KR102537010B1 - Cathode Active Material for Secondary Battery - Google Patents

Abstract

The present invention provides a positive electrode active material including: a particle core including lithium transition metal oxide; and an interface unit formed on at least a part of the surface of the particle core, wherein the Co concentration of the interface unit is less than or equal to the Co concentration of the particle core. The positive electrode active material according to the present invention maintains the stability and overall characteristics of the active material by the special composition of the particle core and the interface unit.

Description

Cathode Active Material for Secondary Battery {Cathode Active Material for Secondary Battery}

The present invention relates to a cathode active material for a secondary battery, and more particularly, includes a particle core containing lithium transition metal oxide and an interface formed on at least a part of the surface of the particle core, wherein the Co concentration of the interface is the particle core. It relates to a positive electrode active material that satisfies the condition of less than or equal to the Co concentration of

One of the main specifications of electronic products in modern society is battery capacity. Factors such as cell phone battery capacity, range of km that can be driven on a single charge of an electric vehicle, and flight time of a drone can have a great impact on product selection. Beyond this, it is very likely to be widely used in the field of UAM (urban air mobility) and aerospace technology in the near future. As human technology develops as described above, the need for a battery having a high energy density in the same volume and the same weight is increasing.

In order to develop a lithium ion battery with high energy density, it is necessary to maximize the energy density of the cathode active material, and for this purpose, Ni, Mn, and Co ternary cathode active materials are mainly used. In particular, in order to derive the highest capacity in the same voltage range, most recent studies on lithium ion positive electrode active materials have been conducted in the direction of increasing the ratio of Ni.

However, in order to use the high Ni-based cathode active material, a process for improving stability and lifetime is additionally required. Among them, the surface modification technology generally increases stability by forming a passivation surface modification layer of metal oxides (Al, B, W, Zr, Ti, etc.) on the surface of the positive electrode active material. This method of surface modification using a metal oxide has a disadvantage in that resistance increases and capacity decreases accordingly, since the metal oxide does not participate in the charge-discharge process.

In order to minimize these disadvantages, a technique for modifying the surface using a composition that creates a layered structure has been proposed. However, since this technology is mainly aimed at improving stability and lifespan, Co-rich compounds and Mn-rich compounds have been tested mainly, and the disadvantages of increased resistance and reduced capacity have not been completely overcome, although stability is still increased.

On the other hand, with the improved production technology of cathode active materials that incorporates many recent studies, it is possible to improve stability even in the bare stage without surface modification. Active material stability improvement technology through additives, active material structure stability improvement technology through firing atmosphere control, in particular, stability improvement technology through single-particle active material, etc. are applied to increase stability and lifespan without surface modification. has become possible

Nevertheless, technology capable of improving the capacity of the active material and lowering the resistance characteristics to a desired level has not been developed.

Therefore, the need for such technology development is high in the art.

An object of the present invention is to solve the problems of the prior art and the technical problems that have been requested from the past.

As a result of repeated in-depth research and experiments to overcome these conventional problems, the inventors of the present invention developed a novel cathode active material that satisfies specific conditions as will be described later, and a secondary battery based on such a cathode active material has high capacity and It has been confirmed that excellent battery characteristics can be exhibited due to remarkably low resistance characteristics, and the present invention has been completed.

Therefore, the positive electrode active material according to the present invention includes a particle core containing lithium transition metal oxide and an interface formed on at least a part of the surface of the particle core, wherein the Co concentration of the interface is equal to or less than the Co concentration of the particle core. characterized by

When the cathode active material satisfies the above condition ('Co concentration of the interface part' ≤ 'Co concentration of the particle core'), the Co content of the interface part is controlled so that the production cost of the active material can be reduced and the content of other elements in the interface part can be increased. Due to this, not only can the capacity be increased, but also the resistance characteristics that directly affect the charge/discharge efficiency can be improved.

For example, in a layered structure, Co can use electrons from Co ⁺³ to Co ⁺⁴ , but Ni can use electrons from Ni ⁺² to Ni ⁺⁴ . Many capacities are available.

