JP2001250534A - Lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery, using a carbon material for a negative electrode, having high capacity even in rapid charge/discharge, less self-discharging and improved cycle characteristics. SOLUTION: The lithium ion secondary battery comprises a coating layer with its carbon-particle surface defining at least a portion of the negative electrode contacting a separator and having a carbonate structure, the coating layer being controlled to be in the range of 5 nm-200 nm in thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention 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 electrolytic solution.
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 comprises a positive electrode, a separator containing a non-aqueous electrolyte, and a negative electrode made of a carbon material capable of inserting and extracting lithium ions. The surface of the carbon particles constituting at least the portion in contact with the separator has a coating layer having a carbonate structure, and the coating layer has a thickness of 5 nm to 20 nm.
A lithium secondary battery having a thickness of 0 nm.

When a carbon material is used for the negative electrode, the reaction of absorbing and releasing lithium into and from the carbon material is largely controlled by the state of a film formed between the electrolytic solution 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, when carbonates are used as a solvent for the electrolytic solution, particularly in the initial charging step of the battery, the electrolytic solution is deposited on the carbon surface of the negative electrode. The thickness of the carbonate-structured film formed by partial decomposition is 5 nm to 200 nm.
It has been found that the charge / discharge cycle performance is significantly improved by controlling the temperature within the range.

As a method of forming a film having a controlled carbonate structure on the surface of the negative electrode carbon material, for example, by controlling the temperature at the time of initial charging of the battery, the speed of the film forming reaction can be controlled. By controlling the time of the initial charge, the thickness of the coating can be controlled.

[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 surfaces 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 produce 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_TwoAcetylene black as a conductive agent and a binder
Polyvinylidene fluoride (PVDF) powder at a weight ratio of 8
The mixture was mixed at a ratio of 5: 10: 5, and N-methylpi
Loridone was added, kneaded and dispersed to prepare a coating solution. Said
Apply coating solution on both sides of 20μm thick aluminum foil current collector
And roll collector to reduce the thickness including the current collector to 230 μm
6.3mAh / cm ^TwoPositive electrode sheet with a capacity of
Was made. The positive electrode sheet has a width of 61 mm and a height of 107
mm at the end of the sheet, thickness 20μm width 1
Attach 0mm aluminum lead plate, and at 150 ℃
After drying under reduced pressure 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.

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)
A battery manufactured in the same manner as the battery 1 of the present invention except that the battery was charged at a constant voltage of 1.5 A and 4.2 V at 0 ° C. and then left at 50 ° C. for 6 days is referred to as a battery 2 of the present invention.

(Battery 3 of the present invention)
A battery prepared in the same manner as the battery 1 of the present invention except that the battery was charged at a constant voltage of 1.5 A and 4.2 V at 0 ° C. and then left at 60 ° C. for 7 days is referred to as a battery 3 of the present invention.

(Battery 4 of the present invention)
A battery manufactured in the same manner as the battery 1 of the present invention except that the battery was charged at a constant voltage of 1.5 A and 4.2 V at 0 ° C. and then left at 60 ° C. for 14 days is referred to as a battery 4 of the present invention.

(Comparative Battery 1) As a film forming step, 20
A battery prepared in the same manner as the battery A of the present invention except that the battery was subjected to constant voltage charging at 1.5 A and 4.2 V at ℃ and left at 60 ° C. for 21 days is referred to as a comparative battery 1.

(Comparative Battery 2) Instead of the film forming step, 2
A battery fabricated in the same manner as the battery 1 of the present invention except that the battery was subjected to a constant voltage charge of 1.5 A and 4.2 V at 0 ° C. and then left at 20 ° C. for 12 hours or less was designated as a comparative battery 2. .

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 and discharge test.

(Analysis of Coating) The batteries 1 to 4 of the present invention and the comparative batteries 1 and 2 were disassembled, and the negative electrode 9 was sufficiently washed with diethyl carbonate. Then, the surface of the negative electrode dried under 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.

Also, 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.

(Charge and Discharge Test) (1) Measurement of 0.1 hour rate discharge capacity Using batteries 1 to 4 of the present invention and comparative batteries 1 and 2,
1.5 A after discharging to 3 V at A (0.1 hour rate)
(0.1 CA) at a constant voltage of 4.2 V for 15 hours,
Five cycles of repetition of charge / discharge for performing constant current discharge up to 3 V at 1.5 A (0.1 hour rate) were performed. 5 obtained
The results of the discharge capacity at the cycle are also shown in Table 1.

(2) Measurement of 2 hour rate discharge capacity Subsequently, constant voltage charging of 4.2 V at 15 A (1 hour rate) for 1.5 hours and constant current discharging of 3 V at 30 A (2 hour rate) are performed. The charge and discharge were performed for one cycle. Table 1 also shows the ratio between the obtained discharge capacity and the 0.1 hour rate discharge capacity.

(3) Measurement of 1 hour rate cycle life Subsequently, constant voltage charging of 4.2 V for 1.5 hours at 15 A (1 hour rate) and constant current discharging up to 3 V at 15 A (1 hour rate) are performed. Charge and discharge were continuously repeated. The cycle number at which the discharge capacity decreased to 80% of the 0.1 hour rate discharge capacity was defined as the cycle life. The results are shown in Table 1.

[0039]

[Table 1]

From the above results, it is preferable that the film thickness is 100 nm or less in order to secure a high high rate discharge capacity, and it is necessary that the film thickness is 5 nm or more in order to obtain a sufficient cycle life. I found it.

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

[0042]

The reason why the cycle life characteristics are improved by controlling the thickness of the film is not necessarily clear, but the present inventors 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 electrolytic solution 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, the thickness of the carbonate structure film is about 1 nm and is brittle, so that the carbonate structure film is broken by repeated charge and discharge. As a result, the carbon surface is exposed to a hydrohalic acid such as hydrogen fluoride generated by a reaction between a 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.

If the thickness of the coating exceeds 200 nm, lithium ion conduction is impaired, which adversely affects high-rate charge / discharge performance. When the thickness of the film is less than 5 nm, the film is easily broken by repeated charge and discharge, and the function as a protective film cannot be exhibited.

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 2 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 the carbon material-containing negative electrode with a coating layer having a carbonate skeleton 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 lithium secondary battery having high rate charge / discharge characteristics and excellent cycle life characteristics can be obtained by a simple method such as controlling the storage temperature and storage time after initial charging. 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 (72) Inventor Hiroshi Yufu 2-3-1-21, Kosobe-cho, Takatsuki-shi, Osaka No. F-term in Yuasa Corporation (reference)

Claims (1)

[Claims] A positive electrode, a separator containing a non-aqueous electrolyte,
A negative electrode made of a carbon material capable of occluding and releasing lithium ions; a surface of carbon particles constituting at least a portion of the negative electrode in contact with the separator has a coating layer having a carbonate structure; and a thickness of the coating layer is 5 nm to 200n
m, a lithium-ion secondary battery.