JP4579588B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery using insertion / extraction of lithium ions.

  Conventionally, lithium ion secondary batteries that use lithium ion storage / release have high voltage and high energy density. Therefore, in the field of information equipment and communication equipment, mainly personal information terminals such as personal computers and mobile phones. Practical use has progressed and it has become widely popular. In other fields, the development of electric vehicles is urgently required due to environmental problems and resource issues, and the use of lithium ion secondary batteries as power sources for electric vehicles is being studied.

A lithium ion secondary battery mainly includes a positive electrode, a negative electrode, and a non-aqueous electrolyte that moves lithium ions between the positive electrode and the negative electrode.
As the positive electrode of the lithium ion secondary battery, for example, one containing lithium cobaltate (LiCoO ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMn ₂ O ₄ ) or the like as a positive electrode active material is used. I came. When these positive electrode active materials are used, a lithium ion secondary battery having a high energy density and a high voltage can be produced.

  However, the positive electrode active material as described above has a metal element having a low Clark number in its composition. Therefore, the lithium ion secondary battery having such a positive electrode active material has a problem that its manufacturing cost is high and stable supply is difficult. In addition, since the positive electrode active material as described above is relatively toxic and has a great influence on the environment, a new positive electrode active material that replaces these has been demanded.

Therefore, in recent years, it has been proposed to use a compound having an olivine structure as a positive electrode active material. An example of such a compound having an olivine structure is LiFePO ₄ . This LiFePO ₄ has a large volume density of 3.6 g / cm ² , can generate a high potential of 3.4 V, and has a large theoretical capacity of 170 mAh / g. In addition, LiFePO ₄ is promising as a positive electrode active material for a lithium ion secondary battery because it contains one detachable Li per electrochemical atom in the initial state.

Furthermore, LiFePO ₄ has Fe, which is a resource-rich and inexpensive material, in its composition. Therefore, compared with the above LiCoO ₂ , LiNiO ₂ , LiMn ₂ O _4, etc., the production cost is low, and the influence on the environment is small because of low toxicity.

Furthermore, recently, by substituting other elements Fe site LiFePO _4, it has been confirmed to be able to improve the characteristics of the positive electrode active material of LiFePO _4. For example, by replacing the Fe site of LiFePO ₄ with Mn, charge / discharge cycle characteristics can be improved (see Patent Document 1).

  However, in the lithium ion secondary battery disclosed in Patent Document 1, the charge / discharge capacity decreases when the lithium ion secondary battery is repeatedly charged and discharged under a high temperature condition of, for example, about 60 ° C. was there. In addition, a lithium ion secondary battery using such a compound having an olivine structure as a positive electrode active material has a problem in that when the charge / discharge cycle is repeated at a high temperature, the internal resistance of the battery is significantly increased. For example, when the charge / discharge cycle is repeated under the condition of 60 ° C., the internal resistance after 500 cycles may jump to more than twice the initial value.

  In particular, portable devices such as a camera-integrated VTR, a mobile phone, and a laptop personal computer may be left in a high temperature state such as in an automobile or may be charged in such a state. Therefore, lithium ion secondary batteries having excellent charge / discharge cycle characteristics at high temperatures are particularly required for applications such as portable devices.

JP 2002-117845 A

  The present invention has been made in view of such conventional problems, and an object of the present invention is to provide a lithium ion secondary battery having high output and excellent charge / discharge cycle characteristics under high temperature conditions.

The present invention relates to a lithium ion having a positive electrode containing an oxide containing lithium as a positive electrode active material, a negative electrode containing a carbon material as a negative electrode active material, and a non-aqueous electrolyte obtained by dissolving an electrolyte in an organic solvent. In secondary batteries,
In the non-aqueous electrolyte, a compound represented by the following formula (α) is added as an additive,
The positive electrode active material, the composition formula _{Li x Fe 1-y Me y} PO 4 ( where, Me is, Mn, Cr, Co, Cu , Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B and The main component is a lithium iron composite oxide having an olivine structure represented by one or more selected from Nb, 0.05 ≦ x ≦ 1.2, 0 ≦ y ≦ 1.0) ,
The lithium ion secondary battery is characterized in that the additive is added to the non-aqueous electrolyte so that the molar ratio with the electrolyte is electrolyte: additive = 97-2: 3-98. (Claim 1).

As described above, the lithium ion secondary battery of the present invention has an olivine-structured lithium iron composite oxide having the above specific composition formula as a positive electrode active material, and a carbon material as a negative electrode active material. . The lithium ion secondary battery has a non-aqueous electrolyte to which a compound represented by the above formula (α) is added.
Therefore, when the lithium ion secondary battery is charged one or more times, all or part of the additive is decomposed, and the surface of the positive electrode or / and the negative electrode, the surface of the positive electrode active material or / and the negative electrode active material, or the like. In addition, a coating covering this is formed.

