JP4240960B2 - Lithium secondary battery and manufacturing method thereof - Google Patents

Description

【００６７】
（比較例）
実施例と同様な方法にて作製した角型リチウム二次電池について、表１に示すような第１熱処理工程、第２熱処理工程を行うことにより電池１〜電池１０を作製した。
【００６８】
ただし、電池８の場合には第１熱処理工程のみを行い、電池９の場合には第２熱処理工程のみを行った。また電池１０については、電池Ａの二つの熱処理工程の順序を入れ替える（第２熱処理工程を先に行い、次いで放電し再度充電して電池の充電深度を調整した後に、第１熱処理工程を実施する）ことで電池を作製した。
【００６９】
このようにして作製した実施例の電池Ａ〜電池Ｏ、比較例の電池１〜電池１０について、低温パルス放電特性、低温充電特性、過充電試験による安全性、１５０℃加熱試験による耐熱性を評価した結果を表１に示す。
【００７０】
低温パルス放電特性は、各１０個の電池を用い、２０℃の環境下において、０．７Ｃ（７００ｍＡ）の定電流充電を行い、電池電圧が４．２Ｖに到達した後は、４．２Ｖの電圧を維持したまま、電流値が減衰して０．０５Ｃ（５０ｍＡ）となるまで定電圧充電を行った。その後、２０℃の環境下において、１．２Ｃ（１２００ｍＡ）６ｍｓと０．１Ｃ（１００ｍＡ）１２ｍｓのパルス放電パターンにて、３．２Ｖの放電終止電圧まで放電した場合の電池容量を初期容量とした。次に、上記と同じ充電条件にて充電後、０℃の環境下において３時間放置した後、上記と同じパルス放電条件にて放電した場合の電池容量を測定し、初期容量に対する放電容量比率を算出した。各１０個の電池についての放電容量比率の平均値を表１に示す。
【００７１】
また低温充電特性は、各１０個の電池を用い、２０℃の環境下において、１Ｃ（１０００ｍＡ）の定電流充電を行い、電池電圧が４．２Ｖに到達した後は、４．２Ｖの電圧を維持したまま、電流値が減衰して０．０５Ｃ（５０ｍＡ）となるまで定電圧充電を行った。その後、２０℃の環境下において、１Ｃ（１０００ｍＡ）の定電流にて、３．０Ｖの放電終止電圧まで放電した場合の電池容量を初期容量とした。次に、０℃の環境下において３時間放置した後、上記と同じ充電条件にて充電を行い、次いで２０℃の環境下において３時間放置した後、上記と同じ放電条件にて放電した場合の電池容量を測定し、初期容量に対する放電容量比率を算出した。各１０個の電池についての放電容量比率の平均値を表１に示す。
【００７２】
過充電時の安全性は、各５個の電池を用い、２０℃の環境下において、電池が宙づり状態になるように治具に取り付け、１Ｃ（１０００ｍＡ）の定電流にて連続充電した場合の発火、破裂の割合を評価した。その結果を表１に示す。
【００７３】
１５０℃の加熱試験による耐熱性は、各５個の電池を用い、−５℃の環境下において、１Ｃ（１０００ｍＡ）の定電流充電を行い、電池電圧が４．３１Ｖに到達した後は、４．３１Ｖの電圧を維持したまま、電流値が減衰して０．０５Ｃ（５０ｍＡ）となるまで定電圧充電を行った。次にこの充電した電池を防爆機能付きの乾燥機中に宙づり状態になるように治具に取り付け、５℃／ｍｉｎの昇温速度で１５０℃まで昇温させ、１５０℃で３時間保持した場合の発火、破裂の割合を評価した。その結果を表１に示す。
【００７４】
表１から明らかなように、電池Ａ〜電池Ｏは、充電深度が１５％〜３０％の状態で低温環境下にて熱処理を行う第１熱処理工程と充電深度を５０％〜１００％の状態にした後に高温環境下にて熱処理を行う第２熱処理工程を順次行っているため、低温パルス放電特性、低温充電特性、過充電時の安全性、耐熱性に優れたリチウム二次電池を得ることができた。
【００７５】
さらに電池Ｏのように、第１熱処理工程後に充放電を行い、次いで第２熱処理工程を行った場合についても、同様の効果が得られることが確認できた。
【００７６】
これに対して、電池１の場合には、第１熱処理工程時の充電深度が高すぎるために、電池３の場合には、第１熱処理工程時の処理温度が高すぎるために、負極活物質表面でのＳＥＩ皮膜の生成速度が速くなり、負極活物質の表面に不均質な厚い皮膜が形成され、この不均質な皮膜が核となり、第２熱処理工程時に皮膜が急激に成長する。その結果、電池の内部抵抗が増大し、低温パルス放電特性、低温充電特性が低下すると共に、低温充電時に負極活物質表面に析出した金属リチウムと電解液溶媒との発熱反応により、１５０℃耐熱性が低下したものと考えられる。
【００７７】
電池２の場合には、第１熱処理工程時の充電深度が低すぎるために、負極活物質表面での電解液の分解を抑制し、リチウム挿入を可能にするＳＥＩ皮膜を効果的に形成させることが困難となる。そのため、第２熱処理工程時に負極活物質表面に局所的に不均質な皮膜が形成されることになる。その結果、電極反応が不均一となり、低温パルス放電特性、低温充電特性、１５０℃耐熱性が低下したものと考えられる。
【００７８】
電池４の場合には、第２熱処理工程時の充電深度が高すぎるために、電池６の場合には、第２熱処理工程時の処理温度が高すぎるために、正極活物質表面の不活性化（電解液溶媒の酸化分解生成物による正極表面の皮膜形成）が著しく進行すると共に、負極活物質表面のＳＥＩ皮膜が極めて厚くなる。その結果、電池の内部抵抗が増大し、低温パルス放電特性、低温充電特性が低下すると共に、低温充電時に金属リチウムが析出し、１５０℃耐熱性が著しく低下したものと考えられる。
【００７９】
電池５の場合には、第２熱処理工程時の充電深度が低すぎるために、電池７の場合には、第２熱処理工程時の処理温度が低すぎるために、正極活物質表面の不活性化が不充分となり、その結果、正極負極間の分極バランスがくずれ、低温充電特性が低下すると共に、過充電時の安全性、１５０℃耐熱性が著しく低下した。
【００８０】
電池８の場合には、第２熱処理工程を行っていないために、正極活物質表面の不活性化及び負極活物質表面への均質なＳＥＩ皮膜の形成が不充分となる。その結果、過充電時の安全性、１５０℃耐熱性が著しく低下したものと考えられる。
【００８１】
電池９の場合には、第１熱処理工程を行っていないために、電池１０の場合には、第１熱処理工程より先に第２熱処理工程を行っているために、極板群への電解液の含浸が充分に行われる前に、負極活物質表面に局所的に不均質な厚い皮膜が形成されることになる。その結果、電極反応が不均一となり、低温パルス放電特性、低温充電特性が低下すると共に、低温充電時に金属リチウムが析出し、１５０℃耐熱性が低下したものと考えられる。
【００８２】
【発明の効果】
以上のように、本発明のリチウム二次電池及びその製造方法によれば、充電深度が１５％〜３０％の状態で３０℃以下の低温環境下にて熱処理を行う第１熱処理工程と充電深度を５０％〜１００％の状態にした後に４０℃〜８０℃の高温環境下にて熱処理を行う第２熱処理工程を少なくとも含み、且つこれらの工程を順次行うことにより、正極活物質表面に不活性皮膜が形成されていると共に、負極活物質表面に均質なＳＥＩ皮膜が形成されている電池を得ることができ、低温パルス放電特性、低温充電特性、過充電時の安全性、耐熱性に優れたリチウム二次電池を提供することができる。
