JP2014049288A - Negative electrode for nonaqueous electrolyte secondary battery, manufacturing method therefor and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery in which cycle characteristics can be enhanced by minimizing degradation in collection performance of SiOeven in the end of discharge.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a negative electrode mixture layer including a negative electrode active material containing SiO(0.8≤x≤1.2) and graphite, a dispersant, and a binder; and a negative electrode collector having the negative electrode mixture layer formed on at least one surface. Dispersant coverage factor Rof SiOshown in equation (1), and dispersant coverage factor Rof graphite shown in equation (2) satisfy a relation R>R. Dispersant coverage factor Rof SiO=(mass of dispersant covering SiO)/(mass of SiO)...(1), Dispersant coverage factor Rof graphite=(mass of dispersant covering graphite)/(mass of graphite)...(2).

Description

  The present invention relates to a negative electrode for a nonaqueous electrolyte secondary battery, a method for producing the same, and a nonaqueous electrolyte secondary battery.

In recent years, mobile information terminals such as mobile phones, notebook computers, and smart phones have been rapidly reduced in size and weight, and batteries as driving power sources are required to have higher capacities. A non-aqueous electrolyte secondary battery that performs charge / discharge by moving lithium ions between the positive and negative electrodes along with charge / discharge has a high energy density and a high capacity. Widely used as a drive power source.
The mobile information terminal has a tendency to further increase power consumption with enhancement of functions such as a video playback function and a game function, and the non-aqueous electrolyte secondary battery, which is a driving power source thereof, can be played back for a long time or improved in output. As an object, further increase in capacity and improvement in charge / discharge performance are strongly desired.

  Here, in the non-aqueous electrolyte 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. It is. For this reason, in the negative electrode, a proposal has been made to increase the capacity of the battery by using a negative electrode active material obtained by mixing silicon oxide having a higher specific capacity than graphite and graphite (Patent Document 1 below).

JP2011-233245A

  However, when silicon oxide and graphite are mixed as proposed in the above-mentioned Patent Document 1, silicon oxide expands greatly during charging as compared with graphite, but greatly contracts during discharging. Accordingly, the silicon oxide is easily isolated at the end of discharge, and the current collection performance of the silicon oxide is reduced. As a result, the cycle characteristics are deteriorated.

The nonaqueous electrolyte secondary battery of the present invention includes a negative electrode active material containing SiO _x (0.8 ≦ x ≦ 1.2) and graphite, a dispersant, and a negative electrode mixture layer including a binder, A negative electrode current collector having the negative electrode mixture layer formed on one surface thereof, and a SiO _X dispersant coating ratio R _SiO expressed by the following formula (1), and expressed by the following formula (2): The relation with the dispersant coating ratio R _{C of} graphite satisfies R _SiO > _RC .
Dispersants coverage _R SiO of SiO _X = [dispersants mass covering the SiO _X] / [SiO _X mass] (1)
Graphite dispersant coverage R _C = [dispersant mass covering graphite] / [graphite mass] (2)

According to the present invention, there is an excellent effect that it is possible to suppress a decrease in the current collecting property of SiO _X even at the end of discharge.

  The nonaqueous electrolyte secondary battery and the like according to the present invention will be described below. In addition, the nonaqueous electrolyte secondary battery in this invention is not limited to what was shown to the following form, In the range which does not change the summary, it can change suitably and can implement.

[Production of positive electrode]
Lithium cobaltate as a positive electrode active material, acetylene black (HS100 manufactured by Denki Kagaku Kogyo) as a conductive agent, polyvinylidene fluoride (PVdF) as a binder, and N-methyl-2-pyrrolidone (as a dispersion medium) NMP) was added so that the mass ratio of the positive electrode active material, the conductive agent and the binder was 95.0: 2.5: 2.5, and then kneaded to prepare a positive electrode slurry. Next, the positive electrode slurry is applied to both surfaces of a positive electrode current collector made of aluminum foil, dried, rolled by a rolling roller, and a positive electrode current collector tab is attached to the positive electrode current collector on both surfaces. A positive electrode having a layer formed thereon was produced. The filling density of the positive electrode mixture layer was 3.60 g / cm ³ .

[Production of negative electrode]
First, carbon was coated on the surface of SiO _X (X = 0.93, average particle diameter of 5 μm) as the negative electrode active material so that the ratio to SiO _X was 10% by mass. The coating was performed by the CVD method. Next, SiO _X coated with the carbon, CMC as a dispersant (# 1380 manufactured by Daicel FineChem, Inc., etherification degree: 1.0 to 1.5), and SBR as a binder, The mixture was stirred in an aqueous solution so that the mass ratio of SiO _X , CMC, and SBR was 96: 3: 1 to prepare a SiO _X slurry. In addition, stirring is performed by T. K. Hibismix was used.

