JP2014007120A - Negative electrode for lithium secondary battery, manufacturing method thereof, and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To mainly provide: a negative electrode for a lithium secondary battery which allows the drastic improvement of charge and discharge characteristics in an early cycle; and a manufacturing method thereof.SOLUTION: A negative electrode comprises: a negative electrode mixture layer which has a negative-electrode active material including SiO(0.8≤X≤1.2) 1 coated with carbon, and graphite 2, an aqueous binder 3 including a dispersant and a binding agent, and a nonionic surfactant 4; and a negative electrode current collector with the negative electrode mixture layer formed on at least one side. It is desired that the ratio of the SiOto the total amount of the negative-electrode active material is 1-10 mass%.

Description

  The present invention relates to a negative electrode for a lithium secondary battery, a method for producing the same, and a lithium secondary battery. More specifically, the present invention relates to improvement of a negative electrode using a material containing silicon oxide and graphite as a negative electrode active material.

  In recent years, mobile information terminals such as mobile phones, notebook personal computers, and PDAs have been rapidly reduced in size and weight, and batteries as drive power sources are required to have higher capacities. A lithium secondary battery that performs charging / discharging by moving lithium ions between the positive and negative electrodes along with charging / discharging has a high energy density and high capacity. As widely used.

  The mobile information terminals tend to increase power consumption with enhancement of functions such as video playback function and game function. The lithium secondary battery, which is the driving power source, has a purpose of long-time playback and output improvement. Therefore, further increase in capacity and improvement in charge / discharge performance are strongly desired.

Here, in the lithium secondary battery, it is common to use lithium cobaltate as the positive electrode active material and graphite as the negative electrode active material, but it is difficult to further increase the capacity with these materials. . For this reason, development of an active material with a higher specific capacity is being promoted. For example, in the case of a negative electrode active material, development of an alloy-based material such as silicon is being actively promoted. When such a material is used, the specific capacity is much higher than that of graphite, but the volume expansion is large and there are problems to be solved in terms of safety. Therefore, at present, development of an oxide negative electrode with less volume expansion and higher safety is prioritized.
For example, a proposal has been made to increase the capacity of a battery by using a negative electrode active material in which a specific capacity is high and a volumetric expansion coefficient is smaller than that of a silicon alloy and graphite is mixed (Patent below). Reference 1).

On the other hand, proposals have been made to improve electrochemical characteristics such as cycle characteristics by adding a surfactant to an electrode.
For example, by increasing the binder concentration near the current collector using a surfactant and increasing the adhesion strength between the negative electrode current collector and the negative electrode active material layer, the cycle characteristics of a battery using graphite as the negative electrode active material can be improved. The proposal to improve is made (the following patent document 2).
Further, a proposal has been made to improve cycle characteristics by trapping HF generated in a battery by utilizing the function of a surfactant (Patent Document 3 below).
Further, it has been proposed to improve cycle characteristics by applying a treatment to the constituent parts of the battery with a surfactant (Patent Document 4 below).

JP2011-233245A JP 2012-38597 A JP 2001-118578 A Special table 2006-520082

  However, in the proposal described in Patent Document 1, since silicon oxide has poor conductivity, it is necessary to increase the conductivity by coating the particle surface with carbon by a method such as CVD. However, silicon oxide whose particle surface is carbon-coated has extremely high water repellency, and in a negative electrode using an aqueous binder, the aqueous binder is difficult to contact in the vicinity of the silicon oxide particles. For this reason, the binding force by the water-based binder does not sufficiently function with respect to the expansion and contraction of silicon oxide accompanying charging / discharging. As a result, there is a problem that the negative electrode active material particles made of silicon oxide are easily isolated in the negative electrode active material layer, and the capacity reduction is particularly large at the beginning of the cycle. That is, the merit of high capacity by adding silicon oxide to the negative electrode active material has a problem that it is lost at the beginning of the cycle. In order to solve such a problem, it is conceivable to use an organic solvent-based binder. However, there is a problem that not only the environmental load increases but also the battery manufacturing cost increases.

