JP7699453B2 - Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery - Google Patents

Description

The present invention relates to a negative electrode that can be used to construct a nonaqueous electrolyte secondary battery with excellent charge-discharge cycle characteristics, and to the nonaqueous electrolyte secondary battery that uses the negative electrode.

In recent years, non-aqueous electrolyte secondary batteries have been widely used as power sources for various devices. Providing non-aqueous electrolyte secondary batteries to society can contribute to achieving Goal 12 (Ensure sustainable consumption and production patterns), Goal 3 (Ensure healthy lives and promote well-being for all at all ages), Goal 7 (Ensure access to affordable, reliable, sustainable and modern energy for all), and Goal 11 (Make cities and human settlements inclusive, safe, resilient and sustainable) out of the 17 Sustainable Development Goals (SDGs) established by the United Nations.

In addition, mobile information terminals such as smartphones have been rapidly becoming smaller and lighter in recent years, and the non-aqueous electrolyte secondary batteries that serve as their driving power sources are being required to have even higher capacities. Therefore, in order to increase the capacity of non-aqueous electrolyte secondary batteries, active studies are being conducted on the use of silicon or silicon oxide as the active material.

However, materials that form alloys between lithium and silicon, such as silicon and silicon oxide, undergo large volume expansion and contraction during charging and discharging. As a result, repeated charge-discharge cycles cause the active material to pulverize, degrading the material and reducing capacity, which is an issue.

In response to this, Patent Document 1 proposes a technology for improving charge-discharge cycle characteristics by using polyimide, polyamideimide, or polyamide, which have excellent binding strength, as a binder to suppress problems caused by volume expansion and contraction that accompany the charging and discharging of a battery when a silicon oxide represented by SiO _x (0.5≦x≦1.5) is used as the active material.

Patent Document 2 describes a lithium secondary battery having a negative electrode in which silicon or a silicon alloy is used as the negative electrode active material particles and polyimide resin is used as the binder, and proposes that by adjusting the viscosity of the negative electrode mixture slurry applied to the negative electrode current collector and the temperature at which the slurry is dried, the conductive agent particles are caused to migrate to the surface layer, and the concentration of the conductive agent in the surface layer of the negative electrode active material layer is made higher than the concentration of the conductive agent in the center, thereby improving cycle characteristics.

International Publication No. 2009/063902 International Publication No. 2012/132958

However, the charge/discharge cycle characteristics required for non-aqueous electrolyte secondary batteries have become increasingly sophisticated in recent years, and there is still room for improvement in the technologies described in Patent Documents 1 and 2.

The present invention has been made in consideration of the above circumstances, and its purpose is to provide a negative electrode capable of forming a nonaqueous electrolyte secondary battery having excellent charge-discharge cycle characteristics, and the nonaqueous electrolyte secondary battery using the negative electrode.

The negative electrode for a non-aqueous electrolyte secondary battery of the present invention is characterized in that it has a negative electrode mixture layer containing a negative electrode active material and a binder on a current collector, and the porosity of the surface of the negative electrode mixture layer on the side opposite the current collector is 40% or more.

The nonaqueous electrolyte secondary battery of the present invention has a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, and is characterized in that the negative electrode is the negative electrode for the nonaqueous electrolyte secondary battery of the present invention.

According to the present invention, it is possible to provide a negative electrode capable of forming a nonaqueous electrolyte secondary battery having excellent charge-discharge cycle characteristics, and the nonaqueous electrolyte secondary battery using the negative electrode. In other words, the nonaqueous electrolyte secondary battery of the present invention has excellent charge-discharge cycle characteristics.

1 is an example of a histogram created by converting a scanning electron microscope image of a negative electrode mixture layer of a negative electrode of a nonaqueous electrolyte secondary battery into numerical values for each brightness. 1 is an example of a scanning electron microscope image of a cut surface of a negative electrode mixture layer. FIG. 2 is a diagram showing a fitting result of the histogram shown in FIG. 1 . FIG. 2 is a diagram showing a result of extracting a luminance distribution due to a gap from the histogram shown in FIG. 1 . 1 is a plan view illustrating a schematic diagram of an example of a nonaqueous electrolyte secondary battery of the present invention. 6 is a cross-sectional view taken along line II in FIG. 5.

<Negative electrode for non-aqueous electrolyte secondary battery>
The negative electrode for a nonaqueous electrolyte secondary battery of the present invention (hereinafter sometimes simply referred to as the "negative electrode") has a negative electrode mixture layer containing a negative electrode active material and a binder on a current collector, and the porosity of the surface portion of the negative electrode mixture layer on the opposite side to the current collector is 40% or more.

For example, as shown in the examples of Patent Document 1 (paragraph [0146]) and Patent Document 2 (paragraph [0051]), when producing a negative electrode having a negative electrode mixture layer containing a negative electrode active material and a binder on a current collector, it is common to form the negative electrode mixture layer on the current collector and then perform a compression process such as rolling.

The negative electrode mixture layer, which contains the negative electrode active material and the binder, is usually a porous body containing voids, but when it is manufactured through the above-mentioned compression treatment, the porosity decreases in the surface portion of the negative electrode mixture layer, which is easily affected by the treatment. According to the study by the present inventors, when the porosity in the surface portion of the negative electrode mixture layer decreases by a certain amount or more, the permeability of the non-aqueous electrolyte (non-aqueous electrolyte solution) into the negative electrode mixture layer decreases too much, resulting in a decrease in the charge-discharge cycle characteristics of the battery. In particular, when a hard material such as silicon or silicon oxide is used as the active material, the decrease in the porosity in the surface portion of the negative electrode mixture layer becomes greater, and the above problem becomes more pronounced.