The covering is low in resistance and stable, and covers the surface of the positive electrode and / or negative electrode and the surface of the positive electrode active material and / or negative electrode active material as described above.
Therefore, in the lithium ion secondary battery, the insertion and extraction of lithium ions is performed smoothly, and the interface between the surface of the positive electrode or / and the negative electrode, the surface of the positive electrode active material or / and the negative electrode active material, and the electrolytic solution. The resistance is reduced and the initial output of the battery can be improved over a wide temperature range. In particular, since the resistance of the electrolytic solution increases at low temperatures, the improvement in output becomes more remarkable.
Moreover, the said covering can suppress formation of the highly resistant film on a negative electrode which occurs by decomposition | disassembly of the electrolyte in a non-aqueous electrolyte, etc. Therefore, an increase in internal resistance of the lithium ion secondary battery can be suppressed, and a decrease in charge / discharge cycle characteristics can be suppressed.

The formation of the high-resistance film is particularly likely to occur when the lithium ion secondary battery is repeatedly used in a high temperature environment of, for example, 60 ° C., and lowers the output voltage, the discharge capacity, and the like.
In the lithium ion secondary battery of the present invention, since the coating can prevent the formation of the high-resistance coating, excellent discharge even when used repeatedly in a high temperature environment of, for example, 60 ° C. Capacitance and output voltage can be exhibited.
For example, when an electrolytic solution using LiPF ₆ that has already been put into practical use as a supporting salt is used, HF may be generated by hydrolysis. This HF may corrode a current collector made of, for example, Al under an environment of, for example, 60 ° C. and 4.1 V. As a result, the resistance increases and the battery characteristics may be deteriorated.
On the other hand, in the lithium ion secondary battery, since the compound represented by the formula (α) is added to the non-aqueous electrolyte, HF is not generated even when hydrolyzed. Therefore, the lithium ion secondary battery can exhibit excellent durability.

Furthermore, in the present invention, the positive electrode active material includes a lithium iron composite oxide having an olivine structure represented by a composition formula Li _x Fe _1-y Me _y PO ₄ . Therefore, the lithium ion secondary battery can exhibit a high charge / discharge capacity, and can maintain a high charge / discharge capacity even when it is repeatedly used in a high temperature environment of 60 ° C., for example. In addition, since the increase in internal resistance can be suppressed, a high output voltage can be maintained.

  As described above, according to the present invention, it is possible to provide a lithium ion secondary battery having high output and excellent charge / discharge cycle characteristics under high temperature conditions.

このような添加剤の具体的な例を次に示す。 In the lithium ion secondary battery, as the additive, it may be added a compound represented by the following general formula (1) in the nonaqueous electrolyte solution. However, in the following general formula (1), M is a transition metal, Group III, IV or V element of the periodic table, A ^{a +} is a metal ion, proton or onium ion, a is 1 to 3, b is 1 to 3, p is b / a, m is 1 to 4, n is 1 to 8, q is 0 or 1, R ¹ is C _{1 to} C ₁₀ alkylene, C _{1 to} C ₁₀ halogenated alkylene, C _{6 to} C ₂₀ arylene, or C _{6 to} C ₂₀ halogenated arylene (these alkylenes and arylenes may have substituents, heteroatoms in their structures, the m R ¹ may be bonded to each present.), R ² is halogen, the C ₁ -C alkyl _10, halogenated alkyl, C ₆ -C ₂₀ of C ₁ -C ₁₀ aryl, C of ₆ -C ₂₀ aryl halide, or X ³ R ³ (the alkyl and aryl its structure Substituent may have a hetero atom, and n pieces R ² each may combine with each other to form a ring ^{present.), X 1, X 2} , X 3 is, O, S, or NR ⁴ , R ³ and R ⁴ are each independently hydrogen, C ₁ -C ₁₀ alkyl, C ₁ -C ₁₀ halogenated alkyl, C ₆ -C ₂₀ aryl, C ₆ -C ₂₀ halogenated Each of which represents aryl (these alkyls and aryls may have a substituent or a heteroatom in the structure, and a plurality of R ³ and R ⁴ may be bonded to each other to form a ring; ).

Specific examples of such additives are shown below.

In the above example, A ^{a +} in the general formula (1) is ^a lithium ion, but as a cation other than lithium ion, for example, sodium ion, potassium ion, magnesium ion, calcium ion, barium ion, cesium Ion, silver ion, zinc ion, copper ion, cobalt ion, iron ion, nickel ion, manganese ion, titanium ion, lead ion, chromium ion, vanadium ion, ruthenium ion, yttrium ion, lanthanoid ion, actinoid ion, tetrabutylammonium Ion, tetraethylammonium ion, tetramethylammonium ion, triethylmethylammonium ion, triethylammonium ion, pyridinium ion, imidazolium ion, proton Tetraethyl phosphonium ion, tetramethyl phosphonium ion, tetraphenylphosphonium ion, triphenylsulfonium ion, triethylsulfonium ion and the like.

Preferably, A ^{a +} in the general formula (1) is ^a lithium ion, a tetraalkylammonium ion, or a proton.

In the general formula (1), the valence a of A ^{a +} cation is 1-3. When a is larger than 3, the crystal lattice energy of the additive increases, so that it becomes difficult to dissolve in the organic solvent.
Therefore, most preferably a = 1. Such cations A ^{a +} include lithium ions, tetraalkylammonium ions, and protons.
Similarly, the valence b of the anion is 1 to 3, and b = 1 is most preferable.
In addition, the constant p representing the ratio of cation to anion is inevitably determined by the valence ratio b / a of both.