In parallel with this, graphite as a negative electrode active material, CMC, and SBR were stirred in an aqueous solution so as to have a mass ratio of 98: 1: 1 to prepare a graphite slurry. In addition, stirring is performed by T. K. Hibismix was used.
Next, the SiO _X slurry and the hourly graphite slurry were mixed so that the mass ratio of the SiO _X and the graphite was 10:90 to prepare a negative electrode slurry. Next, this negative electrode slurry is applied to both surfaces of a negative electrode current collector made of copper foil, further dried in the atmosphere at 105 ° C., rolled, and attached to both surfaces of the negative electrode current collector by attaching negative electrode current collector tabs. A negative electrode on which a negative electrode mixture layer was formed was produced. The packing density of the negative electrode mixture layer was 1.60 g / cc.

Here, the dispersion ratio R _SiO of SiO _X shown in the following formula (1) is 0.03, and the dispersion coverage _RC of graphite shown in the following formula (2) is 0.01, The dispersing agent ratio R _{(SiO + C)} shown in the following formula (3) was 0.012.
Dispersants coverage _R SiO of SiO _X = [dispersants mass covering the SiO _X] / [SiO _X mass] (1)
Graphite dispersant coverage R _C = [dispersant mass covering graphite] / [graphite mass] (2)
Dispersant ratio R _{(SiO + C) of the} whole negative electrode = [dispersant mass covering SiO _X + dispersant mass covering graphite] / [SiO _X mass + graphite mass] (3)

[Preparation of non-aqueous electrolyte]
To a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7, lithium hexafluorophosphate (LiPF ₆ ) has a concentration of 1.0 mol / liter. To prepare a non-aqueous electrolyte.

[Production of battery]
After the positive electrode and the negative electrode are opposed to each other through a separator made of a polyethylene microporous film, the positive electrode tab and the negative electrode tab are wound in a spiral shape so that both the positive electrode tab and the negative electrode tab are located at the outermost periphery in each electrode. The body was made. Next, this electrode body was placed in an aluminum laminate as a battery outer package 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. When the nonaqueous electrolyte secondary battery was charged to 4.40 V and discharged to 2.75 V, the discharge capacity was 800 mAh.

Example 1
A battery was produced in the same manner as in the embodiment for carrying out the invention.
The battery thus produced is hereinafter referred to as battery A1.

(Example 2)
A battery was fabricated in the same manner as in Example 1 except that when preparing the SiO _X slurry, the mass ratio of SiO _X , CMC, and SBR was 94: 5: 1.
The dispersion ratio R _SiO of SiO _X shown in the above formula (1) is 0.05, and the dispersion coverage R _C of graphite shown in the above formula (2) is 0.01. The dispersant ratio R _{(SiO + C)} of the whole negative electrode shown in the formula (3) was 0.014.
The battery thus produced is hereinafter referred to as battery A2.

(Example 3)
A battery was fabricated in the same manner as in Example 1 except that when preparing the SiO _X slurry, the mass ratio of SiO _X , CMC, and SBR was 89: 10: 1.
The dispersion ratio R _SiO of SiO _X shown in the above formula (1) is 0.1, and the dispersion coverage R _C of graphite shown in the above formula (2) is 0.01. The dispersant ratio R _{(SiO + C)} of the whole negative electrode shown in the formula (3) was 0.019.
The battery thus produced is hereinafter referred to as battery A3.

(Comparative Example 1)
A battery was fabricated in the same manner as in Example 1 except that when preparing the SiO _X slurry, the mass ratio of SiO _X , CMC, and SBR was 98: 1: 1.
The dispersion ratio R _SiO of SiO _X shown in the above formula (1) is 0.01, and the dispersion coverage _RC of graphite shown in the above formula (2) is 0.01. The dispersing agent ratio R _{(SiO + C)} of the whole negative electrode shown in the formula (3) was 0.01.
The battery thus produced is hereinafter referred to as battery Z1.

(Comparative Example 2)
When preparing the SiO _X slurry, the mass ratio of SiO _X , CMC, and SBR is 98: 1.4: 1. When preparing the graphite slurry, the mass ratio of graphite, CMC, and SBR is 98: 1.4. : A battery was fabricated in the same manner as in Example 1 except that the ratio was 1.4.
The dispersion ratio R _SiO of SiO _X shown in the above formula (1) is 0.014, and the dispersion coverage _RC of graphite shown in the above formula (2) is 0.014. The dispersant ratio R _{(SiO + C)} of the whole negative electrode shown in the formula (3) was 0.014.
The battery thus produced is hereinafter referred to as battery Z2.

