JP2000113884A - Lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce internal resistance and to increase the battery capacity by using lithium cobaltate or lithium nickalate obtained by adding one kind or more elements of B, Bi, Mo, P, Cr, V and W as a positive electrode active material. SOLUTION: The element adding amount to Co or Ni is 0.1 to 20 mol%, preferably 1 to 10 mol%. The sufficient liquid phase is formed, powder having the specified shape is formed, and the different phase does not exist. The additive element in a composite oxide of Co, Ni or Li contributes for improvement of electron conductivity by solid solution, reduction of contact resistance as sintered auxiliaries, good conductivity of a reaction product, and high efficiency of conductive auxiliaries to reduce internal resistance. Accordingly, heat generation is restrained, active materials are increased by reduction of the amount of conductive auxiliaries, and the charging and discharging cycle characteristics are not deteriorated. A positive electrode active material is manufactured by baking mixture of respective elements having the specified ratio in the oxidizing atmosphere at 500 to 1000 deg.C for 5 to 50 hours, and it is preferable to perform baking in twice or more by increasing the temperature in the next stage higher than that in the last stage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a lithium-transition metal composite oxide for a positive electrode active battery in a secondary battery used as an operating power source of a portable electronic device or a motor driving power source of an electric vehicle or a hybrid electric vehicle. The present invention relates to a high-output lithium secondary battery having a low internal resistance and used as a substance.

[0002]

2. Description of the Related Art In recent years, portable electronic devices such as cellular phones, VTRs, and notebook computers have been rapidly reduced in size and weight, and a lithium transition metal composite oxide has been used as a positive electrode active material as a power supply battery. A secondary battery using an organic electrolytic solution obtained by dissolving a carbonaceous material as a negative electrode active material and a lithium ion electrolyte in an organic solvent as an electrolytic solution has been used.

[0003] Such a battery is generally called a lithium secondary battery or a lithium ion battery, and has a large energy density and a cell voltage of about 4V.
Due to its high degree of characteristics, not only the portable electronic device but also a motor of an electric vehicle or a hybrid electric vehicle which is actively spread as a low-emission vehicle due to recent environmental problems. It is also attracting attention as a drive power supply.

Here, in particular, in a lithium secondary battery used as a power supply for driving a motor of an electric vehicle or the like, in order to obtain a large current output necessary for acceleration, climbing a slope, and the like, and to increase charging / discharging efficiency, It is very important to reduce the internal resistance of the battery. In recent years, it has become clear that the conductivity of the positive electrode active material has a significant effect on the internal resistance of the battery.

As the positive electrode active material, generally, lithium cobaltate (LiCoO ₂ ) and lithium manganate (L
A lithium transition metal composite oxide such as iMn ₂ O ₄ ) or lithium nickel oxide (LiNiO ₂ ) is used. Here, LiCoO ₂ and LiNiO ₂ can be expected to have a high potential and have a large lithium capacity. In view of the fact that it has a simple structure and excellent reversibility, and that it has a layered two-dimensional structure suitable for ion diffusion, it is considered that it has conditions to be satisfied as a positive electrode active material.

However, any of the mixed conductors having both lithium ion conductivity and electron conductivity does not necessarily have high electron conductivity. Therefore, attempts have been made to improve the conductivity by adding conductive fine particles such as acetylene black to the positive electrode active material to lower the internal resistance of the battery.

[0007]

Here, when acetylene black is not added, the contact between the positive electrode active material powders deteriorates, the internal resistance of the battery increases, and the utilization rate of the positive electrode active material decreases. And, generally, battery characteristics.
From this, it is clear that the addition of acetylene black contributes to a reduction in internal resistance of the battery and an improvement in cycle characteristics.

[0008] However, the addition of acetylene black decreases the filling amount of the positive electrode active material, which is a factor that lowers the battery capacity. In addition, acetylene black is a kind of carbon and is a semiconductor, and it is considered that there is a limit to improvement in electron conductivity by acetylene black. Therefore, the addition amount is set to an appropriate amount by comparing the positive effect of reducing the internal resistance with the negative effect of reducing the battery capacity.

