JPH09245795A - Positive electrode active material for lithium battery and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce slip-off of active material due to charging and discharging and ensue a longer service life of a positive plate by providing a super-lattice structure having regularity in an array of lithium ion, manganese ion, and manganese ion in a positive electrode active material. SOLUTION: The composition of a positive electrode material for a lithium battery is Li413 Mu513 O4 . The crystal has a spinel structure attribute to a space group Fd 3m and has a super-lattice structure. In the powder X-ray refraction graph using CuK αray, 2 θ has a refraction peak equivalent to (1/2, 1/2, 1/2), (1/3, 1/3, 1/3), and (1/6, 1/6, 1/6) of a space group Fd at 9.2 deg., 6.3 deg., and 3.1 deg.. This positive electrode active material is mixed with lithium nitrate and manganese oxide to be at stoichiometric ratio and is obtained by burning them at approx. 500 to 600 deg. in the oxygen atmosphere. Thereby, the change of the crystal structure due to repetition of charging and discharging is eliminated, and the lower capacity can be suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spinel type lithium manganese composite oxide as a positive electrode active material used in a lithium battery operating at room temperature and a method for producing the same.

[0002]

2. Description of the Related Art Lithium batteries, which operate at room temperature and use organic electrolytes and polymer electrolytes, have theoretically possessed extremely high energy densities, and various studies have been made since ancient times and they have been extremely Many materials and systems have been proposed. These lithium batteries can be roughly classified into primary batteries and secondary batteries. Primary batteries were put into practical use in the latter half of the 1970s, and generally, metallic lithium is used as the negative electrode, and carbon fluoride, manganese dioxide, etc. are used as the positive electrode. The secondary battery uses metallic lithium or a lithium alloy for the negative electrode, and a lithium-free oxide or sulfide such as manganese dioxide, a vanadium compound, or titanium disulfide for the positive electrode, or lithium.
A metallic lithium secondary battery that uses a material such as vanadium composite oxide into which lithium can be inserted further by discharge, and a material that can be a host of lithium ions such as graphite or carbon with low crystallinity for the negative electrode Lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO
₂ ), lithium manganese spinel compound (LiMn
It can be classified as a so-called lithium ion secondary battery using a composite oxide containing lithium such as ₂ O ₄ ).
At present, the latter lithium-ion secondary battery, which is superior in terms of life and safety, has become the mainstream in practice.

Of the various positive electrode active material materials for lithium secondary batteries, manganese oxide materials are extremely attractive from the viewpoints of resources, cost and safety. As manganese oxide-based materials, materials having various compositions and crystal structures have already been proposed. Among them, lithium-manganese composite oxides having a spinel structure are particularly noteworthy. Examples of spinel-type lithium-manganese composite oxides include MMThackeray et al. (J. Electroche
m.Soc. Vol.139, No.2, P.363, 1992) cites substances having the following compositions and spinel display structures.

[0004]

[Table 1] In addition, all of these substances are shown in the composition in a charged state, and the discharge reaction is as follows. The theoretical capacity density is also calculated based on these reactions.

[0005]

Embedded image On the other hand, the spinel structure is generally designated as AB ₂ O ₄ .
A indicates a cation located on the tetrahedral 8 (a) site and B indicates a cation located on the octahedral 16 (d) site. λ-MnO ₂ and Li ₂ Mn ₄ O ₉ are defect type spinels, and □ indicates a defect. Note that Li [Li
_Since it is clear that _0.33 Mn _1.67 ] O ₄ is expressed as Li [Li _1/3 Mn _5/3 ] O ₄ , this expression will be used hereinafter.

In the above table, λ -MnO ₂ is Li ₂ Mn ₂ O
It is possible to discharge up to ₄ , in which case the theoretical capacity density is 308 mAh / g. However, λ -MnO
_In the range of _{2 to} LiMn ₂ O ₄ , it is _{4 to 3} V, and its crystal structure is cubic, while that of LiMn ₂ O _{4 to} Li ₂
In the range of Mn ₂ O ₄ , the voltage is _{3 to 2} V and the crystal structure is tetragonal. In other words, the discharge curve has two steps, which is inconvenient in practical use. Further, there is a problem that the discharge capacity decreases due to repeated charging and discharging, because the strain of the crystal occurs due to the yarn tailor effect with the change of the crystal structure.

