JP2005332629A - Positive electrode active material for nonaqueous electrolyte secondary battery and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a positive electrode active material providing a nonaqueous electrolyte secondary battery whose properties in use continuously charging at a high temperature or repeating a charging and discharging cycle are improved. <P>SOLUTION: This positive electrode active material is composed of a particle of a lithium containing transition metal oxide expressed by a general formula LiMO<SB>2</SB>. The average valence of the transition metal on the surfaces is lower than the average valence in the particle, and the surface of the particle is covered with a carbonaceous material. The M in the formula is at least one kind selected from a group comprising Co, Ni and Mn. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery and a method for producing the same, and improves characteristics of the non-aqueous electrolyte secondary battery in applications where continuous charging at high temperatures or repeated charge / discharge cycles is performed. Is.

  In recent years, consumer electronic devices have become portable and cordless. Traditionally, nickel cadmium batteries or sealed small lead-acid batteries have played a role as power sources for driving these electronic devices. However, as portable and cordless devices become established, the energy density of secondary batteries that serve as drive power sources will increase. There is an increasing demand for smaller and lighter weight. In recent years, there has been a demand for a battery that can be charged and discharged at a high rate, as represented by the rapid market expansion of notebook personal computers.

  From such a situation, a lithium secondary battery exhibiting a high charge / discharge voltage, for example, in Patent Document 1, a non-aqueous electrolyte secondary battery using lithium cobaltate as a positive electrode active material and utilizing insertion / extraction of lithium ions is used. A battery is disclosed.

  By the way, when a lithium-containing composite oxide showing a voltage of 4 V class is used as the positive electrode active material, when the positive electrode active material reacts with the non-aqueous electrolyte in the charged state and is stored at a high temperature or a charge / discharge cycle is repeated, There were problems such as an increased capacity deterioration rate. Therefore, for example, in Patent Document 2, the positive electrode active material is heat-treated in a reducing atmosphere, and the average valence of the transition metal in the surface portion of the positive electrode active material is determined as the average valence of the transition metal in the entire positive electrode active material. Proposed to be lower than the number. According to this, the active oxygen involved in the reaction with the non-aqueous electrolyte is reduced, the reaction between the positive electrode active material and the non-aqueous electrolyte is suppressed when stored in a charged state at a high temperature, and the discharge characteristics are deteriorated. Is prevented.

  Further, in Patent Document 3, at least a part of the surface layer of the positive electrode active material is selected from the group consisting of 0.5 to 19 wt% nitrogen, 0.5 to 35 wt% sulfur, and 0.5 to 25 wt% oxygen. By including a carbon material layer containing at least one kind of element, it is proposed to suppress the decomposition reaction of the electrolytic solution on the positive electrode active material at a high temperature and remove the adverse effect on the high temperature storage characteristics. ing.

Moreover, in patent document 4, when baking the reaction raw material containing a lithium compound and a transition metal compound, after baking in the presence of the substance which has a carbon atom, baking is performed in high oxygen concentration atmosphere. A method for producing a lithium-containing transition metal oxide has been proposed. However, this method is not a method for producing a lithium-containing transition metal oxide in which the firing process is completed only by a firing process in a low oxygen atmosphere.
JP-A-63-59507 JP-A-10-199530 Japanese Patent Laid-Open No. 10-21912 JP 2002-279990 A

  When the positive electrode active material is heat-treated in the reducing atmosphere and the average valence of the transition metal in the surface portion of the positive electrode active material is lower than the average valence of the transition metal in the entire positive electrode active material, a high voltage is generated. When it was stored in a held state, or when charge storage and charge / discharge were repeated, the capacity decreased or the charge / discharge characteristics decreased, and sufficient storage characteristics could not be obtained.

  The cause is considered as follows. That is, since the transition metal having a low valence is easily eluted, the transition metal on the surface portion having a low average valence is eluted in the non-aqueous electrolyte, resulting in a decrease in capacity. Moreover, since the eluted transition metal is reduced and deposited on the negative electrode and prevents the electrode reaction at the negative electrode, the charge / discharge characteristics are deteriorated.

  In addition, even when a carbon material layer is included in at least a part of the surface layer of the conventional positive electrode active material, there is a sufficient improvement effect in storage characteristics that maintain a high voltage and cycle characteristics in which charge storage and charge / discharge are repeated. It was not obtained. The cause is considered to be the presence of active oxygen involved in the reaction with the non-aqueous electrolyte.

