JP2016072027A - Positive electrode material for lithium secondary battery and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode material for lithium secondary battery, by which a lithium secondary battery can be obtained which shows stable battery performance due to an olivine type lithium manganese iron phosphate of which the surface is coated with carbon hardly absorbing moisture.SOLUTION: A positive electrode material for lithium secondary battery comprises an olivine type lithium manganese iron phosphate of which surface is uniformly coated with carbon, which is obtained by using a lithium carbonate as a lithium source, and a trilithium phosphate aqueous solution including 2.7-3.3 mol of lithium per mol of phosphorus. In the case that the material is left adsorbing moisture under the condition of a temperature of 20°C and a relative humidity of 50% until reaching its equilibrium, then the temperature of the material is raised to 150°C and further, the material is kept at the temperature for 20 minutes, and a moisture adsorption amount measured as an amount of moisture volatilized during a period of time from the time of the achievement of equilibrium to the time when the 20 minutes has elapsed is 2200-3500 ppm.SELECTED DRAWING: None

Description

  The present invention relates to a positive electrode material for a lithium secondary battery capable of obtaining a lithium secondary battery having stable battery performance and a method for producing the same.

Secondary batteries used for portable electronic devices, hybrid cars, electric cars, and the like have been developed. In particular, lithium ion secondary batteries are widely known. Under these circumstances, lithium transition metal phosphate compounds such as Li (Fe, Mn) PO ₄ having an olivine type structure are not greatly affected by resource restrictions and can exhibit high safety. Therefore, it is an optimum positive electrode material for obtaining a high-output and large-capacity lithium ion secondary battery. However, these compounds have properties that make it difficult to sufficiently increase the conductivity derived from the crystal structure, and there is room for improvement in the diffusibility of lithium ions. Has been made.

  For example, in Patent Document 1, an attempt is made to improve the performance of the obtained battery by making primary crystal particles ultrafine particles and shortening the lithium ion diffusion distance in the olivine-type positive electrode active material. In Patent Document 2, a conductive carbonaceous material is uniformly deposited on the particle surface of the positive electrode active material, and a regular electric field distribution is obtained on the particle surface to increase the output of the battery.

  On the other hand, in Patent Document 3, when a lithium iron phosphate-based composite oxide composite coated with carbon is used as a positive electrode active material, the discharge capacity after 10 cycles is affected by the water content of the positive electrode active material. It is described that the discharge capacity can be improved if the water content is reduced, and under such a concept, after the firing treatment of the raw material mixture containing the carbonaceous material, by performing pulverization treatment and classification treatment in a dry atmosphere, A technique for reducing the water content of the positive electrode active material to a certain value or less is disclosed.

  Furthermore, Patent Document 4 considers that an active material obtained by using primary particles having an increased specific surface area may be sensitive to deterioration by moist air if the surface is coated with carbon. In addition, a technique is disclosed in which a predetermined raw material is subjected to a synthesis reaction in a dry atmosphere to maintain a humidity level below a certain level during manufacture, storage, and use of a positive electrode material.

JP 2010-251302 A JP 2001-15111 A JP 2003-292309 A Special table 2010-508234 gazette

  Since the techniques described in Patent Documents 3 to 4 are intended to perform a drying process for reducing the water content of the positive electrode active material until the battery is processed, the process until the battery is obtained. Therefore, there is a demand for a technique that can effectively prevent deterioration in performance of electronic characteristics by simpler means.

  Accordingly, an object of the present invention is to provide a lithium secondary battery that has a property that the olivine-type lithium iron manganese phosphate itself, which is coated with carbon, hardly adsorbs moisture, and exhibits stable battery performance. It is providing the positive electrode material for secondary batteries.

  Therefore, the present inventors have made various studies and found that olivine is obtained by coating carbon on the surface, obtained by using an aqueous solution of trilithium phosphate containing phosphorus and lithium in a specific amount while using lithium carbonate as a lithium source. Type lithium iron manganese phosphate has the property of hardly adsorbing moisture, and it is found that if a positive electrode material containing this is used, a lithium secondary battery capable of exhibiting excellent battery characteristics can be obtained. It came to complete.

