JP2019186164A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

A non-aqueous electrolyte secondary battery including a negative electrode containing graphite and SiO is intended to suppress a reduction in storage characteristics while suppressing a decrease in capacity due to use. A nonaqueous electrolyte secondary battery includes a negative electrode. The negative electrode includes a negative electrode mixture layer. The negative electrode mixture layer includes a negative electrode active material, carbon black, a filler, and a binder. The negative electrode active material contains graphite and silicon oxide. The filler is made of ceramics or cellulose, has a major axis length of 5 μm or more and 10 μm or less, and has an aspect ratio of 16.7 or more and 1000 or less. [Selection diagram] Fig. 1

Description

  The present disclosure relates to a non-aqueous electrolyte secondary battery.

  In Japanese Patent Laid-Open No. 2002-164052 (Patent Document 1), in order to form a conductive path, a negative electrode includes a negative electrode active material and a conductive material, and the negative electrode active material includes graphite and silicon oxide (SiO). The material discloses a non-aqueous electrolyte secondary battery containing both carbon black and fibrous carbon.

JP 2002-140552 A

  Since SiO has a large amount of expansion and contraction during charge and discharge, the conductive path formed by contact between the negative electrode active materials is easily cut and electrically isolated negative electrode active material particles are likely to increase and are used (charge / discharge cycle). There is a tendency that the capacity drop is large in the early stage. For this reason, in the nonaqueous electrolyte secondary battery disclosed in Patent Document 1 (hereinafter may be abbreviated as “battery”), by blending a conductive material containing carbon black and fibrous carbon into the negative electrode mixture, It has been proposed to suppress a decrease in battery capacity due to use. However, in this case, since a coating film is formed on the fibrous carbon, which is a conductive material, by reductive decomposition of the electrolytic solution or the like, the capacity drop after storage is large and the storage characteristics are considered to be insufficient.

  An object of the present disclosure is to suppress a decrease in storage characteristics while suppressing a decrease in capacity due to use in a nonaqueous electrolyte secondary battery including a negative electrode including graphite and SiO.

  Hereinafter, the technical configuration and effects of the present disclosure will be described. However, the mechanism of action of the present disclosure includes estimation. The scope of the claims should not be limited by the correctness of the mechanism of action.

[1] The present disclosure relates to a non-aqueous electrolyte secondary battery including a negative electrode.
The negative electrode includes a negative electrode mixture layer.
The negative electrode mixture layer includes a negative electrode active material, carbon black, a filler, and a binder.
The negative electrode active material includes graphite and silicon oxide.
The filler is made of ceramics or cellulose, has a long axis length of 5 μm or more and 10 μm or less, and has an aspect ratio of 16.7 or more and 1000 or less.

  Generally, SiO has a large expansion / contraction amount during charge / discharge. For this reason, when a negative electrode active material contains SiO, it is thought that the conductive network of negative electrode active materials is easy to be cut | disconnected by charging / discharging. Thereby, it is considered that the negative electrode active material is isolated in the negative electrode mixture layer and the battery capacity is reduced.

On the other hand, with reference to FIG. 1, in the battery of the present disclosure, the negative electrode mixture layer includes a negative electrode active material (graphite 61 and SiO 62), carbon black 64, filler 63, and binder 65. The filler 63 has a long axis length of 5 μm or more and 10 μm or less and a shape (for example, needle shape or fiber shape) having an aspect ratio of 16.7 or more and 1000 or less, thereby charging / discharging. Even when the SiO 62 expands and contracts due to the cycle, the filler 63 follows the expansion and contraction, so that the contact between the negative electrode active materials via the carbon black 64, the filler 63, and the binder 65 is easily maintained. Thereby, isolation of the negative electrode active material in the negative electrode mixture layer is suppressed, and a decrease in battery capacity due to use is suppressed. The conductivity of the filler 63 made of ceramics or cellulose is low (for example, the electrical conductivity is 10 ⁴ S / m or less), but the conductive path (negative electrode) is caused by the carbon black 64 (conductive material) attached around the filler 63. It is considered that the electrical connection between the active materials is maintained.

