Regeneration method for a silicon-containing electrochemical storage cell

Abstract

The invention relates to a regeneration method for a silicon-containing electrochemical storage cell, wherein the electrochemical storage cell is operated in a battery electric vehicle, and an anode active material at least partly contains silicon, having the following steps: - discharging the electrochemical storage cell from an operating charging range to a target threshold value, the target threshold value being a function of the anode potential of the electrochemical storage cell and the phase transition of the silicon from a crystalline form to an amorphous form, and then - charging the electrochemical storage cell into the operating range.

Description

[0001] 24-1258 PIF 1

[0002] REGENERATION PROCESS FOR A

[0003] SILICON-CONTAINING ELECTROCHEMICAL STORAGE CELL

[0004] The following description concerns a regeneration process for a silicon-containing electrochemical storage cell, wherein the electrochemical storage cell is operated in a battery-electric vehicle and an anode active material contains at least some silicon. Furthermore, a test setup for carrying out the regeneration process is described.

[0005] State of the art

[0006] Anodes with mixed active materials, containing silicon (e.g., SiOx or a silicon composite (SiC)) as a secondary material alongside graphite, are gaining increasing interest in research and commercial applications because they promise to increase the capacity of lithium-ion batteries. Silicon has the advantage of a significantly higher specific capacity than graphite, but the disadvantage of lower cycle and structural stability. Accelerated degradation of silicon is due to significant volume expansion. Furthermore, battery cells with silicon anodes exhibit a performance impairment known as the memory effect, which occurs when a cell is repeatedly only partially discharged and recharged.

[0007] Batteries, battery cells, and battery storage systems using lithium-ion as the energy carrier and silicon as the active anode material store some of the lithium in a crystalline phase when the battery is not fully discharged. If the battery is only partially operated (e.g., within the typical operating range of 10 to 100% state of charge for battery electric vehicles), this crystalline phase forms but cannot completely revert to its previous state. This leads to a change in the voltage curve. This change creates estimation errors that give the impression that the battery has aged too much and has a reduced range. This effect intensifies with an increasing number of charge cycles.

[0008] The task is to specify a method that reduces the limitations of a battery cell caused by repeated partial discharge and recharge. 24-1258 PIF 2

[0009] This problem is solved by the regeneration process with the features of the independent and dependent claims. Advantageous embodiments and further developments of the invention are described in the dependent claims.

[0010] It is hereby assumed that each feature described in relation to any embodiment may be used alone or in combination with other features described herein, and may be used in combination with one or more features of any other embodiment, or in any combination of any other embodiment, unless explicitly described as an alternative. Furthermore, equivalents and modifications not described below may be used without departing from the scope of the claimed subject matter.

[0011] Any terms used herein, such as "comprises," "includes," "features," "has," "with," or any other variant thereof, are intended to cover non-exclusive inclusion. For example, a method or apparatus that includes or features a list of elements is not necessarily limited to those elements but may include other elements not expressly listed or inherent in such method or apparatus.

[0012] Furthermore, unless explicitly stated otherwise, "or" refers to an inclusive or and not an exclusive "or". For example, a condition A or B is satisfied by any of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present).

[0013] The terms "a" or "an" as used here are defined as "one or more". The terms "another" and "another", as well as any other variant thereof, are to be understood as "at least one more". The term "plural", as used here, is to be understood as "two or more". 24-1258 PIF 3

[0014] The terms "configured" or "set up" to perform a specific function (and their respective variations) are understood within the meaning of the invention to mean that the corresponding device or apparatus already exists in a configuration or setting in which it can perform the function, or at least that it is adjustable—i.e., configurable—so that it can perform the function after appropriate adjustment. Configuration can be achieved, for example, by adjusting parameters of a process sequence or by using switches or similar devices to activate or deactivate functionalities or settings. In particular, the device or apparatus can have several predetermined configurations or operating modes, so that configuration can be carried out by selecting one of these configurations or operating modes.

[0015] The following describes a regeneration process for a silicon-containing electrochemical storage cell. The electrochemical storage cell is operated in a battery-electric vehicle. An anode active material of the electrochemical storage cell contains at least some silicon. According to one embodiment, the process comprises at least the following steps:

[0016] 1) The electrochemical storage cell is discharged from an operating charge range down to a target threshold. The target threshold is a function of the anode potential of the electrochemical storage cell and a phase transition of the silicon from a crystalline to an amorphous form.

[0017] 2) Subsequently, after reaching the target threshold, the electrochemical storage cell is charged up to the operating charge range.

[0018] The electrochemical storage cell is, for example, installed in a battery-electric vehicle. Typically, the electrochemical storage cell is integrated together with a large number of other storage cells to form a high-voltage storage system, so that the procedure presented here is applied analogously to all electrochemical storage cells, i.e., the high-voltage storage system is discharged and charged.

