JP2002025626A - Aging method for lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a processing method capable of enhancing cycle characteristics at high temperatures to the level of practical use, by preventing cycle deterioration, and cycle deterioration at high temperatures in particular, due to charging/discharging, while achieving a large discharge capacity, for a lithium secondary battery using a low-priced spinel-structure lithium-manganese multiple oxide as a main positive-electrode active material. SOLUTION: A mixture is obtained by mixing the spinel-structure lithium- manganese multiple oxide having a fundamental composition of LiMn2O4 with a stratified rock-salt structure lithium-nickel multiple oxide having a fundamental composition of LiNiO2. A positive electrode using this mixture as the positive-electrode active material is assembled with a negative electrode to form the lithium secondary battery. The secondary battery is aged by preserving it at a preservation temperature not lower than 60 deg.C but not higher than 90 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an aging treatment method for a lithium secondary battery, and more particularly to an aging treatment method for improving cycle characteristics.

[0002]

2. Description of the Related Art Lithium secondary batteries utilizing the insertion and extraction of lithium have a high energy density.
With the miniaturization of mobile phones, personal computers, and the like, they have become widespread in the field of communication devices and information-related devices. Also, in the field of automobiles, the development of electric vehicles is urgent due to resource and environmental issues, and lithium secondary batteries are being studied as power sources for electric vehicles. Lithium secondary batteries are required to have good cycle characteristics such that battery characteristics such as battery capacity and battery internal resistance do not change even after repeated charge and discharge, and are expected to be used in a wide temperature range, such as power supplies for electric vehicles. In such applications, good cycle characteristics, particularly at high temperatures, are required.

At present, as a positive electrode active material of a lithium secondary battery, LiC is required to constitute a 4V-class secondary battery.
Lithium transition metal composite oxides such as oO ₂ , LiNiO ₂ and LiMn ₂ O ₄ are preferably used.
O _2, in addition to synthesis easy and handling is relatively easy, since it is excellent in charge-discharge cycle characteristics, a secondary battery using LiCoO ₂ as the positive electrode active material has become mainstream.

[0004] However, cobalt is a scarce resource, and a secondary battery using LiCoO ₂ as a positive electrode active material is difficult to cope with future mass production and enlargement in view of the power source for electric vehicles, and is expensive. Must also be very expensive. Therefore, instead of cobalt, Li which is abundant as a resource and contains inexpensive manganese as a constituent element
A secondary battery using Mn ₂ O ₄ as a positive electrode active material is expected.

In general, in a lithium secondary battery, the cycle deterioration falls within a practical level at a relatively low temperature of 40 ° C. or less, but when the temperature becomes as high as about 60 ° C. at which the battery reaction is activated, the cycle deterioration occurs. There is a problem that it is below the practical level. This is because, when charge and discharge are repeated at a high temperature of about 60 ° C., the positive electrode and the negative electrode active material undergo structural changes, and the capacitance component in a specific potential region decreases. It is conceivable that the internal resistance of the battery increases due to the decrease in the resistance. However, it is practically difficult to eliminate structural changes of the active material itself, swelling of the positive and negative electrodes, and the like.

In particular, a secondary battery using LiMn ₂ O ₄ as a positive electrode active material has a large structural change due to repeated charging and discharging, and therefore has a large decrease in capacity and poor cycle characteristics compared to LiCoO ₂ . Therefore, in a secondary battery using LiMn ₂ O ₄ as the positive electrode active material, the above-mentioned problem becomes more serious.

[0007]

SUMMARY OF THE INVENTION The present inventor has proposed LiMn.
_In order to solve the above-mentioned problem that is particularly serious in a secondary battery using ₂ O ₄ as a positive electrode active material, the present inventors have conducted intensive studies and repeated experiments. It has been found that by reducing the components in advance and swelling the positive and negative electrodes in advance, it is possible to keep cycle deterioration within the range of a practical level.

The present invention is based on this finding. In a lithium secondary battery using an inexpensive lithium manganese composite oxide having a spinel structure as a main cathode active material, a cycle by charge and discharge while achieving a large discharge capacity is achieved. It is an object of the present invention to provide a processing method capable of suppressing deterioration, particularly cycle deterioration at a high temperature, and improving cycle characteristics at a high temperature to a practical level.

