JP2001319647A - Positive electrode for lithium secondary battery and lithium ion battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode, in which a polyaniline-manganese spinel compound-nickel-substituted lithium cobaltate that is used for a high energy density type lithium secondary battery and an electrically conductive agent are coated on an Al current collector. SOLUTION: A positive electrode is composed of a polyaniline, a spinel structure Li1+xMn2O4 (0.01<=x<=0.2), LiCo1-yNiyO2 (0.1<=x<=0.5), and a carbonaceous conductive material. A spinel compound Li1+xMn2-zO4 (0<=x<=0.2, M may be Al, Co, Cr, Fe, 0.01<=z<=0.2), in which different type of metal (M) is substituted, can be used instead of Li1+xMn2O4. Further, PVDF may also be contained in the electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium secondary battery using an intercalation compound such as lithium-tin alloy or lithium-graphite (lithium-carbon) as a negative electrode active material. Active material and LiCo _1-y Ni _y O ₂ )
And a positive electrode comprising a carbon-based conductive agent.

[0002]

2. Description of the Related Art As a positive electrode active material of a 4 V high energy density type lithium secondary battery, a layered compound such as LiNiO ₂ ,
LiCoO ₂ and LiMn ₂ O ₄ having a spinel structure can be used, and LiCoO ₂ has already been put to practical use as a positive electrode active material for a small secondary battery. However, assuming large batteries for electric vehicles and power storage, the cathode material is a spinel-structured LiM
It is limited to n ₂ O ₄ , and the negative electrode material will also be natural graphite and its chemically treated product and tin oxide.

The biggest problem in electrochemical properties of LiMn ₂ O _{4 having} a spinel structure is that the initial charge capacity and the subsequent charge / discharge capacity are small, and a carbon-based material or tin oxide having irreversible capacity is used as a negative electrode. Then, a certain amount of positive electrode capacity is used to cancel the irreversible capacity of the negative electrode.
The capacity of a lithium ion battery using n ₂ O ₄ as a cathode material is significantly reduced. In the case of a carbon-based negative electrode, the irreversible capacity is used for forming a protective film on the surface of the negative electrode, and a certain amount of irreversible capacity is indispensable for stable operation of a lithium ion battery. Also, in the case of tin-based oxides, since metal tin is generated electrochemically, the existence of a large irreversible capacity at the negative electrode is inevitable.

[0004] Conventionally, in the production of a positive electrode, the use of a binder is unavoidable, and if the binder can be replaced with an active material having a binding ability, the battery capacity will increase by 5 to 10%. .

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems in the prior art, and comprises mixing a positive electrode material having a large initial charge capacity with a manganese spinel compound serving as a main positive electrode material to form a charge capacity of the positive electrode. Thus, the capacity of the lithium ion battery is improved by improving the capacity. Polyaniline, which also acts as a binder, has 15
It has a capacity of 0 mAh / g and changes to an oxidized state with high conductivity during charging, and in this process contributes to increasing the initial charging capacity of the positive electrode.

[0006]

It is desirable that the cathode material mixed with the manganese spinel compound has a large charge capacity, excellent cycle characteristics, and high thermal stability of the charge product. A material that satisfies this is lithium cobalt oxide, which is currently used for the cathode material of small lithium ion batteries.
Lithium cobaltate is a situation in which alternative materials are being sought in view of resource and toxicity issues, and the smaller the amount of cobalt used, the better.

[0007] The material used for this purpose is used to compensate for the irreversible capacity of the negative electrode even if the discharge capacity is slightly inferior as long as the charge capacity is about the same as lithium cobalt oxide. Do not give.

[0008]

DETAILED DESCRIPTION OF THE INVENTION FIG.
LiCo with 25% nickel replaced by nickel _0.75Ni
_0.25O_Two1 shows a first charge / discharge curve. Charging is 4.3V
And then discharged to 1V. Both charging capacities
Are equal at 168 mAh / g, but the discharge capacity,
And the discharge curves are different. Lithium cobaltate during discharge
Voltage decreases monotonically while LiCo_0.75Ni
_0.25O_TwoA polarization point was observed around 3.9 V in the discharge curve of FIG.
Thereafter, the voltage drop accompanying the discharge increases. Up to 3V
Capacity of lithium cobalt oxide is 151 mAh / g
Whereas LiCo_{0. 75}Ni_0.25O_TwoDischarge capacity
Is 134 mAh / g, which is about 10% smaller.