That is, the present invention is obtained by using lithium carbonate as a lithium source and using an aqueous trilithium phosphate solution containing 2.7 to 3.3 mol of lithium with respect to 1 mol of phosphorus, and carbon is uniformly formed on the surface. A cathode material for a lithium secondary battery containing olivine type lithium manganese iron phosphate coated,
The amount of adsorbed water measured as the amount of water volatilized from the time when moisture is adsorbed until reaching equilibrium at a temperature of 20 ° C. and a relative humidity of 50% until the temperature is raised to 250 ° C. and held for 20 minutes, The present invention provides a positive electrode material for a lithium secondary battery that is 2000 ppm or less.
Moreover, this invention uses lithium carbonate as a lithium source, the process (I) which adjusts trilithium phosphate aqueous solution containing 2.7-3.3 mol lithium with respect to 1 mol of phosphorus, and process (I) (II) to obtain an olivine-type lithium manganese iron phosphate by subjecting the aqueous trilithium phosphate solution, manganese compound, and iron compound obtained in the above to hydrothermal reaction
The manufacturing method of the said positive electrode material for lithium secondary batteries provided with these is provided.

  Since the lithium secondary battery positive electrode material of the present invention hardly adsorbs moisture, a lithium secondary battery can be obtained by a simple process without requiring strong drying conditions as a manufacturing environment. In a battery, it is possible to stably exhibit excellent battery characteristics even under various usage environments.

Hereinafter, the present invention will be described in detail.
The positive electrode material for a lithium secondary battery of the present invention is obtained by using lithium carbonate as a lithium source and an aqueous trilithium phosphate solution containing 2.7 to 3.3 mol of lithium with respect to 1 mol of phosphorus, It contains olivine-type lithium manganese iron phosphate whose surface is uniformly coated with carbon.

  In the present invention, in obtaining olivine-type lithium iron manganese phosphate, unnecessary metal hydrates are formed, the viscosity of the mixture increases, and the reaction in the process may not proceed favorably. From the viewpoint of effectively avoiding the possibility that the amount of adsorbed moisture of the positive electrode material cannot be sufficiently reduced, an aqueous trilithium phosphate solution containing 2.7 to 3.3 moles of lithium per mole of phosphorus is used. More preferably, the trilithium phosphate aqueous solution contains 2.79 to 2.97 mol of lithium with respect to 1 mol of phosphorus.

In order to obtain the olivine type lithium manganese iron phosphate of the present invention by using the trilithium phosphate aqueous solution containing the specific amount of phosphorus and lithium, specifically, for example,
Step (I) of preparing an aqueous solution of trilithium phosphate containing 2.7 to 3.3 mol of lithium with respect to 1 mol of phosphorus using lithium carbonate as a lithium source, and phosphoric acid obtained in step (I) Step of obtaining olivine-type lithium manganese iron phosphate by subjecting trilithium aqueous solution, manganese compound and iron compound to hydrothermal reaction (II)
It is good to employ | adopt the manufacturing method provided with.

  Step (I) is a step of preparing an aqueous solution containing trilithium phosphate using lithium carbonate as a lithium source. By using lithium carbonate as the lithium source, an olivine-type lithium manganese iron phosphate capable of easily obtaining a positive electrode material with a sufficiently reduced amount of adsorbed water can be produced.

In this step (I), from the viewpoint of improving the dispersibility of the resulting aqueous solution, it is preferable to previously mix lithium carbonate (Li ₂ CO ₃ ) and water to obtain slurry water. The slurry water obtained by mixing lithium carbonate and water preferably contains 20 to 50 parts by mass of lithium carbonate with respect to 100 parts by mass of water, and preferably contains 25 to 45 parts by mass of lithium carbonate. More preferably, it contains 30 to 40 parts by mass of lithium carbonate.

Next, in obtaining an aqueous solution containing trilithium phosphate from the obtained slurry water, it is preferable to use phosphoric acid and stir while dropping it. By adding phosphoric acid dropwise and stirring in small portions, the reaction proceeds satisfactorily in the resulting aqueous solution, and trilithium phosphate particles are produced while being uniformly dispersed, and these particles aggregate unnecessarily. Can also be effectively suppressed.
Phosphoric acid is so-called orthophosphoric acid (H ₃ PO ₄ ) and is preferably used as an aqueous solution having a concentration of 70 to 90% by mass. The dropping rate of phosphoric acid into the slurry water is preferably 15 to 50 mL / min, more preferably 20 to 45 mL / min, and further preferably 28 to 40 ml / min. The stirring time of the slurry water is preferably 1 to 24 hours, more preferably 5 to 15 hours.

