JP4686131B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

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

  In recent years, electronic devices such as mobile communication devices, notebook computers, palmtop computers, integrated video cameras, portable CD (MD) players, cordless telephones, etc. have been reduced in size and weight. As a power source, a battery having a small size and a large capacity is particularly demanded.

  Examples of batteries that are widely used as power sources for these electronic devices include primary batteries such as alkaline manganese batteries, and secondary batteries such as nickel cadmium batteries and lead storage batteries. Among them, the non-aqueous electrolyte secondary battery using a lithium composite oxide for the positive electrode and a carbonaceous material capable of occluding and releasing lithium ions for the negative electrode is small and light, has a high unit cell voltage, and can obtain a high energy density. It is attracting attention.

  Patent Document 1 discloses that vinylene carbonate (VC) is contained in the range of 0.05% by weight or more and 5% by weight or less with respect to the non-aqueous electrolyte of the gel electrolyte, thereby suppressing swelling due to gas generation. Is described. Further, in Patent Document 1, in the batteries of Samples 1 to 3 to which VC is added by 0.5% by weight, the battery volume immediately after the initial charge with respect to immediately before charging is 103.9%, and VC is added by 5.0% by weight. In the batteries of Samples 13 to 15, it is described that the battery volume immediately after the first charge with respect to immediately before the charge is 100.1%, which indicates that the larger the amount of VC added, the smaller the battery swelling.

Tables 1, 2, 5, and 6 of Patent Document 1 show that the cycle characteristics of the samples 32-34 to which no difluoroanisole was added are the batteries of samples 1-3 to which 1% by weight of difluoroanisole was added. It is described that they are equivalent. Similar trends are shown in Samples 4-6, 35-37.
JP 2001-167797 A

  An object of the present invention is to provide a nonaqueous electrolyte secondary battery that has a small swelling during charge storage and a long charge / discharge cycle life.

A non-aqueous electrolyte secondary battery according to the present invention includes a positive electrode including a positive electrode active material, a negative electrode, and a non-aqueous electrolyte including vinylene carbonate (VC) and 2,4-difluoroanisole (DFA). A secondary battery,
The VC content in the non-aqueous electrolyte after the initial charge / discharge is 0.01% by mass to 0.6% by mass, and the DFA amount per 1 m ² of the positive electrode active material surface area is in the range of 1 to 50 mg.
The negative electrode, the surface spacing d ₀₀₂ of by powder X-ray diffraction measurement (002) plane is not more than 0.336 nm, and the diffraction angle 2θ is 42.8 ° in the powder X-ray diffraction measurement ～44.0 ° and 45.5 ° include ～46.6 graphitic material which peak appears in °, and a BET specific surface area of the surface of the negative electrode is characterized by a range of 0.5m ² / g~5m ² / g.

  As described above in detail, according to the present invention, it is possible to provide a nonaqueous electrolyte secondary battery in which swelling during charge storage is suppressed and the charge / discharge cycle life is long.

  Hereinafter, an embodiment of a non-aqueous electrolyte secondary battery according to the present invention will be described. A positive electrode, a negative electrode, and a nonaqueous electrolyte included in the nonaqueous electrolyte secondary battery will be described.

1) Nonaqueous electrolyte This nonaqueous electrolyte contains a nonaqueous solvent and an electrolyte dissolved in the nonaqueous solvent. Non-aqueous solvents include vinylene carbonate (VC) and 2,4-difluoroanisole (DFA). Further, the VC content in the non-aqueous electrolyte after the initial charge / discharge is in the range of 0.01% by mass to 0.6% by mass, and the amount of DFA per 1 m ^{2 of the} positive electrode active material surface area is 1 to ² %. The range is 50 mg.

Here, “at the start of use” means when the user is charged or discharged for the first time after the battery is shipped. If VC is not present in the non-aqueous electrolyte at the start of use, the efficiency balance between the positive and negative electrodes tends to be lost, so that the positive electrode progresses quickly and the charge / discharge cycle life is reduced. To obtain a sufficient charge-discharge cycle life, although it is desirable that the VC content of the non-aqueous electrolyte after initial charge discharge more than 0.01 mass%, VC of the non-aqueous electrolyte after initial charge discharge If the content exceeds 0.6% by mass, the positive electrode and VC react at the time of charging to generate a large amount of gas, so that the battery swells when stored in a charged state increases. Further, since the reaction between the positive electrode and VC causes the positive electrode to deteriorate, the charge / discharge cycle life is shortened. A more preferable range of the VC content in the non-aqueous electrolyte after the initial charge / discharge is 0.01 to 0.45% by mass.

