JP6741771B2 - Lithium-ion battery formation process - Google Patents

Description

FIELD OF THE DISCLOSURE The present disclosure relates to lithium-ion batteries, and more specifically to improvements in the initial charging (chemical conversion process) method of lithium-ion batteries.

BACKGROUND OF THE DISCLOSURE Lithium-ion batteries belong to the group consisting of various types of storage batteries in which lithium ions move from the cathode to the anode during discharge and from the anode to the cathode during charge.

There are various types of lithium-ion batteries. Generally, the anode 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 fine perforated plastic sheet, through which the ions can pass. The anode, cathode, and separator are immersed in the electrolyte.

Lithium-ion batteries are classified by their cathode material.
After assembling the lithium-ion battery, the lithium-ion battery may be subjected to at least one precise controlled charge/discharge cycle to activate the working material so that the battery is suitable for use. .. This step is called a chemical conversion process. By this conversion process, the battery is in the fully charged state for the first time.

In a chemical conversion process, a solid electrolyte interface (SEI) is formed on the anode. The formation of SEI is important for the life of lithium ion batteries.

A method has been proposed in which a lithium-ion battery is initially charged, that is, a chemical conversion process is performed.

Typically, batteries are charged at a constant charge rate. The charge rate is also referred to as the C rate, which represents the charge or discharge rate equal to the hourly capacity of the battery. It has been found that SEI formation is best at low C-rates, but lower C-rates result in longer initial charge times. In fact, it would take approximately 5 hours to fully charge the battery at a C rate equal to C/5. By charging the battery to the battery full charge voltage at a low C rate, SEI is formed on the carbon anode during this initial charge, and then the battery is held at the full charge voltage until the current value falls below the threshold value. Then, the battery is allowed to rest for 2 hours and then discharged at a low C rate to a preset voltage, that is, to a discharge end voltage. This conversion process may be carried out for at least one cycle.

A so-called dynamic chemical conversion process has been proposed for the purpose of reducing the manufacturing time of a lithium ion battery. This process involves charging the battery at a low C rate until the end of the SEI layer formation on the anode (corresponding to the voltage threshold), then at a high C rate to full charge voltage. For example, US 2015/060290 discloses such a conversion protocol. In this chemical conversion protocol, the battery is charged at least twice to the full charge voltage, and the battery is paused for 2 hours each time between charging and discharging of the battery. The time is over 40 hours. Further, in US2015/060290, the voltage value at the end of SEI layer formation on the anode is measured by the method using the temperature difference, so the measured value is an approximate value.

Also, additives have been added to the electrolyte to improve SEI formation and enhance anode stability.

SUMMARY OF THE DISCLOSURE Under the present circumstances, it is desired to obtain a battery having good characteristics even when a charge/discharge cycle is performed many times while shortening the time required for the chemical conversion process.

Therefore, embodiments of the present disclosure provide a method of performing a conversion process for a lithium-ion battery having an anode, a cathode, an electrolyte, and a separator. The above method
Adding an additive to the electrolyte to improve the deposition of the solid electrolyte interface on the anode,
Determining the voltage value at the end of SEI formation, which is the voltage value at the completion of SEI deposition, by measuring the concentration of the additive in SEI,
Charging the battery to a voltage at the end of SEI formation at a first predetermined rate;
And then charging the battery to a full charge capacity at a second charge rate, the second charge rate being greater than the first charge rate.

By providing such a method, the voltage value at the end of SEI formation is accurately determined. In fact, it becomes possible to measure the additive concentration in SEI in an accurate way. The voltage value at the end of SEI formation is determined before the formation process is applied to the battery.

A plurality of batteries are charged at the first predetermined rate. Individual cells are charged to a given voltage and the additive concentration of SEI formed on a portion of the anode is measured, which can determine the end SEI formation voltage value. The voltage value at the end of SEI formation is determined in this way, and a plurality of batteries having the same configuration and the same components as the plurality of batteries used for determining the voltage value at the end of SEI formation are used by using the voltage value at the end of SEI formation. The battery may be charged for the first time.

By charging the cell containing the electrolyte with the additive at the first predetermined rate, SEI with good electrochemical properties is formed on the anode.

