JP2007026913A - Stacked metal foil for lithium ion battery, and lithium ion battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a reliable lithium ion battery of high charge/discharge characteristics that uses expansive active material by relaxing deformation of an electrode due to voluminal expansion of the active material at charging/discharging, and suppressing degradation in cycle characteristics of the lithium ion battery which is caused by deformation or breakage of the electrode at charging/discharging. <P>SOLUTION: A collector 3 is provided by laminating roughened metal foils in two layers. A layer of high elastic modulus 10 in the collector 3 is formed from a void 4 by stacking, and resin 12. By using the stacked collector, expansion/deformation of the active material is reduced when reacted with lithium ion at charging, compared to a collector of Cu single layer. Since a stress relaxing layer is incorporated, deformation or fracture of the electrode is prevented for improved cycle characteristics of the battery itself. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a laminated metal foil for a lithium ion battery using a nonaqueous electrolyte and a lithium ion battery using the metal foil.

  Lithium ion batteries are mainly used as power sources for driving electronic devices for mobile use, but graphite is mainly used as a negative electrode material for the batteries. Since this graphite has a small capacity per unit mass of 372 mAh / g, other materials are required to realize a higher capacity negative electrode.

  As a material having a higher capacity than graphite, a material containing Al, Si, Ge, Sn, or the like is used as an active material, and a negative electrode is formed by forming an intermetallic compound between these and Li. It has been reported that the energy density of an ion battery can be increased (see, for example, Patent Document 1).

  However, these materials occlude lithium ions during charging and cause large volume expansion. For example, when graphite is used for the negative electrode, the volume increase rate at the time of occlusion of lithium ions is about 1.2 times at maximum, but when Si is used as the active material for the negative electrode, in the occluded state, The volume increase reaches up to 4.12 times.

  When the volume change of the active material is large, peeling at the interface between the active material and the current collector or cracking of the active material itself occurs, resulting in a problem that the cycle life is shortened. In particular, a method of forming a silicon-copper mixed layer is used to reinforce the adhesion strength between the active material thin film and the current collector made of copper so as to relieve stress during expansion in order to prevent separation at the interface. (For example, refer to Patent Document 2). In order to suppress cracking of the active material, a microcrystalline Si thin film is formed on a roughened copper foil current collector, and the volume change is caused by a crack on the microcrystalline Si thin film generated by a charge / discharge cycle. The cycle characteristics are improved by relaxing and absorbing the strain caused by the above and preventing separation of the active material and the current collector.

In addition, the use of SiO _x (0 <x <2) as the negative electrode active material is also studied so that the expansion rate during charging itself is smaller than that of Si (see, for example, Patent Document 3).
Japanese Patent Application Laid-Open No. 11-86854 JP 2002-83594 A JP-A-6-325765

  In the negative electrode of Patent Document 2 described above, since the roughened copper foil is used as the current collector, the active material can be prevented from peeling. However, when the current collector copper foil is very thin, it resists deformation of the active material. However, the problem is that the electrode itself undulates and deforms. Furthermore, when the strength of the current collector is small, the electrode itself may tear and break.

  In addition, if a thin silicon film with large expansion during occlusion of lithium is used as the active material and the thickness of the copper foil as a current collector is increased to prevent deformation of the electrode plate itself, internal stress generated during deposition of the thin film Although the silicon electrode plate deformation itself in the vicinity of the silicon-copper foil interface can be prevented, stress is generated at the interface due to expansion / contraction due to charge / discharge, and the film of the silicon thin film as the active material is peeled off.

  In the negative electrode of Patent Document 3, silicon oxide having a molar ratio of oxygen to silicon of 0 to 2 is used as an active material, and the volume change during charge / discharge is suppressed to be smaller than that of silicon. However, expansion deformation at the time of charging / discharging is still large, and electrode plate deformation and film peeling have not been completely prevented. Further, when the negative electrode is laminated with a positive electrode and a separator, the expansion and contraction deformation during charging / discharging in the stacking direction is still large. That mitigation is a challenge.

