JP6826447B2 - Method for reducing the amount of residual lithium in the positive electrode active material particles - Google Patents

Description

The present invention relates to a method for reducing the amount of residual lithium in the positive electrode active material particles.

As a power source for driving electronic devices such as AV equipment and personal computers, a lithium ion secondary battery that is compact, lightweight, has a high energy density, has a high charge / discharge voltage, and has a large charge / discharge capacity is attracting attention. Useful positive electrode active material the lithium-ion secondary battery, for example, the composition _{formula: Li b (Ni x Co y} M z) lithium composite oxide having a basic composition represented by _{O 2} (M is, Mn , Mg, Al and other metals) have been proposed in various ways.

The lithium composite oxide is usually prepared by preparing a precursor from a nickel compound, a cobalt compound, and a compound of a metal such as Mn, Mg, or Al, and then a mixture of the precursor and a lithium compound such as Li ₂ CO _3. Obtained by firing at an appropriate temperature.

However, so-called residual lithium (residual Li ₂ CO ₃ and residual Li OH) is present in the lithium composite oxide obtained through such a firing step. The residual lithium is derived from the residual Li ₂ CO ₃ derived from unreacted Li ₂ CO ₃ , the precipitation of Li ₂ O from the crystal of the produced lithium composite oxide, and the residue of Li ₂ O. residual LiOH and residual _Li 2 CO _3, and, the residual _Li 2 CO ₃ derived from the carbonation of Li of the lithium composite oxide in the firing step while the temperature is lowered contemplated.

The positive electrode active material particles made of a lithium composite oxide containing a large amount of residual lithium may gel during electrode fabrication. Further, such a lithium ion secondary battery having positive electrode active material particles as a positive electrode has a problem that the amount of carbon dioxide generated increases due to a reaction inside the battery during high temperature storage, and the battery expands.

Therefore, in order to reduce the amount of residual lithium in the lithium composite oxide, for example, the calcined raw material mixture is calcined once to prepare a lithium composite oxide, and then the obtained lithium composite oxide is mixed with an aqueous solution of sulfate. A method of washing or contacting with an aqueous solution of sulfate after washing with water (Patent Document 1), or a method of performing at least a part of the firing step of the firing raw material mixture in a moist atmosphere gas having a dew point of 0 ° C to 70 ° C. (Patent Document 2) has been proposed. Further, a method of performing the firing step of the firing material mixture in an oxygen gas atmosphere has also been proposed.

Japanese Unexamined Patent Publication No. 2011-124086 Japanese Unexamined Patent Publication No. 2016-050120

For example, with the conventional methods described in Patent Documents 1 and 2, the amount of residual lithium in the lithium composite oxide, particularly the amount of residual Li ₂ CO ₃ cannot be sufficiently reduced, and the amount of carbon dioxide gas generated during high-temperature storage is increased. The problem of increased battery expansion cannot be solved. Further, when the method of performing the firing step in an oxygen gas atmosphere is adopted, the production cost increases significantly, so such a method is not practical.

The present invention has been made in view of the above problems, and an object of the present invention is to easily and efficiently reduce the amount of residual Li ₂ CO _{3 in} the positive electrode active material particles used for the positive electrode of a non-aqueous electrolyte secondary battery at a production cost. It is an object of the present invention to provide a method capable of suppressing an increase in the amount of particles and sufficiently reducing the amount of the particles.

In order to achieve the above object, in the present invention, when preparing a lithium composite oxide from a lithium compound and a precursor compound, the atmosphere while lowering the firing temperature (lowering temperature) is carbonic acid as compared with the air. The atmosphere was very low in concentration.

Specifically, the method for reducing the amount of residual lithium in the positive electrode active material particles according to the present invention is a method for reducing the amount of residual Li ₂ CO ₃ in the positive electrode active material particles made of lithium composite oxide.
After setting the firing temperature to 850 ° C. or higher and holding the set firing temperature and air atmosphere for a predetermined time, the atmosphere during the lowering of the set firing temperature is changed from the air atmosphere to the carbon dioxide concentration of the air atmosphere. At least a step of calcining a mixture of a precursor compound containing Ni, Co and Mn and a lithium compound to prepare a lithium composite oxide is provided by switching to a low carbon dioxide atmosphere having a carbon dioxide concentration of 1/60 or less. It is characterized by that.

