JP5190746B2 - Positive electrode of thin film lithium battery and thin film lithium battery - Google Patents

Description

  The present invention relates to a positive electrode of a thin film battery and a thin film battery using the electrode. In particular, the present invention relates to a positive electrode of a thin film battery that can suppress deterioration of a negative electrode facing the positive electrode.

  The lithium ion battery is a battery using an active material thin film that absorbs and releases lithium ions in both positive and negative electrodes. Usually, each positive and negative electrode is composed of a current collector and an active material thin film formed on the current collector. With respect to this lithium battery, it has been pointed out that repeated expansion and contraction of the electrode are caused by repeated charging and discharging, thereby causing structural destruction of the electrode and causing it not to function as a reversible electrode (for example, Non-Patent Document 1). . Specifically, the active material thin film is pulverized or the active material thin film is peeled off from the current collector.

  With respect to such a peeling problem, Patent Document 1 discloses a technique for separating an active material thin film into a columnar shape in the thickness direction for the purpose of relaxing stress generated in the active material thin film due to repeated charge and discharge. .

  On the other hand, in the invention according to Patent Document 2, in order to improve the flexible characteristics of the electrode, the positive electrode active material layer provided on one surface of the positive electrode current collector layer includes a plurality of active material layers at regular intervals by a plurality of recesses. The positive electrode is configured to be divided into units.

  Further, Patent Document 3 forms a narrow part of an electrode limited by a cut made in the electrode for the purpose of suppressing local concentration of a short-circuit current at the time of a short circuit between the electrodes. Has been disclosed as a resistance unit that limits the current path of the short-circuit current.

  Furthermore, in Patent Document 4, when the electrode is roll-pressed, in order to prevent waviness and wrinkles from occurring due to strain concentration at the boundary between the uncoated region and the coated region of the electrode active material thin film, A technique is disclosed in which a slit parallel to the width direction of the current collector substrate is formed in the current collector substrate in the electrode active material uncoated region.

  In addition, in Patent Document 5, for the purpose of increasing the electrode area, an electrode mixture containing a polymer as an active material is formed on a current collector to form an electrode, and the electrode after the film formation is predetermined. The technique which divides | segments into these several parts and dries the electrode divided | segmented into the said several part in predetermined temperature atmosphere is disclosed.

JP 2002-83594 A JP 2002-343340 A JP 2005-149794 JP JP 2000-208129 A JP 2000-208136 A "Examination of stress relaxation structure of Cu-Sn alloy thin film negative electrode for lithium secondary battery" Taku Sugiyama et al. 2004 Proceedings of the 45th Battery Conference, p.300

  On the other hand, the thin film battery has a problem of a decrease in battery capacity due to deterioration of the negative electrode active material thin film accompanying charge and discharge. The deterioration of the negative electrode active material thin film will be described with reference to FIGS. 5 and 6 by taking a thin film lithium battery as an example. In this thin film battery, a negative electrode current collector 1, a negative electrode active material thin film 2, a separator 3, a positive electrode active material thin film 4, and a positive electrode current collector 5 are laminated in order from the bottom as shown in FIGS. Have a configuration. These laminates are impregnated with an electrolytic solution, and a battery reaction is performed between the active material thin films 2 and 4 by occlusion / release of lithium ions.

  Here, when the charge / discharge cycle proceeds, a deteriorated region 2A is formed in the negative electrode active material thin film 2 as shown in FIG. 6 (B). The deteriorated region 2A is initially formed in a very narrow region of the negative electrode active material thin film 2, and expands as the charge / discharge cycle progresses. This is because when a part of the negative electrode active material thin film 2 deteriorates for some reason during the charge / discharge cycle, the deterioration region 2A reduces the amount of the negative electrode active material that contributes to the battery reaction compared to other normal regions. This is because the negative electrode active material around the deteriorated region 2A functions to compensate for the decrease, and the number of battery reactions per unit amount in the negative electrode active material thin film increases. As a result, it is considered that the deterioration of the negative electrode active material thin film 2 around the deteriorated region 2A is promoted, and the deteriorated region 2A expands. When the degradation region 2A is enlarged in this way, the electrode function is lowered and the battery capacity is lowered. Then, when the deteriorated region 2A expands to the vicinity of the periphery of the negative electrode active material thin film 2, the negative electrode active material around the deteriorated region 2A cannot compensate for the decrease amount, and finally the area of the opposing positive electrode active material thin film 4 Expansion of the degraded area stops in a narrower area.

