JP7609562B2 - Electrode material and method for producing the same - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act Publication date: November 14, 2020 Meeting name: 60th Battery Debate

The present invention relates to an electrode material used in power storage devices such as secondary batteries and electrochemical capacitors, and a method for manufacturing the electrode material.

As electrodes for electric storage devices such as secondary batteries and electrochemical capacitors, a positive electrode in which a positive electrode material containing lithium ions and a conductive assistant are fixed to the surface of a metal foil, and a negative electrode in which a negative electrode material capable of inserting and removing lithium ions and a conductive assistant are fixed to the surface of a metal foil are used. In recent years, lithium vanadium phosphate (Li ₃ V ₂ (PO ₄ ) ₃ ) having a Na Super Ionic Conductor structure has been attracting attention as an electrode material.

Lithium vanadium phosphate operates at 3.8V to 4.8V against the Li/Li ⁺ standard. It can also exhibit a large capacity of up to 197 mAh/g depending on each potential plateau. Lithium vanadium phosphate has a crystal structure that allows lithium to diffuse three-dimensionally, making it possible to charge and discharge at high speed. Lithium vanadium phosphate with PO bonds has high thermal stability and is excellent in safety. For these reasons, the use of lithium vanadium phosphate as an electrode material is being promoted.

JP 2008-52970 A

As a result of the research conducted by the inventors of the present invention into lithium vanadium phosphate, it was pointed out that the crystal structure of lithium vanadium phosphate may change due to moisture in the air. It was also thought that this change in lithium vanadium phosphate due to moisture may cause a deterioration in the performance characteristics of electrodes manufactured using lithium vanadium phosphate.

The present invention has been proposed to solve the above problems. Its purpose is to provide an electrode material and a method for manufacturing the electrode material that can suppress changes in crystal structure caused by moisture.

When lithium vanadium phosphate is left in a high humidity environment, for example at a humidity of 80% or more, the XRD spectrum of the lithium vanadium phosphate changes before and after being left there, and it was presumed that the moisture in the air causes a change in the crystal structure of the lithium vanadium phosphate. Therefore, the inventors conducted extensive research to improve the durability of lithium vanadium phosphate against moisture, and discovered that by providing a specific water-resistant layer on lithium vanadium phosphate, changes in the crystal structure of lithium vanadium phosphate can be suppressed.

In order to achieve the above object, the electrode material according to the present invention comprises lithium vanadium phosphate bonded to a conductive carbon material, the lithium vanadium phosphate having a water-resistant layer, the water-resistant layer having a crystal structure of a phosphate-based compound, the water-resistant layer having the crystal structure of the phosphate-based compound being an aluminum phosphate compound, or a titanium phosphate compound, or being represented by XαMβ ₍ PO4 ) γ _, where X=Li, Na, K, or Ca, and M=Al, Mn, Co, Fe, Mg _, Ni _, Ti, Zn, Cr, Ni, Cu, Zr, or Nb, and the water-resistant layer is a water-resistant layer formed before electrode formation .

The aluminum phosphate compound is AlPO ₄ It may be.

The titanium phosphate compound is LiTi ₂ (P.O. ₄ ) ₃ It may be.

The water-resistant layer may be formed to have a concentration of 5 to 25 wt % relative to lithium vanadium phosphate.

The method includes a crystallization step of bonding a conductive carbon material with lithium vanadium phosphate, and a coating step of forming a water-resistant layer on the electrode material obtained by the crystallization step, the coating step including a mixing step of mixing a material source for the water-resistant layer in a solution with the electrode material obtained by the crystallization step, and a firing step of firing the solution obtained by the mixing step, and the water-resistant layer has a crystal structure of a phosphate-based compound.

The water-resistant layer having a crystal structure of the phosphate-based compound is an aluminum phosphate compound, a titanium phosphate compound, or X _α M _β (P.O. ₄ ) _Gamma where X is any one of Li, Na, K, and Ca, and M may be any one of Al, Mn, Co, Fe, Mg, Ni, Ti, Zn, Cr, Ni, Cu, Zr, and Nb.

The present invention provides an electrode material and a method for manufacturing the electrode material that can suppress changes in crystal structure caused by moisture.

