JP2001052754A - Aging treatment method of lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a simple treatment method capable of enhancing the aging effect of a battery characteristic to a practical usage level by preventing high- temperature cycle degradation in a lithium secondary battery. SOLUTION: This lithium secondary battery is provided with: a positive electrode formed by binding a positive electrode active material formed of a lithium transition metal composite oxide with a positive electrode binder, a negative electrode formed by binding a negative electrode active material formed of a carbon material with a negative electrode binder; and a nonauqeous electrolytic solution. In this case, the battery is composed by facing the positive electrode to the negative electrode and by mounting the positive electrode and the negative electrode after impregnating the nonaqueous electrolytic solution into them. This aging treatment for storing the battery at a solidifying temperature of the nonaqueous electrolytic solution or below is applied to the battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a lithium secondary battery utilizing the phenomenon of insertion and extraction of lithium ions, and more particularly to a treatment method for reducing changes over time in battery characteristics.

[0002]

2. Description of the Related Art With the miniaturization of personal computers, video cameras, mobile phones, and the like, in the fields of information-related equipment and communication equipment, lithium secondary batteries are used as power sources for these equipments because of their high energy density. Has been put to practical use and has spread widely. On the other hand, in the field of automobiles, the development of electric vehicles is urgent due to environmental problems and resource problems, and lithium secondary batteries are being studied as power sources for electric vehicles. Lithium secondary batteries include
Performance that battery characteristics such as battery capacity and battery internal resistance do not change for a long time in a wide operating temperature range is required.
In recent years, such applications as power supplies for electric vehicles are expanding, the demands are increasing.

[0003] Among lithium batteries, a rocking chair type so-called lithium ion secondary battery using a carbon material for the negative electrode is currently the mainstream because of its safety. This lithium ion secondary battery generally has two electrodes, a positive electrode and a negative electrode, formed by binding an active material to which a conductive material is added as necessary with a binder, and a lithium salt dissolved in an organic solvent. And a non-aqueous electrolytic solution as a main component.

[0004] In order to improve the performance of not changing for a long time in a wide operating temperature range as described above,
Conventionally, various processing methods such as improvement of the above-mentioned constituent elements such as active materials of the positive electrode and the negative electrode, a binder, an organic solvent of a non-aqueous electrolyte, a conditioning treatment applied to a completed battery, and an aging treatment stored at a high temperature. Many technologies, such as an infinite list, have been considered, and some of them are actually implemented.

[0005]

In a current lithium secondary battery, at relatively low temperatures of 40 ° C. or less, cycle deterioration falls within a practical level. However, at a high temperature of about 60 ° C. at which the battery reaction is activated, there is a problem that the cycle deterioration falls below a practical level. This is because, when charge and discharge are repeated at a high temperature of about 60 ° C., the positive and negative electrode active materials undergo structural changes to decrease the charge and discharge capacity in a specific potential region, and the positive and negative electrodes swell and the electron conductivity decreases. It is conceivable that the internal resistance of the battery increases due to the decrease. However, it is substantially difficult to eliminate structural changes of the active material itself and swelling of the positive and negative electrodes.

[0006] The present inventor has found through repeated experiments that this cycle deterioration was studied, and found that a lithium secondary battery with little change over time can be obtained by cooling the battery itself. The present invention is based on this finding, and is a processing method for a lithium secondary battery that is completely different from the conventional one, and is capable of preventing high-temperature cycle deterioration and increasing the time-dependent change in battery characteristics to a practical level. The challenge is to provide

[0007]

The aging treatment method for a lithium secondary battery according to the present invention comprises a positive electrode formed by binding a positive electrode active material comprising a lithium transition metal composite oxide with a positive electrode binder; A negative electrode formed by binding a negative electrode active material comprising a negative electrode binder, and a non-aqueous electrolyte, a method for aging a lithium secondary battery comprising a non-aqueous electrolyte, wherein the positive electrode and the negative electrode are opposed to each other. A battery assembled by impregnating the positive electrode and the negative electrode with the nonaqueous electrolyte is stored at a temperature lower than a temperature at which the nonaqueous electrolyte solidifies.

That is, the aging process of the present invention is a process of storing a lithium secondary battery in a low temperature state before use. How this aging treatment affects the lithium secondary battery is not clear at present, but the following two are conceivable.

