Negative electrode, negative electrode manufacturing method, and battery

Abstract

A negative electrode 10 according to the present disclosure comprises a negative electrode current collector 11, and a negative electrode mixture layer 12 disposed on the negative electrode current collector 11. The negative electrode mixture layer 12 includes a first negative electrode mixture layer 13 disposed on the negative electrode current collector 11, a second negative electrode mixture layer 14 disposed on the first negative electrode mixture layer 13, and a third negative electrode mixture layer 15 disposed on the second negative electrode mixture layer 14. The third negative electrode mixture layer 15 contains graphite as a negative electrode active material, and the mass ratio of artificial graphite to graphite is 80% or more. The thermal decomposition temperature of the material having the lowest thermal decomposition temperature in a binder contained in the third negative electrode mixture layer 15 is higher than the thermal decomposition temperature of the material having the lowest thermal decomposition temperature in a binder contained in the first negative electrode mixture layer 13 and the thermal decomposition temperature of the material having the lowest thermal decomposition temperature in a binder contained in the second negative electrode mixture layer 14.

Description

Negative electrode, method for manufacturing a negative electrode, and battery

[0001] This disclosure relates to a negative electrode, a method for manufacturing a negative electrode, and a battery.

[0002] In recent years, secondary batteries such as lithium-ion batteries have been widely used in applications requiring high capacity, such as automotive and energy storage. The electrodes that make up such batteries have a significant impact on their performance. For this reason, various studies have been conducted on electrodes.

[0003] Conventionally, in lithium-ion secondary batteries, carbon materials, for example, have been used as the active material for the negative electrode. For example, Patent Document 1 proposes a negative electrode comprising a negative electrode mixture layer composed of a first mixture layer located on the surface side of the negative electrode mixture layer and mainly containing graphite coated with amorphous carbon, and a second mixture layer located on the negative electrode current collector side and mainly containing graphite not coated with amorphous carbon.

[0004] International Publication No. 2018 / 225515

[0005] Conventional batteries had a problem in that the binder in the negative electrode mixture layer deteriorated due to the high temperatures during the battery manufacturing process, reducing its bonding ability and thus lowering its cycle performance.

[0006] This disclosure provides a negative electrode that can improve the cycle characteristics of a battery.

[0007] The negative electrode of the present disclosure comprises a negative electrode current collector and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material and a binder, wherein the negative electrode mixture layer includes a first negative electrode mixture layer disposed on the negative electrode current collector, a second negative electrode mixture layer disposed on the first negative electrode mixture layer, and a third negative electrode mixture layer disposed on the second negative electrode mixture layer, the third negative electrode mixture layer contains graphite as the negative electrode active material, the mass ratio of artificial graphite in the graphite contained in the third negative electrode mixture layer is 80% or more, and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the third negative electrode mixture layer is higher than the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the first negative electrode mixture layer and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the second negative electrode mixture layer.

[0008] According to this disclosure, it is possible to provide a negative electrode that can improve the cycle characteristics of a battery.

[0009] Figure 1 is a cross-sectional view showing the schematic configuration of the negative electrode according to Embodiment 1. Figure 2 is a longitudinal cross-sectional view schematically showing an example of a battery according to Embodiment 2.

[0010] The embodiments of this disclosure will be described in detail below with reference to the drawings. This disclosure is not limited to the embodiments described below.

[0011] (Embodiment 1) Figure 1 is a cross-sectional view showing the schematic configuration of a negative electrode according to Embodiment 1. The negative electrode 10 according to Embodiment 1 comprises a negative electrode current collector 11 and a negative electrode mixture layer 12 disposed on the negative electrode current collector 11. The negative electrode mixture layer 12 contains a negative electrode active material and a binder.

[0012] The negative electrode mixture layer 12 includes a first negative electrode mixture layer 13 disposed on the negative electrode current collector 11, a second negative electrode mixture layer 14 disposed on the first negative electrode mixture layer 13, and a third negative electrode mixture layer 15 disposed on the second negative electrode mixture layer 14. The third negative electrode mixture layer 15 contains graphite as a negative electrode active material, and the mass ratio of artificial graphite in the graphite contained in the third negative electrode mixture layer 15 is 80% or more.

[0013] The thermal decomposition temperature of the binder with the lowest thermal decomposition temperature in the third negative electrode mixture layer 15 is higher than the thermal decomposition temperature of the binder with the lowest thermal decomposition temperature in the first negative electrode mixture layer 13, and higher than the thermal decomposition temperature of the binder with the lowest thermal decomposition temperature in the second negative electrode mixture layer 14.

[0014] The negative electrode 10 according to Embodiment 1, having the above configuration, can improve the battery's cycle characteristics.

[0015] More specifically, in the negative electrode 10 according to Embodiment 1, the negative electrode mixture layer 12 includes a third negative electrode mixture layer 15 on its surface side, and the binder contained in the third negative electrode mixture layer 15 has a higher thermal decomposition temperature than the material with the lowest thermal decomposition temperature among the binders contained in the first negative electrode mixture layer 13 and the material with the lowest thermal decomposition temperature among the binders contained in the second negative electrode mixture layer 14. In this specification, the surface side of the negative electrode mixture layer 12 refers to the side opposite to the negative electrode current collector 11, that is, the surface side of the negative electrode mixture layer 12 that is in contact with the electrolyte when the negative electrode 10 constitutes a battery. The surface side of the negative electrode mixture layer 12 is susceptible to heat during the battery manufacturing process, but with the above configuration, the heat resistance of the surface side of the negative electrode mixture layer 12 is improved, thereby suppressing the deterioration of the binder. As a result, the decrease in binding properties can be reduced, and the expansion and contraction of the electrodes can be suppressed, thereby improving the battery's cycle characteristics. Furthermore, by including artificial graphite in a predetermined amount or more on the surface side of the negative electrode mixture layer 12, it is possible to reduce the decrease in bonding properties and improve cycle characteristics while maintaining other battery characteristics from conventional batteries.

[0016] The configurations of the negative electrode 10 in Embodiment 1 will be described in detail below.

[0017] [Negative electrode current collector] A sheet or film made of a metallic material such as stainless steel, nickel, copper, or alloys thereof may be used as the negative electrode current collector 11. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., may be used as the sheet or film. A carbon material such as carbon may be coated on the surface of the negative electrode current collector 11 as a conductive auxiliary material.

[0018] The thickness of the negative electrode current collector 11 is not particularly limited, but from the viewpoint of balancing the strength and weight reduction of the negative electrode 10, it may be, for example, 1 μm or more and 50 μm or less, or 5 μm or more and 20 μm or less.

[0019] [Negative Electrode Compound Layer] The negative electrode compound layer 12 consists of a first negative electrode compound layer 13 placed on the negative electrode current collector 11, a second negative electrode compound layer 14 placed on the first negative electrode compound layer 13, and a third negative electrode compound layer 15 placed on the second negative electrode compound layer 14, all stacked in this order. That is, the third negative electrode compound layer 15 is located on the surface side of the negative electrode compound layer 12.

[0020] In this specification, the negative electrode active material contained in the first negative electrode mixture layer 13 is referred to as the first negative electrode active material. The negative electrode active material contained in the second negative electrode mixture layer 14 is referred to as the second negative electrode active material. The negative electrode active material contained in the third negative electrode mixture layer 15 is referred to as the third negative electrode active material.

[0021] The first negative electrode active material includes, for example, graphite. The first negative electrode active material may contain graphite as its main component. The main component of the first negative electrode active material means the component with the largest mass ratio in the first negative electrode active material, that is, the component that accounts for the largest proportion of the total mass of the negative electrode active material contained in the first negative electrode mixture layer 13. The proportion of graphite in the total mass of the negative electrode active material contained in the first negative electrode mixture layer 13 may be 50% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or even 95% by mass or more. The first negative electrode active material may consist only of graphite.

