JP5116213B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a nonaqueous electrolyte secondary battery, and in particular, in a nonaqueous electrolyte secondary battery using a positive electrode active material mainly composed of olivine type lithium phosphate as a positive electrode, sufficient capacity is obtained and load characteristics are obtained. It is excellent in that it can be rapidly charged at a high rate of current.

  In recent years, non-aqueous electrolyte secondary batteries using a non-aqueous electrolyte and charging / discharging by moving lithium ions between the positive and negative electrodes are widely used as new secondary batteries with high output and high energy density. It came to be used.

In such a non-aqueous electrolyte secondary battery, LiCoO ₂ is generally used as the positive electrode active material in the positive electrode, and a lithium metal, a lithium alloy, or a carbon material capable of occluding and releasing lithium is used as the negative electrode active material in the negative electrode. In addition, as the non-aqueous electrolyte, an electrolyte made of a lithium salt such as LiBF ₄ or LiPF ₆ dissolved in an organic solvent such as ethylene carbonate or diethyl carbonate is used.

However, Co used for the positive electrode active material LiCoO ₂ has a limited reserve and is a scarce resource, and thus has a problem of high production costs. In addition, in the case of a non-aqueous electrolyte secondary battery using LiCoO ₂ as the positive electrode active material, there is a problem in that the thermal stability is extremely lowered at a high temperature that cannot be considered in a normal use state in a charged state. It was.

For this reason, in recent years, the use of LiMn ₂ O ₄ , LiNiO ₂ or the like as a positive electrode active material replacing the above LiCoO ₂ has been studied.

However, when LiMn ₂ O ₄ is used as the positive electrode active material, it is difficult to obtain a sufficient discharge capacity, and when the battery temperature increases, there are problems such as dissolution of Mn in this LiMn ₂ O _4. It was. Further, when LiNiO ₂ is used as the positive electrode active material, there are problems such as a low discharge voltage.

In recent years, it has been studied to use olivine type lithium phosphate such as lithium iron phosphate LiFePO ₄ as the positive electrode active material.

Here, the olivine-type lithium phosphate is a lithium composite compound represented by a general formula LiMPO ₄ (wherein M is at least one element selected from Co, Ni, Mn, and Fe). The operating voltage varies depending on the type of the core metal element M, the battery voltage can be arbitrarily selected by selecting M, and the theoretical capacity is relatively high at about 140 to 170 mAh / g, and the battery per unit mass. There is an advantage that the capacity can be increased.

Recently, a nonaqueous electrolyte secondary battery using lithium iron phosphate LiFePO ₄ in which the metal element M in the olivine type lithium phosphate is iron has been proposed as a positive electrode active material (for example, a patent) Reference 1).

Here, when the lithium iron phosphate LiFePO ₄ in which the metal element M in the olivine-type lithium phosphate is iron is used as the positive electrode active material, iron is produced in a large amount and is inexpensive. There is an advantage that the cost can be greatly reduced.

In addition, when LiFePO ₄ is used as the positive electrode active material, there is a report that it shows excellent load characteristics in the charging reaction (see, for example, Non-Patent Document 1). And in this report, if the positive electrode thickness is applied to about 70 μm (coating amount 10 mg / cm ² ), a cell using Li metal is prepared for the counter electrode, and constant voltage charging is performed, rapid charging is possible. It is shown.

However, constant voltage charging is difficult to control, and in the nonaqueous electrolyte secondary battery using LiFePO ₄ as the positive electrode active material as described above, when Li metal is used as the negative electrode active material, dendrite is charged and discharged. Has occurred, resulting in a problem that the cycle characteristics become very poor.

Furthermore, in the above non-aqueous electrolyte secondary battery, even when graphite is used as the negative electrode active material, since the insertion reaction of Li into the graphite is slow, the battery is charged rapidly when charged at a high rate. There was a problem that polarization at the time of reaction increased and a sufficient charge capacity could not be obtained, and Li was deposited on graphite as a negative electrode active material, resulting in deterioration of cycle characteristics.
JP 2002-110162 A Electrochemical and Solid-State Letters, 8 (1) A55-A58 (2005)

An object of the present invention is to solve the above problems in a non-aqueous electrolyte secondary battery using a positive electrode active material mainly composed of olivine-type lithium phosphate such as lithium iron phosphate LiFePO ₄ as a positive electrode. In such a non-aqueous electrolyte secondary battery, it is an object to provide a sufficient capacity and an excellent load characteristic so that rapid charging at a high rate of current can be appropriately performed.

