JP2009104983A - Lithium-ion secondary battery and power source for electric automobile using it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium-ion secondary battery which has a wider SOC region capable of estimating based on the battery voltage than a conventional one. <P>SOLUTION: A cylindrical lithium-ion secondary battery is manufactured by using manganese-substituted iron lithium phosphate LiFe<SB>0.8</SB>Mn<SB>0.2</SB>PO<SB>4</SB>as a positive electrode active material, soft carbon as a negative electrode active material, and, as a non-aqueous electrolytic liquid, one in which LiPF<SB>6</SB>as an electrolyte is dissolved in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed in volume ratio of 30/70 so that it may be 1 mol/L. When the SOC dependency of the battery voltage at the time of discharge is measured for this battery under a temperature condition of 20°C by carrying out a constant current constant voltage charge up to a charging upper limit voltage of 4.1V by 1C current and then, carrying out a constant current discharge up to a discharge lower limit voltage of 2.5 V by 0.1 C current, the range of the SOC capable of estimating based on the battery voltage has become a wide range of 0-60%, 75-85%. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a lithium ion secondary battery and a power source for an electric vehicle using the same.

Conventionally, lithium ion secondary batteries using amorphous carbon (hard carbon or soft carbon) as a negative electrode active material are known. Since this type of lithium ion secondary battery slowly changes the battery voltage during discharge, the remaining capacity (SOC) can be displayed based on the battery voltage (see, for example, Patent Document 1).
Japanese Patent Laid-Open No. 10-152231 (paragraph 0006)

  However, in a lithium ion secondary battery using amorphous carbon as a negative electrode active material, it is possible to estimate the SOC based on the battery voltage in a low SOC region (for example, a region where SOC is 0 to 60%). In the SOC region, since the battery voltage is substantially constant regardless of the SOC, there is a problem in that the SOC cannot be estimated based on the battery voltage. In particular, in an electric vehicle, since charging / discharging control of the battery is performed while detecting the SOC, it is desired that a wide range of SOC can be easily detected based on the battery voltage.

  The present invention has been made in view of the above-described problems, and provides a lithium ion secondary battery having a wider SOC region than can be estimated based on battery voltage and a power source for an electric vehicle using the same. Main purpose.

In order to achieve the above-described object, the present inventors made a non-use of a negative electrode using soft carbon as a negative electrode active material and a positive electrode using an olivine type lithium compound represented by LiFe _0.8 Mn _0.2 PO ₄ as a positive electrode active material. When a lithium ion secondary battery arranged via an aqueous electrolyte was fabricated, it was found that the SOC can be estimated based on the battery voltage not only in the low SOC region but also in the high SOC region, and the present invention was completed. It was.

That is, the lithium ion secondary battery of the present invention includes a negative electrode using amorphous carbon as a negative electrode active material and LiMPO ₄ (M is two or more selected from the group consisting of Fe, Ni, Mn, Co, and Mg). In the discharge characteristics of the assembled lithium ion secondary battery, the ratio of each metal element is a region in which the battery voltage decreases due to the decrease in SOC due to the effect of adopting two or more metal elements for M, The olivine type lithium compound represented by the above formula is set so as to be on the higher SOC side than the region where the battery voltage decreases with the decrease in SOC due to the influence of adopting amorphous carbon as the negative electrode active material. And an ion conductive medium that allows exchange of lithium ions between the negative electrode and the positive electrode. The electric vehicle power source of the present invention is formed by connecting the above-described lithium ion secondary batteries in series.

  According to the lithium ion secondary battery of the present invention, the SOC can be estimated based on the battery voltage not only in the low SOC region but also on the higher SOC side. Therefore, it is suitable as a power source for an electric vehicle that performs charge / discharge control of the battery while detecting the SOC. Here, it is considered that the SOC can be estimated based on the battery voltage in the low SOC region because the battery voltage slowly changes during discharge because amorphous carbon is used as the negative electrode active material. In addition, the SOC can be estimated based on the battery voltage on the higher SOC side than the low SOC region because the metal element M of the olivine type lithium compound is composed of two or more types, and a step is generated in the plateau region (flat region). It is considered that the battery voltage has a slope with respect to the SOC at the step portion.

