JP2023015591A - Positive electrode for lithium ion secondary battery - Google Patents

Abstract

To provide means capable of improving an output characteristic in a lithium ion secondary battery.SOLUTION: A positive electrode for a lithium ion secondary battery includes a positive electrode active material layer including a positive electrode active material; a binder; and a conductive assistant. A value of a roughness factor defined as a value of a rate of a BET ratio surface area against a geometrical ratio surface area calculated from a mean particle diameter in the positive electrode active material is 3 or less. The binder is vinylidene fluoride- fluoride-hexafluoropropylene copolymer. At the time, a rate of a construction unit derived from fluoride-hexafluoropropylene against all of the construction unit numbers of the copolymer is 5 mol% or more. The content of the binder is greater than 0.31 mass% and is less than 4.85 mass% with respect to a total solid of the positive electrode active material layer.SELECTED DRAWING: None

Description

The present invention relates to a positive electrode for lithium ion secondary batteries.

2. Description of the Related Art In recent years, various electric vehicles are expected to spread toward solving environmental and energy problems. Development of a secondary battery is earnestly carried out as an in-vehicle power source such as a power source for driving a motor, which holds the key to popularization of these electric vehicles. As a secondary battery, attention is focused on a non-aqueous electrolyte secondary battery such as a lithium ion secondary battery, which is expected to have high energy density and high output.

Here, Patent Literature 1 discloses a technique aimed at improving the safety of lithium ion secondary batteries. Specifically, the technique described in Patent Document 1 is a composite containing polyvinylidene fluoride (PVDF) and a PTC (Positive Temperature Coefficient) functional imparting component (eg, PVDF-HFP) as a binder for the positive electrode active material layer. It is characterized by using a PTC binder. As a result, when the temperature of the lithium ion secondary battery rises, the binder melts and the resistance of the positive electrode active material layer increases, thereby suppressing the heat generation of the battery.

WO2018/128139

However, as a result of investigation by the present inventor, it was found that the lithium-ion secondary battery using the technique described in Patent Document 1 has a problem of insufficient output characteristics.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide means for improving the output characteristics of a lithium ion secondary battery.

The inventor of the present invention made earnest studies to solve the above problems. In this process, by controlling the roughness factor of the positive electrode active material contained in the positive electrode active material layer and the type and content of the binder within a specific range, it is possible to improve the output characteristics of the lithium-ion secondary battery. We have found that and completed the present invention.

That is, according to one aspect of the present invention, there is provided a positive electrode for a lithium ion secondary battery having a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive aid. In the positive electrode, the roughness factor defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle size of the positive electrode active material is 3 or less; the binder is vinylidene fluoride. - A hexafluoropropylene copolymer, in which the ratio of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more; and the content of the binder is , being more than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer;

According to the present invention, output characteristics can be improved in a lithium ion secondary battery.

1 is a cross-sectional view schematically showing a laminated (flat) non-bipolar (internal parallel connection type) secondary battery that is an embodiment of the present invention. FIG. FIG. 2 is a cross-sectional view schematically showing a bipolar secondary battery that is another embodiment of the present invention;

One aspect of the present invention relates to a positive electrode for a lithium ion secondary battery (hereinafter also simply referred to as "positive electrode") having a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive aid. In this embodiment, the roughness factor defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle size of the positive electrode active material is 3 or less; A hexafluoropropylene copolymer (hereinafter also referred to as "PVDF-HFP"), in which the number of structural units derived from hexafluoropropylene (hereinafter also referred to as "HFP") relative to the total number of structural units of the copolymer is 5 mol% or more; the content of the binder is more than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer .

According to the positive electrode for a lithium ion secondary battery according to this embodiment, it is possible to improve the output characteristics of the lithium ion secondary battery. The present inventor presumes the mechanism by which such an effect is exhibited as follows.

In this embodiment, the positive electrode active material having a roughness factor of 3 or less has a small surface area. In addition, PVDF-HFP, which has a ratio of the number of structural units derived from HFP (hereinafter also simply referred to as "proportion of HFP") of 5 mol% or more as a binder, is a gel-forming polymer and is a homopolymer of vinylidene fluoride. Flexibility is high compared to PVDF, which is a polymer, and PVDF-HFP, in which the proportion of HFP is less than 5 mol %. By using such a positive electrode active material and a binder in combination, the surface of the positive electrode active material is coated with the binder more uniformly than the conventional positive electrode. In addition, uniform coating can be maintained even after repeated contraction and expansion of the positive electrode active material due to charging and discharging. Furthermore, PVDF-HFP with a high HFP ratio as described above can retain electrolyte. Therefore, the contact between the surface of the positive electrode active material and the electrolytic solution is improved, and the lithium ion conductivity is improved. As a result, by applying the positive electrode for a lithium ion secondary battery according to the present embodiment to the lithium ion secondary battery, the output characteristics of the lithium ion secondary battery can be improved.

On the other hand, as shown in Comparative Examples 3 to 5 below, when a positive electrode active material having a roughness factor greater than 3 is used, or when PVDF is used, as shown in Comparative Examples 7 to 9 and 11 below. In some cases, or in the case of using PVDF-HFP with an HFP ratio of less than 5 mol %, the output characteristics are not sufficient. This is because the surface area of the positive electrode active material is large and the flexibility of the binder is low, so that the surface of the positive electrode active material cannot be uniformly coated with the binder, and the binder has a low ability to retain the electrolyte. This is probably the reason. Repeated contraction and expansion of the positive electrode active material due to charging and discharging under such conditions creates voids in which the electrolyte does not exist on the surface of the positive electrode active material, and as a result, the lithium ion conductivity decreases, resulting in insufficient output characteristics. Gone.

Hereinafter, the embodiments of the present embodiment described above will be described with reference to the drawings, but the technical scope of the present invention should be determined based on the description of the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may differ from the actual ratios. In this specification, "X to Y" indicating a range means "X or more and Y or less". Unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25° C.) and relative humidity of 40 to 50% RH.

FIG. 1 schematically shows a flat (laminated) non-bipolar (internal parallel connection type) secondary battery (hereinafter also simply referred to as a “laminated secondary battery”) that is an embodiment of the present invention. It is a cross-sectional view.

As shown in FIG. 1, the laminated secondary battery 10a of the present embodiment has a structure in which a substantially rectangular power generation element 21 in which charge/discharge reactions actually progress is sealed inside a laminate film 29. As shown in FIG. Here, the power generation element 21 includes a positive electrode in which the positive electrode active material layer 13 is arranged on both sides of the positive electrode current collector 11 ′, an electrolyte layer 17 made of a separator containing an electrolytic solution, and both sides of the negative electrode current collector 12 It has the structure which laminated|stacked the negative electrode with which the negative electrode active material layer 15 was arrange|positioned. Specifically, the positive electrode, the electrolyte layer, and the negative electrode are laminated in this order such that one positive electrode active material layer 13 and the adjacent negative electrode active material layer 15 face each other with the electrolyte layer 17 interposed therebetween. .

