Electrolyte for lithium ion battery and lithium ion battery containing the same

Abstract

The present invention relates to an electrolyte for lithium ion batteries and a lithium ion battery containing the same. The electrolyte for lithium ion batteries is composed of the following components by weight percentage: 10-18% of lithium hexafluorophosphate, 0.2-2.0% of lithium difluorophosphate, 0.2-1.5% of lithium difluorooxalate borate, 5-15% of ethyl propionate, 30-50% of dimethyl carbonate, 10-20% of diethyl carbonate, 15-30% of ethyl methyl carbonate, 0.5-5% of ethylene carbonate, and 1.5%-5% of fluoroethylene carbonate. The lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a diaphragm and the electrolyte. The positive electrode sheet comprises a positive electrode active material, and the positive electrode active material is at least one of lithium manganate, lithium cobaltate, and nickel cobalt manganate, with a particle size of 6-11 microns. The battery does not undergo lithium precipitation when charged at -30°C, has good low temperature performance, and can be applied to -40°C at the same time, with a stable discharge platform.

Description

Technical Field

[0001] The present invention relates to the technical field of low-temperature lithium-ion batteries, in particular to an electrolyte for lithium-ion batteries and a lithium-ion battery containing the electrolyte. Background Art

[0002] When a lithium-ion battery is charging, lithium ions are extracted from the positive electrode and embedded in the negative electrode. However, when some abnormal conditions occur, such as insufficient lithium embedding space in the negative electrode, too much resistance to lithium ion embedding in the negative electrode, lithium ions are extracted from the positive electrode too quickly but cannot be embedded in the negative electrode in equal amounts, the lithium ions that cannot be embedded in the negative electrode can only obtain electrons on the surface of the negative electrode, thus forming a silvery-white metallic lithium single substance, which is commonly known as "lithium deposition". There are many factors that affect lithium deposition, such as electrolyte wetting problems, lithium battery capacity division, and battery cell deformation.

[0003] Active materials represented by lithium manganese oxide, lithium cobalt oxide, nickel cobalt manganese oxide, etc., will produce lithium precipitation when charged at low temperatures, such as -20°C, and the power will decrease during discharge, resulting in limited application of lithium ion batteries at low temperatures. For example, the invention patent CN108878976B discloses a silicon-carbon system lithium ion battery electrolyte, the electrolyte includes an organic solvent, an electrolyte lithium salt and an additive, the additive is composed of a nitrile compound, propylene sulfite and a tetramethyl diamine compound and fluoroethylene carbonate; the mass percentage of the nitrile compound in the electrolyte is 0.5% to 5%; the mass percentage of the propylene sulfite in the electrolyte is 0.5% to 10%; the mass percentage of the tetramethyl diamine compound in the electrolyte is 1% to 30%; the nitrile compound is hexane trinitrile; the tetramethyl diamine compound is tetramethyl hexamethylene diamine. The lithium ion battery prepared by this scheme has a general capacity retention effect at -30°C and cannot be used at lower operating temperatures. Summary of the invention

[0004] The purpose of the present invention is to overcome the problem that lithium deposition is easy to occur in existing lithium ion batteries under low temperature conditions, and to provide an electrolyte for a lithium ion battery and a lithium ion battery containing the electrolyte.

[0005] Under low temperature conditions, the ion conductivity of the electrolyte will decrease, and the impedance of lithium ions being extracted from the positive electrode and embedded in the negative electrode will increase significantly, and the increase in the impedance of the embedded negative electrode will be even greater, thus causing lithium precipitation. Therefore, optimizing the design of the electrolyte is one of the keys to solving the low-temperature lithium precipitation problem of lithium-ion batteries.

[0006] The present invention provides an electrolyte with high lithium ion conductivity. When discharging at -30°C, the lithium ions can be stably embedded into the negative electrode through the boosting effect of the electrolyte, thereby ensuring a good capacity retention rate.

