Non-aqueous secondary battery

Abstract

Non-aqueous secondary battery, which includes: a negative electrode current collector and a negative electrode active material layer held by the negative electrode current collector, The negative electrode active material layer contains negative electrode active material particles and a binder, and The binder contains a rubber-based binder or a resin that has a binder function, wherein the negative electrode active material layer consists of a near-surface region that comprises 1 / 4 of the thickness of the negative electrode active material layer from a surface opposite to the negative electrode current collector, and a residual region that makes up the remaining 3 / 4 of the thickness of the negative electrode active material layer, The near-surface region contains more of the rubber-based binder or resin that has a binder function than the rest of the region. The rubber-based binder and the resin, which has a binder function, contained in the near-surface region, have a combined mass concentration A, and the rubber-based binder and the resin, which has a binder function, contained in the remaining region, have a combined mass concentration B, such that the ratio A / B satisfies the condition 2.0 ≤ (A / B) ≤ 3.8 wherein the negative electrode active material layer is formed by coating the negative electrode current collector with a negative electrode mixture in which the negative electrode active material particles and the binder are dispersed in an aqueous solvent, and by drying the negative electrode mixture so that the binder penetrates into the near-surface area, such that 2.0 ≤ (A / B) ≤ 3.8 is satisfied.

Description

Technical field

[0001] The invention relates to a non-aqueous secondary battery.

[0002] In this description, "secondary battery" refers to batteries that can generally be recharged multiple times and includes so-called electrical storage devices, such as lithium secondary batteries (typically lithium-ion secondary batteries) and nickel-metal hydride batteries. Also in this description, "active material" refers to a substance that can reversibly intercalate and deintercalate (typically encapsulate and release) the chemical species that become the charge carrier in the secondary battery (e.g., lithium ions in a lithium-ion secondary battery). Additionally, "non-aqueous secondary battery" refers to a secondary battery in which a non-aqueous electrolyte (e.g., a non-aqueous electrolyte solution) has been used as the electrolyte. background

[0003] Japanese patent application Publication No. 2008-84742 (JP 2008 - 84 742 A) discloses, for example, a spiderweb-like film of polyvinylidene fluoride (PVDF) on the surface of an electrode mixture layer. According to this publication, such a spiderweb-like PVDF film prevents the formation and ingress of electrode fragments into the battery interior, thus enabling the production of lithium-ion secondary batteries with a low defect rate and improved reliability.

[0004] That is, according to the publication presented here, electrode powder adheres to the adhesive surface of a tape that has been applied to the surface of the electrode mixture layer if there is no PVDF film formed on the surface of the electrode mixture layer.

[0005] However, if a PVDF film is present on the surface of the electrode mixture, the electrode powder does not adhere to the band applied to the electrode mixture layer surface. Accordingly, this publication discloses that the electrode cannot simply fall off, thus suppressing the generation of electrode fragments.

[0006] In a case where an active material is used that exhibits a large volume change as a result of the absorption and release of lithium, the settling of the active material can be suppressed, thereby providing an electrode for a lithium-ion secondary battery that exhibits good cycleability, if the electrode for the lithium-ion secondary battery according to patent literature 2 comprises: a current collector; and an electrode layer formed on a surface of the current collector and containing a binder resin, an active material, and a conductive additive, wherein the electrode layer comprises: a first electrode layer; and a second electrode layer whose binder resin concentration is higher than a binder resin concentration in the first electrode layer; the first electrode layer is arranged on the surface of the current collector;and the second electrode layer on the surface of the current collector is arranged at least in such a way that it makes contact with the surface of the current collector and at least one side surface of the first electrode layer.

[0007] Patent literature 3 discloses an electrode with a front side furthest from the current collector and a back side closest to the current collector, said electrode being arranged on the current collector, and the electrode having a primary gradient of one of the chemical, physical, and performance properties of the electroactive particle composition between the front and back sides, provided that the primary gradient is not a bulk porosity gradient. Citation list Patent literature Patent literature 1: JP 2008 - 84 742 A Patent literature 2: DE 11 2011 101 607 T5 Patent literature 3: WO 2011 / 109 815 A1 Summary of the invention: Technical problem

[0008] In non-aqueous secondary batteries, such as lithium-ion batteries, a negative electrode is used in which a negative electrode active material layer, containing negative electrode active material particles, is held by a negative electrode current collector. Particularly in cases where the lithium-ion battery is used as the energy source for a motor that drives the drive wheels of a vehicle (vehicle traction battery), discharge at a considerably high rate is required. Furthermore, high-rate charging is necessary for charging, which utilizes energy recovery during braking. During high-rate discharge, the negative electrode active material layer must deintercalate a large quantity of lithium ions immediately. During high-rate charging, the negative electrode active material layer must be able to intercalate a large quantity of lithium ions immediately.

[0009] Because vehicle traction batteries are often left outdoors, it is desirable, depending on the climate and season in the region where the vehicle is used, that the required battery function can be achieved across a wide range of temperature environments, from high temperatures up to approximately 60°C to low temperatures down to -30°C. As noted above, a significant amount of lithium ions are repeatedly deintercaled and intercalated within the negative electrode active material layer when a vehicle traction battery is repeatedly charged and discharged at a high rate. However, particularly in a low-temperature environment, the reaction rate of lithium ion deintercalation and intercalation within this negative electrode active material layer decreases, which is a major limiting factor for the battery's reaction rate. Solution to the problem

[0010] The problem is solved by a non-aqueous secondary battery according to claim 1. This battery has a negative electrode current collector and a negative electrode active material layer held by the negative electrode current collector. The negative electrode active material layer comprises negative electrode active material particles and a binder. The binder comprises a rubber-based binder or a resin exhibiting a binder function. The rubber-based binder or the resin exhibiting a binder function is present in excess in a near-surface region of the negative electrode active material layer.

[0011] In this non-aqueous secondary battery, the near-surface region of the negative electrode active material layer contains an excess amount of rubber-based binder or a resin exhibiting binder function. Such a rubber-based binder or resin allows the reaction rate to be maintained at a high level when the chemical species acting as charge carriers in the secondary battery (e.g., lithium ions in a lithium-ion secondary battery) are deintercalary or intercalary within the negative electrode active material layer. As a result, particularly in a low-temperature environment of approximately -15°C, the resistance rise rate after charge-discharge cycles can be kept low.

[0012] Here, the near-surface region is an area comprising 1 / 4 of the thickness of the negative electrode active material layer from its surface. The rubber-based binder and the resin exhibiting a binder function, contained in the near-surface region of the negative electrode active material layer, have a combined mass concentration A, and the rubber-based binder and the resin exhibiting a binder function, contained in sections of the negative electrode active material layer differing from the near-surface region, have a combined mass concentration B such that the ratio A / B satisfies the condition 2.0 ≤ (A / B) ≤ 3.8.

[0013] The rubber-based binder or the resin exhibiting binder function is preferably a binder dispersible in an aqueous solvent. The rubber binder can, for example, be SBR. The resin exhibiting binder function comprises acrylic binders and imide binders. Here, the acrylic binder is at least one binder selected from polyethylene oxide and polyethylene.

[0014] Preferably, the negative electrode active material particles exhibit a tapping density of at least 1 g / cm³ after 150 tapping cycles. 3 This improves the peel resistance of the negative electrode active material layer.

