Secondary batteries

Abstract

To provide a secondary battery including a negative electrode containing a graphite particle and a Si material as a negative electrode active material, in which an input-output characteristic and a cycle characteristic are both suitably achieved.SOLUTION: A secondary battery disclosed herein includes an electrode body including a positive electrode and a negative electrode 60. The negative electrode includes a negative electrode current collector 62, and a negative electrode active material layer 64 disposed on the negative electrode current collector 62. The negative electrode active material layer 64 contains a graphite particle 66 and a Si-containing particle 68 as a negative electrode active material. The Si-containing particle 68 is a composite body of Si and C, and Si of the Si-containing particle 68 is doped with an element M. The element M is at least one kind of element among elements that belong to Group 15 and Group 16 in the periodic table. The amount of doping of the element M in the Si-containing particle 68 is 0.1 at% or more and 5 at% or less. Here, the weight ratio between the graphite particle 66 and the Si-containing particle 68 is 9:1 to 4:6.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a secondary battery. [Background technology]

[0002] Secondary batteries such as lithium-ion secondary batteries are suitably used as portable power sources for personal computers, mobile terminals, etc., and as power sources for driving vehicles such as electric vehicles (BEVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), etc. The negative electrodes used in such secondary batteries generally have a configuration in which a negative electrode active material layer containing a negative electrode active material is disposed on a negative electrode current collector.

[0003] In recent years, the use of Si-based materials as negative electrode active materials has been investigated in order to increase the capacity of secondary batteries (e.g., Patent Documents 1 and 2). Patent Document 1 discloses a sulfur-doped silicon negative electrode material having internal void channels with an average width of 500 nm to 3 μm and an average diameter of 1 μm to 5 μm. Patent Document 2 also discloses negative electrode active material particles containing silicon, oxygen, and boron, with gradient concentrations of oxygen and boron. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Special Publication No. 2022-507948 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-152213 Summary of the Invention [Problem to be solved by the invention]

[0005] In a negative electrode with a high Si content, there is a possibility that the input / output characteristics will be deteriorated due to an increase in resistance, and that Li deposition will occur. Furthermore, as disclosed in Patent Document 1, when only doped silicon is used as the negative electrode active material, the resistance is low and the input / output characteristics of the secondary battery are high, but the battery tends to expand and contract significantly during charging and discharging. This may result in disconnection of the conductive path or a decrease in capacity due to an increase in side reactions.

[0006] The present invention has been made in view of the above points, and aims to provide a secondary battery having a negative electrode containing a Si-based material and graphite particles as a negative electrode active material, which has a favorable balance between input / output characteristics and cycle characteristics. [Means for solving the problem]

[0007] The secondary battery disclosed herein is a secondary battery including an electrode assembly having a positive electrode and a negative electrode, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer includes graphite particles and Si-containing particles as negative electrode active materials. The Si-containing particles are composites of Si and C, and the Si in the Si-containing particles is doped with element M. The element M is at least one element belonging to Groups 15 and 16 of the periodic table. The doping amount of the element M in the Si-containing particles is 0.1 at% or more and 5 at% or less. Here, the weight ratio of the graphite particles to the Si-containing particles is 9:1 to 4:6.

[0008] According to this configuration, by doping the Si-containing particles containing Si and C with the element M, it is possible to improve input / output characteristics while suitably suppressing the expansion and contraction of Si that accompanies charge and discharge. Furthermore, by including the Si-containing particles and the graphite particles in the above weight ratio, it is possible to suitably suppress the expansion and contraction of the entire negative electrode active material. Therefore, it is possible to realize a secondary battery that achieves both input / output characteristics and cycle characteristics. [Brief explanation of the drawings]

[0009] [Figure 1]FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery according to one embodiment. [Figure 2] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly according to one embodiment. [Figure 3] FIG. 3 is a diagram schematically illustrating a negative electrode according to one embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the technology disclosed herein will be described with reference to the drawings. Matters necessary for implementing the technology disclosed herein, other than those specifically mentioned in this specification (e.g., the general configuration and manufacturing process of a secondary battery that do not characterize the technology disclosed herein), can be understood as design matters for a person skilled in the art based on conventional technology in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and common technical knowledge in the relevant field. Each drawing is a schematic representation, and dimensional relationships (e.g., length, width, thickness) do not necessarily reflect actual dimensional relationships. In the drawings described below, components and parts that perform the same function are designated by the same reference numerals, and redundant descriptions may be omitted or simplified. In this specification, the notation "A to B" (A and B are arbitrary numbers) indicating a range means A or greater and B or less.

