JP2008091041A - Nonaqueous secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a battery of high capacity and superior in output characteristics in a nonaqueous electrolyte secondary battery using a lithium nickel manganese complex oxide containing lithium at a transition metal site as a positive electrode active material. <P>SOLUTION: In the nonaqueous electrolyte secondary battery with a positive electrode containing the positive electrode active material, a negative electrode, and a nonaqueous electrolyte, the lithium nickel manganese complex oxide Li[Li<SB>x</SB>Ni<SB>y</SB>Mn<SB>z</SB>]O<SB>2</SB>(in the formula, x, y, and z satisfy 0.1≤x≤0.28, 0.1≤y/z≤1, and relation of x+y+z=1) containing lithium is used at the transition metal site as the positive electrode active material. When charged until a potential of the positive electrode becomes 4.45 V (vs. Li/Li<SP>+</SP>) or more, a ratio n (negative electrode/positive electrode) of the initial charge capacity of the negative electrode to the initial charge capacity of the positive electrode is 0.78≤n≤0.95. The initial charge is carried out until the potential of the positive electrode reaches 4.45 V (vs. Li/Li<SP>+</SP>) or more. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery using a lithium nickel manganese composite oxide containing lithium at a transition metal site as a positive electrode active material.

  In recent years, in order to solve environmental problems caused by exhaust gas, development of HEV (Hybrid Electric Vehicle) using both a gasoline engine and an electric motor of an automobile has been promoted at an international level. As a power source for HEV, a nickel hydride secondary battery has been conventionally used. However, a practical application of a higher voltage and higher capacity lithium ion secondary battery is expected.

  One of the important issues of lithium ion secondary batteries for HEV applications is cost reduction. A composite oxide containing Co is mainly used as a positive electrode active material for lithium-ion secondary batteries for power supplies such as portable electronic devices such as mobile phones, camcorders, and notebook computers that are already in practical use. In view of the above, in a large-sized lithium secondary battery for HEV, a positive electrode material not containing an expensive metal element such as Co is desirable. Moreover, in HEV use, in order to collect | recover brake regeneration energy efficiently, it is preferable on system design that the charge side output of a battery is high. Since the output on the charging side can be increased due to the large difference between the open circuit voltage of the battery and the upper limit voltage of the battery, a lithium ion secondary battery for HEV use is required to have a low voltage battery.

  In particular, in HEV applications, the entire capacity range of the battery is not used evenly, but is used mainly in the charging area near 50% of the charging depth (hereinafter referred to as SOC). Is needed.

However, the positive electrode active lithium cobaltate LiCoO ₂ or lithium nickel LiNiO ₂ has been used as a material, lithium manganese oxide _{LiMn 2 O 4, Ni-Co} -Mn ternary composite oxide of a conventional lithium ion secondary battery And the like have a problem that the charge / discharge voltage of the battery becomes high and the output on the charge side becomes small because of the high potential in the vicinity of SOC 50%.

In order to solve the above problems, as a positive electrode material for lithium ion secondary batteries for HEV, in recent years, only from elements that can be supplied relatively inexpensively, such as lithium-containing phosphates having an olivine structure, Ni, Mn-based composite oxides, etc. Active materials have been extensively studied. Among these, the Li [LiNiMn] O ₂ composite oxide containing lithium at the 3b transition metal site undergoes structural change due to charge / discharge at 4.45 V (vs. Li / Li ⁺ ) or more, as described above. Compared with a positive electrode material that has been put into practical use, the charge / discharge voltage at 50% SOC can be lowered by about 100 to 200 mV. From these characteristics, Li [LiNiMn] O ₂ composite oxide has recently attracted attention as a positive electrode material that is inexpensive, has high capacity, and has high output characteristics (Patent Document 1).

