JP5314264B2 - Method for producing positive electrode active material for lithium secondary battery, positive electrode active material and lithium secondary battery - Google Patents

Description

  The present invention relates to a method for producing a positive electrode active material for a lithium secondary battery, a positive electrode active material, and a lithium secondary battery.

Conventionally, a positive electrode made using olivine-type lithium M phosphate (M is a divalent metal) as a positive electrode active material, and a lithium metal such as a carbon-based material, a lithium alloy, or lithium ions can be occluded and released as a negative electrode active material. Lithium ion secondary batteries (hereinafter simply referred to as lithium secondary batteries) using a non-aqueous electrolyte as the electrolyte in combination with negative electrodes using various materials are conventional lead secondary batteries and nickel-cadmium secondary batteries. Since it is lighter and has a larger discharge capacity than batteries, it is widely used in various electronic devices. In this case, among divalent metals, in particular, a positive electrode manufactured using a positive electrode active material of an olivine-type lithium iron phosphate (LiFePO ₄ ) system manufactured using low-cost, stable iron as a raw material. The provided lithium secondary battery is preferably used. Patent Documents 1 to 7 relating to the invention relating to olivine-type lithium M phosphate and a lithium secondary battery using the same are exemplified below.
In Patent Document 1, an alkali metal element, a transition metal element, and a phosphoric acid compound are baked to produce a positive electrode active material composed of olivine-type M lithium phosphate. An invention that provides a lithium secondary battery having excellent discharge characteristics is disclosed.
In Patent Document 2, an alkali metal element, an element of Group IV to VII of the periodic table, and an alkali metal-containing iron complex oxide are baked in nitrogen gas or in a reducing atmosphere when iron is trivalent, and LiFePO An invention is disclosed in which a positive electrode active material containing olivine-type lithium M phosphate such as ₄ is produced, and using this as a positive electrode, a low-cost lithium secondary battery excellent in charge / discharge characteristics is provided.
Patent Document 3 describes an olivine-type phosphate compound given by the general formula LiFeXPO ₄ (where X is a phosphate compound in the state where the electrochemical potential is 3 to 4 V with respect to the standard potential of lithium metal). And a lithium secondary battery that can be charged and discharged at a low voltage of 4 V or less using the positive electrode active material composed of a metal element such as cobalt and nickel) Yes.
In Patent Document 4, a metal particle that is conductive and has noble metal particles that are more precious than the redox potential as the positive electrode active material of the lithium iron phosphate material is supported on the olivine type lithium iron phosphate material powder. Producing a positive electrode active material represented by the formula LiFeXPO ₄ (where X is at least one of magnesium, cobalt, nickel, and zinc) by firing, and providing an inexpensive lithium secondary battery using this as a positive electrode The invention is disclosed.
In Patent Document 5, a water-soluble organic reducing agent is mixed with a phosphoric acid aqueous solution containing a lithium salt and an iron salt to prepare a mixed aqueous solution, and an alkaline solution is mixed with the mixed aqueous solution to combine lithium and iron. A method of synthesizing a positive electrode active material comprising LiFePO ₄ by forming a coprecipitate of phosphorous oxide and then firing the coprecipitate, and using this as the positive electrode An invention for providing a secondary battery is disclosed.
Patent Document 6 discloses a method for producing a positive electrode active material comprising a lithium-based iron-phosphorus-based composite oxide carbon composite comprising a LiFePO ₄ particle surface coated with a carbonaceous material and having an average particle size of 0.5 μm. An invention is disclosed that provides a lithium secondary battery having a particularly high discharge capacity by using as a positive electrode.
Patent Document 7 discloses a positive electrode active material composed of LiFe (II) PO ₄ , a solution, dispersion or suspension containing Li source, Fe source, P source, C source, and O source as its component raw materials. Sprayed into a high-temperature atmosphere to make a precursor, and this precursor is heat-treated at 80 to 1000 ° C. in a reducing atmosphere or an inert atmosphere. ), An invention for providing a high-power lithium secondary battery having stable charge / discharge cycle performance is disclosed.
Japanese Patent Laid-Open No. 9-13724 Japanese Laid-Open Patent Publication No. 9-13725 JP 2001-85010 A JP 2001-110414 A JP 2002-117831 A JP 2003-292309 A JP 2005-116392 A