Moreover, the proposal of the said patent document 2 is described about the case where graphite is used independently as a negative electrode active material, and is not considered at all about the negative electrode which mixed silicon oxide and graphite.
Furthermore, it is considered that the proposal described in Patent Document 3 is based on the premise that an organic solvent-based binder is used as the binder in consideration of the description of the effects (the trap function of HF) and other descriptions.

  In addition, since the proposal described in Patent Document 4 is intended to improve the wettability with the electrolytic solution, it is sufficient to treat either the positive electrode or the negative electrode with a surfactant (that is, the negative electrode). In addition, the treatment with the surfactant is performed not after the electrode is produced but after any electrode is produced. Also, it is unclear whether the binder is an aqueous binder or an organic solvent binder.

The present invention relates to a negative electrode active material containing SiO _X (0.8 ≦ X ≦ 1.2) whose surface is carbon-coated and graphite, an aqueous binder composed of a dispersant and a binder, and a nonionic surface activity. And a negative electrode current collector having the negative electrode material mixture layer formed on at least one surface thereof.

  According to the present invention, there is an excellent effect that the charge / discharge characteristics at the beginning of the cycle can be dramatically improved.

It is an explanatory view showing a state of a SiO _X according to the embodiment of the present invention. It is an explanatory view showing a state of a SiO _X of the comparative example. It is explanatory drawing which shows typically the state before the charging / discharging cycle of the positive mix layer which concerns on this invention. It is explanatory drawing which shows typically the state after the charging / discharging cycle of the positive mix layer which concerns on this invention. It is explanatory drawing which shows typically the state before the charging / discharging cycle of the positive mix layer which concerns on a comparative example. It is explanatory drawing which shows typically the state after the charging / discharging cycle of the positive mix layer which concerns on a comparative example.

  Hereinafter, the present invention will be described in more detail based on the following embodiments, but the present invention is not limited to the following embodiments at all, and can be implemented with appropriate modifications within a scope not changing the gist thereof. It is a thing.

[Production of positive electrode]
First, NMP (N-methyl-2-pyrrolidone) as a dispersion medium, lithium cobaltate as a positive electrode active material, acetylene black (HS100 manufactured by Denki Kagaku Kogyo) as a conductive agent, and PVdF (polyfluoride as a positive electrode binder). Is added at a mass ratio of 95: 2.5: 2.5. K. The mixture was mixed (stirred) using Hibismix to prepare a positive electrode mixture slurry. Next, this positive electrode mixture slurry is applied to both sides of a positive electrode current collector made of aluminum foil, dried and rolled to form a positive electrode mixture layer on both sides of the positive electrode current collector, and further, a positive electrode tab A positive electrode was prepared by attaching The packing density in the positive electrode mixture layer was 3.6 g / cc.

[Production of negative electrode]
First, carbon was coated on the surface of SiO _X (X = 0.93) as a negative electrode active material so that the ratio to SiO _X was 10 mass%. The coating was performed by the CVD method. Next, the carbon-coated SiO _X and an aqueous solution containing CMC (# 1380 manufactured by Daicel FineChem, Inc., etherification degree: 1.0 to 1.5) as a dispersant are mixed (stirred). did. At this time, a nonionic surfactant (trade name: SN wet 980, a polyether surfactant manufactured by San Nopco) was added as a surfactant. After thoroughly mixing these, graphite as a negative electrode active material was added and mixed, and the viscosity was adjusted by further adding water. Thereafter, SBR as a binder was added and mixed to prepare a negative electrode mixture slurry. Finally, this negative electrode mixture slurry was applied to both surfaces of a negative electrode current collector made of copper foil, dried at 105 ° C. in the atmosphere, and then rolled to form a negative electrode mixture layer (filled on both surfaces of the negative electrode current collector). The density was 1.60 g / cc), and a negative electrode tab was attached to prepare a negative electrode.

