Negative electrode for non-aqueous electrolyte secondary battery and non-aqueous electrolyte secondary battery

Abstract

To provide a negative electrode which enables the formation of a nonaqueous electrolyte secondary battery superior in charge and discharge cycle characteristics, and a nonaqueous electrolyte secondary battery arranged by use of the negative electrode, and which relates to the goals 12, 3, 7, 11 of Sustainable Development Goals (SDGs).SOLUTION: A negative electrode for a nonaqueous electrolyte secondary battery according to the present invention has, on a collector, a negative electrode mixture layer containing a negative electrode active material and a binder. A surface part of the negative electrode mixture layer on a side opposite to the collector has a porosity of 40% or more. A nonaqueous electrolyte secondary battery according to the invention has a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a nonaqueous electrolyte. The nonaqueous electrolyte secondary battery has, as the negative electrode, a negative electrode for a nonaqueous electrolyte secondary battery according to the invention.SELECTED DRAWING: Figure 6

Description

Technical Field

[0001] The present invention relates to a negative electrode capable of constituting a non-aqueous electrolyte secondary battery having excellent charge-discharge cycle characteristics, and to the non-aqueous electrolyte secondary battery using the negative electrode.

Background Art

[0002] In recent years, non-aqueous electrolyte secondary batteries have been widely applied as driving power sources for various devices. By providing non-aqueous electrolyte secondary batteries to society, among the 17 goals of the Sustainable Development Goals (SDGs) established by the United Nations, Goal 12 (ensuring sustainable production and consumption patterns), Goal 3 (ensuring healthy lives and promoting well-being for all people of all ages), Goal 7 (ensuring access for all people to affordable, reliable and sustainable modern energy), and Goal 11 (achieving inclusive, safe, resilient and sustainable cities and human settlements) can be contributed to.

[0003] Also, recently, the miniaturization and weight reduction of mobile information terminals such as smartphones have been rapidly progressing, and further high-capacityization is required for non-aqueous electrolyte secondary batteries as their driving power sources. Therefore, in order to increase the capacity of non-aqueous electrolyte secondary batteries, studies are actively being conducted on using silicon or silicon oxide as an active material.

[0004] However, materials that form an alloy of lithium and Si, such as silicon and silicon oxide, have a large volume expansion and contraction accompanying charge and discharge. Therefore, when the charge-discharge cycle is repeated, there has been a problem that the active material deteriorates due to pulverization of the active material and the like, and the capacity decreases.

[0005] In contrast, Patent Document 1 proposes a technique for suppressing problems due to volume expansion and contraction accompanying charge and discharge of a battery when a silicon oxide represented by SiO x (0.5 ≦ x ≦ 1.5) is used as an active material, and improving charge-discharge cycle characteristics by using a polyimide, polyamideimide, or polyamide having excellent adhesion as a binder.

[0006] Further, Patent Document 2 describes a lithium secondary battery having a negative electrode using silicon or a silicon alloy as a negative electrode active material particle and using a polyimide resin as a binder. By adjusting the viscosity of the negative electrode mixture slurry applied on the negative electrode current collector and the temperature when drying the slurry, movement to the surface layer of the conductive agent particles is caused, and the concentration of the conductive aid in the surface layer of the negative electrode active material layer is made higher than the concentration of the conductive aid in the central portion, thereby improving the cycle characteristics has been proposed.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, the charge-discharge cycle characteristics required for non-aqueous electrolyte secondary batteries have become increasingly advanced recently, and there is still room for improvement even in the technologies described in Patent Documents 1 and 2.

[0009] The present invention has been made in view of the above circumstances, and an object thereof is to provide a negative electrode capable of constituting a non-aqueous electrolyte secondary battery excellent in charge-discharge cycle characteristics, and the non-aqueous electrolyte secondary battery using the negative electrode.

Means for Solving the Problems

[0010] The negative electrode for a non-aqueous electrolyte secondary battery of the present invention has a negative electrode mixture layer containing a negative electrode active material and a binder on a current collector, and the porosity in the surface portion of the negative electrode mixture layer on the side opposite to the current collector is 40% or more.

[0011] The non-aqueous 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 non-aqueous electrolyte, and is characterized in that the negative electrode has the negative electrode for a non-aqueous electrolyte secondary battery of the present invention.

Advantages of the Invention

[0012] According to the present invention, it is possible to provide a negative electrode capable of constituting a non-aqueous electrolyte secondary battery excellent in charge-discharge cycle characteristics, and the non-aqueous electrolyte secondary battery using the negative electrode. That is, the non-aqueous electrolyte secondary battery of the present invention is excellent in charge-discharge cycle characteristics.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0014] <Negative Electrode for Non-Aqueous Electrolyte Secondary Battery> The negative electrode for a non-aqueous electrolyte secondary battery of the present invention (hereinafter, may be simply referred to as "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 side opposite to the current collector is 40% or more.

[0015] For example, when manufacturing a negative electrode having a negative electrode mixture layer containing a negative electrode active material and a binder on a current collector, as in the examples of Patent Document 1 (paragraph

[0146] ) and Patent Document 2 (paragraph

[0051] ), after forming the negative electrode mixture layer on the current collector, it is common to perform a compression treatment such as a rolling treatment.

[0016] The negative electrode mixture layer containing a negative electrode active material and a binder is usually a porous body containing voids. However, when it is manufactured through the above compression treatment, the porosity decreases in the surface portion of the negative electrode mixture layer that 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, it has been clarified that the decrease in the porosity in the surface portion of the negative electrode mixture layer becomes larger, and the above problem becomes more prominent.

