JP2014235856A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery which can achieve high levels in both of input/output density and durability (especially, superior high-rate pulse charge and discharge cycle characteristics).SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode having a positive electrode active material layer; a negative electrode having a negative electrode active material layer; and a nonaqueous electrolyte. The negative electrode active material layer includes a negative electrode active material, and a resin component including at least a thickener. The percentage of the resin component to the whole negative electrode active material layer is 0.8-1.6% by mass. The negative electrode active material consists of a graphite material, and has a DEP oil absorption of 35-50 mL/100 g. In addition, the thickener includes secondary particles having an average particle diameter of 5-50 μm. A 1%-aqueous solution prepared by dissolving or distributing the thickener in water at a rate of 1 mass% has a viscosity of 1000-5000 mPa s.

Description

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

  Nonaqueous electrolyte secondary batteries such as lithium-ion batteries are lighter and have higher energy density than existing batteries, and thus have been preferably used in recent years for high-output power supplies mounted on vehicles. This type of battery is required to have both high input / output density and durability at a high level. Patent Documents 1 and 2 are examples of improved negative electrodes for this purpose. For example, Patent Document 1 describes that a battery having excellent high-rate pulse charge / discharge cycle characteristics can be realized by using graphite particles having a predetermined property as a negative electrode active material.

JP 2013-030441 A JP 2012-216537 A

  The negative electrode of the battery is generally formed by applying a paste (including slurry and ink) obtained by kneading a negative electrode active material and a resin component together with an appropriate aqueous solvent (for example, water) onto the surface of the negative electrode current collector. It is made by drying. However, according to the study of the present inventor, when using the negative electrode active material described in Patent Document 1, it is difficult to prepare a paste having a viscosity suitable for coating, and high shear (for example, shearing force) is imparted during kneading. In some cases, many resin components (typically thickeners and binders) are required to stabilize the paste. When a high shear is imparted during kneading, graphite particles may collide violently in the paste, and the particles may be cracked or chipped. In such a case, there is a possibility that the reaction active point increases and the durability of the battery decreases. In addition, when the proportion of the resin component increases, the liquid retention property of the negative electrode active material layer may decrease or the resistance may increase, and the input / output density may deteriorate.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a non-aqueous electrolyte that can achieve both high input / output density and durability (for example, excellent high-rate pulse charge / discharge cycle characteristics). The next battery is to provide. Another related object is to provide a method for stably manufacturing such a battery.

In order to achieve the above object, the present invention provides a nonaqueous electrolyte secondary battery comprising a positive electrode having a positive electrode active material layer, a negative electrode having a negative electrode active material layer, and a nonaqueous electrolyte. The negative electrode active material layer includes a negative electrode active material and a resin component including at least a thickener, and satisfies all the following conditions (1) to (3).
(1) The content rate of the said resin component is 0.8-1.6 mass% of the whole negative electrode active material layer.
(2) The negative electrode active material is made of a graphite material and has a DEP oil absorption of 35 to 50 mL / 100 g.
(3) The viscosity of a 1% aqueous solution obtained by dissolving or dispersing the thickener in water at a ratio of 1 mass% is 1000, in which the average particle diameter of secondary particles is 5 to 50 μm. ˜5000 mPa · s.

In the battery having the above configuration, the ratio of the resin component (typically thickener and binder) in the negative electrode active material layer is suppressed to a low value of 0.8 to 1.6 mass%. For this reason, the surface of the negative electrode active material is hardly covered with the resin component, and more pores necessary for insertion and extraction of charge carriers can be secured. Therefore, the resistance of the negative electrode can be kept low, and excellent high input / output characteristics can be realized. Moreover, a high charge / discharge capacity and liquid retention can be realized by using a negative electrode active material that satisfies the above properties. Furthermore, by using a thickener that satisfies the above properties, kneading can be performed with a lower share than in the past. For this reason, it is hard to produce a crack and a chip | tip in a negative electrode active material, and high durability is realizable.
Therefore, according to the present invention, it is possible to realize a non-aqueous electrolyte secondary battery that can achieve both high input / output density and high durability.

