JP2010033786A - Evaluation method and manufacturing method of positive electrode active material for lithium secondary battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a novel evaluation method of a positive electrode active material for a lithium secondary battery capable of precisely evaluating easiness to perform micro short circuit (voltage drop). <P>SOLUTION: Ion-exchange water is added to lithium transition metal oxide powder to make up slurries, into which a magnet of 50 mT to 200 mT is inputted and stirred, and then, an iron volume adhered to the magnet is detected to select lithium transition metal oxide powder of which an iron volume per mass satisfies 0 ppb<iron volume<75 ppb. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method for evaluating and manufacturing a positive electrode active material for a lithium secondary battery, particularly a lithium secondary battery mounted in an electric vehicle or a hybrid electric vehicle.

  Lithium secondary batteries have features such as high energy density and long life, so they are used as power sources for home appliances such as video cameras, portable electronic devices such as notebook computers and mobile phones. Recently, it is also used for large batteries mounted on electric vehicles (EV) and hybrid electric vehicles (HEV).

  A lithium secondary battery is a secondary battery with a structure in which lithium is melted as ions from the positive electrode during charging, moves to the negative electrode and is stored, and reversely, lithium ions return from the negative electrode to the positive electrode during discharging. It is known to be caused by the potential of the positive electrode active material.

Known positive electrode active materials for lithium secondary batteries include lithium manganese oxide (LiMn ₂ O ₄ ) having a spinel structure, and lithium transition metal oxides such as LiCoO ₂ and LiNiO ₂ having a layer structure. For example, since LiCoO ₂ has a large charge / discharge capacity and excellent diffusibility for occluding and desorbing lithium ions, most of the commercially available lithium secondary batteries are LiCoO having a high voltage of 4 V as a positive electrode active material. ₂ is used. However, since Co is extremely expensive, development of a lithium transition metal oxide having a layer structure that can be used as a substitute material for LiCoO ₂ is underway.

  This type of lithium transition metal oxide is generally obtained by mixing a raw material compound such as a lithium compound or a cobalt compound in a dry or wet manner, firing it, crushing, crushing or classifying it. The lithium transition metal oxide obtained as described above is suspended as a positive electrode active material in a solvent together with a conductive agent and a binder to form a slurry, and this slurry is applied onto a current collector to produce a positive electrode.

  In this type of lithium secondary battery, from the viewpoint of maintaining output and ensuring safety, development of a lithium secondary battery that is unlikely to cause a micro short circuit is required. As one of the causes of micro short-circuits in lithium secondary batteries, the presence of metallic foreign substances such as iron (Fe) and SUS mixed in the raw material of the active material and the manufacturing process of the active material has been pointed out. Therefore, conventionally, several manufacturing methods for preventing the contamination of these metallic foreign substances have been proposed for the manufacturing method of the positive electrode active material for a lithium secondary battery.

  For example, Patent Document 1 discloses granulating while adding a binder to a raw material powder obtained by accurately weighing and mixing lithium carbonate and cobalt oxide, and then firing the granulated product in an oxygen-containing atmosphere. When the fired product is pulverized to produce a positive electrode active material for a non-aqueous electrolyte secondary battery made of a lithium cobalt complex oxide, the baked product is pulverized into a cemented carbide pin and a surface-hardened stainless steel. A method for producing a positive electrode active material for a non-aqueous electrolyte secondary battery is disclosed, which is performed using a pin mill composed of a disk.

In Patent Document 2, as a method for producing a positive electrode material for a lithium secondary battery, pure water is added to Mn ₂ O ₃ , AlOOH and LiOH · H ₂ O to prepare a slurry, and a circulating medium stirring is performed while stirring the slurry. By pulverizing until the average particle size of the solid content in the slurry becomes 0.3 μm using a mold wet pulverizer, and installing a magnet in the slurry pipe (magnetic field strength: 2000 gauss, number of times the magnetic field passes : 3 times, passing speed of magnetic field: 0.3 m / sec), magnetic metal impurities in the raw material are removed by magnetic separation, then wet pulverized, then spray dried using a spray dryer, and dried A production method is described in which the particles are baked at 900 ° C. for 10 hours, the obtained fired product is slowly cooled at 1 ° C./min, and then sieved with a sieve having an opening of 20 μm to obtain a positive electrode material.

