JP2006196247A - Negative electrode for lithium secondary battery and lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery holding a conductive network in accordance with the volume change of an active material while conductivity is provided to a negative active material layer, and having high capacity and long life. <P>SOLUTION: A negative electrode for the lithium secondary battery uses material electrochemically storing/releasing lithium as an active material, and comprises: a first layer 2 mainly comprising the active material formed on a current collector 1; a second layer 3 mainly comprising a conductive material formed on the first layer; and a third layer 4 mainly comprising the active material formed on the second layer. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery using the same.

  Lithium secondary batteries are used as power sources for various electronic devices because of their high energy density, but there is a strong demand for improvement of the negative electrode in order to further increase the capacity associated with the reduction in size and weight. At present, the theoretical capacity of graphite widely used as a negative electrode material is 372 mAh / g, but an actual lithium secondary battery is designed to utilize about 80% of the theoretical capacity of graphite. The reason is that some of the lithium ions inserted into the graphite at the time of charging are inactivated, accumulating irreversible capacity that does not contribute to discharge, and lowering the life characteristics. Therefore, in anticipation of this phenomenon, the current situation is to adopt a battery design that does not reflect all of the theoretical capacity of graphite. In order to solve this problem and achieve a high capacity, it is necessary to suppress the irreversible capacity of graphite or to develop a high-capacity material different from graphite.

  In order to suppress the irreversible capacity of graphite, it is conceivable to develop a technique (for example, Patent Document 1) in which a conductive agent layer made of a carbon material, metal powder, or conductive ceramic is provided on the negative electrode surface.

  In addition, as an example of using a material having a higher capacity than graphite, an example of using aluminum, silicon, tin, or the like as a negative electrode active material (for example, Patent Document 2) or a non-equilibrium state in combination with an element that does not alloy with an alkaline earth metal The active material (for example, patent document 3) which consists of a solid solution metal formed by 1 is disclosed.

Furthermore, as a combination of the above-mentioned two technical ideas, a carbon composite electrode material (for example, Patent Document 4) in which a film containing a metal element is coated on the surface of highly crystalline carbon particles and carbon is further coated thereon. An example of improving charge / discharge capacity and charge / discharge cycle characteristics by attaching silicon fine particles to the surface of graphite (for example, Patent Document 5), and further arranging conductive carbon around Si fine particles made of crystalline silicon. An example (for example, Patent Document 6) is disclosed.
JP 2000-331686 A Japanese Patent Laid-Open No. 10-255768 Japanese Patent Laid-Open No. 2002-0775350 JP-A-5-299073 Japanese Patent Laid-Open No. 2001-102047 JP 2002-255529 A

  However, even if the technique of Patent Document 1 is used, the conductivity inside the mixture layer made of the active material is not improved, and thus the fundamental problem cannot be solved. Further, in the negative electrode active materials of Patent Documents 2 and 3, the volume of the negative electrode active material expands and contracts due to repeated insertion and extraction of lithium in charge and discharge, which promotes the miniaturization of the active material. Active material is easy to peel off. Since the active material peeled from the current collector is removed from the conductive network, it cannot contribute to the subsequent charge / discharge, which becomes the source of irreversible capacity and causes a decrease in life characteristics. Even if the material having such problems is simply developed by applying the techniques of Patent Documents 4 to 6 and cannot be followed by the dynamic volume change of the active material accompanied by miniaturization, the life characteristics are eventually obtained. It does not lead to a significant improvement.

  The present invention has been made in view of the above problems, and is a high-capacity and long-life lithium that can maintain the conductive network following the volume change of the active material while imparting conductivity to the negative electrode mixture layer. An object is to provide a secondary battery.

  In order to solve the above problems, the negative electrode for a lithium secondary battery of the present invention is formed on a current collector using a material capable of electrochemically occluding and releasing lithium as an active material and using this active material as a main component. It comprises a first layer, a second layer formed on the first layer using a conductive agent as a main agent, and a third layer formed on the second layer using an active material as a main agent. And

  Unlike the case where the conductive agent is simply formed on the negative electrode surface or the case where the conductive agent is arranged on the surface of the negative electrode active material, the mixture is arranged as a conductive layer (second layer) in the middle of the mixture layer made of the negative electrode active material. The conductivity of the layer is greatly improved, and the active material theoretical capacity can be fully utilized. In addition, when a high-capacity negative electrode material having a large volume change is used, the second layer functions as a buffer material, and thus plays a role of securing a conductive network while suppressing the falling off of the active material. improves.

