JP2015106470A - Positive electrode active material, positive electrode and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a positive electrode active material, a positive electrode and a lithium ion secondary battery, capable of obtaining sufficient discharge capacity and good in cycle characteristics.SOLUTION: A positive electrode active material is a compound represented by a composition formula: LiVOPO[where, x satisfies -0.15≤x≤0.15.], the compound having a pH of 3.0 or more and 6.8 or less.

Description

  The present invention relates to a positive electrode active material, a positive electrode, and a lithium ion secondary battery.

Conventionally, layered compounds such as LiCoO ₂ and LiNi _1/3 Mn _1/3 Co _1/3 O ₂ and spinel compounds such as LiMn ₂ O ₄ have been used as active materials for lithium ion secondary batteries. In recent years, compounds having an olivine type structure typified by LiFePO ₄ have attracted attention. An active material having an olivine structure is known to have high thermal stability at high temperatures and high safety. However, the lithium ion secondary battery using LiFePO ₄ has a drawback that its charge / discharge voltage is as low as about 3.5 V and the energy density is low.

Therefore, LiVOPO ₄ has been proposed as a compound that can realize a charge / discharge voltage of 4 V class among active materials having an olivine structure (Patent Document 1).

JP 2004-303527 A

However, in a battery provided with a positive electrode using LiVOPO ₄ obtained by the method described in Patent Document 1, sufficient cycle characteristics cannot be obtained.

  The present invention has been made in view of the above-described problems of the prior art, and an object of the present invention is to provide a positive electrode active material, a positive electrode, and a lithium ion secondary battery that can obtain good cycle characteristics.

In order to achieve the above object, the positive electrode active material according to the present invention has a composition formula Li _1-x VOPO ₄ [where x is −0.15 ≦ x ≦ 0.15. The pH of the said compound is 3.0 or more and 6.8 or less, It is characterized by the above-mentioned.

  According to the present invention, it is possible to realize better cycle characteristics as compared with the conventional case. Although this reason is not necessarily clear, it is considered as follows. When the composition of the positive electrode active material and the variation in crystallinity are small, the vanadium ions from the positive electrode active material are less likely to elute, so that the pH of the positive electrode active material is likely to be 3.0 or more, and the positive electrode active material contributing to charge / discharge It is considered that the decrease in the amount of substances can be suppressed and the cycle characteristics are improved. In addition, when the pH of the positive electrode active material is 6.8 or less, the corrosion of the positive electrode current collector is suppressed, so that peeling due to a decrease in adhesion between the positive electrode active material layer and the positive electrode current collector is suppressed, and a good cycle is achieved. It is considered that the characteristics can be realized.

  The positive electrode active material layer of the present invention preferably has a pH of 3.5 or more and 7.8 or less. As a result, it was found that better cycle characteristics can be realized. The pH of the positive electrode active material layer is considered to be affected by individual conductive assistants and binders constituting the positive electrode active material layer other than the positive electrode active material. When the pH is 3.5 or more, vanadium ions, etc. When the pH is 7.8 or lower, the corrosion due to the positive electrode current collector can be suppressed to prevent the peeling due to the decrease in the adhesion between the positive electrode active material layer and the positive electrode current collector. It is considered that the characteristics can be realized.

  Moreover, the lithium ion secondary battery which concerns on this invention can acquire favorable cycling characteristics by providing the said positive electrode.

  According to the present invention, it is possible to provide a positive electrode active material, a positive electrode, and a lithium ion secondary battery that can obtain good cycle characteristics.

It is a schematic cross section of the lithium ion secondary battery according to the present embodiment.

  Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. In addition, this invention is not limited to the following embodiment. The constituent elements described below include those that can be easily assumed by those skilled in the art and those that are substantially the same. Furthermore, the constituent elements described below can be appropriately combined.

<Lithium ion secondary battery>
The lithium ion secondary battery according to this embodiment will be briefly described with reference to FIG.

