JP6641755B2 - Manufacturing method of lithium ion secondary battery - Google Patents

Description

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

  As a lithium ion secondary battery, there is a stacked type in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween (for example, see Patent Document 1). Lithium-ion secondary batteries are used in a wide range of fields such as mobile phones and notebook personal computers because they have a large capacity even though they are small. In particular, in the case of a relatively large lithium-ion secondary battery used in a vehicle or the like, generally, a laminated electrode assembly is housed in a metal square case.

  In a lithium ion secondary battery, initial charge and discharge are performed in a manufacturing process for activating an active material of an electrode and the like. During the initial charge and discharge, a load is applied to the case in the stacking direction to bring the positive electrode and the negative electrode of the electrode assembly close to each other. In addition, in order to improve battery performance (for example, cycle characteristics and capacity) of a lithium ion secondary battery, use of a silicon-based material as an active material of a negative electrode has been studied. In the case of the negative electrode of this silicon-based material, it expands / contracts at the time of charge / discharge and has a larger expansion amount than the electrode of another material. In consideration of this expansion, it is preferable to provide a predetermined space between the electrode assembly and the inner surface of the case in the stacking direction.

Patent No. 5,182,477

  However, if the above-mentioned interval is too large, the load applied during the initial charge / discharge may not be applied to the electrode assembly in the case at all, or may not be evenly applied to the electrodes of the electrode assembly. In this case, since the space between the positive electrode and the negative electrode of the electrode assembly is widened, there is a possibility that lithium deposition occurs or electrode wrinkles occur.

  In view of the above, an object of the present invention is to propose a lithium ion secondary battery capable of suppressing the occurrence of lithium deposition and electrode wrinkling during initial charge and discharge, and a method of manufacturing the lithium ion secondary battery.

  A lithium ion secondary battery according to one aspect of the present invention is a lithium ion secondary battery including an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween, and a case for housing the electrode assembly. In the negative electrode, an active material layer containing a silicon-based material active material is formed on a metal foil, and the thickness of the electrode assembly in the stacking direction is a, and the thickness of the electrode assembly in the stacking direction and the inner surface of the case are different. When the interval between them is b, the relationship of b / a <0.04 is satisfied.

  In this lithium ion secondary battery, a load is applied during the initial charge and discharge by defining the upper limit of the interval between the electrode assembly and the inner surface of the case so as to satisfy the relationship of b / a <0.04. In this case, the load is appropriately applied to the electrode assembly in the case, and the positive electrode and the negative electrode of the electrode assembly come close to each other. For this reason, this lithium ion secondary battery can suppress the occurrence of lithium deposition and electrode wrinkling during initial charge and discharge.

  In the lithium ion secondary battery of one embodiment, the interval between the inner surfaces facing each other in the stacking direction of the case may be 15 to 35 mm.

  In 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 the lithium ion secondary battery is improved.

  In 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 the lithium ion secondary battery is improved.

  A method for manufacturing a lithium ion secondary battery according to one aspect of the present invention is directed to a lithium ion secondary battery including: an electrode assembly in which a positive electrode and a negative electrode are stacked with a separator interposed therebetween; and a case accommodating the electrode assembly. The active material layer containing a silicon-based material active material is formed on a metal foil on a negative electrode, the thickness of the electrode assembly in the stacking direction is a, and the electrode assembly and the case in the stacking direction are formed. The method includes a step of housing the electrode assembly in a case so as to satisfy a relationship of b / a <0.04, where b is a distance between the electrode assembly and the inner surface.

  In this method of manufacturing a lithium ion secondary battery, the upper limit of the distance between the electrode assembly and the inner surface of the case is defined so as to satisfy the relationship of b / a <0.04, so that during the initial charging and discharging, When a load is applied, the load is appropriately applied to the electrode assembly in the case, and the positive electrode and the negative electrode of the electrode assembly approach each other. Therefore, according to the method of manufacturing a lithium ion secondary battery, it is possible to suppress the occurrence of lithium deposition and electrode wrinkling during initial charge and discharge.

