NL2035640B1

NL2035640B1 - Battery unit and method for discharging the same

Info

Publication number: NL2035640B1
Application number: NL2035640A
Authority: NL
Inventors: Jacob Legerstee Walter; Alexander De Graaf David
Original assignee: Cb Equity Beheer B V
Priority date: 2023-08-18
Filing date: 2023-08-18
Publication date: 2025-03-04
Also published as: WO2025042276A1

Abstract

The present application concerns a battery unit, comprising one or more positive electrodes, a plurality of negative electrodes and an electrolyte comprising lithium ions, arranged in between the one or more positive electrodes and the plurality of negative electrodes. The battery unit is configured to generate an output current by intermittently allowing the negative electrodes from the plurality of negative electrodes to discharge to a corresponding positive electrode from the one or more positive electrodes. The present application further concerns a method for discharging such a battery unit. [Fig 1B]

Description

BATTERY UNIT AND METHOD FOR DISCHARGING THE SAME

The present application concerns a battery unit. The present application further concerns a method for discharging such a battery unit.

Rechargeable lithium-ion batteries are known in the art. A lithium-ion battery comprises a positive electrode, a negative electrode and an electrolyte comprising lithium ions, Li*!, in between.

The battery generates a current when the negative electrode discharges to the positive electrode. The positive electrode is sometimes referred to as the cathode and the negative electrode is sometimes referred to as the anode.

It is well known that the quality of state of the art rechargeable lithium-ion batteries deteriorates during use. The quality of such a battery can be described using its output voltage, maximum depth of discharge or charge capacity. Every cycle of charging and discharging the battery, these aspects decrease. The expected lifetime of lithium-ion batteries is therefore sometimes described in cycles.

This deterioration is, in part, attributed to lithium trapping. Not wishing to be bound by theory, the applicant find that in the art there are various explanations of what exactly happens when the lithium is trapped. One such explanation is schematically shown in figures 2A-D. In each of these figures shows part of a negative electrode at an interface I with the electrolyte E and a graph, together illustrating the distribution of lithium throughout said section of the negative electrode. In said graphs, the x-axis represents the depth into the negative electrode, i.e. the distance of a point in the negative electrode to interface I. The y-axis indicates the amount of lithium at said point in the negative electrode.

When a lithium-ion battery is charged for the first time, the distribution of lithium may look something like what is shown in figure 2A. The lithium ions Li*! in the electrolyte E absorb electrons, cross the interface I between the electrolyte and the negative electrode and forms an alloy. Depending on how far the battery is charged, lithium becomes part of the negative electrode up to a certain depth. Here, that depth is indicated as dl.

When discharging a lithium-ion battery, lithium in the negative electrode rejects electrons, crosses the interface, and is absorbed into the electrolyte as lithium ions. Discharging of the negative electrode is sometimes also referred to as delithiation, the removal of lithium from said electrode.

The rejected electrons form an output current. The lithium ions move through the electrolyte, forming an internal displacement of positive charge. However, in state-of-the-art Li-ion batteries, even when the battery is fully discharged, lithium stays behind, trapped in the alloy-forming electrode material.

As is shown in figure 2B, part of the negative electrode is not completely stripped of lithium.

Specifically, the part between depths d2 and dl.

This effect worsens upon further use: As shown in figure 2C, charging the battery again reintroduces lithium into the negative electrode to form the alloy, starting from the interface I. As part of the negative electrode is occupied by the trapped lithium, newly introduced lithium becomes part of the electrode up to depth d3. The previously trapped lithium may even move further into the negative electrode in the process. Even if no unused electrode is left, the concentration of trapped lithium can increase. When said negative electrode is discharged, more lithium will be trapped and remain in the electrode than before. Some theories suggest that one big volume of trapped lithium is formed and other suggest that various separate volumes of trapped lithium is formed — the latter is shown here in figure 2D. Specifically, in the part between depths d4 and d3.

Lithium trapping reduces the amount of lithium that can move between the electrolyte and the negative electrode, meaning the maximum capacity of the battery decreases.

It is an object of the present application to provide a battery unit with an increased lifetime, which maintains its quality longer over increasing amounts of charge and discharge cycles, in which the capacity decrease per charge/discharge cycle is smaller and in which lithium trapping occurs less than in state-of-the-art batteries.