  The lithium ion secondary battery 100 mainly includes a laminate 30, a case 50 that accommodates the laminate 30 in a sealed state, and a pair of leads 60 and 62 connected to the laminate 30.

The laminated body 30 is configured such that the positive electrode 10 and the negative electrode 20 are disposed to face each other with the separator 18 interposed therebetween. The positive electrode 10 is a product in which a positive electrode active material layer 14 is provided on a positive electrode current collector 12. The negative electrode 20 is a product in which a negative electrode active material layer 24 is provided on a negative electrode current collector 22. The positive electrode active material layer 14 and the negative electrode active material layer 24 are in contact with both sides of the separator 18. Leads 60 and 62 are connected to the end portions of the positive electrode current collector 12 and the negative electrode current collector 22, respectively, and the end portions of the leads 60 and 62 extend to the outside of the case 50.
<Positive electrode active material>
Then, the positive electrode active material which concerns on this embodiment is demonstrated.

The positive electrode active material according to this embodiment is Li _1-x VOPO ₄ [where x is −0.15 ≦ x ≦ 0.15. ] (Hereinafter sometimes referred to as “lithium vanadium phosphate”). The positive electrode active material has a pH of 3.0 or more and 6.8 or less. When the composition of the positive electrode active material and the variation in crystallinity are small, the vanadium ions from the positive electrode active material are less likely to elute, so that the pH of the positive electrode active material is likely to be 3.0 or more, and the positive electrode active material contributing to charge / discharge It is considered that the decrease in the amount of substances can be suppressed and the cycle characteristics are improved. In addition, when the pH of the positive electrode active material is 6.8 or less, the corrosion of the positive electrode current collector is suppressed, so that peeling due to a decrease in adhesion between the positive electrode active material layer and the positive electrode current collector is suppressed, and a good cycle is achieved. It is considered that the characteristics can be realized. Furthermore, the pH is more preferably 3.5 or more and 6.0 or less. The crystal system of the compound is not particularly limited, and any of triclinic, orthorhombic and tetragonal LiVOPO ₄ may be used.

  Moreover, it is preferable that the average particle diameter of the primary particle of a positive electrode active material is 0.02 micrometer or more and 3 micrometers or less. A lithium ion secondary battery using such an active material has a high capacity. When an active material having an average primary particle size of less than 0.02 μm is used, the powder tends to be difficult to handle, and when an active material of more than 5 μm is used, the capacity tends to be small. More preferably, the average particle size is 0.04 μm or more and 1 μm or less.

  Further, when the particle diameters of the cumulative particle size distribution of the positive electrode active material particles on the volume basis are 10% and 90% from the fine particle diameter side are D10 and D90, D90 / D10 obtained by dividing D90 by D10 is 23 or less. It preferably has a particle size distribution. D90 / D10, that is, the ratio of D90 to D10 indicates the ratio of the coarse portion to the fine portion of the cumulative particle size distribution. When the value of D90 / D10 is small, the variation in the particle diameter is small, There is a tendency for the variation in crystallinity to be reduced. Furthermore, it is more preferable that D90 / D10 has a particle size distribution of 20 or less.

<Method for producing positive electrode active material>
Although the manufacturing method of a positive electrode active material is not specifically limited, It is known that it can synthesize | combine by a solid-phase synthesis, a hydrothermal synthesis, the carbothermal reduction method etc. Among them, lithium vanadium phosphate produced by a hydrothermal synthesis method tends to have a small particle size and excellent rate characteristics, and lithium vanadium phosphate produced by a hydrothermal synthesis method is preferable as a positive electrode active material. The lithium vanadium phosphate synthesized by the hydrothermal synthesis method is considered to be due to factors such as particle size, crystal shape, and crystallinity that are suitable for improving the rate characteristics. Below, the manufacturing method of the positive electrode active material using the hydrothermal synthesis method which concerns on this embodiment is demonstrated.

