KR20220023454A - Method for producing cathode active material for lithium secondary battery coated with lithium-tungsten oxide, positive electrode active material produced therefrom, and lithium secondary battery including the same - Google Patents

Abstract

The present invention relates to a method for manufacturing an anode active material for a lithium secondary battery coated with a lithium-tungsten oxide, an anode active material manufactured therefrom, and a lithium secondary battery containing the same. The method for manufacturing an anode active material for a lithium secondary battery coated with a lithium-tungsten oxide according to the present invention comprises: a step of manufacturing a high-nickel (high-Ni) based lithium-nickel-cobalt-manganese oxide in which a lithium compound is exposed on a surface; a step of manufacturing a coating solution by dissolving a tungsten oxide in a basic solution; and a step of manufacturing the anode active material by mixing and drying the lithium-nickel-cobalt-manganese oxide and the coating solution, and heat-treating the same. The anode active material is characterized in that a coating layer composed of a lithium-tungsten oxide is formed on the outside of the lithium-nickel-cobalt-manganese oxide by combining lithium of the lithium compound with the tungsten oxide through heat treatment.

Description

A method for producing a cathode active material for a lithium secondary battery coated with lithium-tungsten oxide, a cathode active material prepared therefrom, and a lithium secondary battery comprising the same material produced therefrom, and lithium secondary battery including the same}

The present invention relates to a method of manufacturing a cathode active material for a lithium secondary battery coated with lithium-tungsten oxide, a cathode active material prepared therefrom, and a lithium secondary battery including the same.

A lithium secondary battery is composed of a positive electrode and a negative electrode capable of inserting/deintercalating lithium ions, and a lithium salt and a non-aqueous electrolyte. The reason for using the non-aqueous electrolyte is that lithium cannot exist stably because of its high reactivity with water.

At this time, the positive electrode is manufactured by applying a slurry in which a positive electrode active material, an electrolyte, a conductive material, and a binder are mixed on a current collector such as Al foil or Cu foil. As a positive electrode active material constituting the positive electrode, there are LiCoO ₂ based, Li(NiCoMn)O ₂ based, LiMn ₂ O ₄ based, and the like, but there are disadvantages in that the discharge capacity is limited, the thermal stability is low, and the material is expensive. For this reason, in order to obtain a high capacity, the amount of nickel in the Li(NiCoMn)O ₂ based positive electrode active material is gradually increasing.

In order to obtain the high energy density of the lithium secondary battery, the amount of nickel must be maximized. However, if the amount of nickel increases, the nickel has no choice but to take the place of lithium, and the lost lithium cannot find its place, so Lithium protrudes from the surface of the positive electrode active material due to exposure, which causes a side reaction in the electrolyte or difficult electrode manufacturing due to gelation of the slurry during electrode manufacturing.

In addition, the oxidation number of nickel is unstable, and the presence of nickel oxide on the surface of the positive electrode active material may cause deterioration of the electrode lifespan. In particular, when the amount of nickel in the Li(NiCoMn)O ₂ composition is greater than 90 mol%, the deterioration of the electrode lifespan is maximized, which is an obstacle to commercialization by applying it to a lithium secondary battery.

Therefore, it is a time when technology development research is urgently required for a cathode active material that can maintain high discharge capacity and achieve high electrochemical properties such as lifespan and output characteristics to achieve low price, high stability, and high capacity and long life. .

Registered in Korea Patent Publication No. 10-1913939, October 25, 2018.

The present invention was invented to solve the above problems, and high-nickel for lithium secondary batteries coated with lithium-tungsten oxide so that not only discharge capacity can be increased, but also electrochemical properties such as lifespan characteristics and output characteristics can be improved It is a technical solution to provide a method for manufacturing a system positive electrode active material, a positive electrode active material prepared therefrom, and a lithium secondary battery including the same.

In order to solve the above technical problem, the present invention provides a high-nickel (high-Ni)-based lithium-nickel-cobalt-manganese oxide having a lithium compound exposed on the surface thereof; preparing a coating solution by dissolving tungsten oxide in a basic solution; and mixing and drying the lithium-nickel-cobalt-manganese oxide and the coating solution to prepare a positive electrode active material by heat treatment; It provides a method of manufacturing a cathode active material for a lithium secondary battery coated with lithium-tungsten oxide, characterized in that a coating layer made of lithium-tungsten oxide is formed on the outside of the lithium-nickel-cobalt-manganese oxide while the oxide is combined.

