Non-aqueous electrolyte secondary batteries

Abstract

This non-electrolyte secondary battery comprises: a positive electrode which has a positive electrode mixture layer including a positive electrode active material; a negative electrode which has a negative electrode mixture layer including a negative electrode active material; and a separator which is disposed between facing surfaces of the positive electrode and the negative electrode, and through which lithium ions pass. The separator is provided with a masking region in which fine pores are sealed with paraffin to prevent the lithium ions from passing therethrough.

Description

[Technical Field]

[0001] The present disclosure relates to a nonaqueous electrolyte secondary battery including: a positive electrode having a positive electrode mixture layer containing a positive electrode active material; a negative electrode having a negative electrode mixture layer containing a negative electrode active material; and a separator that is disposed between opposing surfaces of the positive electrode and the negative electrode and is permeable to lithium ions. [Background technology]

[0002] Lithium-ion batteries, which are non-aqueous electrolyte secondary batteries, are widely used as secondary batteries, and wound-type batteries in which electrodes are wound to increase capacity are widely used. In such wound-type batteries, there are cases where the negative electrode is not located at a position corresponding to the positive electrode due to miswinding or other reasons. In such cases, lithium ions from the positive electrode that does not have a corresponding negative electrode also migrate to the end of the negative electrode facing the positive electrode, making it easier for metallic lithium to precipitate. For this reason, Patent Document 1 discloses a method of preventing lithium precipitation by providing a region at the end of the separator that is impermeable to lithium ions, thereby reducing the number of lithium ions that reach the end of the negative electrode. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-190785 Summary of the Invention

[0004] In non-aqueous electrolyte secondary batteries, there is a need to suppress battery degradation, and various proposals have been made to address this issue. While the capacity of a battery decreases with repeated charge and discharge, this capacity decrease is not linear; (i) there is a relatively large initial decrease, (ii) there is a plateau where the capacity does not change much for a while, and (iii) thereafter the rate of decrease increases again.

[0005] Therefore, we thought that if we could intentionally set the stable state of (ii), we might be able to suppress the decrease in capacity.

[0006] The nonaqueous electrolyte secondary battery according to the present disclosure is a nonaqueous electrolyte secondary battery including: a positive electrode having a positive electrode mixture layer containing a positive electrode active material; a negative electrode having a negative electrode mixture layer containing a negative electrode active material; and a separator disposed between opposing surfaces of the positive electrode and the negative electrode and through which lithium ions are permeable. By providing the separator with a masking region through which lithium ions are inhibited from permeating, it is possible to limit the release of lithium ions from the negative electrode mixture layer during discharge, and to stop discharging before the negative electrode voltage begins to rise sharply just before full discharge.

[0007] According to the present disclosure, by providing a masking region, Li is unevenly distributed within the electrode, and side reactions in the region close to full discharge are suppressed, thereby suppressing a decrease in capacity.

[0008] Furthermore, by forming the masking region using paraffin, it is possible to easily form an appropriate masking region. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a graph showing the change in capacity with respect to the number of charge / discharge cycles. [Figure 2] FIG. 2 is a diagram showing the electrode potential (V (vs. Li / Li+)) during charge and discharge. [Figure 3] 3A and 3B are diagrams showing the state of Li in the positive and negative electrodes during charging and discharging, where FIG. 3A shows a conventional battery and FIG. 3B shows a battery according to the present disclosure. [Figure 4] FIG. 4 is a longitudinal cross-sectional view of a cylindrical secondary battery 10 as an example of the embodiment. [Figure 5] FIG. 5 is a perspective view showing the configuration of the electrode body. [Figure 6] FIG. 6 is a diagram showing various masking patterns. [Figure 7] FIG. 7 is a front view of the positive electrode and negative electrode that constitute the electrode assembly. [Figure 8] FIG. 8 is a diagram showing the capacity maintenance rate when various masking regions are formed using a coin cell. [Figure 9] FIG. 9 is a diagram showing the change in capacity depending on the number of cycles in Examples 1, 2, and 7 and the comparative example. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. However, the present disclosure is not limited to the embodiments described herein.

[0011] "Summary of the Disclosure" As described above, the nonaqueous electrolyte secondary battery according to the present disclosure achieves a stable period in which the change in the state of charge relative to the number of charge / discharge cycles is small.

