JPH10125307A - Lithium-ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce dispersion of a battery characteristic and improve a cycle life characteristic by applying the constitution that the surface of a specific positive electrode active material be covered with an organic substance containing amino acids. SOLUTION: A positive electrode active material 12 is expressed by a general formula Lix MOz , where M stands for a transition metal selected from Co, Mn, Ni, V, Fe and Cr, and (x) for a value between 0.3 and 1.2, and (z) for a value between 1.4 and 3. In addition, the surface of the positive electrode active material 12 is covered with an organic substance containing amino acids. The organic substance containing amino groups is preferably an organic compound having at least one amino group (NH2 <-> ) in a molecule, and expressed by a general formula of NH2 -R-. Also, the introduction of the organic compound to the positive electrode active material surface is achieved by use of a coupling agent, expressed by a general formula of (R-O)XYq , where (q) stands for a value of 2 or 3, Y for a hydrolytic group for a hydrophilic group and (x) represents Si, Ti, Al, and R for an organic functional group for a hydrophobic group. In this case, a functional group having an amino group is selected specifically for R.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium ion secondary battery. More specifically, the present invention relates to a configuration of a positive electrode active material of a non-aqueous electrolyte lithium ion secondary battery.

[0002]

2. Description of the Related Art In recent years, with the advance of electronic technology, high performance, portable and cordless electronic devices have been rapidly advanced. Along with this, batteries used as power supplies for these portable electronic devices have also become smaller, lighter,
There is an increasing demand for higher energy density. Under such circumstances, non-aqueous electrolyte secondary batteries, especially lithium ion secondary batteries, in addition to high voltage, high energy density, also has the feature that there is little self-discharge and no memory effect, Further, it is known that the battery has high safety.

As for such a positive electrode active material of a lithium ion secondary battery, many lithium transition metal composite oxides have been studied, and good performance has been obtained.

[0004]

However, since the lithium transition metal composite oxide constituting such a positive electrode active material has a substantially large surface energy, aggregation of the positive electrode active materials easily occurs. And conductive material,
In a slurry for electrode coating obtained by mixing a binder and a solvent, the dispersibility of the positive electrode active material is reduced, and the mixture is locally solidified in the slurry and the distribution becomes uneven. For this reason, there is a possibility that the electrode composition becomes uneven, the battery characteristics are deteriorated, and the characteristics of the products vary widely, resulting in a decrease in reliability. In addition, since the positive electrode active material has a large surface energy, moisture is adsorbed. If the adsorbed water is not removed by vacuum drying or the like before use, the battery characteristics, particularly the cycle characteristics, are remarkably deteriorated.

In order to solve these problems, a method of treating a composite oxide powder of lithium and a transition metal with a silane coupling agent (JP-A-8-111243) or a method of treating with a titanate coupling agent (JP-A-Hei. 4-2
72668) has been proposed.

However, in some cases, even if the techniques described in these publications are not sufficiently improved, the characteristics of the positive electrode active material are not improved by amino groups which are described later. It does not positively state the effect of coating and does not objectively regulate the amount of amino groups introduced to the surface of the positive electrode active material. Therefore, these publications do not suggest a technique capable of sufficiently addressing the above-described problem.

The present invention has been made in view of such circumstances, and has been shown to positively demonstrate the effect of introducing an amino group onto the surface of the positive electrode active material, and to reduce the amount of the amino group introduced onto the surface of the positive electrode active material. The objective specification using X-ray photoelectron spectroscopy (XPS) enables precise control of electrode composition, control of variation in battery characteristics, and non-aqueous solution having good battery characteristics, especially good cycle characteristics. It is intended to obtain an electrolyte secondary battery.

[0008]

According to the present invention, there is provided a lithium ion secondary battery comprising a positive electrode active material, a negative electrode active material, and an electrolyte containing a lithium salt. Formula Li _x MO _z (1) where M is a transition metal containing at least one selected from Co, Mn, Ni, V, Fe and Cr, and x = 0.
3 to 1.2, z = 1.4 to 3, and a lithium ion secondary battery characterized in that the surface of the positive electrode active material is coated with an organic substance containing an amino group.

