JP5164260B2 - Method for producing carbon-olivine type lithium iron phosphate composite, and positive electrode material for lithium ion battery - Google Patents

Description

  The present invention relates to a method for producing a carbon-olivine type lithium iron phosphate composite and a positive electrode material for a lithium ion battery containing the composite.

The lithium-ion battery (lithium-ion secondary battery) was commercialized in 1991 as a new battery with higher energy density than before, and now the market is rapidly expanding as an indispensable power source for electronic devices such as mobile phones and laptop computers. It is continuing. However, although the battery performance has been improved by improving and developing negative electrode materials and electrolytes, rock salt type lithium cobalt oxide (LiCoO ₂ ) has been frequently used as the positive electrode material from the beginning to the present. Lithium-ion batteries using LiCoO ₂ have excellent performance as small batteries, but the price of raw materials cobalt is small, and the price fluctuates violently or when an internal short circuit occurs for some reason during charging. It has problems such as the risk of oxygen being released and the electrolyte solution burning and exploding. Therefore, there is a need for a safe and effective alternative positive electrode material for lithium ion batteries.

Under such circumstances, iron-based materials rich in raw materials are expected, and in particular, olivine-type lithium iron phosphate is attracting attention as a next-generation positive electrode material that can solve the above-described problems of LiCoO ₂ (for example, , Patent Document 1, Non-Patent Document 1, etc.).

By the way, the positive electrode of a normal lithium ion battery uses a material such as LiCoO ₂ as an active material, and a conductive aid such as a carbon material for increasing the electron conductivity in the positive electrode, and the active material and conductive aid. Forming a positive electrode mixture having a binder for binding, or forming such a positive electrode mixture layer (positive electrode mixture layer) on one or both sides of a conductive substrate serving as a current collector It consists of

  However, since olivine-type lithium iron phosphate has very low electronic conductivity, it is difficult to ensure excellent battery characteristics by simply forming a positive electrode by coexisting a conductive auxiliary agent. It is.

  In view of this, a technique for improving electron conductivity in a positive electrode material for a lithium ion battery using olivine type lithium iron phosphate has been studied.

  For example, in Patent Document 2, a phosphoric acid aqueous solution containing a lithium salt and an iron salt is mixed with a water-soluble organic reducing agent such as ascorbic acid, a carbon material is further added, an alkali solution is mixed, A production method has been proposed in which a coprecipitate of a composite phosphorous oxide of lithium and iron is produced and calcined to obtain a composite of olivine-type lithium iron phosphate and carbon. And in patent document 2, in this olivine type lithium iron phosphate and carbon composite, since the electron conductivity is improved by the carbon particles adhering to the surface of the olivine type lithium iron phosphate particles, the olivine type It is described that the electronic conductivity is better than that of lithium iron phosphate alone.

Japanese Patent No. 3484003 JP 2002-117831 A “Hydrothermic synthesis of lithium ion phosphate cata- lys”, Electrochemistry Communication, 2002, No. 3, p. 505-508

  However, according to the study of the present inventors, it has been found that the production method disclosed in Patent Document 2 cannot synthesize olivine-type lithium iron phosphate itself.

  Therefore, the applicant firstly (1) a step of producing a carbon-iron phosphate complex by precipitation, and (2) a coprecipitate containing the carbon-iron phosphate complex and lithium phosphate. A carbon-olivine type phosphoric acid suitable as a positive electrode material for a lithium ion battery by producing a carbon-olivine type lithium iron phosphate composite through a step of producing and (3) a step of firing the coprecipitate An iron complex has been produced by a simple and efficient method, and a patent application has already been filed for it (Japanese Patent Laid-Open No. 2007-35295).

  However, in order to increase the capacity of the lithium ion battery, it is desirable to improve the carbon-olivine type iron phosphate composite so as to be suitable for it.

  Accordingly, the present invention provides a method for easily and efficiently producing a carbon-olivine-type lithium iron phosphate composite capable of producing a higher-capacity lithium ion battery, and the carbon-olivine-type obtained by the production method. An object of the present invention is to provide a higher capacity positive electrode material for a lithium ion battery using a lithium iron phosphate composite.

The present invention that achieves the above object includes the following steps (1) to (4): (1) a step of producing a carbon-iron phosphate complex by precipitation;
(2) a step of producing a coprecipitate containing the carbon-iron phosphate complex and lithium phosphate;
(3) A step of adding and mixing an organic substance to be a carbon precursor to the coprecipitate,
(4) A step of firing a mixture of the coprecipitate and an organic substance that becomes a carbon precursor,
And a method for producing a carbon-olivine type lithium iron phosphate complex, characterized by producing a carbon-olivine type lithium iron phosphate complex.

  In the present invention, after the coprecipitate is produced in the step (2), the coprecipitate is dried and calcined, and the step (3) is converted to the calcined product of the coprecipitate. It is good also as a process of adding the organic substance used as a carbon precursor, and making it the process of mixing, and baking the mixture of the calcined product of the said coprecipitate and the organic substance used as a carbon precursor.

  Furthermore, a positive electrode material for a lithium ion battery comprising a carbon-olivine type lithium iron phosphate composite obtained by the production method of the present invention is also included in the present invention, and the carbon-olivine type iron phosphate composite described above, A positive electrode material for a lithium ion battery containing 0.1 to 30% by mass of a carbon material other than carbon derived from the carbon-olivine type lithium iron phosphate composite is also included in the present invention.

  ADVANTAGE OF THE INVENTION According to this invention, the carbon-olivine type lithium iron phosphate composite suitable as a positive electrode material for lithium ion batteries which can manufacture a high capacity | capacitance lithium ion battery can be manufactured simply and efficiently. Moreover, the carbon-olivine type lithium iron phosphate composite obtained by the production method of the present invention, and the positive electrode material for a lithium ion battery of the present invention containing the composite are high capacity and good battery characteristics. Can be configured. The reason why the present invention can achieve such an effect will be sequentially described in the following [Best Mode for Carrying Out the Invention].

  In order to improve the electronic conductivity of the positive electrode material for lithium ion batteries having olivine type lithium iron phosphate, olivine type lithium iron phosphate and carbon functioning as a conductive additive are uniformly mixed. Is required. In the production method of the present invention, the synthesis of olivine-type lithium iron phosphate and the combination with carbon are simultaneously achieved by a simple and efficient method so that carbon and olivine-type lithium iron phosphate are mixed uniformly. In addition, since an organic substance that is a precursor of carbon is added and baked, carbon that is generated from the organic substance added thereafter and carbon that is already complexed with iron phosphate are carbon with high electron conductivity. Thus, it was possible to produce carbon-olivine type lithium iron phosphate having high electron conductivity.

  As a main synthesis method of olivine-type lithium iron phosphate, for example, as described in Patent Document 1, a solid phase method in which raw material compounds are mixed and fired, or as described in Non-Patent Document 1, A hydrothermal method is known in which a mixture of a raw material compound and water is heated in a pressure vessel. Further, as described above, Patent Document 2 describes a coprecipitation method in which an alkaline solution is added to a mixed aqueous solution containing a raw material compound and a reducing agent to obtain a coprecipitate, and the coprecipitate is fired.

  However, the solid phase method requires a long mixing step in order to make the composition uniform when mixing raw material compounds before firing, and cannot be an efficient method. Further, in the hydrothermal method, it is necessary to add a part of the raw material compound in a considerably excessive amount than theoretically required. In the coprecipitation method disclosed in Patent Document 2, as described above, according to the study of the present inventors, it is difficult to synthesize the olivine-type lithium iron phosphate itself.

  Therefore, in the present invention, the carbon-iron phosphate complex and the lithium phosphate are obtained by the steps (1) and (2) using a reaction in a liquid that can be easily and efficiently performed uniform dispersion and mixing. In the subsequent step (3), an organic substance to be a carbon precursor is added to and mixed with the coprecipitate. In the step (4), the coprecipitate and the carbon precursor are mixed. The carbon-olivine-type lithium iron phosphate complex in which a carbon network with high electron conductivity is formed and uniformly complexed by firing a mixture with the organic substance to become a simple and efficient facility Was successfully manufactured. Hereinafter, the method of the present invention will be described in detail.

  Step (1) in the method of the present invention is a step of producing a carbon-iron phosphate complex by precipitation. Specifically, for example, an aqueous solution having a compound containing phosphoric acid and divalent iron is added to an aqueous phase in which a carbon material is dispersed, and then neutralized with a water-soluble base, thereby carbon. -Precipitate the iron phosphate complex.

