JP4843833B2 - Method for improving low temperature characteristics of lithium secondary battery - Google Patents

Description

【００５３】
〈低温における内部抵抗と入出力特性の評価〉
上記各実施例および比較例の処理方法により低温特性改善処理を行ったリチウム二次電池をそれぞれ以下、実施例１のリチウム二次電池等と表す。各実施例および比較例のリチウム二次電池について、−３０℃における内部抵抗と入出力とを測定した。以下に、−３０℃における内部抵抗と入出力の測定方法を説明する。
【００５４】
各実施例および比較例のリチウム二次電池の基準容量を１時間で放電するために必要な電流を１時間率（１Ｃ）とした。各リチウム二次電池の基準容量の３０％まで充電した状態（ＳＯＣ３０％）で、雰囲気温度を−３０℃とし、０．１Ｃで１０秒間放電させ、１０秒目の電圧を測定した。次いで０．３Ｃで１０秒間、１Ｃで１０秒間、３Ｃで１０秒間、１０Ｃで１０秒間放電させ、各１０秒目の電圧を測定した。同様の手順で充電も行い、各１０秒目の電圧を測定した。そして、電圧の電流依存性を求め、電流−電圧直線の勾配を内部抵抗とした。さらに放電側の電流−電圧直線と下限電圧（３Ｖ）とで囲まれる３角形の面積を出力（Ｗ）、充電側の電流−電圧直線と上限電圧（４．１Ｖ）とで囲まれる３角形の面積を入力（Ｗ）とした。
【００５５】
なお、２５℃における内部抵抗と入出力についても、上記−３０℃における内部抵抗と入出力の測定方法において、充放電を行う際の雰囲気温度を２５℃とした以外はその測定方法と同様の方法で測定した。２５℃における内部抵抗の値は充放電の充電上限電圧を変えてもほとんど変化しなかった。
【００５６】
ここで、各実施例および比較例のリチウム二次電池の−３０℃における内部抵抗の値を「−３０℃内部抵抗」、２５℃における内部抵抗の値を「２５℃内部抵抗」と示す。そして、「−３０℃内部抵抗」／「２５℃内部抵抗」を計算して、内部抵抗比とした。また、各実施例および比較例のリチウム二次電池の−３０℃における入出力の値を「−３０℃入出力」、２５℃における入出力の値を「２５℃入出力」と示す。そして、「−３０℃入出力」／「２５℃入出力」を計算して、入出力比とした。
【００５７】
図２に低温特性改善処理における充電上限電圧と内部抵抗比との関係を示す。図２から明らかなように、通常使用上限電圧である４．１Ｖより高い電圧を充電上限電圧とする充放電を行った各実施例のリチウム二次電池は、いずれも内部抵抗比が減少し、−３０℃における内部抵抗が減少していることがわかる。この内部抵抗の減少は、充電上限電圧を高くしたことによって、充放電反応においてより多くのリチウムイオンが脱離・挿入し、その結果、活物質が割れて新たに反応に関与する活物質の表面積が増加し、反応が活性化したためと考えられる。
【００５８】
また、通常使用上限電圧である４．１Ｖより０．２Ｖ高い電圧以上の電圧を充電上限電圧とする充放電を行った実施例２〜５の各リチウム二次電池は、より大きく−３０℃における内部抵抗が減少することが確認できた。
【００５９】
一方、通常使用上限電圧である４．１Ｖより０．４Ｖ高い電圧以上の電圧を充電上限電圧とする充放電を行い、充電上限電圧が４．５Ｖ以上である実施例５および実施例６のリチウム二次電池では、−３０℃における内部抵抗は若干増加した。これは、この電圧領域では電解液の分解が始まったためと考えられる。したがって、通常使用上限電圧より０．４Ｖ高い電圧未満の電圧を充電上限電圧とする充放電を行うことがより望ましいことが確認された。
【００６０】
なお、内部抵抗比が最も低い値となった、実施例２〜４のリチウム二次電池のＬｉ／Ｌｉ⁺に対する正極電位は、表１に示した通り、４．３Ｖ以上４．５Ｖ未満の範囲となっていることから、充電上限電圧は正極電位がＬｉ／Ｌｉ⁺の電位に対して４．３Ｖ以上４．５Ｖ未満となるような電圧とすることがより望ましいことが確認された。
【００６１】
次に、低温特性改善処理における充電上限電圧と入出力比との関係を図３に示す。図３から明らかなように、通常使用上限電圧である４．１Ｖより高い電圧を充電上限電圧とする充放電を行った各実施例のリチウム二次電池は、いずれも入出力比が増加し、−３０℃における入出力特性が向上していることがわかる。これは、上述したように、充電上限電圧を高くしたことによってより多くのリチウムイオンが脱離・挿入し、活物質が割れて、新たに反応に関与する活物質の表面積が増加したためと考えられる。
【００６２】
また、通常使用上限電圧である４．１Ｖより０．２Ｖ高い電圧以上の電圧を充電上限電圧とする充放電を行った実施例２〜４の各リチウム二次電池は、より大きく−３０℃における入出力が増加することが確認できた。
【００６３】
一方、通常使用上限電圧である４．１Ｖより０．４Ｖ高い電圧以上の電圧を充電上限電圧とする充放電を行い、充電上限電圧が４．５Ｖ以上である実施例５および実施例６のリチウム二次電池では、−３０℃における入出力は減少した。これは、この電圧領域では電解液の分解が始まったためと考えられる。したがって、通常使用上限電圧より０．４Ｖ高い電圧未満の電圧を充電上限電圧とする充放電を行うことがより望ましいことが確認された。
【００６４】
なお、入出力比が高い値となった、実施例２〜４のリチウム二次電池のＬｉ／Ｌｉ⁺に対する正極電位は、表１に示した通り、４．３Ｖ以上４．５Ｖ未満の範囲となっていることから、充電上限電圧は正極電位がＬｉ／Ｌｉ⁺の電位に対して４．３Ｖ以上４．５Ｖ未満となるような電圧とすることがより望ましいことが確認された。
【００６５】
〈サイクル耐久試験後の低温における電池の内部抵抗と入出力特性の評価〉
上記各実施例および比較例のリチウム二次電池について、−３０℃における内部抵抗および入出力特性のサイクル耐久性を調べるため、サイクル耐久試験を行った。サイクル耐久試験の方法を以下に説明する。
【００６６】
各リチウム二次電池を、充電終止電圧４．１Ｖ、放電終止電圧３Ｖで充放電した。充放電は、定電流充電−定電流放電方式で行った。電流密度は、充放電ともに１ｍＡ／ｃｍ² で、充放電の雰囲気温度は６０℃とした。この充放電を１サイクルとし、計５００サイクルの耐久試験を実施した。そして、５００サイクルの耐久試験後に、各リチウム二次電池について上記と同様の方法により、−３０℃における内部抵抗と入出力を測定した。