In addition, the c-axis interlayer distance expansion of the layered structure that occurs during the charging reaction of the positive electrode active material increases more when the composition of Co is high or 100%, and this deepening of the c-axis interlayer distance expansion causes the crystal structure of the active material to change to monoclinic. and gradually cause a phase change phenomenon to the H1-3 phase and the P3 phase, which may deteriorate the capacity and resistance characteristics. Therefore, this problem can be solved by the Co concentration condition of the interface part.

Moreover, when Co is appropriately contained in the interface, there is an advantage that the crystal structure is stabilized and the charge and discharge cycle is improved. However, when the proportion of Co increases during active material production, the unit price rises, resulting in a large risk in terms of consumption cost and productivity. there is. Therefore, if the Co concentration of the interface is controlled under the specific conditions of the present invention, there is an advantage in effectively lowering the production cost of the active material while expecting to improve overall physical properties.

Preferably, while having a particle morphology of a single particle, when the condition that the Co concentration of the particle core is greater than the Co concentration of the interface portion ('Co concentration of the interface portion' < 'Co concentration of the particle core') is satisfied, more can be effective

In one specific example, the lithium transition metal oxide may be a compound including a composition represented by Formula 1 below.

Li(Li _x M1 _y M2 _1-xyz D _z )O _2-a Q _a (1)

In the above formula,

0≤x≤0.2, 0<y≤1, 0≤z≤0.2, 0≤a≤0.2;

M1 is Co;

M2 is at least one transition metal element that is stable in tetracoordinate or hexacoordinate;

D is at least one element selected from alkaline earth metals, transition metals, and nonmetals as a dopant;

Q is one or more anions.

For reference, when D is a transition metal, transition metals defined in M1 and M2 are excluded from these transition metals.

In one specific example, M2 is one or more, preferably two, elements selected from the group consisting of Ni and Mn, and D is Al, W, Si, V, B, Ba, Ca, Zr, Ti , Mg, Ta, Nb, and at least one element selected from the group consisting of Mo.

The technology of the present invention is particularly useful for high Ni-based cathode active materials, in which structural instability due to cation mixing is intensified due to high Ni content, for example, Ni content of 60 mol% or more based on the total content of transition metal, preferably 70 mol% or more, particularly preferably 80 mol% or more of the positive electrode active material may be more effective.

Due to the layered structure characteristics of High Ni-based cathode active materials, the higher the firing temperature, the more severe the degradation of properties due to cation mixing. For lithiation, in practice, high Ni-based cathode active materials are produced at temperatures above 700 °C. Among the cathode active materials produced at such high temperatures, the most unstable structure is the dangling bond, which is the end of the layered structure on the surface. Therefore, by using an additional metal raw material prepared in a similar NCM composition ratio to that of the cathode active material and heat-treating again at a temperature lower than the firing temperature of the active material, the unstable layered structure is minimized through the formation of an interface that minimizes the unstable dangling bond. Capacitance and resistance characteristics can be improved because the intercalation and desorption of the layered structure of Li ions becomes easy during charge/discharge.

The interface part may include at least one element selected from the group consisting of Ni, Mn, Al, V, Ti, Zr, Sr, Sn, Mo, Zn, Si, B, P, and Mg in addition to Co. More preferably, it may contain Ni and Mn, which are easy to form a layered structure by reacting with Li. Further, it may be more preferable when the interface portion includes Co and Ni and satisfies a condition in which the Ni concentration is higher than the Co concentration.

The material included in the interface may be in the form of a (transition) metal oxide, lithium (transition) metal oxide, or the like, and preferably has a relatively high content (70 to 99 mol%) of Ni and a low content (1 to 15 mol%) of Ni. ) of Co and optionally containing other elements.

In one specific example, the interface portion may be formed in an area of 1 μm or less in the direction from the surface of the particle core to the center of the particle core.

If the interfacial portion has an area of more than 1 μm, it is undesirable because resistance may increase by affecting ionic conductivity. The minimum size of the interface portion may be determined within a range technically producible while providing desired effects of the present invention, and may be, for example, 0.01 μm or more.

