JP2973830B2 - Lithium secondary battery - Google Patents

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 having a small size and a large charge / discharge capacity, in particular, a lithium metal, a lithium alloy, or a carbon-based material capable of intercalating lithium as a negative electrode active material. The present invention relates to a long-life, high-energy-density lithium secondary battery having a water electrolyte as a main component.

[0002]

2. Description of the Related Art Many proposals have been made on high energy density batteries using lithium as a negative electrode active material. For example, batteries using an intercalation compound of graphite and fluorine as a positive electrode active material and lithium metal as a negative electrode active material are known (for example, US Pat. No. 3,542,337). Further, lithium batteries using manganese dioxide as a positive electrode active material are already commercially available. However, these batteries were primary batteries and could not be charged and discharged.

A secondary battery using lithium as a negative electrode active material is a battery using titanium, zirconium, hafnium, niobium, tantalum, vanadium sulfide, a selenium compound, a tellurium compound, or the like as a positive electrode active material. And batteries using cobalt dioxide or the like have been proposed. However, these batteries have not always been satisfactory in battery characteristics and economy.

The use of V ₂ O ₅ as a positive electrode active material is disclosed in the technical literature (Extended Abstracts of Electrchem. Soc. Me).
eting (Toronto. May 11-16, 1975, No. 2)
7)). However, the charge / discharge cycle characteristics were not sufficient.

Therefore, V ₂ O ₅ contains less than 20 mol% of P ₂ O ₅
Is added and melted, then put into water and quenched to obtain V
It has been proposed to completely amorphize ₂ O ₅ to solve the above-mentioned problem (Japanese Patent Publication No. 4-24828). Although proposals foregoing improvement in charge-discharge cycle characteristics complete amorphization of the V ₂ O ₅ is fulfilled, caused a reduction in the energy density lowers the operating potential of the battery by amorphization of V ₂ O _5.

[0006]

In order to solve the above-mentioned problems, it is necessary to use a positive electrode active material utilizing technique which improves cycle characteristics without lowering the energy density inherent in the positive electrode active material. Development is required.

[0007]

An object of the present invention is to produce a positive electrode active material by coexisting a highly crystalline phase having a high operating potential and a high energy density and an amorphous phase having a long cycle life. Thus, the cycle characteristics can be improved without lowering the energy density.

[0008]

DETAILED DESCRIPTION OF THE INVENTION The present invention, in the active material of positive electrode, a composite oxide of vanadium and lithium such as LiV ₃ O _8, or Cu ₂ V ₂
An amorphous phase and a crystalline phase of a composite oxide of vanadium such as O ₇ and a first transition metal coexist.

In the present invention, an amorphous phase and a crystalline phase can coexist by adding a crystallization inhibitor. The crystallization inhibitor is TeO ₂ , GeO ₂ , BaO, PbO, P ₂ O ₅ ,
0.1 mol% to 10 mol of at least one kind of Sb ₂ O ₅
l% can be added.

The present invention will be described in more detail. For example,
In the case of V ₂ O ₅ , the crystal phase has a high operating voltage of 3.0 V or higher and has a high energy density, and the amorphous phase has a flexible crystal structure and thus has good cycle characteristics. As described above, by allowing the crystalline phase and the amorphous phase to coexist in the positive electrode active material, a lithium secondary battery having high energy density and good cycle characteristics can be obtained. In the present invention, as such a positive electrode active material , a composite oxide of vanadium and lithium , or vanadium and a first transition metal, for example, LiV ₃
O ₈ or Cu ₂ V ₂ O ₇ is used.

A method for producing a positive electrode active material having a positive electrode active material structure of a lithium secondary battery in which two phases, ie, a crystalline phase and an amorphous phase coexist, is exemplified by a vanadium oxide such as P ₂
Addition of a crystallization inhibitor represented by O ₅ is effective. That is, this addition makes it possible to stably and easily obtain a positive electrode active material in which two phases of a crystalline phase and an amorphous phase coexist without using a water-injection method. Compounds added for this purpose include TeO ₂ , GeO ₂ , B in addition to P ₂ O _5.
aO, PbO, Sb ₂ O _{5 and the} like. The ratio of the amorphous phase to the crystalline phase changes depending on the type and amount of the additive compound or the quenching rate from the melting temperature, and thereby the characteristics as the positive electrode active material also change. In other words, when the amount is small, the crystalline phase increases and shows a high capacity, but the cycle characteristics remain problematic, and when it is large, the ratio of the amorphous phase increases and the cycle characteristics improve, but the capacity remains problematic. I will. For example, in the case of P ₂ O ₅ , the range of 0.1 to 10.0 mol% is appropriate. Further, from the results of microstructure observation with an optical microscope, it has been found that the ratio of the amorphous phase to the crystalline phase, which is higher in the crystalline phase, exhibits higher energy density and better cycle characteristics.

