JP2018026261A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery excellent in PTC characteristics.SOLUTION: A nonaqueous electrolyte secondary battery includes a positive electrode collector foil 11 and a positive electrode mixture layer 12. A first layer 1 is disposed between the positive electrode collector foil 11 and the positive electrode mixture layer 12. The first layer 1 contains an expandable particle expanding with an increase in temperature. A second layer 2 is disposed between the first layer 1 and the positive electrode mixture layer 12. The second layer 2 has conductivity. The second layer 2 has a porosity lower than that of the positive electrode mixture layer 12.SELECTED DRAWING: Figure 2

Description

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

  International Publication No. 2015/046469 (Patent Document 1) discloses a battery configuration in which a PTC (Positive Temperature Coefficient) layer is disposed between a positive electrode current collector foil and a positive electrode mixture layer.

International Publication No. 2015/046469

  Various proposals have been made for the purpose of improving the safety of nonaqueous electrolyte secondary batteries. Patent Document 1 proposes to dispose a PTC layer between the positive electrode current collector foil and the positive electrode mixture layer. The PTC layer has a positive resistance temperature coefficient. That is, the PTC layer contains expandable particles that expand as the temperature rises. When the battery generates heat, the PTC layer expands to separate the positive electrode current collector foil and the positive electrode mixture layer. Thereby, the resistance between the positive electrode current collector foil and the positive electrode mixture layer is increased, and the conduction between the positive electrode current collector foil and the positive electrode mixture layer is eventually cut off. However, there remains room for improvement in the following points.

  The positive electrode mixture layer is a particle layer and includes many holes. Particularly in applications where high output is important (for example, in-vehicle applications), the positive electrode mixture layer is designed to have a high porosity. Therefore, the expanded expansible particles may enter the pores in the positive electrode mixture layer. If the expandable particles enter the pores in the positive electrode mixture layer, conduction between the positive electrode current collector foil and the positive electrode mixture layer may not be sufficiently blocked.

  Therefore, the present disclosure aims to provide a non-aqueous electrolyte secondary battery that is excellent in a characteristic (hereinafter referred to as “PTC characteristic”) that interrupts conduction inside the battery when the temperature rises.

The nonaqueous electrolyte secondary battery of the present disclosure includes a positive electrode current collector foil and a positive electrode mixture layer.
A first layer is disposed between the positive electrode current collector foil and the positive electrode mixture layer. The first layer contains expandable particles that expand as the temperature rises.
A second layer is disposed between the first layer and the positive electrode mixture layer. The second layer has conductivity. The second layer has a porosity that is lower than the porosity of the positive electrode mixture layer.

  In the nonaqueous electrolyte secondary battery of the present disclosure, the first layer corresponds to a PTC layer. The first layer expands when the battery generates heat, thereby increasing the resistance between the positive electrode current collector foil and the positive electrode mixture layer. In the nonaqueous electrolyte secondary battery of the present disclosure, the second layer is disposed between the first layer (PTC layer) and the positive electrode mixture layer. The second layer has a lower porosity than the positive electrode mixture layer. Due to the presence of the second layer, the expandable particles contained in the first layer are prevented from entering the positive electrode mixture layer. Therefore, the nonaqueous electrolyte secondary battery of the present disclosure is excellent in PTC characteristics.

FIG. 1 is a schematic cross-sectional view illustrating an example of a configuration of a nonaqueous electrolyte secondary battery according to an embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the positive electrode.

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

<Nonaqueous electrolyte secondary battery>
The nonaqueous electrolyte secondary battery of this embodiment is excellent in PTC characteristics. Therefore, the nonaqueous electrolyte secondary battery of the present embodiment is suitable for in-vehicle applications that require extremely high safety, for example. “In-vehicle use” indicates a power source of an electric vehicle such as a hybrid vehicle (HV). However, the use of the nonaqueous electrolyte secondary battery of the present embodiment is not limited to in-vehicle use. The nonaqueous electrolyte secondary battery of the present embodiment can be applied to all uses.