Therefore, in the negative electrode of the present invention, the porosity of the surface portion of the negative electrode mixture layer opposite the current collector is increased to 40% or more, so that even if a hard material such as silicon or silicon oxide is used as the negative electrode active material, the permeability of the nonaqueous electrolyte into the negative electrode mixture layer can be well maintained, thereby enabling the construction of a nonaqueous electrolyte secondary battery with excellent charge-discharge cycle characteristics.

The negative electrode active material used in the negative electrode is not particularly limited as long as it is a negative electrode active material used in conventionally known non-aqueous electrolyte secondary batteries, that is, a material capable of absorbing and releasing lithium ions. For example, one or a mixture of two or more carbon-based materials capable of absorbing and releasing lithium ions, such as graphite, pyrolytic carbons, cokes, glassy carbons, baked bodies of organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers, can be used. In addition, elements such as silicon (Si), tin (Sn), germanium (Ge), bismuth (Bi), antimony (Sb), and indium (In), their alloys, and their compounds (oxides, etc.); compounds that can be charged and discharged at a low voltage close to that of lithium metal, such as lithium-containing nitrides or lithium-containing oxides; lithium metal and lithium/aluminum alloys; can also be used as the negative electrode active material.

Among these negative electrode active materials, it is preferable to use silicon or silicon oxide, since this can increase the capacity of the battery. As mentioned above, when silicon or silicon oxide is used as the negative electrode active material, the charge/discharge cycle characteristics of the battery are particularly likely to deteriorate, but this can be suppressed with the negative electrode of the present invention.

The silicon oxide may be one represented by the composition formula SiO _x (0.5≦x≦1.5).

SiO _x may contain microcrystalline or amorphous phase of Si, in which case, the atomic ratio of Si to O is the ratio including the Si of the microcrystalline or amorphous phase of Si. That is, SiO _x includes a structure in which Si (e.g., microcrystalline Si) is dispersed in an amorphous SiO ₂ matrix, and the amorphous SiO ₂ and the Si dispersed therein may be combined to satisfy the atomic ratio x of 0.5≦x≦1.5. For example, in the case of a material in which Si is dispersed in an amorphous SiO ₂ matrix and the molar ratio of SiO ₂ to Si is 1:1, x=1, so the structural formula is represented by SiO. In the case of a material with such a structure, for example, in X-ray diffraction analysis, a peak due to the presence of Si (microcrystalline Si) may not be observed, but the presence of fine Si can be confirmed by observation with a transmission electron microscope.

The silicon or silicon oxide used as the negative electrode active material can be made into a composite with a carbon material. Examples of such composites include those in which the surface of silicon or silicon oxide particles is coated with a carbon material, and those in which silicon or silicon oxide particles are granulated with a carbon material. A preferred composite in which the surface of silicon or silicon oxide particles is coated with a carbon material is one in which silicon or silicon oxide particles and a hydrocarbon gas (toluene, benzene, xylene, mesitylene, etc.) are heated in the gas phase to heat the silicon or silicon oxide particles and a hydrocarbon gas (toluene, benzene, xylene, mesitylene, etc.), and the carbon generated by the pyrolysis of the hydrocarbon gas is deposited on the surface of the particles.

In the case of a composite of silicon or silicon oxide and a carbon material, the amount of the carbon material is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, and even more preferably 7 parts by mass or more, and is preferably 20 parts by mass or less, and more preferably 17 parts by mass or less, per 100 parts by mass of silicon or silicon oxide.

The negative electrode active material may have its particle surface coated with polyimide. Silicon and silicon oxide in particular have a large amount of volume expansion and contraction associated with the charging and discharging of a battery, and as described above, they deteriorate by pulverizing or the like when the battery is repeatedly charged and discharged, which is likely to cause a decrease in the charge and discharge cycle characteristics of the battery. However, if the surface of silicon or silicon oxide particles is coated with polyimide, which has a large binding strength, the amount of volume expansion and contraction during charging and discharging of the battery can be reduced, making it less likely to deteriorate due to pulverization even with repeated charging and discharging, and thus making it possible to further improve the charge and discharge cycle characteristics of the battery.

In addition, in batteries using negative electrode active materials such as silicon or silicon oxide, repeated charging and discharging tends to increase the porosity of the negative electrode mixture layer due to volume changes in silicon or silicon oxide. If the porosity of the negative electrode mixture layer is large, for example, its mechanical strength decreases, making it more likely to cause problems such as cracking of the negative electrode mixture layer in the battery. However, when silicon or silicon oxide coated with polyimide is used as the negative electrode active material, even if the porosity of the negative electrode mixture layer is increased in advance so that the porosity of the surface portion of the negative electrode mixture layer satisfies the above value, the increase in the porosity of the negative electrode mixture layer due to repeated charging and discharging of the battery can be suppressed, and from this perspective, it is possible to further improve the charge and discharge cycle characteristics of the battery.

When the surface of the particles of the negative electrode active material is coated with polyimide, the amount of polyimide is preferably 3 parts by mass or more, and more preferably 5 parts by mass or more, per 100 parts by mass of the negative electrode active material, from the viewpoint of ensuring the above-mentioned effect of coating the particles of the negative electrode active material. However, if the amount of polyimide coating the surface of the particles of the negative electrode active material is too large, there is a risk that the efficiency of absorbing and releasing Li ions in the negative electrode active material will decrease, so the amount is preferably 10 parts by mass or less, and more preferably 8 parts by mass or less, per 100 parts by mass of the negative electrode active material.

