JP2008034306A - Manufacturing method of positive electrode active material for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium battery having large charge and discharge capacity, excellent high-rate discharge in particular, and stability for a long period of time, in which a conductive carbon layer is formed by baking water soluble carbonhydrate having reducibility, or mixed water soluble carbonhydrate mixed with the water soluble carbonhydrate having reducibility and the water soluble carbonhydrate having no reducibility on the surface of an olivine type M lithium phosphate particle or between the particles as a positive electrode active material of olivine type M lithium phosphate (M is an element containing at least one or more kinds of iron, cobalt, manganese, and nickel), and threreby, elution of a metal component can be suppressed in order to coat the active material particle, and current collecting effect is enhanced. <P>SOLUTION: Olivine type M lithium phosphate (M is an element containing at least one or more kinds of iron, cobalt, manganese, and nickel), and water soluble carbonhydrate having reducibility or mixed water soluble carbonhydrate mixed with the water soluble carbonhydrate are heat-treated in an inert atmosphere to carbonize the water soluble cabonhydrate or the mixed water soluble carbonhydrate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

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

A lithium secondary battery using a negative electrode active material as a material capable of occluding and releasing lithium metal, lithium alloy or lithium ion is characterized by high voltage and excellent reversibility. In particular, lithium ion secondary batteries using a composite oxide of lithium and a transition metal as a positive electrode active material and a carbon-based material as a negative electrode active material are used in conventional lead secondary batteries and nickel-cadmium secondary batteries. In comparison, it is light in weight and has a large discharge capacity, so it is widely used in electronic equipment.

Currently, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , or LiMn ₂ O ₄ is mainly used as a positive electrode active material for lithium ion secondary batteries that are generally used. In particular, cobalt and nickel are buried. Because it is produced only in a limited area, the positive electrode active material for lithium-ion secondary batteries is expected to increase further in the future, both from the viewpoint of price and the stable supply of raw materials. It is not preferable. From the viewpoint of safety, these active materials may be problematic because of their high reactivity. Manganese is a relatively inexpensive material, but has a problem in stability of cycle characteristics.

In contrast, lithium iron phosphate using low-cost and stable iron as a raw material, or a material obtained by substituting a part of iron in lithium iron phosphate with other elements, is a positive electrode active material for lithium secondary batteries. Is known to work as.

However, these olivine-type M lithium phosphate (M is a transition metal) -based material has a slow lithium insertion / release reaction during battery charging / discharging, and is more electrically than conventional lithium metal oxides such as LiCoO _2. Since the resistance is very large, there is a problem that resistance polarization increases when charging / discharging is performed, a sufficient discharge capacity cannot be obtained, and chargeability is poor. In particular, it becomes remarkable in charge / discharge of a large current.

As a method for solving such a problem, the particles of olivine-type M lithium phosphate material are made finer, the reaction area is increased, lithium ion diffusion is facilitated, and the electricity inside the lithium iron phosphate material particles is increased. It is considered to shorten the flowing distance.

However, the fine particles of the olivine-type M lithium phosphate-based material are liable to cause secondary aggregation when mixed with a conductive material such as carbon black during electrode production. Since the lithium iron phosphate material particles and the conductive agent are in contact with each other inside the aggregated particles, a sufficient current collecting effect cannot be obtained and the electric resistance becomes very large. Therefore, 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.

Therefore, a method of carrying at least one kind of fine particles of silver, carbon, platinum, palladium, etc. that are conductive and nobler than the redox potential on fine particles of lithium iron phosphate material (Patent Document 1), or a conductive agent As a carbon source, and a suspension of carbon black or an organic compound as a carbon source is added during the synthesis of the lithium iron phosphate material and dispersed uniformly on the particle surface (Patent Document 2), or olivine A method using particles in which a conductive path made of carbon is incorporated inside the particles of type lithium phosphate (Patent Document 3), or a synthetic raw material for olivine compounds such as lithium compounds, divalent metal compounds, and phosphate compounds. A method of adding a carbon precursor or a water-soluble organic substance and producing a carbon composite after firing (Patent Document 4) has been proposed.

