JP2012004046A - Manufacturing method of active material, active material and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of active material capable of improving a discharge capacity of a lithium ion secondary battery.SOLUTION: A manufacturing method of active material according to the present invention includes a hydrothermal synthesis step of heating a mixed liquid containing a lithium source, a phosphate source, a vanadium source having pentavalent vanadium, water, and reductant under pressure. A concentration of the reductant in the mixed liquid is 0.005-0.4 mol/L.

Description

  The present invention relates to an active material manufacturing method, an active material, and a lithium ion secondary battery.

Conventionally, a layered compound such as LiCoO ₂ or LiNi _1/3 Mn _1/3 Co _1/3 O ₂ or a spinel compound such as LiMn ₂ O ₄ has been used as a positive electrode material (positive electrode active material) of a lithium ion secondary battery. It was. In recent years, compounds having an olivine type structure typified by LiFePO ₄ have attracted attention. It is known that a positive electrode material having an olivine structure has 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, as a phosphate-based positive electrode material capable of realizing a high charge-discharge voltage, such as LiCoPO4 and LiNiPO ₄ it has been proposed. However, the present situation is that a sufficient capacity is not obtained even in lithium ion secondary batteries using these positive electrode materials. LiVOPO ₄ is known as a compound that can realize a charge / discharge voltage of 4 V class among phosphoric acid positive electrode materials. However, even in a lithium ion secondary battery using LiVOPO ₄ , sufficient reversible capacity and rate characteristics are not obtained. Said positive electrode material is described in the following patent documents 1, 2, and the following nonpatent literatures 1-4, for example. Hereinafter, in some cases, a lithium ion secondary battery is referred to as a “battery”.

JP 2003-68304 A JP 2004-303527 A

J. et al. Solid State Chem. , 95, 352 (1991) N. Dupre et al. , Solid State Ionics, 140, pp. 209-221 (2001) N. Dupre et al. , J .; Power Sources, 97-98, pp. 532-534 (2001) J. et al. Baker et al. J. et al. Electrochem. Soc. , 151, A796 (2004)

  The present invention has been made in view of the above-described problems of the prior art, and provides an active material manufacturing method, an active material, and a lithium ion secondary battery capable of improving the discharge capacity of a lithium ion secondary battery. The purpose is to do.

  In order to achieve the above object, the method for producing an active material according to the present invention heats a mixed solution containing a lithium source, a phosphate source, a vanadium source having pentavalent vanadium, water, and a reducing agent under pressure. A hydrothermal synthesis step is provided, and the concentration of the reducing agent in the mixed solution is 0.005 to 0.4 mol / L.

According to the present invention, nanoscale LiVOPO ₄ β-type crystals (orthorhombic crystals) can be obtained in high yield. Then, in the lithium ion secondary battery comprising a LiVOPO ₄ obtained by the present invention as a cathode active material, a large discharge capacity can be achieved.

In the said invention, it is preferable that a reducing agent is tartaric acid. Thereby, the yield of the β type crystal of LiVOPO ₄ is increased, and the discharge capacity of the battery is easily improved.

In the said invention, it is preferable that the Raman spectrum of a liquid mixture has a peak whose Raman shift is 970-990 cm < ^-1 >. In the mixed solution having such a peak, pentavalent vanadium is reduced by a reducing agent and tetravalent vanadium is generated, and thus LiVOPO ₄ containing tetravalent vanadium as a constituent element is easily generated.

Active material in accordance with the present invention comprises a particle group of LiVOPO _4, is β-type crystal phase content of LiVOPO ₄ is 71 mol% or more based on the total amount of the LiVOPO _4. A lithium ion secondary battery according to the present invention includes a positive electrode having a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector, and the positive electrode active material layer is an active material according to the present invention. Containing.

  The active material according to the present invention can be obtained, for example, by the method for producing an active material according to the present invention. In the lithium ion secondary battery according to the present invention, a large discharge capacity is achieved.

In the active material according to the present invention, in the volume-based particle size distribution of the particle group measured by the laser scattering method, the primary particle size D50 having a volume cumulative rate from the side where the primary particle size is small is 50% is 500 nm or less. It is preferable that In such a battery using LiVOPO ₄ having a small particle size, the discharge capacity is easily improved.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the active material which can improve the discharge capacity of a lithium ion secondary battery, an active material, and a lithium ion secondary battery can be provided.

