JP2011192500A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the cycle performance of a nonaqueous electrolyte secondary battery using an iron phosphate compound as a negative electrode active material. <P>SOLUTION: The cycle performance of the nonaqueous electrolyte secondary battery using an iron phosphate compound as a negative electrode active material is improved by using FePO<SB>4</SB>xH<SB>2</SB>O(0<x) as a negative electrode active material. The nonaqueous electrolyte secondary battery having high discharge capacity can be also provided. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a negative electrode active material for a non-aqueous electrolyte secondary battery, a negative electrode for a non-aqueous electrolyte secondary battery, and a non-aqueous electrolyte secondary battery.

In recent years, non-aqueous electrolyte secondary batteries represented by lithium ion secondary batteries having a relatively high energy density have attracted attention as power sources for portable devices such as mobile phones and notebook computers, or electric vehicles.
Generally, a transition metal oxide such as LiCoO ₂ is used for the positive electrode active material of a lithium ion battery, and a carbon material such as graphite is used for the negative electrode active material.

  In order to further increase the capacity of lithium ion secondary batteries, development of new positive electrode and negative electrode active materials is underway. In addition to having a high capacity, these active materials are required to have excellent cycle performance and storage performance in order to withstand long-term use. Further, from the viewpoint of practical use, an active material that uses abundant elements as resources and can therefore be provided at low cost is desired.

For example, Patent Document 1 discloses an invention in which a compound given by a composition formula FePO ₄ is used as a positive electrode active material. According to Patent Document 1, “the positive electrode active material that is the iron compound of the present invention is less than half the cost of LiMn ₂ O ₄ , which is the cheapest Mn oxide among known 4V class high voltage positive electrodes. (Paragraph 0005), "According to the present invention, a small and high energy density lithium battery with a large reversible capacity can be constructed at an extremely low cost. The battery of the present invention is a coin-type battery. It has an advantage that it can be used in various fields, etc. "(paragraph 0010). In the example of Patent Document 1, “the positive electrode active material was FePO ₄ · xH ₂ O heat-treated at 250 ° C. for 36 hours and dehydrated FePO ₄ ” (paragraph 0007). Cycle characteristics were obtained, “Simply drying the commercially available FePO ₄ × x H ₂ O at 90 ° C. does not remove the water contained in the crystals, and good cycle characteristics cannot be obtained” (paragraph). 0008).

  Patent Document 2 discloses that “a compound that liberates phosphate ions in a solution and metallic iron are mixed, dissolved and reacted with the metallic iron, and then baked to perform ferric phosphate. , And a lithium battery invention using the positive electrode material for lithium battery is described. In the example of Patent Document 2, ferric phosphate synthesized using various phosphorus compounds and iron powder is shown. "In this example, inexpensive starting materials such as iron powder and phosphorus pentoxide are used, As a result of charge / discharge measurement at a synthesis temperature of 100 to 650 ° C., a calcined sample of 350 ° C. or higher showed a maximum capacity value of 115 mAh / g, significantly exceeding the capacity value of 40 mAh / g reported in the previous report. (Page 14, Lines 23 to 26) and "In addition, when applied to a non-aqueous electrolyte battery such as a lithium battery later, it is desirable that there is no residual moisture in the positive electrode material. From the viewpoint of complete removal, high-temperature firing is preferable "(page 8, line 21 to line 23).

Patent Document 3 discloses an invention in which a negative electrode active material mainly composed of a lithium composite oxide represented by a basic composition formula Li ₃ Fe ₂ (PO ₄ ) ₃ is used as a negative electrode active material of an aqueous lithium secondary battery. Has been. The Examples of Patent Document 3, describes a battery using a _{Li 3 Fe 2 (PO 4)} 3 in the negative electrode active material, "therefore, as a negative electrode active _material, the use of _{Li 3 Fe 2 (PO 4)} 3 Thus, it can be seen that a water-type lithium secondary battery having a large capacity and excellent charge / discharge cycle characteristics can be constituted "(paragraph 0052).