[0011] FIG. 1 is a diagram schematically illustrating the internal structure of a secondary battery 100 according to one embodiment. As shown in FIG. 1, the secondary battery 100 includes an electrode assembly 20 having a positive electrode 50 and a negative electrode 60, a non-aqueous electrolyte (not shown), and a battery case 30 that accommodates the electrode assembly 20 and the non-aqueous electrolyte. The secondary battery 100 shown in FIG. 1 is a lithium-ion secondary battery. The negative electrode 60 disclosed herein is preferably used as a negative electrode for a lithium-ion secondary battery.

[0012] The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 that is configured to release internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. The battery case 30 is also provided with an inlet (not shown) for injecting a non-aqueous electrolyte. The positive electrode terminal 42 is electrically connected to a positive electrode current collector plate 42a. The negative electrode terminal 44 is electrically connected to a negative electrode current collector plate 44a. The battery case 30 is made of a lightweight metal material with good thermal conductivity, such as aluminum.

[0013] FIG. 2 is a diagram schematically illustrating the configuration of an electrode assembly 20. Here, the electrode assembly 20 is a flat-shaped wound electrode assembly. As shown in FIG. 2, the electrode assembly 20 has a configuration in which a long sheet-shaped positive electrode 50 (hereinafter also referred to as "positive electrode sheet 50") and a long sheet-shaped negative electrode 60 (hereinafter also referred to as "negative electrode sheet 60") are stacked together with two long separators 70 interposed therebetween and wound in the longitudinal direction. The positive electrode sheet 50 has a configuration in which a positive electrode active material layer 54 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long positive electrode current collector 52. The negative electrode sheet 60 has a configuration in which a negative electrode active material layer 64 is formed along the longitudinal direction on one or both sides (both sides in this case) of a long negative electrode current collector 62. 1 and 2, the positive electrode current collector exposed portion 52a (i.e., a portion where the positive electrode current collector 52 is exposed without the positive electrode active material layer 54) and the negative electrode current collector exposed portion 62a (i.e., a portion where the negative electrode current collector 62 is exposed without the negative electrode active material layer 64) are formed so as to protrude outward from both ends in the winding axis direction (i.e., the sheet width direction perpendicular to the longitudinal direction) of the electrode body 20. A positive electrode current collector plate 42a and a negative electrode current collector plate 44a are joined to the positive electrode current collector exposed portion 52a and the negative electrode current collector exposed portion 62a, respectively.

[0014] The positive electrode current collector 52 constituting the positive electrode sheet 50 may be a known positive electrode current collector used in lithium-ion secondary batteries, and is not particularly limited. For example, a sheet or foil made of a metal with good conductivity (e.g., aluminum, nickel, titanium, stainless steel, etc.) can be used. Aluminum foil is preferred as the positive electrode current collector 52. The dimensions of the positive electrode current collector 52 are not particularly limited and may be determined appropriately depending on the battery design. When aluminum foil is used as the positive electrode current collector 52, its thickness is not particularly limited, but is, for example, 5 μm to 35 μm, and preferably 7 μm to 20 μm.

[0015] The positive electrode active material layer 54 contains a positive electrode active material. The positive electrode active material may be a positive electrode active material of a known composition used in lithium ion secondary batteries. Specific examples of the positive electrode active material include lithium composite oxides and lithium transition metal phosphate compounds (e.g., lithium iron phosphate (LiFePO4), lithium manganese phosphate (LiMnPO4)). The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

[0016] The lithium composite oxide is preferably a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn as a transition metal element, and specific examples thereof include lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide. These positive electrode active materials may be used alone or in combination of two or more. Among them, lithium nickel cobalt manganese composite oxide is preferably used as the positive electrode active material.

[0017] In this specification, the term "lithium nickel cobalt manganese composite oxide" refers to oxides containing Li, Ni, Co, Mn, and O as constituent elements, as well as oxides containing one or more additional elements. Examples of such additional elements include transition metal elements and typical metal elements such as Mg, Ca, Al, Ti, V, Cr, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn, and Sn. The additional element may also be a metalloid element such as B, C, Si, or P, or a nonmetal element such as S, F, Cl, Br, or I. This also applies to the lithium nickel composite oxide, lithium cobalt composite oxide, lithium manganese composite oxide, lithium nickel manganese composite oxide, lithium nickel cobalt aluminum composite oxide, and lithium iron nickel manganese composite oxide.

[0018] The positive electrode active material layer 54 may contain components other than the positive electrode active material, such as a conductive material, a binder, etc. Suitable conductive materials include carbon black such as acetylene black (AB); carbon fibers such as vapor grown carbon fiber (VGCF) and carbon nanotubes (CNT); and other carbon materials (e.g., graphite). Suitable binders include polyvinylidene fluoride (PVdF).