FIG. 1 is a schematic cross-sectional view of a lithium ion secondary battery including a positive electrode active material layer containing an active material according to an embodiment of the present invention.

  Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant description is omitted. Further, the positional relationship such as up, down, left and right is based on the positional relationship shown in the drawings unless otherwise specified. Further, the dimensional ratios and positional relationships in the drawings are not limited to those illustrated.

(Method for producing active material)
<Hydrothermal synthesis process>
The manufacturing method of the active material which concerns on one Embodiment of this invention is equipped with a hydrothermal synthesis process. In the hydrothermal synthesis process, first, a lithium source, a phosphate source, a vanadium source, water and a reducing agent are introduced into a reaction vessel (for example, an autoclave) having a function of heating and pressurizing the inside, and these are dispersed. A mixed solution (aqueous solution) is prepared. The lithium source, phosphate source and vanadium source are preferably dissolved almost completely and uniformly in the mixed solution. That is, it is preferable that the liquid mixture does not suspend and has translucency or transparency. This makes it easy to synthesize LiVOPO ₄ having a high β-type crystal phase ratio and a high capacity density. In preparing the mixed solution, for example, first, a mixture of a phosphate source, a vanadium source, water and a reducing agent may be refluxed, and then a lithium source may be added thereto. By this reflux, a complex of a phosphate source and a vanadium source can be formed.

The vanadium source contains vanadium having a valence of 5. Pentavalent vanadium is reduced by a reducing agent in the mixed solution to vanadium having a valence of 4. Vanadium may exist as ions of V ⁵⁺ , V ^4+, etc. in the mixed solution, or may exist as ions of vanadate (vanadium oxide) composed of tetravalent or pentavalent vanadium.

If the valence of vanadium contained in the vanadium source is 4 or less, vanadium is reduced by a reducing agent in the mixed solution to vanadium having a valence of 3 or less. Since LiVOPO ₄ contains tetravalent vanadium as a constituent element, LiVOPO ₄ is hardly synthesized in a mixed solution containing vanadium having a valence of 3 or less. Further, a vanadium source having a vanadium valence of 4 or less is not preferable because it is more expensive than a vanadium source composed of pentavalent vanadium and increases the production cost of LiVOPO ₄ .

The density | concentration of the reducing agent in a liquid mixture is 0.005-0.4 mol with respect to 1 liter of liquid mixtures. As a result, the vanadium source is sufficiently dissolved in the mixed solution, and the pentavalent vanadium is reliably reduced to tetravalent vanadium to synthesize β-type LiVOPO ₄ . If the concentration of the reducing agent is outside the above numerical range, with the proportion of α-type crystal phase in LiVOPO ₄ is greater than the ratio of β-type crystal phase, the greater the primary particle diameter D50 of LiVOPO _4, the battery The discharge capacity is reduced.

As the reducing agent, for example, tartaric acid, ascorbic acid, citric acid and the like may be used, but tartaric acid is excellent in that the ratio of the β-type crystal phase in LiVOPO ₄ is increased to increase the discharge capacity of the battery.

The Raman spectrum of the mixed solution preferably has a peak with a Raman shift of 970 to 990 cm ⁻¹ . This peak means the presence of V ⁴⁺ or the presence of vanadate ions with tetravalent vanadium. That is, this peak means that the vanadium source is almost completely dissolved in the mixed solution by the action of the reducing agent, and pentavalent vanadium in the vanadium source is reduced to produce tetravalent vanadium. . In this way, in hydrothermal synthesis using a mixed solution having a peak in a region where the Raman shift is 970 to 990 cm ⁻¹ in the Raman spectrum, LiVOPO ₄ having a large discharge capacity is easily obtained. In other words, in hydrothermal synthesis using a mixed solution in which the vanadium source is almost completely dissolved, it is easy to obtain LiVOPO ₄ having a large discharge capacity. The Raman spectrum is a spectrum indicating the intensity of Raman scattered light corresponding to the difference (Raman shift) between the frequency of Raman scattered light and the frequency of incident light, and can be measured by a well-known Raman spectroscopy. it can.

As the lithium source, for example, at least one selected from the group consisting of LiNO ₃ , Li ₂ CO ₃ , LiOH, LiCl, Li ₂ SO _4, Li ₃ PO _4, and CH ₃ COOLi may be used.

