JP2010033924A - Positive electrode for lithium-ion secondary battery, and lithium-ion secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode for a lithium-ion secondary battery capable of sufficiently maintaining quick charge and discharge characteristics even after high temperature is maintained, and the lithium-ion secondary battery using the positive electrode. <P>SOLUTION: A positive electrode active material is formed by mixing a first lithium compound wherein Mn of LiMn<SB>2</SB>O<SB>4</SB>is partially replaced with Li, B, Mg, Al, Co, or Ni, and a second lithium compound wherein Fe of LiFe PO<SB>4</SB>is partially replaced with Mn, Mg, Ni, or Co, and a third lithium compound wherein Ni of LiNiO<SB>2</SB>is partially replaced with Li, Mg, Mn, Al, or Co at respective designated content ratios. A positive electrode active material layer 11 is formed by applying the positive electrode active material to a positive electrode collector 13, and thus, the lithium secondary battery is manufactured. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a positive electrode for a lithium ion secondary battery and a lithium ion secondary battery using the same.

  In recent years, as portable electronic devices such as mobile phones and notebook personal computers have become widespread, these electronic devices have been reduced in size and weight. As a drive power source for a portable electronic device, it is necessary to be able to charge, to be small and to have a long drive time. A secondary battery having a high energy density is generally used as a power source that satisfies such requirements.

  As such a secondary battery having a high energy density, a lithium ion secondary battery is generally known. The lithium ion secondary battery described here has a configuration in which a positive electrode and a negative electrode are opposed to each other via a separator, and each of the positive electrode and the negative electrode includes a positive electrode current collector and a positive electrode active material, and a negative electrode current collector and a negative electrode active material, respectively. It is a thing. Each of these elements is impregnated with a non-aqueous electrolyte solution. When this lithium ion secondary battery is charged or discharged, lithium ions dissolved in the electrolyte solution move between the positive electrode and the negative electrode through the separator, and occlusion of lithium ions in the positive electrode active material and the negative electrode active material, Release takes place, thereby acting as a battery. Here, as the negative electrode active material of the lithium ion secondary battery, a metal material such as a carbon material that occludes and releases lithium ions, aluminum (Al) that forms an alloy with lithium (Li), and metallic lithium. Is used.

On the other hand, regarding the positive electrode active material used for the positive electrode of a lithium ion secondary battery, the material disclosed by patent documents 1-3 is each developed. Among these, Patent Document 1 discloses a composition of a first lithium compound having a composition of LiFePO ₄ or Li (Fe, Mn) PO ₄ and LiMO ₂ (M is at least one of Co or Ni, or Mn). A positive electrode active material comprising a second lithium compound is described. Patent Document 2 describes a positive electrode active material made of a lithium compound having a composition of LiMn ₂ O ₄ . Further, Patent Document 3 discloses that the first and second lithium compounds having compositions of LiCoO ₂ and LiMnO ₂ respectively, and Li (Fe, M) PO ₄ (M is at least Mg, Co, Ni, Mn, Zn). It describes a positive electrode active material comprising a third lithium compound having a composition of (one type). As described in Patent Document 1, a lithium ion secondary battery using a positive electrode active material having these compositions is reduced in cost, improved in energy density that can be stored, operational stability at high temperature, and durability against overdischarge. It has excellent characteristics such as.

JP 2001-307730 A JP 2004-055328 A JP 2003-346799 A

  When a lithium ion secondary battery is used as a driving power source for a portable electronic device or the like, as a request from the user side, it is required to shorten the charging time, that is, to enable rapid charging. Conventional general lithium ion secondary batteries are used under charging conditions such that they are fully charged in 1 to 3 hours, but this charging time can be shortened to about 10 to 15 minutes, for example. If possible, the convenience of the lithium ion secondary battery can be greatly enhanced. However, it is known that when such a quick charge is performed on a conventional lithium ion secondary battery, the characteristics of the lithium ion secondary battery are significantly deteriorated in a short period of use. This deterioration in battery characteristics is manifested as a significant irreversible decrease in the discharge capacity (or energy density) of a battery that can be charged over a short period of time. That is, it is known that there is a trade-off relationship between shortening the charging time of a lithium ion secondary battery and reducing the capacity of the battery. Note that the rate of decrease in the discharge capacity of the battery is generally expressed as a decrease in the capacity maintenance rate.

In general, in order to improve the quick charge characteristics of a lithium ion secondary battery and enable charging in a short time, it is preferable to use a lithium compound having a composition of LiMn ₂ O ₄ or LiNiO ₂ as a positive electrode active material. It has been known. However, when LiMn ₂ O ₄ is used as the positive electrode active material, the average valence of Mn (manganese) ions changes between trivalent and tetravalent before and after charging and discharging, and is therefore referred to as Jahn-Teller strain. Crystal distortion is generated in the crystal. This crystal distortion increases or decreases the volume of the positive electrode active material portion of the battery during charging and subsequent discharging, and therefore structural destruction occurs in the positive electrode of the lithium ion secondary battery when rapid charging is repeated. there is a possibility.

In addition, when a slight amount of water is present in the electrolyte solution in the battery, this water reacts with the supporting salt that is a component of the electrolyte solution to generate H ions (hydrogen ions: H ⁺ ). When LiPF ₆ is used as the supporting salt, it is known that the reaction of the following chemical formula occurs.

LiPF ₆ + H ₂ O → POF ₃ + Li ⁺ + 3F ⁻ + 2H ⁺

When H ⁺ is thus generated in the electrolyte solution, Mn of the LiMn ₂ O ₄ positive electrode active material is ionized and dissolved in the electrolyte solution by the reaction of the following chemical formula.

2LiMn ₂ O ₄ + 4H ⁺ → 3λ-MnO ₂ + Mn ²⁺ + 2Li ⁺ + 2H ₂ O

Here, λ-MnO ₂ is λ-type manganese dioxide. In this reaction, H ⁺ reacts with LiMn ₂ O ₄ , whereby Mn ions (Mn ²⁺ ) are eluted into the electrolyte solution, and λ-MnO ₂ and H ₂ O are generated. That is, H ₂ O present in a minute amount in the electrolyte solution acts as a catalyst, and Mn ions are eluted from LiMn ₂ O _{4 in} the presence of the supporting salt. This eluted Mn ion has been found to be a cause of an irreversible increase in internal impedance in the lithium ion secondary battery. Even when a compound having a composition other than LiPF ₆ is used as the supporting salt, the same reaction occurs when the supporting salt has a lithium fluoride structure.

These phenomena also occur when a lithium compound having a composition of LiNiO ₂ instead of LiMn ₂ O ₄ is used as the positive electrode active material. Accordingly, structural destruction of the positive electrode of the battery due to generation of Jahn-Teller distortion due to repeated charging / discharging, and elution of ionized Ni (nickel) element into the electrolyte solution due to rapid charging, even when LiNiO ₂ is used, LiMn ₂ As in the case of O ₄ , an irreversible increase in internal impedance is caused in the lithium ion secondary battery.

  The increase in the internal impedance of the battery due to the ionization of Mn contained in the positive electrode active material is caused by the movement of the Li ion through the separator and the negative electrode active by the eluted Mn ions being deposited on the surface of the negative electrode active material and the separator. This is thought to be due to the inhibition of ionization and metallization on the material surface. In addition, positive electrode active material triggered by elution and precipitation of Mn ions, inactivation by liberation of negative electrode active material, and electrolyte solution due to the influence of acid generated by water and Mn ions slightly contained in the electrolyte solution It is thought that this is also caused by the deterioration.

  When the internal impedance of the battery increases due to these reasons, in the case of rapid charging of the lithium ion secondary battery, the discharge capacity of the battery that can be charged under the charging condition, that is, the capacity maintenance rate decreases. . This is because, in order to rapidly charge the battery when the internal impedance of the battery increases, it is necessary to increase the applied voltage to increase the current density during charging. However, if this is done, the internal temperature of the battery will rise rapidly, leading to deterioration of the electrolyte in the battery. For this reason, there is an upper limit for the applied voltage when charging the lithium ion secondary battery, and if the internal impedance is high, the applied voltage will soon reach this upper limit in rapid charging and will shift to constant voltage charging. . Therefore, the battery cannot be fully charged within the set rapid charging time, and the capacity of the battery that can be rapidly charged is reduced.

It is known that the increase in internal impedance of the battery due to the above rapid charging hardly occurs when a lithium compound having a composition of LiFePO ₄ is used as the positive electrode active material. In the positive electrode active material using LiFePO ₄ , large crystal distortion does not occur in the crystal due to insertion and extraction of Li ions, and Fe is not ionized and eluted into the electrolyte solution. However, LiFePO ₄ has a remarkably low electric conductivity as compared with the above LiMn ₂ O ₄ and LiNiO ₂ lithium compounds. In general, the electrical conductivity of LiMn ₂ O ₄ is about 10 ⁻³ / Ω / m and the same for LiNiO _2, but the electrical conductivity of LiFePO ₄ is only about 10 ⁻⁷ / Ω / m. The difference appears as an increase in internal impedance when the battery is rapidly charged. Therefore, even if LiFePO ₄ is simply adopted as the positive electrode active material of the lithium ion secondary battery, the capacity of the rechargeable battery in the rapid charge cannot be increased.

