KR102274784B1 - positive material having surface coated positive active material for lithium secondary battery, preparation method thereof and lithium secondary battery comprising the same - Google Patents

Abstract

One embodiment of the present invention provides a cathode material for a secondary battery. The positive electrode material for a secondary battery includes positive electrode active material particles and a surface coating layer located on the surface of the positive electrode active material particles and containing a material represented by the following Chemical Formula 1, where X is OH, F or Cl, and x may be 0 to 1.
[Formula 1]
Ca ₅ (PO ₄ ) ₃ X _1- xLix

Description

A positive material for a lithium secondary battery having a surface-coated positive active material, a manufacturing method thereof, and a lithium secondary battery comprising the same

The present invention relates to a secondary battery, and more particularly, to a lithium secondary battery.

A secondary battery refers to a battery that can be used repeatedly because it can be charged as well as discharged. In a typical lithium secondary battery among secondary batteries, lithium ions contained in the positive electrode active material move to the negative electrode through the electrolyte, and then are inserted into the layered structure of the negative electrode active material (charging), and then the lithium ions inserted into the layered structure of the negative electrode active material are returned again. It works on the principle of returning to the positive pole (discharge). These lithium secondary batteries are currently commercialized and used as small power sources for mobile phones and notebook computers, and are expected to be used as large power sources for hybrid vehicles, and the demand is expected to increase.

However, the composite metal oxide, which is mainly used as a positive electrode active material in a lithium secondary battery, is deteriorated due to a reaction with an electrolyte, and thus shows somewhat insufficient reliability.

An object of the present invention is to provide an active material for a secondary battery with improved stability and a secondary battery including the same.

The technical problems of the present invention are not limited to the technical problems mentioned above, and other technical problems not mentioned will be clearly understood by those skilled in the art from the following description.

In order to achieve the above technical problem, an embodiment of the present invention provides a cathode material for a secondary battery. The positive electrode material for a secondary battery includes positive electrode active material particles and a surface coating layer located on the surface of the positive electrode active material particles and containing a material represented by the following Chemical Formula 1, where X is OH, F or Cl, and x may be 0 to 1.

[Formula 1]

Ca ₅ (PO ₄ ) ₃ X _1- xLix

The cathode active material is represented by the formula Li _1+x (Ni _1-yz Co _y M) _1-x O _{2 In} the formula, M is Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Al, Ga, In, Sn, Bi or Mn, where -0.2 ≤ x ≤ 0.2, 0.01 ≤ y ≤ 0.5, 0.01 ≤ z ≤ 0.5, 0 < y+z < 1.

The thickness of the surface coating layer may be 0.1 to 100 nm.

The surface coating layer represented by Formula 1 may be a layer containing at least one _{of Ca 5} (PO ₄ ) ₃ (OH) _1-x Lix, and Ca ₅ (PO ₄ ) ₃ (F) _{1-x Lix.}

The surface coating layer is a CaPOH _x ^- (x = 3 or 4) peak and LiCaPOH _y ^- (y = 0, 1, 2, 3, or 4) may represent at least one of the peaks.

Another embodiment of the present invention provides a method for manufacturing a cathode material for a secondary battery in order to achieve the above technical problem. The method for preparing a cathode material for a secondary battery may include preparing a coating solution by dissolving phosphoric acid and a calcium salt in a solvent, adding a cathode active material to the coating solution to obtain a precipitate, and heat-treating the precipitate.

The solvent may include water, purified water, or distilled water.

The heat treatment may be performed at a temperature of 200 to 290 °C.

After obtaining a precipitate by adding a positive electrode active material to the coating solution, the method may further include drying the precipitate. The drying may be performed at a temperature of 50 to 100 ℃.

As described above, according to the embodiment of the present invention, a positive electrode active material having improved stability may be obtained through surface treatment.

However, the effects of the present invention are not limited to the above-mentioned effects, and other effects not mentioned will be clearly understood by those skilled in the art from the following description.

