JP7314191B2 - lithium ion secondary battery - Google Patents

Description

The present invention relates to lithium ion secondary batteries.

In recent years, lithium-ion secondary batteries have been favorably used as portable power sources for personal computers, mobile terminals, and the like, and power sources for driving vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV).

A positive electrode active material such as a lithium transition metal composite oxide is generally used for the positive electrode of a lithium ion secondary battery. It is known that when a transition metal is eluted from such a positive electrode active material by an acid or the like generated in a non-aqueous electrolyte, and the eluted transition metal is deposited on the negative electrode, lithium (Li) is easily deposited. In order to suppress the deposition of Li due to the elution of this transition metal, Patent Document 1 proposes to use a positive electrode active material with a low Ni composition at the widthwise ends of the positive electrode active material layer and use a positive electrode active material with a high Ni composition in the central portion.

JP 2019-149269 A

However, in the conventional technology described above, since a positive electrode active material with a high energy density and a high Ni composition is used in combination with a positive electrode active material with a low energy density and a low Ni composition, the energy density of the lithium ion secondary battery is lowered. Therefore, when positive electrode active materials having different compositions are used in combination, the characteristics of the lithium ion secondary battery may deteriorate, and there is room for improvement in this respect.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to suppress deposition of lithium in a lithium ion secondary battery without using a positive electrode active material having a different composition.

A lithium-ion secondary battery disclosed herein includes an electrode body in which a positive electrode and a negative electrode are laminated. The positive electrode has a positive electrode active material layer. The negative electrode has a negative electrode active material layer. The negative electrode active material layer has a dimension in at least one direction larger than that of the positive electrode active material layer. The negative electrode active material layer has a facing region facing the positive electrode active material layer and a non-facing region not facing the positive electrode active material layer. In the direction, the positive electrode active material layer has a central region containing secondary positive electrode active material particles and end regions containing single positive electrode active material particles. In the end regions, the mass ratio of the single positive electrode active material particles to the total mass of the positive electrode active material particles is 30% by mass or more. The mass ratio of the secondary particle-like positive electrode active material particles to the total mass of the positive electrode active material particles in the central region is greater than the mass ratio of the secondary particle-like positive electrode active material particles to the total mass of the positive electrode active material particles in the end regions. A ratio of the dimension of the end region of the positive electrode active material layer to the dimension of the non-facing region of the negative electrode active material layer in the direction is 2 or more and 7.5 or less. According to such a configuration, deposition of lithium can be suppressed in the lithium ion secondary battery without using positive electrode active materials having different compositions.

In a preferred embodiment of the lithium ion secondary battery disclosed herein, the dimension ratio is 2.5 or more and 6.2 or less. According to such a configuration, deposition of lithium can be further suppressed.

In a preferred aspect of the lithium-ion secondary battery disclosed herein, in the end region, the mass ratio of the single-particle active material particles to the total mass of the positive electrode active material particles is 50% by mass or more. According to such a configuration, deposition of lithium can be further suppressed.

In a preferred embodiment of the lithium ion secondary battery disclosed herein, the negative electrode active material layer contains a negative electrode active material layer, and the negative electrode active material is graphite. According to such a configuration, deposition of lithium can be particularly suppressed.

In a preferred embodiment of the lithium ion secondary battery disclosed herein, the positive electrode active material particles contained in the end regions and the positive electrode active material particles contained in the central region are particles of a lithium-nickel-cobalt-manganese-based composite oxide, and the molar ratio of Ni in the metal other than Li in the lithium-nickel-cobalt-manganese-based composite oxide contained in the end regions is 0.9 times or more and 1.1 times or less than the molar ratio of Ni in the metal other than Li in the lithium-nickel-cobalt-manganese-based composite oxide contained in the central region. According to such a configuration, it is possible to prevent the deterioration of the characteristics of the lithium ion secondary battery due to the combined use of positive electrode active materials with greatly different compositions, and it is possible to suppress the fact that the metal elution becomes uneven due to the difference in metal elution behavior due to the difference in the composition of the positive electrode active material, and the lithium deposition and elution effect weakens in part of the positive electrode active material layer.

1 is a cross-sectional view schematically showing the internal structure of a lithium ion secondary battery according to one embodiment of the present invention; FIG. 1 is a schematic exploded view showing the configuration of a wound electrode body of a lithium ion secondary battery according to one embodiment of the present invention; FIG. 1 is a schematic cross-sectional view showing configurations of a positive electrode and a negative electrode of a lithium ion secondary battery according to one embodiment of the present invention; FIG.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings. Matters not mentioned in this specification but necessary for the implementation of the present invention can be grasped as design matters by those skilled in the art based on the prior art in the relevant field. The present invention can be implemented based on the contents disclosed in this specification and common general technical knowledge in the field. Moreover, in the following drawings, members and parts having the same function are denoted by the same reference numerals. Also, the dimensional relationships (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relationships.

