JP2011071017A - Lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve both high energy density and high durability against high temperature when a positive electrode active material containing an iron lithium phosphate compound is used. <P>SOLUTION: A lithium secondary battery 10 includes: a positive electrode sheet 13 in which a positive electrode active material 12 is formed in a current collector 11; a negative electrode sheet 18 in which a negative electrode active material 17 is formed on a surface of the current collector 14; and a nonaqueous electrolytic solution 20 to fill between the positive electrode sheet 13 and the negative electrode sheet 18. As for this lithium secondary battery, the iron lithium phosphate compound is contained in the positive electrode active material 12, and in the negative electrode active material 17, amorphous carbon (for example, easily-graphitizable carbon) is contained within a range of 20 wt.% or more and 40 wt.% or less, and graphite is contained within a range of 80 wt.% or less and 60 wt.% or more. The specific surface area A of the negative electrode active material 17 is formed by 2.38 m<SP>2</SP>/g or more and 3.40 m<SP>2</SP>/g or less. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium secondary battery.

Conventionally, research has been conducted on an active material containing a lithium iron phosphate compound (LiFePO ₄ ) having an olivine structure as a basic skeleton as an active material mainly used for a positive electrode of a lithium secondary battery. Since this material is mainly composed of an inexpensive element such as iron or phosphorus, it is expected as a material that contributes to reducing the cost of battery materials. In addition, since it is difficult to release oxygen even when the temperature is raised, it is expected to be a material that contributes to improving the reliability of the battery because of its low reactivity with the electrolyte at high temperatures. However, this lithium iron phosphate compound has a smaller battery capacity than those containing cobalt, nickel, etc., and Fe in the positive electrode active material may elute into the electrolyte and precipitate at the negative electrode. Due to the fact that durability may be low, various improvements continue. For example, a lithium iron phosphate compound having an olivine structure as a basic skeleton is used as a positive electrode active material, graphite or lithium titanate as a negative electrode active material, and lithium bisoxalatoborate (LiBOB) is used as an electrolyte salt. (For example, refer nonpatent literature 1). In this lithium secondary battery, the cycle characteristics can be improved by using lithium titanate as the negative electrode active material and LiBOB as the electrolyte salt.

Electrochemistry Communications 7 (2005) 669-673

  However, in the lithium ion secondary battery of Non-Patent Document 1 described above, since lithium titanate is used as the negative electrode active material, there is a problem that the battery voltage is about 1.8 V and the energy density is reduced. Further, when graphite is used as the negative electrode active material in order to increase the energy density, the cycle characteristics particularly at high temperatures may be deteriorated. That is, it has been desired to have high energy density and high temperature durability.

  This invention is made | formed in view of such a subject, and provides the lithium secondary battery which has a high energy density and high high-temperature durability in the thing using the positive electrode active material containing a lithium iron phosphate compound. The main purpose.

  As a result of diligent research to achieve the above-described object, the present inventors found that the lithium secondary battery using a positive electrode active material containing a lithium iron phosphate compound has different crystallinity such as graphite and graphitizable carbon. When a plurality of carbon materials are mixed and the specific surface area is within a predetermined range, it has been found that both high energy density and high high temperature durability can be achieved, and the present invention has been completed.

That is, the lithium secondary battery of the present invention includes a positive electrode having a positive electrode active material containing a lithium iron phosphate compound capable of occluding and releasing lithium and a negative electrode active material in which at least two types of carbon materials having different crystallinity are mixed. A negative electrode satisfying 2.38 ≦ A ≦ 3.40, and the positive electrode and the negative electrode, when the specific surface area of the negative electrode active material is A (m ² / g) And an ion conductive medium that conducts lithium ions.

