KR102394000B1 - Phosphate positive electrode active material for lithium secondary battery and a method of manufacturing the same - Google Patents

Abstract

The present invention relates to a phosphate-based positive electrode active material for a lithium secondary battery and a method for manufacturing the same, wherein iron and titanium are co-doped in monoclinic Li ₃ V ₂ (PO ₄ ) ₃ Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ (x≥0.015) to include the technical gist. Accordingly, by co-doping Li ₃ V ₂ (PO ₄ ) ₃ with iron (Fe) and titanium (Ti), Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ Lithium secondary By applying it as a positive electrode active material for a battery, the discharge capacity of a lithium secondary battery is increased, and various electrochemical properties such as high output characteristics and cycle characteristics are improved.

Description

Phosphate positive electrode active material for lithium secondary battery and a method of manufacturing the same}

The present invention relates to a phosphate-based positive electrode active material for a lithium secondary battery and a method for manufacturing the same, and more particularly, to Li ₃ V ₂ (PO ₄ ) ₃ iron (Fe) and titanium (Ti) are co-doped (co-doping) And, Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene is applied to a lithium secondary battery by adding graphene, and relates to a phosphate-based positive electrode active material for a lithium secondary battery and a method for manufacturing the same.

In line with industrial development and improvement of living standards, a high-performance energy storage device capable of realizing small size and high capacity along with research on weight reduction and low power consumption for the purpose of miniaturization and long-term continuous use of portable electronic devices is required. Accordingly, recently, lithium secondary batteries have been used as large-capacity power storage batteries such as electric vehicles and battery power storage systems, and small, high-performance energy sources such as portable electronic devices such as mobile phones, camcorders, and notebook computers.

In particular, the lithium secondary battery has advantages of high energy density, large capacity per area, low self-discharge rate, and long lifespan. In addition, since there is no memory effect, it is convenient for users to use and has a long lifespan. The structure of a lithium secondary battery is composed of an anode, a cathode, a lithium salt, and a non-aqueous electrolyte solution capable of inserting and deintercalating lithium ions. The reason for using a non-aqueous electrolyte is that lithium (Li) cannot stably exist when an aqueous electrolyte is used because lithium (Li) has high reactivity with water.

Representative cathode active materials of lithium secondary batteries include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , and the like. Among them, LiCoO ₂ has excellent lifespan characteristics and potential flatness due to excellent conductivity, excellent high-rate discharge characteristics, and stable charge/discharge behavior. However, the fact that the price of cobalt (Co) is expensive compared to other materials and that the internal temperature of the battery rapidly rises due to poor thermal stability in the charged state when a lithium secondary battery is misused, thereby desorbing lattice oxygen there is. In the case of LiNiO ₂ , compared to LiCoO ₂ , the price is low, the specific capacity is high, and the oxidation of the electrolyte is low because it shows a low discharge voltage. situation. _In addition, LiMn ₂ O ₄ is cheaper and easier to synthesize than other cathode active materials, so it is used in low-cost products _. It is difficult to manufacture high energy density batteries due to low specific capacity and wrap density compared to materials. In addition, not only the diffusion resistance of ions during charging and discharging of the secondary battery is large, but when the manganese (Mn) oxidation number falls below 3.5 in the operating voltage range, a phase transition occurs from a cube to a square due to the Jahn-Heller effect, resulting in a lifespan characteristics are lowered. In particular, there is a problem in that the battery characteristics are rapidly deteriorated due to the elution of manganese according to the side reaction of the electrolyte during charging and discharging at 60°C or higher.

In order to solve the disadvantages of these materials, the prior art 'US Patent No. 6808848' proposes to improve the output characteristics by applying a heterogeneous positive electrode active material composed of LiNiCoMnO ₂ and LiMn ₂ O ₄ . There are no reports of improvement. In addition, in order to solve the low conductivity problem of Li _{1 - x} Fe _x PO ₄ having an olivine structure and Li[Mn ₂ 0 _x M _x ]O ₄ having a cubic structure, 'US Patent Publication No. 2004/ No. 0157126 discloses a method of coating LiFePO ₄ and Li[Mn _{2 - x} M _x ]O ₄ using a carbon raw material gas such as acetylene and butane, but it is difficult to control the atmosphere during manufacturing It can induce volatilization of lithium (Li) during the process, and it is difficult to prepare a uniform coating active material, so satisfactory battery characteristics cannot be realized. In addition to this, a layered-layered composite structure of Li ₂ MnO ₃ -LiMO ₂ (where M is a group consisting of Ni, Co, Mn, and mixtures thereof) has been studied and reported. However, these materials have a very large irreversible capacity during initial charging and discharging, and the problem of swelling due to oxygen generation is also serious. In order to solve this problem, a method of removing lithium and oxygen through acid treatment using strong acids such as nitric acid or sulfuric acid has been reported, however, the surface of the active material is damaged or the crystal structure is changed due to acid treatment, and the electrochemical properties are deteriorated. has a

