JP2011076997A - Lithium-ion secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery capable of rapid charging. <P>SOLUTION: A positive electrode active material has a first lithium compound having a spinel structure represented by chemical formula (1): Li<SB>x1</SB>Mn<SB>2-y1</SB>M1<SB>y1</SB>O<SB>4</SB>(1) (M1 is at least one element selected from among Li, Mg, B, Al, V, Cr, Fe, Co, Ni, W., 0<y1≤0.3, and 0≤x1≤1) and a second lithium compound having a layer structure represented by chemical formula (2): Li<SB>x2</SB>Ni<SB>1-y2</SB>M2<SB>y2</SB>O<SB>2</SB>(2) (M2 is at least one element selected from among Li, Mg, B, Al, Ti, V, C, Mn., 0<y2≤0.7, and 0≤x2≤1). When a content proportion of the first lithium compound to a content proportion of the second lithium compound is denoted by a:b (a, b are respective mass proportions, and a+b=100), 8≤a≤92, 8≤b≤92, and pH of a supernatant solution of the positive electrode active material layer 11 is 7.0 or more and 11.9 or less. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery.

  With the development of technology and demand for mobile devices, the demand for lithium-ion secondary batteries as an energy source has increased rapidly. Recently, as a power source for electric vehicles (EV), hybrid electric vehicles (HEV), etc. The use of lithium ion secondary batteries has become a reality. As a result, many studies have been conducted on lithium ion secondary batteries that can meet various requirements, and in particular, there is a great demand for lithium ion secondary batteries having rapid charge / discharge characteristics.

  The lithium ion secondary battery has a configuration in which a positive electrode and a negative electrode are opposed to each other via a separator, and the positive electrode and the negative electrode are each composed of a positive electrode current collector and a positive electrode active material, and a negative electrode current collector and a negative electrode active material. 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 material that occludes and releases lithium ions, such as a carbon material, is used.

On the other hand, with regard to the positive electrode active material used for the positive electrode of the lithium ion secondary battery, since the operating voltage exceeds 4 V, lithium cobalt oxide (LiCoO ₂ ) has been energetically studied. The adoption of ₂ was the mainstream. LiCoO ₂ is widely used as a positive electrode active material in today's lithium ion secondary batteries because it exhibits good characteristics in terms of total performance such as potential flatness, capacity, discharge potential, and cycle characteristics.

However, since Co is unevenly distributed on the earth and is a scarce resource, there are problems that the cost is high and stable supply is difficult. In addition, since LiCoO ₂ has a layered rock salt structure (α-NaFeO ₂ structure), an oxygen layer having a large electronegativity is adjacent due to lithium detachment during charging. For this reason, it is necessary to limit the amount of lithium extracted during actual use, and if too much lithium is extracted, such as in an overcharged state, the structure will change due to electrostatic repulsion between oxygen layers and heat will be generated. There is much room for improvement. In order to ensure the safety of the battery, a large protection circuit is required outside, and therefore a positive electrode material with higher safety is required.

Therefore, as a resource to replace Co, a positive electrode active material that is abundant on the earth and is based on Ni, Mn, or Fe, such as lithium iron phosphate (LiFePO ₄ ), lithium nickelate (LiNiO ₂ ), A positive electrode material using a lithium-containing transition metal oxide based on lithium manganate (LiMn ₂ O ₄ ) or the like has been proposed and put into practical use.

LiFePO ₄ has an olivine structure and is fixed by covalently bonding oxygen with an element other than iron, so that it does not release oxygen even at high temperatures, and LiCoO ₂ , LiNiO ₂ , LiMn ₂ O It can be inferred that the safety of the battery can be increased compared with the active material such as ₄ . However, it has been pointed out that LiFePO ₄ has a remarkably low electrical conductivity compared to LiNiO ₂ and LiMn ₂ O ₄ and has a rapid charge / discharge characteristic.

LiNiO ₂ has a large capacity per unit mass of 274 mAhg ⁻¹ , is attractive as a battery active material, and is the most expected material for practical use as a power source for electric vehicles. However, LiNiO ₂ has a large capacity deterioration during high-temperature storage like LiMn ₂ O _4, and Ni is dissolved in the electrolytic solution. Therefore, there remains a problem that stability and charge / discharge cycle characteristics are not sufficient. Furthermore, Non-Patent Document 1 reports that LiNiO ₂ deteriorates when exposed to a humidified atmosphere.

Since LiMn ₂ O ₄ has a positive spinel structure and has a space group Fd3m, it has a high potential equivalent to 4V class LiCoO ₂ with respect to the lithium electrode, is easily synthesized, and has a high battery capacity. Therefore, it has attracted attention as a very promising material and has been put into practical use. However, LiMn ₂ O ₄ is an excellent material as described above, but the capacity deterioration during storage at high temperature is large and Mn is dissolved in the electrolytic solution. Therefore, there remains a problem that the charge / discharge cycle characteristics are not sufficient. It was. In addition, a problem that the charge / discharge cycle characteristics deteriorate due to Jahn-Teller distortion of Mn ³⁺ is also pointed out.

Along with this, various methods have been studied to improve the cycle characteristics of lithium ion secondary batteries using LiMn ₂ O ₄ as the positive electrode. For example, cycle characteristics can be improved by improving the reactivity during synthesis, cycle characteristics can be improved by controlling the particle size, and cycle characteristics can be improved by removing impurities. Improvement has not been achieved.

