WO2025014087A1 - Positive electrode and secondary battery including same - Google Patents

Abstract

The present invention provides a positive electrode and a secondary battery including same. The positive electrode comprises: a current collector; a first positive electrode active material layer formed on one surface of the current collector, and a second positive electrode active material layer formed on the other surface of the current collector, wherein the first positive electrode active material layer contains a lithium iron phosphate-based active material doped with zirconium (Zr), and the second positive electrode active material layer contains a lithium iron phosphate-based active material doped with sodium (Na).

Description

Anode and secondary battery containing the same

The present invention relates to a cathode and a secondary battery including the cathode.

As the electronics, communications, and space industries develop, the demand for secondary batteries as energy sources is rapidly increasing. In particular, as the importance of global eco-friendly policies is emphasized, the electric vehicle market is growing rapidly, and research and development on secondary batteries is actively being conducted domestically and internationally.

Among various secondary batteries, lithium secondary batteries with high discharge voltage and energy density are widely used as an energy source for various mobile devices and electronic products.

Lithium transition metal composite oxides are used as positive electrode active materials for lithium secondary batteries, and among these, lithium cobalt composite metal oxides such as LiCoO ₂ , which have high operating voltage and excellent capacity characteristics, are mainly used. However, LiCoO ₂ has low stability and is expensive, making it difficult to mass-produce lithium secondary batteries.

Accordingly, lithium manganese composite metal oxide, lithium iron phosphate, lithium nickel composite metal oxide, etc. have been developed as materials to replace LiCoO ₂ . Among these, lithium iron phosphate (LiFePO ₄ ) with an olivine structure has a high volume density of 3.6 g/cm ³ and a theoretical capacity of approximately 170 mAh/g.

However, lithium iron phosphate (LiFePO ₄ ) has low electrical conductivity, so when LiFePO ₄ is used as a cathode active material, the internal resistance of the battery increases, which causes a problem in that the discharge capacity and life characteristics are reduced.

In particular, lithium secondary batteries are recently being used as power sources for hybrid and electric vehicles, and lithium secondary batteries with improved discharge capacity and excellent life characteristics are required.

The present invention provides a cathode having improved discharge capacity and life characteristics and minimized voltage drop, and a secondary battery including the cathode.

A positive electrode according to the present invention includes a current collector, a first positive electrode active material layer formed on one surface of the current collector, and a second positive electrode active material layer formed on the other surface of the current collector, wherein the first positive electrode active material layer includes a lithium iron phosphate-based active material doped with a zirconium (Zr) element, and the second positive electrode active material layer includes a lithium iron phosphate-based active material doped with a sodium (Na) element.

In one embodiment of the present invention, the first positive electrode active material layer may include an aluminum (Al) element coated on the lithium iron phosphate-based active material.

In one embodiment of the present invention, each of the first positive electrode active material layer and the second positive electrode active material layer may include at least one binder coagulant having a number average molecular weight of 30,000 g/mol to 80,000 g/mol selected from the group consisting of polyacrylamide, polyurethane, and polyacrylonitrile.

A secondary battery according to the present invention includes a cathode, an anode, and a separator interposed between the cathode and the anode, wherein the cathode includes a current collector, a first cathode active material layer formed on one surface of the current collector, and a second cathode active material layer formed on the other surface of the current collector, wherein the first cathode active material layer includes a lithium iron phosphate-based active material doped with a zirconium (Zr) element, and the second cathode active material layer includes a lithium iron phosphate-based active material doped with a sodium (Na) element.

In one embodiment of the present invention, the cathode may include at least one of niobium-doped titanium-tin-oxide and niobium-doped titanium-oxide.

A positive electrode according to the present invention comprises a current collector, and a positive electrode active material layer including a lithium iron phosphate-based active material, wherein the current collector includes pores, and the total volume of the pores is 30% to 80% of the total volume of the current collector, and a flame retardant layer including a flame retardant is formed on at least one surface of the current collector.

In one embodiment of the present invention, the positive electrode active material layer may include a sodium (Na) element coated on the lithium iron phosphate-based active material.

In one embodiment of the present invention, the positive electrode active material layer may include at least one binder coagulant having a number average molecular weight of 30,000 g/mol to 80,000 g/mol selected from the group consisting of polyacrylamide, polyurethane, and polyacrylonitrile.

In one embodiment of the present invention, the flame retardant may include an aliphatic halogen compound.

