JP2013109881A - Electrode for nonaqueous electrolyte battery, nonaqueous electrolyte battery, and electric vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: an electrode for a nonaqueous electrolyte battery, in which the porosity of an active material layer is small, so that battery performance thereof can be improved; and a nonaqueous electrolyte battery.SOLUTION: A nonaqueous electrolyte battery electrode includes an active material layer containing an active material. The active material layer comprises a powder compact obtained by pressure forming an electrode material containing powder of the active material, and has a porosity of 2% or less. The active material layer can be obtained by pressure forming the electrode material at a surface pressure exceeding 490 MPa (5 ton/cm) by cold isostatic pressing (CIP).

Description

  The present invention relates to a nonaqueous electrolyte battery electrode having an active material layer containing an active material, a nonaqueous electrolyte battery using the electrode, and an electric vehicle including the battery.

  Non-aqueous electrolyte batteries have a long life, high efficiency, and high capacity, and are used in mobile devices such as mobile phones, notebook computers, and digital cameras. Typical examples of the nonaqueous electrolyte battery include a lithium battery and a lithium ion secondary battery (hereinafter simply referred to as “lithium battery”) using a lithium ion transfer reaction between the positive electrode and the negative electrode.

  The lithium-based battery includes a positive electrode having a positive electrode active material layer containing a positive electrode active material, a negative electrode having a negative electrode active material layer containing a negative electrode active material, an electrolyte layer interposed between these positive and negative active material layers, Is provided. And charging / discharging is performed by a lithium ion moving through an electrolyte layer between a positive electrode active material layer and a negative electrode active material layer. In recent years, an all-solid battery using an incombustible inorganic solid electrolyte instead of an organic electrolyte has been proposed (see Patent Document 1).

The active material layer of the electrode for nonaqueous electrolyte is usually formed by a wet method or a dry method. In the case of a wet method, a slurry is prepared by adding a solvent to an electrode material containing an active material powder, this slurry is applied on a substrate (current collector) and dried, and then this is rolled or pressed. Press molding. In the case of the dry method, an electrode material containing an active material powder is placed in a mold and pressure-molded by mold molding. For example, Patent Document 1 discloses an active material layer in which an electrode material obtained by mixing a conductive additive or a binder with an active material is pressure-formed at ² ton / cm ² .

JP 2006-202552 A

  However, in the conventional forming method, voids are generated in the active material layer, and the porosity of the active material layer is large. For example, in the method for forming an active material layer described in Patent Document 1 described above, it is considered that voids are likely to occur in the active material layer because the surface pressure during pressure molding is low. When such a nonaqueous electrolyte battery electrode having an active material layer having a large porosity is used for a nonaqueous electrolyte battery, the voids in the active material layer cause a decrease in battery performance. Specifically, the contact area between the active material particles is small, and the battery reaction area is reduced, which leads to an increase in internal resistance. In addition, since there are many voids in the active material layer, the discharge capacity is reduced. Furthermore, the adhesiveness between the active material particles is low, and the cycle characteristics are deteriorated. Therefore, it is desired that the porosity of the active material layer is small. In particular, when the particle size of the active material powder is small (for example, the average particle size D50 (particle size corresponding to a weight ratio of 50% is less than 5 μm)), the bulk density is small and the filling rate is reduced. There is a tendency for voids to form inside.

  The present invention has been made in view of the above circumstances, and one of the objects thereof is a nonaqueous electrolyte battery electrode capable of improving battery performance and a nonaqueous electrolyte layer having a small porosity of an active material layer. The object is to provide an electrolyte battery. Another object is to provide an electric vehicle including the above-described non-aqueous electrolyte battery.

  (1) The electrode for nonaqueous electrolyte batteries of the present invention has an active material layer containing an active material. The active material layer is made of a powder molded body obtained by pressure-molding an electrode material containing an active material powder, and the porosity of the active material layer is 2% or less.