As described above, various approaches have been attempted to improve the cycle characteristics of LiMn ₂ O ₄ , but the cycle characteristics comparable to the Co-based Co, which is currently the mainstream, especially the deterioration mechanism is promoted under high temperature use environment. For this reason, further ingenuity is required to realize cycle characteristics at high temperature use. In particular, considering the expansion of future application fields such as electric vehicles, securing cycle characteristics at high temperatures is becoming increasingly important.

  In Patent Document 1, since a mixture of a lithium / manganese composite oxide and a lithium / nickel composite oxide is used as the positive electrode active material, the properties of the lithium / manganese composite oxide and the lithium / nickel composite oxide are mutually different. The volume change accompanying charging / discharging as the whole positive electrode active material can be reduced. As a result, it is described that the positive electrode active material is hardly subjected to stress due to volume change and the contact with the conductive agent is kept good, so that the cycle life is improved.

  In Patent Document 2, in a lithium secondary battery including a positive electrode active material layer mainly composed of a lithium transition metal composite oxide, the lithium transition metal composite oxide includes a lithium-nickel-manganese-M composite oxide and By using a mixture of lithium-manganese spinel composite oxide, a high-rate positive electrode for a non-aqueous electrolyte secondary battery that can be charged / discharged at a high rate, has a high capacity, and has excellent charge / discharge cycle durability Obtaining materials is described.

JP-A-8-45498 JP 2002-1003008 A

J. et al. Electrochem. Soc. , 144, 4226 (1997)

  However, it is not sufficient for obtaining a lithium ion secondary battery excellent in rapid charge / discharge characteristics, and further improvement is required.

  When a lithium ion secondary battery is used as a driving power source for an electric vehicle or the like, as a request from the user side, firstly, there is a great demand for a lithium ion secondary battery having a rapid charge / discharge characteristic that shortens a charging time. 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 the charging time of a lithium ion secondary battery and 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, it is known that the use of a lithium compound having a composition of LiMn ₂ O ₄ or LiNiO ₂ is suitable as a positive electrode active material of a lithium ion secondary battery. However, when LiMn ₂ O ₄ is used as the positive electrode active material, the average valence of the Mn (manganese) ion changes between trivalent and tetravalent before and after charge and discharge, and is therefore referred to as Jahn-Teller strain. Crystal distortion is generated in the crystal. Therefore, when rapid charging is repeated, structural destruction may occur in the positive electrode of the lithium ion secondary battery.

Further, 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 a supporting salt, it is known that a reaction of the following chemical formula occurs.

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

Thus, when H ⁺ is generated in the electrolyte solution, Mn of LiMn ₂ O ₄ of the positive electrode active material is ionized and dissolved in the electrolyte solution by a 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 ²⁺ is 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.

Experiments have shown that the above degradation mechanism reduces Mn ³⁺ that can contribute to redox and reduces the capacity retention rate of the battery. 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.

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 fact that the insertion / extraction of Li ions to / from the substance is inhibited. In addition, the positive electrode active material triggered by elution and precipitation of Mn ions, the deactivation of the negative electrode active material, the influence of the acid generated by the water and LiPF ₆ slightly contained in the electrolyte solution, and the eluted Mn ions It is thought that this is also caused by the deterioration of the electrolyte solution due to the above.

  If the internal impedance of the battery increases for these reasons, the initial discharge capacity and the discharge capacity after holding, that is, the capacity retention rate of the battery will decrease. This is because if the battery is charged when the internal impedance of the battery is increased, the cell voltage immediately reaches the upper limit voltage according to Ohm's law V = IR, and the battery cannot be fully charged.

  The present invention solves the above-mentioned problems in lithium ion secondary batteries, and the internal impedance at the time of rapid charging does not increase greatly even after long-term use, and therefore the degree of battery capacity reduction at 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.

  That is, the technical problem of the present invention is to provide a lithium ion secondary battery that can be rapidly charged.

In the lithium ion secondary battery of the present invention, the positive electrode active material has the chemical formula (1)
Li _x1 Mn _2-y1 M1 _y1 O ₄ (1)
(M1 represents at least one element selected from Li, Mg, B, Al, V, Cr, Fe, Co, Ni, and W, 0 <y1 ≦ 0.3. X1 represents that Li is 0 ≦ a first lithium compound having a spinel structure represented by the following formula (2):
Li _x2 Ni _1-y2 M2 _y2 O ₂ (2)
(M2 represents at least one element selected from Li, Mg, B, Al, Ti, V, Co, and Mn, 0 <y2 ≦ 0.7. X2 represents that Li is 0 ≦ x2 ≦ 1. The second lithium compound having a layered structure represented by the following formula: and the ratio of the content of the first lithium compound and the second lithium compound respectively. a: b (where a and b are mass ratios and a + b = 100, respectively) 8 ≦ a ≦ 92 and 8 ≦ b ≦ 92, and the pH of the supernatant of the positive electrode active material measured by the pH measurement method Is 7.0 or more and 11.9 or less.

  The lithium ion secondary battery of the present invention is characterized in that the M2 of the second lithium compound is at least one element selected from Li, Mn, and Co.

  In the lithium ion secondary battery of the present invention, when the content ratio of the first lithium compound and the second lithium compound is a: b (a and b are mass ratios, respectively, a + b = 100), 15 ≦ a ≦ 85 and 15 ≦ b ≦ 85.

