JP2009170384A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance charge and discharge capacity and cycle characteristics of a lithium battery using polyimide as a binder of a negative electrode. <P>SOLUTION: The lithium secondary battery has the negative electrode wherein negative electrode active material particles for occluding lithium from a positive electrode active material are bound at the time of initial charging, and a lithium source for supplying lithium to the negative electrode before the initial charging is arranged on the negative electrode or a negative electrode collector by electrically keeping in contact with each other. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery having improved charge / discharge capacity and cycle characteristics.

  In recent years, non-aqueous electrolyte secondary batteries such as lithium secondary batteries have been widely used as power sources for mobile phones, notebook computers, and the like. These non-aqueous electrolyte secondary batteries are larger in volume or weight capacity density than secondary batteries such as conventional alkaline storage batteries and can take out a high voltage. Has been widely adopted as a major contribution to the development of today's mobile devices.

  In lithium secondary batteries, carbon materials such as graphite and amorphous carbon capable of occluding and releasing lithium ions are widely used as negative electrode active materials. When producing a negative electrode using a carbon material, a slurry obtained by mixing a carbon material and a binder in a solvent is applied onto a negative electrode current collector such as a copper foil and dried to form a negative electrode active material layer. Was.

  Conventionally, polyvinylidene fluoride (PVdF) has been widely used as a binder. However, although polyvinylidene fluoride has excellent binding properties between the active material particles, the adhesion between the active material particles and the current collector is low, and when the charge / discharge cycle is repeated, the negative electrode active material layer from the current collector Detachment and dropout were caused, leading to a decrease in cycle capacity retention rate.

  In order to solve these problems, a negative electrode using a polyimide as a binder in a battery using carbon powder as a negative electrode active material has been proposed (for example, Patent Documents 1 and 2). As described in these prior arts, when polyimide is used as a binder, the adhesion between the carbon powder and the metal surface of the current collector is good, and the battery capacity can be maintained even after repeated charge / discharge cycles. Is less likely to decrease, and has an excellent effect even in the cycle life.

In a lithium secondary battery such as a lithium ion battery, a carbon material that does not contain lithium is used as the negative electrode active material, and the lithium contained in the positive electrode active material during the initial charge is occluded in the negative electrode. As a function.
However, the charge / discharge of a lithium secondary battery using polyimide as a binder has a low initial charge / discharge efficiency, and the battery capacity may be greatly reduced.
As a result of intensive studies on this phenomenon, it has been found that a part of lithium occluded in the negative electrode during the first charge remains without being released during the discharge, thereby greatly reducing the battery capacity. It is known that some lithium remains in the carbon material used for the negative electrode active material and cannot be completely taken out at the time of discharge, and there is an irreversible capacity. However, when polyimide is used as a binder, It is considered that the irreversible capacity increases due to an increase in lithium that is taken in and not used for discharge.

Japanese Patent No. 3311402 JP 2004-247233 A

  The present invention focuses on the fact that when a binder containing polyimide is used for the negative electrode of a lithium secondary battery, the irreversible capacity that cannot be taken out from the negative electrode during discharge out of lithium stored in the negative electrode during charging from the positive electrode increases. However, it is an object of the present invention to provide a lithium secondary battery having a large charge / discharge capacity and good cycle characteristics.

The present invention has a negative electrode in which negative electrode active material particles that occlude lithium from the positive electrode active material at the time of initial charge are bound with polyimide, and the negative electrode or the negative electrode current collector is attached to the negative electrode before the initial charge. A lithium secondary battery in which a lithium source for supplying lithium is in electrical contact.
In the lithium secondary battery, the negative electrode active material is graphite.
In the lithium secondary battery, the amount of the lithium source corresponds to 0.6 to 1.0 times the amount of electricity initially charged with respect to the negative electrode.
The lithium source is the lithium secondary battery made of lithium metal.

  According to the present invention, the binder made of polyimide is used as the binder for the negative electrode, and the negative electrode active material contains lithium in the negative electrode active material before the first charge in addition to lithium that is occluded during the first charge from the positive electrode active material. By occluding lithium, a lithium secondary battery with improved cycle characteristics and charge / discharge capacity can be provided.

  In the lithium secondary battery using polyimide as a binder for the negative electrode, the present invention behaves as if the polyimide used as the binder is a negative electrode active material, and the lithium occluded during charging is released during initial discharge. The result is a battery with a large irreversible capacity, resulting in a polyimide binder by arranging a lithium source such as metallic lithium that is in electrical contact with the negative electrode or the negative electrode current collector and releases lithium during discharge. It has been found that an increase in irreversible capacity that occurs in the above case is prevented, a lithium secondary battery having a large charge / discharge capacity and good cycle characteristics is provided.