However, since Li [LiNiMn] O ₂ composite oxide has a large irreversible capacity for reactions occurring at 4.45 V (vs. Li / Li ⁺ ) or more at the first charge, Li [LiNiMn] O ₂ composite oxide was used. In order to produce a battery, the amount of the opposing negative electrode must be used excessively. Therefore, since the amount of positive and negative electrode active materials involved in charge / discharge decreases, the battery capacity decreases. Further, there is a problem that the thickness of the negative electrode plate increases due to an increase in the amount of negative electrode, and the output characteristics deteriorate due to an increase in resistance.
US Patent Publication No. 2003/0108793

  An object of the present invention is a non-aqueous electrolyte secondary battery using a lithium nickel manganese composite oxide containing lithium at a transition metal site as a positive electrode active material, and having a high capacity and excellent output characteristics. Is to provide.

The present invention relates to a non-aqueous electrolyte secondary battery including a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte having lithium ion conductivity. Lithium nickel manganese composite oxide Li [Li _x Ni _y Mn _z ] O ₂ (wherein x, y and z are 0.1 ≦ x ≦ 0.28, 0.1 ≦ y / z ≦ 1, And x + y + z = 1), and the ratio n of the initial charge capacity of the negative electrode to the initial charge capacity of the negative electrode when the positive electrode potential is charged to 4.45 V (vs. Li / Li ⁺ ) or higher. (Negative electrode / positive electrode) is 0.78 ≦ n ≦ 0.95, and the first charge is performed until the potential of the positive electrode becomes 4.45 V (vs. Li / Li ⁺ ) or higher.

  According to the present invention, in a non-aqueous electrolyte secondary battery using a lithium nickel manganese composite oxide containing lithium at a transition metal site as a positive electrode active material, the non-aqueous electrolyte secondary battery having high capacity and excellent output characteristics It can be.

In the lithium nickel manganese composite oxide represented by Li [Li _x Ni _y Mn _z ] O ₂ in the present invention, the Li amount x contained in the transition metal 3b site has a positive electrode potential of 4.45 V (vs. Li / Li ⁺ ) is greatly related to the charge capacity when charged until it becomes equal to or greater than. For this reason, x indicating the amount of Li contained in the transition metal site is within a range of 0.1 ≦ x ≦ 0.28 from the viewpoint of a balance between increasing the battery capacity and decreasing the charge / discharge voltage of the battery. It is preferable that

Further, the ratio (y / z) between y indicating the amount of Ni and z indicating the amount of Mn is greatly related to the capacity when the amount of Ni is less than 4.45 V (vs. Li / Li ⁺ ). In order to reduce the cost and to lower the charge / discharge voltage of the battery, it is necessary to increase the amount, and from the viewpoint of these balances, it is preferably within the range of 0.1 ≦ y / z ≦ 1. X, y, and z representing the amounts of Li, Ni, and Mn at these 3b sites have a relationship of x + y + z = 1. In addition, Li amount x contained in the transition metal site of the lithium nickel manganese composite oxide in the present invention can be measured using an X-ray diffraction method or a neutron diffraction method.

The lithium nickel manganese composite oxide in the present invention needs to be charged for the first time until the potential of the positive electrode becomes 4.45 V (vs. Li / Li ⁺ ) or higher. This causes a structural change in the lithium nickel manganese composite oxide. As a battery voltage when a carbon material is used as the negative electrode active material, it is desirable to charge at 4.5 V or more. After the structural change is caused at the first charging, it is not necessary to perform the subsequent charging so that the potential of the positive electrode becomes 4.45 V (vs. Li / Li ⁺ ) or more. The effect of the present invention can be obtained even when used at about 2V.

