JP3327468B2 - Lithium ion secondary battery and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery, and more particularly to a surface coating of a negative electrode carbon material.

[0002]

2. Description of the Related Art In recent years, the weight and size of electronic devices have been rapidly reduced, and a secondary battery having a small size, light weight, and high energy density has been demanded as a driving power source for these devices. Above all, lithium secondary batteries are expected to have high voltage and high energy density.

[0003] As a negative electrode of a lithium secondary battery, lithium metal and lithium alloy have been conventionally used. However, lithium metal has problems in short-circuiting due to precipitation of resinous lithium (dendrites), safety at high temperatures, and the like. or,
Lithium alloys have problems such as short cycle life and low charge / discharge energy efficiency. Recently, in order to solve these problems, research on using a carbon material for a negative electrode is active.

However, lithium using a carbon material for the negative electrode
In an ion battery, LiPF is used as the electrolyte. _FourIncluding halogen
When a lithium salt is used, the lithium salt is stored inside the battery.
Hydrogen halide such as hydrogen fluoride reacting with mixed water
Generates acid, which affects the negative electrode and increases self-discharge
And the cycle characteristics are deteriorated. JP
Japanese Patent Application Laid-Open No. 5-257553 discloses that S and N are heterogeneous in an electrolyte.
Add a heterocyclic compound as an element and apply these
By forming a film of decomposition products, self-discharge and size
It is reported that the cycle life is improved.

[0005]

SUMMARY OF THE INVENTION The present invention provides a lithium secondary battery using a carbon material for a negative electrode, which is excellent in high-rate charge / discharge characteristics, has little self-discharge, and is excellent in charge / discharge cycle characteristics. The purpose is to do.

[0006]

In order to solve the above-mentioned problems, the present invention provides a positive electrode, a separator containing a non-aqueous electrolyte, and a negative electrode made of a carbon material. Lithium ion secondary battery characterized in that the surface of the constituting carbon particles has a coating layer having a carbonate structure, and the coating layer contains at least one element of boron, sulfur and nitrogen. It is. Further, the thickness of the coating layer is 5 nm to 100 nm.
It is a lithium ion secondary battery characterized by the following. Further, the electrolyte contains at least a carbonate-based solvent, the positive electrode before the initial charge contains one or more elements of boron, sulfur, and nitrogen, and performs the initial charge at a temperature of 40 ° C. or more. A method for producing a lithium ion secondary battery, which is a feature of the present invention.

When a carbon material is used for the negative electrode, the reaction of inserting and extracting lithium into and from the carbon material is largely controlled by the state of a film formed between the electrolyte and the carbon surface. Explaining the lithium metal negative electrode as a model, if a lithium metal surface has a dense and highly ion-conductive coating, it exhibits excellent battery characteristics. The rate characteristics and charge / discharge cycle characteristics deteriorate. Here, it is known that the former film component is lithium carbonate, lithium oxide or the like, and the latter film component is lithium fluoride or the like. The same can be said for a film formed on the surface of a carbon material. That is, one of the factors that increase the interfacial resistance of the carbon material is that the carbon material surface
For example, a film having low ion conductivity such as lithium fluoride may be formed.

The present inventors have conducted research on a film formed on the surface of a carbon material. As a result, one or more of boron, sulfur, and nitrogen were added to a film layer having a carbonate structure formed on the surface of the negative electrode carbon. It has been found that charge / discharge cycle performance is significantly improved by containing an element and controlling the thickness of the coating layer to 5 nm to 100 nm.

As a method of forming the film of the present invention on the surface of the negative electrode carbon, for example, a positive electrode material containing any one element of boron, sulfur and nitrogen is used, and a charging operation is performed. There is a method in which the potential is dissolved in the electrolytic solution while the negative electrode potential decomposes the carbonates in the electrolytic solution to form a film while taking in the element. At this time, by controlling the temperature at the time of the initial charge of the battery, the speed of the film forming reaction can be controlled, and by controlling the time of the initial charge, the thickness of the film can be controlled. As a method for incorporating elements such as boron, sulfur, and nitrogen into the positive electrode material, B ₂ O ₃ , B ₂ S ₃ , BH _3.
A material such as N (CH) ₃ may be mixed with the positive electrode active material, or LiCoO ₂ or LiMn ₂ O ₄
Alternatively, a lithium metal oxide obtained by substituting a part of Co and Mn elements with boron or the like may be used. When a lithium metal oxide substituted with boron or the like is used, the temperature of the initial charging step is set to 4
The effect of the present invention can be obtained by performing the process at 0 ° C. or higher and setting the positive electrode potential to 4.0 V or higher.

