JP2001351686A - Method for improving low temperature characteristics of lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for improving low-temperature characteristics of a lithium secondary battery, in which the surface area involved in the reaction of the positive electrode active material particles is increased, and the charge / discharge reaction is activated to reduce the input at low temperature. Provided is a simple method for improving low-temperature characteristics such as output characteristics and large current characteristics. SOLUTION: The method for improving low temperature characteristics of a lithium secondary battery of 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. A method of performing charging and discharging in which a voltage higher than the normal use upper limit voltage of the battery is set as the charge upper limit voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a processing method for improving low-temperature characteristics of a lithium secondary battery using a phenomenon of inserting and extracting lithium ions.

[0002]

2. Description of the Related Art As mobile phones and personal computers become smaller,
Secondary batteries with high energy density are required, and lithium secondary batteries have come into widespread use in the fields of communication devices and information-related devices. In addition, demands for electric vehicles are increasing in the field of automobiles due to resource problems and environmental problems, and the development of lithium secondary batteries that are inexpensive, have large capacities, and have good cycle characteristics is urgently required.

At present, as a positive electrode active material of a lithium secondary battery, as a lithium transition metal composite oxide, LiCoO ₂ or LiNiO ₂ having a layered rock salt structure, LiMn ₂ O _{4 having a} spinel structure, or the like is 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.

In general, the characteristics of a lithium secondary battery depend on the temperature. For example, at a temperature around 0 ° C. or lower, the input / output characteristics are greatly reduced. In particular, at a low temperature of around −30 ° C., in the case of a lithium secondary battery using a lithium transition metal composite oxide as the positive electrode active material, the output is 1% or less of the output at room temperature. This is a serious problem particularly when, for example, the lithium secondary battery is used as a power source for an electric vehicle. That is, in an electric vehicle using the lithium secondary battery as a power source, it is difficult to start an engine or a motor in an extremely cold place.

[0005]

The deterioration of characteristics such as input / output characteristics and large current characteristics of a lithium secondary battery at a low temperature is as follows.
At low temperatures, it is considered that the cause is that the activity of the lithium ion desorption / insertion reaction, which is the charge / discharge reaction of the battery, decreases. When the activity of the lithium ion desorption / insertion reaction decreases, the utilization rate of the active material decreases, and a sufficient capacity cannot be obtained. This decrease in reaction activity can be regarded as a phenomenon of an increase in the internal resistance of the battery.

As a method for suppressing the reduction 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. Become. When the lithium transition metal composite oxide is used as the active material, the positive electrode is prepared by mixing a powdered active material with a conductive material and a binder to form a paste-like positive electrode mixture, and forming the mixture on the surface of the current collector. 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 desorption / insertion reaction of lithium ions, which is the charge / discharge reaction of the battery, can be smoothly performed, and the reaction can be activated. Conceivable.

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. It is an object of the present invention to provide a simple method for improving low-temperature characteristics such as input / output characteristics at low temperatures and large current characteristics by increasing a surface area to be used and activating a charge / discharge reaction.

[0008]

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. After forming the battery, the battery is charged and discharged with a voltage higher than the normal use upper limit voltage as a charging upper limit voltage. Here, the “normal use upper limit voltage” means the upper limit voltage in a voltage range in which the battery can be charged and discharged reversibly without significantly impairing the characteristics of the battery.

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. With the desorption / insertion of lithium ions in this charge / discharge reaction,
The crystal lattice of the active material particles expands and contracts. For example, when a lithium nickel composite oxide is used as the lithium transition metal composite oxide, when 60% or more of the total lithium is desorbed in the charging reaction, the lattice volume shrinks by 5% or more.

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 active material surfaces that can newly participate in the reaction, that is, new surfaces are generated, and the surface area of the active material particles involved in the reaction greatly increases.

[0011] The cracking of the active material particles is caused by the desorption / insertion of a certain amount or more of lithium in the charge / discharge reaction. The amount of desorbed lithium is controlled by the upper limit potential of the battery. In other words, increasing the upper limit potential of the battery increases the amount of lithium desorbed, resulting in cracks in the active material, generating many new surfaces, and greatly increasing the surface area involved in the reaction of the active material particles. It is thought that it can be done.