  The lithium nickel manganese composite oxide used in the present invention may contain one or more metal elements other than Li, Ni, and Mn. Specifically, B, Mg, Al, Si, P, Ca, Sc, Ti, Cr, Fe, Co, Cu, Zn, Ga, Ge, As, Sr, Y, Zr, Nb, Mo, Tc, Ru , Rh, Pd, In, Sn, Sb, Te, Ba, lanthanoid elements, Hf, Ta, W, Re, Os, Ir, Pt, Pb, Bi, Ra, actinoid elements, and the like may be further included. From the viewpoint of securing the weight energy density (Wh / kg) of the active material, the content of these metal elements is 0.1 or less in terms of molar ratio with respect to the transition metal element contained in the 3b site. Preferably, it is 0.001 or more and 0.05 or less. For the same reason, one or more kinds of halogen elements or chalcogen elements may be contained. Specifically, F, Cl, Br, I, At, S, Se, Te, Po, or the like may be included. In addition, from the viewpoint of securing the important energy density (Wh / kg) of the active material, the content of the halogen element or the chalcogen element is 0.1 or less in terms of molar ratio with respect to O contained in the 6c site. And more preferably 0.001 or more and 0.05 or less.

  In the present invention, a positive electrode active material other than the lithium nickel manganese composite oxide may be mixed as the positive electrode active material. The other positive electrode active material to be mixed is not particularly limited as long as it is a compound capable of reversibly inserting and desorbing Li, but it is a layered structure capable of inserting and desorbing Li while maintaining a stable crystal structure. A positive electrode active material having a rock salt structure, a spinel structure, or an olivine structure is preferable.

As the supporting salt used in the present invention, a lithium salt generally used as an electrolyte of a nonaqueous electrolyte secondary battery can be used. Such a lithium salt preferably contains one or more elements of P, B, F, O, S, N, and Cl. Specifically, LiPF ₆ , LiBF ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ ) ₃ , LiC (C ₂ F ₅ SO ₂ ) ₃ , LiAsF ₆ , LiClO _{4 and the} like and mixtures thereof can be used. Furthermore, in addition to these salts, it is preferable that a lithium salt having an oxalato complex as an anion is contained, and more preferably lithium-bis (oxalato) borate that suppresses an increase in resistance after high-temperature storage.

  Moreover, as a solvent of the non-aqueous electrolyte used in the present invention, those conventionally used as an electrolyte solvent for non-aqueous electrolyte secondary batteries can be used. For example, cyclic carbonates such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate, and chain carbonates such as dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate can be used. In particular, a mixed solvent of cyclic carbonate and chain carbonate having high lithium ion conductivity is preferable. An ionic liquid can also be used as a solvent for the electrolyte. The cationic species and the anionic species are not particularly limited, but from the viewpoint of obtaining low viscosity, electrochemical stability, and hydrophobicity, the cation is a pyridinium cation, an imidazolium cation, or a quaternary ammonium cation as an anion. Is particularly preferably a combination using a fluorine-containing imide anion.

The negative electrode active material used in the present invention is not particularly limited as long as it can reversibly occlude and release lithium, and a carbon material, an alloy, a metal oxide, or the like can be used. In particular, from the viewpoint of cost, it is preferable to use a carbon material. Specific examples of the carbon material include natural graphite, artificial graphite, mesophase pitch-based carbon fiber (MCF), mesocarbon microbeads (MCMB), coke, and hard carbon. , Fullerenes, carbon nanotubes and the like. Among these, a graphitic carbon material is particularly preferably used because of a small potential change accompanying lithium insertion / extraction. By using a graphitic carbon material, the potential of the positive electrode is maintained at 4.45 V (vs. Li / Li ⁺ ) or more during the first charge, causing a structural change of the lithium nickel manganese composite oxide in the battery. Can be made easier.

  The separator used in the present invention is not particularly limited as long as it is a material that prevents a short circuit due to contact between the positive electrode and the negative electrode and that can obtain lithium ion conductivity by impregnating with an electrolytic solution. For example, a polypropylene, polyethylene, a polypropylene-polyethylene multilayer separator, etc. are mentioned.

The initial charge capacity ratio n in the present invention can be measured using a three-electrode test cell in which a positive electrode or a negative electrode to be used is a working electrode and lithium metal is a counter electrode and a reference electrode. That is, n = (negative electrode initial charge capacity) / (positive electrode initial charge capacity), and n is calculated from the positive electrode initial charge capacity measured using a three-electrode test cell and the negative electrode initial charge capacity. Can do. The initial charge capacity of the positive electrode is charged until the potential of the positive electrode becomes 4.45 V (vs. Li / Li ⁺ ) or higher. That is, in the present invention, as described above, since it is necessary to charge the initial charge so that the potential of the positive electrode is 4.45 V (vs. Li / Li ⁺ ) or more, the potential of the positive electrode during the initial charge is The initial charge capacity of the positive electrode is measured with a three-electrode test cell.

