JP3165953B2 - 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, and more particularly to a negative electrode for a lithium secondary battery having a large discharge capacity and a high output density and excellent cycle characteristics. Lithium rechargeable batteries are used in electric vehicles, memory backup,
It is applied as a power supply for driving portable equipment.

[0002]

2. Description of the Related Art Conventionally, lithium (Li) metal and alloys such as Li--Al and Li--Pb have been used as a negative electrode of a lithium secondary battery. Short circuit of both poles and short cycle life,
There was a disadvantage that the energy density was low. Recently, research on using a carbon material for a negative electrode has been actively conducted to solve these problems. This type of negative electrode is disclosed in, for example,
9073 and JP-A-2-121258. Japanese Patent Application Laid-Open No. 5-299073 discloses a method of forming a carbon composite by coating the surface of highly crystalline carbon particles forming a core with a film containing a metal element of group VIII, and further coating carbon thereon. According to the report, the carbon material having a turbostratic structure on the surface assists the intercalation of lithium, and the charge / discharge capacity and the charge / discharge rate are remarkably improved due to the large surface area of the electrode. On the other hand, in Japanese Patent Application Laid-Open No. 2-121258, a carbon material having a hexagonal structure of H / c <0.15, an interplanar spacing of> 3.37 °, and a crystallite size Lc <150 ° in the C-axis direction and Li
It is described that a charge / discharge cycle life is long and a charge / discharge characteristic at a large current is good by using a mixture of a metal and an alloyable metal. However, in each case, the difficulty of the alloy of the negative electrode carbon material and the theoretical capacity of carbon were not drawn out, and the output density was not yet sufficient. Therefore, the energy density and the power density were insufficient for mounting on electric vehicles and motorcycles.

[0003]

As described above, when a carbon material and a composite material are used as a negative electrode, there is a problem that the theoretical capacity of carbon cannot be drawn out and that it is difficult to manufacture an electrode.

The present invention solves the above-mentioned problems,
By using a negative electrode that holds a carbon particle carrying fine particles of a metal that can be alloyed with lithium in a current collector,
Aims to provide a lithium secondary battery with high capacity and excellent charge / discharge cycle characteristics

[0005]

The present inventors have conducted various studies to solve the above-mentioned problems, and as a result, have completed the present invention based on the following findings.

FIG. 1 shows the measurement results of the cycle characteristics of the conventional negative electrode and the improved negative electrode. The carbon used was natural graphite that had been subjected to a high-purification treatment, and had a particle size of about 11 μm. Using a solution obtained by dissolving ethylene propylene terpolymer (hereinafter abbreviated as EPDM) in diethylbenzene as a binder on the carbon, a paste in which the weight ratio of the carbon and the EPDM was 94: 6 was converted to a thick current collector. Separately, the paste was applied to a copper foam metal having a three-dimensional network structure with a thickness of 0.9 mm and a porosity of 93%, which is a current collector. Here, the former is called a conventional type, and the latter is called an improved type. Both were air-dried, vacuum-dried at 80 ° C. for 3 hours, molded at a pressure of 0.5 ton / cm ² , and
It vacuum-dried at 0 degreeC for 2 hours, and each was set as the negative electrode. One of these negative electrodes is combined with a lithium metal counter electrode with a polypropylene microporous membrane serving as a separator interposed therebetween, and 1M LiPF ₆ / ethylene carbonate-dimethoxyethane (hereinafter abbreviated as EC-DME) as an electrolytic solution; A test cell using lithium metal as a reference electrode was assembled. Using this test cell, the conventional negative electrode and the improved negative electrode were subjected to a cycle test at a charging / discharging rate of 120 mA / g of carbon and a potential width of charging / discharging: 0.01 to 1.0 V.

