JPH0878004A - Lithium secondary battery - Google Patents

Abstract

PURPOSE: To provide a nonaqueous electrolyte secondary battery with low cost, high capacity, long charge/discharge cycle life, and high reliability by constituting with a specified negative electrode, a specified nonaqueous electrolyte, and a specified positive electrode. CONSTITUTION: A nonaqueous electrolyte secondary battery comprises a negative electrode capable of absorbing lithium or a lithium ion (example: Li-Pb-La alloy powder), a nonaqueous solvent (example: a mixture solvent of propylene carbonate and 1,2-dimethoxyethane) in which a lithium salt (example: LiPF6 ) is dissolved, and a positive electrode using a lithium-containing composite oxide represented by preferably LiFe1-y My O2 (M is Al, Ga, or In; 0<<=0.5), to which a group 3B element in the periodic table is added as a positive electrode active material. Preferably, crystal structure of space group R3d <5> is held in the structure of the positive electrode active material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of a positive electrode active material for a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art A non-aqueous electrolyte secondary battery using lithium as a negative electrode active material has a high voltage and a high energy density. Therefore, it is not only used as a power source for portable electronic devices but also for distributed power storage. It has attracted attention as a battery and is being actively developed.
Although some batteries have been put to practical use, the battery capacity, charge / discharge cycle life, and reliability are still insufficient.

Conventionally, as non-aqueous electrolyte secondary batteries, many batteries using a positive electrode active material such as a transition metal oxide, a transition metal chalcogenide, or a lithium-containing transition metal oxide have been proposed. Among these positive electrode active materials, LiCoO ₂ , LiNiO ₂ or LiFeO ₂ is expected to have a high battery voltage and a high energy density (Japanese Patent Laid-Open No. 5-174872). However, a battery having a sufficient charge / discharge cycle life and high reliability has not been obtained. Among these, LiCoO ₂ is said to be able to obtain relatively stable performance, but especially Co has a small amount of resources, so
There is also a problem that the cost is extremely high. Whether used as a power source for portable electronic devices or a distributed power storage battery, development of a low-cost and highly reliable non-aqueous electrolyte secondary battery is urgent.

Layered compounds LiCoO ₂ and LiNi
The reason why sufficient cycle life cannot be obtained when O ₂ and LiFeO ₂ based active materials are used as the positive electrode active material is considered as follows. These active materials have a structure in which lithium ions are present between layers of a layered structure formed by a transition metal and oxygen, the valence of the transition metal is likely to change, and lithium ions are easily inserted and desorbed. Therefore, it is used as a positive electrode active material. However, on the other hand, every time the charge / discharge cycle is repeated, the lithium ions between the layers are completely desorbed, the bonding force between the layers is reduced, and the crystal structure is destroyed. The main reason why the charge / discharge cycle life cannot be obtained is the destruction of the crystal structure. It is basically difficult to prevent the above-described deterioration of the crystal structure of the positive electrode active material in the composite oxide composed of lithium and a transition metal. Further, even if stable performance is obtained with LiCoO ₂ and LiNiO ₂ , cost problems still have to be solved in order to popularize these active materials.

[0005]

In the case of a composite oxide composed of lithium and a transition metal, the crystal structure of the positive electrode active material is destroyed due to repeated charge and discharge cycles, and a sufficient cycle life cannot be obtained, and the active material is expensive. There is a problem that is. An object of the present invention is to provide a positive electrode active material that is low in cost and stable with respect to charge / discharge cycles.

[0006]

The present invention comprises a negative electrode capable of occluding lithium or lithium ions, a positive electrode using a lithium-containing iron composite oxide as an active material, and a non-aqueous solvent in which a lithium salt is dissolved. The non-aqueous electrolyte secondary battery is characterized in that the positive electrode active material is a lithium-containing iron composite oxide to which a Group 3B element of the periodic table of elements is added.

The lithium-containing iron composite oxide has a chemical formula of LiFe _1-y M _y O ₂ (M = Al, Ga, In; 0 <y
≦ 0.5).

Further, the structure of the positive electrode active material represented by LiFe _1-y M _y O ₂ has a crystal structure of space group R _3d ⁵ .

In order to solve the above-mentioned problems, it is necessary to design a low-cost compound in which the diffusion coefficient of lithium ions in the crystal is maintained and the lithium ions are not completely desorbed. In a complex oxide composed of lithium and a transition metal, the valence of the transition metal changes according to the desorption of the lithium ion, so that the complete desorption of the lithium ion can easily occur by slightly increasing the battery charging voltage. . For this reason, the lithium between layers is completely lost, and it becomes difficult to maintain the structure between layers, and a sufficient cycle life cannot be obtained. In order to prevent the complete desorption of lithium between layers without lowering the diffusion function of lithium ions in the crystal, it is effective to add a typical element that does not change the valence in the crystal structure. After the desorption of the lithium ion according to the valence change of the transition metal existing in the crystal, the valence of the typical element does not change, so more lithium ions do not desorb even if the battery voltage is increased. . That is, a part of the lithium ions always remains between the layers, and the bonding force between the layers is not lost, so that the crystal structure can be stably maintained even when charging and discharging are repeated.

