JP6195936B2 - Powder for negative electrode of lithium ion secondary battery - Google Patents

Description

  The present invention relates to a powder used for a negative electrode material of a lithium ion secondary battery, and more particularly to a powder containing lithium-doped carbon-coated silicon oxide powder.

  In recent years, with the remarkable development of portable electronic devices, communication devices, etc., there is a strong demand for the development of secondary batteries with high energy density from the viewpoints of economy and miniaturization and weight reduction of the devices. Currently, high energy density secondary batteries include nickel cadmium batteries, nickel metal hydride batteries, lithium ion secondary batteries, and polymer batteries. Among these, lithium ion secondary batteries have a much longer lifespan and higher capacity than nickel cadmium batteries and nickel metal hydride batteries, and thus the demand thereof has shown high growth in the power supply market.

  FIG. 1 is a diagram illustrating a configuration example of a coin-shaped lithium ion secondary battery. As shown in the figure, the lithium ion secondary battery maintains the electrical insulation between the positive electrode 1, the negative electrode 2, the separator 3 impregnated with the electrolyte, and the positive electrode 1 and the negative electrode 2 and seals the battery contents. The gasket 4 is provided. When charging / discharging is performed, lithium ions reciprocate between the positive electrode 1 and the negative electrode 2 through the electrolytic solution of the separator 3.

The positive electrode 1 includes a counter electrode case 1a, a counter electrode current collector 1b, and a counter electrode 1c. The counter electrode 1c is mainly made of lithium cobalt oxide (LiCoO ₂ ) or manganese spinel (LiMn ₂ O ₄ ). Is done. The negative electrode 2 includes a working electrode case 2a, a working electrode current collector 2b, and a working electrode 2c. The negative electrode material used for the working electrode 2c is generally an active material (negative electrode) capable of occluding and releasing lithium ions. Active material), a conductive aid and a binder.

  Conventionally, carbon has been used as the negative electrode active material, but in recent years, attempts have been made to use silicon oxide in order to increase the charge / discharge capacity. Here, “silicon oxide” is an amorphous silicon oxide, for example, obtained by cooling and precipitating silicon monoxide gas generated by heating a mixture of silicon dioxide and silicon. is there.

  Silicon oxide can be a negative electrode active material capable of increasing the effective charge / discharge capacity because there is little deterioration due to structure destruction due to occlusion / release of lithium ions during charge / discharge and generation of irreversible substances. . Therefore, by using silicon oxide as the negative electrode active material, it has a high capacity compared to the case of using carbon, and compared to the case of using a high capacity negative electrode material such as Si or Sn alloy, A lithium ion secondary battery having good cycle characteristics is obtained.

It is known that the use of a silicon oxide powder doped with lithium as the negative electrode powder can improve the initial efficiency and cycle characteristics of the lithium ion secondary battery. For example, Patent Document 1 below discloses a lithium-containing silicon oxide powder represented by the general formula SiLi _x O _y (0 <x <1.0, 0 <y <1.5). It is said that a lithium ion secondary battery having a high capacity and free from cycle deterioration can be obtained by using as a negative electrode material for a lithium ion secondary battery. This lithium-containing silicon oxide powder is produced by heating and reacting a mixture of SiO _z powder (1.0 ≦ z ≦ 1.6) with metallic lithium or a lithium compound.

  By the way, in order to form the negative electrode material into the working electrode 2c in the form of a film, a binder and water or an organic solvent are added to the negative electrode powder to form a slurry. It is applied and dried. The binder includes an aqueous binder used in the form of organic particles dispersed in water, and an organic binder made of a resin and dissolved in an organic solvent. If the negative electrode powder is made into a slurry using only water and an aqueous binder, the viscosity of the slurry becomes too low as compared with an appropriate viscosity, and the applicability to the substrate becomes poor. For this reason, when an aqueous binder is used, a thickener is added so that the slurry has an appropriate viscosity.

  In Patent Document 1 below, when forming a negative electrode of a lithium ion secondary battery, the lithium-containing silicon oxide powder is made into a slurry by adding polyvinylidene fluoride and N-methylpyrrolidone. That is, Patent Document 1 discloses that a lithium-containing silicon oxide powder is made into a slurry using an organic binder.