The organic substance containing an amino group referred to in the present invention is:
An organic compound having at least one amino group (NH ₂ —) in a molecule is represented by the following general formula (2) NH ₂ —R— (2) Where R is, for example, -C _n H _{2n (n} is an integer)
Represents an alkyl chain or the like represented by the following formula, but is not particularly limited to such an alkyl chain: -C ₂ H ₄ -NH-C
NH or CO, such as ₂ H ₄ or —COCHC ₃ H ₆ —
Those containing a CH group can also be used. This R may be linear or branched.

The introduction of the organic compound containing an amino group represented by the general formula (2) onto the surface of the positive electrode active material is achieved by various coupling agents represented by the following general formula (3). (RO) XY _q (q = 2 or 3) (3) In the formula, Y represents a hydrolyzable group of a hydrophilic group.
_{-OCH 3, -OC 2 H5, -i} -C 3 H 7, -OCOC
_{H 3, OC 2 H 4 OCH} 3, -N (CH 3) 2, groups such as Cl and the like. In the formula, X is Si, Ti, Al, or the like. Wherein R represents an organic functional group of the hydrophobic group, such as represented by the general formula (2), but the functional group is chosen to have at least one amino group in the molecule, as an example, NH _{2
C 3 H 6 -, NH 2} C 2 H 4 NHC 3 H 6 -, NH 2 COCHC
_{3 H 6 -, NH 2 C} 2 H 4 NHC 2 H 4 NHC 3 H 6 -, and the like.

Specific examples of the coupling agent include γ-aminopropyltriethoxysilane, γ-aminopropyltriethoxytitanate, γ-aminopropyldiethoxyaluminate, γ-aminopropyltrimethoxysilane, γ-aminopropyltriethoxysilane. Methoxy titanate, γ-
Aminopropyl dimethoxyaluminate, N-β- (aminoethyl) -γ-aminopropyltriethoxysilane, N-β- (aminoethyl) -γ-aminopropyltriethoxytitanate, N-β- (aminoethyl) -γ −
Aminopropyldiethoxyaluminate, N-β- (aminoethyl) -γ-aminopropyltrimethoxysilane, N-β- (aminoethyl) -γ-aminopropyltrimethoxytitanate, N-β- (aminoethyl)- γ
-Aminopropyldimethoxyaluminate, N-β-
(Aminoethyl) -β-aminoethyltriisopropoxysilane, N-β- (aminoethyl) -β-aminoethyltriisopropoxytitanate, (aminoethyl)-
β-aminoethyldiisopropoxyaluminate, etc., preferably γ-aminopropyltriethoxytitanate, γ-aminopropyltrimethoxytitanate, N-β- (aminoethyl) -γ-aminopropyltriethoxy Titanate, N-β- (aminoethyl) -γ-aminopropyltrimethoxytitanate, N
And titanate coupling agents such as -β- (aminoethyl) -β-aminoethyltriisopropoxytitanate.

[0012] The amount of coupling agent used is defined in the state after the final treatment as follows. That is, the amount of amino groups introduced on the surface of the positive electrode active material is defined using X-ray photoelectron spectroscopy (XPS). This coupling agent treatment condition may be any condition, and only the final amino group introduction amount is subject to regulation. Specifically, the amount of amino group introduced on the surface of the positive electrode active material is determined by the sensitivity-corrected peak area intensity (I) of nitrogen (N) obtained by using X-ray photoelectron spectroscopy (XPS). _N ) and the general formula (1)
The ratio I _N / I _M sensitivity corrected peak area intensity of the metal element in _(M) (I M) is 0.01 to 10.0, is preferably defined by the value of 0.1 to 5.0 . The ratio _IN / _IM is a value that varies depending on the length of the R group in the general formula (3) and the amount of the coupling agent introduced in the coupling agent to be used. The ring agent itself becomes difficult to handle, causing a problem in work. If the amount is too small, the effect cannot be expected so much. Conversely, if the amount is too large, the slurry properties change and the coating becomes difficult. Therefore, the amount of the coupling agent introduced is desirably within the above range.

