JP4632017B2 - Nonaqueous electrolyte secondary battery - Google Patents

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 on the positive electrode is a 5V class high voltage battery, it can be used as a battery pack for portable computers, power tools, HEVs, EVs, etc. There is an advantage that the number of cells connected in series can be reduced.

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.

Further, in Patent Document 2, in a non-aqueous electrolyte secondary battery using a 4V class positive electrode active material such as LiCoO ₂ or LiMn ₂ O ₄ , a cyclic carbonate and a chain carbonate are mixed and used as an electrolyte solvent. A technique is disclosed in which a phosphazene derivative is added in an amount of 20 to 90% by volume to obtain a self-digestible or flame-retardant electrolyte and to increase the safety of the battery.

JP 2000-515672 A JP 2001-217011 A Ota et al., 41st Battery Discussion Meeting, 2D16, P452 (2000)

  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. In contrast, since the operating voltage is high, the nonaqueous solvent contained in the electrolyte is oxidized and decomposed at the time of charging the battery not only at a high temperature but also at room temperature. Practical application has become difficult.

  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. In addition, by optimizing the electrolyte composition of the non-aqueous electrolyte secondary battery, the expansion coefficient of the high-voltage non-aqueous electrolyte secondary battery is suppressed to 5% or less. The expansion coefficient described here indicates a change in the thickness of the battery. When the initial battery thickness is Ts and the battery thickness after the test is Te, it is obtained by the following equation.

  Expansion rate (%) = ((Te−Ts) / Ts) × 100

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 < In the non-aqueous electrolyte secondary battery that is a lithium-nickel-manganese composite oxide represented by 0.4), the non-aqueous electrolyte is represented by the general formula (1):

Or the general formula (2) shown in Chemical Formula 2:

(Where R ₁ and R ₂ are fluorine and ethoxy groups, n is an integer of 3 to 10)
The non-aqueous electrolyte secondary battery characterized by including 0.1-20 mass% of phosphazene derivatives represented by these.

Depending on the configuration of the present invention, a stable film-like substance is formed on the negative electrode surface by the phosphazene derivative.

  As a result, the phenomenon that the oxidative decomposition product of the solvent under the high voltage at the positive electrode moves to the negative electrode and undergoes a reduction reaction on the negative electrode, which is characteristic of the 5V class battery, can be suppressed, and the battery can be suppressed. A high-voltage nonaqueous electrolyte secondary battery in which the expansion rate of the battery is suppressed to 5% or less can be obtained. Therefore, the industrial value of the present invention is extremely large.

  In order to solve the problem of battery swelling after a charge / discharge cycle in a 5V class battery, the inventor fabricated a battery including a lithium / nickel / manganese composite oxide on the positive electrode and investigated the mechanism of battery swelling.

  As a result, it was found that the battery thickness was remarkably increased because the product produced by oxidative decomposition of the solvent at the positive electrode was reduced on the negative electrode to generate gas at the beginning of the cycle. It was also found that when charging and discharging were repeated in the presence of gas in the battery, the current distribution was non-uniform, so that the electrode plate deteriorated and the battery thickness further increased. The reason why gas generation occurs mainly at the beginning of the cycle is not clear, but due to the oxidative decomposition reaction at the positive electrode, a film-like substance is formed on the positive electrode as the charge / discharge cycle proceeds, and the reaction with the electrolyte does not occur. The cause is thought to be suppressed.

  Therefore, in order to solve the problem of battery swelling, the inventor has intensively studied the electrolyte composition and the types of additives in the electrolyte with the aim of suppressing the gas generation at the beginning of the cycle that causes the battery swelling. As a result, it was found that when the electrolyte of the nonaqueous electrolyte secondary battery contains 0.1 to 20% by mass of a phosphazene derivative, battery swelling is significantly reduced. The cause of this is not completely elucidated, but is presumed as follows. The addition of 0.1 to 20% by mass of a phosphazene derivative to the electrolyte improved the oxidation resistance of the electrolyte, formed a stable film-like substance on the negative electrode surface, and was generated by oxidative decomposition of the solvent at the positive electrode. It is considered that the reaction between the product and the negative electrode was suppressed.