Upon completion of SEI formation, the battery is charged to full charge capacity at the second predetermined rate. The second charge rate is greater than the first charge rate. This reduces the time taken for the chemical conversion process.

The voltage value at the end of SEI formation may be determined as the voltage when the additive concentration in SEI becomes constant.

In fact, it can be concluded that the SEI formation on the anode is complete if the additive concentration in the SEI does not increase with increasing charging time or increasing charging voltage.

The SEI additive concentration may be measured by XPS.
X-ray photoelectron spectroscopy (XPS) is a qualitative quantitative analysis technique capable of measuring the elemental composition of the sample surface. XPS can also detect light elements such as lithium, and the elemental composition can be measured in units of 1/1000. Another advantage of XPS is that the sample surface chemistry can be analyzed without additional treatment of the sample surface specimen.

The first predetermined rate may be 2C or less, preferably 1C or less, and more preferably 0.5C or less.

SEI may form on the anode at a first predetermined rate. This first predetermined rate allows the formation of SEI with good electrochemical properties without significantly extending the time taken for the overall conversion process.

The second predetermined rate may be 2C or higher, preferably 3C or higher, and more preferably 4C or higher.

Since the second predetermined rate is higher than the first predetermined rate, the time required for the chemical conversion process can be shortened by using the second predetermined rate. In fact, since the second predetermined rate is greater than the first predetermined rate, the end of the peak voltage value is greater at the second predetermined rate than at the first predetermined rate. The charging time to reach the full charge voltage is shortened.

Before the battery is charged to the full charge capacity, the battery may be discharged at the first predetermined rate to the cutoff voltage value of the battery, and at the first predetermined rate to the voltage value at the end of SEI formation. The battery is recharged.

By performing a cycle between the cutoff voltage value and the voltage value at the end of SEI formation, the capacity retention of the battery can be improved.

The additive provided in the electrolyte may be selected from oxalate, ethylene carbonate, or sulfone.

The decomposition voltage of the additive may be lower than the decomposition voltage of the electrolyte.
Additives can improve SEI formation on the anode, and the in-use characteristics of SEI formed can be superior to the in-use characteristics of SEI formed from electrolyte alone. As a result, the life of the lithium ion battery can be improved and the power can be increased.

The first predetermined rate may be selected in relation to the additive.
By selecting the first predetermined rate in relation to the additive, battery performance may be better. For example, the capacity retention of the battery can be improved. The selection of the first predetermined rate may be done by performing a simple test on the test battery before the battery is first charged. In particular, the first derivative (dQ/dV) of the charge capacity with respect to the measured voltage may be calculated from the measured values of the charge capacity and the voltage. The first derivative has at least one peak, the maximum peak value voltage of which decreases when the battery is charged and discharged at a predetermined rate. The first predetermined rate may be selected in correlation with the downward trend of the dQ/dV curve.

The elements described above and the elements described in the specification may be combined unless stated to the contrary.

It is understood that both the above summary and the following detailed description are exemplary and are provided for purposes of illustration and not limitation of the claimed disclosure.

The accompanying drawings are incorporated herein and form a part of the specification. 1 illustrates an embodiment of the present disclosure, and also serves to explain the basic idea of the present specification together with the description of the present specification.

3 shows a lithium-ion battery. FIG. 3 is a block diagram illustrating an exemplary method according to an embodiment of the present disclosure. It is an XPS spectrum of SEI with an additive and without an additive, and shows intensity (unit is arbitrary) in correlation with binding energy (electron volt). It is a graph showing the additive concentration (atomic composition percentage, atomic share percent) in SEI in correlation with voltage (volt).

Description of Embodiments Reference will now be made in detail to exemplary embodiments of the present disclosure. Examples of these embodiments are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used for the same or similar parts or portions throughout the drawings.

FIG. 1 is a schematic diagram showing an example of a lithium ion battery 10. The lithium-ion battery 10 includes an anode 12 fixed on an anode current collector 14 and a cathode 16 fixed on 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.

Typically, the anode 12 is made of a carbonaceous material, the anode current collector 14 is made of copper, the cathode 16 is made of a lithium intercalation compound, and the cathode current collector 18 is made of aluminum. Lithium ions present in the electrolyte 22 move from the anode 12 to the cathode 16 during the discharge of the battery 10 and from the cathode 16 to the anode 12 during the charge of the battery 10.