1 is a flowchart showing a manufacturing procedure of an electrode material. 1 is a flowchart showing a manufacturing procedure for forming a water-resistant layer on an electrode material. 1 is a graph showing a diffraction pattern by XRD (X-ray powder diffraction) of a reference example of an electrode material. HRTEM images of electrode materials of examples and comparative examples, where (a) and (b) are overall images, and (c) and (d) are partially enlarged images. 3A and 3B are HRTEM images of the electrode material of the example, where (a) and (b) are whole images, and (c) and (d) are partially enlarged images. 1 is a graph showing a diffraction pattern by XRD of an electrode material of a comparative example. 1 is a graph showing a diffraction pattern by XRD of an electrode material of an example. 1 is a graph showing a diffraction pattern by XRD of an electrode material of an example.

[composition]
The electrode material of this embodiment includes lithium vanadium phosphate and a conductive carbon material. Lithium vanadium phosphate has a Nasicon structure represented by the general formula LixV ₂ (PO ₄ ) _3. When lithium vanadium phosphate is used as an electrode material for an electricity storage device, 0≦x≦3 and the valence of vanadium ions can be 3 to 5 due to the desorption and insertion of lithium ions accompanying charge and discharge.

The lithium vanadium phosphate is bonded to the conductive carbon material and is coated with a water-resistant layer. "Bonded" means that the lithium vanadium phosphate is in physical contact with the surface of the conductive carbon material, and that the lithium vanadium phosphate and the conductive carbon material are also electrically connected, resulting in a highly conductive state. For example, this includes a state in which the lithium vanadium phosphate is bonded to the surface of the conductive carbon material at the atomic level, sharing a structure.

In addition, it is preferable that the lithium vanadium phosphate is coated with a water-resistant layer having a crystal structure of a phosphate-based compound. The crystal structure of a phosphate-based compound is a system in which a _PO4 tetrahedron is introduced into the crystal, and the lithium vanadium phosphate also has the crystal structure of this phosphate-based compound. Therefore, the water-resistant layer having the crystal structure of a phosphate-based compound has a high affinity with the lithium vanadium phosphate.

Furthermore, the water-resistant layer having a crystal structure of a phosphate-based compound covers the surface of the lithium vanadium phosphate, and more preferably, the contact surface with the lithium vanadium phosphate forms a solid solution with the lithium vanadium phosphate. A solid solution is a state in which the water-resistant layer is dissolved in the crystal structure of the lithium vanadium phosphate, and part of the vanadium element of the lithium vanadium phosphate is replaced by the metal element of the water-resistant layer while maintaining the original crystal structure. In other words, it is presumed that the boundary between the water-resistant layer and the lithium vanadium phosphate is in a state in which two substances having the crystal structure of a phosphate-based compound are mixed together (solid solution).

The water-resistant layer having a crystal structure of a phosphate-based compound may be an aluminum phosphate compound or a titanium phosphate compound. Specifically, _it may contain _AlPO4 or _LiTi2 ( _PO4 ) ₃ . Alternatively, the water-resistant layer may be represented by _XαMβ ( _PO4 ) _γ , where X=Li, Na, K, or Ca, and M=Al, Mn, Co, Fe, Mg, Ni, Ti, Zn, Cr, Ni, Cu, Zr, or Nb. The water-resistant layer is preferably coated with 5 to 25 wt% of the lithium vanadium phosphate contained in the electrode material. If the amount exceeds 25 wt%, not only will the electrode capacity decrease, but the resistance will also increase.

The electrode material as described above can be formed by, first, forming a precursor of lithium vanadium phosphate in a liquid phase reaction such as a complex formation reaction, a hydrolysis reaction, an oxidation reaction, a polymerization reaction, or a condensation reaction, and then heating and baking the precursor. For example, the electrode material can be obtained by mixing a phosphate source, a conductive carbon material, a lithium source, and a vanadium source using the Pechini method, and then baking the mixture.

Secondly, the resulting electrode material is mixed with a source material for a water-resistant layer having a crystal structure of a phosphate-based compound, and then fired. In other words, a water-resistant layer having a crystal structure of a phosphate-based compound is formed on the electrode material obtained by combining lithium vanadium phosphate with a conductive carbon material.