  In Patent Document 3, after the impact pulverization treatment was performed on the bulk fired product of the lithium transition metal composite oxide by an impact pulverizer to obtain a lithium transition metal composite oxide powder having an average particle size of 5 to 25 μm, For the lithium transition metal composite oxide powder, using an air classifier, the classification point excluding the low particle diameter component is any value of 0.6 × D μm or less, and the classification point excluding the high particle diameter component is 1. A positive electrode comprising a lithium-transition metal composite oxide powder having an average particle size of 5 to 25 μm, which is subjected to classification treatment by setting to any value of 2 × D μm or more and from which the low particle size component and high particle size component are removed A method for producing a positive electrode active material for a lithium ion secondary battery characterized by obtaining an active material is disclosed.

Patent Document 4 discloses Li _x MO ₂ (M is a layered crystal structure) as a material capable of electrochemical desorption and insertion of Li by firing a raw material for an active material for a lithium secondary battery. At least one metal selected from the group consisting of Mn, Fe, Co, and Ni, which may further contain Mg and / or Al (0.95 <x <1.05). In the method for producing an active material, the iron quality of the initial raw material is controlled, the metallic iron in the resulting active material for a lithium secondary battery is extracted and measured with a bromine-methanol solution, and the resulting metal Disclosed is a method for producing an active material for a lithium secondary battery, characterized in that the quality of the functional iron is less than 4 ppm.

Claim 1 of JP 2000-58054 A Examples [0122]-[0123] of Japanese Patent Application Laid-Open No. 2004-311297 Claim 1 of JP-A-2008-108574 Claim 1 of Japanese Patent No. 3620744

A lithium secondary battery mounted on an electric vehicle (EV), a hybrid electric vehicle (HEV), etc. is mounted with a large number of batteries connected to one vehicle body. Since there is a possibility of disappearing, it is particularly required to eliminate the possibility of a micro short circuit.
Moreover, since it is assumed that an electric vehicle (EV) and a hybrid electric vehicle (HEV) are left for a long time under the hot weather in summer, the lithium secondary battery is temporarily charged in a high temperature state for a long time while being charged. If the battery is left unattended, the lithium secondary battery is likely to be short-circuited (voltage drop). Therefore, in developing a positive electrode active material for a lithium secondary battery mounted on an electric vehicle (EV) or a hybrid electric vehicle (HEV), It is required that short-circuiting (voltage drop) is difficult even when the battery is kept charged for a long time in a charged state.

  By the way, regarding a positive electrode active material of a lithium secondary battery mounted on an electric vehicle (EV) or a hybrid electric vehicle (HEV), the present applicant has already disclosed a magnet in a patent document (PCT / JP2008 / 051702) that has not been disclosed yet. Paying attention to the relationship between the concentration of magnetic deposits magnetized on the electrode and the minute short circuit (voltage drop) of the lithium secondary battery, a liquid is added to the lithium transition metal oxide powder to form a slurry, and a 450-600 mT magnet is added to this slurry. Disclosed is a method for evaluating the voltage drop characteristics of the positive electrode active material on the basis of the magnetic substance concentration per mass of the lithium transition metal oxide powder by detecting the concentration of the magnetic substance adhering to the magnet. ing.

  The present invention relates to a positive electrode active material used for a lithium secondary battery, particularly a positive electrode active material used for a lithium secondary battery of an electric vehicle (EV) or a hybrid electric vehicle (HEV), based on knowledge obtained through further research. Even if it is difficult to discriminate with the above-mentioned evaluation method, it is possible to accurately evaluate the ease of short-circuiting, and in particular, it is easy to short-circuit (voltage drop) in a state where it is kept at a high temperature for a long time in a charged state. It is intended to provide a new method for evaluating a positive electrode active material for a lithium secondary battery, and a new method for producing a positive electrode active material for a lithium secondary battery using such an evaluation method.

  In the present invention, a liquid is added to a lithium transition metal oxide powder to form a slurry, and a 50 mT to 200 mT magnet is added to the slurry and stirred, and the amount of iron adhering to the magnet is detected. A method for evaluating a positive electrode active material for a lithium secondary battery, wherein the amount of iron per mass of the powder is 0 ppb <iron amount <75 ppb, and a lithium active material for the lithium secondary battery. The present invention proposes a method for producing a positive electrode active material for a lithium secondary battery, characterized by selecting a transition metal oxide powder.