<Method for producing positive electrode active material by hydrothermal synthesis method>
The hydrothermal synthesis method includes a raw material preparation step, a hydrothermal synthesis step, a drying step, and a firing step. However, you may implement a baking process, without performing a drying process. You may implement a grinding | pulverization process as needed after a baking process. Furthermore, if necessary, the elution component removal step of removing the solvent elution component may be carried out by stirring the product obtained by the firing step or the product obtained by the pulverization step in a solvent, filtering and drying. .

  In the raw material preparation step, a lithium source, a vanadium source, a phosphorus source, and water are stirred and mixed to prepare a mixture (mixed solution). In the raw material preparation step, it is preferable to mix a lithium source, a vanadium source, a phosphorus source and water at the same time. Moreover, you may add a reducing agent as needed.

The lithium source is, for example, selected from the group consisting of Li ₂ CO ₃ , LiF, LiNO ₃ , LiOH, LiCl, LiBr, LiI, Li ₂ SO ₄ , Li ₃ PO _4, CH ₃ COOLi, and hydrates thereof. One kind or two or more kinds can be used. In particular, when a water-soluble lithium salt is used, the discharge capacity of the lithium ion secondary battery tends to be improved. Examples of the water-soluble lithium salt include LiNO ₃ , LiOH, LiCl, LiI, Li ₂ SO ₄ and CH ₃ COOLi, and hydrates thereof.

As the phosphorus source, for example, at least one selected from the group consisting of H ₃ PO ₄ , NH ₄ H ₂ PO ₄ and (NH ₄ ) ₂ HPO ₄ can be used. Two or more phosphorus sources may be used in combination.

As the vanadium source, for example, any of metal vanadium, V ₂ O ₃ , V ₂ O _5, or NH ₄ VO ₃ can be used. Two or more vanadium sources may be used in combination.

The mixing ratio of the lithium source, the vanadium source and the phosphorus source is such that the ratio of the number of moles of lithium contained in the lithium source, the number of moles of vanadium contained in the vanadium source, and the number of moles of phosphorus contained in the phosphorus source is 1: 1: It may be adjusted to be 1. That is, the molar ratio of Li, V and P in the mixture may be adjusted so as to be the stoichiometric ratio of LiVOPO ₄ (1: 1: 1). Note that the blending ratio does not necessarily satisfy the above stoichiometric ratio. For example, in order to prevent the loss of Li in the finally obtained active material, a large amount of lithium source may be blended. That is, the molar ratio of Li, V and P in the mixture may be deliberately shifted from 1: 1: 1.

No particular limitation is imposed on the reducing agent, for example, hydrazine _{(NH 2 NH 2 · H 2} O) or hydrogen peroxide _(H 2 _{O 2)} or the like can be used.

  In the hydrothermal synthesis step, first, the above-described lithium source, phosphate source, vanadium source, water, and reducing agent are put into a reaction vessel (for example, an autoclave) having a function of heating and pressurizing the inside. A mixture (aqueous solution) in which is dispersed is prepared. Subsequently, the reaction vessel is sealed and heated while pressurizing the mixture, thereby causing a hydrothermal reaction to proceed in the mixture. In addition, what is necessary is just to adjust suitably the time which heats a mixture, pressurizing according to the quantity of a mixture.

  What is necessary is just to heat at about 80-300 degreeC in a drying process. As a drying method, oven drying, spray dryer, flash jet dryer or the like can be used.

  The heat treatment method of the firing step is arbitrary, and for example, a box furnace, a tubular furnace, a tunnel furnace, a rotary kiln, or the like can be used. The heat treatment is usually divided into three parts of temperature rise, maximum temperature hold, and temperature drop, and the steps of temperature rise, maximum temperature hold, and temperature drop may be repeated twice or more. Further, a crushing step that means eliminating aggregation to such an extent that the secondary particles are not destroyed may be sandwiched between the heat treatments.

  Although the atmosphere of a baking process is not specifically limited, It is preferable that it is an air atmosphere.