In this invention, in order to solve the above problems, a positive electrode mixture layer containing a positive electrode active material mainly composed of olivine type lithium phosphate, a binder, and a conductive agent is formed on the positive electrode current collector. In a nonaqueous electrolyte secondary battery including a formed positive electrode, a negative electrode, and a nonaqueous electrolyte, a negative electrode active material mainly composed of silicon is used for the negative electrode, and the positive electrode current collector is disposed on the negative electrode. The thickness of the positive electrode mixture layer to be formed was adjusted to 25 μm or less per side.

  Here, as an olivine type lithium phosphate used for said positive electrode active material, it is preferable to use lithium iron phosphate, for example.

  The negative electrode is preferably formed by depositing a thin layer mainly composed of silicon on a negative electrode current collector, and separating the thin layer into a columnar shape by a cut in the thickness direction. .

  In the nonaqueous electrolyte secondary battery according to the present invention, a positive electrode in which a positive electrode mixture layer containing a positive electrode active material mainly composed of olivine type lithium phosphate and a binder is formed on a positive electrode current collector is used. In this case, since the thickness of the positive electrode mixture layer formed on the positive electrode current collector is 25 μm or less per side as described above, the load characteristic at the time of charging reaction in this positive electrode is improved, and rapid charging is performed at a high rate of current. Even in such a case, a sufficient charge capacity can be obtained.

  In the nonaqueous electrolyte secondary battery according to the present invention, since the negative electrode active material mainly composed of silicon is used for the negative electrode as described above, the insertion reaction of Li into silicon is faster than that of graphite. Even when fast charging is performed at a high rate of current, polarization during charging reaction is suppressed, and a sufficient charging capacity can be obtained, and Li can be deposited on silicon of the negative electrode active material. It is suppressed and the cycle characteristics are also improved.

  In addition, in the nonaqueous electrolyte secondary battery according to the present invention, when lithium iron phosphate is used as the olivine type lithium phosphate used for the positive electrode active material, the amount of iron produced is large and inexpensive as described above. The production cost of the positive electrode and the nonaqueous electrolyte secondary battery can be greatly reduced.

  Further, in the nonaqueous electrolyte secondary battery according to the present invention, as the negative electrode, a thin layer mainly composed of silicon is deposited on the negative electrode current collector, and the thin layer is formed into a columnar shape by a cut in the thickness direction. When the separated material is used, the Li insertion reaction into silicon can be performed more quickly, and a sufficient charge capacity can be obtained, and the deposition of Li on the negative electrode active material silicon can be further suppressed. Thus, the cycle characteristics are further improved. In addition, in order to perform the Li insertion reaction into silicon more rapidly, the above silicon is preferably amorphous or microcrystalline silicon.

  In addition, as the nonaqueous electrolyte in the nonaqueous electrolyte secondary battery of the present invention, those generally used in nonaqueous electrolyte secondary batteries can be used, for example, nonaqueous electrolytes in which an electrolyte is dissolved in a nonaqueous solvent. An electrolytic solution or the like can be used.

  Here, as the non-aqueous solvent, those generally used can be used. For example, cyclic carbonate, chain carbonate, ester, cyclic ether, chain ether, nitrile, amide Etc. can be used.

  As the cyclic carbonate, for example, ethylene carbonate, propylene carbonate, butylene carbonate and the like can be used, and those in which some or all of these hydrogen groups are fluorinated can also be used. Carbonate, fluoroethyl carbonate, etc. can be used. As the chain carbonate, for example, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl isopropyl carbonate and the like can be used, and some or all of these hydrogen groups are fluorine. It is also possible to use a modified one. Examples of esters that can be used include methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, and γ-butyrolactone. Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,4-dioxane, 1 , 3,5-trioxane, furan, 2-methylfuran, 1,8-cineol, crown ether, and the like can be used. Examples of the chain ethers include 1,2-dimethoxyethane, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, and butyl phenyl. Ether, pentylphenyl ether, methoxytoluene, benzylethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether , It can be used tetraethylene glycol dimethyl like. Moreover, acetonitrile etc. can be used as nitriles, and dimethylformamide etc. can be used as amides. In particular, use cyclic carbonates such as ethylene carbonate and propylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and dipropyl carbonate, and chain ethers such as 1,2-dimethoxyethane. Is preferable from the viewpoint of voltage stability, and a non-aqueous solvent in which ethylene carbonate and 1,2-dimethoxyethane are mixed is more preferable because of excellent voltage stability, low viscosity, and high dielectric constant.

