JP2013120734A - Nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous secondary battery capable of maintaining low resistance and high capacity for the charge/discharge cycle at high rate.SOLUTION: In the nonaqueous secondary battery 100, the density f1 of a positive electrode active material layer 223 is f1≤2.85 g/cm, and the aspect ratio As of the primary particles 800 of positive electrode active material particles 610 is As≥1.5, in the arithmetic average of the positive electrode active material particles 610 contained in the positive electrode active material layer 223.

Description

  The present invention relates to a non-aqueous secondary battery.

  In the present specification, the “secondary battery” generally refers to a battery that can be repeatedly charged and includes so-called storage batteries such as lithium secondary batteries (typically lithium ion secondary batteries) and nickel metal hydride batteries. In the present specification, the “active material” refers to reversibly occlusion and release (typically insertion and removal) of a chemical species that serves as a charge carrier in a secondary battery (for example, lithium ions in a lithium ion secondary battery). A possible substance. Further, among secondary batteries, a secondary battery in which a non-aqueous electrolyte (for example, a non-aqueous electrolyte) is used as an electrolyte is appropriately referred to as a “non-aqueous secondary battery”.

  Regarding a lithium ion secondary battery, for example, in JP-A-2005-228512, at least one of an active material contained in a negative electrode and an active material contained in a positive electrode is a particulate active material having an aspect ratio of 1.02 or more and 3 or less. A non-aqueous secondary battery is disclosed. In particular, the aspect ratio of the positive electrode active material particles is preferably 1.02 or more and 2.2 or less. Here, in a non-aqueous electrolyte secondary battery using a particulate active material having a small aspect ratio, cycle characteristics are improved by combining a porous film made of a thermoplastic resin containing an inorganic filler as a separator. It is said that you can.

Japanese Unexamined Patent Application Publication No. 2009-205893 discloses a lithium nickel cobalt manganese composite oxide represented by a predetermined general formula that is used as positive electrode active material particles of a lithium ion secondary battery. The lithium nickel cobalt manganese composite oxide disclosed here has an average particle size of 5 to 40 μm, a BET specific surface area of 5 to 25 m ² / g, and a tap density of 1.70 g / ml or more. According to the publication, a lithium ion secondary battery capable of exhibiting excellent cycle characteristics and load characteristics can be obtained by using such lithium nickel cobalt manganese composite oxide as positive electrode active material particles.

Moreover, in JP 2007-91573 A, a lithium nickel cobalt manganese composite oxide having a press density of 3.1 to 4.5 g / cm ³ is used as positive electrode active material particles for a lithium ion secondary battery. It is disclosed. According to the publication, it is said that a lithium ion secondary battery having high safety in repeated charging / discharging, high chargeability, and high battery performance can be obtained. In this publication, as an indicator of filling property, the press density when pressed at a pressure of ² t / cm ^{2 in} addition to the tap bulk density (hereinafter referred to as press density) is evaluated. And higher capacity can be achieved.

JP 2005-228512 A JP 2009-205893 A JP 2007-91573 A

  By the way, in a secondary battery for vehicle driving (vehicle driving battery) in which driving wheels are driven by an electric motor, a remarkably high output is required in order to realize smooth acceleration of the vehicle. Moreover, in order to charge efficiently using the regenerative energy at the time of a brake, the charge at high rate is calculated | required. For this reason, a battery for driving a vehicle is required to have a charge / discharge performance at a significantly higher rate than so-called consumer applications such as personal computers and portable devices. Furthermore, it is required to maintain the performance such as maintaining a low resistance and a high capacity with respect to the charge / discharge cycle at such a high rate.

  In addition to charge / discharge characteristics at a high rate, the present inventors have provided a positive electrode current collector in which a positive electrode active material layer containing positive electrode active material particles is formed on a (sheet-like) positive electrode current collector. We are researching how to achieve both high capacity and high battery capacity. Considering the charge / discharge characteristics at a high rate, the density of the positive electrode active material layer is preferably small, and such a tendency is actually obtained. However, even when the density of the positive electrode active material layer is reduced, the resistance of the lithium ion secondary battery may increase and the capacity may decrease with respect to charging and discharging at a high rate.

A non-aqueous secondary battery proposed by the present invention includes a positive electrode current collector, a positive electrode active material layer formed on the positive electrode current collector, and positive electrode active material particles and a conductive material included in the positive electrode active material layer. ing. The density f1 of the positive electrode active material layer is f1 ≦ 2.85 g / cm ³ , and the aspect ratio As of the primary particles of the positive electrode active material particles in the arithmetic average of the positive electrode active material particles contained in the positive electrode active material layer Is As ≧ 1.5. According to such a secondary battery, it is possible to maintain a low resistance and a high capacity against charge / discharge at a high rate.

Further, the density f1 of the positive electrode active material layer may be 2.20 g / cm ³ ≦ f1. The aspect ratio of the primary particles of the positive electrode active material particles is the longest major axis of the primary particles of the positive electrode active material particles, and the minor axis that is the length of the primary particles of the positive electrode active material particles in the direction orthogonal to the major axis. The ratio (major axis / minor axis) may be obtained. In this case, for example, at least one primary particle suitable for evaluating the aspect ratio of the primary particle may be extracted from a cross-sectional image or an appearance image taken so that the primary particle of the positive electrode active material particle can be seen. Then, for the extracted primary particles, a ratio (major axis / minor axis) between the longest major axis in the image and the minor axis which is the length of the primary particles of the positive electrode active material particles in the direction orthogonal to the major axis is obtained. Good. Further, the ratio (major axis / minor axis) may be evaluated by an arithmetic average of the extracted primary particles.

  Further, in the arithmetic average of the positive electrode active material particles contained in the positive electrode active material layer, the circularity M of the primary particles of the positive electrode active material particles may be 0.92 ≦ M ≦ 0.98.

Further, the circularity M of the primary particles of the positive electrode active material particles is determined based on the cross-sectional area S and the peripheral length L of the primary particles of the positive electrode active material particles, respectively. Here, the equation for obtaining the circularity M is preferably M = 4πS / L ² ; In this case, the circularity M of the primary particles of the positive electrode active material particles is used to evaluate the circularity of the primary particles from, for example, a cross-sectional image or an appearance image of the positive electrode active material particles photographed so that the primary particles can be seen. Any suitable at least one primary particle may be extracted. Then, for the extracted primary particles, the circularity M may be obtained based on the cross-sectional area S and the peripheral length L, respectively. Further, the circularity M may be evaluated by an arithmetic average of the circularity M of the extracted primary particles.

The positive electrode active material particles may be a lithium transition metal oxide having a layered structure, for example. In this case, the lithium transition metal oxide may contain Ni, Co, and Mn. The lithium transition metal oxide is a compound having a layered structure including Li _{1 + x} Ni _y Co _z Mn _(1-yz) M _γ O ₂ . Here, 0 ≦ x ≦ 0.2, 0.1 <y <0.9, 0.1 <z <0.4, M is an additive, and 0 ≦ γ ≦ 0.01 , M is at least one additive selected from the group consisting of Zr, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B and F.

  In addition, this non-aqueous secondary battery can maintain a low resistance and a high capacity against charge / discharge at a high rate. Moreover, the assembled battery which combined two or more can be constructed | assembled, and can be employ | adopted for the vehicle drive battery provided with the non-aqueous secondary battery or the battery.

