JP2017027727A - Method of manufacturing lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing a lithium ion secondary battery, capable of preventing lithium deposition and electrode wrinkles during initial charge/discharge.SOLUTION: A lithium ion secondary battery 1 includes: an electrode assembly formed by laminating a positive electrode and a negative electrode via a separator; and a case 2 housing the electrode assembly. A method of manufacturing the lithium ion secondary battery 1 includes: an initial charge/discharge step of performing initial charging/discharging of the lithium ion secondary battery 1; and a load application step of applying a load along a lamination direction D of the electrode assembly to the case 2 during the initial charge/discharge step. In the negative electrode, an active material layer containing an active material of a silicon-based material is formed on a metal foil. In the load application step, a load is applied such that the maximum value of surface pressure applied on the case 2 is 2 MPa or more.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a method for manufacturing a lithium ion secondary battery.

  A lithium ion secondary battery includes a stacked type in which a positive electrode and a negative electrode are stacked via a separator (see, for example, Patent Document 1). Lithium ion secondary batteries are small and have a large capacity, and are therefore used in a wide range of fields such as mobile phones and notebook personal computers. In particular, in the case of a relatively large lithium ion secondary battery used in a vehicle or the like, a stacked electrode assembly is generally housed in a metal square case.

  Lithium ion secondary batteries are initially charged and discharged during the manufacturing process in order to activate the active material of the electrode. During this initial charge / discharge, a load is applied to the case in the stacking direction in order to bring the positive electrode and the negative electrode of the electrode assembly close to each other. Further, in order to improve battery performance (for example, cycle characteristics and capacity), lithium ion secondary batteries have been studied to use a silicon-based material as an active material for a negative electrode. In the case of this negative electrode made of a silicon-based material, it expands / contracts at the time of charging / discharging, and the expansion amount is large as compared with electrodes of other materials. In consideration of this expansion, it is preferable to provide a predetermined interval between the electrode assembly and the inner surface of the case in the stacking direction.

Japanese Patent No. 5182477

  However, if the applied load is small, the load may not be evenly applied to the electrodes of the electrode assembly during the initial charge / discharge. In this case, since the space between the positive electrode and the negative electrode of the electrode assembly spreads, there is a possibility that lithium deposition may occur or electrode wrinkles may occur.

  Therefore, an object of the present invention is to propose a method of manufacturing a lithium ion secondary battery that can suppress the occurrence of lithium deposition and electrode wrinkles during initial charge / discharge.

  A method of manufacturing a lithium ion secondary battery according to one aspect of the present invention includes an electrode assembly in which a positive electrode and a negative electrode are stacked via a separator, and a case that houses the electrode assembly. An initial charging / discharging process for initial charging / discharging of a lithium ion secondary battery, and a load applying process for applying a load along the stacking direction of the electrode assembly to the case during the initial charging / discharging process. In the negative electrode, an active material layer containing an active material of a silicon-based material is formed on the metal foil, and in the load adding step, a load is applied so that the maximum surface pressure received by the case is 2 MPa or more. Append.

  In this manufacturing method, by setting the maximum surface pressure received in the case by the load applied in the load adding process to 2 MPa or more, the load is appropriately applied to the electrode assembly in the case during the initial charge / discharge process, and the electrode assembly A solid positive electrode and a negative electrode are close to each other. Therefore, in this manufacturing method, it is possible to suppress lithium precipitation and electrode wrinkles during initial charge / discharge.

  In the load adding step of the manufacturing method according to one embodiment, the load may be applied so that the maximum surface pressure received by the case is 4.5 MPa or less. In this manufacturing method, by setting the maximum value of the surface pressure received by the case to 4.5 MPa or less, it is possible to suppress an excessive load from being applied to the electrode assembly during the initial charge / discharge process.

  In the initial charging / discharging process of the manufacturing method of one embodiment, the voltage of the lithium ion secondary battery may be increased in a plurality of stages during charging. In this initial charge / discharge process, the state of the maximum voltage raised during charging may be maintained for a predetermined time. Thereby, the battery performance of a lithium ion secondary battery can be improved.

  In the initial charging / discharging process of the manufacturing method of one embodiment, the ambient temperature of the lithium ion secondary battery may be changed to a plurality of temperatures during charging. Thereby, initial charge / discharge can be performed in consideration of the actual use environment of the lithium ion secondary battery.

  In the manufacturing method of one embodiment, the silicon content of the active material layer of the negative electrode is 30% by mass or more. Thereby, the battery performance of a lithium ion secondary battery improves.

  In the manufacturing method of one embodiment, the silicon content of the active material layer of the negative electrode is 80% by mass or less. Thereby, the battery performance of a lithium ion secondary battery improves.

  ADVANTAGE OF THE INVENTION According to this invention, generation | occurrence | production of lithium precipitation and an electrode wrinkle at the time of initial stage charge / discharge can be suppressed.

It is sectional drawing which shows typically the lithium ion secondary battery which concerns on one Embodiment. It is sectional drawing along the II-II line in FIG. It is an example of the charging conditions in an initial stage charging / discharging process, (a) is a table | surface which shows charging conditions, (b) is a graph which shows charging conditions. It is a figure which shows typically the load addition method in a load addition process, (a) is a top view, (b) is a side view. It is a table | surface which shows the Example and the comparative example which changed the maximum value of the surface pressure.

  Hereinafter, with reference to drawings, the manufacturing method of the lithium ion secondary battery which concerns on embodiment of this invention is demonstrated. In addition, the same code | symbol is attached | subjected about the element which is the same or it corresponds in each figure, and the overlapping description is abbreviate | omitted.