Methods for coating the surface of particles of negative electrode active material with polyimide include, for example, placing the particles of negative electrode active material in a solution (polyamic acid solution) in which polyamic acid (polyamide acid) is dissolved in a solvent [water or an organic solvent such as N-methyl-2-pyrrolidone (NMP)], or spraying the polyamic acid solution onto the surface of the particles of negative electrode active material to adhere the polyamic acid solution to the surface of the particles of negative electrode active material, and then heating to remove the solvent while reacting the polyamic acid to form polyimide. For the polyamic acid solution, commercially available products such as DREAMBOND (registered trademark, manufactured by Industrial Summit Technology Corporation) and UPIA (registered trademark, manufactured by Ube Industries, Ltd.) can be used.

Examples of binders to be contained in the negative electrode mixture layer include polysaccharides such as starch, polyvinyl alcohol, polyacrylic acid, carboxymethyl cellulose (CMC), hydroxypropyl cellulose, regenerated cellulose, and diacetyl cellulose, and modified products thereof; thermoplastic resins such as polyvinyl chloride, polyvinylpyrrolidone (PVP), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyamideimide, and polyamide, and modified products thereof; polyimide; polymers having rubber-like elasticity such as ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), butadiene rubber, polybutadiene, fluororubber, and polyethylene oxide, and modified products thereof; and the like. One or more of these can be used.

The negative electrode mixture layer may contain a conductive additive. Examples of conductive additives that may be contained in the negative electrode mixture layer include one or more of the following materials: carbon black (thermal black, furnace black, channel black, ketjen black, acetylene black, etc.), carbon fiber, metal powder (copper, nickel, aluminum, silver, etc. powder), metal fiber, polyphenylene derivatives (as described in JP-A-59-20971), etc.

In the negative electrode mixture layer, the content of the negative electrode active material is preferably 85 to 90 mass %, and the content of the binder is preferably 5 to 10 mass %. In addition, when a conductive additive is used, the content of the conductive additive in the negative electrode mixture layer is preferably 3 to 8 mass %. The thickness of the negative electrode mixture layer is preferably, for example, 35 to 55 μm.

The negative electrode current collector can be made of copper or nickel foil, punched metal, net, expanded metal, foamed metal; carbon sheet; or the like. The thickness of the negative electrode current collector is preferably, for example, 5 to 30 μm.

From the viewpoint of forming a nonaqueous electrolyte secondary battery with excellent charge-discharge cycle characteristics, the porosity of the surface portion of the negative electrode mixture layer opposite the current collector is 40% or more, and preferably 45% or more. However, if the porosity of the surface portion of the negative electrode mixture layer opposite the current collector is too large, for example, the mechanical strength of the negative electrode mixture layer decreases, making it easier for cracks to occur within the battery. Therefore, from the viewpoint of preventing the occurrence of such problems, the porosity of the surface portion of the negative electrode mixture layer opposite the current collector is preferably 55% or less, and more preferably 52% or less.

The porosity of the entire negative electrode mixture layer is preferably 50% or more, more preferably 53% or more, and is preferably 60% or less, more preferably 57% or less.

The porosity of the negative electrode mixture layer referred to in this specification is a value determined by the following method.

The negative electrode mixture layer is cut in the thickness direction, and a scanning electron microscope (SEM) image (reflected electron image) of the cut surface is obtained. The equipment and conditions used can be, for example, as follows (the values described in the examples below are values obtained using the equipment and conditions described below).
Scanning electron microscope: Hitachi S4800
Detector: Reflected electron mode Acceleration voltage: 2 kV
Magnification: 3,000x Capture resolution: 2,560 x 1,920
Minimum dot length: 16.5 nm (equivalent circle diameter: 18.6 nm)
Minimum dot area: 163 nm ²

In order to use the entire information from one end of the negative electrode mixture layer in the thickness direction to the other end in the analysis, if the entire thickness direction of the negative electrode mixture layer cannot fit in one field of view, the entire information is obtained by dividing the thickness direction into two fields of view, etc.

Using the image analysis software "ImageJ," the obtained SEM images are quantified for each brightness to create a histogram. Figure 1 shows an example of a histogram of an SEM image. In the histogram shown in Figure 1, the high brightness peak at a brightness of about 180 is due to the active material, the medium brightness peak at a brightness of about 130 is due to the conductive additive and binder, and the broad peak at a brightness of about 80 is due to voids.

First, the obtained histogram is fitted with a Gaussian distribution to identify the luminance distribution due to the active material and the luminance distribution due to the conductive additive (and binder). Here, fitting is performed assuming that the active material and the conductive additive are each a single material.

Figure 2 shows an example of an SEM image of the cut surface of the negative electrode mixture layer. The SEM image obtained reflects not only the information of the cut surface of the negative electrode mixture layer, but also the brightness information up to a certain depth from the cut surface as shown in Figure 2. Therefore, even in areas that are actually voids, the brightness information of the active material and conductive additive present underneath is picked up, and there are areas that are apparently determined to be occupied by the above materials. In addition, the cut surface of the active material should originally have a uniform brightness, but as shown in Figure 2, due to the influence of minute irregularities present on the cut surface, brightness information may be obtained in which minute regions showing values lower than the actual brightness are scattered.

Figure 3 shows the results of fitting the histogram. The brightness values obtained in the area apparently occupied by the material and the area with low brightness within the cut surface of the active material are almost the same, so a single brightness distribution is newly assumed by combining these, and a Gaussian distribution fitting is performed on the histogram.