[0019] Although not particularly limited, the content of the positive electrode active material in the positive electrode active material layer (i.e., the content of the positive electrode active material relative to the total weight of the positive electrode active material layer 54) is, for example, preferably 70 wt% or more, more preferably 80 wt% to 99 wt% or less, and may be 85 wt% to 98 wt% or less. Furthermore, the content of the conductive material in the positive electrode active material layer is also not particularly limited, but is preferably 0.5 wt% to 5 wt% or less, and more preferably 0.7 wt% to 5 wt% or less. Furthermore, the content of the binder in the positive electrode active material layer is also not particularly limited, but is preferably 0.5 wt% to 5 wt% or less, and more preferably 0.7 wt% to 5 wt% or less.

[0020] The thickness of the positive electrode active material layer 54 per side is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

[0021] As the separator 70, various conventional microporous sheets can be used, such as microporous resin sheets made of resins such as polyethylene (PE) and polypropylene (PP). Such microporous resin sheets may have a single-layer structure or a multi-layer structure of two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). Separator 70 may also have a heat-resistant layer (HRL).

[0022] Conventional nonaqueous electrolytes can be used, for example, nonaqueous electrolytes containing a supporting salt in an organic solvent (nonaqueous solvent). Nonaqueous solvents include aprotic solvents such as carbonates, esters, and ethers. Among these, carbonates, such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), are preferred. Alternatively, fluorine-based solvents, such as fluorinated carbonates, such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyl difluoromethyl carbonate (F-DMC), and trifluorodimethyl carbonate (TFDMC), are preferred. These nonaqueous solvents can be used alone or in appropriate combinations of two or more. Lithium salts, such as LiPF, LiBF, and LiClO, are preferred as supporting salts. The concentration of the supporting salt is not particularly limited, but is preferably about 0.7 mol / L or more and 1.3 mol / L or less. The nonaqueous electrolyte may contain components other than the nonaqueous solvent and supporting salt described above, as long as the effects of the present technology are not significantly impaired. For example, the nonaqueous electrolyte may contain various additives such as a gas generating agent, a film-forming agent, a dispersant, and a thickener.

[0023] The negative electrode 60 of the secondary battery disclosed herein will now be described. FIG. 3 is a schematic diagram showing the negative electrode 60 of the secondary battery 100 disclosed herein. As shown in FIG. 3, the negative electrode 60 includes a negative electrode current collector 62 and a negative electrode active material layer 64 disposed on the negative electrode current collector 62. The negative electrode current collector 62 may be a conventionally known one and is not particularly limited. Examples include a sheet or foil made of a metal such as copper, nickel, titanium, or stainless steel. When copper foil is used as the negative electrode current collector 62, its average thickness is not particularly limited, but is, for example, 5 μm to 30 μm, preferably 5 μm to 20 μm, and more preferably 5 μm to 15 μm.

[0024] The negative electrode active material layer 64 includes at least a negative electrode active material. The negative electrode active material layer 64 includes graphite particles 66 and Si-containing particles 68 as the negative electrode active material. The Si-containing particles 68 are doped with at least one element from Group 15 or Group 16 of the periodic table. In the negative electrode active material layer 64 disclosed herein, the weight ratio of the graphite particles 66 to the Si-containing particles 68 is 9:1 to 4:6. This configuration achieves a secondary battery 100 that achieves both excellent cycle characteristics and excellent input / output characteristics. In this specification, the term "doping" refers to adding a small amount of an element from Group 15 or Group 16 to the Si-containing particles as an additive different from the Si-containing particles.

[0025] While not intending to limit the technology disclosed herein, the reason for such an effect is presumed to be as follows. Doping Si with element M causes a metal phase transition (insulator-to-metal transition) and improves conductivity. This can improve the input / output characteristics of the secondary battery. On the other hand, the negative electrode tends to expand and contract during charging and discharging, which can increase the risk of conductive path disconnection and side reactions, thereby deteriorating the cycle characteristics (e.g., capacity retention rate) of the secondary battery. Therefore, in the secondary battery 100 disclosed herein, by using Si-containing particles 68 containing Si and C instead of Si doped with element M, the expansion and contraction of Si during charging and discharging can be suppressed. Furthermore, in the secondary battery 100 disclosed herein, by including graphite particles 66 and Si-containing particles 68 in a predetermined ratio, the expansion and contraction of the entire negative electrode can be more suitably suppressed. This configuration can realize a secondary battery 100 that satisfies both input / output characteristics and cycle characteristics.

[0026] As the graphite particles 66, for example, artificial graphite, natural graphite, etc. are used. The graphite particles 66 may have a coating layer of amorphous carbon on their surfaces. Although not particularly limited, the graphite particles 66 are preferably substantially spherical. In this specification, the term "substantially spherical" encompasses spherical shapes, rugby ball shapes, etc., and refers to particles having an average aspect ratio (the ratio of the length in the superaxial direction to the length in the minor axis direction in the smallest rectangle circumscribing the particle) of, for example, 1 to 2 (preferably 1 to 1.5).