Incidentally, in order to increase the surface area involved in the reaction of the positive electrode active material particles, it is conceivable to use a lithium transition metal composite oxide having a small particle size as the active material. However, when a lithium transition metal composite oxide having a small particle size is used as the active material, the active material surface is covered with a conductive material, a binder, or a 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 significantly.

Therefore, the method for improving the low-temperature characteristics of a lithium secondary battery according to the present invention is characterized in that, after forming the battery, charge and discharge are performed with a voltage higher than the normal use upper limit voltage as a charge upper limit voltage. Increase the surface area involved,
This is a processing method that activates a charge / discharge reaction and greatly improves input / output characteristics at low temperatures.

Further, the method for improving the low-temperature characteristics of a lithium secondary battery according to the present invention is a method for activating a charge-discharge reaction and reducing the internal resistance of the battery. This is also a processing method in which a decrease in capacity is small, that is, a large current characteristic (rate characteristic) at a low temperature is improved.

Further, the method for improving the low-temperature characteristics of a lithium secondary battery according to the present invention only requires charging and discharging after setting the battery to a voltage higher than a normal use upper limit voltage as a charge upper limit voltage.
This is an extremely simple processing method that can significantly improve low-temperature characteristics such as input / output characteristics and large current characteristics at low temperatures.

Furthermore, as shown in a later experiment, the method for improving the low-temperature characteristics of a lithium secondary battery according to the present invention shows that the improved low-temperature characteristics of the lithium secondary battery can be improved as long as the charging upper limit voltage is not too high. Since the durability is maintained even after the durability test, by optimizing the charging upper limit voltage, a processing method in which low-temperature characteristics are improved without impairing cycle durability is achieved.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a method for improving low temperature characteristics of a lithium secondary battery according to the present invention will be described below in the order of the configuration of a target lithium secondary battery and the method for improving low temperature characteristics.

<Structure of Lithium Secondary Battery> In general, a lithium secondary battery has a positive electrode and a negative electrode that occlude and release lithium ions, a separator sandwiched between the positive electrode and the negative electrode, And a non-aqueous electrolyte solution for moving lithium ions. The lithium secondary battery to which the low-temperature characteristic improving method of the present invention is applied also complies with this configuration. Hereinafter, each component will be described.

The positive electrode is prepared by mixing a conductive material and a binder with a positive electrode active material capable of occluding and releasing lithium ions, adding an appropriate solvent as needed, and forming a paste-like positive electrode mixture into aluminum or the like. On the surface of the metal foil current collector,
It can be formed by drying and then increasing the active material density by pressing. The application of the positive electrode mixture to the surface of the current collector, drying, pressing, and the like may be performed according to an ordinary method.

In the lithium secondary battery to which the method for improving low-temperature characteristics of the present invention is applied, a lithium transition metal composite oxide is used as a positive electrode active material. As the lithium transition metal composite oxide, for example, from the viewpoint that a secondary battery of a 4V class can be configured, a lithium cobalt composite oxide or a lithium nickel composite oxide having a layered rock salt structure having a basic composition of LiCoO ₂ or LiNiO ₂ , Alternatively, the basic composition is LiMn ₂ O ₄
It is preferable to use a lithium manganese composite oxide having a spinel structure.

The basic composition means a typical composition of each of the above composite oxides. In addition to the one represented by the above-mentioned composition formula, for example, a lithium site or a transition metal site may be replaced by one or more other types. It also includes compositions such as those partially substituted with more than one kind of element. Further, the stoichiometric composition is not necessarily limited to the stoichiometric composition. For example, a non-stoichiometric composition in which a cation element such as Li or Ni which is inevitably produced in the manufacturing process is deficient, or an oxygen element is deficient. Including things. Further, one of the lithium transition metal composite oxides may be used, or two or more of them may be used in combination.

In particular, it is less expensive than LiCoO ₂ ,
In addition to being superior in cycle characteristics at high temperatures as compared with iMn ₂ O ₄ , a large change in lattice volume due to desorption / insertion of lithium in a charge / discharge reaction easily causes cracking,
It is preferable to use a lithium nickel composite oxide having a layered rock salt structure having a basic composition of LiNiO ₂ , because a large number of active material surfaces involved in the reaction are easily generated.