  In addition, when the filler 63 is made of ceramic or cellulose having low conductivity, it is difficult to form a coating by reductive decomposition of the electrolytic solution. Therefore, when fibrous carbon or the like having high conductivity (a coating is easily formed) is used. In comparison with the above, a decrease in capacity after storage is suppressed, and a decrease in storage characteristics of the battery is suppressed.

  Therefore, according to the present disclosure, in a nonaqueous electrolyte secondary battery including a negative electrode including graphite and SiO, it is possible to suppress a decrease in storage characteristics while suppressing a decrease in capacity due to use.

It is a conceptual diagram for demonstrating the effect of this indication. It is the schematic which shows an example of a structure of the nonaqueous electrolyte secondary battery of embodiment. It is the schematic which shows an example of a structure of the electrode group of embodiment. It is the schematic which shows an example of a structure of the negative electrode of embodiment. It is the schematic which shows an example of a structure of the positive electrode of embodiment.

  Hereinafter, an embodiment of the present disclosure (referred to as “the present embodiment” in the present specification) will be described. However, the following description does not limit the scope of the claims.

<Configuration of non-aqueous electrolyte secondary battery>
The non-aqueous electrolyte secondary battery of the present disclosure is not particularly limited as long as it includes a negative electrode including a negative electrode mixture layer including a negative electrode active material, a conductive material, and a binder according to the present disclosure. A known configuration can be employed. The conventionally known configuration includes, for example, an electrode group having a positive electrode, a negative electrode, and a separator disposed between the positive electrode and the negative electrode, and this electrode group is used as an exterior material together with an electrolyte having lithium ion conductivity. This refers to the configuration that is arranged.

FIG. 2 is a schematic diagram illustrating an example of the configuration of the battery according to the present embodiment.
The battery 100 includes an exterior material 50. The packaging material 50 is made of, for example, an aluminum laminate film. That is, the battery 100 is a laminate type battery. However, in the present embodiment, the type and form of the battery 100 should not be particularly limited. The battery 100 may be, for example, a square battery or a cylindrical battery. The positive electrode tab 51 and the negative electrode tab 52 communicate with the inside and outside of the exterior material 50, respectively. The positive electrode tab 51 is, for example, an aluminum (Al) thin plate. The negative electrode tab 52 is, for example, a copper (Cu) thin plate. The exterior material 50 may include a CID, a gas discharge valve, a liquid injection hole, and the like (all not shown).

<Electrode group>
FIG. 3 is a schematic diagram illustrating an example of the configuration of the electrode group according to the present embodiment.
The packaging material 50 houses the electrode group 40 and an electrolytic solution (not shown). The electrode group 40 is a stacked type. However, the electrode group 40 may be a wound type. The electrode group 40 includes a positive electrode 10, a negative electrode 20, and a separator 30. That is, the battery 100 includes at least the negative electrode 20.

<Negative electrode>
FIG. 4 is a schematic diagram illustrating an example of the configuration of the negative electrode of the present embodiment.
The battery 100 includes at least the negative electrode 20. The negative electrode 20 can be a strip-shaped sheet. The negative electrode 20 includes a negative electrode current collector 21 and a negative electrode mixture layer 22. That is, the negative electrode 20 includes a negative electrode mixture layer 22. The negative electrode current collector 21 may be, for example, a Cu foil. The negative electrode current collector 21 may have a thickness of, for example, 5 μm or more and 20 μm or less.

<Negative electrode mixture layer>
The negative electrode mixture layer 22 is formed on the surface of the negative electrode current collector 21.
The negative electrode mixture layer 22 may be formed on both the front and back surfaces of the negative electrode current collector 21. The negative electrode mixture layer 22 may have a thickness of, for example, 50 μm or more and 150 μm or less. The negative electrode mixture layer 22 includes a negative electrode active material, a conductive material, a filler, and a binder. The negative electrode mixture layer 22 includes, for example, a negative electrode active material of 90% by mass to 99% by mass, a conductive material of 0.5% by mass to 5% by mass, and a binder of 0.5% by mass to 5% by mass. May be included.

