JP2013073924A - Negative electrode for lithium ion secondary battery, and lithium ion secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a negative electrode for a lithium ion secondary battery ensuring safety in an internal short circuit or the like without impairing the high output characteristics and high capacity characteristics of the battery.SOLUTION: A negative electrode 2 for a lithium ion secondary battery comprises: a collector 10; a first layer 11 formed on the collector 10 and containing an active material; and a second layer 12 formed on the first layer 11 and containing a metallic oxide capable of occluding and discharging lithium ions, the active material is composed of a carbon material, the average particle diameter Dof the metallic oxide is one tenth or less of the average particle diameter Dof the carbon material, and the average film thickness L of the protective layer 12 is three times or more of the average particle diameter Dof the metallic oxide.

Description

  The present invention relates to a negative electrode for a lithium ion secondary battery having a structure in which a protective layer containing lithium titanate is formed on an active material layer containing a carbon material, and a negative electrode for a lithium ion secondary battery using the same.

Lithium ion secondary batteries are widely used as drive power sources for portable electronic devices and communication devices. Generally, in a lithium ion secondary battery, a carbon material capable of occluding and releasing lithium is used for the negative electrode plate, and a composite oxide of lithium and a transition metal such as LiCoO ₂ is used as the active material for the positive electrode plate. As a result, a high-power, high-capacity secondary battery is realized. In recent years, with higher functionality of electronic devices and communication devices, higher output and higher capacity are desired.

  By the way, in a lithium ion secondary battery with a high output and a high capacity, for example, a foreign substance is mixed into the battery for some reason, and thus the separator is damaged, thereby causing an internal short circuit between the positive electrode plate and the negative electrode plate. When this occurs, rapid heat generation occurs due to the concentrated current flowing in the short-circuited part, which may cause decomposition of the positive and negative electrode materials, gas generation due to boiling or decomposition of the electrolyte, and the like. is there.

  To prevent problems caused by such internal short circuits, the surface of the negative electrode active material layer or positive electrode active material layer is covered with a porous protective film containing insulating fine particles such as alumina to prevent the occurrence of internal short circuits. A method has been proposed (see, for example, Patent Document 1).

  However, since the porous protective film containing insulating fine particles such as alumina coated on the surface of the negative electrode active material layer or the positive electrode active material layer has low conductivity, the internal resistance of the battery increases, and the high output characteristics of the battery It becomes a factor to reduce. Moreover, since it does not function as an active material, it becomes a factor which inhibits the high capacity | capacitance of a battery.

  Therefore, in Patent Document 2, a main negative electrode layer containing a carbon material or the like is formed on the surface of the current collector, and further, a surface layer containing lithium titanate having a spinel structure is formed on the surface of the main negative electrode layer. A negative electrode structure is described. Lithium titanate has a property of occluding and releasing lithium, and thus functions as a negative electrode active material. However, lithium titanate having a spinel structure shows low resistance in the state of occluding lithium, but occludes lithium. When it is not, it shows high resistance. Therefore, when an internal short circuit occurs, rapid discharge occurs in the short circuit part, so that lithium titanate near the short circuit part does not occlude lithium. As a result, the resistance of the surface layer at the short-circuit portion increases, so that a large current can be suppressed from flowing through the short-circuit portion.

JP 2009-91461 A JP 2010-97720 A

  The lithium titanate used for the surface layer described in Patent Document 2 has lower conductivity and smaller capacity than the carbon material. Therefore, when the film thickness of the surface layer is increased, there is a problem that high output and high capacity of the battery are hindered. On the other hand, when the film thickness of the surface layer is reduced, there is a problem that the current blocking effect inherent to the lithium titanate having a spinel structure cannot be sufficiently exhibited.

  The present invention has been made in view of such problems, and its main purpose is a negative electrode for a lithium ion secondary battery that ensures safety at the time of an internal short circuit without deteriorating the high output and high capacity characteristics of the battery. Is to provide.