  A lithium ion battery will be described as an example of the lithium secondary battery in the present invention. The positive electrode is composed of a positive electrode active material layer containing a positive electrode active material capable of occluding and releasing lithium ions and a positive electrode current collector, and the negative electrode is composed of polyimide as a negative electrode active material for occluding and releasing lithium ions. It is composed of a negative electrode active material layer and a negative electrode current collector bonded as a binder. These positive electrode and negative electrode are arranged to face each other via a separator to form a battery element made of a laminate, or are wound after being laminated in the order of separator / positive electrode / separator / negative electrode to produce a battery element. Thereafter, after the metallic lithium is disposed in electrical connection with the negative electrode or the negative electrode current collector, the battery element is housed in an outer container and impregnated with a non-aqueous electrolyte and sealed.

Aluminum, titanium, or an alloy thereof can be used as the positive electrode current collector, and copper, stainless steel, nickel, titanium, or an alloy thereof can be used as the negative electrode current collector.
Moreover, as a separator, porous films, such as polyolefin, such as a polypropylene and polyethylene, a fluororesin, are used.

The positive electrode is manufactured by applying a positive electrode active material on a positive electrode current collector, and a lithium-containing composite oxide is used as the positive electrode active material, specifically, a layered structure material represented by LiMO ₂ And a compound having a spinel structure represented by LiMn ₂ O ₄ . Here, M is at least one selected from Co, Ni, Mn, and Fe, and a part thereof may be substituted with other cations such as Mg, Al, and Ti.
A positive electrode active material is dispersed and kneaded in a solvent such as N-methyl-2-pyrrolidone (NMP) together with a conductivity-imparting agent such as carbon black and a binder such as polyvinylidene fluoride. The positive electrode active material layer can be obtained by a method such as drying the solvent after coating on the electric body. The positive electrode on which the positive electrode active material layer is formed can be adjusted to an appropriate density by a method such as pressing.

The negative electrode is manufactured by applying a negative electrode active material on a negative electrode current collector, and contains a conductivity-imparting agent such as graphite, amorphous carbon, carbon black, acetylene black, polyimide, or a precursor thereof. A slurry obtained by mixing an agent with a dispersion medium such as N-methyl-2-pyrrolidone is applied onto a current collector made of copper foil using a doctor blade, and then dried by heating to form a negative electrode active material layer. be able to. The obtained negative electrode can be adjusted to a predetermined density by a roll press or the like.
When a polyimide precursor is used instead of polyimide, it can be converted to polyimide by heat treatment at 200 ° C. to 400 ° C.

  An example of the polyimide is an aromatic polyimide containing an imide bond in the straight chain. Either a thermosetting polyimide soluble in a solvent such as N-methyl-2-pyrrolidone or a thermoplastic polyimide can be used. Moreover, it may replace with a polyimide and may use the polyimide precursor which polyimidizes by heat processing, such as a polyamic acid. For example, Ube Industries U-Varnish (polyamic acid type), Hitachi Chemical HCI series (polyamic acid type), OPI series (polyamideimide resin), Toyobo Viromax (polyamideimide resin), Eye. S. Examples include Pyre-ML (polyamic acid solution) and SKYBOND (polyimide varnish) manufactured by Tei.

  It is preferable that the polyimide in a negative electrode active material layer is the range of 2 mass%-20 mass% with respect to the mass of a negative electrode. When the amount is less than 2% by mass, the adhesion between the active materials or between the active material and the current collector is reduced, resulting in deterioration of cycle characteristics. When the amount is more than 20% by mass, the active material in the negative electrode mixture is reduced. A decrease in the material ratio causes a problem such as an increase in electrode resistance due to a decrease in negative electrode capacity.

By using polyimide as the binder, the adhesion between the negative electrode active material and the current collector is improved as compared with the case where polyvinylidene fluoride is used, and the electricity between the current collector and the negative electrode active material layer is maintained even after repeated charge and discharge. Therefore, good cycle characteristics can be obtained.
Batteries using polyimide may have a lower initial discharge capacity than those using polyvinylidene fluoride. This decrease in the initial discharge capacity is considered to appear in the same manner as an active material having an irreversible capacity because the polyimide itself takes in lithium.
Lithium is captured by functional groups such as the benzene ring, nitrogen atom, and oxygen atom in the polyimide during charging and cannot be separated during discharging, or lithium reacts with these atoms, resulting in consumption of lithium. It is presumed that
As described above, since a phenomenon similar to the irreversible capacity appears, it is considered that the performance of graphite having excellent characteristics as a negative electrode active material cannot be sufficiently utilized.