【００６７】
上記サイクル耐久試験後の、各実施例および比較例のリチウム二次電池の−３０℃における内部抵抗の値を「サイクル後−３０℃内部抵抗」と示し、先に測定したサイクル耐久試験を行っていないものとの比、すなわち、「サイクル後−３０℃内部抵抗」／「−３０℃内部抵抗」を計算して「サイクル後内部抵抗比」とした。また、上記サイクル耐久試験後の、各実施例および比較例のリチウム二次電池の−３０℃における入出力の値を「サイクル後−３０℃入出力」と示し、先に測定したサイクル耐久試験を行っていないものとの比、すなわち、「サイクル後−３０℃入出力」／「−３０℃入出力」を計算して「サイクル後入出力比」とした。
【００６８】
低温特性改善処理における充電上限電圧とサイクル後内部抵抗比との関係を図４に示す。図４から明らかなように、充電上限電圧が４．４５Ｖ以下の電池は、その内部抵抗の上昇が１５％以下でほぼ一定であり、サイクル試験後においても上述した内部抵抗の減少が維持されていることがわかった。一方、充電上限電圧が４．５Ｖ以上になると、内部抵抗の上昇は６０％以上となった。
【００６９】
次に、低温特性改善処理における充電上限電圧とサイクル後入出力比との関係を図５に示す。図５から明らかなように、充電上限電圧が４．４５Ｖ以下の電池は、その入出力の低下は２０％以下でほぼ一定であり、サイクル試験後においても上述した入出力の増加が維持されていることがわかった。一方、充電上限電圧が４．５Ｖ以上になると入出力の低下は４０％以上となった。
【００７０】
以上より、通常使用上限電圧より高い電圧を充電上限電圧とする充放電を行って低温特性が向上したリチウム二次電池は、サイクル耐久性も良好であることが確認できた。一方、低温特性改善処理における充電上限電圧を４．５Ｖ以上とすると、サイクル耐久性は低下することがわかった。これは充電上限電圧を４．５Ｖ以上として充放電を行うと、電解液が分解して正極活物質表面に被膜が形成されるためと考えられる。一旦形成された被膜は、その後に充放電を行う際に上限電圧を低くしても残存し、充放電を繰り返す間にその被膜を核として新たな被膜が成長するために、耐久性が低下したと考えられる。
【００７１】
【発明の効果】
本発明のリチウム二次電池の低温特性改善処理方法によれば、電池形成後に通常使用上限電圧より高い電圧を充電上限電圧とする充放電を行うことで、正極活物質粒子の反応に関与する表面積を増加し、充放電反応を活性化させて、リチウム二次電池の低温特性を大幅に向上することができる。
【００７２】
また、本発明のリチウム二次電池の低温特性改善処理方法によれば、電池形成後に通常使用上限電圧より高い電圧を充電上限電圧として充放電を行うだけで、低温特性を極めて簡便に向上することができる。
【図面の簡単な説明】
【図１】 リチウム二次電池の構造を模式的に示す図である。
【図２】 低温特性改善処理における充電上限電圧と内部抵抗比との関係を示す図である。
【図３】 低温特性改善処理における充電上限電圧と入出力比との関係を示す図である。
【図４】 低温特性改善処理における充電上限電圧とサイクル後内部抵抗比との関係を示す図である。
【図５】 低温特性改善処理における充電上限電圧とサイクル後入出力比との関係を示す図である。
【符号の説明】
１：リチウム二次電池 ２：電池ケース ３：正極 ４：負極
５：セパレータ ６：電極体 ７：正極リード ８：負極リード
９：キャップ １０：絶縁板[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a treatment method for improving the low temperature characteristics of a lithium secondary battery utilizing the phenomenon of insertion / extraction of lithium ions.
[0002]
[Prior art]
With the downsizing of mobile phones, personal computers, etc., secondary batteries with high energy density are required, and lithium secondary batteries have become widespread in the fields of communication equipment and information-related equipment. Further, due to resource problems and environmental problems, there is an increasing demand for electric vehicles in the field of automobiles, and there is an urgent need to develop lithium secondary batteries that are inexpensive, have large capacity, and have good cycle characteristics.
[0003]
Currently, lithium secondary battery positive electrode active materials include LiCoO having a layered rock salt structure as a lithium transition metal composite oxide.₂And LiNiO₂Or LiMn with spinel structure₂O_FourEtc. are known. These positive electrode active materials have already been put into practical use as those capable of constituting a 4V class secondary battery, or attempts have been made to put them into practical use.