  The present invention solves the above-mentioned problems in lithium ion secondary batteries, and the internal impedance during rapid charging does not increase greatly even after long-term use, and thus the degree of battery capacity reduction during rapid charging is small, Accordingly, the present invention proposes a new positive electrode active material configuration of a lithium ion secondary battery with a small decrease in capacity maintenance rate during use, and a lithium ion secondary battery using the same.

The positive electrode of the lithium ion secondary battery according to the present invention comprises a positive electrode active material containing a first lithium compound, a second lithium compound, and a third lithium compound, each having a different composition, and a lithium ion secondary battery. It is used as a positive electrode for an automobile. Here, the first, second, and third lithium compounds each have a part of Mn, Fe, and Ni of the lithium compounds represented by chemical formulas LiMn ₂ O ₄ , LiFePO ₄ , and LiNiO ₂ respectively. Each composition is contained in the positive electrode as fine crystals. By mixing these three types of lithium compounds in a proportion within a specific range, a special effect was obtained as a positive electrode active material.

In the present invention, in the first, second, and third lithium compounds used as the constituent elements of the positive electrode active material, the crystal structure is stabilized by partially replacing specific elements. As a result, crystal distortions such as the Jahn-Teller distortion generated during insertion and extraction of Li ions are alleviated, and displacements such as the shape of the compound particles due to the distortions are reduced, so that the lithium ion secondary battery can be rapidly charged. This reduces the internal impedance. Specifically, for LiMn ₂ O ₄ that is the first lithium compound, one or more elements selected from Li, B, Mg, Al, Co, and Ni are added to replace Mn.

  In order to fully exhibit this effect, it is preferable that the amount of the substitution element added is large. However, if the amount of the substitution element of Mn exceeds 0.3, the first lithium is added. The ability to occlude and release Li ions by the compound is limited, and this appears as an increase in internal impedance during rapid charging. Therefore, the amount of the element added is preferably greater than 0 and less than or equal to 0.3 with respect to Mn2 atoms. The first lithium compound in which a part of Mn is substituted has a high ability to occlude and release lithium ions, and acts as a main component when the lithium ion secondary battery according to the present invention exhibits a charge / discharge function.

In addition, although the Li element is contained in the additive element to the first lithium compound, Li added here replaces Mn, and enters the same site as Mn in the crystal structure. Conceivable. Therefore, the position of the substitution site and the effect of addition differ from the element (Li) in the Li site in the structure of LiMn ₂ O ₄ before Mn substitution.

On the other hand, in the present invention, in LiFePO ₄ which is the second lithium compound, Fe and other elements selected from Mn, Mg, Ni and Co are added as in the case of the first lithium compound. Has been replaced. In the second lithium compound, even in the composition of LiFePO ₄ without element substitution, the crystal distortion at the time of insertion and extraction of Li ions is originally small, and the thermal stability is sufficiently excellent. The reason for substituting the element at the Fe site in the second lithium compound is to improve the ability to occlude and release Li ions during charge / discharge of the battery.

  The second lithium compound has a lower electrical conductivity than the first and third lithium compounds, and this appears as a result of high internal impedance during rapid charging. Here, it is considered that the effect of reducing the internal impedance can be obtained by partially replacing the Fe site element with another element. In addition, if the added amount of the substitution element of Fe (iron) exceeds a ratio of 0.2 with respect to Fe1 atom, the thermal stability is lowered. For this reason, the addition amount of one or more elements selected from Mn, Mg, Ni, and Co is preferably greater than 0 and less than or equal to 0.2 with respect to Fe1 atoms.

Furthermore, for LiNiO ₂ as the third lithium compound, one or more elements selected from Li, Mg, Mn, Al, and Co are added to replace Ni, thereby stabilizing the crystal structure. Yes. This substitution relaxes crystal distortion that occurs during insertion and extraction of Li ions, and as in the case of the first lithium compound, a reduction in internal impedance during rapid charging of the battery is realized. In order to fully exhibit this effect, it is preferable that the amount of substitution element added be large. However, if the amount of substitution element addition of Ni exceeds 0.3 with respect to Ni1 atoms, storage of Li ions is performed. The ability to release will be limited. Therefore, the amount of the element added is preferably greater than 0 and less than or equal to 0.3 with respect to Ni1 atoms. Note that the Li element described here as an additive element replaces Ni, and is considered to enter the same site as Ni in the crystal structure and not enter the Li site. This is the same as in the case of the first lithium compound.

In the present invention, the first, second, and third lithium compounds in which some of the elements are replaced with other elements by the above-described method are further mixed at a specific ratio, thereby rapidly Suppressed increase in internal impedance during charging. The effects of mixing these three types of lithium compounds into a positive electrode active material are considered as follows. First, in the present invention, an electrolyte solution is prepared by mixing a third lithium compound (LiNiO ₂ partially substituted with Ni element) with a first lithium compound (LiMn ₂ O ₄ partially substituted with Mn element). It is considered that the slight moisture contained therein reacts with the third lithium compound to capture H ⁺ in H ₂ O and liberate OH ⁻ according to the following chemical formula.

LiNiO ₂ + H ₂ O → β-NiOOH + Li ⁺ + OH ⁻

Here, β-NiOOH is β-type nickel oxyhydroxide. In the case of only the first lithium compound, H ₂ O slightly present in the electrolyte solution in the battery reacts with the supporting salt as described above to generate H ions, and LiMn ₂ O ₄ to Mn ions Elution occurs. However, by mixing the first lithium compound and the third lithium compound, LiNiO ₂ captures a slight amount of water in the electrolyte solution, so that elution of Mn ions from LiMn ₂ O ₄ is suppressed. An effect is produced. All of the above effects appear as a reduction in internal impedance during rapid charging.

However, just mixing the third lithium compound with the first lithium compound leaves the problem of thermal stability in the positive electrode of the battery. When the internal temperature of a lithium ion secondary battery becomes high, the structure of the interface between the particles that make up the lithium compound, or between the conductivity-imparting agent and the binder generally changes, which causes an irreversible increase in internal impedance. There is. Here, in the present invention, the thermal stability problem is solved by adding LiFePO ₄ , which is a part of the Fe element, which is the second lithium compound, at a constant ratio. The reason for the improvement by this addition is not clear in detail, but by adding the second lithium compound as the positive electrode active material, the structure of the interface between the particles including the fine crystal particles constituting the positive electrode active material layer It seems that there will be an effect of mitigating these changes.

  The second lithium compound has a problem that the electrical conductivity is remarkably lower than that of the first and third lithium compounds as described above. However, this point is that the first and third lithium compounds ( The electrical conductivity of the first lithium compound) can be sufficiently compensated. Thus, by mixing the first, second, and third lithium compounds, the problem of increase in internal impedance at the time of rapid charging, which has existed in the positive electrode of the conventional lithium ion secondary battery, is solved, and charging is performed. Sometimes it becomes possible to obtain a battery with a sufficiently low internal impedance.

  Here, the proportions of the first, second, and third lithium compounds effective for reducing the internal impedance of the battery are the mixing ratios of a: b: c (a, b, c are weight ratios, a + b + c, respectively). = 100), 0 <a ≦ 90, 0 <b ≦ 50, and 0 <c ≦ 50 are preferable. This mixing ratio is obtained by accumulating evaluations of a large number of samples having different compositions and ranges as well as the types of elements to be added to the first, second and third lithium compounds and the ranges of the preferred addition amounts thereof. It was obtained experimentally.

  When a lithium ion secondary battery is produced using a positive electrode active material made of a lithium compound, the release and insertion of Li atoms from the Li site in the crystal structure are repeated by charging and discharging the lithium ion secondary battery. It is known that the content of Li in the lithium compound in the positive electrode active material is lower than the initial value. The Li content in the first, second, and third lithium compounds in the present invention is defined as the composition range in the initial state in which charging and discharging have not yet been performed in the lithium ion secondary battery. is there.

That is, the present invention is an oxide having a crystal structure of the chemical formula LiMn ₂ O ₄ , and the molar ratio of Li, Mn and additive element Ma contained in the oxide is
Li: Mn: Ma = 1: 2-x: x (0 <x <2, Ma is one or more selected from Li, B, Mg, Al, Co, Ni)
A first lithium compound composition is the chemical formula of an oxide having a crystal structure of LiFePO _4, Li contained in the oxide, the molar ratio of Fe and additional element Mb,
Li: Fe: Mb = 1: 1-y: y (0 <y <1, Mb is one or more selected from Mn, Mg, Ni, Co)
A second lithium compound having a composition of: and an oxide having a crystal structure of the chemical formula LiNiO ₂ , and the molar ratio of Li, Ni and additive element Mc contained in the oxide is
Li: Ni: Mc = 1: 1-z: z (0 <z <1, Mc is one or more selected from Li, Mg, Mn, Al, Co)
A positive electrode for a lithium ion secondary battery comprising a positive electrode active material comprising a third lithium compound having a composition of

  In the present invention, the x is 0 <x ≦ 0.3, the y is 0 <y ≦ 0.2, and the z is 0 <z ≦ 0.3. And a positive electrode for a lithium ion secondary battery.