  In the lithium ion secondary battery of the present invention, examples of the amorphous carbon used for the negative electrode active material include soft carbon, hard carbon, a mixture of both carbons, and the like. Soft carbon is an easily graphitizable carbon material that is graphitized by heat treatment at 1000 ° C. or higher, and examples thereof include coke, mesocarbon microspheres, mesophase pitch carbon fibers, and pyrolytic vapor grown carbon fibers. Hard carbon is a non-graphitizable carbon material that hardly undergoes graphitization even by heat treatment at 2800 ° C. or higher. For example, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer resin (PFA resin) or polyacrylonitrile-based carbon fiber ( PAN-based carbon fiber) and glassy carbon (glassy carbon). When such amorphous carbon is used as the negative electrode active material, the SOC can be estimated based on the battery voltage in a low SOC region (for example, a region where the SOC is 0 to 60%). As the amorphous carbon, soft carbon is preferable to hard carbon. This is because soft carbon has less irreversible capacity than hard carbon.

In the lithium ion secondary battery of the present invention, the olivine type lithium compound represented by LiMPO ₄ (M is as described above) used for the positive electrode active material is, for example, a mixture of a Li source, an M source, and a PO ₄ source. After that, the mixture is fired, and the obtained fired product is crushed. When such an olivine type lithium compound is used as the positive electrode active material, a range in which the SOC can be estimated based on the battery voltage can be ensured even on the higher SOC side than the low SOC region. In addition, since an olivine type lithium compound that hardly generates oxygen even when it becomes high temperature or overcharged is adopted as the positive electrode active material, it becomes a thermally and chemically stable battery. Here, as the Li source, for example, a known lithium compound such as lithium carbonate (Li ₂ CO ₃ ) or lithium hydroxide (LiOH) can be used. As the PO ₄ source, for example, a known phosphate such as ammonium dihydrogen phosphate (NH ₄ H ₂ PO ₄ ) or diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ) can be used. Thus, compounds containing Li and PO ₄ individually may be used, but it is preferable to use a compound containing both Li and PO ₄ because the number of weighings can be reduced. Examples of such a compound include lithium dihydrogen phosphate (LiH ₂ PO ₄ ). As the M source, when one of the two or more metal elements M is Fe, for example, iron (II) oxalate dihydrate (FeC ₂ O ₄ .2H ₂ O) or iron chloride (II) ( A known divalent iron compound such as FeCl ₂ ) can be used. At this time, iron (II) oxalate dihydrate is more preferable. This is because iron (II) oxalate dihydrate is less corrosive to the gas generated during firing than iron (II) chloride. When one of the two or more metal elements M is Ni, for example, a known divalent nickel compound such as nickel oxide (NiO) or nickel hydroxide (Ni (OH) ₂ ) can be used. When one of the two or more metal elements M is Mn, for example, a known divalent manganese compound such as manganese carbonate (MnCO ₃ ) or manganese chloride tetrahydrate (MnCl ₂ .4H ₂ O) is used. be able to. When one of the two or more metal elements M is Co, for example, a known divalent cobalt compound such as cobalt oxide (CoO) or cobalt chloride (CoCl ₂ ) can be used. When one of the two or more metal elements M is Mg, for example, a known divalent magnesium compound such as magnesium oxide (MgO) or magnesium hydroxide (Mg (OH) ₂ ) can be used.

The firing of the mixture of the Li source, the M source and the PO ₄ source is not particularly limited. For example, the firing temperature is preferably 400 ° C. or higher, and 600 ° C. or higher and 800 ° C. or lower. Is more preferable. A calcination temperature of less than 400 ° C. is not preferable because the reaction does not proceed sufficiently and the crystallinity deteriorates. Further, firing at 800 ° C. or higher is not preferable because grain growth is promoted and the particle diameter is increased. The firing time is not particularly limited as long as it is sufficient to complete firing of the mixture, but is preferably about 10 to 15 hours, for example. Firing is preferably performed in an environment where oxygen is not present, such as in an inert atmosphere. If firing is performed in an environment where oxygen is present, part of iron becomes trivalent and discharge capacity decreases, which is not preferable. Specifically, it is not particularly limited, and examples thereof include a rare gas group element gas stream such as helium gas, neon gas, and argon gas, and an inert gas stream such as nitrogen gas. At this time, since argon gas is heavier than oxygen, it is more preferable to perform under argon gas flow. In order to use a fired product containing these components as a positive electrode active material, it is preferable to increase the electron conductivity by mixing with a conductive material. It is preferable to crush the fired product in preparation for such mixing. The method for pulverizing the fired product is not particularly limited as long as it can be pulverized to such an extent that it can be sufficiently mixed with a conductive material described later. For example, a ball mill, a mixer, a mortar, etc. The method to use is mentioned.

