JP2006012613A - Charging method of non-aqueous electrolyte secondary battery, and battery system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a charging method enabling rapid charging of a nonaqueous electrolyte secondary battery containing Li<SB>x</SB>FePO<SB>4</SB>(0<x≤1) as a positive electrode active material. <P>SOLUTION: This is a charging method of a non-aqueous electrolyte secondary battery containing a compound as expressed by a formula: Li<SB>x</SB>FePO<SB>4</SB>(0<x≤1) as a positive electrode active material, characterized by impressing the battery with an overvoltage higher than the open circuit potential of the battery by 0.1 V or more. The battery system is equipped with a non-aqueous electrolyte secondary battery containing a compound as expressed by a formula : Li<SB>x</SB>FePO<SB>4</SB>(0<x≤1) as a positive electrode active material and a circuit mechanism which can impress an overvoltage higher than the open circuit potential of the battery by 0.1 V or more. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a method for charging a non-aqueous electrolyte secondary battery containing a compound represented by Li _x FePO ₄ (0 <x ≦ 1) as a positive electrode active material, and a battery having a circuit mechanism that enables the charging method. It is about the system.

In recent years, electronic devices such as video cameras and headphone stereos have been remarkably improved in performance and size, and the demand for higher capacity secondary batteries serving as power sources for these electronic devices has also increased. As the secondary battery, a lead secondary battery, a nickel cadmium secondary battery, and a nickel metal hydride battery are conventionally used. In addition, a non-aqueous electrolyte secondary battery using a carbonaceous material as a negative electrode active material and lithium cobalt oxide (LiCoO ₂ ) as a positive electrode active material is capable of dendrite growth by utilizing lithium doping and dedoping. Since the powdering of lithium and lithium can be suppressed, it has excellent cycle life performance, and can achieve high energy density and high capacity. Examples of the positive electrode active material of the lithium ion secondary battery include LiCoO ₂ , LiNiO ₂ having the same space group R3m / layer structure as LiCoO ₂ , LiMn ₂ O ₄ having a positive spinel structure, and having a space group Fd3m. Has been put to practical use. Moreover, as a charging method, it is performed by flowing a constant current through the cell.

  The lithium ion secondary battery has a high cost for the positive electrode material and is expensive compared to the conventional secondary battery. This is because the transition metal that is a constituent element of the positive electrode material is scarce, and it is desirable to use a material that is based on abundant and inexpensive elements such as iron (Fe).

  In addition, conventional positive electrode materials generally have a problem in operational stability. This is caused by high reactivity with the electrolyte due to high voltage and instability of the crystal structure, and does not show sufficient stability in high temperature cycle characteristics, storage characteristics, self-discharge characteristics, etc. There are many cases.

In recent years, by optimizing the synthesis process of a material based on iron: LiFePO ₄ , various physical properties required for a positive electrode for a lithium battery are controlled, and conventional positive electrode materials such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ are used. (J. Electrochem. Soc., 148, A224, 2001) has been reported. In addition, it has excellent operational stability at high temperatures, and almost no cycle deterioration, storage deterioration or self-discharge is observed even at high temperatures of 80 ° C., making it an ideal material for cost and stability. It has been investigated. Furthermore, LiFePO ₄ has a feature that the generated voltage is 3.4 V and the charge / discharge characteristics are extremely flat (J. Power Sources, 119-121, 232, 2003).

However, LiFePO ₄ has relatively low electronic conductivity, and there is a problem that charge / discharge characteristics deteriorate under high current. In particular, since it takes 2-3 hours to fully charge a lithium battery using LiFePO ₄ as an electrode, realization of rapid charging is extremely important for practical use of the battery, and the characteristics are not deteriorated even under a high current. The technical measures for this are undecided.

JP-A-6-283207 and JP-A-9-134725 describe a nonaqueous electrolyte secondary battery containing a compound represented by LiFePO ₄ as a positive electrode active material. However, there is no recognition at all about constant voltage control for applying an overvoltage as a charging method for such a secondary battery.