In the present invention, the lithium-nickel-cobalt-manganese oxide is co-precipitated using a basic aqueous solution and a nickel raw material, a cobalt raw material and a manganese raw material to prepare a nickel-cobalt-manganese oxide; and mixing lithium hydroxide hydrate (LiOH·H ₂ O) with the nickel-cobalt-manganese oxide, followed by heat treatment and sintering.

In the present invention, the lithium-nickel-cobalt-manganese oxide is characterized in that the nickel content is 90 mol% or more.

In the present invention, the lithium-tungsten oxide is represented by Li _x W _y O _z of Formula 1 (provided that 0.1 ≤ x ≤ 6, 1 ≤ y ≤ 2, 3 ≤ z ≤ 9). do.

In order to solve the above other technical problems, the present invention provides a positive active material, characterized in that manufactured by the above method.

In order to solve the another technical problem, the present invention provides a lithium secondary battery including the positive active material.

According to the method of manufacturing a lithium-tungsten oxide-coated positive electrode active material for a lithium secondary battery of the present invention by means of solving the above problems, by coating the lithium-tungsten oxide on the surface of the high-nickel-based Li(NiCoMn)O ₂ , the structure It has the effect of being able to manufacture a positive electrode active material that can improve surface stability while suppressing side reactions on the surface as well as having a positive stability.

In addition, when the lithium-tungsten oxide-coated positive electrode active material of the present invention is applied to a lithium secondary battery, it facilitates the movement of lithium ions so that lithium ions can be easily inserted and detached from the surface of the positive electrode active material. Since electrochemical characteristics such as lifespan characteristics and output characteristics are improved while maintaining high stability, there is an effect that can be used in various applications other than lithium secondary batteries.

1 is a schematic view showing a method of manufacturing a lithium-tungsten oxide-coated positive electrode active material for a lithium secondary battery according to the present invention;
2 is a flow chart showing a method of manufacturing a lithium-tungsten oxide-coated positive electrode active material for a lithium secondary battery according to the present invention.
3 is a photograph showing Examples 1 to 3;
4 is a photograph showing whether or not the lithium-tungsten oxide is coated according to Example 2. FIG.
5 is a graph showing the XRD patterns of Examples 1 to 3;
6 is a graph showing the XPS spectra of Example 2.
7 is a graph showing the initial charge/discharge curve at 0.1C and cycling performance at 0.5C of Examples 1 to 3;
8 is a graph showing a Nyquist plot after 80 cycles of Examples 1 to 3;
9 is a graph showing the rate characteristics of Examples 1 to 3;

Hereinafter, the present invention will be described in detail.

1 is a schematic diagram showing a method of manufacturing a lithium secondary battery positive electrode active material coated with lithium-tungsten oxide according to the present invention, and FIG. 2 is a method of manufacturing a lithium secondary battery positive electrode active material coated with lithium-tungsten oxide according to the present invention is shown in the flowchart.

1 and 2, the lithium-tungsten oxide-coated positive electrode active material of the present invention is prepared in a high-nickel (high-Ni)-based lithium-nickel-cobalt-manganese oxide with a lithium compound exposed on the surface (S10a), preparing a coating solution by dissolving tungsten oxide in a basic solution (S10b) and mixing and drying the lithium-nickel-cobalt-manganese oxide and the coating solution, followed by heat treatment to prepare a cathode active material (S20) manufactured through

According to the above-described manufacturing method, first, a high-nickel-based lithium-nickel-cobalt-manganese oxide having a lithium compound exposed on the surface is prepared (S10a).

Prior to the description, high-nickel refers to a high nickel content such as 90 mol% of nickel with respect to the total number of moles of transition metals in the positive electrode active material. Although there is little amount exposed to the surface, if the amount of nickel in lithium-nickel-cobalt-manganese oxide increases, lithium displaced by a large amount of nickel protrudes and is exposed to the surface of lithium-nickel-cobalt-manganese oxide do.

