JPWO2013084862A1 - Active material for positive electrode, method for producing the same, and lithium ion secondary battery - Google Patents

Abstract

The present invention proposes an active material for a positive electrode, a method for producing the same, and a lithium ion secondary battery capable of obtaining a large charge / discharge capacity at a higher charge / discharge rate than before, while achieving low cost and safety. In the active material for positive electrodes of the present invention, any one of lithium iron silicate, lithium manganese silicate, and a solid solution containing these is included in at least one of copper, vanadium, and titanium. Thereby, in this positive electrode active material, since it is not using rare elements Co and Ni, low cost and safety can be achieved, and in addition to this, since the electron conductivity can be improved than before, The discharge capacity per volume or the discharge capacity per weight can be increased at a high charge / discharge rate, and thus the charge / discharge rate is faster than before and a large charge / discharge capacity can be obtained.

Description

  The present invention relates to a positive electrode active material, a method for producing the same, and a lithium ion secondary battery, and is suitable for application to a lithium ion secondary battery using lithium ions as an ion conductive medium.

  In recent years, lithium ion secondary batteries have a feature of large capacity per volume and large capacity per weight, and are widely used as power sources for mobile phones, notebook personal computers, electric vehicles, and the like. Here, the lithium ion secondary battery has an active material (positive electrode active material, negative electrode active material) capable of occluding and releasing lithium ions in the positive electrode and the negative electrode, respectively, and lithium ions move in the electrolyte. It is a battery that operates by. As the negative electrode active material, in addition to a material capable of occluding and releasing lithium ions, such as a carbon material, a metal material that forms an alloy with Li, such as Li, Al, or Si can also be used.

On the other hand, a layered structure compound such as LiCoO ₂ , LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ or the like has been used as the positive electrode active material of the lithium ion secondary battery. Conventionally, these positive electrode active materials have a problem that the manufacturing cost is increased because Co or Ni, which are rare elements, are used as the material.

  Moreover, when a large current flows due to overcharge or short circuit, the positive electrode active material using these materials rapidly rises in temperature and further releases oxygen. An electrolyte of a lithium ion secondary battery, which is a flammable organic solvent, may ignite due to this temperature rise and oxygen supply, and may burn explosively. For this reason, lithium ion secondary batteries using positive electrode active materials made of these materials are often marketed with elements incorporating protection circuits and short circuit prevention measures.

However, as long as these positive electrode active materials are used, even if a protective circuit or the like is provided, the essential danger cannot be avoided, and an accident may occur for some reason. Therefore, in recent years, in order to solve the above-mentioned problems, oxygen is not released even when overheating without using a rare element such as Co or Ni as a positive electrode active material of a lithium ion secondary battery. Active materials for positive electrodes such as LiFePO ₄ and Li ₂ FeSiO ₄ have been developed (see, for example, Patent Document 1).

Japanese Patent No. 3484003

However, an active material for a positive electrode made of, for example, LiFePO ₄ or Li ₂ FeSiO ₄ has a low electron conductivity, and at a high charge / discharge rate, the discharge capacity per volume or the discharge capacity per weight is small. There was a problem that it was not possible to meet the demand for charge / discharge capacity.

  Therefore, the present invention was made in consideration of the above points, and while achieving low cost and safety, the active material for a positive electrode capable of obtaining a large charge / discharge capacity at a higher charge / discharge rate than before, It aims at proposing the manufacturing method and a lithium ion secondary battery.

  In order to solve such a problem, claim 1 of the present invention provides lithium iron silicate, lithium manganese silicate, and a solid solution containing them (the crystal structure remains as it is, in which one atom is replaced with another atom) For example, including lithium iron manganese silicate), and at least one of copper, vanadium, and titanium.

Further, the present invention provides a first solution containing at least one of Cu, V, and Ti, a second solution containing at least one of Fe and Mn, and a silicic acid suspension or A solution generation step of mixing any of tetraalkyl orthosilicate (Si (OR) ₄ , R is an alkyl group) with a lithium hydroxide solution to generate a raw material liquid, and hydrothermally treating the raw material liquid in a low oxygen atmosphere And a treatment step of generating a positive electrode active material containing at least one of copper, vanadium, and titanium in any one of lithium iron silicate, lithium manganese silicate, and a solid solution containing these, and A method for producing a positive electrode active material, comprising:

  According to claim 1 of the present invention, since the rare elements Co and Ni are not used, low cost and safety can be achieved, and in addition to this, since the electron conductivity can be improved as compared with the prior art, Active material for positive electrode and lithium capable of increasing discharge capacity per volume or discharge capacity per weight at a high charge / discharge rate, thus providing a higher charge / discharge capacity and a higher charge / discharge capacity than conventional ones An ion secondary battery can be proposed.

  According to the fourth aspect of the present invention, it is possible to manufacture an active material for a positive electrode capable of obtaining a large charge / discharge capacity at a higher charge / discharge rate than the conventional one while maintaining low cost and safety.