When mixing material sources using the Pechini method, an organic compound with multiple carboxyl groups and an alcohol with multiple hydroxyl groups are added. The organic compound with multiple carboxyl groups forms a metal complex when added to an aqueous solution containing vanadium ions. Adding an alcohol with multiple hydroxyl groups to this metal complex polymerizes through an ester reaction.

As conductive carbon materials, carbon nanotubes and conductive carbon black having a hollow shell structure (e.g., Ketjen Black (registered trademark)) are suitable, but carbon blacks such as carbon nanofibers, acetylene black, amorphous carbon, carbon fibers, natural graphite, artificial graphite, activated carbon, mesoporous carbon, nanoporous carbon, graphene, fullerene, or mixtures of multiple of these can also be used.

The vanadium source and phosphate source include metal ammonium salts, acetates, sulfates, nitrates, halogen compounds, and chelating agents, whether the lithium vanadium phosphate is produced by hydrolysis or by complex formation. Specifically, the vanadium source can be NH ₄ VO _3. However, it can also be V ₂ O ₅ , V ₂ O ₃ , metallic vanadium, V ₂ O ₄ , vanadium (III) acetylacetonate, and vanadium (IV) oxyacetylacetonate.

The _phosphate source can be _{H3PO4 ,} although _it can also be any _PO4 -containing _compound such as _NH4H2PO4 , ( _NH4 ) _{2HPO4 , P2O5 ,} and _Li3PO4 .

The lithium source may be _{CH3COOLi ,} but may also be _LiNO3 , _Li2CO3 , _LiOH , _LiOH.H2O , LiCl, _{Li2SO4 , and LIC3H5O3} .

An example of an organic compound having multiple carboxyl groups is the tricarboxylic acid citric acid, but it may also be a dicarboxylic acid such as oxalic acid, malonic acid, or succinic acid. An example of an alcohol having multiple hydroxyl groups is ethylene glycol, but it may also be other dihydric alcohols such as propylene glycol, or trihydric alcohols such as glycerin, or polyalcohols such as polyvinyl alcohol.

The material source of the water-resistant layer includes a metal source and a phosphate source, which is mixed with the electrode material and then re-fired to form a water-resistant layer that covers the _{electrode material} . The phosphate source can be a _{PO4 -} containing compound such as _{H3PO4 , NH4H2PO4} , ( _NH4 ) _{2HPO4 , P2O5 ,} and Li3PO4. When the water-resistant layer contains _AlPO4 , the metal source can be Al( _NO3 ) _{3 ,} but it can also be triisopropoxyaluminum, metallic aluminum, alumina, or other inorganic and organic salts of aluminum.

In addition, when the water-resistant layer contains LiTi ₂ (PO ₄ ) ₃ , the metal source may be Ti(O ⁿ Bu) ₄ , but may also be titanium oxide, titanium alkoxide, etc. When the water-resistant layer contains LiTi ₂ (PO ₄ ) ₃ , in addition to the phosphoric acid source and the metal source, the material source may include a lithium source and an organic compound having multiple carboxyl groups. Examples of the lithium source include CH ₃ COOLi, LiNO ₃ , Li ₂ CO ₃ , LiOH, LiOH.H ₂ O, LiCl, Li ₂ SO _4, and LIC ₃ H ₅ O _3. Examples of the organic compound having multiple carboxyl groups include tricarboxylic acid such as citric acid, but may also be dicarboxylic acids such as oxalic acid, malonic acid, and succinic acid, or polycarboxylic acids such as polyacrylic acid.

[Production method]
The method for producing an electrode material according to the present embodiment includes the following steps.
(1) A crystallization step for bonding a conductive carbon material with lithium vanadium phosphate; (2) A coating step for forming a water-resistant layer on the electrode material obtained by the crystallization step.

(1) Crystallization step The crystallization step is a step of bonding lithium vanadium phosphate to the surface of a conductive carbon material. An example of the crystallization step by the Pechini method will be described below with reference to Fig. 1. However, it is sufficient that the conductive carbon material and lithium vanadium phosphate are bonded in the crystallization step, and the specific method is not limited to the following.

In other words, the order of mixing the material sources and the number and timing of firing can be changed as appropriate. In addition, the liquid phase reaction used to form the lithium vanadium phosphate precursor is not limited to a complex formation reaction, and other reactions such as a hydrolysis reaction, an oxidation reaction, a polymerization reaction, and a condensation reaction can also be used.

