Method for forming a battery cell and device for controlling formation

Abstract

The invention relates to a method for forming a battery cell, in particular for forming a solid-electrolyte interphase, wherein a time-dependent voltage (4, 5) is used to form the battery cell. The method is characterized in that, in order to charge (4) the battery cell during formation, the voltage (4) is increased from a minimum voltage (Vmin) to a maximum voltage (Vmax) in multiple potential steps (42, 43, 44), a constant voltage being associated with each of the potential steps (42, 43, 44). Furthermore, the invention relates to a device for controlling formation of a battery cell.

Description

[0001] Description

[0002] Method for forming a battery cell and device for controlling a formation

[0003] The invention relates to a method according to the preamble of claim 1 and a device according to the preamble of claim 15.

[0004] Forming a battery cell is one of the final production steps in cell manufacturing. Typically, this formation process takes place after the cell has been filled with electrolyte and sealed, and before the battery cell is stored and undergoes end-of-line quality control.

[0005] The formation process encompasses the initial charging and discharging cycles of the finished battery cell and aims to ensure a defined initial state that is as optimal as possible for later use. In lithium-ion cells and related technologies, the formation process aims to create a well-defined solid electrolyte interphase (SEI) on the surface of the anode. This interphase forms through reactions between the electrode and electrolyte additives within specific voltage and, if necessary, temperature ranges, which are traversed during the charging and discharging cycles of the formation process.

[0006] Within a battery cell, several complex reactions occur simultaneously, differing in their reaction kinetics. Each electrochemical reaction, especially those contributing to SEI formation, is dependent on the potential at the reaction site.

[0007] Therefore, if the specified current or applied voltage is incorrectly set over time, potentials may develop at the interfaces that are not suitable for the formation of the optimal SEI, or other reactions, such as the charging of the electrode materials, may be accelerated, which in turn negatively affect SEI formation. It is therefore essential that the formation process uses suitable current, voltage, and / or temperature profiles to generate the desired SEI.

[0008] Furthermore, cell formation is one of the most time- and energy-intensive processes in cell production, so it must be as efficient as possible. The aforementioned technical requirements, as well as the complex underlying chemical reactions, place high technical demands on the definition and control of process parameters during the formation process.

[0009] Known forming processes use constant currents and, if necessary, voltage and temperature profiles based on empirical tests on the produced cells or battery cells. The process parameters are adjusted to achieve the best possible compromise between performance and process costs. If measurements are used to monitor the forming process, they are limited to current and voltage measurements. For example, the process is cycled until the measured differential capacitance falls below a certain threshold. This method requires a very precise understanding of the available process windows and still cannot completely eliminate variations in the quality of the produced cells.

[0010] The present invention is based on the objective of improving the formation of a battery cell.

[0011] The problem is solved by a method with the features of independent claim 1 and by a device with the features of independent claim 15. Advantageous embodiments and further developments of the invention are specified in the dependent claims.

[0012] In the inventive method for forming a battery cell, in particular for forming a solid-electrolyte interface, a time-dependent voltage is used for forming the battery cell. The inventive method is characterized in that, for charging the battery cell during formation, the voltage is increased from a minimum voltage to a maximum voltage in several potential stages, with each potential stage being associated with a constant voltage.

[0013] The battery cell is specifically designed as a lithium-ion battery cell.

[0014] The method according to the invention can preferably be carried out within the framework of a manufacturing process for the battery cell.

[0015] Analogous to and equivalent to the invention, the voltage of the battery cell is reduced from a maximum voltage to a minimum voltage in several potential stages during the formation process, with each potential stage being associated with a constant voltage. The method according to the invention thus constitutes a control / regulation system for the formation process, in which the voltage is controlled or regulated according to the invention, i.e., according to the potential stages. Here, the voltage is fundamentally time-dependent due to the aforementioned increases, but remains constant within the potential stages.

[0016] In other words, according to the invention, the voltage is increased (charging) and / or decreased (discharging) in steps between the minimum and maximum voltages. The potential levels are thus defined by fixed and / or controllable time intervals within which the voltage remains constant. During charging, this constant voltage increases from potential level to potential level. Similarly, during discharging, the same constant voltage decreases from potential level to potential level. The current within the potential levels, particularly during the transition from one potential level to the next, is regulated by the individual battery cell itself, so that, unlike in known methods, it is not kept constant.

