JP2014071947A - Nonaqueous electrolyte secondary battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery sufficiently suppressed in deterioration of capacity and deterioration of durability even when being left or used under a high-temperature environment.SOLUTION: A nonaqueous electrolyte secondary battery comprises: a positive electrode member that includes a positive electrode collector and a positive electrode active material coated on the positive electrode collector; a negative electrode member; and an electrolyte. The positive electrode active material comprises: a positive electrode active material containing at least iron; and an oxide of the positive electrode active material, and the oxide is contained on the side opposite to the positive electrode collector more than on the positive electrode collector side, in the positive electrode active material.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery and a method for manufacturing the same.

  Conventionally, as a non-aqueous electrolyte secondary battery, a battery including a positive electrode member, a negative electrode member, and an electrolytic solution is known. The positive electrode member includes a positive electrode current collector and a positive electrode active material layer laminated on the current collector, and the negative electrode member is laminated on the negative electrode current collector and the current collector. Negative electrode active material layer. In addition, the positive electrode member and the negative electrode member are usually separated by a separator.

  As this type of nonaqueous electrolyte secondary battery, a battery in which the positive electrode active material is a compound containing iron such as a lithium iron phosphate compound is known. Such a non-aqueous electrolyte secondary battery may be installed in an automobile, or may be left in a high temperature environment or used. In such a case, the temperature inside the nonaqueous electrolyte secondary battery becomes high, and iron (Fe) present in the positive electrode active material may be eluted into the electrolyte. And the elution thing precipitates on the surface of a negative electrode member, and there exists a problem that durability fall and the fall of a capacity | capacitance arise because a precipitate coat | covers this surface.

  Therefore, a nonaqueous electrolyte secondary battery in which elution of iron from such a positive electrode active material is suppressed has been proposed. For example, a non-aqueous electrolyte secondary battery manufactured by using an active material from which Fe impurities in an active material or a raw material constituting the active material are removed by a magnetic force generator and Fe impurities are removed has been proposed. (See Patent Document 1).

JP 2009-38013 A

  According to the non-aqueous electrolyte secondary battery described above, impurities in the positive electrode active material are removed. However, when the positive electrode active material contains iron as a constituent component, even if the impurities are removed, the durability decreases as described above when left in a high temperature environment or used. Capacity reduction will still occur.

  In view of the above problems, the present invention provides a non-aqueous electrolyte secondary battery in which a decrease in durability and a decrease in capacity are sufficiently suppressed even when left standing or used in a high temperature environment. The task is to do.

Nonaqueous electrolyte secondary battery according to the present invention,
A positive electrode member including a positive electrode current collector, and a positive electrode active material applied on the positive electrode current collector, a negative electrode member, and an electrolyte solution,
The positive electrode active material is composed of a positive electrode active material containing at least iron and an oxide of the positive electrode active material,
The oxide is contained more on the opposite side of the positive electrode current collector than the positive electrode current collector side of the positive electrode active material.

Here, “containing iron” means containing iron as a component as a part of the material constituting the positive electrode active material layer, and “containing iron” includes iron as an impurity. Including iron inevitably mixed in the positive electrode active material is also included.
In addition, “the oxide of the positive electrode active material” means a material produced by oxidizing (combining with oxygen) the positive electrode active material containing iron.

According to the non-aqueous electrolyte secondary battery having such a configuration, the positive electrode active material containing iron contains more oxide on the side opposite to the positive electrode current collector than on the positive electrode current collector side of the positive electrode active material. The iron contained as a component in the positive electrode active material is sufficiently insolubilized in the electrolyte solution on the opposite side than the positive electrode current collector side, so that the iron is eluted from the opposite side. It is suppressed. Thereby, the fall of the durability in a high temperature environment is fully suppressed. In addition, compared with the case where the oxide is uniformly contained in the whole positive electrode active material, the capacity | capacitance fall resulting from the said oxide is fully suppressed.
Therefore, even when left standing or used in a high temperature environment, a decrease in durability and a decrease in capacity can be sufficiently suppressed.

  In the nonaqueous electrolyte secondary battery, the positive electrode active material is preferably a lithium iron phosphate compound.

  In the non-aqueous electrolyte secondary battery, it is preferable that the oxide is contained up to 5 μm from the surface of the positive electrode active material to the inside.

  Thus, the oxide is contained in the positive electrode active material in a region of 5 μm inward from the surface, so that the oxide is more on the opposite side than the positive electrode current collector side of the positive electrode active material. Is contained more reliably. This more reliably suppresses the decrease in durability under the high temperature environment described above.

In addition, the method for producing a non-aqueous electrolyte secondary battery according to the present invention includes:
Apply the positive electrode active material on the positive electrode current collector,
The surface of the positive electrode active material is oxidized by heating in the presence of oxygen to produce a positive electrode member,
A non-aqueous electrolyte secondary battery is produced using the obtained positive electrode member.

