JPH0878006A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a lithium secondary battery with high energy density, allowable to deep charge/discharge, and with long life by constituting a positive active material with a specified composite oxide. CONSTITUTION: A positive electrode active material is made of a composite oxide (example: Li1.03 Ni0.89 Co0.10 B0.01 O2 ) having layer structure represented by a formula of Lia Nib Mc <1> Md <2> O2 (M<1> is Co, and M<2> is at least one selected from B, Al, In, and Sn.). For example, the active material is mixed with acetylene black and polytetrafluoroethylene powder, the mixture is molded to form a positive electrode 1, the positive electrode 1 is pressed into a positive can 4 with a positive current collector 6, and a negative electrode (example: lithium foil) 2 is pressed in a negative can 5 through a negative current collector 7, then an electrolyte (example: LiPF6 -EC/DEC solution) and a separator 3 are combined to obtain a lithium secondary battery.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium secondary battery, and more particularly to a positive electrode active material thereof.

[0002]

2. Description of the Related Art In recent years, an active material having an operating voltage of about 4 V for high energy density, a battery using a carbon material for a negative electrode for a long life, and the like have been attracting attention. Even when a carbon material is used for the negative electrode to prolong the life, it is difficult to obtain a high energy density battery unless the positive electrode has a high operating voltage. Therefore, LiCoO ₂ or LiNi
Such as O _2, such as a compound or LiMn ₂ O ₄ having a layered structure represented by LiMO _2, a compound having a spinel structure represented by LiM ₂ O ₄ have been proposed and already partially commercialized.

[0003]

However, LiC
oO ₂ has a small amount of cobalt as a resource and is expensive, and has a small capacity and is insufficient. Also, LiNiO ₂ using resource-stable nickel has a larger capacity than LiCoO ₂ but has a cycle. There is a problem in practical use due to the large deterioration of capacity accompanying it and the difficulty in stable synthesis on a mass production scale as compared with LiCoO ₂ .

In order to solve these problems, LiNi
Research and development for substituting a part of Ni in O ₂ to form a composite are also actively conducted. For example, JP-A-62-264560, JP-A-63-114063, JP-A-63-121565,
JP-A-63-299056, JP-A-1-120765,
In JP-A-2-40861 and JP-A-5-325966, L
Although the composite oxide represented by iNi _x Co _1-x O ₂ to be used for the positive electrode has been proposed, the initial capacity is lower than in LiNiO _2.

Further, JP-A-62-256371, JP-A-5-101827, JP-A-5-198301, and JP-A-5-301827.
-283076, JP-A-5-299092, JP-A-6-
In 96768 etc., a part of Ni in LiNiO ₂ is converted to C
Substitution with various transition metals such as o, V, Cr, Fe, Cu, Mg, Ti, and Mn has been proposed, but improvement in cycle characteristics is insufficient.

On the other hand, in JP-A-4-253162, LiC is used.
part Pb of Co oO _2, Bi, it is proposed to replace in B, also the general formula in Kokoku 4-24831 A _x M _y
The transition metal element M such as Ni of N _z O ₂ contains Al, In, Sn.
Substitution with at least one element N of the above is proposed. Further, in JP-A-5-54889, the general formula Li
_{In x} M _y L _z O ₂ , a transition metal element M such as Ni is added to non-metal elements and metalloid elements of Group IIIB, IVB and VB of the periodic table, alkaline earth metal elements and Zn, Cu, Ti and the like. Substitution with one or more elements L selected from metal elements is proposed.

However, in LiCoO ₂ , it was easy to replace a part of Co with the element L, whereas it is difficult to synthesize an active material in which a part of Ni of LiNiO ₂ is replaced with the element L. It was found that L was not incorporated into the structure and remained as an impurity in the active material, which adversely affects the battery performance such as a decrease in charge / discharge efficiency and an increase in self-discharge. Although the reason cannot be determined, in the case of LiNiO ₂ , it is more difficult to form a layered structure than in LiCoO ₂ , and the element L hinders the growth in the C-axis direction at the crystal growth stage, the substitution of the element L is difficult to occur, and it remains as an impurity. Conceivable.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a long-life lithium secondary battery having a large energy density.