  According to the evaluation method of the present invention, a positive active material used for a lithium secondary battery, particularly a positive active material used for a lithium secondary battery of an electric vehicle (EV) or a hybrid electric vehicle (HEV), a micro short circuit (voltage drop). It is possible to accurately evaluate the easiness of short-circuiting (voltage drop) in the state of being kept in a high temperature for a long time while being charged.

In the present invention, the amount of iron is detected using a magnet of 50 mT to 200 mT, so that the positive electrode active material used for the lithium secondary battery, particularly lithium secondary battery of an electric vehicle (EV) or a hybrid electric vehicle (HEV). With regard to the positive electrode active material used in secondary batteries, it is possible to accurately evaluate the ease of short-circuiting (voltage drop), especially the ease of short-circuiting (voltage drop) in the state of being charged and maintained at a high temperature for a long time. As a result, it is possible to provide a positive electrode active material for a lithium secondary battery that is unlikely to cause a minute short circuit (voltage drop) and that can prevent a short circuit (voltage drop) in a state of being maintained at a high temperature for a long time in a charged state. Can now.
The reason why it is possible to accurately evaluate the ease of short-circuiting (voltage drop), especially the ease of short-circuiting (voltage drop) in a state of being kept charged for a long time in a charged state, is the detection of the amount of iron When the strength of the magnet used in the above is weakened, it seems that the evaluation accuracy is usually lowered due to a decrease in the amount of iron detected, but in the case of the present invention, the strength of the magnet is reduced to an appropriate range. , Detection accuracy of iron that influences micro short circuit (voltage drop), that is, iron with strong magnetic force, is improved, and as a result, it is easy to perform micro short circuit (voltage drop), especially in a state where it is kept at a high temperature for a long time in a charged state It can be considered that the ease of short-circuiting (voltage drop) can be accurately evaluated.

BEST MODE FOR CARRYING OUT THE INVENTION

  Hereinafter, although embodiment of this invention is described, this invention is not limited to the following embodiment.

  The method for producing a positive electrode active material for a lithium secondary battery as an example of an embodiment of the present invention detects a specific amount of iron contained in a lithium transition metal oxide powder produced or obtained by a specific method, and the iron It is a method characterized by evaluating and selecting lithium transition metal oxide powder based on the amount.

(Lithium transition metal oxide powder)
The lithium transition metal oxide powder as an object of evaluation / selection is not limited in composition and particle size as long as it can be used as a positive electrode active material of a lithium secondary battery. For example, a spinel-type lithium transition metal oxide, that is, a lithium transition represented by the general formula (1) Li _{1 + x} Me _2−x O _4−δ (Me is a transition element, a typical element, or a combination thereof) Metal oxide and layered lithium transition metal oxide, that is, represented by the general formula (2) Li _{1 + x} Me _1-x O _2-δ (Me is a transition element, a typical element, or a combination thereof) Preferred examples thereof include powders made of lithium transition metal oxide (hereinafter referred to as “the present Li transition metal oxide powder”).

However, special substances having diamagnetism such as LiCoO ₂ cannot be evaluated based on the same criteria as other lithium transition metal oxides, so these are excluded from the target.

In the general formulas (1) and (2), “Me element” is a transition element, a typical element, or a combination thereof. For example, any element of Mn, Co and Ni, or an element composed of a combination of two or more of these elements can be exemplified, and among them, a case where all three elements of Mn, Co and Ni are included is preferably exemplified. be able to.
In addition to any element of Mn, Co and Ni, or an element composed of a combination of two or more of these elements, a transition element, a typical element, or a combination thereof may be used as a substitution element thereof. .
Further, the atomic ratio of oxygen may have some non-stoichiometry (for example, represented by “4-δ” or “2-δ”), or a part of oxygen may be substituted with fluorine.

(Measurement and evaluation of iron content)
A liquid is added to the lithium transition metal oxide powder to make a slurry, and a 50 mT to 200 mT magnet is put into the slurry and stirred, and the amount of iron adhering to the magnet is detected. The present Li transition metal oxide powder can be obtained by evaluating and sorting the lithium transition metal oxide powder on the basis of whether the amount of iron is within a predetermined range.

