KR20230101512A - A cathode active material having a core-double-shell structure, a method for manufacturing the same, a cathode comprising the same, and a secondary battery comprising the same - Google Patents

Abstract

One embodiment of the present invention, the core portion including an active material; a first shell layer formed by doping a metal on a surface of the core part; and a second shell layer coated on the surface of the first shell layer to form a layer and containing phosphate.
The positive electrode active material of the core-double shell structure has excellent electrochemical properties and excellent lifespan characteristics because structural stability and chemical stability are improved through doping and coating.

Description

A cathode active material with a core-double shell structure, a method for manufacturing the same, a cathode including the same, and a secondary battery including the same AND A SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a core-double shell structure cathode active material, and more particularly, to a core-double shell structure cathode active material in which a metal doped layer and a coating layer containing a phosphate form a double shell structure.

Recently, lithium-ion batteries have received a lot of attention because of their advantages in a wide range of applications, including portable devices, electric vehicles, hybrid electric vehicles and uninterruptible power supplies.

The ternary layered oxide LiNi _x Co _y Mn _z O ₂ (NCM), which has emerged to meet higher energy densities in active materials of lithium-ion batteries, is receiving intense research attention due to its remarkable capacity, wide operating voltage range, and excellent structural stability. .

In particular, among the LiNi _x Co _y Mn _z O ₂ family, the Ni-rich LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811) anode is one of the most promising anode candidates for high-energy Li-ion batteries.

However, for sufficient commercialization of NCM811, material problems such as inferior structure and poor chemical stability caused by increasing the nickel content for improved energy density must be improved.

(1) One of the important issues related to structural stability is the conversion from the original layered structure to a disordered spinel structure and an inactive rock-salt structure during cycling. These changes lead to severe structural collapse and capacity loss during cycling.

(2) Another important issue related to structural stability is the spontaneous reduction of Ni ³⁺ to Ni ²⁺ on the anode surface in contact with air, and the resulting Ni ²⁺ tends to occupy Li ⁺ , Ni ²⁺ (0.69 Å) and Li ⁺ (0.76 Å) having similar radii inevitably form a Li ⁺ and Ni ²⁺ mixed layer.

(3) Another important issue related to chemical stability is that Ni-rich NCM materials face the problem of dissolution of transition metals (TMs) in LiPF ₆ ^-based electrolytes, which leads to severe structural instability.

Next, (4) one of the important issues related to chemical stability is that Ni-rich NCM materials are exposed to moisture and CO ₂ in the air during the synthesis process, and many residual lithium compounds such as LiOH and Li ₂ CO ₃ exist on the surface. will be.

The formation of these lithium impurities blocks intercalation/ _{deintercalation} by diffusion of Li ⁺ ions and charge transfer reactions at the interface, and promotes the _formation of _HF. generate in large quantities.

In addition, LiOH readily reacts with lithium hexafluorophosphate (LiPF ₆ ) in the electrolyte, ultimately continuously accumulating insulating LiF on the anode surface during cycling.

In addition, residual lithium is detrimental to good positive electrode production due to gelation of the positive electrode slurry. This is because the pH value of the anode powder in water increased during the electrode manufacturing process, and unfortunately, this problem is significantly aggravated in cathode materials with a high Ni content (≥80%).

Fortunately, the problems related to structural and chemical stability occurring in NCM materials containing a large amount of Ni can be mitigated by introducing a dopant into the NCM material or modifying the surface.

However, since the method of introducing a dopant improves structural stability and the method of surface modification improves chemical stability, each method has a limitation in that an improvement effect can be obtained only in a single aspect.

Therefore, the development of technology that can simultaneously improve structural stability and chemical stability is a very urgent problem for commercialization of high-energy lithium-ion batteries.

Looking at the trend of the prior art from this point of view, for example, Korean Patent Publication No. 10-2019-0086403 (name: cathode active material, manufacturing method thereof, and lithium secondary battery including the same) has a nickel concentration of 79 mol% or more In the cathode active material comprising a, at least one of boron (B), titanium (Ti), zirconium (Zr), tungsten (W), molybdenum (Mo), tin (Sn), or tantalum (Ta) is doped Disclosure of a cathode active material characterized in that it has been attempted to provide a cathode active material with a minimized lifespan reduction characteristic in a high-nickel cathode active material, but the doping is to improve structural stability by minimizing cracks generated during charging and discharging. In view of the limitations, there is still an urgent need to develop a technology that can simultaneously improve structural stability and chemical stability.

Republic of Korea Patent Publication No. 10-2019-0086403

An object of the present invention to solve the above problems is to provide a positive electrode active material having a core-double shell structure in which both structural stability and chemical stability are improved in a positive electrode active material containing a large amount of Ni.

Another object of the present invention is to provide a method for manufacturing a cathode active material having a core-double shell structure in which both structural stability and chemical stability are improved in a cathode active material containing a large amount of Ni.

Another object of the present invention is to provide a positive electrode including a positive electrode active material of a core-double shell structure with improved structural stability and chemical stability in the positive electrode active material containing a large amount of Ni, and a secondary battery including the same.

The technical problem to be achieved by the present invention is not limited to the above-mentioned technical problem, and other technical problems not mentioned can be clearly understood by those skilled in the art from the description below. There will be.

In one embodiment of the present invention for achieving the above technical problem, the core-double shell structure of the positive electrode active material includes a core portion including an active material; a first shell layer formed by doping a metal on a surface of the core part; and a second shell layer coated on the surface of the first shell layer to form a layer and containing phosphate.

In an embodiment of the present invention, the active material may include at least one selected from the group consisting of LiNiMnCoO ₂ , LiNiO ₂ , LiNiCoO ₂ and combinations thereof.

In an embodiment of the present invention, the doped metal may include at least one selected from the group consisting of magnesium, boron, titanium, aluminum, chromium, and combinations thereof.

In an embodiment of the present invention, the phosphate may include at least one selected from the group consisting of FePO ₄ , AlPO ₄ , CoPO ₄ , Li ₃ PO ₄ , CaHPO ₄ and combinations thereof.

In an embodiment of the present invention, the first shell layer may have a thickness of 50 nm or more and 80 nm or less.

In an embodiment of the present invention, the second shell layer may have a thickness of 1.5 nm or more and 2.5 nm or less.

In order to achieve the above technical problem, in another embodiment of the present invention, a method for manufacturing a cathode active material having a core-double shell structure comprises the steps of (i) mixing an active material precursor with a shell layer precursor including a metal and a phosphate; and (ii) calcining the mixture. In this case, in the calcining, the active material precursor forms a core portion and the surface of the core portion is doped with the metal to form a first shell layer, It is characterized in that the second shell layer containing the phosphate is coated on the surface of the first shell layer.

In an embodiment of the present invention, the active material precursor and the shell layer precursor in step (i) may be mixed at a molar ratio of 1:0.01 or more and 1:0.02 or less.

In an embodiment of the present invention, the calcination in step (ii) may be performed at a temperature of 730° C. or more and 780° C. or less.

In an embodiment of the present invention, the active material precursor may include at least one selected from the group consisting of LiNiMnCoO ₂ , LiNiO ₂ , LiNiCoO ₂ and combinations thereof.

In an embodiment of the present invention, the shell layer precursor may include at least one selected from the group consisting of MgHPO ₄ , AlPO ₄ , CaHPO ₄ and combinations thereof.

In order to achieve the above technical problem, another embodiment of the present invention provides a positive electrode characterized by comprising a positive electrode active material of a core-double shell structure.

In order to achieve the above technical problem, another embodiment of the present invention provides a secondary battery comprising the positive electrode.

