JP5359490B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

Conventionally, as a lithium compound used for the positive electrode of a lithium ion secondary battery, a lithium nickel composite oxide, a lithium cobalt composite oxide, or the like is used. In recent years, lithium iron composite oxides (such as LiFePO ₄ ) that have abundant resources and are inexpensive have attracted attention as lithium compounds that can replace cobalt and nickel, which have scarce resources. Since this lithium iron composite oxide tends to be inferior in cycle durability to those containing cobalt, nickel and the like, various improvements have been continued.

  For example, Patent Document 1 proposes using a mixture of lithium iron composite oxide and lithium nickelate in a predetermined ratio as a positive electrode active material and using a graphitized mesophase carbon as a negative electrode active material. Yes. Lithium nickelate is said to have low chemical stability, but with such a battery configuration, cycle durability and chemical stability are good.

JP 2007-317534 A

  However, in the lithium ion secondary battery described in Patent Document 1, although the chemical stability and the cycle durability are excellent, the lithium ion secondary battery is not yet sufficient and has a higher chemical stability and cycle durability. It was desired. It has also been desired to have higher low temperature characteristics.

  The present invention has been made to solve such problems, and has a positive electrode active material including a lithium iron composite oxide and a negative electrode active material including graphite, and has chemical stability and cycle durability. The main object is to provide a lithium ion secondary battery that can be further improved and the low temperature characteristics can be further improved.

  In order to achieve the above-described object, the present inventors include, in a lithium ion secondary battery, a lithium iron composite oxide having an olivine structure and a lithium nickel composite oxide having a layered rock salt structure at a predetermined weight ratio. By having a positive electrode having a positive electrode active material and a negative electrode containing a negative electrode active material having amorphous carbon-coated graphite, cycle durability, chemical stability, and low temperature characteristics can be improved. The inventors have found what can be done and have completed the present invention.

That is, the lithium ion secondary battery of the present invention is
A ratio of the lithium nickel composite oxide to the total weight of the lithium iron composite oxide and the lithium nickel composite oxide, comprising a lithium iron composite oxide having an olivine structure and a lithium nickel composite oxide having a layered rock salt structure A positive electrode having a positive electrode active material of 5 wt% or more and less than 40 wt%;
A negative electrode comprising a negative electrode active material having amorphous carbon-coated graphite;
An ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
It is equipped with.

  In this lithium ion secondary battery, chemical stability, cycle durability, and low temperature characteristics can be improved. The reason why such an effect can be obtained is not clear, but by combining lithium nickel composite oxide with good cycle durability and lithium iron composite oxide with good chemical stability in an appropriate ratio, It is considered that stability and cycle durability can be improved. In addition, in lithium iron composite oxide, local lithium occlusion / release occurs, which may cause non-uniformity of lithium in the positive electrode and reduce chemical stability and cycle durability. This can be mitigated by combining with things. Moreover, since this non-uniformity of lithium tends to become more prominent at low temperatures, it is considered that the low temperature characteristics are also improved by adding lithium nickel composite oxide in a suitable range. Further, by combining with a negative electrode active material containing amorphous carbon-coated graphite, it is considered that the capacity balance between the positive electrode and the negative electrode is improved, and chemical stability, cycle durability, and low-temperature characteristics are improved.

1 is a schematic diagram showing an example of a lithium ion secondary battery 10.

  The lithium ion secondary battery of the present invention includes a lithium iron composite oxide having an olivine structure and a lithium nickel composite oxide having a layered rock salt structure, and is based on the total weight of the lithium iron composite oxide and the lithium nickel composite oxide. A positive electrode having a positive electrode active material in which the proportion of the lithium nickel composite oxide is 5% by weight or more and less than 40% by weight, a negative electrode containing a negative electrode active material having amorphous carbon-coated graphite, and an intermediate between the positive electrode and the negative electrode And an ion conductive medium that conducts lithium ions.