  The carbon material used in the step (1) is not particularly limited as long as it is a powder or a dispersion thereof, but a material having good conductivity is preferred. Specific examples include carbon black (acetylene black, ketjen black, furnace black, lamp black, etc.), graphite, graphite and the like. Carbon materials with shape anisotropy (carbon materials having a fiber-like shape with a certain aspect ratio) such as carbon nanofibers, carbon nanotubes, carbon nanohorns, carbon nanowires, and carbon nanocoils are also used as carbon materials. it can. Among these, acetylene black and ketjen black, which are often used as battery materials, are particularly preferable. In the step (1), the carbon material content in the aqueous dispersion of the carbon material is preferably 0.1 to 30% by mass, for example.

  As phosphoric acid, orthophosphoric acid is preferable. Phosphoric acid may be used as it is, but is preferably used in the form of an aqueous solution. The concentration in the case of an aqueous solution is preferably, for example, 5 to 85% by mass.

  In dispersing the carbon material in water, it is preferable to increase the dispersibility of the carbon material using a known dispersant. As the dispersant, an anionic surfactant, a cationic surfactant, a nonionic surfactant, or the like can be used, and there is no particular limitation, but only carbon remains after firing in the step (4), and no unnecessary components remain. Therefore, a nonionic surfactant is particularly preferable. As for the amount of the dispersant used, it is sufficient if the carbon material can be dispersed well in water. For example, the dispersant is 0.1 parts by mass or more, more preferably 2 parts by mass or more with respect to 100 parts by mass of the carbon material. Thus, it is desirable to use 100 parts by mass or less, more preferably 50 parts by mass or less, and still more preferably 20 parts by mass or less.

  Further, carbon black paste in which a dispersant is dispersed in advance, carbon black whose dispersibility is improved by surface treatment, or the like may be used.

  The iron compound used in the aqueous solution having a compound containing divalent iron is not particularly limited as long as it contains divalent iron and is water-soluble. There are few types, and ferrous sulfate (such as ferrous sulfate and heptahydrate) is industrially easily available. Normally, when synthesis of olivine-type lithium iron phosphate is carried out in the atmosphere using a compound containing divalent iron as a raw material, iron is oxidized from divalent to trivalent. The synthesis of the precursor before firing is performed in an inert gas atmosphere such as nitrogen gas or argon gas, or in the final firing step, firing is performed in a reducing gas atmosphere such as hydrogen to reduce iron. There is a need to. However, in the step (1) of the method of the present invention, the oxidation of iron is prevented because the reaction between phosphoric acid and a compound containing divalent iron is carried out in the presence of carbon. Therefore, the step (1) can be performed in the air, and also in the step (4), it is not necessary to perform firing in a reducing gas atmosphere (however, firing in a reducing gas atmosphere is not necessary). It is not excluded).

  It is preferable that the density | concentration of this compound in the aqueous solution which has a compound containing bivalent iron is 1-50 mass%, for example.

  Examples of the water-soluble base used for neutralization include alkali metal hydroxides such as NaOH and KOH; ammonia; various amines. These water-soluble bases are preferably used as an aqueous solution having a concentration of about 1 to 50% by mass, for example. The amount of the water-soluble base used at the time of neutralization is preferably such that the pH of the reaction solution containing the precipitate after addition of the water-soluble base is 7.0 to 12.0.

  In addition, from the viewpoint of reducing the amount of impurities in the finally obtained carbon-olivine-type lithium iron phosphate complex and favorably producing olivine-type lithium iron phosphate, the carbon-phosphate obtained by the step (1) In the iron complex, the molar ratio of Fe and P is Fe: 1.0, P is 0.3 or more, more preferably 0.5 or more, and 3.0 or less. More preferably, it is 1.0 or less. In the carbon-olivine type lithium iron phosphate composite obtained by the method of the present invention, the carbon content is 0.1% by mass or more, more preferably 1% by mass or more, and 50% by mass or less. The content is preferably 25% by mass or less (details will be described later). Therefore, in the step (1), the content ratio of Fe and P in the carbon-phosphate complex satisfies the above preferred value, and the carbon in the finally obtained carbon-olivine type lithium iron phosphate complex It is preferable to adjust the use ratio of the compound containing the carbon material, phosphoric acid, and divalent iron so that the content becomes the above preferred value.

  In the step (1), the temperature of the reaction solution is preferably 0 to 80 ° C. In the step (1), a precipitate is obtained by neutralizing with a water-soluble base, but from the viewpoint of atomization and homogenization of the precipitate (carbon-iron phosphate complex), a carbon material is contained. While sufficiently stirring the liquid obtained by adding an aqueous solution containing a compound containing phosphoric acid and divalent iron to the dispersion, the water-soluble base (preferably an aqueous solution thereof) is added for about 1 to 60 minutes. It is preferable to dripping over and then to continue stirring for 5 to 60 minutes. For the precipitation, it is possible to change the dropping order of the phosphoric acid, the aqueous solution having a compound containing divalent iron, and the water-soluble base, or perform simultaneous dropping.

  In the step (1), the precipitate of the carbon-iron phosphate complex produced in the reaction solution is taken out by filtration or the like and used for the next step (2). In addition, when many impurities are contained, you may repeat the water washing and filtration of a filter cake.

  As described above, the carbon-iron phosphate complex obtained in the step (1) adopts a method of generating iron phosphate in water in which a carbon material is dispersed in advance, for example, iron phosphate is It is presumed that it has a structure attached so as to cover the surface of the carbon material. However, since carbon black particles generally form a structure and are not monodispersed into individual particles in normal dispersion, a carbon-iron phosphate complex using carbon black as a carbon material is actually It is expected to form a composite precipitate with the carbon black structure as the core. Further, depending on the amount of iron phosphate, it is considered that the entire surface of the carbon material (carbon black) cannot be covered, and a part thereof is exposed.

  The step (2) in the method of the present invention is a step of producing a coprecipitate containing the carbon-iron phosphate complex obtained in the step (1) and lithium phosphate. Examples of the step (2) include the following step (2-1) or step (2-2).

  In the step (2-1), first, the carbon-iron phosphate complex obtained in the step (1) is preferably added to water containing a dispersant, and the mixture is added for a predetermined time (for example, 1 to 60 minutes). About) Stirring to obtain a slurry in which the carbon-iron phosphate complex is uniformly dispersed. Then, phosphoric acid is added to the slurry, and a lithium compound-containing aqueous solution is further added to produce a coprecipitate containing a carbon-iron phosphate complex and lithium phosphate.

  In dispersing the carbon-iron phosphate complex in water, it is preferable to prepare water with a dispersant added in advance, and gradually add the carbon-iron phosphate complex while stirring this. It is preferable to adopt the following method. As the dispersant, an anionic surfactant, a cationic surfactant, a nonionic surfactant, or the like can be used, and there is no particular limitation, but only carbon remains after firing in the step (4), and no unnecessary components remain. Therefore, a nonionic surfactant is particularly preferable.

  The content of the carbon-iron phosphate complex in water is, for example, preferably 1 to 50% by mass. Moreover, as a usage-amount of a dispersing agent, although carbon-iron-phosphate complex should just disperse | distribute favorably in water, for example, with respect to 100 mass parts of carbon-iron-phosphate composites, dispersing agent is 0.1 It is desirable to use it by mass part or more, more preferably 0.2 mass part or more, 10 mass parts or less, more preferably 5 mass parts or less, and further preferably 2.5 mass parts or less.

  As the phosphoric acid used in the step (2-1), orthophosphoric acid is preferable as in the step (1). Moreover, although phosphoric acid may be used as it is, it is preferably used in the form of an aqueous solution. The concentration in the case of an aqueous solution is preferably, for example, 5 to 85% by mass.

  The lithium compound related to the lithium compound-containing aqueous solution used in the step (2-1) is not particularly limited as long as it is water-soluble, but lithium hydroxide is particularly preferable. Moreover, when using lithium hydroxide as a lithium compound, it is preferable that the density | concentration of lithium hydroxide in the lithium compound containing aqueous solution is 0.1-17 mass%, for example.

  In the step (2-1), the temperature of the reaction solution is preferably 0 to 80 ° C. In the step (2-1), a coprecipitate is obtained by adding a lithium compound-containing aqueous solution to water in which the carbon-iron phosphate complex and phosphoric acid are present. From the standpoint of homogenization, the lithium compound-containing aqueous solution is dropped in a time of about 1 to 60 minutes while sufficiently stirring the water containing the carbon-iron phosphate complex and phosphoric acid, and then stirred for 5 to 60 minutes. It is preferable to continue. Furthermore, the coprecipitate can be produced by changing the dropping order of the phosphoric acid and the lithium compound-containing aqueous solution, or simultaneously dropping the phosphoric acid and the lithium compound-containing aqueous solution.

