JP2006351242A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte secondary battery of a high capacity showing an excellent charge/discharge cycle characteristics even under a high-temperature environment, in case nickel-containing lithium complex oxide is used. <P>SOLUTION: In the nonaqueous electrolyte secondary battery provided with a cathode containing nickel-containing lithium complex oxide as a cathode active material, an anode with a material capable of storing and releasing lithium as an active material, a separator intercalated between the cathode and the anode, and nonaqueous solution, the nonaqueous electrolytic solution contains organic acid of 20 ppm or more and 100 ppm or less. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte secondary battery with an improved non-aqueous electrolyte.

In recent years, in the field of non-aqueous electrolyte secondary batteries, research on lithium ion secondary batteries having high voltage and high energy density has been active.
Currently, LiCoO ₂ exhibiting a high charge / discharge voltage is used as the positive electrode active material in most of the commercially available lithium ion secondary batteries. On the other hand, there is a strong demand for higher capacity, and research and development has been actively conducted on a positive electrode active material having a higher capacity in place of LiCoO ₂ . Among them, research on nickel-containing lithium composite oxide (for example, LiNiO ₂ ) whose main component is nickel has been vigorously conducted, and some of them have already been commercialized.

In addition to increasing the capacity of lithium ion secondary batteries, higher reliability and longer life are also desired. However, in general, nickel-containing lithium composite oxides such as LiNiO ₂ have not yet occupied the market because their cycle characteristics and storage characteristics are much worse than LiCoO ₂ . Therefore, in order to improve the characteristics of the nickel-containing lithium composite oxide, such improvements have been actively made. For example, in order to suppress the gas generation in a high temperature environment and improve the storage characteristics, it has been proposed to wash the nickel-containing lithium composite oxide with water (see Patent Document 1). By the water washing treatment, lithium carbonate and lithium sulfate generated in the nickel-containing lithium composite oxide are removed.

Moreover, as a non-aqueous electrolyte used for a non-aqueous electrolyte secondary battery, a solution obtained by dissolving a solute in a non-aqueous solvent is generally used. As the non-aqueous solvent, cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and the like are used. As the solute, lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), or the like is used.

On the other hand, for the purpose of improving battery characteristics, attempts have been made to mix various additives into the positive electrode active material, the negative electrode active material, and the electrolyte. For example, in the case of using a lithium metal-based negative electrode, it has been proposed to add 20 ppm to 400 ppm of hydrogen fluoride to a nonaqueous electrolytic solution in order to improve charge / discharge cycle characteristics (see Patent Document 2). By adding hydrogen fluoride to the non-aqueous electrolyte, a uniform film is formed on the surface of the lithium metal-based negative electrode, and generation of lithium dendrite is suppressed.
JP 2003-017054 A Japanese Patent Laid-Open No. 07-302613

  However, in the case of a battery using a nickel-containing lithium composite oxide as disclosed in Patent Document 1 as a positive electrode active material, a side reaction between the nonaqueous electrolyte and the positive electrode active material occurs violently, particularly in a high temperature environment, and a sufficient cycle Characteristics have not been obtained.

  The present invention has been made in view of the above problems, and when nickel-containing lithium composite oxide is used as a positive electrode active material, it has a high capacity exhibiting good charge / discharge cycle characteristics even in a high temperature environment. An object is to provide a non-aqueous electrolyte secondary battery of the type.

As a result of investigating the cause for the above problem and further studying, the present inventors have found that a sufficient amount of organic acid in the electrolytic solution is particularly effective for the positive electrode active material of the lithium composite oxide containing nickel. It was found that the side reaction between the electrolytic solution and the positive electrode active material can be remarkably suppressed.
That is, the present invention includes a positive electrode containing nickel-containing lithium composite oxide as a positive electrode active material, a negative electrode capable of inserting and extracting lithium, a separator interposed between the positive electrode and the negative electrode, and 20 ppm to 100 ppm. The present invention relates to a non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte solution containing any organic acid. Here, the organic acid represents the organic acid and the conjugate base when the conjugate base is included.