[0012] Figure 1 shows the change in capacity versus the number of charge-discharge cycles for a cylindrical 18650-size graphite battery cell (Ni-Co-Mn ternary (NCM) / graphite) when it is repeatedly charged and discharged at 25°C and 0.5C. As shown, the capacity loss is large at the beginning of the cycle (i), but is small and stable in the middle of the cycle (ii). Thereafter, the rate of loss increases at the end of the cycle (iii), reaching the end of the battery's life.

[0013] Therefore, we disassembled the battery in a discharged state at the end of the initial cycle (i). Observation of the negative electrode revealed that the area near the top and bottom ends had turned blue. The color of the negative electrode changes as it absorbs Li ions, which is why the colors at the ends and the center are different. This shows that Li is unevenly distributed in the negative electrode in the battery at the end of the initial cycle (i). Therefore, it may be possible to suppress the initial capacity loss by actively creating uneven distribution of Li in the electrode.

[0014] Figure 2 shows the electrode potential (V) vs. Li / Li during charging and discharging. +)). The positive electrode potential varies according to the state of charge (SOC), following a relatively smooth, nearly linear curve. In this example, the positive electrode potential is 3.65 V in the discharged state and approximately 4.4 V at full charge. Meanwhile, the negative electrode potential rises rapidly from a few percent of the SOC, close to full discharge. Discharge ends when the interelectrode potential reaches approximately 3.0 V, so the negative electrode potential at this point is approximately 0.6 V, after the negative electrode potential has risen sharply from approximately 0.3 V. This rapid rise in negative electrode potential indicates the possibility of a side reaction occurring in addition to the normal battery charge / discharge reactions. Therefore, it is believed that deterioration can be suppressed by suppressing this side reaction. To achieve this, it is conceivable to stop discharging the negative electrode before the plateau.

[0015] Therefore, in the present disclosure, a masking region that does not allow Li ions to pass through is formed in the separator, which allows Li to be unevenly distributed in the electrode and enables discharge to be stopped before the negative electrode potential rises sharply.

[0016] FIG. 3 shows the state of Li in the positive and negative electrodes during charging and discharging, with FIG. 3(a) showing a conventional battery and FIG. 3(b) showing the battery of the present disclosure.

[0017] Thus, in conventional batteries, in the initial state (discharged state), Li is present at the positive electrode, moves to the negative electrode upon charging, and moves to the positive electrode upon discharging, and this process is repeated with charging and discharging. On the other hand, in the battery of the present disclosure, a masking region is formed on the separator. Therefore, as in conventional batteries, Li is present at the positive electrode in the initial state (discharged state), but when Li moves to the negative electrode upon charging, the masking region gets in the way, and Li in the area corresponding to the masking region on the positive electrode remains without moving to the negative electrode. Then, Li spreads in the positive and negative electrodes, but charging ends in an unevenly distributed state. Note that the voltage gradient in the electrodes is larger at the positive electrode, so the voltage v for Li movement in the positive electrode is + is the voltage for Li migration in the negative electrode, v - Furthermore, when the battery is next discharged, the masked area remains, and the discharge ends with Li unevenly distributed in the electrode, with some Li remaining in the negative electrode.

[0018] In this way, Li is unevenly distributed in the electrode, and discharge can be stopped in a state where the release of Li from the negative electrode is incomplete (before the potential of the negative electrode rises).

[0019] "Description of the Embodiments" An example of an embodiment of a nonaqueous electrolyte secondary battery according to the present disclosure will be described below. Hereinafter, a cylindrical battery in which a wound electrode assembly is housed in a cylindrical outer can with a bottom will be exemplified. However, the outer can is not limited to a cylindrical outer can and may be, for example, a rectangular outer can or an outer can made of a laminate sheet including a metal layer and a resin layer. Furthermore, the electrode assembly may be a laminated electrode assembly in which multiple positive electrodes and multiple negative electrodes are alternately stacked with separators interposed therebetween.

[0020] <Overall structure of wound battery> FIG. 4 is a longitudinal cross-sectional view of a cylindrical secondary battery 10 according to an embodiment. The secondary battery 10 shown in FIG. 4 includes an electrode assembly 14 and a nonaqueous electrolyte housed in an exterior housing 15. The electrode assembly 14 has a wound structure in which a positive electrode 11 and a negative electrode 12 are wound with a separator 13 interposed therebetween. Examples of nonaqueous solvents (organic solvents) for the nonaqueous electrolyte include carbonates, lactones, ethers, ketones, and esters, and two or more of these solvents can be mixed together. When two or more solvents are mixed together, a mixed solvent containing a cyclic carbonate and a chain carbonate is preferably used. For example, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and the like can be used as the cyclic carbonate, and dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and the like can be used as the chain carbonate. Examples of electrolyte salts that can be used for the non-aqueous electrolyte include LiPF, LiBF, LiCF, SO, and mixtures thereof. The amount of electrolyte salt dissolved in the non-aqueous solvent can be, for example, 0.5 to 2.0 mol / L. For ease of explanation, the following description will refer to the sealing body 16 side as "top" and the bottom side of the exterior body 15 as "bottom."