The additive has an ionic metal complex structure, and the central M is selected from a transition metal, a group III, group IV, or group V element of the periodic table.
Preferably, M in the general formula (1) is Al, B, V, Ti, Si, Zr, Ge, Sn, Cu, Y, Zn, Ga, Nb, Ta, Bi, P, As, Sc, It may be either Hf or Sb .
In this case, the additive can be easily synthesized.

  More preferably, M in the general formula (1) is Al, B, or P. In this case, in addition to facilitating the synthesis of the additive, it is possible to obtain effects that the toxicity of the additive is reduced and the manufacturing cost is reduced.

  Next, the ligand part of the additive (ionic metal complex) will be described. Hereinafter, in the above general formula (1), an organic or inorganic part bonded to M is referred to as a ligand.

R ¹ in the general formula (1) is selected from C _{1 to} C ₁₀ alkylene, C _{1 to} C ₁₀ halogenated alkylene, C _{6 to} C ₂₀ arylene, or C _{6 to} C ₂₀ halogenated arylene. It consists of things. These alkylene and arylene may have a substituent or a hetero atom in the structure. Specifically, instead of hydrogen on alkylene and arylene, halogen, chain or cyclic alkyl group, aryl group, alkenyl group alkoxy group, aryloxy group, sulfonyl group, amino group, cyano group, carbonyl group, acyl group , An amide group, a hydroxyl group, and a structure in which nitrogen, sulfur, or oxygen is introduced in place of carbon on alkylene and arylene.
Further, when a plurality of R ¹ are present (when q = 1, m = 2 to 4), each may be bonded, and examples thereof include a ligand such as ethylenediaminetetraacetic acid.

R ² is selected from halogen, C ₁ -C ₁₀ alkyl, C ₁ -C ₁₀ alkyl halide, C ₆ -C ₂₀ aryl, C ₆ -C ₂₀ aryl halide, or X ³ R ³ It consists of things. Similarly to R ¹ , alkyl and aryl may have a substituent or a heteroatom in the structure, and when there are a plurality of R ² (when n = 2 to 8), each R ² is They may combine to form a ring.
Preferably, R ² is preferably an electron-withdrawing group, particularly fluorine. In this case, the solubility and dissociation degree of the additive can be improved, and the ion conductivity can be improved accordingly. Furthermore, in this case, the oxidation resistance is improved, thereby preventing the occurrence of side reactions.

X ¹ , X ² , and X ³ are each independently O, S, or NR ⁴ , and the ligand is bonded to M through these heteroatoms. Here, it is not impossible to combine other than O, S, and N, but it becomes very complicated in synthesis. As a feature of the compound represented by the general formula (1), there is binding to M by X ¹ and X ² in the same in the ligand, these ligands form a M and chelate structure . The constant q in this ligand is 0 or 1. When q = 0, the chelate ring becomes a five-membered ring, and the complex structure of the additive is stabilized. Therefore, in this case, it is possible to prevent the additive from causing a side reaction other than the formation of the coating.

R ³ and R ⁴ are each independently hydrogen, C ₁ -C ₁₀ alkyl, C ₁ -C ₁₀ alkyl halide, C ₆ -C ₂₀ aryl, C ₆ -C ₂₀ aryl halide. Yes, these alkyls and aryls may have a substituent or a heteroatom in the structure, and when a plurality of R ³ and R ⁴ are present, they may combine to form a ring. .

The constants m and n related to the number of ligands described above are determined by the type of M at the center, where m is 1 to 4 and n is 1 to 8.
It in ^{R 1, R 2, R 3} , R 4 described above, C ₁ -C ₁₀ indicates 1 to 10 carbon atoms, C ₆ -C ₂₀ is 6 to 20 carbon atoms Indicates.

As a method for synthesizing the additive, for example, in the case of a compound represented by the following chemical formula (2), oxalic acid is added after reacting LiBF ₄ with 2 moles of lithium alkoxide in a non-aqueous solvent. Thus, there is a method of replacing alkoxide bonded to boron with oxalic acid.

  Next, in the lithium ion secondary battery of the present invention, as described above, all or part of the additive is decomposed by charging the lithium ion secondary battery at least once, so that the positive electrode or The surface of the negative electrode and / or the surface of the positive electrode active material or / and the negative electrode active material is coated to form a coating.

  The coating can be detected by, for example, X-ray photoelectron spectroscopy (XPS) or IR analysis.

  Further, the lithium ion secondary battery includes the positive electrode and the negative electrode, a separator that is sandwiched between the positive electrode and the negative electrode, the non-aqueous electrolyte that moves lithium between the positive electrode and the negative electrode, and the like. It can be configured as a main component.

  For example, the positive electrode is made by mixing a conductive material and a binder into the positive electrode active material and adding a suitable solvent to form a paste-like positive electrode mixture. The surface of a metal foil current collector such as aluminum or stainless steel is used as the positive electrode. It can be applied and dried, and compressed to increase the electrode density as necessary.