JP-A-6-283207 JP-A-9-134725 J. Electrochem. Soc., 148, A224, 2001 J. Power Sources, 119-121, 232, 2003

The present invention comprehensively considers the characteristics of LiFePO ₄ and provides means for solving the problems. The object is (1) while constructing a safe lithium-ion battery system at a lower cost than before. (2) To provide a non-aqueous electrolyte secondary battery having excellent rapid charging characteristics.

In order to achieve the above object, the present invention provides a method for charging a nonaqueous electrolyte secondary battery comprising a compound represented by the composition formula: Li _x FePO ₄ (0 <x ≦ 1) as a positive electrode active material, Provided is a charging method characterized by applying an overvoltage that is 0.1 V or more higher than the open circuit potential of the battery.

Furthermore, the present invention relates to a nonaqueous electrolyte secondary battery containing a compound represented by the composition formula: Li _x FePO ₄ (0 <x ≦ 1) as a positive electrode active material, and 0.1 V higher than the open circuit potential of the battery. There is also provided a battery system including a circuit mechanism that enables application of a high overvoltage.

  According to the present invention, while constructing a lithium-ion battery system that is lower in cost than in the past, the operational stability under special conditions such as high temperature is greatly improved, and a secondary battery system that is extremely excellent in quick charge characteristics is constructed. It is possible.

In general, a nonaqueous electrolyte secondary battery containing Li _x CoO ₂ or Li _x NiO ₂ as a positive electrode active material is charged by constant current control. This is because if such a positive electrode active material is forcibly applied with a high voltage (overvoltage) from the outside, the x value becomes too small and the structure becomes unstable, which adversely affects the safety and cycle characteristics of the battery. In addition, such a material has a high generated voltage of about 4 V, and there is a problem that the electrolyte is oxidized and decomposed when a higher voltage is applied. Therefore, charging by only constant voltage control is not employed in conventional lithium batteries.

On the other hand, Li _x FePO ₄ is extremely stable even in the form of FePO _{4 from} which all Li is extracted. Further, the generated voltage of LiFePO ₄ is 3.4 V, which has a large margin with respect to the oxidative decomposition voltage (about 4.5 V) of the electrolyte, and a larger overvoltage can be applied. Furthermore, the charge / discharge reaction of Li _x FePO ₄ is a two-phase reaction system of LiFePO ₄ / FePO ₄ , and is characterized in that the potential is constant regardless of x. Therefore, even if a large current is supplied during charging, the potential is unlikely to deviate from the equilibrium value, and it is difficult to actually apply a voltage for extracting a large current. Therefore, a large current can be taken out only by forcibly applying a voltage from the outside. Therefore, charging by constant voltage control can be easily and effectively applied to a non-aqueous electrolyte secondary battery containing Li _x FePO ₄ (0 <x ≦ 1) as a positive electrode active material.

The present invention has been made on the basis of the material characteristics of Li _x FePO ₄ (0 <x ≦ 1) as such a positive electrode active material, and is less than the open circuit potential of the battery by constant voltage control. By applying an overvoltage higher than 1V, rapid charging is possible.

  The open circuit potential refers to the voltage of the battery that is observed in a state where no current flows, as recognized in the art. According to the present invention, it is preferable that the overvoltage is 0.2 V or more higher than the open circuit potential, more preferably 0.5 V or more. On the other hand, the upper limit of the overvoltage is mainly determined by the oxidative decomposition voltage of the electrolyte, and is generally about 1.1V.

  As the charging method, a constant voltage control in which the applied voltage is constant, a constant voltage control in which the overvoltage is constant, pulse charging control, and other constant voltage control methods can be employed. The open circuit potential may increase with the charge depth in relation to the negative electrode material described below. Therefore, in the constant voltage control in which the applied voltage is constant, the overvoltage may decrease with the charging time. In the constant voltage control in which the overvoltage is constant, the applied voltage may increase with the charging time. In any charging method, it is preferable that the overvoltage state is established for a certain period, for example, 30% or more, preferably 50% or more, and most preferably 100% of the total charging time.

  According to the present invention, 50% or more of the theoretical capacity of a lithium battery can be charged within 30 minutes, preferably within 10 minutes.