Here, “DBP (dibutyl phthalate) oil absorption” refers to the amount measured by the measuring method defined in JIS K6217-4 (2001). Further, the "average particle size", the volume-based particle size distribution measured by a particle size distribution measurement based on laser diffraction light scattering method, a particle diameter corresponding to cumulative 50% from fine particle side (D ₅₀ particle size, median Also called diameter). The “viscosity” refers to a value measured using a general B-type viscometer at 25 ° C. under the condition of a rotational speed of 60 rpm.

It is a flowchart of the negative electrode paste preparation method which concerns on one Embodiment. It is a graph which shows the relationship between the torque rate (%) of a solid kneading process, and the capacity | capacitance maintenance rate (%) of a cycle test. It is a graph which shows the relationship between the DEP oil absorption amount (ml / g) of a negative electrode active material, and the capacity | capacitance maintenance factor (%) of a cycle test. It is a graph which shows the relationship between the DEP oil absorption amount (ml / g) of a negative electrode active material, and the resistance increase rate (%) after a high-rate pulse charging / discharging cycle test.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for implementation can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field.

The non-aqueous electrolyte secondary battery disclosed herein is characterized by including a negative electrode active material layer that satisfies the above conditions (1) to (3).
The negative electrode active material is made of graphite (graphite). Graphite has a carbon hexagonal network structure developed compared to other materials, and can achieve an excellent energy density. On the other hand, the bond between the layers is weak and it is easy to crack or chip during kneading. For this reason, the significance of application of the technology disclosed herein is particularly great. Among these, amorphous coated graphite in a form in which the surface of graphite particles as a core is coated with an amorphous carbon material can be suitably used. Thereby, the reactivity with a nonaqueous electrolyte can be suppressed lower, and an irreversible capacity | capacitance and resistance can be suppressed further. The DEP oil absorption of the negative electrode active material (graphite) is 35 to 50 mL / 100 g (particularly 35 to 40 mL / 100 g). In general, the negative electrode active material tends to become more familiar with the non-aqueous electrolyte as the DEP oil absorption value increases. That is, it becomes easier to occlude and release charge carriers, and a high input / output density can be realized. On the other hand, if the value of the DEP oil absorption is too large, the viscosity at the time of kneading becomes too high, and the applicability and storage stability of the paste may be reduced (for example, it tends to settle). By setting it as the said range, a high energy density and a high output density can be implement | achieved.

  In the technique disclosed here, the resin component contains a thickener as an essential component. As the thickener, various polymer materials such as carboxymethylcellulose (CMC), methylcellulose, and ethylcellulose can be used, and among them, CMC (typically a sodium salt) can be preferably used. From the viewpoint of reducing the amount of thickener used, a relatively high molecular weight having a weight average molecular weight of about 100,000 to 1,000,000 is suitable. Moreover, the average particle diameter of the secondary particle of a thickener is 5-50 micrometers. In general, thickeners tend to agglomerate due to their molecular weight, and the secondary particle size tends to be relatively large. However, kneading can be carried out with a lower share compared to the prior art by using one having an average particle size within the above range. Further, the 1% viscosity of such a thickener, that is, the viscosity of a 1% aqueous solution in which the thickener is dissolved or dispersed in water at a ratio of 1% by mass is 1000 to 5000 mPa · s.

  In a preferred embodiment, the resin component further contains a binder in addition to the thickener. As the binder, various polymer materials such as styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVdF) can be used, and among them, SBR can be preferably used.

In the technique disclosed here, the ratio of the resin component in the entire negative electrode active material layer is 0.8 to 1.6 mass%. The resistance of the negative electrode active material layer can be reduced by keeping the ratio of the resin component low. In addition, the liquid retention of the negative electrode active material layer can be improved, and a high input / output density can be realized. In other words, the ratio of the negative electrode active material to the whole negative electrode active material layer is preferably 98.4 to 99.2% by mass. The mass of the negative electrode active material layer per unit area can be about 3 to ²⁰ mg / cm ² per one side of the negative electrode current collector. The density of the negative electrode active material layer can be about 1.1 to 1.6 g / cm ³ .

Such a negative electrode active material layer can be formed by applying and drying a negative electrode paste containing a negative electrode active material and a resin component on a negative electrode current collector. As the negative electrode current collector, a conductive material made of a metal having good conductivity (for example, copper) can be suitably used.
The negative electrode paste is prepared, for example, in steps (S10) to (S40) shown in the flowchart of FIG. 1, specifically (S10) grinding step; (S20) solid kneading step; (S30) dilution kneading step; S40) A finish kneading step can be suitably performed.

