the absence of oxidizing agents, O2, air, N2, H2O, and / or organic solvents. This prevents oxidation and / or contamination.

[0019] In one embodiment, the mechanical mixing comprises: a. jointly calendering a part comprising Li and a part comprising Na, b. folding the resulting product, and / or c. calendering the folded product, wherein steps b. and c. are typically repeated several times.

[0020] Calendering means pressing between two rollers. In particular, the rollers are arranged parallel to each other. This allows the thickness of the layer to be reduced. Calendering takes place in the solid state of the two metals. Typically, a part containing Li and a part containing Na are first placed on top of each other. This can be a part consisting largely or entirely of Li and / or a part consisting largely or entirely of Na. In principle, it is desirable for the parts to be made of Li and Na with the highest possible purity of the respective metal, although impurities cannot be avoided.

[0021] In particular, the Na and Li-containing parts are present as flat elements, such as a thin plate or foil. Folding is performed in such a way that the surface area is halved and the thickness doubled. Subsequent calendering in step c. can reduce the thickness and increase the surface area again.

[0022] In particular, steps b. and c. are repeated at least 5 times, preferably at least 7 times, particularly preferably at least 10 times, further preferably at least 13 times or at least 15 times, and in one embodiment at least 18 times. The more frequently the steps are repeated, the more finely distributed the mixture of Li and Na is. Each repetition doubles the number of layers. A particularly finely distributed mixture is desirable in order to obtain particularly uniform three-dimensional contact with the electrode and thus minimize the current density. On the other hand, too frequent repetition requires effort and energy. The stated frequencies of repetition represent an optimum between effort and yield, depending on the application. The mixture is subsequently available, in particular, as a foil.

[0023] In one embodiment, the mixture contains 1 mol% Na to 10 mol% Na or 90 mol% Na to 99 mol% Na.

[0024] The mixture contains, in particular, 3% Na with a tolerance of -2 mol% and +7 mol%, or 97 mol% Na with a tolerance of -7 mol% and +2 mol%. The remainder is, in particular, Li. As explained above, impurities are unavoidable.

[0025] The claimed concentration ranges of 3 mol% and 97 mol% Na correspond to the eutectic points of a mixture or alloy of Na and Li. Such an alloy or mixture thus has a uniform melting point that is below the melting points of the individual components. In the case of 97 mol% Li, the melting point is 170°C, which is lower than the melting point of Li. In the case of 97 mol% Na, the melting point is as low as 92°C. This enables particularly energy-efficient further processing of the mixture and a comparatively low temperature at which components heated together with the mixture remain thermally stable.

[0026] In one embodiment, the mixture contains at least 0.5 mol%, in particular at least 1 mol%, preferably at least 2 mol% Na and / or at most 8 mol%, in particular at most 6 mol%, preferably at most 5 mol% Na or 4 mol%. In one embodiment, the mixture contains at most 99.5 mol%, in particular at most 99 mol%, preferably at most 98 mol% Na and / or at least 92 mol%, in particular at least 94 mol%, preferably at least 95 mol% or 96 mol% Na.

[0027] When a precursor for a metal electrode for a lithium battery is produced, a small portion of sodium is typically mixed with a large portion of lithium. In this way, through suitable further processing, a three-dimensional sodium lattice can be created from which a large amount of lithium ions can be electrochemically removed and then re-enriched. When a precursor for a metal electrode for a sodium battery is produced, a small portion of lithium is typically mixed with a large portion of sodium. It has been shown that the specified concentration ranges enable the production of stable three-dimensional lattices. In one embodiment, the mixture is processed by cutting and / or forming to produce a near-net-shape electrode. The mixture is therefore processed to obtain a shape that corresponds to or is at least close to the shape of the electrode.In particular, this means a shape such that the battery cell can then be manufactured.

[0028] For example, the mixture is formed, particularly at room temperature, to obtain such a shape. For example, the electrode may be required as a foil with a specific thickness. Depending on the design of the battery cell, a foil may, for example, have a thickness of greater than or equal to 5 pm or 10 pm and / or less than or equal to 200 pm or 150 pm, in one example approximately 20 pm. In this case, forming takes place such that the mixture is converted into a corresponding foil, for example by rolling, such as by calendering.

[0029] Alternatively or additionally, the mixture can be processed by separating, for example cutting, to produce a near-net-shape electrode shape.

[0030] If the process involves melting and cooling, this typically occurs after machining. If the process involves the production of a composite, this typically occurs after machining.

[0031] In one embodiment, a composite is produced from the mixture on the one hand and a current collector and / or a separator on the other hand.

[0032] The production of the composite is a further step in the further processing of the mixture on the way to producing a battery cell. This involves bonding the mixture to one or more components of the battery cell. The production of the composite particularly involves the preferably surface contact of the precursor with the current collector and / or the separator. The components of the composite do not necessarily have to be attached to one another. It is sufficient for the components of the composite to contact one another.