  The lithium secondary battery of the present invention can have high energy density and high high temperature durability. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, in general, in a charge / discharge cycle, iron is eluted from an iron phosphate compound, which is a positive electrode active material, and this eluted Fe may be deposited or deposited on the negative electrode, which reduces the function of the negative electrode active material. There is. On the other hand, in the present invention, a plurality of carbon materials having different crystallinities are mixed and the specific surface area is in a suitable range, so that precipitation and adhesion of eluted Fe on the negative electrode is suppressed. It is inferred that

The schematic diagram which shows an example of the lithium secondary battery 10 of this invention. X-ray diffraction measurement results of graphite and graphitizable carbon. Results of Raman spectroscopic measurement of graphite and graphitizable carbon. The discharge curve of Example 2 and Comparative Examples 1 and 4. FIG.

The lithium secondary battery of the present invention includes a positive electrode having a positive electrode active material containing a lithium iron phosphate compound capable of inserting and extracting lithium, and a negative electrode active material in which at least two types of carbon materials having different crystallinity are mixed. When the specific surface area of the negative electrode active material is A (m ² / g), lithium ions are conducted by being interposed between the negative electrode satisfying 2.38 ≦ A ≦ 3.40 and the positive electrode and the negative electrode. An ion conducting medium.

The positive electrode of the lithium secondary battery of the present invention is, for example, a mixture of a positive electrode active material, a conductive material, and a binder, and an appropriate solvent is added to form a paste-like positive electrode material, which is applied to the surface of the current collector. It may be dried and compressed to increase the electrode density as necessary. The lithium iron phosphate compound contained in the positive electrode active material may be a compound having a basic composition represented by LiFePO ₄ , and may be obtained by adding other components such as Mn, Ni, Co, etc. to this Fe site. . Further, the lithium iron phosphate compound is preferably a single phase having an olivine structure. The olivine type structure is based on the hexagonal close-packed packing of oxygen, in which phosphorus is located at the tetrahedral site and lithium and Fe are located at the octahedral site, and such a structure is highly stable. preferable. When this lithium iron phosphate compound having an olivine structure is used as a positive electrode active material for a lithium secondary battery, it is difficult to release oxygen, so that a lithium secondary battery excellent in safety can be manufactured. Fe is preferable because it is abundant as a resource and inexpensive. The positive electrode active material may include a sulfide containing a transition metal element, an oxide containing lithium and a transition metal element, or the like in addition to the iron phosphate lithium compound. Specifically, transition metal sulfides such as TiS ₂ , TiS ₃ , MoS ₃ , FeS ₂ , Li _(1-x) MnO ₂ (0 <x <1, etc., the same shall apply hereinafter), Li _(1-x) Mn Lithium manganese composite oxide such as ₂ O ₄ , lithium cobalt composite oxide such as Li _(1-x) CoO ₂ , lithium nickel composite oxide such as Li _(1-x) NiO ₂ , lithium such as LiV ₂ O ₃ Vanadium composite oxides, transition metal oxides such as V ₂ O _{5, and the} like can be used.

  The conductive material contained in the positive 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, graphite such as natural graphite (scale-like graphite, scale-like graphite) or artificial graphite, acetylene black , Carbon black, ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold, etc.) or a mixture of two or more thereof can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles, for example, a polytetrafluoroethylene (PTFE), a polyvinylidene fluoride (PVDF), a fluorine-containing resin such as 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 positive electrode active material, the conductive material, and the 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 dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with 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.