Japanese Patent No. 4702510

  However, organic binders are more expensive than aqueous binders. Conventionally, when carbon is used as the negative electrode active material, a slurry is prepared using an aqueous binder. For this reason, in the technical field of lithium ion secondary batteries, data on water-based binders has been sufficiently accumulated, but data on organic binders has not been sufficiently accumulated. Therefore, even when the negative electrode powder contains lithium-doped silicon oxide powder, if an aqueous binder can be used, a wealth of knowledge about the aqueous binder obtained so far can be utilized.

  When the lithium-containing silicon oxide powder disclosed in Patent Document 1 is made into a slurry using water and an aqueous binder, even if a thickener is added, the viscosity of the slurry is such that proper coating properties can be obtained. Lower than This is because the lithium component of the lithium-containing silicon oxide powder reacts with water, which inhibits the hydrogen bond between the thickener molecule and water, resulting in no thickening effect by the thickener. It is believed that there is.

  In such a low-viscosity slurry, the components of the slurry tend to be non-uniform. For example, the active material and the binder are localized in the slurry. For this reason, when a negative electrode is formed using such a slurry, an active material, a binder, etc. localize in the negative electrode, and the cycle characteristics of the lithium ion secondary battery provided with the negative electrode deteriorate. This is because, due to this localization, the expansion / contraction of the active material particles due to insertion / extraction of lithium is not buffered by the binder, and the structure of the negative electrode is destroyed.

  The present invention has been made in view of these problems, and is a negative electrode powder for a lithium ion secondary battery, and even if a negative electrode is produced using a slurry containing water, an aqueous binder, and a thickener, It aims at providing the powder which can acquire a favorable battery characteristic.

The gist of the present invention resides in the following negative electrode powders (A) to (C).
(A) A negative electrode powder for a lithium ion secondary battery, which is used together with an aqueous binder,
A mixture of carbon-coated silicon powder, lithium-doped carbon-coated silicon oxide powder, and graphite powder,
The content of the carbon-coated silicon powder is α mass%, the content of the lithium-doped carbon-coated silicon oxide powder is β mass%, the content of the graphite powder is γ mass%, and X = (α + β) / (Α + β + γ) × 100 and Y = α / β × 100, a powder satisfying any of the following formulas (1) to (3).
X <50 (1)
1 ≦ Y ≦ 10 (2)
−9 × X + 19 ≦ Y ≦ −9 / 10 × X + 37 (3)

(B) The negative electrode powder as described in (A) above, wherein both the following formulas (4) and (5) are satisfied.
3 ≦ Y ≦ 8 (4)
−5 / 2 × X + 41/2 ≦ Y ≦ −5 / 4 × X + 38 (5)

(C) The negative electrode powder as described in (A) or (B) above, wherein the carbon-coated silicon powder has a volume median diameter in the range of 0.1 to 2 μm.

  Even when the negative electrode powder of the present invention is made into a slurry using water, an aqueous binder, and a thickener, the slurry viscosity can be sufficiently increased. For this reason, in the slurry, the constituent components of the slurry can be maintained uniformly, and the localization of the binder and the like can be prevented from proceeding. Therefore, when a negative electrode is formed using such a slurry, the active material, the binder, and the like are uniformly distributed in the negative electrode, so that the cycle characteristics of the lithium ion secondary battery provided with the negative electrode can be improved. .

FIG. 1 is a diagram illustrating a configuration example of a coin-shaped lithium ion secondary battery. FIG. 2 is a diagram showing the relationship between the values of X and Y and the overall evaluation of battery characteristics.

1. As described above, the negative electrode powder of the present invention is used for a negative electrode material of a lithium ion secondary battery (hereinafter simply referred to as “battery”). A silicon powder, a lithium-doped carbon-coated silicon oxide powder, and a graphite powder are mixed, the content of the carbon-coated silicon powder is α mass%, and the lithium-doped carbon-coated silicon oxide When the powder content is β mass%, the graphite powder content is γ mass%, X = (α + β) / (α + β + γ) × 100, Y = α / β × 100, the following formula (1) to (3 ) Satisfies all of the relations of the formulas ”.
X <50 (1)
1 ≦ Y ≦ 10 (2)
−9 × X + 19 ≦ Y ≦ −9 / 10 × X + 37 (3)
Both silicon-coated silicon powder (hereinafter referred to as “carbon-coated silicon powder”) and graphite powder are substantially free of lithium.