  The liquid to be slurried by adding the lithium transition metal oxide powder may be any liquid that can disperse the lithium transition metal oxide powder, and preferably includes ion-exchanged water.

  At this time, the amount of the lithium transition metal oxide powder in the slurry is such that the lithium transition metal oxide powder can be sufficiently dispersed in the slurry and the lithium transition metal oxide powder is in sufficient contact with the magnet. From the viewpoint of being able to be produced, it is preferably 10 g / L to 1500 g / L, particularly 50 g / L to 1500 g / L, and particularly preferably 100 g / L to 300 g / L.

It is important that the strength of the magnet used is 50 mT to 200 mT. As described above, even when it is difficult to accurately evaluate the amount of magnetic deposits using a magnet of 450-600 mT, by detecting the amount of iron using a magnet of 50 mT to 200 mT, Iron that has a strong magnetic force that affects a micro short circuit (voltage drop) can be selectively detected. As a result, it is easy to make a micro short circuit (voltage drop), especially in a state where it is kept at a high temperature for a long time in a charged state. It is now possible to accurately evaluate the ease of short-circuiting (voltage drop).
Further, among 50 mT to 200 mT, it is more preferable to select and use a magnet in the range of 100 mT to 150 mT, particularly a magnet in the range of 120 mT to 140 mT, depending on the application, for example, the thickness of the separator.

Regarding the quantitative relationship between the lithium transition metal oxide powder and the magnet, the lithium transition metal oxide powder is particularly 1 to 15 g / cm ² so that the lithium transition metal oxide powder is 0.3 g / cm ² or more with respect to the total surface area of the magnet. as will be ^2, among others as a 2 to 8 g / cm ^2, preferably to adjust the amount of both so that in particular 2 to 3 g / cm ² among them.

  As for the degree and time of stirring, it is preferable to stir so that the amount of iron attached to the magnet does not increase any more within 24 hours.

  The amount of iron adhering to the magnet may be determined by taking JIS G 1258: 1999 into account and dissolving the iron adhering to the magnet with an acid.

  As an evaluation criterion, in other words, as a selection criterion, the amount of iron per mass of the lithium transition metal oxide is 0 ppb <the amount of iron <75 ppb, preferably 1 ppb <the amount of iron ≦ 35 ppb, more preferably May be selected based on whether or not 2 ppb <iron content ≦ 25 ppb, preferably 5 ppb <iron content ≦ 25 ppb.

(Characteristics / Applications)
The Li transition metal oxide powder is crushed and classified as necessary, and then a lithium secondary battery,
In particular, it can be effectively used as a positive electrode active material for a lithium secondary battery mounted on an electric vehicle (EV) or a hybrid electric vehicle (HEV).

For example, a positive electrode mixture can be produced by mixing the present Li transition metal oxide powder as a positive electrode active material, a conductive material made of carbon black or the like, and a binder. Such a positive electrode mixture is used for the positive electrode, for example, a material that can store and desorb lithium such as lithium or carbon is used for the negative electrode, and lithium such as lithium hexafluorophosphate (LiPF ₆ ) is used for the non-aqueous electrolyte. A lithium secondary battery can be formed by using a salt dissolved in a mixed solvent such as ethylene carbonate-dimethyl carbonate. However, the present invention is not limited to the battery having such a configuration.

<Explanation of terms>
A “hybrid vehicle” is a vehicle that uses a combination of two power sources, an electric motor and an internal combustion engine, and includes a plug-in hybrid vehicle that has a larger current value than a normal hybrid vehicle.
Batteries mounted on electric vehicles (EV) and hybrid electric vehicles (HEV) are charged / discharged between the limits of charge / discharge depth, such as batteries for consumer products such as video cameras, notebook computers, and mobile phones. In contrast, it is often charged and discharged mainly in the central area of the charge / discharge depth (SOC (State Of Charge) = 20 to 80%), so it is installed in electric vehicles (EV) and hybrid electric vehicles (HEV). In the development of a positive electrode active material for a lithium secondary battery, it is preferable to study characteristics when charging / discharging in the central region (SOC (State Of Charge) = 20 to 80%) of the charging / discharging depth. .