[0027] The average particle diameter of graphite particles 66 (D 50 The particle diameter is not particularly limited, but is preferably 5 μm or more and 30 μm or less, and more preferably 10 μm or more and 25 μm or less. Within this range, the expansion and contraction of the entire negative electrode is suitably suppressed. In this specification, the "average particle diameter of graphite particles" refers to the particle diameter (D 50 This refers to the particle size.

[0028] The Si-containing particles 68 are not particularly limited as long as they are particles containing Si and C. The Si-containing particles 68 may contain components other than Si and C, as long as they contain Si and C. A Si-C composite is preferably used as the Si-containing particles 68. The Si-C composite can be formed, for example, by supporting Si metal, Si oxide, or the like on a carbon material (graphite, porous carbon material, etc.). As shown in FIG. 3, the Si-containing particles 68 preferably include a porous carbon material 68a including pores 68p and Si nanoparticles 68b arranged in the pores 68p of the porous carbon material 68a.

[0029] Although not particularly limited, the Si-containing particles 68 preferably have a Si content of 30 wt% or more, more preferably 40 wt% or more, and even more preferably 50 wt% or more, when the total weight of the Si-containing particles 68 is 100 wt%. This allows the secondary battery 100 to have a high capacity. Furthermore, the resistance reduction effect due to the doping with element M is more preferably exhibited. Although not particularly limited, the upper limit of the Si content in the Si-containing particles 68 is preferably 75 wt% or less, and may be 70 wt% or less. This allows both input / output characteristics and cycle characteristics to be suitably achieved. The Si content in the Si-containing particles can be confirmed using an inductively coupled plasma optical emission spectroscopy (ICP-OES) device.

[0030] Although not particularly limited, the average particle diameter (D 50 The particle diameter) is preferably, for example, 0.5 μm or more and 20 μm or less, and more preferably 1 μm or more and 15 μm or less. In this specification, the "average particle diameter of the Si-containing particles" refers to the particle diameter (D 50 This refers to the particle size.

[0031] The Si-containing particles 68 preferably have a plurality of pores (voids) 68p therein. The presence of such pores 68p in the Si-containing particles 68 favorably suppresses expansion and contraction during charge and discharge. Although not particularly limited, the Si-containing particles 68 preferably have a plurality of pores 68p with a relatively small average pore diameter. Because the pores 68p are relatively small, the pores 68p tend to remain even when pressed during the production of the negative electrode 60. This makes it possible to more favorably mitigate expansion and contraction of the Si-containing particles 68 during charge and discharge, thereby improving cycle characteristics.

[0032] The porosity of the Si-containing particles 68 is preferably, for example, 5 vol% or more. There is no particular upper limit to the porosity, but it is preferably, for example, 60 vol% or less. The "porosity" can be calculated based on the formula: porosity (%) = 1 - bulk density of Si-containing particles / true density of Si-containing particles × 100.

[0033] Although not particularly limited, the oxygen content in the Si-containing particles 68 is preferably, for example, 7 wt % or less when the entire Si-containing particles are taken as 100 wt %. The oxygen content can be measured using an oxygen analyzer.

[0034] The Si nanoparticles 68b contained in the Si-containing particles 68 are nano-sized (i.e., less than 1 μm) particles containing Si. The Si nanoparticles 68b may be present in the pores 68p of the porous carbon material 68a. The presence of the Si nanoparticles 68b in the pores 68p of the porous carbon material 68a suppresses expansion and contraction of the Si nanoparticles 68b, and also effectively suppresses expansion and contraction of the Si-containing particles themselves. This allows for increased capacity and improved cycle characteristics.

[0035] Although not particularly limited, the Si nanoparticles 68b are preferably 100 nm or less, more preferably 50 nm or less. This reduces the amount of expansion and contraction per Si nanoparticle 68b during charge and discharge, making them less likely to break even with repeated expansion and contraction. Furthermore, although not particularly limited, the average particle diameter of the Si nanoparticles 68b may be, for example, 5 nm or more. In this specification, the "average particle diameter of Si nanoparticles" can be determined as follows. First, the negative electrode active material layer is processed with a focused ion beam (FIB) to prepare a sample for observation with a scanning transmission electron microscope (STEM). Then, the sample is subjected to elemental analysis by EDX element mapping, and then a bright-field (BF) image and a high-angle annular dark-field (HAADF) image are obtained. The diameter of the Si nanoparticles can be determined from the contrast and shape obtained from the BF and HAADF images. The "average particle diameter of Si nanoparticles" here refers to the arithmetic mean of the diameters of at least 10 Si nanoparticles.

