JP2009054462A - Lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery in which elevation of voltage inside the battery caused by formation of hydrogen fluoride in an electrolytic liquid containing a fluorine based electrolyte and damage of an electrode can be suppressed, and rupture of the battery and leakage of the electrolytic liquid do not occur, and which has high safety and a long life. <P>SOLUTION: In the lithium-ion secondary battery which is equipped with a positive electrode that contains lithium ions and is chargeable and dischargeable, a negative electrode composed of a carbon material that stores or releases the lithium ions, a separator composed of porous polyethylene, and a non-aqueous electrolytic liquid in which the fluorine based electrolyte is dissolved, a negative electrode surface is covered with polymer gel having a fluoride ion trapping function. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery using a carbon material as a negative electrode, and relates to a lithium ion secondary battery in which gas generation related to the negative electrode is suppressed.

Currently, lithium ion secondary batteries used in portable devices such widespread cellular phone, as a positive electrode active material, LiCoO ₂
, LiNiO ₂ , LiMn ₂ O ₄ , ternary composite oxides in which a part of these active materials Co, Ni, and Mn is substituted with other metal elements, etc. A carbonate non-aqueous electrolyte containing a fluorine-based electrolyte such as lithium fluorophosphate (LiPF ₆ ) is used.
Such a lithium ion secondary battery is an excellent secondary battery having both high voltage and high energy density.

However, hydrogen fluoride (HF) and three kinds of water are caused by a reaction between a trace amount of water (H ₂ O) contained in the non-aqueous electrolyte and a fluorine-based electrolyte (for example, lithium hexafluorophosphate: LiPF ₆ ) that is an electrolyte anion. Phosphorus fluoride (PF ₃ ) is generated, and phosphorous trifluoride further promotes the decomposition of carbonate, which is the main component of the electrolytic solution, and generates organic fluorophosphoric acid. Such a reaction process occurs in a chain, which increases the battery internal pressure.

Furthermore, it is known that a solid electrolyte interface exists on the negative electrode surface. Its main component is lithium fluoride (LiF) produced by decomposition of lithium hexafluorophosphate in the non-aqueous electrolyte, which hinders effective use of lithium ions in the positive electrode.
In addition, phosphorus pentafluoride (PF ₅ ), which is a by-product at this time, reacts quickly with a small amount of moisture in the battery and generates a gas mainly composed of hydrogen fluoride (HF). Since this phosphorus pentafluoride (PF ₅ ) is unevenly distributed in the vicinity of the negative electrode due to decomposition of the electrolyte anion, hydrogen fluoride (HF) which is a reaction product of water and phosphorus pentafluoride is also generated in the vicinity of the negative electrode. Become. Since this hydrogen fluoride (HF) not only increases the internal pressure of the battery but also promotes the deterioration of the positive and negative electrodes, it is desirable to prevent diffusion to the entire battery.

In response to such problems, a method of reducing the amount of moisture brought into the battery system when manufacturing a battery as much as possible, a method of reducing moisture contained in the electrode by vacuum heating and drying, a non-aqueous electrolyte having a moisture content of 10 ppm or less There have been attempts to use a method for suppressing the decomposition of the electrolyte anion by regulating the operating temperature of the battery.
Further, a method for solving the above problem by providing a film on the carbon-based negative electrode, for example, coating with an acrylic resin film proposed in Patent Document 1, coating with a solid polymer electrolyte in Patent Document 2, and Patent Document 3 Forming a Li _x Si _y O _z film, or an electrode protective film formed from a fluorinated anionic surfactant described in Patent Document 4. Patent Document 5 also proposes a method of adding a lithium compound to the electrolytic solution in order to remove moisture and free acid in the nonaqueous electrolytic solution.

JP-A-8-195197 JP-A-7-235328 JP 2000-67865 A JP-A-9-92280 Japanese Patent Laid-Open No. 11-185811

  In view of such circumstances, the technical problem to be solved by the present invention is similar to the problem presented in the above-mentioned patent document, and the internal pressure of the battery due to the generation of hydrogen fluoride in a non-aqueous electrolyte containing a fluorine-based electrolyte. The purpose of the present invention is to provide a lithium ion secondary battery with high safety and long life, in which the rise of the battery and damage to the electrode are suppressed, and the battery does not rupture or the electrolyte leaks.