The conductive material used for the positive electrode is for ensuring the electron conductivity of the positive electrode active material layer, and is made of carbon material powder such as carbon black, acetylene black, and graphite.
A species or a mixture of two or more species can be used. The binder plays a role of binding the active material particles, and is made of polytetrafluoroethylene, polyvinylidene fluoride, a fluorine-containing resin such as fluororubber, polypropylene,
A thermoplastic resin such as polyethylene can be used.
An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent in which the active material, the conductive material, and the binder are dispersed.

The negative electrode is formed by forming metallic lithium as a negative electrode active material into a sheet in the same manner as that of a general battery, or by pressing the sheet into a collector net made of nickel, stainless steel or the like. . As the negative electrode active material, a lithium alloy or a lithium compound can be used instead of metal lithium.

As another form of the negative electrode, the negative electrode can be formed by using a carbon material capable of inserting and extracting lithium ions as the negative electrode active material. Examples of the carbon substance that can be used include natural or artificial graphite, fired organic compounds such as phenolic resins, and powders such as coke. In this case, a binder is mixed with the negative electrode active material, an appropriate solvent is added, and a paste of the negative electrode mixture is applied to the surface of a metal foil current collector such as copper, dried, and then pressed. Can be formed. In this case, coating, drying, pressing and the like may be performed according to a usual method.

When a carbon material is used as the negative electrode active material, a fluorine-containing resin such as polyvinylidene fluoride or the like is used as the negative electrode binder and an organic solvent such as N-methyl-2-pyrrolidone is used as the solvent, similarly to the positive electrode. Can be.

The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode from the negative electrode, holds the electrolytic solution and allows ions to pass therethrough, and uses a thin microporous membrane such as polyethylene or polypropylene. Can be.

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, and the like.
One kind of γ-butyrolactone, acetonitrile, dimethoxyethane, tetrahydrofuran, dioxolane, methylene chloride and the like, or a mixture of two or more kinds thereof can be used. As the electrolyte to be dissolved, LiI, LiCl which generates lithium ions when dissolved are used.
O ₄ , LiAsF ₆ , LiBF ₄ , LiPF _{6 and the} like can be used.

Instead of using the separator and the non-aqueous electrolyte, a solid polymer electrolyte using a high molecular weight polymer such as polyethylene oxide and a lithium salt such as LiClO ₄ or LiN (CF ₃ SO ₂ ) ₂ is used. Alternatively, a gel electrolyte in which the above non-aqueous electrolyte is trapped in a solid polymer matrix such as polyacrylonitrile can also be used.

The lithium secondary battery constituted as described above can be formed in various shapes such as a coin type, a stacked type and a cylindrical type. In any case, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the electrode body is made to conduct from the positive electrode and the negative electrode to the positive electrode terminal and the negative electrode terminal communicating with the outside, respectively. Can be sealed in a battery case together with the non-aqueous electrolyte to complete the battery.

<Low-temperature characteristic improving method> The low-temperature characteristic improving method for a lithium secondary battery according to the present invention is a method of performing charging / discharging after forming a battery so that a voltage higher than a normal use upper limit voltage of the battery is a charging upper limit voltage. It is. Hereinafter, the charging / discharging in the low-temperature characteristic improvement method of the present invention is referred to as low-temperature characteristic improvement processing.

The low-temperature characteristic improving process is performed after the battery is formed. For example, the process can be performed after the battery is completed in the above-described embodiment. In addition, at the stage where the positive electrode and the negative electrode constituting the battery are formed, the separator is sandwiched between them, and the electrode body is formed, the electrode body may be immersed in the electrolytic solution. The low-temperature characteristic improvement process is particularly performed in the initial stage after battery formation, that is, before the battery is used, considering that the positive and negative electrode potentials shift to the higher potential side during repeated charge and discharge. It is desirable to do it in stages.