In the non-aqueous electrolyte secondary batteries, non-aqueous _{electrolyte, LiPF 6, LiBF 4, LiAsF} 6, LiSbF 6, LiPF 5 (CF 3), LiPF 5 (C 2 F 5), LiPF 3 (C 2 F ₅ ) ₃ , LiBF ₃ (CF ₃ ), LiBF ₃ (C ₂ F ₅ ), and LiBF ₂ (C ₂ F ₅ ) ₂ are preferably included.

  In the non-aqueous electrolyte secondary battery, the organic acid preferably includes a carbon acid. The carbon acid preferably contains at least one selected from the group consisting of dimedone, triacetylmethane, acetylacetone, cyclopentadiene, and 1,3-dioxolane.

In the non-aqueous electrolyte secondary battery, the nickel-containing lithium composite oxide has the following formula:
_{LiNi x M 1-xy L y} O 2
(M is at least one of Co and Mn, and L is Al, Sr, Y, Zr, Ta, Mg, Ti, Zn, B, Ca, Cr, Si, Ga, Sn, P, V, Sb. , Nb, Mo, W and Fe, at least one selected from the group consisting of 0.1 ≦ x ≦ 1.0, 0 ≦ y ≦ 0.1)
It is preferable to be represented by

  When the non-aqueous electrolyte contains a large amount of HF from the beginning, most of it is consumed in the film formation reaction at the negative electrode, and the film on the negative electrode becomes too thick, which may inhibit the charge / discharge reaction. In addition, if most of the HF is consumed first in the negative electrode, it may be difficult to form a film on the positive electrode. On the other hand, in the present invention, by using a non-aqueous electrolyte containing an organic acid of 20 ppm or more and 100 ppm or less, for example, HF is generated in the non-aqueous electrolyte after high temperature storage, and an inactive film is formed on the positive electrode. The Thereby, even when nickel-containing lithium composite oxide is used as the positive electrode active material, it is possible to avoid deterioration of cycle characteristics due to a side reaction between the nonaqueous electrolytic solution and the positive electrode active material in a high temperature environment. Furthermore, since the non-aqueous electrolyte does not contain a large amount of HF from the beginning, it can be further reduced that the coating film of the negative electrode becomes too thick or the coating film is hardly formed on the positive electrode. For this reason, the nonaqueous electrolyte secondary battery which has a favorable characteristic is realizable.

Hereinafter, the best mode for carrying out the present invention will be described in detail.
FIG. 1 shows a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.
The nonaqueous electrolyte secondary battery in FIG. 1 includes a battery case 18 and a power generation element housed in the battery case. The power generation element includes an electrode plate group and a non-aqueous electrolyte (not shown).
The electrode plate group includes a positive electrode plate 11, a negative electrode plate 12, and a separator 13 disposed between the positive electrode plate and the negative electrode plate, and the positive electrode plate 11 and the negative electrode plate 12 are wound in a spiral shape via the separator 13. Produced.
A positive electrode lead 14 is drawn from the positive electrode plate 11 and connected to the back surface of the sealing plate 19 that also serves as a positive electrode terminal. A negative electrode lead 15 is drawn out from the negative electrode plate 12 and connected to the bottom of the battery case 18. An upper insulating plate 16 is provided above the electrode plate group, and a lower insulating plate 17 is provided below the electrode plate group.
The battery case 18 is sealed by caulking the opening end of the battery case 18 to the sealing plate 19 via the gasket 20.