As can be seen from the experimental details to be described later, the cathode active material according to the present invention may have a characteristic in which LCO peaks are not found around 37 to 38 ° and 44.5 to 45.5 ° of 2theta during X-ray diffraction analysis.

In one specific example, the particle morphology of the positive electrode active material according to the present invention may be an independent particle shape of a single particle. That is, the cathode active material may have a monolithic structure. The monolithic structure means a structure in which the particles exist as independent phases without mutual aggregation, and when the monolithic structure is present, the strength of the particles increases, so that the stability of the positive electrode active material during charging and discharging of a battery including the monolithic structure can be improved.

In another specific example, the particle morphology of the positive electrode active material according to the present invention may be an agglomerated particle shape in which primary particles are aggregated to form a particle aggregate of secondary particles. When a plurality of primary particles are aggregated, the Co concentration at the grain boundary of the primary particles in the secondary particles is preferably equal to or less than the Co concentration in the particle core ('Co concentration at the grain boundary' ≤ 'Co concentration in the particle core'). can The shape of the grain boundary is determined according to the shape of the adjacent primary particles, and may be a straight line, a curve, a square, a rectangle, a hexagon, a triangle, or a combination thereof when viewed in cross section.

In the positive electrode active material according to the present invention, in one specific example, 40 to 100% of the surface area of the particle core may be applied to the interface, and if less than 40% of the surface area of the particle core is applied, in the present invention The effect of forming the interface may not appear.

Such an interface portion may be prepared by, for example, heat treatment at 450° C. to 750° C., and this heat treatment temperature may be preferable as a temperature range in which the Lithium-Metal-Oxide layer structure is formed.

The present invention also provides a secondary battery including the cathode active material. Since the specific configuration and structure of the secondary battery is well known in the art, a detailed description thereof is excluded from the present specification.

As described above, the positive electrode active material according to the present invention maintains the stability and overall characteristics of the active material by the special configuration of the particle core and the interface, while increasing the capacity and significantly improving the resistance characteristics that directly affect the charge and discharge efficiency. have an effect that can

1A to 1C are SEM images and EDX images of particle flakes of the positive electrode active material prepared in Example 1;
2 is an EDX image of a particle flake of the positive electrode active material prepared in Comparative Example 4;
Figure 3 is an XRD analysis graph for the cathode active materials prepared in Example 1 and Comparative Examples 1 and 4, respectively;
4a and 4b are graphs showing images and analysis results showing positions of EDX droplet analysis in the EDX droplet analysis of the cathode active material prepared in Comparative Example 4;
5A and 5B are graphs showing images and analysis results showing positions of EDX droplet analysis in the EDX droplet analysis of the cathode active material prepared in Example 1;

Hereinafter, the present invention will be further detailed with reference to examples and the like of the present invention, but the scope of the present invention is not limited thereto.

[Reference Example 1]

Zoomwe's precursor with a composition of 93% Ni, 5% Co, and 2% Mn was mixed with a dry mixer (Henschel Mixer) together with LiOH and additional dopant additives so that the molar ratio of transition metal and Li was 0.95 to 1.05. Mixed for a minute.

The resulting mixture was filled in an alumina container, heat-treated at 880° C. for 15 hours under an O ₂ atmosphere, and then the active materials aggregated during firing were pulverized with a Jet Mill pulverizer to obtain Li(Ni _0.93 Co _0.05 Mn _0.02 )O ₂ . .

[Reference Example 2]

Li(Ni 0.75 Co 0.10 Mn _0.15 )O ₂ was obtained under the same conditions as in Reference Example 1, except that a precursor containing ₇₅ % Ni, ₁₀ % Co, and 15% Mn was used.