Thereby, the positive electrode, the electrolyte layer, and the negative electrode form one cell layer 19 . Therefore, it can be said that the stacked secondary battery 10a shown in FIG. 1 has a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. The positive electrode current collectors of the outermost layers located on both outermost layers of the power generating element 21 have the positive electrode active material layer 13 only on one side, but the active material layer may be provided on both sides. . That is, a current collector having active material layers on both sides may be used as it is as the current collector for the outermost layer, instead of using a current collector exclusively for the outermost layer having an active material layer on only one side. In addition, by reversing the arrangement of the positive electrode and the negative electrode from FIG. 1, the outermost negative electrode current collector is positioned on both outermost layers of the power generation element 21, A negative electrode active material layer may be arranged on both sides.

A positive electrode current collector 25 and a negative electrode current collector 27 electrically connected to each electrode (positive electrode and negative electrode) are attached to the positive electrode current collector 11 ′ and the negative electrode current collector 12 , respectively. It has a structure in which it is led out to the outside of the laminate film 29 so as to be The positive electrode current collector 25 and the negative electrode current collector 27 are connected to the positive electrode current collector 11′ and the negative electrode current collector 12 of each electrode via a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary. It may be attached by ultrasonic welding, resistance welding, or the like.

FIG. 2 is a cross-sectional view schematically showing a bipolar secondary battery that is another embodiment of the present invention. A bipolar secondary battery 10b shown in FIG. 2 has a structure in which a substantially rectangular power generation element 21 in which charge/discharge reactions actually progress is sealed inside a laminate film 29 that is a battery exterior body. In this specification, a bipolar lithium ion secondary battery may be simply referred to as a "bipolar secondary battery", and an electrode for a bipolar lithium ion secondary battery may be simply referred to as a "bipolar electrode".

As shown in FIG. 2, in the power generation element 21 of the bipolar secondary battery 10b of the present embodiment, the positive electrode active material layer 13 electrically coupled to one surface of the current collector 11 is formed. It has a plurality of bipolar electrodes 23 with a negative electrode active material layer 15 electrically coupled to the opposite surface. Each bipolar electrode 23 is laminated via the electrolyte layer 17 to form the power generating element 21 . At this time, the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the one bipolar electrode 23 face each other with the electrolyte layer 17 interposed therebetween. Each bipolar electrode 23 and electrolyte layer 17 are alternately stacked. That is, the electrolyte layer 17 is sandwiched between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the one bipolar electrode 23 . ing.

Adjacent positive electrode active material layer 13 , electrolyte layer 17 , and negative electrode active material layer 15 constitute one cell layer 19 . Therefore, it can be said that the bipolar secondary battery 10b has a structure in which the cell layers 19 are stacked. A sealing portion (insulating layer) 31 is arranged on the outer peripheral portion of the cell layer 19 . This prevents a liquid junction due to leakage of the electrolyte from the electrolyte layer 17, contact between adjacent current collectors 11 in the battery, and slight unevenness at the end of the cell layer 19 in the power generation element 21. This prevents short circuits caused by The positive electrode active material layer 13 is formed only on one side of the outermost current collector 11 a on the positive electrode side located in the outermost layer of the power generating element 21 . In addition, the negative electrode active material layer 15 is formed only on one side of the outermost current collector 11b on the negative electrode side located in the outermost layer of the power generating element 21 .

Furthermore, in the bipolar secondary battery 10b shown in FIG. 2, a positive electrode current collector plate (positive electrode tab) 25 is arranged adjacent to the outermost layer current collector 11a on the positive electrode side, and this is extended to form a battery outer package. It is led out from the laminate film 29 . On the other hand, a negative electrode current collector plate (negative electrode tab) 27 is arranged adjacent to the outermost layer current collector 11 b on the negative electrode side, and is similarly extended to lead out from the laminate film 29 .

Note that the number of times the cell layers 19 are stacked is adjusted according to the desired voltage. Further, in the bipolar secondary battery 10b, the number of stacking of the cell layers 19 may be reduced if sufficient output can be secured even if the thickness of the battery is made as thin as possible. In the bipolar secondary battery 10b as well, in order to prevent external shocks and environmental deterioration during use, the power generation element 21 is enclosed under reduced pressure in a laminate film 29, which is a battery outer body, and a positive electrode collector plate 25 and a negative electrode collector are enclosed. A structure in which the electric plate 27 is taken out of the laminate film 29 is preferable.

Main constituent members of the positive electrode for a lithium ion secondary battery according to this embodiment will be described below. A positive electrode for a lithium ion secondary battery according to this embodiment has a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive aid. The positive electrode active material layer is formed on the surface of an optionally provided current collector.

[Current collector]
The current collector has a function of mediating transfer of electrons from a positive electrode active material layer and a negative electrode active material layer, which will be described later. There are no particular restrictions on the material that constitutes the current collector. As the constituent material of the current collector, for example, a metal or a conductive resin can be used.

Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or the like may be used. Alternatively, a foil in which a metal surface is coated with aluminum may be used. Among them, aluminum, stainless steel, copper, and nickel are preferable from the viewpoint of electronic conductivity, battery operating potential, adhesion of the negative electrode active material to the current collector by sputtering, and the like.

As the latter conductive resin, a resin obtained by adding a conductive filler to a non-conductive polymeric material can be used.

Examples of non-conductive polymer materials include polyethylene (PE; high-density polyethylene (HDPE), low-density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), and polyimide. (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethylacrylate (PMA), polymethylmethacrylate (PMMA) , polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), or polystyrene (PS).

Any electrically conductive filler can be used without particular limitation. Examples of materials having excellent conductivity, potential resistance, or lithium ion blocking properties include metals and conductive carbon. The metal is not particularly limited, but includes at least one metal selected from the group consisting of Ni, Ti, Al, Cu, Pt, Fe, Cr, Sn, Zn, In, and Sb, or these metals. It preferably contains alloys or metal oxides. Also, the conductive carbon is not particularly limited. Preferably, it is selected from the group consisting of acetylene black, Vulcan (registered trademark), Black Pearl (registered trademark), carbon nanofiber, Ketjenblack (registered trademark), carbon nanotube, carbon nanohorn, carbon nanoballoon, and fullerene. At least one type is included.

The amount of the conductive filler to be added is not particularly limited as long as it can impart sufficient conductivity to the current collector, and is generally about 5 to 80% by mass.

The current collector may have a single-layer structure made of a single material, or may have a laminated structure in which layers made of these materials are appropriately combined. From the viewpoint of reducing the weight of the current collector, it is preferable to include at least a conductive resin layer made of a resin having conductivity. Moreover, from the viewpoint of blocking movement of lithium ions between the single cell layers, a metal layer may be provided on a part of the current collector. Furthermore, if the positive electrode active material layer and the negative electrode active material layer to be described later themselves have conductivity and can exhibit a current collecting function, a current collector is used as a member separate from these electrode active material layers. No need. In such a form, the positive electrode active material layer to be described later constitutes the negative electrode as it is, and the negative electrode active material layer to be described later constitutes the positive electrode as it is.