[0007] As is clear from Fig. 1, when the conventional negative electrode 1 is used, the discharge capacity decreases every cycle, and after about 500 cycles, the discharge capacity decreases to about 60% of the initial capacity. did. On the other hand, 5
Even after the 00 cycle, the decrease rate was as small as 4.5%, and the effect of improving the current collector was recognized. This experimental fact is thought to be because the improved electrode with a three-dimensional network structure was able to prevent the current collection effect between carbon particles from decreasing due to electrode swelling due to volume changes due to repeated charge and discharge. Can be Then, the following experiment was conducted to verify the above. That is, it was examined whether the same effect can be obtained by adding metal fibers to the negative electrode mixture. Figure 2 shows the result.
Shown in The experiment of FIG. 2 is almost the same as that of FIG. 1, but the outline of the measurement conditions is shown below. The carbon used has a particle size of about 3μ
m artificial graphite, and a copper fiber with a wire diameter of 10 μm
The mixture was mixed at a weight ratio of 0:10. Polyvinylidene fluoride (hereinafter abbreviated as PVDF) is used as a binder in this mixture.
Of the above mixture and PV using an N-methylpyrrolidone solution of
A paste in which the weight ratio of DF was adjusted to 90:10 was applied to a copper foil having a thickness of 20 μm as a current collector, air-dried, and then dried.
After vacuum-drying at a temperature of 0.5 ° C. for 3 hours and molding at a pressure of 0.5 t / cm ² , vacuum drying was performed at 120 ° C. for 2 hours to obtain a negative electrode. This negative electrode was combined with a lithium metal counter electrode via a polypropylene microporous membrane, and 1 M LiP
A test cell using F ₆ / ethylene carbonate + dimethyl carbonate (hereinafter abbreviated as EC + DMC) and lithium metal as a reference electrode was assembled. The charge / discharge rate is carbon 1
120 mA per g, and the upper and lower limit electric potentials of charge and discharge were 1.0 V and 0.01 V, respectively. The results obtained are shown in FIG. FIG. 2 also shows the characteristics of the negative electrode to which no copper powder was added. As is clear from the results in FIG. 2, it was found that the negative electrode 3 to which the copper powder was added had a larger discharge capacity and the decrease in each cycle became extremely smaller than the negative electrode 4 to which the copper powder was not added. Similar results were obtained for a negative electrode using copper powder instead of copper fiber. From the above results, increasing the current collection performance of the negative electrode mixture layer is an important factor for improving the discharge capacity and cycle characteristics, and as a result of further detailed examination,
Rather than simply mixing carbon and conductive fibers or conductive powder, by supporting fine particles of a metal that forms an alloy with lithium on carbon, compared to a mixed system of carbon and conductive material, Even if the (supporting) amount is small, the same effect can be obtained, at the same time, the alloying capacity with lithium can be used, and the improvement of electric conductivity and thermal conductivity by interposing a metal between carbon particles can be expected. That it brings a new function.

[0008]

[0009]

[0010]

The gist of the present invention will be described below. The present invention includes a positive electrode and a negative electrode, a separator interposed between the positive and negative electrodes, a lithium secondary battery including the electrolyte immersing the positive and negative electrodes and a separator, a negative electrode of this battery, crystalline
Powder that carries a metal that forms an alloy with lithium on carbon particles
Powder , and the crystalline carbon particles have an average particle size of 1 to 20 μm.
m, surface spacing (d002) by X-ray analysis is 3.354
The crystal size in the C-axis direction (Lc) is 300 ° or more, the specific surface area is 0.1 to 30 m ² / g, and the metal forming an alloy with lithium has a particle size of 1000 °. It is characterized by the following.

[0012] The lithium secondary battery of the present invention is an electric vehicle driven by a motor and an automatic vehicle driven by a motor.
Useful as a power source for wheels. At this time, this lithium secondary battery has a charge / discharge rate of 1 coulomb (C).
As described above, a battery having an energy density of 300 wh or more per liter of battery capacity is preferable .

The lithium secondary battery of the present invention has a capacity of 0.5.
wh to 50 kwh, and the battery weight per 1 kg
Discharge for more than 15 minutes at an output with an energy density of 350w or more
Those that allow are preferred.

[0014]

The carbon particles of the present invention can be obtained by heat-treating highly crystalline carbon particles, for example, a graphitizable material obtained from natural graphite, petroleum coke, coal pitch coke or the like at a temperature of 2500 ° C. or higher. The average particle size is 50μ
m or less, preferably 1 to 20 μm. The shape may be spherical, massive, scale-like, fibrous, or a crushed product thereof.

Next, as the supported metal, Al, Sb, B,
Ba, Bi, Cd, Ca, Ga, In, Ir, Pb, H
At least one of g, Si, Ag, Sr, Te, Tl and Sn is selected, but elements satisfying the following conditions are preferable.

(1) an alloy composition having a high lithium content;
(2) relatively low atomic weight and relatively high density;
(3) Easy reduction, (4) Low oxidation-reduction potential of lithium alloy, (5) Less disposal problem, (6) Relatively inexpensive.

As a method for supporting a metal, there are a vapor deposition method, a sputtering method, a wet reduction method, an electrochemical reduction method, a plating method, a gas phase reduction gas treatment method, and the like. Any suitable supporting method may be applied. The amount of metal carried is 30 wt% or less, preferably 1-1.
0 wt% is preferred. Further, the particle size of the supported metal is
Considering the deposition / dissolution rate of the lithium alloy in charging and discharging, the temperature is preferably 1000 ° or less.