[Lithium ion secondary battery]
A lithium ion secondary battery according to an embodiment is a rectangular nonaqueous electrolyte secondary battery, and a stacked electrode assembly in which a plurality of positive electrodes and negative electrodes are stacked via a separator is accommodated in a rectangular case. Has been. A lithium ion secondary battery according to an embodiment is a lithium ion secondary battery having a relatively large size (capacity), for example, as a battery mounted on a vehicle such as an industrial vehicle, an electric vehicle, or a plug-in hybrid car. Used. In one embodiment, after the electrode assembly is accommodated in the case, an initial charge / discharge step of performing initial charge / discharge of the lithium ion secondary battery, and a load applied to the lithium ion secondary battery (case) during the initial charge / discharge It applies to the manufacturing method of a lithium ion secondary battery including the load addition process which adds. Conventional manufacturing processes are applied to the manufacturing process of the lithium ion secondary battery other than these processes.

  Before describing a method for manufacturing a lithium ion secondary battery 1, a lithium ion secondary battery 1 according to an embodiment will be described with reference to FIGS. FIG. 1 is a cross-sectional view schematically showing a lithium ion secondary battery according to an embodiment. FIG. 2 is a cross-sectional view taken along the line II-II in FIG.

  The lithium ion secondary battery 1 includes a case 2 and an electrode assembly 3 accommodated in the case 2. In the lithium ion secondary battery 1, an electrolytic solution is injected into the case 2, and the electrolytic solution is impregnated in the electrode assembly 3. The electrolytic solution is, for example, an organic solvent-based or non-aqueous electrolytic solution.

[Case]
The case 2 has a rectangular shape, for example, a substantially rectangular parallelepiped shape. The case 2 is a metal can, for example, an aluminum or aluminum alloy can. The case 2 includes a bottomed rectangular tube-shaped main body 2a and a lid 2b that covers an opening of the main body 2a. The main body 2a is composed of a substantially rectangular flat plate-like bottom plate and four substantially rectangular flat plate-like side plates respectively extending in the vertical direction from each end of the bottom plate. The lid 2b has a substantially rectangular flat plate shape. The lid 2b has substantially the same shape as the opening of the main body 2a. The lid 2b is disposed in the opening of the main body 2a and joined to the main body 2a by welding or the like. The case 2 has a substantially rectangular parallelepiped sealed space formed by the body 2a and the lid 2b.

  On the lid 2 b of the case 2, the positive terminal 5 and the negative terminal 6 are disposed so as to be separated from each other. The positive electrode terminal 5 is fixed to the lid 2 b through an insulating ring 7. The negative electrode terminal 6 is fixed to the lid 2b through an insulating ring 8.

[Electrode assembly]
The electrode assembly 3 includes a plurality of positive electrodes 11, a plurality of negative electrodes 12, and a plurality of separators 13 disposed between the positive electrodes 11 and the negative electrodes 12. In the electrode assembly 3, positive electrodes 11 and negative electrodes 12 are alternately stacked via separators 13.

[Positive electrode]
The positive electrode 11 includes a metal foil 11a and a positive electrode active material layer 11b formed on at least one surface (both surfaces in the example shown in FIG. 2) of the metal foil 11a. The positive electrode 11 has a tab 11c where the positive electrode active material layer 11b is not formed at the end of the metal foil 11a. The tab 11 c is provided at the end of the positive electrode 11 and is electrically connected to the positive electrode terminal 5 through the conductive member 14.

  The metal foil 11a is, for example, an aluminum foil. The positive electrode active material layer 11b is formed by applying a positive electrode paste to the metal foil 11a. This electrode paste contains a positive electrode active material, a binder, a solvent, and the like. The positive electrode active material is made of, for example, a composite oxide or a sulfur-based material. The composite oxide includes at least one of manganese, nickel, cobalt, and aluminum and lithium. The binder is, for example, a fluorine-containing resin such as polyvinylidene fluoride, polytetrafluoroethylene, or fluorine rubber, a thermoplastic resin such as polypropylene or polyethylene, an imide resin such as polyimide or polyamideimide, or an alkoxysilanol group-containing resin. Examples of the solvent include organic solvents such as NMP (N-methylpyrrolidone), methanol, and methyl isobutyl ketone, and water. The electrode paste may contain a conductive auxiliary such as carbon black, graphite, acetylene black, and ketjen black (registered trademark). The electrode paste may contain a thickening agent such as carboxymethylcellulose (CMC).

[Negative electrode]
The negative electrode 12 includes a metal foil 12a and a negative electrode active material layer 12b formed on at least one surface (both surfaces in the example shown in FIG. 2) of the metal foil 12a. The negative electrode 12 has a tab 12c in which the negative electrode active material layer 12b is not formed at the end of the metal foil 12a. The tab 12 c is provided at the end of the negative electrode 12 and is electrically connected to the negative electrode terminal 6 via the conductive member 15.

The metal foil 12a is, for example, a copper foil. The negative electrode active material layer 12b is formed by applying a negative electrode paste to the metal foil 12a. This electrode paste contains a negative electrode active material, a binder, a solvent, and the like. The negative electrode active material is made of a silicon-based material (silicon-based material) containing Si, SiO _x (0.5 ≦ x ≦ 1.5), or the like. The silicon-based material is, for example, a nano silicon material. The nanosilicon material includes fluorine and nano-sized silicon crystallites. The nanosilicon material may be a composite particle further containing at least one of amorphous silicon, silicon oxide (SiO _x ), or a silicon compound in addition to silicon crystallites. The nanosilicon material can be obtained, for example, by heat-treating a layered silicon compound (for example, silicon nanosheet). For example, the same binder and solvent as those of the positive electrode 11 described above are used. The negative electrode active material layer 12b of the negative electrode 12 may also contain a conductive additive and a thickener as in the positive electrode 11 described above.