  However, none of the above-described conventional techniques disclose any problem of suppressing the deterioration of the negative electrode active material thin film and no specific means for solving the problem.

  The invention according to Patent Document 1 separates the negative electrode active material thin film into a columnar shape in the thickness direction. This separation is performed so that the stress associated with expansion and contraction of the active material thin film during the charge / discharge can be relieved. It is a requirement that a cut is formed between the columnar portions. This cut is formed as follows, for example. First, irregularities are formed on the surface of the current collector, and irregularities are also formed on the surface of the active material thin film formed on the current collector. Then, the active material thin film expands and contracts due to charge and discharge after the first time, and stress acts, thereby forming a cut having the concave and convex valleys on the surface of the active material thin film as ends. Thus, in the invention according to Patent Document 1, the purpose is to absorb the stress accompanying the expansion and contraction of the active material thin film during charge and discharge, so it is sufficient that a break is formed between the columnar portions. There is no suggestion about the purpose of preventing deterioration of the negative electrode active material thin film or what kind of cut is necessary in order to prevent this deterioration.

  In the invention according to Patent Document 2, the purpose is to improve the flexible characteristics of the electrode. For the purpose of preventing the deterioration of the negative electrode active material thin film, and what kind of recess is specifically required to prevent this deterioration. There is no suggestion about.

  In the invention according to Patent Document 3, it is necessary to form a narrow part of the electrode as a resistance part that restricts the current path, and the individual electrodes limited by the cut are connected by the narrow part and separated into a plurality of individual regions. It has not been done.

  In the invention according to Patent Document 4, the slit is formed in the electrode active material uncoated region in the current collecting substrate, and the electrode active material thin film itself is not divided into a plurality of regions.

  The invention according to Patent Document 5 is similar to the invention according to Patent Document 1, and is divided into a plurality of parts, and only discloses that a break is formed between the electrodes. For this reason, there is no suggestion in this document about the purpose of preventing the deterioration of the negative electrode active material thin film and what kind of cut is necessary in order to prevent this deterioration.

  The present invention has been made in view of the above circumstances, and one of its purposes is to provide a positive electrode of a thin film battery that can suppress the deterioration of the negative electrode active material thin film and improve the decrease in battery capacity.

  Another object of the present invention is to provide a thin film battery that can suppress deterioration of the negative electrode active material thin film and improve the decrease in battery capacity.

  The positive electrode of the thin film battery of the present invention is a positive electrode of a thin film battery having a positive electrode active material thin film opposed to the negative electrode active material thin film. In order to suppress the deterioration of the negative electrode active material thin film, at least the surface of the positive electrode active material thin film facing the negative electrode active material thin film is divided into a plurality of minute areas.

  The reason why deterioration of the negative electrode active material thin film can be suppressed by this configuration will be described below. As described above, in the case of a thin film battery in which the areas of the positive electrode active material thin film and the negative electrode active material thin film are substantially the same, the negative electrode active material thin film deteriorates and the battery capacity decreases. Here, the deterioration state of the negative electrode active material thin film accompanying charging / discharging is demonstrated based on FIG. 1 for the example of the thin film lithium battery which made the positive electrode size smaller than the negative electrode size. This thin film battery has a configuration in which a negative electrode current collector 1, a negative electrode active material thin film 2, a separator 3, a positive electrode active material thin film 4, and a positive electrode current collector 5 are laminated in order from the bottom as shown in FIG. Have. These laminates are impregnated with an electrolytic solution, and a battery reaction is performed by occlusion / release of lithium ions between both active material thin films. However, in this battery, the size of the positive electrode composed of the positive electrode current collector 5 and the positive electrode active material thin film 4, that is, the area of the positive electrode when viewed in plan from the thickness direction, the negative electrode current collector 1 and the negative electrode active material thin film 5 and FIG. 6 is different from the battery of FIG.

