JP4884995B2 - Positive electrode active material, positive electrode and secondary battery - Google Patents

Description

  The present invention relates to an improved positive electrode active material made of spinel lithium manganese oxide used for a positive electrode of a secondary battery, a positive electrode containing the positive electrode active material, and a secondary battery. Specifically, by using this improved positive electrode active material for the positive electrode of the secondary battery, the capacity deterioration accompanying the charge / discharge cycle is remarkably improved while maintaining the high charge / discharge capacity of the battery, in particular, by improving the characteristics of the secondary battery. It will be improved.

LiMn ₂ O _4, which is a complex oxide of manganese and lithium, has been proposed as a positive electrode active material for lithium secondary batteries, and has been actively researched. Although it has the characteristics of high voltage and high energy density, it has a problem that the charge / discharge cycle life is short, and has not been used as a practical battery. Until now, as disclosed in JP-A-7-282798 and the like, excess lithium is used to make Li _{1 + X} Mn _2-X O ₄ , or JP-A-3-108261 and JP-A-3-219571. a part of manganese as disclosed in equal Co, and replaced with other metals such as Cr or a _{LiMn 2-X Co X O 4} , LiMn 2-X Cr X O 4, lithium manganese oxide It has been proposed to improve the properties and improve cycle characteristics. In this method, a part of the Mn site is replaced with a metal such as lithium, cobalt, or chromium, so that the crystal structure is densified and the deterioration of the charge / discharge function can be improved.
JP-A-7-282798 JP-A-3-108261 JP-A-3-219571

However, since these reforming methods cause a decrease in charge / discharge capacity, a lithium manganese oxide having improved cycle characteristics without reducing the charge / discharge capacity has been desired. Thus, as a result of intensive investigations focusing on the composition and crystallinity of the spinel lithium manganese oxide, the present inventors have experimentally determined that the conventional synthesis methods are the following (A) to (C). We have reached the present invention.
(A) Oxygen vacancies are generated and an oxide having an extended lattice constant is generated.
(B) The lattice constant of the charging end (n> 0.8) is too contracted.
(C) Due to (A) and (B), the expansion / contraction rate of the crystal accompanying the insertion / release of lithium is large, and the crystal is easily collapsed.
As a conventional technique for the knowledge of these (A) to (C), with respect to (A), the formula

の酸素欠損δの熱処理条件による変化及び酸素欠損のあるマンガン酸リチウムの電気化学的キャラクタリゼーションはＪ．Ｅｌｅｃｔｒｏｃｈｅｍ．Ｓｏｃ．，Ｖｏｌ．１４２，Ｎｏ．７，Ｊｕｌｙ，１９９５．で報告されている。しかし、酸素欠損に伴うサイクル特性の低下については何ら明確な記載がなされていないし、またその電気化学的な測定のされたマンガン酸リチウムは本願発明の範囲には含まれない。

The changes in oxygen deficiency δ in the heat treatment conditions and the electrochemical characterization of lithium manganate with oxygen deficiency are described in Electrochem. Soc. , Vol. 142, no. 7, July, 1995. It is reported in. However, there is no clear description about the deterioration of cycle characteristics due to oxygen deficiency, and lithium manganate whose electrochemical measurement has been performed is not included in the scope of the present invention.

の酸素ノンストイキオメトリーが報告されているが、電気化学的測定及び考察は何ら為されていない。 In the Journal of Alloys and Compounds 235 (1996) 163-169, spinel

Oxygen non-stoichiometry has been reported, but no electrochemical measurements or considerations have been made.

Regarding (C), Mater. Res. Bull. , 18 (1983) 1375. Mater. Res. Bull. , 19 (1984) 179. Mater. Res. Bull. , 18 (1984) 461. , J .; Electrochem. Soc. , Vol. 137, no. 3, March (1990) 769. It is reported by the literature such as that in the region where n = 0.5 or more, that is, the region where lithium is released more than half, a reaction in a two-phase region where two cubic crystals exist occurs.