In the present invention, a nickel-containing lithium composite oxide is used as the positive electrode active material. Among _{them, LiNi x M 1-xy L} y O 2 (M is at least one of Co and Mn, L is Al, Sr, Y, Zr, Ta, Mg, Ti, Zn, B, Ca, It is at least one selected from the group consisting of Cr, Si, Ga, Sn, P, V, Sb, Nb, Mo, W and Fe, and 0.1 ≦ x ≦ 1.0, 0 ≦ y ≦ 0.1 ) Is preferably used. As described above, when the element L is included, the crystal structure is stabilized and the battery characteristics are improved.
In the above formula, x is more preferably in the range of 0.3 ≦ x ≦ 0.9, and most preferably in the range of 0.7 ≦ x ≦ 0.9.

  y is any of L, Al, Sr, Y, Zr, Ta, Mg, Ti, Zn, B, Ca, Cr, Si, Ga, Sn, P, V, Sb, Nb, Mo, W, and Fe Even if it exceeds 0.1, the generation of the inert film on the surface of the positive electrode active material becomes insufficient, and the cycle characteristics at high temperature may be slightly lowered. For this reason, y is preferably 0.1 or less, more preferably 0.05 or less, and particularly preferably 0.01 to 0.05.

  Examples of the negative electrode active material include graphite such as natural graphite (such as flake graphite) and artificial graphite, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black, and carbon fiber. Metal fibers, alloys, lithium metals, tin compounds, silicides, nitrides, and the like can be used.

  Examples of the positive electrode binder and the negative electrode binder include polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), fluorine, and the like. Vinylidene chloride-hexafluoropropylene copolymer and the like are used.

  Examples of the conductive agent included in the electrode include graphites, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and other carbon blacks, carbon fibers, and metal fibers.

  For the positive electrode current collector, for example, a sheet foil made of stainless steel, aluminum, titanium, or the like is used. For the negative electrode current collector, for example, a sheet foil made of stainless steel, nickel, copper, or the like is used. The thicknesses of the positive electrode current collector and the negative electrode current collector are not particularly limited, but are preferably 1 to 500 μm.

  As the separator, a microporous thin film having a large ion permeability, a predetermined mechanical strength, and an insulating property can be used. Examples of such separators include sheets made of olefin polymers such as polypropylene and polyethylene, glass fibers, nonwoven fabrics, and woven fabrics. The thickness of the separator is generally preferably 10 to 300 μm.

  The nonaqueous electrolytic solution includes a nonaqueous solvent and a solute dissolved in the nonaqueous solvent. As the non-aqueous solvent, for example, a cyclic carbonate, a chain carbonate, a cyclic carboxylic acid ester or the like is used. Here, examples of the cyclic carbonate include propylene carbonate (PC) and ethylene carbonate (EC). Examples of the chain carbonate include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of the cyclic carboxylic acid ester include γ-butyrolactone (GBL) and γ-valerolactone (GVL).

Solutes dissolved in the non-aqueous solvent include LiPF ₆ , LiClO ₄ , LiBF ₄ , LiAlCl ₄ , LiSbF ₆ , LiSCN, LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , Li (CF ₃ SO ₂ ) ₂ , LiAsF ₆ , LiB. ₁₀ Cl ₁₀ , lower aliphatic lithium carboxylate, LiCl, LiBr, LiI, chloroborane lithium, bis (1,2-benzenediolate (2-)-O, O ′) lithium borate, bis (2,3-naphthalene) Dioleate (2-)-O, O ′) lithium borate, bis (2,2′-biphenyldiolate (2-)-O, O ′) lithium borate, bis (5-fluoro-2-oleate-1- Borates such as benzenesulfonic acid-O, O ′) lithium borate, lithium bistetrafluoromethanesulfonate imidolithium ((CF ₃ SO ₂ ) ₂ NLi), Tet Nonfluorofluorosulfonic acid nonafluorobutane imide lithium (LiN (CF ₃ SO ₂ ) (C ₄ F ₉ SO ₂ )), lithium bispentafluoroethane sulfonate imide ((C ₂ F ₅ SO ₂ ) ₂ NLi) It may contain imide salts such as. These may be used alone or in combination of two or more.

  In the non-aqueous electrolyte secondary battery of the present invention, the non-aqueous electrolyte contains 20 ppm or more and 100 ppm or less of an organic acid. Examples of organic acids that can be used include Bronsted acids such as carboxylic acids and carbon acids.