  In the pulverization step, for example, a planetary ball mill or a jet mill can be used as a pulverization method. By the pulverization, the primary particles or the secondary particles are micronized. In addition, you may implement a grinding | pulverization process at the time of producing the positive electrode active material layer of a lithium ion secondary battery. In the manufacturing process of the positive electrode active material layer 14, a slurry prepared from an active material, a conductive additive, a binder, a solvent, and the like is applied on the positive electrode current collector 12 and dried to form the positive electrode active material layer 14. In this step, a mixture of the active material and the conductive additive may be pulverized. Further, the slurry itself may be pulverized.

  In the eluted component removal step, the product obtained in the baking step or the product obtained in the pulverization step is stirred in a solvent, filtered to remove the components eluted in the solvent, and the powder remaining on the filter paper Is preferably dried. The solvent is selected depending on the components to be eluted, and examples thereof include distilled water, organic solvents such as acetone and ethanol.

  For the pH measurement of the positive electrode active material, an aqueous suspension of the positive electrode active material powder, that is, a sample in which 5 g of the positive electrode active material powder is suspended in 100 ml of water is used. After stirring well at 25 ° C. for about 1 to 2 hours, measurement is made after confirming that the pH value does not change significantly. The details of other measurements conform to JIS Z8802.

When pH of the obtained positive electrode active material is 3.0 or higher, elution of vanadium ions and the like from the positive electrode active material is suppressed, and when pH is 6.8 or lower, the positive electrode current collector 12 and the positive electrode active material are suppressed. It is considered that the deterioration of the adhesion with the layer 14 is easily suppressed, and the cycle characteristics are improved.
<Positive electrode>
Subsequently, the positive electrode 10 according to the present embodiment will be described.

  As the positive electrode current collector 12 of the positive electrode 10, for example, an aluminum foil or the like can be used. The positive electrode active material layer 14 contains at least the positive electrode active material according to the present embodiment and a conductive additive. The positive electrode active material layer 14 may include a binder that binds the positive electrode active material and the conductive additive.

  Examples of the conductive aid include carbon materials such as carbon blacks, metal powders such as copper, nickel, stainless steel, and iron, a mixture of carbon materials and metal powders, and conductive oxides such as ITO.

  The binder is not particularly limited as long as the positive electrode active material and the conductive additive can be bound to the positive electrode current collector 12, and a known binder can be used. Examples thereof include fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and vinylidene fluoride-hexafluoropropylene copolymer.

  The ratio of the positive electrode active material, the conductive additive, and the binder of the positive electrode active material layer 14 is not particularly limited, but the electrode density tends to decrease when the ratio of the positive electrode active material is small, and the ratio of the positive electrode active material is 80% by weight or more. Is preferred.

  Such a positive electrode 10 is prepared by a known method, for example, a positive electrode active material, a conductive additive and a binder, and a solvent corresponding to the type thereof, for example, N-methyl-2-pyrrolidone, N, N-dimethyl in the case of PVDF. The slurry can be produced by applying a slurry added to a solvent such as formamide on the surface of the positive electrode current collector 12 and drying it.

  To measure the pH of the positive electrode active material layer 14, an aqueous suspension of the positive electrode active material layer powder peeled off from the positive electrode current collector 12 using a scraper or the like, that is, 5 g of the positive electrode active material layer powder was suspended in 100 ml of water. The sample is used. After stirring well at 25 ° C. for about 1 to 2 hours, measurement is made after confirming that the pH value does not change significantly. The details of other measurements conform to JIS Z8802.

  When the obtained positive electrode active material layer 14 has a pH of 3.5 or higher, elution of vanadium ions and the like from the positive electrode active material is suppressed, and when the pH is 7.8 or lower, the positive electrode current collector 12 and the positive electrode It is considered that it is easy to suppress a decrease in adhesion with the active material layer 14 and the cycle characteristics are improved.

<Negative electrode>
As the negative electrode current collector 22, a copper foil or the like can be used. Moreover, as the negative electrode active material layer 24, the thing containing a negative electrode active material, a conductive support agent, and a binder can be used. It does not specifically limit as a conductive support agent, A carbon material, a metal powder, etc. can be used. As the binder used for the negative electrode, fluororesins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP) can be used.

