KR20240062088A - Positive-electrode active material comprising Iron doped Lithium rich oxide for lithium secondary battery and manufacturing method thereof - Google Patents

Abstract

The present invention relates to an iron-doped lithium excess oxide cathode active material for lithium ion secondary batteries and a method for manufacturing the same. The present invention relates to a method for manufacturing Li ₂ MnO ₃ by doping it with inexpensive iron without using expensive transition metals Co and Ni at all, thereby reducing manufacturing costs. , improvements in structural stability and rate characteristics can be expected.

Description

Iron-doped lithium rich oxide cathode active material for lithium ion secondary battery and manufacturing method thereof {Positive-electrode active material comprising Iron doped Lithium rich oxide for lithium secondary battery and manufacturing method thereof}

The present invention relates to a cathode active material for lithium ion secondary batteries and a method of manufacturing the same, and specifically to a lithium excess oxide cathode active material doped with iron excluding cobalt and nickel, and a method of manufacturing the same.

Recently, as the production of renewable energy, an eco-friendly power generation method, has increased to reduce carbon emissions, the power generation is not constant and fluctuates, and the installation of energy storage systems (ESS) that use secondary batteries for stable storage and supply of electric energy is expanding. The demand for batteries has increased rapidly. Among secondary batteries, lithium-ion batteries (LIB) using cathode materials with a layered structure have the highest energy density, so research has been conducted most actively and they have been commercialized.

Among the currently commercialized LIB anode materials, the most widely used battery with high energy density is the NCM battery, which is a ternary battery of nickel (Ni), cobalt (Co), and manganese (Mn). Recently, as a way to further improve the energy density of this NCM battery, the content of nickel (Ni), which has a high reversible capacity among nickel (Ni), cobalt (Co), and manganese (Mn), has been increased, resulting in nickel (Li). ) content has increased to account for more than 60 to 90%. As a result, demand has increased explosively, and the nickel inventory on the London Metal Exchange has recently decreased by one-third over the past two years, causing prices to rise as well. In addition, cobalt, which is used for the stability of batteries, has limited reserves and areas, so it is the most expensive among battery raw materials.

Therefore, research is being conducted on Li ₂ MnO ₃ (LMO), an overlithiated layered oxide (OLO) material, which is one of the next-generation battery anode materials that can reduce dependence on cobalt and nickel among currently used battery raw materials. . This material has a very high theoretical capacity of 458 mAh/g because more Li, which can develop battery capacity, is inserted than the existing layered structure, but it is electrochemically inactive because manganese (Mn) exists in a +4 valence immediately after synthesis. Since the Coulombic efficiency in the initial cycle is low and the lifetime stability is low due to structural collapse during the electrochemical activation process, research is being conducted on various methods such as doping and surface coating to solve this problem. In order to achieve reversible capacity and stability, nickel (Ni) and cobalt (Co) are being used again.

Republic of Korea Patent No. 10-2519556

The purpose of the present invention is to achieve price competitiveness by doping lithium excess oxide cathode active material with inexpensive iron, excluding high-cost cobalt and nickel raw materials, to have excellent electrochemical performance, and to overcome the disadvantages of the existing cathode active material with Li ₂ MnO ₃ structure. The aim is to provide a cathode active material with significantly improved low cycle stability and rapid reversible capacity reduction, and a method for manufacturing the same.

In order to achieve the above object, lithium manganese oxide (LMO) is doped with iron, and a positive electrode active material represented by the following formula (1) is provided.

[Formula 1]

Li ₂ Mn _1-x Fe _x O ₃

In Formula 1, 0 < x < 1.

Additionally, the present invention provides a positive electrode for a lithium ion secondary battery containing a positive electrode active material according to Formula 1 above.

In addition, the present invention includes the step of mixing a lithium (Li) supply compound, a manganese (Mn) supply compound, and an iron (Fe) supply compound in a solvent, and then adding a chelating agent and a basic solution (first step). ; Obtaining a gel by drying the reaction solution obtained in the first step (second step); Obtaining a precursor by heat treating the gel (third step); and heat-treating the precursor (fourth step). It provides a method for producing a positive electrode active material according to Chemical Formula 1, including a step.

The cathode active material according to the present invention does not use any of the expensive transition metals cobalt (Co) and nickel (Ni) in Li ₂ MnO ₃ and dopes it with iron, a cheap and environmentally friendly transition metal, resulting in cost savings and the existing LiNiMnCoO ₂ ( It has the advantage of having excellent performance with a capacity comparable to that of an NCM battery and long-term cycle stability.