Next, each distribution found is subtracted from the overall histogram, and the remaining low brightness areas are assumed to reflect the brightness of voids, and the ratio of these areas to the entire histogram (area ratio) is calculated as the apparent porosity. Figure 4 shows the results of extracting the brightness distribution due to voids from the histogram using the above method.

The "portion that appears to be occupied by the material" that is removed from the histogram by the above subtraction is actually a portion that should be added to the porosity calculation as voids, so the actual porosity is calculated using the correction described below.

Although detailed explanation is omitted, the actual porosity: S (%) and the apparent porosity: T (%) calculated from the histogram are as follows:
T = ^Sn
By determining the value of n, the actual porosity can be calculated.

The actual porosity: S can be calculated using the following formula (1) if the true density: ρi (g/cm ³ ), the ratio of each constituent material of the negative electrode mixture layer expressed in mass %, ai, the thickness: t (cm), and the mass per unit area of the negative electrode mixture layer, m (g/cm ² ), are known. Therefore, by measuring the thickness and mass per unit area of the negative electrode mixture layer in advance, calculating the actual porosity (overall average value) using the following formula (1), and calculating the value of n from the relational expression with the apparent porosity calculated from the histogram, it becomes possible to directly calculate the actual porosity: S from the calculated apparent porosity: T for another histogram obtained for the same negative electrode mixture layer.

S=100-(Σai/ρi)×(m/t) (1)

Note that if the acquisition conditions of the SEM image are different, such as the accelerating voltage or magnification of the electron microscope, the depth at which the brightness information is obtained will differ and the value of n may change. Therefore, if the acquisition conditions of the SEM image are changed, the value of n must be recalculated.

Then, the porosity obtained from the SEM image of the region from the surface of the negative electrode mixture layer opposite the current collector to a position 10 μm in the thickness direction by the above method is defined as the "porosity of the surface portion of the negative electrode mixture layer opposite the current collector." In addition, the actual porosity S obtained by (1) above is defined as the "porosity of the entire negative electrode mixture layer."

In addition, when a conductive assistant is contained in the negative electrode mixture layer, it is preferable that the content (A) of the conductive assistant in the surface portion of the negative electrode mixture layer opposite the current collector is greater than the content (B) of the conductive assistant on the current collector side of the center in the thickness direction of the negative electrode mixture layer. By using a negative electrode that satisfies this, the load characteristics of the battery can be further improved.

In addition, in the negative electrode mixture layer, the ratio of the content (A) of the conductive additive to the content (B) of the conductive additive: (A)/(B) is preferably 2 or more from the viewpoint of ensuring the above-mentioned effect better, and is preferably 3.5 or less from the viewpoint of increasing the conductivity of the entire negative electrode mixture layer to a certain degree.

The content of the conductive additive in the negative electrode mixture layer is calculated from the ratio of the area of the "part of brightness due to conductive additive, etc. (distribution 2 in Figure 3)" extracted by fitting from the histogram for each brightness of the SEM image obtained when measuring the porosity of the negative electrode mixture layer to the area of the entire histogram.

Then, the content of the conductive assistant determined by the above-mentioned method from the SEM image of the region from the surface of the negative electrode mixture layer opposite the current collector to a position 10 μm in the thickness direction is defined as the "conductive assistant content (A)," and the content of the conductive assistant determined from the SEM image of the region from the center of the negative electrode mixture layer in the thickness direction to a position 10 μm toward the current collector is defined as the "conductive assistant content (B)."

The negative electrode can be manufactured, for example, by dispersing a negative electrode mixture containing a negative electrode active material, a binder, and, if necessary, a conductive assistant in a solvent such as an organic solvent such as NMP or water to prepare a paste or slurry-like negative electrode mixture-containing composition (however, the binder may be dissolved in the solvent), applying this to one or both sides of a current collector, and then drying the composition.

As described above, the negative electrode mixture composition is usually applied to the surface of the negative electrode current collector, dried to form a negative electrode mixture layer (a dried coating of the negative electrode mixture composition), and then a compression process such as rolling is performed. In this case, the voids in the surface part of the negative electrode mixture layer are crushed. Therefore, in order to adjust the porosity of the surface part to the above value, after drying the negative electrode mixture composition applied to the current collector to form the negative electrode mixture layer, the compression process is not performed, or if compression process is performed, it is performed under conditions such that the porosity of the surface part of the negative electrode mixture layer does not fall below the above value, and it is more preferable not to perform compression process. Note that the conditions for performing compression process after forming the negative electrode mixture layer vary depending on the device for performing compression process, so the negative electrode can be manufactured after extracting conditions such that the porosity of the surface part of the negative electrode mixture layer in the treated negative electrode satisfies the above value.

In addition, if the negative electrode mixture-containing composition applied to the current collector is dried to form a negative electrode mixture layer and then compressed, the conductive assistant that was present on the surface of the negative electrode mixture layer after drying is pushed into the inside of the negative electrode mixture layer (the current collector side), reducing the amount of conductive assistant on the surface, which may result in a decrease in conductivity near the surface.

However, if the negative electrode mixture-containing composition applied to the current collector is dried to form the negative electrode mixture layer, and then compression is not performed or compression is performed under conditions in which the porosity of the surface portion of the negative electrode mixture layer does not fall below the above value, the decrease in the amount of conductive assistant in the surface portion can be suppressed, and the relationship between the conductive assistant content (A) and the conductive assistant content (B) can be adjusted as described above, thereby obtaining a negative electrode that can further improve the load characteristics of the battery.