  In the present invention, the ratio of the content of the first lithium compound, the second lithium compound, and the third lithium compound contained in the positive electrode active material is set to a: b: c (a, b , C are weight ratios, a + b + c = 100), and 0 <a ≦ 90, 0 <b ≦ 50, 0 <c ≦ 50.

  Furthermore, the present invention is a lithium ion secondary battery using the positive electrode for a lithium ion secondary battery described above.

The positive electrode for a lithium ion secondary battery of the present invention has a positive electrode active material obtained by mixing first, second and third lithium compounds having different compositions. The first, second, and third lithium compounds are compounds represented by LiMn ₂ O ₄ , LiFePO ₄ , and LiNiO ₂ , respectively, and have a composition in which Mn, Fe, and Ni are partially substituted with specific elements. By mixing these three kinds of compounds at a specific ratio to make a positive electrode active material, a lithium ion secondary battery using the compound can suppress an increase in internal impedance during rapid charging even after long-term use. Is possible. Therefore, by using the positive electrode for a lithium ion secondary battery according to the present invention, it is possible to provide a lithium ion secondary battery that can be rapidly charged.

  Moreover, the positive electrode for lithium ion secondary batteries of this invention has the characteristics that thermal stability is high. For this reason, when used in a lithium ion secondary battery, even when the battery is fully charged and kept at a high temperature, the internal impedance can still be kept at a low value. Therefore, even if the inside of the battery is subjected to a high temperature condition, it is possible to continue to maintain the characteristics capable of rapid charging.

  Hereinafter, an embodiment of a lithium ion secondary battery using the positive electrode of the present invention will be described with reference to FIG.

  FIG. 1 shows a cross-sectional view of the shape of a single-plate laminate type battery cell as an example of a lithium ion secondary battery having a positive electrode provided with the positive electrode active material of the present invention. In FIG. 1, the positive electrode region includes a positive electrode active material layer 11 and a positive electrode current collector 13, and the negative electrode region similarly includes a negative electrode active material layer 12 and a negative electrode current collector 14. Here, the positive electrode active material layer 11 and the negative electrode active material layer 12 face each other with the separator 15 interposed therebetween. The positive electrode current collector 13 and the negative electrode current collector 14 are generally made of a metal foil, and the positive electrode active material layer 11 and the negative electrode active material layer 12 are formed on each side by applying and solidifying the positive electrode active material and the negative electrode active material. Is formed. The ends of the positive electrode current collector 13 and the negative electrode current collector 14 are drawn out to the outside of the battery cell as a positive electrode tab 18 and a negative electrode tab 19, respectively. The battery cell is sealed from above and below by exterior laminates 16 and 17. Yes. The sealed battery cell is filled with an electrolyte solution. As this electrolyte solution, a non-aqueous organic electrolyte solution in which a lithium salt is dissolved as a supporting salt is used.

  In addition, in the lithium ion secondary battery using the positive electrode of the present invention, the battery shape is basically not limited, and the electrode shape may be a winding type as long as the positive electrode region and the negative electrode region face each other with the separator interposed therebetween. It is also possible to have a stacked shape. Further, the structure of the battery cell may be not only the single plate laminate type but also a coin type, a laminate pack type, a square cell, a cylindrical cell, or the like.

  Generally, a lithium ion secondary battery has a positive electrode using a lithium compound as a positive electrode active material, and a negative electrode provided with a negative electrode active material capable of occluding and releasing lithium ions. In order to prevent electrical connection from occurring, a non-conductive separator and an electrolyte region are provided. Here, both the positive electrode and the negative electrode are kept immersed in an electrolyte solution having lithium ion conductivity, and these components are sealed in a container. When a voltage is applied from the outside between the positive electrode and the negative electrode constituting the battery, lithium ions are released from the positive electrode active material which is the positive electrode active material, and the lithium ions are occluded in the negative electrode active material through the electrolyte solution. It will be in a charge state. In addition, when the positive electrode and the negative electrode are electrically connected via a load outside the battery, this time, lithium ions are released from the negative electrode active material contrary to the charging, and the lithium ions are occluded in the positive electrode active material. As a result, discharge is performed.

The structure of the lithium ion secondary battery using the positive electrode of the present invention will be described below. The positive electrode in the battery of the present invention has a crystal structure of LiMn ₂ O ₄ , and Mn is partially substituted by the element Ma (Ma is one or more selected from Li, B, Mg, Al, Ni, Co). A second lithium compound having a crystal structure of a first lithium compound having a composition and LiFePO ₄ , wherein Fe is partially substituted with an element Mb (Mb is one or more selected from Mn, Mg, Ni, and Co); A lithium compound and a _third lithium compound having a crystal structure of LiNiO ₂ and partially substituted with Ni by an element Mc (Mc is one or more selected from Li, Mg, Mn, Al, and Co); It is characterized by including the mixture which consists of as a positive electrode active material.

Of these, the following starting materials are preferably used in the production of the first lithium compound. First, Li ₂ CO ₃ , LiOH, Li ₂ O, LiNO are used as starting materials for adding Li as an additive element to the Li site and Li as an element co-added to the Mn site to the first lithium compound. ₃ , Li ₂ SO ₄ or the like can be used. In particular, lithium salts such as Li ₂ CO ₃ and LiOH are highly reactive with transition metal raw materials, and the CO ₃ and OH groups contained in the reaction product are volatilized in the form of CO ₂ and H ₂ O during the firing. Therefore, it is preferable because the possibility of remaining in the positive electrode active material and having an adverse effect is low.

As starting materials for adding Mn element, electrolytic manganese dioxide (EMD), Mn oxides such as Mn ₂ O ₃ and Mn ₃ O ₄ , MnCO ₃ and MnSO ₄ can be used. . As a starting material for adding Mg element to the Mn site, Mg (OH) ₂ or the like can be used. Similarly, Al (OH) ₃ or the like can be used as a starting material for adding Al element. B ₂ O ₃ or the like can be used as a starting material for the B element. As a starting material for Co element, oxides such as Co ₂ O ₃ and Co ₃ O ₄ , Co (OH) ₃ , CoCO ₃ , and CoSO ₄ can be used. NiO, Ni (OH) ₂ , NiSO ₄ , Ni (NO ₃ ) _2, etc. can be used as starting materials for the Ni element.

  These starting materials are weighed so as to have a predetermined metal composition ratio, and pulverized and mixed using a mortar or ball mill. The obtained mixed powder is baked at a temperature of 500 ° C. to 1200 ° C. in an air or oxygen atmosphere to obtain a powder of a first lithium compound that is an oxide. The firing temperature at this time is preferably a high temperature in order to diffuse each element. However, if the firing temperature is too high, oxygen vacancies may be generated in the composition, and the powder particles may be agglomerated. The state may not be maintained. When such a phenomenon occurs, when the produced lithium compound is used as the positive electrode active material of the battery, various adverse effects may occur on various characteristics of the battery. Therefore, the above upper limit exists for the firing temperature, and it is more preferable to perform firing in the range of 500 ° C to 900 ° C. Moreover, in order to prevent oxygen deficiency of the generated first lithium compound, it is more preferable to perform firing in an oxygen atmosphere.

The specific surface area of the obtained powdery first lithium compound is 0.01 m ² / g or more, 10 m ² / g or less, more preferably more preferably 0.1 m ² / g or more, 3m ² / g or less. The larger the specific surface area, the more binders are required for constituting the positive electrode active material layer, and this is disadvantageous in terms of the capacity density of the electrode. On the other hand, when the specific surface area is too small, the ionic conductivity between the electrolyte solution and the positive electrode active material may decrease. When the median diameter measured by the laser diffraction / scattering method is the average particle diameter, the average particle diameter of the powdery first lithium compound is preferably 0.1 μm or more and 70 μm or less, more preferably 1 μm or more. 30 μm or less. If this average particle size is large, uneven irregularities may occur in the positive electrode active material layer when the positive electrode active material layer is formed on the surface of the positive electrode current collector. On the other hand, when the average particle size is small, the binding property of the formed positive electrode active material layer to the positive electrode current collector may be low.

In the production of the second lithium compound, the following starting materials are preferably used. First, as a raw material for synthesizing lithium iron phosphate represented by LiFePO ₄ that does not contain an Fe site substitution element, various lithium compounds, divalent iron compounds, and phosphate compounds are used in appropriate combination. Among these, LiF, LiCl, LiBr, LiI, LiCO ₃ , LiOH, Li ₃ PO _{4 and the} like can be used as starting materials for Li contained. Starting materials for Fe include FeF ₂ , FeCl ₂ , FeBr ₂ , FeI ₂ , FeSO ₄ , Fe ₃ (PO ₄ ) ₂ , Fe (C ₂ O ₄ ) · 2H ₂ O, (CH ₃ COO). ₂ Fe or the like can be used. Starting materials for P include: H ₃ PO ₄ , HPO ₃ , H ₄ P ₂ O ₇ , H ₅ P ₃ O ₁₀ , (NH ₄ ) ₃ PO ₄ , NH ₄ PO ₃ , Li ₃ PO ₄ , Fe ₃ (PO ₄ ) ₂ or the like can be used.