The negative electrode of the nonaqueous electrolyte secondary battery in the present invention contains a negative electrode active material. As the negative electrode active material, a carbonaceous material or an alloy material that does not contain lithium and has a large lithium capacity (amount that can be doped with lithium) is used. The carbonaceous material can be doped and dedoped with lithium, and includes pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, organic high A fired body of a molecular compound (a product obtained by firing and carbonizing a phenol resin, a furan resin or the like at an appropriate temperature), carbon fiber, activated carbon, or the like can be used. The alloy compound here is a chemical element M _x M ′ _y Li _z (M ′ is one or more metal elements other than Li element and M element, x is A numerical value greater than 0, and y and z are numerical values greater than or equal to 0). Further, elements such as B, Si, As and the like, which are semiconductor elements, are included in the metal element. For example, Mg, B, Al, Ga, In, Si, Ge, Sn, Pb, Sb, Bi, Cd, Ag, Zn, Hf, Zr, and Y and their alloy compounds, Li-Al , Li-Al-M (M: composed of one or more of 2A, 3B, and 4B transition metal elements) AlSb, CuMgSb, and the like. As an element capable of forming an alloy with lithium, a group 3B typical element is preferably used, preferably Si or Sn, and more preferably Si. For example, a compound represented by M _x Si, M _x Sn (M is one or more metal elements excluding Si or Sn), specifically, SiB ₄ , SiB ₆ , Mg ₂ Si, _{mg 2 Sn, Ni 2 Si,} TiSi 2, MoSi 2, CoSi 2, NiSi 2, CaSi 2, CrSi 2, Cu 5 Si, FeSi 2, MnSi 2, NbSi 2, TaSi 2, VSi 2, WSi 2, ZnSi 2 Etc. Furthermore, the 4B group compound except carbon containing one nonmetallic element can also be used as the negative electrode in the present invention. The material may contain one or more group 4B elements. Moreover, metal elements other than 4B group containing lithium may be contained. For example, SiC, Si ₃ N ₄ , Si ₂ N ₂ O, Ge ₂ N ₂ O, SiO _x (0 <x ≦ 2), SnO _x (0 <x ≦ 2), LiSiO, LiSnO, and the like. Although the manufacturing method of the said negative electrode material is not limited, The method of heat-processing in an inert atmosphere or reducing atmosphere by mixing a mechanical alloying method and a raw material compound is taken. Two or more kinds of the above materials may be mixed in the negative electrode. The lithium doping of the above materials may be performed electrochemically in the battery after the battery is manufactured, and after being manufactured or before the battery is manufactured, it is supplied from a positive electrode or a lithium source other than the positive electrode and is electrochemically doped. It doesn't matter. Alternatively, like a material in which a transition metal is doped in Li ₃ N, it may be synthesized as a lithium-containing material during material synthesis and contained in the negative electrode during battery production.

  As the current collector for the negative electrode, a metal that does not form an alloy with lithium can be used, and among these, copper and nickel are desirable, and a product obtained by plating these may be used.

  The non-aqueous electrolyte in the present invention is a non-aqueous electrolyte obtained by dissolving an electrolyte salt in a non-aqueous solvent, a solid electrolyte containing an electrolyte salt, a gel electrolyte obtained by impregnating an organic polymer with a non-aqueous solvent and an electrolyte salt, Alternatively, any of inorganic solid electrolytes may be used. The nonaqueous electrolytic solution is prepared by appropriately combining an organic solvent and an electrolyte, and any of these organic solvents may be used as long as it is used for this type of battery. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4 Examples thereof include methyl 1,3 dioxolane, diethyl ether, sulfolane, methyl sulfolane, acetonitrile, propionitrile, anisole, acetate ester, butyrate ester, propionate ester and the like. As the solid electrolyte, either an inorganic solid electrolyte or a polymer solid electrolyte may be used as long as the material has lithium ion conductivity. Examples of the inorganic solid electrolyte include lithium nitride, lithium iodide, crystalline sulfide, and glass sulfide. A polymer solid electrolyte is composed of an electrolyte salt and a polymer compound that dissolves the electrolyte salt. The polymer compound is composed of an ether polymer such as poly (ethylene oxide) or a crosslinked product, a poly (methacrylate) ester, an acrylate, or the like. It can be used alone, copolymerized or mixed in the molecule.

