JP5528564B2 - Nonaqueous electrolyte secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte secondary battery that uses a lithium iron phosphate positive electrode and achieves both high capacity and storage characteristics.

  Conventionally, lithium cobaltate has been the mainstream as a positive electrode active material for nonaqueous electrolyte batteries. However, since cobalt, which is the raw material, is low in production and expensive, the use of lithium cobaltate increases the production cost of the battery. A battery using lithium cobaltate has a problem in safety when the battery temperature rises at the end of charging.

  For this reason, the use of lithium manganate, lithium nickelate, etc. as a positive electrode active material instead of lithium cobaltate is currently being studied. However, lithium manganate cannot realize a sufficient discharge capacity, and the battery temperature is low. When it becomes higher, there are problems such as elution of manganese. On the other hand, lithium nickelate has problems such as a lower discharge voltage and lower thermal stability at the end of charging.

Such that even recently, high stability at high temperature calorific value is low, olivine-type lithium iron phosphate such as metal elution hardly LiFePO ₄ has attracted attention as a positive electrode active material can be used in place of lithium cobaltate ing.

The olivine-type lithium phosphate is represented by a general formula of LiMPO ₄ (M is at least one element selected from Co, Ni, Mn, and Fe), and the battery voltage is arbitrarily determined depending on the type of the constituent metal element M. Can be selected. Further, since the use capacity is relatively high at about 140 to 170 mAh / g, there is an advantage that the battery capacity per unit weight can be increased. And when iron and manganese are selected as M, it has the advantage that the production cost can be greatly reduced because the production amount is large and it is inexpensive.

  In particular, lithium iron phosphate becomes iron phosphate in the charged state, and its structural stability and end-of-charge voltage can be charged almost 100% at the lithium metal standard 3.6V, so it is used as the main component of organic electrolyte. 100% can be charged at 4.2 V or less of the decomposition potential of the cyclic carbonate and the chain carbonate. Therefore, it is expected as a positive electrode active material that can suppress decomposition of the electrolytic solution and has high durability.

  However, lithium iron phosphate has a NASICON structure, which is an ionic conductor, and therefore has a poor electronic conductivity and a strong crystal structure. Therefore, lithium ion diffusion is limited, and a one-dimensional diffusion path. Therefore, it is known that the diffusion of lithium ions is poor. Therefore, the material has a high resistance value and is not suitable for battery materials.

  In order to solve these problems, the surface of lithium iron phosphate particles is coated with a highly conductive carbon material, thereby improving the electronic conductivity, reducing the particle size to 1 μm or less, shortening the reactive path, and reacting. The technique of making it function as a battery material by devising which raises speed has been reported (patent document 1).

JP 2002-110162 A Japanese Patent No. 4183403

W. Li et.al., Electrochem.Solid-State Lett., 10 (4), A115-A117 (2007) M. C. Smart et. Al., J. Electrochem. Soc., 155 (8), A557-A568 (2008)

However, when lithium iron phosphate whose surface is coated with a carbon material is used, there arises a problem that battery characteristics, particularly storage characteristics, deteriorate. When the present inventors examined this problem, since the specific surface area of lithium iron phosphate whose surface was coated with nano-level carbon particles was as high as 10 to 30 m ² / g, it easily adsorbed moisture in the atmosphere. Furthermore, since the moisture is taken into the pores of the nano-sized carbon material, it is difficult to remove it. Therefore, it is considered that the expected battery characteristics cannot be obtained when water mixed in the lithium ion secondary battery reacts with LiPF _{6 as an} electrolyte to generate hydrofluoric acid.

When water is mixed in the lithium ion secondary battery, the water and LiPF ₆ are expressed by the formula (1):
LiPF ₆ + H ₂ O → LiF + PF ₃ O + HF (1)
Thus, hydrofluoric acid is produced together with lithium fluoride and trifluorophosphoric acid. Since hydrofluoric acid is a strong acid, iron may be eluted from the olivine Fe positive electrode and a solid electrolyte interface (SEI) that is a protective layer of the negative electrode may be eluted. Therefore, gas is generated during charging, charging / discharging efficiency is reduced, and durability is reduced.

  Lithium iron phosphate is useful because it has high thermal stability and is difficult to elute metals at high temperatures. However, since it has low reactivity, it needs to have a large specific surface area for application to an electric vehicle battery. However, as described above, when the specific surface area is large, moisture is easily adsorbed, so that moisture is mixed into the battery and battery characteristics are deteriorated. Therefore, an object of the present invention is to achieve both load characteristics and high-temperature storage characteristics of a lithium ion secondary battery using a lithium iron phosphate positive electrode having a large specific surface area by suppressing the influence of moisture.