  According to the present invention, the negative electrode active material can be utilized according to the theoretical capacity, and further, when a high capacity negative electrode material is used, a conductive network can be secured, so that a high capacity and long life lithium secondary battery can be provided. it can.

  The best mode for carrying out the present invention will be described below with reference to the drawings.

  FIG. 1 is a schematic sectional view showing a negative electrode of the present invention. A first layer 2 mainly composed of an active material is formed on a current collector 1, and a second layer 3 mainly composed of a conductive agent is formed thereon, and further an active material is mainly disposed thereon. The third layer 4 is formed, and the negative electrode for a lithium secondary battery of the present invention is configured.

  Here, the active material that is the main component of the first layer 2 may be any particles that can occlude and release lithium ions, such as graphites, amorphous carbons, pyrolytic carbons, cokes, Carbon fiber, metallic lithium, lithium alloy (Li-Al, Li-Pb, etc.), inorganic compound (carbide, oxide, nitride, boride, halide, intermetallic compound, etc.), metal particles such as aluminum and tin Compounds can be used. Among these active materials, in the case of a non-metallic material, an average particle size of 0.01 to 30 μm is preferable, and 10 to 20 μm is particularly preferable. If the particle size is too small, the discharge capacity decreases due to the effect of the film, and if it is too large, the diffusion of lithium in the particles becomes slow, making it difficult to charge and discharge at a high rate. Of these, in the case of graphite, the proportion of rhombohedral crystals is preferably 20% by weight or less, and particularly preferably in the range of 5 to 15% by weight. Moreover, when an active material is a metal material, for the same reason as a nonmetallic material, an average particle diameter of 0.1-100 micrometers is preferable, and 1-10 micrometers is especially preferable. In the case of a metal material, the secondary effect of increasing the conductivity of the mixture layer itself can be imparted.

In addition, the first layer 2 may be formed not only on the current collector 1 but also in the form of a thin film by a dry process such as sputtering. Here, it is possible to reduce the proportion of the conductive agent in the second layer 3 to be described later by making silicon as a main component as an active material and attaching carbon to a part of the surface thereof. In this case, carbon may be adhered to the powder surface by using a physical method such as a mechanical milling method or a chemical method such as a CVD method. As a result, it is possible to increase the packing density of the negative electrode active material mixture layer, and to increase the capacity as an electrode for a lithium secondary battery.

  When the first layer 2 is formed by coating, a binder can be used. The binder may be any resin that is stable when the potential of lithium is in the range of 0 to 2 V, and the addition amount is preferably in the range of 2 to 20% by weight based on the active material.

  The second layer 3 is a layer containing a conductive agent as a main agent, and reduces the volume change of the active material while increasing the conductivity of the mixture layer (the first layer 2 and the third layer 4) made of the active material. Play a role. The second layer 3 may be formed not only on the first layer 2 but also in a thin film by a dry process such as plating or sputtering. Moreover, when forming the 2nd layer 3 by coating, the binder similar to the 1st layer 2 can be used. Furthermore, nickel, cobalt, or carbon is preferable as an element constituting the second layer 3, but it is more preferable to add silicon or tin to these because alloying with lithium is possible and high capacity can be achieved.

  Here, when the second layer 3 is formed by a plating method, in order to make this layer porous, for example, in the case of nickel electroless plating, stannous ions or lead ions having strong reducing power are adsorbed on the carbon surface in advance. Then, after obtaining a tin or lead metal thin film, it is preferable to adopt a method in which a part of the tin or lead is replaced with nickel having a small reducing power by electroless plating. In addition to nickel, the same effect can be obtained by replacing with copper, palladium or silver. Here, in order to omit the substitution with tin or the like, an alloy plating method can also be used. In particular, a plating layer containing boron or phosphorus becomes more porous due to amorphization. The plating layer according to the present invention is for avoiding a side reaction caused by direct contact of the surface of carbon as an active material with the electrolytic solution. Here, in addition to boron or phosphorus, the plating layer can be alloyed with lithium, such as Al, Si, Ga, Ge, In, Sb, Bi, As, Zn, Cd, etc., Group 1 or 2 other than lithium Or a small amount of non-metallic elements such as O, N, F, and S.