  FIG. 1 is a schematic cross-sectional view showing an example of the configuration of the nonaqueous electrolyte secondary battery according to this embodiment. Hereinafter, the nonaqueous electrolyte secondary battery may be abbreviated as “battery”.

  The battery 100 includes a battery case 101. The battery case 101 shown in FIG. 1 has a rectangular shape (flat rectangular parallelepiped). However, the battery case 101 may have a cylindrical shape, for example. The battery case 101 is made of, for example, an aluminum (Al) alloy. However, the battery case 101 may be, for example, an aluminum laminate film bag or the like as long as it has a predetermined sealing property. Although not shown, the battery case 101 may be provided with a liquid injection port, a safety valve, a pressure-sensitive current interruption mechanism (CID), and the like.

  Inside the battery case 101, an electrode group 102 and a non-aqueous electrolyte 103 are disposed. The nonaqueous electrolyte 103 is a liquid electrolyte (electrolytic solution). A part of the nonaqueous electrolyte 103 is stored at the bottom of the battery case 101. The nonaqueous electrolyte 103 is also held inside the electrode group 102.

  The electrode group 102 is led out of the battery case 101 by a current collecting terminal 104. The electrode group 102 includes a positive electrode, a negative electrode, and a separator. The electrode group 102 may be a wound electrode group or a stacked electrode group. The wound electrode group is an electrode group formed by winding a belt-like positive electrode and a belt-like negative electrode in a spiral shape. The stacked electrode group is an electrode group configured by alternately stacking a rectangular positive electrode and a rectangular negative electrode. In any of the electrode groups, a separator is disposed between the positive electrode and the negative electrode.

  The width dimension of the electrode group 102 (dimension in the X-axis direction in FIG. 1) may be about 130 mm, for example. The height dimension of the electrode group 102 (dimension in the Y-axis direction in FIG. 1) may be about 50 mm, for example. In the thickness direction of the electrode group 102 (Z-axis direction in FIG. 1), for example, about 130 separators may be stacked.

《Positive electrode》
The positive electrode can be a strip-like, rectangular or other sheet member. FIG. 2 is a schematic cross-sectional view showing an example of the configuration of the positive electrode. The positive electrode 10 includes a positive electrode current collector foil 11 and a positive electrode mixture layer 12. That is, the battery 100 includes the positive electrode current collector foil 11 and the positive electrode mixture layer 12.

(Positive electrode current collector foil)
The positive electrode current collector foil 11 has a thickness of about 5 to 30 μm, for example. The positive electrode current collector foil 11 may be, for example, an Al foil, an Al alloy foil, or the like.

(Positive electrode mixture layer)
The positive electrode mixture layer 12 has a thickness of about 100 to 200 μm, for example. The positive electrode mixture layer 12 is applied to the surface of the second layer 2 described later. The positive electrode mixture layer 12 contains, for example, 80 to 98% by mass of positive electrode active material particles, 1 to 10% by mass of a conductive material, and 1 to 10% by mass of a binder. The positive electrode active material particles are, for example, a lithium-containing metal oxide. Examples of the lithium-containing metal oxide include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O _{2, and the} like. The positive electrode active material particles have an average particle diameter of about 1 to 30 μm, for example. The average particle size in the present specification indicates a particle size of 50% cumulative from the fine particle side in the volume-based particle size distribution measured by the laser diffraction scattering method.

  From the viewpoint of output characteristics, the positive electrode mixture layer 12 preferably has a porosity of 25% or more. From the viewpoint of volume energy density, the positive electrode mixture layer 12 preferably has a porosity of 35% or less. The porosity of the positive electrode mixture layer 12 can be adjusted by, for example, the composition of the positive electrode mixture layer 12, the compressibility, and the like.

The “porosity” in the present specification has the following formula
Porosity = (V1−V2) ÷ V1
Is calculated by In the formula, V1 represents the apparent volume of the layer (positive electrode mixture layer, second layer described later), and V2 represents the true volume of the layer. The apparent volume is the product of the layer thickness and the layer area. The true volume is calculated by dividing the mass of the layer by the true density of the layer. The true density of the layer is calculated from the true density of each material constituting the layer and the composition (mixing ratio) of the layer.