The effect of the present invention according to the above configuration is,

In the active material containing a large amount of Ni, a positive electrode active material of a core-double shell structure having excellent electrochemical characteristics and lifespan characteristics by suppressing phase transition to a rock salt structure during charge and discharge cycles and suppressing side reactions with electrolytes, It is possible to provide a manufacturing method, a cathode and a secondary battery including the same.

Another effect of the present invention is that doping and coating can be performed at the same time using a single material, so that it can be produced in a simple and efficient way, and since a coating layer is formed using lithium residue in the coating, the electrochemical characteristics are improved and the lifespan is improved. It is possible to provide a positive electrode active material of a core-double shell structure with improved properties, a manufacturing method thereof, and a positive electrode and a secondary battery including the same.

The effects of the present invention are not limited to the above effects, and should be understood to include all effects that can be inferred from the detailed description of the present invention or the configuration of the invention described in the claims.

1 is an image showing the structure of a positive electrode active material having a core-double shell structure, which is an embodiment of the present invention.
2 is a flowchart of a method for manufacturing a cathode active material having a core-double shell structure according to an embodiment of the present invention.
Figure 3 (a) is the XRD pattern analysis data for pristine NCM, M-NCM, P-NCM and MP-NCM.
3(b) is an enlarged view of the (003) peak in XRD patterns for pristine-NCM, M-NCM, P-NCM and MP-NCM.
4 is data summarizing the results of performing Rietveld refinements based on XRD patterns.
5 is FE-SEM images of (a) pristine-NCM, (b) M-NCM, (c) P-NCM and (d) MP-NCM.
6 is the result of SEM-EDS analysis of MP-NCM.
7 is HR-TEM images of (f) pristine-NCM, (g) M-NCM, (h) P-NCM and (i) MP-NCM.
8 is TEM-EDS line scan data of a cross-section of MP-NCM particles.
9 is data obtained by analyzing the surface layers of pristine-NCM and MP-NCM using FT-IR spectroscopy.
10 is data obtained by analyzing surface layer species of pristine-NCM and MP-NCM using time-of-flight secondary-ion mass spectrometry (TOF-SIMS).
11 and 12 are data obtained by comparing and analyzing chemical species of pristine-NCM and MP-NCM by XPS.
13 is XPS depth profile measurement data for MP-NCM.
Figure 14 (a) is a graph showing charge and discharge curves of pristine-NCM, M-NCM, P-NCM and MP-NCM.
14(b) is data obtained by measuring rate performances of pristine-NCM, M-NCM, P-NCM, and MP-NCM.
14(c) is data obtained by measuring cycle characteristics of pristine-NCM, M-NCM, P-NCM, and MP-NCM.
14(d) is data obtained by measuring cycle characteristics of pristine-NCM and MP-NCM at a temperature of 55°C.
FIG. 15(e) is data obtained by measuring the electrochemical behavior after the 1st cycle, and FIG. 15(f) is data obtained by measuring the electrochemical behavior after the 100th cycle.
Figure 15 (g, h) is the data measured in the voltage range of 2.7V to 4.3V pristine-NCM and MP-NCM by cyclic voltammetry (CV).
16 is data obtained by measuring phase changes during the charging process using in situ XRD for pristine-NCM and MP-NCM.
17 is HR-TEM, FFT and SEAD pattern images of pristine-NCM.
18 is HR-TEM, FFT and SEAD pattern images of MP-NCM.
19 is a TEM image of a cross section of pristine-NCM and MP-NCM.

Hereinafter, the present invention will be described with reference to the accompanying drawings. However, the present invention may be embodied in many different forms and, therefore, is not limited to the embodiments described herein. And in order to clearly explain the present invention in the drawings, parts irrelevant to the description are omitted, and similar reference numerals are attached to similar parts throughout the specification.

Throughout the specification, when a part is said to be "connected (connected, contacted, combined)" with another part, this is not only "directly connected", but also "indirectly connected" with another member in between. "Including cases where In addition, when a part "includes" a certain component, it means that it may further include other components without excluding other components unless otherwise stated.

Terms used in this specification are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as "include" or "have" are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, but one or more other features It should be understood that the presence or addition of numbers, steps, operations, components, parts, or combinations thereof is not precluded.

Hereinafter, the cathode active material 10 having a core-double shell structure, which is an embodiment of the present invention, will be described in detail with reference to the accompanying drawings.

In the present invention, the core-double shell structure of the positive electrode active material 10 includes a core portion 100 including an active material; a first shell layer 200 forming a layer by doping a metal on the surface of the core part 100; and a second shell layer 300 coated on the surface of the first shell layer 200 to form a layer and containing phosphate.

Prior to a detailed description of each component, the reason for including the above configuration will be examined first.

The present invention was developed to solve problems occurring in a cathode active material containing a high Ni content.

As described above, in the cathode active material containing a high content of Ni, i) the layered structure is converted into a rock-salt structure leading to structural collapse, ii) transition metal depending on the electrolyte ) is eluted and becomes structurally unstable, iii) lithium residue generates gas (CO ₂ , etc.) and expands the active material and electrode, iv) lithium residue accumulates insulating material on the surface of the active material and electrode Various problems such as deteriorating electrochemical properties and life characteristics occur.

In the present invention, in order to commercialize such a positive electrode active material containing a high content of Ni, a doped layer (first shell layer 200) doped with a metal is formed on the core portion 100 containing the active material, and the doped layer ( 200) is characterized in that a coating layer (second shell layer 300) containing phosphate is formed on it.

Specifically, the doped metal is substituted into the lattice of the active material 100 to improve the structural stability of the lattice, and thus, the active material 100 is suppressed from changing the lattice or structural transition even during continued charging and discharging, thereby improving the structural stability.

In addition, the coating layer 300 blocks side reactions between the active material 100 and the electrolyte to remove elements that adversely affect the electrochemical characteristics and lifespan characteristics of the active material 100. In particular, the coating layer 300 containing phosphate contains a large amount of Ni. Since the coating layer 300 is formed by consuming the lithium residue, which is a major problem in the active material 100, the side reaction caused by the lithium residue is suppressed and the generation of an insulating material generated by the reaction of the lithium residue is suppressed.

Accordingly, the positive active material 10 of the core-double shell structure proposed by the present invention has excellent electrochemical characteristics and significantly improved lifespan characteristics.

Hereinafter, each configuration of the positive electrode active material 10 having a core-double shell structure according to the present invention will be examined.

First, the core part 100 including an active material will be described.

Referring to FIG. 1 , the active material 100 forms a core in the positive electrode active material 10 having a core-double shell structure. In terms of shape, the spherical shown in FIG. 1 is exemplary, and may have various shapes depending on synthesis conditions, but if the structure is covered on the entire surface by the double shell, it should be interpreted as being included in the present invention.

The active material 100 may include at least one selected from the group consisting of LiNiMnCoO ₂ , LiNiO ₂ , LiNiCoO ₂ and combinations thereof.

However, it is not limited thereto, and in the present invention, the solution principle directly related to the active material 100 is i) the doped metal replaces the transition metal in the active material 100 to form a stronger bond and a wider ion channel, ii) in terms of blocking or removing lithium residues,

It should be construed as being included in the present invention as long as it is a) a lithium-based active material, and b) includes a transition metal that forms a weaker bond with a lattice than a doped metal and forms a narrower ion channel with a smaller particle radius. .

For example, the active material 100 may be LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (NCM811).

Next, the first shell layer 200 forming a layer by doping metal on the surface of the core portion 100 will be described .

Referring to FIG. 1 , the first shell layer 200 is positioned on the core part 100 and surrounds the entire surface of the core part 100 to form a layer. Specifically, the doped metal penetrates into the surface layer of the active material 100 constituting the core part 100 and forms a layer in such a way that it is substituted with some elements of the lattice of the active material 100 .

