JPH11339805A - Positive electrode active material for nonaqueous electrolyte secondary battery and nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material not reducing a discharge capacity when a voltage is applied at a high temperature by preparing a spinel lithium-manganese composite oxide constituted of Li, Mn, B, O, a bivalent metal element and a trivalent metal element at specific ratios while the bivalent metal element exists on Li side and the trivalent metal element exists on Mn side. SOLUTION: A spinal lithium-managenese composite oxide Li(x-a) Aa Mn(y-b) Bb O4 is prepared, where A is an element capable of generating bivalent metal ions, B is an element capable of generating trivalent metal ions, 0<x<=1.5, 1.8<=y<=2.2, 0<a<=0.3, 0<=b<=0.5, x>a, the element A replaces Lil existing at the site 8a, and the element B replaces Mn existing at the site 16d. The element Zn and/or Mg is preferably used of the element A, and the element Cr, Fe, Co or Ni is preferably used for the element B. A positive electrode active material for a nonaqueous electrolyte secondary battery having a high operating voltage, a high energy density and a long cycle life is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a positive electrode active material for a non-aqueous electrolyte secondary battery.

[0002]

2. Description of the Related Art A non-aqueous electrolyte secondary battery using lithium or a compound capable of occluding and releasing lithium as a negative electrode is expected to have a high energy density, and much research has been conducted. In recent years, a spinel lithium manganese composite oxide (LiMn ₂ O ₄ ) has been studied as a positive electrode active material of this battery. This material crystallographically has the space group Fd3m
In an ideal crystal where Li: Mn: O = 1: 2: 4, lithium occupies the 8a site and manganese occupies the 16d site.

However, when this material is used as a positive electrode active material, there has been a problem that the discharge capacity is remarkably reduced when charge and discharge are repeated. It is considered that the cause of this is that the Li structure moves or partially breaks due to the movement of Li ions in the crystal during charge and discharge.

[0004] In order to solve the above problems, Cr element, Fe element,
There has been proposed a method of stabilizing a crystal lattice by substituting a Co element, a Ni element, or the like. However, even when a battery is manufactured using the positive electrode active material obtained by this method, when a voltage application test is performed at a high temperature, manganese becomes electrochemically similar to the case where LiMn ₂ O ₄ is used as the positive electrode active material. However, there is a problem in that it is dissolved in an electrolytic solution and the discharge capacity is greatly reduced, and has not been put to practical use.

[0005]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and has a high operating voltage, a high energy density, a long cycle life, and a non-aqueous solution which does not decrease in discharge capacity even when voltage is applied at a high temperature. Positive electrode active material for obtaining an electrolyte secondary battery,
And a non-aqueous electrolyte secondary battery having the positive electrode active material.

[0006]

SUMMARY OF THE INVENTION The present invention provides Li _(xa) A
is represented by _{a Mn (yb) B b O} 4 ( provided that, A is an element which can be a divalent metal ion, B is an element capable of being a trivalent metal ion, 0 <x ≦ 1.5,1.8 ≦ y ≦ 2.
2, 0 <a ≦ 0.3 and 0 ≦ b ≦ 0.5. A) a positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the element A is composed of a spinel lithium manganese composite oxide which is present at an 8a site where lithium is present and the element B is present at a 16d site where manganese is present And a non-aqueous electrolyte secondary battery having the positive electrode active material.

The inventors of the present invention have conducted various studies to solve the above-mentioned problem of reduction in discharge capacity due to application of a voltage at a high temperature. As a result, a part of the 16d site where manganese is present is replaced with another element. Is effective, but it has been found that it is particularly effective to substitute a part of the 8a site where lithium is present with another element, leading to the present invention.

In the present invention, Li _(xa) A _a Mn _(yb) B _b
In O ₄ , the element represented by A is an element that can be a divalent metal ion and can enter the 8a site. Specific examples include elements such as Zn, Mg, Cd, and Cu, and the Mg element and / or the Zn element are particularly preferable. Radius of divalent metal ions of these elements are considered to be replaced with Li ⁺ closer an ionic radius of Li ^+.

The lithium manganese composite oxide to which a voltage is applied as a positive electrode active material is in a state where lithium is partially or completely removed from a state where no voltage is applied. Therefore, the oxide has a significantly reduced number of positively charged elements in the layer where lithium is present,
It is considered that the state becomes more unstable than the state where no voltage is applied, and the performance is deteriorated particularly by the voltage application at a high temperature. In the present invention, by partially substituting the element A at the 8a site where lithium is present in the oxide, it is possible to suppress a significant decrease in the positive charge in the layer where lithium is present when a voltage is applied. It is considered that the stability to the temperature, particularly the stability to the voltage application at a high temperature is improved.