【図面の簡単な説明】
【図１】本発明のリチウム二次電池の縦断面図
【符号の説明】
１１ 電池ケース
１２ 封口板
１３ 正極集電体
１４ 正極板
１５ セパレータ
１６ 負極板
１７ 負極集電体
１８ 注液栓[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a lithium secondary battery, and in particular, a lithium secondary battery excellent in low-temperature pulse discharge characteristics, low-temperature charge characteristics, safety during overcharge, and heat resistance, which is required as a power source for mobile phones, and its It relates to a manufacturing method.
[0002]
[Prior art]
In recent years, with the rapid progress of miniaturization, weight reduction and high performance of portable information devices, development and practical use of lithium secondary batteries that have high operating voltage of 4V class and suitable for high energy density as their driving power supply Is being actively promoted.
[0003]
As a positive electrode active material of a lithium secondary battery, LiCoO having a layered rock salt structure ₂ , LiNiO ₂ LiMn with spinel structure ₂ O _Four Lithium-containing transition metal compounds such as natural graphite, spherical and fibrous artificial graphite, non-graphitizable carbon (hard carbon), graphitizable carbon (soft carbon), etc. The carbon material is adopted.
[0004]
Efficient measures to achieve high energy density include high density packing of positive electrode active material, adoption of high capacity negative electrode active material, thinner separator, optimization of electrode plate group structure and mechanical parts, etc. As a result, the energy density has been improved at an annual rate of 10% or more.
[0005]
Furthermore, in order to ensure the safety and practical reliability of lithium ion batteries, in terms of materials, studies have been actively conducted on improving the functionality of separators and optimizing electrolyte additives. On the other hand, in terms of the manufacturing process, the SEI film that improves the impregnation of the electrolyte solution into the electrode plate group and suppresses the decomposition of the electrolyte solution on the surface of the negative electrode active material (carbon material) and enables lithium insertion is effective. In order to form the target, a method of introducing a heat treatment process called “aging” after injecting the electrolyte into the battery case and sealing it has been proposed.
[0006]
For example, JP 2000-340262 A discloses that a battery is stored at a temperature of 40 ° C. or higher and 90 ° C. or lower. Specifically, when the depth of charge (SOC) of the battery is 30% or more and the storage temperature is 40 ° C. or higher and lower than 70 ° C., LiNiO having a large initial discharge capacity ₂ LiNiO with improved charge / discharge cycle characteristics when a lithium-based lithium battery is obtained, the charge depth (SOC) of the battery is 60% or more, and the storage temperature is 70 ° C or higher and 90 ° C or lower ₂ A method for obtaining a lithium secondary battery has been proposed.
