JP4641375B2 - Method for producing composite of olivine type lithium phosphate and carbon material - Google Patents

Description

The present invention relates to a method for producing a composite of an olivine-type lithium phosphate and a carbon material used for an active material for an electrochemical element capable of intercalating and deintercalating metal cations such as Li ions.

A lithium ion secondary battery uses a carbon material as an active material for a negative electrode, a composite oxide of Li and a transition metal such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ as an active material for a positive electrode, and is a microporous polyolefin. By using a membrane as a separator and using an organic electrolytic solution having a lithium salt such as LiPF ₆ or LiBF ₄ as an electrolyte, Li ions are intercalated into the carbon material of the negative electrode during charging, and Li ions are positive electrode Li during discharging. Charging / discharging is performed by intercalating a composite oxide of bismuth and transition metal (Patent Document 1).
US Pat. No. 4,567,031

  However, the above active material has a problem that it is difficult to charge and discharge at high speed because Li ion intercalation and deintercalation are slow.

In addition to the materials described above, LiFePO ₄ , which is an olivine-type lithium phosphate, has been studied as a lithium secondary battery in addition to the above materials (Patent Document 2).
JP-A-9-134724

However, this LiFePO ₄ has a conductivity as low as about 10 ⁻⁹ mS / cm, and sufficient conductivity cannot be obtained by a method such as adding a conductive auxiliary agent for producing a conventional electrode. The problem was that it was difficult to discharge.

Therefore, it is also considered to composite the LiFePO ₄ and the carbon material to improve it (Patent Documents 3 to 5). In addition, a method of doping LiFePO ₄ with a dissimilar metal such as Zr, Ti, Nb, or Mg has been studied (Non-Patent Document 1).
JP 2002-75364 A Japanese Patent Laid-Open No. 2003-34534 JP 2003-203628 A Nature Materials, Vol. 1 OCT. 2002

However, these methods are all, by firing in synthesizing LiFePO _4, or complexed with carbon material, since the doped dissimilar metals LiFePO _4, the large size of the micron order or more LiFePO ₄ To produce crystals. Thus, the active material complexed even when complexed with carbon materials, Toka particulate carbon material is a structure attached to the surface, becomes the internal containing a carbon material on the structure of the active material, therefore, the LiFePO ₄ The conductivity between crystal grains cannot be sufficiently secured. Also, when different types of metals are doped, the conductivity can be improved, but since the firing method is adopted, a long heating step at a high temperature is required, and the cost required for the process becomes high. There was a point.

The present invention solves the problems of active materials for electrochemical devices such as lithium secondary batteries as described above, and is capable of constituting a high-capacity electrochemical device that can be charged and discharged at high speed. It aims at providing the manufacturing method of the composite_body | complex of the olivine type lithium phosphate used for the active material for elements, and a carbon material.

As a result of intensive studies to solve the above problems, the inventors of the present invention are a composite of an olivine-type lithium phosphate having a chemical formula represented by LiFePO ₄ and a carbon material, and the carbon material is substantially in a form. When the active material for an electrochemical device is configured with a structure in which a part or all of the carbon material is covered with olivine type lithium phosphate, it is possible to charge and discharge at high speed and to have a high capacity It has been found that a chemical element can be constructed, and the present invention has been completed.

That is, by combining the olivine-type lithium phosphate, whose chemical formula is mainly represented by LiFePO ₄ , and the carbon material, the diffusion of Li ions is accelerated, and charging / discharging at high speed becomes possible. In addition, by combining the olivine-type lithium phosphate and carbon material at the nano level, conductivity is ensured, load characteristics are improved, capacity at high-speed charge / discharge is further increased, and high capacity is obtained. It is done.

In the present invention, primarily the chemical formula The olivine-type lithium phosphate represented by LiFePO _4, the main component is an olivine-type lithium phosphate represented by LiFePO _4, as a byproduct when generating the LiFePO ₄ LiFePO ₄ (OH) or Li ₂ Fe ₂ PO ₄ or the like may be contained as a by-product.