A secondary battery according to the present invention comprises a cathode, an anode, and a separator interposed between the cathode and the anode, wherein the cathode comprises a current collector and a cathode active material layer including a lithium iron phosphate-based active material, the current collector comprises pores, and the total volume of the pores is 30% to 80% of the total volume of the current collector, and a flame retardant layer including a flame retardant is formed on at least one surface of the current collector.

The positive electrode according to the present invention includes a lithium iron phosphate-based active material doped with different elements on one side and the other side of the current collector, thereby improving discharge capacity and life characteristics and minimizing voltage enhancement.

The structural or functional descriptions of the embodiments disclosed in this specification or application are merely exemplified for the purpose of explaining embodiments according to the technical idea of the present invention, and the embodiments according to the technical idea of the present invention can be implemented in various forms other than the embodiments disclosed in this specification or application, and the technical idea of the present invention is not construed as being limited to the embodiments described in this specification or application.

A positive electrode according to the present invention includes a current collector, a first positive electrode active material layer formed on one surface of the current collector, and a second positive electrode active material layer formed on the other surface of the current collector, wherein the first positive electrode active material layer includes a lithium iron phosphate-based active material doped with a zirconium (Zr) element, and the second positive electrode active material layer includes a lithium iron phosphate-based active material doped with a sodium (Na) element.

The positive electrode according to the present invention includes a current collector. The current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the secondary battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. The thickness of the positive electrode current collector can be adjusted depending on the required product, and can be, for example, 10 ㎛ to 500 ㎛, 10 ㎛ to 300 ㎛, or 20 ㎛ to 300 ㎛.

The positive electrode according to the present invention is formed on the current collector and may include a coating layer including an associative urethane-based thickener and a binder.

The coating layer can improve the interfacial adhesion between the current collector and the first positive electrode active material layer, or between the current collector and the second positive electrode active material layer. The coating layer can include an associative urethane-based thickener to suppress the occurrence of pinholes on the electrode surface. The associative urethane-based thickener can strengthen the cohesion between positive electrode active materials.

The above associative urethane thickener may be referred to as a copolymer produced by synthesis between a polyalkylene glycol compound and a monomer or condensate referred to as an associative monomer of the alkyl, aryl or arylalkyl type consisting of, for example, a polyisocyanate and a hydrophobic terminal group.

The weight average molecular weight of the above-mentioned associative urethane-based thickener may be 200,000 g/mol to 500,000 g/mol, 250,000 g/mol to 500,000 g/mol, 250,000 g/mol to 450,000 g/mol, 300,000 g/mol to 450,000 g/mol, 350,000 g/mol to 450,000 g/mol, or 350,000 g/mol to 400,000 g/mol.

The coating layer can be prepared from a coating layer composition, and the associative urethane-based thickener can be included in an amount of 0.01 wt% to 3.00 wt%, 0.01 wt% to 1.00 wt%, or 0.1 wt% to 1.00 wt% based on the total weight of the coating layer composition.

The above coating layer may include a binder. The binder may include styrene butadiene rubber (SBR). The binder may further include one or more selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluororubber, styrene (styrene monomer: SM), butadiene (BD), and butyl acrylate (BA).

The positive electrode according to the present invention includes a first positive electrode active material layer formed on one surface of the current collector, and a second positive electrode active material layer formed on the other surface of the current collector, wherein the first positive electrode active material layer includes a lithium iron phosphate-based active material doped with a zirconium (Zr) element, and the second positive electrode active material layer includes a lithium iron phosphate-based active material doped with a sodium (Na) element.

The above lithium iron phosphate-based active material may be a compound represented by the following chemical formula 1.

[Chemical Formula 1]

Li _1+a Fe _1-x M _x (PO _4-b )Y _b

In the chemical formula 1, M is at least one element selected from the group consisting of Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, and Cu, Y is at least one element selected from the group consisting of F, S, and N, and a, b, and x are -0.5≤a≤0.5, 0≤b≤0.1, and 0≤x≤0.5.

The above lithium iron phosphate active material may be LiFePO ₄ .

In general, lithium iron phosphate-based active materials have superior stability compared to nickel cobalt-based metal oxide active materials. However, lithium iron phosphate-based active materials have a problem in that the internal resistance of secondary batteries increases when used as a cathode active material due to low electrical conductivity. In addition, they have problems in that the capacity is small, the cycle characteristics deteriorate rapidly when used for a long period of time, and the thermal stability deteriorates.