[0033] A battery cell with a liquid electrolyte typically comprises a negative electrode, a positive electrode, and a porous separator located between them, which holds the electrolyte and electrically insulates the electrodes from each other. The separator is typically made of plastic. A current collector, for example in the form of a thin foil, particularly made of metal such as Al, Cu, or Ni, is typically arranged on the side of the electrodes facing away from the separator. The mixture is preferably used to provide the negative electrode.

[0034] The components of solid-state battery cells include the electrodes and the separator, which electrically isolates the positive and negative electrodes of a cell from each other. Typically, a solid electrolyte serves as the separator. The negative electrode comprises or consists of, for example, lithium or sodium, and / or the positive electrode comprises or consists of, for example, porous carbon. The electrolyte may comprise or consist of a ceramic, such as NaSICON, glass, or a glass-ceramic composite material. A solid-state battery cell may also have a current collector as described above on the side facing away from the electrolyte. The mixture is preferably used to provide the negative electrode.

[0035] In a battery cell with a liquid electrolyte and / or in a solid-state battery cell, one or more intermediate layers may also be present which are arranged between two adjacent components of the above-mentioned components of the battery cell, for example in order to reduce the contact resistance.

[0036] A battery, regardless of its design, can contain one or more battery cells. A current collector can connect several battery cells together.

[0037] In one embodiment, the mixture is first processed to produce a near-net-shape electrode shape by cutting and / or forming. The processed mixture can then be, for example, a self-supporting metal electrode. A composite is then produced from the thus processed mixture and the current collector and / or separator. For example, the processed mixture can be applied to the separator or current collector. Further processing can then take place, for example, by cutting.

[0038] In another embodiment, a composite is first produced, for example, by applying the mixture to the separator or current collector and then processing it to produce a near-net-shape electrode shape by cutting and / or forming. For example, the mixture on the separator or current collector can be formed, e.g., rolled, and / or cut. Here, too, further processing can subsequently take place, for example, by cutting. Alternatively or additionally, the mixture can also be processed, for example, by cutting, before the composite is produced.

[0039] In one embodiment, the composite comprises the mixture and a current collector. In one embodiment, the composite comprises the mixture and a separator. In one embodiment, the composite comprises the mixture, a current collector, and a separator. The mixture is typically arranged between the current collector and the separator. In particular, there is surface contact between the mixture and the current collector and / or the separator in the composite.

[0040] The composite is preferably produced from the mixture on the one hand and a current collector and a separator on the other. The mixture is arranged between the current collector and the separator. The mixture is then preferably melted in the composite. This enables a particularly low contact resistance. Only in the case that the separator (or the current collector) is not stable at the temperature required for melting, for example, in the case of a conventional plastic separator in a battery cell with a liquid electrolyte, is the corresponding component omitted from the composite production. In the case of a ceramic separator in a solid-state battery cell, this is usually sufficiently temperature-stable.

[0041] In one embodiment, the mixture is melted. Melting the mixture is a further processing step toward producing a battery cell.

[0042] Melting leads to a mixing of the two metals at the atomic level. This creates an at least essentially homogeneous mixture or alloy. If mechanical mixing occurs, for example, through repeated calendering and folding, the metals present in thin layers are also mixed transversely to the plane of extension of the layers through melting. In the case of a eutectic or approximately eutectic mixture, melting leads to the formation of the desired typical eutectic phase patterns.

[0043] The temperature is adjusted so that both Li and Na are in liquid form, at least temporarily. In the case of a eutectic or approximately eutectic mixture, a common melting point is present. In the case of a non-eutectic mixture, a temperature is set that corresponds at least to the higher melting point. It cannot be ruled out that the mixture contains impurities or traces with a higher melting point and thus remains in solid form.

[0044] In particular, the melting takes place in such a way that the melt is present for a period of time < 60 min, preferably < 30 min and particularly preferably < 10 min or < 5 min. In a particularly preferred embodiment, the melt is present for a period of time < 1 min and more preferably < 30 s or < 10 s. In one embodiment, the melt is present for only a few seconds or one second or even less. The melt is typically present for at least 1 ms. If the period is too long, this leads to an undesired enlargement of the individual particles or phases of the mixture. The further the mixing ratio is from the eutectic point, the more important a short melting duration is. For mixing ratios that are very close to the eutectic point, even a longer melting time does not lead to a significant enlargement of the phases.

[0045] This is typically followed by cooling to solidify the melt. Melting preferably occurs at a maximum temperature of 200 °C.

[0046] In one embodiment, the composite is heated so that the mixture melts. The mixture is heated as a component of the composite and thus melted. Preferably, the heating is carried out in such a way that the other component(s) of the mixture, typically the current collector and / or the separator, do not melt.