[0009]

The aging treatment method for a lithium secondary battery according to the present invention comprises a spinel structure lithium manganese composite oxide having a basic composition of LiMn ₂ O ₄ and a layered rock salt structure having a basic composition of LiNiO _2. An aging treatment method for a lithium secondary battery formed by assembling a positive electrode using a mixture obtained by mixing a lithium nickel composite oxide with a positive electrode active material and a negative electrode, wherein the assembled battery is
It is stored at a storage temperature of not less than 90 ° C and not less than 90 ° C. In other words, the aging treatment method of the lithium secondary battery of the present invention uses a spinel-structured lithium manganese composite oxide as the main positive electrode active material by a simple means such as storing the battery in which the positive electrode and the negative electrode have been assembled in a high-temperature atmosphere. It is intended to improve the characteristics of a lithium secondary battery used as a material.

How the aging process works on the lithium secondary battery is not known at present, but the following two are conceivable. Part 1
First, by storing at a relatively high temperature of 60 ° C. or more and 90 ° C. or less, the charge / discharge capacity component in a potential region that is reduced by repeating charge / discharge is reduced in advance. The charge / discharge capacity component in this potential region is a relatively unstable capacity component caused by a structural change of the spinel-structured lithium manganese composite oxide, and decreases preferentially as charge / discharge is repeated. Therefore, it is considered that by reducing the capacity component in advance, although the initial discharge capacity of the battery is reduced, a battery with a small capacity reduction can be obtained by subsequent charging and discharging.

Further, by performing the aging treatment in a state of being charged to some extent, lithium ions in the negative electrode are deactivated at a certain ratio, and the amount of lithium that can reversibly move between the positive and negative electrodes decreases. For this reason, it is considered that the utilization rate of the positive electrode active material is reduced, the load on the positive electrode side is reduced, and a battery with a small capacity reduction can be obtained by subsequent charging and discharging.

The other is that by performing an aging treatment, the electrode is swelled to an extreme state in advance and is brought into a stable state. In general,
Electrode mixes powdered active material with conductive material and binder,
It is formed by applying a paste-like positive electrode mixture to the surface of the current collector or the like. The electrolyte infiltrates the electrode, and the electrode swells. The swelling of the electrode is promoted by the repetitive volume change of the active material accompanying the desorption / insertion of lithium ions in the charge / discharge reaction. It is considered that the internal resistance of the battery increases due to an increase in the DC resistance due to the loss of the electron conduction path inside the electrode due to the swelling of the electrode.
Therefore, if swelling is promoted by keeping the electrode at a predetermined temperature in advance, it is considered that swelling of the electrode does not progress with subsequent charge / discharge, and a change in internal resistance due to loss of the electron conduction path can be suppressed. Can be

Therefore, by performing this aging treatment, a secondary battery having a small decrease in discharge capacity and a small increase in internal resistance even after repeated charging and discharging, that is, a lithium secondary battery having good cycle characteristics can be obtained. Will be obtained. In particular, at high temperatures, cycle deterioration such as a decrease in discharge capacity due to charge / discharge and an increase in internal resistance was remarkable, so that the cycle deterioration was efficiently suppressed, and lithium batteries having good cycle characteristics at high temperatures were obtained. The next battery will be obtained.

As described above, by performing the aging treatment, the initial discharge capacity of the lithium secondary battery is reduced to some extent. However, the lithium secondary battery to which this aging treatment method is applied is characterized by a spinel structure lithium manganese composite oxide as a main active material and a layered rock salt structure lithium nickel composite oxide having a large theoretical capacity as an auxiliary active material. By using a mixture mixed as a positive electrode active material, the reduced discharge capacity can be sufficiently compensated for, and a secondary battery having a large initial discharge capacity can be obtained.

Therefore, the aging treatment method for a lithium secondary battery according to the present invention can be applied to a lithium secondary battery using a lithium manganese composite oxide having a spinel structure and a high safety as a main active material for a positive electrode active material. This is a processing method capable of suppressing the cycle deterioration due to charge and discharge, particularly the cycle deterioration at a high temperature, and improving the cycle characteristics at a high temperature to a practical level while achieving the discharge capacity.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the aging treatment method for a lithium secondary battery according to the present invention will be described in detail. First, the configuration of a lithium secondary battery to which the aging method of the present invention is applied will be described, and then the aging process will be described.