In the crystallization process, first, a phosphoric acid source and a conductive carbon material are added to water to perform a first mechanochemical treatment. Note that a vanadium source may be mixed in place of the phosphoric acid source. The mechanochemical treatment is a treatment in which mechanical energy such as shear stress or centrifugal force is applied using a rotating reaction vessel or the like. The mechanochemical treatment is a process that uses ultra-centrifugal force treatment (Ultra-Centrifugal
In short, it is sufficient to apply shear stress, centrifugal force, or other mechanical energy by using mechanical energy such as a UC process, etc., so that the phosphate source or vanadium source can be attached to the conductive carbon material by obtaining a dispersion of the phosphate source or vanadium source and the conductive carbon material, and base points of lithium vanadium phosphate can be generated on the surface of the conductive carbon material.

In mechanochemical processing, a solution is subjected to shear stress and centrifugal force in a rotating reactor. The reactor is preferably made of an outer cylinder and an inner cylinder, with a through hole on the side of the inner cylinder that can rotate, and a sheathing plate on the opening of the outer cylinder. It is believed that this mechanical energy is converted into chemical energy required for the reaction, that is, activation energy. This allows the reaction to proceed in a short time. To provide a satisfactory amount of mechanical energy, it is desirable to generate a centrifugal force of 1500 N (kgms ^-2 ) or more. Preferably, it is 60,000 N (kgms ^-2 ) or more.

Secondly, a second mechanochemical treatment is performed to generate a precursor of lithium vanadium phosphate based on the lithium vanadium phosphate generated on the surface of the conductive carbon material. For example, a vanadium source, an organic compound having multiple carboxyl groups, and a lithium source are added to a dispersion of a phosphoric acid source and a conductive carbon material. Furthermore, an alcohol having multiple hydroxyl groups is added, and a second mechanochemical treatment is performed.

The above-mentioned mechanochemical treatment can also serve to micronize and highly disperse the lithium source, vanadium source, phosphate source, and conductive carbon material. By subjecting the lithium source, vanadium source, and phosphate source to mechanochemical treatment, it is possible to promote the generation of lithium vanadium phosphate precursors through complex formation reactions, hydrolysis reactions, polymerization reactions, condensation reactions, etc., promote the bonding between the lithium vanadium phosphate precursor and the conductive carbon material, and form the lithium vanadium phosphate precursor into nanoparticles.

In the above example, the mechanochemical treatment is performed twice, but from the viewpoint of achieving fine particle size and high dispersion, the mechanochemical treatment may be performed three or more times by adding the mechanochemical treatment after mixing each material source. Also, the lithium source, vanadium source, phosphate source, conductive carbon material, and other material sources may be mixed at once, and the mixed solution may be subjected to the mechanochemical treatment once.

Thirdly, the solution is concentrated and dried using an evaporator, vacuum dried at 80-130°C, and then pre-baked in an air atmosphere at 250-300°C for 3-5 hours. This heat treatment in pre-baking removes carbon other than the conductive carbon material. It also removes organic compounds with multiple carboxyl groups, alcohols with multiple hydroxyl groups, and polymers formed by esterification of organic compounds with multiple carboxyl groups and alcohols with multiple hydroxyl groups.

Furthermore, the material is sintered in a nitrogen atmosphere at 700 to 900°C for 5 to 120 minutes. During this sintering, the vanadium is oxidized without agglomerating, and the crystallization of lithium vanadium phosphate progresses. This results in a composite powder electrode material in which nanoparticles of lithium vanadium phosphate are bonded to the surface of the conductive carbon material. This completes the crystallization process.

(2) Coating step In the coating step, a water-resistant layer is formed on the electrode material obtained by the crystallization step. The coating step is performed on the electrode material in which the lithium vanadium phosphate is bonded to the surface of the conductive carbon material through crystallization. An example of the coating step for forming a water-resistant layer will be described below with reference to FIG. 2.

First, the source material for the water-resistant layer and the electrode material obtained in the crystallization process are added to a liquid in which the source material for the water-resistant layer is soluble, and mechanochemical treatment is performed. This mechanochemical treatment causes the source material for the water-resistant layer and the electrode material to be finely divided and dispersed in the solution.

