KR100389654B1 - Positive active material for lithium secondary battery and method of preparing same - Google Patents

Abstract

The present invention relates to a positive electrode active material for a lithium secondary battery and a method for manufacturing the same, which method comprises mixing a metal salt solution and a chelating agent solution in which lithium salt, transition metal salt, cobalt salt and phosphorus salt are dissolved, and heating the mixture to gel To prepare a, and to heat the gel.

[Formula 1]

Li _{1 + x} M _1-yz M ' _y P _z O ₄

(In Formula 1, M and M 'is at least one transition metal selected from the group consisting of Co, Ni, Fe, Mn, Al, Cr, Fe, Mg, Sr and V,

x is from 0 to 0.5,

y is 0 to 1,

z is 0 to 1,

0 ≤ y + z ≤ 1.

The production method of the present invention can be heat-treated at a lower temperature than the conventional solid-phase reaction method, it is also possible to synthesize a single phase material, it is possible to produce an active material more excellent electrochemical activity. In addition, the electrochemically excellent activity, when using this positive electrode active material can provide a lithium secondary battery with more excellent power (power).

Description

A cathode active material for a lithium secondary battery and a manufacturing method thereof {POSITIVE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERY AND METHOD OF PREPARING SAME}

[Industrial use]

The present invention relates to a positive electrode active material for a lithium secondary battery and a method for manufacturing the same, and more particularly, to a positive electrode active material for a lithium secondary battery and a method for producing the same showing an excellent amount of power.

[Prior art]

Lithium secondary batteries are prepared by reversibly inserting and detaching lithium ions as a positive electrode and a negative electrode, and filling an organic or polymer electrolyte between the positive electrode and the negative electrode, and lithium ions are inserted / desorbed at the positive electrode and the negative electrode. When produced, electrical energy is generated by oxidation and reduction reactions.

Lithium metal is used as a negative electrode active material of a lithium secondary battery. However, when lithium metal is used, there is a risk of explosion due to a short circuit of the battery due to the formation of dendrite. Thus, lithium metal may be replaced with a carbon-based material such as amorphous carbon or crystalline carbon. It is going to be replaced. In particular, in recent years, boron-coated graphite (BOC) is manufactured by adding boron to a carbon-based material to increase the capacity of the carbon-based material.

As a cathode active material, a chalcogenide compound is used, and for example, a composite metal such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1), LiMnO _2, or the like Oxides are being studied. Among the cathode active materials, Mn-based cathode active materials such as LiMn ₂ O ₄ and LiMnO ₂ are easy to synthesize, are relatively inexpensive, and are less attractive to the environment due to less pollution. However, they have a small capacity. LiNiO ₂ is the cheapest of the above-mentioned positive electrode active materials, and exhibits the highest discharge capacity of battery characteristics, but has a disadvantage of difficulty in synthesizing.

LiCoO ₂ exhibits good electrical conductivity, high battery voltage, and excellent electrode characteristics, and is a representative cathode active material commercialized and marketed by Sony Corp., and has a 95% or more share as a cathode active material of lithium secondary batteries. However, LiCoO ₂ has the disadvantage of being expensive.

Accordingly, research is being conducted to replace LiCoO ₂ , and one of such methods is to increase the amount of power even if the same amount of Co is used, thereby increasing the battery usage time. A method of using LiCoPO ₄ having a structure has been studied.

The operating voltage of LiCoO _{2 in} the conventional layer structure is about 3.7V, while the material in Olivain maintains the operating voltage of 4.8V, and it is possible to maintain a relatively large amount of power by applying such a material as a cathode active material. In addition, there is an advantage that can significantly increase the use time of the battery, it is also considered that there is an advantage that can reduce the number of batteries when configuring the battery pack.

However, the synthesis of LiCoPO ₄ of the olivine structure known to date is 750 after mixing Li, Co, P source, for example Li ₂ CO ₃ , Co ₃ O ₄ , (NH ₄ ) ₂ HPO ₄ in a solid state After the first heat treatment at ℃, LiCoPO ₄ and Li ₅ P ₃ O ₁₀ , a lithium-rich phosphate to prepare a second heat treatment at 350 ℃ for 9 hours, and then the third heat treatment at 750 ℃ for 30 hours It is a solid phase reaction method which heat-processes several times over a long time. Therefore, there is a problem that the manufacturing time is long, and accordingly the manufacturing cost is high. In addition, when measuring the XRD pattern of LiCoPO ₄ prepared by the solid-phase reaction method, not only has a pure oline structure, but there is a problem that a minor phase appears.