  The reason why the content of the phosphazene derivative in the nonaqueous electrolyte was 0.1 to 20% by mass was that the above effect was not sufficiently obtained when the content was less than 0.1% because the content was small. In addition, when the content exceeds 20%, the battery thickness after the charge / discharge cycle is remarkably increased. This is considered to be because when the content of the phosphazene derivative exceeds 20%, a film-like substance is excessively formed on the negative electrode, the resistance of the negative electrode is increased, and lithium electrodeposition is caused. Therefore, the content of the phosphazene derivative in the nonaqueous electrolyte is desirably 0.1 to 20% by mass.

The type of phosphazene derivative used in the present invention is not particularly limited except that it has fluorine and alkoxy groups as substituents , and various phosphazene derivatives can be used as appropriate. Examples of the phosphazene derivative include at least one selected from a chain phosphazene derivative represented by the general formula (1) represented by the chemical formula 1 or a cyclic phosphazene derivative represented by the general formula (2) represented by the chemical formula 2. Seeds can be used.

However, in General Formula (1) and (2), R ₁ and R ₂ represent a monovalent substituent or a halogen group element. n represents an integer of 3 to 10. Examples of the halogen group element include fluorine, chlorine, bromine and the like. Of these, fluorine is preferable, and in the present invention, R ₁ and R ₂ have at least one fluorine. Examples of the monovalent substituent include a hydrogen atom, an alkoxy group, an alkyl group, a carboxyl group, an acyl group, and an aryl group. Among them, the alkoxy group is not preferable. Examples of the alkoxy group include a methoxy group, an ethoxy group, a propoxy group, a butoxy group, and an alkoxy-substituted alkoxy group such as a methoxyethoxy group. Of these, R ₁ and R ₂ are preferably a methoxy group, an ethoxy group, or a methoxyethoxy group. In the present invention, R ₁ and R ₂ have at least one ethoxy group. Furthermore, it is preferable that hydrogen in the monovalent substituent is substituted with a halogen element such as fluorine. Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a butyl group, and a pentyl group. Examples of the acyl group include formyl group, acetyl group, propionyl group, butyryl group, isobutyryl group, and valeryl group. Examples of the aryl group include a phenyl group, a tolyl group, and a naphthyl group.

The positive electrode active material used for the non-aqueous electrolyte secondary battery of the present invention 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, The lithium / nickel / manganese composite oxide represented by 0 ≦ δ <0.4) 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 the discharge curve of the charge / discharge curve of LiMn _1.5 Ni _0.5 O ₄ shown in FIG. 1 on page 453 of Non-Patent Document 1 shown in FIG. As seen, about 4.7V vs. 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.

  There are no particular limitations on the organic solvent of the electrolytic solution used in the nonaqueous electrolyte battery of the present invention. For example, ethers, ketones, lactones, nitriles, amines, amides, sulfur compounds, halogenated hydrocarbons, Esters, carbonates, nitro compounds, phosphate ester compounds, sulfolane hydrocarbons, etc. can be used, but among these ethers, ketones, esters, lactones, halogenated hydrocarbons, Carbonates and sulfolane hydrocarbons are preferred.

  Examples of these include tetrahydrofuran, 2-methyltetrahydrofuran, tetrahydropyran, 1,4-dioxane, anisole, monoglyme, 4-methyl-2-pentanone, ethyl acetate, methyl acetate, methyl propionate, ethyl propionate, 1, 2-dichloroethane, γ-butyrolactone, γ-valerolactone, dimethoxyethane, diethoxyethane, methyl formate, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, propylene carbonate, ethylene carbonate, vinylene carbonate, butylene carbonate, dimethylformamide, dimethyl Sulfoxide, dimethylthioformamide, sulfolane, 3-methyl-sulfolane, trimethyl phosphate, triethyl phosphate, and mixtures thereof Although a solvent etc. can be mentioned, it is not necessarily limited to these. Preferably, the organic solvent is selected from ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, and dimethyl carbonate.

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. Of these, LiPF ₆ is preferably used. Furthermore, the lithium salt concentration is preferably 0.5 to 2.0 mol / dm ³ .

  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 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 amount of the phosphazene derivative added was examined using the produced square battery. The following six types of basic electrolytes were used. In addition, all the mixing ratio of a solvent shows a volume ratio.