In addition, the ratio of SiO _X with respect to the total amount of the negative electrode active material is 5% by mass (SiO _X and graphite are in a mass ratio of 5:95), and the negative electrode active material, CMC, and SBR are in a mass ratio. The ratio is 97.5: 1.0: 1.5. Furthermore, the ratio of the nonionic surfactant with respect to the total amount of solid content (consisting of the negative electrode active material, CMC, and SBR) was 1.0 mass%. Moreover, as a mixer (stirrer) at the time of preparing a negative electrode mixture slurry, T.M. K. Hibismix was used. CMC and SBR constitute an aqueous binder.

[Preparation of non-aqueous electrolyte]
A non-aqueous electrolyte was prepared by dissolving 1 mol / liter of lithium hexafluorophosphate (LiPF ₆ ) in a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7. .

[Battery assembly]
The positive electrode, the negative electrode, and a polyethylene separator disposed between the positive and negative electrodes were spirally wound to produce an electrode body. At this time, both the positive electrode tab and the negative electrode tab were arranged so as to be located on the outermost peripheral portion of each electrode. Next, the electrode body was placed in a battery outer package made of an aluminum laminate and vacuum-dried at 105 ° C. for 2 hours. Finally, after pouring the non-aqueous electrolyte into the battery outer package, a battery was fabricated by sealing the opening of the battery outer package. The design capacity of the battery is 20 mAh.

Example 1
A negative electrode and a battery were produced in the same manner as in the embodiment for carrying out the invention.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a1 and battery A1, respectively.

(Example 2)
A negative electrode and a battery were prepared in the same manner as in Example 1 except that a nonionic surfactant (trade name: Noptex ED-052, a polyether surfactant manufactured by San Nopco) was used as the surfactant. did.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a2 and battery A2, respectively.

(Example 3)
A negative electrode and a battery were prepared in the same manner as in Example 1 except that a nonionic surfactant (trade name: Noptex ED-090, a polyether surfactant manufactured by San Nopco) was used as the surfactant. did.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a3 and battery A3, respectively.

Example 4
As the surfactant, a negative electrode and a non-ionic surfactant (trade name: NAROACTY CL-70, polyether surfactant manufactured by Sanyo Chemical Industries, Ltd.) were used in the same manner as in Example 1 above. A battery was produced.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a4 and battery A4, respectively.

(Example 5)
A negative electrode and a battery were produced in the same manner as in Example 1 except that the ratio of the nonionic surfactant to the total amount of the solid content was 0.1% by mass.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a5 and battery A5, respectively.

(Example 6)
The negative electrode and the battery were manufactured in the same manner as in Example 1 except that the ratio of SiO _{X to} the total amount of the negative electrode active material was 1% by mass (SiO _X and graphite were in a mass ratio of 1:99). Produced.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a6 and battery A6, respectively.

(Example 7)
The negative electrode and the battery were fabricated in the same manner as in Example 1 except that the ratio of SiO _{X to} the total amount of the negative electrode active material was 10% by mass (SiO _X and graphite were 10:90 by mass ratio). Produced.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a7 and battery A7, respectively.

(Example 8)
In mixing SiO _X , an aqueous solution containing CMC, and a nonionic surfactant, graphite is included (that is, an aqueous solution containing SiO _X and CMC as in Example 1). After mixing nonionic surfactant, graphite is not added and mixed, but SiO _X , graphite, an aqueous solution containing CMC, and nonionic surfactant are mixed at the same time. The negative electrode and the battery were produced in the same manner as in Example 1 except that.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a8 and battery A8, respectively.

Example 9
A negative electrode and a battery were produced in the same manner as in Example 8 except that the ratio of the nonionic surfactant to the total amount of the solid content was 0.1% by mass.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode a9 and battery A9, respectively.

(Comparative Example 1)
A negative electrode and a battery were produced in the same manner as in Example 1 except that the nonionic surfactant was not added when preparing the negative electrode mixture slurry.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode z1 and battery Z1, respectively.

(Comparative Example 2)
The negative electrode and the battery were fabricated in the same manner as in Comparative Example 1 except that the ratio of SiO _{X to} the total amount of the negative electrode active material was 1% by mass (SiO _X and graphite were in a mass ratio of 1:99). Produced.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode z2 and battery Z2, respectively.