[0017] Therefore, in the negative electrode of the present invention, by making the porosity of the surface portion of the negative electrode mixture layer on the side opposite to the current collector 40% or more, even when a hard material such as silicon or silicon oxide is used as the negative electrode active material, the permeability of the non-aqueous electrolyte into the negative electrode mixture layer can be maintained well, and a non-aqueous electrolyte secondary battery excellent in charge-discharge cycle characteristics can be configured.

[0018] The negative electrode active material used for the negative electrode is not particularly limited as long as it is a negative electrode active material conventionally known to be used in non-aqueous electrolyte secondary batteries, that is, a material capable of occluding and releasing lithium ions. For example, one or a mixture of two or more carbon-based materials capable of occluding and releasing lithium ions, such as graphite, pyrolytic carbons, cokes, glassy carbons, fired products of organic polymer compounds, mesocarbon microbeads (MCMB), and carbon fibers, is used. In addition, elements such as silicon (Si), tin (Sn), germanium (Ge), bismuth (Bi), antimony (Sb), indium (In), their alloys, and their compounds (such as oxides); compounds capable of charging and discharging 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.

[0019] Among these negative electrode active materials, it is preferable to use silicon or silicon oxide because the battery capacity can be increased. As described above, when silicon or silicon oxide is used as the negative electrode active material, the charge-discharge cycle characteristics of the battery tend to deteriorate particularly easily. However, with the negative electrode of the present invention, this can be suppressed.

[0020] Examples of the silicon oxide include those represented by the composition formula SiO x (0.5 ≦ x ≦ 1.5).

[0021] SiO x may contain microcrystalline or amorphous phases of Si. In this case, the atomic ratio of Si to O is the ratio including Si in the microcrystalline or amorphous phase of Si. That is, SiO xIt includes those having a structure in which Si (for example, microcrystalline Si) is dispersed in an amorphous SiO2 matrix. As long as the amorphous SiO2 and the Si dispersed therein satisfy the atomic ratio x of 0.5 ≦ x ≦ 1.5. For example, in the case of a material having a structure in which Si is dispersed in an amorphous SiO2 matrix and the molar ratio of SiO2 to Si is 1:1, since x = 1, it is represented as SiO in the structural formula. In the case of a material having such a structure, for example, in X-ray diffraction analysis, peaks due to the presence of Si (microcrystalline Si) may not be observed, but when observed with a transmission electron microscope, the presence of fine Si can be confirmed.

[0022] 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 obtained by coating the surface of silicon or silicon oxide particles with a carbon material for composite formation, and those obtained by granulating silicon or silicon oxide particles and a carbon material for composite formation. As for the composite obtained by coating the surface of silicon or silicon oxide particles with a carbon material, it is preferable to heat silicon or silicon oxide particles and a hydrocarbon gas (such as toluene, benzene, xylene, mesitylene) in the gas phase, and deposit the carbon generated by the thermal decomposition of the hydrocarbon gas on the particle surface.

[0023] In the case of a composite of silicon or silicon oxide and a carbon material, it is preferable that the carbon material is 3 parts by mass or more, more preferably 5 parts by mass or more, still more preferably 7 parts by mass or more, and preferably 20 parts by mass or less, more preferably 17 parts by mass or less, based on 100 parts by mass of silicon or silicon oxide.

[0024] The negative electrode active material can have its particle surface coated with polyimide. In particular, silicon and silicon oxides have a large volume expansion and contraction during charge and discharge of the battery, and as described above, when the charge and discharge of the battery are repeated, they deteriorate such as becoming pulverized, so it is easy to cause a decrease in the charge and discharge cycle characteristics of the battery. However, when the surface of silicon or silicon oxide particles is coated with polyimide having a large adhesive force, the volume expansion and contraction during charge and discharge of the battery can be reduced, so that deterioration due to pulverization or the like is less likely to occur even when charge and discharge are repeated, and thus the charge and discharge cycle characteristics of the battery can be further improved.

[0025] Also, in a battery using a negative electrode active material such as silicon or silicon oxide, when charge and discharge are repeated, the porosity of the negative electrode binder layer tends to increase due to the volume change of silicon or silicon oxide. When the porosity of the negative electrode binder layer is large, for example, its mechanical strength becomes small, so problems such as cracks in the negative electrode binder layer are likely to occur inside the battery. However, when silicon or silicon oxide whose surface is coated with polyimide is used as the negative electrode active material, even if the porosity of the negative electrode binder layer is increased in advance so that the porosity in the surface portion of the negative electrode binder layer satisfies the above value, the increase in the porosity of the negative electrode binder layer due to repeated charge and discharge of the battery can be suppressed. Therefore, from this perspective as well, the charge and discharge cycle characteristics of the battery can be further improved.

[0026] When coating the surface of the negative electrode active material particles with polyimide, the amount of polyimide is preferably 3 parts by mass or more, more preferably 5 parts by mass or more, based on 100 parts by mass of the negative electrode active material, from the viewpoint of ensuring the above effects by coating the negative electrode active material particles. However, if the amount of polyimide coating the surface of the negative electrode active material particles is too large, there is a risk that the efficiency of lithium ion occlusion and release in the negative electrode active material will decrease. Therefore, the amount is preferably 10 parts by mass or less, more preferably 8 parts by mass or less, based on 100 parts by mass of the negative electrode active material.