The lithium compound exposed on the surface of the lithium-nickel-cobalt-manganese oxide may be a material such as lithium hydroxide (LiOH) or lithium carbonate (Li ₂ CO ₃ ). In general, in the end, the lithium compound exposed on the surface of lithium-nickel-cobalt-manganese oxide was washed with water and then heat-treated to prepare a positive electrode active material. A coating layer made of lithium-tungsten oxide can be made by inducing a reaction of the lithium compound with lithium using the exposed and protruding lithium compound.

For this purpose, nickel raw materials such as NiSO ₄ ·6H ₂ O, cobalt raw materials such as CoSO ₄ ·7H ₂ O, and manganese raw materials such as MnSO ₄ ·H ₂ O are used, sodium hydroxide (NaOH) aqueous solution, ammonia (NH ₄ OH) ) By using a basic solution such as an aqueous solution as a chelating agent, a Ni _0.90 Co _0.05 Mn _0.05 (OH) ₂ precursor is obtained through a co-precipitation reaction in the reactor. The Ni _0.90 Co _0.05 Mn _0.05 (OH) ₂ precursor thus obtained was mixed with lithium hydroxide hydrate (LiOH·H ₂ O) in a mol ratio of 1:1.05, heat-treated at 450 to 550° C. for 2 to 8 hours, and then in air. LiNi _0.90 Co _0.05 Mn _0.05 O ₂ Synthesis of LiNi 0.90 Co 0.05 Mn 0.05 O 2 is completed by calcination at 600 ~ 700 ℃ under the atmosphere.

At this time, if a lot of moisture is contained in the precursor, since the characteristics of the lithium secondary battery are deteriorated, it is preferable to heat treatment at 450 to 550 ° C. Heat treatment at less than 450 ° C. is insufficient to evaporate moisture in the aqueous solution. Heat treatment at a temperature exceeding 550 ° C. . Subsequently, when calcined at less than 600° C., it is insufficient to firmly maintain the spherical shape of LiNi _0.90 Co _0.05 Mn _0.05 O ₂ , and when calcined at more than 700° C., it may rather cause physical property deformation.

Next, a coating solution is prepared by dissolving tungsten oxide in a basic solution (S10b).

Since tungsten oxide is insoluble in general water, it is preferable to dissolve tungsten oxide in a basic solution. To this end, a coating solution is prepared by dissolving tungsten oxide in a range of 0.1 to 1 wt% in 100 wt% of a basic solution.

When the tungsten oxide content is less than 0.1 wt%, the amount that can react with the lithium compound on the surface of the lithium-nickel-cobalt-manganese oxide decreases, so that the portion where the coating layer is not formed on the surface of the lithium-nickel-cobalt-manganese oxide remains. Therefore, it becomes difficult to achieve a complete coating layer.

Conversely, when tungsten oxide exceeds 1 wt%, residual tungsten that cannot react with the lithium compound on the surface of lithium-nickel-cobalt-manganese oxide remains because the amount of tungsten oxide is excessive. cannot be given.

Finally, after mixing and drying the lithium-nickel-cobalt-manganese oxide and the coating solution, heat treatment is performed to prepare a positive electrode active material (S20).

The coating solution prepared above is slowly added to lithium-nickel-cobalt-manganese oxide and mixed using a mortar. After that, it can be vacuum dried at 70~90℃ for 8~12 hours, and when vacuum drying is completed, it is heat treated at a temperature of 300~500℃ for 1~10 hours under an atmospheric atmosphere in the air to high-nickel lithium-nickel- The lithium compound on the surface of the cobalt-manganese oxide and the tungsten oxide react to form a coating layer made of lithium-tungsten oxide.

In this way, lithium-nickel-cobalt-manganese oxide is disposed on the core, and a coating layer made of lithium-tungsten oxide is disposed as a shell. In particular, lithium and tungsten oxide contained in the lithium compound exposed on the surface of lithium-nickel-cobalt-manganese oxide and Since the lithium-tungsten oxide formed by the reaction of is a lithium ion conductor, it is possible to suppress side reactions on the surface of the positive electrode active material while increasing the structural stability of the positive electrode active material.