Examples of the carboxylic acid include formic acid, acetic acid, benzoic acid, malonic acid, oxalic acid, maleic acid, fumaric acid and the like.
Examples of the carbon acid include dimedone, triacetylmethane, acetylacetone, cyclopentadiene, 1,3-dioxolane, nitroethane, dinitroethane, and ethyl acetoacetate.
In addition, as the organic acid, phenol or the like can be used because protons can be released.
Moreover, the non-aqueous electrolyte may contain the above organic acids independently, and may contain it in combination of 2 or more.

Many lithium compounds such as lithium hydroxide (LiOH) and lithium oxide (Li ₂ O), which are synthetic raw materials, are present on the surface of the nickel-containing lithium composite oxide that is the positive electrode active material. This is because, when synthesizing the nickel-containing lithium composite oxide, a lithium compound as a synthesis raw material is slightly excessively added in order to promote particle growth.

For example, when an organic acid such as Bronsted acid is expressed as HA, HA reacts with a lithium compound on the surface of the positive electrode active material as shown in the following formulas (1) and (2), and moisture Produce. An organic acid has an advantage that it has high oxidation resistance even after protons are eliminated and is not easily decomposed.
LiOH + HA → LiA + H ₂ O (Formula 1)
Li ₂ O + 2HA → 2LiA + H ₂ O (Formula 2)

Moreover, it is preferable that a non-aqueous electrolyte contains what can react with water and can discharge | release HF among the said solutes. As such, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiSbF ₆ , LiPF ₅ (CF ₃ ), LiPF ₅ (C ₂ F ₅ ), LiPF ₃ (C ₂ F ₅ ) ₃ , LiBF ₃ (CF ₃ ), LiBF ₃ (C ₂ F ₅ ), LiBF ₂ (C ₂ F ₅ ) _{2 and the} like. These solutes may be used alone or in combination of two or more.

For example, non-aqueous if electrolyte solution containing LiPF _6, the water newly generated in the above manner, as shown in the following equation (3), LiPF ₆ in the nonaqueous electrolytic solution is hydrolyzed Hydrogen fluoride (HF) is produced.
LiPF ₆ → Li ⁺ + PF ₆ ⁻
PF ₆ ⁻ + H ₂ O → POF ₃ + 2HF + F ⁻ (Formula 3)

In particular, in a high temperature environment, a reaction as shown in the following formula (4) is also added, and the amount of HF generated becomes larger.
LiPF ₆ → PF ₅ + LiF
PF ₅ + H ₂ O → POF ₃ + 2HF (Formula 4)

  When HF is present in the non-aqueous electrolyte, HF reacts with the lithium compound on the surface of the positive electrode active material to form an inactive lithium fluoride coating (LiF). With this LiF coating, side reactions between the non-aqueous electrolyte and the positive electrode active material are suppressed even at high temperatures, and the cycle characteristics can be improved.

  The concentration of the organic acid contained in the non-aqueous electrolyte is 20 ppm or more and 100 ppm or less. If the concentration of the organic acid is less than 20 ppm, the effect of improving the cycle characteristics due to the formation of a film on the positive electrode may not be sufficiently obtained. When the concentration of the organic acid exceeds 100 ppm, the coating becomes thick and the charge / discharge reaction may be inhibited.

  Among the above organic acids, it is preferable to use a carbon acid. Carbon acid is capable of releasing protons, particularly when the positive electrode is charged and at a high temperature, and a conjugate base formed by releasing a proton from carbon acid is electrochemically stable and the conjugate base itself is not easily oxidized. is there. Here, the carbon acid refers to a compound having hydrogen that has a certain degree of strong acidity and is bonded to carbon having no multiple bond. That is, a carbon acid is a compound in which a hydrogen atom bonded to a carbon atom is easily removed as a proton.