The temperature of the slurry water when stirring the slurry water is preferably 20 to 90 ° C, more preferably 20 to 70 ° C.
In addition, when stirring slurry water, it is preferable to cool to below the boiling point temperature of slurry water. Specifically, cooling to 80 ° C. or lower is preferable, and cooling to 20 to 60 ° C. is more preferable.

  In step (II), the aqueous solution containing trilithium phosphate obtained in step (I), a mixture containing a manganese compound and an iron compound is subjected to a hydrothermal reaction. The manganese compound may be a divalent manganese compound, and examples thereof include manganese halide, manganese sulfate, and manganese acetate. These may be used alone or in combination of two or more. Of these, manganese sulfate is preferably used from the viewpoint of improving battery characteristics. The iron compound may be a divalent iron compound or a hydrate thereof, such as a halide such as iron halide; a sulfate such as iron sulfate; an organic acid such as iron oxalate or iron acetate. Salts; and hydrates thereof. Among these, iron sulfate is preferably used from the viewpoint of improving battery characteristics.

  The use molar ratio of these manganese compound and iron compound (manganese compound: iron compound) is preferably 99: 1 to 51:49, more preferably 95: 5 to 70:30, and still more preferably 90:10. ~ 75: 25.

The mixture containing the aqueous solution containing trilithium phosphate, the manganese compound, and the iron compound may further contain a metal (M) compound other than the manganese compound and the iron compound. M in the metal (M) compound is Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and is synonymous with M in formula (A) described later. . As such metal (M) compounds, halides, sulfates, organic acid salts, hydrates thereof, and the like can be used. These may be used alone or in combination of two or more. Especially, it is preferable to use the metal (M) compound whose M is Mg or Zr from a viewpoint of improving a battery characteristic.
When these metal (M) compounds are used, the total addition amount of the manganese compound, iron compound, and metal (M) compound is preferably relative to 1 mol of trilithium phosphate in the aqueous solution obtained in the step (I). Is 0.99 to 1.01 mol, more preferably 0.995 to 1.005 mol.

  The amount of water used for the hydrothermal reaction is the above-mentioned step (I) from the viewpoint of the solubility of the manganese compound, the iron compound, and the metal (M) compound, the ease of stirring, the efficiency of synthesis, and the like. Preferably it is 10-30 mol with respect to 1 mol of phosphate ions in the aqueous solution containing trilithium phosphate obtained in 1. More preferably, it is 12.5-25 mol.

The order of addition of the manganese compound, iron compound, and metal (M) compound is not particularly limited. Moreover, while adding these compounds, you may add antioxidant as needed. As such an antioxidant, sodium sulfite (Na ₂ SO ₃ ), hydrosulfite sodium (Na ₂ S ₂ O ₄ ), aqueous ammonia and the like can be used. From the viewpoint of preventing the formation of lithium manganese iron phosphate from being excessively added, the addition amount of the antioxidant is 1 mol in total for the manganese compound, the iron compound, and the metal (M) compound. , Preferably it is 0.01-1 mol, More preferably, it is 0.03-0.5 mol.

The temperature in hydrothermal reaction should just be 100 degreeC or more, and 110-210 degreeC is preferable. The hydrothermal reaction is preferably performed in a pressure vessel, and when the reaction is performed at 110 to 180 ° C., the pressure at this time is preferably 0.2 to 0.9 MPa, and the reaction is performed at 160 to 210 ° C. The pressure is preferably 0.5 to 1.8 MPa. The hydrothermal reaction time is preferably 0.5 to 24 hours, more preferably 0.5 to 12 hours.
The obtained olivine-type lithium iron manganese phosphate can be isolated by filtration, washed with water, and dried. The amount of water used for washing is preferably 5 to 100 parts by mass, more preferably 5 to 50 parts by mass with respect to 1 part by mass of olivine-type lithium iron manganese phosphate. As the drying means, freeze drying or vacuum drying is used.