In the present invention, the positive electrode active material has a composition formula Li _x Fe _{1 -y} Me _y PO ₄ (where Me is Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, The main component is a lithium iron composite oxide having an olivine structure represented by one or more selected from Mg, B, and Nb, 0.05 ≦ x ≦ 1.2, and 0 ≦ y ≦ 1.0.

In the lithium iron composite oxide having the olivine structure represented by the above composition formula Li _x Fe _1-y Me _y PO ₄ , when x and y are out of the above ranges, the olivine structure collapses during charge and discharge. However, there is a risk that the capacity may suddenly decrease or the resistance may increase.

The positive electrode active material is preferably composed of particles of the lithium iron composite oxide, and carbon particles are preferably combined with the particles.
In this case, the effect that the resistance of the lithium ion secondary battery is reduced can be obtained.

  Examples of the fine particles of the carbon material include ketjen black, acetylene black, and carbon black.

  The conductive material is for ensuring the electrical conductivity of the positive electrode. For example, one or more carbon powder powders such as carbon black, acetylene black, natural graphite, artificial graphite, and cokes are used. Can be used.

The binder serves to bind the active material particles and the conductive material particles. For example, a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. Etc. can be used. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber which is an aqueous binder can be used.
For example, an organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing the active material, the conductive material, and the binder.

  Next, the negative electrode is prepared by mixing a binder with the carbon material, which is the negative electrode active material, and applying a suitable solvent to form a paste of the negative electrode mixture on the surface of a metal foil current collector such as copper. It can be dried and then formed by pressing. Similarly to the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as the binder to be mixed with the negative electrode active material, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent.

  Examples of the carbon material of the negative electrode active material include natural or artificial graphite, mesocarbon microbeads (MCMB), mesophase pitch-based carbon fibers and mixed materials thereof, vapor-grown carbonized fibers, and organic compound fired bodies such as phenol resins. , Cokes, carbon black, pyrolytic carbons, carbon fibers and the like. These carbon materials can be used alone or in combination of two or more.

The carbon material as the negative electrode active material preferably has a specific surface area of 0.8 to ⁵ m ² / g.
In this case, when the lithium ion secondary battery is charged, the additive is decomposed to easily form a low resistance and stable coating on the negative electrode or / and the negative electrode active material. An increase in internal resistance can be further suppressed.
When the specific surface area is less than 0.8 m ² / g or more than 5 m ² / g, the coating is not sufficiently formed, and the increase in the internal resistance may not be sufficiently suppressed.

  The separator to be narrowly attached to the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the non-aqueous electrolyte. For example, a thin microporous film such as polyethylene or polypropylene can be used.

  Next, as the non-aqueous electrolyte, a solution obtained by dissolving the additive and the electrolyte in an organic solvent can be used.

The electrolyte may be A ^{a +} (PF ₆ ⁻ ) _a , A ^{a +} (ClO ₄ ⁻ ) _a , A ^{a +} (BF ₄ ⁻ ) _a , A ^{a +} (AsF ₆ ⁻ ) _a , or A ^{a +} (SbF ₆ ⁻ ) _a , (However, A ^{a +} is a metal ion, a proton, or an onium ion, and a is 1 to 3 ).

  In this case, it is possible to obtain an effect that the ion conductivity is relatively high and electrochemically stable. In this case, the lithium ion secondary battery can be manufactured at a low cost.

Further, in the non-aqueous electrolyte solution, the _{electrolyte, LiPF 6, LiClO 4, LiBF} 4, LiAsF 6 or is preferably at least one selected from LiSbF ₆ (claim 4).
In this case, it is possible to obtain an effect that the ion conductivity is relatively high and electrochemically stable, and further, an effect that the lithium ion of the additive can also contribute to the charge / discharge reaction of the battery. be able to. In this case, the lithium ion secondary battery can be produced at low cost.

  As the organic solvent for dissolving the additive and the electrolyte, an aprotic organic solvent can be used. As such an organic solvent, for example, a mixed solvent composed of one kind or two or more kinds selected from cyclic carbonate, chain carbonate, cyclic ester, cyclic ether, chain ether and the like can be used.

  Here, examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. Examples of the cyclic ester carbonate include gamma butyrolactone and gamma valerolactone. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane and ethylene glycol dimethyl ether. As the organic solvent, any one of these may be used alone, or two or more kinds may be mixed and used.

Moreover, it is preferable that the said additive is added in the said non-aqueous electrolyte so that it may become electrolyte: additive = 97-2: 3-98 by molar ratio with the said electrolyte .
If the molar ratio between the electrolyte and the additive is out of the above range, repeated charging and discharging may increase the internal resistance of the lithium ion secondary battery, making it impossible to obtain sufficient output. There is.

Further, from the viewpoint that the increase in internal resistance of the lithium ion secondary battery can be further suppressed, the molar ratio of the electrolyte to the additive is electrolyte: additive = 95-50: 5-50. More preferred.
In order to improve the initial output of the lithium ion secondary battery, the molar ratio of the electrolyte to the additive is more preferably electrolyte: additive = 95-80: 5-20.
Further, in order to achieve both the effect of suppressing the increase in internal resistance and the effect of improving the initial output as much as possible, the molar ratio of the electrolyte to the additive is electrolyte: additive = 95-80: 5-20. Preferably there is.
Therefore, the mixing ratio of the electrolyte and the additive can be appropriately determined according to battery characteristics required according to the use of the lithium ion secondary battery.