  The third layer 4 can be formed by selecting the same composition as the first layer 2 and by the same method as the first layer 2. Here, the first layer 2 and the third layer 4 may have the same composition, but different compositions may be selected as necessary. Also, the formation method may be the same or different between the first layer 2 and the third layer 4.

  Further, as shown in FIG. 2, by providing a fourth layer 5 similar to the second layer 3 on the third layer 4, film formation at the interface between the electrode plate and the electrolyte is suppressed. More preferred.

  The structural example of each element in the lithium secondary battery of this embodiment is given.

The electrolyte is obtained by dissolving a lithium salt such as LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₄ , LiAsF ₆ as an electrolyte in ethylene carbonate, propylene carbonate, dimethyl carbonate, dimethoxyethane, tetrahydrofuran or the like or a mixture thereof. Is used. Or what added the said organic solvent as a plasticizer to the composite_body | complex of the said lithium salt and polyacrylonitrile as a solid electrolyte may be used.

Although there is no restriction | limiting in particular about a positive electrode, A transition metal type oxide, a metal chalcogen bond, a metal halide, etc. are mentioned. The transition metal is preferably cobalt, nickel, manganese, iron, chromium, titanium, vanadium, or molybdenum, and the compounds are LiCoO ₂ , LiMnO ₂ , LiNiO ₂ , LiNi _1-x Co _x O ₂ , LiNi _1-xy Co _{x. MyO 2} (M is a trivalent metal or a transition metal) is preferred.

As the separator, a polypropylene, polyethylene, or polyolefin-based porous resin film can be used.

  Examples relating to the present invention will be described below.

Example 1
(I) Production of positive electrode Using Li ₂ CO ₃ and CoCO ₃ as starting materials, weigh them so that the atomic ratio of Li: Co is 1: 1, mix them in a mortar, press this, and press-mold After that, it was baked in the air at 800 ° C. for 24 hours to obtain a LiCoO ₂ fired body. This was pulverized with a mortar until the average particle size became 20 μm. The resulting LiCoO ₂ powder was mixed so that it was 100 parts by weight, acetylene black as a conductive agent was 10 parts by weight, and polytetrafluoroethylene as a binder was 10 parts by weight to obtain a paste mixture. This mixture was applied to a positive electrode current collector made of an aluminum foil having a thickness of 20 μm, and then dried and rolled to obtain a positive electrode. After the positive electrode lead was spot welded to the positive electrode, vacuum drying was performed at 110 ° C. for 3 hours.

(Ii) Production of Negative Electrode An electrolytic copper foil (thickness: 14 μm) was used as the current collector 1. The active material is artificial graphite having an average particle diameter of about 20 μm, 100 parts by weight of the graphite, 30 parts by weight of acetylene black as a conductive agent, and N-methyl-2-pyrrolidone of polyvinylidene fluoride (PVdF) as a binder. 4 parts by weight of the (NMP) solution was converted into PVdF weight and kneaded to obtain a paste-like negative electrode mixture. This negative electrode mixture was applied to the current collector with a coating thickness of 100 μm, and then dried at 60 ° C. for 8 hours to form the first layer 2.

  Subsequently, 10 parts by weight of acetylene black and 4 parts by weight of an aqueous solution of carboxymethyl cellulose (hereinafter abbreviated as CMC) as a binder were added and kneaded to obtain a paste-like negative electrode mixture. The negative electrode mixture was applied on the first layer 2 with a coating thickness of 10 μm, and then dried at 60 ° C. for 8 hours to form a second layer 3.

  Further, an active material layer similar to that of the first layer 2 was applied on the second layer 3 with a coating thickness of 80 μm to form a third layer 4. The coated material was dried at 60 ° C. for 8 hours and then rolled, the negative electrode lead was spot welded, and vacuum dried at 110 ° C. for 3 hours. By these procedures, the first layer 2 which is an active material layer is provided on the surface of the current collector 1, the second layer 3 which is a conductive layer thereon, and the third layer which is an active material layer thereon. A negative electrode having the layer 4 was formed.

