JP2005078820A - Non-aqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the charge/discharge cycle performance of a high voltage non-aqueous electrolyte secondary battery where a positive electrode comprises a lithium nickel manganese compound oxide. <P>SOLUTION: The positive electrode active material is a lithium nickel manganese compound oxide expressed by a general formula Li<SB>x</SB>Ni<SB>y</SB>Mn<SB>2-y</SB>O<SB>4-δ</SB>(where, 0<x<1.1, 0.45<y<0.55, and 0≤δ<0.4). The non-aqueous electrolyte contains annular carbonate and chain-like carbonate, and at least either the annular carbonate or chain-like carbonate contains a fluorine element. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a non-aqueous electrolyte secondary battery in which a positive electrode active material is a lithium / nickel / manganese composite oxide.

In recent years, as electric devices have become more portable and smaller, non-aqueous electrolyte secondary batteries having high energy density and light weight have been applied as built-in batteries. At present, lithium cobalt oxide (LiCoO ₂ ) is mainly used as a positive electrode active material for non-aqueous electrolyte secondary batteries that are commercially available. However, in the future, with further increase in production and battery size, material cost, cobalt reserves and environmental regulations may become serious.

Accordingly, lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMn ₂ O ₄ ), and the like have been proposed as positive electrode active materials that replace lithium cobalt oxide. Among them, lithium manganese oxide is expected in terms of low cost and low pollution.

Recently, as reported in Patent Document 1, a lithium-nickel-manganese composite oxide (LiNi _0.5 Mn _1.5 O ₄ ) in which a part of Mn of lithium manganese oxide is replaced with Ni was found. It was done. Since the non-aqueous electrolyte secondary battery equipped with the lithium / nickel / manganese composite oxide as a positive electrode is a 5V class high voltage battery, in applications such as portable computers, power tools, HEVs and EVs, There is an advantage that the number of cells connected in series can be reduced.

JP 2000-515672 A.

A charge / discharge curve of LiMn _1.5 Ni _0.5 O ₄ , which is a lithium / nickel / manganese composite oxide, is shown in Non-Patent Document 1.

Ota et al., 41st Battery Discussion Meeting, 2D16, P452 (2000).

Further, in Patent Document 2, in a nonaqueous electrolyte secondary battery using a 5V class positive electrode active material such as LiMn _1.5 Ni _0.5 O ₄ , a cyclic carbonate and a chain carbonate are mixed and used as an electrolyte solution solvent. Technology is disclosed.

JP 2001-256966 A.

  Furthermore, in Patent Document 3, in a non-aqueous electrolyte secondary battery using a 4V class positive electrode active material such as lithium cobaltate, lithium nickelate, lithium manganate, etc., fluorinated carbonate and chain carbonate are mixed as an electrolyte solvent. And a volume ratio (fluorinated carbonate: chain carbonate) in the range of 5:95 to 90:10 is disclosed, thereby improving the safety against battery destructive testing.

JP-A-10-116630.

  However, a 5V class non-aqueous electrolyte secondary battery provided with a positive electrode containing this lithium / nickel / manganese composite oxide is a non-aqueous electrolyte secondary battery provided with a positive electrode using lithium cobalt oxide or lithium manganese oxide. However, since the operating voltage is high, the non-aqueous solvent contained in the electrolyte is oxidatively decomposed when the battery is charged, and there is a problem that the charge / discharge cycle performance deteriorates due to depletion of the electrolyte, making it difficult to put it to practical use. It has become.

  The present invention has solved the problem that hinders the practical application of this 5V class non-aqueous electrolyte secondary battery by systematic many experiments, and its purpose is to provide a lithium / nickel / manganese composite oxide on the positive electrode. It is another object of the present invention to improve the charge / discharge cycle performance of a high-voltage nonaqueous electrolyte secondary battery by optimizing the electrolyte composition of the nonaqueous electrolyte secondary battery.

According to the first aspect of the present invention, the positive electrode active material has the general formula Li _x Ni _y Mn _2-y O _4-δ (where 0 <x <1.1, 0.45 <y <0.55, 0 ≦ δ < 0.4), a non-aqueous electrolyte secondary battery that is a lithium / nickel / manganese composite oxide, wherein the non-aqueous electrolyte includes a cyclic carbonate and a chain carbonate, and at least one of the cyclic carbonate and the chain carbonate. The seed is characterized by containing a fluorine element.