[0022] The first negative electrode active material may contain natural graphite. The proportion of natural graphite in the graphite included as the first negative electrode active material in the first negative electrode mixture layer 13 may be 70% by mass or more, 80% by mass or more, 90% by mass or more, even 95% by mass or more, or 100% by mass. The first negative electrode active material may contain natural graphite as its main component, or it may consist only of natural graphite. With such a configuration, the binding of the negative electrode mixture layer 12 to the negative electrode current collector 11 can be improved.

[0023] The second negative electrode active material includes, for example, graphite. The second negative electrode active material may contain graphite as its main component. The main component of the second negative electrode active material means the component with the largest mass ratio in the second negative electrode active material, that is, the component that accounts for the largest proportion of the total mass of the negative electrode active material contained in the second negative electrode mixture layer 14. The proportion of graphite in the total mass of the negative electrode active material contained in the second negative electrode mixture layer 14 may be 50% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or even 95% by mass or more. The second negative electrode active material may consist only of graphite.

[0024] The second negative electrode active material may contain artificial graphite. The proportion of artificial graphite in the graphite included as the second negative electrode active material in the second negative electrode mixture layer 14 may be 70% by mass or more, 80% by mass or more, 90% by mass or more, even 95% by mass or more, or 100% by mass. The second negative electrode active material may contain artificial graphite as its main component, or it may consist only of artificial graphite. With such a configuration, the ion resistance of the negative electrode 10 can be reduced, and the charge and discharge characteristics of the battery can be improved.

[0025] Artificial graphite and natural graphite have different ranges of internal porosity. For example, natural graphite has an internal porosity greater than 5%, while artificial graphite has an internal porosity of 5% or less. The internal porosity of natural graphite may be, for example, 8% or more. Preferably, the internal porosity of natural graphite is 20% or less, more preferably 12% or more, and more preferably 18% or less. The internal porosity of artificial graphite may be, for example, 1% or more, and may be 2% or more. Here, the internal porosity of a graphite particle refers to the ratio of the area of ​​internal voids S2 to the area of ​​the graphite particle S1 in a cross-sectional image of the graphite particle (for example, an SEM image or a TEM image), and is calculated by (S2 / S1) × 100. The area of ​​the graphite particle S1 is the area of ​​the region surrounded by the outer circumference of the graphite particle. If there are multiple internal voids, the area of ​​internal voids S2 refers to the total area of ​​the multiple internal voids. The internal porosity of natural graphite can be determined by calculating the average of the internal porosities of 10 natural graphite particles. The internal porosity of artificial graphite can be determined by calculating the average of the internal porosity of 10 artificial graphite particles.

[0026] For example, the first negative electrode active material may contain natural graphite as its main component, and the second negative electrode active material may contain artificial graphite as its main component. In this case, the porosity of the second negative electrode mixture layer 14 may be greater than that of the first negative electrode mixture layer 13. The porosity of the first negative electrode mixture layer 13 is a two-dimensional value obtained from the ratio of the area of ​​voids to the cross-sectional area of ​​the first negative electrode mixture layer 13 in a cross-section of the first negative electrode mixture layer 13. Similarly, the porosity of the second negative electrode mixture layer 14 is a two-dimensional value obtained from the ratio of the area of ​​voids to the cross-sectional area of ​​the second negative electrode mixture layer 14 in a cross-section of the second negative electrode mixture layer 14.

[0027] Therefore, the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 can be distinguished, for example, by using a cross-sectional image of the negative electrode mixture layer 12 to determine the differences in graphite particles and the resulting differences in porosity.

[0028] While graphite has been described as a material for the negative electrode active material that can be included in the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, i.e., a material that can be included in the first negative electrode active material and the second negative electrode active material, the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 may further contain other materials as negative electrode active materials. For example, the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14 may further contain a silicon-containing material as a negative electrode active material. That is, the first negative electrode active material may contain a silicon-containing material. Also, the second negative electrode active material may contain a silicon-containing material. By including a silicon-containing material as a negative electrode active material, it is possible to achieve a higher battery capacity.

[0029] In this specification, silicon-containing materials mean materials containing Si. Examples of silicon-containing materials include Si, Si alloys, Si compounds, and Si-containing composite materials.

[0030] Silicon-containing materials are preferably composite materials containing Si. Si-containing composite materials are, for example, composite particles comprising an ionic conductive phase and a Si phase dispersed within the ionic conductive phase. The ionic conductive phase includes, for example, at least one selected from the group consisting of an aluminate phase, a silicate phase, a carbon phase, a silicide phase, and a silicon oxide phase. The ionic conductive phase may consist of one phase or multiple phases. In the Si phase, for example, Si is formed into fine particles and dispersed within the ionic conductive phase. In Si-containing composite materials, the stress associated with the expansion and contraction of the Si phase during charging and discharging is relieved by the ionic conductive phase, suppressing cracks and fractures in the composite material. Therefore, Si-containing composite materials can achieve both high capacity due to the inclusion of Si and improved charge-discharge cycle characteristics.

[0031] The aluminate phase is, for example, a phase containing alkali aluminate comprising at least one alkali metal element, Al, and O. The alkali metal element is, for example, Li. The composition of lithium aluminate is given by the formula: Li u AlO (3+u) / 2 It can be expressed as follows. From the viewpoint of ease of manufacture, stability, and ionic conductivity, etc., u in the formula may be, for example, greater than 0 and less than or equal to 5, or greater than 0 and less than or equal to 1.

[0032] The silicate phase may satisfy the following conditions (A) and / or (B): (A) The silicate phase contains at least one element selected from the group consisting of alkali metal elements and Group 2 elements (Group 2 elements of the long-period periodic table). (B) The silicate phase contains element L. Element L is at least one element selected from the group consisting of B, Al, Zr, Nb, Ta, V, lanthanides, Y, Ti, P, Bi, Zn, Sn, Pb, Sb, Co, Er, F, and W. Lanthanides are a collective term for 15 elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71.

[0033] Regarding the above condition (A), examples of alkali metal elements include lithium (Li), potassium (K), and sodium (Na). Examples of Group 2 elements include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Inclusion of an alkali metal element and / or a Group 2 element may reduce the irreversible capacity of the silicate phase. A silicate phase containing lithium (hereinafter sometimes referred to as a "lithium silicate phase") is preferred, for example, because it has low irreversible capacity and high initial charge-discharge efficiency.

[0034] The lithium silicate phase only needs to be an oxide phase containing Li, Si, and O, and may also contain other elements. The atomic ratio of O to Si in the lithium silicate phase: O / Si is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase: Li / Si is, for example, greater than 0 and less than 4.

[0035] The lithium silicate phase is represented by formula: Li 2z SiO (2+z) (0<z<2) may include a lithium silicate phase represented by the above formula, or may be composed of said lithium silicate phase. z preferably satisfies the relationship 0<z<1, and z=1 / 2 (i.e., Li2Si2O5) is more preferred.

[0036] The silicide phase is a phase of a compound composed of Si and an element that is more electropositive than Si, and examples thereof include NiSi, Mg2Si, and TiSi2.

[0037] The silicon oxide phase is composed of a compound of Si and O. The main component of the silicon oxide phase may be silicon dioxide. The main component of the silicon oxide phase is, for example, a component that accounts for 95 mass% or more and 100 mass% or less of the silicon oxide phase.

[0038] The ion conductive phase may contain a carbon phase. The carbon phase is a matrix containing carbon, and includes, for example, a carbonaceous material. The carbonaceous material is not particularly limited as long as it is a carbon material capable of occluding lithium ions. The carbonaceous material may be amorphous carbon. Amorphous carbon is, for example, a carbon material having an average interplanar spacing d002 of the (002) plane measured by Kα diffraction that exceeds 0.34 nm. Examples of the carbonaceous material include pitch-derived materials, amorphous carbon, carbon black, and organic polymer-derived materials. The pitch is, for example, coal pitch or petroleum pitch, and the coal pitch is, for example, coal tar pitch.