One is that the low-temperature aging treatment changes the charge / discharge characteristics of the lithium secondary battery. As will be shown in a later example, considering the differential capacity curve, the charge / discharge capacity component in the potential region which decreases by the high-temperature charge / discharge cycle of about 60 ° C. is previously reduced by the aging process. Due to the disappearance of the unstable capacity component, the lithium secondary battery has a small change in capacity even when used at a high temperature thereafter.

The other is an effect by freezing of the non-aqueous electrolyte. The positive electrode and the negative electrode are formed by binding an active material with a binder, and the positive electrode and the negative electrode are impregnated with a non-aqueous electrolyte. A minute space exists in the binder, and the minute space is impregnated with the non-aqueous electrolyte.
When the non-aqueous electrolyte solidifies, crystals are generated so as to expand the minute space. After this crystal is melted, the space where the crystal was present is still expanded, and the binder itself has a sponge-like (sponge-like) appearance. With charge and discharge, the active material in the electrode repeatedly expands and contracts in volume, so that the electrode itself repeats expansion and contraction, but the electrode subjected to this aging treatment has a sponge-like binder. Therefore, the electrode has a structure capable of absorbing the change in the volume of the electrode caused by the charge and discharge. As a result, the swelling of the electrode is suppressed, and the increase in the internal resistance due to the loss of the conductive path is suppressed.

[0011] Due to the above two conceivable actions, the lithium secondary battery subjected to the aging treatment can suppress the decrease in the discharge capacity and the increase in the internal resistance of the lithium secondary battery even after repeated charging and discharging at a high temperature. Is suppressed, and a lithium secondary battery having a small change over time in battery characteristics is obtained.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The structure of a lithium secondary battery to be subjected to the aging process and the mode of the aging process will be described below in detail by dividing the items.

<Structure of Lithium Secondary Battery> A lithium secondary battery to be subjected to the aging treatment has a positive electrode formed by binding a positive electrode active material comprising a lithium transition metal composite oxide with a positive electrode binder, A negative electrode formed by binding a negative electrode active material made of a carbon material with a negative electrode binder and a non-aqueous electrolyte are provided as main components. The battery is a so-called lithium ion secondary battery, which is not particularly limited and may be any battery that adopts its general form. Below, the form is illustrated.

As the lithium transition metal composite oxide serving as the positive electrode active material, a known oxide may be used. When a 4V-class lithium secondary battery is configured, a layered rock salt structure lithium cobalt composite oxide (LiCoO ₂ ), a layered rock salt structure lithium nickel composite oxide (LiNiO ₂ ), a layered rock salt structure lithium manganese composite oxide (LiMnO ₂₎ ), Spinel structure lithium manganese composite oxide (LiMn ₂ O ₄ )
Etc. may be used.

Among them, when it is expensive but wants to have better cycle characteristics and the like, LiCoO ₂
When importance is placed on cost advantages over battery performance, LiMnO ₂ or LiMn ₂ O ₄ may be used. In order to obtain a lithium secondary battery having a good balance between cycle characteristics and battery cost, it is desirable to use LiNiO ₂ . It is also possible to use a mixture of two or more of these or two or more of these with another lithium transition metal composite oxide.

When the lithium nickel composite oxide having a layered rock salt structure is used as a positive electrode active material, the composition formula is LiNiO ₂
The stoichiometric composition represented by the following formula can be used.
Further, in order to improve the cycle characteristics and the like of the secondary battery, Ni
One in which part of a site is replaced with another element can also be used. Among those substituted with other elements, the composition formula Li
_{Ni x M1 y M2 z O 2} (M1 is Co, at least one selected from Mn; M2 is Al, B, Fe, Cr, at least one selected from Mg; x + y + z = 1; 0.5 <x <
0.95; 0.01 <y <0.4; 0.001 <z <
It is desirable to use the one represented by 0.2).

The LiNi _x M1 _y M2 _z O ₂ is obtained by substituting a part of the Ni site with at least two elements M1 and M2 having different roles. If the substitution ratio is defined by the ratio of leaving Ni without substitution, that is, the value of x in the composition formula, 0.5 <x <0.95. If x ≦ 0.5, not only a layered rock salt structure but also a second phase such as a spinel structure is generated. If x ≧ 0.95, the substitution effect is too small. This is because a battery having the desired good cycle characteristics cannot be formed. Note that 0.
More preferably, the range is 7 <x <0.9.