  Conventionally, it has been known that vinylene carbonate (VC) is used as an electrolytic solution additive to improve high-temperature storage characteristics, and a film is formed on the negative electrode. By increasing the content of VC, high-temperature storage is achieved. It has been reported that the properties are improved. However, in this method, since a film is formed on the negative electrode, there is a problem in that the movement of lithium ions from the electrolytic solution into the negative electrode active material is hindered and the direct current resistance value increases.

On the other hand, as a hydrofluoric acid inhibitor for removing the influence of moisture in the electrolyte, tristrifluoroethyl phosphate (TTFP), N, N-dimethylacetamide (DMA), and n-methylpyrrolidone (NMP) are available. Known (Non-Patent Documents 1 and 2). These hydrofluoric acid inhibitors are PF _{5 in} an electrolyte solution using LiPF ₆ as a supporting salt in a battery system in which a cobalt-substituted lithium nickelate LiNi _0.8 Co _0.2 O ₂ positive electrode and a graphite negative electrode are combined. It has been reported that the reaction between ions and moisture is suppressed to suppress the formation of hydrofluoric acid (HF), and the high-temperature storage characteristics are improved as in the case of vinylene carbonate (VC). On the other hand, Non-Patent Document 2 reports that when DMA is used as a part of the mixed solvent, the reaction resistance is increased. In Patent Document 2, as with ethylene carbonate (EC), DMA is expected to improve load characteristics as a high dielectric constant solvent. Thus, the effect of the hydrofluoric acid inhibitor on the battery characteristics varies depending on the battery system to be applied, the type of hydrofluoric acid inhibitor, and the content in the electrolytic solution. Therefore, it is very difficult to predict the efficacy of the hydrofluoric acid inhibitor.

  However, as a result of intensive studies aimed at achieving both load characteristics and high-temperature storage characteristics of lithium ion secondary batteries using lithium iron phosphate positive electrodes, the present inventors have found that vinylene carbonate and a predetermined amount are contained in the non-electrolyte. It was found that the above-mentioned object can be achieved by adding N, N-dimethylacetamide as a hydrofluoric acid inhibitor.

  That is, the present invention includes the following.

(1) formula having an olivine type structure _{LiFe 1-X M x PO 4} :
[Where
M is at least one selected from the group consisting of Ni, Co, Mn, Ti, Zr, and Mo;
x is 0 ≦ x <1]
A positive electrode comprising a positive electrode active material represented by:
A negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions; and a non-aqueous electrolyte;
A non-aqueous electrolyte secondary battery comprising:
The surface of the positive electrode active material is coated with carbon,
The non-aqueous electrolyte includes N, N-dimethylacetamide, vinylene carbonate, a supporting salt, and an organic solvent;
The nonaqueous electrolyte secondary battery, wherein a content of the N, N-dimethylacetamide is 0.01 to 0.7% by weight of the nonaqueous electrolyte.

(2) The amount of carbon covering the surface of the positive electrode active material is 1 to 5% by weight of the carbon-coated positive electrode active material, the specific surface area of the carbon-coated positive electrode active material is 10 to 20 m ² / g, and the carbon The nonaqueous electrolyte secondary battery according to (1), wherein the coated positive electrode active material contains 300 to 1000 ppm of water.

(3) The nonaqueous electrolyte secondary battery according to (1) or (2), wherein the content of vinylene carbonate is 0.5 to 3% by weight of the nonaqueous electrolyte.

(4) The nonaqueous electrolyte secondary battery according to any one of (1) to (3), wherein x is 0.

(5) The nonaqueous electrolyte secondary battery according to any one of (1) to (4), wherein the negative electrode active material capable of occluding and releasing lithium ions is graphite.

(6) supporting salt is LiPF _6, (1) a non-aqueous electrolyte secondary battery according to any one of the - (5).

(7) The organic solvent is a mixed solvent of ethylene carbonate and chain carbonate, and the content of the chain carbonate is 70 to 80% by volume of the mixed solvent. The nonaqueous electrolyte secondary battery as described.

  By applying the present invention, it is possible to reduce the influence of moisture contained while suppressing an increase in the reaction resistance of the lithium iron phosphate positive electrode, while maintaining the load characteristics of the battery using the lithium iron phosphate positive electrode while maintaining a high temperature. Storage characteristics can be improved.

A structural diagram of a cylindrical battery is shown. The relationship between DMA content rate and initial resistance value is shown. The relationship between DMA content rate and the capacity maintenance rate after high temperature storage is shown.

  Hereinafter, the present invention will be described in detail.