[0015] This non-aqueous secondary battery is preferably designed as a lithium-ion battery. Multiple such non-aqueous secondary batteries can be combined to construct a battery pack. Because the above non-aqueous secondary battery and battery pack are able to maintain a low resistance rise ratio in a low-temperature environment at approximately -15°C, they are particularly well-suited for use as vehicle traction batteries. Brief description of the drawings Fig. Figure 1 is a diagram showing an example of the structure of a lithium-ion secondary battery. Fig. Figure 2 is a diagram showing a wound electrode arrangement for a lithium-ion secondary battery. Fig. 3 is a sectional view, extending along III-III in Fig. 2 is taken. Fig. Figure 4 is a cross-sectional view showing the structure of a positive electrode active material layer. Fig. Figure 5 is a cross-sectional view showing the structure of a negative electrode active material layer. Fig. Figure 6 is a side view showing the weld between an uncoated area of ​​the wound electrode assembly and an electrode pole head. Fig. Figure 7 is a schematic view showing the state of a lithium-ion secondary battery during charging. Fig. Figure 8 is a schematic view showing the state of a lithium-ion secondary battery during discharge. Fig. Figure 9 is a diagram showing a 100A lithium-ion secondary battery according to one embodiment of the invention. Fig. Figure 10 is a diagram showing the structure of a negative electrode active material layer 243A in this lithium ion secondary battery 100A. Fig. Figure 11 is a graphical representation showing the relationship between the A / B ratio and the resistance increase ratio (%) after charge-discharge cycles for the test cells of samples A to C. Fig. Figure 12 is a graphical representation showing the relationship between the tapping density after 150 tappings of the negative electrode active material particles and the peel strength. Fig. Figure 13 is a diagram illustrating the 90° peel-off adhesive strength test procedure. Fig. Figure 14 is a schematic side view of a vehicle (automobile) equipped with a non-aqueous secondary battery (vehicle traction battery) according to an embodiment of the invention. Description of the embodiments

[0016] First, an example of the construction of a lithium-ion secondary battery as a non-aqueous secondary battery is described. Then, with suitable reference to this example construction, a lithium-ion secondary battery according to an embodiment of the invention is described. Components and features that have the same function are designated by the same symbols. The diagrams are drawn schematically and do not necessarily represent the actual dimensions. The diagrams show only examples and, unless otherwise stated, are not limiting to the scope of the invention.

[0017] Fig. Figure 1 shows a 100 kWh lithium-ion secondary battery. This 100 kWh lithium-ion secondary battery exhibits, as shown in Fig. Figure 1 shows a wound electrode arrangement 200 and a battery housing 300. Fig. Figure 2 is a diagram showing the wound electrode arrangement 200. Fig. Figure 3 shows a cross-section extending along III-III in Fig. 2. taken.

[0018] The wound electrode arrangement 200 exhibits, as shown in Fig. Figure 2 shows 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 each strips made of sheet material. << Positive electrode sheet 220 >>

[0019] The positive electrode sheet 220 has a positive electrode current collector 221 and a positive electrode active material layer 223, each in the form of a strip. Preferably, a metal foil suitable for the positive electrode can be used as the positive electrode current collector 221. For example, a strip of aluminum foil with a thickness of approximately 15 µm can be used as the positive electrode current collector 221. An uncoated area 222 is established 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 layers 223 are as shown in Fig. Figure 3 shows a positive electrode active material held on both surfaces of the positive electrode current collector 221, except in the uncoated area 222, which is established on the positive electrode current collector 221. A positive electrode active material is contained in the positive electrode active material layers 223. The positive electrode active material layers 223 are formed by coating a positive electrode mixture containing positive electrode active material onto the positive electrode current collector 221. << Positive electrode active material layers 223 and positive electrode active material particles 610 >>

[0020] Fig. Figure 4 is a cross-sectional view of the positive electrode sheet 220. Fig. Figure 4 shows the positive electrode active material particles 610, the conductive material 620, and the binder 630 within the positive electrode active material layers 223, large and schematically illustrated to clarify the structure of the positive electrode active material layers 223. The positive electrode active material layers 223 comprise, as shown in Fig. 4 shown, positive electrode active material particles 610, a conductive material 620 and a binder 630.

[0021] A substance suitable for use as the positive electrode active material in a lithium-ion secondary battery can be called positive electrode active material particle 610. Illustrative examples of positive electrode active material particles 610 include lithium transition metal oxides such as LiNiCoMnO2 (lithium nickel cobalt manganese mixed oxide), LiNiO2 (lithium nickelate), LiCoO2 (lithium cobaltate), LiMn2O4 (lithium manganate), and LiFePO4 (lithium iron phosphate). For example, LiMn2O4 exhibits a spinel structure. LiNiO2 and LiCoO2 also exhibit a layered rock salt structure. LiFePO4, for example, exhibits an olivine structure. LiFePO4 with an olivine structure can be nanometer-sized particles. Alternatively, LiFePO4 with an olivine structure can be additionally coated with a carbon film. << Conductive material 620 >>

[0022] Conductive material 620 is exemplified by carbon materials such as carbon powder and carbon fibers. Conductive material 620 can consist of a single type selected from these conductive materials and be used alone, or it can consist of two or more types used in combination. The carbon powder used, for example, can be one of various carbon blacks (e.g., acetylene black, oil furnace black, graphitized carbon black, carbon black, graphite, Ketjen black) or it can be a graphite powder. << Binder 630 >>

[0023] The binder 630 binds together the positive electrode active material particles 610 and the particles of the conductive material 620, which are contained in the positive electrode active material layers 223, or binds these particles to the positive electrode current collector 221. A polymer that is able to dissolve or disperse in the solvent used can be used as such a binder 630. For example, in a positive electrode mixture composition that uses an aqueous solvent, water-soluble or water-dispersible polymers, such as cellulose-like polymers (e.g., carboxymethylcellulose (CMC), hydroxypropylmethylcellulose (HPMC)), polyvinyl alcohol (PVA), fluoropolymers (e.g., polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP)), and rubbers (e.g.,Vinyl acetate copolymers, styrene-butadiene copolymers (SBR), and acrylic acid-modified SBR resins (SBR latex). In a positive electrode mixture composition using a non-aqueous solvent, a polymer (e.g., polyvinylidene fluoride (PVDF), polyvinylidene chloride (PVDC), polyacrylonitrile (PAN)) may be preferentially used. << Thickener, solvent >>

[0024] To form the positive electrode active material layers 223, a positive electrode mixture is prepared by blending the positive electrode active material particles 610 described above and the conductive material 620 within a solvent to form a paste (slurry). The mixture is then coated onto the positive electrode current collector 221, followed by drying and rolling. Currently, either an aqueous or a non-aqueous solvent can be used as the solvent in the positive electrode mixture. A preferred example of a non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). The polymer materials referred to above as the binder 630 can be used to manifest not only the function of a binder but also the functions of a thickener and other additives in the positive electrode mixture.

[0025] The weight ratio of the positive electrode active material as a proportion of the total positive electrode mixture is generally preferably at least approximately 50 wt.% (typically from 50 to 95 wt.%), and more preferably from approximately 70 to approximately 95 wt.% (e.g., from 75 to 90 wt.%). The ratio of the conductive material as a proportion of the total positive electrode mixture can be adjusted to approximately 2 to approximately 20 wt.%; it is generally preferred that this ratio be adjusted to approximately 2 to approximately 15 wt.%. In the composition in which binder is used, the ratio of binder as a proportion of the total positive electrode mixture can be adjusted to approximately 1 to approximately 10 wt.%; it is generally preferred that this ratio be adjusted to approximately 2 to approximately 5 wt.%. << Negative electrode sheet 240 >>

[0026] The negative electrode sheet 240 shows, as in Fig. Figure 2 shows a negative electrode current collector 241 and a negative electrode active material layer 243, each in the form of a strip. A metal foil suitable for the negative electrode can advantageously be used as the negative electrode current collector 241. For example, a strip of copper foil with a thickness of approximately 10 µm can be used as the negative electrode current collector 241. An uncoated area 242 is established along the edge on one side in the width direction of the negative electrode current collector 241. The negative electrode active material layers 243 are formed on both surfaces of the negative electrode current collector 241 except in the uncoated area 242 established on the negative electrode current collector 241. The negative electrode active material layers 243 are held on the negative electrode current collector 241 and include at least one negative electrode active material.The negative electrode active material layers 243 are formed by coating a negative electrode active material-containing negative electrode mixture onto the negative electrode current collector 241. << Negative electrode active material layer 243 >>

[0027] Fig. Figure 5 is a cross-sectional view of the negative electrode sheet 240 in a lithium-ion secondary battery 100. The negative electrode active material layers 243 include, as shown in Fig. 5 shown, negative electrode active material particles 710, a thickening agent (not shown), a binder 730 and similar. In Fig. 5 The negative electrode active material particles 710 and the binder 730 within the negative electrode active material layers 243 are shown large and schematically to clarify the structure of the negative electrode active material layers 243. << Negative electrode active material particles 710 >>

[0028] One, two, or more types of materials previously used in lithium-ion secondary batteries can be used as the negative electrode active material particles 710 without particular limitation. Such materials are illustrated by particle-like carbon materials (carbon particles) that contain a graphite structure (layered structure) in at least one section. More specifically, the negative electrode active material can be natural graphite, natural graphite coated with an amorphous carbon material, graphite, non-graphitized carbon (hard carbon), graphitable carbon (soft carbon), or a carbon material that is a combination thereof. Of these, the diagrams show one case in which lamellar graphite was used as the negative electrode active material particle 710, although the negative electrode active material particles 710 are not limited to this illustrated example.