At this time, the doped metal has a larger particle radius than the existing element substituted in the lattice of the active material 100 to form a wider ion channel in the lattice of the active material 100, and forms a stronger bond with the lattice of the active material 100 to form an active material ( 100) It is preferable to adopt a material capable of imparting structural stability to the lattice.

In consideration of this, the doped metal may include at least one selected from the group consisting of magnesium, boron, titanium, aluminum, chromium, and combinations thereof. However, it is not limited thereto, and any material that satisfies the above conditions should be construed as being included in the present invention.

Next, the first shell layer 200 may exist from about 2 nm away from the particle surface of the core-double shell structure positive active material 10 to about 80 nm into the particle.

At this time, the thickness of the first shell layer 200 may be 50 nm or more and 80 nm or less, which is that the metal to be doped is normally doped into the active material to increase the space in which lithium ions can penetrate in the crystal lattice and effectively exhibit electrochemical properties. is the thickness to improve

Next, the second shell layer 300 coated on the surface of the first shell layer 200 to form a layer and containing phosphate will be described.

Referring to FIG. 1, the second shell layer 300 is located on the first shell layer 200, and unlike the first shell layer 200 penetrating into the active material 100 to form a layer, the first shell layer 200 forms a layer. It is characterized in that a separate layer is formed on the 1 shell layer 200.

In shape, like the first shell layer 200, it is not limited to the spherical shape shown in FIG. .

As described above, the second shell layer 300 serves to block the reaction with the electrolyte, which adversely affects the electrochemical characteristics and lifespan characteristics of the active material 100 . In addition, it serves to add structural stability by physically maintaining the active material 100 firmly.

In particular, the second shell layer 300 is characterized in that it contains phosphate as a coating material. As will be described in detail in experimental examples to be described later, phosphate binds and consumes lithium residues (LiOH and Li ₂ CO ₃ ) generated by the reaction of a lithium-based active material with moisture or CO ₂ in the air to form a coating layer 300. form

The lithium residue promotes the elution of transition metal ions in the active material 100, generates an insulating material (LiF), and expands the active material 100 and the electrode to contribute to generating gas (CO ₂ ) that harms structural stability. In this regard, the removal of lithium residues is essential.

Therefore, the second shell layer 300 containing the phosphate is formed by removing lithium residues that adversely affect the active material 100, and at the same time, the coating layer 300 is formed to contribute to the structural stability and chemical stability of the active material 100. It can be said that it is very encouraging to improve the lifespan characteristics of lithium-based active materials containing a large amount of Ni.

Based on this, the phosphate may include at least one selected from the group consisting of FePO ₄ , AlPO ₄ , CoPO ₄ , Li ₃ PO ₄ , CaHPO ₄ and combinations thereof. However, it is not limited thereto, and it should be construed as being included in the present invention if it is a material of phosphate synthesized with a metal composed of cations and Li ₃ PO ₄ .

Next, the second shell layer 300 is characterized in that the thickness is 1.5 nm or more and 2.5 nm or less.

Specifically, when the thickness of the second shell layer 300 is less than 1.5 nm, the coating layer 300 does not exist on the surface of the particles relatively or the coating layer 300 is too thin to effectively block side reactions between the active material 100 and the electrolyte It cannot be done, and if it exceeds 2.5 nm, it causes difficulties in smooth lithium ion desulfurization due to the too thick coating layer 300, so the thickness of the second shell layer 300 is preferably 1.5 nm or more and 2.5 nm or less.

Hereinafter, a method for manufacturing a cathode active material having a core-double shell structure, which is another embodiment of the present invention, will be described in detail with reference to the accompanying drawings. In the description, components overlapping with the positive electrode active material 10 of the above-described core-double shell structure should be interpreted in the same way, and overlapping descriptions will be omitted.

In the present invention, the method for manufacturing a core-double shell structure positive active material includes (i) mixing an active material precursor with a shell layer precursor including a metal and a phosphate (S100); and (ii) calcining the mixture (S200). In the calcining, the active material precursor forms the core part 100 by the calcination, and the metal It is doped to form the first shell layer 200, characterized in that the second shell layer 300 containing the phosphate is coated on the surface of the first shell layer 200.

Prior to a detailed description of each step, the reason for including the above step will be examined first.

A first feature of the present invention in the manufacturing method is that a metal doped layer (first shell layer 200) and a coating layer (second shell layer 300) are formed by a single material and a single process. Here, a single material means that there is no need to separately prepare a material for forming the metal doped layer 200 and a material for forming the coating layer 300, and a single process is separate from the step for forming the metal doped layer 200. This means that a separate process for forming the coating layer 300 is not required.

Specifically, through a single process of mixing and burning a shell layer precursor containing a metal and a phosphate with an active material precursor, the metal of the shell layer precursor penetrates into the active material 100 to form a doped layer (first shell layer 200), The phosphate of the shell layer precursor forms a coating layer (second shell layer 300) on the doped layer 200 that is distinct from the doped layer 200.

Therefore, this manufacturing method is expected to have a low manufacturing cost because the process method is simple and very efficient, and it can be considered suitable for commercialization.

Next, in the manufacturing method, the second characteristic of the present invention is that the coating layer (second shell layer 300) is formed while consuming or removing lithium residues (LiOH, Li ₂ CO ₃ ). As described above, the lithium residue adversely affects the electrochemical properties and lifespan characteristics of an active material or an electrode.

In light of this, this manufacturing method is excellent in process efficiency by solving the process required to remove the elements that adversely affect the active material and the process required to improve the active material in a single process, and at the same time, the effect is demonstrated in the experiments described later. As you will see in detail in the example, it is very good.

Hereinafter, each step of the method for manufacturing a positive active material having a core-double shell structure according to the present invention will be described.

First, (i) mixing an active material precursor with a shell layer precursor including a metal and a phosphate (S100); will be described.

In the mixing of step (i) (S100), it is characterized by a liquid phase process. The existing coating process and the doping process are mainly performed by a solid-state method, but the present invention is characterized in that it is performed by a liquid-state process in that it is the most economical and simple.

The content of the desired active material and the first shell layer 200 and the second shell layer 300 are mixed in consideration of the thickness, and accordingly, the active material precursor and the shell layer precursor in step (i) (S100) are 1:0.01 or more 1: They may be mixed in a molar ratio of 0.02 or less.

Specifically, when the active material precursor and the shell layer precursor are mixed at a molar ratio of less than 1:0.01, an area without the first shell layer 200 or the second shell layer 300 partially exists on the surface of the core part 100, , 1: When mixed in excess of 0.02, the first shell layer 200 and the second shell layer 300 exist thickly, and the performance of the cathode material is rather deteriorated. Accordingly, the mixing molar ratio of the active material precursor and the shell layer precursor is preferably 1:0.01 or more and 1:0.02 or less.

In the material, the active material precursor may include at least one selected from the group consisting of LiNiMnCoO ₂ , LiNiO ₂ , LiNiCoO ₂ and combinations thereof.

In addition, the shell layer precursor may include at least one selected from the group consisting of MgHPO ₄ , AlPO ₄ , CaHPO ₄ and combinations thereof. However, it is not limited thereto, and the shell layer precursor is doped with metal cations having a strong bonding force with oxygen on the surface of the core part 100, and phosphate is coated on the doped layer (the first shell layer 200) to perform coating and metal doping. Any material that simultaneously enjoys the synergistic effect of should be interpreted as being included in the present invention.

Next, (ii) calcining the mixture (S200); will be described.

By the calcination, the metal is doped on the surface of the core part 100 to form the first shell layer 200, and the surface of the first shell layer 200 is coated with the second shell layer 300 containing the phosphate. to be characterized

At this time, the calcination of step (ii) (S200) may be performed at a temperature of 730° C. or more and 780° C. or less. Specifically, if the calcination temperature is less than 730 ° C or greater than 780 ° C, the synthesis of the mixture is not smooth.