  By setting the initial charge capacity ratio n within the range of 0.78 ≦ n ≦ 0.95, a nonaqueous electrolyte secondary battery having a high discharge capacity and excellent output characteristics can be obtained.

  Moreover, since lithium contained in the transition metal site of the lithium nickel manganese composite oxide is considered to be irreversible lithium, the ratio of reversible lithium to the total lithium amount is represented by 1 / (1 + x). For this reason, it is preferable that the initial charge capacity ratio n is further in the range of 1 / (1 + x) ≦ n ≦ 0.95.

  In the present invention, the ratio n (negative electrode / positive electrode) of the initial charge capacity of the negative electrode to the initial charge capacity of the positive electrode is in the range of 0.78 ≦ n ≦ 0.95. The battery can have a high capacity and excellent output characteristics.

  Hereinafter, the present invention will be described in detail on the basis of examples. However, the present invention is not limited to the following examples, and can be appropriately modified and implemented without departing from the scope of the present invention. is there.

(Example 1)
[Preparation of positive electrode active material]
Li [LiNiMn] O ₂ composite oxide is a composite hydroxide of Ni and Mn obtained by adding an alkaline solution to a mixed solution of Ni salt and Mn salt to coprecipitate a hydroxide of Ni and Mn. And Li ₂ CO ₃ and Ni—Mn composite hydroxide are mixed so that the molar ratio of Li: Ni: Mn element is 1.22: 0.17: 0.61, and the mixture is mixed in an air atmosphere. Then, after calcining at 500 ° C. for 10 hours, firing was performed at 1000 ° C. for 20 hours. The composition of the obtained Li [LiNiMn] O ₂ composite oxide was Li [Li _0.22 Ni _0.17 Mn _0.61 ] O ₂ .

[Production of positive electrode]
The positive electrode active material produced as described above, carbon as a conductive agent, and an N-methyl-2-pyrrolidone solution in which polyvinylidene fluoride as a binder is dissolved, the positive electrode active material, the conductive agent, and the binder The weight ratio was adjusted to 90: 5: 5 and then kneaded to prepare a positive electrode slurry. The produced positive electrode slurry was applied on an aluminum foil as a current collector and then dried to obtain a positive electrode plate. Thereafter, the obtained positive electrode plate was cut into a size of 30 × 37 mm ² , and the 7 mm coated part was peeled off, rolled using a rolling roller, and the coated part was peeled off and exposed on the aluminum foil. A positive electrode was prepared by attaching a current collecting tab.

(Production of negative electrode)
An aqueous solution in which graphite as a negative electrode active material, styrene butadiene rubber as a binder, and carboxymethyl cellulose as a thickener are dissolved, and the weight ratio of the active material, the binder, and the thickener is 97.5: 1. 5: 1.0, and then kneaded to prepare a negative electrode slurry. The prepared negative electrode slurry was applied on a copper foil as a current collector by adjusting the application amount so that the initial charge capacity ratio n (negative electrode / positive electrode) was 0.78 with respect to the positive electrode capacity, and then dried. A negative electrode plate was obtained. Thereafter, the obtained negative electrode plate was cut out to a size of 31 mm × 37.5 mm, and the 6.5 mm coated part was peeled off, rolled using a rolling roller, and the coated part was peeled off and exposed on the copper foil. A negative electrode was prepared by attaching a current collecting tab made of nickel.

The initial charge capacity of the positive electrode and the negative electrode was measured by separately preparing a three-electrode test cell. Note that the positive electrode was charged until the potential was 4.6 V (vs. Li ^/ Li ⁺ ).