[0047] If the process involves producing a composite and melting the mixture, the melting of the mixture preferably takes place after the composite has been produced. The produced composite is exposed to the corresponding elevated temperature. The other parts of the composite preferably remain in the solid state. Heating the components of the composite together creates a particularly good bond, which enables a particularly low contact resistance. In particular, the mixture is melted while it is on at least one other component of the composite, for example, while it is placed on top of it.

[0048] Although the contact resistance between metals is generally already very low, heating the electrode and current collector together can achieve better wetting, thus improving electron conductivity. Contact resistance is the electrical resistance of an electrical contact surface, for example, between two contacts.

[0049] Heating the electrode and separator together can result in significantly improved ion conduction, particularly in the case of lithium lanthanum zirconium oxide (LLZO). In this application, LLZO also includes LLZTO (e.g., Li6.45AI0.05La3Zr1.6Ta0.4O12).

[0050] In one embodiment, the melt is cooled to a temperature of no more than 0.6 x MP, where MP is the melting point of the mixture. The melt is cooled in such a way that both Li and Na are converted back to the solid state. The cooling takes place to a temperature of 60% of the melting point or below. This ensures that diffusion processes are prevented, which could cause the individual particles or phases of the metals to enlarge. The result is a molten and resolidified mixture with a particularly fine, in particular essentially homogeneous, distribution of the metals.

[0051] In one embodiment, cooling occurs to approximately room temperature.

[0052] In one embodiment, cooling takes place within a period of less than 1 minute.

[0053] In one embodiment, cooling to a temperature of at most 0.6 x MP takes place within a period of <60 min, preferably <30 min, and particularly preferably <10 min or <5 min. In a particularly preferred embodiment, cooling takes place within a period of <1 min, and more preferably <30 s or <10 s. In one embodiment, cooling takes place within a few seconds or one second, or even shorter. For technical reasons, cooling typically takes place within a period of at least 1 ms. Cooling typically takes place at a cooling rate of >1 °C / s, preferably >5 °C / s, and particularly preferably >10 °C / s or >50 °C / s. In one embodiment, the stated periods and / or cooling rates apply to cooling to room temperature. Too long a period leads to an undesired enlargement of the individual particles or phases of the mixture. In other words, quenching occurs.Cooling preferably occurs by contacting a component acting as a heat sink, for example a sufficiently large and / or cooled component such as a metal plate such as copper, over a large area. The component does not have to be directly contacted. It is also possible to bring a carrier material carrying the melt into contact with the component. The carrier material can be, for example, a current collector or a separator of a previously produced composite. It is essential that heat flows from the melt into the component. This allows for particularly rapid cooling, which allows for a particularly short duration of the liquid phase. Undesirable diffusion processes are thus prevented.

[0054] A further aspect of the invention is a method for producing a battery cell, in which the precursor according to the invention is used as a metal electrode. All features, advantages, and effects of the method mentioned above also apply to the method according to this aspect, and vice versa.

[0055] During assembly, the precursor is combined with the other parts of the battery cell described above. In particular, a suitable, at least temporary, mechanical connection of the components is established.

[0056] The precursor is used as a metal electrode, particularly as a negative electrode. The mixture or composite is supplemented with the remaining components of the battery cell, such as electrolyte, current collector, positive electrode, and / or separator, as well as, if necessary, one or more intermediate layers.

[0057] In one embodiment, an electrolyte and / or separator comprising or made from lithium lanthanum zirconium oxide is used to manufacture the battery cell. It has been shown that this material, in combination with an electrode containing Li and Na, achieves particularly low resistances. Alternatively or additionally, an electrolyte and / or separator can comprise or be made from ceramic, glass, glass-ceramic, and / or plastic.

[0058] In one embodiment, the battery cell is formed in such a way that at least 70% of the Na or Li is removed from the metal electrode, at least temporarily. In particular, at least 80% of the Na or Li is removed, preferably at least 90%, and particularly preferably at least 99%. In this way, a three-dimensional lattice structure of the remaining metal with intervening free spaces is created for the first time.

[0059] At least 70% of the electrochemically active metal in the battery cell is removed. In a sodium battery, at least 70% of the sodium is removed. In a lithium battery, at least 70% of the lithium is removed. Percentages refer to mass percentages.

[0060] Formation refers to the first cycle(s) of charging and discharging a manufactured battery cell. This involves, for example, the formation of boundary layers between the electrodes or active materials and the electrolyte.

[0061] It cannot be ruled out that the formation also takes place after the battery has been manufactured.

[0062] Due to the precursor produced according to the invention, a three-dimensional structure (also referred to as self-assembly) is formed during formation, which enables higher cell performance. In the case of Li batteries, this involves a three-dimensional Na structure; in the case of Na batteries, this involves a three-dimensional Li structure. Self-assembly can occur during formation and / or during the first charge / discharge cycles.