1 is a schematic diagram showing a secondary battery according to an embodiment of the present invention.
2 is a flowchart illustrating a method for manufacturing a cathode material according to an embodiment of the present invention.
3 shows an XRD graph of a lithium compound having a surface coating layer formed thereon according to Preparation Example of the present invention and a lithium compound according to Comparative Examples.
4 is a graph showing the results of thermogravimetric analysis (TG) of a lithium compound having a surface coating layer formed according to a preparation example of the present invention.
5 is a graph showing the cyclic voltammetry (CV) measurement results of an electrode using a lithium compound having a surface coating layer formed according to a preparation example of the present invention.
Figure 6a is a graph showing the XRD results of the pellets according to the chemical stability Experimental Example 1 of the present invention.
Figure 6b is a graph showing the XRD results of the pellets according to the chemical stability Experimental Example 2 of the present invention.
6c and 6d are schematic diagrams showing the structures of hydroxyapatite and fluorapatite used in Chemical Stability Experimental Examples 1 and 2, respectively.
7A and 7B are images showing SEM mapping of the positive active material on which the surface coating layer is formed according to Preparation Example of the present invention.
7c is an image showing the TEM mapping of the positive active material with the surface coating layer formed according to the preparation example of the present invention.
8 is a graph showing XRD results of a positive electrode active material having a surface coating layer formed according to Preparation Example of the present invention (treated) and a positive electrode active material (bare) according to Comparative Example.
9 is a time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis result of the surface of the positive electrode active material having a surface coating layer according to Preparation Example of the present invention (treated) and the positive electrode active material (bare) according to Comparative Example. This is the graph shown.
10, 11, and 12 are graphs illustrating charge/discharge characteristics of batteries according to a battery preparation example (treated) and a battery comparative example (bare).
13 is an XRD graph of a positive active material after a charge/discharge cycle of a battery according to Battery Preparation Example.

Since the present invention can have various changes and can have various embodiments, specific embodiments are illustrated in the drawings and described in detail. However, this is not intended to limit the present invention to specific embodiments, and it should be understood to include all modifications, equivalents and substitutes included in the spirit and scope of the present invention.

The terms used herein are used only to describe specific embodiments, and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In the present application, terms such as “comprise” or “have” are intended to designate that a feature, number, step, operation, component, part, or combination thereof described in the specification exists, but one or more other features It should be understood that this does not preclude the existence or addition of numbers, steps, operations, components, parts, or combinations thereof.

In this specification, when a range for "X to Y" is described, all numbers between X and Y should also be construed as being recited together. As an example, when a range of 1 to 10 is described, it should be interpreted that not only 1 and 10 but also numbers therebetween, ie, integers and decimals, are all described.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Terms such as those defined in a commonly used dictionary should be interpreted as having a meaning consistent with the meaning in the context of the related art, and should not be interpreted in an ideal or excessively formal meaning unless explicitly defined in the present application. does not

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings.

1 is a schematic diagram showing a secondary battery according to an embodiment of the present invention.

Referring to FIG. 1 , the secondary battery 100 includes a negative electrode having a negative active material into which lithium can be inserted and removed, a positive electrode containing the above-described positive electrode active material, and an electrolyte 160 disposed between the negative electrode and the positive electrode. can The anode active material layer 120 and the anode current collector 110 may be referred to as an anode, and the cathode active material layer 140 and the cathode current collector 150 may be referred to as a cathode. Specifically, it includes a negative active material layer 120 , a positive active material layer 140 , and a separator 130 interposed therebetween, and between the negative active material layer 120 and the separator 130 , and the positive active material layer 140 . ) and the separator 130 , the electrolyte 160 may be disposed or filled. In addition, the anode active material layer 120 may be disposed on the anode current collector 110 , and the cathode active material layer 140 may be disposed on the cathode current collector 150 .

<Anode>

In order to obtain a positive electrode, a positive electrode active material may be prepared. The positive active material is a particle that reversibly inserts and desorbs lithium, and may be a lithium-transition metal oxide. Any lithium-transition metal oxide may be used as long as it is conventionally used in the art, for example, LiCoO ₂ , LiNiO ₂ , Li(Co _x Ni _1-x )O ₂ (0.5 ≤ x < 1), LiMn ₂ O ₄ , LiMn ₅ O ₁₂ .

