JP2001023611A - Positive electrode of lithium ion secondary battery and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light-weighed lithium ion secondary battery and a positive electrode thereof in which a charge capacity is largely maintained and a charge capacity density and a weight energy density are enhanced. SOLUTION: A positive electrode of a lithium ion secondary electrode is constituted by mixing a LiMn2O4 powder and a large charge capacity positive electrode active substance powder. Here, Li2Mn2O4 and LiNiO2 having a larger charge capacity than that of LiM2O4 are, for example, used as the large charge capacity positive electrode active substance and it is preferable that a composition ratio thereof against the whole positive electrode active substance is 5-20% by weight and 6-21% by weight respectively. The positive electrode can be also constituted as the lithium ion secondary electrode by combining it with a negative electrode constituted by a carbon material. Thereby, since a irreversible capacity part is compensated by the large charge capacity positive electrode active substance and an addition amount thereof is small, a light weight of the positive electrode and the battery is realized and the charge capacity density in the battery is enhanced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a positive electrode of a lithium ion secondary battery, and also relates to the lithium ion secondary battery itself.

[0002]

_2. Description of the Related Art Recently, lithium composite oxides such as LiCoO ₂ and LiMn ₂ O ₄ have been used as a positive electrode active material of a lithium ion secondary battery. In particular, LiMn ₂ O ₄ is promising in terms of resources and cost. Further, there is a material in which lithium manganate having different compositions such as LiMnO ₂ and Li ₂ Mn ₂ O ₄ are added to LiMn ₂ O ₄ as a positive electrode active material.

For example, Japanese Patent Application Laid-Open No. 6-349493 discloses that a lithium composite oxide (LiMn ₂ O ₄ , LiCoO ₂ , LiNiO _2, etc.) is mixed with LiMnO ₂ as a positive electrode active material. As a result, it is stated that as a result, a battery that prevents elution of the negative electrode current collector in overdischarge and that does not significantly deteriorate in performance due to overdischarge is realized.

In Japanese Patent Application Laid-Open No. 10-199528, a composite in which the surfaces of spinel-structured LiMn ₂ O ₄ particles are coated with the same spinel-structured Li ₂ Mn ₂ O ₄ as a positive electrode active material is used. By suppressing the elution of manganese during charging of the positive electrode active material particles and the collapse of the crystal structure during discharging, a battery having excellent discharge characteristics and cycle characteristics is provided.

[0005]

By the way, in a lithium secondary battery having both the positive electrode as in the above two examples and the negative electrode made of a carbon material such as graphite as the negative electrode active material,
Usually, a film of a lithium compound is formed on the surface of the negative electrode material at the time of initial charging, and the same amount of charge capacity cannot be taken out at the time of discharging. This loss is called irreversible capacity. In the current lithium ion secondary battery, a positive electrode active material is added to compensate for this irreversible capacity. Therefore, problems such as a decrease in the charge capacity density and a decrease in the weight energy density of the battery have been caused.

The present invention has been made in view of the above circumstances, and an object thereof is to maintain a large charging capacity,
An object of the present invention is to provide a light-weight positive electrode of a lithium-ion secondary battery and a lithium-ion secondary battery itself capable of improving a charge capacity density and a weight energy density.

[0007]

The present invention employs the following means in order to solve the above-mentioned problems. That is, the positive electrode of the lithium ion secondary battery according to claim 1 is a cubic L
A positive electrode active material obtained by simply mixing iMn ₂ O ₄ powder and a large charge capacity positive electrode active material powder, wherein the composition of the large charge capacity positive electrode active material with respect to the positive electrode active material is 5 to 25% by weight. It is assumed that.

According to this, the irreversible capacity component is compensated for by the large charge capacity positive electrode active material. At this time, the “large charge capacity” of the large charge capacity positive electrode active material is LiMn ₂ O ₄
It makes sense for the charging capacity of For this reason,
The supplement of the irreversible capacity by the large charge capacity positive electrode active material is achieved by adding a small amount. The specific amount of the addition is preferably 5 to 25% by weight as described above.