(Negative electrode active material)
The negative electrode active material electrochemically occludes and releases charge carriers (lithium ions in this embodiment). The negative electrode active material includes graphite and silicon oxide (SiO). The negative electrode active material can be, for example, a mixture of graphite and SiO. When the negative electrode active material contains graphite and SiO, an increase in battery capacity is expected.

  The graphite may be natural graphite or artificial graphite, may be spheroidized, or may be amorphous coated. Graphite may have an average particle size of, for example, 10 to 20 μm.

  For example, SiO may have an average particle diameter of 1 to 10 μm. In the present specification, the “average particle diameter” means a particle diameter (D50) at which the cumulative particle volume from the fine particle side is 50% of the total particle volume in the volume-based particle diameter distribution obtained by the laser diffraction scattering method. Show.

  SiO may be contained in an amount of 1% by mass to 30% by mass with respect to the negative electrode active material composed of graphite and SiO, and is preferably contained in an amount of 2% by mass to 25% by mass with respect to the negative electrode active material, More preferably, the content is 5% by mass or more and 20% by mass or less with respect to the negative electrode active material. When SiO is contained in an amount of less than 1% by mass with respect to the negative electrode active material, the battery capacity may not be increased sufficiently. When SiO is contained in an amount exceeding 30% by mass with respect to the negative electrode active material, even when a small amount of a conductive material is added, the capacity reduction in use cannot be sufficiently improved. On the other hand, when a large amount of conductive material is added, the capacity drop during use can be improved to some extent, but the storage characteristics are lowered.

(Carbon black)
Examples of the carbon black 64 include acetylene black (AB), furnace black, and ketjen black (registered trademark). The carbon black 64 may be vapor grown carbon fiber (VGCF) or the like. The primary particles of carbon black 64 (raw material powder before mixing) may have an average particle diameter of 10 to 40 nm, for example. The carbon black 64 can function as a conductive material, for example.

(Filler)
The filler 63 is made of ceramics or cellulose. The ceramics are not particularly limited, for _{example, ZnO, TiO 2, WO 2} , NiO, and the like _Al 2 _{O 3.} Cellulose is not particularly limited.

  The filler 63 has a long axis length of 5 μm or more and 10 μm or less. The filler 63 has an aspect ratio of 16.7 or more and 1000 or less. The shape of the filler 63 may be, for example, a needle shape or a fiber shape.

(Binder)
The binder 65 should not be specifically limited. The binder 65 may be, for example, carboxymethylcellulose (CMC), styrene butadiene rubber (SBR), polyamide (PA), polyimide (PI), polyamideimide (PAI), polyvinylidene fluoride (PVdF), or the like. The negative electrode 20 may contain one kind of binder alone. Two or more binders may be included in the negative electrode 20. The binder content may be, for example, 0.1 parts by mass or more and 10 parts by mass or less with respect to the total amount of the negative electrode active material of 100 parts by mass (total amount of graphite and SiO).

<Positive electrode>
FIG. 5 is a schematic diagram illustrating an example of the configuration of the positive electrode of the present embodiment.
The battery 100 includes at least the positive electrode 10. The positive electrode 10 can be a strip-shaped sheet. The positive electrode 10 includes a positive electrode mixture layer 12 and a positive electrode current collector 11. The positive electrode current collector 11 may be, for example, an Al foil. The positive electrode current collector 11 may have a thickness of 10 μm or more and 50 μm or less, for example.

<< Positive electrode mixture layer >>
The positive electrode mixture layer 12 is formed on the surface of the positive electrode current collector 11. The positive electrode mixture layer 12 may be formed on both the front and back surfaces of the positive electrode current collector 11. The positive electrode mixture layer 12 may have a thickness of not less than 100 μm and not more than 200 μm, for example. The positive electrode mixture layer 12 includes at least a positive electrode active material. The positive electrode mixture layer 12 may include, for example, 80% by mass to 98% by mass of a positive electrode active material, 1% by mass to 10% by mass of a conductive material, and 1% by mass to 10% by mass of a binder.

(Positive electrode active material, conductive material and binder)
The positive electrode active material, the conductive material, and the binder should not be particularly limited. The positive electrode active material may be, for example, LiCoO ₂ , LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (NCM), LiMnO ₂ , LiMn ₂ O ₄ , LiFePO ₄ or the like. The conductive material may be, for example, acetylene black (AB), furnace black, vapor grown carbon fiber (VGCF), graphite or the like. The binder may be, for example, polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), or the like.

