KR100792157B1 - Cathode active material for lithium secondary batteries and preparation method thereof - Google Patents

Abstract

The present invention provides a cathode active material in which a coating layer composed of the compound of Formula 1 is formed on the surface of a cathode active material, a method of manufacturing the same, and a cathode and a lithium secondary battery using the same. The capacity reduction with the discharge exhibits an improved effect.

MHPO ₄ (1)

In the above, M is a metal and metalloid containing a d orbit.

Description

Cathode active material for lithium secondary battery and manufacturing method thereof {CATHODE ACTIVE MATERIAL FOR LITHIUM SECONDARY BATTERIES AND PREPARATION METHOD THEREOF}

1 to 4 are graphs showing experimental results of coin-type batteries using the positive electrode active materials prepared in Examples 1 to 3 and Comparative Examples of the present invention, respectively, at 0.1, 0.2, 0.5, and 1 C at 3 V and 4.3 V sections. charging and discharging at a rate of 1 C and 50 charging and discharging at a 1 C rate are shown;

5 is an XRD graph of SrHPO ₄ constituting the coating layer in Examples 1 to 3 of the present invention.

The present invention relates to a positive electrode active material, a method of manufacturing the same, a positive electrode and a lithium secondary battery using the same, and more particularly, to provide a positive electrode active material having a coating layer composed of the compound of Formula 1 described later on the surface of the positive electrode active material The positive electrode active material exhibits improved cycle characteristics and improved capacity reduction due to high rate charge and discharge.

As a cathode active material of a lithium secondary battery, composite metal oxides such as LiCoO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1), and LiMnO ₂ are mainly used. Among the cathode active materials, manganese-based cathode active materials such as LiMn ₂ O ₄ and LiMnO ₂ have advantages in terms of synthesis method and cost, but have a disadvantage of low discharge capacity. LiCoO ₂ exhibits good electrical conductivity, high battery voltage, and excellent electrode characteristics, and is a representative cathode active material applied to most batteries currently on the market. However, the disadvantage is that the price is expensive. LiNiO _{2, which} is a nickel-based positive electrode active material, exhibits the highest discharge capacity of battery characteristics among the above-mentioned positive electrode active materials, but has disadvantages such as rapid deterioration of life and rapid drop in characteristics at high temperatures.

As a method for improving the stability of the positive electrode active material during charging and discharging, a method of doping other elements to Ni-based or Co-based lithium oxide has been proposed. As an example of this method, Japanese Patent Publication No. 12-149945 describes the performance of LiNiO ₂ . LiNi _x M _y Co _z O ₂ (M is at least one element of Mn and Al, x + y + z = 1), which is an improved active material, is disclosed.

As another method for improving the stability of the positive electrode active material, a method of modifying the surface of the positive electrode active material is known. Japanese Patent Laid-Open No. 9-55210 discloses a positive electrode active material prepared by coating lithium nickel-based oxide with an alkoxide of Co, Al, Mn, and then heat-treating it, and Japanese Patent Laid-Open No. 11-16566 discloses Ti, Sn, Bi. Lithium-based oxides coated with metals of Cu, Si, Ga, W, Zr, B or Mo and / or oxides thereof are disclosed.

As a method of increasing the structural stability of the positive electrode active material, US Pat. No. 5,705,291 discloses a method of coating a surface with a composition comprising borate, aluminate, silicate, or mixtures thereof.

However, these cathode active materials of the prior art still have poor structural stability and thus have not reached a satisfactory level of life characteristics due to high rate charging and discharging.

Accordingly, an object of the present invention is to solve the problems of the prior art and the technical problem that has been requested from the past.

After repeating in-depth studies and various experiments, the inventors of the present invention improve the structural safety of a positive electrode when a coating layer containing a specific component represented by Formula 1 is formed on the surface of a positive electrode active material used in a lithium secondary battery. It was confirmed that the life characteristics due to high rate charging and discharging can be improved, and the present invention was completed.