  Examples of the shape of the lithium ion secondary battery include a paper type, a coin type, a cylindrical type, and a square type. However, the lithium ion secondary battery of the present invention is not limited to these.

Example 1
Next, an embodiment of the lithium ion secondary battery of the present invention will be described with reference to FIGS.
As shown in FIGS. 1 and 2, the lithium ion secondary battery 1 of this example includes a positive electrode 2 containing an oxide containing lithium as a positive electrode active material 25, and a negative electrode 3 containing a carbon material as a negative electrode active material 35. And a nonaqueous electrolytic solution obtained by dissolving the electrolyte 51 in an organic solvent.

In the non-aqueous electrolyte, a compound represented by the following formula ( α ) is added as an additive 53.
The positive electrode active material 25 is mainly composed of a lithium iron composite oxide having an olivine structure represented by LiFePO ₄ or LiFe _0.85 Mn _0.15 PO ₄ .

Hereinafter, the lithium ion secondary battery 1 of this example will be described in detail with reference to FIGS.
As shown in FIG. 1, the lithium ion secondary battery 1 of this example includes a positive electrode 2, a negative electrode 3, a separator 4, a gasket 59, a battery case 6, and the like. The battery case 6 is an 18650 type cylindrical battery case, and includes a cap 63 and an outer can 65. In the battery case 6, a sheet-like positive electrode 2 and a negative electrode 3 are arranged in a wound state together with a separator 4 sandwiched between the positive electrode 2 and the negative electrode 3.
Further, a gasket 59 is disposed inside the cap 63 of the battery case 6, and a non-aqueous electrolyte is injected into the battery case 6.

As shown in FIGS. 1 and 2, the positive electrode 2 contains a lithium iron composite oxide having an olivine structure represented by LiFePO ₄ or LiFe _0.85 Mn _0.15 PO ₄ as the positive electrode active material 25, and the negative electrode 3 has a negative electrode active material. The substance 35 contains a carbon material.
The positive electrode 2 and the negative electrode 3 are respectively provided with a positive electrode current collecting lead 23 and a negative electrode current collecting lead 33 by welding. The positive electrode current collecting lead 23 is connected by welding to a positive electrode current collecting tab 235 disposed on the cap 63 side. Further, the negative electrode current collecting lead 33 is connected to the negative electrode current collecting tab 335 disposed on the bottom of the outer can 65 by welding.

In addition, the nonaqueous electrolytic solution is obtained by dissolving LiPF ₆ as the electrolyte 51 in an organic solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 30:70 as shown in FIG. Has been injected into. In addition, a compound represented by the above formula ( α ) (LiPF ₂ (C ₂ O ₄ ) ₂ , hereinafter referred to as LPFO as appropriate) is added as an additive 53 to the non-aqueous electrolyte. The additive 53 is decomposed by charging the lithium ion secondary battery 1 one or more times, covering the positive electrode 2 or / and the negative electrode 3, the positive electrode active material 25 or / and the negative electrode active material 35, and covering 55. Form. FIG. 2 shows a state in which the covering 55 is formed on the surface of the negative electrode 3.

Next, the manufacturing method of the lithium ion secondary battery of this example will be described with reference to FIGS.
First, the non-aqueous electrolyte was prepared as follows.
That is, first, an electrolyte solution was prepared by adding LiPF ₆ as an electrolyte to an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7 to a final concentration of 1M. . Further, the compound (LPFO) represented by the above formula ( α ) is added to an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7 so that the final concentration becomes 1M. In addition to the above, an additive solution was prepared.

Next, the electrolyte solution and the additive solution were mixed to prepare a non-aqueous electrolyte. At this time, the electrolyte solution and the additive solution are mixed so that the molar ratio of the electrolyte (LiPF ₆ ) and the additive (LPFO) in the non-aqueous electrolyte is electrolyte: additive = 80: 20. Mixed.

Next, a positive electrode and a negative electrode were prepared as follows.
In the positive electrode, first, a lithium iron composite oxide having an olivine structure represented by LiFePO ₄ is prepared as a positive electrode active material, and the positive electrode active material and carbon black as a conductive material (TB5500, manufactured by Tokai Carbon Co., Ltd.). And polyvinylidene fluoride as a binder (manufactured by Kureha Chemical Industry Co., Ltd., KF polymer), an appropriate amount of n-methyl-2-pyrrolidone is added as a dispersant, kneaded and pasted positive electrode mixture Got. The mixing ratio of the positive electrode active material, the conductive material, and the binder was a weight ratio of positive electrode active material: conductive material: binder = 85: 10: 5.