[0029] Examples of other negative electrode active materials include metal compounds (preferably metal oxides) in which silicon, germanium, tin, lead, aluminum, gallium, indium, arsenic, antimony, bismuth, or similar metals serve as the constituent metal element. Alternatively, lithium titanium oxide (LTO) can be used as the negative electrode active material particles. The negative electrode active material composed of a metal compound can be used in the form of granular bodies of excellent conductivity, obtained, for example, by completely coating the surface of the metal oxide with a carbon film. In this case, no conductive material needs to be included in the negative electrode active material layer, and the conductive material content can be lower than in common practice.Additional forms of such negative electrode active materials and their characteristics, such as particle size, can be appropriately selected according to the required properties. << Thickener, solvent >>

[0030] To form the negative electrode active material layers 243, a negative electrode mixture is prepared by mixing the negative electrode active material particles 710 and the binder 730 described above within a solvent to form a paste (slurry), then coating the mixture onto the negative electrode current collector 241, followed by drying and rolling. Currently, either an aqueous or a non-aqueous solvent can be used as the solvent in the negative electrode mixture. A preferred example of a non-aqueous solvent is N-methyl-2-pyrrolidone (NMP). The above example uses the binder 630 for the positive electrode active material layers 223 (see Fig. 4) The polymer materials described above can be used as the binder 730. The polymer materials described above as an example of the binder 630 for the positive electrode active material layers 223 can also be used to manifest not only the function of a binder but also the function of a thickening agent and other additives in the positive electrode mixture or negative electrode mixture. << Separators 262, 264 >>

[0031] Separators 262 and 264 are, as in Fig. 1 and Fig. Figure 2 shows components that separate the positive electrode sheet 220 and the negative electrode sheet 240. In this example, the separators 262 and 264 are strips of sheet material of predetermined width, which have a plurality of small pores. Separators having a single-layer or multi-layer construction and composed of, for example, a porous polyolefin resin can be used as separators 262 and 264. In this example, as shown in Fig. 2 and Fig. As shown in Figure 3, the negative electrode active material layers 243 have a width b1, which is slightly wider than the width a1 of the positive electrode active material layers 223. Additionally, the separators 262 and 264 each have widths c1 and c2, which are slightly wider than the width b1 of the negative electrode active material layers 243 (c1, c2 > b1 > a1).

[0032] In the embodiment described in Fig. 1 and Fig. As shown in Figure 2, separators 262 and 264 are composed of sheet-like components. These separators both insulate between the positive electrode active material layers 223 and the negative electrode active material layers 243, and allow the movement of electrolyte between them. Therefore, they are not limited to sheet-like components. Instead of sheet-like components, separators 262 and 264 can each be composed of, for example, a layer of particles with electrically insulating properties formed on the surface of the positive electrode active material layers 223 or the negative electrode active material layers 243. Here, the electrically insulating particles can be composed of an electrically insulating inorganic filler (e.g., a filler such as a metal oxide or a metal hydroxide) or electrically insulating resin particles (e.g.,particles made of, for example, polyethylene, polypropylene or similar materials).

[0033] In this wound electrode arrangement 200, as in Fig. 2 and Fig. As shown in Figure 3, the positive electrode sheet 220 and the negative electrode sheet 240 are arranged one above the other with the separators 262 and 264 interposed between them, so that the positive electrode active material layers 223 face the negative electrode active material layers 243. More precisely, in the wound electrode arrangement 200, the positive electrode sheet 220, the negative electrode sheet 240, and the separators 262 and 264 are arranged as layers one above the other in the following order: positive electrode sheet 220, separator 262, negative electrode sheet 240, separator 264.

[0034] Furthermore, at this point, the positive electrode active material layers 223 and the negative electrode active material layers 243 are opposite each other, with separators 262 and 264 interposed between them. A section of the positive electrode current collector 221, on which the positive electrode active material layers 223 are not formed (uncoated area 222), also protrudes on one side of the section of the positive electrode current collector 221 where the positive electrode active material layers 223 face the negative electrode active material layers 243. A section of the negative electrode current collector 241, on which the negative electrode active material layers 243 have not been formed (uncoated area 242), protrudes on one side opposite where this uncoated area 222 protrudes. << Battery housing 300 >>

[0035] In this example, the battery casing is 300, as shown in Fig. Figure 1 shows a so-called prismatic battery housing comprising a container body 320 and a lid 340. The container body 320 is a flat, box-like container in the shape of a square tube, closed at the bottom and open on one side (the top). The lid 340 is a component attached to the opening on the top of the container body 320 and serves to close the opening.

[0036] In secondary batteries for installation in vehicles, it is desirable to increase the battery's weight-to-energy efficiency (battery capacity per unit weight) to improve the vehicle's fuel efficiency. Accordingly, in this embodiment, a lightweight metal, such as aluminum or an aluminum alloy, is used for the container body 320 and the lid 340, which constitute the battery housing 300. By proceeding in this way, the weight-to-energy efficiency can be increased.

[0037] The battery housing 300 has a flat rectangular inner space that serves as the space for housing the wound electrode assembly 200. As shown in Fig. As shown in Figure 1, the flat inner space of the battery housing 300 is slightly wider than the width of the wound electrode assembly 200. In this embodiment, the battery housing 300 has a container body 320 in the form of a square tube, which is closed at the bottom, and a lid 340, which closes the opening in the container body 320. Electrode clamps 420, 440 are also attached to the lid 340 of the battery housing 300. The electrode clamps 420, 440 extend through the battery housing 300 (lid 340) and protrude outside the battery housing 300. Additionally, an electrolyte injection port 350 and a safety valve 360 ​​are provided in the lid 340.

[0038] As in Fig. As shown in Figure 2, the wound electrode arrangement 200 was bent and flattened under an applied force in a direction perpendicular to the winding axis WL. In the Fig. In the example shown, the uncoated area 222 of the positive electrode current collector 221 and the uncoated area 242 of the negative electrode current collector 241 are helically exposed on both sides of the separators 262 and 264, respectively. As shown in Fig. As shown in Figure 6, in this embodiment the central sections 224, 244 of the uncoated areas 222, 242 are gathered together and welded to the tips 420a, 440a of the electrode clamps 420, 440. Currently, due to differences in the respective materials, a technique such as ultrasonic welding is used to weld the electrode clamp 420 to the positive electrode current collector 221, and a technique such as resistance welding is used to weld the electrode clamp 440 to the negative electrode current collector 241. Fig. Figure 6 is a side view showing the weld points between the central section 224 (244) of an uncoated area 222 (242) of the wound electrode arrangement 200 and an electrode clamp 420 (440), and is a cross-sectional view showing along VI-VI in Fig. 1 is taken.

[0039] The wound electrode assembly 200 is attached in its bent and flattened state to the electrode clamps 420, 440, which are fixed to the cover 340. This wound electrode assembly 200 is, as shown in Fig. 1 shows the flat internal space of the container body 320. After the wound electrode arrangement 200 has been placed therein, the container body 320 has been closed with the lid 340. The joint 322 (see Fig. 1) The area between the cover 340 and the container body 320 is welded and sealed by laser welding or similar means. In this example, the wound electrode assembly 200 is positioned inside the battery housing 300 by the electrode clamps 420, 440, which are fixed to the cover 340 (battery housing 300). << Electrolyte solution >>

[0040] Next, an electrolyte solution is injected into the battery casing 300 through the electrolyte injection hole 350 provided in the cover 340. This electrolyte solution is a non-aqueous electrolyte solution, that is, an electrolyte solution in which water is not the solvent. For example, the electrolyte solution used can be one obtained by containing LiPF6 up to a concentration of about 1 mol / L within a mixed solvent of ethylene carbonate and diethyl carbonate (such as a mixture of these solvents in a volume ratio of approximately 1:1). After the electrolyte solution has been injected, a metal sealing cap 352 is attached to the electrolyte injection hole 350 (e.g., by welding), thereby sealing the battery casing 300. The electrolyte solution is not limited to the electrolyte solution described herein.For example, non-aqueous electrolyte solutions that have previously been used in lithium secondary batteries can be used as suitable electrolyte solutions. < < Pores >>