US Patent No. 6808848 US Patent Publication No. 2004/0157126

Accordingly, an object of the present invention is to co-doping iron (Fe) and titanium (Ti) to Li ₃ V ₂ (PO ₄ ) ₃ and adding graphene to Li ₃ V _{2 -2 x} To provide a phosphate-based positive electrode active material for a lithium secondary battery in which Fe _x Ti _x (PO ₄ ) ₃ /graphene is applied to a lithium secondary battery and a method for manufacturing the same.

The above object is, the monoclinic Li ₃ V ₂ (PO ₄ ) ₃ iron and titanium co-doped (co-doped) Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ (x≥0.015) ) is achieved by a positive electrode active material for a phosphate-based lithium secondary battery, characterized in that it contains.

The Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ preferably includes Li ₃ V _{2 -2x} Fe _x Ti _x (PO ₄ ) ₃ /C mixed with a carbon material. .

The above object also includes the steps of preparing a monoclinic Li ₃ V ₂ (PO ₄ ) ₃ ; The Li ₃ V ₂ (PO ₄ ) ₃ by co-doping with iron and titanium through a sol-gel method, Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ (x ≥ 0.015) is also achieved by a method for manufacturing a positive electrode active material for a lithium secondary battery comprising the step of preparing.

The step of preparing the monoclinic Li ₃ V ₂ (PO ₄ ) ₃ is by dissolving vanadium oxide (V ₂ O ₃ ) and oxalic acid (C ₂ H ₂ O ₄ ) in distilled water or water and stirring. step of; Lithium carbonate (Li ₂ CO ₃ ) and ammonium phosphate (Ammonium phosphate, NH ₄ H ₂ PO ₄ ) are mixed and stirred to form a Li ₃ V ₂ (PO ₄ ) ₃ mixed solution. and the co-doping of iron and titanium includes mixing and stirring iron nitrate (Fe(NO ₃ ) ₃ ) and titanium dioxide (TiO ₂ ) in the mixed solution. it is preferable

In addition, after the step of co-doping the iron and titanium, mixing and stirring a carbon material (Carbon material) solution; drying the solution to form a Li ₃ V _{2 -2x} Fe _x Ti _x (PO ₄ ) ₃ /C precursor; It is preferable to heat-treat the Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /C precursor to form Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /C Do.

According to the configuration of the present invention described above, by co-doping iron (Fe) and titanium (Ti) to Li ₃ V ₂ (PO ₄ ) ₃ and adding graphene, Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene is applied as a phosphate-based positive electrode active material for lithium secondary batteries to increase the discharge capacity of lithium secondary batteries, and overall electrochemical properties such as high output characteristics and cycle characteristics are improved, resulting in low cost, high safety and high capacity , it is possible to obtain the effect of manufacturing a long-life secondary battery.

1 is a flowchart of a method for manufacturing a phosphate-based positive electrode active material for a lithium secondary battery according to an embodiment of the present invention;
Figure 2 is a positive electrode active material Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ XRD pattern graph of,
3 is a FE-SEM photograph of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
4 is a TEM photograph of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
5 is a TEM elemetal mapping photograph of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
6 is an XPS spectrum of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
7 is an initial charge-discharge curve of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
8 is a discharge curve according to the current rate of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
9 is a cycle characteristic of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
10 is a cyclic voltammetry curve of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ ,
11 is an impedance curve of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ .

Hereinafter, a phosphate-based positive electrode active material for a lithium secondary battery and a manufacturing method thereof according to an embodiment of the present invention will be described in detail.