[0007]
WO97 / 30487 discloses a battery open circuit voltage of 2.5 V after a low temperature aging of 2 ° C. to 30 ° C. with a battery open circuit voltage of 0.5 V to 3.0 V, and then charging or charging / discharging. A method of performing high temperature aging at 40 ° C. to 70 ° C. in a state of ˜3.8 V has been proposed.
[0008]
Furthermore, in JP-A-11-288712, after being stored at a temperature of 30 ° C. to 70 ° C. under a condition where the open circuit voltage is 2.5V to 3.8V, it is charged to 4.0V or higher, and then 30 ° C. to There has been proposed a method for improving charge / discharge cycle characteristics with a high capacity by storing under conditions where the open circuit voltage is 3.9 V to 4.3 V at a temperature of 70 ° C.
[0009]
[Patent Document 1]
JP 2000-340262 A
[Patent Document 2]
WO97 / 30487
[Patent Document 3]
JP-A-11-288712
[0010]
[Problems to be solved by the invention]
However, even using these proposals, it is possible to provide a lithium secondary battery excellent in low-temperature pulse discharge characteristics, low-temperature charge characteristics, overcharge safety, and heat resistance, which are required as a power source for mobile phones. It was difficult.
[0011]
For example, as disclosed in JP 2000-340262 A, when the depth of charge (SOC) of a battery is 30% or more and the storage temperature is 40 ° C. or higher and lower than 70 ° C., the low-temperature pulse discharge characteristic is Although improved, the film formation on the surface of the positive electrode due to the oxidative decomposition product of the electrolyte solvent became insufficient, resulting in a problem that safety during overcharge was lowered. Furthermore, since the impregnation of the electrolyte into the electrode plate due to expansion of the active material during aging is difficult to promote, the electrode reaction during charge / discharge becomes non-uniform, and the capacity retention rate at the initial stage of the cycle cannot be secured. Was.
[0012]
On the other hand, when the charging depth (SOC) of the battery is 60% or more and the storage temperature is 70 ° C. or higher and 90 ° C. or lower, the charging depth of the battery is high and the aging treatment temperature is high. Thus, a heterogeneous thick film is formed on the surfaces of the positive electrode active material and the negative electrode active material. As a result, the internal resistance (film resistance, charge transfer resistance) of the battery increases and the low-temperature pulse discharge characteristics deteriorate. Occurred.
[0013]
In addition, when charged in a low temperature environment, metallic lithium precipitates on the surface of the negative electrode active material in the form of needles and dendrites, and some of them break off and react with the electrolyte solvent to form an SEI film. As a result, the charging / discharging efficiency of the battery was reduced. Further, when a 150 ° C. heating test was performed using a battery charged in this low temperature environment, ignition and rupture could occur due to an exothermic reaction between the electrolyte solution solvent and metallic lithium deposited on the surface of the negative electrode active material.
[0014]
As disclosed in WO97 / 30487, even when performing a two-stage aging process including a low temperature aging process and a high temperature aging process, the setting of the battery open circuit voltage during the low temperature aging process is inappropriate. In addition, it is impossible to simultaneously promote the impregnation of the electrolytic solution into the electrode plate group and the formation of a dense, thin and uniform SEI film on the negative electrode surface, and locally non-uniformity on the positive electrode and negative electrode active material surfaces during high-temperature aging treatment. As a result, a low-temperature pulse discharge characteristic, heat resistance, and charge / discharge cycle characteristics were deteriorated due to non-uniformity of the electrode reaction.
[0015]
Further, as disclosed in Japanese Patent Application Laid-Open No. 11-288712, even when performing a two-stage high-temperature aging treatment in which the battery open circuit voltage during aging is changed, the surface of the negative electrode active material is exposed during the first high-temperature aging. While suppressing rapid growth of the SEI film, the impregnation of the electrolyte into the electrode plate group cannot be promoted, and a non-uniform thick film is formed on the surface of the negative electrode active material. As a result, the internal resistance of the battery ( Film resistance, charge transfer resistance) increased, and low temperature pulse discharge characteristics, low temperature charge characteristics, and heat resistance significantly decreased.
[0016]
The present invention solves such a conventional problem, and is a lithium secondary excellent in low-temperature pulse discharge characteristics, low-temperature charge characteristics, safety during overcharge, and heat resistance, which is required as a power source for mobile phones. It is an object of the present invention to provide a battery and a manufacturing method thereof.
[0017]
[Means for Solving the Problems]
In order to achieve the above object, a lithium secondary battery and a manufacturing method thereof according to the present invention include a positive electrode and a negative electrode containing an active material capable of reversibly occluding and desorbing lithium ions, a separator, and a non-aqueous electrolyte. A method for manufacturing a lithium secondary battery, wherein at least a charging depth is 15% to 30%. Below 30 ° C After the first heat treatment step in which heat treatment is performed in a low temperature environment and the charging depth is set to 50% to 100% 40 ℃ ~ 80 ℃ It includes a second heat treatment step in which heat treatment is performed in a high temperature environment, and these steps are sequentially performed.
[0018]
like this In Charging depth 15% to 30% In state Below 30 ° C The first heat treatment step that performs heat treatment in a low-temperature environment promotes the impregnation of the electrolyte solution into the electrode plate group, and the surface of the negative electrode active material. 3nm to 5nm thick A homogeneous SEI film can be formed. In addition, this film exhibits lithium ion conductivity, but does not have electronic conductivity, so that rapid film growth due to subsequent decomposition of the electrolytic solution can be suppressed.
