JP2003100295A - Positive electrode material for lithium secondary battery and manufacturing method thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To revise the conventional means for measuring the heat stability of a positively electrode material, and to provide a new means, which is used to provide a positive electrode material with a superior heat stability for improving the safety of a battery, a manufacturing method of the positive electrode material, and a secondary battery using the positive electrode material. SOLUTION: The positive electrode material for the lithium secondary battery is made of a compound represented by the chemical formula Lix Niy Coz Mm O2 , and of which BET (Brunauer-Emmett-Teller) specific surface area value is 0.8 m<2> /g or less, the heat stability is high, and discharge capacity is large. In the above formula, M represents one or more than two elements selected from barium, strontium, and boron, and (x), (y), (z) and (m) represents the molar ratio value of each element, where each value is within the range of: 0.9<=x<=1.1, 0.5<=y<=0.95, 0.05<=z<=0.5, and 0.0005<=m<=0.02.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode material for a lithium secondary battery, particularly a positive electrode material for a lithium secondary battery obtained by blending an alkaline earth metal or the like in nickel-cobalt complex lithium oxide, a method for producing the same and lithium. Regarding secondary batteries.

[0002]

2. Description of the Related Art Lithium nickel oxide, which is a positive electrode material for lithium secondary batteries, is characterized by a large discharge capacity. However, when charge and discharge are repeated, the lithium nickel composite oxide in which lithium ions are deintercalated tends to undergo phase transformation into a state without lithium deficiency. This tendency becomes remarkable as the operating temperature of the battery increases.

Since this phase transformation is an irreversible reaction, the absolute amount of the lithium-nickel composite oxide as the positive electrode active material decreases, resulting in a problem that the discharge capacity decreases. Further, the generated oxygen easily reacts with the electrolytic solution forming the battery, and there is a danger that the battery may burst or ignite when the operating temperature is high.

In order to solve this problem, for example, Japanese Unexamined Patent Publication No. 20
In No. 01-23629, after forming a lithium secondary battery by using a lithium nickel-based composite oxide as a positive electrode material for a lithium secondary battery, the secondary battery is charged / discharged, and the lithium nickel-based composite oxide is used. A method for evaluating the thermal stability of a positive electrode material used in a lithium secondary battery by performing thermogravimetric analysis of the lithium nickel-based composite oxide after generating deintercalation of lithium ions has been proposed. There is. Further, as a lithium ion secondary battery positive electrode active material evaluated by such means, it contains Co at a ratio of 0.05 to 0.3 mol with respect to 1 mol of Ni, and further contains B, Al and M.
A composition containing 0.001 to 0.1 mol in total of one or more elements selected from g, Ca, Sr, Ba, Fe, Ti, Zr, Y, La and Ce is shown.

[0005]

However, it is not necessarily sufficient to evaluate the thermal stability of the positive electrode material and thus the safety of the battery using it by the above-mentioned conventional means. The result is clear. For example, it is considered that oxygen gas is generated in a high temperature range around 170 ° C. or higher in the positive electrode in a state where the battery is charged, and there is a risk that the generated oxygen reacts with the electrolytic solution forming the battery. When the thermal stability of the positive electrode material is evaluated by the amount of weight loss from 200 to 300 ° C., it is possible to predict the swelling or rupture of the battery due to the decomposition gas of the positive electrode or the electrolytic solution. However, when oxygen generated from the positive electrode begins to react with the electrolytic solution, an abnormal change in the battery begins to occur.
Unpredictable due to weight loss up to ~ 300 ° C. Therefore, the above method has a problem that the safety of the battery cannot be sufficiently grasped. The present invention improves the conventional means for measuring the thermal stability of the positive electrode material, using the positive electrode material excellent in thermal stability to contribute to the improvement of the safety of the battery,
It is an object of the present invention to propose a manufacturing method thereof and a secondary battery excellent in stability using the positive electrode material.