Accordingly, the cathode active material layer according to the present invention includes a lithium iron phosphate-based active material doped with a zirconium (Zr) element and a lithium iron phosphate-based active material doped with a sodium (Na) element on one surface and the other surface of the current collector, respectively. Due to the zirconium and sodium doping materials, the first cathode active material layer and the second cathode active material layer each generate voltage drop sections in different SOC sections, so that the resistance increase at the point of voltage drop can be improved, and as a result, the performance of the secondary battery can be enhanced.

The zirconium doped in the lithium iron phosphate-based active material improves the mechanical properties of the cathode active material, so that the phenomenon of the cathode active material being broken during the secondary battery manufacturing process or by external impact can be minimized.

The content of the zirconium relative to the total weight of the first positive electrode active material layer may be included as 500 ppm to 9,000 ppm, 500 ppm to 8,000 ppm, 500 ppm to 7,000 ppm, 500 ppm to 6,000 ppm, 500 ppm to 5,000 ppm, or 500 ppm to 3,000 ppm. When the above range is satisfied, the mechanical characteristics and electrical conductivity of the secondary battery can be improved, so that the swelling phenomenon can be reduced during long-term use of the secondary battery, and the life characteristics can be improved.

The above first positive electrode active material layer may include an aluminum (Al) element coated on the lithium iron phosphate-based active material. By forming the aluminum element on the lithium iron phosphate-based active material, the chemical stability of the surface of the lithium iron phosphate-based active material is improved, so that structural collapse of the lithium iron phosphate-based active material can be suppressed.

In addition, since the aluminum element is oxidized first compared to the metal included in the lithium iron phosphate-based active material, a side reaction between the lithium iron phosphate-based active material and the electrolyte can be suppressed, and thus an increase in resistance due to the movement of lithium ions can be prevented.

As a precursor for forming the aluminum element in the coating layer, at least one selected from the group consisting of Al ₂ O ₃ , Al(OH) ₃ , AlF ₃ , AlBr ₃ , AlPO ₄ , AlCl ₃ , Al(NO) ₃ , Al(H ₂ PO ₄ ) ₃ , and C ₂ H ₅ O ₄ Al can be used.

The sodium doped into the lithium iron phosphate-based active material improves the chemical stability of the surface of the lithium iron phosphate-based active material, so that structural collapse of the lithium iron phosphate-based active material can be suppressed.

The content of the sodium element relative to the total weight of the second positive electrode active material layer may be included as 500 ppm to 5,000 ppm, 500 ppm to 4,000 ppm, 500 ppm to 3,500 ppm, 500 ppm to 3,000 ppm, 600 ppm to 3,000 ppm, 700 ppm to 3,000 ppm, 800 ppm to 3,000 ppm, or 800 ppm to 2,000 ppm. When the above range is satisfied, the phenomenon of the resistance of the secondary battery increasing or the output decreasing due to the formation of an excessive coating layer can be suppressed.

As a precursor for forming the above sodium element, at least one selected from the group consisting of NaOH, Ba(OH) ₂ , Na ₂ CO ₃ , NaCl, CH ₃ COONa, Na ₂ SO ₄ , and NaNO ₂ can be used.

The first positive electrode active material layer, the second positive electrode active material layer, or the first and second positive electrode active material layers may include a structure in which a nitrogen (N) element is doped into the lithium iron phosphate-based active material. The nitrogen element may improve the electrical conductivity of the secondary battery, thereby minimizing the phenomenon in which the initial efficiency of the secondary battery is reduced.

The content of the nitrogen element relative to the total weight of the first and second positive electrode active material layers may be included as 300 ppm to 10,000 ppm, 500 ppm to 5,000 ppm, 500 ppm to 4,000 ppm, 500 ppm to 3,000 ppm, 500 ppm to 2,000 ppm, or 500 ppm to 1,500 ppm. When the above range is satisfied, the electrical conductivity of the secondary battery is improved, and the phenomenon of initial efficiency deterioration can be minimized.

The lithium iron phosphate-based active material included in the first positive electrode active material layer and the lithium iron phosphate-based active material included in the second positive electrode active material layer may include a conductive coating layer.

The conductive coating layer may include a carbon-based material. The carbon-based material may be at least one selected from the group consisting of carbon black, carbon fibers or metal fibers, metal powder, conductive whiskers, conductive metals, activated carbon, polyphenylene derivatives, natural graphite, artificial graphite, Super-P, acetylene black, Ketjen black, channel black, finace black, lamp black, summer black, Denka black, aluminum powder, nickel powder, zinc oxide, gallium titanate, and titanium oxide.