Further, in the method for reducing the amount of residual lithium (residual Li ₂ CO ₃ amount) in the positive electrode active material particles according to the present invention, the low carbon dioxide gas atmosphere is a high oxygen gas atmosphere having an oxygen concentration of 80% by volume or more, or A decarbonized gas atmosphere having a carbon dioxide concentration of 4 ppm or less is preferable.

Further, in the method for reducing the amount of residual lithium (residual Li ₂ CO ₃ amount) in the positive electrode active material particles according to the present invention, the oxygen concentration in the high oxygen gas atmosphere is preferably 85% by volume or more, or the above. The carbon dioxide concentration in the decarbonized gas atmosphere is preferably 3 ppm or less.

Further, in the method for reducing the amount of residual lithium (residual Li ₂ CO ₃ amount) in the positive electrode active material particles according to the present invention, it is preferable to set the firing temperature to 870 ° C. to 1000 ° C.

The amount of residual lithium contained in the positive electrode active material particles obtained by such a reduction method, particularly the amount of residual Li ₂ CO ₃ is sufficiently reduced. Therefore, in the non-aqueous electrolyte secondary battery using the positive electrode active material particles as the positive electrode, the amount of carbon dioxide gas generated by the reaction inside the battery is suppressed even during high temperature storage, so that the battery does not easily expand. .. Further, in the method for reducing the amount of residual lithium in the positive electrode active material particles according to the present invention, the amount of high oxygen gas or decarboxylation gas used is small, and the production cost does not increase significantly.

According to the method for reducing the amount of residual lithium according to the present invention, the amount of residual Li ₂ CO _{3 in} the positive electrode active material particles used for the positive electrode of the non-aqueous electrolyte secondary battery can be easily and efficiently and sufficiently reduced, and the production cost can be reduced. It is possible to suppress the rise of the non-aqueous electrolyte secondary battery and prevent the expansion of the non-aqueous electrolyte secondary battery during high temperature storage. Furthermore, the high temperature cycle characteristics of the non-aqueous electrolyte secondary battery can be improved.

Hereinafter, modes for carrying out the present invention will be described. The following description of preferred embodiments is merely exemplary and is not intended to limit the invention, its application methods or its uses.

[Method of reducing the amount of residual lithium in the positive electrode active material particles]
A method for reducing the amount of residual lithium in the positive electrode active material particles according to the embodiment of the present invention will be described. The positive electrode active material particles obtained by the method for reducing the amount of residual lithium of the present invention are used for the positive electrode of a non-aqueous electrolyte secondary battery.

The positive electrode active material particles obtained by using the method for reducing the amount of residual lithium according to the present embodiment are composed of a lithium composite oxide containing Li, Ni, Co, and Mn. Such a method for reducing the amount of residual lithium in the positive electrode active material particles includes at least the following steps.

That is, in the method for reducing the amount of residual lithium according to the present embodiment, a mixture of a precursor compound containing Ni, Co and Mn and a lithium compound is calcined under specific conditions to prepare a lithium composite oxide. ..

In the precursor compound, for example, the ratio of the nickel compound, the cobalt compound, and the manganese compound is adjusted so that Ni, Co, and Mn are in a desired ratio, and these compounds are reacted to react. It is obtained by washing and drying the reaction product after obtaining the product.

The nickel compound is not particularly limited, and examples thereof include nickel hydroxide, nickel sulfate, nickel oxide, nickel nitrate, nickel carbonate, nickel chloride, nickel iodide, and metallic nickel.

The cobalt compound is not particularly limited, and examples thereof include cobalt hydroxide, cobalt sulfate, cobalt oxide, cobalt nitrate, cobalt carbonate, cobalt chloride, cobalt iodide, and metallic cobalt.

The manganese compound is not particularly limited, and examples thereof include manganese hydroxide, manganese sulfate, manganese oxide, manganese nitrate, manganese carbonate, manganese chloride, manganese iodide, and metallic manganese.

Then, the lithium compound and the precursor compound obtained as described above are mixed in a desired ratio to prepare a mixture.

The lithium compound is not particularly limited, and is, for example, lithium carbonate, lithium hydroxide / monohydrate, anhydrous lithium hydroxide, lithium nitrate, lithium acetate, lithium bromide, lithium chloride, lithium citrate, and fluoride. Examples thereof include lithium, lithium iodide, lithium lactate, lithium oxalate, lithium phosphate, lithium pyruvate, lithium sulfate, lithium oxide and the like, and lithium carbonate, lithium hydroxide and monohydrate are particularly preferable.