  Among the above carbon acids, a carbon acid having a pKa value of 15.7 or less of water is preferable. In particular, the carbon acid preferably contains at least one selected from the group consisting of dimedone, triacetylmethane, acetylacetone, cyclopentadiene, and 1,3-dioxolane. This is because conjugate bases formed by releasing protons from these carbon acids are more electrochemically stable, and the conjugate bases themselves are very difficult to oxidize.

As described above, when the non-aqueous electrolyte contains an organic acid, for example, HF can be generated in the non-aqueous electrolyte after being stored at a high temperature.
When the non-aqueous electrolyte contains a large amount of HF from the beginning, most of it is consumed in the film formation reaction at the negative electrode, and the film on the negative electrode becomes too thick, which may inhibit the charge / discharge reaction. In addition, if most of the HF is consumed first in the negative electrode, it may be difficult to form a film on the positive electrode. However, in the present invention, since the non-aqueous electrolyte does not contain a large amount of HF from the beginning, it is possible to further reduce that the coating film of the negative electrode becomes too thick or the coating film is hardly formed on the positive electrode. it can.

Moreover, it is preferable that the non-aqueous electrolyte contains a cyclic carbonate having at least one carbon-carbon unsaturated bond. It is because it decomposes | disassembles on a negative electrode and forms a film with high lithium ion conductivity, and charge / discharge efficiency becomes high.
Examples of the cyclic carbonate having at least one carbon-carbon unsaturated bond include vinylene carbonate (VC), 3-methyl vinylene carbonate, 3,4-dimethyl vinylene carbonate, 3-ethyl vinylene carbonate, 3,4-diethyl. Examples include vinylene carbonate, 3-propyl vinylene carbonate, 3,4-dipropyl vinylene carbonate, 3-phenyl vinylene carbonate, 3,4-diphenyl vinylene carbonate, vinyl ethylene carbonate (VEC), and divinyl ethylene carbonate. These may be used alone or in combination of two or more. Among these, at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, and divinyl ethylene carbonate is preferable. In the above compound, part of the hydrogen atoms may be substituted with fluorine atoms.

  Further, the non-aqueous electrolyte may contain a known benzene derivative that decomposes during overcharge to form a film on the electrode and inactivate the battery. As the benzene derivative, those having a phenyl group and a cyclic compound group adjacent to the phenyl group are preferable. As the cyclic compound group, a phenyl group, a cyclic ether group, a cyclic ester group, a cycloalkyl group, a phenoxy group, and the like are preferable. Specific examples of the benzene derivative include cyclohexylbenzene, biphenyl, diphenyl ether and the like. These may be used alone or in combination of two or more. However, it is preferable that the content rate of a benzene derivative is 10 volume% or less of the whole non-aqueous solvent.

  Hereinafter, based on an Example, this invention is demonstrated in detail.

(I) Preparation of non-aqueous electrolyte LiPF ₆ was dissolved in a mixed solvent of EC and EMC (volume ratio 1: 4) at a concentration of 1.0 mol / L. Dimedone, which is an organic acid, was added to the obtained solution so as to have an initial concentration of 50 ppm to prepare a nonaqueous electrolytic solution.

(Ii) Preparation of positive electrode plate 85 parts by weight of a positive electrode active material (LiNi _0.8 Co _0.2 O ₂ ) powder, 10 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of PVDF as a binder are mixed, and the mixture is prepared. Obtained. The mixture was dispersed in dehydrated N-methyl-2-pyrrolidone (NMP) to prepare a slurry positive electrode mixture. This positive electrode mixture was applied onto a positive electrode current collector made of an aluminum foil. Next, it was dried and rolled to obtain a positive electrode plate.

(Iii) Production of Negative Electrode Plate 75 parts by weight of artificial graphite powder, 20 parts by weight of acetylene black as a conductive agent, and 5 parts by weight of PVDF as a binder were mixed to obtain a mixture. The mixture was dispersed in dehydrated NMP to prepare a slurry-like negative electrode mixture. This negative electrode mixture was applied onto a negative electrode current collector made of copper foil. Next, it was dried and rolled to obtain a negative electrode plate.