[0004]
In general, the characteristics of a lithium secondary battery depend on the temperature. For example, the input / output characteristics are greatly deteriorated at temperatures below about 0 ° C. In particular, at a low temperature around −30 ° C., in the case of a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material, the output is 1% or less of the output at room temperature. This is a particularly serious problem when, for example, the lithium secondary battery is used as a power source for an electric vehicle. That is, an electric vehicle using the lithium secondary battery as a power source is difficult to start an engine or a motor in an extremely cold region.
[0005]
[Problems to be solved by the invention]
The decrease in input / output characteristics and large current characteristics at low temperatures of lithium secondary batteries is thought to be due to the decrease in the activity of lithium ion desorption / insertion reactions, which are battery charge / discharge reactions at low temperatures. . When the activity of the lithium ion desorption / insertion reaction is reduced, the utilization factor of the active material is reduced, and a sufficient capacity cannot be obtained. This decrease in reaction activity can be considered as a phenomenon of an increase in the internal resistance of the battery.
[0006]
As a method for suppressing the decrease in the reaction activity, when attention is paid to the positive electrode of the battery, it is effective to increase the effective area of the positive electrode, that is, the surface area involved in the reaction of the positive electrode active material particles. When a lithium transition metal composite oxide is used as an active material, the positive electrode is obtained by mixing a conductive material and a binder with a powdered active material to obtain a paste-like positive electrode mixture. It is formed by coating or the like. Therefore, by increasing the surface area involved in the reaction of the particles of the positive electrode active material, the lithium ion desorption / insertion reaction, which is the charge / discharge reaction of the battery, can be performed smoothly and the reaction can be activated. Conceivable.
[0007]
The present invention has been made from such a viewpoint, and is a method for improving the low temperature characteristics of a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material, wherein the surface area involved in the reaction of the positive electrode active material particles is reduced. The purpose is to provide a simple method for improving low temperature characteristics such as input / output characteristics and large current characteristics at low temperatures by increasing and activating charge / discharge reactions.
[0008]
[Means for Solving the Problems]
  The method for improving low temperature characteristics of a lithium secondary battery according to the present invention is a method for improving low temperature characteristics of a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material and a carbon material not containing silicon as a negative electrode active material. There,The lithium transition metal composite oxide has a basic composition of LiNiO. ₂ It is a layered rock salt structure lithium nickel composite oxide andAfter battery formation, the voltage is 0.2V higher than the normal use upper limit voltage of the batteryAnd less than 0.4V higher voltageCharging / discharging is performed using the above voltage as the charge upper limit voltage. Here, the “normal use upper limit voltage” means an upper limit voltage in a voltage range that can be reversibly charged and discharged without greatly impairing the characteristics of the battery.
[0009]
In a positive electrode of a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material, lithium ions are desorbed from the lithium transition metal composite oxide in a charging reaction, and lithium ions are inserted in a discharging reaction. As the lithium ions are desorbed and inserted in the charge / discharge reaction, the crystal lattice of the active material particles expands and contracts. For example, when lithium nickel composite oxide is used as the lithium transition metal composite oxide, the lattice volume shrinks by 5% or more when 60% or more of the total lithium is desorbed in the charging reaction.
[0010]
When the crystal lattice of the active material particles expands and contracts, distortion occurs inside the particles, and cracks occur in the active material particles that cannot withstand the distortion. As a result, it is considered that a large number of new active material surfaces that can participate in the reaction, that is, new surfaces are generated, and the surface area involved in the reaction of the active material particles is greatly increased.
[0011]
The active material particles are cracked when a certain amount or more of lithium is desorbed and inserted in the charge / discharge reaction. The amount of lithium desorbed is controlled by the upper limit potential of the battery. In other words, by increasing the upper limit potential of the battery, the amount of lithium desorption increases, resulting in the active material cracking and generating many new surfaces, greatly increasing the surface area involved in the reaction of the active material particles. It is thought that it can be made.
[0012]
In order to increase the surface area involved in the reaction of the positive electrode active material particles, a lithium transition metal composite oxide having a small particle diameter may be used as the active material. However, when a lithium transition metal composite oxide having a small particle size is used as the active material, the surface of the active material is covered with the conductive material, the binder, or the gas adsorbed on the surface during the synthesis of the active material. The surface area involved in the reaction of the material particles does not increase so much.
[0013]
Therefore, the method for improving the low temperature characteristics of the lithium secondary battery of the present invention is a surface area involved in the reaction of the positive electrode active material particles by performing charge and discharge with a voltage higher than the normal use upper limit voltage after the battery formation. And the charge / discharge reaction is activated to significantly improve the input / output characteristics at low temperatures.
[0014]
In addition, the method for improving the low temperature characteristics of the lithium secondary battery of the present invention activates the charge / discharge reaction and decreases the internal resistance of the battery, so that the capacity is reduced even when the battery is charged / discharged at a high current density at a low temperature. This is a processing method that improves a large current characteristic (rate characteristic) at a low temperature, that is, at a low temperature.
[0015]
Furthermore, the method for improving the low temperature characteristics of the lithium secondary battery of the present invention is such that, after the battery is formed, charging / discharging is performed using a voltage higher than the normal use upper limit voltage as a charge upper limit voltage. This is a very simple processing method that can greatly improve the low temperature characteristics.