[0010] The controlled carbonate structure coating layer comprises:
It is not necessary to form on the surface of all the carbon particles of the negative electrode, and it is sufficient if it is formed on at least the surface of the carbon particles constituting the portion in contact with the separator.

[0011] The "thickness of the coating layer" in the claims means that the battery is disassembled, the negative electrode plate is taken out, the electrolyte is washed and removed with a solvent such as diethyl carbonate, and vacuum drying is performed. It means the value measured after performing.

[0012] In the battery, the coating layer is considered to be swollen by the electrolyte, so that the actual thickness of the coating is considered to be larger than this. In order to measure the actual thickness of the coating layer, a method of embedding the electrode in a resin such as epoxy, cutting the cross section and measuring the distance from the carbon surface to the embedded resin with an electron microscope or the like can be used. However, in this method, the swelling coefficient varies depending on the composition of the electrolytic solution, the structure of the carbonate skeleton to be formed, and the like, and a large error between samples is likely to appear.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is not limited by the following description, and the test method and the positive electrode active material, the negative electrode active material, the positive electrode, the negative electrode, the electrolyte, the separator, the shape of the battery and the like of the battery to be constituted are arbitrary. is there. FIG. 1 shows an example of an embodiment of the battery of the present invention.

The carbon material used for the negative electrode 9 may be any carbon material capable of occluding and releasing lithium. In particular, the plane spacing (d ₀₀₂ ) estimated by X-ray diffraction method is 0.
Carbon particles having a crystallite size (Lc) of 3354 to 0.3369 nm in the c-axis direction of 20 nm or more are preferred.

As the positive electrode 7, LiCoO ₂ , LiNi
Lithium metal oxides such as O ₂ and LiMn ₂ O _4;
Co, Ni of iCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ ,
An oxide or the like in which a part of Mn is substituted with another element can be used. As the other elements, Li, B, V, A
1, Ni, Co, Mg, Cr, Tb and the like.

As the separator 8, an insulating thin film having excellent ion permeability and mechanical strength can be used. From the viewpoint of organic solvent resistance and hydrophobicity, olefin polymers such as polypropylene and polyethylene are used. The pore size of the separator is in a range generally used for a battery, and is, for example, 0.01 to 1 μm. The same applies to the thickness, which is in the range generally used for batteries, for example, 5 to 40 μm.

The electrolyte includes LiPF ₆ , LiBF ₄ , LiAsF ₆ and LiOSO ₂ exhibiting high lithium ion conductivity.
CF _3, etc. and the like can be used. These fluorinated electrolytes are used after being dissolved in a non-aqueous solvent at a concentration of usually 0.1 M to 3.0 M, preferably 0.5 M to 2.0 M.

The non-aqueous solvent is preferably used in a combination of a high dielectric constant solvent and a low viscosity solvent. Examples of the high dielectric constant solvent include ethylene carbonate (E
C) and cyclic carbonates such as propylene carbonate (PC) are preferably used. These high dielectric constant solvents may be used alone or in combination of two or more.

Examples of the low viscosity solvent include dimethyl carbonate (DMC) and methyl ethyl carbonate (M
EC), chain carbonates such as dimethyl carbonate (DMC), and lactones such as γ-butyrolactone. These low-viscosity solvents may be used alone or in combination of two or more.

[0020]

The present invention will be described below in further detail with reference to examples.

(Battery 1 of the Present Invention) Negative electrode 9 was obtained as follows. 10% by weight of polyvinylidene fluoride (PVDF) powder as a binder was added to artificial graphite (particle diameter: 6 μm),
N-methylpyrrolidone is added as a solvent, kneaded and dispersed,
A coating solution was prepared. The coating solution was applied to both sides of a copper foil current collector having a thickness of 10 μm, and the thickness including the current collector was adjusted to 180 μm by a roll press to prepare a negative electrode sheet having a capacity of 7 mAh / cm ² . The negative electrode sheet has a width of 65
The sheet was cut into a shape having a height of 111 mm and a copper lead plate having a thickness of 10 μm and a width of 10 mm was attached to the end of the sheet.
It dried under reduced pressure at 12 degreeC for 12 hours, and was set as the negative electrode 9.