  As described above, according to the present invention, there is provided a nonaqueous electrolyte secondary battery in which a decrease in durability and a decrease in capacity are sufficiently suppressed even when left and used in a high temperature environment. .

1 is a schematic perspective view schematically showing a nonaqueous electrolyte secondary battery according to an embodiment of the present invention. Schematic perspective view showing the internal structure of the nonaqueous electrolyte secondary battery of the present embodiment The schematic side view which shows typically the internal structure of the positive electrode active material layer of this embodiment In the example, the ratio of the positive electrode active material layer on the side opposite to the positive electrode current collector (front side): Fe ³⁺ / Fe ²⁺ is A, and the ratio in the vicinity of the positive electrode current collector (back surface side) of the positive electrode active material layer : Graph showing the relationship between the following ratio (A / B) where Fe ³⁺ / Fe ²⁺ is B, and the discharge capacity ratio and durability performance ratio

  Hereinafter, a non-aqueous electrolyte secondary battery and a manufacturing method thereof according to the present invention will be described with reference to the drawings.

  First, the nonaqueous electrolyte secondary battery of this embodiment will be described.

As shown in FIGS. 1 and 2, the nonaqueous electrolyte secondary battery 1 of the present embodiment includes a container 2 and a power generation including a positive electrode member 11 and a negative electrode member 13 accommodated in the container 2. An element 10 as an element and an electrolytic solution 20 accommodated in the container 2 are provided. The electrolytic solution 20 may be impregnated in the element 10 or may be present at the bottom of the container 2 while being impregnated.
The container 2 includes a container body 3 that can accommodate the element 10 and a lid member 4 that covers the container body 3. The container body 3 and the lid member 4 are made of, for example, a stainless steel plate and are welded to each other.
An external gasket 5 made of an insulating material is disposed on the outer surface of the lid member 4, and the opening of the lid member 4 and the opening of the external gasket 5 are connected. An external terminal 21 is accommodated in the external gasket 5.
The external terminal 21 passes through the opening of the external gasket 5 and the opening of the lid member 4 and protrudes into the container body 3, and the protruding portions of the external terminal 21 are the positive and negative electrode members 11 of the element 10. , 13 is connected to the current collector. The shape of the current collector is not particularly limited, but is, for example, a plate shape. The current collector is formed of the same type of metal member as the electrode member to be connected. The external terminal 21 is made of, for example, an aluminum alloy material such as aluminum or an aluminum alloy.
The external gasket 5 and the external terminal 21 are provided for a positive electrode and a negative electrode. The external gasket 5 and the external terminal 21 for the positive electrode are arranged on one end side in the longitudinal direction of the lid member 4, and the external gasket 5 and the external terminal 21 for the negative electrode are arranged on the other end side in the longitudinal direction.
The lid member 4 has an injection port (not shown) for injecting the electrolytic solution 20 into the container body 3, and this injection port is sealed after the injection of the electrolytic solution 20. .

  The element 10 includes a positive electrode member 11, a negative electrode member 13, and a separator 15 disposed between these electrode members, and these are laminated.

  As shown in FIGS. 1 and 2, an element 10 is accommodated in the container body 3. One element 10 may be accommodated in the container body 3, or a plurality of elements 10 may be accommodated. In the latter case, the plurality of elements 10 are electrically connected in parallel.

The positive electrode member 11 includes a positive electrode current collector 11a such as an aluminum foil and a positive electrode active material layer 11b. A material for forming the positive electrode active material layer 11b is applied on the positive electrode current collector 11a. ing.
The details of the positive electrode active material layer 11b will be described later. Moreover, although this embodiment shows a mode in which the positive electrode active material is arranged in layers on the positive electrode current collector 11a, the positive electrode active material of the present invention is not particularly limited to such a layer shape.

  The negative electrode member 13 includes a negative electrode current collector 13a such as a copper foil and a negative electrode active material layer 13b, and a material for forming the negative electrode active material layer 13b is applied to the negative electrode current collector 13a. .

  The negative electrode active material is a material that can contribute to an electrode reaction of a charge reaction and a discharge reaction in the negative electrode. Examples of the negative electrode active material include carbon-based materials such as amorphous carbon, non-graphitizable carbon, graphitizable carbon, and graphite.

  The negative electrode active material layer 13b includes, for example, a binder such as polyvinylidene fluoride (PVDF) and a conductive auxiliary agent such as acetylene black, in addition to the negative electrode active material, and other components. May be.

  The separator 15 allows passage of the electrolytic solution 20 while blocking electrical connection with the positive and negative electrode members 11 and 13. Examples of the separator 15 include a porous film formed from a polyolefin resin such as polyethylene. Such a porous film may contain additives such as a plasticizer, an antioxidant, and a flame retardant.