In the pulverization step (S10), first, the average particle diameter of the secondary particles of the thickener is pulverized and adjusted to 5 to 50 μm using a pulverizer. As the disperser, for example, a jet mill, a planetary mixer, a disper, or the like can be used. By setting the secondary particle diameter of the thickener within the above range, it is difficult for undissolved matter or aggregation to occur in the subsequent kneading step, and the thickener can be uniformly dissolved or dispersed in the kneaded product. Thereby, it is possible to suppress the occurrence of coating defects such as scales (pinholes) in the negative electrode active material layer.
Next, in the kneading and kneading step (S20), the negative active material and water are added to the pulverized thickener and kneaded (kneaded). For kneading, a planetary mixer, a roll mill or the like can be used. Moreover, what is necessary is just to let the kneading | mixing time be time until the said material disperse | distributes uniformly in water. The solid content ratio at the time of the kneading and kneading is a solid content ratio at which the torque is 50% or less (typically 10 to 50%, for example, 10 to 30%) when the solid content ratio that is the maximum torque is 100%. It is preferable that In other words, the kneading and kneading is preferably performed with a relatively low share as compared with the conventional case. Thereby, the negative electrode active material particles are hardly cracked or chipped, and a battery having further excellent durability can be realized.
Next, in the dilution kneading step (S30), an aqueous solvent is added to the kneaded product after the kneading and kneading to dilute, and further kneading. Thereby, the paste is prepared to have fluidity suitable for coating.
Next, in the final kneading step (S40), a binder is added to the kneaded material after the dilution kneading and further kneading. Thereby, the mechanical strength (shape retainability) of the negative electrode active material layer can be ensured, and more excellent durability can be realized.

The nonaqueous electrolyte secondary battery disclosed here may be the same as the conventional one except that it includes the negative electrode active material layer as described above.
As the positive electrode, a positive electrode active material can be used in which a positive electrode active material layer is formed by adhering a positive electrode active material as a paste together with a conductive material, a binder, or the like onto a positive electrode current collector. As the positive electrode current collector, a conductive member made of a metal having good conductivity (for example, aluminum) can be suitably employed. Examples of the positive electrode active material include lithium composite metal oxides such as layered and spinel (for example, LiNiO ₂ , LiCoO ₂ , LiFeO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ , LiNi _0.5 Mn _1.5 O ₄ , LiCrMnO ₄ , LiFePO _4, etc.) can be suitably employed. As the conductive material, a carbon material such as carbon black (for example, acetylene black or ketjen black) can be adopted. As the binder, various polymer materials such as polyvinylidene fluoride (PVdF) and polyethylene oxide (PEO) can be adopted.

  A separator is typically used for the insulating layer interposed between the positive electrode and the negative electrode. As the separator, a porous resin sheet made of a resin such as polyethylene (PE) or polypropylene (PP) can be suitably used. Note that in a battery using a solid electrolyte (lithium polymer battery), the electrolyte can also serve as a separator.

As the nonaqueous electrolyte, typically, a nonaqueous solvent containing a supporting salt is used. Alternatively, the liquid non-aqueous electrolyte may be added with a polymer to form a solid (typically a so-called gel). As the supporting salt, lithium salt, sodium salt, magnesium salt and the like can be used, and among them, lithium salts such as LiPF ₆ and LiBF ₄ can be preferably used. As the non-aqueous solvent, aprotic solvents such as carbonates, esters, ethers, nitriles, sulfones and lactones can be used. Of these, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) can be preferably used.

  The nonaqueous electrolyte secondary battery disclosed herein can achieve both high input / output density and durability at a high level. Therefore, it can be suitably used as a power source (drive power source) for a motor mounted on a vehicle such as a plug-in hybrid vehicle (PHV) or a hybrid vehicle (HV).

  Hereinafter, the battery disclosed herein will be described in more detail by taking a lithium ion battery as an embodiment as an example.