Further, as the above electrolyte, those generally used in non-aqueous electrolyte secondary batteries can be used, and LiPF ₆ , LiAsF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (C _l F _{2l + 1} SO ₂₎ (in _{C m F 2m + 1 SO 2} ) ( wherein, l, m is an integer of 1 or _{more.), LiC (C p F} 2p + 1 SO 2) (C q F 2q + 1 SO 2) (C _r F _{2r + 1} SO ₂ ) (wherein p, q, and r are integers of 1 or more), lithium difluoro (oxalato) borate represented by the following chemical formula 1, and the like can be used. These electrolytes may be used alone or in combination of two or more. The electrolyte can be used by dissolving in the non-aqueous solvent at a concentration of 0.1 to 1.5M, preferably 0.5 to 1.5M.

  Hereinafter, the nonaqueous electrolyte secondary battery according to the present invention will be specifically described with reference to examples, and in the nonaqueous electrolyte secondary battery according to this example, charging at a high rate of current can be performed appropriately. At the same time, it is clarified that the charging characteristics of the positive electrode are improved. In addition, the nonaqueous electrolyte secondary battery in the present invention is not limited to those shown in the following examples, and can be implemented with appropriate modifications within a range not changing the gist thereof.

(Experiment)
In this experiment, N-methyl- was mixed with a mixture in which LiFePO _{4 as} a positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder were mixed at a weight ratio of 90: 5: 5. 2-Pyrrolidone was added to prepare a slurry of the mixture, and this slurry was applied to one side of a positive electrode current collector made of an aluminum foil by a doctor blade method, and then dried at 80 ° C. with a hot plate. A positive electrode composite containing LiFePO _{4 as} a positive electrode active material, acetylene black as a conductive agent, and polyvinylidene fluoride as a binder on one side of the positive electrode current collector is cut into a size of 2 cm and rolled using a roller. An agent layer was formed and vacuum-dried at 100 ° C.

  Here, in this experiment, the amount of the slurry of the mixture applied to one side of the positive electrode current collector was changed, and in Experimental Example 1, the positive electrode in which the thickness of the positive electrode mixture layer became 17 μm was tested. In Example 2, a positive electrode in which the thickness of the positive electrode mixture layer was 25 μm was produced, and in Experimental Example 3, a positive electrode in which the thickness of the positive electrode mixture layer was 60 μm was produced.

As shown in FIG. 1, each of the positive electrodes described above is used for the working electrode 1, a lithium metal is used for the negative electrode as the counter electrode 2 and the reference electrode 3, and ethylene carbonate and diethyl carbonate are in a volume of 3: 7. The reference electrode 3 is immersed in a cell 5 containing a non-aqueous electrolyte 4 in which 1M LiPF ₆ is dissolved in a solvent mixed in a ratio, and the working electrode 1 and the counter electrode 2 are placed between the working electrode 1 and the counter electrode 2. Each of the test cells using each of the positive electrodes described above was fabricated by immersing the separator 6 in the middle of the sample.

  And after charging each said test cell to 4.2V with the electric current of about 0.2 It, respectively, after performing the charge / discharge cycle which discharges to 2.0V with the electric current of about 0.2 It, respectively, The battery was charged to 4.2 V with a high-rate current of 10 It, and the charge capacity per 1 g of the positive electrode active material in each positive electrode was determined. The results are shown in Table 1 below.

  As is clear from this result, when the positive electrodes of Experimental Examples 1 and 2 in which the thickness of the positive electrode mixture layer formed on one side of the positive electrode current collector was 25 μm or less were used, Compared with the case of using the positive electrode of Experimental Example 3 in which the thickness of the positive electrode mixture layer to be formed is over 60 μm exceeding 25 μm, the charge capacity per 1 g of the positive electrode active material is greatly increased, and the current at a high rate The charging characteristics of the positive electrode were greatly improved.

Example 1
In Example 1, the same positive electrode as in Experimental Example 2 described above, in which the positive electrode material mixture layer having a thickness of 25 μm was formed on one surface of the positive electrode current collector, was used.

