JP6741770B2 - High temperature aging process for lithium-ion batteries - Google Patents

Description

FIELD OF THE DISCLOSURE The present disclosure relates to rechargeable cells, and in particular to lithium ion batteries or cells, and more particularly to initial charging (formation process) of such a battery followed by a high temperature aging process of such battery, Regarding the improved method.

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

There are various types of lithium-ion batteries. The anode generally comprises carbon and the cathode comprises 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 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 step is called the forming process. This formation process provides the 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 lithium ion batteries or cells.

A method of initial charging of the lithium-ion battery, that is, a forming process has been proposed.
Typically, batteries are charged at a constant charging rate. Charging rate C-is also referred to as rate and represents the charging or discharging rate corresponding to the capacity of the battery for one hour. It has been found that SEI is best formed at low C-rates, which means that the initial charge takes place over a long period of time. In fact, it would take approximately 5 hours to fully charge the battery at a C-rate equivalent to C/5. The battery is charged at a low C-rate to the full charge voltage of the battery so that SEI is formed on the carbon anode during the first charge, and then at full charge voltage until the current drops below the threshold. Is kept constant. The battery is then left for 2 hours and discharged at a low C-rate to the set voltage, ie the discharge cut-off voltage. This forming process may be cycled at least once.

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

It is further known to perform a high temperature aging process on the battery to complete the battery after the forming process.

Japanese Unexamined Patent Publication No. 2014-071975, for example, describes a method for manufacturing a secondary battery, in which the resistance of the positive electrode of the battery is reduced to less than 5.5 mΩ due to high temperature aging. However, this method cannot estimate the capacity stability, that is, the capacity retention rate. Furthermore, it is difficult or difficult to distinguish between the resistance of the positive electrode and the resistance of the negative electrode by using impedance measurement depending on the type of battery, so it is difficult to identify the origin of the resistance. ..

SUMMARY OF THE DISCLOSURE Currently, it is still desirable to reliably control the decrease in positive electrode resistance while at the same time reliably controlling the increase in capacity stability.

Thus, according to embodiments of the present disclosure, there is provided a method of performing a high temperature aging process on a rechargeable cell, particularly a lithium ion cell having an anode, a cathode, an electrolyte, and a separator. This method
Supplying lithium fluoride (LiF) to the solid electrolyte interface at a first predetermined concentration;
Heating the anode until the concentration of lithium fluoride (LiF) at the solid electrolyte interface reaches a second predetermined concentration that is lower than the first predetermined concentration.

By providing such a method, lithium fluoride on the anode (negative electrode) can improve capacity stability. At the same time, since the concentration of lithium fluoride (LiF) in the anode can be controlled by controlling the heating temperature and the heating time, the resistance of the anode can also be controlled. Therefore, both capacity stability and anode resistance can be reliably controlled so that these two properties have or are within the desired range at the end of the high temperature aging process.

Desirably, the heating is stopped when the lithium fluoride (LiF) at the solid electrolyte interface reaches the second predetermined concentration.

Instead of heating only the anode, it is also possible to heat the finished cell in this way.

The determination of heating temperature and heating time may be performed prior to subjecting the cell of the particular cell type to the high temperature aging process. For example, test cells of the same cell type may each be heated at a fixed temperature and various heating times. The resulting properties of the test cells with respect to their anode resistance and their capacity stability, ie capacity retention, may then be measured. A property of the test cell having a value of the property in the desired range may be used to determine a target concentration of lithium fluoride (LiF), a second predetermined concentration.

Lithium fluoride (LiF) may be supplied to the SEI of the anode at a first predetermined concentration during the process of forming the lithium-ion cell.

In other words, lithium fluoride (LiF) may be added to the anode during the cell formation process, ie before heating the anode. In particular, the SEI itself can also be formed during the forming process.

This method may be performed to complete the lithium-ion cell after the process of forming the lithium-ion cell. The forming process may include, among other things, a first charging of the cell.

The second predetermined concentration may be between 2 at% (atomic %) and 7 at %.
Within this range, the finished cell can have the desired capacity stability and the desired resistance of its anode.

