JP2001160392A - Nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous secondary battery of improved preservation stability, where there is no decrease in battery capacity during storage. SOLUTION: This nonaqueous secondary battery utilizes a negative electrode, which is composed of an electric collector and a sintered body of silicon and carbon formed on the collector, for which silicon particles are covered with carbonaceous material, and the reaction of electrolyte and silicon particles has been suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous secondary battery having a negative electrode made of a sintered body containing silicon as an active material.

[0002]

2. Description of the Related Art As portable electronic devices are reduced in size and weight, secondary batteries with higher energy density are demanded. Among non-aqueous secondary batteries, lithium secondary batteries are expected to meet such demands. However, since it has been difficult to achieve the required energy density with the conventionally used negative electrode active materials of carbon materials such as graphite, silicon, which can be expected to have a higher energy density, is used as the negative electrode active material instead of carbon materials. It is being considered for use.

For example, Japanese Patent Application Laid-Open No. Hei 7-29602 discloses that silicon is used as a negative electrode active material of a lithium ion secondary battery.

[0004]

However, when a battery is constituted by using silicon as a negative electrode active material and the battery is left after the discharge is completed, the battery is over-discharged, and the elution of silicon in the negative electrode and the current collector, for example, In addition, there is a problem that the elution of copper starts, the battery capacity is reduced, and the life of the battery is shortened.

Therefore, the present invention provides a non-aqueous secondary battery excellent in storage stability without lowering the battery capacity during storage by suppressing the elution of silicon and the current collector of the negative electrode during overdischarge. The task was to do.

[0006]

Means for Solving the Problems In the course of research for suppressing self-discharge, the present inventors have developed a method in which a negative electrode using silicon as a negative electrode active material is mixed with a solvent composed of an alkyl carbonate and Li
When immersed in a typical electrolytic solution using PF ₆ as an electrolyte and analyzing the electrolytic solution and the negative electrode, when the contact area between the silicon particles and the electrolytic solution increases, the compound containing silicon and fluorine and the LiPF ₆ and the solvent Reaction products increase,
A battery using a negative electrode having a large contact area between a silicon particle and an electrolytic solution has poor storage stability, and when silicon or a current collector of a negative electrode active material is coated with a carbonaceous material, these reactions may occur. It has been found that it is suppressed and the storage stability is improved.

That is, the non-aqueous secondary battery of the present invention is a non-aqueous secondary battery including a positive electrode, a negative electrode, and a lithium ion conductive electrolyte containing an active material capable of inserting and extracting lithium ions. The negative electrode comprises a current collector made of a conductive metal, and a sintered body of silicon and carbonaceous material of the negative electrode active material formed on the current collector, and silicon particles in the sintered body are It is characterized by being coated with a carbonaceous material so as to suppress the reaction between the electrolytic solution and silicon particles.

As described above, on the exposed surface of the silicon particles, the silicon and the electrolytic solution react directly to form a compound containing silicon and fluorine, and further, the solvent and the electrolyte LiPF
_It is thought that the reaction with ₆ proceeds. For this reason, the reaction proceeds as the area of the silicon particles in direct contact with the electrolytic solution increases. Therefore, by coating the surface of the silicon particles with the carbonaceous material, it is possible to suppress the reaction of the silicon particles with the electrolyte on the surface of the silicon particles. Therefore, an irreversible reaction can be suppressed, and a decrease in battery capacity can be prevented.

Further, the non-aqueous secondary battery of the present invention forms a coating film containing silicon particles and a carbonaceous material or an organic material which becomes a carbonaceous material by heat treatment on a current collector, and forms a non-oxidizing atmosphere. Can be used as a negative electrode.

Further, the non-aqueous secondary battery of the present invention is characterized in that the surface of the current collector is further coated with a carbonaceous material to suppress the direct contact between the current collector and the electrolyte, thereby improving the current collection. It is possible to use a negative electrode that suppresses elution during overdischarge of the body.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The non-aqueous secondary battery of the present invention can be suitably used for a lithium secondary battery. Hereinafter, embodiments of the present invention will be described in detail for a lithium secondary battery.