[0027] As a method for coating the surface of the particles of the negative electrode active material with polyimide, for example, the particles of the negative electrode active material are placed in a solution (a polyamic acid solution) in which polyamic acid (polyamide acid) is dissolved in a solvent (an organic solvent such as water or N-methyl-2-pyrrolidone (NMP)), or the polyamic acid solution is sprayed onto the surface of the particles of the negative electrode active material, and after the polyamic acid solution is adhered to the surface of the particles of the negative electrode active material, it is heated to remove the solvent and the polyamic acid is reacted to form polyimide. Examples of such methods include. 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 as the polyamic acid solution.

[0028] Examples of the binder to be contained in the negative electrode binder 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, polyvinyl pyrrolidone (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; etc. One or more of these can be used.

[0029] A conductive aid can be contained in the negative electrode binder layer. Examples of the conductive aid to be contained in the negative electrode binder layer include one or more of materials such as carbon black (thermal black, furnace black, channel black, ketjen black, acetylene black, etc.), carbon fiber, metal powder (powder of copper, nickel, aluminum, silver, etc.), metal fiber, and polyphenylene derivative (described in JP-A-59-20971).

[0030] In the negative electrode mixture layer, it is preferable that the content of the negative electrode active material is 85 to 90% by mass and the content of the binder is 5 to 10% by mass. When using a conductive assistant, the content of the conductive assistant in the negative electrode mixture layer is preferably 3 to 8% by mass. The thickness of the negative electrode mixture layer is preferably, for example, 35 to 55 μm.

[0031] For the negative electrode current collector, foils made of copper or nickel, punching metal, mesh, expanded metal, foamed metal; carbon sheets; etc. can be used. The thickness of the negative electrode current collector is preferably, for example, 5 to 30 μm.

[0032] From the viewpoint of enabling the formation of a non-aqueous electrolyte secondary battery with excellent charge-discharge cycle characteristics, the porosity of the surface portion on the side opposite to the current collector in the negative electrode mixture layer is 40% or more, and preferably 45% or more. However, if the porosity of the surface portion on the side opposite to the current collector in the negative electrode mixture layer is too large, for example, the mechanical strength of the negative electrode mixture layer decreases and cracks are likely to occur inside the battery. Therefore, from the viewpoint of suppressing the occurrence of such problems, the porosity of the surface portion on the side opposite to the current collector in the negative electrode mixture layer is preferably 55% or less, and more preferably 52% or less.

[0033] Also, the porosity of the entire negative electrode mixture layer is preferably 50% or more, more preferably 53% or more, preferably 60% or less, and more preferably 57% or less.

[0034] The porosity of the negative electrode mixture layer as referred to in this specification is a value obtained by the following method.

[0035] The negative electrode mixture layer is cut in the thickness direction, and a scanning electron microscope (SEM) image (secondary electron image) of the cut surface is acquired. The equipment and conditions to be used can be, for example, as follows (the values described in the examples below are the values obtained under the following equipment and conditions). Scanning electron microscope: Hitachi S4800 Detector: Secondary electron mode Accelerating voltage: 2 kV Magnification: 3,000 times Capture resolution: 2,560 × 1,920 Minimum dot length: 16.5 nm (equivalent circle diameter: 18.6 nm) Minimum dot area: 163 nm 2

[0036] In order to use all the information from one end to the other end in the thickness direction of the negative electrode composite layer for analysis, if the entire thickness direction of the negative electrode composite layer does not fit within one field of view, the entire information shall be obtained by dividing it into two fields of view in the thickness direction or the like.

[0037] Using the image analysis software "ImageJ", the obtained SEM image is digitized for each luminance value to create a histogram. Figure 1 is a diagram showing an example of a histogram of the SEM image. In the histogram shown in Figure 1, the high-luminance peak around a luminance of about 180 is due to the active material, the medium-luminance peak around a luminance of about 130 is due to the conductive assistant and the binder, and the broad peak around a luminance of about 80 is due to the voids.

[0038] First, for the obtained histogram, in order to specify the luminance distribution due to the active material and the luminance distribution due to the conductive assistant (and the binder), fitting with a Gaussian distribution is performed. Here, the active material and the conductive assistant are each fitted assuming a single material.

[0039] Figure 2 shows an example of an SEM image of the cross-section of the negative electrode composite layer. In the obtained SEM image, not only the information of the cross-section of the negative electrode composite layer but also the luminance information up to a certain depth from the cross-section is reflected as shown in Figure 2. Therefore, for a portion that is originally a void, the luminance information of the active material and the conductive assistant existing below it is also picked up, and there will be a portion that is judged to be occupied by the above-mentioned material apparently. Also, although the cross-section of the active material should originally have a single luminance, as shown in Figure 2, luminance information in which minute regions showing values lower than the original luminance are scattered may be obtained due to the influence of minute unevenness existing on the cross-section.

[0040] Figure 3 shows the result of fitting to the histogram. Since the values of the luminance obtained in the portion that is apparently judged to be occupied by the material and in the portion with low luminance within the cross-section of the active material are approximately the same, these are combined to newly assume a luminance distribution, and fitting to the histogram with a Gaussian distribution is performed.

[0041] Next, each obtained distribution is subtracted from the overall histogram, and the ratio (area ratio) of that region to the entire histogram is calculated as the apparent porosity with the remaining low-luminance region being the one where the luminance of the voids is reflected. The result of extracting the luminance distribution due to voids from the histogram by the above method is shown in Figure 4.

[0042] Note that the "portion that is apparently judged to be occupied by the material" which will be removed from the histogram by the above subtraction is a portion that should originally be added to the calculation of the porosity as a void, so the actual porosity is calculated by the correction described below.