  According to the electrode for a nonaqueous electrolyte battery of the present invention, since the porosity of the active material layer is 2% or less and the porosity of the active material layer is small, the battery performance can be improved. Specifically, there is almost no void in the active material layer, and the contact area between the active material particles (battery reaction area) increases, so that the internal resistance of the battery can be reduced and the discharge capacity of the battery is improved. Can do. In addition, since the adhesion between the active materials is improved, the cycle characteristics of the battery can be improved. Note that the porosity (%) refers to the ratio (%) of the total area of the voids in the observation visual field area by performing cross-sectional observation of the active material layer, obtaining the total area of the voids in the observation visual field. Specifically, the void ratio is measured by taking an image of the observation visual field, and using the image analysis software to binarize the extracted image so that the void is black and the others are white. By doing. The porosity is measured in three or more observation fields, and the average value is used. Further, the observation visual field area is, for example, 20 μm × 20 μm.

  (2) As an embodiment of the electrode for a non-aqueous electrolyte battery of the present invention, the active material layer may be formed by pressing the above electrode material by cold isostatic pressing with a surface pressure exceeding 490 MPa.

An active material layer (powder molded body) with a porosity of 2% or less can be obtained by pressing and molding the above electrode material by cold isostatic pressing (CIP) at a surface pressure exceeding 490 MPa (5 ton / cm ² ). . Since it is cold isostatic pressurization, it is effective for consolidation of the active material layer, and the filling rate is improved as compared with uniaxial pressurization such as ordinary press and mold forming. Further, when the surface pressure exceeds 490 MPa, voids in the active material layer are reduced. In particular, even when the particle size of the active material powder is small (for example, the average particle size D50 (particle size corresponding to 50% by weight) is less than 5 μm), the porosity of the active material layer can be 2% or less.

  (3) As one form of the electrode for nonaqueous electrolyte batteries of the present invention, the active material is lithium titanate.

In particular, when lithium titanate (Li ₄ Ti ₅ O ₁₂ ) is applied to the active material, since lithium titanate has a large specific surface area, voids are easily generated in the active material layer. In the present invention, since the porosity is 2% or less, the effect is large when the active material is lithium titanate. In addition, since lithium titanate has almost no volume change associated with charge / discharge, the absence of voids in the active material layer is very effective in improving battery performance.

  (4) As one form of the electrode for nonaqueous electrolyte batteries of this invention, it is mentioned that an electrode material contains the powder of a solid electrolyte.

  In a battery having a solid electrolyte layer in which an electrolyte layer is formed with a solid electrolyte (solid electrolyte type battery), when the active material layer does not contain a solid electrolyte, lithium ions can be exchanged only at the interface between the active material layer and the solid electrolyte layer. However, the ions are not sufficiently diffused inside the active material layer (parts away from the interface), and the active material inside the active material layer may not be effectively used for the battery reaction. Therefore, the active material layer contains the active material and the solid electrolyte, and the active material and the solid electrolyte are mixed in the active material layer, whereby the solid electrolyte promotes ion diffusion inside the active material layer, and the active material The active material inside the layer can be effectively used for the battery reaction. As a result, the internal resistance of the battery can be further reduced.

Typical examples of the solid electrolyte include sulfide-based solid electrolytes containing Li ₂ S, and oxide-based solid electrolytes such as Li ₃ PO ₄ and Li ₃ PO _4-x N _x (LIPON). Examples of the sulfide-based solid electrolyte include Li ₂ S-P ₂ S ₅ system, Li ₂ S-SiS ₂ system, Li ₂ S-B ₂ S ₃ system, and further P ₂ O ₅ and Li ₃ PO ₄ may be added. A sulfide-based solid electrolyte is preferable because it generally exhibits higher lithium ion conductivity than an oxide-based solid electrolyte. In particular, among sulfide-based solid electrolytes, Li ₂ S—P ₂ S ₅ -based solid electrolytes are more preferable because they exhibit high lithium ion conductivity.

On the other hand, as the active material, a positive electrode active material or a negative electrode active material is used depending on the application (positive electrode or negative electrode). Lithium-containing composite oxides such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiFePO ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.8 Co _0.15 Al _0.05 O ₂ Is mentioned. Examples of the negative electrode active material include carbon materials such as lithium metal and graphite, metals or alloys alloyed with lithium such as Al, Si, Sn, Zn, and In, and metal sulfides such as FeS ₂ and TiS ₂ . Note that lithium titanate can be used for both the positive electrode active material and the negative electrode active material, but is preferably used for the negative electrode active material.