[0007] The present invention also provides a battery, comprising the above electrolyte, preferably at least one of lithium manganese oxide, lithium cobalt oxide, and nickel cobalt manganese oxide as the positive electrode active material, with a particle size of 6-11 microns, and having a good matching effect with the electrolyte. The positive electrode active material is mixed with a conductive agent and a binder, and then coated on the positive electrode metal foil, with a coating thickness of 160-220 g / m 2 After rolling and slitting, the positive electrode is welded to the tabs to obtain the positive electrode sheet, and the compaction density of the positive electrode sheet is 2.8-3.5g / cm 3 At a specific thickness and compaction density, the migration speed of the active lithium source in the positive electrode sheet can reach an optimal state.

[0008] For the negative electrode sheet, since the main material is different from the positive electrode sheet, graphite, conductive agent and binder are mixed and stirred according to the mass ratio and then coated on the negative electrode metal foil. The coating thickness is 80-120g / m 2 After rolling and slitting, the negative electrode sheet is welded to the tabs, and the compaction density of the negative electrode sheet is 1.3-1.55g / cm 3 Under the above thickness and compaction density conditions, the lithium source can be more easily embedded and maintain a high structural stability after the subsequent lithium source is removed.

[0009] The specific plan is as follows:

[0010] The invention discloses an electrolyte for lithium ion batteries, which is composed of the following components by weight percentage: 10-18% of lithium hexafluorophosphate, 0.2-2.0% of lithium difluorophosphate, 0.2-1.5% of lithium difluorooxalatoborate, 5-15% of ethyl propionate, 30-50% of dimethyl carbonate, 10-20% of diethyl carbonate, 15-30% of ethyl methyl carbonate, 0.5-5% of ethylene carbonate and 1.5-5% of fluoroethylene carbonate.

[0011] Furthermore, the invention is composed of the following components by weight percentage: 15-17% lithium hexafluorophosphate, 1-2.0% lithium difluorophosphate, 1-1.5% lithium difluorooxalatoborate, 5-15% ethyl propionate, 30-42% dimethyl carbonate, 10-20% diethyl carbonate, 23-30% ethyl methyl carbonate, 1-5% ethylene carbonate, and 2%-5% fluoroethylene carbonate.

[0012] The present invention also protects a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is located between the positive electrode sheet and the negative electrode sheet, the lithium-ion battery contains an electrolyte for the lithium-ion battery, the positive electrode sheet comprises a positive electrode active material, and the positive electrode active material is at least one of lithium manganese oxide, lithium cobalt oxide, and nickel cobalt manganese oxide, and has a particle size of 6-11 microns.

[0013] Furthermore, the preparation method of the positive electrode sheet is: the positive electrode active material, the conductive agent, and the binder are mixed and stirred according to a mass ratio of (93-96): (0.5-2.5): (1.5-4.5) and then coated on the positive electrode metal foil, and the coating thickness is 160-220 g / m 2 After rolling and slitting, the positive electrode is welded to the tabs to obtain the positive electrode sheet, and the compaction density of the positive electrode sheet is 2.8-3.5g / cm 3 .

[0014] Furthermore, the positive electrode metal foil is aluminum foil with a thickness of 12-20 microns.

[0015] Furthermore, the preparation method of the negative electrode sheet is: graphite, conductive agent, and binder are mixed and stirred according to a mass ratio of (93.5-95): (0.5-1.5): (2.0-4.0) and then coated on the negative electrode metal foil, and the coating thickness is 80-120g / m 2 After rolling and slitting, the negative electrode sheet is welded to the tabs, and the compaction density of the negative electrode sheet is 1.3-1.55g / cm 3 .

[0016] Furthermore, the negative electrode metal foil is copper foil with a thickness of 8-16 microns.

[0017] Furthermore, the conductive agent is at least one of carbon black and carbon nanotubes; and the binder is acrylate or an acrylate derivative.

[0018] Furthermore, the diaphragm is a polyethylene polymer film with a porosity of 42-55%.

