Manufacturing method for lithium-ion secondary batteries

Abstract

To suppress occurrence of internal short-circuiting.SOLUTION: A method of manufacturing a lithium ion secondary battery includes: an assembly step of obtaining a battery assembly by injecting an electrolyte containing fluorine into a sheath body in which the power generation element is accommodated, the power generation element including a positive electrode, a negative electrode and a separator; a first charging step of charging the battery assembly in such a manner that a voltage becomes 3.5 V or higher to 4.0 V or lower; a first aging step of retaining the battery assembly after the first charging step at 40°C or higher and for 15 hours or longer; a second charging step of charging the battery assembly after the first aging step in such a manner that the voltage exceeds 4.0 V; a discharging step of discharging the battery assembly after the second charging step in such a manner that the voltage becomes 3.5 V or lower; and a second aging step of retaining the battery assembly after the discharging step at 40°C or higher and for 0.5 hours or longer.SELECTED DRAWING: Figure 1

Description

[[Technical Field]]

[0001] The present disclosure relates to a method for manufacturing a lithium-ion secondary battery. [[Background Art]]

[0002] Patent Document 1 (Japanese Unexamined Patent Publication No. 2020-91977) discloses a method for manufacturing a lithium-ion secondary battery, comprising: a pre-charging step of charging the lithium-ion secondary battery to a predetermined voltage at a high temperature; and a pre-discharging step of holding the voltage state for a predetermined time after discharging the lithium-ion secondary battery to the predetermined voltage. [[Prior Art Documents]] [[Patent Documents]]

[0003] [[Patent Document 1]] Japanese Unexamined Patent Publication No. 2020-91977 [[Summary of the Invention]] [[Problem to be Solved by the Invention]]

[0004] In the manufacturing process of a lithium-ion secondary battery, metallic foreign matter may be mixed between a positive electrode active material layer and a separator. When the lithium-ion secondary battery is initially charged and the positive electrode potential exceeds the dissolution potential of the metallic foreign matter, the metallic foreign matter begins to dissolve. If the lithium-ion secondary battery is left to stand in this state, the entire metallic foreign matter dissolves into metal ions.

[0005] On the other hand, the potential of the negative electrode of the lithium-ion secondary battery after initial charging is lower than the deposition potential of the metallic foreign matter. Therefore, the metal ions gradually deposit on the negative electrode active material layer and may grow in a dendritic shape. This grown deposited metal can break through the separator, reach the positive electrode active material layer, and cause an internal short circuit.

[0006] In Patent Document 1, the positive electrode potential is pre-charged to a voltage higher than the dissolution potential of the metallic foreign matter to dissolve the metallic foreign matter, and then a voltage lower than the dissolution potential is maintained for a predetermined time to form a passivation film on the surface of the remaining metallic foreign matter. This makes it possible to suppress the occurrence of internal short circuits caused by the metallic foreign matter.

[0007] On the other hand, in the method described in Patent Document 1, sufficient electrical conductivity may not be achieved with the metal foreign matter during manufacturing, and the metal foreign matter may remain undissolved. Therefore, there is a risk that the metal foreign matter will gradually dissolve before use.

[0008] The purpose of this disclosure is to suppress the occurrence of internal short circuits. [Means for solving the problem]

[0009] [1] A method for manufacturing a lithium-ion secondary battery including a power generation element, The aforementioned power generation element includes a positive electrode, a negative electrode, and a separator. The assembly process involves pouring an electrolyte containing fluorine into the outer casing housing the power generation element to obtain a battery assembly, A first charging step involves charging the battery assembly so that its voltage is between 3.5V and 4.0V, A first aging step in which the battery assembly after the first charging step is held at 40°C or higher for 15 hours or more, A second charging step is performed to charge the battery assembly after the first aging step so that its voltage exceeds 4.0V, A discharge step in which the battery assembly after the second charging step is discharged so that the voltage is 3.5V or less, The process includes a second aging step of holding the battery assembly after the discharge step at a temperature of 40°C or higher for 0.5 hours or more.

