JP7153193B2 - Non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to nonaqueous electrolyte secondary batteries such as lithium ion secondary batteries. Specifically, it relates to the structure of a wound electrode assembly for a non-aqueous electrolyte secondary battery.

In recent years, the demand for non-aqueous electrolyte secondary batteries has been increasing more and more as a so-called portable power source for personal computers, mobile terminals, etc. or as a power source for driving vehicles. In particular, lithium-ion secondary batteries, which are lightweight and provide high energy density, are preferably used as high-output power sources for driving vehicles such as electric vehicles and hybrid vehicles.

A typical example of this type of secondary battery includes a flat wound electrode assembly. In general, in a cross section orthogonal to the winding axis of the wound electrode body, two R portions having curved outer surfaces are sandwiched between both R portions at both ends in the longitudinal direction. There is a central longitudinal section, section F, which has two surfaces. The wound electrode body typically has a configuration in which a positive electrode and a negative electrode (hereinafter referred to as "electrodes" when not particularly distinguished between positive and negative) are wound with a separator interposed therebetween. In such a non-aqueous electrolyte secondary battery, a negative electrode active material layer formed on the negative electrode and a positive electrode active material layer formed on the positive electrode (hereinafter referred to as “active material layer” when the positive and negative are not particularly distinguished) form a separator. They are arranged so as to face each other on both sides.
In this type of non-aqueous electrolyte secondary battery, a non-aqueous electrolyte is generally held inside the wound electrode body, and the charge carriers (for example, lithium ions) contained in the non-aqueous electrolyte are Charging and discharging are performed by going back and forth between the positive electrode active material layer containing the positive electrode active material and the negative electrode active material layer containing the negative electrode active material. Focusing on the negative electrode active material layer, charge carriers are occluded in the negative electrode active material layer during charging, and the charge carriers occluded during charging are released from the negative electrode active material layer into the non-aqueous electrolyte during discharging.

JP 2018-85180 A

By the way, one of the problems of the non-aqueous electrolyte secondary battery including the wound electrode assembly having the above-described structure is the expansion and contraction of the active material layer during charging and discharging.
As the active material expands and contracts, the non-aqueous electrolyte inside the wound electrode body may be pushed out of the wound electrode body. Further, in the case of a flat-shaped wound electrode body, stress due to expansion and contraction tends to concentrate on the R portion. The present inventors have found that the non-aqueous electrolytic solution is particularly likely to be discharged from this portion and that the non-aqueous electrolytic solution may be difficult to be retained there.
The non-aqueous electrolyte discharged to the outside of the flat-shaped wound electrode body, for example, is impregnated (penetrated) into the wound electrode body again from the constituent members of the wound electrode body. The battery reaction inside the electrode assembly cannot be supplied with charge carriers, which may lead to a decrease in battery capacity. This is not preferable because it increases the battery resistance and may cause deterioration of the battery performance.

As a means for retaining the non-aqueous electrolyte inside the wound electrode assembly during charging and discharging, the shape of the box-shaped housing (i.e., battery case) containing the wound electrode assembly and the dimensions of the wound electrode assembly are adjusted. For example, preparation of a wound electrode body having a structure in which the non-aqueous electrolyte is less likely to be discharged by adjustment (Patent Document 1). However, according to the studies of the present inventors, although the technique described in Patent Document 1 has a structure for preventing discharge of the non-aqueous electrolyte, impregnation (permeation) of the non-aqueous electrolyte into the wound electrode body is difficult. Regarding ease of use, it was confirmed that there is room for improvement.
Therefore, the present invention was created to solve the problems related to the above-described non-aqueous electrolyte secondary battery equipped with a flat wound electrode body, and even when charging and discharging are repeated at a high rate, Discharge of the non-aqueous electrolyte from the flat wound electrode body is suppressed, and the wound electrode body is provided with permeability of the non-aqueous electrolyte to prevent deterioration of battery performance due to discharge of the non-aqueous electrolyte. It is an object of the present invention to provide a non-aqueous electrolyte secondary battery that can prevent