  Although the VC content in the non-aqueous electrolyte at the start of use tends to increase as the initial addition amount increases, it is not uniquely determined even if the initial addition amount of VC is determined. The negative electrode composition, the surface state of the negative electrode, the initial charging conditions, and the like can be changed by changing within an allowable error range in manufacturing. The present inventors have found that the battery characteristics are not uniquely determined by the initial addition amount of VC, and that there is a correlation between the VC content in the nonaqueous electrolyte at the start of use and the battery characteristics. It led to the invention.

The reason why the amount of DFA per 1 m ^{2 of the} positive electrode active material surface area is limited to the above range is as follows. When the amount of DFA per 1 m ^{2 of the} positive electrode active material surface area is less than 1 mg, the reaction between the positive electrode and VC cannot be suppressed, so that the battery swells during charge storage increases and the charge / discharge cycle life decreases. On the other hand, when the amount of DFA per 1 m ^{2 of} the positive electrode active material surface area exceeds 50 mg, the internal impedance increases due to the resistance film generated by the decomposition of DFA, and therefore the charge / discharge cycle life is reduced. A more preferable range of the DFA amount per 1 m ^{2 of the} positive electrode active material surface area is 5 to 40 mg.

  The non-aqueous solvent preferably contains a solvent other than VC and DFA. Other solvents include, for example, cyclic carbonates such as ethylene carbonate (EC) and propylene carbonate (PC), chain carbonates {eg, methyl ethyl carbonate (MEC), diethyl carbonate (DEC), dimethyl carbonate (DMC)} , Γ-butyrolactone (GBL), vinyl ethylene carbonate (VEC), phenyl ethylene carbonate (phEC), γ-valerolactone (VL), methyl propionate (MP), ethyl propionate (EP), 2-methylfuran (2Me) -F), furan (F), thiophene (TIOP), catechol carbonate (CATC), ethylene sulfite (ES), 12-crown-4 (Crown), tetraethylene glycol dimethyl ether (Ether), cyclohexane Examples include xylbenzene (CHB). The type of the other solvent can be one type or two or more types. Among these, it is desirable to contain at least one of a cyclic carbonate and a chain carbonate in a non-aqueous solvent.

  The non-aqueous solvent desirably contains a sultone compound having at least one double bond in the ring.

Here, as the sultone compound having at least one double bond in the ring, the sultone compound A represented by the general formula shown in the following chemical formula 1 or at least one H of the sultone compound A is substituted with a hydrocarbon group. Sultone compound B can be used. In the present application, sultone compound A or sultone compound B may be used alone, or both sultone compound A and sultone compound B may be used.

In Chemical Formula 1, C _m H _n is a linear hydrocarbon group, and m and n are integers of 2 or more that satisfy 2m> n.

  A sultone compound having a double bond in the ring can form a dense protective film on the surface of the negative electrode without gas generation due to the double bond opening during the reduction reaction with the negative electrode to cause a polymerization reaction. . At this time, if VC is present, a dense protective film excellent in lithium ion permeability can be formed.

Among the sultone compounds, a compound in which sultone compound A has m = 3 and n = 4, that is, 1,3-propene sultone (PRS), or a compound in which m = 4 and n = 6, that is, 1 , 4-butylene sultone (BTS). This is because the protective coating derived from these compounds has the highest effect of suppressing side reactions caused by contact between Li ⁺ in the graphite material and the nonaqueous electrolyte. As the sultone compound, 1,3-propene sultone (PRS) or 1,4-butylene sultone (BTS) may be used alone, or these PRS and BTS may be used in combination.

The content of the sultone compound in the non-aqueous electrolyte is desirably 10% by mass or less, more preferably 5% by mass or less, and most preferably 4% by mass or less. Further, in order to sufficiently obtain the effect of forming a protective film with the sultone compound, it is desirable to ensure a sultone compound ratio of at least 0.01% by mass . Furthermore, when the ratio of the sultone compound is 0.1% by mass or more, a protective function such as suppression of gas generation during the initial charge can be made sufficient.

Examples of the electrolyte dissolved in the non-aqueous solvent include lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium arsenic hexafluoride ( LiAsF ₆ ), lithium trifluorometasulfonate (LiCF ₃ SO ₃ ), bistrifluoromethylsulfonylimide lithium [(LiN (CF ₃ SO ₂ ) ₂ ], LiN (C ₂ F ₅ SO ₂ ) ₂ and other lithium salts The type of electrolyte used can be one type or two or more types, and among them, those containing LiPF ₆ or LiBF ₄ are preferable.

  The amount of the electrolyte dissolved in the non-aqueous solvent is preferably 0.5 to 2.5 mol / L. A more preferable range is 1 to 2.5 mol / L.

  The non-aqueous electrolyte preferably contains a surfactant such as trioctyl phosphate (TOP) in order to improve the wettability with the separator. The addition amount of the surfactant is preferably 3% or less, and more preferably in the range of 0.1 to 1%.