It is the schematic which shows an energy band. It is a graph which shows the mode of the change of the lattice constant when increasing Cu content to Example 2, and the lattice constant of the comparative example 2. FIG. It is a graph which shows the charging / discharging curve obtained by the charging / discharging characteristic test using Example 1 and Comparative Example 1. FIG. It is a graph which shows the discharge curve obtained by the charging / discharging characteristic test using Example 2, Example 3, and Comparative Example 2. FIG. 6 is a graph showing a discharge curve obtained by a charge / discharge characteristic test using Example 4. FIG.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The positive electrode active material according to the present invention includes at least one of copper (Cu), vanadium (V), and titanium (Ti) in a lithium silicate containing at least one of iron and manganese. By using the positive electrode of the lithium ion secondary battery, the capacity of the lithium ion secondary battery and the high-speed charge / discharge operation can be realized. The lithium ion secondary battery in the present invention has the same configuration as that of a general lithium ion secondary battery except for the positive electrode active material, for example, the negative electrode active material is a carbon material or the like. Here, the entire description of the lithium ion secondary battery is omitted, and only the active material for the positive electrode will be described below.

  In practice, the positive electrode active material of the present invention can improve the electronic conductivity by using a lithium silicate having a predetermined chemical composition containing at least one of Cu, V and Ti. Here, first, for the purpose of improving the electronic conductivity of the lithium silicate, a precise electronic state calculation was carried out for the lithium silicate containing various transition metals as additives. In this electronic state calculation, the local spin density approximation method which adopted on-site electron correlation strictly was adopted. Table 1 shows the calculation results obtained with these lithium manganese silicate (hereinafter also referred to as Mn-based). In this electronic state calculation, the Fermi level is 0 [eV].

  Here, for the Mn-based energy band, as shown in FIG. 1, the forbidden band width Eg between the valence band and the conduction band is 3.7 [eV] (the valence band upper end Ev is 0 [eV], the conduction band). The lower end Ec was 3.7 [eV]). From Table 1, when Cu is added to the Mn system, it has an empty defect level in the vicinity of 0 [eV] to 1 [eV] in the vicinity of the valence band upper end Ev. Therefore, when Cu is used as an additive in the Mn system, electrons in the valence band of the Mn system that are clogged with electrons move to an empty defect level of Cu, and holes are generated in the valence band. An increase in electron conductivity due to conduction of electron holes (holes) is expected.

  On the other hand, when Ti, V, and Cr are added to the Mn system from Table 1, there are defect levels occupied by electrons in the vicinity of 3.0 [eV] to 4.0 [eV] near the lower end Ec of the conduction band. Therefore, when these Ti, V, and Cr are used as additives in the Mn system, electrons in the defect level of the additive move to a Mn-based conduction band in which electrons are not clogged. An increase in electronic conductivity due to conduction is expected. However, Co and Ni in Table 1 have free defect levels in the vicinity of 3.0 [eV] to 4.0 [eV] in the vicinity of the conduction band lower end Ec, so that free electrons such as Ti, V, and Cr are used. The increase in electron conductivity due to conduction cannot be expected.

  Therefore, in the Mn system, when the valence band and conduction band of the energy band are compared with the electronic state of the defect level of each additive, Ti, V, and Cr are effective additives for increasing the electron conductivity. And Cu were found to be effective.

  Next, four additives of Ti, V, Cr, and Cu that were effective in increasing the electron conductivity when viewed from the defect level were actually added to the Mn system, and each additive was converted to the Mn system. It was confirmed whether it could be included. As a result, the three additives of Ti, V, and Cu could all be replaced with manganese and included in the Mn system. However, Cr was not substituted with manganese, but became a structure separated from Mn, and could not be included in Mn. Accordingly, it was confirmed that at least one of Cu, V and Ti is preferable as an additive capable of improving the Mn-based electron conductivity.

  Next, for lithium iron silicate (hereinafter also referred to as Fe system), as in the case of Mn system described above, an electronic state calculation is performed by adopting a local spin density approximation method that strictly incorporates on-site electron correlation. It was. As a result, the calculation results shown in Table 2 below were obtained for the Fe system.

  In the Fe system, the width Eg of the forbidden band between the valence band and the conductor (FIG. 1) is 3.2 [eV] (the valence band upper end Ev is 0 [eV] and the conduction band lower end Ec is 3.2 [eV]). It was. Then, the Fe-based valence band and conduction band are compared with the electronic states of the defect levels of each additive, and an additive that is effective in increasing the electron conductivity from the same concept as the Mn system described above. As a result of confirmation, it was confirmed that Ti, V, Cr, and Cu are effective in increasing the electron conductivity even in the Fe system.

  Next, four additives of Ti, V, Cr, and Cu that were effective in increasing the electron conductivity as viewed from the defect level were actually added to the Fe system, and each additive was added to the Fe system. It was confirmed whether or not it can be included. As a result, all of the three additives of Ti, V, and Cu could be replaced with iron and included in the Fe system. However, Cr did not replace iron, but became a structure separated from the Fe system, and could not be included in the Fe system. Accordingly, it was confirmed that at least one of Cu, V and Ti is preferable as an additive capable of improving the Fe-based electronic conductivity.