On the other hand, it is preferable to use the following starting materials for the substitution element at the Fe site of the second lithium compound. First, MnCO ₃ , MnSO ₄ , MnCl ₂ , Mn (NO ₃ ) ₂ or the like can be used as a starting material for Mn. As a starting material for Mg, MgSO ₄ or the like can be used. NiSO ₄ or the like can be used as a starting material for Ni. CoSO ₄ or the like can be used as a starting material for Co.

  These starting materials are weighed so as to have a predetermined metal composition ratio, and pulverized and mixed with a mortar or ball mill. The obtained mixed powder is fired at a temperature of 500 ° C. to 1200 ° C. in a nitrogen or Ar atmosphere to obtain a second lithium compound powder.

The specific surface area of the thus obtained powdery second lithium compound is 0.01 m ² / g or more, is preferably from 100 m ² / g, more preferably 0.1 m ² / g or more, 50 m ² / g or less. The larger the specific surface area, the more binders are required for constituting the positive electrode active material layer, and this is disadvantageous in terms of the capacity density of the electrode. On the other hand, when the specific surface area is too small, the ionic conductivity between the electrolyte solution and the positive electrode active material may decrease. The average particle size of the powdered second lithium compound is preferably 0.1 μm or more and 70 μm or less, more preferably 1 μm or more and 30 μm or less. If this average particle size is large, uneven irregularities may occur in the positive electrode active material layer when the positive electrode active material layer is formed on the surface of the positive electrode current collector. On the other hand, when the average particle size is small, the binding property of the formed positive electrode active material layer to the positive electrode current collector may be low.

Furthermore, in the production of the third lithium compound, it is preferable to use the following starting materials. First, Li ₂ CO ₃ , LiOH, Li ₂ O, LiNO are used as starting materials for adding Li as an additive element to the Li site and Li as an element co-added to the Ni site to the third lithium compound. ₃ , Li ₂ SO ₄ or the like can be used. NiO, Ni (OH) ₂ , NiSO ₄ , Ni (NO ₃ ) _2, etc. can be used as starting materials for adding Ni element. Further, Co (OH) ₃ , Co ₂ O ₃ , Co ₃ O ₄ , CoSO _4, etc. can be used as a starting material for adding Co element to the Ni site. Similarly, Al (OH) ₃ or the like can be used as a starting material for the Al element. As starting materials for the Mn element, electrolytic manganese dioxide (EMD), Mn oxides such as Mn ₂ O ₃ and Mn ₃ O ₄ , MnCO ₃ and MnSO ₄ can be used. As a starting material for Mg element, Mg (OH) ₂ or the like can be used.

  These starting materials are weighed so as to have the desired metal composition ratio, and pulverized and mixed with a mortar or ball mill. The obtained mixed powder is fired at a temperature of 500 ° C. to 1200 ° C. in an air or oxygen atmosphere to obtain a third lithium compound powder. The firing temperature at this time is desirably a high temperature in order to diffuse each element. However, if the firing temperature is too high, aggregation of powder particles may occur and the powder state may not be maintained. For this reason, the above upper limit exists in the firing temperature, and it is more preferable to perform firing in the range of 500 ° C to 1000 ° C.

The specific surface area of the obtained powdery third lithium compound 0.01 m ² / g or more, 10 m ² / g or less, more preferably more preferably 0.1 m ² / g or more, 3m ² / g or less. The larger the specific surface area, the more binders are required for constituting the positive electrode active material layer, and this is disadvantageous in terms of the capacity density of the electrode. On the other hand, when the specific surface area is too small, the ionic conductivity between the electrolyte solution and the positive electrode active material may decrease. The average particle size of the powdery third lithium compound is preferably 0.1 μm or more and 70 μm or less, more preferably 1 μm or more and 30 μm or less. If this average particle size is large, unevenness in the positive electrode active material layer may occur during the formation of the positive electrode active material layer on the surface of the positive electrode current collector. On the other hand, when the average particle size is small, the binding property of the formed positive electrode active material layer to the positive electrode current collector may be low.

Whether the produced powdery first, second, and third lithium compounds each have a predetermined crystal structure can be evaluated by powder X-ray diffraction. Each powdered lithium compound is set in a powder X-ray diffractometer, and the diffraction angle and intensity of the diffracted light obtained by irradiating with characteristic X-rays are measured. Data Cards: Powder X-ray diffraction pattern database card) to identify the crystal structure. In the case of the present invention, the diffraction patterns of the obtained powdery first, second and third lithium compounds are respectively compared with the diffraction patterns in the crystal structures of LiMn ₂ O ₄ , LiFePO ₄ and LiNiO ₂ . By measuring the diffraction intensity, it is possible to specify the crystal structure and the crystallization rate of the generated compound.

  The powdery first, second, and third lithium compounds obtained by the above method are mixed and applied to the surface of a current collector made of a metal foil to form a positive electrode active material layer to obtain a positive electrode. The method is as follows. First, powdery first, second, and third lithium compounds are mixed at a predetermined ratio, respectively, to obtain a positive electrode active material. Next, this positive electrode active material is mixed with a conductivity-imparting material, a binder, and a non-aqueous organic solvent, sufficiently stirred to form a slurry, and then applied to the surface of the current collector, formed into a film, and dried. Solidify. Here, as the conductivity imparting material, a carbon material such as acetylene black, carbon black, graphite, or fibrous carbon is preferably used, and in addition, a metal powder such as aluminum, a conductive oxide powder, or the like is used. Can do. As the binder, polyvinylidene fluoride (PVDF), an acrylic polymer, an imide polymer, or the like is used. The organic solvent is non-aqueous and is appropriately selected in consideration of dispersibility of each solute and ease of drying and solidification. The current collector is preferably a metal foil mainly composed of aluminum, an aluminum alloy, titanium, or the like.

  Here, the preferable addition amount of the conductivity imparting material is 0.5 to 30% by weight with respect to the total amount of the positive electrode active material excluding the organic solvent, the conductivity imparting material and the binder, A preferable addition amount is similarly 1 to 10 weight% with respect to the said total amount. When the content ratio of the conductivity-imparting material and the binder mixed here is too small, the electric conductivity in the formed positive electrode active material layer becomes small, and thereby the charge / discharge rate characteristics (a certain amount of charge) of the battery. The speed for charging / discharging the battery may be reduced, and electrode peeling may occur. Conversely, when the content ratio of the conductivity-imparting material and the binder is too large, the content ratio of the positive electrode active material is reduced, so that the energy density of the lithium ion secondary battery to be produced is reduced, and the unit weight of the battery Per charge capacity may be small. A preferable content ratio of the positive electrode active material is 70% by weight or more and 98.5% by weight or less, and more preferably 85% by weight or more and 97% by weight or less with respect to the total amount.

There is an upper limit and a lower limit for the density of the positive electrode active material layer formed by coating on the surface of the current collector. In general, when the density of the positive electrode active material layer is too high, the electrolyte solution that fills the periphery of the positive electrode of the lithium ion secondary battery is less likely to enter the gap of the positive electrode because there are few voids formed in the positive electrode active material layer. Become. For this reason, the amount of movement of Li ions is reduced, and the charge / discharge rate characteristics of the battery may be reduced. On the other hand, when the density of the positive electrode active material layer is too small, the energy density of the lithium ion secondary battery to be manufactured may be reduced as in the case where the content ratio of the positive electrode active material in the positive electrode active material layer is small. is there. For this reason, the density of the positive electrode active material layer is preferably 1 g / cm ³ or more and 4.5 g / cm ³ or less. More preferably, it is 2 g / cm ³ or more and 4 g / cm ³ or less.

Examples of the negative electrode active material constituting the negative electrode of the lithium ion secondary battery include carbon materials such as graphite, hard carbon, and soft carbon, silicon oxides such as SiOx (x is an O content), silicon, silicon, and silicon other than silicon Silicon composite compounds containing these elements, tin or tin oxide, tin composite oxides containing elements other than tin and tin, aluminum alloys, lithium metals, titanium oxide, and Li ₄ Ti ₅ O ₁₂ Or it can mix and use. The negative electrode is produced by the same method as that of the positive electrode. However, as the current collector, it is preferable to use a metal foil mainly composed of copper, nickel, stainless steel, silver, or the like that does not easily react with Li ions.

  As the electrolyte solution that can be used for the lithium ion secondary battery in the present invention, it is preferable to use one or a mixture of two or more of aprotic organic solvents. The organic solvent is as follows.