In the present specification, when expressed as “X to Y” (X and Y are arbitrary numbers), “X is preferably greater than X” and “ Is less than Y. Further, X and Y at that time are numerical values considering rounding.
Further, when expressed as “X or more” or “Y or less” (X and Y are arbitrary numbers), it means “preferably larger than X” or “preferably smaller than Y” unless otherwise specified. To do. Further, X and Y at that time are numerical values considering rounding.

  Next, although this invention is further demonstrated based on an Example, this invention is not limited to the Example shown below.

<Measurement of iron content 1>
A sample (powder) is made into a slurry, and a magnet coated with tetrafluoroethylene is put into the slurry and iron is attached to the magnet. Then, in accordance with JIS G 1258: 1999, the iron attached to the magnet is removed. The amount of iron was determined by acid dissolution. Next, this will be described in detail.
In addition, since the iron adhering to the magnet is very small, it is necessary to immerse the magnet together in an acidic solution to dissolve the iron with an acid. Therefore, a magnet coated with tetrafluoroethylene (50 mT to 200 mT, cylindrical shape, diameter 2 cm, length 5 cm, surface area 37.7 cm ² ) was used as the magnet, and the strength of each magnet was measured before measurement. The strength of the magnet was measured using a TESLA METER model TM-601 manufactured by KANETEC.

In a 1000 cc polypropylene pot, 100 g of lithium transition metal oxide powder (sample) and 500 cc of ion exchange water are put into a slurry, and one magnet (100 mT to 150 mT) is put into this slurry, and a ball mill is added. The sample was placed on a rotating gantry and rotated at 60 rpm for 30 minutes.
Next, the magnet is taken out, placed in a 300 mL tall beaker, immersed in ion-exchanged water, and 3 by a two-frequency method (simultaneous oscillation of 28 kHz / 38 kHz) using an ultrasonic cleaner (model US-205, manufactured by SND Corporation). The ion-exchanged water immersed in the magnet was then exchanged, and the fine particles adhering to the magnet were removed by repeating the exchange of ion-exchanged water and washing with ultrasonic waves 8 times in this way.

  Next, the magnet is taken out and placed in a 100 mL graduated cylinder, immersed in aqua regia (a liquid in which concentrated hydrochloric acid and concentrated nitric acid are mixed at a volume ratio of 3: 1) so that the magnet can be completely submerged, and the aqua regia is 80 The iron was dissolved by heating at 30 ° C. for 30 minutes. Next, the magnet was taken out from the aqua regia, the aqua regia in which iron was dissolved was diluted with ion-exchanged water, the diluted aqua regia was analyzed by ICP, the amount of Fe was quantified, and the amount of iron per sample weight was calculated. .

Based on the amount of iron per sample weight obtained, lithium transition metal oxide powders were evaluated and selected according to the following criteria.
0 ppb <iron amount <35 ppb: ◎ (preferably applicable)
35 ppb ≤ iron content <75 ppb: ○ (adoptable)
75 ppb ≤ iron amount: × (not applicable)

<Measurement of iron content 2>
Samples (powder) obtained in Examples and Comparative Examples were slurried, and a magnet coated with tetrafluoroethylene was put into the slurry to attach iron to the magnet, and then JIS G 1258: 1999 was referred to. Then, a method of quantifying iron by dissolving the iron adhering to the magnet with an acid was employed. Next, this will be described in detail.
In addition, since the iron adhering to the magnet is very small, it is necessary to immerse the magnet together in an acidic solution to dissolve the iron with an acid. Therefore, a magnet coated with tetrafluoroethylene was used as the magnet, and the strength of each magnet was measured before the measurement. The strength of the magnet was measured using a TESLA METER model TM-601 manufactured by KANETEC.