When the negative electrode mixture layer is not compressed after formation, or when the negative electrode mixture layer is compressed under conditions that result in a porosity of 40% or more in the surface portion, the surface roughness of the negative electrode mixture layer (the surface opposite the current collector) becomes larger than when the negative electrode mixture layer is compressed under conditions that are generally used in the manufacture of negative electrodes. For example, in the negative electrode of the present invention, the arithmetic mean roughness of the unevenness on the surface of the negative electrode mixture layer opposite the current collector can be 4.1 μm or more. Also, the arithmetic mean roughness of the unevenness on the surface of the negative electrode mixture layer opposite the current collector is usually 6 μm or less.

In this specification, the "arithmetic mean roughness of the unevenness on the surface of the negative electrode mixture layer opposite the current collector" refers to the arithmetic mean roughness as defined in Japanese Industrial Standards (JIS) B 0601 (2013), and specifically, it is a value obtained by observing three fields of view of 90 μm × 90 μm at 50x magnification using a confocal laser microscope ("Real-time scanning laser microscope 1LM-21D" manufactured by Lasertec Corporation), measuring 900 × 900 pixels for each field of view, arithmetically averaging the absolute values from the average line of each point, and then arithmetically averaging the values for each field of view.

In addition, in the negative electrode, the negative electrode active material contained in the negative electrode mixture layer can be doped with Li ions.

In non-aqueous electrolyte secondary batteries constructed using a large-capacity negative electrode active material such as silicon oxide, generally, a large proportion of the Li released from the positive electrode upon charging is absorbed into the negative electrode active material and remains without being released during the next discharge, and there is a risk that the battery's inherent capacity cannot be fully utilized. However, by doping the negative electrode active material with Li ions in advance (pre-doping), the proportion of Li that can move between the positive and negative electrodes during charging and discharging can be increased, reducing the irreversible capacity of the battery, making it possible to form a battery with a higher capacity.

Although there is a method of doping the negative electrode active material with Li ions within the non-aqueous electrolyte secondary battery (in-situ pre-doping), to dope the negative electrode active material relating to the negative electrode with Li ions before it is incorporated into the non-aqueous electrolyte secondary battery, for example, the following ex-situ pre-doping (i) or (ii) can be adopted.

Ex situ pre-doping method (i)
A negative electrode produced using a negative electrode active material that is not doped with Li ions is used, and the negative electrode active material is doped with Li ions.

In the ex situ pre-doping method (i), the doping of Li ions into the negative electrode active material in the negative electrode mixture layer of the negative electrode can be carried out by, for example, immersing the negative electrode in a solution in which biphenyl, polycyclic aromatic compounds (anthracene, naphthalene, etc.), p-benzoquinone, metallic Li, etc. are dissolved in a solvent such as tetrahydrofuran or diethyl ether, and then washing and drying with a solvent (hereinafter referred to as ex situ pre-doping method (i-1)). The amount of Li ions doped at this time can be controlled by adjusting the amount of each component in the solution.

In addition, in the external pre-doping method (i), the negative electrode (working electrode) and lithium metal foil (counter electrode, including lithium alloy foil) are immersed in a non-aqueous electrolyte and a current is passed between them, so that the negative electrode active material in the negative electrode mixture layer can be doped with Li ions (hereinafter referred to as the external pre-doping method (i-2)). The non-aqueous electrolyte can be the same as the non-aqueous electrolyte for non-aqueous electrolyte secondary batteries (described in detail later). The amount of Li ions doped at this time can be controlled by adjusting the current density per area of the negative electrode (negative electrode mixture layer) and the amount of electricity passed.

Ex situ pre-doping method (ii)
Li ions are directly doped into the negative electrode active material that is not doped with Li ions. In this case, the negative electrode active material (negative electrode active material before doping with Li ions) is immersed in the solution in which the negative electrode is immersed, instead of the negative electrode, as described above in the explanation of the ex situ pre-doping method (i-1), so that the negative electrode active material can be doped with Li ions.

When this ex situ pre-doping method (ii) is employed, the negative electrode active material (negative electrode active material doped with Li ions) obtained thereby can be used to manufacture a negative electrode by the above-mentioned method or the like, thereby obtaining a negative electrode containing a negative electrode active material doped with Li ions. In this case, the negative electrode active material used to manufacture the negative electrode can be partly or entirely doped with Li ions by the ex situ pre-doping method (ii).

<Nonaqueous electrolyte secondary battery>
The nonaqueous electrolyte secondary battery of the present invention has a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte, and the negative electrode is the negative electrode for the nonaqueous electrolyte secondary battery of the present invention.

(positive electrode)
The positive electrode of the non-aqueous electrolyte secondary battery may have a structure in which a positive electrode mixture layer containing a positive electrode active material, a conductive assistant, and a binder is provided on one or both sides of a current collector.

The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in conventionally known non-aqueous electrolyte secondary batteries, that is, a material capable of absorbing and releasing lithium ions. Specifically, for example, a metal oxide (lithium-containing composite oxide) composed of Li and a metal M other than Li (Co, Ni, Mn, Fe, Mg, Al, etc.) can be used. Examples of such lithium-containing composite oxides include lithium cobalt oxides such as LiCoO ₂ ; lithium manganese oxides such as LiMnO ₂ and Li ₂ MnO ₃ ; lithium nickel oxides such as LiNiO ₂ ; lithium-containing composite oxides having a layered structure such as LiCo _1-x NiO ₂ ; lithium-containing composite oxides having a spinel structure such as LiMn ₂ O ₄ and Li _4/3 Ti _5/3 O ₄ ; lithium-containing composite oxides having an olivine structure such as LiFePO ₄ ; oxides having the above oxides as a basic composition and substituted with various elements; and the like.