  Patent Document 4 is characterized in that “a chargeable / dischargeable positive electrode, a chargeable / dischargeable negative electrode, and a nonaqueous electrolyte are provided, and the battery contains a substance that generates water or carbon dioxide gas due to temperature rise. Non-aqueous electrolyte secondary battery "(Claim 1) is disclosed, and further," a substance that generates water by temperature rise is a compound having crystal water "(Claim 8). Yes. The invention according to Patent Document 4 states that “When a lithium secondary battery is exposed to a high temperature in a charged state, the subsequent characteristics are greatly deteriorated due to a reaction that is considered to involve an active material in a charged state and a nonaqueous electrolyte. In this way, the battery deteriorated due to the high temperature is not recovered and the battery becomes unusable ”(paragraph 0003). If a substance is contained, a slight amount of water is generated in the vicinity of the portion where the above reaction, that is, a reaction in which the active material deteriorates, and this water inhibits the reaction. This suppresses the progression in a chain ”(paragraph 0006). Examples of Patent Document 4 include, as a substance that generates water in the positive electrode, the negative electrode, or the electrolyte when the temperature rises, hydroxides such as zinc hydroxide and iron oxide hydroxide, boric acid, and aluminum oxide hydrate. It has been shown that by adding a compound having water of crystallization, “it was found to have an effect of suppressing a decrease in capacity due to the battery being exposed to a high temperature” (paragraph 0016).

Patent Document 5 discloses a “non-aqueous solid electrolyte battery using a gel electrolyte in which a supporting electrolyte is dissolved and using a phosphoric acid compound containing iron as a main component of a negative electrode active material that absorbs and releases lithium.” The invention of item 1) is disclosed. Examples of Patent Document 5 include: “High purity chemical Co., Ltd. iron phosphate (FePO ₄ .2H ₂ O) is vacuum dried at 200 ° C., and anhydrous iron phosphate (FePO ₄ ) is used as a negative electrode active material. ”,“ Using nickel-lithium cobaltate (LiNi _0.5 Co _0.5 O ₂ ) as a positive electrode active material ”(paragraph 0026) describes a non-aqueous solid electrolyte battery,“ charging end voltage 3. In the case of “0 V and discharge end voltage of 2.0 V” (paragraph 0034), it is described that excellent cycle performance is exhibited as compared with the case where graphite is used as the negative electrode active material.

Non-Patent Document 1 shows that FePO ₄ .2H ₂ O having a monoclinic crystal structure and represented by a space group P2 ₁ / n is used as a positive electrode active material, and the active material It is described that a reversible capacity of 106 mAhg ⁻¹ is shown when a lithium ion battery using a positive electrode is operated between 2.0 and 4.0 V.

JP-A-6-283207 International Publication No. 2004/036672 Pamphlet JP 2007-123094 A JP 11-191417 A Japanese Patent No. 3906944

Zaghib, Julien, `` Structure and electrochemistry of FePO4 ・ 2H2O hydrate '', Journal of Power Sources, Elsevier, 2005, 142, p.279-284

Here, dehydrated FePO ₄ shown in Patent Documents 1, 2 and 5 inserted lithium to a potential lower than 0.8 V with respect to the metal lithium potential, as shown in the examples described later. In some cases, there is a problem that the cycle performance is lowered.

  This invention is made | formed in view of the said problem, Comprising: It aims at improving the cycling performance of the nonaqueous electrolyte secondary battery which used the iron phosphate compound as a negative electrode active material.

As a result of intensive studies to solve the above-mentioned problems, the inventors have found that a nonaqueous electrolyte secondary battery having excellent cycle performance can be obtained by including crystal water in the iron phosphate compound. Invented. That is, the technical configuration and operational effects of the present invention for solving the above-described problems are as follows. However, the action mechanism described in this specification includes estimation, and its correctness does not limit the present invention.
The negative electrode active material for a nonaqueous electrolyte secondary battery of the present invention is a negative electrode active material for a nonaqueous electrolyte secondary battery characterized by containing iron (III) phosphate containing crystal water.
The negative electrode active material for a nonaqueous electrolyte secondary battery of the present invention is characterized by containing FePO ₄ .xH ₂ O (0 <x).
The negative electrode active material for a non-aqueous electrolyte secondary battery of the present invention is characterized in that the value range of x is 0 <x ≦ 1.
Moreover, the negative electrode active material for a non-aqueous electrolyte secondary battery of the present invention is characterized in that a peak derived from crystal water exists in the infrared absorption spectrum of the negative electrode active material.
Furthermore, the negative electrode active material for a non-aqueous electrolyte secondary battery of the present invention is characterized by having an amorphous or orthorhombic (space group Pbca) crystal structure.
The negative electrode for a nonaqueous electrolyte secondary battery according to the present invention is a negative electrode for a nonaqueous electrolyte secondary battery comprising the negative electrode active material for a nonaqueous electrolyte secondary battery.
The nonaqueous electrolyte secondary battery of the present invention is characterized by using the negative electrode for a nonaqueous electrolyte secondary battery.