  The present invention solves the above-mentioned conventional problems, relaxes the volume expansion of the active material during charging, eliminates the separation of the active material from the current collector due to charge and discharge, An object of the present invention is to provide a metal foil for a lithium ion battery capable of improving cycle characteristics while preventing breakage and increasing energy density, and a lithium ion battery using the metal foil as a negative electrode.

  In order to solve the conventional problem, in the laminated metal foil for a lithium ion battery of the present invention, at least one of the first surface and the second surface facing the first surface is roughened. A laminated metal foil for a lithium ion battery in which a plurality of metal foils are laminated, the surfaces where the laminated metal foils are in contact with each other are roughened surfaces, and the two surfaces with which the roughened surfaces are in contact A gap exists between the metal foils.

  With this configuration, even when an active material having a large expansion when occluding lithium ions such as silicon is formed on the laminated metal foil, there is a gap between the two metal foils with which the roughened surface abuts. Therefore, since the void relaxes the expansion, it is possible to eliminate the peeling of the active material from the current collector due to charging / discharging, and to prevent deformation and breakage of the entire electrode.

  Further, the lithium ion battery of the present invention has a negative electrode having a negative electrode active material that occludes and releases lithium ions on the surface of the current collector made of the laminated metal foil for a lithium ion battery, and a current collector surface made of a metal thin plate. A positive electrode having a positive electrode active material that occludes and releases lithium ions and a lithium ion conductive electrolyte or polymer electrolyte are provided.

  With this configuration, it is possible to provide a lithium ion battery capable of improving cycle characteristics while increasing energy density.

  According to the laminated metal foil for a lithium ion battery of the present invention, the deformation of the electrode plate due to the expansion of the active material at the time of reaction with lithium ions is alleviated, and the deformation such as wrinkles and breakage entering the negative electrode plate itself is broken. By preventing this, it becomes possible to improve the charge / discharge cycle characteristics of the lithium ion battery.

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

(Embodiment 1)
FIG. 1 is a schematic longitudinal sectional view of a negative electrode for a lithium ion battery according to Embodiment 1 of the present invention. In FIG. 1, a negative electrode 1 is composed of a current collector 3 and an active material 2 carried thereon. The current collector 3 includes a metal foil 5 and a metal foil 6, and the surfaces on which the metal foil 5 and the metal foil 6 are in contact with each other are roughened surfaces. There is a void 4. The metal foil 5 is a surface roughened also on the surface facing the metal foil 6, and the active material 2 is formed on the surface. The surface of the metal foil 6 that faces the surface that contacts the metal foil 5 is also a roughened surface.

  As metal foil 5 and metal foil 6, what consists of copper which roughened the surface of the surface which at least contacts them can be used. The surface property of the surface of the metal foil 5 that supports the active material 2 has a preferable shape depending on the material of the active material 2, but may be roughened to the extent that the adhesive strength with the active material 2 can be maintained.

  Specifically, the active material 2 includes, as a base material, silicon alone, silicon oxide, Si—Sn alloy, and the like. The active material 2 may contain elements other than the constituent elements of the base material. The thickness 9 of the active material 2 may be about 4 μm to 50 μm, but is not limited thereto. What is necessary is just to decide by the structure of a battery. The thickness 9 of the active material 2 is a thickness obtained by subtracting the thickness of the current collector from the total thickness of the negative electrode plate.

  The layer 10 having a small elastic modulus in the current collector 3 is a region made of only the metal foil 5. The layer 11 having a large elastic modulus in the current collector 3 can be formed as a layered region including the void 4 generated by the lamination of the metal foil 5 and the metal foil 6.

  In addition, it is preferable that the thickness 7 of the metal foil 5 and the thickness 8 of the metal foil 6 are 4 μm or more and 50 μm or less, respectively. Here, the thickness 7 and the thickness 8 mean the total thickness of the metal foil 5 and the metal foil 6. That is, the thickness includes the roughened convex portion. If the thickness 7 and the thickness 8 are less than 4 μm, the strength is insufficient, and if it exceeds 50 μm, the thickness of the current collector 3 itself increases, which may hinder the increase in capacity.