The element M1 selected from Co and Mn mainly serves to stabilize the crystal structure of the lithium nickel composite oxide. By stabilizing the crystal structure at M1, the cycle characteristics of the lithium secondary battery are more favorably maintained, and particularly, deterioration of the battery capacity due to charge / discharge at high temperature and storage at high temperature is suppressed. In order to sufficiently exert the effect of improving the cycle characteristics, the substitution ratio of M1, that is, the value of y in the composition formula, is set to 0.01 <y <0.4. y
When ≦ 0.01, the crystal structure of the formed secondary battery is not sufficiently stabilized, so that the cycle characteristics are not good.
If ≧ 0.4, the crystallinity of the lithium-nickel composite oxide is undesirably reduced. It is more preferable that 0.1 <y <0.3. Further, the replacing element M1 is Co
Is more desirable. In Co, while suppressing the capacity reduction due to the element substitution, the obtained composite oxide Li
This is because (Co, Ni) O ₂ is an all-solid solution type and has an advantage of minimizing a decrease in crystallinity.

The element M2 selected from Al, B, Fe, Cr and Mg mainly serves to suppress the decomposition reaction of the active material due to the release of oxygen and to improve the thermal stability. Due to this role, the substitution ratio of M2, that is, the value of z in the composition formula, is set to 0.001 <z <0.2. z ≦
In the case of 0.001, a sufficient effect on safety cannot be obtained, and in the case of z ≧ 0.2, the capacity of the positive electrode decreases, which is not preferable. In addition, 0.01 <z <0.
It is more preferably set to 1. Further, the substituting element M2
It is more preferable to use Al. This is because Al has an advantage of minimizing a decrease in capacity while improving thermal stability.

For example, to manufacture a layered rock salt structure lithium nickel composite oxide represented by the composition formula LiNi _x Co _y Al _z O ₂ , use LiOH · H ₂ O, Ni (O
This can be synthesized by mixing predetermined amounts of H) ₂ , Co ₃ O ₄ , and Al (OH) ₃ and baking them in an oxygen stream at a temperature of about 850 ° C. for about 20 hours.

The positive electrode is prepared by mixing a powder of the lithium transition metal composite oxide as a positive electrode active material with a conductive material and a binder, and adding an appropriate solvent to form a paste-like positive electrode mixture. A material formed by coating and drying the surface of a current collector made of a metal foil such as aluminum, and compressing it as necessary to increase the electrode density can be used. The conductive material is for ensuring the electrical conductivity of the positive electrode, and is made of a carbon material powder such as carbon black, acetylene black, graphite, or the like.
A species or a mixture of two or more species can be used.
The binding agent plays a role of binding the active material particles and the conductive material particles, and may be a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, or fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. . 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.

As the negative electrode active material, a carbon material capable of inserting and extracting lithium is used. Examples of carbon materials that can be used include natural graphite, spherical or fibrous artificial graphite, non-graphitizable carbon, and organic compound fired substances such as phenolic resins, and powdered substances of easily graphitizable carbon such as coke. Can be mentioned. As in the case of the positive electrode active material, the carbon material serving as the negative electrode active material has respective advantages, and may be selected according to the characteristics of the lithium secondary battery to be manufactured.

Of these, natural and artificial graphites have the advantage of being capable of forming a lithium secondary battery having a large capacity (high energy density) and good power characteristics because of its high true density and excellent conductivity. There is. When a lithium secondary battery utilizing this advantage is manufactured, it is desirable that the graphite used has high crystallinity, the (002) plane spacing d ₀₀₂ is 3.4 ° or less, and the crystallite thickness in the c-axis direction. It is preferable to use one having Lc of 1000 ° or more. In addition, artificial graphite is, for example, 2800 ° C.
It can be manufactured by heat treatment at the above high temperature. In this case, the easily graphitizable carbon as a raw material includes coke and optically anisotropic small spheres (mesocarbon microbeads: MCMB) obtained in the process of heating pitches at about 400 ° C. .

The easily graphitizable carbon is generally made from tar pitch obtained from petroleum or coal, and includes coke,
MCMB, mesophase pitch-based carbon fiber, pyrolytic vapor growth carbon fiber, and the like. An organic compound fired body such as a phenol resin can also be used. Since graphitizable carbon is an inexpensive carbon material, it can be a negative electrode active material that can constitute a lithium secondary battery that is excellent in cost. Among them, coke has the advantages of low cost and relatively large capacity, and considering this point,
It is desirable to use coke. When coke is used, it is preferable to use one having a (002) plane spacing d ₀₀₂ of 3.4 ° or more and a crystallite thickness Lc in the c-axis direction of 30 ° or less.