  In the step (2-2), phosphoric acid and a lithium compound-containing aqueous solution are mixed to produce a lithium phosphate precipitate, and the carbon-iron phosphate complex obtained in the step (1) is produced in this water. And co-precipitate containing a carbon-iron phosphate complex and lithium phosphate is obtained. In the step (2-2), phosphoric acid and a lithium compound-containing aqueous solution are mixed in an aqueous phase in which a carbon material is dispersed in advance, and then the carbon-iron phosphate obtained in the step (1). You may add a composite_body | complex. As the carbon material, various materials exemplified in the step (1) can be used, and acetylene black and ketjen black are particularly preferable.

  The same thing as what can be used at the process of (2-1) can be used for the phosphoric acid and lithium compound containing aqueous solution which concern on the process of (2-2). Moreover, it is preferable to add a dispersing agent in the reaction liquid before adding the carbon-iron phosphate complex obtained in the step (1). The same dispersant as in the step (2-1) can be used as the dispersant, and the amount used is preferably the same as in the step (2-1).

  In the step (2-2), when water in which a carbon material is dispersed in advance is used, it is preferable to add a dispersant to the water. The same dispersant as in the step (2-1) can be used as the dispersant. Moreover, the usage-amount is 0.1 mass part or more with respect to 100 mass parts of total amounts of the carbon material added beforehand, and the carbon-iron-phosphate complex added after that, More preferably, it is 0.00. It is desirable that the amount be 2 parts by mass or more and 10 parts by mass or less, more preferably 5 parts by mass or less.

  In the step (2-2), the temperature of the reaction solution is preferably 0 to 80 ° C. In the step (2-2), a precipitate (lithium phosphate precipitate) is obtained by mixing phosphoric acid and a lithium compound-containing aqueous solution. The lithium phosphate precipitate is atomized and homogenized. From this point of view, for example, phosphoric acid is used in the form of an aqueous solution, while the phosphoric acid aqueous solution is sufficiently stirred, the lithium compound-containing aqueous solution is added dropwise over a period of about 1 to 60 minutes, and the carbon-iron phosphate complex is further added. It is desirable to continue stirring for 5 to 60 minutes after adding. In addition, for the formation of a lithium phosphate precipitate, phosphoric acid (preferably a phosphoric acid aqueous solution) is dropped into a lithium compound-containing aqueous solution, or phosphoric acid (preferably a phosphoric acid aqueous solution) and a lithium compound-containing aqueous solution are added to water. It is also possible to perform simultaneous dripping.

  In either the step (2-1) or the step (2-2), a coprecipitate containing a carbon-iron phosphate complex and lithium phosphate (hereinafter referred to as “carbon-iron phosphate-phosphate”). The pH of the reaction solution at the time of production (referred to as “lithium coprecipitate”) is 7.0 or more, more preferably 9.0 or more, and preferably 12.0 or less, more preferably 11.0 or less.

  The step (3) in the present invention is a slurry containing the carbon-iron phosphate-lithium phosphate coprecipitate obtained through the step (2-1) as described above or the step (2-2). Is a step of adding and mixing an organic substance to be a carbon precursor.

  In the present invention, the above-mentioned “organic substance to be a carbon precursor” constitutes a part of carbon of the “carbon-olivine type lithium iron phosphate complex” through the firing step (4). It means “organic matter” and not “what the organic matter changes to constitute the carbon precursor”. That is, the “organic substance to be a carbon precursor” has the same meaning as “organic substance as a carbon precursor”. Examples of the organic substance that serves as the carbon precursor include monosaccharides such as glucose and fructose, disaccharides such as sucrose (sucrose), polysaccharides including three or more saccharides such as cellulose, dextrin, and starch, and derivatives thereof. The monosaccharides and disaccharides can be preferably used, but are inexpensive and water-soluble, and can be uniformly mixed with the carbon-iron phosphate-lithium phosphate coprecipitate using water as a medium. Are preferred, and sucrose is particularly preferred.

  The organic substance to be the carbon precursor may be added as it is to the slurry containing the carbon-iron phosphate-lithium phosphate coprecipitate, but is preferably added as an aqueous solution previously dissolved in water. Although the density | concentration of the organic substance used as the carbon precursor in that case is not specifically limited, 10-50 mass% is preferable. Further, at that time, it is preferable to add a dispersant such as a nonionic surfactant to thoroughly disperse the organic substance serving as the carbon precursor.

  The addition of the organic substance serving as the carbon precursor or an aqueous solution thereof is preferably performed while stirring and mixing the slurry containing the carbon-iron phosphate-lithium phosphate coprecipitate, and is stirred for about 10 minutes after the addition. It is preferable to continue mixing.

  Then, a slurry containing a mixture of the carbon-iron phosphate-lithium phosphate coprecipitate obtained as described above and an organic substance serving as a carbon precursor is wet pulverized by a wet media pulverizer such as a sand mill or a ball mill. It is preferable to disperse.

  That is, when an organic substance as a carbon precursor is added to and mixed with the carbon-iron phosphate-lithium phosphate coprecipitate, the resulting mixture tends to aggregate and form secondary particles. In the case of (4) firing, it is easier to obtain a high-capacity lithium ion battery by firing the aggregated state by wet grinding with a wet media grinding machine to form primary particles and dispersing.

  A mixture of the carbon-iron phosphate-lithium phosphate coprecipitate and the organic substance that becomes the carbon precursor that has undergone the above-described steps is taken out from the liquid by drying and removing the moisture.

  The drying method and conditions are not particularly limited as long as moisture in the mixture can be removed. From the viewpoint of suppressing oxidation of iron in the carbon-iron phosphate-lithium phosphate coprecipitate, it is considered that drying at a lower temperature in an inert gas such as nitrogen gas or drying by reduced pressure drying is preferable. However, in the case of the carbon-iron phosphate-lithium phosphate coprecipitate according to the method of the present invention, oxidation of iron hardly occurs even when drying using a hot air dryer or the like in the atmosphere. As an example, drying with a hot air dryer can also be employed. And when drying by this hot air dryer, it is preferable to dry with a hot air of 40-150 degreeC for about 1 to 24 hours, for example.

  The carbon-iron phosphate-lithium phosphate coprecipitate obtained in the step (2-1) has, for example, a structure in which lithium phosphate is attached so as to cover the surface of the carbon-iron phosphate complex. Presumed to be. In addition, in the step (2-2), carbon-iron phosphate-lithium phosphate coprecipitation obtained by adopting a method of adding a phosphoric acid aqueous solution and a lithium compound-containing aqueous solution to water in which a carbon material is previously dispersed In the product, for example, it is presumed that a carbon-lithium iron phosphate complex and a carbon material having lithium phosphate attached to the surface coexist. In addition, since carbon black particles are considered to form a structure as described above, in the carbon-iron phosphate-lithium phosphate coprecipitate using carbon black as a carbon material, A carbon-olivine type lithium iron phosphate composite that is believed to form a composite coprecipitate with the carbon black structure as the nucleus, and the nucleus due to the carbon black structure is obtained through the step (4). The organic substance that forms a conductive path and increases its electronic conductivity, and becomes a carbon precursor added to the carbon-iron phosphate-lithium phosphate coprecipitate, is electrically conductive with the carbon of the coprecipitate during firing. It is presumed that a highly carbon network is formed to enhance electron conductivity. Further, depending on the amount of lithium phosphate to be introduced, it is considered that not all of the surfaces of the carbon-iron phosphate complex and the carbon material can be covered, and some of them are exposed.

  In the step (4) of the method of the present invention, a mixture of the carbon-iron phosphate-lithium phosphate coprecipitate obtained in the step (3) and an organic substance serving as a carbon precursor is calcined to obtain carbon-olivine. This is a process for producing a type lithium phosphate composite.

  As a calcination temperature in the process of (4), it is preferable to set it as 500-900 degreeC, for example. If the firing temperature is too low, it may be difficult to produce a carbon-olivine type lithium iron phosphate complex, and if it is too high, some decomposition may occur. Firing is preferably performed, for example, in an inert gas atmosphere such as nitrogen gas or argon gas, but can also be performed in a reducing gas atmosphere such as hydrogen gas.

  The firing time in the step (4) is preferably 3 to 24 hours, for example. Moreover, when performing baking in the process of (4) in nitrogen gas atmosphere, it is preferable that the flow volume of nitrogen gas shall be 0.1-50 L / min, for example.

