JP2016126953A - Method for reducing unevenness in charging secondary battery, and method for manufacturing secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a method for reducing the unevenness in charging a secondary battery; and a method for manufacturing a secondary battery by utilizing the reducing method.SOLUTION: (1) A method for reducing the unevenness in charging a secondary battery including a positive electrode, a negative electrode, and an electrolyte containing an organic solvent comprises: an exhaust step for exhausting gas produced in the battery after the start of initial charging to be performed subsequently to the assembly of the secondary battery before reaching the state of full charge. (2)In the method for reducing the unevenness in charging a secondary battery as described in (1), gas produced by starting the initial charging and charging the battery to a given capacity with the secondary battery sealed is stored in the secondary battery and then, the seal is temporarily cancelled to exhaust the gas in the exhaust step. (3)In the method for reducing the unevenness in charging a secondary battery as described in (1) or (2), after charging the battery to 50-90% of a rating capacity, gas produced in the battery is exhausted.SELECTED DRAWING: None

Description

  The present invention relates to a method for reducing charging unevenness of a secondary battery and a method for manufacturing a secondary battery.

  Secondary batteries are deployed in a wide range of applications such as portable devices, electric vehicles, storage batteries for homes or business facilities. Lithium ion secondary batteries have a high discharge voltage suitable for these applications, and have become the mainstream of current secondary batteries.

  In the manufacturing process of the lithium ion secondary battery, there is a problem that gas is generated in the battery due to decomposition of a part of the organic solvent constituting the electrolyte during the initial charge. The generated gas is a by-product when the SEI (Solid Electrolyte Interface) that stabilizes the electrode surface is formed. If the generated gas stays in the battery and is repeatedly charged and discharged, the battery performance deteriorates. is there.

In order to prevent the above-mentioned problem, Patent Document 1 discloses a method in which the battery outer body is temporarily sealed and subjected to initial charging in order to prevent moisture absorption of the organic solvent, and then a part of the outer body is opened to reduce the pressure. The use of an electrolytic solution to which a hydrophilic solvent has been added has been proposed for the purpose of discharging gas and promoting gas discharge.
Patent Document 2 proposes a method of reducing charging unevenness by applying pressure in the stacking direction of the electrode body during gas discharge and sufficiently discharging until reaching a predetermined battery voltage.

JP 2003-331916 A JP 2010-9983 A

  However, the user's demand for battery performance is increasing year by year, and there is a need to eliminate the remaining charging unevenness even by the conventionally proposed methods.

  This invention is made | formed in view of the said situation, and makes it a subject to provide the manufacturing method of a secondary battery using the charging nonuniformity reduction method of a secondary battery, and this method.

(1) A method for reducing charging unevenness of a secondary battery comprising a positive electrode and a negative electrode, and an electrolyte containing an organic solvent, wherein the battery is fully charged after the start of the first charge performed after the secondary battery is assembled. A method for reducing charging unevenness of a secondary battery, which has an exhaust process of discharging gas generated in the battery before reaching the battery.
(2) The first charging is started in a state where the secondary battery is sealed, and after the gas generated by charging to a predetermined capacity is accumulated in the secondary battery, the sealing is performed in the exhaust process. The method for reducing uneven charging of the secondary battery according to (1), wherein the gas is exhausted by temporarily releasing the gas.
(3) The method for reducing uneven charging of the secondary battery according to (1) or (2), wherein the gas generated in the battery is exhausted after being charged to 50 to 90% of the rated capacity.
(4) Charging even after dQ / dV becomes 1000 or more in the charge curve after the first charge start (horizontal axis: battery voltage V, vertical axis: amount of change in charge / battery voltage change dQ / dV) And after the dQ / dV reaches the maximum value, before the dQ / dV becomes 2000 or less or immediately after 2000, the gas generated in the battery is exhausted (1) to (3) The method for reducing uneven charging of the secondary battery according to any one of the above.
(5) After the start of the initial charging, the charging is continued even after the battery voltage becomes 3.5V or higher, and then before the battery voltage reaches 4.0V or immediately after reaching 4.0V, The method for reducing uneven charging of the secondary battery according to any one of (1) to (4), wherein the generated gas is exhausted.
(6) Reducing charging unevenness of the secondary battery according to any one of (1) to (5), including a discharging step of restarting charging after the exhausting step and discharging after reaching full charge. Method.
(7) A method for manufacturing a secondary battery, including a step of performing the method for reducing charging unevenness according to any one of (1) to (6).