FIG. 1 is a diagram illustrating an example of the structure of a lithium ion secondary battery. FIG. 2 is a view showing a wound electrode body of a lithium ion secondary battery. 3 is a cross-sectional view showing a III-III cross section in FIG. 2. FIG. 4 is a cross-sectional view showing the structure of the positive electrode mixture layer. FIG. 5 is a cross-sectional view showing the structure of the negative electrode mixture layer. FIG. 6 is a diagram schematically illustrating a state of the lithium ion secondary battery during charging. 7A and 7B are electron micrographs of the positive electrode active material particles 610. FIG. FIG. 8 is a schematic diagram showing primary particles 800 of the positive electrode active material particles 610. FIG. 9 shows the relationship between the density of the positive electrode active material layer and the rate of increase in resistance for a non-aqueous secondary battery. FIG. 10 is a diagram showing the relationship between the aspect ratio of the primary particles of the positive electrode active material particles and the capacity retention ratio after the charge / discharge cycle. FIG. 11 is a diagram showing the relationship between the aspect ratio of the primary particles of the positive electrode active material particles and the capacity retention ratio after the charge / discharge cycle. FIG. 12 is a diagram showing the relationship between the circularity M of the primary particles of the positive electrode active material particles and the capacity retention ratio after the charge / discharge cycle. FIG. 13 is a diagram showing the relationship between the circularity M of the primary particles of the positive electrode active material particles and the capacity retention ratio after the charge / discharge cycle. FIG. 14 is a side view schematically showing a vehicle (automobile) provided with a non-aqueous secondary battery (vehicle driving battery) according to an embodiment of the present invention.

  Hereinafter, a non-aqueous secondary battery according to an embodiment of the present invention will be described with reference to the drawings. Here, first, a structure example of a non-aqueous secondary battery will be described by taking a lithium ion secondary battery as an example. Then, the non-aqueous secondary battery which concerns on one Embodiment of this invention is demonstrated, referring this structural example suitably. In addition, the same code | symbol is attached | subjected suitably to the member and site | part which show the same effect | action. Each drawing is schematically drawn and does not necessarily reflect the real thing. Each drawing shows an example only and does not limit the present invention unless otherwise specified.

≪Lithium ion secondary battery 100≫
FIG. 1 shows a lithium ion secondary battery 100. As shown in FIG. 1, the lithium ion secondary battery 100 includes a wound electrode body 200 and a battery case 300. FIG. 2 is a view showing the wound electrode body 200. FIG. 3 shows a III-III cross section in FIG.

  As shown in FIG. 2, the wound electrode body 200 includes a positive electrode sheet 220, a negative electrode sheet 240, and separators 262 and 264. The positive electrode sheet 220, the negative electrode sheet 240, and the separators 262 and 264 are respectively strip-shaped sheet materials.

≪Positive electrode sheet 220≫
The positive electrode sheet 220 includes a strip-shaped positive electrode current collector 221 and a positive electrode active material layer 223. For the positive electrode current collector 221, a metal foil suitable for the positive electrode can be suitably used. For the positive electrode current collector 221, for example, a strip-shaped aluminum foil having a predetermined width and a thickness of approximately 15 μm can be used. An uncoated portion 222 is set along the edge on one side in the width direction of the positive electrode current collector 221. In the illustrated example, the positive electrode active material layer 223 is held on both surfaces of the positive electrode current collector 221 except for the uncoated portion 222 set on the positive electrode current collector 221 as shown in FIG. The positive electrode active material layer 223 contains a positive electrode active material. The positive electrode active material layer 223 is formed by applying a positive electrode mixture containing a positive electrode active material to the positive electrode current collector 221.

<< Positive Electrode Active Material Layer 223 and Positive Electrode Active Material Particles 610 >>
Here, FIG. 4 is a cross-sectional view of the positive electrode sheet 220. In FIG. 4, the positive electrode active material particles 610, the conductive material 620, and the binder 630 in the positive electrode active material layer 223 are schematically illustrated so that the structure of the positive electrode active material layer 223 becomes clear. As shown in FIG. 4, the positive electrode active material layer 223 includes positive electrode active material particles 610, a conductive material 620, and a binder 630.

As the positive electrode active material particles 610, a material that can be used as a positive electrode active material of a lithium ion secondary battery can be used. Examples of the positive electrode active material particles 610 include LiNiCoMnO ₂ (lithium nickel cobalt manganese composite oxide), LiNiO ₂ (lithium nickelate), LiCoO ₂ (lithium cobaltate), LiMn ₂ O ₄ (lithium manganate), LiFePO _And lithium transition metal oxides such as ₄ (lithium iron phosphate). Here, LiMn ₂ O ₄ has, for example, a spinel structure. LiNiO ₂ or LiCoO ₂ has a layered rock salt structure. LiFePO ₄ has, for example, an olivine structure. LiFePO ₄ having an olivine structure includes, for example, nanometer order particles. Moreover, LiFePO _{4 having} an olivine structure can be further covered with a carbon film.

≪Conductive material 620≫
Examples of the conductive material 620 include carbon materials such as carbon powder and carbon fiber. As the conductive material 620, one kind selected from such conductive materials may be used alone, or two or more kinds may be used in combination. As the carbon powder, various carbon blacks (for example, acetylene black, oil furnace black, graphitized carbon black, carbon black, graphite, ketjen black), graphite powder, and the like can be used.

≪Binder 630≫
Further, the binder 630 binds the positive electrode active material particles 610 and the conductive material 620 included in the positive electrode active material layer 223, or binds these particles and the positive electrode current collector 221. As the binder 630, a polymer that can be dissolved or dispersed in a solvent to be used can be used. For example, in a positive electrode mixture composition using an aqueous solvent, a cellulose polymer (carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC), etc.), a fluorine resin (eg, polyvinyl alcohol (PVA), polytetrafluoroethylene, etc.) (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP, etc.), rubbers (vinyl acetate copolymer, styrene butadiene copolymer (SBR), acrylic acid-modified SBR resin (SBR latex), etc.) A water-soluble or water-dispersible polymer such as can be preferably used. In the positive electrode mixture composition using a non-aqueous solvent, a polymer (polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyacrylonitrile (PAN), etc.) can be preferably employed.

≪Thickener, solvent≫
The positive electrode active material layer 223 is prepared, for example, by preparing a positive electrode mixture in which the above-described positive electrode active material particles 610 and the conductive material 620 are mixed in a paste (slurry) with a solvent, applied to the positive electrode current collector 221, and dried. And is formed by rolling. At this time, as the solvent for the positive electrode mixture, either an aqueous solvent or a non-aqueous solvent can be used. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). The polymer material exemplified as the binder 630 may be used for the purpose of exhibiting a function as a thickener or other additive of the positive electrode mixture in addition to the function as a binder.

  The mass ratio of the positive electrode active material in the total positive electrode mixture is preferably about 50 wt% or more (typically 50 to 95 wt%), and usually about 70 to 95 wt% (for example, 75 to 90 wt%). It is more preferable. Moreover, the ratio of the electrically conductive material to the whole positive electrode mixture can be, for example, about 2 to 20 wt%, and is usually preferably about 2 to 15 wt%. In the composition using the binder, the ratio of the binder to the whole positive electrode mixture can be, for example, about 1 to 10 wt%, and usually about 2 to 5 wt% is preferable.

<< Negative Electrode Sheet 240 >>
As illustrated in FIG. 2, the negative electrode sheet 240 includes a strip-shaped negative electrode current collector 241 and a negative electrode active material layer 243. For the negative electrode current collector 241, a metal foil suitable for the negative electrode can be suitably used. The negative electrode current collector 241 is made of a strip-shaped copper foil having a predetermined width and a thickness of about 10 μm. On one side in the width direction of the negative electrode current collector 241, an uncoated part 242 is set along the edge. The negative electrode active material layer 243 is formed on both surfaces of the negative electrode current collector 241 except for the uncoated portion 242 set on the negative electrode current collector 241. The negative electrode active material layer 243 is held by the negative electrode current collector 241 and contains at least a negative electrode active material. In the negative electrode active material layer 243, a negative electrode mixture containing a negative electrode active material is applied to the negative electrode current collector 241.

<< Negative Electrode Active Material Layer 243 >>
FIG. 5 is a cross-sectional view of the negative electrode sheet 240 of the lithium ion secondary battery 100. As shown in FIG. 5, the negative electrode active material layer 243 includes negative electrode active material particles 710, a thickener (not shown), a binder 730, and the like. In FIG. 5, the negative electrode active material particles 710 and the binder 730 in the negative electrode active material layer 243 are schematically illustrated so that the structure of the negative electrode active material layer 243 becomes clear.