  In addition, if necessary, a mixture of carbon-iron phosphate-lithium phosphate coprecipitate before firing and an organic substance serving as a carbon precursor, or a carbon-olivine-type lithium iron phosphate composite obtained after firing may be used. It can also be pulverized or compression molded.

  In the carbon-olivine type lithium iron phosphate composite, among the elements contained therein, the content ratio of Li, Fe and P is more preferably Li: 0.5 or more with respect to Fe: 1.0 in terms of molar ratio. Is 0.8 or more and 1.5 or less, more preferably 1.2 or less, and P: 0.5 or more, more preferably 0.8 or more and 1.5 or less, more preferably 1 .2 or less is desirable. With such a content ratio, the amount of impurities in the composite is small, and the battery characteristics of a battery using this as a positive electrode material can be improved. In the method of the present invention, the reaction proceeds almost according to the theoretical equivalent. Therefore, by adjusting the use ratio of each raw material compound in the steps (1), (2) and (3), the above content ratio in the complex Can be controlled.

  In the carbon-olivine type lithium iron phosphate composite, the carbon content is preferably 0.1% by mass or more, and preferably 50% by mass or less. If the carbon content is too low, the effect of improving the electronic conductivity may be reduced. If the carbon content is too high, the content of the olivine-type lithium iron phosphate is reduced, so that the capacity of the battery using this as a positive electrode material is increased. Can be difficult. The carbon content is more preferably 1% by mass or more and 25% by mass or less from the viewpoint of improving the charge / discharge characteristics of the battery using the composite and the bulk density of the composite. It is more preferable.

In the method of the present invention, the reaction in each step is considered to proceed as in the following formulas (A) to (C). The following formula (A) shows the reaction in the above step (1), formula (B) in the step (2), and formula (C) in the step (4). In addition, the process of adding and mixing the organic substance which becomes a carbon precursor to the coprecipitate of (3) is merely physical mixing, and there is no chemical reaction. In the following formulas (A) to (C), the raw material compounds used are merely examples, and the method of the present invention is not limited to the use of these raw material compounds.
_{3FeSO 4 · 7H 2 O + 2H} 3 PO 4 + 6NaOH
_{→ Fe 3 (PO 4) 2} · 8H 2 O ↓ + 3Na 2 SO 4 + 2H 2 O (A)
Fe ₃ (PO ₄ ) ₂ · 8H ₂ O ↓ + 3LiOH · H ₂ O + H ₃ PO _{4
→ Fe 3 (PO 4) 2} · 8H 2 O ↓ + Li 3 PO 4 ↓ + 2H 2 O (B)
_{Fe 3 (PO 4) 2 ·} 8H 2 O + Li 3 PO 4 → 3LiFePO 4 (C)

  Thus, in the method of the present invention, since the carbon-iron phosphate-lithium phosphate coprecipitate is produced in two stages of the above formula (A) and the above formula (B), divalent iron is contained. In order to remove impurities contained in the compound and by-products contained in the carbon-iron phosphate complex (when ferrous sulfate is used, such as bowel (sodium sulfate), ammonium sulfate, etc.), the above (A ) After the formula [that is, after the step (1)], washing with water may be performed. Even if such water washing is performed, the elution of the lithium compound does not occur, and the molar ratio of each element does not change. Therefore, in the final coprecipitate [coprecipitate obtained in the step (2)] The molar ratio of Li, Fe and P can be easily controlled, and the use of an excess lithium compound is not necessary, so that it can be produced at low cost and is efficient. Furthermore, the method of the present invention is also efficient in that the reaction can be performed in the vicinity of the theoretical equivalent.

Incidentally, in the above-described hydrothermal method and known coprecipitation methods other than the method of the present invention, there are many examples in which the lithium-containing compound serving as the lithium source is used in excess of the theoretically required amount. In solution methods such as hydrothermal method and coprecipitation method using ferrous sulfate, heptahydrate, phosphoric acid and lithium hydroxide, a precipitate is obtained by one-stage precipitation shown in the following formula (D). It is baked into olivine type lithium iron phosphate.
3FeSO ₄ · 7H ₂ O + 3H ₃ PO ₄ + 9LiOH · H ₂ O
_{→ Fe 3 (PO 4) 2} · 8H 2 O ↓ + Li 2 PO 4 ↓ + 3Li 2 SO 4 (D)

The precipitate obtained by this reaction is composed mainly of Fe ₃ (PO ₄ ) ₂ · 8H ₂ O and Li ₃ PO ₄ , and the theoretical molar ratio is Fe ₃ to make olivine-type lithium iron phosphate after firing. (PO ₄ ) ₂ · 8H ₂ O: Li ₃ PO ₄ = 1: 1. However, since simple filtration contains some Li ₂ SO ₄ , it is necessary to wash the precipitate (coprecipitate) with water in order to remove it. When impurities are mixed in, it is expected to cause a capacity drop during the charge / discharge test of the olivine-type lithium iron phosphate finally obtained, and it is desirable to remove impurities as much as possible. However, since Li ₃ PO ₄ or the Li content contained in the precipitate is dissolved by washing with water, the problem is that the molar ratio deviates from the theoretical value when removing impurities.

  On the other hand, in the method of the present invention, as described above, the carbon-iron phosphate-lithium phosphate coprecipitate is generated in a two-step reaction, and thus water washing for removing impurities after the first-step reaction. However, since no lithium-containing compound coexists at that time, there is no weight loss of the lithium-containing compound by washing with water, and a reaction in the vicinity of the theoretical equivalent is possible.

The method for producing olivine-type lithium iron phosphate described in Patent Document 2 is also a method of once producing a precipitate and baking it. In this precipitate production reaction, the following (E ) It is thought that the reaction by the formula is estimated.
3FeSO ₄ · 7H ₂ O + 3H ₃ PO ₄ + 3LiOH · H ₂ O + nNaOH
_{→ Fe 3 (PO 4) 2} · 8H 2 O ↓ + Li 3 PO 4 ↓ + 3Na 2 SO 4 (E)

However, according to the study of the present inventor, in the method described in Patent Document 2, Li content was not included in the precipitate, and olivine-type lithium iron phosphate was not obtained after firing. Described in the example section). This is expected because the reaction of the following formula (F) occurs.
3FeSO ₄ · 7H ₂ O + 3H ₃ PO ₄ + 3LiOH · H ₂ O + nNaOH
_{→ Fe 3 (PO 4) 2} · 8H 2 O ↓ + 3Li 2 SO 4 + Na 3 PO 4 (F)

  That is, as shown in the above formula (F), the Li component forms water-soluble lithium sulfate and was not contained in the precipitate. It is thought that it was not generated.

  The carbon-olivine-type lithium iron phosphate composite obtained by the method of the present invention has the characteristics possessed by the olivine-type lithium iron phosphate and has enhanced electronic conductivity, and is used as a positive electrode material for a lithium ion battery. Suitable for doing. In order to construct a positive electrode for a lithium ion battery using the carbon-olivine type lithium iron phosphate composite according to the present invention, for example, the carbon-olivine type lithium iron phosphate composite is used as it is as a positive electrode material. Others may be formed into a molded body of a positive electrode mixture containing a binder and, if necessary, a conductive additive such as a carbon material, as in the case of a conventionally known positive electrode. Moreover, you may form these positive mixes as a positive mix layer in the single side | surface or both surfaces of the electroconductive base | substrate used as a collector as needed.

  Furthermore, the carbon-olivine type lithium iron phosphate composite according to the present invention and a carbon material other than carbon derived from the carbon-olivine type lithium iron phosphate are included to constitute the positive electrode material for a lithium ion battery of the present invention. Alternatively, a binder or the like may be added to the positive electrode material to form a positive electrode mixture, thereby forming a positive electrode for a lithium ion battery. Examples of the carbon material include various carbon materials exemplified above used for the production of a carbon-olivine type lithium iron phosphate composite [various carbon blacks; various carbon materials having shape anisotropy (having a certain aspect ratio). Carbon material having a shape similar to fiber)] and the like. In particular, in the case of the carbon material having the shape anisotropy exemplified above, a conductive path between the carbon-olivine type lithium iron phosphate composites can be satisfactorily formed from the shape, so that the electron conductivity can be further increased. This is possible, and the charge / discharge characteristics of a lithium ion battery using the same can be further improved.