As a result of various investigations from the above viewpoints, the lithium-containing iron composite oxide has a ionic radius relatively close to that of iron ions as a typical element, and has a valence of 3
It was found that the addition of 3B group elements such as l, Ga and In is effective in stabilizing the structure. In addition, cost problems can be solved by using iron that is rich in resources.
Among the elements of the 3B group, Al has an ionic radius of 0.68.
Å, which is extremely close to the ionic radius of Fe of 0.69Å. Although any of the 3B group elements is effective, it is preferable to add Al having an ionic radius close to that of Fe. Also, Al is abundant in terms of resources, and is therefore preferable in terms of cost.

Even if Al is substituted for the Fe site, the ionic radius is substantially the same, so that the layered structure is maintained and the diffusion function of lithium ions is not impaired. As the substitution amount of Al increases, the amount of lithium ions remaining between layers increases, and the stability of the structure increases, but conversely the capacity density of the active material decreases. When Al is not added, the theoretical capacity density of the active material per gram is 283 Ah / kg, and when half of the Fe sites are replaced with Al, the theoretical capacity density is 167 Ah / kg.
It becomes kg. In order to take advantage of the high energy density of the lithium secondary battery, a capacity density per positive electrode active material weight of 150 Ah / kg or more is required. From the viewpoint of the stability of the crystal structure and the capacity density, the substitution amount of Al for Fe is preferably 0.5 or less.

Layered structure of LiFe used in the present invention
_1-y M y O ₂ is obtained by immersing the NaFe _1-y M y O ₂ in a molten salt of Li ion exchange. On the other hand, NaFe _1-y M
_y O ₂ can be synthesized by mixing and baking Na salt, Fe salt, and M salt. LiFe _1-y M _y obtained by ion exchange
To form a positive electrode using an O ₂ positive electrode active material, a conductivity-imparting powder such as acetylene black is added and kneaded, and the mixture is applied to a support made of aluminum or stainless steel. The object of the present invention can be achieved regardless of whether a graphite-based carbon material, an amorphous carbon material, lithium metal, or lithium alloy is used for the negative electrode.

Further, as the electrolyte, propylene carbonate, 2-methyltetrahydrofuran, dioxolene, tetrahydrofuran, 1,2-dimethoxyethane,
LiClO ₄ , LiA in one or more kinds of aprotic polar organic solvents selected from ethylene carbonate, γ-butyrolactone, dimethyl sulfoxide, acetonitrile, formamide, dimethylformamide, nitromethane and the like.
An organic electrolyte solution in which a lithium salt such as lCl ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ or the like is dissolved, or a solid electrolyte or a molten salt in which lithium ions are used as conduction ions, etc. Generally, a carbon material or lithium metal is used as a negative electrode active material. Known electrolytes used in batteries can be used. Further, the effect of the present invention is not impaired even if a microporous separator is used depending on the structural requirements of the battery.

[0014]

Function LiFe _1-y M _y O ₂ has a layered structure, and lithium between layers is inserted and desorbed with charge and discharge. During repeated charging and discharging, lithium ions between layers are desorbed and the layered structure is destroyed. By substituting a part of Fe with the element M of the 3B group, an amount of lithium ions corresponding to the substitution amount of the element M always remains between the layers, and the crystal structure is kept stable. That is, since the valence of the element M does not change, after the lithium ions corresponding to the valence change amount of Fe are desorbed, the lithium between layers is not desorbed from the layers any more due to the principle of electroneutralism. Therefore, by using the positive electrode active material of the present invention, it is possible to obtain a non-aqueous electrolyte secondary battery with low cost and good cycle characteristics.

[0015]

EXAMPLES The present invention will be described below with reference to examples. still,
The present invention is not limited to the examples below. In the following examples, all batteries were produced and evaluated in a glove box in an argon atmosphere.

Example 1 After mixing a predetermined amount of sodium carbonate, iron oxide, and aluminum nitrate with a ball mill,
Baked at 1000 ° C. for 5 hours in the air atmosphere to give NaFe
A powder of _1-y Al _y O ₂ was obtained. N obtained by X-ray diffraction
It was confirmed that aFe _1-y Al _y O ₂ had a layered crystal structure. The powder obtained was subjected to ion exchange in molten lithium nitrate at 300 ° C. for 2 days to obtain L having a layered structure.
iFe _1-y Al _y O ₂ was obtained. The composition of the LiFe _1-y Al _y O ₂ powder was y = 0.1, 0.2, 0.3, 0.4, 0.5,
And 0.55. For comparison, sodium carbonate and iron oxide having a composition of y = 0 to which Al was not added were also synthesized.