<< Negative Electrode Active Material Particles 710 >>
As the negative electrode active material particles 710, one or two or more materials conventionally used for lithium ion secondary batteries can be used as the negative electrode active material without any particular limitation. For example, a particulate carbon material (carbon particles) including a graphite structure (layered structure) at least in part. More specifically, the negative electrode active material is, for example, natural graphite, natural graphite coated with an amorphous carbon material, graphite (graphite), non-graphitizable carbon (hard carbon), graphitizable carbon ( Soft carbon) or a carbon material combining these may be used. Here, the negative electrode active material particles 710 are illustrated using so-called scaly graphite, but the negative electrode active material particles 710 are not limited to the illustrated example.

≪Thickener, solvent≫
The negative electrode active material layer 243 is prepared, for example, by preparing a negative electrode mixture in which the negative electrode active material particles 710 and the binder 730 described above are mixed in a paste (slurry) with a solvent, and applied to the negative electrode current collector 241 and dried. It is formed by rolling. At this time, any of an aqueous solvent and a non-aqueous solvent can be used as the solvent for the negative electrode mixture. A suitable example of the non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). For the binder 730, the polymer material exemplified as the binder 630 of the positive electrode active material layer 223 (see FIG. 4) can be used. Further, the polymer material exemplified as the binder 630 of the positive electrode active material layer 223 may be used for the purpose of exhibiting a function as a thickener or other additive of the positive electrode mixture in addition to the function as a binder. possible.

<< Separators 262, 264 >>
The separators 262 and 264 are members that separate the positive electrode sheet 220 and the negative electrode sheet 240 as shown in FIG. 1 or FIG. In this example, the separators 262 and 264 are made of a strip-shaped sheet material having a predetermined width and having a plurality of minute holes. As the separators 262 and 264, for example, a single layer structure separator or a multilayer structure separator made of a porous polyolefin resin can be used. In this example, as shown in FIGS. 2 and 3, the width b1 of the negative electrode active material layer 243 is slightly wider than the width a1 of the positive electrode active material layer 223. Furthermore, the widths c1 and c2 of the separators 262 and 264 are slightly wider than the width b1 of the negative electrode active material layer 243 (c1, c2>b1> a1).

  In the example shown in FIGS. 1 and 2, the separators 262 and 264 are made of sheet-like members. The separators 262 and 264 may be members that insulate the positive electrode active material layer 223 and the negative electrode active material layer 243 and allow the electrolyte to move. Therefore, it is not limited to a sheet-like member. The separators 262 and 264 may be formed of a layer of insulating particles formed on the surface of the positive electrode active material layer 223 or the negative electrode active material layer 243, for example, instead of the sheet-like member. Here, as the particles having insulating properties, inorganic fillers having insulating properties (for example, fillers such as metal oxides and metal hydroxides) or resin particles having insulating properties (for example, particles such as polyethylene and polypropylene). ).

  In this wound electrode body 200, as shown in FIG. 2 and FIG. 3, the positive electrode sheet 220 and the negative electrode sheet 240 have a positive electrode active material layer 223 and a negative electrode active material layer 243 with separators 262 and 264 interposed therebetween. Are stacked so that they face each other. More specifically, in the wound electrode body 200, the positive electrode sheet 220, the negative electrode sheet 240, and the separators 262 and 264 are stacked in the order of the positive electrode sheet 220, the separator 262, the negative electrode sheet 240, and the separator 264.

  At this time, the positive electrode active material layer 223 and the negative electrode active material layer 243 are opposed to each other with the separators 262 and 264 interposed therebetween. Then, on one side of the portion where the positive electrode active material layer 223 and the negative electrode active material layer 243 face each other, a portion of the positive electrode current collector 221 where the positive electrode active material layer 223 is not formed (uncoated portion 222) protrudes. Yes. A portion of the negative electrode current collector 241 where the negative electrode active material layer 243 is not formed (uncoated portion 242) protrudes on the side opposite to the side where the uncoated portion 222 protrudes. Further, the positive electrode sheet 220, the negative electrode sheet 240, and the separators 262 and 264 are wound along the winding axis WL set in the width direction of the positive electrode sheet 220 in a state where they are overlapped in this way.

≪Battery case 300≫
In this example, as shown in FIG. 1, the battery case 300 is a so-called square battery case, and includes a container body 320 and a lid 340. The container main body 320 has a bottomed rectangular tube shape and is a flat box-shaped container having one side surface (upper surface) opened. The lid 340 is a member that is attached to the opening (opening on the upper surface) of the container body 320 and closes the opening.

  In a vehicle-mounted secondary battery, it is desired to improve the weight energy efficiency (battery capacity per unit weight) in order to improve the fuel efficiency of the vehicle. In this embodiment, the container main body 320 and the lid body 340 constituting the battery case 300 are made of a lightweight metal such as aluminum or an aluminum alloy. Thereby, weight energy efficiency can be improved.

  The battery case 300 has a flat rectangular internal space as a space for accommodating the wound electrode body 200. Further, as shown in FIG. 1, the flat internal space of the battery case 300 is slightly wider than the wound electrode body 200. In this embodiment, the battery case 300 includes a bottomed rectangular tubular container body 320 and a lid 340 that closes the opening of the container body 320. Electrode terminals 420 and 440 are attached to the lid 340 of the battery case 300. The electrode terminals 420 and 440 pass through the battery case 300 (lid 340) and come out of the battery case 300. The lid 340 is provided with a liquid injection hole 350 and a safety valve 360.

  As shown in FIG. 2, the wound electrode body 200 is flatly pushed and bent in one direction orthogonal to the winding axis WL. In the example shown in FIG. 2, the uncoated part 222 of the positive electrode current collector 221 and the uncoated part 242 of the negative electrode current collector 241 are spirally exposed on both sides of the separators 262 and 264, respectively. In this embodiment, the intermediate portions of the uncoated portions 222 and 242 are gathered and welded to the tip portions 420a and 440a of the electrode terminals 420 and 440. At this time, for example, ultrasonic welding is used for welding the electrode terminal 420 and the positive electrode current collector 221 due to the difference in materials. Further, for example, resistance welding is used for welding the electrode terminal 440 and the negative electrode current collector 241.

  The wound electrode body 200 is attached to the electrode terminals 420 and 440 fixed to the lid body 340 in a state where the wound electrode body 200 is pressed and bent flat. The wound electrode body 200 is accommodated in a flat internal space of the container body 320 as shown in FIG. The container body 320 is closed by the lid 340 after the wound electrode body 200 is accommodated. The joint 322 (see FIG. 1) between the lid 340 and the container main body 320 is welded and sealed, for example, by laser welding. Thus, in this example, the wound electrode body 200 is positioned in the battery case 300 by the electrode terminals 420 and 440 fixed to the lid 340 (battery case 300).

≪Electrolytic solution≫
Thereafter, an electrolytic solution is injected into the battery case 300 from a liquid injection hole 350 provided in the lid 340. As the electrolytic solution, a so-called non-aqueous electrolytic solution that does not use water as a solvent is used. In this example, an electrolytic solution in which LiPF ₆ is contained at a concentration of about 1 mol / liter in a mixed solvent of ethylene carbonate and diethyl carbonate (for example, a mixed solvent having a volume ratio of about 1: 1) is used. Yes. Thereafter, a metal sealing cap 352 is attached (for example, welded) to the liquid injection hole 350 to seal the battery case 300. The electrolytic solution is not limited to the electrolytic solution exemplified here. For example, non-aqueous electrolytes conventionally used for lithium ion secondary batteries can be used as appropriate.

≪Hole≫
Here, the positive electrode active material layer 223 has minute gaps 225 that should also be referred to as cavities, for example, between the positive electrode active material particles 610 and the conductive material 620 (see FIG. 4). An electrolytic solution (not shown) can penetrate into the minute gaps of the positive electrode active material layer 223. In addition, the negative electrode active material layer 243 has minute gaps 245 that should also be referred to as cavities, for example, between the negative electrode active material particles 710 (see FIG. 5). Here, the gaps 225 and 245 (cavities) are appropriately referred to as “holes”. As shown in FIG. 2, the wound electrode body 200 has uncoated portions 222 and 242 spirally wound on both sides along the winding axis WL. On both sides 252 and 254 along the winding axis WL, the electrolytic solution can permeate from the gaps between the uncoated portions 222 and 242. For this reason, in the lithium ion secondary battery 100, the electrolytic solution is immersed in the positive electrode active material layer 223 and the negative electrode active material layer 243.