  In the positive electrode material for a lithium ion battery containing the carbon-olivine type lithium iron phosphate composite and the carbon material, the content of the carbon material other than carbon derived from the carbon-olivine type lithium iron phosphate composite is, for example, It is desirable that the content be 0.1% by mass or more, more preferably 1% by mass or more, and 30% by mass or less, more preferably 20% by mass or less. When the content of the carbon material other than carbon derived from the carbon-olivine type lithium iron phosphate composite is too low, the effect of using the carbon material may be reduced. Also, if the content of carbon material other than carbon derived from the carbon-olivine lithium iron phosphate composite is too high, the amount of olivine lithium iron phosphate acting as an active material in the positive electrode will be reduced, This may cause a decrease in capacity of a lithium ion battery using the same. In particular, in the case of a carbon material having the above shape anisotropy, since the specific surface area is relatively small, if the content of carbon material other than carbon derived from the carbon-olivine type lithium iron phosphate composite is too high The contact point with lithium iron phosphate may be reduced.

  When a lithium ion battery is constituted using a carbon-olivine type lithium iron phosphate composite according to the present invention and a positive electrode for a lithium ion battery using the positive electrode material of the present invention, a negative electrode, a separator, There are no particular limitations on the various configurations such as the electrolytic solution and the outer package, and the same configurations as those of conventionally known lithium ion batteries can be employed.

  In power supply applications for small devices such as mobile phones and personal computers, the use of lithium-ion batteries continues to expand due to their high performance, but in the future, they will be used for large-scale devices such as automobiles and industrial vehicles. The price and safety are the bottleneck. By using olivine-type lithium iron phosphate, which is highly safe, and easily and efficiently manufactured by the method of the present invention, low cost is achieved, and lithium-ion batteries are rapidly deployed for use in large equipment. Expected.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples do not limit the present invention, and all modifications made without departing from the gist of the preceding and following descriptions are included in the technical scope of the present invention. In the following Examples and Comparative Examples, “%” indicating the concentration of a solution, a dispersion, etc. is “%” based on mass unless otherwise indicated.

Example 1
(1-1) Production of Carbon-Iron Phosphate Complex 190 cm ³ of ion-exchanged water was placed in a 2 L separable flask, and a nonionic surfactant [“Emulgen MS-110 (trade name)” manufactured by Kao Corporation] After adding 5 g, acetylene black powder [Denka Black (trade name) manufactured by Denki Kagaku Co., Ltd., particle size 35 nm] 2.4 g was added and stirred for 15 minutes to obtain a carbon black dispersion. Thereafter, an aqueous solution in which 76.9 g of 85% phosphoric acid and 278.0 g of ferrous sulfate heptahydrate were previously dissolved in 540 cm ³ of ion-exchanged water was added to the carbon black dispersion and stirred for 10 minutes. . Further, 227.1 g of a 15% aqueous ammonia solution was added to the carbon black dispersion, and the mixture was stirred for 15 minutes and then filtered to obtain a carbon-iron phosphate complex.

(1-2) Manufacture of carbon-iron phosphate-lithium phosphate coprecipitate Ion-exchanged water 615 cm ³ was placed in a 2 L separable flask and nonionic surfactant [Emulgen MS-110 (trade name, manufactured by Kao Corporation) ]]] After adding 0.5 g, the total amount of the carbon-iron phosphate complex obtained in (1-1) was added and stirred for 15 minutes to obtain a carbon-iron phosphate complex dispersion. Thereafter, 38.4 g of 85% phosphoric acid was added to the carbon-iron phosphate complex dispersion, and an aqueous solution in which 41.9 g of lithium hydroxide monohydrate was previously dissolved in 380 cm ³ of ion-exchanged water was added. Then, the mixture was stirred for 10 minutes to obtain a slurry liquid containing a carbon-iron phosphate-lithium phosphate coprecipitate.

(1-3) Addition of organic substance as carbon precursor To the carbon-iron phosphate-lithium phosphate coprecipitate obtained in (1-2) above, 28.0 g of sucrose as an organic substance as a carbon precursor was added. A 40% aqueous solution dissolved in ion-exchanged water was added with stirring, and the mixture was further stirred for 10 minutes and mixed to obtain a slurry solution of a mixture of carbon-iron phosphate-lithium phosphate coprecipitate and sucrose.

(1-4) Firing The slurry liquid obtained in the above (1-3) is put into a horizontal sand mill [“Dynomill MULTI LAB type (trade name)” manufactured by Shinmaru Enterprises Co., Ltd.], and further 0.5 mm therein. The zirconia beads having a diameter were filled and wet pulverized to disperse a mixture of carbon-iron phosphate-lithium phosphate coprecipitate and sucrose. The dispersed slurry was dried at 70 ° C. under reduced pressure, and the obtained dried product was pulverized by rotating for 72 hours using a 1 L nylon pot containing 2 cm diameter zirconia balls. After pulverization, it was fired in a nitrogen stream at 700 ° C. for 5 hours to obtain a carbon-olivine type lithium iron phosphate composite in the form of particles.

  About the carbon-olivine type lithium iron phosphate complex obtained as described above, analysis of molar ratio by ICP, confirmation of crystalline component using powder X-ray diffractometer, and fully automatic elemental analyzer The carbon content of was measured. The results and measurement methods are shown below.

[Analysis of molar ratio by ICP]
Part of the carbon-olivine type lithium iron phosphate complex obtained in (1-4) above is placed in 20% hydrochloric acid and dissolved by heating. Insoluble matter is removed by filtration, and then the filtrate is diluted. The concentration of hydrochloric acid was adjusted to 1%, and each component of the coprecipitate in the filtrate after the concentration adjustment was determined by ICP emission analysis [Rigaku Corporation “CCD-ICP emission analyzer CIROS-120” (trade name)]. When the ratio (molar ratio) was calculated, it was confirmed that Li: Fe: P = 1: 1: 1.

[Confirmation of crystalline component using powder X-ray diffractometer]
The carbon-olivine type lithium iron phosphate composite obtained in the above (1-4) was used using a powder X-ray diffractometer [“Fully automatic powder X-ray diffractometer X′Pert PRO (trade name)” manufactured by PANalytical]. X-ray diffraction measurement was performed under the conditions of a counter-cathode Cu (Ni filter), a tube voltage of 45 kV, and a current of 40 mA. From the obtained diffraction diagram, it was confirmed that the crystalline component was olivine-type lithium iron phosphate LiFePO ₄ did.

[Measurement of carbon content using a fully automatic elemental analyzer]
The carbon-olivine type lithium iron phosphate complex obtained in (1-4) above was subjected to elemental analysis using a fully automatic elemental analyzer ["vario ELIII (trade name)" manufactured by Elemental Co., Ltd.], and the carbon- It was confirmed that 5.2% by mass of carbon was contained in the olivine-type lithium iron phosphate composite.

Example 2
The carbon-olivine type lithium iron phosphate complex was obtained by operating under the same conditions as in Example 1 except that the amount of sucrose added was 13.7 g.

  The obtained carbon-olivine type lithium iron phosphate complex was analyzed in the same manner as in Example 1. As a result, it was confirmed that 3.1% by mass of carbon was contained in the carbon-olivine-type lithium iron phosphate composite, but the others were the same as in Example 1.

Example 3
The carbon-olivine type lithium iron phosphate complex was obtained by operating under the same conditions as in Example 1 except that the amount of sucrose added was 6.8 g.

  The obtained carbon-olivine type lithium iron phosphate complex was analyzed in the same manner as in Example 1. As a result, it was confirmed that 2.0% by mass of carbon was contained in the carbon-olivine-type lithium iron phosphate composite, but the others were the same as in Example 1.

Example 4
(4-1) Production of carbon-iron phosphate complex The same operation as (1-1) of Example 1 was performed to obtain the same carbon-iron phosphate complex as that of Example 1.

(4-2) Production of carbon-iron phosphate-lithium phosphate coprecipitate Ion-exchanged water 615 cm ³ was placed in a 2 L separable flask, and a nonionic surfactant [Emulgen MS-110 (trade name, manufactured by Kao Corporation) ]]] After adding 0.5 g, the total amount of the carbon-iron phosphate complex obtained in (1-1) was added and stirred for 15 minutes to obtain a carbon-iron phosphate complex dispersion. Thereafter, 38.4 g of 85% phosphoric acid was added to the carbon-iron phosphate complex dispersion, and an aqueous solution in which 41.9 g of lithium hydroxide monohydrate was dissolved in 380 cm ³ of ion-exchanged water in advance. The mixture was added and stirred for 10 minutes, followed by filtration to obtain a carbon-iron phosphate-lithium phosphate coprecipitate.