However, the positive electrode active material composed of olivine-type M lithium phosphate disclosed in Patent Documents 1 to 3 has a much higher electric resistance than lithium metal oxides such as LiCoO ₂ that have been conventionally used as a positive electrode active material. Since it is large, resistance polarization increases when charging and discharging are performed, and a sufficient discharge capacity cannot be obtained. There are also problems such as poor charge acceptance.
As a method for solving such a problem, the powder particles of olivine-type lithium M phosphate are made finer, the reaction area is increased, the lithium ion diffusion is facilitated, and the positive electrode active material powder is carbon black, etc. It is conceivable to mix the conductive agent powder, and to shorten the distance that electrons flow inside the powder particles of lithium M phosphate.
However, the fine primary particles of the olivine-type M lithium phosphate-based material are liable to cause secondary aggregation when mixed with a conductive agent powder such as carbon black during the production of the positive electrode. Inside the agglomerated particles, a sufficient current collecting effect cannot be obtained and the electric resistance becomes very large. As a result, the active material in the central part of the aggregated particles does not cause electron conduction even when the battery is charged / discharged, and the charge / discharge capacity decreases. On the other hand, since the fine primary particles have a large surface area, the amount of the dispersion medium required in the preparation of the slurry for producing the positive electrode is increased, and it is difficult to obtain the required coating amount for the current collecting base material, and cracking occurs during drying. There are problems such as being easy to occur, a high capacity positive electrode cannot be obtained because sufficient compression is difficult, elution of the metal of the positive electrode active material in the electrolyte increases, and the life of the lithium secondary battery is shortened.
In general, it is considered that the particle surface of the olivine-type M lithium phosphate-based material is amorphous because of its lower crystallinity than the bulk. For this reason, the divalent metal is oxidized by being left in the air, and converted to a trivalent phosphate having a higher resistance. As a result, a large polarization is generated at the time of initial charge, so that there are problems that the leaving condition is severe, activation becomes complicated, and resistance components remain.
In addition, referring to the specific problems of Patent Documents 4 to 7, in Patent Document 4, noble metal particles that are more precious than the oxidation-reduction potential are susceptible to chemical modification accompanied by oxidation-reduction, and as a lithium secondary battery. There is a problem with stability.
In Patent Document 5, a positive electrode active material composed of a LiFePO ₄ carbon composite is mixed into a mixed aqueous solution in which carbon black or other water-soluble organic reducing agent is mixed as a carbon source in a phosphoric acid aqueous solution containing a lithium salt and an iron salt. Since the co-precipitate of lithium and iron composite phosphate is mixed and mixed with an alkaline solution, the dispersion effect of the carbon particles on the surface of the LiFePO ₄ particles is insufficient, and a sufficient current collecting effect is obtained. I can't get it.
In Patent Documents 6 and 7, it is difficult to control the particle size as the positive electrode active material.
The present invention manufactures a positive electrode active material comprising olivine-type lithium M phosphate that has solved the above-mentioned problems of the conventional invention, and uses this as a positive electrode for lithium secondary batteries having excellent discharge capacity maintenance characteristics and long-term stability. An object is to provide a secondary battery.

The present invention, as described in claim 1, using only the powder of olivine-type lithium M phosphate (M is a divalent metal) having a primary particle size of less than 1 micron as a raw material, which is used in a vacuum or in an inert atmosphere. The present invention resides in a method for producing a positive electrode active material for a lithium secondary battery comprising a powder of olivine-type lithium M phosphate, characterized by sintering under heating and crushing a mass of the obtained sintered body .
The present invention, as set forth in claim 2, only the powder of the olivine-type phosphate M lithium particles having a particle size of less than 1 micron primary particles (M is a divalent metal) as a raw material, adding and mixing a carbon source to made were made by mixture was sintered by heating under vacuum or under an inert atmosphere, the olivine-type phosphate M lithium powder of the complex of the carbon, characterized by pulverizing a lump of sintered bodies obtained It exists in the manufacturing method of the positive electrode active material for lithium secondary batteries.
Further, according to the present invention, as defined in claim 3, M is iron, nickel, cobalt or manganese.
Furthermore, the present invention provides a positive electrode active material for a lithium secondary battery produced by the production method of the invention as described in claim 4.
Furthermore, as described in claim 5, the present invention resides in a lithium secondary battery comprising a positive electrode manufactured using the positive electrode active material for a lithium secondary battery according to the above invention.
The present invention further comprises a positive electrode prepared using a positive electrode active material for a lithium secondary battery comprising particles of 20 microns or less, as described in claim 6. Next battery.