<Structure of Lithium Secondary Battery> A lithium secondary battery to which the aging treatment method of the present invention is applied includes a lithium manganese composite oxide having a spinel structure having a basic composition of LiMn ₂ O ₄ and a basic composition of LiNiO _2. A lithium secondary battery formed by assembling a positive electrode having a mixture of a layered rock salt structure lithium nickel composite oxide and a negative electrode as a positive electrode active material. In general, a lithium secondary battery mainly includes a positive electrode, a negative electrode, a separator sandwiched between the positive electrode and the negative electrode, and a non-aqueous electrolyte. Therefore, the present lithium secondary battery may follow this configuration.

The positive electrode active material includes a lithium manganese composite oxide having a spinel structure having a basic composition of LiMn ₂ O ₄ as a main active material, and a lithium nickel composite oxide having a layered rock salt structure as an auxiliary active material. Is used.

Here, the term “basic composition is used” means not only a composition represented by the composition formula but also a part of sites such as Li, Mn, and Ni in the crystal structure by another element. It is meant to include substitutions. Further, it is meant that not only those having a stoichiometric composition but also those having a non-stoichiometric composition in which some elements are missing or the like are included.

The lithium manganese composite oxide having a spinel structure having a basic composition of LiMn ₂ O ₄ has a composition of Li _{1 + a Mb} Mn _2-ab O ₄ (M is Ti, V, Cr, Fe,
At least one selected from Co, Ni, Zn, Cu, W, Mg and Al; 0 <a <0.2; 0 <b <0.5) is desirable. This Li _{1 + a Mb} Mn _2-ab O ₄
Has a structure in which part of the Mn site in the crystal is replaced with lithium and another metal M in order to further stabilize the crystal structure. Specifically, it is more preferable to use Ni as the other metal M from the viewpoint of easy synthesis and cost. The substitution ratio is 0 <a <0.2, 0 <b <
0.5. This is because when a ≧ 0.2 or b ≧ 0.5, unreacted materials remain to lower the crystallinity or decrease the capacity too much.

For example, when producing a spinel-structured lithium manganese composite oxide represented by Li _{1 + a} Ni _b Mn _2-ab O ₄ , LiOH, Ni (OH) ₂ , MnO ₂
Can be synthesized by mixing a predetermined amount of each and baking in an oxygen stream at a temperature of about 800 ° C. for about 12 hours.

The lithium nickel composite oxide having a layered rock salt structure having a basic composition of LiNiO ₂ has a composition of LiNi _x M1 _y M2 _z O ₂ (M1 is at least one selected from Co and Mn; M2 is Al , B, Fe, Cr, Mg; x + y + z = 1; 0.5 <
x <0.95; 0.01 <y <0.4; 0.001 <z
<0.2) is desirable.

The LiNi _x M1 _y M2 _z O ₂ is obtained by partially substituting Ni sites with two or more kinds of elements M1 and M2 having different roles. The ratio of Ni remaining without being substituted, that is, the value of x in the composition formula, is 0.5 <x <
Preferably, it is 0.95. When x ≦ 0.5 as compared with the preferable range, not only the layered rock salt structure but also a second phase such as a spinel structure is generated and the capacity is excessively reduced. If ≧ 0.95, the replacement effect is too small to form a battery having the desired good cycle characteristics. Note that 0.7 <x <0.
More preferably, it is in the range of 9.

The element M1 selected from Co and Mn mainly plays a role in stabilizing the crystal structure of the lithium nickel composite oxide. By stabilizing the crystal structure at M1, the cycle characteristics of the non-aqueous electrolyte secondary battery are kept good, and deterioration of the battery capacity due to charge / discharge at high temperature and storage at high temperature is particularly 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 desirably 0.01 <y <0.4. Compared to those in the preferred range, when y ≦ 0.01, the cycle characteristics are not good because the crystal structure of the secondary battery is not sufficiently stabilized, and when y ≧ 0.4, the lithium nickel composite The crystallinity of the oxide is undesirably reduced. It is more preferable that 0.05 <y <0.3. Further, Co has the advantage of suppressing the capacity reduction due to the element substitution and Li (Co, Ni) O ₂ being an all-solid solution type and having the advantage of minimizing the crystallinity reduction. Then, it is more desirable to use Co for M1.