≪Gas escape route≫
In this example, the flat internal space of the battery case 300 is slightly wider than the wound electrode body 200 deformed flat. On both sides of the wound electrode body 200, gaps 310 and 312 are provided between the wound electrode body 200 and the battery case 300. The gaps 310 and 312 serve as a gas escape path. For example, when the temperature of the lithium ion secondary battery 100 becomes abnormally high, for example, when overcharge occurs, the electrolyte may be decomposed and gas may be generated abnormally. In this embodiment, the abnormally generated gas moves toward the safety valve 360 through the gaps 310 and 312 between the wound electrode body 200 and the battery case 300 on both sides of the wound electrode body 200, and from the safety valve 360 to the battery case 300. Exhausted outside.

  In the lithium ion secondary battery 100, the positive electrode current collector 221 and the negative electrode current collector 241 are electrically connected to an external device through electrode terminals 420 and 440 that penetrate the battery case 300. Hereinafter, the operation of the lithium ion secondary battery 100 during charging and discharging will be described.

≪Operation when charging≫
FIG. 6 schematically shows the state of the lithium ion secondary battery 100 during charging. At the time of charging, as shown in FIG. 6, the electrode terminals 420 and 440 (see FIG. 1) of the lithium ion secondary battery 100 are connected to the charger 290. Due to the action of the charger 290, lithium ions (Li) are released from the positive electrode active material in the positive electrode active material layer 223 to the electrolytic solution 280 during charging. In addition, charges are released from the positive electrode active material layer 223. The discharged electric charge is sent to the positive electrode current collector 221 through a conductive material (not shown), and further sent to the negative electrode sheet 240 through the charger 290. In the negative electrode sheet 240, electric charges are stored, and lithium ions (Li) in the electrolytic solution 280 are absorbed and stored in the negative electrode active material in the negative electrode active material layer 243.

<< Operation during discharge >>
Although illustration is omitted, at the time of discharging, electric charge is sent from the negative electrode sheet 240 to the positive electrode sheet 220, and lithium ions stored in the negative electrode active material layer 243 are released into the electrolytic solution 280. In the positive electrode, lithium ions in the electrolytic solution 280 are taken into the positive electrode active material in the positive electrode active material layer 223.

  Thus, in charging / discharging of the lithium ion secondary battery 100, lithium ions travel between the positive electrode active material layer 223 and the negative electrode active material layer 243 through the electrolytic solution 280. At the time of charging, electric charge is sent from the positive electrode active material to the positive electrode current collector 221 through the conductive material. On the other hand, at the time of discharging, the charge is returned from the positive electrode current collector 221 to the positive electrode active material through the conductive material.

  During charging, the smoother the movement of lithium ions and the movement of electrons, the more efficient and rapid charging is considered possible. At the time of discharging, it is considered that the smoother the movement of lithium ions and the movement of electrons, the lower the resistance of the battery, the amount of discharge, and the output of the battery.

≪Other battery types≫
The above shows an example of a lithium ion secondary battery. The lithium ion secondary battery is not limited to the above form. Similarly, an electrode sheet in which an electrode mixture is applied to a metal foil is used in various other battery forms. For example, as another battery type, a cylindrical battery or a laminate battery is known. A cylindrical battery is a battery in which a wound electrode body is accommodated in a cylindrical battery case. A laminate type battery is a battery in which a positive electrode sheet and a negative electrode sheet are stacked with a separator interposed therebetween.

  Hereinafter, a lithium ion secondary battery according to an embodiment of the present invention will be described. Here, the same reference numerals are used as appropriate for members or parts that have the same action as the above-described lithium ion secondary battery 100, and description will be made with reference to the above-described diagram of the lithium ion secondary battery 100 as necessary.

  FIG. 7A is an electron micrograph of the positive electrode active material particles 610 in the positive electrode active material layer 223, and FIG. 7B is an electron expanded to such an extent that the primary particles 800 of the positive electrode active material particles 610 can be observed. It is a micrograph. FIG. 8 is a schematic diagram showing primary particles 800 of the positive electrode active material particles 610.

As shown in FIGS. 1 and 2, the lithium ion secondary battery 100 includes a positive electrode current collector 221 and a positive electrode active material layer 223 held by the positive electrode current collector 221. As shown in FIG. 4, the positive electrode active material layer 223 includes positive electrode active material particles 610, a conductive material 620, and a binder 630. Here, in the lithium ion secondary battery 100, the density f1 of the positive electrode active material layer 223 is f1 ≦ 2.85 g / cm ³ . Furthermore, in the arithmetic average of the positive electrode active material particles 610 included in the positive electrode active material layer 223, the aspect ratio As of the primary particles 800 of the positive electrode active material particles 610 is As ≧ 1.5.

  The inventor has found that a non-aqueous secondary battery can maintain a low resistance and a high capacity against charge / discharge at a high rate by using the positive electrode active material particles 610 having such characteristics. Hereinafter, the lithium ion secondary battery 100 will be described more specifically. Here, the density f1 of the positive electrode active material layer 223 will be described first, and then the aspect ratio As of the primary particles 800 of the positive electrode active material particles 610 will be described. Further, after that, the circularity of the primary particles 800 of the positive electrode active material particles 610 will be mentioned.

<< Density f1 of Positive Electrode Active Material Layer 223 >>
The density f1 of the positive electrode active material layer 223 can be calculated by dividing the mass (weight per unit area) of the positive electrode active material layer 223 per unit area by the thickness of the positive electrode active material layer 223. The mass (weight per unit area) of the positive electrode active material layer 223 may be obtained by subtracting the mass of the positive electrode current collector 221 from the mass of the positive electrode sheet 220 cut into a predetermined area. Further, “the thickness of the positive electrode active material layer 223” may be evaluated by an average value of the thicknesses of the positive electrode active material layer 223.
“Density f1 of positive electrode active material layer 223” = “Mass (weight per unit area) of positive electrode active material layer 223 per unit area” ÷ “Thickness of positive electrode active material layer 223”;
“Mass (weight per unit area) of positive electrode active material layer 223” = “Mass of positive electrode sheet 220” − “Mass of positive electrode current collector 221”;

  According to the knowledge of the present inventor, when a reduction in size and weight and an increase in capacity (increasing energy density) are required, positive electrode active material particles having a high tap density or press density of the positive electrode active material particles are used. It is better to increase the density. In this case, the portion used at a low output of about 0.1 C is durable and can maintain a high capacity.

  However, in applications for driving a vehicle, a remarkably high output is required, and for example, durability against a charge / discharge cycle at a high rate of about 4C or 5C is required. According to the knowledge of the present inventors, when the density of the positive electrode active material layer 223 is high, durability (for example, resistance increase rate) with respect to a charge / discharge cycle at a high rate of about 4C or 5C is deteriorated. In consideration of such durability, the density f1 of the positive electrode active material layer 223 is desirably small, for example, f1 ≦ 2.85. Here, 1C is a current value at which discharge is completed in exactly one hour after constant current discharge of the cell. For example, in a cell with a rated capacity of 2.22 Ah, 1C = 2.2A.

  Here, the present inventor creates evaluation cells having different densities of the positive electrode active material layer, measures the IV resistance for each predetermined cycle with respect to a predetermined charge / discharge cycle, and examines the increasing tendency of the IV resistance. It was.

≪Evaluation cell≫
The evaluation cell prepared here is a so-called 18650 type battery (not shown). Here, the 18650-type battery is illustrated as the evaluation cell, but the same tendency can be obtained in other sizes of cylindrical batteries, batteries of other shapes such as a square type and a laminate type. sell. For this reason, the shape and structure of the evaluation cell do not particularly limit the present invention.

≪Evaluation cell positive electrode≫
A positive electrode mixture was prepared to form a positive electrode active material layer in the positive electrode. Here, the positive electrode mixture includes a ternary lithium transition metal oxide (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ) as a positive electrode active material, acetylene black (AB) as a conductive material, and polyfluoride as a binder. Vinylidene chloride (PVDF) was used. In addition, as acetylene black (AB), Denki Kagaku Denka black powder form was used.