(First layer)
The first layer 1 is disposed between the positive electrode current collector foil 11 and the positive electrode mixture layer 12. The first layer 1 is typically applied to the surface of the positive electrode current collector foil 11. The first layer 1 has a thickness of about 5 to 30 μm, for example. The first layer 1 contains expandable particles. The expandable particles expand as the temperature increases. Thereby, the first layer 1 functions as a PTC layer, that is, a layer whose resistance increases as the temperature rises. The expandable particles may be particles that are non-conductive and expand as the temperature rises. Examples of the expandable particles include polyethylene (PE) particles and polypropylene (PP) particles. The expandable particles have an average particle size of 0.1 to 10 μm, for example.

  The 1st layer 1 needs to show electroconductivity except at the time of expansion. The first layer 1 may further contain, for example, a conductive material and a binder. The first layer 1 contains, for example, 80 to 98% by mass of expandable particles, 1 to 10% by mass of a conductive material, and 1 to 10% by mass of a binder. The conductive material is preferably vapor grown carbon fiber (VGCF). When the 1st layer 1 contains VGCF, it exists in the tendency for the resistance increase accompanying a charging / discharging cycle to be suppressed. However, the conductive material may be, for example, acetyl black (AB) as long as it has conductivity. For example, the binder may be polyvinylidene fluoride (PVdF), an acrylic resin, or the like.

(Second layer)
The second layer 2 is disposed between the first layer 1 and the positive electrode mixture layer 12. The second layer 2 is typically applied to the surface of the first layer 1. The second layer 2 has a thickness of 5 to 20 μm, for example. The second layer 2 has conductivity. The second layer 2 has a porosity that is lower than the porosity of the positive electrode mixture layer 12. Thereby, when the expandable particle contained in the first layer 1 expands, the expandable particle is suppressed from entering the positive electrode mixture layer 12. Therefore, the battery 100 is excellent in PTC characteristics. The second layer 2 has a porosity of 19% or more and less than 25%, for example.

  The second layer 2 preferably contains graphite. Graphite has high conductivity. Graphite is easy to be oriented and densely packed. The second layer 2 may contain, for example, 95 to 99% by mass of graphite and 1 to 5% by mass of a binder. Examples of the binder include PVdF and acrylic resin.

  The second layer 2 may further contain inorganic particles. When the second layer 2 contains inorganic particles, the pores are filled in the inorganic particles. Thereby, the porosity of the 2nd layer 2 can be adjusted low. The second layer 2 may contain, for example, 25 to 94% by mass of graphite, 5 to 70% by mass of inorganic particles, and 1 to 5% by mass of a binder. From the viewpoint of PTC characteristics, the second layer 2 preferably contains 25 to 69% by mass of graphite, 30 to 70% by mass of inorganic particles, and 1 to 5% by mass of a binder.

Examples of the inorganic particles include α-alumina (α-Al ₂ O ₃ ), boehmite (AlOOH), titania (TiO ₂ ), zirconia (ZrO ₂ ), magnesia (MgO), magnesium hydroxide (Mg (OH) ₂ ). Etc. An inorganic particle may be used individually by 1 type, and 2 or more types may be used in combination. Therefore, the inorganic particles may be at least one selected from the group consisting of α-alumina, boehmite, titania, zirconia, magnesia, and magnesium hydroxide. The inorganic particles have an average particle size of about 0.2 to 2 μm, for example.

<Negative electrode>
The negative electrode includes a negative electrode current collector foil and a negative electrode mixture layer. The negative electrode current collector foil has a thickness of about 5 to 30 μm, for example. The negative electrode current collector foil may be, for example, a copper (Cu) foil. The negative electrode mixture layer is applied to the surface of the negative electrode current collector foil. The negative electrode mixture layer has a thickness of about 50 to 150 μm, for example. The negative electrode mixture layer contains 90 to 99% by mass of negative electrode active material particles and 1 to 10% by mass of a binder. The negative electrode active material particles may be, for example, graphite, graphitizable carbon, non-graphitizable carbon, or the like. The surface of graphite may be coated with non-graphitic carbon. The negative electrode active material particles have an average particle diameter of about 1 to 30 μm, for example. The binder may be, for example, carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR). The positive / negative capacity ratio (a value obtained by dividing the negative electrode capacity by the positive electrode capacity) may be about 1.7 to 2.0, for example.