Next, the solution is concentrated and dried using an evaporator, vacuum dried at 80-130°C, and then fired in a nitrogen atmosphere at 500-900°C for 5-120 minutes. This firing causes the source material for the water-resistant layer to coat the electrode material, forming a water-resistant layer. In addition, in the coating process, the electrode material obtained in the crystallization process is added to the solution, and moisture causes a change in the structure of the electrode material. However, by carrying out this firing, the electrode material with the changed structure is re-crystallized. This forms a water-resistant layer on the electrode material obtained in the crystallization process. This completes the coating process.

[Action and Effect]
As described above, the electrode material of the present embodiment includes lithium vanadium phosphate bonded to a conductive carbon material, and the lithium vanadium phosphate has a water-resistant layer. By having the lithium vanadium phosphate have a water-resistant layer in this way, it is possible to suppress structural changes caused by moisture. In addition, when the water-resistant layer has a crystal structure of a phosphate-based compound, it has a high affinity with lithium vanadium phosphate, and it is possible to further suppress changes in the crystal structure caused by moisture.

When the water-resistant layer contains an aluminum phosphate compound or a titanium phosphate compound, the effects of moisture are suppressed, and the elution of vanadium into moisture is more effectively prevented. If the water-resistant layer is formed so that it contains 5 to 25 wt % of lithium vanadium phosphate, there is no risk of a decrease in electrode capacity or an increase in resistance.

The present invention will be described in more detail with reference to the following examples. Note that the present invention is not limited to the following examples.

Example 1
In this Example 1, an electrode material made of a composite of lithium vanadium phosphate and carbon nanotubes (hereinafter sometimes referred to as CNTs) and having a water-resistant layer containing _AlPO4 was produced by the manufacturing procedure shown below.

The material sources of the lithium vanadium phosphate are ammonium metavanadate (NH ₄ VO ₃ ), lithium acetate (CH ₃ COOLi), and phosphoric acid (H ₃ PO ₄ ). The average fiber diameter of the CNTs is 15 to 30 nm. The weight ratio of each material source of the lithium vanadium phosphate to the CNTs is 70:30.

First, a mixed solution was prepared by adding 1.58 g of H ₃ PO ₄ and 0.8 g of CNT to distilled water. This mixed solution was subjected to UC treatment using the above reactor in an environment of 80° C. As the UC treatment, the inner cylinder was rotated at a rotation speed of 50 m/s, and a centrifugal force of 66,000 N (kgms-2) was applied to the mixed solution for 5 minutes.

To the obtained mixture, 1.08g _{of NH4VO3} , 1.77g of citric acid, 0.93g of _CH3COOLi , and 20g of distilled water were added. Furthermore, 2.29g of ethylene glycol and 22g of distilled water were added. As a result, a metal complex was formed between the vanadium ions and the citric acid, and the citric acid further formed a polymer by an ester reaction with the ethylene glycol. The polymer formed by this polymerization reaction penetrates between the metal complexes, dispersing the metal complexes and maintaining the dispersed state. The above mixture was subjected to UC treatment under the same conditions as above.

The mixture was then concentrated and dried using an evaporator, vacuum dried at 80°C, and pre-baked at 300°C for 5 hours. It was then baked at 800°C for 60 minutes in a nitrogen atmosphere. Through the above crystallization process, an electrode material was obtained in which lithium vanadium phosphate was bonded to the surface of the CNTs.

Next, the metal source (Al( _NO3 ) ₃ ) of the water-resistant layer and the phosphate source (( _NH4 ) _{2HPO4 )} of the water-resistant layer dissolved in distilled water were added to the obtained electrode material. 1.19 g of 20 wt% Al( _NO3 ) ₃ aqueous solution and 0.08 _g of ( _NH4 ) _2HPO4 were added to 2.2 g of the electrode material. The above mixture was subjected to UC treatment under the same conditions as above.

The mixture was concentrated and dried using an evaporator, vacuum dried at 80° C., and then baked in a nitrogen atmosphere at 800° C. for 30 minutes. Through the above coating process, lithium vanadium phosphate was bonded to the surface of the CNTs, and an electrode material was obtained in which a water-resistant layer containing 5 wt % _AlPO4 was formed.

Example 2
In this Example 2, an electrode material was produced by the following manufacturing procedure, which was made of a composite of lithium vanadium phosphate and CNT and had a water-resistant layer containing LiTi ₂ (PO ₄ ) _3. The crystallization process was the same as in Example 1.