[0021] The open end of the exterior body 15 is sealed with the sealing body 16, thereby sealing the interior of the secondary battery 10. Insulating plates 17 and 18 are provided above and below the electrode body 14. The positive electrode lead 19 extends upward through a through hole in the insulating plate 17 and is welded to the underside of a filter 22, which is the bottom plate of the sealing body 16. In the secondary battery 10, a cap 26, which is the top plate of the sealing body 16 and is electrically connected to the filter 22, serves as the positive electrode terminal. On the other hand, the negative electrode lead 20 extends through a through hole in the insulating plate 18 toward the bottom of the exterior body 15 and is welded to the inner bottom surface of the exterior body 15. In the secondary battery 10, the exterior body 15 serves as the negative electrode terminal. Note that if the negative electrode lead 20 is installed at the terminal end, the negative electrode lead 20 passes outside the insulating plate 18, extends toward the bottom of the exterior body 15, and is welded to the inner bottom surface of the exterior body 15.

[0022] Exterior body 15 is, for example, a cylindrical metal exterior can with a bottom. A gasket 27 is provided between exterior body 15 and sealing body 16, ensuring the airtightness of the interior of secondary battery 10. Exterior body 15 has a grooved portion 21 that supports sealing body 16, formed, for example, by pressing the side surface from the outside. Grooved portion 21 is preferably formed in an annular shape along the circumferential direction of exterior body 15, and supports sealing body 16 on its upper surface via gasket 27.

[0023] The sealing body 16 includes a filter 22, a lower valve body 23, an insulating member 24, an upper valve body 25, and a cap 26, which are stacked in this order from the electrode body 14 side. Each component of the sealing body 16 has, for example, a disk or ring shape, and all components except for the insulating member 24 are electrically connected to each other. The lower valve body 23 and the upper valve body 25 are connected to each other at their respective centers, with the insulating member 24 interposed between their respective peripheral edges. If the internal pressure of the battery increases due to abnormal heat generation, for example, the lower valve body 23 may break, causing the upper valve body 25 to bulge toward the cap 26 and separate from the lower valve body 25, thereby cutting off the electrical connection between them. If the internal pressure continues to increase, the upper valve body 25 may break, and gas may be released from the opening 26a of the cap 26.

[0024] <Electrode body configuration> Next, the electrode assembly 14 will be described with reference to FIG. 5. FIG. 5 is a perspective view of the electrode assembly 14. As described above, the electrode assembly 14 has a wound structure in which the positive electrode 11 and the negative electrode 12 are spirally wound with the separator 13 interposed therebetween. The positive electrode 11, the negative electrode 12, and the separator 13 are all formed in strip shapes and spirally wound around a winding core disposed along the winding axis 28, resulting in a state in which they are alternately stacked in the radial direction of the electrode assembly 14. In the radial direction, the side of the winding axis 28 is referred to as the inner peripheral side, and the opposite side is referred to as the outer peripheral side. In the electrode assembly 14, the longitudinal direction of the positive electrode 11 and the negative electrode 12 is the winding direction, and the strip width direction of the positive electrode 11 and the negative electrode 12 is the axial direction. The positive electrode lead 19 extends axially from approximately the center in the radial direction between the center and the outermost periphery at the upper end of the electrode assembly 14. Furthermore, the negative electrode lead 20 extends in the axial direction from the vicinity of the winding axis 28 at the lower end of the electrode body 14 .

[0025] <Separator configuration> A porous sheet having ion permeability and insulating properties is used for the separator 13. Specific examples of the porous sheet include a microporous thin film, a woven fabric, and a nonwoven fabric. The material for the separator 13 is preferably an olefin resin such as polyethylene or polypropylene. The thickness of the separator 13 is, for example, 10 μm to 50 μm. Separators 13 tend to be thinner as the capacity and output of batteries increase. The separator 13 has a melting point of, for example, about 130°C to 180°C.

