JP2012195082A - Method for manufacturing positive electrode active material for lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing a positive electrode active material for a lithium ion secondary battery capable of inhibiting the reduction in battery capacity associated with the activation of the positive electrode active material.SOLUTION: The method for manufacturing a positive electrode active material for a lithium ion secondary battery comprises: an acid treatment step in which an active material represented by the composition formula: xLiMO(1-x)LiMO, where Mrepresents one or more kinds of metallic elements necessarily containing tetravalent manganese, Mrepresents one or more kinds of metallic elements, 0≤x<1, and Li may be partly replaced by hydrogen, is brought into contact with an acid solution; and a lithium compensation step in which the active material having undergone the acid treatment is brought into contact with a lithium solution containing a lithium compound.

Description

  The present invention relates to a method for producing a positive electrode active material for a lithium ion secondary battery using a lithium manganese based composite oxide as a positive electrode active material.

In recent years, along with the development of portable electronic devices such as mobile phones and notebook computers, and the practical application of electric vehicles, secondary batteries with small and light weight and high capacity are required. As a high-capacity secondary battery that meets this demand, a non-aqueous secondary battery using lithium cobalt oxide (LiCoO ₂ ) as a positive electrode material and a carbon-based material as a negative electrode material has been commercialized. Such a non-aqueous secondary battery has a high energy density and can be reduced in size and weight, and therefore has attracted attention as a power source in a wide range of fields. However, since LiCoO ₂ is manufactured using Co, which is a rare metal, as a raw material, it is expected that a shortage of resources will become serious in the future. Furthermore, since Co is expensive and has a large price fluctuation, development of a positive electrode material that is inexpensive and stable in supply is desired.

Therefore, the use of a lithium manganese oxide-based composite oxide containing manganese (Mn), whose constituent elements are inexpensive and whose supply is stable, is considered promising. Among them, a substance called Li ₂ MnO ₃ that is composed of tetravalent manganese ions and does not contain trivalent manganese ions that cause manganese elution at the time of charge and discharge has attracted attention. Li ₂ MnO ₃ has been considered to be impossible to charge and discharge until now, but recent research has found that it can be charged and discharged by charging to a high potential, and Li ₂ MnO ₃ and LiMO ₂ (M is Development of xLi ₂ MnO _3. (1-x) LiMO ₂ (0 <x ≦ 1), which is a solid solution with a transition metal element), has been actively conducted. Note that Li ₂ MnO ₃ can also be expressed as a general formula Li (Li _0.33 Mn _0.67 ) O ₂ and belongs to the same crystal structure as LiMO ₂ . Therefore, xLi ₂ MnO _3. (1-x) LiMO ₂ is Li _1.33-y Mn _0.67-z M _{y + z} O ₂ (0 ≦ y <0.33, 0 ≦ z <0.67) May be described.

However, solid solution active materials containing these Li ₂ MnO ₃ and Li ₂ MnO ₃ are required to be further improved with respect to cycle characteristics.

For example, when a secondary battery using a lithium manganese composite oxide having a layered rock salt structure containing Li ₂ MnO ₃ as a positive electrode active material is used, the battery is charged with an irreversible capacity called activation prior to use. There is a need. However, to activate the lithium manganese composite oxide, it is necessary to charge at a high voltage of 4.5 V or more at the Li counter electrode potential. Gas generation due to decomposition of the electrolyte and formation of a high resistance film on the surface of the positive electrode active material There was a problem of adversely affecting the cycle characteristics.

On the other hand, as a method for chemically causing the same reaction as Li elimination occurring in the positive electrode active material during charging, Non-Patent Documents 1 and 2 propose that an acid treatment is performed on a lithium manganese composite oxide. Patent Documents 1 and ₂ disclose a method of chemically desorbing Li ions using lithium manganese-based composite oxide LiMnO ₂ as a starting material and using an acid such as hydrochloric acid or sulfuric acid. Patent Document 3, acid-treated by stirring the lithium manganese composite oxide _{Li 1 + x (Mn y M} 1-y) 1-x O 2 in an aqueous solution of nitric acid, by performing heat treatment after washing with water, the cathode _Li as an active material _{1 + x-a (Mn y} M 1-y) 1-x O 2 ± b (0 <a <0.3,0 <b <0.1,0 <x <0.4,0 < A method for obtaining y ≦ 1, 0.95 <1 + xa <1.15, M is a transition metal) is shown.