  According to this invention, from the formula (1), the proportion of the lithium-doped carbon-coated silicon oxide powder (hereinafter referred to as “carbon-coated lithium-doped silicon oxide powder”) in this negative electrode powder is 50 mass. %. That is, the proportion of the lithium-containing powder in the whole negative electrode powder is reduced in the negative electrode powder of the present invention as compared with the lithium-containing silicon oxide powder that is a conventional negative electrode powder. For this reason, in the slurry prepared using the negative electrode powder, water, an aqueous binder, and a thickener, the reaction between the lithium component in the negative electrode powder and water is reduced, and the thickener molecules and water are reduced. Can be sufficiently hydrogen bonded.

  Therefore, even when the negative electrode powder of the present invention is made into a slurry using water, an aqueous binder, and a thickener, the localization of the binder and the like can be prevented from proceeding in the slurry. The components can be kept uniform. Therefore, when a negative electrode is formed using such a slurry, since the binder and the like are uniformly distributed in the negative electrode, the cycle characteristics of the lithium ion secondary battery provided with the negative electrode can be improved.

  Since not only silicon oxide but also silicon and graphite are active materials that occlude and release lithium, the negative electrode powder of the present invention increases the charge / discharge capacity even if the proportion of silicon oxide is low. be able to.

  Regarding the above formula (2), when the ratio of the carbon-coated silicon powder to the carbon-coated lithium-doped silicon oxide powder is so small as 1> Y, the ratio of the carbon-coated lithium-doped silicon oxide powder in the whole negative electrode powder is sufficiently large And the charge / discharge capacity cannot be increased. When the carbon-coated silicon powder is fine (for example, as described later, the volume median diameter is 2 μm or less), the viscosity of the slurry can be increased by increasing the proportion of the carbon-coated silicon powder. In the case of 1> Y, such an effect cannot be obtained.

  Further, since silicon has a large amount of occlusion of lithium and large expansion / contraction rate at the time of occlusion / release of lithium compared to silicon oxide, the ratio of carbon-coated silicon powder to carbon-coated lithium-doped silicon oxide powder is: When 10 <Y and the amount is large to some extent, expansion / contraction of the negative electrode material at the time of occlusion / release of lithium becomes too large, and the structure of the negative electrode is destroyed, so that the cycle characteristics of the battery are deteriorated.

  In the negative electrode powder, when the ratio of the carbon-coated silicon powder and the carbon-coated lithium-doped silicon oxide powder is small, the ratio of the graphite powder is large. Graphite has a smaller amount of occlusion of lithium than silicon and silicon oxide. When the proportion of the carbon-coated silicon powder and the carbon-coated lithium-doped silicon oxide powder in the negative electrode powder in the above formula (3) is as low as −9 × X + 19> Y, the initial charge / discharge capacity of the battery is Lower.

  On the other hand, when the proportion of the carbon-coated silicon powder and the carbon-coated lithium-doped silicon oxide powder in the negative electrode powder is as large as Y> −9 / 10 × X + 37, the negative electrode material at the time of occlusion / release of lithium Expansion / shrinkage becomes too large, and cycle characteristics deteriorate.

In order to sufficiently obtain the effects of the requirements of the above formulas (2) and (3), it is preferable to satisfy the requirements of the following formulas (4) and (5).
3 ≦ Y ≦ 8 (4)
−5 / 2 × X + 41/2 ≦ Y ≦ −5 / 4 × X + 38 (5)

Further, regarding the carbon-coated silicon powder, from the carbon density ρ (g / m ³ ), the carbon content a (mass%) and the BET specific surface area S (m ² / g) of the carbon-coated silicon powder, a / (ρ -It is preferable that the carbon coating film thickness calculated by S) is 1 * 10 < ^-3 > micrometer-0.1 micrometer. In this case, as the negative electrode material, sufficiently high conductivity can be obtained, and at the time of charging / discharging of the battery, the carbon film can be hardly broken due to expansion / contraction of the negative electrode material. A literature value can be adopted as the density of carbon, for example, 2.2 × 10 ⁶ g / m ³ .

