JP6741769B2 - Lithium-ion battery formation process - Google Patents

Description

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

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 capacity stability (ie capacity retention).

Japanese Patent Laid-Open No. 2013098099(A) describes a method for producing a lithium ion secondary battery in which lithium difluorophosphate (LiPF ₂ O ₂ ) is added as an additive to an electrolyte. The method is controlled based on measuring the amount of LiPF ₂ O ₂ in the electrolyte. However, this method can control the obtained capacity stability of the battery only in a limited way.

SUMMARY OF THE DISCLOSURE Currently, it is still desirable to control the amount of additives in such a way that improved capacity stability can be reliably achieved.

Thus, according to embodiments of the present disclosure, there is provided a method of performing a process of forming a rechargeable cell, particularly a lithium ion cell having an anode, a cathode, an electrolyte, and a separator. This method
Adding lithium difluorophosphate (LiPF ₂ O ₂ ) as an additive to the electrolyte to improve the construction of the solid electrolyte interface on the anode;
Charging the cell to a predetermined cell voltage at a predetermined charging rate so that the lithium difluorophosphate concentration at the solid electrolyte interface reaches a predetermined minimum concentration level.

By providing such a method, the amount of LiPF ₂ O ₂ on the anode surface (ie its concentration in at %) can be determined. Therefore, a direct measurement of surface composition may be made. It has been found that controlling the amount of LiPF ₂ O ₂ on the anode surface (ie, SEI) can increase the capacity stability more reliably than by controlling the amount of LiPF ₂ O ₂ in the electrolyte. It was Further, the additive concentration in SEI may be measured by an accurate method.

The determination of a given cell voltage and/or a given charging rate may be performed prior to subjecting a cell of a particular cell type to a forming process. For example, test cells of the same cell type may each be charged by applying different charging rates. The resulting concentration and capacity stability (ie retention) of SEI LiPF ₂ O ₂ may then be measured. The concentration of LiPF ₂ O ₂ in SEI increases with decreasing charge rate. Since the capacitive stability increases with increasing concentration of LiPF ₂ O ₂ of SEI, by LiPF ₂ O ₂ in the SEI is controlled so as to reach the predetermined minimum concentration level of the desired minimum volume stability be able to.

Thus, once such a predetermined minimum concentration level is determined, the corresponding maximum charge rate can also be determined. Then, when performing a normal cell formation process of the same cell type as the test cell, instead of measuring the concentration of LiPF ₂ O _{2 in} SEI, by controlling the charge rate (and finally the cell voltage) , Charging may be controlled.

However, normal cell formation processes can also be controlled by controlling their SEI LiPF ₂ O ₂ concentration levels.

A given charge rate can be selected as a function of the concentration of lithium difluorophosphate in SEI.

The concentration of lithium difluorophosphate in SEI 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.

The charging rate may be up to 2C, preferably 1C, more preferably 0.5C or less.

The predetermined minimum concentration level of lithium difluorophosphate in SEI may be at least 2 at% (atomic %). The concentration is desirably measured based on the peak area of the second lowest peak between 130 and 142 eV on the XPS spectrogram. The second lowest peak preferably indicates a concentration level of lithium difluorophosphate above 1 at %, so that the smallest peak is excluded from the selection of the second lowest peak.

If the concentration does not reach the desired concentration of 2 at %, the cell may be subjected to several charge cycles in this way.

The predetermined cell voltage may be 3V or higher. However, carbonate type chemicals can react predominantly above 3V. To be precise, a predetermined cell voltage of less than 3V is desirable.

The disclosure further relates to rechargeable cells, in particular
An anode having SEI,
The cathode,
An electrolyte,
A separator and the SEI comprises lithium difluorophosphate (LiPF ₂ O ₂ ) at a concentration of at least 2 at %.

In this concentration range of LiPF ₂ O ₂ , the cell can have an acceptable capacity stability.

The cells may be formed by the forming 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. Shows the LiPF ₂ O ₂ concentration of SEI, which is a diagram of the XPS spectra.

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.

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

Examples of oxalates include lithium bis(oxalato)borate:

Can be mentioned.
In particular, the additive may be lithium difluorophosphate (LiPF ₂ O ₂ ), more specifically 5 wt% (weight percent) added to the electrolyte 22.

The 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 during the charging of the cell 10.

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 performed on test cells of the same cell type in order to evaluate the appropriate cell voltage and charge rate in the disclosed formation process. This evaluation is based on the measured difluorophosphate (LiPF ₂ O ₂ ) concentration. Once these parameters of proper cell voltage and charge rate are known for the test cell, the subsequent formation process of normal cells of the same cell type can be controlled based on these parameters.

In step S1, lithium difluorophosphate (LiPF ₂ O ₂ ) is added to the electrolyte 22 as an additive. This additive serves to improve the construction of the solid electrolyte interface 24 on the anode during the first charge.

In step S2, the first charging of the cell is started. This means that SEI will also begin to be built. This further implies that the concentration of LiPF ₂ O _{2 in} SEI increases during charging.

The battery is charged to a predetermined cell voltage, for example 3V or higher. Further, the charging is performed at a predetermined charging speed C, for example, a maximum charging speed C of 1 C, preferably 0.5 C or less.