[0043] Although detailed explanation is omitted, the actual porosity: S(%) and the apparent porosity: T(%) calculated from the histogram have the T = S n relationship, and by obtaining the value of n, the actual porosity can be calculated.

[0044] The actual porosity: S is the true density of each constituent material of the negative electrode binder layer: ρi (g / cm 3 ) and the ratio of each constituent material expressed in mass%: ai, the thickness of the negative electrode binder layer: t (cm), and the mass per unit area of the negative electrode binder layer: m (g / cm 2) If known, it can be obtained using the following formula (1). Therefore, measure the thickness and mass per unit area of the negative electrode binder layer in advance, obtain the actual porosity (overall average value) using the following formula (1), and calculate the value of n from the relational expression with the apparent porosity calculated from the histogram. Then, for another histogram obtained for the same negative electrode binder layer, it becomes possible to directly calculate the actual porosity S from the calculated apparent porosity: T.

[0045] S = 100 - (Σai / ρi)×(m / t) (1)

[0046] Note that when the acquisition conditions of the SEM image are different, such as when the acceleration voltage and magnification of the electron microscope are different, the depth at which luminance information can be obtained is different, and the value of n may change. Therefore, when changing the acquisition conditions of the SEM image, it is necessary to recalculate the value of n.

[0047] Then, the porosity obtained from the SEM image of the region from the surface on the side opposite to the current collector of the negative electrode binder layer to a position 10 μm in the thickness direction is defined as the "porosity of the surface portion on the side opposite to the current collector of the negative electrode binder layer". Also, the actual porosity S obtained by the above (1) is defined as the "porosity of the entire negative electrode binder layer".

[0048] Also, when a conductive assistant is contained in the negative electrode binder layer, it is preferable that the content (A) of the conductive assistant in the surface portion on the side opposite to the current collector of the negative electrode binder layer is higher than the content (B) of the conductive assistant on the current collector side from the center in the thickness direction of the negative electrode binder layer. By using a negative electrode that satisfies this, the load characteristics of the battery can be further improved.

[0049] Note that in the negative electrode binder layer, the ratio of the content (A) of the conductive assistant to the content (B) of the conductive assistant: (A) / (B) is preferably 2 or more from the viewpoint of ensuring the above effects more favorably, and preferably 3.5 or less from the viewpoint of increasing the conductivity of the entire negative electrode binder layer to a certain extent.

[0050] The content of the conductive aid in the negative electrode mixture layer is calculated from the ratio of the area of "the portion of luminance due to the conductive aid, etc. (distribution 2 in Fig. 3)" extracted by fitting from the histogram for each luminance of the SEM image obtained when measuring the porosity of the negative electrode mixture layer to the area of the entire histogram.

[0051] Then, by the above method, the content of the conductive aid obtained from the SEM image of the region from the surface on the side opposite to the current collector of the negative electrode mixture layer to a position 10 μm in the thickness direction is defined as "content of conductive aid (A)", and the content of the conductive aid obtained from the SEM image of the region from the center in the thickness direction of the negative electrode mixture layer to a position 10 μm on the current collector side is defined as "content of conductive aid (B)".

[0052] 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 aid, etc. in a solvent such as an organic solvent like NMP or water to prepare a paste-like 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 drying it.

[0053] Normally, as described above, after applying the negative electrode mixture-containing composition to the surface of the negative electrode current collector and drying it to form a negative electrode mixture layer (dry coating film of the negative electrode mixture-containing composition), a compression treatment such as rolling is performed. In this case, however, the voids in the surface portion of the negative electrode mixture layer will be crushed. Therefore, in order to adjust the porosity of the surface portion to the above value, after drying the negative electrode mixture-containing composition applied to the current collector to form a negative electrode mixture layer, either no compression treatment is performed, or if a compression treatment is performed, it should be carried out under conditions such that the porosity of the surface portion of the negative electrode mixture layer does not fall below the above value. It is more preferable not to perform a compression treatment. Note that since the conditions for performing a compression treatment after forming the negative electrode mixture layer vary depending on the apparatus for performing the compression treatment, after extracting conditions such that the porosity of the surface portion of the negative electrode mixture layer in the processed negative electrode can satisfy the above value, the negative electrode can be manufactured.

[0054] In addition, when the negative electrode mixture-containing composition applied to the current collector is dried to form a negative electrode mixture layer and then subjected to a compression treatment, the conductive auxiliary agent present on the surface portion of the negative electrode mixture layer at the stage after drying is pushed into the inner side (current collector side) of the negative electrode mixture layer. Therefore, the amount of the conductive auxiliary agent on the surface portion decreases, and there is a risk that the conductivity in the vicinity of the surface portion decreases.

[0055] However, when the negative electrode mixture-containing composition applied to the current collector is dried to form a negative electrode mixture layer and then either not subjected to a compression treatment or subjected to a compression treatment under the condition that 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 the conductive auxiliary agent on the surface portion can be suppressed. Therefore, the relationship between the content (A) of the conductive auxiliary agent and the content (B) of the conductive auxiliary agent can be adjusted as described above, and thereby, a negative electrode capable of further enhancing the load characteristics of the battery can be obtained.

[0056] In addition, when the compression treatment is not performed after the formation of the negative electrode mixture layer or when the compression treatment is performed under the condition that the porosity of the surface portion of the negative electrode mixture layer is 40% or more, the surface (the surface opposite to the current collector) of the negative electrode mixture layer has a larger surface roughness than when the compression treatment is performed under the conditions employed during general negative electrode production. 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 to the current collector can be 4.1 μm or more. Further, the arithmetic mean roughness of the unevenness on the surface of the negative electrode mixture layer opposite to the current collector is usually 6 μm or less.