  The olivine-type lithium iron manganese phosphate used in the present invention is obtained using the above-mentioned trilithium phosphate aqueous solution, and the surface is uniformly coated with carbon. When the positive electrode material for a lithium secondary battery of the present invention contains such olivine type lithium manganese iron phosphate, it is possible to obtain a battery exhibiting superior battery characteristics while effectively reducing the amount of adsorbed moisture. . That is, to uniformly coat carbon on the surface of the olivine-type lithium manganese iron phosphate used in the present invention and to produce a positive electrode material for a lithium secondary battery containing the same, the above steps (I) to (II) are performed. In addition, it is preferable to further include a step of coating carbon on the surface of the olivine-type lithium manganese iron phosphate obtained in the step (II) as the step (III).

  In order to coat carbon on the surface of olivine type lithium manganese iron phosphate, carbon sources such as glucose, fructose, polyethylene glycol, polyvinyl alcohol, carboxymethylcellulose, saccharose, starch, dextrin, citric acid, or acetylene black are used in a conventional manner. Carbon sources of carbon black such as ketjen black, channel black, furnace black, lamp black, thermal black, and water, and water may be added and then fired.

The firing conditions are 400 ° C. or higher, preferably 400 to 800 ° C. for 10 minutes to 3 hours, preferably 0.5 to 1.5 hours under an inert gas atmosphere or reducing conditions. By this treatment, it is possible to obtain olivine-type lithium manganese iron phosphate in which carbon is uniformly coated on the particle surface. The amount of the carbon source used is preferably 3 to 15 parts by mass as carbon contained in the carbon source and more preferably 5 to 10 parts by mass as carbon contained in the carbon source with respect to 100 parts by mass of the olivine-type lithium iron manganese phosphate. .
In the present invention, it is preferable to use a positive electrode material for a lithium secondary battery obtained by coating the surface of olivine-type lithium manganese iron phosphate with carbon and then firing it.

Specifically, the olivine-type lithium iron manganese phosphate used in the present invention is, for example, the following formula (A):
LiFe _a Mn _b M _c PO ₄ (A)
(In the formula, M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. A, b and c are 0 <a <0.5, 0 .5 <b <1, and 0 ≦ c ≦ 0.2, and 2a + 2b + (valence of M) × c = 2.
It is represented by

The olivine-type lithium iron manganese phosphate represented by the above formula (A) contains at least manganese (Mn) and iron (Fe) which are transition metals. In the formula (A), M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd, or Gd, and is preferably Mg or Zr. a is 0 <a <0.5, and preferably 0.1 ≦ a ≦ 0.3. b is 0.5 <b <1, and preferably 0.7 ≦ b ≦ 0.9. c satisfies 0 ≦ c ≦ 0.2, and preferably 0 ≦ c ≦ 0.1. These a, b and c are numbers satisfying 2a + 2b + (M valence) × c = 2. Specific examples of the olivine-type lithium iron manganese phosphate represented by the above formula (A) include LiMn _0.8 Fe _0.2 PO ₄ , LiMn _0.75 Fe _0.15 Mg _0.1 PO ₄ , LiMn _0.75 Fe _0.19 Zr _0.03 PO _{4 and the} like. Is mentioned.

The BET specific surface area of the olivine-type lithium iron iron phosphate used in the present invention is preferably 5 to 40 m from the viewpoint of effectively reducing the amount of adsorbed water, coupled with having an appropriate crystallite diameter described later. ^{It is 2} / g, More preferably, it is 5-20 m < ² > / g.
In addition, the crystallite size of the olivine-type lithium iron manganese phosphate used in the present invention is a fine particle having the above-mentioned appropriate BET specific surface area, ensuring excellent battery characteristics while reducing the amount of adsorbed water. From this viewpoint, the thickness is preferably 10 to 150 nm, and more preferably 10 to 100 nm. The crystallite diameter is a value obtained by applying Scherrer's equation.