<Examination I: Examination of secondary particle diameter of thickener and torque ratio during kneading>
(Examples 1 to 10)
First, carboxymethylcellulose (CMC, weight average molecular weight 330,000) as a thickener was pulverized by a jet mill to prepare secondary particles having an average particle size of 50 μm and 20 μm, respectively. Next, amorphous coated graphite powder (C, average particle size of 20 μm) as a negative electrode active material and the above-mentioned pulverized CMC (Examples 1 to 5 having an average particle size of 50 μm), Examples 6 to 10 having an average particle size Were mixed together with ion-exchanged water so that the mass ratio was 98.4: 0.8, and each was kneaded and kneaded with a biaxial planetary kneader in a 5 L container. It was. The torque at the time of kneading is defined by a relative value (torque ratio) when the solid content ratio that is the maximum torque is 100, and here, five types of torque ratios of 15% to 100% are examined. That is, in Examples 1 to 5 (or Examples 6 to 10), only the torque rates are different from each other. The torque rate was adjusted by adjusting the amount of water to be added.
After adding water to this kneaded material and further diluting and kneading, the same amount of styrene butadiene rubber (SBR) as a binder is added to the above CMC and finishing kneading to obtain a negative electrode paste (composition ratio on a mass basis). However, C: CMC: SBR = 98.4: 0.8: 0.8) was prepared. This negative electrode paste was applied to a long copper foil (negative electrode current collector) having a thickness of about 10 μm with a basis weight of 10 mg / cm ² to form a negative electrode active material layer. This was dried and then pressed with a 3T press (Hosen) to produce a sheet-like negative electrode (negative electrode sheet) having an electrode density of 1.5 g / cm ³ .

Next, Li _1.00 Ni _0.33 Co _0.33 Mn _0.33 O ₂ powder as the positive electrode active material powder, acetylene black (AB) as the conductive material, and polyvinylidene fluoride (PVdF) as the binder in a mass ratio of 90: A paste was prepared by mixing with N-methylpyrrolidone (NMP) so as to be 5: 5. This paste was applied to a long aluminum foil (positive electrode current collector) having a thickness of about 15 μm to form a positive electrode active material layer. The obtained positive electrode was dried and pressed to produce a sheet-like positive electrode (positive electrode sheet).

Next, the positive electrode sheet and the negative electrode sheet prepared above are used as a separator (here, a three-layer structure in which a polypropylene (PP) layer is laminated on both sides of a polyethylene (PE) layer and having a thickness of 20 μm). And then stacked on top of each other. And the positive electrode terminal was joined to the edge part of the positive electrode electrical power collector of this laminated body (electrode body), and the negative electrode terminal was joined to the edge part of the negative electrode electrical power collector, respectively.
This electrode body was accommodated in a battery case made of laminate, and a non-aqueous electrolyte was injected. As the non-aqueous electrolyte, a mixed solvent containing ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) at a volume ratio of 3: 3: 4, and LiPF ₆ as an electrolyte are approximately. Those dissolved at a concentration of 1 mol / L were used. Then, the laminate was sealed by thermocompression bonding to construct a lithium ion battery.

(Comparative Examples 1-5)
In the above Examples, lithium ion batteries (Comparative Examples 1 to 5) were similarly constructed except that the thickener was used as it was without being pulverized during the preparation of the negative electrode paste. In Comparative Examples 1 to 5, only the torque rate during kneading and kneading was made different from each other by adjusting the amount of water to be added.

(Electrical characteristics evaluation)
The battery thus constructed was subjected to an initial charging process at a constant current of 1 C at 25 ° C. until the positive electrode was changed from 0 V (vs. Li / Li ⁺ ) to 4.1 V (vs. Li / Li ⁺ ). Next, this battery was discharged to 3 V with a constant current of 1 C, and the capacity at the time of discharge was defined as the initial capacity. The battery after the initial capacity measurement was transferred to a constant temperature bath of 60 ° C., and a durability test of 1000 cycles was performed in a voltage range of SOC 0% to 100% at a constant current of 2C. After the temperature of the battery after the test was lowered to 25 ° C., the battery capacity was measured by the same method as described above. Then, the capacity retention rate (%) was calculated by dividing the battery capacity after the cycle test by the initial capacity and multiplying by 100. The results are shown in FIG.

  As shown in FIG. 2, the capacity retention rate was able to be relatively improved as compared with the prior art by reducing the secondary particle diameter of the thickener in advance by pulverization. In particular, in the examples, when the torque rate during kneading and kneading was 50% or less, the capacity retention rate after 1000 cycles could be 85% or more. The reason for this is considered that cracking of the negative electrode active material was suppressed by reducing the torque rate, and side reactions such as decomposition of the nonaqueous electrolyte were reduced. In the comparative example, when the torque rate during kneading and kneading was less than 50%, many coating defects such as skeins (pinholes) were observed in the negative electrode active material layer. The cause of this is thought to be that the thickener did not melt or aggregated due to the low share during kneading and kneading.