  The lithium ion secondary battery of the present invention is characterized in that the pH measured by the pH measurement method is 8.0 or more and 10.0 or less.

The positive electrode for a lithium ion secondary battery of the present invention has a positive electrode active material obtained by mixing first and second lithium compounds having different compositions. The first and second lithium compounds are represented by Li _x1 Mn _2-y1 M1 _y1 O ₄ and Li _x2 Ni _1-y2 M2 _y2 O ₂ , respectively, and a composition in which Mn and Ni are partially substituted with specific elements It is a compound of this. By mixing these two kinds of compounds at a specific ratio and adjusting the pH as the positive electrode active material, the lithium-ion secondary battery using the compound has an internal impedance during rapid charging even after long-term use. Can be suppressed. 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.

  According to the present invention, it is possible to provide a lithium ion secondary battery that can be rapidly charged.

Sectional drawing of the example of the lithium ion secondary battery of this invention.

  An embodiment of the present invention will be described.

A positive electrode of a lithium ion secondary battery according to the present invention is characterized in that a positive electrode active material containing a first lithium compound and a second lithium compound having different compositions is used as a positive electrode for a lithium ion secondary battery. Is. Here, the first and second lithium compounds are represented by chemical formulas Li _x1 Mn _2-y1 M1 _y1 O ₄ and Li _x2 Ni _1-y2 M2 _y2 O ₂ , respectively. In this lithium compound, a part of Ni is substituted with another element. By mixing the lithium compounds of these two compositions at a ratio in a specific range and further using the pH of the supernatant of the positive electrode active material measured by the pH measurement method in a specific range, a special effect as a positive electrode active material is obtained. Is obtained.

In the present invention, in the first and second lithium compounds used as 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 Jahn-Teller distortion generated during insertion and extraction of Li ions are alleviated, and changes in crystal structure due to the distortions are reduced, so that the internal impedance at the time of rapid charging of the lithium ion secondary battery can be reduced. Reduction is realized. Specifically, for Li _x1 Mn _2-y1 M1 _y1 O ₄ which is the first lithium compound, one kind selected from Li, Mg, B, Al, V, Cr, Fe, Co, Ni, and W The above elements are added to replace Mn.

  In order to fully exhibit this effect, it is better that the amount of the substitution element added is large. However, when the addition amount y1 of the substitution element of Mn exceeds 0.3, occlusion of Li ions by the first lithium compound, Release capability may be limited. Therefore, the amount of element added is preferably greater than 0 and less than or equal to 0.3 with respect to M1 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 substitution element to the 1st lithium compound, Li added here substitutes Mn, and thinks that it will enter the same site as Mn in crystal structure. It is done. Therefore, the position of the substitution site and the effect of addition are different from the element (Li) in the Li site in the structure of LiMn ₂ O ₄ before Mn substitution.

  X1 in the first lithium compound indicates that Li can be desorbed and inserted in the range of 0 ≦ x1 ≦ 1.

On the other hand, in the present invention, Li _x2 Ni _1-y2 M2 _y2 O ₂ which is the second lithium compound is also Li, Mg, B, Al, Ti, V, Co, as in the case of the first lithium compound. Ni is substituted by adding one or more elements selected from Mn. This substitution relaxes the 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 better that the amount of the substitution element added is large. However, when the amount of the substitution element of Ni exceeds 0.7, the ability to occlude and release Li ions is sufficient. May be restricted. Therefore, the amount of element added is preferably greater than 0 and less than or equal to 0.7 with respect to Ni 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.

  X2 in the second lithium compound indicates that Li can be desorbed and inserted in the range of 0 ≦ x2 ≦ 1.

In the present invention, the first and second lithium compounds in which some of the elements are replaced with other elements by the above method are further mixed at specific ratios, respectively, so that the battery can be rapidly charged after long-term use. Suppressing the increase in internal impedance of The effects of mixing these two types of lithium compounds into a positive electrode active material are considered as follows. First, in the present invention, by mixing the second lithium compound with the first lithium compound, a slight amount of water contained in the electrolyte solution reacts with the second lithium compound, and H ₂ is reacted according to the following chemical formula. It is presumed that the effect of capturing H ⁺ in O and releasing OH ⁻ occurs.

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, mixing of the first lithium compound and the second lithium compound suppresses elution of Mn ions from LiMn ₂ O ₄ because LiNiO ₂ captures a slight amount of water in the electrolyte solution. An effect is produced. All of the above effects appear as a reduction in internal impedance during rapid charging.

The reaction presumed that LiNiO ₂ captures H ⁺ in H ₂ O and liberates OH ⁻ is related to the pH of the reaction field. In order to promote the reaction, the pH is 7.0 or more, The range is preferably 11.9 or less. This pH value is a value obtained by the method described in the pH measurement method described later. This pH was obtained experimentally by accumulating a number of different samples. The pH should be 7.0 or higher, but when the pH exceeds 11.9, the slurry made of the active material, polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP) increases the viscosity. Experimentation revealed that gelation occurred and it was difficult to stably coat the current collector. The pH varies depending not only on the active material composition, but also on the surface impurities and the manufacturing method, but when the pH is in the range of 7.0 or more and 11.9 or less, a special effect is obtained. It is. And when pH is the range of 8.0 or more and 10.0 or less, a more preferable effect can be acquired.

  In the pH measurement method, the positive electrode active material was washed with diethyl carbonate, the supporting salt contained in the positive electrode active material was removed, and then vacuum dried at 100 ° C. for 24 hours. It is immersed in distilled water having a pH of 7.0 or more and 8.0 or less distilled three times, the positive electrode active material is stirred, and the pH of the supernatant after 12 hours is measured.