The electrolytic solution 20 is configured by dissolving an electrolyte in an organic solvent. There is no restriction | limiting in particular in the organic solvent used for the said electrolyte solution 20, For example, ethers, ketones, lactones, nitriles, amines, amides, a sulfur compound, halogenated hydrocarbons, ester, carbonates Nitro compounds, phosphate ester compounds, sulfolane hydrocarbons, etc. can be used, among these ethers, ketones, esters, lactones, halogenated hydrocarbons, carbonates, sulfolane series Compounds are preferred. Examples of these are tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, anisole, monoglyme, 4-methyl-2-pentanone, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, 1,2-dichloroethane. , Γ-butyrolactone, dimethoxyethane, methyl formate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, vinylene carbonate, dimethylformamide, dimethyl sulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfolane, phosphorus Examples thereof include trimethyl acid, triethyl phosphate, and mixed solvents thereof, but are not necessarily limited thereto. Cyclic carbonates and cyclic esters are preferred. Most preferably, the organic solvent is a mixture of one or more of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, and diethyl carbonate.
The electrolyte salt used in the electrolytic solution 20 is not particularly limited, but LiClO ₄ , LiBF ₄ , LiAsF ₆ , CF ₃ SO ₃ Li, LiPF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiI, LiAlCl _{4 and the} like and mixtures thereof. Preferably, it is a lithium salt obtained by mixing one or more of LiBF ₄ and LiPF ₆ .
In addition, the electrolyte solution 20 is not specifically limited to the electrolyte solution containing said organic solvent and electrolyte salt.
In addition to the above, a membrane (solid electrolyte membrane) that is supplementarily formed from a solid ion conductive material can also be used as the electrolyte. In this case, the non-aqueous electrolyte secondary battery 1 can be formed by the positive electrode member 11, the negative electrode member 13, the separator 15 disposed therebetween, the solid electrolyte membrane, and the electrolyte 20. . In addition, the nonaqueous electrolyte secondary battery 1 may be formed of the positive electrode member 11, the negative electrode member 13, the solid electrolyte membrane disposed therebetween, and the electrolytic solution 20.
Further, when the solid electrolyte membrane is an organic solid electrolyte formed from polyethylene oxide, polyacrylonitrile or polyethylene glycol, and modified products thereof, it is lightweight and flexible, and therefore is wound. Is advantageous. Furthermore, in addition to the above, the solid electrolyte membrane can be formed using an inorganic solid electrolyte or a mixed material of an organic solid electrolyte and an inorganic solid electrolyte.

  And in this embodiment, the said positive electrode active material layer 11b is comprised from the positive electrode active material which contains at least iron, and the oxide of this positive electrode active material, and the said oxide is the said positive electrode active material layer 11b. It is contained more on the side opposite to the positive electrode current collector 11a than on the positive electrode current collector 11a side.

A specific embodiment is shown in FIG. FIG. 3 is a diagram schematically showing a configuration in the thickness direction of the positive electrode active material layer 11b in the present embodiment. As schematically shown in this figure, a first region 11c made of a positive electrode active material containing iron is formed on the positive electrode current collector 11a side (lower side) of the positive electrode active material layer 11b. There is a second region 11d made of the oxide adjacent to the first region 11c and on the opposite side (upper side) of the positive electrode current collector 11a.
The thickness of the positive electrode active material layer 11b is not particularly limited, but is usually about 20 μm to 200 μm.
Note that FIG. 3 shows a mode in which two regions are laminated in this way, but the positive electrode active material layer 11b of the present invention is not limited to this mode, and the positive electrode current collector of the positive electrode active material layer 11b is not limited thereto. An aspect in which the content of the oxide increases from the body 11a side toward the opposite side may be employed.

The positive electrode active material is a substance that contributes to the electrode reactions of the charge reaction and the discharge reaction in the positive electrode, and is a compound containing iron. Such a positive electrode active material, e.g., lithium iron phosphate compounds may be mentioned. As the lithium phosphate compound, other LiFePO _4, the general formula _{LiMn (1-xy) Fe x} M y PO 4 (0 ≦ x ≦ 0.5, 0 ≦ y ≦ 0.1, M = Mg, Co, Cr, Ti, Y, Mo, or a compound represented by Nb).
When the positive electrode active material is a compound represented by the above general formula, there is an advantage that the discharge capacity of the non-aqueous electrolyte secondary battery 1 becomes larger.
The positive electrode active material described above can be appropriately manufactured using a conventionally known method. For example, the compound represented by the above general formula is prepared by, for example, dissolving manganese acetate tetrahydrate and iron acetate in purified water, adding dropwise an aqueous phosphoric acid solution while stirring this mixed solution, Then, lithium hydroxide monohydrate powder was added to the mixed solution, and ammonia water was added while stirring the obtained mixed solution in a water bath at 60 ° C. to adjust the pH to 9.5. By changing the temperature to 80 ° C. and stirring, a liquid mixture in which solid matter is precipitated is obtained. After the obtained liquid mixture is vacuum-dried, the obtained solid content is pulverized and nitrogen gas is circulated as necessary. It can be manufactured by firing under.