As the negative electrode active material, carbon materials such as graphite and non-graphitizable carbon, metals that can be combined with lithium such as Al, Si and Sn, and amorphous materials mainly composed of oxides such as SiO ₂ and SnO ₂ Examples thereof include particles containing a compound, lithium titanate (Li ₄ Ti ₅ O ₁₂ ), and the like.

  The negative electrode 20 may be manufactured by adjusting the slurry and applying it to the negative electrode current collector 22 in the same manner as the positive electrode 10.

<Electrolyte>
The electrolytic solution is not particularly limited. For example, in the present embodiment, an electrolytic solution containing a lithium salt in an organic solvent can be used. Examples of the lithium _salt, LiPF _6, LiClO 4, salts of LiBF ₄ or the like can be used. In addition, these salts may be used individually by 1 type, and may use 2 or more types together.

  Preferable examples of the organic solvent include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, and methyl ethyl carbonate. These may be used alone or in combination of two or more at any ratio.

  The separator 18 is at least one component selected from the group consisting of a monolayer of a film made of polyethylene, polypropylene or polyolefin, a stretched film of a laminate or a mixture of the above resins, or a group consisting of cellulose, polyester and polypropylene. The fiber nonwoven fabric which consists of can be used.

  The case 50 seals the laminated body 30 and the electrolytic solution therein. The case 50 is not particularly limited as long as it can prevent leakage of the electrolytic solution to the outside and entry of moisture and the like into the lithium ion secondary battery 100 from the outside. For example, a metal laminate film can be used. .

  The leads 60 and 62 are made of a conductive material such as aluminum.

  This active material can also be used as an electrode material for electrochemical elements other than lithium ion secondary batteries. As such an electrochemical element, a secondary battery other than a lithium ion secondary battery, such as a metal lithium secondary battery (an electrode including the composite particles of the present invention as a positive electrode and metal lithium as a negative electrode). And electrochemical capacitors such as lithium capacitors.

  EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.

Example 1
[Production of evaluation cell]
V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so as to have a molar ratio of about 1: 2: 2, poured into distilled water, and stirred with a magnetic stirrer for 1 hour. Hydrazine monohydrate (NH ₂ NH ₂ .H ₂ O) was added dropwise little by little with vigorous stirring, and the mixture was further stirred for 1 hour, and then the mixed solution was transferred to a glass container for autoclave. The container was sealed, heated with stirring at 160 ° C. for 8 hours, and the obtained paste was dried in an oven at 100 ° C. for 12 hours. The obtained dry powder was crushed for 10 minutes using a mortar and then baked in a box furnace at 600 ° C. for 4 hours in the air.

The active material thus obtained was subjected to composition analysis by an inductively coupled plasma method (hereinafter, ICP method), and as a result, it was confirmed that the active material composition was LiVOPO ₄ .

  About the pH of the obtained positive electrode active material, it measured by the above-mentioned method. The result was pH = 3.0.

LiVOPO ₄ particles and acetylene black of pH = 6.5 were weighed at a weight ratio of 80:10, and subjected to a pulverization treatment for 10 minutes using a planetary ball mill. The rotation speed of the planetary ball mill was set to 530 rpm.

  A mixture of the obtained mixture and polyvinylidene fluoride (PVDF, Kureha Chemical KF7305) as a binder was dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. The weight ratio of the mixture and PVDF in the slurry was adjusted to 90:10. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to produce a positive electrode on which a positive electrode active material layer was formed.

  The positive electrode active material layer was peeled off from the obtained positive electrode, and the pH of the positive electrode active material layer was measured. The measurement method was performed as described above, and the result was pH = 3.8.