The negative electrode of the lithium secondary battery of the present invention is prepared by mixing a negative electrode active material, a conductive material, and a binder, and adding a suitable solvent to form a paste-like negative electrode mixture on the surface of the current collector. It may be formed by coating and drying, and compressing to increase the electrode density as necessary. The negative electrode active material includes a mixture of at least two types of carbon materials having different crystallinity, but is not particularly limited as long as it can occlude and release lithium. For example, coke, glassy carbon One or more carbon materials such as carbon, non-graphitizable carbon, graphitizable carbon, pyrolytic carbon, carbon fiber, graphite, hard carbon, and soft carbon may be included. Among these, it is preferable to contain at least graphite. Moreover, it is preferable to contain carbon materials, such as an amorphous carbon, as active materials other than graphite. In the negative electrode of the present invention, it is preferable that at least graphite and amorphous carbon are mixed as a carbon material of the negative electrode active material. At this time, the proportion of amorphous carbon in the negative electrode active material is preferably in the range of 20 wt% to 40 wt%, and more preferably in the range of 25 wt% to 35 wt%. If the proportion of amorphous carbon is 20% by weight or more, the charge / discharge cycle characteristics can be further improved, and if this proportion is 40% by weight or less, the energy density can be further increased. Further, the proportion of graphite in the negative electrode active material is preferably in the range of 60 wt% to 80 wt%, and more preferably in the range of 65 wt% to 75 wt%. If the proportion of graphite is 60% by weight or more, the energy density can be further increased, and if this proportion is 80% by weight or less, the charge / discharge cycle characteristics can be further enhanced. The negative electrode of the lithium secondary battery of the present invention preferably contains graphitizable carbon as amorphous carbon in the negative electrode active material, and more preferably contains graphitizable carbon with d ₀₀₂ = 0.34 nm or more. preferable. The graphitizable carbon having d ₀₀₂ = 0.34 nm or more is preferable because the lithium insertion potential becomes a noble potential as compared with the graphite negative electrode, and the ability to reduce iron of the positive electrode through the electrolytic solution is weak. The graphitizable carbon preferably has a half-value width of about 2 ° = 26 ° in the vicinity of 2θ = 26 ° as measured by X-ray diffraction. This X-ray diffraction measurement is performed using Cu-Kα rays. The graphitizable carbon preferably has a Raman R value of 0.20 or more as measured by Raman spectroscopy. The Raman spectroscopic measurement is performed using an Ar ⁺ ion laser having a wavelength of 532 nm. This Raman R value refers to the value of the intensity ratio I ₁₃₆₀ / I ₁₅₈₀ which is the peak intensity of the 1360 cm ⁻¹ region (D band) with respect to the peak intensity of the ₁₅₈₀ cm ⁻¹ region (G band). The negative electrode of the lithium secondary battery of the present invention satisfies 2.38 ≦ A ≦ 3.40 when the specific surface area of the negative electrode active material is A (m ² / g). more preferably 2.50m ² / g or more, and more preferably not more than 3.20 m ² / g. When the specific surface area A is 2.50 m ² / g or more, the charge / discharge cycle characteristics can be further improved, and when it is 3.20 m ² / g or less, the energy density can be further increased. The specific surface area of this negative electrode active material shall mean the BET specific surface area measured by adsorbing nitrogen gas at the liquid nitrogen temperature.

  In addition, as the conductive material, binder, solvent, and the like used for the negative electrode, those exemplified for the positive electrode can be used. The negative electrode current collector includes copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as improved adhesion, conductivity and reduction resistance. For the purpose, for example, a copper surface treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. The shape of the current collector can be the same as that of the positive electrode.

  As the ion conduction medium of the lithium secondary battery of the present invention, a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, or the like can be used. Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Chain carbonates such as butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; Examples include furans such as lan, methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, the combination of cyclic carbonates and chain carbonates is preferable. According to this combination, not only the cycle characteristics representing the battery characteristics in repeated charge and discharge are excellent, but also the viscosity of the electrolyte, the electric capacity of the obtained battery, the battery output, etc. should be balanced. it can. In addition, it is thought that cyclic carbonates are improving the electroconductivity of electrolyte solution, and chain carbonates are considered that the viscosity of electrolyte solution is suppressed.