<Separator>
As shown in FIG. 3, the battery 100 may include a separator 30. The separator 30 is a strip-shaped film. The separator 30 is disposed between the positive electrode 10 and the negative electrode 20. The separator 30 is an electrically insulating porous film. For example, the separator 30 may have a thickness of 5 to 30 μm. The separator 30 can be made of, for example, polyethylene (PE), polypropylene (PP), or the like.

  Separator 30 may have a laminated structure. For example, the separator 30 may be configured by laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order. The separator 30 may have a heat resistant layer on the surface. The heat-resistant layer can include, for example, an oxide material such as alumina and a resin material such as polyimide.

<Electrolyte>
The electrolytic solution includes a solvent and a supporting electrolyte salt. The solvent is aprotic. The solvent may be, for example, a mixture of cyclic carbonate and chain carbonate. The mixing ratio may be, for example, cyclic carbonate: chain carbonate = 1: 9 to 5: 5 (volume ratio). Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Examples of the chain carbonate include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). A cyclic carbonate and a chain carbonate may be used individually by 1 type, and 2 or more types may be used in combination. The supporting electrolyte salt may be, for example, LiPF ₆ , LiBF ₄ , Li [N (FSO ₂ ) ₂ ] or the like. In the electrolytic solution 60, the supporting electrolyte salt may have a concentration of 0.5 to 2.0 mol / l, for example. The supporting electrolyte salt may be used alone, or two or more supporting electrolyte salts may be used in combination.

(Other ingredients)
Other components may be further contained in addition to the solvent and the lithium salt. As other components, for example, additives such as a film forming agent can be considered. Examples of the film forming agent include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), Li [B (C ₂ O ₄ ) ₂ ] (commonly called “LiBOB”), LiPO ₂ F _2, propane sultone (PS). And ethylene sulfite (ES). The electrolytic solution may contain, for example, 0.1 to 5% by mass of other components.

  Examples of the present disclosure are described below. However, the following description does not limit the scope of the claims.

<Manufacture of non-aqueous electrolyte secondary batteries>
<< Reference Examples 1, 2, 10 and Examples 3-9, 11-14 >>
1. Production of positive electrode The following materials were prepared.
Cathode active material: NCM
Conductive material: AB
Binder: PVdF
Solvent: N-methyl-2-pyrrolidone (NMP)
Positive electrode current collector: Al foil

  A positive electrode mixture slurry was prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent. The mixing ratio of the solid content is “positive electrode active material: conductive material: binder = 98: 1: 1 (mass ratio)”. The positive electrode mixture slurry was applied to the surface (both front and back surfaces) of the positive electrode current collector 11 and dried. As a result, the positive electrode mixture layer 12 was formed. The positive electrode mixture layer 12 was rolled. Thereby, the positive electrode 10 was manufactured. The positive electrode 10 was cut into a sheet shape.

2. Production of negative electrode The following materials were prepared.
Negative electrode active material: Mixture of graphite (amorphous coated spherical natural graphite, average particle size: 15 μm) and SiO (average particle size: 5 μm) [graphite / SiO = 95/5 (mass ratio)]
Carbon black (conductive material): AB (average particle size of primary particles: 35 nm)
Ceramic filler or cellulose fiber (In Reference Examples 1, 2, 10 and Examples 3 to 9, 11 to 14, any one shown in Table 1 was used.)
Binder: CMC
Solvent: Water Negative electrode current collector: Cu foil

  A negative electrode mixture slurry was prepared by mixing a negative electrode active material, AB, ceramic filler or cellulose fiber, a binder, and a solvent. The mixing ratio of the solid content is “negative electrode active material: AB: ceramic filler or cellulose fiber: binder = 97: 1: 1: 1 (mass ratio)”. The negative electrode mixture slurry was applied to the surface (front and back surfaces) of the negative electrode current collector 21 and dried. Thereby, the negative electrode mixture layer 22 was formed. The negative electrode mixture layer 22 was rolled. Thereby, the negative electrode 20 was manufactured. The negative electrode 20 was cut into a sheet.

