JP2019506717A - Lithium-ion battery formation process - Google Patents

Abstract

A method for performing a formation process of a lithium ion cell (10) comprising an anode (12), a cathode (16), an electrolyte (22), and a separator (20), the forming process performing a charge / discharge cycle Charging the cell to a first predetermined voltage level (V1) and discharging the cell to a second predetermined voltage level (V2) lower than the first predetermined voltage level; A step of measuring the resistance R of the cell, R = (V1−V2) / I, where I is the discharge current, and − the measured resistance of the cell reaches a predetermined lower resistance limit (Rmin) And repeating the charge / discharge cycle.

Description

FIELD OF THE DISCLOSURE The present disclosure relates to rechargeable cells, in particular lithium ion batteries or cells, and more particularly to an improved method for initial charging (formation process) of such batteries.

BACKGROUND OF THE DISCLOSURE Lithium ion batteries are part of a rechargeable battery type family in which lithium ions move from the negative electrode to the positive electrode during discharge and move from the positive electrode to the negative electrode during charging.

  There are various types of lithium ion batteries. The anode generally contains carbon and the cathode contains a lithium compound. The anode and cathode are separated by a separator made of a porous polymer such as a micro-perforated plastic sheet, which allows ions to pass through. The anode, cathode, and separator are immersed in the electrolyte.

Lithium ion batteries are classified according to the cathode material.
Once the lithium ion battery is assembled, the lithium ion battery may be subjected to at least one precisely controlled charge / discharge cycle to activate the functional material before the battery is suitable for use. . This process is called a forming process. This formation process achieves an initial full charge of the battery.

  During the formation process, a solid electrolyte interface (SEI) is formed on the anode. SEI formation is important in the life of a lithium ion battery or cell.

Methods for initial charging of lithium ion batteries, i.e. forming processes, have been proposed.
Typically, the battery is charged at a constant charge rate. Charge rate C—also expressed as a rate, represents a charge or discharge rate corresponding to the capacity of the battery for 1 hour. SEI has been found to be best formed at low C-rates, which means that the initial charge takes place over a long period of time. In fact, it takes about 5 hours to fully charge the battery at a C-speed corresponding to C / 5. The battery is charged to the full charge voltage of the battery at a small C-rate so that SEI is formed on the carbon anode during the first charge, and then the battery is at full charge voltage until the current drops below the threshold. Kept constant. The battery is then left for 2 hours and discharged at a low C-rate to the set voltage, i.e. to the discharge cut-off voltage. This forming process may be cycled at least once.

  In order to reduce the manufacturing time of lithium ion batteries, so-called dynamic formation processes have been proposed. In such a process, the battery is charged at a small C-rate until the end of SEI layer formation on the anode, which corresponds to the threshold voltage value, and then the battery is charged to full charge voltage using a large C-rate. Is done. For example, US2015 / 060290 discloses such a formation procedure that further includes at least further steps of charging the battery at least twice to full charge voltage and resting the cell for 2 hours between each charge / discharge of the cell. And the total time of the dynamic forming process is over 40 hours. However, in US Pat. No. 2015/060290, the measurement is approximate because at the end of the SEI layer formation, the voltage value of the anode is measured by a method using the temperature difference.

  Additives have also been added to the electrolyte to improve SEI formation and thus increase anode stability.

  A further method for charging a secondary battery is known from JP2011-222358A. In this method, the lithium ion secondary battery is initially charged during high temperature aging. The SEI film formation is determined by the impedance change. If the SEI film formation is not completed, the charging method is changed. However, impedance measurements require additional equipment such as sensors and may therefore lead to higher costs.

SUMMARY OF THE DISCLOSURE Currently, it is still desirable to perform the forming process reliably and efficiently while maintaining a battery that will exhibit good characteristics over multiple charge / discharge cycles.

Thus, according to embodiments of the present disclosure, a method for performing a formation process of a rechargeable cell, particularly a lithium ion cell having an anode, a cathode, an electrolyte, and a separator is provided. This method
- a step of performing charge and discharge cycles, charging the cell to a first predetermined voltage level V _1, second to predetermined discharge to a voltage level V ₂ lower than the first predetermined voltage level, the steps ,
Measuring the resistance R of the cell, R = (V ₁ −V ₂ ) / I, where I is the discharge current;
-Repeating the charge / discharge cycle until the measured resistance of the cell reaches a predetermined lower resistance limit.

  By providing such a method, repeated charge / discharge cycles can improve capacity retention, i.e., prolong cell life.