[Example 1]

In a weight ratio of 0.5 wt% of the weight of Li(Ni _0.93 Co _0.05 Mn _0.02 )O ₂ and Li(Ni _0.93 Co _0.05 Mn _0.02 )O ₂ obtained in Reference Example 1, Ni(OH) ₂ , Co(OH) ₂ , After mixing Al ₂ O ₃ with a dry mixer (Henschel Mixer) to a Ni:Co:Al=90:5:5 ratio, the resulting mixture was filled in an alumina container and heat-treated at 600°C for 10 hours under an O ₂ atmosphere. , A positive electrode active material having an interface portion including an oxide described in Table 1 below was prepared, and a positive electrode and a lithium secondary battery were prepared according to the following manufacturing method.

[Manufacture of anode]

The positive electrode active material prepared above, Super-C65 as a conductive material, and PVdF KF1100 as a binder were mixed with N-Methyl-2-pyrrolidone as a solvent in a weight ratio of 95:2:3, respectively, to prepare a paste for forming a positive electrode. The positive electrode forming paste was applied on an aluminum current collector, dried at 130° C., and then rolled to prepare a positive electrode.

[Production of Lithium Secondary Battery]

The positive electrode prepared above is prepared according to the size of a coin cell, Li metal is used as a negative electrode, and an electrode assembly is prepared by interposing porous polyethylene, which is a separator between the positive electrode and the negative electrode, and the electrode assembly is placed in a cell case. After placing it inside, an electrolyte solution was injected into the cell case to manufacture a lithium secondary battery. As the electrolyte solution, an electrolyte solution of Welcos Co., Ltd., in which 1.0M concentration of lithium hexafluorophosphate (LiPF ₆ ) was dissolved in an organic solvent consisting of ethylene carbonate/dimethyl carbonate (mixed volume ratio of EC/DMC = 1/1) was used.

[Example 2]

The positive electrode active material, the positive electrode, and lithium under the same conditions as in Example 1, except that Ni(OH) ₂ , Co(OH) ₂ , and Al ₂ O ₃ were mixed in a ratio of Ni:Co:Al=92:3:5. Secondary batteries were each manufactured.

[Example 3]

The positive electrode active material, the positive electrode, and lithium under the same conditions as in Example 1, except that Ni(OH) ₂ , Co(OH) ₂ , and Al ₂ O ₃ were mixed in a ratio of Ni:Co:Al=94:1:5. Secondary batteries were each manufactured.

[Example 4]

A cathode active material, a cathode, and a lithium secondary battery under the same conditions as in Example 1, except that Ni(OH) ₂ , Co(OH) ₂ , and MnCO ₃ were mixed in a ratio of Ni:Co:Mn=90:5:5. were prepared, respectively.

[Comparative Example 1]

A positive electrode active material, a positive electrode, and a lithium secondary battery were prepared without performing an interface forming process on Li(Ni _0.93 Co _0.05 Mn _0.02 )O ₂ obtained in Reference Example 1, respectively.

[Comparative Example 2]

The positive electrode active material, the positive electrode, and lithium under the same conditions as in Example 1, except that Ni(OH) ₂ , Co(OH) ₂ , and Al ₂ O ₃ were mixed in a ratio of Ni:Co:Al=85:10:5. Secondary batteries were each manufactured.

[Comparative Example 3]

A cathode active material, a cathode, and a lithium secondary battery were prepared under the same conditions as in Example 1, except that Ni(OH) ₂ and Al ₂ O ₃ were mixed in a ratio of Ni:Al=95:5.

[Comparative Example 4]

A cathode active material, a cathode, and a lithium secondary battery were prepared under the same conditions as in Example 1, except that Co(OH) ₂ and Al ₂ O ₃ were mixed in a ratio of Co:Al=95:5.

[Comparative Example 5]

A cathode active material, a cathode, and a lithium secondary battery were prepared under the same conditions as in Example 1, except that MnCO ₃ and Al ₂ O ₃ were mixed in a ratio of Mn:Al=95:5.