Further, since the positive electrode active material powder undergoes a volume change due to desorption / insertion of lithium ions during charge / discharge, the added acetylene black causes the volume change of the positive electrode active material powder due to the volume change. It may not contribute to the electrical connection between them or the electrical connection between the positive electrode active material powder and the current collector, which may result in an increase in internal resistance over time.

Therefore, in order to improve the conductivity of the positive electrode active material, the electric resistance of the positive electrode active material itself is reduced,
It is considered that the addition of acetylene black is preferably limited to auxiliary conductivity improvement. However, a method for greatly reducing the electric resistance of the positive electrode active material itself has not been found so far.

[0011]

Means for Solving the Problems The present invention has been made in view of the above-mentioned problems of the prior art, and an object thereof is to use LiCoO ₂ and LiNiO ₂ as a positive electrode active material. It is an object of the present invention to reduce the resistance of a positive electrode material, thereby providing a lithium secondary battery having a large output and a large capacity.

That is, according to the present invention, B, Bi,
At least one selected from Mo, P, Cr, V, W
There is provided a lithium secondary battery using lithium cobalt oxide or lithium nickel oxide to which at least two or more kinds of elements are added as a positive electrode active material.

Here, the added element, that is, the added amount of the added element is 0.1 to 0.1 moles of Co or Ni in lithium cobalt oxide or lithium nickel oxide.
preferably at least 20 mol% and not more than 20 mol%,
In particular, it is more preferable that the content be 1 mol% or more and 10 mol% or less. The production of such a positive electrode active material is preferably performed by:
The mixture of the compounds of each element adjusted to a predetermined ratio is fired in an oxidizing atmosphere at a temperature of 500 ° C. to 1000 ° C. for 5 hours to 50 hours. An oxidizing atmosphere generally refers to an atmosphere having an oxygen partial pressure at which a sample in a furnace causes an oxidation reaction. In addition, when performing a baking process in two or more times,
It is preferable that the firing temperature in the next step is higher than the firing temperature in the previous step.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION In the lithium secondary battery of the present invention, B, Bi, Mo, P, Cr, V, and W (hereinafter, these element groups are collectively referred to as “additional elements”). Lithium cobaltate (hereinafter, referred to as “LiCoO ₂ ”) or lithium nickelate (hereinafter, “LiNiO”) added with at least one element selected from the group consisting of
O ₂ ”. ) Is used as the positive electrode active material. Each of these has a layered structure.

Although LiCoO ₂ and LiNiO ₂ have stoichiometric compositions, the lithium cobaltate and lithium nickelate used in the present invention do not always have a stoichiometric composition represented by such a chemical formula. It is not necessary to have the cation, and the cation may be deficient or the oxygen ion may be deficient in a composition range capable of maintaining the crystal structure. In particular, for LiNiO ₂ , Ni
Is difficult to become a trivalent ion, and it is difficult to produce a sample in which Li and Ni are regularly arranged. For this reason, in this description, for convenience, the stoichiometric composition will be used.

The additive element may be LiCoO
₂ and LiNiO ₂ (hereinafter referred to as “LiCoO ₂ etc.”) are not presently clear. As an embodiment, when the additive element is dissolved in LiCoO ₂ or the like, that is, when the element conductivity between the cations is improved to improve the electron conductivity, the oxide of the additive element or the additive element and LiCoO 2 are added. compounds with ₂ or the like, as strong a bond between between the primary particles, such as sintering material to LiCoO ₂ and / or secondary particles, the contact resistance between the particles of LiCoO ₂ or the like and / or particles When the amount is reduced, it is presumed that the reaction product of the added element and LiCoO ₂ or the like shows good conductivity and contributes to the reduction of the resistance of the entire positive electrode active material. Therefore, it is not clear whether the additive element exists as a crystalline phase or as an amorphous phase.