The lithium-tungsten oxide constituting the coating layer may have a composition represented by the following formula (1). Examples of the lithium-tungsten oxide according to this may include LiWO ₃ , Li ₂ WO ₄ , and Li ₆ W ₂ O ₉ .

[Formula 1]

Li _x W _y O _z

(However, 0.1 ≤ x ≤ 6, 1 ≤ y ≤ 2, 3 ≤ z ≤ 9.)

At this time, the reason for vacuum drying at 70~90℃ for 8~12 hours is to remove the solvent of the coating solution so that the lithium of the lithium compound can react well with tungsten oxide. When vacuum-drying, moisture and ammonia are not evaporated, and the characteristics of the lithium secondary battery are eventually deteriorated due to residual impurities. Physical properties may be altered.

During heat treatment, if the temperature is less than 300°C or less than 1 hour, the lithium and tungsten oxide of the lithium compound do not react and the combination of lithium and tungsten cannot be induced, so that a coating layer made of lithium-tungsten oxide cannot be formed, If the time is exceeded, there is a need to be careful because a good effect on the reaction of lithium and tungsten is not derived compared to the case of heat treatment at a temperature or time lower than that, and if the temperature is raised, it becomes an inefficient process. In addition, the complete reaction of lithium and tungsten oxide of the lithium compound can be achieved only when an atmospheric atmosphere is created during the heat treatment.

In summary, the positive electrode active material manufactured according to the above-described method has an advantage in that it is possible to manufacture a lithium secondary battery with high stability and low cost due to improved electrochemical characteristics such as lifespan characteristics and output characteristics while maintaining high discharge capacity.

Hereinafter, an embodiment of the present invention will be described in more detail as follows. However, the following examples are merely illustrative to aid the understanding of the present invention, and the scope of the present invention is not limited thereby.

<Example 1>

First, a Ni _0.90 Co _0.05 Mn _0.05 (OH) ₂ precursor was synthesized by a co-precipitation method. As raw materials, NiSO ₄ ·6H ₂ O, CoSO ₄ ·7H ₂ O, and MnSO ₄ ·H ₂ O were used, and NaOH and NH ₄ OH solutions were used as chelating agents. The obtained spherical Ni _0.90 Co _0.05 Mn _0.05 (OH) ₂ precursor was mixed with LiOH·H ₂ O in a 1:1.05 mol ratio and calcined at 500° C. for 5 hours and then at 680° C. for 15 hours in air, followed by LiNi _0.90 Co _0.05 Mn _0.05 O ₂ was synthesized. LiNi _0.90 Co _0.05 Mn _0.05 O ₂ synthesized in this way will be referred to as pristine NCM.

Meanwhile, a coating solution was prepared by dissolving tungsten oxide powder into 0.1 wt% of 100 wt% of NH ₄ OH solution. The coating solution was slowly added to the pristine NCM and mixed using a mortar. In order to remove the solvent of the pristine NCM mixed with the coating solution, it was vacuum dried at 80° C. for 10 hours. Subsequently, lithium-tungsten oxide-coated LiNi _0.90 Co _0.05 Mn _0.05 O ₂ was synthesized by heat treatment at 400° C. for 5 hours in an atmospheric atmosphere. The synthesized one will be referred to as "0.1wt% LWO-NCM".

<Example 2>

In the same manner as in Example 1, tungsten oxide powder was added so as to be 0.5 wt% in 100 wt% of NH ₄ OH solution to synthesize lithium-tungsten oxide-coated LiNi _0.90 Co _0.05 Mn _0.05 O ₂ . The synthesized one will be referred to as "0.5wt% LWO-NCM".

<Example 3>

In the same manner as in Example 1, tungsten oxide powder was added to 1wt% of 100wt% of NH ₄ OH solution to synthesize lithium-tungsten oxide-coated LiNi _0.90 Co _0.05 Mn _0.05 O ₂ . The synthesized one will be referred to as "1wt% LWO-NCM".

The positive active materials prepared according to Examples 1 to 3 were checked for shape using field emission scanning electron microscopy (FESEM) and component distribution was checked through energy dispersive X-ray detector (EDX) analysis.