  Next, artificial graphite (FSN manufactured by BTR) and N methylpyrrolidone (NMP) 5 wt% solution of polyvinylidene fluoride (PVdF) as a negative electrode were mixed so that the ratio of artificial graphite: polyvinylidene fluoride = 93: 7 was obtained. A slurry paint was prepared. The negative electrode was produced by apply | coating a coating material to the copper foil which is a collector, and drying and rolling.

A positive electrode and a negative electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate (element body). This laminate was placed in an aluminum laminate pack. As the electrolytic solution, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratio of 3: 7, and LiPF ₆ was dissolved as a supporting salt to a concentration of 1 mol / L.

  The above electrolyte was poured into an aluminum laminate pack containing the laminate, and then vacuum-sealed to produce an evaluation cell of Example 1.

[Measurement of battery characteristics]
The evaluation cell of Example 1 was charged at a constant current of up to 4.3 V at 25 ° C. with a current value of 18 mA / g and then discharged at a constant current of 2.8 V at a current value of 18 mA / g. At this time, the discharge capacity of Example 1 was 134 mAh / g (initial discharge capacity). A cycle test was repeated for 100 cycles of this charge / discharge cycle. Assuming that the initial discharge capacity of the evaluation cell of Example 1 is 100%, the discharge capacity after 100 cycles was 91.8%. Hereinafter, the ratio of the discharge capacity after 100 cycles when the initial discharge capacity is 100% is referred to as cycle characteristics. A high cycle characteristic indicates that the battery is excellent in charge / discharge cycle durability.

(Example 2)
The obtained positive electrode active material was stirred with a magnetic stirrer of 10 ml per 1 g of distilled water for 1 hour, then filtered, and the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. The active material composition was measured, the pH was measured, the evaluation cell was prepared, and the battery characteristics were evaluated in the same manner as in Example 1 except that the material was used. The results are shown in Table 1.

(Example 3)
The obtained positive electrode active material was stirred with a magnetic stirrer of 10 ml per 1 g of distilled water for 1 hour, then filtered, and the dried powder obtained by performing the elution component removing step of drying the powder remaining on the filter paper three times. The active material composition was measured, the pH was measured, the evaluation cell was prepared, and the battery characteristics were evaluated in the same manner as in Example 1, except that the positive electrode active material was used. The results are shown in Table 1.

Example 4
In the raw material adjustment step, V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 1.8: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. The mixture was stirred for 1 hour with an underwater magnetic stirrer, then filtered and the same as in Example 1 except that the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. By the method, the active material composition was measured, the pH was measured, the evaluation cell was produced, and the battery characteristics were evaluated. The results are shown in Table 1.

(Example 5)
In the raw material adjustment step, V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 2.2: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. After stirring for 1 hour with an underwater magnetic stirrer and filtering, the elution component removal step of drying the powder remaining on the filter paper was performed three times, except that the dry powder was used as the positive electrode active material. In the same manner, active material composition measurement, pH measurement, evaluation cell preparation and battery characteristic evaluation were performed. The results are shown in Table 1.

(Example 6)
In the raw material adjustment step, V ₂ O ₅ , LiOH · H ₂ O and H ₃ PO ₄ were weighed so as to have a molar ratio of about 1: 1.7: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. The mixture was stirred for 15 minutes with an underwater magnetic stirrer, then filtered and the same as in Example 1 except that the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. By the method, the active material composition was measured, the pH was measured, the evaluation cell was produced, and the battery characteristics were evaluated. The results are shown in Table 1.

(Example 7)
In the raw material adjustment step, V ₂ O ₅ , LiOH · H ₂ O and H ₃ PO ₄ were weighed so as to have a molar ratio of about 1: 2.3: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. The mixture was stirred for 1 hour with an underwater magnetic stirrer, then filtered and the same as in Example 1 except that the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. By the method, the active material composition was measured, the pH was measured, the evaluation cell was produced, and the battery characteristics were evaluated. The results are shown in Table 1.