Here, the mass ratio of the positive electrode active material, the conductive material, and the binder was positive electrode active material: conductive material: binder = 91: 6: 3. A positive electrode mixture paste was prepared by mixing the positive electrode active material, the conductive material, and the binder with N-methyl-2-pyrrolidone (NMP). Next, the positive electrode mixture paste was applied to the positive electrode current collector and dried. Here, an aluminum foil (thickness 15 μm) as a positive electrode current collector was used. The positive electrode active material layer was formed on both surfaces of the positive electrode current collector. Here, the basis weight after drying of the positive electrode active material layer was about 30 mg / cm ² in total on both sides of the positive electrode current collector. The positive electrode sheet is formed by adjusting the amount of the positive electrode mixture paste applied to the positive electrode current collector, the amount of rolling when the roller sheet is rolled after drying, the density of the positive electrode active material layer, and the positive electrode active material. The layer thickness was adjusted.

≪Negative electrode of evaluation cell≫
Here, first, graphite (for example, natural graphite particles at least partially covered with an amorphous carbon film) was used as the negative electrode active material particles in the negative electrode mixture. Further, carboxymethylcellulose (CMC) was used as a thickener, and styrene-butadiene rubber (SBR) was used as a binder. Here, the mass ratio of the negative electrode active material particles, the thickener (CMC), and the binder (SBR) was negative electrode active material particles: CMC: SBR = 98: 1: 1. A negative electrode mixture paste was prepared by mixing these negative electrode active material particles, CMC, and SBR with ion-exchanged water. Next, the negative electrode mixture paste was applied to the negative electrode current collector and dried. Here, a copper foil (thickness 10 μm) as a negative electrode current collector was used. The basis weight after drying of the negative electrode active material layer was set so that the theoretical capacity ratio of the negative electrode to the positive electrode (the capacity of the negative electrode / the capacity of the positive electrode) was 1.4. Moreover, the negative electrode active material layer was formed on both surfaces of the negative electrode current collector. The negative electrode sheet can be adjusted by adjusting the amount of the negative electrode mixture paste applied to the negative electrode current collector, the amount of rolling when the roller press is rolled after drying, the density of the negative electrode active material layer, and the negative electrode active material. The layer thickness was adjusted. Here, the density of the negative electrode active material layer was set to 1.4 g / cm ³ .

≪Evaluator cell separator≫
As the separator, a separator made of a porous sheet having a three-layer structure (PP / PE / PP) of polypropylene (PP) and polyethylene (PE) was used. Here, the mass ratio of polypropylene (PP) and polyethylene (PE) was PP: PE: PP = 3: 4: 3.

≪Assembly of evaluation cell≫
A test 18650 type cell (lithium ion secondary battery) was constructed using the negative electrode produced above, the positive electrode, and the separator. Here, a wound electrode body was produced by laminating and winding a positive electrode sheet and a negative electrode sheet with a separator interposed therebetween. And the winding electrode body was accommodated in the cylindrical battery case, the nonaqueous electrolyte solution was poured and sealed, and the cell for evaluation was constructed. Here, as the non-aqueous electrolyte, ethylene carbonate (EC) and dimethyl carbonate (DMC) are mixed in a predetermined volume ratio (EC: DMC = 3: 7) and 1 mol / L as a lithium salt. An electrolytic solution in which LiPF ₆ was dissolved was used. Here, the evaluation cell was prepared so that the rated capacity was about 1 Ah.

  Here, evaluation cells having different densities of the positive electrode active material layer or primary particles of the positive electrode active material particles are obtained. The evaluation cells here have almost the same configuration except that the positive electrode is different. And about this evaluation cell, after giving predetermined conditioning, the predetermined charging / discharging cycle was given, and the resistance increase rate and capacity maintenance rate by the charging / discharging cycle were evaluated.

<< conditioning >>
Here, conditioning is performed by the following procedures 1 and 2.
Procedure 1: After reaching 4.1 V with a constant current charge of 1 C, pause for 5 minutes.
Procedure 2: After Procedure 1, charge for 1.5 hours by constant voltage charging and rest for 5 minutes.
In such conditioning, a required reaction occurs due to initial charging, and gas is generated. Moreover, a required film formation is formed on the negative electrode active material layer or the like.

≪Measurement of rated capacity≫
After the conditioning, the rated capacity is measured for the evaluation cell. The rated capacity is measured by the following procedures 1 to 3. Here, in order to make the influence of temperature constant, the rated capacity is measured in a temperature environment of 25 ° C. In this case, the SOC for the evaluation cell was set to 3.0V SOC and 3.0% SOC to 4.1V.
Procedure 1: After reaching 3.0 V by constant current discharge of 1C, discharge at constant voltage discharge for 2 hours, and then rest for 5 minutes.
Procedure 2: After reaching 4.1 V by constant current charging at 1 C, charge for 2.5 hours by constant voltage charging, and then rest for 5 minutes.
Procedure 3: After reaching 3.0 V by constant current discharge of 0.5 C, discharge at constant voltage discharge for 2 hours, and then rest for 5 minutes.
Here, the discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge to the constant voltage discharge in the procedure 3 is defined as “rated capacity”.

≪SOC adjustment≫
The SOC adjustment is performed by the following procedures 1 and 2. Here, the SOC adjustment may be performed after the conditioning process and the measurement of the rated capacity. Here, in order to make the influence of temperature constant, SOC adjustment is performed in a temperature environment of 25 ° C.
Procedure 1: Charge from 3V to 1C at a constant current to a charge state of approximately 60% of the rated capacity (in this case, charge at a constant current up to a voltage of SOC 60%, 3.72V).
Procedure 2: After step 1, charge at a constant voltage for 2.5 hours (here, charge at a constant voltage of 3.72 V for 2.5 hours).
Thereby, the cell for evaluation can be adjusted to a predetermined charge state. In addition, although the case where SOC is adjusted to 60% is described here, it can be adjusted to an arbitrary charged state by changing the charged state in procedure 1. For example, when adjusting to SOC 90%, in the procedure 1, the evaluation cell may be in a charged state (SOC 90%) of 90% of the rated capacity.

≪Relationship between positive electrode active material layer density and resistance increase rate≫
FIG. 9 shows the relationship between the density of the positive electrode active material layer and the rate of increase in resistance for the lithium ion secondary battery. Here, for the evaluation cell, after the conditioning, a predetermined IV resistance is measured and set as an initial resistance. Next, a predetermined charge / discharge cycle was performed, and a predetermined IV resistance was measured for each predetermined cycle by the same method as the initial resistance. Thereby, about the cell for evaluation from which the density of a positive electrode active material layer differs, the tendency for resistance to rise, respectively was investigated.

≪IV resistance≫
Here, in the measurement of the IV resistance, each evaluation cell is adjusted to SOC 30% in a temperature environment of 25 ° C. Then, after resting for 10 minutes, the evaluation cell was discharged with a constant current of 20 C (here, about 20 A) for 10 seconds (CC discharge). Then, a voltage drop (voltage drop amount ΔV) after 10 seconds was obtained, and an IV resistance (R = ΔV / I) was obtained therefrom. Here, I was 20C (about 20A).

≪Charge / discharge cycle≫
Here, the evaluation cell is first adjusted to SOC 60% in a temperature environment of 25 ° C. Next, in a temperature environment of 60 ° C., the battery is discharged at a constant current of 4C, and when the voltage drops to 3.0V, it is paused for 10 minutes, and then charged at a constant current of 4C, and the voltage becomes 4.1V. Then, 10 minutes of rest, which is one cycle, is repeatedly discharged and charged in the range of 3.0V to 4.1V. Here, the IV resistance was measured every 100 cycles, and the charge / discharge cycle was repeated 2000 times while the evaluation cell was adjusted to SOC 60% in an environment of 25 ° C. Thereby, the rise tendency of IV resistance with respect to a high-rate charging / discharging cycle is obtained.