A negative electrode is prepared using the metal-supported carbon particles obtained as described above. In this case, a binder is used. The binder is not particularly limited as long as it does not react with an electrolytic solution such as EPDM, PVDF, and polytetrafluoroethylene. The amount of the binder is 1 to carbon.
30 wt%, preferably 5 to 15 wt% is suitable. As the negative electrode shape using the aforementioned mixture, a sheet shape,
It is possible to adapt to the shape of the battery by filling the film or foamed metal on a film or metal foil. The thickness of the mixture layer is preferably in the range of 10 to 200 μm.

By combining the negative electrode thus obtained with a commonly used positive electrode, a separator and an electrolytic solution, an optimum lithium secondary battery can be obtained. As the active material used for the positive electrode, LiCoO ₂ , LiNiO ₂
Or a composite oxide containing lithium such as LiMn ₂ O ₄ may be used, and a mixture of a conductive agent such as carbon black or carbon and an adhesive is applied to a current collector such as an Al foil to form a positive electrode. I do.

As the separator, a porous film of polypropylene, polyethylene or polyolefin is used. Examples of the electrolytic solution include propylene carbonate (PC), ethylene carbonate (EC), 1,2-dimethoxyethane (DME), and dimethyl carbonate (D
Two or more types of mixed solvents such as MC), diethyl carbonate (DEC), and methyl ethyl carbonate (MEC) are used. As the electrolyte, LiPF ₆ , Li
There are BF ₄ , LiClO _{4 and the} like, and those dissolved in the above solvents are used.

With the aim of improving the carbon negative electrode for lithium secondary batteries and increasing discharge capacity, power density and cycle characteristics, by using carbon particles carrying fine particles of a metal forming an alloy with lithium, (1) increase of discharge capacity, (2) improvement of electric conductivity, (3) improvement of output density,
(4) Improved cycle characteristics, (5) Improved heat dissipation in the assembled battery, and (6) High-speed charge / discharge were enabled.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION

[Embodiment 1] Natural graphite that has been subjected to a high-purification treatment (particle size 1)
(1 μm) are suspended in 450 ml of water containing 25 ml of ethyl alcohol. This is heated to about 60 ° C., and 1.73 silver nitrate (AgNO ₃ ) is added and dissolved with vigorous stirring. A 0.5% by weight aqueous solution of sodium tetrahydride borate (NaBH ₄ ) is dropped by a microtube pump, and the reduction reaction is completed in about 3 hours. Thereafter, the mixture was filtered, washed with water, and vacuum dried at 300 ° C. for 6 hours. According to the chemical analysis, the amount of the obtained powder A was 9.9% by weight with respect to 10.0% by weight of the charged composition, which was a good amount. Also, by X-ray diffraction, Ag
Inspection of the presence state revealed that only silver diffraction lines in the metallic state were detected. Next, when the dispersion state of Ag was observed by energy dispersive electron probe microanalysis,
The Ag particles were distributed over the entire surface of the graphite particles, and were slightly concentrated on the end surfaces of the graphite particles. Further, when the size of the Ag particles was observed with a transmission electron microscope, it was found that particles of several hundreds of degrees were substantially uniformly dispersed.

[Embodiment 2] Artificial graphite (particle size: 3 μm)
9.0 g of a commercially available tin plating solution (GL, manufactured by Kojundo Chemical Co., Ltd.)
S-500B), and the mixture was stirred while being heated to about 65 ° C., and electrolessly plated with tin on carbon powder. The mixture was filtered, washed with water, and vacuum-dried at 300 ° C. for 6 hours to obtain Powder B. As a result of chemical analysis, the amount of supported Sn was 4.6% by weight.

Embodiment 3 9.0 g of highly purified natural graphite (particle size 11 μm) is suspended in 450 ml of water containing 25 ml of ethyl alcohol. About 6
The mixture is heated to 0 ° C., and 2.32 g of bismuth nitrate (Bi (NO ₃ ) ₃ .xH ₂ O) is added and dissolved with vigorous stirring.
To this, (1: 4) aqueous ammonia is dropped by a micro tube pump until the solution becomes alkaline. Thereafter, the mixture was filtered, washed with water, and vacuum dried at 300 ° C. for 6 hours. This powder was subjected to a reduction treatment by heating at 600 ° C. for 3 hours in a helium gas stream containing 4% by volume of hydrogen to obtain powder C. The powder C was subjected to a chemical analysis, and an analysis result of 9.6% by weight was obtained with respect to 10.0% by weight of the charged composition.