  A negative electrode for a lithium ion secondary battery according to the present invention includes a current collector, a first layer containing an active material formed on the current collector, and a lithium ion formed on the first layer. The active material is made of a carbon material, and the average particle size of the metal oxide is 1/10 or less of the average particle size of the carbon material. The average film thickness of the protective layer is 3 times or more the average particle diameter of the metal oxide.

  In a preferred embodiment, the metal oxide comprises lithium titanate.

  In another preferred embodiment, the metal oxide has an average particle size of 1 μm or more. Further, the 90% cumulative particle size (D90) of the metal oxide is preferably 4 μm or less.

  In another preferred embodiment, the protective layer has a thickness in the range of 3 to 30 μm.

  ADVANTAGE OF THE INVENTION According to this invention, the negative electrode for lithium ion secondary batteries which ensured safety | security at the time of an internal short circuit etc. can be provided, without reducing the high output of a battery, and a high capacity | capacitance characteristic.

It is sectional drawing which showed the structure of the negative electrode for lithium ion secondary batteries in one Embodiment of this invention. It is sectional drawing which showed the structure of the lithium ion secondary battery using the negative electrode in one Embodiment of this invention. It is sectional drawing which showed typically the arrangement | sequence of the carbon material which comprises an active material layer, and the arrangement | sequence of the metal oxide which comprises a protective layer. 2 is an enlarged cross-sectional electron micrograph of a negative electrode with defects. It is sectional drawing which showed the manufacturing method of the negative electrode in other embodiment of this invention.

  Before explaining the present invention, the background to the idea of the present invention will be described first.

  As described above, lithium titanate has lower conductivity and smaller capacity than carbon materials, while the diffusion rate of lithium ions in lithium titanate is larger than the diffusion rate of lithium ions in carbon material. There is a characteristic.

  Therefore, the negative electrode having a structure in which a protective layer containing lithium titanate is formed on an active material layer containing a carbon material is formed on the active material layer containing a carbon material when the output of the lithium ion secondary battery is increased. An effect of suppressing lithium deposition can be expected. This makes it possible to effectively prevent the occurrence of an internal short circuit or the like due to lithium deposition in a lithium ion secondary battery with high output.

  On the other hand, since lithium titanate has lower conductivity and smaller capacity than carbon materials, the thickness of the protective layer containing lithium titanate should be reduced in order to suppress a decrease in capacity of the active material layer containing carbon materials. It is necessary to make it as thin as possible.

  However, when the protective layer is thinned, if a part of the active material layer is exposed, lithium may be deposited on the exposed portion. In particular, when the output of the battery is increased, the safety is reduced and the cycle characteristics are increased. There is a risk of lowering.

  Inventors of the present application have been studying thinning of the protective layer in the negative electrode structure in which the protective layer containing lithium titanate is formed, because the protective layer is lost and part of the active material layer is not exposed. Has found that the relationship between the particle size of the carbon material constituting the active material layer and the particle size of lithium titanate constituting the protective layer must be considered, and the present invention has been conceived.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to the following embodiment. Moreover, it can change suitably in the range which does not deviate from the range which has the effect of this invention. Furthermore, combinations with other embodiments are possible.

  FIG. 1 is a cross-sectional view showing a configuration of a negative electrode 2 for a lithium ion secondary battery according to an embodiment of the present invention.

  As shown in FIG. 1, the negative electrode 2 includes a current collector 10, an active material layer (first layer) 11 containing an active material formed on the current collector 10, and an active material layer 11. And a protective layer (second layer) 12 containing a metal oxide capable of inserting and extracting lithium ions.

  Here, the active material contained in the active material layer 11 is made of a carbon material, and examples thereof include graphite, carbon black, acetylene black, and carbon fiber. Examples of the metal oxide included in the protective layer 12 include molybdenum oxide in addition to lithium titanate.

  The active material layer 11 may contain a predetermined amount of binder. Examples of the binder include polyvinylidene fluoride (PVDF), fluorine-based rubber, styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), acrylic rubber (ACM), and the like.

  Further, the protective layer 12 may contain a predetermined amount of a conductive agent and a binder. Examples of the conductive agent include carbon materials such as graphite. Examples of the binder include polyvinylidene fluoride (PVDF), fluorine rubber, styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), and acrylic rubber (ACM).