  In the lithium ion secondary battery of the present invention, the positive electrode is, for example, a mixture of a positive electrode active material, a conductive additive, and a binder and an appropriate solvent added to form a slurry-like positive electrode mixture. It may be formed by applying and drying on the surface and compressing to increase the electrode density as necessary. The positive electrode includes a lithium iron composite oxide having an olivine structure and a lithium nickel composite oxide having a layered rock salt structure, and the lithium nickel composite oxide with respect to the total weight of the lithium iron composite oxide and the lithium nickel composite oxide. A positive electrode active material having a ratio of 5 wt% or more and less than 40 wt% is included. The ratio of the lithium nickel composite oxide to the total weight of the lithium iron composite oxide and the lithium nickel composite oxide may be 5% by weight or more and less than 40% by weight. Among these, it is preferable that it is 10 to 35 weight%, and it is more preferable that it is 25 to 35 weight%. If it is 5% by weight or more, the effect of lithium nickel composite oxide having good cycle durability can be obtained, and local lithium occlusion and release that may occur in lithium iron composite oxide can be further suppressed. it is conceivable that. Moreover, if it is less than 40 weight%, lithium nickel complex oxide is not excessive, and it is thought that higher chemical stability is obtained. Moreover, if it is 10 weight% or more and 35 weight% or less, since a low-temperature characteristic becomes favorable, it is preferable, since cycling durability becomes more favorable if it is 25 weight% or more and 35 weight% or less. In addition, for example, it is possible to use a large amount of lithium iron composite oxide with an emphasis on cost, or suitable for high-speed charging using a large amount of lithium nickel composite oxide with high lithium diffusibility and good electron conductivity. It is also good. As described above, the ratio of the lithium nickel composite oxide can be appropriately selected according to the required characteristics as long as it is in the range of 5 wt% or more and less than 40 wt%.

The lithium iron composite oxide having an olivine structure, for example, those represented by the general formula _{Li a Fe 1-b M1 b} PO 4. Here, M1 is not particularly limited, but is preferably at least one selected from Mn, Mg, Ni, Co, Cu, Zn, Ge, Cr, V, Mo, and Ti. Among these, it is more preferable that Mn, Mg, Ni, and Co are included. Further, a preferably satisfies 0.5 ≦ a ≦ 1.2, b preferably satisfies 0 ≦ b <1.0, and 0.01 ≦ b ≦ 0.3. More preferably. If a ≧ 0.5, the amount of lithium to be occluded on the negative electrode side at the first charge is sufficient, and if a ≦ 1.2, the amount of lithium does not become excessive, so It is because it is thought that a short circuit can be suppressed. The lithium iron composite oxide having an olivine structure is not limited to that specified by the basic formula described above. For example, it may be LiFePO ₄ , Li, P, or M1 may be partially substituted with other elements, and not only the stoichiometric composition but also some elements may be deficient or excessive. It may be of a non-stoichiometric composition.

Examples of the lithium nickel composite oxide having a layered rock salt structure include those represented by the general formula LiNi _x M2 _1-x O ₂ . Here, M2 is not particularly limited, but is preferably at least one selected from Mg, Co, Mn, and Al. Further, x preferably satisfies 0.4 <x <0.95. In particular, it is represented by the general formula LiNi _x M2 _1-xy Al _y O ₂ , x satisfies 0.5 <x <0.95, and y satisfies 0.001 <y <0.2. Is preferred. This is because, when a part of Ni is substituted with Al in this way, the layered rock salt structure is stabilized, and it is considered that cycle durability and thermal stability are improved. The lithium nickel composite oxide having a layered rock salt structure is not limited to that specified by the basic formula described above. For example, it may be LiNiO ₂ , Li, Ni, M2 may be partially substituted with other elements, and not only the stoichiometric composition but also some elements may be deficient or excessive. It may be of a non-stoichiometric composition.