  In the lithium secondary battery of the present invention, the battery element including the positive electrode and the negative electrode is provided with a lithium source different from the positive electrode in the battery, and discharged from the positive electrode into the negative electrode during initial charging. It is equipped with a lithium source that compensates for the irreversible capacity of lithium that is sometimes not extracted. As a result, the charge / discharge efficiency can be improved and the capacity loss of the battery can be suppressed.

The amount of the lithium source disposed in the battery for the purpose of lithium supplementation is preferably 0.6 to 1.0 times, more preferably 0.8 to 1.0 times the irreversible capacity of the whole negative electrode. It is.
When the amount of the lithium source is larger than the irreversible capacity of the negative electrode, that is, 1 or more times, the lithium source remains in the battery without being occluded in the negative electrode. The remaining lithium source is not only useless, but lithium dendrite may grow, and there may be a problem in terms of safety, such as a short circuit when the battery is dropped or shocked. In addition, when charging from the positive electrode to the negative electrode, excess lithium that is not occluded by the negative electrode may be deposited on the surface of the negative electrode, leading to deterioration of cycle characteristics.

On the other hand, when the amount of the lithium source is less than 0.6 times the irreversible capacity, the effect of supplementing the irreversible capacity is insufficient.
For the irreversible capacity, a lithium source corresponding to the irreversible capacity determined from the difference between the initial charge amount of the produced lithium secondary battery and the charge amount that can be discharged for the first time can be arranged.

The lithium source arranged for irreversible capacity compensation is preferably metallic lithium. Lithium metal is preferable because its electric capacity is as large as 3860 mAh / g and its density is as small as 0.534 g / cm ³ . As metallic lithium, it is preferable to use a thin lithium foil.
The irreversible capacity is as follows:
W: Ratio of polyimide in negative electrode mixture (%)
α: Irreversible capacity of the polyimide in the negative electrode mixture (mAh / g)
β: Irreversible capacity of graphite (mAh / g)
D: Negative electrode mixture weight per unit area of negative electrode (g / cm ² )
S: Total area of the negative electrode in the lithium secondary battery (cm ² )
Irreversible capacity (mAh) = {W · α / 100 + (1−W / 100) · β} · D · S

  Lithium metal is electrically isolated from the positive electrode and placed in contact with the electrolyte solution, and is left in electrical contact with at least one of the negative electrode active material layer and the current collector. By doing so, it is possible to electrochemically introduce lithium into the negative electrode before charging the battery. Moreover, since the introduction of lithium into the negative electrode proceeds faster as the temperature increases, it is preferably carried out while keeping the temperature of the battery elevated.

In the lithium secondary battery of the present invention, a nonaqueous electrolytic solution containing a lithium salt as a supporting electrolyte in a nonaqueous solvent can be used.
The lithium salt, lithium imide _{salt, LiPF 6, LiAsF 6, LiAlCl} 4, LiClO 4, LiBF 4, LiSbF 6 , and the like. Of these, LiPF ₆ and LiBF ₄ are particularly preferable. LiN The lithium imide salt _{(C k F 2k + 1SO 2} ) (C m F 2m + 1SO 2) (k, m are each independently 1 or 2). These can be used alone or in combination of two or more.

  The non-aqueous solvent is at least selected from organic solvents such as cyclic carbonates, chain carbonates, aliphatic carboxylic acid esters, γ-lactones, cyclic ethers, chain ethers and their fluorinated derivatives. One kind of organic solvent is used. More specifically, cyclic carbonates: propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (BC), and their derivative chain carbonates: dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl Methyl carbonate (EMC), dipropyl carbonate (DPC), and derivatives thereof, aliphatic carboxylic acid esters: methyl formate, methyl acetate, ethyl propionate, and derivatives thereof, γ-lactones: γ-butyrolactone, and These derivatives, cyclic ethers: tetrahydrofuran, 2-methyltetrahydrofuran chain ethers: 1,2-ethoxyethane (DEE), ethoxymethoxyethane (EME), diethyl ether, and derivatives thereof, Others: dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylformamide, dioxolane, acetonitrile, propyl nitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1, 3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ethyl ether, 1,3-propane sultone, anisole, N-methylpyrrolidone, fluorinated carboxylic acid ester A seed or a mixture of two or more can be used. In addition to these, vinylene carbonate (VC) may be added.

  The lithium secondary battery of the present invention can have a cylindrical shape, a square shape, a coin shape, or the like, which is sealed with a metal can or a film-like exterior material in which a synthetic resin film and a metal foil are laminated. It can be used appropriately according to the purpose of use of the battery.