According to the secondary battery charging unevenness reducing method of the present invention, it is possible to sufficiently reduce the charging unevenness caused by the gas generated during the initial charging. In addition, since the conventional method for reducing charging unevenness can be used in combination, the present invention can be easily applied without significantly changing the conventional manufacturing platform.
According to the method for producing a secondary battery of the present invention, the performance inherent in the secondary battery can be sufficiently exhibited.

It is a dQ / dV curve in the first charge of the secondary battery used in the preliminary test. It is an OCV curve in the first charge of the secondary battery used in the preliminary test.

  As a result of studying the solution of the above problems, the present inventors have found that all the conventional methods are fully charged at the first charge. Therefore, a charge curve indicating the relationship between “charge amount change / battery voltage change (dQ / dV)” and battery voltage (V) from the start of the initial charge to full charge was analyzed (see FIG. 1). ). As a result, a peak was considered that gas was generated during the charging period of 3.6 to 3.8 V before reaching 4.2 V representing the fully charged state. From this, it was found that during the charging period of 3.8 to 4.2 V, charging was performed in a state where a large amount of generated gas stayed. By continuing the charging until the gas is fully charged in the state where the gas stays around the electrode, a portion where the charging reaction proceeds on the surface of the electrode and a portion where the charging reaction is hindered by the gas occur, resulting in charging unevenness. Furthermore, it has been found that there is a problem that lithium metal is deposited on the separator under the influence of the charging unevenness.

  From the above examination, it has been found that the charging unevenness that remains even by the conventionally disclosed method is caused by full charge in the initial charging process. Furthermore, the present inventors have clarified that the occurrence of uneven charging at the end of the first charge can be prevented by discharging the gas after the gas is generated in the first charge and before reaching the full charge.

The present invention has been made based on the above findings, and the present invention will be described below based on preferred embodiments. However, the present invention is not limited to such embodiments.
The present invention is not limited to lithium ion secondary batteries and can be implemented in various known secondary batteries. Hereinafter, a lithium ion secondary battery will be described as an example of an embodiment of the present invention.

<Method for reducing uneven charging>
1st embodiment of this invention is a method of reducing the charge nonuniformity of the secondary battery provided with the positive electrode and the negative electrode, and the electrolyte containing a lithium ion and an organic solvent.

<Configuration of secondary battery>
The kind of said positive electrode, negative electrode, and electrolyte is not specifically limited, The member used for a well-known lithium ion secondary battery is used. That is, the method for reducing charging unevenness according to the present embodiment can be applied when manufacturing a known lithium ion secondary battery.
Examples of the configuration of the positive electrode include a configuration in which a known positive electrode material composition containing a positive electrode active material, a binder, and a conductive additive is applied onto an aluminum current collector having an arbitrary shape.
As a structure of the said negative electrode, the structure formed by apply | coating the well-known negative electrode material composition containing a negative electrode active material, a binder, and a conductive support agent on copper collectors of arbitrary shapes is mentioned, for example.

  As the electrode active material, a known carbon material, silicon compound or the like in which lithium ions are reversibly intercalated is suitable. When a carbon material is used as one of the negative electrode active materials, SEI is generated on the surface of the negative electrode active material layer during the initial charge and the electrode performance is improved, but part of the organic solvent is decomposed as the SEI is generated. Therefore, gas is easily generated. In this embodiment, since the occurrence of charging unevenness due to the gas can be sufficiently reduced, there is no problem in using a carbon material as the electrode active material.

  The method for manufacturing the electrode is not particularly limited, and a known method is applied. For example, an electrode active material, a conductive additive, a binder and a dilution solvent are mixed with a Henschel mixer or the like, and the resulting composition is applied onto a current collector by a blade coating method, and then the dilution solvent is evaporated to obtain a desired An electrode is obtained. The thickness of the formed electrode active material layer is not particularly limited, and can be set to a thickness of 5 μm to 500 μm, for example. You may perform a press and a rolling process when adjusting this thickness.

  When the positive electrode and the negative electrode are housed in the battery cell, it is preferable to install an insulating separator between the two electrodes in order to prevent a short circuit. The type of the separator is not particularly limited, and examples thereof include a porous resin film that can be inserted between both electrodes, and an insulating layer in which insulating particles and a binder are bound to the electrode surface.