  Therefore, the present invention covers the surface portion with a low average valence with a carbonaceous material, so that it can be stored in a state where a high voltage is maintained, or even when charging and discharging are repeated, the lithium content Suppresses the reaction between the transition metal oxide and the non-aqueous electrolyte, and suppresses the transition metal on the surface portion with a low average valence from leaching into the non-aqueous electrolyte to prevent the capacity and discharge characteristics from deteriorating. It is an object of the present invention to provide a non-aqueous electrolyte secondary battery excellent in high temperature continuous charge storage characteristics and high temperature charge storage cycle life characteristics.

In order to solve the above problems, the positive electrode active material for a non-aqueous electrolyte secondary battery of the present invention has a general formula LiMO ₂ (wherein M is at least one selected from the group consisting of Co, Ni, and Mn). )), The average valence of the transition metal on the surface is lower than the average valence inside the particle, and the surface of the particle is coated with a carbonaceous material. It is characterized by.

In the present invention, a salt of at least one transition metal selected from the group consisting of Co, Ni, and Mn, a lithium salt, and a substance containing a carbon atom are mixed, and the mixture has an oxygen concentration of 1% to 5%. The manufacturing method of the positive electrode active material for non-aqueous-electrolyte secondary batteries which has the process baked in said atmosphere.
The ratio of the number of moles of carbon atoms contained in the carbon atom-containing substance in the mixture to the number of moles of transition metal atoms of the transition metal salt is preferably in the range of 0.05 to 0.30.

  According to the present invention, when stored in a state in which a high voltage is maintained, or when repeated charge storage and charge / discharge, the reaction between the lithium-containing transition metal oxide and the non-aqueous electrolyte is suppressed, To suppress the transition metal of the surface part with a low average valence from eluting into the non-aqueous electrolyte, and obtain a non-aqueous electrolyte secondary battery with excellent high-temperature continuous charge storage characteristics and high-temperature charge storage cycle life characteristics Can do.

The positive electrode active material for a non-aqueous electrolyte secondary battery according to the present invention contains a lithium represented by the general formula LiMO ₂ (wherein M is at least one selected from the group consisting of Co, Ni, and Mn). It consists of particles of transition metal oxide, the average valence of the transition metal on the surface is lower than the average valence inside the particle, and the surface of the particle is coated with a carbonaceous material.

  Here, the lithium-containing transition metal oxide particles basically refer to primary particles, and when used as an active material, they may be in secondary particles or in a higher-order aggregated state. In the positive electrode active material according to the present invention, the average valence of the transition metal on the particle surface is lower than the average valence inside the particle, and the surface of the particle is coated with a carbonaceous material. However, the average valence of the transition metal on the particle surface and the inside may be substantially the same and may include a minor component that is not coated with a carbonaceous material.

This positive electrode active material is a mixture of at least one transition metal salt selected from the group consisting of Co, Ni, and Mn, a lithium salt, and a material containing a carbon atom, and the mixture has an oxygen concentration of 1% to It can be obtained by firing in a 5% atmosphere.
The firing temperature is preferably 600 ° C to 1100 ° C. The ratio of the number of moles of carbon atoms contained in the carbon-containing substance in the mixture to the number of moles of transition metal atoms in the transition metal salt is preferably in the range of 0.05 to 0.30.

Next, embodiments for obtaining the positive electrode active material of the present invention will be described in detail below.
As a lithium salt used as a raw material, lithium carbonate, lithium hydroxide, lithium nitrate, lithium sulfate, lithium oxide, or the like can be used. As the cobalt salt, cobalt oxide, cobalt hydroxide, cobalt carbonate, or the like can be used. As the nickel salt, nickel oxide, nickel hydroxide, nickel carbonate or the like can be used. As the manganese salt, manganese oxide, manganese hydroxide, manganese carbonate, or the like can be used.

  As a substance containing a carbon atom, it is preferable to use water-soluble polymers such as pitch coke, carboxymethyl cellulose, polyvinyl alcohol, and polyethylene oxide.

  Examples of the present invention will be described below. FIG. 1 shows the structure of a cylindrical lithium ion secondary battery produced in the example. In addition, although the cylindrical battery was produced here, the shape of the battery of this invention is not limited to this. The present invention can be applied to, for example, a rectangular battery, a coin battery, a button battery, a sheet battery, a stacked battery, a flat battery, a large battery used in an electric vehicle, and the like.

Example 1
In this example, the effects of the present invention were confirmed in lithium-containing transition metal oxides LiCoO ₂ , LiNiO ₂ , LiNi _0.5 Co _0.5 O ₂ , and LiNi _0.3 Co _0.4 Mn _0.3 O ₂ .