<< Separator >>
The separator separates the positive electrode and the negative electrode, and holds the nonaqueous electrolyte in the internal void. For example, the separator has a thickness of about 5 to 30 μm. The separator is, for example, a polyolefin porous film. The separator can be, for example, a PE porous film, a PP porous film, or the like. The separator may have a multilayer structure. For example, the separator may be configured by laminating a porous film of PP, a porous film of PE, and a porous film of PP in this order.

  The separator may have a heat resistant layer on its surface. The heat-resistant layer has a thickness of about 3 to 10 μm, for example. The heat-resistant layer contains, for example, inorganic particles such as α-alumina, and a heat-resistant resin such as an aramid resin.

《Nonaqueous electrolyte》
The non-aqueous electrolyte contains an aprotic solvent and a supporting electrolyte salt. The aprotic solvent may be, for example, a mixture of a cyclic carbonate and a chain carbonate. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, and the like. Examples of the chain carbonate include ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC). The mixing ratio of the cyclic carbonate and the chain carbonate may be, for example, about “cyclic carbonate: chain carbonate = 1: 9 to 5: 5” in volume ratio. The aprotic solvent has, for example, a composition of EC: EMC: DMC = 3: 3: 4 (v: v: v).

The nonaqueous electrolyte contains, for example, a supporting electrolyte salt of 0.5 mol / l or more and 2.0 mol / l or less. The supporting electrolyte salt may be a Li salt such as LiPF ₆ , LiBF ₄ , Li [N (SO ₂ F) ₂ ], Li [N (SO ₂ CF ₃ ) ₂ ], for example.

The non-aqueous electrolyte can contain various additives in addition to the above components. Examples of the additive include Li [B (C ₂ O ₄ ) ₂ ], Li [BF ₂ (C ₂ O ₄ )], Li [PF ₂ (C ₂ O ₄ ) ₂ ], LiPO ₂ F ₂ , vinylene. Examples include carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfite (ES), and propane sultone (PS).

  Examples will be described below. However, the following examples do not limit the scope of the invention of the present disclosure.

<Preparation of nonaqueous electrolyte secondary battery>
Nonaqueous electrolyte secondary batteries according to Examples and Comparative Examples were produced as follows.

Example 1
91 parts by mass of expandable particles (PE particles), 5 parts by mass of a conductive material (AB), and 4 parts by mass of a binder (PVdF) were mixed using N-methyl-2-pyrrolidone (NMP) as a solvent. Thereby, the 1st paste was prepared. An Al foil having a thickness of 15 μm was prepared as a positive electrode current collector foil. The 1st paste was coated on the surface (front and back both surfaces) of Al foil with the gravure coater. Thereby, the first layer was formed.

  97 parts by mass of a conductive material (graphite) and 3 parts by mass of a binder (PVdF) were mixed using NMP as a solvent. Thereby, the 2nd paste was prepared. The second paste was applied to the surface of the first layer with a gravure coater. Thereby, the second layer was formed.

92 parts by mass of positive electrode active material particles (LiNi _1/3 Co _1/3 Mn _1/3 O ₂ ), 4.5 parts by mass of conductive material (AB) and 3.5 parts by mass of binder (PVdF) As a solvent. Thereby, a positive electrode mixture paste was prepared. The positive electrode mixture paste was applied to the surface of the second layer by a die coater. Thereby, the positive electrode mixture layer was formed. From the above, a positive electrode was produced.

  A negative electrode containing graphite as a negative electrode active material was prepared. A separator was prepared. This separator has a three-layer structure of PP film / PE film / PP film. The positive electrode and the negative electrode were wound with a separator between them, thereby forming an electrode group. The electrode group was stored in a battery case. A non-aqueous electrolyte (electrolytic solution) was injected into the battery case. From the above, a nonaqueous electrolyte secondary battery according to Example 1 was produced.

