KR20240115462A - An olivine cathode active material including a hybrid coating film and its production method - Google Patents

Abstract

The present invention relates to an olivine positive electrode active material including a hybrid coating film and a method of manufacturing the same. By forming a hybrid coating layer containing amorphous carbon and a solid electrolyte on the surface of the positive electrode active material, excellent rate capability and cycle stability are achieved. By achieving this, there is the advantage of improving the overall electrochemical properties.

Description

An olivine cathode active material including a hybrid coating film and its production method}

The present invention relates to an olivine positive electrode active material including a hybrid coating film and a method of manufacturing the same.

Recently, portable electronic devices for information and communication such as mobile phones, personal digital assistants (PDAs), and notebook PCs, portable electronic devices such as digital cameras, camcorders, and MP3s, and electric bicycles and electric vehicles are continuously charged and discharged. The demand for used secondary batteries is increasing exponentially. Currently commercially available lithium batteries use LiCoO ₂ for the anode and carbon for the cathode.

However, cobalt, the starting material for positive electrode active materials, has limited reserves and is expensive, and there is a need to develop alternative positive electrode materials due to problems with toxicity to the human body and environmental pollution. Positive electrode active materials currently being actively researched and developed include LiNiO ₂ , LiCo _x Ni _1-x O ₂ and LiMn ₂ O ₄ . LiNiO ₂ , which has a layered structure like LiCoO _{2 ,} is not commercialized due to difficulties in synthesizing materials at stoichiometric ratios and problems with thermal stability, while some of LiMn ₂ O ₄ are commercialized in low-priced products. . LiMn ₂ O ₄ , a 4V-class spinel positive electrode active material, has the advantage of using manganese as a starting material, but has poor lifespan characteristics due to a structural variation called Jahn-Teller distortion caused by Mn ³⁺ .

Therefore, there is a need for a positive electrode active material that is more economical, stable, has high capacity, and has good cycle characteristics. Currently _, compounds with _an olivine structure as positive electrode active materials for lithium batteries have _the chemical formula Li ) can be given as an example.

_{Among the} compounds represented _{by Li} LiFePO ₄ is environmentally friendly, has abundant reserves, and the raw material price is very cheap. In addition, low power and low voltage can be realized more easily than existing positive electrode active materials, and the battery capacity is also excellent with a theoretical capacity of 170 mAh/g.

However, the solid-phase reaction method and wet reaction method, which are the synthesis methods of olivine-type LiFePO _{4 ,} which is the precursor of LiFePO ₄ , are unable to control particle size and particle shape, and it is difficult to synthesize a powder composed of single particles. In other words, a new method for synthesizing olivine-type positive electrode active materials that suppresses a small specific surface area and has a high volumetric energy density is needed.

LiMnPO ₄ (4.4 V vs. Li/Li ⁺ ), LiCoPO ₄ (4.8 V vs. Li/Li ⁺ ) and LiNiPO ₄ (5.1 V vs. Li/Li ⁺ ) have higher operating voltages and are superior to commercial LiFePO ₄ (3.4 V vs. Li/Li + ). It has a higher energy density than V vs. Li/Li ⁺ ). Therefore, if a solid solution of olivine material combining Fe, Mn, Co, and Ni is developed, not only can it have relatively high conductivity due to Fe, but it can also have higher energy density due to ions such as Mn, Co, and Ni.

Olivine materials have inherently low conductivity, so lithium ions and electrons become unbalanced. This limits lithium ion insertion and extraction and deteriorates the electrochemical properties of the olivine cathode material. Lithium ion conductive coatings are important because the lithium ion conductivity provided by carbon coatings is much less than the electronic conductivity. The electron and lithium ion hybrid conductive layer will balance the conductivity of lithium ions and electrons. Therefore, the coated material can improve electrochemical performance.

Republic of Korea Patent No. 10-2245002 (registered on April 21, 2021)

The purpose of the present invention is to form a hybrid coating layer containing amorphous carbon and solid electrolyte on the surface of a positive electrode active material, thereby achieving excellent rate capability and cycle stability, thereby improving the overall electrochemical properties and a positive electrode active material thereof. The purpose is to provide a manufacturing method.