[Preparation of positive electrode active material]
As raw materials for LiCoO ₂ , lithium carbonate and tricobalt tetroxide were used, and each was measured and mixed so that the molar ratio of Li to Co in lithium carbonate and tricobalt tetroxide was Li: Co = 1: 1. Moreover, as a raw material of LiNiO ₂ , lithium carbonate and nickel hydroxide were used and weighed and mixed so that the molar ratio of Li to Ni of lithium carbonate and nickel hydroxide was Li: Co = 1: 1. As a raw material for LiNi _0.5 Co _0.5 O ₂ , lithium carbonate, nickel hydroxide, and tricobalt tetroxide are used, and the molar ratio of Li, Ni, and Co of lithium carbonate, nickel hydroxide, and tricobalt tetroxide is Li: Ni: Each was measured and mixed so that Co = 1: 0.5: 0.5. LiNi _0.3 Co _0.4 Mn _0.3 O ₂ is made of lithium carbonate, nickel hydroxide, tricobalt tetroxide and manganese oxide. Li, Ni and Co of lithium carbonate, nickel hydroxide, tricobalt tetroxide and manganese oxide are used. Each was weighed and mixed so that the molar ratio of Mn was Li: Ni: Co: Mn = 1: 0.3: 0.4: 0.3.

  Next, a 1% aqueous solution of carboxymethyl cellulose, which is a raw material of the carbonaceous material, to 1 kg of these mixtures is a molar ratio of carbon atoms in carboxymethyl cellulose to the number of moles of transition metal atoms present in 1 kg of the mixture. Were mixed so as to have the amounts shown in Table 1. These mixtures were preliminarily dried at 100 ° C., then calcined at 900 ° C. for 24 hours in an oxygen concentration atmosphere shown in Table 1, pulverized and classified to prepare a positive electrode active material having an average particle diameter of about 10 μm.

  The transition metal on the particle surface of the positive electrode active material thus obtained was analyzed using ESCA. Table 1 shows the thickness of the layer having a low average valence of the transition metal obtained by this analysis.

[Preparation of positive electrode plate]
100 parts by weight of the positive electrode active material obtained by the above method is mixed with 1.5 parts by weight of acetylene black as a conductive agent, and further a polyvinylidene fluoride (PVdF) N-methyl-2-pyrrolidone solution as a binder. 2 parts by weight of resin was added and stirred and mixed to obtain a paste-like positive electrode mixture. The positive electrode mixture was applied to both surfaces of a current collector of aluminum foil having a thickness of 15 μm, dried, rolled, cut into a predetermined size, and a positive electrode plate was produced.

[Preparation of negative electrode plate]
A negative electrode plate was prepared using flaky graphite having an average particle diameter of about 20 μm. 1 part by weight of a carboxymethyl cellulose aqueous solution as a thickener is mixed with 100 parts by weight of the flake graphite, and 1 part by weight of styrene butadiene rubber is added as a binder, followed by stirring and mixing. A mixture was obtained. The negative electrode mixture was applied to both sides of a 10 μm thick copper foil current collector, dried, rolled, and cut into a predetermined size to prepare a negative electrode plate.

[Production of battery]
The produced positive electrode plate 1 and negative electrode plate 2 were spirally wound through a separator 3 made of microporous polyethylene having a thickness of 20 μm to constitute an electrode plate group. A positive electrode lead 4 and a negative electrode lead 5 were welded to the positive electrode plate 1 and the negative electrode plate 2, respectively. Insulating rings 6 and 8 made of polyethylene resin were attached to the lower and upper parts of the electrode plate group, respectively, and as shown in FIG. 1, they were housed in a battery case 7 processed with an organic electrolyte resistant stainless steel plate. The other end of the negative electrode lead 5 was spot welded to the bottom of the battery case 7. The other end of the positive electrode lead 4 was spot welded to an aluminum sealing plate 10 provided with a safety valve. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1.0 mol / l in a mixed solvent of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1: 3 and injecting a predetermined amount thereof into the electrode plate group is used. After that, the opening of the battery case 7 was sealed with an insulating gasket 9 and a sealing plate 10 to obtain a test battery of Example 1.