In step S3, the concentration of LiPF ₂ O ₂ is measured. This may be done by XPS measurements. For this purpose, the cell 10 is preferably disassembled.

It is further determined in step S3 whether the measured LiPF ₂ O ₂ concentration is above a predetermined concentration (predetermined minimum concentration level), in particular 2 at% (atomic %). If this is not the case, charging is continued by returning to step S2.

However, charging may be stopped if the measured LiPF ₂ O ₂ concentration is above a predetermined minimum concentration level. In any case, for example, if the battery has not yet reached its fully charged state, it is possible to continue charging without stopping charging. In such a case, the charging may be continued, but finally at various charging rates, especially at higher charging rates, eg 3C.

The LiPF ₂ O ₂ concentration in SEI can be increased by continuing charging. The cell capacity stability (ie capacity retention) is a function of LiPF ₂ O ₂ concentration in SEI. Therefore, the LiPF ₂ O ₂ concentration can be used as a threshold to determine if the cell has reached sufficient capacity stability. Therefore, a particular cell type of test cell may be used to determine a target concentration of LiPF ₂ O ₂ .

It has been found that the capacity stability is within the desired range when the LiPF ₂ O ₂ concentration is at least 2 at %. 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 were each charged to the same cell voltage, eg 3V, but at different charging rates, eg 0.01C-5C. All test cells were supplied with the same additive LiPF ₂ O ₂ to the electrolyte at the same concentration before charging.

Table 1 summarizes the resulting properties of the cell after the formation process.

As can be seen in this test, Samples 1-3 have a capacity retention within the acceptable range, i.e. at least 97.5% (see Sample 3). These samples are charged at a maximum charging rate of 1C (see sample 3). Their minimum LiPF ₂ O ₂ concentration is 3.08 at% (see sample 3). Therefore, particularly further corresponding tests have shown that a suitable LiPF ₂ O ₂ concentration at the end of the formation process is at least 2 at %.

LiPF ₂ O ₂ concentration was measured by XPS measurement. The sample anode was immersed in a solution of ethyl methyl 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 LiPF ₂ O ₂ concentration of SEI.
The LiPF ₂ O ₂ concentration relevant to the present disclosure can be measured by the lowest peak between 130 eV and 124 eV in the XPS spectrum of FIG. This is shown as "Peak A" in FIG. Atomic% can be calculated by measuring the peak area. Therefore, this peak represents the LiPF ₂ O ₂ concentration and may therefore be used to control the capacity retention rate (ie capacity stability) of the cell.

For the determination of the LiPF ₂ O ₂ concentration, the total proportion (concentration) of the elements is calculated by 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 (peak A and the other peak). The following equation can be used to determine the concentration of peak A.
(Phosphorus concentration)*(Ratio of peak A)/100=concentration of peak A 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, peak A is derived from LiPF ₂ O ₂ while the other peak is derived from another phosphorus component.

The capacity retention 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 500 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 (11)

A method of performing a process for forming a rechargeable cell (10) comprising an anode (12), a cathode (16), an electrolyte (22) and a separator (20), said forming process comprising:
Adding lithium difluorophosphate (LiPF ₂ O ₂ ) as an additive to the electrolyte (22) to improve the construction of the solid electrolyte interface (SEI) (24) on the anode (12);
Charging the cell (10) to a predetermined cell voltage (Vc) at a predetermined charging rate (C) so that the concentration of the lithium difluorophosphate at the solid electrolyte interface (24) reaches a predetermined minimum concentration level. And a step of performing. The method of claim 1, wherein the predetermined charge rate (C) is selected as a function of the concentration of the lithium difluorophosphate in the SEI. The method of claim 1 or 2, wherein the concentration of the lithium difluorophosphate of the SEI (24) is measured by XPS. The method according to any one of claims 1 to 3, wherein the charging rate (C) is at most 2C, preferably 1C, more preferably 0.5C or less. The predetermined minimum concentration level of the lithium difluorophosphate in the SEI is at least 2 at %, and the concentration is desirably measured based on the peak area of the second lowest peak between 130 and 142 eV on the XPS spectrogram. A method according to any one of claims 1 to 4. The method according to any one of claims 1 to 5, wherein the predetermined cell voltage (V _C ) is at most 3V. An anode (12) having a solid electrolyte interface (SEI) (24),
A cathode (16),
An electrolyte (22),
A separator (20), wherein the SEI comprises lithium difluorophosphate (LiPF ₂ O ₂ ) at a concentration of at least 2 at %. The cell (10) of claim 7 , wherein the anode (12) comprises graphite and/or the cathode (16) comprises Li Ni _1/3 Co _1/3 Mn _1/3 O ₂ . A cell (10) according to claim 7 or 8 , wherein the separator (20) is made of a film comprising polyethylene. Said electrolyte (22), ethylene carbonate acid includes dimethyl carbonate, and a mixture of ethyl methyl carbonate, prior Symbol electrolyte (22) comprises LiPF _6, cells according to any one of claims 7 9 (10). The electrolyte (22) comprises a mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate, present in equal volume ratios, wherein the electrolyte (22) is LiPF ₆ The cell (10) according to claim 10, which comprises 1 mol/L.