The positive electrode active material oxide is produced by oxidizing the positive electrode active material. Examples of the oxide include iron oxide (for example, Fe ₂ O ₃ and Fe ₃ O ₄ ) obtained by oxidizing iron contained in the positive electrode active material and iron oxide hydroxide (for example, FeO (OH)). These iron oxides are hardly dissolved in the electrolytic solution 20.
Thus, in the positive electrode active material layer 11b, the second region 11d containing the oxide is insolubilized with iron in the positive electrode active material. Therefore, the second region 11d does not contain the oxide. Iron is less likely to elute into the electrolyte solution 20 than in the region 11c.

The positive electrode active material layer 11b has, for example, a binder such as polyvinylidene fluoride (PVDF) and a conductive auxiliary agent such as acetylene black in addition to the positive electrode active material and the oxide. You may have the component of.
The positive electrode member 11 may further include a carbon coat layer laminated on the surface of the positive electrode active material layer 11b. Examples of such a carbon coat layer include fibrous carbon as disclosed in Japanese Patent Application Laid-Open No. 2008-111749. Thus, when the positive electrode member 11 includes the carbon coat layer, there is an advantage that a battery excellent in high rate charge / discharge performance can be supplied with improved electron conductivity.

As described above, the positive electrode active material layer 11b contains a positive electrode active material containing iron and an oxide of the positive electrode active material, and the oxide collects the positive electrode current collector of the positive electrode active material layer. The iron contained in the positive electrode active material layer 11b as a constituent component more than the positive electrode current collector 11a than on the positive electrode current collector 11a side. Since the side is sufficiently insolubilized with respect to the electrolytic solution 20, elution of iron from the opposite side is suppressed. Thereby, the fall of the durability in a high temperature environment is fully suppressed. In addition, compared with the case where the oxide is uniformly contained in the whole positive electrode active material layer 11b, the capacity | capacitance fall resulting from the said oxide is fully suppressed.
Therefore, even when left standing or used in a high temperature environment, a decrease in durability and a decrease in capacity are sufficiently suppressed.

For example, in the present embodiment, a configuration in which the positive electrode active material is a lithium iron phosphate compound (LiFePO ₄ ) and the electrolytic solution 20 includes a component capable of generating an acid such as LiPF ₆ is employed. In such a case, there is a possibility that LiPF ₆ is decomposed and acid is generated in a high temperature environment. In this case, when the generated acid reacts with iron in the lithium iron phosphate compound, iron is eluted from the positive electrode active material, which may lead to a decrease in capacity and a decrease in durability.
However, as described above, since the oxide is contained more on the opposite side than the cathode current collector 11a side of the cathode active material layer 11b, the iron on the opposite side is, for example, Fe ₂ O. ₃ , converted to Fe ₃ O ₄ , FeO (OH) or the like, and sufficiently insolubilized with respect to the acid in the electrolytic solution 20. Thereby, since elution of iron from the opposite side is suppressed, a decrease in capacity and a decrease in durability under the high temperature environment described above are suppressed.

The oxide is preferably contained at least 5 μm from the surface of the positive electrode active material layer 11b.
As described above, when the oxide is contained in at least the positive electrode active material layer 11b in the depth region, the positive electrode active material layer 11b is positioned closer to the opposite side than the positive electrode current collector 11a side. The oxide is contained more reliably. This more reliably suppresses the decrease in durability under the high temperature environment described above.
In the embodiment shown in FIG. 3, the positive electrode member 11 does not have the carbon coat layer as described above. However, when the positive electrode member 11 has the carbon coat layer, the positive electrode active member disposed below the carbon coat layer is used. A region up to 5 μm from the surface of the substance 11b corresponds to the region.

In addition, it can be confirmed by applying colorimetric analysis by the o-phenanthroline coloring method that the oxide is contained in the cathode active material layer 11b on the side opposite to the cathode current collector 11a.
In this analysis, first, from the surface of the positive electrode active material 11b to the inside, up to 1/10 of the thickness of the positive electrode active material 11b is taken as a first sample, and the back surface of the positive electrode active material 11b (on the positive electrode current collector 11a side) From the surface) to the inside, 1/10 of the thickness is taken as the second sample.
Next, the content of ferrous iron (divalent iron) and the content (concentration) of ferric iron (trivalent iron) in the first sample are the same as that of ferrous ions (Fe ²⁺ ), respectively. Measured as content and content of ferric ion (Fe ³⁺ ). The ferrous content corresponds to the content of the positive electrode active material that has not been oxidized, and the ferric content corresponds to the content of the oxide. Based on this measurement result, the ratio of ferric iron to ferrous iron (Fe ³⁺ / Fe ²⁺ ) is calculated.
In addition, the said measurement and calculation are performed by the method described in the Example.