These batteries were subjected to a continuous charge storage test and a charge storage cycle life test under a high temperature environment.
In the high-temperature continuous charge storage test, first, the battery was charged at a constant voltage for 2 hours under the conditions of a charge voltage of 4.20 V and a maximum charge current of 800 mA, and then a constant current discharge of a discharge current of 800 mA and a discharge end voltage of 3.0 V was performed. Two cycles were performed under a 20 ° C. environment, and the discharge capacity at the second cycle was confirmed. Thereafter, the battery was stored with a constant voltage charged for 7 days at a charging voltage of 4.20 V and a maximum charging current of 800 mA in a 60 ° C. environment. The battery which was continuously charged and stored at a constant voltage for 7 days was charged and discharged again at 20 ° C. under the same conditions as described above, and the discharge capacity at the second cycle was confirmed. The ratio of the discharge capacity after the continuous charge storage to the discharge capacity before the continuous charge storage was evaluated as a capacity recovery rate (hereinafter expressed as a capacity recovery rate under Condition 1).

  In the high-temperature charge storage cycle life test, first, the battery was charged at a constant voltage for 2 hours under the conditions of a charging voltage of 4.20 V and a charging maximum current of 800 mA, and then a constant current discharge with a discharging current of 800 mA and a discharge end voltage of 3.0 V was performed. Two cycles were performed under an environment of 20 ° C., and the discharge capacity at the second cycle was confirmed. After that, the battery was charged at a constant voltage for 2 hours under the same conditions and left for 24 hours as it was, and then a charge / discharge cycle for discharging with a discharge current of 800 mA and a discharge end voltage of 3.0 V under a 45 ° C. environment at 100 cycles. went. The battery charged and discharged for 100 cycles in a 45 ° C. environment was charged and discharged again at 20 ° C. under the same conditions as before the cycle, and the discharge capacity at the second cycle was confirmed. The ratio of the discharge capacity after 100 cycles of high-temperature charge storage to the discharge capacity before the high-temperature charge storage cycle was evaluated as a capacity recovery rate (hereinafter referred to as a capacity recovery rate of Condition 2).

  Table 1 shows the evaluation results of the continuous charge storage test and the charge storage cycle life test under the high temperature environment obtained as described above.

  As is apparent from the results in Table 1, on the surface of the lithium-containing transition metal oxide particles, the average valence of the transition metal is low and the surface is coated with a carbonaceous material. The batteries of 1-3, 1-7, 1-11, and 1-15 are No. 1 that do not include both the above-described low average valence layer and the carbonaceous film. Compared with the batteries of 1-1-2, 1-4-6, 1-8-10, 1-12-14, the continuous charge storage characteristics and the charge storage cycle life characteristics in a high temperature environment were remarkably improved. . This is because the surface portion with a low average valence is coated with a carbonaceous material, so that the lithium-containing transition metal can be stored even when stored in a state where a high voltage is maintained, or when charging and discharging are repeated. This is because the reaction between the oxide and the non-aqueous electrolyte is suppressed, and the transition metal on the surface portion with a low average valence is prevented from eluting into the non-aqueous electrolyte, thereby preventing the capacity and discharge characteristics from being reduced. Conceivable.

Example 2
In this example, in lithium-containing transition metal oxide LiCoO ₂ , the average valence of the transition metal on the surface of the particle is lower than the average valence inside the particle, and the surface is coated with a carbonaceous material In order to obtain an active material, the oxygen concentration during firing and the amount of carbon to be mixed were examined.

Lithium carbonate and tricobalt tetroxide were used as raw materials for the positive electrode active material LiCoO ₂ . Lithium carbonate and tricobalt tetroxide were weighed and mixed so that the molar ratio of Li and Co was Li: Co = 1: 1. Next, the molar ratio of carbon atoms in carboxymethyl cellulose to 1 kg aqueous solution of carboxymethyl cellulose, which is the raw material of the carbonaceous material, with respect to 1 kg of the mixture is shown in Table 2 with respect to the number of moles of Co atoms present in 1 kg of the mixture. It mixed so that it might become the quantity shown in. The mixture thus obtained was pre-dried at 100 ° C., then calcined at 900 ° C. for 24 hours in an oxygen concentration atmosphere shown in Table 2, pulverized and classified to prepare a positive electrode active material.
Hereinafter, a positive electrode was produced in the same manner as in Example 1, a battery was assembled, and a test battery of Example 2 was obtained.

  About the battery of Example 2, the continuous charge preservation | save test in the high temperature environment and the charge preservation | save cycle life test were done like Example 1, and each initial capacity and capacity | capacitance recovery rate were calculated | required. Table 2 shows the initial discharge capacities of these batteries and the evaluation results of the continuous charge storage test and the charge storage cycle life test under a high temperature environment.