  Put 100g of lithium transition metal oxide powder (sample) in a 1000cc polypropylene pot, put 500cc of ion exchange water and 450-600mT magnet covered with tetrafluoroethylene, put on a ball mill rotating base, and adjust in advance Rotation was performed for 30 minutes at 60 rpm. Next, the magnet was taken out, placed in a 100 mL beaker, immersed in ion-exchanged water, washed with an ultrasonic cleaner (model US-205, manufactured by SND Co., Ltd.) for 3 minutes at an output switching frequency setting, and adhered to the magnet. Excess powder was removed. The exchange of ion-exchanged water soaking the magnet and washing with ultrasonic waves were repeated 8 times. Then, the magnet is taken out and placed in a 50 mL graduated cylinder, immersed in aqua regia (a liquid in which concentrated hydrochloric acid and concentrated nitric acid are mixed at a volume ratio of 3: 1) so that the magnet is completely submerged, and is 80 ° C. in aqua regia. For 30 minutes to dissolve the iron. The magnet was taken out from the aqua regia, and the aqua regia in which iron was dissolved was diluted with ion exchange water. The diluted aqua regia was analyzed by ICP, the amount of Fe was quantified, and the amount of iron per sample weight was calculated.

<Battery evaluation>
Considering the charge / discharge depth (SOC = 20 to 80%) at which the battery is actually used in HEV, the following battery evaluation was performed.

(About samples 1 to 5)
In 8.80 g of lithium transition metal oxide powder (sample) as the positive electrode active material, 0.60 g of acetylene black (manufactured by Denki Kagaku Kogyo) as the conductive material, and N-methyl-2-pyrrolidone (NMP) 5.00 g of a solution in which 12 wt% of polyvinylidene fluoride (PVDF, manufactured by Kishida Chemical Co., Ltd.) is dissolved and 5 mL of N-methyl-2-pyrrolidone (NMP) are mixed, and a planetary stirring and defoaming device (Mazerustar KK manufactured by Kurabo Industries) -50S) to obtain a paste-like positive electrode mixture.
This paste-like positive electrode mixture was applied onto an aluminum foil as a current collector using an applicator having a clearance adjusted to 250 μm, vacuum-dried at 120 ° C. overnight, punched out with a 16 mmφ punch, and 4 t / cm ^2. A positive electrode plate was obtained by press-consolidating at a pressure of

In producing the battery, immediately before producing the battery, the positive electrode was vacuum-dried at 120 ° C. for 120 minutes to remove the adhering moisture and incorporated in the battery.
In addition, the weight of an aluminum foil having a diameter of 16 mmφ was obtained in advance, the weight of the aluminum foil was subtracted from the weight of the positive electrode plate, and the weight of the applied positive electrode mixture was obtained. Further, the content of the positive electrode active material was determined from the mixing ratio of the positive electrode active material, acetylene black and PVDF.

(About Samples 6-7)
In 8.00 g of lithium transition metal oxide powder (sample) as a positive electrode active material, 1.00 g of acetylene black (manufactured by Denki Kagaku Kogyo) as a conductive material, and N-methyl-2-pyrrolidone (NMP) 8.30 g of a solution containing 12 wt% of polyvinylidene fluoride (PVDF, manufactured by Kishida Chemical Co., Ltd.) and 5 mL of N-methyl-2-pyrrolidone (NMP) are mixed, and a planetary stirring and deaerator (Mazerustar KK manufactured by Kurabo Industries Co., Ltd.) is mixed. -50S) to obtain a paste-like positive electrode mixture.
This paste-like positive electrode mixture was applied onto an aluminum foil as a current collector using an applicator with a clearance adjusted to 350 μm, vacuum-dried at 120 ° C. overnight, punched out with a 14 mmφ punch, and 4 t / cm ^2. A positive electrode plate was obtained by press-consolidating at a pressure of

In producing the battery, immediately before producing the battery, the positive electrode was vacuum-dried at 120 ° C. for 120 minutes to remove the adhering moisture and incorporated in the battery.
Further, the weight of an aluminum foil having a diameter of 14 mmφ was obtained in advance, the weight of the aluminum foil was subtracted from the weight of the positive electrode plate, and the weight of the applied positive electrode mixture was obtained. Further, the content of the positive electrode active material was determined from the mixing ratio of the positive electrode active material, acetylene black and PVDF.