JP 2004-59379 A JP 2007-77018 A JP 2009-4285 A Japanese National Patent Publication No. 11-506721 Japanese Patent Application Laid-Open No. 07-073882

Journal of the Electrochemical Society, 153, (6) A1186-A1192 (2006) Journal of the Electrochemical Society, 157, (11) A1177-A1182 (2010)

By applying the various documents to the technique disclosed in Li ₂ MnO ₃ lithium-manganese-based composite oxide containing, for activation by charging to a high voltage required in Li ₂ MnO ₃ lithium-manganese-based composite oxide containing It is expected that the stage can be omitted. However, in that case, when the treatment is performed with an acidic aqueous solution, lithium in the lithium manganese composite oxide is exchanged for protons in the acidic aqueous solution, and lithium flows out into the acidic elementary solution. For this reason, the lithium content in the lithium manganese composite oxide decreases, which causes a decrease in battery capacity.

  Patent Documents 4 and 5 disclose that a lithium compound is mixed with manganese dioxide and heat-treated to obtain a lithium-containing manganese composite oxide. However, this technique replaces ion-exchangeable hydrogen in manganese dioxide with Li ions. Since the target oxide is different from the lithium manganese composite oxide having the layered rock salt structure, the techniques of Patent Documents 4 and 5 cannot be applied to the lithium supplementation of the lithium manganese composite oxide as they are.

  This invention is made | formed in view of this situation, and makes it a subject to provide the manufacturing method of the positive electrode active material for lithium ion secondary batteries which can suppress the reduction | decrease of the battery capacity by activation of a positive electrode active material. .

(1) The method for producing a positive electrode active material for a lithium ion secondary battery of the present invention has a composition formula: xLi ₂ M ¹ O _3. (1-x) LiM ² O ₂ (M ¹ is essentially tetravalent manganese. One or more metal elements, M ² is one or more metal elements, 0 <x ≦ 1, and Li may be partially substituted with hydrogen.) An acid treatment step to be carried out, and a lithium supplementation step in which a lithium solution containing a lithium compound is brought into contact with the acid-treated active material.

According to the said structure, the acid solution is made to contact the active material. Therefore, Li ₂ O is extracted from the active material xLi ₂ MnO _3. (1-x) LiMO ₂ to activate the active material.

By bringing the active material into contact with the acid solution, Li ions are extracted as Li ₂ O from the active material, so that the battery capacity is reduced as it is.

  Therefore, in the present invention, a lithium solution containing a lithium compound is brought into contact with the active material in contact with the acid solution. For this reason, Li ion is supplemented to the activated active material, and battery capacity increases.

  (2) The acid solution is preferably composed of any one of an aqueous sulfuric acid solution, an aqueous nitric acid solution, and an aqueous ammonium sulfate solution. In this case, the active material is sufficiently activated.

  (3) It is preferable that the said lithium compound contains at least 1 sort (s) of lithium hydroxide and lithium nitrate. In this case, Li ions are sufficiently supplemented to the activated active material, and the battery capacity is increased.

  (4) It is preferable that the lithium compound is lithium hydroxide, and the concentration of the lithium hydroxide in the lithium solution is 0.1 mol / L (M) or more and 5 M or less. In this case, Li ions are sufficiently supplemented to the activated active material, and the battery capacity is increased.

(5) In the acid treatment step, it is preferable that Li ₂ O be extracted from the active material by bringing an acid solution into contact with the active material. In this case, the active material is effectively activated.

  According to the method for manufacturing a lithium ion secondary battery of the present invention, the acid solution is brought into contact with the active material and then brought into contact with the lithium solution. Therefore, the active material is activated and can be used as a battery, and Li ions are supplemented in the lithium supplementation process, thereby increasing the capacity of the secondary battery.