[0017] The method according to the invention therefore does not provide for a constant current during the charging and / or discharging of the battery cell during its formation, but rather a step-like or staircase-like increase or decrease of the voltage. The height of the respective steps or staircases can be the same or different.

[0018] This allows the battery cell to cycle between the two defined operating points V. m in and V ma x. It is ensured that, analogous to the voltage, the amounts of charge supplied and removed are also comparable to a known continuous, i.e., non-step-like, process. Since the currents vary in this case, and preferably correspond at least to the value of the known continuous process, a considerable time saving results compared to known forming methods.

[0019] Furthermore, the invention has the advantage that, through the step-like and thus gradual formation, the respective current strengths, voltages, cell internal resistances, charge quantities and other quantities within the potential stages can be recorded and thus, in principle, made available or used for controlling / regulating the formation.

[0020] The invention significantly reduces the formation time, thereby saving time, energy, and costs, particularly within the manufacturing process of the battery cell. The device according to the invention for controlling / regulating the formation of a battery cell, especially for the formation of a solid-electrolyte interface, comprises a control unit, wherein a time-dependent voltage for forming the battery cell can be controlled by means of the control unit. The device according to the invention is characterized in that the control unit is configured to increase the voltage from a minimum voltage to a maximum voltage in several potential stages for charging the battery cell during formation, with each potential stage being associated with a constant voltage.

[0021] Similar advantages and / or embodiments of the device according to the invention result from the method according to the invention, which are equivalent and have the same effect.

[0022] According to an advantageous embodiment of the invention, a new potential stage begins when, after the preceding potential stage, the current has dropped to a defined minimum current.

[0023] Typically, the current decreases after reaching the voltage value or the voltage of a potential stage. When the current reaches the minimum current with a defined accuracy, the voltage is increased to the voltage associated with the next potential stage. This process can be repeated for each potential stage. It is particularly preferred that the minimum current is set to correspond to the current used in known continuous forming processes (current-controlled forming).

[0024] In an advantageous further development of the invention, the minimum current is determined specifically for each battery cell.

[0025] In other words, the minimum current is preferably determined based on the battery cell, particularly its type. This enables advantageous conditioning tailored to the specific battery cell or cell type. In particular, the individual C-rate of the battery cell can be used for this purpose. Furthermore, individual, i.e., battery cell-specific, electrical parameters can be employed.

[0026] According to an advantageous embodiment of the invention, charging of the battery cell is terminated when the maximum voltage is reached. Thus, the maximum voltage is the voltage used for the formation of the battery cell. This can also be set specific to the battery cell. This advantageously ensures that no excessively high voltages are used that would endanger the SEI formation and / or the battery cell. After charging the battery cell to the maximum voltage, a stepwise discharge of the battery cell, similar to charging, can preferably be carried out during the formation process. Here, the voltage is reduced stepwise until the minimum voltage is reached.

[0027] In an advantageous embodiment of the invention, the battery cell is discharged after charging, wherein, during the formation phase, the voltage of the battery cell is reduced from the maximum voltage to the minimum voltage in several potential steps, with each potential step being associated with a constant voltage. Preferably, several such charging and discharging cycles according to the aforementioned potential steps are repeated successively.

[0028] This advantageously allows the discharge of the battery cell during its formation to also be carried out in stages, i.e., in potential or voltage stages. This further improves the formation of the battery cell. In particular, it allows the formation time to be further reduced.

[0029] It is particularly preferred if the same potential levels are used for discharging as for charging. Alternatively, different potential levels are provided for charging and discharging.

[0030] In other words, when the battery cell is discharged, the potential stages of charging are symbolically traversed in reverse. This advantageously enables improved SEI formation.

[0031] According to an advantageous embodiment of the invention, the voltage of one of the potential stages is determined depending on a preceding potential stage.

[0032] In other words, physical parameters of the preceding potential stage are preferably measured, in particular voltages, currents, and / or cell internal resistances, and the level or voltage of the subsequent potential stage is determined or adjusted based on the measured values ​​of these parameters. This advantageously results in a dynamic, step-like charging or discharging of the battery cell during its formation, whereby the level of the potential stages is not predetermined but rather determined or adjusted during the process based on battery cell-specific physical parameters. This allows for further improvement in the formation or development of the state of electrical activity (SEI) of the respective battery cell.