[0009]

As a result of diligent study on the above problems, in NiNiO ₂ , a part of Ni was
It has been found that when substituting with elements of Al, In and Sn, it becomes very easy by adding Co. The reason for this cannot be determined, but Co is likely to have the same LiMO ₂ type layered structure as Ni and is replaced by the addition of Co without hindering the growth in the C-axis direction. In addition, LiCoO ₂
Among them, the elements of B, Al, In and Sn can be easily replaced with Co to form a layered structure. Therefore, LiNi
It is considered that Ni in O ₂ could be uniformly replaced without inhibiting growth in the C-axis direction only by adding elements of B, Al, In and Sn at the same time as Co.

Further, a part of Ni in LiNiO _{2 is}
The reason why the substitution with the elements such as Al, In, and Sn is selected is shown below.

It is known that B, Al and In have trivalence and Sn has tetravalence, but such elements do not contribute to the battery reaction.

In the portion substituted with the trivalent elements B, Al and In, lithium exists in a fixed form. This portion plays a pillar role of the Li layer, and suppresses repulsion between oxygen layers in a charged state and suppresses a change in crystal structure. Further investigation revealed that these elements B, Al, and In were uniformly present in the crystal due to the presence of Co, and exhibited their effects. As a result, the lithium remaining between the oxygen layers is evenly dispersed, enhancing its effect.

Further, since the portion substituted with the tetravalent element Sn is strongly bound to oxygen, it suppresses repulsion between oxygen layers in a charged state and suppresses a change in crystal structure. Further examination revealed that the Sn element was uniformly present in the crystal due to the presence of Co and exhibited its effect. As a result, the repulsion is suppressed between the oxygen layers as a whole, which enhances the effect.

Therefore, the active material of the present invention can be charged and discharged deeper than the conventional LiNiO ₂ by the above effects, so that the capacity is increased, and it is considered that the capacity decrease after the lapse of cycles is small.

[0015]

Operation: In the presence of Co in LiNiO ₂ , B, Al, I
The reason why the capacity and the cycle characteristics are improved by substituting with elements such as n and Sn is considered as follows.

Generally, when LiNiO ₂ is charged at a deep depth, the crystal structure changes, and further the crystal structure collapses. By removing Li in the layered structure,
Repulsion between the oxygen layers occurs and changes to a more stable crystal structure, or the crystal cannot collapse due to the inability to withstand the repulsion.

On the other hand, by substituting a part of Ni in LiNiO _{2 with} an element such as B, Al, In or Sn in the presence of Co, a portion where Li does not move is formed in the layered structure. Since the repulsive force between oxygen and oxygen can be suppressed,
It is possible to prevent the crystal structure from changing or collapsing. Therefore, as compared with the conventional LiNiO ₂ , it seems that even if deep charge / discharge is performed, excellent cycle stability is exhibited.

[0018]

EXAMPLES Examples of the present invention will be described below.

[0019] In the preparation of (Example 1) lithium composite oxide having a layered structure, LiOH · H ₂ 0, Ni
₂ CO ₃ , CoCO ₃ , and B ₂ O ₃ are used, and Li: Ni:
The molar ratio of Co: B is 1.03: 0.89: 0.10:
Weigh and mix so as to be 0.01, in oxygen, 750 ℃
It was baked for 20 hours. After firing, the product was cooled in dry air and ground in a dry atmosphere to obtain a positive electrode active material.

From the X-ray diffraction pattern of the obtained positive electrode active material, it was found that crystals were obtained in a single phase.

Using this active material, a coin-type lithium secondary battery shown in FIG. 1 was manufactured as follows. The active material was mixed with acetylene black and polytetrafluoroethylene powder at a weight ratio of 85: 10: 5, toluene was added, and the mixture was sufficiently kneaded. This is 0.8mm thick by roller press
Was formed into a sheet shape. Next, this was punched into a circle having a diameter of 16 mm and heat-treated at 200 ° C. for 15 hours under reduced pressure to obtain a positive electrode 1. The positive electrode 1 was used by pressure bonding to a positive electrode can 4 having a positive electrode current collector 6. For the negative electrode 2, a 0.3 mm-thick lithium foil was punched into a circular shape having a diameter of 15 mm, and the negative electrode can 5 was pressure bonded to the negative electrode can 5 via the negative electrode current collector 7. LiP was added to a mixed solvent of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1.
An electrolytic solution in which F ₆ was dissolved at 1 mol / l was used, and a polypropylene microporous film was used as the separator 3. The positive electrode,
A coin-type lithium battery having a diameter of 20 mm and a thickness of 1.6 mm was produced using the negative electrode, the electrolytic solution and the separator. This battery is designated as A1. In FIG. 1, 8 is an insulating packing.