[0026] <Masking area> In this embodiment, strip-shaped masking regions 13a are provided at the upper and lower ends of the separator 13. The pores of the masking regions 13a are sealed with, for example, paraffin, so that they cannot pass through Li ions.

[0027] Paraffin is solid at room temperature but becomes liquid at temperatures above 80°C, for example. Therefore, the masking region 13a can be formed by impregnating the separator 13 with liquid paraffin. The temperature of the liquid paraffin is set lower than the melting point of the separator. Various types of paraffin with different properties are commercially available. For example, if the drying temperature of the battery cell is 105°C and the melting point of the separator is about 135°C, paraffin with a melting point of about 100 to 120°C is considered suitable.

[0028] Liquid paraffin quickly penetrates into the resin separator 13, making it very easy to process. Therefore, masking areas of various patterns can be formed by printing. In addition, paraffin can be used in a variety of ways, such as (C n H 2n+2 ), and its main skeleton is the same as that of polyethylene, making it resistant to oxidation and reduction, and it is also inexpensive. Furthermore, because it is non-polar, it does not dissolve when mixed with the electrolyte. Furthermore, the masked area simply becomes transparent, with no change in thickness, making it less likely to develop unevenness or wrinkles. In particular, using polyethylene for the separator 13 makes it easier to seal the pores with paraffin.

[0029] The masking area can be formed by applying Li-ion impermeable tape or by thermal dissolution, but care must be taken as tape is prone to creating unevenness and thermal dissolution is prone to creating wrinkles.

[0030] <Masking pattern> In this embodiment, the provision of a masking region allows Li to be unevenly distributed within the electrode. Therefore, a masking region may be provided inside the battery as long as it partially restricts the movement of Li ions. In particular, if the area without the masking region becomes large, the area in which the effect of restricting the movement of Li ions is not achieved increases, so it is preferable to provide masking regions at some intervals inside the battery.

[0031] The area of ​​the masking region is, for example, 0.1% or more, preferably 0.5% or more, more preferably 1% or more, and particularly preferably 5% or more, of the area of ​​the separator 13. For example, if the area of ​​the masking region is 5% or more, the effect of improving the capacity retention rate becomes more significant. On the other hand, the upper limit of the area of ​​the masking region is preferably 50% or less, more preferably 30% or less, from the viewpoint of ensuring battery capacity, etc.

[0032] FIG. 6 shows various masking patterns. As such, the position of the masking regions is not limited to the edges of the battery, but may be provided discretely within the battery, or may be in a grid pattern. The masking regions within the separator may be uniformly distributed, more near the center, or more near the edges, and may be varied to improve the characteristics. Furthermore, in the case of a grid pattern, the non-masked regions are surrounded by the masking regions, so that in the event of a short circuit within these regions, the masking regions can function as a barrier to prevent thermal runaway, thereby improving safety.

[0033] <Positive and negative electrode configuration> FIG. 7 is a front view of the positive electrode 11 and negative electrode 12 that constitute the electrode assembly 14. FIG. 7 shows the positive electrode 11 and negative electrode 12 in a developed state. In this electrode assembly 14, the negative electrode 12 is formed larger than the positive electrode 11 to prevent lithium deposition on the negative electrode 12. Specifically, the length of the negative electrode 12 in the strip width direction (axial direction) is larger than the length of the positive electrode 11 in the strip width direction. Furthermore, the length of the negative electrode 12 in the longitudinal direction is larger than the length of the positive electrode 11 in the longitudinal direction. As a result, when wound into the electrode assembly 14, at least the portion of the positive electrode 11 where the positive electrode mixture layer 32 is formed is disposed opposite the portion of the negative electrode 12 where the negative electrode mixture layer 42 is formed, with the separator 13 interposed therebetween.

[0034] <Positive electrode> The positive electrode 11 has a strip-shaped positive electrode current collector 30 and a positive electrode mixture layer 32 formed on the positive electrode current collector 30. The positive electrode mixture layer 32 is formed on at least one of the inner peripheral side and the outer peripheral side of the positive electrode current collector 30. For the positive electrode current collector 30, a metal foil such as aluminum or a film having the metal disposed on the surface layer is used. A preferred positive electrode current collector 30 is a metal foil mainly composed of aluminum or an aluminum alloy. The thickness of the positive electrode current collector 30 is, for example, 10 μm to 30 μm.