  Incidentally, conventionally, Mn-based and Fe-based lithium silicates generally have low electronic conductivity, and lithium ions cannot be taken in and out of the structure at high speed, and this is used as a positive electrode active material. However, it has been difficult to realize a large-capacity lithium ion secondary battery capable of high-speed charge / discharge.

  On the other hand, in the positive electrode active material of the present invention, by adding at least one of Cu, V, and Ti to the Mn-based and / or Fe-based lithium silicate, a new electron conduction level is obtained. As a result, the electron conductivity can be increased. Therefore, in the lithium ion secondary battery using the positive electrode active material according to the present invention, the insertion / desorption rate of the lithium ion from the positive electrode active material responding to the transfer of electrons is increased, and the charge / discharge can be performed at a high speed up to a large capacity. A possible lithium ion secondary battery can be realized. Here, the Mn-based and / or Fe-based lithium silicate includes a solid solution of lithium iron silicate and lithium manganese silicate in addition to either lithium iron silicate or lithium manganese silicate. It is a waste.

Further, the positive electrode active material uses at least one of Cu, V, and Ti, and Mn-based and / or Fe-based lithium silicate as a constituent material, and uses conventional Co and Ni at all. Since it is not used, a layered structure compound such as LiCoO ₂ , LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O ₂ or the like, which has been widely used as an active material for positive electrodes of lithium ion secondary batteries. In contrast, even if an abnormal state such as overcharge or short circuit occurs, it does not generate heat and supply oxygen. Therefore, the positive electrode active material of the present invention can ignite the flammable organic electrolyte and cause no explosion, and can significantly improve safety compared to the prior art.

Moreover, since the positive electrode active material of the present invention uses at least one of Cu, V, and Ti, and Mn-based and / or Fe-based lithium silicates as constituent materials, Unlike layered structure compounds such as LiCoO ₂ , LiNiO ₂ , LiNi _1/3 Co _1/3 Mn _1/3 O _{2 and the} like that are frequently used as active materials for lithium ion secondary batteries, rare and expensive Co and Ni are used. Since it is not included, a cheaper lithium ion secondary battery can be provided.

  Actually, in the positive electrode active material according to the present invention, the total of Cu, V, and Ti added as constituent materials (when only one of Cu, V, and Ti is added, Additive) and the total of Fe-based iron and Mn-based manganese (if composed of either one of Fe-based and Mn-based, one atomic or iron) atomic ratio However, it is preferable that it is 0.005: 1.0 to 0.333: 1.0. By adjusting the positive electrode active material to such an atomic ratio, the electron conductivity can be further improved than before.

Further, specifically, the positive electrode active material according to the present invention has a chemical formula of Li _x A _y D _z SiO ₄ (A is a transition metal ion including at least one of Cu, V, and Ti, and D is Fe and / or Mn, x can be represented by a chemical formula of 0 ≦ x ≦ 2.5, y is 0.005 ≦ y ≦ 0.25, and z is 0.75 ≦ z ≦ 1.0). In the lithium ion secondary battery, lithium ions are released from the positive electrode active material during charging, and on the other hand, lithium ions are occluded in the positive electrode active material during discharging. From this, the chemical formula representing the positive electrode active material can be A _y D _z SiO ₄ (ie, 0 = x) in which Li is not present during charging, and Li _x A _y D _z SiO ₄ (ie, during discharging) 0 <x ≦ 2.5).

  Here, in practice, when D in the above chemical formula is Fe, the ratio y of A is preferably 0.005 ≦ y ≦ 0.05, and the electronic conductivity of the Fe-based positive electrode active material can be further improved. . On the other hand, when D in the chemical formula is Mn, 0.05 ≦ y ≦ 0.20 is desirable when A is Cu, and 0.01 ≦ y ≦ 0.05 is desirable when A is V or Ti. The electron conductivity can be further improved for the positive electrode active material.

  Such a positive electrode active material according to the present invention can be produced by the following procedure. Here, as an example, first, a first solution containing at least one of Cu, V, and Ti and a second solution containing at least one of Fe and Mn are prepared. The first solution is either a nitric acid solution containing at least one of Cu, V, and Ti, or a citric acid solution containing at least one of Cu, V, and Ti. Is preferred. The second solution is preferably either a chloride solution containing at least one of Fe and Mn or a sulfuric acid solution containing at least one of Fe and Mn.

Separately from this, either one of silicic acid suspension or tetraalkyl orthosilicate is prepared. Then, the first solution, the second solution, and either silicic acid suspension or tetraalkoxysilane are mixed with the lithium hydroxide solution to produce a raw material solution. Incidentally, here, tetraalkylsilane (Si (OCH ₃ ) ₄ ), tetraethoxysilane (Si (OCH ₂ CH ₃ ) ₄ ), tetrapropoxysilane (Si (OCH ₂ CH ₂ CH _{3) 4} ), tetrabutoxysilane (Si (OCH ₂ CH ₂ CH ₂ CH ₃ ) ₄ ) and other various Si (OR) ₄ (where R is a C _n H _{2n + 1} alkyl group (preferably n is A tetraalkyl orthosilicate represented by any one of 1 to 6)) may be applied.