The active material for an electrochemical device comprising a composite of an olivine type lithium phosphate and a carbon material produced by the production method of the present invention can be charged / discharged at high speed, and Li ions can be intercalated. High capacity can be obtained because deintercalation is possible. Therefore, by using this active material, a high-capacity electrochemical device capable of high-speed charge / discharge can be constructed.

In the present invention, the carbon material to be combined with the olivine-type lithium phosphate mainly represented by the chemical formula LiFePO ₄ is a substance that can impart electronic conductivity to the composite as an active material by combining. If there is no particular limitation, for example, artificial graphite, acetylene black, ketjen black, carbon black, vapor grown carbon fiber (VGCF), amorphous carbon, carbon fiber, carbon nanotube, fullerenes, etc. A carbonaceous material can be used suitably.

The composite of the olivine-type lithium phosphate mainly represented by the chemical formula LiFePO ₄ and the carbon material may substantially retain the form of the carbon material in order to make use of the electronic conductivity of the carbon material. preferable. The term “substantially retaining the form” as used herein means that the form of the carbon material before compounding and the active material after compounding have a similar structure when morphological observation is performed using an electron microscope or the like. It means doing. For example, when it is observed that crystals of olivine-type lithium phosphate are deposited on the micron order, such as a conventional composite of olivine-type lithium phosphate and a carbon material, the carbon material is clearly a composite. The form is different from that before the conversion, and it cannot be said that the form of the carbon material is substantially maintained.

The composite of the olivine type lithium phosphate and the carbon material mainly represented by the chemical formula LiFePO ₄ of the present invention is FeSO ₄ or its hydrate, FeSO ₄ .nH ₂ O (nH ₂ O is hydration water). FeCl ₂ or its hydrate FeCl ₂ .nH ₂ O (where nH ₂ O is hydration water) and (NH ₄ ) ₂ Fe (SO ₄ ) ₂ or its hydrate (NH ₄ ) ₂ Fe (SO ₄ ) ₂ · nH ₂ O (nH ₂ O is hydration water) and at least one selected from the group consisting of LiOH or hydrates thereof LiOH · nH ₂ O (nH ₂ O is hydration water) ) And H ₃ PO ₄ are raw materials for olivine-type lithium phosphate, and the raw materials for olivine-type lithium phosphate and carbon materials are stirred and mixed, and then a precursor of LiFePO ₄ by a hydrothermal synthesis method. LiFePO ₄ (OH) is mainly phosphorus Sanli Synthesizing a complex of um and a carbon material, then, it can be produced by annealing by heat treatment in an inert gas. The hydrothermal synthesis method at that time is not particularly limited, and a conventionally known method can be employed. For example, a method of heat-treating the mixture of the raw material for the olivine-type lithium phosphate and the carbon material, which are stirred and mixed, in a pressure resistant vessel, preferably at 130 to 170 ° C., more preferably 150 ° C. can be employed. And the time of the heat processing in that case is preferably 1 to 5 hours. If the heat treatment time is less than 1 hour, the LiFePO ₄ precursor is not sufficiently produced, and the amount of LiFePO ₄ produced after annealing is small, and good characteristics may not be obtained. Further, when the heat treatment time is more than 5 hours, it is sufficiently exploit that good electron conductivity possessed by the carbon material to cause crystal growth in the crystal precipitated micron order of LiFePO ₄ when generating precursor LiFePO ₄ There is a risk that it will not be possible. The produced LiFePO ₄ precursor can be annealed in a subsequent inert gas to produce LiFePO ₄ . As temperature at the time of annealing treatment, 400-600 degreeC is preferable, 450-550 degreeC is more preferable, About 500 degreeC is further more preferable. If the annealing temperature is too low, a large amount of LiFePO ₄ (OH) remains, and if the annealing temperature is too high, Li ₂ Fe ₂ (PO) ₄ is generated. In either case, there is an adverse effect such as reducing the capacity of the active material, which is not preferable. The annealing treatment time is preferably 30 minutes to 5 hours. Further, when hydrothermal synthesis is performed, a fine particle agent, a surfactant and the like can be appropriately added. As the micronizing agent, for example, polyethylene glycol can be used, and as the surfactant, for example, acetone can be used. By synthesizing by a hydrothermal synthesis method, it is possible to synthesize olivine type lithium phosphate complexed with a carbon material at a low temperature in a short time.