In the coating process, the metal source (Ti(O ⁿ Bu) ₄ ) of the water-resistant layer dissolved in ethanol, the phosphate source (NH ₄ H ₂ PO ₄ ), the lithium source (CH ₃ COOLi), and citric acid of the water-resistant layer dissolved in distilled water were added to the obtained electrode material. 0.04 g of Ti(O ⁿ Bu) ₄ and 10 g of ethanol were added to 0.45 g of the electrode material. Furthermore, 6 g of distilled water, 0.02 g of NH ₄ H ₂ PO ₄ , 0.004 g of CH ₃ COOLi, and 0.01 g of citric acid were added. The above mixture was subjected to UC treatment under the same conditions as above.

The mixture was concentrated and dried using an evaporator, vacuum dried at 80° C., and then re-fired in a nitrogen atmosphere at 800° C. for 30 minutes. Through the above coating process, lithium vanadium phosphate was bonded to the surface of the CNTs, and an electrode material was obtained in which a water-resistant layer containing 5 wt % LiTi ₂ (PO ₄ ) ₃ was formed.

Example 3
The manufacturing method of the electrode material of Example 3 was the same as that of Example 2. However, 0.21 g of Ti(O ⁿ Bu) ₄ , 0.10 g of NH ₄ H ₂ PO ₄ , 0.02 g of CH ₃ COOLi, and 0.06 g of citric acid were added to 0.45 g of the electrode material to form a water-resistant layer containing 25 wt % LiTi ₂ (PO ₄ ) ₃ relative to the electrode material.

(Comparative Example 1)
The manufacturing method of the electrode material of Comparative Example 1 was the same as the crystallization step of the above-mentioned Example 1. However, the electrode material of Comparative Example 1 was not subjected to the coating step, and no water-resistant layer was formed.

(Comparative Example 2)
The electrode material of Comparative Example 1 was left in a low humidity environment at room temperature of 20 to 25° C. and humidity of about 0% for 6 days to prepare an electrode material of Comparative Example 2.

(Comparative Example 3)
The electrode material of Comparative Example 1 was left in a high humidity environment at room temperature of 20 to 25° C. and humidity of about 80% for 6 days to obtain an electrode material of Comparative Example 3.

<Water resistance of lithium vanadium phosphate>
The water resistance of lithium vanadium phosphate not forming a water-resistant layer was examined using the above Comparative Examples 1 to 3. That is, the crystal structure of Comparative Example 1, which is an electrode material immediately after production, Comparative Example 2, which is an electrode material after being left at low humidity, and Comparative Example 3, which is an electrode material after being left at high temperature, was analyzed by XRD (X-ray powder diffraction method). The results are shown in FIG.

As is clear from Figure 3, in the diffraction pattern of Comparative Example 1 immediately after production, peaks characteristic of lithium vanadium phosphate were confirmed at around 2θ = 21 degrees, 24 degrees, and 29 degrees. Furthermore, in Comparative Example 2 after being left at low humidity, peaks similar to those of Comparative Example 1 were confirmed. From these results, it was confirmed that the crystal structure of the electrode material combining lithium vanadium phosphate and a conductive carbon material does not change even when left in a low humidity environment with a humidity of around 0%.

On the other hand, in Comparative Example 3 after being left at high humidity, the characteristic peaks of lithium vanadium phosphate disappeared. In addition, as shown by the * and diamond marks in the figure, peaks that were not confirmed in Comparative Example 1 were detected. Of these, the * peak was presumed to be a peak derived from Li ₃ PO ₄ , but the diamond peak was unknown. From this result, it was confirmed that the crystal structure of Comparative Example 3 had changed and decomposed due to being left at high humidity. In other words, it was revealed that lithium vanadium phosphate is easily affected by moisture in the air and has low stability.

Observation of crystal structure
For Examples 1 to 3 and Comparative Example 1, HRTEM images were taken by focusing on the crystals of the electrode material using a high-resolution transmission electron microscope. FIG. 4 shows HRTEM images of Comparative Example 1 and Example 1. FIGS. 4(a) and (b) are HRTEM images taken at a magnification of 20,000 times. At this magnification, the overall crystal structure of Comparative Example 1 and Example 1 was confirmed, and it was found that both were electrode materials in which lithium vanadium phosphate was bound to CNTs.

