Method and device for producing a battery cell

Abstract

A method for producing a battery cell comprising an electrolyte, a sulfur-containing cathode and a lithium or silicon anode, wherein the electrolyte (9) is enriched with polysulfides before the battery cell is put into operation, characterized by the following steps: a) providing an electrolyte (9) for lithium-sulfur battery cells, b) providing a lithium anode (5) and a sulphur-containing cathode (6), both connected to an adjustable voltage source (7), c) contacting the electrolyte with the lithium anode (5) and with the sulphur-containing cathode (6) to form a preparation cell (8), d) Operating the voltage source (7) to discharge the preparation cell (8) in such a way that polysulfides are formed at the cathode (6) and dissolve in the electrolyte to enrich it, the electrolyte (9) then being removed from the preparation cell (8) and inserted into the battery cell.

Description

[0001] The invention relates to a method for producing a battery cell comprising an electrolyte, a sulfur-containing cathode and a lithium or silicon anode, wherein the electrolyte is enriched with polysulfides before the electrolytic cell is put into operation.

[0002] Furthermore, the invention relates to a device for producing a battery cell having a sulfur-containing cathode and a lithium or silicon anode.

[0003] Methods and devices of the type mentioned above are known from the prior art. Due to growing concerns about greenhouse gases in the atmosphere, there is an increasing need to replace fossil fuels with alternative energy sources. While some progress has been made in the use of alternative energy sources for stationary applications, their use in mobile applications, such as motor vehicles, still presents a challenge. One current development focus is the electrification of motor vehicles. However, this approach is confronted with several problems, such as the need for batteries with high energy density and long lifetimes while maintaining low costs. In this context, the so-called lithium-sulfur battery, whose theoretical energy density is approximately 2300 mWh / g (Li 2S) is more than four times that of lithium-ion batteries, making it an interesting candidate for mobile applications. In addition, lithium-sulfur batteries require inexpensive materials such as sulfur instead of expensive transition metals.

[0004] The advantages of the gravimetrically higher energy density and the lower costs of the active material "sulfur" are offset by the serious disadvantage of low cycle stability. According to current knowledge, the low cycle stability of lithium-sulfur batteries is due to various mechanisms. Firstly, during discharge, polysulfides form on the cathode side of lithium-sulfur batteries. These polysulfides dissolve in the electrolyte and migrate from the cathode to the anode by diffusion or migration. On the anode side, the polysulfides formed at the beginning of the discharge on the cathode side, particularly the long-chain polysulfides, are further reduced to short-chain polysulfides on the anode. The short-chain polysulfides, such as Li 2 S 2 and Li 2S, which are no longer soluble electrolytes, precipitate due to their poor solubility or deposit as an inactive layer on the anode. These precipitated polysulfide species or those deposited on the anode are no longer available for the oxidation reaction on the cathode during cell charging. This leads to a loss of capacity at the cathode and thus to a severe degradation of the battery cell in terms of its cell capacity.

[0005] To avoid the problem described above, the published patent application WO 2011 / 134613 A1, for example, proposes enriching the electrolyte with polysulfide, which is added to the electrolyte before the battery cell is put into operation. This is intended to prevent polysulfides from forming on the sulfur cathode and dissolving in the electrolyte. The published patent application does not disclose how the polysulfides are to be added to the electrolyte. However, other publications, such as DE 31 36 820 C2, propose enriching the electrolyte with polysulfides by adding additives to the electrolyte and stirring them in. This process is time-consuming and requires complex production, which leads to corresponding manufacturing costs.

[0006] The published patent application US 6,200,704 B1 and the publication “Celine Barchasz, Jean-Claude Lepêtre, Sébastien Patoux, Fannie Alloin: Electrochemical properties of ether-based electrolytes for lithium / sulfur rechargeable batteries. 89. Electrochimica Acta: Elsevier, 2013. 737-743. - ISBN 0013-4686” also disclose processes for producing electrolytes for lithium-sulfur battery cells.

[0007] The published patent application US 2005 / 0136327 A1 discloses a battery cell and a method for producing a battery cell with a sulfur-containing cathode, a lithium anode and an electrolyte for lithium-sulfur battery cells, wherein the battery cell is electrochemically characterized by repeated charging / discharging.

[0008] The invention is based on the object of creating a method and a device for producing a battery cell which can be carried out cost-effectively and with little effort.