[0057] The "arithmetic mean roughness of the unevenness on the surface of the negative electrode mixture layer opposite to the current collector" as used in this specification is the arithmetic mean roughness defined in Japanese Industrial Standard (JIS) B 0601 (2013). Specifically, using a confocal laser microscope ("Real-time Scanning Laser Microscope 1LM-21D" manufactured by Lasertec Corporation), three fields of view of 90 μm × 90 μm are observed at a magnification of 50 times, measured at 900 × 900 pixels for each field of view, and the absolute values from the average line of each point are arithmetically averaged to obtain the numerical values of each field of view, and then the values are further arithmetically averaged.

[0058] In the negative electrode, Li ions can be doped into the negative electrode active material contained in the negative electrode mixture layer.

[0059] In a non-aqueous electrolyte secondary battery configured using a negative electrode active material with a large capacity such as silicon oxide, generally, among the Li released from the positive electrode during charging, the proportion that is taken into the negative electrode active material and remains without being released during the next discharge is large, and there is a risk that the capacity originally possessed by the battery cannot be fully extracted. However, by pre-doping (pre-doping) Li ions into the negative electrode active material, the proportion of Li that can move back and forth between the positive electrode and the negative electrode during charge and discharge can be increased, reducing the irreversible capacity of the battery, and thus a higher-capacity battery can be formed.

[0060] To dope Li ions into the negative electrode active material, there is a method of performing it in a non-aqueous electrolyte secondary battery (in-system pre-doping). However, to dope Li ions into the negative electrode active material related to the negative electrode before it is incorporated into the non-aqueous electrolyte secondary battery, for example, the following out-of-system pre-doping (i) or (ii) can be adopted.

[0061] Out-of-system pre-doping method (i) Use a negative electrode manufactured using a negative electrode active material not doped with Li ions, and dope Li ions into the negative electrode active material.

[0062] In the out-of-system pre-doping method (i), doping of Li ions into the negative electrode active material in the negative electrode mixture layer of the negative electrode can be carried out, for example, by immersing the negative electrode in a solution in which biphenyl, polycyclic aromatic compounds (such as anthracene and naphthalene), p-benzoquinone, metallic Li, etc. are dissolved in a solvent such as tetrahydrofuran or diethyl ether, and then washing and drying with the solvent [hereinafter referred to as the out-of-system pre-doping method (i-1)]. The doping amount of Li ions at this time can be controlled by adjusting the amounts of each component in the solution.

[0063] In addition, in the off-system pre-doping method (i), Li ions can also be doped into the negative electrode active material in the negative electrode mixture layer by immersing the negative electrode (working electrode) and a lithium metal foil (counter electrode, including a lithium alloy foil) in a non-aqueous electrolyte solution and applying an electric current between them [hereinafter referred to as the off-system pre-doping method (i-2)]. As the non-aqueous electrolyte solution, the same solution as the non-aqueous electrolyte solution for non-aqueous electrolyte secondary batteries (described in detail later) can be used. The amount of Li ion doping at this time can be controlled by adjusting the current density per unit area of the negative electrode (negative electrode mixture layer) and the amount of electricity applied.

[0064] Off-system pre-doping method (ii) Li ions are directly doped into the negative electrode active material that has not been doped with Li ions. In this case, by immersing the negative electrode active material (negative electrode active material before Li ion doping) in the solution described above for immersing the negative electrode, instead of the negative electrode, Li ions can be doped into the negative electrode active material.

[0065] When adopting this off-system pre-doping method (ii), by using the obtained negative electrode active material (negative electrode active material doped with Li ions) to manufacture the negative electrode by the above method or the like, a negative electrode containing the negative electrode active material doped with Li ions can be obtained. In this case, for the negative electrode active material used in the manufacture of the negative electrode, a part or all of it can be the one doped with Li ions by the off-system pre-doping method (ii).

[0066] <Non-aqueous electrolyte secondary battery> The non-aqueous 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 non-aqueous electrolyte, and the negative electrode is the negative electrode for the non-aqueous electrolyte secondary battery of the present invention.

[0067] (Positive electrode) Examples of the positive electrode for a non-aqueous electrolyte secondary battery include those having 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.

[0068] The positive electrode active material is not particularly limited as long as it is a positive electrode active material used in a conventionally known non-aqueous electrolyte secondary battery, that is, a material capable of occluding and releasing lithium ions. Specifically, for example, metal oxides (lithium-containing composite oxides) composed of Li and a metal M other than Li (such as Co, Ni, Mn, Fe, Mg, Al, etc.) can be used. Examples of such lithium-containing composite oxides include lithium cobalt oxides such as LiCoO2; lithium manganese oxides such as LiMnO2 and Li2MnO3; lithium nickel oxides such as LiNiO2; lithium-containing composite oxides with a layered structure such as LiCo 1-x NiO2; lithium-containing composite oxides with a spinel structure such as LiMn2O4 and Li 4 / 3 Ti 5 / 3 O4; lithium-containing composite oxides with an olivine structure such as LiFePO4; oxides obtained by substituting various elements with the above oxides as the basic composition; and the like.