However, as shown in Examples described later, in a battery using a positive electrode active material obtained by incorporating these additional elements in a raw material preparation stage, a remarkable effect of reducing internal resistance was obtained. Is a fact confirmed experimentally. Considering that the additive element forms a liquid phase near the synthesis temperature of LiCoO ₂ or the like,
During the synthesis of O ₂ and the like, a phenomenon similar to liquid phase sintering occurs, mass transfer in the liquid phase is activated, and grain growth is promoted.
Particles having a smooth surface can be easily obtained, and the synthesized LiC
It is considered that the decrease in the specific surface area of oO ₂ or the like allows the acetylene black, which is a conductive auxiliary material added to the positive electrode active material at the time of producing a battery, to act efficiently, thereby improving the electron conductivity of the positive electrode material.

The addition amount of the additive element to LiCoO ₂ and the like, preferably greater than or equal to 0.1 mol% 20 mol% or less with respect to the number of moles of Co or Ni of LiCoO ₂ Hitoshichu, further, 1 mol% or more 10 mol% Particularly remarkable effects can be obtained in the following, which is preferable. 0.1mo addition amount
When the amount is less than 1% or more than 20 mol%, the effect of reducing the internal resistance in the battery is not recognized.

The reason for this is that when the amount of the added element is small, a liquid phase that is not sufficient to activate mass transfer and promote grain growth is not formed, and a predetermined form (particle size, specific surface area) is not obtained. And the like), that is, the effect of the added element does not appear. On the other hand, when the amount of the added element is large, it may be difficult to obtain a target composition. Specifically, when the amount of the added element is large, the powder X-ray diffraction image of the synthesized product shows that LiCoO ₂
And the like, the intensity of the peaks indicating the decrease of the peaks, the peak shape becomes broader, and the peaks indicating the presence of the hetero phase appear more strongly. This suggests that the crystallinity of LiCoO ₂ or the like is reduced and the amount of the generated product is reduced. In addition, when a battery is manufactured, the internal resistance increases and the battery capacity is reduced.

Now, the positive electrode active material LiC of the present invention
For the production of oO _{2 and the} like, a mixture of compounds of each element (additional element and Li, Co or Ni) adjusted to a predetermined ratio is used as a raw material in an oxidizing atmosphere at a temperature of 500 to 1000 ° C. for 5 hours to 50 hours. It is preferable to carry out the firing by taking a long time. Note that the oxidizing atmosphere generally refers to an atmosphere having an oxygen partial pressure at which a sample in a furnace causes an oxidation reaction. LiC
In the synthesis of oO _{2 and the} like, the oxygen partial pressure is preferably 10% or more, and specifically, an air atmosphere, an oxygen atmosphere, or the like is applicable.

Here, in the case where the baking treatment is performed twice or more, it is preferable that the baking temperature in the next step is higher than the baking temperature in the previous step. In the present invention,
As described above, it is considered important to generate a liquid phase of the additional element, and the firing temperature (synthesis temperature) is determined in consideration of the melting point of the additional element, the eutectic in the material system to be synthesized, and the like. Such a firing method uses commercially available inexpensive raw materials,
Moreover, it is preferable in that the synthesis can be easily performed without special equipment.

The compound of each element is not particularly limited, but it is needless to say that a high-purity and inexpensive material can be preferably used as a raw material. It is preferable to use various salts such as carbonates, acetates, etc., hydroxides, oxides, and peroxides, which do not generate water, from the viewpoint of health and safety and maintenance of the device, but nitrates, hydrochlorides, sulfates, etc. Can also be used. Note that the Li raw material, usually, oxides Li ₂ O
Is rarely used because of its chemical instability.

Generally, in the synthesis of LiCoO _{2 and the} like, it is known that the synthesis temperature is lowered by using a salt instead of an oxide as a raw material. By utilizing this property, it is possible to easily match the liquid-phase formation temperature of the additive element and the synthesis temperature of LiCoO ₂ or the like so that the effect of the additive element can be easily obtained. When the material system to which the additive element is added has a eutectic composition, it is possible to lower the synthesis temperature.

The material other than the positive electrode active material used for manufacturing the battery is not particularly limited, and various conventionally known materials can be used. For example, as the negative electrode active material, an amorphous carbon material such as soft carbon or hard carbon, artificial graphite such as highly graphitized carbon material, or a carbon material such as natural graphite is used.