E1: EC: DEC (3: 7) / LiPF ₆ (1.0 mol / dm ³ )
E2: EC: EMC (2: 8) / LiPF ₆ (1.0 mol / dm ³ )
E3: EC: DMC: DEC (5: 3: 2) / LiPF ₆ (1.0 mol / dm ³ )
E4: EC: γ-BL: DEC (3: 3: 4) / LiPF ₆ (1.0 mol / dm ³ )
E5: EC: PC: DEC (1: 2: 7) / LiPF ₆ (1.0 mol / dm ³ )
E6: EC: DEC (3: 7) / LiBF ₄ (1.0 mol / dm ³ )
As the phosphazene derivative, a chain phosphazene derivative [P1] represented by Chemical formula 3 or a cyclic phosphazene derivative [P2] represented by Chemical formula 4 was used.

Specifically, a predetermined amount of phosphazene derivatives [P1] and [P2] are added to a mixed solvent in which 1.0 mol / dm ^{3 of} LiPF ₆ as a solute is dissolved to prepare a basic electrolytic solution used for the test. And it mixes so that the mass% of a basic electrolyte solution and the mass% of a phosphazene derivative may become 100% in total, and it is set as the adjusted non-aqueous electrolyte. Next, 2.17 g of the adjusted non-aqueous electrolyte was injected into the prismatic battery, and then the inlet was sealed to prepare a test rectangular non-aqueous electrolyte secondary battery.

  About these nonaqueous electrolyte secondary batteries, the charge / discharge cycle test was done at 25 degreeC. 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.

  For each battery, the initial battery thickness (Ts), the battery thickness after 10 cycles (Te (10)), and the battery thickness after 100 cycles (Te (100)) were measured. In all the batteries, Ts = 4.2 mm.

[Examples 1-6 and Comparative Examples 1-3]
As the non-aqueous electrolyte, prismatic non-aqueous electrolyte secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 3 using a mixed electrolyte of the basic electrolyte E1 and the phosphazene derivative P1 were produced. However, Comparative Example 1 does not include the phosphazene derivative P1. The amount of phosphazene derivative contained in the nonaqueous electrolyte of the produced battery, the battery thickness after 10 cycles (Te (10)), the battery thickness after 100 cycles (Te (100)), and the battery expansion coefficient after 100 cycles Is shown in Table 1. In the following table, the phosphazene derivative is represented by “PD”.

[Examples 7 to 12 and Comparative Examples 4 to 6]
As the non-aqueous electrolyte, the rectangular non-aqueous electrolytes 2 of Examples 7 to 12 and Comparative Examples 4 and 5 were used in the same manner as Example 1, except that a mixed electrolyte of basic electrolyte E1 and phosphazene derivative P2 was used. A secondary battery was produced. The amount of phosphazene derivative contained in the nonaqueous electrolyte of the produced battery, the battery thickness after 10 cycles (Te (10)), the battery thickness after 100 cycles (Te (100)), and the battery expansion coefficient after 100 cycles Are shown in Table 2.

  From Tables 1 and 2, the following was found. In the batteries of Comparative Example 1 and Comparative Example 4 with no phosphazene derivative added (0 wt% added), the initial battery thickness was 4.2 mm, but after 100 cycles, the battery thickness expanded to 5 mm or more. Was.

  In the batteries of Examples 1 to 6 and Examples 7 to 12 containing 0.1 to 20% by mass of the phosphazene derivative, the battery swelling was suppressed to 4.41 mm or less and the expansion rate to 5% or less.

  However, in the case of the batteries of Comparative Example 2 and Comparative Example 4 in which the addition amount of the phosphazene derivative is 0.05% by mass, and the batteries of Comparative Example 3 and Comparative Example 5 in which the addition amount is 25% by mass, The expansion rate was greater than 5%.

  The cause of this is not clear yet, but when the electrolyte contains a phosphazene derivative of 0.1 to 20% by mass, a thin, stable film-like substance is formed on the negative electrode surface to suppress the expansion of the battery thickness due to gas generation. Is done. However, when the content of the phosphazene derivative is less than 0.1% by mass, the film formation is insufficient, and when the content exceeds 20% by mass, the film becomes thick and the resistance increases. It is considered that the negative electrode deterioration due to lithium electrodeposition or the like was promoted, and the battery thickness expanded due to the expansion of the negative electrode plate.