As described above, the upper limit voltage for normal use is the upper limit voltage of the operating voltage range of the battery, which is determined as a voltage range in which the battery can be charged and discharged reversibly without greatly impairing the characteristics of the battery. Is different depending on the configuration. For example, in the case of a battery using a lithium-cobalt composite oxide or a lithium-nickel composite oxide capable of forming 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 as follows. Since the voltage is about 3 to 4.1 V, the upper limit voltage is about 4.1 V. The charge upper limit voltage means a charge end voltage in the low temperature characteristic improvement processing.

For charging in the low-temperature characteristic improving process, a voltage higher than the normal use upper limit voltage may be set 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 cracks in the positive electrode active material and generating a large number of active material surfaces that can newly participate in the reaction. It is desirable that the charging upper limit voltage is a voltage 0.2 V higher than the normal use upper limit voltage.

In the case where the above-mentioned 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 electrolytic solution is decomposed. There is a possibility that the problem that the durability of the battery is reduced and the problem that the product generated by the decomposition of the electrolytic solution covers the electrode and the large current characteristic is reduced. Further, as will be understood from experiments described later, if the positive electrode potential is too high, there arises a problem that the low-temperature characteristics improved by performing the low-temperature characteristics improvement treatment deteriorate after the cycle durability test. From such a point of view, the charging upper limit voltage is more than the normal use upper limit voltage by 0.1.
It is desirable that the voltage be lower than the voltage higher by 4V.

More specifically, a basic set is used as a positive electrode active material.
LiNiO_TwoLayered rock salt structure lithium nickel
The maximum charge voltage when using a composite oxide is the positive electrode potential
Is Li / Li⁺4.3 V or more and 4.5 V or less with respect to the potential of
It is desirable to set the voltage to be full. That is,
Since the battery voltage is the potential difference between the positive and negative electrodes,
Depending on the combination of active materials used for the positive and negative electrodes
Things that change. Thus, for example, the cathode active material
The basic composition is LiNiO_TwoLayered rock salt structure
Rechargeable lithium battery using a nickel oxide composite oxide
Is the voltage of the positive electrode alone, regardless of the type of active material of the negative electrode.
So that the upper limit voltage for charging is within the above range
It is desirable that the voltage be used. Note that L
i / Li ⁺Positive electrode potential is simulated with Li reference electrode
It can be measured using a battery.

For charging and discharging, a general power supply capable of controlling the current density and the upper and lower limits of voltage during charging and discharging may be used. For example, charging is performed with a constant current until a predetermined voltage is reached, and charging is completed. It can be charged and discharged by a constant current charge-constant current discharge method, after which it is discharged until a predetermined voltage is reached with a constant current, or after the battery is charged with a constant current until a predetermined voltage is reached, the voltage is maintained. The battery can be charged and discharged for a predetermined period of time and then discharged at a constant current until a predetermined voltage is reached after the charging is completed.

The current density during charging is not particularly limited, and charging may be performed at 0.01 to ² mA / cm ² . Similarly, the current density at the time of discharge is 0.01 to 2
The discharge may be performed at mA / cm ² .

The discharge may be performed until a predetermined voltage is reached. In particular, from the viewpoint that a larger capacity can be obtained, it is desirable to set the lower limit voltage of the operating voltage range of the battery as the discharge end voltage.

The temperature at which the low-temperature characteristic improving process is performed is not particularly limited, and may be, for example, 0 to 60 ° C. In addition, the low-temperature property improving treatment is desirably performed 1 to 5 times because a new surface of the active material is sufficiently generated and the electrolyte is hardly decomposed.

The embodiment of the structure of the lithium secondary battery and the method for improving the low-temperature characteristics described above are merely examples, and the method for improving the low-temperature characteristics of the lithium secondary battery of the present invention is based on the above-described embodiment. First, various modifications and improvements can be made based on the knowledge of those skilled in the art.

[0042]

EXAMPLES Based on the above embodiment, a lithium secondary battery using a lithium transition metal composite oxide as a positive electrode active material was manufactured, and a low-temperature characteristic improvement treatment was performed to obtain the internal resistance and input / output of the lithium secondary battery. Characteristics and the like were measured. Hereinafter, formation of a lithium secondary battery, low-temperature characteristic improvement processing, evaluation of the internal resistance and input / output characteristics of the battery at a low temperature, and evaluation of the internal resistance and input / output characteristics of the battery at a low temperature after a cycle durability test will be described.