This object is achieved in a battery unit according to claim 1, which comprises one or more positive electrodes, a plurality of negative electrodes, and an electrolyte comprising lithium ions, arranged in between the one or more positive electrodes and the plurality of negative electrodes. The battery is configured to generate an output current by intermittently allowing the negative electrodes from the plurality of negative electrodes to discharge to a corresponding positive electrode from the one or more positive electrodes.

When a negative electrode is allowed to discharge, lithium at its interface rejects electrons and is absorbed into the electrolyte. This doesn’t happen to lithium deeper into the negative electrode, so a lithium-gradient occurs in the negative electrode. At and near the interface, a lithiam-poor volume is formed, while deeper into the negative electrode, lithium remains.

Not wishing to be bound by theory, the applicant finds that lithium trapping occurs when the battery unit is discharged faster than diffusion of lithium in the alloy can undo the occurring gradient.

Because, during normal discharge conditions, the diffusion speed of lithium in the alloy is low with respect to the rate at which lithium is absorbed into the electrolyte, the gradient increases while the negative electrode is allowed to discharge.

Interrupting the discharge process introduces rest periods and allows redistribution of lithium in the negative electrode to take place. The amount of lithium that becomes trapped in a negative electrode when it is discharged intermittently, is much smaller than when the negative electrode is discharged under continuous load. Therefore, in the battery unit according to claim 1, less lithium is lost to lithium trapping in the negative electrode. Such battery units have smaller capacity decrease per charge / discharge cycle, meaning it maintains its quality longer and has an increased lifetime.

In a preferred embodiment, the one or more positive electrodes comprises one positive electrode for each negative electrode from the plurality of negative electrodes and there is a one-to- one relationship between the negative electrodes from the plurality of negative electrodes and the positive electrodes from the one or more positive electrodes.

In some embodiments, the battery unit further comprises switching means and a controller.

The switching means are arranged to allow or disallow each of the negative electrodes from the plurality of negative electrodes to discharge. The controller configured to control the switching means, and configured to generate an output current by intermittently allowing the negative electrodes from the plurality of negative electrodes to discharge.

Specifically, the controller can be further configured to allow a first negative electrode from the plurality of negative electrodes to discharge for a first amount time and subsequently disallow the first negative electrode from the plurality of negative electrodes to discharge for a second amount of time. Embodiments are conceivable wherein first amount of time is between about 1 minute and about 10 minutes, preferably about 5 minutes, and/or wherein the second amount of time is between 1 minute and about 30 minutes, preferably about 15 minutes, and/or wherein a ratio between the first amount of time and the second amount of time is between 10:1 and 1:10, and preferably around 1:3.

To intermittently discharge the negative electrodes, the controller may be further configured to pulse-width modulate the discharging of the negative electrodes from the plurality of negative electrodes. In such embodiments, a duty cycle of each negative electrode is equal to one over the number of negative electrodes in the plurality of negative electrodes. In such embodiments, the negative electrodes discharge in a current pulse having either a rectangular shape, a sinus-shape, a cosine-shape, or a triangle-shape.

The output current that the battery unit according to the invention is able to provide, amy be a constant and/or continuous output current.

In some embodiments, the battery unit comprises a positive terminal and a negative terminal.

The positive terminal is then electrically connected or connectable to the one or more positive electrodes, and the negative terminal is then electrically connected or connectable to the plurality of negative electrodes.

Preferably, the battery unit is configured to generate current from the negative terminal, through the discharging negative electrode and the corresponding positive electrode, to the positive terminal. In such embodiments, the switching means may be arranged to electrically connect the positive terminal to one or more of the one or more positive electrodes and/or the negative terminal to one or more of the plurality of negative electrodes.

The negative electrodes may be made of various alloy forming materials, preferably comprising silicon (Si), gold (Ag), aluminium (Al), antimony (Sb), tin (Sn), or zinc (Zn).

According to a further aspect of the invention, a method for discharging a battery unit if according to claim 16 is provided. This battery unit comprises one or more positive electrodes, a plurality of negative electrodes, and an electrolyte comprising lithium ions, arranged in between the one or more positive electrodes and the plurality of negative electrodes. the method comprising: generating a output current by intermittently allowing the negative electrodes from the plurality of negative electrodes to discharge to a corresponding positive electrode from the one or more positive electrodes.