  An object of the present invention is to provide a positive electrode active material made of a spinel-type lithium manganese oxide that maintains a high charge / discharge capacity as a positive electrode active material for a secondary battery and has little capacity deterioration associated with a charge / discharge cycle. The present invention provides a positive electrode for a secondary battery including the positive electrode active material and a secondary battery including the positive electrode. The gist of the present invention is the positive electrode active material, the positive electrode, and the secondary battery described in 1 to 7 below. Exist.

1. A positive electrode active material for a secondary battery comprising a lithium-containing metal oxide capable of inserting and releasing lithium, wherein the lithium-containing metal oxide is represented by the general formula LiMn ₂ O ₄ and is a lattice before lithium dedoping The constant a is in the range of 8.220Å ≦ a ≦ 8.235Å, and the following formula (1)
[Chemical formula 4]
LiMn ₂ O ₄ ⇔Li _1-n Mn ₂ O ₄ + nLi + + ne− (1)
In the reversible reaction shown in FIG. 1, the rate of change of the lattice constant accompanying lithium insertion / release during charging / discharging in the range of 0 ≦ n ≦ 1 is 1.3 × 10 ⁻³ Å or less per 1 mAh / g, A positive electrode active material having a lattice constant value of 8.07 （or more at the end (potential 4.35V counter electrode Li) and a lattice constant value of 8.235Å or less at the discharge end (potential 3.2V counter electrode Li).

2. Lithium, a compound capable of inserting and releasing lithium ions or a lithium alloy is used as the negative electrode active material, and the electrolyte is LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr. The above 1 item, which is used as a positive electrode active material of a secondary battery including an electrolytic solution in which at least one compound selected from the group consisting of CH ₃ SO ₃ Li and CF ₃ SO ₃ Li is dissolved in an organic solvent. Positive electrode active material.
3. 3. The positive electrode active material according to item 1 or 2, wherein the lithium-containing metal oxide has an initial discharge capacity (at the time of Li counter electrode) of 100 mAh / g or more.

4). 4. The positive electrode active material according to any one of 1 to 3 above, wherein the lithium-containing metal oxide is produced using a lithium compound and Mn ₂ O ₃ produced by heat treatment of MnCO ₃ or MnO ₂ as a starting material. material.
5. The lithium-containing metal oxide is manufactured by mixing a lithium compound and Mn ₂ O ₃ prepared by heat-treating MnCO ₃ or MnO ₂ , firing in the air, and then gradually cooling. 5. The positive electrode active material according to any one of items 1 to 4.
6). A positive electrode for a secondary battery including the positive electrode active material according to any one of items 1 to 5.
7). A secondary battery comprising the positive electrode for a secondary battery as described in 6 above.

  By using the positive electrode active material comprising the lithium manganese oxide of the present invention, a battery having a large charge / discharge capacity and improved cycle characteristics can be obtained. As a result, since lithium manganese oxide, which is an inexpensive material, can be used as a positive electrode material, a high-performance, safe and inexpensive lithium secondary battery can be supplied to a wide range of applications, and its industrial value is great. .

Hereinafter, the present invention will be described in more detail.
The greatest feature of the present invention is that the lithium-containing metal oxide, which is the positive electrode active material of the secondary battery, has a lattice constant a before the lithium dedoping of the cubic spinel crystal represented by the general formula LiMn ₂ O ₄ is 8.220４. ≦ a ≦ 8.235mm,
The following formula (1)
[Chemical formula 5]
LiMn ₂ O ₄ ⇔Li _1-n Mn ₂ O ₄ + nLi + + ne− (1)
In the reversible reaction shown in FIG. 1, the rate of change of the lattice constant accompanying lithium insertion / release during charging / discharging in the range of 0 ≦ n ≦ 1 is 1.3 × 10 ⁻³ Å or less per 1 mAh / g, This is a lithium manganese oxide having a lattice constant value of 8.07 に お け る or more at the end (potential 4.35V counter electrode Li) and a lattice constant value of 8.235Å or less at the discharge end (potential 3.2V counter electrode Li). That is. Here, the general formula LiMn ₂ O ₄ does not necessarily mean that the ratio of lithium, manganese, and oxygen is strictly 1: 2: 4, as will be apparent from the following description. There may be a gap. Such lithium / manganese nonstoichiometric compounds and oxygen non-stoichiometry are well known. Cubic spinel crystals, specifically the following space groups

の構造を有するスピネル型のマンガン酸リチウムは、本発明で規定する一般式ＬｉＭｎ₂Ｏ₄の概念に含まれる。

The spinel-type lithium manganate having the structure: is included in the concept of the general formula LiMn ₂ O ₄ defined in the present invention.