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. Because of this role, the substitution rate of M2, that is, z in the composition formula,
Is preferably 0.001 <z <0.2.
Compared with those in this preferred range, when z ≦ 0.001,
A sufficient effect on safety cannot be obtained, and z ≧ 0.
In the case of 2, the capacity of the positive electrode is undesirably reduced. It is more preferable that 0.004 <z <0.1. Furthermore, Al has improved thermal stability,
With the advantage of minimizing capacity loss,
Considering this, it is more desirable to use Al for M2.

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

Lithium manganese composite oxide and lithium nickel
Mixing with a nickel composite oxide is used for normal mixing.
It can be done by the method that is used. For example, L synthesized by the above method
i_{1+ a}Ni_bMn_2-abO_FourAnd LiNi_xCo_yAl_zO_TwoWhen
Is crushed and classified using a device such as a ball mill.
What is necessary is just to make it uniform by a device such as a V-type mixer.
No. In addition, mixing is performed by simultaneously crushing and classifying both.
It is possible. Mixing is either wet or dry
Can also be adopted. LiNi_xCo_yAl_zO_TwoPossibility of decomposition
In consideration of the above, it is desirable to carry out the dry method.

The mixing ratio of the lithium manganese composite oxide and the lithium nickel composite oxide is 100
wt%, lithium nickel composite oxide is 20%
It is desirable that the content be mixed in the range of not less than wt% and not more than 50 wt%.
When the content of the lithium nickel composite oxide is less than 20 wt%, the initial discharge capacity when the aging treatment is performed is smaller than that in the proper range, and the 50 wt%
%, The cost is higher than that in the proper range, and the safety of the lithium nickel composite oxide is slightly inferior due to low thermal stability.

For the positive electrode, a conductive material and a binder are mixed with a powder of a mixture of the above-mentioned lithium manganese composite oxide and lithium nickel composite oxide serving as a positive electrode active material, and an appropriate solvent is added thereto to form a paste. The positive electrode mixture can be 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. In this case, application, drying, pressing and the like may be performed according to a usual method.

The conductive material is for ensuring the electrical conductivity of the positive electrode, and may be a mixture of one or more powdered carbon materials such as carbon black, acetylene black and graphite. it can. The binder plays a role of binding the active material particles and the conductive material particles, and it is possible to use a fluororesin such as polytetrafluoroethylene, polyvinylidene fluoride, fluororubber, or a thermoplastic resin such as polypropylene or polyethylene. it can. As a solvent for dispersing these active material, conductive material and binder, N-
An organic solvent such as methyl-2-pyrrolidone can be used.

The negative electrode can be formed by sheet-forming metal lithium as the negative electrode active material or by pressing the sheet-shaped metal to a current 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 constituted 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 the carbon material is used as the negative electrode active material, a fluorine-containing resin such as polyvinylidene fluoride or the like can be used as the negative electrode binder, and an organic solvent such as N-methyl-2-pyrrolidone can be used as the solvent, similarly to the positive electrode.

The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode and the negative electrode and holds the electrolyte, and a thin microporous film of polyethylene, polypropylene or the like 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.

As the organic solvent for dissolving the lithium salt, an aprotic organic solvent is used. For example, a solvent mixture 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 linear 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. Any one of these can be used alone, or two or more can be used as a mixture.

Instead of using the separator and the nonaqueous 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. It is also possible to use a gel electrolyte in which the above non-aqueous electrolyte is trapped in a solid polymer matrix such as polyacrylonitrile.

The lithium secondary battery constituted by the above-mentioned components can be of 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. Through such an assembly process, a lithium secondary battery is completed.

<Aspect of Aging Process> This aging process 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 completes 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, for example, the formed electrode body may be impregnated with a non-aqueous electrolyte in another container other than the battery case, and the aging treatment may be performed in the container or the like. It is possible.