Similarly, for the second sample, the ferrous content and the ferric content are measured, and based on the measurement results, the ratio of ferric iron to ferrous (Fe ³⁺ / Fe ²⁺ ) is calculated.
When the ratio of the first sample is A and the ratio of the second sample is B, when the ratio of A to B is greater than 1 (A / B> 1), the oxide is a positive electrode collector. It is contained more on the opposite side than the electric body 11a.

When the non-aqueous electrolyte secondary battery 1 is not charged, the positive electrode active material 11b may be analyzed as described above. When the non-aqueous electrolyte secondary battery 1 is charged, after discharging to 2.0V, Disassemble the battery, take out the positive electrode member, and adjust it with a beaker cell or the like so that the discharge state is the same as when uncharged (for example, LizFePO ₄ (0 <z <1) becomes LiFePO ₄ ). The analysis may be performed as described above. That is, the above analysis may be performed with the charge amount of the nonaqueous electrolyte secondary battery 1 being 2.0 V or less.

  Further, A / B is not particularly limited as long as it is larger than 1. However, the larger A / B, the more the oxide is contained on the opposite side, and the durability tends to be further improved. On the other hand, as A / B becomes smaller, the decrease in capacity tends to be suppressed. Therefore, for example, in view of such a viewpoint, it is preferable that A / B is 2 to 10.

Next, the manufacturing method of the nonaqueous electrolyte secondary battery of this embodiment will be described.
In the manufacturing method of the non-aqueous electrolyte secondary battery 1 of the present embodiment, a positive electrode active material is applied on the positive electrode current collector 11a, and the surface of the positive electrode active material is heated in the presence of oxygen, so that the positive electrode The positive electrode member 11 is produced by oxidizing the surface side of the active material, and the non-aqueous electrolyte secondary battery 1 is produced using the positive electrode member 11, the negative electrode member 13, and the electrolytic solution 20 obtained.

Specifically, first, the positive electrode active material, the binder, the conductive additive, and an organic solvent such as N-methylpyrrolidone are mixed to form a paste-like mixture (positive electrode mixture paste). Is obtained.
This mixture is applied onto the positive electrode current collector 11a and dried.
Next, the surface of the dried positive electrode mixture is heated at, for example, 80 to 150 ° C. under conditions where oxygen is present (for example, in air), so that the surface side of the positive electrode active material layer is oxidized and the surface is oxidized. An oxide of the positive electrode active material is generated, whereby the positive electrode member 11 is produced.

  Next, the obtained positive electrode member 11, separator 15, negative electrode member 13 and separator 15 are laminated in this order, and then wound to form the element 10. Then, the element 10 is inserted into the container body 3, the current collector is connected to the positive and negative electrode members 11, 13, and the container body 3 is covered with the lid member 4 to which the external gasket 5 and the external terminal 21 are attached. The electrical part and the external terminal 21 are connected, and in this state, the container main body 3 and the lid member 4 are welded, the electrolytic solution 20 is injected through an injection port (not shown), and the injection port is closed, thereby non-water An electrolyte secondary battery is produced.

  In addition, the manufacturing method of the non-aqueous-electrolyte secondary battery 1 of this embodiment is not specifically limited to the said manufacturing method.

  As described above, the nonaqueous electrolyte secondary battery 1 of the present embodiment includes the positive electrode current collector 11a and the positive electrode active material (here, the positive electrode active material layer 11b) applied on the positive electrode current collector 11a. The positive electrode member 11, the negative electrode member 13, and the electrolytic solution 20 are provided, and the positive electrode active material layer 11 b contains a positive electrode active material containing iron and an oxide of the positive electrode active material, More oxide is contained on the opposite side of the positive electrode current collector 11a than on the positive electrode current collector 11a side of the positive electrode active material layer 11b.

According to the nonaqueous electrolyte secondary battery 1 having such a configuration, the oxide of the positive electrode active material containing iron is more on the opposite side of the positive electrode current collector 11a than the positive electrode current collector 11a side of the positive electrode active material layer 11b. By being contained, iron contained as a component in the positive electrode active material can be sufficiently insolubilized in the electrolyte solution on the opposite side of the positive electrode current collector 11a side. The elution of iron from the opposite side can be suppressed. Thereby, the fall of durability in a high temperature environment can fully be suppressed. In addition, compared with the case where the oxide is uniformly contained in the whole positive electrode active material layer 11b, the capacity | capacitance fall resulting from the said oxide can fully be suppressed.
Therefore, even when left standing or used in a high temperature environment, a decrease in durability and a decrease in capacity can be sufficiently suppressed.