[0063] In one embodiment, the forming process involves charging and / or discharging with a current that is gradually increased. Typically, the current density is also gradually increased, since the area remains the same.

[0064] In one embodiment, the battery cell is a battery cell in which the positive electrode is already discharged after assembly, i.e., has the full Li or Na content. This can be the case, for example, with a positive electrode made of or with LiCoCl, LiCoO.sub.33NiO.sub.33MnO.sub.33O.sub.2, NaMnO.sub.2, NaVPO.sub.4, etc. In this case, the first formation step is typically a charging step, in which Li or Na is sent towards the negative electrode. In this case, a comparatively thin negative electrode can preferably be produced or used and / or a material composition can be selected which contains less electrochemically active metal, for example in the region of the eutectic point of Na and Li with 3 mol% of the electrochemically active metal.

[0065] In an alternative embodiment, the battery cell is a battery cell in which the positive electrode is charged after assembly, i.e., contains no or hardly any Li or Na. This can be the case, for example, with a positive electrode made of or with sulfur or air, i.e., an oxygen electrode. In this case, a comparatively thick negative electrode can preferably be manufactured or used and / or a material composition can be selected that contains a large amount of electrochemically active metal, for example, in the region of the eutectic point of Na and Li with 97 mol% of the electrochemically active metal. In this way, the capacity of the negative electrode is maximized.

[0066] A further aspect of the invention is a method for producing a battery using at least one battery cell according to the invention. All features, advantages, and effects of the method mentioned above also apply to the method according to this aspect, and vice versa.

[0067] A battery is manufactured or assembled from one or more battery cells. The battery may additionally include electrical terminals and / or a housing, and / or the battery cells may be electrically connected to one another.

[0068] A further aspect of the invention is a battery cell. The battery cell comprises a negative electrode, an electrolyte, and a positive electrode, wherein the negative electrode comprises Li and Na. The negative electrode comprises a three-dimensional lattice structure of Li, in particular in a Na matrix, and / or a three-dimensional lattice structure of Na, in particular in a Li matrix. All features, advantages, and effects of the method mentioned above also apply to the method according to this aspect, and vice versa.

[0069] A Na or Li matrix refers to a particularly coherent phase of Na or Li. A lattice structure can, for example, be a lattice made of the respective metal at the atomic level. Between the particles of the respective metal there are free spaces or particles of another metal, in particular the other metal. The negative electrode comprises, in particular, a region comprising Na or consisting essentially of Na, which borders the lattice structure made of Li, or a region comprising Li or consisting essentially of Li, which borders the lattice structure made of Na. "Essentially" means that impurities are possible, as described.

[0070] Such a battery cell can only be manufactured using the method according to the invention, since no conventional process is currently known for producing such a lattice structure from Li or Na. In the charged state, the three-dimensional lattice structure of the metal is located within a matrix of the other metal. In the fully discharged state, there are open spaces between the three-dimensional lattice structure.

[0071] In particular, the three-dimensional lattice structure is sponge-like and / or has a pore size of at least 1 pm, in particular at least 5 pm, typically at least 10 pm, preferably at least 20 pm, in particular at least 30 pm, particularly preferably at least 40 pm and / or at most 70 pm, in particular at most 60 pm, preferably at most 50 pm.

[0072] Below, exemplary embodiments of the invention are explained in more detail with reference to experiments and figures. Features of the exemplary embodiments can be combined individually or in multiples with the claimed subject matter, unless otherwise stated. The claimed scope of protection is not limited to the exemplary embodiments.

[0073] They show:

[0074] Fig. 1 : a schematic representation of a battery cell,

[0075] Fig. 2: a schematic sequence of a method according to the invention,

[0076] Fig. 3: a method for mechanical mixing,

[0077] Fig. 4: a series of measurements of a critical current density of a battery cell,

[0078] Fig. 5: a series of measurements of the total resistance of a battery cell,

[0079] Fig. 6: a series of measurements with decomposition of the total resistance of a

[0080] battery cell,

[0081] Fig. 7: microscopic representations of a three-dimensional

[0082] lattice structure,

[0083] Fig. 8: a schematic representation of the formation of a three-dimensional

[0084] lattice structure, as well as

[0085] Fig. 9: Measurements of the complex impedance of another battery cell. Figure 1 shows a simplified, schematic, and not-to-scale representation of a battery cell 2. Since a battery 3 can also consist of a single battery cell 2, Figure 3 also shows a battery 3. Typically, however, a battery 3 contains a plurality of battery cells 2, which are connected in series and / or parallel.