The supporting salt contained in the lithium secondary battery of the present invention is, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , Examples include LiSbF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, LiClO ₄ , LiCl, LiF, LiBr, LiI, and LiAlCl ₄ . Among these, from the group consisting of inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _3. It is preferable from the viewpoint of electrical characteristics to use a combination of one or two or more selected salts. Further, lithium bisoxalatoborate (LiBOB) may be used as the supporting salt. The supporting salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. If the concentration of the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and if it is 5 mol / L or less, the electrolytic solution can be made more stable. Moreover, you may add flame retardants, such as a phosphorus type, a halogen type, and a boron type, to this non-aqueous electrolyte.

  Further, instead of the liquid ion conducting medium, a solid ion conducting polymer may be used as the ion conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, and polyvinylidene fluoride and a supporting salt can be used. Further, an ion conductive polymer and a non-aqueous electrolyte can be used in combination. In addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, an inorganic solid powder bound by an organic binder, or the like can be used as the ion conductive medium.

  The lithium 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 lithium secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or a thin fine olefin resin such as polyethylene or polypropylene is used. A porous membrane is mentioned. These may be used alone or in combination.

The shape of the lithium 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 diagram showing an example of a lithium secondary battery 10 of the present invention. The lithium 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, and the positive electrode sheet 13 and the negative electrode sheet. 18 and a non-aqueous electrolyte solution 20 that fills the space between the positive electrode sheet 13 and the negative electrode sheet 18. In this lithium secondary battery 10, the separator 19 is sandwiched between the positive electrode sheet 13 and the negative electrode sheet 18, and these are wound and inserted into the cylindrical case 22, and the positive electrode terminal 24 connected to the positive electrode sheet 13 and the negative electrode sheet are connected. A connected negative electrode terminal 26 is provided. Here, the positive electrode active material 12 contains a lithium iron phosphate compound, and the negative electrode active material 17 contains 20% by weight to 40% by weight of amorphous carbon (for example, graphitizable carbon) and 80% by weight of graphite. Below 60% by weight is contained. Further, the specific surface area A of the negative electrode active material 17 is formed to be 2.38 m ² / g or more and 3.40 m ² / g or less.

  The lithium secondary battery of the present embodiment described in detail above can have high energy density and high high temperature durability. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, in general, in a charge / discharge cycle, iron is eluted from an iron phosphate compound, which is a positive electrode active material, and this eluted Fe may be deposited or deposited on the negative electrode, which reduces the function of the negative electrode active material. There is. On the other hand, in the present invention, a plurality of carbon materials having different crystallinities are mixed, and the specific surface area is in a suitable range. Therefore, a negative electrode active material surface film called SEI formed on the negative electrode It is inferred that the components and shape are good. As a result, it is speculated that precipitation and adhesion of the eluted Fe on the negative electrode can be suppressed, and both high energy density and high high temperature durability performance can be achieved.

  Here, the thing containing the mixture of graphite and graphitizable carbon as a negative electrode active material is considered. Graphite has a region (plateau region) where the potential change is as small as about 0.1 V on the basis of Li metal. On the other hand, the graphitizable carbon negative electrode, which is one of amorphous carbon, changes its potential linearly from 1V to 0V. That is, on average, the graphite negative electrode has a lower potential than the graphitizable carbon negative electrode. As described above, the graphitizable carbon negative electrode has a weaker ability to reduce the positive electrode Fe through the electrolytic solution than the graphite, and thus seems to suppress the precipitation of the positive electrode Fe on the negative electrode. Moreover, it is guessed that the difference in the surface state of graphite and graphitizable carbon has an influence. For example, when the mixing ratio of the graphitizable carbon negative electrode is too low, the force for reducing Fe in the negative electrode is increased, and therefore, precipitation of Fe on the negative electrode is increased, resulting in poor cycle durability. On the other hand, if the mixing ratio of the graphitizable carbon negative electrode is too high, the operating voltage becomes low, and the energy density becomes small. That is, by controlling the mixing ratio of the graphitizable carbon negative electrode to an appropriate value, the SEI formed on the negative electrode is improved, and as a result, the precipitation and adhesion of the eluted Fe on the negative electrode is suppressed. Therefore, it is presumed that a battery having a high energy density can be obtained by improving the high temperature durability (cycle characteristics) and suppressing the operating voltage drop.