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

1 is a cross-sectional view schematically illustrating a lithium ion secondary battery according to one embodiment. FIG. 2 is a sectional view taken along the line II-II in FIG. 1. It is an example of the charging conditions of an initial charge / discharge, (a) is a table | surface which shows a charging condition, (b) is a graph which shows a charging condition. It is a figure which shows typically the load addition method during initial charge / discharge, (a) is a top view, (b) is a side view. 4 is a table showing examples and comparative examples in which the intervals of the lithium ion secondary batteries were changed.

  Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described with reference to the drawings. In each drawing, the same or corresponding elements are denoted by the same reference numerals, and redundant description will be omitted.

[Lithium ion secondary battery]
The lithium ion secondary battery according to one embodiment is a prismatic nonaqueous electrolyte secondary battery, and is a stacked type in which a plurality of positive electrodes and negative electrodes are stacked with a separator interposed therebetween. The lithium ion secondary battery according to one embodiment is a lithium ion secondary battery having a relatively large size (capacity), such as a battery mounted on a vehicle such as an industrial vehicle, an electric vehicle, or a plug-in hybrid car. Used.

  A lithium ion secondary battery 1 according to one embodiment will be described with reference to FIGS. FIG. 1 is a sectional view schematically showing a lithium ion secondary battery according to one embodiment. FIG. 2 is a 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 housed 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 inside the electrode assembly 3. The electrolyte is, for example, an organic solvent-based or non-aqueous electrolyte.

[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 square tubular main body 2a and a lid 2b that covers an opening of the main body 2a. The main body 2a includes a substantially rectangular flat bottom plate and four substantially rectangular flat side plates extending vertically 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 arranged in the opening of the main body 2a, and is joined to the main body 2a by welding or the like. In the case 2, a substantially rectangular parallelepiped closed space is formed inside the main body 2a and the lid 2b.

  On the lid 2b of the case 2, a positive electrode terminal 5 and a negative electrode terminal 6 are arranged separately from each other. The positive electrode terminal 5 is fixed to the lid 2b via an insulating ring 7. The negative electrode terminal 6 is fixed to the lid 2b via 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 arranged between the positive electrodes 11 and the negative electrodes 12. In the electrode assembly 3, a positive electrode 11 and a negative electrode 12 are alternately stacked via a separator 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 at the end of the metal foil 11a where the positive electrode active material layer 11b is not formed. The tab 11 c is provided at an end of the positive electrode 11, and is electrically connected to the positive electrode terminal 5 via 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 includes, for example, a composite oxide and a sulfur-based material. The composite oxide contains 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 fluororubber, a thermoplastic resin such as polypropylene or polyethylene, an imide-based resin such as polyimide or polyamideimide, or a resin containing an alkoxysilinole group. The solvent is, for example, an organic solvent such as NMP (N-methylpyrrolidone), methanol, methyl isobutyl ketone, or water. In addition, the electrode paste may include a conductive auxiliary such as carbon black, graphite, acetylene black, and Ketjen black (registered trademark). Further, the electrode paste may include a thickener 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 at the end of the metal foil 12a where the negative electrode active material layer 12b is not formed. The tab 12c is provided at an end of the negative electrode 12, and is electrically connected to the negative 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) and the like. The silicon-based material is, for example, a nanosilicon material. Nanosilicon materials include fluorine and nanosized silicon crystallites. Nano silicon material, in addition to silicon crystallites, amorphous silicon, silicon oxide (SiO _x), or a silicon compound may be a composite particle further comprises at least one. The nanosilicon material is obtained by, for example, heat-treating a layered silicon compound (for example, a silicon nanosheet). As the binder and the solvent, for example, those similar to the above-described positive electrode 11 are used. The negative electrode active material layer 12b of the negative electrode 12 may also contain a conductive additive and a thickener, similarly to 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 equal to or less than 80% by mass. If the silicon content is greater than 80% by mass, the conductivity and the 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 capacity of the lithium ion secondary battery 1 can be made higher than that of 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 during charge and 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 in a fully charged state.