According to the aging treatment method of the present invention, the assembled lithium secondary battery is stored at a storage temperature of 60 ° C. or more and 90 ° C. or less. If the storage temperature is lower than 60 ° C., the effect of aging, that is, the effect of reducing the discharge capacity or suppressing the increase in the internal resistance of the battery cannot be expected. If the storage temperature is higher than 90 ° C., the deterioration, decomposition, Short circuit due to softening of the separator and liquid leakage due to increase in internal pressure occur, which is not preferable. The storage temperature is desirably 60 ° C. or more and 80 ° C. or less because the effect of aging is greater. It is clear from the results obtained by experiments that the storage time should be 3 hours or more, but it is more effective to set the storage time to 6 hours or more. Also, the storage method is not particularly limited, and any storage method that can be generally used may be adopted.

In the aging process, the state of charge (SOC) of the battery also affects the cycle characteristic improving effect.
That is, by pre-charging the assembled battery, that is, charging it before storing it, the subsequent aging process becomes more effective. The method of charging the lithium secondary battery before storage is not particularly limited,
What is necessary is just to charge using a charger generally used.

Here, the “state of charge (SOC)” is defined as 100%, that is, a fully charged state, where the upper limit of the battery voltage can be obtained in the range of battery voltage that can be charged and discharged reversibly. The state of charge at which the lower limit of the battery voltage is obtained is 0%, that is, the state of charge (S
OC: State of Charge).

The state of charge (SOC) may be any state. For example, it has been found from experiments that it is desirable to store the battery in a state where the SOC is 60% or more. This is because when a certain amount of lithium is occluded in the negative electrode active material, or when the aging treatment is performed in a state where the amount of lithium in the positive electrode active material is small to some extent,
This is because the charge / discharge capacity component in a certain potential region described above can be effectively reduced, and lithium ions can be effectively depleted in the negative electrode.

<Other Embodiments> The lithium secondary battery to which the aging treatment method of the present invention is applied and the embodiment of the aging treatment method have been described above. However, the above-described embodiment is merely one embodiment, and the present invention is not limited thereto. The aging treatment method for a lithium secondary battery according to
The present invention can be implemented in various forms with various modifications and improvements based on the knowledge of those skilled in the art.

[0044]

EXAMPLES A lithium secondary battery was fabricated based on the above embodiment, and the aging treatment was performed to evaluate its effect. Hereinafter, the produced lithium secondary battery, aging treatment, cycle charge / discharge test, evaluation of initial discharge capacity with respect to the mixing ratio of lithium nickel composite oxide, and evaluation of cycle characteristics will be described.

<Prepared Lithium Secondary Battery> The positive electrode active material includes a lithium manganese composite oxide having a spinel structure represented by a composition formula Li _1.05 Ni _0.1 Mn _1.85 O ₄ and a composition formula Li
A mixture was used in which lithium nickel composite oxides represented by Ni _0.8 Co _0.15 Al _0.05 O ₂ were mixed at various ratios shown below. Li _1.05 Ni _0.1 Mn _1.85 O ₄ is LiO
H, Ni (OH) ₂ and MnO ₂ are mixed in predetermined amounts, respectively.
It is synthesized by firing at a temperature of 800 ° C. for 12 hours in an oxygen stream. LiNi _0.8 Co
_0.15 Al _0.05 O ₂ is LiOH · H ₂ O, Ni (O
H) ₂ , Co ₃ O ₄ , and Al (OH) ₃ are each mixed in a predetermined amount, and the mixture is fired at a temperature of 850 ° C. in an oxygen stream for 20 hours. The lithium nickel composite oxide was mixed so as to be 10, 20, 35, and 50 wt%, respectively, when the total of both composite oxides was 100 wt%. The mixing was performed by a dry method using a mixer.

The positive electrode was prepared by first mixing 90 parts by weight of the above mixture serving as a positive electrode active material, 7 parts by weight of acetylene black as a conductive material, and 9 parts by weight of polyvinylidene fluoride as a binder. N-methyl-2-pyrrolidone was added to prepare a paste-like positive electrode mixture, and then the paste-like positive electrode mixture was applied to both sides of a 20-μm-thick aluminum foil current collector and dried, Then, it is compressed by a roll press and the thickness of the positive electrode mixture is 40 per side.
A sheet having a thickness of μm was prepared. This sheet-shaped positive electrode was cut into a size of 54 mm × 450 mm for use.