The first predetermined concentration may be above 7 at %, preferably above 10 atom %. The SEI may be supplied with this concentration during the formation process.

The anode may be heated at a predetermined temperature, for example above 30°C, in particular at 60°C. At this temperature level the desired results were obtained with regard to capacity stability and resistance of the anode, while at the same time the heating time could be limited to an acceptable range.

The anode may be heated for a predetermined heating time. This heating time can be selected as a function of the lithium fluoride (LiF) concentration of SEI. Therefore, the heating time can be calculated based on the determined concentration of lithium fluoride (LiF). Such a determination may be made in a preliminary experiment for a particular cell type, for example by heating several test cells for different heating times.

The predetermined heating time may in particular be between 30 minutes and 200 hours, preferably between 5 and 24 hours.

The concentration of lithium fluoride (LiF) may be measured by XPS.
X-ray photoelectron spectroscopy (XPS) is a qualitative and quantitative analytical technique that allows the measurement of elemental composition on the surface of a sample. XPS can detect light elements such as lithium and can measure the elemental composition in the range of thousandths. XPS also has the advantage that the surface chemistry of the sample can be analyzed without the need for further processing of surface preparation.

Cells, especially test cells, may be disassembled for XPS measurements. To analyze their resulting lithium fluoride (LiF) concentration, decomposition may be performed after the heating procedure has been performed in the test cell.

The disclosure further relates to rechargeable cells, in particular
An anode having a solid electrolyte interface (24),
A cathode (16),
An electrolyte (22),
A separator (20),
And a solid electrolyte interface comprising lithium fluoride (LiF) at a concentration between 2 at% and 7 at%.

In this concentration range of lithium fluoride (LiF), the cell can have the desired capacity stability and resistance of its anode.

The anode may be formed by a high temperature aging process as described above.
The anode may include graphite.

The cathode may include LiNo _1/3 Co _1/3 Mn _1/3 O ₂ .
The separator may be made of a film containing polyethylene.

The electrolyte may comprise a mixture of ethylene carbonate, dimethyl carbonate, ethylmethyl carbonate, which is present in a particularly equal volume ratio.

The electrolyte may contain LiPF ₆ , especially at 1 mol/L.
It is contemplated that the above elements and combinations within the specification may be combined, as long as there is no contradiction.

It should be understood that both the foregoing general description and the following detailed description are merely exemplary and explanatory and are not intended to limit the disclosure as set forth in the claims.

The accompanying drawings, which are incorporated in and constitute a part of this specification, serve together with the description to illustrate the disclosed embodiments and to 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. It is a figure of the XPS spectrum which shows the lithium fluoride (LiF) density|concentration of SEI.

DESCRIPTION OF EMBODIMENTS Reference will now be made in detail to the disclosed exemplary embodiments, 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 the cathode 16 are separated by a separator 20, and the anode 12, the cathode 16 and the separator 20 are immersed in an electrolyte 22.

Anode 12 is typically made of carbonaceous material and/or graphite. The anode current collector 14 may be made of copper. The cathode 16 may be made of an intercalated 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. The 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, which are present in equal volume ratios. The electrolyte may also include 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, ie, during the initial charge of the cell. This means that the SEI 24 is desirably formed before the disclosed high temperature aging process is performed.

In addition, SEI 24 includes lithium fluoride (LiF). After the formation process, LiF may have a concentration of >7 at %, eg 10 at% or higher. LiF may be added to SEI 24 during the formation process. LiF may be derived from the electrolyte 22. As will be explained below, the concentration of LiF in SEI 24 decreases during the disclosed high temperature aging process.

FIG. 2 shows a flow chart illustrating an exemplary method according to embodiments of the present disclosure. For this cell type, the method is preferably carried out in test cells of the same cell type in order to evaluate the appropriate heating time and temperature in the disclosed high temperature aging process. This evaluation is based on the measured LiF concentration. Once these parameters of appropriate heating time and heating temperature are known for the test cell, the subsequent high temperature aging process of a normal cell of the same cell type can be controlled based on these parameters.

In step S1, a cell forming process is performed. This involves the initial charging of the cell and thus preferably the formation of SEI 24.