[0009] The object underlying the invention is achieved by the method having the features of claim 1. The method according to the invention has the advantage that an electrolyte enriched with polysulfides is provided cost-effectively and with relatively little effort, wherein, moreover, the type and amount of polysulfides dissolved in the electrolyte can be optimally adjusted. To this end, the method according to the invention provides that an electrolyte for lithium-sulfur battery cells is first provided. The electrolyte is preferably a non-aqueous electrolyte. LiTFSi in DOL / DME is particularly preferably provided as the electrolyte. The electrolyte can be in liquid or nearly solid form. Furthermore, a lithium anode and a sulfur-containing cathode are provided, both of which are connected to an adjustable voltage source.The electrolyte is contacted with the lithium anode and the sulfur-containing cathode to form a preparation cell. The voltage source is operated to discharge the preparation cell, so that polysulfides form at the cathode and dissolve in the electrolyte to enrich it. As soon as a desired amount of dissolved polysulfides is present, the electrolyte is removed from the preparation cell and is available for use in the battery cell. The method according to the invention thus enriches the electrolyte with polysulfides in a simple manner. The invention ensures that the enrichment of the electrolyte does not take place in the battery cell itself, but beforehand, so that when the battery cell is first discharged, the electrolyte is already enriched with polysulfides, thereby at least reducing the loss of sulfur from the cathode of the battery cell.If the electrolyte in the battery cell is enriched to saturation before the first discharge, less sulfur is lost from the cathode for the reaction. Diffusion losses driven by concentration gradients are also minimized by the prior enrichment with polysulfides. This increases the cycling stability of the battery cell.

[0010] According to an advantageous development of the invention, it is provided that in step d) the preparation cell is discharged at least once. The severe degradation of the cell described above occurs particularly in the first discharge-charge cycles of the battery cell. Thus, even a single discharge of the preparation cell can increase the cycle stability of the battery cell. Even a single discharge to a potential of 2.2 V or 2 V has proven advantageous. The preparation cell is therefore preferably discharged to a potential of 2.0 V to 2.4 V, particularly preferably to 2.2 V, 2.1 V, or 2.0 V.

[0011] Preferably, however, in step d), the preparation cell is discharged several times. After each discharge, the preparation cell is recharged using the voltage source. The repeated discharge further enriches the electrolyte with polysulfides and increases its stability, which leads to corresponding advantages in the subsequent battery cell.

[0012] Particularly preferably, in step d), the preparatory cell is discharged and recharged until the electrolyte is saturated with polysulfides. This provides the highest possible stability of the electrolyte or the subsequent battery cell, whereby the stability of the battery cell's capacity is maintained or at least virtually maintained even after several charging and discharging cycles.

[0013] At least compared to previously known lithium-sulfur battery cells, this significantly improves cycle stability and cell capacity.

[0014] According to an advantageous development of the invention, a reaction voltage of the voltage source, particularly for discharging, is adjusted depending on a desired chain length of the polysulfides dissolved in the electrolyte. A potentiostat or a galvanostat is preferably used as the energy source, by means of which either the voltage or the current conducted through the electrolyte can be kept constant. By appropriately adjusting the reaction voltage, which leads to the desired voltage or current, the polysulfides are influenced with regard to their chain length.As a result, the proposed method not only makes it possible to enrich or saturate the electrolyte with polysulfides, but it is now also possible to adjust the type of polysulfides, which allows corresponding freedom in the production process of the electrolyte and / or the battery cell, for example, enabling the production of different polysulfides depending on the desired battery cell.

[0015] According to an advantageous development of the invention, the reaction voltage or the voltage applied to the electrolyte is set to a value of 2.0 volts to 2.4 volts, in particular to a value of 2.2 volts. This results in polysulfides with long to medium chain lengths, which are particularly well suited for cycle stability.

[0016] Furthermore, it is preferably provided that the preparation cell is heated at least during the execution of step d). This allows reactions occurring in the preparation cell to be accelerated and thus shortens the preparation time of the electrolyte. This leads to corresponding advantages in production, as it enables a higher throughput.

[0017] The method according to the invention is further characterized in that the electrolyte is produced by the method described above, removed from the preparation cell, and inserted into the battery cell. This results in the aforementioned advantages for the battery cell with regard to cycle stability and capacity.