[Examples 13 to 18 and Comparative Examples 7 to 10]
In the same manner as in Example 1 except that a mixed electrolytic solution of the basic electrolytic solution E2 and the phosphazene derivative P1 or P2 was used as the nonaqueous electrolytic solution, the non-square electrolytic solutions of Examples 13 to 18 and Comparative Examples 7 to 10 were used. A water electrolyte secondary battery was produced. The type and content of the phosphazene derivative contained in the non-aqueous electrolyte of the produced battery, battery thickness after 10 cycles (Te (10)), battery thickness after 100 cycles (Te (100)), and after 100 cycles The battery expansion rate is shown in Table 3.

[Examples 19 to 24 and Comparative Examples 11 to 14]
In the same manner as in Example 1, except that a mixed electrolytic solution of the basic electrolytic solution E3 and the phosphazene derivative P1 or P2 was used as the nonaqueous electrolytic solution, the square nonaqueous solutions of Examples 19 to 24 and Comparative Examples 10 and 11 were used. An electrolyte secondary battery was produced. Types and contents of phosphazene derivatives contained in the non-aqueous electrolyte of the produced battery, battery thickness after 10 cycles (Te (10)), battery thickness after 100 cycles (Te (100)), and after 100 cycles The battery expansion rate is shown in Table 4.

[Examples 25-30 and Comparative Examples 15-18]
In the same manner as in Example 1 except that a mixed electrolyte of basic electrolyte E4 and phosphazene derivative P1 or P2 was used as the nonaqueous electrolyte, the square nonaqueous solutions of Examples 25-30 and Comparative Examples 15-18 were used. An electrolyte secondary battery was produced. Types and contents of phosphazene derivatives contained in the non-aqueous electrolyte of the produced battery, battery thickness after 10 cycles (Te (10)), battery thickness after 100 cycles (Te (100)), and after 100 cycles The battery expansion rate is shown in Table 5.

[Examples 31 to 36 and Comparative Examples 19 to 22]
As the nonaqueous electrolyte, the rectangular nonaqueous solutions of Examples 31 to 36 and Comparative Examples 19 to 22 were used in the same manner as in Example 1 except that a mixed electrolyte of the basic electrolyte E5 and the phosphazene derivative P1 or P2 was used. An electrolyte secondary battery was produced. Types and contents of phosphazene derivatives contained in the non-aqueous electrolyte of the produced battery, battery thickness after 10 cycles (Te (10)), battery thickness after 100 cycles (Te (100)), and after 100 cycles The battery expansion rate is shown in Table 6.

[Examples 37 to 42 and Comparative Examples 23 to 26]
In the same manner as in Example 1, except that a mixed electrolytic solution of the basic electrolytic solution E6 and the phosphazene derivative P1 or P2 was used as the nonaqueous electrolytic solution, the square nonaqueous solutions of Examples 37 to 42 and Comparative Examples 23 to 26 were used. An electrolyte secondary battery was produced. The type and content of the phosphazene derivative contained in the non-aqueous electrolyte of the produced battery, battery thickness after 10 cycles (Te (10)), battery thickness after 100 cycles (Te (100)), and after 100 cycles The battery expansion rate is shown in Table 7.

  As shown in Tables 3 to 7, when 0.1 to 20% by mass of the phosphazene derivative was contained, the battery swelling was suppressed to 4.41 mm or less and the expansion rate was 5% or less, but the content of the phosphazene derivative was 0. When the amount is less than 1% by mass or exceeds 20% by mass, the swelling of the battery cannot be reduced to 5% or less.

Shows the charge and discharge of LiMn _1.5 Ni _0.5 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 is represented by the general formula (1):
Or the general formula (2) shown in Chemical Formula 2:
(Where R ₁ and R ₂ are fluorine and ethoxy groups, n is an integer of 3 to 10)
The non-aqueous electrolyte secondary battery characterized by including 0.1-20 mass% of phosphazene derivatives represented by these.