As the organic electrolyte, ethylene carbonate (EC), diethyl carbonate (DE)
C), carbonates such as dimethyl carbonate (DMC), propylene carbonate (PC) and γ
- butyrolactone, dissolved in tetrahydrofuran, alone or a mixed solvent of an organic solvent such as acetonitrile, lithium complex fluorine compound such as LiPF ₆ and LiBF ₄ as an electrolyte, or one or two or more kinds of LiClO ₄ lithium halides such as Can be used.

As described above, a battery manufactured using the positive electrode active material based on LiCoO ₂ or the like containing the additive element of the present invention has good electrical characteristics with reduced internal resistance. . Accordingly, it is not necessary to increase the amount of the conductive additive, and it is possible to increase the filling amount of the positive electrode active material itself, thereby improving the energy density. Further, as described later, there is no adverse effect on the cycle test.

Such a battery having a small internal resistance and a large energy density, and a battery having good cycle characteristics are preferably used as a power source for driving a motor of an electric vehicle (EV) or a hybrid electric vehicle (HEV). Can be. This is because EVs and HEVs require a large current discharge during acceleration or climbing a hill, so that the internal resistance is reduced to suppress heat generation and battery temperature rise due to the internal resistance and prevent deterioration of battery characteristics. Is very important. When the internal resistance is small, the charge / discharge efficiency is improved. Further, when the energy density is large, there is an advantage that the continuous running distance per charge is kept long.

[0028]

EXAMPLES Hereinafter, Mo, W, B, and V are added as additional elements.
The present invention will be described in more detail with reference to Examples in which is used. However, it goes without saying that the present invention is not limited to the following Examples.

(Preparation of LiCoO ₂ -based W / Mo / B-added positive electrode active material) Commercially available Li ₂ CO ₃ , C
Using a powder of o ₃ O ₄ , WO ₃ or MoO ₃ or B ₂ O ₃ , as shown in Table 1, the molar ratio of W or Mo or B to the molar ratio of Co in LiCoO ₂ With 0.
Weighed to be 1, 1, 10, 20, 30 mol%,
The mixture was mixed and fired in an air atmosphere at 900 ° C. for 20 hours to synthesize a positive electrode active material. In addition, Li to which no additional element is added
CoO ₂ was also synthesized under the same conditions.

[0030]

[Table 1]

(Preparation of Battery) Next, acetylene black powder as a conductive additive and polyvinylidene fluoride as a binder were added to each of the obtained positive electrode active materials in a weight ratio of 50: 2: 3. To produce a positive electrode material. 0.02 g of the positive electrode material was press-formed at a pressure of 300 kg / cm ² into a disc having a diameter of 20 mm to obtain a positive electrode. LiPF ₆ as an electrolyte was added to the thus prepared positive electrode and an organic solvent in which EC and DEC were mixed at an equal volume ratio of 1 mol / L.
A coin cell was manufactured using an electrolyte dissolved in a ratio of L, a negative electrode made of carbon, and a separator separating the positive electrode and the negative electrode.

(Measurement of Internal Resistance of Battery) Subsequently, with respect to the produced coin cell, 1 depending on the capacity of the positive electrode active material.
A charge-discharge test is performed for only one cycle, in which the battery is charged to 4.1 V at a constant current of C rate-constant voltage and discharged to 2.5 V at a constant current of 1 C rate. The internal resistance of the battery was determined by dividing the difference from the voltage immediately after (voltage difference) by the discharge current. The internal resistivity was determined by dividing the internal resistance of each battery by the internal resistance of the battery using LiCoO ₂ to which no additional element was added. The results are shown in Table 1. When W, Mo or B is added in an amount of 0.1 to 20 mol% as compared with LiCoO ₂ containing no additional element, the internal resistance is reduced. In particular, the effect is improved at an addition amount of 1 to 10 mol%. It turned out to be big.