As the positive electrode active material, a lithium-containing transition metal oxide which can be doped and dedoped with lithium is used. For example, as such a lithium-containing transition metal oxide, a lithium-containing manganese oxide (L
iMn ₂ O ₄ ), lithium-containing cobalt oxide (LiCo)
O ₂ and the like), lithium-containing nickel oxide (LiNiO
₂ ) as well as lithium-containing iron oxides, lithium-containing chromium oxides, lithium-containing vanadium oxides, and lithium-containing transition metals containing at least two transition metals selected from the group consisting of these transition metals. A composite oxide (0 <x <1 such as LiNi _x Co _1-x O ₂ ) is exemplified. Alkali metals other than lithium (elements IA and IIA of the periodic table), metalloids Al, Ga, In,
Ge, Sn, Pb, Sb, Bi and the like may be mixed. The mixing amount is preferably 0 to 10 mol%.

On the other hand, as the negative electrode material, a carbon material capable of doping and undoping lithium is used. For example, as such carbon materials, pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, organic polymer compound fired bodies (furan resin, etc.) are suitable. Fired at a temperature and carbonized), carbon fiber, activated carbon and the like can be used. Especially,
(002) spacing of planes is 3.70 or more, true density 1.70 g
/ Cc, and a carbon material that does not have an exothermic peak at 700 ° C. or higher in differential thermal analysis in an air stream.

As the electrolyte, a non-aqueous electrolyte obtained by dissolving a lithium salt as a supporting electrolyte in a non-aqueous solvent is used.

The non-aqueous solvent is not particularly limited.
Propylene carbonate, ethylene carbonate, 1,
A mixed solvent of 2-dimethoxyethane, γ-butyl lactone, tetrahydrofuran, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, dipropyl carbonate, or a mixture of two or more thereof is used.

As the electrolyte, those commonly used in lithium batteries can be used.
O ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiCl,
LiBr, CH ₃ SO ₃ Li, CF ₃ SO ₃ Li, etc. are used alone or in combination of two or more.

Further, a solid electrolyte may be used instead of the non-aqueous electrolyte.

As described above, by introducing an amino group to the surface of the positive electrode active material, the surface energy of the surface of the positive electrode active material is reduced, and aggregation of the positive electrode active materials is suppressed. For this reason, the dispersibility of the positive electrode active material in the slurry is improved, and as a result, the composition of the electrode becomes more homogeneous, and the difference and dispersion of the battery characteristics are suppressed. Also, by introducing an amino group to the outermost surface, adsorption of moisture on the surface of the positive electrode active material can be suppressed, and since this amino group traps an acid component which is an impurity in the electrolytic solution, the positive electrode active material due to the impurity is trapped. The decomposition reaction on the surface can be suppressed. Thereby, the battery characteristics are comprehensively improved, and particularly, the cycle characteristics can be improved.

[0020]

1 (A), 1 (B) and 1 (C) are explanatory views of the configuration of a coin-type lithium ion secondary battery according to an embodiment of the present invention. As shown in FIG. 1A, a coin sail 1 composed of a disc-shaped housing is provided with a current collector (not shown) on a negative electrode terminal 2 on the upper surface.
A negative electrode 3 composed of a negative electrode active material joined via a positive electrode, a positive electrode 5 composed of a positive electrode active material 12 disposed opposite to the negative electrode 3 with a separator 4 interposed therebetween, and this positive electrode 5 with a current collector interposed therebetween And a positive electrode terminal 6 joined thereto. An electrolytic solution 7 is filled between the negative electrode terminal 2 and the positive electrode terminal 6, and the electrolytic solution is impregnated or held in the pores of the porous film to perform ion transfer between the positive and negative electrodes during the electromotive reaction. The gasket 8 seals between the negative electrode terminal 2 and the positive electrode terminal 6 around the coin cell.

FIG. 2B is a schematic view showing the structure of the positive electrode portion. The positive electrode active material 12 is provided on a current collector 10 made of aluminum foil or the like bonded to the positive electrode terminal 6 as a positive electrode mixture 9 containing carbon powder as a conductive material and a binder binder. This positive electrode mixture 9 is once dissolved in a solvent such as NMP, and this solution (slurry) is
After being applied on top, it is heated to evaporate the solvent and solidify.

Each particle of the positive electrode active material 12 is covered with an organic film 11 as shown in FIG. The organic film 11 is introduced into the surface of each particle of the positive electrode active material by chemically reacting a coupling agent of an organic material containing an amino group on the surface of the positive electrode active material 12 and partially bonding the same.