On the other hand, the positive electrode active material tends to have a higher energy density when incorporated in a lithium battery as the potential thereof is higher (noble). However, the decomposition of the organic electrolytic solution and the safety of the battery are important. When giving priority to
It is generally advantageous that the potential is low (base). Of the 3V active material shown in the above table, Mn ₃ O ₄ has a low capacity density, and LiMn ₂ O ₄ has a significantly lower capacity density with repeated charge and discharge cycles, so it cannot be said to be an appropriate material. . In the case of Li ₂ Mn ₄ O ₉ and Li ₄ Mn ₅ O ₁₂ , when the charge and discharge cycle was repeated, the discharge capacity density of 150 mAh / g was obtained in the first cycle, but when the charge and discharge of 10 cycles were repeated. , 125-130 mAh / g. In addition, Li _6.0 Mn ₅ O obtained by chemically (not electrochemically) inserting lithium into Li ₄ Mn ₅ O _12
12 (equivalent to 109 mAh / g) still has the same pure cubic crystal as before insertion, but up to Li _6.3 Mn ₅ O ₁₂ (equivalent to 156 mAh / g), it becomes a mixture of cubic and tetragonal crystals, It has been reported that crystal structure changes occur. Note that the Li described here
₄ Mn ₅ O ₁₂ is prepared by mixing powders of lithium carbonate (Li ₂ CO ₃ ) and manganese carbonate (MnCO ₃ ) in a stoichiometric ratio and firing the mixture at 400 ° C. in air.

On the other hand, MN Richard et al. (Solid State Ioni
cs, Vol73, Page81, 1994) is spinel type Li ₄ Mn ₅ O
₁₂ (Li [Li _1/3 Mn _5/3 ] O ₄ ) to the above MMThackeray
The same method and conditions as those described above, or γ-manganese oxyhydroxide (γ-MnOOH) and lithium hydroxide monohydrate (LiOH.
H ₂ O) and then calcined in air at 450 ° C. to synthesize it. First, 4.2 V (vs. Li /
After charging to Li ⁺ ), we tried an operation of discharging to 2.5V and charging again, and reported that a discharge capacity density of about 110mAh / g was obtained at around 2.8V. Moreover, it is estimated that the capacity density of about 150 mAh / g can be reversibly obtained by starting from discharging instead of charging and repeating charging and discharging between 2.5 and 3.6 V. However, this is a speculation, it is not an actual result, and it does not specifically mention whether or not the crystal structure changes with charge and discharge.

[0009]

As described above, the conventional Li
_4/3 Mn _5/3 O ₄ shows a discharge capacity that is fairly close to the theoretical value of about 150 mAh / g (163 mAh / g) at the beginning of charging / discharging, but at the 10th cycle, 125 to 130 mAh / g It is a problem to decrease. This is thought to be closely related to the method of synthesizing this material and its crystal structure. The present invention seeks to solve this problem.

[0010]

The present invention provides Li _4/3 Mn _5/3
By giving O ₄ a superlattice structure in which the arrangement of its constituent lithium ions, manganese ions, and oxygen ions has regularity, changes in the crystal structure due to repeated charging, discharging, and charging / discharging It is to eliminate it as much as possible and prevent the capacity from decreasing.

Also, Li _4/3 showing such a superlattice structure
The feature of the present invention is that Mn _5/3 O ₄ is synthesized by firing a mixture of lithium nitrate and a manganese compound at 500 to 600 ° C. in an oxygen atmosphere.