In this specification, the term “secondary battery” refers to an electricity storage device that can be repeatedly charged and discharged, and is a term that includes so-called storage batteries and electricity storage elements such as electric double layer capacitors. In this specification, the term “lithium ion secondary battery” refers to a secondary battery that utilizes lithium ions as a charge carrier and is charged/discharged by the transfer of charge associated with the lithium ions between the positive and negative electrodes.

Hereinafter, the present invention will be described in detail by taking as an example a flat prismatic lithium ion secondary battery having a flat wound electrode body and a flat battery case, but the present invention is described in the embodiments. It is not intended to be limited.

The lithium ion secondary battery 100 shown in FIG. 1 is a sealed battery constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte 80 in a flat rectangular battery case (that is, an outer container) 30. The battery case 30 is provided with a positive terminal 42 and a negative terminal 44 for external connection, and a thin safety valve 36 that is set to release the internal pressure of the battery case 30 when it rises above a predetermined level. Further, the battery case 30 is provided with an injection port (not shown) for injecting the non-aqueous electrolyte 80 . The positive terminal 42 is electrically connected to the positive collector plate 42a. The negative terminal 44 is electrically connected to the negative collector plate 44a. As the material of the battery case 30, for example, a metal material such as aluminum that is lightweight and has good thermal conductivity is used. Note that FIG. 1 does not accurately represent the amount of the non-aqueous electrolyte 80 .

As shown in FIGS. 1 and 2, the wound electrode body 20 has a configuration in which a positive electrode sheet 50 and a negative electrode sheet 60 are superimposed with two long separator sheets 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one side or both sides (here, both sides) of a long positive electrode current collector 52 . The negative electrode sheet 60 has a configuration 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 . The positive electrode active material layer non-formed portion 52a (i.e., the portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54 formed) and the negative electrode active material layer non-formed portion 62a (i.e., the portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64 formed) are formed to protrude outward from both ends of the wound electrode body 20 in the winding axial direction (i.e., the sheet width direction perpendicular to the longitudinal direction). A positive collector plate 42a and a negative collector plate 44a are joined to the positive electrode active material layer non-formed portion 52a and the negative electrode active material layer non-formed portion 62a, respectively.

The configurations of the positive electrode sheet 50 and the negative electrode sheet 60 will be described in more detail with reference to FIG. FIG. 3 shows cross-sectional views of the positive electrode sheet 50 and the negative electrode sheet 60 along the thickness direction (D direction in FIG. 3) and the width direction (W direction in FIG. 3).

As shown in FIG. 3 , the negative electrode active material layer 64 has a dimension in the width direction (ie, width) that is larger than the dimension in the width direction (ie, width) of the positive electrode active material layer 54 . The negative electrode active material layer 64 faces the positive electrode active material layer 54 . However, since the negative electrode active material layer 64 is wider than the positive electrode active material layer 54, it has a facing region 64A that faces the positive electrode active material layer 54 and non-facing regions 64B and 64C that do not face the positive electrode active material layer 54. The non-opposing regions 64B and 64C are located at the ends of the negative electrode active material layer 64 in the width direction.

Although the dimension in the W direction of the non-facing regions 64B and 64C of the negative electrode active material layer 64 is not particularly limited, it is preferably 1 mm or more and 3 mm or less, and more preferably 1 mm or more and 2 mm or less. The width of the negative electrode active material layer 64 is, for example, more than 1.00 times and 1.10 times or less (preferably 1.01 times or more and 1.06 times or less) the width of the positive electrode active material layer 54, but is not limited thereto.

On the other hand, in the width direction, the positive electrode active material layer 54 has a central region 54A and end regions 54B and 54C. The central region 54A is a region sandwiched between the end regions 54B and 54C.

A central region 54A of the positive electrode active material layer 54 contains secondary positive electrode active material particles. Secondary particles are particles formed by agglomeration of primary particles. On the other hand, the end regions 54B and 54C of the positive electrode active material layer 54 contain single-particle positive electrode active material particles.

A "single grain" as used herein is a grain produced by the growth of a single crystal nucleus, and is thus a grain of a single crystal that does not contain grain boundaries. Whether the particles are single crystals can be confirmed, for example, by analysis of electron beam diffraction images with a transmission electron microscope (TEM).