Figure 1 is a schematic diagram of manufacturing an iron-doped lithium excess oxide cathode active material.
Figure 2 is an X-ray diffraction analysis (XRD) graph of Preparation Example 1 and Comparative Example 1 according to the present invention.
Figure 3 is an XRD analysis graph of Example and Comparative Example 2 according to the present invention.
Figure 4 is a scanning electron microscope (SEM) image of Example and Comparative Example 2 of the present invention.
Figure 5 is a graph of imaging and X-ray photoelectron spectroscopy (XPS) of Example and Comparative Example 2 according to the present invention.
Figure 6 shows the electrochemical performance evaluation results of Example and Comparative Example 2 according to the present invention.
Figure 7 is a graph of the galvanostatic intermittent titration technique (GITT) and lithium ion diffusion coefficient of Example 3 and Comparative Example 2 according to the present invention.
Figure 8 is a graph of electrochemical impedance spectroscopy (EIS) results of Example 3 and Comparative Example 2 according to the present invention.
Figure 9 is a high-resolution transmission electron microscope (HRTEM) image of Example 3 and Comparative Example 2 according to the present invention after a 100-cycle electrical charge and discharge test.
Figure 10 is a limited-field electron diffraction pattern (SAED) image of Example 3 and Comparative Example 2 according to the present invention.

Hereinafter, the present invention will be described in detail.

The present invention provides a cathode active material in which lithium manganese oxide (LMO) is doped with iron and represented by the following formula (1).

[Formula 1]

Li ₂ Mn _1-x Fe _x O ₃

In Formula 1, 0 < x < 1.

In Formula 1, x may be 0.02 to 0.08.

If x is outside the range, the effect of expanding the interplanar distance between the interlayers, which helps improve the Li ⁺ diffusion rate, is reduced, and the length of the plane increases, which may cause a problem in which the reaction is inhibited due to an increase in the Li ⁺ diffusion distance. there is.

Additionally, the present invention provides a positive electrode for a lithium ion secondary battery containing a positive electrode active material according to Formula 1 above.

In addition, the present invention includes the step of mixing a lithium (Li) supply compound, a manganese (Mn) supply compound, and an iron (Fe) supply compound in a solvent, and then adding a chelating agent and a basic solution (first step). : Obtaining a gel by drying the reaction solution obtained in the first step (second step): Obtaining a precursor by heat-treating the gel (third step): and heat-treating the precursor (fourth step): It provides a method for producing a positive electrode active material according to Chemical Formula 1, including.

The lithium (Li) supply compound in the first step may be selected from one or more of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium acetate, but is limited thereto. That is not the case.

The manganese (Mn) supply compound in the first step may be selected from one or more of manganese nitrate, manganese acetate, manganese chloride, and manganese sulfate, but is limited thereto. That is not the case.

The iron (Fe) supply compound in the first step may be selected from one or more of iron sulfate, iron chloride, and iron nitrate, but is not limited thereto.

The chelating agent in the first step may be one or more selected from EDTA (ethylene-diamine-tetraacetic acid), DHEG (Dihydroxyethyl Glycine), and citric acid, but is not limited thereto.

The solution in the first step can be mixed by stirring at 60 to 100 °C for 30 to 80 hours. If stirred below 60°C, reaction may not be possible in a uniform state because gelation does not occur and the particles settle during the temperature increase during heat treatment. If stirring exceeds 100°C, the solution may boil and dry, forming a gel with a non-uniform composition.

The third step may be heat treatment at 400 to 500°C for 3 to 7 hours. If heat treatment is performed below 400°C, the layered structure may not be properly formed. If heat treatment exceeds 500°C, lithium ions may be inserted incompletely.

In the fourth step, heat treatment may be performed at 850 to 950° C. for 3 to 7 hours in an O ₂ atmosphere. When heat treated below 850°C, life stability may be drastically reduced. If heat treatment exceeds 950℃, activation may be difficult and battery capacity may decrease.

Hereinafter, the present invention will be described in detail through examples to aid understanding. However, the following examples only illustrate the content of the present invention and the scope of the present invention is not limited to the following examples. Examples of the present invention are provided to more completely explain the present invention to those skilled in the art.