[0039] The Si phase is a phase of elemental Si, and repeats occlusion and release of Li ions as the battery is charged and discharged.

[0040] In each of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, from the viewpoint of improving cycle characteristics, the content ratio of the silicon-containing material is, for example, 40% by mass or less based on the total mass of the negative electrode active material, may be 35% by mass or less, and may be 30% by mass or less. In each of the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14, from the viewpoint of increasing capacity, the content ratio of the silicon-containing material is, for example, 5% by mass or more based on the total mass of the negative electrode active material. An example of a suitable range for the content ratio of the silicon-containing material is 5% by mass or more and 35% by mass or less based on the total mass of the negative electrode active material, may be 10% by mass or more and 30% by mass or less, and may be 5% by mass or more and 15% by mass or less.

[0041] The third negative electrode active material contains graphite.

[0042] The third negative electrode active material may contain graphite as a main component. The main component of the third negative electrode active material means the component having the largest mass proportion in the third negative electrode active material, that is, the component accounting for the largest proportion in the total mass of the negative electrode active materials contained in the third negative electrode mixture layer 15. From the viewpoints of cycle characteristics and charge-discharge characteristics, the proportion of graphite in the total mass of the negative electrode active materials contained in the third negative electrode mixture layer 15 may be 50% by mass or more, may be 70% by mass or more, 80% by mass or more, 90% by mass or more, and may be 95% by mass or more. The third negative electrode active material may be composed only of graphite.

[0043] The third negative electrode active material contains artificial graphite, and the mass ratio of artificial graphite in the graphite contained as the negative electrode active material in the third negative electrode mixture layer 15 is 80% or more. The proportion of artificial graphite in the graphite contained as the third negative electrode active material in the third negative electrode mixture layer 15 may be 85% by mass or more, 90% by mass or more, 95% by mass or more, even 98% by mass or more, or 100% by mass. The third negative electrode active material may contain artificial graphite as its main component, or it may consist only of artificial graphite. With such a configuration, the charge and discharge characteristics of the battery can be improved.

[0044] The average particle size of the artificial graphite contained in the third negative electrode mixture layer 15 may be smaller than the average particle size of the graphite contained in the first negative electrode mixture layer 13 and the average particle size of the graphite contained in the second negative electrode mixture layer 14. The average particle sizes can be compared, for example, by observing the cross-section of the negative electrode active material using a scanning electron microscope (SEM) or a transmission electron microscope (TEM). That is, the average particle size of the artificial graphite in the third negative electrode mixture layer 15 can be measured from the SEM image or TEM image of the cross-section of the third negative electrode mixture layer 15 in which the cross-section of the artificial graphite is exposed. The average particle size can be determined by averaging the particle sizes of 100 artificial graphite particles arbitrarily extracted from the above SEM image or TEM image. The diameter of the circumscribed circle of the artificial graphite particle is defined as the particle size of the artificial graphite. The average particle size of graphite contained in the first negative electrode mixture layer 13 and the average particle size of graphite contained in the second negative electrode mixture layer 14 are determined from SEM or TEM images of the cross-section of the first negative electrode mixture layer 13, where the cross-section of graphite, i.e., the first negative electrode active material, is exposed, and from SEM or TEM images of the cross-section of the second negative electrode mixture layer 14, where the cross-section of graphite, i.e., the second negative electrode active material, is exposed, using the same method as for the average particle size of artificial graphite in the third negative electrode mixture layer 15.

[0045] The average particle size of the artificial graphite contained in the third negative electrode mixture layer 15 may be 15 μm or less, 10 μm or less, 0.5 μm or more and 10 μm or less, or 1 μm or more and 7 μm or less. In this way, by having a small average particle size of the artificial graphite contained in the third negative electrode mixture layer 15, the thickness of the third negative electrode mixture layer 15 can be reduced. Therefore, the battery's cycle characteristics can be improved while maintaining the charge-discharge characteristics.

[0046] From the above, the second negative electrode mixture layer 14 and the third negative electrode mixture layer 15 can be distinguished, for example, by using a cross-sectional image of the negative electrode mixture layer 12 to determine the difference in the size of the graphite particles and the resulting difference in porosity.

[0047] The third negative electrode mixture layer 15 may consist substantially of graphite and a binder only.

[0048] The negative electrode mixture layer 12 contains a binder. Hereinafter, the binder contained in the first negative electrode mixture layer 13 will be referred to as the first binder. The binder contained in the second negative electrode mixture layer 14 will be referred to as the second binder. The binder contained in the third negative electrode mixture layer 15 will be referred to as the third binder.

[0049] Examples of binders include fluororesins such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyimide, acrylic resins, polyolefins, and styrene-butadiene rubber (SBR). These resins may also be used in combination with carboxymethylcellulose (CMC) or its salts, polyacrylic acid (PAA) or its salts, polyvinyl alcohol (PVA), etc.

[0050] The thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the third binder is higher than the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the first binder, and higher than the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the second binder. Hereinafter, the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the first binder may be referred to as TD1, the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the second binder as TD2, and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the third binder as TD3.

[0051] The thermal decomposition temperature indicates the temperature at which weight loss due to thermal decomposition begins. The thermal decomposition temperature can be measured, for example, by simultaneous thermogravimetric and differential calorimetry (TG-DTA). Measurement conditions include, for example, heating from 25°C to 500°C at a heating rate of 10°C / min under a nitrogen atmosphere.

[0052] TD3 may be greater than 260°C, 280°C or higher, or 300°C or higher. TD1 and TD2 may each be 260°C or lower.

[0053] The first binder may contain CMC, and the thermal decomposition temperature of the CMC may be TD1. The second binder may contain CMC, and the thermal decomposition temperature of the CMC may be TD2. The third binder may contain CMC, and the thermal decomposition temperature of the CMC may be TD3. That is, the thermal decomposition temperature of the CMC as the third binder may be higher than the thermal decomposition temperature of the CMC as the first binder and the thermal decomposition temperature of the CMC as the second binder.

[0054] The CMC contained in the third binder may have a smaller degree of substitution of carboxymethyl groups per anhydrous glucose unit compared to the CMC contained in the first binder and the CMC contained in the second binder.

[0055] The third binder may contain CMC as its main component. The main component of the third binder means the component with the largest mass proportion in the third binder. The proportion of CMC in the total mass of the binder contained in the third negative electrode mixture layer 15 may be 50% by mass or more, 70% by mass or more, 80% by mass or more, 90% by mass or more, or even 95% by mass or more. The third binder may consist only of CMC.

[0056] The negative electrode mixture layer 12 may further contain a conductive agent. Examples of conductive agents include carbon black such as acetylene black and Ketjenblack, graphite, carbon nanotubes (CNTs), carbon nanofibers, and graphene.

[0057] The thickness of the third negative electrode mixture layer 15 relative to the total thickness of the negative electrode mixture layer 12 may be 20% or less, 15% or less, 10% or less, or 5% or less. By reducing the thickness of the third negative electrode mixture layer 15 in this way, the battery's cycle characteristics can be improved while maintaining the charge-discharge characteristics.

[0058] [Method for manufacturing a negative electrode] The method for manufacturing the negative electrode 10 according to Embodiment 1 includes: applying a first negative electrode mixture slurry containing a first binder and a first negative electrode active material onto the main surface of the negative electrode current collector 11 to form a first negative electrode mixture layer 13; applying a second negative electrode mixture slurry containing a second binder and a second negative electrode active material onto the first negative electrode mixture layer 13 to form a second negative electrode mixture layer 14; and applying a third negative electrode mixture slurry containing a third binder and a third negative electrode active material onto the second negative electrode mixture layer 14 to form a third negative electrode mixture layer 15.