Further, as the negative electrode, a copper alloy foil (arithmetic average roughness Ra: 0.25 μm, thickness: 25 μm) in which the surface of the heat resistant rolled copper alloy foil is roughened by depositing copper by an electrolytic method is used. Used as a negative electrode current collector. Then, under the conditions of a DC pulse frequency of 100 kHz, a DC pulse width of 1856 ns, a DC pulse power of 2000 W, an argon gas flow rate of 60 sccm (standard cubic centimeter permanents), a gas pressure of 2.0 to 2.5 × 10 ⁻¹ Pa, and a formation time of 30 minutes. Sputtering was performed to deposit a negative electrode active material layer made of an amorphous silicon thin film having a thickness of 1 μm on the negative electrode current collector. When the negative electrode active material layer made of the amorphous silicon thin film is deposited in this way, the negative electrode current collector made of the amorphous silicon thin film is made rough because the surface of the negative electrode current collector is roughened. The active material layer was in a state of being separated into a columnar shape by the cut in the thickness direction. In this embodiment, a DC pulse is supplied as power for sputtering. However, sputtering can be performed under the same conditions with DC and high frequency.

  And what deposited the negative electrode active material layer which consists of an amorphous silicon thin film on the negative electrode collector as mentioned above was cut out to the size of 2.5 cm x 2.5 cm, and the negative electrode was produced.

As the non-aqueous electrolyte, the same solution as in the above experiment, in which 1M LiPF ₆ was dissolved in a solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7, was used.

  And in producing the nonaqueous electrolyte secondary battery of this Example 1, as shown in FIG.2 and FIG.3 (A), (B), the positive electrode in which the positive mix layer 11a was formed as mentioned above The positive electrode current collector tab 11c is attached to the positive electrode current collector 11b, and the negative electrode current collector tab 12b of the negative electrode 12 on which the negative electrode active material layer 12a made of an amorphous silicon thin film is formed as described above. 12c is attached, and a separator 13 made of porous polyethylene is sandwiched between the positive electrode 11 and the negative electrode 12, and the separator 13 is inserted into an exterior body 14 made of an aluminum laminate film. 500 μl of the nonaqueous electrolyte solution was added, and then the positive electrode current collector tab 11 c and the negative electrode current collector tab 12 c were taken out to seal the opening of the outer package 14. It was. The capacity of the nonaqueous electrolyte secondary battery of Example 1 was about 1.65 mAh.

  Next, Example 1 manufactured in this way was charged to 4.2 V at each charging current of 0.6 mA, 17 mA (about 10 It), 34 mA (about 20 It), and the charging characteristics at each charging current were examined. Discharge characteristics were examined by discharging to 2.0 V at a discharge current of 0.6 mA, and the results are shown in FIG.

  As a result, the nonaqueous electrolyte secondary battery of Example 1 can be charged properly even at a high current of 17 mA (about 10 It) or 34 mA (about 20 It), and a sufficient charging capacity can be obtained. I found out that

It is a schematic explanatory drawing of the test cell produced in the experiment of this invention. 1 is a schematic perspective view of a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. It is a schematic sectional drawing of the nonaqueous electrolyte secondary battery which concerns on the Example. It is the figure which showed the charging / discharging characteristic of the nonaqueous electrolyte secondary battery which concerns on the same Example.

Explanation of symbols

1 Working electrode (positive electrode)
2 Counter electrode (negative electrode)
DESCRIPTION OF SYMBOLS 3 Reference electrode 4 Non-aqueous electrolyte 5 Cell 6 Separator 11 Positive electrode 11a Positive electrode mixture layer 11b Positive electrode collector 11c Positive electrode current collection tab 12 Negative electrode 12a Negative electrode active material layer 12b Negative electrode current collector 12c Negative electrode current collection tab 13 Separator 14 Exterior body

Claims (3)

A positive electrode mixture layer including a positive electrode active material mainly composed of olivine-type lithium phosphate, a binder, and a conductive agent is provided on a positive electrode current collector, a negative electrode, and a nonaqueous electrolyte. In the non-aqueous electrolyte secondary battery, a negative electrode active material mainly composed of silicon is used for the negative electrode, and the thickness of the positive electrode mixture layer formed on the positive electrode current collector is 25 μm or less per side. A non-aqueous electrolyte secondary battery. 2. The nonaqueous electrolyte secondary battery according to claim 1, wherein the olivine type lithium phosphate is lithium iron phosphate. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode is formed by depositing a thin layer mainly composed of silicon on a negative electrode current collector, and the thin layer is formed in a thickness direction. A non-aqueous electrolyte secondary battery, wherein the non-aqueous electrolyte secondary battery is separated into columns by the cuts.

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