The invention according to claim 1 is a non-charged solution in which a chargeable / dischargeable positive electrode containing lithium ions, a negative electrode made of a carbon material that absorbs and releases lithium ions, a separator made of porous polyethylene, and a fluorine-based electrolyte are dissolved. A lithium ion secondary battery comprising a water electrolyte, wherein the negative electrode surface is coated with a polymer gel having a fluoride ion (F ⁻ ) trapping function.

  The invention described in claim 2 is the lithium ion secondary battery according to claim 1, wherein the polymer gel is a copolymer of monomers having a fluoride ion trap function.

  The invention according to claim 3 is the lithium ion secondary battery according to claim 1, wherein the polymer gel is a copolymer of methyl methacrylate, ethylene dimethacrylate, and a monomer having a fluoride ion trap function. is there.

  According to a fourth aspect of the present invention, the monomer having a fluoride ion trap function has an amide group, a nitrile group, an aryl group, or a heterocyclic ring having a double bond capable of addition polymerization without destroying the structure of the functional group. 4. The lithium ion secondary battery according to claim 2, wherein the lithium ion secondary battery is a compound.

  The invention according to claim 5 is the lithium ion secondary battery according to any one of claims 2 to 3, wherein the monomer having a fluoride ion trap function is an acrylamide compound monomer.

  The invention according to claim 6 is the lithium ion secondary battery according to claim 5, wherein the acrylamide compound monomer is composed of one or more of acrylamide, methacrylamide and polyacrylic acid ester.

  The invention according to claim 7 is the lithium ion secondary battery according to claim 6, wherein the substance amount ratio of the acrylamide compound monomer before the synthesis of the polymer gel is 15 to 25%.

  In the present invention, the surface of the carbon negative electrode is coated with a polymer gel having a function of trapping fluoride ions, whereby fluoride ions are obtained by secondary interaction between the functional groups provided in the polymer gel and fluoride ions. It suppresses the gas generation reaction in the battery and suppresses the increase in the internal pressure of the battery, and at the same time, suppresses the damage of the electrode, and is highly safe lithium without battery damage such as battery rupture or electrolyte leakage. An ion secondary battery is provided.

  In the lithium ion secondary battery according to the present invention, the carbon negative electrode is coated with a polymer gel having a fluoride ion trap function, thereby suppressing the hydrogen fluoride generation reaction that occurs on the surface of the negative electrode. A lithium ion secondary battery having a small amount and high safety can be obtained.

  The main point of the present invention is to fix hydrogen fluoride by chemical equilibrium, and in fixing hydrogen fluoride, the fluoride ion concentration in the battery system does not change beyond a certain concentration. Since hydrogen ions are also fixed at the same time, no hydrogen is generated.

  The method focuses on the secondary interaction between a fluoride ion and a functional group, which is also widely known in the field of organic synthesis. For example, in a lithium secondary battery, an electrolytic solution that is a source of hydrogen fluoride Patent Document 5 proposes a method for removing free acid and moisture in the electrolyte by adding a lithium compound to the electrolytic solution. In the present invention, a functional group that reacts with fluoride ions (an ion trap functional functional group). In this method, the generation of hydrogen fluoride is suppressed by trapping fluoride ions and hydrogen ions in the vicinity of the negative electrode by coating the negative electrode with a polymer gel having a base.

  In the present invention, fluoride ions and hydrogen ions are selectively trapped in the vicinity of the negative electrode. Therefore, there are high concentrations of fluoride ions and hydrogen ions in the vicinity of the ion trap functional substituent, which are in contact with the positive electrode. In this case, since the positive electrode is dissolved, the polymer gel having a fluoride ion trap function is provided away from the positive electrode.

Hereinafter, it demonstrates in detail using an Example. In addition, this invention is not limited only to a following example.
The positive electrode uses lithium cobaltate (LiCoO ₂ ) as the positive electrode active material, carbon black as the conductive agent, and polyvinylidene fluoride (PVdF) as the binder, and these are mixed sufficiently in a weight ratio of 90: 5: 5. Further, an appropriate amount of N-methyl-2-pyrrolidone (NMP) was added to form a slurry, which was then applied onto a 20 μm thick aluminum foil. After drying, it was rolled and cut so that the active material surface was a square with a side of 40 mm.

  The negative electrode uses artificial graphite powder as the negative electrode active material, carboxymethylcellulose sodium salt (CMC) aqueous solution as the thickener, and SBR latex as the binder, and sufficiently mixed at a weight ratio of 97: 2: 1. After adding water to form a slurry, the slurry was applied onto a copper foil, dried, and then rolled and cut so that the active material surface was a square having a side of 45 mm.