In the lithium ion secondary battery of the present invention, the positive _{electrode, LiFe x Mn 1-x PO} 4 (x is in the discharge characteristics of the lithium ion secondary battery assembled, the SOC due to the effects obtained by the two-component system of Fe and Mn The region in which the battery voltage decreases with the decrease is set to be higher on the SOC side than the region in which the battery voltage decreases with the decrease in SOC due to the influence of adopting amorphous carbon as the negative electrode active material) The represented olivine type lithium compound is preferably used as the positive electrode active material. In this case, since the Fe and Mn are relatively abundant, the cost of the positive electrode active material is not increased. Here, x is preferably 0.65 or more and 0.95 or less. In this way, the region where the battery voltage decreases as the SOC decreases due to the influence of the two-component system of Fe and Mn, the battery voltage decreases as the SOC decreases due to the influence of adopting amorphous carbon as the negative electrode active material. It is surely on the high SOC side than the region. For example, when a lithium ion secondary battery is used as a power source for an electric vehicle, the value of x is preferably determined in consideration of battery control performed in the electric vehicle.

  In the lithium secondary battery of the present invention, the positive electrode may contain a conductive material. The conductive material is not particularly limited as long as it is a conductive material. For example, carbon blacks such as ketjen black, acetylene black, channel black, furnace black, lamp black and thermal black may be used, and natural graphite such as flake graphite, graphite such as artificial graphite and expanded graphite may be used. Further, conductive fibers such as carbon fibers and metal fibers, metal powders such as copper, silver, nickel, and aluminum, or organic conductive materials such as polyphenylene derivatives may be used. These may be used alone or in combination.

  In the lithium ion secondary battery of the present invention, the positive electrode may contain a binder. Although it does not specifically limit as a binder, A thermoplastic resin, a thermosetting resin, etc. are mentioned. For example, polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, fluoro rubber, tetrafluoroethylene-hexafluoroethylene copolymer, tetrafluoroethylene-hexafluoropropylene copolymer ( FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-chlorotrifluoroethylene copolymer, ethylene-tetrafluoroethylene copolymer ( ETFE resin), polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymer, propylene-tetrafluoroethylene copolymer, ethylene- Rollotrifluoroethylene copolymer (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymer, vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymer, ethylene-acrylic acid copolymer, etc. Is mentioned. These materials may be used alone or in combination.

In the lithium ion secondary battery of the present invention, the ion conductive medium is not particularly limited. For example, an electrolytic solution containing a supporting salt, a gel electrolyte, a solid electrolyte, or the like can be used. Examples of the supporting salt include known supports such as LiPF ₆ , LiClO ₄ , LiBF ₄ , Li (CF ₃ SO ₃ ), LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , and LiN (C ₂ F ₅ SO ₂ ). A salt can be used. As a solvent for the electrolytic solution, for example, an aprotic organic solvent can be used. Examples of such an organic solvent include cyclic carbonates, chain carbonates, cyclic esters, cyclic ethers, chain ethers, and the like. Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, and vinyl carbonate. Examples of the chain carbonate include dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate. Examples of the cyclic ester carbonate include gamma butyrolactone and gamma valerolactone. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane and ethylene glycol dimethyl ether. These may be used alone or in combination. 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 is a composition that can withstand the range of use of the lithium ion secondary battery. A microporous membrane 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. Further, a plurality of such lithium ion secondary batteries may be connected in series to serve as an electric vehicle power source. Examples of the electric vehicle include a battery electric vehicle driven only by a battery, a hybrid electric vehicle combining an internal combustion engine and a motor drive, a fuel cell vehicle generating power by a fuel cell, and the like.

  Hereinafter, specific examples of the present invention will be described using examples.