[Embodiment 4] N-methylpyrrolidone solution of PVDF was used as a binder for powder A, powder B and powder C obtained in the above embodiments 1 to 3, and each powder was mixed with PVDF 90: A paste having a weight ratio of 10 was applied to a copper foil having a thickness of 20 μm as a current collector, air-dried, and then dried at 80 ° C. for 3 hours.
After vacuum drying for 0.5 hours and molding at a pressure of 0.5 t / cm ² ,
Furthermore, it vacuum-dried at 120 degreeC for 2 hours, and obtained the negative electrode A, the negative electrode B, and the negative electrode C. These negative electrodes were combined with a lithium metal counter electrode through a polyester film, and 1 MLiPF ₆
/ EC + DMC, a test cell using lithium metal for the reference electrode was assembled. The charge and discharge rate is 80 mA per g of carbon,
The upper and lower limit potentials of charge and discharge were set to 1.0 V and 0.01 V, respectively. The obtained results are summarized in Table 1 in comparison with carbon not supporting a metal. Here, the powder of Comparative Example 1 is natural graphite (particle diameter 11 μm) subjected to a high-purification treatment, and the powder of Comparative Example 2 is artificial graphite (particle diameter 3 μm).

[0027]

[Table 1]

The powders A, B, and C exhibit values exceeding the theoretical capacity density of graphite of 372 mAh / g, because the alloying capacity of metal and lithium is added. Further, the results of the cycle test of the negative electrode of powder A are shown in FIG.
h / g discharge capacity is maintained.

Fifth Embodiment A negative electrode of powder A obtained in the fourth embodiment, a positive electrode using lithium cobalt oxide (LiCoO ₂ ) as a positive electrode material, a polyester film separator, and 1M LiPF ₆ / EC + DMC AA batteries were constructed using the electrolytic solution and evaluated. This battery is
It shows a discharge capacity of 350 Wh / 1 from the first cycle.
The value did not decrease even after 30 cycles.

[Embodiment 6] In the same manner as in Embodiment 1, highly purified natural graphite (particle size: 11 μm)
Was treated with silver nitrate (AgNO ₃ ), sodium tetrahydride borate (NaBH ₄ ), and the like. However, powder D was obtained by adding 0.02 mol / l potassium hydrogen phthalate to control the pH before adding the reducing agent (NaBH ₄ ). Examination of the Ag metal particle diameter and the dispersion state revealed that the powder D had a Ag particle diameter (100
Å200 °) is small and the dispersion state is remarkably improved.

[Embodiment 7] Artificial graphite (surface spacing (d0
02) = 3.359 °, crystal size Le = 5 in the c-axis direction
46 °, specific surface area = 10.5 m ² / g), and the same preparation as in Embodiment 1 was carried out to obtain a powder E carrying Ag particles.

[Embodiment 8] Artificial graphite (surface spacing (d0
02) = 3.359 °, crystal size Le = 5 in the c-axis direction
46 °, specific surface area = 10.5 m ² / g), and the same operation as in Embodiment 6 was performed to obtain a powder F carrying Ag particles having a smaller particle diameter.

[Embodiment 9] Artificial graphite (surface spacing (d00
2) = 3.359 °, crystal size Le = 54 in the c-axis direction
6Å, 9.0 g of specific surface area = 10.5 m ² / g) in 25 ml
Is suspended in 200 ml of water containing C ₂ H ₅₆ H. 1.9 g of S was added to 50 ml of water containing 0.5 ml of HCl.
ncl obtained by dissolving a ₂ · 2H ₂ O and water was added to a total volume of 450 ml. While heating and stirring the mixture at 50 to 60 ° C., a 0.5% NaBH ₄ aqueous solution is added dropwise using a microtube pump, and a reduction reaction is performed for about 3 hours. Thereafter, filtration and water washing are performed, and drying is performed by heating at 350 ° C. in vacuum for 6 hours or more. The powder G obtained here was subjected to ICP analysis for Sn.
It showed a good value of 9.76 wt% to wt%. When the Sn form was examined by X-ray diffraction, it was found that the Sn form was
Was found to be completely metal.

[Embodiment 10] Powders D, E, F and G
Then, a solution obtained by dissolving PVDF in N-methylpyrrolidone as a binder was added, and the mixture was kneaded and mixed so that PVDF became 10 wt% with respect to the powder to prepare various pastes. This paste was applied to a copper foil having a thickness of 20 μm as a current collector, air-dried, and then dried in vacuum at 80 ° C. for 3 hours.
Then, after molding at a pressure of 0.5 ton / cm ² , the negative electrodes D, E, F and G were prepared by drying at 120 ° C. for 2 hours in a vacuum. These negative electrodes were combined with a lithium metal counter electrode and a polyethylene-based porous membrane as a separator, and 1 M was used as an electrolyte.
LiPF ₆ / EC-DMC, the reference electrode using Li metal, the charge and discharge rate of carbon 1g per 80 mA, the potential width cycled tested 0.01V～1.0V. The results are shown in Table 1 above, using carbon not supporting metal as a comparative example.
Are shown together.