Hereinafter, a cathode and a secondary battery as another embodiment of the present invention will be described.

Characterized in that the positive electrode includes the above-described positive electrode active material 10 of the core-double shell structure, and the secondary battery includes a positive electrode including the positive electrode active material 10 of the core-double shell structure do.

Since the positive electrode and the secondary battery include the positive electrode active material 10 having the core-double shell structure, structural stability and chemical stability are excellent, and thus, electrochemical characteristics and lifespan characteristics are remarkably excellent. Detailed characteristics will be described later in detail in the following experimental examples.

manufacturing example

(1) Manufacturing overview

In order to confirm the characteristics of the cathode active material in which the doped layer and the coating layer presented in the present invention form a core-double shell structure, pure NCM (pristine NCM), NCM doped with Mg (M-NCM), and Li ₃ PO ₄ coated NCM (P-NCM) and NCM (MP-NCM) treated with MgHPO4 were prepared, and electrodes and secondary batteries including the active material of the core-double shell structure proposed in the present invention were additionally prepared.

(2) Preparation of pristine NCM

Commercial LiNi _0.8 Co _0.1 Mn _0.1 (OH) ₂ from Cosmo Material Co. was used as a precursor, and LiOH·H ₂ O (Li/stoichiometry = 1.05) was uniformly mixed with it.

Then, the mixture was preheated at 500 °C for 5 hours and calcined at 750 °C for 15 hours in an air conduit to prepare pristine NCM.

(3) Preparation of M-NCM

Precursor LiNi _0.8 Co _0.1 Mn _0.1 (OH) ₂ and LiOH were mixed at a ratio of 1:1.05, and then mixed with magnesium source Mg(OH) ₂ at a molar ratio of 1 wt%, followed by calcination at 750 °C for 15 hours in an air atmosphere. Thus, M-NCM was prepared.

(4) Preparation of P-NCM

After adding 5 polyphosphoric acid (PPA, 0.4 g) to dimethyl sulfoxide (DMSO, 10 g), NCM (5 g) was added.

Thereafter, the slurry was stirred at 80° C. until only solids remained, and thereafter, vacuum drying was performed at 80° C. to completely remove residual moisture to prepare Li ₃ PO ₄ coated NCM (P-NCM).

(5) Preparation of MP-NCM

The LiNi _0.8 Co _0.1 Mn _0.1 (OH) ₂ precursor was thoroughly mixed with MgHPO ₄ .3H ₂ O in a molar ratio of 1:1.015.

Thereafter, the resulting mixture was calcined through the same method as the above-mentioned heating method (750° C. for 15 hours) to prepare NCM (denoted MP-NCM) treated with MgHPO ₄ .

(6) Preparation of a positive electrode containing an active material of a core-double shell structure

N-methyl pyrrolidone with 96wt% NCM811 powder, 2wt% Super-P carbon black and 2wt% polyvinylidene fluoride (PVDF) binder on aluminum (Al) foil loaded with ~14.6mg cm ⁻² of active material NMP) slurry was cast.

Thereafter, the NMP solvent was evaporated by drying in a vacuum at 120° C. for 24 hours or more to prepare a positive electrode including an active material having a core-double shell structure.

(7) Manufacturing of a secondary battery including the positive electrode

The battery was prepared by adding 1M LiPF ₆ to a solution in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 1:1:1 as an electrolyte, and a lithium metal disk was used as an anode. Polyethylene (PE) was prepared as a separator and assembled in an Ar-filled glove box.

Experimental example 1

Crystallographic analysis experiments using XRD and Rietveld refinements

(1) Experiment outline

In this experiment, XRD pattern analysis was performed to confirm the phase and crystal structure of the NCM prepared according to the above Preparation Example.

In addition, the results were analyzed through Rietveld refinements for further analysis of the crystal structure.

(2) Result analysis 1 - XRD pattern analysis

Figure 3 (a) is the XRD pattern analysis data for pristine NCM, M-NCM, P-NCM and MP-NCM.

According to Figure 3(a), all the diffraction peaks of pristine NCM, M-NCM, P-NCM and MP-NCM are in good agreement with the hexagonal α-NaFeO ₂ layered structure belonging to the space group R-3m, resulting in high It was confirmed that it exhibits crystallinity.

At the same time, it was confirmed that the prepared NCM had a well-ordered layered structure through clear peak splitting of (006)/(102) and (108)/(110) peaks.

3(b) is an enlarged view of the (003) peak in XRD patterns for pristine-NCM, M-NCM, P-NCM and MP-NCM.

According to FIG. 3(b), the M-NCM and MP-NCM peaks doped with Mg ²⁺ are slightly shifted to the left in the 2θ axis compared to the peaks of pristine-NCM and P-NCM ^undoped with Mg 2+ . appeared.

This is a clear result indicating an increase in the lattice parameter and lattice spacing ^, so Mg ²⁺ doping increases the lattice parameter and lattice spacing through Fig. This can be seen as a result of confirming that the structural stability of the active material can be improved during cycling.

(3) Analysis of results 2 - Rietveld refinements

4 is data summarizing the results of performing Rietveld refinements based on XRD patterns.

In FIG. 4, parameter a corresponds to the inter-layer metal-to-metal distance, and parameter c corresponds to three times the inter-slab distance.

According to FIG. 4, it can be confirmed that the values of parameter a and parameter c of Mg ²⁺ doped M-NCM and MP-NCM are relatively larger than those of Mg ²⁺ undoped pristine-NCM and P-NCM. .

These results can be interpreted as follows.

1) A small amount of Mg ²⁺ (0.72Å) with a large ionic radius replaces Ni ²⁺ (0.69Å) with a small ionic radius at the lithium site, not only increasing the c-axis, This is the result of increasing the a-axis at transition metal sites (Ni ³⁺ (0.56Å), Co ³⁺ (0.54Å), Mn ⁴⁺ (0.53Å)).

2) In the layered structure, the bond length of Mg ²⁺ -O is greater than that of Ni-O.

In other words, it can be seen that Mg doping can increase the lattice parameters a and c of the Li layer and the transition metal layer of the NCM material to promote Li+ ion diffusion and improve structural stability during cycling.

Additionally, the I(003)/I(104) value summarized in FIG. 4 can be used to ^determine the extent ^of cation disorder in the layered compound.

It is known to suffer from undesirable cation disorder when the value of I(003)/I(104) is less than 1.2. Based on this, all NCMs measured in this experiment have values significantly greater than 1.2, indicating that they have a distinct layered structure divided into distinct layers.

Experimental Example 2

Structural analysis experiment of Mg doped layer and coating layer using SEM and TEM

(1) Experiment outline

Confirm the synthesis state and distribution of component elements of the NCM prepared according to the preparation example through SEM, and check the state of doping and coating through TEM to confirm the double layer structure of the anode material of the present invention core-double shell structure This experiment was conducted.

(2) Result analysis 1 - Confirmation of synthesis state and component element distribution state through SEM

5 is FE-SEM images of (a) pristine-NCM, (b) M-NCM, (c) P-NCM and (d) MP-NCM.

According to FIG. 5, each particle of all the synthesized NCMs is a spherical particle with an average diameter of 8.0 μm to 12.0 μm (this is called a secondary particle), and these secondary particles are again about 250 nm in size and are tightly (tightly). ), it can be confirmed that it is composed of agglomerated particles (which are referred to as primary particles).

In order to more closely analyze the synthesis state of MP-NCM presented in the present invention, EDS analysis was performed.

6 is the result of SEM-EDS analysis of MP-NCM.