  Here, when the charge / discharge cycle proceeds, a deteriorated region 2A is formed in the negative electrode active material thin film 2 as shown in FIG. As described with reference to FIG. 6, in the deteriorated region 2A, when some deteriorated region 2A is generated in a part of the negative electrode active material thin film 2, the deteriorated region 2A expands to the surroundings to form the deteriorated region 2A. However, when the deteriorated region 2A expands to the vicinity of the region without the positive electrode active material thin film 4, the battery reaction per unit amount in the negative electrode active material thin film 2 does not increase, and the expansion of the deteriorated region 2A stops. That is, when viewed as the entire negative electrode active material thin film, the formation of the deteriorated region 2A is suppressed to a very narrow range, and the influence on the decrease in capacity of the entire battery can be reduced.

  As is clear from this explanation, if at least the surface of the positive electrode active material thin film facing the negative electrode active material thin film is divided into minute areas, the positive electrode active material having a smaller area than the negative electrode active material thin film when viewed in units of individual small areas. Since the material thin films are combined, the deterioration of the negative electrode active material thin film can be suppressed as in the case described with reference to FIG.

  The division into the minute areas is preferably performed by arranging a dividing member having a pattern corresponding to the divided pattern of the minute areas on the surface of the positive electrode active material thin film facing the negative electrode active material thin film. For example, a thin film battery has a configuration in which a separator is interposed between a positive electrode active material thin film and a negative electrode active material thin film. At this time, by disposing a dividing member between the separator and the positive electrode active material thin film, the surface of the positive electrode active material thin film with which the dividing member is in contact is suppressed from moving ions between the negative electrode active material thin film and dividing. Since the surface of the positive electrode active material thin film with which the member is not in contact is allowed to move ions between the negative electrode active material thin film, the surface of the positive electrode active material thin film facing the negative electrode active material thin film is minute with a simple configuration. Can be divided into areas.

  This division member shall have a pattern according to the division pattern of a micro area. For example, by using a dividing member formed in a lattice shape, the surface of the positive electrode active material facing the negative electrode active material can be divided into a mesh shape.

  Moreover, as a material of this division member, the material which can suppress a movement of ion between a positive electrode active material thin film and a negative electrode active material thin film can be utilized suitably. For example, Fe, Co, Ni, Cu, Zn, Mn, Ti, Zr, etc., which are metals that do not easily store Li, are preferable as the material of the divided member.

  As described above, the positive electrode active material thin film only needs to be divided at least on the surface facing the negative electrode active material thin film. Therefore, the positive electrode active material thin film may be divided only in the surface facing the negative electrode active material thin film in the positive electrode active material thin film, and may not be divided in the thickness direction, or may be divided in the thickness direction. Any of these may be used.

  In order to divide only the surface facing the negative electrode active material thin film in the positive electrode active material thin film, it is preferable to use the dividing member as described above because it can be realized with a simple configuration. In addition, in order to divide the positive electrode active material thin film not only in the surface facing the negative electrode active material thin film but also in the thickness direction, the positive electrode active material thin film can be formed by using a mask having a division pattern of a minute area. After forming a typical planar positive electrode active material thin film, a part of this thin film may be removed with an appropriate jig or the like to form a gap.

  Moreover, the pattern which divides | segments a positive electrode active material thin film is not specifically limited. Typically, there is a division pattern in which a lattice-like gap can be formed, but a division pattern in which a gap inclined with respect to the vertical side and the horizontal side of the positive electrode active material thin film may be formed.

  On the other hand, the thin film battery of the present invention is characterized by using the positive electrode described above. This battery typically has a configuration in which an electrolyte solution is impregnated with a laminate in which a negative electrode current collector, a negative electrode active material thin film, a separator, a positive electrode active material thin film, and a positive electrode current collector are sequentially laminated. According to this thin film battery, deterioration of the negative electrode active material thin film can be suppressed to prevent a decrease in battery capacity.

  In particular, it is preferable that the area of the negative electrode active material thin film facing the positive electrode active material thin film is larger than the total area of the minute areas of the positive electrode active material thin film. In the invention according to Patent Document 1, the active material thin film expands and contracts due to charge and discharge, stress acts, and a cut is formed with the valleys of the irregularities on the surface of the active material thin film as the ends. Is approximately equal to the area of the active material thin film before being divided. On the other hand, if the area of the negative electrode active material thin film facing the positive electrode active material thin film is made larger than the total area of the fine areas of the positive electrode active material thin film, the deterioration region can be expanded more reliably in the negative electrode active material thin film. Can be suppressed.