Single particles have the property of being difficult to agglomerate, and single particles constitute positive electrode active material particles by themselves, but single particles may be aggregated to form positive electrode active material particles. However, when single particles are aggregated to form positive electrode active material particles, the number of aggregated single particles is 2 or more and 10 or less. Therefore, one positive electrode active material particle is composed of 1 to 10 single particles, and the positive electrode active material particle may be composed of 1 to 5 single particles, may be composed of 1 to 3 single particles, and may be composed of 1 single particle. The number of single particles in one positive electrode active material particle can be confirmed by observation with a scanning electron microscope (SEM) at a magnification of 10,000 to 30,000.

Thus, a single particle is different from a polycrystalline particle composed of a plurality of crystal grains or a secondary particle composed of a large number of aggregated fine particles (primary particles). A single-particle positive electrode active material can be produced according to a known method for obtaining single-crystal particles (for example, a molten salt method).

Also, the single particles are usually larger than the primary particles in the case where the primary particles constituting the secondary particles are single crystals. Therefore, it is difficult to agglomerate. The maximum diameter of a single particle may be 0.5 μm or more, may be more than 1 μm, may be more than 2 μm, and may be 3 μm or more and 7 μm or more. Moreover, the average maximum diameter of the single particles may be 3 μm or more and 7 μm or more. The maximum diameter of a single particle can be determined as the distance between the two furthest points on the contour line of the single particle in the SEM image of the single particle. This SEM image may be a two-dimensional projection image of a single particle, or may be a cross-sectional image. The average maximum diameter of single particles can be obtained as the average value of the maximum diameters of 100 or more arbitrarily selected single particles in the SEM image.

The shape of the single particle is not particularly limited, and may be spherical, columnar, plate-like, or amorphous.

The primary particles that make up the secondary particles may be single crystals having no crystal grain boundaries or polycrystals having crystal grain boundaries. One secondary particle is composed of 11 or more primary particles, and may be composed of 50 or more, or 100 or more primary particles. The secondary particles may also consist of 10,000 or less, or 1,000 or less primary particles. The number of primary particles in one secondary particle can be confirmed by SEM observation at a magnification of 10,000 to 30,000.

Regarding the maximum diameter of the primary particles that constitute the secondary particles, when the primary particles are single crystals, the maximum diameter of the primary particles is usually smaller than the maximum diameter of the single particles. For this reason, it tends to agglomerate. The maximum diameter of the primary particles may be less than 0.5 μm, may be 0.05 μm or more and 0.2 μm or less, or may be 0.1 μm or more and 0.2 μm or less. Also, the average maximum diameter of the primary particles may be 0.1 μm or more and 0.2 μm or less.

When the primary particles are polycrystalline, the maximum diameter of the primary particles may be 0.01 μm or more and 2 μm or less, and may be 0.05 μm or more and 1 μm or less. The average maximum diameter of the primary particles may be 0.05 μm or more and 1 μm or less.

In one secondary particle, when the maximum diameter of 10 or more arbitrarily selected primary particles is within a predetermined range, the maximum diameter of all primary particles contained in the one secondary particle can be considered to be within the predetermined range. The maximum diameter and average maximum diameter of primary particles can be measured by the same method as the method for measuring the maximum diameter and average maximum diameter of single particles.

The shape of the primary particles that make up the secondary particles is not particularly limited, and may be spherical, columnar, plate-like, or irregular.

When the positive electrode active material particles are secondary particles, the average diameter (median diameter D50) of the secondary particles is not particularly limited, but is, for example, 0.05 μm or more and 25 μm or less, preferably 0.5 μm or more and 23 μm or less, more preferably 3 μm or more and 22 μm or less. The average particle diameter (median diameter D50) of the secondary particles can be obtained by, for example, a laser diffraction scattering method.

The single-particle positive electrode active material is more resistant to elution of the transition metal than the secondary-particle positive electrode active material. Therefore, by using the single-particle positive electrode active material in the end regions 54B and 54C, an effect of suppressing lithium deposition can be obtained. However, in the end regions 54B and 54C, if the mass ratio of the single-particle positive electrode active material particles to the total mass of the positive electrode active material particles is too small, the lithium deposition suppressing effect cannot be sufficiently obtained. Therefore, the mass ratio is 30% by mass or more, and preferably 50% by mass or more from the viewpoint of a higher lithium deposition suppressing effect. The mass proportion may be 100 mass %. That is, all the positive electrode active material particles contained in the end regions 54B and 54C may be single particles.

When the mass ratio of the end regions 54B and 54C is less than 100% by mass, the end regions 54B and 54C contain secondary positive electrode active material particles in addition to single positive electrode active material particles. Therefore, the mass ratio of the secondary positive electrode active material particles to the total mass of the positive electrode active material particles in the end regions 54B and 54C is 70% by mass or less, preferably 50% by mass or less.