  The content of silicon (silicon) in the negative electrode active material layer 12b is 30% by mass or more, and preferably 50% by mass or more. The silicon content of the negative electrode active material layer 12b is preferably 80% by mass or less. If the silicon content is higher than 80% by mass, the conductivity and binding force of the negative electrode active material layer 12b may be insufficient. By using an active material of a silicon-based material for the negative electrode 12, the lithium ion secondary battery 1 can have a higher capacity than using an active material of another material. In particular, the finer the silicon crystallite of the nanosilicon material, the better the cycle characteristics of the lithium ion secondary battery 1. However, in the negative electrode 12 using an active material of a silicon-based material, the active material expands / contracts at the time of charge / discharge, and the amount of expansion of the negative electrode active material layer 12b is larger than that of an electrode using an active material of another material. This expansion amount becomes the largest when the lithium ion secondary battery 1 is fully charged.

[Separator]
The separator 13 separates the positive electrode 11 and the negative electrode 12 and allows lithium ions to pass through while preventing a short circuit of current due to contact between the two electrodes. For example, as shown in FIG. 2, the separator 13 is formed in a bag shape and accommodates the positive electrode 11 inside, and is formed in a bag shape and accommodates the positive electrode 11, the negative electrode 12, and the separator 13 a inside. And a separator 13b. The separator 13 is formed of, for example, a porous film made of a polyolefin resin such as polyethylene (PE) or polypropylene (PP), a woven fabric or a nonwoven fabric made of polypropylene, polyethylene terephthalate (PET), methyl cellulose, or the like. The separator 13 is not limited to a bag shape, and a sheet shape may be used.

[Silicon sheet]
Here, the above-described silicon sheet will be further described. The negative electrode for a non-aqueous secondary battery of one embodiment of the present invention includes a current collector (metal foil 12a) and a negative electrode active material layer (12b) bonded to the current collector. A current collector is a chemically inert electronic high conductor that keeps current flowing through an electrode during discharging or charging. The current collector can adopt a shape such as a foil or a plate, but is not particularly limited as long as it has a shape according to the purpose. What is necessary is just to use what is generally used for a lithium ion secondary battery etc. as a collector. For example, aluminum foil, aluminum mesh, punched aluminum sheet, aluminum expanded sheet, stainless steel foil, stainless steel mesh, punched stainless steel sheet, stainless steel expanded sheet, nickel foam, nickel non-woven fabric, copper foil, copper mesh, punched copper sheet, Examples include a copper expanded sheet, a titanium foil, a titanium mesh, a carbon nonwoven fabric, and a carbon woven fabric.

  The negative electrode active material layer includes nano silicon, that is, a first active material including nano-sized silicon particles, and a second active material including graphite. Nanosilicon is produced by heat-treating a layered polysilane represented by the composition formula (SiH) n in a non-oxidizing atmosphere having a structure in which a plurality of six-membered rings composed of silicon atoms are connected. It is obtained by heat-treating a layered polysilane described in Patent Literature 1 (Physical Review B (1993), vol 48, 8172-8189) in a non-oxidizing atmosphere.

  The first active material includes aggregated particles containing nanosilicon (hereinafter, also referred to as “nanosilicon aggregated particles”). Note that in this specification, “aggregated particles containing nanosilicon” means particles composed of silicon crystallites attached to each other, and other silicon crystallites containing amorphous silicon or impurities. Particles formed by adhering to the material, and particles obtained by adhering these particles to each other, and a plurality of plate-like silicon bodies in which silicon particles having a particle size of nanometer order are arranged in layers are arranged in the thickness direction. It has a laminated structure. This structure is confirmed by SEM observation. The plate-like silicon body is observed to have a thickness of about 10 nm to about 100 nm. From the viewpoint of strength and ease of insertion / removal of lithium ions and the like, the thickness of the plate-like silicon body is preferably in the range of 20 nm to 90 nm, and more preferably in the range of 20 nm to 50 nm. The length of the plate-like silicon body in the major axis direction was 0.1 μm to 50 μm. In addition, it is considered that the plate-like silicon body preferably has an aspect ratio (length / thickness in the major axis direction) of 2 to 1000.

  According to the result of TEM (Transmission Electron Microscopy) observation, a striped structure with shading is recognized in the plate-like silicon body. The nano-sized silicon particles have a long shape in the major axis direction of 5 nm to 20 nm, a minor axis direction length of 2 nm to 5 nm, and a flat shape with a major axis / minor axis ratio of 2.5 to 10. The thickness of the voids and / or silicon oxide is 2 nm to 10 nm, and the ratio of the thickness of the voids and / or silicon oxide to the thickness of the nano-sized silicon particles is 0.5 to 2.

  The nanosilicon agglomerated particles can be produced by heat-treating the layered polysilane described in Non-Patent Document 1 at a temperature exceeding 100 ° C. in a non-oxidizing atmosphere. This layered polysilane can be obtained by reacting calcium disilicide with an acid. For example, as described in Non-Patent Document 1, it can be produced by reacting hydrogen chloride with calcium disilicide. Calcium disilicide forms a layered crystal in which a Ca atom layer is inserted between (111) faces of diamond-type Si, and a layered polysilane is obtained by extracting calcium by reaction with an acid.

  The Si crystallite size of nanosilicon in the nanosilicon aggregated particles is preferably 0.5 nm to 300 nm, particularly preferably 1 nm to 100 nm, 1 nm to 50 nm, and more preferably 1 nm to 10 nm for use as an electrode active material of a power storage device. . The Si crystallite size is calculated by the Scherrer equation from the half width of the diffraction peak of the Si (111) plane as a result of the X-ray diffraction measurement.

