Negative electrode and lithium ion secondary battery

Abstract

The invention relates to a negative electrode and a lithium ion secondary battery. The negative electrode contains a negative electrode active material having an average particle diameter D50 of 2 [mu] m < = D50 < = 25 [mu] m and a proportion of secondary particles of 50% or more, and has a curve rate T of 1 < = T < = 20 and a migration index Ka of 1.0 < = Ka < = 1.6.

Description

Technical Field

[0001] The present disclosure relates to a negative electrode and a lithium ion secondary battery. Background Art

[0002] Lithium-ion secondary batteries, represented by non-aqueous electrolyte secondary batteries and solid-state batteries, have been widely used in recent years as power sources for automobiles, mobile phones, laptop computers, digital cameras, etc. The electrodes used in lithium-ion secondary batteries are manufactured by, for example, applying a slurry composed of a dispersion of an active material (positive electrode active material or negative electrode active material), a binder, and a conductive additive to a conductive collector and drying the resulting slurry.

[0003] Japanese Patent Application Laid-Open No. 2015-041434 discloses the following: In order to maintain the capacity retention rate at 70% or more and 100% or less, in an electrode in which an active material layer including an active material and a binder is formed on a current collector, the curvature is adjusted to 1.1 or more and 1.5 or less. In addition, Japanese Patent Application Laid-Open No. 2015-041434 discloses that in order to keep the curvature within the above range, the densities of the positive and negative electrodes are adjusted to a specified range. Furthermore, Japanese Patent Application Laid-Open No. 2009-199730 discloses the following technology: the insulating layer arranged between the positive and negative electrodes is made into a two-layer structure with different curvatures, thereby preventing the voltage drop caused by a small short circuit and improving the output of the secondary battery.

[0004] Meanwhile, Japanese Patent Publication No. 2023-511881 discloses that at least a portion of the artificial graphite used as the negative electrode active material forms secondary particles, with the number of secondary particles accounting for 50% or more of the total number of particles in the negative electrode active material. Furthermore, Japanese Patent Publication No. 2023-511881 discloses that the average particle size D50 of the artificial graphite used as the negative electrode active material is 15 μm or greater. According to Japanese Patent Publication No. 2023-511881, the use of such a negative electrode active material can achieve excellent rapid charging performance and a long cycle life. Summary of the Invention

[0005] However, the above-mentioned technology still has the following problem: in order to enable rapid charging of lithium-ion secondary batteries, the tortuosity of the electrodes is reduced to reduce diffusion resistance, but even when foreign matter is mixed in, local lithium deposition occurs, sometimes causing a short circuit.

[0006] Therefore, an object of the present disclosure is to provide a negative electrode that can be quickly charged, has low diffusion resistance, and can prevent the occurrence of short circuits, and a lithium ion secondary battery having the negative electrode.

[0007] The present disclosure to achieve the above-mentioned objectives includes the following contents.

[0008] <1> A negative electrode comprising a negative electrode active material having an average particle size D50 of 2 μm ≤ D50 ≤ 25 μm, a secondary particle ratio of 50% or more, a tortuosity T of 1 ≤ T ≤ 20, and a migration index Ka of 1.0 ≤ Ka ≤ 1.6.

[0009] <2> The negative electrode according to <1>, wherein the negative electrode active material includes artificial graphite.

[0010] <3> A lithium-ion secondary battery comprising: a positive electrode containing a positive electrode active material; the negative electrode according to <1> or <2>; and an electrolyte layer disposed between the positive electrode and the negative electrode.

[0011] According to the present disclosure, it is possible to provide a negative electrode having low diffusion resistance and capable of preventing occurrence of a short circuit, and a lithium ion secondary battery having the negative electrode. DETAILED DESCRIPTION

[0012] The following describes embodiments of the present disclosure, but the description is for illustration only and does not limit the scope of the present disclosure.

[0013] In this specification, a numerical range expressed using “to” indicates a range including the numerical values ​​described before and after “to” as the minimum value and the maximum value, respectively.