  As the non-aqueous electrolyte, one having a substantially liquid or gel-like form can be used.

  The amount of non-aqueous electrolyte is preferably 0.2 to 0.6 g per 100 mAh of battery unit capacity. A more preferable range of the nonaqueous electrolytic mass is 0.25 to 0.55 g / 100 mAh.

2) Positive electrode The positive electrode includes a current collector and a positive electrode layer that is supported on one or both surfaces of the current collector and includes an active material.

  The positive electrode layer includes a positive electrode active material, a binder, and a conductive agent.

Examples of the positive electrode active material include various oxides such as manganese dioxide, lithium manganese composite oxide, lithium-containing nickel oxide, lithium-containing cobalt oxide, lithium-containing nickel cobalt oxide, lithium-containing iron oxide, and lithium. Examples thereof include vanadium oxide and chalcogen compounds such as titanium disulfide and molybdenum disulfide. Among them, when a lithium-containing cobalt oxide (for example, LiCoO ₂ ), a lithium-containing nickel cobalt oxide (for example, LiNi _0.8 Co _0.2 O ₂ ), or a lithium manganese composite oxide (for example, LiMn ₂ O ₄ , LiMnO ₂ ) is used. This is preferable because a high voltage can be obtained. As the positive electrode active material, one kind of oxide may be used alone, or two or more kinds of oxides may be mixed and used.

  Examples of the conductive agent include acetylene black, carbon black, and graphite.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethersulfone, ethylene-propylene-diene copolymer (EPDM), and styrene-butadiene rubber (SBR). be able to.

The mixing ratio of the positive electrode active material, a conductive agent and a binder is the positive electrode active material 85 to 98% by weight, the conductive agent 1 to 10 mass%, is preferably in the range of binder 1 to 5 mass%.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, aluminum, stainless steel, or nickel.

  The positive electrode is produced, for example, by suspending a conductive agent and a binder in an appropriate solvent in a positive electrode active material, applying the suspension to a current collector, drying it, and applying a press.

3) Negative electrode The negative electrode includes a current collector and a negative electrode layer supported on one side or both sides of the current collector. The negative electrode layer includes a carbonaceous material that occludes / releases lithium ions and a binder.

The carbonaceous material has an interplanar spacing d ₀₀₂ of 0.336 nm or less by powder X-ray diffraction measurement, and a diffraction angle 2θ of 42.8 ° to 44.0 ° and 45.5 ° to 46.46 in powder X-ray diffraction measurement. Includes graphite materials that have a peak at 6 °.

Since such a graphite material can continue to react with VC during the charge / discharge cycle and maintain the function of the protective coating on the negative electrode surface, the charge / discharge efficiency of the negative electrode can be increased. Moreover, the reaction opportunity of a positive electrode and VC can be decreased because a negative electrode and VC react. As a result, it is possible to suppress positive electrode deterioration accompanying the progress of the charge / discharge cycle. The surface spacing d ₀₀₂ is spacing of (002) plane in complete graphite crystal d _002, i.e. it is preferably larger than 0.3354 nm.

  The graphite material preferably has an R value measured by Raman spectrum of 0.3 or more in intensity ratio and 1 or more in area ratio. Such a graphitic material has a part or all of the surface having lower crystallinity than the inside, and can suppress the decomposition reaction of a non-aqueous solvent such as propylene carbonate (PC). The secondary battery can be improved, and both the discharge capacity and the cycle life can be realized. Further, when the strength ratio is larger than 1.5 and the area ratio is larger than 4, the ratio of the low crystalline structure region in the graphite material is increased, so that the protective film on the negative electrode surface may be insufficient. There is. In addition, an irreversible reaction in which lithium is trapped in a low crystalline portion is known, which may reduce efficiency. Therefore, it is desirable to set the upper limit of the intensity ratio to 1.5 and the upper limit of the area ratio to 4. A more preferable range of the area ratio is 1 to 3.

  The graphite material is produced, for example, by the method described below. That is, carbonaceous material such as coke, pitch, thermosetting resin, or carbon precursor is mixed with natural graphite in liquid form or pulverized and then molded into a graphite material, and then 2500 ° C. or less in an inert atmosphere. It can be obtained by heat treatment at a temperature of Further, a material obtained by performing chemical vapor deposition using benzene, toluene or the like as a graphite material and depositing a carbon layer having low crystallinity on the surface may be used.

The specific surface area according to the BET method of the carbonaceous material containing the graphite material is desirably in the range of 1.5 m ² / g or more and 10 m ² / g or less. That is, if the specific surface area is less than 1.5 m ² / g, the protective coating may act as a resistance component against the lithium intercalation reaction, and the discharge capacity of the secondary battery may be reduced. On the other hand, when the specific surface area exceeds 10 m ² / g, the uniformity of the distribution of the protective film may be lowered. A more preferable range of the specific surface area is 1.5 to 10 m ² / g, and a more preferable range is 1.5 to ⁶ m ² / g.