As the negative electrode used in the present invention, a sintered body obtained by molding and sintering a raw material powder can be used, but it is preferable to use a sintered body obtained by sintering a coating film containing the raw material powder.
For example, a negative electrode manufactured by the following method can be used. That is, a coating liquid is prepared by adding a silicon powder as a negative electrode active material and a carbonaceous material or an organic material to be a carbonaceous material by heat treatment to a solvent, and applying the coating liquid on a current collector made of a conductive metal. After drying, the solvent is removed to prepare a coating film. Next, this coating film is sintered in a non-oxidizing atmosphere to obtain a negative electrode in which the current collector and the sintered body are integrated.

The silicon powder used in the present invention may be either crystalline silicon or amorphous silicon, and is a silicon compound which can be decomposed or reduced to silicon by heat treatment in a non-oxidizing atmosphere. Is also good. Examples of the silicon compound include an inorganic silicon compound such as silicon oxide, and an organic silicon compound such as a silicone resin and a silicon-containing polymer compound. Of these, silicon alone is particularly preferred. The particle size of the silicon powder is not particularly limited, but from the viewpoint of reducing the contact area with the electrolytic solution, and further, from the viewpoint of operability, raw material price, and uniformity of the negative electrode material, the average particle size is 0.5.
Those having a size of at least 100 μm can be preferably used. The silicon particles in the sintered body are coated with a carbonaceous material, and have a structure in which a dispersed phase of silicon is present in a continuous phase of the carbonaceous material. Further, it is preferable that the sintered body does not substantially contain silicon compounds such as SiC which does not occlude or release lithium ions and has no capacity or a reaction product of silicon and a current collector.

Examples of the carbonaceous material used for the negative electrode of the present invention include coke, glassy carbon, graphite, pitch carbides, and mixtures thereof. Organic materials that become carbonaceous materials by heat treatment include thermosetting resins such as phenol resins, phenol formaldehyde resins, epoxy resins, unsaturated polyester resins, furan resins, urea resins, melamine resins, alkyd resins, and xylene resins. resin,
Naphthalene, acenaphthylene, phenanthrene, anthracene, triphenylene, pyrene, chrysene, naphthacene, picene, perylene, pentaphen, pentacene, and the like, or a condensed polycyclic hydrocarbon compound or a derivative thereof, or a pitch containing a mixture thereof as a main component. And a phenol formaldehyde resin or a xylene resin.

When a negative electrode is prepared by forming a coating film containing the raw material powder on a current collector and sintering the same in a non-oxidizing atmosphere, the firing temperature is set to a reaction temperature between SiC or silicon and the current collector. In order to suppress the production of silicon compounds such as 1400 ℃ or less, and the melting point of the current collector used to suppress the reaction between silicon and the current collector is desirably not more than, for example, when copper is used for the current collector Has a melting point of 1083 ° C. or less, more preferably 700 to 850 ° C., for copper.

In preparing the negative electrode, the following method can be used. That is, after the coating film is formed on the current collector, the coating film and the current collector are immersed together in a coating liquid containing a carbonaceous material or an organic material that becomes a carbonaceous material by heat treatment, or the current collector The coating solution is applied to the resultant, and dried to remove the solvent to prepare a coating film. Next, the current collector can be covered with a carbonaceous material by sintering the coating film and the current collector in a non-oxidizing atmosphere. This suppresses the contact between the current collector and the electrolytic solution, so that the progress of the elution reaction of the current collector metal can be suppressed.

Further, as the positive electrode used in the present invention, any conventionally known materials can be used as the positive electrode active material.
_{i x CoO 2, Li x NiO} 2, MnO 2, LiMnO 2, L
_{i x Mn 2 O 4, Li} x Mn 2-y O 4, α-V 2 O 5, TiS 2
Etc. can be used.

The non-aqueous electrolyte used in the present invention may be a non-aqueous electrolyte in which a lithium compound such as LiPF ₆ is dissolved as an electrolyte in an organic solvent such as ethylene carbonate or dimethyl carbonate, or a lithium compound solid in a polymer. A solid polymer electrolyte in which an organic solvent in which a dissolved or lithium compound is dissolved is held can be used.