Examples of conductive assistants for the positive electrode mixture layer include graphite (graphite carbon materials) such as natural graphite (e.g., flake graphite) and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; and carbon fibers; and other carbon materials. In addition, PVDF, PTFE, vinylidene fluoride-chlorotrifluoroethylene copolymer [P(VDF-CTFE)], SBR, carboxymethyl cellulose (CMC), and the like are preferably used as binders for the positive electrode mixture layer.

The positive electrode is manufactured, for example, by dispersing the positive electrode active material, conductive additive, binder, etc. in a solvent such as NMP to prepare a paste or slurry-like positive electrode mixture-containing composition (however, the binder may be dissolved in the solvent), applying this to one or both sides of the current collector, drying, and then carrying out a compression process such as rolling as necessary. However, the positive electrode is not limited to those manufactured by the above manufacturing method, and may be manufactured by other methods.

The positive electrode current collector can be made of a metal foil such as aluminum or stainless steel, a punched metal, a mesh, an expanded metal, a foamed metal, a carbon sheet, or the like. The thickness of the positive electrode current collector is preferably 5 to 30 μm.

In the positive electrode mixture layer, the content of the positive electrode active material can be 60 to 95 mass%, the content of the binder can be 1 to 15 mass%, and the content of the conductive additive can be 3 to 20 mass%. In addition, the thickness of the positive electrode mixture layer (thickness per side of the current collector) is preferably 30 to 150 μm.

(Separator)
The separator of the nonaqueous electrolyte secondary battery should have sufficient strength and be capable of retaining a large amount of nonaqueous electrolyte, and may be a microporous membrane made of polyolefin such as polyethylene (PE) or polypropylene (PP). The microporous membrane constituting the separator may be, for example, one made of only PE or only PP, may contain an ethylene-propylene copolymer, or may be a laminate of a microporous membrane made of PE and a microporous membrane made of PP.

Furthermore, the separator may be a laminated separator composed of a porous layer mainly made of a resin with a melting point of 140°C or less and a porous layer mainly made of a resin with a melting point of 150°C or more or an inorganic filler with a heat resistance temperature of 150°C or more. Here, "melting point" refers to the melting temperature measured using a differential scanning calorimeter (DSC) in accordance with the provisions of Japanese Industrial Standards (JIS) K 7121, and "heat resistance temperature of 150°C or more" means that no deformation such as softening is observed at least at 150°C.

The thickness of the separator (a separator made of a microporous polyolefin film or the laminated separator) is more preferably 10 to 30 μm.

(Non-aqueous electrolyte)
A non-aqueous liquid electrolyte (nonaqueous electrolyte) is usually used as the non-aqueous electrolyte of a non-aqueous electrolyte secondary battery. As the non-aqueous electrolyte, an organic solvent containing an electrolyte salt such as a lithium salt is used. The organic solvent is not particularly limited, but examples thereof include chain esters such as dimethyl carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (MEC), and methyl propyl carbonate; cyclic esters having a high dielectric constant such as ethylene carbonate (EC), propylene carbonate, butylene carbonate, and vinylene carbonate; mixed solvents of chain esters and cyclic esters; and the like. In particular, a mixed solvent of a chain ester and a cyclic ester as the main solvent is suitable.

Examples of electrolyte salts to be dissolved in an organic solvent in preparing a nonaqueous electrolyte include _LiPF6 , LiBF4, _LiAsF6 , LiSbF6, _LiCF3SO3 , _LiC4F9SO3 , _LiCF3CO2 , _{Li2C2F4 (} SO3 _{) 2 , LiCnF2n +} 1SO3 (n≧ ₂ ), LiN _{( RfSO2 )} ( _Rf'SO2 ), LiC( _{RfSO2 ) 3} , _and LiN( _RfOSO2 ) ₂ (wherein Rf and _{Rf '} are fluoroalkyl groups) may be used alone or in combination of two or more. The concentration of the electrolyte salt in the nonaqueous electrolyte is not particularly limited, but is preferably 0.3 mol/L or more, more preferably 0.4 mol/L or more, and is preferably 1.7 mol/L or less, more preferably 1.5 mol/L or less.

Additives such as vinylene carbonate (VC), vinyl ethylene carbonate, acid anhydride, sulfonic acid ester, dinitrile, 1,3-propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, and t-butylbenzene (including derivatives thereof) can also be added to the non-aqueous electrolyte as appropriate to further improve the charge/discharge cycle characteristics of the battery and to improve safety, such as high-temperature storage properties and overcharging prevention.

In non-aqueous electrolyte secondary batteries, in addition to the non-aqueous electrolyte, a gel electrolyte obtained by gelling the non-aqueous electrolyte with a gelling agent such as a polymer, or a known solid electrolyte can also be used as the non-aqueous electrolyte.

(electrode body)
In a nonaqueous electrolyte secondary battery, a negative electrode and a positive electrode are used in the form of a laminate (laminated electrode body) in which they are laminated with a separator interposed therebetween, or in the form of a wound body (wound electrode body) in which this laminate is further wound in a spiral shape.

(Form of non-aqueous electrolyte secondary battery, etc.)
There is no particular limitation on the shape of the nonaqueous electrolyte secondary battery, and it may be any shape, such as a small cylindrical shape, coin shape, button shape, flat shape, rectangular shape, or a large shape for use in electric vehicles.

Figures 5 and 6 show schematic diagrams illustrating an example of a nonaqueous electrolyte secondary battery of the present invention. Figure 5 is a plan view of the nonaqueous electrolyte secondary battery, and Figure 6 is a cross-sectional view taken along line I-I in Figure 5.