  The method for housing the electrode laminate (electrode body) having the positive electrode, the separator and the negative electrode in the outer package is not particularly limited. For example, a rampack in which an electrode laminate formed by laminating flat electrodes is enclosed in an aluminum laminate bag. Examples thereof include a winding method in which a film-shaped electrode laminate is wound into a concentric roll and enclosed in a cylindrical battery case.

  After housing the electrode stack in the outer package, the electrolyte is injected into the outer package so that the electrolyte containing lithium ions and an organic solvent (non-aqueous solvent) immerses the electrode stack or the separator. The electrolyte may be a liquid electrolyte or a gel-like gel electrolyte.

The electrolyte is preferably an electrolyte salt containing lithium ions. The type of the electrolyte salt is not particularly limited, and a known lithium salt can be applied. For example, LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ CFO, LiSO ₃ CF ₃ , LiCF ₂ CF ₂ SO ₃ , LiClO ₄ , Examples include lithium salts such as LiN (COCF ₂ CF ₃ ) ₂ and LiN (SO ₂ CF ₃ ) ₂ . One of these lithium salts may be used alone, or two or more thereof may be used in combination. The density | concentration of electrolyte salt is not specifically limited, For example, about 0.3 mol / L-3.0 mol / L is mentioned.

  The kind of the organic solvent constituting the electrolyte is not particularly limited, and a known organic solvent (non-aqueous solvent) can be used as long as it is an organic solvent that enables conduction of lithium ions between the positive electrode and the negative electrode.

<Gas emission>
In the charging unevenness reducing method of the present embodiment, first, the electrode laminate is housed in the exterior body as described above, and after injecting the electrolyte, the terminal tab connected to each electrode is exposed to the outside. The exterior body is sealed and a lithium ion secondary battery is assembled. Next, after starting the first charge, the gas generated in the battery is exhausted before reaching the fully charged state.

  Here, “full charge” refers to a state where the amount of electricity (unit: Ah) corresponding to the rated capacity of the secondary battery is 100% charged (SOC: State Of Charge). The rated capacity is determined by a known method. In the initial charging, charging is started from a state where the charging rate is about 0% to about 100%. In addition, when the lithium pre-doping process is performed to the negative electrode active material in advance, the charge rate may exceed 0% before the start of the first charge. The relationship between the amount of electricity (charge amount) from the start of charging to full charge and the battery voltage is represented by an OCV curve with the open circuit voltage of the battery on the vertical axis and the charge amount on the horizontal axis (see Fig. 2). ).

  In this embodiment, the initial charging is started in a state where the assembled secondary battery is sealed, and after the gas generated by charging to a predetermined capacity is accumulated in the secondary battery, the sealing is temporarily performed. It is preferable that the gas accumulated in the battery is exhausted.

  When the assembled battery is charged for the first time without sealing, there is a problem that the electrolyte absorbs moisture (moisture) from the outside air during the charging period. In order to prevent moisture absorption of the electrolyte, it is necessary to charge in a dry atmosphere. However, since the charging process takes several hours even if it is short, performing the charging process in a dry atmosphere significantly reduces the manufacturing efficiency of the secondary battery. There is a case. Therefore, it is preferable to perform the charging step in a state where the secondary battery is once sealed and moisture absorption from the outside air is prevented.

  When initial charging is performed in a state where the assembled secondary battery is sealed, gas is generated in the battery mainly in the later stage of the charging period. In the present embodiment, the generation of gas is allowed, and it is allowed that gas is accumulated in the secondary battery by charging to a predetermined capacity. In the early stage of gas generation, the gas tends to exist as many fine bubbles adhering to the electrode and the separator. It may be difficult to efficiently discharge such a fine gas. However, in the present embodiment, the gas is sufficiently generated and a relatively large gas reservoir formed by combining the bubbles is formed in the battery, so that the efficiency of the gas discharge process performed later can be improved.

  After gas accumulates in the battery charged to a predetermined capacity, the charging is stopped at an appropriate time and the process proceeds to the exhaust process. In the exhaust process, the sealing of the secondary battery is temporarily released by a known method, and the gas in the battery is discharged. Since the internal pressure is increased by the gas generated in the battery, there is little risk of moisture from the outside air entering the battery even when the sealing is released, but it is preferable to perform the exhaust process in a dry atmosphere. For example, exhaustion can be completed in several minutes to several tens of minutes by using a known method for promoting gas discharge such as pressurizing the exterior body from the outside or placing it in a reduced-pressure atmosphere.