It is a figure which shows the charging / discharging curve of the secondary battery which used the samples 1 and 2 for the positive electrode, and used metallic lithium for the negative electrode. It is a figure which shows the charging / discharging curve of the secondary battery which used the samples 1 and 2 for the positive electrode, and used carbon for the negative electrode. It is a figure which shows the charging / discharging curve of the secondary battery which used the samples 1-4 for the positive electrode and used metallic lithium for the negative electrode. It is a figure which shows the charging / discharging curve of the secondary battery which used the samples 1-4 for the positive electrode and used carbon for the negative electrode. It is a figure which shows the charging / discharging curve of the secondary battery which used the samples 2 and 5 for the positive electrode and used carbon for the negative electrode. It is a figure which shows the charging / discharging curve of the secondary battery which used sample 2,5,6 for the positive electrode and used carbon for the negative electrode. It is a figure which shows the charging / discharging curve of the secondary battery which used sample 1,2,5,7 for the positive electrode and used metallic lithium for the negative electrode. It is a figure which shows the charging / discharging curve of the secondary battery which used sample 1,2,5,7 for the positive electrode and used carbon for the negative electrode.

  The manufacturing method of the positive electrode active material for lithium ion secondary batteries of this invention has an acid treatment process and a lithium supplementation process.

(1) In the acid treatment step acid treatment step, the composition ^{_{formula: xLi 2 M 1 O 3 ·}} (1-x) LiM 2 O 2 (M 1 is tetravalent manganese least one metallic element as essential, ^{M 2} Is one or more metal elements, 0 <x ≦ 1, and a part of Li may be replaced with hydrogen.) An acid solution is brought into contact with the active material.

The active material is a solid solution composed of xLi ₂ M ¹ O ₃ and (1-x) LiM ² O ₂ . Li ₂ M ¹ O ₃ and LiM ² O ₂ both have a layered rock salt structure (α-NaFeO ₂ type). Li ₂ M ¹ O ₃ has a large battery capacity, and LiM ² O ₂ is said to have excellent battery cycle characteristics. Li ₂ M ¹ O ₃ —LiM ² O ₂ containing both Li ₂ M ¹ O _{3 and} LiM ² O ₂ has a large battery capacity and excellent battery cycle characteristics.

Li ₂ M ¹ O ₃ can also be expressed as a general formula Li (Li _0.33 M ¹ _0.67 ) O ₂ and is said to belong to the same layered crystal structure as LiM ² O ₂ . Therefore, xLi ₂ M ¹ O _3. (1-x) LiM ² O ₂ is Li _1.33-x M ¹ _0.67-y M ² _{x + y} O ₂ (M ¹ is essential for tetravalent Mn. One or more metal elements, M ² may be described as a metal element, 0 ≦ x ≦ 0.33, 0 ≦ y <0.67).

^{M 1} in the li ₂ M ¹ O ₃ is an essential component tetravalent Mn. Most of M ¹ is preferably tetravalent Mn, but less than 50% or even less than 80% may be substituted with another metal element. The metal element other than Mn that can constitute M ² is, for example, at least one selected from Ni, Al, Co, Fe, Mg, and Ti from the viewpoint of chargeable / dischargeable capacity when an electrode material is used. Preferably it is a seed.

Note that composite oxides slightly deviating from the above composition formula due to defects of Li, Ni, M ¹ , M ^2, or O that are inevitably generated are also included. Therefore, the average oxidation number of the ^{M 1} may be slightly shifted from the tetravalent, it is allowed to 3.8 to 4.2 valence.

M ² in LiM ² O ₂ is a metal element, and is preferably at least one of Ni, Al, Mn, Co, Fe, Mg, and Ti, for example.

The oxidation number of M ² is preferably trivalent. When M ² is composed of a plurality of metal elements, the average oxidation number of the metal elements constituting M ² is preferably trivalent. Examples of the trivalent element include Ni, Co, Al, Mg, Fe, and Sn. When M ² consists of Mn and Ni, Mn becomes tetravalent, Ni becomes divalent, and Mn and Ni as M ² are included in the same number of moles.

Note that composite oxides slightly deviating from the above composition formula due to unavoidable loss of Li, M ¹ , M ² or O are also included. Accordingly, the oxidation number of ^{M 2} may be slightly shifted from trivalent, it is allowed to 2.8 to 3.2 valence.