The present invention is to solve the above-described problems, an object of the present invention is to provide a method for producing a positive electrode active material for a lithium secondary battery that can be heat-treated at a low temperature to economically produce a positive electrode active material.

Another object of the present invention is to provide a method for producing a positive electrode active material for a lithium secondary battery, which can produce a single phase active material without an impurity phase even if heat treatment is performed at a low temperature.

Still another object of the present invention is to provide a positive electrode active material for a lithium secondary battery having excellent electrochemical activity prepared by the above method.

Still another object of the present invention is to provide a positive electrode active material for a lithium secondary battery, which can provide a battery having excellent electric power.

1 is a SEM photograph of LiCoPO ₄ prepared according to Example 1 of the present invention.

2 is a SEM photograph of LiCoPO ₄ prepared according to Example 3 of the present invention.

Figure 3a is a SEM photograph of LiCoPO ₄ prepared according to Comparative Example 3

FIG. 3B is a SEM photograph at 4 times magnification of the SEM photograph of FIG. 2A; FIG.

4 is a graph showing an XRD pattern of LiCoPO ₄ prepared according to Comparative Examples 1 to 3.

Figure 5 is a graph showing the XRD pattern of the first and the second heat-treated precursor LiCoPO ₄ and LiCoPO ₄ of Example 3 in Example 2 of the present invention.

6 is a graph showing CV voltamogram of LiCoO ₂ .

7 is a graph showing a CV voltammogram of the positive electrode active material for lithium secondary batteries prepared according to Example 2 and Comparative Example 3 of the present invention.

8 is a graph showing the first charge-discharge cycle characteristics of a coin battery prepared using a lithium secondary battery positive electrode active material and LiCoO ₂ prepared according to Example 2 of the present invention.

In order to achieve the above object, the present invention is to mix a metal solution and a chelating agent solution in which lithium salt, transition metal salt, cobalt salt and phosphorus salt is dissolved; Heating the mixture to prepare a gel; It provides a method for producing a cathode active material for a lithium secondary battery of the formula (1) comprising the step of heat-treating the gel.

[Formula 1]

Li _{1 + x} M _1-yz M ' _y P _z O ₄

(In Formula 1, M and M 'are at least one transition metal selected from the group consisting of Co, Ni, Fe, Mn, Al, Cr, Fe, Mg, Sr and V,

x is from 0 to 0.5,

y is 0 to 1,

z is 0 to 1,

0 ≦ y + z ≦ 1.).

The present invention also provides a cathode active material for a lithium secondary battery of Formula 1 having a single-phase olivine structure prepared by the above method.

Hereinafter, the present invention will be described in detail.

The present invention slightly changes the structure of LiCo _1-x MPO ₄ (0 ≦ _x ≦ 1, where M is a metal or a nonmetal), which is being studied as a material that can replace expensive LiCoO ₂ having excellent electrochemical properties, Provided is a method of preparing a cathode active material for a lithium secondary battery of Chemical Formula 1 using a chelating method which may be prepared at a lower temperature than a conventional solid phase reaction method.

[Formula 1]

Li _{1 + x} M _1-yz M ' _y P _z O ₄

(In Formula 1, M and M 'are at least one transition metal selected from the group consisting of Co, Ni, Fe, Mn, Al, Cr, Fe, Mg, Sr and V,

x is from 0 to 0.5,

y is 0 to 1,

z is 0 to 1,

0 ≦ y + z ≦ 1.).

In the production method of the present invention, first, a metal salt solution in which lithium salt, transition metal salt, cobalt salt and phosphorus salt is dissolved, and a chelating agent solution are mixed.