According to Figure 6, it can be seen that Ni, Co, Mn, Mg and P are uniformly dispersed in MP-NCM. Through this, it was confirmed that the NCMs were uniformly synthesized and sufficient as samples prior to the analysis to be performed below, and it is expected that the method for manufacturing a cathode active material of a core-double shell structure proposed by the present invention produces uniform synthesis results. can

(3) Result Analysis 2 - Confirmation of doping and coating layer formation through TEM

7 is HR-TEM images of (f) pristine-NCM, (g) M-NCM, (h) P-NCM and (i) MP-NCM.

According to FIG. 7, (f) pristine-NCM and (g) M-NCM show clean and smooth particle surfaces free from impurities. On the other hand, (h) P-NCM and (i) MP-NCM have a thin layer with a thickness of about 2 nm.

This difference is the result of Mg penetrating into the inner surface of the NCM particle to form a layer by doping, but Li ₃ PO ₄ being coated on the outer surface of the NCM particle to form a separate layer. It can also be interpreted as a result indicating that the Li ₃ PO ₄ layer is formed by combining lithium residues (Li ₂ CO ₃ and LiOH) formed by eluting out of the NCM during the heat treatment process and MgHPO ₄ .

TEM-EDS line scans of MP-NCM particle cross-sections were performed to analyze this double layer structure in more detail.

8 is TEM-EDS line scan data of a cross-section of MP-NCM particles.

According to FIG. 8, the P element is distributed in a low level content only within 2 nm from the outermost layer, while the Mg element is concentrated in the MP-NCM bulk lattice in the range of 2 nm to 5 nm from the outermost layer, so that Li ₃ PO ₄ It can be seen that the coated portion and the Mg-doped portion are clearly distinguished.

Through the above results, when NCM is treated using MgHPO ₄ proposed by the present invention, the doping of the metal layer and the formation of the coating layer are clearly separated and performed with a single material, which means that the treatment using MgHPO ₄ contains a large amount of Ni. This can be said to be a result confirming that the structural stability provided by the doping layer and the chemical stability provided by the coating layer can be provided to the active material.

Experimental Example 3

Mg doping and Li through surface layer analysis ₃ PO ₄ Experiment to confirm the effect of coating

(1) Experiment outline

In this experiment, pristine-NCM and MP were analyzed using Fourier transform infrared spectroscopy (FT-IR spectroscopy) and time-of-flight secondary-ion mass spectrometry (TOF-SIMS). - The elemental composition of the surface layer of NCM was analyzed, and through this, the effects of Mg doping and Li ₃ PO ₄ coating on NCM containing a large amount of Ni were confirmed.

(2) Analysis of results 1 - Analysis of surface layer elements of pristine-NCM and MP-NCM

9 is data obtained by analyzing the surface layers of pristine-NCM and MP-NCM using FT-IR spectroscopy.

Referring to FIG. 9, peaks or bands at 865.6 cm ^-1 , 1442.3 cm ^-1 , 1502.2 cm ^-1 and 3200 to 3650 cm ^-1 strongly appearing in pristine-NCM are different from those of MP-NCM.

Specifically, the peak at 865.6 cm ^-1 is related to out-plane flexural vibration of CO ₃ ^2- , and the peaks at 1442.3 cm ^-1 and 1502.2 cm ^-1 are related to asymmetric stretching of CO ₃ ^2- . Associated with vibration (anti-symmetric stretching vibration), it represents Li ₂ CO ₃ .

And the band at 3200 to 3650 cm ^-1 represents LiOH in relation to the stretching vibration of OH.

In the case of MP-NCM, the above-mentioned peaks (865.6 cm ^-1 , 1442.3 cm ^-1 , 1502.2 cm ^-1 and 3200~3650 cm ^-1 ) are absent or weak compared to pristine-NCM, and the amount of Li ₂ CO ₃ and LiOH It can be seen that this is remarkably small.

On the other hand, MP-NCM exhibits strong peaks at 1035.8 cm ^-1 and 1085.4 cm ^-1 , showing a difference from pristine-NCM, which is associated with asymmetric stretching vibration of PO and represents PO ₄ ^3- .

Summarizing the above results, it can be seen that in the case of MP-NCM, a Li ₃ PO ₄ coating layer is formed and lithium residual compounds (Li ₂ CO ₃ and LiOH) are removed or reduced.

(3) Analysis of results 2 - Analysis of surface layer species

10 is data obtained by analyzing surface layer species of pristine-NCM and MP-NCM using time-of-flight secondary-ion mass spectrometry (TOF-SIMS).

Specifically, FIG. 10 shows typical species of cathode-electrolyte interphases (CEI) generated by electrolyte oxidation or transition metal elution in pristine-NCM and MP-NCM.

The positive electrolyte interphase layer (CEI) is divided into an inner layer and an outer layer. Generally, the outer layer is POF ₂ ^- , C ₂ F ^- which is an organic and inorganic interphase species generated by oxidative decomposition of electrolyte and LiPF ₆ . and C ₂ HO ^- , and the inner layer is mainly a result of active material decomposition, especially active material decomposition exacerbated by HF and unstable from layered structure to rock-salt structure. It is considered a layer containing LiF ₂ ^- , MnF ₃ ^- and NiF ₃ ^- originating from transition metal migration.

Referring to FIG. 10 based on the foregoing, MP-NCM has all chemical species (POF ₂ ^- , C ₂ F ^- , C ₂ HO ^- , LiF ₂ ^- , MnF ₃ ^- and NiF ₃ ^- compared to pristine-NCM). ) can be found to be small.

In addition, in the case of pristine-NCM, POF ₂ ^- , C ₂ F ^- , C ₂ HO ^- and LiF ₂ ^- are detected in large quantities even after 1600s of sputtering, which means that electrolyte penetrates into the bulk of pristine-NCM. .

In addition, the MnF ₃ ^- and NiF ₃ ^- detected in MP-NCM were smaller than those in pristine-NCM, which indicates that the parasitic reaction between the anode and the electrolyte was reduced, as well as the final disappearance of the cathode-electrolyte interface layer (CEI).

In summary of the above analysis, the core-double shell structure by Mg doping and Li ₃ PO ₄ coating proposed by the present invention reduces the occurrence of cracks in the active material containing a large amount of Ni, and prevents unintended side reactions between the active material and the electrolyte. can be reduced, and ultimately, it was confirmed that active material decomposition and electrolyte decomposition can be effectively alleviated.

Experimental Example 4

Chemical species analysis experiment using XPS

(1) Experiment outline

This experiment was carried out in a way to comparatively analyze pristine-NCM and MP-NCM using XPS (X-ray photoelectron spectroscopy) in order to understand the chemical state of MP-NCM more closely.

(2) Analysis of results

11 and 12 are data obtained by comparing and analyzing chemical species of pristine-NCM and MP-NCM by XPS.

1) O 1s XPS spectrum

11(a) is an O 1s XPS spectrum, and a peak having a binding energy of 528.9 eV represents lattice oxygen (O _lattice ) and a peak of 531.5 eV represents impurity oxygen (O _impurity ).

According to FIG. 11(a), the O _lattice peak intensity is stronger and the O _impurity peak intensity is weaker in MP-NCM than in pristine-NCM. A strong O _lattice peak means strong Mg-O bonding, and a strong O _impurity peak means strong side reactions with active oxygen (O ^- , O ^2- and CO ₃ ^2- ) on the surface of the active material.

Therefore, FIG. 11 (a) can be seen as a result showing that MP-NCM has excellent structural stability due to a strong Mg-O bond compared to pristine-NCM, and excellent air stability due to less side reactions on the surface.