A positive electrode was prepared using these positive electrode materials, and a separator and a negative electrode were superposed on the positive electrode to construct a test battery. Ethylene cathode material _{LiFe 1-y Al y O 2} -
4.0 wt of propylene-diene copolymer (EPDM)
% Xylene solution as a binder and acetylene black as a 9.0 wt% conductive material. A paste obtained by kneading these was applied to an expanded metal made of SUS430 and vacuum dried at room temperature to obtain a positive electrode. Li-Pb-La alloy powder was used for the negative electrode. Li-Pb
The composition of the -La alloy is 3.5: 1.0: 0.03 in atomic ratio. As the electrolytic solution, a mixed solvent solution of propylene carbonate (PC) with a concentration of 1.0 M of LiPF ₆ and 1,2-dimethoxyethane (DME) was used. As the separator, a polypropylene non-woven fabric and a microporous film were stacked and used. The charge / discharge test is performed at a constant current and the current density is 0.
It was set to 05 mA / cm ² . The final voltage was 4.5 V during charging and 2.5 V during discharging. Further, the number of cycles when the capacity density per weight of the positive electrode active material became 100 Ah / kg or less was used as an index of cycle life. Table 1 shows the first cycle capacity density and cycle life of each tested battery.

[0018]

[Table 1]

As shown in Table 1, the battery 1 using the active material to which Al was not added had a capacity of 190 Ah in the first cycle.
/ Kg is large, but the cycle life is as short as 109 cycles.
On the other hand, in the case where Al was added, the capacity at the first cycle decreased as the amount of Al added increased, but the service life increased, and all of them except the battery 7 exhibited a cycle life of 200 cycles or more. Since the battery 7 has a short life, the capacity of the first cycle is originally as small as 133 Ah / kg,
It is thought that it reached 100 Ah / kg early. It was y.ltoreq.0.5 that the capacity density of 150 Ah / kg per weight of the positive electrode active material capable of taking advantage of the high capacity density of the lithium secondary battery was satisfied. Especially the battery 2,
Sample No. 3 had a large capacity at the first cycle, and the crystal structure was stabilized by Al.

Example 2 After mixing sodium carbonate, iron oxide, and gallium oxalate in predetermined amounts with a ball mill,
Baked at 900 ° C for 6 hours in the air to give NaFe _1-y
A powder of Ga _y O ₂ was obtained. Na obtained by X-ray diffraction
Fe _1-y Ga _y O ₂ was confirmed to have a layered crystal structure. The obtained powder was subjected to ion exchange in molten lithium nitrate at 300 ° C. for 2 days to obtain a layered structure of Li.
To obtain a _{Fe 1-y Ga y O 2} . _{Incidentally, LiFe 1-y Ga y O} 2 composition of the powder is y = 0.1,0.2,0.3,0.35, and was 0.4. A test battery was prepared in the same manner as in Example 1 using the obtained powder, and the positive electrode active material was evaluated.
Table 2 collectively shows the results of each test battery. For comparison, Table 2 also shows the results of the test battery (Battery 1 of Example 1) in which Ga was not added.

[0021]

[Table 2]

As shown in Table 2, the battery 1 using the positive electrode active material to which Ga is not added has a large discharge capacity of 190 Ah / kg in the first cycle but a short cycle life of 109 cycles. However, the batteries 2 to 5 using the Ga-added positive electrode active material also have good cycle life. Battery 6 is 1
Since the capacity at the cycle was as small as 137 Ah / kg, the cycle life was a short value of 169 cycles. Also, Ga
Since it is heavier than Al of Example 1, it was y ≦ 0.35 that the weight capacity density of 150 Ah / kg or more was obtained.

[0023]

As described above, according to the present invention,
It is possible to prepare a positive electrode active material having a low capacity, a large capacity density, and good cycle characteristics, and thus a highly reliable non-aqueous electrolyte secondary battery can be provided.

Claims (3)

[Claims] 1. A negative electrode capable of occluding lithium or lithium ions, a positive electrode using a lithium-containing iron composite oxide as an active material, and a non-aqueous solvent in which a lithium salt is dissolved.
In the non-aqueous electrolyte secondary battery, the positive electrode active material is a lithium-containing iron composite oxide to which a Group 3B element of the periodic table of elements is added, which is a non-aqueous electrolyte secondary battery. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein
The lithium-containing iron composite oxide has a chemical formula of LiFe _{1- y My} O
₂ (M = Al, Ga, In; 0 <y ≦ 0.5) A non-aqueous electrolyte secondary battery. 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein
A non-aqueous electrolyte secondary battery, wherein the structure of the positive electrode active material represented by LiFe _{1 -y My} O ₂ has a crystal structure of space group R _3d ⁵ .