The positive electrode of the lithium ion secondary battery according to the second aspect is characterized in that cubic LiMn ₂ O ₄ powder is mixed with tetragonal Li ₂ M
and characterized in that it is composed of a positive electrode active material obtained by simply mixing n ₂ O ₄ powder, and in particular, the Li ₂ Mn ₂ O ₄
The composition of the powder with respect to the positive electrode active material is 5 to 25% by weight (claim 3).

The positive electrode of the lithium ion secondary battery according to claim 4 is characterized in that cubic LiMn ₂ O ₄ powder is mixed with hexagonal L
It is characterized by being composed of a positive electrode active material obtained by simply mixing iNiO ₂ powder.
_{2 The} composition of the powder with respect to the positive electrode active material is 5 to 25% by weight.
(Claim 5).

Further, in the lithium ion secondary battery according to the present invention, a cubic LiMn ₂ O ₄ powder is added to a tetragonal Li ₂
It is characterized by comprising a positive electrode active material obtained by simply mixing Mn ₂ O ₄ powder and hexagonal LiNiO ₂ powder.

According to these positive electrodes, LiMn_TwoO_FourTo
Then, as the large charge capacity positive electrode active material described above, Li_TwoMn_Two
O_FourPowder or LiNiO_TwoBy adding powder
Thus, the irreversible capacity is compensated. At this time, Li
_TwoMn_TwoO_FourOr LiNiO_TwoIs LiMn_TwoO_Fourcompared to
Claim 1 because the charging capacity is large.
From the section, it can be seen that the addition amount may be small. Its amount
As Li_TwoMn_TwoO _FourOr LiNiO_TwoAny of
Even in this case, 5 to 25 weight of the entire positive electrode active material is used.
%, Especially Li_TwoMn_TwoO_FourAbout
5-20% by weight, LiNiO_TwoAbout 6 to 2
More preferably, it is 1% by weight. It should be noted that such small
The fact that the amount is added means that the irreversible capacity is compensated.
In addition to obtaining a positive effect, the cathode of the present invention can be made lightweight.
And that it is possible. Apart from these, Li
Mn_TwoO_FourFor the powder, the above Li_TwoMn_TwoO_FourPowder and L
iNiO_TwoAn embodiment in which powder is added simultaneously may be used.
No. Even in such a case, the same effect as above can be obtained.
It is clear that

Finally, in the positive electrode as described above, if the lithium ion secondary battery (claim 7) having both the positive electrode and the negative electrode made of a carbon material is constituted, the lightweight In addition to the above, it is possible to obtain the effect of improving the charge capacity density and the weight energy density.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. Positive electrode, negative electrode, electrolyte,
In a non-aqueous electrolyte type lithium ion secondary battery having a separator or the like as a main component, a positive electrode is made of an active material obtained by mixing LiMn ₂ O ₄ particles and large charge capacity positive electrode active material particles.

Among the lithium composite oxides that release lithium ions, Li ₂ Mn ₂ O ₄ or LiNiO ₂ ,
Like Li ₃ V ₂ O ₅ , the initial charge capacity is large, but there is a case where the capacity is remarkably reduced with the charge / discharge cycle. This is hereinafter referred to as “large charge capacity positive electrode active material”. Since these materials are not practical as a positive electrode active material of a lithium ion secondary battery, they have not received attention. On the other hand, the use of LiMn ₂ O ₄ as a positive electrode active material is advantageous in terms of resources as compared with other lithium composite oxides, the cost is low, and a stable charge capacity of about 120 mAh / g can be obtained. Therefore, it is considered preferable.

Therefore, in the present invention, by utilizing these material characteristics, the capacity reduction at the time of the initial charge when LiMn ₂ O ₄ is used for the positive electrode, that is, the large charge capacity for the amount corresponding to the amount of irreversible dissolution. The main purpose is to add a positive electrode active material in advance and compensate for the decrease in capacity. Note that cubic LiMn, which is a main component of the positive electrode active material,
_{For 2} O ₄ , LiMn ₂ -xAxO ₄ (element A is 2
Group A to Group 7A, Group 8 and Group 1 to 4B metals, 0 ≦ x <
As shown by the composition of 1), a composition in which the Mn site is replaced by another metal element is also included.