  The resistance is desirably measured during discharge, for example based on one or several measurements of the current being discharged. The current may thus be based on constant current discharge. The current is usually most stable during discharge. However, the resistance may also be calculated at the end of discharge or after discharge.

  Resistance may also be measured based on the average value of current during discharge. This measurement method usually provides the most accurate resistance calculation. For example, the average may be based on a value between 1 second and 10 seconds (last) after the start of discharge. The starting point of the discharge (0 seconds) may be avoided because the discharge current is usually unstable at this point.

  The measurement of the predetermined lower limit of resistance may be performed before the formation process is performed on the cell. For example, each test cell may be charged by subjecting the test cell to various number of charge / discharge cycles. The resulting resistance of the test cell may then be measured. SEI is formed by applying a charge / discharge cycle and reduces resistance. The resistance saturates as the number of charge / discharge cycles increases, indicating, for example, the completion of the SEI formation process after the third cycle. Therefore, the lower limit of resistance can be determined to correspond to the saturation value of this resistance or the value at which the resistance starts to saturate. The latter value may be determined, for example, by adding a tolerance range, eg 1, 2, or 3% to the actual saturation value.

  Thus, the lower resistance limit can be used to initially charge a cell having the same configuration and the same components as the cell used to determine the lower resistance limit. By proper selection of the lower resistance limit, the determination of the required number of charge / discharge cycles until formation is complete is accurate because resistance can be reliably measured without the need for expensive measurement equipment.

  Thus, charge / discharge cycles can be limited to the minimum number required to complete the formation process. This limitation makes it possible to reduce the time of the forming process.

  When the measured resistance of the cell reaches a predetermined lower resistance limit, the cell may be charged to a predetermined final voltage level that is higher than the first predetermined voltage level.

The cell may reach full charge capacity at a predetermined final voltage level.
Therefore, once SEI formation is complete, the cell may be charged to full charge capacity.

  In the charge / discharge cycle, the cell may be charged at a first predetermined charge rate and discharged at a second charge rate, the second charge rate being greater than the first charge rate.

  When the cell is charged to a first predetermined voltage level, in particular at a first predetermined charge rate, SEI formation on the anode is possible. This first predetermined rate allows the formation of SEI with good electrochemical properties without unduly extending the time of the entire formation process.

  Since the second predetermined rate is greater than the first predetermined rate, the second predetermined charging rate allows a reduction in the time of the forming process.

Each of the charge / discharge rates may be any value, preferably less than 5C.
The first charging rate may be less than 2C, preferably 1C or less.

The second charging rate may be 2C or higher, in particular or 3C or higher.
Furthermore, the step of charging the cell to a predetermined final voltage level at a third predetermined charging rate greater than the first predetermined charging rate can also be performed. The third predetermined charging rate may be 2C or higher, particularly 3C or higher.

  The first predetermined voltage level may be less than the fully charged voltage of the cell, for example, less than 4V. In particular, the first predetermined voltage level may be between 2V and 3.5V, more specifically 3.2V.

The second predetermined voltage level may be greater than 0V and less than 3V, in particular 2V.
Such values are preferred because SEI can be formed early in the formation. The SEI is stacked on the surface of the anode. Charging / discharging electrochemistry occurs on the anode surface. In addition, the additive creates SEI on the surface and improves electrochemistry like a catalyst. The additive mainly makes SEI below 3V. From these facts, SEI can be formed early in the formation to improve performance. SEI cannot grow indefinitely. When SEI reaches saturation thickness, growth stops.

The predetermined final voltage level may be 4V.
The second predetermined voltage level is determined when the cell has a first predetermined voltage level, particularly during a charge / discharge cycle at a predetermined time interval of between 5 seconds and 30 seconds, more specifically 10 seconds. It may be determined by discharging.

  Thus, the second predetermined voltage level may be determined based on the discharge time. Such an embodiment requires only a simple control method. This is possible because the second predetermined voltage level has an arbitrary level lower than the first predetermined voltage level.

  The predetermined resistance lower limit can be determined as a value at which the resistance measured when the charge / discharge cycle is repeated is constant.

  In other words, the predetermined resistance lower limit may correspond to a measured saturation value of the resistance when the number of charge / discharge cycles is increased.

  In fact, if the measured resistance does not decrease further with increasing number of charge / discharge cycles, it can be concluded that SEI formation on the anode is complete.