According to the invention of claim 1, sintering does not cause secondary agglomeration. Therefore, not only the current collection effect is improved, but also crystal growth has occurred, as compared with the positive electrode active material before sintering. Thus, a positive electrode active material composed of olivine-type lithium M phosphate that yields a positive electrode with significantly improved conductivity and ultimate density can be obtained. A stable and robust positive electrode active material comprising a sintered composite of an olivine type lithium M phosphate and a carbon source produced by the invention according to claim 2 is obtained.
According to the invention of claim 3, in the method for producing a lithium secondary battery according to claim 1 or 2, it is preferable to use a divalent metal such as iron or nickel.
A lithium secondary battery comprising the positive electrode according to claim 5 produced using the positive electrode active material according to the invention according to claim 4, wherein the positive electrode active material according to the present invention used for the positive electrode is a positive electrode active material before sintering. Compared to lithium secondary batteries equipped with a positive electrode made of a material, the elution of the metal of the positive electrode active material into the electrolyte solution can be significantly reduced by float charging or standing, thus improving the capacity retention rate during charge and discharge. Resulting in a lithium secondary battery that is stable over a long period of time.
Further, according to the invention according to claim 6, the amount of the dispersion medium is small in the preparation of the slurry at the time of producing the positive electrode using the positive electrode active material of 20 microns or less, the required coating amount is sufficient, and at the time of drying It is possible to produce a high-capacity positive electrode that is highly densely compressed without causing cracks.

Embodiments of the present invention will be described in detail below.
The present invention relates to a positive electrode active material powder composed of olivine-type M lithium phosphate (where M is a divalent metal) synthesized using a liquid phase reaction or solid phase reaction method, which is a conventionally known arbitrary method. It is used as a raw material for the positive electrode active material manufacturing method of the invention.
That is, according to the conventional desired synthesis method, the target olivine-type lithium M phosphate is synthesized. For example, as the composition component material, for example, at least one of lithium phosphate, divalent or trivalent transition metal A desired solution is added to or mixed with the metal compound, and the mixture is synthesized by firing in the atmosphere or an inert atmosphere such as argon. A positive electrode active material composed of synthesized olivine-type lithium M phosphate (M is a divalent metal) is obtained.
The divalent metal represented by M is at least one selected from the group consisting of Co, Ni, Fe, Mn, Cu, Mg, Zn, Ca, Cd, Sr, and Ba. In particular, when a trivalent transition metal is used as a material for synthesizing olivine-type lithium M phosphate, during the heating reaction, it is baked in a reducing atmosphere such as hydrogen gas to obtain a divalent metal.
Examples of the carbon source include powders such as acetylene black, ketjen black, graphite carbon, and carbon black, and water-soluble organic reducing agents such as phenol derivatives such as ascorbic acid, phenol, and pyrogallol.

The present invention uses, as a raw material, the positive electrode active material powder composed of olivine-type lithium M phosphate synthesized by the conventional desired synthesis method as described above, and sintering this in an inert atmosphere or in vacuum, A lithium secondary battery including a positive electrode produced using the positive electrode active material produced and using the positive electrode active material powder before sintering is manufactured for a longer time than a lithium secondary battery including a positive electrode produced using the positive electrode active material powder before sintering. It was confirmed that a lithium secondary battery with improved stability, discharge capacity and the like can be obtained. In this method for producing a positive electrode active material, the inventors of the present application can sinter the particles constituting the positive electrode active material only to particles smaller than 1 micron, but olivine-type phosphoric acid M having a size of 1 micron or more. It has been found that the positive electrode active material particles of lithium do not cause a sintering phenomenon. Based on this finding, a particular feature of the present invention therefore resides in sintering the positive electrode active material particles of less than 1 micron. The sintering temperature is in the range of 400 ° C to 900 ° C, preferably in the range of 500 ° C to 800 ° C.
Therefore, if all of the positive electrode active material powder synthesized by the conventional desired synthesis method is a particle of less than 1 micron, if it is used as a raw material as it is or mixed with particles of 1 micron or more, The powder is sieved and only the powder of less than 1 micron is collected and used as a raw material, and the remaining particles of 1 micron or more are pulverized by a ball mill etc. until they become particles of less than 1 micron. When all of the powder consists of particles of 1 micron or more, all of the powder is milled and pulverized into particles of less than 1 micron, which is used as a raw material. In addition, when synthesized by a solid phase reaction method, it is usually synthesized in the form of a nodule, so this is pulverized with a ball mill or the like until it becomes less than 1 micron to obtain a raw material.

The following is a more detailed embodiment of the present invention, as olivine-type M lithium phosphate M, especially olivine-type lithium iron phosphate (LiFePO ₄ ) synthesized using resource-rich and inexpensive iron. A method for producing the positive electrode active material of the present invention using as a raw material will be described.