The specific surface area of the negative electrode surface by the BET method is desirably in the range of 0.5 to ⁵ m ² / g. This is due to the reason explained below. When the specific surface area is less than 0.5 m ² / g, the reactivity between the negative electrode and VC is lowered, and thus there is a possibility that the charge / discharge cycle life cannot be improved. On the other hand, when the specific surface area exceeds 5 m ² / g, VC in the non-aqueous electrolyte may disappear at the initial stage of the charge / discharge cycle. A more preferable range of the specific surface area is 1 to 4 m ² / g.

  Examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), ethylene-propylene-diene copolymer (EPDM), styrene-butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like. Can be used.

Mixing ratio of the carbonaceous material and the binder, carbonaceous material 90-98 wt%, is preferably in the range of the binder 2-10% by weight.

  The negative electrode can contain a conductive filler. Examples of the conductive filler include carbon black such as flaky graphite and acetylene black.

  As the current collector, a conductive substrate having a porous structure or a non-porous conductive substrate can be used. These conductive substrates can be formed from, for example, copper, stainless steel, or nickel.

  The negative electrode is, for example, kneaded with a carbonaceous material that occludes / releases lithium ions and a binder in the presence of a solvent, and the resulting suspension is applied to a current collector and dried, followed by a desired pressure. It is produced by pressing once or multistage pressing 2-5 times.

  The nonaqueous electrolyte secondary battery according to the present invention preferably has a structure in which an electrode group in which a separator is interposed between a positive electrode and a negative electrode is housed in a container.

  For example, (i) the positive electrode and the negative electrode are wound in a flat shape or a spiral shape with a separator interposed therebetween, or (ii) the positive electrode and the negative electrode are wound in a spiral shape with a separator interposed therebetween. Rotating and then compressing in the radial direction, (iii) bending the positive electrode and the negative electrode one or more times with a separator interposed therebetween, or (iv) laminating the positive electrode and the negative electrode with a separator interposed therebetween It is produced by.

  The electrode group need not be pressed, but may be pressed to increase the integrated strength of the positive electrode, the negative electrode, and the separator. It is also possible to heat at the time of pressing.

  As this separator, a microporous film, a woven fabric, a non-woven fabric, a laminate of the same material or different materials among these can be used. Examples of the material for forming the separator include polyethylene, polypropylene, ethylene-propylene copolymer, and ethylene-butene copolymer. As a material for forming the separator, one type or two or more types selected from the types described above can be used.

  The thickness of the separator is preferably 30 μm or less, and more preferably 25 μm or less. Moreover, it is preferable that the lower limit of thickness is 5 micrometers, and a more preferable lower limit is 8 micrometers.

  The separator preferably has a heat shrinkage rate of 20% or less at 120 ° C. for 1 hour. The heat shrinkage rate is more preferably 15% or less.

  The separator preferably has a porosity in the range of 30 to 60%. A more preferable range of the porosity is 35 to 50%.

The separator preferably has an air permeability of 600 seconds / 100 cm ³ or less. The air permeability means time (seconds) required for 100 cm ³ of air to pass through the separator. The upper limit value of the air permeability is more preferably 500 seconds / 100 cm ³ . The lower limit value of the air permeability is preferably 50 seconds / 100 cm ^3, and a more preferable lower limit value is 80 seconds / 100 cm ³ .

  Moreover, the shape of a container can be made into a bottomed cylindrical shape, a bottomed rectangular cylinder shape, a bag shape, a cup shape, etc., for example.

  This container can be formed from, for example, a sheet including a resin layer, a metal plate, a metal film, or the like.

  The resin layer contained in the sheet can be composed of, for example, polyolefin or polyamide.

  The metal plate and the metal film can be formed of, for example, iron, stainless steel, or aluminum.

  The thickness of the container (the thickness of the container wall) is desirably 0.3 mm or less. This is because if the thickness is greater than 0.3 mm, it is difficult to obtain a high weight energy density and volume energy density. A preferable range of the thickness of the container is 0.25 mm or less, a more preferable range is 0.15 mm or less, and a most preferable range is 0.12 mm or less. Moreover, since it will become easy to deform | transform and break when thickness is thinner than 0.05 mm, it is preferable that the lower limit of the thickness of a container shall be 0.05 mm.