On the other hand, the mass ratio of the secondary positive electrode active material particles to the total mass of the positive electrode active material particles in the central region 54A is greater than the mass ratio of the secondary positive electrode active material particles to the total mass of the positive electrode active material particles in the end regions 54B and 54C. Therefore, the mass ratio of the secondary particle-like positive electrode active material particles in the central region 54A is more than 70% by mass, preferably 80% by mass or more, more preferably 85% by mass or more, still more preferably 90% by mass or more, and most preferably 100% by mass. That is, most preferably, all positive electrode active material particles contained in central region 54A are in the form of secondary particles.

In the present embodiment, the ratio (wp/wn) of the dimension (dimension wp in FIG. 3) of the end region 54B of the positive electrode active material layer 54 to the dimension (dimension wn in FIG. 3) of the non-facing region 64B of the negative electrode active material layer 64 in the width direction is 2 or more and 7.5 or less. When the ratio (wp/wn) is within this range, it is possible to appropriately obtain the effect of suppressing lithium deposition by the single positive electrode active material particles. The ratio (wp/wn) is preferably 2.5 or more and 6.2 or less from the viewpoint of a higher lithium deposition suppressing effect.

As described above, in the present embodiment, focusing on the particle form of the positive electrode active material, the deposition of lithium in a lithium ion secondary battery is suppressed by arranging an appropriate amount of a single-particle positive electrode active material having an effect of suppressing lithium deposition in the positive electrode active material layer, focusing on the particle form of the positive electrode active material. Therefore, in the present embodiment, deposition of lithium can be suppressed in the lithium ion secondary battery without using positive electrode active materials having different compositions.

In this embodiment, the composition of the positive electrode active material is not particularly limited. The composition of the positive electrode active material contained in the central region 54A and the composition of the positive electrode active material contained in the end regions 54B and 54C may be the same or different, but are preferably the same or similar. When the compositions are the same or similar, it is possible to prevent the deterioration of the characteristics of the lithium ion secondary battery due to the combined use of positive electrode active materials having greatly different compositions. In addition, it is possible to prevent the metal elution from becoming non-uniform due to the difference in metal elution behavior based on the difference in the composition of the positive electrode active material, and the weakening of the lithium deposition and elution effect in a part of the positive electrode active material layer. When the compositions are the same or similar, specifically, for example, the positive electrode active material contained in the end regions 54B and 54C and the positive electrode active material contained in the central region 54A are nickel-containing lithium composite oxides (particularly lithium-nickel-manganese-cobalt-based composite oxides), and the molar ratio of Ni in the metal other than Li in the positive electrode active material contained in the end regions 54B and 54C is the same as the molar ratio of Ni in the metal other than Li in the positive electrode active material contained in the central region 54A. For example, it may be 0.9 times or more and 1.1 times or less.

The type of positive electrode active material may be the same as known positive electrode active materials used in lithium ion secondary batteries. Specifically, for example, a lithium composite oxide, a lithium transition metal phosphate compound, or the like can be used as the positive electrode active material. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxides, lithium cobalt composite oxides, lithium manganese composite oxides, lithium nickel manganese composite oxides, lithium nickel cobalt manganese composite oxides, lithium nickel cobalt aluminum composite oxides, and lithium iron nickel manganese composite oxides. These positive electrode active materials may be used singly or in combination of two or more.

In this specification, the term "lithium-nickel-cobalt-manganese-based composite oxide" is a term that includes oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additive elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn and Sn. Also, the additive elements may be metalloid elements such as B, C, Si and P, and nonmetal elements such as S, F, Cl, Br and I. The same applies to the lithium-nickel-based composite oxide, lithium-cobalt-based composite oxide, lithium-manganese-based composite oxide, lithium-nickel-manganese-based composite oxide, lithium-nickel-cobalt-aluminum-based composite oxide, lithium-iron-nickel-manganese-based composite oxide, and the like.

As the positive electrode active material, a lithium-nickel-cobalt-manganese-based composite oxide is preferable because it can impart various advantageous characteristics to the lithium-ion secondary battery 100 . As the lithium-nickel-cobalt-manganese composite oxide, one having a composition represented by the following formula (I) is preferable.
Li _1+x Ni _y Co _z Mn _(1-yz) M _α O _2-β Q _β (I)

In formula (I), x, y, z, α, and β satisfy −0.3≦x≦0.3, 0.1≦y≦0.95, 0.02≦z≦0.5, 0≦α≦0.1, and 0≦β≦0.5. M is at least one element selected from the group consisting of Al, Zr, B, Mg, Fe, Cu, Zn, Sn, Na, K, Ba, Sr, Ca, W, Mo, Nb, Ti, and Si. Q is at least one element selected from the group consisting of F, Cl and Br. x preferably satisfies 0≦x≦0.3, more preferably 0≦x≦0.15, and still more preferably 0. From the viewpoint of energy density, y preferably satisfies 0.3≦y≦0.95, more preferably 0.5≦y≦0.95, still more preferably 0.6≦y≦0.95, and most preferably 0.7≦y≦0.95. z preferably satisfies 0.02≦z<0.4, more preferably 0.02≦z≦0.2, and still more preferably 0.02≦z≦0.15. α preferably satisfies 0≦α≦0.05, more preferably 0. β preferably satisfies 0≦β≦0.1, more preferably 0.