Hereinafter, specific examples of each constituent material will be further described.

Example 1 LiCoO was used as a positive electrode active material.
₂ as a coupling agent, N-β- (aminoethyl) -β-aminoethyltriisopropoxy titanate coupling agent (Plenact KR44, manufactured by Ajinomoto Co.)
An example using is described. First, a 1 wt% solution of N-β- (aminoethyl) -β-aminoethyl triisopropoxy titanate coupling agent was prepared using a predetermined amount of methyl ethyl ketone as a solvent, and LiCoO ₂ powder was added to this solution, For about 10 minutes. Thereafter, the mixture was allowed to stand for several hours to precipitate LiCoO ₂ , and then the methyl ethyl ketone in the supernatant was removed.

An appropriate amount of pure methyl ethyl ketone was added thereto, and the mixture was stirred for about 10 minutes and then left for several hours to give LiCoO ₂
Is subjected to a rinsing step of removing methyl ethyl ketone in the supernatant, and unreacted N-β- (aminoethyl)-
The β-aminoethyl triisopropoxy titanate coupling agent was removed. The thus-coupled LiCoO ₂ powder thus obtained was dried in an oven for several hours to completely remove methyl ethyl ketone, thereby obtaining a final coupled LiCoO ₂ powder. The treated LiCoO ₂ powder thus obtained was subjected to X-ray photoelectron spectroscopy (XP
The ratio I _N / I _Co was determined using S), and the value was 0.572.

The treated LiCoO ₂ is used as a positive electrode active material, metallic lithium is used as a negative electrode active material, and LiPF is used as an electrolyte.
_A coin-type battery was prepared using 1M-propylene carbonate / 1,2-dimethyl carbonate mixed non-aqueous solution using No. ₆ as an electrolyte. Produce 10 coin-shaped batteries,
The upper limit voltage during charging is 4.2 V, and the final voltage during discharging is 3.
The charge / discharge capacity was measured at a constant current of 0.5 mA / cm ² at 0 V. The results were converted to the capacity per 1 g of the positive electrode active material, and the ratio R = (standard deviation / average value) × 100 (%) of the average value of 10 samples and the standard deviation thereof was determined.
0.04 (%) for the first charge and R = 0.08 (%) for the first discharge.

Further, in order to examine the charge / discharge cycle characteristics, treated LiCoO ₂ was used as a positive electrode active material, non-graphitizable carbon was used as a negative electrode active material, and LiPF ₆ was used as an electrolyte.
A coin-type battery was manufactured using an M-propylene carbonate / 1,2-dimethyl carbonate mixed non-aqueous solution as an electrolytic solution. The upper limit voltage at the time of charging was 4.2 V, the final voltage at the time of discharging was 3.0 V, and charging and discharging were performed at a constant current of a current density of 0.5 mA / cm ² . FIG. 2 shows the cycle characteristics obtained as a result. The horizontal axis represents the number of cycles, and the vertical axis represents the ratio of the discharge capacity of each cycle to the maximum value of the discharge capacity (discharge capacity maintenance ratio). Assuming that the discharge capacity at the first cycle is 100 (%), the discharge capacity maintenance rate at the 200th cycle is 90.0%.
(%)Met.

(Example 2) As a coupling agent, N-
β- (aminoethyl) -γ-aminopropyltrimethoxysilane coupling agent (A-120 manufactured by Nippon Unicar Co., Ltd.)
0) under the same conditions as in Example 1 except that LiCoO
_Two powders were surface treated. When the ratio I _N / I _Co was determined using X-ray photoelectron spectroscopy (XPS), the value was 1.12.
It was 5. The charge / discharge capacity was measured under the same conditions as in Example 1 except that this treated LiCoO ₂ was used as the positive electrode active material. The ratio of the standard deviation to the resulting average,
R = (standard deviation / average value) × 100 (%) indicates that R = 0.09 (%) for the first charge and R = 0.15 (%) for the first discharge. )Met.

The charge-discharge cycle characteristics were examined under the same conditions as in Example 1 except that this treated LiCoO ₂ was used as the positive electrode active material. FIG. 2 shows the cycle characteristics obtained as a result.
Shown in Assuming that the discharge capacity at the first cycle is 100 (%), the discharge capacity retention rate at the 200th cycle is 88.9.
(%)Met.