In order to achieve the above object, according to the present invention, a long sheet-like positive electrode having a positive electrode active material layer containing a positive electrode active material and a long sheet-like negative electrode having a negative electrode active material layer containing a negative electrode active material are provided. Provided is a non-aqueous electrolyte secondary battery comprising a wound electrode assembly that is superimposed and wound around a winding shaft with a long sheet-like separator interposed therebetween, and a non-aqueous electrolyte.
In the non-aqueous electrolyte secondary battery disclosed herein, the wound electrode body has a flat shape, and in a cross section orthogonal to the winding axis, both ends in the longitudinal direction, and the outer surface is curved. and an F portion, which is a central portion in the longitudinal direction sandwiched between the R portions and has two surfaces.
In the cross section of the wound electrode body, a flat center line L is defined as a straight line PP' connecting the outer curved apexes P and P' on the outer surfaces of the two R portions farthest from the center. When defined, in the cross section of the wound electrode body, along the flat center line L, a straight line VV connecting the inner vertices V and V' of the innermost positive electrode or negative electrode in the two R portions ' is the length of the F long side length A, and the thickness VP between the inner vertex V and the outer curved vertex P closer to the vertex V is the central thickness D of the R portion, A/ (A+D) is 0.64 or more and 0.92 or less.
Moreover, the porosity of the positive electrode active material layer is 10 vol % or more and 50 vol % or less.

In the non-aqueous electrolyte secondary battery having such a configuration, expansion tends to concentrate on the R portion of the wound electrode body due to the shape of the laminated surface of the flat wound electrode body and the porosity of the positive electrode active material layer. Contraction stress can be relaxed. As a result, the electrolytic solution is less likely to be discharged from the wound electrode assembly due to the expansion and contraction, and the non-electrolyte solution can efficiently permeate into the wound electrode assembly. As a result, the non-aqueous electrolyte is easily retained inside the wound electrode body, so that even if the expansion and contraction are repeated, the battery capacity does not decrease, and an increase in battery resistance can be suppressed. As a result, deterioration of battery performance of the non-aqueous electrolyte secondary battery can be prevented.

1 is a perspective view schematically showing the outer shape of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. 1 is a schematic diagram showing a layer structure of a wound electrode body of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. 1 is a cross-sectional view schematically showing a cross-section of a wound electrode body of a non-aqueous electrolyte secondary battery according to one embodiment; FIG. 4 is a graph showing the results of a high-rate charge/discharge cycle test of a non-aqueous electrolyte secondary battery according to one embodiment.

Preferred embodiments of the secondary battery disclosed herein will be described below with appropriate reference to the drawings. Matters other than those specifically mentioned in this specification, which are necessary for carrying out the present invention, can be grasped as design matters by those skilled in the art based on the prior art in the field. The present invention can be implemented based on the contents disclosed in this specification and common general technical knowledge in the field. Further, in each drawing, the same reference numerals are given to the members and parts having the same action. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

As used herein, the term "non-aqueous electrolyte secondary battery" refers to a secondary battery in which the solvent constituting the electrolyte is mainly composed of a non-aqueous solvent (that is, an organic solvent). Here, the term "secondary battery" refers to a chargeable and dischargeable power storage device from which predetermined electrical energy can be taken out repeatedly. For example, lithium-ion secondary batteries, sodium-ion secondary batteries, etc., in which alkali metal ions in a non-aqueous electrolyte carry charge transfer, are typical examples included in the non-aqueous electrolyte secondary batteries referred to herein.
"Electrode body" refers to a structure that constitutes the main body of a battery, including a positive electrode, a negative electrode, and a porous insulating layer that can function as a separator between the positive and negative electrodes. A “positive electrode active material” or “negative electrode active material” can reversibly absorb and release chemical species that act as charge carriers (for example, lithium ions in lithium ion secondary batteries and sodium ions in sodium ion secondary batteries). compound (positive electrode active material or negative electrode active material).

In the following, as a typical example of a non-aqueous electrolyte secondary battery, the case of applying the present invention to a lithium ion secondary battery having a configuration in which a wound electrode body and a non-aqueous electrolyte are accommodated in a battery case will be mainly described. Embodiments will be specifically described. It should be noted that the embodiments described below are not intended to limit the present invention to those described in such embodiments.

As shown in FIG. 1, in a lithium ion secondary battery 100 according to the present embodiment, a flat wound electrode body is housed in a flat rectangular (box-shaped) battery case 30 together with a non-aqueous electrolyte. configuration.
The battery case 30 includes a flat rectangular parallelepiped battery case main body 32 with an open upper end, and a lid 34 that closes the opening. On the upper surface of the battery case 30 (that is, the lid 34), a positive electrode terminal 42 for external connection electrically connected to the positive electrode of the wound electrode body and a negative electrode terminal 44 electrically connected to the negative electrode of the wound electrode body are provided. is provided. The lid 34 is also provided with a safety valve 36 for discharging gas generated inside the battery case 30 to the outside of the battery case 30, like the battery case of a conventional lithium-ion secondary battery.
Examples of materials for the battery case 30 include metal materials such as aluminum and resin materials such as polyimide resin. The shape of the case (outer shape of the container) may be circular (cylindrical, coin-shaped, button-shaped), hexahedral (rectangular parallelepiped, cubic), bag-shaped, or a shape obtained by processing and deforming them. good.

