JPH07114942A - Non-aqueous electrolyte lithium secondary battery - Google Patents

Abstract

PURPOSE:To improve a cycle characteristic so as to realize high electric power and a high capacity by including composite oxide composed of lithium and nickel of monocrystal powder having an average particle diameter less than a specific value into a positive electrode. CONSTITUTION:Polycrystal powder of LiNiO2 is used as a positive electrode active material for a positive electrode 1. Acetylene black of a conducting agent and a polyfluoroethylene resin of a binding agent are mixed with the positive electrode active material in predetermined ratios, followed by drying, thus obtaining a positive electrode mixture. The resultant mixture is pressure- molded into the positive electrode 1. The positive electrode 1 is press-fitted to the inner surface of a case 3 disposed in a current collector 2. A negative electrode 4 is constituted of a lithium plate, and is press-fitted to a sealing plate 6 having a negative electrode current collector 5. A separator 7 is made of a porous propylene film, and a gasket 8 is made of propylene. Propylene carbonate dissolved with lithium perchlorate is used as a non-aqueous electrolyte. An average particle diameter of the polycrystal powder of LiNiO2 is 10mum or less.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte lithium secondary battery, which has been actively developed recently, and more particularly to improvement of its positive electrode.

[0002]

2. Description of the Related Art Non-aqueous electrolyte secondary batteries using lithium or a lithium compound as a negative electrode are expected to have high energy density at high voltage, and many studies have been conducted. So far, LiCoO ₂ , L has been used as a positive electrode active material of a non-aqueous electrolyte secondary battery.
_{iMn 2 O4, LiFeO 2, LiNiO} 2, V 2 O 5, Cr
Oxides of transition metals such as ₂ O ₅ , MnO ₂ , TiS ₂ , MoS ₂ and chalcogen compounds have been proposed. These compounds have a layered structure or a tunnel structure, and have a crystal structure that allows lithium ions to enter and exit. In particular,
LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ are attracting attention as positive electrode active materials for 4V class non-aqueous electrolyte lithium secondary batteries. However, among these, LiCoO ₂ , which is the most promising positive electrode active material in terms of characteristics, is expensive because cobalt is an expensive element. In addition, there are concerns about the supply of raw materials, and it is possible that supply shortages due to changes in the world situation and price hikes may occur. In addition, LiMn ₂ O ₄
Although LiNiO ₂ is characteristically slightly inferior to LiCoO ₂ , the raw material compounds of manganese and nickel are stably supplied at a very low cost.
There are no worries in terms of cost or raw material supply. In addition, LiN
iO ₂ has the same composition and structure as LiCoO ₂ ,
It is a material that can obtain high capacity and high voltage as a positive electrode active material for a lithium secondary battery. However, LiNiO ₂
Is a material whose properties vary greatly depending on the synthesis conditions, and it has been very difficult to synthesize a crystalline phase having excellent properties as a positive electrode active material material for lithium secondary batteries. However, as a result of investigating various starting materials, firing conditions, etc., it became possible to obtain a material having a higher capacity than LiCoO ₂ , and in the future, it is highly expected as a positive electrode material for a lithium secondary battery which replaces LiCoO _2. It is a material.

[0003]

When LiNiO ₂ is used as the positive electrode active material, the initial capacity greatly exceeds that of LiCoO ₂ , and a high capacity positive electrode for a lithium secondary battery can be obtained, but there is a problem in terms of cycle characteristics. was there. Particularly, in order to obtain a high capacity, the capacity per active material weight is 150 mAh.
When charging / discharging is performed at a deep depth exceeding / g, the capacity of each cycle is greatly reduced, and the cycle characteristics are extremely deteriorated. Thus, the capacity per active material weight is 15
In order to obtain a positive electrode material for a lithium secondary battery having a high capacity exceeding 0 mAh / g, it is essential to improve cycle characteristics. The present invention solves such a problem and provides a positive electrode for a lithium secondary battery having excellent cycle characteristics.

[0004]

The present invention provides a positive electrode containing a composite oxide of lithium and nickel as a main component, a negative electrode containing lithium or a material capable of reversibly inserting and extracting lithium, and a non-aqueous electrolyte. In the provided non-aqueous electrolyte lithium secondary battery, a single crystal powder having an average particle size of 10 μm or less is used as the lithium-nickel composite oxide. The present invention also uses, as the lithium-nickel composite oxide, a polycrystalline powder having a maximum grain size of 30 μm or less and a crystallite size of 30% or more of the grain size.