The metal salt solution is prepared by dissolving lithium salt, transition metal salt, cobalt salt and phosphorus salt in a solvent. Lithium nitrate, lithium acetate, lithium carbonate or lithium hydroxide may be used as the lithium salt, and the transition metal salt may be a transition of Co, Ni, Fe, Mn, Al, Cr, Fe, Mg, Sr or V. One or more nitrates, acetates, carbonates or hydroxides comprising metals can be used. Cobalt nitrate, cobalt hydroxide, cobalt carbonate or cobalt acetate may be used as the cobalt salt, and HPO ₃ or ammonium phosphate may be used as the phosphate salt. Distilled water, ethanol or methanol may be used as the solvent. The lithium salt, transition metal salt, cobalt salt and phosphorus salt are not limited to the compound.

It is preferable to use a water-soluble polymer as the chelating agent, and use a polymer having -OH, -O-, -COOH or an ester functional group capable of chelating metal ions on the side of the polymer chain. More preferred. Representative examples of the chelating agent include polyvinyl alcohol, polyethylene glycol, polyacrylic acid, polymethyl methacrylate or polyethylene oxide.

The chelating agent solution is prepared by measuring and dissolving the chelating agent in a solvent so as to be 0.1 to 10 times the total number of moles of metal ions of the metal salt solution. In preparing the solution, the temperature may be dissolved at about 70-80 ° C. to facilitate dissolution. The chelating agent is used by calculating the molecular weight of the monomer unit of the polymer to 1 mole, and 0.1 to 10 times the number of moles of metal ions used. If the polymer is less than 0.1 times the total number of moles of metal ions, the phase of the desired material does not form and if it is more than 10 times, the viscosity is too large to be difficult to synthesize the gel is undesirable. The solvent may dissolve the chelating agent, and any solvent may be used as long as it is compatible with the solvent of the metal salt solution. As a representative example, an alcohol such as water, ethanol, methanol, and a mixture of water and alcohol may be used. .

When the metal salt solution and the chelating agent solution are mixed, the chelating agent is chelated to the metal ions so that the metal ions and the chelating agent are uniformly distributed in the solution, and the mixed solution is heated at 70 to 80 ° C. Evaporation forms a gel.

This gel is then heat treated. The heat treatment process may be performed at 200 to 1000 ° C. for 5 to 20 hours. More preferably, the gel is first heat-treated at 300 to 500 ° C. for 3 to 5 hours to prepare an amorphous precursor, and then the second precursor is heat-treated at 400 to 800 ° C. for 5 to 15 hours. Low risk of explosion At this time, the temperature increase rate is preferably as slow as possible at a rate of 0.1 to 3 ℃ / min. If the temperature increase rate is higher than 3 ° C / min, it is not preferable because there is a risk of exploding the gel due to rapid pyrolysis reaction upon heating.

According to the heat treatment process, a cathode active material for a lithium secondary battery of Chemical Formula 1 having a single phase olivine structure is prepared.

[Formula 1]

Li _{1 + x} M _1-yz M ' _y P _z O ₄

(In Formula 1, M and M 'are at least one transition metal selected from the group consisting of Co, Ni, Fe, Mn, Al, Cr, Fe, Mg, Sr and V,

x is from 0 to 0.5,

y is 0 to 1,

z is 0 to 1,

0 ≦ y + z ≦ 1.).

As the organic polymer is used as a chelating agent, the uniformity of the medium is ensured, the amount of the chelating agent, the type of the metal salt, and the temperature and time of the heat treatment process are controlled to adjust the size of the active material particles, the shape of the particles, the surface properties, etc. Can be arbitrarily controlled and a single phase active material can be prepared without the formation of a minor phase.

The cathode active material of Chemical Formula 1 prepared by the above method has only a single phase olivine structure without an impurity phase in the XRD pattern, and thus is distinguished from a cathode active material having both an impurity phase and an olivine structure prepared by a conventional solid phase method. Compared with the positive electrode active material manufactured by the solid phase method, the electrochemical activity is very excellent, and thus a lithium secondary battery having excellent power may be provided.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are merely examples of the present invention and the present invention is not limited to the following examples.