As the phosphoric acid source, for example, at least one selected from the group consisting of H ₃ PO ₄ , NH ₄ H ₂ PO ₄ , (NH ₄ ) ₂ HPO ₄ and Li ₃ PO ₄ may be used.

As the vanadium source, for example, at least one selected from the group consisting of V ₂ O ₅ and NH ₄ VO ₃ may be used.

  Two or more lithium sources, two or more phosphoric acid sources, or two or more vanadium sources may be used in combination.

  In the hydrothermal synthesis step, the ratio [P] / [V] of the number of moles of phosphorus element [P] contained in the liquid mixture before heating to the number of moles [V] of vanadium element contained in the liquid mixture is set to 0.9. What is necessary is just to adjust to -1.2. [P] / [V] may be adjusted by the blending ratio of the phosphate source and the vanadium source. However, even if [P] / [V] is outside the above range, the effect of the present invention is achieved.

  In the hydrothermal synthesis step, the ratio [Li] / [V] of the number of moles of lithium element [Li] and [V] contained in the mixed solution before heating may be adjusted to 0.9 to 1.2. [Li] / [V] may be adjusted according to the blending ratio of the lithium source and the vanadium source. However, even if [Li] / [V] is outside the above range, the effect of the present invention is achieved.

In the hydrothermal synthesis step, the hydrothermal reaction is advanced in the mixed liquid by heating the mixed liquid in the sealed reactor while applying pressure. Thus, LiVOPO ₄ is hydrothermally synthesized as an active material.

In the hydrothermal synthesis step, the mixed solution may be heated to 150 to 300 ° C. under pressure. This makes it easy to obtain LiVOPO ₄ having a small particle size on the nm scale and having a high Li diffusibility. Incidentally, when the heating temperature of the mixture is too low, the generation and crystal growth of LiVOPO ₄ does not proceed sufficiently. If the heating temperature of the mixed solution is too high, LiVOPO ₄ tends to grow too much and its particle size tends to increase. Moreover, when the heating temperature of a liquid mixture is too high, high heat resistance is calculated | required by the reaction container, and there exists a tendency for the manufacturing cost of an active material to increase. However, the effect of the present invention is achieved even when the heating temperature of the mixed liquid is outside the above range.

The pressure applied to the mixed solution in the hydrothermal synthesis step may be 0.2 to 1 MPa. If the pressure applied to the mixed solution is too low, the crystallinity of LiVOPO ₄ finally obtained tends to decrease, and the capacity density tends to decrease. If the pressure applied to the mixed solution is too high, the reaction vessel is required to have high pressure resistance, and the production cost of the active material tends to increase. However, the effects of the present invention can be achieved even when the pressure applied to the mixture is outside the above range.

<Heat treatment process>
The manufacturing method of the active material which concerns on this embodiment may be equipped with the heat processing process which further heats the liquid mixture after a hydrothermal synthesis process. By the heat treatment step, the reaction of the lithium source, the phosphate source, and the vanadium source that did not react in the hydrothermal synthesis step can be advanced, or the crystal growth of LiVOPO ₄ generated in the hydrothermal synthesis step can be promoted. As a result, the capacity density of LiVOPO ₄ is improved, and the discharge capacity and rate characteristics of a battery using the LiVOPO ₄ tend to be improved.

In the present embodiment, when the mixed liquid is heated in a high temperature region of 200 to 300 ° C. in the hydrothermal synthesis step, it becomes easy to form a sufficiently large LiVOPO ₄ crystal by the hydrothermal synthesis step alone. Moreover, in this embodiment, even if it is a case where a liquid mixture is heated in a low-temperature area | region below 200 degreeC in a hydrothermal synthesis process, it is possible to form a desired active material by a hydrothermal synthesis process alone. However, when the mixed liquid is heated in the low temperature region in the hydrothermal synthesis process, the heat treatment process following the hydrothermal synthesis process promotes the synthesis and crystal growth of LiVOPO ₄ and further improves the effects of the present invention. Tend to.

  What is necessary is just to heat the liquid mixture after a hydrothermal synthesis process at the heat processing temperature of 400-700 degreeC when implementing a heat processing process. The heat treatment time of the mixed solution may be 3 to 20 hours. The heat treatment atmosphere of the mixed solution may be a nitrogen atmosphere, an argon atmosphere, or an air atmosphere.