When a graphite-based carbon material is used for the negative electrode, the irreversible capacity is 60 mAh / g or more. When the active material weight ratio of the positive and negative electrodes is 2-3 (positive electrode weight / negative electrode weight), 20 to 30 mAh of the positive electrode capacity is used. / G will be consumed to form a protective film on the negative electrode. The charge / discharge capacity of a manganese spinel compound with excellent high-temperature cycle characteristics is 1
It is about 10 mAh / g, and 18 to 27% of the capacity of the positive electrode is lost in the process of forming the protective film on the negative electrode. As can be seen in Example 1, the proportion of polyaniline in the positive electrode mixture is about 5%, and even when the capacity is 150 mAh / g, the amount of electricity supplemented by the charging of polyaniline is 150 × in terms of the oxide-based positive electrode material. 0.05 = 7.5 mAh /
g, and the capacity corresponding to 12.5 to 22.5 mAh / g is still lost from the positive electrode. In the case of a lithium ion battery with a regulated positive electrode capacity, the capacity of the battery is obtained by subtracting the capacity of forming the protective film of the negative electrode and multiplying by the weight of the active material.

As shown in Example 2, the weight ratio of the positive and negative electrode active materials was 2.5, and the negative electrode graphite had an irreversible capacity of 70 m.
Ah / g, reversible capacity 340 mAh / g. The positive electrode active material is a lithium-rich spinel having a capacity of 110 mAh / g, a charge capacity of 168 mAh / g, and a discharge capacity of 134 mAh / g.
Is a mixture of LiCo _0.75 Ni _0.25 O ₂ . When the mixing ratio of the two is 10: 1, the capacity as a lithium ion battery per 1 g of the positive electrode active material is 87.3 mAh / g.
The measured discharge capacity obtained in Example 2 was 91.6 mAh / g.
The capacity is several mAh / g higher than the calculated value due to the contribution of polyaniline. As shown in Example 3, even if LiCoO ₂ is used instead of LiCo _0.75 Ni _0.25 O ₂ , there is no significant difference in discharge capacity.

When the mixing ratio of the positive electrode active material is increased to 5: 1, the calculated capacity of the lithium ion battery per gram of the positive electrode active material is 91.6 mAh / g. When the weight ratio of the positive and negative electrode active materials is set to 3, the increase in capacity becomes even more remarkable. The calculated capacity at a positive electrode mixing ratio of 10: 1 is also 91.8 mAh / g, and at 5: 1 the calculated capacity is 96.3 mAh / g. These results are the same even when LiCoO ₂ is used. Example 4
The discharge capacity of a lithium ion battery having a positive / negative electrode active material weight ratio of 3 was 93.3 mAh / g, which shows almost expected battery characteristics.

As described above, the active material to be mixed with the positive electrode does not need to be lithium cobalt oxide having a large discharge capacity, but a nickel-substituted lithium cobalt oxide (LiCo _1-y Ni _y O ₂ ) whose charge amount is almost equal to lithium cobalt oxide. : 0.1 ≦ y
≦ 0.5) is sufficient. Since these materials have a small cobalt ratio, they are low-cost materials having the same performance but low cobalt content. Further, since polyaniline, which is a 3V class positive electrode material, is also used as a binder, an increase in capacity corresponding to the charge capacity of polyaniline is recognized, and polyaniline itself also contributes to an increase in capacity.

The manganese-based spinel compound used for the positive electrode material is preferably a dissimilar metal-substituted spinel (Li _{1 + x} Mn _2-z M _z O ₄ : Al, Fe, Cr or Co) having excellent high-temperature cycle characteristics. In the case of Examples 5-8 using these compounds instead of the lithium-rich spinel compounds, an increase in the discharge capacity of the lithium-ion battery was observed, and it is clear that this method can be applied to the heterometal-substituted spinel compounds.

The positive electrode shown in Example 2 has poor flexibility, and if the electrode is bent, the electrode may crack and peel off may occur between the Al foil of the current collector and the positive electrode mixture. In the case of a flat plate type battery, since no bending stress is applied, there is no difficulty, and conversely, the shape retention ability is high and the operation at the time of battery production becomes easy. However, in the case of a cylindrical battery, a large shading stress acts, and there is a fear that the positive electrode mixture may peel off from the current collector when the electrode is wound. In the positive electrode containing 4 wt% of polyvinylidene fluoride (PVDF) shown in Example 9, the flexibility of the electrode is improved, and the positive electrode mixture is not cracked and peeled off from the current collector even when wound on a round bar having a diameter of 5 mm. I was not able to admit.

