JP2699176B2 - Lithium secondary battery - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a lithium secondary battery, and more particularly, to improvement of a positive electrode active material thereof.

[Conventional technology]

Conventionally, metal sulfides such as titanium disulfide and molybdenum disulfide have been used as positive electrode active materials for lithium secondary batteries.

However, these metal sulfide-based positive electrode active materials have a problem that the battery voltage is low from the viewpoint of obtaining a battery having a battery voltage of 3 V or less and a high energy density.

Therefore, to obtain batteries with higher energy density,
Use of LiCoO ₂ as a positive electrode active material has been studied (for example, US Pat. No. 4,567,031).

Although the relationship between the charge / discharge cycle and capacity degradation when using this LiCoO ₂ in a secondary battery has not yet been reported, LiCoO ₂
When O ₂ is used as the positive electrode active material, the voltage is as high as 4.5 to 3.9 V, so the decomposition of the electrolytic solution (more precisely, the decomposition of the organic solvent used as the solvent of the electrolytic solution by oxidation reaction or polymerization) may occur. It is thought to occur.

Among the organic solvents used as the electrolyte solvent, even propylene carbonate, which has excellent oxidation resistance, is superior to platinum.
At 25 ° C, the lithium electrode is oxidized from 4.2V to 4.5V and starts to decompose, generating carbon dioxide (CO ₂ ) [G. Eggert et al., Electrochimica Acta., 31 (11), 144
3 (1986) and others].

In addition, many polymer electrolytes are decomposed from around 4V.

Therefore, to use the currently known organic electrolyte, it is necessary to limit the upper limit of the battery voltage to around 4V,
It is desirable to prevent decomposition of the electrolyte.

Then, the present inventors charged and discharged a lithium secondary battery using LiCoO ₂ as a positive electrode active material in a voltage range of 4 V or less, and found that the available charge / discharge capacity was small.

[Problems to be solved by the invention]

The present invention is used when a lithium secondary battery using LiCoO ₂ which is expected as a high energy density battery as a positive electrode active material is charged and discharged in a voltage range of 4 V or less from the viewpoint of preventing decomposition of an electrolyte solution. It is an object of the present invention to provide a lithium secondary battery that solves the problem that the charge / discharge capacity that can be obtained is small and that can obtain a large charge / discharge capacity even in a charge / discharge voltage range in which decomposition of an electrolytic solution can be prevented.

[Means for solving the problem]

The present invention reduces the open circuit voltage by dissolving Ni in LiCoO ₂ so that a large charge / discharge capacity can be obtained even in a charge / discharge voltage range of 4 V or less.

That is, according to the present invention, as the positive electrode active material, coprecipitated into a state represented by the formula (I) Li (Co _1-y Ni _y ) O ₂ (I) (where y is 0.75 <y ≦ 0.9). Formula (II) Li _x (Co _1-y Ni _y ) O ₂ (II) wherein lithium is partially removed by charging from lithium (cobalt-nickel) oxide synthesized by the method, wherein x is 0 < The present invention relates to a lithium secondary battery using a lithium (cobalt-nickel) oxide represented by x <1 and y is 0.75 <y ≦ 0.9.

The reason why the use of the lithium (cobalt-nickel) oxide represented by the above formula (II) lowers the open circuit voltage and enables a large charge / discharge capacity to be obtained in a voltage range of 4 V or less is as follows. Can be considered.

The open circuit voltage of a lithium secondary battery using Li _x CoO ₂ as a positive electrode active material is in the range of 4.6 to 3.9V. That is, x is 0 to 1
The open circuit voltage fluctuates in the range of 4.6 to 3.9 V with the change in the range. The range of the amount of Li falling within the voltage range of 4 V or less is only 0.7 ≦ x ≦ 1. Moreover, since the polarization cannot be used greatly in the range where x approaches 1, the actually usable range is further narrowed. Therefore, Li can be changed only in a very narrow range, and the range available for charge and discharge is very narrow, so that the obtained charge and discharge capacity is small.

On the other hand, in the lithium secondary battery using Li _x (Co _1-y Ni _y ) O ₂ as the positive electrode active material, the open circuit voltage is in the range of slightly more than 4 V to 3.5 V (x is 0 to 0 as in the above case). The open-circuit voltage fluctuates in the range of slightly more than 4V to 3.5V as it changes between 1). And
The region of x that falls within the voltage range of 4 V or less (that is, the range of the amount of Li) differs depending on the amount of solid solution of Ni (that is, the y value).
When y = 0.8, the range is 0.45 ≦ x ≦ 1, and the range where x can be changed is wider than in the case of Li _x CoO ₂ . Therefore,
The range that can be charged and discharged in a voltage range of 4 V or less is widened, and the obtained charge and discharge capacity is increased.