(4-2-1) Drying and calcination The coprecipitate obtained in (4-2) was dried in a vacuum dryer at 40 ° C. for 24 hours. The obtained dried product was analyzed for the molar ratio by ICP in the same manner as in Example 1. As a result, the molar ratio of each component was Li: Fe: P = 1: 1: 1.

  The dried product obtained as described above was placed in a 1 L nylon pot filled with 2 cm of zirconia balls and rotated for 72 hours for pulverization. After pulverization, in a nitrogen stream at 500 ° C. for 5 hours. It was calcined under the conditions of

(4-3) Addition of organic substance as carbon precursor Disperse 150 g of the calcined product obtained in (4-2-1) above in an aqueous solution in which 22.4 g of sucrose was previously dissolved in 425 cm ³ of ion-exchanged water. Thus, a slurry liquid of a mixture of calcined carbon-iron phosphate-lithium phosphate coprecipitate and sucrose was obtained.

(4-4) Firing A slurry of a mixture of the calcined carbon-iron phosphate-lithium phosphate coprecipitate obtained in (4-3) above and sucrose is mixed with a horizontal sand mill [manufactured by Shinmaru Enterprises Co., Ltd. DYNOMILL MULTI LAB type (trade name) "], filled with zirconia beads and wet pulverized to disperse a mixture of carbon-iron phosphate-lithium phosphate co-precipitate and sucrose. The dispersed slurry was dried under reduced pressure at 70 ° C., and the obtained pulverized powder was fired in a nitrogen stream at 700 ° C. for 5 hours to obtain a carbon-olivine type lithium iron phosphate composite.

The obtained carbon-olivine type lithium iron phosphate composite was analyzed in the same manner as in Example 1. As a result, the molar ratio of each component was Li: Fe: P = 1: 1: 1, and the crystalline component Was confirmed to be olivine-type lithium iron phosphate LiFePO ₄ and 5.1% by mass of carbon was contained in the carbon-olivine-type lithium iron phosphate composite.

Example 5
(5-1) Production of carbon-iron phosphate complex The same operation as (1-1) in Example 1 was performed to obtain the same carbon-iron phosphate complex as in Example 1.

(5-2) Production of carbon-iron phosphate-lithium phosphate coprecipitate The same operation as (1-2) in Example 1 was carried out, and the same carbon-iron phosphate-phosphate as in Example 1 A slurry solution of lithium coprecipitate was obtained.

(5-3) Addition of organic substance as carbon precursor A 40% aqueous solution prepared by dissolving 28.0 g of sucrose in ion-exchanged water in the carbon-iron phosphate-lithium phosphate coprecipitate obtained above. After the addition with stirring, the mixture was further stirred for 10 minutes and mixed to obtain a slurry liquid of a mixture of carbon-iron phosphate-lithium phosphate coprecipitate and sucrose.

(5-4) Firing The slurry liquid of the mixture of carbon-iron phosphate-lithium phosphate coprecipitate and sucrose obtained as described above was dried at 70 ° C. under reduced pressure, and in a nitrogen stream, 700 Firing was performed at 5 ° C. for 5 hours to obtain a carbon-olivine type lithium iron phosphate composite.

When the carbon-olivine type lithium iron phosphate composite obtained as described above was analyzed in the same manner as in Example 1, the molar ratio of each component of the carbon-olivine type lithium iron phosphate composite was Li: Fe: P = 1: 1: 1, the crystalline component was confirmed to be olivine-type lithium iron phosphate LiFePO ₄ , and the carbon content in the carbon-olivine-type lithium iron phosphate composite was 5%. It was confirmed that 1 mass% was contained.

Comparative Example 1
In Comparative Example 1, a carbon-olivine type lithium iron phosphate complex was produced using a conventionally known solid phase method as a method for producing olivine type lithium iron phosphate.

Ferrous phosphate octahydrate “” Fe ₃ (PO ₄ ) ₂ · 8H ₂ O] and lithium phosphate (Li ₃ PO ₄ ) have an element ratio of lithium to iron of 1: 1. The acetylene black [“DENKA BLACK (trade name)” manufactured by Denki Kagaku Co., Ltd.] was added to this so that the carbon content after firing would be 5.0% by mass, and the dry planetary ball mill (manufactured by Lecce) The mixture was mixed for 10 hours using “PM-100 (trade name).” The resulting mixture was fired in a nitrogen stream at 700 ° C. for 5 hours, and the resulting fired product was put into a simple cutter mill. To obtain a composite.

When the obtained carbon-olivine type lithium iron phosphate composite was analyzed in the same manner as in Example 1, the molar ratio of each component of the carbon-olivine type lithium iron phosphate composite was Li: Fe: P = 1: 1: 1, and it was confirmed that most of the crystalline component was olivine-type lithium iron phosphate LiFePO ₄ , and the carbon content in the carbon-olivine-type lithium iron phosphate composite was 5.1 mass. % Was confirmed.

Comparative Example 2
Also in this comparative example 2, the carbon-olivine type lithium iron phosphate composite was manufactured using the conventionally well-known solid-phase method as a manufacturing method of olivine type lithium iron phosphate. However, in Comparative Example 2, while acetylene black was used as the carbon component in Comparative Example 1, a carbon-olivine type lithium iron phosphate composite was produced using sucrose as a carbon precursor. It was.

Ferrous phosphate octahydrate [Fe ₃ (PO ₄ ) ₂ · 8H ₂ O] and lithium phosphate (Li ₃ PO ₄ ) so that the element ratio of lithium to iron is 1: 1 Sucrose is added to this so that the carbon content after firing is 5.0% by mass, and mixed for 10 hours using a dry planetary ball mill ["PM-100 (trade name)" manufactured by Lecce) did. The obtained mixture was baked in a nitrogen stream at 700 ° C. for 5 hours, and the obtained baked product was pulverized by a simple cutter mill to obtain a composite.

When the obtained carbon-olivine type lithium iron phosphate composite was analyzed in the same manner as in Example 1, the molar ratio of each component of the carbon-olivine type lithium iron phosphate composite was Li: Fe: P = 1: 1: 1, and it was confirmed that most of the crystalline component was olivine-type lithium iron phosphate LiFePO ₄ , and the carbon content in the carbon-olivine-type lithium iron phosphate composite was 5.1 mass. % Was confirmed.

Comparative Example 3
Also in this comparative example 3, the carbon-olivine type lithium iron phosphate composite was manufactured using the conventionally well-known solid-phase method as a manufacturing method of olivine type lithium iron phosphate. However, in Comparative Example 3, as the carbon component, Comparative Example 1 used acetylene black and Comparative Example 2 used sucrose, whereas acetylene black and sucrose were used in combination, and carbon-olivine type phosphoric acid was used. An iron-lithium composite was produced.

Ferrous phosphate octahydrate [Fe ₃ (PO ₄ ) ₂ · 8H ₂ O] and lithium phosphate (Li ₃ PO ₄ ) so that the element ratio of lithium to iron is 1: 1 And sucrose: acetylene black [Denka Black (trade name) manufactured by Denki Kagaku Co., Ltd.] = 11: 1 (mass ratio), and the carbon content after firing is 5.0% by mass. In addition, the mixture was mixed for 10 hours using a planetary ball mill ("PM-100 (trade name)" manufactured by Lecce) in a dry manner. The resulting mixture was calcined in a nitrogen stream at 700 ° C for 5 hours. The obtained fired product was pulverized with a simple cutter mill to obtain a composite.

When the obtained carbon-lithium iron phosphate composite was analyzed in the same manner as in Example 1, the molar ratio of each component of the carbon-olivine-type lithium iron phosphate composite was Li: Fe: P = 1: 1: 1 and the majority of the crystalline component was confirmed to be olivine-type lithium iron phosphate LiFePO ₄ , and the carbon-olivine-type lithium iron phosphate composite contained 5.2% by mass of carbon. It was confirmed that

Comparative Example 4
In this comparative example 4, the organic substance which becomes the carbon precursor after the step of producing the coprecipitate containing the carbon-iron phosphate complex and lithium phosphate of (2) and before the firing step of (4). The carbon-lithium iron phosphate composite [acetylene black: olivine-type lithium iron phosphate = 20: 80 (mass ratio) carbon-lithium iron phosphate composite] was manufactured without adding and mixing An example is shown.