[Separator]
The separator 13 separates the positive electrode 11 and the negative electrode 12 and allows lithium ions to pass therethrough while preventing a current short circuit 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 therein, and the separator 13 is formed in a bag shape and accommodates the positive electrode 11, the negative electrode 12, and the separator 13 a therein. 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), or a woven or nonwoven fabric made of polypropylene, polyethylene terephthalate (PET), methylcellulose, or the like. Note that the separator 13 is not limited to a bag shape, and a sheet shape may be used.

[Silicon sheet]
Here, the silicon sheet described above will be further described. The negative electrode for a nonaqueous secondary battery of one embodiment of the present invention includes a current collector (metal foil 12a) and a negative electrode active material layer (12b) bound to the current collector. The current collector refers to a chemically inert high-electron conductor for continuously supplying a current to an electrode during discharging or charging. The shape of the current collector may be a foil, a plate, or the like, but is not particularly limited as long as the shape is in accordance with the purpose. As the current collector, one generally used for a lithium ion secondary battery or the like may be used. 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, foamed nickel, nickel nonwoven 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 nanosilicon, that is, a first active material including nano-sized silicon particles, and a second active material including graphite. Nanosilicon has a structure in which a plurality of six-membered rings composed of silicon atoms are connected, and is manufactured by heat-treating a layered polysilane represented by a composition formula (SiH) n in a non-oxidizing atmosphere. It is obtained by heat-treating a layered polysilane described in Patent Document 1 (Physical Review B (1993), vol. 48, 8172-8189) in a non-oxidizing atmosphere.

  The first active material includes aggregated particles containing the nanosilicon (hereinafter, sometimes referred to as “nanosilicon aggregated particles”). In this specification, “agglomerated particles containing nanosilicon” refer to particles formed by attaching silicon crystallites to each other, and other particles in which a plurality of silicon crystallites contain amorphous silicon or impurities. Particles formed by adhering to a 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 a layered manner in a thickness direction. It has a laminated structure. This structure is confirmed by SEM observation. Plate-like silicon bodies are observed with a thickness of about 10 nm to about 100 nm. 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, from the viewpoints of strength, ease of insertion and removal of lithium ions and the like. The length in the major axis direction of the plate-like silicon body was 0.1 μm to 50 μm. It is considered that the plate-like silicon body preferably has an aspect ratio (length / thickness in the long axis direction) of 2 to 1,000.

  According to TEM (Transmission Electron Microscopy) observation results, a striped structure with shading is observed in the plate-like silicon body. The nano-sized silicon particles have a length of 5 nm to 20 nm in the major axis direction and a length of 2 nm to 5 nm in the minor axis direction, and have a flat shape with a ratio of the major axis to the minor axis 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.

  Nanosilicon aggregated particles can be produced by subjecting the layered polysilane described in Non-Patent Document 1 to a heat treatment 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 layered crystals in which a Ca atomic layer is inserted between the (111) planes of diamond-type Si, and 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, 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 from the half value width of the diffraction peak of the Si (111) plane as a result of the X-ray diffraction measurement according to Scherrer's formula.

<Si / C composite>
In the Si / C composite, the carbon layer covers at least a part of the nanosilicon aggregated particles, and as one embodiment, it can be said that the carbon layer covers at least the surface of the plate-like silicon body. The carbon layer may be formed on the surface of the nano-sized silicon particles of the plate-shaped silicon body, between the layers of the plate-shaped 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 over the nanosilicon aggregated particles is preferably in the range of 1 to 100 nm, more preferably in the range of 10 to 50 nm, and more preferably in the range of 5 to 50 nm. More preferred. If the thickness of the carbon layer is too small, the BET specific surface area of the Si / C composite becomes too large, and the cycle characteristics of the secondary battery may decrease due to the generation of SEI. Efficiency may decrease. If the carbon layer is too thick, the initial capacity of the secondary battery may be reduced, and insertion / removal of Li may be difficult, and charging / discharging may be difficult. The Si / C composite also preferably has a structure in which nanosilicon aggregated particles are dispersed in a matrix of a carbon layer.