As an example, the positive electrode compound is represented by the formula Li _1+x (Ni _1-yz Co _y M) _1-x O ₂ , where M is Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Zn , Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, Al, Ga, In, Sn, Bi or Mn. Also, -0.2 ≤ x ≤ 0.2, 0.01 ≤ y ≤ 0.5, 0.01 ≤ z ≤ 0.5, 0 < y+z < 1). Specifically, the cathode active material may be [Ni _1/3 Co _1/3 Mn _1/3 ]O ₂ , Li[Ni _0.6 Co _0.2 Mn _0.2 ]O ₂ , Li[Ni _0.85 Co _0.1 Al _0.5 ]O ₂ , However, the present invention is not limited thereto.

The surface coating layer may be positioned on the surface of the positive active material particles and contain a phosphoric acid compound represented by the following Chemical Formula 1.

[Formula 1]

Ca ₅ (PO ₄ ) ₃ (X) _1-x Li _x

In Formula 1, X may be OH, F or Cl, and x may be 0 to 1.

Specifically, the surface coating layer is Ca ₅ (PO ₄ ) ₃ (OH) _1-x Lix, Ca ₅ (PO ₄ ) ₃ (Cl) _1-x Lix or Ca ₅ (PO ₄ ) ₃ (F) _1-x Lix In particular, it may contain at least one of _{Ca 5} (PO ₄ ) ₃ (OH) _1-x Lix, and Ca ₅ (PO ₄ ) ₃ (F) _{1-x Lix (here, x} is 0 to 1). That is, the surface coating layer contains hydroxyapatite, fluoroapatite, or chloroapatite, and the apatite may be doped with lithium.

_{In addition, the surface coating layer was obtained from the CaPOH x} ^- (x = 3 or 4) peak and LiCaPOH _y ^- (y = 0, 1, 2) in Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS) analysis. , 3, or 4) peaks.

The surface coating layer may be formed in a layered structure on the surface of the positive active material particles, and may be attached in the form of an island. Here, the island shape may mean a shape in which a hemispherical, non-spherical, or atypical shape having a predetermined volume is discontinuously located.

Such a surface coating layer may serve to protect the positive active material, thereby improving stability. In addition, the surface coating layer is doped with lithium, and may not interfere with the movement of lithium metal ions.

The thickness of the surface coating layer may be 0.1 to 100 nm, for example, 1 to 80 nm, more specifically, 5 to 20 nm. When the thickness of the surface coating layer is too thick, the amount of active lithium per unit volume of the electrode is reduced, which may cause a decrease in the capacity of a secondary battery including the same. In addition, when the thickness of the surface coating layer is too thin, the effect of suppressing the side reaction between the active material and the electrolyte may be insignificant. Accordingly, the thickness range may be satisfied.

2 is a flowchart illustrating a method for manufacturing a cathode material according to an embodiment of the present invention.

Referring to FIG. 2 , it may include a step (S10) of preparing a coating solution by dissolving phosphoric acid and calcium salt in a solvent.

The solvent may be water. Specifically, it may be distilled water or purified water, preferably deionized water. The phosphoric acid may be H ₃ PO ₄ . The calcium salt may be calcium nitrate, calcium chloride, calcium acetate, calcium sulfide, or a combination of two or more thereof. The calcium salt may be an acidic raw material capable of being slightly acidified so that the treatment solution can exhibit weak acidity.

It may include a step (S20) of obtaining a precipitate by adding a cathode active material to the coating solution.

As an example, phosphoric acid and calcium salt in the coating solution may react with residual lithium metal compound by the positive electrode active material to form calcium lithium phosphate as a precipitate through Scheme 1 and/or Scheme 2 below. However, it is not limited to the following reaction.

[Scheme 1]

[Scheme 2]

Specifically, on the surface of the positive electrode active material, which is a lithium-transition metal oxide, a lithium metal oxide remaining without forming an oxide with the transition metal may be present. The residual lithium metal compound may be, for example, lithium carbonate (Li ₂ CO ₃ ), lithium hydroxide (LiOH), or the like. Such residual lithium metal compound may react with the material in the electrolyte in the secondary battery. The reactant obtained by this may accumulate on the surface of the positive electrode active material, and may interfere with the movement of lithium metal ions. As an example, the residual lithium metal compound may react with hydrofluoric acid in the electrolyte to generate lithium fluoride (LiF).