[0086] The battery cell 2 contains a negative electrode 8 and a positive electrode 9, which are arranged on opposite sides of the separator 6. It is thus a full cell. The solid electrolyte acts as the separator 6 and is ion-conductive, but not, or only very slightly, electron-conductive (electrically conductive).

[0087] The battery cell 2 can be symmetrical or asymmetrical, i.e., the negative electrode 8 and the positive electrode 9 can be made of the same material or different materials. The electrodes 8, 9 can have the same or different structures. For example, a sodium metal anode can be present. The separator 6 can be made of LLZO, for example. The battery cell 2 was manufactured, in particular, using the method according to the invention.

[0088] The battery cell can optionally have a current collector 21, for example in the form of a copper foil, on the side of the negative electrode 8 facing away from the separator 6 and / or on the side of the positive electrode 9 facing away from the separator 6. A current collector 21 can electrically connect several battery cells 2 of a battery 3 to one another.

[0089] Figure 2 schematically shows a method according to the invention for producing a precursor 1. The method comprises mechanical mixing 14 of Li and Na in the solid state, whereby a mixture 15 is produced. The mixture 15 can be used directly as precursor 1. Alternatively, one or more optional additional steps can be carried out to produce the precursor 1 from the mixture 15. Each of the optional additional steps, which are shown as dashed boxes in Figure 2, can be carried out individually and / or in any combination with other steps. Each of the optional additional steps can be omitted individually and / or in any combination with other steps.The dashed arrows from box 15 to box 17, box 18, box 19 and box 1, from box 16 to box 18, box 19 and box 1, from box 17 to box 19 and box 1 and from box 18 to box 1 have been omitted to increase the clarity of Figure 2.

[0090] For example, separation and / or forming 16 can occur. In this process, a near-net-shape electrode shape can be produced. For example, a composite 17 can be produced from the mixture 15, on the one hand, and a current collector 5 and / or a separator 6, on the other. For example, melting 18 of the mixture can occur. For example, cooling 19 of the melt can occur, typically to a temperature of at most 0.6 x MP, where MP is the melting point of the mixture.

[0091] Figure 3 schematically shows a process for mechanical mixing to produce a mixture of Li and Na. The process comprises joint calendering 10 of a part comprising Li and a part comprising Na, folding 11 of the resulting product, and calendering 12 of the folded product. Folding 11 and calendering 12 are repeated several times.

[0092] Figure 4 shows a series of measurements of the critical current density of a battery cell. The battery cell is a symmetrical battery cell with Li and Na electrodes and a lithium lanthanum zirconium oxide separator. The battery cell was manufactured according to Example 1 below:

[0093] Example 1

[0094] Stoichiometric amounts of lithium (99.99%, Alfa Aesar) and sodium (99.8%, Alfa Aesar) were mechanically mixed. The metals were calendered into foils and the foils folded. This process was repeated at least 20 times until a homogeneous distribution was achieved. A mixture of Li and 3 mol% (5.36 vol%, 9.29 wt%) Na was chosen because this composition has a eutectic melting point of -170 °C.

[0095] Symmetric LLZO cells were prepared as follows by solid-state reaction of Li645AI005La3Zr16Ta04O12. LLZO powder was pressed into pellets and sintered in a closed alumina crucible at 1175 °C in air for 10 h in a high-temperature muffle furnace. The sintered pellets had a diameter of -7 mm. XRD shows a pure cubic LLZO phase. The crystallographic density of p(XRD) = 5.348 g ern -3was determined by Rietveld refinement using the Fullprof program package. On average, the pellets had a geometric density of p(geo) = 5.31 (3) g ern -3, which corresponds to a relative density of 99.3(7)%. The pellets were cut into 0.7 mm thick slices using a diamond saw (IsoMet, Bühler, ITW Test & Measurement GmbH, Leinfelden-Echterdingen, Germany) and manually pre-polished under ambient atmosphere. The separators were then placed in a glove box. To produce symmetrical cells, both sides of the LLZO separators were manually polished with 800 grit SiC sandpaper. The metal electrodes were then manually coated by pressing freshly calendered foils onto the freshly polished separators. The symmetrical cell was placed between two nickel current collectors and heated to 300 °C for five minutes. The hot cell was then quenched on a metal plate at room temperature. After cooling to room temperature, the symmetrical cell was transferred to a Swagelok housing, sealed, and removed from the glove box for analysis.

[0096] The sample and other samples mentioned in this application were characterized as follows: X-ray diffraction (XRD) analysis was performed using an Empyrean diffractometer (Malvern Panalytical GmbH, Kassel, Germany) with Cu-Ko radiation. The diffractometer was equipped with a PIXcel 3D The XRDs were measured in reflection geometries from 10 to 140° with a step size of 0.026°. Rietveld refinement of the XRD patterns was performed using the Fullprof package.