[Positive electrode active material layer]
The positive electrode active material layer contains a positive electrode active material, a binder and a conductive aid. In addition to these, the positive electrode active material layer may contain optional components such as an electrolytic solution (liquid electrolyte).

(Positive electrode active material)
The positive electrode active material has a function of releasing ions such as lithium ions during charging and absorbing ions such as lithium ions during discharging. In the positive electrode for a lithium ion secondary battery of the present embodiment, the type of positive electrode active material is not particularly limited, but it preferably has a space group of R3m because it has a higher capacity. The positive electrode active material belonging to the space group R3m has a layered structure (layered rock salt structure) in which lithium atomic layers and transition metal atomic layers are alternately stacked. Therefore, by using such a positive electrode active material, the battery capacity of the lithium ion secondary battery can be improved.

Positive electrode active materials belonging to space group R3m include, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , Li(Ni—Mn—Co)O ₂ , Li(Ni—Co—Al)O ₂ and their transition metals. Lithium-transition metal composite oxides such as those in which a part of is replaced with other elements. In some cases, two or more positive electrode active materials may be used together. More preferably, a composite oxide containing lithium and nickel is used, and more preferably Li(Ni--Mn--Co) O ₂ and those in which a portion of these transition metals are replaced with other elements (hereinafter referred to as Also simply referred to as "NMC composite oxide") or Li (Ni-Co-Al) O ₂ and those in which a part of these transition metals are replaced by other elements (hereinafter simply referred to as "NCA composite oxide" ) is used, and NMC composite oxide is particularly preferably used. That is, according to a preferred embodiment of the present invention, the positive electrode active material is a lithium-nickel-manganese-cobalt composite oxide having a layered structure. NMC composite oxides and NCA composite oxides have a layered crystal structure in which lithium atomic layers and transition metal atomic layers are alternately stacked via oxygen atomic layers, and one Li atom per transition metal M atom is The amount of Li that can be contained and taken out is twice that of the spinel-based lithium manganese oxide, that is, the supply capacity is doubled, and a high capacity can be obtained.

NMC composite oxides and NCA composite oxides also include composite oxides in which a portion of the transition metal element is replaced with another metal element, as described above. Other elements in that case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, and Cu. , Ag, Zn, etc., preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, more preferably Ti, Zr, P, Al, Mg, Cr, more preferably Ti, Zr, Al, Mg, or Cr from the viewpoint of improving cycle characteristics. However, other metal elements that can substitute for the transition metal elements of the NCA composite oxide are those other than Al.

Since the NMC composite oxide has a high theoretical discharge capacity, it is preferably represented by the general formula (1): _{LiaNibMncCodMxO2 (} wherein a, _b , _c , _d , _x satisfies 0.9≦a≦1.2, 0<b<1, 0<c≦0.5, 0<d≦0.5, 0≦x≦0.3, and b+c+d+x=1. at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. represents From the viewpoint of cycle characteristics, it is preferable that 0.4≦b≦0.92 in general formula (1). The composition of each element can be measured, for example, by plasma (ICP) emission spectrometry.

Nickel (Ni), cobalt (Co) and manganese (Mn) are generally known to contribute to capacity and output characteristics from the viewpoint of improving material purity and electronic conductivity. Ti or the like partially replaces the transition metal in the crystal lattice. From the viewpoint of cycle characteristics, part of the transition metal element may be substituted with another metal element. Since at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr and Cr dissolves in a solid solution, the crystal structure is stabilized. It is considered that the battery capacity can be prevented from decreasing even if the cycle is repeated, and excellent cycle characteristics can be realized.

As a more preferred embodiment, in general formula (1), b, c and d are 0.44≦b≦0.92, 0.05≦c≦0.31, and 0.03≦d≦0.26. is preferable from the viewpoint of improving the balance between capacity and life characteristics. For example, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ is LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ etc. which have been proven in general consumer batteries. Compared to , the capacity per unit mass is large, and it is possible to improve the energy density, so it has the advantage that a compact and high-capacity battery can be produced, and it is also preferable from the viewpoint of cruising distance. _{LiNi0.8Co0.1Al0.1O2 , LiNi0.8Mn0.1Co0.1O2 , and LiNi0.88Mn0.06Co0.88Mn0.06Co0 . _ _ _ _ 06} O ₂ , LiNi _0.86 Mn _0.08 Co _0.06 O ₂ are more preferred. On the other hand, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ has life characteristics as excellent as LiNi _1/3 Mn _1/3 Co _1/3 O ₂ .

The positive electrode for a lithium ion secondary battery according to this embodiment is characterized in that the roughness factor of the positive electrode active material is 3 or less. Here, the "roughness factor" is a parameter that serves as an index of the smoothness of the surface of the positive electrode active material. It is calculated as a value of the ratio of "BET specific surface area" to "geometric specific surface area" (= BET specific surface area/geometric specific surface area). According to this embodiment, the surface of the positive electrode active material can be uniformly coated with the binder, which will be described later, by controlling the roughness factor to be 3 or less. Since the binder has a function of retaining the electrolyte, it is considered that uniform coating of the binder improves the lithium ion conductivity near the surface of the positive electrode active material. As a result, according to this embodiment, it is possible to improve the output characteristics of the lithium ion secondary battery. The roughness factor value is preferably 2.7 or less, more preferably 2.4 or less. On the other hand, the lower limit of the roughness factor is 1 or more. In addition, when the positive electrode active material is a mixture of two or more kinds, first, the BET specific surface area and the geometric specific surface area of each positive electrode active material constituting the mixture are measured by the above method, and the obtained values are The BET specific surface area and the geometric specific surface area of the mixture are calculated by summing the mixture ratios. Then, the roughness factor is calculated from these calculated values by the same method as described above.

The technique for controlling the roughness factor of the positive electrode active material to be 3 or less is not particularly limited, and conventionally known knowledge capable of controlling the smoothness of the surface of the positive electrode active material particles can be appropriately referred to. For example, as an example of a technique for producing an NMC composite oxide, first, (1) the precursor _{( NixCoyMnz} )( _OH ) ₂ is synthesized by a coprecipitation method. Specifically, an aqueous solution containing raw materials such as nickel sulfate, cobalt sulfate, manganese sulfate, sodium hydroxide, and ammonium hydroxide in a desired stoichiometric ratio is stirred for a predetermined time. After separating the precipitate by filtration, it is dried at a temperature of about 80° C. for about 8 to 15 hours to obtain a precursor. Next, (2) the obtained precursor is calcined at a temperature of about 600 to 800° C. for about 3 to 7 hours to obtain Ni _x Co _y Mn _z O ₂ . Then, (3) a lithium salt (for example, lithium carbonate) is added to the obtained oxide so as to have a stoichiometric ratio of 1 to 1.2 times, and the mixture is pulverized and mixed with a ball mill. (4) After firing at a temperature of about 600 to 800° C. for about 3 to 5 hours in the air or in an oxygen atmosphere, firing is further performed at a temperature of about 750 to 1000° C. for about 2 to 12 hours. Finally, (5) the fired powder is washed with water to remove the remaining lithium salt, and then dried at a temperature of about 80° C. for about 10 to 15 hours to obtain an NMC composite oxide. can. In addition, (6) if necessary, after pulverizing the sample with a ball mill, it may be refired at 500 to 1000° C. for 3 to 5 hours in the air or in an oxygen atmosphere. At this time, in order to control the value of the roughness factor of the NMC composite oxide, (2) firing temperature and time, (3) degree of pulverization, (4) firing time and temperature, (6) pulverization By adjusting the degree of , the size of the primary particles can be controlled. Further, by performing the re-baking in (6) at a high temperature for a long time, the surface after pulverization becomes smoother and the roughness factor can be lowered.