1, in order to prepare Li ₃ V _{2 -2x} Fe _x Ti _x (PO ₄ ) ₃ , which is a phosphate-based positive electrode active material for a lithium secondary battery, first monoclinic Li ₃ V ₂ (PO ₄ ) ₃ is prepared. The step of dissolving vanadium oxide (V ₂ O ₃ ) and oxalic acid (C ₂ H ₂ O ₄ ) in distilled water or water and stirring, and lithium carbonate (Li ₂ CO ₃ ) and Ammonium phosphate (Ammonium phosphate, NH ₄ H ₂ PO ₄ ) is obtained through mixing and stirring. After that, Li ₃ V ₂ (PO ₄ ) ₃ to co-doping iron and titanium through a sol-gel method, which is iron nitrate (Iron nitrate, Fe(NO ₃ ) ₃ ) and titanium dioxide (TiO ₂ ). Finally, Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ is obtained, and in some cases, a carbon material is mixed with Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /C can also be formed. Here, the carbon material is activated carbon, graphite, graphene, soft carbon, hard carbon, carbon black, carbon nano tube, CNT), carbon nanofiber (CNF), modified carbon (Modified carbon), carbon composite material (Carbon composite) and selected from the group consisting of mixtures thereof, among them, graphene is most preferable.

This will be described in more detail through the following examples.

<Example>

In this embodiment, vanadium oxide (V ₂ O ₃ ), oxalic acid (C ₂ H ₂ O ₄ ), lithium carbonate (Li ₂ CO ₃ ), ammonium phosphate (Ammonium phosphate, NH ₄ H) Li ₃ V _{2 -2 x} Fe _x Ti _x using ₂ PO ₄ ), titanium dioxide (TiO ₂ ), iron nitrate (Fe(NO ₃ ) ₃ ), and graphene oxide (PO ₄ ) ₃ /graphene was synthesized.

First, a mixed solution was formed by dissolving vanadium oxide and oxalic acid in distilled water at a stoichiometry composition ratio of 1:3, and stirred until the mixed solution had a blue color. Lithium carbide, ammonium phosphate, titanium oxide, and iron nitrate were sequentially added to the stirred mixed solution and stirred for 1 hour until they were completely dissolved in the mixed solution. A solution containing graphene was added thereto, followed by stirring at room temperature for 24 hours. After the solution was dried at 100° C., Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene precursor (Precursor) was synthesized.

After heat-treating the Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene precursor obtained through the above method in an argon (Ar) atmosphere in a tubular sintering furnace at 350° C. for 4 hours, The final material, Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene was synthesized by heat treatment at 800° C. for 8 hours.

X-ray diffraction analysis (Philips Co., X-pert PW3710) was performed to observe the crystal structure and impurities of the synthesized Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene cathode active material. Measurements were made using Cu-kα rays at a scanning speed of 0.04°/sec and a scanning range of 10 to 80° (2θ) under the conditions of 40 kV and 30 mA. In addition, FE-SEM (Field emission scanning electron microscope, Hitachi Co., S-4800) analysis and TEM (Transition electron microscopy, TF30ST) analysis were performed to measure the particle size and surface shape. In order to observe the oxidation number of the oxide, XPS (X-ray photoelectron spectroscopic, VG Multilab ESCA 2000 system, ThermoVG Scientific) was used. In addition, the carbon content was measured using a CHNS Elemental Analyzer (EA, 2400 series 2, PerkinElmer).

Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene In order to investigate the electrochemical properties of the positive electrode active material, the positive electrode active material, Super P Black, a conductive material, and PVDF (Polyvinylidene fluoride) as a binder, positive electrode active material: conductive material :Binder=70:20:10 A slurry was prepared by dispersing it in NMP (n-Methyl pyrrolidone) in a weight ratio of 10. This slurry was thinly applied to an aluminum foil and dried at 100° C. for 12 hours to prepare an electrode. The dried electrode was compressed at 110° C. using a hot roll press. A coin battery (CR2032) was completed by using the compressed electrode as a positive electrode, and stacking a lithium negative electrode and a separator (Celguard 3501). As the electrolyte, a solution in which 1M LiPF ₆ salt was dissolved in EC (Ethylene carbonate):DMC (Dimethyl carbonate) = 1:1 by volume was used.

The electrochemical properties of the synthesized cathode active material were measured using a charge/discharge tester (TOYO system, TOCAT-3100). Battery characteristics such as discharge capacity, discharge rate characteristics, cycle life, etc. were charged under a constant current-constant voltage condition in a voltage range of 3.0 to 4.8V. Impedance behavior was measured at 100 kHz to 10 mHz (amplitude 5 mV) using VMP3 (Biologic).