First, the requirements regarding the lattice constant will be described. Oxygen deficiency increases the initial capacity by increasing the number of Mn ³⁺ yarn teller ions, but the crystal structure is distorted and the lattice constant is increased. As a result, the charge / discharge cycle characteristics are degraded, and when the average valence of manganese is 3.5 or less (the ratio of Mn ³⁺ is more than half), it is quantitatively reduced as the amount of defects increases. It was. Furthermore, when the average valences of manganese are almost the same, the deterioration of the cycle characteristics due to oxygen deficiency is remarkable in the oxide having a lattice constant exceeding 8.235%, and is suppressed to 8.235% or less. It was slight in the oxide. Therefore, the value of the lattice constant at the discharge end (3.2 V counter electrode Li) was set to 8.235 Å or less. More preferable value is 8.233 mm or less, and most preferable is 8.231 mm or less.

By reducing oxygen vacancies as described above, the cycle characteristics can be considerably improved as shown in Comparative Examples 1 and 2 to be described later. However, the present invention is not satisfied with this, and the aforementioned findings (B ) To limit the lattice constant of the charging end. In other words, structural changes have a very important meaning in realizing the large charge capacity of spinel LiMn ₂ O ₄ and the reversibility of stable charge / discharge. It was found that the shrinkage of the lattice constant was large at the charging end (potential 4.35V counter lithium), and conversely, the good one suppressed the shrinkage of the lattice constant. Based on these results, the value of the lattice constant at the charging end is set to 8.07 mm or more. More preferably, it is 8.075 mm or more, and most preferably 8.080 mm or more.

This suppresses an unstable structural change in the region of n = 0.5 or more in the reversible reaction formula of the battery, and can reduce excessive shrinkage of the lattice constant at the charging end resulting from this. The crystal expansion / contraction rate associated with the insertion / release of lithium in the range of 0 ≦ n ≦ 1 in combination with the definition in FIG. Based on these results, it is necessary that the rate of change of the lattice constant accompanying the insertion / release of lithium is 1.3 × 10 ⁻³ Å or less per 1 mAh / g, more preferably 1.25 per 1 mAh / g. × 10 ⁻³ Å or less, most preferably 1.20 × 10 ⁻³当 た り or less per 1 mAh / g.

In the present invention, the lattice constant of LiMn ₂ O ₄ which is a positive electrode active material before lithium dedoping is defined as 8.220Å ≦ a ≦ 8.235Å, but this range is from the positive electrode active material to the lithium in the battery system. Shows the initial state before undoping. If it deviates from this range, desired battery characteristics cannot be obtained. Here, the more preferable lower limit of the lattice constant is 8.225Å, the upper limit is 8.234Å, the most preferable lower limit is 8.228Å, and the upper limit is 8.233Å.
When a lithium-containing metal oxide represented by the general formula LiMn ₂ O ₄ having such a lattice constant (hereinafter referred to as lithium manganese oxide of the present invention) is used as a counter electrode, the discharge capacity is usually 100 mAh / g or more can be obtained, a more preferable value is 110 mAh / g or more, and a most preferable value is 120 mAh / g or more.