Nonaqueous electrolyte secondary battery according to the present invention,
Formula having the olivine structure _{LiFe 1-x M x PO 4} :
[Where
M is at least one selected from the group consisting of Ni, Co, Mn, Ti, Zr, and Mo;
x is 0 ≦ x <1]
A positive electrode comprising a positive electrode active material represented by:
A negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions; and a non-aqueous electrolyte;
Furthermore, the surface of the positive electrode active material is coated with carbon, and the non-aqueous electrolyte contains N, N-dimethylacetamide, vinylene carbonate, a supporting salt, and an organic solvent, The content of N-dimethylacetamide is 0.01 to 0.7% by weight of the nonaqueous electrolyte.

  Compared with the conventional positive electrode active material, the positive electrode active material having an olivine structure has high thermal stability, and the metal does not easily elute at a high temperature. Moreover, it is possible to increase the battery capacity per unit weight, and furthermore, by covering the surface with carbon, the electronic conductivity can be improved. However, when the specific surface area is increased by coating with carbon, moisture in the air is easily adsorbed, and the battery characteristics are deteriorated. However, the above-mentioned drawback can be overcome by using a non-aqueous electrolyte containing N, N-dimethylacetamide and vinylene carbonate. That is, by combining the positive electrode active material having an olivine type structure and the non-aqueous electrolyte, it is possible to sufficiently exert the function of the positive electrode active material having many advantages over the conventional positive electrode active material. Become.

  As an embodiment of the present invention, a nonaqueous electrolyte battery having the configuration shown in FIG. 1 can be mentioned. The nonaqueous electrolyte battery includes a positive electrode in which a positive electrode active material, a carbon material of a conductive additive and a binder are formed in a film shape on an aluminum foil, and a negative electrode active material and a binder in a film shape on a copper foil. The structure is filled with a non-aqueous electrolyte solution in a state where the negative electrodes formed on the surface are opposed and electrically isolated by a separator. The positive and negative electrodes are wound as shown in FIG. 1 and stored in a predetermined metal container. There are two types of battery structures, one that is housed in cylindrical and square metal containers as shown in the figure, and the other that is a sheet-like positive and negative electrode layered without being wound. The battery structure shown in FIG.

1. In the non-aqueous electrolyte secondary battery according to the present invention, a composition formula LiFe _1-X M _x PO ₄ having an olivine type structure:
[Where
M is at least one selected from the group consisting of Ni, Co, Mn, Ti, Zr, and Mo;
x is 0 ≦ x <1]
The positive electrode containing the positive electrode active material represented by these is used.

For example, lithium iron manganese phosphate (composition formula: LiFe _1-x Mn _x PO ₄ where 0 <x <1) in which a part of iron of lithium iron phosphate is substituted with manganese has the same characteristics as lithium iron phosphate. Since it has, it can be used as a positive electrode active material. In addition, since lithium iron phosphate or a part of iron or manganese of lithium manganese phosphate is replaced with Ti, Zr and / or Mo, and the material having improved reactivity basically has the same characteristics, It can be used as a positive electrode active material. Further, lithium iron phosphate, or lithium manganese phosphate in which iron or a part of manganese is substituted with Ni and / or Co can also be used as a positive electrode active material (hereinafter, a part of iron is another metal). The lithium iron phosphate substituted with the element M may also be simply expressed as “lithium iron phosphate”). Note that lithium manganese phosphate (composition formula: LiMnPO ₄ ) in which all of iron of lithium iron phosphate is substituted with manganese also has the same characteristics as lithium iron phosphate, and can be used as a positive electrode active material. .

  From the viewpoint of production cost, x is preferably 0 ≦ x ≦ 0.5, more preferably 0 ≦ x ≦ 0.3, and particularly preferably x is 0. Moreover, when substituting a part of iron, it is preferable that M is Mn.

  In order to activate the reaction of lithium iron phosphate, the average particle size is preferably 0.5 μm or less.

  In order to impart electronic conductivity, the surface of lithium iron phosphate is coated with carbon. Here, “coating” means that the entire surface is coated or only a part of the surface is coated. The amount of carbon to be coated (hereinafter also simply referred to as “coated carbon”) is not particularly limited, but 1 to 5 of lithium iron phosphate coated with carbon (hereinafter also simply referred to as “carbon-coated positive electrode active material”). It is preferable that it is weight%, and it is especially preferable that it is 1-3 weight%. If the amount of the coated carbon is less than 1% by weight, sufficient electron conductivity cannot be imparted and load characteristics may not be obtained. On the other hand, the amount of coated carbon and electronic conductivity are in a proportional relationship, but if the amount of coated carbon exceeds 5% by weight, the specific surface area also increases, so the amount of moisture adsorbed increases and the storage characteristics can be reduced. There is sex. In addition, it is preferable that the quantity of coating | coated carbon is 1 to 6 weight% with respect to the positive mix after drying, and it is especially preferable that it is 1 to 4 weight%.