Preparation of cathode active material raw material consisting of lithium iron phosphate:
First, before carrying out the present invention, for example, a positive electrode active material composed of lithium iron phosphate composed of only nano-order particles having a relatively high purity was synthesized by a hydrothermal method. That is, as its synthetic material, 463 g of lithium phosphate and 795 g of divalent iron chloride tetrahydrate, for example, as divalent iron compound, are put into a pressure vessel (autoclave) together with 2000 ml of distilled water, and stirred. After mixing, the inside of the vessel was replaced with argon gas and sealed. This pressure-resistant airtight container was reacted in an oil bath at 180 ° C. for 48 hours. Subsequently, after cooling to room temperature, the reaction product was taken out and dried at 100 ° C. to obtain a powder. The obtained powder was confirmed to be lithium iron phosphate by an X-ray diffraction pattern. Further, it was confirmed by scanning electron microscope (SEM) observation that the powder had a particle size in the range of 20 nm to 200 nm.
Example 1
10 g of the lithium iron phosphate powder synthesized above was used as a raw material, put in a crucible, and then put in a vacuum gas replacement furnace. The inside of the furnace was evacuated after nitrogen gas substitution, subjected to a baking treatment at 300 ° C. for 3 hours, and then a sintering treatment at 800 ° C. for 3 hours. Next, after cooling to room temperature, the crucible was taken out of the furnace, and a sintered sample of the positive electrode active material therein was collected. The sample was a dark green mass. This was pulverized with a coffee mill. Next, this was sieved to obtain a positive electrode active material powder composed of particles of 20 microns or less. The powder was charged with 5% of acetylene black and mixed for 1 hour in a ball mill having a ball diameter of 10 mm. Thus, a positive electrode active material composed of a mixed composite of lithium iron phosphate powder and carbon was produced. Hereinafter, this sample powder is referred to as an implementation product 1.

Fig. 1 is a scanning electron microscope (SEM) photograph of a positive electrode active material powder that is a raw material before sintering at a magnification of 10,000, and Fig. 2 is a scan at the same magnification as Fig. 1 after sintering of the positive electrode active material shown in Fig. 1. An electron microscope (SEM) photograph is shown.
As shown in FIG. 2, the nano particles of the powder before sintering shown in FIG. 1 grew to a micron order as shown in FIG. 2, and were confirmed to be round and smooth large particle crystals.
FIG. 3 shows respective X-ray diffraction patterns of the powder before sintering and the powder after sintering. As is clear from this, it was confirmed that the basic diffraction pattern did not change before and after sintering. For example, the crystallite size by sintering increased from 413 to 524 mm in (020).
By such a sintering process, the surface area of the particles is reduced and the crystallinity is improved. As a result, as shown below, the effect of reducing the elution amount of the metal in the positive electrode active material into the electrolyte is brought about. When the paste is applied to the current-collecting metal substrate during the production of the positive electrode, the positive electrode active material has an increased bulk density and an effect of increasing the capacity by increasing the density of the coating layer.
Example 2
Using 10 g of the synthesized lithium iron phosphate powder as a raw material and mixing it with 1 g of commercially available sugar containing sucrose as a main ingredient and invert sugar added as a carbon source, and adding 10 ml of distilled water to the mixture After thoroughly stirring and mixing, it was dried at 100 ° C. for 2 hours. Next, after putting this in a crucible, this was put in a vacuum gas replacement furnace, replaced with nitrogen gas, and then evacuated, fired at 300 ° C. for 5 hours, and then sintered at 800 ° C. for 5 hours. Processed. Subsequently, after cooling this to room temperature, the crucible was taken out from the furnace and the sintered body inside was collected. The sintered body was a black lump. The powder was pulverized with a coffee mill and sieved to obtain a powder comprising a sintered composite of the positive electrode active material of the present invention and carbon comprising particles of 20 microns or less. Hereinafter, this sample powder is referred to as an implementation product 2. The amount of carbon contained in the sample powder by thermogravimetric analysis was 1.5%.
Comparative Example 1
5% of acetylene black is added to 10 g of the synthesized lithium iron phosphate powder, mixed for 1 hour in a ball mill with a ball diameter of 10 mm, and pulverized to form a mixed composite of lithium iron phosphate and carbon A positive electrode active material powder was obtained. Hereinafter, this sample powder is referred to as Comparative Product 1.
Comparative Example 2
10 g of ferrous phosphate hydrate (Fe ₃ (PO ₄ ) ₂ · 8H ₂ O) and 2.4 g of the above lithium phosphate (Li ₃ PO ₄ ) were sufficiently dry mixed with a mixer. Subsequently, it was immersed in a solution of 3 g of polyethylene glycol in 10 ml of distilled water for 1 hour with stirring, and then dried under reduced pressure to remove the solvent. This mixture was pulverized with alumina beads having a particle diameter of 8 mm using a dry bead mill apparatus to obtain a pre-reaction body. Next, 10 g of this reaction precursor was press-molded at 44 MPa by a hand press. Next, the compression molded product was calcined at 600 ° C. for 5 hours in a nitrogen atmosphere, cooled, pulverized and classified to obtain a positive electrode active material powder composed of a calcined composite in which the surface of lithium iron phosphate particles was coated with carbon. It was. Hereinafter, this sample powder is referred to as Comparative Product 2. The particle size of lithium iron phosphate, the main component of this sample powder, was found to be about 0.2 to 2 microns, which is significantly larger than the nano particle size of the lithium iron phosphate raw material synthesized by the hydrothermal method. did. The carbon content of the comparative product was confirmed to be 5.3% by thermogravimetric analysis.