A solid electrolyte interface (SEI) 24 is formed on the anode 12. The formation of SEI 24 occurs during the battery formation process, ie during the initial charge of the battery.

Additives may be added to the electrolyte 22 to improve SEI formation.
In some embodiments, the additive provided in the electrolyte may be selected from oxalate, ethylene carbonate, or sulfone.

Examples of oxalates include salts of the following with lithium.

(1) is difluorobis(oxalato)phosphate .
(2) is difluoro(oxalato)borate.

(3) is bis(oxalato)borate.
(4) is tetrafluoro(oxalato)phosphate.

(5) is tris(oxalato)phosphate.
The following may be mentioned as examples of ethylene carbonate.

(6) is vinylene carbonate.
(7) is fluoroethylene carbonate.

(8) is (fluoromethyl)ethylene carbonate.
The following can be mentioned as examples of the sulfone.

(9) is sulfolane.
(10) is ethyl methyl sulfone.

FIG. 2 is a block diagram illustrating an exemplary method according to an embodiment of the present disclosure.
In step 26, the voltage value V _SEI at the end of SEI formation is determined.

In step 28, an additive is added to the electrolyte 22 in the cell 10 to improve the deposition of SEI 24 on the anode 12.

In step 30, the battery 10 is charged at the first predetermined rate C ₁ to the SEI formation end voltage value V _SEI determined in step 26.

For example, in the battery 10, the anode 12 is made of graphite, the cathode 16 is made of LiNo _1/3 Co _1/3 Mn _1/3 O ₂ , and the separator 20 is made of a thin film containing polyethylene. The electrolyte 22 is an equal volume ratio mixture of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate. The electrolyte further contains 1 mol/L (mol/liter) LiPF ₆ . The additive is lithium difluorobis(oxalato)phosphate, which is added to the electrolyte 22 at 5% by weight.

For example, the first predetermined rate C ₁ may be equal to 0.3C. According to this first predetermined rate C ₁ , the SEI 24 having good electrochemical characteristics can be formed without significantly extending the time required for the entire chemical conversion process.

Next, the battery 10 is charged from the voltage value V _SEI at the end of SEI formation to the full charge capacity at the _second predetermined rate C ₂ .

As shown in FIG. 2, after the battery 10 is charged to the SEI formation end voltage value V _SEI , the battery 10 is cut off from the SEI formation end voltage value V _SEI at the first predetermined rate C1. _It may be discharged to _cut .

For example, the cutoff voltage value _Vcut may be equal to 3V.
Next, the battery 10 is charged to the voltage value V _SEI at the end of SEI formation at the _first predetermined rate C ₁ (step 30).

This charge/discharge cycle, step 30 and step 32, may be repeated, which results in better SEI formation.

Next, the battery 10 is charged from the voltage value V _SEI at the end of SEI formation to the full charge capacity at the _second predetermined rate C ₂ .

By repeating Step 30 and Step 32, the capacity retention of the battery 10 can be improved, but at the same time, the time required for the chemical conversion process can be lengthened.

The voltage value V _SEI at the end of SEI formation may be determined as follows.
A given number of batteries 10 having the same components were charged to different voltages at a _first predetermined rate C ₁ . The voltages of the battery 10 reached by charging at the _first predetermined rate C ₁ are summarized in Table 1.

The anodes of Samples AH were then immersed in the ethylmethyl carbonate solution for 10 minutes and dried. The anodes of Samples A to H were ready for XPS analysis.

The X-ray intensity in the XPS analysis was 1500 eV (electron volt), and the X-ray diameter was 200 μm (micrometer).

As shown in FIG. 3, the XPS spectrum (dashed line) of SEI24 formed on the anode 12 without the addition of the additive to the electrolyte 22 has no peak, but the lithium difluorobis(oxalato)phosphate on the electrolyte 22 does not. added there is XPS spectrum of SEI24 formed on the anode 12 of Feto additive (solid line) has a peak at 134.85EV.

This peak corresponds to the additive and is therefore used to measure the lithium difluorobis(oxalato)phosphate concentration in SEI24.