In the present invention, Li _x (C) of the formula (II) indicating lithium (cobalt-nickel) oxide used as a positive electrode active material is used.
o _1-y Ni _y ) In O ₂ , the reason why y is in the range of 0.75 <y ≦ 0.9 is that when y exceeds 0.9 (that is, when the amount of solid solution of Ni increases), the polarization increases, This is because the charge / discharge capacity decreases, and if y becomes too small (that is,
The lower the solid solution amount of Ni), the smaller the solid solution amount of Ni, the lower the open circuit voltage is reduced, and the range in which x can be changed in the voltage range of 4 V or less is narrowed, and the obtained charge / discharge capacity is small. Because it becomes.

The above formula (II) used as a positive electrode active material in the present invention
The lithium (cobalt-nickel) oxide represented by
It is obtained based on synthesizing a lithium (cobalt-nickel) oxide represented by the formula (I) by a coprecipitation method. That is, when the coprecipitation method is used, the formula (I) or the formula (I
Even when y in I) is a high value of 0.75 <y ≦ 0.9, a lithium (cobalt-nickel) oxide having a large charge / discharge capacity can be obtained.
When it is increased to about 0.9, a lithium (cobalt-nickel) oxide having a large charge / discharge capacity cannot be obtained.

In the present invention, the co-precipitation method for obtaining lithium (cobalt-nickel) oxide refers to a method in which Co (cobalt) and Ni (nickel) are co-precipitated as a carbonate from an aqueous solution containing Co ions and Ni ions to obtain a uniform precipitate. It means a method of synthesizing lithium (cobalt-nickel) oxide through a process of forming a mixture.

In the battery of the present invention, lithium or a lithium alloy is used for the negative electrode. Examples of the lithium alloy used for such a purpose include a lithium-aluminum alloy, a lithium-tin alloy, a lithium-zinc alloy, and a lithium-lead alloy. , Lithium-bismuth alloy, lithium-silicon alloy, lithium-antimony alloy, lithium-magnesium alloy, lithium-indium alloy, lithium-gallium alloy, lithium-germanium alloy, lithium-
Gallium-indium alloy and the like can be mentioned. Further, those obtained by adding a small amount of another metal to these lithium alloys can also be used for the negative electrode.

As the electrolytic solution, those commonly used for this type of battery can be used without any particular limitation. Examples of the electrolyte include, for example, organic solvents such as 1,2-dimethoxyethane, ethylene carbonate, propylene carbonate, γ-butyrolactone, tetrahydrofuran, 1,3-dioxolan, and 4-methyl-1,3-dioxolan, alone or in combination. In the above mixed solvent, for example, LiClO ₄ , LiPF
₆ , one prepared by dissolving one or more of electrolytes such as LiAsF ₆ , LiSbF ₆ , LiBF ₄ and LiB (C ₆ H ₅ ) ₄ .

〔Example〕

Next, the present invention will be described in more detail with reference to examples.

Example 1 Li (Co _1-y Ni _y ) O ₂ was synthesized. y is 0.8. When this is expressed according to the formula (I), Li (Co _0.2 Ni _0.8 ) O ₂
It is.

The synthesis was performed as shown below. First, Co (cobalt) and Ni (nickel) were coprecipitated from an aqueous solution containing Co ions and Ni ions as a carbonate (under ordinary conditions, a basic carbonate) to form a uniform mixture. This coprecipitation method will be described later in detail. The precipitate obtained as above is washed with water, dried at 140 ° C. in argon, mixed with Li ₂ CO _3, and dried in air (N ₂ / O ₂ = 80/20), 920
The reaction was carried out by heating at a temperature of 3 ° C. for 3 hours, and Li (Co _0.2 Ni _0.8 ) O ₂ was obtained by air quenching (a method of taking out the heated sample into a room temperature atmosphere and rapidly cooling it).

The co-precipitation of Co and Ni from an aqueous solution containing Co ions and Ni ions was performed as follows.

NiCl ₂ .6H ₂ O and CoCl ₂ .6H ₂ O were saturated with carbon dioxide gas such that the molar ratio of Ni and Co was 80:20 [Ni / Co = 80/20 (molar ratio)]. The solution was dissolved in pure water, and an aqueous solution of NaHCO ₃ was added to the solution, and allowed to stand for coprecipitation.