(Comparative Example 3)
The negative electrode and the battery were manufactured in the same manner as in Comparative Example 1 except that the ratio of SiO _{X to} the total amount of the negative electrode active material was 10% by mass (SiO _X and graphite were 10:90 in mass ratio). Produced.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode z3 and battery Z3, respectively.

(Comparative Example 4)
The negative electrode and the battery were fabricated in the same manner as in Comparative Example 1 except that the ratio of SiO _{X to} the total amount of the negative electrode active material was 20% by mass (SiO _X and graphite were 20:80 in mass ratio). Produced.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode z4 and battery Z4, respectively.

(Reference Example 1)
Only graphite was used as the negative electrode active material (specifically, graphite and an aqueous solution containing CMC were mixed at the same time, and then SBR as a binder was added and mixed to obtain a negative electrode mixture slurry. A negative electrode and a battery were produced in the same manner as in Comparative Example 1 except that the above was prepared.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode r1 and battery R1, respectively.

(Reference Example 2)
Only graphite was used as the negative electrode active material (specifically, graphite, an aqueous solution containing CMC, and a nonionic surfactant were mixed at the same time, and then SBR as a binder was added and mixed. Thus, a negative electrode and a battery were produced in the same manner as in Example 1 except that a negative electrode mixture slurry was prepared.
The negative electrode and battery thus produced are hereinafter referred to as negative electrode r2 and battery R2, respectively.

(Experiment 1)
Since the adhesion strength of the electrode plates in the negative electrodes a1 to a9, z1 to z4, r1, and r2 was examined, the results are shown in Table 1. The experiment was performed as follows.
A negative electrode of 100 mm × 25 mm size is pasted on a 120 mm × 30 mm acrylic plate using a 70 mm × 20 mm double-sided tape (Nichiban NW-20 manufactured by Nichiban Co., Ltd.), and the end of the pasted negative electrode is The film was pulled 50 mm at a constant speed (100 mm / min) (perpendicular to the electrode bonding surface), and the strength during peeling was measured. As an experimental apparatus, a small tabletop testing machine (trade names: FGS-TV and FGP-5) manufactured by Nidec Sympo Corporation was used. Each battery was measured three times, and the average value is shown in Table 1.

(Experiment 2)
Since the batteries A1 to A9, Z1 to Z4, R1, and R2 were charged and discharged up to 30 cycles under the following charge and discharge conditions, the capacity maintenance rate at the 10th cycle and the capacity maintenance rate at the 30th cycle of each battery were examined. The results are shown in Table 1.

[Charging / discharging conditions]
After constant current charging up to 4.4 V with a current of 1.0 It (20 mA), constant voltage charging was performed until the current became (1/20) It (1 mA) at a constant voltage of 4.4 V. After resting for 10 minutes, constant current charging was performed up to 2.75 V with a current of 1.0 It (20 mA).

(Experiment 3)
After charging / discharging the batteries A1 to A9, Z1 to Z4, R1, and R2 up to 30 cycles under the above charging / discharging conditions, each battery is disassembled and the negative electrode is peeled off (a part of the negative electrode mixture layer is peeled off) The results are shown in Table 1.

(About the result of Experiment 1)
Compare the negative electrodes a1 to a5, a8, a9, and z1 in which the proportions of SiO _X are all 5% by mass. As a result, it was confirmed that the negative electrode z1 to which the nonionic surfactant was not added had higher adhesion strength than the negative electrodes a1 to a5, a8, and a9 to which the nonionic surfactant was added. It was. Further, when comparing the negative electrodes a6 and z2 whose SiO _X ratios are all 1% by mass, the negative electrode z2 to which the nonionic surfactant is not added is compared with the negative electrode a6 to which the nonionic surfactant is added. It was confirmed that the adhesion strength was increased. Further, when comparing the negative electrodes a7 and z3 whose SiO _X ratios are all 10% by mass, the negative electrode z3 to which the nonionic surfactant is not added is compared with the negative electrode a7 to which the nonionic surfactant is added. It was confirmed that the adhesion strength was increased.

