JPH0883606A - Lithium secondary battery - Google Patents

Abstract

(57) [Summary] [Structure] A positive electrode in which a crystalline phase and an amorphous phase coexist is prepared by first adding V ₂ O ₅ powder to P ₂ as a crystallization inhibitor of V ₂ O _5.
O ₅ of 20.0, 10.0, 5.0, 3.0, 1.0, 0.1 m
The ol% was changed in 6 ways, mixed and added, melted at 1000 ° C., held for 2 hours, then poured onto a graphite block at room temperature for rapid cooling and solidification. [Effect] An excellent lithium secondary battery having a large charge / discharge capacity and a high energy density and a long cycle life can be constructed.

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 carbonaceous material capable of intercalating lithium as a negative electrode active material. The present invention relates to a long-life and high-energy-density lithium secondary battery having a water electrolyte as a main component.

[0002]

2. Description of the Related Art Conventionally, many proposals have been made for a high energy density battery using lithium as a negative electrode active material. For example, a battery using graphite and fluorine intercalation compounds as a positive electrode active material and lithium metal as a negative electrode active material is known (for example, US Pat. No. 35142337). Furthermore, a lithium battery using manganese dioxide as a positive electrode active material is already on the market. However, these batteries were primary batteries and could not be charged or discharged.

Secondary batteries using lithium as the negative electrode active material include batteries using titanium, zirconium, hafnium, niobium, tantalum, vanadium sulfides, selenium compounds, tellurium compounds, etc. as the positive electrode active material, and further manganese dioxide. A battery using cobalt oxide, cobalt dioxide, or the like has been proposed. However, it cannot be said that these batteries have sufficient battery characteristics and economical efficiency.

The use of V ₂ O ₅ as a positive electrode active material has been reported in the Extended Abstracts of Electrchem. Soc. Me.
eting (Toronto. May 11-16, 1975, No. 2
7)). However, the charge / discharge cycle characteristic was not sufficient.

[0005] Therefore, V ₂ O ₅ to 20 mol% or less of P ₂ O ₅
Was added and melted, then poured into water and rapidly cooled to V
It has been proposed to completely amorphize ₂ O ₅ to solve the above 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 problems, there is a need for a technique for utilizing a positive electrode active material that can improve the cycle characteristics without lowering the energy density originally possessed by the positive electrode active material. Development is needed.

[0007]

The above-mentioned problem is to prepare a positive electrode active material by coexisting a phase having a high operating potential and a high energy density with good crystallinity and an amorphous phase having a long cycle life. Thus, the cycle characteristics can be improved without lowering the energy density.

[0008]

According to the present invention, vanadium oxide such as V ₂ O ₅ or vanadium-lithium composite oxide such as LiV ₃ O ₈ or vanadium such as Cu ₂ V ₂ O _{7 is} contained in the active material of the positive electrode. The amorphous phase and the crystalline phase of the first transition metal composite oxide coexist.

In the present invention, a crystallization inhibitor may be added so that the amorphous phase and the crystalline phase coexist. The crystallization suppressing substance is TeO ₂ , GeO ₂ , BaO, PbO, P ₂ O ₅ ,
At least one of Sb ₂ O ₅ is contained in an amount of 0.1 mol% to 10 mo.
l% can be added.

The present invention will be described in more detail. For example,
In the case of V ₂ O ₅ , the crystalline phase has a high operating potential of 3.0 V or higher and a high energy density, and the amorphous phase has a flexible crystal structure, and thus exhibits good cycle characteristics. The present invention is V
_The coexistence of the crystalline phase and the amorphous phase in the ₂ O ₅ positive electrode active material enables a lithium secondary battery having high energy density and good cycle characteristics. Such a positive electrode active material may be a composite oxide of vanadium and lithium or vanadium and a first transition metal other than V ₂ O ₅ , such as L.
Examples thereof include iV ₃ O ₈ and Cu ₂ V ₂ O ₇ .