[0014] In the numerical ranges described in this specification, the upper limit or lower limit of one numerical range may be replaced by the upper limit or lower limit of another numerical range described in this specification. In addition, in the numerical ranges described in this specification, the upper limit or lower limit of the numerical range may be replaced by the value shown in the Examples.

[0015] In this specification, the term "step" includes not only independent steps but also steps that cannot be clearly distinguished from other steps as long as the intended purpose of the step is achieved.

[0016] In this specification, each component may include a plurality of corresponding substances. In the present embodiment, when the amount of each component in the composition is mentioned, if there are multiple substances corresponding to each component in the composition, unless otherwise specified, it means the total amount of the multiple substances present in the composition.

[0017] The negative electrode disclosed herein includes a negative electrode active material having an average particle size (D50) of 2 μm ≤ D50 ≤ 25 μm and a secondary particle ratio of 50% or greater. The negative electrode disclosed herein has a tortuosity (T) of 1 ≤ T ≤ 20 and a mobility index (Ka) of 1.0 ≤ Ka ≤ 1.6. By including a negative electrode active material having the aforementioned specified average particle size D50 and secondary particle ratio, and having a tortuosity and a mobility index within the aforementioned ranges, the negative electrode disclosed herein can achieve low diffusion resistance with excellent fast charging performance while preventing the occurrence of short circuits.

[0018] [Negative electrode active material]

[0019] In the negative electrode of the present disclosure, the negative electrode active material may be selected from natural graphite, artificial graphite, soft carbon, hard carbon, Si, SiOx (0 < x < 2), Si-based alloys, Sn, SnOx (0 < x < 2), Li, Li-based alloys, and Li4Ti5O 12 In particular, in the negative electrode of the present disclosure, artificial graphite is preferably included as the negative electrode active material.

[0020] [Average particle size (D50)]

[0021] The average particle size (D50), also known as the median diameter, indicates the particle size at which the cumulative frequency from the smaller particle size side in the particle size distribution of the primary particles on a volume basis reaches 50%. The particle size D50 of the negative electrode active material is a value measured by a laser diffraction method and can be measured using a laser diffraction scattering intensity distribution measuring device. Furthermore, as a negative electrode active material having an average particle size (D50) of 2μm≤D50≤25μm, a commercially available product with a specified particle size can be appropriately used. The average particle size (D50) is particularly preferably 2μm≤D50≤18μm.

[0022] [Secondary particles]

[0023] The negative electrode active material is in the form of either primary particles or secondary particles. The so-called primary particles of the negative electrode active material refer to individual particles that have not yet become aggregated. The so-called secondary particles refer to particles in an aggregated state composed of a plurality of particles formed by the aggregation of two or more primary particles. Primary particles and secondary particles can be easily distinguished by taking an SEM image with a scanning electron microscope. The number ratio of primary particles or secondary particles in the negative electrode active material can be measured, for example, using a scanning electron microscope. The ratio of secondary particles in the present disclosure means the ratio (number %) of the number of secondary particles in all particles of the negative electrode active material composed of primary particles and secondary particles. The ratio of secondary particles can be measured, for example, as described below.

[0024] First, a sheet is prepared in which the particles of the negative electrode active material to be measured are attached without overlapping. Using a scanning electron microscope, the number of primary particles and secondary particles contained in a portion of the sheet is counted. The ratio of the number of secondary particles to the total number of primary and secondary particles is then calculated to determine the proportion of secondary particles in the negative electrode active material. Alternatively, the proportion of secondary particles can be determined at multiple locations on the sheet, for example, four locations. The average value of these four locations can be used as the proportion of secondary particles in the negative electrode active material.

[0025] Furthermore, the negative electrode active material having a secondary particle ratio of 50% or more can be prepared by using primary particles of the negative electrode active material, for example, preparing secondary particles by spray drying, and mixing them with primary particles so that the secondary particles account for 50% or more.