The existence of the element A at the 8a site where lithium is present can be confirmed by Rietveld analysis of the powder X-ray diffraction pattern and EXAFS analysis of the X-ray absorption spectrum of the element A. That is, in the Rietveld analysis, it can be determined whether or not the element A exists at the 8a site from the convergence of the calculation result. In the EXAFS analysis, the distance between the element A and Mn, which is the second close peak, is calculated, and when the value is close to the distance between the 8a site and the 16d site, the element A exists at the 8a site. Is obtained.

In the lithium manganese composite oxide Li _(xa) A _a Mn _(yb) B _b O _{4 according to the} present invention, 0 <a ≦
0.3. If the value of a exceeds 0.3, LiMn ₂ O
_Since the spinel structure of No. ₄ cannot be maintained, the discharge capacity of a secondary battery having the oxide as a positive electrode active material is extremely reduced. In consideration of the effect of the stability of voltage application at a high temperature due to the presence of the element represented by A, a is preferably 0.01 or more, and more preferably 0.01 ≦ a ≦ 0.15.

In the present invention, the element represented by B is an element which can be a trivalent metal ion and can enter the 16d site. Specifically, Al, Ti, V, Cr,
Elements such as Fe, Co, and Ni are mentioned, and in particular, a Cr element,
It is preferably at least one selected from the group consisting of Fe element, Co element and Ni element. Although the effect of the present invention can be obtained even if the element represented by B is not contained, the element represented by B
By entering the site, the bonding state between Mn and O becomes strong, and Mn can be effectively prevented from electrochemically dissolving in the electrolytic solution by applying a voltage at a high temperature.
It is preferred that

In the lithium manganese composite oxide Li _(xa) A _a Mn _(yb) B _b O _{4 according to the} present invention, 0 ≦ b
≦ 0.5. When b exceeds 0.5, the discharge capacity is extremely reduced. More preferably, 0.1 ≦ b ≦ 0.3.

In the above oxide, the values of x and y are 0
<X ≦ 1.5 and 1.8 ≦ y ≦ 2.2. x and y
Deviates from this range, the spinel structure of LiMn ₂ O ₄ cannot be maintained. The more preferable range of x is 0.5 ≦ x
≦ 1.2, and a more preferable range of y is 1.9 ≦ y ≦
2.1. In the present invention, x> a.

The method for producing the lithium-manganese composite oxide Li _(xa) A _a Mn _(yb) B _b O _{4 according to} the present invention comprises: as a starting material, an oxide of a metal element constituting the oxide;
Hydroxides, carbonates, nitrates, organic acid salts and the like can be used.
The firing atmosphere preferably has an oxygen concentration of 50% or more, particularly preferably 70% or more. When firing is performed at an oxygen concentration of 50% or more, the battery using the fired product as a positive electrode active material has a very small decrease in discharge capacity even when a voltage is applied at a high temperature. Although the reason for this is not clear, firing in an atmosphere having a high oxygen concentration _yields the general formula Li _(xa) A _a Mn.
_(yb) element represented by A in the B _b O _4, becomes stronger the bond between the element and Mn and oxygen, represented by B, to suppress the Mn be energized at a high temperature is dissolved in the electrolyte It is considered possible.

The firing temperature is 600 to 1000.
C is preferable, and particularly preferably 700 to 900C. When the temperature is lower than 600 ° C., crystal growth is insufficient,
The charge / discharge capacity tends to be small. When the temperature exceeds 1000 ° C., the crystals grow too much, and the cycle characteristics tend to deteriorate.

The firing time is preferably from 10 to 50 hours, particularly preferably from 15 to 45 hours. If the time is less than 10 hours, the crystal growth is insufficient, and the charge / discharge capacity tends to be small. If the time exceeds 50 hours, the crystals grow too much, and the cycle characteristics tend to deteriorate. More preferably 20 to
40 hours.

In order to uniformly grow the crystal, it is preferable to include a step of once taking out a proper firing time from the firing furnace, pulverizing and mixing the fired product, and firing again. Then, it is preferable to gradually cool after completion of the firing.

The negative electrode of the non-aqueous electrolyte secondary battery of the present invention is lithium or a substance capable of inserting and extracting lithium. Materials that can absorb and release lithium include lithium alloys,
Examples include carbon materials such as graphite.