In relation to this, Figure 3 shows the photos of Examples 1 to 3 to confirm the surface uniformity, Figure 3 (a) is a FESEM image of the pristine NCM, Figure 3 (b) is 0.1wt% LWO- NCM is shown as a FESEM image, Figure 3 (c) is shown as a FESEM image of 0.5wt% LWO-NCM, Figure 3 (d) is shown as a FESEM image of 1wt% LWO-NCM.

As in Figs. 3(a), 3(b), 3(c) and 3(d), all samples had a similar spherical shape, and no significant change in particle shape was observed after coating. . However, when the content of tungsten oxide was increased to 1 wt%, small particles were observed on the surface. On the other hand, in 0.1wt% LWO-NCM and 0.5wt% LWO-NCM, a coating layer was not observed on the surface, which is thought to be because the coating layer was uniformly formed on the surface. At this time, FIG. 3(e) shows that when 0.5wt% LWO-NCM was observed by EDX mapping, the presence of tungsten was confirmed, and thus the distribution of tungsten could be confirmed.

4 is a photograph showing a field emission transmission electron microscope (FETEM) image in order to confirm whether lithium-tungsten oxide is coated according to Example 2, and the micro-shape of the sample surface coating layer can be confirmed using FETEM.

Figure 4 (a) shows the pristine NCM as a FETEM image, Figure 4 (b) is a FETEM image of 0.5wt% LWO-NCM. In the pristine NCM of FIG. 4(a), it is observed that no other material is present on the surface, but in the 0.5wt% LWO-NCM as in the LWO layer of FIG. 4(b), the presence of a coating layer with a thickness of about 5 nm on the surface can be checked

<Test Example 1>

In Test Example 1, an XRD pattern (X-ray diffraction) was analyzed to confirm the crystal structure between the pristine NCM according to Examples 1 to 3 and the LWO-NCM according to this.

In relation to this, Figure 5 is a graph showing the XRD patterns of pristine NCM and LWO-NCM according to Examples 1 to 3, and looking at this, it can be seen that the 2θ value is measured within the range of 10 to 80 °.

Referring to the (003) peak of FIG. 5 , it can be seen that Li-O-M-O-Li-O-M-0-Li is repeatedly formed and the layered structure is well developed. When tungsten enters lithium or metal, the (104) peak moves. Judging from the fact that the (104) peak remains unchanged, tungsten oxide reacts with lithium from the surface of lithium-nickel-cobalt-manganese oxide to the surface It is treated and confirmed that it can exist as a coating layer without getting inside the lattice of the crystal.

m 공간군에 속하고 있으며, 모들 샘플들의 (006)/(102)와 (108)/(110) 피크가 분명하게 분리되어 있어 층상구조가 잘 발달되었다는 것을 알 수 있다. 단, 리튬-텅스텐 산화물은 소량 포함되어 있어서 XRD 패턴에서는 나타나지 않았다.In other words, the XRD patterns of all samples showed a layered hexagonal α-NaFeO ₂ structure, and R

It belongs to the m space group, and the (006)/(102) and (108)/(110) peaks of all samples are clearly separated, indicating that the layered structure is well developed. However, since a small amount of lithium-tungsten oxide was included, it did not appear in the XRD pattern.

Therefore, the shift of the peak did not occur in all samples, which can support that tungsten did not diffuse into the NCM crystal structure, and it is confirmed that it exists in the form of a coating layer on the surface of the NCM rather than doping. At this time, the values of I ₍₀₀₃₎ /I ₍₁₀₄₎ of pristine NCM and LWO-NCM, which are cation mixtures, are shown in Table 1 below.

Cationic mixing means that the ionic radii of Li ⁺ (0.76 Å) and Ni ²⁺ (0.69 Å) are similar, meaning that Ni ²⁺ is located at the Li ⁺ 3a site, which leads to a decrease in lifetime. After the coating process, it is confirmed that the cation mixture values are similar, and it is confirmed that Li in the NCM crystal is not consumed and there is no change in the crystal structure. As such, it can be seen that the lithium-tungsten oxide coating process reacted with the residual lithium compound (LiOH, Li ₂ CO ₃ ) of the pristine NCM without affecting the crystal structure of the pristine NCM to form a coating layer.