The thickness of the conductive coating layer may be 1 nm to 500 nm, 1 nm to 300 nm, 1 nm to 250 nm, 1 nm to 200 nm, 5 nm to 200 nm, 5 nm to 100 nm, or 5 nm to 50 nm. When the above range is satisfied, the movement of lithium ions is not impeded, and the electrical conductivity can be increased.

The content of carbon included in the conductive coating layer may be 1 wt% to 3 wt%, 1 wt% to 2.5 wt%, or 1 wt% to 2 wt% based on the total weight of the first and second positive electrode active material layers. When the above range is satisfied, the problem of increased resistance due to decreased electrical conductivity does not occur, and the phenomenon of the positive electrode active material being delaminated during the manufacture of the positive electrode can be minimized.

The average particle diameter (D ₅₀ ) of the lithium iron phosphate-based active material included in the first positive electrode active material layer and the second positive electrode active material layer may be 0.3 ㎛ to 10.0 ㎛, 0.3 ㎛ to 9.0 ㎛, or 0.6 ㎛ to 9.0 ㎛. The average particle diameter (D _{50 ) may be defined as a particle diameter at 50% of the particle diameter distribution of the positive electrode active material. In addition, the average particle diameter (D 50 )} may be measured using a laser diffraction method. When the above range is satisfied, uniform mixing with the conductive material and the binder is possible, and process efficiency may be increased.

The specific surface area (BET) of the lithium iron phosphate-based active material included in the first positive electrode active material layer and the second positive electrode active material layer may be 30 m ² /g or less, 20 m ² /g or less, or 5 m ² /g to 15 m ² /g. The specific surface area (BET) can be calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77 K) using BELSORP-mino II of BEL Japan. When the above range is satisfied, a decrease in adhesion to the positive electrode current collector may not occur.

The tap density of the lithium iron phosphate-based active material included in the first positive electrode active material layer and the second positive electrode active material layer is 0.5 g/cm ³ or more, 0.6 g/cm ³ or more, or 0.6 g/cm ³ to It can be 1.5 g/cm ³ . The tap density refers to the apparent density of the lithium iron phosphate-based active material powder, and can be measured using KYT-5000 from Seishin Co., Ltd. When the above range is satisfied, the packing density of the positive electrode is improved, the thickness of the positive electrode is improved to be thin, and the breakage phenomenon of the positive electrode active material can be improved.

Each of the first positive electrode active material layer and the second positive electrode active material layer may include a conductive material and a binder. The binder may improve the bonding strength between the lithium iron phosphate-based active material and the conductive material, and the bonding strength between the first and second positive electrode active material layers and the positive electrode current collector. The binder may be added in an amount of 1 to 30 parts by weight, 1 to 20 parts by weight, or 1 to 15 parts by weight based on 100 parts by weight of the lithium iron phosphate-based active material. The type of the binder is not particularly limited, but for example, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), fluororubber, styrene butadiene rubber (SBR, cellulose-based resin), etc. may be used.

The conductive agent can improve the conductivity of the lithium iron phosphate-based active material. The conductive agent can be added in an amount of 1 to 40 parts by weight, 1 to 20 parts by weight, or 1 to 10 parts by weight based on 100 parts by weight of the lithium iron phosphate-based active material. The type of the conductive agent is not particularly limited, but for example, Super-P, graphite, acetylene black, etc. can be used.

Each of the first positive electrode active material layer and the second positive electrode active material layer may include at least one binder coagulant having a number average molecular weight of 30,000 g/mol to 80,000 g/mol selected from the group consisting of polyacrylamide, polyurethane, and polyacrylonitrile. The binder coagulant may appropriately control the dispersibility of the binder, thereby suppressing a phenomenon in which the binder is concentratedly distributed on the current collector and electrical conductivity is lowered.

A secondary battery according to the present invention includes a cathode, an anode, and a separator interposed between the cathode and the anode, wherein the cathode includes a current collector, a first cathode active material layer formed on one surface of the current collector, and a second cathode active material layer formed on the other surface of the current collector, wherein the first cathode active material layer includes a lithium iron phosphate-based active material doped with a zirconium (Zr) element, and the second cathode active material layer includes a lithium iron phosphate-based active material doped with a sodium (Na) element.

The above positive electrode can be used in the same manner as the current collector, first and second positive electrode active material layers, etc. described in relation to the above positive electrode.

The above secondary battery may include, in addition to the positive electrode of the present invention, an anode and a separator. The anode may include an anode current collector and a negative electrode active material layer formed on the anode current collector.