When 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. In addition, the composite has a resistivity, which is the reciprocal of the electrical conductivity, of 100 Ω · cm or less.

The carbon layer preferably has an average thickness (R) and a standard deviation (σ) of the 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 is too large, the characteristics of the negative electrode active material become unstable, and the cycle characteristics of the power storage device may be deteriorated. Further, as the average thickness (R) of the carbon layer is smaller, the instability of the characteristics of the negative electrode active material due to the variation in the thickness of the carbon layer 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 the standard deviation (σ) of the thickness of the carbon layer satisfy the relational expression (1): R / 3σ> 1 As described above, a Si / C composite having a thin and uniform carbon layer is particularly preferable. 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 a rise in resistance, and to suppress the generation of SEI by reducing the specific surface area of the negative electrode active material. Becomes possible. Further, the carbon layer suppresses the 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, which is a ratio of G-band to D-band, of 0.5 or more in the Raman spectrum of the carbon. 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 that the crystallinity of carbon is higher.

  According to experiments performed by the present inventors, it has become clear that the initial efficiency of a power storage device including a Si / C composite having a carbon layer having a low G / D ratio is reduced. That is, the carbon in the carbon layer of the Si / C composite preferably has a G / D ratio, which is the ratio of G-band to D-band in the Raman spectrum, of 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 of the power storage device is reduced and the initial efficiency is improved.

  The composition of silicon and carbon in the Si / C composite, carbon is preferably in the range of 1 to 40% by mass, 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 is more than 40% by mass, the initial capacity becomes low when used for a negative electrode of a power storage device, which may not be practical. If the amount of carbon is less than 1% by mass, the effect of compounding the carbon layer may not be obtained.

  TEM (Transmission Electron Microscopy) observation was performed on the Si / C composite. As a result, a structure in which dark gray particles having a long diameter of about 10 nm were arranged in a layered manner oriented perpendicular to the long side and a structure in which light gray portions were layered in a layered manner was observed. Was.

  The carbon layer may include at least one metal atom selected from transition metals. The conductivity of lithium ions and the like at the negative electrode is improved because the metal atoms improve the conductivity in the carbon layer. Therefore, the storage / release characteristics of lithium and the like during charge and discharge of the secondary battery are improved, and the transfer resistance of Li can be reduced, so that the initial efficiency and the 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 content of metal atoms in the carbon layer is preferably in the range of 0.1 to 10% by mass. If the content of metal atoms is less than 0.1% by mass, it may be difficult to achieve the effect of the addition, 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.

[Method of manufacturing lithium ion secondary battery]
A method for manufacturing the lithium ion secondary battery 1 (particularly, an initial charge / discharge step and a load applying step) will be described. When the lithium ion secondary battery 1 is manufactured, when the positive electrode 11, the negative electrode 12, and the separator 13 are manufactured, a step of housing the positive electrode 11 in the bag-shaped separator 13 a, The electrode assembly 3 is formed by a process of alternately laminating the anode and the negative electrode 12 and a process of accommodating the laminated body in the bag-shaped separator 13b. Furthermore, the steps of inserting the electrode assembly 3 into the main body 2a of the case 2, injecting the electrolytic solution into the case 2 to impregnate the electrolytic solution inside the electrode assembly 3, and sealing the case 2 make lithium possible. The ion secondary battery 1 is formed. Then, an initial charging / discharging step and a load applying step are performed on the lithium ion secondary battery 1. In addition, an aging step and the like are performed.