For example, high-rate 9, the density of the positive electrode active material ^{layer, 3.24g / cm 3, 2.85g /} cm 3, 2.52g / cm 3, the cell for evaluation of 2.21 g / cm ^3, the above-mentioned It is the result of having measured the rise tendency of IV resistance by a charging / discharging cycle.

As a result, when the density of the positive electrode active material layer 223 was 3.24 g / cm ³ , the resistance tended to increase markedly with respect to the charge / discharge cycle at a predetermined high rate. In contrast, the density of the positive electrode active material layer 223 is ^{2.85g / cm 3, 2.52g / cm} 3, the evaluation cell of 2.21 g / cm ^3, with respect to the charge-discharge cycle at a predetermined high rate Resistance rise was kept low. For this reason, in a non-aqueous secondary battery such as a lithium ion secondary battery in which the discharge voltage can be 4 V or higher, the density of the positive electrode active material layer 223 is 3.3 for a charge / discharge cycle at a high rate of about 4C. When it is 24 g / cm ³ , the resistance tends to increase, and the required cycle durability cannot be obtained. On the other hand, if it is 2.85 g / cm ³ or less, an increase in resistance tends to be suppressed, and a required cycle durability can be obtained for a charge / discharge cycle at a high rate of about 4C.

For this reason, in applications where required cycle durability is required for charge / discharge cycles at a high rate of about 4C (for example, vehicle driving batteries), the density of the positive electrode active material layer is 2.85 g / cm ³ or less. There should be. More preferably, the density of the positive electrode active material layer is 2.80 g / cm ³ or less, more preferably the density of the positive electrode active material layer is 2.75 g / cm ³ or less. Thereby, more stable cycle durability can be obtained with respect to a charge / discharge cycle at a high rate of about 4C.

As the inventors infer, when the density of the positive electrode active material layer is high, the salt concentration of the electrolytic solution tends to be biased in the positive electrode active material layer with respect to a charge / discharge cycle at a high rate of about 4C. On the other hand, the density of the positive electrode active material layer is preferably set as small as 2.85 g / cm ³ or less. In this case, a required gap is secured in the positive electrode active material layer, and a sufficient electrolyte solution permeates the positive electrode active material layer. For this reason, it is thought that resistance rise can be suppressed with respect to the charge / discharge cycle at a high rate of about 4C.

In the lithium ion secondary battery 100, the density f1 of the positive electrode active material layer 223 is as small as f1 ≦ 2.85 g / cm ³ . For this reason, a resistance rise can be suppressed with respect to a charge / discharge cycle at a high rate of about 4C. Furthermore, if the density f1 of the positive electrode active material layer 223 is too small, there is a possibility that the required rigidity is not ensured for the expansion / contraction of charge / discharge. In this case, the density f1 of the positive electrode active material layer 223 is preferably about 2.20 g / cm ³ or more (2.20 g / cm ³ ≦ f1). Thereby, the positive electrode active material layer 223 has a required rigidity.

Thus, the density f1 of the positive electrode active material layer was set to f1 ≦ 2.85 g / cm ^3, and the density f1 of the positive electrode active material layer was reduced. As a result, an increase in resistance of the lithium ion secondary battery 100 can be suppressed against a charge / discharge cycle at a high rate of about 4C. However, even when the density f1 of the positive electrode active material layer 223 is reduced to 2.85 g / cm ³ or less, the capacity may decrease with respect to a charge / discharge cycle at a high rate of about 4C.

  The inventor examined the positive electrode active material particles 610 in more detail. As a result, it was found that the shape of the primary particles 800 of the positive electrode active material particles 610 is involved in the event that the capacity decreases with respect to the charge / discharge cycle at a high rate of about 4C. Furthermore, the primary particle 800 of the positive electrode active material particles 610 was found to have a condition that can maintain a high capacity with respect to a charge / discharge cycle at a high rate of about 4C.

≪Primary particles of positive electrode active material particles 610≫
Here, the positive electrode active material particles 610 are secondary particles. The primary particles 800 of the positive electrode active material particles 610 form the positive electrode active material particles 610 as the secondary particles, and are considered to be unit particles (ultimate particles) as judged from the apparent geometric form. The form is a primary particle 800. Further, as shown in FIG. 8, the primary particles 800 are aggregates of crystallites 810 of a positive electrode active material (for example, lithium transition metal oxide).

  The primary particles 800 can be observed based on, for example, TEM images or SEM images of the particle surfaces of the positive electrode active material particles 610. Electron micrographs of the positive electrode active material particles 610 and TEM images and SEM images of the surface of the positive electrode active material particles 610 can be obtained by, for example, VE-9800 manufactured by Keyence or Hitachi ultra-high resolution field emission scanning microscope S5500. . In this case, for example, the primary particles 800 can be automatically determined by using image analysis software (for example, Nikkiso Viewtrac) using a difference in shade and color tone. Further, the peripheral length and area of the primary particles 800, the short diameter and the long diameter, and the aspect ratio and the circularity M described later can be automatically measured by programming in advance. Here, FIG. 7A is an SEM image of a cross section obtained by bending and splitting the positive electrode active material layer 223. According to the SEM image, as shown in FIG. 7B, the primary particles 800 of the positive electrode active material particles 610 in the positive electrode active material layer 223 can be observed.

<< Aspect Ratio As of Primary Particle 800 of Positive Electrode Active Material Particle 610 >>
Here, the aspect ratio As of the primary particles 800 of the positive electrode active material particles 610 is evaluated. In the evaluation of the aspect ratio As, for example, the primary particles 800 of the positive electrode active material particles 610 have the longest long diameter L1 and the primary particles of the positive electrode active material particles 610 in the direction orthogonal to the long diameter L1, as shown in FIG. The ratio (major axis / minor axis) to the minor axis L2, which is 800 in length, may be obtained.

  For example, at least one primary particle 800 suitable for evaluating the aspect ratio As of the primary particle 800 may be extracted from a cross-sectional image or an appearance image taken so that the primary particle 800 of the positive electrode active material particle 610 can be seen. Then, for the extracted primary particles 800, as shown in FIG. 8, the longest long diameter L1 in the image and the length of the primary particles 800 of the positive electrode active material particles 610 in the direction orthogonal to the long diameter L1, respectively. The ratio (major axis / minor axis) to the minor axis L2 may be obtained. For the positive electrode active material particles 610, the arithmetic average of the ratio (major axis / minor axis) of the extracted primary particles 800 may be the aspect ratio As of the primary particles 800 of the positive electrode active material particles 610.

  The major axis L1 of the primary particle 800 can be measured based on an electron micrograph of the positive electrode active material particle 610, a TEM image or an SEM image of the particle surface of the positive electrode active material particle 610, and the like. When measuring the major axis L1 of the primary particle 800, the primary particle 800 suitable for specifying the major axis L1 of the primary particle 800 from the TEM image or SEM image of the particle surface of the positive electrode active material particle 610 as the secondary particle. Is identified.

  For example, a plurality of primary particles 800 are shown in the TEM image or SEM image of the particle surface of the positive electrode active material particles 610 that are secondary particles. Among these, primary particles 800 are arranged in descending order of area, and a plurality of primary particles 800 having a large area are extracted. Thereby, in the SEM image of the TEM image of the particle surface, primary particles 800 divided substantially along the center can be extracted. And as shown in FIG. 8, let the longest width | variety (major axis) in the extracted primary particle 800 be the major axis L1 of the primary particle 800. FIG. In addition, the longest width in the direction orthogonal to the major axis L1 is defined as the minor axis L2 of the primary particle 800.

  Here, when the positive electrode active material particles 610 are referred to as the major axis L 1 and the minor axis L 2 of the primary particles 800, the arithmetic average of a plurality of primary particles 800 included in the single positive electrode active material particle 610 may be evaluated. In the positive electrode active material layer 223, the arithmetic average of the plurality of positive electrode active material particles 610 included in the positive electrode active material layer 223 may be further evaluated.