In the lithium ion secondary battery of the present invention, the negative electrode is, for example, a mixture of a negative electrode active material, a conductive additive, and a binder and an appropriate solvent added to form a slurry-like negative electrode mixture. It may be formed by applying and drying on the surface and compressing to increase the electrode density as necessary. The negative electrode includes a negative electrode active material having amorphous carbon-coated graphite. Amorphous carbon-coated graphite refers to a material in which all or part of its surface is coated with an amorphous carbon material having low crystallinity, with graphite having high crystallinity as a nucleus. In general, when a graphite-based material is used as the negative electrode, the charge amount in the initial charge / discharge cycle includes a capacity that cannot be taken out during discharge, which is called irreversible capacity (retention), but it is reversible in the subsequent charge / discharge cycle. Has excellent cycle durability. In addition, it has low and flat discharge characteristics. On the other hand, graphite has an active crystallite end face that is easily oriented on the crystal surface and tends to react with the electrolyte. Therefore, it is considered that the cycle durability is good and the chemical stability is better by covering the surface of graphite with amorphous carbon that hardly reacts with the electrolytic solution. Furthermore, by covering the surface of the graphite with amorphous carbon, it is possible to suppress an excessive specific surface area of the graphite involved in the battery reaction, reduce the irreversible capacity, and increase the capacity of the positive electrode and the negative electrode. It is considered that the balance can be improved. The graphite as the core is preferably natural graphite from the viewpoint of cost. Examples of such natural graphite include scaly graphite and scaly graphite. Natural graphite may be used after being subjected to mechanical shape control. For example, the graphite may be used by taking the corners of the graphite particles, rounding them into a spherical shape, or pulverizing them. It is considered that such shape control can suppress the selective orientation of flake graphite and the like generated in the pressing process, and can suppress the inhibition of lithium ion insertion / extraction. Thereby, it is considered that the subsequent decrease in charge / discharge capacity due to the increase in initial retention can be suppressed. Further, when the specific surface area of the particles is reduced, the bulk density can be prevented from becoming too low. The raw material and production method of amorphous carbon covering graphite are not particularly limited, but coal-based or petroleum-based tar, pitch, asphalt (hereinafter also referred to as heavy oil), core, After thoroughly mixing and stirring with the graphite, the unnecessary heavy oil component is removed, dried and fired to produce a coating on the graphite surface. In addition, baking may be performed at a temperature at which the heavy oil becomes amorphous, for example, 600 ° C. to 2000 ° C., or after the hard oil layer that has been coated is subjected to non-graphitization treatment, It may be fired at a high temperature. Examples of the non-graphitizing treatment include a method in which heat treatment is performed at a low temperature (for example, 50 ° C. to 400 ° C.) in an oxidizing gas atmosphere such as oxygen, ozone, carbon dioxide, and ionic oxide. This amorphous carbon preferably covers the entire surface of the graphite uniformly. This is because it can be considered that a material with less variation can be obtained and the capacity of the negative electrode can be controlled more appropriately. The thickness of the amorphous carbon coating is not particularly limited, but is preferably 1/2 or less, more preferably 1/4 or less of the particle diameter of the graphite to be coated. The amorphous carbon-coated graphite has an average particle size (D50%) of preferably 1 μm or more and 100 μm or less, more preferably 10 μm or more and 50 μm or less, and further preferably 15 μm or more and 25 μm or less. The true specific gravity is preferably 1.5 to 2.3, and more preferably 2.0 to 2.3. Further, the specific surface area measured by the BET method is preferably 0.1 m ² / g or more and 10 m ² / g or less, and more preferably 0.5 m ² / g or more and 2.0 m ² / g or less.