The non-graphitizable carbon is a so-called hard carbon, and is a carbon material having a structure close to amorphous, such as glassy carbon. Generally, it is a material obtained by carbonizing a thermosetting resin, and does not develop a graphite structure even when the heat treatment temperature is increased. The non-graphitizable carbon has the advantages of high safety and relatively low cost. In view of this, it is desirable to use non-graphitizable carbon as the negative electrode active material. Specifically, for example, a phenol resin fired body, a polyacrylonitrile-based carbon fiber, pseudo isotropic carbon, a furfuryl alcohol resin fired body, or the like can be used. More preferably, (00
2) It is preferable to use those having a plane spacing d ₀₀₂ of 3.6 ° or more and a crystallite thickness Lc in the c-axis direction of 100 ° or less.

The above-mentioned graphite, easily graphitizable carbon, hardly graphitizable carbon and the like can be used alone or as a mixture of two or more kinds. As an embodiment in which two or more kinds are mixed, for example, while securing safety at the time of overcharging, the amount of lithium absorbed and released by the lithium transition metal composite oxide as the positive electrode active material is limited to improve cycle characteristics. For the purpose of improving the quality, a case where graphite is mixed with a non-graphitizable carbon material such as non-graphitizable carbon and easily graphitizable carbon can be exemplified. When a mixture of graphite and a non-graphitized carbonaceous material is used for the negative electrode active material, the mixing ratio of the two may be determined by the balance between cycle characteristics and discharge capacity.

The negative electrode is prepared by mixing a binder into the above carbon material powder, adding an appropriate solvent if necessary, and forming a paste-like negative electrode mixture, like the positive electrode, into a metal such as copper. It is formed by coating and drying on the surface of a current collector made of foil, and then increasing the density of the negative electrode mixture by pressing or the like as necessary. Like the positive electrode, a fluorine-containing resin such as polyvinylidene fluoride can be used as the binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent.

In the formed positive electrode and negative electrode, active material particles (including conductive material particles in the case of the positive electrode) are bound with a binder. A minute space is created between the agent and the active material particles. This space is a space to be impregnated with a non-aqueous electrolyte, which will be described in detail later.

Between the positive electrode and the negative electrode, a separator serving to separate the positive electrode and the negative electrode and hold the electrolytic solution is interposed. As this separator, a thin microporous film such as polyethylene or polypropylene can be used.

The non-aqueous electrolyte is obtained by dissolving a lithium salt as an electrolyte in an organic solvent. The lithium salt is dissociated by dissolving in an organic solvent and forms lithium ions in the electrolyte. Examples of usable lithium salts include LiBF ₄ , LiPF ₆ , LiClO ₄ , and LiCF ₃
SO ₃ , LiAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN
(C ₂ F ₅ SO ₂ ) _{2 and the} like. Each of these lithium salts may be used alone, or two or more of these lithium salts may be used in combination.

An aprotic organic solvent is used as the organic solvent for dissolving the lithium salt. For example, a mixed solvent of one or more of cyclic carbonate, chain carbonate, cyclic ester, cyclic ether or chain ether can be used. Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate.Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate.Examples of the cyclic ester include gamma butyrolactone and gamma valero. Examples of lactones and cyclic ethers include tetrahydrofuran and 2-methyltetrahydrofuran, and examples of chain ethers include dimethoxyethane and ethylene glycol dimethyl ether.

The lithium secondary battery constituted as described above can have various shapes such as a cylindrical type, a stacked type, a coin type and the like. In any case, the separator is sandwiched between the positive electrode and the negative electrode to form an electrode body. The positive electrode terminal and the negative electrode terminal leading to the outside from the positive electrode current collector and the negative electrode current collector are connected to each other using a current collecting lead or the like, and the electrode body is sealed in a battery case together with the non-aqueous electrolyte. The battery is completed through such an assembly process.

The impregnation of the nonaqueous electrolyte into the positive electrode and the negative electrode
This is performed in the assembling step. It may be performed after the electrode body is inserted into the battery case, or may be performed on the electrode body in a container different from the battery case. It is desirable that the impregnation is repeated several times of pressurization and decompression so that the nonaqueous electrolyte sufficiently infiltrates the above-mentioned minute space existing in the electrode.