  In addition to the active material and the solid electrolyte, the active material layer may contain a conductive additive or a binder (binder) as necessary. Examples of the conductive assistant include carbon black (CB) such as acetylene black (AB) and ketjen black (KB). Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and a silicone resin.

  (5) The nonaqueous electrolyte battery of the present invention has a positive electrode, a negative electrode, and a solid electrolyte layer interposed between these two electrodes. And a positive electrode or a negative electrode is the electrode for non-aqueous electrolyte batteries of the present invention described above.

  The electrode for a nonaqueous electrolyte battery of the present invention can be applied to either a positive electrode or a negative electrode of a nonaqueous electrolyte battery. When applied to a positive electrode, a positive electrode active material is used, and when applied to a negative electrode, A negative electrode active material is used. According to the nonaqueous electrolyte battery of the present invention, battery performance is improved by using the above-described electrode for nonaqueous electrolyte battery of the present invention for the positive electrode or the negative electrode as compared with the conventional battery.

  (6) The electric vehicle of the present invention includes the above-described non-aqueous electrolyte battery of the present invention as a power source for a driving motor.

  The electric vehicle is a vehicle including a motor as a drive source, and the electric vehicle of the present invention has high battery performance by using the above-described non-aqueous electrolyte battery of the present invention as a power source of the drive motor. A non-aqueous electrolyte battery is provided as a power source for the drive motor. The electric vehicle includes a hybrid vehicle and an electric vehicle.

  Since the nonaqueous electrolyte battery electrode and nonaqueous electrolyte battery of the present invention have a porosity of the active material layer of 2% or less, the battery performance can be improved.

[Example 1]
A nonaqueous electrolyte battery including the electrode for a nonaqueous electrolyte battery of the present invention was produced, and the battery performance was evaluated.

  In this example, the non-aqueous electrolyte battery has a structure in which a positive electrode current collector, a positive electrode active material layer, a sulfide-based solid electrolyte layer, a negative electrode active material layer, and a negative electrode current collector are sequentially laminated. The positive electrode body and the negative electrode body manufactured in (1) were stacked and bonded together.

[Preparation of positive electrode body]
An Al foil was used for the substrate to be the positive electrode current collector. LiNi _0.8 Co _0.15 Al _0.05 O ₂ powder (average particle size (D50) 6 μm) was used as the positive electrode active material. And the powder of LiNi _0.8 Co _0.15 Al _0.05 O _{2 and} the powder of Li ₂ S-P ₂ S ₅ solid electrolyte (Li ₂ S: P ₂ S ₅ molar ratio 80:20) in a mass ratio of 70:30 The positive electrode material mixed at the ratio was put into a mold and pressure-molded by mold molding to prepare a positive electrode active material layer made of a powder molded body. The pressure molding was performed under an atmosphere of 200 ° C. and a surface pressure of 353 MPa (3.6 ton / cm ² ). The thickness of the positive electrode active material layer was 80 μm. On the Al foil serving as the positive electrode current collector, the above-described positive electrode active material layer formed by mixing LiNi _0.8 Co _0.15 Al _0.05 O ₂ and Li ₂ S-P ₂ S ₅ based solid electrolyte is laminated, and the positive electrode is Produced. Furthermore, a Li ₂ S-P ₂ S ₅ solid electrolyte (Li ₂ S: P ₂ S ₅ molar ratio 80:20) was deposited on the positive electrode active material layer by vacuum deposition, and the amorphous state A solid electrolyte layer made of a Li ₂ S-P ₂ S ₅ solid electrolyte was formed to obtain a positive electrode body. The thickness of the solid electrolyte layer was 10 μm.