Further, in another aspect of the present invention, a mixture of the single crystal powder and the polycrystalline powder is used as the composite oxide. Here, the polycrystalline powder preferably has a maximum particle size of 5 μm or less.

[0006]

The present inventors have found that the above structure can improve the cycle characteristics of a positive electrode using a composite oxide of lithium and nickel. That is, when a positive electrode was prepared using LiNiO ₂ synthesized by using lithium nitrate and nickel carbonate as an active material and a charge / discharge test was performed, cycle deterioration was large, and particularly the capacity per active material weight was 150.
It has been found that such a tendency is remarkably exhibited when it exceeds mAh / g. When the cause of this cycle deterioration was investigated in detail, it was found that a large cycle deterioration occurred due to charging and discharging using a plateau region of about 4.2 V with respect to lithium. Furthermore, when the change in the crystal structure with the depth of charge was examined by X-ray diffraction measurement, it was found that a sharp shrinkage of the crystal lattice occurred in the plateau region. Moreover, when the particle shape of the active material was examined by SEM observation, etc., the particle size was several μm.
It was found that each particle is not a single crystal but a secondary particle formed by aggregating fine crystal particles, although it is only to a degree.

From the above, by repeating the charge and discharge cycle, the crystal grains are distorted, which causes cracks in the grain boundaries between the crystal grains inside the secondary particles, resulting in defective current collection. It is considered that the utilization rate of the active material is reduced and the capacity is reduced. Therefore, first, by suppressing the particle growth of the secondary particles and decreasing the particle size, the contact area between the conductive agent and the active material particles was increased, and a positive electrode for a lithium secondary battery with good current collection was prepared and charged and discharged. The test was conducted.
As a result, current collection failure due to cracks generated due to crystal strain during charge / discharge is reduced, and cycle deterioration is suppressed when charge / discharge at a deep depth is repeated so that a high capacity of 150 mAh / g or more is obtained. I was able to.

Further, by promoting grain growth of crystal grains, an active material powder composed of secondary particles formed by aggregation of single crystal grains having a relatively large size is obtained, and using this, an active material powder is prepared and filled. A discharge test was conducted. As a result, 150
It was possible to significantly suppress cycle deterioration when charging / discharging at a deep depth such that a high capacity of mAh / g or more was obtained. This is because the crystal particles that form the secondary particles are large, and each crystal particle is exposed on the surface of the secondary particle and comes into contact with the conductive agent to have an electrical contact, so that the distortion of the crystal particles due to charging and discharging is caused. It is considered that this is because even if a crack occurs at the grain boundary, it is not electrically isolated and current collection failure does not occur. Furthermore, a positive electrode was prepared using a powder composed of single crystal particles having no grain boundary, which was obtained by further promoting the grain growth of crystal particles, and was subjected to a charge / discharge test. As a result, 150 mAh
It was possible to significantly suppress the cycle deterioration when charging / discharging at a deep depth such that a high capacity of / g or more was obtained. It is considered that this is because the use of the active material powder composed of single crystal particles did not cause the current collection failure due to the generation of cracks at the grain boundaries accompanying the strain of the crystals during charge and discharge.

[0009]

The present invention will be described in detail below with reference to examples, but the present invention is not limited to these examples. [Embodiment 1] In this embodiment, a case will be described where LiNiO ₂ which is a polycrystalline powder having a maximum grain size of 5 μm or less and a crystallite size of 30% or less of the grain size is used as a positive electrode active material. . First, the molar ratio of nickel to lithium is 1.0: 1.
The resulting mixture was weighed out to give a dispersion medium of 1, and a small amount of water was added as a dispersion medium, mixed thoroughly, dried and calcined in an oxygen stream for 12 hours to obtain a black calcined product. The firing temperature was 650 ° C. The fired product obtained was pulverized and classified to an average particle size of 5μ.
A powder of m or less was obtained. When SEM observation and particle size distribution measurement were performed, the obtained powder had a maximum particle size of 5 μm or less, and the size of the primary particles of the single crystal forming each particle was 1.5 μm or less. Next, a positive electrode was produced using the fired product obtained as described above as an active material. An active material, acetylene black as a conductive agent, and polyfluorinated ethylene resin as a binder were mixed at a weight ratio of 7: 2: 1, and sufficiently dried to be a positive electrode mixture, and 0.15 g of this positive electrode mixture was added. A positive electrode was obtained by pressure molding into a pellet having a diameter of 17.5 mm at 2 ton / cm ² .