(Example 8)
In the raw material adjustment step, V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 1.8: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. The mixture was stirred for 15 minutes with an underwater magnetic stirrer, then filtered and the same as in Example 1 except that the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. By the method, the active material composition was measured, the pH was measured, the evaluation cell was produced, and the battery characteristics were evaluated. The results are shown in Table 1.

Example 9
In the raw material adjustment step, V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 2.2: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. After stirring for 1 hour with an underwater magnetic stirrer, filtration was performed and the elution component removal step of drying the powder remaining on the filter paper was performed twice. In the same manner, active material composition measurement, pH measurement, evaluation cell preparation and battery characteristic evaluation were performed. The results are shown in Table 1.

(Example 10)
In the raw material adjustment step, V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 1.8: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. After stirring for 1 hour with an underwater magnetic stirrer and filtering, the elution component removal step of drying the powder remaining on the filter paper was performed three times, except that the dry powder was used as the positive electrode active material. In the same manner, active material composition measurement, pH measurement, evaluation cell preparation and battery characteristic evaluation were performed. The results are shown in Table 1.

(Example 11)
In the raw material adjustment step, V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 2.2: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. The mixture was stirred for 15 minutes with an underwater magnetic stirrer, then filtered and the same as in Example 1 except that the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. By the method, the active material composition was measured, the pH was measured, the evaluation cell was produced, and the battery characteristics were evaluated. The results are shown in Table 1.

(Example 12)
The active material composition was measured, the pH was measured, the evaluation cell was prepared, and the battery characteristics were evaluated in the same manner as in Example 1 except that acetylene black having a pH of 3.5 was used. The results are shown in Table 1.

(Example 13)
The obtained positive electrode active material was stirred with a magnetic stirrer of 10 ml per 1 g of distilled water for 1 hour, then filtered, and the dried powder obtained by performing the elution component removing step of drying the powder remaining on the filter paper three times. The active material composition was measured, the pH was measured, the evaluation cell was prepared, and the battery characteristics were evaluated in the same manner as in Example 1 except that acetylene black having a pH of 8.5 was used as the positive electrode active material. It was. The results are shown in Table 1.

(Comparative Example 1)
The active material composition was measured, the pH was measured and evaluated in the same manner as in Example 1 except that the liquid mixture was transferred to a glass container for autoclave, the container was sealed, and heated at 160 ° C. for 1 hour with stirring. Cells were prepared and battery characteristics were evaluated. The results are shown in Table 1.

(Comparative Example 2)
The obtained positive electrode active material was crushed with a planetary ball mill for 30 minutes, stirred for 1 hour with a magnetic stirrer in 10 ml of distilled water per gram, filtered, and the powder remaining on the filter paper was dried. The active material composition was measured, the pH was measured, the evaluation cell was prepared, and the battery characteristics were evaluated in the same manner as in Example 1 except that the dry powder obtained by performing the component removal step three times was used as the positive electrode active material It was. The results are shown in Table 1.

(Comparative Example 3)
The obtained positive electrode active material was crushed with a planetary ball mill for 30 minutes, stirred for 1 hour with a magnetic stirrer in 10 ml of distilled water per gram, filtered, and the powder remaining on the filter paper was dried. The component removal step was performed three times, and the same method as in Example 1 was used, except that acetylene black having a pH of 8.5 was used. Went. The results are shown in Table 1.

(Comparative Example 4)
In the raw material adjustment step, V ₂ O ₅ , LiOH · H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 1.64: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. The mixture was stirred for 1 hour with an underwater magnetic stirrer, then filtered and the same as in Example 1 except that the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. By the method, the active material composition was measured, the pH was measured, the evaluation cell was produced, and the battery characteristics were evaluated. The results are shown in Table 1.

(Comparative Example 5)
In the raw material adjustment step, V ₂ O ₅ , LiOH · H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 2.36: 2, and the obtained positive electrode active material was distilled at 10 ml per gram. The mixture was stirred for 1 hour with an underwater magnetic stirrer, then filtered and the same as in Example 1 except that the dried powder that had been subjected to the elution component removal step of drying the powder remaining on the filter paper was used as the positive electrode active material. By the method, the active material composition was measured, the pH was measured, the evaluation cell was produced, and the battery characteristics were evaluated. The results are shown in Table 1.

