JP2010086681A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery with rise of inner resistance restrained due to repetition of charge and discharge. <P>SOLUTION: The nonaqueous electrolyte secondary battery 1 includes a cathode containing a cathode active material, an anode containing an anode active material and nonaqueous electrolyte. As the cathode active material, a compound expressed in a general formula: Li<SB>v</SB>M1<SB>w</SB>PO<SB>4</SB>(provided, 0≤v≤2, 0.8≤w≤1.2, and M1 is 3d transition metal) is used, and the nonaqueous electrolyte contains 3.0% by mass or less of vinylene carbonate and 4.0% by mass or less of organic silicide with an Si-N bond to a total mass of the nonaqueous electrolyte. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

Non-aqueous electrolyte secondary batteries (hereinafter also simply referred to as “batteries”) have high voltage and high energy density, and are therefore widely used, for example, as mobile phones and notebook personal computer power supplies.
In such a non-aqueous electrolyte secondary battery, a carbon material is generally used as a negative electrode active material, and a lithium transition metal composite oxide is used as a positive electrode active material. LiPF ₆ is used as a non-aqueous electrolyte in a solvent such as ethylene carbonate (EC). A non-aqueous electrolyte solution in which a supporting salt such as is dissolved is used.

Then, by adding vinylene carbonate (vinylene carbonate) to the non-aqueous electrolyte as described above, a stable SEI film (solid electrolyte interface) is formed on the negative electrode by the first charge / discharge, and the electrolyte solution is formed by this film. It is known that decomposition is suppressed (see, for example, Patent Document 1 and Patent Document 2).
Further, Patent Document 3 and Patent Document 4 include a non-aqueous electrolyte containing an organosilicon compound having a Si—N bond, thereby reacting the compound with water or hydrogen fluoride in the electrolyte, and A technique for changing to a compound that does not adversely affect the material is disclosed.
JP 2003-331927 A JP-A-8-96852 Japanese Patent Laid-Open No. 11-16602 JP 2003-7332 A

By the way, a compound represented by the general formula Li _v M1 _w PO ₄ (where 0 ≦ v ≦ 2, 0.8 ≦ w ≦ 1.2, and M1 is a 3d transition metal) as a positive electrode active material, such as phosphorus When lithium iron oxide (LiFePO ₄ ) is used, since the compound contains more water than lithium cobaltate (LiCoO ₂ ) or the like, water is likely to be brought into the battery. In addition, when lithium hexafluorophosphate (LiPF ₆ ) is used as the electrolyte, it reacts with water contained in the battery to generate hydrofluoric acid as shown in the following formula.
LiPF ₆ + H ₂ O → 2HF + LiF + POF ₃

In such a case, even if vinylene carbonate is added to the non-aqueous electrolyte, the vinylene carbonate is consumed by reacting with water or acid contained in the electrolyte, and it is difficult to form a stable SEI film on the negative electrode. was there.
Further, even when the SEI film is formed, there is a problem that the internal resistance increases due to repeated charge and discharge. One possible cause of such a problem is that, for example, the SEI film has a high electrical resistance, so that the film is destroyed by repeated charge and discharge.

On the other hand, when an organic silicon compound having a Si—N bond was added to the non-aqueous electrolyte without adding vinylene carbonate, decomposition of the electrolyte on the negative electrode could not be suppressed, and internal resistance There was a problem of rising.
The present invention has been completed based on the above circumstances, and an object thereof is to provide a nonaqueous electrolyte secondary battery in which an increase in internal resistance due to repeated charge and discharge is suppressed.

As a result of intensive studies to solve the above problems, the present inventors have determined that the positive electrode active material has a general formula Li _v M1 _w PO ₄ (where 0 ≦ v ≦ 2, 0.8 ≦ w ≦ 1.2). And M1 is a 3d transition metal) by adding a predetermined amount of vinylene carbonate and a predetermined amount of an organosilicon compound having a Si—N bond to the non-aqueous electrolyte. The inventors have found that the increase in the internal resistance of the battery can be remarkably suppressed as compared with the case where these are added alone.