[Production of carbon-phosphate complex]
Into a ² L separable flask, 650 cm ³ of ion-exchanged water was added, and 4.0 g of a nonionic surfactant (“Emulgen MS-110 (trade name)” manufactured by Kao Corporation) was added, followed by acetylene black powder [manufactured by Denki Kagaku “ "Denka Black (trade name)", particle size 35 nm] 28.1 g was added and stirred for 15 minutes to obtain a carbon black dispersion. Thereafter, an aqueous solution in which 53.1 g of 85% phosphoric acid and 192.1 g of ferrous sulfate heptahydrate were previously dissolved in 430 cm ³ of ion-exchanged water was added to the carbon black dispersion and stirred for 10 minutes. . Further, 184.3 g of a 30% aqueous sodium hydroxide solution was added to the carbon black dispersion, and the mixture was stirred for 15 minutes and filtered to obtain a carbon-iron phosphate complex.

[Production of carbon-iron phosphate-lithium phosphate coprecipitate]
Into a ² L separable flask, 650 cm ³ of ion-exchanged water was added, and 4.0 g of a nonionic surfactant [“Emulgen MS-110 (trade name)” manufactured by Kao Corporation] was added, then (1-1) of Example 1 The total amount of the carbon-iron phosphate complex obtained by performing the same operation as above was added and stirred for 15 minutes to obtain a carbon-iron phosphate complex dispersion. Thereafter, an aqueous solution in which 25.6 g of 85% phosphoric acid was added to the carbon-iron phosphate complex dispersion, and 29.0 g of lithium hydroxide monohydrate was dissolved in 165 cm ³ of ion-exchanged water in advance. The mixture was added and stirred for 10 minutes, followed by filtration to obtain a carbon-iron phosphate-lithium phosphate coprecipitate.

[Baking]
The coprecipitate obtained as described above was dried in a vacuum dryer at 40 ° C. for 24 hours. The dried coprecipitate was fired in a nitrogen stream at 600 ° C. for 5 hours, and then pulverized with a simple cutter mill to obtain a carbon-olivine type lithium iron phosphate composite.

When the carbon-olivine type lithium iron phosphate composite obtained as described above was analyzed in the same manner as in Example 1, the molar ratio of each component of the carbon-olivine type lithium iron phosphate composite was as follows. Li: Fe: P = 1: 1: 1, and it was confirmed that the crystalline component was olivine-type lithium iron phosphate LiFePO ₄ , and the carbon content in the carbon-olivine-type lithium iron phosphate complex was Was confirmed to be 20.2 mass%.

Comparative Example 5
In this comparative example 5, instead of producing the carbon-lithium iron phosphate composite of acetylene black: olivine lithium iron phosphate = 20: 80 (mass ratio) in comparative example 3, acetylene black: The olivine type lithium iron phosphate = 5: 95 (mass ratio) carbon-lithium iron phosphate composite was manufactured.

[Production of carbon-phosphate complex]
Into a ² L separable flask, 650 cm ³ of ion-exchanged water was added, 1.3 g of a nonionic surfactant (“Emulgen MS-110 (trade name)” manufactured by Kao Corporation) was added, and acetylene black powder [manufactured by Denki Kagaku “ Denka Black (trade name) ”, particle size 35 nm] 6.3 g was added and stirred for 15 minutes to obtain a carbon black dispersion. Thereafter, an aqueous solution in which 53.1 g of 85% phosphoric acid and 192.1 g of ferrous sulfate heptahydrate were previously dissolved in 430 cm ³ of ion-exchanged water was added to the carbon black dispersion and stirred for 10 minutes. . Furthermore, 156.7 g of a 15% aqueous ammonia solution was added to the carbon black dispersion, and the mixture was stirred for 15 minutes and filtered to obtain a carbon-iron phosphate complex.

[Production of carbon-iron phosphate-lithium phosphate coprecipitate]
650 cm ³ of ion-exchanged water was put into a 2 L separable flask, and after adding 1.3 g of nonionic surfactant [“Emulgen MS-110 (trade name)” manufactured by Kao Corporation], (1-1) of Example 1 The total amount of the carbon-iron phosphate complex obtained by performing the same operation as above was added and stirred for 15 minutes to obtain a carbon-iron phosphate complex dispersion. Thereafter, an aqueous solution in which 25.6 g of 85% phosphoric acid was added to the carbon-iron phosphate complex dispersion, and 29.0 g of lithium hydroxide monohydrate was dissolved in 165 cm ³ of ion-exchanged water in advance. The mixture was added and stirred for 10 minutes, followed by filtration to obtain a carbon-iron phosphate-lithium phosphate coprecipitate.

[Baking]
The coprecipitate obtained as described above was dried in a vacuum dryer at 40 ° C. for 24 hours. The dried coprecipitate was fired in a nitrogen stream at 700 ° C. for 5 hours, and then pulverized by a simple cutter mill to obtain a carbon-olivine type lithium iron phosphate composite.

When the carbon-olivine type lithium iron phosphate composite obtained as described above was analyzed in the same manner as in Example 1, the molar ratio of each component of the carbon-olivine type lithium iron phosphate composite was as follows. Li: Fe: P = 1: 1: 1, and it was confirmed that the crystalline component was olivine-type lithium iron phosphate LiFePO ₄ , and the carbon content in the carbon-olivine-type lithium iron phosphate complex was Was confirmed to be 5.2% by mass.

Comparative Example 6
In Comparative Example 6, iron phosphate was synthesized in the first step without synthesizing the carbon-iron phosphate complex as in step (1) in the present invention, and thereafter, carbon was synthesized in the same manner as in the present invention. -The olivine type lithium iron phosphate composite was produced.

[Production of iron phosphate]
540 cm ³ of ion-exchanged water was placed in a 2 L separable flask, and 76.9 g of 85% phosphoric acid and 278.0 g of ferrous sulfate heptahydrate were dissolved. Further, 227.1 g of a 15% aqueous ammonia solution was added, and the mixture was stirred for 15 minutes and then filtered to obtain iron phosphate.

[Production of iron phosphate and lithium phosphate coprecipitate]
Into a 2 L separable flask, 615 cm ³ of ion-exchanged water was added, and the total amount of iron phosphate obtained above was added and stirred for 15 minutes to obtain an iron phosphate dispersion. Thereafter, 38.4 g of 85% phosphoric acid was added, an aqueous solution in which 41.9 g of lithium hydroxide monohydrate was previously dissolved in 380 cm ³ of ion-exchanged water was added and stirred for 10 minutes, and iron phosphate-phosphorus A slurry of lithium acid coprecipitate was obtained.

[Addition of organic substance as carbon precursor]
After adding a 40% aqueous solution in which 34.6 g of sucrose was dissolved in ion-exchanged water to the slurry solution of iron phosphate-lithium phosphate coprecipitate obtained as described above, the mixture was stirred for 10 minutes and mixed to obtain a slurry. Liquid.

[Baking]
The slurry solution of the mixture of phosphoric acid, lithium phosphate coprecipitate and sucrose obtained as described above is put in a horizontal sand mill [Shinmaru Enterprises Co., Ltd. “Dynomill MULTI LAB type (trade name)”] Then, 0.5 mm diameter zirconia beads were filled and wet pulverized. The obtained slurry was dried at 70 ° C. under reduced pressure, and the obtained dried product was pulverized by rotating for 72 hours using a 1 L nylon pot containing 2 cm diameter zirconia balls. After pulverization, it was fired in a nitrogen stream at 700 ° C. for 5 hours to obtain an olivine-type lithium iron phosphate composite.

When the obtained carbon-olivine type lithium iron phosphate composite was analyzed in the same manner as in Example 1, the molar ratio of each component was Li: Fe: P = 1: 1: 1, and the crystalline component Was confirmed to be olivine-type lithium iron phosphate LiFePO ₄ , and 5.1% by mass of carbon was confirmed to be contained in the carbon-olivine-type lithium iron phosphate composite.

Comparative Example 7
In Comparative Example 7, an attempt was made to produce a carbon-olivine type lithium iron phosphate composite by the olivine type lithium iron phosphate synthesis method (coprecipitation method) disclosed in Patent Document 2.

A 2 L separable flask was charged with 472.4 cm ³ of ion-exchanged water and 95.6 g of 85% phosphoric acid to prepare a 10% by volume diluted phosphoric acid aqueous solution. To the phosphoric acid aqueous solution, 192.1 g of ferrous sulfate heptahydrate and 29.0 g of lithium hydroxide monohydrate were added so that the molar ratio of Li to Fe was 1: 1. Dissolved to prepare a mixed solution. To this mixed solution, 7.9 g of ascorbic acid (1% by mass of the mixed solution) was added and stirred for 10 minutes. Next, 4.0 ml of acetone (0.5% by volume of the mixed solution) is added to the mixed solution, and 10 g of acetylene black powder [“DENKA BLACK (trade name)” manufactured by Denki Kagaku Co., Ltd.] is added. The mixture was stirred for 1 minute, and then an aqueous sodium hydroxide solution having a concentration of 1 mol / l was added dropwise, stirred for 10 minutes, and filtered to obtain a coprecipitate.