<Preparation Example 1> Preparation of cathode active material precursor

20.5 mmol of lithium nitrate, 9.8 mmol of manganese nitrate, and 0.2 mmol of iron nitrate were added to 50 ml of distilled water and stirred for 1 hour. EDTA (Ethylenediaminetetraacetic Acid), a chelating agent, was added to the mixed solvent in the same molar quantity as the metal salt and stirred for another 1 hour. Afterwards, ammonia water (NH ₄ OH solution) was added to adjust the pH to 8, stirred at 80°C for 48 to 60 hours, and dried to gelate and prepare a Xerogel-type wet gel. Afterwards, the temperature was raised to 400°C and 5°C/min in a muffle furnace, and heat treatment was performed for 5 hours to prepare a positive electrode active material precursor.

<Comparative Example 1>

A positive electrode active material precursor was prepared in the same manner as in Preparation Example 1 except that it was heat treated at 300° C. in a muffle furnace.

<Example 1> Production of positive electrode active material

The cathode active material precursor was heat-treated on an alumina boat using a tube furnace in an O ₂ atmosphere at 200 sccm, 850 ℃, 5 ℃/min for 5 hours to prepare the cathode active material (Li ₂ Mn _0.98 Fe _0.02 O ₃ , LMFO-2).

<Example 2> Production of positive electrode active material

A positive electrode material precursor was prepared in the same manner as in Preparation Example 1 except that 9.6 mmol of manganese nitrate and 0.4 mmol of iron nitrate were added, and a positive electrode active material was prepared according to Example 1 (Li ₂ Mn _0.96 Fe _0.04 O ₃ , LMFO-4).

<Example 3> Production of positive electrode active material

A positive electrode material precursor was prepared in the same manner as in Preparation Example 1 except that 9.4 mmol of manganese nitrate and 0.6 mmol of iron nitrate were added, and a positive electrode active material was prepared according to Example 1 (Li ₂ Mn _0.94 Fe _0.06 O ₃ , LMFO-6).

<Example 4> Production of positive electrode active material

A positive electrode material precursor was prepared in the same manner as in Preparation Example 1 except that 9.2 mmol of manganese nitrate and 0.8 mmol of iron nitrate were added, and a positive electrode active material was prepared according to Example 1 (Li ₂ Mn _0.92 Fe _0.08 O ₃ , LMFO-8).

<Comparative Example 2>

A positive electrode material precursor was prepared in the same manner as in Preparation Example 1 except that 10 mmol of manganese nitrate was added and iron nitrate was excluded, and a positive electrode active material was prepared according to Example 1 (Li ₂ MnO ₃ , LMFO-0).

<Experimental Example 1> Evaluation of structural properties of cathode active material precursor

In Figure 2, X-ray diffraction (XRD) analysis was performed on Preparation Example 1 and Comparative Example 1 to confirm the characteristics of the precursor. The heat treatment process is a process of removing the EDTA polymer that formed the matrix with metal ions to obtain only the oxide. Since EDTA polymer begins to spontaneously ignite around 300 ℃, the minimum required temperature for this process can be said to be 300 ℃. In Comparative Example 1, in which heat treatment was performed at 300°C, not only overlithiated layered oxide (OLO) with a layered structure was formed, but also LiMn ₂ O ₄ with a spinel structure. However, in Preparation Example 1, an oxide with a spinel structure was not formed. When heat treatment of the cathode active material precursor was performed at 400°C, Li ₂ MnO ₃ with a high level of crystallinity in a layered structure could be obtained.

<Experimental Example 2> Evaluation of structural characteristics of positive electrode active material

To analyze the structure of the embodiment of the present invention, X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were performed, and the results are shown in Figures 3 to 5.

Figure 3(a) is an XRD analysis graph of Example and Comparative Example 2 according to the present invention. Examples 1 to 4 and Comparative Example 2 confirmed the formation of a layered structure through peaks on the (001), (-133), and (33-1) planes. The formation of a Li _1/3 Mn _2/3 superstructure was confirmed through the peaks of the (020) and (110) planes, confirming the formation of Li ₂ MnO ₃ with an overall monoclinic C2/m structure. . In addition, it was confirmed that iron was not separated and was evenly doped, as no new peaks appeared even after adding iron. Figure 3(b) is an enlarged graph of the peak of the (001) plane among the XRD analysis graphs of Example and Comparative Example 2 according to the present invention. As the doping concentration of iron increased, the (001) plane, which can determine the distance between the plate planes, moved to the left, confirming that the size of the passage through which lithium ions move increased.