[0036] As described above, the Si-containing particles 68 are doped with at least one element M from Groups 15 and 16 of the periodic table. That is, the Si-containing particles 68 contain at least one element M from Groups 15 and 16 of the periodic table. Specific examples of the element M include sulfur (S), phosphorus (P), nitrogen (N), arsenic (As), antimony (Sb), bismuth (Bi), selenium (Se), tellurium (Te), and polonium (Po). Of these, S and P are preferably used as the element M. The Si-containing particles 68 may be doped with one of the above elements alone or a combination of two or more of them.

[0037] The doping amount (i.e., content) of element M in the Si-containing particles 68 is preferably 0.1 at% or more, more preferably 0.5 at% or more, and even more preferably 1 at% or more. This allows the resistance-reducing effect of doping with element M to be suitably exhibited, thereby improving input / output characteristics. On the other hand, if the doping amount is too high, the resistance-reducing effect tends to be reduced. The doping amount of element M is preferably 5 at% or less, more preferably 4 at% or less, and even more preferably 3 at% or less. Note that the doping of element M in the Si-containing particles and the doping amount can be confirmed using an inductively coupled plasma optical emission spectroscopy (ICP-OES) device.

[0038] In the Si-containing particle 68, the element M may be contained only on the surface of the Si nanoparticle 68b, or may be uniformly diffused inside the Si nanoparticle 68b. Preferably, in the Si-containing particle 68, the element M is uniformly diffused inside the Si nanoparticle 68b. This can more suitably improve the resistance reduction effect. Although not particularly limited, it is preferable that the element M does not form a compound with Si and / or C in the Si-containing particle 68.

[0039] The Si-containing particles 68 according to this embodiment can be suitably produced by the following method. Note that the following description will be given taking as an example a case where the Si-containing particles 68 are doped with sulfur (S) as the element M. However, the Si-containing particles 68 according to this embodiment are not limited to those produced by the following production method.

[0040] A suitable method for producing the Si-containing particles 68 according to this embodiment may include at least a step of mixing an SiO-C composite (porous carbon material-SiO), a sulfate compound, and a metal reducing agent (mixing step), and a step of heating the mixture (heating step).

[0041] In the mixing step, the SiO-C composite, the sulfate compound, and the metal reducing agent are mixed to prepare a mixture. The mixing method in the mixing step is not particularly limited, but for example, mixing using a mortar is preferable. In addition, the sulfate compound used in the mixing step is not particularly limited, but examples thereof include magnesium sulfate, barium sulfate, and sodium sulfate. The metal reducing agent is not particularly limited, and may be Mg, Al, or the like.

[0042] Next, in the heating step, the prepared mixture is heated to carry out a reduction reaction. In the heating step, sulfur (S) is selectively reduced, and Si in the SiO-C composite is doped with S. The heating step is preferably carried out, for example, in an inert atmosphere (e.g., an argon atmosphere, a nitrogen atmosphere, etc.) at a heating temperature of 200°C to 500°C. The heating time is not particularly limited, but may be, for example, about 0.1 to 10 hours. Note that the method for producing the Si-containing particles 68 according to this embodiment may include a step of dispersing the reactant in a solvent (e.g., water, etc.) after the heating step (after completion of the reduction reaction) and removing sulfate chloride and the like.

[0043] Although not particularly limited, the weight ratio of the graphite particles 66 to the Si-containing particles 68 is preferably adjusted to 9:1 to 4:6, more preferably 9:1 to 5:5, and may be adjusted to 7:3 to 5:5. By adjusting the weight ratio of the graphite particles 66 to the Si-containing particles 68 to fall within the above range, expansion and contraction of the negative electrode as a whole is suitably suppressed. This allows the secondary battery 100 to have both improved cycle characteristics and improved input / output characteristics.

[0044] The negative electrode active material layer 64 may contain, as the Si-containing particles, Si-containing particles (second Si-containing particles) other than Si-containing particles containing Si and C, as long as the effect of the present technology is not significantly impaired. Examples of the second Si-containing particles include SiOx and porous particles in which Si nanoparticles not doped with a foreign element are dispersed.

[0045] The negative electrode active material layer 64 preferably contains a conductive material in addition to the above-described negative electrode active material (graphite particles 66 and Si-containing particles 68). Examples of the conductive material that can be used include carbon nanotubes such as single-walled carbon nanotubes (SWCNT), double-walled carbon nanotubes (DWCNT), and multi-walled carbon nanotubes (MWCNT), carbon black such as acetylene black (AB), and carbon fibers. Among these, carbon nanotubes are preferred, and single-walled carbon nanotubes are more preferred. By using carbon nanotubes as the conductive material, the conductive path can be more suitably maintained, and the cycle characteristics of the secondary battery 100 can be more suitably improved.