In the case of a manganese-based spinel compound, when overcharge occurs, the potential of the positive electrode quickly reaches 4.6 V or more, and decomposition of the electrolytic solution occurs. At this time, an increase in the internal pressure due to the generated gas may lead to an accident. L coexisting in the positive electrode mixture
About 40% of Li which can be desorbed still remains even when charged to 4.3 V (vs. Li) in iCo _1-y Ni _y O ₂ , and LiCo accompanied by dedope of lithium even when overcharge occurs
The oxidation reaction of _1-y Ni _y O ₂ occurs, and even if overcharge occurs, it does not directly lead to the decomposition of the solvent, which leads to an increase in the internal pressure of the battery.
That is, the coexistence of LiCo _1-y Ni _y O ₂ in the positive electrode also improves the safety against overcharging.

[0016]

EXAMPLES Example 1 Ni-substituted lithium cobalt oxide mixed with a manganese-based spinel compound was synthesized by the following procedure. A predetermined amount of _{LiOH-H 2 O, Co 3} O 4 and Ni
(OH) ₂ is pulverized and mixed, and calcined in air at 500 ° C. for 5 hours. Finally fired in air at 850 ° C for 20 hours, Ni
Substituted lithium cobaltate LiCo _1-y Ni _y O ₂ (0 ≦ x
≤ 0.5). No impurities could be confirmed by XRD measurement, and all peaks were single-phase compounds that could be indexed by surface cubic system.

The charge / discharge capacity of these compounds was evaluated by a battery test using lithium metal as a counter electrode. The positive electrode kneads 20 mg of the active material and 10 mg of the conductive binder,
It was made by crimping on a stainless steel mesh. 1 for electrolyte
M LiPF ₆ -EC (ethylene carbonate) -DM
C (dimethyl carbonate) (EC: DMC in a volume ratio of 1: 2) was used. The charging and discharging were performed at a constant current of 0.2 mA, and the charging end voltage was 4.3 V. Thereafter, the battery was discharged to 1V.

FIG. 1 shows a charge / discharge curve of a representative sample.
When the value of y was in the range of 0.5 or less, the charge capacity showed a value of 164-168 mAh / g, regardless of the Ni substitution amount, and was almost constant. In the discharge curve, when y exceeds 0.2, the discharge potential tends to decrease. The discharge capacity up to 3 V is 1 even for LiCo _0.75 Ni _0.25 O _{2 with} the smallest capacity.
34 mAh / g, giving a higher discharge capacity than the manganese-based spinel compound.

Example 2 5 g of Li _1.1 Mn ₂ O ₄ , L
iCo _0.75 Ni _0.25 O ₂ 500mg, carbon-based conductive agent 5
00 mg, and 250 mg of polyaniline.
-Methylpyrrolidone was added and kneaded to prepare a positive electrode paste. This paste was applied on an Al foil by a doctor blade and dried.

Further, 3 g of graphite, 300 mg of polyvinylidene fluoride and N-methylpyrrolidone were kneaded to prepare a negative electrode paste. The negative electrode paste was applied on a Cu foil by a doctor blade similarly to the positive electrode, and dried.

The positive electrode was 14 mm in diameter (active material weight: 19.2).
mg), the negative electrode was cut into a diameter of 15 mm (7.7 mg of the negative electrode active material), and dried at 100 ° C. under vacuum. Using these electrodes, a coin-type lithium ion battery was prepared. The positive / negative electrode active material weight ratio of this battery is 2.5.

The electrolyte is 1M LiPF ₆ -EC-DM
C (EC: DMC was 1: 2 by volume ratio) was used. Charging is performed with a constant current (0.8 mA) and a constant voltage (4.3 V).
The discharge was up to 3V. The charge capacity was 2.30 mAh, and the discharge capacity up to 3 V was 1.76 mAh. The discharge capacity per 1 g of the positive electrode active material (oxide only) was 91.
It became 6 mAh / g.

Example 3 5 g of Li _1.1 Mn ₂ O ₄ , L
500 mg of iCoO _2, 500 mg of a carbon-based conductive agent, and 250 mg of polyaniline were mixed, and N-methylpyrrolidone was added and kneaded to prepare a positive electrode paste. This paste was applied on an Al foil by a doctor blade and dried. The negative electrode used in Example 2 was used.