According to the present invention, two phases of a crystalline phase and an amorphous phase coexist by adding P ₂ O ₅ to the positive electrode active material in the range of 0.1 to 10.0 mol% and suppressing the cooling rate after melting. It is intended to obtain such a positive electrode active material. For the present invention achieve the foregoing reasons quenching techniques from the melting temperature of the positive electrode active material is important. Generally, as a method of rapidly cooling a high-temperature melt to obtain a solidified body, (A) a method of spraying a molten state into a coolant and spraying and solidifying the same, and (B) a method of solidifying the melt by sandwiching the same between the coolants And (C) a method in which the melt is poured into water and solidified. Among these three methods, in the method C, the water molecules destroy both the crystalline phase and the amorphous phase, so that the energy density and the life are reduced. On the other hand, in the methods A and B, the object of the present invention is achieved because there is no such a fear as described above. When no additive is used, it is possible to obtain a positive electrode in which a crystalline phase and an amorphous phase coexist by the method A in which the cooling rate is extremely high. For example, positive
The object of the present invention can be achieved by spraying a polar active material and then partially crystallizing the material by heat treatment. When a crystallization inhibitor is used, a large amount of a positive electrode active material in which a crystalline phase and an amorphous phase coexist can be obtained by the method B, which is industrially effective. In the lithium secondary battery using the positive electrode active material produced in this way, the battery capacity shows a high capacity value of the crystalline phase, and the charge / discharge cycle characteristics are greatly improved by the excellent reversibility of the amorphous phase. Be improved.

To form a positive electrode using this positive electrode active material, a binder powder and a conductivity-imparting powder such as acetylene black are added and kneaded, and the mixture is coated on a support made of stainless steel or the like. Used as positive electrode. The object of the present invention can be achieved by using any material into which lithium can be inserted, such as lithium-carbon or lithium metal or lithium alloy.

Further, the electrolyte is propylene carbonate,
One or more aprotic polar organic solvents such as 2-methyltetrahydrofuran, dioxolen, tetrahydrofuran, 1,2-dimethoxyethane, ethylene carbonate, γ-butyrolactone, dimethyl sulfoxide, acetonitrile, formamide, dimethylformamide, nitromethane, and the like may be LiClO ₄ , LiAlCl. ₄ , LiBF ₄ ,
LiPF ₆ , an organic electrolyte solution in which a solute such as a lithium salt such as LiAsF ₆ is dissolved, or a solid electrolyte or a molten salt using lithium ions as a conductor, such as a known electrolyte generally used in a battery using lithium as a negative electrode active material. An electrolyte can be used.

The effect of the present invention is not impaired even if a microporous separator is used as required in the structure of the battery. In addition,
Various methods are available for confirming that the amorphous phase and the crystalline phase coexist, but as a reliable method, the presence of the crystalline phase is determined by X-ray diffraction, and the presence of the crystalline phase is determined by differential thermal analysis. Since the presence of the crystalline phase can be confirmed, it can be confirmed that the two phases coexist using these two techniques. In addition, as a simple method, it can be easily confirmed by observing the structure of the positive electrode active material with an optical microscope or a transmission electron microscope.

[0016]

The present invention will be described below in detail with reference to comparative examples and examples. In the following comparative examples and examples , all of the production and measurement of the evaluation cell were performed in an argon atmosphere.

(Comparative Example) Table 1 shows V ₂ produced in this comparative example.
O ₅ positive electrode active material and the amount of P ₂ O ₅ added for crystallization suppression,
And the characteristics of a lithium secondary battery evaluation cell made using this positive electrode active material.