  The surface roughness Rz (based on JIS B0601) of the surface of the metal foil 5 that contacts the metal foil 6 and the surface of the metal foil 6 that contacts the metal foil 5 is preferably 0.1 μm or more and 10 μm or less. If the surface roughness Rz is less than 0.1 μm, it is difficult to form the layer 11 having a large elastic modulus including the voids 4, and if it exceeds 10 μm, the thickness of the current collector 3 itself increases and the capacity increases. This is because it may cause trouble.

  Further, the layer 11 having a large elastic modulus may contain a resin 12 in addition to the gap 4. Such a case will be described with reference to FIG.

  In FIG. 2, the same components as those in FIG. In FIG. 2, the layer 11 having a large elastic modulus includes voids 4 and a resin 12. The resin 12 can be formed by, for example, applying a resin material to the surface of the metal foil 6 that contacts the metal foil 5.

  The resin 12 preferably contains at least one of conductive particles and filler 13 therein. Examples of the conductive particles include carbon. Furthermore, it becomes possible to control the layer 11 with a large elastic modulus by adjusting the content of the filler 13.

  By including a resin between the metal foil 5 and the metal foil 6 to be laminated, layers having different elastic moduli can be formed as in the case of the simple gap 4 alone. The layer 11 having a large elastic modulus can be formed by the surface convex portion of the metal foil 5 or the metal foil 6 and the surrounding empty wall 4, the resin 12, the conductive particles or the filler 13, and a current collector having a laminated structure The body 3 can be formed.

  As long as the layer 11 having a large elastic modulus functions as a stress relaxation layer against deformation, a plurality of layers may be formed. That is, the effect can be further increased by alternately laminating the layer 10 having a small elastic modulus and the layer 11 having a large elastic modulus. That is, such a laminated composite current collector 3 can alleviate electrode plate deformation caused by expansion of the active material by reacting with lithium ions during charging. Therefore, the strength of the current collector 3 is improved, and the negative electrode 1 in which the strength is hardly deteriorated is obtained.

  In addition, by using the negative electrode 1 including the current collector 3 as described above, a lithium ion battery having excellent cycle characteristics can be obtained.

  Next, the manufacturing method of the negative electrode for lithium ion batteries of this invention is demonstrated.

  For example, in the negative electrode for a lithium ion battery of the present invention shown in FIG. 1, for example, only the current collector 3 is prepared in advance as a laminated structure, and then the surface of the current collector 3 is applied by a coating process or a vacuum deposition process. Examples thereof include a method of forming an active material 2 and producing a negative electrode, and a method of laminating a metal foil 5 on which a surface of the active material 2 is previously formed by a coating process or a vacuum deposition process with another metal foil 6.

  As a method for forming the active material 2 on the surface of the current collector 3, a thin film process using a vacuum vapor deposition apparatus can be exemplified. The vacuum deposition apparatus includes a vacuum chamber and a gas inlet for controlling the atmosphere. In the vacuum chamber, a metal foil to be a substrate (current collector) and a silicon raw material are arranged. When a long metal foil is used, mechanisms such as a roll for winding the metal foil, a can, and a take-up roller may be provided. In the vapor deposition apparatus, the vapor deposition source is heated by an electron beam (EB) as means for heating the silicon raw material.

  The method for producing the negative electrode for a lithium ion battery of the present invention when such a vapor deposition apparatus and a roller press are used will be described more specifically below.

(Manufacturing method 1)
First, a method for manufacturing the negative electrode shown in FIG. 1 will be described. In order to produce the laminated current collector 3, the surface of the copper foil having two smooth surfaces is roughened. As a roughening method, copper particles having an average particle diameter of 2 μm are deposited on the surface of the copper foil by electrodeposition so that the surface roughness Rz of the copper foil becomes 1 μm or more. A roughened copper foil having a surface roughness Rz of 1 to 10 μm can be obtained by adjusting the conditions for electrodeposition of copper particles on the copper foil. As another roughening method, roughened copper having a surface roughness Rz of 0.1 to 10 μm is obtained by evaporating copper particles to a thickness of 0.1 μm or more on the surface of the copper foil by a resistance heating vacuum deposition method. A foil can be obtained.