(About samples 1-7)
The electrochemical evaluation cell “TOMCELL (registered trademark)” in FIG. 1 was produced using metal Li having a diameter of 20 mm and a thickness of 1.0 mm as the negative electrode.
That is, as shown in FIG. 1, the positive electrode plate 3 was arranged in the center of the inside of the lower body 1 made of organic electrolyte resistant stainless steel. On the upper surface of the positive electrode plate 3, a polypropylene separator (“Celguard 2400”) 4 impregnated with an electrolytic solution is disposed, and the separator 4 is fixed by a Teflon spacer 5 (“Teflon” is a registered trademark of DUPONT USA). . Further, on the upper surface of the separator 4, the negative electrode 6 is disposed below, the spacer 7 also serving as a negative electrode terminal is disposed, the upper body 2 is placed on the upper surface, the screws are tightened, the battery is sealed, and the electrochemical An evaluation cell was produced.
As the electrolytic solution, a mixture of EC and DMC in a volume ratio of 3: 7 was used as a solvent, and a solution obtained by dissolving 1 mol / L of LiPF ₆ as a solute was used.

(Voltage drop confirmation test)
Using the electrochemical evaluation cell “TOMCELL (registered trademark)” produced as described above, a voltage drop confirmation test was performed.

  At 25 ° C., charging / discharging was repeated twice in the range of electrode potential of 3.0 to 4.3V. Next, the battery was charged up to 80% of the discharge capacity at the second cycle (SOC = 80%), removed from the apparatus, and then the potential was measured. And the battery of such a charge state was put into a 65 degreeC thermostat, and was preserve | saved for 30 days. Thereafter, the potential after 30 days was measured, and those having a potential drop of 200 mV or more were judged as “defective”, and those having a potential drop of less than 200 mV were judged as “good”, and the failure rate in 100 pieces was determined.

<Sample 1-5>
Lithium carbonate 18.15 (g) and electrolytic manganese dioxide 81.85 (g) were weighed and mixed to obtain a mixed raw material. The obtained mixed raw material is placed in a firing container (alumina crucible size = vertical * horizontal * height = 10 * 10 * 5 (cm)) and the ratio of open area to filling height (open area cm ² / The filling was performed such that the filling height cm) was 100. The raw material apparent density at this time was 1.1 g / cm ³ .
Then, using a static electric furnace, the temperature was raised from room temperature to the firing set temperature at a heating rate = 150 ° C./hr, held at a firing temperature (holding temperature) of 750 ° C. for 20 hours, and then from the holding temperature to 600 ° C. The temperature was decreased at a rate of temperature decrease of 20 ° C./hr until it was naturally cooled to room temperature. The temperature variation within the holding time was controlled within the range of 740 ° C to 760 ° C.
The fired powder obtained by firing in this way is crushed in a mortar, classified with a sieve having an opening of 75 μm, and the powder under the sieve is further separated by a magnetic separator (Magnetec Japan, MODEL: SGC-010232 ) At a rate of 1 kg / min, and the magnetic deposit was removed to obtain a sample. The number of magnetic separations was repeated as shown in Table 1.

<Sample 6-7>
Lithium carbonate with an average particle size (D50) of 8 μm, electrolytic manganese dioxide with an average particle size (D50) of 22 μm, nickel hydroxide with an average particle size (D50) of 25 μm, and cobalt oxyhydroxide with an average particle size (D50) of 14 μm In a molar ratio of Li: Mn: Ni: Co = 1.07: 0.31: 0.31: 0.31, water was added, mixed and stirred, and the solid content concentration was 50 wt%. A slurry was prepared.
To the resulting slurry (20 kg of raw material powder), 6 wt% of a polycarboxylic acid ammonium salt (SN Dispersant 5468 manufactured by San Nopco Co., Ltd.) as a dispersant was added, and pulverized with a wet pulverizer at 1300 rpm for 29 minutes. The average particle size (D50) was 0.7 μm.
The obtained pulverized slurry was granulated and dried using a thermal spray dryer (spray dryer, OC-16 manufactured by Okawahara Chemical Co., Ltd.). At this time, a rotating disk was used for spraying, and granulation drying was performed by adjusting the temperature so that the rotation speed was 21,000 rpm, the slurry supply amount was 24 kg / hr, and the outlet temperature of the drying tower was 100 ° C.
The obtained granulated powder was baked at 975 ° C. for 20 hours in the air using a stationary electric furnace. The fired powder obtained by firing is classified with a sieve having an opening of 75 μm, and the powder under the sieve is rotated by a classification rotor using a collision type pulverizer with a classification mechanism (Counterjet mill “100AFG / 50ATP” manufactured by Hosokawa Micron). Number: 14900 rpm, pulverization air pressure: 0.6 MPa, pulverization nozzle φ: 2.5 × 3 used, powder supply: 4.5 kg / h, pulverization to obtain lithium transition metal oxide powder It was.
The lithium transition metal oxide powder thus obtained was put into a magnetic separator (Magnec Tech Japan Co., Ltd., MODEL: SGC-010232) at a rate of 1 kg / min, and the magnetic deposit was removed to prepare a sample. Obtained.
The number of magnetic separations was repeated as shown in Table 1.