As shown in FIG. 2, the wound electrode body 20 is formed by a positive electrode active material layer along the longitudinal direction on one side or both sides (here, both sides) of a long sheet-like positive electrode current collector 52 made of a metal such as aluminum. 54, and a negative electrode 60 in which a negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long negative electrode current collector 62 made of a metal such as copper. are superimposed via a long sheet-like separator 70 and wound in the longitudinal direction to form a flat shape.
In the central portion of the wound electrode body 20 in the winding axial direction, the wound core portion (that is, the positive electrode active material layer 54 of the positive electrode 50, the negative electrode active material layer 64 of the negative electrode 60, and the separator 70 are densely laminated. part) is formed. At both ends of the wound electrode assembly 20 in the winding axial direction, the positive electrode active material layer non-formed portion 52a of the positive electrode 50 and the negative electrode active material layer non-formed portion 62a of the negative electrode 60 extend outward from the wound core portion. It sticks out. A positive current collector plate and a negative current collector plate are attached to the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a, respectively, and the positive electrode terminal 42 (see FIG. 1) and the negative electrode terminal 44 (see FIG. 1) are provided. ) are electrically connected to each other.

As is clear from FIG. 3 showing a cross section perpendicular to the winding axial direction, the wound electrode body 20 according to the present embodiment has both ends in the longitudinal direction of the cross section, and the outer surface is curved from the curved surface. and an F portion 24 sandwiched between the R portions 22a and 22b in the longitudinal direction and having two surfaces. .

In the cross section of the wound electrode body 20, a straight line PP' connecting the outer curved apexes P and P' on the outer surfaces of the two R portions 22a and 22b farthest from the center of the cross section is drawn at the flat center. Define line L.
In the cross section, along the flat center line L, the length of a straight line VV' that connects the innermost inner vertexes V and V' of the positive electrode or the negative electrode in the two R portions 22a and 22b is the length of the F portion. The length of the long side of is A. Furthermore, the thickness VP between one inner vertex V and the outer curved vertex P closer to the vertex V (the thickness V between the inner vertex V' and the outer curved vertex P' closer to the vertex V''-P') is defined as the thickness D at the center of the R portion. At this time, preferably, the ratio (proportion) of A to (A+D), that is, A/(A+D) is set to be 0.64 or more and 0.92 or less (for example, 0.88 or more and 0.92 or less). .

Materials and members constituting the positive electrode 50, the negative electrode 60, and the separator 70 of the wound electrode assembly 20 can be the same as those used in conventional general non-aqueous electrolyte secondary batteries.
Preferred examples of the positive electrode active material contained in the positive electrode active material layer 54 include layered transition metal oxides containing lithium. For example, a lithium-nickel-cobalt-manganese composite oxide (eg, LiNi _0.38 Co ₀ ) having a layered structure containing at least Li, Ni, Co and Mn as constituent elements (typically, a layered rocksalt structure belonging to the hexagonal system). _.32 Mn _0.3 O ₂ ) is preferred.

The proportion of the positive electrode active material in the entire positive electrode active material layer 54 is suitably about 60% by mass or more (typically 60% by mass or more, 70% by mass or more), and usually about 82% by mass or more. It is preferably 99% by mass.
The porosity (porosity, porosity) of the positive electrode active material layer 54 is preferably, for example, 10 to 50 vol % (eg, 20 to 40 vol %). When the positive electrode active material layer 54 satisfies such properties, appropriate voids can be maintained in the positive electrode active material layer 54, and the non-aqueous electrolyte can be sufficiently permeated. As used herein, the term “porosity” refers to a value obtained by dividing the total pore volume (cm ³ ) obtained by measurement with a mercury porosimeter by the apparent volume (cm ³ ) of the active material layer and multiplying by 100. Say.