<Formation of Lithium Secondary Battery> The structure of the lithium secondary battery formed in this embodiment is schematically shown in FIG.
In FIG. 1, a lithium secondary battery 1 has an electrode body 6 formed by spirally winding a sheet-shaped positive electrode 3 and a sheet-shaped negative electrode 4 through a separator 5 in a battery case 2. It was done. A positive electrode lead 7 connected to the center of the sheet-shaped positive electrode 3 is connected to a cap 9 attached to the battery case 2, and a negative electrode lead 8 connected to the outer periphery of the sheet-shaped negative electrode 4. Is connected to the battery case 2. Further, insulating plates 10 are mounted on the inner bottom surface of the battery case 2 and the upper portion of the electrode body 6, respectively.

The sheet-shaped positive electrode 3 is made of L as a positive electrode active material.
It was formed using iNi _0.8 Co _0.15 Al _0.05 O ₂ . First, the active material LiNi _0.8 Co _0.15 Al _0.05 O ₂ 9
0 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 was added, and the mixture was kneaded and kneaded to obtain a paste-like 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-shaped positive electrode 3. The size of the positive electrode 3 was 45 mm × 900 mm, and the coating thickness of the positive electrode mixture after dry pressing was 30 μm per side.

The sheet-like negative electrode 4 was formed using artificial graphite as a negative electrode active material. First, polyvinylidene fluoride as a binder was added to 95 parts by weight of artificial graphite as an active material.
Parts by weight were mixed, N-methyl-2-pyrrolidone was added as a solvent, and the mixture was kneaded to prepare a paste-like negative electrode mixture. Next, the negative electrode mixture was applied to both surfaces of a copper foil current collector having a thickness of 10 μm, dried, and roll-pressed to obtain a sheet-shaped negative electrode 4. The size of the negative electrode 4 is 49 mm × 920 m
m, the coating thickness of the negative electrode mixture after dry pressing is 3 per side.
The thickness was 5 μm.

The above-mentioned positive electrode 3 and negative electrode 4 are sandwiched between a polyethylene separator 5 having a thickness of 25 μm and a width of 52 mm.
To form a roll-shaped electrode body 6.
The insulating plate 10 was mounted below 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 also mounted on the electrode body 6, and a non-aqueous electrolyte was injected. As the non-aqueous electrolyte, a solution obtained by dissolving LiPF ₆ at a concentration of 1 M in a mixed solvent obtained by mixing ethylene carbonate and diethyl carbonate at a volume ratio of 3: 7 was used. Then, after the positive electrode lead 7 was connected to the cap 9, the cap was closed with the cap 9 to form the lithium secondary battery 1.

<Low-temperature characteristic improvement processing> Using the completed lithium secondary battery, low-temperature characteristic improvement processing was performed by changing the charging upper limit voltage. From the result of the preliminary charge / discharge test, the operating voltage range of the lithium secondary battery used in this treatment was 3 to 4.1 V.
1 V was set. Hereinafter, the processing method of each example and comparative example will be described.

(1) Processing Method of Example 1 As low-temperature characteristic improvement processing, charging and discharging were performed five times in total. In the first time, charging was performed at a constant current of 0.25 mA / cm ² until the voltage reached 4.2 V, and charging was further performed at a constant voltage of 4.2 V, also serving as conditioning of the battery. . The total charging time was 6 hours. Then
The battery was discharged at a constant current of 0.2 mA / cm ² until the voltage reached 3 V. In the second to fifth charge / discharge operations, a constant current / 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 as in the first time. However, the current density was 1 mA / cm ^{2 for} both charging and discharging, and the charging time was 2 hours in total. The ambient temperature of these five charge / discharge cycles was all 25 ° C. The discharge capacity at the time of the fifth charge / discharge was set as the reference capacity.

(2) Processing method of Examples 2 to 6 Charge and discharge were performed in the same manner as in Example 1 except that the upper limit voltage of charging was changed. Here, each charging upper limit voltage is 4.3 V in Example 2, 4.4 V in Example 3, and 4.45 in Example 4.
V, 4.5 V for Example 5, and 4.6 V for Example 6.