  ADVANTAGE OF THE INVENTION According to this invention, the cycle performance of the nonaqueous electrolyte secondary battery using the iron phosphate compound as the negative electrode active material can be made excellent.

Weight change during thermal analysis of FePO ₄ xH ₂ O (x = 3.33) up to 750 ° C. X-ray diffraction of the _{FePO 4 · xH 2 O (x} = 3.33,0.84) , and FePO ₄ (XRD) pattern

  Embodiments of the present invention are illustrated below, but the present invention is not limited to these descriptions.

  Since the negative electrode active material according to the present invention exhibits excellent cycle performance by containing crystal water, all iron phosphate compounds containing crystal water are within the scope of the present invention. However, when a large amount of crystal water is contained, the charge / discharge efficiency and the discharge capacity are lowered, and therefore the value of x is preferably 4 or less, and particularly preferably 1 or less. Particularly, when x is 1 or less, the discharge capacity is excellent, which is particularly preferable.

The iron phosphate compound that is the negative electrode active material according to the present invention can be represented by the composition formula FePO ₄ · xH ₂ O (0 <x), and a part of Fe, P, or O in the above general formula is Fe It does not exclude those substituted with other transition metal elements, metal elements such as Al, or other non-metal elements. Even if the surface of the active material particles is coated with a conductive material such as carbon for the purpose of improving the electronic conductivity or the conductive material is encapsulated inside the particles, the effect of the present invention is inhibited. It can be used suitably without doing, and the case where such a thing is used is also within the scope of the present invention.

The iron phosphate compound according to the present invention is characterized by having an amorphous or orthorhombic (space group Pbca) crystal structure, and is composed of Li _y FePO ₄ .xH ₂ O by occlusion of lithium. Even if it becomes a composition, it has a crystal structure different from lithium iron phosphate having an olivine type crystal structure conventionally used as a positive electrode active material. The one obtained by electrochemically extracting Li from LiFePO ₄ having an olivine type structure is an orthorhombic crystal, but is a heterosite of the space group Pnma, and the orthorhombic crystal (space group Pbca) referred to in the present invention. Is different. In addition, even if the active material of the present invention is partially amorphous after charging and discharging, it does not affect the effect of the present invention, and thus such a material is also within the scope of the present invention. .

The method for producing a negative electrode active material according to the present invention is not limited, but basically, a raw material containing a metal element constituting the active material and a raw material containing a raw material to be a phosphoric acid source are mixed and reacted. Can be obtained. At this time, the composition of the compound actually obtained may slightly vary compared to the composition calculated from the raw material composition ratio. The present invention can be carried out without departing from the technical idea or main features thereof, and the scope of the present invention is only that the composition of the product obtained as a result of the production does not exactly match the above composition formula. It should not be interpreted as not belonging to.
Commercially available iron (III) phosphate n hydrate may be partially dehydrated by heating and drying under normal pressure or reduced pressure.

The iron phosphate compound of the present invention is preferably a powder having an average particle size of 100 μm or less. Particularly, in order to effectively bring out the effects of the present invention, it is preferable that the particle size is small, the average particle size of the secondary particles is 0.2 to 20 μm, and the particle size of the primary particles is 0.02 to 2 μm. It is more preferable.
The average particle size of the secondary particles of the iron phosphate compound is determined by particle size distribution measurement by liquid phase precipitation method or laser diffraction / scattering method, and the average particle size of the primary particles is the result of observation with a transmission electron microscope (TEM). Can be obtained by image analysis.

  Further, the iron phosphate compound of the present invention preferably has a crystallite diameter of 100 nm or less. In particular, by reducing the crystallite size, the ion diffusion rate in the particles can be improved, so the crystallite size is preferably 60 nm or less. The crystallite diameter of the iron phosphate compound is calculated by fitting the half width of the peak attributed to the (111) plane of X-ray diffraction to Scherrer's equation.