JP 2001-110414 A JP 2005-116392 A JP 2003-203628 A JP 2003-292309 A

However, the method described in Patent Document 1 requires the use of a reducing agent because of the support of metal particles, requires ball milling or bead milling, and the process is complicated. It is easily denatured and has a problem with stability. In addition, since the particles are connected to each other, the above problem of current collection cannot be sufficiently solved.
Moreover, the dispersion effect is also insufficient in the method described in Patent Document 2, and a sufficient current collecting effect cannot be expected.
Moreover, although the method described in Patent Document 3 makes it possible to improve battery performance and conductivity by forming a conductive path made of carbon inside the particles, it does not satisfy market demands. .
Moreover, since the method described in Patent Document 4 requires a high degree of particle size control as a battery active material, there is a problem that the control becomes remarkably difficult when added to the raw material.
In addition, it is considered that the particle surface of the olivine-type M lithium phosphate 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 is converted into a trivalent phosphate having 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.

Under such a background, the present invention has been made in order to solve the above-described problems, and the purpose thereof is to discharge a lithium secondary battery using an inexpensive lithium iron phosphate material as a positive electrode at a large current. It aims to improve the characteristics or charging characteristics and to provide a stable supply.

The present invention relates to olivine-type lithium M phosphate (M is an element containing at least one of iron, cobalt, manganese and nickel) and a water-soluble carbohydrate having a reducing property or a mixed water-soluble carbohydrate obtained by mixing the water-soluble carbohydrate. Is heated in an inert atmosphere to carbonize the water-soluble carbohydrate or mixed water-soluble carbide.

In addition, a carbon-based conductive agent is further mixed in the positive electrode active material.

In the present invention, as a positive electrode active material of olivine-type lithium M phosphate (M is an element containing at least one of iron, cobalt, manganese, and nickel), the surface of the olivine-type lithium phosphate M particles is reducible. The active material particles are coated by firing a water-soluble carbohydrate having a water content or a mixed water-soluble carbide in which a water-soluble carbohydrate having a reducing property and a water-soluble carbohydrate not having a reducing property are mixed to form a conductive carbon layer. Therefore, it is possible to provide a long-term stable olivine-type M lithium phosphate battery that can suppress elution of metal components, has an improved current collection effect, has a large charge / discharge capacity, and particularly has a high rate of discharge. Is possible.

Since olivine-type lithium M phosphate (M is an element containing at least one of iron, cobalt, manganese, and nickel) has very low conductivity, the grain size and crystallinity of crystal grains are precisely controlled. There is a need.
Therefore, the present invention forms conductive carbon obtained from a water-soluble carbohydrate having reducibility or a mixed water-soluble carbohydrate in which the water-soluble carbohydrate is mixed between the surfaces or particles of the olivine-type lithium M phosphate.

According to the present invention, phosphoric acid M is obtained by heat-treating a powder obtained from a water-soluble carbohydrate having reducibility to lithium M phosphate particles or a mixed water-soluble carbohydrate obtained by mixing the water-soluble carbohydrate in an inert atmosphere. It is possible to obtain particles in which the particle size and crystallinity of the crystal particles are well controlled from those obtained by mixing lithium powder and a carbon source during synthesis. Further, by applying a carbon-based conductive agent to the heat-treated powder, it is possible to obtain an M lithium phosphate powder with further improved characteristics. By doing so, it is possible to deposit carbon having conductivity while suppressing the oxidation of the surface of the fine olivine-type M lithium phosphate particles having a large surface area synthesized by controlling the particles.

In the present invention, the water-soluble carbohydrate having a reducing property or the mixed water-soluble carbohydrate mixed with the water-soluble carbohydrate is limited to spread the water-dissolved carbohydrate on the surface of the particles uniformly. Thus, the particles can be coated with an organic substance.
Oxidation of crystal particles can be suppressed by reducing carbohydrates. Examples of reducing carbohydrates include glucose, fructose, sucrose hydrolysates, and cellulose hydrolysates, as well as aldehyde groups and ketone groups. As long as it has a reducing functional group of Examples of the carbon source used in the present invention include water-soluble hydrocarbons, and sugars such as sucrose, oligosaccharides and dextrin, gelatinized starch, glycogen, and the like can be used. Since these carbohydrates are inexpensive and are biomass, it is desirable to use them from the viewpoint of environmental considerations.

Examples of the present invention will be described below. In addition, this invention is not limited only to a following example.