Figures 4(c) and (d) are HRTEM images taken at a magnification of 300,000 times. The HRTEM image of Comparative Example 1 in Figure 4(c) is an image of lithium vanadium phosphate particles, but no layer formation can be confirmed on the particle surface. On the other hand, in the HRTEM image of Example 1 in Figure 4(d), it can be confirmed that a layer has been formed on the surface of the lithium vanadium phosphate particles, as partially indicated by the arrow in the figure.

HRTEM images of Examples 2 and 3 are shown in Figure 5. Figures 5(a) and (b) are HRTEM images at a magnification of 150,000 times, and Figures 5(c) and (d) are HRTEM images at a magnification of 300,000 times. Figures 5(a) and (c) confirm that a granular layer is formed on the surface of the lithium vanadium phosphate particles of Example 2. Figures 5(b) and (d) also confirm that a uniform layer is formed on the surface of the lithium vanadium phosphate particles of Example 3.

<Method for Evaluating Water Resistance of Lithium Vanadium Phosphate>
A test method for evaluating the water resistance of lithium vanadium phosphate was examined, and the test was carried out as follows. The test method was as follows: 0.1 g of the electrode material immediately after production was added to 1 cc of distilled water and stirred for 10 minutes by ultrasonic treatment. Then, the mixture was left at 60 degrees for 47 hours using a heating device. The mixture after the heat treatment was filtered, and the obtained powder was subjected to XRD analysis, and the filtrate was subjected to ICP emission spectroscopy to measure the amount of vanadium eluted. XRD analysis was also performed on the electrode material before the test, and the results were compared with those after the test.

FIG. 6 shows the results of XRD analysis performed before and after the test on the electrode material of Comparative Example 1. A characteristic peak of lithium vanadium phosphate was confirmed in the electrode material before the test. However, in the electrode material after the test, the characteristic peak disappeared, and only the peak derived from CNT was confirmed. From this result, it was confirmed that the electrode material of Li ₃ V ₂ (PO ₄ ) ₃ /CNT such as Comparative Example 1 is vulnerable to moisture, and when this test was performed, almost all of the lithium vanadium phosphate was dissolved in the filtrate.

Next, Fig. 7 shows the results of XRD analysis performed before and after the test on the electrode material of Example 1. For the electrode material of Example 1, there was no significant change in the peak value of the diffraction pattern before and after the test. From this result, it was confirmed _that the electrode material of _Li3V2 ( _PO4 ) ₃ /CNT coated with a water-resistant layer containing _AlPO4 as in Example 1 is not easily affected by moisture, that the crystal structure does not change significantly even when this test is performed, and that the dissolution of lithium vanadium phosphate in the filtrate is suppressed.

Next, the results of ICP emission spectroscopic analysis performed on the electrode materials of Comparative Example 1 and Example 1 before and after the test are shown in Table 1.

As seen from Table 1, in Comparative Example 1 having no water-resistant layer, 86.6% of the vanadium added was dissolved into the filtrate. On the other hand, in Example 1 having a water-resistant layer containing _AlPO4 , 31.3% of the vanadium added was dissolved into the filtrate. From the above results, it was revealed that the electrode material provided with a water-resistant layer containing _AlPO4 has the effect of moisture suppressed by the water-resistant layer, and the dissolution of vanadium into moisture is suppressed.

<Consideration of the amount of addition>
The above-mentioned tests to evaluate the water resistance were also conducted for Examples 2 and 3. FIG. 8(a) shows the results of XRD analysis performed before and after the test for the electrode material of Example 2. FIG. 8(b) shows the results of XRD analysis performed before and after the test for the electrode material of Example 3. For the electrode materials of Examples 2 and 3, the peak values of the diffraction patterns of the electrode materials before and after the test did not change significantly. From these results, it was confirmed that the electrode material of Li ₃ V ₂ (PO ₄ ) ₃ /CNT coated with a water-resistant layer containing LiTi ₂ (PO ₄ ) ₃ as in Examples 2 and 3 is not easily affected by moisture, and the crystal structure does not change significantly even when this test is performed, and the dissolution of lithium vanadium phosphate into the filtrate is suppressed.