A part of Li contained in Li ₂ M ¹ O ₃ and LiM ² O ₂ may be substituted with hydrogen (H). For example, in Li, 60% or less, further 45% or less in atomic ratio may be substituted with H.

A composite oxide represented by a composition formula xLi ₂ M ¹ O ₃ · (1-x) LiM ² O ₂ is used as a basic composition, and a lithium nickel-based composite oxide having a layered rock salt structure includes, for example, x in the composition formula When Li is 1, Li ₂ M ¹ O ₃ is obtained, and when x in the composition formula is 0.5, Li ₂ M ¹ O ₃ .LiM ² O ₂ is obtained.

Examples of Li ₂ M ¹ O ₃ include Li ₂ MnO ₃ , LiMn _1/3 Ni _1/3 Co _1/3 O ₂ , and LiMn _1/2 Ni _1/2 O ₂ . The ^{_{Li 2 M 1 O 3 · LiM}} 2 O 2, for _{example, Li 2 MnO 3 · LiNiO 2} , Li 2 MnO 3 · LiMn 1/3 Ni 1/3 Co 1/3 O 2, Li 2 MnO 3 · LiMn _½ Ni _½ O _{2 and the} like can be mentioned.

  A part of Ni, Mn, and Co, which are constituent elements of the composite oxides listed above, may be substituted with other metal elements. The obtained composite oxide as a whole may have the basic composition as the exemplified oxide, and may slightly deviate from the above composition formula due to unavoidable metal element or oxygen deficiency.

Li in M ¹ and M ² may be substituted with H in an atomic ratio of 60% or less, further 45% or less.

When the acid solution is brought into contact with the active material, Li ₂ O is extracted from the active material as shown in the following reaction formula (1).
^{_{xLi 2 M 1 O 3 · (}} 1-x) LiM 2 O 2 → xLi 2-y M 1 O 3-y / 2 · (1-x) LiM 2 O 2 + y / 2Li 2 O ···· (1 )
(0 <x ≦ 1, 0 <y <2)
The extracted Li ₂ O is eluted into the acid solution as Li ions, and can be confirmed by an increase in the amount of Li by plasma emission spectroscopic analysis (ICP).

  The acid solution is preferably composed of any one of an aqueous sulfuric acid solution, an aqueous nitric acid solution, and an aqueous ammonium sulfate solution. Among these, a sulfuric acid aqueous solution is preferable. The reaction formula when the active material is brought into contact with the sulfuric acid aqueous solution is shown in the following formula.

_{^{xLi 2 M 1 O 3 · (}} 1-x) LiM 2 O 2 + H 2 SO 4 → xLi 2-y H y M 1 O 3 · (1-x) LiM 2 O 2 + LiSO 4 ... (2)
_{^{xLi 2-y H y M 1}} O 3 · (1-x) LiM 2 O 2 → xLi 2-y M 1 O 3-y / 2 · (1-x) LiM 2 O 2 +1/2 H 2 O ... (3)
The concentration of the acid product such as acid and salt contained in the acid solution is slightly different depending on the type of the acid product, but is preferably about 0.01 M or more and 5 M or less. When the concentration of the acid product in the sulfuric acid aqueous solution is less than 0.01M, the active material may be hardly activated, and when it exceeds 5M, the structure of the active material may change.

  When the acid solution is an aqueous sulfuric acid solution, the concentration of sulfuric acid in the aqueous sulfuric acid solution is preferably 0.01 M or more and 5 M or less, and more preferably 0.1 M or more and 2 M or less. When the concentration of sulfuric acid in the aqueous sulfuric acid solution is less than 0.01M, the active material may be difficult to activate, and when it exceeds 5M, the structure of the active material may change.

  When the acid solution is an aqueous ammonium sulfate solution, the concentration of ammonium sulfate in the aqueous ammonium sulfate solution is preferably 0.01 M or more and 5 M or less, and more preferably 0.05 M or more and 1 M or less. When the concentration of ammonium sulfate in the aqueous ammonium sulfate solution is less than 0.01M, the active material may be difficult to activate, and when it exceeds 5M, the structure of the active material may change.