In order to achieve the above object, the present invention provides a positive electrode active material having an olivine structure and a coating layer comprising a lithium ion conductive solid electrolyte and a carbon material applied on the surface of the positive electrode active material and satisfying the following formula (1). Provided is a positive electrode active material comprising:

[Formula 1]

Li _1+x R _x M _2-x (PO ₄ ) ₃

In Formula 1, R is one or more metals selected from the group consisting of Al, B, Sn, and Ge, and M is one or more transition metals selected from the group consisting of Ti, Ge, and Hf, and 0≤x<2.

Additionally, the present invention provides an electrode containing the positive electrode active material.

Additionally, the present invention provides a lithium secondary battery including the electrode.

In addition, the present invention includes a first step of preparing an olivine precursor, a solid electrolyte precursor, and a carbon precursor; A second step of synthesizing olivine precursor; A third step of mixing a coating precursor including the solid electrolyte precursor and a carbon precursor with the olivine precursor; and a fourth step of synthesizing the coating precursor with the olivine precursor.

The present invention forms a hybrid coating layer containing amorphous carbon and solid electrolyte on the surface of the olivine positive electrode active material, provides a movement path for electrons and lithium ions between the surface or particles of the olivine positive electrode active material, and provides excellent rate characteristics. By achieving (rate capability) and cycle stability, we provide a positive electrode active material with improved overall electrochemical properties.

Figure 1 is an (b) pristine LFMCP, LFMCP@C and LFMCP@C_LATP; and (c) LATP.
Figure 2 is an EDS image and a high-resolution transmission electron microscopy (HR-TEM) image of a positive electrode active material manufactured according to an embodiment of the present invention, (a) is LFMP according to the distribution of each element This is an EDS layered image of @C_LATP in which (b) Fe, (c) Mn, (d) Ti, (e) P, and (f) C are distributed, respectively. (g) is the TEM image at the same location as the EDS image, and (h) is the HR-TEM image of the material in the region marked with a square in (a, b, d, f and g).
Figure 3 shows an EDS image and a TEM image of the positive electrode active material manufactured according to an embodiment of the present invention, (a) shows an EDS layered image of LFMCP@C_LATP according to the distribution of each element, (b) Fe, (c) Mn, respectively. , (d)Co, (e)Ti, (f)P, (g)C. (h) is the TEM image in the region marked with a square in (a, b, e and g).
Figure 4 is a graph showing Raman spectra (a, c) and thermogravimetric analysis (TGA) profiles (b, d) of a positive electrode active material manufactured according to an embodiment of the present invention. Figures 4(a) and 4(b) are LFMP@C and LFMP@C_LATP, and Figures 4(c) and 4(d) are result graphs of LFMCP@C and LFMCP@C_LATP.
Figure 5 is a graph evaluating the electrochemical properties of an electrode using pristine LFMP, LFMP@C, and LFMP@C_LATP, which are cathode active materials manufactured according to an embodiment of the present invention, (a) at a scan rate of 100 mV/s. Current-voltage profile, (b) charge/discharge profile at 0.05 C, (c) C-rate performance, and (d) long-term cycle performance at 1.0 C (operating voltage range: 2.0∼4.5 V, vs Li/Li) ⁺ ).
Figure 6 is a graph evaluating the electrochemical properties of an electrode using pristine LFMCP, LFMCP@C, and LFMCP@C_LATP, which are cathode active materials manufactured according to an embodiment of the present invention, (a) at a scan rate of 100 mV/s. Current-voltage profile, (b) charge/discharge profile at 0.05 C, (c) C-rate performance, and (d) long-term cycle performance at 1.0 C (operating voltage range: 2.0∼5.0 V, vs Li/Li) ⁺ ).

Hereinafter, in order to explain the present invention in more detail, preferred embodiments according to the present invention will be described in more detail with reference to the attached drawings. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms.