A cylindrical battery shown in FIG. 1 was produced.
The positive electrode plate 11 and the negative electrode plate 12 were wound in a spiral shape via the separator 13 to produce an electrode plate group. Subsequently, the electrode plate group was housed in a nickel-plated iron battery case 18. An aluminum positive electrode lead 14 was pulled out from the positive electrode plate 11 and connected to the back surface of the sealing plate 19 that also served as a positive electrode terminal. Further, a nickel negative electrode lead 15 was pulled out from the negative electrode plate 12 and connected to the bottom of the battery case 18. An insulating plate 16 is provided above the electrode plate group, and an insulating plate 17 is provided below the electrode plate group. Then, a predetermined amount of the non-aqueous electrolyte prepared as described above was injected into the battery case 18. Next, the opening end of the battery case 18 was caulked to the sealing plate 19 via the gasket 20, and the opening of the battery case 18 was sealed to produce a non-aqueous electrolyte secondary battery. The obtained battery was designated as battery 1.

(V) Measurement of free acid concentration contained in non-aqueous electrolyte at initial stage (immediately after preparation) The amount of free acid contained in the non-aqueous electrolyte immediately after preparation was measured by a neutralization titration method.
20 g of the prepared non-aqueous electrolyte was added to ice water (about 100 g of ice + about 150 g of ion-exchanged water) to obtain a mixture. Bromothymol blue was added to the mixture as an indicator, and the mixture was titrated with a 0.1 mol / l NaOH solution to determine the free acid concentration. At this time, since it is considered that almost no HF is generated in the nonaqueous electrolytic solution, the obtained free acid concentration can be regarded as the concentration of the organic acid.

(Vi) Measurement of free acid concentration (free acid concentration after charge storage) after being stored at high temperature in a charged state Free acid included in nonaqueous electrolyte solution after being stored at high temperature in a charged state The amount of was measured by the pH measurement method.
The manufactured battery was charged to a fully charged state using constant current-constant voltage charging for 2 hours and 30 minutes at a maximum current of 1050 mA and an upper limit voltage of 4.2 V. Here, the fully charged state means a state where the battery is charged to the nominal capacity.
After a 10-minute rest, it was stored in a 45 ° C. environment for 1 hour. A hole was made in the battery after storage, and a non-aqueous electrolyte was collected from the battery by a centrifugal separation operation (5000 rpm / 10 min). 1 g of the collected non-aqueous electrolyte was added to ice water (10 g) to obtain a mixture. The pH of the mixture was measured with a pH meter.

  An aqueous HF solution is prepared to have a predetermined concentration (for example, 10 ppm, 100 ppm, 1000 ppm) by a neutralization titration method, pH of various aqueous HF solutions is measured with a pH meter, and a pH-free acid concentration calibration curve It was created. Using this calibration curve, the concentration of protons (that is, free acids) contained in the non-aqueous electrolyte after storage was determined. In this case, since the non-aqueous electrolyte contains HF or the like as a free acid in addition to the organic acid, the total amount of the free acid is determined by the pH measurement.

(Vii) Battery Evaluation For the manufactured battery, the charge / discharge cycle of the battery was repeated at 45 ° C., the discharge capacity at the third cycle was regarded as 100%, and the capacity retention rate of the battery after 500 cycles was calculated. The cycle maintenance rate was used. The results are shown in Table 1.
Here, as charging / discharging conditions, a constant current / constant voltage charging is performed for 2 hours and 30 minutes at a maximum current of 1050 mA and an upper limit voltage of 4.2 V, and after a pause of 10 minutes, a discharge current is 1500 mA and a discharge end voltage is 3.0 V. A constant current discharge was performed and a pause of 10 minutes was performed.

Comparative Example 1

A non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that dimedone was not added to the non-aqueous electrolyte. The obtained battery was designated as comparative battery 2.
The free acid concentration contained in the nonaqueous electrolyte solution was determined in the same manner as in Example 1 after initial storage and after storage. Moreover, the charge / discharge cycle was performed on the same conditions as the said Example 1 using the comparative battery 2. FIG. The obtained results are shown in Table 1.