The positive electrode active material according to the present invention for achieving this object is composed of a multi-component coating layer represented by the following formula (1) on part or all of the surface of the positive electrode active material that can occlude and release lithium.

MHPO ₄

In the above, M is a metal and metalloid containing a d orbit.

The present invention also provides a method for producing the positive electrode active material, such a production method,

a) preparing a coating solution by dissolving a precursor of M and a phosphorus precursor compound in Formula 1 in a solvent;

b) adding and stirring the positive electrode active material to the coating solution; And

c) heat treating the coated cathode active material;

It is configured to include.

The present invention also provides a cathode for a lithium secondary battery comprising the cathode active material.

Furthermore, the present invention provides a lithium secondary battery comprising the above positive electrode, a negative electrode including a negative electrode active material capable of occluding and releasing lithium, a separator, and an electrolyte.

Hereinafter, the present invention will be described in detail.

As the cathode active material of the present invention, a conventional lithium transition metal composite oxide capable of occluding and releasing lithium may be used, but is not limited thereto, for example, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn _2. O ₄ , LiNi _1-X Co _X M _Y O ₂ (M = Al, Ti, Mg, Zr, 0 <X≤1, 0≤Y≤0.2) or LiNi _X Co _Y Mn _1-XY O ₂ (0 < X≤0.5, 0 <Y≤0.5) and the like may be used.

The present invention seeks to improve the cycle characteristics of the battery and the life characteristics due to high rate charging and discharging by a surface modification method of forming a coating layer of a multicomponent compound as in Formula 1 on the surface of the positive electrode active material.

Of the elements constituting the compound coating layer formed on the surface of the positive electrode active material, M is an element containing a d orbit on the orbital structure of the atom, for example, Al, Sr, Zr, Co, Cr, Ga, Ge, Fe, etc. Among these, Al, Zr, Sr, Co, etc. which are easy to doping on a positive electrode active material are more preferable. Especially, Sr is especially preferable.

In addition, P, an element constituting the compound coating layer formed on the surface of the positive electrode active material, has a strong bonding force with oxygen. However, as long as it is a compound capable of bonding with an oxygen atom as shown in Formula 1, other elements may be possible, and all of them should be interpreted as being included in the scope of the present invention.

The amount of the coating layer in the positive electrode active material is preferably in the range of 0.1 to 10% by weight based on 100% by weight of the positive electrode active material. When less than 0.1% by weight, it is difficult to effectively achieve the structural stability of the positive electrode according to the increase of the Li intercalation potential, and when the amount exceeds 10% by weight, the amount of the positive electrode active material is relatively lowered, and thus the battery charge and discharge capacity is not preferable.

The coating layer may cover the entire surface of the positive electrode active material, but in some cases, only a part thereof may be coated. Preferably, the coating layer covers the whole surface of the positive electrode active material.

The coating layer preferably comprises a berlinite, cristobalite or tridymite phase, and preferably comprises a berlinite phase.

The coating layer may be in amorphous, crystalline or mixed form thereof.

As a method of manufacturing the positive electrode active material including the multicomponent coating layer of the present invention, a conventional coating method known in the art or a method of applying the same may be used. Preferred examples are as follows.

First, the coating solution may be prepared by dissolving the precursor compound and phosphorus precursor compound of M in the formula (1) in a solvent.

The precursor compounds are water-soluble or water-insoluble compounds each containing the above elements, but are not limited to, for example, alkoxides, nitrates, acetates, halides, hydroxides, oxides containing each element. , Carbonate, oxalate, sulfate, or a salt containing the above-mentioned elements include compounds capable of ionizing. In particular, strontium alkoxide, strontium nitrate, strontium hydroxide, strontium oxide, strontium acetate, strontium sulfate, strontium chloride, monododecyl phosphate, diammonium hydrogen phosphate, phosphoric acid and the like are preferable. In addition, a compound containing one or more of the above elements or a combination thereof may also be used.

As the solvent, an organic solvent such as water or alcohol may be used.