[0019] The term "battery electric vehicle" used here refers specifically to a passenger car, including all types of hybrid and battery-powered electric vehicles, as well as 24-1258 PIF 4

[0020] Vehicles such as sedans, vans, buses, trucks, delivery vans, and the like. The term "electric vehicle" refers specifically to electric or hybrid vehicles, particularly vehicles that are at least partially powered by an electric motor. An electric vehicle can be a passenger car, but also a vehicle such as a van, bus, truck, delivery van, and the like, or a two-wheeler such as an (electric) motorcycle, (electric) scooter, e-bike, e-scooter, and the like.

[0021] The term "active material," as used here, refers in particular to a material that can be electrochemically active and is suitable for coating electrodes for electrode windings in battery cells, and into which ions, especially lithium ions, can be incorporated. The active material for the anode may, in particular, be graphite, SiOx, SiC, Si, or another material.

[0022] The improved concept presented here is based in particular on the considerations outlined below. Silicon, as an anode active material, exhibits a phase transition from an amorphous to a crystalline structure when alloyed with lithium ions. This phase transition forms at a comparatively high cell voltage and can only revert to a lower cell voltage. The improved concept presented here is based on the experimentally observed fact that a complete discharge of a silicon-containing electrochemical storage cell reverts the phase transition, and the cell returns from the crystalline structure to its original amorphous structure. This effect can be reversed by a single deep discharge to a suitably chosen target threshold.

[0023] The inventors investigated, among other things, the effects of the described phase transition on the operation and state estimation of silicon-containing mixed anodes, including graphite / SiOx mixed anodes, in full cells. They found that continuous cycling without complete battery discharge leads to lithium becoming trapped in the crystalline phase, which has a significantly higher delithiation potential than amorphous silicon. This results in changes to the voltage profile at lower states of charge (SoC), where silicon is active during discharge, and a reduction in discharge energy, which can limit the range of battery-electric vehicles. Furthermore, voltage-based SoC and state-of-health (SoH) estimates lead to misleading values ​​and uncertainties in battery operation. 24-1258 PIF 5

[0024] The proposed discharge and charge process, depending on the anode potential, achieves a phase transition of the cell from a crystalline structure back to its original amorphous structure. Upon reaching the target threshold, this phase transition is sufficiently complete that the original amorphous structure has essentially reverted. The lithium trapped by partial charge and discharge cycles is then released.

[0025] In one embodiment, the battery-electric vehicle is put into a regeneration operating mode, and the discharging and charging follow each other immediately.

[0026] The regeneration process should not be carried out during normal ferry operation, as this could result in the vehicle breaking down. The regeneration process can be performed, for example, during a service check at a vehicle workshop or with a permanent connection to a bidirectional wallbox. The regeneration mode can be activated, for example, via a control unit in the vehicle, which can then automatically initiate and carry out the necessary charging and discharging measures.

[0027] In one embodiment, the battery-electric vehicle is discharged in regeneration mode using a charging and discharging device in a test stand, a bidirectional wallbox and / or by switching consumers on and off on the vehicle side.

[0028] In one embodiment, the target threshold is determined by determining the anode potential of the electrochemical storage cell.

[0029] The anode potential can be determined in advance, for example at the factory, by measuring it against a reference electrode for the electrochemical storage cell. From this reference measurement, the anode potential can then be indirectly determined during the regeneration process. The phase transition is complete at a specific anode potential and can be indicated by a corresponding measured anode potential. The attainment of the anode potential can be determined by indirect measurement, for example, of the cell voltage. This is equivalent to reaching the target threshold. 24-1258 PIF 6

[0030] In one embodiment, the target threshold is predetermined by a model. This model can be based on known methods, such as electrochemical or material models. Based on these models, the attainment of the anode potential can be measured indirectly, for example, by measuring the cell voltage. This means the target threshold has been reached.

[0031] In one embodiment, a discharge curve is defined for the electrochemical storage cell. The discharge curve specifies the cell voltage as a function of the state of charge. During discharge of the electrochemical storage cell from its operating range, the current cell voltages are measured. These current cell voltages are compared with the discharge curve. The target threshold is reached by continuously discharging the electrochemical storage cell. The target threshold is finally reached when the current cell voltage is equal to the corresponding cell voltage on the discharge curve for a given state of charge, within a predetermined tolerance.

[0032] In one embodiment, the target threshold is set at a charge level of less than 10%, less than 5%, or equal to 0%.

[0033] The described phase transition is complete for a given electrochemical storage cell when discharged to a characteristic value. The target threshold can therefore be set to a value at which the phase transition can be considered reliably complete. This has the practical advantage that the target threshold does not necessarily have to be individually determined for each cell installed in a vehicle. Experience with a single cell type may suffice. For example, a vehicle's battery can be discharged to 0% state of charge (SoC) during an inspection or service, since the vehicle will not be driven for the duration of the service anyway.