  As a suitable time for stopping the first charge and moving to the exhaust process, it is preferable to move to the exhaust process after the start of the first charge and charging to a predetermined charging rate of the rated capacity of the secondary battery. Here, although the predetermined charging rate depends on the degree of gas generation, for example, 50 to 90% of the rated capacity is preferable, 60 to 85% is more preferable, and 70 to 80% is more preferable.

By charging to a charge rate of 50% or more in the initial charge and then moving to the exhaust process, sufficient gas is generated in the battery, so that the exhaust can be efficiently performed. On the other hand, if the charge rate is less than 50% and the process proceeds to the exhaust process, a large amount of new gas may be generated in the charging period after resealing, which may cause uneven charging.
By charging to a discharge rate of 90% or less in the initial charge and then proceeding to the exhaust process, the generation of gas in the battery has almost ended, and charging unevenness can be sufficiently reduced. On the other hand, if the initial charging is continued even in a state where the charging rate exceeds 90%, charging unevenness may be caused.

  A suitable time for stopping the initial charge and moving to the exhaust process can be determined with reference to a charge curve examined in advance. For example, refer to the dQ / dV curve where the horizontal axis is the open circuit battery voltage (V) and the vertical axis is the amount of change in battery charge / battery voltage (dQ / dV). After charging reaches 1000 or more, charging continues and sufficient gas is generated in the battery, and after the dQ / dV value reaches the maximum value, the dQ / dV value that has fallen below 2000 or 1500 or less Alternatively, immediately after becoming, there is a method of stopping charging and moving to an exhaust process.

  After the dQ / dV value becomes 1000 or more in the first charge, the gas generation becomes active, and after the dQ / dV value shows the maximum value, the gas generation ends, and the dQ / dV value It is considered that the generation of gas has almost ended by the time when the gas reaches 2000 or less or 1500 or less. By moving to the exhaust process immediately after the generation of gas ends, charging unevenness can be sufficiently reduced and the battery performance inherently possessed by the secondary battery can be maximized.

  A suitable time for stopping the initial charging and moving to the exhaust process can be determined while monitoring the battery voltage during charging. For example, the first charge is started while measuring the battery voltage (V) of the open circuit, the charge is continued even after the battery voltage becomes 3.5 V or higher, and then the battery voltage reaches or reaches 4.0 V. Immediately after this, there is a method of stopping charging and moving to an exhaust process.

  After the battery voltage becomes 3.5 V or more in the first charge, gas generation becomes active, and then it is considered that the gas generation has almost ended by the time the battery voltage reaches 4.0 V. By moving to the exhaust process immediately after the generation of gas ends, charging unevenness can be sufficiently reduced and the battery performance inherently possessed by the secondary battery can be maximized.

In the present embodiment, it is preferable to have a discharging step in which charging is resumed after the secondary battery is resealed in the exhausting step and discharged after reaching full charging.
By completing the charge / discharge cycle by discharging after the initial charge, the cycle characteristics of the secondary battery can be improved. After the discharge, several charge / discharge cycles may be repeated. By repeating this, the electrode surface may be stabilized, and the cycle characteristics of the battery may be further improved. In the second and subsequent charging / discharging cycles, if gas is generated, the charging unevenness reducing method of this embodiment can be performed again.

<Method for manufacturing secondary battery>
The second embodiment of the present invention is a method for manufacturing a secondary battery including the step of performing the method for reducing charging unevenness of the first embodiment described above. The second embodiment may include other steps, for example, a positive electrode and negative electrode preparation step, an electrolyte preparation step, a secondary battery assembly step, and the like. Known methods can be applied as specific manufacturing methods in other steps.

  EXAMPLES Next, although an Example demonstrates this invention further in detail, this invention is not limited by these examples.