Figure 2 shows the XRD analysis results of the Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ sample. The obtained powder exhibited a monoclinic structure having a p2 ₁ /n space group, and no peak due to carbon (C) or graphene was observed. It is considered that this is because carbon exists in an amorphous form and is also present in a small amount. Moreover, oxalic acid used as a raw material is expected to act as a reducing agent that is reduced from V ^{5 +} to V ^{3 +} during the heat treatment process. Comparing with the XRD pattern of the sample to which no doping element was added, the sample doped with iron and titanium does not have an impurity peak, and it is assumed that all of the doping elements entered the vanadium site of Li ₃ V ₂ (PO ₄ ) ₃ . These results are consistent with Liu (H. Liu, P. Gao, J. Fang and G. Yang, Chem. Commun., 47, 9110 (2011)) and Fei (L. Fei, W. Lu, Li Sun, J. Wang). , J. Wei, HLW Chan and Y. Wang, RSC Adv., 3, 1297 (2013)).

3 is a result of observing the surface shape of the Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ sample by FE-SEM. The upper photo is a photo of samples doped with iron and titanium, the lower photo is a photo of an undoped sample, and the undoped sample and the iron and titanium doped sample are different from each other. showing shape. The undoped sample shows an irregular shape, and particles of non-uniform size are agglomerated with each other. In contrast, it can be seen that the sample doped with iron and titanium has a small and uniform particle size. For example, the Li ₃ V _1.98 Fe _0.01 Ti _{0.01 (} PO ₄ ) ₃ sample shows a particle size distribution of ₁ to ₂ μm, and these small and uniform particles have a large surface area in the electrode reaction and the Because the diffusion path is short, it acts as a factor to improve the electrochemical properties. That is, in the present invention, the doping element serves as a surfactant to limit the growth of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ particles, and Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ) ₃ plays an important role in controlling the surface shape to improve the electrochemical properties.

4 is a Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ /graphene (x=0.01) sample was observed by TEM. Looking at the HR-TEM picture, graphene is about 100 nm thick and covers the surface of Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ . Therefore, graphene and doping materials with high electrical conductivity are expected to facilitate the movement of electrons during battery charging and discharging. Moreover, as can be seen in FIG. 4b , the monoclinic Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ grains exhibit single crystal characteristics.

The result of EDS elemetal mapping in order to confirm the distribution state of each element can be confirmed in FIG. 5 . Through this, _it was confirmed that iron and titanium were uniformly and uniformly doped in the Li ₃ V _1.98 Fe _0.01 Ti _{0.01 (} PO ₄ ) ₃ sample _.

₆ is an XPS spectra analyzed to confirm the oxidation state of _the Li ₃ V ₂ (PO ₄ ) ₃ Li ₃ V _1.98 Fe _0.01 Ti _0.01 (PO ₄ ) ₃ sample doped with iron and titanium _. is shown. The XPS spectra of V2p showed two peaks due to spin-orbit coupling. The main peak observed at 516.5 eV and the satellite peak observed at 522.7 eV are due to V2p. The binding energy of the two peaks indicates that vanadium is in the +3 oxidation state. Since the Ti2p peak seen at 454.2eV is the same as the TiO ₂ peak reported in the paper, it can be confirmed that titanium is in the +4 oxidation state. In addition, the Fe2p _{3 /2} peak seen at 710.5 eV is the same as that reported for LiFePO ₄ , so it is assumed to be an oxidation state of +2. Accordingly, in Li ₃ V _1.98 Fe _0.01 Ti _{0.01 (} PO ₄ ) ₃ , vanadium, iron, _and titanium exist in oxidation states of ₊₃ , +2, and +4, respectively. Here, the ionic diameter of Fe ²⁺ is 0.78 Å, which is larger than that of V ^{3 +} , 0.64 Å, ^and the ionic diameter of Ti ⁴⁺ is 0.61 Å, which is slightly smaller ^than the ionic diameter of V ^{3+ .} Therefore, it is assumed that Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ has a larger crystal lattice due to the introduction of Fe ^{2+ .}