First, lithium iron phosphate was synthesized by a hydrothermal method which is a known method. Lithium iron phosphate was charged with 4.63 g of lithium phosphate and 7.95 g of divalent iron chloride tetrahydrate as a divalent iron compound together with 200 ml of distilled water in a pressure vessel (autoclave). The air was replaced with argon gas and sealed. The pressure vessel was reacted in an oil bath at 180 ° C. for 48 hours. Thereafter, the mixture was allowed to cool to room temperature, and the contents were taken out and dried at 100 ° C. to obtain 6.47 g of a powder sample.
The obtained powder sample was confirmed to be lithium iron phosphate by an X-ray diffraction pattern, and it had a particle size of 20 to 200 nm from observation with a scanning electron microscope (SEM). It could be confirmed.

(Preparation of powder A)
Then, 6.47 g of lithium iron phosphate obtained by the above method and 1.54 g of commercially available sugar containing sucrose as a main component and invert sugar (a mixture of glucose and fructose having an aldehyde group of a reducing functional group). And mixed. The sugar used in this example is a water-soluble carbohydrate having reducibility and has carbon corresponding to 10% with respect to lithium iron phosphate. Next, 10 ml of distilled water was added to the mixture, kneaded well, and dried at 100 ° C. for 2 hours. Thereafter, baking was performed at 600 ° C. for 2 hours in a nitrogen atmosphere, and after cooling to room temperature, a black lump reaction product was obtained. In this process, the action of the reducing carbohydrate coated on the surface can suppress the oxidation of the mixed particles from divalent to trivalent. At the same time, it is considered that the mixed particles are changed to conductive carbon by firing. The reaction product was pulverized by a ball mill using zirconia balls having a diameter of 10 mm to obtain a powder having an average particle diameter of 2 μm (powder A).
In addition, when the powder A produced by mixing and reacting lithium iron phosphate and sugar was observed with a scanning electron microscope (SEM), it was a fine powder having a large surface area, and the produced carbon was porous. In addition, it was confirmed that the produced carbon was in close contact with the periphery of the lithium iron phosphate particles, and the particles were bound by carbon. From this, the crystal grain size of the obtained powder A is large, it becomes easy to produce a slurry for coating, and it is possible to suppress elution of iron from the lithium iron phosphate particles. .
Powder A was calculated from the result of quantitative analysis of Fe by inductively coupled plasma (ICP) and had a carbon content of 6%.

(Preparation of powder B)
Preparation of powder A except that 6.47 g of lithium iron phosphate obtained by the same method as in Example 1, 0.08 g of glucose as a water-soluble carbohydrate having a reducing property, and 1.39 g of sucrose were mixed. A powder having an average particle diameter of 2 μm was obtained in the same manner as (Powder B).
The powder was calculated from the results of quantitative analysis of Fe by ICP and had a carbon content of 5.8%.

(Preparation of powder C)
6.47 g of lithium iron phosphate obtained by the same method as in Example 1 was mixed with 0.08 g of glucose as a water-soluble carbohydrate having a reducing property. A powder having an average particle diameter of 2 μm was obtained in the same manner as in the preparation of Powder A (Powder C) except that 10 ml of a 100 ° C. gelatinized starch aqueous solution containing 1.39 g of potato starch as a water-soluble carbohydrate was added.
The powder was calculated from the results of quantitative analysis of Fe by ICP and had a carbon content of 5.5%.

(Comparative example)
(Preparation of powder D)
First, 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. Next, the mixture was immersed in a solution of 3 g of polyethylene glycol in 10 ml of distilled water for 1 hour with stirring, and 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 reaction precursor. And 10 g of this reaction precursor was press-molded at 44 MPa by hand press. Next, this press-molded product was fired at 600 ° C. for 5 hours in a nitrogen atmosphere, cooled, pulverized and classified to obtain lithium iron phosphate coated with a carbonaceous material (powder D).
The powder was calculated from the results of quantitative analysis of Fe by ICP and had a carbon content of 5.6%.
Moreover, the particle size of the lithium iron phosphate portion of this composite was about 0.2 to 2 μm, which was found to be larger than the particle size of lithium iron phosphate obtained by the hydrothermal method. Moreover, although it tried to obtain a small particle size product by changing manufacturing conditions, it was difficult to stably obtain a small particle size lithium iron phosphate.