Moreover, in the non-aqueous electrolyte secondary battery 1, the positive electrode active material is preferably a lithium iron phosphate compound.
In the non-aqueous electrolyte secondary battery 1, the oxide is preferably contained at least 5 μm from the surface of the positive electrode active material layer 11 b toward the inside. In this way, the oxide is contained in at least the region up to 5 μm inward from the surface, so that the oxide on the opposite side of the positive electrode current collector 11a side of the positive electrode active material layer 11b. Is contained more reliably. Thereby, the above-mentioned fall of durability in a high temperature environment can be suppressed more reliably.

  Moreover, the manufacturing method of the non-aqueous electrolyte secondary battery 1 of the present embodiment is applied by applying a positive electrode active material on the positive electrode current collector 11a, and oxidizing the surface of the positive electrode active material by heating in the presence of oxygen. Thus, a positive electrode member 11 is produced, and a nonaqueous electrolyte secondary battery is produced using the obtained positive electrode member 11.

  Moreover, it is preferable that the non-aqueous-electrolyte secondary battery 1 of this embodiment is manufactured with an above-described manufacturing method. That is, the non-aqueous electrolyte secondary battery 1 according to the present embodiment has a positive electrode active material coated on the positive electrode current collector 11a, and the surface of the positive electrode active material is oxidized by heating in the presence of oxygen. It is preferable that the electrode member 11 is produced and produced using the obtained positive electrode member 11.

  The non-aqueous electrolyte secondary battery and the manufacturing method thereof of the present invention are as described above, but the present invention is not limited to the above-described embodiment, and can be appropriately modified within the intended scope of the present invention. Moreover, the effect of this invention is not limited to the said embodiment.

  EXAMPLES Next, although an Example is given and this invention is demonstrated in more detail, this invention is not limited to these.

<Preparation of lithium iron phosphate (LiFePO ₄ )>
LiFePO ₄ as a positive electrode active material was produced. Iron oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O) that is an organic acid iron (II) salt, ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) that is an ammonium phosphate salt, and a lithium salt Lithium carbonate (Li ₂ CO ₃ ) was measured so as to have a molar ratio of 2: 2: 1, and these were mixed by a dry method. Ethanol was added to the resulting mixture to form a paste, and wet pulverization and mixing were performed for 2 hours using a ball mill (planetary mill manufactured by FRITSCH, ball diameter: 1 cm) under a nitrogen atmosphere. Thus, the mixture containing an iron source, a phosphorus source, and a lithium source was prepared.

The mixture was put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and the moisture content at a dew point of −20 ° C. using an atmosphere substitution type firing furnace (Desken tabletop vacuum gas substitution furnace KDF-75). The mixture was calcined under a flow of hydrogen / nitrogen mixed gas containing 0.5% by volume of hydrogen adjusted to a flow rate of 2.0 liter / min. The firing temperature was 750 ° C., and the firing time (the time for maintaining the firing temperature) was 5 hours. The rate of temperature increase was 5 ° C./minute, and the temperature was naturally cooled. In this way, a lithium iron phosphate compound (LiFePO ₄ ) as a positive electrode active material was produced.

<Preparation of positive electrode member>
Using the positive electrode active material produced as described above, 86% by weight of each positive electrode active material, 7% by weight of acetylene black as a conductive auxiliary agent, and 7% by weight of PVDF were mixed, and N-methyl as a solvent was added to this mixture. Pyrrolidone was added to form a positive electrode mixture paste. 3.2 g / 100 cm ₂ of positive electrode mixture paste was applied to both sides of a 20 μm-thick aluminum foil as a positive electrode current collector, dried, and then applied with a 500 kgf / cm load by a roll press. Compression molded. Thereby, a strip-like positive electrode member was produced. The size of each positive electrode member was 210 cm in length, 13 cm in width, and the total thickness of the positive electrode current collector and the positive electrode active material layers on both sides was 190 μm.
After such drying, the positive electrode current collector and the positive electrode active material are placed in a dryer (air atmosphere) at 150 ° C., 0 (no standing, ie, no oxidation treatment), 0.1, 0.5, 1, 2, 5 The positive electrode member was produced by leaving it for 10, 15, 20, 30, 40, and 50 hours, respectively.