The content of the positive electrode active material is not particularly limited, but is preferably 70% by mass or more, more preferably 80% by mass or more, and still more preferably 85% by mass or more in the positive electrode active material layer 54 (that is, with respect to the total mass of the positive electrode active material).

The positive electrode active material layer 54 may contain components other than the positive electrode active material. Examples thereof include lithium phosphate (Li ₃ PO ₄ ), conductive materials, binders, and the like.

The content of lithium phosphate in the positive electrode active material layer 54 is not particularly limited, but is preferably 1% by mass or more and 15% by mass or less, more preferably 2% by mass or more and 12% by mass or less.

Carbon black such as acetylene black (AB) and other carbon materials (eg, graphite) can be suitably used as the conductive material. The content of the conductive material in the positive electrode active material layer 54 is not particularly limited, but is, for example, 0.1% by mass or more and 20% by mass or less, preferably 1% by mass or more and 15% by mass or less, and more preferably 2% by mass or more and 10% by mass or less.

As the binder, for example, polyvinylidene fluoride (PVdF) or the like can be used. The content of the binder in the positive electrode active material layer 54 is not particularly limited, but is, for example, 0.5% by mass or more and 15% by mass or less, preferably 1% by mass or more and 10% by mass or less, and more preferably 1.5% by mass or more and 8% by mass or less.

Although the thickness of the positive electrode active material layer 54 is not particularly limited, it is, for example, 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less.

As the positive electrode current collector 52 constituting the positive electrode sheet 50, a sheet or foil-like body made of metal such as aluminum, nickel, titanium, or stainless steel can be used, and aluminum foil is preferably used. When an aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

In the lithium-ion secondary battery 100, both of the end regions 54B and 54C contain single-particle positive electrode active material particles, but only one of them may contain single-particle positive electrode active material particles as described above. When both of the end regions 54B and 54C contain single-particle positive electrode active material particles, the lithium deposition suppressing effect is enhanced. In the present embodiment, the widthwise end of the positive electrode active material layer 54 is set to the end region containing the single-particle positive electrode active material particles. Both the widthwise end of the positive electrode active material layer 54 and the end in the direction orthogonal thereto may be set as end regions containing single-particle positive electrode active material particles.

On the other hand, as the negative electrode current collector 62 constituting the negative electrode sheet 60, a sheet or foil-like body made of metal such as copper, nickel, titanium, or stainless steel can be used, and copper foil is preferably used. When a copper foil is used as the negative electrode current collector 62, its thickness is not particularly limited, but is, for example, 5 μm or more and 35 μm or less, preferably 7 μm or more and 20 μm or less.

As the negative electrode active material, a carbon material-based negative electrode active material or the like can be used, and among them, graphite is preferable. Graphite may be natural graphite, artificial graphite, or amorphous carbon-coated graphite in which graphite is coated with an amorphous carbon material.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited, but is preferably 90% by mass or more, more preferably 95% by mass or more.

The negative electrode active material layer 64 may contain components other than the negative electrode active material, such as binders and thickeners.

As the binder, for example, styrene-butadiene rubber (SBR) and its modified products, acrylonitrile-butadiene rubber and its modified products, acrylic rubber and its modified products, fluororubber and the like can be used. Among them, SBR is preferable. The content of the binder in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.1% by mass or more and 8% by mass or less, and more preferably 0.2% by mass or more and 3% by mass or less.

Examples of thickeners that can be used include cellulose-based polymers such as carboxymethylcellulose (CMC), methylcellulose (MC), cellulose acetate phthalate (CAP), and hydroxypropylmethylcellulose (HPMC); polyvinyl alcohol (PVA) and the like. Among them, CMC is preferred. The content of the thickening agent in the negative electrode active material layer 64 is not particularly limited, but is preferably 0.3% by mass or more and 3% by mass or less, and more preferably 0.4% by mass or more and 2% by mass or less.

Although the thickness of the negative electrode active material layer 64 is not particularly limited, it is, for example, 10 μm or more and 300 μm or less, preferably 20 μm or more and 200 μm or less.

Examples of the separator 70 include porous sheets (films) made of resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, and polyamide. Such a 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). A heat-resistant layer (HRL) may be provided on the surface of the separator 70 .