Comparative Example 2

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that lithium cobaltate (LiCoO ₂ ) was used instead of LiNi _0.8 Co _0.2 O ₂ . The obtained battery was designated as comparative battery 3.
The free acid concentration contained in the nonaqueous electrolyte solution was determined in the same manner as in Example 1 after initial storage and after storage. Moreover, the charge / discharge cycle was performed on the same conditions as the said Example 1 using the comparative battery 3. FIG. The obtained results are shown in Table 1.

Comparative Example 3

A non-aqueous electrolyte secondary battery was fabricated in the same manner as in Example 1 except that the non-aqueous electrolyte did not contain an organic acid and lithium cobaltate was used as the positive electrode active material. The obtained battery was designated as comparative battery 4.
The free acid concentration contained in the nonaqueous electrolyte solution was determined in the same manner as in Example 1 after initial storage and after storage. Moreover, the charge / discharge cycle was performed on the same conditions as the said Example 1 using the comparative battery 4. FIG. The obtained results are shown in Table 1.

Comparative Example 4

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that hydrogen fluoride was added to the nonaqueous electrolyte instead of dimedone, which is an organic acid. The obtained battery was designated as comparative battery 5.
The free acid concentration contained in the nonaqueous electrolyte solution was determined in the same manner as in Example 1 after initial storage and after storage. Moreover, the charge / discharge cycle was performed on the same conditions as the said Example 1 using the comparative battery 5. FIG. The obtained results are shown in Table 1.

  As shown in Table 1, it can be seen that the cycle characteristics are improved only in the case of the battery 1 in which the nickel-containing lithium composite oxide is used as the positive electrode active material and the non-aqueous electrolyte contains dimedone. This is presumably because a protective film was formed on the positive electrode by the reaction between the lithium compound on the surface of the nickel-containing lithium composite oxide and a sufficient amount of HF in the non-aqueous electrolyte.

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that dimedone was added to the nonaqueous electrolyte at an initial concentration as shown in Table 2. The obtained batteries were designated as batteries 5 to 11, respectively. Here, the battery 5 is a comparative battery.
Using batteries 5 to 11, the free acid concentration after charge storage was determined in the same manner as in Example 1. Moreover, the charge / discharge cycle was performed on the same conditions as the said Example 1 using the batteries 5-11. The obtained results are shown in Table 2.

  As shown in Table 2, when the concentration of the free acid contained in the non-aqueous electrolyte after charge storage is less than 20 ppm, and when the free acid concentration is greater than 100 ppm, the effect of improving the cycle characteristics is sufficient. It turns out that it is not obtained. If the free acid concentration is less than 20 ppm, film formation does not occur sufficiently. If the free acid concentration exceeds 100 ppm, the coating becomes thick and the charge / discharge reaction is inhibited. Therefore, the free acid concentration contained in the nonaqueous electrolytic solution is preferably 20 ppm or more and 100 ppm or less.

A nonaqueous electrolyte secondary battery was produced in the same manner as in Example 1 except that the compound shown in Table 3 was added as an organic acid so as to occupy 1% by weight of the nonaqueous electrolyte. The obtained batteries were designated as batteries 12 to 26, respectively.
The free acid concentration contained in the nonaqueous electrolyte solution was determined in the same manner as in Example 1 after initial storage and after storage. Moreover, the charging / discharging cycle was performed on the same conditions as the said Example 1 using the batteries 12-26. The obtained results are shown in Table 3.

It can be seen that the batteries 12 to 26 in which the nickel-containing lithium composite oxide is used as the positive electrode active material and various organic acids shown in Table 3 are contained in the nonaqueous electrolytic solution are all excellent in high-temperature cycle characteristics. This is because the compounds shown in Table 3 release moisture as shown in the above formula, particularly when the positive electrode is in a charged state and at a high temperature, and LiPF ₆ is decomposed by the moisture to produce HF. This is probably because a protective coating was formed on the top.