[0034] In one embodiment, the operating charging range is between 10% and 100% state of charge.

[0035] Furthermore, a test rig for carrying out a regeneration process for a silicon-containing electrochemical storage cell of a battery-electric vehicle is described. According to one embodiment, the test rig includes a charging and discharging device, wherein the charging and discharging device performs a process according to one of the foregoing aspects. 24-1258 PIF 7

[0036] A battery-powered vehicle with battery cells containing mixed anodes of graphite and silicon undergoes a regeneration process during a workshop inspection, for example, to recover lithium from the crystalline phase of the silicon. For this purpose, the battery is completely discharged for a period of time, such as overnight (e.g., via a bidirectional wallbox or by activating electrical consumers in the vehicle) and then fully recharged. This process provides the vehicle with more effective energy.

[0037] The following describes exemplary embodiments with reference to the accompanying drawings. Further details, preferred embodiments, and refinements will be derived from these. Identical or functionally equivalent components are identified by the same reference numerals in the figures. The components shown, as well as their relative sizes, are not to be considered to scale. Where components and parts function identically across different figures, their descriptions will not necessarily be repeated for each subsequent figure.

[0038] In detail:

[0039] Figure 1 shows an exemplary flowchart of a regeneration process for a silicon-containing electrochemical storage cell, and

[0040] Figure 2 shows an example measurement curve / discharge curve for determining a target threshold.

[0041] Detailed

[0042] Figure 1 shows an exemplary flowchart of a regeneration process for a silicon-containing electrochemical storage cell. The process is suitable for regenerating electrochemical storage cells in a battery-electric vehicle, for example, in a high-voltage storage system of a battery-electric vehicle. The electrochemical storage cell (or several such cells) contains an anode active material that contains at least some silicon. 24-1258 PIF 8

[0043] Silicon, as an anode active material, exhibits a phase transition from an amorphous to a crystalline structure during lithium ionization. This phase transition forms at a comparatively high cell voltage and can only revert to a lower cell voltage. The improved concept presented here is based on the experimentally confirmed observation that a complete discharge of a silicon-containing electrochemical storage cell leads to a phase transition, causing the cell to revert from its crystalline structure back to its original amorphous state. This effect can be reversed by a single deep discharge. The method presented here utilizes this principle, enabling the anode material to undergo a transformation back from the crystalline to the amorphous phase. For clarity, a single storage cell is described below.The method is not limited to a single cell, but can be applied analogously to many storage cells, for example, when the cells are arranged and set up to form a high-voltage storage system.

[0044] In the first step, S1, the electrochemical storage cell is discharged. Typically, this cell is installed in a battery-electric vehicle. For discharge step S1, the vehicle is placed, for example, on a test bench equipped with a suitable charging and discharging device for carrying out the regeneration process. This charging and discharging device can be used, for instance, in a workshop as part of a service procedure. Alternatively, a bidirectional wallbox with charging and discharging functionality can be used. The discharge process can also be supported by selectively switching consumers on and off within the vehicle, for example, by putting the vehicle into a regeneration mode.

[0045] At the beginning of discharge step S1, the electrochemical storage cell is in an operating charge range, typically between 10% and 100% state of charge. Discharge then proceeds from this operating charge range down to a target threshold, which is below the operating charge range. The target threshold is a function of the anode potential of the electrochemical storage cell and corresponds to a state of charge of the electrochemical storage cell outside the operating charge range, for example, less than 10%, less than 5%, or equal to 0%. The target threshold is specifically chosen or determined such that, upon reaching the target threshold or the corresponding state of charge, a phase transition of the silicon of the cell's anode active material from a crystalline to an amorphous form (24-1258 PIF 9) is substantially (or entirely) complete or can be considered to be complete.

[0046] The target threshold represents a material parameter of the electrochemical storage cell and can be modeled as such or estimated by measurement, for example, of a discharge curve. The target threshold can be predefined by creating a model or a discharge curve for a specific type of storage cell. If this type is installed in the battery-electric vehicle, measurements of the current cell voltages during discharge can indicate whether the target threshold has been reached. Furthermore, it is possible to compare the current cell voltages with a previously determined discharge curve, for example, one set at the factory. Through ongoing measurements, the current cell voltages yield a current discharge curve, which, due to a memory effect, may deviate from a modeled or previously measured discharge curve.The target threshold is reached when the current cell voltage is equal to the corresponding cell voltage of the discharge curve for a given state of charge, within a predetermined tolerance. In other words, the target threshold is reached when the measured and the predetermined discharge curves intersect.