3. Assembling Each of the plurality of negative electrodes 20 and each of the plurality of positive electrodes 10 are alternately stacked with the separators 30 interposed therebetween so that the negative electrodes 20 are positioned on both outer edges in the stacking direction, and a stacked electrode group 40 is produced. It was done. The separator 30 is configured by laminating a porous film made of PP, a porous film made of PE, and a porous film made of PP in this order.

The electrode group 40 was accommodated in a rectangular exterior material 50 made of an aluminum laminate film. An electrolyte solution was injected into the exterior material 50. The electrolytic solution contains the following Li salt and solvent.
Li salt: LiPF ₆ (1.0 mol / L)
Solvent: [EC: DMC: EMC = 3: 4: 3 (volume ratio)]
The packaging material 50 was sealed.
From the above, the batteries (laminate type batteries) 100 of Reference Examples 1, 2, 10 and Examples 3-9, 11-14 were manufactured.

<< Comparative Examples 1-7 >>
Comparative Examples 1 to 7 were carried out in the same manner as in Example 1 except that the filler 63 used as a conductive material in the production of the negative electrode 20 was changed to fibrous carbon nanotubes (CNT) shown in Table 1 below. The battery 100 was manufactured.

<< Comparative Example 8 >>
The negative electrode mixture slurry is prepared by adding SBR to the slurry obtained by mixing the negative electrode active material, CMC and the solvent (water) without using carbon black and filler 63 in the production of the negative electrode 20, and further mixing them. It was. The mixing ratio of the solid content is “negative electrode active material: CMC: SBR = 98: 1: 1 (mass ratio)”. Otherwise, the battery 100 of Comparative Example 8 was manufactured in the same manner as in the example.

<Evaluation>
(Initial capacity measurement)
At 25 ° C., the initial capacity (discharge) of the batteries according to the respective examples, reference examples, and comparative examples is obtained by the following constant voltage-constant current (CV-CC) charging and constant voltage-constant current (CV-CC) discharging. Capacity) was confirmed.
CV-CC charging: CV = 4.2V, CC = 12A (0.3C), end current = 2A
CV-CC discharge: CV = 2.5V, CC = 12A (0.3C), end current = 2A

(Charge / discharge cycle test)
At 25 ° C., the battery according to each example, reference example and comparative example was subjected to 50 charge / discharge cycles consisting of the following constant voltage-constant current method charge (CV-CC charge) and constant current method discharge (CC discharge). Repeated.
CV-CC charging: CV = 4.2V, CC = 20A (0.5C), end current = 2A
CC type discharge: current = 20A (0.5C), end voltage = 2.5V

  After 50 cycles, the capacity (discharge capacity) of the battery is measured under the same conditions as the initial capacity measurement, and the capacity retention rate after the cycle is obtained by dividing the discharge capacity after the cycle by the initial capacity (initial discharge capacity). It was. The results are shown in the column of “Cycle capacity maintenance ratio” in Table 1.

(Preservation test)
The battery was fully charged, and a storage test (high temperature storage test) for 14 days was performed in a 60 ° C. environment. After the storage test, the capacity after storage was measured under the same conditions as the initial capacity measurement, and the discharge capacity after storage was divided by the initial discharge capacity to obtain the capacity retention rate after storage. The results are shown in the column “Capacity retention after storage” in Table 1.

(Measurement of short axis length, long axis length, aspect ratio)
From the SEM or TEM image, the major axis length (corresponding to the length) and the minor axis length (corresponding to the diameter) of the needle shape and the fiber shape were measured. At this time, measurements were made at 20 arbitrary locations, and the average value was taken as the measured value. The major axis length / minor axis length was defined as the aspect ratio.

<Result>
As a result of the “cycle capacity retention ratio” in Table 1, as a material of the negative electrode mixture layer, an implementation using ceramic filler (a filler made of ceramics) or cellulose fiber (a filler made of cellulose) and AB which is carbon black. In Examples 3 to 9 and 11 to 14, the cycle capacity retention rate was higher than that of Comparative Example 8 in which no filler and conductive material were added. Moreover, compared with the comparative examples 1-7 using CNT and AB as a material of a negative electrode compound-material layer, the same cycle capacity maintenance factor was shown.