[0041] The positive electrode active material layers 223 have a small gap 225 (see Fig. 4) which can also be called cavities between the positive electrolyte active material particles 610, particles of the conductive material 620, and similar components. The electrolyte solution (not shown) is able to infiltrate these small gaps in the positive electrode active material layers 223. Similarly, the negative electrode active material layers 243 exhibit small gaps 245 (see Fig. 5) which can also be called cavities between, for example, the positive electrolyte active material particles 710. These gaps 225, 245 (cavities) are appropriately referred to here as "pores". In the wound electrode arrangement 200, which is in Fig. As shown in Figure 2, the uncoated areas 222, 242 are wound helically on both sides along the winding axis WL. The electrolyte solution is able to infiltrate through gaps in the uncoated areas 222, 242 on both sides 252, 254 along this winding axis WL. The electrolyte solution thus penetrates the positive electrode active material layers 223 and the negative electrode active material layers 243 throughout the interior of the lithium-ion secondary battery 100. << Gas ventilation routes >>

[0042] In this example, the flat inner space of the battery housing 300 is slightly wider than the wound electrode assembly 200, which has been flattened. Gap spaces 310, 312 are provided on both sides of the wound electrode assembly 200 between the wound electrode assembly 200 and the battery housing 300. These gaps 310, 312 serve as pathways for gas venting. In cases where overcharging has occurred, for example, the temperature of the lithium-ion secondary battery 100 becomes abnormally high, causing the electrolyte solution to decompose and potentially leading to abnormal gas generation.In this embodiment, such abnormally generated gases move towards the safety valve 360 ​​by passing through the gaps 310, 312 between the wound electrode arrangement 200 and the battery housing 300 on both sides of the wound electrode arrangement 200 and are discharged through the safety valve 360 ​​to the outside of the battery housing 300.

[0043] In this 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 via the electrode terminals 420, 440, which pass through the battery housing 300. The operation of the lithium-ion secondary battery 110 during charging and discharging is described below. << Operation during charging >>

[0044] Fig. Figure 7 schematically shows the state during charging of the lithium-ion secondary battery 100. During charging, as shown in Fig. Figure 7 shows the electrode terminals 420, 440 of the lithium-ion secondary battery 100 (see Fig. 1) connected to a charger 290. Due to the action of the charger 290 during charging, lithium ions (Li) are released into the electrolyte solution 280 from the positive electrode active material within the positive electrode active material layer 223. At the same time, electrical charges are released from the positive electrode active material layer 223. The released charges pass through the conductive material (not shown) and are sent to the positive electrode current collector 221, from where they then pass through the charger 290 and are sent to the negative electrode sheet 240. The charges are collected at the negative electrode sheet 240, together with the lithium ions (Li) within the electrolyte solution 280, which are absorbed and stored by the negative electrode active material within the negative electrode active material layer 243. << Operation during unloading >>

[0045] Fig. Figure 8 schematically shows the state during the discharge of the lithium-ion secondary battery 100. During discharge, as shown in Fig. As shown in Figure 8, electrical charges are sent from the negative electrode sheet 240 to the positive electrode sheet 220, along which lithium ions stored in the negative electrode active material layer 243 are released into the electrolyte solution 280. At the positive electrode, lithium ions from the electrolyte solution 280 are also absorbed into the positive electrode active material within the positive electrode active material layer 223.

[0046] Lithium ions then move back and forth between the positive electrode active material layer 223 and the negative electrode active material layer 243 by passing through the electrolyte solution 280 during charging and discharging of the lithium-ion secondary battery 100. During charging, electrical charges are also sent from the positive electrode active material to the positive electrode current collector 221 by passing through the conductive material. Conversely, during discharging, electrical charges are returned to the positive electrode active material at the positive electrode current collector 221 by passing through the conductive material.

[0047] It is assumed that more uniform movement of lithium ions and electrons during charging enables efficient and rapid charging. More uniform movement of lithium ions and electrons during discharging is assumed to lower battery resistance, increase the discharge rate, and increase battery current. << Other battery configurations >>

[0048] An example of a lithium-ion secondary battery was described above, although lithium secondary batteries are not limited to the preceding configuration. Electrode sheets obtained by similarly coating an electrode mixture onto a metal foil can also be used in various other battery configurations. For example, other known battery configurations include cylindrical batteries and laminated batteries. Cylindrical batteries are batteries in which the wound electrode array is housed in a cylindrical battery casing. Laminated batteries are batteries in which the positive electrode sheet and the negative electrode sheet are laminated together with a separator between them.

[0049] A lithium secondary battery is described below as a non-aqueous secondary battery according to an embodiment of the invention. In the following description, components and sections that have the same functions as in the lithium secondary battery 100 described above are represented by the same symbols, and reference is made, where necessary, to the diagram of the lithium secondary battery 100 described above. << Lithium secondary battery 100A >>

[0050] Fig. Figure 9 shows a lithium-ion secondary battery 100A, which serves as an embodiment of the non-aqueous secondary battery disclosed herein. Fig. Figure 10 shows the structure of the negative electrode active material layer 243A in this lithium ion secondary battery 100A.

[0051] As in Fig. As shown in Figure 9, the lithium-ion secondary battery 100A has a negative electrode current collector 241A and a negative electrode active material layer 243A, which is held by the negative electrode current collector 241A. As shown in Fig. As shown in Figure 10, the negative electrode active material layer 243A comprises negative electrode active material particles 710 and a binder 730. In this embodiment, the binder 730 comprises a rubber-based binder or a resin exhibiting binder function (e.g., an acrylic binder or an imide binder). Additionally, the rubber-based binder or the resin exhibiting binder function is present in excess within the negative electrode active material layer 243A in a near-surface region. With such a non-aqueous secondary battery, the reaction rate of lithium ion deintercalation and intercalation in the negative electrode active material layer 243A can be maintained at a high level. This makes it possible to keep the resistance rise ratio low after charge-discharge cycles in a low-temperature environment at approximately -15°C.The non-aqueous secondary battery mentioned above is explained in detail below. << Negative electrode active material layer 243A >>

[0052] The negative electrode active material layer 243A contains a rubber-based binder or a resin that exhibits binder function (e.g., acrylic binder, imide binder, polyethylene oxide (PEO), and polyethylene (PE)). An example of a rubber-based binder is styrene-butadiene gum (SBR). SBR, PEO, and PE are all binders that are dispersible in an aqueous solvent. The imide binder can be any imide binder capable of being used as the binder in the negative electrode active material layer 243A of a non-aqueous secondary battery. Such imide binders are exemplified by polyamide imides.

[0053] In this embodiment, as in Fig. As shown in Figure 10, the binder 730, which is a rubber-based binder or a resin exhibiting binder function, is present in excess in the near-surface region A1 of the negative electrode active material layer 243A. That is, in the near-surface region A1 of the negative electrode active material layer 243A, the mass concentration of the rubber-like binder or resin exhibiting binder function is higher than in other sections of the negative electrode active material layer 243A.

[0054] Here, the near-surface region A1 of the negative electrode active material layer 243A is defined as a region comprising 1 / 4 of the thickness of the negative electrode active material layer 243A from its surface. The rubber-based binder and the resin exhibiting a binder function, contained within the near-surface region A1 of the negative electrode active material layer 243A, have a combined mass concentration A, and the rubber-based binder and the resin exhibiting a binder function, contained in sections of the negative electrode active material layer 243A that differ from the near-surface region A1, have a combined mass concentration B, such that the ratio A / B is from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8).

[0055] In this lithium-ion secondary battery 100A, the rubber-based binder or the resin that has a binder function is therefore distributed disproportionately in the near-surface area A1 of the negative electrode active material layer 243A.As mentioned above, if the near-surface region A1 of the negative electrode active material layer 243A is defined as a region comprising 1 / 4 of the thickness of the negative electrode active material layer 243A from its surface, the ratio A / B of the combined mass concentration A of the rubber-based binder and the resin having a binder function included in the near-surface region A1 of the negative electrode active material layer 243A to the combined mass concentration B of the rubber-based binder and the resin having a binder function included in sections of the negative electrode active material layer 243A that differ from the near-surface region A1 is approximately 2.0 to approximately 3.8.This means that the rubber-based binder and the resin, which has a binder function, are present in a considerable excess in the near-surface region A1 of the negative electrode active material layer 243A, which is from approximately 2 times to approximately 3.8 times the level in sections of the negative electrode active material layer 243A that differ from the near-surface region A1.