  Next, the raw material liquid is placed in the pressure vessel, and hydrothermal treatment is performed to heat the raw material solution with the inside of the pressure vessel in a low oxygen atmosphere. The present invention which contains at least any one of copper, vanadium, and titanium in any one of lithium iron silicate, lithium manganese silicate, and a solid solution containing these from a raw material liquid by such hydrothermal treatment. The positive electrode active material can be produced.

  Here, the hydrothermal treatment is to heat the raw material liquid in a low oxygen atmosphere in which the oxygen concentration is much lower than that in the air. When the raw material liquid is heated by this, the elements and oxygen in the raw material liquid are heated. The positive electrode active material having the composition according to the present invention can be generated from the raw material liquid. In order to achieve such a low oxygen atmosphere, for example, it is desirable to heat to a temperature of 100 [° C.] or higher in a state where hydrogen gas is filled in a pressure vessel at a pressure of 0.1 [MPa] or higher.

By the way, in such a positive electrode active material, iron and / or manganese are contained in the lithium silicate. For example, when lithium iron silicate of Li ₂ FeSiO ₄ is used as the positive electrode active material, Li is charged by charging. ⁺ and desorption occurs and the oxidation of ^{Fe 2+} to ^{Fe 3+,} whereas, insertion and Li ⁺ by the discharge, and reduction of ^{Fe 3+} to ^{Fe 2+} occurs. Here, when Fe ³⁺ is generated in the positive electrode active material at the time of manufacturing, since desorption of Li ⁺ is suppressed, charging does not proceed sufficiently, and as a result, the discharge capacity is reduced. End up.

Therefore, in the positive electrode active material according to the present invention, hydrogen gas or the like is introduced into the pressure vessel at the time of manufacture, and hydrothermal heat treatment is performed to heat the raw material liquid in a low oxygen atmosphere. Fe ²⁺ and Mn ²⁺ are reliably generated in the active material, and reduction of charge / discharge capacity can be avoided.

  In the above configuration, in the positive electrode active material according to the present invention, at least one of copper, vanadium, and titanium is added to any one of lithium iron silicate, lithium manganese silicate, and a solid solution containing these. The configuration included. Thereby, in this positive electrode active material, since it is not using rare elements Co and Ni, low cost and safety can be achieved, and in addition to this, since the electron conductivity can be improved than before, The discharge capacity per volume or the discharge capacity per weight can be increased at a high charge / discharge rate, and thus the charge / discharge rate is faster than before and a large charge / discharge capacity can be obtained.

  The present invention is not limited to the present embodiment, and various modifications can be made within the scope of the present invention. If the above-described positive electrode active material according to the present invention can be manufactured, for example, hydrothermal treatment Various conditions may be applied to the occasional low oxygen atmosphere.

  Examples of the positive electrode active material of the present invention described above will be described below. In addition, this invention is not limited to a following example.

(1) Synthesis of active materials for positive electrodes of Examples 1 to 4 and Comparative Examples 1 and 2 (1-1) Synthetic example of active materials for positive electrodes made of lithium iron silicate containing copper (Example 1)
First, in a sealed container, 0.4320 [g] of lithium hydroxide monohydrate was added and dissolved in 10 [ml] of pure water substituted with Ar gas to obtain an aqueous lithium hydroxide solution. Separately, 0.5009 [g] of iron (II) chloride tetrahydrate was added to 5 [mL] of pure water substituted with Ar gas in a sealed container, and dissolved by stirring to obtain iron (II) chloride. An aqueous solution (second solution) was obtained. Separately, in a sealed container, 0.0093 [g] of copper citrate 2.5 hydrate was added to ethanol 5 [mL] that had been boiled under reduced pressure and then purged with Ar gas, and stirred to obtain a copper citrate solution ( A first solution was obtained. Furthermore, tetraethoxysilane (tetraethyl orthosilicate) 0.600 [mL] was added to this copper citrate aqueous solution and stirred, and this was added to the aqueous iron (II) chloride solution and stirred to obtain a mixed solution. Furthermore, a lithium hydroxide aqueous solution was dropped into the mixed solution and mixed with stirring to obtain a final raw material liquid.

  Next, this raw material liquid was put in a pressure vessel, heated at 225 [° C.] for 24 hours, and hydrothermally treated in a low oxygen atmosphere. Specifically, in this hydrothermal treatment, after degassing the pressure vessel with a rotary vacuum pump, hydrogen gas was added at room temperature so that the total pressure was 0.99 [MPa]. Next, solid-liquid separation was performed by centrifugal separation after hydrothermal treatment, and only the solid component was recovered. And after adding pure water to this solid component and washing | cleaning, only the solid component was obtained by centrifugation. The obtained solid component was dried under reduced pressure at room temperature, and then pulverized with a mortar to obtain an active material for positive electrode of Example 1 made of lithium silicate containing copper and iron.