[0033] It is particularly preferred if the voltage is determined as a function of the cell's internal resistance, in particular as a function of the ohmic resistance of the preceding potential stage.

[0034] For this purpose, the cell's internal resistance at the preceding potential stage is measured. This is done, for example, for various frequencies using electrochemical impedance spectroscopy. In particular, the ohmic resistance R is measured. ohm The value obtained from high-frequency impedance spectroscopy is recorded and used. The height of the subsequent potential step is then determined according to At / = R ohm max determined or set, whereby / max This denotes the maximum permissible current. Thus, the i-th potential level has the voltage U. t = U i-1 + At / = U i-1 + / ? ohm max Alternatively or additionally, the ohmic resistance can be determined using / ? ohm= At / / / are determined, where At / and / are the measured potential jumps and current intensities of the preceding potential stage.

[0035] In an advantageous embodiment of the invention, a maximum current is specified for the formation process, wherein the potential levels are defined such that the current within each of the potential levels is less than or equal to the maximum current.

[0036] This advantageously ensures that the current induced by the respective voltage of the potential stage does not exceed the permissible maximum current for the battery cell.

[0037] According to an advantageous embodiment of the invention, the charge throughput associated with one or more of the potential levels is determined.

[0038] This advantageously allows the amount of charge associated with the formation process, or the associated charge throughput, to be determined. This can be achieved in particular by integrating the current's time course.

[0039] In an advantageous embodiment of the invention, the time course of the current is evaluated for one or more of the potential levels to determine the kinetics of the battery cell. In particular, the relaxation times of the current can be used for this purpose to obtain information about the kinetics of the battery cell.

[0040] According to an advantageous embodiment of the invention, the current associated with the maximum voltage is recorded and used as a quality indicator for the formation of the solid-electrolyte interface phase.

[0041] The maximum current, provided it is set below the specified limit (maximum current strength), provides information about reactive processes within the battery cell and can therefore be used as a quality indicator for SEI formation.

[0042] Furthermore, current, voltage, resistance and / or charge quantity per potential level can be used as a quality indicator.

[0043] In an advantageous further development of the invention, the internal cell resistance is detected and / or determined for one or more potential levels.

[0044] This is achieved primarily through electrochemical impedance spectroscopy. This advantageously allows the amplitude or voltage of the potential stages to be dynamically adjusted or fixed for each battery cell. The cell's internal resistance is fundamentally complex, meaning it has a real and an imaginary part, or equivalently, a magnitude and a phase. The cell's internal resistance is frequency-dependent, allowing it to be measured at one or more frequencies or within a defined frequency range.

[0045] Another advantage of electrochemical impedance spectroscopy is that it can be performed immediately before the next potential step, i.e., before the next potential level. This allows the resistance of the battery cell to be measured as accurately and in real time as possible.

[0046] According to an advantageous embodiment of the invention, the potential levels are thus determined in a battery cell-specific manner.

[0047] This allows for further improvement in the formation of a battery cell and thus the development of the respective SEI. Further advantages, features, and details of the invention will become apparent from the exemplary embodiments described below and from the drawings. These show, schematically:

[0048] Figure 1 shows a charging and discharging cycle during a formation according to an embodiment of the invention;

[0049] Figure 2 shows a corresponding stress curve during a forming process according to the embodiment of the invention; and

[0050] Figure 3 A corresponding current profile during a formation process according to the embodiment of the invention.

[0051] Similar, equivalent or equivalent elements may be provided with the same reference symbols in one or more of the figures.

[0052] Figure 1 shows a schematic charging cycle 4 and discharging cycle 5 of a battery cell during its formation according to an embodiment of the invention.

[0053] Furthermore, Figure 1 shows a schematic charging cycle 2 and discharging cycle 3 of a battery cell during its formation according to a known method.

[0054] Time in arbitrary units is plotted on the abscissa 100 of the diagram shown.

[0055] The voltage is plotted in arbitrary units on the ordinate 101 of the diagram shown.