[0026] By making the proportion of secondary particles in the negative electrode active material 50% or more, the diffusion resistance is reduced and the occurrence of short circuits can be prevented. In the negative electrode of the present disclosure, the proportion of secondary particles in the negative electrode active material is made to be 50% or more, and there is no particular upper limit, and 100% can be secondary particles. That is, in the negative electrode of the present disclosure, the proportion of secondary particles in the negative electrode active material can be made to be 50% to 100%, preferably 50% to 90%, more preferably 50% to 80%, further preferably 50% to 70%, and most preferably 55% to 65%.

[0027] [Manufacturing of negative electrode]

[0028] The negative electrode disclosed in the present invention includes a negative electrode collector and a layer (negative electrode active material layer) containing the above-mentioned negative electrode active material provided on the negative electrode collector. There is no particular limitation on the negative electrode collector, and it may be in the form of foil, plate, mesh, punched metal or foam. Examples of metals constituting the negative electrode collector include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, stainless steel and the like. In particular, from the viewpoint of ensuring reduction resistance and the viewpoint of being difficult to alloy with lithium, the negative electrode collector may include at least one metal selected from Cu, Ni and stainless steel. When manufacturing the negative electrode, for example, a slurry prepared by dispersing the negative electrode active material, binder and other components in a dispersion medium is applied to the negative electrode collector and dried.

[0029] [Curvature rate (T)]

[0030] In the negative electrode manufactured as described above, the tortuosity (T) of the negative electrode active material layer satisfies 1≤T≤20. The tortuosity (T) is calculated, for example, by the method described in Japanese Patent No. 5815617. Specifically, the tortuosity (T) can be calculated using the following formula (1).

[0031] T=f / s(1)

[0032] In formula (1), T is tortuosity, f is the total length of a path passing through the gap between the two opposing surfaces and having an opening between the two opposing surfaces, and s is the distance between the two opposing surfaces (thickness of the negative electrode).

[0033] The f and s in the above formula (1) can be obtained as follows. First, a three-dimensional image is obtained for the negative electrode of the measurement object, and the three-dimensional image is binarized into the area of ​​the negative electrode active material and the area of ​​the gap. The path having an opening on one main surface and the other main surface of the negative electrode and a continuous gap in the thickness direction is thinned. In this way, the total length (f) of the path penetrating the negative electrode of the measurement object and the distance (s) between one main surface and the other main surface of the negative electrode can be obtained.

[0034] Furthermore, the tortuosity is usually a value greater than 1. If the tortuosity is 1, it means that it is a straight path, and the closer the tortuosity is to 1, the closer the path is to a straight line. If the tortuosity exceeds 20, the diffusion resistance becomes large, and the negative electrode may not be suitable for fast charging. In addition, if the tortuosity exceeds 20, a short circuit may also occur. As the tortuosity, 1≤T≤9.0 is particularly preferred. In addition, in order to adjust the tortuosity of the negative electrode, for example, the following method can be listed: a slurry in which the negative electrode active material, a binder and other components are dispersed in a dispersion medium is applied to the negative electrode collector, and after drying, the pressure during rolling is changed to adjust the density of the negative electrode active material.

[0035] [Migration Index (Ka)]

[0036] In addition, in the negative electrode manufactured as described above, the migration index (Ka) of the negative electrode active material layer becomes 1.0≤Ka≤1.6. The so-called migration index (Ka) is a numerical value that quantitatively represents the phenomenon of the bias of the binder component in the thickness direction, which is called binder migration. The so-called migration index in the negative electrode is a numerical value that shows the imbalance (bias) of the binder component contained in the negative electrode active material layer for the negative electrode active material layer formed on the negative electrode collector. The negative electrode active material layer is divided into two equal parts in the thickness direction, with the negative electrode collector side as the lower part and the upper part of the lower part as the upper part, and the mass concentration (α) of the binder in the upper part and the mass concentration (β) of the binder in the lower part are calculated. The migration index in the negative electrode can be calculated as α / β.