In addition, as described above, the crystallite diameter of the positive electrode active material is controlled to a large value such as 1 μm or more, or the number ratio of particles composed of a single crystallite in the particles constituting the positive electrode active material is set in the above preferable range. As a method of controlling to, patent documents such as Patent No. 6574222 and Patent No. 5702289, Solid State Ionics, Volume 345, February 2020, 115200, Journal of The Electrochemical Society, 165 (5) A10458 (A10 2018) can be referred to.

In the positive electrode for a lithium ion secondary battery according to the present embodiment, the positive electrode active material has a crystallite diameter calculated by the Williamson-Hall method of X (nm) and an average particle diameter (D50) calculated by the laser diffraction method. When Y (nm), it is preferable to satisfy Y/X≦70. Here, Y/X is an index of the number of crystal grains forming the positive electrode active material particles, and the smaller the value, the smaller the number of crystal grains forming the particles. The crystallite size X (nm) and the average particle size (D50) can be measured by the methods described in the Examples section below. Y/X is more preferably 50 or less, still more preferably 40 or less. On the other hand, the lower limit of Y/X is 1 or more. When Y/X is within the above range, the binder, which will be described later, can be more uniformly coated on the surface of the positive electrode active material. Since the binder has a function of retaining the electrolytic solution, it is considered that the lithium ion conductivity in the vicinity of the surface of the positive electrode active material is further improved by coating the binder more uniformly. As a result, it is possible to further improve the output characteristics of the lithium ion secondary battery using the positive electrode according to the present embodiment.

In the positive electrode for a lithium ion secondary battery according to the present embodiment, the positive electrode active material is composed of the R3m space group described above, and in addition, the (003) plane for the diffraction peak of the (104) plane obtained by X-ray diffraction measurement The peak intensity ratio ((003)/(104)) of the diffraction peak of is preferably 1.35 or more. The peak intensity ratio ((003)/(104)) is more preferably 1.4 or more, still more preferably 1.45 or more, and particularly preferably 1.48 or more. Although the upper limit of the peak intensity ratio ((003)/(104)) is not particularly limited, it is preferably 2.1 or less. The peak intensity ratio is an index of the crystallinity of the positive electrode active material, and the larger the peak intensity ratio, the higher the crystallinity. When the peak intensity ratio is within the above range, the number of defects in the crystal is reduced, and a decrease in battery charge/discharge capacity and a decrease in durability can be suppressed. The peak intensity ratio can be controlled by raw materials, composition, firing conditions, and the like. The crystal structure and peak intensity ratio of the positive electrode active material described above can be determined by the measurement methods described in Examples below.

In the positive electrode for lithium ion secondary battery according to the present embodiment, the tap density of the positive electrode active material is preferably 2 g/cm ³ or less, more preferably 1.9 g/cm ³ or less, and 1.8 g/cm 3 or less. cm ³ or less is more preferable. Although the lower limit of the tap density is not particularly limited, it is preferably 1.2 g/cm ³ or more. The smaller the tap density value, the smaller the volume of the space of the irregularities on the particle surface and the volume of the gap between the particles. When the tap density is within the above range, the binder, which will be described later, can be more uniformly coated on the surface of the positive electrode active material. Since the binder has a function of retaining the electrolytic solution, it is considered that the lithium ion conductivity in the vicinity of the surface of the positive electrode active material is further improved by coating the binder more uniformly. As a result, it is possible to further improve the output characteristics of the lithium ion secondary battery using the positive electrode according to the present embodiment.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited.

(Binder)
The binder (binding agent) has a function of maintaining the structure of the positive electrode active material layer by binding together the members contained in the positive electrode active material layer. In the positive electrode for a lithium ion secondary battery according to the present embodiment, the binder is a vinylidene fluoride-hexafluoropropylene copolymer, and the number of structural units derived from hexafluoropropylene with respect to the total number of structural units of the copolymer is is 5 mol % or more. The ratio is preferably 6 mol % or more and 20 mol % or less, more preferably 6.5 mol % or more and 10 mol % or less, and still more preferably 6.9 mol % or more and 8 mol % or less. If the ratio is less than 5 mol%, the flexibility of the binder and the swelling property in the electrolyte are insufficient, so that the surface of the positive electrode active material cannot be uniformly coated with the binder, or the amount of electrolyte retained is reduced. Sometimes. As a result, the lithium ion conductivity in the vicinity of the surface of the positive electrode active material is lowered, and there is a possibility that sufficient output characteristics cannot be exhibited.

Further, in the positive electrode for a lithium ion secondary battery according to the present embodiment, the content of the binder is more than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer. characterized by The binder content is preferably 0.50% by mass or more and 2.97% by mass or less, more preferably 1.01% by mass or more and 2.00% by mass or less. If the binder content is 0.31% by mass or less, the electrode structure may not be maintained due to insufficient binding properties. On the other hand, if the binder content is 4.85% by mass or more, the coating of the surface of the positive electrode active material with the binder may become uneven, and the output characteristics may not be improved.

(Conductivity aid)
The conductive aid has a function of forming an electron conductive path (conductive path) in the positive electrode active material layer. When such electron conducting paths are formed in the positive electrode active material layer, the internal resistance of the battery can be reduced and the output characteristics at a high rate can be improved. In the positive electrode for a lithium ion secondary battery according to the present embodiment, the surface of the positive electrode active material is uniformly coated with the binder, so that the electronic conduction paths between adjacent positive electrode active materials and between the current collector and the positive electrode active material From the viewpoint of ensuring the conductivity aid is essential.

Examples of conductive aids include particulate carbon materials such as acetylene black, carbon black, channel black, thermal black, Ketjenblack (registered trademark), carbon nanotubes (single-walled carbon nanotubes and multi-walled carbon nanotubes), and carbon nanofibers. , vapor-grown carbon fiber, electrospun carbon fiber, polyacrylonitrile-based carbon fiber, pitch-based carbon fiber, and other fibrous carbon materials. Only one type of conductive aid may be used alone, or two or more types may be used in combination. Among them, the conductive aid is preferably a fibrous carbon material, more preferably a carbon nanotube, because it can form a good electron conduction path.