  When the acid solution is an aqueous nitric acid solution, the concentration of nitric acid in the aqueous nitric acid solution is preferably 0.01 M or more and 5 M or less, and more preferably 0.1 M or more and 2 M or less. If the concentration of nitric acid in the aqueous nitric acid solution is less than 0.01M, the active material may be less likely to be activated, and if it exceeds 5M, the structure of the active material may change.

(2) Lithium filling step In the lithium filling step, a lithium solution containing a lithium compound is brought into contact with the acid-treated active material. Then, the active material ^{_{xLi 2-y M 1 O 3}} -y / 2 · (1-x) LiM 2 O 2 in the formula was subjected to acid treatment by proton exchange (1), Li ions are introduced. An increase in battery capacity is observed due to the introduction of Li ions.

  As the lithium compound, it is preferable to use a lithium compound capable of eluting Li ions into the solvent. As such a lithium compound, lithium hydroxide, a lithium salt, or the like may be used. For example, lithium hydroxide or lithium nitrate may be used as the lithium salt. As a solvent of the lithium solution, water is preferable.

For ^{_{xLi 2-y M 1 O 3}} -y / 2 · (1-x) LiM 2 O 2 itself is alkaline, strongly alkaline under easily introduce lithium salt or Li ions. For this reason, as conditions of the lithium solution in which Li ions are easily introduced, for example, the concentration of the lithium compound is high, the pH of the lithium compound is high, and the solubility of the lithium compound is high.

  The concentration of the lithium compound in the lithium solution is preferably from 0.1 M to 5 M, and more preferably from 1 M to 3 M. When the concentration of the lithium compound in the lithium solution is less than 0.1M, Li ion supplementation may be difficult, and when it exceeds 5M, the structure of the active material may change.

For example, when the lithium compound is lithium hydroxide (LiOH / H ₂ O), lithium hydroxide has high alkalinity, and thus it is easy to introduce Li ions into the active material. Lithium nitrate or the like has high solubility in water, but since it is neutral, the ability to introduce Li ions into the active material is not as high as that of lithium hydroxide.

  For example, when the lithium compound is lithium hydroxide, the concentration of lithium hydroxide in the lithium solution is preferably from 0.1 M to 5 M, and more preferably from 1 M to 3 M. If the concentration of lithium hydroxide in the lithium solution is less than 0.1M, Li ion supplementation may be difficult, and if it exceeds 5M, the structure of the active material may change.

  When the lithium compound is lithium nitrate, the concentration of lithium nitrate in the lithium solution is preferably from 0.1 M to 5 M, and more preferably from 1 M to 3 M. If the concentration of lithium nitrate in the lithium solution is less than 0.1M, Li ion supplementation may be difficult, and if it exceeds 5M, the structure of the active material may change.

  The pH of the lithium solution is preferably alkaline, and more preferably 7.1 or more and 13 or less. When the pH is 7.1 or more, supplementation of Li ions to the active material is likely to occur.

  The positive electrode active material obtained by the production method of the present invention described above covers the current collector surface together with a binder, a conductive auxiliary agent, etc. as necessary to constitute a positive electrode. This positive electrode is used for a lithium ion secondary battery together with a negative electrode, a separator, and a nonaqueous electrolyte. The lithium ion secondary battery is a kind of non-aqueous electrolyte secondary battery, and is a secondary battery in which Li ions in the electrolyte bear electric conduction. Lithium ion secondary batteries can be suitably used in the field of automobiles in addition to the fields of communication devices and information-related devices such as mobile phones and personal computers. For example, if this lithium ion secondary battery is mounted on a vehicle, the lithium ion secondary battery can be used as a power source for an electric vehicle.

(Synthesis of active material <sample 1>)
As described below, an active material made of Li ₂ MnO ₃ .LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was prepared by a molten salt method.

A raw material mixture was prepared by mixing 0.20 mol of lithium hydroxide-hydrate LiOH.H ₂ O as a lithium compound (molten salt raw material) and 0.02 mol of nickel manganese cobalt oxide as a nickel compound. The raw material mixture was put in a crucible, transferred into an electric furnace at 700 ° C., and heated in the atmosphere at 700 ° C. for 2 hours. At this time, the raw material mixture melted to form a molten salt, and a black product was precipitated.