  Laminated surfaces of the two obtained copper foils are laminated so as to face each other, and are laminated by pressing through a roller press to have a gap 4 between them. The current collector 3 can be obtained.

  A silicon oxide as the active material 2 is formed on the surface of the current collector 3 by using a coating process or a thin film process. In the case of the coating process, the silicon oxide powder is kneaded together with a binder and a solvent, passed through a slit, and applied to the surface of the current collector 3. In the case of a thin film process, the current collector 3 is mounted as a substrate in a vacuum chamber, the silicon raw material is heated by EB heating means, and silicon atoms constituting a single silicon are collected while oxygen gas is introduced into the chamber. Deposit on the surface of the body 3. Note that the surface of the current collector 3 forming the active material 2 is preferably roughened. As the roughening method, the same method as described above can be applied, or roughening may be performed by roughening the roller surface when the pressure is applied by a roller press.

  In this way, the negative electrode 1 in which the active material is formed can be formed on the surface of the current collector 3 that is formed into a laminate and has the voids 4.

(Manufacturing method 2)
Next, a method for manufacturing the negative electrode shown in FIG. 2 will be described. The resin 12 is applied to the roughened surface of one copper foil of the two copper foils having the roughened surface obtained by any one of the methods described in the production method 1, The laminated current collector 3 is obtained by facing the other roughened surface and pressing with a roller press.

  The resin 12 to be applied may be mixed with conductive particles such as dispersed metal powder or carbon nanofibers, or a filler may be added to improve the strength.

  As a method for forming silicon oxide as an active material on the surface of the current collector 3, the same method as the manufacturing method 1 can be applied. Note that the surface of the current collector 3 forming the active material 2 is preferably roughened. As the roughening method, the same method as described above can be applied, or roughening may be performed by roughening the roller surface when the pressure is applied by a roller press.

  In this way, the negative electrode 1 in which the active material is formed can be formed on the surface of the current collector 3 that is formed into a laminate and has the resin 12 in the void 4.

  In addition to the production method 1 or 2 as described above, the negative electrode for lithium ion batteries of the present invention can also be produced by the following production method 3.

(Manufacturing method 3)
In this manufacturing method, first, two copper foils are roughened by the method described in Manufacturing Method 1. One copper foil roughens the surface opposite to the roughened surface by the same method. Silicon oxide, which is an active material, is formed on the surface of the copper foil roughened on both sides. The method similar to the manufacturing method 1 can be applied to the method for forming silicon oxide.

  Thereafter, another copper foil is stacked so that the roughened surfaces are in contact with each other, and is pressed by a roller press or the like. At that time, a resin may be applied to the gap between the roughened surfaces.

  Hereinafter, the present invention will be described in more detail by way of specific examples. In addition, this invention is not limited to the Example shown below.

Example 1
First, a positive electrode combined with a negative electrode was produced. First, 3% by weight of acetylene black as a conductive material is mixed with lithium cobalt oxide (LiCoO ₂ ) having an average particle size of 10 μm, and N-methyl-2-pyrrolidone (NMP) of poly (vinylidene fluoride) (PVdF) as a binder. The solution was kneaded by adding 4% by weight with respect to the PVdF weight to prepare a paste-like positive electrode mixture. This positive electrode mixture was applied to a current collector sheet made of an aluminum foil, dried and then rolled to prepare a positive electrode plate. The obtained positive electrode plate was cut into a predetermined size to obtain a positive electrode.

  Next, a negative electrode was produced. First, in order to produce a laminated current collector, copper particles having a particle diameter of 2 to 5 μm are deposited on both the front and back surfaces of the copper foil by electrodeposition, the copper foil thickness is 18 μm, and the front and back surface roughness Rz is 5 μm. A roughened copper foil was produced. The obtained two roughened copper foils were stacked facing each other and integrated by pressing through a roller press to a thickness of 35 μm. The larger the surface roughness between the copper foils, the larger the thickness of the part including the voids, but it was confirmed from the result of microscopic observation that it was 3 μm. Further, a silicon oxide film was formed on the surface of the stacked current collector. The laminated current collector is mounted as a substrate in a vacuum chamber, the silicon raw material is heated by EB heating means, and silicon atoms constituting the silicon simple substance are thickened on the surface of the current collector while introducing oxygen gas into the chamber. It was deposited as a 10 μm thick silicon oxide. Note that a silicon single crystal having a purity of 99.9999% was used as a deposition material. The acceleration voltage of the electron beam applied to the silicon single crystal was −8 kV, and the emission was 300 mA. Finally, the obtained electrode plate was cut into a predetermined size to obtain a negative electrode.