With respect to the carbon-coated silicon powder having such a thickness, the volume-median diameter of the carbon-coated silicon powder is in the range of 0.01 to 2 μm, and the carbon content of the carbon-coated silicon powder is 0.00. It is easy to obtain when it is in the range of 6 to 10.0% by mass. The volume median diameter (hereinafter referred to as “D ₅₀ ”) is a particle size of 50% cumulative from the fine particle side (or coarse particle side) of the volume-based cumulative particle size distribution, and is an index of the average particle size of the powder. It becomes. The cumulative particle size distribution can be measured by, for example, a laser diffraction particle size distribution measuring apparatus.

Further, as described above, the D ₅₀ of the carbon-coated silicon powder, by a 2μm or less, compared with the case 2μm greater than the effect of increasing the viscosity of the slurry using the anode for powder. Further, the D ₅₀ of the carbon-coated silicon powder, by a 2μm or less, (in FIG. 1, the working electrode current collector 2b) electrode due to expansion and contraction of the negative electrode material during charging and discharging of the battery peeling of the negative electrode material from Can be suppressed. By the D ₅₀ of the carbon-coated silicon powder with more than 0.01 [mu] m, the negative electrode powder can easily be slurried. Silicon powder D ₅₀ is less than 0.01μm, there is the rapid oxidation occurs, it is difficult to manufacture.

The _{D 50} of the carbon-coated silicon powder d, the carbon-coated lithium-doped silicon oxide _{D 50} of the powder e, if the _{D 50} of the graphite powder is f,
0.1 <d / e <0.6 and 0.2 <e / f <0.6
It is preferable that In this case, handling of the powder is easy, and the cycle characteristics of the battery can be improved. That is, when at least one of d / e ≦ 0.1 and e / f ≦ 0.2 is satisfied, the carbon-coated silicon powder, the carbon-coated lithium-doped silicon oxide powder, and the graphite powder are not sufficiently mixed and segregated. May occur. On the other hand, when at least one of d / e ≧ 0.6 and e / f ≧ 0.6 is satisfied, the cycle characteristics of the battery deteriorate.

Further, D ₅₀ (d) of the carbon-coated silicon powder is more preferably in the range of 0.1 to 2 μm. In this case, handling of the carbon-coated silicon powder can be further facilitated.

2. Method for Producing Negative Electrode Powder of the Present Invention Hereinafter, an example of a method for producing the negative electrode powder of the present invention will be described.
First, carbon-coated lithium-doped silicon oxide powder, carbon-coated silicon powder, and graphite powder are prepared.

Carbon-coated lithium-doped silicon oxide can be produced as follows. First, as a whole, a silicon oxide powder having an average composition of Si: O = 1: y (0.4 <y <1.5) and a lithium raw material powder are prepared in a molar ratio. As the silicon oxide powder, for example, silicon oxide and silicon dioxide powder are mixed and heated to deposit silicon oxide on the substrate from the SiO gas generated by the sublimation reaction, and this silicon oxide is pulverized. For example, a D ₅₀ adjusted to 3.0 to 30 μm by grinding with a ball mill can be used. As lithium raw material powder, what contains metallic lithium or a lithium compound (for example, LiH) can be used.

These silicon oxide powder and lithium raw material powder are mixed, and this mixed powder is fired at a temperature in the range of 200 to 1200 ° C. (preferably 350 to 900 ° C.) in an inert gas atmosphere. Thereby, silicon oxide and lithium are combined (reacted) to obtain silicon oxide powder doped with lithium. The lower the firing temperature, the harder the compounding proceeds, and at temperatures below 200 ° C., the compounding does not proceed substantially. When the firing temperature is higher than 900 ° C., disproportionation of silicon oxide (decomposition by the reaction of 2SiO → Si + SiO ₂ ) proceeds. As the disproportionation of silicon oxide proceeds, battery characteristics often deteriorate.

  Thereafter, the silicon oxide powder doped with lithium is coated with carbon by a thermal CVD (Chemical Vapor Deposition) reaction at 500 to 1200 ° C. (preferably 600 to 900 ° C.). At this time, for example, hydrocarbon (methane, propane, acetylene, etc.) gas can be used as the carbon source.