[0016]
Furthermore, the method for improving the low temperature characteristics of the lithium secondary battery according to the present invention is, as will be shown in a later experiment, the low temperature characteristics of the improved lithium secondary battery are the cycle durability test unless the charge upper limit voltage is set too high. Since it is maintained later, it becomes a processing method for improving the low temperature characteristics without deteriorating the cycle durability by optimizing the upper limit voltage for charging.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the method for improving the low-temperature characteristics of a lithium secondary battery according to the present invention will be described in the order of the configuration of the target lithium secondary battery and the method for improving the low-temperature characteristics.
[0018]
<Configuration of lithium secondary battery>
Generally, a lithium secondary battery includes a positive electrode and a negative electrode that occlude and release lithium ions, a separator that is sandwiched between the positive electrode and the negative electrode, a non-aqueous electrolyte that moves lithium ions between the positive electrode and the negative electrode, The lithium secondary battery to which the low-temperature characteristic improving treatment method of the present invention is applied also follows this configuration. Hereinafter, each component will be described.
[0019]
For the positive electrode, a conductive material and a binder are mixed with a positive electrode active material capable of occluding and releasing lithium ions, and an appropriate solvent is added as necessary to obtain a paste-like positive electrode mixture, which is a metal foil such as aluminum. It can be formed by applying and drying on the surface of the current collector, and then increasing the active material density by pressing. In addition, what is necessary is just to follow the application | coating, drying, press, etc. to the collector surface of a positive electrode compound material.
[0020]
  In the lithium secondary battery targeted by the method for improving low temperature characteristics of the present invention, a lithium transition metal composite oxide is used as the positive electrode active material. As a lithium transition metal complex oxide, for example, a basic composition can be formed from the viewpoint that a 4V class secondary battery can be constituted.LiNiO₂Layered rock salt structureNoTitanium nickel composite oxidationThingsUseThe
[0021]
The basic composition means a representative composition of each of the above complex oxides. In addition to those represented by the above composition formula, for example, lithium sites and transition metal sites may be included in one or more other types. Also includes compositions such as those partially substituted with elements. In addition, it is not necessarily limited to a stoichiometric composition, for example, a non-stoichiometric composition in which a cation element such as Li or Ni that is inevitably produced in production is deficient or oxygen element is deficient. Including things. Furthermore, one type of lithium transition metal composite oxide can be used, or two or more types can be mixed and used.
[0022]
  In particular, LiCoO₂Lower price, LiMn₂O_FourIn addition to the excellent cycle characteristics at high temperatures compared to the above, the change in the lattice volume due to lithium desorption / insertion is large in the charge / discharge reaction, so it is easy to crack, and many new active material surfaces that are involved in the reaction are generated. The basic composition is LiNiO because it is easy₂Using lithium nickel composite oxide with layered rock salt structureThe
[0023]
The conductive material used for the positive electrode is for ensuring the electron conductivity of the positive electrode active material layer, and is a mixture of one or more carbon material powders such as carbon black, acetylene black, and graphite. Can be used. The binder plays a role of anchoring the active material particles, and a fluorine-containing resin such as polytetrafluoroethylene, polyvinylidene fluoride, and fluororubber, and a thermoplastic resin such as polypropylene and polyethylene can be used. An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent for dispersing these active material, conductive material, and binder.
[0025]
The negative electrodeA carbon material that can absorb and desorb lithium ions is used for the negative electrode active material.TeCompletionDo. Examples of the carbon material that can be used include natural or artificial graphite, a fired organic compound such as phenol resin, and a powder such as coke.AndThe negative electrode mixture, which is made by mixing a binder with the negative electrode active material and adding an appropriate solvent to form a paste, is applied to the surface of a metal foil current collector such as copper, dried, and then pressed. be able to. Application, drying, pressing, etc. in this caseIsWhat is necessary is just to follow a normal method.
[0026]
When a carbon material is used as the negative electrode active material, a fluorine-containing resin such as polyvinylidene fluoride can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent.
[0027]
The separator sandwiched between the positive electrode and the negative electrode retains the electrolytic solution while isolating the positive electrode and the negative electrode and allows ions to pass through. A thin microporous film such as polyethylene or polypropylene can be used.
[0028]
The non-aqueous electrolyte is obtained by dissolving an electrolyte in an organic solvent. Examples of the organic solvent include aprotic organic solvents such as ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, acetonitrile, dimethoxyethane, and tetrahydrofuran. , Dioxolane, methylene chloride or the like, or a mixture of two or more thereof can be used. Further, as the electrolyte to be dissolved, LiI and LiClO that generate lithium ions when dissolved are used._Four, LiAsF₆, LiBF_Four, LiPF₆Etc. can be used.
[0029]
In place of the separator and the non-aqueous electrolyte, a high molecular weight polymer such as polyethylene oxide and LiClO are used._FourAnd LiN (CF_ThreeSO₂)₂It is also possible to use a solid polymer electrolyte using a lithium salt such as a gel electrolyte, and a gel electrolyte obtained by trapping the nonaqueous electrolytic solution in a solid polymer matrix such as polyacrylonitrile.
[0030]
Although it is a lithium secondary battery comprised from the above, the shape can be made into various things, such as a coin type, a laminated type, and a cylindrical type. Regardless of which shape is adopted, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the electrode body is electrically connected between the positive electrode and the negative electrode to the positive electrode terminal and the negative electrode terminal. Can be sealed in a battery case together with a non-aqueous electrolyte to complete the battery.
[0031]
<Low temperature characteristics improvement treatment method>
The method for improving the low-temperature characteristics of the lithium secondary battery of the present invention is a method of performing charge / discharge with a voltage higher than the normal use upper limit voltage of the battery as a charge upper limit voltage after the battery is formed. Hereinafter, the charging / discharging in the low temperature characteristic improving method of the present invention is referred to as a low temperature characteristic improving process.