A cylindrical battery was produced using the obtained positive electrode and negative electrode. First, the positive electrode and the negative electrode were spirally wound through a 25 μm thick polyethylene porous separator. At that time, the positive electrode active material and the negative electrode active material were opposed to each other with a separator interposed therebetween. The electrode plate group was housed in a nickel-plated battery case, the positive electrode and the positive electrode terminal were connected via a lead, and the negative electrode was connected to the case by a lead. Next, an electrolytic solution in which LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 1: 3 so as to have a concentration of 1 mol / L is used. A predetermined amount thereof was poured into the battery case. Finally, the case was sealed with a sealing plate to produce a battery.

(Comparative Example 1)
As Comparative Example 1, a current collector having the same thickness (35 μm) as Example 1 (surface roughness Rz = 5 μm on both sides) was used as a current collector without using a current collector with a laminated structure for the negative electrode. A comparative battery 1 was produced in the same manner as in Example 1 except that.

(Example 2)
A battery was fabricated in the same manner as in Example 1 except that the negative electrode described below was used as the negative electrode.

  As a method for producing the negative electrode, first, copper particles having a particle size of 2 to 5 μm were deposited on both the front and back surfaces by electrodeposition so that the surface roughness Rz was 5 μm, and a roughened copper foil having a copper foil thickness of 18 μm (1) And roughened copper foil (2) was produced. An epoxy resin is applied to the surface of one of the two roughened copper foils (1), and the roughened copper foil (2) is stacked on the surface of the copper foil. Then, a laminated copper foil having a thickness of 35 μm was produced. Next, a silicon oxide film was formed on the roughened copper foil (1) in the same manner as in Example 1 to produce a negative electrode plate (1).

  Using the obtained negative electrode, a battery was produced in the same manner as in Example 1.

(Example 3)
A battery was fabricated in the same manner as in Example 1 except that the negative electrode described below was used as the negative electrode.

  As a method for producing the negative electrode, first, copper particles were electrodeposited on both the front and back surfaces so that the surface roughness Rz was 5 μm, and the roughened copper foil (1) and the roughened copper foil having a copper foil thickness of 18 μm. (2) was produced. Next, a silicon oxide film was formed on the roughened copper foil (1) in the same manner as in Example 1 to produce a negative electrode plate (1). Further, an epoxy resin in which 5 wt% of particulate acetylene black is mixed with the binder on the roughened copper foil (2) is applied, and the negative electrode plate (1) is stacked thereon to produce a negative electrode. did. The thickness of the negative electrode after lamination was 45 μm as the total of the laminated copper foil and silicon oxide.

  Using the obtained negative electrode, a battery was produced in the same manner as in Example 1.

(Evaluation methods)
About the battery of Example 1, Example 2 and Example 3 manufactured as described above and the comparative battery 1, an ambient temperature was set to 25 ° C., and a charge / discharge test was performed using a continuous charge / discharge device. The test conditions were a charge current of 40 mA and a constant current charge up to a battery voltage of 4.2 V. After resting for 20 minutes, a discharge current of 40 mA was discharged to a final voltage of 2.5 V. This charge / discharge cycle was repeated twice, and the second discharge capacity was defined as the initial capacity. Regarding the cycle characteristics, the ratio of the initial capacity to the discharge capacity after the 100-cycle test was measured as a maintenance factor. The obtained results are shown in Table 1 and FIG.

  As shown in Table 1 and FIG. 4, the batteries of Example 1, Example 2 and Example 3 have a large capacity maintenance rate, and have a capacity similar to that of the comparative battery 1 and a capacity maintenance rate after 100 cycles. It can be seen that there is also an improvement.