The positive electrode active material layer 54 may contain a conductive aid, a binder, and the like, if necessary.
Examples of conductive aids include acetylene black (AB), vapor-grown carbon, and ketjen black. The proportion of the conductive aid in the entire positive electrode active material layer 54 is suitably approximately 0.5% by mass or more (typically 1% by mass or more), and usually approximately 20% by mass or less (typically 1% by mass or more). In terms, it is preferably 18% by mass or less).
Examples of binders include styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVdF), butyl rubber (BR), and acrylonitrile-butadiene rubber (ABR). The proportion of the binder in the entire positive electrode active material layer 54 is suitably approximately 0.1% by mass or more (typically 0.5% by mass or more), and usually approximately 5.0% by mass or less. (typically 3.0% by mass or less).

Examples of the negative electrode active material contained in the negative electrode active material layer 64 include graphite-based materials such as natural graphite (plumbago) and artificial graphite; graphite, mesocarbon microbeads, carbon-based negative electrode active materials such as carbon black; Tin and their compounds are included.

The negative electrode active material layer 64 may contain the above-described conductive aid, binder, and the like, if necessary. In addition, an additive such as a thickening agent can be used as appropriate. Examples of thickening agents include carboxymethyl cellulose (CMC) and methyl cellulose (MC).

Examples of the separator sheet 70 include porous sheets (films) made of resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. The porous sheet may have a single-layer structure or a laminated structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer).

As the non-aqueous electrolytic solution, typically, a non-aqueous electrolytic solution containing a supporting salt (that is, an electrolyte) in a non-aqueous solvent (organic solvent) can be used.
As the non-aqueous solvent, for example, one of ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) is used alone, or Two or more kinds can be used in appropriate combination.
As the supporting electrolyte, one or more of lithium compounds (lithium salts) such as LiPF ₆ , LiBF ₄ and LiClO ₄ can be used. LiPF ₆ is particularly preferably used.

The lithium ion secondary battery 100 is manufactured using the various materials and members described above. Lithium ion secondary battery 100 concentrates on R portions 22a and 22b even during charging and discharging of wound electrode body 20 due to the shape of the cross section of the flat wound electrode body and the configuration of positive electrode active material layer 54 described above. It is possible to relax the stress of expansion and contraction that is easy to do. This suppresses the discharge of the non-aqueous electrolyte due to the expansion and contraction, and improves the permeability of the non-electrolyte to the wound electrode body 20 . As a result, the non-aqueous electrolyte is easily retained inside the wound electrode body 20, and even if the wound electrode body 20 repeats expansion and contraction due to high-rate charge/discharge, the battery capacity of the lithium ion secondary battery 100 does not become small. Therefore, an increase in battery resistance can be prevented. Therefore, according to the present invention, good battery performance of the battery can be maintained.

In addition, depending on the state of use, the non-aqueous electrolyte discharged to the outside of the wound electrode body may deposit a substance derived from the charge carrier (for example, metallic lithium) on the negative electrode active material layer 64 . In the lithium-ion secondary battery 100 according to the present embodiment, discharge of the non-aqueous electrolyte from the wound electrode body 20 is suppressed, so deposition of metallic lithium or the like on the negative electrode active material layer 64 can be prevented. can.

Several test examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in such specific examples.
<Test Example 1: Construction of Lithium Ion Secondary Battery>
LiNi _0.38 Co _0.32 Mn _0.30 O ₂ (LNCM) was prepared as a positive electrode active material powder. This LNCM, AB as a conductive material, and PVdF as a binder are put into a kneader so that the weight ratio of LNCM: AB: PVdF = 90.4: 8.1: 1.5, and N-methylpyrrolidone (NMP) to prepare a slurry for forming a positive electrode active material layer. This slurry was applied to both sides of an aluminum foil (positive electrode current collector), dried and then pressed to produce a positive electrode. Table 1 shows the porosities (vol %) of the positive electrode active material layers according to Samples 1 to 14 of this example measured by a mercury porosimeter.
Natural graphite was prepared as a negative electrode active material powder. The natural graphite, SBR as a binder, and CMC as a thickener are put into a kneader so that the weight ratio of natural graphite: SBR: CMC = 98: 1: 1, and kneaded in ion-exchanged water. , a slurry for forming a negative electrode active material layer was prepared. This slurry was applied to both sides of a copper foil (negative electrode current collector), dried and then pressed to produce a negative electrode.