  Here, the ratio of the first and second lithium compounds effective for reducing the internal impedance of the battery is such that the mixing ratio is a: b (a and b are mass ratios and a + b = 100, respectively) It is preferable to set the ranges of 8 ≦ a ≦ 92 and 8 ≦ b ≦ 92. Particularly preferably, 15 ≦ a ≦ 85 and 15 ≦ b ≦ 85. This mixing ratio is experimentally determined by accumulating evaluations of a large number of samples having different compositions and ranges as well as the types of elements added to the first and second lithium compounds and the ranges of the preferred addition amounts thereof. It is obtained.

  FIG. 1 is a cross-sectional view of an example of a lithium ion secondary battery of the present invention. As an example of a lithium ion secondary battery having a positive electrode provided with a positive electrode active material, the shape of a single-plate laminated battery cell is shown as a cross-sectional view. In FIG. 1, the positive electrode region includes a positive electrode active material 11 and a positive electrode current collector 13, and the negative electrode region similarly includes a negative electrode active material 12 and a negative electrode current collector 14. Here, the positive electrode active material 11 and the negative electrode active material 12 face each other with a separator 15 interposed therebetween. The positive electrode current collector 13 and the negative electrode current collector 14 are generally made of metal foil, and the positive electrode active material 11 and the negative electrode active material 12 are applied and solidified on one side of each. The ends of the positive electrode current collector 13 and the negative electrode current collector 14 are drawn out of the battery cell as a positive electrode tab 18 and a negative electrode tab 19, respectively, and the battery cell is sealed from above and below by exterior laminates 16 and 17. ing. 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 is not limited to the single plate laminate type, but may be a coin type, a laminate pack type, a square cell, a cylindrical cell, or the like.

  In general, a lithium ion secondary battery has a positive electrode using a lithium compound as a positive electrode active material, and a negative electrode including a negative electrode active material capable of occluding and releasing lithium ions. Non-conductive separators and electrolyte regions are provided to prevent electrical connection. 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 Li _x1 Mn _2-y1 M1 _y1 O ₄ , and Mn is an element M1 (M1 is Li, Mg, B, Al, V, Cr, Fe, Co, Ni, A first lithium compound partially substituted by one or more selected from W) and a crystal structure of Li _x2 Ni _1-y2 M2 _y2 O ₂ , wherein Ni is an element M2 (M2 is Li, Mg) , B, Al, Ti, V, Co, or Mn), and a mixture of the second lithium compound having a composition partially substituted with the second active compound 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 ₃ 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. 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 CO ₃ groups and OH groups contained in the reaction product are volatilized in the form of CO ₂ and H ₂ O during 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 , MnSO _{4 and the} like can be used. . Further, Li ₂ CO ₃ , LiOH, or the like can be used as a starting material for adding Li element to the Mn site. Similarly, Mg (OH) ₂ or the like can be used as a starting material for adding Mg element. B ₂ O ₃ or the like can be used as a starting material for the B element. As a starting material for adding Al element, Al (OH) ₃ or the like can be used. As a starting material for the V element, VO ₂ or the like can be used. As a starting material for the Cr element, CrO ₃ or the like can be used. The starting materials for the Fe _{element, 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. As a starting material for the 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. WO ₃ or the like can be used as a starting material for the W 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 500 ° C. or higher in order to diffuse each element. However, if the firing temperature is too high, oxygen vacancies may occur in the composition. May occur and the powder state cannot be maintained. When such a phenomenon occurs, when the produced lithium compound is used as a positive electrode active material of a battery, various adverse effects may occur on various characteristics of the battery. For this reason, the firing temperature has an upper limit of 1200 ° C., and it is more preferable to perform firing in the range of 500 ° C. to 900 ° C. Further, in order to prevent oxygen deficiency in the generated first lithium compound, baking in an oxygen atmosphere is more preferable.

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. When the specific surface area is larger than 10 m ² / g, a larger amount of the binder for forming the positive electrode active material layer is required, which is disadvantageous in terms of the capacity density of the electrode. On the other hand, if the specific surface area is smaller than 0.01 m ² / g, the ionic conductivity between the electrolyte solution and the positive electrode active material may be lowered. 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 the average particle size is larger than 70 μm, unevenness in the positive electrode active material layer may be produced in 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 smaller than 0.1 μm, the binding property of the formed positive electrode active material layer to the positive electrode current collector may be lowered.