Preparation of the fluoride ion trap functional polymer gel was performed by the following synthesis method using acrylamide, 4-hydroxystyrene, acrylonitrile, or 4-vinylpyridine as the fluoride ion trap functional monomer.
The secondary interaction when using an amide is due to the hydrogen bond between the amino group and the fluoride ion, and this hydrogen bond has increased intramolecular polarization due to the induced effect derived from carbonyl oxygen. It is a stronger bond.

2-2 is a polymerization initiator for mixing a fluoride ion trap functional monomer, methyl methacrylate, and ethylene dimethacrylate as a cross-linking material in a substance ratio of 15:75:10 and dissolving them. An appropriate amount of an organic solvent in which 2000 ppm of '-azobisisobutyronitrile (AIBN) was dissolved was added to obtain a monomer solution. The organic solvent used was a mixture of ethylene carbonate, ethyl methyl carbonate and a volume ratio of 3: 7.
The fluoride ion trap functional polymer gel was copolymerized by heating the monomer solution in an electric furnace to prepare a three-dimensional network polymer. This polymer is gelled by infiltrating the electrolytic solution due to the presence of cross-linked and copolymerized methyl methacrylate.

  The ion trap functional polymer gel is coated on the negative electrode by dropping the above monomer solution on the negative electrode plate, applying it uniformly by spin coating, and then polymerizing it in an electric furnace at 80 ° C. for 20 minutes. Thus, a negative electrode according to the present invention was produced.

The test cell uses an aluminum laminate type cell, one positive / negative electrode with tabs each coated with an active material on one side, and a porous polyethylene separator layered between them to form an electrode element. Wrapped with a sufficiently large aluminum laminate, the liquid injection part was removed, and it was heat sealed. Thereafter, a solution prepared by dissolving 1.0 mol / L of lithium hexafluorophosphate in a solvent in which ethylene carbonate and ethyl methyl carbonate are mixed at a volume ratio of 3: 7 is used as an electrolytic solution (total acid amount of 10 ppm or less). 1.72 g was injected into the cell from the part.
The polymer covering the negative electrode is gelled by the injection of the electrolytic solution, and the coating becomes an ion trap functional polymer gel.

  After injecting the liquid, seal the infusion part with a vacuum heat sealer and attach an insulating tape to the back of the tab to prevent external short circuit. The sample was sandwiched between two acrylic plates and subjected to a swelling test.

  Invention cell No. No. 1 uses acrylamide as a fluoride ion trap functional monomer, and cell No. 1 of the present invention. 2 is 4-hydroxystyrene, cell No. of the present invention. 3 is acrylonitrile, cell No. of the present invention. 4 is one using 4-vinylpyridine as a fluoride ion trap functional monomer, and each cell was prepared in the same manner as in the above example except that the fluoride ion trap functional monomer was changed.

  As a comparative example, 2 parts by weight of polyacrylonitrile (PAN) is added to the artificial graphite powder 100, and an appropriate amount of N, N dimethylformamide (DMF) is added to form a paste, followed by vacuum drying at a temperature of 200 ° C. Then, the carbon material was lightly pulverized in a mortar to obtain a carbon material coated with PAN. A carbon material coated with an acrylic resin was mixed with polyvinylidene fluoride (PVdF) as a binder, and an appropriate amount of NMP was added to form a paste, which was applied onto a copper foil. After vacuum drying at 200 ° C., a rolled product was used as the negative electrode, and a cell was prepared in the same manner as the cell of the present invention except that, and comparative cell No. It was set to 10.

  As a conventional example, a negative electrode not coated with a monomer solution was used, and other than that, a cell was prepared in the same manner as the cell of the present invention. It was set to 20.

  The negative electrode of the cell of the present invention is covered with a polymer gel infiltrated with an electrolyte solution, whereas the comparative cell No. No. 10, a carbon material coated with an acrylic resin (polymer) was applied on an electrode and then vacuum-dried at 200 ° C. to obtain a negative electrode plate. . No. 3 uses acrylonitrile as a fluoride ion trap functional monomer. 10 polyacrylonitrile is used, and as described above, the structure of the negative electrode is composed of a material covered with a polymer gel infiltrated with an electrolyte and a carbon material coated with an acrylic resin (polymer). It is very different.