Next, in step S2, the high temperature aging process of the cell 10 is started. For example, the cell 10 or at least its anode 12 may be heated, for example at 60°C.

In step S3, the LiF concentration of SEI 24 is measured. This may be done by XPS measurements. For this purpose, the cell 10 is preferably disassembled. For the purposes of degradation, several samples (ie test cells) may be obtained from the same production lot, ie from the same cell type. In a cell manufacturing process (including especially high temperature aging processes), cells are manufactured from a particular manufacturing lot having multiple cells (eg, 20 cells). In the case of high temperature aging, there are variations in temperature and time among multiple lots. Therefore, even if the temperature and time are predetermined, the LiF measurement is required to make it more accurate for each lot. Thus, one or preferably several samples (ie test cells) may be obtained from the same production lot.

It is further determined in step S3 whether the measured LiF concentration is below a predetermined concentration (the second predetermined concentration according to the disclosure). If this is not the case, heating is continued by returning to step S2.

However, if the measured LiF concentration is below a predetermined concentration, heating is stopped. Complete the high temperature aging process.

By heating the anode, the LiF concentration decreases. The characteristics of the test cell with respect to the resistance of the test cell anode and the capacity stability of the test cell are a function of the LiF concentration. Therefore, the LiF concentration can be used as a threshold to determine whether to complete the high temperature aging process. A LiF concentration in a test cell having a value for the property within the desired range may be used to determine a target concentration of lithium fluoride (LiF), a second predetermined concentration.

It was found that the above properties were within the desired range when the LiF concentration was within the range of 2 to 7 at% at the end of the high temperature aging process. This was determined in trials, as outlined below.

A given number of test cells 10 (ie samples) of the same type having the same components are subjected to different heating times, eg 0 to 0, in a high temperature aging process at the same temperature, in particular above 30° C., eg 60° C. Heated for 100 hours. All test cells undergo the same formation process, eg they are charged to 4V each at a charging rate of 1C.

Table 1 summarizes the resulting properties of the cell after the high temperature aging process.

As can be seen in this test, samples 1-4 have a capacity retention within the acceptable range, ie at least 98% (acceptable values are highlighted). This first candidate group of samples has been heated for up to 24 hours (see Sample 4). Their minimum LiF concentration is 2.5 at %.

However, only samples 3-5 have a response resistance in the acceptable range, i.e. up to 3.5 mΩ (acceptable values are highlighted). This second candidate group of samples has been heated for a minimum of 5 hours (see Sample 3). Their maximum LiF concentration is 6.3 at %. Therefore, especially further corresponding tests have shown that a suitable LiF concentration at the end of the high temperature aging process is in the range of 2-7 at %.

The LiF concentration was measured by XPS measurement. The decomposed anode of the sample was immersed in a solution of ethylmethyl carbonate for 10 minutes and dried. They were set in a glove box and subjected to measurement in a closed chamber. The anode was then ready for XPS analysis.

The X-ray intensity used during XPS analysis was 1500 eV (electron volt) and the X-ray diameter was 200 μm (micrometer). The detected angle of photoelectrons was 45°.

FIG. 3 shows an XPS spectrum showing the lithium fluoride (LiF) concentration of SEI.
The LiF concentration relevant to the present disclosure can be measured by the lowest peak between 680 eV and 700 eV, in particular between 680 eV and 695 eV in the XPS spectrum of FIG.

Therefore, this peak represents the LiF concentration and may therefore be used to characterize the test cells with respect to their resistance and their capacitive stability.

For the determination of the LiF concentration, the total ratio (concentration) of the elements is calculated with the total energy. From this calculation, the phosphorus ratio (concentration) can be obtained. However, phosphorus consists of two peaks. Therefore, separation by Gaussian fitting is required. Thus, the ratio of the two peaks can be obtained (the lowest peak of interest and the other peak). The following formula can be used to determine the concentration of the lowest peak.
(Phosphorus concentration)*(Ratio of the lowest peak)/100=Concentration of the lowest peak Since the XPS peak follows a normal distribution (Gaussian distribution), the fitting curve of FIG. 3 can be used. When multiple peaks overlap, Gaussian fitting is needed to calculate the areas separately. For example, the peak of LiF is on the right side (the lowest peak) between 680 and 700 eV. The peak of PF is on the left (highest peak).