Furthermore, in the production of the second lithium compound, it is preferable to use the following starting materials. First, Li ₂ CO ₃ , LiOH, Li ₂ O, LiNO ₃ 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 second lithium compound. Li ₂ SO ₄ or the like can be used. Further, NiO, Ni (OH) ₂ , NiSO ₄ , Ni (NO ₃ ) ₂ or the like can be used as a starting material for adding Ni element. Further, Mg (OH) ₂ or the like can be used as a starting material for adding Mg element to the Ni site. B ₂ O ₃ or the like can be used as a starting material for the B element. As a starting material for adding Al element, Al (OH) ₃ or the like can be used. Ti (OH) ₂ or the like can be used as a starting material for adding the Ti element. As a starting material for the V element, VO ₂ or the like can be used. As a starting material for adding Co element, Co (OH) ₃ , Co ₂ O ₃ , Co ₃ O ₄ , CoSO _{4 and the} like can be used. 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 , MnSO _{4 and 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 second lithium compound powder. The firing temperature at this time is preferably 500 ° C. or higher in order to diffuse each element. However, if the firing temperature is too high, there is a risk that the powder particles may be aggregated and the powder state cannot be maintained. There is. For this reason, the firing temperature has an upper limit of 1200 ° C., 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 second 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. When the specific surface area is larger than 10 m ² / g, a larger amount of the binder for forming the positive electrode active material layer is required, which is disadvantageous in terms of the capacity density of the electrode. On the other hand, if the specific surface area is smaller than 0.01 m ² / g, the ionic conductivity between the electrolyte solution and the positive electrode active material may be lowered. 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 larger than 70 μm, uneven unevenness 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 smaller than 0.1 μm, the binding property of the formed positive electrode active material layer to the positive electrode current collector may be lowered.

Whether the produced powdery first and second lithium compounds each have a predetermined crystal structure can be evaluated by powder X-ray diffraction. Each powdery lithium compound is set in a powder X-ray diffractometer, and the result obtained by measuring the diffraction angle and intensity of the diffracted light obtained by irradiating characteristic X-rays is obtained as an ICDD Cards (International Center for Diffraction Data). The crystal structure is identified by querying Cards: powder X-ray diffraction pattern database card). In the case of the present invention, the diffraction patterns of the obtained powdery first and second lithium compounds are expressed by the crystal structures of Li _x1 Mn _2-y1 M1 _y1 O ₄ and Li _x2 Ni _1-y2 M2 _y2 O _2. It is possible to specify the crystal structure and the crystallization rate of the produced compound by comparing the diffraction pattern with each of the above and measuring the diffraction intensity.

  The powdered first and second 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, thereby forming a positive electrode. The method is as follows. First, the powdery first and second lithium compounds are mixed at a predetermined ratio 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 to form a film. Dry and 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 mass with respect to the total amount of the positive electrode active material excluding the organic solvent, the conductivity-imparting material, and the binder, The preferable addition amount is similarly 1 to 10% by mass with respect to the total amount. If the addition amount of the conductivity-imparting material and the binder mixed here is less than 0.5% by mass and 1% by mass, respectively, the electrical conductivity in the formed positive electrode active material layer becomes small, and thereby the battery There is a risk that the charge / discharge rate characteristics (speed for charging and discharging a certain amount of charge) may be reduced, and the problem of electrode peeling may occur. On the contrary, if the addition amount of the conductivity-imparting material and the binder is larger than 30% by mass and 10% by mass, respectively, the content ratio of the positive electrode active material becomes small. May decrease, and the charge capacity per unit mass of the battery may be reduced. A preferable content ratio of the positive electrode active material is 70% by mass or more and 98.5% by mass or less, and more preferably 85% by mass or more and 97% by mass 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 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 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 Each of the 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-diethoxyethane (DEE) ), Chain ethers such as ethoxymethoxyethane (EME), cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, acetamide, dimethylphenol Rumamide, 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, you may use what added the polymer etc. with respect to the aprotic organic solvent, and solidified the electrolyte solution in the gel form. Further, 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 carboxylates, 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 higher than 1.5 mol / l, it is considered that the density and viscosity of the electrolyte solution are increased and the movement of Li ions is hindered. Conversely, if the electrolyte concentration is lower than 0.5 mol / l, the electrical conductivity of the electrolyte solution may be reduced.

  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, the positive electrode active material and the negative electrode active material are formed on the surfaces of the positive electrode current collector and the negative electrode current collector, respectively, as a positive electrode and a negative electrode, and a separator is interposed between the two. To form an electrode body. By sealing this electrode body together with an electrolyte solution inside a film structure or the like 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 rolled to form a rolled body, which is also stored in a battery can in a dry air or inert gas atmosphere, filled with an electrolyte solution, and sealed to form a cylindrical or rectangular 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.

  Examples of the present invention are described in detail below.

  Various compositions and mixing ratios of lithium ion secondary batteries can be obtained by changing the composition of each of the first and second lithium compounds to be mixed as the positive electrode active material and the mixing ratio between the first and second lithium compounds. Each of them 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 and second 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 LiOH and Mg (OH) ₂ , B ₂ O ₃ , Al (OH) ₃ , VO ₂ are used as starting materials for the elements added to the Mn site as necessary. , CrO ₃ , FeSO ₄ , Co ₃ O ₄ , and Ni (OH) ₂ were used and weighed so that the composition ratio of the first lithium compound was the target. 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 the total composition produced, 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 (the mass of the powder material having the crystal structure of LiMn ₂ O ₄ Ratio).

(Production of second lithium compound)
LiOH and Ni (OH) ₂ are used as starting materials, and LiOH and Mg (OH) ₂ , B ₂ O ₃ , Al (OH) ₃ , Ti () are optionally used as starting materials for elements added to the Ni site. OH) ₂ , VO ₂ , Co (OH) ₃ , MnO ₂ (EMD), Co (OH) ₃ , Al (OH) ₃ , MnO ₂ , Mg (OH) ₂ The lithium compound was weighed so as to have a composition ratio of 2 lithium compounds. 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 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 having the total composition, 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 powder materials of the first and second lithium compounds, which are the positive electrode active material materials obtained in the above steps, and the conductivity-imparting material are dispersed and kneaded in a solution in which a binder is dissolved in an organic solvent. A slurry was formed. 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 mass 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. In this way, 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, thereby obtaining a laminate. At that time, the thickness of the negative electrode active material 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 a 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 the negative electrode of 1 sheet used for one battery.