It is preferable that the BET specific surface area by the nitrogen adsorption method of the iron phosphate compound according to the present invention is 0.5 to 50 m ² .

  In order to obtain the iron phosphate compound powder in a predetermined shape, a pulverizer or a classifier can be used. For example, a mortar, a ball mill, a sand mill, a vibrating ball mill, a planetary ball mill, a jet mill, a counter jet mill, a swirling air flow type jet mill, a sieve, or the like can be used. At the time of pulverization, wet pulverization in which an organic solvent such as water or alcohol or hexane coexists may be used. The classification method is not particularly limited, and a sieve, an air classifier or the like can be used in a dry or wet manner as necessary.

  The coefficient of crystal water x contained in the composition formula of the iron phosphate compound according to the present invention is determined by examining the amount of water in the iron phosphate compound using various analytical methods such as a thermogravimetry. The crystal structure of the iron phosphate compound can be examined by X-ray diffraction measurement.

The infrared absorption spectrum of the negative electrode active material according to the present invention can be measured using an infrared spectrophotometer such as a Fourier transform infrared spectrophotometer (FT-IR) or a dispersion infrared spectrophotometer. . The measurement may be performed using a powder of the negative electrode active material, or the negative electrode active material may be peeled off from the current collector to be in a powder state, or may be measured with the shape of the electrode plate. When measuring powder, the Nujol method, which is measured by dispersing the sample in a solvent that transmits infrared rays, or mixed with potassium bromide powder, pressed into tablets, and then crystallized with potassium bromide. It can be measured by a permeation measuring method such as a tablet method (KBr method) for sandwiching and measuring. When measuring the electrode plate as it is, a reflection measurement method such as an attenuated total reflection method (ATR method) is used in which the negative electrode plate is brought into close contact with a medium crystal such as ZnSe, Ge, or diamond and the incident light is totally reflected. be able to. Further, in order to prevent moisture in the atmosphere from being taken in as crystal water, it is necessary to perform measurement in a place where the moisture content is extremely low, such as a dry room or an argon box having a dew point of −50 ° C. or less.
When crystal water is included in the sample, a peak derived from the stretching vibration of water is observed in the region of 3300 cm ⁻¹ to 3500 cm ^{−1 in} the infrared absorption spectrum. Note that a peak exists when the sample preparation and measurement conditions are optimized so that the maximum peak intensity in the spectrum is about 10% in transmittance (absorbance is about 1 Abs). This is something that can be identified.

  The nonaqueous electrolyte secondary battery of the present invention is configured using a negative electrode containing at least the iron phosphate compound, a positive electrode, a separator, a nonaqueous electrolyte containing an electrolyte salt and a nonaqueous solvent. .

  There is no restriction | limiting in particular as a positive electrode active material used for the nonaqueous electrolyte secondary battery of this invention, Arbitrary things can be used suitably. Examples thereof include transition metal oxides, transition metal sulfides, lithium transition metal composite oxides, lithium transition metal polyanion composite compounds, and the like. Examples of the transition metal oxide include manganese oxide, iron oxide, copper oxide, nickel oxide, vanadium oxide, and examples of the transition metal sulfide include molybdenum sulfide and titanium sulfide. Examples of the lithium transition metal composite oxide include lithium manganese composite oxide, lithium nickel composite oxide, lithium cobalt composite oxide, lithium nickel cobalt composite oxide, lithium nickel manganese composite oxide, and lithium nickel cobalt manganese composite oxide. Examples of the lithium transition metal polyanion composite compound include lithium iron phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium cobalt phosphate, lithium lithium phosphate, lithium iron silicate, lithium manganese silicate, boric acid Iron lithium etc. are mentioned. Moreover, the metal, oxygen, sulfur, or polyanion in these compounds may be partially substituted. Furthermore, conductive polymer compounds such as disulfide, polypyrrole, polyaniline, polyparastyrene, polyacetylene, and polyacene materials, pseudographite-structured carbonaceous materials, and the like are exemplified, but the invention is not limited thereto. Moreover, these compounds may be used independently and may mix and use 2 or more types.