[Example 1]
Li source, a lithium dihydrogen phosphate LiH ₂ PO ₄ as a PO ₄ source, the iron (II) oxalate dihydrate FeC ₂ O ₄ · 2H ₂ O as Fe source, Mn source as manganese acetate (II) Mn (CH ₃ COO) ₂ was used and precisely weighed so that the molar ratio of Li: Fe: Mn: PO ₄ was 1: 0.8: 0.2: 1. After thoroughly mixing the raw material with ethanol using a ball mill, the powder obtained by volatilizing ethanol and pulverizing with a mortar was pelletized at a pressure of 1 t / cm ² was used as a firing sample. The firing sample was put in a highly airtight tubular furnace, the pressure was reduced, and the inside of the furnace was replaced with argon gas, followed by firing at 600 ° C. for 12 hours. Thereafter, the temperature was lowered to room temperature, and the fired product was crushed with a mortar to obtain a manganese-substituted olivine-structured lithium iron phosphate (LiFe _0.8 Mn _0.2 PO ₄ ) powder. During firing, argon gas was flowed into the tubular furnace at a flow rate of 30 mL / min.

Next, a cylindrical lithium ion secondary battery was produced as follows. First, LiFe _0.8 Mn _0.2 PO ₄ obtained earlier was used as the positive electrode active material. 85% by weight of this positive electrode active material, 10% by weight of carbon black as a conductive material, 5% by weight of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone as a dispersing agent is added and dispersed to form a slurry. The positive electrode composite material was used. This positive electrode mixture was applied to both sides of a 20 μm thick aluminum foil current collector, dried, then densified with a roll press, and cut into a 52 mm wide × 450 mm long shape as a positive electrode sheet. In addition, the adhesion amount of the positive electrode active material was about 7 mg / cm ² per side. Next, soft carbon (product name GP-5 manufactured by Nippon Carbon Co., Ltd.) was used as the negative electrode active material. A slurry-like negative electrode mixture was prepared by mixing 95% by weight of this negative electrode active material, 5% by weight of polyvinylidene fluoride as a binder, and adding and dispersing an appropriate amount of N-methyl-2-pyrrolidone as a dispersant. The negative electrode mixture was applied to both sides of a 10 μm thick copper foil current collector, dried, then densified with a roll press, and cut into a 54 mm wide × 500 mm long shape as a negative electrode sheet. In addition, the adhesion amount of the negative electrode active material was about 4 mg / cm ² per side. The positive electrode sheet and the negative electrode sheet were wound by sandwiching a polyethylene separator having a width of 56 mm and a thickness of 25 μm to produce a roll electrode body. This electrode body was inserted into a 18650 type cylindrical case, impregnated with a non-aqueous electrolyte, and then sealed to produce a cylindrical lithium ion secondary battery. In addition, in the non-aqueous electrolyte solution, LiPF ₆ as an electrolyte was dissolved in a mixed solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 30/70 so as to be 1 mol / L. A thing was used.

  Next, the cylindrical lithium ion secondary battery produced in this way was subjected to a charge / discharge test. In the charge / discharge test, a constant current and constant voltage charge is performed up to a charging upper limit voltage of 4.1 V with a current of 1 C under a temperature condition of 20 ° C., and then a constant current discharge is performed with a current of 0.1 C to a discharge lower limit voltage of 2.5 V The SOC dependence of the battery voltage during discharge was measured. The result is shown in FIG. As is apparent from FIG. 1, in the SOC range of 0 to 60% (low SOC range), the battery voltage decreases as the SOC decreases, so that it is possible to estimate the SOC based on the battery voltage. Further, in the region where the SOC is 75 to 85%, a step (part where the battery voltage decreases as the SOC decreases) due to the replacement of iron in the olivine lithium iron phosphate with manganese occurs. The SOC can be estimated. Thus, the range of SOC that can be estimated based on the battery voltage is as wide as 0 to 60% and 75 to 85%.

[Example 2]
The Li: Fe: Mn: PO _{4 of} Example 1 was precisely weighed so that the molar ratio was 1: 0.95: 0.05: 1 and manganese-substituted olivine-structured lithium iron phosphate (LiFe _0.95 Mn _0.05 PO ₄ ) Was synthesized and this was used as the positive electrode active material, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1, and a charge / discharge test was performed. The result is shown in FIG. As apparent from FIG. 2, in the region where the SOC is 0 to 60% (low SOC region), the battery voltage decreases as the SOC decreases. Therefore, it is possible to estimate the SOC based on the battery voltage. Further, in the region where the SOC is 93 to 100%, a step (a portion where the battery voltage decreases as the SOC decreases) due to the replacement of iron in the lithium iron phosphate of the olivine structure with manganese occurs. The SOC can be estimated. Thus, the range of SOC that can be estimated based on the battery voltage is as wide as 0 to 60% and 93 to 100%.