Of the present invention, the positive electrode active material is the formula _{Li x Ni y Mn 2-y} O 4-δ ( where, 0 <x <1.1,0.45 <y <0.55,0 ≦ δ <0.4 ), A non-aqueous electrolyte secondary battery that is a lithium-nickel-manganese composite oxide, wherein the non-aqueous electrolyte includes a cyclic carbonate and a chain carbonate, and at least one of the cyclic carbonate or the chain carbonate is fluorine. In a nonaqueous electrolyte secondary battery containing an element, an oxidative decomposition reaction of a nonaqueous solvent under a high voltage is suppressed.

  As a result, the problem of deterioration of charge / discharge cycle performance due to electrolyte depletion due to oxidative decomposition of electrolyte under high voltage at the positive electrode, which is peculiar to 5V class batteries, has been improved, and high energy density and long life High voltage non-aqueous electrolyte secondary battery can be obtained. Therefore, the industrial value of the present invention is extremely large.

  In order to solve the problems of the 5V class battery, such as an increase in internal resistance of the battery accompanying charge / discharge cycles and poor life performance, the non-aqueous electrolyte secondary battery comprising a lithium / nickel / manganese composite oxide on the positive electrode Were made and repeated intensive experiments on the electrolyte composition.

  As a result, it was found that when the nonaqueous electrolyte contains a mixed solvent of a cyclic carbonate and a chain carbonate, a charge / discharge cycle of 50 cycles or more is possible. However, even in the battery using the mixed solvent, charge / discharge cycle deterioration due to oxidative decomposition of the solvent was remarkable.

  As a result of further investigation of the solvent, it was found that the charge / discharge cycle performance was improved by substituting at least part of the hydrogen element of the cyclic carbonate or chain carbonate with a fluorine element. Hereinafter, “a compound obtained by substituting at least a part of a hydrogen element of a cyclic carbonate or a chain carbonate with a fluorine element” will be referred to as “fluorinated carbonate”.

  The reason for this has not yet been clarified, but the following reasons are possible. In a 5V class battery, when fluorinated carbonate is contained in a non-aqueous solvent, a stable oxidation-resistant film is formed on the surface of the positive electrode active material, and as a result, the reaction between the positive electrode and the solvent is suppressed, and charge / discharge cycle characteristics Will be improved.

  Furthermore, by substituting at least a part of the hydrogen element of the cyclic carbonate or chain carbonate with the fluorine element, the molecular structure is stabilized and the oxidation resistance is improved. As a result, the oxidative decomposition of the nonaqueous solvent is suppressed, and the charge is reduced. Discharge cycle characteristics are improved.

  Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and vinylene carbonate (VC). Examples of the chain carbonate include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC). One or more cyclic carbonates or chain carbonates are used in combination, and at least one of the mixed solvents is a solvent containing a fluorine element.

  There is no restriction | limiting in particular as a kind of the cyclic carbonate and chain carbonate (henceforth fluorinated carbonate) containing the fluorine element used by this invention, A various fluorinated carbonate can be used suitably. The fluorinated carbonate is selected from, for example, a fluorinated cyclic carbonate represented by the general formula (1) represented by the chemical formula 1 or a fluorinated chain carbonate represented by the general formula (2) represented by the chemical formula 2. At least one of the above can be used.

In the general formula (1), R ₁ , R ₂ and R ₃ represent a hydrogen atom, R 4 represents a hydrogen atom or an alkyl group, and a part or all of the hydrogen atoms of R ₁ to R ₄ are substituted with an F element. Yes. In addition, R ₅ and R ₆ in the general formula (2) represent an alkyl group, and part or all of the hydrogen atoms in R ₅ and R ₆ are substituted with the F element.