However, according to the present invention, as the residual lithium metal compound is consumed in the process of forming the surface coating layer, it is possible to suppress deterioration of the positive electrode active material due to a side reaction with the electrolyte.

The precipitate obtained from the above may further perform a drying step (not shown). Drying may be performed at a temperature of 50 to 100 ℃, for example, 60 to 95 ℃, as another example, 70 to 90 ℃. The precipitate may be obtained by obtaining a surface coating layer containing the material according to the present invention by further comprising the step of drying at a temperature lower than the heat treatment temperature before heat treatment.

It may include the step of heat-treating the precipitate (S30). A surface coating layer may be formed on the surface of the positive active material particles through the heat treatment step, and the surface coating layer may contain hydroxyapatite, fluoroapatite, or chloroapatite, and the apatite may be doped with lithium.

As an example, calcium lithium phosphate, which is a precipitate obtained from Schemes 1 and/or 2, may be heat-treated to form a material represented by Formula 1 through Scheme 3 below.

[Scheme 3]

The heat treatment may be performed at a temperature of 200 to 290 °C, for example, 230 to 280 °C. Specifically, it may be carried out at a temperature of 240 to 260° C. for 1 hour to 3 hours. When the above temperature range is satisfied, it may be possible to obtain a surface coating layer containing the material according to the present invention without including the residual lithium metal compound.

The positive electrode material can be obtained by mixing the positive electrode active material containing the surface coating layer obtained in this way, the conductive material, and the binder. In this case, the conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, or graphene. The binder is a thermoplastic resin, for example, polyvinylidene fluoride, polytetrafluoroethylene, ethylene tetrafluoride, vinylidene fluoride-based copolymer, fluororesin such as propylene hexafluoride, and/or polyolefin resin such as polyethylene or polypropylene. may include

A positive electrode material may be applied on a positive electrode current collector to form a positive electrode. The positive electrode current collector may be a conductor such as Al, Ni, or stainless steel. Applying the positive electrode material on the positive electrode current collector may be performed by pressure molding or a method of making a paste using an organic solvent and then applying the paste on the current collector and pressing the paste to fix it. The organic solvent is an amine-based solvent such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; and an aprotic polar solvent such as dimethylacetamide or N-methyl-2-pyrrolidone. Applying the paste on the positive electrode current collector may be performed using, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

<Cathode>

Anode active materials include metals, metal alloys, metal oxides, metal fluorides, metal sulfides, and natural graphite, artificial graphite, cokes, carbon black, carbon nanotubes, and graphite that can cause lithium ion deintercalation or conversion reaction. It can also be formed using carbon materials, such as a pin.

A negative electrode material can be obtained by mixing the negative electrode active material, the conductive material, and the binder. In this case, the conductive material may be a carbon material such as natural graphite, artificial graphite, coke, carbon black, carbon nanotubes, or graphene. The binder is a thermoplastic resin, for example, polyvinylidene fluoride, polytetrafluoroethylene, ethylene tetrafluoride, vinylidene fluoride-based copolymer, fluororesin such as propylene hexafluoride, and/or polyolefin resin such as polyethylene or polypropylene. may include

A negative electrode material may be applied on a positive electrode current collector to form a positive electrode. The positive electrode current collector may be a conductor such as Al, Ni, or stainless steel. Applying the negative electrode material on the positive electrode current collector may be performed by pressure molding, or a method in which a paste is made using an organic solvent or the like and then the paste is applied on the current collector and fixed by pressing. The organic solvent is an amine-based solvent such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; and an aprotic polar solvent such as dimethylacetamide or N-methyl-2-pyrrolidone. Applying the paste on the negative electrode current collector may be performed using, for example, a gravure coating method, a slit die coating method, a knife coating method, or a spray coating method.