[0019]
Next, set the charging depth 50% to 100% After making a state 40 ℃ ~ 80 ℃ By the second heat treatment step in which heat treatment is performed in a high temperature environment, formation of an inert film on the surface of the positive electrode active material by the oxidative decomposition product of the electrolyte solvent, and formation on the surface of the negative electrode active material Thickness 10nm-40nm The homogeneous SEI film can be simultaneously formed.
[0020]
Therefore, the lithium secondary battery in which an inert film is formed on the surface of the positive electrode active material and a homogeneous SEI film is formed on the surface of the negative electrode active material by such a manufacturing method is used for the internal resistance (film) of the battery. Resistance, charge transfer resistance), and uniformity of electrode reaction, ensuring a lithium secondary battery with excellent low-temperature pulse discharge characteristics, low-temperature charge characteristics, overcharge safety, and heat resistance it can.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
The shape of the lithium secondary battery of the present invention is not limited to a square shape, a flat shape, a cylindrical shape, or the like, but a vertical cross-sectional view of the rectangular nonaqueous electrolyte battery shown in FIG. 1 is used. Embodiments of the present invention will be described.
[0022]
As shown in FIG. 1, an electrode plate group in which a positive electrode plate 14 and a negative electrode plate 16 are wound in an elliptical shape with a separator 15 interposed therebetween is accommodated in a bottomed rectangular battery case 11, and a sealing plate After the sealing plate 12 and the battery case 11 are laser-welded, the nonaqueous electrolyte is injected from the injection hole provided in the sealing plate 12, and then the injection plug 18 is sealed with a laser.
[0023]
This positive electrode plate 14 is obtained by kneading and dispersing a positive electrode active material and a binder on one side or both sides of a current collector 13 made of an aluminum foil, a lath-processed or etched foil, and a conductive agent as necessary. The paste can be applied, dried and rolled. The thickness of the positive electrode plate 14 is preferably 130 μm to 200 μm and flexible.
[0024]
As the positive electrode active material, for example, a lithium-containing transition metal compound that can accept lithium ions as a guest is used. For example, a composite metal oxide of at least one metal selected from cobalt, manganese, nickel, chromium, iron and vanadium and lithium, LiCoO ₂ LiMnO ₂ , LiNiO ₂ LiCo _x Ni _(1-x) O ₂ (0 <x <1), LiCrO ₂ , ΑLiFeO ₂ , LiVO ₂ Etc. are preferred.
[0025]
The binder is not particularly limited as long as it can be kneaded and dispersed in a solvent. For example, a fluorine-based binder, acrylic rubber, modified acrylic rubber, styrene-butadiene rubber (SBR), acrylic heavy A polymer, a vinyl polymer or the like can be used alone or as a mixture or copolymer of two or more. As the fluorine-based binder, for example, polyvinylidene fluoride, a copolymer of vinylidene fluoride and propylene hexafluoride, and a dispersion of polytetrafluoroethylene resin are preferable.
[0026]
If necessary, a conductive agent and a thickener can be added. As the conductive agent, acetylene black, graphite, carbon fiber or the like is used alone, or a mixture of two or more kinds is preferable. As the thickener, ethylene-vinyl alcohol is used. A polymer, carboxymethylcellulose, methylcellulose and the like are preferable.
[0027]
As the solvent, a solvent capable of dissolving the binder is suitable. In the case of an organic binder, N-methyl-2-pyrrolidone, N, N-dimethylformamide, tetrahydrofuran, dimethylacetamide, dimethylsulfoxide, hexamethyl An organic solvent such as sulforamide, tetramethylurea, acetone or methyl ethyl ketone is preferably used alone or as a mixed solvent thereof. In the case of an aqueous binder, water or warm water is preferred.
[0028]
In addition, various dispersants, surfactants, stabilizers, and the like can be added as necessary when the paste is kneaded and dispersed.
[0029]
The coating and drying is not particularly limited, and the slurry mixture kneaded and dispersed as described above, for example, a slit die coater, a reverse roll coater, a lip coater, a blade coater, a knife coater, a gravure coater, It can be easily applied using a dip coater or the like, and drying close to natural drying is preferable, but considering productivity, it is preferable to dry at a temperature of 70 ° C. to 200 ° C. for 5 hours to 10 minutes.
[0030]
Rolling is preferably performed several times at a linear pressure of 1000 to 2000 kg / cm or by changing the linear pressure until a predetermined thickness is reached by a roll press.
[0031]
The negative electrode plate 16 can be prepared by applying, drying, and rolling a paste obtained by kneading and dispersing a negative electrode active material, a binder, and, if necessary, a conductive agent in a solvent on one side or both sides of the current collector 17. . And the thickness of a negative electrode plate is 140 micrometers-210 micrometers similarly to a positive electrode plate, and it is preferable that it has a softness | flexibility.
[0032]
The copper or copper alloy used as the negative electrode current collector 17 is not particularly limited, and examples thereof include rolled foil, electrolytic foil, and the shape thereof is foil, perforated foil, expanded material, lath material, and the like. It does not matter.