According to the present invention described above, the VC content in the non-aqueous electrolyte after initial charge / discharge is 0.01% by mass or more and 0.6% by mass or less, and the amount of DFA per 1 m ² of the positive electrode active material surface area is 1. In the range of ˜50 mg, and the powder X-ray diffractometry on the negative electrode, the diffraction angle 2θ is 42.8 ° to 44.0 ° and 45.5 ° to 46.6 °, and the interplanar spacing d ₀₀₂ is 0.336 nm. include the following graphitic material, and specific surface area by the BET method of the surface of the negative electrode is in the range of 0.5m ² / g~5m ² / g, suppresses the deterioration of the positive electrode with the progress of charge-discharge cycles Therefore, it is possible to realize a non-aqueous electrolyte secondary battery that has less battery swelling when stored in a charged state and has a long charge / discharge cycle life.

  The secondary battery according to the present invention suppresses the deterioration of the positive electrode because (1) the function of the protective film can be maintained by the reaction between the negative electrode and the VC during the charge / discharge cycle. (2) The reaction opportunity between the VC and the positive electrode is reduced by the amount of the reaction between the negative electrode and the VC, and (3) the effect of suppressing the reaction between the positive electrode and the VC by DFA contributes. It is guessed.

  In addition, the charge / discharge cycle life of the secondary battery can be further improved by incorporating a sultone compound having at least one double bond in the ring in the nonaqueous electrolyte. This is presumably because the negative electrode characteristics were maintained in a good state for a long period during the charge / discharge cycle, and as a result, the deterioration of the positive electrode was further suppressed.

  The nonaqueous electrolyte secondary battery according to the present invention can have various forms such as a cylindrical shape, a rectangular shape, and a thin shape. Among them, a thin lithium ion secondary battery and a prismatic lithium ion secondary battery will be described in detail with reference to FIGS.

  First, a thin lithium ion secondary battery will be described with reference to FIGS.

  As shown in FIG. 1, an electrode group 2 is accommodated in a container body 1 having a long box-like cup shape. The electrode group 2 has a structure in which a laminate including a positive electrode 3, a negative electrode 4, and a separator 5 disposed between the positive electrode 3 and the negative electrode 4 is wound into a flat shape. The nonaqueous electrolyte is held in the electrode group 2. A part of the edge of the container body 1 is wide and functions as the lid plate 6. The container body 1 and the cover plate 6 are each composed of a laminate film. The laminate film includes an external protective layer 7, an internal protective layer 8 containing a thermoplastic resin, and a metal layer 9 disposed between the external protective layer 7 and the internal protective layer 8. A lid 6 is fixed to the container body 1 by heat sealing using a thermoplastic resin of the inner protective layer 8, whereby the electrode group 2 is sealed in the container. A positive electrode tab 10 is connected to the positive electrode 3, and a negative electrode tab 11 is connected to the negative electrode 4, and each is pulled out of the container and serves as a positive electrode terminal and a negative electrode terminal.

  Next, the prismatic nonaqueous electrolyte secondary battery will be described.

  As shown in FIG. 3, an electrode group 13 is accommodated in a bottomed rectangular cylindrical container 12 made of metal such as aluminum. In the electrode group 13, a positive electrode 14, a separator 15 and a negative electrode 16 are laminated in this order and wound in a flat shape. A spacer 17 having an opening near the center is disposed above the electrode group 13.

  The nonaqueous electrolyte is held in the electrode group 13. A sealing plate 18 b having a liquid injection port 18 a and having a circular hole opened near the center is laser welded to the opening of the container 12. The liquid injection port 18a is covered with a sealing lid (not shown). The negative electrode terminal 19 is disposed in a circular hole of the sealing plate 18b via a hermetic seal. The negative electrode tab 20 drawn out from the negative electrode 16 is welded to the lower end of the negative electrode terminal 19. On the other hand, a positive electrode tab (not shown) is connected to a container 12 that also serves as a positive electrode terminal.

[Example]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Examples 1-4)
<Preparation of positive electrode>
First, lithium cobalt oxides; in (Li _x CoO ₂ where, X is 0 <X ≦ 1 a is) powder 90 wt%, and 5 wt% of acetylene black, polyvinylidene fluoride (PVdF) of 5 wt% dimethyl formamide (DMF) solution was added and mixed to prepare a slurry. The slurry was applied to both sides of a current collector made of an aluminum foil having a thickness of 15 μm, then dried and pressed to produce a positive electrode having a structure in which the positive electrode layer was supported on both sides of the current collector. The thickness of the positive electrode layer was 60 μm per side.

<Production of negative electrode>
The natural graphite was spheroidized and then subjected to chemical vapor deposition at 1000 ° C. in a benzene / N ₂ stream to obtain a graphite material.

The obtained graphite material has an (002) plane spacing (d ₀₀₂ ) of 0.3356 nm by powder X-ray diffraction, and a diffraction angle 2θ of 43.35 ° and 46 in an X-ray diffraction measurement using CuKα rays. A peak appeared at 19 °. In addition, the space _| interval d002 of (002) plane is the value calculated | required by the half-value-width midpoint method from the powder X-ray diffraction spectrum. At this time, scattering correction such as Lorentz scattering was not performed.