As 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, a crystalline phase and an amorphous phase, coexist, vanadium-based oxides include, for example, P ₂
It is effective to add a crystallization inhibitor represented by O ₅ . That is, this addition makes it possible to stably and easily obtain a positive electrode active material in which two phases, a crystalline phase and an amorphous phase, coexist even if the method is not the method of charging in water. Compounds added for this purpose include TeO ₂ , GeO ₂ , B in addition to P ₂ O _5.
Examples include 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 added compound or the quenching rate from the melting temperature, which also changes the characteristics as the positive electrode active material. That is, when the addition amount is small, the crystalline phase increases and the capacity is high, but there is a problem in the cycle characteristics. When the addition amount is large, the ratio of the amorphous phase is high and the cycle characteristics are improved, but the capacity remains a problem. I will end up. For example, in the case of P ₂ O ₅ , the range of 0.1 to 10.0 mol% is appropriate. Further, from the result of microscopic observation of the structure with an optical microscope, it has been found that the ratio of the amorphous phase to the crystalline phase is higher in the crystalline phase and exhibits a good cycle characteristic with a high energy density.

According to the present invention, V ₂ O ₅ is added with P ₂ O _{5 in the range} of 0.1 to 10.
It is intended to obtain a positive electrode active material in which two phases, a crystalline phase and an amorphous phase, coexist by adding in the range of 0 mol% and suppressing the cooling rate after melting. For the above reasons, the technique of quenching from the melting temperature of the positive electrode active material such as V ₂ O ₅ is important for achieving the present invention. Generally, as a method of rapidly cooling a high-temperature melt to obtain a solidified body, (A) a method of spraying the melted body and spraying it on a coolant, and (B) sandwiching the melted body in the coolant to solidify There is a method, (C) a method of pouring the melt into water to solidify it. Among these three methods, in the C method, water molecules destroy both the structures of 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, there is no fear as described above, and the object of the present invention is achieved. When no additive is used, it is possible to obtain a positive electrode in which a crystalline phase and an amorphous phase coexist by method A, which has an extremely high cooling rate. For example, the object of the present invention can be achieved by spraying V ₂ O ₅ and then partially crystallizing it by heat treatment. When the crystallization inhibitor is used, a large amount of the positive electrode active material in which the crystalline phase and the amorphous phase coexist can be obtained by the B method, 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 significantly improved due to 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 this is applied onto a support made of stainless steel or the like. Used as a positive electrode. The object of the present invention can be achieved regardless of whether the negative electrode is made of a material into which lithium can be inserted, such as a lithium-carbon system or a lithium metal or a lithium alloy.

Further, the electrolyte is propylene carbonate,
LiClO ₄ , LiAlCl in one or more aprotic polar organic solvents such as 2-methyltetrahydrofuran, dioxolene, tetrahydrofuran, 1,2-dimethoxyethane, ethylene carbonate, γ-butyrolactone, dimethyl sulfoxide, acetonitrile, formamide, dimethylformamide and nitromethane. ₄ , LiBF ₄ ,
Known organic electrolytes in which solutes such as lithium salts such as LiPF ₆ and LiAsF ₆ are dissolved or solid electrolytes or molten salts in which lithium ions are used as conductors are generally used in batteries 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 if necessary in the structure of the battery. In addition,
Various methods are possible as a method for confirming that the amorphous phase and the crystalline phase coexist, but as a reliable method, the presence of the crystalline phase can be confirmed by the differential thermal analysis by the X-ray diffraction method. Since it is possible to confirm the existence of the crystalline phase, it is possible to confirm that they coexist by using these two methods. 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 examples. In the following examples, all evaluation cells were manufactured and measured in an argon atmosphere.

(Example 1) Table 1 shows V prepared in this example.
₂ O ₅ positive electrode active material and amount of P ₂ O ₅ added to suppress crystallization,
The characteristics of a lithium secondary battery evaluation cell made using this positive electrode active material are shown.

[0018]

[Table 1]

To prepare a positive electrode in which a crystalline phase and an amorphous phase coexist, first, V ₂ O ₅ powder was mixed with P ₂ O ₅ as a crystallization inhibitor of V ₂ O _{5 in} an amount of 20.0, 10.0, 5.0, 3.0, 1.0, 0.
The content was changed to 1 mol% in 6 different ways, added and mixed, melted at 1000 ° C., held for 2 hours, poured onto a graphite block at room temperature, and rapidly solidified. 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 added amount of P ₂ O ₅ was 0.1
It was confirmed that two phases coexist in the range of 10.0 mol%. Next, these coagulated bodies are crushed under an argon atmosphere and X
It was confirmed from the diffraction image by the line diffraction method that the crystalline phase consisted of V ₂ O ₅ . The presence of an amorphous phase was also confirmed by a thermal analysis method. The evaluation cell was prepared by adding EPDM (abbreviation of ethylene-propylene-diene copolymer) as a binder powder to the obtained positive electrode active material powder in an amount of 4.0 wt% and acetylene black powder in an amount of 9.0 wt% as a conductivity-imparting powder. %
Was added and kneaded with xylene to prepare a positive electrode paste. Next, this is applied on the expanded metal made of SUS304, and after holding and drying in vacuum at room temperature for 5.0 hours,
It was used as the positive electrode.