[0012]

BEST MODE _FOR CARRYING OUT THE INVENTION The present invention is based on the findings obtained as a result of examining various synthetic methods and synthetic conditions of Li _4/3 Mn _5/3 O ₄ . That is, conventionally, when synthesizing the substance, a method and conditions have been adopted in which a lithium compound and a manganese compound are mixed and fired in air at 400 to 450 ° C. On the other hand, it was found that when oxygen was used as the atmospheric gas instead of air, a crystal believed to have a superlattice structure was obtained. The sample having the composition of Li _4/3 Mn _5/3 O ₄ synthesized by the inventors of the present application was _confirmed to have a space group Fd3m by X-ray diffraction measurement and Rietveld analysis.
Was found to belong to. Also, the (1,1,1) of this crystal
To the lower angle side (1 / (2θ = 18.5 °))
2,1 / 2,1 / 2) surface (2θ = 9.2 °), (1 / 3,1 / 3,1 / 3) surface (2
It was discovered that X-ray diffraction peaks corresponding to the (1 / 6,1 / 6,1 / 6) plane (2θ = 3.1 °) were observed.

When this crystal structure is viewed from the direction perpendicular to the (1,1,1) plane of Li _4/3 Mn _5/3 O ₄ , it appears to have a layered structure as shown in FIG. The tetrahedral 8 (a) sites are occupied by lithium ions and the octahedral 16 (d) sites are occupied by lithium ions and manganese ions. The X-ray diffraction peak observed on the low angle side is the octahedron 16
(d) Lithium ions and manganese ions in the site are 1:
It suggests that they are regularly arranged at a ratio of 5, that is, they have a superlattice structure.

Li _4/3 Mn _5/3 O ₄ having such a structure
An electrode was prepared by a conventionally known method using, and a charge / discharge test (range of 2.0 to 3.5 V, discharge →
After repeated charging), an extremely flat discharge curve was obtained at about 2.8 V, and a discharge capacity density of 150 mAh / g at the first cycle and 142 mAh / g at the 100th cycle was obtained. Almost no change was observed in the X-ray diffraction pattern during the charge / discharge process, and it was confirmed that the lattice constant of the sample in which lithium was inserted (discharged) by about 0.95 atoms only increased by about 0.05 A. Thus, the degree of decrease in discharge capacity density with the progress of charge / discharge cycles is extremely smaller than that of the conventional one because the positive electrode active material according to the present invention has the superlattice structure as described above. It is thought that. The above-mentioned MNR Richards et al. Mainly used the tetrahedron 8 (a) for insertion (discharge) and desorption (charge) of lithium ions.
It seems that the lithium ions at the site are involved, but when starting from the discharge process, lithium ions are found at the 16 (c) site, which is equivalent to the octahedral 16 (d) site in Fig. 1. It is considered to have been inserted. In this case, the discharge / charge reaction can be described as follows.

[0015]

Embedded image

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to preferred embodiments. [Example] After chemically synthesized manganese dioxide powder (IC No. 22) and lithium nitrate powder were mixed at a molar ratio of 5: 4, the temperature was changed to 550 ° C, 500 ° C, 450 ° C for 48 hours, Heat treatment was performed under an oxygen atmosphere. Then, it was naturally cooled to room temperature to obtain a positive electrode active material according to the present invention.

Next, 87 wt% of lithium-containing spinel-type manganese composite oxide as a positive electrode active material, 5 wt% of carbon black as a conductive material, and 3 wt% of n-methyl-2pyrrolidone containing 5 wt% of polyvinylidene fluoride as a binder. The mixed solution is mixed in a dry room to form a paste, which is then applied to the titanium net of the current collector, dried at 80 ° C., and the positive electrode plate according to the present invention having a size of 25 mm × 25 mm × 0.25 mm (Positive electrode active material: 91 mg, theoretical capacity 13.5 mAh) was manufactured. The positive plate when the heat treatment temperature was 550 ° C and 500 ° C respectively
(A) and (B).

A test battery was manufactured using one positive electrode plate and two lithium metal plates of the same size as the counter electrode, and 300 ml of a mixed solution of ethylene carbonate and diethyl carbonate containing 1 M lithium perchlorate as an electrolyte. did. A metallic lithium reference electrode was used for measuring the potential of the positive electrode.
A cycle test was conducted in which this battery was discharged to 2.0 V at 25 ° C. and a current density of 0.5 mA / cm ² , and charged to 3.3 V at the same current density.