The present invention provides a positive electrode active material having an olivine structure; and a coating layer coated on the surface of the positive electrode active material and including a lithium ion conductive solid electrolyte and a carbon material that satisfies the following formula (1).

[Formula 1]

Li _1+x R _x M _2-x (PO ₄ ) ₃

In Formula 1, R is one or more metals selected from the group consisting of Al, B, Sn, and Ge, and M is one or more transition metals selected from the group consisting of Ti, Ge, and Hf, and 0≤x<2.

In addition to the lithium ion conductive solid electrolyte that satisfies the above formula (1), it can be applied if it has high lithium ion conductivity, stability, and compatibility with olivine materials such as metal oxides, phosphates, and other solid electrolytes.

The positive electrode active material having the olivine structure may satisfy the following formula (2).

[Formula 2]

LiM _x M′ _y M″ _Z N _1-xyz PO ₄

In Formula 2, M, M′, M″, and N are elements independently selected from the group consisting of Fe, Mn, Co, Ni, and combinations thereof, 0≤x,y,z≤1, x+ y+z≤1.

At this time, the positive electrode active material may be doped with one or more metal elements selected from the group consisting of Cr, Zr, Nb, Cu, V, Mo, Ti, Zn, and Al.

The positive electrode active material having an olivine structure can be mixed with a lithium-containing compound and fired to produce a positive electrode active material having an olivine structure for a lithium secondary battery. The positive electrode active material having the olivine structure is preferably spherical. Here, the sphere may be selected from the group consisting of a circular shape, an oval shape, and a combination thereof, but is not limited thereto. When manufacturing a positive electrode active material for a lithium secondary battery using the positive electrode active material having a spherical olivine structure, it is preferable to manufacture a positive electrode active material having a spherical olivine structure.

The solid electrolyte may satisfy the following formula (3).

[Formula 3]

Li _1+y Al _y Ti _2-y (PO ₄ ) ₃

In Formula 3, 0≤y<2.

The carbon precursor is a carbonaceous compound such as sucrose, glucose, lactose, or polymer, or an inorganic source such as acetylene black, carbon nanotube, or graphene, and further oxalate. , carbon-containing precursors such as acetate, carbonate, and citrate may be used.

Additionally, the present invention provides an electrode containing the positive electrode active material.

Additionally, the present invention provides a lithium secondary battery including the electrode.

In addition, the present invention includes a first step of preparing an olivine precursor, a solid electrolyte precursor, and a carbon precursor; A second step of synthesizing olivine precursor; A third step of mixing a coating precursor including the solid electrolyte precursor and a carbon precursor with the olivine precursor; and a fourth step of synthesizing the coating precursor with the olivine precursor.

The olivine precursor may satisfy the following formula (2).

[Formula 2]

LiM _x M′ _y M″ _z N _1-xyz PO ₄

In Formula 2, M, M′, M″, and N are elements independently selected from the group consisting of Fe, Mn, Co, Ni, and combinations thereof, 0≤x,y,z≤1, x+ y+z≤1.

At this time, the olivine precursor may be doped with one or more metal elements selected from the group consisting of Cr, Zr, Nb, Cu, V, Mo, Ti, Zn, and Al.

The solid electrolyte precursor may satisfy the following formula (3).

[Formula 3]

Li _1+y Al _y Ti _2-y (PO ₄ ) ₃

In Formula 3, 0≤y<2.

The carbon precursor is a carbonaceous compound such as sucrose, glucose, lactose, or polymer, or an inorganic source such as acetylene black, carbon nanotube, or graphene, and further oxalate. , carbon-containing precursors such as acetate, carbonate, and citrate may be used.

In the first and third steps, the precursor may be prepared using a ball mill at a rotation speed of 200 to 400 rpm for 2 to 4 hours.

In the second and fourth steps, the synthesis may be performed through a solid-state method, but is not limited thereto, and may be performed under argon gas at 150 to 200 ° C. for 2 to 4 hours.

The present invention uses a solid-state method to synthesize and coat olivine materials, but the synthesis process of olivine materials is not limited to the solid-state method, and includes sol-gel methods, coprecipitation methods, and hydrothermal methods. and solvothermal processes may be utilized.