[0035] The positive electrode mixture layer 32 is preferably formed over the entire area excluding the positive electrode exposed portion 34 described later on both surfaces of the positive electrode current collector 30. The positive electrode mixture layer 32 preferably contains a positive electrode active material, a conductive agent, and a binder. The positive electrode mixture layer 32 is formed by applying a positive electrode mixture slurry containing a positive electrode active material, a conductive agent, a binder, and a solvent such as N-methyl-2-pyrrolidone (NMP) onto both surfaces of the positive electrode current collector 30 and drying it. Thereafter, the positive electrode mixture layer 32 is compressed.

[0036] Examples of the positive electrode active material include lithium-containing transition metal oxides containing transition metal elements such as Co, Mn, and Ni. The lithium-containing transition metal oxide is not particularly limited, but is preferably a composite oxide represented by the general formula Li 1+x MO2 (where -0.2 < x ≦ 0.2 and M contains at least one of Ni, Co, Mn, and Al). For example, NCA in which a part of nickel is substituted with cobalt and aluminum is added is used.

[0037] Examples of the conductive agent contained in the positive electrode mixture layer 32 include carbon materials such as carbon black (CB), acetylene black (AB), ketjen black, and graphite.

[0038] Examples of binders contained in the positive electrode mixture layer 32 include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. When preparing the positive electrode mixture slurry using an aqueous solvent, styrene butadiene rubber (SBR), nitrile rubber (NBR), CMC or its salt, polyacrylic acid or its salt, polyvinyl alcohol, and the like can be used. From the viewpoint of flexibility of the positive electrode 11, rubber-based resins having a molecular structure of repeated double bonds and single bonds, such as SBR and NBR, are preferred as binders. These may be used alone or in combination of two or more. The content of the binder in the positive electrode mixture layer 32 is 0.5% by mass to 10% by mass, and preferably 1% by mass to 5% by mass.

[0039] The positive electrode 11 is provided with a positive electrode exposed portion 34, where the surface of the positive electrode current collector 30 is exposed. The positive electrode exposed portion 34 is a portion to which the positive electrode lead 19 is connected, and is a portion of the surface of the positive electrode current collector 30 that is not covered with the positive electrode mixture layer 32. The positive electrode exposed portion 34 is formed to be wider in the longitudinal direction than the positive electrode lead 19. The positive electrode exposed portions 34 are preferably provided on both sides of the positive electrode 11 so as to overlap in the thickness direction of the positive electrode 11. The positive electrode lead 19 is joined to the positive electrode exposed portion 34 by, for example, ultrasonic welding.

[0040] <Negative electrode> The negative electrode 12 has a strip-shaped negative electrode current collector 40 and a negative electrode mixture layer 42 formed on both sides of the negative electrode current collector 40. The negative electrode current collector 40 may be, for example, a foil of a metal such as copper, or a film having such a metal disposed on its surface. The thickness of the negative electrode current collector 40 is, for example, 5 μm to 30 μm.

[0041] The negative electrode mixture layer 42 is preferably formed on the entire surface of each of the negative electrode current collectors 40, excluding a negative electrode exposed portion 44, which will be described later. The negative electrode mixture layer 42 preferably contains a negative electrode active material and a binder. The negative electrode mixture layer 42 is formed by applying a negative electrode mixture slurry containing a negative electrode active material, a binder, and a solvent such as water to each of the two surfaces of the negative electrode current collectors 40 and drying the slurry. Thereafter, the negative electrode mixture layer 42 is compressed.

[0042] The negative electrode active material is not particularly limited as long as it can reversibly absorb and release lithium (Li) ions, and examples thereof include carbon materials such as natural graphite and artificial graphite, metals that alloy with lithium such as Si and Sn, or alloys or oxides containing these. For example, graphite may be used in combination with a first silicon material (hereinafter referred to as "SiO") containing a silicon oxide phase and silicon dispersed within the silicon oxide phase, and a second silicon material (hereinafter referred to as "LSX") containing a lithium silicate phase and silicon dispersed within the lithium silicate phase, as the silicon-based active material.