[0056] In the 100A lithium-ion secondary battery, as shown above, the rubber-based binder and the resin, which has a binding function, are present in a highly disproportionately high concentration in the near-surface region A1 of the negative electrode active material layer 243A. Therefore, the reaction rate of lithium ion deintercalation and intercalation in the negative electrode active material layer 243A can be maintained at a high rate. Particularly in a low-temperature environment at approximately -15°C, the resistance of the 110A lithium-ion secondary battery can be kept low in applications where charging and discharging are continuously repeated at a high rate.

[0057] The inventors suspect the following regarding this effect.

[0058] Normally, the rubber-based binder and the resin, which has a binder function, are not present to a high degree disproportionately in the near-surface region A1 of the negative electrode active material layer 243A, as described above. In this case, the sites where lithium ions deintercalate and intercalate are not uniform on the surface of the negative electrode active material layer 243 (see Fig. 7 or Fig. 8) present. That is, sites where lithium ions deintercalate and intercalate with relative ease, and sites where lithium ions deintercalate and intercalate with relative difficulty, are both present on the surface of the negative electrode active material layer 243A. Such a given lack of uniformity in the intercalation and deintercalation of lithium ions on the surface of the negative electrode active material layer 243A occurs in the same way in the opposite positive electrode active material layer 223 (see Fig. 7 or Fig. 8) both locations where the battery reaction proceeds with relative ease and locations where the battery reaction proceeds with relative difficulty. Furthermore, if there is a lack of uniformity in the intercalation and deintercalation of lithium ions on the surface of the negative electrode active material layer 243A, this will be a factor in the resistance rise ratio of the entire battery. It is not exactly known what causes the lack of uniformity in the intercalation and deintercalation of lithium ions on the surface of the negative electrode active material layer 243.

[0059] In the 100A lithium-ion secondary battery described above, the rubber-based binder and the resin, which acts as a binder, are present in a highly disproportionately high concentration in the near-surface region A1 of the negative electrode active material layer 243A. In this case, the flux of lithium ions deintercalating from the negative electrode active material particles 710 into the negative electrode active material layer 243A is enhanced by the rubber-based binder and the resin, which are present in a highly disproportionately high concentration in the near-surface region A1 of the negative electrode active material layer 243A. Furthermore, the points at which the lithium ions are intercalated appear to be generally evenly distributed across the surface of the negative electrode active material layer 243A.The lack of uniformity in the intercalation and deintercalation of lithium ions at the surface of the negative electrode active material layer 243A is thereby eliminated, and the intercalation and deintercalation of lithium ions proceeds with relative uniformity at the surface of the negative electrode active material layer 243A. When the intercalation and deintercalation of lithium ions is carried out with general uniformity at the surface of the negative electrode active material layer 243A, the battery reactions also proceed more uniformly in the opposite positive electrode active material layer 223 (see ). Fig. 7 or Fig. 8) forward, which in turn allows the resistance of the battery as a whole to be kept low. This is certainly what the inventors believe will happen.

[0060] To form the negative electrode active material layer 243A, a negative electrode mixture is prepared, for example, by mixing negative electrode active material particles 710 and a binder 730 within a solvent to form a paste (slurry). The mixture is then coated onto the negative electrode current collector 241, followed by drying and rolling. Migration occurs during the drying step in this formation process. Due to migration, the rubber-based binder and the resin, which has a binder function, move somewhat towards the near-surface region A1 of the electrode active material layer 243A. However, in general, the rubber binder and the resin, which has a binder function, do not reach a level in the near-surface region A1 of the negative electrode active material layer 243A that is approximately 2 to 3.8 times higher than the level of these components in other sections of the negative electrode active material layer 243A.

[0061] In this embodiment, a negative electrode mixture is prepared by mixing the negative electrode active material particles 710 and the binder 730 within a solvent to form a paste (slurry). An aqueous solvent was used, and the amount of solvent was relatively large compared to the negative electrode active material particles 710 and the binder 730. Furthermore, a high drying temperature was used in the drying step. This facilitated migration and adjusted the levels of the rubber-based binder and resin, which exhibits binder function, in the near-surface region A1 of the negative electrode active material layer 243A to approximately 2 to approximately 3.8 times the levels in other sections of the negative electrode active material layer 243A.

[0062] In this embodiment, the mass ratio of the negative electrode active material particles 710, CMC as a thickening agent, and the binder 730 was further refined to negative electrode active material particles 710 : CMC : binder 730 = 98 : 1 : 1. Pure water was added as the solvent for the negative electrode mixture. The viscosity of the negative electrode mixture to be coated was adjusted using a Brookfield viscometer; the viscosity of the negative electrode mixture at room temperature (here 25°C) was approximately 3000 mPa·s (20 rpm). Migration during the drying step was also promoted by immediately placing the negative electrode mixture in a drying atmosphere after coating.

[0063] Furthermore, the migration was greatly enhanced here, increasing the level of rubber-based binder resin and the resin exhibiting a binder function in the near-surface region A1 of the negative electrode active material layer 243A to approximately 2 to 3.8 times the level found in other regions of the negative electrode active material layer 243A. The formation of the negative electrode active material layer 243A is not limited to this process.

[0064] Here, a process that significantly enhances migration was mentioned as the process for forming the negative electrode active material layer 243A, but the process for forming the negative electrode active material layer 243A is not limited to this method. Another exemplary process for forming the negative electrode active material layer 243A involves multiple applications of the negative electrode mixture (in other words, at least two). In such a case, it is desirable to increase the proportion of rubber-based binder and resin, which has a binder function, in the negative electrode mixture that is applied the second and subsequent times and can form in the near-surface region of the negative electrode active material layer 243A.

[0065] As a result, in this embodiment, the rubber-based binder and the resin having a binder function were set in the near-surface region A1 of the negative electrode active material layer 243A to a level of approximately 2 times to approximately 3.8 times that in other sections of the negative electrode active material layer 243A.

[0066] To determine how the rubber-based binder and the resin exhibiting binder function are distributed within the negative electrode active material layer 243A, it is advantageous to use EDX analysis (energy-dispersive X-ray spectroscopy or “EDAX”) based on a sectional scanning electron microscope (SEM) image of the formed negative electrode active material layer 243A. Based on this technique, it is possible to identify how the rubber-based binder and the resin exhibiting binder function are distributed within the negative electrode active material layer 243A.

[0067] For example, in a case where SBR is included as a rubber-based binder in the negative electrode active material layer 243A, the SBR is first stained with bromine (Br), followed by determining the distribution of Br within the negative electrode active material layer 243A by energy-dispersive X-ray analysis. This makes it possible to determine how SBR is distributed within the negative electrode active material layer 243A.

[0068] The energy-dispersive X-ray analyzer (EDX analyzer) used here was a MACHS 200 manufactured by Shimadzu Corporation. • Acceleration voltage: 15 kV • Working distance (WD): approximately 10 mm • Sample current: 60 nA to 70 nA • Magnification: 1000x

[0069] The distribution of not only SBR but also binder within the negative electrode active material layer can be determined by EDX analysis. Here, A is the combined mass concentration of the rubber-based binder and the resin exhibiting binder function contained in the near-surface region A1 of the negative electrode active material layer 243A, and B is the combined mass concentration of the rubber-based binder and the resin exhibiting binder function contained in sections of the negative electrode active material layer 243A that differ from the near-surface region A1; the ratio A / B should be from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8). In this way, it is possible, for example, to keep the resistance rise ratio low after charge-discharge cycles in which charging and discharging are continuously repeated. < <testzelle>>

[0070] Test cells with different A / B ratios were fabricated. The resistance rise ratio after a given number of charge / discharge cycles was evaluated for each test cell. The test cells were constructed as type 18650 cells. Additionally, samples A through C with different negative electrode active material layer structures were prepared for the test cells. Sample A is described below, and samples B and C are also described sequentially. <<Positivelektrode von Testzellen> >

[0071] A positive electrode mixture was prepared for use in forming the positive electrode active material layer in the positive electrode. A ternary lithium transition metal oxide (LiNi) was included in the positive electrode mixture. 1 / 3 Co 1 / 3 Mn 1 / 3 O2) was used as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder. Accordingly, the positive electrode active material, the conductive material, and the binder were adjusted to a mass ratio of positive electrode active material : conductive material : binder = 91 : 6 : 3. The positive electrode mixture was prepared by combining the positive electrode active material, the conductive material, and the binder with ion-exchanged water. Next, the positive electrode mixture was coated on both sides of a positive electrode current collector and dried. Here, an aluminum foil (15 µm thick) was used as the positive electrode current collector. This resulted in a positive electrode (positive electrode sheet) with a positive electrode active material layer on both sides of the positive electrode current collector.The positive electrode sheet was dried, then rolled to a thickness of 110 µm using a roller press. The coating weight of the positive electrode mixture on the positive electrode current collector was adjusted so that, after the positive electrode mixture had dried, the weight of the positive electrode active material layer per surface unit of the positive electrode current collector was 25 mg / cm². 2 became. <<Negativelektrode von Testzelle (Probe A)> >

[0072] This will first be described with regard to Sample A. In Sample A, the negative electrode mixture was prepared using natural graphite in the form of flakes as the negative electrode active material, carboxymethylcellulose (CMC) as the thickener, and a binder. Styrene-butadiene gum (SBR), a rubber-based binder, was used as the binder in Sample A.