<Test Example 2>

In Test Example 2, XPS (X-ray photoelectron spectroscopy) spectra of the pristine NCM according to Example 1 and Ni2p, Co2p, Mn2p, C1s and W4f of the LWO-NCM were analyzed.

Through this XPS analysis, the chemical bonding state of each element on the sample surface can be confirmed. In this regard, Figure 6 is a graph showing the XPS spectra of pristine NCM and LWO-NCM according to Example 2. At this time, FIG. 6(a) is the XPS spectrum of Ni2p, FIG. 6(b) is the XPS spectrum of Co2p, FIG. 6(c) is the XPS spectrum of Mn2p, FIG. 6(d) is the XPS spectrum of C1s, FIG. 6(e) shows the XPS spectrum of W4f.

In detail, XPS analysis was performed to confirm the chemical bonding state of each element on the surface of pristine NCM and LWO-NCM, and Ni2p, Co2p, Mn2p, C1s, W4f spectra of pristine NCM and 0.5wt% LWO-NCM can be confirmed.

It is confirmed that the positions of the binding energy peaks of Ni2p, Co2p, and Mn2p are not changed after coating, which means that there is no change in the oxidation numbers of each Ni, Co, and Mn. The binding energy peaks of Ni2p are located at 854.9 eV and 872.5 eV, which correspond to Ni2p _3/2 and Ni2p _1/2 . In addition, 861.1eV (Nip _3/2 ) and 880.0eV (Nip _1/2 ) correspond to satellite. Usually, the binding energy peak of Ni2p _3/2 appears in the range of 854-856 eV, and it is reported that Ni ²⁺ and Ni ³⁺ from NiO and Ni ₂ O ₃ bond exist. Co ³⁺ is observed at 780.3 eV from Co2p _3/2 , and Mn ⁴⁺ is observed at 642.3 eV from Mn ₂ p _3/2 .

In the case of 0.5wt% LWO-NCM, W4f is located at 34.9eV and 36.7eV, corresponding to W4f _7/2 and W4f _5/2 , respectively. This means that W exists as W ⁵⁺ . Moreover, there is one peak at 37.8 eV, indicating the presence of W ⁶⁺ .

The presence of tungsten could not be confirmed by XRD due to the small amount of tungsten, but the presence of tungsten could be confirmed in LWO-NCM through XPS analysis. In addition, the bonding state of the coating layer could be confirmed through the XPS result, and it seems that it exists in the form of Li _x WO _y such as LiWO ₃ , Li ₂ WO ₄ . Since Li _x WO _y is produced by reacting with residual lithium compounds (LiOH, Li ₂ CO ₃ ), it can be seen that various lithium-tungsten compounds are produced due to the locally present residual lithium. This Li _x WO _y coating layer can facilitate the movement of lithium ions through insertion and desorption of lithium ions from the surface of the positive electrode active material.

Looking at the C1s spectrum of FIG. 6(d), the intensity of the peak of Li ₂ CO ₃ observed at 288.8 eV decreases after coating, which is a lithium-tungsten oxide reaction with the residual lithium compound present on the surface of the pristine NCM. to confirm that it was created. Through the lithium-tungsten oxide produced in this way, it is possible to improve the lifespan characteristics and electrical conductivity of the NCM.

<Test Example 3>

In Test Example 3, the electrochemical characteristics of the lithium secondary batteries to which the positive active materials according to Examples 1 to 3 were applied were analyzed.

In relation to this, a 2032 coin cell type was used for electrochemical property evaluation, and a slurry was prepared by mixing a positive electrode active material, a conductive material (Super-P black), and a binder (PVDF: polyvinylidene fluoride) in a weight ratio of 96: 2: 2 prepared, coated on an Al current collector (thickness 16 μm), and vacuum-dried at 100° C. for 10 hours to remove the solvent. The loading amount of the electrode and the density of the mixture were adjusted to 15mg/cm ² and 3.3g/cm ³ , respectively. After the positive electrode plate was punched with a diameter of 14 mm, it was vacuum-dried at 120° C. for 10 hours. A lithium metal (thickness of 500 μm, diameter of 16 mm) was used for the negative electrode, and a PE film (thickness of 20 μm) was used as the separator. 1M LiPF ₆ EC/DMC/EMC (1 : 1 : 1, v/v/v) was used as the electrolyte. The coin cell was assembled in an argon (Ar) atmosphere glove box.