The non-aqueous electrolyte as a cathode active material for use in secondary battery general formula _{Li x Ni y Mn 2-y} O 4-δ ( although the present invention, 0 <x <1.1,0.45 <y <0.55 , 0 ≦ δ <0.4), the lithium-nickel-manganese composite oxide is 4.5 to 4.9 V vs. It has a discharge potential flat part in the range of Li / Li ⁺ . Here, the “discharge potential flat portion” refers to FIG. 1 on page 453 of Non-Patent Document 1 shown in FIG. As shown in the discharge curve of LiMn _1.5 Ni _0.5 O ₄ shown in FIG. A portion where the discharge voltage hardly changes with respect to the discharge capacity (time in the case of constant current discharge) like a voltage plateau of Li / Li ⁺ is shown.

  The lithium / nickel / manganese composite oxide of the present invention can be synthesized by, for example, a solid phase method in which, for example, a lithium source, a manganese source, and a nickel source are mixed and fired. It can also be synthesized by a sol-gel method as disclosed in Patent Document 1.

  Examples of the lithium source include lithium hydroxide monohydrate, lithium nitrate, lithium carbonate, lithium acetate, lithium bromide, lithium chloride, lithium citrate, lithium fluoride, lithium iodide, lithium lactate, and lithium oxalate. Lithium phosphate, lithium pyruvate, lithium sulfate, lithium oxide and the like. Examples of the manganese source include manganese dioxide, manganese oxide, manganese hydroxide, manganese carbonate, manganese nitrate, manganese sulfate, and manganese oxalate. Among these, manganese dioxide is particularly preferable. Furthermore, examples of the nickel source include nickel nitrate, nickel carbonate, and nickel oxide.

  Further, the positive electrode active material of the present invention is composed of two transition metal elements, Ni and Mn, but without changing the intention of the invention, the positive electrode active material is made of Al, Ti, Fe, Nb, Mo or the like. Other metal elements such as W may be included.

  As the negative electrode material used in the nonaqueous electrolyte secondary battery of the present invention, a substance capable of inserting / extracting lithium ions is used. Examples of the substance capable of inserting / extracting lithium ions include carbon materials such as graphite and amorphous carbon, oxides, nitrides, and lithium alloys. As the lithium alloy, for example, an alloy of lithium and aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, or the like can be used. In addition, oxides, nitrides, and lithium alloys can be used by being mixed with or supported on various carbon materials.

  As the separator used in the nonaqueous electrolyte battery of the present invention, a microporous membrane made of a polyolefin resin such as polyethylene or polypropylene, polyvinylidene fluoride, or the like is used, and a plurality of microporous membranes having different materials, weight average molecular weights and porosity May be laminated, or those microporous membranes may contain appropriate amounts of various plasticizers, antioxidants, flame retardants and other additives.

Examples of the organic solvent for the electrolyte used in the nonaqueous electrolyte battery of the present invention include a mixed solvent of carbonates according to claim 1, lactones, sulfur compounds, phosphate ester compounds, sulfolane hydrocarbons, phosphazene derivatives, and the like. Can be mixed and used.
Moreover, there is no restriction | limiting in particular as a solute of the electrolyte used for this invention, A various solute can be used suitably. For example, LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiPF ₆ , LiCF (CF ₃ ) ₅ , LiCF ₂ (CF ₃ ) ₄ , LiCF ₃ (CF ₃ ) ₃ , LiCF ₄ (CF ₃ ) ₂ , LiCF ₅ (CF ₃ ), LiCF ₃ (C ₂ F ₅ ) ₃ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (C ₂ F ₅ CO) ₂ , LiI, LiAlCl ₄ , LiBC ₄ O ₈ or the like can be used alone or in admixture of two or more.

  In addition, a solid or gel ion conductive electrolyte may be used as the electrolyte. In this case, the configuration of the nonaqueous electrolyte battery includes a combination of a positive electrode, a negative electrode, and a separator, an organic or inorganic solid electrolyte and the above nonaqueous electrolyte, or an organic or inorganic solid electrolyte membrane as the positive electrode, the negative electrode, and the separator. A combination with the non-aqueous electrolytic solution is mentioned. Examples of the ion conductive electrolyte include polyethylene oxide, polypropylene oxide, polyacrylonitrile, polyethylene glycol, and derivatives thereof.