(1-2) Synthesis example of active material for positive electrode made of lithium iron silicate containing no copper (Comparative Example 1)
First, in a sealed container, 0.42 [g] of lithium hydroxide monohydrate was added to 10 [mL] of pure water substituted with Ar gas and dissolved by stirring to obtain an aqueous lithium hydroxide solution. Separately, 0.695 [g] of iron sulfate heptahydrate was added to 5 [mL] of pure water substituted with Ar gas in an airtight container and dissolved by stirring to obtain an iron (II) sulfate aqueous solution. . Next, Ar gas-substituted silica sol 0.75 [mL] is added to an iron (II) sulfate aqueous solution in an airtight container and stirred. Further, an aqueous lithium hydroxide solution is added dropwise thereto and mixed with stirring to obtain a final raw material solution. Obtained.

  Next, this raw material liquid was put in a pressure vessel, heated at 325 [° C.] for 2 hours, and hydrothermally treated in a low oxygen atmosphere. Again, in this hydrothermal treatment, after degassing the pressure vessel with a rotary vacuum pump, hydrogen gas was added at room temperature so that the total pressure was 0.99 [MPa]. Next, after this hydrothermal treatment, solid-liquid separation was performed by centrifugation to recover only the solid components. The solid component was washed by adding pure water, and then only the solid component was obtained by centrifugation. The obtained solid component was dried under reduced pressure at room temperature and pulverized with a mortar to obtain a positive electrode active material of Comparative Example 1 made of lithium iron silicate.

(1-3) Synthesis example of positive electrode active material comprising lithium manganese silicate containing copper (Example 2)
First, in a sealed container, 0.431 [g] of lithium hydroxide monohydrate was added and dissolved in 10 [mL] of pure water substituted with Ar gas to obtain an aqueous lithium hydroxide solution. Separately, in a sealed container, 0.499 [g] of manganese (II) chloride tetrahydrate was added to 5 [mL] of pure water substituted with Ar gas, and dissolved by stirring to obtain manganese (II) chloride. An aqueous solution (second solution) was obtained. Subsequently, after adding 0.0093 [g] of copper citrate 2.5 hydrate to ethanol 5 [mL] in which the Ar gas was substituted after boiling under reduced pressure in a closed container, the mixture was stirred (copper citrate solution (first solution)). To this, tetramethoxysilane (tetraethyl orthosilicate) 0.600 [mL] was added and stirred, and further a manganese (II) chloride aqueous solution was added thereto to obtain a mixed solution. Finally, a lithium hydroxide aqueous solution was dropped into the mixed solution and mixed by stirring to obtain a final raw material liquid.

  Next, this raw material liquid was put in a pressure vessel and heated at 225 [° C.] for 24 hours, and hydrothermally treated in a low oxygen atmosphere. Here, in this hydrothermal treatment, after degassing the pressure vessel with a rotary vacuum pump, hydrogen gas was added so that the total pressure was 0.99 [MPa] at room temperature. Next, after this hydrothermal treatment, solid-liquid separation was performed by centrifugation to recover only the solid components. The solid component was washed by adding pure water, and then only the solid component was obtained by centrifugation. The obtained solid component was dried under reduced pressure at room temperature and then pulverized with a mortar to obtain an active material for positive electrode of Example 2 made of lithium manganese silicate containing copper.

(1-4) Synthesis example of active material for positive electrode made of lithium manganese silicate containing copper (Example 3)
First, in a sealed container, 0.431 [g] of lithium hydroxide monohydrate was added and dissolved in 10 [mL] of pure water substituted with Ar gas to obtain an aqueous lithium hydroxide solution. Separately, in a sealed container, 0.499 [g] of manganese (II) chloride tetrahydrate was added to 5 [mL] of pure water substituted with Ar gas, and dissolved by stirring to obtain manganese (II) chloride. An aqueous solution (second solution) was obtained. Then, after adding 0.0093 [g] of copper citrate hemihydrate to ethanol 5 [mL] after boiling under reduced pressure and replacing with Ar gas in a sealed container (copper citrate solution (first solution)) To this, tetramethoxysilane (tetraethyl orthosilicate) 0.600 [mL] was added and stirred, and further an aqueous manganese (II) chloride solution was added to obtain a mixed solution. And lithium hydroxide aqueous solution was dripped at this mixed solution, and it stirred and mixed, and obtained the raw material liquid.

  Next, this raw material liquid was put in a pressure vessel and heated at 225 [° C.] for 24 hours, and hydrothermally treated in a low oxygen atmosphere. Here, in this hydrothermal treatment, after degassing the pressure vessel with a rotary vacuum pump, hydrogen gas was added so that the total pressure was 0.99 [MPa] at room temperature. Subsequently, after this hydrothermal treatment, solid-liquid separation was performed by centrifugation, and only the solid component was recovered. The solid component was washed by adding pure water, and then only the solid component was obtained by centrifugation. The obtained solid component was dried under reduced pressure at room temperature and pulverized with a mortar.