Wherein A desired proportion of the lithium compound and the precursor compound, the basic composition of the lithium composite oxide to be described later (composition _{formula: Li a (Ni p Co q} Mn r) O 2) molar ratio based on [Li / ( Ni + Co + Mn)] (where p + q + r = 1). Specific examples of the desired ratio of the lithium compound to the precursor compound (range a in the formula) will be described later.

Then, the mixture obtained as described above is calcined under specific conditions to prepare a lithium composite oxide.

As described above, residual lithium (residual Li ₂ CO ₃ , residual LiOH) is present in the lithium composite oxide obtained through the firing step, and the residual lithium is derived from unreacted Li ₂ CO ₃ . Residual Li ₂ CO ₃ , residual Li OH derived from the precipitation of Li ₂ O from the crystal of the formed lithium composite oxide on the surface and residual Li ₂ O, and residual Li ₂ CO ₃ , and the temperature decrease in the firing step. Residual Li ₂ CO ₃ derived from the carbonation of Li in the lithium composite oxide while allowing it to occur is conceivable.

Among these, we focused on the fact that Li ₂ CO ₃ is generated by contacting Li with the carbonic acid component by lowering the temperature from a high temperature in the firing step. The inventors have found that by suppressing the contact between Li and the carbonic acid component as much as possible, carbonation of Li can be prevented and the amount of residual Li ₂ CO ₃ can be sufficiently reduced. The condition for suppressing the contact between Li and the carbonic acid component as much as possible is the specific condition.

That is, the specific condition is that the firing temperature is set to 850 ° C. or higher, the set firing temperature and the atmospheric atmosphere are maintained for a predetermined time, and then the atmosphere during the lowering of the set firing temperature is the atmospheric atmosphere. Therefore, the carbon dioxide concentration is switched to a low carbon dioxide gas atmosphere in which the carbon dioxide concentration is 1/60 or less of the carbon dioxide concentration in the air atmosphere.

The firing temperature is set to 850 ° C. or higher, preferably 870 ° C. or higher, more preferably 890 ° C. or higher, preferably 1000 ° C. or lower, still more preferably 990 ° C. or lower. When the firing temperature is set in this range, Li and Ni, Co, and Mn tend to be uniform, and there is no risk of oxygen defects increasing. If the calcination temperature is lower than 850 ° C., the desired lithium composite oxide crystals cannot be obtained. On the other hand, if the firing temperature exceeds 1000 ° C., crystal growth proceeds too much, and the lithium composite oxide tends to cause cation mixing.

After setting the firing temperature in this way, the set firing temperature and air atmosphere are maintained for a predetermined time to proceed with firing of the mixture. The holding time at this time is not particularly limited and may be appropriately changed according to the composition of the mixture and the like, but is preferably about 3 hours to 10 hours, for example.

Next, the mixture is continuously fired while lowering the set firing temperature until it approaches, for example, about room temperature (temperature lowering operation). At this time, the atmosphere during the temperature lowering is switched from the atmospheric atmosphere to a low carbon dioxide gas atmosphere in which the carbon dioxide concentration is much lower than the carbon dioxide concentration in the atmospheric atmosphere, and firing is performed. The carbonic acid concentration in the low carbon dioxide atmosphere is 1/60 or less, preferably 1/100 or less of the carbonic acid concentration in the atmospheric atmosphere. By proceeding with the temperature lowering operation in such a low carbon dioxide gas atmosphere, the contact between Li and the carbonic acid component is sufficiently suppressed, and the carbonation of Li can be sufficiently prevented.

As the low carbon dioxide gas atmosphere, for example, a high oxygen gas atmosphere (atmosphere with a low carbon dioxide amount) or a decarbonation gas atmosphere is preferably exemplified. The high oxygen gas atmosphere is a substantially oxygen gas atmosphere having an oxygen concentration of 80% by volume or more, preferably 85% by volume or more. Further, the decarboxylation gas atmosphere is an atmosphere in which the carbon dioxide concentration is 4 ppm or less, preferably 3 ppm or less, and substantially does not contain carbon dioxide gas. By switching to such a high oxygen gas atmosphere or a decarbonated gas atmosphere having an extremely low carbon dioxide concentration and firing the mixture while lowering the firing temperature, the contact between Li and the carbon dioxide component is further suppressed. Carbonation of Li is hindered.

At the time when the lowering of the set firing temperature is started, the lowering of the set firing temperature is started so that the atmosphere of the firing furnace is switched from the atmospheric atmosphere to the low carbon dioxide gas atmosphere. It is preferable to switch the atmosphere of the firing system about 3 hours before to about 1 hour before.