  The coprecipitate was dried overnight in a 40 ° C. vacuum dryer. This was dissolved in 1% hydrochloric acid by heating and dissolved, and the insoluble matter was removed by filtration. Then, ICP emission analysis was performed in the same manner as in Example 1, and the ratio of each component of the coprecipitate in the filtrate (molar ratio). ) Was calculated, Li: Fe: P = 0.01: 1: 0.8, and it was found that Li was hardly contained in the coprecipitate. Since it was clear that the olivine-type lithium iron phosphate was not generated even when the coprecipitate was fired, the subsequent operation was stopped.

[Evaluation as a cathode material for lithium ion batteries]
A lithium ion battery having a positive electrode using the carbon-olivine type lithium iron phosphate composites obtained in Examples 1 to 5 and Comparative Examples 1 to 6 as a positive electrode material was prepared, the discharge capacity was measured, and the carbon -The olivine type lithium iron phosphate composite was evaluated as a positive electrode material. The details are as follows.

  In Comparative Example 7, since the intended carbon-olivine type lithium iron phosphate composite was not obtained, evaluation as a positive electrode material of this lithium ion battery was not performed.

  20 mg each of the carbon-olivine type lithium iron phosphate composites obtained in Examples 1 to 5 and Comparative Examples 1 to 6 and polytetrafluoroethylene as a binder ["Teflon (registered trademark) 6" manufactured by DuPont -J "] 2 mg was kneaded in a mortar, formed into a film of about 1 cm square, placed on a nickel wire mesh, and pressurized for 5 minutes at a pressure of 1 ton to produce a working electrode (positive electrode).

In a glove box substituted with argon, two metal lithium foils pressed against a nickel wire mesh were fixed, and these were used as a counter electrode and a reference electrode. 30 ml of electrolytic solution (propylene carbonate solution in which LiClO ₄ was dissolved at a concentration of 1 mol / l) was placed in a beaker type triode cell, and the above working electrode, counter electrode and reference electrode were attached, and a test cell (lithium ion battery) Configured.

About said test cell, the charging / discharging test was done using the battery charging / discharging apparatus ["HJ-1001SM8 (brand name)" by Hokuto Denko Co., Ltd.]. The test was performed at a current density of 17 mA / g with respect to LiFePO ₄ , setting the upper limit of the voltage to 4.3 V and the lower limit to 2.5 V, and discharging per gram of olivine lithium iron phosphate as the positive electrode active material. The capacity (mAh / g) was calculated. The results are shown in Table 1.

  As shown in Table 1, when the carbon-olivine type lithium iron phosphate composites of Examples 1 to 5 were used as the positive electrode material, the carbon-olivine type lithium iron phosphate composites of Comparative Examples 1 to 6 were used as the positive electrode. Compared with the case where it used as material, it was clear that the discharge capacity was large and it was excellent as a positive electrode material of the lithium ion batteries of Examples 1 to 5 with high capacity.

  On the other hand, the carbon-olivine type lithium iron phosphate composite of Comparative Example 1 has a small discharge capacity because Comparative Example 1 employs a solid phase method, so that each component can be uniformly mixed. Therefore, it is considered that the reaction between iron phosphate and lithium phosphate and the composite with carbon were not sufficiently performed during firing.

  Moreover, the carbon-olivine type | mold iron phosphate complex of the comparative example 2 using the sucrose used as a carbon precursor as a carbon component, and the carbon-olivine type phosphorus of the comparative example 3 which used acetylene black and sucrose together as a carbon component. Since the lithium iron oxide composite also had a small discharge capacity, even in these Comparative Examples 2 to 3, since the solid phase method was adopted as in Comparative Example 1, each component could not be uniformly mixed. For this reason, it is considered that the reaction between iron phosphate and lithium phosphate at the time of firing and the combination with carbon were not sufficiently performed.

  Then, after the step of producing the coprecipitate containing the carbon-iron phosphate complex and lithium phosphate in (2), and before the firing step in (4), addition and mixing of an organic substance serving as a carbon precursor It was a carbon precursor that the discharge capacity of the carbon-olivine type lithium iron phosphate composite of Comparative Example 4 and the carbon-olivine type lithium iron phosphate composite of Comparative Example 5 that were produced was small. This is probably because a carbon network with acetylene black could not be formed because there was no organic substance, and therefore sufficient electronic conductivity could not be obtained.

  Moreover, the discharge capacity of the carbon-olivine type lithium iron phosphate composite of Comparative Example 6 was small because iron phosphate was synthesized without synthesizing the carbon-iron phosphate composite in the first step. Therefore, it is considered that despite the addition of sucrose and baking, a carbon network was not formed, and therefore sufficient electronic conductivity could not be obtained.

Claims (14)

A method for producing a carbon-olivine type lithium iron phosphate composite,
(1) a step of producing a carbon-iron phosphate complex by precipitation;
(2) a step of producing a coprecipitate containing the carbon-iron phosphate complex and lithium phosphate;
(3) A step of adding and mixing an organic substance to be a carbon precursor to the coprecipitate,
(4) A step of firing a mixture of the coprecipitate and an organic substance that becomes a carbon precursor,
A method for producing a carbon-olivine-type lithium iron phosphate composite characterized by comprising:   The manufacturing method according to claim 1, wherein the mixture of the coprecipitate and the organic substance to be the carbon precursor is pulverized and dispersed by a wet media pulverizer between the step (3) and the step (4). . A method for producing a carbon-olivine type lithium iron phosphate composite,
(1) a step of producing a carbon-iron phosphate complex by precipitation;
(2) A step of producing a coprecipitate containing the carbon-iron phosphate complex and lithium phosphate, drying the coprecipitate, and calcining.
(3) A step of adding and mixing an organic substance that becomes a carbon precursor to the co-precipitate calcined product,
(4) A step of firing a mixture of the co-precipitate calcined product and an organic material to be a carbon precursor,
A method for producing a carbon-olivine-type lithium iron phosphate composite characterized by comprising:   The mixture of the calcined product of the coprecipitate and the organic material to be a carbon precursor is pulverized and dispersed by a wet media pulverizer between the step (3) and the step (4). Manufacturing method.   In the step (2), the carbon-iron phosphate complex obtained in the step (1) is dispersed in water, then phosphoric acid is added to the water, and a lithium compound-containing aqueous solution is further added. The manufacturing method of Claim 1 or 3 which obtains the coprecipitate containing a carbon-iron-phosphate complex and lithium phosphate by doing.   In the step (2), the carbon-iron phosphate complex obtained in the step (1) is added to water in which phosphoric acid and a lithium compound-containing aqueous solution are mixed to form a lithium phosphate precipitate. The manufacturing method of Claim 1 or 3 which obtains the coprecipitate containing a carbon-iron-phosphate complex and lithium phosphate by disperse | distributing.   The manufacturing method of Claim 6 which mixes phosphoric acid and lithium compound containing aqueous solution in the water which disperse | distributed the carbon material, and also adds the carbon-iron-phosphate complex obtained at the process of said (1).   In the step (1), an aqueous solution containing a compound containing phosphoric acid and divalent iron is added to the water in which the carbon material is dispersed, and then neutralized with a water-soluble base, whereby carbon-phosphorus The production method according to any one of claims 1 to 7, wherein an iron oxide complex precipitate is obtained.   The manufacturing method according to claim 1 or 3, wherein in the step (3), the organic substance serving as the carbon precursor added to the coprecipitate is a saccharide.   The production method according to claim 9, wherein the saccharide is a monosaccharide or a disaccharide.   The manufacturing method according to any one of claims 1 to 10, wherein in the step (4), the firing temperature is 500 to 900 ° C.   The carbon content in the carbon-olivine type lithium iron phosphate composite obtained through the steps (1), (2), (3) and (4) is 0.1 to 50% by mass. The manufacturing method in any one of 1-11.   It consists of a carbon-olivine type lithium iron phosphate composite obtained by the manufacturing method in any one of Claims 1-12, The positive electrode material for lithium ion batteries characterized by the above-mentioned.   A carbon-olivine-type lithium iron phosphate composite obtained by the production method according to claim 1 and a carbon material other than carbon derived from the carbon-olivine-type lithium iron phosphate composite is set to 0.000. 1-30 mass% containing, The positive electrode material for lithium ion batteries characterized by the above-mentioned.