  In the lithium ion secondary battery of the present invention, the conductive additive contained in the positive electrode and the negative electrode is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, natural graphite (scale graphite, 1 or 2 types of graphite such as (flaky graphite) and artificial graphite, acetylene black, carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) What mixed the above can be used. Among these, carbon black and acetylene black are preferable as the conductive aid from the viewpoints of electron conductivity and coatability. The binder plays a role of holding the active material particles and the conductive auxiliary particles, and for example, a fluorine-containing resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine rubber, or polypropylene. , Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. Examples of the solvent for dispersing the active material, conductive additive, and binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropyl. Organic solvents such as amine, ethylene oxide, and tetrahydrofuran can be used. Moreover, a dispersant, a thickener, or the like may be added to water, and the active material may be slurried with a latex such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Current collectors include aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, and aluminum, copper, etc. for the purpose of improving adhesion, conductivity, and oxidation resistance. A surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

In the lithium ion secondary battery of the present invention, the ion conductive medium may be a nonaqueous electrolytic solution or ionic liquid in which a supporting salt is dissolved in an organic solvent, a gel electrolyte, a solid electrolyte, or the like. Of these, a non-aqueous electrolyte is preferable. Examples of the supporting salt include known LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ , Li (CF ₃ SO ₂ ) ₂ N, Li (CF ₃ SO ₃ ), LiN (C ₂ F ₅ SO ₂ ), and the like. Supporting salts can be used. The concentration of the supporting salt is preferably 0.1 to 2.0M, and more preferably 0.8 to 1.2M. Examples of the organic solvent include conventional secondary batteries such as ethylene carbonate (EC), ethyl methyl carbonate (EMC), propylene carbonate (PC), γ-butyrolactone (GBL), diethyl carbonate (DEC), and dimethyl carbonate (DMC). And organic solvents used in capacitors. These may be used alone or in combination. The ionic liquid is not particularly limited, and 1-methyl-3-propylimidazolium bis (trifluorosulfonyl) imide, 1-ethyl-3-butylimidazolium tetrafluoroborate, or the like may be used. it can. The gel electrolyte is not particularly limited. For example, a polymer such as polyvinylidene fluoride, polyethylene glycol, or polyacrylonitrile, or a saccharide such as an amino acid derivative or sorbitol derivative is added with an electrolyte containing a supporting salt. And a gel electrolyte. Examples of the solid electrolyte include inorganic solid electrolytes and organic solid electrolytes. Well-known inorganic solid electrolytes include, for example, Li nitrides, halides, oxyacid salts, and the like. Among _{them, Li 4 SiO 4, Li 4} SiO 4 -LiI-LiOH, xLi 3 PO 4 - (1-x) Li 4 SiO 4, Li 2 SiS 3, Li 3 PO 4 -Li 2 S-SiS 2, sulfide Examples thereof include phosphorus compounds. These may be used alone or in combination. Examples of the organic solid electrolyte include polyethylene oxide, polypropylene oxide, polyvinyl alcohol, polyvinylidene fluoride, polyphosphazene, polyethylene sulfide, polyhexafluoropropylene, and derivatives thereof. These may be used alone or in combination.

  The lithium ion secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the range of use of the secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a microporous film of an olefin resin such as polyethylene or polypropylene Is mentioned. These may be used alone or in combination.

  The shape of the lithium ion secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc. FIG. 1 is a schematic view showing an example of a lithium ion secondary battery 10 of the present invention. The lithium ion secondary battery 10 includes a positive electrode sheet 13 in which a positive electrode active material 12 is formed on a current collector 11, a negative electrode sheet 18 in which a negative electrode active material 17 is formed on the surface of the current collector 14, a positive electrode sheet 13 and a negative electrode A separator 19 provided between the sheet 18 and a nonaqueous electrolytic solution 20 that fills between the positive electrode sheet 13 and the negative electrode sheet 18 are provided. In this lithium ion secondary battery 10, a separator 19 is sandwiched between a positive electrode sheet 13 and a negative electrode sheet 18, and these are wound and inserted into a cylindrical case 22, and a positive electrode terminal 24 and a negative electrode sheet connected to the positive electrode sheet 13. And a negative electrode terminal 26 connected to each other.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the lithium ion secondary battery of this invention concretely is demonstrated as an Example.