(Preparation of powder E)
In a solution in which water and ethanol are mixed at a volume ratio of 1: 1, the lithium iron phosphate powder obtained by the hydrothermal method is sufficiently stirred, and silver nitrate is added while the stirring is continued. Weighed and added to 5% of the weight of lithium iron phosphate. While further stirring, 20 ml of acetaldehyde was added per 1 g of silver nitrate to deposit silver on the lithium iron phosphate powder. This was filtered and dried to obtain a powder (powder E).

(Preparation of powder F)
Lithium nitrate (LiNO ₃ ), iron nitrate (Fe (NO ₃ ) ₃ ), and phosphoric acid (H ₃ PO ₄ ) are mixed at a molar ratio of 1: 1: 1 to obtain lithium iron phosphate. It was dissolved in water so that it was converted to 0.1M. After 20 g of acetylene black was added to 1000 cc of this solution and sufficiently stirred, the solution was atomized with an ultrasonic atomizer and then introduced into a heat treatment furnace at 300 ° C. using air as a carrier gas for thermal decomposition. The precursor was obtained. Subsequently, this precursor was heat-treated at 450 ° C. for 1 hour in a nitrogen gas atmosphere to obtain a powder (powder F).

(Preparation of powder G)
6.47 g of lithium iron phosphate obtained by the same method as in Example 1 and 20 g of acetylene black were mixed to obtain a powder (powder G). Acetylene black has 6% of carbon relative to lithium iron phosphate.

(Preparation of positive electrode)
First, polyvinylidene fluoride (PVdF) as a binder was mixed with the powder A (additionless lithium iron phosphate) prepared in Example 1 at a weight ratio of 95: 5, and N-methyl-2 -Pyrrolidone (NMP) was added and sufficiently kneaded to obtain a positive electrode slurry. The positive electrode paste was applied to a 20 μm thick aluminum foil current collector and dried at 120 ° C. for 30 minutes. Thereafter, it was rolled with a roll press and punched into a 2 cm ² disk shape to obtain a positive electrode.

(Preparation of negative electrode)
Next, artificial graphite (average particle size 5 μm, d ₀₀₂ = 0.337 nm, Lc = 58 nm) and polyvinylidene fluoride (PVdF) as a negative electrode material were mixed at a weight ratio of 95: 5, and N-methyl-2 -Pyrrolidone (NMP) was added and sufficiently kneaded to obtain a negative electrode paste. And the said negative electrode paste was apply | coated on the 20-micrometer-thick copper foil electrical power collector, and it dried at 130 degreeC under pressure reduction for 12 hours after natural drying in 25 degreeC normal temperature. Thereafter, it was rolled with a roll press and punched into a 2 cm ² disk shape to obtain a negative electrode.

(Preparation of electrolyte)
Then, a volume of ethylene carbonate and diethyl carbonate ratio of 1: 1 solvent mixture in a ratio of the LiPF ₆ was dissolved at a concentration of 1M, to prepare an electrolyte solution. The amount of water in the electrolyte was less than 15 ppm.

(Production of lithium secondary battery)
Using these positive electrode, negative electrode, electrolyte and separator, a coin-type lithium secondary battery having a diameter of 25 mm and a thickness of 1.6 mm was produced by a known method (Invention 1).
The negative electrode can, the positive electrode can, the negative electrode current collector, and the positive electrode current collector are made of stainless steel plates, and the insulating packing is made of polyphenylene sulfide. As the separator, a polyphenylene sulfide non-woven fabric was used, and this was impregnated with an electrolytic solution.
The atmosphere at the time of producing the coin-type lithium secondary battery was a dew point of −50 ° C. or lower.

Next, a positive electrode was prepared in the same manner as described above except that acetylene black as a carbon-based conductive agent was mixed with the various powders (powder A to G) obtained above so that the total carbon amount was 10%. Then, a negative electrode and an electrolytic solution were produced to produce various coin-type lithium secondary batteries (in which powders A to C are used, the present inventions 2 to 4 and powders D to G are used in comparative examples 1 to 4). .

(Measurement of Fe elution amount of each positive electrode)
It is known that a battery using lithium iron phosphate as a positive electrode has a problem of capacity reduction due to iron eluting into the electrolytic solution by float charging at high temperature or standing and depositing on the negative electrode. Therefore, 100 cc of the electrolyte used in the battery was placed in a heat-resistant plastic container so that the lithium iron phosphate was 5 g, and the iron concentration eluted in the electrolyte after standing at 80 ° C. for 10 days was atomic absorption. Measured by analytical method.
Table 1 shows the amount of Fe elution from various positive electrodes.