The active material for an electrochemical device comprising a composite of the olivine-type lithium phosphate represented by the chemical formula LiFePO ₄ and the carbon material of the present invention thus obtained is obtained by combining the olivine-type lithium phosphate and the carbon material. It is compounded at the nanometer scale, and even after the compounding, the structure of the carbon material is substantially maintained and the carbon material is coated with olivine type lithium phosphate. The coating with the olivine-type lithium phosphate may extend over the entire carbon material or may be a partial coating. The ratio between the olivine-type lithium phosphate and the carbon material is not particularly limited, but is preferably 40:60 to 90:10 by mass ratio. When the ratio of the olivine type lithium phosphate is less than the above range, the discharge capacity of the active material is decreased. When the ratio is more than the above range, the ratio of the carbon material is too small and sufficient electronic conductivity may not be obtained.

The obtained composite of olivine-type lithium phosphate mainly represented by the chemical formula LiFePO ₄ and a carbon material can form an electrode by a method such as application to a conventionally known electrode current collector, pressure bonding, or pelletization. . Although there is no restriction | limiting in particular in these methods, In order to utilize a high-speed charge / discharge characteristic more appropriately, it is preferable to employ | adopt the method of apply | coating to a collector or crimping | bonding. The base material used for the current collector may be any material that is generally used for current collectors of electrochemical devices. For example, when used as the positive electrode of a Li-ion battery, Al, Ni, etc. Are preferably used. And as a collector, what processed those base materials into foil shape, mesh shape, etc. can be used. Further, a polymer binder or the like may be added for the purpose of improving the adhesion to the current collector. The polymer binder may be any compound as long as it has the necessary adhesion, is not soluble in the electrolyte, and is stable without causing side reactions such as oxidation or reduction in the voltage range used. However, for example, fluoropolymers such as polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, and tetrafluoroethylene are preferably used from the viewpoint of being chemically stable.

An electrochemical device can be formed using the composite of the olivine-type lithium phosphate and the carbon material obtained as described above, in which the chemical formula is mainly LiFePO ₄ , as an active material. Considering that the potential of the active material made of this composite is about 3.4 V as compared to Li, it is preferable to use it as the positive electrode active material. As a counter electrode when used as a positive electrode, for example, metal such as lithium, LiAl alloy, Sn, Si, or other metal that forms an alloy with Li, or lithium such as amorphous carbon, artificial graphite, natural graphite, fullerene, or nanotube is used. Lithium titanate capable of occluding and releasing lithium, such as a carbon-based material that can be occluded and released (intercalated and deintercalated), Li ₄ Ti ₅ O ₁₂ , and Li ₂ Ti ₃ O ₇ can be used.

In constituting an electrochemical element using the active material for an electrochemical element of the present invention, an electrolytic solution in which a Li salt is dissolved in an organic solvent is used. The Li salt may be any one as long as it dissociates in a solvent to form Li ⁺ ions and does not cause a side reaction such as separation in the voltage range used as the device. For example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiClO ₄ and other inorganic compounds, LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), LiC ( _{SO 2 CF 2) 3, LiC} (SO 2 C 2 F 5) 2, LiPF 6-n (C 2 F 5) n ( n is an integer from 1 to _{6), LiSO 2 CF 3,} LiSO 3 C 2 F ₆ , organic compounds such as LiSO ₂ C ₄ F ₈ can be used.