Next, the results of ICP emission spectroscopic analysis performed on the electrode materials of Examples 2 and 3 before and after the test are shown in Table 2.

From Table 2, in Example 2 having a water-resistant layer containing 5 wt % _LiTi2 ( _PO4 ) ₃ relative to lithium vanadium phosphate, 37.7% of the vanadium added was dissolved in the filtrate, which was equivalent to the result in Example 1 having a water-resistant layer containing 5 wt % _AlPO4 relative to lithium vanadium phosphate.

In Example 3 having a water-resistant layer containing 25 wt % _LiTi2 ( _PO4 ) ₃ relative to lithium vanadium phosphate, 23.3% of the vanadium added was dissolved in the filtrate. From the above, it was found that by increasing the ratio of the water-resistant layer to the lithium vanadium phosphate, a more uniform coating of the water-resistant layer is possible, the effect of moisture is further suppressed, and the dissolution of vanadium into moisture can be suppressed.

<Cell capacity maintenance rate>
The electrode materials of Example 1, Example 2, Example 3, and Comparative Example 1 were made into electrodes using PVDF, and full cells were made using L ₄ T ₅ O ₁₂ cells as the counter electrode and 1M LiPF ₆ /EC+DEC 1:1 (in volume) as the electrolyte, and the capacity retention rate was evaluated. As a result, the capacity retention rate of Comparative Example 1 without a water-resistant layer was 77%, the capacity retention rate of Example 1 having a water-resistant layer containing 5 wt% AlPO ₄ relative to lithium vanadium phosphate was 89%, and the capacity retention rate of Example 2 having a water-resistant layer containing 5 wt% LiTi ₂ (PO ₄ ) ₃ relative to lithium vanadium phosphate was 87%. In addition, the capacity retention rate of Example 3 having a water-resistant layer containing 25 wt% LiTi ₂ (PO ₄ ) ₃ relative to lithium vanadium phosphate was 91%.

From this result, it was found that even when a water-resistant layer containing _AlPO4 or _LiTi2 ( _PO4 ) ₃ was formed, the charge/discharge characteristics were equivalent to those of an electrode material that did not have a water-resistant layer. It was also confirmed that the capacity retention rate did not change depending on the content of _LiTi2 ( _PO4 ) ₃ . In other words, it was confirmed that the presence of a water-resistant layer containing _AlPO4 or _LiTi2 ( _PO4 ) ₃ did not cause deterioration in the charge/discharge characteristics.

Claims (6)

comprising lithium vanadium phosphate bound to a conductive carbon material;
The lithium vanadium phosphate has a water-resistant layer,
the water-resistant layer has a crystal structure of a phosphate-based compound,
The water-resistant layer having a crystal structure of the phosphate-based compound is an aluminum phosphate compound, or a _titanium phosphate compound, or is represented by _XαMβ ( _PO4 ) _γ , where X is any one of Li, Na, K, and Ca, and M is any one of Al, Mn, Co, Fe, Mg, Ni, Ti, Zn, Cr, Ni, Cu, Zr, and Nb;
The water-resistant layer is a water-resistant layer formed before forming an electrode. 2. The electrode material according to claim 1, wherein the aluminum phosphate compound is _AlPO4 . 2. The electrode material according to claim 1, wherein the titanium phosphate compound is _LiTi2 ( _PO4 ) ₃ . 4. The electrode material according to claim 1, wherein the water-resistant layer is formed so as to have a content of 5 to 25 wt % relative to lithium vanadium phosphate. a crystallization step of combining the conductive carbon material with lithium vanadium phosphate;
A coating step of forming a water-resistant layer on the electrode material obtained by the crystallization step,
The coating step includes a mixing step of mixing a material source of a water-resistant layer in a solution with the electrode material obtained by the crystallization step;
A baking step of baking the solution obtained in the mixing step,
The water-resistant layer has a crystal structure of a phosphate-based compound. 6. The method for producing an electrode material according to claim 5, wherein the water-resistant layer having a crystal structure of a phosphate-based compound is an aluminum phosphate compound, or a _titanium phosphate compound, or is represented by _XαMβ ( _PO4 ) _γ , where X is any one of Li, Na, K, and Ca, and M is any one of Al, Mn, Co, Fe, Mg, Ni, Ti, Zn, Cr, Ni, Cu, Zr, and Nb.