  According to the thin film lithium battery of the present invention, it is possible to suppress the deterioration of the negative electrode active material thin film by dividing at least the negative electrode side surface of the positive electrode active material thin film into minute areas. As a result, it is possible to improve the decrease in battery capacity associated with the charge / discharge cycle.

  Embodiments of the present invention will be described below.

Example 1
First, the embodiment of the present invention will be described with reference to FIG. 2 by taking a lithium ion battery having a positive electrode active material thin film divided in a lattice shape as an example.

  As shown in FIG. 2A, this battery has a structure in which a negative electrode current collector 1, a negative electrode active material thin film 2, a separator 3, a positive electrode active material thin film 4, and a positive electrode current collector 5 are laminated. These laminates are impregnated with an electrolytic solution, and a battery reaction is performed between the active material thin films 2 and 4 by occlusion / release of lithium ions.

  For the negative electrode current collector 1, a metal foil having excellent corrosion resistance against the negative electrode electrochemical reaction can be used. Specific examples include one selected from copper (Cu), nickel (Ni), iron (Fe), chromium (Cr), and alloys thereof. Since these metals do not form an intermetallic compound with lithium (Li), the negative electrode active material thin film 2 causes structural breakdown due to problems due to the intermetallic compound with lithium, specifically, expansion / contraction due to charge / discharge. It is possible to prevent such problems that the current collecting property is lowered or the joining property of the negative electrode active material thin film 2 is lowered and the negative electrode active material thin film 2 is easily detached from the current collector 1.

  The negative electrode active material thin film 2 is composed of a layer containing an active material capable of occluding lithium. For example, in addition to graphite and silicon, one selected from the group consisting of Li metal and a metal capable of forming an alloy with Li metal, or a mixture or alloy thereof can be suitably used. Examples of the metal capable of forming an alloy with Li include at least one selected from the group consisting of aluminum (Al), silicon (Si), tin (Sn), bismuth (Bi), and indium (In). For example, after silicon is dispersed in a solvent, it is applied to the negative electrode current collector 1 and dried to form the negative electrode active material thin film 2. As this solvent, PVDF (polyvinylidene fluoride) or PTFE (polytetrafluoroethylene) dissolved in NMP (N-methylpyrrolidone) can be used.

  The formation method of the negative electrode active material thin film 2 is preferably a vapor deposition method. Examples of the vapor deposition method include a PVD (physical vapor phase synthesis) method and a CVD (chemical vapor phase synthesis) method. Specifically, examples of the PVD method include a vacuum deposition method, a sputtering method, an ion plating method, and a laser ablation method, and examples of the CVD method include a thermal CVD method and a plasma CVD method. In addition, a plating method can be used as a method for forming the negative electrode active material thin film 2.

For the separator 3, a porous material impregnated with an electrolytic solution in which a lithium salt is dissolved in an organic solvent is usually used. As specific lithium salts, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiCF ₃ SO ₃ , LiCF ₃ CO _{2 and the} like are used. Organic solvents include ethylene carbonate (EC), diethyl carbonate (DEC), propylene carbonate (PC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane (DME), tetrahydrofuran (THF) Organic solvents such as diethyl ether (DEE) and acetonitrile (AN) can be used singly or in combination. As the porous material, a porous body made of a resin such as polypropylene or polytetrafluoroethylene can be used. Needless to say, known lithium salts and organic solvents other than those described above can be used.

The positive electrode active material thin film 4 is composed of a layer containing an active material that occludes and releases lithium ions. In particular, one selected from the group consisting of oxides such as lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ) and olivine-type lithium iron phosphate (LiFePO ₄ ). Or a mixture thereof can be preferably used. More specifically, after the LiCoO ₂ is dispersed in a solvent, the positive electrode active material thin film 4 can be formed by applying it to the positive electrode current collector 5 and drying it. As this solvent, PVDF or PTFE dissolved in NMP can be used.