In addition, you may preheat the liquid mixture obtained at a hydrothermal synthesis process at about 60-150 degreeC for about 1 to 30 hours, before heating at a heat processing process. Preheating removes excess moisture and organic solvent from the mixed solution, and the mixed solution becomes powder. What is necessary is just to implement the heat processing process with respect to this powder. As a result, it is possible to prevent impurities from being taken into LiVOPO ₄ in the heat treatment step and to make the particle shape uniform.

(Active material and lithium ion secondary battery)
The active material according to the present embodiment can be obtained by the above-described method for producing an active material according to the present embodiment. Active material according to the present embodiment includes a particle group of LiVOPO _4, is β-type crystal phase content of LiVOPO ₄ is 71 mol% or more based on the total amount of the LiVOPO _4. The upper limit of the content rate of the β-type crystal phase is not particularly limited, and may be 100 mol% or 95 mol%. When the β-type crystal phase content is less than 71 mol%, the discharge capacity of the battery is small. Since the β-type crystal of LiVOPO ₄ has a linear and short ion conduction path as compared with the α-type crystal (triclinic crystal), it has excellent characteristics for reversibly inserting and desorbing lithium ions. Therefore, a battery using an active material containing a high percentage of LiVOPO ₄ β-type crystals achieves a higher charge / discharge capacity than a battery using α-type crystals.

In the volume-based particle size distribution of the LiVOPO ₄ particle group measured by the laser scattering method, the primary particle size D50 having a volume cumulative rate from the smaller primary particle size of 50% is preferably 500 nm or less, It is more preferably 260 nm or less, and particularly preferably 127 nm or less. The lower limit value of D50 is not particularly limited, but is about 47 nm. Thus, the particle size of LiVOPO ₄ obtained by the method for producing an active material according to this embodiment described above is smaller than that of conventional LiVOPO ₄ . Therefore, in LiVOPO _{4 of the} present embodiment, the density of the ion conduction path is increased and the diffusion distance of lithium ions in the particles is shortened and the lithium ion diffusing capacity is increased as compared with the conventional active material. . In the present embodiment, as LiVOPO ₄ becomes smaller, the specific surface area becomes larger than the conventional one. Accordingly, the reversibility of Li in LiVOPO ₄ is improved, and the contact area between the current collector and LiVOPO ₄ and the contact area between the conductive agent in the active material layer and LiVOPO ₄ are increased, and the density of the electron conduction path is increased. Will increase. For the above reasons, the LiVOPO _{4 of the} present embodiment improves the conductivity and capacity density of ions and electrons as compared with the conventional active material. Therefore, in the lithium ion secondary battery using LiVOPO _{4 of the} present embodiment, the discharge capacity is improved.

  As shown in FIG. 1, a lithium ion secondary battery 100 according to the present embodiment is disposed adjacent to each other between a plate-like negative electrode 20 and a plate-like positive electrode 10 facing each other, and the negative electrode 20 and the positive electrode 10. A plate-like separator 18, an electrolyte solution containing lithium ions, a case 50 containing these in a sealed state, and one end of the negative electrode 20 being electrically connected. A negative electrode lead 62 whose other end protrudes outside the case, and a positive electrode lead 60 whose one end is electrically connected to the positive electrode 10 and whose other end protrudes outside the case are provided. .

  The negative electrode 20 includes a negative electrode current collector 22 and a negative electrode active material layer 24 formed on the negative electrode current collector 22. The positive electrode 10 includes a positive electrode current collector 12 and a positive electrode active material layer 14 formed on the positive electrode current collector 12. The separator 18 is located between the negative electrode active material layer 24 and the positive electrode active material layer 14.

  The positive electrode active material layer 14 contains the active material according to the present embodiment.

  As mentioned above, although one suitable embodiment of the present invention was described in detail, the present invention is not limited to the above-mentioned embodiment.

For example, in the hydrothermal synthesis step, carbon particles may be added to the mixed solution before heating. Thereby, at least a part of LiVOPO ₄ is generated on the surface of the carbon particles, and the LiVOPO ₄ can be supported on the carbon particles. As a result, it is possible to improve the electrical conductivity of the obtained active material. Examples of the material constituting the carbon particles include carbon black such as acetylene black, graphite, activated carbon, hard carbon, and soft carbon.