[Production of winding electrode body]
A wound electrode body was fabricated by winding the positive electrode and the negative electrode fabricated as described above facing each other through a polyethylene separator.

(Preparation of electrolyte)
1 mol / liter of LiPF ₆ as a supporting salt is dissolved in a solvent prepared by mixing ethylene carbonate (EC), methyl ethyl carbonate (MEC), and dimethyl carbonate (DMC) in a volume ratio of 3: 3: 4, An electrolytic solution was prepared by dissolving 1% by weight of vinylene carbonate (VC) as a film forming agent.

[Production of battery]
After inserting the wound electrode body produced as described above into a battery can, drying under reduced pressure, pouring the electrolyte solution in a glove box under an argon (Ar) atmosphere, and sealing the non-water An electrolyte secondary battery A was produced.

(Charge / discharge test)
The battery was charged at 0.8 mA at 25 ° C. for 5 hours and allowed to stand at 25 ° C. for 5 days for stabilization. Thereafter, constant current charging was performed at 25 mA at 3.2 mA to 4.5 V, 4.5 V was constant voltage charging to 0.32 mA, and then 1.6 mA was discharged to 2.4 V. The discharge capacity after repeating this cycle 5 times was defined as the discharge capacity of the battery.

[DC resistance measurement test]
After adjusting the SOC to 80% from the result of the discharge capacity obtained from the above charge / discharge test, the following measurement shows the slope when each current value is plotted on the horizontal axis and the voltage is plotted on the vertical axis, and each point is linearly approximated. From this, the DC resistance was calculated.

(1) 1 mA discharge (10 seconds) → pause (5 minutes) → 1 mA charge (10 seconds) → pause (5 minutes)
(2) 5 mA discharge (10 seconds) → pause (5 minutes) → 1 mA charge (50 seconds) → pause (5 minutes)
(3) 10 mA discharge (10 seconds) → pause (5 minutes) → 1 mA charge (100 seconds) → pause (5 minutes)
(4) 20 mA discharge (10 seconds) → pause (5 minutes) → 1 mA charge (200 seconds) → pause (5 minutes)
The charge / discharge tests (1) to (4) were sequentially performed at room temperature, the voltage after 10 seconds at each discharge was measured, and the direct current resistance was determined from the slope of the change in voltage due to the current value.

(Example 2)
In Example 1, a nonaqueous electrolyte secondary battery B was prepared in the same manner except that n = 0.81, and the results of discharge capacity and DC resistance were obtained by the same measurement.

(Example 3)
In Example 1, a nonaqueous electrolyte secondary battery C was prepared in the same manner except that n = 0.92, and the results of discharge capacity and DC resistance were obtained by the same measurement.

Example 4
In Example 1, a nonaqueous electrolyte secondary battery D was prepared in the same manner except that n = 0.95, and the results of discharge capacity and DC resistance were obtained by the same measurement.

(Comparative Example 1)
In Example 1, a nonaqueous electrolyte secondary battery E was prepared in the same manner except that n = 0.65, and the results of discharge capacity and DC resistance were obtained by the same measurement.

(Comparative Example 2)
In Example 1, a nonaqueous electrolyte secondary battery F was similarly manufactured except that n = 0.72, and the results of discharge capacity and DC resistance were obtained by the same measurement.

(Comparative Example 3)
In Example 1, a nonaqueous electrolyte secondary battery G was similarly manufactured except that n = 0.96, and the results of discharge capacity and DC resistance were obtained by the same measurement.

(Comparative Example 4)
In Example 1, a nonaqueous electrolyte secondary battery H was similarly manufactured except that n = 1.03, and the results of discharge capacity and DC resistance were obtained by the same measurement.

  The results of Examples 1 to 4 and Comparative Examples 1 to 4 are shown in Table 1 below, and FIG. 1 shows the relationship between the negative electrode / positive electrode capacity ratio (initial charge capacity ratio) n, the discharge capacity, and the DC resistance. Show.