[Initial charge / discharge step]
In the manufacturing process of the lithium ion secondary battery 1, the electrode assembly 3 impregnated with the electrolytic solution is accommodated in the case 2, and after the case 2 is sealed, an initial charge and discharge (also referred to as “conditioning”) is performed. Will be implemented. This initial charge / discharge will be described with reference to FIGS. 3A and 3B show an example of charging conditions for initial charging and discharging. FIG. 3A is a table showing charging conditions, and FIG. 3B is a graph showing charging conditions. In this graph, the horizontal axis represents time (h), and the vertical axis represents voltage (V). 4A and 4B are diagrams schematically illustrating a method of applying a load during initial charge and discharge, where FIG. 4A is a plan view and FIG. 4B is a side view.

  The initial charge / discharge is performed for the purpose of activating the active materials of the electrodes 11 and 12 and suppressing deterioration of the active materials by forming a film on the electrodes 11 and 12. In the initial charge / discharge, the lithium-ion secondary battery 1 is charged to a predetermined voltage (full charge state) by supplying a current thereto, and the lithium-ion secondary battery 1 is discharged after charging. An example of the charging method in the initial charge / discharge 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, is increased to 3 V in the first step, is increased to 3.9 V in the second step, and The voltage is increased to 4.3 V in three steps. 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 the voltage reaches 3 V. In the second stage, the charging condition is 0.4C_CCCV_4h, and charging is performed at a constant current at a charging current rate of 0.4 from 3V to 3.9V, and at 3.9V, charging is performed at a constant voltage for 4 hours. In the third stage, the charging condition is 0.4C_CCCV_2h, charging is performed at a constant current at a charging current rate of 0.4 from 3.9V to 4.3V, and charging is performed for 2 hours at a constant voltage when the voltage reaches 4.3V. . During constant voltage charging, the amount of current supplied gradually decreases. When the voltage of the lithium ion secondary battery 1 reaches 4.3 V, the lithium ion secondary battery 1 is fully charged. Therefore, in this example, the fully charged state of the lithium ion secondary battery 1 is maintained for 2 hours.

  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 conditions, the ambient temperature is set to 25 ° C. as the standard temperature of the atmosphere (normal temperature). During the third stage of the charging condition, the cycle temperature is set to 60 ° C. as a use environment temperature when the lithium ion secondary battery 1 is used in a vehicle or the like (particularly, a temperature at a high temperature which is a severe use environment).

  The voltage, time, temperature, and the like in the initial charge / discharge step are merely examples, and may be variously changed. For example, the voltage of the lithium ion secondary battery 1 is increased to 2.8 V (lower limit voltage) in the first stage, to 3.95 V in the second stage, and to 4.3 V (upper limit voltage) in the third stage. It may be raised to. 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 or four or more stages, or may be charged at a constant voltage without being divided into a plurality of stages. Further, 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. Alternatively, the ambient temperature may be kept constant without changing the ambient temperature during charging.

[Load application process]
During the initial charge / discharge, a load is applied in the stacking direction D from both sides of the case 2 of the lithium ion secondary battery 1. Thus, when a load is applied to the electrode assembly 3, 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 method for applying the load will be described with reference to FIG.

  In the example shown in FIG. 4, a pair of plates 20 and 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 in the stacking direction D, and are disposed in contact with the outer side surface 2 d of the case 2. The plate 20 has a predetermined thickness and is a substantially rectangular flat plate. The plate 20 has a through hole. The bolt 21 is inserted into this through hole. The number of through holes is the same as the number of bolts 21. The bolt 21 is a long bolt longer than the distance between the pair of plates 20. As shown in FIG. 4B, the bolts 21 are arranged at, for example, four corners of the plate 20 and have four bolts. 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 into the through holes of the pair of plates 20 and screwed to the nuts 22 outside the plates 20. Due to the tightening force between the plurality of bolts 21 and the nuts 22, a load is applied from the pair of plates 20 to the outer surfaces 2d, 2d of the case 2. The magnitude of the applied load is, for example, several tens of kN.