The powders D, E and F exhibit values exceeding the theoretical capacity of carbon, because the capacity of alloying metal and lithium is added.

Incidentally, the cycle test result of the negative electrode of the powder D shows the same tendency as shown in FIG. 3, and the discharge capacity of 370 mAh / g is maintained even after 300 cycles.

Embodiment 11 A mixture of LiCoO ₂ active material, artificial graphite and PVDF in a weight ratio of 87: 9: 4 was applied to both sides of a 20 μm-thick aluminum foil to a thickness of 90 μm. And dried and rolled positive electrode 15 and thickness 2
Powder D obtained in Embodiment 6 on a copper foil of 0 μm on one side 5
Negative electrode 1 coated on both sides to a thickness of 8 μm, dried and rolled
7 and a 25 μm thick polyethylene porous membrane separator 19 were wound as shown in FIG.
Stored in a 47 mm battery can and used 1 M LiPF as electrolyte
The characteristics were evaluated using ₆ / EC-DMC.

The test conditions were as follows: charge / discharge rate: 1 C, end-of-charge voltage: 4.2 V, end-of-discharge voltage: 2.5 V. As a result, an energy density of 300 wh / l (battery volume) was obtained, and stable performance was shown up to 300 cycles.

[0039]

The effect of using the negative electrode obtained by the present invention, that is, the powder in which fine particles of a metal forming an alloy with lithium are supported on carbon particles is used for the negative electrode.

First, by interposing a metal between carbon particles, (1) the electric conductivity is improved, and the speed of the charge / discharge reaction is improved. (2) Since the charge / discharge capacity of an alloy formed with lithium as an additional metal can be used, the theoretical capacity of graphite is 372 m.
A value exceeding Ah / g is obtained. (3) The irreversible capacity is reduced because the supported metal covers the reaction site on the surface of the carbon particles that causes the irreversible capacity. (4) Since the discharge capacity increases, the output density of the battery naturally increases. (5) (1)
As a result, the cycle characteristics are improved, and the heat dissipation of the assembled battery can be improved.

[Brief description of the drawings]

FIG. 1 is a cycle characteristic diagram of a conventional negative electrode and an improved negative electrode.

FIG. 2 is a cycle characteristic diagram of a copper fiber-added negative electrode and a non-added negative electrode.

FIG. 3 is a cycle characteristic diagram of a negative electrode according to the present invention.

FIG. 4 is a configuration diagram of a cylindrical battery of the present invention.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 Cycle characteristics of conventional negative electrode 2 Cycle characteristics of improved negative electrode 3 Cycle characteristics of copper fiber-added negative electrode 4 Cycle characteristics of non-added negative electrode 5 Cycle characteristics of negative electrode according to the present invention 15 Positive electrode 16 Positive terminal 17 Negative electrode 18 Negative terminal 19 Separator

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takeo Yamagata 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Within Hitachi Research Laboratory, Hitachi, Ltd. (72) Inventor Tatsuo Horiba 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture No. 1 Hitachi, Ltd., Hitachi Research Laboratory (72) Inventor Ren Muranaka 7-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Hitachi, Ltd. Hitachi Research Laboratory (56) References JP-A-5-286763 (JP, A) JP-A-2-121258 (JP, A) JP-A-6-318454 (JP, A) JP-A-5-82171 (JP, A)

Claims (2)

(57) [Claims] 1. A lithium secondary battery comprising a positive electrode and a negative electrode, a separator interposed between the positive and negative electrodes, and an electrolyte for immersing the positive and negative electrodes and the separator, wherein the negative electrode comprises crystalline carbon particles and lithium and an alloy. The crystalline carbon particles have an average particle diameter of 1 to 20 μm, and have a plane spacing (d00) determined by X-ray analysis.
2) is 3.354 to 3.369 °, and the crystal size in the C-axis direction is large.
The size (Lc) is 300 ° or more, and the specific surface area is 0.1 to 30.
m ² / g, and the metal forming an alloy with lithium has a particle size of 1000 ° or less. 2. The lithium secondary battery according to claim 1 , wherein the metal forming an alloy with lithium is Ag,
A lithium secondary battery having a particle size of 100 to 200 °.