  Although the Example of this invention is shown below, it is not limited to this.

  First, a lithium / nickel / manganese composite oxide, which is a 5V class positive electrode active material, was synthesized by a solid phase method. As starting materials, lithium hydroxide monohydrate, various types of electrolytic manganese dioxide, and nickel nitrate were used. These starting materials were weighed so as to have a molar ratio of Li: Mn: Ni = 1: 1.5: 0.5, mixed, and calcined at 500 ° C. in air. Then, the positive electrode active material of this invention was obtained by baking at 700 degreeC in oxygen for 20 hours.

For sample identification, powder X-ray diffraction measurement, ion chromatography and atomic absorption analysis were used. As a result, it was confirmed that all the obtained samples were lithium / nickel / manganese composite oxides having a basic composition of LiNi _0.5 Mn _1.5 O ₄ .

  Next, a positive electrode plate using a lithium / nickel / manganese composite oxide as a positive electrode active material was produced. To 90% by mass of lithium / nickel / manganese composite oxide, 4% by mass of acetylene black as a conductive agent, 6% by mass of polyvinylidene fluoride (PVdF) as a binder, and N-methyl-2-pyrrolidone as a solvent were added. Wet mixed to form a slurry. This slurry-like coating solution was applied to both sides of an aluminum foil having a thickness of 15 μm, dried at 120 ° C., and pressed to obtain a positive electrode plate.

  Next, a negative electrode plate was produced. Graphite 50 wt%, PVdF 5 wt%, and NMP 45 wt% were mixed to form a paste. This paste was applied to a 10 μm thick copper foil as a current collector, dried at 130 ° C., and pressed to obtain a negative electrode plate.

  Next, using these positive and negative electrode plates and a polypropylene microporous separator having a thickness of 25 μm, a wound type power generation element is formed. The wound type power generation element is placed in a rectangular battery case, and has a height of 48 mm, a width of A square nonaqueous electrolyte secondary battery having a thickness of 30 mm, a thickness of 4.2 mm, and a nominal capacity of 600 mAh was produced.

  The non-aqueous electrolyte was examined using the produced square battery. Specifically, after injecting 2.17 g of various nonaqueous electrolytes into the prismatic battery, the inlet was sealed to prepare a test prismatic nonaqueous electrolyte secondary battery.

  Next, a charge / discharge cycle test was performed at 25 ° C. for these nonaqueous electrolyte secondary batteries. The charging conditions are as follows. The battery was charged to 4.8 V with a constant current of 1 CmA (= 600 mA), further charged at a constant voltage of 4.8 V, and the charging was terminated when the total charging time was 3 hours. The discharge condition was a discharge of up to 3.4 V with a constant current of 1 CmA.

Here, as the fluorinated cyclic carbonate used in the present invention, fluorinated ethylene carbonate represented by Chemical Formula 3 (FEC-1) and Chemical Formula 4 (FEC-2), and fluorinated represented by Chemical Formula 5 (FPC). Propylene carbonate was used. As the fluorinated chain carbonate, fluorinated ethyl methyl carbonate represented by Chemical Formula 6 (FEMC-1) and Chemical Formula 7 (FEMC-2) and fluorinated diethyl carbonate represented by Chemical Formula 8 (FDEC) are used. did. In all cases of the following Examples and Comparative Examples, LiPF ₆ was used as a solute and dissolved in a mixed solvent at a concentration of 1.0 mol / dm ³ .

[Examples 1 to 6 and Comparative Example 1]
As the non-aqueous electrolyte, an electrolyte containing at least one of a fluorinated cyclic carbonate represented by Chemical Formula 3 or Chemical Formula 4 or a fluorinated chain carbonate represented by Chemical Formula 6 or Chemical Formula 7 is used. The square nonaqueous electrolyte secondary battery of Examples 1-6 which made the mixing ratio with a chain carbonate 4: 6 (volume ratio) was produced. The battery of Comparative Example 1 uses a mixed solvent of EC and EMC (volume ratio 4: 6). Table 1 shows the composition of the nonaqueous solvent and the initial discharge capacity, the discharge capacity after 100 cycles, and the capacity retention rate after 100 cycles with respect to the initial discharge capacity of the produced battery.