The crucible containing the molten salt was cooled to room temperature in the electric furnace and then taken out from the electric furnace. After the molten salt was sufficiently cooled and solidified, the crucible was immersed in 200 ml of distilled water, and the molten salt solidified by stirring was dissolved in water. Since the black product was insoluble in water, the water became a black suspension. Filtration of the black suspension gave a clear filtrate and a black solid filtrate on the filter paper. The obtained filtrate was further filtered while thoroughly washing with distilled water. The black solid after washing was vacuum-dried at 120 ° C. for 12 hours and then pulverized using a mortar and pestle. X-ray diffraction (XRD) using CuKα rays was measured for the obtained black powder, and it was confirmed that an active material Li ₂ MnO ₃ .LiNi _1/3 Co _1/3 Mn _1/3 O ₂ was generated. It was done. The composition ratio of Li ₂ MnO ₃ .LiNi _1/3 Co _1/3 Mn _1/3 O ₂ is a molar ratio, Li ₂ MnO ₃ : LiNi _1/3 Co _1/3 Mn _1/3 O ₂ = 50: 50. This was designated as Sample 1.

(Implementation of acid treatment process for active material <samples 2, 3, 4>)
The sample 1 was subjected to an acid treatment step by the following three methods.

First, in the first acid treatment step, Li ₂ MnO ₃ .LiNi _1/3 Co _1/3 Mn _1/3 O ₂ (300 mg) of Sample 1 obtained by synthesis of the active material was added to 0.1 M sulfuric acid. Mixed in aqueous (H ₂ SO ₄ ) solution (5 ml) at room temperature overnight. Thereafter, it was washed twice with distilled water and dried in vacuum at 120 ° C. for 6 hours. This was designated as Sample 2.

In the second acid treatment step, the sample 1 (300 mg) was mixed overnight at room temperature in a 0.1 M nitric acid (HNO ₃ ) aqueous solution (5 ml). Thereafter, it was washed twice with distilled water and dried in vacuum at 120 ° C. for 6 hours. This was designated as Sample 3.

In the third acid treatment step, the sample 1 (300 mg) was mixed overnight in an aqueous 0.1 M ammonium sulfate ((NH ₄ ) ₂ SO ₄ ) solution (5 ml) at room temperature. Thereafter, it was washed twice with distilled water and dried in vacuum at 120 ° C. for 6 hours. This was designated as Sample 4.

(Positive electrode active material <Sample 5>)
An active material (sample 2) that had been subjected to an acid treatment step with an aqueous sulfuric acid solution was subjected to a lithium supplementation step. In the lithium supplementation step, the active material (sample 2) subjected to the acid treatment step was mixed overnight in a 1M lithium hydroxide aqueous solution at room temperature. Thereafter, it was washed twice with distilled water and again dried in a vacuum at 120 ° C. for 6 hours to obtain a target positive electrode active material. This was designated as Sample 5.

(Positive electrode active material <Sample 6>)
A positive electrode active material was prepared in the same manner as Sample 5 except that a 1M lithium nitrate aqueous solution was used instead of the lithium hydroxide aqueous solution in the lithium filling step of the production method of Sample 5. This was designated as Sample 6.

(Positive electrode active material <Sample 7>)
A positive electrode active material was prepared in the same manner as Sample 5 except that a 3M lithium hydroxide aqueous solution was used instead of the 1M lithium hydroxide aqueous solution in the lithium supplementation step of the production method of Sample 5. This was designated as Sample 7.

  The characteristics of the production methods of Samples 1 to 7 are shown in Table 1.

(Production of secondary battery)
Each of Samples 1 to 7 was mixed at a mass ratio of each sample: Ketjen black: conductive binder (TAB) = 50: 20: 30. The conductive binder (TAB) is a mixture of acetylene black (AB) and polytetrafluoroethylene (PETF) mixed at AB: PTFE = 2: 1 (mass ratio). Subsequently, this mixture was crimped | bonded to the aluminum mesh which is a collector. Then, it vacuum-dried at 120 degreeC for 12 hours or more, and was set as the electrode (positive electrode: (phi) 14mm). The negative electrode facing the positive electrode was metallic lithium (φ14 mm, thickness 400 μm) or carbon (graphite).