Further, as a large charge capacity positive electrode active material, L
i ₂ Mn ₂ O ₄ (initial charge capacity 220 mAh / g), LiNiO
2 (initial charge capacity _{200mAh / g), Li 3 V} 2 O 5 is (initial charge capacity 250 mAh / g) and the like are considered, LiMn ₂ O ₄
In the present invention, basically any lithium composite oxide may be selected as long as the lithium composite oxide has a value larger than the initial charge capacity. Specifically, it is considered that the most preferable material should be determined in consideration of these factors including the viewpoints of environmental resistance, cost, and life required for practical use. For example, among the above, Li ₃ V ₂
O _5, since the material cost and production cost can be expected to be high, if chosen from the above examples, it is desirable that preferably employs a _{Li 2 Mn 2 O 4, LiNiO} 2.

Regarding the composition of the positive electrode active material, LiMn ₂ O ₄
(100-x)% by weight, and add the large charge capacity positive electrode active material to x
If the composition ratio x is 5% by weight in any case of Li ₂ Mn ₂ O ₄ to LiNiO ₂ , the composition ratio x may be 5 to 25% by weight. since capacity is about 10 to 30% of the initial charge capacity, about 5 to 20 wt% in Li ₂ Mn ₂ O _4,
More preferably, the content of LiNiO ₂ is about 6 to 21% by weight.

For example, the initial charge capacity of the negative electrode active material is 360 m
Ah / g and an irreversible capacity of 60 mAh / g, LiMn ₂ O ₄
If used alone as the positive electrode active material, 360/120 = 3 g of Li
Mn ₂ O ₄ is required. On the other hand, if the irreversible capacity at the time of the first charge is covered by Li ₂ Mn ₂ O ₄ , the necessary addition amount is 0.27 g (= 60/220 = (irreversible capacity) / (Li ₂ Mn ₂ O ₄ alone) Initial charge capacity)), 2.5 g of LiMn ₂ O ₄ (= 300/120 =
(Initial charge capacity of positive electrode active material−irreversible capacity) / (LiMn ₂
With O ₄ initial charge capacity alone)), the positive electrode active material weight is a total 2.77 g. Therefore, in this case, Li
It is lighter than the positive electrode of Mn ₂ O ₄ alone, and the weight energy density of the battery is improved. Further, since the large-capacity positive electrode active material to be added is small, the cost of the material is almost the same as that of the conventional one. The large-capacity positive-electrode active material used here does not need to be limited to single use, and may be used as a mixture of two or more.

Hereinafter, a method for manufacturing the above-described positive electrode or a lithium ion secondary battery including the positive electrode, or suitable materials constituting the same will be described. As a method for producing the positive electrode active material LiMn ₂ O ₄ was mixed with a large charge capacity positive electrode active material Li2Mn2O4 predetermined amount, a conductive material such as acetylene black, and mixing and stirring together with a binder such as polyvinylidene fluoride vinylidene After the film is formed by a doctor blade method, the film is dried. Also, LiNiO as a large charge capacity positive electrode active material
_Even in the case of using ₂ , the same manufacturing method may be used.

As the negative electrode material of the lithium ion secondary battery, various carbon materials can be used as long as they can smoothly take in and release lithium ions.
For example, graphite is a typical material.

Further, the electrolyte of the lithium ion secondary battery
For example, propylene carbonate, ethylene carbonate
-Carbonate, γ-butyrolactone, tetrahydrofura
2-methyltetrahydrofuran, dioxolane, 4-
Methyl dioxolane, sulfolane, 1,2-dimethoxye
Tan, dimethyl sulfoxide, acetonitrile, N, N-
Dimethylformamide, diethylene glycol, dimethyl
Aprotic solvents such as
LiBF as an electrolyte in a mixed solvent of two or more solvents _Four, L
iCLF_Four, LiAsF₆, LiSbF₆, LiAlO_Four,
LiAlCl_Four, LiPF₆Dissolves LiCl, LiI, etc.
What was made can be used. The separator is
Polypropylene such as a commonly used porous polypropylene nonwoven fabric
An olefin-based porous membrane can be used.