That is, the present invention is a non-aqueous electrolyte secondary battery including a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte, and the positive electrode active material includes a general formula Li _v M1 _w A compound represented by PO ₄ (where 0 ≦ v ≦ 2, 0.8 ≦ w ≦ 1.2, and M1 is a 3d transition metal), and the nonaqueous electrolyte is a total of the nonaqueous electrolytes. A non-aqueous electrolyte secondary battery comprising 3.0% by mass or less of vinylene carbonate and 4.0% by mass or less of an organosilicon compound having a Si—N bond with respect to mass.

  The details of why the increase in the internal resistance of the battery can be remarkably suppressed by adding a predetermined amount of vinylene carbonate and a predetermined amount of an organosilicon compound having a Si—N bond to the nonaqueous electrolyte are not clear. However, it is guessed as follows.

  The organosilicon compound having a Si-N bond added to the non-aqueous electrolyte reacts preferentially with water or acid contained in the battery, and suppresses a decrease in film forming ability due to the reaction between vinylene carbonate and water or acid. It is thought that the rise in the internal resistance of the battery was suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, the nonaqueous electrolyte secondary battery which suppressed the raise of internal resistance by repetition of charging / discharging can be provided.

<Embodiment 1>
Embodiment 1 of the present invention will be described with reference to FIG.
FIG. 1 is a schematic cross-sectional view of a prismatic nonaqueous electrolyte secondary battery 1 according to an embodiment of the present invention. This non-aqueous electrolyte secondary battery 1 (hereinafter also simply referred to as “battery”) includes a positive electrode plate 3 (positive electrode) obtained by applying a positive electrode mixture to a positive electrode current collector made of aluminum foil, and a negative electrode made of copper foil. A power generation element 2 in which a negative electrode plate 4 (negative electrode) formed by applying a negative electrode mixture to a current collector is spirally wound via a separator 5 and a nonaqueous electrolyte are housed in a battery case 6. .

  A battery lid 7 provided with a safety valve 8 is attached to the battery case 6 by laser welding, the negative electrode plate 4 is connected to a negative electrode terminal 9 at the upper part of the battery case 6 via a negative electrode lead 11, and the positive electrode plate 3 is a positive electrode. The battery lid 7 is connected via the lead 10.

  The positive electrode plate 3 includes a positive electrode mixture layer containing a positive electrode active material capable of occluding and releasing lithium ions on both surfaces of a positive electrode current collector formed of a metal such as aluminum. A positive electrode lead 10 is welded to a portion of the positive electrode current collector where the positive electrode mixture layer is not formed.

In the nonaqueous electrolyte secondary battery of the present invention, as the positive electrode active material, the general formula Li _v M1 _w PO ₄ (where 0 ≦ v ≦ 2, 0.8 ≦ w ≦ 1.2, and M1 is a 3d transition metal) And a compound having an olivine structure represented by: In the present invention, as the positive electrode active material, a material having the olivine structure coated with carbon or amorphous carbon may be used. When coated with carbon, a large amount of water is likely to be brought into the battery, but the effect of the present invention is particularly prominent when such a battery having a large amount of water is used.
Among the compounds having an olivine structure represented by the general formula Li _v M1 _w PO ₄ , LiFePO ₄ is particularly preferable from the viewpoint of being an inexpensive material.

  A conductive agent, a binder, or the like can be added to the positive electrode active material. As the conductive agent, an inorganic compound or an organic compound can be used. As the inorganic compound, carbon black, graphite or the like can be used, and as the organic compound, for example, a conductive polymer such as polyaniline can be used. As the binder, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, styrene-butadiene rubber, polyacrylonitrile, or the like can be used alone or in combination.

  Next, the negative electrode plate 4 will be described. The negative electrode plate 4 includes a negative electrode mixture layer containing a negative electrode active material capable of occluding and releasing lithium ions on both surfaces of a negative electrode current collector formed of a metal such as copper. A negative electrode lead 11 is welded by ultrasonic welding to a portion of the negative electrode current collector where the negative electrode mixture layer is not formed.

As the negative electrode active material contained in the negative electrode mixture layer, carbonaceous materials such as graphite, non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), Al, Si, Pb, Sn, Zn And an alloy compound of lithium with Cd and the like, metal Li, and general formula M5O _z (where M5 is at least one element selected from W, Mo, Si, Cu, and Sn, 0 ≦ z ≦ 2). Metal oxides or mixtures thereof can be used. Similarly to the positive electrode active material, a binder such as polyvinylidene fluoride can be added to the negative electrode active material.