The specific surface area of the carbon-coated positive electrode active material is dependent like the amount of coated carbon is preferably 10 to 20 m ² / g, particularly preferably 10 to 15 m ² / g.

In order to give sufficient electronic conductivity, a conductive auxiliary agent having a large specific surface area may be added to the positive electrode mixture separately from the coated carbon. As the conductive assistant, it is preferable to use a carbon material, and it is particularly preferable to use a carbon material having a specific surface area of 10 m ² / g or more. The amount of the conductive auxiliary agent is not particularly limited, but is preferably 3 to 8% by weight of the positive electrode mixture after drying.

  The content of the positive electrode active material in the positive electrode mixture is preferably 83 to 92% by weight, and particularly preferably 85 to 92% by weight. If it is less than 83% by weight, the content of lithium iron phosphate decreases, the content of carbon having a low true density increases, the electrode density decreases, and the energy density may decrease. On the other hand, if it exceeds 92% by weight, the lithium iron phosphate particles are fine and have a high specific surface area, so the adhesiveness between the particles and between the particles and the current collector foil is weakened, and a structure having sufficient strength is formed. Can be difficult.

  In order to maintain the adhesion strength between the particles and the interface of the current collector foil, the binder is preferably used in an amount of 4 to 8% by weight with respect to the positive electrode mixture after drying. The binder is not particularly limited, but it is preferable to use polyvinylidene fluoride (PVdF).

The water content in the carbon-coated positive electrode active material depends on the specific surface area and the amount of coated carbon. When the specific surface area of the carbon-coated positive electrode active material is 10 to 20 m ² / g and the amount of coated carbon is 3 to 5% by weight of the carbon-coated positive electrode active material, the water content is 500 to 2000 ppm. The lower the moisture content, the better, but the lithium iron phosphate particles whose surface is coated with carbon are easy to adsorb moisture in the atmosphere and contain 500 ppm or more of moisture. On the other hand, depending on the amount of coated carbon, the water content is almost saturated at 2000 ppm. When it contains more moisture, lithium iron phosphate reacts with moisture to produce iron phosphide and the like, and its properties as a positive electrode active material are degraded. In addition, since the moisture content depends on the storage condition of the material, it is preferable that the moisture content is 300 to 1000 ppm in a state where vacuum drying is performed at 80 ° C. for 6 hours.

The positive electrode paint was first mixed with lithium iron phosphate and acetylene black, impregnated with NMP, then added with a PVdF binder solution, kneaded, added with NMP, and adjusted to a predetermined viscosity. The prepared positive electrode paint was applied on an aluminum foil in the range of 100 to 160 g / m ² and dried at 120 ° C. for 15 minutes to obtain a positive electrode coating film. Then, it pressed and what adjusted the electrode density between 1.6-2.2 g / cm < ³ > was used as the positive electrode.

2. The negative electrode active material contained in the negative electrode is not particularly limited as long as it is a substance capable of occluding and releasing lithium ions, and various substances can be used. For example, it is preferable to use graphite as the negative electrode active material.

As one embodiment of the present invention, a predetermined amount of a paint in which graphite material, binder carboxymethyl cellulose and styrene butadiene rubber are dispersed in an aqueous solution is applied on a copper foil, and dried at 100 ° C. for 15 minutes. Is used as the negative electrode. The coating amount of the negative electrode is preferably 1/2 to 2/3 of the coating amount of the positive electrode. When less than 1/2 of the coating amount of the positive electrode, the charging depth of the negative electrode is too deep, and the cycle and the storage life are reduced. On the other hand, when it is more than 2/3, the energy density becomes small. The electrode density of the negative electrode is preferably in the range of 1.3 to 1.7 g / cm ³ . If it is less than 1.3 g / cm ³ , the energy density becomes small. On the other hand, when it is larger than 1.7 g / cm ³ , the number of vacancies in the electrode decreases, the content of the electrolytic solution decreases, the amount of lithium reaction decreases, and the current hardly flows.

  The ratio of the active material in the positive electrode and the negative electrode varies depending on the type of the negative electrode active material, but in general, (weight of the positive electrode active material) / (weight of the negative electrode active material) is 1.5 to 3.5. Preferably there is. Within this range, the characteristics of the lithium iron phosphate can be used well. However, as an anode active material, an alloy containing an element that can be alloyed with lithium, or an alloy mainly containing such an element, a lithium-containing composite nitride, and other components such as those materials and a carbonaceous material When the composite is used, since the capacity of the negative electrode becomes too large at the above ratio, it is desirable that (weight of the positive electrode active material) / (weight of the negative electrode active material) be 4 to 7.