The content of the conductive aid contained in the positive electrode active material layer (the total amount when two or more are included) is 2% by mass or less with respect to 100% by mass of the total solid content of the positive electrode active material layer. It is preferably 1% by mass or less, and more preferably 1% by mass or less. At such an upper limit value, the aggregation of the conductive aids is suppressed, whereby an electron conduction path is favorably formed, so that the output characteristics can be further improved. Moreover, it becomes possible to further improve the energy density of the lithium ion secondary battery. Although the lower limit of the content of the conductive aid is not particularly limited, it exceeds 0% by mass, preferably 0.1% by mass or more, preferably 0.2% by mass or more, and 0.3 % by mass or more is more preferable. At such a lower limit, there is a sufficient amount of the conductive aid to form an electron conduction path, so the output characteristics can be further improved.

In the positive electrode for a lithium ion secondary battery of this embodiment, the thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be appropriately referred to. For example, the thickness of the positive electrode active material layer is usually about 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, still more preferably 40 to 200 μm. As the thickness of the positive electrode active material layer increases, it becomes possible to hold the positive electrode active material for exhibiting a sufficient capacity (energy density). On the other hand, the smaller the thickness of the positive electrode active material layer, the better the discharge rate characteristics.

The porosity of the positive electrode active material layer is preferably 20-50%, more preferably 20-45%. When the porosity of the positive electrode active material layer is within the above range, it is possible to maintain sufficient contact between the electron conductive materials (conductive aid, positive electrode active material, etc.) in the positive electrode active material layer, and electron transfer An increase in resistance can be prevented. Moreover, since sufficient electrolyte exists between the positive electrode active material particles, an increase in lithium ion transfer resistance can be prevented. As a result, it is possible to further improve the output characteristics of the lithium ion secondary battery.

The density of the positive electrode active material layer is preferably 2.10 to 3.00 g/cm ³ , more preferably 2.15 to 2.85 g/cm ³ , still more preferably 2.20 to 2.80 g/cm 3 . ^cm3 . If the density of the positive electrode active material layer is equal to or higher than the above lower limit, a battery with sufficient energy density can be obtained. On the other hand, if the density of the positive electrode active material layer is equal to or lower than the above-mentioned upper limit value, the electrolytic solution that fills the voids is sufficiently secured, and an increase in lithium ion transfer resistance in the positive electrode active material layer can be prevented. As a result, it is possible to further improve the output characteristics of the lithium ion secondary battery. In addition, in this specification, the density of the positive electrode active material layer shall be measured by the following method.

The density of the positive electrode active material layer is calculated according to the following formula.
Positive electrode active material layer density (g/cm ³ )=solid material mass (g)/positive electrode active material layer volume (cm ³ ).

The solid material mass is calculated by adding only the mass of the solid material among the masses of the materials in the positive electrode active material layer. The positive electrode active material layer volume is calculated from the thickness and application area of the positive electrode active material layer.

The positive electrode for a lithium ion secondary battery according to this embodiment can improve the output characteristics of the lithium ion secondary battery by being applied to the lithium ion secondary battery. Therefore, according to another aspect of the present invention, there is provided a lithium ion secondary battery including a power generation element having the positive electrode for a lithium ion secondary battery according to one aspect of the present invention described above. Components of the lithium ion secondary battery other than the positive electrode will be briefly described below.

[Negative electrode active material layer]
(Negative electrode active material)
The negative electrode active material has a function of releasing ions such as lithium ions during discharging and absorbing ions such as lithium ions during charging.

Examples of the negative electrode active material include carbon materials such as graphite (graphite), soft carbon and hard carbon, lithium-transition metal composite oxides (eg, Li ₄ Ti ₅ O ₁₂ ), metal materials (tin, silicon), silicon containing alloy-based negative electrode materials (eg, Si ₆₀ Sn ₁₀ Ti ₃₀ ), lithium alloy-based negative electrode materials (eg, lithium-tin alloys, lithium-silicon alloys, lithium-aluminum alloys, lithium-aluminum-manganese alloys, etc.), and the like. . In some cases, two or more kinds of negative electrode active materials may be used together. Silicon-containing alloy negative electrode materials, carbon materials, lithium-transition metal composite oxides, and lithium alloy negative electrode materials are preferably used as negative electrode active materials from the viewpoint of capacity and output characteristics. Needless to say, negative electrode active materials other than those described above may be used.

Although the average particle diameter (D ₅₀ ) of the negative electrode active material is not particularly limited, it is preferably 1 to 100 μm, more preferably 1 to 20 μm, from the viewpoint of high output.

The content of the negative electrode active material in the negative electrode active material layer is, for example, 60% by mass or more and less than 100% by mass, preferably 80% by mass or more and 99.5% or less, relative to 100% by mass of the total solid content. It is preferably more than 95% by mass and 99.0% by mass or less, more preferably 97% by mass or more and 98.5% by mass or less. If the content of the negative electrode active material is within the above range, both battery capacity and output characteristics can be achieved.

In addition, the negative electrode active material layer further contains other additives such as conductive aids and binders similar to those described above for the positive electrode active material layer, if necessary.

The thickness of the negative electrode active material layer is not particularly limited, and the same thickness as described above for the positive electrode active material layer can be adopted.

[Electrolyte layer]
The electrolyte layer preferably has a structure in which a separator is impregnated with an electrolytic solution (liquid electrolyte).

(Electrolyte)
The electrolyte has a function as a carrier of lithium ions. The electrolytic solution has a form in which a lithium salt is dissolved in a non-aqueous solvent.

Non-aqueous solvents include, for example, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), methyl formate (MF ), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO ), and γ-butyrolactone (GBL). Among them, the non-aqueous solvent is preferably a chain carbonate, more preferably diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate ( DMC), more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Lithium salts include Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C ₂ F ₅ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF _{3 SO3} and the like. Among them, the lithium salt is preferably Li(FSO ₂ ) ₂ N from the viewpoint of battery output and charge/discharge cycle characteristics.

The concentration of the lithium salt in the electrolytic solution is preferably 0.1-3.0 mol/L, more preferably 0.8-2.2 mol/L.

The electrolytic solution may further contain additives other than the components described above. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2- Divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethylethylene carbonate, allyloxymethylethylene carbonate, acryloxymethylethylene carbonate, methacryloxymethylethylene carbonate, ethynylethylene carbonate, propargylethylene carbonate, ethynyloxymethylethylene carbonate, propargyloxyethylene carbonate, methyleneethylene carbonate, 1 , 1-dimethyl-2-methylene ethylene carbonate and the like. These additives may be used alone or in combination of two or more. In addition, when the additive is used in the electrolytic solution, the amount used can be appropriately adjusted.

(separator)
The separator that constitutes the electrolyte layer has a function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and a function of a partition wall between the positive electrode and the negative electrode.

Examples of the form of the separator include porous sheet separators and non-woven fabric separators made of a polymer or fiber that absorbs and retains the electrolytic solution.