[0032]
The low temperature characteristic improving process is performed after the battery is formed, and can be performed, for example, after the battery is completed according to the above embodiment. Moreover, you may carry out in the state which formed the positive electrode and negative electrode which comprise a battery, the electrode body was immersed in electrolyte solution in the stage which put the separator between them and used as the electrode body. In consideration of the fact that the positive and negative electrode potentials shift to the higher potential side during repeated charging and discharging, the low temperature characteristic improving treatment is particularly in the initial stage after battery formation, that is, before using the battery. It is desirable to do this in stages.
[0033]
As described above, the normal use upper limit voltage is the upper limit voltage of the operating voltage range of the battery, which is determined as a voltage range that can be reversibly charged and discharged without greatly impairing the characteristics of the battery. Is different. For example, in the case of a battery using a lithium cobalt composite oxide or a lithium nickel composite oxide that can constitute the above-described 4V class battery as the positive electrode active material and a carbon material as the negative electrode active material, the operating voltage range is Since it is about 3 to 4.1 V, the upper limit voltage is about 4.1 V. Further, the charge upper limit voltage means a charge end voltage in the low temperature characteristic improving process.
[0034]
  Charging in the low temperature characteristic improving process may be performed with a voltage higher than the normal use upper limit voltage as the charge upper limit voltage. In particular, from the viewpoint that the surface area involved in the reaction of the active material particles can be significantly increased by generating more active material surfaces that can be involved in the reaction by causing more cracks in the positive electrode active material. The upper limit voltage for charging is a voltage that is 0.2 V higher than the upper limit voltage for normal use.The
[0035]
  In addition, when the above-described lithium cobalt composite oxide, lithium nickel composite oxide, or the like is used as the positive electrode active material to form a 4V class battery, if the positive electrode potential becomes too high, the electrolyte is decomposed and the battery There is a possibility that a problem that durability is lowered and a problem that a large current characteristic is lowered when a product generated by decomposition of the electrolytic solution covers the electrode. Further, as is understood from the experiment shown later, when the positive electrode potential becomes too high, there arises a problem that the low temperature characteristics improved by performing the low temperature characteristic improvement treatment deteriorate after the cycle durability test. From this point of view, the charging upper limit voltage is set to a voltage less than 0.4 V higher than the normal use upper limit voltage.The
[0036]
More specifically, the basic composition of the positive electrode active material is LiNiO.₂When the layered rock salt structure lithium nickel composite oxide is used, the charge upper limit voltage is such that the positive electrode potential is Li / Li⁺It is desirable that the voltage be 4.3 V or more and less than 4.5 V with respect to the potential. That is, since the voltage of a battery is a potential difference between positive and negative electrodes, it changes with the combination of the active material used for the positive electrode and negative electrode which comprise a battery. Thus, for example, the basic composition is LiNiO as a positive electrode active material.₂In the lithium secondary battery using the layered rock salt structure lithium nickel composite oxide to be used, the upper limit charging voltage is within the above range when expressed by the potential of the positive electrode alone, regardless of the type of the active material of the negative electrode. It is desirable to do. Li / Li⁺The positive electrode potential can be measured using a simulated battery with a Li reference electrode.
[0037]
The charging / discharging method may be a general power source that can control the upper and lower limits of the current density and voltage at the time of charging / discharging. For example, charging is performed until a predetermined voltage is reached with a constant current, and after charging is completed It is possible to charge / discharge with a constant current charge-constant current discharge method, which is discharged until a predetermined voltage is reached with a current, or after charging until a predetermined voltage is reached with a constant current, the voltage is maintained and predetermined It is possible to charge and discharge by a constant current constant voltage charge-constant current discharge method in which the battery is charged for a predetermined time and discharged until a predetermined voltage is reached with a constant current after the end of charging.
[0038]
The current density at the time of charging is not particularly limited, and is 0.01 to 2 mA / cm.²As long as it is charged. Similarly, the current density during discharge is 0.01 to 2 mA / cm.²As a discharge.
[0039]
The discharge only needs to be performed until a predetermined voltage is reached. In particular, from the viewpoint that a larger capacity can be obtained, it is desirable that the lower limit voltage of the battery operating voltage range is the discharge end voltage.
[0040]
The temperature at which the low temperature characteristic improving treatment is performed is not particularly limited, and may be, for example, 0 to 60 ° C. Moreover, it is desirable to perform the low temperature characteristic improving treatment 1 to 5 times because the new surface of the active material is sufficiently generated and the electrolytic solution is hardly decomposed.
[0041]
In addition, the configuration of the lithium secondary battery and the embodiment of the low temperature characteristics improvement processing method described so far are merely examples, and the low temperature characteristics improvement processing method of the lithium secondary battery of the present invention includes the above embodiment, The present invention can be implemented in various modifications and improvements based on the knowledge of those skilled in the art.
[0042]
【Example】
Based on the above embodiment, a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material is produced, and the low-temperature characteristics improvement treatment is performed to measure the internal resistance and input / output characteristics of the lithium secondary battery. did. Hereinafter, formation of a lithium secondary battery, low temperature characteristic improvement processing, evaluation of internal resistance and input / output characteristics of the battery at low temperatures, and evaluation of internal resistance and input / output characteristics of the battery at low temperatures after the cycle durability test will be described.