Next, the positive electrode mixture obtained as described above was applied to both surfaces of an aluminum foil current collector having a thickness of 20 μm and dried. Thereafter, the sheet was densified with a roll press and cut into a shape having a width of 52 mm and a length of 450 mm to produce a sheet-like positive electrode. The thickness of the positive electrode mixture on the aluminum foil current collector was 35 μm per side, and the amount of positive electrode active material deposited was about 7 mg / cm ² per side.

  On the other hand, in the negative electrode, artificial spheroidal graphite (manufactured by Osaka Gas Chemical Co., Ltd., MCMB) is prepared as a negative electrode active material, and polyvinylidene fluoride (manufactured by Kureha Chemical Industries, Ltd. Polymer) was mixed, an appropriate amount of n-methyl-2-pyrrolidone was added as a dispersant, and kneaded to obtain a paste-like negative electrode mixture. The mixing ratio of the negative electrode active material and the binder was a weight ratio of negative electrode active material: binder = 95: 5.

Next, the negative electrode mixture obtained as described above was applied to both sides of a 10 μm thick copper foil current collector and dried. Thereafter, the sheet was densified with a roll press and cut into a shape having a width of 54 mm and a length of 500 mm to produce a sheet-like negative electrode. Note that the thickness of the negative electrode mixture on the copper foil current collector was 25 μm per side, and the amount of the negative electrode active material deposited was about 5 mg / cm ² per side.

  Next, as shown in FIG. 1, a positive electrode current collecting lead 23 and a negative electrode current collecting lead 33 were welded to the sheet-like positive electrode 2 and negative electrode 3 obtained as described above, respectively. These positive electrode 2 and negative electrode 3 were wound in a state where a polyethylene separator 4 (manufactured by Tonen Tarpis Co., Ltd.) having a width of 56 mm and a thickness of 25 μm was sandwiched between them to produce a spiral wound electrode.

  Subsequently, the wound electrode was inserted into an 18650-type cylindrical battery case 6 including an outer can 65 and a cap 63. At this time, the positive electrode current collecting lead 25 is connected to the positive electrode current collecting tab 235 disposed on the cap 63 side of the battery case 6 by welding, and the negative electrode current collecting lead is disposed on the negative electrode current collecting tab 335 disposed on the bottom of the outer can 6. 33 was connected by welding.

  Next, the battery case 6 was impregnated with the non-aqueous electrolyte prepared as described above. A gasket 59 is disposed inside the cap 63, and the cap 63 is disposed in the opening of the outer can 65. Subsequently, the battery case 6 was hermetically sealed by caulking the cap 63, and the lithium ion secondary battery 1 was manufactured. This was designated as Sample E1.

  Further, in this example, the sample E1 refers to three types of lithium ion secondary batteries in which the mixing ratio of the electrolyte and the additive in the nonaqueous electrolytic solution or the composition of the positive electrode active material is different. These were prepared in the same manner as E1, and these were designated as Sample E2 to Sample E4, respectively. The lithium ion secondary batteries of Samples E2 to E4 are the same as Sample E1 except that the mixing ratio of the electrolyte and the additive in the non-aqueous electrolyte or the composition of the positive electrode active material was changed. It was produced in the same way.

Specifically, in the sample E2, the molar ratio of the electrolyte (LiPF ₆ ) and the additive (LPFO) in the nonaqueous electrolytic solution is such that the electrolyte: additive = 95: 5. The electrolyte solution and the additive solution were mixed to produce a non-aqueous electrolyte, and a lithium ion secondary battery (sample E2) was produced using this.

Further, in Sample E3, the molar ratio of the electrolyte of the nonaqueous electrolyte solution and (LiPF ₆₎ above and additives (LPFO) is, the electrolyte: additive = 50: so that 50, and the electrolyte solution A non-aqueous electrolyte was prepared by mixing with the additive solution, and a lithium ion secondary battery (sample E3) was prepared using this.

In sample E4, a positive electrode was prepared using a lithium iron composite oxide having an olivine structure represented by LiFe _0.85 Mn _0.15 PO ₄ as the positive electrode active material, and a lithium ion secondary battery (sample E4) was used. ) Was produced.
The molar ratio of the electrolyte to the additive and the composition of the positive electrode active material contained in the nonaqueous electrolyte solutions of the samples E1 to E4 and the composition of the positive electrode active material are shown in Table 1 described later.

  In the lithium ion secondary batteries of Sample E1 to Sample E4 produced in this example, as shown in FIG. 2, an additive 53 is added together with the electrolyte 51 in the nonaqueous electrolytic solution. For this reason, in the lithium ion secondary battery 1 of each sample, all or part of the additive 53 is decomposed by charging it once or more, and the surface of the negative electrode 3 and / or the negative electrode active material 35 is coated. 55 is formed.

(Comparative example)
This example is an example in which a comparative lithium ion secondary battery was manufactured in order to clarify the excellent characteristics of the lithium ion secondary batteries (samples E1 to E4) manufactured in Example 1. Specifically, for comparison, two types of lithium ion secondary batteries in which the non-aqueous electrolyte did not contain the additive were prepared.