  As the separator 5, a woven fabric, a non-woven fabric, a synthetic resin microporous film, or the like can be used, and a synthetic resin microporous film can be suitably used. Among the synthetic resin microporous membranes, polyolefin microporous membranes such as polyethylene microporous membranes, polypropylene microporous membranes, heat-resistant resins processed from aramids, etc., or microporous membranes composed of these are particularly suitable. Used for.

The non-aqueous electrolyte is obtained by dissolving an electrolyte salt in a non-aqueous solvent.
Examples of the electrolyte salt include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ CO ₂ , LiCF ₃ (CF ₃ ) ₃ , LiCF ₃ (C ₂ F ₅ ) ₃ , LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ CF ₂ CF ₃ ) ₂ , LiN (COCF ₃ ) ₂ , LiN (COCF ₂ CF ₃ ) ₂ , LiPF ₃ (CF ₂ CF ₃ ) ₃ or the like alone or in combination Can be used as a mixture.

  Nonaqueous solvents for dissolving the electrolyte salts are ethylene carbonate, propylene carbonate, γ-butyrolactone, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, sulfolane, dimethyl sulfoxide, acetonitrile, dimethylformamide, dimethylacetamide, 1,2-dimethoxyethane. , 1,2-diethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dioxolane, methyl acetate and the like can be used alone or in admixture of two or more.

  In the present invention, the nonaqueous electrolyte contains 3.0% by mass or less of vinylene carbonate and 4.0% by mass or less of an organosilicon compound having a Si—N bond with respect to the total mass.

  When the amount of vinylene carbonate (hereinafter also simply referred to as “VC”) exceeds 3.0 mass% or less with respect to the total mass of the nonaqueous electrolyte, the internal resistance of the battery increases. In addition, it is preferable that the quantity of VC is 0.02 mass% or more with respect to the total mass of a nonaqueous electrolyte. When the amount of VC is less than 0.02% by mass with respect to the total mass of the nonaqueous electrolyte, the coating film may not be sufficiently formed.

  When the amount of the organosilicon compound having a Si—N bond (hereinafter also simply referred to as “organosilicon compound”) exceeds 4.0 mass% with respect to the total mass of the nonaqueous electrolyte, the internal resistance of the battery increases. . In addition, it is preferable that the quantity of an organosilicon compound is 0.01 mass% or more with respect to the total mass of a nonaqueous electrolyte. If the amount of the organosilicon compound is less than 0.01% by mass with respect to the total mass of the nonaqueous electrolyte, the effect of suppressing the reaction between vinylene carbonate and water may not be exhibited.

  In the present invention, by using a non-aqueous electrolyte containing VC and an organosilicon compound in the above amounts, the increase in the internal resistance of the battery is remarkably suppressed as compared with a battery using a non-aqueous electrolyte containing these alone. Can do it.

Although it is not clear about the mechanism which suppresses the raise of the internal resistance of the battery which has nonaqueous electrolyte containing VC and an organosilicon compound, it estimates as follows.
The organosilicon compound having a Si-N bond added to the non-aqueous electrolyte reacts preferentially with water or acid contained in the battery, and suppresses a decrease in film forming ability due to the reaction between vinylene carbonate and water or acid. It is thought that the rise in the internal resistance of the battery was suppressed.

  In the present invention, examples of the organosilicon compound having a Si—N bond include 1,3-diphenyl-1,1,3,3-tetramethyldisilazane, 1,1,3,3,5,5. -Hexamethylcyclotrisilazane, 1,1,1,3,3,3-hexamethyldisilazane, 1,3-bis (chloromethyl) tetramethyldisilazane, 1,3-divinyl-1,1,3 3-tetramethyldisilazane, heptamethyldisilazane, octamethylcyclotetrasilazane, (N, O-bistrimethylsilyl) acetamide, (N, N-diethylamino) trimethylsilazane, bis (trimethylsilyl) -1,4-butanediamine, N-trimethylsilylimidazole etc. are mentioned.