For the negative electrode to be opposed, artificial graphite was used as an active material. First, 90 parts by weight of artificial graphite serving as a negative electrode active material, 10 parts by weight of polyvinylidene fluoride as a binder were mixed, and an appropriate amount of N-methyl-2-pyrrolidone was added as a solvent, and a paste-like negative electrode mixture was added. Prepare the material, then
This paste-like negative electrode mixture is applied to both sides of a copper foil current collector having a thickness of 10 μm, dried, and then compressed by a roll press to obtain a sheet-like negative electrode mixture having a thickness of 50 μm per side. Produced. This sheet-shaped negative electrode is 56 mm x 5
It was cut into a size of 00 mm and used.

The above-described positive electrode and negative electrode were wound with a polyethylene separator having a thickness of 25 μm and a width of 58 mm interposed therebetween to form a roll-shaped electrode body. Then, the electrode body was inserted into a 18650-type cylindrical battery case (outside diameter 18 mmφ, length 65 mm), a non-aqueous electrolyte was injected, and the battery case was sealed to produce a cylindrical lithium secondary battery. . 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 1: 1 was used.

<Aging Treatment> Each of the produced lithium secondary batteries was first subjected to conditioning. Conditioning is performed at 0.2 mA / 20 ° C.
The test was performed by charging the battery at a constant current of 4.2 V at a current density of cm ² and then discharging the battery at a constant current of 3.0 mA at a current density of 0.2 mA / cm ² . With the discharge capacity at the time of this one-cycle charge / discharge, SOC = 100% and SO
It was assumed that it corresponded to the difference capacity of the charged state with C = 0%. Next, an aging treatment was applied to each of the conditioned secondary batteries. Note that, depending on the conditions of the aging process, the respective charging states (S
OC). The preliminary charging was performed by constant current charging at a current density of 0.2 mA / cm ² . The conditions of the aging process are as follows: the state of charge (SOC) is 0%,
%, 60%, and 100%, and stored in a constant temperature bath at a storage temperature of 60 ° C., 70 ° C., 80 ° C., and 90 ° C. in each SOC state. 3 hours, 6 hours,
The treatment was performed for 24 hours, 72 hours, and 3 weeks.

<Cycle charge / discharge test> After the aging treatment, a charge / discharge cycle test was performed to evaluate the cycle characteristics of each lithium secondary battery. The charge / discharge cycle test was conducted at a temperature of 60 ° C., which is regarded as the upper limit of the actual operating temperature range of the battery, and the high-temperature cycle characteristics were evaluated. The conditions are as follows: charge at a constant current of 1.0 mA / cm ^{2 to} a charge upper limit voltage of 4.2 V, and then discharge at a constant current of 1.0 mA / cm ² at a discharge lower limit voltage of 3.0 V
The charge / discharge for discharging up to one cycle was defined as one cycle, and this cycle was repeated 500 cycles.

For each lithium secondary battery, the discharge capacity in each cycle and the DC resistance of the battery were measured. The discharge capacity at the first cycle is defined as the initial discharge capacity, and the capacity retention rate is calculated as {discharge capacity after each cycle /
It was calculated using the formula: initial discharge capacity × 100%｝. In the cycle, a value obtained by calculation using the formula {(average charge voltage−average discharge voltage) / (charge / discharge current × 2)} was adopted as the DC resistance. And 1
Using the DC resistance at the cycle as the initial DC resistance, the resistance increase rate is expressed as ｛(DC resistance after each cycle / initial DC resistance−
1) × 100%}.

<Evaluation of Initial Discharge Capacity with Mixing Ratio of Lithium Nickel Composite Oxide> Changes in the initial discharge capacity with and without the aging treatment were examined for the above various lithium secondary batteries having different mixing ratios of the lithium nickel composite oxide. Was. As an example, FIG. 1 shows the initial discharge capacities of a lithium secondary battery that has not been subjected to aging treatment and a lithium secondary battery that has been subjected to aging treatment by storing at a storage temperature of 60 ° C. for 3 weeks in a state of SOC 100%. . In FIG. 1, the mark ● indicates a lithium secondary battery not subjected to the aging treatment, and the mark ○ indicates the one subjected to the aging treatment.