A microporous polyethylene film having a thickness of 20 μm was sandwiched between the positive electrode and the negative electrode as a separator to obtain an electrode body battery. This electrode body battery was accommodated in a battery case (CR2032 coin cell manufactured by Hosen Co., Ltd.). In addition, a non-aqueous electrolyte in which LiPF ₆ is dissolved at a concentration of 1.0 mol / L is injected into a mixed solvent in which ethylene carbonate and diethyl carbonate are mixed at a ratio of 1: 1 (volume ratio) to the battery case. A battery was obtained. The produced secondary battery was subjected to the following charge / discharge test and subjected to evaluation.

<Charge / discharge test>
About each produced secondary battery, the charging / discharging test was done under 25 degreeC constant temperature. Charging of the fabricated secondary battery was performed at a rate of 0.2 C, and when using lithium metal for the negative electrode (described later in <Evaluation 1>, <Evaluation 3>, <Evaluation 7> in FIG. 7), 4.5 V was applied to the negative electrode. When carbon (graphite) is used (Fig. 8 of <Evaluation 2>, <Evaluation 4>, <Evaluation 5>, <Evaluation 6>, <Evaluation 7> described later), constant-current charging is performed up to 4.2V. Thereafter, charging was performed at a constant voltage of 4.5 V when the negative electrode was metallic lithium and a constant voltage of 4.2 V when the negative electrode was carbon (graphite) up to a current value of 0.02 C. The discharge was performed at a rate of 0.2 C up to 2.0V.

<Evaluation 1>
FIG. 1 shows a charge / discharge curve of a secondary battery using Samples 1 and 2 as the positive electrode and metal lithium as the negative electrode. In FIG. 1, a curve rising to the right is a charging curve, and a curve falling to the right is a discharge curve. The charge / discharge at the first cycle of the secondary battery using the active material of Sample 1 is (1), (6), the charge / discharge at the second cycle is (7), (2), and the charge / discharge at the third cycle is ( 5) and (4). The charge / discharge at the first cycle of the secondary battery using the active material of Sample 2 is indicated by 1, 6, the charge / discharge at the second cycle is 3, and the charge / discharge at the third cycle is indicated by 5, 2.

As shown in the oval part of FIG. 1, in the secondary battery using the active material of Sample 1 before the acid treatment step, a flat portion (plateau) appears around 4.5 V when charging in the first cycle. Yes. This is a curve that appears when Li ₂ MnO ₃ is activated. This indicates that the active material before the acid treatment step is not activated.

  On the other hand, in the secondary battery using the active material after the acid treatment step of Sample 2, the active material was active because the flat portion was not seen in the vicinity of 4.5 V when charging in the first cycle. You can see that

<Evaluation 2>
FIG. 2 shows a charge / discharge curve of a secondary battery using Samples 1 and 2 as the positive electrode and carbon as the negative electrode.

  Since the capacity decreased after the acid treatment step, it can be confirmed that the capacity decreased due to the elution of lithium in the active material.

<Evaluation 3>
FIG. 3 shows a charge / discharge curve of a secondary battery in which samples 1 to 4 are used for the positive electrode and metallic lithium is used for the negative electrode. As shown in FIG. 2, the charging curve of the secondary battery using the active material (samples 2 to 4) subjected to the acid treatment is more than that of the secondary battery using the active material (sample 1) not subjected to the acid treatment. However, there are few flat parts, and it turns out that the active material has been activated by performing acid treatment. Further, among the active materials subjected to the acid treatment, the decrease in battery capacity was the smallest in the sample 4 which was subjected to the acid treatment with ammonium sulfate. Thus, it turns out that battery capacity changes with the kind of acid which performs acid treatment.

<Evaluation 4>
FIG. 4 shows a charge / discharge curve of a secondary battery in which samples 1 to 4 are used for the positive electrode and carbon is used for the negative electrode. When the treatment was performed with ammonium sulfate, which is a weak acid, the lithium ion elution was small, so that the capacity decrease was small. In the case of nitric acid, a large amount of lithium ion elution occurred, and a considerable capacity decrease was observed. The battery capacity decreased in the order of ammonium sulfate (sample 4), sulfuric acid (sample 2), and nitric acid (sample 3).