In preferred embodiments of this method, generating the current comprises allowing a first negative electrode from the plurality of negative electrodes to discharge for a first amount time and subsequently disallowing the first negative electrode from the plurality of negative electrodes to discharge for a second amount of time. The first amount of time may be between about 1 minute and about 10 minutes, preferably about 5 minutes and/or wherein the second amount of time may be between 1 minute and about 30 minutes, preferably about 15 minutes. Or, the ratio between the first amount of time and the second amount of time may be between 10:1 and 1:10, and preferably around 1:3.

The approach to allowing the negative electrodes to discharge may also be considered pulse- width modulation of the discharging of the negative electrodes from the plurality of negative electrodes.

Further details and advantageous effects of embodiment according to the invention will be elucidated, referring to the accompanying figures. Likewise reference numbers refer to likewise elements. In the accompanying figures: figures 1A-C each show an embodiment of a battery unit according to the invention;

figures 2A-D illustrate lithinm-trapping in a negative electrode in state-of-the-art rechargeable lithium-ion batteries; figures 3A-H illustrate lithium trapping in a negative electrode in a battery unit according to the invention; 5 figures 4A-B each show how currents from different cells can combine to a continuous current; figure SA shows a layout for electrodes as may be used in an embodiment according to the invention and figure 5B shows a layout for terminals as may be used for a layout of electrodes as shown in figure SA.

Figures 1A-C each shown an embodiment of a battery unit 100 according to the invention.

Unit 100 comprises silicon negative electrodes 10A, 10B, 10C, … 10n and positive electrodes 11A, 11B, 11C, … 10n, in between which electrolyte E is arranged. The silicon negative electrodes are made of and the positive electrodes can be made of any one of a number of well-known materials. Electrolyte E is arranged between corresponding negative and positive electrodes and contains lithium ions.

In figures 1 A-C, each battery unit 100 comprises a common negative terminal 14 and a common positive terminal 15 to function as electrical connections. Embodiments are also conceivable in which this electrical connection is provided for differently. The various silicon negative electrodes and positive electrodes may each have their own terminal, or may be divided into any other number of groups.

In these figures 1A-C specifically, there is a one-to-one relationship between the silicon negative electrodes and positive electrodes. Each pair of electrodes 10A/11A, 10B/11B, 10C/11C, ... 10n/11n, form what may also be referred to as an electrode pair or an anode-cathode pair. In these embodiments, all pairs are combined to act as one cell. However, embodiments are also conceivable in which the electrode pairs are included in multiple cell. Embodiments are also conceivable in which the relationship between silicon negative electrodes and positive electrodes is not one-to-one.

In these figures 1A-C specifically, battery unit 100 comprises switches 12A, ... 13nand a controller 101. Such switches can be implemented by any appropriate switching means such as transistors, e.g. MOSFETs. In these embodiments, the components are included in one housing and together perform as if they are one battery cell.

In figure 1A, switches 12A, 12B, 12C, ... 12n, are arranged between silicon negative electrodes 10A, 10B, 10C, ... 10n and common negative terminal 14, and switches 13A, 13B, 13C, 35... 13n, are arranged between positive electrodes 11A, 11B, 11C, ... lln and common positive terminal 15. In figure 1B, switches 12A, 12B, 12C, ... 12n, are arranged between the silicon negative electrodes 10A, 10B, 10C, ... 10n and the common negative terminal 14. In figure 1C, switches 13A, 13B, 13C, … 13n are arranged between positive electrodes positive electrodes 11A, 11B, 11C, … 11n and the common positive terminal. In each of these embodiments, each switch is arranged between one electrode and the corresponding common terminal.

In each of the figures 1A-C, an output current is generated by allowing the silicon negative electrode 10A to discharge. Controller 101 has closed the corresponding switch or switches, forming an electrical path from common negative terminal 14, via silicon negative electrode 10A, via positive electrode 11A, to common positive terminal 15. Discharge of the further silicon negative electrodes 10B, 10C, … 10n is disallowed. Controller 101 has opened the corresponding switch or switches.

To discharge silicon negative electrode 10A intermittently, controller 101 can interrupt the discharge by open the corresponding switch or switches, breaking the electrical path between the common terminals through said silicon negative electrode. While discharge is interrupted, lithium in silicon negative electrode 10A diffuses and redistributes, thus preventing trapping. One possible definition of silicon negative electrode 10A being discharged intermittently is that the discharge of silicon negative electrode 10A , for example when during discharge of the battery unit, the discharge to said silicon negative electrode 10A is interrupted at least one.