According to the experiments of the present inventors, it was confirmed that the acidity (pH) of the coating liquid affects the dissociation degree of the precursor compound, and as a result, the crystal structure of the coating layer affects the electrical properties of the positive electrode active material. Preferred acidity of the coating liquid is preferably pH 6-10, more preferably 7-8. Acidity control of a coating liquid can be performed, for example by adding ammonia water to a coating liquid.

Coating is performed by adding, mixing, and stirring a positive electrode active material, that is, a lithium intercalation compound, to the coating solution prepared as described above, and the coating process may use a general coating method commonly used in the art. For example, solvent evaporation, co-precipitation, precipitation, sol-gel method, post-sorption filter method, sputtering, CVD and the like can be used.

As such, the heat treatment temperature range of the positive electrode active material coated with the precursor of the multicomponent compound is preferably 100 to 700 ° C. The heat treatment time is preferably 1 to 20 hours, more preferably 2 to 5 hours.

The positive electrode for a lithium secondary battery including the positive electrode active material according to the present invention, as is known in the art, is configured to include a conductive agent together with a binder, preferably a binder, in the positive electrode active material.

The binder is added in an amount of 1 to 50% by weight based on the total weight of the mixture including the positive electrode active material, and examples thereof include polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, and hydroxy. Propylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various And copolymers. Preferably, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), or the like can be used.

The conductive agent is added in an amount of 1 to 50% by weight based on the total weight of the mixture including the positive electrode active material, and is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, natural graphite or Graphite such as artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

In some cases, in addition to the component may further include a filler for inhibiting the expansion of the positive electrode, the filler is not particularly limited as long as it is a fibrous material without causing chemical changes in the battery, for example, polyethylene, poly Olefinic polymers such as propylene; Fibrous materials, such as glass fiber and carbon fiber, can be used.

The positive electrode can be prepared by conventional methods known in the art. For example, after applying the positive electrode slurry prepared by mixing the positive electrode active material of the present invention with a binder to a current collector, the solvent or dispersion medium is removed by drying, and the active material is bound to the current collector and the active materials are bound to each other. It can manufacture.

The positive electrode current collector is generally made to a thickness of 1 to 500 μm. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric. Preferably aluminum sheets can be used.

The method of applying the slurry to the current collector is also not particularly limited. For example, it may be applied by a doctor blade, dipping, brushing, etc., the coating amount is not particularly limited, but the thickness of the active material layer formed after removing the solvent or the dispersion medium is usually 0.005 to 5 mm, preferably 0.05 to 2 mm range The amount to which it becomes is preferable. The method of removing the solvent or the dispersion medium is not particularly limited, but the solvent or the dispersion medium is volatilized as quickly as possible within the speed range in which stress concentration occurs and cracks occur in the active material layer or the active material layer does not peel off from the current collector. It is preferable to use a method of adjusting to remove.

In addition, the present invention provides a lithium secondary battery comprising the positive electrode, the negative electrode and the electrolyte, the lithium secondary battery includes a lithium metal secondary battery, lithium ion secondary battery, lithium polymer secondary battery or lithium ion polymer secondary battery, etc. .

Lithium secondary battery of the present invention can be prepared by putting a porous separator between the positive electrode and the negative electrode in a conventional manner known in the art to put the electrolyte.

The negative electrode is manufactured by coating and drying a negative electrode active material on a negative electrode current collector, and optionally, components as described above with respect to the positive electrode may be further included as necessary.

The negative electrode active material may be, for example, carbon such as hardly graphitized carbon or graphite carbon; Li _x Fe ₂ O ₃ (0 ≦ _x ≦ 1), Li _x WO ₂ (0 ≦ _x ≦ 1), Sn _x Me _1-x Me ' _y O _z (Me: Mn, Fe, Pb, Ge; Me' Metal complex oxides such as Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen, 0 <x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8); Lithium metal; Lithium alloys; Silicon-based alloys; Tin-based alloys; SnO, SnO ₂ , PbO, PbO ₂ , Pb ₂ O ₃ , Pb ₃ O ₄ , Sb ₂ O ₃ , Sb ₂ O ₄ , Sb ₂ O ₅ , GeO, GeO ₂ , Bi ₂ O ₃ , Bi ₂ O ₄ , Metal oxides such as Bi ₂ O ₅ ; Conductive polymers such as polyacetylene; Li-Co-Ni-based materials and the like can be used. Preferably carbon, lithium metal or alloys thereof can be used.