  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.

  Hereinafter, an example in which the lithium secondary battery of the present invention was specifically manufactured will be described as an example.

[Example 1]
As examples demonstrating the present invention, a positive electrode containing a lithium iron phosphate compound (LiFePO ₄ ) having an olivine structure as a basic skeleton, a negative electrode containing graphite and graphitizable carbon, a carbonate-based solvent and LiPF ₆ A lithium secondary battery using an electrolytic solution containing bismuth was studied. As the positive electrode active material, a lithium iron phosphate compound (LiFePO ₄ ) having an olivine type structure as a basic skeleton, carbon as a conductive material, and polyvinylidene fluoride (Kureha KF polymer) as a binder are used. A positive electrode mixture in which the binder was mixed at 78.5 / 13.8 / 7.7% by weight was prepared. This positive electrode mixture is dispersed with N-methyl-2-pyrrolidone (NMP) to form a paste. This positive electrode mixture paste is coated and dried on both surfaces of an aluminum foil having a thickness of 20 μm, and roll-pressed. Used as. The positive electrode sheet electrode was 54 mm × 450 mm. Then, by mixing the following graphitization graphitizable carbon and d ₀₀₂ = 0.388nm for d ₀₀₂ = 0.340 nm so that the 40:60 weight ratio, which was used as a negative electrode active material. When the specific surface area of this negative electrode active material was calculated using the specific surface area of graphite and the specific surface area of graphitizable carbon, it was 3.40 m ² / g. Negative electrode active material and binder used in this mixture are polyvinylidene fluoride (Kureha KF polymer), the negative electrode active material and the binder are mixed at 95/5% by weight, respectively, and dispersed with NMP. A paste was prepared. This negative electrode mixture paste was coated and dried on both sides of a 10 μm thick copper foil, and roll-pressed to adjust the porosity of the negative electrode mixture layer to 36% by volume as a negative electrode sheet electrode. The active material density of the negative electrode mixture layer determined from the volume (area × thickness) of the negative electrode mixture layer and the weight of the active material was 1.0 g / cm ³ . In addition, the active material density of the positive electrode mixture layer determined from the volume (area × thickness) of the positive electrode mixture layer and the weight of the active material was 1.3 g / cm ³ . The negative electrode sheet electrode was 56 mm × 500 mm. The electrolytic solution used was LiPF ₆ dissolved in a mixed solvent (volume ratio 3: 7) of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) at a concentration of 1 mol / L. The prepared positive and negative electrode sheet electrodes are rolled into a roll through a separator (product of Tonen Tapirs, PE 25 μm thickness, width 58 mm), inserted into a 18650 battery can, and the above electrolyte solution is injected. A lithium secondary battery produced by crimping and sealing was designated as Example 1.

In addition, the porosity of the negative electrode mixture layer was calculated as follows. For example, when the electrode material includes an active material, a conductive material, and a binder, the weight of the active material is A (g), the true density of the active material is X (g / cm ³ ), and the weight of the conductive material is B. (G), Y (g / cm ³ ) for the true density of the conductive material, C (g) for the weight of the binder, Z (g / cm ³ ) for the true density of the binder, When the material density was M (g / cm ³ ), the porosity V (volume%) was calculated using the following formula (1).

  V = 100− (100M) / [A / (A / X + B / Y + C / Z)] (1)

[Examples 2-3]
Example 2 A lithium secondary battery obtained through the same steps as Example 1 except that a mixture of graphitizable carbon and graphite in a weight ratio of 30:70 was used as the negative electrode active material. It was. The specific surface area of this negative electrode active material was 2.89 m ² / g. Also, a lithium secondary battery obtained through the same steps as in Example 1 was used except that a graphitized carbon and graphite mixed at a weight ratio of 20:80 were used as the negative electrode active material. Example 3 was used.
The specific surface area of this negative electrode active material was 2.38 m ² / g.