(3) Processing Methods of Comparative Examples 1 to 3 Charge and discharge were performed in the same manner as in Example 1 except that the upper limit voltage of charging was changed. Here, each charging 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.

Table 1 summarizes the charging upper limit voltages in the processing methods of the examples and comparative examples. In Table 1, the positive electrode potential with respect to Li / Li ⁺ was obtained from measurement of a simulated battery with a Li reference electrode.

[0052]

[Table 1]

<Evaluation of Internal Resistance and Input / Output Characteristics at Low Temperature> Lithium secondary batteries that have been subjected to low-temperature characteristic improvement processing by the processing methods of the above Examples and Comparative Examples are as follows.
This is referred to as a lithium secondary battery of Example 1. With respect to the lithium secondary batteries of Examples and Comparative Examples, the internal resistance and the input / output at −30 ° C. were measured. Hereinafter, a method of measuring the internal resistance and the input / output at −30 ° C. will be described.

The current required to discharge the reference capacity of each of the lithium secondary batteries of the Examples and Comparative Examples in one hour was defined as an hourly rate (1 C). In a state where each lithium secondary battery is charged to 30% of the reference capacity (SOC 30%), the ambient temperature is set to −30 ° C., and the battery is discharged at 0.1 C for 10 seconds.
The voltage at the second was measured. Then at 0.3C for 10 seconds,
C was discharged for 10 seconds, 3C was discharged for 10 seconds, and 10C was discharged for 10 seconds, and the voltage at each 10 seconds was measured. Charging was performed in the same manner, and the voltage at the 10th second was measured. Then, the current dependence of the voltage was determined, and the gradient of the current-voltage straight line was defined as the internal resistance. Furthermore, the current-voltage straight line on the discharge side and the lower limit voltage (3
V) is the output (W), and the area surrounded by the charging-side current-voltage straight line and the upper limit voltage (4.1 V) is the output (W).
The area of the square was defined as input (W).

As for the internal resistance and the input / output at 25 ° C., in the method for measuring the internal resistance and the input / output at −30 ° C., the ambient temperature at the time of charging / discharging was set to 25 ° C.
The measurement was carried out in the same manner as the measurement method except that the measurement was performed. 2
The value of the internal resistance at 5 ° C. hardly changed even when the charge upper limit voltage for charge and discharge was changed.

Here, the value of the internal resistance at −30 ° C. of each of the lithium secondary batteries of Examples and Comparative Examples is expressed as “−30 ° C.
The value of the internal resistance at 25 ° C. is referred to as “25 ° C. internal resistance”. Then, “-30 ° C. internal resistance” / “2
"5 ° C. internal resistance" was calculated and used as an internal resistance ratio. Also,
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”. And "-
“30 ° C. input / output” / “25 ° C. input / output” was calculated to obtain an input / output ratio.

FIG. 2 shows the relationship between the charging upper limit voltage and the internal resistance ratio in the low temperature characteristic improvement processing. As is clear from FIG. 2, the lithium secondary batteries of the respective examples in which charging and discharging were performed with a voltage higher than the normal use upper limit voltage of 4.1 V as a charge upper limit voltage all had a reduced internal resistance ratio, It can be seen that the internal resistance at −30 ° C. has decreased. This decrease in internal resistance is caused by increasing the charging upper limit voltage.
More lithium ions are desorbed in the charge / discharge reaction
It is considered that as a result of the insertion, the active material was cracked and the surface area of the active material involved in the reaction newly increased, and the reaction was activated.

In addition, each of the lithium secondary batteries of Examples 2 to 5 in which charging and discharging were performed with a voltage equal to or higher than 0.2 V higher than the normal upper limit voltage of 4.1 V as the charging upper limit voltage was larger. It was confirmed that the internal resistance at 30 ° C. decreased.

On the other hand, the charging and discharging were performed with a voltage higher than the normal use upper limit voltage of 4.1 V by 0.4 V or higher as the charging upper limit voltage, and the charging upper limit voltage was 4.5 V or higher. In the lithium secondary battery of No. 6, the internal resistance at -30 ° C increased slightly. This is considered to be because decomposition of the electrolyte started in this voltage region. Therefore, it was confirmed that it is more desirable to perform charging / discharging with a voltage lower than 0.4 V higher than the normal use upper limit voltage as the charge upper limit voltage.