[0043]
<Formation of lithium secondary battery>
The structure of the lithium secondary battery formed in this example is schematically shown in FIG. In FIG. 1, a lithium secondary battery 1 has an electrode body 6 configured by winding a sheet-like positive electrode 3 and a sheet-like negative electrode 4 in a spiral shape via a separator 5 in a battery case 2. It is a thing. The positive electrode lead 7 connected to the winding center of the sheet-like positive electrode 3 is connected to the cap 9 attached to the battery case 2, and the negative electrode lead 8 connected to the outer periphery of the sheet-like negative electrode 4. Is connected to the battery case 2. Insulating plates 10 are respectively attached to the inner bottom surface of the battery case 2 and the upper portion of the electrode body 6.
[0044]
The sheet-like positive electrode 3 has LiNi as a positive electrode active material._0.8Co_0.15Al_0.05O₂Formed using. First, the active material LiNi_0.8Co_0.15Al_0.05O₂90 parts by weight, 5 parts by weight of graphite as a conductive material and 5 parts by weight of polyvinylidene fluoride as a binder, N-methyl-2-pyrrolidone as a solvent is added, kneaded and pasted positive electrode The mix was adjusted. Next, this positive electrode mixture was applied to both surfaces of an aluminum foil current collector having a thickness of 15 μm, dried, and roll-pressed to obtain a sheet-like positive electrode 3. The size of the positive electrode 3 was 45 mm × 900 mm, and the coating thickness after the dry pressing of the positive electrode mixture was 30 μm per side.
[0045]
The sheet-like negative electrode 4 was formed using artificial graphite as a negative electrode active material. First, 95 parts by weight of artificial graphite as an active material is mixed with 5 parts by weight of polyvinylidene fluoride as a binder, N-methyl-2-pyrrolidone is added as a solvent, and the mixture is kneaded and paste-like negative electrode mixture Adjusted. Next, this negative electrode mixture was applied to both sides of a 10 μm thick copper foil current collector, dried, and roll-pressed to obtain a sheet-like negative electrode 4. The size of the negative electrode 4 was 49 mm × 920 mm, and the coating thickness of the negative electrode mixture after dry pressing was 35 μm per side.
[0046]
The positive electrode 3 and the negative electrode 4 were wound by sandwiching a polyethylene separator 5 having a thickness of 25 μm and a width of 52 mm therebetween to form a roll-shaped electrode body 6. An insulating plate 10 was mounted under the electrode body 6, and the electrode body 6 was inserted into the battery case 2. Then, the negative electrode lead 8 was connected to the battery case 2, the insulating plate 10 was mounted on the electrode body 6, and a non-aqueous electrolyte was injected. The non-aqueous electrolyte is LiPF in a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a volume ratio of 3: 7.₆Was dissolved at a concentration of 1M. And after connecting the positive electrode lead 7 to the cap 9, it sealed with the cap 9, and the lithium secondary battery 1 was formed.
[0047]
<Low temperature characteristics improvement treatment>
Using the completed lithium secondary battery, the low-temperature characteristic improvement treatment was performed by changing the charging upper limit voltage. In addition, since the operating voltage range of the lithium secondary battery used by this process was 3 to 4.1V from the result of the charging / discharging test done preliminary, the normal use upper limit voltage was set to 4.1V. Below, the processing method of each Example and a comparative example is demonstrated.
[0048]
  (1)testProcessing method of Example 1
  As the low temperature characteristic improving treatment, charging / discharging was performed 5 times in total. The first round is also used for battery conditioning, with a current density of 0.25 mA / cm.²After charging until the voltage reached 4.2V at a constant current of, charging was further performed at a constant voltage of 4.2V. The total charging time was 6 hours. Then, current density 0.2mA / cm²The battery was discharged until a voltage of 3 V was reached at a constant current of. In the charge and discharge from the second time to the fifth time, as in the first time, constant current and constant voltage charge-constant current discharge was performed at a charge end voltage of 4.2 V and a discharge end voltage of 3 V. However, the current density is 1 mA / cm for both charge and discharge.²The charging time was 2 hours in total. The atmosphere temperature for these five charge / discharge cycles was 25 ° C. Further, the discharge capacity at the time of the fifth charge / discharge was used as the reference capacity.
[0049]
  (2) Processing method of Examples 2-6
  Except for changing the maximum charging voltage,testCharge / discharge was performed in the same manner as in Example 1. Here, each charge upper limit voltage was 4.3 V in Example 2, 4.4 V in Example 3, 4.45 V in Example 4, 4.5 V in Example 5, and 4.6 V in Example 6. .
[0050]
  (3) Treatment method of Comparative Examples 1 to 3
  Except for changing the maximum charging voltage,testCharge / discharge was performed in the same manner as in Example 1. Here, each charge upper limit voltage was set to 3.9 V in Comparative Example 1, 4.0 V in Comparative Example 2, and 4.1 V in Comparative Example 3.
  The charging upper limit voltages in the processing methods of the respective examples and comparative examples are collectively shown in Table 1. In Table 1, Li / Li⁺The positive electrode potential was obtained from measurement of a simulated battery with a Li reference electrode.
[0051]
The charging upper limit voltages in the processing methods of the respective examples and comparative examples are collectively shown in Table 1. In Table 1, Li / Li⁺The positive electrode potential was obtained from measurement of a simulated battery with a Li reference electrode.
[0052]
[Table 1]

[0053]
<Evaluation of internal resistance and input / output characteristics at low temperatures>
The lithium secondary batteries that have been subjected to the low-temperature characteristic improving process by the processing methods of the above Examples and Comparative Examples will be hereinafter referred to as the lithium secondary battery of Example 1 and the like. About the lithium secondary battery of each Example and the comparative example, the internal resistance and input / output in -30 degreeC were measured. Hereinafter, a method for measuring internal resistance and input / output at −30 ° C. will be described.