<Electrolyte>

The electrolyte may be a lithium salt. Lithium salt is lithium perchlorate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoromethane cellphonate (LiCF ₃ SO ₃ ), lithium hexafluoroacetate (LiAsF) ₆ ), lithium trifluoromethanesulfonylimide (Li(CF ₃ SO ₂ ) ₂ N), or a mixture of two or more thereof. Among these, an electrolyte containing fluorine can be used. In addition, the electrolyte can be dissolved in an organic solvent and used as a non-aqueous electrolyte. Examples of the organic solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4- carbonates such as trifluoromethyl-1,3-dioxolan-2-one and 1,2-di(methoxycarbonyloxy)ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropylmethyl ether, 2,2,3,3-tetrafluoropropyldifluoromethyl ether, tetrahydrofuran, 2-methyltetrahydro ethers such as furan; esters such as methyl formate, methyl acetate, and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; amides such as N,N-dimethylformamide and N,N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone; Alternatively, a fluorine substituent may be introduced into the organic solvent described above.

Alternatively, a solid electrolyte may be used. The solid electrolyte may be an organic solid electrolyte such as a polyethylene oxide-based polymer compound or a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain. Also, a so-called gel-type electrolyte in which a non-aqueous electrolyte is supported on a polymer compound may be used. On the other hand, an inorganic solid electrolyte may be used. There are cases where the safety of the sodium secondary battery can be further improved by using these solid electrolytes. In addition, the solid electrolyte may serve as a separator to be described later, and in that case, a separator may not be required.

<Separator>

A separator may be disposed between the anode and the cathode. The separator may be a material having the form of a porous film, a nonwoven fabric, or a woven fabric made of a material such as a polyolefin resin such as polyethylene or polypropylene, a fluororesin, or a nitrogen-containing aromatic polymer. The thickness of the separator is preferably thinner as long as the mechanical strength is maintained from the viewpoint of increasing the bulk energy density of the battery and decreasing the internal resistance. The thickness of the separator may be generally about 5 to 200 μm, and more specifically, 5 to 40 μm.

<Method for manufacturing alkali metal secondary battery>

After the positive electrode, the separator, and the negative electrode are sequentially stacked to form an electrode group, if necessary, the electrode group is rolled up and stored in a battery can, and the secondary battery can be manufactured by impregnating the electrode group with a non-aqueous electrolyte. Alternatively, after forming an electrode group by laminating a positive electrode, a solid electrolyte, and a negative electrode, if necessary, the electrode group may be rolled up and stored in a battery can to manufacture a secondary battery.

Hereinafter, a preferred experimental example (example) is presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

[Experimental Examples]

Preparation example of lithium compound with surface coating layer formed

Phosphoric acid and calcium nitrate, which are coating raw materials, were added to 100 ml of deionized water in a molar ratio of 1:1, and then sufficiently stirred to obtain a solution. Thereafter, lithium carbonate (Li ₂ CO ₃ ) as a lithium compound was put into the solution in a molar ratio of 1:0.5 with the coating raw material, and the amount of the solution was the same as that of the cathode material. At this time, the obtained precipitate is sufficiently dried and heat-treated at 250 ° C. for 2 hours. The surface coating layer is a lithium compound with a surface coating layer formed thereon. The surface coating layer contains hydroxyapatite (Ca ₁₀ (PO ₄ ) ₆ (OH) ₂ ) and The hydroxyapatite was obtained by doping lithium (Li-HAP).

Lithium Compound Comparative Example 1

It was prepared in the same manner as in Preparation Example of the lithium compound in which the surface coating layer was formed, except that the precipitate was dried sufficiently and heat-treated at 300° C. for 2 hours.

Lithium Compound Comparative Example 2

It was prepared in the same manner as in Preparation Example of the lithium compound in which the surface coating layer was formed, except that the precipitate was dried sufficiently and heat-treated at 400° C. for 2 hours.

Lithium Compound Comparative Example 3

The precipitate was sufficiently dried and prepared in the same manner as in Preparation Example of the lithium compound in which the surface coating layer was formed, except that heat treatment was not performed.