[0056] The process according to the present embodiment proceeds in stages, meaning that the voltage is increased (charging) and / or decreased (discharging) in a step-like or staircase-like manner according to several potential levels. This step-like behavior of the voltage is symbolized in the figure by discrete dots / crosses. Thus, each dot or cross corresponds to a potential level within which the voltage is essentially kept constant. The time-dependent voltage induced by the potential levels is labeled in Figure 1 with reference numeral 4 for charging and with reference numeral 5 for discharging. For the known process, the voltage is also time-dependent, with charging labeled with reference numeral 2 and discharging with reference numeral 3. The voltage profiles 2, 3 for the known process are not quasi-discrete in steps, but continuous.In this process, the charging current and the discharging current are kept constant for charging 2 and discharging 3, respectively, in contrast to the method according to the invention.

[0057] Figure 1 shows that the method according to the invention, or according to one of its embodiments, leads to a significantly shorter formation time. This allows for a reduction in the time, energy, and costs associated with formation, particularly within a manufacturing process.

[0058] Typically, at the beginning of the formation process, the battery cell is in an equilibrium state with an open-circuit voltage (OCV) of V. m in.

[0059] The voltage is then increased to the new value of the first potential stage, for example V01, using a potentiostat and / or cycler. Since an immediate increase in potential is associated with a high current peak, a maximum current, i.e., a maximum current intensity, can be set. This advantageously ensures that the battery cell is not damaged. The battery cell is then supplied with current until the new voltage is reached.

[0060] Alternatively or additionally to the aforementioned limitation of the maximum current, the potential steps can be set or adjusted so low that impermissible currents or current intensities are prevented. Furthermore, the potential steps can be adjusted by means of a control / regulation of the formation process, which analyzes the parameters of the last potential step to set the magnitude of the next potential step.

[0061] Once the new voltage value is reached, the current typically decreases. When it falls to a fixed value I m If the value is in, then the next potential level V02 can be set. Imin corresponds in particular to the value that would be used in a continuous, current-controlled process.

[0062] The gradual increase of the potential can continue until a predetermined maximum voltage V is reached. max is reached. For a subsequent discharge process, the direction of the voltage change can be reversed. In the diagram of Figure 1, next to the minimum voltage V, m in and the maximum voltage V ma The potential level Vos is marked on the ordinate 101 as an example. Here, the potential level Vos starts at time tos, which is marked on the abscissa 100 as an exemplary time point.

[0063] Figure 2 shows a voltage curve 4 corresponding to Figure 1 during the charging 4 of the battery cell during its formation.

[0064] The abscissa 100 of the diagram shown in Figure 2 represents time in arbitrary units.

[0065] The voltage is plotted in arbitrary units on the ordinate 101 of the diagram.

[0066] The diagram shows three potential levels 42, 43, 44, which are associated with the voltages V04, Vos, and Vos. Within each of the potential levels 42, 43, 44, the respective associated voltage V04, Vos, and Vos is constant.

[0067] In other words, the fourth potential stage 42 has a voltage V04. At time tos, the fifth potential stage 43 begins; that is, the voltage is increased from Vo4 to Vos and remains constant within the fifth potential stage 43. At time tos, the fifth potential stage 43 ends, and the sixth potential stage 44 begins, exhibiting the voltage Vos. This results in a step-like or staircase-like voltage profile 4.

[0068] The voltage increase from one potential stage 42, 43, 44 to the next potential stage 42, 43, 44 is relatively abrupt, but typically not instantaneous. However, it can also be (quasi) instantaneous. How quickly the voltage can be increased depends primarily on the maximum permissible current. The timing for increasing (charging) or decreasing (discharging) the voltage from potential stage 42, 43, 44 to potential stage 42, 43, 44 can be triggered by the current dropping to its minimum value. This is further illustrated in Figure 3.

[0069] Figure 3 shows a current curve corresponding to Figure 2, specifically the current strength 6.

[0070] Time, in arbitrary units, is plotted on the abscissa 100 of the diagram. Current, or amperage, in arbitrary units, is plotted on the ordinate 102 of the diagram.

[0071] The current waveform across the potential steps is jagged, with peaks originating from the beginning and thus causing a sudden voltage increase at the start of a new potential step. These current peaks must remain below the maximum permissible current. This can be ensured, for example, by adjusting the rate of voltage increase.