  Further, here, an equal amount of the obtained powder and sucrose, 20 [mL] of pure water were weighed, and planetary ball milling was pulverized for 1 hour at a rotational speed of 100 [rpm]. Then, the hydrothermally treated solution and the ball are separated by filtration, and the hydrothermally treated solution is frozen in a preliminary freezer. The frozen hydrothermally treated solution is then dried overnight in a freeze dryer. The obtained dry powder was placed on an alumina boat and inserted into the center of the tubular furnace. After the furnace was sealed, the furnace was heated at 700 [° C.] for 12 hours and heat-treated in an Ar atmosphere.

  Subsequently, the powder taken out after the heat treatment was pulverized with a mortar, thereby obtaining the positive electrode active material of Example 3 made of lithium manganese silicate containing copper. In addition, by mixing and heat-treating sucrose in this manner, the surface of the positive electrode active material is coated with conductive carbon, and the electron conduction inside the particles can be effectively utilized.

(1-5) Synthesis example of active material for positive electrode made of lithium manganese silicate not containing copper (Comparative Example 2)
First, 0.431 [g] of lithium hydroxide monohydrate was added to 10 [mL] of pure water substituted with Ar gas in a sealed container, and dissolved by stirring to obtain an aqueous lithium hydroxide solution. Separately, 0.509 [g] of manganese (II) chloride tetrahydrate was added to 5 [mL] of pure water substituted with Ar gas in a sealed container, dissolved by stirring, and a manganese (II) chloride aqueous solution was added. Obtained. Next, 0.6000 [mL] of tetramethoxysilane (tetraethyl orthosilicate) was added to ethanol 5 [mL] that had been boiled under reduced pressure in an airtight container and purged with Ar gas, and then an aqueous manganese (II) chloride solution was added thereto. To obtain a mixed solution. Next, an aqueous lithium hydroxide solution was added dropwise to the mixed solution and mixed by stirring to obtain a raw material liquid.

  Next, the raw material liquid was put in a pressure vessel and heated at 325 [° C.] for 2 hours, and hydrothermal treatment was performed in a low oxygen atmosphere. Here, in this hydrothermal treatment, after degassing the pressure vessel with a rotary vacuum pump, hydrogen gas was added so that the total pressure was 0.99 [MPa] at room temperature. Next, after this hydrothermal treatment, solid-liquid separation was performed by centrifugation to recover only the solid components. The solid component was washed by adding pure water, and only the solid component was obtained by centrifugation after washing. The obtained solid component was dried under reduced pressure at room temperature and pulverized with a mortar to obtain a positive electrode active material of Comparative Example 2 made of lithium manganese silicate.

(1-6) Synthesis Example of Active Material for Positive Electrode Consisting of Titanium-Containing Lithium Silicate (Iron / Manganese) (Example 4)
First, 0.431 [g] of lithium hydroxide monohydrate was added to 10 [mL] of pure water purged with Ar gas in a sealed container and dissolved by stirring to obtain an aqueous lithium hydroxide solution. Separately, in pure water [mL] substituted with Ar gas in a sealed container, iron (II) chloride tetrahydrate 0.250 [g] and manganese chloride (II) tetrahydrate 0.249 [g] An aqueous solution (second solution) in which TAS-FINE (water-soluble Ti powder) 0.0123 [g] is dissolved in 1 [mL] pure water is added and stirred, and iron chloride ( II) + manganese chloride (II) + Ti aqueous solution was obtained. Next, tetramethoxysilane (tetraethyl orthosilicate) 0.600 [mL] was added to ethanol 5 [mL] which was boiled under reduced pressure in an airtight container and purged with Ar gas, and stirred, and then iron (II) chloride + manganese chloride was added thereto. (II) + Ti aqueous solution was added to obtain a mixed solution. Next, an aqueous lithium hydroxide solution was added dropwise to the mixed solution and mixed by stirring to obtain a raw material liquid.

  Next, the raw material liquid was put in a pressure vessel and heated at 225 [° C.] for 24 hours to perform hydrothermal treatment in a low oxygen atmosphere. Here, in this hydrothermal treatment, after degassing the pressure vessel with a rotary vacuum pump, hydrogen gas was added at room temperature so that the total pressure was 0.99 [MPa]. Next, after this hydrothermal treatment, solid-liquid separation was performed by centrifugation to recover only the solid components. The solid component was washed by adding pure water, and only the solid component was obtained by centrifugation after washing. The obtained solid component was dried under reduced pressure at room temperature and then pulverized with a mortar to obtain a positive electrode active material composed of lithium iron manganese silicate containing titanium.