The size and shape of the electrode is 1.5 × 2.0 × 0.03c
It has a square plate shape of m and has one positive electrode. The negative electrode active material used in this example is a Li-Pb-La alloy, and the composition thereof has an atomic ratio of 3.5: 1.0: 0.03. This was pulverized and classified to 45 μm or less, and acetylene black powder and EPDM were added in the same manner as the positive electrode to prepare an electrode. The number of negative electrodes used was two, and an evaluation cell was assembled by sandwiching the positive electrode with this. The cell container was manufactured using a laminated film of aluminum. A polypropylene non-woven fabric and a microporous film were stacked and used as the separator. The electrolyte contained 1.0 M LiPF ₆ propylene carbonate (PC) and 1,2-dimethoxyethane (DM).
A mixed solvent solution of E) was used.

The charge / discharge cycle test was a constant current test, the current density was 1.0 mA / cm ² , discharge start, and the charge / discharge end voltage during the test was 3.5V and 1.5V. The battery characteristic evaluation item of the battery of the present invention is the energy density obtained by converting the discharge energy value for one cycle obtained by integrating the product of the operating voltage and the current value of the battery by the discharge time per 1.0 kg of the active material, The energy density at the beginning of the cycle was calculated 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 addition amount of P ₂ O ₅ of No. 6 in Table 1 was 20.0 m.
In the case of ol%, the number of cycles until the energy density becomes 400 Wh / kg or less is 302 times, which is excellent in cycle life, but the energy density at the initial stage of the test is 510 Wh / kg.
As little as kg. It is considered that this is because the amorphization progressed so much that no clear V ₂ O ₅ diffraction line was observed in the X-ray diffraction measurement as described above. On the other hand, P ₂ O ₅
In the range of addition of 0.1 to 10.0 mol%, the crystalline phase and the amorphous phase coexist as described above, and the charge / discharge cycle of these evaluation cells until reaching 400 Wh / kg. The number of samples was good, about 300 times, and the energy density at the beginning 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 characteristics that the crystalline material has a high capacity and that the amorphous material has excellent cycle characteristics A positive electrode active material having both of the above can be obtained.

Example 2 Table 2 shows L produced in this example.
The iV ₃ O ₈ positive electrode active material, the amount of P ₂ O ₅ added to suppress crystallization, and the characteristics of a lithium secondary battery evaluation cell made using this positive electrode active material are 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 Example 1. The observation of the texture of the obtained solidified body is the same as in Example 1. As a result, a structure in which two phases coexist was exhibited in the range of 0.1 to 10.0 mol% of P ₂ O ₅ added. Furthermore, it was confirmed that the crystalline phase consisted of LiV ₃ O ₈ . The presence of an amorphous phase was also confirmed by a thermal analysis method. The manufacturing method of the evaluation cell and the characteristic evaluation of the manufactured battery are the same as in Example 1.

In the evaluation cell test made from 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, which is excellent in cycle life, but at the beginning of the test. Has an energy density of 420
As little as Wh / kg. On the other hand, the addition amount of P ₂ O ₅ is 0.1 to 1
In the range of 0.0 mol%, the crystalline phase and the amorphous phase coexist as described above. The results of this charge / discharge cycle test show that all the samples have a good value of 300 times or more, and the energy density at the initial stage of the charge / discharge cycle test is as high as around 700 Wh / kg.

Example 3 Table 3 shows C prepared in this example.
u ₂ V ₂ O ₇ positive electrode active material and P ₂ O ₅ added to suppress crystallization
And the characteristics of a lithium secondary battery evaluation cell made by using this positive electrode active material.

[0028]

[Table 3]

The method for producing a positive electrode in which a crystalline phase and an amorphous phase coexisted is the same as in Example 1 except that Cu ₂ V ₂ O ₇ powder and P ₂ O ₅ as a crystallization inhibitor are added to 20.0,10. .0, 5.0, 3.0,
It was prepared by adding and mixing while changing the amount to 1.0 and 0.1 mol% in 6 ways. The amount of P ₂ O ₅ added was 0.1 to 10.0 mol% by microscopic observation, X-ray diffraction, and thermal analysis of the obtained solidified body.
It was confirmed that within the range, the structure had two phases coexisting, the crystalline phase was Cu ₂ V ₂ O ₇ , and the amorphous phase was present. The manufacturing method of the evaluation cell, the characteristic evaluation method, and the like are the same as in Example 1.