  Next, in addition to the carbon content of Example 1, Example 2, Comparative Example 1 and Comparative Example 2 obtained above, (020) crystallite length by X-ray diffraction, 20 kN pressurization by powder resistance meter The reached density and conductivity were measured. That is, the weighed powder sample was put in a cylinder, and the direct current resistance when the sample was compressed with a piston with a set load was measured with a powder resistance meter, whereby the conductivity and the density reached were measured by the displacement during compression. For comparison, the above-mentioned measurement items were also measured for the lithium iron phosphate synthesized and used as a raw material before sintering. The results are shown in Table 1 below.

As can be seen from Table 1, when Examples 1 and 2 obtained according to Examples 1 and 2 of the present invention are compared with Comparative Examples 1 and 2 obtained from Comparative Examples 1 and 2, respectively, crystals accompanying sintering are obtained. Growth (Å) was observed for (020) crystallite growth. In addition, the ultimate density (g / cc) was 2.4 or more, which was significantly higher than that of Comparative Products 1 and 2. Regarding conductivity, in particular, the product 2 has a carbon content of 1.5%, which is significantly lower than the carbon content of the product 1, the comparison product 1 and 2, and the carbon content of 5%. It was confirmed that it increased. This is presumably because the carbon particles produced by sugar sintering efficiently coat the surfaces of the olivine type lithium iron phosphate particles.
The effect of the sintering treatment by the production method of the present invention is further apparent when compared with the respective numerical values of the same measurement items as above for the synthesized lithium iron phosphate before sintering used as the raw material of the production method. is there.

Next, in order to test the amount of Fe elution with respect to the high-temperature electrolyte solution, 5 g of lithium iron phosphate in each powder was obtained for Example Product 1, Example Product 2, Comparative Product 1, Comparative Product 2 and raw material lithium iron phosphate. Into a nonaqueous electrolyte solution at 80 ° C. (EC: EMC was dissolved in 1 mol of LiPF ₆ as a solute in 20 g of a solvent in a ratio of 3: 7) and left at the same temperature for 10 days, The concentration of Fe eluted in each electrolyte was measured by atomic absorption spectrometry. The results are shown in Table 2 below.

  As apparent from Table 2 above, in the products 1 and 2 produced by the production method of the present invention, the surface area of the lithium iron phosphate powder particles was significantly reduced by sintering, so iron was eluted into the electrolyte. Compared to comparative products 1 and 2, it is presumed that they could be greatly suppressed. This brings about the effect that a stable battery can be obtained over a long period of time when the positive electrode active material produced according to the present invention is used as the positive electrode of a lithium secondary battery.

The positive electrodes were produced as follows using the four kinds of the working products 1 and 2 and the comparative products 1 and 2, respectively.
That is, acetylene black as a conductive agent was added and mixed with each of the above four types of powders so that the total carbon amount was 10%. Each of the mixed powders and polyvinylidene fluoride (PVdF) as a binder were mixed at a weight ratio of 95: 5, and N-methyl-2-pyrrolidone (NMP) was added to this, and kneaded thoroughly. Thus, each positive electrode slurry was prepared. Next, each positive electrode slurry was applied to a current collector made of an aluminum foil having a thickness of 15 μm at a coating amount of 100 g / m ² , and then dried at 120 ° C. for 30 minutes. Thereafter, this was rolled to a density of 2.0 g / cc with a roll press, and then punched into a 2 cm ² disk shape to produce each positive electrode.
The four types of positive electrodes thus obtained differ in the bulk density of the four types of positive electrode active materials used above, and therefore the coating thicknesses thereof differ, and the higher the reaching density shown in Table 1, the higher the coating density. A thin positive electrode was obtained. That is, the positive electrode produced using the working products 1 and 2 was thinner than the coating thickness of the comparative products 1 and 2. On the other hand, it was difficult to increase the density of the positive electrode produced using the comparative products 1 and 2, and reached a predetermined density, but peeling of the coated film from the current collector was observed in part. In the battery test described below, a portion having no separation was selected and used.
In general, a positive electrode active material made of olivine-type lithium iron phosphate has a true density of only about 2/3 as compared with other types of positive electrode active materials. Therefore, a positive electrode having a thicker and higher packing density as a practical positive electrode. Therefore, by using a positive electrode active material such as Examples 1 and 2 produced by the production method such as Example 1 and Example 2 according to the present invention as the positive electrode, it is possible to easily meet the requirements of the positive electrode. Industrial production can be realized. In addition, it was found that it is preferable to use the positive electrode active material of the present invention pulverized to 20 microns or less in consideration of positive electrode slurry adjustment and coating properties.