Figure 4 is a scanning electron microscope (SEM) image of Example and Comparative Example 2 according to the present invention. Figure 4(a) is Comparative Example 2, Figure 4(b) is Example 1, Figure 4(c) is Example 2, Figure 4(d) is Example 3, and Figure 4(e) is Example 4. This is an SEM image. It was confirmed that the positive electrode active material manufactured according to an example of the present invention was mainly formed in a large plate shape and was evenly dispersed throughout. This dispersed state forms a wider electrode/electrolyte interface, enabling rapid insertion/desorption of lithium ions. In addition, it was confirmed that the shape of individual particles was not deformed even when iron was doped, confirming that the doping was done without affecting the existing structure.

Figure 5 is an XPS graph of Example and Comparative Example 2 according to the present invention. Figure 5(a) is the Mn 2p spectrum, and Figure 5(c) is the O 1s spectrum. It was confirmed that when iron, which exists as 2+/3+, is doped instead of Mn, which exists as 3+/4+, the ratio of Mn ⁴⁺ increases and the ratio of oxygen vacancies (O _vac ) increases to match the charge load. . The oxygen vacancies formed in this way help activate the positive electrode active material and contribute to improving reversible capacity. Figure 5(b) is the Fe 2p spectrum, confirming that the doping was uniform as a constant Fe ^2+/3+ ratio was formed regardless of the iron doping concentration. Since the bond dissociation energy of iron and oxygen is greater than that of manganese and oxygen, the bond length is shortened, reducing the gap between the transition metal layers, and increasing the gap between the lithium layers, which improves structural stability and rate characteristics.

<Experimental Example 3> Evaluation of electrochemical properties of positive electrode active material

1. Half-cell electrode assembly

In order to confirm the electrochemical properties of the positive electrode active material according to the present invention, the positive electrode active material prepared in Example and Comparative Example 2 was mixed with a conductive material (Ketjen black) and a binder (Polyvinylidene fluoride, PVdF) in a ratio of 7:2:1, respectively. A slurry was prepared with N-Methyl-2-pyrrolidone (NMP) as a solvent. The prepared slurry was uniformly cast on the current collector using a doctor blade. Afterwards, the thickness was measured after drying at 100°C for 2 hours, and pressing was performed at 80% of the measured thickness. The pressed electrode was punched into a circular shape measuring 13 pi. Coin cell assembly for half-cell testing was performed in an argon-filled glove box. 1 M LiPF ₆ dissolved in ethylene carbonate and diethyl carbonate was used as the liquid electrolyte, polypropylene was used as a separator, and lithium metal was used as the cathode. Half-cell electrode assembly was performed using the method.

2. Electrochemical performance evaluation

Figure 6 shows the results of evaluating the electrochemical performance of the half-cell electrode assembled using Example and Comparative Example 2 according to the present invention. Figure 6 shows the results of the cycle test (left) and rate characteristic test (right). The cycle test was conducted at a current density of 20 mA/g. Rate characteristic tests were conducted at current densities of 20, 40, 80, 100, 200, and 20 mA/g. Through iron doping, the reversible capacity was 174 (Comparative Example 2), 184 (Example 1), 195 (Example 2), 222 (Example 3), and 191 (Example 4) mAh/g, Example 3 This showed the best results. In terms of cycle stability and rate characteristics, Example 3 showed the best performance, and the doping concentration of iron was optimized.

Figure 7 is a graph of galvanostatic intermittent titration technique (GITT) (left) and average lithium ion diffusion coefficient (right). In order to confirm the effect of iron doping, Example 3 and Comparative Example 2 were subjected to galvanostatic intermittent titration technique (GITT). It was confirmed that the average discharge voltage drop (iRdrop) occurring during the charging and discharging process was significantly reduced as iron was doped. Based on the graph of Example 3 and Comparative Example 2 analyzed by galvanostatic titration technique (GITT), the lithium ion diffusion coefficient was calculated for each 10% remaining capacity (state of charge), and the average lithium ion diffusion coefficient was shown. . The average lithium ion diffusion coefficient was confirmed to be 6.9x10 ^-11 cm ² /s in Comparative Example 2 and 2.5x10 ^-10 cm ² /s in Example 3. It was confirmed that the lithium ion diffusion coefficient of Example 3 was more than 3 times improved than the lithium ion diffusion coefficient of Comparative Example 2.