[0037] More specifically, for example, the negative electrode is cut in the thickness direction using an ion milling device, and the resulting cut surface is analyzed using EPMA (Electron Probe Micro Analyzer). If the binder contains polyvinylidene fluoride (PVdF), the mass concentration of the binder in the upper and lower portions can be measured using fluorine as an indicator. Furthermore, if the binder does not contain a specific element, the mass concentration of the binder in the upper and lower portions can be measured by selectively dyeing the binder.

[0038] When the migration index (Ka) exceeds 1, it means that the binder is biased towards the upper part of the negative electrode active material layer. When the migration index (Ka) is less than 1, it means that the binder is biased towards the lower part of the negative electrode active material layer. A migration index (Ka) of 1 means that the binder is uniformly present in the upper and lower parts of the negative electrode active material layer. The migration index can be adjusted by applying a slurry obtained by dispersing the negative electrode active material, the binder and other components in a dispersion medium to the negative electrode current collector and then drying it. For example, after the negative electrode current collector is coated with the above-mentioned slurry, the migration index (Ka) can be controlled to a higher value by increasing the drying temperature and / or rapid drying.

[0039] The migration index is set to 1.0≤Ka≤1.6, and more preferably 1.0≤Ka≤1.2 from the viewpoint of preventing the occurrence of short circuits.

[0040] [Lithium-ion secondary battery]

[0041] The negative electrode of the present disclosure described above can be used as the negative electrode of a lithium ion secondary battery. The lithium ion secondary battery of the present disclosure comprises: a positive electrode comprising a positive electrode active material, the negative electrode described above, and an electrolyte layer disposed between the positive electrode and the negative electrode. The electrolyte layer of the lithium ion secondary battery of the present disclosure may contain a liquid electrolyte instead of a solid electrolyte, the electrolyte layer may contain a solid electrolyte instead of a liquid electrolyte, or the electrolyte layer may contain a liquid electrolyte and a solid electrolyte. In the case where the electrolyte layer contains a liquid electrolyte, the electrolyte layer preferably has a separator for preventing contact between the positive electrode and the negative electrode while maintaining the liquid electrolyte. In addition, in the case where the electrolyte layer contains a solid electrolyte, the electrolyte layer may optionally contain a binder and the like in addition to the solid electrolyte.

[0042] Solid electrolytes can generally be used without limitation for solid batteries. As such solid electrolytes, crystalline nitrides, oxides, sulfides and oxyacid salts, as well as materials with non-crystalline glass structures can be used. Specifically, sulfide solid electrolytes that can be used as solid electrolytes can be listed, for example, as at least one selected from LiI-LiBr-Li3PS4, Li2S-SiS2, LiI-Li2S-SiS2, LiI-Li2S-P2S5, LiI-Li2O-Li2S-P2S5, LiI-Li2S-P2O5, LiI-Li3PO4-P2S5, Li2S-P2S5, Li3PS4, LiCl-LiBr-Li3PS4, LiCl-LiBr-Li2S-P2S5 and LiCl-LiBr-Li2S-SiS2. In addition, as oxide-based solid electrolytes, for example, Li 0.34 La 0.56 TiO3、Li 3 / 8 Sr 7 / 16 Ta 3 / 4 M 1 / 4 O3 (M=Zr or Hf), Li7La3Zr2O 12 、Li 1.3 Al 0.7 Ti 1.3 (PO4)3、Li 1.5 Al 0.5 Ge 1.5 (PO4)3、Li 3.5 Ge 0.5 V 0.5 O.Li 2.88 PO 3.73 N 0.14 , and Li 2.9 Si 0.45 PO 1.6 N 1.3 In addition to these, complex hydride-based lithium ion conductors and halide-based lithium ion conductors can be used as solid electrolytes.

[0043] Liquid electrolyte can usually use the non-aqueous electrolyte used in non-aqueous lithium ion secondary battery without limitation. The non-aqueous electrolyte can be a composition containing a supporting salt in a non-aqueous solvent. As the non-aqueous solvent, it is possible to list a material selected from an organic electrolyte, a fluorine-based solvent, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and two or more combinations thereof.