The active material of this embodiment can also be used as an electrode material for electrochemical devices other than lithium ion secondary batteries. As such an electrochemical element, other than lithium ion secondary batteries such as a metal lithium secondary battery (the electrode containing LiVOPO ₄ obtained according to the present invention is used as a positive electrode and metal lithium is used as a negative electrode). Examples include secondary batteries and electrochemical capacitors such as lithium capacitors. These electrochemical elements can be used for power sources such as self-propelled micromachines and IC cards, and distributed power sources arranged on or in a printed circuit board.

  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
<Composition of mixture>
In the production of LiVOPO ₄ of Example 1, a mixed solution containing the following raw materials was prepared.

Lithium source: 1.70 g (0.04 mol) of LiOH.H ₂ O (molecular weight = 41.96, manufactured by Nacalai Tesque, special grade, purity 99% by weight).

Phosphoric acid source: 4.69 g (0.04 mol) of H ₃ PO ₄ (molecular weight = 98.00, manufactured by Nacalai Tesque, first grade, purity: 85% by weight).

Vanadium source: 3.67 g (0.02 mol) of V ₂ O ₅ (molecular weight = 181.88, manufactured by Nacalai Tesque, special grade, purity: 99% by weight).

  200 g of distilled water (manufactured by Nacalai Tesque, for HPLC (high performance liquid chromatography)). Separately, 20 g of distilled water was also used between the glass inner cylinder and the autoclave.

Reducing agent: 1.51 g (0.01 mol) of L-(+)-tartaric acid (molecular formula = C ₄ H ₆ O ₆ , molecular weight = 150.09, manufactured by Nacalai Tesque, special grade, purity: 99.5% by weight) .

  Concentration of reducing agent (tartaric acid) in the mixed solution: 0.05 mol / L.

  Ratio of reducing agent (tartaric acid) to 1 mol of vanadium in the mixed solution: 25 mol vs. V.

As apparent from the contents of the phosphoric acid source and the vanadium source, the ratio [P] of the number of moles of phosphorus element [P] contained in the mixed solution and the number of moles [V] of vanadium element contained in the mixed solution [P] ] / [V] was adjusted to 1. Further, as is clear from the contents of the above lithium source and vanadium source, the ratio [Li] / [V] of the number of moles [Li] and [V] of the lithium element contained in the mixed solution is adjusted to 1. did. Further, as apparent from the content of the lithium source and the amount of distilled water, the concentration of Li ^{+ in} the mixed solution was adjusted to 0.2 mol / L. Each amount of the raw materials is equivalent to a yield of about 6.756 g (0.04 mol) of LiVOPO ₄ in terms of stoichiometry when converted to LiVOPO ₄ (molecular weight: 168.85).

<Adjustment of liquid mixture>
The above mixture was prepared by the following procedure. First, a 35 mm football-type rotator is placed in a 0.5 L glass inner cylinder of an autoclave, and V ₂ O ₅ , distilled water and H ₃ PO ₄ are introduced in this order, and these are put in a magnetic stirrer for 2.5 hours. Stir. L-(+)-tartaric acid was added to the mixture obtained by this stirring. Immediately after adding L-(+)-tartaric acid, the hue of the mixture did not change immediately. The mixture stirred for 1 hour after the addition of L-(+)-tartaric acid became a bright yellow paste with a pH of 1. LiOH · H ₂ O was added to this bright yellow paste to obtain a grass green mixed solution of Example 1 having the above composition. The pH of the bright yellow paste immediately after the addition of LiOH.H ₂ O was 3. As a result of visual observation, the raw materials (lithium source, phosphate source, vanadium source and reducing agent) were completely dissolved in the mixed solution of Example 1, and no solid was confirmed. That is, it was confirmed that the mixed solution of Example 1 was transparent without being suspended.

<Hydrothermal synthesis process>
The glass inner cylinder containing the mixed liquid of Example 1 was sealed, and heating of the mixed liquid was started by predetermined PID control while stirring the mixed liquid in the glass inner cylinder with a strong magnetic stirrer. The internal pressure in the glass inner cylinder sealed with water vapor generated by heating was increased. In this way, in the hydrothermal synthesis step, the mixed solution in the glass inner cylinder was maintained at 250 ° C. under pressure for about 10 hours.