Carbon-coated silicon powder can also be obtained by coating carbon on silicon powder by the same method. Silicon powder, for _{example, D 50} is, can be used for 0.1 to 30 [mu] m. Depending carbon coating, a silicon oxide powder and silicon D ₅₀ of the powder, it will not change substantially.

_{D 50} of the graphite powder, for example, can be 10 to 40 [mu] m.

  And carbon covering silicon powder, carbon covering lithium dope silicon oxide, and graphite powder are the ratio which satisfies the requirements of the above-mentioned (1)-(3) formula, Preferably, the above-mentioned (1) formula and (4) formula And the powder for negative electrodes of this invention can be obtained by mixing in the ratio which satisfy | fills the requirements of (5) Formula.

  Carbon-coated silicon powder, carbon-coated lithium-doped silicon oxide powder, and graphite powder were prepared, and these powders were mixed at a ratio falling within the scope of the present invention (Examples), and a ratio not falling within the scope of the present invention (Comparative example) were prepared, batteries were prepared using the respective powders, and the characteristics were measured.

The carbon-coated lithium-doped silicon oxide powder was produced as follows. First, 8.8 g (0.2 mol) of silicon oxide powder and 0.8 g (0.1 mol) of lithium hydride (LiH) powder were prepared as raw material powders. Silicon oxide powder has an average composition, in molar ratio, Si: O = 1: with _{1.02, D 50} is a 4.9 [mu] m, BET specific surface area was of 3.0 m ² / g .

These raw material powders were mixed and fired at 750 ° C. for 360 minutes in an Ar atmosphere at atmospheric pressure to react silicon oxide with lithium to obtain lithium-doped silicon oxide powder. The obtained powder was rotated at 5 rpm using a kiln (rotary heat treatment furnace) and heated to 850 ° C. while flowing Ar gas mixed with propane (C ₃ H ₈ ) gas. Thereby, carbon coating by thermal CVD reaction was performed on the particles of lithium-doped silicon oxide powder. The carbon content of the lithium-doped silicon oxide powder was 1.0% by mass.

The carbon-coated silicon powder was produced as follows. A silicon powder having a D ₅₀ of 1.9 μm was rotated at 5 rpm using a kiln, and heated to 850 ° C. while flowing Ar gas mixed with propane (C ₃ H ₈ ) gas. Thereby, carbon coating by thermal CVD reaction was performed on the silicon powder particles. The carbon content of the carbon-coated silicon powder was 10% by mass.

As the graphite powder, SWF15P2 manufactured by Chuo Electric Industry Co., Ltd. was used. The _{D 50} of the graphite powder was 17.7Myuemu. 10% cumulative from the cumulative particle size distribution fine side of the volume-based, and cumulative 90% particle size, respectively _{D 10,} and when the _{D 90, D 10} of the graphite powder, and _{D 90,} respectively, 11.9, It was 28.6 μm. Moreover, about this graphite powder, the specific surface area was 3.7 m < ² > / g and the tap density was 1.11 g / cm < ³ >.

  The above carbon-coated silicon powder, carbon-coated lithium-doped silicon oxide powder, and graphite powder were mixed at various ratios to obtain negative electrode powder samples. Table 1 shows the mixing ratio of these powders by X and Y. X = (α + β) / (α + β + γ) × 100, Y = α / β × 100, and α, β, and γ are carbon-coated silicon powder, carbon-coated lithium-doped silicon oxide powder in each negative electrode powder, and It is the content rate (mass%) of graphite powder. The values of X and Y satisfy all the requirements of the above formulas (1) to (3) are examples of the present invention, and any one of the requirements of the above formulas (1) to (3) However, what is not satisfied is a comparative example.

  A negative electrode was produced using each sample of the obtained negative electrode powder, and a battery (coin cell) was produced using the negative electrode.

A slurry containing a sample of each negative electrode powder was applied to a copper foil having a thickness of 20 μm, dried at 120 ° C. for 120 minutes, and then punched into 1 cm ² (1 cm × 1 cm) to obtain a negative electrode. The slurry is a ratio of 50: 0.75: 0.75: 0.5 in terms of a mass ratio of each negative electrode powder sample, acetylene black (AB), styrene butadiene rubber (SBR), and carboxymethyl cellulose (CMC). Further, each negative electrode powder was prepared by adding ion exchange water at a ratio of 48.6 to 50.0 by mass ratio. In general, the larger the value of α, the higher the viscosity of the slurry.