  Any organic solvent may be used as long as it dissolves the Li salt and does not cause side reactions such as decomposition in the voltage range used as the device. For example, ethylene carbonate, propylene carbonate, butylene carbonate, vinylene Cyclic carbonates such as carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, cyclic esters such as γ-butyrolactone, chain ethers such as dimethoxyethane, diglyme, triglyme and tetraglyme, dioxane, tetrahydrofuran, 2-methyl Cyclic ethers such as tetrahydrofuran, nitriles such as acetonitrile, propionitrile, and methoxypropionitrile can be used alone or in combination of two or more. In order to obtain particularly good characteristics, it is preferable to employ a combination capable of obtaining high conductivity such as a mixed solvent of ethylene carbonate and chain carbonate.

  These electrolytes contain additives such as vinylene carbonate or derivatives thereof, benzene or derivatives thereof, 1,3-propane sultone, diphenyl for the purpose of improving safety, cycleability, high temperature storage properties and the like. -Additives such as disulfide or a derivative thereof, biphenyl or a derivative thereof can be appropriately added.

  Instead of organic solvents, room temperature molten salts such as ethyl-methylimidazolium trifluoromethylsulfonium imide, heptyl-trimethylammonium trifluoromethylsulfonium imide, pyridinium trifluoromethylsulfonium imide, guanidinium trifluoromethylsulfonium imide, etc. It can also be used.

  Further, the above electrolyte solution is crosslinked with polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer, polyacrylonitrile, polyethylene oxide, polypropylene oxide, ethylene oxide-propylene oxide copolymer, ethylene oxide chain in the main chain or side chain. An electrolytic solution obtained by adding a host polymer capable of forming a gel electrolyte such as a polymer to be gelled can also be used.

  Also, instead of electrolyte, polymers such as polyethylene oxide, siloxane polymers containing ethylene oxide groups in the side chains, (meth) acrylates containing ethylene oxide groups in the side chains, polyethylene carbonates, polypropylene carbonates, polycarbonates containing anhydroglucitol groups It is also possible to use a polymer electrolyte comprising lithium salt.

Further, instead of the electrolytic solution, Li ₂ S—SiS ₂ —Li ₄ SiO ₄ , Li ₂ S—SiS ₂ —P ₂ S ₅ —LiI, LiO ₂ —Al ₂ O ₃ —TiO ₂ —P ₂ O _5, etc. Inorganic glass electrolytes, NASICON types such as LiTi ₂ (PO ₄ ) ₃ , perovskite types such as La _0.57 Li _0.28 TiO ₃ , LISICON types such as Li _3.25 Ge _0.25 P _0.75 S ₄ and Li _3.4 Si _0.4 P _0.6 S ₄ An inorganic solid electrolyte can also be used.

  An electrochemical element is constructed using these constituent materials, and the form of the element at that time may be any shape and form such as a conventionally known cylindrical shape, square shape, coin shape or laminate type. There is no particular limitation. For example, when producing a cylindrical electrochemical element, a sheet-like positive electrode and negative electrode produced through application or pressure bonding to a current collector are wound through a separator made of a polyolefin microporous film. The wound electrode body is inserted into a bottomed cylindrical can made of a material such as stainless steel or aluminum, attached with positive and negative terminals, injected with an electrolyte, and sealed. As a sealing method, a method such as caulking through a gasket or laser welding can be employed. In addition, when producing a laminate type electrochemical element having an aluminum laminate film or the like having aluminum as a core material as an exterior material, the electrode body having the winding structure described above is wound not in a cylindrical shape but in an oval shape to form a bag. It is produced by inserting the aluminum laminate film into a bag, taking out the terminals, injecting an electrolyte, and sealing the laminate film by heat fusion or the like. When producing a coin-shaped electrochemical device, a pellet-shaped electrode formed by pressing and a counter electrode are placed in a can via a separator, an electrolyte is injected, a counter electrode can is covered, and a sealing gasket is inserted. It is made by caulking.

  Next, an Example is given and this invention is demonstrated more concretely in detail. However, the present invention is not limited to these examples. In the following examples and the like,% indicating the concentration of a solution or the like is mass% unless otherwise noted.