<Aspect of Aging Treatment> The aging treatment of the present invention is performed on the assembled lithium secondary battery. "Attached" does not mean only the state in which the battery is completed by sealing the electrode body together with the non-aqueous electrolyte in the battery case, but also forms the electrode body by laminating a positive electrode and a negative electrode. It means that it includes the state where it was set. Even before the electrode body is housed in the battery case, the formed electrode body is
It is also possible to impregnate the non-aqueous electrolyte in another container other than the battery case and perform the aging treatment in the container or the like.

The aging process is not particularly limited. Any method may be used as long as the battery can be kept in a low temperature state at an arbitrarily set storage temperature for an arbitrarily set storage time.

The storage temperature in the aging treatment is lower than the temperature at which the non-aqueous electrolyte impregnated in the electrode of the target lithium secondary battery is solidified so that the function of freezing of the non-aqueous electrolyte can be exerted. The solidification temperature of the non-aqueous electrolyte is
It depends on the concentration of the lithium salt and the type of the organic solvent of the non-aqueous electrolyte. When the organic solvent is a mixture of two or more kinds of organic solvents, the solidification start temperature and the solidification end temperature may be different. In this case, it is desirable to store the solidification at a temperature lower than the temperature at which solidification is completely completed.

In the embodiment described later in detail, immersion in liquid nitrogen is performed. In this case, the storage temperature is as low as about -196 ° C. The treatment with liquid nitrogen may be a treatment in which the assembled battery is merely immersed therein, and is a treatment method that can be stored in a low temperature state by an extremely simple method.

The storage time in the aging process depends on the storage temperature. Although this time is not clear at present, it is considered that the longer the storage temperature, the longer the storage is required. For example, in the case of the above immersion treatment in liquid nitrogen, the storage time is desirably 1 hour or more. In this case, if the time is less than 1 hour, the effect of improving the high-temperature cycle characteristics may not be sufficiently obtained. In this case, it is more preferable to set the storage time to 3 hours or more.

In the aging treatment, the state of charge of the lithium secondary battery to be treated also affects the high-temperature cycle characteristics and the effect of reducing the internal resistance increase rate. Here, the “state of charge (SOC)” is a lower limit of 100%, that is, a fully charged state, where the upper limit of the battery voltage is obtained in a range of battery voltages that can be charged and discharged reversibly. This means a state of charge (SOC) when the state of charge at which the battery voltage is obtained is 0%, that is, the state of charge is an idle state.

The degree of the state of charge (SOC) of the lithium secondary battery charged in the aging process is more effective depending on the storage temperature, storage time, and the like. The conditions have not been deterministic. For example, as is evident from experiments, in the case of the above-described immersion treatment in liquid nitrogen, it is desirable that SOC is 60% or more. This is because when the cooling treatment is performed in a state where a certain amount of lithium is occluded in the negative electrode active material or a certain amount of lithium is contained in the positive electrode active material, the aforementioned unstable capacity component is effectively reduced. It is thought that it can be done.

[0041]

EXAMPLE An experiment was conducted to evaluate the aging process of the present invention based on the above embodiment. The content of this experiment and its evaluation are described below as examples.

<Prepared Lithium Secondary Battery> The lithium secondary battery used in this experiment is shown below.

The positive electrode active material has a composition formula of LiNi _0.8 Co _0.15
A layered rock-salt lithium nickel composite oxide represented by Al _0.05 O ₂ was used. LiNi _0.8 Co _0.15 Al _0.05 O ₂
Are LiOH.H ₂ O, Ni (OH) ₂ , Co ₃ O ₄ , Al
(OH) ₃ are mixed in a predetermined amount, and 85
It was synthesized by firing at a temperature of 0 ° C. for 20 hours. The positive electrode is first made of this LiNi _0.8 Co _0.15 Al
_To 85 parts by weight of _0.05 O ₂ , 10 parts by weight of acetylene black (HS-100: manufactured by Denki Kagaku Kogyo) as a conductive material,
Polyvinylidene fluoride (PVDF) (KF
5 parts by weight of a polymer: Kureha Chemical Industry), and an appropriate amount of N-methyl-2-pyrrolidone (NMP) as a solvent was added to obtain a paste-like positive electrode mixture.
It was applied to both sides of a positive electrode current collector made of 0 μm aluminum foil, dried, and then compression-molded by a roll press to produce a sheet. The size of the positive electrode sheet is 54 mm x
450mm, the thickness per side of the positive electrode mixture layer,
It was 40 μm.