(Comparative Example 1)

LiOH, Co ₃ O ₄ and (NH ₄ ) ₂ HPO ₄ as starting materials were measured so that the equivalence ratio of Li / Co / P is 1/1/1, and then ethanol was used as a grounding media, The mixture was prepared by mixing for about 30 minutes at. The mixture thus prepared was placed in a heat treatment crucible and subjected to primary heat treatment at 450 ° C. for 5 hours and then ground. Subsequently, the first heat-treated product was subjected to a second heat treatment while blowing dry air at 600 ° C. for 10 hours. The heat-treated product was ground in a mortar and pestle to make fine powder, and then classified using a sieve of 325 mesh or less to prepare LiCoPO ₄ , which is a cathode active material for a 5V lithium secondary battery.

(Comparative Example 2)

A material subjected to heat treatment at 450 ° C. for 5 hours was performed in the same manner as in Comparative Example 1 except that the heat treatment was performed at 700 ° C. for 10 hours under a flow of air.

(Comparative Example 3)

It carried out similarly to the said comparative example 2 except having changed the secondary heat processing temperature from 700 degreeC to 800 degreeC.

(Example 1)

LiOH, Co ₃ O ₄ , (NH ₄ ) ₂ HPO ₄ was measured so that the equivalence ratio of Li / Co / P is 1/1 / 1 and dissolved in distilled water to prepare a metal salt solution. In another beaker, a polyvinyl alcohol solution was prepared by dissolving polyvinyl alcohol corresponding to 0.5 times the total moles of metal ions (moles of polyvinyl alcohol / moles of metal (Li + Co + P) = 0.5) in distilled water.

The metal salt solution and the polyvinyl alcohol solution were mixed with each other and maintained at a temperature of about 70 ° C., and the solvent was evaporated. When the solvent was almost evaporated to a gel state, the solvent was transferred to a heat treatment vessel to prepare a precursor by performing a first heat treatment in an air flowing atmosphere at 350 ° C. for 3 hours.

The prepared precursor was ground finely in a mortar and then subjected to a second heat treatment at 400 ℃ for 10 hours. After the heat treatment powder was again pulverized finely in a bowl and classified using a sieve of 325 mesh or less to prepare a LiCoPO ₄ positive electrode active material for 5V class lithium secondary battery.

(Example 2)

Polyvinyl alcohol was carried out in the same manner as in Example 1 except that polyvinyl alcohol was used in an amount corresponding to 1 times the total moles of metal ions (moles of polyvinyl alcohol / moles of metal (Co + Li + P) = 1).

(Example 3)

A second heat treatment was performed in the same manner as in Example 2 except that the heat treatment was performed at 800 ° C. for 10 hours.

(Example 4)

Polyvinyl alcohol was carried out in the same manner as in Example 1 except that polyvinyl alcohol was used in an amount corresponding to three times the total moles of metal ions (moles of polyvinyl alcohol / moles of metal (Li + Co + P) = 3).

(Example 5)

Polyvinyl alcohol was carried out in the same manner as in Example 1 except that polyvinyl alcohol was used in an amount corresponding to 4 times the total moles of metal ions (moles of polyvinyl alcohol / moles of metal (Li + Co + P) = 4).

SEM photographs of LiCoPO ₄ prepared by the method of Examples 1 and 3 are shown in FIGS. 1 and 2. A SEM photograph of LiCoPO ₄ prepared by the method of Comparative Example 3 is shown in FIG. 3A, and the SEM photograph of FIG. 3A is enlarged 4 times and shown in FIG. 3B. As shown in FIGS. 1, 2 and 3A-3B, LiCoPO ₄ subjected to secondary heat treatment at a low temperature (Examples 1 and 400 ° C.) exhibits an amorphous structure, but high temperature (Examples 3 and Comparative Examples). It can be seen that LiCoPO 4 heat-treated at 3, 800 ° C.) has a crystalline structure.

The XRD pattern of LiCoPO ₄ prepared by the solid phase reaction method was measured while varying the heat treatment temperature of Comparative Examples 1 to 3, and is shown in FIG. 4. In Example 2, the precursor prepared by 350 ° C. first heat treatment, and the precursor were 400 XRD patterns of the active material prepared by secondary heat treatment at ° C and the active material prepared by secondary heat treatment at 800 ° C of Example 3 (LiCoPO ₄ ) are shown in FIG. 5. As shown in FIGS. 4 and 5, LiCoPO ₄ compounds prepared by the methods of Comparative Examples 1 to 3 are observed in addition to the LiCoPO ₄ phase, while other phases are observed, whereas the precursor and active material prepared by the method of Example 2 and Example 3 It can be seen that in the active material prepared by the method, only a single phase of LiCoPO ₄ is observed.