Such a result is considered to be due to the following reasons. Since no nonionic surfactant is added to the negative electrodes z1 to z3, as shown in FIG. 2, the water-based binder does not adhere so much around the SiO _X 1. Therefore, the amount of the aqueous binder in the vicinity of the negative electrode current collector increases accordingly. On the other hand, since the nonionic surfactant is added to the negative electrodes a1 to a9, the nonionic surfactant 4 is adsorbed around the SiO _X 1 as shown in FIG. For this reason, the hydrophilicity of SiO _X 1 increases, and the water-based binder 3 adheres around the SiO _X 1. As a result, the amount of the aqueous binder in the vicinity of the negative electrode current collector is reduced by the amount of the aqueous binder 3 adhering to the periphery of the SiO _X 1.

Also, the negative electrode a1 and the negative electrode a8 are different in only the substances present in the aqueous solution when the ratio of SiO _X and the kind and ratio of the nonionic surfactant are all the same, and when the nonionic surfactant is added and mixed. As a result, it was confirmed that the adhesion strength of the negative electrode a1 was smaller than that of the negative electrode a8. Such a result is considered to be due to the following reasons. In the negative electrode a1, adding a non-ionic surfactant, only SiO _X and CMC are present in mixing, a non-ionic surfactant only on the surface of the SiO _X is adsorbed, thereby, the surface of the SiO _X There is a desired amount of CMC and a predetermined amount of SBR added in a subsequent step (ie, a desired amount of aqueous binder is present on the surface of SiO _X ). On the other hand, in the negative electrode a8, when adding and mixing the nonionic surfactant, not only SiO _X and CMC but also graphite exists, so the nonionic surfactant is not only the surface of SiO _X , It is also adsorbed on the surface of graphite. Therefore, it is considered that the amount of nonionic surfactant present on the surface of SiO _X is reduced as compared with the case of the negative electrode a1, and the aqueous binder existing on the surface of SiO _X is also reduced. For the same reason, it was recognized that the adhesion strength of the negative electrode a5 was smaller than that of the negative electrode a9.

From the above, in order to fully exert the effect of the present invention with a small amount of addition, the substances present in the aqueous solution when adding and mixing the nonionic surfactant are only SiO _X and CMC, and thereafter It can be seen that it is preferable to add graphite.

Here, when forming the anode a1, after mixing the non-ionic surfactant and a SiO _X and CMC, added graphite, in the manufacturing method of mixing, because the external force is applied during mixing of the graphite, the surface of the SiO _X It is also considered that the nonionic surfactant once adsorbed on the surface of the SiO 2 peels off from the SiO _X and the peeled nonionic surfactant is adsorbed on the surface of the graphite. However, since the adsorption power of the nonionic surfactant once adsorbed on the surface of SiO _X is large (that is, it is difficult to peel off from the SiO _X ), the amount of nonionic surfactant adsorbed on the surface of graphite is very small. it is conceivable that. Therefore, when the negative electrode a1 is manufactured by the manufacturing method, a desired amount of an aqueous binder is present on the surface of SiO _X.

Also, the ratio of SiO _X , the type of nonionic surfactant, and the addition of nonionic surfactant, the substances present in the aqueous solution when mixing are all the same, only the ratio of nonionic surfactant When comparing the negative electrode a1 and the negative electrode a5, the adhesion strengths of the negative electrode a1 and the negative electrode a5 were found to be substantially the same.
From this, it is considered that the effect of the present invention can be manifested if the addition amount is higher than the critical micelle concentration. Therefore, the lower limit of the ratio of the nonionic surfactant to the total amount of the negative electrode mixture layer may be 0.1% by mass or more. On the other hand, the upper limit of the ratio is preferably 2.0% by mass or less (particularly 1.0% by mass or less). If this value is exceeded, the amount of the negative electrode active material decreases, and thus the negative electrode capacity decreases.