[0054]
The current required for discharging the reference capacity of the lithium secondary battery of each Example and Comparative Example in 1 hour was set to 1 hour rate (1C). With the battery charged to 30% of the standard capacity of each lithium secondary battery (SOC 30%), the ambient temperature was set to −30 ° C., the battery was discharged at 0.1 C for 10 seconds, and the voltage at 10 seconds was measured. Next, the battery was discharged at 0.3 C for 10 seconds, 1 C for 10 seconds, 3 C for 10 seconds, and 10 C for 10 seconds, and the voltage at each 10 second was measured. Charging was performed in the same procedure, and the voltage at the 10th second was measured. Then, the current dependency of the voltage was obtained, and the slope of the current-voltage straight line was defined as the internal resistance. Further, the area of the triangle surrounded by the current-voltage line on the discharge side and the lower limit voltage (3V) is output (W), and the area of the triangle surrounded by the current-voltage line on the charge side and the upper limit voltage (4.1V) The area was taken as input (W).
[0055]
As for the internal resistance and input / output at 25 ° C., the measurement method of the internal resistance and input / output at −30 ° C. is the same as the measurement method except that the ambient temperature when charging / discharging is 25 ° C. Measured with The value of the internal resistance at 25 ° C. hardly changed even when the charge upper limit voltage of charge / discharge was changed.
[0056]
Here, the value of the internal resistance at −30 ° C. of the lithium secondary batteries of Examples and Comparative Examples is indicated as “−30 ° C. internal resistance”, and the value of the internal resistance at 25 ° C. is indicated as “25 ° C. internal resistance”. Then, “−30 ° C. internal resistance” / “25 ° C. internal resistance” was calculated as the internal resistance ratio. In addition, the input / output values at −30 ° C. of the lithium secondary batteries of the examples and comparative examples are indicated as “−30 ° C. input / output”, and the input / output values at 25 ° C. are indicated as “25 ° C. input / output”. Then, “−30 ° C. input / output” / “25 ° C. input / output” was calculated and used as the input / output ratio.
[0057]
FIG. 2 shows the relationship between the charging upper limit voltage and the internal resistance ratio in the low temperature characteristic improving process. As is clear from FIG. 2, the lithium secondary batteries of the respective examples that were charged / discharged with a voltage higher than the normal use upper limit voltage of 4.1 V as the charge upper limit voltage were all reduced in internal resistance ratio, It can be seen that the internal resistance at −30 ° C. decreases. This decrease in internal resistance is caused by increasing the charge upper limit voltage, so that more lithium ions are desorbed / inserted in the charge / discharge reaction, and as a result, the active material breaks and the surface area of the active material newly involved in the reaction is increased. This is probably because the reaction was activated.
[0058]
Moreover, each lithium secondary battery of Examples 2-5 which performed charging / discharging which used the voltage more than the voltage 0.2V higher than normal use upper limit voltage 4.1V as a charge upper limit voltage is larger at -30 degreeC. It was confirmed that the internal resistance decreased.
[0059]
On the other hand, the lithium of Example 5 and Example 6 in which charge / discharge is performed with a voltage higher than the normal use upper limit voltage of 4.1V by 0.4V or more as the charge upper limit voltage, and the charge upper limit voltage is 4.5V or higher. In the secondary battery, the internal resistance at −30 ° C. slightly increased. This is presumably because the decomposition of the electrolyte started in this voltage region. Therefore, it was confirmed that it is more desirable to perform charging / discharging which uses a voltage less than 0.4V higher than the normal use upper limit voltage as a charge upper limit voltage.
[0060]
In addition, Li / Li of the lithium secondary battery of Examples 2 to 4 having the lowest internal resistance ratio⁺As shown in Table 1, the positive electrode potential is in the range of 4.3 V to less than 4.5 V.⁺It has been confirmed that it is more desirable to set the voltage to 4.3 V or more and less than 4.5 V with respect to the potential.
[0061]
Next, FIG. 3 shows the relationship between the charge upper limit voltage and the input / output ratio in the low temperature characteristic improving process. As is clear from FIG. 3, the lithium secondary batteries of the respective examples that were charged / discharged with a voltage higher than the normal use upper limit voltage of 4.1 V as the charge upper limit voltage had an increased input / output ratio. It can be seen that the input / output characteristics at −30 ° C. are improved. As described above, this is thought to be due to the fact that, by increasing the charging upper limit voltage, more lithium ions were desorbed / inserted, the active material cracked, and the surface area of the active material newly involved in the reaction increased. .
[0062]
Moreover, each lithium secondary battery of Examples 2-4 which performed charging / discharging which uses the voltage more than 0.2V higher than normal use upper limit voltage 4.1V as a charge upper limit voltage is larger at -30 degreeC. It was confirmed that input and output increased.
[0063]
On the other hand, the lithium of Example 5 and Example 6 in which charge / discharge is performed with a voltage higher than the normal use upper limit voltage of 4.1V by 0.4V or more as the charge upper limit voltage, and the charge upper limit voltage is 4.5V or higher. In the secondary battery, the input / output at −30 ° C. decreased. This is presumably because the decomposition of the electrolyte started in this voltage region. Therefore, it was confirmed that it is more desirable to perform charging / discharging which uses a voltage less than 0.4V higher than the normal use upper limit voltage as a charge upper limit voltage.
[0064]
In addition, Li / Li of the lithium secondary battery of Examples 2-4 with which the input-output ratio became a high value.⁺As shown in Table 1, the positive electrode potential is in the range of 4.3 V to less than 4.5 V.⁺It has been confirmed that it is more desirable to set the voltage to 4.3 V or more and less than 4.5 V with respect to the potential.