A positive electrode material for a lithium secondary battery of the present invention is obtained using the above-mentioned trilithium phosphate aqueous solution, contains olivine-type lithium manganese iron phosphate whose surface is uniformly coated with carbon, and has a temperature of 20 ° C. and The amount of adsorbed water measured as the amount of water volatilized from the time when water is adsorbed until reaching equilibrium at 50% relative humidity until the temperature is raised to 250 ° C. and held for 20 minutes is the lithium of the present invention. It is 2000 ppm or less in the positive electrode material for secondary batteries. That is, in the present invention, the amount of adsorbed moisture of the positive electrode material for a lithium secondary battery and the time when water is adsorbed until equilibrium is reached at a temperature of 20 ° C. and a relative humidity of 50%, the temperature is raised to 250 ° C. for 20 minutes. The amount of water volatilized by the time it is held is regarded as the same amount, and the measured value of the volatilized water amount is taken as the adsorbed water amount of the positive electrode material for lithium secondary batteries. As described above, when the amount of adsorbed water measured under a predetermined condition is within a limited range, it is possible to stably exhibit excellent battery characteristics even under various usage environments. The amount of adsorbed moisture is preferably not more than 1990 ppm, more preferably not more than 1985 ppm in the positive electrode material for a lithium secondary battery of the present invention.
The amount of water volatilized from the time when moisture is adsorbed until reaching equilibrium at a temperature of 20 ° C. and a relative humidity of 50% to when the temperature is raised to 250 ° C. and held for 20 minutes is, for example, the Karl Fischer moisture meter Can be measured.

  The lithium secondary battery to which the positive electrode for a lithium secondary battery including the positive electrode material for a lithium secondary battery of the present invention can be applied is not particularly limited as long as the positive electrode, the negative electrode, the electrolytic solution, and the separator are essential components.

  Here, as long as lithium ions can be occluded at the time of charging and released at the time of discharging, the material structure is not particularly limited, and a known material structure can be used. For example, a carbon material such as lithium metal, graphite, or amorphous carbon. It is preferable to use an electrode formed of an intercalating material capable of electrochemically inserting and extracting lithium, particularly a carbon material.

  The electrolytic solution is obtained by dissolving a supporting salt in an organic solvent. The organic solvent is not particularly limited as long as it is an organic solvent that is usually used for an electrolyte solution of a lithium ion secondary battery. For example, carbonates, halogenated hydrocarbons, ethers, ketones, nitriles, lactones An oxolane compound or the like can be used.

The type of the supporting salt is not particularly limited, but an inorganic salt selected from LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ , a derivative of the inorganic salt, LiSO ₃ CF ₃ , LiC (SO ₃ CF ₃ ) ₂ and LiN (SO ₃ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ and LiN (SO ₂ CF ₃ ) (SO ₂ C ₄ F ₉ ), and organic salt derivatives It is preferable that it is at least 1 type of these.

  The separator plays a role of electrically insulating the positive electrode and the negative electrode and holding the electrolytic solution. For example, a porous synthetic resin film, particularly a polyolefin polymer (polyethylene, polypropylene) porous film may be used.

  EXAMPLES Hereinafter, although this invention is demonstrated concretely based on an Example, this invention is not limited to these Examples.

[Example 1]
11.084 g of Li ₂ CO ₃ and 30 mL of water were mixed to obtain slurry water. Next, 112.929 g of 85% aqueous phosphoric acid solution was added dropwise at 35 mL / min while stirring the resulting slurry water at a temperature of 25 ° C. for 2 to 3 minutes, followed by 12 hours of phosphorus. An aqueous solution containing trilithium acid was obtained. Such an aqueous solution contained 2.97 moles of lithium per mole of phosphorus.

Next, 19.286 g of MnSO ₄ .5H ₂ O and 5.560 g of FeSO ₄ .7H ₂ O were added to the total amount of the aqueous solution containing the obtained trilithium phosphate to obtain a mixed solution. At this time, the molar ratio of MnSO ₄ and FeSO ₄ added (manganese compound: iron compound) was 80:20.
Subsequently, the obtained liquid mixture was thrown into the autoclave and hydrothermal reaction was performed at 170 degreeC for 0.5 hour. The pressure in the autoclave was 0.8 MPa. The produced crystal was filtered, and then washed with 12 parts by mass of water with respect to 1 part by mass of the crystal. The washed crystal was vacuum-dried at 60 ° C. and 1 Torr to obtain lithium iron manganese phosphate (LiMn _0.8 Fe _0.2 PO ₄ ).
The obtained lithium iron manganese phosphate was confirmed to be a single phase by powder X-ray diffraction measurement.