(2) Evaluation Results (2-1) About Compositions of Examples 1 to 4 For the positive electrode obtained in “(1) Synthesis of active materials for positive electrodes of Examples 1 to 4 and Comparative Examples 1 and 2” described above. When the composition of the active material was examined by inductively coupled plasma optical emission spectrometry, the element added for improving the electron conductivity (copper in Examples 1 to 3 and titanium in Example 4) was included. It was confirmed that However, the atomic ratio of copper in the positive electrode active material is lower than the atomic ratio of copper in the manufacturing raw material. For example, in Example 2, the atomic ratio of Cu: Mn: Si is 0.15 in the manufacturing raw material. Was 0.85: 1.00, but was 0.016: 0.95: 1.00 in the finally obtained positive electrode active material.

  In Example 4, the atomic ratio of titanium in the positive electrode active material was approximately the same as the atomic ratio of titanium in the manufacturing raw material. Ti: Fe: Mn in the manufacturing raw material was 0.02: 0.49: 0.49, and was 0.017: 0.49: 0.47 in the positive electrode active material finally obtained. .

(2-2) About the lattice constants of Example 1 and Comparative Example 1 Table 3 shows the results of examining the lattice constants of the positive electrode active material of Example 1 and the positive electrode active material of Comparative Example 1 by X-ray diffraction analysis. Indicates. In addition, since X-ray diffraction peaks other than lithium iron silicate are not observed in both Example 1 and Comparative Example 1, it is a single phase having no subcomponents of different structures and has the same crystal structure. I was able to confirm. Further, as shown in Table 3, the lattice constants a, b, and c of both the positive electrode active material of Example 1 and the positive electrode active material of Comparative Example 1 are all statistically significantly different from each other. It shows that the positive electrode active material of Example 1 contains copper.

Incidentally, the positive electrode active material of Example 1 is Li ₂ Cu _0.02 Fe _0.98 SiO ₄ containing copper, while the positive electrode active material of Comparative Example 1 is Li ₂ FeSiO not containing copper. ₄ .

Next, with respect to the positive electrode active material represented by Li ₂ Cu _y Mn _z SiO ₄ of Example 2, the lattice constant when copper addition amount (Cu doping amount) is increased and copper is not included. When the lattice constant of the positive electrode active material of Comparative Example 2 was examined by X-ray diffraction analysis, the results shown in FIG. 2 were obtained ((In FIG. 2, Example 2 is represented by ■, Comparative Example 2 represents each of the positive electrode active materials having different Cu contents in Example 2 and the positive electrode active material in Comparative Example 2 with no X-ray diffraction peaks other than lithium manganese silicate. From the results shown in Fig. 2, it was confirmed that the lattice constants a, b, and c decreased as the Cu content increased. Such a decrease in lattice constant is because Cu has a smaller ionic radius than Mn. u have shown that you are replacing the Mn.

(2-3) Charge / Discharge Characteristic Test Next, each of the positive electrode active materials 10.0 [mg] of Examples 1 to 4 and Comparative Examples 1 and 2 described above was substituted with acetylene black 10.0 [mg] and polytetrafluoroethylene 1.0, respectively. [Mg] was added and mixed in an agate mortar to prepare a plurality of mixtures. Subsequently, each mixture was pressure-bonded to a Ni mesh, and this was used as a working electrode (positive electrode). For the counter electrode (negative electrode) and the reference electrode, metal lithium bonded to Ni mesh was used. As the electrolyte, a 1M lithium perchlorate / propylene carbonate solution was used.

  Using these working electrode, counter electrode, reference electrode, and electrolyte, a plurality of types of three-electrode cells were prepared in which different types of positive electrode active materials were provided on each working electrode. Then, using each three-electrode cell, a constant current discharge characteristic test was performed at 60 [° C.] in a potential range of 1.5 to 4.3 [V] and a current density of 10 [mA / g] to 100 [mA / g]. . When the charge / discharge characteristic test was performed on the three-electrode cell using the positive electrode active material of Example 1 and the three-electrode cell using the positive electrode active material of Comparative Example 1, the charge / discharge as shown in FIG. Characteristics were obtained.

  In FIG. 3, the horizontal axis represents the capacity of the positive electrode (working electrode), which is the product of the discharge current value and the discharge time, while the vertical axis represents the potential of the working electrode. And in FIG. 3, the charging / discharging curve of the active material for positive electrodes of Example 1 measured by the current density of 10 [mA / g] is shown by the solid line, and the comparative example similarly measured by the current density of 10 [mA / g] The charge / discharge curve of the positive electrode active material 1 is shown by a broken line. As shown in FIG. 3, the positive electrode active material of Example 1 to which copper was added had a discharge capacity per weight of 173 [mAh / g]. On the other hand, the positive electrode active material of Comparative Example 1 to which copper was not added had a discharge capacity per weight of 123 [mAh / g]. Thus, in the positive electrode active material of Example 1 according to the present invention, it was confirmed that the discharge capacity per weight was improved as compared with the positive electrode active material of Comparative Example 1. In addition, it was confirmed that the positive electrode active material of Example 1 according to the present invention also improved the charge capacity per weight as compared with the positive electrode active material of Comparative Example 1.