2) C 1s XPS spectrum

Figure 11 (b) is a C 1s XPS spectrum, a peak with a binding energy of 284.6 eV represents CC bonding and a peak of 289.5 eV represents CO ₃ ^2- .

According to FIG. 11(b), it can be seen that the peak of pristine-NCM is broader overall, which is because the surface of the active material is covered with lithium impurities (LiOH, LiHCO ₃ and Li ₂ CO ₃ ) due to chemical weathering. It means surrounded.

In addition, in the case of MP-NCM, the peak representing CO ₃ ^2- is rather low, which means that MgHPO ₄ consumes lithium impurities absorbed on the surface of active material particles during the annealing process.

Through the above results, it was confirmed that the treatment using MgHPO ₄ can improve the lithium storage capacity by suppressing residual lithium that has a negative effect.

3) Ni 2p XPS spectrum

11(c) is the Ni 2p XPS spectrum, and the peak with a binding energy of 855.2 eV represents Ni 2p _3/2 .

Referring to FIG. 11(c), the weak Ni 2p _3/2 of pristine-NCM means that the surface of the active material is covered with residual lithium impurities.

In addition, in the case of MP-NCM, the Ni 2p _3/2 spectrum shows a Ni ³⁺ (856.5eV) peak with stronger intensity than that of pristine-NCM, indicating that most of the Ni present on the surface of MP-NCM is Ni ³⁺ means to exist.

Summarizing the above results, it can be seen that the MgHPO ₄ treatment has excellent structural stability of MP-NCM because the Li ⁺ /Ni ²⁺ bond, which adversely affects structural stability, is reduced as described above.

4) Co 2p XPS spectrum

11(d) is a Co 2p XPS spectrum, and a peak having a binding energy of 779.5 eV represents 2p _3/2 and a peak having a binding energy of 794.4 eV represents 2p _1/2 .

Referring to FIG. 11 (d), it can be seen that the peak position of Co 2p for MP-NCM is slightly shifted relative to pristine-NCM. These differences indicate that the atomic state of Co is affected by MgHPO ₄ treatment.

5) Mn 2p XPS spectrum

12(e) is the Mn 2p XPS spectrum, and the peak with a binding energy of 642.2 eV represents Mn 2p _3/2 and the peak with 653.9 eV represents Mn 2p _1/2 .

According to FIG. 12(e), the Mn 2p _3/2 peak appears dominant, which means the dominant Mn ⁴⁺ cation.

6) Mg 1s XPS spectrum

12(f) is the Mg 1s XPS spectrum, and a peak representing Mg 1s is observed when the binding energy is 1302.7 eV in MP-NCM. These results correspond to strong evidence that Mg ²⁺ ions replace the crystal lattice of NCM materials by doping.

7) P2p XPS spectrum

12(g) is a P 2p XPS spectrum, and a peak indicating 2p _3/2 is observed when the binding energy is 132.7 eV in MP-NCM. Since the 2p _3/2 peak represents Li ₃ PO ₄ , it was confirmed that the Li ₃ PO ₄ coating layer was effectively formed by the MgHPO ₄ treatment.

8) XPS depth profile measurement for MP-NCM

13 is XPS depth profile measurement data for MP-NCM.

13, P is detected when the etching depth is 0 nm to 2 nm, while Mg is detected at an etching depth of 2 nm to 80 nm. These results are very consistent with FIG. 8 of Experimental Example 2 described above.

9) Overall

Summarizing the above results, Mg ions serving as pillar ions were incorporated into NCM by treatment with MgHPO ₄ , and at the same time, a Li ₃ PO ₄ layer resulting from the reaction between phosphate ions and lithium impurities was successfully coated on NCM. confirmed that it is.

Experimental Example 5

Electrochemical property measurement experiment

(1) Experiment outline

Electrochemical properties were measured for the electrode prepared according to the above-described Preparation Example. As a common condition for each experiment, measurement was performed for an electrode thickly loaded with about 14.6 mg cm ⁻² .

(2) charge and discharge profile

1) Measurement conditions

This charge/discharge profile measurement experiment was performed under conditions of 25°C, 3.0V to 4.3V, and 0.1C (1C=202mAh g ^-1 ).

2) Result analysis

Figure 14 (a) is a graph showing charge and discharge curves of pristine-NCM, M-NCM, P-NCM and MP-NCM.

According to FIG. 14 (a), no additional potential plateau or polarization curve was observed in all measured charge/discharge curves. This means that the doped Mg ²⁺ and the coated Li ₃ PO ₄ do not participate in the electrochemical reaction.

In addition, pristine-NCM showed the lowest initial discharge capacity of 199.1mAh g ^-1 and the lowest coulombic efficiency of 85.5%, and N-NCM and P-NCM showed slightly higher initial discharge capacity than pristine-NCM. , MP-NCM showed the highest initial discharge capacity of 203.5mAh g ^-1 and the highest coulombic efficiency of 89.4%.

Looking at the cause of MP-NCM showing the highest initial discharge capacity and highest coulombic efficiency, doping with Mg ²⁺ and coating with Li ₃ PO ₄ on the NCM-based active material containing a large amount of Ni resulted in a faster rate than other NCMs. It is analyzed that this is due to the endowment of charge transfer and greater Li ⁺ storage capacity.

(3) Measure rate performances

1) Measurement conditions

Measured under C-rate conditions of 3.0V to 4.3V and 0.5C to 6.0C

2) Result analysis

14(b) is data obtained by measuring rate performances of pristine-NCM, M-NCM, P-NCM, and MP-NCM.

Looking at Figure 14 (b), the capacity retentions in all NCMs were inversely proportional to the C-rate.

Also, at low C-rates (0.5C, 1.0C, and 2.0C), the difference in retention capacity between NCMs was subtle. However, the difference in retention capacity increased as the C-rate increased, and the difference in retention capacity became clear at high C-rates (4.0C and 6.0C).

Specifically, in the case of pristine-NCM, it showed a capacity of 151.3mAh at a high C-rate of 6.0C, showing a clear capacity drop and low rate characteristics. It appears to be due to a decrease in electronic and ionic conductivities due to the occurrence of decay.

On the other hand, M-NCM, P-NCM, and MP-NCM with surface-modified active materials showed higher capacity retention rates compared to pristine-NCM.

In particular, MP-NCM showed the highest capacity retention rate at all measured C-rates, and the highest discharge capacity at 6.0C with 169.4mAh g-1. In addition, when the C-rate returned from 6.0C to 2.0C, it was the only NCM investigated to recover to the same initial capacity as the initial one.

These results show that i) Mg is substituted in the NCM lattice by doping to form Mg-O bonds, and the length of Mg-O bonds is longer than that of Ni-O bonds, which expands the diffusion path of lithium ions and electrons, thereby increasing the conductivity of lithium ions. It seems to be because the movement of electrons and electrons became faster. ii) Also, it seems that the Li ₃ PO ₄ coating layer can promote the diffusion of lithium even at a high C-rate due to its ionic conductivity. Iii) Also, it seems that the MgHPO ₄ treatment effectively reduces residual lithium, which hinders the penetration of Li ⁺ , by forming a Li ₃ PO ₄ coating layer.

In summary of the above results, it was confirmed that the treatment using MgHPO ₄ according to the present invention reduces interfacial resistance and can act as an accelerator for rapid movement of lithium ions through NCM.

(4) Cycling test 1

1) Measurement conditions

After 100 cycles for pristine-NCM, M-NCM, P-NCM, and MP-NCM, it was measured under the speed conditions of 3.0V to 4.3V, 25℃ and 0.5C.

2) Result analysis

14(c) is data obtained by measuring cycle characteristics of pristine-NCM, M-NCM, P-NCM, and MP-NCM.