Although the thickness of the separator 70 is not particularly limited, it is, for example, 5 μm or more and 50 μm or less, preferably 10 μm or more and 30 μm or less.

The non-aqueous electrolyte 80 typically contains a non-aqueous solvent and an electrolyte salt (in other words, a supporting salt). As the non-aqueous solvent, organic solvents such as various carbonates, ethers, esters, nitriles, sulfones, lactones, etc., which are used in electrolyte solutions for general lithium ion secondary batteries, can be used without particular limitation. Among them, carbonates are preferable, and specific examples thereof include ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC), trifluorodimethyl carbonate (TFDMC), and the like. Such non-aqueous solvents can be used singly or in combination of two or more.

As the electrolyte salt, for example, lithium salts such as LiPF ₆ , LiBF ₄ and lithium bis(fluorosulfonyl)imide (LiFSI) can be used, among which LiPF ₆ is preferred. The concentration of the electrolyte salt is not particularly limited, but is preferably 0.7 mol/L or more and 1.3 mol/L or less.

The nonaqueous electrolytic solution 80 may contain various additives such as film forming agents such as oxalato complexes, gas generating agents such as biphenyl (BP) and cyclohexylbenzene (CHB), thickeners, and the like, as long as the effects of the present invention are not significantly impaired.

In the lithium ion secondary battery 100 configured as described above, deposition of lithium due to elution of the transition metal from the positive electrode active material by the acid or the like generated in the non-aqueous electrolyte is suppressed. Therefore, the lithium ion secondary battery 100 has excellent durability.

The lithium ion secondary battery 100 can be used for various purposes. Suitable applications include drive power supplies mounted in vehicles such as electric vehicles (EV), hybrid vehicles (HV), and plug-in hybrid vehicles (PHV). Moreover, the lithium ion secondary battery 100 can be used as a storage battery such as a small power storage device. The lithium ion secondary battery 100 can also be used typically in the form of an assembled battery in which a plurality of batteries are connected in series and/or in parallel.

As an example, the prismatic lithium ion secondary battery 100 including the flattened wound electrode body 20 has been described. However, the lithium ion secondary battery disclosed herein can also be configured as a lithium ion secondary battery including a laminated electrode body (that is, an electrode body in which a plurality of positive electrodes and a plurality of negative electrodes are alternately laminated). The nonaqueous electrolyte secondary battery disclosed herein can also be configured as a cylindrical lithium ion secondary battery, a laminate case type lithium ion secondary battery, a coin type lithium ion secondary battery, or the like.

Moreover, in the above example, the non-aqueous electrolytic solution 80 is used as the electrolyte, but it is also possible to construct an all-solid secondary battery using a solid electrolyte.

EXAMPLES Examples relating to the present invention will be described below, but the present invention is not intended to be limited to those shown in the examples.

<Example 1>
Secondary particles of LiNi _0.8 Co _0.1 Mn _0.1 O ₂ were prepared as first positive electrode active material particles. In this first positive electrode active material, the average diameter of primary particles was 0.5 μm, and the average diameter of secondary particles was 12 μm. The secondary particulate LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed at a mass ratio of NCM:AB:PVDF=100:1:1 in N-methyl-2-pyrrolidone (NMP) to prepare a slurry for forming a first positive electrode active material layer.

Single particles of LiNi _0.8 Co _0.1 Mn _0.1 O ₂ were prepared as second positive electrode active material particles. The average diameter of single particles in this second positive electrode active material was 4 μm. This secondary particulate LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM), acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder were mixed at a mass ratio of NCM:AB:PVDF=100:1:1 in N-methyl-2-pyrrolidone (NMP) to prepare a slurry for forming a second positive electrode active material layer.

A multi-layer coating die was provided with an upper slit and a lower slit. By masking the central portion of the lower slit of the die, coating can be performed from the lower slit on both sides of the coating region of the upper slit. Using this die, the first positive electrode active material layer forming slurry was applied to the central portion of the aluminum foil, and the second positive electrode active material layer forming slurry was applied to both outer sides of the central portion so as to be in contact with the central portion. At this time, a positive electrode active material layer non-formation portion was provided at the end of the aluminum foil where the slurry was not applied. After that, it was dried and roll-pressed to a predetermined thickness to prepare a positive electrode sheet.

Further, graphite (C) as a negative electrode active material, styrene-butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener were mixed in ion-exchanged water at a mass ratio of C:SBR:CMC=100:1:1 to prepare slurry for forming a negative electrode active material layer. This negative electrode active material layer forming slurry was applied onto a copper foil. At this time, a negative electrode active material layer non-formation portion was provided at the end of the copper foil where the slurry was not applied. After that, it was dried and roll-pressed to a predetermined thickness to prepare a negative electrode sheet.