The positive electrode 7 is made of LiCo as a positive electrode active material.
O ₂ , B ₂ O ₃ , acetylene black as a conductive agent and polyvinylidene fluoride (PVDF) powder as a binder were mixed at a weight ratio of 84: 1: 10: 5, and N was used as a solvent.
-Methylpyrrolidone was added and kneaded and dispersed to prepare a coating solution. The coating solution was applied to both sides of a 20 μm-thick aluminum foil current collector, and the thickness including the current collector was adjusted to 2 by a roll press.
The thickness was adjusted to 30 μm to prepare a positive electrode sheet having a capacity of 6.3 mAh / cm ² . The positive electrode sheet was cut into a shape having a width of 61 mm and a height of 107 mm.
Attach an aluminum lead plate with a width of 10 mm
After drying under reduced pressure at 50 ° C. for 12 hours, a positive electrode 7 was obtained.

A polyethylene microporous membrane bag is 65 mm wide.
× It was formed into a bag shape having a height of 111 mm and used as the separator 8. The positive electrode 7 inserted in the bag of the separator 8 and the negative electrode 9 were alternately laminated to obtain an electrode group including 40 positive electrodes 7 and 41 negative electrodes 9. This electrode group was wrapped in an insulating film made of a polyethylene resin, and housed in a rectangular battery case 10 made of aluminum. The lead plates of the positive electrode 7 and the negative electrode 9 were connected to the positive electrode terminal 5 and the negative electrode terminal 4 attached to the lid 2 by bolts, respectively. The terminals are insulated using a gasket 6 made of polypropylene resin.

A lid 2 made of aluminum having a safety valve 1 and a battery case 19 are welded by a laser to a width of 70 mm and a height of 1 mm.
A prismatic battery having a width of 30 mm (136 mm including terminals) and a width of 22 mm was produced. 3 is a laser weld.

In a solvent in which fully purified ethylene carbonate and sufficiently purified diethyl carbonate are mixed at a volume ratio of 1: 1, 1 well purified LiPF ₆ is added.
The solution dissolved in mol / l was used as an electrolyte, and 65 g of the electrolyte was injected into the prismatic battery and sealed.

Next, as a film forming step, the prismatic battery was charged at a constant voltage of 1.5 A and 4.2 V at 20 ° C., and then left at 50 ° C. for 3 days. The potential of the positive electrode during standing is 4.0 V or more (vsLi / Li ⁺ ), and the potential of the negative electrode is 0.
It was 3 V or less (vsLi / Li ⁺ ).

In this way, the capacitance 15Ah shown in FIG.
The prototype non-aqueous electrolyte battery was manufactured. This battery is referred to as Battery 1 of the invention.

(Battery 2 of the Present Invention) Among the positive electrode materials, B ₂ O ₃
A battery manufactured in the same manner as the battery 1 of the present invention except that B ₂ S ₃ was used in place of After charging, the potential of the positive electrode during standing was 4.0 V or more (vsLi / Li ⁺ ), and the potential of the negative electrode was 0.3 V or less (vsLi / Li ⁺ ).

(Battery 3 of the Present Invention) Among the positive electrode materials, B ₂ O ₃
A battery manufactured in the same manner as the battery 1 of the present invention except that BH ₃ .N (CH ₃ ) ₃ was used instead of
After charging, the potential of the positive electrode during standing was 4.0 V or more (vsLi / Li ⁺ ), and the potential of the negative electrode was 0.3 V or less (vsLi / Li ⁺ ).

(Battery 4 of the Present Invention) The film forming step was performed at 20 ° C.
After applying a constant voltage charge of 1.5 A and 4.2 V at
Battery 1 of the present invention except that it was left at 60 ° C. for 4 days
A battery manufactured in the same manner as in the above is referred to as Battery 4 of the invention. The potential of the positive electrode during standing was 4.0 V or more (vsLi / Li ⁺ ), and the potential of the negative electrode was 0.3 V or less (vsLi / Li ⁺ ).

(Battery 5 of the Present Invention) The film forming step was performed at 20 ° C.
After applying a constant voltage charge of 1.5 A and 4.2 V at
Battery 1 of the present invention except that it was left at 60 ° C. for 7 days
A battery manufactured in the same manner as in the above is referred to as Battery 5 of the invention. The potential of the positive electrode during standing was 4.0 V or more (vsLi / Li ⁺ ), and the potential of the negative electrode was 0.3 V or less (vsLi / Li ⁺ ).

(Comparative Battery 1) In the film forming step, after applying a constant voltage of 1.5 A and 4.2 V at 20 ° C.,
Battery 1 of the present invention except that it was left at 0 ° C. for 14 days
A battery fabricated in the same manner as in Comparative Example 1 is referred to as Comparative Battery 1. The potential of the positive electrode during standing was 4.0 V or more (vsLi / Li ⁺ ), and the potential of the negative electrode was 0.3 V or less (vsLi / Li ⁺ ).