  Prior to performing the charge / discharge cycle, an additive may be added to the electrolyte to improve the construction of the solid electrolyte interface on the anode, in particular the additive is selected from oxalate, and the additive is more specific. Lithium bis (oxalato) borate.

The additive may exhibit a decomposition potential at a lower voltage than the electrolyte.
These additives improve the formation of SEI on the anode and can achieve SEI with better in-use properties than SEI formed from electrolyte alone.

  It is intended that the above elements and those within the scope of the specification may be combined as long as there is no contradiction.

  It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure as recited in the claims.

  The accompanying drawings, which are incorporated in and constitute a part of this specification, together with the description serve to illustrate the disclosed embodiments and explain the principles thereof.

It is a figure which shows a lithium ion cell. 6 is a flowchart illustrating an exemplary method according to an embodiment of the present disclosure. FIG. 3 is an exemplary and schematic voltage-time diagram of a charge / discharge cycle applied to a cell.

DESCRIPTION OF EMBODIMENTS Reference will now be made in detail to the exemplary embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

  FIG. 1 shows a schematic diagram of an exemplary lithium ion cell 10. The lithium ion cell 10 includes an anode 12 fixed to an anode current collector 14 and a cathode 16 fixed to a cathode current collector 18. The anode 12 and cathode 16 are separated by a separator 20, and the anode 12, cathode 16, and separator 20 are immersed in an electrolyte 22.

Typically, the anode 12 is made of a carbonaceous material and / or graphite. The anode current collector 14 may be made of copper. The cathode 16 may be made of an interlayer lithium compound, such as LiNi _1/3 Co _1/3 Mn _1/3 O ₂ . The cathode current collector 18 may be made of aluminum. Separator 20 may be made of a film containing polyethylene.

The electrolyte 22 may be a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate present in equal volume ratios. The electrolyte may also contain LiPF _{6 at} 1 mol / L (mol / liter).

  At the anode 12, a solid electrolyte interface (SEI) 24 is formed. The SEI 24 is formed during the cell formation process, i.e. during the initial charging of the cell.

Additives may be added to the electrolyte 22 to improve the formation of SEI.
According to some embodiments, the additive provided in the electrolyte may be selected from oxalate.

  Examples of oxalates include lithium salt of bis (oxalato) borate:

Can be mentioned.
In particular, the additive may be lithium bis (oxalato) borate, and more specifically may be added to the electrolyte 22 at 5 wt% (weight percent).

  Lithium ions present in the electrolyte 22 move from the anode 12 to the cathode 16 during the discharge of the cell 10 and move from the cathode 16 to the anode 12 when the cell 10 is charged.

FIG. 2 shows a flowchart illustrating an exemplary method according to an embodiment of the present disclosure.
In step S1, the cell 10 first to a predetermined voltage level _{V 1} at a first predetermined speed _{C 1,} charges for example, up to 3.2 V.

For example, the first predetermined speed C ₁ may be corresponded to 1C. This first predetermined rate C ₁ allows the formation of SEI 24 with good electrochemical properties without unduly extending the time of the entire formation process.

Next, in step S2, the cell 10 is discharged at a _second predetermined speed C2 until it reaches a _second predetermined voltage level V2, for example 2V.

During or after discharging the cell 10 in step S2, the resistance R of the cell is measured. For this purpose, the formula R = (V ₁ −V ₂ ) / I is used. I is a discharge current, and may be measured during the discharge in step S2.

In step S3, it is determined whether or not the resistance R exceeds a predetermined resistance lower limit _Rmin . If so, the method returns to step S1, i.e. to charge up to a first predetermined voltage level V ₁ again cell. Therefore, the charge / discharge cycle is repeated.

However, resistance R (or less i.e. _{R min)} If it has reached the predetermined resistance limit _{R min} does not exceed a predetermined resistance limit _{R min,} at step S4 to a predetermined final voltage level _{V fin} cells You may charge. V _fin may be higher than the first predetermined voltage level, for example 4V. This voltage level may correspond to the fully charged state of the cell. Thus, when this voltage level is reached, the cell is fully charged for the first time.

The formation of SEI further proceeds by each charging procedure in step S1. The formation of SEI reduces the resistance until SEI formation is complete. At that time, the saturation level of the resistance is reached. Thus, the resistance can be used as a threshold to determine when SEI formation is complete. A predetermined resistance lower limit R _min (this threshold) may be determined based on the saturation level.

The predetermined resistance lower limit R _min may be determined as follows.
A given number of test cells 10 (i.e., samples) having the same components were each charged with varying numbers of charge / discharge cycles (i.e., repeated charge / discharge cycles). Table 1 summarizes the resulting resistance of the cell.