(Preparation of electrolyte solution)
The electrolyte solution used for the lithium ion secondary battery is a mixture of propylene carbonate (PC), ethylene carbonate (PC), and ethyl methyl carbonate (EMC) as an organic solvent in a volume ratio of 5:25:70. In this, LiPF ₆ was dissolved and used as a supporting salt. 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 produced by the above method were placed so that the positive and negative electrode active materials face each other, and were placed in a battery cell made of a laminate cell with a separator between them. 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 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 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 to a lower limit voltage of 3.0 V with a constant current of 8 mA, and the discharge capacity at that time was measured. This is defined as the discharge capacity after holding. When the ratio of the discharge capacity value after holding to the initial discharge capacity in each battery is measured and the composition and mixing ratio of the first and second lithium compounds in the positive electrode active material are changed, the ratio of the discharge capacity Each of the values was evaluated.

Here, as described above, the initial charge capacity of each battery is set to be a constant value of 2.0 mAh / cm ² , and the size of the positive electrode for capturing and releasing Li ions is set for each battery. Both are 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 reaching the upper limit voltage, the charging is switched to constant voltage charging. When charging for a total of 15 minutes, the charging is performed at a constant current and a constant voltage of 10 C for 15 minutes. . 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 C for 15 minutes.

  Lithium ion secondary batteries in which the compositions and mixing ratios of the first and second lithium compounds in the positive electrode active material were changed under the above production conditions were 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.

(Evaluation criteria)
In Tables 1 to 7 shown below, the case where the capacity retention rate in each of the examples and comparative examples is 50% or more is determined as good (◯ determination), and the case where it is less than 50% is determined as poor (× determination). 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 for the lithium ion secondary battery of the present invention with the evaluation method described in JIS C8711, the ambient temperature during measurement of the battery storage period (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 at 10 C for 15 minutes 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 of the present invention is set to be a stricter standard than the general method defined in the Japanese Industrial Standards is that lithium ion This is because the secondary battery demands 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 superior 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.

(PH measurement method)
After measuring the capacity retention rate, the lithium ion secondary battery was disassembled, and the positive electrode plate was washed with diethyl carbonate to remove the supporting salt contained in the positive electrode active material. Thereafter, the positive electrode active material was vacuum dried at 100 ° C. for 24 hours. After drying, it was immersed in distilled water having a pH of 7 to 8 that was distilled three times, 20 times the electrode mass, the positive electrode active material was stirred, and the pH of the supernatant after 12 hours was measured.

(Examples 1-16, Comparative Examples 1-8)
The composition of the first and second 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 a battery is produced by changing the mixing ratio of each lithium compound. And Examples 1 to 16 and Comparative Examples 1 to 8, respectively. The compositions of the first and second lithium compounds are LiMn _1.95 Mg _0.05 O ₄ and LiNi _0.95 Co _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 Table 1, the mixing ratio of the first and second lithium compounds is shown as a percentage (mass ratio, unit%), and the total of the two is 100%. The capacity retention rate (unit%) is an average value of the measured values of the 10 batteries. Moreover, as a determination, according to the above-described determination criteria, whether it was good (◯) or defective (×) was indicated by a symbol. The pH after the measurement was 3.0 V vs. It is the value which measured the electrode discharged to Li / Li + by the method as described in pH measuring method.

  Table 1 shows the first lithium compound (mass%), the second lithium compound (mass%), the capacity retention rate (%), the pH after determination and measurement in Examples 1 to 16 and Comparative Examples 1 to 8. .

From the evaluation results of Table 1, in Comparative Examples 1 to 8 in which the first lithium compound LiMn _1.95 Mg _0.05 O ₄ and the second lithium compound LiNi _0.95 Co _0.05 O ₂ are mixed, The capacity retention rate of the lithium ion secondary battery after holding at a high temperature of 60 ° C. was lower than that of Examples 1-16. This is presumably because Mn of LiMn ₂ O ₄ was eluted into the electrolyte during storage at high temperature and deposited on the negative electrode, thereby increasing the cell resistance. On the other hand, by mixing the first and second lithium compounds of the present invention at a specific ratio to obtain a positive electrode active material, the capacity maintenance rate of the lithium ion secondary battery is 50 even after holding at a high temperature of 60 ° C. It was found that it is possible to make it more than%. This capacity retention ratio is 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 the rapid charge characteristics are greatly improved as compared with the conventional lithium ion secondary battery. It shows that. Further, in this case, the mixing ratio constituting the positive electrode active material is a + b = 100, 8 ≦ a ≦ 92, where the mass ratio of the contents of the first lithium compound and the second lithium compound is a and b, respectively. It was confirmed that the capacity retention rate was good when 8 ≦ b ≦ 92.

(Examples 17 to 28, Comparative Examples 9 to 13)
While the mixing ratio of the first and second lithium compounds was fixed, the composition of the second lithium compound was also fixed. Further, the substitution element of Mn of the first lithium compound is also fixed to either Li (lithium) or Al (aluminum), and a lithium ion secondary battery is manufactured by changing only the substitution ratio with respect to Mn. Was evaluated how it changed. As a comparative example, the case where Mn was not substituted at all was designated as Comparative Example 9. The mixing ratio of the first and second lithium compounds is 90% by mass and 10% by mass, respectively, and each composition is Li _x1 Mn _2-y1 M1 _y1 O ₄ (M1 is Li or Al, y1 is substitution by M1) Amount), LiNi _0.8 Co _0.2 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 the comparative examples is 10 each, and the capacity retention rate is an average value thereof.