  Then, 3 g of the obtained lithium iron iron phosphate, 0.15 g of glucose, 28 mL of ethanol, and 2 mL of water were pulverized and mixed in a ball mill for 1 hour, and then at 700 ° C. for 1 hour in an argon hydrogen atmosphere (hydrogen concentration 3%). Baking was performed to obtain lithium manganese iron phosphate whose surface was uniformly coated with carbon.

[Example 2]
A lithium manganese iron phosphate whose surface is uniformly coated with carbon is obtained in the same manner as in Example 1 except that the pressure in the autoclave is 0.5 MPa and the hydrothermal reaction is performed at 150 ° C. for 0.5 hours. It was.
The lithium iron manganese phosphate obtained after the hydrothermal reaction was confirmed to be a single phase by powder X-ray diffraction measurement.

[Example 3]
A lithium manganese iron phosphate whose surface is uniformly coated with carbon was obtained in the same manner as in Example 1 except that the pressure in the autoclave was 0.25 MPa and the hydrothermal reaction was carried out at 130 ° C. for 0.5 hour. It was.
The lithium iron manganese phosphate obtained after the hydrothermal reaction was confirmed to be a single phase by powder X-ray diffraction measurement.

[Example 4]
Lithium iron manganese phosphate having a uniformly coated carbon surface was obtained in the same manner as in Example 1 except that the pressure in the autoclave was 0.8 MPa and the hydrothermal reaction was performed at 170 ° C. for 12 hours.
The lithium iron manganese phosphate obtained after the hydrothermal reaction was confirmed to be a single phase by powder X-ray diffraction measurement.

[Example 5]
Lithium iron manganese phosphate having a surface uniformly coated with carbon was obtained in the same manner as in Example 1 except that the pressure in the autoclave was 1.5 MPa and the hydrothermal reaction was performed at 200 ° C. for 12 hours.
The lithium iron manganese phosphate obtained after the hydrothermal reaction was confirmed to be a single phase by powder X-ray diffraction measurement.

[Example 6]
The surface was uniformly coated with carbon in the same manner as in Example 1, except that Li ₂ CO ₃ was 10.972 g, the pressure in the autoclave was 1.5 MPa, and the hydrothermal reaction was performed at 200 ° C. for 12 hours. The resulting lithium manganese iron phosphate was obtained.
The aqueous solution containing trilithium phosphate used contained 2.94 mol of lithium with respect to 1 mol of phosphorus, and the lithium iron manganese phosphate obtained after the hydrothermal reaction was subjected to powder X-ray diffraction. The single phase was confirmed by measurement.

[Example 7]
Except that Li ₂ CO ₃ was changed to 10.860 g, in the same manner as in Example 1, lithium iron iron phosphate having a surface uniformly coated with carbon was obtained.
The aqueous solution containing trilithium phosphate used contained 2.91 moles of lithium with respect to 1 mole of phosphorus, and the lithium manganese iron phosphate obtained after the hydrothermal reaction was subjected to powder X-ray diffraction. The single phase was confirmed by measurement.

[Example 8]
Except that Li ₂ CO ₃ was changed to 11.195 g, in the same manner as in Example 1, lithium iron manganese phosphate having a surface uniformly coated with carbon was obtained.
The aqueous solution containing trilithium phosphate used contained 3 moles of lithium with respect to 1 mole of phosphorus, and lithium manganese iron phosphate obtained after the hydrothermal reaction was measured by powder X-ray diffraction measurement. It was confirmed to be a single phase.

[Example 9]
Except that Li ₂ CO ₃ was changed to 11.307 g, in the same manner as in Example 1, lithium iron iron phosphate having a uniformly coated carbon surface was obtained.
The aqueous solution containing trilithium phosphate used contained 3.03 mol of lithium with respect to 1 mol of phosphorus, and the lithium iron manganese phosphate obtained after the hydrothermal reaction was subjected to powder X-ray diffraction. The single phase was confirmed by measurement.