<Example>
Hereinafter, although the Example and comparative example of this invention are shown, this invention is not limited to this.
1. Production of Battery of Example 1 A nonaqueous electrolyte secondary battery having the form shown in FIG. 1 was produced by the following method.
(1) Production of positive electrode plate 5 parts by mass of polyvinylidene fluoride as a binder, 5 parts by mass of acetylene black as a conductive agent, and 90 parts by mass of LiFePO ₄ coated with amorphous carbon as a positive electrode active material were mixed. N-methyl-2-pyrrolidone was added to the product to prepare a paste, and this was applied to both sides of a positive electrode current collector made of aluminum foil having a thickness of 20 μm and dried to produce positive electrode plate 3 The positive electrode lead 10 was provided.

(2) Production of Negative Electrode Plate 90 parts by mass of non-graphitizable carbon as a negative electrode active material and 10 parts by mass of polyvinylidene fluoride as a binder were prepared in a paste form by adding to N-methyl-2-pyrrolidone. Thereafter, this was applied to both sides of a negative electrode current collector made of copper foil having a thickness of 10 μm, and dried to prepare the negative electrode plate 4, and the negative electrode lead 11 was provided.

(3) Fabrication of battery A polyethylene microporous membrane was used as the separator.
A non-aqueous electrolyte was prepared by the following method. In a mixed solvent of ethylene carbonate (EC): diethyl carbonate (DMC): ethyl methyl carbonate (EMC) = 3: 2: 5 (volume ratio), LiPF ₆ was dissolved to 1 mol / L after preparation, 0.01% by mass of 1,3-diphenyl-1,1,3,3-tetramethyldisilazane (DPTMSZ) and 0.02% by mass of vinylene carbonate (VC) based on the total mass of the nonaqueous electrolyte Was added to prepare a non-aqueous electrolyte.
Using these materials, five cells of the nonaqueous electrolyte secondary battery of Example 1 having a capacity of 400 mAh were produced.

2. Production of Batteries of Examples 2 to 48 and Comparative Examples 1 to 58 0.01% by mass of 1,3-diphenyl-1,1,3,3-tetramethyldisilazane (based on the total mass of the nonaqueous electrolyte) Examples 2-36 and Comparative Example 1 except that DPTMSZ and VC were added in place of DPTMSZ) and 0.02% by mass of vinylene carbonate (VC). Five non-aqueous electrolyte secondary batteries of Examples 1 to 28 were produced.

  The amount of 1,1,3,3,5,5-hexamethyl shown in Table 2 is used instead of 0.01 wt% DPTMSZ and 0.02 wt% VC based on the total weight of the nonaqueous electrolyte. Except that cyclotrisilazane (HM-CTSZ) and VC were added, the non-aqueous electrolyte secondary batteries of Examples 37 to 42 and Comparative Examples 29 to 43 were produced in units of 5 in the same manner as Example 1.

  Instead of 0.01 wt% DPTMSZ and 0.02 wt% VC, the amount of 1,1,1,3,3,3-hexamethyl shown in Table 3 with respect to the total weight of the nonaqueous electrolyte Five non-aqueous electrolyte secondary batteries of Examples 43 to 48 and Comparative Examples 44 to 58 were produced in the same manner as in Example 1 except that disilazane (HM-DS) and VC were added.

3. Evaluation Test (1) Charge / Discharge Cycle Life Test An initial discharge capacity confirmation test was performed by the following method using 5 cells of each of Examples 1-48 and Comparative Examples 1-58.
By charging each battery to 3.6 V at a constant current of 400 mA at 25 ° C. and further charging for 3 hours at a constant voltage of 3.6 V, and then discharging at a constant current of 400 mA and a final voltage of 2.0 V. The initial discharge capacity was measured.

  About each battery after initial stage discharge capacity measurement, the charge / discharge cycle life test at 60 degreeC was done with the following method. In a constant temperature bath of 60 ° C., each battery was charged with a constant current of 3.6 mA and a constant voltage of 3.6 V for 30 minutes, and then discharged under a condition of a final voltage of 2.0 V with a constant current of 800 mA. 1000 cycles were repeated. Next, each battery was cooled at 25 ° C. for 5 hours, and each battery after cooling was subjected to a discharge capacity confirmation test at 25 ° C. in the same manner as in the initial discharge capacity confirmation test.