[0010] After the initial charge of the battery assembly, further charging to a voltage above a predetermined level causes the fluorine in the electrolyte to react with hydrogen to form hydrogen fluoride. If metal foreign matter is present in the battery assembly, discharging the resulting hydrogen fluoride to a voltage below a predetermined level and then aging it at a temperature above a predetermined level forms a passive fluoride film on the surface of the metal foreign matter. As a result, it is expected that the occurrence of internal short circuits during use will be suppressed.

[0011] [2] The method for manufacturing a lithium-ion secondary battery according to [1], wherein the electrolyte comprises at least one selected from the group consisting of LiPF6 and LiBF4.

[0012] [3] The method for manufacturing a lithium-ion secondary battery according to [1] or [2], wherein the negative electrode contains Li3PO4. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a schematic flowchart of the manufacturing method for a lithium-ion secondary battery in this embodiment. [Figure 2] Figure 2 is a table showing the sample composition and evaluation results for the examples and comparative examples. [Modes for carrying out the invention]

[0014] Embodiments of the present disclosure (hereinafter abbreviated as "Embodiments") and examples of the present disclosure (hereinafter abbreviated as "Examples") are described below. However, these embodiments and examples do not limit the technical scope of the present disclosure.

[0015] In this specification, the magnitude of the time rate of current may be represented by the symbol "C". A current of 1C discharges the rated capacity of the battery in one hour.

[0016] In this specification, "SOC (State of Charge)" refers to the percentage of the current charge capacity relative to the full charge capacity.

[0017] <Method for Manufacturing Lithium Ion Secondary Battery> FIG. 1 is a schematic flow chart of a method for manufacturing a lithium ion secondary battery (hereinafter sometimes simply referred to as "battery") according to the present embodiment. Hereinafter, "the method for manufacturing a battery according to the present embodiment" may be abbreviated as "the present manufacturing method". The present manufacturing method includes at least (a) an assembling step, (b) a first charging step, (c) a first aging step, (d) a second charging step, (e) a discharging step, and (f) a second aging step.

[0018] <<(a) Assembling Step>> In the assembling step, a fluorine-containing electrolytic solution is injected into an outer package that houses a power generation element, thereby obtaining a battery assembly.

[0019] The power generation element may also be referred to as an electrode body or an electrode group. The power generation element includes a positive electrode, a separator, and a negative electrode, and may have any structure. For example, the power generation element may be of a wound type. The positive electrode, the separator, and the negative electrode may all be strip-shaped sheets. The power generation element may be formed, for example, by laminating a positive electrode, a (first) separator, a negative electrode, and a (second) separator in this order. After winding, the power generation element may be formed into a flat shape.

[0020] In the process of forming the power generation element, metallic foreign matter may be mixed between the positive electrode and the separator. Examples of the metallic foreign matter include stainless steel foreign matter made of stainless steel such as SUS304, copper foreign matter made of copper, iron foreign matter made of iron, brass foreign matter made of brass, and the like.

[0021] (Positive Electrode) This step includes preparing a positive electrode. The positive electrode may include a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector may include, for example, aluminum (Al) foil or the like. The positive electrode active material layer contains a positive electrode active material. For example, the positive electrode active material layer may be formed by applying a slurry containing a positive electrode active material to the surface of the positive electrode current collector.

[0022] The positive electrode active material may be, for example, particulate. The positive electrode active material may have, for example, an average particle size D50 of 1 to 30 μm. The positive electrode active material may contain, for example, at least one selected from the group consisting of LiCoO2, LiNiO2, LiMnO2, Li(NiCoMn)O2, and Li(NiCoAl)O2. For example, in "Li(NiCoMn)O2", "(NiCoMn)" indicates that the sum of the composition ratios in parentheses is 1. As long as the sum is 1, the amount of individual components is arbitrary. In this specification, the average particle size D50 is the particle size at which the cumulative frequency of the smallest particle size in the volume-based particle size distribution reaches 50%. The volume-based particle size distribution can be measured by a laser diffraction particle size distribution analyzer.

[0023] The positive electrode active material layer may contain hydroxides. The inclusion of hydroxides in the positive electrode active material layer promotes the generation of hydrogen fluoride (HF) in the second charging process described later. Examples of hydroxides include metal hydroxides. Examples of metal hydroxides include alkali metal hydroxides such as lithium hydroxide, sodium hydroxide, and potassium hydroxide.