The present invention will be described below with reference to examples and comparative examples of the present invention.
Example 1
(Preparation of polyimide-containing negative electrode)
Natural graphite powder having an average particle diameter of 20 μm as a negative electrode active material and polyimide varnish (U-Vanice-A manufactured by Ube Industries) made of polyamic acid as a polyimide precursor as a binder at a mass ratio of 95: 5 A slurry was prepared by uniformly dispersing in -2-pyrrolidone (NMP). After applying this slurry on a 15 μm thick copper foil serving as a negative electrode current collector, N-methyl-2-pyrrolidone (NMP) was evaporated at 125 ° C. for 10 minutes to form a negative electrode active material layer. After pressing, heat treatment was performed at 350 ° C. for 1 hour in a nitrogen atmosphere to prepare a negative electrode, and the amount of the negative electrode mixture per unit area after drying was 0.008 g / cm ² .
(Preparation of positive electrode)
Li-Mn ₂ O ₄ powder having an average particle size of 10 μm as a positive electrode active material, polyvinylidene fluoride as a binder, and carbon black as a conductivity-imparting agent at a mass ratio of 92: 4: 4 and N-methyl-2-pyrrolidone ( NMP) was uniformly dispersed to prepare a slurry. The slurry was applied on an aluminum foil having a thickness of 20 μm serving as a positive electrode current collector, and then a positive electrode active material layer was formed by evaporating NMP at 125 ° C. for 10 minutes, followed by pressing to produce a positive electrode. The amount of positive electrode mixture per unit area after drying was 0.02 g / cm ² .
The electrolyte used was a solution of 1 mol / L LiPF ₆ as an electrolyte in ethylene carbonate (EC): diethyl carbonate (DEC) = 30: 70 (volume%) as a solvent.

(Production of coin-type battery)
The previously produced negative electrode was punched into a circle with a diameter of 12 mm, placed in one of a stainless steel coin-shaped case, a separator laminated with polyethylene and polypropylene was placed on the negative electrode active material layer, and impregnated with an electrolytic solution. Thereafter, metallic lithium was laminated on the separator, and the other coin-type case was covered via a gasket and caulked to produce a coin-type battery.
The charge / discharge capacity of the negative electrode of the coin-type battery was measured by a constant current charge / discharge test method. At this time, the final voltage during charging was 0 V, the final voltage during discharging was 1.0 V, and the current value was 40 mA / g with respect to the negative electrode mixture amount.
Similarly, the charge / discharge capacity of the negative electrode prepared using polyvinylidene fluoride as a binder was measured in the same manner. From the difference between the two, the irreversible capacity of polyimide was 1100 mAh / g.

(Production of stacked battery)
The positive electrode and the negative electrode prepared previously were cut into 5 cm × 5 cm, respectively, and used as electrodes. An aluminum tab having a width of 5 mm, a length of 3 cm, and a thickness of 0.1 mm was used as a positive electrode current collector, and a nickel tab of the same size was used. Each of the negative electrode current collectors was ultrasonically welded. A negative electrode having a tab connected to both sides of a separator in which polyethylene and polypropylene of 6 cm × 6 cm were laminated and a positive electrode were laminated so that the active material layer faces each other.
A battery composed of two 8 cm × 8 cm film-like packaging materials laminated in the order of polyethylene film / aluminum foil / polyethylene film by heat-sealing, and comprising a negative electrode, a separator, and a positive electrode through an opening on one side A stack of elements was inserted.

The proportion of polyimide in the negative electrode active material layer is 5%, the irreversible capacity of polyimide in the negative electrode material layer is 1100 mAh / g, the irreversible capacity of graphite is 35 mAh / g, the negative electrode active material mass per negative electrode area is 0.008 g / cm ² , When the irreversible capacity of the whole negative electrode is calculated with the total area of the negative electrode active material layer being 25 cm ² , {5 · 1100/100 + (1-5 / 100) · 35} × 0.008 × 25 = 17.7 mAh
This was converted to the mass of metallic lithium, and a value of 0.00456 g was obtained.
Therefore, when the ratio of the negative electrode to the irreversible capacity is 1, the amount of lithium metal is 0.00456 g.

Further, as a lithium source, a lithium metal foil having a thickness of 0.1 mm and a mass of 0.00456 g is inserted between the negative electrode current collector and the outer package, impregnated with a non-aqueous electrolyte, and then opened under vacuum. Was sealed by thermal fusion to produce a laminated battery covered with a film-like exterior material. Since the inside of the film-like packaging material is always pressed at a surface pressure close to atmospheric pressure by sealing under vacuum, the electrical contact between the attached lithium metal foil and the negative electrode current collector is maintained, It is also in contact with a non-aqueous electrolyte.
The laminated battery covered with the above-described film-shaped packaging material was aged at 25 ° C. for 3 days to dope lithium from the metal lithium to the negative electrode.