FIG. 1 shows that the initial discharge capacity increases as the mixing ratio of the lithium nickel composite oxide increases regardless of the presence or absence of the aging treatment. This is because the theoretical capacity of the lithium nickel composite oxide is larger than the theoretical capacity of the lithium manganese composite oxide,
It is considered that the effect was exhibited by mixing. Further, the lithium secondary battery subjected to the aging treatment has a lower initial discharge capacity as a whole than the battery not subjected to the aging treatment. This is presumably because the aging treatment reduced the charge / discharge capacity component in the potential region, which was reduced by repeated charge / discharge. Therefore, the mixing ratio of the lithium-nickel composite oxide should be 20 wt% or more and 50 wt% or less when the total of both composite oxides is 100 wt% from the viewpoint of compensating for the discharge capacity reduced by performing the aging treatment. Was confirmed to be desirable.

<Evaluation of Cycle Characteristics> As a typical result of the above-described charge / discharge cycle test, the mixing ratio of the lithium manganese composite oxide and the lithium nickel composite oxide was 6%.
A cycle characteristic of a lithium secondary battery using a mixture of 5 wt%: 35 wt% as a positive electrode active material when stored at a storage temperature of 60 ° C. for 3 weeks in a state of SOC = 100% will be described.

FIG. 2 shows the cycle dependency of the discharge capacity per unit weight of the positive electrode active material of the secondary battery stored at a storage temperature of 60 ° C. for 3 weeks in the state of SOC = 100%, and FIG. 4 shows the cycle dependency of the capacity retention rate of the sample. FIG. 4 shows the cycle dependence of the DC resistance of the battery. 2 to 4 also show the cycle dependency of the secondary battery that has not been subjected to the aging process.

In the case of a secondary battery not subjected to the aging treatment, the initial discharge capacity was 12% per unit weight of the positive electrode active material.
At 4 mAh / g, the discharge capacity decreases as the cycle proceeds thereafter, and the capacity retention after 500 cycles becomes about 60%. Further, the resistance increase rate after 500 cycles is as high as about 90%.

On the other hand, a secondary battery stored at a storage temperature of 60 ° C. for 3 weeks under the condition of SOC = 100% is as follows:
Although the initial discharge capacity was 107 mAh / g, which was about 14% lower than that of the case where aging was not performed, the decrease in the discharge capacity thereafter was small, and the capacity retention after 500 cycles was about 75%. %, Which is a very good capacity retention rate. Furthermore, the increase in DC resistance that occurs as the cycle proceeds is small, and the resistance increase rate after 500 cycles is only 40%.

As is apparent from the results, by performing the aging treatment of storing at a storage temperature of about 60 ° C. for a predetermined time, the initial discharge capacity is reduced, but the capacity reduction due to the progress of the cycle is greatly suppressed. It can be seen that the discharge capacity finally exceeds the discharge capacity of the lithium secondary battery not subjected to the aging 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, this aging treatment improves the cycle characteristics of a lithium secondary battery using a mixture of a spinel structure lithium manganese composite oxide and a layered rock salt structure lithium nickel composite oxide as a positive electrode active material. It can be confirmed that the
Since the charge-discharge cycle test is performed at such a high temperature, it can be confirmed that the cycle characteristics at high temperatures can be particularly improved.

Next, as a result of a charge / discharge cycle test on the secondary batteries aged under the above various 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. Here, as a representative example, a result of performing a charge / discharge cycle test on a lithium secondary battery using a mixture in which a mixture ratio of a lithium manganese composite oxide and a lithium nickel composite oxide is 65 wt%: 35 wt% as a positive electrode active material. Is shown. In each data in the table, the numerical value on the left is the initial discharge capacity (mAh / g), the numerical value on the center is the capacity retention rate (%) after 500 cycles, and the numerical value on the right is the DC resistance increase rate (%) after 500 cycles. ) Respectively. In the data column,-indicates that a battery failure has occurred or that charging / discharging for 500 cycles could not be performed. Incidentally, the initial discharge capacity of the secondary battery that was not subjected to the aging treatment was 124 m.
Ah / g, and the capacity retention rate after 500 cycles is 60
%, The DC resistance increase rate after 500 cycles was 90%.