7 is a graph showing the initial charge and discharge curves of the Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ sample, (a) is x=0, (b) is x=0.005, (c) is x = 0.010, (d) shows the curve of each sample in the case of x = 0.015. The charge and discharge curves of all samples regardless of iron and titanium doping showed typical charge and discharge behavior of Li ₃ V ₂ (PO ₄ ) ₃ . Three lithium ions escape from the Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ sample during charging to 4.8V, and voltage flatness can be confirmed at voltages of 3.60V, 3.68v, 4.08V, and 4.58V. could The first oxide peaks seen around 3.60V and 3.68V indicate that the first lithium ion escapes in the second stage. The peak seen as the second lithium ion escapes around 4.08V is due to oxidation from V ^{3 +} to V ^{4 +} . The oxide peak seen near 4.58V is due to the escape of the third lithium ion, and at this time, a phase transition occurs from LiV ₂ (PO ₄ ) ₃ to V ₂ (PO ₄ ) ₃ . Although this basic behavior is almost constant regardless of doping, it can be seen that the doped samples have a higher discharge capacity. In addition, there was a slight difference in the charge and discharge curves of the samples doped with iron and titanium. That is, when the doping amount x was increased, the flat portions at 3.60 V and 3.68 V were slightly inclined, and the two flat portions merged into one. This is presumed to be because V ^{3 +} decreases while inactive Ti ^{4 +} increases. In addition, the difference between the charge and discharge voltages of the doped sample was further reduced. The discharge capacity of the x=0.01 sample was 188.8 mAh/g, showing the highest value. In the case of the doped sample, the discharge capacity increased as the period in which the third lithium ion was emitted became longer.

8 shows the discharge rate characteristics of the Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ sample. All samples were charged at 0.1C, and the results are shown when discharging at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, and 10C. As the current rate increased, the discharge capacity due to the polarization of the electrode decreased. 188.8 mAh/g, 179.6 mAh/g, 171.0 mAh/g, 161.3 mAh/g, 150.5 mAh/g, 138.9 at 0.1C, 0.2C, 0.5C, 1C, 2C, 5C, 10C for samples with x=0.01 mAh/g, and 124.6 mAh/g. This is a result of greatly improved output characteristics compared to the undoped sample, because, as described above, the crystal lattice expands and the particle size decreases due to co-doping of iron and titanium.

9 shows the charge and discharge cycle characteristics of the Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ sample. After 100 cycles, the sample of x=0.01 showed a high discharge capacity of 130.9 mAh/g. This is a capacity retention rate of about 77% of the initial capacity of 170.2 mAh/g. Meanwhile, in the case of the undoped x=0 sample, the discharge capacity after 100 cycles was 106.5 mAh/g, which showed a 70% capacity retention rate compared to the initial capacity of 150.1 mAh/g. These results are because the excellent cycle performance of the x=0.010 sample was changed to facilitate insertion and desorption of lithium ions by iron and titanium co-doping. The ion diameters of V ^{3 +} , V ^{4 +} , and V ^{5 +} are 0.64 Å, 0.58 Å, and 0.335 Å, respectively. During the charging and discharging process, the oxidation state of vanadium repeatedly increases and decreases, and the crystal lattice repeats expansion and contraction. Fe ^{2+ and Fe 3+ are} electrochemically inert ^and have ionic diameters of 0.78 Å and 0.645 Å, respectively. The change between Fe ²⁺ and Fe ^{3+ has a smaller structural change than the change between V 3+ and V 5+ .} In addition, it is known that the ^{ion diameter of Ti 4+ does} not change during the charging and discharging process. Therefore, it is inferred that iron and titanium co-doping reduces the expansion and contraction changes of the crystal lattice during the charging and discharging process, and thus the charging and discharging characteristics are improved while the structure is maintained stably.

10 is a Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ Cyclic voltammetry (CV) of the sample shows the results. The CV curve was measured in the second cycle so that the electrolyte sufficiently penetrated into the electrode and a solid electrolyte interface (SEI) layer was formed. All samples exhibited four anode current peaks and three cathode electrode peaks in a voltage range of 3.0 to 4.8V, which is consistent with the results of the charge and discharge curves of FIG. 3 . The peak voltages during charging and discharging were reduced with iron and titanium co-doping, which is consistent with the results of the initial charging and discharging curves shown in FIG. 6 . 10 also shows that the x=0.01 sample has the smallest voltage difference.