As the matrix of the gel electrolyte, various polymers can be used as long as they absorb and absorb the non-aqueous electrolyte. For example, fluorine-based polymers such as poly (vinylidene fluoride) and poly (vinylidene fluoride-co-hexafluoropropylene), ether-based polymers such as poly (ethylene oxide) and cross-linked products, and poly (acrylonitrile) Can be used. In particular, it is desirable to use a fluorine-based polymer from the viewpoint of redox stability. By containing an electrolyte salt, ionic conductivity is imparted. Any electrolyte salt may be used as long as it is used in this type of battery. For example, LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, LiCl, LiBr, etc.

According to the present invention, a non-aqueous electrolyte secondary battery containing a compound represented by the composition formula: Li _x FePO ₄ (0 <x ≦ 1) as a positive electrode active material, and 0.1 V higher than the open circuit potential of the battery. There is provided a battery system including a circuit mechanism that enables application of a high overvoltage. The battery system according to the present invention may be one in which the nonaqueous electrolyte secondary battery and the circuit mechanism are integrated, or may be an element in which both are separate and independent. As the circuit mechanism, any one of various internal circuits known in the technical field can be used. An example of such an internal circuit is shown in FIG. A person skilled in the art can construct a battery system by appropriately combining the nonaqueous electrolyte secondary battery according to the present invention with various circuit mechanisms that enable the charging method according to the present invention.

Embodiments to which the present invention is applied will be described below with reference to FIGS.
A study was made using a coin cell using lithium metal as the negative electrode as shown in FIG. Unless otherwise specified, electrode fabrication was performed under the following conditions. That is, 90 wt% of the dried active material, 8 wt% of acetylene black as a conductive agent, 2 wt% of polyvinylidene fluoride (PVDF) (Aldrich # 1300) as a binder are kneaded using dimethylformamide (DMF) to form a paste, It pelletized with the aluminum mesh used as an electric body, and dried at 100 degreeC for 1 hour (in dry argon stream). Charging is carried out by incorporating a pellet carrying 10 mg of active material into a 2016 coin cell with lithium metal disposed at the opposite electrode, and using propylene carbonate (PC) + dimethyl carbonate (DMC) (1: 1) / 1M LiPF ₆ as the electrolyte. went. In order to apply a constant voltage, a potentiostat (Hokuto Denko HA-501) having a circuit as shown in FIG. 6 was used. A constant voltage was applied to the cell, and the time dependency of the induced current was measured. Moreover, the time dependence of the capacity consumption was calculated by integrating this. The measurement was performed at normal temperature (23 ° C.).

Example 1 (Invention)
LiFePO ₄ as a positive electrode active material was synthesized as follows. Iron oxalate FeC ₂ O ₄ . 2H ₂ O, ammonium phosphate NH ₄ H ₂ PO ₄ and lithium carbonate Li ₂ CO ₃ are weighed to a molar ratio of 2: 2: 1, and finally 8 wt% of carbon remains. After carbon black was added, the mixture was mixed thoroughly and baked in a nitrogen atmosphere at 600 ° C. for 24 hours under a nitrogen atmosphere. When X-ray diffraction of this powder was performed, it was confirmed that single-phase LiFePO ₄ was synthesized.

Example 2 (comparative example)
LiMnPO ₄ as a positive electrode active material was synthesized as follows. Manganese oxalate MnC ₂ O ₄ . 0.5H ₂ O, ammonium phosphate NH ₄ H ₂ PO ₄ and lithium carbonate Li ₂ CO ₃ are weighed in a molar ratio of 2: 2: 1, and finally 8 wt% of carbon remains. After adding carbon black as described above, the mixture was sufficiently mixed and calcined in a nitrogen atmosphere at 600 ° C. for 24 hours in a nitrogen atmosphere. When X-ray diffraction of this powder was performed, it was confirmed that single-phase LiMnPO ₄ was synthesized.

Example 3 (comparative example)
LiMn ₂ O ₄ as a positive electrode active material was synthesized as follows. Manganese dioxide MnO ₂ and lithium hydroxide LiOH. H ₂ O was weighed so as to have a molar ratio of 2: 1 and thoroughly mixed, and then calcined at 470 ° C. in air and further calcined at 750 ° C. in air. When X-ray diffraction of this powder was performed, it was confirmed that single-phase LiMn ₂ O ₄ was synthesized.