Not wishing to be bound by theory, one possible description and/or explanation of this redistribution of lithium in a silicon negative electrode (e.g. negative electrode 10A) is given in figures 3A-3E. In each of these figures shows part of a silicon negative electrode at an interface 1 with the electrolyte E and a graph, together illustrating the distribution of lithium throughout said section of the silicon negative electrode. In said graphs, the x-axis represents the depth into the silicon negative electrode, i.e. the distance of a point in the silicon negative electrode to interface L

The y-axis indicates the amount of lithium at said point in the silicon negative electrode.

Figure 3A shows a lithium distribution in a silicon negative electrode that may occur after a battery unit is charged for the first time. Lithium forms an alloy with the silicon in volume 1, up to depth d1. Beyond depth d1, a volume of silicon 2 remains.

Figure 3B shows a lithium distribution in a silicon negative electrode that may occur while a silicon negative electrode is allowed to discharge. As seen in the graph of figure 3B, a lithium- gradient forms at interface I. To avoid a volume of lithium-poor silicon being formed and to avoid lithium from getting trapped, discharge of the silicon negative electrode should be interrupted.

Figure 3C shows a lithium distribution in a silicon negative elective that may occur after the discharge of the silicon negative electrode was interrupted. The time for which the discharge is interrupted may be referred to as the rest time. In figure 3C, there is no more lithium-gradient at interface L The overall level of lithium in the negative electrode, i.e. the level of lithium from interface I up to d1 is reduced. While in this embodiment, the rest time is long enough for the gradient to disappear completely, this is not necessary. The lithium trapping can already be reduced by interrupting the discharge by a rest time long enough for the gradient to lessen, i.e. at least be less prevalent than at the time the discharge is interrupted.

Figures 3D and 3E show lithium distributions in a silicon negative elective that may occur after a second discharge time and after a second rest time, respectively. In figure 3D, a lithium- gradient is formed again and in figure 3E, the lithium-gradient has disappeared and the overall level of lithium is reduced.

Figure 3F shows a lithium distribution that can be achieved in a battery unit according to the invention, and/or using a method for discharging according to the invention. Only a very small amount of lithium remains in the silicon and therefore the performance of the battery unit is not affected by trapping.

Figures 3G and 3H show lithium distributions after the battery unit 100 is recharged and discharged, respectively. As shown here, no substantial amounts of lithium become trapped in the negative electrode.

Figures 4A and 4B illustrate methods for discharging battery units according to the invention. Both figures illustrate how four cells enumerated 1, 2, 3, and n can generate currents intermittently. In these embodiments specifically, the cells generate current one by one. Generating current in other orders is possible as well. The sum of the currents generated by the individual cells achieves a constant or continuous common output current, while also providing each cell (or, more specifically, the silicon negative electrode or electrodes in said cell) with rest time. Each cell can be said to provide current in a pulse during a discharge time. The rest time may also be referred to as relaxation time.

In figure 4A, during the discharge time cell 1, indicated with “current,” said cell provides a constant current, i.e. a rectangular pulse. Because the current is equally high over the discharge time, and one cell is sufficient to provide the desired output current, discharge times of the various cells do not have to overlap. Embodiments are conceivable in which cells discharge with rectangular pulses and in which discharge times do overlap. The group of cells (or more specifically, the plurality of negative electrodes) is preferably capable of providing a constant output current. The rest time of cell 1, indicated with “rest,” is as long as the discharge time of cells 2, 3, and n combined. The discharge time and the rest time of one cell together can be said to form one period, as after the rest time, the next discharge time commences. Unless the battery unit is turned off or common output current demand is halted completely, in which case there is not necessarily a next discharge time.

The applicant finds that in some practical embodiments, the discharge time can be anywhere between 1-10 minutes, such as 5 minutes. The rest time can be anywhere between 1-30 minutes, such as 15 minutes. However, discharge times and rest times in the order of seconds are also possible. The relation between the discharge time and the rest time can also be expressed as ratio’s. The applicant finds that in some practical embodiments, the ratio discharge time : rest time can be anywhere between 10:1, 1:1, 1:2. 1:3, 1:10, or even 1:100. However, depending on the particulars of an embodiment, other amounts of time and ratios can also be achieved.

The methods for discharging illustrated in figures 4A and 4B are advantageous when applied to battery cells that are similar or the same, or are at least similar in operation. The cells are discharged according to the same, or at least similar, discharge times and rest times.