[Example 1]
The positive electrode was produced as follows. First, as a positive electrode active material, a lithium iron composite oxide (LiFePO ₄ ) having an olivine structure and a lithium nickel composite oxide (Li _1.05 Ni _0.75 Co _0.15 Al _0.05 Mg _0.05 O ₂ ) at a weight ratio of 95: 5. A mixture was used. The positive electrode active material, carbon black (TB5500 manufactured by Tokai Carbon Co., Ltd.) as a conductive additive, and polyvinylidene fluoride (KF polymer manufactured by Kureha Chemical Industry Co., Ltd.) as a binder were used in a ratio of 78.5: 13.8: 11.7 A positive electrode mixture was prepared by mixing at a weight ratio. The positive electrode mixture thus prepared is dispersed in N-methyl-2-pyrrolidone to form a paste, coated on both sides of an aluminum foil having a thickness of 20 μm, dried, and roll pressed to form a positive electrode of 54 mm × 450 mm. A sheet was obtained. The negative electrode was produced as follows. Amorphous carbon-coated graphite (OMAC-2 manufactured by Osaka Gas Chemical) as a negative electrode active material and polyvinylidene fluoride as a binder were mixed at a weight ratio of 95: 5 to obtain a negative electrode mixture. The prepared negative electrode mixture was dispersed in N-methyl-2-pyrrolidone to form a paste, coated on both sides of a 10 μm thick copper foil, dried, and roll pressed to obtain a 56 mm × 500 mm negative electrode sheet. . The positive electrode sheet and the negative electrode sheet thus produced were wound with a 25 μm thick polyethylene separator interposed therebetween, and a roll-shaped electrode body was produced. This was inserted into a 18650 type cylindrical case, and adjusted by adding 1M LiPF ₆ to a solution (made by Kishida Pharmaceutical Co., Ltd.) in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 3: 7. The non-aqueous electrolyte solution was impregnated and sealed to prepare a cylindrical battery. Thus, the battery of Example 1 was obtained. The amorphous carbon-coated graphite used in the experiment had an average particle diameter (D50) of 21.2 μm, a true specific gravity of 2.16, and a specific surface area measured by the BET method of 1.03 m ² / g.

[Examples 2 to 7]
A battery of Example 2 was obtained in the same manner as Example 1 except that the weight ratio of the lithium iron composite oxide to the lithium nickel composite oxide was 90:10. Further, the battery of Example 3 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide to the lithium nickel composite oxide was 85:15. Further, the battery of Example 4 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide to the lithium nickel composite oxide was 80:20. Further, the battery of Example 5 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide to the lithium nickel composite oxide was 75:25. Further, the battery of Example 6 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide to the lithium nickel composite oxide was set to 70:30. Further, the battery of Example 7 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide to the lithium nickel composite oxide was 65:35.

[Comparative Examples 1 to 7]
A battery of Comparative Example 1 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide and the lithium nickel composite oxide was 100: 0. Further, a battery of Comparative Example 2 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide and the lithium nickel composite oxide was 98: 2. Further, a battery of Comparative Example 3 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide and the lithium nickel composite oxide was 60:40. Further, a battery of Comparative Example 4 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide and the lithium nickel composite oxide was 50:50. Further, a battery of Comparative Example 5 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide and the lithium nickel composite oxide was 40:60. Further, a battery of Comparative Example 6 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide and the lithium nickel composite oxide was 20:80. Further, a battery of Comparative Example 7 was obtained in the same manner as in Example 1 except that the weight ratio of the lithium iron composite oxide and the lithium nickel composite oxide was 0: 100.