  That is, cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), vinylene carbonate (VC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), Chain carbonates such as dipropyl carbonate (DPC), aliphatic carboxylic acid esters such as methyl formate, methyl acetate and ethyl propionate, γ-lactones such as γ-butyrolactone, 1,2-ethoxyethane (DEE) , Chain ethers such as ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylform Muamide, acetonitrile, propylnitrile, nitromethane, ethyl monoglyme, phosphoric acid triester, trimethoxymethane, dioxolane derivative, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, Examples thereof include propylene carbonate derivatives, tetrahydrofuran derivatives, ethyl ether, 1,3-propane sultone, anisole, N-methyl-2-pyrrolidone (NMP), and fluorinated carboxylic acid esters.

  Moreover, a polymer or the like may be added to the aprotic organic solvent to solidify the electrolyte solution into a gel. Furthermore, a room temperature molten salt or an ionic liquid represented by a cyclic ammonium cation or the same anion may be used. Among these electrolyte solutions, a method of using a mixture of cyclic carbonates and chain carbonates is particularly suitable from the viewpoints of electrical conductivity and stability under high voltage.

In these electrolyte solutions, lithium salts are dissolved and used as supporting salts. Examples of the lithium salt to be dissolved include LiPF ₆ , LiAsF ₆ , LiAlCl ₄ , LiClO ₄ , LiBF ₄ , LiSbF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiC (CF ₃ SO ₂ ) ₃ , LiN (CF _{3 SO 2) 2, LiN (C} 2 F 5 SO 2) 2, LiCH 3 SO 3, LiC 2 H 5 SO 3, LiC 3 H 7 SO 3, lower aliphatic lithium carboxylate and other lithium carboxylate, chloroborane Examples include lithium, lithium tetraphenylborate, LiBr, LiI, LiSCN, LiCl, and LiF. The electrolyte concentration of the dissolved support salt is preferably 0.5 mol / l or more and 1.5 mol / l or less. When the electrolyte concentration of the supporting salt is too high, it is considered that the density and viscosity of the electrolyte solution increase and the movement of Li ions is hindered. Conversely, when the electrolyte concentration is too low, the electrical conductivity of the electrolyte solution may decrease.

  The separator used in the lithium ion secondary battery of the present invention is suitably a polymer film such as a propylene film. According to the lithium ion secondary battery of the present invention, a positive electrode active material layer and a negative electrode active material layer are formed on the surfaces of a positive electrode current collector and a negative electrode current collector, respectively. It laminates through a separator to make an electrode body. By sealing this electrode body together with an electrolyte solution inside a film structure formed by laminating a synthetic resin and a metal foil in dry air or an inert gas atmosphere, lithium ions having a single plate laminate type cell A secondary battery can be manufactured. Alternatively, the electrode body is further wound to form a wound body, which is also housed in a battery can in a dry air or inert gas atmosphere, filled with an electrolyte solution, and sealed to form a cylindrical or square cell. The lithium ion secondary battery which has can be produced.

  The potential of the positive electrode of the lithium ion secondary battery manufactured here needs to be 5.5 V or less with respect to the potential of Li. In general, lithium ion secondary batteries have the property that decomposition of the electrolyte solution proceeds when the positive electrode potential is high, and the reliability of the battery is maintained particularly when charging / discharging is repeated or stored at a high temperature of 60 ° C. or higher. Therefore, the positive electrode potential is preferably 5.2 V or less, more preferably 4.5 V or less. On the other hand, the potential of the negative electrode needs to be 0 V or more with respect to the potential of Li.

  Various compositions and mixtures can be obtained by changing the composition of each of the first, second and third lithium compounds to be mixed as the positive electrode active material and the mixing ratio between the first, second and third lithium compounds. Proportion of lithium ion secondary batteries were prepared, each of which was used as an example and a comparative example. A series of lithium ion secondary batteries all have single-plate laminated cells of the same shape. These lithium ion secondary batteries were held at a high temperature and evaluated for a reduction in discharge capacity. In the following, methods for producing and evaluating lithium ion secondary batteries of Examples and Comparative Examples will be described. In each of the fabricated lithium ion secondary batteries of Examples and Comparative Examples, the conditions other than the compositions of the first, second, and third lithium compounds used and the mixing ratio thereof are all the same.

(Preparation of first lithium compound)
LiOH and MnO ₂ (EMD) are used as starting materials, and the LiOH and Ni (OH) ₂ , Mg (OH) ₂ , Al (OH) ₃ , if necessary, as starting materials for elements added to the Mn site, Each of Co ₃ O ₄ , B ₂ O ₃ , and Fe ₂ O ₃ was used and weighed so as to achieve the desired composition ratio of the first lithium compound. Next, these raw materials were mixed and pulverized and mixed for 1 hour or more in a mortar, and then baked in air at 900 ° C. for 12 hours. After the first firing, the mixture was pulverized and mixed again, and further fired in an oxygen atmosphere at 700 ° C. for 12 hours. Thereafter, the coarse particles were removed from the mixture through a 50 μm mesh sieve to obtain a powder of a positive electrode active material having the composition of the first lithium compound. The powder material obtained here has a specific surface area of 0.1 m ² / g to 5 m ² / g in the first lithium compound of all compositions prepared, and an average particle size of 0.5 μm to 40 μm. Range. Further, by powder X-ray diffraction, the obtained powder material has a crystal structure of LiMn ₂ O _{4 and} a crystallization ratio of 90% or more (weight ratio of the powder material having a crystal structure of LiMn ₂ O ₄ ) It was confirmed that it has.

(Production of second lithium compound)
Li ₂ CO ₃ , (CH ₃ COO) ₂ Fe, and NH ₄ H ₂ PO ₄ are used as starting materials, and the MnCO ₃ , MgCO ₃ , and NiSO ₄ are optionally used as starting materials for elements added to the Fe site. Each of CoSO ₄ was weighed so that the composition ratio of the second lithium compound was the target. Next, these raw materials were mixed and pulverized and mixed in a mortar for 1 hour or more, and then pre-baked for 12 hours in a nitrogen atmosphere at 300 ° C., and further under the conditions of 600 ° C. and 24 hours in a nitrogen atmosphere. Firing was performed. Then, coarse particles were removed from the mixture through a 50 μm mesh sieve to obtain a powder of a positive electrode active material having the composition of the second lithium compound. The powder material obtained here has a specific surface area of 0.1 m ² / g to 5 m ² / g in the second lithium compound of the total composition produced, and an average particle size of 0.5 μm to The range was 40 μm. Further, it was confirmed by powder X-ray diffraction that the obtained powder material had a crystal structure of LiFePO _{4 and} the crystallization rate was 90% or more.

(Preparation of third lithium compound)
LiOH and Ni (OH) ₂ are used as starting materials, and the LiOH and Co (OH) ₃ , Al (OH) ₃ , MnO ₂ , Mg (OH are optionally used as starting materials for the elements added to the Ni site. ) Each of ₂ was used and weighed so that the composition ratio of the target third lithium compound was obtained. Next, these raw materials were mixed and pulverized and mixed for 1 hour or more in a mortar, and then baked in air at 900 ° C. for 12 hours. After the first firing, the product was pulverized and mixed again, and further fired in the oxygen atmosphere at 700 ° C. for 12 hours. Then, coarse particles were removed from the mixture through a 50 μm mesh sieve to obtain a powder of a positive electrode active material having a composition of a third lithium compound. The powder material obtained here has a specific surface area of 0.1 m ² / g to 5 m ² / g in the third lithium compound of all compositions prepared, and an average particle diameter of 0.5 μm to 5 μm. The range was 40 μm. Further, it was confirmed by powder X-ray diffraction that the obtained powder material had a crystal structure of LiNiO _{2 and} the crystallization rate was 90% or more.

(Preparation of positive electrode)
The first, second, and third lithium compound powder materials, which are the positive electrode active material materials obtained in the above-described steps, and the conductivity imparting material are dispersed in a solution in which a binder is dissolved in an organic solvent. And kneaded to form a slurry. Here, carbon black, which is a carbon material, was used as a conductivity imparting material, polyvinylidene fluoride (PVDF) was used as a binder, and N-methyl-2-pyrrolidone (NMP) was used as an organic solvent. Here, the weight ratio of the positive electrode active material, the conductivity-imparting material, and the binder was 90: 6: 4, respectively. The slurry thus produced was applied on a positive electrode current collector made of an aluminum (Al) foil having a thickness of 20 μm to form a positive electrode active material layer, thereby obtaining a laminate. At that time, the initial charge capacity per unit area of the positive electrode to be produced (the amount of charge accumulated in the battery when the fully charged non-charged battery is first fully charged) is 2.0 mAh / cm ² Thus, the thickness of the positive electrode active material layer to be applied was adjusted. Then, the produced laminated body was dried in vacuum for 12 hours and solidified to obtain a positive electrode material. This positive electrode material was cut into a 20 mm long and 20 mm wide square. Then, it pressure-molded with the pressure of 3 t / cm < ² >, and produced one positive electrode used for one battery.