[0069] Examples of the conductive assistant in the positive electrode binder layer include graphite (graphite-based carbon materials) such as natural graphite (flake graphite, etc.) and artificial graphite; carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon fibers; and other carbon materials. In addition, PVDF, PTFE, vinylidene fluoride-chlorotrifluoroethylene copolymer [P(VDF-CTFE)], SBR, carboxymethyl cellulose (CMC), etc. are preferably used as the binder in the positive electrode binder layer.

[0070] The positive electrode is manufactured, for example, by preparing a paste-like or slurry-like positive electrode binder-containing composition in which a positive electrode active material, a conductive assistant, a binder, etc. are dispersed in a solvent such as NMP (however, the binder may be dissolved in the solvent), applying this to one or both sides of a current collector, drying, and then, if necessary, subjecting it to a compression treatment such as a rolling treatment. However, the positive electrode is not limited to those manufactured by the above manufacturing method, and those manufactured by other methods may also be used.

[0071] For the positive electrode current collector, metal foils such as aluminum and stainless steel, punching metal, mesh, expanded metal, foamed metal; carbon sheets; etc. can be used. The thickness of the positive electrode current collector is preferably 5 to 30 μm.

[0072] In the positive electrode mixture layer, the content of the positive electrode active material can be 60 to 95% by mass, the content of the binder can be 1 to 15% by mass, and the content of the conductive assistant can be 3 to 20% by mass. Also, the thickness of the positive electrode mixture layer (thickness per side of the current collector) is preferably 30 to 150 μm.

[0073] (Separator) As the separator of the non-aqueous electrolyte secondary battery, those with sufficient strength and capable of retaining a large amount of non-aqueous electrolyte are preferable, and microporous membranes made of polyolefins such as polyethylene (PE) and polypropylene (PP) can be used. The microporous membrane constituting the separator may be, for example, one using only PE or only PP, may contain an ethylene-propylene copolymer, or may be a laminate of a PE microporous membrane and a PP microporous membrane.

[0074] Furthermore, as the separator, a laminated separator composed of a porous layer mainly made of a resin with a melting point of 140°C or lower and a porous layer mainly containing a resin with a melting point of 150°C or higher or an inorganic filler with a heat-resistant temperature of 150°C or higher can be used. Here, the "melting point" means the melting temperature measured using a differential scanning calorimeter (DSC) according to the provisions of Japanese Industrial Standard (JIS) K 7121, and "heat-resistant temperature of 150°C or higher" means that no deformation such as softening is observed at least at 150°C.

[0075] The thickness of the separator (separator made of a microporous polyolefin membrane or the laminated separator) is more preferably 10 to 30 μm.

[0076] (Non-aqueous electrolyte) For the non-aqueous electrolyte of a non-aqueous electrolyte secondary battery, a non-aqueous liquid electrolyte (non-aqueous electrolyte solution) is usually used. As the non-aqueous electrolyte solution, a solution obtained by dissolving an electrolyte salt such as a lithium salt in an organic solvent is used. The organic solvent is not particularly limited. For example, chain esters such as dimethyl carbonate, diethyl carbonate (DEC), ethyl methyl carbonate (MEC), and methyl propyl carbonate; cyclic esters with high dielectric constants such as ethylene carbonate (EC), propylene carbonate, butylene carbonate, and vinylene carbonate; a mixed solvent of a chain ester and a cyclic ester; and the like can be mentioned. In particular, a mixed solvent of a cyclic ester with a chain ester as the main solvent is suitable.

[0077] As the electrolyte salt dissolved in the organic solvent in the preparation of the non-aqueous electrolyte solution, for example, LiPF6, LiBF4, LiAsF6, LiSbF6, LiCF3SO3, LiC4F9SO3, LiCF3CO2, Li2C2F4(SO3)2, LiC n F 2n+1 SO3(n≧2), LiN(RfSO2)(Rf’S O2), LiC(RfSO2)3, LiN(RfOSO2)2 [where Rf and Rf’ are fluoroalkyl groups] and the like are used alone or in a mixture of two or more. The concentration of the electrolyte salt in the non-aqueous electrolyte solution is not particularly limited, but it is preferably 0.3 mol / l or more, more preferably 0.4 mol / l or more, and preferably 1.7 mol / l or less, more preferably 1.5 mol / l or less.

[0078] In addition, for the non-aqueous electrolyte solution, for the purpose of further improving the charge-discharge cycle characteristics of the battery and improving safety such as high-temperature storage properties and overcharge prevention, additives (including derivatives thereof) such as vinylene carbonate (VC), vinyl ethylene carbonate, anhydrous acid, sulfonic acid ester, dinitrile, 1,3-propane sultone, diphenyl disulfide, cyclohexylbenzene, biphenyl, fluorobenzene, t-butylbenzene can be appropriately added.

[0079] In a non-aqueous electrolyte secondary battery, as the non-aqueous electrolyte, in addition to the non-aqueous electrolyte solution, a gelled electrolyte obtained by gelling the non-aqueous electrolyte solution with a gelling agent made of a polymer or the like, or a known solid electrolyte can also be used.

[0080] (Electrode body) In a non-aqueous electrolyte secondary battery, the negative electrode and the positive electrode are used in the form of a laminate (laminated electrode body) laminated via a separator, or a wound body (wound electrode body) obtained by further winding this laminate in a spiral shape.

[0081] (Form of non-aqueous electrolyte secondary battery, etc.) There is no particular limitation on the form of the non-aqueous electrolyte secondary battery. For example, it may be any form such as a small cylindrical shape, coin shape, button shape, flat shape, rectangular shape, or a large one used for an electric vehicle or the like.