The firing time of the mixture in a low carbon dioxide atmosphere may be a time during which the firing temperature decreases until it approaches, for example, about room temperature, but it is usually preferably about 5 hours to 10 hours.

[Positive electrode active material particles]
Wherein as the, the steps of the method for reducing the residual amount of lithium in accordance with the present embodiment are at least over in preparation, a lithium composite oxide as a positive electrode active material particles, for example, the composition _{formula: Li a (Ni p Co q} Mn r ) It has a basic composition represented by O ₂ . Here, p + q + r = 1.

In the composition formula, a indicates the amount (mol) of Li with respect to 1 mol of (Ni + Co + Mn) when preparing the mixture. The range of a is preferably 0.96 ≦ a ≦ 1.15, more preferably 0.98 ≦ a ≦ 1.10, and further preferably 1.00 ≦ a ≦ 1.08. preferable.

In the composition formula, p indicates the amount (mol) of Ni when preparing the precursor compound. The range of p is preferably 0 <p ≦ 0.80, more preferably 0.20 ≦ p ≦ 0.75, and even more preferably 0.20 ≦ p ≦ 0.65.

In the composition formula, q indicates the amount (mol) of Co when preparing the precursor compound. The range of q is preferably 0 <q ≦ 0.40, more preferably 0.05 ≦ q ≦ 0.35.

In the composition formula, r indicates the amount (mol) of Mn when preparing the precursor compound. The range of r is preferably 0 <r ≦ 0.50, more preferably 0.10 ≦ r ≦ 0.45.

The positive electrode active material particles prepared by the method for reducing the amount of residual lithium according to the present embodiment include, for example, Al, Mg, P, Ca, Ti, Y, Sn, Bi, Ce, in addition to Ni, Co, and Mn. Dissimilar metals such as Zr, La, Mo, Sc, Nb, and W can be contained. The content form is not particularly limited, and may be present in place of the main element in the crystal lattice, or may be present on the surface and grain boundaries of the secondary particles in which the primary particles are aggregated.

In the positive electrode active material particles prepared through the method for reducing the residual lithium amount according to the present embodiment, the residual lithium amount, particularly the residual Li ₂ CO ₃ amount, is the same as that of the positive electrode active material particles obtained by, for example, the conventional production method. In comparison, it is greatly reduced. That is, in the method for reducing the amount of residual lithium according to the present embodiment, the atmosphere during the lowering of the set firing temperature is switched from the atmospheric atmosphere to the low carbon dioxide gas atmosphere to fire the mixture. However, in the conventional manufacturing method, the mixture is fired. The atmosphere during the lowering of the set firing temperature is not switched as it is.

For example, when the residual Li ₂ CO ₃ in the positive electrode active material particles obtained by such a conventional production method is 100, the positive electrode active material particles prepared through the method for reducing the amount of residual lithium according to the present embodiment. The amount of residual Li ₂ CO _{3 in the above} is 70 or less, further 60 or less. Specifically, the amount of residual Li ₂ CO ₃ in the positive electrode active material particles prepared through the method for reducing the amount of residual lithium according to the present embodiment depends on the firing conditions such as the firing temperature and the composition of the positive electrode active material particles. However, for example, in the case of Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ , it is about 0.05% by weight or less. It is recognized that this is a very low value as compared with about 0.06% by weight or more, which is the amount of residual Li ₂ CO _{3 in} the positive electrode active material particles having the same composition obtained by the conventional production method.

The positive electrode active material particles prepared via the method of reducing the residual amount of lithium according to the present embodiment preferably has a BET specific surface area of the particles is 0.05m ² /g~1.0m ² / g. When positive electrode active material particles whose BET specific surface area is adjusted to such a range are used for the positive electrode of a non-aqueous electrolyte secondary battery, the positive electrode active material particles play a catalytic role, and thus react with the electrolytic solution. The sex can be reduced.

The positive electrode active material particles prepared through the method for reducing the amount of residual lithium according to the present embodiment preferably contain the primary particle diameter of the particles in the range of 0.1 μm to 4 μm. When positive electrode active material particles whose primary particle size is within such a range are used for the positive electrode of a non-aqueous electrolyte secondary battery, gas is generated in the laminated non-aqueous electrolyte secondary battery using graphite for the negative electrode. A few good high temperature charge / discharge characteristics can be obtained. When the primary particle size is less than 0.1 μm, smaller primary particles are generated due to particle destruction due to compression during electrode fabrication, and the reaction with the electrolytic solution at the particle interface tends to be intense. When the primary particle size exceeds 4 μm, the diffusion resistance of lithium ions becomes high, so that the initial discharge capacity of the non-aqueous electrolyte secondary battery tends to decrease.