The nonaqueous electrolyte secondary battery 1 shown in Figures 5 and 6 contains an electrode assembly in which a positive electrode 5 and a negative electrode 6 are laminated with a separator 7 between them, and a nonaqueous electrolyte (not shown) in a laminate film exterior 2 made of two sheets of metal laminate film, and the laminate film exterior 2 is sealed at its outer periphery by heat fusing the upper and lower metal laminate films.

In FIG. 6, in order to avoid complicating the drawing, the layers constituting the laminate film exterior body 2 and the layers of the positive electrode 5 and negative electrode 6 are not shown separately.

The positive electrode 5 is connected to the positive electrode external terminal 3 within the battery 1, and although not shown, the negative electrode 6 is also connected to the negative electrode external terminal 4 within the battery 1. One end of each of the positive electrode external terminal 3 and the negative electrode external terminal 4 is extended to the outside of the laminate film exterior body 2 so that they can be connected to external devices, etc.

In a non-aqueous electrolyte secondary battery, as described above, a negative electrode containing at least a negative electrode active material doped with Li ions may be used, but a battery may also be assembled using a negative electrode containing a negative electrode active material that is not doped with Li ions, and Li ions may be doped into the negative electrode active material in the negative electrode mixture layer of the negative electrode in the battery (in-situ pre-doping).

The in-situ pre-doping of the negative electrode active material in the negative electrode mixture layer of the negative electrode can be carried out, for example, by assembling a battery using an electrode having a Li supply source (lithium metal foil, lithium alloy foil, etc.) separate from the positive electrode and negative electrode, i.e., a pre-doping electrode for doping Li ions into the negative electrode active material, and by passing current through this pre-doping electrode, Li ions are doped from the Li supply source into the negative electrode active material in the negative electrode mixture layer within the battery. Therefore, in the case of a battery in which Li ions are doped into the negative electrode active material by in-situ pre-doping, even at the time when the doping of Li ions is completed, the pre-doping electrode remains in the battery with some or all of the Li supply source remaining.

On the other hand, in the case of a non-aqueous electrolyte secondary battery assembled using a negative electrode containing a negative electrode active material that has been doped with Li ions in advance by ex-situ pre-doping, there is no internal pre-doping electrode that is used (or has been used) for doping with Li ions.

The nonaqueous electrolyte secondary battery of the present invention can be used in the same applications as conventionally known nonaqueous electrolyte secondary batteries. In addition, the negative electrode for the nonaqueous electrolyte secondary battery of the present invention can constitute the nonaqueous electrolyte secondary battery of the present invention.

The present invention will be described in detail below based on examples. However, the following examples do not limit the present invention.

Example 1
<Preparation of negative electrode>
A negative electrode mixture-containing paste was prepared by uniformly mixing 87 parts by mass of silicon oxide (SiO) as a negative electrode active material, 5 parts by mass of acetylene black as a conductive additive, and 8 parts by mass of polyacrylic acid as a binder using water as a solvent.

This paste containing the negative electrode mixture was applied to one side of a 6 μm-thick copper foil that served as a negative electrode current collector, and then dried to produce a negative electrode having a 46 μm-thick negative electrode mixture layer on one side of the negative electrode current collector. In the obtained negative electrode, the size of the negative electrode mixture layer in plan view was 2.5 cm x 4.1 cm. In addition, when applying the paste containing the negative electrode mixture to the negative electrode current collector, an exposed portion was provided on part of the negative electrode current collector where no negative electrode mixture layer was formed, and this was used as the negative electrode external terminal.

In the negative electrode, the porosity of the surface portion and the entire negative electrode mixture layer was 47.8% and 55.3%, respectively. The content (A) of the conductive assistant in the surface portion of the negative electrode mixture layer was greater than the content (B) of the conductive assistant on the collector side of the center in the thickness direction of the negative electrode mixture layer, and the ratio (A)/(B) was 2.9. Furthermore, the arithmetic mean roughness of the unevenness of the surface of the negative electrode mixture layer was 5.5 μm, and the maximum peak height of the convex portions measured when determining the arithmetic mean roughness was 38.8 μm.

<Preparation of Positive Electrode>
A positive electrode mixture-containing paste was prepared by uniformly mixing 85 parts by mass of LiCoO ₂ as a positive electrode active material, 10 parts by mass of acetylene black as a conductive additive, and 5 parts by mass of PVDF as a binder using NMP as a solvent.

This paste containing the positive electrode mixture was applied to one side of an 11 μm thick aluminum foil that would serve as a negative electrode current collector, dried, and compressed using a rope press to produce a positive electrode having a 58 μm thick positive electrode mixture layer on one side of the positive electrode current collector. In the resulting positive electrode, the size of the positive electrode mixture layer in plan view was 2.7 cm x 4.2 cm. In addition, when applying the paste containing the positive electrode mixture to the positive electrode current collector, an exposed portion was provided on part of the positive electrode current collector where no positive electrode mixture layer was formed, and this was used as the positive electrode external terminal.

<Battery assembly>
The positive electrode and the negative electrode were stacked with a PE microporous membrane separator (thickness 16 μm, porosity 40%) interposed therebetween to form a laminated electrode body. The laminated electrode body was inserted into a bag-shaped exterior body made of an 8 cm x 7 cm aluminum laminate film. A part of the positive electrode external terminal and the negative electrode external terminal of the laminated electrode body were made to protrude from the opening of the exterior body. Next, LiPF ₆ was dissolved at a concentration of 1 mol/L in a solution in which EC, DEC, and MEC were mixed in a volume ratio of 1:1:1, and then VC was further dissolved in an amount of 1 mass% to prepare a nonaqueous electrolyte solution, which was then injected into the exterior body. Thereafter, the opening of the exterior body was sealed to produce a nonaqueous electrolyte secondary battery having the appearance shown in FIG. 5 and the cross-sectional structure shown in FIG. 6.