[0033]
As the negative electrode active material, for example, a material containing graphite having a graphite crystal structure capable of reversibly occluding and desorbing lithium ions, such as natural graphite, spherical and fibrous artificial graphite, non-graphitizable carbon (hard carbon) ) And carbon materials such as graphitizable carbon (soft carbon) are preferable, and in particular, the spacing between lattice planes (002) (d ₀₀₂ ) Is more preferably a carbon material having a graphite type crystal structure of 0.3350 to 0.3400 nm.
[0034]
As the binder, the solvent, and the conductive agent and thickener that can be added as necessary, the same materials as those for the positive electrode can be used.
[0035]
The separator 15 is preferably a single layer of microporous polyolefin resin such as polyethylene resin or polypropylene resin having a thickness of 15 μm to 30 μm or a laminate of polypropylene resin on both sides of the polyethylene resin.
[0036]
The battery case 11 is a bottomed rectangular case with an open upper end, and the material thereof is an aluminum alloy containing a trace amount of metal such as manganese or copper or inexpensive nickel plating from the viewpoint of pressure strength. A steel plate is preferred.
[0037]
The electrode plate group in which the positive electrode plate 14 and the negative electrode plate 16 thus manufactured are wound in a flat shape with the separator 15 interposed therebetween is dried and then stored in the battery case 11 or the electrode plate. The group is stored in the battery case 11 and then dried.
[0038]
The drying condition is preferably an atmosphere of low humidity and high temperature. However, if the temperature is too high, the separator may be thermally contracted or the microporous pores may be crushed, which adversely affects battery characteristics. The dew point is -30 to -80 ° C, and the temperature is preferably 80 to 120 ° C.
[0039]
The electrolyte is adjusted by dissolving the electrolyte in a non-aqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-dichloroethane, 1,3-dimethoxypropane, 4- Methyl-2-pentanone, 1,4-dioxane, acetonitrile, propionitrile, butyronitrile, valeronitrile, benzonitrile, sulfolane, 3-methyl-sulfolane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethylformamide, dimethylsulfoxide, dimethylformamide, Trimethyl phosphate, triethyl phosphate, and the like can be used, and these nonaqueous solvents can be used alone or as a mixed solvent of two or more kinds.
[0040]
As the electrolyte contained in the non-aqueous electrolyte, for example, a lithium salt having a strong electron withdrawing property is used, for example, LiPF. ₆ , LiBF _Four LiClO _Four , LiAsF ₆ , LiCF _Three SO _Three , LiN (SO ₂ CF _Three ) ₂ , LiN (SO ₂ C ₂ F _Five ) ₂ , LiC (SO ₂ CF _Three ) _Three Etc. These electrolytes may be used alone or in combination of two or more. These electrolytes are preferably dissolved at a concentration of 0.5 to 1.5 M in the non-aqueous solvent.
[0041]
In the production method of the present invention, the lithium secondary battery manufactured in this way is in a state where the charging depth of the battery is 15% to 30%. Below 30 ° C A first heat treatment step is performed in which heat treatment is performed in a low temperature environment. The heat treatment temperature is , 0 A range of from 20 ° C to 20 ° C is optimal. When the first heat treatment is performed in an environment exceeding 30 ° C., the generation rate of the film is high, and thus a non-uniform thick film is formed on the surface of the negative electrode active material. The film grows rapidly during the process. As a result, the internal resistance of the battery is increased, and the low temperature pulse discharge characteristics, the low temperature charge characteristics, and the heat resistance are deteriorated.
[0042]
Further, when the charging depth of the battery is less than 15%, it becomes difficult to effectively form an SEI film that suppresses decomposition of the electrolyte solution on the surface of the negative electrode active material and enables lithium insertion. On the other hand, if it exceeds 30%, the generation rate of the SEI film on the negative electrode active material surface increases, 3nm to 5nm thick A homogeneous SEI film cannot be formed. In any of the above cases where the charging depth of the battery is not appropriate, a locally inhomogeneous film is formed on the surface of the negative electrode active material during the second heat treatment step. As a result, the electrode reaction becomes non-uniform, and the low-temperature pulse discharge characteristics, the low-temperature charge characteristics, and the heat resistance are deteriorated.
[0043]
The heat treatment time is appropriately determined depending on the heat treatment temperature and the battery charging depth, but is preferably in the range of about 6 hours to 10 days in consideration of mass productivity.
[0044]
Next, after setting the battery charge depth to 50% to 100% 40 ℃ ~ 80 ℃ A second heat treatment step is performed in which heat treatment is performed in a high temperature environment.
[0045]
In addition, in order to make the charging depth of the battery 50% to 100%, either the method of performing only charging or the method of charging after charging / discharging may be used, and the time setting in the constant current charging method may be used. Or it can obtain easily by the voltage setting (voltage setting: 3.81V-4.20V) by a constant voltage charging system.
[0046]
The charging condition is not particularly limited, and the maximum current is preferably 0.5 C (2 hour rate) or less.
[0047]
When the charging depth of the battery is less than 50%, the inactivation of the surface of the positive electrode active material (film formation on the surface of the positive electrode due to the oxidative decomposition product of the electrolyte solvent) is insufficient, so the safety during overcharge is reduced. Therefore, when the charging depth of the battery exceeds 100%, the deactivation of the surface of the positive electrode active material proceeds remarkably, and the internal resistance of the battery increases because the SEI film on the surface of the negative electrode active material becomes too thick. Since low temperature pulse discharge characteristics, low temperature charge characteristics, and heat resistance are deteriorated, it is not preferable.