In the present invention, ball milling was performed to prepare the precursor. However, the preparation process of precursor materials is not limited to ball milling.

In addition, the second and fourth steps may additionally include heat treatment at 600 to 700° C. for 4 to 6 hours. In order to improve the overall electrochemical performance of the cathode material, stable carbon is added to the surface of the olivine particles. and a solid electrolyte layer may be required.

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.

<Example> Manufacture of positive electrode active material

The technical scheme of material synthesis includes the following first to fourth steps.

Step 1: Preparation of olivine precursor, the olivine precursor is LiFe _0.4 Mn _0.6 PO ₄ and LiFe _0.4 Mn _0.3 Co _0.3 PO ₄ .

Step 2: Synthesizing olivine precursors.

Step 3: Preparation of precursor for coating, specifically mixing Li _1.3 Al _0.3 Ti _1.7 ( _{PO 4} ) _{3 solid} electrolyte precursor _for coating _and carbon precursor _.

Step 4: Synthesizing precursor for coating and olivine precursor.

Precursor Preparation Method (Steps 1 and 3)

In the first and third steps, all precursors were prepared through a ball mill. LiFe_0.4Mn_0.6P.O.₄, LiFe_0.4Mn_0.3Co_0.3P.O.₄ and Li_1.3Al_0.3Ti_1.7(PO₄)₃The raw materials were weighed according to the mole fraction of the final product. LiFe_0.4Mn_0.6P.O._4,LiFe_0.4Mn_0.3Co_0.3P.O.₄ Precursor preparation To Li₂C.O.₃(Aldrich, 99.0%), FeC₂O₄·2H₂O (Junsei, 99.0%), MnC₂O₄·2H₂O (AlfaAesar), CoC₂O₄·2H₂O (Thermo Fisher Scientific), NH₄H₂P.O.₄(Junsei, 99.5%) was used as the starting material. Li_1.3Al_0.3Ti_1.7(PO₄)₃For (LATP) precursor preparation, Li₂C.O.₃(Aldrich, 99.0%), AlOH(OOCCH₃)₂(Aldrich, 100.0%), TiO₂(Aldrich, 100.0%), and N.H.₄H₂P.O.₄(Junsei, 99.5%) was used as the starting material. Sucrose (Junsei, 100%) was ground into fine particles and used as a carbon source.

The raw materials for each final product were mixed by a ball mill (Pulverisette, Fritsch) in acetone (Samcheon, 99.7%) solvent. The mass ratio of zirconia balls (Ø=5 mm, SciLab) compared to the raw materials was set at 20:1. At this time, the rotation speed of the ball mill was 300 rpm, and the ball mill ran for about 3 hours.

Material synthesis method (steps 2 and 4)

A simple and highly scalable solid-state reaction was used for the second and fourth steps of the olivine material synthesis process and for the subsequent coating process. The precursors were purged in a tube furnace under argon gas at 120°C for 3 hours, then treated at 370°C for 4 hours, and heat treated at 670°C for 6 hours.

<Experimental example>

Determination of material properties

The formation of the hybrid layer was determined ^by Microscope (High-resolution transmission electron microscope, HR-TEM, JEM-2100F With Cs Corrector) and Energy dispersive spectroscopy (EDS), Raman spectroscopy (DXR™ 3, Thermo Fisher Scientific Inc., Woodward, Austin, AZ, USA), thermogravimetric analysis (TGA, DT Q600 heated from room temperature to 750°C at a rate of 10°C/min in air condition). there was.