[Preparation of negative electrode body]
Al foil was used for the substrate used as the negative electrode current collector. Li ₄ Ti ₅ O ₁₂ powder (average particle size (D50) 4 μm) was used as the negative electrode active material. And Li ₄ Ti ₅ O ₁₂ powder and Li ₂ S-P ₂ S ₅ solid electrolyte (Li ₂ S: P ₂ S ₅ molar ratio 70:30) powder and conductive auxiliary agent acetylene black (AB) A negative electrode material mixed with a mass ratio of 62: 38: 3.6 was cold isostatically pressed with a surface pressure of 3922 MPa (40 ton / cm ² ) using a cold isostatic pressing (CIP) device. A negative electrode active material layer made of a powder compact was produced by pressure molding. The thickness of the negative electrode active material layer was 120 μm. On the Al foil as the negative electrode current collector, the negative electrode active material layer formed by mixing Li ₄ Ti ₅ O ₁₂ , Li ₂ S-P ₂ S ₅ solid electrolyte and AB is laminated to produce a negative electrode did. Furthermore, a Li ₂ S-P ₂ S ₅ solid electrolyte (Li ₂ S: P ₂ S ₅ molar ratio 80:20) was deposited on the negative electrode active material layer by vacuum deposition, and the amorphous state A solid electrolyte layer composed of a Li ₂ S—P ₂ S ₅ solid electrolyte was formed, and this was used as a negative electrode body. The thickness of the solid electrolyte layer was 10 μm.

(Analysis of negative electrode active material layer)
When the cross section in the thickness direction of the produced negative electrode active material layer was observed with a scanning electron microscope and the porosity of the negative electrode active material layer was measured, the porosity of the negative electrode active material layer was 0.5%. The porosity was binarized using image analysis software for any three observation fields (20 μm × 20 μm), the porosity was measured, and the average value was taken as the porosity.

[Production of battery]
In a dry atmosphere with a dew point temperature of −40 ° C., the positive electrode body and the negative electrode body are overlapped so that the solid electrolyte layers face each other, and the solid electrolyte layers are bonded to each other by pressurizing and heating treatment. Was made. This pressurizing and heating treatment was performed by applying a pressure of 16 MPa in the overlapping direction of the positive electrode body and the negative electrode body, heating to 190 ° C. and holding for 130 minutes. By this pressure heat treatment, the solid electrolyte layer in an amorphous state is crystallized, and the solid electrolyte layer on the positive electrode side and the solid electrolyte layer on the negative electrode side are joined and integrated. Here, since the solid electrolyte layer on the positive electrode side and the solid electrolyte layer on the negative electrode side are bonded by utilizing the mutual diffusion of atoms when the amorphous crystallizes, a bonding interface is formed between both layers. Therefore, the ion transfer resistance in the solid electrolyte layer can be reduced. Moreover, integration can be promoted by applying pressure.

Although the reason is not clear, crystallization of an amorphous Li ₂ S-P ₂ S ₅ solid electrolyte layer formed by depositing a Li ₂ S-P ₂ S ₅ solid electrolyte by a vapor phase method The temperature is different from the crystallization temperature of the solid electrolyte layer formed by pressing the amorphous Li ₂ S—P ₂ S ₅ solid electrolyte powder. Specifically, in the case of a Li ₂ S-P ₂ S ₅ solid electrolyte having a molar ratio of Li ₂ S to P ₂ S ₅ of 80:20, the crystallization temperature of the solid electrolyte layer formed by the vapor phase method Is about 150 ° C., and the crystallization temperature of the solid electrolyte layer of the powder compact is about 220 ° C. In this example, since the solid electrolyte layer on the positive electrode side and the solid electrolyte layer on the negative electrode side are respectively formed by a gas phase method, heating at the time of joining the solid electrolyte layer on the positive electrode side and the solid electrolyte layer on the negative electrode side is performed. If the temperature is 150 ° C. or higher, both layers can be crystallized and bonded.

(Evaluation of battery performance)
Above manner embedded in the evaluation test for the case the nonaqueous electrolyte battery fabricated (lithium-based batteries), the cut-off voltage: 3.5 V-1.0 V, current density: subjected to a charge-discharge test under the condition of 0.3 mA / cm ² The battery performance was evaluated.