[0065]
<Evaluation of internal resistance and input / output characteristics of battery at low temperature after cycle endurance test>
For the lithium secondary batteries of the above Examples and Comparative Examples, a cycle durability test was conducted in order to investigate the cycle resistance of the internal resistance and input / output characteristics at −30 ° C. The cycle durability test method will be described below.
[0066]
Each lithium secondary battery was charged and discharged at a charge end voltage of 4.1 V and a discharge end voltage of 3 V. Charging / discharging was performed by a constant current charge-constant current discharge method. Current density is 1 mA / cm for both charge and discharge.²Thus, the charge / discharge ambient temperature was set to 60 ° C. This charging / discharging was made into 1 cycle, and the durability test of a total of 500 cycles was implemented. After 500 cycles of endurance test, the internal resistance and input / output at −30 ° C. were measured for each lithium secondary battery by the same method as described above.
[0067]
The value of the internal resistance at −30 ° C. of the lithium secondary batteries of the examples and comparative examples after the cycle endurance test is indicated as “−30 ° C. internal resistance after cycle”, and the cycle endurance test previously measured is performed. The ratio to the non-existing one, ie, “-30 ° C. internal resistance after cycle” / “− 30 ° C. internal resistance” was calculated as “post cycle internal resistance ratio”. In addition, after the cycle endurance test, the input / output value at −30 ° C. of each of the lithium secondary batteries of Examples and Comparative Examples is indicated as “−30 ° C. input / output after cycle”, and the cycle endurance test previously measured The ratio to the one not performed, that is, “30 ° C. input / output after cycle” / “− 30 ° C. input / output” was calculated as “input / output ratio after cycle”.
[0068]
FIG. 4 shows the relationship between the charge upper limit voltage and the cycle internal resistance ratio in the low temperature characteristic improving process. As is clear from FIG. 4, the battery whose upper limit of charge voltage is 4.45 V or less has an almost constant increase in internal resistance of 15% or less, and the above-described decrease in internal resistance is maintained even after the cycle test. I found out. On the other hand, when the charge upper limit voltage was 4.5 V or more, the increase in internal resistance was 60% or more.
[0069]
Next, FIG. 5 shows the relationship between the charge upper limit voltage and the post-cycle input / output ratio in the low temperature characteristic improving process. As is clear from FIG. 5, the battery whose charge upper limit voltage is 4.45 V or less has a substantially constant decrease in input / output of 20% or less, and the increase in input / output described above is maintained even after the cycle test. I found out. On the other hand, when the charge upper limit voltage was 4.5 V or more, the decrease in input / output was 40% or more.
[0070]
From the above, it was confirmed that the lithium secondary battery improved in low-temperature characteristics by performing charge / discharge using a voltage higher than the normal use upper limit voltage as the charge upper limit voltage has good cycle durability. On the other hand, it was found that when the charge upper limit voltage in the low temperature characteristic improving process is 4.5 V or more, the cycle durability is lowered. This is considered to be because when the charge upper limit voltage is 4.5 V or higher and the charge and discharge are performed, the electrolytic solution is decomposed and a film is formed on the surface of the positive electrode active material. Once formed, the film remains even if the upper limit voltage is lowered during charge / discharge, and a new film grows with the film as a core during repeated charge / discharge, resulting in decreased durability. it is conceivable that.
[0071]
【The invention's effect】
According to the method for improving the low-temperature characteristics of the lithium secondary battery of the present invention, the surface area involved in the reaction of the positive electrode active material particles by performing charge / discharge with a voltage higher than the normal use upper limit voltage as the charge upper limit voltage after the battery is formed. And the charge / discharge reaction can be activated to significantly improve the low-temperature characteristics of the lithium secondary battery.
[0072]
In addition, according to the method for improving the low temperature characteristics of the lithium secondary battery of the present invention, the low temperature characteristics can be improved extremely simply by performing charging / discharging with a voltage higher than the normal use upper limit voltage as the charge upper limit voltage after the battery is formed. Can do.
[Brief description of the drawings]
FIG. 1 is a diagram schematically showing the structure of a lithium secondary battery.
FIG. 2 is a diagram showing a relationship between a charge upper limit voltage and an internal resistance ratio in a low temperature characteristic improving process.
FIG. 3 is a diagram showing a relationship between a charge upper limit voltage and an input / output ratio in the low temperature characteristic improving process.
FIG. 4 is a diagram showing a relationship between a charge upper limit voltage and a cycle internal resistance ratio in a low temperature characteristic improving process.
FIG. 5 is a diagram showing the relationship between the charge upper limit voltage and the post-cycle input / output ratio in the low temperature characteristic improving process.
[Explanation of symbols]
1: Lithium secondary battery 2: Battery case 3: Positive electrode 4: Negative electrode
5: Separator 6: Electrode body 7: Positive electrode lead 8: Negative electrode lead
9: Cap 10: Insulating plate

Claims (2)

A method for improving low temperature characteristics of a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material and a carbon material not containing silicon as a negative electrode active material,
The lithium transition metal composite oxide is a layered rock salt structure lithium nickel composite oxide having a basic composition of LiNiO ₂ .
A low temperature of a lithium secondary battery, characterized in that after the battery is formed, charging / discharging is performed with a voltage that is 0.2 V higher than the normal use upper limit voltage of the battery and less than 0.4 V higher than the normal use upper limit voltage. Characteristic improvement processing method. Before SL charging upper limit voltage, low temperature properties improved processing method of a lithium secondary battery according to claim 1 is a voltage that the positive electrode potential is less than 4.3V or 4.5V relative to Li / Li ⁺ potential .

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