<Evaluation 5>
FIG. 5 shows a charge / discharge curve of a secondary battery in which samples 2 and 5 were used for the positive electrode and carbon was used for the negative electrode. As shown in FIG. 5, the battery capacity was higher when the lithium supplementation process was performed (sample 5) than when the active material was not subjected to the lithium supplementation process (sample 2). This is because lithium ions are returned to the active material by lithium supplementation of the active material with lithium hydroxide, and hydrogen introduced into the active material in the acid treatment step is exchanged with Li ions for the inside of the active material. This is probably because the battery capacity has increased.

<Evaluation 6>
FIG. 6 shows a charge / discharge curve of a secondary battery in which samples 2, 5, and 6 are used for the positive electrode and carbon is used for the negative electrode. As shown in FIG. 6, the battery capacity is higher when the acid treatment step and the lithium supplementation step with the lithium hydroxide aqueous solution (sample 5) are performed than with the active material subjected to only the acid treatment step (sample 2). it was high. This indicates that the battery capacity has been recovered by treating the active material subjected to the acid treatment step with an aqueous lithium hydroxide solution.

  When the active material after the acid treatment step was treated with lithium nitrate (sample 6), the battery capacity was lower than when only the acid treatment step was carried out (sample 2). This is presumably because when the active material was treated with lithium nitrate, lithium ions were eluted from the precursor preferentially rather than lithium supplementation in the precursor, and the battery capacity was reduced.

<Evaluation 7>
FIG. 7 shows charge / discharge curves of a secondary battery using Samples 1, 2, 5, and 7 for the positive electrode and metallic lithium for the negative electrode. FIG. 8 shows a charge / discharge curve of a secondary battery using Samples 1, 2, 5, and 7 for the positive electrode and carbon for the negative electrode.

  As shown in FIGS. 7 and 8, the acid treatment and the lithium supplementation process are both performed when the metal lithium is used for the negative electrode and when the carbon is used as compared with the case where only the acid treatment is performed (Sample 2). In the case of performing (Samples 5 and 7), the battery capacity was higher. However, as indicated by the oval portion of FIG. 7, when the lithium hydroxide concentration of the lithium hydroxide aqueous solution used in the lithium supplementation process is high (sample 7), the lithium hydroxide concentration is low (sample 5). A flat part that was not observed appeared. This means that when the lithium hydroxide concentration in the lithium hydroxide aqueous solution is increased, the active material once activated by the acid treatment is inactivated. Therefore, it can be said that the concentration of lithium hydroxide in the lithium solution is preferably 1 M or less.

Claims (5)

Composition formula: xLi ₂ M ¹ O _3. (1-x) LiM ² O ₂ (M ¹ is one or more metal elements essential for tetravalent manganese, M ² is one or more metal elements, 0 <x ≦ 1, Li may be partially substituted with hydrogen.) An acid treatment step of bringing an acid solution into contact with an active material represented by:
And a lithium filling step in which a lithium solution containing a lithium compound is brought into contact with the acid-treated active material. A method for producing a positive electrode active material for a lithium ion secondary battery.   2. The method for producing a positive electrode active material for a lithium ion secondary battery according to claim 1, wherein the acid solution is one of a sulfuric acid aqueous solution, a nitric acid aqueous solution, and an ammonium sulfate aqueous solution.   The said lithium compound is a manufacturing method of the positive electrode active material for lithium ion secondary batteries of Claim 1 or 2 containing at least 1 sort (s) of lithium hydroxide and lithium nitrate.   4. The positive electrode active for a lithium ion secondary battery according to claim 3, wherein the lithium compound is lithium hydroxide, and the concentration of the lithium hydroxide in the lithium solution is 0.1 mol (M) or more and 5 mol or less. A method for producing a substance. 5. The positive electrode active material for a lithium ion secondary battery according to claim 1, wherein Li ₂ O is extracted from the active material by bringing the acid solution into contact with the active material in the acid treatment step. Manufacturing method.

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