<Internal resistance>
When the internal resistance of the battery after charging in the first cycle was measured with an impedance analyzer, the internal resistance (total resistance) of the battery was 65 Ωcm ² .
<Discharge capacity>
When the discharge capacity at the first cycle was measured, the discharge capacity was 2.90 mAh / cm ² .
<Cycle characteristics>
The retention rate of the discharge capacity at the 100th cycle relative to the discharge capacity at the 1st cycle was determined to be 99%.

[Example 2]
A negative electrode active material layer was produced in the same manner as in Example 1 except that the surface pressure by cold isostatic pressing during production of the negative electrode active material layer was 1961 MPa (20 ton / cm ² ), and a battery was produced. did. And when the negative electrode active material layer was analyzed similarly to Example 1, the porosity of the negative electrode active material layer was 1.5%. Further, when the battery performance was evaluated in the same manner as in Example 1, the internal resistance (total resistance) was 80 Ωcm ² , the discharge capacity was 2.75 mAh / cm ² , and the discharge capacity maintenance rate at the 100th cycle was 99%. .

[Comparative Example 1]
The negative electrode active material layer was formed by pressure molding by die molding in the same manner as the positive electrode active material layer, and the pressure molding conditions were 200 ° C. under a surface pressure of 490 MPa (5 ton / cm ² ). A negative electrode active material layer was produced in the same manner as in Example 1, and a battery was produced. And when the negative electrode active material layer was analyzed similarly to Example 1, the porosity of the negative electrode active material layer was 10%. Further, when the battery performance was evaluated in the same manner as in Example 1, the internal resistance (total resistance) was 800 Ωcm ² , the discharge capacity was 2.00 mAh / cm ² , and the discharge capacity maintenance rate at the 100th cycle was 85%. .

[Comparative Example 2]
A negative electrode active material layer was produced in the same manner as in Comparative Example 1 except that the surface pressure by mold forming when producing the negative electrode active material layer was 196 MPa ( ² ton / cm ² ). In this case, the surface pressure at the time of pressure molding was low, and the negative electrode active material layer could not be sufficiently retained, so the shape of the negative electrode active material layer collapsed after pressure molding, and a battery could not be produced. It was.

  From the above results, the batteries of Examples 1 and 2 including the negative electrode having a negative electrode active material layer having a porosity of 2% or less were compared with the battery of Comparative Example 1 in terms of internal resistance, discharge capacity, and cycle characteristics ( It can be seen that the discharge capacity retention ratio is excellent in all points.

Note that the present invention is not limited to the above-described embodiment, and can be modified as appropriate without departing from the gist of the present invention. For example, LiCoO ₂ may be used as the positive electrode active material.

  The electrode for nonaqueous electrolyte batteries of the present invention can be suitably used for, for example, an electrode of a lithium battery. Further, the nonaqueous electrolyte battery of the present invention can be used, for example, as a power source for an electric vehicle such as an electric vehicle as well as a mobile phone, a notebook computer, and a digital camera.

Claims (6)

An electrode for a non-aqueous electrolyte battery having an active material layer containing an active material,
The active material layer comprises a powder molded body obtained by pressure molding an electrode material containing the active material powder,
A nonaqueous electrolyte battery electrode, wherein the active material layer has a porosity of 2% or less.   2. The electrode for a non-aqueous electrolyte battery according to claim 1, wherein the active material layer is formed by pressure forming the electrode material at a surface pressure exceeding 490 MPa by cold isostatic pressing.   The electrode for a nonaqueous electrolyte battery according to claim 1, wherein the active material is lithium titanate.   The said electrode material contains the powder of a solid electrolyte further, The electrode for nonaqueous electrolyte batteries as described in any one of Claims 1-3 characterized by the above-mentioned. A non-aqueous electrolyte battery having a positive electrode, a negative electrode, and a solid electrolyte layer interposed between the two electrodes,
The said positive electrode or the said negative electrode is a nonaqueous electrolyte battery electrode as described in any one of Claims 1-4, The nonaqueous electrolyte battery characterized by the above-mentioned.   An electric vehicle comprising the nonaqueous electrolyte battery according to claim 5 as a power source for a drive motor.