  In this positive electrode active material thin film 4, at least the surface facing the negative electrode active material thin film 2, that is, the contact surface with the separator 3 in this example, is divided into a plurality of minute areas 4A. In FIG. 2, a plurality of microscopic areas 4 </ b> A are formed by forming lattice-like gaps 6 in the positive electrode active material thin film 4. A perspective view of this active material thin film is shown in FIG. A grid-like gap 6 is formed in the positive electrode active material thin film 4, and the gap 6 has a depth extending to the front and back of the thin film 4. As a result, a large number of block-like minute areas 4A are formed on the positive electrode current collector 5 to constitute the positive electrode active material thin film 4. Further, since the contour area of the positive electrode active material thin film 4 is equal to that of the negative electrode active material thin film 2 (FIG. 2), the total area of the contact surface with the separator 3 in each block-like minute area 4A is This is smaller than the area of the contact surface with the separator 3.

  The positive electrode active material thin film 4 can be formed using a wet method or a dry method. Examples of the wet method include a sol-gel method, a colloid method, and a casting method. Examples of the dry method include a vapor deposition method that is a vapor deposition method, an ion plating method, a sputtering method, a laser ablation method, and the like. At the time of the film formation, for example, the positive electrode active material thin film 4 is formed using a mask having a lattice pattern, or the flat positive electrode active material thin film 4 is formed, and then the lattice-like pattern that passes through the thin film 4 from the front and back is formed. The positive electrode active material thin film 4 divided into a lattice shape can be formed by attaching the streaks with an appropriate instrument.

  For the positive electrode current collector 5, a metal foil having excellent corrosion resistance against the positive electrode electrochemical reaction can be used. Specific examples include one selected from aluminum (Al), nickel (Ni), alloys thereof, and stainless steel.

  And such a laminated body is comprised as a battery by being immersed in electrolyte solution. As described in the above description of the “separator”, an electrolytic solution in which a lithium salt is dissolved in an organic solvent can be suitably used as the electrolytic solution.

  When charging / discharging with such a thin film battery, as shown in FIG. 2 (B), the negative electrode active material thin film 2 may be partially deteriorated for some reason to form a deteriorated region 2A. This deterioration includes (1) peeling of the negative electrode active material thin film 2 from the negative electrode current collector 1, (2) pulverization of the negative electrode active material thin film 2, and (3) immobilization of lithium ions. When this deteriorated area 2A occurs, in the negative electrode active material thin film 2, the amount of negative electrode active material that contributes to the battery reaction is reduced compared to other normal areas, so the negative electrode active material around the deteriorated area 2A decreases. The deterioration region 2A expands to the surroundings. However, since the positive electrode active material thin film 4 is divided into a plurality of minute areas 4A by the lattice-like gaps 6, there is a gap 6 without the positive electrode active material thin film 4 at a position opposite to the negative electrode active material thin film 2, The battery reaction does not substantially occur in the negative electrode active material thin film portion facing the gap 6. For this reason, the negative electrode active material thin film portion facing the gap 6 cannot guarantee the amount of decrease in the negative electrode active material in the deteriorated region 2A, and is only up to a region smaller than the bonding surface with the separator 3 in each minute region 4A. The deteriorated area 2A does not expand. As a result, the deteriorated region 2A of the negative electrode active material thin film 2 can be suppressed to a much smaller region than the case where there is no gap in the positive electrode active material thin film 4, and reduction in battery capacity can be reduced.

(Example 2)
Next, an embodiment of the present invention in which only the contact surface of the positive electrode active material thin film with the separator is divided into minute areas will be described with reference to FIG.

  The difference between this example and Example 1 is that the positive electrode active material thin film 4 is not divided in the thickness direction, and a mesh 7 (divided member) is used to divide the thin film 4 into minute areas 4A. That is. Since other configurations are the same as those in the first embodiment, the description thereof will be omitted, and the following description will be made focusing on these differences.

