JP2511667B2 - Lithium secondary battery - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a lithium secondary battery, and more particularly to improvement of a positive electrode active material thereof.

[Conventional technology]

Conventionally, sulfides such as titanium disulfide and molybdenum disulfide have been proposed as positive electrode active materials for lithium secondary batteries, and some have already been put into practical use.

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

Therefore, to obtain a battery with higher energy density,
The use of LiCoO ₂ as a positive electrode active material has been studied (eg, US Pat. No. 4,567,031).

However, the relationship between the charge / discharge cycle and the deterioration of capacity when this LiCoO ₂ is used in a secondary battery has not been reported at all.

Then, the present inventors examined the charge / discharge cycle characteristics of the lithium secondary battery using this LiCoO ₂ as the positive electrode active material, and found that the capacity deterioration associated with the charge / discharge cycle was large.

[Problems to be solved by the invention]

An object of the present invention is to solve the problem that a battery using a LiCoO _{2 based} active material causes capacity deterioration with charge / discharge cycles, and to provide a lithium secondary battery with good charge / discharge cycle characteristics. .

[Means for solving problems]

According to the present invention, a small amount of Ni is dissolved in LiCoO ₂ so as to suppress capacity deterioration due to charge / discharge cycles.

That is, according to the present invention, as the positive electrode active material, lithium synthesized in a state represented by the formula (I) Li (Co _1-y N _y ) O ₂ (I) (where y is 0.1 ≦ y ≦ 0.4). Formula (II) Li _x (Co _1-y N _y ) O ₂ (II) obtained by removing a part of lithium from (cobalt-nickel) oxide by charging (where x is 0 <x <1 and y Is 0.1 ≦ y ≦ 0.4)
By using the lithium (cobalt-nickel) oxide represented by the above, it is possible to suppress the capacity deterioration due to the charge / discharge cycle and provide a lithium secondary battery having good charge / discharge cycle characteristics. Further, in the battery using the lithium (cobalt-nickel) oxide represented by the above formula (II) as the positive electrode active material, the battery voltage is about 3.5 to 4.6 V (Li is 0 to 0).
Since the voltage fluctuates in the range of 1, the battery voltage also fluctuates accordingly, and a higher energy density can be expected compared to a battery using a sulfide-based active material.

In the present invention, as described above, a small amount of Ni is dissolved in LiCoO ₂ to suppress the capacity deterioration due to the charge / discharge cycle, but Li _x (Co _1-y
The reason why Ni _y ) O ₂ suppresses the deterioration of the capacity due to the charge / discharge cycle is considered to be as follows.

First, considering the main causes of the capacity deterioration of the secondary battery with charge / discharge cycles, the following three points can be mentioned.

 Physical changes such as pulverization of active material particles.

 Changes in the crystal structure, bonding state, composition, etc. of the active material.

A reaction between an active material and an electrolytic solution (for example, in Li _x CoO ₂ , oxygen is dissociated and released to oxidize or polymerize the electrolytic solution).

However, in Li _x (Co _1-y Ni _y ) O ₂ containing Ni, LiC
The open circuit voltage is lower than that of oO ₂ , and this lowering of the voltage reduces the oxidizing power, and as a result, the reactivity with the electrolytic solution decreases,
It is considered that the capacity deterioration due to the charge / discharge cycle is suppressed. That is, it is considered that Li _x (Co _{1 -y} Ni _y ) O ₂ suppresses the capacity deterioration due to the charge / discharge cycle by suppressing the occurrence of the capacity deterioration cause.

However, if the addition amount of Ni is increased, the charge / discharge cycle characteristics are rather deteriorated. The cause of this is not clear at present, but it is considered that the deterioration due to the above-mentioned change becomes large in the Ni-rich region.

In the present invention, the value of x in the formula of the lithium (cobalt-nickel) oxide represented by Li _x (Co _1-y Ni _y ) O ₂ is set to 0 <x <1 by the charging and discharging. , Li can be freely changed within the above range, and any value can be used. With respect to the value of y, the value of y is set to 0.1 ≦ y ≦ 0.4 due to the above-mentioned relationship. However, when the value of y is smaller than 0.1, N is solidified.
Since the amount of i is too small, the effect of suppressing capacity deterioration due to charge / discharge cycles is not sufficiently exerted. On the other hand, when the value of y is larger than 0.4, the cause of deterioration is large as described above due to the increase in the amount of Ni. This is because the charge / discharge cycle characteristics are rather deteriorated.

〔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, and y was 0.3. When this is displayed according to formula (I), Li (Co _0.7 Ni _0.3 ) O ₂
Is.