When 10 hours have passed since the temperature of the mixed liquid in the glass inner cylinder reached 250 ° C., the heating was stopped, the mixed liquid was air-cooled for about 4.5 hours, and after the temperature dropped to room temperature, the mixed liquid was placed in the glass. Removed from the cylinder. The mixed liquid taken out from the glass inner cylinder was a suspension containing a maroon precipitate. No foaming was observed in the suspension. The pH of the suspension was 6. The glass inner cylinder was left still and the supernatant in the container was filtered. The filtrate was slightly cloudy. About 200 ml of distilled water was put into the glass inner cylinder, and the inside of the glass inner cylinder was stirred and washed. The pH of the distilled water after washing was 7. Immediately thereafter, the distilled water after washing was suction filtered. The brown or maroon precipitate obtained by the above filtration was washed with about 200 ml of acetone, and then suction filtered again using a filter having an opening of 52 μm. The filtrate (acetone) was slightly cloudy. The brown precipitate after filtration was dried with warm air to obtain 6.04 g of a brown solid. Weight of brown solid which was converted to LiVOPO _4, it corresponds to a 89.4% yield of LiVOPO _4, which has been assumed when charging the raw material 6.756g was confirmed.

<Heat treatment process>
Of the dried brown solid, 1.00 g was placed in an alumina crucible. A heat treatment step of heating the solid in the alumina crucible using a heating furnace was performed. In the heat treatment step, the solid in the alumina crucible was heated in an air atmosphere. In the heat treatment step, the temperature in the furnace was raised from room temperature to 450 ° C. over 45 minutes, the solid in the alumina crucible was heated at 450 ° C. for 4 hours, and then the heating furnace was naturally cooled. By this heat treatment step, 0.98 g of bright green powder was obtained as the active material of Example 1. The residual ratio of the solid in the heat treatment step was 98% by mass.

(Examples 2-9, Comparative Examples 1-3)
In Examples 2-9 and Comparative Examples 1-3, the compounds shown in Table 1 were used as reducing agents. In Examples 2 to 9 and Comparative Examples 1 to 3, the concentration of the reducing agent in the mixed solution (hereinafter referred to as “concentration X”) was adjusted to the values shown in Table 1. In Examples 2 to 9 and Comparative Examples 1 to 3, the ratio of the reducing agent to 1 mol of vanadium in the mixed solution (hereinafter referred to as “ratio Y”) was adjusted to the values shown in Table 1.

  Except for the above, the active materials of Examples 2 to 9 and Comparative Examples 1 to 3 were obtained in the same manner as in Example 1.

[Solubility of raw materials in liquid mixture]
In the same manner as in Example 1, the solubility of the raw materials (lithium source, phosphate source, vanadium source and reducing agent) in the mixed solution of each Example and Comparative Example was examined. The results are shown in Table 1.

[Raman spectrum measurement]
The Raman spectrum of the mixed solution of Example 1 before heating in the hydrothermal synthesis step was measured by Raman spectroscopy. For measurement of the Raman spectrum, a 532 type Raman spectroscopic system manufactured by Kaiser was used. In the measurement of the Raman spectrum, the mixed solution was irradiated with a laser having a wavelength of 532 nm. In the Raman spectrum of Example 1, a peak having a Raman shift of 982 cm ⁻¹ was confirmed. This is a peak derived from V ⁴⁺ possessed by vanadate ions in the mixed solution. In the Raman spectrum of Example 1, a peak having a Raman shift of 1035 to 1130 cm ⁻¹ was confirmed. This is a ^{V 4+} Raman shift corresponding to the peak is 982 cm ^-1 is a peak derived from the different ^{V 4+} vibration state. From the measurement result of the Raman spectrum, it was confirmed that in the mixed solution of Example 1, the vanadium source was completely dissolved and pentavalent vanadium was reduced and changed to tetravalent vanadium.

In the same manner as in Example 1, the Raman spectra of the mixed liquids of each Example and Comparative Example before heating in the hydrothermal synthesis step were measured, and the peak having a Raman shift of 982 cm ⁻¹ (hereinafter, “Raman peak”). .) Was confirmed. The results are shown in Table 1.