3. Nonaqueous electrolyte The nonaqueous electrolyte used in the nonaqueous electrolyte secondary battery according to the present invention includes N, N-dimethylacetamide (DMA), vinylene carbonate (VC), a supporting salt, and an organic solvent (other than DMA and VC). including. The combined use of N, N-dimethylacetamide and vinylene carbonate can improve the high-temperature storage characteristics while maintaining the load characteristics of the secondary battery. As the non-aqueous electrolyte, an organic solvent-based liquid electrolyte in which a supporting salt is dissolved in an organic solvent, that is, an electrolytic solution, a polymer electrolyte in which the electrolytic solution is held in a polymer, or the like can be used.

  N, N-dimethylacetamide functions as a hydrofluoric acid inhibitor and can reduce the influence of moisture mixed in the secondary battery. The content of N, N-dimethylacetamide is 0.01 to 0.7% by weight of the nonaqueous electrolyte from the viewpoint of improving the high-temperature storage characteristics while maintaining the load characteristics of the secondary battery, and 0.05 It is preferable that it is -0.7 weight%, and it is especially preferable that it is 0.1-0.7 weight%. When the content is less than 0.01% by weight, the high-temperature storage characteristics tend to decrease, and when the content exceeds 0.7% by weight, both the load characteristics and the high-temperature storage characteristics tend to decrease.

  The content of vinylene carbonate is not particularly limited, but from the viewpoint of improving high-temperature storage characteristics while maintaining the load characteristics of the secondary battery, and from the viewpoint of forming good SEI when using a graphite negative electrode, It is preferably 0.5 to 3% by weight, more preferably 0.5 to 2% by weight, and particularly preferably 0.5 to 1.5% by weight. If it exceeds 3% by weight, the service life is improved, but the SEI film is formed too much, and the load characteristics may not be obtained.

  The organic solvent contained in the non-aqueous electrolyte is not particularly limited, but preferably contains a chain ester from the viewpoint of load characteristics. Examples of such chain esters include chain carbonates typified by dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC), ethyl acetate, and methyl propionate. These chain esters may be used alone or in admixture of two or more. In particular, in order to improve the low temperature characteristics, the chain ester may occupy 50% by volume or more of the total organic solvent. It is particularly preferred that the chain ester occupies 65% by volume or more of the total organic solvent.

  However, as an organic solvent, in order to improve the discharge capacity, compared with the chain ester alone, an ester having a high induction rate (induction rate: 30 or more) is mixed with the chain ester. Is preferred. Specific examples of such esters include, for example, cyclic carbonates represented by ethylene carbonate, propylene carbonate, butylene carbonate, γ-butyrolactone, ethylene glycol sulfite, and the like, particularly ethylene carbonate, propylene carbonate, and the like. Cyclic esters are preferred.

  Such an ester having a high dielectric constant is preferably contained in an amount of 10% by volume or more, particularly 20% by volume or more of the total organic solvent from the viewpoint of discharge capacity. Moreover, from the point of load characteristics, 40 volume% or less is preferable and 30 volume% or less is more preferable.

  Examples of the solvent that can be used together with the ester having a high dielectric constant include 1,2-dimethoxyethane, 1,3-dioxolane, tetrahydrofuran, 2-methyl-tetrahydrofuran, and diethyl ether. In addition, a fluorine-containing organic solvent can also be used.

The supporting salt dissolved in an organic solvent, for _{example, LiClO 4, LiPF 6, LiBF} 4, LiAsF 6, LiSbF 6, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiCF 3 CO 2, Li 2 C 2 F 4 (SO ₃ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiC _n F _{2n + 1} SO ₃ (n ≧ 2) are used alone or in combination of two or more. Among these, LiPF ₆ and LiC ₄ F ₉ SO ₃ that can obtain good charge / discharge characteristics are preferably used. The concentration of the supporting salt in the nonaqueous electrolyte is not particularly limited, but is preferably about 0.3 to 1.7 mol / dm ³ , particularly about 0.4 to 1.5 mol / dm ³ .

Among these, LiPF ₆ salt is desirable as a supporting salt because it exhibits good load characteristics. Other supporting salts include selective branches such as LiBF ₄ , but it is desirable that LiPF ₆ be the main component. Moreover, in order to give a practical load characteristic, it is preferable that the mixed organic solvent contains ethylene carbonate and a chain carbonate such as 70 to 80% by volume of dimethyl carbonate or ethyl methyl carbonate.