[Example 5]

In a weight ratio of 0.5 wt% of the weight of Li(Ni _0.75 Co _0.10 Mn _0.15 )O ₂ and Li(Ni _0.75 Co _0.10 Mn _0.15 )O ₂ obtained in Reference Example 2, Ni(OH) ₂ , Co(OH) ₂ , After mixing MnCO ₃ and Al ₂ O ₃ with a dry mixer (Henschel Mixer) to a ratio of Ni:Co:Mn:Al=75:10:10:5, the resulting mixture was filled in an alumina container and heated at 600 °C under an O ₂ atmosphere. After heat treatment at °C for 10 hours, a positive electrode active material, a positive electrode, and a lithium secondary battery formed with an interface portion containing an oxide described in Table 1 below were prepared according to the above preparation method.

[Example 6]

The same conditions as in Example 5 except that Ni(OH) ₂ , Co(OH) ₂ , MnCO ₃ , Al ₂ O ₃ were mixed in a ratio of Ni:Co:Mn:Al=80:5:10:5 A cathode active material, a cathode, and a lithium secondary battery were respectively prepared.

[Comparative Example 6]

A positive electrode active material, a positive electrode, and a lithium secondary battery were prepared without performing an interface forming process on Li(Ni _0.75 Co _0.10 Mn _0.15 )O ₂ obtained in Reference Example 2, respectively.

[Comparative Example 7]

A cathode active material, a cathode, and a lithium secondary battery were prepared under the same conditions as in Example 5, except that Co(OH) ₂ and Al ₂ O ₃ were mixed in a ratio of Co:Al=95:5.

The elemental composition of the core and the interface of the positive electrode active material prepared in the Examples and Comparative Examples is shown in Table 1 below.

[Experimental Example 1]

Scanning electron microscope (SEM) images and energy dispersive X-ray spectroscopy (EDX) images were taken for the particle flakes of the cathode active material prepared in Example 1, and the results are shown in FIGS. 1A to 1C.

As shown in FIG. 1, it can be seen that a plurality of active material particles exist in an independent form, and some particles adjacent to each other form grain boundaries.

In addition, as shown in FIG. 1B, Co shows a rather low content at the grain boundary and shows a similar Co content distribution to the core, which is in contrast to FIG. 2, which discloses an EDX image of the cathode active material of Comparative Example 3 . That is, in the case of FIG. 2, a relatively high Co content distribution is shown at the grain boundary rather than at the core.

In the case of Ni, as shown in FIG. 1c, a uniform Ni content distribution is shown throughout the particle.

[Experimental Example 2]

XRD analysis was performed on the cathode active materials prepared in Example 1 and Comparative Examples 1 and 4 under the following measurement conditions, and the results are shown in FIG. 3 .

<Measurement conditions>

Power source: CuKα (line focus), wavelength: 1.541836Å, control axis: 2θ/θ,

Measurement method: continuous, counting unit: cps, opening angle: 10.0°,

End angle: 80.0°, number of integrations: 1 time, sampling width: 0.01°,

Scan speed: 1.3°/min, Voltage: 40kV, Current: 40Ma,

Divergence slit: 0.2mm, divergence limiting slit: 10mm,

Scattering slit: open, light receiving slit: open, offset angle: 0°,

Goniometer radius: 285 mm, optical system: concentrating method, attachment: ASC-48,

Slit: D/teX, Ultra Slit detector: D/teX Ultra,

Filter: None; Rotation speed: 30 rpm;

Incident Monochrome: CBO Ni-Kβ

As shown in FIG. 3, LCO XRD peaks of 37 to 38 ° and 44.5 to 45.5 ° were observed in the cathode active material of Comparative Example 4, and LCO XRD peaks of 44.5 to 45.5 ° were observed in the cathode active material of Comparative Example 1, Peaks were not observed in the cathode active material of Example 1. This means that the cathode active material of Example 1 is not a co-rich composition.

[Experimental Example 3]

EDX droplet analysis was performed on the cathode active material prepared in Comparative Example 4, and graphs showing the position of the EDX droplet analysis and the analysis results are shown in FIGS. 4a and 4b, respectively. Similarly, graphs showing the location and analysis results of the EDX droplet analysis of the cathode active material prepared in Example 1 are shown in FIGS. 5A and 5B , respectively.

As can be seen from these data, it can be seen that the positive active material of Comparative Example 4 has a high Co content in the interface portion compared to the core, whereas the positive active material of Example 1 does not.