Referring to FIG. 14(c), the discharge capacity of pristine-NCM rapidly decreased to 134.9mAh g ^-1 after 100 cycles, showing a capacity retention of 72.4%, showing the lowest cycle characteristics among all NCMs.

On the other hand, M-NCM, P-NCM and MP-NCM showed capacity retention of 82.4%, 80.7% and 86.3% after 100 cycles, respectively.

Analyzing the results below,

First of all, oxygen release, Li ₂ CO ₃ , and generation of unwanted gases due to electrolyte decomposition are major causes that adversely affect cycle characteristics, ultimately resulting in a serious capacity decrease.

Specifically, the release of lattice oxygen (O _lattice ) causes cathodic oxidation of Ethylene Carbonate (EC)/Dimethyle Carbonate (DMC)-based solvents to release CO and CO ₂ .

O _lattice + EC/DMC → 2CO ₂ + CO + 2H ₂ O (1)

2O _lattice → O ₂ (2)

In addition, the reaction between LiPF ₆ and Li ₂ CO ₃ produces POF ₃ and CO ₂ .

LiPF ₆ + Li ₂ CO ₃ → POF ₃ + CO ₂ + 3LiF (3)

In addition, unstable LiPF ₆ , an electrolyte salt, is easily decomposed to generate HF in the presence of moisture.

LiPF ₆ → LiF + PF ₅ (4)

PF ₅ +H ₂ O → POF ₃ +2HF (5)

POF ₃ + Li ₂ O → 6LiF + P ₂ O ₅ (6)

In addition, the LiPF ₆ decomposition reaction causes dissolution of transition metal ions by HF, and the produced insulating LiF is precipitated on the surface of the anode to increase resistance. Accordingly, the structure of the NCM is deformed, resulting in continuous performance degradation.

Based on this, the MP-NCM formation using MgHPO ₄ proposed by the present invention can effectively solve the above problems and improve the electrochemical performance of NCM by simultaneously performing Mg ²⁺ doping and Li ₃ PO ₄ coating. .

Specifically, looking at the role of the Li ₃ PO ₄ coating layer, it not only inhibits the formation of internal cracks in P-NCM and MP-NCM, but also prevents the dissolution of transition metals and side reactions between electrodes and electrolytes.

In addition, the Li ₃ PO ₄ coating layer serves to remove residual moisture and HF from the electrolyte near the NCM surface.

Li ₃ PO ₄ + H ₂ O = Li _x H _y PO ₄ (or PO _x H _y ) + Li ₂ O (7)

Li ₃ PO ₄ + HF = Li _x H _y PO ₄ (or PO _x H _y ) + LiF (8)

Accordingly, the propagation of HF during cycling is suppressed, the acidity of the electrolyte is lowered, and the deterioration of the NCM surface is greatly suppressed. In particular, the most important thing is that when the Li ₃ PO ₄ layer is formed on the NCM surface, as mentioned in the above equations (1)-(3), Li ₂ O, LiOH and Li ₂ CO ₃ that adversely affect the cycle characteristics can be reduced. there is.

Next, looking at the role of the Mg doped layer, in the case of M-NCM and MP-NCM, Mg ²⁺ is bonded to the NCM structure to provide excellent structural stability. This is reasonable in that the bond dissociation energy of Mg-O is 394 kJ/mol and the bond dissociation energy of Ni-O is 391 kJ/mol.

In addition, Mg ²⁺ reduces the binding of Li ⁺ /Ni ²⁺ ions during cycling due to the pillaring effect in the transition metal-slab, not only maintaining the NCM lattice structure but also preventing anisotropic contraction and local collapse. .

In addition, the lithium ion diffusion channel widened by doping with Mg ²⁺ (0.72 Å), which has a larger ionic radius than Ni ²⁺ (0.69 Å), enables rapid diffusion of lithium ions.

Considering the effects of the Li ₃ PO ₄ coating and Mg ²⁺ doping, the core-double shell structure proposed by the present invention protects an active material containing a large amount of Ni from reaction with an electrolyte and prevents corrosion by HF. mitigating and ultimately protecting against severe structural collapse.

Therefore, it can be seen that the best cycle characteristics of MP-NCM shown in Fig. 14(c) are due to the core-double shell structure composed of Mg doped layer and Li ₃ PO ₄ coating layer.

(5) Cycling test 2

1) Measurement conditions

The cycling test was performed at a temperature of 55 ° C., which was different from the aforementioned cycling test 1, and through this, the core-double shell structure presented in the present invention was performed even at high temperatures, where detrimental effects on cycling characteristics such as structural decomposition and gas generation strongly act. We want to confirm that the MP-NCM of shows excellent cycle characteristics.

2) Result analysis

14(d) is data obtained by measuring cycle characteristics of pristine-NCM and MP-NCM at a temperature of 55°C.

Referring to FIG. 14(d), both pristine-NCM and MP-NCM exhibited a lower capacity retention rate than the aforementioned cycling measurement at 25°C.

In particular, pristine-NCM showed a severe dose decline with a low dose retention rate of 54.0%. On the other hand, MP-NCM maintained a capacity of 163.4mAh g-1 even after 100 cycles at 55°C, showing an excellent retention rate of 85.4%.

Through the above results, it was confirmed that the MP-NCM of the double shell structure proposed by the present invention showed excellent effects on cycling properties by improving structural stability and chemical stability even at high temperatures.

(6) Electrochemical behavior analysis

1) Measurement conditions

It was measured by electrochemical impedance spectroscopy (EIS), and was performed under conditions of a voltage range of 3.0V to 4.3V.

2) Result analysis

FIG. 15(e) is data obtained by measuring the electrochemical behavior after the 1st cycle, and FIG. 15(f) is data obtained by measuring the electrochemical behavior after the 100th cycle.

In the electrochemical behavior measurement data, the semicircle is the result of combining the contribution of the rate performance by the short diffusion method and the low resistance of lithium ion migration through the surface layer.

Specifically, the semicircle in the high frequency region represents the surface film resistance (Rs), the semicircle in the middle region represents the charge-transfer resistance (Rct), and the slanted line in the low frequency region represents the NCM structure. represents the Warburg impedance (W) resulting from the lithium ion behavior of

Circuits inserted in Fig. 15(e, f) correspond to equivalent circuits of EIS results.

15(e, f), the Rct value of pristine-NCM significantly increases after cycling, which is analyzed as a result of the formation of CEI with low ionic conductivity on the surface of NCM particles.

On the _contrary , MP-NCM shows a significantly smaller increase in Rct, which is because i) electrode and electrolyte are separated by Li ₃ PO ₄ coating layer (isolation effect) and ii) structural stability is improved by Mg doping. is analyzed as

Through the above results, it can be confirmed that the MP-NCM of the core-double shell structure proposed by the present invention suppresses unwanted side reactions at the interface and exhibits excellent cycle performance by preserving the conductivity of ions and electrons during long-term cycling.

(7) Analysis of capacity reduction mechanism

1) Measurement conditions

To confirm the capacity reduction mechanism occurring in NCM, pristine-NCM and MP-NCM were analyzed by cyclic voltammetry (CV).

2) Analysis of results - analysis using cyclic voltammetry (CV)

Figure 15 (g, h) is the data measured in the voltage range of 2.7V to 4.3V pristine-NCM and MP-NCM by cyclic voltammetry (CV).

Looking at Figure 15 (g, h), three pairs of redox peaks appear in the voltage range of 2.7V to 4.3V. It appears in a hexagonal phase (H1), a monoclinic phase (M), and then two other hexagonal phases (H2 and H3).

It is known that the phase transition from H2 to H3 is associated with a severe anisotropic volume change and the generation and propagation of microcracks in the bulk.