Figure 8(a) shows electrochemical impedance spectroscopy (EIS) performed after coin cell assembly using Example 3 and Comparative Example 2. The charge transfer resistance (R _ct ), which is the resistance corresponding to the diameter of the semicircle, was confirmed to be 730 Ω in Example 3 and 616 Ω in Comparative Example 2. R _ct decreased due to the doping effect of iron. Figures 8(b) and (c) show the results of EIS analysis again after a long-term charge/discharge test of 100 cycles at a current density of 20 mA/g. In Figure 8(c), the section 1 to 5k in Figure 8(b) is enlarged. R _ct of Comparative Example 2 increased so significantly that it was not fully specified in the same test frequency range and exceeded 60 ㏀. In Example 3, R _ct was approximately 3.7 kΩ, confirming that the increase in resistance was suppressed when compared to Comparative Example 2. The reason why R _ct decreased is because the structural stability was greatly improved by doping with iron.

<Experimental Example 4> Structural analysis after cycle test

Example 3 and Comparative Example 2 were subjected to a long-term charge/discharge test of 100 cycles at a current density of 20 mA/g. After the above test, the cells were disassembled to obtain particles of Example 3 and Comparative Example 2, and then high-resolution transmission electron microscopy (HRTEM) was taken. Figure 9(a) is an HRTEM image of Comparative Example 2, in which a very deep structural change occurred with a phase transition depth of about 70 to 170 nm. Figure 9(b) is an HRTEM image of Example 3, and it was confirmed that the phase transition depth was about 26 nm, so the occurrence of irreversible phase transition was very small. These results confirmed that iron doping significantly contributed to improving structural stability.

Figure 10 is a limited-field electron diffraction pattern (SAED) image, and the occurrence of the phase transition observed in Figure 9 was confirmed by the diffraction pattern. Figure 10(a) shows the results before a 100 cycle long-term charge/discharge test, confirming that the reciprocal lattice points are clear. Figure 10(b) shows the SAED results of Comparative Example 2 after the 100 cycle long-term charge/discharge test. Figure 10(c) shows the SAED results of Example 3 after the 100 cycle long-term charge/discharge test. In Figures 10(b) and (c), it was confirmed that in Example 3 and Comparative Example 2, specific reciprocal lattice points disappeared due to spinel phase formation and superlattice structure collapse due to phase transition on the particle surface.

As the specific parts of the present invention have been described in detail above, it is clear to those skilled in the art that these specific techniques are merely preferred embodiments and do not limit the scope of the present invention. do. That is, the practical scope of the present invention is defined by the appended claims and their equivalents.

Claims (11)

Lithium manganese oxide (LMO) is doped with iron, and the positive electrode active material is represented by the following formula 1:
[Formula 1]
Li ₂ Mn _1-x Fe _x O ₃
In Formula 1, 0 < x < 1. The cathode active material of claim 1, wherein in the formula, x is 0.02 to 0.08. A positive electrode for a lithium ion secondary battery containing the positive electrode active material according to claim 1. Mixing a Li supply compound, a Mn supply compound, and an Fe supply compound in a solvent, then adding a chelating agent and a basic solution (first step);
Obtaining a gel by drying the reaction solution obtained in the first step (second step);
Obtaining a precursor by heat treating the gel (third step); and
A method for producing a positive electrode active material according to claim 1, comprising: heat-treating the precursor (fourth step). The method of claim 4, wherein the Li supply compound in the first step is at least one selected from the group consisting of lithium hydroxide, lithium carbonate, lithium nitrate, and lithium acetate. A method of producing a positive electrode active material comprising. The method of claim 4, wherein the Mn supply compound in the first step is one selected from the group consisting of manganese nitrate, manganese acetate, manganese chloride, and manganese sulfate. A method for producing a positive electrode active material including the above. The method of claim 4, wherein the Fe supply compound in the first step includes at least one selected from the group consisting of iron sulfate, iron sulfide, iron chloride, and iron nitrate. Method for manufacturing cathode active material. The method of claim 4, wherein the chelating agent in the first step is at least one selected from ethylene-diamine-tetraacetic acid (EDTA), dihydroxyethyl glycine (DHEG), and citric acid. The method of claim 4, wherein the solution of the first step is mixed by stirring at 60 to 100° C. for 30 to 80 hours. The method of claim 4, wherein the third step of heat treatment is performed at 400 to 500° C. for 3 to 7 hours. The method of claim 4, wherein the heat treatment in the fourth step is performed at 850 to 950° C. for 3 to 7 hours in an O ₂ atmosphere.