[Evaluation of cycle durability]
Using the batteries of Examples 1 to 7 and Comparative Examples 1 to 7, the battery was fully charged up to 4.1 V at a constant current of 2.0 mA / cm ² under a temperature condition of 60 ° C. Charging / discharging which discharges to 2.5 V with a constant current of density 2.0 mA / cm ² is defined as one cycle, and this cycle is performed for a total of 500 cycles. At this time, the discharge capacity at the first cycle and the discharge capacity at the 500th cycle were measured, and the capacity retention ratio W _HF (%) represented by (discharge capacity at the 500th cycle / discharge capacity at the first cycle) × 100 was calculated. Asked. Using the batteries of Examples 1 to 7 and Comparative Examples 1 to 7 that were produced, partial charging was performed to 3.6 V at a constant current of 2.0 mA / cm ² under a temperature condition of 60 ° C., and then Charging / discharging for discharging to 2.5 V at a constant current of a current density of 2.0 mA / cm ^{2 was} defined as one cycle, and this cycle was performed for a total of 500 cycles. At this time, the discharge capacity at the first cycle and the discharge capacity at the 500th cycle were measured, and the capacity maintenance ratio W _HP (%) represented by (discharge capacity at the 500th cycle / discharge capacity at the first cycle) × 100 was calculated. Asked. Further, using the batteries of Examples 1 to 7 and Comparative Examples 1 to 7, the battery was fully charged up to 4.2 V at a constant current of 5.0 mA / cm ² under a temperature condition of 0 ° C., and then The charging / discharging which discharges to 2.5V with the constant current of 5.0 mA / cm < ² > of current density was made into 1 cycle, and this cycle was performed 200 times in total. At this time, the discharge capacity at the first cycle and the discharge capacity at the 200th cycle were measured, and the capacity maintenance ratio W _LF (%) represented by (discharge capacity at the 200th cycle / discharge capacity at the first cycle) × 100 was obtained. Asked.

[Evaluation of low temperature characteristics]
The initial low temperature output was determined as follows. Using the batteries of Examples 1 to 7 and Comparative Examples 1 to 7, after charging to 50% of the battery capacity (SOC = 50%), under a temperature condition of −30 ° C., 0.1 A, 0.5 A, 1.0 A, 2.0 A, 3.0 A, and 5.0 A currents were passed to measure the battery voltage after 10 seconds. The applied current value and voltage value were extrapolated by linear approximation, and the current value at which the battery voltage after 10 seconds would be 2.5 V was obtained. Then, the initial low temperature output which is an output value at −30 ° C. was obtained by multiplying the obtained current value by 2.5V. Moreover, the low temperature output after a high temperature cycle was calculated | required similarly to the initial low temperature output using the battery used for the charge / discharge cycle test (full charge / discharge) under high temperature.

[Evaluation of chemical stability]
The initial overcharge test was conducted as follows. Using the batteries of Examples 1 to 7 and Comparative Examples 1 to 7, charging was performed at a constant current of ² mA / cm ^{2 at} a current density of 50 ° C. This charging was continued for 2.5 hours, and the presence or absence of ignition was confirmed. Moreover, the overcharge test after low-temperature endurance was done like the initial overcharge test using the battery used for the above-mentioned low-temperature charge / discharge cycle test.

[Experimental result]
Table 1 summarizes the experimental results of Examples 1-7 and Comparative Examples 1-7. Table 1 shows initial low temperature output, low temperature output after high temperature durability, capacity maintenance rate in high temperature full charge / discharge cycle, capacity maintenance rate in high temperature partial charge / discharge cycle, capacity maintenance rate in low temperature full charge / discharge cycle, The test results of the initial overcharge test and the overcharge test after low temperature durability were shown. When full charge / discharge was repeated at 60 ° C., the capacity retention ratio became better as the proportion of the lithium nickel composite oxide increased. In addition, when partial charge / discharge was repeated at 60 ° C., the capacity retention ratio became better as the proportion of the lithium nickel composite oxide increased. On the other hand, when full charge / discharge is repeated at 0 ° C., the ratio of the lithium nickel composite oxide is within a range of 50 wt% or less (Examples 1 to 7 and Comparative Examples 1 to 4). The capacity retention rate improved as the amount increased, but when exceeded (Comparative Examples 5 to 7), the capacity retention rate decreased. Of these, the lithium nickel composite oxide ratio was in the range of 5 wt% to 60 wt%, and the capacity retention rate was 40% or more, which was favorable. From the above, the capacity retention rate may be good regardless of the low temperature and high temperature in the range of 5% by weight to 60% by weight of the lithium nickel composite oxide (Examples 1 to 7, Comparative Examples 3 to 5). I understand. In such a range, the capacity retention rate was good in high-temperature and low-temperature charge / discharge cycles where the capacity retention rate is likely to decrease, and it is speculated that the capacity retention rate will be good at room temperature as well. It was done.