Electrochemical property evaluation

Electrodes were prepared to evaluate electrochemical properties. Synthetic olivine material, a positive electrode active material, a conductive material (Super-P), and a binder (PVDF: Polyvinylidene fluoride, Aldrich, M _W = 534,000) were mixed with a solvent (NMP, N-methyl-2-) at a ratio of 8:1:1. A slurry was prepared by mixing with pyrrolidone, Duksan, HPLC grade), applied to a carbon-coated Al foil as a current collector, and dried in a vacuum oven at 80°C for 12 hours. At this time, the mass of the active material is about 3.0 mg·cm ^-2 . The 2032-type cell was used and was assembled in an Ar atmosphere glove box. In the construction of the battery, ED (ethylene carbonate)/DMC (dimethyl carbonate) = 1/1 (vol.%) in which 1.0M LiPF ₆ was dissolved was used as the electrolyte (Panax Etec Co. Ltd, Korea). A porous polypropylene membrane was used as the separator. The counter electrode, the cathode, used lithium foil. Charge and discharge characteristics were confirmed using the manufactured coin cell. The charge/discharge measurement device was measured at room temperature using WonAtech WBCS3000, an automatic battery cycler. The voltage range of LiFe _0.4 Mn _0.6 PO ₄ based material is 2.0-4.5V vs.Li/Li ^{+ , and} the voltage range of LiFe _0.4 Mn _0.3 Co _0.3 PO ₄ based material is 2.0-4.5V vs.Li/Li + . 2.0-5.0 V vs. Measurements were made by fixing Li/Li ⁺ . Charge/discharge capacity according to current value can be obtained within a fixed voltage range, and charge/discharge current values (1C = 170 mAh/g) were measured at various C-rates.

<Experimental Example 1> X-ray diffraction (XRD) measurement

X-ray diffraction (XRD, Rigaku Ultima IV, Japan) was measured to determine the structure and crystallinity of the olivine material and the coated olivine material. At this time, the scanning range (2θ) was 10 to 80°, the scanning speed was 10°/min, and the X-ray trend was analyzed using Cu Kα rays. Figure 1(a) shows the XRD patterns of pristine LFMP, LFMP@C, and LFMP@C_LATP. It can be seen that all peaks of LFMP match well with the peaks of LiFePO ₄ and LiMnPO ₄ . It can be seen that all XRD peaks of LFMP@C and LFMP@C_LATP match well with the pristine LFMP peak. This means that the two-step synthesis method for coating does not affect the structure of LFMP. In addition, it can be confirmed that crystallization was successful through the appearance of sharp peaks. Peaks for graphitic C cannot be identified in the three XRD patterns. This is because the amount of carbon after coating is small and carbon with an amorphous structure is dominant.

When comparing the XRD pattern of LFMP@C_LATP with that of pristine LFMP and LFMP@C, an additional small peak can be seen around 24.5 ^° of the pattern 2θ of LFMP@C_LATP. Similar patterns can be seen in pristine LFMCP, LFMCP@C, and LFMCP@C_LATP (Figure 1(b)). It can be confirmed that the structure of LFMCP is not affected by the two-step synthesis process and maintains its original structure. Peaks for graphitic C cannot be identified in the three XRD patterns. In addition, when comparing the XRD pattern of LFMCP@C_LATP with that of pristine LFMCP and LFMCP@C, an additional small peak can be seen around 24.5 ^o of the pattern 2θ of LFMCP@C_LATP.

To investigate the additional small peak around 24.5 ^o observed only in LFMP@C_LATP and LFMCP@C_LATP, LATP was synthesized under the same conditions and the XRD pattern was confirmed (Figure 1(c)). When comparing the XRD pattern of LATP with the reference material (ICSD#14585), it can be confirmed that it matches well, which shows that the synthesis conditions presented in the present invention are suitable for forming LATP. In addition, a strong peak appears around 24.6° of LATP 2θ and It can be seen that small peaks appearing in LFMP@C_LATP and LFMCP@C_LATP appear in similar positions.

Therefore, it can be confirmed from XRD measurement that it can be successfully synthesized through a two-step synthesis method of synthesizing olivine material and then coating it with LATP solid electrolyte and carbon. It can be confirmed that neither the synthesis process nor the coating process affects the crystal structure of the olivine material.