<Si / C composite>
In the Si / C composite, the carbon layer covers at least a part of the nanosilicon aggregated particles, and as one aspect, the carbon layer covers at least the surface of the plate-like silicon body. The carbon layer may be formed between the surface of the nano-sized silicon particles of the plate-like silicon body or between the layers of the plate-like silicon body, or may be formed between the nano-sized silicon particles. The carbon layer preferably covers the entire surface of the plate-like silicon body.

  The thickness of the carbon layer composited covering the nanosilicon agglomerated particles is preferably in the range of 1 to 100 nm, more preferably in the range of 10 to 50 nm, and in the range of 5 to 50 nm. Further preferred. If the thickness of the carbon layer is too thin, the BET specific surface area of the Si / C composite becomes too large, and the cycle characteristics of the secondary battery may deteriorate due to the generation of SEI. Efficiency may be reduced. If the carbon layer is too thick, the initial capacity of the secondary battery may be reduced, and Li insertion / removal may be difficult, which may make charge / discharge difficult. The Si / C composite is preferably one having a structure in which nanosilicon aggregated particles are dispersed in a matrix of a carbon layer.

If the thickness of the carbon layer is in the range of 1 nm to 100 nm, the carbon contained in the Si / C composite is 1 to 30% by mass, and the BET specific surface area of the composite is 1 m ² / g to 10 m ² /. g. The composite has a resistivity that is the reciprocal of the conductivity of 100 Ω · cm or less.

The carbon layer preferably has an average thickness (R) and a standard deviation (σ) of thickness satisfying the following relational expression (1).
Relational expression (1): R / 3σ> 1

  If the standard deviation (σ) of the thickness of the carbon layer becomes too large, the characteristics of the negative electrode active material may become unstable, and the cycle characteristics of the power storage device may deteriorate. In addition, as the average thickness (R) of the carbon layer is smaller, the instability of the negative electrode active material characteristics due to variations in the carbon layer thickness becomes more prominent. Therefore, the carbon layer preferably satisfies the relational expression (1).

  The thickness of the carbon layer is in the range of 1 nm to 100 nm, and the average thickness (R) and standard deviation (σ) of the carbon layer satisfy the relational expression (1): R / 3σ> 1 Thus, a Si / C composite having a thin and uniform carbon layer is particularly preferred. In a non-aqueous secondary battery using the Si / C composite as a negative electrode active material, it is possible to improve conductivity while suppressing an increase in resistance, and to suppress the generation of SEI by reducing the specific surface area of the negative electrode active material. Is possible. Further, the carbon layer suppresses pulverization of the Si / C composite due to repeated expansion and contraction during charge and discharge as a non-aqueous secondary battery.

The carbon layer desirably has a G / D ratio of 0.5 or more in the Raman spectrum of the carbon, which is the ratio of G-band to D-band. In the Raman spectrum of the carbon, the peak of G-band is around 1590 cm ^-1, a peak of D-band is observed around 1350 cm ^-1. G-band is derived from graphite, and D-band is derived from defects. Therefore, the higher the G / D ratio, which is the ratio of G-band to D-band, means higher carbon crystallinity.

  According to the experiments by the present inventors, it has been clarified that the initial efficiency of the power storage device including the Si / C composite of the carbon layer having a low G / D ratio is lowered. That is, the carbon of the carbon layer in the Si / C composite has a G / D ratio, which is a ratio of G-band to D-band in the Raman spectrum, preferably 0.2 or more, and more preferably 0.5 or more. By using such a Si / C composite as the negative electrode active material, the irreversible capacity in the power storage device is reduced and the initial efficiency is improved.

  The composition of silicon and carbon in the Si / C composite is preferably in the range of 1 to 40% by mass of carbon, more preferably in the range of 1 to 30% by mass, and in the range of 3 to 7% by mass. Is particularly preferred. If the amount of carbon exceeds 40% by mass, the initial capacity may be lowered when used for the negative electrode of the power storage device, which may not be practical. If the amount of carbon is less than 1% by mass, the effect of combining the carbon layers may not be obtained.

  TEM (Transmission Electron Microscopy) observation was performed on the Si / C composite. As a result, a structure was observed in which dark gray particles having a major axis particle size of about 10 nm were aligned perpendicularly to the long side and arranged in layers, and a thin gray portion was laminated in layers. It was.

  The carbon layer may contain at least one metal atom selected from transition metals. Since conductivity in the carbon layer is improved by this metal atom, conductivity of lithium ions and the like in the negative electrode is improved. Therefore, the occlusion / release characteristics of lithium and the like during charging / discharging of the secondary battery are improved and the movement resistance of Li can be reduced, so that the initial efficiency and initial capacity of the secondary battery are improved.

  As a metal atom selected from transition metals, Cu, Fe, Ni and the like are preferable, and Cu is particularly preferable. The metal atom content in the carbon layer is preferably in the range of 0.1 to 10% by mass. If the content of the metal atom is less than 0.1% by mass, it may be difficult to develop the added effect, and if it exceeds 10% by mass, the strength of the carbon layer may decrease and the cycle characteristics of the secondary battery may decrease. is there.

[Expansion characteristics of lithium ion secondary batteries]
In the lithium ion secondary battery 1, the electrodes 11 and 12 of the electrode assembly 3 in the case 2 expand / contract during charging / discharging. In particular, as described above, the negative electrode 12 expands more than the positive electrode 11. As shown in FIG. 2, in the electrode assembly 3, since the plurality of negative electrodes 12 are stacked, the amount of expansion in the stacking direction D increases. The amount of expansion is greatest when the lithium ion secondary battery 1 is fully charged (maximum voltage). In addition, when the ambient temperature of the lithium ion secondary battery 1 is increased, thermal expansion of a material such as an active material is also applied, so that the amount of expansion of the electrode assembly 3 is increased.