The synthesis was performed as shown below. First, Co and Ni
Co-precipitate as a carbonate (under normal conditions it becomes a basic carbonate) from an aqueous solution containing Ni and Ni ions to form a uniform mixture, wash the precipitate thus obtained with water and then in argon. After drying at 140 ℃, it is mixed with Li ₂ CO ₃ and heated in air (N ₂ / O ₂ = 80/20) at 920 ℃ for 3 hours to react, air quench (heated sample at 25 ℃ Of the Li (Co)
_0.7 Ni _0.3 ) O ₂ was synthesized. The boat used to store the sample for heating the sample is mainly composed of Al ₂ O ₃ .

The coprecipitation of Co and Ni in the aqueous solution was performed as follows.

The molar ratio of nickel and cobalt is 30:70 [Ni / Co = 30/7
0 was dissolved NiCl · 6H ₂ O and CoCl ₂ · 6H ₂ O (mole ratio)] in pure water saturated with carbon dioxide gas, the solution NaHCO ₃ aqueous solution was added, the coprecipitated standing.

Using Li (Co _0.7 Ni _0.3 ) O ₂ synthesized as described above, 10% by weight of flake graphite was added as an electron conduction aid.
After adding and mixing at the ratio of, the mixture is pressure-molded at 3 t / cm ² ,
A molded body having a diameter of 9 mm and a thickness of about 0.3 mm was produced. Using the obtained molded body as a positive electrode, the battery shown in FIG. 1 (model cell)
Was produced.

In FIG. 1, part A is an enlarged view of a main part of the battery, in which 1 is a negative electrode, and this negative electrode 1 is 1% Li _0.1 V ₂ O ₅ powder.
0% by weight of scaly graphite and 5% by weight of polytetrafluoroethylene were added, mixed and pressure-molded. Diameter 16
mm, and a thickness of about 2 mm. And
Li _0.1 V ₂ O ₅ used as the negative electrode active material
It was synthesized by reacting ₂ O ₅ with η-butyllithium (η-C ₄ H ₉ Li). 2 is a positive electrode, and this positive electrode 2 is Li _x (Co _0.7 Ni _0.3 ) O ₂ (where the formula is Li (Co _0.7 Ni _0.3 ) O ₂ synthesized as described above, in which a part of lithium is extracted by charging. And x is 0 <x <1) as a positive electrode active material.

3 is a solution obtained by dissolving 1 mol / LiBF ₄ of a mixed solvent of propylene carbonate and 1,2-dimethoxyethane in a volume ratio of 2: 1, and 4 is a separator made of polypropylene nonwoven fabric. Reference numeral ₅ is a reference electrode made of a pressure-molded body using Li _0.1 V ₂ O ₅ as an active material, 6 is a polypropylene container, and 7 is a collector made of a platinum expanded mesh spot-welded with a platinum lead wire. Is.

And the theoretical quantity of electricity of the positive electrode of this battery is in the charge / discharge region.
Li _x (Co _1-y N _y ) O ₂ (0 <x <1) is 15 mAh, and the theoretical amount of electricity of the negative electrode is 70 mAh when the charge / discharge region is Li _x V ₂ O ₅ (0 ≦ x ≦ 1). The amount of electricity of the negative electrode is set to be more than the amount of electricity of the positive electrode.

Example 2 The y value of Li (Co _1-y Ni _y ) O ₂ was synthesized to 0.1, and this Li (C
o _0.9 Ni _0.1 ) L with some lithium removed from O ₂ by charging
A battery was prepared in the same manner as in Example 1 except that i _x (Co _0.9 Ni _0.1 ) O ₂ (where x in the formula is 0 <x <1) was used as the positive electrode active material.

Example 3 The y value of Li (Co _1-y Ni _y ) O ₂ was synthesized to be 0.2, and this Li (C
o _0.8 Ni _0.2 ) L with some lithium removed from O ₂ by charging
A battery was prepared in the same manner as in Example 1 except that i _x (Co _0.8 Ni _0.2 ) O ₂ (where x in the formula is 0 <x <1) was used as the positive electrode active material.

Example 4 The y value of Li (Co _1-y Ni _y ) O ₂ was synthesized to 0.4, and this Li (C
o _0.6 Ni _0.4 ) L with the lithium partially removed from O ₂
A battery was produced in the same manner as in Example 1 except that i _x (Co _0.6 Ni _0.4 ) O ₂ (where x in the formula is 0 <x <1) was used as the positive electrode active material.

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

Comparative Example 2 The y value of Li (Co _1-y Ni _y ) O ₂ was synthesized to 0.5, and this Li (C
o _0.5 Ni _0.5 ) L with some lithium removed from O ₂ by charging
A battery was produced in the same manner as in Example 1 except that i _x (Co _0.5 Ni _0.5 ) O ₂ (where x in the formula is 0 <x <1) was used as the positive electrode active material.