First, LiPF ₆ as an electrolyte is added to an organic solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 3: 7 so that the final concentration is 1 M, and a non-aqueous electrolyte is added. Produced.
Subsequently, in the same manner as in Example 1 above, a positive electrode and a negative electrode were prepared, and the positive electrode, the negative electrode, and the non-aqueous electrolyte were placed in a battery case to produce a lithium ion secondary battery. This was designated as Sample C1.
Sample C1 is the same as Sample E1 to Sample E3 except that the additive is not added to the non-aqueous electrolyte.

Further, in this example, the cathode active material contains an olivine-structured lithium iron composite oxide represented by LiFe _0.85 Mn _0.15 PO ₄ similar to that of the sample E4, and an additive is added to the non-aqueous electrolyte. A lithium ion secondary battery that was not used was prepared.

Specifically, first, in the same manner as in Comparative Example 1, a nonaqueous electrolytic solution to which the additive was not added was prepared.
Subsequently, LiFe _0.85 Mn _0.15 PO ₄ is prepared as a positive electrode active material, the positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride as a binder are mixed, and n-methyl is used as a dispersant. An appropriate amount of -2-pyrrolidone was added and kneaded to obtain a paste-like positive electrode mixture. The mixing ratio of the positive electrode active material, the conductive material, and the binder was a weight ratio of positive electrode active material: conductive material: binder = 85: 10: 5.

  Next, as in Example 1, the positive electrode mixture obtained as described above was applied to both sides of an aluminum foil current collector having a thickness of 20 μm, dried, and then densified with a roll press. The sheet-like positive electrode was produced by cutting into a shape having a width of 54 mm and a length of 450 mm.

Next, in the same manner as Sample E1 in Example 1, a negative electrode was prepared, and the positive electrode, the negative electrode, and the non-aqueous electrolyte were placed in a battery case to produce a lithium ion secondary battery. This was designated as Sample C2.
Sample C2 is the same as Sample C1 except that it contains LiFe _0.85 Mn _0.15 PO ₄ as the positive electrode active material.
The molar ratio of the electrolyte and the additive contained in the non-aqueous electrolytes of the samples C1 and C2 and the composition of the positive electrode active material are shown in Table 1 described later.

(Experimental example)
Next, in this example, the following initial output test was performed using the samples E1 to E4 prepared in Example 1 and the samples C1 and C2 prepared in the comparative example, and the low temperature (−30 ° C.) The initial output performance of each sample was evaluated. Moreover, while performing the following charging / discharging cycle test about each sample, the capacity | capacitance maintenance factor and the resistance increase rate were measured.

"Initial output test"
Each sample (samples E1-E4, C1 and C2) was kept at −30 ° C. After that, the battery capacity is adjusted to 50% (SOC = 50%), 0.12A, 0.4A, 1.2A, 2.4A, 4.8A current is applied and the battery voltage after 10 seconds is measured. The output value was calculated. The measurement was performed by preparing three samples similar to each sample, and obtaining the average. The output values of the samples E1 to E3 are expressed as relative values when the value of the sample C1 is 1, that is, a value normalized based on the value of the sample C1. The output value of the sample E4 is expressed as a relative value when the value of the sample C2 is 1, that is, a value normalized based on the value of the sample C2. The results are shown in Table 1 below.

"Charge / discharge cycle test"
Under the temperature condition of 60 ° C., which is considered to be the upper limit of the actual use temperature range of the battery, the above sample E1 to sample E5 and sample C1 are charged to a maximum charge voltage of 4.2 V at a constant current of 2.0 mA / cm ² Charging and discharging, which was then performed at a constant current with a current density of 2.0 mA / cm ² and discharging to a discharge lower limit voltage of 3 V, was defined as one cycle, and this cycle was performed for a total of 500 cycles.

"Capacity maintenance rate"
When the discharge capacity before the charge / discharge cycle test was capacity A (initial discharge capacity) and the discharge capacity after the charge / discharge cycle test was capacity B, it was calculated by the following equation (a).
Capacity maintenance rate (%) = capacity B / capacity A × 100 (a)

  In calculating the capacity retention rate, the discharge capacity before and after the charge / discharge cycle test was measured once for each sample, and the capacity retention rate was calculated from the above equation (a). The measurement was performed by preparing three samples similar to each sample, and obtaining the average. The results are shown in Table 1.

In addition, the resistance increase rate before and after the charge / discharge cycle test was calculated as follows.
"Evaluation of resistance increase rate"
Each sample was adjusted to 50% of the battery capacity (SOC = 50%), and 0.12A, 0.4A, 1.2A, 2.4A, 4.8A current was passed and the battery voltage after 10 seconds was measured. did. The applied current and voltage were linearly approximated, and IV resistance was obtained from the slope.
The rate of increase in resistance can be calculated by the following equation (b), where IV resistance after the charge / discharge test is resistance B and IV resistance before the charge / discharge test is resistance A (initial IV resistance).
Resistance increase rate (%) = (resistance B−resistance A) × 100 / resistance A (b)

  In calculating the resistance increase rate, the IV resistance before and after the charge / discharge cycle test was measured once for each sample, and the resistance increase rate was calculated from the above equation (b). The measurement was performed by preparing three samples similar to each sample, and obtaining the average. The results are shown in Table 1.