[0006]

Means for Solving the Problems The present inventors have found that the DTG curve (Derivative Thermogravim
etry; primary differential curve with respect to temperature of thermogravimetric curve) was found to be different among the materials, and it was found that the one that begins to change at higher temperatures is the positive electrode material with excellent thermal stability. To maintain the specific surface area of the positive electrode material having a constant composition small,
Furthermore, they have found that the DTG increase start temperature in the charged state of the positive electrode material is important and completed the present invention.

The positive electrode material for lithium secondary batteries of the present invention comprises a compound represented by the chemical formula Li _x Ni _y Co _z M _m O ₂ and has a BET specific surface area value of 0.8 m ² / g or less. It has high thermal stability and a large discharge capacity. Here, in the above chemical formula, M is Ba, Sr and one or more elements selected from B, x, y, z and m are the values of the molar ratio of each element, 0.9 ≤ x ≦ 1.1, 0.5 ≦ y ≦ 0.95,
0.05 ≦ z ≦ 0.5 and 0.0005 ≦ m ≦ 0.02.

The above chemical formula Li_xNi_yCo_zM_mO₂Compound represented by
In the charged state, the chemical formula Li_aNi _bCo_cM_nO₂Represented by
Which is a compound that increases the DTG of the compound in the charged state.
The large starting temperature should be 215 ° C or higher, preferably 230 ° C or higher.
And is desirable. Here, in the above chemical formula, M is Ba, Sr,
And one or more elements selected from B, x, y, z,
m, a, b, c and n are the values of the molar ratio of each element,
0.9 ≦ x ≦ 1.1, 0.5 ≦ y ≦ 0.95, 0.05 ≦ z ≦ 0.5, 0.0005 ≦ m
≤0.02, 0.2≤a≤0.4, 0.5≤b≤0.95, 0.05≤c≤0.5,
0.0005 ≦ n ≦ 0.02.

In the above positive electrode material, the BET specific surface area value is
It is preferably less than 0.5 m ² / g, and the tap density is 1.5 g / cm ³ or more, the amount of the positive electrode material filled in the battery is increased to increase the charge / discharge capacity per unit volume of the battery. It is suitable for increasing the size.

Such a positive electrode material for a lithium secondary battery is prepared by adding a lithium salt and a salt containing the element M to a compound represented by Ni _y Co _z (OH) ₂ and mixing them, followed by firing and crushing to obtain a chemical formula. L
In producing a lithium secondary battery positive electrode material represented by i _x Ni _y Co _z M _m O ₂ , the compound represented by Ni _y Co _z (OH) ₂ has a tap density of 1.8 g / cm ³ or more, Powder with an average particle size of 5 to 20 μm,
It can be preferably produced by using a spherical powder. In this case, the firing is performed in an oxygen atmosphere.
Pre-baking to hold for 2 to 6 hours at 300 ~ 500 ℃, heating step to raise the temperature at 5 to 30 ℃ / min after the pre-baking, and 650 to 900 ℃ for 2 to 30 hours subsequent to the heating step. It is preferable to carry out the final firing steps for holding in sequence. Where the above elements
M is one or more elements selected from Ba, Sr and B, x, y, z and m are the values of the molar ratio of each element, 0.9 ≤ x ≤ 1.1, 0.5 ≤ y ≦ 0.95, 0.05 ≦ z ≦ 0.5, 0.0005
≦ m ≦ 0.02. The average particle size is measured by a laser diffraction method.

The lithium secondary battery of the present invention is configured such that the active material of the battery contains the positive electrode material described above or is composed of the positive electrode material, and thus the safety of the secondary battery is remarkably high. The danger that the battery will ignite or burst even when exposed to high temperatures is avoided.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have developed a number of positive electrode materials for lithium secondary batteries in which DTG of a compound in a charged state is used.
Was measured, and the relationship between the DTG increase start temperature and the safety of the battery was investigated. Here, the positive electrode material for a lithium secondary battery refers to a positive electrode material compound before charging, and the compound in a charged state is subjected to a charge / discharge test on the positive electrode material compound by the DTG measurement method described later, and in the charged state. A positive electrode material.