As can be seen in this example, the resistance begins to saturate at 293 mΩ in this example after the second iteration, i.e., when three charge / discharge cycles have been applied. Therefore, in this type of cell, an appropriate value for the predetermined lower resistance limit R _min may be 293 mΩ.

  FIG. 3 shows an exemplary and schematic voltage-time diagram of charge and discharge cycles applied to the cell.

At t = 0, the battery is charged starting from 0V. This charging procedure corresponds to step S1 in FIG. When the voltage reaches a first predetermined voltage level V ₁ , for example 3.2V, the cell is discharged again to 2V, for example, until it reaches a _second predetermined voltage level V ₂ . This discharge procedure corresponds to step S2 in FIG. When it reaches the discharge time or the second predetermined voltage level V _2, it may be measured resistance.

  Such a charge / discharge cycle may be repeated here several times until the measured resistance is below a predetermined lower resistance limit.

  Therefore, if the required number of charge / discharge cycles is accurately determined based on the predetermined resistance lower limit, it is proved that a cell having a good capacity maintenance ratio and a relatively short formation process can be manufactured. .

  Throughout the description, including the claims, the term “comprising” should be understood to be synonymous with “including at least one” unless stated otherwise. Moreover, any range indicated in the description, including the claims, should be understood to include its end value (s) unless otherwise indicated. It should be understood that specific values for the elements described are within acceptable manufacturing tolerances or industry tolerances known to those skilled in the art, and are “substantially” and / or “approximately” and / or “ Any use of the term “generally” should be understood to mean falling within such acceptable tolerances.

  Although the present disclosure herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present disclosure.

  It is intended that the specification and examples be considered as exemplary only, with a true scope of disclosure being indicated by the following claims.

  The method has been described with respect to one cell. However, this can be easily applied to a battery having a plurality of cells. Furthermore, it can be applied to cell types other than lithium ion cells.

Claims (10)

A method of performing a formation process of a rechargeable cell (10) comprising an anode (12), a cathode (16), an electrolyte (22), and a separator (20), wherein the formation process comprises:
A charge / discharge cycle, wherein the cell is charged to a _first predetermined voltage level (V ₁ ) and a second predetermined voltage level (V ₂ ) lower than the first predetermined voltage level; Discharging until, and
Measuring the resistance R of the cell during discharge, R = (V ₁ −V ₂ ) / I, where I is the discharge current;
Repeating the charge / discharge cycle until the measured resistance of the cell reaches a predetermined lower resistance limit (R _min ). The method further comprises charging the cell to a predetermined final voltage level (V _fin ) higher than the first predetermined voltage level when the measured resistance of the cell reaches the predetermined lower resistance limit. The method according to 1. The method according to claim 1, wherein the cell reaches a full charge capacity at the predetermined final voltage level (V _fin ). In the charge / discharge cycle, the cell is charged at a first predetermined charge rate (C ₁ ) and discharged at a _second charge rate (C ₂ ) greater than the first charge rate (C ₁ ). 4. A method according to any one of claims 1 to 3. 5. The method according to claim 1, wherein the first charging rate (C ₁ ) is less than 2C, preferably 1C or less. 6. The method according to claim 1, wherein the second charging rate (C ₂ ) is 2C or higher, in particular 3C or higher. The first predetermined voltage level (V ₁ ) is less than 4V, in particular between 2V and 3.5V, more specifically 3.2V, and / or the second predetermined voltage level (V ₂ ) is Greater than 0V and less than 3V, in particular 2V, and / or the predetermined final voltage level (V _fin ) is 4V,
The method according to any one of claims 1 to 6. The second predetermined voltage level (V ₂ ) is a predetermined value of 5 seconds to 30 seconds, more particularly 10 seconds, especially when the cell has the first predetermined voltage level (V ₁ ). The method according to any one of claims 1 to 7, wherein the method is determined by discharging the cell in the charge / discharge cycle in a time interval. The method according to claim 1, wherein the predetermined lower resistance limit (R _min ) is determined as a value at which the measured resistance becomes constant when the charge / discharge cycle is repeated.   Prior to performing the charge / discharge cycle, an additive is added to the electrolyte (22) to improve the construction of the solid electrolyte interface (24) on the anode (12), in particular the additive is from oxalate. 10. A method according to any one of claims 1 to 9, wherein the additive selected is more particularly lithium bis (oxalato) borate.

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