As the negative electrode active material, graphitized mesocarbon microspheres (MCMB) (MCMB25-28:
Osaka Gas Chemicals). The negative electrode is first
To 95 parts by weight of CMB, 5 parts by weight of PVDF was mixed as a binder, and an appropriate amount of NMP was added as a solvent to obtain a paste-like negative electrode mixture. This negative electrode mixture was used as a 10-μm-thick copper foil positive electrode. It was applied to both sides of the current collector, dried, and then compression molded by a roll press to produce a sheet. The size of the negative electrode sheet was 56 mm × 500 mm, and the thickness per side of the negative electrode mixture layer was 50 μm.

As the separator, a microporous polyethylene film having a thickness of 25 μm (manufactured by Tonen Talpis) was used. As the non-aqueous electrolyte, a mixture of ethylene carbonate and diethyl carbonate at a volume ratio of 1: 1 was used as an electrolyte. LiPF ₆ dissolved at a concentration of 1 M was used.
The positive electrode and the negative electrode were wound with a separator interposed therebetween to form an electrode body, and this electrode body was sealed in a 18650 type cylindrical battery can with the above nonaqueous electrolyte to complete a cylindrical lithium secondary battery.

<Aging Treatment> A considerable number of the above lithium secondary batteries were manufactured in order to perform aging treatment under various aging conditions. Each prepared battery,
First, it was subjected to conditioning. Conditioning was performed at 20 ° C. at a current density of 0.2 mA / cm ^{2 at} a constant current of 4.1 V, followed by 0.2 mA / cm 2
^The test was performed by discharging at a constant current to 3.0 V at a current density of ² . With the discharge capacity at that time, SOC =
It was assumed to correspond to the difference capacity between the state of charge of 100% and the state of charge of SOC = 0%.

Next, the conditioned secondary battery was subjected to an aging treatment. The conditions of the aging process are as follows: the state of charge (SOC) is 0%, 40%, 60%.
% And 100%, and in each SOC state,
Each lithium secondary battery was immersed in liquid nitrogen (about -196 ° C) and stored for 10 minutes, 1 hour, 3 hours, and 12 hours, respectively.

<Cycle charge / discharge test> After the aging treatment, a charge / discharge cycle test was performed to evaluate the cycle characteristics of each secondary battery. The charge / discharge cycle test was performed at a temperature of 60 ° C., which is considered to be the upper limit of the actual operating temperature range of the battery, and the high-temperature cycle characteristics were evaluated. In addition, the charge / discharge cycle test was carried out in a charge upper limit voltage of 4.1 V to a discharge lower limit voltage of 3.0 V and a charge / discharge current density of 2.0 mA / cm ^2.
Under the above condition, the cycle of repeating the constant current charging and the constant current discharging was repeated up to 500 cycles.

What was measured in this charge / discharge cycle test was the discharge capacity in each cycle and the DC resistance of the secondary battery in each cycle. The DC resistance of the secondary battery is
It was determined by the following equation.

DC resistance = (average charge voltage−average discharge voltage) / (2 × charge / discharge current) <Evaluation> As a typical result of the above charge / discharge cycle test, SOC = 100% in liquid nitrogen for 12 hours Of a lithium secondary battery stored and aged,
FIG. 1 shows the cycle dependency of the discharge capacity per unit weight of the positive electrode active material, and FIG. 2 shows the cycle dependency of the battery DC resistance. 1 and 2 also show the dependence of the secondary battery on which the aging process was not performed.

In the case of the secondary battery not subjected to the aging treatment, the initial discharge capacity (discharge capacity at the first cycle) is 144 mAh / g per unit weight of the positive electrode active material, and the discharge capacity decreases as the cycle proceeds thereafter. The capacity retention rate after 500 cycles (discharge capacity at 500th cycle / initial discharge capacity × 100) is about 60%. Also,
As for the DC resistance, the resistance value after 500 cycles is about 120% of the initial resistance value (the resistance value in the first cycle).
Has also increased.

On the other hand, the secondary battery which was stored in liquid nitrogen for 12 hours in the state of SOC = 100% and aged was found to have an initial discharge capacity of 132 mAh / g and not subjected to the aging process. About 8 in comparison
%, But the decrease in the discharge capacity is small even after the subsequent cycles, and the capacity retention rate after 500 cycles is improved to about 80%. In addition, the increase in DC resistance that occurs as the cycle proceeds is small, and the resistance value after 500 cycles is only about 40% of the initial resistance value.