As shown in FIG. 4, the additive concentration (%) in SEI in equilibrium with the voltage (volt) is in equilibrium. That is, the additive concentration in SEI does not exceed a given voltage value. This voltage value is the voltage value V _SEI at the end of SEI formation at a given first predetermined rate C ₁ for a given battery. In this example, the voltage value V _SEI at the end of SEI formation is equal to 2.5 V (volts).

For the purpose of demonstrating the performance of cell 10 having SEI 24 formed on anode 12 according to an exemplary method of the present disclosure, various conversion processes were applied to similar cells, ie, cells having the same components.

The various chemical conversion processes described above are summarized in Table 2. The table also describes the time required for the chemical conversion process and the capacity retention of the battery obtained by the chemical conversion processes listed in the table.

Sample 1 was charged to full charge capacity at 3C.
Sample 3 was charged to full charge capacity at 0.3C.

Sample 2 is charged to a voltage value V _SEI at the end of SEI formation at a first predetermined rate C1 which is equal to 0.3 C in the example, and then at a second predetermined rate C ₂ which is equal to 3 C in the example. , SEI formation end voltage value V _SEI was charged to full charge capacity.

As shown in Table 2, Sample 1 takes a relatively short time for the chemical conversion process, but has a relatively poor capacity retention.

The capacity retention of Samples 2 and 3 is better than the above. Samples 2 and 3 have almost the same capacity retention, but the time required for the chemical conversion process of sample 2 is shorter than 1/6 of the time required for the chemical conversion process of sample 3.

Therefore, by accurately determining the voltage value V _SEI at the end of SEI formation and using different predetermined charge rates, it is possible to manufacture a battery having good capacity retention and a relatively short time required for the formation process. Shown.

Throughout this description, including the claims, the term "comprising" is understood to be synonymous with "including at least one" unless stated otherwise. Further, all ranges shown in the present description (including claims) are understood to include the values at both ends of the range unless otherwise specified. Particular values for the recited elements are understood to be within the range of acceptable manufacturing or industry tolerances known to those of ordinary skill in the art, and also “substantially” and/or “approximately” and Where the term "generally" is used, the meaning of these terms is to be understood as being within the above tolerances.

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

The specification and examples are provided by way of illustration only, and the true scope of the disclosure is set forth by the following claims.

The method is described for a single cell. However, the method can be easily applied to a battery including a plurality of cells.

Claims (8)

A method of performing a conversion process for a lithium ion battery (10) including an anode (12), a cathode (16), an electrolyte (22), and a separator (20), the conversion process comprising:
Adding an additive to the electrolyte (22) to improve the deposition of a solid electrolyte interface (24) on the anode (12);
And determining the solid electrolyte interface (24) Additives solid electrolyte interface at completion of forming voltage is a voltage value at the time of the solid electrolyte interface deposited completed by the determining the concentration (V _SEI) value in,
Charging the battery (10) to a voltage (V _SEI ) at the end of solid electrolyte interface formation at a _first predetermined rate (C ₁ );
Then a step of charging to the full charge capacity in the battery (10) a second charge rate (C _2), said second charge rate (C ₂₎ than the first charge rate (C ₁₎ And a method, including the steps of: The method according to claim 1, wherein the voltage at the end of solid electrolyte interface formation (V _SEI ) value is determined as the voltage at which the concentration of the additive in the solid electrolyte interface (24) becomes constant. The method according to claim 1 or 2, wherein the additive concentration at the solid electrolyte interface (24) is measured by XPS. The method according to claim 1, wherein the first charge rate (C ₁ ) is 2 C or less, preferably 1 C or less, more preferably 0.5 C or less. The method according to claim 1, wherein the second charging rate (C ₂ ) is 2C or higher, preferably 3C or higher, more preferably 4C or higher. Before the battery (10) is charged to the full charge capacity, the battery (10) discharges to the cut-off voltage value (V _cut ) of the battery (10) at the first predetermined rate (C ₁ ). And the battery (10) is recharged at the first predetermined rate (C ₁ ) to the solid electrolyte interface formation end voltage (V _SEI ) value. The described method. 7. The method according to any one of claims 1 to 6, wherein the additive provided in the electrolyte (22) is selected from oxalates, ethylene carbonates or sulfones. It said first predetermined rate (C ₁₎ is chosen in correlation with the additive A method according to any one of claims 1 to 7.

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