[0073] In sample A, the negative electrode active material, the thickening agent (CMC), and the binder (SBR) were adjusted to a mass ratio of negative electrode active material : CMC : SBR = 98 : 1 : 1. A negative electrode mixture was prepared by combining the negative electrode active material, CMC, and SBR with ion-exchanged water. Next, the negative electrode mixture was coated on both sides of the negative electrode current collector and dried. A copper foil (10 µm thick) was used as the negative electrode current collector. This produced a negative electrode (negative electrode sheet) with a negative electrode active material layer on both sides of the negative electrode current collector. The negative electrode sheet was dried and then rolled to a thickness of 100 µm using a roller press.The thickness of the negative electrode active material layer formed on each side of the negative electrode current collector was set to 45 µm. The coating weight of the negative electrode mixture on the negative electrode current collector was set such that, after the negative electrode mixture had dried, the weight of the negative electrode active material layer per surface unit of the negative electrode current collector was 13 mg / cm². 2 became. < <testzellenseparator>>

[0074] A separator composed of a porous sheet with a three-layer structure (PP / PE / PP) of polypropylene (PP) and polyethylene (PE) was used. <<Montage von Testzellen> >

[0075] Type 18650 cells (lithium-ion batteries) for testing were fabricated using the negative electrode, positive electrode, and separator as described above. A cylindrical wound electrode assembly was produced by laminating and winding the positive and negative electrode sheets with a separator sandwiched between them. The wound electrode assembly was placed in a cylindrical battery case, followed by injection of a non-aqueous electrolyte solution and sealing of the battery case, thus completing the construction of the test cell.The non-aqueous electrolyte solution used here was an electrolyte solution prepared by dissolving 1 mol / L (lithium salt-based) of LiPF6 in a mixed solvent composed of ethylene carbonate (EC), diethyl carbonate (DMC) and ethyl methyl carbonate (EMC) in a given volume ratio (EC : DMC : EMC = 3 : 4 : 3).

[0076] Samples A to C are each described in detail below. <<Probe A (Bindemittel: SBR; Dicke an Negativelektrodenaktivmaterialschicht: 45 µm)> >

[0077] As described above, in sample A, the rubber-based binder styrene-butadiene rubber (SBR) is used as the binder in the negative electrode active material layer. The thickness of the negative electrode active material layer formed on each side of the negative electrode current collector was 45 µm.

[0078] A number of Probe A test cells were produced in which the ratio A / B of the mass concentration A of SBR contained in the near-surface region A1 of the negative electrode active material layer to the mass concentration B of SBR contained in sections of the negative electrode active material layer differing from the near-surface region A1 was varied. The resistance rise ratio after continuous charging and discharging was measured for each of the test cells.

[0079] The degree of migration and the A / B ratio were regulated by preparing the negative electrode mixture during the formation of the negative electrode active material layer and by appropriately adjusting the conditions in the subsequent drying step. Here, the near-surface region A1 of the negative electrode active material layer was defined as an area comprising 1 / 4 of the thickness of the negative electrode active material layer from the surface. This also applies to samples B and C. <<Probe B (Bindemittel : PVDF; Dicke an Negativelektrodenaktivmaterialschicht : 45 um)> >

[0080] In sample B, PVDF was used instead of SBR as the binder in the negative electrode active material layer. With the use of PVDF, the non-aqueous solvent N-methyl-2-pyrrolidone (NMP) was used as the solvent for preparing the negative electrode mixture. The negative electrode active material and the binder (PVDF) were set to a mass ratio of negative electrode active material : PVDF = 98 : 2. Here, apart from the use of PVDF as a binder, the design was largely the same as in examples obtained using SBR.

[0081] A number of sample B test cells were prepared in which the ratio A / B of the mass concentration A of PVDF contained in the near-surface region A1 of the negative electrode active material layer to the mass concentration B of PVDF contained in sections of the negative electrode active material layer differing from the near-surface region A1 was varied. The resistance rise ratio after continuous charging and discharging was measured for each of the test cells. <<Probe C (Bindemittel: SBR; Dicke an Negativelektrodenaktivmaterialschicht: 75 µm)> >

[0082] The samples were prepared so that the thickness of the negative electrode active material layer on each side of the negative electrode current collector was approximately 75 µm. The coating weight of negative electrode mixture on the negative electrode current collector was adjusted so that, after drying, the weight of the negative electrode material layer per unit surface area of ​​the negative electrode current collector was 20 mg / cm². 2 became.

[0083] A number of sample C test cells were prepared in which the ratio A / B of the mass concentration A of SBR contained in the near-surface region A1 of the negative electrode active material layer to the mass concentration B of SBR contained in sections of the negative electrode active material layer differing from the near-surface region A1 was varied. The resistance rise ratio after continuous charging and discharging was measured for each of the test cells. <<Verhältnis A / B> >

[0084] Here, the ratio A / B of the mass concentration A of binder contained in the near-surface region A1 of the negative electrode active material layer 243A to the mass concentration B of binder contained in sections of the negative electrode active material layer 243A that differ from the near-surface region A1 was determined for each sample. This ratio A / B can be measured by EDX analysis (EDAX) as described above. <<Widerstandsanstiegsverhältnis (Widerstandsanstiegsverhältnis (%) nach Lade- / Entladezyklen)> >

[0085] Each of the test cells for samples A to C is set to a state of charge (SOC) of 60% after being subjected to predetermined conditions, and the initial resistance V1 is measured. Next, charging and discharging are continuously repeated for a specified number of charge and discharge cycles in a -15°C temperature environment. The resistance (resistance after cycles V2) is measured before and after the implementation of charge / discharge cycles. Here, the resistance rise ratio V is the ratio (V2 / V1) of the resistance after cycles V2 to the initial resistance V1. < <konditionierung>>

[0086] Conditioning is performed here through the following steps 1 and 2. Step 1: Constant current charging at 1C until 4.1V is reached, followed by 5 minutes of standby mode. Step 2: After step 1, constant voltage charging for 1.5 hours, followed by 5 minutes of standby mode.

[0087] With this type of conditioning, the necessary reactions occur due to initial charging, and a gas is generated. Additionally, the desired film forms, for example, on the negative electrode active material layer. <<Messung der bewerteten Kapazität> >

[0088] After the conditioning described above, the rated capacity (nominal capacity) of the test cell is measured. The rated capacity is measured using the following steps 1 to 3. To ensure a consistent influence of temperature, the rated capacity is measured in a 25°C temperature environment. Step 1: Constant current discharge at 1C until 3.0V is reached, followed by 2 hours of constant voltage discharge, then 10 seconds of standby. Step 2: Constant current charging at 1C until 4.1V is reached, followed by 2.5 hours of constant voltage discharge, then 10 seconds of standby. Step 3: Constant current discharge at 0.5 C until 3.0 V is reached, followed by 2 hours of constant voltage discharge, then 10 seconds of standby.

[0089] Here, the discharge capacity in discharge from constant current discharge to constant voltage discharge in step 3 (CCCV discharge capacity) is treated as the "rated capacity". < <soc-einstellung>>

[0090] The state of charge (SOC) is set by steps 1 and 2 below. The SOC setting should be performed here, followed by the conditioning step described above and the measurement of the rated capacity. To ensure a consistent effect of temperature, the SOC setting is performed in a 25°C temperature environment. Step 1: Charging from 3V at a constant current of 1C is performed, thereby setting the charge level to approximately 60% of the rated capacity (SOC 60%). Step 2: After step 1, constant voltage charging is performed for 2.5 hours.