<Preparation of secondary battery materials>
<Electrolyte>
Lithium oxalate-boron trifluoride complex (LOX-BF ₃ ) solution (lithium salt concentration 1 mol / kg, dimethyl carbonate: ethylene carbonate (2: 1, volume ratio) mixed solvent) was mixed and stirred to prepare an electrolyte solution. Obtained.
<Separator>
A non-woven fabric (PP, porosity: 76%, thickness: 30 μm, HOP6 manufactured by Hirose Paper Co., Ltd.) was cut into 225 × 285 mm and used.
<Positive electrode>
After preparing a positive electrode mixture by mixing 93 parts by mass of LiCoO ₂ (lithium cobaltate), 3 parts by mass of PVDF (polyvinylidene fluoride) and 4 parts by mass of carbon black as a conductive additive, N is a solvent. A positive electrode mixture slurry was prepared by dispersing in -methylpyrrolidone (NMP). The positive electrode mixture slurry is coated on both sides of a 15 μm aluminum foil, leaving a portion with no active material layer 15 mm wide, and further dried under reduced pressure (100 ° C., −0.1 MPa, 10 hours) and roll-pressed. Thus, a positive electrode was obtained.
<Negative electrode>
A negative electrode mixture slurry was prepared by mixing 95 parts by mass of graphite and 5 parts by mass of PVDF (polyvinylidene fluoride) to prepare a negative electrode mixture and then dispersing it in N-methylpyrrolidone (NMP) as a solvent. The negative electrode was obtained by coating the both sides of 15-micrometer copper foil, leaving the part which does not have an active material layer 15 mm width, drying under reduced pressure (100 degreeC, -0.1 MPa, 10 hours), and roll-pressing.
The above positive electrode and negative electrode were cut as follows. Including the portion without active material layer (15 mm width), the negative electrode is 235 × 280 mm (the portion with the active material layer is 220 × 280 mm), the positive electrode is 235 × 280 mm (the portion with the active material layer is 220 × 280 mm) Molded to size.

<Assembly of secondary battery>
Two negative electrodes, one positive electrode, and two separators were alternately laminated. Specifically, a separator was placed on the negative electrode so as to cover the negative electrode active material layer. The positive electrode was covered so that the positive electrode and the negative electrode were not in direct contact with each other with the separator in between so that the portion of the negative electrode without the active material layer was opposed to the portion of the negative electrode without the active material layer. Further, a separator was placed on the positive electrode so as to cover the positive electrode active material layer. In addition, with the separator in between, place the positive electrode without the active material layer on the opposite side of the negative electrode without the active material layer so that the negative electrode is not in direct contact with the negative electrode. It was.
Terminal tabs were electrically connected by ultrasonic welding or the like to the positive electrode and negative electrode portions of the laminate without the active material layer.
The laminate was sandwiched between aluminum laminate films (300 × 350 mm) so that the terminal tabs protruded to the outside, and the three sides were sealed by lamination. Sealing was performed at a portion of about 2 mm from the electrode end.
A lithium ion secondary battery before initial charge was obtained by injecting an electrolyte solution from one side left without sealing and vacuum-sealing. At this time, sealing was performed leaving a 2 cm blank from the electrode end.

<Preliminary test (comparative example)>
The lithium ion secondary battery assembled as described above was initially charged at a constant current (200 mA) until full charge as in the conventional method. The OCV curve at this time is shown in FIG. In the OCV curve of FIG. 2, the battery voltage immediately after the start of charging is about 2.5 V, and the battery voltage rapidly rises to about 3.6 V in the initial stage of charging (˜180 mAh, SOC about 14%), and then reaches the full level. The battery voltage gradually increased to about 4.2 V in the charged state (about 1300 mAh, charging rate about 100%).

  For the initial charge of the preliminary test, as shown in FIG. 1, draw a charge curve (dQ / dV curve) with the battery voltage V on the horizontal axis and the amount of change in battery charge / battery voltage on the vertical axis. The charging characteristics were analyzed. As a result, as described above, gas is generated in the charging period of 3.6 to 3.8 V before reaching 4.2 V representing the fully charged state, and in the charging period of 3.8 to 4.2 V thereafter. It was thought that gas generation had almost ended.

  The secondary battery that was fully charged in the preliminary test was disassembled and the state of each member was confirmed. Lithium metal deposition was observed on the surface of the separator. Such precipitation is not preferable because it may promote the generation of lithium dendride. Next, when the surface of the negative electrode was observed, it was confirmed that uneven charging occurred because there was a difference in color between the location where lithium ions were occluded by the charging reaction and the location where the charging reaction was delayed. This uneven charging may not be resolved even in the subsequent discharging process, which causes a decrease in battery performance. The reason for the occurrence of this charging unevenness is that during the charging period of 3.8 to 4.2 V when the generation of gas has ended, charging is performed in a state where a large amount of generated gas stays in the battery. It was thought that there was.