[0046] The content of the negative electrode active material in the negative electrode active material layer (i.e., the content of the negative electrode active material relative to the total weight of the negative electrode active material layer 64) is not particularly limited, but is preferably 80 wt% or more, more preferably 90 wt% to 99 wt%, and may be 95 wt% to 99 wt%. The content of the conductive material in the negative electrode active material layer is not particularly limited, but is preferably 0.01 wt% to 1 wt%, and more preferably 0.05 wt% to 0.5 wt%.

[0047] The negative electrode active material layer 64 may contain a binder in addition to the above-described negative electrode active material. Conventionally known binders can be used. Examples of binders include carboxymethyl cellulose (CMC), polyacrylic acid (PAA), styrene butadiene rubber (SBR), and polyvinylidene fluoride (PVDF). Among these, CMC, PAA, and SBR are preferably used. Although not particularly limited, it is more preferable to use a combination of CMC, PAA, and SBR. The content of the binder in the negative electrode active material layer is not particularly limited, but is preferably 1 wt% to 10 wt%, and more preferably 1.5 wt% to 7 wt%.

[0048] The thickness of the negative electrode active material layer 64 per side is not particularly limited, but is, for example, 20 μm or more, and preferably 50 μm or more. On the other hand, the thickness is, for example, 300 μm or less, and preferably 200 μm or less.

[0049] The negative electrode active material layer 64 can be formed by dispersing graphite particles 66 and Si-containing particles 68 as the negative electrode active material, and materials used as needed (for example, a conductive material or a binder) in an appropriate solvent (for example, water) to prepare a paste-like (or slurry-like) composition, applying the composition to the surface of the negative electrode current collector 62, and drying it. Thereafter, the thickness and density of the negative electrode active material layer 64 can be adjusted by pressing as needed.

[0050] The configuration of the negative electrode 60 and the configuration of the secondary battery 100 according to one embodiment have been described above. The negative electrode 60 is preferably used in nonaqueous electrolyte secondary batteries. The conductivity of the negative electrode 60 is improved by doping Si with element M. Furthermore, the negative electrode 60 is preferably prevented from disconnecting its conductive path due to expansion and contraction caused by repeated charge and discharge. Therefore, the secondary battery 100 is realized with favorable balance between input / output characteristics and cycle characteristics. The secondary battery 100 can be used for various purposes, and is preferably used, for example, as a power source (driving power source) for a motor mounted on a vehicle such as a passenger car or truck. The type of vehicle is not particularly limited, and examples include a plug-in hybrid electric vehicle (PHEV), a hybrid electric vehicle (HEV), and an electric vehicle (BEV). The secondary battery 100 can also be preferably used in the construction of a battery pack.

[0051] Furthermore, in the above-described secondary battery 100, a wound electrode body is exemplified as the electrode body 20, but this is not limited thereto, and the electrode body 20 may be, for example, a laminated electrode body, which is an electrode body in which a plurality of approximately rectangular positive electrodes and a plurality of approximately rectangular negative electrodes are alternately stacked with separators interposed therebetween.

[0052] Test examples relating to the present invention will be described below, but it is not intended that the present invention be limited to those shown in the following test examples.

[0053] <Example 1> First, as the negative electrode active material, sulfur (S)-doped Si-containing particles (Si-C composite, Si content: 50 wt%, doping amount: 3 at%) and graphite particles (D 50 Particle diameter: 15 μm) were prepared. Single-walled carbon nanotubes (SWCNT) were also prepared as a conductive material. Carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and styrene butadiene rubber (SBR) were also prepared as binders. These were kneaded with water as a solvent in a weight ratio of graphite particles:Si-containing particles:SWCNT:CMC:PAA:SBR=70:30:0.1:1:1:1.5 to prepare a slurry for forming a negative electrode active material layer.

[0054] Specifically, the mixing and kneading of the slurry for forming the negative electrode active material layer was carried out as follows. First, Si-containing particles, a paste-like SWCNT (solid content 2%), and a dispersion medium were added, and a disperser was used to disperse and mix at 3000 rpm to prepare a paste. Next, graphite particles, CMC, and PAA were dry-mixed using a stirring granulator. The paste, the dry-mixed mixed powder, and the dispersion medium were then kneaded together. The solid content during the kneading was 65%. SBR and a solvent (water) were further added to the kneaded mixture and mixed. In this way, a slurry for forming the negative electrode active material layer was prepared. This slurry was applied in strips to both sides of a copper foil (thickness 10 μm). The slurry on the copper foil was then dried, pressed to a predetermined thickness, and processed to a predetermined dimension to prepare a negative electrode sheet.

[0055] Next, lithium nickel cobalt manganese composite oxide (NCM) was prepared as the positive electrode active material, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVDF) as the binder. These were mixed with N-methylpyrrolidone (NMP) as the solvent in a weight ratio of NCM:AB:PVDF = 100:1:1 to prepare a slurry for forming the positive electrode active material layer. This slurry was applied in strips to both sides of an aluminum foil (thickness 15 μm). The slurry on the aluminum foil was then dried, pressed to the specified thickness, and processed to the specified dimensions to produce a positive electrode sheet.