  Moreover, among the compounds shown in Table 3, the batteries 20 to 26 containing carbon acid were excellent in high temperature cycle characteristics. This is presumably because the conjugate base formed by releasing protons from the carbon acid is electrochemically stable and the conjugate base itself is not easily oxidized.

  In particular, it can be seen that the batteries 22 to 26 containing dimedone, triacetylmethane, acetylacetone, cyclopentadiene, or 1,3-dioxolane are particularly excellent in cycle characteristics. This is thought to be because conjugate bases formed by releasing protons from these carbon acids are more electrochemically stable and the conjugate bases themselves are very difficult to oxidize.

Dimedone was added to the non-aqueous electrolyte at an initial concentration as shown in Tables 4 and 5, the concentration of LiPF ₆ was 1.0 mol / L, and the materials shown in Tables 4 and 5 were used as the positive electrode active material. Except for this, a non-aqueous electrolyte secondary battery was produced in the same manner as in Example 1. The obtained batteries were designated as batteries 27 to 62, respectively.
The free acid concentration contained in the non-aqueous electrolyte was determined in the same manner as in Example 1 for the batteries 27 to 62 at the initial stage and after storage under charge. Moreover, the charge / discharge cycle was performed on the same conditions as the said Example 1 using the batteries 27-62. The results obtained are shown in Tables 4 and 5.

As shown in Table 4 and 5, the general formula _{LiNi x M 1-xy L y} O 2 (M is at least one of Co and Mn, L is Al, Sr, Y, Zr, Ta, Mg , Ti, Zn, B, Ca, Cr, Si, Ga, Sn, P, V, Sb, Nb, Mo, W, and Fe, and at least 0.1 ≦ x ≦ 1 0.0, 0 ≦ y ≦ 0.1) and a non-aqueous electrolyte containing an organic acid are used in combination to obtain a battery having excellent high-temperature cycle characteristics.
Further, from the results of Table 4, the Ni content in the positive electrode active material is preferably 0.1 ≦ x ≦ 0.9, more preferably 0.3 ≦ x ≦ 0.9, It can be seen that 0.7 ≦ x ≦ 0.9 is most preferable.

  Since the nonaqueous electrolyte secondary battery of the present invention has a high capacity and a long life, it is useful as a power source for small portable devices.

1 is a longitudinal sectional view schematically showing a nonaqueous electrolyte secondary battery according to an embodiment of the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Positive electrode plate 12 Negative electrode plate 13 Separator 14 Positive electrode lead 15 Negative electrode lead 16 Upper insulating plate 17 Lower insulating plate 18 Battery case 19 Sealing plate 20 Gasket

Claims (5)

A non-aqueous solution comprising a positive electrode including a nickel-containing lithium composite oxide as a positive electrode active material, a negative electrode capable of inserting and extracting lithium, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte An electrolyte secondary battery,
The non-aqueous electrolyte secondary battery in which the non-aqueous electrolyte contains 20 ppm or more and 100 ppm or less of an organic acid. The non-aqueous electrolyte secondary battery according to claim 1, wherein the non-aqueous electrolyte contains LiPF ₆ .   The non-aqueous electrolyte secondary battery according to claim 1, wherein the organic acid includes a carbon acid.   The non-aqueous electrolyte secondary battery according to claim 3, wherein the carbon acid includes at least one selected from the group consisting of dimedone, triacetylmethane, acetylacetone, cyclopentadiene, and 1,3-dioxolane. The nickel-containing lithium composite oxide has the following formula:
_{LiNi x M 1-xy L y} O 2
(M is at least one of Co and Mn, and L is Al, Sr, Y, Zr, Ta, Mg, Ti, Zn, B, Ca, Cr, Si, Ga, Sn, P, V, Sb. , Nb, Mo, W and Fe, at least one selected from the group consisting of 0.1 ≦ x ≦ 1.0, 0 ≦ y ≦ 0.1)
The nonaqueous electrolyte secondary battery of Claim 1 represented by these.

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