FIG. 1 shows Nos. 3, 6, 9, 11 and 14 shown in Table 1.
2 is a result of measuring a DTG curve of a compound of the positive electrode material for a lithium secondary battery in a charged state. As can be seen from FIG. 1, the DTG of the compound in the charged state starts to increase from approximately 190 ° C. and has a peak between 220 and 290 ° C.

[0014]

[Table 1]

The DTG measuring method and the initial discharge capacity of the compound in the above charged state were measured according to the following procedures. 90 mass% of positive electrode material powder for lithium secondary battery, 5 mass% of acetylene black and 5 mass% of polyvinylidene fluoride
N-Methyl-2pyrrolidone was added to, and after sufficiently kneading, applied to an aluminum current collector to a thickness of about 150 μm, and 200 kg / cm
^After pressurizing at about ² , 150 pieces punched into a disc with a diameter of 14 mm
It was vacuum dried at 15 ° C. for 15 hours to obtain a positive electrode. A lithium metal sheet was used for the negative electrode, and a polypropylene porous membrane (trade name Celgard # 2400) was used for the separator. Also,
Ethylene carbonate (EC) / Dimethyl carbonate (DM
LiClO ₄ (1 mol) was dissolved in a mixed solution (1 l) having a volume ratio of 1: 1 to prepare a nonaqueous electrolytic solution.

These were assembled into a test cell in a glove box substituted with argon, and the current density was 1 mA / cm 2.
^The initial discharge capacity was measured with a constant value of ² and charging / discharging in the voltage range of 2.75 to 4.2V. After charging and discharging in this way, the positive electrode was taken out from the test cell in a 4.2V charged state, the positive electrode material powder for a lithium secondary battery was peeled off from the aluminum current collector, washed with dimethyl carbonate, and 100 ° C. It was dried in vacuum.

The chemical formula of the compound in the charged state of the positive electrode material for a lithium secondary battery thus obtained is represented by Li _a Ni _b
When the molar ratio of each element was calculated by chemical analysis as Co _c M _n O ₂ , the molar ratio a of Li was in the range of 0.2 to 0.4 mol with respect to the total amount of Ni and Co. The compound powder in this charged state was placed under an argon atmosphere using a thermogravimetric measuring device.
The temperature was raised at a rate of 10 ° C./min to measure DTG. The DTG increase start temperature means the temperature at which DTG exceeds 0.015% / ° C.

The test on the safety of the battery was conducted as follows. Lithium-nickel-based composite oxide 90 mass%, carbon black 5 mass%, polyvinylidene fluoride 5 mass%,
N-Methyl-2pyrrolidone was added to form a paste, which was applied on an aluminum foil and dried to obtain a positive electrode. A paste consisting of graphite powder and N-methyl-2pyrrolidone was applied onto a copper foil and dried to give a negative electrode. With a separator interposed between the positive electrode and the negative electrode, 1 mol of LiPF ₆ was dissolved in 1 l of a mixed solution of ethylene carbonate (EC) and diethyl carbonate (DEC) at a volume ratio of 1: 1 as an electrolytic solution. A cylindrical lithium ion secondary battery having a size was used. Ten cylindrical batteries each having the same positive electrode material were produced and subjected to a high temperature storage test and a nail insertion test, five each.

In the high temperature storage test, 150V at 4.2V charge state
It was investigated whether the battery was deformed, ruptured, or ignited by the rupture when stored in an atmosphere of 5 ° C for 5 hours. The nailing test is conducted with a 4.4V charged state and in an air atmosphere.
A 2.5 mm nail was pierced and the presence or absence of ignition from the battery was investigated.

As a result, as shown in Table 1, there is a correlation between the DTG increasing start temperature of the compound in the charged state of the positive electrode material and the safety of the battery according to the above evaluation criteria, and the DTG increasing start of the compound in the charged state is started. It was found that the higher the temperature, the better the safety of the battery. The DTG of the compound in the charged state is considered to be an index of the thermal decomposition rate in the state where the positive electrode material in the temperature rising process is charged, so it is presumed that the one with a high increase initiation temperature brought about the improvement of the safety of the battery. To be done.