The internal resistance ratio became the lowest value.
The positive electrode potential with respect to Li / Li ⁺ of the lithium secondary batteries of Examples 2 to 4 was 4.3 V or more and 4.5 as shown in Table 1.
Since it is in the range of less than V, the charging upper limit voltage is 4.3 V or more with respect to the potential of the positive electrode Li / Li ⁺ .
It was confirmed that it is more desirable to set the voltage to be less than 5 V.

Next, FIG. 3 shows the relationship between the charging upper limit voltage and the input / output ratio in the low temperature characteristic improvement processing. As is clear from FIG. 3, the lithium secondary batteries of the respective examples in which charging and discharging were performed with the charging upper limit voltage being higher than the normal use upper limit voltage of 4.1 V, the input / output ratio increased, -30 ° C
It can be seen that the input / output characteristics of the sample are improved. It is considered that this is because, as described above, more lithium ions are desorbed and inserted by increasing the charging upper limit voltage, the active material is cracked, and the surface area of the active material newly participating in the reaction is increased. .

In addition, each of the lithium secondary batteries of Examples 2 to 4 in which charging and discharging were performed with a voltage equal to or higher than the normal use upper limit voltage of 4.1 V being 0.2 V or higher as the charge upper limit voltage was larger. It was confirmed that the input and output at 30 ° C. increased.

On the other hand, the charging and discharging were performed with a voltage higher than the normal use upper limit voltage of 4.1 V by 0.4 V or higher as the charging upper limit voltage, and the charging upper limit voltage was 4.5 V or higher. In the lithium secondary battery of No. 6, input / output at −30 ° C. decreased. This is considered to be because decomposition of the electrolyte started in this voltage region. Therefore,
It was confirmed that it is more desirable to perform charging / discharging with a voltage lower than 0.4 V higher than the normal use upper limit voltage as a charge upper limit voltage.

The positive electrode potential with respect to Li / Li ⁺ of the lithium secondary batteries of Examples 2 to 4 having a high input / output ratio was 4.3 V or more and less than 4.5 V as shown in Table 1. , It was confirmed that it is more desirable to set the charging upper limit voltage to a voltage at which the positive electrode potential is 4.3 V or more and less than 4.5 V with respect to the Li / Li ⁺ potential.

<Evaluation of Internal Resistance and Input / Output Characteristics of Battery at Low Temperature after Cycle Durability Test> For the lithium secondary batteries of the above Examples and Comparative Examples, cycle durability of internal resistance and input / output characteristics at −30 ° C. A cycle endurance test was performed to determine The method of the cycle durability test will be described below.

Each lithium secondary battery was charged at a charge end voltage of 4.
The battery was charged and discharged at 1 V and a discharge end voltage of 3 V. Charging and discharging were performed by a constant current charging-constant current discharging method. The current density was 1 mA / cm ² for both charging and discharging, and the ambient temperature for charging and discharging was 60 ° C.
And This charge / discharge was defined as one cycle, and a durability test was performed for a total of 500 cycles. Then, after a 500 cycle 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.

The value of the internal resistance at −30 ° C. of the lithium secondary batteries of the respective Examples and Comparative Examples after the above cycle endurance test is indicated as “−30 ° C. internal resistance after cycle”, and the cycle endurance test previously measured The ratio with those that do not,
That is, “−30 ° C. internal resistance after cycle” / “− 30
° C internal resistance ”was calculated as“ the internal resistance ratio after the cycle ”. In addition, the input / output value at −30 ° C. of the lithium secondary batteries of the respective Examples and Comparative Examples after the above cycle endurance test is referred to as “−30 ° C. input / output after cycle”, and the cycle endurance test measured earlier was performed. The ratio to that not performed, that is, “-30 ° C. input / output after cycle” / “− 30 ° C. input / output” was calculated to be “input / output ratio after cycle”.

FIG. 4 shows the relationship between the charge upper limit voltage and the internal resistance ratio after the cycle in the low temperature characteristic improvement processing. As is clear from FIG. 4, in the battery having the charge upper limit voltage of 4.45 V or less, the increase in the internal resistance is almost constant at 15% or less, and the decrease in the internal resistance is maintained even after the cycle test. I knew it was there. On the other hand, when the charging upper limit voltage became 4.5 V or more, the increase in the internal resistance became 60% or more.