[0044] As supporting salts, materials that can be listed include lithium compounds (lithium salts) selected from Li(FSO2)2N, LiPF6, LiBF4, LiClO4, LiAsF6, LiCF3SO3, LiC4F9SO3, LiN(CF3SO2)2, LiC(CF3SO2)3, LiI, and combinations of two or more of these.

[0045] Examples of the binder used in the solid electrolyte include butadiene rubber (BR)-based binders, butylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, and polyimide (PI)-based binders. The binder may be used alone or in combination of two or more.

[0046] The separator used in the liquid electrolyte can be any separator commonly used in non-aqueous lithium ion secondary batteries, for example, separators containing resins such as polyethylene (PE), polypropylene (PP), polyester and polyamide. The separator can be a single-layer structure or a multi-layer structure. As a multi-layer separator, for example, a separator of a 2-layer structure of PE / PP or a separator of a 3-layer structure of PP / PE / PP or PE / PP / PE can be listed. The separator can be a non-woven fabric such as a cellulose non-woven fabric, a resin non-woven fabric, or a glass fiber non-woven fabric.

[0047] The positive electrode includes a positive electrode current collector and a positive electrode active material layer containing a positive electrode active material. The positive electrode current collector is not particularly limited and may be in the form of a foil, a plate, a mesh, a punched metal, or a foam. Examples of metals constituting the positive electrode current collector include Cu, Ni, Cr, Au, Pt, Ag, Al, Fe, Ti, Zn, Co, and stainless steel. In particular, the positive electrode current collector may include Al to ensure oxidation resistance.

[0048] There are no particular limitations on the positive electrode active material, and conventionally known materials can be used as appropriate. Examples of positive electrode active materials include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li(NiCoMn)O2, Li(NiCoAl)O2, and LiFePO4. Furthermore, the positive electrode active material particles may be Hi-Nickel (high nickel, i.e., a positive electrode active material with a high Ni ratio), a Li-Ni-Co-Mn composite oxide, or a ternary positive electrode active material.

[0049] Furthermore, a lithium-ion secondary battery can include a structure other than the above-mentioned positive electrode, negative electrode, and electrolyte layer. For example, a lithium-ion secondary battery includes: an insulating film enclosure that accommodates the positive electrode, negative electrode, and electrolyte layer, a positive terminal component that is conductive with the positive electrode collector, a negative terminal component that is conductive with the negative electrode collector, and a battery case that accommodates the insulating film enclosure that accommodates the positive electrode, negative electrode, and electrolyte layer while making the ends of these positive terminal components and negative terminal components face outward. Furthermore, in the case where the lithium-ion secondary battery is a non-aqueous lithium-ion secondary battery using a liquid electrolyte, a battery case having an opening for injecting the liquid electrolyte can be used.

[0050] The lithium-ion secondary battery of the present invention constructed as described above improves the fast charging performance by using the above-mentioned negative electrode, suppresses the occurrence of short circuit, and has excellent safety. In the lithium-ion secondary battery of the present invention, by using a negative electrode with a tortuosity within a prescribed range as described above, ion diffusion can be promoted, diffusion resistance and overvoltage during charging can be suppressed, and it is therefore suitable for fast charging. The so-called fast charging is not particularly limited and refers to a charging method that is faster than ordinary charging. The so-called fast charging can, for example, be set to charge with a power supply of 20kW, 50kW, 120kW or more.

[0051] In addition, in the lithium-ion secondary battery disclosed herein, even if the negative electrode has a curvature within the above-mentioned range, the migration index is limited to the above-mentioned range, that is, the binder is segregated on the surface of the negative electrode active material layer (the surface on the side opposite to the negative electrode collector side), thereby reducing the electronic conductivity of the surface and preventing the occurrence of short circuits.