Li (Co _0.2 Ni _0.8 ) O ₂ synthesized as described above was used, and flaky graphite was added as an electron conduction aid in an amount of 10% by weight.
After adding and mixing 5% by weight of polytetrafluoroethylene as a binder, press molding at 3t / cm ² is performed to form a disc having a diameter of 9mm and a thickness of about 0.3mm. A molded body was produced. Using the obtained molded body as a positive electrode, a battery (model cell) shown in FIG. 1 was produced.

In FIG. 1, part A is an enlarged view of only a main part of the battery. In the figure, reference numeral 1 denotes a negative electrode, and this negative electrode 1 denotes Li
_0.1 V ₂ O ₅ powder is mixed with 10% by weight of flaky graphite and 5% by weight of polytetrafluoroethylene, mixed and then pressed to produce a disk having a diameter of 16 mm and a thickness of about 2 mm. It is made of a shaped body. Li _0.1 V ₂ O ₅ used as the negative electrode active material was synthesized by reacting V ₂ O ₅ with n-butyllithium (nC ₄ H ₉ Li) in hexane. Reference numeral 2 denotes a positive electrode. This positive electrode 2 is Li _x (Co _0.2 Ni _0.8 ) O _{2 obtained} by extracting a part of lithium from Li (Co _0.2 Ni _0.8 ) O ₂ synthesized as described above, provided that the formula X inside is 0
<X <1) is used as a positive electrode active material, and is a pressure-formed body to which flaky graphite and polytetrafluoroethylene are added.

3 propylene carbonate and 1,2-dimethoxyethane and volume ratio of 2: electrolyte obtained by dissolving 1 mol / l of LiBF ₄ in a mixed solvent of 1, 4 is a separator made of polypropylene nonwoven. Reference numeral ₅ denotes a reference electrode made of a press-molded body using Li _0.1 V ₂ O ₅ as an active material. Reference numeral 6 denotes a polypropylene container. Reference numeral 7 denotes a current collector formed of a platinum expanded net formed by spot welding platinum lead wires. It is.

The theoretical amount of electricity of the positive electrode of this battery
Li _x (Co _1-y Ni _y ) O ₂ (0 <x <1), less than 15 mAh, and the theoretical charge of the negative electrode is Li _x V ₂ O ₅ (0 <x ≦ 1)
Is set to be 70 mAh, and the amount of electricity of the negative electrode is larger than the amount of electricity of the positive electrode.

Example 2 Li (Co _1-y Ni _y ) O ₂ was synthesized by a coprecipitation method with the value of y set to 0.9, and Li _{x obtained} by extracting a part of lithium from this Li (Co _0.1 Ni _0.9 ) O ₂ by charging. (Co _0.1 Ni _0.9 ) O ₂ (where x in the formula is
Is <0 <x <1) as in Example 1 except that a positive electrode active material was used.

Comparative Example 1 The value of y of Li (Co _1-y Ni _y ) O ₂ was set to 0, that is, LiCoO ₂ was synthesized, and part of lithium was extracted from the LiCoO ₂ by charging.
A battery was fabricated in the same manner as in Example 1, except that Li _x CoO ₂ (where x in the formula was 0 <x <1) was used as the positive electrode active material.

Comparative Example 2 Li (Co _1-y Ni _y ) O ₂ was synthesized by coprecipitation with the value of y being 0.4, and a part of lithium was extracted from Li (Co _0.6 Ni _0.4 ) O ₂ by charging to obtain Li _x. (Co _0.6 Ni _0.4 ) O ₂ (where x in the formula is
Is <0 <x <1) as in Example 1 except that a positive electrode active material was used.

Comparative Example 3 Li (Co _1-y Ni _y ) O _{2 having} a y value of 1.0, ie, LiN
Example 1 except that iO ₂ was synthesized and a part of lithium was extracted from LiNiO ₂ by charging to use Li _x NiO ₂ (where x in the formula is 0 <x <1) as a positive electrode active material. A battery was produced in the same manner as in Example 1.

Next, the batteries of Examples 1 and 2 and Comparative Examples 1 to 3
Was charged and discharged. The charge and discharge were performed at a charge current and a discharge current of 0.636 mA (1.0 mA / cm ² per unit sectional area of the positive electrode) in a voltage range of +0.6 V to −0.2 V with respect to the reference electrode. Li _0.1 V ₂ O _{5 of} reference pole is 3.4 against Li
Since it has a voltage of V, this voltage range is in a range of 4.0 V to 3.2 V when Li is used as a negative electrode.