Next, FIG. 5 shows the relationship between the charging upper limit voltage and the post-cycle input / output ratio in the low-temperature characteristic improvement processing. FIG.
As is clear from the above, in the battery having the charging upper limit voltage of 4.45 V or less, the decrease in the input / output is almost constant at 20% or less, and the above-mentioned increase in the input / output is maintained even after the cycle test. I understood. On the other hand, when the charging upper limit voltage was 4.5 V or more, the decrease in input / output was 40% or more.

From the above, it was confirmed that a lithium secondary battery having improved low-temperature characteristics by performing charge / discharge with a voltage higher than the normal use upper limit voltage as a charge upper limit voltage had good cycle durability. On the other hand, it was found that when the charging upper limit voltage in the low-temperature characteristic improvement processing was 4.5 V or more, the cycle durability was reduced. This raises the charging upper limit voltage to 4.5V
It is considered that when the charge and discharge are performed as described above, the electrolyte is decomposed and a film is formed on the surface of the positive electrode active material. Once formed, the film remains even when the upper limit voltage is lowered when performing charge and discharge thereafter, and a new film grows with the film as a nucleus during repetition of charge and discharge, so that the durability is reduced. it is conceivable that.

[0071]

According to the method for improving low-temperature characteristics of a lithium secondary battery of the present invention, charge / discharge is performed by setting a voltage higher than a normal use upper limit voltage to a charge upper limit voltage after forming the battery.
By increasing the surface area involved in the reaction of the positive electrode active material particles and activating the charge / discharge reaction, the low temperature characteristics of the lithium secondary battery can be significantly improved.

Further, according to the method for improving low-temperature characteristics of a lithium secondary battery of the present invention, the low-temperature characteristics can be extremely easily achieved only by performing charging / discharging with a voltage higher than a normal use upper limit voltage as a charge upper limit voltage after forming the battery. Can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a structure of a lithium secondary battery.

FIG. 2 is a diagram showing a relationship between a charging upper limit voltage and an internal resistance ratio in a low-temperature characteristic improvement process.

FIG. 3 is a diagram showing a relationship between a charging upper limit voltage and an input / output ratio in a low-temperature characteristic improvement process.

FIG. 4 is a diagram illustrating a relationship between a charging upper limit voltage and an internal resistance ratio after a cycle in a low-temperature characteristic improvement process.

FIG. 5 is a diagram showing a relationship between a charging upper limit voltage and a post-cycle input / output ratio in a low-temperature characteristic improvement 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

 ────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Yukimasa Nishide 1 Toyota Town, Toyota City, Aichi Prefecture Toyota Motor Corporation F-term (reference) 5H029 AJ00 AK03 AL07 AM03 AM04 AM05 AM07 BJ02 BJ14 CJ16 DJ16 DJ17 HJ02 HJ18 5H050 AA06 BA17 CA08 CB08 DA02 EA09 EA24 FA17 FA18 GA18 HA02 HA18

Claims (4)

[Claims] 1. 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, wherein after forming the battery, a voltage higher than a normal use upper limit voltage of the battery is defined as a charge upper limit voltage. A method for improving the low-temperature characteristics of a lithium secondary battery, comprising performing charge and discharge. 2. The method for improving low-temperature characteristics of a lithium secondary battery according to claim 1, wherein the charging upper limit voltage is a voltage higher than a normal use upper limit voltage by 0.2V or more. 3. The method for improving low temperature characteristics of a lithium secondary battery according to claim 1, wherein the charging upper limit voltage is a voltage lower than a voltage 0.4 V higher than a normal use upper limit voltage. 4. The lithium transition metal composite oxide is a layered rock-salt structure lithium nickel composite oxide having a basic composition of LiNiO _2, and the upper limit charging voltage is determined based on the positive electrode potential with respect to the Li / Li ⁺ potential. The method for improving the low-temperature characteristics of a lithium secondary battery according to claim 1, wherein the voltage is 4.3 V or more and less than 4.5 V.