  Moreover, in order to improve the safety | security and storage characteristic of a battery, you may make an non-aqueous electrolyte contain an aromatic compound. As the aromatic compound, benzenes having an alkyl group such as cyclohexylbenzene or t-butylbenzene, biphenyl, or fluorobenzenes are preferably used.

4). As the separator, a separator having sufficient strength and capable of holding a large amount of non-aqueous electrolyte is preferable. From such a viewpoint, the thickness is 5 to 50 μm, polypropylene, polyethylene, a copolymer of propylene and ethylene, etc. A polyolefin microporous film or non-woven fabric is preferably used. In particular, when a thin separator of 5 to 20 μm is used, the characteristics of the battery are likely to deteriorate during charge / discharge cycles and high-temperature storage, but the lithium iron phosphate of the present invention is excellent in stability. Even if such a thin separator is used, the battery can function stably.

  Since the lithium iron phosphate positive electrode has high thermal stability, it is possible to provide a lithium ion battery with high thermal stability even when the above polyolefin separator is used. Furthermore, by applying a functional separator that suppresses thermal shrinkage at 150 ° C. or higher by applying 3 to 5 μm of an oxide such as magnesium oxide or silicon dioxide, the thermal stability of the lithium ion battery is further improved. Can do.

  Examples of the present invention will be described below. However, this invention is not limited only to those Examples.

Example 1
Lithium iron phosphate represented by the composition formula LiFePO ₄ consists of 107 g of LiH ₂ PO ₄ (manufactured by Aldrich), 175 g of FeC ₂ O ₄ .2H ₂ O (manufactured by Kojundo Kagaku) and 16.4 g of dextrin (sum) The zirconia grinding balls were put into a zirconia pot and mixed with a planetary ball mill (manufactured by Frichche) at a rotation speed of 3 levels for 30 minutes. The mixed powder was put into an alumina crucible and calcined at 400 ° C. for 10 hours under an argon flow of 0.3 L / min. Again, the zirconia grinding balls were put into the zirconia pot, rotated at 1 level for 1 minute, crushed, and again put into the alumina crucible, under an argon flow of 0.3 L / min, 700 After firing for 10 hours at ℃, the obtained powder was put into a zirconia pot with zirconia grinding balls, rotated at 1 level for 1 minute, crushed, and screened with a 45 μm mesh. The desired particle size was obtained by adjusting the particle size. As a result of conducting a composition analysis by ICP measurement (manufactured by Shimadzu Corporation), it was Li _1.0 Fe _0.98 P _1.02 O ₄ (carbon content: 3.0 wt%). The coating with amorphous carbon was confirmed using a scanning electron microscope (manufactured by Hitachi, Ltd.) and a powder X-ray diffractometer (manufactured by Rigaku Corporation). B. E. The specific surface area was measured from the amount of nitrogen adsorbed under low temperature and low humidity conditions by the T method, and as a result, it was 15 m ² / g. For the measurement, a fully automatic BET specific surface area measuring device (manufactured by Mountec Co., Ltd.) was used, and vacuum deaeration was performed as a pretreatment at 300 ° C. for 6 hours, and the nitrogen adsorption amount was determined at a liquid nitrogen (77 ° K) temperature. Further, the moisture content was measured using a Karl Fischer moisture meter (manufactured by Kyoto Electronics Industry Co., Ltd.). As a result, the moisture content after vacuum drying at 80 ° C. was 600 ppm. Here, lithium carbonate may be used instead of lithium hydroxide.

  In order to blend lithium iron phosphate, carbon, and binder at 85 wt%, 8 wt%, and 7 wt%, respectively, carbon coated lithium iron phosphate is 87 wt%, and acetylene is used as a conductive aid other than the coated carbon. 6% by weight of black and 7% by weight of PVdF binder were used.

Using a small kneader, carbon coated lithium iron phosphate and acetylene black were mixed, PVdF-containing NMP solution was added, and the resulting coating was applied to aluminum foil at 150 g / m ² at 120 ° C. Dried. Then, it pressed so that an electrode density might be set to 1.7 g / cm < ³ >, and it cut | judged to the predetermined size, and produced the positive electrode.

As the negative electrode, a natural graphite material having a particle diameter of 20 μm was used, and 80 g / m ^{2 of} a negative electrode slurry in which carboxymethyl cellulose and styrene butadiene rubber were dispersed in an aqueous solution as a binder was applied and dried at 100 ° C. Then, it pressed so that it might become 1.5 g / cm < ³ >, and it cut | judged to the predetermined size, and produced the negative electrode.