(Experiment)
The batteries A1 to A3, Z1, and Z2 were charged and discharged under the conditions shown below, and the cycle characteristics (capacity maintenance ratio after 30 cycles) were examined. The results are shown in Table 1.

-Charging / discharging conditions After charging with constant current until the battery voltage reaches 4.4 V at 1.0 It (800 mA) current, the current value becomes (1/20) It (40 mA) at a constant voltage of 4.4 V. The battery was charged until After resting for 10 minutes, constant current discharge was performed until the battery voltage became 2.75 V at a current of 1.0 It (800 mA). Under such conditions (temperature is 25 ° C.), charge and discharge were performed for 30 cycles, and the capacity retention rate after 30 cycles shown in the following formula (4) was calculated.
Capacity retention rate after 30 cycles (%) = [Discharge capacity at 30th cycle / Discharge capacity at 1st cycle] × 100 (4)

As is clear from Table 1 above, the batteries A1 to A3 in which R _SiO > _RC have a higher capacity maintenance ratio than the batteries Z1 and Z2 in which R _SiO = _RC . I understand. This can suppress the isolation of SiO _X during shrinkage by concentrating and arranging the dispersing agent on the surface of SiO _{X that} is larger than graphite in expansion and contraction during charge and discharge. Therefore, even if the charge / discharge cycle is repeated, it is possible to suppress a decrease in the current collecting property of SiO _X , and as a result, cycle characteristics are improved. In addition, when comparing the battery A2 and the battery Z2 having the same R _{(SiO + C)} , the battery A2 in which R _SiO > _RC is maintained in capacity compared to the battery Z2 in which R _SiO = _RC. The rate is high. Therefore, it can be seen that the effect of the present invention cannot be achieved simply by increasing the ratio of the dispersant.
The R _SiO is preferably 0.01 ≦ R _SiO ≦ 0.15, the R _C is preferably 0.005 ≦ R _C ≦ 0.02, and R _{(SiO + C)} is It is desirable that 0.008 ≦ R _{(SiO + C)} ≦ 0.1. If R 2 _{SiO 3} , R _C , and R 2 _{(SiO + C)} are too small, the effect of adding the dispersing agent is not sufficiently exhibited. On the other hand, if R 2 _{SiO 3} , R _C , and R 2 _{(SiO + C)} are too large, the surface of the negative electrode active material is dispersed. This is because the amount of the agent becomes too large and the acceptability of lithium ions is lowered.

(Other matters)
(1) The ratio of SiO _X in the negative electrode mixture layer is preferably 0.5% by mass or more and 25% by mass or less, and particularly preferably 1.0% by mass or more and 20% by mass or less. _If the content of SiO _X is too small, the negative electrode capacity may not be increased. On the other hand, if the content of SiO _X is too large, the expansion in the negative electrode increases, so that the negative electrode mixture layer is peeled off and the negative electrode current collector is collected. The body may be deformed and the cycle characteristics may be deteriorated. Further, it is preferred that the particle size of the SiO _X is 1 to 20 [mu] m.

(2) The lithium transition metal composite oxide used in the present invention includes the above lithium cobaltate, Ni—Co—Mn lithium composite oxide, Ni—Mn—Al lithium composite oxide, Ni—Co—Al A lithium composite oxide containing nickel or manganese, such as lithium composite oxide, or an olivine type lithium phosphate represented by lithium iron phosphate LiFePO ₄ may be used. Moreover, you may use them individually or in mixture.

(3) As the solvent of the non-aqueous electrolyte used in the present invention, solvents and additives conventionally used in non-aqueous electrolyte secondary batteries can be used at the same time. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, propionic acid Compounds containing esters such as ethyl and γ-butyrolactone, compounds containing sulfone groups such as propane sultone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 1,2-dioxane, 1,4 -Compounds containing ethers such as dioxane and 2-methyltetrahydrofuran, butyronitrile, valeronitrile, n-heptanenitrile, succinonitrile, glutaronitrile, adiponitrile, pimeronite Le, 1,2,3-propanetriol-carbonitrile, 1,3,5-pentanetricarboxylic carbonitrile compounds containing nitrile such as nitrile or can be used compounds comprising an amide such as dimethylformamide. In particular, a solvent in which a part of these H is substituted with F is preferably used. Moreover, these can be used individually or in combination of several, The solvent which combined these with the compound containing a small amount of nitriles and the compound containing ether is preferable.

Furthermore, as a solute used in the non-aqueous electrolyte, a known lithium salt that is conventionally used in a non-aqueous electrolyte secondary battery can be used. As such a lithium salt, a lithium salt containing one or more elements among P, B, F, O, S, N, and Cl can be used. Specifically, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (FSO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), Lithium salts such as LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO ₄ and mixtures thereof can be used. In particular, LiPF ₆ is preferably used in order to enhance the high rate charge / discharge characteristics and durability of the nonaqueous electrolyte secondary battery.