<Experimental Example 2> High-resolution transmission electron microscope (HR-TEM) and energy dispersive spectroscopy (EDS) result analysis

To observe the size, shape, and distribution of powder particles of the synthesized material, a high-resolution transmission electron microscope (HR-TEM, JEM-2100F With Cs Corrector) and an energy dispersive spectrometer were used. It was confirmed using spectroscpoy (EDS). Figures 2a to 2f show EDS layer images corresponding to Fe, Mn, Ti, P, and C of LFMP@C_LATP, and the distribution of each element can be confirmed. As it was confirmed that Fe and Mn were well distributed, it was confirmed that the solid solution LiFe _0.4 Mn _0.6 PO ₄ (LFMP) with an olivine structure was successfully formed.

Since the Al content in Li _1.3 Al _0.3 Ti _1.7 (PO ₄ ) ₃ (LATP) is very small compared to Ti, the distribution of LATP can be confirmed by the Ti element. LATP can be well distributed on the surface of olivine particles or can form as individual particles that are strongly connected to olivine particles. Because LATP is synthesized at relatively high temperatures, the formation of individual LATP particles is expected. These synthesis conditions can make it thermodynamically more stable by inducing the movement of some small-sized particles to form larger-sized particles.

The distribution of P element with respect to Fe-Mn and Ti elements confirms the formation of phosphate-based compounds LFMP and LATP, as in the previous XRD measurement. The formation of carbon on LFMP particles can be confirmed by the good distribution of C elements with respect to Fe-Mn elements. In addition, it can be seen that the intensity of the C element with respect to the Fe-Mn element is relatively higher than that of the Ti element, and this confirms that carbon and LATP are simultaneously coated on the LFMP particles. Simultaneous coating of carbon and LATP on LFMP particles is important for application to lithium-ion batteries because it can simultaneously provide electron and lithium ion transport channels during the charging/discharging process.

As shown in Figure 2g, LFMP@C_LATP particles are synthesized in a wide range of particle sizes and in aggregated form, which is a natural result of the solid-phase method. Additionally, the HR-TEM image of the material in Figure 2h, a region containing not only C elements but also Fe-Mn and Ti elements, confirms strong connections between particles in the amorphous phase of carbon.

Meanwhile, Figures 3a to 3g show EDS layered images corresponding to Fe, Mn, Co, Ti, P, and C of LFMCP@C_LATP, and the distribution of each element can be confirmed. As it was confirmed that Fe, Mn, and Co were well distributed, it was confirmed that the solid solution LiFe _0.4 Mn _0.3 Co _0.3 PO ₄ (LFMCP) with an olivine structure was successfully formed. LATP can be well distributed on the surface of LFMCP particles, as observed for LFMP@C_LATP, or can be formed as individual particles that are strongly connected to olivine particles. The distribution of P element with respect to Fe-Mn-Co and Ti elements confirms the formation of phosphate-based LFMCP and LATP, as in the previous XRD measurement.

In addition, it can be seen that the intensity of the C element with respect to the Fe-Mn-Co element is relatively higher than that of the Ti element, and through this, it can be confirmed that carbon and LATP are simultaneously coated on the LFMCP particles. It can be seen in Figure 3h that LFMCP@C_LATP particles are synthesized in a wide range of particle sizes and in an aggregated form, which is a natural result of the solid-phase method. The formation of amorphous carbon can be confirmed by the formation of a relatively disordered matrix in LFMCP and LATP particles.

<Experimental Example 3> Raman spectroscopy result analysis

The properties of the carbon can be confirmed once again through Raman spectroscopy (DXR™ 3, Thermo Fisher Scientific Inc., Woodward, Austin, AZ, USA), as shown in FIGS. 4a and 4c. of amorphous carbon at approximately 1334 cm ^-1 and 1592 cm ^-1 in the Raman spectra of LFMP@C, LFMP@C_LATP, LFMCP@C and LFMCP@C_LATP, respectively. A _{1 g} mode and graphite Both the D and G bands of carbon corresponding to the E _{2 g} vibration mode can be confirmed. Also relatively high By checking the I _D /I _G values, it can be seen that amorphous carbon is dominant for all samples.