As a porous sheet separator made of polymer or fiber, for example, a microporous (microporous membrane) can be used. Specific forms of the porous sheet made of the polymer or fiber include, for example, polyolefins such as polyethylene (PE) and polypropylene (PP); structural laminates, etc.), hydrocarbon resins such as polyimide, aramid, and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and microporous (microporous membrane) separators made of glass fibers.

The thickness of the microporous (microporous membrane) separator cannot be defined uniquely because it varies depending on the intended use. As an example, in applications such as secondary batteries for driving motors such as electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell vehicles (FCV), the single layer or multiple layers may have a thickness of 4 to 60 μm. desirable. The fine pore size of the microporous (microporous membrane) separator is desirably 1 μm or less at maximum (usually, the pore size is about several tens of nanometers).

As the non-woven fabric separator, conventionally known materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide and aramid are used singly or in combination. Also, the bulk density of the non-woven fabric is not particularly limited as long as the impregnated polymer gel electrolyte provides sufficient battery characteristics. Furthermore, the thickness of the nonwoven fabric separator may be the same as that of the electrolyte layer, preferably 5 to 200 μm, particularly preferably 10 to 100 μm.

The separator is preferably a separator in which a heat-resistant insulating layer is laminated on a porous substrate (separator with a heat-resistant insulating layer). A heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. As the separator with a heat-resistant insulating layer, one having high heat resistance with a melting point or thermal softening point of 150° C. or higher, preferably 200° C. or higher is used. By having the heat-resistant insulating layer, the internal stress of the separator, which increases when the temperature rises, is relieved, so that the effect of suppressing thermal shrinkage can be obtained. As a result, it is possible to prevent the induction of short-circuiting between the electrodes of the battery, so that the battery structure is such that the deterioration in performance due to the temperature rise is less likely to occur. Moreover, the presence of the heat-resistant insulating layer improves the mechanical strength of the separator with the heat-resistant insulating layer, and the separator is less likely to break. Furthermore, due to the effect of suppressing heat shrinkage and high mechanical strength, the separator is less likely to curl during the battery manufacturing process.

[Positive collector plate and negative collector plate]
The material constituting the current collectors (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collectors for lithium ion secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable as the constituent material of the current collector plate. From the viewpoint of light weight, corrosion resistance and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The same material or different materials may be used for the positive collector plate 25 and the negative collector plate 27 .

[Positive lead and negative lead]
Although not shown, the current collector 11 and the current collector plates (25, 27) may be electrically connected via a positive lead or a negative lead. Materials used in known lithium-ion secondary batteries can also be employed as the constituent materials of the positive and negative electrode leads. In addition, the part taken out from the exterior body should be heat-resistant and insulative so that it does not affect the product (for example, automobile parts, especially electronic equipment, etc.) by contacting peripheral equipment or wiring and causing electric leakage. Covering with a shrinkable tube or the like is preferable.

[Seal part]
The sealing portion (insulating layer) 31 is a member unique to a bipolar secondary battery (series stacked battery) and has a function of preventing leakage of electrolyte from the electrolyte layer. In addition, the sealing portion 31 has a function of preventing contact between current collectors and short circuiting at the ends of the cell layers. The material constituting the sealing portion should have insulating properties, sealing properties against dropout of the solid electrolyte, sealing properties (sealing properties) against permeation of moisture from the outside, and heat resistance under the battery operating temperature. good. For example, acrylic resin, urethane resin, epoxy resin, polyethylene resin, polypropylene resin, polyimide resin, rubber (ethylene-propylene-diene rubber: EPDM), and the like can be used. Alternatively, an isocyanate-based adhesive, an acrylic resin-based adhesive, a cyanoacrylate-based adhesive, or the like may be used, or a hot-melt adhesive (urethane resin, polyamide resin, polyolefin resin) or the like may be used. Among them, from the viewpoint of corrosion resistance, chemical resistance, ease of production (film forming property), economic efficiency, etc., polyethylene resin and polypropylene resin are preferably used as the constituent material of the seal part, and amorphous polypropylene resin is the main component. It is more preferable to use a resin obtained by copolymerizing ethylene, propylene, and butene.

[Battery exterior body]
As the battery exterior body, a known metal can case can be used, and as shown in FIGS. 1 and 2, a bag-like case using a laminate film 29 containing aluminum that can cover the power generation element is used. can be The laminated film may be, for example, a laminated film having a three-layer structure in which PP, aluminum and nylon are laminated in this order, but is not limited thereto. A laminate film is desirable from the viewpoint that it is excellent in high power output and cooling performance and can be suitably used for batteries for large equipment for EV and HEV. Further, the exterior body is more preferably an aluminate laminate because the group pressure applied to the power generating element from the outside can be easily adjusted, and the thickness of the electrolytic solution layer can be easily adjusted to a desired thickness.

The lithium ion secondary battery according to this embodiment can exhibit excellent output characteristics. Therefore, the lithium ion secondary battery according to this embodiment is suitably used as a power source for driving EVs and HEVs.

[Assembled battery]
An assembled battery is configured by connecting a plurality of batteries. Specifically, at least two or more are used, and serialization or parallelization or both of them are used. By connecting in series and in parallel, it is possible to freely adjust the capacity and voltage.

A plurality of batteries can be connected in series or in parallel to form a small detachable assembled battery. A plurality of these detachable small battery packs are further connected in series or in parallel to form a large-capacity, large-capacity battery suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric output density. It is also possible to form assembled batteries (battery modules, battery packs, etc.) with output. How many batteries are connected to make an assembled battery, and how many stages of small assembled batteries are stacked to make a large-capacity assembled battery, depends on the battery capacity of the vehicle (electric vehicle) in which it is installed. It should be decided according to the output.

[vehicle]
The lithium ion secondary battery of this embodiment has excellent output characteristics. Furthermore, it has a high volumetric energy density. Vehicle applications such as electric vehicles, hybrid electric vehicles, fuel cell vehicles, and hybrid fuel cell vehicles require high capacity and large current compared to electrical and portable electronic device applications. Therefore, the lithium-ion secondary battery can be suitably used as a power source for vehicles, for example, a power source for driving a vehicle or an auxiliary power source.

Specifically, a battery or an assembled battery formed by combining a plurality of these batteries can be mounted on a vehicle. In the present invention, a battery having a large capacity and excellent output characteristics can be configured. Therefore, when such a battery is mounted on a vehicle, a plug-in hybrid electric vehicle with a long EV driving range or an electric vehicle with a long driving distance per charge can be configured. Examples of vehicles include hybrid vehicles, fuel cell vehicles, and electric vehicles (both of which include four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, and buses, light vehicles, etc.), as well as two-wheeled vehicles (motorcycles) and three-wheeled vehicles. ). However, the application is not limited to automobiles, and it can be applied to various power sources for other vehicles such as trains and other moving bodies, and can be used as a power source for installation such as an uninterruptible power supply. It is also possible to

EXAMPLES The present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited only to the following examples.