[0059] An example of a method for manufacturing the negative electrode 10 is described below. A first negative electrode slurry for the first negative electrode mixture layer 13 is prepared by adding a first negative electrode active material containing natural graphite, a first binder, and an optional component such as a conductive agent to a dispersion medium (e.g., water) in a predetermined ratio. Similarly, a second negative electrode active material containing artificial graphite, a second binder, and an optional component such as a conductive agent to a dispersion medium (e.g., water) in a predetermined ratio to prepare a second negative electrode slurry for the second negative electrode mixture layer 14. Then, a third negative electrode slurry for the third negative electrode mixture layer 15 is prepared by adding a third negative electrode active material containing artificial graphite, a third binder containing CMC, and an optional component such as a conductive agent to a dispersion medium (e.g., water) in a predetermined ratio. In this case, the mass ratio of artificial graphite in the graphite of the third negative electrode active material is set to 80% or more, and the CMC used in the third binder has a thermal decomposition temperature higher than that of TD1 and TD2.

[0060] A first negative electrode mixture slurry is applied to at least one main surface of the negative electrode current collector 11, and the coating is dried to form a first negative electrode mixture layer 13. A second negative electrode mixture slurry is applied to the first negative electrode mixture layer 13, and the coating is dried to form a second negative electrode mixture layer 14. A third negative electrode mixture slurry is applied to the second negative electrode mixture layer 14, and the coating is dried to form a third negative electrode mixture layer 15. At this time, the third negative electrode mixture layer 15 is made to be thinner than the first negative electrode mixture layer 13 and the second negative electrode mixture layer 14. After that, the first negative electrode mixture layer 13, the second negative electrode mixture layer 14, and the third negative electrode mixture layer 15 are rolled at a predetermined linear pressure using a rolling mill or the like. In this way, a negative electrode mixture layer 12 is produced, and the negative electrode 10 is produced. The rolling process may be carried out after forming a first negative electrode mixture layer 13 on the negative electrode current collector 11, followed by additional rolling after forming a second negative electrode mixture layer 14 on the first negative electrode mixture layer 13, and then further additional rolling after forming a third negative electrode mixture layer 15 on the second negative electrode mixture layer 14. The negative electrode mixture layer 12 may be formed on one main surface of the negative electrode current collector 11, or on both main surfaces.

[0061] (Embodiment 2) The battery in Embodiment 2 comprises a positive electrode, a negative electrode, and an electrolyte. The negative electrode is the same as the negative electrode in Embodiment 1. With this configuration, the battery in Embodiment 2 can improve its cycle characteristics.

[0062] Figure 2 is a schematic longitudinal cross-sectional view showing an example of a battery according to Embodiment 2. The battery 100 is a cylindrical battery comprising a cylindrical battery case, a wound electrode group 24, and an electrolyte (not shown). The electrode group 24 is housed inside the battery case and is in contact with the electrolyte.

[0063] The battery case consists of a case body 25, which is a bottomed cylindrical metal container, and a sealing body 26 that seals the opening of the case body 25. A gasket 37 is placed between the case body 25 and the sealing body 26. The gasket 37 ensures that the battery case is airtight. Inside the case body 25, insulating plates 27 and 28 are placed at both ends of the electrode group 24 in the winding axis direction of the electrode group 24, respectively.

[0064] The case body 25 has, for example, a stepped portion 31. The stepped portion 31 can be formed by partially pressing the side wall of the case body 25 from the outside. The stepped portion 31 may be formed in an annular shape on the side wall of the case body 25 along the circumferential direction of a virtual circle defined by the case body 25. In this case, the sealing body 26 is supported, for example, by the opening side surface of the stepped portion 31.

[0065] The sealing body 26 comprises a filter 32, a lower valve body 33, an insulating member 34, an upper valve body 35, and a cap 36. In the sealing body 26, these members are stacked in this order. The sealing body 26 is installed in the opening of the case body 25 such that the cap 36 is located on the outside of the case body 25 and the filter 32 is located on the inside of the case body 25.

[0066] Each of the above-mentioned components constituting the sealing body 26 is, for example, disc-shaped or ring-shaped. Except for the insulating member 34, each of the above-mentioned components is electrically connected to one another.

[0067] The electrode group 24 includes a positive electrode 21, a separator 22, and a negative electrode 23. The positive electrode 21, the separator 22, and the negative electrode 23 are all strip-shaped. The width direction of the strip-shaped positive electrode 21 and negative electrode 23 is, for example, parallel to the winding axis of the electrode group 24. The separator 22 is positioned between the positive electrode 21 and the negative electrode 23. The positive electrode 21 and the negative electrode 23 are wound in a spiral shape with the separator 22 interposed between them.

[0068] When observing a cross-section of the battery 100 in a direction perpendicular to the winding axis of the electrode group 24, the positive electrode 21 and the negative electrode 23 are alternately stacked in the radial direction of a virtual circle defined by the case body 25, with a separator 22 interposed between them.

[0069] The positive electrode 21 is electrically connected to the cap 36, which also serves as the positive electrode terminal, via a positive electrode lead 29. One end of the positive electrode lead 29 is connected, for example, near the center of the positive electrode 21 in the longitudinal direction. The positive electrode lead 29 extends from the positive electrode 21 to the filter 32 through a through hole formed in the insulating plate 27. The other end of the positive electrode lead 29 is welded, for example, to the electrode group 24 side of the filter 32.

[0070] The negative electrode 23 is electrically connected to the case body 25, which also serves as the negative electrode terminal, via a negative electrode lead 30. One end of the negative electrode lead 30 is connected, for example, to the end of the negative electrode 23 in the longitudinal direction of the negative electrode 23. The other end of the negative electrode lead 30 is welded, for example, to the inner bottom surface of the case body 25.

[0071] The components of battery 100 will be described in detail below.

[0072] The positive electrode 21 includes a material having the property of intercalating and releasing metal ions (e.g., lithium ions). The positive electrode 21 includes, for example, a positive electrode active material. The positive electrode 21 comprises, for example, a positive electrode current collector and a positive electrode mixture layer supported on the surface of the positive electrode current collector.

[0073] As the positive electrode current collector, a sheet or film made of a metallic material such as aluminum, stainless steel, titanium, or their alloys can be used. Aluminum and its alloys are suitable as materials for positive electrode current collectors because they are inexpensive and easy to make into thin films. The sheet or film may be porous or non-porous. Metal foil, metal mesh, etc., can be used as the sheet or film. A carbon material such as carbon may be coated on the surface of the positive electrode current collector as a conductive auxiliary material.

[0074] The positive electrode mixture layer contains a positive electrode active material. The positive electrode active material may be a material that has the ability to intercept and release metal ions (e.g., lithium ions). As the positive electrode active material, lithium-containing transition metal oxides, lithium-containing transition metal phosphates, transition metal fluorides, polyanionic materials, fluorinated polyanionic materials, transition metal sulfides, transition metal oxysulfides, transition metal oxynitrides, etc., can be used. In particular, when lithium-containing transition metal oxides or lithium-containing transition metal phosphates are used as the positive electrode active material, the manufacturing cost of the battery can be reduced and the average discharge voltage can be increased. Examples of lithium-containing transition metal oxides include lithium cobalt oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese oxide, and lithium nickel manganese oxide. Examples of lithium-containing transition metal phosphates include lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, and lithium nickel phosphate.

[0075] The positive electrode mixture layer may further contain a binder. As the binder, the material described in Embodiment 1 as a binder usable for the negative electrode mixture layer can also be used for the positive electrode mixture layer.

[0076] The positive electrode mixture layer may further contain a conductive agent. As the conductive agent, the material described in Embodiment 1 as a conductive agent usable in the negative electrode mixture layer can also be used in the positive electrode mixture layer.

[0077] The negative electrode 23 includes a material having the property of intercalating and releasing metal ions (e.g., lithium ions). The negative electrode 23 is the negative electrode 10 according to Embodiment 1.