[0052] The present disclosure will be described in more detail below using examples, but the technical scope of the present disclosure is not limited to the following examples.

[0053] [Example 1]

[0054] Artificial graphite (average particle size (D50): 16.4 μm) was used as the negative electrode active material. The granulation rate in the secondary granulation step was controlled to achieve a secondary particle ratio of 60%. A slurry containing artificial graphite was then prepared to have the following composition.

[0055] Artificial graphite 90% by mass

[0056] Binder (PVdF) 5 mass%

[0057] Conductive material (acetylene black) 5% by mass

[0058] The obtained slurry was applied on a copper foil, rolled with a press position of 0.1 mm, and dried at 100° C. for 3 hours.

[0059] [Example 2]

[0060] A negative electrode was produced in the same manner as in Example 1 except that the drying condition was changed to 120° C. for 2 hours.

[0061] [Example 3]

[0062] A negative electrode was produced in the same manner as in Example 1 except that the drying condition was changed to 150° C. for 1 hour.

[0063] [Example 4]

[0064] A negative electrode was produced in the same manner as in Example 2 except that artificial graphite (average particle size (D50): 2.4 μm) was used.

[0065] [Example 5]

[0066] A negative electrode was produced in the same manner as in Example 2 except that artificial graphite (average particle size (D50): 4.9 μm) was used.

[0067] [Example 6]

[0068] A negative electrode was produced in the same manner as in Example 2 except that artificial graphite (average particle size (D50): 10.7 μm) was used.

[0069] [Example 7]

[0070] A negative electrode was produced in the same manner as in Example 2 except that artificial graphite (average particle size (D50): 24.3 μm) was used.

[0071] [Example 8]

[0072] A negative electrode was produced in the same manner as in Example 2 except that the ratio of the secondary particles was changed to 50%.

[0073] [Example 9]

[0074] A negative electrode was produced in the same manner as in Example 2 except that the roll pressing conditions were changed to conditions in which the roll pressing was performed at a pressing depth of 0.05 mm.

[0075] [Example 10]

[0076] A negative electrode was produced in the same manner as in Example 2 except that the roll pressing conditions were changed to conditions in which the roll pressing was performed at a pressing depth of 0.15 mm.

[0077] [Comparative Example 1]

[0078] A negative electrode was produced in the same manner as in Example 1 except that the drying condition was changed to 60° C. for 4 hours.

[0079] [Comparative Example 2]

[0080] A negative electrode was produced in the same manner as in Example 1 except that the drying condition was changed to 200° C. for 30 minutes.

[0081] [Comparative Example 3]

[0082] A negative electrode was produced in the same manner as in Example 2 except that artificial graphite (average particle size (D50): 1.9 μm) was used.

[0083] [Comparative Example 4]

[0084] A negative electrode was produced in the same manner as in Example 2 except that artificial graphite (average particle size (D50): 26.7 μm) was used.

[0085] [Comparative Example 5]

[0086] A negative electrode was produced in the same manner as in Example 2 except that the ratio of the secondary particles was changed to 30%.

[0087] [Comparative Example 6]

[0088] A negative electrode was produced in the same manner as in Example 2 except that the ratio of the secondary particles was changed to 40%.

[0089] [Comparative Example 7]

[0090] A negative electrode was produced in the same manner as in Example 2 except that the roll pressing conditions were changed to conditions in which the roll pressing was performed at a pressing depth of 0.2 mm.

[0091] [Curvature measurement]

[0092] For the negative electrodes of Examples 1 to 10 and Comparative Examples 1 to 7, the curvature (T) was calculated using the method described in Japanese Patent No. 5815617. That is, a three-dimensional image was obtained for the negative electrode to be measured, and the three-dimensional image was binarized into the region of the negative electrode active material and the region of the void. The path having an opening on one main surface and the other main surface of the negative electrode and a continuous void in the thickness direction was thinned. From this, the total length (f) of the path penetrating the negative electrode to be measured and the distance (s) between one main surface and the other main surface of the negative electrode were determined, and T = f / s was calculated.