  The specific surface area by the BET method and the Raman R value were measured for the graphite material by the method described below.

(Measurement of specific surface area by BET method)
As a measuring device, a product name manufactured by Yuasa Ionics used Kantasorb. The sample amount was set to around 0.5 g, and the sample was deaerated at 120 ° C. for 15 minutes as a pretreatment. The specific surface area of the graphite material was 2.7 m ² / g.

(Measurement of R value)
Peak separation is performed on the Raman spectrum of the graphite material, and D (A1g): peak derived from the disorder of the graphite structure near 1360 cm ⁻¹ , D ′ (A1g): the disorder of the graphite structure near 1620 cm ⁻¹ Derived peak, D: Peak derived from disorder of graphite structure of amorphous carbon, G (E2g): Peak derived from graphite structure in the vicinity of 1580 cm ⁻¹ , G: Peak derived from graphite structure of amorphous carbon were obtained.

Calculating the intensity of each peak, intensity and I _D the sum of the intensity of a peak derived from the D-band, the ratio of I _G the sum of the intensity of a peak derived from the G band (I _D / I _G) The ratio was 0.35. Also, the area of each peak is calculated, and the ratio of the sum of the peak area values derived from the _D band to the sum of the peak area values derived from the G band, S _G (S _D / S _The area ratio of _G ) was 1.1.

A slurry was prepared by mixing 95% by mass of this graphite material and a 5% by mass polyvinylidene fluoride (PVdF) dimethylformamide (DMF) solution. The slurry was applied to both surfaces of a current collector made of a copper foil having a thickness of 12 μm, dried, and pressed to prepare a negative electrode having a structure in which the negative electrode layer was supported on the current collector. The thickness of the negative electrode layer was 55 μm per side. Moreover, it was 1.68 m < ² > / g when the specific surface area of the negative electrode surface was measured on the conditions similar to the case of a negative electrode active material.

<Separator>
A separator made of a microporous polyethylene film having a thickness of 25 μm was prepared.

<Preparation of non-aqueous electrolyte>
The concentration of lithium hexafluorophosphate (LiPF ₆ ) is 1 mol / L in a solvent in which ethylene carbonate (EC) and methyl ethyl carbonate (MEC) are mixed so that the volume ratio EC: MEC is 1: 2. to that dissolved as, VC content in the nonaqueous electrolytic solution, DFA content and PRS content, respectively 2%, 2 wt%, vinylene carbonate to be 0.25 wt% (VC), 2,4-Difluoroanisole (DFA) and 1,3-propene sultone (PRS) were added, and these were mixed to prepare a non-aqueous electrolyte (liquid non-aqueous electrolyte).

The mass of the lithium cobalt oxide as the positive electrode active material 5g, and a specific surface area by the BET method because it is 0.3m ² / g, DFA per positive electrode active material surface area 1 m ² was 30 mg. In addition, the measurement of the specific surface area by BET method was performed by the method similar to the case of a negative electrode active material.

<Production of electrode group>
After the positive electrode lead made of a strip-shaped aluminum foil (thickness 100 μm) is ultrasonically welded to the positive electrode current collector, and the negative electrode lead made of a strip-shaped nickel foil (thickness 100 μm) is ultrasonically welded to the negative electrode current collector The positive electrode and the negative electrode were spirally wound through the separator therebetween, and then formed into a flat shape to produce an electrode group.

  An aluminum sheet having a thickness of 300 μm was formed into a rectangular can having a thickness of 5 mm, a width of 30 mm, and a height of 48 mm, and the electrode group was housed in the obtained container.

  Subsequently, the electrode group in the container was vacuum dried at 80 ° C. for 12 hours to remove moisture adsorbed on the electrode group and the aluminum can.

  By injecting the liquid nonaqueous electrolyte into the electrode group in the container so that the amount per battery capacity of 1 Ah is 3.4 g and sealing it, it has the structure shown in FIG. 3 and has a thickness of 5 mm. A rectangular nonaqueous electrolyte secondary battery having a width of 30 mm and a height of 48 mm was assembled.

  The following treatment was applied to the non-aqueous electrolyte secondary battery as an initial charge / discharge process. First, after being left in a high temperature environment of 45 ° C. for 2 hours, constant current / constant voltage charging was performed for 15 hours at 0.2 C to 4.2 V in that environment. Thereafter, it was left at 20 ° C. for 7 days. Furthermore, it discharged to 3.0V at 0.2C in a 20 degreeC environment, and the nonaqueous electrolyte secondary battery was manufactured. By this method, 10 lots of samples were prepared for 5 lots.

  The obtained secondary battery is in a state before the start of use which is not discharged or charged by the user. The amount of remaining VC in the non-aqueous electrolyte before the start of use was measured by the method described below, and at the same time, PRS in the non-aqueous electrolyte was identified.