[0048]
Heat treatment temperature Special The preferred range is 40 ° C to 70 ° C.
[0049]
When the heat treatment temperature is less than 40 ° C., the surface of the positive electrode active material is not sufficiently deactivated, so that the safety during overcharge is reduced, which is not preferable. When the heat treatment temperature exceeds 80 ° C., the positive electrode A heterogeneous thick film is formed on the surface of the active material and the negative electrode active material, the internal resistance of the battery is increased, and the low-temperature pulse discharge characteristics, the low-temperature charge characteristics, and the heat resistance are not preferable.
[0050]
The heat treatment time is appropriately determined depending on the heat treatment temperature and the battery charging depth, but is preferably in the range of about 12 hours to 10 days in consideration of mass productivity.
[0051]
As above, the charging depth 15% to 30% In state Below 30 ° C The first heat treatment step in which heat treatment is performed in a low-temperature environment promotes the impregnation of the electrolyte solution into the electrode plate group and exhibits lithium ion conductivity on the negative electrode active material surface, but does not have electron conductivity, and thereafter 3 nm to 5 nm thickness capable of suppressing rapid film growth due to decomposition of electrolyte Average A quality SEI film can be formed. The And Charge depth 50% to 100% After making a state 40 ℃ ~ 80 ℃ By the second heat treatment step in which heat treatment is performed in a high temperature environment, formation of an inactive film having a thickness of 0.5 nm to 1.0 nm on the surface of the positive electrode active material by the oxidative decomposition product of the electrolyte solvent, and to the surface of the negative electrode active material The uniform SEI film having a thickness of 10 nm to 40 nm can be simultaneously formed.
[0052]
Therefore, by such a manufacturing method, an inert film is formed on the surface of the positive electrode active material, and a homogeneous SEI film is formed on the surface of the negative electrode active material. Therefore, low temperature pulse discharge characteristics, low temperature charge characteristics, safety during overcharge Lithium secondary battery excellent in heat resistance and heat resistance can be provided.
[0053]
【Example】
The present invention will be described in detail using examples and comparative examples, but these do not limit the present invention.
[0054]
(Example)
First, the positive electrode plate 14 is composed of 100 parts by weight of lithium cobalt oxide as a positive electrode active material, 3 parts by weight of acetylene black as a conductive agent, and 4 parts by weight of polytetrafluoroethylene (PTFE) resin as a binder in a solid content. 0.8 parts by weight of methylcellulose was added and kneaded and dispersed using water as a solvent to prepare a paste. The paste was continuously applied intermittently to a current collector 13 made of a strip-shaped aluminum foil having a thickness of 20 μm, dried, heat-treated at 250 ° C. for 10 hours, and then rolled three times at a linear pressure of 1000 kg / cm. Went.
[0055]
Then, a positive electrode plate 14 having a width dimension of 42 mm, a length of 300 mm, and a thickness of 0.180 mm is attached by spot welding of an aluminum positive electrode lead and further applying an insulating tape made of polypropylene resin for preventing an internal short circuit. Was made.
[0056]
Next, the negative electrode plate 16 is 100 parts by weight of scaly graphite capable of occluding and releasing lithium as a negative electrode active material, and 4 parts by weight of a water-soluble dispersion of styrene butadiene rubber (SBR) as a binder, 0.8 parts by weight of carboxymethyl cellulose as a thickener and water as a solvent were added and kneaded and dispersed to prepare a paste mixture. The paste was continuously applied intermittently to a current collector 17 made of a strip-shaped copper foil having a thickness of 14 μm, dried at 110 ° C. for 30 minutes, and rolled three times at a linear pressure of 110 kg / cm.
[0057]
Then, the negative electrode plate 16 having a width of 44 mm, a length of 400 mm, and a thickness of 0.196 mm is attached by spot welding of a negative electrode lead made of nickel and further applying an insulating tape made of polypropylene resin for preventing an internal short circuit. Was made.
[0058]
In this manner, the positive electrode plate 14 and the negative electrode plate 16 are insulated from each other through the polypropylene resin microporous separator 15 having a thickness of 20 μm, and the electrode group wound in an elliptical shape is heated at 60 ° C. A flat plate group was obtained by pressing at a temperature of 6.5 MPa under a pressure condition of 6.5 MPa for 30 seconds.
[0059]
This flat electrode plate group is made of a 3000 series aluminum alloy containing a trace amount of metals such as manganese and copper, has a thickness of 0.25 mm, a width dimension of 6.3 mm, a length dimension of 34.0 mm, and a total height of 50. It was housed in a bottomed rectangular battery case 11 produced by press molding into a 0.0 mm shape.
[0060]
By drying for 2 hours at a dew point of −30 ° C. and a temperature of 90 ° C., the moisture content of the electrode plate group was reduced from 500 ppm to 70 ppm as measured using a Karl Fischer moisture meter.