[0097] Scanning electron microscopy (SEM) images were taken using a Zeiss EVO 15 (Carl Zeiss AG, Oberkochen, Germany) at an accelerating voltage of 15 kV. Energy-dispersive X-ray spectra (EDX) were measured using an ULTIM MAX 100 detector and analyzed using the AZtec software package (both Oxford Instruments plc, England).

[0098] Electrochemical characterization was performed using a BioLogic VMP-300 multipotentiostat (Bio-Logic Sciences Instruments Ltd, Claix, France) and a VT 4002EMC climatic chamber (Vötsch Industrietechnik GmbH, Balingen, Germany). Impedance spectra were recorded at 25 °C using the EC-Lab program at frequencies between 7 MHz and 1 Hz with an electric field perturbation of 10 mV mnr. 1The measured impedance spectra were fitted with RelaxlS3 in the frequency range from 3.49 MHz to 1 Hz and 2.77 MHz to 5 Hz at 25 °C and 60 °C, respectively. The impedance spectra were fitted with an inductance element L, three R-CPE elements, and a final CPE element. The inductance element Lcabie was necessary due to the measurement setup and corresponds to the inductance of the cables used. To match the R-CPE elements to their corresponding physical counterparts, the effective capacitance C was calculated from the fitted resistance R, the CPE coefficient Q, and the exponential parameter α as follows:

[0099] The capacities fit well with the values ​​reported in the literature for the mass (10 -11 F), the grain boundaries (GB, 10 -8 F) of an ionic conductor and the interface resistance (IF, 10 -5 F) between electrolyte and electrode.

[0100] To determine the critical current density (CCD) for lithium dendrite formation, lithium dissolution and deposition experiments were performed using the BioLogic VMP-300 multipotentiostat. The cells were heated to 60 °C, and capacity-controlled galvanostatic cycles were performed at 0.1 mAh cm -2 carried out, starting with 50 pA-cnrr 2 and increasing by 50 pA cm -2 per cycle. One data point was recorded every 30 seconds. Additional data points were recorded if the measured voltage deviated by more than 10 mV from the previous data point to detect micro-short circuits. In addition, after every 0.1 mA-cnr 2 Dissolution and deposition step impedance spectra were measured. Total dissolution experiments were performed with symmetric cells at different current densities (0.1 to 1.0 mA-cnr 2) and temperatures (25 °C and 60 °C). The cells were polarized until the termination criterion of 1.0 V was reached. Every 0.5 mAh cm -2 an impedance spectrum was measured.

[0101] Plotted in Figure 4 are the current densities in mA / cm 2 l / A (mA / cm 2 ) and the voltage in mV U (mV), which describes the changes in cell polarization, over time in hours t (h). Alternating currents and voltages are applied in opposite directions, each of which can have an approximately rectangular shape. Pauses during which no current or voltage is applied can occur between each application. The application frequency can be increased.

[0102] The area shown on the left, for example the first 40 or 50 hours, shows the battery cell's forming cycles. Low current densities, for example below 1 mA / cm 2or below 0.6 mA / cm 2 applied and / or low voltages, for example, below 50 mV or below 25 mV, are measured. The current or current density and / or voltage is increased gradually. This means that with each subsequent application, a higher current or current density and / or voltage is applied, at least on average. The current or current density or voltage can remain constant during the application or can be increased.

[0103] During formation, the battery cell is alternately charged and discharged. Once the negative electrode is discharged after assembly, formation typically begins with a charging step. Once the negative electrode is charged after assembly, formation typically begins with a discharging step.

[0104] The left side shows charge and discharge cycles. Even at the highest applied current density of 5.0 mA / cm 2and after 300 cycles (dissolution-deposition cycles), no dendrite formation was observed, and the potential increased only by 2.4 times (from -102 mV to -240 mV), which is well below the usual stop criterion of 1.0 V.

[0105] Figure 5 shows a series of measurements of the total resistance of a battery cell. The imaginary part of the complex impedance is plotted in Q -Zi mag (O) via the real part of the complex impedance in QZ Reai (Q). The measurement points A refer to the freshly manufactured battery cell and correspond to the leftmost area of ​​Figure 4. The measurement points B refer to the battery cell after a test with a critical current density (CCD) of up to 1.0 mA crrr 2 and correspond to the middle area of ​​Figure 4. The measurement points C refer to the battery cell after a CCD test at 5.0 mA cm -2 and correspond to the rightmost area of ​​Figure 4.

[0106] It can be seen that the real part of the impedance decreases significantly during the forming process and is minimal after the forming process. A slight increase is observed afterward.