According to the examples, the resistance of the test cell (ie the sample of Table 1) was measured by electrical impedance spectroscopy. For this purpose, the test cell was cooled to 0°C. The cell voltage for measurement was 3.5V. The voltage amplitude was 5 mV. The measurement frequency was 0.01 Hz to 100 kHz. The reaction resistance was obtained from the semicircle fitting on the Cole-Cole plot.

The capacity retention rate (that is, capacity stability) was measured by performing a charge/discharge cycle test on the test cell. The test involved charging and discharging between 3V and 4V at a charging rate of 2C, preferably 300 cycles at room temperature. The capacity retention rate can be calculated by the formula of (capacity retention rate)=((discharge capacity after cycle test)/(discharge capacity after formation))*100(%).

Throughout the description, including the claims, the term "comprising" is to be understood as being 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 extreme value(s), unless stated otherwise. It should be understood that the particular values for the listed elements are within the permissible manufacturing or industry tolerances known to those of ordinary skill in the art, "substantially" and/or "approximately" and/or " Any use of the term "generally" should be understood to mean included within such acceptable tolerances.

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

The specification and examples are considered exemplary only and the true scope of the disclosure is intended to be indicated by the following claims.

The method is described with respect to one cell. However, this is easily applicable to batteries with multiple cells. Furthermore, it can be applied to cell types other than lithium ion cells.

Claims (13)

A method of performing a high temperature aging process of a rechargeable cell (10) comprising an anode (12), a cathode (16), an electrolyte (22) and a separator (20), wherein a solid electrolyte interface (24) is on the anode. Are formed on the
Supplying lithium fluoride (LiF) to the solid electrolyte interface (24) at a first predetermined concentration;
- until reaching the second predetermined concentration lower than concentration of the first predetermined concentration of lithium fluoride (LiF) of the solid electrolyte interface (24), viewed including the step of heating the anode ,
The method of heating the anode, wherein the anode is heated for a predetermined heating time selected as a function of the lithium fluoride (LiF) concentration at the solid electrolyte interface (24) . The method of claim 1, wherein lithium fluoride (LiF) is provided to the solid electrolyte interface (24) at the first predetermined concentration during the process of forming the rechargeable cell (10). Said method, wherein done to complete the rechargeable cell (10) after the formation process of the rechargeable cell (10), said forming process, in particular the first charge of the cell, according to claim 2 The method described in. 4. The method according to any one of claims 1 to 3, wherein the second predetermined concentration is between 2 atomic% and 7 at%. 5. A method according to any one of claims 1 to 4, wherein the first predetermined concentration is above 7 atom %, desirably above 10 atom %. Method according to any one of claims 1 to 5, wherein the anode is heated at a predetermined temperature above 30°C, in particular 60°C. Wherein the predetermined heating time, 3 between 0 minutes to 200 hours, preferably is between 5-24 hours, The method according to any one of the 請 Motomeko 1 6. The method according to claim 1, wherein the concentration of the lithium fluoride (LiF) is measured by XPS. The method according to any one of claims 1 to 8, wherein the cell (10) is disassembled for the XPS measurement. An anode (12) having a solid electrolyte interface (24),
A cathode (16),
An electrolyte (22),
A rechargeable cell (10) comprising a separator (20), wherein the solid electrolyte interface (24) comprises lithium fluoride (LiF) at a concentration between 2 at% and 7 at %. The cell (10) according to claim 10 , wherein the anode (12) comprises graphite and/or the cathode (16) comprises LiN i _1/3 Co _1/3 Mn _1/3 O ₂ . The cell (10) according to claim 10 or 11 , wherein the separator (20) is made of a film comprising polyethylene. Said electrolyte (22) is present in an equal volume of ethylene carbonate, dimethyl carbonate, and comprise a mixture of ethyl methyl carbonate, and / or the electrolyte (22), including LiPF _6, claim 10 12 The cell (10) according to any one of the preceding claims.