[Measurement of crystal structure]
As a result of analysis by powder X-ray diffractometry (XRD), it was confirmed that the active materials of all Examples and Comparative Examples were LiVOPO ₄ . The content (unit: mol%) of the β-type crystal phase in LiVOPO _{4 of} each Example and Comparative Example was determined by Rietveld analysis based on powder X-ray diffraction (XRD). The results are shown in Table 1.

[Measurement of particle size distribution]
The particle size distribution of LiVOPO ₄ of each example and comparative example was measured by a laser scattering method (dynamic light scattering method). An apparatus made by Malvern was used for measurement of the particle size distribution. Then, to determine the primary particle diameter D50 on the volume basis of LiVOPO ₄ of Examples and Comparative Examples. The results are shown in Table 1.

<Production of evaluation cell>
A mixture of the active material of Example 1, polyvinylidene fluoride (PVDF) as a binder, and acetylene black was dispersed in N-methyl-2-pyrrolidone (NMP) as a solvent to prepare a slurry. The slurry was prepared so that the weight ratio of the active material, acetylene black, and PVDF was 84: 8: 8 in the slurry. This slurry was applied onto an aluminum foil as a current collector, dried, and then rolled to obtain an electrode (positive electrode) on which an active material layer containing the active material of Example 1 was formed.

Next, the obtained electrode and the Li foil as the counter electrode were laminated with a separator made of a polyethylene microporous film interposed therebetween to obtain a laminate (element body). This laminate was put in an aluminum laminator pack, and 1M LiPF ₆ solution was injected as an electrolyte into the aluminum laminate pack, followed by vacuum sealing to produce an evaluation cell of Example 1.

  In the same manner as in Example 1, evaluation cells were produced using each of the active materials of Examples 2 to 9 and Comparative Examples 1 to 3 alone.

[Measurement of discharge capacity]
Using the evaluation cell of Example 1, the discharge capacity (unit: mAh / unit) when the discharge rate is 0.01 C (current value at which discharge is completed in 100 hours when constant current discharge is performed at 25 ° C.) g) was measured. The measurement results are shown in Table 1.

  In the same manner as in Example 1, the discharge capacities of the evaluation cells of Examples 2 to 9 and Comparative Examples 1 to 3 were measured. The results are shown in Table 1.

As apparent from Table 1, in Examples 1 to 9, the concentration of the reducing agent in the mixed solution was 0.005 to 0.4 mol / L. It was confirmed that the content of the β-type crystal phase in LiVOPO ₄ of Examples 1 to 9 was 71 mol% or more. It was confirmed that the discharge capacities of the evaluation cells of Examples 1 to 9 were larger than those of Comparative Examples 1 to 3 in which the concentration of the reducing agent in the mixed solution was outside the range of 0.005 to 0.4 mol / L. It was.

Each of the mixed solutions of Comparative Examples 1 and 3 was a paste having a non-uniform composition because the raw material was suspended without completely dissolving. In each of the mixed liquids of Comparative Examples 1 and 3, a peak having a Raman shift of 982 cm ⁻¹ was not confirmed. It was confirmed that LiVOPO ₄ of Comparative Examples 2 and 3 contains an α-type crystal phase as a main component.

  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 ... power generation element, 50 ... case, 60, 62 ... lead, 100 ... lithium ion secondary battery.

Claims (6)

Comprising a hydrothermal synthesis step of heating a mixed solution containing a lithium source, a phosphate source, a vanadium source having pentavalent vanadium, water and a reducing agent under pressure,
The concentration of the reducing agent in the mixed solution is 0.005 to 0.4 mol / L.
A method for producing an active material. The reducing agent is tartaric acid,
The manufacturing method of the active material of Claim 1. The Raman spectrum of the mixture has a peak with a Raman shift of 970 to 990 cm ⁻¹ .
The manufacturing method of the active material of Claim 1 or 2. Comprising a particle group of LiVOPO ₄
Is at least 71 mol% of the total amount content of the LiVOPO ₄ of β-type crystal phase of LiVOPO _4,
Active material. In the volume-based particle size distribution of the particles measured by the laser scattering method,
The primary particle size D50 having a volume cumulative rate of 50% from the smaller primary particle size is 500 nm or less.
The active material according to claim 4. A positive electrode having a positive electrode current collector and a positive electrode active material layer provided on the positive electrode current collector;
The positive electrode active material layer contains the active material according to claim 4 or 5.
Lithium ion secondary battery.

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