From 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.
The number of cycles until it becomes less than / kg is 250 times,
It has excellent cycle life, but the energy density at the beginning of the test is as low as 540 Wh / kg. On the other hand, when the amount of P ₂ O ₅ added is in the range of 0.1 to 10.0 mol%, the crystalline phase and the amorphous phase coexist as described above. The results of this charge / discharge cycle test show good values in comparison with the results of only the crystal phase of 50 times, about 200 times for all samples, and the energy density at the beginning of the charge / discharge cycle test is as high as 820 Wh / kg or more. Showed the value.

(Example 4) Table 4 shows V prepared in this example.
₂ O ₅ cathode active material and the amount of GeO ₂ added to suppress crystallization,
The characteristics of a lithium secondary battery evaluation cell made by using this positive electrode active material are shown.

[0032]

[Table 4]

The positive electrode in which the crystalline phase and the amorphous phase coexist is V ₂
GeO ₂ as a crystallization inhibitor is added to O ₅ powder in an amount of 30.0,
It was prepared in the same manner as in Example 1 by adding and mixing the contents in 10.0, 5.0, 1.0, 0.1 mol% in 5 different ways. According to the same method as in Examples 1 to 3, the added amount of GeO ₂ is 0.1 to 1.
In the range of 0.0 mol%, two phases, a crystalline phase and an amorphous phase, coexisted. The manufacturing method of the evaluation cell, the characteristic evaluation method, and the like are the same as in Example 1.

From Table 4, the number of cycles until the energy density obtained from the evaluation cell of the lithium secondary battery made using No. 6 as the positive electrode active material becomes 400 Wh / kg or less is shown.
The cycle life is 398 times, which is excellent in cycle life, but the energy density at the initial stage of the test is low at 480 Wh / kg. On the other hand, Ge
When the amount of O ₂ added is in the range of 0.1 to 10.0 mol%, the crystalline phase and the amorphous phase coexist as described above. The results of this charge / discharge cycle test show that all the samples have a good value of 300 times or more, and the energy density at the initial stage of the charge / discharge cycle test is a high value of 620 Wh / kg or more.

[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 constructed.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Mamoru Mizumoto 7-1-1 Omika-cho, Hitachi-shi, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Tatsuo Horiba 7-chome, Omika-cho, Hitachi-shi, Ibaraki No. 1 in Hitachi, Ltd. Hitachi Research Laboratory

Claims (7)

[Claims] 1. A negative electrode and a positive electrode made of a carbon-based material capable of intercalating lithium metal, a lithium alloy, or lithium, and a non-aqueous electrolytic solution as main components, and an amorphous phase in an active material of the positive electrode. A lithium secondary battery characterized in that a crystalline phase coexists. 2. The lithium secondary battery according to claim 1, wherein an amorphous phase and a crystalline phase of vanadium oxide such as V ₂ O ₅ coexist in the active material of the positive electrode. 3. The lithium secondary battery according to claim 1, wherein an amorphous phase and a crystalline phase of a composite oxide of vanadium and lithium such as LiV ₃ O ₈ coexist in the active material of the positive electrode. 4. The lithium secondary according to claim 1, wherein an amorphous phase and a crystalline phase of a composite oxide of vanadium and a first transition metal such as Cu ₂ V ₂ O ₇ coexist in the active material of the positive electrode. battery. 5. The lithium secondary battery according to claim 1, 2, 3 or 4, wherein a crystallization suppressing substance is added to allow an amorphous phase and a crystalline phase to coexist in the active material of the positive electrode. 6. The crystallization suppressing material according to claim 5, wherein the crystallization suppressing substance is TeO ₂ , GeO ₂ , BaO, PbO, P ₂ O ₅ , Sb ₂ O _5.
Rechargeable lithium battery consisting of at least one of the above. 7. The crystallization inhibiting material according to claim 5, wherein TeO ₂ , GeO ₂ , BaO, PbO, P ₂ O ₅ and Sb _{2 are used.}
At least one of O ₅ is 0.1 mol% to 10.0 mo
Lithium secondary battery added in the range of l%.