The skilled person will appreciate that the reduction in lithium trapping can be optimized for a particular output current. Aspects that can be considered are the number of available cells, the specific types of cells, appropriate discharge times and rest times, etc.

In the embodiment shown in figure 4A, the method for discharging may also be described as a sort of pulse width modulation. In this context, the discharge time can be considered the pulse width. The ratio discharge time : (discharge time + rest time) can be considered the duty cycle. In the embodiment shown in figure 4A, there are four battery cells having identical discharge times and rest times, making the duty cycle of each battery cell one over four or 25%. This can be done for any number of negative electrodes. Generally, the duty cycle of each negative electrode is preferably equal to one over the number of negative electrodes in the battery unit.

The approach shown in figure 4A may also be described as alternatingly discharging each of the negative electrodes from the plurality of negative electrodes. The cells 1, 2, 3, n can be said to be discharged in a particular order, or can be said to be discharged according to a particular rotation. The order in which the battery cells are discharged can be the same over the time that the battery unit as a whole is discharged, but embodiments are also conceivable in which that order is changed over time, perhaps giving priority to allowing some battery cells to discharge again, or delaying having to discharge others.

However, the invention is not limited thereto. Embodiments are also conceivable in which the battery cells differ from one another and the methods for discharging them can be adapted based thereon, assigning different discharge and rest times different battery cells, or even vary the discharge and test times assigned to a battery cell over time. For example, if it is detected that quality of a battery has already deteriorated, perhaps due to lithium-tapping resulting from earlier operation not according to the invention, the discharge time may be reduced for that cell, rest time may be increased for that cell, or the ratio discharge time : rest time may be reduced. In the context of this application, ‘different’ battery cells may mean that the cell has a different, or even multiple silicon negative electrodes; the battery cells may comprise different electrolytes; or the battery cells may be of identical make. while one has been through more cycles than the other, meaning that forms of degradation are more present. The skilled person will be aware of how such varying discharge times may be scheduled.

In figure 4B, the individual battery cells 1, 2, 3, n provide current per pulse having a sine (or cosine) pulse. Other shapes can be used as well. such as a triangle shape. (The skilled person will understand that this does not mean that the current provided by said cell overal! has the shape of a sine or cosine.) In this embodiment, to provide a continuous common output, the discharge times of the cells do overlap. The discharge time for cell 1 is approximately indicated by “C” and the rest time is approximately indicated by “R.” However, as the current provided by individual cells increases and decreases smoothly over time, where the discharge time ends and the rest time begins is more of a grey area.

The rectangular pulse of figure 4A and the sine pulse of figure 4B are both what can be described as current profiles. Embodiments are also conceivable in which other current profiles are used.

In the embodiment shown in figures SA and SB, the battery unit according to the invention comprises one positive electrode and four negative electrodes. Any other plurality of negative electrodes may be provided as well. These figures show an embodiment in which these electrodes are provided in one battery cell. Figure 5A shows a configuration of four silicon negative electrodes 10A, 10B, 10C, 10D and one positive electrode 11. In between, electrolyte E is arranged. Each of the four silicon negative electrodes is electrically connected to a corresponding terminal T1-T4. The common positive electrode 11 is electrically connected to terminal T5. Figure 5B shows the outside of the battery cell, and shows that in this embodiment battery unit 100 further comprises a housing. The terminals T1-T5 can be electrically connected to from outside of the housing and allow for electrical connection to the corresponding electrode.

Embodiments are also conceivable in which controller 101 in itself does not form a part of battery unit 100. In such embodiments, control signals may be provided to the switches in other ways such as via a wired connection or wireless connection.

The skilled person will appreciate that the negative electrodes can also be made of other materials than silicon. Negative electrodes can be made of any material that can form an alloy with lithium. Examples are gold (Ag), aluminium (Al), antimony (Sb), tin (Sn), zinc (Zn). Other examples are silicon-oxide (e.g. S102) or tin-oxide (e.g. SnO). The negative electrodes do not have to be made from this alloy-forming material alone, but may be made of a composition incorporating one of these materials. An example thereof is a negative electrode made of Si-C, a composite of silicon and carbon .f

It is to be understood that the invention is not limited to particular embodiments described inthe above and that aspects thereof may vary in further conceivable embodiments, while staying within the scope of the claims. Tt is also to be understood that the terminology used herein is for the purpose of describing particular aspects only, and is not intended to be limiting.