[0018] The device according to the invention with the features of claim 8 enables the production of the electrolyte by the method according to the invention and thus leads to the correspondingly mentioned advantages with regard to the battery cell subsequently used. The device according to the invention has a preparation cell which has a lithium anode and a sulfur-containing cathode, both of which are connected to a voltage source. Furthermore, the device has a receptacle for an exchangeable electrolyte and a control unit which operates the voltage source. The control unit is designed such that it operates the voltage source in such a way that polysulfides form at the cathode, which dissolve in the respective electrolyte contained in the receptacle to enrich it. The electrolyte contained in the receptacle is in contact with the lithium anode and the sulfur-containing cathode.The receptacle can, for example, be a container into which the electrolyte can be poured in liquid form. Alternatively, the receptacle can also be designed to hold or retain an electrolyte in solid form. It is important that the receptacle allows the electrolyte to be replaced. The receptacle preferably does not hold the electrolyte directly, but rather a container carrying the electrolyte, so that contamination of the receptacle by the electrolyte or the electrolyte already enriched with polysulfides is avoided. The device also has means for introducing the polysulfide-enriched electrolyte into the battery cell. The means can, for example, be a transport device or a pipette-like device that can hold the electrolyte and release it into the battery cell.In particular, the means comprise a container carrying the electrolyte, which can be inserted into the receptacle of the preparation cell.

[0019] According to a preferred embodiment of the invention, the voltage source is designed as a potentiostat or a galvanostat, although the potentiostat can also be operated as a galvanostat. It is important that the reaction voltage provided by the potentiostat is adjustable in order to generate a desired voltage or current in the electrolyte, which is preferably adjusted or adjustable depending on the chain length of the polysulfides to be produced.

[0020] In the following, the invention will be explained in more detail using an exemplary embodiment. Fig. 1 a cycle stability diagram of a lithium-sulfur battery cell, Fig. 2 a preparation cell for producing an electrolyte for a lithium-sulfur battery cell and Fig. 3 a diagram explaining the production of different polysulfides depending on a set reaction voltage.

[0021] Fig. Figure 1 shows a diagram of a typical discharge curve 1 of a lithium-sulfur battery cell over its number of cycles X, where a cycle is understood to be the single discharge and charge of the battery cell. The typical discharge curve 1 shows the discharge capacity K of the battery cell over the number of cycles X, where the discharge capacity is related to the amount of sulfur used in the cell (mAh / g Schwefel). It can be seen that the capacity of the battery cell decreases sharply, especially during the first few cycles. This degradation of the battery cell is due to a loss of capacity at the cathode, which occurs through the formation of polysulfides on the cathode side during discharge. The polysulfides dissolve in the electrolyte and migrate from the cathode to the anode. On the anode side, the long-chain polysulfides formed on the cathode side at the beginning of the discharge are further reduced to short-chain polysulfides, which are no longer soluble in the electrolyte. These then precipitate due to their poor solubility or deposit as an inactive layer on the anode. This reduces the capacity or discharge capacity of the battery cell accordingly, especially during initial commissioning.

[0022] Furthermore, Fig. Figure 1 shows a discharge curve 2 for a lithium-sulfur battery cell whose electrolyte was enriched and, in particular, saturated with polysulfides before the battery cell was put into operation. It is clearly visible that the degradation at the beginning of commissioning is less severe than in the typical discharge curve 1, and that the battery cell exhibits a higher overall capacity over its lifetime or number of cycles X.

[0023] Fig. Figure 2 shows a schematic representation of a device 3 by means of which the aforementioned electrolyte is produced or saturated with polysulfides. For this purpose, the device 3 has a receptacle 4 into which an electrolyte for lithium-sulfur battery cells, in particular in liquid form, can be introduced. In this case, the receptacle 4 is a corresponding container for holding liquid electrolyte.

[0024] The device 3 further comprises a lithium anode 5 and a sulfur-containing cathode 6, both of which can be arranged or are arranged in the receptacle 4. The anode 5 and the cathode 6 are both connected to a voltage source 7, which is designed as a DC voltage source and in particular as a potentiostat, and which is operated by a control unit 10. The sulfur-containing cathode is in particular a sulfur-filled gas diffusion electrode (GDL). The receptacle 4, the lithium anode 5, and the sulfur-containing cathode 6 together form a preparation cell 8, into which the electrolyte 9 is introduced. Preferably, the device 3 or the preparation cell 8 is sealed hermetically under argon gas. The preparation cell is charged and discharged by means of the potentiostat, so that polysulfides form on the sulfur cathode and are dissolved in the electrolyte in the receptacle 4.If necessary, the preparation cell 8 is tempered during this process. The preparation cell 8 is expediently charged and discharged until the electrolyte is saturated with polysulfides.