(Preparation of negative electrode)
A graphite powder material as a negative electrode active material and a conductivity imparting material were dispersed in a solution in which a binder was dissolved in an organic solvent and kneaded to form a slurry. Here, carbon black as a carbon material was used as the conductivity imparting material, polyvinylidene fluoride (PVDF) was used as the binder, and N-methyl-2-pyrrolidone (NMP) was used as the organic solvent. Next, the prepared slurry was applied on a negative electrode current collector made of a copper (Cu) foil having a thickness of 10 μm to form a negative electrode active material layer, thereby obtaining a laminate. At that time, the thickness of the negative electrode active material layer to be applied was adjusted so that the initial charge capacity per unit area of the negative electrode to be manufactured was 2.4 mAh / cm ² . Thereafter, as in the case of the positive electrode material, the produced laminate was dried and solidified in vacuum for 12 hours to obtain a negative electrode material. This negative electrode material was cut into a square of 22 mm length and 22 mm width. Then, it pressure-molded with the pressure of 1 t / cm < ² >, and produced one negative electrode used for one battery.

(Preparation of electrolyte solution)
Moreover, the electrolyte solution used for a lithium ion secondary battery uses what mixed ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in the ratio of 30:70 by the volume ratio as an organic solvent, and uses this as supporting salt. LiPF ₆ was dissolved and used. The concentration of the supporting salt LiPF ₆ was 1 mol / l.

(Assembly of lithium ion secondary battery)
The positive electrode and the negative electrode prepared by the above method were placed so that the positive and negative active material layers face each other, and were placed in a battery cell made of a laminate cell with a separator interposed therebetween. Here, a polypropylene film as an insulator was used as the separator, and the shape of the separator was larger than that of either the positive electrode or the negative electrode. For this reason, the positive electrode and the negative electrode are insulated from each other by the separator. Next, an Al tab, which is a lead leading out of the battery, is joined to the end of the positive electrode current collector, and a nickel (Ni) tab of the lead lead is similarly joined to the end of the negative electrode current collector. It was pulled out of the laminate cell of the battery. Next, an electrolyte solution was filled in the battery laminate cell, the laminate cell was sealed, and a lithium ion secondary battery having a single-plate laminate type battery cell was assembled. In these series of assembly steps, ten lithium ion secondary batteries each having a positive electrode active material having the same composition are produced.

(Evaluation of lithium ion secondary battery)
The following evaluation method was implemented with respect to the lithium ion secondary battery produced by the said process. First, the prepared lithium ion secondary battery was charged by a constant current and constant voltage method with an upper limit voltage of 4.2 V and a current value of 8 mA, and was fully charged (initial charge). Next, the discharge was performed at a constant current with the lower limit voltage set to 3.0 V (initial discharge). The discharge capacity at this time (the amount of charge taken out from the battery by the initial discharge) is defined as the initial discharge capacity.

  After the first charge / discharge, the battery was charged again under the same charging conditions as the first charge to obtain a fully charged state, and the battery was kept at 60 ° C. and stored for 90 days. After the end of storage, after discharging once to 3.0 V of the same lower limit voltage as described above at a constant current of 8 mA in a temperature atmosphere of 20 ° C., the upper limit voltage is 4.2 V and the current density is 10 times that of the initial charge. Rapid charging was performed for 15 minutes by a constant current constant voltage method at a current value of 80 mA. Thereafter, discharging was performed again at a constant current of 8 mA to a lower limit voltage of 3.0 V, and the discharge capacity at that time was measured. This is defined as the discharge capacity after holding. When the ratio of the value of the discharge capacity after holding to the initial discharge capacity in each battery is measured, and the composition and mixing ratio of the first, second, and third lithium compounds in the positive electrode active material are changed, this discharge capacity We evaluated how the ratio value of each changed.

Here, as described above, the initial charge capacity of each battery is set to a constant value of 2.0 mAh / cm ² , and the area of the positive electrode that captures and releases Li ions is the same for each battery. It is 20 mm long and 20 mm wide. Therefore, the initial charge capacity of each battery is the same at 8 mAh. Therefore, if each battery is charged at a constant current of 8 mA, it can be almost fully charged in one hour. Hereinafter, charging at a constant current of 8 mA is referred to as 1C. However, since the upper limit voltage is set at the time of charging, the actual charging of each battery is constant current constant voltage charging that shifts to constant voltage charging when the charging voltage reaches the upper limit voltage. Therefore, for example, charging is initially performed at a constant current of 80 mA, and when the upper limit voltage is reached, the charging is switched to constant voltage charging. The fast charge for 15 minutes at the current value of 80 mA after the discharge after storage at 60 ° C. for 90 days in the evaluation of the lithium ion secondary battery is this constant current / constant voltage charge at 10 C15.

  A lithium ion secondary battery in which the compositions and mixing ratios of the first, second, and third lithium compounds in the positive electrode active material were changed under the above production conditions was used as Examples and Comparative Examples. Next, after each battery is initially charged, initially discharged and recharged, held at a high temperature of 60 ° C. for 90 days, discharged and rapidly charged and further discharged, and the value of the discharged capacity after holding relative to the initial discharge capacity was measured. The ratio (hereinafter, capacity retention rate) was evaluated. The evaluation results are shown in Tables 1 to 19, respectively. Each table shows the composition or mixing ratio (percentage, unit%) of each lithium compound in the positive electrode active material constituting each lithium ion secondary battery, and the capacity maintenance ratio value (unit%) for each composition. , And evaluation results (◯ and ×). The number of lithium ion secondary batteries evaluated for each composition and mixing ratio is 10 each, and the capacity retention rate of the batteries of each composition is the average value of the measured values of these 10 batteries.

(Evaluation criteria)
In Tables 1 to 19 shown below, the case where the capacity retention ratio value in each of the examples and comparative examples is 50% or more is judged as good (◯ judgment), and the case where it is less than 50% is judged as bad (x judgment). This criterion is based on a standard standardized as JIS (Japanese Industrial Standard) C8711 “Lithium Secondary Battery for Portable Devices”. In this standard, as a standard for capacity recovery after long-term storage that should be satisfied by a lithium ion secondary battery, the battery is charged and discharged once, and then charged until it reaches a 50% charge state. A test method is described in which the sample is stored for 90 days at 2 ° C., discharged once, and then discharged at a constant current (0.2 C) at an ambient temperature of 20 ± 5 ° C. It is a requirement of the same standard regarding long-term storage that this last discharge capacity is 50% or more (capacity recovery after long-term storage is 50% or more) with respect to the discharge capacity at the time of first charge / discharge.

  Comparing the above-described evaluation method relating to the lithium ion secondary battery of the present invention with the evaluation method described in JIS C8711, the ambient temperature at the time of measuring the storage period of the battery (90 days) and the discharge capacity after holding the battery Conditions such as (20 ° C.) are the same. However, in terms of storage temperature (evaluation according to the present invention: 60 ° C., JIS C8711: 40 ° C.) and storage capacity during storage (the present invention: full charge, JIS C8711: 50% charge), JIS C8711 In comparison, it can be said that the evaluation criteria according to the present invention are more severe.

  This is because, in general, in a lithium ion secondary battery, it has been confirmed that deterioration proceeds faster in the case of holding at a higher temperature and storing in a state where the charging rate is high. In JIS C8711, the charging method after 90 days of holding at high temperature is not specified, but if it is interpreted as a general charging method, it can be considered as charging at 1C. On the other hand, in the evaluation according to the present invention, rapid charging for 10 C15 is performed as a charging method after holding at high temperature. In this case, if the internal impedance of the battery is greatly increased by maintaining the high temperature for 90 days, the capacity of the battery charged within 15 minutes will decrease, and this will appear as a large decrease in capacity maintenance rate. Become. Therefore, the evaluation method for the capacity retention rate before and after holding at high temperature in the present invention is equivalent to JIS C8711 for the evaluation criteria for determining the case of 50% or more as good, but due to the severity of the actual test contents, It substantially exceeds the standard of JIS C8711.

  As described above, the reason why the battery evaluation method according to the present invention is made to be a stricter standard than the general method defined in the Japanese Industrial Standard is that in recent years, lithium ion This is because the secondary battery requires a product that satisfies a stricter level than ever before in terms of maintaining the charging capacity during long-term use. A lithium ion secondary battery that satisfies the evaluation criteria employed in the present invention has more excellent characteristics with respect to capacity retention during long-term storage than conventional batteries. The present invention specifically specifies the structure of the positive electrode active material in order to satisfy such a high requirement level regarding the capacity retention rate, and thereby obtains characteristics superior to those of the conventional battery. is there.