11 is a Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ EIS (Electrochemical impedance spectroscopy) measurement results of the sample is shown. All samples were prepared identically, and batteries that were charged and discharged 20 times before EIS measurement were used. All Nyquist plots show semicircles and sloped lines. The semicircle represents the charge transfer reaction, and the slanted line represents the diffusion of lithium ions in the solid phase. As a result of the measurement, all of the co-doping samples showed a low Rct (Charge Transfer Resistance) value, and in particular, the x=0.010 sample showed the lowest value at 12.0Ω. This is also consistent with the results of the charging and discharging characteristics shown in FIGS. 7 to 10 .

Li ₃ V _{2 -2 x} Fe _x Ti _x (PO ₄ ) ₃ of the present invention is co-doped with iron and titanium to act as a surfactant, so it is formed into smaller and more uniform particles than the non-co-doped sample, and The same particle size improves the electrochemical properties because the surface area is large and the diffusion path of lithium ions is short. In addition, iron and titanium have high electrical conductivity, which facilitates the movement of electrons during charging and discharging of the battery. can get

Claims (10)

In the cathode active material for a lithium secondary battery,
Monoclinic Li ₃ V ₂ (PO ₄ ) ₃ is co-doped with iron and titanium, and the particle size is 1 to 2 μm Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ (x =0.005~0.015);
Activated carbon, graphite, graphene, soft carbon, hard carbon, carbon black, carbon nanotube (CNT), carbon Nanofiber (Carbon nano fiber, CNF), modified carbon (Modified carbon), carbon composite material (Carbon composite), and a carbon material selected from the group consisting of mixtures thereof;
The carbon material has a structure that covers the surface of the Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ ,
A cathode active material for a lithium secondary battery, characterized in that the discharge capacity is 175 to 190 mAh/g, and the Rct (Charge transfer resistance) value is 10 to 30 Ω.
delete delete The method of claim 1,
The Li ₃ V ₂ (PO ₄ ) ₃ is vanadium oxide (V ₂ O ₃ ), oxalic acid (C ₂ H ₂ O ₄ ), lithium carbonate (Li ₂ CO ₃ ) and ammonium phosphate. (Ammonium phosphate, NH ₄ H ₂ PO ₄ ) A cathode active material for a lithium secondary battery, characterized in that it is formed by mixing. The method of claim 1,
The iron is iron nitrate (Iron nitrate, Fe(NO ₃ ) ₃ ), and the titanium is a cathode active material for a lithium secondary battery, characterized in that it is co-doped using titanium dioxide (TiO ₂ ). In the method for manufacturing a cathode active material for a lithium secondary battery,
Dissolving vanadium oxide (V ₂ O ₃ ) and oxalic acid (C ₂ H ₂ O ₄ ) in distilled water or water to prepare a stirred mixture;
Lithium carbonate (Li ₂ CO ₃ ) and ammonium phosphate (Ammonium phosphate, NH ₄ H ₂ PO ₄ ) in the mixture were mixed and stirred to prepare monoclinic Li ₃ V ₂ (PO ₄ ) ₃ ;
The Li ₃ V ₂ (PO ₄ ) ₃ is co-doped with iron and titanium through a sol-gel method to have a particle size of 1 to 2 μm Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ (x = 0.005 ~ 0.015) to prepare;
mixing and stirring the Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ with a carbon material solution;
drying the solution in which the carbon material is mixed to form a Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ /C precursor; and
Heat treatment of the Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ /C precursor to form a structure in which the carbon material covers the surface of the Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ step; including,
Discharge capacity is 175 ~ 190 mAh / g, Rct (Charge transfer resistance) to provide a positive electrode active material for a secondary battery of 10 ~ 30Ω,
A method for manufacturing a cathode active material for a lithium secondary battery.
delete 7. The method of claim 6,
The co-doping of iron and titanium comprises:
The Li ₃ V ₂ (PO ₄ ) ₃ for a lithium secondary battery comprising the step of mixing and stirring iron nitrate (Iron nitrate, Fe(NO ₃ ) ₃ ) and titanium dioxide (TiO ₂ ) A method for manufacturing a cathode active material.
delete 7. The method of claim 6,
Heating the Li ₃ V _2-2x Fe _x Ti _x (PO ₄ ) ₃ /C precursor is,
A method of manufacturing a cathode active material for a lithium secondary battery, characterized in that heat treatment is performed at 350 to 800° C. in an argon (Ar) atmosphere.

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