(Preparation of LiNiO ₂ -based B / V-added positive electrode active material) Next, commercially available Li ₂ CO ₃ , N
Using iCO ₃ , B ₂ O ₃ or V ₂ O ₅ powder, as shown in Table 2, the ratio of B to the number of moles of Ni in LiNiO ₂
Or the number of moles of V is 0.1, 1, 10, 20, 3
The mixture was weighed and mixed so as to be 0 mol%, and first calcined at 600 ° C. for about 16 hours in an oxygen atmosphere. Next, a pulverizing process is performed, and the pulverized material is further heated to 750 ° C.
For 24 hours to synthesize a positive electrode active material. LiNiO ₂ to which no additional element was added was also synthesized under the same conditions.

[0034]

[Table 2]

(Production of Battery and Measurement of Internal Resistance) The production of the battery using the obtained LiNiO ₂ -based B / V-added positive electrode active material and the measurement of the internal resistance were performed by the above-described LiCoO ₂ -based W /
It carried out by the same method as when the Mo / B-added positive electrode active material was used. Table 2 shows the measurement results of the internal resistivity. Compared to LiNiO ₂ containing no additional element, B or V
Shows that when 0.1 to 20 mol% is added, the internal resistance is reduced, and the effect is particularly large when the addition amount is 1 to 10 mol%. In addition, comparing Tables 1 and 2, it can be seen that the effect of reducing internal resistance is particularly large when B is used as an additive element.

(Cycle Operation Test) Next, in the batteries prepared using the various positive electrode active materials shown in Tables 1 and 2, a charge / discharge cycle was performed under the same conditions as the charge / discharge when measuring the internal resistance. Repeated 100 times. As a result, regarding the state of the change in the internal resistance value, no deterioration in the cycle characteristics due to the addition of the additional element was observed.

(Synthesis of Positive Electrode Active Material Using a Plurality of Additive Elements, Fabrication of Battery, and Evaluation of Internal Resistance) Based on the results of the above-described test, Li to which W and Mo were added simultaneously
CoO ₂ and LiNiO ₂ were synthesized and used as a positive electrode active material, and a similar test was performed. As a result, the total added amount of W and Mo was Co or N of LiCoO ₂ or LiNiO _2.
0.1 mol% or more and 20 mol% with respect to the number of moles of i
It was confirmed that the effect of reducing the internal resistance was obtained within the following range. Further, the same was true when at least one or more of B, Bi, P, Cr, and V were selected.

[0038]

As described above, according to the lithium secondary battery of the present invention, the internal resistance of the battery is reduced by using lithium cobaltate and lithium nickelate obtained by adding a predetermined additive element as the positive electrode active material. A significant reduction is realized. In this case, since it is not necessary to increase the amount of the conductive additive, it is possible to increase the filling amount of the positive electrode active material to increase the battery capacity. Thus, a remarkable effect of providing a battery having a large output, a large capacity and excellent charge / discharge cycle characteristics can be obtained.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H01M 10/40 C04B 35/64 C (72) Inventor Kennobu Kitou 2-56 Sudacho, Mizuho-ku, Nagoya-shi, Aichi Japan F-term (reference) in Insulator Co., Ltd. 5H029 AJ03 AJ05 AJ06 AK03 AL06 AM02 AM03 AM05 AM07 BJ03 CJ28 HJ00 HJ01 HJ14

Claims (5)

[Claims] 1. A positive electrode active material comprising lithium cobaltate or lithium nickelate to which at least one element selected from the group consisting of B, Bi, Mo, P, Cr, V and W is added. A lithium secondary battery, characterized in that: 2. The amount of the added element is 0.1 mol% or more and 20 mol% or less with respect to the number of moles of cobalt (Co) or nickel (Ni) in the lithium cobaltate or lithium nickelate. The lithium secondary battery according to claim 1, wherein: 3. The amount of the additional element is more preferably 1 mol% or more and 10 mol% or less with respect to the number of moles of Co or Ni in the lithium cobaltate or lithium nickelate. Item 4. A lithium secondary battery according to Item 2. 4. A method according to claim 1, wherein said positive electrode active material is mixed in a oxidizing atmosphere at 500 ° C. to 1
The lithium secondary battery according to any one of claims 1 to 3, wherein the battery is obtained by firing in a temperature range of 000C for 5 hours to 50 hours. 5. The lithium secondary battery according to claim 4, wherein the firing is performed in two or more times, and the firing temperature in the next step is higher than the firing temperature in the previous step.