  As is known from Table 1, the lithium ion secondary batteries of Sample E1 to Sample E4 in which the electrolyte and the additive are added to the non-aqueous electrolyte solution are those in which only the electrolyte is added to the non-aqueous electrolyte solution. Compared with the sample C1 and the sample C2, the output at the initial stage, particularly the output at a low temperature was high, and a high capacity retention rate could be exhibited even after the charge / discharge cycle test under a high temperature condition. In addition, the rate of increase in IV resistance of samples E1 to E4 after the charge / discharge cycle test is very low compared to samples C1 and C2, and the increase in internal resistance is suppressed in samples E1 to E4. Recognize.

Thus, a lithium ion secondary battery containing a lithium iron composite oxide having an olivine structure represented by the above composition formula in the positive electrode active material and containing the electrolyte and the additive in a non-aqueous electrolyte is: It can be seen that the output at the initial stage, particularly at the low temperature, is high, and the charge / discharge cycle characteristics at the high temperature are excellent.

  As described above, the lithium ion secondary batteries of Samples E1 to E4 have high output at a low temperature of about −30 ° C. and excellent charge / discharge cycle characteristics at a high temperature of about 60 ° C. As shown, in these lithium ion secondary batteries 1 (sample E1 to sample E4), in addition to the positive electrode active material 25 containing a lithium iron composite oxide having a specific composition of olivine structure, non-aqueous electrolysis This is probably because the electrolyte 51 and the additive 53 are added to the liquid. As shown in the figure, at least a part of the additive 53 in the non-aqueous electrolyte is decomposed during the first charge, and the surface of the negative electrode active material 35 and / or the negative electrode 3 has a low resistance and a stable coating 55. It is thought that it forms.

Further, when the voltage-charge capacity curves at the time of initial charge of the samples E1 to E4 and the samples C1 and C2 are examined, in the samples E1 to E4 in which the additive is added to the non-aqueous electrolyte together with the electrolyte, A capacity component that was considered to decompose the additive to form a coating on the negative electrode was observed in the vicinity of 1.8V.
On the other hand, in Sample C1 and Sample C2 to which no additive is added, there is no capacity component near 1.8 V as described above, and it is considered that no coating is formed on the negative electrode.

As is known from Table 1, when the amount of the additive added to the non-aqueous electrolyte is a molar ratio with the electrolyte, that is, electrolyte: additive = 95-80: 5-20, the temperature is low. It can be seen that the initial output at can be significantly improved. Moreover, when electrolyte: additive = 95-50: 5-50, it turns out that the raise of IV resistance can be suppressed notably.
Although not shown in the table, when the amount of the additive is a molar ratio with the electrolyte and is electrolyte: additive = 97-2: 3-98, the effect of improving the initial output at low temperature is improved. In addition, it has been confirmed that the effect of suppressing the increase in the IV resistance of the charge / discharge cycle at a high temperature can be sufficiently obtained.

BRIEF DESCRIPTION OF THE DRAWINGS Explanatory drawing which shows the structure of the lithium ion secondary battery concerning Example 1. FIG. The elements on larger scale of the positive electrode of the lithium ion secondary battery concerning Example 1, and a negative electrode.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Lithium ion secondary battery 2 Positive electrode 25 Positive electrode active material 3 Negative electrode 35 Negative electrode active material 51 Electrolyte 53 Additive 55 Covering material

Claims (4)

In a lithium ion secondary battery having a positive electrode containing an oxide containing lithium as a positive electrode active material, a negative electrode containing a carbon material as a negative electrode active material, and a non-aqueous electrolyte obtained by dissolving an electrolyte in an organic solvent ,
In the non-aqueous electrolyte, a compound represented by the following formula (α) is added as an additive,
The positive electrode active material has a composition formula Li _x Fe _1-y Me _y PO ₄ (where Me is Mn, Cr, Co, Cu, Ni, V, Mo, Ti, Zn, Al, Ga, Mg, B, and The main component is a lithium iron composite oxide having an olivine structure represented by one or more selected from Nb, 0.05 ≦ x ≦ 1.2, 0 ≦ y ≦ 1.0) ,
The lithium ion secondary battery is characterized in that the additive is added to the non-aqueous electrolyte so that the molar ratio with the electrolyte is electrolyte: additive = 97-2: 3-98. .

  2. The lithium ion secondary battery according to claim 1, wherein the positive electrode active material is composed of particles of the lithium iron composite oxide, and carbon particles are complexed with the particles. 3. The electrolyte according to claim 1, wherein the electrolyte includes A ^{a +} (PF ₆ ⁻ ) _a , A ^{a +} (ClO ₄ ⁻ ) _a , A ^{a +} (BF ₄ ⁻ ) _a , A ^{a +} (AsF ₆ ⁻ ) _a , or A ^{a +.} A lithium ion secondary battery characterized by being one or more selected from (SbF ₆ ⁻ ) _a , wherein A ^{a +} is a metal ion, proton, or onium ion, and a is 1 to 3 . In any one of claims 1 to 3, the _{electrolyte, LiPF 6, LiClO 4, LiBF} 4, LiAsF 6 , lithium-ion secondary battery which is characterized in that at least one selected from LiSbF _6,.

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