However, in the comparison of the negative electrode a1, a6, a7, since the adhesion strength as the ratio of the SiO _X increases increases, when the proportion of SiO _X is to generous addition of non-ionic surfactants preferable. Further, in the comparison between the negative electrode a1 and the negative electrode a8, when not only SiO _X but also graphite is present in the aqueous solution when adding and mixing the nonionic surfactant, the adhesion strength is increased. When producing by this method, it is preferable to add a large amount of a nonionic surfactant.

  In addition, the measurement of the adhesion strength described above was performed to examine where the aqueous binder is located, and is not directly related to the battery performance unless the strength is extremely small. Specifically, in all the batteries used in the above experiments, since the adhesion strength exceeds 100 mN, there is no practical inconvenience due to the adhesion strength.

(About the result of Experiment 2)
The batteries A1 to A5, A8, A9, and Z1 in which the ratio of SiO _X is 5% by mass are all compared. As a result, the batteries A1 to A5, A8, and A9 in which the nonionic surfactant is added to the negative electrode have improved cycle characteristics compared to the battery Z1 in which the nonionic surfactant is not added to the negative electrode. (The capacity retention rate at 10 cycles and 30 cycles is high. That is, the battery characteristics at the beginning of the cycle are improved). Further, when comparing the batteries A6 and Z2 whose SiO _X ratios are all 1% by mass, the battery A6 in which the nonionic surfactant is added to the negative electrode is a battery in which the nonionic surfactant is not added to the negative electrode. It was confirmed that the cycle characteristics were improved as compared with Z2. Further, when comparing the batteries A7 and Z3, in which the proportions of SiO _X are all 10% by mass, the battery A7 in which the nonionic surfactant is added to the negative electrode is the battery in which the nonionic surfactant is not added to the negative electrode. It was confirmed that the cycle characteristics were improved as compared with Z3.

Such a result is considered to be due to the following reasons. In the negative electrodes z1 to z3 of the batteries Z1 to Z3, as shown in FIG. 2, the water-based binder does not adhere to the periphery of the SiO _X 1 so much. On the other hand, in the negative electrodes a1 to a9 of the batteries A1 to A9, the nonionic surfactant 4 is adsorbed around the SiO _X 1 as shown in FIG. The water-based binder 3 adheres around the SiO _X 1.

Therefore, the negative electrode z1~z3 batteries Z1 to Z3, the charge and discharge cycles ago (see FIG. 5) are present in a state SiO _X 1 is dispersed in the negative electrode mixture layer, the SiO _X 1 during charge and discharge When it expands and contracts, the interparticle contact cannot be maintained by the binding force of the aqueous binder 3. For this reason, after the charge / discharge cycle, SiO _X 1 exists in an isolated state in the negative electrode mixture layer (see FIG. 6), and SiO _X 1 does not function as the negative electrode active material, so that the cycle characteristics are deteriorated. .
On the other hand, in the negative electrodes a1 to a9 of the batteries A1 to A9, not only the SiO _X 1 is dispersed before the charge / discharge cycle (see FIG. 3), but the SiO _X 1 expands and contracts during charge / discharge. Even so, the interparticle contact can be maintained by the binding force of the aqueous binder 3. For this reason, even after the charge / discharge cycle, SiO _X 1 exists in a dispersed state in the negative electrode mixture layer (see FIG. 4), and SiO _X 1 functions as a negative electrode active material, so that the cycle characteristics are improved.

Here, a cycle characteristic test was not performed on the battery in which the proportion of SiO _X was 20% by mass and the nonionic surfactant was added. However, the cycle characteristics of the battery A7 in which the proportion of SiO _X is 10% by mass and the nonionic surfactant is added are that the proportion of SiO _X is 1% by mass or 5% by mass, and the nonionic surfactant is Considering that the cycle characteristics of the batteries A6 and A1 added are greatly reduced, the cycle characteristics of the battery with the non-ionic surfactant added with the proportion of SiO _x being 20% by mass is It is estimated that it will be considerably worse. Therefore, the ratio of SiO _{X to} the total amount of the negative electrode active material is preferably 1% by mass or more and 10% by mass or less.