  Table 1 shows the first lithium compounds, capacity retention rate (%), pH after determination and measurement in Examples 17 to 28 and Comparative Examples 9 to 13.

  From the evaluation results of Table 2, when the amount of Mn substitution 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 faster than the conventional battery. It was found that charging characteristics can be obtained. However, such an effect did not occur when the substitution amount of Mn was 0 (zero). Moreover, when the substitution amount of Mn was the said range, it turned out that a favorable capacity | capacitance maintenance factor is obtained even if a substitution element is any of Li and Al.

(Examples 29-38, Comparative Examples 14-17)
Similarly to the case of Table 2, the mixing ratio of the first and second lithium compounds and the composition of the second lithium compound are fixed, and only the Mn substitution element of the first lithium compound is changed to change the lithium ion secondary battery. And the change in capacity retention rate was evaluated. The substitution element of Mn was 10 types of Li, Mg, B, Al, V, Cr, Fe, Co, Ni, and W. As a comparative example, a battery was produced when substitution with Ba, La, and Ce was performed. The amount of Mn substitution by each substitution element was also fixed at 0.1. The mixing ratio of the first and second lithium compounds is 90% by mass and 10% by mass, respectively, and each composition is Li _x1 Mn _2-y1 M1 _y1 O ₄ (M1 is Li, Mg, B, Al, V) , Cr, Fe, Co, Ni, W and Ba, La, Ce, and y1 are M1 substitution amounts), and LiNi _0.8 Co _0.2 O ₂ . 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 2.

  Table 1 shows the first lithium compounds, capacity retention rate (%), pH after determination and measurement in Examples 29 to 38 and Comparative Examples 14 to 17.

According to the evaluation results in Table 3, when Mn of the first lithium compound is replaced by 0.1 by any element of Li, Mg, B, Al, V, Cr, Fe, Co, Ni, and W, As in the case of Li or Al in Table 2, the capacity retention rate of the lithium ion secondary battery was 50% or more. Therefore, it was found 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, La, or Ce, 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⁺ ), La (La ³⁺ ), and Ce (Ce ⁴⁺ ) is larger than twice the ion radius of Mn ³⁺ to be substituted. This shows that there are certain restrictions on the substitution elements.

(Examples 39 to 51, Comparative Example 18)
As in Tables 2 and 3, the mixing ratio of the first and second lithium compounds and the composition of the second lithium compound were fixed. And while substituting Mn of the 1st lithium compound with multiple types of elements, the substitution ratio was changed, the lithium ion secondary battery was produced, and the change of the capacity maintenance rate was evaluated. Substitution elements of Mn are Li, Mg, B, Al, V, Cr, Fe, Co, Ni, and W. Mn was substituted by two or three kinds of elements selected from these. The mixing ratio of the first and second lithium compounds is 90% by mass and 10% by mass, respectively, and the composition of the first lithium compound is Li _x1 Mn _2-y1 M1 _y1 O ₄ (M1 is Li, Mg, B, At least one element selected from Al, V, Cr, Fe, Co, Ni, and W, y1 is the total amount of substitution by M1), and the composition of the second lithium compound is LiNi _0.8 Mn _0.2 O ₂ . The 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 2 and 3.

  Table 1 shows the first lithium compounds, capacity retention ratio (%), pH after determination and measurement in Examples 39 to 51 and Comparative Example 18.

  In Table 4, the case where Mn of the 1st lithium compound was substituted by each combination of Li, Mg, B, Al, V, Cr, Fe, Co, Ni, and W element was evaluated, respectively. In any combination, it was found that the capacity retention rate of the lithium ion secondary battery was 50% or more, and good quick charge characteristics were obtained as compared with the conventional battery. However, as shown in Comparative Example 18, such an effect did not occur when the substitution amount of Mn was 0 (zero).

(Examples 52 to 75, Comparative Examples 19 to 21)
While the mixing ratio of the first and second lithium compounds was fixed, the composition of the first lithium compound was also fixed. Furthermore, the substitution element of Ni in the second lithium compound is also fixed to either Co or Mn, 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. Evaluated what to do. As a comparative example, the case where Ni was not substituted at all was designated as Comparative Example 19. The mixing ratios of the first and second lithium compounds are 80% by mass and 20% by mass, respectively, and the respective compositions are LiMn _1.95 Li _0.05 O ₄ , Li _x2 Ni _1-y2 M2 _y2 O ₂ ( M2 is Co or Mn, and y2 is the amount of substitution by M2. The negative electrode active material of each battery is graphite. The number of evaluated lithium ion secondary batteries in each of the examples and the comparative examples is 10 each, and the capacity retention rate is an average value thereof.

  Table 5 shows the second lithium compounds, capacity retention rate (%), pH after determination and measurement in Examples 52 to 75 and Comparative Examples 19 to 21.

  According to the evaluation results in Table 5, when the amount of Ni substitution of the second lithium compound is 0.7 or less, the capacity maintenance rate of the lithium ion secondary battery is 50% or more, which is better than the conventional battery. It was found that quick charge characteristics can be obtained. However, such an effect did not occur when the substitution amount of Ni was 0 (zero). It was also found that when the amount of substitution of Ni is in the above range, a good capacity retention rate can be obtained regardless of whether the substitution element is Co or Mn.