The positive electrode active material particles prepared through the method for reducing the amount of residual lithium according to the present embodiment preferably have an average secondary particle diameter (D50) of 4 μm to 30 μm. When the average secondary particle size is less than 4 μm, the packing density at the time of electrode fabrication tends to decrease, and the reactivity with the electrolytic solution tends to increase. When the average secondary particle size exceeds 30 μm, it tends to be difficult to produce industrially.

When the positive electrode active material particles prepared through the method for reducing the amount of residual lithium according to the present embodiment are used as the positive electrode of the non-aqueous electrolyte secondary battery and the negative electrode is Li (condition: 3.0V to 4.3V, charging). The initial discharge capacity of the cc-cv), non-aqueous electrolyte secondary battery varies depending on the composition of the positive electrode active material particles, but for example, the above Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ When is used, it is about 163 mAh / g to about 171 mAh / g. It is recognized that this is about the same as the case where the positive electrode active material particles having the same composition obtained by the conventional production method are used.

Similarly, when the positive electrode active material particles prepared through the method for reducing the amount of residual lithium according to the present embodiment are used for the positive electrode of the non-aqueous electrolyte secondary battery, the mechanism is not clear, but the non-aqueous electrolyte secondary battery High temperature cycle characteristics are also improved. The high temperature cycle characteristics differ depending on the composition of the positive electrode active material particles, but for example, when the above Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ is used, the conventional production method is used. An improvement of about 2% or more (increase in numerical value) is observed as compared with the case where the obtained positive electrode active material particles having the same composition are used.

[Non-aqueous electrolyte secondary battery]
A non-aqueous electrolyte secondary battery can be manufactured by using the positive electrode active material particles prepared through the method for reducing the amount of residual lithium according to the present embodiment as the positive electrode. The non-aqueous electrolyte secondary battery will be described.

The non-aqueous electrolyte secondary battery is composed of a positive electrode containing the positive electrode active material particles, a negative electrode, and an electrolyte.

The positive electrode is not particularly limited, but is usually obtained by kneading positive electrode active material particles, a conductive agent, and a binder. Examples of the conductive agent include acetylene black, graphite, carbon black, graphite and the like. Examples of the binder include polytetrafluoroethylene and polyvinylidene fluoride.

The negative electrode is made of a negative electrode active material. Examples of the negative electrode active material include metallic lithium, lithium / aluminum alloy, lithium / tin alloy, silicon, silicon / carbon composite, graphite and the like.

Examples of the electrolyte include at least one type of lithium salts such as lithium perchlorate (LiClO ₄ ) and lithium tetrafluoroborate (LiBF ₄ ), in addition to lithium hexafluorophosphate (LiPF ₆ ). , These can be dissolved in a solvent to obtain an electrolytic solution.

Examples of the solvent for the electrolytic solution include carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), and diethyl carbonate (DEC), and ethers such as dimethoxyethane (DME). An organic solvent containing at least one type can be used.

[Action]
An important point in the present invention is that in the method for reducing the amount of residual lithium according to the present invention, a mixture of a lithium compound and a precursor compound containing Ni, Co, and Mn is fired under specific conditions to perform lithium composite oxidation. The point is to prepare things. The specific condition is a condition for suppressing the contact between Li and the carbon dioxide component as much as possible, that is, after the firing temperature is set to 850 ° C. or higher and the set firing temperature and air atmosphere are maintained for a predetermined time. The atmosphere during which the set firing temperature is lowered to room temperature is switched from the atmospheric atmosphere to a low carbon dioxide gas atmosphere in which the carbon dioxide concentration is 1/60 or less of the carbon dioxide concentration in the atmospheric atmosphere.

The amount of residual lithium contained in the positive electrode active material particles obtained through such a reduction method, particularly the amount of residual Li ₂ CO ₃ is sufficiently reduced. Therefore, in the non-aqueous electrolyte secondary battery using the positive electrode active material particles as the positive electrode, the amount of carbon dioxide gas generated by the reaction inside the battery is suppressed even during high temperature storage, and the battery does not easily expand. It also has excellent high temperature cycle characteristics. Further, in such a reduction method, the amount of low carbon dioxide gas such as high oxygen gas used is small, and the production cost does not increase significantly.