The electrolyte salt used in the non-aqueous electrolyte secondary battery of the present invention is not particularly limited, and lithium salts that are stable in a wide potential region generally used in non-aqueous electrolyte secondary batteries can be used. For example, LiBF ₄ , LiPF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ), LiC (CF ₃ SO ₂ ) ₃ , LiB (C ₂ O ₄ ) ₂ , LiC (C ₂ F ₅ SO ₂ ) ₃ , and the like. These may be used alone or in combination of two or more.

The concentration of the electrolyte salt in the nonaqueous electrolyte, in order to ensure the non-aqueous electrolyte secondary battery having excellent output ^performance, preferably ^{0.1mol / dm 3 ~5.0mol / dm 3} , more preferably, 0.8 mol / dm ^{3 to} 2.0 mol / dm ³ .

The non-aqueous solvent constituting the non-aqueous electrolyte is not limited, and an organic solvent generally used for a non-aqueous electrolyte used for a non-aqueous electrolyte secondary battery can be used.
For example, cyclic carbonates such as propylene carbonate, ethylene carbonate, butylene carbonate, chloroethylene carbonate, styrene carbonate, catechol carbonate, 1-phenyl vinylene carbonate, 1,2-diphenyl vinylene carbonate, γ-butyrolactone, γ-valerolactone, propio Cyclic carboxylic acid esters such as lactone, chain carbonates such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and diphenyl carbonate, chain carboxylic acid esters such as methyl acetate and methyl butyrate, tetrahydrofuran or derivatives thereof, 1,3-dioxane, Ethers such as dimethoxyethane, diethoxyethane, methoxyethoxyethane, methyldiglyme, acetonitrile, benzonitrile Examples thereof include, but are not limited to, nitriles such as sulfur, dioxalane or a derivative thereof alone or a mixture of two or more thereof.
Moreover, these organic solvents can be mixed and used in arbitrary ratios.

Furthermore, the non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery of the present invention may be any other compound than the above-mentioned electrolyte salt and non-aqueous solvent, if necessary, as long as the effects of the present invention are not impaired. Can be contained in an amount of.
Examples of such other compounds include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; Partially fluorinated products of the aromatic compounds such as -fluorobiphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene; 2,4-difluoroanisole, 2,5-difluoroanisole, 2,6-difluoroanisole, 3, 5 -Overcharge inhibitors such as fluorine-containing anisole compounds such as difluoroanisole; vinylene carbonate, vinyl ethylene carbonate, fluoroethylene carbonate, trifluoropropylene carbonate, succinic anhydride, glutar anhydride Negative electrode film forming agents such as maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, cyclohexanedicarboxylic anhydride; ethylene sulfite, propylene sulfite, dimethyl sulfite, propane sultone, propene sultone, butane sultone, methyl methanesulfonate Cathodic protective agents such as, busulfan, methyl toluene sulfonate, dimethyl sulfate, ethylene sulfate, sulfolane, dimethyl sulfone, diethyl sulfone, dimethyl sulfoxide, diethyl sulfoxide, tetramethylene sulfoxide, diphenyl sulfide, thioanisole, diphenyl disulfide, dipyridinium disulfide, etc. Is mentioned.

  Two or more of the above compounds may be used in combination. The combined use of a negative electrode film forming agent and a positive electrode protective agent, or the combined use of an overcharge inhibitor, a negative electrode film forming agent and a positive electrode protective agent is particularly preferred.

  The content ratio of these other compounds in the non-aqueous electrolyte is not particularly limited, but is preferably 0.01% by mass or more, more preferably 0.1% by mass or more, and still more preferably, with respect to the whole non-aqueous electrolyte. The upper limit is preferably 5% by mass or less, more preferably 3% by mass or less, and still more preferably 2% by mass or less. By adding these compounds, safety can be improved, and capacity maintenance performance and cycle performance after high-temperature storage can be improved.

  As the current collector used in the nonaqueous electrolyte secondary battery of the present invention, a known one can be arbitrarily used. For example, metal materials such as copper, aluminum, nickel, stainless steel, nickel-plated steel and the like can be mentioned. Among them, aluminum or copper is particularly preferable because of its high thermal conductivity and electronic conductivity. Examples of the shape include a sheet, a foam, a sintered porous body, and an expanded lattice. Further, a current collector having a hole in an arbitrary shape can be used.