  The initial low-temperature output showed a high value of 5.5 W or more in the range of lithium nickel composite oxide in the range of 5 wt% to 40 wt% (Examples 1 to 7, Comparative Example 3). Moreover, the low-temperature output after high temperature durability was favorable in the range whose lithium nickel complex oxide is 5 weight% or more (Examples 1-7, Comparative Examples 3-7). From the above, it was found that the low-temperature output is good when the lithium nickel composite oxide is in the range of 5 wt% to 40 wt% (Examples 1 to 7, Comparative Example 3).

  In the initial overcharge test, the lithium nickel composite oxide was good in the range of 0 wt% or more and less than 40 wt% (Examples 1 to 7, Comparative Examples 1 and 2) without ignition and smoke generation. When the lithium nickel composite oxide was in the range of 40 wt% or more and less than 80 wt% (Comparative Examples 3 to 5), smoke was emitted, and in the range of 80 wt% or more (Comparative Examples 6 and 7), ignition occurred. Further, in the overcharge test after low temperature durability, the lithium nickel composite oxide was good in the range of 5 wt% or more and less than 40 wt% (Examples 1 to 7) with no ignition / smoke, but lithium nickel composite oxidation. In the range of 40 wt% or more and less than 60 wt% (Comparative Examples 3 and 4), smoke was generated, and the range of 0 wt% or more and less than 5 wt% (Comparative Examples 1 and 2) and the range of 60 wt% or more (Comparative In Examples 6 and 7, ignition occurred. From the above, considering the chemical stability of the battery, the proportion of the lithium nickel composite oxide is in the range of 0 wt% to less than 80 wt% that does not cause ignition in at least the initial overcharge test (Examples 1 to 7). Comparative Examples 1 to 5) are necessary, and it is considered that it is more preferably in the range of 5% by weight or more and less than 40% by weight (Examples 1 to 7) that does not generate smoke or ignition after low temperature durability. It was.