Each battery after the discharge capacity confirmation test at 25 ° C. is charged for 3 hours with a constant current-constant voltage charge with a charge current of 1 CmA (400 mA) and a charge voltage of 3.2 V, and the SOC of the battery is set to 50%. The voltage (E1) when discharged at 80 mA for 10 seconds and the voltage (E2) when discharged at 0.5 CmA for 10 seconds were measured.
Here, “SOC 50%” represents that the charge electricity amount is 50% with respect to the capacity of the battery.

Using the above measured values, the DC resistance value (R) was calculated by the following equation.
R = (E1-E2) / Discharge current (I)

  The DC resistance value was calculated for each battery by 5 cells, and the average values are shown in Tables 1 to 3. If the DC resistance value was 300 mΩ or less, it was determined that the increase in internal resistance due to repeated charge and discharge could be suppressed.

  In Tables 1 to 3, the amounts of the organosilicon compound and VC added to the nonaqueous electrolyte (% by mass with respect to the total mass of the nonaqueous electrolyte) are also shown. In the table, the resistance value means a DC resistance value.

4). Results and Discussion The present invention is provided with a nonaqueous electrolyte containing an organosilicon compound having a Si—N bond of 4.0% by mass or less and VC of 3.0% by mass or less with respect to the total mass of the nonaqueous electrolyte. In the batteries (Examples 1 to 48), the DC resistance value was 290 mΩ or less, and an increase in internal resistance due to repeated charge and discharge could be suppressed.
Among the batteries of the present invention, those containing 1.0 to 2.0% by mass of an organosilicon compound and 0.5 to 2.0% by mass of VC (Examples 16, 17, 22, 23, 28, 29, 40, 41, 46, 47) particularly favorable results were obtained.

  However, in batteries (Comparative Examples 1 to 58) in which at least one of the organosilicon compound and VC is out of the above content range, the DC resistance value is high, and the increase in internal resistance due to repeated charge and discharge cannot be suppressed. It was.

Specifically, in the batteries (Comparative Examples 2-7, Comparative Examples 29-34, and Comparative Examples 44-49) to which only 0.01% by mass to 4.0% by mass of the organic silicon compound was added, Although the resistance value was smaller than that of the battery not added with both VCs (Comparative Example 1), it was very small.
Moreover, in the battery (Comparative Examples 9, 11, 13, 15, 17, 19) in which only VC is added in 0.02 mass% to 3.0 mass%, both the organosilicon compound and VC are not added (Comparative Example). Although the resistance value was smaller than 1), it was very small.

As described above, LiFePO ₄ is used as the positive electrode active material, and an organosilicon compound having 4.0% by mass or less of Si—N bonds with respect to the total mass of the nonaqueous electrolyte, and 3.0% by mass or less of VC and It has been found that the use of a non-aqueous electrolyte containing a remarkably effective effect of suppressing an increase in internal resistance due to repeated charge and discharge.

  <Other embodiments>

  The present invention is not limited to the embodiments described with reference to the above description and drawings. For example, the following embodiments are also included in the technical scope of the present invention.

  (1) Although the rectangular battery is shown in the above embodiment, the shape of the battery may be, for example, a cylindrical shape.

  (2) In the above embodiment, non-graphitizable carbon is used as the negative electrode active material, but carbon black, graphite, graphitizable carbon (soft carbon), or the like may be used as the negative electrode active material.

Sectional drawing of the battery of Embodiment 1.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Nonaqueous electrolyte secondary battery 3 ... Positive electrode plate 4 ... Negative electrode plate 5 ... Separator 6 ... Battery case

Claims (1)

A non-aqueous electrolyte secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte,
For the positive electrode active material, a compound represented by the general formula Li _v M1 _w PO ₄ (where 0 ≦ v ≦ 2, 0.8 ≦ w ≦ 1.2, and M1 is a 3d transition metal) is used,
The nonaqueous electrolyte contains 3.0% by mass or less of vinylene carbonate and 4.0% by mass or less of an organosilicon compound having a Si—N bond with respect to the total mass of the nonaqueous electrolyte. Non-aqueous electrolyte secondary battery.

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