In the lithium ion secondary battery 100, the density f1 of the positive electrode active material layer 223 is f1 ≦ 2.85 g / cm ³ . According to the knowledge of the present inventor, the aspect ratio As of the primary particles of the positive electrode active material particles 610 is preferably As ≧ 1.5. In short, in the lithium ion secondary battery 100, it is preferable that the density f1 of the positive electrode active material layer 223 is small and the primary particles 800 of the positive electrode active material particles 610 of the positive electrode active material layer 223 are vertically long (horizontally long). Thereby, a capacity | capacitance can be maintained high with respect to the charge / discharge cycle in about 4C high rate.

  As described above, by using the positive electrode active material particles 610 having the primary particles 800 having an aspect ratio As of about 1.5 or more, the capacity can be maintained high with respect to a charge / discharge cycle at a high rate of about 4C. The reason for this is not clear. As the inventors infer, lithium ions are repeatedly released and occluded by a high-rate charge / discharge cycle of about 4 C, so that the primary particles 800 of the positive electrode active material particles 610 expand and contract. In the positive electrode active material layer 223 having a small density, the force for pressing the positive electrode active material particles 610 is small. Due to the stress accompanying the expansion and contraction, the positive electrode active material particles 610 are cracked between the primary particles 800. When the positive electrode active material particles 610 are cracked between the primary particles 800, the conductive path in the positive electrode active material particles 610 is cut, and primary particles 800 separated and independent from the conductive paths are generated in the positive electrode active material particles 610. The generation of primary particles 800 that are separated and independent from the conductive path in the positive electrode active material particles 610 is considered to be one of the causes of the capacity reduction of the lithium ion secondary battery 100.

  On the other hand, when the primary particles 800 of the positive electrode active material particles 610 are vertically long (horizontally long), when viewed at the level of the primary particles 800, the contact between the primary particles 800 is equivalent to the length of the primary particles 800 (longitudinal). More points. For this reason, if the primary particles 800 are vertically long (landscape), even if cracks occur between the primary particles 800, a conductive path is easily secured. For this reason, the capacity | capacitance of the lithium ion secondary battery 100 can be maintained high with respect to the charging / discharging cycle at about 4C high rate.

  Although the above has been inferred by the present inventors, the lithium ion secondary battery 100 actually employs the cathode active material particles 610 whose primary particles 800 are vertically long (landscape) even when the density of the cathode active material layer 223 is small. By doing so, the increase in resistance after the charge / discharge cycle at a high rate can be kept small, and the capacity can be kept high.

For example, FIGS. 10 and 11 show the relationship between the aspect ratio of the primary particles 800 of the positive electrode active material particles 610 and the capacity retention ratio after the charge / discharge cycle. FIG. 10 shows a case where discharging and charging are repeated at a constant current of 0.1 C within a temperature range of 3.0 V to 4.1 V in a temperature environment of 60 ° C. (1000 cycles). FIG. 11 shows a case where discharging and charging are repeated at a constant current of 4 C in a temperature environment of 60 ° C. in a range of 3.0 V to 4.1 V (here, 1000 cycles). In FIGS. 10 and 11, when the density of the positive electrode active material layer 223 is 2.52 g / cm ³ , the plot is indicated by “◯”. When the density of the positive electrode active material layer 223 is 2.85 g / cm ³ , the density is plotted with “x”. When the density of the positive electrode active material layer 223 is 3.24 g / cm ³ , the density is plotted with “Δ”.
Here, based on the image of the cross section of the positive electrode active material layer 223 obtained by VE-9800 manufactured by Keyence, an image of the primary particles 800 of the positive electrode active material particles 610 in the positive electrode active material layer 223 is observed, and the positive electrode active material layer 223 is observed. The aspect ratio of the primary particle 800 of the material particle 610 was determined.

≪Capacity maintenance ratio (capacity maintenance ratio after cycle) ≫
Here, the capacity maintenance rate (capacity maintenance rate after cycle) evaluates the capacity based on evaluation cells adjusted to a predetermined charge state before and after a predetermined charge / discharge cycle. Here, the capacity of the evaluation cell before a predetermined charge / discharge cycle is defined as “initial capacity”, and the capacity of the evaluation cell after a predetermined charge / discharge cycle is defined as “post-cycle capacity”. “Capacity maintenance ratio” is a value obtained by dividing “capacity after cycle” by “initial capacity”.
“Capacity maintenance ratio” = “Capacity after cycle” / “Initial capacity”;

  Here, the capacity of the evaluation cell is a discharge capacity measured based on the evaluation cell adjusted to SOC 100% (4.1 V) in a temperature environment of 25 ° C. Here, the “discharge capacity” is a discharge at a constant current of 1 C from 4.1 V (SOC 100%) to 3.0 V (SOC 0%) in a temperature environment of 25 ° C., respectively, followed by a total discharge time of 2 hours. It is the integrated capacity (integrated discharge amount) measured when discharging at a constant voltage until the time.

As shown in FIG. 10, in the charge / discharge cycle (charge / discharge cycle at a low rate) in which discharge and charge are repeated at a constant current of 0.1 C, the density of the positive electrode active material layer 223 is approximately 2.52 g / cm ³ . In some cases, the capacity maintenance rate tends to decrease. Even in this case, if the aspect ratio of the primary particles 800 of the positive electrode active material particles 610 is 1.5 or more, the capacity of the lithium ion secondary battery can be maintained high.

  As shown in FIG. 11, in a charge / discharge cycle (charge / discharge cycle at a high rate) in which discharge and charge are repeated at a constant current of 4 C, when the density of the positive electrode active material layer 223 is high, The electrolyte is insufficient (withered). For this reason, the capacity tends to decrease. Moreover, even when the density of the positive electrode active material layer 223 is low, cracking occurs. For this reason, when the aspect ratio is close to 1, the capacity tends to decrease. In contrast, when the aspect ratio of the positive electrode active material particles 610 is increased to 1.5 or more, the capacity tends to be maintained high. For this reason, the aspect ratio of the positive electrode active material particles 610 is 1.5 or more, more preferably 2 or more. Thereby, the capacity of the lithium ion secondary battery 100 can be maintained high.

≪Circularity M of primary particles 800≫
Here, the circularity M of the primary particles 800 of the positive electrode active material particles 610 is determined based on the cross-sectional area S of the primary particles 800 and the peripheral length L of the primary particles 800 with respect to the primary particles 800. It may be obtained based on the formula of L ² ;

For example, the circularity M of the primary particles 800 of the positive electrode active material particles 610 is evaluated from the cross-sectional image or the appearance image taken so that the primary particles 800 of the positive electrode active material particles 610 can be seen. At least one primary particle 800 suitable for the extraction may be extracted. And about the extracted primary particle 800, based on the cross-sectional area S of the primary particle 800 and the peripheral length L of the primary particle 800,
M = 4πS / L ² ;
It may be obtained based on the following formula.

  Here, when a plurality of primary particles 800 are extracted from the positive electrode active material particles 610, the circularity M of the primary particles 800 of the positive electrode active material particles 610 may be evaluated by an arithmetic average of the extracted primary particles. Here, from the cross-sectional image taken so that the primary particles 800 of the positive electrode active material particles 610 can be seen, the short diameter of the cross-sectional area of the primary particles 800 is the average of the short diameters when the above-described aspect ratio is calculated. Extract some primary particles 800 close to the value. Among them, the circularity M of the primary particle 800 having the circularity M of the primary particle 800 closest to 1 is set to the circularity M of the primary particle 800 of the positive electrode active material particle 610. In addition, some primary particles 800 that are close to the average value of the short diameter when calculating the aspect ratio described above are extracted, and some primary particles 800 having a circularity of the primary particles 800 close to 1 (for example, five). The arithmetic average of the circularity of the extracted primary particles 800 may be set to the circularity M of the positive electrode active material particles 610.

As the circularity M of the primary particle 800 is closer to 1, the cross section perpendicular to the major axis of the primary particle 800 is closer to a perfect circle. According to the knowledge obtained by the present inventor, the density of the positive electrode active material layer 223 is 2.85 g / cm ³ or less, especially for the charge / discharge cycle at a high rate of about 4C, and the positive electrode active material In the arithmetic average of the positive electrode active material particles contained in the layer, the circularity M of the primary particles of the positive electrode active material particles is preferably 0.92 ≦ M ≦ 0.98. Thereby, the capacity maintenance rate of the lithium ion secondary battery 100 can be maintained high.