(Measurement of residual amount of VC and identification of PRS)
For the secondary battery of Example 1, after the initial charge / discharge step, the circuit was opened for 5 hours or more to sufficiently settle the potential, and then the Ar concentration was 99.9% or more and the dew point was −50 ° C. or less. It decomposed | disassembled in the glove box and took out the electrode group. The electrode group was put in a centrifuge tube, dimethyl sulfoxide (DMSO) -d ₆ was added and sealed, removed from the glove box, and centrifuged. Thereafter, a mixed solution of the electrolytic solution and the DMSO-d ₆ was collected from the centrifuge tube in the glove box. About 0.5 ml of the mixed solvent was put into a 5 mmφ NMR sample tube and subjected to NMR measurement. The apparatus used for the NMR measurement is JNM-LA400WB manufactured by JEOL Ltd., the observation nucleus is ¹ H, the observation frequency is 400 MHz, and the residual proton signal slightly contained in dimethyl sulfoxide (DMSO) -d ₆ is used as an internal reference. (2.5 ppm). The measurement temperature was 25 ° C. ^{In the 1} HNMR spectrum, a peak corresponding to EC was observed near 4.5 ppm, and a peak corresponding to VC was observed near 7.7 ppm. On the other hand, peaks corresponding to PRS were observed around 5.1 ppm (P ₁ ), 7.05 ppm (P ₂ ), and 7.2 ppm (P ₃ ) as shown in the spectrum of FIG. From these results, it was confirmed that VC and PRS were contained in the nonaqueous solvent present in the secondary battery of Example 1 after the initial charge / discharge step.

Further, when ¹³ CNMR measurement was performed with an observation frequency of 100 MHz and dimethyl sulfoxide (DMSO) -d ₆ (39.5 ppm) as an internal reference substance, a peak corresponding to EC was around 66 ppm, and a peak corresponding to VC was 133 ppm. In the vicinity, peaks corresponding to PRS were observed at around 74 ppm, around 124 ppm and around 140 ppm. From this result as well, VC and PRS were found in the nonaqueous solvent present in the secondary battery of Example 1 after the initial charge / discharge process. It was confirmed that it was included.

  In Examples 2 to 4 as well, measurement was performed under the same conditions as described in Example 1. As a result, the same results as in Example 1 could be obtained.

Furthermore, for 2 of each lot, the amount of residual VC in the non-aqueous electrolyte was calculated by determining the ratio of the NMR integrated intensity of VC to the NMR integrated intensity of EC in the ¹ H NMR spectrum. Due to fluctuations, there was a difference in the amount of remaining VC between lots. Since there is almost no difference in the remaining amount of VC in the lot, the battery characteristics test of each example was performed using the remaining cells of the lot whose VC remaining amount was the value described in Table 1 below, and the average value was obtained. Is shown in Table 1 below.

(Example 5)
Ten lots of nonaqueous electrolyte secondary batteries were produced in the same manner as described in Example 1 except that the VC content in the nonaqueous electrolyte at the time of assembly was 1.5% by mass. When the remaining amount of VC in the non-aqueous electrolyte after initial charge and discharge was measured for each lot, a value of 0.25 to 0.15% by mass was obtained, of which 0.2% by mass was taken as Example 5. .

(Comparative Example 1)
As described above except that the VC content in the non-aqueous electrolyte at the time of assembly is 0.8% by mass, so that the VC remaining amount in the non-aqueous electrolyte after initial charge / discharge is 0.0% by mass. A nonaqueous electrolyte secondary battery was produced in the same manner as described in Example 1.

(Comparative Example 2)
Ten lots of nonaqueous electrolyte secondary batteries were produced in the same manner as described in Example 1 except that the VC content in the nonaqueous electrolyte at the time of assembly was 3.2 mass%. When the amount of remaining VC in the non-aqueous electrolyte after initial charge / discharge was measured for each lot, a value of 1.4 to 1.6% by mass was obtained, of which 1.5% by mass was used as Comparative Example 2. .

(Examples 6 to 10 and Comparative Examples 3 to 4)
Except for changing the DFA content per 1 m ² of the positive electrode active material surface area as shown in the following Table 1 by changing the DFA content in the non-aqueous electrolyte, the same as described in Example 1 above. Thus, a non-aqueous electrolyte secondary battery was manufactured.

(Examples 11-14)
A nonaqueous electrolyte secondary battery was manufactured in the same manner as described in Example 1 except that the PRS content in the nonaqueous electrolyte was changed as shown in Table 1 below.