[0056] The negative electrode sheet and the positive electrode sheet prepared above were stacked with a separator interposed therebetween to produce a laminated electrode assembly. Current-collecting leads were attached to the positive and negative electrode plates, respectively, and the laminated electrode assembly was inserted into an exterior housing made of an aluminum laminate sheet. A nonaqueous electrolyte was poured into the interior of the exterior housing, and the opening of the exterior housing was sealed to produce a test battery of Example 1. A porous polyolefin sheet with a three-layer structure of PP / PE / PP was used as the separator. The nonaqueous electrolyte was prepared by dissolving LiPF6 as a supporting electrolyte at a concentration of 1 mol / L in a mixed solvent of ethylene carbonate (EC), fluoroethylene carbonate (FEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of EC:FEC:EMC:DMC=15:5:40:40.

[0057] <Example 2> The evaluation battery of Example 2 was fabricated in the same manner as in Example 1, except that Si-containing particles doped with phosphorus (P) (Si-C composite, Si content: 50 wt%, doping amount: 3 at%) were prepared as the Si-containing particles.

[0058] <Example 3> An evaluation battery for Example 3 was fabricated in the same manner as in Example 1, except that the compounding ratio of each material was changed to a weight ratio of graphite particles:Si-containing particles:SWCNT:CMC:PAA:SBR=90:10:0.1:1:1:1.5.

[0059] <Example 4> The evaluation battery of Example 4 was fabricated in the same manner as in Example 1, except that the compounding ratio of each material was changed to a weight ratio of graphite particles:Si-containing particles:SWCNT:CMC:PAA:SBR=50:50:0.1:1:1:1.5.

[0060] <Example 5> An evaluation battery of Example 5 was produced in the same manner as in Example 1, except that Si-containing particles with an Si content of 25 wt % were prepared as the Si-containing particles.

[0061] <Example 6> An evaluation battery of Example 6 was produced in the same manner as in Example 1, except that Si-containing particles with an Si content of 75 wt % were prepared as the Si-containing particles.

[0062] <Example 7> An evaluation battery of Example 7 was produced in the same manner as in Example 1, except that undoped Si-containing particles were prepared as the Si-containing particles.

[0063] <Example 8> The evaluation battery of Example 8 was fabricated in the same manner as in Example 1, except that undoped Si-containing particles were prepared as the Si-containing particles, and the compounding ratio of each material was changed to a weight ratio of graphite particles:Si-containing particles:SWCNT:CMC:PAA:SBR=50:50:0.1:1:1:1.5.

[0064] <Example 9> An evaluation battery of Example 9 was produced in the same manner as in Example 1, except that Si-containing particles with a doping amount of S of 6 at % were prepared as the Si-containing particles.

[0065] <Example 10> An evaluation battery of Example 10 was produced in the same manner as in Example 1, except that Si-containing particles doped with P in an amount of 6 at % were prepared as the Si-containing particles.

[0066] <Example 11> An evaluation battery of Example 11 was fabricated in the same manner as in Example 1, except that the blending ratio of each material was changed so that the weight ratio of graphite particles: Si-containing particles: SWCNT: CMC: PAA: SBR was 30:70:0.1:1:1:1.5.

[0067] <Evaluation of cycle capacity retention rate> In an environment at 25°C, after performing CCCV charging (up to 4.2V at a rate of 0.4C and then cutting at 0.1C), CC discharging (cutting at 2.5V at a rate of 0.4C) was defined as one cycle, and the capacity at this time was taken as the initial capacity. A cycle test was conducted by repeating 200 cycles of charge and discharge. The discharge capacity of the first cycle (initial capacity) and the discharge capacity of the 200th cycle were measured, and the cycle capacity retention rate was determined by the following formula (1). The higher the cycle capacity retention rate, the better the cycle characteristics of the secondary battery. The results are shown in Table 1. Cycle capacity retention rate (%) = ((Discharge capacity of the 200th cycle) / (Discharge capacity of the first cycle)) × 100 ··· Formula (1)

[0068] <Evaluation of internal resistance> In an environment at 25°C, with the initial capacity of each of the above evaluation batteries taken as SOC100%, each evaluation battery was charged in a thermostat at 25°C until the SOC reached 50%. Then, in a thermostat at 25°C, charging was performed for 10 seconds at current values of 0.1C, 0.2C, 0.5C, 1C, and 2C, and the battery voltage after charging at each current value was measured. The I-V characteristics during charging were obtained by plotting each current value and each battery voltage, and the internal resistance (mΩ) during discharge was determined from the slope of the obtained straight line. The results are shown in Table 1. Note that the smaller the internal resistance value, the better the input / output characteristics.