[0078] The electrolyte solution used as the electrolyte may contain a non-aqueous solvent and a lithium salt dissolved in the non-aqueous solvent. The concentration of the lithium salt in the electrolyte solution may be, for example, 0.5 mol / liter or more and 2 mol / liter or less. By controlling the lithium salt concentration within the above range, an electrolyte solution with excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0079] As non-aqueous solvents, cyclic carbonate esters, linear carbonate esters, cyclic ethers, linear ethers, nitriles, amides, etc., may be used. One of these solvents may be used, or two or more may be used in combination.

[0080] Examples of lithium salts that can be used include lithium hexafluoride phosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bisperfluoroethylsulfonylimide (LiN(SO2C2F5)2), LiAsF6, LiCF3SO3, and lithium difluoro(oxalato)borate. One of these electrolyte salts may be used, or two or more may be used in combination.

[0081] Typically, it is desirable to interpose a separator between the positive and negative electrodes. The separator 22 has high ion permeability and appropriate mechanical strength and insulating properties. As the separator 22, a microporous thin film, woven fabric, and nonwoven fabric can be used. As the material of the separator 22, for example, a polymer can be used. The polymer may be polyolefin such as polypropylene and polyethylene.

[0082] In the battery according to Embodiment 2, the electrolyte may be impregnated into a polymer provided as a separator, for example. That is, the battery according to Embodiment 2 may have a structure in which both the electrolyte and the polymer are used in combination.

[0083] The battery according to Embodiment 2 may further contain a solid electrolyte as the electrolyte. That is, the battery of this disclosure may have a hybrid structure in which an electrolyte and a solid electrolyte are used in combination. Examples of solid electrolyte materials are halide solid electrolytes, sulfide solid electrolytes, oxide solid electrolytes, or organic polymer solid electrolytes. In this disclosure, "halide solid electrolyte" means a solid electrolyte in which a halogen element is the main component of the anions. "Sulfide solid electrolyte" means a solid electrolyte in which sulfur is the main component of the anions. "Oxide solid electrolyte" means a solid electrolyte in which oxygen is the main component of the anions. The main component of the anions means the anion with the largest amount of substance among all the anions constituting the solid electrolyte.

[0084] As an example of the structure of the battery according to Embodiment 2, the configuration example shown in Figure 2, namely a cylindrical non-aqueous electrolyte secondary battery in which a wound-type electrode group, in which the positive electrode and negative electrode are wound around a separator, and an electrolyte are housed in an outer casing, is described. However, the battery according to this disclosure is not limited to this configuration example. The battery according to Embodiment 2 may take any form, such as prismatic, coin-type, button-type, laminate-type, etc. Furthermore, as the electrode group in the battery according to Embodiment 2, other forms of electrode groups may be used instead of the wound-type electrode group, such as an electrode group in which the positive electrode and negative electrode are stacked with a separator.

[0085] (Other Embodiments) (Note) The above description of embodiments discloses the following technologies.

[0086] (Technical 1) A negative electrode comprising: a negative electrode current collector; and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material and a binder, wherein the negative electrode mixture layer includes a first negative electrode mixture layer disposed on the negative electrode current collector; a second negative electrode mixture layer disposed on the first negative electrode mixture layer; and a third negative electrode mixture layer disposed on the second negative electrode mixture layer, wherein the third negative electrode mixture layer contains graphite as the negative electrode active material; the mass ratio of artificial graphite in the graphite contained in the third negative electrode mixture layer is 80% or more; and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the third negative electrode mixture layer is higher than the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the first negative electrode mixture layer, and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the second negative electrode mixture layer.

[0087] This configuration allows the negative electrode according to Technology 1 to reduce the decrease in the binding properties of the negative electrode mixture layer and improve the battery's cycle characteristics.

[0088] (Technical 2) The anode according to Technical 1, wherein the first anode mixture layer and the second anode mixture layer contain graphite as the anode active material.

[0089] This configuration allows the negative electrode related to Technology 2 to improve its cycle characteristics while maintaining excellent charge-discharge characteristics.

[0090] (Technical 3) The negative electrode according to Technical 2, wherein the average particle size of the artificial graphite contained in the third negative electrode mixture layer is smaller than the average particle size of the graphite contained in the first negative electrode mixture layer and the average particle size of the graphite contained in the second negative electrode mixture layer.

[0091] This configuration allows the negative electrode related to Technology 3 to improve its cycle characteristics while maintaining excellent charge-discharge characteristics.

[0092] (Technical 4) When the negative electrode active material contained in the third negative electrode mixture layer is the third negative electrode active material, the negative electrode according to any one of Technical 1 to 3, wherein the third negative electrode active material mainly contains artificial graphite.

[0093] This configuration allows the negative electrode according to Technology 4 to reduce the decrease in the binding properties of the negative electrode mixture layer and improve the battery's cycle characteristics. Furthermore, this configuration can provide excellent charge and discharge characteristics.

[0094] (Technical 5) A negative electrode according to any one of Technical 1 to 4, wherein the negative electrode active material contained in the first negative electrode mixture layer is defined as the first negative electrode active material, and the negative electrode active material contained in the second negative electrode mixture layer is defined as the second negative electrode active material, wherein the first negative electrode active material mainly contains natural graphite, and the second negative electrode active material mainly contains artificial graphite.

[0095] This configuration allows the negative electrode according to Technology 5 to reduce the decrease in the binding properties of the negative electrode mixture layer and improve the battery's cycle characteristics. Furthermore, this configuration can provide excellent charge and discharge characteristics.

[0096] (Technical 6) The negative electrode according to any one of Technical 1 to 5, wherein the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the third binder is 280°C or higher.

[0097] This configuration allows the negative electrode according to technology 6 to reduce the decrease in the binding properties of the negative electrode mixture layer and improve the battery's cycle characteristics.

[0098] (Technical 7) The anode according to any one of Technical 1 to 6, wherein the thickness of the third anode mixture layer is 10% or less of the total thickness of the anode mixture layer.

[0099] This configuration allows the negative electrode related to technology 7 to improve its cycle characteristics while maintaining excellent charge-discharge characteristics.

[0100] (Technical 8) A method for manufacturing a negative electrode, comprising: applying a first negative electrode mixture slurry containing a first binder and a first negative electrode active material onto the main surface of a negative electrode current collector to form a first negative electrode mixture layer; applying a second negative electrode mixture slurry containing a second binder and a second negative electrode active material onto the first negative electrode mixture layer to form a second negative electrode mixture layer; and applying a third negative electrode mixture slurry containing a third binder and a third negative electrode active material onto the second negative electrode mixture layer to form a third negative electrode mixture layer, wherein the third negative electrode active material contains graphite, the mass ratio of artificial graphite in the graphite is 80% or more, and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the third binder is higher than the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the first binder and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the second binder.

[0101] With this configuration, the manufacturing method relating to Technology 8 can provide a negative electrode that can improve the battery's cycle characteristics.

[0102] (Technical 9) A battery comprising a negative electrode, a positive electrode, and an electrolyte as described in any one of Technical 1 to 7.

[0103] This configuration allows the battery according to technology 9 to have improved cycle characteristics.

[0104] The present disclosure will be described in more detail below with reference to examples. The following examples are merely illustrative and not limited to any one aspect.

[0105] <Fabrication of the negative electrode> [Example 1] (Preparation of the first negative electrode active material) Natural graphite and a silicon-containing material were prepared as the first negative electrode active material. The natural graphite and the silicon-containing material were mixed in a mass ratio of natural graphite:silicon-containing material of approximately 90:10, and this was used as the negative electrode active material for the first negative electrode mixture layer. The D50 of the above natural graphite was 18 μm. The silicon-containing material was as follows.

[0106] (Preparation of the second negative electrode active material) As the second negative electrode active material, artificial graphite and a silicon-containing material were prepared. The artificial graphite and the silicon-containing material were mixed in a mass ratio of approximately 90:10, and this was used as the negative electrode active material for the first negative electrode mixture layer. The D50 of the above artificial graphite was 17 μm. The silicon-containing material was as follows.