(Iii) Production of Battery The obtained positive electrode and negative electrode were wound in an oval shape through a 25 μm thick polyethylene separator to produce an electrode group. This electrode group was inserted into an aluminum battery case, and an electrolytic solution in which 1 mol / liter of LiPF ₆ was dissolved in an equal volume mixed solvent of ethylene carbonate and diethyl carbonate was injected. Thus, a prismatic lithium secondary battery having a width of 5.2 mm, a length of 34 mm, and a total height of 36 mm was produced. This is referred to as the battery of Example 1.

(Example 2)
For the battery of Example 1, the active material of the first layer 2 in the negative electrode is carbonized on the surface of silicon particles by CVD (for details, see Journal of The Electrochemical Society, Vol. 149, A1598 (2002)). A prismatic lithium secondary battery similar to that of Example 1 was fabricated except that the surface-modified silicon was deposited with a coating thickness of 60 μm. This is referred to as the battery of Example 2.

(Example 3)
For the battery of Example 2, the active material of the third layer 4 in the negative electrode was changed to the same surface-modified silicon as that of the first layer 2 and the coating thickness was set to 60 μm. A similar prismatic lithium secondary battery was produced. This is referred to as the battery of Example 3.

Example 4
A prismatic lithium secondary battery similar to that of Example 3 was produced, except that the second layer 3 of the negative electrode was formed by nickel plating as described below with respect to the battery of Example 3. This nickel plating is based on a Watt bath of 170 g / L nickel sulfate, 40 g / L nickel chloride, and 30 g / L boric acid, with a pH of 4.5, and electrolysis for 2 seconds at 1 A / dm ² at a bath temperature of 40 ° C. In addition, a nickel thin film layer (approximately 5 μm) is formed on the surface of the first layer 2 made of surface-modified silicon. This is the battery of Example 4.

(Example 5)
A prismatic lithium secondary battery similar to that of Example 3 was produced, except that the second layer 3 in the negative electrode was formed by cobalt plating shown below with respect to the battery of Example 3. This cobalt plating is based on an electrolytic bath of cobalt sulfate 0.08M, sodium hypophosphite 0.2M, sodium citrate 0.2M, and boric acid 0.5M, with a pH of 7, and 10 at a bath temperature of 80 ° C. Electroless plating is performed for 1 minute, and a cobalt thin film layer (approximately 1 μm) is formed on the surface of the first layer 2 made of surface-modified silicon. This is the battery of Example 5.

(Example 6)
For the battery of Example 3, the same as Example 3 except that the fourth layer 5 in the negative electrode was formed on the third layer 4 with the same composition and thickness as the second layer 3. A square lithium secondary battery was prepared. This is the battery of Example 6.

(Example 7)
For the battery of Example 4, the same as Example 4 except that the fourth layer 5 in the negative electrode was formed on the third layer 4 with the same manufacturing method and thickness as the second layer 3. A square lithium secondary battery was prepared. This is the battery of Example 7.

(Example 8)
When forming the second layer 3 in the negative electrode for the battery of Example 4, silicon particles having an average particle diameter of 1 μm (the raw material is the same as that of Example 2 and the CVD process is not performed as in Example 2). A prismatic lithium secondary battery similar to that of Example 4 was produced except that the amount thereof was 15 parts by weight with respect to 100 parts by weight of nickel. This is the battery of Example 8.

Example 9
A prismatic lithium secondary battery similar to that in Example 8 was produced, except that the particles coexisting in the second layer 3 in the negative electrode were tin having an average particle diameter of 1 μm with respect to the battery of Example 8. This is the battery of Example 9.

(Comparative Example 1)
For the battery of Example 1, square lithium similar to Example 1 except that the second layer 3 and the third layer 4 in the negative electrode are not provided, and only the first layer 2 having a coating thickness of 180 μm is used. A secondary battery was produced. This is referred to as the battery of Comparative Example 1.

(Comparative Example 2)
For the battery of Example 3, the rectangular lithium similar to Example 3 except that the second layer 3 and the third layer 4 in the negative electrode are not provided and only the first layer 2 having a coating thickness of 120 μm is used. A secondary battery was produced. This is referred to as the battery of Comparative Example 2.

(Comparative Example 3)
A prismatic lithium secondary battery similar to that of Comparative Example 1 except that only the second layer 3 identical to that of Example 1 was provided on the first layer 2 of the negative electrode with respect to the battery of Comparative Example 1. Produced. This is referred to as the battery of Comparative Example 3.