  Styrene butadiene rubber is an aqueous binder. CMC is a thickener and dissolves in water in the slurry to increase the viscosity of the slurry as compared to the case where CMC is not added.

The battery is manufactured by using the negative electrode and a lithium foil as a counter electrode, and placing a 30 μm thick polyethylene porous film separator impregnated with an electrolyte between the negative electrode and the counter electrode. did. The electrolyte is a solution obtained by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1: 1, and lithium hexafluoride (LiPF ₆ ) is in a ratio of 1 mol / L. Thus, it was dissolved.

About the obtained battery, discharge capacity ratio and cycle characteristics were measured as battery characteristics.
The discharge capacity ratio is the ratio of the initial discharge capacities of Comparative Examples 2 to 6 and Examples 1 to 8 to the initial discharge capacity of the battery of Comparative Example 1. The initial discharge capacity is the discharge capacity at the first cycle in the cycle characteristic measurement method described below.

  The cycle characteristics were measured using a secondary battery charge / discharge tester manufactured by Nagano Corporation. Charging was performed at a constant current of 0.1 C until the voltage reached 0V, and after the voltage reached 0V, the cell voltage was maintained at 0V. Charging was terminated when the current value fell below 20 μA. Discharging was performed at a constant current of 0.1 C until the voltage reached 1.5V. The above charge / discharge cycle was performed 50 times, and the ratio (%) of the discharge capacity at the 50th cycle when the initial discharge capacity was 100 was defined as the cycle characteristics.

When calculating the current value, the value of 1C was calculated assuming that the discharge capacity of SiO was 1500 mAh / g and the discharge capacity of Si was 2400 mAh / g. For example, when the weight of SiO as the active material in the negative electrode is M (mg), the current value I of 0.1 C is
I = 1500 mAh / g × M × 10 ⁻³ × 0.1
Calculated as

Table 1 shows the measurement results of the discharge capacity ratio and the cycle characteristics. The column of “Comprehensive evaluation” in Table 1 is as follows according to the sum of the discharge capacity ratio (%) and the cycle characteristics (%).
×: Impossible; 200% or less ○: Good; higher than 200% and less than 220% ◎: Particularly good; higher than 220% FIG. 2 shows the relationship between the values of X and Y and the overall evaluation of the battery evaluation. FIG. In FIG. 2, the range surrounded by the thick line is the range of the present invention, and the range surrounded by the broken line is a more preferable range of the range of the present invention.

  As is clear from Table 1 and FIG. 2, when using a comparative example, that is, a powder that does not satisfy any one of the requirements of the above formulas (1) to (3), the discharge capacity ratio ( %), And the sum of the cycle characteristics (%) is 200% or less. On the other hand, in the example, that is, the powder satisfying all the requirements of the above formulas (1) to (3), the total discharge capacity ratio (%) and cycle characteristics (%) exceeded 200%. In particular, in the powder satisfying all the requirements of the above formulas (1), (4), and (5), the total discharge capacity ratio (%) and cycle characteristics (%) exceeded 220%.

Claims (3)

A powder for a negative electrode of a lithium ion secondary battery, used with an aqueous binder,
A mixture of carbon-coated silicon powder, lithium-doped carbon-coated silicon oxide powder, and graphite powder,
The content of the carbon-coated silicon powder is α mass%, the content of the lithium-doped carbon-coated silicon oxide powder is β mass%, the content of the graphite powder is γ mass%, and X = (α + β) / (Α + β + γ) × 100 and Y = α / β × 100, a powder satisfying any of the following formulas (1) to (3).
X <50 (1)
1 ≦ Y ≦ 10 (2)
−9 × X + 19 ≦ Y ≦ −9 / 10 × X + 37 (3) The negative electrode powder according to claim 1, wherein both of the following formulas (4) and (5) are satisfied.
3 ≦ Y ≦ 8 (4)
−5 / 2 × X + 41/2 ≦ Y ≦ −5 / 4 × X + 38 (5)   3. The negative electrode powder according to claim 1, wherein a volume median diameter of the carbon-coated silicon powder is in a range of 0.1 to 2 μm.

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