  Moreover, the collector 10 can use copper foil or copper alloy foil, for example.

  In the present embodiment, the protective layer 12 includes a metal oxide capable of inserting and extracting lithium ions, and thus functions as an active material layer. However, the metal oxide contained in the protective layer 12 is an active material layer. 11 is made of a material having a lower conductivity than the carbon material contained in the carbon material 11, and for convenience, is distinguished from the active material layer 11 and is called a protective layer 12.

  FIG. 2 is a cross-sectional view showing a configuration of a lithium ion secondary battery 100 using the negative electrode 2 in the present embodiment.

  As shown in FIG. 2, an electrode group 4 in which a positive electrode 1 and a negative electrode 2 are wound through a separator 3 is housed in a battery case 7 together with a non-aqueous electrolyte (not shown). The positive electrode 1 is joined to a sealing body 8 that also serves as a positive electrode terminal via a positive electrode lead 5, and the negative electrode 2 is joined to the bottom of a battery case 7 that also serves as a negative electrode terminal via a negative electrode lead 6. The opening of the battery case 7 is sealed with a sealing body 8 via a gasket 9.

  In this embodiment, there are no particular limitations on the other components constituting the lithium ion secondary battery 100 other than the negative electrode 2, and members that are normally used can be used.

  Next, the relationship between the average particle diameter of the metal oxide and the average particle diameter of the carbon material when the film thickness of the protective layer 12 is reduced with reference to FIGS. Will be explained.

  Here, the “average particle size” means a particle size (D50) at which the cumulative volume is 50% in the particle size distribution of the particles. The particle size distribution of the particles can be measured by, for example, a laser diffraction type particle size distribution measuring apparatus.

  3A and 3B are cross-sectional views schematically showing the arrangement of the carbon material 21 constituting the active material layer 11 and the arrangement of the metal oxide 20 constituting the protective layer 12. In addition, the particle | grains of the carbon material 21 and the metal oxide 20 are displayed using the magnitude | size of an average particle diameter.

As shown in FIGS. 3A and 3B, the surface of the active material layer 11 has irregularities having a size substantially corresponding to the average particle diameter D ₂ of the carbon material 21. Therefore, the surface of the active material layer 11, the average particle diameter D ₁ of the metal oxide 20 are arranged so as to fill the unevenness, the protective layer 12 is formed.

Here, when it is assumed that the protective layer 12 is formed on the surface of the carbon material 21 on the active material layer 11 with such a thin thickness that the metal oxides 20 are arranged in about one piece, FIG. as shown in a), the average particle diameter D ₁ of the metal oxide 20 is sufficiently smaller than the average particle diameter D ₂ of the carbon material 21, on the carbon material 21, without the metal oxide 20 is lost, The protective layer 12 can be formed by filling the unevenness. However, as shown in FIG. 3B, if the average particle diameter D ₁ of the metal oxide 20 is relatively larger than the average particle diameter D _{2 of} the carbon material 21, a metal is formed on the carbon material 21. The possibility that the oxide 20 is lost increases.

Moreover, since the protective layer 12 thinned as described above is formed on the active material layer 11 with unevenness, the film thickness is not uniform. Therefore, when the average film thickness L of the protective layer 12 is defined by the average value of the film thickness measured in a certain range as shown in FIGS. 3A and 3B, as shown in FIG. the average particle diameter D ₁ of the metal oxide 20 is less than the average thickness L of the protective layer 12, on the carbon material 21, the metal oxide 20 is not deficient, and FIG. 3 (b) As shown in FIG. 4, if the average particle diameter D ₁ of the metal oxide 20 is relatively larger than the average film thickness L of the protective layer 12, the metal oxide 20 may be lost on the carbon material 21. Becomes higher.

From the above consideration, in the structure of the negative electrode 2 in which the protective layer 12 including the metal oxide is formed on the active material layer 11 including the carbon material, the protective layer 12 is lost even if the protective layer 12 is thinned. In order not to expose part of the active material layer 11, the average particle diameter D ₁ of the metal oxide 20 constituting the protective layer 12 and the average particle diameter D ₂ of the carbon material 21 constituting the active material layer 11 relationship, and the average particle diameter D ₁ of the metal oxide 20, must take into account the relationship between the average thickness L of the protective layer 12.