The initial charge/discharge capacity is charged at 25℃ at 0.1C (1C=210mAh g ^-1 ) until the constant current (CC) becomes 4.3V, and then is charged at a constant voltage (CV) of 4.3V so that the charging current becomes 0.01C. was charged once until The lifespan was repeated 80 cycles with constant current-constant voltage charging and constant current discharging with a current of 0.5C. Cyclic voltammetry (CV) was performed at a scanning rate of 0.1 mV s ⁻¹ in a voltage range of 3.0 to 4.3 V at 25°C. Impedance (EIS: electrochemical impedance spectroscopy) was measured in a frequency range of 1 MHz to 10 mHz with an amplitude of 5 mV.

Initial charge/discharge and cycling performance test

7 is a graph showing the initial charge/discharge curve at 0.1C and the cycling performance at 0.5C of Examples 1 to 3, and FIG. 7(a) shows the initial charge/discharge curve at 0.1C, FIG. (b) shows the life characteristics of cycling performance at 0.5C. The initial charge/discharge characteristics were confirmed with a current density of 0.1C in the voltage range of 3.0~4.3V at 25℃, and then the lifespan characteristics were confirmed with a current density of 0.5C. First, the initial charging and discharging results are shown in Table 2 below.

Referring to Table 2, all samples showed similar charge-discharge curves, which indicates that the coating layer does not affect the charge-discharge behavior. Although pristine NCM showed the highest charge capacity (239.5 mAh g ^-1 ), it exhibited the lowest discharge capacity (214.5 mAh g -1) with a low coulombic efficiency (89.5%). On the other hand, the LWO-NCM samples showed a higher coulombic efficiency than the pristine NCM, and although the initial charge was low due to the weight of the coating layer, the initial discharge capacity was higher than that of the pristine NCM. It can be seen that this is because the coating layer serves as a conductor of lithium ions.

Referring to Figure 7 (a), it can be confirmed the life characteristics of the pristine NCM and LWO-NCM. LWO-NCM samples had improved lifespan characteristics than pristine NCM, and in particular, 0.5wt% LWO-NCM samples showed the best lifespan characteristics. After 80 cycles, pristine NCM showed a capacity retention rate of 76.6%, 0.1wt% LWO-NCM showed 81.6%, 0.5wt% LWO-NCM showed 84.6%, and 1wt% LWO-NCM showed 83.2%. It can be seen that this is because the formation of the coating layer reduces side reactions due to the reduction of residual lithium compounds such as LiOH and Li ₂ CO ₃ , and acts as a protective film to suppress the decomposition of the electrolyte from the oxidation power at high voltage.

EIS exam

8 is a graph showing the Nyquist plot after 80 cycles of Examples 1 to 3, and shows the EIS after 80 cycles to confirm the reaction state on the surface by lithium-tungsten oxide coating.

The EIS graph consists of two semicircles and a straight line, and shows its equivalent circuit. In the equivalent circuit, R _s is the resistance of the electrolyte, the first semicircle R _SEI is the film resistance, the second semicircle R _ct is the charge transfer from the surface to the bulk, and Z _w shown as a straight line is related to the diffusion of lithium ions. means the Warburg resistance. After 80 cycles, LWO-NCM samples showed lower R _SEI and R _ct values than pristine NCM. The pristine NCM showed the highest R _SEI and R _ct values of 29.7 Ω and 567.2 Ω, whereas the 0.5wt% LWO-NMC showed the lowest R _SEI and R _ct values of 14.0 Ω and 42.7 Ω.

According to the EIS results, it can be confirmed that the coating layer suppresses an increase in electrical resistance during charging and discharging, and in particular, suppresses an increase in R _ct resistance. This is because lithium ions in the Li _x WO _y layer play a role of smoothly moving to the bulk of the positive electrode active material through insertion and desorption. Each component value of EIS after 80 cycles can be confirmed in Table 3 above.