(Comparative Example 4)
A prismatic lithium secondary battery similar to that of Comparative Example 2 except that only the second layer 3 identical to that of Example 3 was provided on the first layer 2 of the negative electrode with respect to the battery of Comparative Example 2. Produced. This is referred to as the battery of Comparative Example 4.

  Each battery described above was evaluated as follows. The results are shown in (Table 1).

(Design capacity)
Adjust the length of positive and negative electrodes and separator according to the negative electrode theoretical capacity of each battery, and unify the ratio of the cross-sectional area of the electrode group in the cross-sectional area of the battery case to 92% and the design utilization rate of the negative electrode to 93%. Thus, an electrode group of each battery was configured. The positive electrode theoretical capacity at this time is shown in Table 1 as the design capacity.

(Life characteristics)
Each battery described above is charged at a constant current rate of 2.5 hours (depending on the design capacity of each battery) at 20 ° C. until 4.2 V is reached, and then at a constant 4.2 V until it reaches 10 hours. A constant voltage charge was performed. After resting for 20 minutes, constant current discharge was performed at a rate of 1 hour until 3.0V was reached. The discharge capacity at the 200th cycle when the charge / discharge cycle is repeated as one cycle is shown in (Table 1).

When the design utilization of the negative electrode is increased to 93%, even if the negative electrode uses graphite as the negative electrode active material, the influence of lithium deposition at the end of charging becomes significant. The discharge capacity of is low. Here, when surface-modified silicon is used as the negative electrode active material, miniaturization of the active material is promoted by repeated charge and discharge, so that the discharge capacity at the 200th cycle is further reduced. Further, in Comparative Examples 3 and 4 in which the conductive layer is formed only on the surface of such a negative electrode, the problems of Comparative Examples 1 and 2 cannot be fundamentally solved.

  In contrast to these comparative examples, Examples 1 to 5 in which a conductive layer was provided between the mixture layers as the second layer 3 exhibited good life characteristics. Among them, the reason why the life characteristics of Examples 2 to 5 using the surface-modified silicon are good is that the second layer 3 not only functions as a mere conductive layer, but also the volume change of the surface-modified silicon accompanying charge / discharge. It can be mentioned that it also functions as a buffer layer.

  In Examples 6 and 7, in which the fourth layer 5 was further provided with respect to Examples 3 and 4, there was a tendency to prolong the lifetime, which seems to be based on further improvement in conductivity. Further, in Examples 8 and 9 in which a material capable of being alloyed with lithium is coexisted in the second layer 3 as compared with Example 4, there is a slight tendency to improve the design capacity and extend the life. It was.

  According to the present invention, it is possible to make maximum use of the theoretical capacity of the negative electrode active material and to use a new material having a large theoretical capacity without any problem, so that the lithium secondary having high capacity and excellent cycle life characteristics can be used. The applicability of the present invention is great because a battery can be provided.

Schematic sectional view showing the negative electrode of claim 1 of the present invention Schematic sectional view showing the negative electrode of claim 3 of the present invention

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Current collector 2 1st layer 3 2nd layer 4 3rd layer 5 4th layer

Claims (6)

An anode for a lithium secondary battery using an electrochemically active material capable of inserting and extracting lithium,
A first layer formed mainly on the active material and formed on the current collector;
A second layer formed on the first layer with a conductive agent as a main agent;
A negative electrode for a lithium secondary battery comprising the active material as a main ingredient and a third layer formed on the second layer. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein the second layer contains at least one selected from nickel, cobalt, silicon, tin, and carbon. 2. The negative electrode for a lithium secondary battery according to claim 1, wherein a fourth layer mainly comprising the conductive agent is provided on the third layer. The negative electrode for a lithium secondary battery according to claim 3, wherein the second and fourth layers contain at least one selected from nickel, cobalt, silicon, tin and carbon. 5. The negative electrode for a lithium secondary battery according to claim 1, wherein the active material contains silicon as a main component, and carbon is attached to a part of the surface thereof. A lithium secondary battery comprising a negative electrode comprising the electrode according to any one of claims 1 to 5, a positive electrode, and a nonaqueous electrolyte.