(Discussion)
Compared to the case of detecting and evaluating the amount of iron magnetized on a magnet using a 450-600 mT magnet (Iron amount measurement 2), the amount of iron magnetized on the magnet using a magnet of 50 mT to 200 mT When it is detected and evaluated (Iron amount measurement 1), it is easier to make a short circuit (voltage drop), especially when it is kept charged for a long time at a high temperature (voltage drop). It was found that the accuracy can be accurately evaluated. In particular, comparing Samples 2 and 3, in Sample 2, the amount of iron in Sample 2 was detected to be less, but in Sample 1, the amount of iron was detected to be high and matched the defective rate of voltage drop. Was the result.
The reason for this seems to be that if the strength of the magnet used to detect the amount of iron is weakened, the amount of iron detected is usually reduced and the evaluation accuracy is lowered. By reducing the strength of the iron to an appropriate range, the detection accuracy of iron that affects micro short-circuit (voltage drop), that is, iron with strong magnetic force, will be increased. As a result, the ease of micro short-circuit (voltage drop), In particular, it can be considered that the ease of short-circuiting (voltage drop) in a state of being kept at a high temperature for a long time while being charged can be accurately evaluated.

  In addition, although the strength of the magnet used this time for measuring the amount of iron is in the range of 130 to 140 mT according to the values in the table, the strength of the magnet is not stable, so the magnet in the range of 100 to 150 mT is used. I can say that there is. From the experience of the tests so far, it is considered that the same effect as the magnets in the range of 100 to 150 mT used in this test can be obtained if a magnet of 50 mT to 200 mT is used.

It is the figure which showed the structure of the cell for electrochemical evaluation produced for the battery evaluation of the sample.

Claims (5)

  A liquid is added to the lithium transition metal oxide powder to make a slurry, and a 50 mT to 200 mT magnet is put into the slurry and stirred, and the amount of iron adhering to the magnet is detected, and the mass of the lithium transition metal oxide powder. A method for evaluating a positive electrode active material for a lithium secondary battery, wherein a lithium transition metal oxide powder having an iron content of 0 ppb <iron content <75 ppb is selected.   A liquid is added to the lithium transition metal oxide powder to make a slurry, and a 50 mT to 200 mT magnet is put into the slurry and stirred, and the amount of iron adhering to the magnet is detected, and the mass of the lithium transition metal oxide powder. A method for evaluating a positive electrode active material for a lithium secondary battery, wherein a lithium transition metal oxide powder having an iron content of 1 ppb <iron content ≦ 35 ppb is selected.   A liquid is added to the lithium transition metal oxide powder to make a slurry, and a 50 mT to 200 mT magnet is put into the slurry and stirred, and the amount of iron adhering to the magnet is detected, and the mass of the lithium transition metal oxide powder. A method for evaluating a positive electrode active material for a lithium secondary battery, wherein a lithium transition metal oxide powder having an iron content of 2 ppb <iron content ≦ 25 ppb is selected.   A liquid is added to the lithium transition metal oxide powder to make a slurry, and a 50 mT to 200 mT magnet is put into the slurry and stirred, and the amount of iron adhering to the magnet is detected, and the mass of the lithium transition metal oxide powder. A method for evaluating a positive electrode active material for a lithium secondary battery, wherein a lithium transition metal oxide powder having an iron content of 5 ppb <iron content ≦ 25 ppb is selected. A method for producing a positive electrode active material for a lithium secondary battery, wherein the lithium transition metal oxide powder is selected by the evaluation method according to claim 1.

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