[Comparative Example 1]
Lithium iron iron phosphate having a uniformly coated carbon surface was obtained in the same manner as in Example 1 except that 12.85 g of LiOH.H ₂ O was used instead of 11.084 g of Li ₂ CO ₃ .
The aqueous solution containing trilithium phosphate used contained 2.97 mol of lithium with respect to 1 mol of phosphorus, and the lithium iron manganese phosphate obtained after the hydrothermal reaction was subjected to powder X-ray diffraction. The single phase was confirmed by measurement.

[Comparative Example 2]
Lithium phosphate (Li ₃ PO ₄ ) 5.7897 g and iron phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) 25.08 g were placed in 25 mL of water and stirred with a magnetic stirrer for 12 hours. A mixed aqueous solution was obtained.
Next, 7.5 g of ethylene glycol was added to this mixed aqueous solution, stirred for 3 hours, and vacuum dried to obtain a precursor powder. Subsequently, the precursor powder was wet pulverized using a planetary ball mill. Specifically, a container made of zirconia (80 cc) was used as a container, 30 g of φ1 mm zirconia balls were used as grinding media, and 5 g of the precursor sample and 30 mL of ethanol were placed in the container and pulverized for 3 hours.
Next, the obtained slurry was dried in a constant temperature bath at 80 ° C. for 24 hours to obtain a precursor pulverized powder. The precursor pulverized powder was calcined at 600 ° C. for 5 hours in an argon atmosphere, pulverized using a planetary ball mill after cooling, and then classified using a 75 μm sieve to obtain a lithium iron phosphate active material. The obtained lithium iron phosphate was confirmed to be a single phase by powder X-ray diffraction measurement.

<< Measurement of crystallite diameter and BET specific surface area >>
In Examples 1 to 9 and Comparative Example 1, lithium iron manganese phosphate obtained after the hydrothermal reaction was used. In Comparative Example 2, carbon iron obtained after the firing reaction was supported. A powder X-ray diffraction measurement sample pressed at a pressure of 70 kg was prepared using a molding machine (TK-750, manufactured by Tokyo Kagaku). Next, for each of the obtained samples for measurement, a diffraction angle 2θ of 10 ° to 80 ° was set to a step size of 0 with a powder X-ray diffraction measurement apparatus (D8 Advance, manufactured by Bruker) with a tube voltage-current set to 35 kV-350 mA. X-ray diffraction pattern was obtained by measuring at 0.023 ° and a measurement speed of 0.13 sec / step. The crystallite diameter was determined by applying the Scherrer equation to all angles of the X-ray diffraction pattern. Further, the BET specific surface area was measured by a nitrogen adsorption method with a BET specific surface area measuring apparatus (Flow Soap II 2300, manufactured by Shimadzu Corporation).
The results are shown in Table 1.

<Measurement of adsorbed water content>
The lithium iron manganese phosphate obtained in Examples 1 to 9 and Comparative Example 1 with the surface uniformly coated with carbon, and the phosphorus obtained in Comparative Example 2 with the surface uniformly coated with carbon. About lithium iron oxide, moisture was adsorbed until it reached equilibrium after standing in an environment of temperature 20 ° C. and relative humidity 50% for 1 day. The amount was measured with a Karl Fischer moisture meter (MKC-610, manufactured by Kyoto Electronics Industry Co., Ltd.) and determined as the amount of adsorbed moisture in the positive electrode material for a lithium secondary battery.
The results are shown in Table 1.
Here, in Comparative Example 2, the adsorbed water content is as high as 3510 ppm, and as described in Patent Document 3, a lithium iron phosphorus-based composite oxide carbon composite having a low water content of 2000 ppm or less. It can be seen that a drying step is essential after producing the lithium iron phosphorus-based composite oxide carbon composite in order to obtain On the other hand, it can be seen that the example of the present invention can realize a positive electrode material having a low amount of adsorbed moisture without going through a drying process at such a stage.