[0107] (Preparation of silicon-containing material) A mesoporous silicon-containing material containing Si and C was prepared. The median diameter (D50) of the silicon-containing material was 10 μm.

[0108] (Preparation of the third negative electrode active material) Artificial graphite was used as the negative electrode active material for the third negative electrode mixture layer. The median diameter (D50) of the artificial graphite used as the negative electrode active material in the third negative electrode mixture layer, as measured by laser diffraction scattering, was 10 μm. The measurement was performed using the "MT3000II" manufactured by Microtrac-Bell Co., Ltd., with water as the dispersion medium, and the particle size at which the volume integrated value in the particle size distribution became 50% was defined as D50.

[0109] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode slurry for the first negative electrode slurry layer. The thermal decomposition temperature of the CMC used in the negative electrode slurry for the first negative electrode slurry layer was measured by TG-DTA and was found to be 260°C. The measurement was performed using a thermogravimetric differential thermal analyzer (manufactured by Hitachi High-Tech Science Corporation, product name "TG / DTA7200") under a nitrogen atmosphere, with the temperature raised from 25°C to 500°C at a heating rate of 10°C / min. The thermal decomposition temperature of the CMC was the same as the thermal decomposition temperature (TD1) of the binder with the lowest thermal decomposition temperature in the first negative electrode mixture slurry.

[0110] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0111] A third negative electrode active material and carboxymethylcellulose (CMC) were mixed in a mass ratio of third negative electrode active material:CMC = 100:3, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the third negative electrode mixture layer. The thermal decomposition temperature (TD3) of the CMC used in the negative electrode mixture slurry for the third negative electrode mixture layer was 300°C.

[0112] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied onto the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the third negative electrode mixture layer was applied onto the second negative electrode mixture layer, and the coating film was dried and compressed to form the third negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer, the slurry for the second negative electrode mixture layer, and the slurry for the third negative electrode mixture layer was set to: slurry for the first negative electrode mixture layer: slurry for the second negative electrode mixture layer: slurry for the third negative electrode mixture layer = 55:35:10. The fabricated negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, with the thickness of the negative electrode mixture layer being 60 μm on each side. Furthermore, the ratio T3 / (T1+T2), calculated from the thicknesses of the first negative electrode mixture layer (T1), the second negative electrode mixture layer (T2), and the third negative electrode mixture layer (T3), was 10 / 90. In other words, the thickness of the third negative electrode mixture layer was 10% of the total thickness of the negative electrode mixture layer.

[0113] The negative electrode of Example 1 was obtained as described above. An exposed portion of the negative electrode was provided, where the surface of the negative electrode current collector was exposed.

[0114] [Example 2] (Preparation of the first negative electrode active material) The first negative electrode active material was prepared in the same manner as in Example 1.

[0115] (Preparation of the second negative electrode active material) The second negative electrode active material was prepared in the same manner as in Example 1.

[0116] (Preparation of the third negative electrode active material) Artificial graphite with a D50 of 7 μm was used as the third negative electrode active material.

[0117] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the first negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD1) of the material with the lowest thermal decomposition temperature among the binders in the first negative electrode mixture slurry.

[0118] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0119] A third negative electrode active material and carboxymethylcellulose (CMC) were mixed in a mass ratio of third negative electrode active material:CMC = 100:3, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the third negative electrode mixture layer. The thermal decomposition temperature (TD3) of the CMC used in the negative electrode mixture slurry for the third negative electrode mixture layer was 300°C.

[0120] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied onto the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the third negative electrode mixture layer was applied onto the second negative electrode mixture layer, and the coating film was dried and compressed to form the third negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer, the slurry for the second negative electrode mixture layer, and the slurry for the third negative electrode mixture layer was set to: slurry for the first negative electrode mixture layer: slurry for the second negative electrode mixture layer: slurry for the third negative electrode mixture layer = 60:35:5. The fabricated negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, with the thickness of the negative electrode mixture layer being 60 μm on each side. Furthermore, the ratio T3 / (T1+T2), calculated from the thicknesses of the first negative electrode mixture layer (T1), the second negative electrode mixture layer (T2), and the third negative electrode mixture layer (T3), was 5 / 95. In other words, the thickness of the third negative electrode mixture layer was 5% of the total thickness of the negative electrode mixture layer.

[0121] In this manner, the negative electrode of Example 2 was obtained. A portion of the negative electrode was provided with an exposed area where the surface of the negative electrode current collector was exposed.

[0122] [Example 3] (Preparation of the first negative electrode active material) The first negative electrode active material was prepared in the same manner as in Example 1.

[0123] (Preparation of the second negative electrode active material) The second negative electrode active material was prepared in the same manner as in Example 1.

[0124] (Preparation of the third negative electrode active material) The same artificial graphite used in the third negative electrode mixture layer of Example 1 was used as the negative electrode active material for the third negative electrode mixture layer.

[0125] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the first negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD1) of the material with the lowest thermal decomposition temperature among the binders in the first negative electrode mixture slurry.

[0126] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0127] A third negative electrode active material and carboxymethylcellulose (CMC) were mixed in a mass ratio of third negative electrode active material:CMC = 100:3, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the third negative electrode mixture layer. The thermal decomposition temperature (TD3) of the CMC used in the negative electrode mixture slurry for the third negative electrode mixture layer was 280°C.

[0128] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied onto the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the third negative electrode mixture layer was applied onto the second negative electrode mixture layer, and the coating film was dried and compressed to form the third negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer, the slurry for the second negative electrode mixture layer, and the slurry for the third negative electrode mixture layer was set to: slurry for the first negative electrode mixture layer: slurry for the second negative electrode mixture layer: slurry for the third negative electrode mixture layer = 55:35:10. The fabricated negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, with the thickness of the negative electrode mixture layer being 60 μm on each side. Furthermore, the ratio T3 / (T1+T2), calculated from the thicknesses of the first negative electrode mixture layer (T1), the second negative electrode mixture layer (T2), and the third negative electrode mixture layer (T3), was 10 / 90. In other words, the thickness of the third negative electrode mixture layer was 10% of the total thickness of the negative electrode mixture layer.

[0129] In this manner, the negative electrode of Example 3 was obtained. A portion of the negative electrode was provided with an exposed area where the surface of the negative electrode current collector was exposed.

[0130] [Example 4] (Preparation of the first negative electrode active material) The first negative electrode active material was prepared in the same manner as in Example 1.

[0131] (Preparation of the second negative electrode active material) The second negative electrode active material was prepared in the same manner as in Example 1.

[0132] (Preparation of the third negative electrode active material) The same artificial graphite used in the third negative electrode mixture layer of Example 2 was used as the negative electrode active material for the third negative electrode mixture layer.

[0133] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the first negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD1) of the material with the lowest thermal decomposition temperature among the binders in the first negative electrode mixture slurry.

[0134] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0135] A third negative electrode active material and carboxymethylcellulose (CMC) were mixed in a mass ratio of third negative electrode active material:CMC = 100:3, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the third negative electrode mixture layer. The thermal decomposition temperature (TD3) of the CMC used in the negative electrode mixture slurry for the third negative electrode mixture layer was 280°C.

[0136] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied onto the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the third negative electrode mixture layer was applied onto the second negative electrode mixture layer, and the coating film was dried and compressed to form the third negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer, the slurry for the second negative electrode mixture layer, and the slurry for the third negative electrode mixture layer was set to: slurry for the first negative electrode mixture layer: slurry for the second negative electrode mixture layer: slurry for the third negative electrode mixture layer = 60:35:5. The fabricated negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, with the thickness of the negative electrode mixture layer being 60 μm on each side. Furthermore, the ratio T3 / (T1+T2), calculated from the thicknesses of the first negative electrode mixture layer (T1), the second negative electrode mixture layer (T2), and the third negative electrode mixture layer (T3), was 5 / 95. In other words, the thickness of the third negative electrode mixture layer was 5% of the total thickness of the negative electrode mixture layer.