  During this initial charge and discharge, the electrodes 11 and 12 of the electrode assembly 3 in the case 2 expand and contract. In particular, the negative electrode 12 expands more than the positive electrode 11. As shown in FIG. 2, in the electrode assembly 3, since a plurality of negative electrodes 12 are stacked, the amount of expansion in the stacking direction D increases. This expansion amount becomes maximum when the lithium ion secondary battery 1 is in a fully charged state (maximum voltage). In addition, when the ambient temperature of the lithium ion secondary battery 1 increases, thermal expansion of a material such as an active material is added, so that the expansion amount of the electrode assembly 3 increases. For example, when the battery is charged under the charging conditions shown in FIG. 3, the expansion amount of the electrode assembly 3 (particularly, the negative electrode 12) is largest during the constant voltage charging for 2 hours in the third stage.

  Assuming that the distance c between the inner surfaces 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 larger the expansion amount of the electrode assembly 3, the larger the amount of expansion. A large stress is generated between the case 2 and the electrode assembly 3. Due to this stress, battery performance such as the battery life of the lithium ion secondary battery 1 may be reduced.

  Therefore, in the lithium ion secondary battery 1, an interval b having a predetermined size is provided 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 of the electrode assembly 3, but the interval may be provided on both sides of the electrode assembly 3.

  However, as the distance b increases, the dimensional margin of the electrode assembly 3 with respect to the case 2 increases, so that the positive electrode 11 and the negative electrode 12 are easily separated from each other in the electrode assembly 3. In particular, if the distance b is too large, even if the electrode assembly 3 expands during the initial charge and discharge (especially, even when fully expanded in the fully charged state), the load applied to the outer surface 2d of the case 2 May not be applied to the electrode assembly 3 at all or not evenly applied to the electrodes 11 and 12 of the electrode assembly 3. In this case, the expansion is suppressed in a portion where the load is properly applied, and the expansion is easily performed in a portion where the load is not applied or a portion where the load is small, and 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 on a part of the electrode assembly 3 during charging. In addition, since the distance between the positive electrode 11 and the negative electrode 12 increases, the resistance between the positive electrode 11 and the negative electrode 12 increases, and when a charging current flows, the voltage between the positive electrode 11 and the negative electrode 12 increases. These factors may cause lithium deposition or electrode wrinkles on the electrodes 11 and 12 of the electrode assembly 3.

  Therefore, in the lithium ion secondary battery 1, the interval b is set so that the thickness a and the interval b of the electrode assembly 3 in the stacking direction D satisfy the relationship of 0.008 <b / a <0.04. I have. That is, in the manufacturing process of the lithium ion secondary battery 1, the electrode assembly 3 is accommodated in the case 2 so as to satisfy the relationship of 0.008 <b / a <0.04. Accordingly, the upper limit of the interval b is determined from the relationship of b / a <0.04, and the lower limit of the interval b is determined from the relationship of 0.008 <b / a, in accordance with the thickness a of the electrode assembly 3 in the stacking direction D. Since the value is determined, the interval b is a value between the upper limit and the lower limit.

  For example, when the battery capacity is determined by the design specification of the lithium ion secondary battery 1, the outer dimensions of the lithium ion secondary battery 1 (the outer dimensions of the case 2 (thickness in the stacking direction D, etc.)) are determined according to the battery capacity. Can be Thereby, the distance c between the inner side surfaces 2c of the case 2 is also determined. Since the lithium ion secondary battery 1 is a relatively large lithium ion secondary battery, the interval c between the inner side surfaces 2c of the case 2 is, for example, 15 to 35 mm. From the relationship c (= a + b) between the inner side surfaces 2c, 2c of the case 2 and 0.008 <b / a <0.04, the thickness a and the interval b of the electrode assembly 3 in the stacking direction D are set. You. The thickness a of the electrode assembly 3 in the stacking direction D is measured as follows. First, the positive electrode 11 and the negative electrode 12 are laminated via the separator 13 to obtain the electrode assembly 3. Next, a compressive load of 5 kN is applied to the electrode assembly 3 by a pressing machine, and the thickness a of the electrode assembly 3 in the stacking direction D after being pressed is measured. The distance c between the inner side surfaces 2c of the case 2 is measured using, for example, a caliper.