The manufacturing method of the lithium manganese oxide of this invention is demonstrated below. Examples of the lithium compound used as a starting material include Li ₂ CO ₃ , LiNO ₃ , LiOH, LiOH · H ₂ O, LiCl, CH ₃ COOLi, Li ₂ O, lithium dicarboxylate, and the like, among which LiOH · H ₂ O LiOH or lithium dicarboxylate is preferably used.
In addition, examples of manganese compounds used as starting materials include manganese oxides such as Mn ₂ O ₃ and MnO ₂ , manganese salts such as MnCO ₃ , Mn (NO ₃ ) ₂ , and manganese dicarboxylate. Mn ₂ O _3, it is preferable to use the dicarboxylic acid manganese, Mn ₂ O ₃ in this case may be used those prepared by heat-treating compound such as MnCO ₃ and MnO _2.

  Next, in the present invention, the aforementioned manganese compound and lithium compound are mixed. Mixing may be a normal method, for example, a method of dry mixing both raw materials, a method of wet mixing, a method of suspending a manganese compound in an aqueous lithium salt solution, and then a method of drying the suspension, a method of coprecipitation, Alternatively, any method capable of uniformly mixing such as a method of pulverizing and mixing with a ball mill may be used.

As an example of a specific production method of the lithium manganese oxide of the present invention, γ-MnO ₂ was heated at 750 ° C. for 24 hours in the air, and Mn ₂ O ₃ and LiOH · H ₂ O were mixed with lithium. The mixture mixed at a molar ratio of manganese of 1: 2 was calcined and then calcined at 750 ° C. in the air, and the temperature was increased to 450 ° C. at 0.2 ° C./min. And a method in which a non-aqueous solution of a lithium salt and a manganese salt is coprecipitated by a dicarboxylate coprecipitation method, and a method of firing the powder. As a method for cooling the sample at this time, rapid quenching is not preferable because oxygen deficiency tends to occur.

In the secondary battery, the spinel system represented by the general formula LiMn ₂ O ₄ of the present invention as described above, which defines the rate of change of the lattice constant accompanying the insertion and release of lithium and the range of the lattice constant between the charge end and the discharge end As a negative electrode active material used in combination with a positive electrode including a positive electrode active material made of lithium manganese oxide, any material that is normally used for this type of secondary battery can be used. Examples include lithium and lithium alloys, but a carbon material capable of inserting and releasing lithium with higher safety is preferable. The carbon material is not particularly limited, but graphite, coal-based coke, petroleum-based coke, coal-based pitch carbide, petroleum-based pitch carbide, needle coke, pitch coke, phenolic resin, crystalline cellulose, etc. Examples thereof include partially graphitized carbon materials, furnace black, acetylene black, and pitch-based carbon fibers.

  As the negative electrode, a negative electrode active material and a binder (binder) slurryed with a solvent can be applied and dried. Similarly, the positive electrode can be obtained by applying a slurry of a positive electrode active material, a binder (binder), and a conductive agent with a solvent and drying it. Examples of the binder (binder) for the negative electrode and the positive electrode active material include polyvinylidene fluoride, polytetrafluoroethylene, EPDM (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene rubber), and NBR (acrylonitrile). -Butadiene rubber), fluorine rubber and the like, but are not limited thereto.

  As the conductive agent for the positive electrode, graphite fine particles, carbon black such as acetylene black, and amorphous carbon fine particles such as needle coke are preferably used, but are not limited thereto. As the separator, a microporous polymer film is used, and one made of a polyolefin polymer such as nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, polypropylene, polyethylene, polybutene, or the like is used. The chemical and electrochemical stability of the separator is an important factor. In this respect, a polyolefin-based polymer is preferable, and it is desirable that the polymer is made of polyethylene in view of the self-occluding temperature which is one of the purposes of the battery separator.

  In the case of a polyethylene separator, ultrahigh molecular weight polyethylene is preferable from the viewpoint of high temperature shape maintenance, and the lower limit of the molecular weight is preferably 500,000, more preferably 1,000,000, and most preferably 1.5 million. On the other hand, the upper limit of the molecular weight is preferably 5 million, more preferably 4 million, and most preferably 3 million. This is because if the molecular weight is too large, the pores of the separator may not close when heated because the fluidity is too low.