3 shows an XRD graph of a lithium compound having a surface coating layer formed thereon according to Preparation Example of the present invention and a lithium compound according to Comparative Examples.

Referring to FIG. 3 , in the lithium compound according to Comparative Examples, a surface coating layer containing hydroxyapatite is not formed on the surface of the lithium compound, but in the lithium compound according to Preparation Example, lithium carbonate is not detected on the surface of the lithium compound. As a result, it can be confirmed that a surface coating layer containing hydroxyapatite was formed, and that the hydroxyapatite was doped with lithium (Li-HAP).

<Thermal stability>

4 is a graph showing the results of thermogravimetric analysis (TG) of a lithium compound having a surface coating layer formed according to Preparation Example of the present invention.

In detail, the weight loss rate was measured by raising the temperature from room temperature to 700° C. under an argon atmosphere, and it can be seen from FIG. 4 that the weight loss rate at 700° C. is less than 10% compared to the total compound at room temperature. Accordingly, it can be confirmed that the lithium compound having the surface coating layer (Li-HAP) formed according to the present invention has very good thermal stability.

<Electrochemical stability>

5 is a graph showing the cyclic voltammetry (CV) measurement results of an electrode using a lithium compound having a surface coating layer formed according to a preparation example of the present invention.

In detail, the lithium compound, the super P carbon conductive material, and the PVdF binder having the surface coating layer formed according to the above Preparation Example are stirred in NMP at a weight ratio of 9:0.5:0.5 to form an electrode active material slurry composition, and the electrode active material slurry composition was cast on an aluminum foil current collector and dried to form an electrode. The electrode was scanned at a voltage rate of 1 mV/s. Referring to FIG. 5 , it can be seen that cycle driving is performed very stably in a voltage window of 2.5 to 4.5 V.

<Physical stability>

Table 1 below summarizes the mechanical properties of hydroxyapatite contained in the surface coating layer of the present invention.

[Table 1]

<Chemical Stability>

Chemical Stability Experimental Example 1-1

1 g of the lithium compound having the surface coating layer formed according to the preparation example is pelletized, 1M LiPF ₆ is dissolved in a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC), and vinylene carbonate in a 2 weight ratio with respect to the total weight of the electrolyte ( VC) was prepared. 0.1 g of water was added to 10 g of the solution obtained above, and after storage for one week, the pellets were labored and dried.

Chemical Stability Experimental Example 1-2

Pellets were obtained in the same manner as in Chemical Stability Experiment 1-1 except that 0.1 g of hydrofluoric acid (HF) was added instead of 0.1 g of water.

Figure 6a is a graph showing the XRD results of the pellets according to the chemical stability Experimental Example 1 of the present invention.

The solution used in the chemical stability test example has the same composition as the electrolyte contained in the battery, and by further including water in the solution, the initial cycle state of the battery can be implemented through this experiment. Referring to FIG. 6A , it can be confirmed that the lithium compound having the surface coating layer formed has very similar peaks on the XRD as compared to before the addition of water. This may mean that the lithium compound including the surface coating layer is not decomposed even when reacting with water, that is, the surface coating layer may have stability with respect to water by containing hydroxyapatite.

In addition, by further including hydrofluoric acid in the solution, as the battery is driven in this experiment, the electrolyte salt is decomposed and hydrofluoric acid is formed in the electrolyte. Referring to FIG. 6A , it can be seen that most of the hydroxyapatite contained in the surface coating layer reacts with hydrofluoric acid to be changed to fluoroapatite.

Chemical Stability Experimental Example 2-1

Pellets were obtained in the same manner as in Chemical Stability Experiment 1-1, except that 1 g of a lithium compound having a surface coating layer containing fluorapatite was used instead of hydroxyapatite.

Chemical Stability Experimental Example 2-2

Pellets in the same manner as in Chemical Stability Experiment 1-1 except that 1 g of a lithium compound having a surface coating layer containing fluoroapatite was used instead of hydroxyapatite, and 0.1 g of hydrofluoric acid was added instead of 0.1 g of water. got

Figure 6b is a graph showing the XRD results of the pellets according to the chemical stability Experimental Example 2 of the present invention.