As the non-aqueous electrolyte, LiBF ₄ ,
A solution in which a lithium salt such as LiPF ₆ or LiClO ₄ is dissolved in a non-aqueous solvent such as a carbonate ester can be used as the electrolytic solution. Further, a polymer electrolyte containing a matrix made of a polymer substance and the above-mentioned electrolytic solution may be used.

[0021]

The following examples (Examples 1 to 4) and comparative examples (Example 5)
The present invention will be described in detail with reference to (7), but the present invention is not limited to these.

Example 1 Li ₂ CO ₃ , MnCO ₃ , Fe
₂ O ₃ and ZnO were mixed in a ball mill at a molar ratio of 19: 68: 5: 2, fired at 800 ° C. for 2 hours in an atmosphere of 70% oxygen and 30% nitrogen, and then taken out. After pulverizing this calcined product, it is again 85% in an atmosphere of 100% oxygen.
The mixture was baked at 0 ° C. for 20 hours, then cooled to room temperature over 24 hours, and the baked product was ground in a mortar. As a result of analyzing this by an elemental absorption method, Li _0.95 Zn _0.05 Mn _1.7 Fe _0.25
O ₄ .

The pulverized product was subjected to X-ray diffraction measurement, and the obtained diffraction pattern was subjected to Rietveld analysis. However, in the analysis, Mn, Fe, and Zn were considered to be difficult to distinguish in X-ray diffraction, and were collectively treated as element M. As a result, 8a
The calculation result showed that the occupation ratio of the element M in the site was about 5%, which was almost equal to the addition ratio of Zn. Also,
EXAFS analysis of the X-ray absorption spectrum of each element of Mn, Fe, and Zn of this pulverized product showed that Mn and Fe were 1
The analysis result showed that Zn exists at the 6d site and Zn exists at the 8a site.

85 parts by weight of the pulverized material as a positive electrode active material, 10 parts by weight of Ketjen black as a conductive material and 5 parts by weight of polyvinylidene fluoride resin as a binder were added.
A slurry was formed by dispersing in N-methyl-2-pyrrolidone. This slurry is uniformly applied to an aluminum foil having a thickness of 30 μm to form a coating film, dried at 180 ° C., and then rolled by a roller press until the thickness of the coating film becomes 100 μm, and then 20 mm × 20 mm To obtain a positive electrode.

Next, a metal lithium foil having a thickness of 0.6 mm was cut into a size of 25 mm × 25 mm, and a nickel lead was crimped to an end to obtain a negative electrode. In a container filled with a sufficient amount of electrolyte, a negative electrode and a positive electrode are opposed to each other such that the coated surface of the positive electrode faces the negative electrode, and a microporous film made of polypropylene having a thickness of 25 μm is interposed therebetween as a separator. Then, the sample was immersed in the electrolytic solution. In this state, a charge / discharge power supply was connected, and a voltage application test was performed in an argon atmosphere at 60 ° C. As the electrolytic solution, a mixed solvent of ethylene carbonate and propylene carbonate (volume ratio of 1:
In 1), a solution in which LiPF ₆ was dissolved at a concentration of 1 mol / L was used.

The conditions of the voltage application test were as follows: charge voltage up to 4.3 V, discharge voltage up to 3.0 V, and 1 mA /
Charge / discharge was performed at a constant current at a current density of cm ² , and the initial discharge capacity was measured. Next, the battery was charged again to 4.3 V, and the voltage was continuously applied at 4.3 V for one month. Then 3.0
The discharge capacity was measured by discharging to V, and the maintenance ratio of the discharge capacity with respect to the initial value was examined. Table 1 shows the results.

[0027] [Example 2] except for changing the mixing ratio of the raw materials in the same manner as in Example _{1 Li 1.0 Zn 0.1 Mn 1.7 Fe} 0.2 O 4
I got In this oxide, Zn has an 8a site, Fe
Was confirmed to be at the 16d site. A sample was prepared in the same manner as in Example 1 except that this oxide was used as a positive electrode active material, and a voltage application test was performed in the same manner as in Example 1 to examine a discharge capacity retention ratio. Table 1 shows the results.