Hereinafter, the present invention will be specifically described with reference to typical examples and comparative examples of the present invention, but the present invention is not limited to these examples.

(Composition of positive electrode active material particles)
The composition of the positive electrode active material particles was determined as follows. A 1.0 g sample was heated and dissolved in 25 ml of a 20% hydrochloric acid solution, cooled, transferred to a 100 ml volumetric flask, and pure water was added to prepare a preparation solution. ICAP [Optima 8300, manufactured by PerkinElmer Co., Ltd.] was used for the measurement of this adjusting solution, and each element was quantified and determined.

(Amount of residual lithium in positive electrode active material particles)
The amount of residual lithium in the positive electrode active material particles was measured using the Walder method. Specifically, it was obtained by adding 20 g of particle powder of a lithium composite oxide as positive electrode active material particles to 100 ml of water, stirring at room temperature for 20 minutes, and then filtering and removing the solid content. The supernatant was determined by titrating with 0.2N hydrochloric acid. On the pH curve drawn by plotting the titration amount (ml) on the horizontal axis and the pH of the supernatant liquid on the vertical axis, the two points with the largest inclination are the first titration point and the second titration point from the one with the smallest titration. The titration points were used, and the respective amounts were calculated from the titrations at these points using the following formulas to obtain the residual lithium amount. The abbreviations in the calculation formula are as follows.
T ₁ : Titration up to the first titration point (ml)
T ₂ : Titration up to the second titration point (ml)
C _HCl : Concentration of hydrochloric acid used for titration (mol / l)
F _HCl : Hydrochloric acid factor used for titration M _Li2CO3 : Molecular weight of Li ₂ Co ₃ M _LiOH : Molecular weight of _LiOH W: Weight of positive electrode active material particles (g)

Residual Li ₂ Co ₃ amount (% by weight)
= (T _2- T ₁ ) × C _HCl × F _HCl × M _Li2CO3 × 2 × 100 / (W × 1000)
Residual LiOH amount (% by weight)
= {T ₂ -2 x (T _2- T ₁ )} x C _HCl x F _HCl x M _LiOH x 2 x 100 / (W x 1000)

(BET specific surface area of positive electrode active material particles)
The specific surface area of the positive electrode active material particles by the BET method was measured by drying and degassing the sample under nitrogen gas at 120 ° C. for 45 minutes, and then using a BET specific surface area measuring device [MONOSORB, manufactured by Yuasa Ionics Co., Ltd.]. ..

(Average secondary particle diameter (D50) of precursor compound and positive electrode active material particles)
Average secondary particle size of precursor compound particles and lithium composite oxide particle powder as positive electrode active material particles by wet laser method using a laser particle size distribution measuring device [Microtrack HRA, manufactured by Nikkiso Co., Ltd.] (Volume basis) was measured.

<Making coin cells>
The coin cell related to the battery evaluation was produced as follows. First, 90% by weight of the particle powder of the lithium composite oxide as the positive electrode active material particles, 3% by weight of acetylene black and 3% by weight of graphite (KS-16) as the conductive agent, and polyfluore dissolved in N-methylpyrrolidone as the binder. After mixing with 4% by weight of vinylidene, the mixture was applied to an Al metal foil and dried at 150 ° C. to prepare a sheet. This sheet was punched to 16 mmΦ and then crimped at 1 t / cm ² to a thickness of 50 μm to obtain a positive electrode. The negative electrode was metallic lithium having a thickness of 500 μm punched to 16 mmΦ. The electrolytic solution was a solution in which EC and DMC in which 1 mol / L LiPF ₆ was dissolved were mixed at an EC: DMC = 1: 2 (volume ratio). A 2032 type coin cell was produced using these positive electrode, negative electrode, and electrolytic solution.

(Initial discharge capacity of non-aqueous electrolyte secondary battery)
Using the coin cell, charging was performed at a current density of 0.2 C rate from a voltage of 3.0 V to 4.3 V (cc-cv) in an environment of 25 ° C. Then, under the same environment, discharge was performed from a voltage of 4.3 V to 3.0 V (cc) at a current density of 0.1 C rate, and the initial discharge capacity (mAh / g) was measured.