[0018]

[Table 1]

[0019] Preparation of positive electrode crystalline phase and an amorphous phase coexist first, the P ₂ O ₅ as crystallization inhibitors of V ₂ O ₅ to V ₂ O ₅ powder 20.0,10.0, 5.0, 3.0, 1.0, 0.
The mixture was added and mixed at 1 mol% in 6 ways, melted at 1000 ° C., kept for 2 hours, and then poured on a graphite block at room temperature for rapid solidification. The surface of these solidified bodies was polished, and the structure was observed with an optical microscope to examine the presence or absence of a structure in which two phases coexist. As a result, the amount of P ₂ O ₅ added was 0.1.
It was recognized that the two phases coexist in the range of mol10.0 mol%. Next, these coagulates were pulverized in an argon atmosphere to
It was confirmed from a diffraction image by a line diffraction method that the crystalline phase was composed of V ₂ O ₅ . In addition, the presence of an amorphous phase was confirmed by a thermal analysis method. The evaluation cell was prepared by adding 4.0 wt% of EPDM (abbreviation of ethylene-propylene-diene copolymer) as a binder powder and 9.0 wt% of acetylene black powder as a conductivity-imparting powder to the obtained positive electrode active material powder. %
Was added and kneaded using xylene to prepare a positive electrode paste. Next, this was coated on an expanded metal made of SUS304, and dried in a vacuum at room temperature for 5.0 hours.
The positive electrode was used.

The dimensions of the electrodes are 1.5 × 2.0 × 0.03c.
m and a single positive electrode. The negative electrode active material used in this example is a Li-Pb-La alloy, and its composition is 3.5: 1.0: 0.03 in atomic ratio. This was pulverized and classified to 45 μm or less, and acetylene black powder and EPDM were added similarly to the positive electrode to prepare an electrode. The number of negative electrodes used was two, and an evaluation cell was assembled using the two negative electrodes so as to sandwich the positive electrode. The cell container was manufactured using an aluminum laminate film. A nonwoven fabric made of polypropylene and a microporous film were used as a separator. The electrolyte solution was a 1.0 M concentration of LiPF ₆ propylene carbonate (PC) and 1,2-dimethoxyethane (DM).
The mixed solvent solution of E) was used.

The charge / discharge cycle test was a constant current test. Discharge was started at a current density of 1.0 mA / cm ² , and charge / discharge end voltages during the test were 3.5 V and 1.5 V. The battery characteristics evaluation item of the battery of the present invention is defined as an energy density obtained by integrating the product of the operating voltage and the current value of the battery by the discharge time, and the discharge energy value for one cycle obtained by integrating the product with respect to 1.0 kg of the active material. The energy density at the beginning of the cycle was determined from the average value for 10 cycles. The energy density of the positive electrode active material is 40
The number of cycles when it became 0 Wh / kg or less was used as an index of cycle life.

The amount of P ₂ O ₅ of No. 6 in Table 1 was 20.0 m.
ol%, the number of cycles until the energy density becomes 400 Wh / kg or less is 302 times, and the cycle life is excellent, but the energy density at the initial stage of the test is 510 Wh / kg.
kg and small. This is considered to be due to the fact that amorphization has progressed to such an extent that no clear diffraction line of V ₂ O ₅ is observed in the X-ray diffraction measurement as described above. On the other hand, P ₂ O ₅
In the range of 0.1 to 10.0 mol%, the two phases of the crystalline phase and the amorphous phase coexist as described above, and the charge / discharge cycle until these evaluation cells reach 400 Wh / kg. The number of samples was as good as about 300 times, and the energy density at the initial stage of the charge / discharge cycle test was 700 Wh /
It shows a high value of around kg. From the above, by coexisting the amorphous phase and the crystalline phase in the positive electrode active material, the characteristic that the crystalline material has a high capacity and the characteristic that the amorphous material has excellent cycle characteristics Thus, a positive electrode active material having both of the above is obtained.

(Embodiment 1 ) Table 2 shows the Ls produced in this embodiment.
The amount of iV ₃ O ₈ positive electrode active material and the amount of P ₂ O ₅ added for suppressing crystallization, and the characteristics of a lithium secondary battery evaluation cell made using this positive electrode active material were shown.