Example 2
The same silicon oxide as used in Example 1 was mixed with a polyamic acid solution "DREAMBOND" (registered trademark, manufactured by Industrial Summit Technology Corporation) and heated at 200°C for 10 hours to obtain silicon oxide having a surface coated with polyimide (silicon oxide: 95 parts by mass, polyimide: 5 parts by mass). A negative electrode was produced in the same manner as in Example 1 except that this was used as the negative electrode active material, and a nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that this negative electrode was used.

In the negative electrode prepared in Example 2, the porosity of the surface portion and the entire negative electrode mixture layer was 50.3% and 55.7%, respectively. The content (A) of the conductive assistant in the surface portion of the negative electrode mixture layer was greater than the content (B) of the conductive assistant on the collector side of the center in the thickness direction of the negative electrode mixture layer, and the ratio (A)/(B) was 2.3. Furthermore, the arithmetic mean roughness of the unevenness of the surface of the negative electrode mixture layer was 4.3 μm, and the maximum peak height of the convex portion measured when determining the arithmetic mean roughness was 38.5 μm.

Comparative Example 1
A negative electrode was produced in the same manner as in Example 2, except that the negative electrode mixture-containing paste was applied to the surface of the negative electrode current collector, dried, and then compressed with a roll press. A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 2, except that this negative electrode was used.

In the negative electrode produced in Comparative Example 1, the porosity of the surface portion and the entire negative electrode mixture layer was 38.2% and 45.7%, respectively. In addition, the content (A) of the conductive assistant in the surface portion of the negative electrode mixture layer was equal to the content (B) of the conductive assistant on the collector side from the center in the thickness direction of the negative electrode mixture layer, and the ratio (A)/(B) was 0.9. Furthermore, the arithmetic mean roughness of the unevenness of the surface of the negative electrode mixture layer was 4.0 μm, and the maximum peak height of the convex portion measured when determining the arithmetic mean roughness was 26.8 μm.

For the nonaqueous electrolyte secondary batteries of the examples and comparative examples, a series of operations was repeated 1000 cycles, in which a constant current charge was performed at a current value of 0.5 C until the voltage reached 4.35 V relative to the design capacity, and then a constant current discharge was performed at a current value of 0.5 C until the voltage reached 2.5 V. Then, for each battery, the capacity retention rate was calculated as a percentage by dividing the discharge capacity at the 1000th cycle by the discharge capacity at the 1st cycle, and the charge/discharge cycle characteristics were evaluated.

The evaluation results are shown in Table 1 for the negative electrodes used in each battery, together with the porosity of the surface of the negative electrode mixture layer, the arithmetic mean roughness of the surface irregularities of the negative electrode mixture layer, and the maximum peak height of the protrusions on the surface of the negative electrode mixture layer, all determined by the above method.

As shown in Table 1, the nonaqueous electrolyte secondary batteries of Examples 1 and 2, which used a negative electrode in which the porosity of the surface portion of the negative electrode mixture layer was adjusted to a suitable value, had a larger capacity retention rate during the charge-discharge cycle characteristic evaluation and had excellent charge-discharge cycle characteristics compared to the battery of Comparative Example 1, which used a negative electrode in which the porosity of the surface portion was inappropriate.

Comparing the battery of Example 1 with the battery of Example 2, the battery of Example 2, which used a negative electrode active material whose surface was coated with polyimide, had better evaluation results for the charge-discharge cycle characteristics than the battery of Example 1, which used a negative electrode active material whose surface was not coated. This is thought to be because the use of polyimide in the battery of Example 2 reduced the volume change of the negative electrode active material due to repeated charging and discharging, and reduced the decrease in the conductivity of the negative electrode mixture layer compared to the battery of Example 1.

REFERENCE SIGNS LIST 1 nonaqueous electrolyte secondary battery 2 laminate film exterior 3 positive electrode external terminal 4 negative electrode external terminal 5 positive electrode 6 negative electrode 7 separator

Claims (5)

A negative electrode for a non-aqueous electrolyte secondary battery having a negative electrode mixture layer on a current collector, the negative electrode mixture layer including a negative electrode active material and a binder,
When a region of the negative electrode mixture layer extending from the surface opposite to the current collector to a position 10 μm in a thickness direction is defined as a surface portion of the negative electrode mixture layer,
the porosity of the surface portion of the negative electrode mixture layer is 40% or more,
The negative electrode mixture layer contains a conductive assistant,
a content (A) of the conductive assistant in the surface portion of the negative electrode mixture layer is greater than a content (B) of the conductive assistant in a region from a center in a thickness direction of the negative electrode mixture layer to a position 10 μm toward the current collector, and the content (B) of the conductive assistant is a positive value,
a ratio (A)/(B) of the content (A) of the conductive assistant to the content (B) of the conductive assistant is 2 or more and 3.5 or less . The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material contains silicon or silicon oxide. 3. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 1, wherein the arithmetic mean roughness of the surface of the negative electrode mixture layer opposite to the current collector is 4.1 [mu]m or more. 4. The negative electrode for a non-aqueous electrolyte secondary battery in accordance with claim 1 , wherein the surface of the negative electrode active material is covered with a polyimide. A non-aqueous electrolyte secondary battery having a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,
A non-aqueous electrolyte secondary battery comprising the negative electrode for non-aqueous electrolyte secondary batteries according to any one of claims 1 to 4 as the negative electrode.

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