  As described above, the negative electrode active material includes amorphous carbon-coated graphite, and the positive electrode active material has a ratio of the lithium nickel composite oxide to the lithium iron composite oxide having an olivine structure of 5 wt% or more and less than 40 wt%. As a result, it was found that the chemical stability during overcharging was good, and the low-temperature output and capacity retention rate were also good. The reason why such a result was obtained was presumed as follows. First, it was speculated that the capacity retention rate in the high-temperature cycle was further improved by adding a lithium nickel composite oxide exhibiting suitable performance to the lithium iron composite oxide. Moreover, about the capacity maintenance rate in the high temperature cycle in the partial charging / discharging which narrowed the potential window at the time of charging / discharging, the lithium iron complex oxide has a flat charging / discharging curve except the region where the SOC is 0% or 100%. If the shape is shown and partial charge / discharge is repeated, local charge / discharge (non-uniform charge / discharge) may occur and deterioration may be accelerated. This is probably due to the low electronic conductivity of the lithium iron composite oxide. On the other hand, lithium nickel composite oxide has better electronic conductivity than lithium iron composite oxide, so by adding lithium nickel composite oxide to lithium iron composite oxide, non-uniform charge and discharge can be achieved. It was speculated that it was possible to suppress the decrease in capacity maintenance rate in partial charge / discharge. In addition, as for the capacity retention rate in the low temperature cycle, the lithium iron composite oxide has little fluctuation in resistance value at the end of charging, so the potential drop at the negative electrode side becomes large at the end of charging and reaches the lithium deposition potential In some cases, precipitation of lithium may occur, resulting in a decrease in capacity and chemical stability. On the other hand, in the lithium-nickel composite oxide, the resistance increase at the end of charging is large, and by adding this, the potential fluctuation at the end of charging can be further reduced. It was inferred that the capacity maintenance rate could be further increased. However, if the addition amount of the lithium nickel composite oxide exceeds 60% by weight, the capacity retention rate in the low-temperature cycle is lowered, so that mixing of the lithium iron composite oxide and the lithium nickel composite oxide in a suitable addition range It was inferred that there was some synergistic effect. Further, regarding the overcharge test of the initial battery, the lithium nickel composite oxide may have a problem such that lithium is extracted in an overcharged state, and the extracted lithium is deposited on the negative electrode. In addition, oxygen may be generated at high temperatures to generate heat. On the other hand, the lithium iron composite oxide has high overcharge stability and a stable structure, and for example, release of oxygen hardly occurs even at high temperatures. For this reason, it was speculated that chemical stability could be improved by adding lithium nickel composite oxide in a suitable addition range to lithium iron composite oxide. As for the overcharge test after the low-temperature cycle, the lithium iron composite oxide has a remarkable local charge / discharge during the low-temperature charge / discharge cycle, and thermally unstable metal Li is deposited on the negative electrode. Therefore, chemical stability may be reduced. On the other hand, when lithium nickel complex oxide was added, it was guessed that chemical stability could be improved, for example, precipitation of metal Li was suppressed. However, if the addition amount of the lithium nickel composite oxide exceeds 60% by weight, the chemical stability after the low-temperature cycle is lowered, so mixing of the lithium iron composite oxide and the lithium nickel composite oxide within a suitable addition range It was inferred that there was some synergistic effect.

  The lithium ion secondary battery of the present invention is different from the battery described in Japanese Patent Application Laid-Open No. 2007-317534 (Patent Document 1) in that the carbon used as the negative electrode active material is amorphous carbon-coated graphite. . The amorphous carbon-coated graphite in the present invention may be mesophase graphite coated with amorphous carbon, but there is no description or suggestion of coating with amorphous carbon. In addition, if amorphous carbon-coated graphite is used, cycle durability under more severe conditions becomes better than the conditions described in Patent Document 1, and the chemical stability of the battery can be further increased. A positive electrode can be selected. Such an effect is not naturally predicted.

  DESCRIPTION OF SYMBOLS 10 Lithium ion secondary battery, 11 Current collector, 12 Positive electrode active material, 13 Positive electrode sheet, 14 Current collector, 17 Negative electrode active material, 18 Negative electrode sheet, 19 Separator, 20 Nonaqueous electrolyte, 22 Cylindrical case, 24 Positive electrode Terminal, 26 Negative terminal.

Claims (3)

A ratio of the lithium nickel composite oxide to the total weight of the lithium iron composite oxide and the lithium nickel composite oxide, comprising a lithium iron composite oxide having an olivine structure and a lithium nickel composite oxide having a layered rock salt structure A positive electrode having a positive electrode active material having a content of 15 wt% or more and less than 40 wt%
A negative electrode comprising a negative electrode active material having amorphous carbon-coated graphite;
An ion conductive medium interposed between the positive electrode and the negative electrode and conducting lithium ions;
Lithium ion secondary battery equipped with. The lithium nickel composite oxide is represented by the general formula _{LiNi x M2 1-x O 2} (M2 is Mg, Co, Mn, and at least one selected from Al, x a is 0.4 <x <0.95 The lithium ion secondary battery according to claim 1, which is represented by: The positive electrode active material according to claim 1 or 2, wherein a ratio of the lithium nickel composite oxide to a total weight of the lithium iron composite oxide and the lithium nickel composite oxide is 15 wt% or more and 35 wt% or less. The lithium ion secondary battery as described.

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