  By setting the interval b that is equal to or less than the upper limit value of the interval b defined by the relationship of b / a <0.04, the load applied at the time of initial charge / discharge causes the load of the electrodes 11, 3 of the electrode assembly 3 in the case 2 to increase. 12 is applied almost evenly. Thereby, the electrodes 11 and 12 (particularly, the negative electrode 12) can be uniformly expanded in the electrode assembly 3. In addition, 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. Further, the distance between the positive electrode 11 and the negative electrode 12 is reduced, and the resistance between the positive electrode 11 and the negative electrode 12 is reduced. As a result, the deposition of lithium on the electrodes 11 and 12 of the electrode assembly 3 and the occurrence of electrode wrinkles are suppressed.

  Further, by setting the interval b equal to or greater than the lower limit value defined by the relationship of b / a> 0.008, the expansion of the electrodes 11 and 12 (particularly, the negative electrode 12) of the electrode assembly 3 during the initial charge and discharge is reduced. It is acceptable in the case 2 and no large stress occurs between the case 2 and the electrode assembly 3. Further, when the electrode assembly 3 is accommodated in the case 2, it can be accommodated smoothly.

[Example]
With reference to FIG. 5, Examples 1-4 and Comparative Examples 1-3 in which the interval b of the lithium ion secondary battery 1 is changed will be described. In this example and the comparative example, the distance c between the inner side surfaces 2c of the case 2 was set to a constant value (24.6 mm), and the thickness a and the distance b of the electrode assembly 3 were changed. In Examples 1 to 4, the intervals b satisfying the relationship of 0.008 <b / a <0.04 were set. In Comparative Examples 1 to 3, intervals b satisfying b / a> 0.04 (not satisfying the relationship of 0.008 <b / a <0.04) were set. In this example and the comparative example, initial charge / discharge was performed on the lithium ion secondary battery 1 at each interval b, and after the completion of the initial charge / discharge, the electrode assembly 3 was taken out of the case 2 and during the initial charge / discharge. The presence or absence of lithium deposition and electrode wrinkling was confirmed. If neither lithium deposition nor electrode wrinkling has occurred, the determination result is “OK” without abnormality, and if either lithium deposition or electrode wrinkling has occurred, the determination result is “NG”. " In the initial charging and discharging, charging was performed under the charging conditions shown in FIG. 3, and a load was applied during the initial charging and discharging by the method shown in FIG. 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-based separator, and the thickness of the separator 13 was 25 μm. As the insulating film of the separator 13, a polyolefin resin was used, and the thickness of the insulating film was 150 μm.

  In the lithium ion secondary battery 1 of Example 1, the interval b was 0.2 mm, and the thickness a of the electrode assembly 3 was 24.4 mm. b / a is about 0.0082. In the case of this lithium ion secondary battery 1, neither lithium precipitation nor electrode wrinkling 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 0.4 mm, and the thickness a of the electrode assembly 3 was 24.2 mm. b / a is about 0.017. In the case of this lithium ion secondary battery 1, neither lithium precipitation nor electrode wrinkling 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 0.6 mm, and the thickness a of the electrode assembly 3 was 24.0 mm. b / a is 0.025. In the case of this lithium ion secondary battery 1, neither lithium deposition nor electrode wrinkling occurred during the initial charging. Therefore, the determination result is no abnormality.