Specifically, in the case of a positive electrode material having a high DTG increasing start temperature of the compound in a charged state at a boundary of 215 ° C., the high temperature storage test result of the battery is good. Further, in the nailing test, the results are different when the DTG increase starting temperature of the compound in the charged state is 215 ° C as a boundary, and it is good at 215 ° C or higher. Furthermore, when the DTG increase start temperature of the compound in the charged state is 230 ° C. or higher, very good results are obtained. Thus, it can be presumed that the safety of the battery is improved by using the positive electrode material having a higher DTG increasing start temperature of the compound in the charged state.

FIG. 2 shows the chemical formula Li _x Ni _y Co _z M _m O ₂ shown in Table 1.
3 is a graph showing the relationship between the BET specific surface area value of each positive electrode material for lithium secondary batteries represented by and the DTG increase start temperature of the compound in the charged state. As shown here, the DTG increase start temperature of the compound in the charge state of the lithium secondary battery positive electrode material of this system is closely related to the BET specific surface area of the lithium secondary battery positive electrode material before charging, and the BET ratio When the surface area value is 0.8 m ² / g or less, the DTG increase start temperature of the compound in the charged state rises to 215 ° C. or higher. In the above chemical formula, M is one or more elements selected from Ba, Sr and B, x, y, z and m are values of the molar ratio of each element, 0.9 ≦ x, respectively. ≦ 1.1, 0.5 ≦ y ≦ 0.95, 0.05 ≦ z
≦ 0.5 and 0.0005 ≦ m ≦ 0.02. Further, the specific surface area is measured by the BET method obtained from the adsorption amount of nitrogen gas.

The most important point of the present invention is to use the DTG increasing start temperature of the compound in the charged state as an index for evaluating the thermal stability of the positive electrode material for a lithium secondary battery as described above, and the positive temperature of the positive electrode material before charging. The relationship with the BET specific surface area is clarified. In the present invention, this relationship is expressed by the chemical formula Li
_It is applied to a positive electrode material for a lithium secondary battery, which is a compound represented by _x Ni _y Co _z M _m O ₂ .

The reason why the chemical formula Li _x Ni _y Co _z M _m O ₂ is used is as follows. First, the present invention is based on a lithium nickel composite oxide having a large discharge capacity. However, while LiNiO ₂ itself has a high discharge capacity among the positive electrode materials, it has a problem in its thermal stability. So Ni
Substituting 0.05 to 0.5 mol of Co with Co to improve thermal stability. When Co is 0.05 mol or more, the thermal stability is improved, but when it is more than 0.5 mol, the discharge capacity decreases.

Further, B, Sr or Ba is blended in a proportion of 0.0005 to 0.02 mol with respect to the total amount of Ni and Co to improve thermal stability and obtain a sufficient discharge capacity. If the amount of these elements is less than 0.0005 mol, the improvement of thermal stability is insufficient, and if it is more than 0.2 mol, the discharge capacity decreases. In addition, L
When i is small, i has a crystal structure with many lithium deficiencies, and the capacity decreases. Further, if the amount of Li is too large, hydrates and carbonates are generated, and a gelled state occurs when an electrode is produced, and the handling property deteriorates, so the range is 0.9 to 1.1.

In the above invention, the tap density is 1.5 g / cm ³
The above is preferable because the amount of the positive electrode material filled in the battery is increased and the discharge capacity per unit volume of the battery is increased. The tap density was measured by using a powder tester manufactured by Hosokawa Micron Co., Ltd. and tapping 200 times using a 100 ml container for tap density measurement.

The positive electrode material for such a lithium secondary battery is
It can be manufactured as follows. First the starting material
The molar ratio of Co to the total amount of Ni and Co is 0.05.
Ni adjusted to ~ 0.5_yCo_z(OH)₂To manufacture. In its manufacture
For this purpose, for example, a dense Ni_yC
o_z(OH)₂Of powder, the average particle size is 5 to 20μ
m, and tap density is 1.8g / cm³Adjust to be above
Is desirable. In addition, in general, lithium composite oxide
When synthesizing, the starting material is Ni_yCo_z(OH) ₂Shape of
Since the fineness and compactness are inherited as they are,
It is particularly preferable that the form is spherical. With this,
Clearly has a small specific surface area and a large tap density
A positive electrode material can be obtained.