(Examples 1-99, Comparative Examples 1-95)
The composition of the first, second and third lithium compounds constituting the positive electrode active material constituting the lithium ion secondary battery in the present invention is fixed to a specific value, and the mixing ratio of each lithium compound is changed. Batteries were produced and designated as Examples 1 to 99 and Comparative Examples 1 to 95, respectively. Tables 1 to 4 show the evaluation results of the capacity retention rates. Here, the compositions of the first, second, and third lithium compounds are LiMn _1.95 Al _0.05 O ₄ , LiFe _0.9 Mn _0.1 PO ₄ , and LiNi _0.95 Al _0.05 O ₂ , respectively. In addition, the negative electrode active material of each battery is graphite, and the number of lithium ion secondary batteries evaluated in each of the examples and comparative examples is ten. In each table, the mixing ratio of the first to third lithium compounds is shown as a percentage (weight ratio, unit%), and the total of the three is 100%. The capacity retention rate (unit%) is an average value of the measured values of the 10 batteries. Further, as a determination, according to the determination criterion, whether it was good (◯) or defective (×) was indicated by a symbol.

  Moreover, each composition of Examples 1-99 described in the said Tables 1-4 and Comparative Examples 1-95 was described in FIG. 2 as a ternary figure. In FIG. 2, the case where the capacity maintenance ratio is 50% or more (Examples 1 to 99) is indicated by “◯”, and the case where the capacity maintenance rate is less than 50% (Comparative Examples 1 to 95) is indicated by “X”. As can be seen from FIG. 2, the condition for the capacity retention rate to be 50% or more (determination is “◯”) is that the mixing ratio of the first, second, and third lithium compounds at the time of manufacturing the positive electrode active material is a: When b: c (a + b + C = 100%, a, b, c are weight ratios), the mixing ratio of each lithium compound is in the range of 0 <a ≦ 90, 0 <b ≦ 50, 0 <c ≦ 50 This is the case.

(Examples 100 to 198, Comparative Examples 96 to 190)
When the additive elements of the first, second, and third lithium compounds are different from those in Examples 1 to 99 and Comparative Examples 1 to 95, the composition of each lithium compound is fixed to a specific value, Batteries were produced by changing only their mixing ratios, and Examples 100 to 198 and Comparative Examples 96 to 190 were used, respectively. Tables 5 to 8 show the evaluation results of the capacity retention rates. Here, the compositions of the first, second, and third lithium compounds are LiMn _1.95 Mg _0.05 O ₄ , LiFe _0.9 Ni _0.1 PO ₄ , and LiNi _0.95 Co _0.05 O ₂ , respectively. The negative electrode active material of each battery is graphite. The number of evaluated lithium ion secondary batteries in each of the examples and comparative examples is 10 each, and the capacity retention rate values in Table 9 are average values. In addition, the description of each item of Table 5-Table 8 is the same as the said Table 1-Table 4.

  The compositions of Examples 100 to 198 and Comparative Examples 96 to 190 described in Tables 5 to 8 are shown in FIG. 3 as a ternary diagram. Also in FIG. 3, the case where the capacity maintenance ratio is 50% or more (Examples 100 to 198) is “◯”, and the case where the capacity maintenance rate is less than 50% (Comparative Examples 96 to 190) is “×”. It shows. In FIG. 3, the condition for the capacity retention rate to be 50% or more (determination is “◯”) is that the mixing ratio of the first, second, and third lithium compounds at the time of manufacturing the positive electrode active material is a: b: c ( a + b + C = 100%, a, b, and c are weight ratios), as in the case of FIG. 2, the mixing ratio of each lithium compound is 0 <a ≦ 90, 0 <b ≦ 50, 0 < This is a case where the range of c ≦ 50 is satisfied.

  From the evaluation results shown in Tables 1 and 8 and FIGS. 2 and 3, by mixing the first, second and third lithium compounds of the present invention at a specific ratio to obtain a positive electrode active material, a high temperature of 60 ° C. It can be seen that the capacity maintenance rate of the lithium ion secondary battery can be 50% or more even after the holding. This capacity retention ratio indicates that the internal impedance value is still low even after the lithium ion secondary battery of the present invention is kept at a high temperature, and that the rapid charging characteristics are greatly improved compared to the conventional battery. Show. In this case, the mixing ratio of the first, second, and third lithium compounds constituting the positive electrode active material is 90% by weight or less for the first lithium compound, 50% by weight or less for the second lithium compound, 3 lithium compound is 50 weight% or less, and all do not include the case of 0%.

(Examples 199 to 210, Comparative Examples 191 to 195)
While the mixing ratio of the first, second, and third lithium compounds was fixed, the compositions of the second and third lithium compounds were also fixed. Further, the Mn substitution element of the first lithium compound is also fixed to either Co (cobalt) or Mg (magnesium), and only the substitution ratio with respect to Mn is changed to produce a lithium ion secondary battery. Was evaluated how it changed. As a comparative example, the case where Mn was not substituted at all was designated as Comparative Example 191. The evaluation results are shown in Table 9 as Examples 199 to 210 and Comparative Examples 191 to 195. In Table 9, the mixing ratios of the first, second, and third lithium compounds are 70 wt%, 10 wt%, and 20 wt%, respectively, and the respective compositions are LiMn _2-x (Co, Mg) _x O _4. (X is a substitution amount by Co or Mg), LiFe _0.9 Ni _0.1 PO ₄ , LiNi _0.9 Mn _0.1 O ₂ . The negative electrode active material of each battery is graphite. The number of evaluated lithium ion secondary batteries in each of the examples and comparative examples is 10 each, and the capacity retention ratio values shown in Table 9 are average values. In addition, the description of each item of Table 9 is the same as the said Table 1-Table 4.

  According to Table 9, when the substitution amount of Mn of the first lithium compound is 0.3 or less, the capacity retention rate of the lithium ion secondary battery is 50% or more, which is good quick charge characteristics as compared with the conventional battery. It can be seen that However, such an effect does not occur when the substitution amount of Mn is 0 (zero). In addition, when the substitution amount of Mn is in the above range, it can be seen that a good capacity retention ratio can be obtained regardless of whether the substitution element is Co or Mg.

(Examples 211 to 214, Comparative Example 196)
As in Table 9, the mixing ratio of the first, second, and third lithium compounds and the composition of the second and third lithium compounds are fixed, and only the Mn substitution element of the first lithium compound is changed. A lithium ion secondary battery was produced by changing the capacity retention rate. There were four types of substitution elements of Mn, Ni, B (boron), Al, and Li, and a battery was produced when substitution with Ba (barium) was performed as a comparative example. The amount of Mn substitution by each substitution element was also fixed at 0.1. These evaluation results are shown in Table 10 as Examples 211 to 214 and Comparative Example 196. The composition of the first lithium compound in Table 10 is LiMn _1.9 Ma _0.1 O ₄ (Ma is one of Ni, B, Al, Li, and Ba). All other conditions such as the evaluation number of the lithium ion secondary batteries of the examples and comparative examples are the same as those of the examples and comparative examples shown in Table 9.

According to the result of Table 10, when Mn of the first lithium compound is replaced by 0.1 by any element of Ni, B, Al, Li, as in the case of Co or Mg, The capacity retention rate of the lithium ion secondary battery is 50% or more. Therefore, it can be seen that good quick charge characteristics can be obtained as compared with the conventional battery. However, a predetermined effect could not be obtained when the element was replaced by Ba which is an element outside the scope of the present invention. This result is related to the fact that the ion radius of ionized Ba (Ba ²⁺ ) is larger than twice the ion radius of Mn ³⁺ to be substituted, and that Mn substitution elements have certain restrictions. Is shown.

(Examples 215 to 287, Comparative Examples 197 to 270)
As in Tables 9 and 10, the mixing ratio of the first, second, and third lithium compounds and the composition of the second and third lithium compounds were fixed. And while substituting Mn of the 1st lithium compound with several types of elements, the substitution ratio was changed, the lithium ion secondary battery was produced, and the change of a capacity | capacitance maintenance factor was evaluated. Substitution elements for Mn were Co, Mg, and Al, and Mn was substituted with two or three kinds of elements selected from these. These evaluation results are shown in Tables 11 to 13 as Examples 215 to 287 and Comparative Examples 197 to 270. In Tables 11 to 13, the composition of the first lithium compound is LiMn _2-x (Co, Mg, Al) _x O ₄ (x is the total amount of substitution by Co, Mg, Al). Other conditions such as the evaluation number of the lithium ion secondary batteries of the examples and comparative examples are all the same as those of the examples and comparative examples shown in Tables 9 and 10 above.

  In Tables 11-13, evaluation is performed for the case where Mn of the first lithium compound is substituted by each combination of Co and Al, Co and Mg, and all elements of Co, Al, and Mg. According to Tables 11-13, in any combination of these, when the amount of substitution is 0.3 or less, the capacity maintenance rate of the lithium ion secondary battery is 50% or more. It can be seen that good quick charge characteristics can be obtained. However, as shown in Comparative Example 191 of Table 9, such an effect does not occur when the amount of Mn substitution is 0 (zero).