<Positive electrode active material>
As the positive electrode active material, the following two commercial products were prepared.

Single crystal NMC composite oxide (manufactured by Eastspring, ME88SC, LiNi _0.86 Mn _0.08 Co _0.06 O ₂ , roughness factor: 2.4, crystallite diameter (X): 110 nm, average particle diameter (D50) ( Y): 4000 nm, Y/X: 36.4, crystal structure: space group R3m, peak intensity ratio (003)/(104): 1.48, tap density: 1.76 g/cm ³ ).

Polycrystalline NMC composite oxide (manufactured by Ecopro, NMC811, LiNi _0.8 Mn _0.1 Co _0.1 O ₂ , roughness factor: 16.5, crystallite diameter (X): 80 nm, average particle diameter (D50) ( Y): 11500 nm, Y/X: 144, crystal structure: space group R3m, peak intensity ratio (003)/(104): 1.29, tap density: 2.68 g/cm ³ ).

In addition, the physical property value of each positive electrode active material was measured by the following methods.

[Roughness Factor]
First, the powder of the positive electrode active material was observed using a scanning electron microscope (SEM). , and the arithmetic average value of the particle diameters of the particles observed in several tens of fields was calculated as the average particle diameter.

Next, from the value of the average particle diameter calculated above, the volume of particles [m ³ ] and the surface area [m ² ] of a true sphere having the average particle diameter as the particle diameter are calculated. By multiplying the specific gravity of the substance (here, 4.78 [g/cm ³ ]), the spherical particle mass [g] was calculated. Then, the "geometric specific surface area [m ² /g] calculated from the average particle diameter" was calculated by dividing the particle surface area calculated above by the particle mass calculated above.

On the other hand, according to JIS Z8830: 2013 (ISO 9277: 2010) "Method for measuring specific surface area of powder (solid) by gas adsorption", measurement was performed using nitrogen gas as an adsorption gas by a static volumetric method. The BET specific surface area [m ² /g] was calculated by analyzing by the point method. Then, the roughness factor was calculated as the ratio of the "BET specific surface area" to the "geometric specific surface area calculated from the average particle diameter" calculated above (=BET specific surface area/geometric specific surface area).

[Y/X]
(Crystallite diameter X)
The crystallite diameter X of the positive electrode active material is determined by the Williamson-Hall method (Williamson-Hall method, Hall, W.II., J.Inst.Met., 75, 1127 (1950); idem, Proc.Phys.Soc., A62 , 741 (1949)). An X-ray diffraction (XRD) apparatus (manufactured by Rigaku) using CuKα rays was used for the measurement. According to the Williamson-Hall method, the following relationship holds between the diffraction peak θ obtained by X-ray diffraction measurement of the positive electrode active material and its integrated value β.

βcos θ/λ=2η(sin θ/λ)+(1/ε)
Here, ε corresponds to the crystallite diameter, λ corresponds to the wavelength of the CuKα ray, and η corresponds to the strain of the positive electrode active material. Take sin θ on the X axis and β cos θ on the Y axis, plot the diffraction peak θ of the obtained positive electrode active material and its integral width β on the X axis and the Y axis, and use the least squares method to obtain an approximate straight line. pull. Obtain the crystallite size ε from the intersection with the Y axis.

(Average particle size (D50))
The average particle size (D50) of the positive electrode active material was measured by a laser diffraction method. D50 represents the particle diameter when the cumulative value of the particle size distribution on a volume basis is 50%.

[Crystal structure, peak intensity ratio (003)/(104)]
Powder X-ray diffraction measurement for calculating the crystal structure and peak intensity ratio (I(003)/I(104)) of the positive electrode active material was performed using an X-ray diffractometer using CuKα rays (manufactured by Rigaku). , Fundamental Parameters were employed for the analysis. Analysis was performed using analysis software Topas Version 3 using X-ray diffraction patterns obtained from a range of diffraction angles 2θ = 15 to 120°.

[Tap density]
The positive electrode active material was placed in a 10 mL glass graduated cylinder, and the powder packing density after tapping 200 times was measured and taken as the tap density of the positive electrode active material.

<Preparation of positive electrode>
[Example 1]
97 parts by mass (97.98% by mass) of the above single-crystal NMC composite oxide as a positive electrode active material, and 1 mass of carbon nanotubes (abbreviation: CNT, fiber diameter: 10 nm, fiber length: 20 μm, aspect ratio: 2000) as a conductive aid. part (1.01% by mass), PVDF-HFP as a binder (manufactured by Arkema, Kyner Flex 2501, ratio of number of structural units derived from hexafluoropropylene: 6.9 mol%) from 1 part by mass (1.01% by mass) A solid content was prepared. N-methyl-2-pyrrolidone (NMP) as a solvent was added to this solid content and mixed to obtain a positive electrode slurry having a solid content concentration of 70% by mass. Using a doctor blade and a coating machine (manufactured by Tester Sangyo) whose gap is adjusted so that the basis weight is 2.5 g/cm ³ , the positive electrode slurry is applied onto an aluminum foil having a thickness of 20 μm, and then coated. The membrane was dried for 1 hour on a hot plate adjusted to 80°C. The dried coating film was pressed using a tabletop small roll press machine (manufactured by Tester Sangyo Co., Ltd.) so as to have a porosity of 20%. After that, this was placed in a vacuum dryer and dried at 130° C. for 8 hours in a vacuumed state to obtain the positive electrode of this example.

[Example 2]
A positive electrode of this example was obtained in the same manner as in Example 1, except that the amount of PVDF-HFP used as a binder was changed to 2 parts by mass (2.00% by mass).

[Example 3]
A positive electrode of this example was obtained in the same manner as in Example 1, except that the amount of PVDF-HFP as a binder was changed to 3 parts by mass (2.97% by mass).

[Comparative Example 1]
A positive electrode of this comparative example was obtained in the same manner as in Example 1, except that the amount of PVDF-HFP used as a binder was changed to 0.3 parts by mass (0.31% by mass).

[Comparative Example 2]
A positive electrode of this comparative example was obtained in the same manner as in Example 1, except that the amount of PVDF-HFP as a binder was changed to 5 parts by mass (4.85% by mass).

[Comparative Example 3]
A positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 97 parts by mass (97.98% by mass) of the polycrystalline NMC composite oxide was used as the positive electrode active material.

[Comparative Example 4]
A positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the amount of PVDF-HFP as a binder was changed to 2 parts by mass (2.00% by mass).

[Comparative Example 5]
A positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the amount of PVDF-HFP as a binder was changed to 3 parts by mass (2.97% by mass).

[Comparative Example 6]
A positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that the amount of PVDF-HFP as a binder was changed to 5 parts by mass (4.85% by mass).

[Comparative Example 7]
A positive electrode of this comparative example was obtained in the same manner as in Example 1, except that 1 part by mass (1.01% by mass) of PVDF (Kureha KF Polymer W#7200, manufactured by Kureha) was used as the binder.