Next, when the composition of each positive electrode active material of Examples 2 and 3 and the composition of the positive electrode active material of Comparative Example 2 was examined, each of the positive electrode active materials of Examples 2 and 3 was Li ₂ containing copper. It was Cu _0.016 Mn _0.95 SiO ₄ , and the positive electrode active material of Comparative Example 2 was Li ₂ MnSiO ₄ not containing copper. And about the three-electrode cell which each used the active material for positive electrodes of each of these Examples 2 and 3, and the three-electrode cell which used the active material for positive electrodes of the comparative example 2, respectively, a charging / discharging characteristic test was each performed, and about discharge characteristics As a result of the examination, the discharge characteristics as shown in FIG.

  In FIG. 4, the discharge curves of the positive electrode active materials of Examples 2 and 3 measured at a current density of 10 [mA / g] are indicated by solid lines and alternate long and short dash lines, and are also measured at a current density of 10 [mA / g]. The discharge curve of the positive electrode active material of Comparative Example 2 is shown by a broken line. From the results shown in FIG. 4, each positive electrode active material of Example 2 to which copper was added had a discharge capacity of 132 [mAh / g] per weight, and each positive electrode active material of Example 3 to which copper was also added. The discharge capacity per weight was 233 [mAh / g]. On the other hand, the positive electrode active material of Comparative Example 2 to which copper was not added had a discharge capacity per weight of 50 [mAh / g]. As described above, it was confirmed that each positive electrode active material of Examples 2 and 3 according to the present invention had an improved discharge capacity per weight as compared with the positive electrode active material of Comparative Example 2.

  From the above results, in the positive electrode active material, low cost and safety can be maintained without using rare elements such as Co and Ni by adding copper to lithium iron silicate or lithium manganese silicate. However, the discharge capacity per volume of the positive electrode active material or the discharge capacity per weight can be improved.

We also examined the composition for the positive electrode active material of Example 4, this Example 4 was _{Li 2 Ti 0.017 Fe 0.49 Mn 0.47} SiO 4 containing titanium. And when the charge / discharge characteristic test was done about the three electrode cell using the active material for positive electrodes of this Example 4 and the discharge characteristic was investigated, the discharge characteristic as shown in FIG. 5 was obtained. Thus, in the positive electrode active material of Example 4 according to the present invention, it was confirmed that the discharge capacity per weight was improved as compared with the positive electrode active material of Comparative Example 2. From the above results, in the positive electrode active material, a structure in which titanium is added to lithium iron manganese silicate allows the positive electrode to maintain low cost and safety without using rare elements such as Co and Ni. It was confirmed that the discharge capacity per volume or the discharge capacity per weight of the active material can be improved.

By the way, in these examples, the charge / discharge characteristic test is explained for the positive electrode active material in which copper is added to lithium iron silicate or lithium manganese silicate, and the positive electrode active material in which titanium is added to lithium manganese manganese silicate. However, the same charge and discharge as described above is also applied to the positive electrode active material in which vanadium or titanium is added to lithium iron silicate or lithium manganese silicate, and the positive electrode active material in which copper or vanadium is added to lithium iron manganese silicate. A characteristic test is conducted. As a result, for any positive electrode active material to which copper, vanadium or titanium is added, the discharge capacity per volume or the discharge capacity per weight of the positive electrode active material can be improved, as in the above-described examples. Confirmed.

Claims (6)

A positive electrode active material, wherein at least one of copper, vanadium, and titanium is contained in any one of lithium iron silicate, lithium manganese silicate, and a solid solution containing these. A total of the copper, the vanadium, and the titanium;
The atomic ratio of the iron of the lithium iron silicate and the total manganese of the lithium manganese silicate is 0.005: 1.0 to 0.333: 1.0. Active material for positive electrode. The chemical formula is Li _x A _y D _z SiO ₄ (A is a transition metal ion containing at least one of Cu, V, and Ti, D is Fe and / or Mn, x is 0 ≦ x ≦ 2.5 The positive electrode active material according to claim 1, wherein y is made of lithium silicate wherein 0.005 ≦ y ≦ 0.25 and z is 0.75 ≦ z ≦ 1.0. A first solution containing at least one of Cu, V, and Ti; a second solution containing at least one of Fe and Mn; and a silicic acid suspension or a tetraalkyl orthosilicate (Si ( OR) ₄ , R is an alkyl group) and a lithium hydroxide solution to produce a raw material solution;
The raw material liquid is hydrothermally treated in a low oxygen atmosphere, and any one of lithium iron silicate, lithium manganese silicate, and a solid solution containing these includes at least one of copper, vanadium, and titanium. And a processing step of producing a positive electrode active material. A method for producing a positive electrode active material. The first solution includes at least one of a nitric acid solution and a citric acid solution,
The positive electrode active material manufacturing method according to claim 4, wherein the second solution includes at least one of a chloride solution and a sulfuric acid solution. A lithium ion secondary battery comprising a positive electrode using the positive electrode active material according to any one of claims 1 to 3.

Images