[0043] The binder contained in the negative electrode mixture layer 42 is typically made of resin (resin binder), and examples thereof include fluorine-based resins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide (PI), acrylic resins, and polyolefin resins. When preparing the negative electrode mixture slurry using an aqueous solvent, styrene butadiene rubber (SBR), nitrile rubber (NBR), CMC or a salt thereof, polyacrylic acid or a salt thereof, polyvinyl alcohol, and the like can be used. From the viewpoint of flexibility of the negative electrode 12, rubber-based resins having a molecular structure of repeated double bonds and single bonds, such as SBR and NBR, are preferred as the binder. These may be used alone or in combination of two or more. The binder content in the negative electrode mixture layer 42 is 0.5% by mass to 10% by mass, and preferably 1% by mass to 5% by mass.

[0044] 7, the starting end 42a of the negative electrode mixture layer 42 is a portion adjacent to the negative electrode exposed portion 44. On the other hand, the ending end 42b of the negative electrode mixture layer 42 is the same as the ending end of the negative electrode 12. The negative electrode mixture layer 42 exists continuously from the starting end 42a to the ending end 42b.

[0045] <Example> The present disclosure will be further described below with reference to examples, but the present disclosure is not limited to these examples.

[0046] Figure 8 shows the capacity retention rate when various masking regions are formed using a coin cell. 0.91 Co ** Al ** The negative electrode active material was a mixture of graphite and (SiO+LSX) in a mass ratio of 94:6.

[0047] In the cycle test, each battery of the example and comparative example was charged at a constant current of 0.3 C in a temperature environment of 25°C until the battery voltage reached 4.0 V, and then charged at a constant voltage until the current value reached 1 / 50 C at 4.0 V. Subsequently, the battery was discharged at a constant current of 0.5 C until the battery voltage reached 2.5 V. This charge / discharge cycle was repeated 100 times.

[0048] The masking area (%) is the ratio of the masked area to the area where the positive and negative electrodes function as electrodes face each other, the capacity (%) is the ratio when no masking is taken as 100, and the 100-cycle capacity retention rate is the ratio of the battery capacity after 100 charge / discharge cycles to the initial capacity. The masking pattern is a schematic representation of the masking area.

[0049] The comparative example has no masking region, Example 1 has a masking region only on the outer periphery, Examples 2 to 6 have masking regions on the outer periphery and inside, and Example 7 has a masking region only on the inside.

[0050] These results show that providing a masking area increases the capacity retention rate regardless of the location. Furthermore, even a small masking area of ​​0.1% is effective. On the other hand, if the masking area exceeds 30%, the capacity becomes too small, so the upper limit should be around 30%.

[0051] 9 shows the change in capacity with the number of cycles in Examples 1, 2, and 7, and the Comparative Example. As shown, the Examples show less capacity loss than the Comparative Example. Furthermore, providing a masking region on the inside also reduces capacity loss. [Explanation of symbols]

[0052] 10 Secondary battery 11 Positive electrode 12 Negative electrode 13 Separator 14 Electrode body 15 Exterior body 16 Sealing body 17,18 Insulating plate 19 Positive lead 20 Negative lead 21 Grooved part 22 filters 23 Lower valve body 24 Insulating material 25 Upper valve 26 Cap 26a opening 27 Gasket 28 Winding shaft 30 Positive electrode current collector 32 Positive electrode mixture layer 34 Positive electrode exposed part 40 Negative electrode current collector 42 negative electrode mixture layer 42-1 Outer negative electrode mixture layer 42-2 Inner negative electrode mixture layer 44 Negative electrode exposed part

Claims

1. A non-aqueous electrolyte secondary battery including: a positive electrode having a positive electrode mixture layer containing a positive electrode active material; a negative electrode having a negative electrode mixture layer containing a negative electrode active material; and a separator that is disposed between opposing surfaces of the positive electrode and the negative electrode and through which lithium ions can pass, By providing the separator with a plurality of masked regions and an unmasked region so that a plurality of masked regions in which lithium ion permeation is suppressed are provided at intervals, lithium ions are unevenly distributed in the positive electrode and the negative electrode, and release of lithium ions from the negative electrode mixture layer during discharge is restricted, and discharge can be stopped before the negative electrode voltage begins to rise sharply just before complete discharge; the masked area in the separator is set to 5% or more and less than 30%; the masking region is formed by sealing the pores of the separator with paraffin impregnated in the pores.

2. A non-aqueous electrolyte secondary battery as described in claim 1, wherein the masking area of ​​the separator is 12% or more and 21% or less.

3. The positive electrode and the negative electrode form an electrode body wound with the separator interposed therebetween, 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the masking region is provided inside the separator and away from an end portion thereof, the end portion being an end portion in a winding axis direction of the electrode body of the nonaqueous electrolyte secondary battery.

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