The scope of the present invention will be limited only by the appended claims.

Claims

CONCLUSIONS

1. A battery pack comprising: one or more positive electrodes; a plurality of negative electrodes; an electrolyte comprising Hthium tones disposed between the one or more positive electrodes and the plurality of negative electrodes; the battery pack being adapted to generate an output current by intermittently allowing the negative electrodes of the plurality of negative electrodes to discharge to a corresponding positive electrode of the one or more positive electrodes.

2. The battery unit of claim 1, wherein the one or more positive electrodes comprises a positive electrode for each negative electrode of the plurality of negative electrodes and there is a one-to-one relationship between the negative electrodes of the plurality of negative electrodes and the positive electrodes of the one or more positive electrodes.

3. The battery pack of claim 1 or 2, further comprising: switching means arranged to allow or disallow each of the negative electrodes of the plurality of negative electrodes to discharge; a driver arranged to control the switching means and arranged to generate an output current by intermittently allowing the negative electrodes of the plurality of electrodes to discharge.

4. The battery pack of claim 3, wherein the control device is further configured to allow a first negative electrode of the plurality of negative electrodes to discharge for a first amount of time and subsequently not allowing the first negative electrode of the plurality of negative electrodes to discharge for a second amount of time.

5. The battery pack of claim 4, wherein the first amount of time is between about 1 minute and about 10 minutes, and preferably about 5 minutes.

6. The battery unit of claim 4 or 5, wherein the second amount of time is between about | minute and about 30 minutes, and preferably about 15 minutes.

7. The battery pack of claim 4, 5, or 6, wherein a ratio of the first amount of time to the second amount of time is between 10:1 and 1:10, and preferably about 1:3.

8. The battery pack of any one of claims 3 to 7, wherein, to provide the output current, the driving device is further configured to pulse-width modulate the discharging of the negative electrodes of the plurality of negative electrodes.

9. The battery pack of claim 8, wherein a duty cycle of each negative electrode is equal to one over the number of negative electrodes of the plurality of negative electrodes.

10. The battery unit of claim 8 or 9, wherein the negative electrodes discharge in a current pulse having a rectangular shape, sinusoidal shape, cosine shape or a triangle shape.

11. Battery unit according to any of claims 3 to 10, wherein the output current is a constant and/or continuous output current.

12. The battery unit of any preceding claim, further comprising: a positive terminal electrically connected or connectable to the one or more positive electrodes, and a negative terminal electrically connected or connectable to the plurality of negative electrodes.

13. The battery unit of claim 12, wherein the battery unit is further configured to generate a current from the negative terminal, through the discharging negative electrode and the corresponding positive electrode, to the positive terminal.

14. A battery pack according to any one of claims 3 to 8, and claim 12 or 13, wherein the switching means is arranged to electrically connect the positive terminal to the one or more of the one or more positive electrodes and/or the negative terminal to the one or more of the plurality of negative electrodes.

15. A battery unit according to any preceding claim, wherein the negative electrodes are made of a leach-forming material, preferably comprising one of silicon (Si), gold (Ag), aluminium (Al), antimony (Sb), tin (Sn), and zinc (Zn).

16. A method of discharging a battery pack comprising one or more positive electrodes, a plurality of negative electrodes, and an electrolyte comprising lithium ions disposed between the one or more positive electrodes and the plurality of negative electrodes, the method comprising: generating an output current by intermittently allowing the negative electrodes of the plurality of negative electrodes to discharge to a corresponding positive electrode of the one or more positive electrodes.

17. The method of claim 16, wherein generating the current comprises: allowing a first negative electrode of the plurality of negative electrodes to discharge for a first amount of time and subsequently not allowing the first negative electrode of the plurality of negative electrodes to discharge for a second amount of time.

18. The battery unit of claim 17, wherein the first amount of time is between about 1 minute and about 10 minutes, and preferably about 5 minutes, and/or wherein the second amount of time is between about 1 minute and about 30 minutes, and preferably about 15 minutes.

19. A battery unit according to claim 17 or 18, wherein a ratio between the first amount of time and the second amount of time is between 10:1 and 1:10, and preferably about 1:3.

20. The battery pack of any one of claims 16 to 19, wherein generating the current further comprises pulse-width modulating the discharging of the negative electrodes of the plurality of negative electrodes.