[0093] [Migration index determination]

[0094] The migration index of the negative electrodes of Examples 1 to 10 and Comparative Examples 1 to 7 was measured under the following conditions. Specifically, the negative electrodes were dyed with PVdF, cut in the thickness direction using an ion milling device, and the cut surfaces were analyzed using EPMA (Electron Probe Micro Analyzer) to measure the migration index.

[0095] [Resistance measurement]

[0096] The resistance values ​​of the negative electrodes of Examples 1 to 10 and Comparative Examples 1 to 7 were measured under the following conditions. Test cells were prepared using the negative electrodes of Examples 1 to 10 and Comparative Examples 1 to 7, and the resistance values ​​were calculated from the voltage drop ΔV and the current value during the initial discharge, when discharged at a rate equivalent to 1C for 10 seconds. The resistance value of the test cell using the negative electrode of Comparative Example 1 was set to 1.00, and the resistance values ​​of the test cells using the negative electrodes of Examples 1 to 10 and Comparative Examples 2 to 7 were normalized.

[0097] [Short circuit test]

[0098] Using the negative electrodes of Examples 1 to 10 and Comparative Examples 1 to 7, test batteries were made, and a short-circuit test was performed using the following experimental device. The experimental device for the short-circuit test includes: a drill plate with a fixed spike, an XYZ stage, a resistance measuring device, a micrometer, a thermocouple, and a computer. In this device, the structure is as follows: the spike is fixed at the top of the drill plate, and the test battery is fixed on the XYZ stage below the drill plate. While measuring the height of the test battery with a micrometer, the test battery is raised at a certain speed, and the temperature, DC resistance, and displacement are measured and input into the computer. A thermocouple is set and fixed on the surface of the test battery, and the test battery is clamped with an insulating plate. The nail is pierced 4mm, and its displacement and temperature change are measured. When the rising temperature is below 150°C, the result of the short-circuit test is set as good (○), and when the rising temperature exceeds 150°C, the result of the short-circuit test is set as poor (×).

[0099] [result]

[0100] The resistance values ​​and the results of the short-circuit test when the migration index was changed are shown in Table 1 (Comparative Examples 1 and 2, Examples 1 to 3).

[0101]

Table 1

[0102]

[0103] Next, the resistance values ​​and the results of the short-circuit test when artificial graphites having different D50s were used are shown in Table 2 (Comparative Examples 3 and 4, Example 2, and Examples 4 to 7).

[0104]

Table 2

[0105]

[0106] Next, the resistance values ​​and the results of the short-circuit test when the ratio of the secondary particles was changed are shown in Table 3 (Comparative Examples 5 and 6, Example 2, and Example 8).

[0107]

Table 3

[0108]

[0109] Next, the resistance values ​​and the results of the short-circuit test when the curvature ratio was changed are shown in Table 4 (Comparative Example 7, Example 2, Example 9, and Example 10).

[0110]

Table 4

[0111]

[0112] From the results of Examples 1 to 10 and Comparative Examples 1 to 7, it can be seen that if a negative electrode active material is used that includes an average particle size (D50) of 2μm≤D50≤25μm, a proportion of secondary particles of 50% or more, and a tortuosity (T) of 1≤T≤20 and a migration index (Ka) of 1.0≤Ka≤1.6, the negative electrode will have low resistance and can suppress the occurrence of short circuits.

Claims

1. A negative electrode comprising a negative electrode active material having an average particle size D50 of 2 μm ≤ D50 ≤ 25 μm and a secondary particle ratio of 50% or more, The tortuosity T is 1≤T≤20, and the migration index Ka is 1.0≤Ka≤1.

6.

2. The negative electrode according to claim 1, wherein The negative electrode active material includes artificial graphite.

3. A lithium-ion secondary battery comprising: A positive electrode including a positive electrode active material, The negative electrode according to claim 1 or 2, and An electrolyte layer is disposed between the positive electrode and the negative electrode.

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