  As the separator, it is preferable to use a microporous membrane or a nonwoven fabric alone or in combination. Examples of the material constituting the separator include polyolefin resins typified by polyethylene and polypropylene, polyester resins typified by polyethylene terephthalate and polybutylene terephthalate, polyvinylidene fluoride, and vinylidene fluoride-hexafluoropropylene copolymer. , Vinylidene fluoride-perfluorovinyl ether copolymer, vinylidene fluoride-tetrafluoroethylene copolymer, vinylidene fluoride-trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-hexafluoro Acetone copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-trifluoropropylene copolymer, vinylidene fluoride-tetrafluoro Ethylene - hexafluoropropylene copolymer, vinylidene fluoride - ethylene - can be mentioned tetrafluoroethylene copolymer. In particular, in the present invention, a microporous membrane mainly composed of a polyolefin resin typified by polyethylene, polypropylene, or the like is preferable.

  Other battery components include a terminal, an insulating plate, a battery case, and the like, but these components may be used as they are.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely using an Example and a comparative example, this invention is not limited to these Examples, unless the summary is exceeded.

[Synthesis of negative electrode active material]
Example 1
As a pale yellow powder FePO ₄ · xH ₂ O was used as it was purchased from Nacalai Tesque. FePO ₄ · xH ₂ O is put in an alumina sagger (outside dimension 90 × 90 × 50 mm), and an atmosphere substitution type firing furnace (a tabletop vacuum gas substitution furnace KDF-75 manufactured by Denken, volume 2.4 l) is used. The heat treatment was performed under a flow of argon gas (flow rate: 10 sccm). The heat treatment temperature was 600 ° C., and the heat treatment time (time for maintaining the heat treatment temperature) was 3 hours. At this time, the rate of temperature increase was 5 ° C./min, and the temperature was naturally cooled. Nitrogen gas was always circulated from the time of temperature rise to the time of removal after the sample was introduced into the furnace. The active material thus prepared is referred to as an active material a of the present invention.

(Example 2)
A heat treatment of FePO ₄ .xH ₂ O was performed in the same manner as in Example 1 except that the heat treatment temperature was 490 ° C. Let the produced active material be this invention active material b.

(Example 3)
A heat treatment of FePO ₄ .xH ₂ O was performed in the same manner as in Example 1 except that the heat treatment temperature was 200 ° C. Let the produced active material be this invention active material c.

Example 4
FePO ₄ · xH ₂ O that is not subjected to heat treatment is defined as an active material d of the present invention.

(Comparative Example 1)
The heat treatment of FePO ₄ .xH ₂ O was performed in the same manner as in Example 1 except that the heat treatment temperature was 750 ° C. The produced active material is referred to as a comparative active material e.

[Infrared absorption analysis]
This invention active material ad or the comparative active material e and potassium bromide powder were mixed by the mass ratio of 1: 3, and it grind | pulverized using the agate mortar. The pulverized powder was pressed using a tablet pellet molding machine to produce a tablet having a diameter of 11 mm and a thickness of about 1 mm. The obtained tablets were sandwiched between crystals of potassium bromide, and infrared absorption analysis was performed in the range of 2800 to 3800 cm ⁻¹ using a Fourier transform infrared spectrophotometer (System 2000 FTIR manufactured by PerkinElmer). The resolution was 8 cm ⁻¹ , the number of integrations was 3, and the detector was TGS.
As a result, in the active materials other than the comparative active material e, a peak derived from crystal water was observed in the vicinity of 3400 cm ⁻¹ . Therefore, it was found that by firing FePO ₄ .xH ₂ O at 750 ° C., water of crystallization was desorbed and dehydrated.