The spherical and dense Ni _y Co _z (O
H) ₂ is mixed with a lithium salt and a salt containing M, followed by firing and crushing to obtain a lithium secondary battery positive electrode material represented by the chemical formula Li _x Ni _y Co _z M _m O ₂ , at that time, Pre-baking is performed by holding the firing conditions in an oxygen atmosphere at 300 to 500 ° C. for 2 to 6 hours, a temperature raising step of raising the temperature at 5 to 30 ° C./min after the preliminary firing, and 650 to 900 subsequent to the temperature raising step. It is advisable to carry out a final firing step in which the holding is carried out for 2 to 30 hours at ℃.

The pre-baking is intended to completely remove the water of crystallization water in the raw material while suppressing the reaction between lithium and nickel, so it is preferable to hold the temperature at 300 to 500 ° C. for 2 hours or more. However, if it exceeds 6 hours, productivity is lowered, which is not preferable. The rate of temperature rise in the temperature raising process is 5 to 30 ° C / mi from the viewpoint of protection and productivity of the crucible for firing and refractory for firing.
It should be n. If the final firing temperature is less than 650 ° C, the reaction is difficult to proceed, and if it exceeds 900 ° C, lithium is scattered, which is not preferable. The holding time is preferably 2 to 30 hours in consideration of reactivity and productivity.

[0030]

EXAMPLES General formula Li _x Ni _y Co _z M _m O having the composition shown in Table 2
A positive electrode material having ₂ was manufactured, and its specific surface area, tap density, discharge capacity and DTG increasing start temperature were measured. Except that the using powdered reagents under tap density 1.8 g / cm ³ as the starting material in per No.11,12,15 and 16 in the manufacture of dense spherical tap density 1.9~2.1g / cm ³ Ni _y Co _z (OH) ₂ was used. As the firing conditions, short-time firing or long-term firing described below in an oxygen atmosphere was adopted. The measurement results are also shown in Table 2.

Short-time firing Pre-baking: Hold at 400 ℃ for 4h Temperature rising rate: 10 ℃ / min Final firing: Hold at 800 ℃ for 4 hours Long time firing Pre-baking: Hold at 500 ℃ for 6h Temperature rising rate: 20 ℃ / min Final firing: 750 ℃ 12h hold

[0032]

[Table 2]

As shown here, the positive electrode material having the composition of the present invention and having a predetermined specific surface area has a high discharge capacity and a high DTG increasing start temperature. When the tap density is high, the amount of the positive electrode material that can be filled in the battery can be increased, and the charge / discharge capacity per unit volume of the battery can be increased accordingly.

[0034]

As described above, the present invention reduces the BTG specific surface area with respect to the thermal stability of the positive electrode material for lithium secondary batteries, thereby increasing the DTG increasing start temperature of the compound in the charged state, thereby increasing the secondary temperature. The safety of the battery is remarkably improved, and even when the secondary battery is exposed to a high temperature state, the risk of the battery exploding or igniting can be avoided.

[Brief description of drawings]

FIG. 1 is a graph showing changes in DTG when the positive electrode materials for various lithium secondary batteries in a charged state are heated in an argon atmosphere at 10 ° C./min.

FIG. 2 is a graph showing the relationship between the DTG increase initiation temperature and the specific surface area of a positive electrode material for a lithium secondary battery, which is composed of a compound represented by the chemical formula Li _x Ni _y Co _z M _m O ₂ .