  In this example, a mesh 7 is interposed between the separator 3 and the positive electrode active material thin film 4. This mesh 7 is a lattice formed of Fe, Co, Ni, Cu, Zn, Mn, Ti, Zr, which is a metal that suppresses the movement of ions between the positive and negative electrode active material thin films 2, 4 and does not occlude Li. It is a member. The width of the strip-like line constituting the mesh 7 corresponds to the width of the gap 6 (FIGS. 2 and 3) in the first embodiment. That is, by using this mesh 7, only the contact surface of the positive electrode active material thin film 4 with the separator 3 can be divided into the minute areas 4A, and the positive electrode active material thin film 4 that is not divided in the thickness direction can be obtained. .

  According to the battery having this configuration, the portion of the surface of the positive electrode active material thin film 4 covered with the mesh 7 is inhibited from the battery reaction with the negative electrode active material thin film 2. Expansion of the deteriorated area 2A can be stopped in an area smaller than each configured grid. Accordingly, a decrease in battery capacity can be suppressed. Further, in this example, the positive electrode active material thin film 4 can be divided into a plurality of minute areas with a simple configuration in which the mesh 7 is simply sandwiched between the separator 3 and the positive electrode active material thin film 4, and the productivity is excellent. In particular, the positive electrode active material thin film 4 does not need to be divided into minute areas by providing a gap, and a planar thin film can be used.

<Test Example 1>
Each thin film lithium battery having the configuration of Example 1 and Example 2 and a thin film lithium battery of a comparative example in which the positive electrode active material thin film was not divided into minute areas were produced, and a charge / discharge cycle test was performed. The specifications of the battery used for the test were as follows: current collector for positive electrode: Al foil, thin film for positive electrode active material: LiCoO ₂ film, separator: LiPF ₆ dissolved in organic solvent (a mixture of DEC and EC) made of polypropylene A porous sheet impregnated, negative electrode active material thin film: Si. The test conditions are a charge end voltage: 4.2 V, a discharge end voltage: 2.75 V, and a discharge current: 1 mA / cm ² .

  As a result of this test, the capacity retention after 100 cycles was about 60% in Example 1 and Example 2, and about 30% in the comparative example. From this test result, it was confirmed that each example battery had a smaller decrease in battery capacity than the comparative example.

  The positive electrode and the battery of the present invention can be suitably used as a secondary battery that can be charged and discharged, particularly a lithium secondary battery that can suppress deterioration of the negative electrode active material thin film and has a small decrease in battery capacity. The battery is expected to be used as a power source for various electric / electronic devices such as a mobile type and a portable type.

In the principle explanatory view of the battery of the present invention, (A) shows the state of the battery before deterioration, and (B) shows the state of the battery after deterioration. It is explanatory drawing which shows this invention battery of Example 1, (A) shows the state of the battery before deterioration, (B) after deterioration. 3 is a schematic perspective view of a positive electrode used in the battery of Example 1. FIG. 4 is a schematic perspective view showing a battery of the present invention of Example 2. FIG. It is a model perspective view which shows the conventional battery. In the explanatory view showing the conventional battery, (A) shows the state of the battery before deterioration, and (B) shows the state of the battery after deterioration.

Explanation of symbols

1 Negative electrode current collector
2 Negative electrode active material thin film 2A Degraded area
3 Separator
4 Cathode active material thin film 4A Micro area
5 Positive current collector
6 Clearance
7 mesh

Claims (5)

A positive electrode of a thin film lithium battery having a positive electrode active material thin film facing the negative electrode active material thin film,
In order to suppress the deterioration of the negative electrode active material thin film, at least the positive electrode active material thin film is disposed on the surface of the positive electrode active material thin film facing the negative electrode active material thin film, so as to have at least a positive electrode active material. A positive electrode of a thin film lithium battery, wherein a surface of the material thin film facing the negative electrode active material thin film is divided into a plurality of micro areas. The positive electrode of the thin film lithium battery according to claim 1, wherein the dividing member is formed in a lattice shape. The material of the said division member is Fe, Co, Ni, Cu, Zn, Mn, Ti, Zr, The positive electrode of the thin film lithium battery of Claim 1 or 2 characterized by the above-mentioned. A thin film lithium battery using the positive electrode according to any one of claims 1 to 3. 5. The thin film lithium battery according to claim 4, wherein an area of a surface of the negative electrode active material thin film facing a positive electrode active material thin film is larger than a total area of the microscopic areas.

Images