[0082] Figs. 5 and 6 show drawings schematically showing an example of the non-aqueous electrolyte secondary battery of the present invention. Fig. 5 is a plan view of the non-aqueous electrolyte secondary battery, and Fig. 6 is a cross-sectional view taken along line I-I of Fig. 5.

[0083] The non-aqueous electrolyte secondary battery 1 shown in Figs. 5 and 6 houses an electrode body in which a positive electrode 5 and a negative electrode 6 are laminated via a separator 7, and a non-aqueous electrolyte (not shown) inside a laminate film exterior body 2 composed of two metal laminate films. The laminate film exterior body 2 is sealed by heat-sealing the upper and lower metal laminate films at its outer peripheral portion.

[0084] In Fig. 6, in order to avoid complication of the drawing, each layer constituting the laminate film exterior body 2 and each layer of the positive electrode 5 and the negative electrode 6 are not shown separately.

[0085] The positive electrode 5 is connected to the positive electrode external terminal 3 inside the battery 1. Also, although not shown, the negative electrode 6 is also connected to the negative electrode external terminal 4 inside the battery 1. And the positive electrode external terminal 3 and the negative electrode external terminal 4 are drawn out to the outside of the laminate film exterior body 2 at one end side so as to be connectable to an external device or the like.

[0086] 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. However, a battery may be assembled using a negative electrode containing a negative electrode active material not doped with Li ions, and Li ions may be doped (in-system pre-doping) into the negative electrode active material in the negative electrode mixture layer of the negative electrode in the battery.

[0087] In-system 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 (such as a lithium metal foil or a lithium alloy foil) separate from the positive electrode and the negative electrode, that is, a pre-doping electrode for doping Li ions into the negative electrode active material, and energizing this pre-doping electrode to dope Li ions from the Li supply source into the negative electrode active material in the negative electrode mixture layer in the battery. Therefore, in the case of a battery in which Li ions are doped into the negative electrode active material by in-system pre-doping, even at the time when the doping of Li ions is completed, a part of the Li supply source remains or a pre-doping electrode in which all has disappeared remains in the battery.

[0088] 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 pre-doped with Li ions by out-of-system pre-doping, it does not have a pre-doping electrode used for (or used in) the doping of Li ions inside.

[0089] The non-aqueous electrolyte secondary battery of the present invention can be applied to the same uses as conventionally known non-aqueous electrolyte secondary batteries. Further, the negative electrode for a non-aqueous electrolyte secondary battery of the present invention can constitute the non-aqueous electrolyte secondary battery of the present invention.

Examples

[0090] Hereinafter, the present invention will be described in detail based on examples. However, the following examples do not limit the present invention.

[0091] Example 1 <Fabrication of negative electrode> 87 parts by mass of silicon oxide (SiO) as the negative electrode active material, 5 parts by mass of acetylene black as the conductive assistant, and 8 parts by mass of polyacrylic acid as the binder were mixed uniformly with water as the solvent to prepare a negative electrode mixture-containing paste.

[0092] This negative electrode mixture-containing paste was applied to one side of a copper foil with a thickness of 6 μm serving as the negative electrode current collector, dried, and a negative electrode having a negative electrode mixture layer with a thickness of 46 μm on one side of the negative electrode current collector was produced. In the obtained negative electrode, the size of the negative electrode mixture layer in plan view was 2.5 cm × 4.1 cm. Also, when applying the negative electrode mixture-containing paste to the negative electrode current collector, an exposed portion where no negative electrode mixture layer was formed was provided in a part of the negative electrode current collector, and this was used as the negative electrode external terminal.

[0093] In the negative electrode, the porosity of the surface portion and the whole of the negative electrode mixture layer was 47.8% and 55.3% respectively. Also, the content (A) of the conductive assistant in the surface portion of the negative electrode mixture layer was more than the content (B) of the conductive assistant on the current collector side from 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 on the surface of the negative electrode mixture layer was 5.5 μm, and the maximum peak height of the convex portion measured when obtaining the arithmetic mean roughness was 38.8 μm.

[0094] <Fabrication of the positive electrode> 85 parts by mass of LiCoO2 as the positive electrode active material, 10 parts by mass of acetylene black as the conductive assistant, and 5 parts by mass of PVDF as the binder were mixed uniformly with NMP as the solvent to prepare a positive electrode mixture-containing paste.

[0095] This positive electrode active material-containing paste was applied to one side of an aluminum foil with a thickness of 11 μm serving as a negative electrode current collector, dried, and then subjected to a compression treatment using a rope press machine to produce a positive electrode having a positive electrode active material layer with a thickness of 58 μm on one side of the positive electrode current collector. In the obtained positive electrode, the size of the positive electrode active material layer in plan view was 2.7 cm × 4.2 cm. Further, when applying the positive electrode active material-containing paste to the positive electrode current collector, an exposed portion where no positive electrode active material layer was formed was provided on a part of the positive electrode current collector, and this was used as the positive electrode external terminal.

[0096] <Assembly of the battery> The above positive electrode and the above negative electrode were overlapped with a PE microporous membrane separator (thickness 16 μm, porosity 40%) interposed therebetween to form a laminated electrode body. This laminated electrode body was inserted into a bag-shaped exterior body made of an 8 cm × 7 cm aluminum laminate film. Note that a part of the positive electrode external terminal and the negative electrode external terminal of the laminated electrode body protruded from the opening of the exterior body. Next, after dissolving LiPF6 in a solution obtained by mixing EC, DEC, and MEC at a volume ratio of 1:1:1 at a concentration of 1 mol / L, VC was further dissolved in an amount of 1% by mass to prepare a non-aqueous electrolyte, which was then injected into the exterior body. Thereafter, the opening of the exterior body was sealed to produce a non-aqueous electrolyte secondary battery having the appearance shown in FIG. 5 and the cross-sectional structure shown in FIG. 6.