[Comparative Examples 1-4]
Comparative Example 1 is a lithium secondary battery obtained through the same steps as in Example 1 except that a mixture of graphitizable carbon and graphite in a weight ratio of 100: 0 is used as the negative electrode active material. It was. The specific surface area of this negative electrode active material (graphitizable carbon) was 6.48 m ² / g. In addition, a lithium secondary battery obtained through the same process as in Example 1 was compared except that a mixture of graphitizable carbon and graphite in a weight ratio of 50:50 was used as the negative electrode active material. Example 2 was adopted. The specific surface area of this negative electrode active material was 3.92 m ² / g. Further, a lithium secondary battery obtained through the same process as in Example 1 was compared except that a mixture of graphitizable carbon and graphite in a weight ratio of 10:90 was used as the negative electrode active material. Example 3 was used. The specific surface area of this negative electrode active material was 1.86 m ² / g. Further, a lithium secondary battery obtained through the same process as in Example 1 was compared except that a mixture of graphitizable carbon and graphite so that the weight ratio was 0: 100 was used as the negative electrode active material. Example 4 was adopted. The specific surface area of this negative electrode active material (graphite) was 1.35 m ² / g.

[X-ray diffraction measurement]
X-ray diffraction measurement of graphite and graphitizable carbon used for the negative electrode mixture was performed using an X-ray diffractometer (RINT-2200, manufactured by Rigaku). The measurement conditions were scanning from 10 ° to 70 ° at 40 kV-30 mA with Cu-Kα rays. FIG. 2 shows X-ray diffraction measurement results of graphite and graphitizable carbon used for the negative electrode. As shown in FIG. 2, in graphitizable carbon, a broad peak was observed in the 2θ = 26 ° region where the graphite peak exists. The full width at half maximum in the 2θ = 26 ° region was 0.49 ° for graphite and 3.28 ° for graphitizable carbon.

[Raman spectroscopy measurement]
Raman spectroscopy measurement of graphite and graphitizable carbon used for the negative electrode mixture was performed using a laser Raman spectroscopy system (manufactured by JASCO Corporation, NRS-3300). Using an Ar ⁺ ion laser, Raman spectroscopic measurement is performed with excitation light having a wavelength of 532 nm, and a peak intensity ratio I ₁₃₆₀ between a peak in the vicinity of 1580 cm ⁻¹ representing the carbon stack structure and a peak in the vicinity of 1360 cm ⁻¹ representing the carbon turbulence structure. / I ₁₅₈₀ was calculated as the Raman R value. FIG. 3 shows the results of Raman spectroscopic measurement of graphite and graphitizable carbon used for the negative electrode. This Raman R value was 0.12 for graphite and 0.99 for graphitizable carbon.

[Specific surface area measurement]
The specific surface areas of graphite and graphitizable carbon used for the negative electrode active material were measured using a specific surface area measuring device (AUTOSORB-1 manufactured by Yuasa Ionics). The specific surface area was measured by adsorbing nitrogen gas to the sample at the liquid nitrogen temperature.

[Energy density measurement]
Using the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4, energy density was measured. Energy density of 20 ° C. under ambient temperature, was charged at a constant current density of 0.2 mA / cm ² until the charge voltage 4.1 V, the current density of 0.2 mA / cm ² until the discharge end voltage 2.5V It was obtained by discharging at a constant current and multiplying the discharge capacity at this time by the average voltage. The average voltage was obtained by averaging the voltages in a range where the voltage changes linearly.