Next, four types of lithium secondary batteries were manufactured as follows using the four types of positive electrodes thus prepared, the negative electrode described below, a separator, and a non-aqueous electrolyte.
That is, artificial graphite (average particle size 5 μm, d ₀₀₂ = 0.337 nm, Lc = 58 nm) was used as the negative electrode material, and this was mixed with polyvinylidene fluoride (PVdF) at a weight ratio of 95: 5. Methyl-2-pyrrolidone (NMP) was added and sufficiently kneaded to prepare a negative electrode paste. Next, the negative electrode paste was applied onto a current collector of copper foil having a thickness of 15 μm, naturally dried at room temperature of 25 ° C., and further dried at 130 ° C. under reduced pressure for 12 hours. Thereafter, it was rolled with a roll press and punched into a disk shape of 2 cm ² which was the same as the positive electrode to produce a negative electrode.
As the electrolytic solution, LiPF ₆ was dissolved at a concentration of 1M in a mixed solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 1 to prepare an electrolytic solution. The amount of water in the electrolyte was 15 ppm.
Four types of coin-type lithium secondary batteries were manufactured using the above four types of positive electrode, negative electrode, and electrolyte. In this case, as the separator interposed between the positive and negative electrodes, a polymer porous film such as polypropylene and other known ones are selectively used. Note that the atmosphere during the production was a dew point of −50 ° C. or lower. The positive and negative electrodes were used by being crimped to a battery case with a current collector. Thus, four types of coin-type lithium secondary batteries having a diameter of 25 mm and a thickness of 1.6 mm were manufactured. The batteries manufactured using the above-mentioned implementation product 1, implementation product 2, comparison product 1 and comparison product 2 are referred to as implementation product 1 battery, implementation product 2 battery, comparison product 1 battery, and comparison product 2 battery, respectively. Called.

Lithium secondary battery discharge capacity test:
A large number of each of the above four types of batteries were prepared, and 10 cycles of charging / discharging at a low rate were performed. The charging conditions at this time were constant current and constant voltage charging with a current of 0.1 CA and a voltage of 4.1 V, and the discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 2.0 V. All temperatures were 25 ° C. The discharge capacity at the 10th cycle was measured. In the 11th cycle, a high rate discharge test was performed and a high rate discharge capacity of 5 CA was measured. The capacity was the capacity per gram of filled lithium iron phosphate. All test temperatures were 25 ° C. The measurement results for each of the above are shown in Table 3 below.

  As shown in Table 3, the batteries of the working products 1 and 2 have a slightly lower discharge capacity than the batteries of the comparative products 1 and 2, but practically comparable rate-specific discharge characteristics, etc. Confirmed to show.

High temperature float charge characteristics test of lithium battery:
Each of the above four types of batteries was tested as follows. That is, a high temperature float charge characteristic test was conducted at 60 ° C. for 3 months with a charge setting voltage of 4.1V. After returning to room temperature after the test, the capacity was measured by discharging at 0.1 CA.
The results are shown in Table 4 below.

As is clear from Table 4, the battery of Example Product 1 and the battery of Example Product 2 showed significantly superior capacity retention ratios as compared with the batteries of Comparative Product 1 and Comparative Product 2. This is presumed to be because the amount of Fe transferred to the negative electrode was reduced to suppress the functional failure of SEI due to the Fe elution suppression effect of the sintering treatment of the present invention.
As described above, when the olivine-type M lithium phosphate produced according to the present invention is used as a positive electrode active material composed of a composite of this and the conductive agent carbon, the slurry coatability during the production of the positive electrode is improved. In addition, the effect of improving the packing density is achieved, and a lithium secondary battery using this as a positive electrode provides a stable battery for a long period of time.

In addition, it goes without saying that the same positive electrode active material can be obtained by sintering in an inert atmosphere such as argon gas instead of sintering under vacuum.
Further, in the above examples, the positive electrode active material composed of particles of less than 1 micron in which M of the olivine-type lithium lithium M is Fe is used as a raw material, but the production cost is higher than that of Fe and the resources are scarce. , Ni, Co, Mn, and other positive electrode active materials obtained by replacing Fe with a positive electrode active material having the same characteristics and effects as those obtained using Fe can be obtained. A lithium secondary battery including a positive electrode active material composed of a mixed or sintered composite and a positive electrode produced using the positive electrode active material is obtained.