  When the interval c between the inner side surfaces 2c and 2c facing each other in the stacking direction D of the case 2 and the thickness a of the electrode assembly 3 in the stacking direction D are substantially the same, the case 2 increases as the expansion amount of the electrode assembly 3 increases. A large stress is generated between the electrode assembly 3 and the electrode assembly 3. Due to this stress, battery performance such as battery life of the lithium ion secondary battery 1 may be deteriorated.

  Therefore, the lithium ion secondary battery 1 is provided with a predetermined interval b between the inner surface 2 c of the case 2 and the electrode assembly 3 in the stacking direction D. In the example shown in FIG. 2, the interval is provided on one side surface 3 a side of the electrode assembly 3, but the interval may be provided on both sides of the electrode assembly 3.

[Method for producing lithium ion secondary battery]
A method for manufacturing the lithium ion secondary battery 1 (particularly, an initial charge / discharge step and a load addition step) will be described. When manufacturing the lithium ion secondary battery 1, when the positive electrode 11, the negative electrode 12, and the separator 13 are manufactured, the step of accommodating the positive electrode 11 in the bag-shaped separator 13, the positive electrode 11 accommodated in the bag-shaped separator 13. The electrode assembly 3 is formed by alternately stacking the negative electrode 12 and the negative electrode 12 and storing the stacked body in the bag-shaped separator 13b. Further, the step of inserting the electrode assembly 3 into the main body 2a of the case 2, the step of injecting the electrolyte into the case 2 and impregnating the inside of the electrode assembly 3 with the electrolyte, and the step of sealing the case 2 An ion secondary battery 1 is formed. Then, an initial charge / discharge step and a load addition step are performed on the lithium ion secondary battery 1. In addition, an aging process or the like is performed.

[Initial charge / discharge process]
The initial charge / discharge process will be described with reference to FIG. FIG. 3 is an example of charging conditions in the initial charging / discharging process, (a) is a table showing charging conditions, and (b) is a graph showing charging conditions. In this graph, the horizontal axis represents time (h) and the vertical axis represents voltage (V).

  In the initial charge / discharge step, the lithium ion secondary battery 1 is charged to the maximum voltage (full charge state) by supplying current to the lithium ion secondary battery 1, and the lithium ion secondary battery 1 is discharged after charging. This initial charge / discharge is performed for the purpose of, for example, activating the active material of the electrodes 11 and 12 and suppressing deterioration of the active material by forming a film on the electrodes 11 and 12. An example of the charging method in the initial charging / discharging process will be described with reference to FIG.

  In the example shown in FIG. 3, the voltage of the lithium ion secondary battery 1 is increased in three steps to 4.3 V (maximum voltage), increased to 3 V in the first step, and up to 3.9 V in the second step. Increase to 4.3V in the third stage. In the first stage, the charging condition is 0.1 C_CC, and charging is performed at a constant current of a charging current rate (C rate) of 0.1 until 3V is reached. In the second stage, the charging condition is 0.4C_CCCV_4h, and charging is performed at a constant current of a charging current rate of 0.4 from 3V to 3.9V, and charging is performed at a constant voltage for 4 hours when it reaches 3.9V. In the third stage, the charging condition is 0.4C_CCCV_2h, and charging is performed at a constant current of a charging current rate of 0.4 from 3.9V to 4.3V, and charging is performed at a constant voltage for 2 hours at 4.3V. . During constant voltage charging, the amount of current supplied gradually decreases. The lithium ion secondary battery 1 is fully charged at 4.3V. Therefore, in this example, the fully charged state (maximum voltage) of the lithium ion secondary battery 1 is maintained for 2 hours. During the constant voltage charging in the fully charged state, the expansion amount of the electrode assembly 3 (particularly, the negative electrode 12) becomes the largest.

  In the example shown in FIG. 3, the ambient temperature of the lithium ion secondary battery 1 is also changed. During the first stage and the second stage of the charging condition, the ambient temperature is set to 25 ° C. as the standard temperature (normal temperature) of the atmosphere. During the third stage of the charging condition, the ambient temperature is set to 60 ° C. as a use environment temperature (particularly a temperature at a severe high temperature as the use environment) when the lithium ion secondary battery 1 is used in a vehicle or the like.

  In addition, the voltage, time, temperature, etc. in the said initial charging / discharging process are examples, and may be changed variously. For example, the voltage of the lithium ion secondary battery 1 is increased to 2.8 V (use lower limit voltage) in the first stage, raised to 3.95 V in the second stage, and 4.3 V (use upper limit voltage) in the third stage. May be raised. In the first stage, the charging condition may be 0.2C_CC, in the second stage, the charging condition may be 0.8C_CCCV_3h, and in the third stage, the charging condition may be 0.8C_CCCV_3h. Further, the voltage of the lithium ion secondary battery 1 may be increased in two stages or four or more stages, or may be charged with a constant voltage without being divided into a plurality of stages. Alternatively, the ambient temperature during the first stage of the charging condition may be 25 ° C., and the ambient temperature during the third stage of the charging condition may be 25 ° C. Further, the ambient temperature may be constant without changing the ambient temperature during charging.

[Load application process]
With reference to FIG. 4, a load addition process is demonstrated. FIG. 4 is a diagram schematically showing a load application method in the load application process, in which (a) is a plan view and (b) is a side view. This load adding step is performed in parallel with the initial charge / discharge step.