The positive electrode was 14 mm in diameter (active material weight: 19.3).
mg), the negative electrode was cut into a diameter of 15 mm (7.7 mg of the negative electrode active material), and dried at 100 ° C. under vacuum. Using these electrodes, a coin-type lithium ion battery was prepared. The positive / negative electrode active material weight ratio of this battery is 2.5.

The electrolyte and the charging and discharging conditions are the same as in the second embodiment. The charge capacity was 2.31 mAh, and the discharge capacity up to 3 V was 1.74 mAh. The discharge capacity per 1 g of the positive electrode active material (only oxide) is 90.1 mAh / g.
It became.

Example 4 The positive electrode prepared in Example 2 was cut to a diameter of 14 mm (active material weight: 19.5 mg), and the negative electrode was cut to a diameter of 14 mm (negative electrode active material: 6.5 mg).
Vacuum dried at ℃. Using these electrodes, a coin-type lithium ion battery was prepared. The positive / negative electrode active material weight ratio of this battery was 3. The electrolyte and the charge / discharge conditions were the same as in Example 2.
Is the same as The charge capacity is 2.32 mAh and 3V
Discharge capacity up to 1.86 mAh. The discharge capacity per 1 g of the positive electrode active material (oxide only) is 95.4 mAh.
/ G.

Example 5 LiMn_1.9Co_0.1O_Two 5
g, LiCo_0.75Ni_0.25O_Two 500mg, carbon-based
Mix 500mg of electric agent and 250mg of polyaniline
And add N-methylpyrrolidone and knead the mixture to form a positive electrode paste.
Was made. This paste is A
1 on a foil and dried. LiMn used here
_1.9Co_0.1O _TwoHas a charge / discharge capacity of 115 mAh / g.
You.

The positive electrode was 14 mm in diameter (active material weight: 19.1).
mg), and the negative electrode prepared in Example 2 was cut to a diameter of 15
mm (negative electrode active material: 7.6 mg), and vacuum dried at 100 ° C. Using these electrodes, a coin-type lithium ion battery was prepared. The positive / negative electrode active material weight ratio of this battery is approximately 2.5. The electrolyte and the charge / discharge conditions were the same as in Example 2. The charging capacity is 2.30 mAh,
The discharge capacity up to 3 V was 1.84 mAh. As in the case of Example 2, the discharge capacity per 1 g of the positive electrode active material (only oxide) was 96.3 mAh / g.

Example 6 LiMn_1.9Al_0.1O_Two 5
g, LiCo_0.75Ni_0.25O_Two 500mg, carbon-based
Mix 500mg of electric agent and 250mg of polyaniline
And add N-methylpyrrolidone and knead the mixture to form a positive electrode paste.
Was made. This paste is A
1 on a foil and dried. LiMn used here
_1.9Al_0.1O _TwoHas a charge / discharge capacity of 125 mAh / g.
You.

The positive electrode was 14 mm in diameter (active material weight: 19.1).
mg), and the negative electrode prepared in Example 2 was cut to a diameter of 15
mm (negative electrode active material: 7.6 mg), and vacuum dried at 100 ° C. Using these electrodes, a coin-type lithium ion battery was prepared. The positive / negative electrode active material weight ratio of this battery is approximately 2.5. The electrolyte and the charge / discharge conditions were the same as in Example 2. The charging capacity is 2.54 mAh,
The discharge capacity up to 3 V was 2.01 mAh. The discharge capacity per 1 g of the positive electrode active material (oxide only) was 105.2.
mAh / g.

Example 7 Li _1.08 Mn _1.9 Cr _0.1 O ₂
5 g, LiCo _0.75 Ni _0.25 O ₂ 500 mg, carbon-based conductive agent 500 mg, and polyaniline 250 mg were mixed, N-methylpyrrolidone was added and kneaded to prepare a positive electrode paste. This paste was applied on an Al foil by a doctor blade and dried. Li used here
The charge / discharge capacity of _1.08 Mn _1.9 Cr _0.1 O ₂ is 108 mAh /
g.