(Examples 76 to 83, Comparative Example 22)
Similarly to the case of Table 5, the mixing ratio of the first and second lithium compounds and the composition of the first lithium compound are fixed, and only the Ni substituting element of the second lithium compound is changed to change the lithium ion secondary battery. And the change in capacity retention rate was evaluated. There were eight types of substitution elements for Ni: Li, Mg, B, Al, Ti, V, Co, and Mn. Further, the amount of substitution of Ni by each substitution element was also fixed at 0.3. The mixing ratios of the first and second lithium compounds are 80% by mass and 20% by mass, respectively, and the compositions are LiMn _1.75 Li _0.1 Al _0.1 B _0.05 O ₄ and Li _x2 Ni, respectively. _1-y2 M2 _y2 O ₂ (M2 is one element selected from Li, Mg, B, Al, Ti, V, Co, and Mn, and Ba and y2 are substitution amounts by M2). 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 5.

  Table 6 shows the second lithium compounds, capacity retention rate (%), pH after determination and measurement in Examples 76 to 83 and Comparative Example 22.

According to the evaluation result of Table 6, when Ni of the second lithium compound was replaced by 0.3 by any element of Li, Mg, B, Al, Ti, V, Co, and Mn, As in the case of Co or Mn, the capacity retention rate of the lithium ion secondary battery was 50% or more. Therefore, it was found 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 Ni ³⁺ to be substituted, and that there are certain restrictions on the Ni substitution element. It is shown.

(Examples 84 to 93, Comparative Example 23)
As in Table 6, the mixing ratio of the first and second lithium compounds and the composition of the first lithium compound were fixed. And while substituting Ni of a 2nd 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. The substitution elements for Ni are Li, Mg, B, Al, Ti, V, Co, and Mn. Ni was substituted with two or three kinds of elements selected from these. The mixing ratios of the first and second lithium compounds are 80% by mass and 20% by mass, respectively, and the compositions are LiMn _1.75 Li _0.1 Al _0.1 B _0.05 O ₄ and Li _x2 Ni, respectively. _1-y2 M2 _y2 O ₂ (M2 is at least one element selected from Li, Mg, B, Al, Ti, V, Co, and Mn, and y2 is the total amount of substitution by M2). 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 5 and 6.

  Table 7 shows the second lithium compounds, capacity retention rate (%), pH after determination and measurement in Examples 84 to 93 and Comparative Example 23.

  In Table 7, the case where Ni of the second lithium compound was replaced by each combination of elements of Li, Mg, B, Al, Ti, V, Co, and Mn was evaluated. According to the evaluation results in Table 7, the capacity retention rate of the lithium ion secondary battery is 50% or more in any combination of these, and good quick charge characteristics are obtained as compared with the conventional battery. I found out that However, as shown in Comparative Example 23, such an effect did not occur when the substitution amount of Ni was 0 (zero).

The series of evaluations described in Tables 1 to 7 confirmed the following. In order to stabilize the crystal structure, a first lithium compound, Li _x2 Ni _1-y2 M2 _y2 O, in which a part of Mn having a composition of Li _x1 Mn _2-y1 M1 _y1 O ₄ is substituted with another element. _{The two} lithium compounds obtained by substituting a part of Ni having the composition ₂ with other elements are mixed at a specific ratio to form a positive electrode active material, and a lithium ion secondary battery is produced using each of these. However, the lithium ion secondary battery produced in this way was able to maintain a sufficiently large capacity retention 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. That is, it was found that the present invention can provide a lithium ion secondary battery that can be rapidly charged.

  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.

DESCRIPTION OF SYMBOLS 11 Positive electrode active material 12 Negative electrode active material 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 positive electrode active material has the chemical formula (1)
Li _x1 Mn _2-y1 M1 _y1 O ₄ (1)
(M1 represents at least one element selected from Li, Mg, B, Al, V, Cr, Fe, Co, Ni, and W, 0 <y1 ≦ 0.3. X1 represents that Li is 0 ≦ a first lithium compound having a spinel structure represented by the following formula (2):
Li _x2 Ni _1-y2 M2 _y2 O ₂ (2)
(M2 represents at least one element selected from Li, Mg, B, Al, Ti, V, Co, and Mn, 0 <y2 ≦ 0.7. X2 represents that Li is 0 ≦ x2 ≦ 1. The second lithium compound having a layered structure represented by the following formula: and the ratio of the content of the first lithium compound and the second lithium compound respectively. a: b (where a and b are mass ratios and a + b = 100, respectively) 8 ≦ a ≦ 92 and 8 ≦ b ≦ 92, and the pH of the supernatant of the positive electrode active material measured by the pH measurement method The lithium ion secondary battery is characterized in that is 7.0 or more and 11.9 or less.   The lithium ion secondary battery according to claim 1, wherein the M2 of the second lithium compound is at least one element selected from Li, Mn, and Co.   When the content ratios of the first lithium compound and the second lithium compound are a: b (a and b are mass ratios, a + b = 100, respectively), 15 ≦ a ≦ 85, 15 ≦ b ≦ The lithium ion secondary battery according to claim 1, wherein the lithium ion secondary battery is 85.   The lithium ion secondary battery according to any one of claims 1 to 3, wherein the pH measured by the pH measurement method is 8.0 or more and 10.0 or less.

Images