Further, the olivine-type M lithium phosphate comprising particles of less than 1 micron according to the present invention is sintered with a mixture of a carbon source such as carbon powder in a vacuum or under an inert atmosphere, and the resulting sintered body is pulverized. An example of a method for producing a positive electrode active material composed of a sintered composite comprising olivine-type lithium M phosphate particles and carbon according to the present invention will be described.
In carrying out the production methods of Examples 3 to 5 below, olivine type lithium iron phosphate powder synthesized by the above hydrothermal method having the same particle size of 20 nm to 200 nm as used in Example 1 is prepared as a raw material. .
Example 3
Using 10 g of the lithium iron phosphate powder synthesized above as a raw material, 5% of the total acetylene black as a carbon source was mixed and ground in a ball mill with a ball diameter of 10 mm for 1 hour to obtain a black powder. This was put in a crucible, placed in a furnace, fired at 300 ° C. for 5 hours in a nitrogen atmosphere, and then sintered at 800 ° C. for 5 hours. Next, after cooling to room temperature, the crucible was taken out and a sample inside was collected. This was pulverized with a coffee mill. Subsequently, a positive electrode active material powder comprising a sintered composite of lithium iron phosphate powder of the present invention and carbon comprising particles of 20 microns or less was produced by sieving. This sample powder is referred to as an implementation product 3.

FIG. 4 shows a scanning electron microscope (SEM) photograph of Example Product 3 at a magnification of 10,000. As a result, it was confirmed that the particles of the positive electrode active material powder after sintering grew to a micron order and became large particle crystals as in the case of Example 1 in FIG.
Further, it was confirmed from the X-ray diffraction pattern of the powder of Example Product 3 that the basic pattern was not changed by sintering. The crystallite size by sintering increased to, for example, 415 to 615 mm at (020).

Example 4
Using 10 g of the lithium iron phosphate powder synthesized above as a raw material, and 5% of the total acetylene black as a carbon source in wet dispersion in acetone to form a uniform mixed solution, this was placed in a crucible and then dried. A black mixed powder was obtained. Next, this was put in a furnace, fired at 300 ° C. for 5 hours in a nitrogen atmosphere, and then sintered at 800 ° C. for 5 hours. Next, after cooling to room temperature, the crucible was taken out and a sample inside was collected. This was pulverized with a coffee mill. Subsequently, a positive electrode active material powder comprising a sintered composite of lithium iron phosphate powder of the present invention and carbon comprising particles of 20 microns or less was produced by sieving. This sample powder is hereinafter referred to as Example Product 4.
Example 5
Using 10 g of the lithium iron phosphate powder synthesized above as a raw material, mixing this with 5% of the total acetylene black and 10% of the total sucrose as a carbon source, wet-dispersing this in distilled water and mixing uniformly The resulting mixture was put into a crucible and then dried to obtain a black mixed powder. Next, this was put in a furnace, fired at 300 ° C. for 5 hours in a nitrogen atmosphere, and then sintered at 800 ° C. for 5 hours. Next, after cooling to room temperature, the crucible was taken out and a sample inside was collected. This was pulverized with a coffee mill. Subsequently, a positive electrode active material powder comprising a sintered composite of lithium iron phosphate powder of the present invention and carbon comprising particles of 20 microns or less was produced by sieving. This sample powder is hereinafter referred to as Example Product 5.

  Next, the carbon content of the product 3, the product 4 and the product 5 obtained above, (020) crystallite length by X-ray diffraction, reached density and conductivity at 20 kN pressurization by a powder resistance meter It was measured. That is, the weighed powder sample was put in a cylinder, and the direct current resistance when the sample was compressed with a piston with a set load was measured with a powder resistance meter, whereby the conductivity and the density reached were measured by the displacement during compression. The results are shown in Table 5 below.

  As can be seen from Table 5, when Examples 3, 4 and 5 obtained by Examples 3, 4 and 5 of the present invention were compared with Comparative Examples 1 and 2 obtained from Comparative Examples 1 and 2, the results were as follows. The crystal growth (Å) associated with crystallization was observed as (020) crystallite growth. In addition, greater growth was observed than the products 1 and 2 shown in Table 1. In addition, the ultimate density (g / cc) is 2.4 or more, which is significantly higher than that of the comparative products 1 and 2, and the conductivity of the implementation products 3 and 4 is the carbon content of the comparative products 1 and 2 and Although the amount was smaller than that of the product 1, the increase in conductivity was confirmed.

Next, in order to test the amount of Fe elution with respect to the high-temperature electrolyte solution for each of Example 3, Example 4, and Example 5, the non-aqueous solution at 80 ° C. was adjusted so that the amount of lithium iron phosphate in each powder was 5 g. Electrolyte solution (EC: EMC was dissolved in 20 g of solvent in a ratio of 3: 7 and 1 mol of LiPF ₆ was dissolved as a solute) and left at the same temperature for 10 days, and then the Fe eluted in each electrolyte solution The concentration was measured by atomic absorption spectrometry. The results are shown in Table 6 below.