Example 1
A 1 mol / l LiOH.H ₂ O aqueous solution, a 1 mol / l FeSO ₄ .7H ₂ O aqueous solution, and H ₃ PO ₄ have a molar ratio of Li: Fe: P = 3.0: 1.0: 1. were mixed so that 0, was added polyethylene glycol (1/3 of the volume of LiFePO ₄ for the raw material solution) as fine particles accelerator. Furthermore, acetone (same volume as the raw material liquid for LiFePO ₄ ) as the surfactant and acetylene black (average particle diameter 35 nm) as the carbon material have the same mass as the amount calculated assuming that the raw materials are all LiFePO _4. The mixture was stirred and mixed for 15 hours. The obtained mixture was put into a pressure vessel and hydrothermal synthesis was performed at 150 ° C. for 3 hours to obtain a composite of a precursor of LiFePO ₄ and a carbon material. The composite was washed and then annealed in N ₂ at 500 ° C. for 1 hour to obtain a composite of an olivine type lithium phosphate having a chemical formula mainly represented by LiFePO ₄ and a carbon material.

Example 2
Except that the annealing temperature was 400 ° C., a composite of an olivine type lithium phosphate whose chemical formula was mainly represented by LiFePO ₄ and a carbon material was produced in the same manner as in Example 1.

Example 3
Except that the annealing temperature was 550 ° C., a composite of an olivine type lithium phosphate whose chemical formula was mainly represented by LiFePO ₄ and a carbon material was manufactured in the same manner as in Example 1.

Example 4
Except that the annealing temperature was 600 ° C., a composite of an olivine type lithium phosphate whose chemical formula was mainly represented by LiFePO ₄ and a carbon material was manufactured in the same manner as in Example 1.

Comparative Example 1
A composite of a LiFePO ₄ precursor and a carbon material was produced in the same manner as in Example 1 except that annealing was not performed.

Comparative Example 2
An olivine type lithium phosphate was produced in the same manner as in Example 1, except that acetylene black as a carbon material was not mixed.

1 is a composite of an olivine-type lithium phosphate having a chemical formula mainly represented by LiFePO ₄ and a carbon material manufactured in Examples 1 to ₄ , and a composite of a precursor of LiFePO ₄ and a carbon material manufactured in Comparative Example 1. The measurement result of the X-ray diffraction spectrum of a body is shown.

As shown in FIG. 1, in Example 1, it can be seen that almost 100% olivine-type lithium phosphate is LiFePO ₄ , and a good composite with high purity is obtained. Also, in Examples 2 to 4, it can be seen that LiFePO ₄ is mainly produced although a peak due to impurities is slightly observed.

On the other hand, in Comparative Example 1, it can be seen that almost no LiFePO ₄ peak is generated.

2 shows an SEM (scanning electron microscope) image photograph of the composite produced in Example 1, and FIG. 3 shows an SEM image photograph of acetylene black (AB) as a carbon material used for the composite. FIG. 4 shows a SEM image photograph of LiFePO ₄ produced in Comparative Example 2.

As can be seen from FIGS. 2 to 3, the composite of the olivine type lithium phosphate and the carbon material mainly represented by the chemical formula LiFePO ₄ in Example 1 is a good composite of the olivine type lithium phosphate and the carbon material. And maintains the complexed acetylene black form. On the other hand, as shown in FIG. 4, LiFePO ₄ that is not complexed with the carbon material exhibits a rhomboid crystal structure, and the olivine-type lithium phosphate and the carbon material in which the chemical formula of Example 1 is mainly represented by LiFePO ₄ The difference with the complex is obvious.

Example 5
Polytetrafluoroethylene as a binder was mixed with the composite of olivine type lithium phosphate and the carbon material mainly represented by the chemical formula LiFePO ₄ manufactured in Example 1 at a mass ratio of 90:10. An electrode was prepared, and the resulting mixture was pressure-bonded to a Ni mesh to produce an electrode. The mixture mass on the electrode was 20 mg / cm ³ .