Rate characteristic test

The rate-rate characteristic was tested to confirm the diffusion of lithium ions, which can be confirmed through FIG. 9 . That is, FIG. 9 is a graph showing the rate characteristics of Examples 1 to 3, and the discharge rate rates of the pristine NCM and LWO-NCM samples of 0.5C, 1C, 2C, 3C and 5C in the voltage range of 3.0 to 4.3V at 25°C. Characteristics are confirmed and indicated.

Referring to FIG. 9 , it was confirmed that the discharge capacity values of the pristine NCM and LWO-NCM samples were significantly different as the current density increased. Based on these results, it is confirmed that the NCM surface has a low resistance under the influence of the coating layer, which facilitates the movement of lithium ions from the surface to the bulk. This shows that LWO-NCM maintains excellent structural stability even at high current density.

As described above, the present invention relates to a method for manufacturing a cathode active material for a lithium secondary battery coated with lithium-tungsten oxide, a high-nickel-based lithium-nickel-cobalt-manganese oxide having a lithium compound exposed on the surface thereof, and a basic solution A cathode active material in which lithium-nickel-cobalt-manganese oxide is coated on the outside of lithium-nickel-cobalt-manganese oxide is coated with a cathode active material by mixing and drying a coating solution in which tungsten oxide is dissolved and then heat-treating. There is this.

As such, the lithium-tungsten oxide-coated cathode active material according to the present invention has great significance in that it can improve surface stability while suppressing side reactions on the surface as well as having structural stability.

Therefore, when the positive electrode active material as in the present invention is applied to a lithium secondary battery, high stability can be satisfied by maintaining high discharge capacity and improving electrochemical characteristics such as lifespan characteristics and output characteristics, so it can be used in various fields other than lithium secondary batteries is expected to

The above description is merely illustrative of the technical spirit of the present invention, and various modifications and variations will be possible without departing from the essential characteristics of the present invention by those skilled in the art to which the present invention pertains. Therefore, the embodiments disclosed in the present invention are not intended to limit the technical spirit of the present invention, but to explain, and the scope of the technical spirit of the present invention is not limited by these embodiments. The protection scope of the present invention should be construed by the claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present invention.

Claims (6)

Preparing a high-nickel (high-Ni)-based lithium-nickel-cobalt-manganese oxide with a lithium compound exposed on the surface;
preparing a coating solution by dissolving tungsten oxide in a basic solution; and
The lithium-nickel-cobalt-manganese oxide and the coating solution are mixed and dried, followed by heat treatment to prepare a positive electrode active material;
The positive active material is
Lithium-tungsten oxide-coated, characterized in that a coating layer made of lithium-tungsten oxide is formed on the outside of the lithium-nickel-cobalt-manganese oxide while lithium of the lithium compound and the tungsten oxide are combined through the heat treatment A method of manufacturing a cathode active material for a lithium secondary battery. According to claim 1,
The lithium-nickel-cobalt-manganese oxide is
preparing a nickel-cobalt-manganese oxide by coprecipitating a basic aqueous solution with a nickel raw material, a cobalt raw material and a manganese raw material; and
Lithium-tungsten oxide-coated cathode active material for lithium secondary battery, characterized in that it is prepared by mixing lithium hydroxide hydrate (LiOH·H ₂ O) with the nickel-cobalt-manganese oxide, followed by heat treatment and sintering; method. According to claim 1,
The lithium-nickel-cobalt-manganese oxide is
A method for producing a cathode active material for a lithium secondary battery coated with lithium-tungsten oxide, wherein the nickel content is 90 mol% or more. According to claim 1,
The lithium-tungsten oxide is
A method of manufacturing a cathode active material for a lithium secondary battery coated with lithium-tungsten oxide, characterized in that it is represented by the following formula (1).
[Formula 1]
Li _x W _y O _z
(However, 0.1 ≤ x ≤ 6, 1 ≤ y ≤ 2, 3 ≤ z ≤ 9.) A positive active material, characterized in that produced by the method of any one of claims 1 to 4. A lithium secondary battery comprising the positive active material according to claim 5.

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