[0137] In this manner, the negative electrode of Example 4 was obtained. A portion of the negative electrode was provided with an exposed section where the surface of the negative electrode current collector was exposed.

[0138] [Example 5] (Preparation of the first negative electrode active material) The first negative electrode active material was prepared in the same manner as in Example 1.

[0139] (Preparation of the second negative electrode active material) The second negative electrode active material was prepared in the same manner as in Example 1.

[0140] (Preparation of the third negative electrode active material) The same artificial graphite used in the third negative electrode mixture layer of Example 2 was used as the negative electrode active material for the third negative electrode mixture layer.

[0141] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the first negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD1) of the material with the lowest thermal decomposition temperature among the binders in the first negative electrode mixture slurry.

[0142] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0143] A third negative electrode active material and carboxymethylcellulose (CMC) were mixed in a mass ratio of third negative electrode active material:CMC = 100:3, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the third negative electrode mixture layer. The thermal decomposition temperature (TD3) of the CMC used in the negative electrode mixture slurry for the third negative electrode mixture layer was 300°C.

[0144] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied onto the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the third negative electrode mixture layer was applied onto the second negative electrode mixture layer, and the coating film was dried and compressed to form the third negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer, the slurry for the second negative electrode mixture layer, and the slurry for the third negative electrode mixture layer was set to: slurry for the first negative electrode mixture layer: slurry for the second negative electrode mixture layer: slurry for the third negative electrode mixture layer = 50:30:20. The fabricated negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, with the thickness of the negative electrode mixture layer being 60 μm on each side. Furthermore, the ratio T3 / (T1+T2), calculated from the thicknesses of the first negative electrode mixture layer (T1), the second negative electrode mixture layer (T2), and the third negative electrode mixture layer (T3), was 20 / 80. In other words, the thickness of the third negative electrode mixture layer was 20% of the total thickness of the negative electrode mixture layer.

[0145] In this manner, the negative electrode of Example 5 was obtained. A portion of the negative electrode was provided with an exposed section where the surface of the negative electrode current collector was exposed.

[0146] [Comparative Example 1] (Preparation of the first negative electrode active material) The first negative electrode active material was prepared in the same manner as in Example 1.

[0147] (Preparation of the second negative electrode active material) The second negative electrode active material was prepared in the same manner as in Example 1.

[0148] (Preparation of the third negative electrode active material) Natural graphite with a D50 of 10 μm was used as the negative electrode active material for the third negative electrode mixture layer.

[0149] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the first negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD1) of the material with the lowest thermal decomposition temperature among the binders in the first negative electrode mixture slurry.

[0150] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0151] The third negative electrode active material and carboxymethylcellulose (CMC) were mixed in a mass ratio of third negative electrode active material:CMC = 100:3, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the third negative electrode mixture layer. The thermal decomposition temperature (TD3) of the CMC used in the negative electrode mixture slurry for the third negative electrode mixture layer was 300°C.

[0152] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied onto the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the third negative electrode mixture layer was applied onto the second negative electrode mixture layer, and the coating film was dried and compressed to form the third negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer, the slurry for the second negative electrode mixture layer, and the slurry for the third negative electrode mixture layer was set to: slurry for the first negative electrode mixture layer: slurry for the second negative electrode mixture layer: slurry for the third negative electrode mixture layer = 55:35:10. The fabricated negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, with the thickness of the negative electrode mixture layer being 60 μm on each side. Furthermore, the ratio T3 / (T1+T2), calculated from the thicknesses of the first negative electrode mixture layer (T1), the second negative electrode mixture layer (T2), and the third negative electrode mixture layer (T3), was 10 / 90. In other words, the thickness of the third negative electrode mixture layer was 10% of the total thickness of the negative electrode mixture layer.

[0153] In this manner, the negative electrode of Comparative Example 1 was obtained. A portion of the negative electrode was provided with an exposed section where the surface of the negative electrode current collector was exposed.

[0154] [Comparative Example 2] (Preparation of the first negative electrode active material) The first negative electrode active material was prepared in the same manner as in Example 1.

[0155] (Preparation of the second negative electrode active material) The second negative electrode active material was prepared in the same manner as in Example 1.

[0156] (Preparation of the third negative electrode active material) The same artificial graphite used in the third negative electrode mixture layer of Example 2 was used as the negative electrode active material for the third negative electrode mixture layer.

[0157] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the first negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD1) of the material with the lowest thermal decomposition temperature among the binders in the first negative electrode mixture slurry.

[0158] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0159] A third negative electrode active material and carboxymethylcellulose (CMC) were mixed in a mass ratio of third negative electrode active material:CMC = 100:3, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the third negative electrode mixture layer. The thermal decomposition temperature (TD3) of the CMC used in the negative electrode mixture slurry for the third negative electrode mixture layer was 260°C.

[0160] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied onto the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the third negative electrode mixture layer was applied onto the second negative electrode mixture layer, and the coating film was dried and compressed to form the third negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer, the slurry for the second negative electrode mixture layer, and the slurry for the third negative electrode mixture layer was set to: slurry for the first negative electrode mixture layer: slurry for the second negative electrode mixture layer: slurry for the third negative electrode mixture layer = 55:35:10. The fabricated negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, with the thickness of the negative electrode mixture layer being 60 μm on each side. Furthermore, the ratio T3 / (T1+T2), calculated from the thicknesses of the first negative electrode mixture layer (T1), the second negative electrode mixture layer (T2), and the third negative electrode mixture layer (T3), was 10 / 90. In other words, the thickness of the third negative electrode mixture layer was 10% of the total thickness of the negative electrode mixture layer.

[0161] In this manner, the negative electrode of Comparative Example 2 was obtained. A portion of the negative electrode was provided with an exposed section where the surface of the negative electrode current collector was exposed.

[0162] [Comparative Example 3] (Preparation of the first negative electrode active material) The first negative electrode active material was prepared in the same manner as in Example 1.

[0163] (Preparation of the second negative electrode active material) The second negative electrode active material was prepared in the same manner as in Example 1.

[0164] (Preparation of the negative electrode) The first negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of first negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the first negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the first negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD1) of the material with the lowest thermal decomposition temperature among the binders in the first negative electrode mixture slurry.

[0165] The second negative electrode active material, carboxymethylcellulose (CMC), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) were mixed in a mass ratio of second negative electrode active material:CMC:SBR:PAA = 100:1:1:0.5, and an appropriate amount of water was added to prepare a negative electrode mixture slurry for the second negative electrode mixture layer. The thermal decomposition temperature of the CMC used in the negative electrode mixture slurry for the second negative electrode mixture layer was 260°C. This thermal decomposition temperature of the CMC was the thermal decomposition temperature (TD2) of the material with the lowest thermal decomposition temperature among the binders in the second negative electrode mixture slurry.

[0166] Next, the prepared negative electrode mixture slurry for the first negative electrode mixture layer was applied to the surface of a negative electrode current collector made of copper foil, and the coating film was dried and compressed to form the first negative electrode mixture layer. Furthermore, the prepared negative electrode mixture slurry for the second negative electrode mixture layer was applied on the first negative electrode mixture layer, and the coating film was dried and compressed to form the second negative electrode mixture layer. At this time, the coating mass ratio of the slurry for the first negative electrode mixture layer and the slurry for the second negative electrode mixture layer was set to slurry for the first negative electrode mixture layer:slurry for the second negative electrode mixture layer = 60:40. The prepared negative electrode had a three-layer negative electrode mixture layer on the surface of the negative electrode current collector, and the thickness of the negative electrode mixture layer was 60 μm on each side. The negative electrode of Comparative Example 3 did not have a third negative electrode mixture layer.