Referring to FIG. 6b , when the surface coating layer contains fluoroapatite as a lithium compound having a surface coating layer formed thereon, even when exposed to water and hydrofluoric acid, it can be confirmed that there is no significant change in the structure of the lithium compound on XRD compared to before exposure. have.

6c and 6d are schematic diagrams showing the structures of hydroxyapatite and fluorapatite used in Chemical Stability Experimental Examples 1 and 2, respectively.

That is, referring to FIGS. 6A and 6B , when the surface coating layer contains hydroxyapatite as a lithium compound having a surface coating layer formed thereon, as the secondary battery is driven, it is exposed to hydrofluoric acid by decomposition of the electrolyte salt, and hydroxyl in the surface coating layer A portion of the apatite may be fluorinated to form fluoroapatite. At this time, even if fluorapatite is included in the surface coating layer, it can be expected that the lithium compound in which the surface coating layer is still formed will be quite stable to water and acid components.

Preparation example of a positive active material with a surface coating layer formed

Phosphoric acid and calcium nitrate, which are coating raw materials, were added to 100 ml of deionized water in a molar ratio of 1:1, and then sufficiently stirred to obtain a solution. Thereafter, Li[Ni _0.85 Co _0.1 Al _0.5 ]O ₂ as a cathode active material was put into the solution in a weight ratio of 1:0.5 with the coating raw material, and the amount of the solution was the same as that of the cathode material. At this time, the obtained precipitate was sufficiently dried at 80° C. and heat-treated at 250° C. for 2 hours to obtain a positive electrode active material having a surface coating layer formed thereon.

Comparative Example of Positive Active Material

_{Li[Ni 0.85} Co _0.1 Al _0.5 ]O ₂ was used as the positive electrode active material, and the surface treatment was not performed to form a surface coating layer.

7A and 7B are images showing SEM mapping of the positive active material on which the surface coating layer is formed according to Preparation Example of the present invention. In addition, FIG. 7c is an image showing TEM mapping of the positive active material with the surface coating layer formed according to the Preparation Example of the present invention.

8 is a graph showing XRD results of the positive electrode active material with a surface coating layer formed according to Preparation Example of the present invention (treated) and the positive electrode active material according to Comparative Example (bare).

Referring to FIG. 8 , it can be seen that the same peaks are confirmed on the XRD images of the Comparative Example before and after the surface treatment and the Preparation Example after the surface treatment, so that there is no structural difference between the active material itself before and after the surface treatment. On the other hand, since the surface coating layer formed by the surface treatment is a very thin layer, it may not appear on the XRD image.

9 is a time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis result of the surface of the positive electrode active material having a surface coating layer according to Preparation Example of the present invention (treated) and the positive electrode active material (bare) according to Comparative Example. This is the graph shown.

Referring to FIG. 9 , it can be seen that the LiOH _x ⁺ (x = 1 or 2) peak and the LiCO ₂ ⁺ peak intensities decrease in the Preparation Example after surface treatment compared to the Comparative Example before surface treatment. The LiOH _x ⁺ peak is related to LiOH on the surface of the lithium-transition metal oxide, and the LiCO ₂ ⁺ peak is _{related to Li 2} CO ₃ on the surface of the lithium-transition metal oxide. Li ₂ CO ₃ That is, it can be estimated that the amount of the residual lithium metal compound is reduced.

On the other hand, _{it can be seen that the intensity of CaPOH x} ^- (x = 3 or 4) peak and LiCaPOH _y ^- (y = 0, 1, 2, 3, or 4) peak was newly detected after surface treatment compared to before surface treatment. These CaPOH _x ^- peaks and LiCaPOH _y ^- peaks can be estimated to be detected from the surface coating layer formed on the surface of the active material obtained in Preparation Example.

Battery manufacturing example

The positive active material with the surface coating layer according to Preparation Example, the super P carbon conductive material, and the PVdF binder were stirred in NMP at a weight ratio of 94:3:3 to form a positive electrode active material slurry composition. The positive electrode active material slurry composition was cast on an aluminum foil current collector and dried to form a positive electrode.