<Confirmation of oxide distribution in positive electrode active material>
(Sample collection)
In each positive electrode member produced as described above, in the region of 30 mm length × 30 mm width of the surface, the first sample up to 5 μm inward from the surface and the back surface (surface on the positive electrode current collector 11 a side) 30 mm length. X A second sample of up to 5 μm was taken from the back surface to the inner side at a width of 30 mm.
(Colorimetric analysis by o-phenanthroline coloring method)
-Preparation of ferrous ion standard solution 0.7022 g of ferrous ammonium sulfate hexahydrate was dissolved in water, 0.2 mL of 6N hydrochloric acid was added, and the volume was adjusted to 100 mL to prepare a stock solution. The stock solution contains 1000 ppm of ferrous ions (Fe ²⁺ ). A standard solution was prepared from this stock solution. Specifically, first, the stock solution was diluted with water to 5 ppm to prepare a diluted solution. Next, the ratios of water and diluting solution are mixed so that the volume ratio is 2.5: 0, 2: 0.5, 1.5: 1, 0.5: 2, 0: 2.5. A group of standard solutions for preparing a calibration curve was prepared.
-Preparation of o-phenanthroline solution 0.025 g of o-phenanthroline hydrochloride monohydrate was dissolved in 20 mL of water, 5.76 g of glacial acetic acid (5.5 mL with a specific gravity of 1.05) was added, and dissolved in 200 mL. An o-phenanthroline solution was prepared.
-Preparation of buffer solution (pH 4.9) 13.6 g of sodium acetate (anhydrous) was dissolved in 100 mL of water, 5.76 g of glacial acetic acid (5.5 mL with a specific gravity of 1.05) was added, and the volume was adjusted to 200 mL. Thus, a buffer solution having a pH of 4.9 was prepared.
-Preparation of hydroxylamine hydrochloride solution 1 g of hydroxylamine hydrochloride was dissolved in 10 g of water to prepare a hydroxylamine hydrochloride solution.
-3N hydrochloric acid solution 3N hydrochloric acid solution was prepared by diluting concentrated hydrochloric acid with water.
6N ammonia water 30% by weight ammonia water (17.6N) was diluted 2.9 times with water to prepare 6N ammonia water.
・ Preparation of sample solution 10 mg of each first sample was added to 10 mL of 3N hydrochloric acid solution that had been sufficiently purged with nitrogen, and dissolved by irradiation with ultrasonic waves for 30 minutes. Was prepared. Each second sample solution was prepared in the same manner except that 10 mg of each second sample was added.
・ Measurement of ferrous ion content After mixing 0.25 mL of o-phenanthroline solution and 0.25 mL of buffer solution, 2.5 mL of each first sample solution was added, and 2 mL of water was further added and stirred. After leaving for 30 minutes, the absorbance at a wavelength of 510 nm of the obtained mixed solution was measured using a general absorptiometer, and compared with a calibration curve prepared using the above standard curve standard solution, The ferrous ion (Fe ²⁺ ) concentration in each first sample solution was obtained.
Further, the absorbance was measured in the same manner except that 2.5 mL of each second sample solution was added, and the ferrous ion (Fe ²⁺ ) concentration of each second sample solution was obtained.
Measurement of total iron ions (total amount of ferrous ions and ferric ions) 0.5 mL of 3N hydrochloric acid solution was added to 2.5 mL of each first sample solution, heated for 5 minutes, and then cooled to room temperature. To the resulting solution, 0.1 mL of hydroxylamine hydrochloride solution and 0.25 mL of o-phenanthroline solution were added and stirred, and then 6N ammonium water was added to adjust the pH to 3.5. To the solution whose pH was adjusted, 0.25 mL of buffer solution was added, and water was further added to adjust the volume to 5 mL. The obtained mixed solution is stirred and allowed to stand for 30 minutes after stirring, and then the absorbance at a wavelength of 510 nm of the obtained mixed solution is measured using a general absorptiometer, and the above standard curve standard solution is used. Compared with the calibration curve prepared in this way, the total iron ion (Fe ²⁺ , Fe ³⁺ total) concentration in each first sample solution was obtained.
Further, the total iron ion concentration in each second sample solution was obtained in the same manner as above except that 2.5 mL of each second sample solution was used.
In addition, what is necessary is just to remove turbidity by filtration, when the solution is turbid after cooling in the above.
-Calculation of ratio (A, B) of ferric ion content with respect to ferrous ion content in 1st sample and 2nd sample Ferrous ion concentration is calculated from the total iron ion concentration obtained above. By subtracting, the concentration of ferric ion (Fe ³⁺ ) in the first sample solution was calculated. Similarly, the ferric ion (Fe ³⁺ ) concentration in the second sample solution was calculated.
Next, by calculating the ratio of ferric ion concentration to ferrous ion concentration in the first sample solution, the ratio of ferric iron to ferrous iron in the first sample (Fe ³⁺ / Fe ^2+). ) Was calculated. This ratio was A.
Similarly, the ratio of ferric iron to ferrous iron (Fe ³⁺ / Fe ²⁺ ) in the second sample was calculated. This ratio was B.
And about each positive electrode active material, the ratio (A / B) of the said ratio A of the 1st sample with respect to the said ratio B of the 2nd sample was computed. The results are shown in FIG.