[Thermogravimetry]
Thermogravimetry was performed in order to know the coefficient of crystal water contained in the composition formulas of the iron phosphate compounds of the present active materials a to d and the comparative active material e. In a dry room with a dew point of −50 ° C. or lower, the temperature is raised from room temperature to 750 ° C. at a heating rate of 5 ° C./minute using a thermogravimetric measurement device (TG / DTA 3200 manufactured by Seiko Instruments Inc.), and that temperature is 1 hour. Retained. The result is shown in FIG. The amounts of the active materials a to d of the present invention and the comparative active material e were 10 mg each. The mass of the iron phosphate compound after heating was 71.54% of the mass before heating. As described above, in the infrared absorption analysis, in the comparative active material e fired at 750 ° C., no peak derived from the crystal water was observed, so it is considered that all the crystal water was desorbed. Therefore, x, which is the coefficient of crystal water contained in FePO ₄ .xH ₂ O, was determined from the mass difference before and after heating, and x = 3.33.
Except that the heating upper limit temperature was 600 ° C., 490 ° C., or 200 ° C., the ratio of the weight change of FePO ₄ .xH ₂ O was similarly examined. The mass after heating was the same as the mass before heating. They were 76.33%, 78.69% and 84.14%, respectively. The coefficient of crystal water x at each heating upper limit temperature obtained from the mass difference before and after heating is as shown in Table 1.

[X-ray diffraction measurement]
X-ray diffraction (XRD) measurement in the range of 15 ≦ 2θ ≦ 40 ° was performed on the active materials b and d of the present invention or the comparative active material e using MiniFlex II manufactured by Rigaku. The result is shown in FIG. From this figure, the active material e which is FePO ₄ .3.33H ₂ O without heat treatment is in an amorphous state, and FePO ₄ .0.84H ₂ O heat treated at 490 ° C. is orthorhombic (space group Pbca). include FePO ₄ · 2H ₂ O, and, FePO ₄ was heat-treated at 750 ° C. was found to contain a FePO ₄ orthorhombic (space group Pbca). In addition, these X-ray diffraction (XRD) patterns include Heterosite type FePO ₄ produced by extracting Li from LiFePO ₄ , monoclinic FePO ₄ .2H ₂ O (space group shown in Non-Patent Document 1). P2 ₁ / n) and monoclinic FePO ₄ (space group P2 ₁ / n) were not detected, and it was revealed that these were not included.

[Production of battery]
The following operations were all performed in a dry room having a dew point dew point of −50 ° C. or lower. The present active material a, acetylene black as a conductive auxiliary agent, and polyimide as a binder are contained in a mass ratio of the present active material a: acetylene black: polyimide = 80: 10: 10, N— A negative electrode paste using methyl-2-pyrrolidone (NMP) as a solvent was prepared. The negative electrode paste was applied to one side of a 20 μm thick copper foil using an applicator, and then dried at 80 ° C. for 30 minutes in order to remove NMP. In addition, the gap of the applicator was appropriately adjusted so that the coating mass after drying was 5 mgcm ⁻² . The obtained negative electrode plate was processed so as to have an electrode area of 12 cm ² to obtain a negative electrode.

A counter electrode was prepared by bonding a lithium metal foil with a thickness of 300 μm on both sides of a current collector made of stainless steel (product name: SUS316) to which a nickel terminal was attached. Using the negative electrode and the counter electrode, a 10 mAh-class lithium ion secondary battery using an aluminum laminate film as the outer package was produced. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ to 1 mol / dm ³ in a non-aqueous solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 3: 7 was used. The lithium ion secondary battery thus obtained is referred to as the present invention battery A.
Inventive batteries B to D and comparative battery E were produced in the same manner as described above except that the inventive active materials b to d and the comparative active material e were used instead of the inventive active material a.

[Initial charge / discharge process]
Invention batteries A to D and comparative battery E produced in the above steps were subjected to an initial charge / discharge step in a 25 ° C. environment prior to a cycle performance test and a capacity confirmation test.

With respect to the manufactured present invention batteries A to D and the comparative battery E, a charging current of 0.1 mAcm ⁻² , a charging voltage of 0.02 V, a constant current and constant voltage charging with a total charging time of 48 hours, and a discharging current of 0.1 mAcm ^{− 2} , 1 cycle charge / discharge consisting of constant current discharge with a final voltage of 2.0V was performed. The discharge capacity at this time is defined as the initial capacity.

[Cycle performance test]
About this invention battery AD after the said initial stage charging / discharging process and the comparison battery E, charging current 0.5mAcm ^<-2 >, charging voltage 0.02V, constant current constant voltage charge of the total charging time 10 hours, and discharge current 0 10 cycles of charge / discharge consisting of constant current discharge of 0.5 mAcm ⁻² and final voltage of 2.0 V were performed.