The electrolyte is a nonaqueous electrolyte containing a lithium salt and an electrolyte compound. As the nonaqueous electrolyte, a nonaqueous electrolyte, a solid electrolyte, an inorganic solid electrolyte, and the like are used.

As said non-aqueous electrolyte, N-methyl- 2-pyrrolidinone, a propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyl Low lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxorone, aceto Nitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative Aprotic organic solvents such as tetrahydrofuran derivatives, ethers, methyl pyroionate and ethyl propionate can be used.

Examples of the organic solid electrolytes include polyethylene derivatives, polyethylene oxide derivatives, polypropylene oxide derivatives, phosphate ester polymers, polyedgetion lysine, polyester sulfides, polyvinyl alcohols, polyvinylidene fluorides, Polymers containing ionic dissociating groups and the like can be used.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

The lithium salt is a good material to be dissolved in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphate triamide, etc. Nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N, N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxy ethanol, aluminum trichloride, etc. It may be. In some cases, in order to impart nonflammability, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics.

The separator is interposed between the cathode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The lithium secondary battery of the present invention is not limited in appearance, but may be a cylindrical, square, pouch type or coin type using a can.

Although the present invention will be described in more detail based on the following Examples and Experimental Examples, the Examples and Experimental Examples are for illustrating the present invention, and the scope of the present invention is not limited thereto.

Example 1 Preparation of Positive Electrode Active Material-1

Sr nitrate (1 g) and H ₃ PO ₄ (0.4 g) were dissolved in water (50 g) in a molar ratio of 1: 1, stirred with 100 g of LiCoO ₂ , dried at 130 ° C. for 6 hours, and then at 700 ° C. Heat treatment was performed for 5 hours. The temperature increase rate at the time of heat processing was 200 degreeC per hour. The pH of the solution before putting the positive electrode active material of LiCoO ₂ are shown four. SrHPO ₄ powder precipitated in this process forms a coating layer on the surface of the LiCoO ₂ powder. This powder was used to assemble a coin-shaped half cell. The positive electrode was composed of Super P carbon black as a conductive agent and 3 wt% of PVDF as a binder and 94 wt% of a positive electrode active material, respectively. This half cell was charged and discharged once at 0.1, 0.2, 0.5 and 1 C rates, respectively, at 3 V and 4.3 V sections, and then charged and discharged 50 times at 1 C rate. The results are shown in FIG.

Example 2: Preparation of Positive Electrode Active Material-2

Experiment was carried out in the same manner as in Example 1 except that the acidity of the coating solution was adjusted to pH 8 using ammonia water, the results are shown in FIG.

Example 3: Preparation of Positive Electrode Active Material-3

The experiment was conducted in the same manner as in Example 1 except that the acidity of the coating solution was adjusted to pH 11 using ammonia water, and the results are shown in FIG. 3.

Comparative Example 1

The experiment was conducted in the same manner as in Example 1, except that LiCoO _2, which had not been surface-coated, was used as the positive electrode active material, and the results were shown in FIG. 4.

Experimental Example 1: Structural Analysis (XRD) Experiment

In order to analyze the structure of the coating layer prepared according to the present invention, the precipitated coating solution was dried and then subjected to heat treatment at 700 ° C. as follows. As samples, coating precipitates of the conditions of Examples 1, 2, and 3 were used.

As a result of the structural analysis, it was confirmed that the structure of the coating layer was changed according to the acidity of the coating solution. It can be seen that the composition of the solution, in particular, the acidity is an important factor among the conditions for forming the coating layer, because the dissociation degree of the compound of each precursor is related to the acidity of the solution and the structure of the coating layer is changed due to the difference. Can be.