<< Comparative Example 1 >>
According to the same procedure as in Example 1, except that the first layer and the second layer are not formed, that is, the positive electrode mixture layer is formed directly on the surface of the positive electrode current collector foil. A water electrolyte secondary battery was produced.

<< Example 2, Example 3, Comparative Example 2 >>
As shown in Table 1 below, Example 2, Example 3 and Comparative Example 2 were performed in the same procedure as Example 1 except that the porosity of the second layer and the positive electrode mixture layer was changed. The nonaqueous electrolyte secondary battery which concerns on this was produced.

Example 4
In Example 4, the positive electrode was fabricated so that the first layer contained VGCF instead of AB. Except for this, a non-aqueous electrolyte secondary battery according to Example 4 was produced by the same procedure as in Example 2.

<< Examples 5 to 12 >>
In Examples 5-12, the positive electrode was produced so that the second layer further contained inorganic particles. Except for this, non-aqueous electrolyte secondary batteries according to Examples 5 to 12 were produced by the same procedure as in Example 4. The types of inorganic particles used are shown in Table 1 below.

<Evaluation>
Each battery was evaluated as follows.

《Resistance increase rate》
The increase in resistance with high load charge / discharge was evaluated. The following “charge → pause → discharge → pause” was defined as one cycle, and the cycle was repeated 1000 times. Here, the magnitude of the current is represented by “C”. The current of “1C” is defined as a current at which the SOC (State Of Charge) of the battery reaches 0% to 100% after charging for 1 hour.

Charging: 2.5C x 240 seconds Pause: 120 seconds Discharge: 30C x 20 seconds Pause: 120 seconds

  The resistance increase rate was calculated by dividing the battery resistance after 1000 cycles by the battery resistance before the cycle. The results are shown in Table 1 above.

<< PTC characteristics >>
The SOC of the battery was adjusted to 50%. The battery temperature was adjusted to 30 ° C. The battery resistance at 30 ° C. was measured. The battery temperature was then adjusted to 120 ° C. The battery resistance at 120 ° C. was measured. The resistance ratio was calculated by dividing the battery resistance at 120 ° C. by the battery resistance at 30 ° C. The results are shown in Table 1 above. The larger the resistance ratio, the better the PTC characteristics.

<Result>
From the results of Comparative Example 2 and Examples 1 to 3 in Table 1, it can be seen that the PTC characteristics are improved when the second layer has a porosity lower than that of the positive electrode mixture layer.

  From the results of Example 4, it can be seen that, when the first layer contains VGCF, the resistance increase rate can be reduced while ensuring a resistance ratio of 200 or more.

  From the result of Examples 5-7, it turns out that a PTC characteristic improves because a 2nd layer contains 5-70 mass% inorganic particle | grains. This is probably because the pores of the second layer are filled with inorganic particles, and the porosity of the second layer is lowered.

  From the results of Examples 8 to 12, it can be seen that boehmite, titania, zirconia, magnesia and magnesium hydroxide also have the same effect as α-alumina.

  The embodiments and examples disclosed herein are illustrative in all aspects and should not be construed as being restrictive. The scope of the invention of the present disclosure is shown not by the above description but by the scope of claims, and is intended to include all modifications within the meaning and scope equivalent to the scope of claims.

  DESCRIPTION OF SYMBOLS 1 1st layer, 2nd layer, 10 positive electrode, 11 positive electrode current collection foil, 12 positive electrode compound material layer, 100 battery (nonaqueous electrolyte secondary battery), 101 battery case, 102 electrode group, 103 nonaqueous electrolyte, 104 Current collector terminal.

Claims (1)

A positive electrode current collector foil, and a positive electrode mixture layer;
A first layer is disposed between the positive electrode current collector foil and the positive electrode mixture layer,
The first layer contains expandable particles that expand as the temperature increases,
A second layer is disposed between the first layer and the positive electrode mixture layer;
The second layer has conductivity,
The second layer has a porosity that is lower than the porosity of the positive electrode mixture layer.
Non-aqueous electrolyte secondary battery.

Images