The above negative electrode current collector is not particularly limited as long as it is conductive and does not cause a chemical change in the secondary battery, and for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the bonding strength of the negative electrode active material can be strengthened by forming fine unevenness on the surface, and can be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc.

The above negative active material layer may include a negative active material.

The above negative active material may be a carbonaceous material such as artificial graphite, natural graphite, graphitized carbon fiber, amorphous carbon, or high-crystalline carbon; or a metallic compound capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloy, Sn alloy, or Al alloy. Low-crystalline carbon may include soft carbon and hard carbon, and high-crystalline carbon may include natural graphite, kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesophase pitch-based carbon microspheres, mesophase pitches, and high-temperature calcined carbon such as petroleum or coal tar pitch derived cokes. The negative active material may also use a composite including a metallic compound and a carbonaceous material, as needed.

Preferably, the negative active material may include at least one of niobium-doped titanium-tin-oxide and niobium-doped titanium-oxide to suppress lithium dentride generation and improve the life characteristics of the secondary battery.

The above separator may be interposed between the cathode and the anode. The separator is configured to prevent an electrical short circuit between the cathode and the anode and to allow ion flow. The separator may include a porous polymer film or a porous non-woven fabric. The porous polymer film may be configured as a single layer or multiple layers including a polyolefin polymer such as an ethylene polymer, a propylene polymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, and an ethylene/methacrylate copolymer. The porous non-woven fabric may include high-melting-point glass fibers and polyethylene terephthalate fibers. However, the present invention is not limited thereto, and according to an embodiment, the separator may be a high-heat-resistant separator (CCS; Ceramic Coated Separator) including ceramic.

The above positive electrode, negative electrode, and separator can be manufactured into an electrode assembly by a winding, lamination, folding, or zigzag stacking process. The electrode assembly can be manufactured into a secondary battery according to the present invention by being provided with an electrolyte. The secondary battery can be any one of a cylindrical shape using a can, a square shape, a pouch shape, and a coin shape, but is not limited thereto.

The electrolyte may be a non-aqueous electrolyte. The electrolyte may include a lithium salt and an organic solvent. The organic solvent may include at least one of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate (MPC), dipropyl carbonate (DPC), vinylene carbonate (VC), dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone, propylene sulfide, and tetrahydrofuran.

Hereinafter, the present invention will be described in more detail based on examples and comparative examples. However, the following examples and comparative examples are only examples for describing the present invention in more detail, and the present invention is not limited to the following examples and comparative examples.

Example

Example 1

<Manufacturing of the first cathode active material>

A positive electrode slurry was prepared by adding ammonia water to a mixture of an aqueous solution containing iron sulfate (FeSO ₄ ㆍ7H ₂ O), phosphoric acid (H ₃ PO ₄ ), zirconia (ZrO ₂ ), and an antioxidant and a lithium aqueous solution (LiOHㆍH ₂ O) to obtain a pH of 6. Thereafter, the positive electrode slurry was introduced at a constant rate under the conditions of 400 ℃ and 270 bar in a continuous process supercritical reactor to prepare a LiFePO ₄ solution, and the LiFePO ₄ solution was washed and filtered to obtain Zr-doped LiFePO ₄ particles.

The above LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were mixed in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent to prepare a first cathode active material.

<Manufacturing of the second cathode active material>

A positive electrode slurry was prepared by adding ammonia water to a mixture of an aqueous solution containing iron sulfate (FeSO ₄ ㆍ7H ₂ O), phosphoric acid (H ₃ PO ₄ ), sodium nitrate (NaNo ₃ ), and an antioxidant and an aqueous lithium solution (LiOHㆍH ₂ O) to obtain a pH of 6. Thereafter, the positive electrode slurry was introduced at a constant rate under the conditions of 400 ℃ and 270 bar in a continuous process supercritical reactor to prepare a LiFePO ₄ solution, and the LiFePO ₄ solution was washed and filtered to obtain LiFePO ₄ particles doped with Na.

The above LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were mixed in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent to prepare a second positive electrode active material.

<Polar manufacturing>

The first positive electrode active material was applied to one surface of aluminum (Al) having a thickness of 15 ㎛, and the second positive electrode active material was applied to the other surface of the aluminum, followed by drying. Thereafter, the dried first positive electrode active material and the second positive electrode active material were rolled to manufacture a positive electrode having a first positive electrode active material layer and a second positive electrode active material layer formed on the aluminum.