As is clear from Table 2 above, the products 3, 4 and 5 produced by the production method of the present invention.
Then, since the surface area of lithium iron phosphate particles was significantly reduced by sintering, it is presumed that iron elution into the electrolytic solution could be greatly suppressed as compared with Comparative Example 2 in which pressure firing was performed. This suggests that when the positive electrode active material produced according to the present invention is used as the positive electrode of a lithium battery, a stable battery can be obtained for a long period of time.

Each positive electrode was produced by the same manufacturing method as that produced using the powders of Examples 1 and 2 using the three types of powders of Examples 3, 4 and 5, respectively.
The three types of positive electrodes obtained in this way were obtained as positive electrodes having a coating thickness thinner than the coating thicknesses of comparative products 1 and 2, as in the case of the positive electrodes prepared using Examples 1 and 2. It was.

  Next, the three types of positive electrodes produced using the working products 3, 4 and 5 in this way, and the same negative electrode and non-same as those used in the case of manufacturing a battery using the working products 1 and 2 of the previous period described below. Using the water electrolyte, three types of lithium secondary batteries including the battery of Example Product 3, the battery of Example Product 4, and the battery of Example Product 5 were manufactured.

Lithium secondary battery discharge capacity test:
A large number of each of the above three types of batteries were prepared, and 10 cycles of charging / discharging at a low rate were performed. The charging conditions at this time were constant current and constant voltage charging with a current of 0.1 CA and a voltage of 4.1 V, and the discharging conditions were constant current discharging with a current of 0.1 CA and a final voltage of 2.0 V. All temperatures were 25 ° C. The discharge capacity at the 10th cycle was measured. In the 11th cycle, a high rate discharge test was performed and a high rate discharge capacity of 5 CA was measured. The capacity was the capacity per gram of filled lithium iron phosphate. All test temperatures were 25 ° C. The measurement results for each of the above are shown in Table 7 below.

  As is clear from Table 7, the discharge capacities of the batteries of the implementation products 3, 4, and 5 are substantially equal to or greater than those of the comparative products 1 and 2.

High temperature float charge characteristics test of lithium battery:
Each of the above four types of batteries was tested as follows. That is, a high temperature float charge characteristic test was conducted at 60 ° C. for 3 months with a charge setting voltage of 4.1V. After returning to room temperature after the test, the capacity was measured by discharging at 0.1 CA.
The results are shown in Table 8 below.

  As is clear from Table 8, the battery of Example Product 3, the battery of Example Product 4, and the battery of Example Product 5 showed significantly superior capacity retention ratios as compared with the batteries of Comparative Product 1 and Comparative Product 2. This is presumed to be because the amount of Fe transferred to the negative electrode was reduced to suppress the functional failure of SEI due to the Fe elution suppression effect of the sintering treatment of the present invention.

FIG. 5 is a scanning electron microscope (SEM) photograph, which substitutes for a drawing, of a positive electrode active material powder used as a raw material in the method for producing a positive electrode active material of the present invention before sintering. The drawing substitute scanning electron microscope (SEM) photograph after sintering of said positive electrode active material powder. The comparison figure which shows the X-ray-diffraction pattern before sintering after sintering of said positive electrode active material. The scanning electron microscope (SEM) photograph which substitutes for drawing of the positive electrode active material powder after the sintering process manufactured in the other Example of the manufacturing method of the positive electrode active material of this invention.

Claims (6)

Baked olivine phosphate M lithium particle size less than 1 micron primary particles (M is a divalent metal) only powder as a raw material, which was sintered by heating under vacuum or inert atmosphere, the resulting A method for producing a positive electrode active material for a lithium secondary battery, comprising a powder of olivine-type lithium M phosphate, characterized by crushing a lump of ligated body. Only powder olivine phosphate M lithium particle size less than 1 micron primary particles (M is a divalent metal) as a raw material, which under vacuum the mixture formed by adding and mixing a carbon source or under an inert atmosphere A method for producing a positive electrode active material for a lithium secondary battery comprising a composite of olivine-type lithium M phosphate and carbon, which comprises sintering by heating and crushing the mass of the obtained sintered body.   3. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein M is iron, nickel, cobalt, or manganese.   A positive electrode active material for a lithium secondary battery obtained by the production method according to claim 1, 2 or 3.   5. A lithium secondary battery comprising a positive electrode produced using the positive electrode active material for a lithium secondary battery according to claim 4.   6. The lithium secondary battery according to claim 5, comprising a positive electrode produced using a positive electrode active material for a lithium secondary battery comprising particles of 20 microns or less.

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