[0097] Example 2 The same silicon oxide used in Example 1 was mixed with a polyamic acid solution “DREAMBOND” (registered trademark, manufactured by Industrial Summit Technology Corporation), heated at 200° C. for 10 hours, and silicon oxide coated with polyimide (5 parts by mass of polyimide with respect to 95 parts by mass of silicon oxide) was obtained. 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 non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that this negative electrode was used.

[0098] In the negative electrode produced in Example 2, the porosity of the surface portion and the whole of the negative electrode mixture layer was 50.3% and 55.7%, respectively. Further, the content (A) of the conductive auxiliary agent in the surface portion of the negative electrode mixture layer was larger than the content (B) of the conductive auxiliary agent on the current collector side from 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 on the surface of the negative electrode mixture layer was 4.3 μm, and the maximum peak height of the convex portions measured when obtaining the arithmetic mean roughness was 38.5 μm.

[0099] Comparative Example 1 A negative electrode was produced in the same manner as in Example 2 except that a negative electrode mixture-containing paste was applied to the surface of the negative electrode current collector, dried, and then subjected to a compression treatment with a roll press machine, and a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 2 except that this negative electrode was used.

[0100] In the negative electrode produced in Comparative Example 1, the porosity of the surface portion and the whole of the negative electrode mixture layer was 38.2% and 45.7%, respectively. Further, the content (A) of the conductive auxiliary agent in the surface portion of the negative electrode mixture layer was equivalent to the content (B) of the conductive auxiliary agent on the current 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 on the surface of the negative electrode mixture layer was 4.0 μm, and the maximum peak height of the convex portions measured when obtaining the arithmetic mean roughness was 26.8 μm.

[0101] Regarding the non-aqueous electrolyte secondary batteries of the examples and comparative examples, a series of operations of constant current charging at a current value of 0.5C until 4.35V was reached with respect to the design capacity, and then constant current discharging at a current value of 0.5C until 2.5V was reached were repeated 1000 cycles. Then, for each battery, the capacity retention rate, which is the value obtained by dividing the discharge capacity at the 1000th cycle by the discharge capacity at the 1st cycle and expressed as a percentage, was determined to evaluate the charge-discharge cycle characteristics.

[0102] The above evaluation results are shown in Table 1 together with the porosity of the surface portion of the negative electrode mixture layer, the arithmetic mean roughness of the unevenness on the surface of the negative electrode mixture layer, and the maximum peak height of the convex portions on the surface of the negative electrode mixture layer obtained by the above method for the negative electrodes used in each battery.

[0103]

Table 1

[0104] As shown in Table 1, the non-aqueous electrolyte secondary batteries of Examples 1 and 2 using 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 excellent charge-discharge cycle characteristics compared to the battery of Comparative Example 1 using a negative electrode with an inappropriate porosity of the surface portion.

[0105] When comparing the battery of Example 1 and the battery of Example 2, the battery of Example 2 using a negative electrode active material coated with polyimide had better evaluation results of charge-discharge cycle characteristics than the battery of Example 1 using a negative electrode active material without coating on the surface. This is presumably because in the battery of Example 2, the volume change of the negative electrode active material due to repeated charge and discharge was small due to the adoption of polyimide, and the decrease in the conductivity of the negative electrode mixture layer was suppressed compared to the battery of Example 1.

Explanation of Reference Numerals

[0106] 1 Non-aqueous electrolyte secondary battery 2 Laminate film exterior body 3 Positive electrode external terminal 4 Negative electrode external terminal 5 Positive electrode 6 Negative electrode 7 Separator

Claims

1. A negative electrode for a non-aqueous electrolyte secondary battery having a negative electrode mixture layer containing a negative electrode active material and a binder on a current collector, When a region from the surface of the negative electrode mixture layer on the side opposite to the current collector to a position 10 μm in the thickness direction is defined as the surface portion of the negative electrode mixture layer, The porosity in the surface portion of the negative electrode mixture layer is 40% or more, The negative electrode mixture layer contains a conductive auxiliary agent, The content (A) of the conductive auxiliary agent in the surface portion of the negative electrode mixture layer is greater than the content (B) of the conductive auxiliary agent in a region from the center in the thickness direction of the negative electrode mixture layer to a position 10 μm toward the current collector side, and the content (B) of the conductive auxiliary agent is a positive value, A negative electrode for a non-aqueous electrolyte secondary battery, wherein the ratio (A) / (B) of the content (A) of the conductive auxiliary agent to the content (B) of the conductive auxiliary agent is 2 or more and 3.5 or less.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to Claim 1, wherein the negative electrode active material contains silicon or a silicon oxide.

3. The negative electrode for a non-aqueous electrolyte secondary battery according to Claim 1 or 2, wherein the arithmetic mean roughness of the unevenness on the surface of the negative electrode mixture layer on the side opposite to the current collector is 4.1 μm or more.

4. The negative electrode for a non-aqueous electrolyte secondary battery according to any one of Claims 1 to 3, wherein the surface of the negative electrode active material is coated with polyimide.

5. 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, The non-aqueous electrolyte secondary battery, wherein the negative electrode is the negative electrode for a non-aqueous electrolyte secondary battery according to any one of Claims 1 to 4.

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