The positive electrode for a lithium ion secondary battery of the present invention manufactured as described above is formed by combining the above-described negative electrode made of a carbon material and various non-aqueous electrolytes with lithium. It can be used as an ion secondary battery. The positive electrode obtained by adding Li ₂ Mn ₂ O _4, which is a positive electrode active material having a large charge capacity, to LiMn ₂ O ₄ provides a high initial charge capacity offset by irreversible capacity, and also has excellent cycle characteristics. A lightweight lithium ion secondary battery can be obtained.

The above-mentioned positive electrode can be manufactured and used in various forms depending on the shape and use of the battery. For example, the form is a disk shape,
Plate-like, film-like, film-like, sheet-like, etc. are conceivable. Also, the thickness of the positive electrode can be appropriately selected according to the form, application, and shape.

[0025]

EXAMPLES Hereinafter, examples based on the present invention confirmed by the present inventors will be described more specifically. First,
The case where Li ₂ Mn ₂ O ₄ is used as the large charge capacity positive electrode active material to be added will be described. This Li ₂ Mn ₂ O ₄ was synthesized as follows. That is, after slowly adding lithium iodide and LiMn ₂ O ₄ to acetonitrile, mixing and stirring, heating and circulating for 4 hours, filtering the solid substance under reduced pressure using a 0.45 μm filter, and washing well with acetonitrile. , Dried with hot air at 110 ° C and stored in a dry state.

Next, the production of the positive electrode was performed as follows.
And the average particle size of 10 [mu] m LiMn ₂ O ₄ grains average particle size of 10μm Li ₂ Mn ₂ O ₄ was manufactured as the in (manufactured by Tosoh Corporation) of was added 10 to 40 wt%. These powders were mixed with N-methylpyrrolidone dissolved in polyvinyldene fluoride (Kureha) as a binder to form a slurry. This slurry is 20m thick into a 20μm aluminum foil by the doctor blade method.
A film was formed at a film thickness of about 400 μm by g / cm ^{2 and} dried at 70 ° C. for 3 hours. Further, the sample was dried at 110 ° C. overnight while reducing the pressure with an oil rotary vacuum pump to obtain a positive electrode sample.

On the other hand, for the negative electrode, a natural graphite raw material having an average particle size of 10 μm was used. The natural graphite granules are mixed with a solution of N-methylpyrrolidone dissolved in polyvinyldene fluoride (Kureha) to form a slurry, and a 7 mg / cm ² film is formed on a copper foil using a doctor blade method at 70 ° C. Dried for 3 hours. Further, the sample was dried at 110 ° C. overnight while reducing the pressure with an oil rotary vacuum pump to obtain a negative electrode sample.

FIG. 1 is a schematic diagram of a cell for charge / discharge test. The positive electrode 1 was manufactured as described above, and a nickel positive electrode lead 3 was welded to an aluminum foil 2 provided on one surface of the formed positive electrode to obtain a positive electrode. The negative electrode 5 has a thickness of 20
A negative electrode 7 was produced by pressure bonding to a μm copper foil 6, and a nickel negative electrode lead 8 was welded to the copper foil 6. Further, an electrolytic solution 9 was prepared by dissolving LiBF4 in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio 1: 2) to a concentration of 1 mol / l.

Using the obtained positive electrode 4, negative electrode 7, and electrolytic solution 9, a cell 10 for charge / discharge test shown in FIG. 1 was prepared. The charge / discharge test cell 10 is mainly composed of a glass container 11 and a silicone rubber stopper 12, and the glass container 11 can be hermetically sealed by the silicone rubber stopper 12.

The positive electrode 4 and the negative electrode 7 are made of silicon rubber by piercing the positive electrode lead 3 and the negative electrode lead 8 into a silicone rubber stopper 12 so that the tips of the respective leads 3 and 8 come out of the charge / discharge test cell. It was fixed to the stopper 12. The positive electrode lead 3 and the negative electrode lead 8 were connected to a potentiogalvanostat (not shown).