[0024] The positive electrode active material layer may further contain, for example, a conductive material, a binder, etc. The conductive material may include, for example, AB. The binder may include, for example, PVDF. The conductive material and binder may be present in amounts of, for example, 0.1% by mass or more and 10% by mass or less relative to the positive electrode active material layer.

[0025] (Negative electrode) This step includes preparing a negative electrode. The negative electrode may include a negative electrode current collector and a negative electrode active material layer. The negative electrode current collector may include, for example, copper (Cu) foil. The negative electrode active material layer includes a negative electrode active material. For example, the negative electrode active material layer may be formed by coating the surface of the negative electrode current collector with a slurry containing the negative electrode active material.

[0026] The negative electrode active material may include, for example, at least one selected from the group consisting of graphite, soft carbon, and hard carbon.

[0027] The negative electrode active material layer may contain organic acid salts. HF generated in the second charging process, described later, may affect the battery's performance. Therefore, if excessive HF is generated, this effect can be reduced by reacting the excess HF with organic acid salts. Examples of organic acid salts include phosphates and borates. Examples of phosphates include Li3PO4, (NH4)3PO4, and AlPO4. Examples of borates include LiB4O7 and LiBO3.

[0028] The negative electrode active material layer may further contain, for example, a conductive material, a binder, etc. The conductive material may contain, for example, carbon nanotubes (CNTs). The binder may contain, for example, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), etc. The amount of conductive material and binder blended may be, for example, 0.1 to 10 parts by mass per 100 parts by mass of negative electrode active material.

[0029] (Separator) The separator is porous. The separator can permeate the electrolyte. The separator separates the positive electrode and the negative electrode. The separator is electrically insulating. The separator may contain, for example, polyolefin resins such as polyethylene (PE) and polypropylene (PP). The separator may have, for example, a single-layer structure or a multi-layer structure. The separator may consist substantially of PE layers, or it may be formed by laminating PP layers, PE layers, and PP layers in that order.

[0030] (electrolyte) The electrolyte contains a solvent and a lithium salt. The solvent is aprotic. The solvent may contain any components. For example, the solvent may contain at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC).

[0031] Li salt is a supporting electrolyte. Li salt is dissolved in a solvent. Li salt contains fluorine. Li salt may contain at least one selected from the group consisting of, for example, LiPF6 and LiBF4. Li salt may have a molar concentration of, for example, 0.5 mol / L or more and 2.0 mol / L or less.

[0032] The electrolyte may further contain any additives. For example, the electrolyte may contain additives in an amount of 0.01% to 5% by mass. The additives may include at least one selected from the group consisting of vinylene carbonate (VC) and vinylethylene carbonate (VEC), for example.

[0033] (Battery assembly) The battery assembly includes an outer casing. The outer casing houses the power generation element and the electrolyte. The outer casing can have any form. For example, the outer casing may be a metal case or a pouch made of metal foil laminate film. The outer casing may contain, for example, aluminum.

[0034] 《(b) First charging process》 In the first charging process, the battery assembly is charged to a voltage of 3.5V to 4.0V.

[0035] In this process, constant voltage (CV) charging is performed. The voltage in this process may be 3.7V or higher. The voltage in this process may be 3.9V or lower.

[0036] For example, constant current (CC) charging may be performed until the battery voltage reaches a predetermined voltage. That is, this process may include CC-CV charging. The current in this process may be, for example, 0.1C or more and 1C or less, or 0.3C or more and 0.7C or less.

[0037] This process may be carried out at room temperature (25±10℃) or at a temperature above room temperature. A temperature above room temperature may be, for example, 40℃ to 70℃ or 55℃ to 65℃.

[0038] (c) First aging process In the first aging process, the battery assembly after the first charge is aged (held) at a temperature of 40°C or higher for 15 hours or more.

[0039] This process may be carried out at, for example, a temperature of 40°C to 70°C, or at a temperature of 55°C to 65°C. This process may be carried out for, for example, 15 hours to 48 hours, or at a time of 18 hours to 24 hours.

[0040] 《(d) Second charging process》 In the second charging process, the battery assembly, which has undergone the first aging process, is charged until its voltage exceeds 4.0V. This causes the fluorine contained in the electrolyte to become HF.