In the swelling test, the charge / discharge test of each produced cell was performed for 30 cycles, and the thickness of the cell before and after the charge / discharge test was measured was measured.
The measurement was performed at a portion other than the laminated portion where the acrylic plate was not applied, that is, the portion where the electrode element was located, and the test results are shown in Table 1.
In addition, in order to make the test easier, the cell of the present invention, the comparative example cell, and the conventional cell are below the position opposite to the position where the tabs projecting in the same direction of the electrode element are attached. The electrode element was wrapped with a sufficiently large aluminum laminate material so that a relatively wide range of aluminum laminate material existed, and the portion where this electrode element was not located was measured as a laminating portion.

  The initial charging conditions of the bulge test were constant current and constant voltage charging with a current of 0.1 CA and a voltage of 4.2 V, and the initial discharging conditions were constant current discharge with a current of 0.1 CA and a final voltage of 2.7 V. The subsequent 29 cycles were constant current and constant voltage charging with a current of 0.5 CA and a voltage of 4.2 V, the discharge conditions were a current of 0.5 CA and a final voltage of 2.7 V, and the test temperatures were all 25 ° C.

  As is clear from Table 1, the change in thickness at each part was measured before and after the test, but the amount of change in thickness was different. 1 is 0.33 mm, cell No. 1 of the present invention. 2 is 0.31 mm, the cell No. of the present invention. 3 is 0.43 mm, the cell No. of the present invention. 4 is 0.28 mm, cell No. 3, except for showing a slightly large difference, it is as small as 0.3 mm.

  On the other hand, the comparison cell No. No. 10 is 0.53 mm, which is clearly larger than the cell of the present invention. For example 20, a larger difference of 0.70 mm was measured.

From the results shown in Table 1, the cells of the comparative example and the conventional example show that the internal pressure is higher than that of the cell of the present invention.
The cause of the increase in the internal pressure is apparently due to the generation of gas due to the decomposition of the non-aqueous electrolyte. 1 to No. 4 is a comparison cell No. 10 and conventional cell no. Compared to 20, the amount of gas generated is reduced.

Conventional cell No. Compared with 20 of the present invention, cell No. 1 to No. 4 and comparative cell no. 10 is considered to have an effect of suppressing the amount of gas generated. It can be seen that the cell of the present invention is higher than 10.
About the reason, it is thought that the difference in the amount of gas generation is that the intermolecular force between the functional group and the fluoride ion and the contact area between them differ.

  Furthermore, the conventional cell No. in which nothing is applied to the cell of the present invention and the negative electrode. Comparison of 20 shows that gas generation is clearly suppressed, and it can be seen that the polymer gel covering the negative electrode shows the effect of suppressing the side reaction gas generation.

  In addition, since the decomposition reaction of the electrolyte in a lithium ion secondary battery is a chain reaction, once the reaction starts, it is impossible to stop it, so a substance that causes a chain at the time of the first reaction is created. The present invention, which uses no method, is an effective means of suppressing gas generation.

Claims (7)

  Lithium ions comprising a chargeable / dischargeable positive electrode containing lithium ions, a negative electrode made of a carbon material that absorbs and releases lithium ions, a separator made of porous polyethylene, and a non-aqueous electrolyte in which a fluorine-based electrolyte is dissolved A lithium ion secondary battery, wherein the negative electrode surface is coated with a polymer gel having a fluoride ion trap function in a secondary battery.   The lithium ion secondary battery according to claim 1, wherein the polymer gel is a copolymer of monomers having a fluoride ion trap function.   The lithium ion secondary battery according to claim 1, wherein the polymer gel is a copolymer of methyl methacrylate, ethylene dimethacrylate, and a monomer having a fluoride ion trap function.   The monomer having a fluoride ion trap function is a compound having an amide group, a nitrile group, an aryl group or a heterocycle having a double bond capable of addition polymerization without breaking the structure of the functional group. Item 5. The lithium ion secondary battery according to any one of Items 3 to 4.   The lithium ion secondary battery according to claim 2, wherein the monomer having a fluoride ion trap function is an acrylamide compound monomer.   The lithium ion secondary battery according to claim 5, wherein the acrylamide compound monomer is composed of one or more of acrylamide, methacrylamide, and polyacrylate.   The lithium ion secondary battery according to claim 6, wherein a substance amount ratio of the acrylamide compound monomer before synthesis of the polymer gel is 15 to 25%.