[0167] In this manner, the negative electrode of Comparative Example 3 was obtained. A portion of the negative electrode was provided with an exposed section where the surface of the negative electrode current collector was exposed.

[0168] Table 1 shows the negative electrode active materials and binders used in each negative electrode mixture layer of Examples 1 to 5 and Comparative Examples 1 to 3. Table 2 shows the percentage of the thickness of the negative electrode mixture layers.

[0169] [Evaluation of Binding Properties] For the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, a peel test was performed as follows to evaluate the binding properties. Measurement was carried out by the following method using a Tensilon universal material testing machine (manufactured by A&D, model RTH-1225). First, the negative electrode was cut into a size of 15 mm × 100 mm to obtain a test piece. The negative electrode mixture layer was peeled from the negative electrode current collector at a peel angle of 90° and a tensile speed of 5 mm / min. The measured value (unit: N) when the negative electrode mixture layer was completely peeled off from the negative electrode current collector was taken as the peel strength. The results are shown in Table 2. Table 2 shows the relative values when the peel strength of Example 1 is taken as the reference (100%).

[0170] <Preparation of Test Cell> (Preparation of Positive Electrode Active Material) [Ni 0.88 Co 0.09 Al 0.03 (OH)₂, the composite hydroxide represented by this formula was calcined at 500°C for 8 hours to obtain an oxide (Ni 0.88 Co 0.09 Al 0.03 O₂). Next, LiOH and the composite oxide were mixed such that the molar ratio of Li to the total amount of Ni, Co, and Al was 1.03:1, to obtain a mixture. This mixture was calcined under an oxygen stream with an oxygen concentration of 95% (at a flow rate of 2 mL per 10 cm 3 and 5 L per 1 kg of the mixture) from room temperature to 650°C at a heating rate of 2.0°C / min, and then calcined from 650°C to 780°C at a heating rate of 0.5°C / min to obtain LiNi 0.88 Co 0.09 Al 0.03 O₂, the lithium-containing composite oxide represented by this formula was obtained.

[0171] (Preparation of the positive electrode) The positive electrode active material, acetylene black, and polyvinylidene fluoride were mixed in a mass ratio of positive electrode active material:acetylene black:polyvinylidene fluoride = 95:2.5:2.5, and a positive electrode mixture slurry was prepared using N-methyl-2-pyrrolidone (NMP) as the dispersion medium. Next, the positive electrode mixture slurry was applied to a positive electrode current collector made of aluminum foil, the coating was dried and compressed, and then the positive electrode current collector was cut to a predetermined electrode size to obtain a positive electrode in which positive electrode mixture layers were arranged on both sides of the positive electrode current collector. An exposed portion was provided on a part of the positive electrode in which the surface of the positive electrode current collector was exposed.

[0172] (Preparation of non-aqueous electrolyte) A non-aqueous electrolyte was prepared by dissolving LiPF6 at a concentration of 1.2 mol / liter in a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:EMC:DMC = 3:3:4 (at 25°C).

[0173] (Fabrication of Secondary Batteries) Using the above-mentioned positive electrode and electrolyte, and the negative electrodes of Examples 1 to 5 and Comparative Examples 1 to 3, a secondary battery was assembled as follows: An aluminum lead was attached to the exposed portion of the positive electrode, and a nickel lead was attached to the exposed portion of the negative electrode. A wound electrode body was fabricated by spirally winding the positive and negative electrodes together via a polyolefin separator. Insulating plates were placed above and below the electrode body, and the electrode body was housed in an outer casing. The negative electrode lead was welded to the bottom of the bottomed cylindrical outer casing, and the positive electrode lead was welded to the sealing body. The electrolyte was injected into the outer casing, and the opening of the outer casing was sealed with the sealing body via a gasket. In this way, secondary batteries serving as test cells for Examples 1 to 5 and Comparative Examples 1 to 3 were fabricated.

[0174] [Evaluation of Capacity Retention Rate] The capacity retention rate of the cells was evaluated using the following method with respect to the test cells of each example and comparative example.

[0175] Under a temperature of 25°C, the test cells of each example and comparative example were charged with a constant current at 1C until the voltage reached 4.2V. Then, they were charged at a constant voltage of 4.2V until the current was reduced to 1 / 50C. Finally, they were discharged with a constant current at 0.5C until the voltage reached 2.5V. This charge-discharge process constituted one cycle, and 500 cycles were performed.

[0176] The capacity retention rate of the test cells in each example and comparative example during the charge-discharge cycle was determined using the following formula: Capacity retention rate = (Discharge capacity at 500 cycles / Discharge capacity at 1 cycle) × 100

[0177] The results are shown in Table 2. Table 2 shows the relative values ​​with the results of Example 1 set as the baseline (100%).

[0178]

[0179]

[0180] The batteries of Examples 1 to 5 exhibited superior capacity retention compared to the batteries of Comparative Examples 1 to 3. Furthermore, the batteries of Examples 1 to 5 also showed sufficient peel strength and improved bonding compared to Comparative Example 3, which lacked a third negative electrode mixture layer. It was confirmed that a negative electrode satisfying the configuration of this disclosure can improve the battery's cycle characteristics.

[0181] The technology disclosed herein is useful for batteries such as lithium-ion secondary batteries.

Claims

1. A negative electrode comprising: a negative electrode current collector; and a negative electrode mixture layer disposed on the negative electrode current collector and containing a negative electrode active material and a binder, wherein the negative electrode mixture layer includes a first negative electrode mixture layer disposed on the negative electrode current collector; a second negative electrode mixture layer disposed on the first negative electrode mixture layer; and a third negative electrode mixture layer disposed on the second negative electrode mixture layer, wherein the third negative electrode mixture layer contains graphite as the negative electrode active material, the mass ratio of artificial graphite in the graphite contained in the third negative electrode mixture layer is 80% or more, and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the third negative electrode mixture layer is higher than the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the first negative electrode mixture layer, and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the second negative electrode mixture layer.

2. The anode according to claim 1, wherein the first anode mixture layer and the second anode mixture layer contain graphite as the anode active material.

3. The negative electrode according to claim 2, wherein the average particle size of the artificial graphite contained in the third negative electrode mixture layer is smaller than the average particle size of the graphite contained in the first negative electrode mixture layer and the average particle size of the graphite contained in the second negative electrode mixture layer.

4. The negative electrode according to claim 1, wherein the negative electrode active material contained in the third negative electrode mixture layer is the third negative electrode active material, and the third negative electrode active material mainly contains artificial graphite.

5. The negative electrode according to claim 1, wherein the negative electrode active material contained in the first negative electrode mixture layer is defined as the first negative electrode active material, and the negative electrode active material contained in the second negative electrode mixture layer is defined as the second negative electrode active material, wherein the first negative electrode active material mainly contains natural graphite, and the second negative electrode active material mainly contains artificial graphite.

6. The negative electrode according to claim 1, wherein the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the binder contained in the third negative electrode mixture layer is 280°C or higher.

7. The negative electrode according to claim 1, wherein the thickness of the third negative electrode mixture layer is 10% or less of the total thickness of the negative electrode mixture layer.

8. A method for manufacturing a negative electrode, comprising: applying a first negative electrode mixture slurry containing a first binder and a first negative electrode active material onto the main surface of a negative electrode current collector to form a first negative electrode mixture layer; applying a second negative electrode mixture slurry containing a second binder and a second negative electrode active material onto the first negative electrode mixture layer to form a second negative electrode mixture layer; and applying a third negative electrode mixture slurry containing a third binder and a third negative electrode active material onto the second negative electrode mixture layer to form a third negative electrode mixture layer, wherein the third negative electrode active material contains graphite, the mass ratio of artificial graphite in the graphite is 80% or more, and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the third binder is higher than the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the first binder and the thermal decomposition temperature of the material with the lowest thermal decomposition temperature in the second binder.

9. A battery comprising a negative electrode, a positive electrode, and an electrolyte, as described in any one of claims 1 to 7.

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