As the electrolyte, an electrolyte containing lithium is used. For example, LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr, CH ₃ SO ₃ Li, CF ₃ SO ₃ An electrolytic solution in which Li or the like is dissolved in an organic solvent is used. The organic solvent is not particularly limited. For example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, ethers, amines, esters, amides. And phosphoric acid ester compounds can be used. Typical examples of these are listed: propylene carbonate, ethylene carbonate, vinylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 4-methyl-2-pentanone, 1,2-dimethoxyethane, 1,2 -Diethoxyethane, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile, benzonitrile, butylnitrile, valeronitrile, 1, Single or two or more kinds of mixed solvents such as 2-dichloroethane, dimethylformamide, dimethyl sulfoxide, trimethyl phosphate and triethyl phosphate can be used.
Moreover, you may make it use not only such a non-aqueous electrolyte but a solid electrolyte.

The method of the present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to these examples.
In this example, when evaluating the initial discharge capacity and the capacity retention rate, metallic lithium is used as the negative electrode, and the initial discharge capacity is converted per 1 g of the positive electrode active material.

Example 1
gamma-MnO 24 hours ₂ at 750 ° C., and Mn ₂ O ₃ and LiOH · H ₂ O was obtained by heating in air was used as a starting material, the atomic ratio of lithium and manganese 1: blended so 2 did. Ethanol was added to this blend and ground well in a mortar to make a uniform mixture. The obtained mixture was calcined in the atmosphere at 500 ° C. for 24 hours. Next, after baking at 750 ° C. for 24 hours in the air, 0.2 ° C./min. Was slowly cooled to obtain LiMn ₂ O ₄ .

(Comparative Example 1)
After mixing LiOH.H ₂ O and γ-MnO ₂ at a ratio of Li / Mn = 1/2 by an automatic mortar for 8 hours, the mixture was heated in oxygen at 750 ° C. for 24 hours, and then 0.5 to 450 ° C. ° C / min. Was slowly cooled to obtain LiMn ₂ O ₄ .
(Comparative Example 2)
The sample obtained in Comparative Example 1 was again heated in nitrogen at 550 ° C. for 24 hours, and then rapidly cooled by withdrawing the sample from the furnace to obtain LiMn ₂ O ₄ having a large oxygen deficiency.

(Comparative Example 3)
Li ₂ CO ₃ and Mn ₂ O ₃ were wet mixed using a mortar at a ratio of Li / Mn = 1/2, and the mixture was calcined at 650 ° C. for 24 hours in the air. Next, after baking at 750 ° C. for 24 hours in the air, the heating furnace was turned off and cooled in the furnace to obtain LiMn ₂ O ₄ . In this case, the cooling rate to 450 ° C. is 10 ° C./min. Met.

After confirming that the samples synthesized in the above examples and comparative examples were all cubic single-phase by precise measurement of powder X-ray diffraction, the lattice constant a of the crystal (cubic system) was expressed by the following formula: 6]
a = d · (h ² + k ² + l ² ) ^1/2
(H, k, l are face indices, d is the face spacing of face indices (hkl))
Is used to calculate the seven face indices of (hkl) = (311), (222), (400), (331), (511), (440), (531), The lattice constant is a.

Next, a battery manufacturing method and charge / discharge conditions will be described with reference to FIG.
Each positive electrode active material synthesized in Example 1 and Comparative Examples 1 to 3, acetylene black as a conductive agent, and polytetrafluoroethylene resin as a binder were mixed in a weight ratio of 75: 20: 5. A positive electrode mixture was obtained. Further, 0.1 g of the positive electrode mixture was press-molded at a diameter of 16 mm at 1 ton / cm ² to obtain a positive electrode 1. A porous polypropylene film was placed on the positive electrode 1 as the separator 3. A lithium plate having a diameter of 16 mm and a thickness of 0.4 mm as the negative electrode 4 was pressure-bonded to a sealing tube 6 provided with a polypropylene 5 gasket. A solution (50 vol%: 50 vol%) of ethylene carbonate and 1,2-dimethoxyethane in which 1 mol / 1 lithium perchlorate is dissolved is used as a non-aqueous electrolyte, and this is added to the separator 3 and the negative electrode 4. It was. Thereafter, the battery was sealed.