  In the load applying step, a load along the stacking direction D is applied to the case 2 of the lithium ion secondary battery 1 during the initial charge / discharge. By applying a load to the electrode assembly 3 in the case 2 by applying this load, the adjacent positive electrode 11 and negative electrode 12 in the electrode assembly 3 come close to each other, and the reaction between the positive electrode 11 and the negative electrode 12 is promoted. . An example of a load applying method in this load applying step will be described with reference to FIG.

  In the example shown in FIG. 4, a pair of plates 20, a plurality of bolts 21 and nuts 22 are used to apply a load. The pair of plates 20 are members that sandwich the lithium ion secondary battery 1 (case 2) in the stacking direction D, and are disposed in contact with the outer surface 2d of the case 2 (the outer surface substantially orthogonal to the stacking direction D). The The plate 20 has a predetermined thickness and has a substantially rectangular flat plate shape. A through hole is formed in a portion of the plate 20 that does not contact the outer surface 2d. Bolts 21 are inserted through the through holes. The number of through holes is the same as the number of bolts 21. The bolt 21 is a long bolt longer than between the pair of plates 20. As shown in FIG. 4B, for example, the bolts 21 are arranged at the four corners of the plate 20 and the number of bolts is four. The plurality of bolts 21 extend in the stacking direction D and connect the pair of plates 20 to each other. The plurality of bolts 21 are respectively inserted through the through holes of the pair of plates 20, and are screwed to the nuts 22 on the outside of the plates 20. Due to the tightening force of the plurality of bolts 21 and nuts 22, a load is applied from the pair of plates 20 to the outer surfaces 2 d and 2 d of the case 2.

  If the load applied in this load application step (the load received on the outer surface 2d of the case 2) is too small, the electrode assembly 3 expands during the initial charge / discharge (especially, it expands most in the fully charged state). Sometimes, the load received on the outer surface 2d of the case 2 may not be applied evenly to the electrodes 11 and 12 of the electrode assembly 3 in the case 2. In this case, the expansion is suppressed at a place where the load is properly applied, and the expansion is easily caused at a place where the load is not applied or a place where the load is small, so that a difference in expansion occurs in the electrode assembly 3. In addition, the reaction between the positive electrode 11 and the negative electrode 12 varies in the electrode assembly 3, and current concentrates in a part of the electrode assembly 3 during charging. Moreover, since the space | interval between the positive electrode 11 and the negative electrode 12 of the electrode assembly 3 spreads, the electrical resistance between the positive electrode 11 and the negative electrode 12 becomes large, and when the charging current flows, the voltage between the positive electrode 11 and the negative electrode 12 is increased. growing. Due to these factors, there is a possibility that lithium deposition and electrode wrinkles may occur on the electrodes 11 and 12 of the electrode assembly 3.

  On the other hand, if the load applied in the load applying process is too large, an excessive load may be applied to the electrode assembly 3 in the case 2. In particular, when the electrode assembly 3 is most expanded in a fully charged state, an excessive load may be applied. When an excessive load is applied, the electrical resistance between the positive electrode 11 and the negative electrode 12 of the electrode assembly 3 increases. Further, the holes of the separator 13 of the electrode assembly 3 may be crushed so that lithium ions cannot pass through the separator 13.

Therefore, in the load adding step, the load is applied so that the maximum value of the surface pressure received by the case 2 is 2 MPa or more and 4.5 MPa or less. The surface pressure received by the case 2 reaches the maximum value when the electrode assembly 3 in the case 2 is most expanded. The electrode assembly 3 expands most during the initial charge / discharge when the battery is fully charged. Therefore, the surface pressure received in case 2 increases as the amount of charge of the lithium ion secondary battery 1 increases (voltage increases) during initial charging, and the surface pressure received in case 2 when fully charged (maximum voltage). Is the maximum value. The surface pressure (MPa (= N / mm ² )) is a load (N) per unit area. Therefore, even when the surface pressure is the same value, the load increases as the area of the outer surface 2d of the case 2 increases.

  An example of a method for adjusting the maximum value of the surface pressure received in Case 2 will be described. The surface pressure received by the case 2 changes according to the size of the interval b provided between the inner surface 2c of the case 2 and the electrode assembly 3, and increases as the interval b decreases. Therefore, the maximum value of the surface pressure received by the case 2 is adjusted by setting the interval b of the lithium ion secondary battery 1 to a predetermined size. The interval b is, for example, 0. It is several mm.

  An example of a method for setting the interval b will be described. At the design stage of the lithium ion secondary battery 1, a plurality of lithium ion secondary batteries having different intervals b are produced. Then, initial charging / discharging is performed for each lithium ion secondary battery at each interval b, and a load is applied by the method shown in FIG. 4 during the initial charging / discharging. At this time, when the surface pressure is measured for each lithium ion secondary battery at each interval b in a fully charged state (maximum voltage), and the measured surface pressure is 2 MPa or more and 4.5 MPa or less Interval b is obtained.

  An example of a method for measuring the surface pressure when a load is applied by the method shown in FIG. 4 will be described. As described above, a load is applied from the pair of plates 20 to the outer surface 2 d of the case 2 by the tightening force of the four bolts 21 and the nut 22. For each bolt 21, the axial force of the bolt 21 is measured by an axial force meter, the axial forces of the four bolts 21 are integrated, and the load received on the entire outer surface 2 d of the case 2 is calculated from the integrated value. Then, by dividing this load by the area of the outer surface 2d of the case 2, the surface pressure received by the outer surface 2d of the case 2 is acquired. Incidentally, during the initial charge / discharge, the axial force of each bolt 21 increases as the charge amount of the lithium ion secondary battery 1 increases (voltage rises), and the axis of each bolt 21 when fully charged (maximum voltage). The force reaches its maximum value.