A cross-sectional view of a battery manufactured by using the positive electrode manufactured as described above is shown in FIG. Reference numeral 1 denotes a positive electrode, which is pressure-bonded to the inner surface of the case 3 provided with the current collector 2. Reference numeral 4 denotes a negative electrode, which is made of a lithium plate having a thickness of 0.8 mm and a diameter of 17.5 mm, and is pressed onto a sealing plate 6 having a negative electrode current collector 5 attached thereto. Reference numeral 7 is a separator made of porous polypropylene fill, and 8 is a polypropylene gasket. As the non-aqueous electrolyte, propylene carbonate in which 1 mol / l lithium perchlorate was dissolved was used. This battery is referred to as battery A. Further, as Comparative Example 1, a positive electrode and a battery were prepared by the same method as described above, using the active material powder that was fired by the same method as this Example and classified so that the maximum particle size was 45 μm or less.

Ten batteries were prepared for each of the above samples and their initial capacities were compared. Charge and discharge conditions are
The voltage was regulated at a constant current of 0.5 mA in a voltage range of 3.0 V to 4.3 V. Table 1 shows the average initial capacity and 50
The capacity retention rate at the cycle is shown.

[0012]

[Table 1]

As shown in Table 1, the capacity retention ratio at the 50th cycle is 72% when the active material obtained by classifying the maximum particle size of Comparative Example 1 to 45 μm or less is used, whereas the capacity retention ratio is 72%. When an active material having a maximum particle size of A of 5 μm or less is used, it is 89%, which shows that it is effective in improving the cycle property.

Example 2 In this example, a positive electrode was produced using LiNiO ₂ powder having a maximum grain size of 30 μm or less and a crystallite size of 30% or more of the grain size. The case will be described. First, lithium nitrate and basic nickel carbonate were weighed out so that lithium and nickel were in a molar ratio of 1.0: 1.1, a small amount of water was added as a dispersion medium, and the mixture was thoroughly mixed and dried. A black fired product was obtained by firing in an oxygen stream for 30 hours. The firing temperature was 650 ° C. The fired product obtained was thoroughly pulverized and classified to obtain a powder having a particle size of 10 μm or less. When the powder after classification was observed by SEM, the particle size was 10 μm or less,
The size of the crystallite forming each particle was 3 μm or more. Next, a positive electrode and a battery were produced in the same manner as in Example 1 using the fired product powder obtained as described above as an active material. This battery is referred to as battery B. Further, as Comparative Example 2, a powder obtained by pulverizing a calcined product obtained by calcining in the same manner as in this Example and then classifying the powder so that the maximum particle size was 45 μm or less was used as an active material. A positive electrode and a battery were produced in the same manner as in. When the SEM observation was performed on the fired product powder of Comparative Example 2, the particle size was 45 μm or less, and the size of the crystallite forming each particle was 3 μm or more.

Ten batteries were prepared for each of the above samples and their initial capacities were compared. Charge and discharge conditions are
The voltage was regulated at a constant current of 0.5 mA in a voltage range of 3.0 V to 4.3 V. Table 2 shows the average initial capacity and 50
The capacity retention rate at the cycle is shown.

[0016]

[Table 2]

As shown in Table 2, the capacity retention ratio at the 50th cycle was 72% in Comparative Example 1, whereas the maximum particle size in this Example was 30 μm or less and the size of the crystallite forming each particle. Battery B using an active material having a thickness of 10 μm or more is 9
It was 6%, which was a great effect in improving the cycleability. Comparative Example 2 was 70%, and no effect of improving cycleability was observed. This is because the particle size is large relative to the size of the crystallites, so cracks that occur between crystallites inside the particles due to charge / discharge, resulting in poor current collection of internal crystals that are not exposed on the particle surface. It is thought to be because of this.