FIG. 2 shows the time dependence of current relaxation and capacity consumption under a constant overvoltage (0.1 V, 0.2 V, 0.3 V) of the battery thus fabricated for LiFePO ₄ and LiMnPO ₄ . It can be seen that the greater the overvoltage, the greater the induced current and the higher the capacity consumption rate. Further, when an overvoltage of 0.1 V or more is applied, the reaction rate can be rapidly increased, which supports the effectiveness of the present invention. It can also be seen that the reaction rate of Fe is much faster than that of Mn.

FIG. 3 shows the result of comparing the time dependency of capacity consumption when a constant voltage of 4.2 V is applied with respect to LiMn ₂ O ₄ , LiFePO ₄ , and LiMnPO ₄ . LiFePO ₄ has been said to have a slow reaction rate due to its electronic insulation, but by applying a large overvoltage forcibly, it exhibits a quick charge characteristic superior to that of LiMn ₂ O _4. Direct support. Since LiMnPO ₄ has a high voltage and very low intrinsic activity, the reaction hardly proceeds.

Next, FIG. 4 shows the discharge characteristics measured after charging for 30 minutes under a constant voltage of 4.2V. LiFePO ₄ can be discharged with a theoretical capacity, and it was confirmed that rapid charging for 30 minutes or less functions sufficiently with respect to LiFePO ₄ .

Furthermore, as a Li extraction acceleration test, x-ray diffraction patterns of LiFePO ₄ , LiMnPO ₄ , and LiMn ₂ O ₄ before and after an oxidation reaction for 30 minutes with an acetonitrile solution of NO ₂ BF ₄ are shown in FIG. For LiFePO ₄ and LiMn ₂ O ₄ , the reaction is completely completed, which corresponds well to FIGS. 3 and 4. For LiMnPO ₄ , the reaction does not proceed sufficiently, and this also corresponds well to FIG. 3 and FIG.

  The present invention is not only applicable to the coin-type secondary battery as in this embodiment, but can also be applied to a rectangular stacked secondary battery, and is also applicable to button-type and cylindrical-type secondary batteries. Is also applicable.

It is a general | schematic cross-sectional view which shows the concept of the charging method by this invention. It is a graph which shows the time dependence of the current relaxation under fixed overvoltage and capacity consumption. It is LiFePO _4, LiMnPO _4, a graph showing the time dependence of capacity consumption of LiMn ₂ O ₄ at the time of applying a constant voltage of 4.2 V. It is a graph which shows the discharge curve after charge by the constant voltage of 4.2V for 30 minutes. LiFePO _4, the LiMnPO _4, LiMn ₂ O _4, wherein x-ray diffraction diagram before and after the oxidation reaction for 30 minutes with acetonitrile solution of NO ₂ BF _4. It is a circuit diagram which shows an example of the circuit which can be used for the constant voltage control by this invention.

Claims (8)

Composition method: Li _x FePO ₄ (0 <x ≦ 1) A method for charging a non-aqueous electrolyte secondary battery containing as a positive electrode active material, which is 0.1 V higher than the open circuit potential of the battery. A charging method characterized by applying a high overvoltage.   The method of claim 1, wherein the overvoltage is 0.2 V or more higher than the open circuit potential of the battery.   The method of claim 1, wherein the overvoltage is 0.5 V or more higher than the open circuit potential of the battery.   The method according to any one of claims 1 to 3, wherein the time for applying the overvoltage accounts for 30% or more of the total charging time.   The method according to claim 1, wherein the application is performed by constant voltage control in which an applied voltage is constant.   The method according to claim 1, wherein the application is performed by constant voltage control in which the overvoltage is constant.   The method according to any one of claims 1 to 6, wherein 50% of the theoretical capacity of the battery is charged within 30 minutes. Nonaqueous electrolyte secondary battery containing a compound represented by the composition formula: Li _x FePO ₄ (0 <x ≦ 1) as a positive electrode active material, and application of an overvoltage higher by 0.1 V or more than the open circuit potential of the battery A battery system comprising a circuit mechanism that makes it possible.

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