In the case of pristine-NCM, it is noteworthy that the intensity of the initial redox peak appearing around 4.2 V gradually decreases due to the loss of H3 while the cycle lengthens, which is caused by the rapid capacity decrease and the gradual structural degradation of pristine-NCM. is analyzed as

In contrast, in the case of MP-NCM, even if the cycle is prolonged, the change in peak intensity is negligible, and the profile is also maintained very similar. This is a result indicating that the phase transition between H2 and H3 is reduced by the core-double shell structure proposed by the present invention.

Therefore, through the above results, it was confirmed that the synergy between Mg doping and Li ₃ PO ₄ applied to the NCM of the core-double shell structure proposed by the present invention improves structural stability and improves electrochemical activity during repeated charging and discharging. .

Experimental Example 6

Phase analysis experiments

(1) Experiment outline

It was performed to confirm the effect of the core-double shell structure proposed by the present invention by checking the phase change and distribution.

Specifically, in situ XRD analysis was performed to confirm the effect of the core-double shell structure through phase change, and HR-TEM (High-resolution transmission electron microscope), FFT to confirm the effect of the core-double shell structure through phase distribution (Fast Fourier transformed), and selected area electron diffraction (SEAD) pattern analysis were performed.

(2) Analysis of results 1 - Phase change analysis using in situ XRD

16 is data obtained by measuring phase changes during the charging process using in situ XRD for pristine-NCM and MP-NCM.

Figure 16 shows the (003) reflection from in situ XRD measurements, and (003) reflection represents the unit cell size in the c-axis direction, which is a function of the charging voltage.

Referring to FIG. 16, both prisitne-NCM and MP-NCM show that the (003) reflection shifts to a low diffraction angle up to 4.1V. 4.1V is the starting point of the phase transition from H2 to H3, and the diffraction angles of both prisitne-NCM and MP-NCM increase from 4.1V.

However, in terms of degree, in the case of MP-NCM, the diffraction angle change of (003) reflection moves smoothly, so it can be expected that the phase transition from H2 to H3 also proceeds smoothly, but in the case of pristine-NCM, the change in diffraction angle is relatively looks sudden

These results are analyzed to be due to delay of anisotropic change and phase transition in the c-axis direction in the NCM lattice by Mg doping and Li ₃ PO ₄ coating.

In view of the fact that the anisotropic change in the c-axis direction in the NCM lattice not only destabilizes the structure but also contributes to the generation of microcracks, which is ultimately the main cause of capacity decay in NCM containing a large amount of Ni, It can be seen that the NCM of the core-double shell structure proposed by the present invention is an excellent solution.

(3) Result analysis 2 - Phase distribution analysis using HR-TEM, FFT and SEAD patterns

Phase distribution analysis was performed by cycling the pristine-NCM and MP-NCM, checking the NCM particle cross-section with HR-TEM, and subsequently performing FFT and SEAD pattern analysis.

17 is HR-TEM, FFT and SEAD pattern images of pristine-NCM,

18 is HR-TEM, FFT and SEAD pattern images of MP-NCM,

19 is a TEM image of a cross section of pristine-NCM and MP-NCM.

Referring to FIG. 17, in the case of pristine-NCM, the structure appeared in the order of rock-salt phase, mixed phase, and layered phase from the outermost surface (Region 1) to the interior (Region 3). This result indicates that structural destruction occurred from the outermost surface to the inside of pristine-NCM particles during the cycle, and is caused by an unwanted interfacial reaction between pristine-NCM and electrolyte.

On the other hand, looking at FIG. 18, in the case of MP-NCM, it can be confirmed that the entire surface (Region 1) to the inside (Region 3) is composed of only layered phases, and thus the structural stability is excellent.

The above results are also consistent with FIG. 19 . Referring to FIG. 19, in the case of pristine-NCM, it can be seen that the rock-salt phase is distributed not only on the outermost surface but also inside the pristine-NCM. In contrast, in the case of MP-NCM, a weak rock-salt phase was observed only on the outermost surface, but the layered phase was uniformly distributed throughout.

(4) Synthesis of analysis results

It is known that the presence of a rock-salt phase in NCM containing a large amount of Ni inhibits charge transfer and increases charge-transfer impedance at the interface between the electrode and the electrolyte. In addition, the phase change (H2 → H3) increases the pressure inside the active material or electrode accompanied by oxygen release.

Accordingly, the above results show that the MP-NCM of the core-double shell structure proposed by the present invention forms strong Mg-O with Mg doping to suppress structural deformation during cycling, and the Li ₃ PO ₄ coating removes lithium residues and It can be said that it is a result confirming that it can effectively suppress the phase transition of NCM to the rock-salt phase by suppressing undesirable side reactions.

The above description of the present invention is for illustrative purposes, and those skilled in the art can understand that it can be easily modified into other specific forms without changing the technical spirit or essential features of the present invention. will be. Therefore, the embodiments described above should be understood as illustrative in all respects and not limiting. For example, each component described as a single type may be implemented in a distributed manner, and similarly, components described as distributed may be implemented in a combined form.

The scope of the present invention is indicated by the following claims, and all changes or modifications derived from the meaning and scope of the claims and equivalent concepts thereof should be construed as being included in the scope of the present invention.

10: Cathode active material of core-double shell structure
100: active material or core
200: doped layer or first shell layer
300: coating layer or second shell layer

Claims (13)

a core portion containing an active material;
a first shell layer formed by doping a metal on a surface of the core part; and
A second shell layer coated on the surface of the first shell layer to form a layer and containing a phosphate; characterized in that it comprises a core-double shell structure of the positive electrode active material. According to claim 1,
The active material is characterized in that it comprises at least one selected from the group consisting of LiNiMnCoO ₂ , LiNiO ₂ , LiNiCoO ₂ and combinations thereof, the core-double shell structure of the cathode active material. According to claim 1,
The doped metal comprises at least one selected from the group consisting of magnesium, boron, titanium, aluminum, chromium, and combinations thereof. According to claim 1,
The phosphate is FePO ₄ , AlPO ₄ , CoPO ₄ , Li ₃ PO ₄ , CaHPO ₄ and combinations thereof, characterized in that it comprises at least one selected from the group consisting of, a core-double shell structure cathode active material. According to claim 1,
The first shell layer has a thickness of 50 nm or more and 80 nm or less, the core-double shell structure of the cathode active material. According to claim 1,
The second shell layer has a thickness of 1.5 nm or more and 2.5 nm or less, the core-double shell structure of the cathode active material. (i) mixing an active material precursor with a shell layer precursor including a metal and a phosphate; and
(ii) calcining the mixture;
In the calcining, the active material precursor forms a core portion by the calcination, the metal is doped on the surface of the core portion to form a first shell layer, and the surface of the first shell layer is coated with a second shell layer containing the phosphate. Characterized in that, the core-double shell structure of the cathode active material manufacturing method. According to claim 7,
The active material precursor and the shell layer precursor of step (i) are mixed in a molar ratio of 1: 0.01 or more and 1: 0.02 or less. According to claim 7,
The calcination of the step (ii) is characterized in that carried out at a temperature of 730 ℃ or more and 780 ℃ or less, the core-double shell structure of the cathode active material manufacturing method. According to claim 7,
The active material precursor comprises at least one selected from the group consisting of LiNiMnCoO ₂ , LiNiO ₂ , LiNiCoO ₂ and combinations thereof, a core-double shell structure positive electrode active material manufacturing method. According to claim 7,
The shell layer precursor comprises at least one selected from the group consisting of MgHPO ₄ , AlPO ₄ , CaHPO ₄ and combinations thereof, the core-double shell structure of the cathode active material manufacturing method. A positive electrode comprising the positive electrode active material of claim 1 having a core-double shell structure. A secondary battery comprising the positive electrode of claim 12.

Images