Since the amount of carbon can generally be expressed as a decrease in the weight of the material at high temperature, it was measured using a thermogravimetric analysis (TGA, DT Q600), and the temperature was measured from room temperature to 750°C at a rate of 10°C/min in air condition. Heated. Since the TGA measurements were performed in air, some of the weight loss due to decomposition and combustion of carbon was offset by weight gain due to partial oxidation of the Fe-Mn-Co elements. Therefore, the actual amount of carbon may be slightly higher than the weight loss observed by TGA.

As shown in Figures 4b and 4d, weight reductions of 2.15%, 2.84%, 1.56%, and 2.39% can be confirmed for LFMP@C, LFMP@C_LATP, LFMCP@C, and LFMCP@C_LATP, respectively. Although the mass of carbon for olivine materials is small, when applied to lithium-ion batteries, their presence is very important to protect particles, preserve electrolyte molecules and improve the electronic conductivity of olivine materials.

<Experimental Example 4> Electrochemical property evaluation

Electrodes were prepared to measure electrochemical properties. Synthetic olivine material, a positive electrode active material, conductive material (Super-P), and binder (PVDF: Polyvinylidene fluoride, Aldrich, M _W = 534,000) were mixed with a solvent (NMP, N-methyl-2-) at a ratio of 8:1:1. A slurry was prepared by mixing with pyrrolidone, Duksan, HPLC grade), applied to a carbon-coated Al foil as a current collector, and dried in a vacuum oven at 80°C for 12 hours. At this time, the mass of the active material is about 3.0 mg·cm ^-2 . The 2032-type cell was used and was assembled in an Ar atmosphere glove box. In the construction of the battery, the electrolyte (Panax Etec Co. Ltd, Korea) is 1.0M LiPF ₆ dissolved in ED ₍ ethylene carbonate)/DMC (dimethyl carbonate)=1/1 (vol.%) was used. The separator was a porous polypropylene membrane. The negative electrode, which is the counter electrode, used lithium foil.

Charge and discharge characteristics were confirmed using the manufactured coin cell. The charge/discharge measurement device was measured at room temperature using WonAtech WBCS3000, an automatic battery cycler. The voltage range of LiFe _0.4 Mn _0.6 PO ₄ based material is 2.0-4.5V vs.Li/Li ⁺ , The voltage range of LiFe _0.4 Mn _0.3 Co _0.3 PO ₄ based material is 2.0-5.0 V vs. Measurements were made by fixing Li/Li ⁺ . Charge/discharge capacity according to current value can be obtained within a fixed voltage range, and charge/discharge current values (1C=170 mAh/g) were measured at various C-rates.

Figures 5a and 6a show the current-voltage profiles of pristine LFMP, LFMP@C, LFMP@C_LATP and pristine LFMCP, LFMCP@C, LFMCP@C_LATP, respectively. It can be seen that the coating improves the electrochemical performance at relatively higher operating voltages than usual for olivine materials, especially LFMCP. Compared to pristine materials and materials with only carbon coating, LFMP@C_LATP and LFMCP@C_LATP can be seen to have an electrochemical peak in the low voltage range of 2.2~2.8V, which may be due to LATP.

LATP plays an important role in the overall performance of olivine materials, as shown in Figures 5(b) and 6(b). Through the LATP coating, the olivine material begins to charge at a relatively low voltage. Extraction of lithium ions at low voltage can induce a gradient that leads to sustained transport of lithium ions from the core of the olivine material to the particle surface. During the charging process of LFMP@C_LATP and LFMCP@C_LATP, more lithium ions are transported, and accordingly, lithium ions are stored in more space during the discharging process.

As a result, LFMP@C_LATP and LFMCP@C_LATP show improved C-rate and long cycle performance as shown in Figures 5c, 5d, 6c, and 6d, respectively. LFMP@C_LATP shows the highest discharge capacity of 162.1 mAh g ^-1 in the first cycle of 0.05C, and 132.9 mAh g ^-1 in the first cycle of 1.0C. Additionally, it can be confirmed that there is a retention rate of 92% after 50 cycles.