  By applying a load so that the maximum value of the surface pressure received by the case 2 in the load applying process is 2 MPa or more, the applied load is applied substantially evenly to the electrodes 11 and 12 of the electrode assembly 3 in the case 2. . Thereby, the electrodes 11 and 12 (particularly, the negative electrode 12) can be uniformly expanded in the electrode assembly 3. Further, variation in the reaction between the positive electrode 11 and the negative electrode 12 in the electrode assembly 3 is suppressed, and current easily flows through the entire electrode assembly 3 during charging. Moreover, the space | interval between the positive electrode 11 and the negative electrode 12 becomes narrow, and the electrical resistance between the positive electrode 11 and the negative electrode 12 becomes small. Thus, lithium deposition and electrode wrinkles on the electrodes 11 and 12 of the electrode assembly 3 are suppressed.

  By applying the load so that the maximum value of the surface pressure received by the case 2 in the load application process is 4.5 MPa or less, even when the electrode assembly 3 is most expanded during the initial charge / discharge, an excessive load is applied. It does not hang on the electrode assembly 3 in the case 2. Thereby, the electrical resistance between the positive electrode 11 and the negative electrode 12 of the electrode assembly 3 becomes small. Further, the holes of the separator 13 of the electrode assembly 3 are not easily crushed.

[Example]
With reference to FIG. 5, Examples 1-4 and Comparative Examples 1-3 which changed the maximum value of the surface pressure received in case 2 are demonstrated. In this example and the comparative example, the interval c between the inner side surfaces 2c of the case 2 was set to a constant value (24.6 mm), and the interval b and the thickness a of the electrode assembly 3 were changed. In Examples 1 to 4, the interval b was set so that the maximum value of the surface pressure was 2 MPa or more. In Comparative Examples 1 to 3, the interval b was set so that the maximum value of the surface pressure was less than 2 MPa. In this example and the comparative example, the initial charge / discharge process and the load application process are performed on the lithium ion secondary battery 1 at each interval b, and the electrode assembly 3 is taken out from the case 2 after the initial charge / discharge process is completed. The presence or absence of generation of lithium precipitation and electrode wrinkles during initial charge / discharge was confirmed. If neither lithium deposition nor electrode wrinkle has occurred, the determination result is “OK” with no abnormality, and if either lithium precipitation or electrode wrinkle has occurred, the determination result is “NG”. " In the initial charge / discharge process, charging was performed under the charging conditions shown in FIG. 3, and in the load adding process, a load was applied by the method shown in FIG. The area of the outer surface 2d of the case 2 is 14000 mm. The composition ratio of the positive electrode 11 was NCM111: LFP: AB: PVDF = 69: 25: 3: 3. The basis weight of the positive electrode 11 was 27 mg / cm ^2, and the thickness of the positive electrode 11 was 200 μm. The composition ratio of the negative electrode 12 was nanosilicon: graphite: AB: PAI = 58: 25: 6: 11. The basis weight of the negative electrode 12 was 4.7 mg / cm ^2, and the thickness of the negative electrode 12 was 90 μm. The separator 13 was a polyolefin separator, and the thickness of the separator 13 was 25 μm. A polyolefin resin was used as the insulating film of the separator 13 and the thickness of the insulating film was 150 μm.

  In the lithium ion secondary battery 1 of Example 1, the interval b was set to 0.2 mm. In the case of this lithium ion secondary battery 1, the maximum value of the load received on the entire outer surface 2d of the case 2 was about 44 kN, and the maximum value of the surface pressure received on the outer surface 2d of the case 2 was about 3.14 MPa. . In this case, neither lithium deposition nor electrode wrinkles occurred during the initial charge / discharge. Therefore, the determination result is no abnormality.

  In the lithium ion secondary battery 1 of Example 2, the interval b was set to 0.4 mm. In the case of this lithium ion secondary battery 1, the maximum value of the load received on the entire outer surface 2d of the case 2 was about 40 kN, and the maximum value of the surface pressure received on the outer surface 2d of the case 2 was about 2.85 MPa. . In this case, neither lithium deposition nor electrode wrinkles occurred during the initial charge / discharge. Therefore, the determination result is no abnormality.

  In the lithium ion secondary battery 1 of Example 3, the interval b was set to 0.6 mm. In the case of this lithium ion secondary battery 1, the maximum value of the load received on the entire outer surface 2d of the case 2 was about 32 kN, and the maximum value of the surface pressure received on the outer surface 2d of the case 2 was about 2.28 MPa. . In this case, neither lithium deposition nor electrode wrinkles occurred during initial charging. Therefore, the determination result is no abnormality.

  In the lithium ion secondary battery 1 of Example 4, the interval b was set to 0.8 mm. In the case of this lithium ion secondary battery 1, the maximum value of the load received on the entire outer surface 2d of the case 2 was about 30 kN, and the maximum value of the surface pressure received on the outer surface 2d of the case 2 was about 2.14 MPa. . In this case, neither lithium deposition nor electrode wrinkles occurred during the initial charge / discharge. Therefore, the determination result is no abnormality.

  In the lithium ion secondary battery of Comparative Example 1, the interval b was 1 mm. In the case of this lithium ion secondary battery 1, the maximum value of the load received on the entire outer surface 2d of the case 2 was about 25 kN, and the maximum value of the surface pressure received on the outer surface 2d of the case 2 was about 1.78 MPa. . In this case, only lithium deposition occurred during the initial charge / discharge. Therefore, the determination result is abnormal.