(High temperature cycle characteristics of non-aqueous electrolyte secondary batteries)
Using the coin cell, charging at a current density of 0.2 C rate from a voltage of 3.0 V to 4.3 V (cc-cv) and a voltage of 4.3 V to 3.0 V (cc) in an environment of 60 ° C. Obtain the ratio of the 101st discharge capacity to the 1st discharge capacity (maintenance rate = 101st discharge capacity / 1st discharge capacity) when discharging at a current density of 0.1 C rate is repeated 101 times. , High temperature cycle characteristics (%).

<Reference example>
First, nickel hydroxide, cobalt hydroxide, and manganese hydroxide are weighed so that the ratio (molar ratio) of Ni, Co, and Mn is Ni: Co: Mn = 50:20: 30. After reacting these compounds to obtain a reactant, the reactant was washed with water and dried to obtain a precursor compound. The average secondary particle size of the precursor compound was about 11.6 μm.

Next, lithium carbonate and the obtained precursor compound were weighed so that the ratio (molar ratio) of Li to Ni, Co and Mn was Li / (Ni + Co + Mn) = 1.02. Was mixed to obtain a uniform mixture.

<Example 1>
The mixture obtained in the reference example was set in a box furnace, and the firing temperature was set to 930 ° C. The firing was carried out by holding the firing temperature and the atmospheric atmosphere for 6 hours. Next, the atmosphere while lowering the firing temperature (lowering temperature) is switched from the atmospheric atmosphere to the high oxygen gas atmosphere, and the temperature is lowered to room temperature (about 25 ° C.) over about 8 hours to complete the firing, and the lithium composite oxide is completed. Positive electrode active material particles were obtained. The atmosphere of the firing system was switched 20 minutes before the start of lowering the firing temperature. The oxygen concentration in the high oxygen gas atmosphere 1 hour after switching was 88.6% by volume.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Example 2>
In Example 1, positive electrode active material particles which are lithium composite oxides were obtained in the same manner as in Example 1 except that the firing temperature was set to 950 ° C.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Example 3>
In Example 1, positive electrode active material particles which are lithium composite oxides were obtained in the same manner as in Example 1 except that the firing temperature was set to 910 ° C.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Example 4>
In Example 1, positive electrode active material particles which are lithium composite oxides were obtained in the same manner as in Example 1 except that the firing temperature was set to 870 ° C.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Example 5>
In Example 1, the positive electrode active material particles which are lithium composite oxides are used in the same manner as in Example 1, except that the atmosphere while lowering the firing temperature (lowering temperature) is switched from the atmospheric atmosphere to the decarboxylation gas atmosphere. Obtained. The atmosphere of the firing system was switched 20 minutes before the start of lowering the firing temperature. The carbon dioxide concentration in the decarboxylation gas atmosphere 1 hour after switching was 3 ppm.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Comparative example 1>
In Example 1, positive electrode active material particles which are lithium composite oxides were obtained in the same manner as in Example 1 except that the atmosphere while lowering the firing temperature (lowering temperature) was not switched to the atmospheric atmosphere.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Comparative example 2>
In Example 2, positive electrode active material particles which are lithium composite oxides were obtained in the same manner as in Example 2 except that the atmosphere while lowering the firing temperature (lowering temperature) was not switched to the atmospheric atmosphere.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Comparative example 3>
In Example 3, positive electrode active material particles which are lithium composite oxides were obtained in the same manner as in Example 3 except that the atmosphere while lowering the firing temperature (lowering temperature) was not switched to the atmospheric atmosphere.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Comparative example 4>
In Example 4, positive electrode active material particles which are lithium composite oxides were obtained in the same manner as in Example 4, except that the atmosphere while lowering the firing temperature (lowering temperature) was not switched to the atmospheric atmosphere.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

<Comparative example 5>
In Example 1, the lithium composite oxide is the same as in Example 1, except that the atmosphere while lowering the firing temperature (lowering temperature) is switched from the atmospheric atmosphere to an atmosphere having a carbonic acid concentration lower than that of the atmospheric atmosphere. Positive electrode active material particles were obtained. The atmosphere of the firing system was switched 20 minutes before the start of lowering the firing temperature. The carbonic acid concentration in the furnace atmosphere 1 hour after the switching was 143 ppm.

The obtained positive electrode active material particles were crushed to obtain particle powder. The positive electrode active material particle powder had a composition represented by the composition formula: Li _1.02 (Ni _0.5 Co _0.2 Mn _0.3 ) O ₂ .