[0072] After reaching the respective constant voltage of the potential stages, the current within the potential stage continuously decreases. When it reaches its minimum value Imin, this is used as the time trigger for the start of the next potential stage. The current in the next potential stage exhibits a similar curve, with the next potential stage again starting when the current decreases to the value I. m in begins. This continues until the maximum voltage V is reached.ma x continues, whereby a discharge then takes place according to the same scheme in descending potential levels.

[0073] Although the invention has been illustrated and described in detail by the preferred embodiments, the invention is not limited by the disclosed examples, nor can other variations be derived from them by a person skilled in the art without leaving the scope of protection of the invention.

[0074] Reference symbol list

[0075] 2 time-dependent standard voltage (charging)

[0076] 3 time-dependent standard voltage (discharge)

[0077] 4. Time-dependent voltage (charging)

[0078] 5. Time-dependent voltage (discharging)

[0079] 6. Time-dependent current (charging)

[0080] 42, 43, 44 Potential level

[0081] 100 Abscissa (time)

[0082] 101 Ordinates (voltage)

[0083] 102 Ordinate (current intensity)

[0084] Vmin minimum voltage

[0085] Vmax maximum voltage

[0086] I min minimum current

Claims

Patent claims 1. Method for forming a battery cell, in particular for forming a solid-electrolyte interface phase, in which a time-dependent voltage (4, 5) is used for forming the battery cell, characterized in that the voltage (4) is increased from a minimum voltage (V) to charge the battery cell during formation. m in) up to a maximum voltage (V ma x) is increased in several potential levels (42, 43, 44), with each of the potential levels (42, 43, 44) being associated with a constant voltage.

2. Method according to claim 1, characterized in that a new potential stage (42, 43, 44) begins when, after the preceding potential stage (42, 43, 44), the current has dropped to a defined minimum current (in).

3. Method according to claim 2, characterized in that the minimum current (Lin) is determined in a battery cell-specific manner.

4. Method according to one of the preceding claims, characterized in that the charging (4) of the battery cell is terminated when the maximum voltage (V) ma x) is reached.

5. Method according to one of the preceding claims, characterized in that after charging (4) the battery cell, a discharge (5) of the battery cell takes place, wherein for discharging (5) the battery cell during the formation the voltage (5) is reduced from the maximum voltage (V) ma x) up to the minimum voltage (V m in) is reduced in several potential steps (42, 43, 44), with each of the potential steps (42, 43, 44) being associated with a constant voltage.

6. Method according to claim 5, characterized in that the same potential levels (42, 43, 44) are used for discharging (5) as for charging (4).

7. Method according to one of the preceding claims, characterized in that the voltage of one of the potential stages (42, 43, 44) is determined as a function of a preceding potential stage (42, 43, 44).

8. Method according to one of the preceding claims, characterized in that the voltage is determined as a function of a cell internal resistance of the preceding potential stage (42, 43, 44).

9. Method according to one of the preceding claims, characterized in that a maximum current is set for the formation, wherein the potential levels (42, 43, 44) are set such that the current within each of the potential levels (42, 43, 44) is less than or equal to the maximum current.

10. Method according to one of the preceding claims, characterized in that the charge throughput associated with one or more of the potential levels (42, 43, 44) is determined.

11. Method according to one of the preceding claims, characterized in that for one or more of the potential levels (42, 43, 44) an evaluation of the time course of the current intensity is carried out to determine the kinetics of the battery cell.

12. Method according to one of the preceding claims, characterized in that the maximum voltage (V) ma x) associated current strength is recorded and used as a quality indicator for the formation of the solid-electrolyte interface phase.

13. Method according to one of the preceding claims, characterized in that a cell internal resistance is detected and / or determined for one or more potential levels (42, 43, 44).

14. Method according to one of the preceding claims, characterized in that the potential levels (42, 43, 44) are determined in a battery cell-specific manner.

15. Device for controlling the formation of a battery cell, in particular for the formation of a solid-electrolyte interface phase, comprising a control unit, wherein a time-dependent voltage (4) for the formation of the battery cell can be controlled by means of the control unit, characterized in that the control unit is configured to increase the voltage (4) from a minimum voltage (V) for charging (4) the battery cell during the formation. m in) up to a maximum voltage (V ma x) to increase in several potential levels (42, 43, 44), with each of the potential levels (42, 43, 44) being associated with a constant voltage.

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