  In the lithium ion secondary battery of Comparative Example 2, the interval b was set to 2 mm. In the case of this lithium ion secondary battery 1, the maximum value of the load received on the entire outer surface 2d of the case 2 was about 20 kN, and the maximum value of the surface pressure received on the outer surface 2d of the case 2 was about 1.43 MPa. . In this case, lithium deposition and electrode wrinkles occurred during initial charge / discharge. Therefore, the determination result is abnormal.

  In the lithium ion secondary battery of Comparative Example 3, the interval b was 3 mm. In the case of this lithium ion secondary battery 1, the maximum value of the load received on the entire outer surface 2d of the case 2 was about 15 kN, and the maximum value of the surface pressure received on the outer surface 2d of the case 2 was about 1.07 MPa. . In this case, lithium deposition and electrode wrinkles occurred during initial charge / discharge. Therefore, the determination result is abnormal.

  According to Examples 1 to 4 and Comparative Examples 1 to 3, when a load was applied so that the maximum value of the surface pressure received in Case 2 in the load application process was 2 MPa or more, lithium deposition and electrode wrinkle during initial charge / discharge were performed. It was confirmed that the occurrence of was suppressed. In Examples 1 to 4, the maximum value of the surface pressure is about 3.14 MPa, which is 4.5 MPa or less.

  According to the method for manufacturing the lithium ion secondary battery 1, the maximum value of the surface pressure received by the case 2 due to the load applied in the load adding process is set to 2 MPa or more, so that the load is applied in the case 2 during the initial charge / discharge process. The electrodes 11 and 12 of the electrode assembly 3 are applied substantially evenly, and the positive electrode 11 and the negative electrode 12 of the electrode assembly 3 are close to each other. Therefore, this lithium ion secondary battery 1 can suppress generation | occurrence | production of lithium precipitation and an electrode wrinkle at the time of initial stage charge / discharge.

  Further, according to this manufacturing method, the maximum value of the surface pressure received by the case 2 due to the load applied in the load adding process is set to 4.5 MPa or less, so that it is excessive with respect to the electrode assembly 3 during the initial charge / discharge process. Can prevent a heavy load from being applied.

  In addition, according to this manufacturing method, the voltage is increased by constant current charging in three stages in the initial charge / discharge process, and each voltage (particularly, the maximum voltage (full charge state)) is maintained for a predetermined time by constant voltage charging in each stage. By doing, the battery performance of the lithium ion secondary battery 1 can be improved. Further, according to this manufacturing method, the initial temperature is changed in consideration of the actual use environment of the lithium ion secondary battery 1 by changing the ambient temperature of the lithium ion secondary battery 1 to a plurality of temperatures in the initial charge / discharge process. Charging / discharging can be performed.

  As mentioned above, although embodiment of this invention was described, it is implemented in various forms, without being limited to the said embodiment.

  For example, in the above-described embodiment, the nano silicon material is shown as an example of the negative electrode active material, but other silicon-based materials including Si, SiO x, and the like may be used. Further, the separator 13b may not be provided.

  In the above embodiment, the maximum value of the surface pressure received by the case is adjusted according to the size of the interval, but the maximum value of the surface pressure received by the case is adjusted by another method without changing the interval. May be. Other methods include, for example, a method of adjusting the tightening force between the bolt and the nut, and a method of adjusting the pressing force by pressing a pair of plates with hydraulic pressure or the like without using the bolt and the nut. is there.

  DESCRIPTION OF SYMBOLS 1 ... Lithium ion secondary battery, 2 ... Case, 2a ... Main body, 2b ... Cover, 2c ... Inner side surface, 2d ... Outer side surface, 3 ... Electrode assembly, 3a ... Side surface, 5 ... Positive electrode terminal, 6 ... Negative electrode terminal, 7, 8 ... Insulating ring, 11 ... Positive electrode, 11a ... Metal foil, 11b ... Positive electrode active material layer, 11c ... Tab, 12 ... Negative electrode, 12a ... Metal foil, 12b ... Negative electrode active material layer, 12c ... Tab, 13 ... Separator , 14, 15 ... conductive members, 20 ... plates, 21 ... bolts, 22 ... nuts.

Claims (7)

A method of manufacturing a lithium ion secondary battery comprising: an electrode assembly formed by laminating a positive electrode and a negative electrode via a separator; and a case for housing the electrode assembly,
An initial charge / discharge step of performing initial charge / discharge of the lithium ion secondary battery;
A load applying step of applying a load along the stacking direction of the electrode assembly to the case during the initial charge / discharge step;
Including
In the negative electrode, an active material layer containing an active material of a silicon-based material is formed on a metal foil,
In the load application step, the load is applied so that the maximum value of the surface pressure received by the case is 2 MPa or more.   2. The method of manufacturing a lithium ion secondary battery according to claim 1, wherein in the load application step, the load is applied so that a maximum value of a surface pressure received by the case is 4.5 MPa or less.   3. The method of manufacturing a lithium ion secondary battery according to claim 1, wherein in the initial charge / discharge step, the voltage of the lithium ion secondary battery is increased in a plurality of stages during charging.   The method for manufacturing a lithium ion secondary battery according to any one of claims 1 to 3, wherein in the initial charge / discharge step, the state of the maximum voltage raised during charging is maintained for a predetermined time.   5. The method of manufacturing a lithium ion secondary battery according to claim 1, wherein in the initial charge / discharge step, the ambient temperature of the lithium ion secondary battery is changed to a plurality of temperatures during charging. .   The method for producing a lithium ion secondary battery according to any one of claims 1 to 5, wherein a content of silicon in the active material layer of the negative electrode is 30% by mass or more.   The manufacturing method of the lithium ion secondary battery as described in any one of Claims 1-6 whose content rate of the silicon of the said active material layer of the said negative electrode is 80 mass% or less.

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