  As a result of disassembling the battery after the test and observing the electrode plate, the negative electrode of the comparative battery 1 was not relieved from deformation due to expansion / contraction of the active material due to insertion / desorption of lithium ions during charging / discharging. It was thought that deformation such as wrinkles on the plate itself and breakage such as tearing occurred, which caused capacity reduction. On the other hand, the negative electrodes of Example 1, Example 2 and Example 3 had little deformation after the charge / discharge test, and the electrode plate did not break. In particular, in the copper foil that was not in direct contact with the active material, no breakage occurred, and it was confirmed that the continuity of the negative electrode group was kept good.

  In other words, by laminating the current collector, the deformation of the electrode plate due to the expansion of the active material during the reaction with lithium ions is alleviated, and the deformation and breakage of the negative electrode plate itself can be prevented. Is considered to have improved.

  In addition, in the battery of Example 1, since the deformation | transformation of the copper foil which is a collector is smaller than the comparative battery 1, it is thought that the capacity | capacitance maintenance factor was favorable.

  Moreover, in the batteries of Example 2 and Example 3, it is presumed that the progress of cracks entering the electrode plate stopped at the lamination interface, and the decrease in the capacity retention rate was smaller.

  Further, in the battery of Example 3, it is presumed that the change in the electrode plate resistance was small after the cycle test and the decrease in the capacity maintenance rate was smaller than that in Example 2.

  In addition, when making the laminated structure of the negative electrode of Example 3, a binder mixed with carbon particles was applied, but in addition to the carbon particles, a current collector integrated with a metal foil such as metal particles, carbon fibers, and inorganic fillers. However, this does not apply if it functions as

  In the present embodiment, the metal foil is 35 μm, but this thickness is a value determined by the thickness of the active material formed on the negative electrode plate, and practically the same thickness corresponding to the active material thickness of 4 to 50 μm. The reliability was confirmed and it was confirmed that the negative electrode plate was not broken.

  According to the present invention, it is possible to provide a lithium ion battery having a high capacity and excellent cycle characteristics.

Schematic longitudinal cross-sectional view of the negative electrode for lithium ion batteries in Embodiment 1 of this invention Schematic longitudinal sectional view of another negative electrode for a lithium ion battery according to Embodiment 1 of the present invention The figure which shows the charging / discharging cycling characteristics of the battery produced on the conditions shown to embodiment of this invention, and a comparative example

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Negative electrode 2 Active material 3 Current collector 4 Space | gap 5,6 Metal foil 7, 8 Metal foil thickness 9 Active material thickness 10 Low elastic modulus layer 11 High elastic modulus layer 12 Resin 13 Conductive particle or filler

Claims (7)

A laminated metal foil for a lithium ion battery in which a plurality of metal foils, each of which is a roughened surface of at least one of the first surface and the second surface facing the first surface,
The surface on which the laminated metal foils abut each other is a roughened surface,
There is a gap between the two metal foils with which the roughened surface abuts,
A laminated metal foil for a lithium ion battery. The thickness of the metal foil is 4 μm or more and 50 μm or less,
The laminated metal foil for a lithium ion battery according to claim 1. The surface roughness Rz of the roughened surface is 0.1 μm or more and 10 μm or less,
The laminated metal foil for a lithium ion battery according to claim 1. At least part of the gap between the two metal foils has a resin;
2. The laminated metal foil for a lithium ion battery according to claim 1, wherein The resin includes at least one of conductive particles and a filler;
The laminated metal foil for a lithium ion battery according to claim 4. The conductive particles include carbon;
The laminated metal foil for a lithium ion battery according to claim 5. A negative electrode having a negative electrode active material that occludes and releases lithium ions on the surface of the current collector made of the laminated metal foil for a lithium ion battery according to any one of claims 1 to 6,
A positive electrode having a positive electrode active material that absorbs and releases lithium ions on the surface of a current collector made of a thin metal plate;
A lithium ion conductive electrolyte or polymer electrolyte;
Lithium ion battery with

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