  As shown in Table 1, the present invention 1-4 has a high carbon coating effect on the surface of lithium iron phosphate particles and a very high effect of suppressing iron elution into the electrolytic solution, as compared with Comparative Examples 1-4. I understand that. Therefore, elution of iron into the electrolytic solution can be suppressed, which suggests that a battery that is stable for a long time can be obtained. As is clear from the present inventions 1 and 2, it can be seen that the presence or absence of the additionally mixed conductive agent does not affect the elution of iron.

(Battery test of lithium secondary battery)
10 cycles of charge / discharge cycle tests were performed using various coin-type lithium secondary batteries (Inventions 1 to 4 and Comparative Examples 1 to 4). The test conditions at this time were charging with a constant current and constant voltage with a charging current of 0.1 CA and a voltage of 4.5 V, and discharging with a constant current with a discharging current of 0.1 CA and a discharge end voltage of 2.0 V. The test was performed in an environment of 25 ° C. Table 2 shows various discharge capacities at this time.
In the 11th cycle, charging is performed at a constant current and a constant voltage with a charging current of 5.0 CA and a voltage of 4.5 V, and discharging is performed at a constant current of a discharging current of 5.0 CA and a discharge end voltage of 2.0 V. A test was conducted. Various discharge capacities at this time are also shown in Table 2. Table 2 also shows the ratio of 5.0 CA and 0.1 CA in order to see the high rate discharge characteristics. It shows that the ratio of 5.0CA to 0.1CA is higher for a battery that is superior in high rate discharge characteristics.
In addition, each capacity | capacitance of 0.1CA and 5.0CA was made into the capacity | capacitance per 1 g of the filled lithium iron phosphate.

As shown in Table 2, it can be seen that the present inventions 1 to 4 have good discharge performance. In particular, it can be seen that the present inventions 1 to 4 are superior to Comparative Examples 1 to 4 in a discharge capacity of 5 CA. In addition, the present invention 2 to 4 in which a conductive agent is mixed with lithium iron phosphate powder improves the current collection efficiency from the lithium iron phosphate particles, and the conductive matrix made of carbon is stretched all over, and further has discharge characteristics. It can be seen that is improved.
On the other hand, Comparative Example 1 uses divalent iron of the amorphous layer on the surface of lithium iron phosphate particles because polyethylene glycol having no reducing action is used as the carbonaceous precursor and the immersion time in the aqueous solution is as long as 1 hour. It is presumed that the discharge characteristics were inferior because the surface resistance was increased by oxidation to trivalent. In Comparative Examples 2 and 3, contact with the active material particles is point contact, and in Comparative Example 4 is simple physical mixing, it is estimated that sufficient current collection effect cannot be obtained and discharge characteristics are poor. .

From the above results, as a positive electrode active material of olivine-type lithium M phosphate, the water-soluble carbohydrate having reducibility, or the water-soluble carbohydrate having reducibility and the reducibility between the surfaces of the olivine-type lithium M phosphate particles or between the particles. By firing the mixed water-soluble carbide mixed with the water-soluble carbohydrate not present to form the conductive carbon layer, it is possible to suppress the elution of iron into the electrolytic solution, and thus a battery that is stable for a long period of time can be obtained. In addition, a lithium battery using this improves the current collection efficiency, and provides a long-term stable lithium battery with particularly good high rate discharge.
In this example, an example of lithium iron phosphate was shown, but similar results were obtained when cobalt, manganese, nickel, or a mixture thereof was used instead of iron.

Claims (2)

Inactive olivine-type lithium M phosphate (M is an element containing at least one of iron, cobalt, manganese and nickel) and a water-soluble carbohydrate having a reducing property or a mixed water-soluble carbohydrate mixed with the water-soluble carbohydrate A method for producing a positive electrode active material for a lithium secondary battery, wherein the water-soluble carbohydrate or mixed water-soluble carbide is carbonized by heat treatment in an atmosphere. 2. The method for producing a positive electrode active material for a lithium secondary battery according to claim 1, wherein a carbon-based conductive agent is further mixed in the positive electrode active material.