<Example 1>
A new lithium ion secondary battery was assembled by the above-described method, and the initial charge was similarly started. The battery voltage of about 3.8 V in which the maximum peak of the dQ / dV curve shown in FIG. Immediately after the charging rate reached about 54%, the charging was stopped, and the next gas discharging step was started.
Specifically, a needle is inserted into the blank portion of the outer package made of a laminate film sealed at the time of assembly to form a hole, the outer package is pressurized under a reduced pressure atmosphere, and the gas generated in the battery is discharged. . Thereafter, the opening was resealed by heat sealing, and the battery was charged until the battery voltage reached 4.2V. The amount of new gas generated during the charging period after resealing was small.

  The secondary battery that was fully charged in Example 1 was disassembled and the state of each member was confirmed. Lithium metal was not deposited on the surface of the separator. When the color of the surface of the negative electrode was observed, it was confirmed that the locations where lithium ions were occluded by the charging reaction were spread uniformly over the entire electrode surface. That is, it has been found that charging unevenness does not occur and the battery performance inherent in the secondary battery can be maximized. The reason why such a good result was obtained may be that the timing of degassing generated in the battery by the initial charge was optimal. Moreover, since there was little generation | occurrence | production of the new gas in the charge period performed after discharging | emitting gas and resealing, it can be said that the influence which it has on battery performance is small.

<Reference example>
A new lithium ion secondary battery is assembled by the above-described method, the initial charge is similarly started, and the battery voltage before the large peak of the dQ / dV curve shown in FIG. When 5V and the charging rate reached about 15%, charging was stopped and the next gas discharging step was started. The method for discharging gas was the same as in Example 1. Thereafter, the opening was resealed by heat sealing, and the battery was charged until the battery voltage reached 4.2V. The amount of new gas generated during the charging period after resealing was large.

  When the secondary battery that was fully charged in the reference example was disassembled and the state of each member was confirmed, the result was the same as the preliminary test. That is, lithium metal deposition was observed on the surface of the separator. Moreover, there was a color difference indicating that charging unevenness occurred on the surface of the negative electrode. The reason for this uneven charging is that the gas discharge process was performed before the generation of gas became active, so in the charging period after resealing, a large amount of generated gas was retained until full charge was reached. It was thought that this was a factor.

  The configurations and combinations thereof in the embodiments described above are examples, and the addition, omission, replacement, and other modifications of the configurations can be made without departing from the spirit of the present invention. Further, the present invention is not limited by each embodiment, and is limited only by the scope of the claims.

  The present invention can be widely used in the electrochemical field such as lithium ion secondary batteries.

Claims (7)

A method for reducing uneven charging of a secondary battery comprising a positive electrode and a negative electrode, and an electrolyte containing an organic solvent,
A method for reducing uneven charging of a secondary battery, comprising: an exhausting step of discharging gas generated in the battery before full charge after starting the first charge performed after assembling the secondary battery.   The initial charging is started in a state where the secondary battery is sealed, and after the gas generated by charging up to a predetermined capacity is accumulated in the secondary battery, the sealing is temporarily performed in the exhaust process. 2. The method for reducing uneven charging of the secondary battery according to claim 1, wherein the gas is exhausted after being released.   The method for reducing charging unevenness of a secondary battery according to claim 1 or 2, wherein the gas generated in the battery is exhausted after being charged to 50 to 90% of the rated capacity.   In the charge curve after the initial charge start (horizontal axis: battery voltage V, vertical axis: amount of change in storage amount / battery voltage change amount dQ / dV), charging continues even after dQ / dV reaches 1000 or more. The gas generated in the battery is exhausted before dQ / dV becomes 2000 or less or immediately after 2000 after dQ / dV shows the maximum value. The method for reducing charging unevenness of the described secondary battery.   After the start of the initial charging, the charging is continued even after the battery voltage becomes 3.5V or higher, and then the gas generated in the battery before the battery voltage reaches 4.0V or immediately after reaching 4.0V. The method for reducing uneven charging of the secondary battery according to any one of claims 1 to 4, wherein:   6. The method for reducing charging unevenness of a secondary battery according to claim 1, further comprising: a discharging step of restarting charging after the exhausting step and discharging after reaching full charging.   The manufacturing method of a secondary battery which has a process of performing the charging nonuniformity reduction method as described in any one of Claims 1-6.

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