[Experimental Example 4]

Electrochemical evaluation of the 2032 coin-type half cells prepared in the above Examples and Comparative Examples, respectively, was performed. Specifically, after aging the coin-type half cell prepared above at room temperature for 10 hours, a charge-discharge test was performed. Capacity evaluation was based on 200 mAh/g at 0.2C Rate, and charge-discharge conditions were performed in the range of 4.25 ~ 2.5 Voltage with constant current (CC) / constant voltage (CV). After measurement, the initial charge-discharge capacities are shown in Table 2 below.

In addition, after aging the coin-type half cell prepared above at room temperature for 10 hours, a hybrid pulse power characterization (HPPC) test was performed with the coin-type half cell in which charge-discharge formation was performed for one cycle. The resistance value was measured using the following equation with the voltage difference obtained by pulsed discharge current at 1C in the SOC 90-10% range, and the results are shown in Table 2 below.

As shown in Table 2, it can be seen that the secondary batteries manufactured using the cathode active materials of Examples have significantly higher battery capacity and significantly lower resistance than the secondary batteries based on Comparative Examples.

Those skilled in the art in the field to which the present invention pertains will be able to make various applications and modifications within the scope of the present invention based on the above information.

Claims (16)

It includes a particle core containing lithium transition metal oxide and an interface formed on at least a part of the surface of the particle core,
The positive electrode active material, characterized in that the Co concentration of the interface portion is less than or equal to the Co concentration of the particle core. The positive electrode active material according to claim 1, wherein the lithium transition metal oxide comprises a composition represented by Formula 1 below:
Li(Li _x M1 _y M2 _1-xyz D _z )O _2-a Q _a (1)
In the above formula,
0≤x≤0.2, 0<y≤1, 0≤z≤0.2, 0≤a≤0.2;
M1 is Co;
M2 is at least one transition metal element that is stable in tetracoordinate or hexacoordinate;
D is at least one element selected from alkaline earth metals, transition metals, and nonmetals as a dopant;
Q is one or more anions;
If D is a transition metal, the transition metals defined for M1 and M2 in this transition metal are excluded. The cathode active material according to claim 1, wherein the Co concentration of the particle core is greater than the Co concentration of the interface portion. The method of claim 1, wherein the interface comprises at least one element selected from the group consisting of Ni, Mn, Al, V, Ti, Zr, Sr, Sn, Mo, Zn, Si, B, P, and Mg in addition to Co. Cathode active material, characterized in that. 5. The cathode active material according to claim 4, wherein the interface portion includes Co and Ni, and the Ni concentration is higher than the Co concentration. The cathode active material according to claim 1, wherein the Ni content is 60 mol% or more based on the total content of the transition metal. The positive electrode active material according to claim 1, wherein the interface portion includes a layered structure in a crystal structure. The cathode active material according to claim 1, wherein the particle morphology is an independent particle form of a single particle. The cathode active material according to claim 1, wherein the particle morphology is an agglomerated particle form in which primary particles are aggregated to form a particle agglomerate of secondary particles. 10. The cathode active material according to claim 9, wherein the Co concentration at the grain boundary of the primary particles in the secondary particles is equal to or less than the Co concentration in the particle core. The method of claim 1, wherein the interface portion is characterized in that it comprises an oxide containing a relatively high content (70 to 99 mol%) of Ni and a low content (1 to 15 mol%) of Co and optionally other elements. positive electrode active material. The positive electrode active material according to claim 1, wherein the interface portion is formed in an area of 1 μm or less from the surface of the particle core. The cathode active material according to claim 1, wherein 40 to 100% of the surface area of the particle core is applied to the interface portion. The cathode active material according to claim 1, wherein the interface portion is prepared by heat treatment at 450°C to 750°C. The cathode active material according to claim 1, wherein no LCO peaks are found around 37 to 38° and 44.5 to 45.5° of X-ray diffraction analysis 2theta. A secondary battery comprising the cathode active material according to claim 1.

Images