For example, FIGS. 12 and 13 show the relationship between the circularity M of the primary particles 800 of the positive electrode active material particles 610 and the capacity retention ratio after the charge / discharge cycle. FIG. 12 shows the circularity M and the capacity retention rate of the lithium ion secondary battery (cycle) when a charge / discharge cycle in which discharge and charge are repeated at a constant current of 0.5 C in the range of 3.0 V to 4.1 V is performed. The relationship with the rear capacity maintenance rate) is shown. FIG. 12 shows the case where the density of the positive electrode active material layer is 2.52 g / cm ³ . FIG. 13 shows the circularity M and the capacity retention rate of the lithium ion secondary battery (capacity after cycle) when a charge / discharge cycle in which discharge and charge are repeated at a constant current of 4 C in the range of 3.0 V to 4.1 V is performed. The maintenance ratio is shown. In Figure 13, the density of the positive electrode active material layer is ^{2.52g / cm 3, 2.85g / cm} 3, are shown for the case of 3.24 g / cm ^3.
Here, based on the image of the cross section of the positive electrode active material layer 223 obtained by VE-9800 manufactured by Keyence, an image of the primary particles 800 of the positive electrode active material particles 610 in the positive electrode active material layer 223 is observed, and the positive electrode active material layer 223 is observed. The circularity M of the primary particle 800 of the material particle 610 was determined.

As shown in FIG. 12, in applications where discharging and charging are repeated at a relatively low current of about 0.5 C, when the density of the positive electrode active material layer is as low as 2.52 g / cm ³ , the circularity M is The capacity maintenance rate of the lithium ion secondary battery tends to be higher when the circularity M is smaller and is not a perfect circle, rather than approaching 1. On the other hand, as shown in FIG. 13, when the density of the positive electrode active material layer is 2.85 g / cm ³ or less, for example, the density of the positive electrode active material layer is 2.52 g / cm ³ , When the circularity M of the primary particles of the material particles is 0.92 ≦ M ≦ 0.98, the capacity of the lithium ion secondary battery can be maintained high.

  The lithium ion secondary battery 100 according to the embodiment of the present invention has been described above, but the lithium ion secondary battery 100 according to the embodiment of the present invention can be variously modified.

<< Positive electrode active material particles 610 >>
For example, the positive electrode active material particles 610 may be a lithium transition metal oxide having a layered structure. In this case, the lithium transition metal oxide may contain Ni, Co, and Mn. By using such positive electrode active material particles 610 made of a lithium transition metal oxide, the above-described tendency for the density f1, the aspect ratio, or the circularity M of the positive electrode active material layer 223 can be obtained more reliably.

More specifically, the lithium transition metal oxide is a compound having a layer structure including Li _{1 + x} Ni _y Co _z Mn _(1-yz) M _γ O ₂ , where 0 ≦ x ≦ 0. .2, 0.1 <y <0.9, 0.1 <z <0.4, M is an additive, 0 ≦ γ ≦ 0.01, and M is Zr, W, It may be at least one kind of additive selected from the group consisting of Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B and F.

  Moreover, the tendency about the density f1, the aspect ratio, or the circularity M of the positive electrode active material layer 223 described above is not limited to the 18650 type evaluation cell described above, and other types of cylindrical batteries, square types, and laminate types. The same tendency can be obtained in other shapes of batteries. Therefore, the present invention is not limited to the 18650 type evaluation cell, and can be widely applied to cylindrical batteries of other sizes, batteries of other shapes such as a square type and a laminate type.

  The lithium ion secondary battery according to one embodiment of the present invention has been described above, but the non-aqueous secondary battery according to the present invention is not limited to the above-described lithium ion secondary battery unless otherwise specified.

  The non-aqueous secondary battery disclosed here can keep the rate of increase in resistance particularly low in a low temperature environment. In particular, in applications where charging and discharging at high rates are repeated, for example, in hybrid vehicles (including plug-in hybrid vehicles) and electric vehicles, it is suitable as a vehicle driving battery in which driving wheels are driven by an electric motor. In the use of such a vehicle driving battery, it is required to discharge at a high output during acceleration, and to rapidly charge the regenerated energy during deceleration. The non-aqueous secondary battery disclosed herein can keep the increase in resistance low in applications where charge and discharge are repeated at such a high rate, and can maintain a high capacity. Therefore, as shown in FIG. 14, the non-aqueous secondary battery 10 (which may be in the form of an assembled battery formed by connecting a plurality of non-aqueous secondary batteries 10 in series) is used as a power source (vehicle drive). A vehicle 1000 (typically, an automobile including an electric motor such as an automobile, in particular, a hybrid automobile or an electric automobile) can be provided.

10 Nonaqueous secondary battery 100 Lithium ion secondary battery 200 Winding electrode body 220 Positive electrode sheet 221 Positive electrode current collector 222 Uncoated part 223 Positive electrode active material layer 240 Negative electrode sheet 241 Negative electrode current collector 242 Uncoated part 243 Negative electrode Active material layer 262, 264 Separator 280 Electrolyte 290 Battery charger 300 Battery case 310 Crevice 320 Container body 340 Cover body 350 Injection hole 352 Sealing cap 360 Safety valve 420 Electrode terminal 440 Electrode terminal 610 Positive electrode active material particle 620 Conductive material 630 Binder 710 Negative electrode active material particles 730 Binder 800 Primary particles 1000 Vehicle WL Winding shaft

Claims (10)

A positive electrode current collector;
A positive electrode active material layer formed on the positive electrode current collector;
Positive electrode active material particles and conductive material contained in the positive electrode active material layer;
With
The positive electrode active material layer has a density f1 of f1 ≦ 2.85 g / cm ³ ,
The non-aqueous secondary battery in which the aspect ratio As of the primary particles of the positive electrode active material particles is As ≧ 1.5 in the arithmetic average of the positive electrode active material particles included in the positive electrode active material layer. The non-aqueous secondary battery according to claim 1, wherein a density f1 of the positive electrode active material layer is 2.20 g / cm ³ ≦ f1. The aspect ratio of the primary particles of the positive electrode active material particles is
For the primary particles of the positive electrode active material particles, the ratio of the longest major axis to the minor axis which is the length of the primary particles of the positive electrode active material particles in the direction orthogonal to the major axis (major axis / minor axis) is determined.
The non-aqueous secondary battery according to claim 1 or 2, wherein the ratio (major axis / minor axis) is an arithmetic average of the extracted primary particles.   The arithmetic average of the positive electrode active material particles contained in the positive electrode active material layer, the circularity M of the primary particles of the positive electrode active material particles is 0.92 ≦ M ≦ 0.98. The non-aqueous secondary battery described in any one of the items. The circularity M of the primary particles of the positive electrode active material particles is
For the primary particles of the positive electrode active material particles, the circularity M is obtained based on the cross-sectional area S and the perimeter L, where the equation for obtaining the circularity M is M = 4πS / L ² ; Item 5. A nonaqueous secondary battery according to Item 4.   The non-aqueous secondary battery according to any one of claims 1 to 5, wherein the positive electrode active material particles are a lithium transition metal oxide having a layered structure.   The non-aqueous secondary battery according to claim 6, wherein the lithium transition metal oxide includes Ni, Co, and Mn. A lithium transition metal oxide is a compound having a layered structure including Li _{1 + x} Ni _y Co _z Mn _(1-yz) M _γ O ₂ .
Here, 0 ≦ x ≦ 0.2, 0.1 <y <0.9, 0.1 <z <0.4, M is an additive, and 0 ≦ γ ≦ 0.01 ,
8. M is at least one additive selected from the group consisting of Zr, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, Al, B, and F. Non-aqueous secondary battery.   An assembled battery in which a plurality of the non-aqueous secondary batteries according to any one of claims 1 to 8 are combined.   A vehicle driving battery comprising the non-aqueous secondary battery according to any one of claims 1 to 8, or the assembled battery according to claim 9.