[0069]

Table 1

[0070] As shown in Table 1, the test batteries of Examples 1 to 6 have a capacity retention rate of 76% or more and an IV resistance of 103 mΩ or less. These results show that a secondary battery containing graphite particles and Si-containing particles as negative electrode active materials, the Si-containing particles containing Si and C, the Si of the Si-containing particles doped with at least one element belonging to Groups 15 and 16 of the periodic table, and a weight ratio of the graphite particles to the Si-containing particles of 9:1 to 4:6 can be realized, thereby achieving both excellent cycle characteristics and excellent input / output characteristics.

[0071] Furthermore, as shown in Table 1, in Examples 1 to 4 and 6 in which the Si content in the Si-containing particles is 30 wt% or more and 75 wt% or less, the capacity retention rate is 76% or more and the IV resistance is 100 mΩ or less, and it can be seen that the input / output characteristics and cycle characteristics of the secondary battery are more suitably compatible.

[0072] Although several embodiments of the present invention have been described above, the above embodiments are merely examples. The present invention can be implemented in various other forms. The present invention can be implemented based on the contents disclosed in this specification and the common general technical knowledge in the relevant field. The technology described in the claims includes various modifications and alterations of the above-exemplified embodiments. For example, it is possible to replace part of the above-described embodiments with other modified embodiments, and it is also possible to add other modified embodiments to the above-described embodiments. Furthermore, if a technical feature is not described as essential, it may be deleted as appropriate.

[0073] As described above, specific aspects of the technology disclosed herein include those described in the following sections. Item 1: A secondary battery including an electrode assembly having a positive electrode and a negative electrode, wherein the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer including graphite particles and Si-containing particles as negative electrode active materials, the Si-containing particles being composites of Si and C, the Si of the Si-containing particles being doped with element M, the element M being at least one element belonging to Group 15 and Group 16 of the periodic table, the doping amount of the element M in the Si-containing particles being 0.1 at% or more and 5 at% or less, and wherein the weight ratio of the graphite particles to the Si-containing particles is 9:1 to 4:6. Item 2: The secondary battery according to item 1, wherein the Si-containing particles have a Si content of 30 wt % or more and 75 wt % or less when the total weight of the Si-containing particles is taken as 100 wt %. Item 3: The secondary battery according to Item 1 or 2, wherein the Si-containing particles include a porous carbon material having pores and Si nanoparticles disposed in the pores of the porous carbon material. Item 4: The average particle diameter D of the graphite particles 50 Item 4. The secondary battery according to any one of items 1 to 3, wherein the thickness of the first electrode is 10 μm or more and 25 μm or less. Item 5: The secondary battery according to any one of Items 1 to 4, wherein the negative electrode active material layer further contains a conductive material, and the content of the conductive material is 0.01 wt% or more and 1 wt% or less when the total weight of the negative electrode active material layer is 100 wt%. [Explanation of symbols]

[0074] 20 Electrode body 30 Battery case 36 Safety valve 42 Positive terminal 42a Positive current collector plate 44 Negative terminal 44a Negative current collector plate 50 Positive electrode (positive electrode sheet) 52 Positive electrode current collector 52a Exposed part of positive electrode current collector 54 Cathode active material layer 60 Negative electrode (negative electrode sheet) 62 Negative electrode current collector 62a Exposed part of negative electrode current collector 64 Negative electrode active material layer 66 graphite particles 68 Si-containing particles 68a Porous carbon material 68b Si nanoparticles 68p pore 70 Separator 100 Secondary battery

Claims

1. A secondary battery including an electrode assembly having a positive electrode and a negative electrode, the negative electrode includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector, the negative electrode active material layer contains, as a negative electrode active material, graphite particles and Si-containing particles, the Si-containing particle is a composite of Si and C, including a porous carbon material having pores and Si nanoparticles disposed in the pores of the porous carbon material; The Si nanoparticles of the Si-containing particles are doped with sulfur and / or phosphorus as the element M; the doping amount of the element M in the Si-containing particle is 0.1 at% or more and 5 at% or less; Here, in the secondary battery, the weight ratio of the graphite particles to the Si-containing particles is 9:1 to 4:

6.

2. 2. The secondary battery according to claim 1, wherein the Si-containing particles have a Si content of 30 wt % to 75 wt % when the total weight of the Si-containing particles is taken as 100 wt %.

3. 2. The secondary battery according to claim 1, wherein the graphite particles have an average particle diameter D50 of 10 μm or more and 25 μm or less.

4. the negative electrode active material layer further contains a conductive material, 2. The secondary battery according to claim 1, wherein the content of the conductive material is 0.01 wt % or more and 1 wt % or less when the total weight of the negative electrode active material layer is taken as 100 wt %.

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