  For the battery evaluation, a 18650 size cylindrical battery (diameter: 18 mm, height: 65 mm) was used. Current collector leads made of aluminum and nickel were welded to the positive electrode and negative electrode cut to a predetermined size, respectively, and the separator was wound and assembled using a polyolefin-based porous film having a thickness of 25 μm.

As an electrolytic solution, a 1M LiPF ₆ EC / DMC (1/3) 1 wt% VC solution in which 0.5 wt% of N, N-dimethylacetamide was added as an HF inhibitor was examined. The amount of the electrolyte was adjusted to 5 to 6 ml.

  Using the 18650 cylindrical evaluation battery obtained in the above configuration, the DC resistance value was measured by the following method in order to evaluate the load characteristics, the capacity retention rate during high temperature storage was determined, and the storage characteristics were evaluated. .

  Charging at room temperature was carried out until the current value was 0.5 CA, the upper limit voltage was 3.6 V, and the final current value was 0.1 CA. Discharge was conducted up to 2.0 V with a current value of 1.0 CA. The capacity at that time was defined as 0% discharge depth, and the capacity when charged again under the same conditions was defined as 100% charge depth. After discharging at a discharge depth of 50% and leaving it for 1 hour to obtain an open circuit voltage, 1CA, 2CA and 3CA pulses were discharged at room temperature, and a DC resistance was obtained by linear approximation using a closed circuit voltage at the 5th second.

  The high-temperature storage characteristics are charged to the fully charged state (charging depth 100%) under the above-mentioned charging conditions, stored in a constant temperature bath at 50 ° C. for 10 days, then measured at 1 CA, and the initial capacity as 100%. The maintenance rate was determined.

  Hereinafter, although an Example and a comparative example are shown, the same positive electrode and negative electrode as Example 1 were used, and the manufacturing method and evaluation method of both electrodes were also the same as Example 1.

Example 2
In the same procedure as in Example 1, 0.1% by weight of N, N-dimethylacetamide as an HF inhibitor was added to 1M LiPF ₆ EC / DMC (1/3) 1% by weight VC solution as an electrolyte. investigated. The produced 18650 type prototype battery was evaluated for DC resistance and high-temperature storage characteristics by the method described in Example 1.

Comparative Example 1
In the same procedure as in Example 1, a 1M LiPF ₆ EC / DMC (1/3) 1 wt% VC solution was examined as an electrolytic solution. The produced 18650 type prototype battery was evaluated for DC resistance and high-temperature storage characteristics by the method described in Example 1.

Comparative Example 2
In the same procedure as in Example 1, 1.0% by weight of N, N-dimethylacetamide as an HF inhibitor was added to a 1M LiPF ₆ EC / DMC (1/3) 1% by weight VC solution as an electrolyte. investigated. The produced 18650 type prototype battery was evaluated for DC resistance and high-temperature storage characteristics by the method described in Example 1.

Comparative Example 3
In the same procedure as Example 1, as electrolyte, 1M LiPF ₆ EC / DMC (1/3) 1 wt% VC solution containing 2.0 wt% of N, N-dimethylacetamide as HF inhibitor investigated. The produced 18650 type prototype battery was evaluated for DC resistance and high-temperature storage characteristics by the method described in Example 1.

Comparative Example 4
In the same procedure as in Example 1, an electrolytic solution in which 0.5% by weight of N, N-dimethylacetamide was added to a 1M LiPF ₆ EC / DMC (1/3) solution was examined. The produced 18650 type prototype battery was evaluated for DC resistance and high-temperature storage characteristics by the method described in Example 1.

  The results of Examples and Comparative Examples are summarized in Table 1. Moreover, the direct-current resistance value was obtained by comparing the resistance values with Comparative Example 1 as a reference (1.00). The capacity retention rate was summarized as the high temperature storage characteristics. FIG. 2 shows the relationship between the DMA content and the DC resistance value for Example 1 to Comparative Example 3 in order to see the effect of the DMA content with 1% VC as an additive in the electrolyte. FIG. 3 shows the relationship with the capacity maintenance rate.

  As can be seen from FIG. 2, when the DMA content is 0% (Comparative Example 1), the DC resistance value is the lowest. The DC resistance value is not significantly increased up to 0.5% and is maintained at 1.03. When the DMA content is 1% (Comparative Example 2), the value is 1.10. When the DMA content is 2% (Comparative Example 3), the DC resistance value is rapidly increased to 1.25. I found out that That is, if the DMA content is 0.5% or less, the direct current resistance value due to DMA is slightly increased, but no significant increase is observed, and the load characteristics are not deteriorated. On the other hand, when the DMA content is higher than 0.5%, the direct current resistance value is extremely increased, and the load characteristics tend to be significantly decreased. This is because a predetermined amount of DMA functions to suppress hydrofluoric acid, and the remaining DMA decomposes on the electrode to form a film.