[0091] This allows the test cell to be set to a desired state of charge. A case was mentioned where the SOC is set to 60%, although it is possible to set it to any state of charge by changing the state of charge in step 1. For example, if the state of charge is to be set to 80%, the test cell should be set to a state of charge that corresponds to 80% of its rated capacity (SOC 80%). <<Ladungs- / Entladungszyklen> >

[0092] In charge / discharge cycles, the test cell is first set to 60% state of charge (SOC). A charge / discharge cycle consists of 10 seconds of constant current discharge at 30C and 1 minute (60 seconds) of constant current charge at 5C. The test cell is left in a quiescent state for 10 minutes between each discharge and charge. Charge / discharge cycles are performed for 3000 cycles, with the test cell being set to 60% SOC every 500 cycles. <<IV Widerstandsmessung> >

[0093] In IV resistance measurement, the initial resistance before charge / discharge cycles and the resistance after charge / discharge cycles are measured. The resistance is evaluated as the IV resistance. Resistance measurement is performed with each test cell set to 60% state of charge (SOC) in a 25°C temperature environment. The test cell is then left at rest for 10 minutes and subsequently discharged for 10 seconds at a constant current of 30C (CC discharge). Here, the lower voltage limit during discharge was set to 3.0 V.

[0094] Fig. Figure 11 shows the relationship between the A / B ratio described above and the resistance increase ratio (%) after charge / discharge cycles for test cells of the respective samples A to C. Here, diamonds (◊) are used to plot the sample A test cells, triangles (Δ) are used to plot the sample B test cells, and rectangles (□) are used to plot the sample C test cells. <<Probe A> >

[0095] Sample A uses SBR as the binder in the negative electrode active material layer, and the negative electrode active material layers have a thickness of approximately 45 µm. Fig. Figure 11 shows test cells of Probe A, plotted with “◊” symbols. In this case, in Probe A, if the ratio A / B of the mass concentration A of SBR contained in the near-surface region A1 of the negative electrode active material layer to the mass concentration B of SBR contained in the sections of the negative electrode active material layer that differ from the near-surface region is from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8), the resistance rise ratio (%) after charge / discharge cycles tends to be kept low. If this ratio A / B is lower than 2.0 or if A / B is greater than 3.8, the resistance rise ratio (%) tends to increase after charge / discharge cycles. <<Probe B> >

[0096] Sample B uses PVDF as the binder incorporated in the negative electrode active material layer, and the negative electrode active material layers have a thickness of 45 µm. Fig. Figure 11 shows the test cells of Probe B, plotted with “Δ” symbols. In this case, when the A / B ratio is from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8), a tendency for the resistance rise ratio (%) to be kept low after charge / discharge cycles is not apparent. <<Probe C> >

[0097] Sample C uses SBR as the binder incorporated in the negative electrode active material layer, and the negative electrode active material layers have a thickness of 75 µm. Fig. Figure 11 shows test cells of Probe C plotted with “□” symbols. In this case, even for Probe C, when the A / B ratio is from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8), a tendency for the resistance rise ratio (%) to be kept low after charge / discharge cycles is visible. <<Trend in Widerstandsanstiegsverhältnis (%) nach Ladungs- / Entladungszyklen > >

[0098] As shown with samples A and B, when SBR is used as the binder for the negative electrode active material layer, the resistance rise ratio (%) tends to be kept low after charge / discharge cycles, regardless of the thickness of the negative electrode active material layer, in cases where the A / B ratio is from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8). In contrast, as shown with sample C, when the binder in the negative electrode active material layer is PVDF, even if the A / B ratio is from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8), no tendency for the resistance rise ratio (%) to be kept low after charge / discharge cycles is apparent.

[0099] As noted above, a vehicle traction battery undergoes repeated high-rate charging and discharging, which necessitates the instantaneous intercalation and deintercalation of a large quantity of lithium ions by the negative electrode active material layer. Particularly because lithium-ion reactions decrease in low-temperature environments of -15°C and below, it is desirable for high-rate charging and discharging to occur smoothly in such a low-temperature environment. For this reason, it is desirable to use SBR as the binder in the negative electrode active material layer and, furthermore, to distribute the SBR disproportionately in the near-surface region A of the negative electrode active material layer, such that the above A / B ratio ranges from approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8).

[0100] An example in which SBR was used as the binder has been described here, although a similar tendency can be observed for other rubber-based binders. According to the inventors' findings, a similar tendency is observed not only for rubber-based binders but also in cases where a resin is used that exhibits binder function, such as an acrylic binder, polyethylene oxide (PEO), polyethylene (PE), or an imide binder. Additionally, it is preferred for such rubber-based binders or resins that incorporate a binder resin to be binders capable of dispersing in aqueous solvents.By using a binder that is able to disperse in an aqueous solvent in cases where the A / B ratio falls within a specific range (approximately 2.0 to approximately 3.8 (2.0 ≤ (A / B) ≤ 3.8)), a tendency for the resistance rise ratio (%) to be kept low after charge / discharge cycles can be observed.

[0101] Here, in the negative electrode active material layer formation step, conditions for easier migration than previously achieved in practice were created by ensuring a high solvent content in the negative electrode mixture and setting a high drying temperature. As a result, the A / B ratio increased to more than 2.0, and the binder is disproportionately present in the near-surface region of the negative electrode active material layer.In cases where migration has been strongly favored and the level of rubber-based binder and resin exhibiting a binding function in the near-surface region A1 of the negative electrode active material layer 243A has been adjusted to approximately 2 to 3.8 times the level in other sections of the negative electrode material layer 243A, the level of rubber-based binders and resins exhibiting a binding function decreases at the boundary between the negative electrode active material layer 243A and the negative electrode current collector 241A. For this reason, there appears to be an increased probability of the negative electrode active material layer 243A detaching from the negative electrode current collector 241A.

[0102] As described above, the lithium-ion secondary battery uses 100A (see Fig. 10) a rubber-based binder or a resin having a binder function, as the binder in the negative electrode active material layer 243A, and the ratio A / B is 2.0 ≤ (A / B) ≤ 3.8. In such a case, it is desirable that the negative electrode active material layer 243A does not simply detach from the negative electrode current collector 241A.

[0103] Regarding the negative electrode active material particles 710, which are desirable for use in the negative electrode active material layer 243A, the inventors have proposed that the negative electrode active material particles 710 have a tapping density of at least 1 g / cm³ after 150 tapping cycles. 3 exhibit. Fig. Figure 12 shows the relationship between the knock density after 150 knocks and the peel strength for the negative electrode active material particles 710.

[0104] Here, the percussion density is determined after 150 taps by placing negative electrode active material particles in a measuring cylinder, then mechanically tapping the cylinder 150 times with a tapping device, thus reducing the detectable volume of the negative electrode active material particles. The peel strength was determined as described in Fig. Figure 13 shows measurements taken in accordance with the 90° peel-off adhesive strength test method (JIS K 6854-1). Fig. Figure 13 is a diagram showing the 90° peel-off adhesive strength test procedure.

[0105] Here, a test piece 120 was cut out to a size of 15 mm (W) x 120 mm (L) and prepared by attaching a pressure-sensitive adhesive strip 105 (No. 3303N from Nitto Denko Corporation) to the negative electrode active material layer 64 on one side of a negative electrode sheet 66. In this cut-out test piece 120, the adhesive strip 105 was detached from one end over a length of 40 mm. Next, double-sided tape (No. 501F from Nitto Denko Corporation) was attached to a platform 115. The test piece 120 was attached to this platform by placing the pressure-sensitive adhesive strip 105 face down onto the double-sided tape 110. The detached 40 mm section of the test piece 120 was then secured in a clamping device 125.The clamping device 125 was then pulled at an angle of 90° to the platform 115, and the tensile load was measured when the negative electrode active material layer 64 was detached from the negative electrode current collector 62. A universal test machine manufactured by Minebea Co., Ltd. was used to pull the clamping device 125 at a pulling rate of 20 m / min. The detachment strength (N / m) was determined by dividing the obtained tensile load (N) by the width (15 mm) of the test piece 120.