Comparative Example 3 The y value of Li (Co _1-y Ni _y ) O ₂ was synthesized to be 0.6, and this Li (C
o _0.4 Ni _0.6 ) L with some lithium removed from O ₂ by charging
A battery was made in the same manner as in Example 1 except that i _x (Co _0.4 Ni _0.6 ) O ₂ (where x in the formula is 0 <x <1) was used as the positive electrode active material.

Next, the batteries of Examples 1 to 4 and Comparative Examples 1 to 3 above.
The battery was charged and discharged. Charge and discharge are 0.318A (0.5mA / cm ² per unit area of positive electrode) for both charging and discharging currents.
Done between 1.0 and 0V voltage.

The batteries of Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1.
The number of cycles that can be charged / discharged in the charge / discharge cycle test of the battery is shown. However, the number of cycles that can be charged and discharged shown in Table 1 is determined as the end point when the capacity deteriorates to 50% of the charge and discharge capacity value at the relatively early stage of charging and discharging (specifically, the fifth cycle). It is a thing. In addition, in Table 1, in order to clarify the change in the number of cycles with the change in the Ni content, they are arranged and displayed in the order of the Ni content (that is, the value of y). Therefore, the display order is different from the order described in the examples and comparative examples described above.

As shown in Table 1, the batteries of Examples 1 to 4 had a larger number of chargeable and dischargeable cycles than the battery of Comparative Example 1 in which LiCoO ₂ containing no Ni was used as the positive electrode active material. The effect of addition and solid solution is recognized. However, in the batteries of Comparative Example 2 and Comparative Example 3 in which the amount of Ni was large, the number of cycles was rather small. It is considered that this is because, as described above, the change in the crystal structure, bonding state, composition, etc. of the active material became large due to charge / discharge due to the increase in the amount of Ni.

From the above results, Ni is a solid solution amount in a composition region of 10 to 40 mol%, that is, Li _x (Co _1-y N _y ) O ₂ (0.1 ≦ y ≦ 0.4), and cycle deterioration is small, which is preferable as a positive electrode active material. It became clear that it was a thing.

In the above example, a model cell test was conducted to investigate the capacity deterioration due to charge / discharge cycles.This shows that in the mounted battery, the influence of battery constituent members other than the positive electrode active material such as the negative electrode appears, This is because it is difficult for the difference in capacity deterioration due to the charge / discharge cycle due to the difference in active material to appear accurately.

In addition, in the above examples and the like, Li (Co _1-y Ni _y ) O ₂ was synthesized by coprecipitation of Co and Ni with a carbonate, but an oxide containing Ni (for example, NiO) and an oxide containing Co ( For example, CoO) may be mixed by a ball mill or the like, then mixed with lithium carbonate or lithium oxide, and heated to synthesize. Alternatively, it may be synthesized using an oxide containing both Co and Ni, such as NiCoO ₂ .

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 (CoNi) portion may be slightly in excess of the stoichiometric ratio in the above formula (within a range of about 5% or less). Further, in oxides, O ₂ may become O _2−δ (δ ≦ 0.3) due to oxidation defects, and these are also included in the scope of the present invention.

Further, in the examples, as the electrolytic solution was used a solution of LiBF ₄ dissolved in a mixed solvent of propylene carbonate and 1,2-dimethoxyethane, instead, other electrolytic solution, for example LiBF ₄ in propylene carbonate. A dissolved electrolytic solution may be used.

The above shows the case where Ni is dissolved in Co, but the same effect is expected when two or more kinds of metals obtained by adding transition metals such as Ti, V, Cr, and Fe to Co are dissolved in Co. it can.

〔The invention's effect〕

As described above, in the present invention, ₁₀ to 40mo
Li (Co _1-y Ni _y ) O ₂ (I) in which 1% of Ni is solid-solved (wherein y is 0.1 ≦ y ≦ 0.4), lithium (cobalt is synthesized) -(Ni) oxide (II) Li _x (Co _1-y Ni _y ) O ₂ (II) in which a part of lithium is removed by charging (where x is 0 <x <1 and y is 0.1 ≦ y ≦ 0.4)
By using the lithium (cobalt-nickel) oxide represented by as the positive electrode active material, the charge-discharge cycle characteristics could be improved as compared with the case where LiCoO ₂ was used as the positive electrode active material.

[Brief description of 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

Claims (1)

(57) [Claims] 1. A lithium secondary battery is synthesized into a state represented by the formula (I) Li (Co _1-y Ni _y ) O ₂ (I) (where y is 0.1 ≦ y ≦ 0.4). (II) Li _x (Co _1-y Ni _y ) O ₂ (II) (where x is 0 <x <1) in which a part of lithium is removed from the lithium (cobalt-nickel) oxide by charging. , Y is 0.1 ≦ y ≦ 0.4)
A lithium secondary battery characterized by using a lithium (cobalt-nickel) oxide represented by as a positive electrode active material.