(Comparative Battery 2) The cathode material is LiCo
A battery manufactured in the same manner as in Example 1 except that O ₂ , acetylene black and PVDF powder were mixed at a weight ratio of 85: 10: 5 is referred to as Comparative Battery 2. The potential of the positive electrode during standing was 4.0 V or more (vsLi / Li ⁺ ), and the potential of the negative electrode was 0.3 V or less (vsLi / Li ⁺ ).

(Comparative Battery 3) The procedure of Example 1 was repeated except that the film-forming step was performed after a constant voltage charge of 1.5 A and 4.2 V at 20 ° C. and a standing time at 20 ° C. of 12 hours or less. A battery similar to that described above is referred to as Comparative Battery 3. The potential of the positive electrode during standing is 4.0 V or more (vsLi /
Li ⁺ ), and the potential of the negative electrode was 0.3 V or less (vsLi / Li ⁺ ).

Each of the batteries 1 to 4 of the present invention and the comparative batteries 1 and 2 was prepared in a plural number, a part of which was subjected to the analysis of the coating, and a part was subjected to the charge / discharge test.

(Analysis of Coating) The batteries 1 to 5 of the present invention and the comparative batteries 1 to 3 were disassembled, and the negative electrode 9 was sufficiently washed with diethyl carbonate. Then, the surface of the negative electrode dried in vacuum was processed by a focused ion beam processing apparatus. did. The cross section of the negative electrode after processing was confirmed by an electron microscopic observation photograph, and the thickness of the coating layer was measured. Table 1 shows the measurement results.

Further, by X-ray photomolecular spectroscopy,
The main components of the coating were analyzed. As a result, it was confirmed that the coatings of all the batteries had a carbonate skeleton. In addition, elements other than C (carbon) detected in the coating of each battery are also shown in Table 1.

[0038]

[Table 1]

(Charge / Discharge Test) (1) Measurement of 0.1 It Discharge Capacity Using batteries 1 to 5 of the present invention and comparative batteries 1 to 3,
1.5 A after discharging to 3 V at A (0.1 It )
(0.1 It ) for 15 hours at a constant voltage of 4.2 V,
Five cycles of repetition of charge / discharge for performing constant current discharge up to 3 V at 1.5 A (0.1 It ) were performed. Table 2 shows the results of the obtained discharge capacity at the fifth cycle. (2) Measurement of 2 It Discharge Capacity Subsequently, a charge / discharge cycle of performing a constant voltage charge of 4.2 V for 1.5 hours at 15 A (1 It ) and a constant current discharge of 3 V at 30 A (2 It ) is performed. Was. Table 2 also shows the ratio between the obtained discharge capacity and the 0.1 It discharge capacity. (3) 1 It subsequently measuring the cycle life, 15A and constant voltage charge for 1.5 hours 4.2V at (1 It), the charge and discharge continuously for at 15A (1 It) performs constant current discharge to 3V Repeated. The cycle number at which the discharge capacity decreased to 80% of the 0.1 It discharge capacity was defined as the cycle life. The results are shown in Table 2.

[0040]

[Table 2]

Comparing the results of the batteries 1 to 3 of the present invention and the comparative battery 2 under the same film forming process, the battery 1 of the present invention in which B ₂ O ₃ was added to the positive electrode did not contain B ₂ O _3. The battery exhibited better cycle performance than Comparative Battery 2. Also, B
_The battery 2 of the present invention and the battery 3 of the present invention in which B ₂ S ₃ or BH ₃ .N (CH) ₃ was added to the positive electrode instead of ₂ O ₃ exhibited better cycle performance than the battery 1 of the present invention.

Next, when B ₂ O ₃ was added to the positive electrode and the results of the batteries 1, 4 and 5 of the present invention and the comparative batteries 1 and 3 having different coating thicknesses were compared, the comparative battery 3 having a coating thickness of 1 nm was compared. In comparison, batteries having a coating thickness of 5 nm or more show good cycle performance. However, the 2 It discharge capacity decreases as the film thickness increases, and in the comparative battery 1 having a film thickness of 200 nm,
It has reached 76% of the 0.1 It discharge capacity.

In the above example, LiCoO ₂ and artificial graphite were used as the positive electrode material and the negative electrode material, respectively, but similar effects were confirmed for batteries using other positive electrode materials and negative electrode materials.

[0044]

According to the above embodiment, B ₂ O added to the positive electrode
₃ absorbs moisture in the air in the process of making the positive electrode sheet,
It is considered that boric acid was formed, and it was converted into boric acid via metaboric acid by a vacuum drying process at 150 ° C. When these substances are applied with a potential of at least 4.0 V,
It is known to be eluted in the electrolyte. It is considered that the eluted boron compound reaches the negative electrode, and a film having a carbonate structure in which boron is incorporated is generated at the negative electrode at a potential of 0.3 V or less.