[0061]
Furthermore, after laser-welding the sealing plate 12 and the battery case 11, from a liquid injection hole provided in the sealing plate 12, to a mixed solvent in which ethylene carbonate (EC) and ethyl methyl carbonate (MEC) are mixed at a ratio of 2: 1. LiPF ₆ After injecting a non-aqueous electrolyte solution dissolved at a concentration of 1.0 M, the battery is charged to a state where the charging depth is 20% by a constant current charging method of 0.2 C (200 mA) for 1 hour in an environment of 30 ° C. To do. Next, the liquid injection plug 18 was sealed with a laser, and a prismatic lithium secondary battery having a battery capacity of 1000 mAh as a design value was produced.
[0062]
The first heat treatment step was performed on the prismatic lithium secondary battery thus produced under a treatment condition of storage for 12 hours in an environment of 10 ° C.
[0063]
Next, after increasing the charging depth to 87% under a constant voltage of 4.07 V and a maximum current of 0.2 C (200 mA) in a 30 ° C. environment, store for 48 hours in a 60 ° C. environment. A battery that was subjected to the second heat treatment step under the treatment conditions was designated as Battery A.
[0064]
Similarly, batteries B to O were manufactured by performing the first heat treatment step and the second heat treatment step as shown in Table 1.
[0065]
In the case of the battery O, after performing the first heat treatment step in the same manner as the battery A, the battery O is discharged to a discharge end voltage of 2.75 V at a discharge current of 1 C (1000 mA) in an environment of 30 ° C. Subsequently, the battery was charged to a state where the charging depth was 100% under a constant voltage of 4.20 V and a maximum current of 0.2 C (200 mA), and then discharged to a discharge end voltage of 2.75 V at a discharge current of 1 C (1000 mA). The charge / discharge treatment is performed for two cycles. Further, after the charge depth is set to 87% under a constant voltage of 4.07 V and a maximum current of 0.2 C (200 mA), the second heat treatment step is performed under a storage condition of 48 hours in an environment of 60 ° C. Went.
[0066]
[Table 1]

[0067]
(Comparative example)
About the square lithium secondary battery produced by the method similar to an Example, the 1st heat treatment process as shown in Table 1 and the 2nd heat treatment process were performed, and the batteries 1-10 were produced.
[0068]
However, in the case of the battery 8, only the first heat treatment step was performed, and in the case of the battery 9, only the second heat treatment step was performed. For battery 10, the order of the two heat treatment steps of battery A is changed (the second heat treatment step is performed first, and then the first heat treatment step is performed after discharging and recharging to adjust the charge depth of the battery. ) To produce a battery.
[0069]
The battery A to battery O of the example thus fabricated and the batteries 1 to 10 of the comparative example were evaluated for low temperature pulse discharge characteristics, low temperature charge characteristics, safety by overcharge test, and heat resistance by 150 ° C. heating test. The results are shown in Table 1.
[0070]
The low-temperature pulse discharge characteristics are 10 batteries each, and the battery is charged with a constant current of 0.7 C (700 mA) in an environment of 20 ° C. After the battery voltage reaches 4.2 V, it is 4.2 V. While maintaining the voltage, constant voltage charging was performed until the current value attenuated to 0.05 C (50 mA). Thereafter, in an environment of 20 ° C., the battery capacity when discharged to a discharge end voltage of 3.2 V with the pulse discharge pattern of 1.2 C (1200 mA) 6 ms and 0.1 C (100 mA) 12 ms was used as the initial capacity. . Next, after charging under the same charging conditions as described above, after standing for 3 hours in an environment of 0 ° C., the battery capacity is measured when discharged under the same pulse discharge conditions as above, Calculated. Table 1 shows the average discharge capacity ratio for each of the 10 batteries.
[0071]
The low-temperature charge characteristics are 10 batteries each, and a constant current charge of 1 C (1000 mA) is performed in an environment of 20 ° C. After the battery voltage reaches 4.2 V, a voltage of 4.2 V is applied. While maintaining, constant voltage charging was performed until the current value attenuated to 0.05 C (50 mA). Thereafter, the battery capacity when discharged to a discharge end voltage of 3.0 V at a constant current of 1 C (1000 mA) in an environment of 20 ° C. was defined as the initial capacity. Next, after being left for 3 hours in an environment of 0 ° C., charging was performed under the same charging conditions as described above, and then being left for 3 hours in an environment of 20 ° C. and then discharged under the same discharging conditions as described above. The battery capacity was measured, and the discharge capacity ratio relative to the initial capacity was calculated. Table 1 shows the average discharge capacity ratio for each of the 10 batteries.
[0072]
The safety at the time of overcharge is that when 5 batteries each are used and the battery is mounted in a jig so that the battery is suspended in an environment of 20 ° C., and continuously charged at a constant current of 1 C (1000 mA). The rate of ignition and rupture was evaluated. The results are shown in Table 1.
[0073]
The heat resistance by a heating test at 150 ° C. is 5 after each battery is charged at a constant current of 1 C (1000 mA) in an environment of −5 ° C. and the battery voltage reaches 4.31 V. While maintaining the voltage of .31 V, constant voltage charging was performed until the current value decreased to 0.05 C (50 mA). Next, when this charged battery is mounted on a jig so as to be suspended in an explosion-proof dryer, the temperature is raised to 150 ° C. at a rate of 5 ° C./min and held at 150 ° C. for 3 hours. The rate of ignition and rupture was evaluated. The results are shown in Table 1.