[0047] The target threshold is a value largely derived from experience. The values ​​suggested here should therefore be considered examples. The target threshold can also incorporate a buffer; that is, the described phase inversion from crystalline to amorphous phase may be completed at a different threshold. This is difficult to measure directly, so the buffer provides a safety margin.

[0048] In a second step, S2, the electrochemical storage cell is charged. Charging takes place within the operating range, for example, immediately after discharging to the target threshold. The operating charging range is typically between 10% and 100% state of charge of the cell.

[0049] Figure 2 shows an example of a measurement curve / discharge curve for determining a target threshold. A predetermined discharge curve E1 and a current discharge curve E2, derived from measurements of the current cell voltages, are shown. The example relates to a specific memory cell with a very high silicon content, which was measured by the inventors. Due to the previously described memory effect, deviations occur at lower charge levels, starting at approximately 30% charge. This is evident from the fact that discharge curves E1 and E2 do not coincide.

[0050] As an example, the regeneration process begins at a state of charge (SoC) of 15%. In this example, this corresponds to a current cell voltage of approximately 3.12 V. Due to the previously described memory effect, the current discharge curve E2 for the 15% SoC does not coincide with the predetermined discharge curve E1. At a 10% SoC, the current cell voltage is approximately 3.03 V. At this value, the current discharge curve E2 and the predetermined discharge curve E1 are so close to each other, or within a tolerance, that the phase transition from a crystalline anode material structure to an amorphous structure can be considered complete. Accordingly, the cell voltage of approximately 3.03 V, or the 10% SoC, can be used as the target threshold. If a buffer is taken into account, the phase transition can be considered to have occurred. This is the case, for example, at the point where the discharge curves E1 and E2 intersect after the start of the discharge step S1.In this example, this applies to a cell voltage of approximately 2.93 V or a state of charge (SoC) of approximately 6%, which can therefore be used as a target threshold with a buffer. A target threshold of 0% SoC can also be used as a target threshold with a buffer.

[0051] The cell voltage must fall below a certain threshold at least once to complete the phase transition. This voltage threshold (and the corresponding state of charge, or SoC) differs for each battery cell. It has been determined that the threshold depends on the proportion of silicon in the total capacity of the battery cell and is therefore a function of its anode potential. The target threshold forms a lower cutoff criterion for the energy storage system (where the storage is defined as "empty"). The regeneration process allows for the recovery of capacity, enabling the battery cell to return to its original voltage curve E1. This allows for easier estimation of the SoC, easier estimation of the state of health, and a higher available discharge energy.

[0052] Although the improved concept has been illustrated and described in detail using exemplary embodiments, it is not limited by these embodiments. Rather, other variations of the improved concept can be derived by a person skilled in the art without departing from the scope of protection defined by the claims. 24-1258 PIF 11

[0053] List of reference signs

[0054] E1 discharge curve

[0055] E2 discharge curve S1 charging

[0056] S2 Unload

Claims

24-1258 PIF 12 1. A regeneration process for a silicon-containing electrochemical storage cell, wherein the electrochemical storage cell is operated in a battery-electric vehicle and an anode active material contains at least partially silicon, comprising the steps: - Discharging the electrochemical storage cell from an operating charge range to a target threshold, where the target threshold is a function of the anode potential of the electrochemical storage cell and a phase transition of the silicon from a crystalline to an amorphous form, and then - Charging the electrochemical storage cell to operating range.

2. The method according to claim 1, wherein the battery-electric vehicle is placed in a regeneration operating mode and the discharging and charging follow each other immediately.

3. The method according to one of the preceding claims, wherein the battery-electric vehicle is discharged in regeneration mode using a charging and discharging device in a test stand, a bidirectional wallbox and / or by switching consumers on and off on the vehicle side.

4. The method according to one of the preceding claims, wherein the target threshold is determined by determining the anode potential of the electrochemical storage cell.

5. The method according to one of the preceding claims, wherein the target threshold is predetermined by modeling.

6. The method according to one of the preceding claims, wherein a discharge curve is determined for the electrochemical storage cell, wherein the discharge curve specifies a cell voltage as a function of the state of charge, - when discharging the electrochemical storage cell from the operating range, current cell voltages are measured, - the current cell voltages are compared with the discharge curve, and - the target threshold is reached by discharging the electrochemical storage cell when the current cell voltage is within a predetermined tolerance for a state of charge 24-1258 PIF 13 of the corresponding cell voltage of the discharge curve for the state of charge.

7. The method according to any of the preceding claims, wherein the target threshold indicates a charge level of less than 10%, less than 5%, or equal to 0%.

8. The method according to one of the preceding claims, wherein the operating charging range is between 10% and 100% state of charge.

9. A test rig for carrying out a regeneration process for a silicon-containing electrochemical storage cell of a battery electric vehicle, comprising a charging and discharging device, wherein the charging and discharging device carries out a method according to one of the preceding claims.

Images