<Evaluation of charge / discharge characteristics>
The positive electrode of the lithium ion secondary battery was produced using the lithium iron manganese phosphate or lithium iron phosphate obtained by Examples 1-9 and Comparative Examples 1-2 whose carbon was uniformly coated on the surface. Specifically, lithium iron manganese phosphate or lithium iron phosphate, ketjen black and polyvinylidene fluoride were mixed at a weight ratio of 75:15:10, and this was mixed with N-methyl-2-pyrrolidone. Was added and kneaded sufficiently to prepare a positive electrode slurry. The positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 20 μm using a coating machine, and vacuum dried at 80 ° C. for 12 hours. Thereafter, it was punched into a disk shape of φ14 mm and pressed at 16 MPa for 2 minutes using a hand press to obtain a positive electrode.
Next, a coin-type lithium ion secondary battery was constructed using the positive electrode. A lithium foil punched to φ15 mm was used for the negative electrode. As the electrolytic solution, a solution obtained by dissolving LiPF ₆ at a concentration of 1 mol / l in a mixed solvent in which ethylene carbonate and ethyl methyl carbonate were mixed at a volume ratio of 1: 1 was used. As the separator, a known one such as a polymer porous film such as polypropylene was used. These battery components were assembled and housed in a conventional manner in an atmosphere with a dew point of −50 ° C. or lower to produce a coin-type lithium secondary battery (CR-2032).

Using the manufactured lithium secondary battery, the initial discharge capacity of 0.1 C (17 mAh / g) and the discharge capacity after 10 cycles in a 30 ° C. temperature environment with a discharge capacity measuring device (HJ-1001SD8, manufactured by Hokuto Denko) Was measured.
The results are shown in Table 2.

  From the above results, the lithium manganese iron phosphate of the example is a fine particle having a specific surface area reduced while having a crystallite diameter of 100 nm or less as compared with the lithium manganese iron phosphate of the comparative example. It can be seen that the positive electrode material using is effectively reduced in the amount of adsorbed moisture. And if it is a lithium secondary battery using the positive electrode material of these Examples, it can express the battery characteristic equivalent to or more than the lithium ion battery using the positive electrode material of the comparative example under the use environment under the above conditions. I understand.

Claims (7)

An olivine type obtained by using lithium carbonate as a lithium source and an aqueous trilithium phosphate solution containing 2.7 to 3.3 moles of lithium with respect to 1 mole of phosphorus and having a surface uniformly coated with carbon. A positive electrode material for a lithium secondary battery containing lithium manganese iron phosphate,
The amount of adsorbed water measured as the amount of water volatilized from the time when moisture is adsorbed until reaching equilibrium at a temperature of 20 ° C. and a relative humidity of 50% until the temperature is raised to 250 ° C. and held for 20 minutes, The positive electrode material for lithium secondary batteries which is 2000 ppm or less. ^{2. The} positive electrode material for a lithium secondary battery according to claim 1, wherein the olivine-type lithium iron manganese phosphate has a BET specific surface area of 9 to 17 m ² / g. The olivine-type lithium manganese iron phosphate has the following formula (A):
LiFe _a Mn _b M _c PO ₄ (A)
(In the formula, M represents Mg, Ca, Sr, Y, Zr, Mo, Ba, Pb, Bi, La, Ce, Nd or Gd. A, b and c are 0 <a <0.5, 0 .5 <b <1, and 0 ≦ c ≦ 0.2, and 2a + 2b + (valence of M) × c = 2.
The positive electrode material for lithium secondary batteries of Claim 1 or 2 represented by these.   The positive electrode material for a lithium secondary battery according to any one of claims 1 to 3, wherein a crystallite diameter of the olivine-type lithium iron manganese phosphate is 50 to 100 nm.   The positive electrode for lithium secondary batteries containing the positive electrode material for lithium secondary batteries of any one of Claims 1-4. Step (I) of preparing an aqueous solution of trilithium phosphate containing 2.7 to 3.3 mol of lithium with respect to 1 mol of phosphorus using lithium carbonate as a lithium source, and phosphoric acid obtained in step (I) Step of obtaining olivine-type lithium manganese iron phosphate by subjecting trilithium aqueous solution, manganese compound and iron compound to hydrothermal reaction (II)
The manufacturing method of the positive electrode material for lithium secondary batteries of Claim 1 provided with these. Further, the surface of the olivine type lithium manganese iron phosphate obtained in the step (II) is coated with carbon (III)
The manufacturing method of the positive electrode material for lithium secondary batteries of Claim 5 provided with these.