[0074]
As is apparent from Table 1, the batteries A to O have a first heat treatment step in which heat treatment is performed in a low temperature environment with a charge depth of 15% to 30% and a charge depth of 50% to 100%. After that, the second heat treatment process in which heat treatment is performed in a high temperature environment is sequentially performed, so that a lithium secondary battery excellent in low temperature pulse discharge characteristics, low temperature charge characteristics, overcharge safety and heat resistance can be obtained. did it.
[0075]
Furthermore, it was confirmed that the same effect was obtained when the charge and discharge were performed after the first heat treatment step and then the second heat treatment step was performed as in the battery O.
[0076]
On the other hand, in the case of the battery 1, the charging depth at the first heat treatment step is too high, and in the case of the battery 3, the treatment temperature at the first heat treatment step is too high. The generation rate of the SEI film on the surface is increased, and a heterogeneous thick film is formed on the surface of the negative electrode active material. This heterogeneous film serves as a nucleus, and the film grows rapidly during the second heat treatment step. As a result, the internal resistance of the battery is increased, the low-temperature pulse discharge characteristics and the low-temperature charge characteristics are decreased, and the heat resistance at 150 ° C. is caused by the exothermic reaction between the lithium metal deposited on the negative electrode active material surface and the electrolyte solvent during low-temperature charge Is thought to have been reduced.
[0077]
In the case of the battery 2, since the charging depth during the first heat treatment step is too low, the decomposition of the electrolytic solution on the surface of the negative electrode active material is suppressed, and an SEI film capable of inserting lithium is effectively formed. It becomes difficult. Therefore, a locally inhomogeneous film is formed on the negative electrode active material surface during the second heat treatment step. As a result, the electrode reaction becomes non-uniform, and the low-temperature pulse discharge characteristics, the low-temperature charge characteristics, and the 150 ° C. heat resistance are considered to have decreased.
[0078]
In the case of the battery 4, the charge depth during the second heat treatment step is too high, and in the case of the battery 6, the treatment temperature during the second heat treatment step is too high, so that the surface of the positive electrode active material is deactivated. (The film formation on the surface of the positive electrode due to the oxidative decomposition product of the electrolyte solvent) proceeds significantly, and the SEI film on the surface of the negative electrode active material becomes extremely thick. As a result, the internal resistance of the battery is increased, the low-temperature pulse discharge characteristics and the low-temperature charge characteristics are lowered, and metallic lithium is precipitated during low-temperature charge, and the heat resistance at 150 ° C. is remarkably lowered.
[0079]
In the case of the battery 5, the charging depth at the second heat treatment step is too low, and in the case of the battery 7, the treatment temperature at the second heat treatment step is too low, so that the surface of the positive electrode active material is deactivated. As a result, the polarization balance between the positive electrode and the negative electrode was lost, and the low-temperature charge characteristics deteriorated, and the safety during overcharge and the 150 ° C. heat resistance were significantly reduced.
[0080]
In the case of the battery 8, since the second heat treatment step is not performed, inactivation of the surface of the positive electrode active material and formation of a uniform SEI film on the surface of the negative electrode active material are insufficient. As a result, it is considered that safety at the time of overcharge and heat resistance at 150 ° C. are remarkably lowered.
[0081]
In the case of the battery 9, since the first heat treatment process is not performed, in the case of the battery 10, the second heat treatment process is performed before the first heat treatment process. Before the impregnation is sufficiently performed, a locally non-uniform thick film is formed on the surface of the negative electrode active material. As a result, the electrode reaction becomes non-uniform, the low-temperature pulse discharge characteristics and the low-temperature charge characteristics deteriorate, and metal lithium is precipitated during low-temperature charge, resulting in a decrease in 150 ° C. heat resistance.
[0082]
【The invention's effect】
As described above, according to the lithium secondary battery and the manufacturing method thereof of the present invention, the charging depth is 15% to 30%. Below 30 ° C After the first heat treatment step in which heat treatment is performed in a low temperature environment and the charging depth is set to 50% to 100% 40 ℃ ~ 80 ℃ It includes at least a second heat treatment step for performing heat treatment in a high-temperature environment. By sequentially performing these steps, an inert film is formed on the surface of the positive electrode active material, and a homogeneous SEI film is formed on the surface of the negative electrode active material. Can be obtained, and a lithium secondary battery excellent in low-temperature pulse discharge characteristics, low-temperature charge characteristics, overcharge safety, and heat resistance can be provided.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view of a lithium secondary battery of the present invention.
[Explanation of symbols]
11 Battery case
12 Sealing plate
13 Positive current collector
14 Positive electrode plate
15 Separator
16 Negative electrode plate
17 Negative electrode current collector
18 Injection stopper

Claims (2)

A method for manufacturing a lithium secondary battery comprising a positive electrode and a negative electrode containing an active material capable of reversibly occluding and desorbing lithium ions, a separator, and a non-aqueous electrolyte, wherein the charging depth is 15% to 30% First heat treatment step of performing heat treatment in a low temperature environment of 30 ° C. or less in a state and second heat treatment step of performing heat treatment in a high temperature environment of 40 ° C. to 80 ° C. after setting the charging depth to 50% to 100% A method for manufacturing a lithium secondary battery, comprising: A lithium secondary battery, wherein an inert film is formed on the surface of the positive electrode active material and a homogeneous SEI film is formed on the surface of the negative electrode active material by the manufacturing method according to claim 1 .

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