[0061]

[Table 1]

As can be seen from the data shown in Table 1, the effects of maintaining the capacity and suppressing the increase in the DC resistance are obtained regardless of the aging treatment performed under any conditions. In particular, S
It can be confirmed that the effect is large when the aging treatment is performed for 6 hours or more in the charged state of OC = 60% or more. When the storage temperature is 90 ° C., the SOC is high,
If the storage time is long, a battery failure or the like may occur and caution is required. Therefore, it can be confirmed that the most preferable storage temperature range is 60 ° C to 80 ° C.

[0063]

According to the aging treatment method for a lithium secondary battery of the present invention, a positive electrode active material is prepared by mixing a mixture obtained by mixing a lithium nickel composite oxide having a large theoretical capacity with an inexpensive and highly safe lithium manganese composite oxide having a spinel structure. This is a method for improving the battery characteristics by a simple process of storing the lithium secondary battery used in the above at a predetermined temperature. According to the aging treatment method of the present invention, while achieving a large discharge capacity, a decrease in the discharge capacity due to repeated charging and discharging is small, and an increase in internal resistance is small. Obtainable.

[Brief description of the drawings]

FIG. 1 is a diagram showing a change in initial discharge capacity depending on the presence or absence of aging treatment in lithium secondary batteries having different mixing ratios of lithium nickel composite oxide.

FIG. 2 is a diagram showing the cycle dependence of the discharge capacity per unit weight of a positive electrode active material of a lithium secondary battery that has been stored and aged at a storage temperature of 60 ° C. for 3 weeks at an SOC of 100%. .

FIG. 3 is a diagram showing the cycle dependency of the capacity retention ratio of a lithium secondary battery that has been stored and aged for 3 weeks at a storage temperature of 60 ° C. with SOC = 100%.

FIG. 4 is a diagram showing the cycle dependency of the battery DC resistance of a lithium secondary battery that has been stored and aged for 3 weeks at a storage temperature of 60 ° C. in a state of SOC = 100%.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takuaki Okuda 41-Cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. (72) Inventor Hideyuki Nakano Nagakute-cho, Aichi-gun, Aichi Prefecture No. 41, Chochu Yokomichi, Toyota Central Research Institute, Inc. (72) Inventor Koji Takeuchi 41, Nagakute-cho, Aichi-gun, Aichi Prefecture, Japan No. 41, Toyota Chuo Research Institute, Inc. (72) Inventor Tetsuro Kobayashi 41 Toyota Chuo R & D Co., Ltd., No. 41, Nagachute-cho, Nagakute-cho, Aichi-gun, Japan (72) Inventor: Takeshi Sasaki 41-Cho, Toyota-Chuo-Yokohama, Nagakute-cho, Aichi-gun, Aichi, Japan 72) Inventor Masao Kanzaki 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Institute, Inc. (72) Inventor Ukyo Male 41-Chome, Toyoda Central Research Institute, Inc., 41-Chome, Toyoda Central R & D Co., Ltd. 5F029 AJ05 AK03 AL06 AM03 AM04 AM05 AM07 CJ02 CJ28 HJ01 HJ02 HJ14 5H050 AA07 BA17 CA08 CA09 CB07 EA08 FA17 FA GA02 GA27 HA01 HA02 HA14 HA19

Claims (3)

[Claims] 1. A spinel-structured lithium manganese composite oxide having a basic composition of LiMn ₂ O ₄ and a basic composition of LiN
An aging method for a lithium secondary battery formed by assembling a positive electrode having a mixture of a layered rock salt structure lithium nickel composite oxide as iO ₂ and a negative electrode as a positive electrode active material, comprising the steps of: Aging the lithium secondary battery at a storage temperature of 60 ° C. or more and 90 ° C. or less. 2. The mixing ratio of the lithium-nickel composite oxide in the mixture is such that the sum of the lithium-nickel composite oxide and the lithium-manganese composite oxide is 100 watts.
The aging treatment method for a lithium secondary battery according to claim 1, wherein the amount is 20 wt% or more and 50 wt% or less when t% is used. 3. The aging treatment method for a lithium secondary battery according to claim 1, wherein the assembled battery is pre-charged, and a state of charge (SOC) of the battery is 60% or more. .