  As is apparent from the results in Table 2, the amount of carbon to be mixed is 5 mol% or more, and the oxygen concentration during firing is in the range of 0.5% to 5%. In the batteries of 2-1 to 5, 2-7 to 9, 2-12 to 14, and 2-17 to 19, the continuous charge storage characteristics and the charge storage cycle life characteristics under a high temperature environment were remarkably improved. No. 1 in which the oxygen concentration during firing was 10%. The batteries of 2-21 to 25 could not obtain a sufficient improvement effect. This is probably because when the oxygen concentration was 10%, a layer having a low average valence of the transition metal could not be formed on the surface of the particles. When the transition metal on the particle surface was analyzed using ESCA, a layer having a low average valence of the transition metal was not observed.

By the way, no. No. 2-1-5 batteries and No. 1 having an oxygen concentration of 1% to 5% and a carbon content of 50 mmol%. In the batteries 2-10, 2-15, and 2-20, the continuous charge storage characteristics and the charge storage cycle life characteristics under a high temperature environment were remarkably improved, but the initial capacity was lowered. This is because the oxygen concentration at the time of firing is too low and the amount of carbon acting as a reducing agent increases, so that the oxygen necessary for synthesis is insufficient, and a layer with a low average valence of transition metal is formed to the inside. This is presumably because the crystallinity of the lithium-containing transition metal oxide LiCoO ₂ was lowered. In order to suppress a decrease in the initial capacity to a minimum, the thickness of the layer having a low average valence of the transition metal is preferably 100 nm or less.

  Further, No. 2 having a carbon content of 2 mol% was mixed. In the batteries of 2-1, 2-6, 2-11, and 2-16, the remarkable effect of improving the continuous charge storage characteristics and the charge storage cycle life characteristics under a high temperature environment was not obtained. This is presumably because the amount of carbon to be coated is small.

  From these results, in order to obtain a positive electrode active material that significantly improves continuous charge storage characteristics and charge storage cycle life characteristics in a high temperature environment without causing an initial capacity reduction, the oxygen concentration during firing is 1%. It was found that the optimal amount of carbon to be mixed is 5 to 30 mol%.

In the above embodiment, showing a lithium-containing transition metal oxide _{LiCoO 2, LiNiO 2, LiNi 0.5} Co 0.5 O 2, and LiNi _0.3 Co _0.4 Mn _0.3 O ₂ of the experimental results. However, the same effect can be obtained as long as it is a lithium-containing transition metal oxide represented by another general formula LiMO ₂ and in which M is at least one selected from the group consisting of Co, Ni, and Mn. .

Furthermore, in the said Example, although the experimental result using carboxymethylcellulose as a raw material of the carbonaceous material to coat | cover was shown, the same effect is acquired also with pitch coke, polyvinyl alcohol, polyethylene oxide, etc.
In each of the above examples, the salt concentration of the electrolyte is 1 mol / l, but the same effect can be obtained even when the salt concentration is 0.5 to 2.0 mol / l.

  In each of the above examples, a mixed solvent of ethylene carbonate and diethyl carbonate was used as the electrolytic solution. However, other nonaqueous solvents such as cyclic esters such as propylene carbonate, cyclic ethers such as tetrahydrofuran, and chain ethers such as dimethoxyethane. The same effect can be obtained by using a non-aqueous solvent such as a chain ester such as methyl propionate or a multicomponent mixed solvent.

  The non-aqueous electrolytic secondary battery using the positive electrode active material according to the present invention is useful for various applications including portable electronic devices.

It is the front view which made a part of cylindrical battery in the example of the present invention a section.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode plate 2 Negative electrode plate 3 Separator 4 Positive electrode lead 5 Negative electrode lead 6 Lower insulating ring 7 Battery case 8 Upper insulating ring 9 Gasket 10 Sealing plate

Claims (3)

It is composed of lithium-containing transition metal oxide particles represented by the general formula LiMO ₂ (wherein M is at least one selected from the group consisting of Co, Ni, and Mn), and the average of transition metals on the surface thereof A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the valence is lower than the average valence inside the particle, and the surface of the particle is coated with a carbonaceous material.   A salt of at least one transition metal selected from the group consisting of Co, Ni, and Mn, a lithium salt, and a substance containing a carbon atom are mixed, and the mixture is mixed in an atmosphere having an oxygen concentration of 1% to 5%. The manufacturing method of the positive electrode active material for nonaqueous electrolyte secondary batteries which has the process to bake.   The ratio between the number of moles of carbon atoms contained in the substance containing carbon atoms in the mixture and the number of moles of transition metal atoms in the salt of the transition metal is 0.05 to 0.30. The manufacturing method of the positive electrode active material for nonaqueous electrolyte secondary batteries.

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