Using the electrode obtained as described above, the metal lithium used as a counter electrode and a reference electrode, a polyethylene microporous membrane separator, the LiClO ₄ in 1M LiClO ₄ PC solution [propylene carbonate (PC) in the electrolyte A tripolar cell was assembled using 1 mol / l dissolved solution], and its electrochemical characteristics were measured as described below.

Comparative Example 3
Acetylene black was mixed at a mass ratio of 1: 1 with the olivine-type lithium phosphate produced in Comparative Example 2, and polytetrafluoroethylene as a binder was blended at a mass ratio of 90:10. A three-electrode cell was assembled in the same manner as in Example 5 except that an electrode was prepared by pressure-bonding the resulting mixture to a Ni mesh, and the electrochemical characteristics thereof were measured. .

  The load characteristics of the cells of Example 5 and Comparative Example 3 manufactured as described above were measured. The result is shown in FIG. The load characteristics were measured by changing the current density from 0.02 A / g to 10 A / g, with a current density during charging of 30 mA / g, a final voltage of 4 V, and a final voltage of 2.5 V during discharging.

  As shown in FIG. 5, in Example 5, compared to Comparative Example 3, there is little decrease in discharge capacity when discharged at a high current density, excellent load characteristics, and high speed charge / discharge is possible. And it showed that a high capacity was obtained.

  Moreover, about the cell of Example 5, the cyclic voltagram (CV) measured in the range of 2.5-4.3V at the sweep rate of 0.1 mV / s at 25 degreeC is shown in FIG. As shown in FIG. 6, the cell of Example 5 has no cyclic voltagram deformation after 5 charge / discharge cycles, exhibits good cycle characteristics, and is suitable for secondary electrochemical devices such as lithium secondary batteries. It had sufficient cycle characteristics as required.

It is a figure which shows the X-ray-diffraction spectrum of the composite_body | complex of the olivine type lithium phosphate of Examples 1-4 and the comparative example 1, and a carbon material. 1 is a diagram showing an SEM image of a composite of an olivine-type lithium phosphate and a carbon material produced mainly in Example 1 and whose chemical formula is mainly represented by LiFePO _4. FIG. 1 is a diagram showing an SEM image of acetylene black used in the manufacture of a composite of an olivine-type lithium phosphate whose chemical formula is mainly represented by LiFePO ₄ and a carbon material in Example 1. FIG. Primarily the chemical formula was prepared in Comparative Example 2 is a view showing an SEM image of the olivine-type lithium phosphate represented by LiFePO _4. It is a figure which shows the load characteristic of the electrochemical element produced in Example 5 and Comparative Example 3. 6 is a diagram showing a cyclic voltammogram of the electrochemical device produced in Example 5. FIG.

Claims (2)

FeSO ₄ or its hydrate FeSO ₄ .nH ₂ O (nH ₂ O is hydrated water), FeCl ₂ or its hydrate FeCl ₂ .nH ₂ O (nH ₂ O is hydrated water) and At least one selected from the group consisting of (NH ₄ ) ₂ Fe (SO ₄ ) ₂ or a hydrate thereof (NH ₄ ) ₂ Fe (SO ₄ ) ₂ .nH ₂ O (nH ₂ O is hydrated water) The seed, LiOH or its hydrate, LiOH.nH ₂ O (nH ₂ O is hydration water) and H ₃ PO ₄ are used as raw materials for olivine-type lithium phosphate, and for the olivine-type lithium phosphate After stirring and mixing the raw material and the carbon material, a precursor of LiFePO ₄ is generated by a hydrothermal synthesis method and annealed at 400 to 600 ° C. in an inert gas. The chemical formula is LiFePO ₄ . Represented Method for producing a complex of the olivine-type lithium phosphate and a carbon material. The carbon material includes at least one selected from the group consisting of artificial graphite, acetylene black, ketjen black, carbon black, vapor grown carbon fiber, amorphous carbon, carbon fiber, carbon nanotube, and fullerenes. The manufacturing method of the composite_body | complex of the olivine type lithium phosphate and carbon material of Claim 1 .

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