As the solute, a lithium salt having an oxalato complex as an anion can also be used. As a lithium salt having this oxalato complex as an anion, in addition to LiBOB [lithium-bisoxalate borate], a lithium salt having an anion in which C ₂ O ₄ ²⁻ is coordinated to the central atom, for example, Li [M (C ₂ O ₄ ) _x R _y ] (wherein M is a transition metal, an element selected from groups IIIb, IVb, and Vb of the periodic table, R is selected from a halogen, an alkyl group, and a halogen-substituted alkyl group) Group, x is a positive integer, and y is 0 or a positive integer). Specifically, there are Li [B (C ₂ O ₄ ) F ₂ ], Li [P (C ₂ O ₄ ) F ₄ ], Li [P (C ₂ O ₄ ) ₂ F ₂ ], and the like. However, it is most preferable to use LiBOB in order to form a stable film on the surface of the negative electrode even in a high temperature environment.
In addition, the said solute may be used not only independently but in mixture of 2 or more types. Further, the concentration of the solute is not particularly limited, but is preferably 0.8 to 1.7 mol per liter of the electrolytic solution. Furthermore, in applications that require discharge with a large electric current, the concentration of the solute is desirably 1.0 to 1.6 mol per liter of the electrolyte.

(4) As a separator used for this invention, the separator conventionally used can be used. Specifically, not only a separator made of polyethylene, but also a material in which a layer made of polypropylene is formed on the surface of a polyethylene layer, or a material in which a resin such as an aramid resin is applied to the surface of a polyethylene separator is used. Also good.

(5) At the interface between the positive electrode and the separator or at the interface between the negative electrode and the separator, a layer made of an inorganic filler that has been conventionally used can be formed. As the filler, it is possible to use oxides or phosphate compounds using titanium, aluminum, silicon, magnesium, etc., which have been used conventionally, or those whose surfaces are treated with hydroxide or the like. .
The filler layer can be formed by directly applying a filler-containing slurry to a positive electrode, a negative electrode, or a separator, or by attaching a sheet formed of a filler to the positive electrode, the negative electrode, or the separator. it can.

  The present invention can be expected to be deployed in, for example, driving power sources for mobile information terminals such as mobile phones, notebook computers, and smartphones, high power driving power sources such as electric vehicles, HEVs, and power tools, and power sources related to power storage.

Claims (6)

A negative electrode mixture layer comprising a negative electrode active material comprising SiO _X (0.8 ≦ x ≦ 1.2) and graphite, a dispersant, and a binder;
A negative electrode current collector in which the negative electrode mixture layer is formed on at least one surface;
Have
Non-aqueous electrolyte in which the relationship between the dispersion ratio R _{SiO of} SiO _{X represented by} the following formula (1) and the dispersion coverage R _{C of} graphite represented by the following formula (2) satisfies R _SiO > _RC Negative electrode for secondary battery.
Dispersants coverage _R SiO of SiO _X = [dispersants mass covering the SiO _X] / [SiO _X mass] (1)
Graphite dispersant coverage R _C = [dispersant mass covering graphite] / [graphite mass] (2) The negative electrode for a nonaqueous electrolyte secondary battery according to claim 1, wherein the SiO _X dispersant coating ratio R _SiO is 0.01 ≦ R _SiO ≦ 0.15. The dispersant coverage _{R C} of the graphite is 0.005 ≦ _{R C} ≦ 0.02, the non-aqueous electrolyte secondary battery negative electrode according to claim 1 or 2. The non-aqueous electrolyte 2 according to any one of claims 1 to 3, wherein a dispersion ratio R _{(SiO + C) of the} whole negative electrode represented by the following formula (3) is 0.008 ≦ R _{(SiO + C)} ≦ 0.1. Negative electrode for secondary battery.
Dispersant ratio R _{(SiO + C) of the} whole negative electrode = [dispersant mass covering SiO _X + dispersant mass covering graphite] / [SiO _X mass + graphite mass] (3) Mixing a negative electrode active material SiO _X (0.8 ≦ x ≦ 1.2), a dispersant, and a binder to prepare a SiO _X slurry;
Mixing a negative electrode active material graphite, a dispersant, and a binder to prepare a graphite slurry;
Mixing the SiO _X slurry and the graphite slurry to prepare a negative electrode slurry;
Applying the negative electrode 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;
With
The manufacturing method of the negative electrode for nonaqueous electrolyte secondary batteries in which the ratio of the mass of the dispersant to the mass of SiO _X in the SiO _X slurry is higher than the ratio of the mass of the dispersant to the mass of graphite in the graphite slurry. 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 non-aqueous electrolyte secondary battery.