LFMCP@C_LATP shows the highest discharge capacity of 142.8 mAh g ^-1 in the first cycle of 0.05 C, and 131.9 mAh g ^-1 in the first cycle of 1.0 C. It can also be confirmed that it has a retention rate of 54% after 50 cycles. The low capacity retention observed for LFMCP@C_LATP at 1.0 C may be due to electrolyte decomposition at a higher than normal operating voltage range of 2.0–5.0 V.

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 (15)

A positive electrode active material having an olivine structure; and
A coating layer applied on the surface of the positive electrode active material and including a lithium ion conductive solid electrolyte and a carbon material that satisfies the following formula (1);
A positive electrode active material comprising:
[Formula 1]
Li _1+x R _x M _2-x (PO ₄ ) ₃
In Formula 1, R is one or more metals selected from the group consisting of Al, B, Sn, and Ge, and M is one or more transition metals selected from the group consisting of Ti, Ge, and Hf, and 0≤x<2. According to claim 1,
The positive electrode active material having the olivine structure is characterized in that it satisfies the following Chemical Formula 2:
[Formula 2]
LiM _x M′ _y M″ _z N _1-xyz PO ₄
In Formula 2, M, M′, M″, and N are elements independently selected from the group consisting of Fe, Mn, Co, Ni, and combinations thereof, 0≤x,y,z≤1, x+ y+z≤1. According to claim 2,
The positive electrode active material is characterized in that it is doped with one or more metal elements selected from the group consisting of Cr, Zr, Nb, Cu, V, Mo, Ti, Zn, and Al. According to claim 1,
The solid electrolyte is a positive electrode active material characterized in that it satisfies the following formula 3:
[Formula 3]
Li _1+y Al _y Ti _2-y (PO ₄ ) ₃
In Formula 3, 0≤y<2. According to claim 1,
The carbon materials include sucrose, glucose, lactose, acetylene black, carbon nanotubes, graphene, oxalate, acetate, carbonate, and citric acid. A positive electrode active material characterized in that it is one or more selected from the group consisting of salts (citrates). An electrode comprising the positive electrode active material according to any one of claims 1 to 5. A lithium secondary battery comprising the electrode of claim 5. A first step of preparing an olivine precursor, a solid electrolyte precursor, and a carbon precursor;
A second step of synthesizing olivine precursor;
A third step of mixing a coating precursor including the solid electrolyte precursor and a carbon precursor with the olivine precursor; and
A method for producing an olivine positive electrode active material comprising a fourth step of synthesizing the precursor for the coating with the olivine precursor. According to claim 8,
The olivine precursor is a method of producing an olivine positive electrode active material, characterized in that it satisfies the following Chemical Formula 2:
[Formula 2]
LiM _x M′ _y M″ _z N _1-xyz PO ₄
In Formula 2, M, M′, M″, and N are elements independently selected from the group consisting of Fe, Mn, Co, Ni, and combinations thereof, 0≤x,y,z≤1, x+ y+z≤1. According to clause 9,
A method of producing an olivine positive electrode active material, characterized in that the olivine precursor is doped with one or more metal elements selected from the group consisting of Cr, Zr, Nb, Cu, V, Mo, Ti, Zn, and Al. According to claim 8,
A method of producing an olivine positive electrode active material, wherein the solid electrolyte precursor satisfies the following formula 3:
[Formula 3]
Li _1+y Al _y Ti _2-y (PO ₄ ) ₃
In Formula 3, 0≤y<2. According to claim 8,
The carbon precursors include sucrose, glucose, lactose, acetylene black, carbon nanotubes, graphene, oxalate, acetate, carbonate, and citric acid. A method of producing an olivine positive electrode active material, characterized in that it contains at least one selected from the group consisting of salts (citrates). According to claim 8,
A method of producing an olivine positive electrode active material, characterized in that the first and third steps are performed through a ball mill at a rotation speed of 200 to 400 rpm for 2 to 4 hours. According to claim 8,
A method for producing an olivine positive electrode active material, characterized in that the synthesis in the second and fourth steps is performed through a solid-state method. According to claim 8,
The fourth step is a method of producing an olivine positive electrode active material, characterized in that it further includes the step of heat treatment at 600 to 700 ° C. for 4 to 6 hours.