<Examination II: Examination of DEP oil absorption amount and resin component content of negative electrode active material>
(Examples 11 to 15)
In the Example of Study I, amorphous coated carbon having a predetermined oil absorption amount was used as the negative electrode active material, and the composition ratio in the negative electrode paste was C: CMC: SBR = 99.2: 0.4: 0.4 (that is, resin Lithium ion batteries (Examples 11 to 15) were constructed in the same manner except that the total amount of components was 0.8% by mass) and the torque rate during kneading was fixed at 30%. In Examples 11 to 15, only the DEP oil absorption amount of the amorphous coated carbon is different from each other.
(Examples 16 and 17)
In the example of the examination II, except that the composition ratio in the negative electrode paste was C: CMC: SBR = 98.4: 0.8: 0.8 (that is, the total of the resin components was 1.6% by mass). Similarly, lithium ion batteries (Examples 11 to 15) were constructed. In Examples 16 and 17, only the DEP oil absorption of the amorphous coated carbon is different from each other.
(Comparative Examples 11-15)
In the example of the study II, the composition ratio in the negative electrode paste was C: CMC: SBR = 98: 1.0: 1.0 (that is, the total of the resin components was 2.0% by mass). Lithium ion batteries (Examples 11 to 15) were constructed. In Comparative Examples 11 to 15, only the DEP oil absorption amount of the amorphous coated carbon is different from each other.

(Electrical characteristics evaluation)
The capacity retention rate after 1000 cycles was measured as in Study I above. The result which concerns on Examples 11-15 and Comparative Examples 11-15 is shown in FIG. Moreover, the high rate pulse charging / discharging cycle test was done using the battery produced separately. Specifically, first, after the initial charging process as described above, the initial resistance was measured. Next, the battery voltage was adjusted to a SOC of 30%, charged for 10 seconds at a constant current of 10 C, and then discharged for 50 seconds at a constant current of 2 C, and a durability test of 30000 cycles was performed. About the battery after a test, battery resistance was measured similarly to the above. Then, the resistance increase rate (%) was calculated by dividing the battery resistance after the high-rate pulse charge / discharge cycle test by the initial resistance and multiplying by 100. The result which concerns on Examples 11-14, 16, 17 and Comparative Examples 11-15 is shown in FIG.

  As shown in FIGS. 3 and 4, the DEP oil absorption amount of the negative electrode active material is set to 35 to 50 mL / 100 g, and the ratio of the resin component in the negative electrode active material layer is reduced, thereby greatly improving the durability. I was able to. For example, by setting the content ratio of the resin component to 1.6% by mass or less (typically 0.8 to 1.6% by mass), a capacity maintenance rate of 91% or more is realized even after 1000 cycles of charge and discharge. In addition, the rate of increase in resistance after the high-rate pulse charge / discharge cycle test could be suppressed to 140% or less (particularly 130% or less).

  As described above, the nonaqueous electrolyte secondary battery (for example, lithium ion battery) according to the present invention can input and output a large current, and is excellent in a high-rate pulse charge / discharge cycle and durability. For this reason, it can be suitably used as a vehicle drive battery that requires extremely high performance for high-rate charge / discharge.

  As mentioned above, although the specific example of this invention was demonstrated in detail, these are only illustrations and do not limit a claim. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.

Claims (1)

A non-aqueous electrolyte secondary battery comprising a positive electrode having a positive electrode active material layer, a negative electrode having a negative electrode active material layer, and a non-aqueous electrolyte,
The negative electrode active material layer includes a negative electrode active material and a resin component including at least a thickener, and the content ratio of the resin component is 0.8 to 1.6% by mass of the entire negative electrode active material layer. ,
The negative electrode active material is made of a graphite material, and has a DEP oil absorption of 35 to 50 mL / 100 g.
The thickener has an average particle diameter of secondary particles of 5 to 50 μm, and a viscosity of 1% aqueous solution in which the thickener is dissolved or dispersed in water at a ratio of 1% by mass is 1000 to 5000 mPa · s. s, a non-aqueous electrolyte secondary battery.

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