<Secondary battery manufacturing>

An electrode assembly was manufactured by interposing a 25 ㎛ thick polyethylene separator between an anode including TiNb ₂ O ₇ (TNO) as an anode active material and the manufactured anode, and then the electrode assembly was placed in a case, and an electrolyte solution containing 1 M LiPF ₆ and 2.5 wt% vinyl carbonate mixed in a volume ratio of ethylene carbonate and methyl ethyl carbonate was injected into the case to manufacture a secondary battery.

Example 2

In the above Example 1, Al(OH) ₃ was mixed as a coating material into each of the Zr-doped _LiFePO4 particles and the Na-doped _LiFePO4 particles, and then heat-treated at 700°C for 5 hours to obtain Al-coated particles, respectively.

In the above Example 1, a secondary battery was manufactured by the same process as in the above Example 1, except that the Al-coated and Zr-doped LiFePO ₄ particles were used instead of the Zr-doped LiFePO ₄ particles, and the Al-coated and Na-doped LiFePO ₄ particles were used instead of the Na-doped LiFePO ₄ particles.

Example 3

A secondary battery was manufactured by the same process as in Example 1, except that instead of manufacturing the first positive electrode active material by mixing the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent, the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were manufactured by mixing the first positive electrode active material in a weight ratio of 96:2:2 in an N-methyl pyrrolidone solvent.

Example 4

A secondary battery was manufactured by the same process as in Example 2, except that instead of manufacturing the first positive electrode active material by mixing the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent, the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were manufactured by mixing the first positive electrode active material in a weight ratio of 96:2:2 in an N-methyl pyrrolidone solvent.

Example 5

A secondary battery was manufactured by the same process as in Example 1, except that instead of manufacturing the second positive electrode active material by mixing the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent, the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were manufactured by mixing the second positive electrode active material in a weight ratio of 96:2:2 in an N-methyl pyrrolidone solvent.

Example 6

A secondary battery was manufactured by the same process as in Example 2, except that instead of manufacturing the second positive electrode active material by mixing the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent, the LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were manufactured by mixing the second positive electrode active material in a weight ratio of 96:2:2 in an N-methyl pyrrolidone solvent.

Comparative Example 1

<Manufacturing of positive electrode active material>

A positive electrode slurry was prepared by adding ammonia water to a mixture of an aqueous solution containing iron sulfate (FeSO ₄ ㆍ7H ₂ O), phosphoric acid (H ₃ PO ₄ ), and an antioxidant and a lithium aqueous solution (LiOHㆍH ₂ O) to obtain a pH of 6. Thereafter, the positive electrode slurry was introduced at a constant rate under the conditions of 400 ℃ and 270 bar in a continuous process supercritical reactor to prepare a LiFePO ₄ solution, and the LiFePO ₄ solution was washed and filtered to obtain LiFePO ₄ particles.

The above LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were mixed in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent to prepare a cathode active material.

<Polar manufacturing>

The positive electrode active material was applied to both sides of 15 ㎛ thick aluminum (Al) and then dried. Afterwards, the dried positive electrode active material was rolled to manufacture a positive electrode having a positive electrode active material layer formed on the aluminum.

<Secondary battery manufacturing>

An electrode assembly was manufactured by interposing a 25 ㎛ thick polyethylene separator between a carbon-based negative electrode and the manufactured positive electrode, and then the electrode assembly was placed in a case, and an electrolyte solution containing 1 M LiPF ₆ and 2.5 wt% vinyl carbonate mixed in a volume ratio of ethylene carbonate and methyl ethyl carbonate was injected into the case to manufacture a secondary battery.

Comparative Example 2

<Manufacturing of the first cathode active material>

A positive electrode slurry was prepared by adding ammonia water to a mixture of an aqueous solution containing iron sulfate (FeSO ₄ ㆍ7H ₂ O), phosphoric acid (H ₃ PO ₄ ), zirconia (ZrO ₂ ), and an antioxidant and a lithium aqueous solution (LiOHㆍH ₂ O) to obtain a pH of 6. Thereafter, the positive electrode slurry was introduced at a constant rate under the conditions of 400 ℃ and 270 bar in a continuous process supercritical reactor to prepare a LiFePO ₄ solution, and the LiFePO ₄ solution was washed and filtered to obtain Zr-doped LiFePO ₄ particles.

The above LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were mixed in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent to prepare a first cathode active material.