(Comparative Example 5)
In the X-ray diffraction measurement using CuKα rays, the ( ₀₀₂ ) plane spacing (d ₀₀₂ ) by powder X-ray diffraction is 0.3362 nm, and the diffraction angle 2θ does not appear as peaks at 43.35 ° and 46.19 °. In addition, 10 lots of nonaqueous electrolyte secondary batteries were produced in the same manner as described in Example 1 except that a graphite material having a specific surface area of 1.4 m ² / g by the BET method was used. When the remaining amount of VC in the non-aqueous electrolyte after initial charge / discharge was measured for each lot, a value of 1.1 to 1.3% by mass was obtained, of which 1.2% by mass was used as Comparative Example 5. .

  About the obtained secondary battery of Examples 1-14 and Comparative Examples 1-5, the charge high temperature storage characteristic and the capacity maintenance rate at the time of the charge / discharge cycle in a 45 degreeC environment are measured on the conditions demonstrated below. The results are shown in Table 1 below.

(Charge high temperature storage characteristics)
Each of the secondary batteries, carried 4.2V to constant current and constant voltage charge for 15 hours at 0.2C at room temperature and then discharged to 3.0V at 0.2C at room temperature.

  Next, the thickness of the battery container was measured after charging at a charge rate of 1C and an end-of-charge voltage of 4.2 V and stored in an environment at a temperature of 80 ° C. for 120 hours. Asked.

{(T ₁ −t ₀ ) / t ₀ } × 100 (%)
Where t ₀ is the thickness of the battery container just before storage, and t ₁ is the thickness of the battery container after 120 hours of storage.

2) Capacity maintenance rate after 200 cycles in a 45 ° C. environment Charge / discharge cycle in which 4.2V constant current / constant voltage charging at a current of 1C is performed for 3 hours and then discharged to 3.0V at a current of 1C. Is repeated in an environment of 45 ° C., and the maximum discharge capacity and the capacity retention after 200 cycles are measured. The results are also shown in Table 1 below. The capacity maintenance rate after 200 cycles is a value when the discharge capacity at the first cycle is 100%.

  As is clear from Table 1, the secondary batteries of Examples 1 to 14 had a smaller swelling when stored in a charged state in a high temperature environment than the secondary batteries of Comparative Examples 2, 3, and 5, and It can be understood that the charge / discharge cycle life is longer than that of the secondary batteries of Comparative Examples 1 and 4.

In the case where the residual amount of VC exceeds 0.6 mass % as in Comparative Examples 2 and 5, and in the case where DFA is not added as in Comparative Example 3, the reaction between the positive electrode and VC proceeds during charging, and swelling occurs. It seems that it has grown. On the other hand, in the case where VC does not exist in the non-aqueous electrolyte at the start of use as in Comparative Example 1, and in the case where the amount of DFA per 1 m ² of active material surface area exceeds 30 mg as in Comparative Example 4, the positive electrode It is presumed that the charge / discharge cycle life was shortened due to the rapid progress of deterioration.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The perspective view which shows the thin nonaqueous electrolyte secondary battery which is one Embodiment of the nonaqueous electrolyte secondary battery which concerns on this invention. FIG. 2 is a partial cross-sectional view of the nonaqueous electrolyte secondary battery in FIG. 1 cut along the line II-II. The partial notch perspective view which shows the square nonaqueous electrolyte secondary battery which is one Embodiment of the nonaqueous electrolyte secondary battery which concerns on this invention. FIG. 3 is a characteristic diagram showing a ¹ HNMR spectrum of PRS contained in the nonaqueous electrolyte of the nonaqueous electrolyte secondary battery of Example 1.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Container main body, 2 ... Electrode group, 3 ... Positive electrode, 4 ... Negative electrode, 5 ... Separator, 6 ... Cover plate, 7 ... External protective layer, 8 ... Internal protective layer, 9 ... Metal layer, 10 ... Positive electrode terminal, 11 ... negative terminal.

Claims (2)

A nonaqueous electrolyte secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode, and a nonaqueous electrolyte including vinylene carbonate (VC) and 2,4-difluoroanisole (DFA),
The VC content in the non-aqueous electrolyte after the initial charge / discharge is 0.01% by mass to 0.6% by mass, and the DFA amount per 1 m ² of the positive electrode active material surface area is in the range of 1 to 50 mg.
The negative electrode, the surface spacing d ₀₀₂ of by powder X-ray diffraction measurement (002) plane is not more than 0.336 nm, and the diffraction angle 2θ is 42.8 ° in the powder X-ray diffraction measurement ～44.0 ° and 45.5 ° include ～46.6 graphitic material which peak appears in °, and a nonaqueous that BET specific surface area of the surface of the negative electrode is characterized by a range of 0.5m ² / g~5m ² / g Electrolyte secondary battery.   The nonaqueous electrolyte contains 0.01% by mass to 10% by mass of a sultone compound composed of at least one of 1,3-propene sultone and 1,4-butylene sultone. Non-aqueous electrolyte secondary battery.

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