Residual lithium amount (residual Li ₂ Co ₃ amount and residual LiOH amount) of the positive electrode active material particles obtained in Examples 1 to 5 and Comparative Examples 1 to 5, specific surface area by the BET method, and average secondary particle diameter (D50). , And the initial discharge capacity and high temperature cycle characteristics of the non-aqueous electrolyte secondary battery provided with the positive electrode active material particles were examined according to the above methods. The results are shown in Table 2 below. The contrast value of the amount of residual Li ₂ Co ₃ is the value when Comparative Example 1 is 100 for Examples 1 and 5, and the value when Comparative Example 2 is 100 for Example 2. For Example 3, the value when Comparative Example 3 was set to 100, for Example 4, the value when Comparative Example 4 was set to 100 was shown, and for Comparative Example 5, Comparative Example 1 was set to 100. Indicates the hour value. In addition, Table 1 summarizes the firing conditions (set firing temperature and firing system atmosphere) in each Example and Comparative Example.

As shown in the firing conditions in Table 1 and the results in Table 2, Example 1 and Comparative Example 1 having the same set firing temperature, Example 2 and Comparative Example 2, and Example 3 and Comparative Example 3 were used. Comparing 4 and Comparative Example 4 respectively, it can be seen that the difference is clear. That is, the positive electrode active material particles of each example obtained by switching the atmosphere of the firing system from the atmospheric atmosphere to the high oxygen gas atmosphere while lowering the set firing temperature all have the atmosphere of the firing system in the atmospheric atmosphere. The amount of residual lithium is significantly reduced as compared with the positive electrode active material particles of each Comparative Example obtained without switching. In each example, the amount of residual Li ₂ CO ₃ is more than half that of each comparative example.

Further, comparing Example 5 and Comparative Example 1 having the same set firing temperature, Example 5 obtained by switching the atmosphere of the firing system while lowering the set firing temperature from the atmospheric atmosphere to the decarbonizing gas atmosphere. It can be seen that the amount of residual Li ₂ CO ₃ is significantly reduced in the positive electrode active material particles of No. 1 as compared with the positive electrode active material particles of Comparative Example 1 obtained without switching the atmosphere of the firing system in the air atmosphere.

Further, the non-aqueous electrolyte secondary battery using the positive electrode active material particles obtained in Examples 1 to 5 as the positive electrode is a non-aqueous electrolyte secondary battery using the positive electrode active material particles obtained in Comparative Examples 1 to 5 as the positive electrode. It can be seen that it has the same initial discharge capacity as the next battery. Further, the non-aqueous electrolyte secondary battery using the positive electrode active material particles obtained in Examples 1 to 5 as the positive electrode is a non-aqueous electrolyte secondary battery using the positive electrode active material particles obtained in Comparative Examples 1 to 5 as the positive electrode. It can be seen that the high temperature cycle characteristics are improved by about 2% or more (the numerical value is increased) as compared with the next battery.

The positive electrode active material particles obtained by the method according to the present invention have a very small amount of residual lithium and are suitable as an active material used for the positive electrode of a non-aqueous electrolyte secondary battery.

Claims (5)

A method for reducing the amount of residual lithium in positive electrode active material particles made of a lithium composite oxide.
After setting the firing temperature to 850 ° C. or higher and holding the set firing temperature and air atmosphere for a predetermined time, the atmosphere during the lowering of the set firing temperature is changed from the air atmosphere to the carbon dioxide concentration of the air atmosphere. At least a step of calcining a mixture of a precursor compound containing Ni, Co and Mn and a lithium compound to prepare a lithium composite oxide is provided by switching to a low carbon dioxide atmosphere having a carbon dioxide concentration of 1/60 or less. A method for reducing the amount of residual lithium in the positive electrode active material particles, which comprises the above. The amount of residual lithium in the positive electrode active material particles according to claim 1, wherein the low carbon dioxide atmosphere is a high oxygen gas atmosphere having an oxygen concentration of 80% by volume or more, or a decarboxylation gas atmosphere having a carbon dioxide concentration of 4 ppm or less. Reduction method. The method for reducing the amount of residual lithium in the positive electrode active material particles according to claim 2, wherein the oxygen concentration in the high oxygen gas atmosphere is 85% by volume or more. The method for reducing the amount of residual lithium in the positive electrode active material particles according to claim 2, wherein the carbon dioxide concentration in the decarboxylation gas atmosphere is 3 ppm or less. The method for reducing the amount of residual lithium in the positive electrode active material particles according to any one of claims 1 to 4, wherein the firing temperature is set to 870 ° C. to 1000 ° C.