(Comparative Example 6)
The active material was prepared in the same manner as in Example 1, except that V ₂ O ₅ , LiOH.H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 1.64: 2. Measurement of composition, measurement of pH, preparation of evaluation cells and evaluation of battery characteristics were performed. The results are shown in Table 1.

(Comparative Example 7)
The active material was prepared in the same manner as in Example 1 except that V ₂ O ₅ , LiOH · H ₂ O and H ₃ PO ₄ were weighed so that the molar ratio was about 1: 2.36: 2 in the raw material adjustment step. Measurement of composition, measurement of pH, preparation of evaluation cells and evaluation of battery characteristics were performed. The results are shown in Table 1.

  As shown in Table 1, the positive electrode active materials of Examples 1 to 13 have a pH of 3.0 to 6.8, and the positive electrode active material layers of Examples 1 to 11 have a pH of 3.5 to 7. .8 or less was confirmed. Furthermore, it was confirmed that the pH of Examples 12 and 13 positive electrode active material layers was 3.4 and 7.9, respectively. Moreover, it was confirmed that the cycle characteristics of the evaluation cells of Examples 1 to 13 tend to be better than the cycle characteristics of all the comparative examples.

  The cross section of the positive electrode obtained in Example 1 was processed, 300 positive electrode active material particles were imaged with a scanning electron microscope, the area of each particle of the obtained image was calculated, and then the equivalent circle diameter D10 and D90, which are the particle sizes of 10% and 90% of the cumulative particle size distribution, were calculated, and D90 / D10 was calculated by dividing D90 by D10. The results are shown in Table 2.

(Example 14)
An active material was produced in the same manner as in Example 1 except that the paste obtained by the hydrothermal synthesis step was dried in an oven at 100 ° C. for 12 hours and pulverized for 20 minutes using a mortar. D90 / D10 was calculated | required by the above-mentioned method, and also by the method similar to Example 1, the active material composition measurement, pH measurement, preparation of the cell for evaluation, and battery characteristic evaluation were performed. The results are shown in Table 2.

(Example 15)
An active material was produced in the same manner as in Example 1, except that the dry powder obtained by drying the paste obtained by the hydrothermal synthesis step in an oven at 100 ° C. for 12 hours was crushed using a mortar for 30 minutes. D90 / D10 was calculated | required by the above-mentioned method, and also by the method similar to Example 1, the active material composition measurement, pH measurement, preparation of the cell for evaluation, and battery characteristic evaluation were performed. The results are shown in Table 2.

  As shown in Table 2, the positive electrode active materials of Examples 1, 14, and 15 have a pH of 3.0 or more and 6.8 or less, and the positive electrode active material layer has a pH of 3.5 or more and 7.8 or less. It was confirmed that D90 / D10 was 23 or less. It was also confirmed that the cycle characteristics of the evaluation cells of Examples 1, 14 and 15 tend to be better than those of all comparative examples.

DESCRIPTION OF SYMBOLS 10 ... Positive electrode, 20 ... Negative electrode, 12 ... Positive electrode collector, 14 ... Positive electrode active material layer, 18 ... Separator, 22 ... Negative electrode collector, 24 ... Negative electrode active material layer, 30 ... Laminate, 50 ... Case, 60 62 ... Lead, 100 ... Lithium ion secondary battery.

Claims (3)

Composition formula Li _1-x VOPO ₄ [where x is −0.15 ≦ x ≦ 0.15. A positive electrode active material, wherein the pH of the compound is 3.0 or more and 6.8 or less.   A positive electrode current collector and a positive electrode active material layer comprising the positive electrode active material according to claim 1 and having a pH of 3.5 or more and 7.8 or less provided on the positive electrode current collector .   A lithium ion secondary battery comprising the positive electrode according to claim 2.

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