Example 3 In this example, the average particle size is 1
A case in which a positive electrode is manufactured using LiNiO ₂ powder composed of a single crystal of 0 μm or less will be described. First, lithium nitrate and basic nickel carbonate were weighed out so that the molar ratio of nickel and lithium was 1.0: 1.1, a small amount of water was added as a dispersion medium, and the mixture was thoroughly mixed and dried. After firing in an oxygen stream at 550 ° C. for 50 hours, firing was performed again in an oxygen stream at 650 ° C. for 6 hours to obtain a black fired product. The obtained fired product was sufficiently pulverized and classified to obtain a powder having an average particle size of 10 μm or less. When the powder after classification was subjected to SEM observation and particle size distribution measurement, it was confirmed that the powder was a single crystal having an average particle size of 10 μm or less. Next, a positive electrode and a battery were produced in the same manner as in Example 1 using the fired product powder obtained as described above as an active material. This battery is referred to as battery C. further,
As Comparative Example 3, a fired product obtained by firing in the same manner as in the present Example was pulverized, and then powders classified to have a maximum particle size of 45 μm or less were used as an active material. A positive electrode and a battery were produced by the method described above.

Ten batteries were prepared for each of the above samples and their initial capacities were compared. Charge and discharge conditions are
The voltage was regulated at a constant current of 0.5 mA in a voltage range of 3.0 V to 4.3 V. Table 3 shows the average initial capacity and 50
The capacity retention rate at the cycle is shown.

[0020]

[Table 3]

As shown in Table 3, the capacity retention ratio at the 50th cycle was 72% in Comparative Example 1, whereas the active material of this Example was a single crystal powder having an average particle size of 10 μm or less. The content of Battery C was 97%, which was a great effect in improving the cycleability. Further, Comparative Example 3 is 81%, and the effect of improving the cycleability is not so great. It is considered that this is because the grain size is large even though it is a single crystal, so that strain due to charge / discharge is large and cracks occur in the crystal, resulting in poor current collection inside the crystal.

In the above examples, lithium nitrate and basic nickel carbonate were used as starting materials for the synthesis, but lithium carbonate, lithium hydroxide,
Similar effects can be obtained even when a lithium salt or lithium oxide such as lithium oxalate or lithium acetate is used as a nickel source, and a nickel salt or nickel oxide such as nickel hydroxide or nickel nitrate is used. Further, in the above examples, only the case of unsubstituted nickel lithium oxide was described,
The same effect can be obtained when nickel is replaced with another element such as aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, zinc, niobium, molybdenum, or tungsten.

[0023]

As described above, according to the present invention, the cycle property of a non-aqueous electrolyte lithium secondary battery using a positive electrode containing a composite oxide of lithium and nickel as a main component is improved, and high voltage and high capacity are obtained. A secondary battery can be obtained.

[Brief description of drawings]

FIG. 1 is a vertical sectional view of a battery used in an example of the present invention.

[Explanation of symbols]

 1 Positive Electrode 2 Positive Electrode Current Collector 3 Case 4 Negative Electrode 5 Negative Current Collector 6 Sealing Plate 7 Separator 8 Gasket

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasuhiko Mito 1006 Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. Within the corporation

Claims (3)

[Claims] 1. A positive electrode containing a composite oxide of lithium and nickel as a main component, a negative electrode containing lithium or a material capable of reversibly occluding and releasing lithium, and a non-aqueous electrolyte. The non-aqueous electrolyte lithium secondary battery is characterized in that the complex oxide is a single crystal powder having an average particle size of 10 μm or less. 2. A positive electrode containing a composite oxide of lithium and nickel as a main component, a negative electrode containing lithium or a material capable of reversibly inserting and extracting lithium, and a non-aqueous electrolyte. 2. The non-aqueous electrolyte lithium secondary battery, wherein the complex oxide is a polycrystalline powder having a maximum grain size of 30 μm or less and a crystallite size of 30% or more of the grain size. 3. A positive electrode containing a composite oxide of lithium and nickel as a main component, a negative electrode containing lithium or a material capable of reversibly inserting and extracting lithium, and a non-aqueous electrolyte. Is a mixture of a single crystal powder having an average particle size of 10 μm or less and a polycrystalline powder having a maximum particle size of 30 μm or less and a crystallite size of 30% or more of the particle size. Characteristic non-aqueous electrolyte lithium secondary battery.