The positive electrode was 14 mm in diameter (active material weight: 19.2).
mg), and the negative electrode prepared in Example 2 was cut to a diameter of 15
mm (7.7 mg of the negative electrode active material), and vacuum dried at 100 ° C. Using these electrodes, a coin-type lithium ion battery was prepared. The positive / negative electrode active material weight ratio of this battery is 2.5. The electrolyte and the charge / discharge conditions were the same as in Example 2.
Is the same as The charging capacity is 2.26 mAh and 3V
Discharge capacity up to 1.72 mAh. The discharge capacity per 1 g of the positive electrode active material (oxide only) is 89.6 mAh.
/ G.

Example 8 LiMn_1.9Fe_0.1O_Two 5
g, LiCo_0.75Ni_0.25O_Two 500mg, carbon-based
Mix 500mg of electric agent and 250mg of polyaniline
And add N-methylpyrrolidone and knead the mixture to form a positive electrode paste.
Was made. This paste is A
1 on a foil and dried. LiMn used here
_1.9Fe_0.1O _TwoHas a charge / discharge capacity of 122 mAh / g.
You.

The positive electrode was 14 mm in diameter (active material weight: 19.2).
mg), and the negative electrode prepared in Example 2 was cut to a diameter of 15
mm (7.7 mg of the negative electrode active material), and vacuum dried at 100 ° C. Using these electrodes, a coin-type lithium ion battery was prepared. The positive / negative electrode active material weight ratio of this battery is 2.5. The charge capacity is 2.51 mAh, 3
The discharge capacity up to V was 1.97 mAh. The discharge capacity per 1 g of the positive electrode active material (only oxide) is 102.6 m.
Ah / g.

Example 9 Li _1.1 Mn ₂ O ₄ 5 g, L
iCo _0.75 Ni _0.25 O ₂ 500mg, carbon-based conductive agent 5
00 mg, polyaniline 250 mg, PVDF 200 m
g was mixed, and N-methylpyrrolidone was added and kneaded to prepare a positive electrode paste. This paste was applied on an Al foil by a doctor blade and dried. This electrode had high flexibility, and unlike the electrode of Example 2, even when wound on a round bar having a diameter of 5 mm, no crack was generated in the positive electrode mixture and no peeling from the current collector was observed.

The positive electrode was 14 mm in diameter (the active material weight was 18.6).
mg), and the negative electrode prepared in Example 2 was cut to a diameter of 15
mm (negative electrode active material: 7.6 mg), and vacuum dried at 100 ° C. The charge capacity is 2.22 mAh and 3V
Discharge capacity up to 1.67 mAh. There was no change in the battery characteristics, and it was confirmed that the addition of PVDF increased the flexibility of the electrode and could be used as a positive electrode of a cylindrical battery.

[0037]

As described above, according to the present invention, a manganese-based spinel compound—Ni-substituted lithium cobaltate—
Polyaniline composite cathodes can provide high capacity lithium ion batteries.

[Brief description of the drawings]

FIG. 1 is a charge / discharge curve of LiCo _0.75 Ni _0.25 O ₂ and LiCoO ₂ .

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H01M 4/62 H01M 4/62 Z 10/40 10/40 Z (72) Inventor Shunji Taniguchi Fukuoka, Fukuoka 2-82, Watanabe-dori, Chuo-ku Kyushu Electric Power Co., Inc. (72) Inventor Kazuyuki Adachi 2-182, Watanabe-dori, Chuo-ku, Fukuoka City, Fukuoka F-term (reference) 5H029 AJ03 AK03 AK16 AK18 AL02 AL07 AL08 AM03 AM05 AM07 DJ08 DJ16 DJ17 EJ12 HJ02 5H050 AA08 BA17 CA09 CA22 CA29 CB02 CB08 CB09 DA02 DA09 EA24 FA17 FA19 GA10 HA02

Claims (4)

[Claims] 1. Polyaniline, spinel structure Li _{1 + x} M
n ₂ O ₄ (0.01 ≦ x ≦ 0.2), LiCo _1-y Ni _y O
_{2 A} positive electrode for a lithium secondary battery comprising (0.1 ≦ x ≦ 0.5) and a carbon-based conductive material. Wherein Li ₁ + x Mn ₂ O spinel compounds of different metals (M) substituted for _{4 Li 1 + x Mn 2-} z M z O 4 (0
≦ x ≦ 0.2, M is Al, Co, Cr, Fe, 0.01
≤ z ≤ 0.2). The positive electrode for a lithium secondary battery according to claim 1, wherein 3. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode material contains a binder such as PVDF. 4. A lithium ion battery having the positive electrode according to claim 1 and a negative electrode made of carbon, graphite or tin oxide.