(Battery charge / discharge test)
The manufactured laminated battery was measured for the initial charge / discharge capacity at 20 ° C. at a current value of 6.0 mA by a constant current charge / discharge test method, and then the discharge capacity and capacity retention rate after 500 cycles at 45 ° C. at a current value of 50 mA. Was measured. In addition, the final voltage at the time of charge is 4.2V, the final voltage at the time of discharge is 3.0V, and is shown in Table 1.

In Table 1, initial discharge capacity (mAh / g), charge / discharge efficiency (%), capacity retention after 500 cycles, and battery capacity (mAh / g) are shown. Further, the ratio of the amount of metallic lithium to the irreversible capacity of the negative electrode was expressed as a compensation magnification. The capacity retention ratio (%) is calculated by multiplying the discharge capacity after the cycle by the initial discharge capacity and multiplying by 100.
Further, FIG. 1 shows changes in the initial discharge capacity with respect to the respective filling rates when polyimide and polyvinylidene fluoride are used as the binder.
FIG. 2 shows the change in capacity after 500 cycles with respect to the filling rate when polyimide and polyvinylidene fluoride are used as the binder.

Example 2-4
Instead of the ratio and mass of the capacity of metallic lithium mounted inside the battery in Example 1 to the irreversible capacity, the ratio and mass were Example 2: Ratio 0.9 / mass 0.00410 g, Example 3: Ratio 0, respectively. .8 / mass 0.00365 g, and Example 4: A test battery was produced in the same manner as in Example 1 except that the ratio was 0.6 / mass 0.00274 g, and evaluation was conducted in the same manner as in Example 1. The results are shown in Table 1.

Comparative Example 1-4
Instead of the ratio and mass of the capacity of metallic lithium attached to the inside of the battery in Example 1 to the irreversible capacity, the ratio and mass were respectively Comparative Example 1: Ratio 0.4 / mass 0.00182 g, Comparative Example 2: Ratio 0 .2 / mass 0.00091 g, Comparative Example 3: Same as Example 1 except that no metallic lithium was mounted inside, and Comparative Example 4: a ratio 1.1 / mass 0.00502 g was produced. Comparative test batteries were prepared and evaluated in the same manner as in Example 1, and the results are shown in Table 1.

Comparative Example 5-9
The irreversible capacity of the negative electrode produced by using polyvinylidene fluoride as a binder for the negative electrode without performing the heat treatment after pressing the negative electrode is based on the irreversible capacity of 35 mAh / g of graphite. Was determined by the following equation.
35 x 0.008 x 25 = 0.000448 mAh
Next, the ratio of the amount of metallic lithium attached to the inside of the battery to the irreversible capacity of the negative electrode, and the mass were Comparative Example 5: Ratio 1.0 / mass 0.00160 g, Comparative Example 6: Ratio 0.8 / mass 0.00128 g, Comparative Example 7: Ratio 0.6 / mass 0.00096 g, Comparative Example 8: Ratio 0.4 / mass 0.00064 g, and Comparative Example 9: Same as Example 1 except that metal lithium was not attached. A battery sealed with a film sheathing material was produced by the method, and the characteristics were evaluated. The results are shown in Table 1.

  In the lithium secondary battery of the present invention, in a battery using polyimide as a binder, the irreversible capacity of the negative electrode is compensated by arranging a lithium source such as lithium metal, and both initial capacity and post-cycle capacity It is possible to provide an excellent battery.

FIG. 1 is a diagram for explaining changes in the initial discharge capacity with respect to the respective filling rates when polyimide and polyvinylidene fluoride are used as binders. FIG. 2 is a diagram for explaining the change in capacity after 500 cycles with respect to the filling rate when polyimide and polyvinylidene fluoride are used as binders.

Claims (4)

  It has a negative electrode in which negative electrode active material particles that occlude lithium from the positive electrode active material during the first charge are bound with polyimide, and lithium is supplied to the negative electrode or the negative electrode current collector before the initial charge. A lithium secondary battery, wherein a lithium source is disposed in electrical contact.   The lithium secondary battery according to claim 1, wherein the negative electrode active material is graphite.   3. The lithium secondary battery according to claim 1, wherein the amount of the lithium source corresponds to 0.6 to 1.0 times the amount of electricity initially charged to the negative electrode.   The lithium secondary battery according to claim 1, wherein the lithium source is made of lithium metal.

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