[Examples 7 to 11 and Comparative Example 2]
As the non-aqueous electrolyte, an electrolyte containing at least one of a fluorinated cyclic carbonate represented by Chemical Formula 3 or a fluorinated chain carbonate represented by Chemical Formula 6 and Chemical Formula 8 is used. Square type nonaqueous electrolyte secondary batteries of Examples 7 to 11 in which the mixing ratio of the chain carbonate and the second chain carbonate was 4: 3: 3 (volume ratio) were produced. The battery of Comparative Example 2 uses a mixed solvent of EC, EMC, and DEC (volume ratio 4: 3: 3). Table 2 shows the composition of the non-aqueous solvent and the initial discharge capacity, the discharge capacity after 100 cycles, and the capacity retention rate after 100 cycles with respect to the initial discharge capacity.

[Examples 12 to 16 and Comparative Example 3]
As the nonaqueous electrolytic solution, an electrolytic solution containing at least one of a fluorinated cyclic carbonate represented by Chemical Formula 3 or Chemical Formula 5 or a fluorinated chain carbonate represented by Chemical Formula 8 is used, and the first cyclic carbonate and Square type nonaqueous electrolyte secondary batteries of Examples 12 to 16 in which the mixing ratio of the second cyclic carbonate and the chain carbonate was 2: 2: 6 (volume ratio) were produced. The battery of Comparative Example 3 uses a mixed solvent of EC, PC, and DEC (volume ratio 2: 2: 6). Table 3 shows the composition of the non-aqueous solvent and the initial discharge capacity, the discharge capacity after 100 cycles, and the capacity retention rate after 100 cycles with respect to the initial discharge capacity.

[Examples 17 to 22 and Comparative Example 4]
As the non-aqueous electrolyte, an electrolyte containing at least one of a fluorinated cyclic carbonate represented by Chemical Formula 3 or Chemical Formula 4 or a fluorinated chain carbonate represented by Chemical Formula 6 or Chemical Formula 7 is used. Square type non-aqueous electrolyte secondary batteries of Examples 17 to 22 having a mixing ratio with the chain carbonate of 2: 8 (volume ratio) were produced. The battery of Comparative Example 4 uses a mixed solvent of EC and EMC (volume ratio 2: 8). Table 4 shows the composition of the non-aqueous solvent and the initial discharge capacity, the discharge capacity after 100 cycles, and the capacity retention rate after 100 cycles with respect to the initial discharge capacity.

[Examples 23 to 27 and Comparative Example 5]
As the nonaqueous electrolytic solution, an electrolytic solution containing at least one of a fluorinated cyclic carbonate represented by Chemical Formula 3 or Chemical Formula 5 or a fluorinated chain carbonate represented by Chemical Formula 8 is used, and the first cyclic carbonate and Square type nonaqueous electrolyte secondary batteries of Examples 23 to 27 in which the mixing ratio of the second cyclic carbonate and the chain carbonate was 1: 4: 5 (volume ratio) were produced. The battery of Comparative Example 5 uses a mixed solvent of EC, PC, and DEC (volume ratio 1: 4: 5). Table 5 shows the composition of the nonaqueous solvent and the initial discharge capacity, the discharge capacity after 100 cycles, and the capacity retention rate after 100 cycles with respect to the initial discharge capacity of the produced battery.

  As shown in Tables 1 to 5, the batteries of the present invention of Examples 1 to 27 improved the discharge capacity retention after 100 cycles to 80% or more as compared with the batteries of Comparative Examples 1 to 5.

Shows the discharge characteristics of LiNi _0.5 Mn _1.5 O _4.

Claims (1)

The positive electrode active material is represented by the general formula Li _x Ni _y Mn _2-y O _4-δ (where 0 <x <1.1, 0.45 <y <0.55, 0 ≦ δ <0.4). In a non-aqueous electrolyte secondary battery that is a lithium-nickel-manganese composite oxide, the non-aqueous electrolyte includes a cyclic carbonate and a chain carbonate, and at least one of the cyclic carbonate or the chain carbonate includes a fluorine element. A non-aqueous electrolyte battery.

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