_{After that, 1M LiPF 6} is dissolved in a mixed solvent of the obtained positive electrode, the lithium metal negative electrode, ethylene carbonate (EC) and dimethyl carbonate (DMC), and vinylene carbonate (VC) in a 2 weight ratio with respect to the total weight of the electrolyte By preparing an electrolyte and a separator, a 2032-type coin cell was manufactured according to a conventional manufacturing process for a secondary battery.

Battery Comparative Example

A battery was obtained in the same manner as in Battery Preparation Example except that the positive electrode active material according to Comparative Example was used instead of the positive electrode active material having a surface coating layer according to Preparation Example.

10, 11, and 12 are graphs showing charge/discharge characteristics of batteries according to a battery preparation example (treated) and a battery comparative example (bare), and FIG. 13 is a positive active material after a charge/discharge cycle of a battery according to the battery preparation example is the XRD graph of

Referring to FIG. 10 , the difference in initial battery performance due to the Comparative Example of the battery before the surface treatment and the Preparation Example of the battery after the surface treatment is not very large, but the battery according to the Preparation Example compared to the Comparative Example exhibits rather high reversibility.

Referring to FIG. 11 , as a result of the experiment while increasing the C-rate from 0.1C to 5C, it can be confirmed that the battery according to the Preparation Example has excellent discharge capacity even at a high rate compared to the battery according to the Comparative Example.

12, as a result of a capacity evaluation experiment with a current density of 3C-rate (1C = 200 mAh/g), the battery according to the Preparation Example showed a higher capacity degradation rate as the cycle progressed compared to the battery according to the Comparative Example. It can be seen that it is small, and the number of drivable cycles is also much higher, confirming that the life characteristics are improved.

13 is, in detail, after the batteries according to Comparative Examples and Preparation Examples were run for 300 cycles at 1C in a voltage range of 3 to 4.3V, the batteries were disassembled to measure the structure of the cathode active material. In the positive electrode active material of the electrode according to the comparative example, the structure of the initial active material is hardly found to remain at 1%, and it can be confirmed that the lithium-deficient phase is 99%. On the other hand, in the case of the electrode according to Preparation Example, even after the cycle of the above conditions is performed, the positive active material maintains the structure of the initial active material at 91%, and has 9% of the lithium-deficient phase, most of which is the phase of the initial active material. It can be confirmed that it is found.

That is, due to the surface coating layer, it is possible to suppress the generation of by-products that interfere with the movement of lithium ions between charging and discharging, and to protect the surface of the positive electrode active material, so the stability of the positive electrode active material is relatively improved, and the secondary battery including the same It is interpreted that the electrochemical performance such as characteristics and capacity retention rate is improved.

Although described with reference to the above embodiments, those skilled in the art can sufficiently modify or change the present invention within the scope without departing from the spirit and scope of the present invention described in the claims below. You will understand.

Claims (10)

delete delete delete delete delete preparing a coating solution by dissolving phosphoric acid and a calcium salt in a solvent;
adding a cathode active material to the coating solution to obtain a precipitate in which calcium lithium phosphate is formed on the surface of the cathode active material through the reaction of the phosphoric acid, the calcium salt, and a residual lithium metal compound on the surface of the cathode active material; and
Method for producing a positive electrode material for a secondary battery comprising the step of heat-treating the precipitate to change the calcium lithium phosphate on the surface of the positive electrode active material into a surface coating layer containing a material represented by the following Chemical Formula 1:
[Formula 1]
Ca ₅ (PO ₄ ) ₃ X _1-x Li _x
In Formula 1, X is OH, F or Cl, and x is 0 to 1. 7. The method of claim 6,
The solvent is a method for producing a cathode material for a secondary battery comprising water, purified water, distilled water. 7. The method of claim 6,
The heat treatment is a method for producing a cathode material for a secondary battery is performed at a temperature of 200 to 290 ℃. 7. The method of claim 6,
After obtaining a precipitate by adding a positive active material to the coating solution, the method further comprising drying the precipitate. 10. The method of claim 9,
The drying is a method for producing a cathode material for a secondary battery is performed at a temperature of 50 to 100 ℃.

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