  As is apparent from Table 1 and FIG. 1, the discharge capacity and the direct current resistance greatly vary depending on the initial charge capacity ratio n. When n is less than 0.78, the discharge capacity decreases (Batteries E and F). Further, when n is 0.78 or more, the discharge capacity shows a substantially constant value (batteries A, B, C, D, G, and H). Further, the direct current resistance has a substantially constant value when n is 0.95 or less (batteries A, B, C, D, E, and F). When n exceeds 0.95, the direct current resistance increases rapidly (batteries G and H). Therefore, it can be seen that high capacity and high output characteristics can be obtained when the initial charge capacity ratio n is in the range of 0.78 ≦ n ≦ 0.95 (batteries A, B, C, and D).

In a positive electrode active material such as LiCoO ₂ and Li [Ni _1/3 Co _1/3 Mn _1/3 ] O ₂ used in conventional lithium ion secondary batteries, all the Li released from the positive electrode is on the negative electrode side. In order to react, the initial charge capacity ratio n had to be greater than 1.0. When n is less than 1.0, Li released from the positive electrode side during the initial charge cannot be used for the reaction on the negative electrode side, and lithium that is not inserted into the negative electrode precipitates, which greatly impairs safety. Because. Further, when n is less than 1.0, the lithium deposition potential is lower than the potential at which the negative electrode undergoes reversible Li insertion / extraction, so when lithium deposition starts to occur, the positive electrode potential in the battery is The battery capacity becomes smaller than the positive electrode capacity assumed from the positive electrode amount.

However, in the Li [LiNiMn] O ₂ composite oxide used in the present invention, the capacity shows a constant value when 0.78 ≦ n, and the behavior is clearly different from that of the conventional positive electrode active material. This is considered to be because Li desorbed from the 3b site among the Li desorbed from the Li [LiNiMn] O ₂ composite oxide becomes irreversible. Therefore, unlike the single electrode evaluation in which the counter electrode is Li, the Li [LiNiMn] O ₂ composite oxide positive electrode does not reversibly return to the positive electrode, so that n <1.0. However, there is no precipitation of lithium, and charging / discharging becomes possible.

  The details of the phenomenon in which the direct current resistance increases when n ≦ 0.95 are unknown, but the increase in the initial charge capacity ratio n increases the increase in the direct current resistance and the change in the charging depth due to the increase in the negative electrode coating amount. This is considered to be due to a synergistic effect.

  According to the present invention, by setting the initial charge capacity ratio n (negative electrode / positive electrode) within the range of 0.78 ≦ n ≦ 0.95, the amount of negative electrode not involved in charge / discharge can be greatly reduced. Thus, a battery having a high capacity and excellent output characteristics can be obtained.

The figure which shows the relationship between initial charge capacity ratio n (negative electrode / positive electrode), discharge capacity, and DC resistance.

Claims (3)

In a non-aqueous electrolyte secondary battery comprising a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and a non-aqueous electrolyte having lithium ion conductivity,
As the positive electrode active material, lithium nickel manganese composite oxide Li [Li _x Ni _y Mn _z ] O ₂ containing lithium at a transition metal site (wherein x, y and z are 0.1 ≦ x ≦ 0. 28, 0.1 ≦ y / z ≦ 1, and x + y + z = 1).
The ratio n (negative electrode / positive electrode) of the initial charge capacity of the negative electrode to the initial charge capacity of the negative electrode when charged until the potential of the positive electrode is 4.45 V (vs. Li / Li ⁺ ) or more is 0.78 ≦ n ≦ 0.95,
The non-aqueous electrolyte secondary battery is characterized in that the first charge is performed until the potential of the positive electrode becomes 4.45 V (vs. Li / Li ⁺ ) or more.   The nonaqueous electrolyte secondary battery according to claim 1, wherein a carbon material is used as the negative electrode active material. 3. The nonaqueous electrolyte secondary battery according to claim 1, wherein the initial charge capacity ratio n (negative electrode / positive electrode) is 1 / (1 + x) ≦ n ≦ 0.95.

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