  In the lithium ion secondary battery 1 of Example 4, the interval b was 0.8 mm, and the thickness a of the electrode assembly 3 was 23.8 mm. b / a is about 0.034. In the case of this lithium ion secondary battery 1, neither lithium precipitation nor electrode wrinkling 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, and the thickness a of the electrode assembly 3 was 23.6 mm. b / a is about 0.042. In the case of this lithium ion secondary battery, only lithium precipitation 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 2 mm, and the thickness a of the electrode assembly 3 was 22.6 mm. b / a is about 0.088. In the case of this lithium ion secondary battery, lithium deposition and electrode wrinkling occurred during initial charge and discharge. Therefore, the determination result is abnormal.

  In the lithium ion secondary battery of Comparative Example 3, the interval b was 3 mm, and the thickness a of the electrode assembly 3 was 21.6 mm. b / a is about 0.14. In the case of this lithium ion secondary battery, lithium deposition and electrode wrinkling occurred during initial charge and discharge. Therefore, the determination result is abnormal.

  According to Examples 1 to 4 and Comparative Examples 1 to 3, in the case of the lithium ion secondary battery 1 that satisfies the relationship of 0.008 <b / a <0.04, lithium precipitation and generation of electrode wrinkles during initial charge / discharge. Was confirmed to be suppressed.

  According to the lithium ion secondary battery 1, by defining the upper limit of the interval b so as to satisfy the relationship of b / a <0.04, the case where a load is applied during the initial charging / discharging is reduced. The electrodes 11 and 12 of the electrode assembly 3 in the electrode assembly 3 are substantially evenly applied, and the positive electrode 11 and the negative electrode 12 of the electrode assembly 3 are close to each other. Therefore, the lithium ion secondary battery 1 can suppress lithium precipitation and electrode wrinkling during initial charge and discharge.

  Further, according to the lithium ion secondary battery 1, by defining the lower limit of the interval b so as to satisfy the relationship of 0.008 <b / a, the electrode assembly 3 (particularly, The electrode assembly 3 can be smoothly accommodated in the case 2 while allowing the negative electrode 12) to expand.

  As described above, the embodiments of the present invention have been described. However, the present invention is not limited to the above embodiments, but may be implemented in various forms.

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

  In the above embodiment, the lower limit of the interval b is also defined by the relationship of 0.008 <b / a, but this relationship does not have to be provided, and at least the interval sufficient to smoothly accommodate the electrode assembly in the case. May be provided.

  DESCRIPTION OF SYMBOLS 1 ... Lithium ion secondary battery, 2 ... case, 2a ... main body, 2b ... lid, 2c ... inner surface, 2d ... outer 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 member, 20 ... plate, 21 ... bolt, 22 ... nut.

Claims (4)

An electrode assembly formed by laminating a positive electrode and a negative electrode with a separator interposed therebetween, and a method for manufacturing a lithium ion secondary battery including a case accommodating the electrode assembly,
On the negative electrode, an active material layer containing a silicon-based active material is formed on a metal foil,
The case is a rectangular parallelepiped aluminum or aluminum alloy square can,
When the thickness of the electrode assembly in the stacking direction is a and the distance between the electrode assembly and the inner surface of the case in the stacking direction is b, the relationship of b / a <0.04 is satisfied. Storing the electrode assembly in the case, a lithium ion secondary battery forming step of forming the lithium ion secondary battery,
An initial charge / discharge step of performing an initial charge / discharge of the lithium ion secondary battery formed in the lithium ion secondary battery forming step,
A load applying step of applying a load along the stacking direction of the electrode assembly to the case during the initial charging / discharging step,
In the load applying step, a method for manufacturing a lithium ion secondary battery, wherein the load is applied to the case by sandwiching the case in the stacking direction by a pair of sandwiching members.   The method for manufacturing a lithium ion secondary battery according to claim 1, wherein an interval between inner surfaces of the case facing each other in the stacking direction is 15 to 35 mm.   3. The method for manufacturing a lithium ion secondary battery according to claim 1, wherein the content of silicon in the active material layer of the negative electrode is 30% by mass or more. 4.   The method for manufacturing a lithium ion secondary battery according to any one of claims 1 to 3, wherein the silicon content of the active material layer of the negative electrode is 80% by mass or less.

Images