The positive electrode 4 and the negative electrode 7 were covered with a porous film made of polypropylene as the separator 13.
Further, after the electrolytic solution 9 was injected into the glass container 11, the glass container 11 was sealed with a silicone rubber stopper 12.

The positive electrode was charged to 4.3 V at a constant current of 50 mA / g per positive electrode active material using a potentiogalvanostat. Next, at a constant current of 50 mA / g per positive electrode active material, 3.0 V
The battery was charged and discharged in a voltage range of 3.0 to 4.3 V.

The results are shown in FIG. 2 and FIG. FIG.
Accordingly, the LiMn ₂ O ₄ positive electrode, a large charge capacity positive electrode active material L
By adding i ₂ Mn ₂ O ₄ , the initial charge capacity of the positive electrode increases linearly. FIG. 3 shows that Li ₂ Mn ₂ O
The discharge capacity of the positive electrode is increased by increasing the addition amount of ₄ , and the discharge capacity does not decrease due to the increase in the number of cycles, and the relationship due to the difference in the addition amount is maintained. It can be seen that it also contributes to improvement.

As described above, the addition amount of Li ₂ Mn ₂ O ₄ is sufficient if it is added in an amount corresponding to the irreversible capacity, and further addition may cause deterioration of the positive electrode and deterioration of cycle characteristics. There is.

Next, FIGS. 4 and 5 will be described. FIG.
Shows the relationship between the initial charge capacity of a positive electrode in which LiNiO ₂ is added to LiMn ₂ O ₄ instead of the above Li ₂ Mn ₂ O ₄ and the amount of LiNiO ₂ added. FIG. 5 also shows the relationship between the number of discharge capacity density cycles of the positive electrode obtained by adding LiNiO ₂ to LiMn ₂ O ₄ . From FIG. 4, it can be seen that the initial charge capacity of the positive electrode increases linearly as in the case shown in FIG. Also, from FIG. 5, as in the case shown in FIG.
It increases with an increase in the amount of iNiO ₂ added, and does not decrease with an increase in the number of cycles, indicating that this also contributes to an improvement in cycle characteristics.

In the above case, the large charge capacity positive electrode active material LiNiO _{2 to be} added is different from that used by Fuji Chemical with an average particle diameter of 10 μm. The configuration, charge / discharge test conditions, etc.
Is the same as the embodiment relating to ₂ Mn ₂ O _4.

[0037]

As described above, according to the positive electrode of the lithium ion secondary battery according to the first aspect, by adding a small amount of the high-charge-capacity positive-electrode active material to LiMn ₂ O ₄ , it is possible to reduce the initial time. This will compensate for the irreversible capacity associated with charging and discharging. In other words, the irreversible capacity is mainly covered by the large charge capacity positive electrode active material and the like, so that the charge capacity can be kept large. Also, since a small amount of the large charge capacity positive electrode active material or the like is sufficient, the positive electrode can be made lightweight, and when used as a lithium ion secondary battery, the charge capacity density and the weight energy density can be improved. it can. In addition,
The amount of the large-capacity positive electrode active material to be added is preferably 5 to 25% by weight based on the composition of the entire positive electrode active material.

As the positive active material for both high charging and charging, Li ₂ Mn ₂ O ₄ powder (Claim 2) and LiNiO ₂ powder (Claim 4) may be selected. In these cases, the composition of Li ₂ Mn ₂ O ₄ with respect to the whole positive electrode active material is 5 to 25% by weight (Claim 3), and the composition of LiNiO 2 powder is also 5 to 25% by weight (Claim 5). Good. Furthermore, a mode in which the above Li ₂ Mn ₂ O ₄ and LiNiO ₂ are simultaneously added is also possible (claim 6).
In any case, according to these inventions, various effects such as the above-described lightening of the positive electrode, the improvement of the charge capacity density and the improvement of the weight energy density can be similarly enjoyed.

Further, the positive electrode according to any one of claims 1 to 6 may be used in combination with a negative electrode made of a carbon material to form a lithium ion secondary battery having these electrodes as both electrodes (claim 7). Of course you can.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a configuration example of a lithium ion secondary battery according to the present invention.