Experimental Example 2: Performance Evaluation of Lithium Secondary Battery

Performance evaluation of the lithium secondary battery using the positive electrode active material including the multicomponent coating layer prepared according to the present invention was performed as follows.

In order to measure the capacity of the battery manufactured using the positive electrode active material including the multicomponent coating layers prepared in Examples 1 to 3, respectively, experiments were performed as follows. As a control, a battery of Comparative Example 1 using an uncoated LiCoO ₂ positive electrode active material was used.

Each battery was charged and discharged once at 0.1 C, 0.2 C and 0.5 C at 3 V to 4.3 V, and then charged and discharged at 1 C 50 times. The results are shown in Table 1 below.

battery 0.1 C discharge capacity (mAh / g) 1 C initial capacity (mAh / g) Capacity after 1C 50 times of charge and charge (mAh / g) Example 1 152 138 122 Example 2 148 142 135 Example 3 150 142 128 Comparative Example 1 152 124 110

As shown in Table 1, the batteries of Examples 1 to 3 showed an initial charge and discharge capacity similar to that of Comparative Example 1, but at a discharge capacity after 50 cycles at 1 C, a significantly higher capacity and capacity retention rate were observed. Showed. This indicates that the structural stability of the electrode is improved by the coating layer compound on the positive electrode active material. In particular, the SrHPO ₄ coating layer shows a large difference in the initial capacity of 1 C and the capacity after 50 charge / discharge cycles according to the pH change. pH It can be seen that 8 shows the best life characteristics.

To confirm this, an X-ray structure analysis for each pH change of SrHPO ₄ constituting the coating layer was performed. The results are shown in FIG. As can be seen from the XRD of FIG. 5, it can be seen that the life characteristics are better when the Berlinite phase is present as the main phase (Examples 2 and 3). In addition, it can be seen that the positive electrode active material coated with SrHPO ₄ having a berninite structure is improved by 20% or more as compared with the positive electrode active material (Comparative Example 1) without a coating layer.

Therefore, it can be seen that the positive electrode active material coated part or all of the surface with the compound according to the present invention improves the structural stability of the electrode to improve the high rate charge and discharge characteristics.

As described above, the positive electrode active material of the present invention optimizes the conditions for forming a coating layer composed of MHPO ₄ compound on part or all of the surface of the existing positive electrode active material to improve the structural stability by adjusting the crystal structure to improve the lifespan characteristics according to high rate charge and discharge And a lithium secondary battery having improved cycle characteristics.

Claims (10)

 A cathode active material comprising a multicomponent coating layer represented by the following Chemical Formula 1 on part or all of the surface of the cathode active material capable of occluding and releasing lithium: MHPO ₄ (1) Wherein M is a metal and metalloid containing a d orbit. The cathode active material of claim 1, wherein the coating layer comprises a berlinite, cristobalite, or tridymite phase. The cathode active material according to claim 2, wherein the coating layer comprises a berlinite phase. The cathode active material according to claim 1, wherein M is Al, Zr, Sr or Co. The cathode active material according to claim 4, wherein M is Sr. The cathode active material according to claim 1, wherein the amount of the coating layer in the cathode active material is in a range of 0.1 to 10% by weight based on the weight of the cathode active material.  a) preparing a coating solution by dissolving a precursor of M and a phosphorus precursor compound in Formula 1 in a solvent; b) adding and stirring the positive electrode active material to the coating solution; And c) heat treating the coated cathode active material; Method for producing a positive electrode active material according to claim 1, which comprises. 8. The method of claim 7, wherein the acidity of the coating solution is adjusted to 6 to 10. A cathode for a lithium secondary battery, comprising the cathode active material according to claim 1. A lithium secondary battery comprising the positive electrode according to claim 9, a negative electrode comprising a negative electrode active material capable of occluding and releasing lithium, a separator, and an electrolyte.

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