[0019]

[Embodiment 1] (1) Preparation of Negative Electrode 70% of a commercially available silicon powder [99.9% product from Kojundo Chemical, average particle size about 1 μm] 3% pitch-containing toluene solution 1
After impregnating the powder with 00 g and drying, it was baked at 1100 ° C. for 3 hours in a nitrogen atmosphere to cover the surface of the silicon powder with a carbon film. 20 g of the carbon-coated silicon powder and natural graphite (NG2 manufactured by Kansai Thermochemical Co., Ltd., average particle size of about 2 μm) were mixed with phenol formaldehyde resin n-methyl-2-pyrrolidone (N
MP) solution (20% solution), and uniformly dispersed in a vibration mill for 30 minutes to prepare a coating solution. This coating solution was applied on a copper foil having a thickness of 40 μm using an applicator having a gap of 450 μm, and dried at 90 ° C. for 30 minutes.
It was punched out at 19φ and pressed at 1.5 × 10 ⁸ Pa.
Next, baking was performed at 850 ° C. for 3 hours in a nitrogen atmosphere.
Thus, the thickness is 150 μm and the density is 1.3 g / cm ^3.
Was prepared.

(2) Preparation of Positive Electrode Lithium carbonate and cobalt carbonate were weighed at a molar ratio of 1: 1 and uniformly wet-mixed with a ball mill using isopropyl alcohol as a solvent, and dried and calcined at 800 ° C. for 5 hours. The product was pulverized with a vibration mill. Next, this powder was pressed at 1.3 × 10 ⁸ Pa and baked at 900 ° C. for 10 hours to obtain a thickness of 300 μm and a density of 3.0 g / c.
LiCoO ₂ pellets of m ³ and 19φ were prepared, and these pellets were used as a positive electrode.

(3) Production of Battery A battery was produced by using a separator having an electrolyte of ethylene carbonate / dimethyl carbonate = 1: 1 (volume ratio) and an electrolyte of 1 M LiPF _{6 with} a separator interposed between both electrodes.

(4) Cycle test A battery was prepared, left for 1 day, and charged and discharged once at a constant current of 3 mA between a charge end voltage of 4.1 V and a discharge end voltage of 2.0 V, and then charged. 10mA to 4.1V cutoff voltage
The battery was stored at room temperature for 30 days in a discharged state after repeating a cycle test in which the battery was discharged at a constant current and a constant voltage of 8 mA and a constant current was discharged at 8 mA to a discharge end voltage of 2.0 V.
The first battery capacity value of the cycle test is used as the initial capacity value,
The ratio of the battery capacity value after storage for 30 days to the initial capacity value was defined as a capacity retention rate (%). Here, in order to investigate the storage stability in the overdischarge state, the charging / discharging voltage range was set to 4.1 V to 2.5 V, and the discharge end potential was lowered.
It was set to 4.1V to 2.0V.

(5) Analysis Whether or not the silicon particles of the negative electrode were covered with the carbonaceous material was determined by analyzing the surface elements of the negative electrode with an X-ray photoelectron spectrometer (ESCA). Further, regarding the presence or absence of decomposition of the electrolytic solution, after immersing the negative electrode in the electrolytic solution for a predetermined time, the electrolytic solution was subjected to gas chromatography mass spectrometry (G
(C-MS).

The presence or absence of a short circuit in the battery was examined based on the disturbance of the voltage or current profile during charging. Table 1 shows the above measurement results and judgment results.