Comparative Example 1 A battery was manufactured under the same conditions as in Example 1 except that LiCoO ₂ not subjected to the treatment with the coupling agent was used as the positive electrode active material. When R is obtained in the same manner as in the first embodiment, R = 0.48 (%) for the first charge, and R = 0.66 (%) for the first discharge. Met. FIG. 2 shows the results of the charge / discharge cycle test. Set the discharge capacity at the first cycle to 1
Assuming 00 (%), the discharge capacity maintenance ratio at the 200th cycle was 87.4 (%).

Comparative Example 2 A LiCoO ₂ powder was coated on the surface under the same conditions as in Example 1 except that a heptadecanoate triisopropoxy titanate coupling agent (manufactured by Ajinomoto Co., Prenact KR TTS) was used as a coupling agent. Processed.

The charge / discharge capacity was measured under the same conditions as in Example 1 except that this treated LiCoO ₂ was used as the positive electrode active material. The resulting ratio, R = (standard deviation / mean) ×
100 (%) is R = 0.
25 (%), R = 0.32 for the first discharge
(%)Met.

The charge / discharge cycle characteristics were examined under the same conditions as in Example 1 except that this treated LiCoO ₂ was used as the positive electrode active material. FIG. 2 shows the cycle characteristics obtained as a result.
Shown in Assuming that the discharge capacity at the first cycle is 100 (%), the discharge capacity retention rate at the 200th cycle is 88.1.
(%)Met.

The above results are summarized in Table 1 below.

[0035]

[Table 1]

As can be seen from Table 1, in both Examples 1 and 2, the ratio R of the standard deviation of charge and discharge is much smaller than in the comparative example, and the variation in the capacitance characteristics is smaller.

Further, as shown in FIG.
In both cases, it can be seen that the decrease in capacity when the number of discharges is increased is suppressed to be smaller than that in the comparative example, and that stable capacity characteristics are maintained.

[0038]

As described above, in the lithium ion secondary battery obtained according to the present invention, the dispersibility of the positive electrode active material in the electrode is improved, and the electrode composition becomes more homogeneous. That is, characteristics with a small difference between solids and excellent cycle life are realized.

[Brief description of the drawings]

FIG. 1 is a structural explanatory view of a coin-type lithium ion secondary battery according to an embodiment of the present invention.

FIG. 2 is a graph of a discharge capacity retention ratio with respect to the number of cycles in Examples and Comparative Examples.

[Explanation of symbols]

1: coin cell, 2: negative electrode terminal, 3: negative electrode, 4: separator, 5: positive electrode, 6: positive electrode terminal, 7: electrolytic solution, 8: gasket, 9: positive electrode mixture, 10: current collector, 11: organic Coating, 12: positive electrode active material.

Claims (5)

[Claims] 1. A lithium ion secondary battery comprising a positive electrode active material, a negative electrode active material, and an electrolyte containing a lithium salt, wherein the positive electrode active material is represented by the following general formula (1): Li _x MO _z (1) Is a transition metal containing at least one selected from Co, Mn, Ni, V, Fe and Cr, and x =
A lithium ion secondary battery represented by 0.3 to 1.2 and z = 1.4 to 3, wherein the surface of the positive electrode active material is coated with an organic material containing an amino group. 2. The lithium according to claim 1, wherein the organic substance containing an amino group is introduced using a silane coupling agent, a titanate coupling agent, or an aluminate coupling agent. Ion secondary battery. 3. The lithium ion secondary battery according to claim 2, wherein the coupling agent is a coupling agent having at least one amino group in a molecule. 4. The amount of amino group introduced on the surface of the positive electrode active material is determined by the sensitivity-corrected peak area intensity (I _N ) of nitrogen ( _N ) and the general formula (X) in X-ray photoelectron spectroscopy. 1) The ratio I _N / I _{M of} the peak area intensity (I _M ) of the metal element (M) in which sensitivity is corrected is 0.01 to 10.
4. The lithium ion secondary battery according to claim 1, wherein the value is 0. 5. The lithium ion according to claim 1, wherein the electrolyte is a non-aqueous electrolyte obtained by dissolving a lithium salt as a supporting electrolyte in a non-aqueous solvent. Rechargeable battery.