(Preparation of negative electrode member)
90% by weight of graphite as a negative electrode active material and 10% by weight of polyvinylidene fluoride (PVDF) as a binder were mixed, and N-methylpyrrolidone as a solvent was added to this mixture to form a negative electrode mixture paste. . A negative electrode mixture paste of 1.5 g / 100 cm ² was applied to both sides of a 10 μm thick copper foil as a negative electrode current collector, dried, and then applied with a load of 100 kgf / cm by a roll press. Compression molded. Thereby, a strip-like negative electrode member was produced. The size of the negative electrode member was 220 cm in length, 13 cm in width, and the total thickness of the negative electrode current collector and the negative electrode active material layers on both sides was 120 μm.

(Production of test battery)

A positive electrode tab and a negative electrode tab were attached to each of the positive electrode member and the negative electrode member produced as described above.
Further, a microporous polyolefin membrane having a width of 12 cm and a thickness of 25 μm was prepared as a separator.

  Next, each positive electrode member, separator, negative electrode member and separator were laminated in this order and wound into a long cylindrical shape. This produced the element (electrode group). Each of the test batteries was fabricated by housing the element in a container, capping the container, and injecting an electrolyte solution.

(Capacity retention test)
A capacity retention test was performed using each test battery.
Specifically, after charging to a voltage of 3.6 V with a current of 1 CA, the discharge capacity when discharging to a voltage of 2.0 V with a current of 1 CA is measured, and the initial discharge capacity per gram of the positive electrode active material is calculated. did.
Next, the discharge capacity after charging / discharging this test battery for 300 cycles under the same charge / discharge conditions was determined, and the discharge capacity ratio (%) was calculated. Here, the “discharge capacity ratio (%)” is a value obtained by multiplying the value obtained by dividing the discharge capacity after 300 cycles by the initial discharge capacity by 100.
FIG. 4 shows the relationship between each ratio A / B and the discharge capacity ratio.

(Durability test)
Furthermore, the durability test was done using another battery produced similarly to the battery used for the charge / discharge cycle test. After charging again to a voltage of 3.6 V with a current of 1 CA, the battery is stored for 10 days in an environment of 60 ° C. After the storage, the battery is repeatedly charged and discharged three times under the same charge and discharge conditions as the initial stage. The capacity was defined as the discharge capacity after storage, and the durability performance ratio was calculated. The “durability performance ratio (%)” is a value obtained by dividing the discharge capacity after storage by the initial discharge capacity and multiplying by 100.
FIG. 4 shows the relationship between each ratio A / B and the durability performance ratio.

  As shown in FIG. 4, the discharge capacity ratio (capacity) is larger than that in the case where the oxide is contained more uniformly on the surface side of the positive electrode active material than on the positive electrode current collector side. It has been found that both a decrease in durability and a decrease in durability performance ratio (durability) can be suppressed. Further, it is understood that when the A / B ratio is 2 or more and 10 or less, the decrease in the discharge capacity ratio (capacity) and the decrease in the durability performance ratio (durability) are further suppressed than when the ratio is outside this range. It was. As a result, by containing the oxide of the positive electrode active material on the surface side of the positive electrode active material layer, it is possible to further suppress the decrease in the discharge capacity ratio while suppressing the decrease in the durability performance ratio due to the oxide. all right.

1: non-aqueous electrolyte secondary battery, 10: element, 11: positive electrode member, 11a: positive electrode current collector, 11b: positive electrode active material layer, 11c: first region, 11d: second region, 13: Negative electrode member, 13a: negative electrode current collector, 13b: negative electrode active material layer, 20: electrolyte solution

Claims (4)

A positive electrode member including a positive electrode current collector, and a positive electrode active material applied on the positive electrode current collector, a negative electrode member, and an electrolyte solution,
The positive electrode active material is composed of a positive electrode active material containing at least iron and an oxide of the positive electrode active material,
The non-aqueous electrolyte secondary battery in which the oxide is contained more on the opposite side of the positive electrode current collector than on the positive electrode current collector side of the positive electrode active material.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode active material is a lithium iron phosphate compound.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the oxide is contained at least up to 5 μm inward from the surface of the positive electrode active material. Apply the positive electrode active material on the positive electrode current collector,
The surface of the positive electrode active material is oxidized by heating in the presence of oxygen to produce a positive electrode member,
A method for producing a nonaqueous electrolyte secondary battery, wherein a nonaqueous electrolyte secondary battery is produced using the obtained positive electrode member.

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