[0107] In contrast to reference cells with pure Li electrodes, where the interface resistance typically increases with increasing polarization current, the cells with LiNa electrodes show an opposite trend. In fact, all contributions to the cell impedance decrease with repeated cycling, after an initial small increase at low polarization currents up to 1.0 mA crn. -2 Figure 6 shows a development of the various contributions to the total impedance during the CCD measurements on the cell from Figure 4, determined by fitting the impedance spectra of Figure 5. The different contributions to the total resistance are plotted against the current density in mA / cm 2 l / A. The total resistance Rtot and the grain resistance R buik are plotted on the left vertical axis; the grain boundary resistance RGB and the interfacial resistance RIF are plotted on the right vertical axis. It can be seen that the interfacial resistance remains practically constant even at high polarization currents over several hundred dissolution and deposition cycles, which distinguishes the electrode according to the invention from all previously reported LLZO / Li cells. Similar to the cells with pure Li electrodes, the initial increase in impedance of cells with LiNa electrodes can be explained by the reduction in contact area due to the dissolution of lithium with the formation of voids. The subsequent decrease in resistance, however, is only observed with LiNa electrodes and is therefore due to the presence of the Na metal. It has been found that cycles with low currents up to 1.0 mA cm -2 reduce the LiNa / LLZO interfacial resistance.

[0108] Figure 7 shows electron microscopic images of a three-dimensional lattice structure obtained by the method according to the invention. Figures 7a and 7b show images taken by scanning electron microscopy (SEM) of the LLZO surface at different magnifications after a symmetrical cell was fully polarized (Li was completely removed). The bar at the bottom right in Figure 7a shows a length of 50 pm, while in Figure 7b it is 3 pm. The surface is almost completely covered with sodium. Figures 7c and 7d show SEM images at different magnifications of the surface of the Ni current collector after lithium was completely removed. The bar at the bottom right in Figure 7c shows a length of 100 pm, while in Figure 7b it is 30 pm. The surface is covered with a sodium framework that formed during the quenching process and subsequent formation.A three-dimensional lattice structure of sodium has formed.

[0109] The surface of the LLZO separator was found to be completely covered by a porous sodium layer, even though no sodium was intentionally applied (Figures 7a and 7b). Therefore, it can be assumed that this sodium layer formed during the first cycles from the evenly distributed Na in the LiNa foil, which was melted and quenched on the LLZO pellet during cell fabrication. The presence of the Na layer explains the very low interfacial resistance of our cells, as Na reacts with any resistant impurities on the LLZO surface, such as lithium oxides or carbonates. Furthermore, Na remains electrochemically inert at the potentials applied during electrochemical cycling but provides electronic conductivity, which is why the sodium layer can establish a morphologically stable electrical contact between the LLZO pellet and the metal anode.

[0110] This hypothesis was confirmed by a post-mortem analysis of the surface of the nickel current collector, which could be separated from the LLZO pellet because it no longer contained any lithium after complete Li dissolution. The analysis revealed that it was completely covered by a uniform layer of highly porous sodium (Figure 4c and Figure 4d). This demonstrates that the sodium layer forms a coherent 3D framework bridging the LLZO and the Ni current collector after complete Li removal and exhibits a sponge-like morphology with a disordered but uniform porosity with a pore size of approximately 40–50 pm. This structure increases the effective surface area of ​​the electrode, ensuring electrical contact and faster surface diffusion, thus enabling higher available capacities compared to a dense 2D lithium morphology.

[0111] Fig. 8 shows a schematic representation of the formation of a three-dimensional lattice structure. The left-hand area, marked (A), shows the negative electrode side of a cell according to the invention with a metal electrode after melting and cooling, particularly after cell assembly. The middle area, marked (B), shows the initial discharge of the anode with pore formation, and the right-hand area, marked (C), shows the formed three-dimensional structure of the electrochemically inactive metal at the end of formation (fully discharged state).

[0112] First, the negative metal electrode 7 is located on the separator. It is made of Li or Na and contains evenly and finely distributed impurities 24 of the other metal. This was achieved by mechanically mixing Li and Na. This structure is a very good or homogeneous mixture of the finely distributed metal in the other metal. A current collector 5 is arranged on the metal electrode 7. In the middle image, the first pores are forming because the predominant material of the metal electrode 7, Li or Na, is removed by the discharge. The finely distributed material of the impurities 24 can remain unchanged in its position, or the layer thickness can be reduced. The image on the right shows that the predominant material of the metal electrode has been completely or almost completely removed.For example, at least 80% of the material has been removed, preferably at least 90% or at least 95%, particularly preferably at least 99%. A three-dimensional lattice structure of the previously finely distributed material of the admixtures 24 remains, with gaps or pores in between. This lattice structure was formed, in particular, by cycling. A further reduction in the layer thickness may have occurred. The lattice structure produced in this way has been shown to have good mechanical stability.