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Toshihiko Funabashi             River iron ore, 1 Niihama-cho, Chuo-ku, Chiba-shi, Chiba Prefecture             Engineering Co., Ltd. F-term (reference) 4G048 AA04 AA05 AB01 AC06 AD03                       AE05                 5H029 AJ03 AJ12 AK03 AL07 AM03                       AM05 AM07 BJ02 CJ02 CJ08                       CJ28 HJ00 HJ02 HJ05 HJ07                       HJ08 HJ14                 5H050 AA08 AA15 BA17 CA08 CB08                       GA02 GA05 GA10 GA26 GA27                       HA02 HA05 HA07 HA08 HA14                       HA20

Claims (10)

[Claims] 1. A high thermal stability characterized by comprising a compound represented by the chemical formula Li _x Ni _y Co _z M _m O ₂ and having a BET specific surface area value of 0.8 m ² / g or less, Also, a positive electrode material for a lithium secondary battery having a large discharge capacity. Here, in the above chemical formula, M is Ba, Sr and one or more elements selected from B, x, y, z and m are the values of the molar ratio of each element, 0.9 ≤ x ≤1.1, 0.5 ≤ y ≤ 0.95, 0.05 ≤ z ≤ 0.5, 0.0
005 ≦ m ≦ 0.02. 2. The compound represented by the chemical formula Li _x Ni _y Co _z M _m O ₂ has the chemical formula Li _a Ni _b Co _c M _n O ₂ in the charged state, and the DTG increase starting temperature of the compound in the charged state. 2. The positive electrode material for lithium secondary batteries according to claim 1, having a high thermal stability and a large discharge capacity. Here, in the above chemical formula, M is Ba, Sr, and one or more elements selected from B, and x, y, z, m, a, b, c and n are molar ratios of each element. Values, 0.9 ≦ x ≦ 1.1, 0.5 ≦ y ≦ 0.95, 0.05, respectively
≦ z ≦ 0.5, 0.0005 ≦ m ≦ 0.02, 0.2 ≦ a ≦ 0.4, 0.5 ≦ b ≦
0.95, 0.05 ≤ c ≤ 0.5, and 0.0005 ≤ n ≤ 0.02. 3. The positive electrode material for a lithium secondary battery, which has a high thermal stability and a large discharge capacity, according to claim 2, which has a DTG increase start temperature of 230 ° C. or higher. 4. A lithium secondary battery with high thermal stability and large discharge capacity according to any one of claims 1 to 3, which has a BET specific surface area value of less than 0.5 m ² / g. Positive electrode material. 5. The positive electrode material for a lithium secondary battery having high thermal stability and large discharge capacity according to claim 1, wherein the tap density is 1.5 g / cm ³ or more. . 6. A compound represented by Ni _y Co _z (OH) ₂ is added with and mixed with a lithium salt and a salt containing the element M, followed by firing and crushing to obtain a chemical formula Li _x Ni _y Co _z M _m O ₂ In producing a positive electrode material for a lithium secondary battery represented by, the compound represented by Ni _y Co _z (OH) ₂ has a tap density of 1.8 g / cm 3.
³ or more, a powdery material having an average particle size of 5 to 20 μm, having high thermal stability according to any one of claims 1 to 5,
And a method for producing a positive electrode material for a lithium secondary battery having a large discharge capacity. Here, the element M is Ba, Sr and one or more elements selected from B, x, y, z and m are the values of the molar ratio of each element, 0.9 ≤ x ≤ 1.1, 0.5 ≦ y ≦ 0.9
5, 0.05 ≦ z ≦ 0.5 and 0.0005 ≦ m ≦ 0.02. 7. The positive electrode for a lithium secondary battery having high thermal stability and large discharge capacity according to claim 6, wherein the compound represented by Ni _y Co _z (OH) ₂ is a spherical powder. Material manufacturing method. 8. The firing is performed in an oxygen atmosphere at 300 to 500 ° C.
Pre-baking to hold for 2-6 hours, and 5-30 ° C / m after the pre-baking
a temperature raising step of raising the temperature in, and 650-9 after the temperature raising step.
The positive electrode material for a lithium secondary battery with high thermal stability and large discharge capacity according to claim 6 or 7, characterized in that a final firing step of holding at 00 ° C for 2 to 30 hours is sequentially performed. Production method. 9. A lithium secondary battery in which the positive electrode active material contains the positive electrode material according to any one of claims 1 to 5. 10. A lithium secondary battery in which the positive electrode active material is the positive electrode material according to any one of claims 1 to 5.