Example 3 Li _1.0 Zn was prepared in the same manner as in Example 1 except that CoO was used instead of Fe ₂ O ₃ as a raw material.
_0.1 Mn _1.7 Co _0.2 O ₄ was obtained. In this oxide, it was confirmed that Zn was present at the 8a site and Co was present at the 16d site. A sample was prepared in the same manner as in Example 1 except that this oxide was used as a positive electrode active material, and a voltage application test was performed in the same manner as in Example 1 to examine a discharge capacity retention ratio. Table 1 shows the results.

Example 4 Li _1.0 Zn _0.05 Mn _1.7 Fe in the same manner as in Example 1 except that the atmosphere for firing the positive electrode active material at 850 ° C. was not 100% oxygen but air.
_0.25 O ₄ was obtained. In this oxide, it was confirmed that Zn was present at the 8a site and Fe was present at the 16d site. A sample was prepared in the same manner as in Example 1 except that this oxide was used as a positive electrode active material, and a voltage application test was performed in the same manner as in Example 1 to examine a discharge capacity retention ratio. Table 1 shows the results.

Example 5 Li _1.0 Mn _1.9 F was used in the same manner as in Example 1 except that the mixing ratio of the raw materials was changed without using ZnO as the raw material.
e _0.1 O ₄ was obtained. In this oxide, Fe is 16d
Confirmed that it exists on the site. A sample was prepared in the same manner as in Example 1 except that this oxide was used as a positive electrode active material, and a voltage application test was performed in the same manner as in Example 1 to examine a discharge capacity retention ratio. Table 1 shows the results.

Example 6 Fe ₂ O ₃ and ZnO as raw materials
Was used, and Li _1.0 Mn _2.0 O ₄ was obtained in the same manner as in Example 1 except that the mixing ratio of the raw materials was changed. A sample was prepared in the same manner as in Example 1 except that this oxide was used as a positive electrode active material,
A voltage application test was performed in the same manner as in Example 1, and the maintenance ratio of the discharge capacity was examined. Table 1 shows the results.

Example 7 Li ₂ CO ₃ , MnCO ₃ , Fe
Using Li ₂ O ₃ and ZnO, Li _0.6
A sample was prepared in the same manner as in Example 1 except that the mixture was adjusted so that the composition became Zn _0.4 Mn _1.7 Fe _0.25 O _4, and this was used as the positive electrode active material. When the capacity was measured, only about 55% of the capacity of Example 1 was obtained. Therefore, when the positive electrode active material was examined by X-ray diffraction without performing a voltage application test on this sample, it was found that the peak was considered to be based on ZnMn ₂ O ₄ of the tetragonal spinel. The large drop in capacity is believed to be due to the formation of this compound.

After the completion of the voltage application test, black deposits were found on the negative electrode lithium of the batteries of Examples 5 and 6, and the deposit was particularly remarkable in Example 6. The sediment was examined by X-ray fluorescence analysis and found to be a compound containing Mn. This is considered to be because Mn ions dissolved from the positive electrode active material were deposited on the negative electrode lithium during voltage application. Examples 1-4
Almost no black deposits were found.

[0034]

[Table 1]

[0035]

According to the non-aqueous electrolyte secondary battery of the present invention, the positive electrode active material is stable even when a voltage is applied, and the dissolution of the positive electrode active material by applying a voltage can be prevented. Even though the capacity is maintained, it is extremely reliable.

Claims (6)

[Claims] (1) Li _(xa) A _a Mn _(yb) B _b O ₄ (where A is an element which can be a divalent metal ion, B is an element which can be a trivalent metal ion, and 0 <X
≦ 1.5, 1.8 ≦ y ≦ 2.2, 0 <a ≦ 0.3, 0 ≦
b ≦ 0.5. ), Element A is the presence of lithium 8
A positive electrode active material for a non-aqueous electrolyte secondary battery, wherein the element B is composed of a spinel-based lithium manganese composite oxide existing at the 16d site where manganese exists at the a site. 2. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the value of a is 0.01 ≦ a ≦ 0.15. 3. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the element represented by A is a Zn element and / or a Mg element. 4. The non-aqueous electrolyte secondary battery according to claim 1, wherein the element represented by B is at least one element selected from the group consisting of Cr element, Fe element, Co element and Ni element. For positive electrode active material. 5. The positive electrode active material for a non-aqueous electrolyte secondary battery according to claim 1, wherein the composite oxide is fired in an atmosphere having an oxygen concentration of 50% or more. 6. A non-aqueous electrolyte secondary comprising lithium or a substance capable of occluding and releasing lithium as a negative electrode active material, and comprising the positive electrode active material according to claim 1, 2, 3, 4, or 5. battery.