Embodiment 2 FIG. A negative electrode was prepared by the following method. Commercially available silicon powder (99.9% high purity chemical product,
70 g of an average particle diameter (about 1 μm) and 20 g of natural graphite (NG2 manufactured by Kansai Thermal Chemical Co., Ltd., an average particle diameter of about 2 μm) were mixed with 200 g of an NMP solution of phenol formaldehyde resin, and dispersed by a vibration mill for 30 minutes to prepare a coating liquid. This coating solution was applied to a copper foil having a thickness of 40 μm, dried at 90 ° C. for 30 minutes, punched out to 19φ, and pressed at 1.5 × 10 ⁸ Pa. This was impregnated with a 3% pitch-containing toluene solution and dried, and then fired at 850 ° C. for 3 hours in a nitrogen atmosphere.
Thus, a negative electrode having a thickness of 150 μm and a density of 1.4 g / cm ³ in which the copper foil carrying the active material was covered with the carbonaceous material was prepared. Others were performed similarly to Example 1. Table 1 shows the results
Shown in

Embodiment 3 FIG. The procedure was performed in the same manner as in Example 1 except that a negative electrode was prepared using silicon powder having an average particle diameter of 0.5 μm. Table 1 shows the results.

Comparative Example 1 A negative electrode was prepared by the following method. Commercially available silicon powder [99.9% high purity chemical product,
70 g of an average particle diameter of about 1 μm] and 20 g of natural graphite (NG2 manufactured by Kansai Thermochemical Co., Ltd., average particle diameter of about 2 μm) 200 g of an NMP solution (20% solution) of phenol formaldehyde resin
, And uniformly dispersed in a vibration mill for 30 minutes to prepare a coating liquid. This coating solution was coated on a copper foil having a thickness of 40 μm with a gap of 4 μm.
Apply using a 50 μm applicator,
After drying for 1 minute, punch out to 19φ and 1.5 × 10 ⁸
Pressed at Pa. This was used as a negative electrode without firing. Other than that, it carried out similarly to Example 1. Table 1 shows the results.

Comparative Example 2 The procedure was performed in the same manner as in Example 1 except that silicon powder having an average particle diameter of 0.3 μm was used. Table 1 shows the results.

Comparative Example 3 The procedure was performed in the same manner as in Comparative Example 1, except that silicon powder having an average particle diameter of 0.3 μm was used. Table 1 shows the results.

Comparative Example 4 The firing temperature of the coating film of the negative electrode was 150
A battery was assembled in the same manner as in Example 1 except that the temperature was set to 0 ° C., and a cycle test was performed. The formation of the silicon compound was confirmed from the fact that the color of the negative electrode changed to gray by firing. On the other hand, in Examples 1, 2, and 3 and Comparative Examples 1 to 3, the negative electrode after firing was black, and it was confirmed that no silicon compound was generated. Also, the back side of the negative electrode (copper foil side)
When the part discolored to gray was analyzed by X-ray diffraction, a peak attributed to a compound consisting of Cu and Si was recognized. It is considered that silicon and the copper foil of the current collector also reacted. The charge / discharge capacity of the battery using this negative electrode was a very small value. Table 1 shows the results.

According to the analysis by ESCA, Examples 1, 2 and
3 and Comparative Example 2 confirmed that the silicon particles were not exposed on the surface of the negative electrode and were covered with the carbonaceous material. In Example 1, the initial capacity and the capacity storage rate were greatly improved as compared with Comparative Example 1. Further, by coating the current collector with a carbonaceous material (Example 2), the capacity retention rate was further improved.

When the electrolyte in which the electrode of Comparative Example 1 was immersed was analyzed by GC-MS, methyl difluorophosphate generated by the reaction of dimethyl carbonate as a solvent and LiPF _{6 as an} electrolyte on the silicon surface was detected. Was. further,
When the immersed electrode was analyzed by ESCA, the structure was not clear, but a compound containing silicon and fluorine was detected, and it was confirmed that the silicon particles reacted directly with the electrolytic solution.

In Comparative Example 2 in which silicon powder having a particle diameter of 0.3 μm was used as a raw material, the ratio of short-circuiting between the positive electrode and the negative electrode was increased as compared with Example 1 in which silicon powder having a particle diameter of 1 μm was used.
Furthermore, in Comparative Example 3 in which silicon was not coated with the carbonaceous material, a reaction product of the solvent and the electrolyte LiPF ₆ was detected, and both the initial capacity and the capacity retention were reduced. It is considered that when the particle diameter of the silicon powder as the raw material is small, the contact area with the electrolytic solution increases, so that not only the reaction with the electrolytic solution easily progresses but also the separator easily penetrates.