[0106] According to the inventors' findings, the negative electrode active material layer 243A, which is in Fig. Figure 10 shows that the negative electrode active material particles 710 have a knock density of at least approximately 1 g / cm³ after 150 knocks. 3 exhibiting high peel resistance. Consequently, in cases where a negative electrode active material layer 243A is formed with a binder disproportionately distributed in the near-surface region A1, it is desirable to use negative electrode active material particles 710 with a tapping density after 150 taps preferably at least approximately 1 g / cm³. 3 , preferably at least 1.08 g / cm² 3 and even more preferably at least 1.10 g / cm² 3 to be used. This ensures a suitable residue level in the negative electrode active material layer 243A.

[0107] In a non-aqueous secondary battery, such as the one described above, with a negative electrode sheet 240A in which a negative electrode active material layer 243A is held by a negative electrode current collector 241A, as in Fig. As shown in Figure 10, a rubber-based binder or a resin exhibiting a binder function (e.g., an acrylic binder or an imide binder) can be included in the binder 730 within the negative electrode active material layer 243A. In this case, it is preferable for the rubber-based binder or the resin exhibiting a binder function to be present in excess (disproportionately distributed) within the negative electrode active material layer 243A in the near-surface region A1 of the negative electrode active material layer 243A.

[0108] In such a non-aqueous secondary battery, a rubber-based binder or a resin exhibiting binder function is present in excess in the near-surface region A1 of the negative electrode active material layer 243A. With such a rubber-based binder or resin exhibiting binder function, the reaction rate during deintercalation or intercalation within the negative electrode active material layer 243A can be maintained at a high level by the chemical species that become the charge carriers within the secondary battery (e.g., lithium ions in a lithium-ion secondary battery). As a result, it is possible to keep the resistance rise ratio after charge / discharge cycles low in a low-temperature environment of approximately -15°C.

[0109] In this case, the near-surface region A1 can be defined as a region comprising 1 / 4 of the thickness of the negative electrode active material layer 243A from its upper surface. At this point, the ratio A / B of the combined mass concentration A of the rubber-based binder and the resin exhibiting a binder function contained in the near-surface region A1 of the negative electrode active material layer 243A, to the combined mass concentration B of the rubber-based binder and the resin exhibiting a binder function contained in sections of the negative electrode active material layer 243A that differ from the near-surface region A1, satisfies the condition 2.0 ≤ (A / B) ≤ 3.8. This allows the resistance rise ratio after charge / discharge cycles to be kept even more reliably low in a low-temperature environment of approximately -15°C.In such a case, the A / B ratio preferentially satisfies the condition 2.1 ≤ (A / B), and more preferentially satisfies the condition 2.2 ≤ (A / B). The A / B ratio also preferentially satisfies the condition (A / B) ≤ 3.7, and more preferentially satisfies the condition (A / B) ≤ 3.6. By fulfilling these conditions, the resistance increase ratio after charge / discharge cycles can be kept even more reliably low.

[0110] It is also desirable for the rubber-based binder or the resin that has a binder function to be a binder capable of dispersion in an aqueous solvent. In this case, it is desirable for the rubber-based binder to be, for example, SBR. The resin that has a binder function includes acrylic binders. The acrylic binder preferably includes at least one binder selected from polyethylene oxide and polyethylene.

[0111] In this case, a tapping density of at least 1 g / cm³ after 150 taps is desirable for the negative electrode active material particles. 3 This enables the peel strength of the negative electrode active material layer 243A to be improved. By disproportionately distributing the binder 730 in the near-surface region A1 of the negative electrode active material layer 243A, a decrease in the peel strength of the negative electrode active material layer 243A can be suppressed.

[0112] This non-aqueous secondary battery can be designed as a lithium-ion battery. Furthermore, this invention is not limited to any one of the embodiments described above. For example, although the non-aqueous secondary battery was exemplified by a lithium-ion secondary battery in the embodiments described above, this invention can be applied to non-aqueous secondary batteries other than lithium-ion secondary batteries. A plurality of the non-aqueous secondary batteries can together constitute a battery pack. Also, in the non-aqueous secondary battery of the invention, the resistance rise ratio after charge / discharge cycles can be kept low, particularly in a low-temperature environment at -15°C.Accordingly, the foregoing non-aqueous secondary batteries and battery packs are particularly preferred for the designs of vehicle traction batteries, which are required to suppress an increase in resistance in such a low-temperature environment.

[0113] Non-aqueous secondary batteries according to the embodiments of this invention have been described above; although the non-aqueous secondary battery of the invention is not limited to the embodiments described above, various modifications are possible.

[0114] The non-aqueous secondary battery disclosed here is capable of keeping the resistance rise ratio low in low-temperature environments and, in particular, enables the provision of non-aqueous secondary batteries, such as lithium-ion secondary batteries, that exhibit high performance in low-temperature environments. This is achieved, as described in Fig. As shown in Figure 14, this invention enables a vehicle 1000 (typically an automobile and in particular an automobile with an electric motor, such as a hybrid vehicle or electric vehicle) equipped with this non-aqueous secondary battery 10 (which may be in the form of a battery pack formed by connecting a plurality of such non-aqueous secondary batteries 10 in series) to be provided as the energy source (vehicle traction battery). Reference symbol list 10 Vehicle traction battery 100, 100A Lithium secondary battery (non-aqueous secondary battery) 200 wound electrode arrangement 220 positive electrode sheets 221 Positive electrode current collector 222 uncoated area 223 Positive electrode active material layer 224 middle section 225 gap 240, 240A Negative electrode sheet 241, 241A Negative electrode current collector 242, 242A uncoated area 243, 243A Negative electrode active material layer 262, 264 Separator 280 electrolyte 290 charger 300 battery cases 320 container bodies 340 lids 350 electrolyte injection hole 352 Sealing cap 360 safety valve 420 Electrode clamp 440 Electrode clamp 610 positive electrode active material particles 620 conductive material 630 binders 710 negative electrode active material particles 730 binders 1000 vehicles < / konditionierung> < / testzellenseparator> < / testzelle>

Claims

[1] Non-aqueous secondary battery comprising: a negative electrode current collector and a negative electrode active material layer held by the negative electrode current collector, The negative electrode active material layer contains negative electrode active material particles and a binder, and The binder contains a rubber-based binder or a resin that has a binder function, wherein the negative electrode active material layer consists of a near-surface region that comprises 1 / 4 of the thickness of the negative electrode active material layer from a surface opposite to the negative electrode current collector, and a residual region that makes up the remaining 3 / 4 of the thickness of the negative electrode active material layer, The near-surface region contains more of the rubber-based binder or resin that has a binder function than the rest of the region. The rubber-based binder and the resin, which has a binder function, contained in the near-surface region, have a combined mass concentration A, and the rubber-based binder and the resin, which has a binder function, contained in the remaining region, have a combined mass concentration B, such that the ratio A / B satisfies the condition 2.0 ≤ (A / B) ≤ 3.8 wherein the negative electrode active material layer is formed by coating the negative electrode current collector with a negative electrode mixture in which the negative electrode active material particles and the binder are dispersed in an aqueous solvent, and by drying the negative electrode mixture so that the binder penetrates into the near-surface area, such that 2.0 ≤ (A / B) ≤ 3.8 is satisfied. [2] Non-aqueous secondary battery according to claim 1, wherein the rubber-based binder or the resin having a binder function is a binder that is dispersible in an aqueous solvent. [3] Non-aqueous secondary battery according to claim 1 or 2, comprising SBR as the rubber-based binder. [4] Non-aqueous secondary battery according to any one of claims 1 to 3, comprising an acrylic binder or an imide binder as the resin having a binder function. [5] Non-aqueous secondary battery according to any one of claims 1 to 4, comprising at least one binder selected from the group consisting of polyethylene oxide and polyethylene, as the resin having a binder function. [6] Non-aqueous secondary battery according to any one of claims 1 to 5, wherein the negative electrode active material particles have a tapping density of at least 1 g / cm³ after 150 tapping cycles. 3 exhibit. [7] Non-aqueous secondary battery according to any one of claims 1 to 6, which is designed as a lithium-ion battery. [8] Battery pack obtained by combining a plurality of the non-aqueous secondary batteries according to any one of claims 1 to 7. [9] Vehicle traction battery comprising the non-aqueous secondary battery according to any one of claims 1 to 7 or the battery pack according to claim 8.

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