<Manufacturing of the second cathode active material>

A positive electrode slurry was prepared by adding ammonia water to a mixture of an aqueous solution containing iron sulfate (FeSO ₄ ㆍ7H ₂ O), phosphoric acid (H ₃ PO ₄ ), and an antioxidant and a lithium aqueous solution (LiOHㆍH ₂ O) to obtain a pH of 6. Thereafter, the positive electrode slurry was introduced at a constant rate under the conditions of 400 ℃ and 270 bar in a continuous process supercritical reactor to prepare a LiFePO ₄ solution, and the LiFePO ₄ solution was washed and filtered to obtain LiFePO ₄ particles.

The above LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were mixed in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent to prepare a second positive electrode active material.

<Polar manufacturing>

The first positive electrode active material was applied to one surface of aluminum (Al) having a thickness of 15 ㎛, and the second positive electrode active material was applied to the other surface of the aluminum, followed by drying. Thereafter, the dried first positive electrode active material and the second positive electrode active material were rolled to manufacture a positive electrode having a first positive electrode active material layer and a second positive electrode active material layer formed on the aluminum.

<Secondary battery manufacturing>

An electrode assembly was manufactured by interposing a 25 ㎛ thick polyethylene separator between a carbon-based negative electrode and the manufactured positive electrode, and then the electrode assembly was placed in a case, and an electrolyte solution containing 1 M LiPF ₆ and 2.5 wt% vinyl carbonate mixed in a volume ratio of ethylene carbonate and methyl ethyl carbonate was injected into the case to manufacture a secondary battery.

Comparative Example 3

<Manufacturing of the first cathode active material>

A positive electrode slurry was prepared by adding ammonia water to a mixture of an aqueous solution containing iron sulfate (FeSO ₄ ㆍ7H ₂ O), phosphoric acid (H ₃ PO ₄ ), and an antioxidant and a lithium aqueous solution (LiOHㆍH ₂ O) to obtain a pH of 6. Thereafter, the positive electrode slurry was introduced at a constant rate under the conditions of 400 ℃ and 270 bar in a continuous process supercritical reactor to prepare a LiFePO ₄ solution, and the LiFePO ₄ solution was washed and filtered to obtain LiFePO ₄ particles.

The above LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were mixed in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent to prepare a first cathode active material.

<Manufacturing of the second cathode active material>

A positive electrode slurry was prepared by adding ammonia water to a mixture of an aqueous solution containing iron sulfate (FeSO ₄ ㆍ7H ₂ O), phosphoric acid (H ₃ PO ₄ ), sodium nitrate (NaNo ₃ ), and an antioxidant and an aqueous lithium solution (LiOHㆍH ₂ O) to obtain a pH of 6. Thereafter, the positive electrode slurry was introduced at a constant rate under the conditions of 400 ℃ and 270 bar in a continuous process supercritical reactor to prepare a LiFePO ₄ solution, and the LiFePO ₄ solution was washed and filtered to obtain LiFePO ₄ particles doped with Na.

The above LiFePO ₄ particles, Super-P, and polyvinylidene fluoride were mixed in a weight ratio of 95:2.5:2.5 in an N-methyl pyrrolidone solvent to prepare a second positive electrode active material.

<Polar manufacturing>

The first positive electrode active material was applied to one surface of aluminum (Al) having a thickness of 15 ㎛, and the second positive electrode active material was applied to the other surface of the aluminum, followed by drying. Thereafter, the dried first positive electrode active material and the second positive electrode active material were rolled to manufacture a positive electrode having a first positive electrode active material layer and a second positive electrode active material layer formed on the aluminum.

<Secondary battery manufacturing>

An electrode assembly was manufactured by interposing a 25 ㎛ thick polyethylene separator between a carbon-based negative electrode and the manufactured positive electrode, and then the electrode assembly was placed in a case, and an electrolyte solution containing 1 M LiPF ₆ and 2.5 wt% vinyl carbonate mixed in a volume ratio of ethylene carbonate and methyl ethyl carbonate was injected into the case to manufacture a secondary battery.

Experimental example

Experimental Example 1 - High-rate characteristic evaluation

Each of the secondary batteries manufactured in Examples 1 to 6 and Comparative Examples 1 to 3 was charged at a constant current of 0.1C rate in a voltage range of 2.5 V to 4.1 V versus lithium metal at room temperature, and the discharge capacity was obtained according to the increase in the current density during discharge, and the charge/discharge efficiency at each rate was calculated therefrom. The current densities during discharge were 0.1C, 0.2C, 0.5C, 1C, and 2C rates, respectively. The charge/discharge efficiency at 2C was calculated by Equation 1 below, and the results are shown in Table 1 below.