[Example 3]
The Li: Fe: Mn: PO _{4 of} Example 1 was precisely weighed so that the molar ratio was 1: 0.65: 0.35: 1, and manganese-substituted olivine structure lithium iron phosphate (LiFe _0.65 Mn _0.35 PO ₄ ) Was synthesized and this was used as the positive electrode active material, and a cylindrical lithium ion secondary battery was produced in the same manner as in Example 1, and a charge / discharge test was performed. The result is shown in FIG. As is clear from FIG. 3, in Example 3, in the region where the SOC is 0 to 60% (low SOC region), the battery voltage decreases as the SOC decreases. Therefore, the SOC is estimated based on the battery voltage. Is possible. Moreover, in the region where the SOC is 60 to 70%, a step (a portion where the battery voltage decreases as the SOC decreases) due to the replacement of iron in the lithium iron phosphate of the olivine structure with manganese occurs. The SOC can be estimated. Thus, the range of SOC that can be estimated based on the battery voltage is as wide as 0 to 70%. As apparent from Examples 1 to 3, since the position of the step generated by the manganese replacement changes according to the amount of manganese to be replaced, it is based on the SOC detectable range required for battery control of the electric vehicle. The amount of manganese to be replaced may be determined.

[Comparative Example 1]
A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that artificial graphite was used as the negative electrode active material of Example 1, and a charge / discharge test was performed. The result is shown in FIG. As is clear from FIG. 4, in Comparative Example 1, the SOC cannot be estimated based on the battery voltage over a wide range of 10 to 75%.

[Comparative Example 2]
The olivine-structured lithium iron phosphate (LiFePO ₄ ) was synthesized by accurately weighing Li: Fe: Mn: PO _{4 in} Example 1 so that the molar ratio was 1: 1: 0: 1, and this was used as the positive electrode active material. A cylindrical lithium ion secondary battery was produced in the same manner as in Example 1 except that the charge / discharge test was performed. The result is shown in FIG. As is clear from FIG. 5, in the region where the SOC is 0 to 60% (low SOC region), the battery voltage decreases as the SOC decreases. Therefore, it is possible to estimate the SOC based on the battery voltage. On the higher SOC side, the SOC cannot be estimated based on the battery voltage.

  In each of the above-described embodiments, the case where the metal element M is a binary system of Fe and Mn has been described. However, if the metal element M is two or more selected from the group consisting of Fe, Ni, Co, Mn, and Mg, It can be easily inferred that the same effect as the embodiment can be obtained.

It is a graph showing the relationship between SOC of Example 1, and a battery voltage. It is a graph showing the relationship between SOC of Example 2, and battery voltage. It is a graph showing the relationship between SOC of Example 3, and battery voltage. It is a graph showing the relationship between SOC of Comparative Example 1 and battery voltage. It is a graph showing the relationship between SOC of Comparative Example 2 and battery voltage.

Claims (5)

A negative electrode using amorphous carbon as a negative electrode active material;
LiMPO ₄ (M is two or more metal elements selected from the group consisting of Fe, Ni, Mn, Co and Mg, and the ratio of each metal element is the discharge characteristics of the assembled lithium ion secondary battery. The region where the battery voltage decreases due to the decrease in SOC due to the effect of employing two or more metal elements in M, the region where the battery voltage decreases due to the decrease in SOC due to the effect of employing amorphous carbon as the negative electrode active material A positive electrode having an olivine-type lithium compound represented by a positive electrode active material as shown in FIG.
An ion conducting medium that allows the exchange of lithium ions between the negative electrode and the positive electrode;
Lithium ion secondary battery equipped with. The positive _{electrode, LiFe x Mn 1-x PO} 4 (x is in the discharge characteristics of the lithium ion secondary battery assembled, the battery voltage with a decrease in the SOC by impact of a two-component system of Fe and Mn is reduced area Is set to be on the higher SOC side than the region where the battery voltage decreases due to the decrease in SOC due to the influence of adopting amorphous carbon as the negative electrode active material). Active material,
The lithium ion secondary battery according to claim 1. The positive electrode, the positive electrode active material an olivine-type lithium compounds represented by _{LiFe x Mn 1-x PO 4} (0.65 ≦ x ≦ 0.95),
The lithium ion secondary battery according to claim 1. The negative electrode uses soft carbon as a negative electrode active material,
The lithium ion secondary battery of any one of Claims 1-3. The lithium ion secondary battery according to any one of claims 1 to 4 is connected in series.
Power source for electric vehicles.

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