[0019] Furthermore, the lithium-ion battery does not undergo lithium deposition when charged and discharged 100 times at -30°C to 25°C, and has a stable discharge platform at -40°C.

[0020] Beneficial effect: The present invention provides an electrolyte for lithium-ion batteries, which can be used at a relatively low temperature. By optimizing the electrolyte formula design, lithium difluorophosphate and lithium difluorooxalatoborate are used in combination with an electrolyte system of lithium hexafluorophosphate, so that positive electrode active materials such as lithium manganate, lithium cobaltate, and nickel cobalt lithium manganate can easily reach the negative electrode through the mass transfer effect of the electrolyte during charging, and can meet normal charging and discharging at -30 degrees without lithium precipitation.

[0021] Furthermore, by optimizing the preparation process of the positive and negative electrodes, the thickness and compaction density are guaranteed, so that lithium ions can be easily embedded and removed, and the electrode structure is stable, with good capacity retention effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solution of the present invention, the accompanying drawings will be briefly introduced below. Obviously, the accompanying drawings in the following description only relate to some embodiments of the present invention, but are not intended to limit the present invention.

[0023] Figure 1 is a battery discharge curve diagram provided by an embodiment of the present invention;

[0024] Figure 2 This is a battery charging curve diagram provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0025] The preferred embodiments of the present invention will be described in more detail below. Although the preferred embodiments of the present invention are described below, it should be understood that the present invention can be implemented in various forms and should not be limited by the embodiments set forth herein. In the examples, those without specifying specific techniques or conditions are carried out according to the techniques or conditions described in the literature in this area or according to the product specification. Those without specifying the manufacturer of reagents or instruments used are all conventional products that can be obtained commercially. In the following examples, if not clearly stated, "%" refers to weight percentage.

[0026] Example 1

[0027] A lithium ion battery comprises a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is located between the positive electrode sheet and the negative electrode sheet, and the lithium ion battery is filled with an electrolyte, the composition of the electrolyte is shown in Table 1, and during preparation, each raw material is weighed according to weight percentage, mixed, and stirred to obtain an evenly distributed electrolyte.

[0028] The preparation method of the positive electrode sheet is as follows: the positive electrode active material, the conductive agent and the binder are mixed and stirred in a mass ratio of 94:2:4 and then coated on the aluminum foil. The coating thickness is 200g / m 2 After rolling and slitting, the positive electrode is welded to the tabs, and the compaction density of the electrode is 3.0g / cm 3 The positive electrode active material is a mixture of lithium manganese oxide, lithium cobalt oxide, and nickel cobalt manganese oxide in a mass ratio of 1:1, and the particle size is 8-9 microns; the conductive agent includes carbon black and carbon nanotubes, which are mixed in a mass ratio of 1:1; the binder is acrylate and its derivatives; the thickness of the aluminum foil is 15 microns.

[0029] Negative electrode: Graphite, conductive agent and binder are mixed in a mass ratio of 95:1:4 and then coated on copper foil. The coating thickness is 100g / m 2 After rolling and slitting, the negative electrode is welded to the tabs, and the compaction density of the electrode is 1.40g / cm 3 The conductive agent includes carbon black and carbon nanotubes, which are mixed in a mass ratio of 1:1; the binder is acrylate and its derivatives; and the copper foil thickness is 10 microns.

[0030] A polyethylene polymer film is used as a separator with a porosity of 42-43%. The positive electrode sheet, separator, and negative electrode sheet are stacked and wound into a circle in sequence to obtain a bare battery cell, which is then packaged in a steel shell, baked, injected with the above-prepared electrolyte, sealed, left to stand, and formed to complete the production of a lithium-ion battery.

[0031] Table 1 Electrolyte raw material ratio table (weight %)

[0032]

[0033]

[0034] Embodiment 2-5

[0035] To prepare a lithium-ion battery, the amounts of the raw materials for the electrolyte are shown in Table 1, and the rest of the preparation method is the same as in Example 1.