The above positive electrode sheet and negative electrode sheet were cut into predetermined dimensions. In this predetermined dimension, the dimension in the width direction of the negative electrode active material layer was made larger than the dimension in the width direction of the positive electrode active material layer. In addition, the ratio of the width direction dimension of the region formed by the second positive electrode active material layer forming slurry (dimension wp in FIG. 3, hereinafter also referred to as “positive electrode end portion width”) at the end of the positive electrode active material layer to the width direction dimension of the region of the negative electrode active material layer not facing the positive electrode active material layer (dimension wn in FIG. 3, hereinafter also referred to as “negative electrode non-facing portion width”) was 5.1.

A porous polyolefin sheet having a three-layer structure of PP/PE/PE was prepared as a separator. As shown in FIG. 1, these were superimposed so that the positive electrode active material layer non-formed portion and the negative electrode active material layer non-formed portion projected in opposite directions to obtain a laminate. Next, the laminated body was wound to obtain a wound body, which was pressed into a flat shape to obtain a flat wound electrode body.

An aluminum plate as a positive electrode collector plate was attached by welding to the portion of the wound electrode body where the positive electrode active material layer was not formed, and a copper plate as a negative electrode collector plate was attached by welding to the portion where the negative electrode active material layer was not formed.

This was inserted into a case made of an aluminum laminate film, welded, and then injected with a non-aqueous electrolyte. The non-aqueous electrolytic solution used was obtained by dissolving LiPF ₆ at a concentration of 1.0 mol/L in a mixed solvent containing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a volume ratio of 1:3. Then, the evaluation lithium ion secondary battery of Example 1 was obtained by sealing the laminate case.

<Comparative Example 1>
A lithium ion secondary battery for evaluation of Comparative Example 1 was obtained in the same manner as in Example 1, except that the entire positive electrode active material layer was formed using the first positive electrode active material layer forming slurry.

<Comparative Example 2>
A lithium ion secondary battery for evaluation of Comparative Example 2 was obtained in the same manner as in Example 1, except that the entire positive electrode active material layer was formed using the second positive electrode active material layer forming slurry.

<Examples 2 and 3 and Comparative Example 3>
Evaluation lithium ion secondary batteries of Examples 2 and 3 and Comparative Example 3 were obtained in the same manner as in Example 1, except that a mixture of monoparticulate LiNi _0.8 Co _0.1 Mn _0.1 O ₂ and secondary particulate LiNi _0.8 Co _0.1 Mn _0.1 O ₂ was used as the second positive electrode active material particles contained in the slurry for forming the second positive electrode active material layer. The mixing ratio (mass ratio) between the single-particle LiNi _0.8 Co _0.1 Mn _0.1 O ₂ and the secondary-particle LiNi _0.8 Co _0.1 Mn _0.1 O ₂ was the value shown in parentheses in Table 1.

<Examples 4 to 6 and Comparative Example 4>
Lithium ion secondary batteries for evaluation of Examples 4 to 6 and Comparative Example 4 were obtained in the same manner as in Example 1, except that the masking width of the multilayer coating die and the like were changed so that the ratio of the width of the positive electrode end portion to the width of the negative electrode non-facing portion of the negative electrode active material layer was changed as shown in Table 1.

<Cycle characteristics evaluation>
Each lithium ion secondary battery for evaluation was subjected to constant current charging at a current value of 0.1 C to 4.2 V at room temperature, and then constant current discharging to 3.0 V at a current value of 0.1 C. The discharge capacity at this time was determined and taken as the initial capacity.

Subsequently, each lithium ion secondary battery for evaluation was charged to 4.2 V and stored in an environment of 60° C. for 8 weeks. After that, each lithium ion secondary battery for evaluation was placed at room temperature, and charging and discharging were repeated 1000 cycles, with constant current charging to 4.2 V at 2 C and constant current discharging to 3.0 V at 2 C being one cycle. The discharge capacity after 1000 cycles was determined in the same manner as the initial capacity. As an index of metal lithium deposition resistance, the capacity retention rate (%) was obtained from (discharge capacity after 1000 charge/discharge cycles/initial capacity)×100. Table 1 shows the results.

<Evaluation of Metallic Lithium Deposition>
Each evaluation lithium ion secondary battery that had been charged and discharged for 1000 cycles was disassembled, and the degree of deposition of metallic lithium on the negative electrode was visually examined. Table 1 shows the results.