The positive electrode and the negative electrode prepared above are stacked in the longitudinal direction with a three-layer structure (PP/PE/PP) made of PE and PP with two separator sheets having a thickness of 20 μm interposed therebetween. The resulting A/(A+D) is wound in the longitudinal direction with a commercially available winding machine while adjusting the winding tension as appropriate, and formed into a flat shape to produce a wound electrode assembly. did.
Next, the obtained wound electrode body was housed inside a battery case, and a non-aqueous electrolyte was injected from the opening of the battery case. As the non-aqueous electrolyte, a mixed solution containing EC, DMC, EMC and LiPF ₆ in a weight ratio of EC:DMC:EMC:LiPF ₆ =30:28:28:14 was used. Then, the opening was sealed to obtain lithium ion secondary batteries for evaluation test (ie, samples 1 to 14).

<High rate charge/discharge cycle test: measurement of IV resistance>
For the lithium ion secondary batteries according to Examples 1 to 14, after conditioning treatment, the SOC (state of charge) of each evaluation test battery was adjusted to 80% in an environment of 25 ° C., and charged at 200 A for 10 seconds. 500 cycles of high rate rectangular wave charging and discharging were performed. Then, the resistance increase rate was calculated from the IV resistance (initial resistance of the battery) before the charge-discharge cycle test and the IV resistance after the charge-discharge cycle test. Here, the rate of increase in resistance was determined by "IV resistance after charge/discharge cycle test/IV resistance before charge/discharge cycle test".
The test results are shown in Table 1 and FIG. Here, in FIG. 4, the resistance increase rate of each battery for evaluation test is shown in parentheses, and when this numerical value is 1, it means that the resistance did not increase.

As is clear from the results shown in Table 1 and FIG. 4, the porosity of the positive electrode active material layer was adjusted to 10 vol% or more and 50 vol% or less, and A/(A+D) was adjusted to 0.64 or more and 0.92 or less. In the lithium ion batteries according to samples 1 to 6, the battery resistance is lower than that of the lithium ion batteries of samples 7 to 14, in which the porosity and A/(A+D) of the positive electrode active material layer are not set within such ranges. was able to suppress the increase in
From the above results, the lithium ion secondary battery according to the present embodiment has excellent high-rate cycle characteristics (that is, maintenance of good battery performance).

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. Even in that case, the same effect as the effect illustrated above can be exhibited.

20 Wound electrode assembly 22a(b) R portion 24 F portion 30 Battery case 32 Battery case main body 34 Lid 36 Safety valve 42 Positive electrode terminal 44 Negative electrode terminal 50 Positive electrode (positive electrode sheet)
52 positive electrode current collector 52a positive electrode active material layer non-formed portion 54 positive electrode active material layer 60 negative electrode (negative electrode sheet)
62 Negative electrode current collector 62a Negative electrode active material layer non-formed portion 64 Negative electrode active material layer 70 Separator 100 Lithium ion secondary battery P, P' Outer curved vertices V, V' Inner apex L Flat center line AF Long side length of F part Thickness at center of D R part

Claims (1)

A long sheet-shaped positive electrode having a positive electrode active material layer containing a positive electrode active material and a long sheet-shaped negative electrode having a negative electrode active material layer containing a negative electrode active material are interposed with a long sheet-shaped separator. a wound electrode body that is overlapped and wound around a winding axis;
Non-aqueous electrolyte and
A non-aqueous electrolyte secondary battery comprising
The positive electrode active material is composed of a lithium-nickel-cobalt-manganese composite oxide,
The negative electrode active material is composed of a graphite-based material and does not contain a lithium-titanium composite oxide,
The wound electrode body has a flat shape, and in a cross section orthogonal to the winding axis,
Two R portions having curved outer surfaces at both ends in the longitudinal direction,
an F section having two surfaces, which is a central portion in the longitudinal direction sandwiched between both R sections;
and
In the transverse section of the wound electrode body, a flat center line L is defined as a straight line PP' connecting the outer curved apexes P and P' on the outer surfaces of the two R portions farthest from the center. when
In the cross section of the wound electrode body, along the flat center line L, the length of a straight line VV' that connects the inner vertexes V and V' of the positive electrode or the negative electrode that are innermost in the two R portions is the F long side length A, and the thickness VP between one of the inner vertices V and the outer curved apex P closer to the vertex V is the R part center thickness D,
A / (A + D) is 0.64 or more and 0.92 or less,
Here, the porosity of the positive electrode active material layer is 10 vol% or more20A non-aqueous electrolyte secondary battery that is vol % or less.

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