Further, the battery A1 and the battery A8 are different in only the substances present in the aqueous solution when the ratio of SiO _X , the kind and the ratio of the nonionic surfactant are all the same, and the nonionic surfactant is added and mixed As a result, it was confirmed that the battery A1 had improved cycle characteristics compared to the battery A8. Such a result is considered to be due to the following reasons. In the negative electrode a1 of the battery A1, since a desired amount of an aqueous binder is present on the surface of SiO _X as described above, it is possible to suppress the isolation of SiO _X. Therefore, SiO _X functions as a negative electrode active material. On the other hand, in the negative electrode a8 of the battery A8, as described above, the amount of the water-based binder present on the surface of SiO _X decreases, so that SiO _X may be isolated and not function as the negative electrode active material. .
From the above, in order to sufficiently exert the effect of the present invention with a small amount of addition, the substances present in the aqueous solution when adding and mixing the nonionic surfactant are only SiO _X and CMC, and thereafter It is preferable to add graphite.

Also, the ratio of SiO _X , the type of nonionic surfactant, and the addition of nonionic surfactant, the substances present in the aqueous solution when mixing are all the same, only the ratio of nonionic surfactant When the battery A1 and the negative electrode A5 having different values were compared, it was found that the cycle characteristics of the battery A1 and the negative electrode A5 were substantially equivalent. For this reason, the lower limit of the ratio of the nonionic surfactant to the total amount of the negative electrode mixture layer may be 0.1% by mass or more for the same reason as described in Experiment 1. For the same reason as described in Experiment 1, the upper limit of the ratio is preferably 2.0% by mass or less (particularly 1.0% by mass or less).

However, in the comparison of the battery A1, A6, A7 as described above, since the cycle characteristics as the ratio of the SiO _X increases is reduced, when the proportion of SiO _X is a nonionic surfactant generous added It is preferable to do this. In addition, in the comparison between the battery A1 and the battery A8, when not only SiO _X but also graphite is present in the aqueous solution when adding and mixing the nonionic surfactant, the cycle characteristics deteriorate, When producing by this method, it is preferable to add a large amount of a nonionic surfactant.

From the above results, in the negative electrode using both carbon-coated SiO _X and graphite as a negative electrode active material and using a water-based binder as a binder, the hydrophilicity of the SiO _X particles is improved to improve the inter-particle size of the water-based binder. It was found that the binding force can be increased and the cycle characteristics can be improved. Moreover, since a water-based binder is used, it is excellent in terms of environment and cost can be reduced as compared with the case where a solvent-based binder is used.

In addition, when comparing the batteries R1 and R2 to which SiO _X as the negative electrode active material is not added, it can be seen that the cycle characteristics do not change depending on whether or not a nonionic surfactant is added. Therefore, the effect of the present invention is found to be peculiar effect that occurs only in the case of adding SiO _X as a negative electrode active material. This is different from the invention described in Patent Document 2. The following points are also different from the invention described in Patent Document 2.

(1) Although the degree of etherification of CMC used in the present invention is 1.0 to 1.5, in the invention described in Patent Document 2, the degree of etherification of CMC is regulated to 0.8 or less.
(2) Even if it is the nonionic surfactant (brand name: NAROACTY CL-70, manufactured by Sanyo Chemical Industries, Ltd.) used in Comparative Example 1 of Patent Document 2, the effect of the present invention is exhibited. (See Example 4).
(3) In Patent Document 2, the adhesion strength is improved when a nonionic surfactant is added, whereas in the present invention, the adhesion strength is reduced when a nonionic surfactant is added. .
From the above, it can be considered that the present invention and the invention described in Patent Document 2 have completely different operational effects.

(About the results of Experiment 3)
Regardless of the presence or absence of the addition of a nonionic surfactant, the negative electrode peeling did not occur in the batteries A1 to A6, A8, A9, Z1, and Z2 in which the proportion of SiO _X was 1% by mass and 5% by mass, In the batteries A7 and Z3 having a SiO _X ratio of 10% by mass, the negative electrode peels slightly. However, in the battery Z4 in which the ratio of SiO _X is 20% by mass, the negative electrode is peeled off. Thus from the viewpoint of suppressing the negative electrode peeling, the proportion of SiO _X is the total amount of the negative electrode active material is preferably not more than 10 mass% to 1 mass%.