[0034]

[Table 1]

[0035]

As described above, in the non-aqueous secondary battery of the present invention, the negative electrode is a sintered body of silicon and carbon as the negative electrode active material, and the silicon particles are coated with the carbonaceous material. Therefore, the reaction between silicon and the electrolyte can be suppressed, and the storage stability of the battery can be improved.

Further, the non-aqueous secondary battery of the present invention forms a coating film containing silicon particles and a carbonaceous material or an organic material which becomes a carbonaceous material by heat treatment on a current collector and fires the coating in a non-oxidizing atmosphere. Since the thus prepared negative electrode is used, the silicon particles can be easily coated with the carbonaceous material.

Further, in the non-aqueous secondary battery of the present invention, since the current collector is coated with the carbonaceous material, elution of the current collector during overdischarge can be suppressed, and the storage stability of the battery is further improved. be able to.

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[Procedure amendment]

[Submission date] August 4, 2000 (200.8.4)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0007

[Correction method] Change

[Correction contents]

Namely, the nonaqueous secondary battery of the present invention, Li
A positive electrode containing an active material capable of occluding and releasing titanium ions;
Non-aqueous containing negative electrode and lithium ion conductive electrolyte
A secondary battery, wherein the negative electrode is made of a conductive metal
Current collector coated with a porous material, and formed on the current collector,
A sintered body of silicon as a negative electrode active material and a carbonaceous material.
And the silicon particles in the sintered body are covered with a carbonaceous material.
Characterized in that that.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0008

[Correction method] Change

[Correction contents]

As described above, on the exposed surface of the silicon particles, the silicon and the electrolytic solution react directly to form a compound containing silicon and fluorine, and further, the solvent and the electrolyte LiPF
_It is thought that the reaction with ₆ proceeds. For this reason, the reaction proceeds as the area of the silicon particles in direct contact with the electrolytic solution increases. Therefore, by coating the surface of the silicon particles with the carbonaceous material, it is possible to suppress the reaction of the silicon particles with the electrolyte on the surface of the silicon particles. Therefore, an irreversible reaction can be suppressed, and a decrease in battery capacity can be prevented. In addition, the surface of the current collector is coated with a carbonaceous material to collect the current.
Current collection by suppressing direct contact between the conductor and electrolyte
Elution during overdischarge of the body can be suppressed.

[Procedure amendment 4]

[Document name to be amended] Statement

[Correction target item name] 0009

[Correction method] Change

[Correction contents]

[0009] The non-aqueous secondary battery of the present invention, the negative
The pole is made of silicon particles and carbonaceous material or carbon
Film on the current collector containing organic material to be a porous material
Then, the coating film and the current collector are
Immersion or current collector in a coating liquid containing an organic material to be a base material
The coating liquid is applied to the coating solution, dried, and sintered in a non-oxidizing atmosphere, and a sintered body can be used as the negative electrode.

[Procedure amendment 5]

[Document name to be amended] Statement

[Correction target item name] 0010

[Correction method] Deleted

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Ryuichi Akagi 1334 Minato, Wakayama, Wakayama Pref. Inventor Tada Hirabayashi 1334 Minato, Wakayama-shi, Wakayama Pref.F-term in Kao Corporation Research Laboratory

Claims (3)

[Claims] 1. A non-aqueous secondary battery comprising a positive electrode, a negative electrode, and a lithium ion conductive electrolyte containing an active material capable of inserting and extracting lithium ions, wherein the negative electrode is made of a conductive metal. A non-aqueous system comprising a current collector and a sintered body of silicon and a carbonaceous material as a negative electrode active material formed on the current collector, wherein silicon particles in the sintered body are coated with a carbonaceous material. Rechargeable battery. 2. A negative electrode obtained by forming a coating film containing silicon particles and a carbonaceous material or an organic material which becomes a carbonaceous material by heat treatment on a current collector and sintering in a non-oxidizing atmosphere. The non-aqueous secondary battery according to claim 1. 3. The non-aqueous secondary battery according to claim 1, wherein the current collector after the formation of the sintered body is further coated with a carbonaceous material.