  The charge / discharge cycle characteristics of these batteries manufactured as described above were compared. In addition, the charging / discharging cycle test in a present Example and a comparative example was done by charging / discharging with constant current between 235 mA of charging / discharging electric current and a voltage range between 4.35V and 3.2V. To determine the lattice constant (first cycle) at the charge and discharge ends, after charging the upper limit potential of 4.35V during charging, and during discharging, after resting for 1 hour after reaching the lower limit potential of 3.2V, pay careful attention to avoid short circuits. After disassembling, the positive electrode pellet was taken out and X-ray diffraction measurement was performed. The results are shown in Table 1.

In the embodiment according to the present invention, the initial discharge capacity is large (initial discharge capacity ≧ 100 mAh / g), and in addition, the cycle characteristics are excellent (capacity maintenance ratio after 100 cycles ≧ 95%, capacity maintenance ratio after 500 cycles ≧ 80). %). FIG. 1 shows the relationship between the lattice constant change rate (ｈ / mAh · g ⁻¹ ) and cycle characteristics (capacity retention rate after 100 cycles%) in Example 1 and Comparative Examples 1 to 3.
In this embodiment, metallic lithium is used as the negative electrode material of the battery, but the same result can be obtained when a lithium alloy or a compound capable of inserting and releasing lithium is used.

FIG. 1 is a correlation diagram between the lattice constant change rate and the cycle characteristics associated with lithium insertion / release of LiMn ₂ O ₄ . FIG. 2 is an explanatory view of a longitudinal section of a coin-type battery used for testing a positive electrode active material for a secondary battery according to an example of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Case 3 Separator 4 Negative electrode 5 Gasket 6 Sealing can

Claims (7)

A positive electrode active material for a secondary battery comprising a lithium-containing metal oxide capable of inserting and releasing lithium, wherein the lithium-containing metal oxide is represented by the general formula LiMn ₂ O ₄ and is a lattice before lithium dedoping The constant a is in the range of 8.220Å ≦ a ≦ 8.235Å, and the following formula (1)
[Chemical 1]
LiMn ₂ O ₄ ⇔Li _1-n Mn ₂ O ₄ + nLi + + ne− (1)
In the reversible reaction shown in FIG. 1, the rate of change of the lattice constant accompanying lithium insertion / release during charging / discharging in the range of 0 ≦ n ≦ 1 is 1.3 × 10 ⁻³ Å or less per 1 mAh / g, A positive electrode active material characterized in that the value of the lattice constant at the end (potential 4.35V counter electrode Li) is 8.07Å or more and the value of the lattice constant at the discharge end (potential 3.2V counter electrode Li) is 8.235Å or less. . Lithium, a compound capable of inserting and releasing lithium ions or a lithium alloy is used as the negative electrode active material, and the electrolyte is LiClO ₄ , LiAsF ₆ , LiPF ₆ , LiBF ₄ , LiB (C ₆ H ₅ ) ₄ , LiCl, LiBr. And a positive electrode active material of a secondary battery including an electrolytic solution in which at least one compound selected from the group consisting of CH ₃ SO ₃ Li and CF ₃ SO ₃ Li is dissolved in an organic solvent. The positive electrode active material according to 1.   The positive electrode active material according to claim 1 or 2, wherein an initial discharge capacity (at the time of Li counter electrode) of the lithium-containing metal oxide is 100 mAh / g or more. The positive electrode according to any one of claims 1 to 3, wherein the lithium-containing metal oxide is produced using Mn ₂ O ₃ produced by heat treatment of a lithium compound and MnCO ₃ or MnO ₂ as a starting material. Active material. The lithium-containing metal oxide is produced by mixing a lithium compound and Mn ₂ O ₃ prepared by heat-treating MnCO ₃ or MnO ₂ , firing in the air, and then gradually cooling. The positive electrode active material according to any one of claims 1 to 4   A positive electrode for a secondary battery including the positive electrode active material according to claim 1.   A secondary battery comprising the positive electrode according to claim 6.

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