The change in the value of the capacity with the progress of cycles is shown in FIG. 2 as the capacity per unit weight of the active material. For reference, it was obtained by mixing powders of manganese carbonate powder and lithium carbonate powder with a molar ratio of 5: 2 as reported previously, and the heat treatment temperature was 850 ° C and 750 ° C. In that case, the conventional positive electrode plates were designated as (C) and (D), respectively. From the figure, the positive plates (A) and (B) according to the present invention are the same as the conventional positive plates.
It can be seen that the capacity is higher than those of (C) and (D), and that the capacity decrease with charge / discharge cycle is small. Further, the charging / discharging behavior was very smooth (FIG. 3), and it was found that the potential difference between the initial stage of charging and the final stage of charging and the initial stage of discharging and the final stage of discharging was extremely small compared to the conventional one.

In order to investigate why the positive electrode plate according to the present invention has a higher capacity and a better charging / discharging cycle performance than the conventional one, an X-ray diffraction analysis of each active material was conducted. Was focused on the low diffraction angle side. As a result, the active material according to the present invention after the heat treatment was
The (1 / 2,1 / 2,1 / 2) plane, (1 / 3,1 / 3,1 / 3) plane and (1 / 6,1 / 6,1) plane of _4/3 Mn _5/3 O ₄ It was found that while the peaks showing the regular arrangement of the / 6) plane are shown, the conventional one does not. This clearly exceeds the order of one unit lattice of the spinel structure and suggests a superlattice structure. FIG. 4 shows an X-ray diffraction pattern of the positive electrode active material synthesized at the heat treatment temperatures of 550 ° C. and 500 ° C. according to the present invention.

Further, when the state of the electrode plate was examined after completion of the 100-cycle charge / discharge test, peeling of the electrode was observed in the conventional one, but not in the present invention. From the above, it can be said that the effect is remarkable.

[0022]

INDUSTRIAL APPLICABILITY As described above, the positive electrode active material according to the present invention has an ordered structure and a superlattice structure which are beyond the unit crystal lattice frame, and is an active material obtained by a very novel and unconventional idea. Is. Moreover, as can be seen from the charge and discharge characteristics of the positive electrode plate using the positive electrode active material according to the present invention, it is possible to obtain a remarkable improvement effect such that the capacity decrease is extremely small, the discharge potential is stable, and the fluctuation is small. In addition, since there is no expansion or contraction of the active material due to charge / discharge, the positive electrode plate to which this active material is applied has less detachment or peeling off of the active material due to charge / discharge, and the life of the positive electrode plate can be extended. The industrial value of the present invention is extremely large.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a crystal arrangement when viewed from a direction perpendicular to a (1,1,1) plane of a spinel structure according to the present invention.

FIG. 2 is a diagram showing a capacity transition of a positive electrode active material according to an example of the present invention during charge / discharge cycles.

FIG. 3 is a diagram showing charge / discharge characteristics of the positive electrode active material according to the present invention.

FIG. 4 is an X-ray diffraction pattern showing a superlattice peak of the positive electrode active material according to the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Zenichiro Takehara 6-10-14 Oeda Nishishinbayashi-cho, Nishikyo-ku, Kyoto City, Kyoto Prefecture

Claims (3)

[Claims] 1. A lithium battery having a composition of Li _4/3 Mn _5/3 O _4, a crystal thereof having a spinel structure belonging to a space group Fd3m, and a superlattice structure. Positive electrode active material. 2. In a powder X-ray diffraction pattern using CuKα rays, 2θ of 9.2 °, 6.3 °, 3.1 ° in the space group Fd3m (1
/ 2,1 / 2,1 / 2), (1 / 3,1 / 3,1 / 3), (1 / 6,1 / 6,1 / 6) The positive electrode active material for a lithium battery according to claim 1. 3. Lithium nitrate and manganese oxide are mixed in a stoichiometric ratio, and the mixture is mixed in an oxygen atmosphere at 500-6.
The method for producing a positive electrode active material for a lithium battery according to claim 1, wherein the positive electrode active material is fired at 00 ° C.