Comparing between Examples 1 to 6 and Comparative Examples 1-4, the positive electrode active material layer has a central region containing secondary particle -shaped positive electrode active substance particles, an end region containing a single particles -like positive electrode active particles, and the end area of the single particles is 30 % by mass or more, and the central area is in the end. It can be seen that if the ratio of the positive edge width is 2 or more and 7.5 or less for the negative extreme untidthetomatic width of 2 or less than the area, it can be effectively suppressed. Further, from a comparison of Examples 1, 4 to 6, and Comparative Example 1, when the ratio of the positive electrode end width to the negative electrode non-facing width is 2.5 or more and 6.2 or less, Li deposition can be extremely effectively suppressed. From the comparison of Examples 1 to 3 and Comparative Example 3, in the end region, when the mass ratio of the monoparticulate active material particles to the total mass of the positive electrode active material particles is 50% by mass or more, it can be seen that Li deposition can be suppressed extremely effectively.

<Example 7>
A lithium ion secondary battery for evaluation of Example 7 was obtained in the same manner as in Example 1, except that the composition of the first and second positive electrode active materials was changed to LiNi _0.5 Co _0.2 Mn _0.3 O ₂ .

<Comparative Example 5>
A lithium ion secondary battery for evaluation of Comparative Example 5 was obtained in the same manner as in Comparative Example 1, except that the composition of the first and second positive electrode active materials was changed to LiNi _0.5 Co _0.2 Mn _0.3 O ₂ .

<Comparative Example 6>
A lithium ion secondary battery for evaluation of Comparative Example 6 was obtained in the same manner as in Comparative Example 2, except that the composition of the first and second positive electrode active materials was changed to LiNi _0.5 Co _0.2 Mn _0.3 O ₂ .

<Evaluation of cycle characteristics and evaluation of metallic lithium deposition>
The evaluation lithium ion secondary batteries of Example 7 and Comparative Examples 5 and 6 were subjected to cycle characteristics evaluation and metallic lithium deposition evaluation in the same manner as described above. Table 2 shows the results.

From the results in Table 2, it can be seen that even if the composition of the positive electrode active material is changed, the results with the same tendency as in Example 1 and Comparative Examples 1 and 2 in Table 1 are obtained. From this, it can be seen that only the particle form of the positive electrode active material contributes to the suppression of Li deposition.

From the above, according to the lithium ion secondary battery disclosed herein, it can be seen that deposition of lithium can be suppressed in the lithium ion secondary battery without using a positive electrode active material with a different composition.

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.

20 Wound electrode assembly 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector 44 Negative electrode terminal 44a Negative electrode current collector 50 Positive electrode sheet (positive electrode)
52 positive electrode current collector 52a positive electrode active material layer non-formed portion 54 positive electrode active material layer 60 negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formation portion 64 Negative electrode active material layer 70 Separator sheet (separator)
80 Non-aqueous electrolyte 100 Lithium ion secondary battery

Claims (4)

A lithium ion secondary battery comprising an electrode body in which a positive electrode and a negative electrode are laminated,
The positive electrode has a positive electrode active material layer,
The negative electrode has a negative electrode active material layer,
The negative electrode active material layer has a dimension in at least one direction larger than that of the positive electrode active material layer,
The negative electrode active material layer has a facing region facing the positive electrode active material layer and a non-facing region not facing the positive electrode active material layer,
In the direction, the positive electrode active material layer has a central region containing secondary positive electrode active material particles and end regions containing single positive electrode active material particles,
In the end region, the mass ratio of the single-particle positive electrode active material particles to the total mass of the positive electrode active material particles is 30% by mass or more,
The mass ratio of the secondary particle-like positive electrode active material particles to the total mass of the positive electrode active material particles in the central region is larger than the mass ratio of the secondary particle-like positive electrode active material particles to the total mass of the positive electrode active material particles in the end regions,
a ratio of the dimension of the end region of the positive electrode active material layer to the dimension of the non-facing region of the negative electrode active material layer in the direction is 2 or more and 7.5 or less;
the positive electrode active material particles contained in the end regions and the positive electrode active material particles contained in the central region are particles of a lithium-nickel-cobalt-manganese-based composite oxide;
A lithium ion secondary battery in which the molar ratio of Ni in the metal other than Li in the lithium-nickel-cobalt-manganese-based composite oxide contained in the end region is 0.9 times or more and 1.1 times or less than the molar ratio of Ni in the metal other than Li in the lithium-nickel-cobalt-manganese-based composite oxide contained in the central region. 2. The lithium ion secondary battery according to claim 1, wherein said dimension ratio is 2.5 or more and 6.2 or less. 3. The lithium ion secondary battery according to claim 1, wherein the mass ratio of said single-particle active material particles to the total mass of said positive electrode active material particles in said end region is 50% by mass or more. the negative electrode active material layer contains a negative electrode active material ,
The lithium ion secondary battery according to any one of claims 1 to 3, wherein said negative electrode active material is graphite.

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