[Comparative Example 8]
A positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the amount of PVDF as a binder was changed to 2 parts by mass (2.00% by mass).

[Comparative Example 9]
A positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the amount of PVDF as a binder was changed to 3 parts by mass (2.97% by mass).

[Comparative Example 10]
A positive electrode of this comparative example was obtained in the same manner as in Comparative Example 7, except that the amount of PVDF as a binder was changed to 5 parts by mass (4.85% by mass).

[Comparative Example 11]
Example 1 except that 3 parts by mass (2.97% by mass) of PVDF-HFP (Kyner Flex 2851, manufactured by Arkema, ratio of number of structural units derived from hexafluoropropylene: 2.4 mol%) was used as the binder. A positive electrode of this comparative example was obtained in the same manner as above.

[Comparative Example 12]
A positive electrode of this comparative example was obtained in the same manner as in Comparative Example 3, except that 1 part by mass (1.01% by mass) of PVDF (Kureha KF Polymer W#7200, manufactured by Kureha) was used as the binder.

<Production of negative electrode>
A solid content of 95.5% by mass of graphite (average particle size: 20 μm) as a negative electrode active material, 0.5% by mass of acetylene black as a conductive aid, and 4% by mass of PVDF as a binder was prepared. N-methyl-2-pyrrolidone (NMP) was added to this solid content and mixed to obtain a negative electrode slurry having a solid content concentration of 70%. Using a doctor blade and a coating machine (manufactured by Tester Sangyo Co., Ltd.) with a gap adjusted so that the basis weight is 2.5 g/cm ³ , the negative electrode slurry is applied onto a copper foil having a thickness of 20 μm, and then coated. The membrane was dried for 1 hour on a hot plate adjusted to 80°C. The dried coating film was pressed using a tabletop small roll press machine (manufactured by Tester Sangyo Co., Ltd.) so as to have a porosity of 20%. Then, this was placed in a vacuum dryer and dried at 130° C. for 8 hours in a vacuumed state to obtain a negative electrode.

<Production of lithium ion secondary battery>
The obtained positive electrode was cut into 12 cm ² pieces, and the negative electrode into 13 cm ² pieces. In the positive electrode, an aluminum foil with an Al terminal was laminated on the current collector foil side of the cut positive electrode. For the negative electrode, a copper foil with a Ni terminal was laminated on the collector foil side of the cut negative electrode. A separator (manufactured by Celgard, manufactured by PP) was inserted into the electrode active material layer side of the positive electrode and the negative electrode to form a laminate. This laminate was sandwiched between heat-sealable aluminum laminate films, and three sides were sealed. 2 mol/L of Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI) was added to a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio 3:7 (EC:EMC)) as a solvent. 280 μL of the electrolytic solution obtained by dissolving was injected from the remaining one side, and then vacuum-sealed with a vacuum sealer (manufactured by TOSPACK) at a degree of vacuum of 99.7%. In this way, a pouch-type lithium ion battery was produced in which the positive electrode and the negative electrode were laminated so as to face each other with the separator interposed therebetween.

[Measurement of direct current resistance (DCR)]
Under the conditions of 25 ° C., 1 C = 220 mAh / g, the first and second charge and discharge are 0.1 C, the voltage range is 2.5 to 4.6 V, charging CC-CV mode, discharging CC mode (between charging and discharging rest time of 8 hours). At SOC 50% (voltage 3.9 V), discharge is performed at 0.1 C, 0.5 C, and 1 C, and the current value (1 sec, 10 sec, 30 sec) flowing at that time is used to obtain the direct current resistance (DCR) value ( Ωcm) was calculated. Then, the DCR value in Comparative Example 12 is X ₀ , the DCR value in each example is X ₁ , and the reduction rate (%) of X ₁ when X ₀ is used as a reference is expressed by the formula: {(X ₀ −X ₁ ) /X ₀ }×100.

The results are shown in Table 1 below.

As can be seen from the results in Table 1, the positive electrode for a lithium ion secondary battery of the present invention has a significantly lower DC resistance (DCR) than Comparative Example 12. Therefore, by applying the positive electrode to a lithium ion secondary battery, it is possible to improve the output characteristics of the lithium ion secondary battery.

On the other hand, in Comparative Example 1, since the content of the binder was small, the structure of the positive electrode active material layer could not be maintained, and electrode cracks occurred. In Comparative Example 2, since the content of the binder was large, the direct current resistance increased.

In Comparative Examples 3 to 5, in which the polycrystalline NMC composite oxide was used, the effect of reducing the DC resistance was not observed, or the extent of reduction was slight, as compared with Comparative Example 12.

In Comparative Examples 7 to 9 using PVDF and Comparative Example 11 using PVDF-HFP with a low HFP ratio, the effect of reducing the direct current resistance is seen compared to Comparative Example 12, but the reduction width is I couldn't say enough.

10a stacked secondary battery,
10b bipolar secondary battery,
11 current collector,
11a outermost current collector on the positive electrode side,
11b outermost current collector on the negative electrode side,
11′ positive electrode current collector 12 negative electrode current collector 13 positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21 power generation element,
23 bipolar electrodes,
25 positive collector plate (positive tab),
27 negative electrode current collector (negative electrode tab),
29 laminate film,
31 seal.

Claims (7)

A positive electrode for a lithium ion secondary battery having a positive electrode active material layer containing a positive electrode active material, a binder, and a conductive aid,
The roughness factor defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle size of the positive electrode active material is 3 or less,
The binder is a vinylidene fluoride-hexafluoropropylene copolymer, wherein the ratio of the number of structural units derived from hexafluoropropylene to the total number of structural units of the copolymer is 5 mol% or more,
A positive electrode for a lithium ion secondary battery, wherein the content of the binder is more than 0.31% by mass and less than 4.85% by mass with respect to the total solid content of the positive electrode active material layer. In the positive electrode active material, when X (nm) is the crystallite diameter calculated by the Williamson-Hall method and Y (nm) is the average particle diameter (D50) calculated by the laser diffraction method, Y/X ≤ 70. The positive electrode for a lithium ion secondary battery according to claim 1, which satisfies 70. The positive electrode active material has a space group of R3m, and the peak intensity ratio (003)/(104) of the diffraction peak of the (003) plane to the diffraction peak of the (104) plane obtained by X-ray diffraction measurement is 1.35. 3. The positive electrode for a lithium ion secondary battery according to claim 1, comprising the above. 4. The positive electrode for a lithium ion secondary battery according to claim 1, wherein said positive electrode active material has a tap density of 2 g/cm ³ or less. 5. The positive electrode for a lithium ion secondary battery according to claim 1, wherein said positive electrode active material is a lithium-nickel-manganese-cobalt composite oxide having a layered structure. The positive electrode for a lithium ion secondary battery according to any one of claims 1 to 5, wherein said conductive aid is a carbon nanotube. A lithium ion secondary battery comprising a power generation element having the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 6.

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