  Therefore, based on such qualitative suggestions, the negative electrode 2 as shown in FIG. 1 was actually produced, and the relationship between the above parameters and the occurrence of defects in the protective layer 12 was quantitatively evaluated. Furthermore, in order to evaluate the defect | deletion degree of the protective layer 12, the lithium ion secondary battery shown in FIG. 2 was produced, and the internal short circuit test was done.

  Table 1 shows the results. Here, the metal oxide 20 used lithium titanate (LixTiyOz: LTO), and the carbon material 21 used graphite. Note that lithium nickelate was used as the positive electrode active material of the lithium ion secondary battery.

In the battery 1-4 shown in Table 1, the average particle diameter _{D 2} of the graphite in a certain size (17 .mu.m), changing the average particle diameter _{D 1} of the LTO to a range of 1.2～1.9Myuemu, Moreover, the negative electrode produced by making the average film thickness L of the protective layer 12 into 10 micrometers in the battery 1, and 5 micrometers in the batteries 2-4 was used, respectively.

  The coverage shown in Table 1 is obtained by photographing the surface of the protective layer 12 of the negative electrode 2 with an optical microscope photograph, binarizing a portion where the defect occurred, and having an area where no defect occurs with respect to an area of 5 mm × 5 mm. The ratio is calculated. FIG. 4 is an enlarged cross-sectional electron micrograph of a negative electrode in which a defect has occurred. The defect A is observed in a part of the protective layer 12 that has been thinned.

  In addition, the internal short-circuit test in Table 1 is the compulsory internal in JIS C 8712 “Safety of sealed small-sized secondary batteries” and JIS C 8714 “Safety tests of single-cell batteries and assembled batteries of lithium ion storage batteries for portable electronic devices”. Evaluation was performed by the number of batteries ignited by the short-circuit test.

  As shown in Table 1, in the negative electrodes used for the batteries 1 and 2, the coverage was as high as 99.8% or more, and no ignition was observed even in the internal short circuit test. On the other hand, in the negative electrode used for the batteries 3 and 4, the coverage was 98% or less, and ignition was observed in the internal short circuit test.

From this result, the average particle diameter _{D 1} of the LTO, 1/10 in the following average particle diameter _{D 2} of the graphite, the average thickness L of the protective layer 12, by more than 3 times the average particle size of LTO Even if the protective layer 12 is thinned, it is possible to prevent the protective layer 12 from being lost and a part of the active material layer 11 from being exposed. In addition, since the LTO constituting the protective layer 12 functions as an active material and the diffusion rate of lithium ions is high, the battery can have high output and high capacity at the same time. Thereby, even if the protective layer 12 is made thin, it is possible to realize a lithium ion secondary battery that ensures safety in the case of an internal short circuit without reducing the high output and high capacity characteristics of the battery.

Furthermore, as shown in Table 1, 1.9 .mu.m average particle diameter _{D 1} of the LTO, the average particle diameter _{D 2} of graphite and 21 [mu] m, an average thickness L of the protective layer 12, a negative electrode was 10μm and 8μm respectively The batteries 5 and 6 were produced using the same, and the coverage and the internal short circuit test were evaluated in the same manner as the batteries 1 to 4. As a result, as shown in Table 1, in both batteries 5 and 6, the coverage was 99.8% or more, and no smoke was observed in the internal short circuit test.

  From the viewpoint of suppressing gas generation due to side reaction with the electrolytic solution, the average particle size of LTO is preferably 1 μm or more.

  By the way, in the schematic diagrams shown in FIGS. 3A and 3B, the metal oxide 20 and the carbon material 21 are displayed using the average particle diameter (D50). The particle size is dispersed with a certain particle size distribution. If the particle size distribution is widened, the metal oxide 20 having a large particle size is present, which may reduce the coverage of the active material layer 11 surface. Similarly, due to the presence of the carbon material 21 having a large particle size, the unevenness of the surface of the active material layer 11 becomes large, and this may also reduce the coverage of the surface of the active material layer 11.