Example 2 Instead of B ₂ O ₃ , Al (N
O ₃₎ using the _{3 · 9H 2 O, Li:} Ni: Co: Al molar ratio of 1.03: 0.89: 0.10: except that weighed so that 0.01 Example 1 A battery was prepared in the same manner as in. From the X-ray diffraction pattern of the obtained positive electrode active material, it was found that crystals were obtained in a single phase.
This battery is designated as A2.

Example 3 In (N) was used instead of B ₂ O _3.
Example 3 above except that O ₃ ) ₃ · xH ₂ O was used and weighed so that the molar ratio of Li: Ni: Co: In was 1.03: 0.89: 0.10: 0.01. A battery was prepared in the same manner as in. From the X-ray diffraction pattern of the obtained positive electrode active material, it was found that crystals were obtained in a single phase.
This battery is designated as A3.

Example 4 SnO was used instead of B ₂ O ₃ , and the molar ratio of Li: Ni: Co: Sn was 1.03:
A battery was made in the same manner as in Example 1 except that the weight was adjusted to 0.89: 0.10: 0.01. From the X-ray diffraction pattern of the obtained positive electrode active material, it was found that crystals were obtained in a single phase. This battery is designated as A4.

(Comparative Example 1) LiOH.H ₂ O, NiCO
_A battery was produced in the same manner as in Example 1 except that ₃ was used and weighed so that the molar ratio of Li: Ni was 1.03: 1.00. From the X-ray diffraction pattern of the obtained positive electrode active material, it was found that crystals were obtained in a single phase. This battery is designated as B1.

[0026] (Comparative Example _{2) LiOH · H 2 0,} NiCO
_A battery was prepared in the same manner as in Example 1 above, except that ₃ and CoCO ₃ were used and weighed so that the molar ratio of Li: Ni: Co was 1.03: 0.90: 0.10. From the X-ray diffraction pattern of the obtained positive electrode active material, it was found that crystals were obtained in a single phase. This battery is designated as B2.

[0027] (Comparative Example _{3) LiOH · H 2 0,} NiCO
₃ , B ₂ O _3, and the molar ratio of Li: Ni: B is 1.0.
A battery was produced in the same manner as in Example 1 except that the weight was adjusted to 3: 0.90: 0.10. From the X-ray diffraction pattern of the obtained positive electrode active material, it was confirmed that the layered crystal growth of LiNiO ₂ was poor and the mixture was a compound that could not be sufficiently specified. Furthermore, when the obtained positive electrode active material was chemically analyzed, it was inferred that divalent Ni remained and that sufficient oxidation of Ni did not occur. This battery is designated as B3.

Batteries A1, A2 thus produced
A charge / discharge cycle test was performed using A3, A4, B1, B2 and B3. The test conditions are a charging current of 3 mA, a charging end voltage of 4.2 V, a discharging current of 3 mA, and a discharging end voltage of 3.0 V.
And

Table 1 shows the results of the charge / discharge test of the batteries thus manufactured.

[0030]

[Table 1]

As can be seen from Table 1, Battery A according to the invention
1, A2, A3, A4 had a larger initial charge / discharge capacity than the comparative batteries B1, B2, B3, and the decrease after 10 cycles was small.

In this way, Ni of LiNiO ₂ is replaced by Co
Only by substituting with B, Al, In, and Sn in the coexistence, the capacity increase and the cycle stability can be realized.

The present invention is not limited to the starting materials, manufacturing methods, positive electrodes, negative electrodes, electrolytes, separators and battery shapes of the active materials described in the above examples. Further, it is also applicable to those using a carbon material for the negative electrode, those using a solid electrolyte instead of the electrolyte or separator, and the like.

[0034]

As described above, the present invention can provide a long-life lithium secondary battery having a large discharge capacity and excellent reversibility.

[Brief description of drawings]

FIG. 1 is a sectional view of a coin-type lithium secondary battery according to a first embodiment of the present invention.

Claims (1)

[Claims] 1. The positive electrode active material is Li _a Ni _b M ¹ _c M ² _d.
Consisting of a composite oxide having a layered structure represented by O ₂ .
M ¹ is Co and M ² is at least B, Al, In,
A lithium secondary battery comprising one or more elements selected from Sn.