  From these results, by combining a ceramic filler (inorganic filler) such as needle-like or fibrous or cellulose fiber and carbon black with high conductivity, a decrease in capacity maintenance rate after the charge / discharge cycle is suppressed, It turns out that the improvement effect of cycling characteristics is acquired. Moreover, it turns out that the same cycle characteristic improvement effect as the case where CNT and AB are added is acquired.

  This is because the conductive path formed by adhering AB and binder to ceramic filler such as needles and fibers or cellulose fibers causes expansion and contraction of the negative electrode active material (especially SiO) accompanying the charge / discharge cycle. In addition, it is presumed that this is because the current collecting property between the active material particles is secured. In addition, the conductive path between the negative electrode active material particles could be confirmed also by SEM observation of the electrode.

  However, in the case of Reference Examples 1 and 2 in which the major axis length of the filler 63 was 2 μm or less and the aspect ratio was smaller than 16.7, the effect of improving the cycle characteristics was reduced. It is presumed that this is because the long axis length of the filler is short and the filler was difficult to follow the expansion and contraction of the negative electrode active material accompanying the charge / discharge cycle. On the other hand, in the case of Reference Example 10 in which the major axis length of the filler was 100 μm and the aspect ratio was 5000, the effect of improving cycle characteristics was reduced. This is presumed to be because the major axis length of the filler is too long and the proportion of the conductive paths existing between the negative electrode active material particles apparently decreases.

  From the above, a negative electrode composite material layer is produced using a ceramic filler such as needle-like or fibrous or cellulose fiber and a conductive material (carbon black), and the major axis length of the ceramic filler or cellulose fiber is 5 to 10 μm, When the aspect ratio is 16.7 to 1000, it is considered that the effect of improving the cycle characteristics can be obtained.

  Furthermore, from the results of “Capacity maintenance ratio after storage” in Table 1, in Comparative Examples 1 to 7 in which CNT and AB were added to the negative electrode mixture, the capacity maintenance ratio after storage was as low as 76 to 86%. In Examples 1 to 14 using ceramic filler or cellulose fiber and AB (carbon black), the capacity retention rate after high-temperature storage is as high as 90 to 92%, and Comparative Example 8 in which no CNT or conductive material is added The capacity retention rate was close to 94%.

  This is because in Comparative Examples 1-7, the film formation reaction due to reductive decomposition or the like of the electrolytic solution occurred due to the high conductivity of the CNT, and the capacity decreased, whereas in Examples 3-9, 11-14, Since the ceramic filler or the cellulose fiber has low conductivity, no film is formed on the filler composed of these, and it is considered that the decrease in the capacity retention rate after storage was suppressed.

  As described above, according to the present disclosure, in a nonaqueous electrolyte secondary battery including a negative electrode including graphite and SiO, it is considered that a decrease in storage characteristics can be suppressed while a decrease in capacity due to use is suppressed.

  The embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The technical scope determined by the description of the scope of claims includes meanings equivalent to the scope of claims and all modifications within the scope.

  DESCRIPTION OF SYMBOLS 10 Positive electrode, 11 Positive electrode collector, 12 Positive electrode compound material layer, 20 Negative electrode, 21 Negative electrode current collector, 22 Negative electrode compound material layer, 30 Separator, 40 Electrode group, 50 Exterior material, 51 Positive electrode tab, 52 Negative electrode tab, 61 Graphite, 62 SiO, 63 filler, 64 carbon black, 65 binder, 100 battery (non-aqueous electrolyte secondary battery).

Claims (1)

A non-aqueous electrolyte secondary battery comprising a negative electrode,
The negative electrode includes a negative electrode mixture layer,
The negative electrode mixture layer includes a negative electrode active material, carbon black, a filler, and a binder,
The negative electrode active material includes graphite and silicon oxide,
The non-aqueous electrolyte secondary battery, wherein the filler is made of ceramics or cellulose, has a major axis length of 5 μm or more and 10 μm or less, and has an aspect ratio of 16.7 or more and 1000 or less.

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