In addition, in order to determine the difference between the charging and discharging potentials of LiCoO ₂ and LiCoPO ₄ , CV voltamograms of LiCoO ₂ (C-10 by Nippon Chem) are shown in FIG. 6, in Comparative Examples 3 and 2, respectively. CV voltamograms of LiCoPO ₄ prepared by the method are shown in FIG. 7. 6 and 7, the charge and discharge potentials of LiCoO ₂ are 4.03-4.05V and 3.8 to 3.85V, respectively, whereas the charge and discharge potentials of LiCoPO ₄ are 5.3V (Example 2) and 5.2V ( Comparative Example 3) and about 4.55 V (Example 2) and 4.6 V (Comparative Example 3) show that the charge and discharge potential of LiCoPO ₄ is higher than that of LiCoO ₂ . As a result, it can be seen that LiCoO ₂ is a 4V class positive electrode active material, and LiCoPO ₄ is a 5V class positive electrode active material. In addition, it can be seen that As can be seen in 7, LiCoPO ₄ Example 2 Comparative Example 3 because you left the strength of the oxidation-reduction peak is much bigger than LiCoPO ₄ the electrochemically active compound is greater in the.

The charge and discharge characteristics of the first charge and discharge cycles of LiCoPO ₄ and LiCoO ₂ (C-10 by Nippon Chem) of Example 2 are shown in FIG. 8. As shown in FIG. 8, it can be seen that LiCoPO ₄ of Example 2 has a much higher charge and discharge potential than LiCoO ₄ .

As described above, the production method of the present invention can be heat-treated at a lower temperature than the conventional solid phase reaction method, it is possible to synthesize a single phase material, it is possible to produce an active material with more excellent electrochemical activity. In addition, the electrochemically excellent activity, when using this positive electrode active material can provide a lithium secondary battery with more excellent power (power).

Claims (6)

(delete) (Tablet) a metal salt solution in which lithium salt, transition metal salt, cobalt salt and phosphorus salt are dissolved and a water-soluble polymer chelating agent solution are mixed; Heating the mixture to prepare a gel; Heat treating the gel; Method for producing a positive electrode active material for lithium secondary batteries of the general formula (1) comprising a step. [Formula 1] Li _{1 + x} M _1-yz M ' _y P _z O ₄ (In Formula 1, M and M 'is at least one transition metal each selected from the group consisting of Co, Ni, Fe, Mn, Al, Cr, Fe, Mg, Sr and V) x is from 0 to 0.5, y is 0 to 1, 0 <z ≤ 1, 0 ≤ y + z ≤ 1. (Correction) The method according to claim 2, wherein the water-soluble polymer comprises a functional group selected from the group consisting of -OH, -O-, ether, -COOH, and an ester capable of chelating metal ions at the branch of the polymer chain. The manufacturing method. (Correction) The production method according to claim 2, wherein the heat treatment is performed at 200 to 1000 ° C for 5 to 20 hours. (Correction) The production method according to claim 2, wherein the heat treatment is performed first at 300 to 500 ° C. for 3 to 5 hours and second at 400 to 800 ° C. for 5 to 15 hours. Mixing a metal salt solution in which lithium salt, transition metal salt, cobalt salt, and phosphorus salt is dissolved with a water-soluble polymer chelating agent solution; Heating the mixture to prepare a gel; Prepared by the process of heat-treating the gel A cathode active material for a lithium secondary battery of Formula 1 having a single phase olivine structure. [Formula 1] Li _{1 + x} M _1-yz M ' _y P _z O ₄ (In Formula 1, M and M 'is at least one transition metal each selected from the group consisting of Co, Ni, Fe, Mn, Al, Cr, Fe, Mg, Sr and V) x is from 0 to 0.5, y is 0 to 1, 0 <z ≤ 1, 0 ≤ y + z ≤ 1.