  On the other hand, as shown in FIG. 3, when the DMA content is increased from 0% (Comparative Example 1) to 0.5% (Example 1), the capacity retention rate is 85 to 92%. When the DMA content is further increased, the capacity retention rate is decreased, and the DMA content is 80% when the DMA content is 1% (Comparative Example 2) and 2% (Comparative Example 3). In some cases, it has been found to decrease to 70%. From the above, when the DMA content is 0.5% or less, the high-temperature storage characteristics are improved, but when the DMA content is more than that, it tends to decrease. When the DMA content was about 1.0%, the effect of DMA disappeared, and the capacity retention rate was almost the same as when the content was 0%. In other words, the DMA functions to suppress hydrofluoric acid, and the remaining DMA increases the direct current resistance as shown in FIG. 2, so that the capacity that can be taken out with 1 CA current decreases, and as a result, the capacity maintenance rate decreases.

  Furthermore, from the results of Example 1 and Comparative Example 4 summarized in Table 1, when the VC contains or does not contain VC, the VC itself affects the DC resistance value and the high temperature storage characteristics. The effect is different. When the DMA content is 0.5%, the dependence of the VC on the increase of the DC resistance value is large. Therefore, when the VC is not contained, the DC resistance value is low. On the other hand, since the dependence of VC on the high temperature storage characteristics is high, the capacity maintenance rate decreases when VC is not included. From the above, it has been found that by including both VC and DMA, the DC resistance can be effectively lowered and the capacity retention rate can be improved.

Examining the above results, in a battery system composed of an olivine-type lithium iron phosphate positive electrode and a graphite negative electrode, N, N-dimethylacetamide (DMA) as a hydrofluoric acid inhibitor in an electrolyte solution containing VC was reduced to 0.000. 01 to 0.7% by weight, preferably 0.05 to 0.7% by weight, particularly preferably 0.1 to 0.7% by weight to maintain a direct current resistance value, output characteristics and high temperature storage characteristics Can be provided.

  All publications, patents and patent applications cited herein are incorporated herein by reference in their entirety.

1 ... Battery can
2 ... Gasket
3 ... Top cover
4 ... Upper lid case
5 ... Positive current collector
6 ... Negative current collector
7 ... shaft core
8 ... Electrode group
12 ... Positive electrode tab
13 ... Negative electrode tab
14 ... Positive electrode
15 ... Negative electrode
16 ... Positive electrode mixture
17 ... Negative electrode mixture
18a, 18b ・ ・ ・ Separator
19 ・ ・ ・ Tape

Claims (7)

Formula having the olivine structure _{LiFe 1-X M x PO 4} :
[Where
M is at least one selected from the group consisting of Ni, Co, Mn, Ti, Zr, and Mo;
x is 0 ≦ x <1]
A positive electrode comprising a positive electrode active material represented by:
A negative electrode containing a negative electrode active material capable of occluding and releasing lithium ions; and a non-aqueous electrolyte;
A non-aqueous electrolyte secondary battery comprising:
The surface of the positive electrode active material is coated with carbon,
The non-aqueous electrolyte includes N, N-dimethylacetamide, vinylene carbonate, a supporting salt, and an organic solvent;
The nonaqueous electrolyte secondary battery, wherein a content of the N, N-dimethylacetamide is 0.01 to 0.7% by weight of the nonaqueous electrolyte. The amount of carbon covering the surface of the positive electrode active material is 1 to 5% by weight of the carbon-coated positive electrode active material, the specific surface area of the carbon-coated positive electrode active material is 10 to 20 m ² / g, and the carbon-coated positive electrode active material The nonaqueous electrolyte secondary battery according to claim 1, wherein the substance contains 300 to 1000 ppm of water.   The nonaqueous electrolyte secondary battery according to claim 1 or 2, wherein the content of vinylene carbonate is 0.5 to 3% by weight of the nonaqueous electrolyte.   The nonaqueous electrolyte secondary battery according to claim 1, wherein x is 0.   The nonaqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material capable of occluding and releasing lithium ions is graphite. The nonaqueous electrolyte secondary battery according to claim 1, wherein the supporting salt is LiPF ₆ .   The nonaqueous electrolyte according to any one of claims 1 to 6, wherein the organic solvent is a mixed solvent of ethylene carbonate and a chain carbonate, and the content of the chain carbonate is 70 to 80% by volume of the mixed solvent. Secondary battery.

Images