[Charge / discharge cycle test]
Using the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4, a high temperature charge / discharge cycle test at 60 ° C. was performed. In the high-temperature charge / discharge cycle test, charge / discharge is performed with a constant current charge of up to 4.1 V at a 2C rate (about 1.0 A) at an ambient temperature of 60 ° C. and a constant current discharge of up to 2.0 V at a 2C rate. This cycle was repeated for a total of 500 cycles. Using the test results, the capacity retention rate C _k (%) was obtained by the following equation (2), assuming that the discharge capacity at the first cycle was C ₁ and the discharge capacity at the 500th cycle was C ₅₀₀ .

Capacity maintenance rate C _k (%) = C ₅₀₀ / C ₁ × 100 (2)

[Power density measurement after charge / discharge cycle test]
Moreover, after performing a high temperature charging / discharging cycle, the power density was calculated | required at 20 degreeC and -30 degreeC. The power density was calculated by conducting a discharge test on the lithium secondary battery after performing a high-temperature charge / discharge cycle at an environmental temperature of 20 ° C. or −30 ° C. In the discharge test, in a state of charge of SOC 50% (a state in which 50% of the rated capacity is charged), the battery has a constant current of 1C to 10C, assuming that the rated capacity of the battery is 1C. The battery was discharged for 10 seconds, and the change in battery voltage in those cases was measured. From the obtained results, extrapolated values of changes in battery voltage at different currents are obtained, and a maximum current value is obtained when it is assumed that the discharge end voltage reaches 3.0 V in 10 seconds, and the discharge end voltage 3. The value obtained by multiplying 0V was the power density of the lithium secondary battery.

(Experimental result)
Specific surface area A (m ² / g) of negative electrode active material of lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4, weight ratio of negative electrode active material, average voltage (V), capacity of charge / discharge cycle test Table 1 summarizes the maintenance rate (%), energy density (Wh), power density (W) at 20 ° C. after the charge / discharge cycle test, and power density (W) at 20 ° C. after the charge / discharge cycle test. It was. Moreover, the discharge curve of Example 2 and Comparative Examples 1 and 4 is shown in FIG. As shown in Table 1, in Comparative Example 1 using graphitizable carbon as the negative electrode active material, the capacity retention ratio and power density after the charge / discharge cycle were high, but the average voltage was low and the energy density was small. In Comparative Example 4 using graphite as the negative electrode active material, the energy density was large, but the capacity retention rate and power density after the charge / discharge cycle were low. On the other hand, in Examples 1-3, it turned out that the capacity | capacitance maintenance factor and power density after a charging / discharging cycle are excellent, an average voltage is high, and an energy density is also excellent. That is, in Examples 1-3, it turned out that high energy density and the performance of high high temperature durability are compatible. Thus, in a lithium secondary battery using a lithium iron phosphate compound as a positive electrode active material, a negative electrode having a specific surface area in the range of 2.38 to 3.40 m ² / g by mixing a plurality of carbon materials having different crystallinity. By using an active material, or a negative electrode active material mixed with graphitizable carbon in the range of 20 wt% or more and 40 wt% or less and graphite, both high energy density and high temperature durability can be achieved. Became clear.

  DESCRIPTION OF SYMBOLS 10 Lithium 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 positive electrode having a positive electrode active material containing a lithium iron phosphate compound capable of inserting and extracting lithium;
When having a negative electrode active material in which at least two kinds of carbon materials having different crystallinity are mixed, and the specific surface area of the negative electrode active material is A (m ² / g), 2.38 ≦ A ≦ 3. A negative electrode satisfying 40,
An ion conductive medium that is interposed between the positive electrode and the negative electrode and conducts lithium ions;
Rechargeable lithium battery.   In the negative electrode, at least graphite and amorphous carbon are mixed as a carbon material of the negative electrode active material, and the proportion of the amorphous carbon in the negative electrode active material is in the range of 20 wt% to 40 wt%. The lithium secondary battery according to claim 1, wherein The lithium secondary battery according to claim 2, wherein the negative electrode includes graphitizable carbon having d ₀₀₂ = 0.34 nm or more as the amorphous carbon in the negative electrode active material.

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