[0036] Comparative Example 1-2

[0037] A comparative battery was prepared. The amounts of the raw materials for the electrolyte were shown in Table 1. The rest of the preparation method was the same as in Example 1.

[0038] Performance Testing

[0039] The prepared batteries were subjected to charge and discharge tests. The batteries prepared in Example 1, Comparative Example 1 and Comparative Example 2 were selected for testing, and the tests were carried out at -40°C, -30°C, -20°C, -10°C and 20°C, respectively. They were charged to 4.2V at a constant current of 0.5C, then switched to constant voltage charging until the current was less than 0.02C, and then discharged to 2.5V at a current of 0.5C, and the cycle was repeated 100 times.

[0040] After the cycle is completed, the battery is fully charged, the battery is dissected, and the lithium deposition on the negative electrode surface is observed. The lithium deposition of the battery is shown in Table 2. It can be seen that the lithium ion battery obtained in Example 1 does not deposit lithium when charged and discharged 100 times at -30°C to 25°C.

[0041] Table 2 Lithium deposition in charge and discharge at different temperatures

[0042]

[0043] The discharge curve and charge curve of the battery at different temperatures are shown in Figure 1 and Figure 2 ,from Figure 1 and Figure 2 It can be seen that the battery prepared in Example 1 has a stable charging and discharging platform at -30°C, -20°C and -10°C, and the battery can continuously discharge at -40°C, thereby working normally.

[0044] The preferred embodiments of the present invention are described in detail above. However, the present invention is not limited to the specific details in the above embodiments. Within the technical concept of the present invention, a variety of simple modifications can be made to the technical solution of the present invention, and these simple modifications all belong to the protection scope of the present invention.

[0045] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the present invention will not further describe various possible combinations.

[0046] In addition, various embodiments of the present invention may be arbitrarily combined, and as long as they do not violate the concept of the present invention, they should also be regarded as the contents disclosed by the present invention.

Claims

1. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is located between the positive electrode sheet and the negative electrode sheet, characterized in that: The lithium ion battery contains an electrolyte for lithium ion batteries, and the positive electrode sheet includes a positive electrode active material. The preparation method of the positive electrode sheet is: the positive electrode active material, the conductive agent, and the binder are mixed and stirred according to a mass ratio of 94:2:4, and then coated on an aluminum foil, and the coating thickness is 200g / m 2 After rolling and slitting, the positive electrode is welded to the tabs, and the compaction density of the electrode is 3.0g / cm 3 The positive electrode active material is a mixture of lithium manganese oxide, lithium cobalt oxide, and nickel cobalt manganese oxide in a mass ratio of 1:1:1, and the particle size is 8-9 microns; the conductive agent includes carbon black and carbon nanotubes, which are mixed in a mass ratio of 1:1; the binder is acrylate and its derivatives; the thickness of the aluminum foil is 15 microns; The lithium ion battery electrolyte is composed of the following components by weight percentage: 12% lithium hexafluorophosphate, 1% lithium difluorophosphate, 1% lithium difluorooxalatoborate, 8% ethyl propionate, 35% dimethyl carbonate, 20% diethyl carbonate, 18% ethyl methyl carbonate, 2% ethylene carbonate, and 3% fluoroethylene carbonate; The preparation method of the negative electrode sheet is as follows: graphite, conductive agent and binder are mixed and stirred in a mass ratio of 95:1:4 and then coated on copper foil with a coating thickness of 100g / m 2 After rolling and slitting, the negative electrode is welded to the tabs, and the compaction density of the electrode is 1.40g / cm 3 , wherein the conductive agent includes carbon black and carbon nanotubes, which are mixed in a mass ratio of 1:1; the binder is acrylate and its derivatives; the copper foil thickness is 10 microns; The lithium-ion battery does not undergo lithium precipitation when charged and discharged 100 times at -30°C to 25°C, and has a stable discharge platform at -40°C.

2. The lithium-ion battery according to claim 1, characterized in that: The diaphragm is a polyethylene polymer film with a porosity of 42-55%.

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