(Other matters)
(1) In the above examples, CMC was used as a dispersant and SBR was used as a binder. However, the present invention is not limited to this, and polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), and the like are not limited thereto. A mixture can be used, and an acrylic resin can be used as a binder. Further, as the nonionic surfactant, polyester surfactants and the like can be used in addition to those described above.
(2) In the above embodiment, _X of SiO _X is set to 0.93. However, the present invention is not limited to this, and X may be in the range of 0.8 ≦ X ≦ 1.2.

(3) The positive electrode active material is not limited to the lithium cobalt oxide, but is Ni—Co—Mn lithium composite oxide, Ni—Mn—Al lithium composite oxide, Ni—Co—Al lithium composite. A lithium composite oxide containing nickel or manganese such as an oxide, or an olivine type lithium phosphate represented by lithium iron phosphate (LiFePO ₄ ) may be used. Moreover, they may be used alone or in combination.

(4) The lithium salt of the non-aqueous electrolyte is not limited to LiPF ₆ described above, but LiBF ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiPF _{6− X (C n F 2n + 1} ) X [ where, 1 <X <6, n = 1 or 2] and the like, can be used as a mixture of one or more of these. The concentration of the supporting salt is not particularly limited, but is preferably 0.8 to 1.8 mol per liter of the electrolyte. The solvent species are preferably carbonate solvents such as EC, FEC, PC, GBL, DEC, EMC, DMC, and more preferably a combination of a cyclic carbonate and a chain carbonate. Further, when FEC is contained, a film is formed on the surface of the silicon oxide, so that the electrolytic solution on the surface of the silicon oxide can be prevented from being decomposed. Therefore, it is preferable that the non-aqueous electrolyte contains FEC.

  The present invention can be applied to a drive power source of a mobile information terminal such as a mobile phone, a notebook personal computer, and a PDA, for example, in applications that require a particularly high capacity. In addition, it can be expected to be used in high output applications that require continuous driving at high temperatures and applications where the battery operating environment is severe, such as HEVs and electric tools.

1: SiO _X
2: Graphite 3: Aqueous binder 4: Nonionic surfactant

Claims (6)

A negative electrode active material containing SiO _X (0.8 ≦ X ≦ 1.2) and graphite coated with carbon on the surface, an aqueous binder composed of a dispersant and a binder, and a nonionic surfactant A negative electrode mixture layer;
A negative electrode current collector in which the negative electrode mixture layer is formed on at least one surface;
A negative electrode for a lithium secondary battery, comprising: 2. The negative electrode for a lithium secondary battery according to claim 1, wherein a ratio of the SiO _{X to} the total amount of the negative electrode active material is 1% by mass or more and 10% by mass or less.   3. The lithium secondary battery according to claim 1, wherein a ratio of the nonionic surfactant to a total amount of the negative electrode active material and the aqueous binder is 0.1% by mass or more and 2.0% by mass or less. Negative electrode.   The negative electrode for a lithium secondary battery according to any one of claims 1 to 3, wherein the nonionic surfactant includes a polyether surfactant. Mixing a carbon-coated SiO _X (0.8 ≦ X ≦ 1.2) surface, a dispersant, and a nonionic surfactant in an aqueous solution;
After adding and mixing graphite in the aqueous solution, preparing a negative electrode mixture slurry by adding and mixing a binder;
Applying the negative electrode mixture slurry to at least one surface of the negative electrode current collector to form a negative electrode mixture layer on at least one surface of the negative electrode current collector;
A method for producing a negative electrode for a lithium secondary battery. The negative electrode according to any one of claims 1 to 4, and
A positive electrode having a positive electrode mixture layer formed on at least one surface of the positive electrode current collector;
A separator disposed between the positive electrode and the negative electrode;
A non-aqueous electrolyte,
A lithium secondary battery comprising:

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