[Capacity confirmation test]
For the present invention batteries A to D and the comparative battery E after the cycle performance test, a charging current of 0.1 mAcm ⁻² , a charging voltage of 0.02 V, a constant current and constant voltage charging with a total charging time of 48 hours, and a discharging current One cycle of charge / discharge consisting of constant current discharge of 0.1 mAcm ⁻² and final voltage of 2.0 V was performed.

  The capacity retention rate was calculated by dividing the discharge capacity obtained in the capacity confirmation test by the initial capacity. Table 2 shows the initial capacity and capacity retention rate.

As shown in Table 2, the present invention batteries A to F using FePO ₄ .xH ₂ O (0 <x) containing crystal water as compared to the comparative battery E using FePO ₄ containing no crystal water as an active material. D showed a high capacity retention rate. Furthermore, in the present invention battery A and the present invention battery B in which the range of the value of x is 0 <x ≦ 1, both the initial capacity and the capacity retention rate are significantly superior to the comparative battery E. It was.

As described above, the reason why the cycle performance and the discharge capacity of the iron phosphate compound are improved by including the crystal water is not clear, but can be considered as follows.
FePO ₄ that does not contain crystal water has a high true density, and when lithium is occluded to 0.8 V or less with respect to the metal lithium potential, the volume changes greatly with the occlusion / release of lithium, and thus charge and discharge are repeated. The capacitance decreases due to the collapse of the crystal structure and the increase of the contact resistance with the current collector and the conductive additive accompanying the volume change of the active material.
On the other hand, FePO ₄ .xH ₂ O (0 <x) has a structure that is bulkier by the amount of crystallization water than FePO ₄ that does not contain crystallization water. Since the decomposition potential of water is about 0.8 V with respect to the metal lithium potential, when lithium is inserted into FePO ₄ .xH ₂ O (0 <x) until the potential is lower than 0.8 V with respect to the metal lithium potential. The crystal water in the crystal is decomposed, but at the same time, it is presumed that Li storage materials such as Li ₂ O and Li ₃ P are generated. Since these Li storage materials play the same role as crystal water in FePO ₄ .xH ₂ O (0 <x) and maintain a bulky crystal structure, FePO ₄ .xH ₂ O (0 <x) is crystal water. Compared to FePO ₄ containing no FePO ₄ , the volume change associated with charge / discharge is reduced. As a result, FePO ₄ .xH ₂ O (0 <x) is considered to exhibit excellent cycle performance without a decrease in capacity even after repeated charge and discharge. Further, since the amount of charged water consumed by the decomposition of crystal water is suppressed because the ratio of crystal water contained in the iron phosphate compound crystal is in the range of 0 <x ≦ 1, the discharge capacity is excellent. It is estimated that the non-aqueous electrolyte secondary battery will be obtained.

From the above, by using FePO ₄ .xH ₂ O (0 <x) as the negative electrode active material, a nonaqueous electrolyte secondary battery excellent in cycle performance can be provided. Further, by using FePO ₄ .xH ₂ O (0 <x ≦ 1) as the negative electrode active material, the discharge capacity of the nonaqueous electrolyte secondary battery using the iron phosphate compound as the negative electrode active material is made excellent. be able to.

Claims (7)

A negative electrode active material for a non-aqueous electrolyte secondary battery, comprising iron (III) phosphate containing crystal water. A negative electrode active material for a non-aqueous electrolyte secondary battery, comprising FePO ₄ .xH ₂ O (0 <x). The negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 2, wherein the range of the value of x is 0 <x≤1. 4. The negative electrode active material for a nonaqueous electrolyte secondary battery according to claim 1, wherein a peak derived from crystal water exists in an infrared absorption spectrum of the negative electrode active material for the nonaqueous electrolyte secondary battery. 5. The negative electrode active material for a non-aqueous electrolyte secondary battery is a simple substance having an amorphous or orthorhombic (space group Pbca) crystal structure, or a mixture of these having a crystal structure. The negative electrode active material for nonaqueous electrolyte secondary batteries according to 1 to 4. A negative electrode for a non-aqueous electrolyte secondary battery comprising the negative electrode active material for a non-aqueous electrolyte secondary battery according to claim 1. A nonaqueous electrolyte secondary battery using the negative electrode for a nonaqueous electrolyte secondary battery according to claim 6.