It is not clear why the cycle characteristics are improved by adding an element such as boron or nitrogen to the carbonate structure film formed on the carbon surface of the negative electrode and controlling the film thickness. They consider as follows.

When the negative electrode carbon material absorbs lithium by charging, the carbon surface is placed in a reducing atmosphere close to the lithium potential. At this time, a film having high ion conductivity such as lithium carbonate is formed at the contact interface between the electrolyte and the carbon material. However, when carbonates are used as the solvent of the electrolytic solution, a coating layer having a carbonate structure with high ion conductivity is formed. However, since the film having the carbonate structure is fragile, the film having the carbonate structure is broken by repeated charge and discharge. As a result, the carbon surface is exposed to hydrohalic acid generated by the reaction between the lithium salt and a trace amount of water, and a coating layer having low ion conductivity such as lithium fluoride is formed. In addition, when the coating layer is broken, the decomposition of the electrolytic solution also proceeds.

Here, it is considered that when the coating layer contains an element such as boron, sulfur, or nitrogen, there is an effect of strengthening the coating having the carbonate structure.

Referring to the results of the examples, the effect is as follows.
When the element is contained alone, it is highest for boron and lowest for sulfur and nitrogen. However, when sulfur and nitrogen are mixed with boron, they exhibit more excellent effects than when each is used alone.

It was also found that lithium fluoride formed on the coating layer of the present invention had little effect on lowering the properties of the carbon anode.

The negative electrode of Comparative Battery 2 was analyzed for its negative electrode surface coating using an X-ray photo-molecular spectrometer while performing argon etching. As a result, the lowermost layer of the carbonate skeleton coating layer, that is, graphite-derived carbon Many lithium fluorides (LiF) were detected in the vicinity of the surface of the carbon negative electrode to the extent that the peak of was detected. The same analysis was performed on the negative electrode of the battery 1 of the present invention. As a result, the lowermost layer of the carbonate skeleton coating layer, that is,
Although lithium fluoride (LiF) was detected in the vicinity of the surface of the carbon negative electrode at which a carbon peak derived from graphite was detected, the amount was much smaller than that of the comparative battery 2.

From these facts, by providing a coating layer having a carbonate skeleton on the negative electrode using a carbon material in a thickness of 5 to 100 nm, an undesired coating such as LiF is less likely to be generated on the carbon surface, and excellent cycle performance is exhibited. It is considered something.

As described above, according to the present invention, a cycle having a high rate charge / discharge characteristic can be obtained by a simple method of adding a small amount of additive to the positive electrode and controlling the storage temperature and storage time after the initial charge. A lithium secondary battery having excellent life characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a battery of the present invention.

[Explanation of symbols]

 7 positive electrode 8 separator 9 negative electrode

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Akihiro Fujii 2-3-1-21, Kosobe-cho, Takatsuki-shi, Osaka Inside Yuasa Corporation Co., Ltd. (72) Inventor Hiroshi Yufu, 2-3-1-21, Kosobe-cho, Takatsuki-shi, Osaka Examiner at Yuasa Corporation Inc. Mitsuji Uemae (56) References JP-A 7-14572 (JP, A) JP-A 8-45509 (JP, A) JP-A 2001-43858 (JP, A) JP JP-A-7-176323 (JP, A) JP-A-9-147864 (JP, A) JP-A-11-144711 (JP, A) JP-A-7-135021 (JP, A) A) JP-A-9-259885 (JP, A) JP-A-10-261406 (JP, A) JP-A-10-270089 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) ) H01M 4/02 H01M 4/62 H01M 10/40

Claims (3)

(57) [Claims] An anode comprising a positive electrode, a separator containing a non-aqueous electrolyte, and a negative electrode made of a carbon material, wherein at least a portion of the carbon particles constituting a part of the negative electrode in contact with the separator has a coating layer having a carbonate structure. A lithium ion secondary battery wherein the coating layer contains at least one element of boron, sulfur and nitrogen. 2. The method according to claim 1, wherein said coating layer has a thickness of 5 nm to 100 nm.
The lithium ion secondary battery according to claim 1, wherein 3. The electrolyte solution contains at least a carbonate-based solvent, and the positive electrode before the initial charge contains one or more elements of boron, sulfur and nitrogen, and performs an initial charge at a temperature of 40 ° C. or more. A method for producing a lithium ion secondary battery.