As is clear from these results, by subjecting the lithium secondary battery to the aging treatment at a low temperature, although the initial discharge capacity is reduced, a decrease in the capacity with the progress of the cycle is greatly suppressed. It can be seen that the discharge capacity exceeds the discharge capacity of the lithium secondary battery not subjected to the heat treatment. Further, it can be seen that the increase in the DC resistance of the battery is greatly suppressed by performing the aging process. Therefore, it can be confirmed that this aging treatment improves the cycle characteristics of the lithium secondary battery, and the charge / discharge cycle test is performed at a high temperature of 60 ° C. It can be confirmed that it can be improved.

Next, as a result of the charge / discharge cycle test for the lithium secondary batteries aged under the respective aging conditions, the initial discharge capacity per unit weight of the positive electrode active material of each lithium secondary battery and the capacity maintenance after 500 cycles The rate and the DC resistance increase rate after 500 cycles are summarized in Table 1 below.

[0055]

[Table 1]

As can be seen from Table 1, the effect of the aging treatment at a low temperature is obtained under any of the conditions. In particular, it can be confirmed that the effect is large when the aging process is performed for 1 hour or more in the state of charge of SOC = 60% or more, and that the effect is more significant when the aging process is performed for 3 hours or more.

Next, the cause of the improvement in the cycle characteristics will be investigated from the differential capacity curve. FIG. 3 shows a typical example of a lithium secondary battery that has been stored and aged in liquid nitrogen for 12 hours at a state of SOC = 100%.
3 shows a differential capacity curve of a charge / discharge curve before and after an aging process.

As is apparent from FIG. 3, the aging treatment at a low temperature reduces the charge capacity in the battery voltage range of 3.5 to 3.9 V. By preliminary examination,
It has been confirmed that the decrease in the battery capacity in the high-temperature cycle at 60 ° C. is a decrease in the charge capacity in the 3.5 to 3.9 V region due to a structural change of the positive electrode active material, that is, deterioration of the positive electrode active material itself. From these results, it is considered that the low-temperature aging treatment can reduce the capacity component in this region in advance, and can reduce the change in capacity during the subsequent high-temperature charge / discharge cycle. Therefore, the differential capacity curve also supports that the aging treatment method of the present invention is a treatment method capable of reducing the change over time of the battery characteristics of the lithium secondary battery.

[0059]

According to the aging treatment of the present invention, in a lithium secondary battery, the assembled battery is stored at a temperature lower than a temperature at which the nonaqueous electrolyte solidifies. By performing such an aging treatment, the unstable capacity component is eliminated in advance, and swelling of the electrode is suppressed by freezing of the non-aqueous electrolyte. As a result, in the lithium secondary battery subjected to the aging treatment, the discharge capacity is suppressed from being reduced even by repeated charge and discharge at a high temperature thereafter, the increase in the internal resistance is suppressed, and the battery characteristic over time is suppressed. A lithium secondary battery with small changes.

[Brief description of the drawings]

FIG. 1 shows the cycle dependence of the discharge capacity per unit weight of a positive electrode active material of a lithium secondary battery stored in liquid nitrogen for 12 hours under an SOC of 100% and aged.

FIG. 2 shows the cycle dependence of the battery DC resistance of a lithium secondary battery that has been stored in liquid nitrogen for 12 hours and subjected to aging treatment at an SOC of 100%.

FIG. 3 shows a differential capacity curve of a charge / discharge curve before and after an aging treatment of a lithium secondary battery stored in liquid nitrogen for 12 hours in an SOC = 100% state and subjected to an aging treatment.

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Claims (2)

[Claims] 1. A positive electrode formed by binding a positive electrode active material comprising a lithium transition metal composite oxide with a positive electrode binder, and a negative electrode active material comprising a carbon material by binding with a negative electrode binder. An aging treatment method for a lithium secondary battery comprising a negative electrode and a non-aqueous electrolyte, wherein the positive electrode and the negative electrode are opposed to each other, and the positive electrode and the negative electrode are impregnated with the non-aqueous electrolyte. An aging treatment method for a lithium secondary battery, wherein the attached battery is stored at a temperature lower than a temperature at which the nonaqueous electrolyte solidifies. 2. The aging treatment method for a lithium secondary battery according to claim 1, wherein the assembled battery is precharged, and a state of charge (SOC) of the battery is 60% or more.