Table 1 shows the batteries of Examples 1 and 2 and Comparative Examples 1 to 3.
Shows the charge / discharge capacity of the battery in the above voltage range. However,
This charge / discharge capacity is a value in the third cycle at which the charge / discharge is stabilized and the capacity becomes a substantially constant value. In the third cycle, the charge capacity and the discharge capacity have substantially the same value. In FIG. 1, in order to clarify the change in the charge / discharge capacity due to the change in the Ni amount, they are arranged and displayed in the order of the Ni amount (that is, the value of y). Therefore, the display order is different from the description order of the above-described embodiments and comparative examples.

As shown in Table 1, the batteries of Examples 1 and 2 using the positive electrode active material in which Ni was dissolved in solid solution were LiCo in which Ni was not dissolved.
Compared to the battery of Comparative Example 1 using O ₂ as the positive electrode active material,
Large charge / discharge capacity in the voltage range of 4.0 to 3.2V. This is N
As a result of solid solution of i, the open circuit voltage is reduced, and x changes within a voltage range of 4 V or less [that is, lithium ion (Li
⁺ ) Can be entered and exited]. However, when the solid solution amount of Ni (that is, the y value) is too large, the charge / discharge capacity decreases with an increase in the Ni amount. This is considered to be because if the amount of Ni is excessively increased, the polarization is increased and the capacity is decreased.

In the above example, a test using a model cell was performed to check the capacity during the charge / discharge cycle. However, in a mounted battery, the influence of battery components other than the positive electrode active material such as the negative electrode appears, This is because it becomes difficult for a difference in charge / discharge cycle capacity due to the difference to appear accurately. In addition, Li _x V ₂ O ₅ is used instead of lithium for the negative electrode. In the case of lithium, it easily reacts with oxygen and moisture, and the surface is
It changes to Li ₂ O and LiOH, etc., and the battery characteristics are easily affected by the Li ₂ O film and LiOH film formed on this lithium surface, but in the case of Li _x V ₂ O ₅ it is affected This is because the difference in battery characteristics due to the difference in the positive electrode active material can be accurately grasped.

In the present invention, the lithium (cobalt-nickel) oxide used as the positive electrode active material is Li _x (Co _1-y Ni _y ) O ₂
However, the transition metal portion (CoNi portion) sometimes deviates slightly from the stoichiometric ratio in the above formula (within a range of ± 5%). Further, oxygen may be shifted due to oxygen deficiency or the like, but since they hardly affect battery characteristics, they are also included in the category of the present invention.

Further, in the examples, a solution obtained by dissolving LiBF _{4 in} a mixed solvent of propylene carbonate and 1,2-dimethoxyethane was used as the electrolytic solution, but instead, another electrolytic solution, for example, LiBF ₄ was used in propylene carbonate. A dissolved electrolytic solution may be used.

〔The invention's effect〕

As described above, in the present invention, a specific amount of LiCoO ₂
Ni was dissolved in a solid solution. Formula (I) Li (Co _1-y Ni _y ) O ₂ (I) (where y is 0.75 <y ≦ 0.9) and was synthesized by a coprecipitation method. Formula (II) Li _x (Co _1-y Ni _y ) O ₂ (II) wherein lithium is partially removed by charging from lithium (cobalt-nickel) oxide (where x is 0 <x <1, y is 0.75 <y ≦ 0.9)
Using lithium (cobalt-nickel) oxide as a positive electrode active material, the charge / discharge capacity in a voltage range of 4 V or less that can ensure the stability of the electrolyte solution, compared to using LiCoO ₂ as the positive electrode active material Could be improved.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a lithium secondary battery according to the present invention. 1 ... negative electrode, 2 ... positive electrode

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Toshikatsu Mabe 1-1-88 Ushitora, Ibaraki-shi, Osaka Hitachi Maxell, Ltd. (56) References JP-A-63-299056 (JP, A) JP-A-62 −256371 (JP, A)

Claims (1)

(57) [Claims] In a lithium secondary battery, coprecipitated into a state represented by the formula (I) Li (Co _1-y Ni _y ) O ₂ (I) (where y is 0.75 <y ≦ 0.9). Formula (II) Li _x (Co _1-y Ni _y ) O ₂ (II) wherein lithium is partially removed by charging from lithium (cobalt-nickel) oxide synthesized by the method, wherein x is 0 < A lithium secondary battery using a lithium (cobalt-nickel) oxide represented by the formula x <1 and y is 0.75 <y ≦ 0.9) as a positive electrode active material.