[0041] In this process, CV charging is performed. The voltage in this process may be 4.2V or higher, or 4.3V or higher. The voltage in this process may be 4.5V or lower.

[0042] For example, CC charging may be performed until the battery voltage reaches a predetermined voltage. That is, this step may include CC-CV charging. The current in this step may be, for example, 0.1C to 10C or 0.5C to 5C. In this step, the battery assembly may be charged to, for example, 50% to 100% SOC, 70 to 100% SOC, or 80 to 90% SOC.

[0043] This process may be carried out at room temperature or at a temperature above room temperature. The temperature above room temperature is as described above.

[0044] 《(e) Discharge process》 In the discharge process, the battery assembly after the second charging process is discharged until its voltage is 3.5V or less.

[0045] In this process, CV discharge is performed. The voltage in this process may be 3.0V or less. The voltage in this process may be 2.5V or more.

[0046] For example, CC discharge may be performed until the battery voltage reaches a predetermined voltage. That is, this process may include CC-CV discharge. The current in this process may be, for example, 0.1C to 5C or 0.5C to 3.0C.

[0047] This process may be carried out at room temperature or at a temperature above room temperature. The temperature above room temperature is as described above.

[0048] (f) Second Aging Process In the second aging process, the battery assembly after the discharge process is aged at 40°C or higher for 0.5 hours or more. This causes a passive fluoride film to form on the surface of any metal foreign matter. As a result, it is expected that the occurrence of internal short circuits during use will be suppressed.

[0049] This process may be carried out at, for example, a temperature of 40°C to 70°C, or at a temperature of 55°C to 65°C. This process may be carried out for, for example, 0.5 hours to 12 hours, or for 1 hour to 6 hours. [Examples]

[0050] <Manufacturing of test batteries> 《No.1》 LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (89 mass%), AB (8 mass%) as a conductive material, and PVDF (3 mass%) as a binder were prepared. By mixing and stirring these materials in NMP, a positive electrode slurry was obtained. The obtained positive electrode slurry was applied to an Al foil, which was to be used as the positive electrode current collector, and dried to obtain the positive electrode.

[0051] For the negative electrode, graphite (98% by mass) was prepared as the negative electrode active material, SBR (1% by mass) as the binder, and CMC (1% by mass) as the thickener. By mixing and stirring these materials with water, a negative electrode slurry was obtained. The obtained negative electrode slurry was applied to a Cu foil, which was the negative electrode current collector, and dried to obtain the negative electrode.

[0052] As a separator, a porous resin (PP / PE / PP) was prepared, in which PP layers were laminated on both sides of a PE layer. The power generation element was formed by alternately laminating the positive and negative electrodes with the separator in between.

[0053] A laminated film pouch was prepared as the outer casing. The power generation element was housed in the outer casing. As the electrolyte, a mixed solvent containing EC (30 vol%), EMC (40 vol%), and DMC (30 vol%) was prepared, in which LiPF6 was dissolved at a concentration of 1 mol / L as a fluorine-containing support salt. The electrolyte was injected into the outer casing. After the electrolyte was injected, the outer casing was sealed. From the above, battery assembly No. 1 was obtained.

[0054] The resulting battery assembly was charged at a constant current of 1 / 3C under a temperature of 25°C until the voltage reached 3.9V (first charging step).

[0055] The battery assembly after the first charging process was subjected to aging for 20 hours at a temperature of 60°C (first aging process). This resulted in the production of test battery No. 1.

[0056] 《No.2》 The same materials as in No. 1 were prepared. As a metallic foreign object, a stainless steel foreign object made of SUS304 with a diameter of 100 μm and a thickness of 5 μm was prepared. The metallic foreign object was placed between the positive electrode active material layer and the separator. At this time, a portion of the positive electrode active material layer was removed. Except for the placement of the metallic foreign object, the battery assembly No. 2 was obtained in the same manner as in No. 1.

[0057] The first charging process and the first aging process were performed under the same conditions as for No. 1. This resulted in the production of test battery No. 2.

[0058] 《No.3》 The same battery assembly as No. 2 was prepared. The first charging process and the first aging process were performed under the same conditions as No. 1.