(Examples 288 to 295, Comparative Examples 271 to 275)
While the mixing ratio of the first, second and third lithium compounds was fixed, the composition of the first and third lithium compounds was fixed this time. And only the substitution element of Fe of the 2nd lithium compound was changed, the lithium ion secondary battery was produced, and the change of the capacity | capacitance maintenance factor was evaluated. As a comparative example, the case where Fe was not substituted at all was designated as Comparative Example 271. The evaluation results are shown in Table 14 as Examples 288 to 295 and Comparative Examples 271 to 275. In Table 14, the mixing ratios of the first, second, and third lithium compounds are 70 wt%, 10 wt%, and 20 wt%, respectively, and the compositions are LiMn _1.95 Al _0.05 O ₄ , LiFe _{1-x, respectively.} Mb _x PO ₄ (Mb is any of Mn, Mg, Ni, Co, and x is the amount of substitution thereof), LiNi _0.9 Mn _0.1 O ₂ . The negative electrode active material of each battery is graphite. The number of evaluations of the lithium ion secondary batteries of each example and comparative example is 10 each, and the capacity retention rate values shown in Table 14 are average values thereof. In addition, the description of each item of Table 14 is the same as the said Table 1-Table 4.

  According to Table 14, when the amount of substitution of Fe in the second lithium compound is 0.2 or less, the capacity retention rate of the lithium ion secondary battery is 50% or more, which is good quick charge characteristics as compared with the conventional battery. It can be seen that However, such an effect does not occur when the substitution amount of Fe is 0 (zero). In addition, when the amount of substitution of Fe is within the above range, it can be seen that a good capacity retention ratio can be obtained regardless of whether the substitution element is Mn or Mg. Further, according to Examples 294 and 295, it can be seen that a good capacity retention rate is obtained even when Ni and Co are selected as the substitution elements for Fe.

(Examples 296 to 303, Comparative Examples 276 to 280)
While the mixing ratio of the first, second, and third lithium compounds was fixed, the compositions of the first and second lithium compounds were also fixed. Furthermore, the substitution element of Ni in the third lithium compound is also fixed to either Mn or Al, and only the substitution ratio with respect to Ni is changed to produce a lithium ion secondary battery, and how the capacity maintenance ratio changes. It was evaluated. As a comparative example, the case where Ni was not substituted at all was designated as comparative example 276. The evaluation results are shown in Table 15 as Examples 296 to 303 and Comparative Examples 276 to 280. In Table 15, the mixing ratios of the first, second, and third lithium compounds are 70 wt%, 10 wt%, and 20 wt%, respectively, and the respective compositions are LiMn _1.95 Li _0.05 O ₄ , LiFe _0.9 Mn _0.1 PO ₄ , LiNi _1-x (Mn, Al) _x O ₂ (x is a substitution amount by Mn or Al). The negative electrode active material of each battery is graphite. The number of evaluations of the lithium ion secondary batteries of each example and comparative example is 10 each, and the capacity retention rate values shown in Table 15 are average values. In addition, the description of each item of Table 15 is the same as the said Table 1-Table 4.

  According to Table 15, when the amount of substitution of Ni in the third lithium compound is 0.3 or less, the capacity retention rate of the lithium ion secondary battery is 50% or more, which is good quick charge characteristics as compared with the conventional battery. It can be seen that However, such an effect does not occur when the substitution amount of Ni is 0 (zero). It can also be seen that when the amount of substitution of Ni is in the above range, a good capacity retention ratio can be obtained regardless of whether the substitution element is Mn or Al.

(Examples 304 to 306)
As in the case of Table 15, the mixing ratio of the first, second and third lithium compounds and the composition of the second and third lithium compounds are fixed, and only the Ni substitution element of the third lithium compound is obtained. A lithium ion secondary battery was produced by changing the capacity retention rate. There are three types of substitution elements for Ni: Mg, Co, and Li. Further, the amount of substitution of Ni by each substitution element was also fixed at 0.1. These evaluation results are shown in Table 16 as Examples 304 to 306. The composition of the third lithium compound in Table 16 is LiNi _0.9 Mc _0.1 O ₂ (Mc is any one of Mg, Co, and Li). All other conditions such as the evaluation number of the lithium ion secondary batteries of the examples and comparative examples are the same as those of the examples and comparative examples shown in Table 15 above.

  According to the results of Table 16, when Ni in the third lithium compound is replaced by 0.1 with any element of Mg, Co, and Li, lithium ions are obtained as in the case of Mn or Al. The capacity maintenance rate of the secondary battery is 50% or more. Therefore, it can be seen that good quick charge characteristics can be obtained as compared with the conventional battery.

(Examples 307 to 379, Comparative Examples 281 to 354)
As in Tables 15 and 16, the mixing ratio of the first, second, and third lithium compounds and the composition of the first and second lithium compounds were fixed. Then, while replacing Ni of the third lithium compound with a plurality of kinds of elements, lithium ion secondary batteries were produced by changing the substitution ratio, and the change in capacity retention rate was evaluated. Ni substitution elements were Al, Co, and Mn, and Ni was substituted with two or three kinds of elements selected from these. These evaluation results are shown in Tables 17 to 19 as Examples 307 to 379 and Comparative Examples 281 to 354. In Tables 17 to 19, the composition of the third lithium compound is LiNi _1-x (Al, Co, Mn) _x O ₂ (x is the total amount of substitution by Al, Co, Mn). Other conditions such as the evaluation number of the lithium ion secondary batteries of the examples and comparative examples are all the same as those of the examples and comparative examples shown in Tables 15 and 16 above.

  In Tables 17-19, evaluation is performed for the case where Ni of the third lithium compound is substituted by each combination of Al and Co, Al and Mn, and all elements of Co, Al, and Mn. According to Tables 17-19, in any combination of these, when the amount of substitution is 0.3 or less, the capacity retention rate of the lithium ion secondary battery is 50% or more, It can be seen that good quick charge characteristics can be obtained. However, as shown in Comparative Example 276 in Table 15, such an effect does not occur when the amount of Ni substitution is 0 (zero).

The series of evaluations described in Tables 1 to 19 confirmed the following. In order to stabilize the crystal structure, a first lithium compound in which a part of Mn having a composition of LiMn ₂ O ₄ is replaced with another element, and a part of Fe having a composition of LiFePO ₄ is replaced with another element. The third lithium compound obtained by substituting a part of Ni having a composition of LiNiO ₂ with another element was mixed at a specific ratio to form a positive electrode active material, and these were used. Lithium ion secondary batteries were produced respectively. The lithium ion secondary battery produced in this manner was able to maintain a sufficiently large capacity maintenance rate even after holding at a high temperature of 60 ° C. for 90 days.

  From the above, it is considered that the lithium ion secondary battery according to the present invention can maintain sufficient rapid charging characteristics even after long-term use by the user. Therefore, according to this invention, the lithium ion secondary battery provided with the high reliability for a user's actual use can be provided. Further, the above description is for explaining the effect in the case of the embodiment of the present invention, and is not intended to limit the invention described in the claims or to reduce the scope of the claims. Absent. Moreover, each part structure of this invention is not restricted to the said embodiment, A various deformation | transformation is possible within the technical scope as described in a claim.

Sectional drawing of the example of the lithium ion secondary battery of this invention. Each composition of the Examples and Comparative Examples in Tables 1 to 4 is represented as a ternary diagram. Each composition of Examples and Comparative Examples in Tables 5 to 8 is represented as a ternary diagram.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Positive electrode active material layer 12 Negative electrode active material layer 13 Positive electrode collector 14 Negative electrode collector 15 Separator 16, 17 Exterior laminate 18 Positive electrode tab 19 Negative electrode tab

Claims (4)

The chemical formula is an oxide having a crystal structure of LiMn ₂ O ₄ , and the molar ratio of Li, Mn and additive element Ma contained in the oxide is
Li: Mn: Ma = 1: 2-x: x (0 <x <2, Ma is one or more selected from Li, B, Mg, Al, Co, Ni)
A first lithium compound having a composition of
The chemical formula is an oxide having a crystal structure of LiFePO ₄ , and the molar ratio of Li, Fe and additive element Mb contained in the oxide is
Li: Fe: Mb = 1: 1-y: y (0 <y <1, Mb is one or more selected from Mn, Mg, Ni, Co)
A second lithium compound of the composition
The chemical formula is an oxide having a crystal structure of LiNiO ₂ , and the molar ratio of Li, Ni and additive element Mc contained in the oxide is
Li: Ni: Mc = 1: 1-z: z (0 <z <1, Mc is one or more selected from Li, Mg, Mn, Al, Co)
A positive electrode for a lithium ion secondary battery, comprising a positive electrode active material comprising a third lithium compound having a composition of X is 0 <x ≦ 0.3,
Y is 0 <y ≦ 0.2;
The positive electrode for a lithium ion secondary battery according to claim 1, wherein z is 0 <z ≦ 0.3.   The ratios of the contents of the first lithium compound, the second lithium compound, and the third lithium compound contained in the positive electrode active material are respectively a: b: c (a, b, c are weight ratios, respectively) , A + b + c = 100), 0 <a ≦ 90, 0 <b ≦ 50, 0 <c ≦ 50, The positive electrode for a lithium ion secondary battery according to claim 1 or 2, .   A lithium ion secondary battery comprising the positive electrode for a lithium ion secondary battery according to any one of claims 1 to 3.

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