  Table 2 shows the results of measuring the particle size distribution of LTO and graphite for batteries 1 and 5 in which ignition was not observed in the internal short-circuit test of Table 1, with 10% cumulative particle diameter (D10), 50 The values of% cumulative particle diameter (D50) and 80% cumulative particle diameter (D90) are shown respectively.

  As shown in Table 2, when the 90% cumulative particle diameter (D90) of LTO is 4 μm or less, a highly safe battery without a reduction in coverage can be obtained. Similarly, if the 90% cumulative particle diameter (D90) of graphite is 40 μm or less, a highly safe battery without a reduction in coverage can be obtained.

  In the present embodiment, the negative electrode 2 is formed by applying and drying the active material layer 11 on the current collector 10 by a commonly used method such as a die coating method or a gravure coating method, and then further on the active material layer 11. The protective layer 12 can be applied and dried.

  FIG. 5 is a cross-sectional view showing a method for manufacturing the negative electrode 2 with improved productivity. In the above method, it is necessary to apply twice, whereas in this method, the active material layer 11 and the protective layer 12 can be simultaneously formed on the current collector 10 by one application. .

  As shown in FIG. 5, the slurry A containing the carbon material and the metal oxidation are moved from the slits 31 and 32 while the die head 30 in which the two slits 31 and 32 are formed is moved in parallel to the current collector 10. The active material layer 11 and the protective layer 12 can be simultaneously formed on the current collector 10 by supplying each of the slurry B containing the material onto the current collector 10.

  Here, the film thickness of the active material layer 11 and the film thickness of the protective layer 12 can be controlled by adjusting the distance between the lip at the tip of the slits 31 and 32 and the current collector 10, respectively. . In addition, since it is necessary to control and form the film thickness of the protective layer 12 to 3-30 micrometers, More preferably, it is 3-10 micrometers in thickness, in order not to produce a coating stripe in the protective layer 12, it is a slit. The lip diameter at the tip of 32 is preferably larger than the 90% cumulative particle diameter (D90) of the metal oxide.

  As mentioned above, although this invention was demonstrated by suitable embodiment, such description is not a limitation matter and of course various modifications are possible. For example, although the active material layer 11 and the protective layer 12 are formed on one surface of the current collector 10 in the above embodiment, the negative electrode 2 may be formed on both surfaces of the current collector 10.

  The present invention is useful as a power source for driving automobiles, electric motorcycles, electric playground equipment and the like.

1 Positive electrode
2 Negative electrode
3 Separator
4 Electrode group
5 Positive lead
6 Negative lead
7 Battery case
8 Sealing body
9 Gasket
10 Current collector
11 Active material layer (first layer)
12 Protective layer (second layer)
20 Metal oxide (LTO)
21 Carbon material (graphite)
30 die head
31, 32 Slit
100 Lithium ion secondary battery

Claims (8)

A current collector,
A first layer including an active material formed on the current collector;
A second layer containing a metal oxide capable of inserting and extracting lithium ions formed on the first layer;
The active material is made of a carbon material,
The average particle diameter of the metal oxide is 1/10 or less of the average particle diameter of the carbon material,
The negative electrode for a lithium ion secondary battery, wherein the average film thickness of the protective layer is at least three times the average particle diameter of the metal oxide.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the metal oxide is made of lithium titanate.   3. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the metal oxide has an average particle size of 1 μm or more.   The negative electrode for a lithium ion secondary battery according to claim 3, wherein the 90% cumulative particle size (D90) of the metal oxide is 4 µm or less.   2. The negative electrode for a lithium ion secondary battery according to claim 1, wherein the protective layer has a thickness in the range of 3 to 30 μm.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the carbon material is made of graphite.   The negative electrode for a lithium ion secondary battery according to claim 1, wherein the carbon material has a 90% cumulative particle diameter (D90) of 40 µm or less.   The lithium ion secondary battery provided with the negative electrode for lithium ion secondary batteries in any one of Claims 1-7.