[0113] It has been shown that such a grid structure enables a significantly higher critical current density (CCD) than the conventional cell design without the three-dimensional grid structure. Compared to pure lithium electrodes, which have a CCD of 2.2 mA cm 2achieved, the cell according to the invention with a three-dimensional lattice structure, here made of sodium, a CCD of more than 5.0 mA cm" 2 Even at this value, neither dendrite formation nor the termination criterion of 1.0 V was reached.

[0114] Example 2:

[0115] LiNa was prepared as described and melted between two nickel foils, each 25 μm thick. One foil was then peeled off, two anodes were punched out, and pressed onto the LLZO by hand. The impedance A was then measured at room temperature (only after pressing). The total resistance was approximately 3900 ohms. The interface was poor, as expected. This is shown in Figure 9, where a zoomed-in or enlarged view was chosen to better distinguish the impedance curves.

[0116] Subsequently, the pressing process was improved by applying pressure using hand calenders, press rolls, and / or press-calendering. The impedance B was measured (pressed and calendered). The total resistance was approximately 1000 ohms. This was followed by additional heating to 170 °C, which corresponds to the melting point of the eutectic used (pressed, calendered, and heated). The total resistance was approximately 340 ohms. The visible semicircle represents the grain boundary. For a control measurement, a higher heating temperature of 300 °C was applied (pressed, calendered, and heated). The curve remains virtually identical. The total resistance was minimally lower, which is presumably due to the fact that the cell temperature was slightly higher than 25 °C. It can be seen that reaching the melting point is sufficient to minimize the resistance.

[0117] List of reference symbols

[0118] Pre-product

[0119] Battery cell

[0120] battery

[0121] electrolyte

[0122] current collector

[0123] separator

[0124] metal electrode

[0125] Negative electrode

[0126] Positive electrode

[0127] Joint calendering

[0128] Fold

[0129] Calendering

[0130] Mix

[0131] mixture

[0132] Separating and / or forming

[0133] Association

[0134] Melt

[0135] Cool

[0136] Lattice structure

[0137] current collector

[0138] Metal admixture

[0139] pores

[0140] current

[0141] Current density in mA / cm 2

[0142] Voltage in mV

[0143] Time in hours

[0144] Imaginary part of the complex impedance in Q

[0145] Real part of the complex impedance in Q

[0146] Resistance in Q

[0147] Total resistance Rtot

[0148] Mass resistance T bu ik

[0149] Grain boundary resistance RGB

[0150] Interface resistance RIF

Claims

Claims 1. A process for producing a precursor (1) for a battery cell (2), in which Li and Na are mechanically mixed together in the solid state.

2. Method according to the preceding claim, characterized in that the mechanical mixing (14) comprises: a. joint calendering (10) of a part comprising Li and a part comprising Na, b. folding (11) the product thus obtained, c. calendering (12) the folded product, wherein steps b. and c. are repeated several times.

3. Method according to one of the preceding claims, characterized in that the mixture (15) Contains 1 mol% Na to 10 mol% Na, or - Contains 90 mol% Na to 99 mol% Na.

4. Method according to one of the preceding claims, characterized in that the mixture (15) is processed by cutting and / or forming (16) in order to produce an electrode shape close to the final shape.

5. Method according to one of the preceding claims, characterized in that a composite (17) is produced from the mixture (15) on the one hand and a current collector (5) and / or a separator (6) on the other hand.

6. Method according to one of the preceding claims, characterized in that the mixture (15) is melted.

7. Method according to the preceding two claims, characterized in that the composite (17) is heated so that the mixture (15) melts.

8. Method according to one of the preceding two claims, characterized in that the melt is cooled to a temperature of at most 0.6 x MP, where MP is the melting point of the mixture (15).

9. Method according to the preceding claim, characterized in that the cooling (19) takes place within a period of less than 1 minute.

10. A method for producing a battery cell (2), wherein the precursor (1) according to one of claims 1 to 9 is used as a metal electrode (7).

11. Method according to the preceding claim, characterized in that an electrolyte (4) and / or a separator (6) comprising lithium lanthanum zirconium oxide is used to produce the battery cell (2).

12. Method according to one of the two preceding claims, characterized in that the battery cell (2) is formed in such a way that at least temporarily the Na or the Li is removed from the metal electrode (7) by at least 70%.

13. Method according to the preceding claim, characterized in that during the forming process, charging and / or discharging takes place with a current intensity (I) which is gradually increased.

14. A method for producing a battery (3) using at least one battery cell (2) produced according to one of claims 10 to 13.

15. Battery cell (2) comprising a negative electrode (8), an electrolyte (4) and a positive electrode (9), wherein the negative electrode (8) comprises Li and Na, characterized in that the negative electrode (8) comprises: - a three-dimensional lattice structure (20) made of Li, in particular in a Na matrix, and / or - a three-dimensional lattice structure (20) of Na, in particular in a Li matrix.

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