FIG. 2 Li ₂ Mn ₂ O ₄ as a large charge capacity positive electrode active material
4 is a graph showing the relationship between the amount of addition and the initial charge capacity when added.

FIG. 3 Li ₂ Mn ₂ O ₄ as a large charge capacity positive electrode active material
5 is a graph showing the relationship between the cycle characteristics of the positive electrode and the discharge capacity when Li was added as a parameter, with the amount of Li ₂ Mn ₂ O ₄ added as a parameter.

FIG. 4 is a graph showing the relationship between the amount of LiNiO ₂ added as a large charge capacity positive electrode active material and the initial charge capacity.

FIG. 5 is a graph showing the relationship between the cycle characteristics of the positive electrode and the discharge capacity when LiNiO ₂ is added as a large charge capacity positive electrode active material, using the amount of LiNiO ₂ added as a parameter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Aluminum foil 3 Positive electrode lead 4 Positive electrode 5 Negative electrode 6 Copper foil 7 Negative electrode 8 Negative lead 9 Electrolyte 10 Charge / discharge test cell 11 Glass container 12 Silicon rubber stopper

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H01M 10/40 H01M 10/40 Z (72) Inventor Kazuyuki 2-1-1, Shiobara, Minami-ku, Fukuoka City, Fukuoka Prefecture Inside Kyushu Electric Power Co., Inc. (72) Inventor Tsutomu Hashimoto 1-1, Akunouracho, Nagasaki City, Nagasaki Prefecture Mitsubishi Heavy Industries, Ltd.Nagasaki Shipyard (72) Inventor Hidehiko Tajima 1-1-1, Akunouracho, Nagasaki City, Nagasaki Prefecture Inside Mitsubishi Heavy Industries, Ltd.Nagasaki Shipyard (72) Inventor Kazuo Akiyama 1-1, Akunouramachi, Nagasaki City, Nagasaki Prefecture Mitsubishi Heavy Industries, Ltd.Nagasaki Shipyard (72) Inventor Katsuo Hashizaki 2-5-1 Marunouchi, Chiyoda-ku, Tokyo No. F-term (reference) in 3 RHI Heavy Industries Ltd. 4G048 AA04 AB01 AC06 AD06 5H003 AA02 BA03 BB04 BB05 BC01 BC06 BD03 BD04 5H014 AA02 BB06 EE10 HH01 5H029 AJ03 AK03 AL06 AM02 AM03 AM04 AM05 AM07 CJ08 DJ17 HJ01

Claims (7)

[Claims] 1. A cathode active material obtained by simply mixing a cubic LiMn ₂ O ₄ powder and a large charge capacity cathode active material powder,
The positive electrode of a lithium ion secondary battery, wherein the composition of the large charge capacity positive electrode active material with respect to the positive electrode active material is 5 to 25% by weight. 2. A cubic LiMn ₂ O ₄ powder containing tetragonal L
A positive electrode for a lithium ion secondary battery, comprising a positive electrode active material obtained by simply mixing i ₂ Mn ₂ O ₄ powder. 3. The positive electrode of a lithium ion secondary battery according to claim 2, wherein the composition of the Li ₂ Mn ₂ O ₄ powder with respect to the positive electrode active material is 5 to 25% by weight. 4. A cubic LiMn ₂ O ₄ powder containing hexagonal L
A positive electrode for a lithium ion secondary battery, comprising a positive electrode active material obtained by simply mixing iNiO ₂ powder. 5. The positive electrode of a lithium ion secondary battery according to claim 4, wherein the composition of the LiNiO ₂ powder with respect to the positive electrode active material is 5 to 25% by weight. 6. A cubic LiMn ₂ O ₄ powder containing tetragonal L
A positive electrode for a lithium ion secondary battery, comprising a positive electrode active material obtained by simply mixing iMn ₂ O ₄ powder and hexagonal LiNiO ₂ powder. 7. A lithium ion secondary battery comprising the positive electrode of the lithium ion secondary electrode according to claim 1 and a negative electrode made of a carbon material.