[Formula 1]

Charge/discharge efficiency at 2C (%) = [2C discharge capacity/0.1C charge capacity] Х100

Experimental Example 2 - Evaluation of Life Characteristics

Each of the secondary batteries manufactured in Examples 1 to 6 and Comparative Examples 1 to 3 was charged and discharged at a constant current of 1C rate in a voltage range of 2.5 V to 4.1 V versus lithium metal at room temperature, while measuring the capacity retention ratio and the discharge capacity at the 500th cycle. The capacity retention ratio at room temperature was calculated by Equation 2 below, and the results are shown in Table 1 below.

[Formula 2]

Capacity retention rate (%) = [discharge capacity at the 500th cycle/discharge capacity at the 1st cycle] Х100

Experimental Example 3 - Voltage Drop Evaluation

The voltage drop according to SOC was measured for each of the secondary batteries manufactured in Examples 1 to 6 and Comparative Examples 1 to 3 in a voltage range of 4.5 V to 2.5 V based on the cathode potential. The section where a rapid voltage drop occurred in the section of SOC 10% to SOC 90% was evaluated based on the following criteria, and the results are shown in Table 1 below.

- ○: The voltage drop is gradual throughout the entire section.

- △: The voltage drop occurs rapidly in one section.

- ×: Voltage drop occurs rapidly in two or more sections.

division Whether two types of positive electrode active materials are included Whether the first cathode active material is Zr doped Whether the second cathode active material is Na-doped In 2C
Charge/discharge efficiency (%) volume
Maintenance rate
(%) Discharge capacity at 500th cycle
(mAh/g) Voltage drop evaluation Example 1 ○ ○ ○ 95.2 88.6 132.1 ○ Example 2 ○ ○ ○ 97.3 91.2 135.7 ○ Example 3 ○ ○ ○ 95.3 87.5 133.2 ○ Example 4 ○ ○ ○ 97.3 90.8 136.1 ○ Example 5 ○ ○ ○ 94.9 85.7 131.3 ○ Example 6 ○ ○ ○ 97.2 90.9 133.2 ○ Comparative Example 1 × × × 82.5 80.2 119.1 × Comparative Example 2 ○ ○ × 92.3 82.1 120.4 △ Comparative Example 3 ○ × ○ 88.7 87.2 123.4 △

As can be confirmed in Table 1 above, Examples 1 to 6, which include a lithium iron phosphate-based active material doped with zirconium on one side of the positive electrode collector and a lithium iron phosphate-based active material doped with sodium on the other side of the positive electrode collector, showed minimized voltage enhancement, excellent high-rate characteristics, and excellent capacity retention according to an increase in discharge rate (C-rate) compared to Comparative Examples 1 to 3. In addition, it was confirmed that Examples 1 to 6 showed significantly improved life characteristics and discharge capacity compared to Comparative Examples 1 to 3. That is, it was confirmed that Examples 1 to 6 including the positive electrode according to the present invention showed minimized voltage enhancement, excellent high-rate characteristics, and improved life characteristics and discharge capacity.

The invention can be applied to a cathode and a secondary battery including the same, wherein the discharge capacity and life characteristics are improved and the voltage enhancement is minimized.

Claims (5)

The whole house; A first positive electrode active material layer formed on one side of the above-mentioned collector; and It includes a second positive electrode active material layer formed on the other side of the above-mentioned collector, The above first cathode active material layer includes a lithium iron phosphate-based active material doped with a zirconium (Zr) element, The second positive electrode active material layer is a positive electrode including a lithium iron phosphate-based active material doped with sodium (Na) element. In the first paragraph, A cathode, wherein the first cathode active material layer includes an aluminum (Al) element coated on the lithium iron phosphate-based active material. In the first paragraph, A positive electrode, wherein each of the first positive electrode active material layer and the second positive electrode active material layer includes at least one binder coagulant having a number average molecular weight of 30,000 g/mol to 80,000 g/mol selected from the group consisting of polyacrylamide, polyurethane, and polyacrylonitrile. A secondary battery comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, The above anode is, The whole house; A first positive electrode active material layer formed on one side of the above-mentioned collector; and It includes a second positive electrode active material layer formed on the other side of the above-mentioned collector, The above first cathode active material layer includes a lithium iron phosphate-based active material doped with a zirconium (zr) element, A secondary battery in which the second cathode active material layer includes a lithium iron phosphate-based active material doped with sodium (Na) element. In paragraph 4, A secondary battery, wherein the negative electrode includes a negative electrode active material layer including a negative electrode active material, and the negative electrode active material includes at least one of titanium-tin-oxide doped with niobium and titanium-oxide doped with niobium.