[0024]

[Table 2]

The method for producing a positive electrode in which a crystalline phase and an amorphous phase coexist is the same as in the comparative example . Observation of the structure of the obtained solidified body
It is the same as the comparative example . As a result, the amount of P ₂ O ₅ added was 0.1.
In the range of 10.0 mol%, a structure in which two phases coexist was exhibited. Further, it was confirmed that the crystalline phase was composed of LiV ₃ O ₈ . In addition, the presence of an amorphous phase was confirmed by a thermal analysis method. How to make the evaluation cell and the evaluation of the characteristics of the manufactured battery
It is the same as the comparative example .

In the evaluation cell test made of the positive electrode active material of No. 6 in Table 2, the number of cycles until the energy density becomes 400 Wh / kg or less is 350 times, and the cycle life is excellent. Has an energy density of 42
It is as small as 0 Wh / kg. On the other hand, when the added amount of P ₂ O ₅ is 0.1
As described above, the crystalline phase and the amorphous phase coexist in the range of mol10.0 mol%. The charge / discharge cycle test results show that all the samples show good values of 300 times or more, and the energy density at the initial stage of the charge / discharge cycle test is 700 Wh / kg.
It shows high values before and after. The characteristics obtained in Example 1
The energy density is the same as the comparative example, but the cycle life is
Compare Tables 1 and 2 with the fact that it is superior to the comparative example.
It can be understood by:

(Example 2 ) Table 3 shows that C prepared in this example was used.
u ₂ V ₂ O ₇ Positive electrode active material and P ₂ O ₅ added for crystallization suppression
And the characteristics of a lithium secondary battery evaluation cell made using this positive electrode active material.

[0028]

[Table 3]

As in the comparative example , a positive electrode having both a crystalline phase and an amorphous phase was prepared by adding P ₂ O ₅ as a crystallization inhibiting substance to Cu ₂ V ₂ O ₇ powder at 20.0 and 10. 0, 5.0, 3.0,
It was prepared by adding and mixing 1.0 and 0.1 mol% in 6 ways. According to the structure observation, X-ray diffraction, and thermal analysis of the obtained solidified body, the added amount of P ₂ O ₅ was 0.1 to 10.0 mol%.
In the range, it was confirmed that the two phases coexisted, that the crystalline phase was composed of Cu ₂ V ₂ O _7, and that an amorphous phase was present. The method for producing the evaluation cell and the method for evaluating the characteristics are the same as those in the comparative example .

As shown in Table 3, in the test of the evaluation cell made using No. 6 as the positive electrode active material, the energy density was 400 Wh.
/ Kg or less is 250 cycles,
Although excellent in cycle life, the initial energy density of the test is as low as 540 Wh / kg. On the other hand, when the added amount of P ₂ O ₅ is in the range of 0.1 to 10.0 mol%, two phases, a crystalline phase and an amorphous phase, coexist as described above. The results of this charge / discharge cycle test were about 200 times for all the samples, and a good value as compared with 50 times of the result of only the crystal phase, and the energy density at the initial stage of the charge / discharge cycle test was as high as 820 Wh / kg or more. The value was shown.

The characteristics obtained in Example 2 are as follows.
Less than or equal to Comparative Example, but excellent in initial energy density
It can be seen by comparing Table 1 and Table 3.

[0032]

[0033]

[0034]

[0035]

According to the battery of the present invention, an excellent lithium secondary battery having a large charge / discharge capacity, a high energy density and a long cycle life can be constituted.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mamoru Mizumoto 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Within Hitachi Research Laboratory, Hitachi, Ltd. No. 1 Hitachi, Ltd. Hitachi Research Laboratory (56) References JP-A-1-128355 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01M 4/02 H01M 4/48 -4/58 H01M 10/40

Claims (2)

(57) [Claims] 1. A negative electrode and a positive electrode comprising lithium metal, a lithium alloy, or a carbon-based material capable of intercalating lithium, and a nonaqueous electrolyte as main constituents, and a composite of vanadium and lithium in an active material of the positive electrode. A lithium secondary battery characterized in that an amorphous phase and a crystalline phase of an oxide coexist. 2. A lithium metal, a lithium alloy, or a lithium alloy.
A carbon-based material that can intercalate titanium
The main components of the electrode and the positive electrode, and a non-aqueous electrolyte,
Complex oxidation of vanadium and first transition metal in active material of positive electrode
A lithium secondary battery , wherein an amorphous phase and a crystalline phase of a material coexist .