[0025] Fig. 3 shows a simplified representation of the reaction taking place during the discharge for one cycle in the preparation cell 8. Fig. 3 via the specific capacity or discharge capacity K of the battery cell, the voltage U set by the voltage source 7 7 in volts, as well as the reactions generated by them. In the first area, the sulfur species S 8the sulfur-containing cathode 6 to long-chain polysulfides. These long-chain polysulfides dissolve in all electrolytes suitable for lithium-sulfur batteries. The long-chain polysulfides are further reduced to short-chain polysulfides upon further discharge at the anode. A mixture of various polysulfides with different chain lengths is formed in the electrolyte. The establishment of the equilibrium of polysulfides in the electrolyte takes several cycles. Depending on the set potential and the reaction time (time to reach equilibrium), as in Fig. 3, different types of polysulfides are produced. Polysulfides with a chain length of Li are preferred. 2 S 8 to Li 2 S 4produced, since short-chain polysulfides are not soluble in the electrolyte and can precipitate. Advantageously, polysulfides are produced here with a potential of approximately 2.2 volts, so that polysulfides with long to medium chain lengths are produced.

[0026] The saturated electrolyte is removed from preparation cell 8 and introduced into the actual battery cell. The advantage of this is that the electrolyte is not saturated with polysulfides in the battery cell, which would remove sulfur from the cathode side of the battery cell. It is evident that the degradation behavior was improved by the saturated electrolyte.

[0027] With the aid of the advantageous device 3, different types of polysulfides with different chain lengths can be generated depending on the potential until the electrolyte is saturated accordingly. This makes it possible to create a defined equilibrium of polysulfides with defined chain lengths in the electrolyte before assembly of the battery cell. The preferred electrolyte, for example, is LiTFSi in DOL / DME (1 / 1). Vol% with polysulfides. The electrolyte produced in this way can be used in battery cells with lithium anodes as well as in battery cells with silicon anodes.

Claims

[1] Method for producing a battery cell comprising an electrolyte, a sulphur-containing cathode and a lithium or silicon anode, wherein the electrolyte (9) is enriched with polysulphides before the battery cell is put into operation, characterized by following steps: a) providing an electrolyte (9) for lithium-sulfur battery cells, b) providing a lithium anode (5) and a sulphur-containing cathode (6), both connected to an adjustable voltage source (7), c) contacting the electrolyte with the lithium anode (5) and with the sulphur-containing cathode (6) to form a preparation cell (8), d) Operating the voltage source (7) to discharge the preparation cell (8) in such a way that polysulfides are formed on the cathode (6) and dissolve in the electrolyte to enrich it, the electrolyte (9) then being removed from the preparation cell (8) and inserted into the battery cell. [2] Method according to claim 1, characterized by that in step d) the preparation cell (8) is discharged at least once. [3] Method according to claim 1, characterized by that in step d) the preparation cell (8) is discharged several times. [4] Method according to claim 1, characterized by that in step d) the preparation cell (8) is discharged until the electrolyte (9) is saturated with polysulfides. [5] Method according to one of the preceding claims, characterized bythat a reaction voltage of the voltage source (7) is set depending on a desired chain length of the polysulfides dissolved in the electrolyte (9). [6] Method according to claim 5, characterized by that the reaction voltage is set to a value of 2.0 volts to 2.4 volts, in particular to a value of 2.2 volts. [7] Method according to one of the preceding claims, characterized by that the preparation cell (8) is heated at least during the execution of step d). [8] Device (3) for producing a battery cell, in particular a lithium-sulfur battery cell, which has a sulfur-containing cathode and a lithium or silicon anode, with a preparation cell (8) which has a lithium anode (5) and a sulfur-containing cathode (6), both of which are connected to a voltage source (7), with a receptacle (4) for an exchangeable electrolyte (9) and with a control unit (10) which operates the voltage source (7) in such a way that polysulfides are formed on the cathode (6), which dissolve in the respective electrolyte (9) located in the receptacle (4) to enrich it, and with means for introducing the polysulfide-enriched electrolyte (9) into the battery cell. [9] Device according to claim 8, characterized by that the voltage source (7) is designed as a potentiostat or as a galvanostat.

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