[0059] The battery assembly, after the first aging process, was charged at a temperature of 25°C until the voltage reached 4.2V (second charging process).

[0060] The battery assembly after the second charging process was discharged at a temperature of 60°C until the voltage reached 3.0V (discharge process).

[0061] The battery assembly after the discharge process was aged for 1 hour at a temperature of 60°C (second aging process). This produced test battery No. 3.

[0062] 《No.4》 Test battery No. 4 was manufactured using the same method as No. 3, except that the second charging process involved charging until the voltage reached 4.3V.

[0063] 《No.5》 For the negative electrode, graphite (97% by mass) was prepared as the negative electrode active material, SBR (1% by mass) as the binder, CMC (1% by mass) as the thickener, and Li3PO4 (1% by mass) as the organic acid salt. These materials were mixed and stirred with water to obtain a negative electrode slurry. The obtained negative electrode slurry was applied to a Cu foil, which was the negative electrode current collector, and dried to obtain the negative electrode. Test battery No. 5 was manufactured in the same manner as No. 4, except for the use of the above negative electrode.

[0064] <Rating> Short-circuit inspection Short-circuit testing was performed on each test battery No. Specifically, each test battery No. was placed flat (with the stacking direction vertical), a load was applied to the test battery from above, and the percentage of short circuits was calculated based on the voltage change after 3 or 10 days. The results are shown in Figure 2. The percentage of short circuits was determined using the voltage of test battery No. 1 as a reference, with the value of the following formula classified as "A" if it was 0% or more and less than 5%, "B" if it was 5% or more and less than 10%, and "C" if it was 10% or more.

[0065] Short circuit rate (%) = (Voltage of test battery No. 1 - Voltage of test battery No. 1) / Voltage of test battery No. 1 × 100 《Discharge capacity》 For each test battery (number), the discharge capacity (initial capacity) was measured under the following conditions at a temperature of 25°C. The results are shown in Figure 2. The discharge capacity is shown as a relative value with the voltage of test battery No. 1 set to 100.

[0066] CC charging: CC current: 1 / 3C, 4.1V cutoff CC discharge: 1 / 3C, 3.0V cutoff <Result> As shown in Figure 2, in No. 2, where only the first charging process and the first aging process were performed, many short circuits were confirmed in the short-circuit tests after 3 days and 10 days. In No. 3, almost no short circuits were confirmed in the short-circuit test after 3 days, but some short circuits were confirmed in the short-circuit test after 10 days. In Nos. 4 and 5, almost no short circuits were confirmed in the short-circuit tests after 3 days and 10 days.

[0067] Furthermore, as shown in Figure 2, the discharge capacity of No. 4 was smaller compared to the other test batteries. This is thought to be due to an increase in the amount of HF generated by increasing the voltage value in the second charging process. On the other hand, No. 5 had a larger discharge capacity than No. 4. This is thought to be due to a decrease in the amount of HF generated by the reaction between HF and the organic acid salt Li3PO4.

[0068] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of this disclosure is indicated by the claims rather than by the description of the embodiments above, and all modifications within the meaning and scope equivalent to the claims are intended to be included.

Claims

1. A method for manufacturing a lithium-ion secondary battery including a power generation element, The aforementioned power generation element includes a positive electrode, a negative electrode, and a separator. The assembly process involves pouring an electrolyte containing fluorine into the outer casing housing the power generation element to obtain a battery assembly, A first charging step involves charging the battery assembly so that its voltage is between 3.5V and 4.0V, A first aging step in which the battery assembly after the first charging step is held at 40°C or higher for 15 hours or more, A second charging step involves charging the battery assembly after the first aging step so that its voltage exceeds 4.0V, A discharge step in which the battery assembly after the second charging step is discharged so that the voltage becomes 3.5V or less, A method for manufacturing a lithium-ion secondary battery, comprising: a second aging step of holding the battery assembly after the discharge step at 40°C or higher for 0.5 hours or more.

2. The electrolyte is LiPF 6 and LiBF 4 A method for manufacturing a lithium-ion secondary battery according to claim 1, comprising at least one selected from the group consisting of the following.

3. The negative electrode is Li ３ PO ４ A method for manufacturing a lithium-ion secondary battery according to claim 1 or 2, including the method described above.

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